ANSYS Mechanical APDL Contact Technology Guide

ANSYS Mechanical APDL Contact Technology Guide

ANSYS, Inc. Southpointe 275 Technology Drive Canonsburg, PA 15317 [email protected] http://www.ansys.com (T) 724-746-3304 (F) 724-514-9494

Release 15.0 November 2013 ANSYS, Inc. is certified to ISO 9001:2008.

Copyright and Trademark Information © 2013 SAS IP, Inc. All rights reserved. Unauthorized use, distribution or duplication is prohibited. ANSYS, ANSYS Workbench, Ansoft, AUTODYN, EKM, Engineering Knowledge Manager, CFX, FLUENT, HFSS and any and all ANSYS, Inc. brand, product, service and feature names, logos and slogans are registered trademarks or trademarks of ANSYS, Inc. or its subsidiaries in the United States or other countries. ICEM CFD is a trademark used by ANSYS, Inc. under license. CFX is a trademark of Sony Corporation in Japan. All other brand, product, service and feature names or trademarks are the property of their respective owners.

Disclaimer Notice THIS ANSYS SOFTWARE PRODUCT AND PROGRAM DOCUMENTATION INCLUDE TRADE SECRETS AND ARE CONFIDENTIAL AND PROPRIETARY PRODUCTS OF ANSYS, INC., ITS SUBSIDIARIES, OR LICENSORS. The software products and documentation are furnished by ANSYS, Inc., its subsidiaries, or affiliates under a software license agreement that contains provisions concerning non-disclosure, copying, length and nature of use, compliance with exporting laws, warranties, disclaimers, limitations of liability, and remedies, and other provisions. The software products and documentation may be used, disclosed, transferred, or copied only in accordance with the terms and conditions of that software license agreement. ANSYS, Inc. is certified to ISO 9001:2008.

U.S. Government Rights For U.S. Government users, except as specifically granted by the ANSYS, Inc. software license agreement, the use, duplication, or disclosure by the United States Government is subject to restrictions stated in the ANSYS, Inc. software license agreement and FAR 12.212 (for non-DOD licenses).

Third-Party Software See the legal information in the product help files for the complete Legal Notice for ANSYS proprietary software and third-party software. If you are unable to access the Legal Notice, please contact ANSYS, Inc. Published in the U.S.A.

Table of Contents 1. Contact Overview .................................................................................................................................... 1 1.1. General Contact Classification ........................................................................................................... 1 1.2. Contact Capabilities .......................................................................................................................... 2 1.2.1. Surface-to-Surface Contact Elements ........................................................................................ 3 1.2.2. Node-to-Surface Contact Elements ........................................................................................... 4 1.2.3. 3-D Line-to-Line Contact .......................................................................................................... 4 1.2.4. Line-to-Surface Contact ............................................................................................................ 4 1.2.5. Node-to-Node Contact Elements .............................................................................................. 5 2. GUI Aids for Contact Analyses ................................................................................................................ 7 2.1. The Contact Manager ........................................................................................................................ 7 2.2. The Contact Wizard ........................................................................................................................... 8 2.3. Managing Contact Pairs .................................................................................................................... 9 3. Surface-to-Surface Contact ................................................................................................................... 11 3.1. Using Surface-to-Surface Contact Elements ..................................................................................... 11 3.2. Steps in a Contact Analysis .............................................................................................................. 11 3.3. Creating the Model Geometry and Mesh ......................................................................................... 12 3.4. Identifying Contact Pairs ................................................................................................................. 12 3.5. Designating Contact and Target Surfaces ........................................................................................ 13 3.5.1. Asymmetric Contact vs. Symmetric Contact ............................................................................ 14 3.5.1.1. Introduction .................................................................................................................. 14 3.5.1.2. Using KEYOPT(8) ............................................................................................................ 15 3.6. Defining the Target Surface ............................................................................................................. 15 3.6.1. Pilot Nodes ............................................................................................................................ 15 3.6.2. Primitives ............................................................................................................................... 15 3.6.3. Element Types and Real Constants .......................................................................................... 16 3.6.3.1. Defining Target Element Geometry ................................................................................ 16 3.6.4. Using Direct Generation to Create Rigid Target Elements ......................................................... 16 3.6.5. Using Meshing Tools to Create Rigid Target Elements .............................................................. 18 3.6.5.1. Some Modeling and Meshing Tips ................................................................................. 20 3.6.5.2. Verifying Nodal Number Ordering (Contact Direction) of Target Surface .......................... 21 3.7. Defining the Deformable Contact Surface ....................................................................................... 22 3.7.1. Element Type ......................................................................................................................... 22 3.7.2. Real Constants and Material Properties ................................................................................... 24 3.7.3. Generating Contact Elements ................................................................................................. 24 3.8. Geometry Correction for Contact and Target Surfaces ...................................................................... 25 3.8.1. Surface Smoothing ................................................................................................................ 25 3.8.2. Simplified Bolt Thread Modeling ............................................................................................. 27 3.9. Set the Real Constants and Element KEYOPTS .................................................................................. 29 3.9.1. Real Constants ....................................................................................................................... 30 3.9.1.1. Positive and Negative Real Constant Values .................................................................... 33 3.9.1.2. Defining Real Constants in Tabular Format ..................................................................... 34 3.9.1.3. Defining Real Constants via a User Subroutine ............................................................... 35 3.9.2. Element KEYOPTS ................................................................................................................... 35 3.9.3. Selecting a Contact Algorithm (KEYOPT(2)) ............................................................................. 38 3.9.3.1. Lagrange Multiplier Method Considerations ................................................................... 38 3.9.4. Determining Contact Stiffness and Allowable Penetration ....................................................... 39 3.9.4.1. Introduction .................................................................................................................. 39 3.9.4.2. Using FKN and FTOLN .................................................................................................... 39 3.9.4.3. Using FKT and SLTO ....................................................................................................... 41 3.9.4.4. Additional Considerations for Specifying Contact Stiffness ............................................. 41 Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

iii

Contact Technology Guide 3.9.4.5. Using KEYOPT(10) .......................................................................................................... 42 3.9.4.6. Using KEYOPT(6) ............................................................................................................ 43 3.9.4.7. Chattering Control Parameters ....................................................................................... 43 3.9.5. Choosing a Friction Model ...................................................................................................... 44 3.9.5.1. Introduction .................................................................................................................. 44 3.9.5.2. Coefficient of Friction .................................................................................................... 45 3.9.5.3. Using TAUMAX, FACT, DC, and COHE ............................................................................... 45 3.9.5.4. Static and Dynamic Friction Coefficients ......................................................................... 46 3.9.5.5. Forced Frictional Sliding Using Velocity Input ................................................................. 47 3.9.6. Selecting Location of Contact Detection ................................................................................. 49 3.9.6.1. Introduction .................................................................................................................. 49 3.9.6.2. Using KEYOPT(4) and TOLS ............................................................................................. 49 3.9.6.3. Using the Surface Projection Based Contact Method (KEYOPT(4) = 3) .............................. 51 3.9.7. Adjusting Initial Contact Conditions ........................................................................................ 52 3.9.7.1. Introduction .................................................................................................................. 52 3.9.7.2. Using PMIN, PMAX, CNOF, ICONT, STRM. KEYOPT(5), and KEYOPT(9) .................................. 53 3.9.8. Physically Moving Contact Nodes Towards the Target Surface .................................................. 59 3.9.9. Determining Contact Status and the Pinball Region ................................................................ 60 3.9.9.1. Introduction .................................................................................................................. 60 3.9.9.2. Using PINB .................................................................................................................... 61 3.9.10. Avoiding Spurious Contact in Self Contact Problems ............................................................. 61 3.9.11. Selecting Surface Interaction Models .................................................................................... 62 3.9.11.1. Introduction ................................................................................................................ 62 3.9.11.2. Using KEYOPT(12) ........................................................................................................ 63 3.9.11.3. Using FKOP .................................................................................................................. 64 3.9.11.4. Using TBND ................................................................................................................. 64 3.9.11.5. Bonded Contact for Shell-Shell Assemblies ................................................................... 65 3.9.12. Modeling Contact with Superelements ................................................................................. 65 3.9.12.1. Introduction ................................................................................................................ 66 3.9.12.2. Using KEYOPT(3) in a 2-D Analysis with Superelements ................................................. 66 3.9.13. Applying Contact Stabilization Damping .............................................................................. 66 3.9.13.1. Introduction ................................................................................................................ 67 3.9.13.2. Using FDMN, FDMT, and KEYOPT(15) ............................................................................ 67 3.9.13.3. Checking Results After Applying Contact Stabilization .................................................. 68 3.9.14. Accounting for Thickness Effect ............................................................................................ 69 3.9.14.1. Introduction ................................................................................................................ 69 3.9.14.2. Using KEYOPT(11) ........................................................................................................ 69 3.9.15. Using Time Step Control and Impact Constraints ................................................................... 70 3.9.15.1. Introduction ................................................................................................................ 70 3.9.15.2. Using KEYOPT(7) .......................................................................................................... 70 3.9.16. Using the Birth and Death Option ......................................................................................... 70 3.10. User-Defined Subroutines for Contact Interfacial Behaviors ............................................................ 71 3.10.1. Defining Your Own Real Constant (USERCNPROP) ................................................................. 72 3.10.2. Writing Your Own Friction Law (USERFRIC) ............................................................................ 75 3.10.3. Defining Your Own Contact Interaction (USERINTER) ............................................................. 77 3.11. Controlling the Motion of the Rigid Target Surface ......................................................................... 82 3.12. Applying Necessary Boundary Conditions to the Deformable Elements .......................................... 84 3.13. Applying Fluid Pressure-Penetration Loads .................................................................................... 84 3.13.1. Applying Fluid Penetration Pressure ..................................................................................... 85 3.13.2. Specifying Fluid Penetration Starting Points .......................................................................... 86 3.13.3. Specifying a Pressure-Penetration Criterion ........................................................................... 87 3.13.4. Specifying a Fluid Penetration Acting Time ........................................................................... 87

iv

Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

Contact Technology Guide 3.13.5. Redefining or Modifying the Pressure-Penetration Loads ....................................................... 89 3.13.6. Postprocessing Fluid Pressure-Penetration Loads .................................................................. 89 3.14. Defining Solution and Load Step Options ...................................................................................... 90 3.15. Solving the Problem ...................................................................................................................... 91 3.16. Reviewing the Results ................................................................................................................... 92 3.16.1. Points to Remember ............................................................................................................. 92 3.16.2. Reviewing Results in POST1 .................................................................................................. 92 3.16.3. Reviewing Results in POST26 ................................................................................................ 94 4. Node-to-Surface Contact ...................................................................................................................... 95 4.1. The Node-to-Surface Contact Element ............................................................................................. 95 4.2. Performing a Node-to-Surface Contact Analysis ............................................................................... 96 4.2.1. CONTA175 KEYOPTS ............................................................................................................... 96 4.2.1.1. KEYOPT(3) ..................................................................................................................... 97 4.2.1.2. KEYOPT(4) ..................................................................................................................... 97 4.2.2. CONTA175 Real Constants ...................................................................................................... 97 4.3. Using CONTA175 for Multiphysics Contact ....................................................................................... 97 5. 3-D Beam-to-Beam Contact .................................................................................................................. 99 5.1. The 3-D Line-to-Line Contact Element ............................................................................................. 99 5.2. Modeling Beam-to-Beam Contact ................................................................................................. 100 5.3. Performing a 3-D Beam-to-Beam Contact Analysis ......................................................................... 101 5.3.1. KEYOPTs and Real Constants ................................................................................................. 102 5.3.1.1. Real Constants R1, R2 ................................................................................................... 102 5.3.1.2. KEYOPT(3) ................................................................................................................... 103 5.3.1.3. Real Constant TOLS ...................................................................................................... 104 5.3.1.4. KEYOPT(4) ................................................................................................................... 104 6. Line-to-Surface Contact ...................................................................................................................... 105 6.1. The 3-D Line-to-Surface Contact Element ...................................................................................... 105 6.2. Performing a 3-D Line-to-Surface Contact Analysis ........................................................................ 106 6.2.1. KEYOPTs and Real Constants ................................................................................................. 106 6.2.1.1. KEYOPT(3) ................................................................................................................... 107 6.2.1.2. Accounting for Thickness Effect (CNOF and KEYOPT(11)) ............................................... 107 6.2.1.3. Real Constant TOLS ...................................................................................................... 107 6.2.1.4. KEYOPT(4) ................................................................................................................... 107 7. Multiphysics Contact ........................................................................................................................... 109 7.1. Modeling Thermal Contact ............................................................................................................ 109 7.1.1. Thermal Contact Behavior vs. Contact Status ......................................................................... 109 7.1.2. Free Thermal Surface ............................................................................................................ 110 7.1.3. Temperature on Target Surface ............................................................................................. 110 7.1.4. Modeling Conduction .......................................................................................................... 110 7.1.4.1. Using TCC .................................................................................................................... 110 7.1.4.2. Using the Quasi Solver Option ..................................................................................... 111 7.1.5. Modeling Convection ........................................................................................................... 111 7.1.6. Modeling Radiation .............................................................................................................. 112 7.1.6.1. Background ................................................................................................................. 112 7.1.6.2. Using SBCT and RDVF .................................................................................................. 112 7.1.7. Modeling Heat Generation Due to Friction ............................................................................ 113 7.1.7.1. Background ................................................................................................................. 113 7.1.7.2. Using FHTG and FWGT ................................................................................................. 113 7.1.8. Modeling External Heat Flux ................................................................................................. 114 7.2. Modeling Electric Contact ............................................................................................................. 114 7.2.1. Modeling Surface Interaction ................................................................................................ 114 7.2.1.1. Background ................................................................................................................. 114 Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

v

Contact Technology Guide 7.2.1.2. Using ECC .................................................................................................................... 115 7.2.2. Modeling Heat Generation Due to Electric Current ................................................................ 115 7.3. Modeling Magnetic Contact .......................................................................................................... 116 7.3.1. Using MCC ........................................................................................................................... 116 7.3.2. Modeling Perfect Magnetic Contact ...................................................................................... 117 8. Node-to-Node Contact ........................................................................................................................ 119 8.1. Node-to-Node Contact Elements ................................................................................................... 119 8.2. Performing a Node-to-Node Contact Analysis ................................................................................ 120 8.2.1. Creating Geometry and Meshing the Model .......................................................................... 120 8.2.2. Generating Contact Elements ............................................................................................... 120 8.2.2.1. Generating Contact Elements Automatically at Coincident Nodes ................................. 121 8.2.2.2. Generating Contact Elements Automatically at Offset Nodes ........................................ 121 8.2.2.3. Node Ordering ............................................................................................................ 121 8.2.3. Defining the Contact Normal ................................................................................................ 122 8.2.4. Defining the Initial Interference or Gap ................................................................................. 123 8.2.5. Selecting the Contact Algorithm ........................................................................................... 123 8.2.6. Applying Necessary Boundary Conditions ............................................................................. 123 8.2.7. Defining the Solution Options .............................................................................................. 124 8.2.8. Solving the Problem ............................................................................................................. 125 8.2.9. Reviewing the Results ........................................................................................................... 125 9. Multipoint Constraints and Assemblies .............................................................................................. 127 9.1. Modeling Solid-Solid and Shell-Shell Assemblies ........................................................................... 128 9.1.1. Choosing the Nodal Detection Method ................................................................................. 128 9.1.2. Controlling Behavior at the Contact Surface .......................................................................... 129 9.1.3. Controlling Degrees of Freedom Used in the MPC Constraint ................................................. 129 9.1.4. Using the MPC Approach for Multiphysics Applications ......................................................... 130 9.2. Modeling a Shell-Solid Assembly ................................................................................................... 130 9.3. Surface-Based Constraints ............................................................................................................. 134 9.3.1. Defining Surface-Based Constraints ...................................................................................... 136 9.3.2. Defining Influence Range (PINB) ........................................................................................... 137 9.3.3. Degrees of Freedom of Surface-Based Constraints ................................................................. 137 9.3.4. Specifying a Local Coordinate System ................................................................................... 138 9.3.5. Additional Guidelines for a Force-Distributed Constraint ....................................................... 139 9.3.6. Additional Guidelines for a Rigid Surface Constraint .............................................................. 140 9.3.7. Additional Guidelines for a Coupling Constraint .................................................................... 140 9.3.8. Modeling a Beam-Solid Assembly ......................................................................................... 141 9.4. Modeling Rigid Bodies .................................................................................................................. 142 9.4.1. Modeling Contact between Rigid Bodies .............................................................................. 142 9.5. Overconstraint Detection and Elimination ..................................................................................... 143 9.6. Restrictions and Recommendations for Internal MPC ..................................................................... 143 10. Dynamic Contact and Impact Modeling ........................................................................................... 145 10.1. Energy and Momentum Conserving Contact ............................................................................... 145 10.1.1. Energy Conservation .......................................................................................................... 146 10.1.2. Automatic Time Stepping ................................................................................................... 146 10.1.3. Penetration and Relative Velocity ........................................................................................ 146 10.1.4. Impact Between Rigid Bodies ............................................................................................. 147 11. Spot Welds ......................................................................................................................................... 149 11.1. Defining a Spot Weld Set ............................................................................................................. 150 11.1.1. Creating a Basic Spot Weld Set with SWGEN ........................................................................ 150 11.1.2. The Components of a Spot Weld ......................................................................................... 154 11.1.3. Adding Surfaces to a Basic Set ............................................................................................ 156 11.2. Listing and Deleting Spot Welds .................................................................................................. 159

vi


Contact Technology Guide 12. Debonding ........................................................................................................................................ 161 12.1. Including Debonding in a Contact Analysis .................................................................................. 161 12.1.1. Cohesive Zone Materials Used for Debonding ..................................................................... 162 12.1.1.1. Bilinear Material Behavior with Tractions and Separation Distances (TBOPT = CBDD) .... 162 12.1.1.2. Bilinear Material Behavior with Tractions and Critical Fracture Energies (TBOPT = CBDE) ..................................................................................................................................... 162 12.1.2. Debonding Modes ............................................................................................................. 163 12.1.3. Other Considerations for Debonding .................................................................................. 163 12.1.4. Postprocessing ................................................................................................................... 164 13. Contact Surface Wear ........................................................................................................................ 165 13.1. Including Wear in a Contact Analysis ............................................................................................ 165 13.1.1. Wear Models ...................................................................................................................... 165 13.1.1.1. Archard Wear Model .................................................................................................. 165 13.1.1.2. User-Defined Wear Model (USERWEAR) ...................................................................... 166 13.1.2. Geometry Update Due to Wear ........................................................................................... 167 13.1.3. Assumptions and Restrictions ............................................................................................. 168 13.1.4. Postprocessing ................................................................................................................... 168 A. Example 2-D Contact Analysis with Fluid Pressure-Penetration Loading .................................................. 171 A.1. Problem Description ..................................................................................................................... 171 A.2. Input File ...................................................................................................................................... 174 Index ........................................................................................................................................................ 179


vii

viii


List of Figures 2.1. Contact Manager Toolbar ........................................................................................................................ 7 2.2. Example of a Contact Wizard Dialog ........................................................................................................ 9 3.1. Localized Contact Zones ....................................................................................................................... 13 3.2. Geometric Entities and Their Corresponding Rigid Target Elements ........................................................ 18 3.3. A Single Circular Target Segment Created From Arc Line Segments ........................................................ 19 3.4. Meshing Patterns for Arbitrary Target Surfaces ....................................................................................... 20 3.5. Smoothing Convex Corner .................................................................................................................... 21 3.6. Correct Node Ordering .......................................................................................................................... 21 3.7. Contact Element Types .......................................................................................................................... 23 3.8. Specification of the Contact Surface's Outward Normal .......................................................................... 25 3.9. Geometry Correction Applied to Revolute and Spherical Surfaces .......................................................... 27 3.10. Bolt Thread Dimensions ...................................................................................................................... 28 3.11. Depth of the Underlying Element ........................................................................................................ 33 3.12. Sliding Contact Resistance .................................................................................................................. 46 3.13. Friction Decay ..................................................................................................................................... 47 3.14. Contact Detection Located at Gauss Point ........................................................................................... 49 3.15. Contact Detection Point Location at Nodal Point ................................................................................. 50 3.16. Node Slippage Using Nodal Integration KEYOPT(4) = 1 or 2 .................................................................. 50 3.17. Surface Projection Based Contact ........................................................................................................ 51 3.18. Contact Surface Adjustment With ICONT ............................................................................................. 54 3.19. Contact Surface Adjustment (PMIN, PMAX) .......................................................................................... 55 3.20. A Scenario in Which Initial Adjustment Will Fail .................................................................................... 56 3.21. Ignoring Initial Penetration, KEYOPT(9) = 1 ........................................................................................... 58 3.22. Components of True Penetration ......................................................................................................... 58 3.23. Ramping Initial Interference ................................................................................................................ 59 3.24. Effect of Moving Contact Nodes .......................................................................................................... 60 3.25. Auto Spurious Prevention ................................................................................................................... 62 3.26. Penalty-Based Shell-Shell Assembly ..................................................................................................... 65 3.27. Path Dependent Fluid Penetration Loading ......................................................................................... 84 3.28. Free End Points (2-D) and Free Open Edges (3-D) ................................................................................. 86 3.29. Fluid Penetration Acting Time (FPAT) Greater Than Substep ................................................................. 88 3.30. Fluid Penetration Acting Time (FPAT) Less Than Substep ...................................................................... 89 4.1. Node-to-Surface Contact Elements ........................................................................................................ 95 5.1. Line-to-Line Contact Elements .............................................................................................................. 99 5.2. Internal Contact (One Beam Sliding Inside Another) ............................................................................. 100 5.3. External Contact (Two Beams Roughly Parallel) .................................................................................... 100 5.4. External Contact (Two Beams Cross Each Other) .................................................................................. 101 5.5. Continuous Line Segments ................................................................................................................. 102 5.6. Equivalent Circular Cross Section ......................................................................................................... 103 6.1. Line-to-Surface Contact Elements ........................................................................................................ 105 6.2. Continuous Line Segments ................................................................................................................. 106 7.1. Target Temperature ............................................................................................................................. 110 8.1. Node-to-Node Contact Element .......................................................................................................... 119 8.2. Contact Between Two Concentric Pipes ............................................................................................... 121 8.3. Two Concentric Pipes, Normals Rotated Properly .................................................................................. 122 8.4. Example of Overconstrained Contact Problem ..................................................................................... 124 9.1. Example of Shell-Solid Assembly ......................................................................................................... 131 9.2. Shell-Solid Assembly (Original Mesh) ................................................................................................... 132 9.3. Shell-Solid Assembly with Shell-Solid Constraint Option ...................................................................... 133 9.4. Shell-Solid Constraint - No Intersection (use KEYOPT(5) = 4 or 5) ........................................................... 134 Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

ix

Contact Technology Guide 9.5. Rigid Surface Constraint ...................................................................................................................... 135 9.6. Force-Distributed Constraint ............................................................................................................... 135 9.7. Coupling Constraint ............................................................................................................................ 136 9.8. Slider Link ........................................................................................................................................... 139 9.9. Free Radial Expansion Under Torque Load ........................................................................................... 139 9.10. Defining Symmetry Conditions for Force-Distributed Constraints ....................................................... 140 9.11. Beam-Solid Assembly Defined by Rigid Surface Constraint ................................................................. 141 9.12. Beam-Solid Assembly Defined by Force-distributed Constraint ........................................................... 142 11.1. Example Spot Weld Configuration ..................................................................................................... 149 11.2. Default Projection Direction for Node 1 ............................................................................................. 151 11.3. Default Projection Direction for Nodes 1 and 2 ................................................................................... 152 11.4. User-specified Projection Direction, Node 1 ....................................................................................... 152 11.5. User-specified Projection Direction, Nodes 1 and 2 ............................................................................. 153 11.6. Search Radius for Spot Weld .............................................................................................................. 154 11.7. Nodes Included in Constraint Equations ............................................................................................ 155 11.8. Beam Element Created for Spot Weld ................................................................................................ 155 11.9. Surfaces Added to Basic Spot Weld Set .............................................................................................. 157 11.10. Node Reordering for Beam Elements ............................................................................................... 158 1. Diagram of Planar Seal Model ................................................................................................................ 171 2. Meshed Planar Seal Model ..................................................................................................................... 172 3. Fluid Pressure Loading on Planar Seal ..................................................................................................... 172 4. Intermediate Fluid Pressure Distribution ................................................................................................ 173 5. Final Fluid Pressure Distribution ............................................................................................................. 173 6. Time History of Fluid Pressure for Three Contact Elements ...................................................................... 174

x


List of Tables 1.1. Contact Support ..................................................................................................................................... 2 3.1. Summary of Real Constant Defaults in Different Environments ............................................................... 31 3.2. Real Constants and Corresponding Primary Variables for CONTA171-CONTA177 ..................................... 34 3.3. Summary of KEYOPT Defaults in Different Environments ........................................................................ 37


xi

xii


Chapter 1: Contact Overview Contact problems are highly nonlinear and require significant computer resources to solve. It is important that you understand the physics of the problem and take the time to set up your model to run as efficiently as possible. Contact problems present two significant difficulties. First, you generally do not know the regions of contact until you've run the problem. Depending on the loads, material, boundary conditions, and other factors, surfaces can come into and go out of contact with each other in a largely unpredictable and abrupt manner. Second, most contact problems need to account for friction. There are several friction laws and models to choose from, and all are nonlinear. Frictional response can be chaotic, making solution convergence difficult. In addition to these two difficulties, many contact problems must also address multi-field effects, such as the conductance of heat, electrical currents, and magnetic flux in the areas of contact. If you do not need to account for friction in your model, and the interaction between the bodies is always bonded, you may be able to use the internal multipoint constraint (MPC) feature (available for certain contact elements) to model various types of contact assemblies and surface-based constraints (see Multipoint Constraints and Assemblies (p. 127) for more information). Another alternative is to use constraint equations or coupled degrees of freedom instead of contact to model these situations (see Coupling and Constraint Equations in the Modeling and Meshing Guide for more information). The external constraint equations or coupling equations are only suitable for small strain applications. In addition to the implicit contact capabilities discussed in this guide, ANSYS also offers explicit contact capabilities with the ANSYS LS-DYNA explicit dynamics product. Explicit capabilities are ideally suited for short-duration contact-impact problems. For more information on the ANSYS LS-DYNA product and its contact capabilities, see the ANSYS LS-DYNA User's Guide.

1.1. General Contact Classification Contact problems fall into two general classes: rigid-to-flexible and flexible-to-flexible. In rigid-to-flexible contact problems, one or more of the contacting surfaces are treated as rigid (i.e., it has a much higher stiffness relative to the deformable body it contacts). In general, any time a soft material comes in contact with a hard material, the problem may be assumed to be rigid-to-flexible. Many metal forming problems fall into this category. The other class, flexible-to-flexible, is the more common type. In this case, both (or all) contacting bodies are deformable (i.e., have similar stiffnesses). An example of a flexible-to-flexible contact is bolted flanges.


1

Contact Overview

1.2. Contact Capabilities Five contact models are available: node-to-node, node-to-surface, surface-to-surface, line-to-line, and line-to-surface. Each type of model uses a different set of contact elements and is appropriate for specific types of problems as shown in the following table: Table 1.1: Contact Support NotetoNode

Nodeto- Surface

Surface-to-Surface

LinetoLine

Lineto- Surface

Contact Element No.

178

175

171, 172

173, 174

176

177

Target Element No.

--

169, 170

169

170

170

170

2-D

Y

Y

Y

--

--

--

3-D

Y

Y

--

Y

Y

Y

small

large

large

large

large

large

Sliding Cylindrical Gap

Y

Spherical Gap

Y

Pure Lagrange Multiplier

Y

Y

Y

Y

Y

Y

Augmented Lagrange Multiplier

Y

Y

Y

Y

Y

Y

Lagrange Multiplier on Normal and Penalty on Tangent

Y

Y

Y

Y

Y

Y

Internal Multipoint Constraint (MPC)

--

Y

Y

Y

Y

Y

Contact Stiffness

semiauto

semiauto

semiauto

semiauto

semiauto

semiauto

Auto-meshing Tools

EINTF

ESURF

ESURF

ESURF

ESURF

ESURF

Lower-Order

Y

Y

Y

Y

Y

Y

Higher-Order

--

Y (2-D only)

Y

Y

Y

Y

Rigid-Flexible

Y

Y

Y

Y

Y

Y

Flexible-Flexible

Y

Y

Y

Y

Y

Y

Thermal Contact

Y

Y

Y

Y

--

--

2


Contact Capabilities NotetoNode

Nodeto- Surface

Surface-to-Surface

LinetoLine

Lineto- Surface

Contact Element No.

178

175

171, 172

173, 174

176

177

Target Element No.

--

169, 170

169

170

170

170

Electric Contact

Y

Y

Y

Y

--

--

Magnetic Contact

--

Y

Y

Y

--

--

To model a contact problem, you first must identify the parts to be analyzed for their possible interaction. If one of the interactions is at a point, the corresponding component of your model is a node. If one of the interactions is at a surface, the corresponding component of your model is an element: either a beam, shell, or solid element. The finite element model recognizes possible contact pairs by the presence of specific contact elements. These contact elements are overlaid on the parts of the model that are being analyzed for interaction. The various contact elements, and procedures for using them, are described in this guide. An overview of the contact elements and their capabilities follows. For detailed information on any of these elements, refer to the Element Reference and the Mechanical APDL Theory Reference.

1.2.1. Surface-to-Surface Contact Elements Support is available for both rigid-to-flexible and flexible-to-flexible surface-to-surface contact elements. These contact elements use a "target surface" and a "contact surface" to form a contact pair. • The target surface is modeled with either TARGE169 or TARGE170 (for 2-D and 3-D, respectively). • The contact surface is modeled with elements CONTA171, CONTA172, CONTA173, and CONTA174. To create a contact pair, assign the same real constant number to both the target and contact elements. You can find more details on defining these elements and their shared real constant sets in Surface-toSurface Contact (p. 11). These surface-to-surface elements are well-suited for applications such as interference fit assembly contact or entry contact, forging, and deep-drawing problems. The surface-to-surface contact elements have several advantages over the node-to-node element CONTA175. These elements: • Support lower and higher order elements on the contact and target surfaces (in other words, cornernoded or midside-noded elements). • Provide better contact results needed for typical engineering purposes, such as normal pressure and friction stress contour plots. • Have no restrictions on the shape of the target surface. Surface discontinuities can be physical or due to mesh discretization. • Allow modeling of fluid pressure penetration loads. Using these elements for a rigid target surface, you can model straight and curved surfaces in 2-D and 3-D, often using simple geometric shapes such as circles, parabolas, spheres, cones, and cylinders. More Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

3

Contact Overview complex rigid forms or general deformable forms can be modeled using special preprocessing techniques (see Defining the Target Surface (p. 15) for more information). Surface-to-surface contact elements are not well-suited for point-to-point, point-to-surface, edge-tosurface, or 3-D line-to-line contact applications, such as pipe whip or snap-fit assemblies. You should use the node-to-surface, node-to-node, or line-to-line elements in these cases. You also can use surfaceto-surface contact elements for most contact regions and use a few node-to-surface contact elements near contact corners. The surface-to-surface contact elements only support general static and transient analyses, buckling, harmonic, modal or spectrum analyses, or substructure analyses.

1.2.2. Node-to-Surface Contact Elements CONTA175 is a node-to-surface contact element. It supports large sliding, large deformation, and different meshes between the contacting components. Contact occurs when the element penetrates one of the target segment elements (TARGE169, TARGE170) on a specified target surface. CONTA175 is typically used to model point-to-surface contact applications, such as the corners of snap-fit parts sliding along the mating surface. You can also use CONTA175 to model surface-to-surface contact, if the contacting surface is defined by a group of nodes and multiple elements are generated. The surfaces can be either rigid or deformable. An example of this type of contact problem is a wire inserted into a slot. Unlike the node-to-node contact elements, you do not need to know the exact location of the contacting area beforehand, nor do the contacting components need to have a compatible mesh. Large deformation and large relative sliding are allowed, although this capability can also model small sliding. CONTA175 does not support 3-D higher-order elements on the contact surface side. The element can fail if the target surface is severely discontinuous. No contour plots are available for contact results.

1.2.3. 3-D Line-to-Line Contact The line-to-line contact element, CONTA176, is typically used to model 3-D beam-to-beam contact (crossing beams or beams that are parallel to each other) or a pipe sliding inside another pipe. Some practical applications are woven fabric and tennis racquet strings. CONTA176 can be attached to 3-D beam or pipe elements and supports both low-order and higherorder elements on the contact surface. The target surface is modeled with 3-D line segments (TARGE170 straight line or parabolic line elements). This element supports large sliding and large displacement applications. For more information on how to use CONTA176, see 3-D Beam-to-Beam Contact (p. 99).

1.2.4. Line-to-Surface Contact The line-to-surface contact element, CONTA177, can be used to model a 3-D beam or shell edge contacting solid or shell elements. CONTA177 supports both low-order and higher-order elements on the contact surface. The target surface is modeled with 3-D target segment elements (TARGE170). This element is also suitable for large sliding and large displacement applications. For more information on how to use CONTA177, see Line-to-Surface Contact (p. 105).

4


Contact Capabilities

1.2.5. Node-to-Node Contact Elements Node-to-node contact elements, CONTA178, are typically used to model point-to-point contact applications. To use node-to-node contact elements, you need to know the location of contact beforehand. These types of contact problems usually involve small relative sliding between contacting surfaces (even in the case of geometric nonlinearities). An example of a node-to-node contact application is the traditional pipe whip model, where the contact point is always located between the pipe tip and the restraint. Node-to-node contact elements can also be used to solve a surface-to-surface problem if the nodes of the two surfaces line up, the relative sliding deformation is negligible, and deflections (rotations) of the two surfaces remain small. These are typically problems with faceted and simple geometry. An interference fit problem is an example of a surface-to-surface problem where the use of node-to-node contact may be sufficient. Another use of node-to-node contact elements is in extremely precise analysis of surface stresses, such as in turbine blade analysis. In addition to unidirectional contact behavior, CONTA178 offers a cylindrical gap option to model contact between two parallel pipes with small relative sliding. (For large sliding applications, see 3-D Line-toLine Contact (p. 4).) The two pipes can be adjacent to one another, or one pipe can be inside of another hollow pipe. Also, CONTA178 can model contact between two rigid spheres; this can be either two adjacent spheres or one sphere inside of a hollow sphere.


5

6


Chapter 2: GUI Aids for Contact Analyses Several GUI aids are available to help you create and manage contact pairs. The Contact Manager allows you to define, view, and edit contact pairs. It provides a convenient way to manage all contact pairs for your entire model. The Contact Wizard, which is accessed from the Contact Manager, leads you through the process of creating contact pairs. The following contact-related GUI topics are available: 2.1.The Contact Manager 2.2.The Contact Wizard 2.3. Managing Contact Pairs

2.1. The Contact Manager You can access the Contact Manager via the Contact Manager icon in the ANSYS Standard Toolbar, or via the menu path Preprocessor> Modeling> Create> Contact Pair. The Contact Manager is available at the Begin level and in the following processors: preprocessor (PREP7), solution (SOLU), and general postprocessor (POST1). The Contact Manager Toolbar provides an intuitive interface for the creation and management of contact pairs. The manager supports surface-to-surface contact analysis, node-to-surface contact analysis (using CONTA175), and the internal multipoint constraint (MPC) method of contact . Figure 2.1: Contact Manager Toolbar

• Contact Wizard - Accesses the Contact Wizard GUI described in The Contact Wizard (p. 8). Allows you to manually define target and contact surfaces. Supports both 2-D and 3-D geometries as well as rigid-flexible (with optional pilot node) or flexible-flexible contact. (Primitives are not supported by contact wizard.) The wizard also supports surface-based constraint contact pairs. • Contact Properties - Allows you to specify the properties of the contact pair(s) via real constants and KEYOPTs for the contact elements used. • Delete Contact Pairs - Deletes the contact pairs selected in the contact pair list. • Contact Selection Options - Specifies display of contact elements, target elements, or both. • Plot Elements/Results - Displays the elements of selected contact pairs; or displays the contact results, if available (POST1 only), specified in the Contact Results field for selected contact pairs. Results are displayed if Model Context is specified as "Result-"; otherwise, elements are displayed. The display can be limited to contact, or target, or both as specified in the Contact Selection Options field. (Note that CONTA175 results will not display graphically, but can be listed with the List Elements/Results icon.) Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

7

GUI Aids for Contact Analyses • Show Normals - Specifies whether or not to display the normals on the elements when plotting contact pairs • Flip Normals - Flips the element normals of the selected contact pair. This action is limited to the elements specified in the Contact Selection Options field. • Switch Contact and Target - Inverts the target surface and the contact surface with each other. This is applicable only to flexible-to-flexible surface-to-surface contact pairs. • List Elements/Results - Lists the elements of the selected contact pairs; or lists the contact results, if available (POST1 only), specified in the Contact Results field for selected contact pairs. Results are listed if Model Context is specified as "Result-"; otherwise, elements are listed. The listing is restricted to the elements specified in the Contact Selection Options field. • Model Context - Displays the contact pairs in the context of the entire model using a translucent plot, or shows only the contact pairs. If set to "Result-" (POST1 only), controls display/listing of contact pair results. • Check Contact Status - Provides contact status information for selected contact pairs. Several options are available (click and hold down the Check Contact Status button to access these options): – Display a detailed listing of status information for each contact pair. – Run a partial solution of the initial contact state (CNCHECK,POST command) that can be subsequently postprocessed from the contact manager. (See CNCHECK for more information.) – Physically move contact nodes to the target surface (CNCHECK,ADJUST command) in order to close a gap or reduce a penetration (see Physically Moving Contact Nodes Towards the Target Surface (p. 59).) – Reset target element and contact element key options and real constants to their default values (CNCHECK,RESET command). • Contact Results - Shows the contact result items for subsequent viewing using the Plot Elements/Results and List Elements/Results icons. The bar below the toolbar icons minimizes or maximizes the Contact Pair list box below it. The Contact Pair list box displays the defined contact pairs. You can select contact pairs from this list for displaying or editing purposes.

2.2. The Contact Wizard This Contact Wizard will lead you through the process of manually creating contact pairs. The wizard supports rigid-flexible (with optional pilot node) and flexible-flexible contact, and supports both surfaceto-surface and node-to-surface configurations. (Note that the wizard does not support rigid target primitives.) The Contact Wizard also supports surface-based constraint contact pairs. To use the Contact Wizard, access the Contact Manager Toolbar and click on the Contact Wizard icon. The wizard steps you through setting up the contact analysis. Below is an example of the screens you will see.

8


Managing Contact Pairs Figure 2.2: Example of a Contact Wizard Dialog

The Contact Wizard remains unavailable (dimmed) if you haven't meshed any portion of your model. If you wish to create a rigid-flexible model, mesh only those parts of the model which will be used as flexible contact surfaces (do not mesh the rigid target surfaces) before launching the wizard. If you wish to create a flexible-flexible model, mesh all parts of the model which will be used as contact surfaces (including target surfaces) before launching the wizard. You can specify target and contact surfaces using lines, areas, volumes, selected set of nodes, or node components. Note that the wizard allows you to choose more than one area for the target and contact surfaces, thus allowing multiple areas to form a single contact surface. If you specify a rigid target surface, you will then have the option to define a pilot node for that contact pair. (The pilot node step is mandatory if you define a surface-based constraint contact pair by picking the Pilot Node Only option.) After you specify the target and contact surfaces, you can specify properties of the contact pair (real constants and KEYOPTs) before creating the contact pair. When you finish specifying all the required data, click the Create button to create the contact and target element types, a unique real constant set with the real constant values, and the elements that make up the contact and target surfaces.

2.3. Managing Contact Pairs As discussed in Surface-to-Surface Contact (p. 11) it is paramount that the contact elements be oriented correctly for proper contact detection. The contact manager provides tools that help you 1. Verify that the normals of the contact and target surfaces are in the correct direction 2. Reverse normals of elements that are not oriented correctly You can choose to display one or more contact pairs on which to perform the above listed operations. Then you have an additional option of only displaying the contact surface or the target surface when verifying or reversing the element normals. In addition these elements can be displayed independently or in the context of your entire model. In the later case the contact elements are highlighted in a translucent plot of your model. Another important function is to edit the properties of the contact pair(s) as needed. The properties include real constant values and key option values as discussed earlier. The Contact Properties button in the contact manager provides a simple to use interface that allows the properties of the selected contact pair(s) to be reviewed and modified if needed.


9

GUI Aids for Contact Analyses Note that when you have multiple contact pairs, it is possible to have conflicts in the real constant values or in the KEYOPT settings. When you have such conflicts, the properties dialog for those real constants or KEYOPT settings is left blank. Finally, you can display or list specific contact result items (contact status, penetration, pressure, etc.) for the selected contact pairs. This option is only available in POST1, and only if a result set is available. The contact results can be displayed independently or in the context of your entire model. (For CONTA175, results can be listed but not displayed graphically.)

Note Prior to displaying or listing result items associated with the initial contact configuration resulting from the CNCHECK,POST command, you need to issue the appropriate FILE and SET,FIRST commands to read the results from the proper results file (see CNCHECK for details).

10


Chapter 3: Surface-to-Surface Contact You can use the surface-to-surface contact elements to model either rigid-flexible or flexible-flexible contact between surfaces. The Contact Manager provides an easy-to-use interface to help you construct and manage contact definitions. You can access the manager via the Contact Manager icon in the Standard Toolbar, or via the menu path Main Menu> Preprocessor> Modeling> Create> Contact Pair. See GUI Aids for Contact Analyses (p. 7) for more information on using the Contact Manager. This chapter explains how to set up a surface-to-surface contact analysis using both the command and GUI approaches. The following topics are available: 3.1. Using Surface-to-Surface Contact Elements 3.2. Steps in a Contact Analysis 3.3. Creating the Model Geometry and Mesh 3.4. Identifying Contact Pairs 3.5. Designating Contact and Target Surfaces 3.6. Defining the Target Surface 3.7. Defining the Deformable Contact Surface 3.8. Geometry Correction for Contact and Target Surfaces 3.9. Set the Real Constants and Element KEYOPTS 3.10. User-Defined Subroutines for Contact Interfacial Behaviors 3.11. Controlling the Motion of the Rigid Target Surface 3.12. Applying Necessary Boundary Conditions to the Deformable Elements 3.13. Applying Fluid Pressure-Penetration Loads 3.14. Defining Solution and Load Step Options 3.15. Solving the Problem 3.16. Reviewing the Results The procedures described in this chapter also apply to node-to-surface contact analyses using CONTA175, 3-D beam-to-beam contact analyses using CONTA176, and 3-D line-to-surface contact using CONTA177 unless otherwise noted.

3.1. Using Surface-to-Surface Contact Elements In problems involving contact between two boundaries, one of the boundaries is conventionally established as the "target" surface, and the other as the "contact" surface. For rigid-flexible contact, the target surface is always the rigid surface, and the contact surface is the deformable surface. For flexible-toflexible contact, both contact and target surfaces are associated with the deformable bodies. These two surfaces together comprise the "contact pair." Use TARGE169 with CONTA171, CONTA172, or CONTA175 to define a 2-D contact pair. For 3-D contact pairs, use TARGE170 with CONTA173, CONTA174, CONTA175, CONTA176, or CONTA177. Each contact pair is identified via the same real constant number.

3.2. Steps in a Contact Analysis Following are basic steps for performing a typical surface-to-surface contact analysis: 1. Create the model geometry and mesh


11

Surface-to-Surface Contact 2. Identify the contact pairs 3. Designate contact and target surfaces 4. Define the target surface 5. Define the contact surface 6. Set the element KEYOPTS and real constants 7. Define/control the motion of the target surface (rigid-to-flexible only) 8. Apply necessary boundary conditions 9. Apply fluid pressure-penetration loads 10. Define solution options and load steps 11. Solve the contact problem 12. Review the results Each contact-specific step also has a corresponding GUI approach where you use functions and features on the Contact Toolbar.

3.3. Creating the Model Geometry and Mesh First, create solid model entities that represent the geometry of the contacting bodies. Set element types, real constants, and material properties as you would for any analysis. Mesh the contacting bodies by meshing the areas or volumes with the element type that you have chosen. For more information, see the Modeling and Meshing Guide. Command(s): AMESH, VMESH GUI: Main Menu> Preprocessor> Meshing> Mesh

Note You should avoid midside-noded elements for 3-D contact surfaces when using node-tosurface element CONTA175.

3.4. Identifying Contact Pairs You must identify where contact might occur during the deformation of your model. Once you've identified potential contact surfaces, you define them via target and contact elements, which will then track the kinematics of the deformation process. Target and contact elements that make up a contact pair are associated with each other via a shared real constant set. The contact zone can be arbitrary; however, for the most efficient solution (primarily in CPU time), you may want to define smaller, localized contacting zones, but be sure your zones are adequate to capture all necessary contact. Different contact pairs must be defined by a different real constant set, even if the element real constant values do not change. There is no limit on the number of surfaces allowed.

12


Designating Contact and Target Surfaces Figure 3.1: Localized Contact Zones

Depending on the geometry of the model (and the potential deformation), multiple target surfaces could interact with the same zone of the contact surface. In such cases, you must define multiple contact pairs (using multiple overlapping contact elements), each with its own real constant number. See Figure 3.1: Localized Contact Zones (p. 13). The geometry-based contact pair generation tools (see the Contact Wizard) may create unnecessary contact and target elements which are never in contact during solution. For modeling small sliding contact or assembly contact, you can issue CNCHECK,TRIM to remove contact and target elements which are initially in far-field (that is, open and not near contact). The trimming is based on the contact pinball size; the smaller the pinball radius, the greater the number of contact and target elements that will be deleted. The contact trimming option is useful in Distributed ANSYS runs to achieve better performance.

3.5. Designating Contact and Target Surfaces Contact elements are constrained against penetrating the target surface. However, target elements can penetrate through the contact surface. For rigid-to-flexible contact, the designation is obvious: the target surface is always the rigid surface and the contact surface is always the deformable surface. For flexibleto-flexible contact, the choice of which surface is designated contact or target can cause a different amount of penetration and thus affect the solution accuracy. Consider the following guidelines when designating the surfaces: • If a convex surface is expected to come into contact with a flat or concave surface, the flat/concave surface should be the target surface. • If one surface has a fine surface mesh and, in comparison, the other has a coarse mesh, the fine mesh should be the contact surface and the coarse mesh should be the target surface. Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

13

Surface-to-Surface Contact • If one surface is stiffer then the other, the softer surface should be the contact surface and the stiffer surface should be the target surface. • If higher-order elements underly one of the external surfaces and lower-order elements underly the other surface, the surface with the underlying higher-order elements should be the contact surface and the other surface should be the target. However, for 3-D node-to-surface contact, the lower-order elements should be the contact surface. The higher-order elements should be the target surface. • If one surface is markedly larger than the other surface, such as in the instance where one surface surrounds the other surface, the larger surface should be the target surface. • In the case of 3-D internal beam-to-beam contact modeled by CONTA176 (a beam or pipe sliding inside another hollow beam or pipe), the inner beam should be considered the contact surface and the outer beam should be the target surface. However, when the inner beam is much stiffer than the outer beam, the inner beam can be the target surface. These guidelines are true for asymmetric contact; however, asymmetric contact may not perform satisfactorily for your model. The following section details the difference between asymmetric and symmetric contact and outlines some of the situations that require symmetric contact.

3.5.1. Asymmetric Contact vs. Symmetric Contact The following topics related to asymmetric contact vs. symmetric contact are available: 3.5.1.1. Introduction 3.5.1.2. Using KEYOPT(8)

3.5.1.1. Introduction Asymmetric contact is defined as having all contact elements on one surface and all target elements on the other surface. This is sometimes called "one-pass contact." This is usually the most efficient way to model surface-to-surface contact. However, under some circumstances asymmetric contact does not perform satisfactorily. In such cases, you can designate each surface to be both a target and a contact surface. You can then generate two sets of contact pairs between the contacting surfaces (or just one contact pair; for example, a self-contact case). This is known as symmetric contact (or "two-pass contact"). Obviously, symmetric contact is less efficient than asymmetric contact. However, many analyses will require its use (typically to reduce penetration). Specific situations that require symmetric contact include models where • The distinction between the contact and target surfaces is not clear. • Both surfaces have very coarse meshes. The symmetric contact algorithm enforces the contact constraint conditions at more surface locations than the asymmetric contact algorithm. If the meshes on both surfaces are identical and sufficiently refined, the symmetric contact algorithm may not significantly improve performance and may, in fact, be more "expensive" in CPU time. In such circumstances, pick one surface to be the target and the other the contact surface. For a symmetric contact definition, the program may find one side of a contact surface as closed and the other side of the surface as open. In this case, it can be difficult to interpret the results. The total contact pressure acting on both sides is the average of the contact pressures on each side of the surface.

14


Defining the Target Surface

3.5.1.2. Using KEYOPT(8) When there are several contact pairs involved in the model, and the graphical picking of contact and target surfaces is difficult, you can just define the symmetric contact pairs and, by setting KEYOPT(8) = 2, the program will internally select which asymmetric pair is to be used at the solution stage based on the guidelines mentioned above in Designating Contact and Target Surfaces.

Note In any contact model, you can mix different types of contact pairs: rigid-to-flexible or flexibleto-flexible contact; symmetric contact or asymmetric contact. However, only one type can exist with a contact pair.

3.6. Defining the Target Surface The target surface can be 2-D or 3-D and either rigid or deformable. For deformable target surfaces, you will normally use the ESURF command to generate the target elements along the boundaries of an existing mesh. You can follow the same method to generate the deformable contact surface (see Defining the Deformable Contact Surface (p. 22) for details). If you are modeling 3-D line-to-line contact and the underlying elements are a part of shell edges, issue ESURF,,,LINE to generate 3-D line or parabola segments along the shell edges. You should not use the following rigid target segments for a deformable target surface: ARC, CARC, CIRC, CYL1, CONE, SPHE, POINT, or PILO. For rigid target surfaces, the following provides general guidelines. In 2-D cases, the shape of the target surface is described by a sequence of straight lines, circular arcs, and parabolas, all of which can be represented with the target segment element TARGE169. You can use any combination to define the complex target surface geometry. In 3-D cases, the shape of the target surfaces is described by a sequence of triangles, quadrilaterals, straight lines, parabolas, cylinders, cones, and spheres, which can be represented with TARGE170. You can use any reasonable combination of low/high-order triangles and quadrilaterals to model a target surface with a complex, arbitrary geometry.

3.6.1. Pilot Nodes The rigid target surface can also be associated with a "pilot node," which is really an element with one node, whose motion governs the motion of the entire target surface. You can think of a pilot node as a handle for the rigid target surface. Forces/moments or rotations/displacements for the entire target surface usually should be prescribed on the pilot node. The pilot node can be one of the nodes on the target element or a node at any arbitrary location. The location of the pilot node is important only when rotation or moment loading is required.

3.6.2. Primitives You can use circle, cylinder, cone, and sphere primitives to model the target (which require real constants to define the radius). You can combine primitive segments with general segments (such as lines, parabolas, triangles, and quadrilaterals) to define a target surface. Primitives cannot be defined directly in the Contact Wizard. The primitives do not support MPC based bonded or no-separation contact.


15

Surface-to-Surface Contact

3.6.3. Element Types and Real Constants Before generating the target element, first define the element type (TARGE169 for 2-D or TARGE170 for 3-D): Command(s): ET GUI: Main Menu> Preprocessor> Element Type> Add/Edit/Delete

3.6.3.1. Defining Target Element Geometry You define characteristics of the target element geometry through real constants R1 and R2 as follows: • For CONTA171 and CONTA172: – R1 is the radius if the target shape (TARGE169) is a circle. – R2 is the element thickness if the underlying element is a superelement set as plane stress with thickness (KEYOPT(3) = 3). The default value is 1. • For CONTA173 and CONTA174 (also applies to node-to-surface element CONTA175, and to line-tosurface element CONTA177): – R1 is the radius if the target shape (TARGE170) is a cylinder, cone, or sphere. – R2 is the radius of a cone at the second node. • For CONTA176, used to model 3-D beam-to-beam contact: – R1 is the radius of circular beams on the target side (target radius). Use a positive value when modeling external beam-to-beam contact. Use a negative value to represent the inner radius of the outer beam (or pipe) when modeling internal beam-to-beam contact. – R2 is the radius of circular beams on the contact side (contact radius). Use a positive value for both external and internal beam-to-beam contact. To set the real constant number for the target elements: Command(s): REAL GUI: Main Menu> Preprocessor> Real Constants For TARGE169 and TARGE170, you need only set real constants R1 and R2 (if required). For a complete description of the target elements, element shapes, and real constants, see the description of TARGE169 and TARGE170 in the Element Reference.

Note Specifying real constants (R1, R2) manually is necessary only if you use direct generation to create your target elements, or if you model 3-D beam-to-beam contact with CONTA176. You can also use the meshing tools to create the elements, or use the Contact Manager Toolbar.

3.6.4. Using Direct Generation to Create Rigid Target Elements To generate target elements directly, use the following command or GUI path: Command(s): TSHAP

16


Defining the Target Surface GUI: Main Menu> Preprocessor> Modeling> Create> Elements> Elem Attributes You then specify the element shape. Possible shapes are: • Straight line (2-D and 3-D) • Parabola (2-D and 3-D) • Clockwise arc (2-D) • Counterclockwise arc (2-D) • Circle (2-D) • Three-node triangle (3-D) • Six-node triangle (3-D) • Four-node quadrilateral (3-D) • Eight-node quadrilateral (3-D) • Cylinder (3-D) • Cone (3-D) • Sphere (3-D) • Point (2-D or 3-D) • Pilot node (2-D and 3-D) Once you specify a target element shape, all subsequent elements will have that shape until you specify another shape.

Note You cannot mix 2-D and 3-D target elements on the same target surface. You cannot mix rigid target elements with deformable target elements on the same target surface. During solution, the program assigns a deformable status to target elements with underlying elements and assigns a rigid status to target elements without underlying elements. If a portion of the underlying elements of a deformable surface are deleted, an error will occur in solution. You can generate the nodes and elements using standard direct-generation techniques. For more information on direct generation modeling techniques, see Direct Generation in the Modeling and Meshing Guide. Command(s): N, E GUI: Main Menu> Preprocessor> Modeling> Create> Nodes Main Menu> Preprocessor> Modeling> Create> Elements You can then verify your element shapes by listing the elements. Command(s): ELIST GUI: Utility Menu> List> Elements> Nodes + Attributes Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

17


3.6.5. Using Meshing Tools to Create Rigid Target Elements You can allow the program generate the elements automatically using standard meshing capabilities. The program recognizes the proper target element shape based on the solid model and ignores the TSHAP setting. To generate a pilot node, use the following command or GUI path: Command(s): KMESH GUI: Main Menu> Preprocessor> Meshing> Mesh> Keypoints

Note KMESH always creates pilot nodes. To generate POINT segments, use the Direct Generation method or use ESURF,,,POINT command on selected nodes. To generate 2-D rigid target elements or 3-D rigid line/parabola segments, use the following command or GUI path. The program creates a single line over each line, parabolic segments over B-splines, and arc segments over each arc and line fillet (see Figure 3.2: Geometric Entities and Their Corresponding Rigid Target Elements (p. 18)). If all the arcs form a closed circle, the program creates a single circular segment (see Figure 3.3: A Single Circular Target Segment Created From Arc Line Segments (p. 19)). However, if the arcs that form a closed circle are created from imported or archived geometry (such as IGES), the program might not create a single circular segment. Command(s): LMESH GUI: Main Menu> Preprocessor> Meshing> Mesh> Lines Figure 3.2: Geometric Entities and Their Corresponding Rigid Target Elements

18


Defining the Target Surface Figure 3.3: A Single Circular Target Segment Created From Arc Line Segments

To generate 3-D rigid target elements, use the following command or GUI path. Command(s): AMESH GUI: Main Menu> Preprocessor> Meshing> Mesh> Areas If the surface segments on the solid model form a complete sphere, cylinder, or cone, then the program automatically generates a single primitive 3-D target element through the AMESH command. By creating fewer elements, the analysis becomes more computationally efficient. For arbitrary surfaces, you should use AMESH to generate target elements. In these cases, the quality of the meshed target shape is not important. It is more important that the target elements represent the rigid surface geometry well. We recommend using mapped meshing on all possible areas. If there is no curvature on the edges of the surface, assign one division on that edge. TARGE169 with a rigid specification will always mesh with one element division, per line, ignoring any LESIZE setting. The default target element shape is quadrilateral. If you want a triangular target element shape, use MSHAPE,1. Figure 3.4: Meshing Patterns for Arbitrary Target Surfaces (p. 20) shows the meshing patterns for arbitrary target surfaces. The following command or GUI path will generate a mapped mesh wherever possible (otherwise, if not possible, it will generate a free mesh). Command(s): MSHKEY,2 GUI: Main Menu> Preprocessor> Meshing> Mesh> Areas> Target Surf If the target surface is flat (or nearly flat), you may select low-order target elements (3-node triangular or 4-node quadrilateral elements). If the target surface is curved you should select high-order target elements (6-node triangular or 8-node quadrilateral). By doing so, set KEYOPT(1) = 1 in the target element definition.

Note Low-order target elements result in "cheaper" CPU usage in getting penetration and gap; however, the meshed surface may not be smooth. Higher-order target elements are more


19

Surface-to-Surface Contact "expensive" to use in getting the penetration and gap, but they need many fewer elements to discretize the whole curved target surface.

Note If target elements are created via program meshing (through the KMESH, LMESH, or ESURF commands) the TSHAP command is ignored and the program chooses the correct shape codes automatically. Figure 3.4: Meshing Patterns for Arbitrary Target Surfaces

3.6.5.1. Some Modeling and Meshing Tips A target surface can be made up of two or more disconnected regions. Where possible, you should localize the contact zone by defining multiple target surfaces (each with a different real constant number). There are no restrictions on the shape of the rigid surfaces. Smoothing is not required. However, you must ensure that the mesh discretization of the curved surfaces on the rigid target surface is adequate. Excessively coarse discretization can cause numerical convergence problems. It can be difficult to obtain a converged solution in a large sliding simulation if the target surface has sharp convex corners. To avoid such modeling problems, use line or area fillet functions on the solid model to smooth out the sharp corner, use a more refined mesh, or use high-order element in the region of abrupt curvature changes (see Figure 3.5: Smoothing Convex Corner (p. 21)).

20


Defining the Target Surface Figure 3.5: Smoothing Convex Corner

3.6.5.2. Verifying Nodal Number Ordering (Contact Direction) of Target Surface The node order of the target surface elements is critical because it defines contact direction. For 2-D contact, the associated (deformable) contact elements must lie to the right of the target surface when moving from the first node to the second node along the target surface line (see Figure 3.6: Correct Node Ordering (p. 21)). Figure 3.6: Correct Node Ordering

For 3-D contact, the target element numbering should be such that the rigid surface's outward normal points toward the contact surface. The outward normal is determined by the right-hand rule. For 3-D line segments, the target nodes must be numbered in a sequence that defines a continuous line. The line can be made up of linear or parabolic segments, depending on whether the underlying beam is made up of first order or second order elements. To check the direction of the normals, turn on the element coordinate systems. Command(s): /PSYMB,ESYS,1 GUI: Utility Menu> PlotCtrls> Symbols


21

Surface-to-Surface Contact If the element normals do not point toward the contact surface, select this element and reverse the direction of the surface normals. Command(s): ESURF,,REVE GUI: Main Menu> Preprocessor> Modeling> Create> Elements> Surf/Contact> Surf to Surf or, reorient the element normals: Command(s): ENORM GUI: Main Menu> Preprocessor> Modeling> Move/Modify> Elements> Shell Normals

Note Contact on target primitives (such as a complete circle, cylinder, cone, or sphere), can occur only on the outside surfaces of such target bodies.

3.7. Defining the Deformable Contact Surface To create the deformable contact surface, you must define that surface using one of the following contact elements: • CONTA171 or CONTA172 (for 2-D) • CONTA173 or CONTA174 (for 3-D) • CONTA175 (for 2- or 3-D) • CONTA176 or CONTA177 (for 3-D line) The contact surface is defined by the set of contact elements that comprise the surface of the deformable body. These contact elements have the same geometric characteristics as the underlying elements of the deformable body. The contact surface elements are of the same order as the underlying elements (lower- or higher-order), with compatible nodes along the edges. The higher-order contact elements can match lower-order underlying elements by dropping the midside nodes. The underlying elements can be solid, shell, beam, or pipe elements. The contact surface can be on either side of the shell or beam elements. The underlying elements may also be a superelement. However, axisymmetric harmonic elements may not be used as underlying elements. As with the target surface elements, you must define the contact surface element type, then select the correct real constant number (the real constant number must be the same as the one used for the target surface for each contact pair), and finally generate the elements.

3.7.1. Element Type The four surface-to-surface contact element types are shown in the figure below, followed by a brief description of each element. For complete information on these element types, see the Element Reference.

22


Defining the Deformable Contact Surface Figure 3.7: Contact Element Types

• CONTA171: This is a 2-D, 2-node, lower order line element that can be located on the surfaces of 2-D solid, shell, or beam elements (such as SHELL208). • CONTA172: This is a 2-D, 3-node, higher order parabolic element that can be located on the surfaces of 2-D solid or beam elements with midside nodes (such as PLANE183). • CONTA173: This is a 3-D, 4-node, lower order quadrilateral element that can be located on the surfaces of 3-D solid or shell elements (such as SHELL181). It can be degenerated to a 3-node triangular element. • CONTA174: This is a 3-D, 8-node, higher order quadrilateral element that can be located on the surfaces of 3-D solid or shell elements with midside nodes (such as SHELL281). It can be degenerated to 3- to 7node quadrilateral/triangular shapes. The following elements can also be defined using the method described here. • CONTA175: This is a 2- or 3-D, 1-node element that can be located on the surface of 2-D lower order and higher order solid or beam elements or 3-D lower order solid or shell elements. Use CONTA175 for nodeto-surface contact. • CONTA176: This is a 3-D line element that can be located on the surface of 3-D beam and pipe elements. The element can be a 2-node line or a 3-node parabola, depending on whether the underlying element is lower or higher order. Use CONTA176 to model 3-D beam-to-beam contact. • CONTA177: This is a 3-D line element that can be located on the surface of 3-D beam and pipe elements or on the edge of 3-D shell elements. The element can be a 2-node line or a 3-node parabola, depending on whether the underlying element is lower or higher order. Use CONTA177 to model 3-D line-to-surface contact. Use the following command to define the contact element type. Command(s): ET GUI: Main Menu> Preprocessor> Element Type> Add/Edit/Delete


23


3.7.2. Real Constants and Material Properties After defining the element type, you need to select the correct real constant set. The real constant set for each contact surface must be the same one used for the corresponding target surface for each contact pair. Each contact pair must reference its own real constant number. The program uses the material properties of the underlying elements to calculate an appropriate contact (or penalty) stiffness. The program automatically defines a default value for tangent (sliding) contact stiffness that is proportional to MU and the normal stiffness. If the underlying element is a superelement, the material property set for the contact elements must be the same as that of the original structural elements used during the formation of the superelement.

3.7.3. Generating Contact Elements You can generate contact elements either through direct generation or by generating the surface automatically from the exterior faces of the underlying elements. We recommend that you use automatic generation; this approach is simpler and more reliable. To automatically generate contact elements, follow these steps: 1. Select the nodes on the meshed deformable body. For each surface, view the node list. If you are certain that particular nodes will never come into contact, you can omit those nodes and reduce CPU time. However, you should always include more nodes than you think you'll need so that you don't miss unexpected areas of contact. Command(s): NSEL GUI: Utility Menu> Select> Entities 2. Generate the contact elements. Command(s): ESURF GUI: Main Menu> Preprocessor> Modeling> Create> Elements> Surf/Contact> Surf to Surf If the contact surface is attached to areas or volumes that are meshed with solid elements, the program automatically determines the outward normal needed for contact calculations. If the underlying elements are beam or shell elements, you must indicate which surface (top or bottom) is the target surface. Command(s): ESURF,,TOP or BOTTOM GUI: Main Menu> Preprocessor> Modeling> Create> Elements> Surf/Contact> Surf to Surf Use the TOP setting (default) to generate contact elements with their outward normals the same as the beam or shell elements' normals. Use the BOTTOM setting to generate contact elements with their outward normals opposite the beam or shell elements' normals. You must make sure that all elements in the beam or shell element selection have their normals consistently oriented. If the underlying elements are solid elements, then the TOP or BOTTOM setting has no effect. 3. Check the direction of the outward normals for the contact elements. The direction of the contact surface's outward normal is critical for proper contact detection. For 3-D surface-to-surface contact elements CONTA173 and CONTA174, the node numbering follows the right hand rule to define its outward normal. The contact surface's outward normal should point toward the target surface. Otherwise, the program may detect over-penetration of the surfaces at the beginning of the analysis and have difficulty finding an initial solution. In most of these cases, the analysis will fail immediately. For the 3-D line-to-line contact element CONTA176 and the 3-D line-to-surface contact element CONTA177, the contact nodes must be ordered in a sequence that defines a continuous line. Fig-

24


Geometry Correction for Contact and Target Surfaces ure 3.8: Specification of the Contact Surface's Outward Normal (p. 25) illustrates both proper and improper specification of the contact surface's outward normal. Command(s): /PSYMB,ESYS GUI: Utility Menu> PlotCtrls> Symbols Figure 3.8: Specification of the Contact Surface's Outward Normal

If the surface normals are specified incorrectly, you must either change them by reversing the node number order of the selected elements. Command(s): ESURF,,REVE GUI: Main Menu> Preprocessor> Modeling> Create> Elements> Surf/Contact> Surf to Surf Or, you can reorient the element normals. Command(s): ENORM GUI: Main Menu> Preprocessor> Modeling> Move/Modify> Elements> Shell Normals

Note ESURF,,TOP or BOTTOM and checking the direction of outward normals will not apply to node-to-surface contact (CONTA175) because it is a single-node element with no surface normal associated with it.

3.8. Geometry Correction for Contact and Target Surfaces Two types of geometry correction are available: 3.8.1. Surface Smoothing 3.8.2. Simplified Bolt Thread Modeling

3.8.1. Surface Smoothing In general, curved contact and target surfaces can be well approximated by linear (lower order) or quadratic (higher order) contact and target elements when the mesh is sufficiently refined. However, in certain circumstances this is not the case; for example, when linear elements are used or when the midside nodes of quadratic elements do not lie exactly on the initial curved geometry because a third party mesh generator was used. Thus, in some contact applications, using a faceted surface in place of the true curved geometry can significantly affect the accuracy of contact stresses. To avoid this problem, you can use an optional geometric correction for circular segments and for spherical and revolute (cylindrical) surfaces via SECTYPE and SECDATA section commands. Geometry


25

Surface-to-Surface Contact correction is available for surface-to-surface contact elements; TARGE169, TARGE170, CONTA171, CONTA172, CONTA173, and CONTA174. A circular correction can be applied to 2-D segments approximating a portion of a circular (or nearly circular) arc. Use the commands: SECTYPE,SECID,CONTACT,CIRCLE SECDATA,X0,Y0 ! circle center location in global Cartesian coordinates (XY plane)

A spherical correction can be applied to surfaces approximating a portion of a sphere (or nearly a sphere). Use the commands: SECTYPE,SECID,CONTACT,SPHERE SECDATA,X0,Y0,Z0 ! sphere center location in global Cartesian coordinates

A revolute (cylindrical) correction can be applied to surfaces approximating a portion of a revolution (or nearly a revolution), such as a cylinder or cone. Use the commands: SECTYPE,SECID,CONTACT,CYLINDER SECDATA,X1,Y1,Z1,X2,Y2,Z2 ! location of two ends of cylindrical axis in global Cartesian coordinates

The circular correction requires that you specify the circle's center location in global Cartesian coordinates. The spherical correction requires that you specify the sphere's center location in global Cartesian coordinates. The cylindrical correction requires that you input the axis of revolution in global Cartesian coordinates. The defined geometry correction can be applied to specific contact and target elements via a section ID. Use the SECNUM command prior to creating contact and target elements to associate the elements with a particular section ID, or use the EMODIF command to assign a section ID to existing elements. You can apply more than one geometry correction to the same contact or target surface. In the peg and slot example below, the peg is one contact surface. Two geometry corrections are applied to this contact surface: a revolute correction on the conical body and a spherical correction on the spherical tip. The cylindrical slot (target surface) requires only a revolute correction.

26


Geometry Correction for Contact and Target Surfaces Figure 3.9: Geometry Correction Applied to Revolute and Spherical Surfaces

spherical correction

revolute correction

Original model configuration

Finite element model showing results

This geometry correction technique assumes that the initial locations of the contact or target element corner nodes (but not necessarily the midside nodes of quadratic elements) lie on the true initial surface geometry. Geometry correction is most beneficial for small deflection analyses (small or large sliding).

3.8.2. Simplified Bolt Thread Modeling When simulating bolts, detailed modeling of the bolt threads may not be practical since it requires a refined mesh discretization on both the bolt and the bolt hole in order to accurately model the thread geometry and achieve solution convergence. This detailed mesh discretization can also be computationally expensive. For system level modeling where the primary bolt function is to transfer a load, the thread detail is sometimes not necessary. The bolt thread modeling technique available via the contact elements allows you to include the behavior of bolt threads without the added thread geometry and mesh discretization detail. Calculations are performed internally to approximate the behavior of the bolts. This feature offers simplified modeling with the near accuracy of a true threaded-bolt model. Bolt thread modeling is available for contact elements CONTA171, CONTA172, CONTA173, CONTA174, and CONTA175. This technique can be applied to a 3-D model or a 2-D axisymmetric model. The mesh discretization of the threads is not needed. Instead, you model smooth cylindrical surfaces on both the bolt and the bolt hole geometries, which helps to ensure good quality element meshes. The contact normal is computed internally based on the user-specified thread geometry data and the two end points of the


27

Surface-to-Surface Contact bolt axis. The following figure shows a bolt geometry with the primary parameters needed to define the thread. Figure 3.10: Bolt Thread Dimensions

Pitch

dm

dr d

(x1 , y 1,z 1)

(x 2, y 2,z 2 )

Thread Angle 2α Bolt Geometry

Hole Geometry

Use the section commands SECTYPE and SECDATA to input the required data: SECTYPE,SECID,CONTACT,BOLT SECDATA,Dm,P,ALPHA,N,X1,Y1,Z1,X2,Y2,Z2

where Dm = Mean pitch diameter, dm P = Pitch distance, p ALPHA = Half-thread angle, α N = Number of starts (defaults to 1) X1, Y1, Z1, X2, Y2, Z2 = Two end points of the bolt axis in global Cartesian coordinates Use the SECNUM command to associate the contact elements with a particular section ID, or use the EMODIF command to assign a section ID to existing contact elements. Assign the section ID only to the contact elements on the thread region of the bolt geometry. Either the bolt or bolt hole can be the contact surface. However, it is highly recommended that you apply the contact surface to the bolt and the target surface to the bolt hole. Symmetric contact should not be used for bolt thread modeling. If symmetric contact is defined, the program will assume asymmetric contact and will usually select the bolt surface as the contact surface. The value of the mean pitch diameter, dm, should be between the values of the major diameter (d) and the minor diameter (dr); see Figure 3.10: Bolt Thread Dimensions (p. 28) for details. If you only know the major diameter, you can input a rough estimation for the mean pitch diameter by using this equation: dm = d - 0.65p In order to achieve better accuracy, the mesh size over the thread region should be smaller than a quarter of the pitch distance, p. The half-thread angle, α, should be smaller than 90 degrees and larger than 0 degrees (α defaults to 30 degrees). The number of thread starts, n, defaults to 1 for a single28


Set the Real Constants and Element KEYOPTS threaded bolt; specify n = 2 for a double-threaded bolt. For a 3-D contact model, input a positive halfthread angle for the right-hand thread and input a negative angle for the left-hand thread. Either a general bolt-thread model or an unidirectional bolt-thread model can be defined. Input a positive mean pitch diameter dm for a general bolt thread model in which contact can occur on both side of the threads (the bolt is either in compression or in tension); input a negative mean pitch diameter dm for the unidirectional bolt-thread model. Specify X1, Y1, Z1 and X2, Y2, Z2 in Global Cartesian coordinates for the two end points of the axis of the bolt. For any 2-D axisymmetric model, X1 = Z1 = X2 = Z2 = 0 must be specified. The order of the two end points is not important for the general bolt thread model. However, it must be chosen appropriately for the unidirectional bolt thread model. The vector (from the first point to the second point) should be oriented to point from the tip of the bolt to the head of the bolt if the bolt is in tension. The bolt axis should be flipped (that is, from the head to the tip) if the bolt is in compression. Contact may not be engaged if an incorrect bolt axis direction is specified. You should always check the stresses in the bolt to make sure that the contact surface is correctly constrained if the unidirectional bolt thread model is defined. Keep the following points in mind when using this bolt thread modeling technique: • The implementation of bolt thread modeling is based on a small strain formulation. The solution accuracy may be affected when the bolt rotates by a relatively large angle with respect to its original position. • This technique is valid only for standard straight threads. It is not applicable for nonstandard threads (such as tapered threads and buttress threads). • The maximum stress in the thread region may vary with mesh density. However, the overall stress distribution remains similar in pattern. • Using Gauss point as the location of contact detection (KEYOPT(4) = 0) is not recommended unless the mesh is sufficiently refined. • Using nodal point, normal to target, as the location of contact detection (KEYOPT(4) = 2) is not recommended.

3.9. Set the Real Constants and Element KEYOPTS The program uses several real constants and KEYOPTs to control contact behavior using surface-tosurface contact elements. For more information in addition to what is presented here, refer to the individual contact element descriptions in the Element Reference. If you decide the real constant and KEYOPT settings you have specified for a particular contact pair are not appropriate, you can use the CNCHECK,RESET command to reset all values back to their default settings. Some real constants and key options are not affected by this command; see CNCHECK for details. In many cases, certain default settings may not be appropriate for your specific model. You can issue the CNCHECK,AUTO command to obtain optimized KEYOPT and real constant settings in terms of robustness and efficiency. Usually, only the undefined or default KEYOPT settings and real constants are changed. Refer to the CNCHECK command description for details of which settings are typically changed. You should always verify these changes by issuing the CNCHECK,DETAIL command to list current contact pair properties. If necessary, you can overwrite the optimized settings by redefining specific KEYOPTs (KEYOPT command) and real constants (RMODIF command). Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

29


3.9.1. Real Constants Two real constants, R1 and R2, are used to define the geometry of the target surface elements. The remaining are used by the contact surface elements. • R1 and R2 define the target element geometry. • FKN defines a normal contact stiffness factor. • FTOLN is a factor based on the thickness of the element which is used to calculate allowable penetration. • ICONT defines an initial closure factor (or adjustment band). • PINB defines a "pinball" region. • PMIN and PMAX define an allowable penetration range for initial penetration. • TAUMAX specifies the maximum contact friction. • CNOF specifies the positive or negative offset value applied to the contact surface. • FKOP specifies the stiffness factor applied when contact opens. • FKT specifies the tangent contact stiffness factor. • COHE specifies the cohesion sliding resistance. • TCC specifies the thermal contact conductance coefficient. • FHTG specifies the fraction of frictional dissipated energy converted into heat. • SBCT specifies the Stefan-Boltzmann constant. • RDVF specifies the radiation view factor. • FWGT specifies the weight factor for the distribution of heat between the contact and target surfaces for thermal contact or for electric contact. • ECC specifies the electric contact conductance or capacitance per unit area. • FHEG specifies the fraction of electric dissipated energy converted into heat. • FACT specifies the ratio of static to dynamic coefficients of friction. • DC specifies the decay coefficient for static/dynamic friction. • SLTO controls maximum sliding distance when MU is nonzero and the tangent contact stiffness (FKT) is updated at each iteration (KEYOPT(10) = 2) or when KEYOPT(2) = 3. • TNOP specifies the maximum allowable tensile contact pressure. • TOLS adds a small tolerance that extends the edge of the target surface. • MCC specifies the magnetic contact permeance (3-D only).

30


Set the Real Constants and Element KEYOPTS • PPCN specifies the pressure-penetration criterion (surface contact elements only). • FPAT specifies the fluid penetration acting time (surface contact elements only). • COR specifies the coefficient of restitution for impact between rigid bodies using impact constraints (KEYOPT(7) = 4). • STRM specifies load step number for ramping penetration. • FDMN specifies the stabilization damping factor in the normal direction. • FDMT specifies the stabilization damping factor in the tangential direction. • FDMD specifies the destabilizing squeal damping factor (3-D only). • FDMS specifies the stabilizing squeal damping factor (3-D only). • TBND specifies the critical bonding temperature. Real constant defaults can vary depending on the environment you are working in. The following table compares the program default values with those of the ANSYS Workbench product. See your ANSYS sales representative for more information about ANSYS Workbench. Table 3.1: Summary of Real Constant Defaults in Different Environments Real Constants

Description

ANSYS Mechanical APDL Default

ANSYS Workbench Default

No.

Name

1

R1

Radius associated with target geometry [5]

0

n/a

2

R2

Radius associated with target geometry

0

n/a

1 Superelement thickness [5] 3

FKN

Normal penalty stiffness factor

1

[1]

4

FTOLN

Penetration tolerance factor

0.1

0.1

5

ICONT

Initial contact closure

0

0

6

PINB

Pinball region

[2]

[2]

7

PMAX

Upper limit of initial penetration

0

0

8

PMIN

Lower limit of initial penetration

0

0

9

TAUMAX

Maximum friction stress

1.00E+20

1.00E+20

10

CNOF

Contact surface offset

0

0

11

FKOP

Contact opening stiffness

1

1

12

FKT

Tangent penalty stiffness factor

1

1

13

COHE

Contact cohesion

0

0

14

TCC

Thermal contact conductance

0

[3]

15

FHTG

Frictional heating factor

1

1


31

Surface-to-Surface Contact Real Constants

Description

ANSYS Mechanical APDL Default

ANSYS Workbench Default

No.

Name

16

SBCT

Stefan-Boltzmann constant

0

n/a

17

RDVF

Radiation view factor

1

n/a

18

FWGT

Heat distribution weighing factor

0.5

0.5

19

ECC

Electric contact conductance

0

[7]

20

FHEG

Joule dissipation weighting factor

1

n/a

21

FACT

Static/dynamic ratio

1

1

22

DC

Exponential decay coefficient

0

0

23

SLTO

Allowable elastic slip

1%

1%

24

TNOP

Maximum allowable tensile contact pressure

[6]

[6]

25

TOLS

Target edge extension factor

[4]

[4]

26

MCC

Magnetic contact permeance

0

n/a

27

PPCN

Pressure-penetration criterion

0

n/a

28

FPAT

Fluid penetration acting time

0.01

n/a

29

COR

Coefficient of restitution

1

1

30

STRM

Load step number for ramping penetration

1

1

31

FDMN

Normal stabilization damping factor

1

n/a

32

FDMT

Tangential stabilization damping factor

0.001

n/a

33

FDMD

Destabilizing squeal damping factor

1

n/a

34

FDMS

Stabilizing squeal damping factor

0

n/a

35

TBND

Critical bonding temperature

n/a

n/a

1. FKN = 10 for bonded. For all other, FKN = 1.0, but if bonded and other contact behavior exists, FKN = 1 for all. 2. Depends on contact behavior (rigid vs. flex target), NLGEOM,ON or OFF, KEYOPT(9) setting, KEYOPT(12) setting, and the value of CNOF (see Using PINB (p. 61)). 3. Calculated as a function of highest conductivity and overall model size. 4. 10% of target length for NLGEOM,OFF. 2% of target length for NLGEOM,ON. 5. R1 and R2 are used to define the target element geometry. See Defining Target Element Geometry (p. 16) and the target element descriptions (TARGE169 and TARGE170) for details on how they are used for different geometries. 6. TNOP defaults to the force convergence tolerance divided by contact area at contact nodes.

32


Set the Real Constants and Element KEYOPTS 7. Calculated as a function of lowest resistivity and overall model size. Command(s): R GUI: Main Menu> Preprocessor> Real Constants

3.9.1.1. Positive and Negative Real Constant Values For the real constants FKN, FTOLN, ICONT, PINB, PMAX, PMIN, FKOP, FKT, SLTO, and TNOP, you can specify either a positive or negative value. The program interprets a positive value as a scaling factor and interprets a negative value as the absolute value. The program uses the depth of the underlying element as the reference value to be used for ICONT, FTOLN, PINB, PMAX, and PMIN. For example, a positive value of 0.1 for ICONT indicates an initial closure factor of 0.1 x depth of the underlying element. However, a negative value of 0.1 indicates an actual adjustment band of 0.1 units. Contact related settings (ICONT, FTOLN, PINB, PMAX, PMIN, FKN, FKT, SLTO) are always averaged across all contact elements in a contact pair. Figure 3.11: Depth of the Underlying Element (p. 33) shows the depth of the underlying element for a solid element. If the underlying elements are shell or beam elements, the depth will usually be 4 times the element thickness. The final contact depth may also be adjusted based on the average contact length when the shape of the underlying element is relatively thin. Figure 3.11: Depth of the Underlying Element

Each contact pair has a pair-based depth which is obtained by averaging the depth of each contact element across all the contact elements in a contact pair. This can avoid the problem of very different element-based depths when there are meshes with large variations in element sizes.

Note When the contact pair depth is too small (for example, 10-5), the machine precision may not guarantee the accuracy of penetration to be calculated. You should scale the length unit in the model.


33


3.9.1.2. Defining Real Constants in Tabular Format For certain real constants, you can define the real constant as a function of primary variables by using tabular input. Table 3.2: Real Constants and Corresponding Primary Variables for CONTA171-CONTA177 (p. 34) lists real constants that allow tabular input and their associated primary variables. Use the *DIM command to dimension the table and identify the variables. Keep in mind that you should follow the positive and negative convention for the real constant values. The possible primary variables used in tabular input are: Time (TIME) X location (X) in local/global coordinates Y location (Y) in local/global coordinates Z location (Z) in local/global coordinates Temperature (TEMP degree of freedom) Contact pressure (PRESSURE); specify positive index values for compression, negative index values for tension Geometric contact gap/penetration (GAP); specify positive index values for penetration, negative index values for an open gap Note that the primary variables X, Y, and Z represent the coordinates of the contact detection points at the beginning of solution (undeformed configuration). Coordinate system applicability is determined by the *DIM command. When defining the tables, the primary variables must be in ascending order in the table indices (as in any table array). When defining real constants via the R, RMORE, or RMODIF command, enclose the table name in % signs (i.e., %tabname%). For example, given a table named CNREAL3 that contains normal contact stiffness (FKN) values, you would issue a command similar to the following: RMODIF,NSET,3,%CNREAL3%

! NSET is the real constant set ID associated with the contact pair

For more information and examples of using table inputs, see TABLE Type Array Parameters in the ANSYS Parametric Design Language Guide; Applying Loads Using TABLE Type Array Parameters in the Basic Analysis Guide; and the *DIM command in the Command Reference. Table 3.2: Real Constants and Corresponding Primary Variables for CONTA171-CONTA177 Real Constants

TIME

Location (X,Y,Z) [1]

TEMP

PRESSURE [2]

GAP [3]

%_CNPROP% [6]

Number

Name

3

FKN

x

x

x

x [4]

x

x

9

TAUMAX

x

x

x

x

x

x

10

CNOF

x

x

11

FKOP

x

x

x

x

x

12

FKT

x

x

x

x

x

x

14

TCC

x

x

x

x [5]

x

x

17

RDVF

x

x

x

x

x

34


Set the Real Constants and Element KEYOPTS Real Constants

Location (X,Y,Z) [1]

TIME

TEMP

19

ECC

x

x

x

26

MCC

x

x

x

27

PPCN

x

x

x

31

FDMN

x

x

32

FDMT

x

35

TBND

x

PRESSURE [2]

x [5]

GAP [3]

%_CNPROP% [6]

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

1. Coordinates of the contact detection points at the beginning of solution (undeformed configuration). 2. Contact normal pressure via the primary variable PRESSURE. For PRESSURE index values, specify positive values for compression; negative value for tension. 3. Geometric contact gap/penetration via the primary variable GAP. For GAP index values, specify positive values for penetration; negative values for an open gap. 4. Contact pressure at the end of the previous iteration, if it exists. 5. When the contact element type does not contain any structural degrees of freedom, the real constants TCC, ECC, and MCC should not be defined as a function of the contact pressure. 6. See Defining Real Constants via a User Subroutine (p. 35).

3.9.1.3. Defining Real Constants via a User Subroutine You can write a USERCNPROP.F subroutine to define your own real constants. The real constants that can be defined via this user subroutine are noted in the real constant table found in each contact element description. When defining real constants with the R, RMORE, or RMODIF command, you must use the reserved table name _CNPROP and enclose it in % signs (%_CNPROP%). When a value field on one of these commands is set to %_CNPROP%, the program internally calls the user-defined subroutine USERCNPROP.F. For example, to specify your own normal contact stiffness, FKN, you would issue a command similar to the following: RMODIF,NSET,3,%_CNPROP%


If several real constant values are defined via %_CNPROP% for a given contact pair, the subroutine will be called for each of these real constant entries. Therefore, you can use a single USERCNPROP subroutine to modify multiple real constant values. For details, see Defining Your Own Real Constant (USERCNPROP) (p. 72).

3.9.2. Element KEYOPTS Each contact element includes several KEYOPTS. We recommend using the default settings, which are suitable for most contact problems. For some specific applications, you can override the defaults. The element KEYOPTS allow you to control several aspects of contact behavior. • Degrees of freedom (KEYOPT(1))


35

Surface-to-Surface Contact • Contact algorithm (defaults to augmented Lagrangian) (KEYOPT(2)) • Unit control for normal contact stiffness (KEYOPT(3)) of surface-to-surface contact (see the note below for other meanings of KEYOPT(3)) • Location of contact detection point (KEYOPT(4)) • CNOF Automated adjustment (KEYOPT(5)) • Contact stiffness variation range (KEYOPT(6)) • Time step control and impact constraints (KEYOPT(7)) • Asymmetric contact selection (KEYOPT(8)) • Effect of initial penetration or gap (KEYOPT(9)) • Contact stiffness update (KEYOPT(10)) • Shell thickness effect (KEYOPT(11)) (not applicable to CONTA176 or CONTA177) • Behavior of contact surface (rough, bonded, etc.) (KEYOPT(12)) • Behavior of fluid penetration load (KEYOPT(14)) • Effect of stabilization damping (KEYOPT(15)) • Squeal damping controls (KEYOPT(16))

Note KEYOPT(3) has different meanings, depending on the element type and other circumstances: • For 2-D and 3-D surface-to-surface contact (CONTA171, CONTA172, CONTA173, CONTA174), KEYOPT(3) can be used to control the units of the absolute normal contact stiffness (that is, when a negative value is specified for FKN) under certain conditions. See Using FKN and FTOLN (p. 39). • For 2-D surface-to-surface contact (CONTA171, CONTA172), if superelements are present in the model, KEYOPT(3) specifies the stress state. See Using KEYOPT(3) in a 2-D Analysis with Superelements (p. 66). • For node-to-surface contact (CONTA175), line-to-line contact (CONTA176), and line-tosurface contact (CONTA177), KEYOPT(3) specifies the contact model (force-based or tractionbased). • For line-to-line contact (CONTA176), KEYOPT(3) also specifies the type of beam-to-beam contact.

KEYOPT defaults can vary depending on the environment you are working in. The following table compares the default values between the program, the Contact Wizard, and the ANSYS Workbench.

36


Set the Real Constants and Element KEYOPTS (See your ANSYS, Inc. sales representative for more information about ANSYS Workbench.) Rows that are highlighted in bold indicate different behaviors between the environments. Table 3.3: Summary of KEYOPT Defaults in Different Environments KEYOPT

Description

ANSYS Mechanical APDL

Contact Wizard

ANSYS Workbench Default Linear (bonded, no sep)

ANSYS Workbench, Default Nonlinear (standard, rough)

1

Selects DOF*

Manual

Auto

Auto

Auto

2

Contact Algorithm

Aug. Lagr.

Aug. Lagr.

Aug. Lagr.

Aug. Lagr.

3

Unit control for normal contact stiffness

No unit control

No unit control

n/a

n/a

4

Location of contact detection point

gauss

gauss

gauss

gauss

5

CNOF/ICONT adjustment

No adjust

No adjust

No adjust

No adjust

6

Contact stiffnes variation

Use default range

Use default range

Use default range

Use default range

7

Element level time increment control

No control

No control

No control

No control

8

Asymmetric contact selection

No action

No action

No action

No action

9

Effect of initial penetration Include or gap all

Include all

Exclude all

Include all

10

Contact stiffness update

Between Between Between iterload iteraations steps tions

Between iterations

11

Beam/shell thickness effect

Exclude

Exclude

Exclude

Exclude

12

Behavior of contact surface

Standard

Standard

Bonded

n/a

14

Behavior of fluid penetration load

Iterationbased

Iterationbased

n/a

n/a

15

Effect of stabilization damping

Active only in first load step

n/a

n/a

n/a

16

Squeal damping controls

Damping scaling factor

n/a

n/a

n/a

*Manual: Requires user to define. Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

37

Surface-to-Surface Contact Auto: Selection is based on DOF of underlying element. Command(s): KEYOPT, ET GUI: Main Menu> Preprocessor> Element Type> Add/Edit/Delete

3.9.3. Selecting a Contact Algorithm (KEYOPT(2)) For surface-to-surface contact elements, the program offers several different contact algorithms: • Penalty method (KEYOPT(2) = 1) • Augmented Lagrangian (default) (KEYOPT(2) = 0) • Lagrange multiplier on contact normal and penalty on tangent (KEYOPT(2) = 3) • Pure Lagrange multiplier on contact normal and tangent (KEYOPT(2) = 4) • Internal multipoint constraint (MPC) (KEYOPT(2) = 2) The penalty method uses a contact "spring" to establish a relationship between the two contact surfaces. The spring stiffness is called the contact stiffness. This method uses the following real constants: FKN and FKT for all values of KEYOPT(10), plus FTOLN and SLTO if KEYOPT(10) = 2. The augmented Lagrangian method (which is the default) is an iterative series of penalty methods. The contact tractions (pressure and frictional stresses) are augmented during equilibrium iterations so that the final penetration is smaller than the allowable tolerance (FTOLN). Compared to the penalty method, the augmented Lagrangian method usually leads to better conditioning and is less sensitive to the magnitude of the contact stiffness. However, in some analyses, the augmented Lagrangian method may require additional iterations, especially if the deformed mesh becomes too distorted. The pure Lagrange multiplier method enforces zero penetration when contact is closed and "zero slip" when sticking contact occurs. The pure Lagrange multiplier method does not require contact stiffness, FKN and FKT. Instead it requires chattering control parameters, FTOLN and TNOP. This method adds contact traction to the model as additional degrees of freedom and requires additional iterations to stabilize contact conditions. It often increases the computational cost compared to the augmented Lagrangian method. An alternative algorithm is the Lagrange multiplier method applied on the contact normal and the penalty method (tangential contact stiffness) on the frictional plane. This method enforces zero penetration and allows a small amount of slip for the sticking contact condition. It requires chattering control parameters, FTOLN and TNOP, as well as the maximum allowable elastic slip parameter SLTO. Another method, the internal multipoint constraint (MPC) algorithm, is used in conjunction with bonded contact (KEYOPT(12) = 5 or 6) and no separation contact (KEYOPT(12) = 4) to model several types of contact assemblies and kinematic constraints. See Multipoint Constraints and Assemblies (p. 127) for more information about how to use this feature.

3.9.3.1. Lagrange Multiplier Method Considerations The Lagrange multiplier methods (KEYOPT(2) = 3, 4) and MPC approach (KEYOPT(2) = 2) do not support the Gauss point detection option (KEYOPT(4) = 0) for surface-to-surface contact. They support the nodal detection options for surface-to-surface contact and node-to-surface contact. When using these options, be careful not to overconstrain the model. The model is overconstrained when a contact node has prescribed boundary conditions, CE and CP equations. The program usually detects and eliminates the overconstraints. However, there is no guarantee that the program will eliminate all the cases of

38


Set the Real Constants and Element KEYOPTS overconstraint. You should always verify your model carefully to address this issue. The Lagrange multiplier also introduces more degrees of freedom which may result in spurious modes for modal and linear eigenvalue buckling analyses. The augmented Lagrangian method would be a better choice for these analysis types. The Lagrange multiplier methods (KEYOPT(2) = 3, 4) introduce zero diagonal terms in the stiffness matrix. Any iterative solver (e.g., PCG) will encounter a preconditioning matrix singularity with these methods. Therefore, you should switch to sparse solver. If overconstraint occurs in bonded shell-shell assemblies when using the MPC algorithm, you can switch to the penalty method or the augmented Lagrangian method. See Bonded Contact for Shell-Shell Assemblies (p. 65) for more information. For 3-D higher order contact elements (CONTA174), the Lagrange multiplier method is applied at each contact node (including mid-side nodes), but the penalty method is applied on the center of the contact elements, even when KEYOPT(2)=3,4 is set.

3.9.4. Determining Contact Stiffness and Allowable Penetration The following topics related to determining contact stiffness and allowable penetration are available. 3.9.4.1. Introduction 3.9.4.2. Using FKN and FTOLN 3.9.4.3. Using FKT and SLTO 3.9.4.4. Additional Considerations for Specifying Contact Stiffness 3.9.4.5. Using KEYOPT(10) 3.9.4.6. Using KEYOPT(6) 3.9.4.7. Chattering Control Parameters

3.9.4.1. Introduction For the augmented Lagrangian method and penalty method, normal and tangential contact stiffnesses are required. The amount of penetration between contact and target surfaces depends on the normal stiffness. The amount of slip in sticking contact depends on the tangential stiffness. Higher stiffness values decrease the amount of penetration/slip, but can lead to ill-conditioning of the global stiffness matrix and to convergence difficulties. Lower stiffness values can lead to a certain amount of penetration/slip and produce an inaccurate solution. Ideally, you want a high enough stiffness that the penetration/slip is acceptably small, but a low enough stiffness that the problem will be well-behaved in terms of convergence. The program provides default values for contact stiffnesses (FKN, FKT), allowable penetration (FTOLN), and allowable slip (SLTO). In most cases, you do not need to define the contact stiffness. In addition, we recommend that you use KEYOPT(10) = 2 to allow the program to update the contact stiffness automatically.

3.9.4.2. Using FKN and FTOLN For certain contact problems, you may choose to use the real constant FKN to define a normal contact stiffness factor. The usual factor range is from 0.01 - 1.0, with a default of 1.0. The default value is appropriate for bulk deformation. If bending deformation dominates, we recommend using a smaller value (0.1). You can also define an absolute normal contact stiffness by specifying a negative value for FKN. In addition, FKN can be defined as a function of primary variables by using tabular input (i.e., %tabname%). The possible primary variables include time, temperature, initial contact detection point location Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

39

Surface-to-Surface Contact (at the beginning of solution), contact pressure of the previous iteration (positive PRESSURE index values indicate compression, negative PRESSURE index values indicate tension), and current geometric penetration (positive GAP index values indicate penetration, negative GAP index values indicate an open gap). For more information, see Defining Real Constants in Tabular Format (p. 34). The user subroutine USERCNPROP.F is also available for defining FKN. To use this subroutine, you must specify the table name %_CNPROP% as the real constant value. For more information, see Defining Real Constants via a User Subroutine (p. 35). If you input an absolute normal contact stiffness by specifying a negative value for FKN, you can control the units by using KEYOPT(3). By default, the units of the user-specified absolute normal contact stiffness is FORCE/LENGTH3. You can change the units to FORCE/LENGTH by setting KEYOPT(3) = 1. This KEYOPT(3) = 1 setting is valid only when a penalty-based algorithm is used (KEYOPT(2) = 0 or 1) and the absolute normal contact stiffness is explicitly specified. Using KEYOPT(3) = 1 is not recommended in conjunction with nodal detection options (KEYOPT(4) = 1 or 2) if any midside nodes exist for the CONTA174 3-D contact element. (KEYOPT(3) has different meanings in other situations. For more information, see the description of KEYOPT(3) in Element KEYOPTS (p. 35).)

Note The default contact normal stiffness is affected by defined material properties, element size, the user-defined penetration tolerance (FTOLN), and the total number of degrees of freedom in the model. Many factors may be applied to the actual contact normal stiffness during the solution. The default contact stiffness listed in the Contact Manager or by the CNCHECK command may be different from the actual contact stiffness reported by the ETABLE command. You should check the value reported by ETABLE to confirm that the appropriate contact normal stiffness is used. Use real constant FTOLN in conjunction with the augmented Lagrangian method. FTOLN is a tolerance factor to be applied in the direction of the surface normal. The range for this factor is less than 1.0 (usually less than 0.2), with a default of 0.1, and is based on the depth of the underlying solid, shell, or beam element (see Figure 3.11: Depth of the Underlying Element (p. 33)). This factor is used to determine if penetration compatibility is satisfied. Contact compatibility is satisfied if penetration is within an allowable tolerance (FTOLN times the depth of underlying elements). The depth is defined by the average depth of each individual contact element in the pair. If the program detects any penetration larger than this tolerance, the global solution is still considered unconverged, even though the residual forces and displacement increments have met convergence criteria. You can also define an absolute allowable penetration by specifying a negative value for FTOLN. In general, the default contact normal stiffness is inversely proportional to the final penetration tolerance; the tighter the tolerance, the higher the contact normal stiffness. When you use real constant FTOLN in conjunction with KEYOPT(10) = 2, the normal contact stiffness (used for the penalty method and the augmented Lagrange method) will be updated at each iteration

40


Set the Real Constants and Element KEYOPTS based on the current mean stress of the underlying elements and the allowable tolerance (FTOLN times the depth of the underlying elements). See also Using KEYOPT(10) (p. 42).

Note When the contact stiffness is too large (for example, 1016), the machine precision may not guarantee good conditioning of the global stiffness matrix. In this case, you should scale the force unit in the model if possible.

Note FTOLN is also used in the Lagrange multiplier methods (KEYOPT(2) = 3, 4) as a chattering control parameter.

3.9.4.3. Using FKT and SLTO The program automatically defines a default tangential contact stiffness that is proportional to MU and the normal stiffness FKN. The default tangential stiffness corresponds to a default value of FKT = 1.0. A positive value for FKT is a factor; a negative value indicates an absolute value of tangential stiffness. For KEYOPT(10) = 2, or when the Lagrange multiplier on normal and penalty on tangent option is used (KEYOPT(2) = 3), the program updates tangential contact stiffness based on current contact normal pressure, PRES, and maximum allowable elastic slip, SLTO (KT = FKT*MU* PRES/SLTO). The real constant SLTO is used to control maximum sliding distance when FKT is updated at each iteration. The program provides default tolerance values which work well in most cases. You can override the default values for SLTO (1% of average contact length in pair) by defining a scaling factor (positive value when using command input) or an absolute value (negative value when using command input). A larger value will enhance convergence but compromise accuracy. Based on the tolerance, current normal pressure, and friction coefficient, the tangential contact stiffness FKT can be obtained automatically. In certain cases users can override FKT by defining a scaling factor (positive input value) or absolute value (negative input value) (see Positive and Negative Real Constants for more information). In addition, FKT can be defined as a function of primary variables by using tabular input (i.e., %tabname%). The possible primary variables include time, temperature, initial contact detection point location (at the beginning of solution), current contact pressure (positive PRESSURE index values indicate compression, negative PRESSURE index values indicate tension), and current geometric penetration (positive GAP index values indicate penetration, negative GAP index values indicate an open gap). For more information, see Defining Real Constants in Tabular Format (p. 34). The user subroutine USERCNPROP.F is also available for defining FKT. To use this subroutine, you must specify the table name %_CNPROP% as the real constant value. For more information, see Defining Real Constants via a User Subroutine (p. 35).

3.9.4.4. Additional Considerations for Specifying Contact Stiffness FKN, FTOLN, FKT, and SLTO can be modified from one load step to another. They can also be adjusted in a restart run. Determining a good stiffness value may require some experimentation on your part. To arrive at a good stiffness value, you can try the following procedure as a "trial run": Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

41

Surface-to-Surface Contact 1.

Use a low value for the contact stiffness to start. In general, it is better to underestimate this value rather than overestimate it. Penetration problems resulting from a low stiffness are easier to fix than convergence difficulties that arise from a high stiffness.

2.

Run the analysis up to a fraction of the final load (just enough to get the contact fully established).

3.

Check the penetration and the number of equilibrium iterations used in each substep. If the global convergence difficulty is caused by too much penetration (rather than by residual forces and displacement increments), FKN may be underestimated or FTOLN may be too small. If the global convergence requires many equilibrium iterations for achieving convergence tolerances of residual forces and displacements rather than the resulting penetration, FKN or FKT may be overestimated.

4.

Adjust FKN, FKT, FTOLN, or SLTO as necessary and run the full analysis. If the penetration control becomes dominant in the global equilibrium iterations (that is, if more iterations were used to converge the problem to within the penetration tolerance than to converge the residual forces), you may increase FTOLN to permit more allowable penetration or increase FKN.

In addition, keep these points in mind when specifying contact stiffness: • For bonded contact and rough contact, the program uses MU = 1.0 to calculate tangential contact stiffness. The default tangential contact stiffness is proportional to the normal stiffness when the contact status is closed or open (near-field). When an absolute value of tangential stiffness is specified (a negative value input for FKT), the tangential stiffness remains unchanged regardless of the contact status. • Generally, the contact stiffness, FKN and FKT, has units of FORCE/LENGTH3. However, for contact force-based models the contact stiffness has units FORCE/LENGTH. This applies to CONTA175 with KEYOPT(3) = 0, CONTA176 with KEYOPT(3) = 0 or 1, and CONTA177 with KEYOPT(3) = 0. In addition, the absolute normal contact stiffness (i.e., a negative value input for FKN) can have units of FORCE/LENGTH under some circumstances; see Using FKN and FTOLN (p. 39) for more information.

3.9.4.5. Using KEYOPT(10) The normal and tangential contact stiffness can be updated during the course of an analysis, either automatically (due to large strain effects that change the underlying element's stiffness) or explicitly (by user-specified FKN or FKT values). KEYOPT(10) governs how the normal and tangential contact stiffness is updated when the augmented Lagrangian or penalty method is used. In most cases we recommend that you use KEYOPT(10) = 2 to allow the program to update contact stiffnesses automatically. The possible settings for KEYOPT(10) are: • KEYOPT(10) = 0: the contact stiffness will be updated at each load step if FKN or FKT is redefined by the user. Stiffness and other settings (ICONT, FTOLN, SLTO, PINB, PMAX, and PMIN) are averaged across contact elements in a contact pair. The default contact stiffness is determined by underlying element depth and material properties. • KEYOPT(10) = 2: the normal contact stiffness will be updated at each iteration based on the current mean stress of the underlying elements and the allowable penetration, FTOLN, except in the very first iteration. The default normal contact stiffness for the first iteration is the same as described for KEYOPT(10) = 0. If bisections occur in the beginning of the analysis, the normal contact stiffness will be reduced by a factor

42


Set the Real Constants and Element KEYOPTS of 0.2 for each bisection. The tangential contact stiffness will be updated at each iteration based on the current contact pressure, MU, and allowable slip (SLTO).

Note When a Lagrange multiplier method (KEYOPT(2) = 3, 4) or MPC algorithm (KEYOPT(2) = 2) is used, KEYOPT(10) is ignored.

3.9.4.6. Using KEYOPT(6) The default method of updating normal contact stiffness is suitable for most applications. However, the variational range of the contact stiffness may not be wide enough to handle certain contact situations. For example: • In the case of a very small penetration tolerance (FTOLN), a larger normal contact stiffness is often needed. • If convergence difficulties are encountered (due to nonlinear materials, localized contact, etc.), a downward adjustment to stiffness may be helpful. • To stabilize the initial contact condition and to prevent rigid body motion, a smaller normal contact stiffness is required. The allowed contact stiffness variation is intended to enhance stiffness updating when KEYOPT(10) = 2 by calculating an optimal allowable range in stiffness for use in the updating scheme. To increase the stiffness variational range, set KEYOPT(6) = 1 to make a nominal refinement to the allowable stiffness range, or KEYOPT(6) = 2 to make an aggressive refinement to the allowable stiffness range. Use of KEYOPT(6) is not recommended when contact convergence is relatively easy as it might produce an unnecessary drop in contact stiffness.

3.9.4.7. Chattering Control Parameters The Lagrange multiplier methods (KEYOPT(2) = 3, 4) do not require contact stiffness, FKN and FKT. Instead they require chattering control parameters, FTOLN and TNOP, by which the program assumes that the contact status remains unchanged. FTOLN is the maximum allowable penetration and TNOP is the maximum allowable tensile contact pressure.

Note A negative contact pressure occurs when the contact status is closed. A tensile contact pressure (positive) refers to a separation between the contact surfaces, but not necessarily an open contact status. However, the sign of the contact pressure is switched during postprocessing.

Note For contact force-based models, TNOP is the maximum allowable tensile contact force. This applies to CONTA175 with KEYOPT(3) = 0, CONTA176 with KEYOPT(3) = 0 or 1, and CONTA177 with KEYOPT(3) = 0. The behavior can be described as follows: Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

43

Surface-to-Surface Contact • If the contact status from the previous iteration is open and the current calculated penetration is smaller than FTOLN, then contact remains open. Otherwise the contact status switches to closed and another iteration is processed. • If the contact status from the previous iteration is closed and the current calculated contact pressure is positive but smaller than TNOP, then contact remains closed. If the tensile contact pressure is larger than TNOP, then the contact status changes from closed to open and the program continues to the next iteration. The program provides reasonable defaults for FTOLN and TNOP. FTOLN defaults to the displacement convergence tolerance. TNOP defaults to 10% of the force convergence tolerance divided by the contact area at contact nodes. When providing values for FTOLN and TNOP: • A positive value is a scaling factor applied to the default values. • A negative value is used as an absolute value (which overrides the default). The objective of FTOLN and TNOP is to provide stability to models which exhibit contact chattering due to changing contact status. If the values you use for these tolerances are too small, the solution will require more iterations. If the values are too large, however, the accuracy of the solution will be affected as a certain amount of penetration or tensile contact force is allowed.

3.9.5. Choosing a Friction Model The following topics related to choosing a friction model are available: 3.9.5.1. Introduction 3.9.5.2. Coefficient of Friction 3.9.5.3. Using TAUMAX, FACT, DC, and COHE 3.9.5.4. Static and Dynamic Friction Coefficients 3.9.5.5. Forced Frictional Sliding Using Velocity Input

3.9.5.1. Introduction In the basic Coulomb friction model, two contacting surfaces can carry shear stresses up to a certain magnitude across their interface before they start sliding relative to each other. This state is known as sticking. The Coulomb friction model defines an equivalent shear stress τ, at which sliding on the surface begins as a fraction of the contact pressure p (τ = µp + COHE, where µ is the friction coefficient and COHE specifies the cohesion sliding resistance). Once the shear stress is exceeded, the two surfaces will slide relative to each other. This state is known as sliding. The sticking/sliding calculations determine when a point transitions from sticking to sliding or vice versa. As an alternative to the program-supplied friction model, you can define your own friction model with the USERFRIC subroutine (see Writing Your Own Friction Law (USERFRIC) (p. 75)). Furthermore, you can define your own contact interaction behavior with the USERINTER subroutine (see Defining Your Own Contact Interaction (USERINTER) (p. 77)). For frictionless, rough, and bonded contact, the contact element stiffness matrices are symmetric. Contact problems involving friction produce unsymmetric stiffnesses. Using an unsymmetric solver is more computationally expensive than a symmetric solver for each iteration. For this reason, the program uses a symmetrization algorithm by which most frictional contact problems can be solved using solvers for symmetric systems. If frictional stresses have a substantial influence on the overall displacement field and the magnitude of the frictional stresses is highly solution dependent, the symmetric approx44


Set the Real Constants and Element KEYOPTS imation to the stiffness matrix may provide a low rate of convergence. In such cases, choose the unsymmetric solution option (NROPT,UNSYM) to improve convergence.

3.9.5.2. Coefficient of Friction The interface coefficient of friction, MU, is used for the Coulomb friction model. You can input MU as a material property for the contact elements. Use MU = 0 for frictionless contact. For rough or bonded contact (KEYOPT(12) = 1, 3, 5, or 6; see Selecting Surface Interaction Models), the program assumes infinite frictional resistance regardless of the specified value of MU. MU can have dependence on temperature, time, normal pressure, sliding distance, or sliding relative velocity. Suitable combinations of up to two fields can be used to define dependency; for example, temperature only, temperature and sliding distance, or sliding relative velocity and normal pressure. If the underlying element is a superelement (MATRIX50), the material property set must be the same as the one used for the original elements that were assembled into the superelement. Note that if the coefficient of friction is defined as a function of temperature, the program always uses the contact surface temperature as the primary variable (not the average temperature from the contact and target surfaces). The program provides two models for Coulomb friction: isotropic friction (2-D and 3-D contact) and orthotropic friction (3-D contact). The isotropic friction model is based on a single coefficient of friction, MU. You can use either TB command input (recommended method) or the MP command to specify MU. The orthotropic friction model is based on two coefficients of friction, MU1 and MU2. Use TB command input to specify MU1 and MU2 in two principal directions (see the element descriptions for CONTA173, CONTA174, CONTA175, CONTA176, and CONTA177 for a description of the principal directions for each individual element). See Contact Friction in the Material Reference for details on how to specify the coefficients of friction.

3.9.5.3. Using TAUMAX, FACT, DC, and COHE The program provides one extension of classical Coulomb friction: real constant TAUMAX is maximum contact friction with units of stress. This maximum contact friction stress can be introduced so that, regardless of the magnitude of normal contact pressure, sliding will occur if the friction stress reaches this value. You typically use TAUMAX when the contact pressure becomes very large (such as in bulk metal forming processes). TAUMAX defaults to 1.0e20. Empirical data is often the best source for TAUMAX. σy Its value may be close to

, where σy is the yield stress of the material being deformed.

You have an option to define TAUMAX as a function of primary variables by using tabular input (i.e., %tabname%). The possible primary variables include time, temperature, initial contact detection point location (at the beginning of solution), current contact pressure (positive PRESSURE index values indicate compression, negative PRESSURE index values indicate tension), and current geometric penetration (positive GAP index values indicate penetration, negative GAP index values indicate an open gap). For more information, see Defining Real Constants in Tabular Format (p. 34). The user subroutine USERCNPROP.F is also available for defining TAUMAX. To use this subroutine, you must specify the table name %_CNPROP% as the real constant value. For more information, see Defining Real Constants via a User Subroutine (p. 35).


45

Surface-to-Surface Contact Another real constant used for the friction law is the cohesion, COHE (default COHE = 0), which has units of stress. It provides sliding resistance, even with zero normal pressure (see Figure 3.12: Sliding Contact Resistance (p. 46)). Figure 3.12: Sliding Contact Resistance

Two other real constants, FACT and DC are involved in specifying static and dynamic friction coefficients, as described in the next section.

3.9.5.4. Static and Dynamic Friction Coefficients The coefficient of friction can depend on the relative velocity of the surfaces in contact. Typically, the static coefficient of friction is higher than the dynamic coefficient of friction. The program provides the following exponential decay friction model:

µ=

×

+

−

−

× rel

where: µ = coefficient of friction. MU = dynamic coefficient of friction. FACT = ratio of static to dynamic coefficients of friction. It defaults to the minimum value of 1.0 DC = decay coefficient, which has units of time/length. Therefore, time has some meaning in a static analysis. DC defaults to zero. When DC is zero, the equation is rewritten to be µ = MU for the case of sliding and µ = FACT*MU for the case of sticking. Vrel = slip rate calculated by the program. For the isotropic friction model, MU is input using the MP command or the TB command as explained above. For orthotropic friction, MU is the equivalent coefficient of friction computed from MU1 and MU2 which are specified with TB command input: µ12 + µ22

µ=

Figure 3.13: Friction Decay (p. 47) shows the exponential decay curve where the static coefficient of friction is given by:

µs

46

=

×


Set the Real Constants and Element KEYOPTS Figure 3.13: Friction Decay

µ µ

s

MU V rel You can determine the decay coefficient if you know the static and dynamic coefficients of friction and at least one data point (µ1 ; Vrel1). The equation for friction decay can be rearranged to give:

=−

 ×ℓ   rel1

µ − −

×

  

If you do not specify a decay coefficient and FACT is greater than 1.0, the coefficient of friction will change suddenly from the static to the dynamic value as soon as contact reaches the sliding state. This behavior is not recommended because the discontinuity may lead to convergence difficulties.

3.9.5.5. Forced Frictional Sliding Using Velocity Input In a static analysis, you can model steady-state frictional sliding between two flexible bodies or between a flexible and a rigid body with different velocities. In this case the sliding velocities no longer follow the nodal displacements, and they are predefined through the CMROTATE command. This command sets the velocities on the nodes of the element component as an initial condition at the start of a load step. The program determines the sliding direction based on the direction of the sliding velocities. This feature is applicable to 3-D contact (CONTA173, CONTA174, CONTA175) and is primarily used for generating sliding contact at frictional contact interfaces in a brake squeal analysis. In this case, the contact pair elements (either the contact elements or the target elements) on the brake rotor need to be included in the rotating element component (CM command) that is specified on the CMROTATE command. We recommend that you include only the contact elements or only the target elements in the element component. Velocities defined by CMROTATE will be ignored for the following contact definitions: Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

47

Surface-to-Surface Contact Frictionless contact Rough contact (KEYOPT(12) = 1) Bonded contact (KEYOPT(12) = 3, 5, 6) MPC contact (KEYOPT(2) = 2)

Note You should always verify the sliding direction when the velocities defined by CMROTATE are applied on nodes that are shared by more than one frictional contact pair. In this situation, you can redefine any contact elements that have a potentially incorrect sliding direction as frictionless contact. The amplitude of the sliding velocity defined by CMROTATE will affect the solution when the friction coefficient is specified as a function of sliding velocity via the command TB,FRIC, or when static and dynamic friction is defined via the real constants FACT and DC. In a complex eigenvalue extraction analysis using the QRDAMP or DAMP methods (see MODOPT), the effects of squeal damping will contribute to the damping matrix. The squeal damping can be identified as two parts: destabilizing damping and stabilizing damping. You can activate destabilizing squeal damping by one of the following methods: 1. Define friction as a function of sliding velocity via the TB,FRIC command. 2. Define static/dynamic friction via real constants FACT and DC. 3. Define a constant friction-sliding velocity gradient via real constant FDMD in conjunction with KEYOPT(16) = 1. 4. Specify the destabilizing squeal damping coefficient directly (either a positive or negative value) in conjunction with KEYOPT(16) = 2. When the destabilizing squeal damping is included by method (1) or (2), you can study its effects by using FDMD as a scaling factor (KEYOPT(16) = 0); FDMD defaults to 1.0. The program multiplies the internally calculated destabilizing damping by this factor. You can specify a constant friction-sliding velocity gradient directly via FDMD by setting KEYOPT(16) = 1. The defined gradient has units of TIME/LENGTH and it is negative in general. You can also specify the destabilizing squeal damping coefficient directly via FDMD by setting KEYOPT(16) = 2. The defined damping coefficient has units of MASS/(AREA*TIME) and it is negative in general. In a linear non-prestressed modal analysis, this is the only way to take the destabilizing squeal damping effects into account. The stabilizing squeal damping is deactivated by default. To activate it, you must specify the scaling factor via the real constant FDMS. FDMS defaults to 0.0. The program multiplies the internally calculated stabilizing damping by this factor. By setting KEYOPT(16) = 1 or KEYOPT(16) = 2 you can specify the stabilizing squeal damping coefficient directly via FDMS. The defined damping coefficient has units of MASS/(AREA*TIME) and it is positive in general. In a linear non-prestressed modal analysis, this is the only way to take the stabilizing squeal damping effects into account. If squeal damping is included in a brake squeal modal analysis that uses the QR Damp eigensolver (MODOPT,QRDAMP command), care should be taken not to generate a damping matrix with large values (coefficients) relative to the values of the stiffness matrix. The accuracy of the QRDAMP eigen48


Set the Real Constants and Element KEYOPTS solver is based on the assumption that the values in the damping matrix are at least an order of magnitude smaller than the stiffness matrix values. If large squeal damping matrix values are generated in conjunction with a QRDAMP modal solution, then the QRDAMP eigensolver could produce spurious zero modes, which can generally be ignored. In this case, the non-zero eigenvalues from the QRDAMP modal solution are still accurate. It is recommended that you use the DAMP eigensolver (MODOPT,DAMP) to check the final solution.

3.9.6. Selecting Location of Contact Detection The following topics related to selecting the location of contact detection are available: 3.9.6.1. Introduction 3.9.6.2. Using KEYOPT(4) and TOLS 3.9.6.3. Using the Surface Projection Based Contact Method (KEYOPT(4) = 3)

3.9.6.1. Introduction Contact detection points are located at the integration points of the contact elements which are interior to the element surface. The contact element is constrained against penetration into the target surface at its integration points. However, the target surface can, in principle, penetrate through into the contact surface, see Figure 3.14: Contact Detection Located at Gauss Point (p. 49). Figure 3.14: Contact Detection Located at Gauss Point

Surface-to-surface contact elements use Gauss integration points as a default, which generally provide more accurate results than the nodal detection scheme, which uses the nodes themselves as the integration points. The node-to-surface contact element, CONTA175, the line-to-line contact element, CONTA176, and the line-to-surface contact element, CONTA177, always use the nodal detection scheme.

3.9.6.2. Using KEYOPT(4) and TOLS The nodal detection algorithms require the smoothing of the contact surface (KEYOPT(4) = 1) or the smoothing of the target surface (KEYOPT(4) = 2), which is quite time consuming. You should use this option only to deal with corner, point-surface, or edge-surface contact (see Figure 3.15: Contact Detection Point Location at Nodal Point (p. 50)). KEYOPT(4) = 1 specifies that the contact normal be perpendicular to the contact surface. KEYOPT(4) = 2 specifies that the contact normal be perpendicular to the target surface. Use this option (KEYOPT(4) = 2) when the target surface is smoother than the contact surface.


49

Surface-to-Surface Contact Figure 3.15: Contact Detection Point Location at Nodal Point

Be aware, however, that using nodes as the contact detection points can lead to other convergence difficulties, such as "node slippage," where the node slips off the edge of the target surface, see Figure 3.16: Node Slippage Using Nodal Integration KEYOPT(4) = 1 or 2 (p. 50). In order to prevent node slippage, you can use real constant TOLS to extend the target surface when the default setting still cannot avoid the problem. For most point-to-surface contact problems, we recommend using CONTA175; see Node-to-Surface Contact (p. 95) later in this guide. Figure 3.16: Node Slippage Using Nodal Integration KEYOPT(4) = 1 or 2

Smoothing is required for nodal detection algorithms, and it is performed by averaging surface normals connected to the node. As a result, the variation of the surface normal is continuous over the surface, which leads to a better calculation of friction behavior and a better convergence. Real constant TOLS is used to add a small tolerance that will internally extend the edge of the target surface when you define the contact detection at the nodal point (KEYOPT(4) = 1 or 2). TOLS is useful for problems where contact nodes are likely to lie on the edge of targets (as at symmetry planes or for models generated in a node-to-node contact pattern). In these situations, the contact node may repeatedly "slip" off the target surface and go completely out of contact, resulting in convergence difficulties from oscillations. Units for TOLS are percent (1.0 implies a 1.0% increase in the target edge length). A

50


Set the Real Constants and Element KEYOPTS small value of TOLS will usually prevent this situation from occurring. The default value is 10 for small deflection and 2 for large deflection (NLGEOM, ON). The definition of KEYOPT(4) in node-to-surface contact element CONTA175 is different. KEYOPT(4) = 1 for surface-to-surface contact is equivalent to KEYOPT(4) = 1 for node-to-surface contact. However, KEYOPT(4) = 2 for surface-to-surface contact is equivalent to KEYOPT(4) = 0 for node-to surface contact. See KEYOPT(4) (p. 97). For the 3-D line-to-line contact element CONTA176 and the 3-D line-to-surface contact element CONTA177, KEYOPT(4) is not used to select the location of contact detection, and the contact normal is always perpendicular to both the contact and target surfaces. For CONTA176 and CONTA177, KEYOPT(4) is used to specify a surface-based constraint type.

3.9.6.3. Using the Surface Projection Based Contact Method (KEYOPT(4) = 3) You can define the surface projection contact method by setting KEYOPT(4) = 3 for surface-to-surface contact elements (CONTA171 through CONTA174). For this method, the contact detection remains at contact nodal points. This option enforces a contact constraint on an overlapping region of the contact and target surfaces (see Figure 3.17: Surface Projection Based Contact (p. 51)) rather than on individual contact nodes (KEYOPT(4) = 1, 2) or Gauss points (KEYOPT(4) = 0). The contact penetration/gap is computed over the overlapping region in an average sense. The advantages of using this option are the following: • In general, it provides more accurate contact tractions and stresses of underlying elements compared with other KEYOPT(4) settings. • The results are less sensitive to the designation of the contact and target surface. • It satisfies moment equilibrium when an offset exists between contact and target surfaces with friction. • Contact forces do not jump when contact nodes slide off the edge of target surfaces. The real constant TOLS is not used with this option. Figure 3.17: Surface Projection Based Contact

Contact element n Target element

Initial Element Positions

Overlapping Region after Surface Projection

There are certain disadvantages to using surface projection based contact, as follows: • This method is computationally more expensive since more nodes are included in each contact constraint condition, especially if 3-D higher order contact/target elements are used in the model. This effect will be more obvious when a modal has a large percentage of contact/target elements, or when the target elements are much more refined than the contact elements. • This method calculates the penetration/gap over the contacting area in an average sense. When a model has corner or edge contact, the averaged penetration/gap could be quite different than the


51

Surface-to-Surface Contact real geometric penetration observed at contact nodes. In this situation, mesh refinement is usually required in order to achieve an accurate solution.

Note The surface projection contact method currently does not support the following: 1. Thermal contact modeled via a “free thermal surface” definition. 2. Rigid surfaces defined by primitive target segments 3. 3-D contact/target elements with partially dropped midside nodes (dropping all midside nodes is supported)

3.9.7. Adjusting Initial Contact Conditions The following topics related to adjusting initial contact conditions are available: 3.9.7.1. Introduction 3.9.7.2. Using PMIN, PMAX, CNOF, ICONT, STRM. KEYOPT(5), and KEYOPT(9)

3.9.7.1. Introduction Rigid body motion is usually not a problem in dynamic analyses. However, in static analyses, rigid body motion occurs when a body is not sufficiently restrained. "Zero or negative pivot" warning messages and impractical, excessively large displacements indicate unconstrained motion in a static analysis. In simulations where rigid body motions are constrained only by the presence of contact, you must ensure that the contact pairs are in contact in the initial geometry. In other words, you want to build your model so that the contact pairs are "just touching." However, you can encounter various problems in doing so: • Rigid body profiles are often complicated, making it difficult to determine where the first point of contact might occur. • Small gaps between element meshes on both sides of the element pair can be introduced by numerical round-off, even if the solid model is built in an initially-contacting state. • Small gaps can exist between the integration points of the contact elements and target surface elements. • For surface projection based contact, a numerical gap distance can exist even though geometric penetration is observed at a contact node. This can occur because the numerical distance is obtained over the overlapping area in an average sense. For the same reasons, too much initial penetration between target and contact surfaces can occur. In such cases, the contact elements may overestimate the contact forces, resulting in nonconvergence or in breaking-away of the components in contact. The definition of initial contact is perhaps the most important aspect of building a contact analysis model. Therefore, you should always issue the CNCHECK command before starting the solution to verify the initial contact status. You may find that you need to adjust the initial contact conditions. The program offers several ways to adjust the initial contact conditions of a contact pair.

52


Set the Real Constants and Element KEYOPTS

3.9.7.2. Using PMIN, PMAX, CNOF, ICONT, STRM. KEYOPT(5), and KEYOPT(9) The following techniques can be performed independently or in combinations of one or more at the beginning of the analysis. They are intended to eliminate small gaps or penetrations caused by numerical round-off due to mesh generation. They are not intended to correct gross errors in either the mesh or in the geometric data. 1. Use real constant CNOF to specify a contact surface offset. Specify a positive value to offset the entire contact surface towards the target surface. Use a negative value to offset the contact surface away from the target surface.

Note If user-defined values are input for both CNOF and PINB, you must ensure that PINB is greater than CNOF. Otherwise, CNOF will be ignored. However, if a user-defined CNOF is input and the PINB value is left at its default value, the PINB value will be adjusted so that it is larger than the CNOF value, as described in Using PINB.

Note For the CONTA177 line-to-surface element, CNOF can be used to model thickness of the underlying beam elements. Input half of the beam thickness for CNOF to properly model the thickness effects. See Accounting for Thickness Effect (CNOF and KEYOPT(11)) (p. 107) for more information. The program can automatically provide the CNOF value to either just close the gap or reduce initial penetration. Set KEYOPT(5) as follows: 1: Closes the gap 2: Reduces initial penetration 3: Either closes the gap or reduces initial penetration Tabular input can also be used to define CNOF. The tabular input can be defined as a function of time and/or initial contact detection point location (at the beginning of solution) in global or local coordinates. For more information, see Defining Real Constants in Tabular Format (p. 34). As an example of when tabular input may be useful, consider the case of a CAD geometry based on nominal values. The geometry may lack a slight curvature variation that is important for analysis purposes. Moving nodes to the actual positions can be a tedious process, yet using the original geometry and neglecting the slight variation in curvature will result in a different contacting area. Consequently, use of CNOF as a function of location allows you to easily include curvature that varies with location without having to modify the original CAD geometry. 2. Use the real constant ICONT to specify a small initial contact closure. This is the depth of an "adjustment band" around the target surface. A positive value for ICONT indicates a scaling factor relative to the depth of the underlying elements. A negative value indicates an absolute contact closure value. The value of ICONT defaults to zero if KEYOPT(5) = 0, 1, 2, or 3. (The ICONT default is different when KEYOPT(12) = 6 for bonded-initial contact; see Selecting Surface Interaction Models (p. 62) for more information). If KEYOPT(5) = 4, the program provides a small (but meaningful) value for ICONT according to the geometric dimensions, and prints a warning message stating what value was assigned. Any contact detection points that fall within this adjustment band are internally shifted


53

Surface-to-Surface Contact to be on the target surface (see Figure 3.18: Contact Surface Adjustment With ICONT (p. 54)(a)). Only a very small correction is suggested; otherwise, severe discontinuity may occur (see Figure (b)). The difference between CNOF and ICONT is that the former shifts the entire contact surface with the distance value CNOF, the latter moves all initially open contact points which are inside of adjustment band ICONT onto the target surface. Figure 3.18: Contact Surface Adjustment With ICONT

(a) before adjustment

Contact Surface arget Surface

(b) after adjustment

Potential discontinuity

I ONT

arget Surface

Contact Surface

3. Use real constants PMIN and PMAX to specify an initial allowable penetration range. When either PMAX or PMIN is specified, the program brings the target surface into a state of initial contact at the beginning of the analysis (see Figure 3.19: Contact Surface Adjustment (PMIN, PMAX) (p. 55)). If the initial penetration is larger than PMAX, the program adjusts the target surface to reduce penetration. If the initial penetration is smaller than PMIN (and within the pinball region), the program adjusts the target surface to ensure initial contact. Initial adjustment for contact status is performed only in translational modes.

54


Set the Real Constants and Element KEYOPTS Such adjustment of initial contact status will be performed for a rigid target surface that has either prescribed loads or displacements. Similarly, a target surface that has no boundary conditions specified may also be adjusted for initial contact. When all the target surface nodes have a prescribed value of zero, the initial adjustment using PMAX and PMIN will not be performed. The program treats applicable degrees of freedom for target surface nodes independently. For example, if you specify the UX degree of freedom to be "zero," then no initial adjustment is possible along the X direction. However, the PMAX and PMIN options will still be activated in the Y and Z directions. The initial status adjustment is an iterative process. The program uses a maximum of 20 iterations. If the target surface cannot be brought into an acceptable penetration range (i.e., in the range of PMIN to PMAX), the analysis proceeds with the original geometry. The program issues a warning message in such circumstances, and you may need to manually adjust your initial geometry. Figure 3.20: A Scenario in Which Initial Adjustment Will Fail (p. 56) illustrates a problem in which initial contact adjustment iteration will fail. The UY degree of freedom for the target has been restrained. Therefore, the only possible adjustment for initial contact is in the X direction. However, in this problem, any movement of the rigid target surface in the X direction will not establish initial contact. For flexible-to-flexible contact, this technique not only moves the entire target surface but also moves the whole deformable body which attaches to the target surface. Make sure there is no other contact surface or target surface connecting with the deformable body. Figure 3.19: Contact Surface Adjustment (PMIN, PMAX)


55

Surface-to-Surface Contact Figure 3.20: A Scenario in Which Initial Adjustment Will Fail

4. Set KEYOPT(9) to adjust initial penetration or gap; see Figure 3.21: Ignoring Initial Penetration, KEYOPT(9) = 1 (p. 58). True initial penetration includes two parts: • Penetration or gap due to geometry • Penetration or gap due to user-defined contact surface offset (CNOF). See Figure 3.22: Components of True Penetration (p. 58). Keep in mind that the effects of KEYOPT(9) are dependent on settings for other KEYOPTs. The indicated initial gap effect is considered only if KEYOPT(12) = 4 or 5 (no separation always or bonded always). KEYOPT(9) offers the following settings for adjusting initial penetration or gap: • To include initial penetration or gap from both geometry and contact surface offset, set KEYOPT(9) = 0. This is the default. • To ignore initial penetration or gap from both geometry and contact surface offset, set KEYOPT(9) = 1. • To include the defined contact surface offset (CNOF) but ignore the initial penetration or gap due to geometry, set KEYOPT(9) = 3. • Set KEYOPT(9) = 5 to include the defined contact surface offset (CNOF) but ignore the initial penetration due to geometry. Even when KEYOPT(12) is not set to 4 or 5, this setting for KEYOPT(9) will also ignore the initial force in open-gap springs as long as contact is detected within the pinball region. For problems such as an interference fit, over-penetration is expected. These problems often have convergence difficulties if the initial penetration is step-applied in the first load step. You may overcome convergence difficulties by ramping the initial penetration over the first load step, see Figure 3.23: Ramping Initial Interference (p. 59). The following KEYOPT(9) settings provide ramping capabilities:

56


Set the Real Constants and Element KEYOPTS • To ramp the total initial penetration or gap (CNOF + the offset due to geometry), set KEYOPT(9) = 2. • To ramp the defined contact surface penetration or gap (CNOF), but ignore the penetration or gap due to geometry, set KEYOPT(9) = 4. • Set KEYOPT(9) = 6 to ramp the defined contact offset (CNOF) but ignore the initial penetration due to geometry. . Even when KEYOPT(12) is not set to 4 or 5, this setting for KEYOPT(9) will also ignore the initial force in open-gap springs as long as contact is detected within the pinball region. For the above KEYOPT(9) settings, you should also set KBC,0 and not specify any external loads in the first load step. In addition, make sure that the pinball region is big enough to capture the initial interference. By default, the ramping options are active only within the first load step. However, you may have a situation where there are multiple interference fits that you want to model sequentially (that is, the interference present in each contact pair will be resolved in different load steps). You can define the load step number in which the ramping option will take place for a given contact pair by using the real constant STRM. For example, you may want to perform a different loading in load step 1, then resolve an interference fit in load step 2 for one contact pair, and finally resolve an interference fit in load step 3 for another contact pair. To accomplish this, you would set STRM = 2 for the first contact pair and set STRM = 3 for the second contact pair. Note that contact will still be active prior to the load step specified by the STRM real constant. You can use the above techniques in conjunction with each other. For example, you may wish to set a very precise initial penetration or gap, but the initial coordinates of the finite element nodes may not be able to provide sufficient precision. To accomplish this, you could: 1. Use CNOF to specify a penetration (positive value) or gap (negative value). 2. Use KEYOPT(9) = 5 to resolve the initial penetration in the first substep (or KEYOPT(9) = 6 to gradually resolve the initial penetration).


57

Surface-to-Surface Contact Figure 3.21: Ignoring Initial Penetration, KEYOPT(9) = 1

Figure 3.22: Components of True Penetration

58


Set the Real Constants and Element KEYOPTS Figure 3.23: Ramping Initial Interference

The program provides a printout (in the output window or file or via the CNCHECK) of the model's initial contact state for each target surface at the beginning of the analysis. This information is helpful for determining the maximum penetration or minimum gap for each target surface. If no contact is detected for a specific target surface, the program issues a warning. This occurs when the target surface is far from contact (i.e., outside of the pinball region), or when the contact/target elements have been killed. See Positive and Negative Real Constants for more information on these real constants.

3.9.8. Physically Moving Contact Nodes Towards the Target Surface You can adjust the initial contact status in order to close the gap by doing one of the following: (a) Define an initial contact adjustment via real constant ICONT. (ICONT may change the shape of the contact detection surface.) (b) Define a contact offset via real constant CNOF. (CNOF does not change the shape of the contact detection surface.) (c) Ignore the penetration by setting KEYOPT(9) = 1. (KEYOPT(9) = 1 does change the shape of the contact detection surface.) However, these adjustment methods do not truly change the physical locations of contact nodes; rather, the contact detection locations are adjusted.


59

Surface-to-Surface Contact The initial adjustment due to (a) is applied only once in the beginning of the contact analysis, where each contact detection point within the ICONT range is made to be in initial contact with the target surface. The contact adjustments due to (b) and (c) offset the entire contact detection surface to close any gap that is present. In doing so, methods (b) and (c) introduce a "rigid region" between the contact and target surfaces during the entire analysis, which can cause a certain amount of residual force if a large rotation appears at the contact surface. This problem can be alleviated by issuing the CNCHECK,ADJUST command, which physically moves contact nodes towards the target surface under the following circumstances: • Only when using the nodal detection option (KEYOPT(4) = 1 or 2), or when using CONTA175 or CONTA177, or when using CONTA176 with KEYOPT(3) = 0 (parallel beams). • Initially open contact nodes inside the ICONT zone. • Initially penetrated nodes with KEYOPT(9) = 1. After issuing the CNCHECK,ADJUST command, the coordinates of the nodes that have been moved are modified as shown in Figure 3.24: Effect of Moving Contact Nodes (p. 60). You can change other contact related settings in PREP7 (for example, set KEYOPT(4) = 0 to use the Gauss detection option) and save the Jobname.DB file. Issuing the SAVE command before issuing the CNCHECK,ADJUST command is recommended in order to resume the Jobname.DB file with the original contact configuration. For those contact pairs whose contact nodes you do not wish to physically move towards target surface, do not define KEYOPT(4) = 1 or 2. Figure 3.24: Effect of Moving Contact Nodes

3.9.9. Determining Contact Status and the Pinball Region The following topics related to determining contact status and the pinball region are available: 3.9.9.1. Introduction 3.9.9.2. Using PINB

3.9.9.1. Introduction The position and motion of a contact element relative to its associated target surface determines the contact element status. The program monitors each contact element and assigns a status: • STAT = 0 Open far-field contact (open and not near contact) • STAT = 1 Open near-field contact

60


Set the Real Constants and Element KEYOPTS • STAT = 2 Sliding contact • STAT = 3 Sticking contact A contact element is considered to be in near-field contact when its integration points (Gauss points or nodal points) are within a code-calculated (or user-defined) distance to the corresponding target surface. This distance is referred to as the pinball region. The pinball region is a circle (in 2-D) or a sphere (in 3-D) centered about the Gauss point.

3.9.9.2. Using PINB Use real constant PINB to specify a scaling factor (positive value for PINB when using command input) or absolute value (negative value for PINB when using command input) for the pinball region. You can specify PINB to have any value. By default, and assuming that large deflection effects apply (NLGEOM,ON), the program defines the pinball region as a circle for 2-D or a sphere for 3-D of radius 4*depth (if rigidto-flexible contact) or 2*depth (if flexible-to-flexible contact) of the underlying element. (See the discussion of element depth in Positive and Negative Real Constant Values (p. 33).) If you include no largedeflection effects (NLGEOM,OFF), the default pinball region is half that of the large-deflection case. (For the no-separation (KEYOPT(12) = 4) and bonded-always (KEYOPT(12) = 5 options, the PINB default is different than described here. See Selecting Surface Interaction Models (p. 62) for more information.)

Note If you input a value for real constant CNOF (contact surface offset) and the default PINB value (as described above) is less than the absolute value of CNOF, the default for PINB will be set to the absolute value of (1.1*CNOF). The computational cost of searching for contact depends on the size of the pinball region. Far-field contact (open and not near contact) element calculations are simple and add little computational demands. The near-field calculations (for contact elements that are nearly or actually in contact) are slower and more complex. The most complex calculations occur once the elements are in actual contact. Setting a proper pinball region is useful to overcome spurious contact definitions if the target surface has several convex regions. However, the default setting should be appropriate for most contact problems.

Note The program will warn you when there is an abrupt change in status (for example, from farfield to closed) during a contact analysis. This may indicate that the substep increment is too large, or possibly (but not likely) that the pinball value (PINB) is too small. See Positive and Negative Real Constants for more information on this real constant.

3.9.10. Avoiding Spurious Contact in Self Contact Problems In some cases of self contact, the program may erroneously assume contact between a contact and target surface that are in very close geometrical position as shown below.


61

Surface-to-Surface Contact Figure 3.25: Auto Spurious Prevention

Contact normal

Location where spurious contact most likely occurs

Target normal

The program alerts you when it first detects spurious contact in each load step. If the program encounters such contact on the first load step, you'll see the following error message: Contact element x has too much penetration related to target element y. We assume it (may be more elements) is spurious contact.

If the program encounters an abrupt change in contact that it classifies as spurious contact, you'll see the following message: Contact element x status changed abruptly with target element y. We assume it (may be more elements) is spurious contact.

The program issues such messages only once per load step. It does not notify you of additional cases of spurious contact that were ignored during the load step.

3.9.11. Selecting Surface Interaction Models The following topics related to selecting surface interaction models are available: 3.9.11.1. Introduction 3.9.11.2. Using KEYOPT(12) 3.9.11.3. Using FKOP 3.9.11.4. Using TBND 3.9.11.5. Bonded Contact for Shell-Shell Assemblies

3.9.11.1. Introduction The surface-to-surface contact elements support normal unilateral contact models as well as other mechanical surface interaction models. As an alternative to the program-supplied contact interactions, you can define your own interfacial model with the USERINTER subroutine (see User-Defined Subroutines for Contact Interfacial Behaviors (p. 71)).

62



3.9.11.2. Using KEYOPT(12) Use KEYOPT(12) to model different contact surface behaviors. • KEYOPT(12) = 0 models standard unilateral contact; that is, normal pressure equals zero if separation occurs. • KEYOPT(12) = 1 models perfectly rough frictional contact where there is no sliding. This case corresponds to an infinite friction coefficient and ignores the material property MU. • KEYOPT(12) = 2 models no separation contact, in which the target and contact surfaces are tied (although sliding is permitted) for the remainder of the analysis once contact is established. • KEYOPT(12) = 3 models "bonded" contact, in which the target and contact surfaces are bonded in all directions (once contact is established) for the remainder of the analysis. • KEYOPT(12) = 4 models no separation contact, in which contact detection points that are either initially inside the pinball region or that once involve contact always attach to the target surface along the normal direction to the contact surface (sliding is permitted). • KEYOPT(12) = 5 models bonded contact, in which contact detection points that are either initially inside the pinball region or that once involve contact always attach to the target surface along the normal and tangent directions to the contact surface (fully bonded). • KEYOPT(12) = 6 models bonded contact, in which the contact detection points that are initially in a closed state will remain attached to the target surface and the contact detection points that are initially in an open state will remain open throughout the analysis. For all types of bonded contact (KEYOPT(12) = 2, 3, 4, 5, and 6), separation of contact can be modeled using the debonding feature. For more information, see Debonding (p. 161). For the no-separation option (KEYOPT(12) = 4) and the bonded-always option (KEYOPT(12) = 5), a relatively small PINB value (pinball region) may be used to prevent any false contact. For these KEYOPT(12) settings, the default for PINB is 0.25 (25% of the contact depth) for small deformation analysis (NLGEOM,OFF) and 0.5 (50% of the contact depth) for large deformation analysis (NLGEOM,ON). (The default PINB value may differ from what is described here if CNOF is input. See Using PINB (p. 61) for more information.) For the bonded-initial option (KEYOPT(12) = 6), a relatively large ICONT value (initial contact closure) may be used to capture the contact. For this KEYOPT(12) setting, the default for ICONT is 0.05 (5% of the contact depth) when KEYOPT(5) = 0 or 4. See Positive and Negative Real Constants for more information on the real constants mentioned above.

Note For bonded contact definitions (KEYOPT(12) = 5 or 6), if the contact is not in a “just touching” position, you may find that no zero modes appear for free vibration. To avoid this issue, use the MPC approach instead of other contact algorithms.


63


3.9.11.3. Using FKOP Real constant FKOP is the stiffness factor applied when contact opens. FKOP is only used for no separation or bonded contact (KEYOPT(12) = 2 through 6). If FKOP is input as a scaling factor (positive value for command input), the true contact opening stiffness equals FKOP times the contact stiffness applied when contact closes. If FKOP is input as an absolute value (negative value for command input), the value is applied as an absolute contact opening stiffness. The default FKOP value is 1. In addition, FKOP can be defined as a function of primary variables by using tabular input (i.e., %tabname%). The possible primary variables include time, temperature, initial contact detection point location (at the beginning of solution), and current geometric gap distance (positive GAP index values indicate penetration, negative GAP index values indicate an open gap). For more information, see Defining Real Constants in Tabular Format (p. 34). The user subroutine USERCNPROP.F is also available for defining FKOP. To use this subroutine, you must specify the table name %_CNPROP% as the real constant value. For more information, see Defining Real Constants via a User Subroutine (p. 35). No separation and bonded contact generate a "pull-back" force when contact opening occurs, and that force may not completely prevent separation. To reduce separation, define a larger value for FKOP. Also, in some cases separation is expected while connection between the contacting surfaces is required to prevent rigid body motion. In such instances, you can specify a small value for FKOP to maintain the connection between the contact surfaces (this is a "weak spring" effect). For bonded contact, the default contact opening stiffness is affected by the normal contact stiffness. However, the tangential stiffness remains unchanged when an absolute value of tangential stiffness (a negative value input for FKT) is specified, regardless of the contact status or the value of FKOP.

3.9.11.4. Using TBND In most welding processes, after the materials around contacting surfaces exceed a critical temperature, the surfaces start to melt and bond with each other. To model this, you can specify the critical temperature using real constant TBND; as soon as the temperature at the contact surface (Tc) for closed contact exceeds this melting temperature, the contact will change to “bonded.” The contact status will remain bonded for the rest of analysis, even if the temperature subsequently decreases below the critical value. The contact surface temperature Tc is obtained either from a coupled thermal-structural solution in which the TEMP degree of freedom is present or from a temperature body load applied to a model having structural degrees of freedom only. In addition, TBND can be defined as a function of primary variables by using tabular input (i.e., %tabname%). The possible primary variables include time, temperature, initial contact detection point location (at the beginning of solution), current contact pressure (positive PRESSURE index values indicate compression, negative PRESSURE index values indicate tension), and current geometric penetration (positive GAP index values indicate penetration, negative GAP index values indicate an open gap). For more information, see Defining Real Constants in Tabular Format (p. 34). The user subroutine USERCNPROP.F is also available for defining TBND. To use this subroutine, you must specify the table name %_CNPROP% as the real constant value. For more information, see Defining Real Constants via a User Subroutine (p. 35). When defining TBND via tabular input or the user subroutine, the TBND real constant should be defined as a bonding state other than the critical temperature: as soon as the value of TBND returned from the

64


Set the Real Constants and Element KEYOPTS table definition or from the user subroutine is equal to or larger than 1.0, the contact is changed to “bonded.” The contact status will remain bonded for the rest of the analysis, even if the TBND value obtained from the table or the user subroutine subsequently falls below 1.0.

3.9.11.5. Bonded Contact for Shell-Shell Assemblies The bonded contact options (KEYOPT(12) = 5 or 6) can be used with the MPC approach (KEYOPT(2) = 2) to model various types of assemblies (see Multipoint Constraints and Assemblies (p. 127)). When this method is used to model shell-shell assemblies, there may be cases where the MPC approach causes the model to be overconstrained. To alleviate this problem, you can use a penalty-based method for shell-shell assemblies. Using the penalty-based method constrains rotational DOFs in addition to translational DOFs. This capability is available for contact elements CONTA173, CONTA174, and CONTA175 in conjunction with TARGE170. To use this method, first set KEYOPT(2) = 0 or 1 (augmented Lagrangian or penalty function) and KEYOPT(12) = 5 or 6 (bonded always or bonded initial contact) in the contact elements. Setting KEYOPT(5) = 2 (shell-shell constraint) for the target elements will cause this penalty-based method to be used. The penalty stiffness used for rotational DOFs is equal to (contact stiffness used for translational DOFs) * (contact length). The contact stiffness for translational DOFs is input by real constant FKN, or defaults to an internal value. The contact length is always calculated internally and it is printed in the output file. The figure below shows the difference in using a conventional penalty-based shell-shell assembly and this method. Figure 3.26: Penalty-Based Shell-Shell Assembly

Note In the case of a penalty-based shell-shell assembly, spurious rotational energy exists if there are gaps or penetrations between the contact and target surfaces. This can affect the accuracy of the solution. In this case it is recommended that you use a shell-solid constraint type by setting KEYOPT(5) = 3, 4, or 5 on the target element, which requires use of the MPC algorithm (KEYOPT(2) = 2 on the contact element).

3.9.12. Modeling Contact with Superelements The following topics related to modeling contact with superelements are available: 3.9.12.1. Introduction 3.9.12.2. Using KEYOPT(3) in a 2-D Analysis with Superelements


65


3.9.12.1. Introduction The surface-to-surface contact elements can model a rigid body (or one linear elastic body) contacting another linear elastic body undergoing small motions. These elastic bodies can be modeled using superelements, which greatly reduces the number of degrees of freedom involved in the contact iteration. Remember that any contact or target nodes must be either all master nodes of the superelements or all slave nodes of the superelements. When the contact pair is built in original elements used to generate superelements, the contact status will not change from its initial status. Because the superelement consists only of a group of retained nodal degrees of freedom, it has no surface geometry on which the program can define a contact and target surface. Therefore, the contact and target surface must be defined on the surface of the original elements before they are assembled into a superelement. Information taken from the superelement includes nodal connection and assembly stiffness, but no material property or stress states (whether axisymmetric, plane stress, or plane strain). One restriction is that the material property set used for the contact elements must be the same as the one used for the original elements before they were assembled into superelements.

3.9.12.2. Using KEYOPT(3) in a 2-D Analysis with Superelements Use KEYOPT(3) to provide information for a 2-D analysis with superelements. In elements CONTA171 and CONTA172, the options are as follows: • No superelement used (KEYOPT(3) = 0) • Axisymmetric, use with superelements only (KEYOPT(3) = 1) • Plane strain or plane stress with unit thickness, use with superelements only (KEYOPT(3) = 2) • Plane stress with thickness input, use with superelements only (KEYOPT(3) = 3). Note that for this case, use real constant R2 to specify the thickness. In a 3-D contact analysis (CONTA173, CONTA174), KEYOPT(3) is not used to specify the stress state when superelements are present. The program automatically detects whether the underlying element is a superelement. For 2-D and 3-D analyses without superelements, KEYOPT(3) of the contact elements can be used to control the units of normal contact stiffness in some circumstances. Note that when superelements are present, normal contact stiffness always has the units FORCE/LENGTH3.

Note KEYOPT(3) has different meanings in other situations. For more information, see the description of KEYOPT(3) in Element KEYOPTS (p. 35).

3.9.13. Applying Contact Stabilization Damping The following topics related to applying contact stabilization damping are available: 3.9.13.1. Introduction 3.9.13.2. Using FDMN, FDMT, and KEYOPT(15) 3.9.13.3. Checking Results After Applying Contact Stabilization

66



3.9.13.1. Introduction Rigid body motion often occurs in the beginning of an analysis due to the fact that the initial contact condition is not well established. The causes may include: • Small gaps between element meshes on both sides of the element pair can be introduced by numerical round-off, even if the solid model is built in an initially-contacting state. • Small gaps can exist between the integration points of the contact elements and target surface elements. • For surface projection based contact, a numerical gap distance can exist even though geometric penetration is observed at a contact node. This can occur because the numerical distance is obtained over the overlapping area in an average sense. For standard contact (KEYOPT(12) = 0) or rough contact (KEYOPT(12) = 1), you can use real constants FDMN and FDMT to define contact damping scaling factors along contact normal and tangential directions. The primary goal of this contact stabilization technique is to damp relative motions between the contact and target surfaces for open contact. It provides a certain amount of resistance to reduce the risk of rigid body motion. The damping forces are calculated by: ∗ n∗ c dn = ∫ dt

=∫

∗ t∗

c

where Vn and Vt are the relative velocities along the normal and tangential directions, and Ac is the area domain of the contact surface. The units of the damping coefficients of FDMN and FDMT are FORCE/(AREA*VELOCITY). For the contact force-based model (used by CONTA175, CONTA176, and CONTA177), the units are FORCE/VELOCITY. The specified damping coefficients should be large enough to prevent rigid body motion, but small enough to insure a solution. The ideal values are fully dependent on the specific problem, the time of the load step, and the number of substeps.

3.9.13.2. Using FDMN, FDMT, and KEYOPT(15) The program computes the damping coefficients internally based on several factors: • Contact stiffness • Pinball radius • Gap distance • Number of substeps • Size of time increment for the current substep In general, contact stabilization damping should only be used for preventing rigid body motion when other initial contact adjustment techniques are not efficient or not suitable for a particular situation. Therefore, contact stabilization is deactivated by default. As an exception, however, the automatic contact stabilization technique will be enabled if all of the following conditions are encountered: Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

67

Surface-to-Surface Contact 1. Gauss point based contact (KEYOPT(4) = 0) or surface projection based contact (KEYOPT(4) = 3) is used. 2. The entire contact pair is in open status. 3. A geometric penetration is detected at any contact nodal point, despite condition 2 above. Without this automatic contact stabilization, rigid body motion may occur. If you wish to deactivate automatic contact stabilization, you can simply set KEYOPT(15) = 1. You can activate contact stabilization damping manually by specifying real constants FDMN and FDMT. Use a positive value of FDMN to specify a damping scaling factor in the normal direction. FDMN defaults to 1.0. You can overwrite the internal normal damping coefficient by specifying a negative value. The units of the damping coefficient are FORCE/(AREA*VELOCITY). For the contact force-based model (used by CONTA175, CONTA176, and CONTA177), the units are FORCE/VELOCITY. Use a positive value of FDMT to specify a damping scaling factor in the tangential direction. The tangential contact damping is activated only when the normal damping is activated. FDMT defaults to 0.001. You can overwrite the internally calculated tangential damping coefficient by specifying a user defined value (use a negative value for command input). The units of the damping coefficient are FORCE/(AREA*VELOCITY). For the contact force-based model (used by CONTA175, CONTA176, and CONTA177), the units are FORCE/VELOCITY. Both FDMN and FDMT can be defined as a function of primary variables by using tabular input (i.e., %tabname%). The possible primary variables include time, temperature, initial contact detection point location (at the beginning of solution), and current geometric gap distance (positive GAP index values indicate penetration, negative GAP index values indicate an open gap). For more information, see Defining Real Constants in Tabular Format (p. 34). The user subroutine USERCNPROP.F is also available for defining FDMN and FDMT. To use this subroutine, you must specify the table name %_CNPROP% as the real constant value. For more information, see Defining Real Constants via a User Subroutine (p. 35). The contact stabilization damping is activated if all of the following conditions are met: • Standard contact (KEYOPT(12) = 0) or rough contact (KEYOPT(12) = 1) • Contact status is near-field • The first load step, unless KEYOPT(15) = 2 or 3 • The entire contact pair has an open contact status in the previous substep, unless KEYOPT(15) = 3 When KEYOPT(15) = 0, 1, or 2, stabilization damping will not be applied in the current substep if any contact detection point had a closed status in the previous substep. However, when KEYOPT(15) = 3, stabilization damping is always applied as long as the current contact status is near-field, regardless of the contact status of the previous substep.

3.9.13.3. Checking Results After Applying Contact Stabilization Stabilization can alleviate convergence problems, but it can also affect solution accuracy if the applied stabilization energy or damping forces are too large. In most cases, the program automatically activates and deactivates contact stabilization damping and estimates reasonable damping forces. However, it

68


Set the Real Constants and Element KEYOPTS is good practice to check the stabilization energy and forces to determine whether or not they are excessive. The contact stabilization energy (accessed via the AENE label on the ETABLE command) should be compared to element potential energy. The energies can be output in the Jobname.OUT file (via the OUTPR command). You can also access the energies as follows: • In POST1 via PRENERGY, PRESOL, PLESOL, and ETABLE commands. • In POST26 via ENERSOL and ESOL commands. If the contact stabilization energy is much less than the potential energy (for example, within a 1.0 percent tolerance), the results should be acceptable and there should be no need to check the stabilization forces or tractions further. You can check the contact pair based damping tractions (maximum normal damping pressure and maximum tangential damping stress) via the NLHIST and NLDIAG commands. If the maximum damping tractions are much smaller than the maximum contact pressures (for example, within a 0.5 percent tolerance), the results are still acceptable.

3.9.14. Accounting for Thickness Effect The following topics related to thickness effects are available: 3.9.14.1. Introduction 3.9.14.2. Using KEYOPT(11)

3.9.14.1. Introduction You can account for the thickness of shells (2-D and 3-D) and beams (2-D) using KEYOPT(11). (This does not apply to 3-D beam-to-beam contact.) For rigid-to-flexible contact, the program automatically shifts the contact surface to the bottom or top of the shell/beam surface. For flexible-to-flexible contact, the program automatically shifts both the contact and target surfaces which are attached to shell/beam elements. By default, the program does not account for the element thickness, and beams and shells are discretized at their mid-surface in which penetration distance is calculated from the mid-surface.

3.9.14.2. Using KEYOPT(11) When you set KEYOPT(11) = 1 to account for beam or shell thickness, the contact distance is calculated from either the top or the bottom surface as specified previously in Steps in a Contact Analysis (p. 11).

Note Only use KEYOPT(11) = 1 to account for thickness when you have shell or beam elements with nodes located at the middle surface. When building your model geometry, if you are going to account for thickness, remember the offsets which may come from either the contact surface or target surface or from both. When you specify a contact offset (CNOF) along with setting KEYOPT(11) = 1, it is defined from the top or bottom of the shell/beam, not the mid-surface. When used with SHELL181, SHELL208, SHELL209, SHELL281, or ELBOW290, changes in thickness during deformation are also taken into account.


69

Surface-to-Surface Contact Note that KEYOPT(11) is not used with the line-to-line contact element (CONTA176). For the line-tosurface contact element (CONTA177) KEYOPT(11) is only used for shell elements on the target side.

Note For shell and beam contact, the penetration and gap distances are always measured from the midsurface of the shell or beam element. Any defined offset of the shell or beam element is ignored by the contact elements.

3.9.15. Using Time Step Control and Impact Constraints The following topics concerning time step control and impact constraints are available: 3.9.15.1. Introduction 3.9.15.2. Using KEYOPT(7)

3.9.15.1. Introduction Time step control is an automatic time stepping feature that predicts when the status of a contact element will change and cuts the current time step back. Impact constraints are used in a transient dynamic analysis to satisfy the momentum and energy balance at the contact and target interface. See Dynamic Contact and Impact Modeling (p. 145) for more information.

3.9.15.2. Using KEYOPT(7) Use KEYOPT(7) = 0, 1, 2, or 3 to control time stepping, where KEYOPT(7) = 0 provides no control (the default), and KEYOPT(7) = 3 provides the most control. • KEYOPT(7) = 0: No control. The time step size is unaffected by the prediction. This setting is appropriate for most analyses when automatic time stepping is activated and a small time step size is allowed. • KEYOPT(7) = 1: Time step size is bisected if too much penetration occurs during an iteration, or if the contact status changes dramatically. • KEYOPT(7) = 2: Predict a reasonable increment for the next substep. • KEYOPT(7) = 3: Predict a minimal time increment for the next substep. Use KEYOPT(7) = 4 to activate impact constraints. • KEYOPT(7) = 4: Use impact constraints for standard or rough contact (KEYOPT(12) = 0 or 1) in a transient dynamic analysis with automatic adjustment of the time increment.

3.9.16. Using the Birth and Death Option The surface-to-surface contact and target elements allow birth and death. The elements can be removed for part of an analysis and then reactivated for a later stage. This feature is useful for modeling complex metal forming processes where multiple rigid target surfaces need to interact with the contact surface at different stages of the analysis. Springback modeling often requires removing the rigid tools at the end of the forming processes. This option cannot be used with "no separation" or bonded contact.

70


User-Defined Subroutines for Contact Interfacial Behaviors

3.10. User-Defined Subroutines for Contact Interfacial Behaviors Three user subroutines are available for modifying or monitoring contact elements (CONTA171 to CONTA178): • USERCNPROP.F - Allows you to perform a user-defined operation on real constants based upon your own nonlinear function • USERFRIC.F - Allows you to write your own friction laws • USERINTER.F - Allows you to write your own contact interaction behaviors The user subroutines are called for each contact detection point of active contact pairs in each iteration during the solution. The input to these user subroutines includes the current and previous contact status, contact stresses, normal penetration/gap, tangential slip increments, and temperatures at the contact detection point and the corresponding interacting point on the target surface. You can specify the number of extra state variables (NSTV) to be saved for the user subroutines via the NSVR command. The maximum number of extra state variables should not exceed 840. The total number of integration points of an element (nintp) is a supplied input argument for the subroutines. Therefore, the number of additional state variables per integration point is NSTV/nintp. Use the USEROU subroutine to store user-supplied element outputs in the non-summable miscellaneous (NMISC) record. You must enter the USRCAL command in order to activate the USEROU subroutine. The maximum number of user-defined output items cannot exceed 120 and should be less than or equal to NSTV on the NSVR command. The number of NMISC records can be obtained by this function call: etyiqr(ITYPE,-110). Keep the following points in mind when using any of these subroutines to define contact interfacial behaviors: • The user subroutines USERCNPROP, USERFRIC, and USERINTER all modify the contact behavior, so only a single subroutine should be used for a given contact pair. However, you can use all three subroutines in a single model as long as each subroutine acts on a unique contact pair. • In order to implement the USERFRIC or USERINTER subroutine in your model, you must define related properties via the TB family of commands and assign the associated material ID to the contact elements. See Writing Your Own Friction Law (USERFRIC) and Defining Your Own Contact Interaction (USERINTER) for more information. • The use of USERFRIC or USERINTER overrides the behavior of program-supplied interfacial features (for example, debonding defined via the TB,CZM command, impact constraints defined by KEYOPT(7) = 4, surface interaction models defined by KEYOPT(12)). Therefore, you must take care if behaviors implemented by such features are desired. In this case, you must program the desired behavior into the USERFRIC or USERINTER subroutine instead of relying on program input. • The user subroutines USERFRIC and USERINTER should not be used if the fast thermal transient solver option (THOPT,QUASI) is being used. • You can use the /UPF command in batch mode to link user subroutines into ANSYS. The /UPF command method cannot be used when the program is run in interactive mode.


71

Surface-to-Surface Contact For more information on the /UPF command and other methods of implementing user subroutines, see Compiling and Linking UPFs on Linux Systems and Compiling and Linking UPFs on Windows Systems in the Guide to User-Programmable Features.

3.10.1. Defining Your Own Real Constant (USERCNPROP) The USERCNPROP subroutine allows you to define your own contact properties via real constants. For example, you can define a nonlinear contact pressure-penetration relationship instead of utilizing a linear one (that is, using a constant contact stiffness). The contact stiffness can be varied due to pressure, penetration, temperature, and your own state variables. The real constants that can be defined via this user subroutine are noted in the real constant table found in each contact element description (CONTA171, CONTA172, CONTA173, CONTA174, CONTA175, CONTA176, CONTA177, and CONTA178). Some of the most commonly used input and output arguments for USERCNPROP are discussed below. For a complete list of input and output arguments, see the description of USERCNPROP in the Guide to User-Programmable Features. When defining user-defined real constants with the R, RMORE, or RMODIF command, you must use the ANSYS reserved table name _CNPROP and enclose it in % signs (%_CNPROP%). When a value field in one of these commands is set to %_CNPROP%, the program internally calls the USERCNPROP subroutine. For example, to specify your own normal contact stiffness, FKN, you would issue a command similar to the following: RMODIF,NSET,3,%_CNPROP%


You can define the real constant as a function of certain input arguments as described in the following table: USERCNPROP Input Argument

Description

kstat

Contact status

stress(1)

Tangential stress in direction 1

stress(2)


stress(3)

Contact normal pressure (positive value for compression; negative value for tension)

stress(4)

Heat flux flowing into contact surface

stress(5)

Heat flux flowing into target surface

stress(6)

Electric current density flowing from contact surface to target surface

Certain passed-in arguments such as contact status (kstat), contact pressure (stress(3)), and tangential stresses (stress(1) and stress(2)) are not current (that is, from the current iteration) for real constants FKN, TAUMAX, CNOF, FKOP, FKT, PPCN. FDMN, FDMT. In this case, the values from the previous iteration are used instead. However, the contact status, contact pressure, and tangential stresses are current for all non-structural related real constants (such as TCC, ECC, MCC, RDVF). Other input arguments include those listed in the following table. These arguments are all current.

72


User-Defined Subroutines for Contact Interfacial Behaviors USERCNPROP Input Argument

Description

strain(1)

Tangential slip increment in direction 1

strain(2)

Tangential slip increment in direction 2

strain(3)

Normal penetration/gap (positive value represents penetration, negative value represents gap)

strain(4)

Temperature at the contact detection point

strain(5)

Temperature at the target point

strain(6)

Voltage at the contact point

strain(7)

Voltage at the target point

strain0(i)

Array variable containing the above values at the end of the previous substep

intIn(1) to intIn(21)

Integer values such as element number, analysis type, and so on

realIn(1) to realIn(12)

Real values such as contact element length, pinball radius, and so on

Real constants can be defined in the time or frequency domain, depending on the analysis type. For static and transient analyses, the input argument realIn(7) represents the current time value. For harmonic analysis, realIn(7) represents the current frequency value. You can also define your real constants as a function of extra solution-dependent state variables. The extra state variables can be updated and used within the user subroutine. Use the NSVR command to specify the number of extra state variables that need to be saved for the USERCNPROP subroutine, as described in the table of input/output arguments shown below. USERCNPROP Input/Output Argument

Description

Notes

usvr

User-defined state variables

Use the NSVR command to size the usvr array and to set nuval to the same value as the number of variables on NSVR. This array is passed in as the values at the beginning of the substep. If any of the solution-dependent state variables are being used for the user-defined interaction behavior, they must be updated to the values at the end of the substep (when kupdhis = 1).

keyerr

Element level error key

Key to indicate if there are any element formulation errors such as abrupt changes in contact status or too much penetration. The error could be caused by an incremental step that is too large or an illegal model. = 0, no error (preset value before calling) = 1, error(s) occurred


73

Surface-to-Surface Contact USERCNPROP Input/Output Argument

Description

Notes

The program decides to stop the analysis or cutback the substep (bi-section) based on other user input and information at a higher level. The user subroutine passes in the position of the real constant in the real constant table (kcnprop), and you need to return the value of the relevant real constant, cnprop(1). If the real constant is a function of contact gap/penetration, contact pressure, or temperature, you can also return the associated derivatives as described in the following table: USERCNPROP Output Argument

Description

Notes

cnprop(1)

Updated value of the real constant

cnprop(2)

Derivative of the real constant with respect to the gap function

For example, the thermal conductance TCC can be defined as a function of geometric penetration/gap (positive value for penetration, negative value for gap). If the real constant is not a function of gap, you simply return cnprop(2) = 0.0.

cnprop(3)

Derivative of the real constant with respect to contact pressure

For example, the thermal conductance TCC can be defined as a function of contact pressure (positive value for compression, negative value for tension). If the real constant is not a function of pressure, you simply return cnprop(3) = 0.0.

cnprop(4)

Derivative of the real constant with respect to temperature at the contact detection point

If the real constant is not a function of temperature at the contact point, you simply return cnprop(4) = 0.0.

cnprop(5)

Derivative of the real constant with If the real constant is not a function of temperatrespect to temperature at the target ure at the target, you simply return cnprop(5) = 0.0.

The subroutine can be called multiple times for a contact detection point during each iteration, based on how many real constants are defined through the label %_CNPROP%. Be careful to define the correct position of the user-defined variables (usvr) during each call. Otherwise, the previously defined variables could be overwritten during subsequent access to the subroutine within an iteration. You can preassign the contact pair ID (RID in the example below) to a parameter (‘PAIRID’ in the example below) and then call subroutine PAREVL inside of USERCNPROP to get the contact pair ID. Follow the example below as a template for using this function inside of USERCNPROP: cName = 'PAIRID' subc(1) = 0.0d0 call parevl(cName,0,subc(1),2,RID,string,kerr)

In this way, one unified USERCNPROP routine can handle contact properties not only for different real constants, but also for different contact pairs.

74


User-Defined Subroutines for Contact Interfacial Behaviors

3.10.2. Writing Your Own Friction Law (USERFRIC) The USERFRIC subroutine allows you to program your own friction model for 2-D and 3-D contact elements (CONTA171, CONTA172, CONTA173, CONTA174, CONTA175, CONTA176, CONTA177, and CONTA178) when the existing program-supplied friction models are not adequate. The frictional stresses can be defined as a function of variables such as slip increments, sliding rate, temperature, and other input arguments. You can specify a number of properties or constants associated with your friction model, and you can introduce extra solution-dependent state variables that can be updated and used within the subroutine. Some of the most commonly used input and output arguments for USERFRIC are discussed below. For a complete list of input and output arguments, see the description of USERFRIC in the Guide to UserProgrammable Features. After writing a USERFRIC subroutine to program your friction model, you incorporate the model in your analysis by using the TB,FRIC command with TBOPT = USER and specifying the friction properties on the TBDATA command, as shown below. The number of friction properties is defined through NPTS on the TB,FRIC command. TB,FRIC,1,,2,USER TBDATA,1,PROP1,PROP2

! Activate user defined friction model; NPTS = 2 ! Define friction properties

Field variables specified with the TBFIELD command are not available for TB,FRIC,,,,USER. Use the NSVR command to specify the number of extra state variables that need to be saved for the USERFRIC subroutine. Use the USEROU subroutine to store user-supplied element outputs in non-summable miscellaneous (NMISC) records. You must enter the USRCAL command in order to activate the USEROU subroutine. The maximum number of user-defined output items should not exceed the number of user-defined state variables. The USERFRIC subroutine is called at the contact detection point when the contact is closed. Some of the input arguments are described in the following table: USERFRIC Input Argument

Description

dslip(ncomp)

Slip increment for the current subset

pres

Current normal contact pressure (positive value for compression; negative value for tension)

tau(ncomp)

Frictional stress component at the beginning of the substep.

kn

Current normal contact stiffness

kt

Tangential contact stiffness

Notes

The passed-in values include the contribution from Lagrange multipliers if the Lagrange multiplier method (KEYOPT(2) = 4) is specified. You can use the passed-in values for computing the frictional stresses. However, you can also pick a suitable value that allows minimal tangential elastic slip during sticking without adversely affecting the convergence. A possible choice could


75

Surface-to-Surface Contact USERFRIC Input Argument

Description

Notes be kt = mu*kn. For the Lagrange multiplier method (KEYOPT(2) = 4), you should use a small kt (several orders of magnitude smaller than mu*pres).

tcont

Current temperature at the contact point

ttarg

Current temperature at the target point

uprop(npropu)

User-defined frictional properties defined through the TB,FRIC command

kupdhis

Key to indicate if history-dependent variables (user-defined) need to be updated after the substep has converged

kupdhis = 1: update (converged) kupdhis = 0: do not update (not converged)

Some of the input/output arguments are described in the following table: USERFRIC Input/Output Argument kstat

Description

Contact status: far-field (kstat = 0) near-field (kstat = 1) sliding (kstat = 2) sticking (kstat = 3)

Notes

You can make decisions about the sticking/sliding condition based on the slip increments and the updated frictional stresses.

tau(ncomp)

Updated frictional stress

This is passed in as the values of the frictional stress component at the beginning of the substep and must be updated to the values at the end of the substep. The passed-in values include the contribution from Lagrange multipliers, if the Lagrange multiplier method (KEYOPT(2) = 4) is defined. In the user subroutine, you can define the friction stresses as a function of slip increment (dslip), normal contact pressure (pres), and other input arguments or user-defined state variables.

slip(ncomp)

Total accumulated slip

This is the accumulated value of dslip from the previous substeps.

mu

Friction coefficient

This is passed in as the value at the beginning of the current substep. It is updated to be the value at the end of the current substep.

You must define the following output arguments:

76


User-Defined Subroutines for Contact Interfacial Behaviors USERFRIC Output Argument

Description

Notes

dt(i,j)

Partial derivative of frictional stresses and normal pressure with respect to the slip increments, normal gap distance, and temperatures

The partial derivative of frictional stresses with respect to the temperatures dt(1,4), dt(1,5), dt(2,4), and dt(2,5) can be used only for coupled thermalstructural and coupled thermal-electrical-structural analyses in which the frictional stresses are defined as a function of the surface temperatures.

dtdp(ncomp)

Partial derivative of frictional stresses with respect to the normal pressure

This contributes to the element stiffness matrix when a Lagrange multiplier method (KEYOPT(2)= 3 or 4) is used.

usvr(nuval)


Use the NSVR command to size the usvr array and to set nuval to the same value as the number of variables on NSVR. This array is passed in as the values at the beginning of the substep. If any of the solution-dependent state variables are being used for the user-defined friction behavior, they must be updated to the values at the end of the substep (when kupdhis = 1).

fdiss

Incremental frictional dissipation

The units are energy per unit area. For purely structural analysis, this argument is used for output only and has no effect on the solution. In coupled thermal-structural and coupled thermalelectrical-structural analyses, the dissipation is converted into heat.

elener

Incremental elastic stored energy density

This argument is used for output only and has no effect on the solution.

3.10.3. Defining Your Own Contact Interaction (USERINTER) As an alternative to the USERFRIC subroutine that allows for user-defined frictional behavior, subroutine USERINTER is available for defining complex interactions between contacting surfaces, including interactions in the normal and tangential directions, and coupled-field interactions. You can write a USERINTER subroutine to program your own contact interactions for 2-D and 3-D contact elements (CONTA171, CONTA172, CONTA173, CONTA174, CONTA175, CONTA176, CONTA177, and CONTA178) when the existing interaction models are not adequate. The contact normal stress, tangential stress, and heat flux can be defined as a function of variables such as slip increment, sliding rate, temperature, and other input arguments. You can specify a number of properties or constants associated with your interaction model, and you can introduce extra solution-dependent state variables that can be updated and used within the subroutine. Some of the most commonly used input and output arguments for USERINTER are discussed below. For a complete list of input and output arguments, see the description of USERINTER in the Guide to User-Programmable Features. After writing a USERINTER subroutine to program your interaction model, you incorporate the model in your analysis by using the command TB,INTER with TBOPT = USER and specifying the interaction properties on the TBDATA command as shown below. The number of interaction properties is defined through NPTS on the TB,INTER command. Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

77

Surface-to-Surface Contact TB,INTER,1,,2,USER TBDATA,1,PROP1,PROP2

! Activate user defined contact interaction; NPTS = 2 ! Define interaction properties

Field variables specified with the TBFIELD command are not available for TB,INTER,,,,USER. Use the NSVR command to specify the number of extra state variables that need to be saved for the USERINTER subroutine. Use the USEROU subroutine to store user-supplied element outputs in non-summable miscellaneous (NMISC) records. You must enter the USRCAL command in order to activate the USEROU subroutine. The maximum number of user-defined output items should not exceed the number of user-defined state variables. The USERINTER subroutine is called at each contact detection point in each iteration. In a structural analysis, you must define the current stresses. In a coupled thermal-structural analysis or a coupled thermal-electrical-structural analysis, you must also define the heat flux and electric current density, respectively, across the interface for the current time. The interaction calculation is based on (a) the relative motion between the contact detection point and the associated intersecting target point, (b) temperatures, and (c) user-defined state variables. The state variables can be used and updated within this subroutine. You must also define the material tangent modulus (interface stiffness matrix) to ensure proper convergence behavior. If effects of contact damping are taken into account, you need to define the interface damping matrix. The interface damping matrix can be used in a complex modal analysis, a full transient structural analysis, or a harmonic analysis at a given frequency. Some of the input arguments are described in the following table: USERINTER Input Argument

Description

Notes

dslip(ncomp)

Current strain component

strain(1)

Slip increment in direction 1

Mechanical component.

strain(2)

Slip increment in direction 2 (3-d only)

Mechanical component.

strain(3)

Contact normal penetration/gap Mechanical component. (positive value represents penetration, negative value represents gap)

strain(4)


strain(5)

Temperature at the target intersect- This argument is generally used for coupled ing point thermal-structural or thermal-electric-structural analysis. The temperature components strain(4) and strain(5) can also be used for a structural analysis as long as they are defined through temperature loadings.

strain(6)

Voltage difference between contact This argument is used for coupled thermal-elecand target tric-structural analysis.

78

This argument is generally used for coupled thermal-structural or thermal-electric-structural analysis. The temperature components strain(4) and strain(5) can also be used for a structural analysis as long as it is defined through temperature loadings.


User-Defined Subroutines for Contact Interfacial Behaviors USERINTER Input Argument

Description

uprop(npropu)

User-defined interfacial properties (defined through the TB,INTER command)

kupdhis

Key to indicate if history-dependent variables (user-defined) need to be updated after the substep has converged

intIn(1) to intIn(20)

Integer values such as element number, analysis type, and so on

realIn(1) to realIn(14)

Real values such as contact element length, pinball radius, and so on

Notes

kupdhis = 1: update (converged) kupdhis = 0: do not update (not converged)

The USERINTER subroutine can be used in the time or frequency domain, depending on the analysis type. For static and transient analyses, the input argument realIn(7) represents the current time value. For harmonic analysis, the input argument realIn(7) represents the current frequency value. Some of the input/output arguments are described in the following table: USERINTER Input/Output Argument kstat

Description

Contact status: far-field (kstat = 0) near-field (kstat = 1) sliding (kstat = 2) sticking (kstat = 3)

Notes

You can make decisions about the contact status based on the relative position of the contact detection point with respect to the target surface, the slip increments, and other state variables.

stress(1)


The passed-in values include the contribution from Lagrange multipliers if the pure Lagrange multiplier method (KEYOPT(2) = 4) is specified.

stress(2)

Tangential stress in direction 2 (3-D only)

The passed-in values include the contribution from Lagrange multipliers if the pure Lagrange multiplier method (KEYOPT(2) = 4) is specified.

stress(3)

Contact normal pressure (positive value for compression, negative value for tension)

The passed-in values include the contribution from Lagrange multipliers if a Lagrange multiplier method (KEYOPT(2) = 3 or 4) is specified.

stress(4)

Heat flux (per area) flowing into the Defined for a coupled thermal-structural or a contact surface (positive value is thermal-electric-structural analysis. heat flowing out of the surface; negative value is heat flowing into the surface)

stress(5)

Heat flux (per area) flowing into the Defined for a coupled thermal-structural or a target surface (positive value is heat thermal-electric-structural analysis. flowing out of the surface; negative value is heat flowing into the surface)


79

Surface-to-Surface Contact USERINTER Input/Output Argument

Description

Notes

stress(6)

Electric current density (per area) flowing from contact to target element (positive value is current flowing from contact surface to target surface; negative value is current flowing from target surface to contact surface)

Defined for a coupled thermal-electric-structural analysis.

usvr(nuval)


Use the NSVR command to size the usvr array and to set nuval to the same value as the number of variables on NSVR. This array is passed in as the values at the beginning of the substep. If any of the solution-dependent state variables are being used for the user-defined interaction behavior, they must be updated to the values at the end of the substep (when kupdhis = 1).

mu

Friction coefficient

This is passed in as the value at the beginning of the current substep. It is updated to be the value at the end of the current substep.

keycnv

Element level convergence

Key to flag if this element satisfies the user defined element convergence criterion. = 1, yes, the criterion is satisfied or there is no criterionl (preset value before calling) = 0, no, the element does not satisfy the element convergence criterion. If this is the case, the iteration will not converge but will continue, even when both force and displacement converge.

You must define the following output arguments: USERINTER Output Argument

Description

Notes

dt(ncomp,ncomp)Partial derivative of the ith stress component with respect to the jth component of the relative strain increment array to ensure proper convergence behaviors

Terms dt(i,,j) (i > 3 or j > 3) are only used for a coupled thermal-structural or a coupled thermalelectrical-structural analysis in which the stress component is a function of the surface temperatures/voltage or the flux is a function of mechanical strain increments (gap/slip).

dtdp(ncomp)

Partial derivative of the frictional stresses with respect to the normal pressure

This argument contributes to the element stiffness matrix when a Lagrange multiplier method (KEYOPT(2)= 3 or 4) is used.

damp(3,3)

Interface damping matrix (structure This argument contributes to the element damponly) ing matrix.

80


User-Defined Subroutines for Contact Interfacial Behaviors USERINTER Output Argument

Description

Notes

fdiss

Incremental frictional dissipation

The units are energy per unit area. For purely structural analysis, this argument is used for output only and has no effect on the solution. In coupled thermal-structural and coupled thermalelectrical-structural analyses, the dissipation is converted into heat.

elener

Incremental elastic stored energy density

This argument is used for output only and has no effect on the solution.

keyerr

Element level error key

Key to indicate if there are any element formulation errors such as abrupt changes in contact status or too much penetration. The error could be caused by an incremental step that is too large or an illegal model. = 0, no error (preset value before calling) = 1, error(s) occurred The program decides to stop the analysis or cutback the substep (bi-section) based on other user input and information at a higher level.

When using a Lagrange multiplier method for normal constraint (KEYOPT(2) = 3 or 4), you should pick a very small normal contact stiffness, kn (several orders of magnitude smaller than the contact pressure). The passed-in normal pressure (stress(3)) includes the contribution from the normal Lagrange multiplier. When using the Lagrange multiplier method for tangential constraint (KEYOPT(2) = 4), you should pick a very small tangential contact stiffness, kt (several orders of magnitude smaller than mu*pressure). The passed-in tangential stresses (stress(1), stress(2)) include the contributions from the tangential Lagrange multipliers. Use the following example as a template to activate the user subroutines. You can preassign the contact pair ID (RID in the example below) to a parameter (‘PAIRID’ in the example below) and then call subroutine PAREVL inside of USERINTER to get the contact pair ID. Follow the example below as a template for using this function inside of USERINTER: cName = 'PAIRID' subc(1) = 0.0d0 call parevl(cName,0,subc(1),2,RID,string,kerr)

In this way, one unified USERINTER routine can handle different interactions based on contact pair ID. Example 3.1: Using the /UPF Command to Link USERINTER ! The /UPF command offers the simplest method for linking ! user-programmable features into ANSYS. ! It cannot be used when ANSYS is run in interactive mode ! /UPF,userinter.F /UPF,userou.F


81

Surface-to-Surface Contact /prep7 … ! ! define contact type ! ITYPE = 4 et,ITYPE,172 ! !Define user interactions ! TB,INTER,1, ,1,USER tbdata,1,0.42 ! define friction coefficient. !Additional saved variables ! The size of this array is: ! no of variables(nuval)* no of integration points(nintp). ! nsvr,ITYPE,5*2 … /solu … ! ! Activate user subroutine userou ! usrcal,userou solve /post1 … ! output additional state variables ! ! You can find the maximum number of NMISC for contact elements by ! issuing the following function call ! maxnmisc=etyiqr(ITYPE,-110) etab,MU1,NMISC,maxnmisc+1 etab,AASR1,NMISC,maxnmisc+2 etab,AASS1,NMISC,maxnmisc+3 etab,RASR1,NMISC,maxnmisc+4 etab,RASS1,NMISC,maxnmisc+5 etab,MU2,NMISC,maxnmisc+6 etab,AASR2,NMISC,maxnmisc+7 etab,AASS2,NMISC,maxnmisc+8 etab,RASR2,NMISC,maxnmisc+9 etab,RASS2,NMISC,maxnmisc+10 ! ! Output the user defined variables ! pretab,MU1,AASR1,AASS1,RASR1,RASS1 pretab,MU2,AASR2,AASS2,RASR2,RASS2

3.11. Controlling the Motion of the Rigid Target Surface Rigid target surfaces are defined in their original configuration, and the motion of the entire surface is then defined by the motion on the pilot node (or the different nodes of the target surface if no pilot node was defined). You must use a pilot node in any of the following situations to control the boundary conditions (and motion) of the entire target surface: • The target surface is subjected to applied forces.

82


Controlling the Motion of the Rigid Target Surface • The target surface is subjected to rotations. • The target surface is connected to other elements (e.g., structural mass element MASS21). • The motion of the target surface is adjusted by the equilibrium condition. • When modelling surface-based constraints or a rigid body. The degrees of freedom of the pilot node represent the motion of the entire rigid surface, including two translational and one rotational degree of freedom in 2-D, and three translational and three rotational degrees of freedom in 3-D. You can apply boundary conditions (displacement, initial velocity), concentrated loads, rotations, etc. to the pilot node. To account for a rigid body's mass, define a mass element (MASS21) on the pilot node. You can also define a follower element (FOLLW201) on the pilot node; the element-specified external forces and moments will follow the motion of the pilot node. By default, KEYOPT(2) = 0 for the target element, the program checks the boundary conditions for each target surface. If all of the following conditions are met, then the program treats the target nodes along the respective degree of freedom as fixed: • There are no explicit boundary conditions or prescribed forces for target surface nodes. • Target surface nodes are not connected to other elements. • Neither constraint equations nor node coupling have been used to constrain such nodes. At the end of each load step, the program releases the constraint conditions that were set internally. The constraint conditions stored in the results file (Jobname.RST) and the database file (Jobname.DB) may be updated due to this change. You should carefully verify whether the current constraint conditions are expected before restarting an analysis or resolving the problem in interactive mode. If you wish, you can control the constraint conditions of target nodes by setting KEYOPT(2) = 1 in the target element definition. Keep in mind the following restrictions on the target surface when using a pilot node: • Each target surface can have only one pilot node. • The pilot node can be one of the nodes on the target elements or a node at any arbitrary location. However, it should not be a node on the contact element. The location of the pilot node becomes important only when rotations or moments are to be applied. For each pilot node, the program automaticallys define an internal node and an internal constraint equation. The rotational DOF of the pilot node is connected to the translational DOF of the internal node by the internal constraint equation. Generally, you should not apply external constraint equations (CE) or node coupling (CP) to the pilot node. • The program ignores all boundary conditions on all nodes other than the pilot node when KEYOPT(2) = 0 (default) for the target elements. • When KEYOPT(2) = 0 (default), only the pilot node can connect to other elements. • By setting KEYOPT(2) = 1 for the target elements, you can apply boundary conditions on any rigid target nodes rather than only on the pilot node. In this case, it is your responsibility to make sure


83

Surface-to-Surface Contact the rigid target surface is not under-constrained or over-constrained. It is still recommended that you apply all boundary conditions on the pilot node, even when KEYOPT(2) = 1.

Note For flexible-to-flexible contact, no special boundary conditions treatment is performed, and KEYOPT(2) = 0 should be used.

3.12. Applying Necessary Boundary Conditions to the Deformable Elements You can apply any necessary boundary conditions as you would for any analysis. For more information about applying boundary conditions, see the appropriate analysis descriptions in the Structural Analysis Guide.

3.13. Applying Fluid Pressure-Penetration Loads Pressure-penetration loads can simulate surrounding fluid or air penetrating into the contact interface, based on the contact status. You can apply pressure-penetration loads to flexible-to-flexible or rigid-toflexible contact pairs. 2-D and 3-D surface-to-surface contact elements (CONTA171, CONTA172, CONTA173, CONTA174) support pressure-penetration loading. Fluid pressure can penetrate into the contact interface from one or multiple locations. The fluid pressurepenetration load has a path dependent nature. The penetrating path can propagate and vary, and it will be determined iteratively. At the beginning of each iteration, the program first detects starting points which are exposed to the fluid pressure. Among the starting points, the program then finds fluid penetrating points where the contact status is open or lost, or where the contact pressure is smaller than the user defined pressure-penetration criterion. When a contact detection point has a contact condition of “penetrating,” it is subjected to the fluid pressure, and its nearest neighboring nodes are considered to be the starting points which are exposed to the fluid pressure as well. The fluid pressure will not be applied to an area having a contact status of open unless the edges/ends of the area belong to the starting points. Figure 3.27: Path Dependent Fluid Penetration Loading Fluid pressure is applied

Starting Point

84


Applying Fluid Pressure-Penetration Loads The fluid pressure starts to penetrate into the interface between contact and target surfaces from the penetrating points. The fluid penetration can be cut off when contact between the surfaces is reestablished or when contact pressure is larger than the fluid penetration criterion. To model fluid penetration loads, you need to specify the following quantities: • fluid pressure • fluid penetration starting points • fluid penetration criterion • fluid penetration acting time An example analysis showing how to apply fluid penetrating loading is presented in Appendix A (p. 171).

3.13.1. Applying Fluid Penetration Pressure The fluid pressure must be applied to contact and target elements using the SFE command: SFE,ELEM,1,PRES,,VAL1,VAL2,VAL3,VAL4

The pressure is applied only on the corner nodes of the contact and target elements. The pressure on the midside nodes of CONTA172, CONTA174, TARGE169, and TARGE170 will be averaged using the pressures of two adjacent corner nodes. VAL3 and VAL4 are not used for 2-D contact and target elements. Pressure value VALi, which is applied to the ith node (where i = 1, 2, 3, 4 indicates node I, J, K, L, respectively) of the contact or target element, can be a constant numerical value or a table name. If it is constant, the magnitude of pressure will be step-applied or ramped based on the KBC command setting. To specify a table, enclose the table name in percent signs (i.e., %tabname%). Use the *DIM command to define the table. Only one table can be specified, and it must be specified in the VAL1 position; tables specified in the VAL2, VAL3, and VAL4 positions will be ignored. The fluid pressure-penetration load will be automatically applied to the penetrating points on contact and target surfaces based on the contact status. By default (KEYOPT(14) = 0 on the contact element), the fluid pressure-penetration load varies during iterations based on the current contact status. In certain cases, the default can cause an unstable convergence pattern since the contact status and the resulting applied fluid penetration load keep changing during iterations. When KEYOPT(14) = 1, the fluid pressure-penetration load will be applied to the contact and target elements at the beginning of each substep and will remain constant over that substep even if the contact status keeps changing during iterations. Small increments are often needed to obtain accurate results. Keep the following points in mind when defining fluid penetration loads: • For flexible-to-flexible contact with a symmetric contact pair definition (including a self-contact pair), you should apply the fluid pressure only to the contact elements. • For flexible-to-flexible contact with an asymmetric contact pair definition, you should generally apply fluid pressure on both contact and target elements which are currently or will potentially be exposed to the surrounding fluid. The program ignores fluid penetration loads applied to a target surface if there are Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

85

Surface-to-Surface Contact no fluid penetration loads applied to the associated contact surface within the contact pair. When the fluid pressures are applied to both contact and target elements, the program has to identify the penetration paths for both the contact surface and the target surface. The iterative process of determining the penetration path on the target surface is very time-consuming, particularly for 3-D contact models. Use a symmetric contact pair definition, therefore, as it does not require the specification of fluid penetration pressure on the target surface. • For rigid-to-flexible contact, you should apply the fluid pressure only to the contact elements. The program automatically applies equivalent forces to the rigid target surface to balance out the applied pressure on the contact surface. Fluid pressure applied to the rigid target surface will be ignored. • For situations in which multiple contact pairs are defined on the same surface, contact elements may overlap each other. In this case, be careful to apply the fluid penetration pressure only once in areas where contact elements overlap. • The fluid penetration pressure can only be applied to the contact and target elements using the SFE command. Other pressure load commands (SF, SFL, SFA) can not be used. In addition, you should not apply the pressure on the underlying elements. • The program ignores any fluid penetration pressures applied to MPC based contact pairs. • The effects of pressure load stiffness are automatically included. If an unsymmetric matrix is needed to achieve convergence for pressure load stiffness effects, issue the NROPT,UNSYM command.

3.13.2. Specifying Fluid Penetration Starting Points When a fluid pressure-penetration load is applied, the fluid pressure penetrates to the surface from defined starting points. There can be one or multiple starting points. The program automatically finds the default starting points by selecting free end points of 2-D contact/target surfaces or nodes of free open edges on 3-D contact/target surfaces ("free" meaning that the element is not fully surrounded by adjacent elements; see figure below). Figure 3.28: Free End Points (2-D) and Free Open Edges (3-D) Free End Points Contact Surface

Contact Surface

Free Open Edge Target Surface

Free End Points

2D contact: Free End Point

86

Target Surface

3D contact: Free Open Edge


Applying Fluid Pressure-Penetration Loads The starting points are initially exposed to the fluid and are potentially subjected to the penetration pressure. There are no default starting points if the contact or target surface is continuous with a closed loop. The default starting points can be overwritten using the SFE command. You can specify starting points, specify penetrating points, and remove the default starting points with the SFE command and STAi values, as described below. Be sure to set LKEY = 2 on the SFE command in order to specify the STAi settings. The command format is: SFE,ELEM,2,PRES,,STA1,STA2,STA3,STA4

STAi = 0 (default)

The program determines whether the ith node is a starting point based on the contact status. The ith node can be a default starting point if the node is a 2-D free point or is on a 3-D free edge.

STAi = 1

The ith node is the starting point which is initially exposed to the fluid. It can be a penetrating point if the initial contact status is "open." The node may no-longer be the starting point when the contact status changes during the deformation process.

STAi = 2

The ith node is a penetrating point. The node is always subjected to the fluid pressure in spite of any contact status change.

STAi = -1

The ith node will not be a default starting point even though it belongs to a 2-D free point or a 3-D free edge node.

Note If only STA1 is specified and the other STAi values are blank, STA2, STA3, and STA4 will default to STA1.

3.13.3. Specifying a Pressure-Penetration Criterion You can specify a pressure-penetration criterion using the contact element real constant PPCN. When the contact pressure is less than the criterion, the starting point turns into the penetrating point; that is, fluid pressure starts to penetrate. Thus, a higher criterion value will allow the fluid to penetrate more easily. When the contact pressure is greater than the criterion, the penetrating point returns back to the starting point; that is, fluid penetration is cut off. By default, the penetration criterion (PPCN) is zero. In this case the fluid penetration occurs only when the contact is open, and the cutoff of fluid penetration occurs only when the contact is reestablished. You can input PPCN as a constant value or as a table of values. The tabular input can be a function of initial contact detection point location (at the beginning of solution), contact pressure (positive index values for PRESSURE), geometric penetration (positive index values for GAP), time, or temperature. To input a table name, you must enclose the name in % symbols (i.e., %tabname%). Use the *DIM command to define the table. For more information, see Defining Real Constants in Tabular Format (p. 34). The user subroutine USERCNPROP.F is also available for defining PPCN. To use this subroutine, you must specify the table name %_CNPROP% as the real constant value. For more information, see Defining Real Constants via a User Subroutine (p. 35).

3.13.4. Specifying a Fluid Penetration Acting Time When the fluid pressure penetration occurs, the fluid pressure is applied normal to the contact/target surfaces. As with conventional pressure loading, the current amount of fluid pressure at a given substep will depend on whether the pressure is input as a constant value or a table of values, and whether the pressure is ramp- or step-applied (KBC command). If the total amount of fluid pressure is applied inRelease 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

87

Surface-to-Surface Contact stantaneously, convergence difficulties may arise due to large changes in stresses near the contact interface. This is also true when the fluid penetration pressure is removed instantaneously, as when the fluid penetration is cut off. To help stabilize the solution, the program offers an option to ramp the fluid pressure linearly over a time period, during one or several substeps. To implement this ramping option, specify the fluid penetration acting time using the contact element real constant FPAT. Input a positive number to define the fraction of the time increment of the load step; input a negative number to define the absolute acting time. The default penetration acting time is 0.01 times the time increment of the current load step. At each penetrating point, if the time increment of the current substep is less than the fluid penetration acting time (FPAT > (tn - tn-1)), the fluid pressure is ramped up linearly from the actual applied pressure of the previous substep to the full current amount of the fluid pressure over the penetration acting time period; otherwise (FPAT ≤ (tn - tn-1)), the full amount of current fluid pressure will be applied. (See figures below.) At the pressure-penetration cutoff points, if the time increment of the current substep is less than the fluid penetration acting time, the fluid pressure is ramped down linearly from the applied pressure of the previous substep to a zero magnitude over the penetration acting time period; otherwise, the fluid pressure will be immediately removed. Figure 3.29: Fluid Penetration Acting Time (FPAT) Greater Than Substep Pressure (tn-1)

FPAT

Current Applied Pressure (input via SFE,,,PRES) Actual Applied Pressure (reported as output item FPRS)

Time tn-1

tn

Acual applied pressure is ramped to the full current applied pressure when FPAT>(tn-tn-1)

88


Applying Fluid Pressure-Penetration Loads Figure 3.30: Fluid Penetration Acting Time (FPAT) Less Than Substep Pressure

FPAT

Current Applied Pressure (input via SFE,,,PRES)

Actual Applied Pressure (reported as output item FP

Time tn-1

tn

Acual applied pressure is ramped to the full current applied pressure when FPAT<(tn-tn-1)

3.13.5. Redefining or Modifying the Pressure-Penetration Loads You can redefine or modify fluid pressure-penetration loads and fluid penetration starting points between load steps using the SFE command. You can also modify the pressure-penetration criterion (real constant PPCN) and acting time (real constant FPAT) using the RMODIF command. To remove a fluid pressure-penetration load, use one of the following methods: • Delete the pressure load using the SFEDELE command. • Apply zero pressure using the SFE command. • Deactivate the contact elements using the EKILL command. Because these methods remove the fluid penetration pressure immediately, convergence difficulties may occur. To avoid this problem you can add an extra load step which applies a small fluid pressure (for example, VALi = 1e-8) via the SFE command instead of removing the pressure.

3.13.6. Postprocessing Fluid Pressure-Penetration Loads You can list and display the actual fluid penetration pressure applied on contact and target elements using FPRS as a contact result item on the PLNSOL, PLESOL, PRNSOL, and PRESOL commands. For example:


89

Surface-to-Surface Contact PLESOL,CONT,FPRS PLNSOL,CONT,FPRS

You can also list and display the fluid penetration pressure applied on target elements by using the ETABLE command.

3.14. Defining Solution and Load Step Options Convergence behavior for contact problems depends strongly on the particular problem. The options listed below are either typical or recommended for most surface-to-surface contact analyses. Please see the Command Reference for further details. The time step size must be small enough to capture the proper contact zone. The smooth transfer of contact forces is disrupted if the time step size is too large. The time step size is specified by a number of substeps or the time step size itself. The following commands adjust these parameters. Command(s): NSUBST GUI: Main Menu> Preprocessor> Loads> Load Step Opts> Time/Frequenc> Freq and Substps Main Menu> Preprocessor> Loads> Load Step Opts> Time/Frequenc> Time and Substps Main Menu> Solution> Analysis Type> Sol'n Control ( : Basic Tab) Main Menu> Solution> Unabridged Menu> Load Step Opts> Time/Frequenc> Freq and Substps Main Menu> Solution> Unabridged Menu> Load Step Opts> Time/Frequenc> Time and Substps Command(s): DELTIM GUI: Main Menu> Preprocessor> Loads> Load Step Opts> Time/Frequenc> Time-Time Step Main Menu> Solution> Analysis Type> Sol'n Control ( : Basic Tab) Main Menu> Solution> Unabridged Menu> Load Step Opts> Time/Frequenc>Time-Time Step

Note A reliable way to set an accurate time step size is to turn automatic time stepping on. The following options are automatically invoked, but may override them if needed. Command(s): AUTOTS,ON GUI: Main Menu> Solution> Analysis Type> Sol'n Control ( : Basic Tab) Main Menu> Solution> Unabridged Menu> Load Step Opts> Time/Frequenc> Time-Time Step or Time and Substps If the contact status changes during the iteration process, discontinuity can occur. To avoid a slow convergence rate and use an updated stiffness matrix, set the Newton-Raphson option to FULL. Command(s): NROPT,FULL,,OFF GUI: Main Menu> Solution> Unabridged Menu> Analysis Type> Analysis Options Also, do not use adaptive descent. Adaptive descent usually does not provide any help for surface-tosurface contact applications, and we recommend turning it off. In cases where frictional sliding dominates, set the unsymmetric solver option (NROPT, UNSYM,,OFF) to avoid slow convergence or divergence. Set the number of equilibrium equations to a number that is appropriate for a reasonable time step size. This command defaults to between 15 and 26 iterations, depending upon the physics of the problem. Command(s): NEQIT GUI: Main Menu> Solution> Analysis Type> Sol'n Control ( : Nonlinear Tab) Main Menu> Solution> Unabridged Menu> Load Step Opts> Nonlinear> Equilibrium Iter

90


Solving the Problem Because the iterations tend to become unstable for large increments, use the line search option to stabilize the calculations. Command(s): LNSRCH GUI: Main Menu> Solution> Analysis Type> Sol'n Control ( : Nonlinear Tab) Main Menu> Solution> Unabridged Menu> Load Step Opts> Nonlinear> Line Search Turn the predictor-corrector option on, except for dynamic analyses. Command(s): PRED GUI: Main Menu> Solution> Analysis Type> Sol'n Control ( : Nonlinear Tab) Main Menu> Solution> Unabridged Menu> Load Step Opts> Nonlinear> Predictor Many convergence failures in contact analyses are the result of using too large a value for contact stiffness (real constant FKN). Be sure to follow the recommendations given earlier in this chapter for estimating contact stiffness. If such estimated values lead to a convergence failure, reduce the contact stiffness and restart. Conversely, if overpenetration occurs in your contact analysis, you probably need a larger value of FKN. In this case, gradually increase the contact stiffness value to an appropriate level by redefining it using RMODIF commands over several load steps in a restart.

Note For most small strains, small displacements, and small sliding problems, set NLGEOM, OFF. This setting will speed up the searching time; however, if the contact problem involves large sliding, set NLGEOM, ON.

Note By default, displacement (U) and rotation (ROT) convergence checking are not active. Issue the CNVTOL,U or CNVTOL,ROT command to add displacement or rotation convergence checking, if necessary.

3.15. Solving the Problem You can now solve the analysis the same as you would for any nonlinear analysis. Keep the following points in mind: • Always check the real constant sets which are related to contact pairs and check the constraint conditions on the target surfaces. Any previous "trial runs" could have changed the settings. • Always check the target surface contact status in the beginning of the analysis. If you detect any unexpected gap (or no contact) or overestimated penetration, abort the analysis and then check your geometric model. You can issue the CNCHECK command to verify the initial contact status. • If your model is experiencing convergence difficulties due to contact, use the NLHIST command as a debugging tool to monitor contact information during the solution. Before starting the solution, issue NLHIST to specify the pair-based contact items (such as contact penetration or gap, contact normal stiffness, etc.) to be tracked. The resulting data are written to a file named Jobname.nlh. NLDIAG,CONT can also be used to monitor contact information. When turned on, this feature will track the same contact information available through NLHIST for all defined contact pairs


91

Surface-to-Surface Contact according to the specified writing frequency (each iteration, substep, or load step). The resulting output is stored in a text file called Jobname.cnd. • By default, contact pair output quantities are written to the output file (Jobname.OUT) or to the screen, as specified by the /OUTPUT command. You can redirect contact pair output to a text file named Jobname.CNM by issuing the command CNTR,OUT,YES before the solution begins. • Always check your results carefully using standard engineering guidelines. See Solve the Analysis in the Structural Analysis Guide. If you are restarting a contact analysis, follow the normal restart procedures as discussed in Restarting an Analysis in the Basic Analysis Guide. However, be aware that the constraint conditions of target surfaces may have been set internally. Verify the constraints carefully before restarting an analysis. Only the real constants FKN, FTOLN, PINB, and FKOP can be changed, and they can only be changed at the point of restart or at the beginning of a new load step.

3.16. Reviewing the Results Results from a contact analysis consist mainly of displacements, stresses, strains, reaction forces, and the contact information (e.g., contact pressure, sliding, etc.). You can review these results in POST1, the general postprocessor, or in POST26, the time-history postprocessor. For contact-related results, you can select CONT as a plotting or list item. While in POST1, you can also review the results from within the Contact Manager (via the Contact Manager icon in the Standard Toolbar). See the "Output Data" section of the element descriptions (the Element Reference) for the available output components. Remember that in POST1, only one substep can be read in at a time, and that the results from that substep should have been written to Jobname.RST (or Jobname.RCN for the initial contact configuration calculated by CNCHECK,POST). (The load step option command OUTRES controls which substep results are stored on Jobname.RST.) A typical POST1 postprocessing sequence is described below.

3.16.1. Points to Remember Following are important guidelines for postprocessing contact analysis results: • When postprocessing results obtained from a contact analysis, the database must be saved after the first solution is finished because the nodal connectivity of contact elements is updated and internal nodes are added during the solution. • In order to reduce the size of the results file, contact element results (such as temperature, ETABLE quantities, contact pressure, sliding, and so on) are not stored in the results file if the contact status is farfield. In this case, to correctly plot or list nodal temperatures at contact surfaces, you must select only the underlying elements before the plot or list operation. See also Points to Remember in the Structural Analysis Guide.

3.16.2. Reviewing Results in POST1 The steps for reviewing results in POST1 are the same as those for a typical nonlinear analysis (see Reviewing Results in POST1 in the Structural Analysis Guide) with the following exceptions: For step 4, the following shows the various CONT options for the PLNSOL and PLESOL commands. You can set these options through the Main Menu> General Postproc> Plot Results> Contour Plot>

92


Reviewing the Results Nodal Solu or Element Solu menu item. Choose Contact for Item, Comp and you'll see a list of the options detailed below. CONT

STAT

Contact status[1]: 3-closed and sticking 2-closed and sliding 1-open but near contact (near-field) 0-open and not near contact (far-field)

“

PENE

Contact penetration

“

PRES[2]

Contact pressure

“

SFRIC[2]

Contact friction stress

“

STOT[2]

Contact total stress (pressure plus friction)

“

SLIDE

Contact sliding distance

“

GAP

Contact gap distance

“

FLUX

Heat flux at contact surface

“

CNOS

Total number of contact status changes during substep

“

FPRS

Fluid penetration pressure (surface-to-surface contact only)

1. For MPC-based contact definitions, the value of STAT can be negative. This indicates that one or more contact constraints were intentionally removed to prevent over-constraint. STAT = -3 is used for MPC bonded contact; STAT = -2 is used for MPC no-separation contact. Negative values are valid for CONTA171, CONTA172, CONTA173, CONTA174, CONTA175, CONTA176, and CONTA177. 2. For the contact force-based model (used for CONTA175 with KEYOPT(3) = 0, CONTA176 with KEYOPT(3) = 0 or 1, and CONTA177 with KEYOPT(3) = 0), PRES, SFRIC, and STOT are the contact normal force, contact friction force, and total contact force, respectively. For rigid-to-flexible contact or asymmetric flexible-to-flexible contact, the contact element provides the true pressure and friction stress. However, for symmetric flexible-to-flexible contact, the true pressure and friction stress is the average of the pressures and friction stresses from both sides of the contact elements.

Note The contact results are only reported at the corner nodes of contact elements. Therefore, for higher order contact elements, mid-side node contact can occur without pressure being reported. In this case, the element’s contact status and contact pressure reported via ETABLE may give more precise information. Note that the contact-specific information (CONT) plots as follows. For 2-D contact analyses, the model will plot in gray and the requested item will be contoured as an area (trapezoid) along the edge of the model where the contact elements are located. Use the FACT item to scale 2-D contour size. For 3-D contact analyses, the model will plot in gray and the requested item will be contoured as a 2-D surface where the contact elements overlay the model. For tabular listings, you may also list contact-specific information by using the CONT label and its arguments with the PRNSOL or PRESOL commands or their related menu items.


93

Surface-to-Surface Contact You should not use the PLESOL or PRESOL command to obtain contact forces for contact elements. The force values reported by these commands may not be accurate for these elements. Instead, use the ETABLE command to obtain contact force values. Various sliding/slip quantities are reported through the ETABLE command. The table below described these quantities for the 3-D case. TASS/TASR

Algebraic sum of sliding components in S and R directions when the contact status is sliding or sticking (STAT = 2, 3).

AASS/AASR

Absolute sum of sliding components in S and R directions when the contact status is sliding or sticking F(STAT = 2, 3).

SLIDE

Amplitude of total accumulated sliding [SQRT*(TASS**2 + TASR**2)] when the contact status is sliding or sticking (STAT = 2, 3).

GSLID

Amplitude of total accumulated sliding when the contact status is near-field, sliding, or sticking (STAT = 1, 2, 3).

ELSI

Total equivalent elastic slip. This represents the reversible tangential motion from the point of zero tangential stresses.

PLSI

Total equivalent tangential plastic slip. This represents the accumulated irreversible frictional sliding when the contact status is sliding (STAT = 2).

VREL

Equivalent sliding velocity (slip rate).

You can also animate contact results over time. See the ANTIME command for details. Command(s): ANTIME GUI: Utility Menu> PlotCtrls> Animate> Over Time If you used CNCHECK,POST to evaluate the initial contact state, you can view the contact results items for the initial contact configuration as you would for any other load step. To do so, you must explicitly read the results of the first load step from the results file Jobname.RCN by issuing the FILE and SET,FIRST commands before postprocessing. Otherwise, the result file may be read improperly.

3.16.3. Reviewing Results in POST26 The steps for reviewing results in POST26 are the same as those for a typical nonlinear analysis See Reviewing Results in POST26 in the Structural Analysis Guide.

94


Chapter 4: Node-to-Surface Contact You can use the node-to-surface contact element CONTA175 to model flexible-flexible or rigid-flexible contact between a surface and a node. Additionally, you can use these elements to represent contact between two surfaces by specifying one surface as a group of nodes. Node-to-surface contact is a phenomenon that occurs in most engineering applications: fasteners (nuts, bolts, rivets, pins), metal forming, rolling operations, dynamic pipe whip, etc. Engineers are interested in the stresses, deflections, forces, and temperature changes that occur due to contact between structural parts. The following node-to-surface contact topics are available: 4.1.The Node-to-Surface Contact Element 4.2. Performing a Node-to-Surface Contact Analysis 4.3. Using CONTA175 for Multiphysics Contact

4.1. The Node-to-Surface Contact Element Node-to-surface contact is represented by following the positions of points on one surface (the contact surface, modeled by CONTA175) relative to lines or areas of another surface (the target surface, modeled by TARGE169 or TARGE170). CONTA175, in two or three dimensions, is depicted in Figure 4.1: Node-to-Surface Contact Elements (p. 95). Figure 4.1: Node-to-Surface Contact Elements

I Y (or Axial)

CONTA175

Y I CONTA175

target normal

target normal

2-D associated target surface (TARGE169)

3-D associated target surface (TARGE170) X (or Radial)

X z

Presented below are characteristics of CONTA175: • Has one node and its target surface is defined by TARGE169 or TARGE170. CONTA175 uses the same real constant set as the surface-to-surface contact elements. See Real Constants for more information. • Supports 2-D/3-D rigid-flexible and flexible-flexible contact.


95

Node-to-Surface Contact • Does not support 3-D contact surfaces with midside nodes, but can support 2-D/3-D target surfaces with midside nodes or 2-D contact surfaces with midside nodes. • Generates elements using the ESURF command, as do the surface-to-surface contact elements. • Supports structural, thermal, electric, and magnetic analyses.

Note CONTA175 is recommended for point-to-surface or edge-to-surface problems. It can also be used to supplement a surface-to-surface pair at strategic locations where edge contact exists. For general surface-to-surface contact, CONTA171 through CONTA174 are recommended.

4.2. Performing a Node-to-Surface Contact Analysis CONTA175 follows the contact pair concept used by surface-to-surface elements CONTA171 through CONTA174. CONTA175 is paired off with target elements TARGE169 and TARGE170. See Identifying Contact Pairs for more information. CONTA175 uses most of the same element KEYOPTS and real constants as the surface-to-surface contact elements. These are described below. You should avoid midside-noded underlying elements of the contact surface, especially in 3-D. The “effective stiffness” at the contact surface nodes is very nonuniform. For instance, for a 20-node brick (SOLID186), the corner nodes have a negative stiffness associated with them. However, the node-tosurface contact algorithm assumes that the stiffness is uniformly distributed across all the surface nodes when contact is made. This condition can lead to convergence difficulties when using midside-noded elements in contact. The midside-noded elements can only be used when bonded or no-separation contact is defined. You can still use midside nodes on 2-D contact surfaces or on 2-D/3-D target surfaces. The basic steps for performing a node-to-surface contact analysis using CONTA175 are the same as those used for a typical surface-to-surface analysis using CONTA171 through CONTA174. See Steps in a Contact Analysis for details. The Contact Manager provides an easy-to-use interface to help you construct and manage contact definitions. You can access the manager via the Contact Manager icon in the ANSYS Standard Toolbar, or via the menu path Main Menu> Preprocessor> Modeling> Create> Contact Pair. See GUI Aids for Contact Analyses (p. 7) for more information on using the Contact Manager. As mentioned, CONTA175 uses the ESURF command to generate elements between corresponding contact pairs, similar to the surface-to-surface contact elements. The GUI path is: Main Menu> Preprocessor> Modeling> Create> Elements> Surf/Contact> Node to Surf Since CONTA175 is a one node element, you are not able to plot the contact results. However, you can list the results using the PRESOL,CONT or PRETAB commands.

4.2.1. CONTA175 KEYOPTS CONTA175 uses most of the same KEYOPTS that are used by the surface-to-surface contact elements CONTA171 through CONTA174. KEYOPT(3) and KEYOPT(4) are used but have different meanings when used with CONTA175. See Element KEYOPTS (p. 35) for a listing of the remaining KEYOPTS.

96


Using CONTA175 for Multiphysics Contact

4.2.1.1. KEYOPT(3) KEYOPT(3) in CONTA175 allows you to choose between a contact force-based model (KEYOPT(3) = 0, default), and a contact traction-based model (KEYOPT(3) = 1). For the contact traction-based model, the program determines the area associated with the contact node. For the single point contact case, a unit area is used which is equivalent to the contact force-based model. When the traction-based model is defined, the real constants FKN, FKT, TNOP, TCC, ECC, and MCC have the same units used in surface-to-surface contact elements (CONTA171 through CONTA174), as do postprocessing items such as PRES, TAUR, TAUS, SFRIC, and so on. When the force-based model is defined, the units of these quantities have a factor of AREA with respect to those used in the traction-based model. For example, contact stiffness FKN has units FORCE/LENGTH for the force-based model, but FORCE/LENGTH3 for the traction-based model; PRES is the contact normal force in the force-based model, but contact pressure in the traction-based model. See the CONTA175 element description for detailed information on the units for these quantities. The traction-based model (KEYOPT(3) = 1) should not be used if the underlying elements are 3-D beam or pipe elements.

4.2.1.2. KEYOPT(4) KEYOPT(4) in CONTA175 allows you to choose the contact normal direction. The contact normal can be either perpendicular to the target surface (KEYOPT(4) = 0, default or KEYOPT(4) = 3), or perpendicular to the contact surface (KEYOPT(4) = 1, 2). When contact occurs on the bottom surface of a shell or beam, and shell thickness effect is included (KEYOPT(11) = 1), or CNOF is defined, KEYOPT(4) = 2 or 3 should be used in order to capture the contact. Real constant TOLS is used to add a small tolerance that will internally extend the edge of the target surface. TOLS is useful for problems where contact nodes are likely to lie on the edge of the target (as at symmetry planes or for models generated in a node-to-node contact pattern). In these situations, the contact node may repeatedly “slip” off the target surface and so completely out of contact, resulting in convergence difficulties from oscillations. Units for TOLS are percent (1.0 implies a 1.0% increase in the target edge length). A small value of TOLS will usually prevent this situation from occurring. The default value is 10 for small deflection and 2 for large deflection (NLGEOM,ON).

4.2.2. CONTA175 Real Constants CONTA175 uses the same real constants used by the surface-to-surface contact elements CONTA171 through CONTA174), except the units of FKN, FKT, TNOP, TCC, ECC, and MCC for the contact force-based model. See a listing of the real constants in Real Constants (p. 30).

4.3. Using CONTA175 for Multiphysics Contact You can use node-to-surface contact element CONTA175 to model thermal contact, electric contact, and magnetic contact as you would use the surface-to-surface contact elements. For multiphysics contact, we recommend that you use the contact traction-based model (KEYOPT(3) = 1), which allows you to use real constants TCC, ECC, and MCC consistently with the surface-to-surface contact elements. If you use the contact force-based model (KEYOPT(3) = 0), you should adjust the values of TCC, ECC, and MCC up or down as you make the element mesh coarser or finer. Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

97

98


Chapter 5: 3-D Beam-to-Beam Contact Contact between beams undergoing large displacements can be encountered in many practical applications. Some examples are hydrogen sensors, water supply lines, nuclear power plant piping, cable wires and coils, woven fabrics, and tennis racquet strings. You can model 2-D beam-to-beam contact by using 2-D surface-to-surface contact elements, CONTA171 and CONTA172. (See Surface-to-Surface Contact (p. 11) for details on how to use these elements.) You can model 3-D beam-to-beam contact by using the 3-D line-to-line contact element, CONTA176. The following beam-to-beam contact topics are available: 5.1.The 3-D Line-to-Line Contact Element 5.2. Modeling Beam-to-Beam Contact 5.3. Performing a 3-D Beam-to-Beam Contact Analysis

5.1. The 3-D Line-to-Line Contact Element 3-D line-to-line contact is represented by following the positions of lines on the contact surface, modeled by CONTA176 elements, relative to lines on the target surface, modeled by TARGE170 elements. Lineto-line contact in three dimensions is depicted in the figure below. Figure 5.1: Line-to-Line Contact Elements CONTA176

Y J X Z

K

I

3-D associated target line segments (TARGE170) The CONTA176 element type: • Has two nodes or three nodes. • Is paired with a target surface defined by TARGE170 elements. • Uses the same real constant set as used for the surface-to-surface contact elements. See Real Constants (p. 30) for more information. • Supports 3-D rigid-flexible and flexible-flexible contact. • Supports 3-D beams and pipes with or without midside nodes. • Can be generated by the ESURF command, similar to the surface-to-surface contact elements.


99

3-D Beam-to-Beam Contact

5.2. Modeling Beam-to-Beam Contact There are three different scenarios for 3-D beam-to-beam contact: • Internal contact, where one beam slides inside another hollow beam (Figure 5.2: Internal Contact (One Beam Sliding Inside Another) (p. 100)). • External contact, where two beams are roughly parallel and contact each other along their outer surfaces (Figure 5.3: External Contact (Two Beams Roughly Parallel) (p. 100)). • External contact, where contact between the exterior surfaces of two beams is pointwise (Figure 5.4: External Contact (Two Beams Cross Each Other) (p. 101)). Figure 5.2: Internal Contact (One Beam Sliding Inside Another)

rt

TARGE170

n rc d

CONTA176 Figure 5.3: External Contact (Two Beams Roughly Parallel) CONTA176

n

rc

d

rt

TARGE170

100


Performing a 3-D Beam-to-Beam Contact Analysis Figure 5.4: External Contact (Two Beams Cross Each Other) n CONTA176

rc

d

rt

TARGE170 For all three scenarios shown above, use 3-D line segments (straight line or parabola) to define the target surface (TARGE170). You can attach the contact elements and target elements to 3-D beams or pipes, which can be first or second order elements. Both flexible-flexible and rigid-flexible contact between two beams can be considered. When modeling internal contact, the inner beam (or pipe) should usually be considered the contact surface and the outer beam (or pipe) should be the target surface. The inner beam can be considered as the target surface only when the inner beam is much stiffer than the outer beam. When modeling external contact, the target surface should be associated with the stiffer beam or with the beam having the coarser mesh.

5.3. Performing a 3-D Beam-to-Beam Contact Analysis Contact element CONTA176 follows the contact pair concept used by the surface-to-surface elements (CONTA171 through CONTA174). You must pair CONTA176 with 3-D target line segments (TARGE170) to model 3-D beam-to-beam contact. See Identifying Contact Pairs (p. 12) for more information. CONTA176 uses most of the same element KEYOPTs and real constants as the surface-to-surface contact elements. Any differences are described in the next section. The basic steps for performing a 3-D beam-to-beam contact analysis are the same as those used for a typical surface-to-surface analysis using CONTA171 through CONTA174. See Steps in a Contact Analysis (p. 11) for details. Use the ESURF command to generate CONTA176 elements between corresponding contact pairs. (In the GUI, pick Main Menu> Preprocessor> Modeling> Create> Elements> Surf/Contact> Surf to Surf.) This procedure is similar to that used for the surface-to-surface contact elements. If the underlying elements of target element TARGE170 are part of shell edges, use ESURF,,,LINE to generate 3-D line or parabola target segments.


101

3-D Beam-to-Beam Contact When using the line segments to form the target surface or using line-to-line contact elements to form the contact surface, the nodes must be entered in a sequence that defines a continuous line, as shown in the figure below. Figure 5.5: Continuous Line Segments

The line can be made up of linear or parabolic segments, depending on whether the attached beam is made up of first order or second order elements. If the nodal ordering of the underlying beam elements is not consistent, you must either change them by reversing the node number order of the selected elements (ESURF,,REVERSE command) or make a consistent element ordering (ENORM command). You can list the CONTA176 results using the PRESOL,CONT or PRETAB commands. Since CONTA176 is a 3-D line element, you can also use the PLLS command to display element table items.

5.3.1. KEYOPTs and Real Constants CONTA176 uses most of the same KEYOPTs that are used by the surface-to-surface contact elements, CONTA171 through CONTA174. KEYOPT(3) and KEYOPT(4) differ from the other contact elements, and KEYOPT(11) is not used by CONTA176. See Element KEYOPTS (p. 35) for a listing of the remaining KEYOPTs. CONTA176 uses the same real constants used by the surface-to-surface contact elements (CONTA171 through CONTA174), except for the units of FKN and FKT. See a listing of the real constants in Real Constants (p. 30).

5.3.1.1. Real Constants R1, R2 For beam-to-beam contact, an important assumption is that of constant circular beam cross section. The contact radius is assumed to be the same for all CONTA176 elements in the contact pair. Likewise, the target radius is assumed to be the same for all TARGE170 elements in the contact pair. You supply the target and contact radii through real constants R1 and R2, respectively. For a general beam cross section, you can use an equivalent circular beam in the contact definition (see figure below). Use these guideline to define the equivalent circular cross section: • Determine the smallest cross section along the beam axis. • Determine the largest circle embedded in that cross section.

102


Performing a 3-D Beam-to-Beam Contact Analysis Figure 5.6: Equivalent Circular Cross Section

Actual beam

Contact surface

Use the first real constant, R1, to define the radius on the target side (target radius rt). Use the second real constant, R2, to define the radius on the contact side (contact radius rc). The contact radius R2 is always a positive number. The target radius R1 can be entered as either a negative or positive value. Use a negative value when modeling internal contact (a beam or pipe sliding within another beam or pipe), with the input value equal to the inner radius of the outer beam. Use a positive value when modeling external contact between the exterior surfaces of two cylindrical beams. For beam-to-beam contact, the thickness effect is accounted for through the contact and target radii. Therefore, KEYOPT(11) (which is used for the surface-to-surface contact elements to include or exclude thickness effects) is not used for CONTA176.

5.3.1.2. KEYOPT(3) For CONTA176, KEYOPT(3) determines the type of beam-to-beam contact as follows: • Use KEYOPT(3) = 0 or 2 to model external contact between parallel beams or internal contact between beams (a beam inside a hollow beam or a pipe inside a pipe). For this case, the sign of the target radius, R1, will determine whether you are modeling parallel beams (R1 > 0) or a beam inside a beam (R1 < 0). • Use KEYOPT(3) = 1 or 3 to model beams that cross. The target radius must be a positive value. KEYOPT(3) also allows you to choose between a contact force-based model (KEYOPT(3) = 0 or 1), and a contact traction-based model (KEYOPT(3) = 2 or 3). For the contact traction-based model, the program determines the area (based on beam element length and the contact radius, R2) associated with the contact node. When the traction-based model is specified, certain real constants (FKN, FKT, TNOP) and postprocessing items (PRES, TAUR, TAUS, SFRIC, and so on) have the same units as in the surface-to-surface contact elements (CONTA171 through CONTA174). When the force-based model is specified, the units of these quantities have a factor of AREA with respect to those used in the traction-based model. For example, contact stiffness FKN has units FORCE/LENGTH for the force-based model, but FORCE/LENGTH3 for the traction-based model; PRES is the contact normal force in the force-based model, but contact pressure in the traction-based model. See the CONTA176 element description for detailed information on the units for these quantities.


103

3-D Beam-to-Beam Contact

5.3.1.3. Real Constant TOLS For CONTA176, the contact normal is uniquely defined and is perpendicular to both the contact and the target surfaces (two contacting beams). Real constant TOLS is used to add a small tolerance that will internally extend the edge of the target surface. TOLS is useful for problems where contact nodes are likely to lie on the edge of the target (as at symmetry planes or for models generated in a node-tonode contact pattern). In these situations, the contact node may repeatedly slip off the target surface and be completely out of contact, resulting in convergence difficulties from oscillations. Units for TOLS are percent (1.0 implies a 1.0% increase in the target edge length). A small value of TOLS will usually prevent this situation from occurring. The default value is 10 for small deflection and 2 for large deflection (NLGEOM,ON).

5.3.1.4. KEYOPT(4) You can use CONTA176 with the multipoint constraint (MPC) approach (KEYOPT(2) = 2) to define surfacebased constraints. The KEYOPT(4) setting will determine the type of surface-based constraint. Set KEYOPT(4) = 1 for a force-distributed constraint, or set KEYOPT(4) = 0 for a rigid surface constraint. See Surface-based Constraints for more information.

104


Chapter 6: Line-to-Surface Contact You can use the 3-D line-to-surface contact element, CONTA177, to model flexible-flexible or rigidflexible contact between a 3-D beam and a surface or between a shell edge and a surface. The following line-to-surface contact topics are available: 6.1.The 3-D Line-to-Surface Contact Element 6.2. Performing a 3-D Line-to-Surface Contact Analysis

6.1. The 3-D Line-to-Surface Contact Element 3-D line-to-surface contact is represented by following the positions of lines on the contact surface, modeled by CONTA177 elements, relative to 3-D surface segments on the target surface, modeled by TARGE170 elements. Line-to-surface contact in three dimensions is depicted in the figure below. Figure 6.1: Line-to-Surface Contact Elements Y CONTA177

X Z

J K

I I

K J

CONTA177

Edge of shell elements

3-D target surface (TARGE170) The CONTA177 element type: • Has two nodes or three nodes. • Is paired with a target surface defined by TARGE170 elements. • Uses the same real constant set as used for the surface-to-surface contact elements. See Real Constants (p. 30) for more information. • Supports 3-D rigid-flexible and flexible-flexible contact. • Supports 3-D beams and shell edges with or without midside nodes. • Can be generated by the ESURF command (similar to the surface-to-surface contact elements) in many cases. See Performing a 3-D Line-to-Surface Contact Analysis (p. 106) for details. Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

105

Line-to-Surface Contact

6.2. Performing a 3-D Line-to-Surface Contact Analysis Contact element CONTA177 follows the contact pair concept used by the surface-to-surface elements (CONTA171 through CONTA174). You must pair CONTA177 with 3-D target surface segments (TARGE170) to model 3-D beam-to-surface or 3-D shell edge-to-surface contact. See Identifying Contact Pairs (p. 12) for more information. CONTA177 uses most of the same element KEYOPTs and real constants as the surface-to-surface contact elements. Any differences are described in the next section. The basic steps for performing a 3-D line-to-surface contact analysis are the same as those used for a typical surface-to-surface analysis using CONTA171 through CONTA174. See Steps in a Contact Analysis (p. 11) for details. When the underlying elements are beams or edges of shell elements you can use the ESURF command to create CONTA177 elements. (In the GUI, pick Main Menu> Preprocessor> Modeling> Create> Elements> Surf/Contact> Surf to Surf.) This procedure is similar to that used for the surface-to-surface contact elements. When using line-to-surface contact elements to form the contact surface, the nodes must be entered in a sequence that defines a continuous line, as shown in the figure below. Figure 6.2: Continuous Line Segments

The line can be made up of linear or parabolic segments, depending on whether the attached beam or shell edge is made up of first order or second order elements. If the nodal ordering of the underlying beam elements is not consistent, you must either change them by reversing the node number order of the selected elements (ESURF,,REVERSE command) or make a consistent element ordering (ENORM command). You can list the CONTA177 results using the PRESOL,CONT or PRETAB commands. Since CONTA177 is a 3-D line element, you can also use the PLLS command to display element table items.

6.2.1. KEYOPTs and Real Constants CONTA177 uses most of the same KEYOPTs that are used by the surface-to-surface contact elements, CONTA171 through CONTA174. KEYOPT(3) and KEYOPT(4) differ from the other contact elements. KEYOPT(8) is not used by CONTA177. See Element KEYOPTS (p. 35) for a listing of the remaining KEYOPTs. CONTA177 uses the same real constants used by the surface-to-surface contact elements (CONTA171 through CONTA174), except for the units of FKN, FKT, and TNOP. See Real Constants (p. 30) for a complete list.

106


Performing a 3-D Line-to-Surface Contact Analysis

6.2.1.1. KEYOPT(3) For CONTA177, KEYOPT(3) allows you to choose between a contact force-based model (KEYOPT(3) = 0), and a contact traction-based model (KEYOPT(3) = 1). For the contact traction-based model, the program determines the area (based on the beam element length and beam section radius) associated with the contact node. When the traction-based model is specified, certain real constants (FKN, FKT, TNOP) and postprocessing items (PRES, TAUR, TAUS, SRIC, and so on) have the same units as in the surface-to-surface contact elements (CONTA171 through CONTA174). When the force-based model is specified, the units of these quantities have a factor of AREA with respect to those used in the traction-based model. For example, contact stiffness FKN has units FORCE/LENGTH for the force-based model, but FORCE/LENGTH3 for the traction-based model. Furthermore, PRES is the contact normal force in the force-based model, but contact pressure in the traction-based model. See the CONTA177 element description for detailed information on the units for these quantities.

6.2.1.2. Accounting for Thickness Effect (CNOF and KEYOPT(11)) For line-to-surface contact, the thickness effect of underlying beam elements (on the contact side) is accounted for through the contact offset real constant, CNOF. The thickness effect of underlying shell elements (on the target side) is accounted for by setting KEYOPT(11). For the case of a beam contacting a solid, input half of the beam thickness for CNOF; KEYOPT(11) is ignored. For the case of a beam contacting a shell surface, input half of the beam thickness for CNOF and set KEYOPT(11) = 1 to include the shell thickness effect. CNOF remains constant for the entire contact pair during the solution. For more information on using CNOF, see Adjusting Initial Contact Conditions (p. 52). For more information on using KEYOPT(11), see Using KEYOPT(11) (p. 69).

6.2.1.3. Real Constant TOLS For CONTA177, the contact normal is uniquely defined and is perpendicular to the target surfaces. Real constant TOLS is used to add a small tolerance that will internally extend the edge of the target surface. TOLS is useful for problems where contact nodes are likely to lie on the edge of the target (as at symmetry planes or for models generated in a node-to-node contact pattern). In these situations, the contact node may repeatedly slip off the target surface and be completely out of contact, resulting in convergence difficulties from oscillations. Units for TOLS are percent (1.0 implies a 1.0% increase in the target edge length). A small value of TOLS will usually prevent this situation from occurring. The default value is 10 for small deflection and 2 for large deflection (NLGEOM,ON).

6.2.1.4. KEYOPT(4) You can use CONTA177 with the multipoint constraint (MPC) approach (KEYOPT(2) = 2) to define surfacebased constraints. The KEYOPT(4) setting will determine the type of surface-based constraint. Set KEYOPT(4) = 1 for a force-distributed constraint, or set KEYOPT(4) = 0 for a rigid surface constraint. See Surface-based Constraints for more information.


107

108


Chapter 7: Multiphysics Contact Multiphysics applications (involving two or more fields) often require the inclusion of contact interaction between two or more surfaces or between a surface and its surroundings. These contact interactions can be modeled by using the thermal, electric, and magnetic capabiities of the surface-to-surface and node-to-surface contact elements (CONTA171, CONTA172, CONTA173, CONTA174, and CONTA175) . The following multiphysics contact topics are available: 7.1. Modeling Thermal Contact 7.2. Modeling Electric Contact 7.3. Modeling Magnetic Contact

7.1. Modeling Thermal Contact You can use surface-to-surface contact elements and the node-to-surface contact element, in combination with thermal-structural coupled field solid elements or thermal elements, to model heat transfer that occurs in the contact surface. To activate both the structural and thermal DOFs, set KEYOPT(1) = 1. To activate the thermal DOF only, set KEYOPT(1) = 2. The following thermal contact features are supported. • Thermal contact conduction between two contacting surfaces. • Heat convection from a “free surface” to the environment or between two open surfaces separated by small gap (“near field” convection). • Heat radiation from a “free surface” to the environment or between two open surfaces separated by a small gap (“near field” radiation). • Heat generation due to frictional dissipation. • Heat flux input.

Note When KEYOPT(1) is set to 2 (thermal DOF only), the program ignores heat generation due to friction.

7.1.1. Thermal Contact Behavior vs. Contact Status Each contact pair can cover one or more thermal contact features. Which feature is active depends on the contact status: Closed Contact: Thermal contact conduction transfers heat between two contacting surfaces. Frictional Sliding: Frictional dissipated energy generates the heat to both the contact and target surfaces. Near-Field Contact: Both heat convection and radiation between the contact and target surfaces are taken into account.The external flux value contributes to the contact surface.


109

Multiphysics Contact Free-Surface Contact: Heat convection and radiation between the contact surface and the environment are taken into account.The external flux value only contributes to both contact and target surfaces. Free-surface contact refers to either “far-field contact” or a user-specified “free thermal surface” (KEYOPT(3) = 1 on the target element).

7.1.2. Free Thermal Surface If you wish to model free surface convection, free surface radiation, or a surface with a supplied heat flux value, you can define a “free” thermal surface. A free thermal surface can be a contact surface with no associated target (that is, the contact pair lacks target elements). You can also set KEYOPT(3) = 1 of the target element type definition to define a free thermal surface. When this KEYOPT is set, both free surface radiation and convection are considered as long as open contact is detected. In this case, there is no convective and radiative heat transfer between the contact and target surfaces.

7.1.3. Temperature on Target Surface For interface heat conduction, near field convection, or near field radiation, a temperature for both the contact and target surfaces is required. For a general target surface, the temperature varies along the surface (see figure below). In this case, the temperature at the intersection between the target surface and the normal from the contact detection point represents the target temperature. For a rigid target, the temperature on the pilot node represents the entire rigid target surface temperature, if the pilot node exists. Figure 7.1: Target Temperature

Target Target temperature at the intersection Far-field contact Contact

7.1.4. Modeling Conduction To take into account the conductive heat transfer between contact and target surfaces, you need to specify the thermal contact conductance coefficient which is real constant TCC.

7.1.4.1. Using TCC The conductive heat transfer between two contacting surfaces is defined by q = TCC X (Tt -Tc) where: 110


Modeling Thermal Contact q: is the heat flux per area. TCC:is the thermal contact conductance coefficient, having units of HEAT/(TIME * TEMPERATURE) for force-based node-to-surface contact, or units of HEAT/(TIME * TEMPERATURE * AREA) for the traction-based model. Tt and Tc: are the temperatures of the contact points on the target and contact surfaces. The TCC value is input through a real constant, which can be made a function of temperature [(Tc + Tt)/2], pressure (positive PRESSURE index values indicate compression, negative PRESSURE index values indicate tension), geometric penetration (positive GAP index values indicate penetration, negative GAP index values indicate an open gap), time, and initial contact detection point location (at the beginning of solution) by using tabular input (i.e., %tabname%). For more information, see Defining Real Constants in Tabular Format (p. 34). The user subroutine USERCNPROP is also available for defining TCC. To use this subroutine, you must specify the table name %_CNPROP% as the real constant value. For more information, see Defining Real Constants via a User Subroutine (p. 35). TCC has units of heat/(time x area x temp). If contact occurs, a small value of TCC yields a measured amount of imperfect contact and a temperature discontinuity across the interface. For large values of TCC, the resulting temperature discontinuity tends to vanish and perfect thermal contact is approached. When not in contact, however, it is assumed that no heat is transferred across the interface. To model contact conduction between two surfaces where a small gap exists, use KEYOPT(12) = 4 or 5 to define either the “bonded contact” or “no-separation contact” options (see Selecting Surface Interaction Models).

Note If CONV is not defined through the SFE command, real constant TCC will be used instead for modeling convection between two opening surfaces only if tabular input or a user subroutine is used. In this case, TCC can be made a function of temperature [(T(I) + T(J))/2], geometric gap, time, and initial contact detection point location by using the tabular input as described above.

7.1.4.2. Using the Quasi Solver Option You can take advantage of the fast thermal transient solver option (THOPT,QUASI) in the contact analysis. (See Nonlinear Options in the Thermal Analysis Guide for more information on this solver option.) To do so, you must use the following contact element key option: KEYOPT(1) = 2 - Temperature DOF only The following solver options must also be set: ANTYPE,TRANS THOPT,QUASI EQSLV,JCG/ICCG When using the fast thermal transient solver, the user subroutines USERFRIC and USERINTER should not be used.

7.1.5. Modeling Convection To model convective heat transfer, you must specify the heat convection coefficient CONV using the SFE command (with KVAL = 1 and CONV as a table parameter). CONV can be a constant value (only Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

111

Multiphysics Contact uniform is allowed) or a function of temperature, time, or location as specified through tabular input. For free surface convection (see Thermal Contact Behavior vs. Contact Status (p. 109) for a definition of “free-surface contact”), you must also specify bulk temperature through the SFE command (with KVAL = 2 and CONV as a table parameter). You can access this command through the following GUI paths: Main Menu> Preprocessor> Loads> Define Loads> Apply> Thermal> Convection> On Elements> Uniform Main Menu> Solution> Define Loads> Apply> Thermal> Convection> On Elements> Uniform The SFE surface load must be applied to the contact elements only. If either the convection coefficient or bulk temperature is not specified, the convection loading will not be active.

Note If CONV is not defined through the SFE command, real constant TCC will be used instead only if it is defined by tabular input or a user subroutine. TCC can be made a function of temperature [(T(I) + T(J))/2], geometric gap (positive GAP index values indicate penetration, negative GAP index values indicate an open gap), time, and initial contact detection point location by using tabular input (i.e., %tabname%).

7.1.6. Modeling Radiation 7.1.6.1. Background To model radiative heat transfer, specify the following: • Emissivity value EMIS, specified through the material property definition. • Stefan-Boltzmann constant SBCT through a real constant. • Offset temperature TOFFST. If you define your data in terms of degrees Fahrenheit or degrees Celsius, you must specify a temperature offset using the TOFFST command. You can access this command through the following GUI paths: Main Menu> Preprocessor> Loads> Analysis Type> Analysis Options Main Menu> Preprocessor> Material Props • Radiation view factor RDVF, specified through a real constant (defaults to 1). • Environment (ambient) temperature. It is only used for modeling radiation between a portion of the contact surface to the environment when the contact status is “free-surface contact” (see Thermal Contact Behavior vs. Contact Status (p. 109)). The temperature is only applied on the contact elements and is input on the SFE command with KVAL = 2 and CONV as a table parameter (this is the same as the bulk temperature in free surface convection modeling). If the environment temperature is not specified through the SFE command, the free surface radiation loading will not be active.

7.1.6.2. Using SBCT and RDVF When contact is open, heat transfer through radiation can occur. The equation is defined by q = RDVF x EMIS x SBCT [(Te + TOFFST)4 - (Tc + TOFFST)4] Where 112


Modeling Thermal Contact TOFFST: The temperature offset from absolute zero to zero (defined through the TOFFST command.) EMIS: The surface emissivity (input as material property). SBCT: The Stefan-Boltzmann constant (input as a real constant).There is no default for SBCT; if it is not defined the radiation effect is excluded.The Stefan-Boltzmann constant should correspond to the units of the FE model to be solved (e.g., 0.1190E-10 Btu/hr-in2-R4; 5.67E-8 W/m2-K4) RDVF: The radiation view factor input as a real constant (defaults to 1). RDVF can be defined as a function of temperature, geometric gap distance (negative GAP index values indicate an open gap), time, and initial contact detection point location (at the beginning of solution) by using tabular input (i.e., %tabname%).The user subroutine USERCNPROP is also available for defining RDVF (specify the table name %_CNPROP% as the real constant value). RDVF is only used for near field radiation. For far field radiation, RDVF is set to 1.0 and a user-assigned value is ignored. Other free-surface conditions recognize user-specified RDVF. For “near-field” radiation, when an intersection from a contact detection point to the target surface (in the direction of normal to the contact point) is detected, and the gap distance is smaller than the pinball radius, Te is the target temperature at the intersection. The radiation modeling here assumes that the radiative heat transfer occurs in the direction of the normal between two surfaces with a small gap. By defining RDVF as a function of gap, you can account for geometry effects. Use the Radiosity Solver method for more generalized radiation problems (see the Thermal Analysis Guide for more information). For “free-surface contact” radiation, Te becomes the “ambient” temperature defined by “bulk temperature” input on the SFE command (with KVAL = 2 and CONV as the table). For a definition of free-surface contact, see Thermal Contact Behavior vs. Contact Status (p. 109).

7.1.7. Modeling Heat Generation Due to Friction 7.1.7.1. Background In order to model heat generation due to frictional dissipated energy, you would typically perform a coupled transient thermal-structural analysis. If inertia effects are negligible, you can turn off transient effects on structural DOFs by using the command TIMINT,STRUC,OFF. However, you must include transient effects on the thermal DOF. Two real constants are required: • FHTG is the frictional dissipated energy converted into heat. • FWGT is the weight factor for the distribution of heat between contact and target surfaces.

7.1.7.2. Using FHTG and FWGT In the coupled thermal-structural contact modeling, the rate of frictional dissipation is given by q = FHTG x τ x V Where τ: The equivalent frictional stress. V: The sliding rate. FHTG: The fraction of frictional dissipated energy converted into heat.This value defaults to 1 and can be input as a real constant. For an input of true 0, you must enter a very small value (for example, 1E-8). If you enter 0, the program interprets this as an input of the default value. The amount of frictional dissipation on contact and target surfaces is defined by qc = FWGT x FHTG x τ x V


113

Multiphysics Contact and qT = (1 - FWGT) x FHTG x τ x V Where qc is the contact side and qT is the target side, and FWGT is the weight factor for the distribution of heat between the contact and target surfaces (input as a real constant). By default, FWGT = 0.5. For an input of true 0, you must enter a very small value (for example, 1E-8). If you enter 0, the program interprets this as an input of the default value.

7.1.8. Modeling External Heat Flux You can apply heat flux on the contact elements through the SFE command. Only uniform flux can be applied. Heat flux cannot be applied on target elements. However, for near field contact, the external flux is applied on contact and will contribute to target elements. For a free thermal surface, if KEYOPT(3) of the target element is set to 1, the external flux is only applied on the contact side. On a given contact element either CONV or HFLUX (but not both) may be specified. However, you can define two different contact pairs: one models convection and the other models heat flux.

7.2. Modeling Electric Contact You can use surface-to-surface contact elements or the node-to-surface contact element, in combination with thermal-electric elements and solid coupled field elements to model electric current conduction. You can also use surface-to-surface contact elements, in combination with piezoelectric and electrostatic elements, to model electric charge across a contacting interface. KEYOPT(1) provides degree of freedom options for modeling electric contact. For combined structural/thermal/electric contact, set KEYOPT(1) = 3 to activate the structural, thermal, and electric current DOFs. For pure thermal/electric contact, set KEYOPT(1) = 4 to activate the thermal and electric current DOFs. For piezoelectric contact, set KEYOPT(1) = 5 to activate the structural and piezoelectric DOFs. For electrostatic contact, set KEYOPT(1) = 6 to activate the electrostatic DOF. The electric contact features are: • Electric conduction between two contacting surfaces. • Heat generation due to electric dissipation. • Electric charge across the contacting interface.

7.2.1. Modeling Surface Interaction 7.2.1.1. Background To take into account the surface interaction for electric contact, you need to specify the electric contact conductance per unit area if you are using the electric current degree of freedom, or the electric contact capacitance per unit area if you are using the piezoelectric or electrostatic degrees of freedom. For either case, this parameter is input as real constant ECC. You can use tabular input to define ECC as a function of contact pressure (pressure as a table), average temperature on contact detection point (temperature as a table), time, and other primary variables. If the “bonded contact” or “no-separation contact” option is set, contact interaction can occur between two surfaces separated by a narrow gap.

114


Modeling Electric Contact

7.2.1.2. Using ECC The interaction between two contacting surfaces is defined by J = ECC x (Vt - Vc) where: J = current density for the electric potential (VOLT) degree of freedom (KEYOPT(1) = 3 or 4), or the electric charge density (KEYOPT(1) = 5, or 6). ECC = electric contact conductance for the electric potential (VOLT) degree of freedom (KEYOPT(1) = 3 or 4), or the electric contact capacitance per unit area for (KEYOPT(1) = 5, or 6). Vt and Vc = voltages at the contact points on the target and contact surfaces. The ECC value is input through a real constant, which can be a function of temperature [(Tt + Tc)/2], contact pressure (positive PRESSURE index values indicate compression, negative PRESSURE index values indicate tension), geometric penetration (positive GAP index values indicate penetration, negative GAP index values indicate an open gap), time, and initial contact detection point location (at the beginning of solution) by using tabular input (i.e., %tabname%). The user subroutine USERCNPROP is also available for defining ECC (specify the table name %_CNPROP% as the real constant value). For the current conduction option, the electric contact conductance ECC has units of electric conductivity/length. For the piezoelectric and electrostatic options, the electric contact capacitance ECC has units of capacitance per unit area. To model surface interaction between two surfaces where a small gap exists, use KEYOPT(12) = 4 or 5 to define either the “bonded contact” or “no-separation contact” options (see Selecting Surface Interaction Models).

Note For force-based node-to-surface contact, ECC has units of (electric conductivity)*(LENGTH) or the capacitance.

7.2.2. Modeling Heat Generation Due to Electric Current For electric current field analyses (KEYOPT(1) = 3 or 4), the heat generation due to electric current is given by q = FHEG x J x (VT - VC) Where FHEG: The fraction of electric dissipated energy converted into heat (Joule heating).This value defaults to 1 and can be input as a real constant. For an input of true 0, you must enter a very small value (for example, 1E-8). If you enter 0, the program interprets this as an input of the default value. J: The current density. Vt and Vc: The voltages at the contact points on the target and contact surfaces. The amount of electric heat dissipation on contact and target surfaces is defined by qc = FWGT x q and


115

Multiphysics Contact qT = (1 - FWGT) x q Where qc is the contact side and qT is the target side, and FWGT is the weight factor for the contact heat dissipation between the contact and target surfaces (input as a real constant). FWGT is the same real constant used for frictional heat generation. By default, FWGT = 0.5. For an input of true 0, you must enter a very small value (for example, 1E-8). If you enter 0, the program interprets this as an input of the default value.

7.3. Modeling Magnetic Contact You can use surface-to-surface contact elements or the node-to-surface contact element to model magnetic flux across two contacted bodies. The following situations are possible. • Non-perfect contact to account for the effects of a small air gap between mating components. This typically occurs at the interface between adjoining bodies. In this situation, there is a gap permeance effect where an MMF drop occurs. You can model this effect by inputting the gap permeance real constant, MCC. This option works best if the magnetic flux is normal to the gap interface. • Perfect contact across dissimilar meshes. This is typically used to model the air gap in a machine, for example, where the rotor and stator meshes meet. For both types of magnetic contact, you must set KEYOPT(1) = 7 to select the degree-of-freedom option. For the 2-D case, the magnetic potential degree of fredom, AZ, is active. For the 3-D case, only the scalar potential degree of freedom, MAG, is active, and scalar potential formulations (reduced (RSP), difference (DSP), or general (GSP)) are available (see MAGOPT).

Note 3-D magnetic contact is not supported for the MVP formulation (AX, AY, AZ), and the edgebased formulation (AZ).

Note Non-perfect magnetic contact is only available for the 3–D contact elements, CONTA173 and CONTA174. For more information on which element types should be used for a particular analysis, see the element discussions in the appropriate chapter of the Low-Frequency Electromagnetic Analysis Guide. For information on the use of the AZ degree of freedom, see Specifying Element Types and Options. For more information on the use of the MAG DOF, see Build the Model. For details on how to set up a contact analysis, see Steps in a Contact Analysis. For an example input listing showing a 2-D static magnetic contact analysis, see Example: 2-D Static Magnetic Contact Analysis.

7.3.1. Using MCC The magnetic flux across the contacting interface is defined by: MFLUX = MCC x (Mt - Mc)

116


Modeling Magnetic Contact where: MFLUX = magnetic flux density Mt , Mc = magnetic potential at the contact points on the target and contact surfaces MCC = contact permeance coefficient (Henries/meters2 in MKS units) The MCC value is input through a real constant, which can be a function of temperature [(Tt + Tc)/2], geometric penetration (positive GAP index values indicate penetration, negative GAP index values indicate an open gap), time, and initial contact detection point location (at the beginning of solution) by using tabular input (i.e., %tabname%). The user subroutine USERCNPROP is also available for defining MCC (specify the table name %_CNPROP% as the real constant value). MCC values can be approximated as µ/t, where µ is the gap permeability and t is the gap width. If the "no-separation contact" or "bonded contact" option is set (KEYOPT(12) = 4 or 5), contact interaction can occur between two surfaces separated by a narrow gap.

7.3.2. Modeling Perfect Magnetic Contact Perfect magnetic contact supports dissimilar meshes on both sides of the contacting interface (MCC = infinity). You must use the internal MPC approach by setting KEYOPT(2) = 2. You must also set KEYOPT(4) = 1 or 2 for contact nodal detection, and KEYOPT(12) = 5, 6 for bonded contact.


117

118


Chapter 8: Node-to-Node Contact You can use node-to-node contact elements to model point-to-point contact (flexible-flexible or rigidflexible). Additionally, you can use these elements to represent contact between two surfaces by specifying individual node-to-node contact between the opposing nodes of each surface. This use requires that the nodes of the two opposing surfaces match up geometrically and that the relative sliding between the two surfaces is negligible. In addition, the deflections (rotations) of the two surfaces must remain small. The following node-to-node contact topics are available: 8.1. Node-to-Node Contact Elements 8.2. Performing a Node-to-Node Contact Analysis

8.1. Node-to-Node Contact Elements The most commonly used node-to-node element is shown in the following figure. Figure 8.1: Node-to-Node Contact Element

In terms of solution CPU time, the CONTA178 element shown is the simplest and least expensive of all contact element types. When modeling conditions warrant its use, it can be an effective tool for modeling a variety of contact situations. Following are some notable features offered with CONTA178: • Several contact algorithms, including the Lagrange method (KEYOPT(2)) • Semiautomatic contact stiffness (real constants FKN-FKS) • Flexibility for defining contact normal • Many contact behaviors (KEYOPT(10)) • Cylindrical and spherical gaps with friction (KEYOPT(1)) • Damper (real constants CV1, CV2) Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

119

Node-to-Node Contact • Multiphysics contact (KEYOPT(6),TCC, ECC) • Real constants in tabular format • User subroutines for user-defined friction, interface interactions, and real constants (USERFRIC, USERINTER, and USERCNPROP)

8.2. Performing a Node-to-Node Contact Analysis The procedure for performing a node-to-node contact analysis is similar to that described for the nodeto-surface contact elements in Node-to-Surface Contact (p. 95). The following describes the typical steps in a node-to-node contact analysis. Each of these steps is explained in detail. 1. Create the geometry and mesh the model. 2. Generate the contact elements. 3. Define the contact normal. 4. Define the initial interference or gap. 5. Select the contact algorithm. 6. Apply the necessary boundary conditions. 7. Define the solution options. 8. Solve the problem. 9. Review the results.

8.2.1. Creating Geometry and Meshing the Model Node-to-node contact elements transmit forces at the nodes (compared to surface-to-surface contact elements which transmit pressures at Gauss points). This feature limits their use to low order elements only. You must identify where contact might occur during the deformation of your model. The nodes on the two surfaces of potential contact must line up. Once you've identified potential contact surfaces and created an adequate mesh you can create the contact elements.

8.2.2. Generating Contact Elements You can generate node-to-node contact element in one of two ways: 1. Use the direct generation method: Command(s): E GUI: Main Menu> Preprocessor> Modeling> Create> Elements> User Numbered> Thru Nodes 2. Use the EINTF command to generate the contact elements automatically at coincident nodes or offset nodes. This is discussed in detail in the following subsections.

120


Performing a Node-to-Node Contact Analysis

8.2.2.1. Generating Contact Elements Automatically at Coincident Nodes If the two bodies are in the “just touching” position, you can use the EINTF command to automatically generate the contact elements. In this case, the menu path is Main Menu> Preprocessor> Modeling> Create> Elements> Auto Numbered> At Coincident Nodes. Only nodes within the tolerance value set in the first argument (TOLER) are considered as coincident. If you wish to check a subset of nodes for coincidence, first select all the nodes you wish to check using the NSEL command.

8.2.2.2. Generating Contact Elements Automatically at Offset Nodes If the two bodies are separated by a gap, the EINTF command can be used to create elements between offset nodes by specifying the node location increments (DX, DY, DZ) in the coordinate system KCN. The GUI path is Main Menu> Preprocessor> Modeling> Create> Elements> Auto Numbered> Offset Nodes. If KOPT is set to 1, the nodes belonging to the contact elements created are rotated into the coordinate system KCN. For example, the following figure shows contact between two concentric pipes separated by a gap. In this example, KCN would be a cylindrical coordinate system centered in O, and DX would be set in ∆ (∆± TOLER). Figure 8.2: Contact Between Two Concentric Pipes

8.2.2.3. Node Ordering Node ordering can be critical in determining the contact normal. You can use EINTF,,,LOW or EINTF,,,HIGH to control node ordering. When using the LOW argument, the 2-node elements are generated from the lowest numbered node to the highest numbered node. When using HIGH, 2-node elements are generated from the highest to the lowest. To check the contact normal, use /PSYMB,ESYS. If you discover that the ordering is incorrect, you can reverse it using EINTF,,,REVE. To determine which side of the interface contains the I nodes, use the following commands: ESEL,,ENAME,,178 NSLE,,POS,1 ESLN NSLE EPLOT Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

121

Node-to-Node Contact

8.2.3. Defining the Contact Normal The contact normal direction is of primary importance in a contact analysis with CONTA178. By default [KEYOPT(5) = 0 and NX, NY, NZ = 0], the program will calculate the contact normal direction based on the initial positions of the I and J nodes, such that a positive displacement (in the element coordinate system) of node J relative to node I opens the gap. However, you must specify the contact normal direction for any of the following conditions: • If nodes I and J have the same initial coordinates. • If the model has an initial interference condition in which the underlying elements' geometry overlaps. • If the initial open gap distance is very small. In the above cases, the ordering of nodes I and J is critical. The correct contact normal usually points from node I toward node J unless contact is initially overlapped. You can specify the contact normal by means of real constants NX, NY, NZ (direction cosines related to the global Cartesian system) or element KEYOPT(5). The following lists the various options for KEYOPT(5): KEYOPT(5) = 0 The contact normal is either based on the real constant values of NX, NY, NZ or on node locations when NX, NY, NZ are not defined. For two dimensional contact, NZ = 0. KEYOPT(5) = 1 (2,3) The contact normal points in a direction which averages the direction cosines of the X (Y, Z) axis of the nodal coordinates on both nodes I and J. The direction cosines on nodes I and J should be very close. This option may be supported by the NORA and NORL commands, which rotate the X axis of the nodal coordinate system to point to the surface normal of solid models. The GUI paths for these commands are, respectively: Main Menu> Preprocessor> Modeling> Move/Modify> Rotate Node CS> To Surf Normal> On Areas Main Menu> Preprocessor> Modeling> Move/Modify> Rotate Node CS> To Surf Normal> On Lines Main Menu> Preprocessor> Modeling> Move/Modify> Rotate Node CS> To Surf Normal> With Area Figure 8.3: Two Concentric Pipes, Normals Rotated Properly

For a pipe-within-a-pipe, this portrays the normals rotated properly.

122


Performing a Node-to-Node Contact Analysis KEYOPT(5) = 4 (5,6) The contact normal points to X (Y, Z) of the element coordinate system issued by the ESYS command. If you use this option, make sure that the element coordinate system specified by ESYS is the Cartesian system.

8.2.4. Defining the Initial Interference or Gap With CONTA178, the gap size can be automatically calculated from the GAP real constant plus the node locations (projection of vector points from node I to J on the contact normal). This is the default (KEYOPT(4) = 0) This means that if you want the initial gap to be determined by the node locations only, set KEYOPT(4) = 0 and the real constant GAP to 0. If KEYOPT(4) = 1, the initial gap size is only based on the real constant GAP (it ignores the node location). A negative value for the gap size can be used to model initial interference. You can ramp initial interference using KEYOPT(9) = 1.

8.2.5. Selecting the Contact Algorithm You can choose from among the following contact algorithms for CONTA178: • Pure Lagrange Multiplier • Lagrange multiplier on the contact normal, penalty on the frictional (tangential) direction. • Augmented Lagrange method • Pure penalty method. When using CONTA178 with the pure penalty method or the augmented Lagrange method, a “semiautomatic” setting is provided for the contact normal and tangential stiffnesses. The program provides a default normal contact stiffness FKN, which is based on Young's modulus E and the size of the underlying elements. FKN and FKS are factors. If you want to import absolute values for FKN and FKS, use negative values.

8.2.6. Applying Necessary Boundary Conditions You can now apply necessary boundary conditions as you would do for any analysis type. For more information on applying boundary conditions, see the appropriate analysis description in the Structural Analysis Guide. When using the Lagrange multiplier method, be careful not to overconstrain the model. The model is overconstrained when a contact constraint at a node conflicts with a prescribed boundary condition on that degree of freedom at the same node. The following figure illustrates this issue:


123

Node-to-Node Contact Figure 8.4: Example of Overconstrained Contact Problem

Both nodes I and J are fixed in the X direction. The model is overconstrained only when the two bodies are in contact and bonded contact or rough friction has been defined. It can also occur if the contact status is “sticking,” In these cases, the constraint in the X-direction is duplicated which causes an overconstrained model. “Zero Pivot” and “Numerical Singularity” warning messages indicate overcontraints in a model. Overconstraints can lead to convergence difficulties and/or inaccurate results. They can be easily avoided by changing the contact definition or the boundary conditions.

8.2.7. Defining the Solution Options Convergence behavior for contact problems depends strongly on the particular problem. The options listed below are either typical or recommended for most node-to-node contact analyses. • Set the appropriate auto time step behavior using KEYOPT(7). The SOLCONTROL command turns this off by default. • The time step size must be small enough to capture the proper contact zone. The smooth transfer of contact forces is disrupted if the time step size is too large. A reliable way to set an accurate time step size is to turn automatic time stepping on. The SOLCONTROL command turns this on by default. Command(s): AUTOTS,ON GUI: Main Menu> Solution> Analysis Type> Sol'n Control ( : Basic Tab) Main Menu> Solution> Unabridged Menu> Load Step Opts> Time/Frequenc> Time-Time Step Main Menu> Solution> Unabridged Menu> Load Step Opts> Time/Frequenc> Time and Substps • Set the number of equilibrium equations to a number that is appropriate for a reasonable time step size. With SOLCONTROL,ON, this command defaults to between 15 and 26 iterations, depending upon the physics of the problem. Command(s): NEQIT GUI: Main Menu> Solution> Analysis Type> Sol'n Control ( : Nonlinear Tab) Main Menu> Solution> Unabridged Menu> Load Step Opts> Nonlinear> Equilibrium Iter • Turn the predictor-corrector option on. Command(s): PRED GUI: Main Menu> Solution> Analysis Type> Sol'n Control ( : Nonlinear Tab) Main Menu> Solution> Unabridged Menu> Load Step Opts> Nonlinear> Predictor • Set the Newton-Raphson option to FULL, with adaptive descent on. Command(s): NROPT,FULL,,ON 124


Performing a Node-to-Node Contact Analysis GUI: Main Menu> Solution> Unabridged Menu> Analysis Type> Analysis Options • For analysis involving friction, using NROPT, UNSYM is useful (and sometimes required if the coefficient of friction µ is > 0.2) for problems where the normal and tangential (sliding) motions are strongly coupled. • NLGEOM, ON is supported but the contact normal is not updated during the analysis. Make sure that only small rotations occur along the contact surfaces (except for the cylindrical gap option). • Many convergence failures in contact analysis are the result of using too large a value for contact stiffness (real constant KN). Be sure to follow the recommendations given previously in this section for an initial estimate for contact stiffness. If such estimated values lead to a convergence failure, reduce the contact stiffness and restart. (You must also explicitly define the tangent stiffness - see the Note below.) • Conversely, if overpenetration occurred in your contact analysis, you probably used a value of KN that was too small. In this case, gradually increase the contact stiffness value to an appropriate level by redefining it using new R commands over several load steps in a restart. (You must also explicitly define the tangent stiffness - see the Note statement below.)

Note Although you can change the contact stiffness value (real constant KN), you cannot change any other real constants between load steps. Therefore, if you plan to change KN in a restart (or from one load step to the next), you cannot allow the value of the tangent (or "sticking") contact stiffness (real constant KT) to be defined by default, because the program would then attempt to redefine the tangent stiffness as the contact stiffness changed. You must explicitly define the tangent stiffness whenever you change the contact stiffness to maintain a consistent value for the tangent stiffness throughout all load steps.

8.2.8. Solving the Problem You can now solve the analysis the same as you would for any linear or nonlinear analysis. See Solve the Analysis in the Structural Analysis Guide.

8.2.9. Reviewing the Results Postprocessing of contact result items for these elements requires the use of ETABLE commands. The steps for reviewing results in POST26 are the same as those for a typical nonlinear analysis See Reviewing Results in POST26 in the Structural Analysis Guide. For general information on postprocessing, refer to the Basic Analysis Guide.


125

126


Chapter 9: Multipoint Constraints and Assemblies You can use the internal multipoint constraint (MPC) approach (KEYOPT(2) = 2), in conjunction with certain bonded and no separation contact definitions (KEYOPT(12) = 4, 5, or 6), to define various contact assemblies and kinematic constraints. This capability is available for contact elements CONTA171, CONTA172, CONTA173, CONTA174, CONTA175, CONTA176, and CONTA177. By this method, the program builds MPC equations internally based on the contact kinematics. You can use this method to model the following contact assemblies and surface-based constraints: • Solid-solid assembly - both contact and target surfaces paste onto solid element faces • Shell-shell assembly - both contact and target surfaces paste onto shell element faces • Shell-solid assembly - the contact surface pastes onto shell element faces and the target surfaces paste onto solid element faces • Rigid surface constraint - the contact nodes are constrained to the rigid body motion defined by the pilot node (similar to the CERIG command) • Force-distributed constraint - the forces or displacements applied to the pilot node are distributed to contact nodes, in an average sense, through shape functions (similar to the RBE3 command) • Coupling constraint - the degrees of freedom at contact nodes are forced to have the same solution as at the pilot node (similar to the CP command) • Beam-solid assembly - one beam end-node is the pilot node which connects to the solid or shell surface (use the rigid surface constraint or force-distributed constraint type of MPC) The internal MPC approach can overcome the drawbacks of the traditional contact algorithms and other multipoint constraint tools. For example: • Degrees of freedom of the contact surface nodes are eliminated, reducing the wave front size of the system equation solver. • No contact stiffness is required as input. For a small deformation problem, this represents true linear contact behavior; no iterations are needed to solve the system of equations. For large deformation problems, the MPC equations are updated during each iteration, overcoming the small strain restriction in conventional constraint equations. • Both translational and rotational degrees of freedom can be constrained. • Generation of internal MPC is simple because it uses contact pair definitions. • Shape functions are taken into account automatically; no weight factor is needed for a force-distributed multipoint constraint (which is similar to the RBE3 command) if you use higher order elements or axisymmetric elements. In addition to forces, displacements can be applied on the pilot node for this type of MPC. The following MPC topics are available: Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

127

Multipoint Constraints and Assemblies 9.1. Modeling Solid-Solid and Shell-Shell Assemblies 9.2. Modeling a Shell-Solid Assembly 9.3. Surface-Based Constraints 9.4. Modeling Rigid Bodies 9.5. Overconstraint Detection and Elimination 9.6. Restrictions and Recommendations for Internal MPC Before using the internal MPC feature, be sure to review the last topic listed above on restrictions and recommendations.

9.1. Modeling Solid-Solid and Shell-Shell Assemblies To define solid-solid or shell-shell assemblies using the internal MPC approach, you must set the following key options on the contact elements: KEYOPT(2) = 2

MPC based approach

KEYOPT(12) = 4, 5, or 6

No separation (see note below), bonded always, or bonded initial

KEYOPT(4) = 1, 2, or 3

Nodal detection or surface projection based method for CONTA171, CONTA172, CONTA173, CONTA174

KEYOPT(4) = 0 or 1

Contact normal direction for CONTA175

Note When used with the MPC approach, the no separation option (KEYOPT(12) = 4) is only valid for modeling solid-solid assemblies that represent a slider line or slider plane. The following key options are ignored: KEYOPT(8) (it is always set to 2 internally), KEYOPT(10). The following real constants are used: R1, R2, ICONT, PINB, CNOF, PMAX, PMIN, TOLS. All other real constants are ignored. This function works similarly to the CEINTF command for small deformation analysis (NLGEOM,OFF). Comparing it to the CEINTF command, the contact surface acts as "region A nodes," and the target surface acts as "region B elements." The contact surface must be defined on the deformable bodies, and the target surface must be defined on either deformable or rigid bodies in the contact pair. In order to prevent overconstraint, only asymmetric contact is supported. If symmetric pairs are defined, the program automatically selects one pair and ignore the other pair (acting as KEYOPT(8) = 2). The self-contact pair definition is not supported.

9.1.1. Choosing the Nodal Detection Method The nodal detection method (KEYOPT(4) = 1) and the surface projection based method (KEYOPT(4) = 3) are well suited for contact pairs in which the contact surface normal direction is nearly opposite to the target surface normal direction. These methods, however, should not be used when the projection along the contact normal does not intersect the target surface. KEYOPT(4) = 2 can be used for all cases, regardless of the normal directions. This option is also well suited for the case when the contact normal direction is nearly perpendicular to the target normal direction.

128


Modeling Solid-Solid and Shell-Shell Assemblies The surface projection based method (KEYOPT(4) = 3) enforces contact constraints on the overlapping area of contact and target elements in an average sense, rather than on each contact node as the other nodal detection methods do (KEYOPT(4) = 1 and 2). Therefore, each internal constraint equation due to the projection based method can involve multiple contact nodes and target nodes. In general, this method provides much more accurate and smoother stress distributions near the contact interface involving dissimilar meshes than the other contact detection options. This holds true even more so for higher order elements involved in contact. However, the projection based method may significantly increase computational cost. The cost can be more expensive when the target element meshes are much more refined than the contact element meshes. This method should only be used for contact regions where the accuracy of local stresses is critical. When the contact node does not exactly lie on the target surface (i.e., offsets exist), using the projection based method for a solid-solid assembly may not represent rigid body rotation correctly. To ensure the correct behavior for rigid body rotation, you can move the contact nodes onto the target surface by issuing the CNCHECK,ADJUST command in the beginning of the analysis.

9.1.2. Controlling Behavior at the Contact Surface For the bonded always option (KEYOPT(12) = 5) and no separation option (KEYOPT(12) = 4), any contact node that lies inside the pinball region (PINB) can be the constrained node in the MPC definition if an intersection with the target surface is detected in the contact normal direction. This holds true at the beginning of deformation, as well as during the deformation process. A relatively small PINB may be used to prevent any false contact. When KEYOPT(12) is set to the bonded always or no separation option, PINB defaults to 0.25 (25% of the contact depth) for small deformation analysis (NLGEOM,OFF), and 0.5 (50% of the contact depth) for large deformation analysis (NLGEOM,ON). (The default PINB value may differ from what is described here if CNOF is input. See Using PINB (p. 61) for more information.) For the "bonded initial" option (KEYOPT(12) = 6), only those contact nodes that are initially in contact or have a very small gap but lie inside the adjustment zone (ICONT) are always constrained via internal MPC. Those contact nodes that are initially open will never be constrained, even though they may later penetrate into the target surface during the deformation process. In order to capture contact, you should specify proper ICONT or CNOF values. Use CNCHECK in conjunction with ICONT to move all the contact nodes that are inside the ICONT zone onto the target surface in the initial configuration, without causing any strain. When the "bonded initial" option is set and KEYOPT(5) = 0 or 4, ICONT defaults to 0.05. See Adjusting Initial Contact Conditions (p. 52) for more information on using KEYOPT(5), CNOF, and ICONT.

9.1.3. Controlling Degrees of Freedom Used in the MPC Constraint You can use KEYOPT(4) on the target element (TARGE169 or TARGE170) to control individual degrees of freedom for the constraint. For example, if you are using TARGE170 elements with 3-D contact elements, you might specify that only UX, UY, and ROTZ be used in the constraint. You can do this by entering a six digit value for KEYOPT(4). The first to sixth digits represent ROTZ, ROTY, ROTX, UZ, UY, UX, respectively. The number 1 (one) indicates the DOF is active, and the number 0 (zero) indicates the DOF is not active. Therefore, to specify that UX, UY, and ROTZ be used in the constraint, you would enter 100011 as the KEYOPT(4) value. For the 3-D case, you can also use KEYOPT(5) of the target element (TARGE170) to explicitly define the type of constraint. In most cases, the program automatically constrains the translational degrees of freedom for a solid-solid assembly and constrains both translational and rotational degrees of freedom for a shell-shell assembly. The constraint types available through KEYOPT(5) are as follows: KEYOPT(5) = 0 - Auto constraint type detection (default) Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

129

Multipoint Constraints and Assemblies KEYOPT(5) = 1 - Solid-solid constraint (no rotational DOFs are constrained) KEYOPT(5) = 2 - Shell-shell constraint (both translational and rotational DOFs are constrained, but they are decoupled) For a shell-shell assembly, you can also use the shell-solid constraint types (KEYOPT(5) = 3, 4, and 5). See Modeling a Shell-Solid Assembly (p. 130) for more information on the shell-solid constraint types. Keep these important points in mind when determining the type of constraint to use for the assembly: • For the 3-D case, if you specify KEYOPT(4), only the degrees of freedom that are common to both the KEYOPT(4) degree-of-freedom set and the KEYOPT(5) degree-of-freedom set will be considered in the constraint equations. • When the no separation option (KEYOPT(12) = 4 on the contact element) is used with the MPC approach to model solid-solid assemblies, only the KEYOPT(5) = 0 and 1 options (auto detection or solid-solid constraint) described above are valid. If the auto detection option is set and the program finds a shell-shell or shell-solid constraint in this situation, the solution will terminate. • If overconstraint occurs in bonded shell-shell assemblies when using the MPC algorithm, you can switch to the penalty method or the augmented Lagrangian method. See Bonded Contact for Shell-Shell Assemblies (p. 65) for more information. • The shell-shell constraint type (KEYOPT(5)=2) constrains the rotational and translational degrees of freedom from both sides. However, the rotational and translational degrees of freedom are decoupled. This option can model situations where one shell edge lines up well with another shell surface. This provides a valid answer which is closer to a matching mesh solution. If a certain amount of gap or penetration exists in the assembly interface, the shell-shell constraint option causes artificial constraints to be added, which can result in an inaccurate solution. In this case, it is recommended that you use a shell-solid constraint type by setting KEYOPT(5) = 3, 4, or 5 on the target element. One drawback of using the shell-solid constraint type is that the initial gap or penetration will remain during the solution (similar to setting KEYOPT(9) = 1 on the contact element).

9.1.4. Using the MPC Approach for Multiphysics Applications The MPC approach can be used for multiphysics applications as well as structural applications. The degrees of freedom specified for the contact elements via KEYOPT(1) determine the physics fields that will be considered for contact. See Multiphysics Contact for more information on multiphysics contact capabilities. If the temperature degree of freedom is active in the model (KEYOPT(1) = 1 or 2), the program builds MPC equations not only for structural degrees of freedom, but also for the temperature DOF. In this case, the real constant TCC is ignored. If only the temperature DOF is set (KEYOPT(1) = 2) and other solution options are defined (ANTYPE,,TRANS; THOPT,QUASI; EQSLV,JCG/ICCG), the internal MPC approach can support fast thermal transient analysis (see Nonlinear Options in the Thermal Analysis Guide). Internal MPC equations for temperature DOF are built to support heat transfer between the two bonded surfaces.

9.2. Modeling a Shell-Solid Assembly The 3-D shell-solid assembly provides a transition from a shell element region to a solid element region. This approach is useful when local modeling requires a full three-dimensional model with a relatively fine mesh, but other parts of the structure can be represented by shell elements (see Figure 9.1: Example of Shell-Solid Assembly (p. 131)). No alignment is required between the solid element mesh and the 130


Modeling a Shell-Solid Assembly shell element mesh. The contact surface or edge must be built on the shell element side. The target surface must be built on the solid elements side. Figure 9.1: Example of Shell-Solid Assembly

Constraint interface Shell elements

Solid elements

To define a shell-solid assembly using the internal MPC approach, you must set the following key options on the contact elements: KEYOPT(2) = 2

MPC-based approach

KEYOPT(12) = 5 or 6

Bonded always or bonded initial

KEYOPT(4) = 1 or 2

Nodal detection for CONTA171, CONTA172, CONTA173, CONTA174 [1]

KEYOPT(4) = 0 or 1

Contact normal direction for CONTA175

1. The surface projection based method for contact detection (KEYOPT(4) = 3) is usually not suitable for modeling shell-solid assemblies. The following real constants are also used: ICONT, FTOL, PINB, CNOF, PMAX, PMIN, TOLS. The following key options are ignored: KEYOPT(8), KEYOPT(10), KEYOPT(1) > 0. In most cases, the program automatically constrains both translational and rotational degrees of freedom for a shell-solid assembly (see Figure 9.2: Shell-Solid Assembly (Original Mesh) (p. 132)). However, you can use KEYOPT(5) of the target element (TARGE170) to explicitly define the type of constraint:


131

Multipoint Constraints and Assemblies KEYOPT(5) = 0 - Auto constraint type detection (default). Based on the underlying element type, this option automatically identifies the appropriate constraint type: solid-solid constraint, shell-shell constraint, or shell-solid constraint. KEYOPT(5) = 3 - Shell-solid constraint; both translational and rotational DOFs are constrained on shell nodes; only translational DOFs are constrained on solid nodes (see Figure 9.3: Shell-Solid Assembly with Shell-Solid Constraint Option (p. 133)). KEYOPT(5) = 4 - Shell-solid constraint, all directions. This option acts the same as KEYOPT(5) = 3 if an intersection is found from the contact normal to the target surface. Otherwise, constraint equations are still built as long as contact nodes and target segments are inside the pinball region. (See Figure 9.4: Shell-Solid Constraint - No Intersection (use KEYOPT(5) = 4 or 5) (p. 134) later in this section.) KEYOPT(5) = 5 - Shell-solid constraint, anywhere inside the pinball region. Constraint equations are always built as long as contact node(s) and target segments are inside the pinball region, regardless of whether an intersection exists between the contact normal and the target surface. (See Figure 9.4: Shell-Solid Constraint - No Intersection (use KEYOPT(5) = 4 or 5) (p. 134) later in this section.) Figure 9.2: Shell-Solid Assembly (Original Mesh)

Shell elements (thickness t )

Contact elements (pre-existing) Solid elements

Target elements TARGE170 (on top of solid)

132


Modeling a Shell-Solid Assembly Figure 9.3: Shell-Solid Assembly with Shell-Solid Constraint Option

MPC equations for shell edge solid surface

FTOLN (influence distance)

For the shell-solid constraint option (KEYOPT(5) = 3 on TARGE170), the program automatically creates an internal set of force-distributed constraints (similar to the RBE3 command) between nodes on the shell edges and nodes on the solid surface. The program uses the pinball region (PINB), initial adjustment zone (ICONT), and influence distance (FTOLN) to determine which nodes on the shell edge will be constrained with which nodes on the solid surface. Each shell node acts as the master node, and associated solid nodes act as slave nodes. For the bonded always option (KEYOPT(12) = 5), any shell node that lies inside the pinball region (PINB) will be included in the constraint if an intersection with the target surface is detected in the contact normal direction. This holds true at the beginning of deformation, as well as during the deformation process. A relatively small PINB value may be used to prevent any false contact. The default for PINB is 0.25 (25% of the contact depth) for small deformation analysis (NLGEOM,OFF), and 0.5 (50% of the contact depth) for large deformation analysis (NLGEOM,ON). (The default PINB value may differ from what is described here if CNOF is input. See Using PINB (p. 61) for more information.) For the bonded initial option (KEYOPT(12) = 6), only the shell nodes that initially lie inside the adjustment zone (ICONT) are included in the constraint sets. Shell nodes that lie outside ICONT are not constrained with the solid nodes. The default for ICONT is 0.05 (5% of the contact depth). The influence distance (FTOLN) is used for the shell-solid constraint option (KEYOPT(5) =3 on TARGE170). Each solid node is included in the constraint set if the perpendicular distance from the solid node to any shell edge is smaller than the influence distance. FTOLN defaults to half the thickness of the shell. A positive FTOLN value represents a scaling factor on the shell half-thickness, and a negative value represents an absolute distance value. A shell-solid assembly can be used in a substructure analysis. However, if a superelement is defined to represent the shell elements, shell thickness is unknown in the use pass. In this case, you must overwrite the default setting of FTOLN (originally a factor of the shell thickness) to account for the zero thickness of the superelement. Input an absolute value for FTOLN (that is, input a negative value) to capture all constrained nodes when the constraint equations are built. Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

133

Multipoint Constraints and Assemblies The KEYOPT(5) = 4 option (shell-solid constraint, all directions) can be used for the case when the shell node does not overlap or intersect the contact surface (see figure below). However, if any intersection is detected from the contact normal to the target surface, this option may not be the best choice because it only builds very localized constraints between the shell node and the target segment which is intersected. The same is always true for KEYOPT(5) = 3. The KEYOPT(5) = 5 option (shell-solid constraint, anywhere inside pinball) is the recommended shellsolid constraint because constraint equations are always built between a shell node and all target segments which are inside the pinball region, regardless of the contact normal and contact intersection. Thus, the pinball size is important since a larger pinball will result in a larger constraint set. This option is useful when you wish to completely constrain one contact side to another. The stresses are more evenly distributed at the shell-solid interface for this option than they would be for other shell-solid constraint types. Figure 9.4: Shell-Solid Constraint - No Intersection (use KEYOPT(5) = 4 or 5) Shell elements

Pinball region Target normal

Contact normal Shell node

Solid elements Keep the following points in mind when modeling shell-solid assemblies: • For the shell-solid constraint type (KEYOPT(5) = 3, 4, or 5), KEYOPT(9) is ignored, and initial penetration or gap always remains constant. To close the initial penetration or gap, issue the command CNCHECK,ADJUST in the beginning of the analysis. • The shell-solid constraint (KEYOPT(5) = 3, 4, or 5) is not always capable of transmitting all components of the moment at the shell nodes due to transverse shear locking of shell elements; a typical example is the moment component that is parallel to the normal of the solid surface.

9.3. Surface-Based Constraints A surface-based constraint can be used to couple the motion of nodes on the contact surface to a single pilot node on the target surface. The multipoint constraint (MPC) capability of the contact elements (KEYOPT(2) = 2) allows you to define three types of surface-based constraints: • Rigid surface constraint - In this type of constraint, the contact nodes are constrained to the rigid body motion defined by the pilot node (see Figure 9.5: Rigid Surface Constraint (p. 135)), similar to a constraint defined by the CERIG command. • Force-distributed constraint - In this type of constraint, forces or displacements applied on the pilot node are distributed to contact nodes (in an average sense) through shape functions (see Figure 9.6: ForceDistributed Constraint (p. 135)), similar to a constraint defined by the RBE3 command. 134


Surface-Based Constraints • Coupling constraint - In this type of constraint, the degrees of freedom of contact nodes are constrained to have the same solution as the degrees of freedom of the pilot node (see Figure 9.7: Coupling Constraint (p. 136)), similar to a constraint defined by the CP command. Figure 9.5: Rigid Surface Constraint

Figure 9.6: Force-Distributed Constraint


135

Multipoint Constraints and Assemblies Figure 9.7: Coupling Constraint

In Figure 9.7: Coupling Constraint (p. 136), the pilot node's x-direction has been rotated by 45 degrees. Only UX is included in the coupling constraint, so UY on the contact nodes are left free. The resulting deformation shows that UX (rotated 45 degrees from global x) is constant on the contact nodes, but UY is nonuniform. These surface-based constraints can be used in the following applications: • To apply loads and boundary conditions to the pilot node (such as torque load or drill rotation). Example: a bolt head submitted to a torque force using a force-distributed constraint. • To model rigid bodies. Example: rigid body definition in multi-body dynamics. • To model rigid end conditions. Example: using a rigid surface constraint to model a rigid end plate or rigid plane section of 3-D solid elements. • To model interactions with other joints. Example: two flexible parts linked by a hinge. This can be modeled by two force-distributed constraint definitions whose pilot nodes are connected by a revolute joint element. • To define transitions between solid and structure elements. Example: a beam element connected to a solid element face.

9.3.1. Defining Surface-Based Constraints The contact surface can be generated via the ESURF command. The contact nodes on the contact surface are the slave nodes of the MPC equations. The pilot node is the only target segment on the target surface side. It is the master node of the MPC equations. Forces and displacements can be applied on the pilot node and the contact nodes. You can define a follower element (FOLLW201) on the pilot node so that the element-specified external forces and moments on the follower element will follow the motion of the pilot node. 136


Surface-Based Constraints For a force-distributed constraint, use the following contact element key options: Force-distributed Constraint KEYOPT Settings KEYOPT(2) = 2

MPC based approach

KEYOPT(12) = 5 or 6


KEYOPT(4) = 1

Indicates a force-distributed constraint for CONTA171, CONTA172, CONTA173, CONTA174, CONTA175, CONTA176, and CONTA177

For a rigid surface constraint, use the following contact element key options: Rigid Surface Constraint KEYOPT Settings KEYOPT(2) = 2

MPC based approach

KEYOPT(12) = 5 or 6


KEYOPT(4) = 2

Indicates a rigid surface constraint for CONTA171, CONTA172, CONTA173, and CONTA174

KEYOPT(4) = 0

Indicates a rigid surface constraint for CONTA175 , CONTA176, and CONTA177

For a coupling constraint, use the following contact element key options: Coupling Constraint KEYOPT Settings KEYOPT(2) = 2

MPC based approach

KEYOPT(12) = 5 or 6


KEYOPT(4) = 3

Indicates a coupling constraint for CONTA171, CONTA172, CONTA173, CONTA174, CONTA175, CONTA176, and CONTA177

The following key options are ignored for surface-based constraints: KEYOPT(8), KEYOPT(5), KEYOPT(7), KEYOPT(10). With the exception of PINB, none of the standard contact real constants are used for surface-based constraints using internal MPC.

9.3.2. Defining Influence Range (PINB) By default, all the contact nodes are included in the surface-based constraints. You can select a subset of nodes from these contact nodes by defining a radius of influence range, PINB. The nodes that lie within the spherical range (radius = PINB) centered about the pilot node are selected for the definition of the surface-based constraints.

9.3.3. Degrees of Freedom of Surface-Based Constraints Use KEYOPT(1) of the contact elements to specify the degrees of freedom to be used in the constraint set. You may include other field degrees of freedom in addition to the structural DOFs. The pilot node has both translational and rotational degrees of freedom. The active degrees of freedom at the pilot node depend on the defined type of target elements. Use TARGE169 for 2-D surface-based constraints that contain UX, UY, and ROTZ degrees of freedom. Use TARGE170 for 3-D surface-based Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

137

Multipoint Constraints and Assemblies constraints that contain UX, UY, UZ, and ROTX, ROTY, ROTZ degrees of freedom. Generally, you should always set KEYOPT(2)=1 for the target element to indicate that boundary conditions for rigid target nodes will be user-specified; otherwise, the program may apply internal constraints on the pilot node. The degrees of freedom of the surface-based constraints can also be controlled by using KEYOPT(4) of the target element (TARGE169 or TARGE170). For example, for the 3-D case (TARGE170), you might specify that only UX, UY, and ROTZ be used in the constraint. You can do this by entering a six digit value for KEYOPT(4). The first to sixth digits represent ROTZ, ROTY, ROTX, UZ, UY, UX, respectively. The number 1 (one) indicates the DOF is active, and the number 0 (zero) indicates the DOF is not active. Therefore, to specify that UX, UY, and ROTZ be used in the constraint, you would enter 100011 as the KEYOPT(4) value. The basic formulation for the rigid surface constraint is similar to the MPC184 rigid beam and rigid link elements. However, this constraint type offers additional flexibility when you fully or partially constrain the degrees of freedom. For examples, the following are possible configurations: • All six degrees of freedom are selected, which is equivalent to the MPC184 rigid beam. • One rotational degree of freedom is excluded, which is equivalent to the MPC184 revolute joint. • Only three translational degrees of freedom are selected, which is equivalent to the MPC184 spherical joint or MPC184 rigid link.

9.3.4. Specifying a Local Coordinate System You can specify the surface-based constraint in a local coordinate system. For the rigid surface constraint, rotate the contact nodes into a local coordinate system. For the force-distributed constraint and the coupling constraint, rotate the pilot node into a local coordinate system. When KEYOPT(12) = 5 is set on the contact elements, the coordinate system does not rotate and it keeps its initial configuration. When the rotation is finite and KEYOPT(12) = 6 is set on the contact elements, the coordinate system in which the constrained degrees of freedom are specified will be co-rotated according to the rotation of the pilot node. The degrees of freedom of the surface-based constraints will be assigned with the co-rotated system. This is true even for the constrained degrees of freedom specified in the global coordinate system.

Note If all degrees of freedom are included in the constraint equations, there will be no difference between the KEYOPT(12) = 5 and KEYOPT(12) = 6 settings. Figure 9.8: Slider Link (p. 139) shows an example of a slider link modeled with contact. A local cylindrical coordinate system is defined at a contact node such that the x-direction is coincident with the line connecting the contact node and pilot node. KEYOPT(4) = 10 is set for the target element type to specify that only the y degree of freedom is constrained and other degrees of freedom are free. The coordinate system at the contact node will be co-rotated according to the rotation of the pilot node.

138


Surface-Based Constraints Figure 9.8: Slider Link

Contact node Rigid body

x y Pilot node Apply rotation In another example, a cylindrical ring is clamped at one end and loaded by a torque at the other end (Figure 9.9: Free Radial Expansion Under Torque Load (p. 139)). A rigid surface constraint is used with all the contact nodes rotated into a cylindrical coordinate system. If the x-direction constraint is free (KEYOPT(4) = 110), the ring is allowed to expand in the radial direction. Figure 9.9: Free Radial Expansion Under Torque Load

9.3.5. Additional Guidelines for a Force-Distributed Constraint • The pilot node is a dependent node (meaning the degrees of freedom for this node are removed). The contact nodes are independent nodes (the degrees of freedom are retained). If the pilot node has constraints applied to it, internally-generated MPC equations are rewritten so that the degrees of freedom of the pilot node are no longer dependent DOF. • KEYOPT(4) of TARGE169 and TARGE170 controls the number of degrees of freedom of the pilot node. • The number of internally-generated MPCs is equal to the number of degrees of freedom defined by KEYOPT(4) of TARGE169 and TARGE170. • When the constrained surface is built on a symmetric geometry model instead of the “full” geometry model, you must define the symmetry conditions using KEYOPT(6) of the target element (TARGE169 or TARGE170). Otherwise, you might get unexpected results (as shown by (c) in the figure below). When defining symmetric conditions, the pilot node must be located on the symmetry plane/edge. Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

139

Multipoint Constraints and Assemblies Figure 9.10: Defining Symmetry Conditions for Force-Distributed Constraints

9.3.6. Additional Guidelines for a Rigid Surface Constraint • The pilot node is an independent (retained) node in the constraint equation. The contact nodes are the dependent (removed) nodes. Use great caution when you apply any displacement constraints, coupling (CP command), or constraint equations (CE command) on the contact nodes since redundant constraints are likely to occur. • KEYOPT(4) of TARGE169 and TARGE170 controls the DOF set (the number of DOF) of the contact (dependent) nodes used in the internally-generated MPCs. • The number of internally-generated MPCs is equal to the number of contact nodes times the number of DOF. • The contact surface does not require underlying elements.

9.3.7. Additional Guidelines for a Coupling Constraint • The pilot node is an independent (retained) node in the constraint equation. The contact nodes are the dependent (removed) nodes. It is strongly recommended that you do not apply any displacement constraints, coupling (CP command), or constraint equations (CE command) on the contact nodes in the same DOF direction as the coupling constraint since redundant constraints are likely to occur. • KEYOPT(4) of TARGE169 and TARGE170 controls the DOF set (the number of DOF) of the pilot node (the independent DOF) used in the internally-generated MPCs. • The DOF set defined by KEYOPT(4) follows the nodal coordinate system of the pilot node. Each contact node can have its own nodal coordinate system, which does not affect the solution.

140


Surface-Based Constraints • The number of internally-generated MPCs is equal to the number of contact nodes times the number of DOF.

9.3.8. Modeling a Beam-Solid Assembly The surface-based constraint technique can be used to apply transitions between solid and structure elements; for example, a beam element connected to the solid or shell element face. One beam end node must be the pilot node and the solid/shell nodes must be the contact nodes. The rigid surface constraint is generally well-suited for the solid beam to solid surface case (see Figure 9.11: Beam-Solid Assembly Defined by Rigid Surface Constraint (p. 141)), and the force-distributed constraint is well-suited for the flexible beam (such as a thin wall beam) to solid/shell surface case (see Figure 9.12: Beam-Solid Assembly Defined by Force-distributed Constraint (p. 142)). Figure 9.11: Beam-Solid Assembly Defined by Rigid Surface Constraint Solid elements Pilot node

Beam elements

Contact elements


141

Multipoint Constraints and Assemblies Figure 9.12: Beam-Solid Assembly Defined by Force-distributed Constraint

Shell elements Pilot node

Beam elements

Contact elements

9.4. Modeling Rigid Bodies Rigid bodies are widely used for numerical simulation of multibody dynamic applications. The definition of a rigid body is similar to the definition of a rigid surface constraint, except that all the elements are defined by target elements. The motion of the rigid body is governed by the degrees of freedom at a pilot node which allows accurate representation of the geometry, mass, and rotary inertia of the rigid body. Each rigid body is only associated with one pilot node. The pilot node can be one of the nodes on a target element or a node at any arbitrary location. It can be connected to point mass, follower (FOLLW201), and deformable elements. For transient analysis you should locate the pilot node at the gravity center of the rigid body if the center of mass is known or can be approximated. You can add a point mass element, MASS21, to the gravity center. See Modeling Rigid Bodies in a Multibody Analysis in the Multibody Analysis Guide for more details.

9.4.1. Modeling Contact between Rigid Bodies You can model contact between two rigid bodies by specifying a contact pair consisting of a contact surface on one rigid body and a target surface on another rigid body. These contact and target elements should overlap the existing contact elements defined for the rigid bodies. Use either the augmented Lagrange algorithm or penalty algorithm (KEYOPT(2) on the contact elements) to avoid overconstraint between rigid body constraints and contact constraints. You cannot use the multipoint constraint (MPC) algorithm (KEYOPT(2) = 2) with bonded or no-separation contact behavior (KEYOPT(12)) to connect two rigid bodies; doing so would cause the model to be overconstrained. Instead, add a third rigid body which connects the first two rigid bodies at their pilot nodes.

142


Restrictions and Recommendations for Internal MPC MPC bonded contact between a flexible body and a rigid body is allowed. For this case, the contact surface of the MPC bonded contact pair must belong to the flexible body. Otherwise, the MPC bonded constraints and rigid body constraints would be redundant. It is possible to model two rigid bodies that are connected or overlapping through shared rigid body nodes or pilot nodes. To prevent overconstraint, the program internally merges the two rigid bodies into one rigid body and treats the second pilot node as a regular rigid body node.

9.5. Overconstraint Detection and Elimination When a degree of freedom is subjected to multiple constraints, overconstraint occurs. Overconstraint can be caused by the combination of the following constraints within the model: • User-specified constraints, including displacement constraints, constraint equations (CE command), and coupling (CP command) • Lagrange multiplier-based contact definitions • MPC-based contact definitions • Constraints introduced through joint elements Overconstraint may result in convergence difficulties or inaccurate solutions. In order to prevent this problem, the program tries to detect and eliminate overconstraints automatically. You can verify the eliminated constraints during postprocessing. At the beginning of a contact analysis, warning messages may appear outlining the cause of the potential overconstraints. Use the CNCHECK,DETAIL command to identify the overconstraints indicated by the messages. Manually remove as many unnecessary constraints as possible. Some overconstraints occur only during the solution due to new deformed configurations or due to changes in contact status. The program eliminates a limited set of overconstraints detected during solution; appropriate messages are issued when this occurs. For MPC-based contact pairs, the contact statuses (contact result item STAT) are set to negative values if one or more contact constraints are removed. STAT is set to -3 for bonded contact; STAT is set to -2 for no-separation contact. Be aware that, due to complexities, certain overconstraints may not be easily detected. The way the program removes the overconstraint is not unique. The order of the redundant constraints will influence how they are removed (the first encountered is kept, the next is removed). You should always verify the modified model carefully. You can list and display the contact status using STAT as a contact result item for the PLESOL and PRESOL commands; for example: PLESOL,CONT,STAT

9.6. Restrictions and Recommendations for Internal MPC The following restrictions and guidelines apply when using the internal MPC approach. • To prevent overconstraint, displacement boundary conditions and other constraint equations or coupling equations (ideally) should not be applied on the contact nodes for solid-solid, shell-shell, and shell-solid assemblies, or for rigid surface constraints and coupling constraints. • The Sparse and PCG solvers are recommended to solve models with constraint equations.


143

Multipoint Constraints and Assemblies • The MPC-based algorithm must be used with nodal detection (KEYOPT(4) = 1, 2, or 3) for CONTA171, CONTA172, CONTA173, and CONTA174. If nodal detection is not set, the program issues a warning message and automatically sets KEYOPT(4) = 2. • MPC no-separation contact (KEYOPT(12) = 4) may cause a discontinuity when contact nodes slide from one target segment to another. However this is not the case for MPC bonded contact (KEYOPT(12) = 5 or 6). • The internal MPC option does not support rigid-flexible contact when rigid surfaces are modeled by any primitive segments (such as circle, cylinder, cone, or sphere). • The shell-solid assembly is usually used for the case where the solid mesh is fine with respect to the shell thickness. The shell-solid interface should be located in a region of the structure where shell theory is valid for an approximation. The accuracy of local stresses near the shell-solid interface (at least within the shell thickness range) is not guaranteed. We recommend you include at least two solid elements along the layer of the shell-solid interface. • Using the force-distributed constraint type of MPC with a large number of contact nodes can result in large and dense submatrices for the global stiffness matrix. This may significantly increase the peak memory required during element stiffness assembly. Consider reducing the number of contact nodes if real memory or virtual memory is limited. • The force-distributed constraint creates internal constraint equations such that the motion of the pilot node is the average of the contact nodes. For the rotations, a least-squares approach is used to define the "average rotation" at the pilot node from the translations of the contact nodes. If the contact nodes are colinear, then one of the rotations on the pilot node that is parallel to the colinear direction can not be determined in terms of the translations of the contact nodes. Therefore, the associated moment component on the pilot node in that direction can not be transmitted. When this case occurs, a warning message is issued. • The contact related postprocessing items (ETABLE items, PRESOL,CONT, etc.) are not supported for the internal MPC option. • MPC contact only generates internal constraint equations. It does not calculate element internal force and stiffness matrices. Therefore, during postprocessing the option ITEM = CONT is not valid on the PRNLD, NFORCE, and FSUM commands. • You can use the CELIST command to list the internally-generated constraint equations associated with MPC contact . You can also use CELIST to convert the internal constraint equations to external constraint equation after a SOLVE command has been issued. (See CELIST for details.) • MPC contact generates constraint equations that can include constant terms. These terms are included on the right hand side of the system equation. Thus, they are scaled by the LVSCALE command when it is used in a mode superposition analysis. These terms are considered as harmonically varying loads when included in a harmonic analysis.

144


Chapter 10: Dynamic Contact and Impact Modeling Performing contact analysis in a transient dynamic procedure is always challenging due to the presence of contact chattering (frequent change in contact status). One of the reasons for contact chattering is that the contact constraints enforce constraints on nodal displacements (penetration) but do not impose any constraints on nodal velocities. Since nodal velocities and nodal accelerations are dependent on nodal displacements (see Section 17.2 Transient Analysis in the Mechanical APDL Theory Reference), any constraints on nodal displacements make the nodal velocities and nodal accelerations inconsistent. This introduces numerical errors in the transient dynamic solution in subsequent time increments and, if left untreated, leads to numerical instability (non-convergence or incorrect solution). Numerical damping is usually added to the time integration scheme to suppress such numerical errors. However, addition of numerical damping does not work in several cases. For example, in problems with multiple or repeated impacts there is constant growth of numerical error in the solution, and the analysis eventually fails to converge in spite of large numerical damping. In other situations the analysis may converge with addition of numerical damping, but the system response may be underdamped or overdamped depending on the growth of numerical error. A more appropriate solution to the contact chattering problem is to treat nodal displacement and nodal velocities consistently. One such solution is provided by Energy and Momentum Conserving Contact (p. 145).

10.1. Energy and Momentum Conserving Contact In transient dynamic analysis with contact, if the contact and target surfaces impact each other with nonzero relative velocities, it is important to satisfy momentum and energy balance for the contact/target interface. This helps to more accurately predict the duration of impact and the rebound velocities after separation. This can be achieved by using impact constraints with any of the 2D or 3D contact elements: CONTA171, CONTA172, CONTA173, CONTA174, CONTA175, CONTA176, CONTA177, and CONTA178. Impact constraints include constraints on penetration and relative velocity (see Energy and Momentum Conserving Contact in the Mechanical APDL Theory Reference). To activate the impact constraints the following contact options must be defined for a contact element: • Contact algorithm must be one of the following: – Augmented Lagrangian (KEYOPT(2) = 0) – Penalty function (KEYOPT(2) = 1) – Lagrange multiplier on contact normal and penalty on tangent (KEYOPT(2) = 3) – Pure Lagrange multiplier on contact normal and tangent (KEYOPT(2) = 4) • Standard or rough contact (KEYOPT(12) = 0 or 1) • Impact constraints (KEYOPT(7) = 4)


145

Dynamic Contact and Impact Modeling The impact constraints can be used with both frictionless and frictional contact.

10.1.1. Energy Conservation The impact constraints are active only on the contact/target interface, so energy conservation is enforced only for contact elements. The underlying finite elements defining the interior of impacting bodies do not satisfy energy conservation. To ensure energy conservation, the following conditions must be satisfied for contact elements: • Relative velocity constraint should be satisfied exactly. • No numerical damping should be used. • No friction should be specified. Energy conservation is relatively easy to satisfy for rigid impact (when both contact and target surfaces are rigid) as compared to flexible impact (when the target surface is rigid and the contact surface is flexible, or both surfaces are flexible). This is because the underlying finite elements for flexible bodies excite higher frequencies, which can make the time integration scheme unstable unless some numerical damping is used. For rigid bodies undergoing only translation motion and impact, numerical damping is generally not needed; however, when rigid bodies are undergoing large rotations, a small amount of numerical damping is necessary to keep the time integration scheme from becoming unstable.

10.1.2. Automatic Time Stepping An automatic time stepping scheme is provided to adjust the time increment size. This can be activated by SOLCONTROL,ON,ON at the procedure level when AUTOTS,ON is set. This scheme starts adjusting size of the time increments before impact such that there is minimum penetration in the substep where contact is first detected. This is important because the contact algorithm will enforce the relative velocity constraints to only prevent further penetration. If the automatic time stepping scheme is not used, there may be some uncontrolled penetration of the surfaces depending on the size of the time increment for the substep where contact is first detected before the relative velocity constraint prevents further penetration. When using fixed time increment, fixed number of substeps, or automatic time stepping (AUTOTS,ON) without SOLCONTROL,ON,ON, you should make sure that the penetrations remain sufficiently small.

10.1.3. Penetration and Relative Velocity When using the impact constraints, the penetration value depends on two factors: • Size of the time increment where contact is first detected • Accuracy of relative velocity constraint The “initial” penetration value depends on the size of the time increment, as explained above. There is no contact force associated with this value. The remaining penetration value comes from the enforcement of relative velocity constraint using the contact algorithm. The value of this penetration depends on the choice of contact algorithm, contact normal stiffness (FKN), and/or allowable penetration (FTOLN). The contact normal force is directly related to this penetration value.

146


Energy and Momentum Conserving Contact

10.1.4. Impact Between Rigid Bodies When using impact constraints to model impact between rigid bodies, a coefficient of restitution (real constant COR) can be used to model loss of energy during impact. The coefficient of restitution defines the ratio of relative velocity of rigid bodies after impact to relative velocity of rigid bodies before impact , so its value varies between 0 and 1. A value of 0 indicates that the rigid bodies stick to each other after impact, while a value of 1 indicates that the rigid bodies rebound after impact with the magnitude of relative velocity after impact being the same as before impact.


147

148


Chapter 11: Spot Welds The traditional way of modeling spot welds involves matching the meshes of different parts at the spot weld connection points, which introduces meshing difficulties and often requires parts to be meshed manually. In addition, this approach does not take into account the effects of the spot weld radius, and it underestimates the strength of the spot weld connection when the radius of the spot weld is the same or lesser order of magnitude than the mesh size. The ANSYS spot weld feature, based on the internal multipoint constraint (MPC) approach, offers a robust alternative to the traditional method. This feature allows you to easily model thin sheet components that are connected with spot welds, rivets, or fasteners. The spot weld can be located anywhere between the parts that are to be connected, independent of the mesh and the node locations. The figure below shows an example of the spot weld configuration. Each spot weld set connects two or more surfaces. Figure 11.1: Example Spot Weld Configuration

Node 3

Surface 3

Spot weld radius

Node 2

Surface 2

Basic Spot Weld Set

Node 1

Surface 1


149

Spot Welds There are several advantages to using this mesh independent spot weld capability: • Parts can be meshed independently. • A basic spot weld set can be easily defined by specifying two surfaces to be connected and a single node called the spot weld node near the surfaces. The spot weld node determines the location of the spot weld. • This approach takes into account effects of the spot weld radius input by the user. The program creates multipoint constraint equations (MPCs) internally through two contact pair definitions, one on each spot weld surface. The internal constraint equations (force-distributed constraints) couple the motion of the surface nodes to the motion of the spot weld node in an average sense. • The spot weld can be rigid (default) or deformable. To specify a deformable spot weld, you simply define a deformable beam element type prior to creating the spot weld. The following spot weld topics are available: 11.1. Defining a Spot Weld Set 11.2. Listing and Deleting Spot Welds

11.1. Defining a Spot Weld Set The general procedure for defining a spot weld set consists of the following steps: 1. Define the basic spot weld set (SWGEN command) 2. Define any additional surfaces for the existing spot weld set (SWADD command) In addition, the SWLIST and SWDEL commands are available for listing and deleting spot weld sets.

11.1.1. Creating a Basic Spot Weld Set with SWGEN Use the SWGEN command to define a basic spot weld set (in the GUI, pick Main Menu> Preprocessor> Modeling> Create> Element > SpotWeld> Create New Set). The required arguments are: • Name of the spot weld set • Spot weld radius • Spot weld surfaces • First spot weld node You can optionally specify the following: • Second spot weld node • Spot weld projection direction • Search radius • Contact and target element types All of these inputs are discussed in detail below.

150


Defining a Spot Weld Set Spot Weld Set Name (Ecomp): Use the Ecomp argument to assign a name to a new basic spot weld set. The spot weld set is a component composed of new beam, target, and contact elements. The name entered for the Ecomp argument will be used to list, delete, output, or add new surfaces to the spot weld set. Spot Weld Surfaces (NCM1, NCM2): Each basic spot weld set contains two surfaces, surface 1 and surface 2, to be connected together. Use arguments NCM1 and NCM2 to specify these surfaces. You can input the name of a predefined node component or a meshed area number. If no nodal component names matching NCM1 and NCM2 are found, they are assumed to be meshed area numbers. Spot Weld Nodes (SND1, SND2): You must specify the first spot weld node, SND1. The location of each spot weld depends on SND1. If you specify the second spot weld node (SND2), it is used to determine the spot weld projection direction. The second node will be generated automatically if it is not specified. You should locate SND1 and SND2 (referred to as node 1 and node 2 in figures that follow) as close to the corresponding surfaces as possible. They can be nodes on the corresponding surfaces or independent nodes in space. If they are not on the corresponding surfaces, node 1 will be moved to surface 1 and node 2 will be moved to surface 2. The program determines the actual location for the spot weld nodes by first projecting node 1 onto surface 1 along the spot weld projection direction. Next, the program projects node 2 to surface 2 along the spot weld projection direction. The spot weld nodes are moved to the surfaces in the beginning of the analysis. Spot Weld Projection Direction: By default, the projection direction is a surface normal vector from node 1 to surface 1 (see figure below). If you specify node 2 (SND2), the projection direction is in line with the two spot weld nodes. If node 2 is not specified, you can still explicitly define the projection direction with the DIRX, DIRY, and DIRZ arguments. Figure 11.2: Default Projection Direction for Node 1

Original position of Node 1

Node 1 after projection

Surface 1

Once the actual location of node 1 has been determined, the program projects node 2 to surface 2 along the spot weld projection direction that was used for node 1. The figure below shows the projection locations of nodes 1 and 2 given a default projection normal to surface 1.


151

Spot Welds Figure 11.3: Default Projection Direction for Nodes 1 and 2

Node 2

Surface 2

Node 1

Surface 1

Original position of Node 1 The figure below shows spot weld node 1 projected as specified by the user (either through the input of DIRX, DIRY, and DIRZ, or by the specification of a second spot weld node). Figure 11.4: User-specified Projection Direction, Node 1

Original position of Node 1

Node 1 after projection

Surface 1

The figure below is another example showing the projection location of spot weld nodes 1 and 2 given a user-specified projection direction.

152


Defining a Spot Weld Set Figure 11.5: User-specified Projection Direction, Nodes 1 and 2

Node 2

Surface 2

User-defined projection direction

Node 1

Surface 1

Original position of Node 1 Spot Weld Radius (SWRD): Each spot weld is assumed to have a circular projection onto the spot weld surfaces. The projection is defined by the required spot weld radius argument SWRD. For each spot weld surface, the program builds six force-distributed constraint equations through a contact pair. The spot weld radius determines the influence range of the constraint equations. Each spot weld node couples with a group of nodes on the corresponding surface within the radius. (See Figure 11.7: Nodes Included in Constraint Equations (p. 155) later in this section.) Search Radius (SHRD): The program creates a contact pair for each spot weld surface. The contact pair contains one contact element defined by the spot weld node, and target elements overlaid fully or partially on the corresponding spot weld surface. Only the surface nodes that fall within a sphere of radius, called the search radius, with its center at the original position of the corresponding spot weld node are used to form target elements (see figure below). The search radius must be larger than the spot weld radius. If the search radius argument (SHRD) is not defined, the program uses a default search radius that is four times the spot weld radius.


153

Spot Welds Figure 11.6: Search Radius for Spot Weld Search radius Spot weld radius

Original position of node 1

TARGE170 elements within search radius

Surface 1

CONTAC175 (spot weld node 1)

Target and Contact Element Types (ITTY, ICTY): By default, the program automatically creates the target and contact element types required for the spot weld. However, you may use the ITTY and ICTY arguments to specify the target and contact element types, respectively. You must also set certain key options if you specify the element types to be used. See The Components of a Spot Weld (p. 154) for more information.

11.1.2. The Components of a Spot Weld Each new spot weld set created by the SWGEN command consists of a beam element and two nodeto-surface MPC contact pairs (see Modeling a Shell-Solid Assembly (p. 130) for more information). Each node-to-surface pair will generate 6 constraint equations. You can choose either a rigid MPC184 element (the default) or a deformable BEAM188 element to link the spot weld surfaces. Below are detailed explanations of how these items are generated: 1. Contact Pairs: Two node-to-surface contact pairs are created for each new spot weld set. One pair is for spot weld surface 1 and the other is for surface 2. Each contact pair has only one contact element, CONTA175, which is defined by the associated spot weld node. The target elements (TARGE170) are formed by a group of surface nodes lying within the region of the search radius. The program creates an independent real constant ID for each contact pair, and proper contact and target element type IDs. If you prefer, you can specify the contact or target element type IDs to be used. To specify a contact element type, use the ICTY argument on SWGEN and set the following contact element options for that element type: KEYOPT(2) = 2 (MPC approach), KEYOPT(12) = 5 (bonded always). To specify a target element type, use the ITTY argument on SWGEN and set the target element option KEYOPT(5) = 4 (shell-solid constraint) for that element type. 2. Force-distributed constraint equations: For each contact pair, the program internally forms force-distributed constraint equations that distribute the internal forces of the spot weld node (that is, the contact node) to the surface nodes (that is, the target nodes) lying within the region of the spot weld radius. This includes all elements that fall entirely or partially within the radius (see the figure below). In other words, it couples the motion of certain surface nodes to the motion of the spot weld node (in an average sense). There are six constraint equations generated for each spot weld surface (that is, each contact pair).

154


Defining a Spot Weld Set Figure 11.7: Nodes Included in Constraint Equations CONTAC175

Spot weld radius Nodes to be constrained

TARGE170 elements

Surface 1

3. Beam element: A beam element is created to connect spot weld surface 1 and surface 2. The two beam end-nodes are the spot weld nodes, node 1 and node 2 (see Figure below). The spot weld set can be rigid or deformable. By default, the program creates a rigid beam element (MPC184 with KEYOPT(1)=1) to represent a rigid spot weld. However, if the currently defined element type is BEAM188 with a solid circular cross-section, the program assumes the spot weld set is deformable and a BEAM188 element is created. In this case, you must input the material properties with the MP command and define the geometry of the beam section using the SECTYPE and SECDATA commands (see the example listing below). Generally, BEAM188 is capable of handling a short beam situation. Figure 11.8: Beam Element Created for Spot Weld

Node 2

Surface 2 Beam element (MPC184 or BEAM188)

Node 1

Surface 1

The following example input shows typical commands used to define a spot weld with a deformable BEAM188 element having a circular cross section.


155

Spot Welds MP,EX,3,200000000000. ! spot weld material properties MP,NUXY,3,0.3 SECTYPE,3,beam,csolid ! define a circular solid beam section SECDATA,2.75e-002 ! beam circular radius ET,3,188 ! element type for a deformable spot weld TYPE,3 MAT,3 SECNUM,3 *SET,NODE1,9000 ! define parameter for node number N,NODE1,0.1,0.5,10.2 ! define spot weld node SWGEN,SWELD1,2.75e-2,2,3,NODE1 ! Spot weld name = SWELD1 ! Radius = 2.75e-2 ! Spot weld surfaces = areas 2 and 3

Note All limitations that are documented for the MPC184 link/beam element in the Element Reference also apply when the element is used in a rigid spot weld definition. Please read the MPC184-Link/Beam description carefully, especially the assumptions and restrictions.

11.1.3. Adding Surfaces to a Basic Set Once a basic spot weld set has been created, additional surfaces can be added to the existing spot weld set. Use the SWADD command to define the additional surfaces (in the GUI, pick Main Menu> Preprocessor> Modeling> Create> Element > SpotWeld> Add More Surfaces). Each added surface (input as NCM1 through NCM9) can be specified by an existing nodal component name or by a meshed area number. In the example command below, four surfaces are added to an existing spot weld set named SWELD1. The four surfaces include area number 3, area number 4, nodal component FAC5, and nodal component FAC6. The search radius is 0.8. SWADD,SWELD1,0.8,3,4,FAC5,FAC6

! Adds 4 surfaces to spot weld SWELD1.

For each new surface, the program creates a new spot weld node, a new contact pair, and a new beam element (see figure below). The location of the new node is determined by projection of one of the closest existing spot weld nodes onto the newly added surface along the spot weld projection direction. The new node-to-surface contact pair contains the new spot weld node as a single contact node in the pair, and surface nodes within the range of the search radius (SHRD) specified on the SWADD command. The search radius defaults to 4 times the spot weld radius (SWRD) defined for the basic spot weld set. Each new contact pair will create 6 force-distributed constraint equations internally. A new beam is created to link the new surface to the closest existing surface through the spot weld nodes.

156


Defining a Spot Weld Set Figure 11.9: Surfaces Added to Basic Spot Weld Set More surfaces

Node 4

Surface 4

Beam 3

Node 3

Surface 3

Beam 2

Spot weld radius

Node 2

Surface 2

Basic spot weld Beam 1

Node 1

Surface 1

Original position of Node 1 After all spot weld nodes are created, the nodes of the beam elements may be reordered to link every two adjacent spot weld nodes (see Figure below). The ordering is important for the output, which is calculated for each beam in a spot weld set. Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

157

Spot Welds Figure 11.10: Node Reordering for Beam Elements Reordered surfaces

Node 3

Surface 3

Beam 3

Node 4

Surface 4 (inserted between 2 and 3)

Beam 2

Node 2

Surface 2

Basic spot weld Beam 1

Node 1

Surface 1

Assuming the basic spot weld set is named SWELD, below is an example of commands that could have produced the above figure. SWADD,SWELD, ,3 ! Add Area 3 to spot weld set SWELD SWADD,SWELD, ,4 ! Add Area 4 (between surfaces 2 and 3) to SWELD

158


Listing and Deleting Spot Welds Initially, when spot weld surface 3 was added, a beam was created between surfaces 2 and 3. When surface 4 was added between surfaces 2 and 3, the beam that went between surfaces 2 and 3 was redefined to go between surfaces 2 and 4, and a new beam was created to link surfaces 4 and 3. You can add more surfaces by repeating the SWADD command. However, the maximum number of allowable surfaces for each spot weld set (including the two surfaces used for the basic set) is 11.

11.2. Listing and Deleting Spot Welds Use the SWLIST command to list the spot weld sets for a specific spot weld or for all spot weld sets (in the GUI, pick Main Menu> Preprocessor> Modeling> Create> Element > SpotWeld> List). To ensure that all defined spotwelds are listed, issue CMSEL,ALL (to select all components) before issuing the SWLIST command. The listing will include nodes, beams, and contact pair information. If the SWLIST command is issued in POST1 (in the GUI, pick Main Menu> General Postproc> List Results> SpotWeld Solution), the beam forces and moments, are also output in the beam local coordinate system. The output includes axial force, shear forces, torque, and bending moments. For a deformed spot weld set, axial stress and bending stress are also reported. Use the SWDEL command to delete a specified spot weld set or all spot weld sets. In the GUI, pick Main Menu> Preprocessor> Modeling> Create> Element > SpotWeld> Delete.


159

160


Chapter 12: Debonding The debonding capability refers specifically to separation of bonded contact. It can be used to simulate interface delamination where the interface is modeled using bonded contact with the augmented Lagrangian method or the pure penalty method. A cohesive zone material must be used to define the traction separation behavior of the interface. The following contact elements support debonding: CONTA171, CONTA172, CONTA173, CONTA174, CONTA175, CONTA176, and CONTA177. An alternative method of modeling interface delamination is to use interface elements with a cohesive zone material (see Interface Delamination and Failure Simulation). However, debonding with contact elements has the following advantages over delamination with the interface elements: • Parts forming the interface can be meshed independently. • Existing models with contact definitions can be easily modified for debonding. • Standard contact and debonding can be simulated with the same contact definitions. • Debonding can be used for various applications; for example, delamination, spot weld failure, and stitch failure.

12.1. Including Debonding in a Contact Analysis Debonding can be defined in any model that includes surface-to-surface (CONTA171 through CONTA174), node-to-surface (CONTA175), line-to-line (CONTA176), or line-to-surface (CONTA177) contact. For a detailed discussion on how to set up a contact analysis, see Surface-to-Surface Contact (p. 11). To activate debonding for a contact pair, the following contact options must be defined for the contact element: • Augmented Lagrangian method or pure penalty method (KEYOPT(2) = 0 or 1) • Bonded contact (KEYOPT(12) = 2, 3, 4, 5, or 6) In addition, you must specify a cohesive zone material model with bilinear behavior for the contact elements. Two ways to specify the material data are available: • Bilinear material behavior with tractions and separation distances (TB,CZM command with TBOPT = CBDD) • Bilinear material behavior with tractions and critical fracture energies (TB,CZM command with TBOPT = CBDE) This material model is discussed in more detail in the following section. Once you have defined the required input parameters, you can solve the analysis the same as you would for any nonlinear analysis (see Solving the Problem (p. 91)). After debonding is completed, the surface interaction is governed by standard contact constraints for normal and tangential directions. Frictional contact is used if friction is specified for contact elements.


161

Debonding

12.1.1. Cohesive Zone Materials Used for Debonding A cohesive zone material model is needed to model debonding in a contact analysis. This material is defined using the data table method (TB and TBDATA commands). Temperature dependent data is also allowed (TBTEMP command). On the TB command, use Lab = CZM to denote a cohesive zone material, and use TBOPT = CBDD or CBDE to indicate the specific material data you will provide. The two data definitions are described below. For more information on this material, see Cohesive Zone Material (CZM) Model in the Mechanical APDL Theory Reference.

12.1.1.1. Bilinear Material Behavior with Tractions and Separation Distances (TBOPT = CBDD) This is a linear elastic material behavior with linear softening characterized by maximum traction and maximum separation. To define this material, use the TB,CZM command with TBOPT = CBDD. Specify the material constants as data items C1 through C6 on the TBDATA command: Constant

Symbol

Meaning

C1

σmax

C2

δnc

C3

τmax

C4

δct

tangential slip at the completion of debonding

C5

η

artificial damping coefficient

C6

β

flag for tangential slip under compressive normal contact stress; must be 0 (off ) or 1 (on)

maximum normal contact stress [1] contact gap at the completion of debonding maximum equivalent tangential contact stress [1]

1. For contact elements using the force-based model (see the description of KEYOPT(3) for CONTA175, CONTA176, and CONTA177), input a contact force value for this quantity. Sample command input for this material is shown below. TB,CZM,1,2,,CBDD TBDATA,1,σmax,

δnc

,τmax,

δct

,η,β

12.1.1.2. Bilinear Material Behavior with Tractions and Critical Fracture Energies (TBOPT = CBDE) This is a linear elastic material behavior with linear softening characterized by maximum traction and critical energy release rate. To define this material, use the TB,CZM command with TBOPT = CBDE. Specify the material constants as data items C1, through C6 on the TBDATA command: Constant

Symbol

C1

σmax

maximum normal contact stress [1]

C2

Gcn

critical fracture energy density (energy/area) for normal separation [2]

162

Meaning


Including Debonding in a Contact Analysis Constant

Symbol

Meaning

C3

τmax

C4

Gct

C5

η

artificial damping coefficient

C6

β

flag for tangential slip under compressive normal contact stress; must be 0 (off ) or 1 (on)

maximum equivalent tangential contact stress [1] critical fracture energy density (energy/area) for tangential slip [2]

1. For contact elements using the force-based model (see the description of KEYOPT(3) for CONTA175, CONTA176, and CONTA177), input a contact force value for this quantity. 2. For contact elements using the force-based model (see the description of KEYOPT(3) for CONTA175, CONTA176, and CONTA177), this quantity is critical fracture energy. Sample command input for this material is shown below. TB,CZM,1,2,,CBDE TBDATA,1,σmax,Gcn,τmax,Gct,η,β

12.1.2. Debonding Modes Debonding involves separation of surfaces forming an interface. The direction of separation determines the debonding mode. The program detects the debonding mode based on material data that you input for normal and tangential directions: • Mode I debonding involves separation normal to the interface. It is activated by inputting data items C1, C2, and C5 on the TBDATA command. • Mode II debonding involves slip tangent to the interface. It is activated by inputting data items C3, C4, and C5 on the TBDATA command. • Mixed mode debonding involves both normal separation and tangential slip. It is activated by inputting data items C1, C2, C3, C4, and C5, and C6 on the TBDATA command.

12.1.3. Other Considerations for Debonding Artificial Damping Debonding is generally accompanied by convergence difficulties in the Newton-Raphson solution. Artificial damping can be used to stabilize the numerical solution. It is activated by specifying the damping coefficient η (input on TBDATA command as C5 using TB,CZM). The damping coefficient has units of time and should be smaller than the minimum time step size so that the maximum traction and maximum separation (or critical fracture energy) values are not exceeded in debonding calculations. Tangential Slip under Normal Compression An option is available to control tangential slip under compressive normal contact stress for mixed mode debonding. By default, no tangential slip is allowed for this case, but it can be activated by setting the flag β to 1 (input on TBDATA command as C6 using TB,CZM). Pinball Radius and Mesh Density Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

163

Debonding When using a fine mesh for underlying elements of bonded surfaces, you may need to increase pinball radius (PINB) for contact elements so that it is greater than the maximum separation value in the normal direction (contact gap when normal contact stress goes to zero). The default value for PINB is based on the depth of the underlying element. If PINB is smaller than the maximum separation value, debonding calculations will be bypassed when the contact gap exceeds PINB.

12.1.4. Postprocessing All applicable output quantities for contact elements are also available for debonding: normal contact stress (PRES), tangential contact stress (TAUR, TAUS, SFRIC), contact gap (GAP), tangential slip (TASR, TASS, SLIDE), etc. In addition, the following debonding specific output quantities are available as NMISC data: debonding time history (DTSTART), debonding parameter (DPARAM), and critical fracture energy (DENERI, DENERII). For more information on how to review results in a contact analysis, see Reviewing the Results (p. 92) in Surface-to-Surface Contact (p. 11).

164


Chapter 13: Contact Surface Wear Wear is the progressive loss of material from the surface of a solid body when in contact with another body. Although wear is a complex phenomenon that involves both mechanical and chemical processes, it can be approximated by models that relate various quantities at the contact surface to material loss. In Mechanical APDL, the material loss due to wear is approximated by repositioning the contact nodes at the contact surface. The new coordinates of the nodes are determined by a wear model. The following models are available for defining wear: • Archard wear model • User-defined wear model Since the contact nodes are moved to new positions, the contact variables (for example, contact pressure) change. The underlying continuum elements also experience a loss in material (and volume), thus simulating the wear. See Including Wear in a Contact Analysis (p. 165) for details on implementing these models.

13.1. Including Wear in a Contact Analysis The following contact elements support modeling wear: CONTA171, CONTA172, CONTA173, CONTA174, and CONTA175. To activate contact surface wear, define wear as a material model via the TB,WEAR command and assign it to the contact elements. You must also define the wear properties via the TBDATA command. You can use the TBFIELD command in conjunction with TBDATA to define the properties as a function of temperature and/or time. The TBTEMP command can also be used to define temperature dependent wear data. The implementation of wear involves two stages. First, the amount of wear is calculated by a wear model. Next, the geometry is updated to account for wear. This process is explained below.

13.1.1. Wear Models The wear models calculate how much and in what direction a contact node is to be moved to simulate wear based on the contact results at the contact nodes. A generalized form of the Archard wear model is available. In addition, you can specify a user-defined wear model using the USERWEAR subroutine.

13.1.1.1. Archard Wear Model ɺ , to contact pressure, sliding velocity, and material The Archard wear model relates the rate of wear, w hardness. The wear is in the direction opposite to the contact normal. The Archard model is defined by the TB,WEAR command with TBOPT = ARCD. The material constants required by the model are specified as data items C1 through C4 on the TBDATA command:


165

Contact Surface Wear Constant

Meaning

C1

Wear coefficient, K

C2

Material hardness, H

C3

Pressure exponent, m

C4

Velocity exponent, n

A sample command input for the Archard wear model is shown below. TB,WEAR,1,,,ARCD TBDATA,1,K,H,m,n

Material hardness and yield stress of the material are often closely related. If the material model of the underlying solid element is a plastic model defined using the TB command with Lab = BISO, MISO, BKIN, MKIN, the element average value of the current yield stress can be used to estimate the hardness as H = yield stress/3. To activate this option, the hardness value (C2) on TBDATA should be entered as -99. This option is only available for the material models mentioned above. In addition, the underlying solid element should be one of the following types: PLANE182, PLANE183, SOLID185, SOLID186, SOLID187, or SOLID285. At each substep of the solution, the yield stress at the previous substep is used for estimating the hardness. Thus, there is a lag of one substep for updating the hardness from the yield stress, and the yield stress data is not available at the first substep of the solution. For this reason, no wear can take place at the first substep if this option is used. There is also an option to use the Archard model as a postprocessor only. To activate this option, enter an additional fifth constant (C5) with a value of -99 on the TBDATA command. In this case, the Archard model is used to calculate the wear, but the contact nodes are not moved; thus wear is just a postprocessing variable and does not affect the solution. This option can be of help in deciding if the geometric correction calculated by the wear law is of intended magnitude or not. This can potentially help if solution convergence becomes a problem due to large wear. This option is not available for analyses involving rezoning. For more information on the Archard model, see the discussion of contact surface wear in the Mechanical APDL Theory Reference.

13.1.1.2. User-Defined Wear Model (USERWEAR) You can specify a user-defined wear option which utilizes the USERWEAR subroutine. This option is activated by the TB,WEAR command with TBOPT = USER. The material constants required by the model are specified as data items on the TBDATA command. A sample command input is shown below: TB,WEAR,1,,4,USER TBDATA,1,C1,C2,C3,C4

For this option, you must write a USERWEAR subroutine to define the wear model. USERWEAR is called at contact detection points that are closed. For efficiency reasons, this subroutine is not called if the contact detection point is open. Some of the input arguments are described below.

166


Including Wear in a Contact Analysis USERWEAR Input Argument

Description

strain(1)

Slip increment in direction 1

strain(2)

Slip increment in direction 2 (3-D only)

strain(3)

Contact normal penetration/gap (positive value represents penetration, negative value represents gap)

stress(1)


stress(2)

Tangential stress in direction 2 (3-D only)

stress(3)

Contact normal pressure

temperature


dtime

Time increment

ndim

Set to 2 for 2-D; set to 3 for 3-D

YieldStress

Current yield stress value (element average) of the underlying material if the material is defined by the TB command with Lab = BISO, MISO, ANISO, BKIN, MKIN, DP, or ANAND; and underlying solid element is of type: 182, 183, 185,186,187 or 285

nTbprop

Number of TBDATA for TB,WEAR per field

Tbprop(nTbprop)

The data defined by TBDATA at a given temperature and current time

coor(6)

Coor(1-3) - current x,y,z coordinate of the contact detection point Coor (4-6) - initial x,y,z coordinate of the contact detection point

localr(3,3)

The direction cosines of the local surface coordinate system at contact detection. localr(1,1), localr(1,2), localr(1,3) in slip direction 1 localr(2,1), localr(2,2), localr(2,3) in slip direction 2 (3-D only) localr(3,1), localr(3,2), localr(3,3) in normal direction

TotWearOld(ndim)

Total wear at the contact point at the previous substep

You also need to define the following output arguments: USERWEAR Output Argument

Description

WearInc

Increment of wear

WearDir (ndim) (Optional)

Direction cosines in the global coordinate system in which the wear increment will be applied. If WearDir is not defined, ANSYS applies wear in the direction opposite to the contact normal.

13.1.2. Geometry Update Due to Wear ɺ x ∆t) calculated by the wear model The wear increment (the rate of wear times the time increment: w is used to move the contact node along the direction opposite to the contact normal at that node. During the nonlinear solution process, at the iteration in which the force and displacement convergence criteria set by the user are met for a particular substep, the wear increment and direction at that iteration are used to move the contact nodes. Since this repositioning of contact nodes results in a loss of equiRelease 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information of ANSYS, Inc. and its subsidiaries and affiliates.

167

Contact Surface Wear librium, additional iterations are required to achieve convergence. If the solution fails to converge after the wear is applied, the usual procedure of bisection is used. The wear increment is discarded and the increment is repeated, applying the bisection. Since wear is a material removal process, the underlying solid element does not experience any strain or stress due to the movement of the contact nodes undergoing wear. A large wear increment can result in the opening of an initially closed contact pair. This may result in convergence problems, especially if rigid body motion occurs. Thus, it is highly recommended that you use very small time increments when modeling wear.

13.1.3. Assumptions and Restrictions You should keep the following assumptions and restrictions in mind when modeling wear. • Wear is only active for quasi-static and transient dynamic analyses (ANTYPE,0 or 4). If a linear perturbation analysis follows a static or transient dynamic base analysis that included wear, the affect of wear is also included in the linear perturbation analysis. • Wear (TB,WEAR) must be defined before the first SOLVE command is issued. You can modify the wear data by redefining TB,WEAR between load steps, but wear must be defined before the first load step. • If the wear data is changed between load steps, TB,WEAR must be redefined to the current values if you restart the analysis. This is due to the multiframe restart architecture, whereby the restart database is based on the beginning of the solution (see Multiframe Restart in the Basic Analysis Guide for more information). • It is recommended that you use wear only with the following contact algorithms: Augmented Lagrangian or Penalty function (KEYOPT(2) = 0 or 1). Using wear with the pure Lagrangian contact algorithm can result in convergence problems and is not recommended. • Wear is only available when the contact detection point is a nodal point (KEYOPT(4) = 1, 2, or 3). • Wear is only available for the following contact surface behaviors: standard and rough (KEYOPT(12) = 0 and 1). • When modeling wear, it is recommended that the underlying elements (beneath the contact elements) are structural solid elements or structural coupled-field solid elements. • Wear is not available for layered solids. • Wear should be used with asymmetric contact pairs. Using wear with symmetric contact pairs can result in convergence problems and is not recommended. • During rezoning, since the geometry is updated with accumulated wear, the wear is initialized to zero (similar to displacement) upon a subsequent restart.

13.1.4. Postprocessing The total wear vector components at a node of a contact element are available as NMISC data. To help in the visualization of the surface that has undergone wear, the wear is also added to the nodal displacements in the Jobname.RST file. The total volume lost due to wear for a contact pair can be tracked via the NLHIST command.

168


Including Wear in a Contact Analysis For more information on how to review results in a contact analysis, see Reviewing the Results (p. 92) in Surface-to-Surface Contact (p. 11).


169

170


Appendix A. Example 2-D Contact Analysis with Fluid Pressure-Penetration Loading This example represents a planar seal compression with applied fluid pressure-penetration loading at the contact interface. It demonstrates the application of the fluid pressure loads and the propagation of the fluid penetration path from one starting point into the contact interface.

A.1. Problem Description The model represents a half symmetry planar hyperelastic seal. Figure 1: Diagram of Planar Seal Model (p. 171) shows a diagram of the model, and Figure 2: Meshed Planar Seal Model (p. 172) shows the meshed half symmetry model. Figure 1: Diagram of Planar Seal Model

0.865 inches applied displacement

1/2 symmetry


171

Example 2-D Contact Analysis with Fluid Pressure-Penetration Loading Figure 2: Meshed Planar Seal Model

The seal is compressed by a displacement-controlled load in the first load step. Fluid pressure is applied to all contact elements in the second load step (see Figure 3: Fluid Pressure Loading on Planar Seal (p. 172)) so that the fluid opens and penetrates into the contact interface that was previously closed by compression. The fluid pressure is applied using the SFE command with the load key set to 1 (LKEY = 1 on SFE). Using SFE with the load key set to 2 (LKEY = 2 on SFE), all default starting points are suppressed (STA1 = -1) and two elements are chosen as starting locations initially exposed to the fluid (STA1 = 1). (See Specifying Fluid Penetration Starting Points (p. 86) for more information on the STA values.) From this location the fluid penetrates gradually by opening the bottom part of the contact surface, while the top part of the contact surface completely closes. Figure 3: Fluid Pressure Loading on Planar Seal

At the end of the first load step (compression load step) most of the contact is closed as shown in the above figure. Plots of contact fluid pressure at an intermediate substep of load step 2 and at the end of load step 2 are shown in Figure 4: Intermediate Fluid Pressure Distribution (p. 173) and Figure 5: Final Fluid Pressure Distribution (p. 173).

172


Problem Description Figure 4: Intermediate Fluid Pressure Distribution

Figure 5: Final Fluid Pressure Distribution

Three contact elements at the bottom of the seal are used for time history postprocessing, representing each region that undergoes fluid penetration. Time history results for fluid contact pressure and gap are printed and plotted to show the path of fluid penetration along the bottom contact elements. The fluid pressure history is shown in Figure 6: Time History of Fluid Pressure for Three Contact Elements (p. 174).


173

Example 2-D Contact Analysis with Fluid Pressure-Penetration Loading Figure 6: Time History of Fluid Pressure for Three Contact Elements

A.2. Input File /title,Seal compression with fluid pressure-penetration loading /com ===================================================================== /com OBJECTIVE: /com The objective of this test is to verify the path of fluid /com penetration pressure with rigid-flex contact /com and CONTA171 default keyoptions. /com /com DESCRIPTION: /com The model represents a planar seal that is compressed. Fluid /com pressure is applied to all contact elements so that the fluid /com penetrates and opens the contact that used to be closed by /com compression. /com /com The fluid pressure is applied after the compression, and one /com starting point is chosen to be initially in fluid (STA=1 on SFE). /com From this location the fluid penetrates gradually at the bottom /com surface. /com /com TEST SPECIFICATIONS: /com SOLID ELEMENTS: PLANE182 - PLANE STRESS /com TARGET ELEMENT: TARGE169 /com CONTACT ELEMENT: CONTA171, K(14)=0, K(2)=0, K(4)=0 /com MATERIAL MODEL: HYPERELASTIC /com /com /com RESULTS: /com At the end of load step 1 there are 3 regions where contact is /com closed at the bottom surface. One element from each of these /com regions is chosen, and the time history results for fluid /com contact pressure and gap are printed and plotted to show the /com path of fluid penetration along the bottom contact elements. /com Notice that at the time when the contact opens /com (gap is not zero), the FPRS is nonzero. /com ===================================================================== /prep7

174


Input File et,1,182

! PLANE182 with plane stress option

tb,HYPER,1,1,2,MOON tbtemp,0 tbdata,,80,20,0,,, /com

**** Build the model ****

k,1 k,2,0.333,0 k,3,0.867,0.867 k,4,1.1,0.867 k,5,1.1,1 k,6,0.8,1 k,7,0.267,0.133 k,8,0,0.133 l,1,2 *repeat,7,1,1 l,8,1 lfillt,1,2,0.20 lfillt,2,3,0.15 lfillt,5,6,0.20 lfillt,6,7,0.15 lfillt,7,8,0.05 lfillt,8,1,0.05 al,all k,98,-1.0,0 k,99,1.1,0 lstr,99,98 k,100,-1.0,1.0 k,101,1.1,1.0 lstr,100,101 lcomb,8,13 lcomb,8,14 esize,0.02 type,1 mat,1 smrtsize,5 amesh,all allsel

lsel,s,,,4 nsll,s,1 d,all,ux,0 allsel /com

! Set element attributes and meshing parameters

! Mesh the model

! BCs to model half symmetry

**** Contact pair creation ****

et,2,169 et,3,171 keyopt,3,10,2 mp,mu,2,0.2 ! Friction r,2 /com

**** Top and bottom rigid targets ****

type,2 real,2 mat,2 lesize,15,,,1 lesize,16,,,1 lmesh,15,16 /com

**** Contact on the planar seal ****

lsel,s,line,,1,12 lsel,u,line,,4


175

Example 2-D Contact Analysis with Fluid Pressure-Penetration Loading nsll,s,1 type,3 esurf allsel,all finish /solu allsel esel,s,ename,,169 nsle nsel,r,loc,y,1 d,all,uy,-0.865 ! Move top rigid target nlgeom,on time,1 nsubst,25,2000,5 outres,all,all allsel nropt,unsym ! Unsymmetric due to friction solve esel,s,ename,,171 ! Select only contact elements sfe,all,1,PRES, ,150 ! Apply fluid pressure of 150 to all contact elements sfe,all,2,pres,,-1 ! Suppress default starting points (STA1 = -1) nsel,s,loc,x ! Select a location for starting points esln esel,r,ename,,171 ! Reselect only the contact elements sfe,all,2,pres,,1 ! Specify the above location to be initially in fluid (STA1 = 1] allsel nsubst,100,1000,10 solve finish /com

**** Post1 postprocessing ****

/post1 /show set,2,last finish /com

**** Post26 time history postprocessing ****

/post26 timerange,1,2 esol,2,523,138 ,cont,fprs,CONTFPRS1 esol,3,523,138 ,cont,gap,CONTGAP1 /com ***************************************************************************** /com History of contact gap and fluid pressure for an element in the /com region that opens first /com ***************************************************************************** prvar,2,3 esol,4,578,53 ,cont,fprs,CONTFPRS2 esol,5,578,53 ,cont,gap,CONTGAP2 /com **************************************************************************** /com History of contact gap and fluid pressure for an element in the /com region that opens later /com ***************************************************************************** prvar,4,5 esol,6,517,6,cont,fprs,CONTFPRS3 esol,7,517,6,cont,gap,CONTGAP3 /com **************************************************************************** /com History of contact gap and fluid pressure for contact element /com that opens last /com ***************************************************************************** prvar,6,7

176


Input File /title,Time history of fluid pressure for three contact elements /axlab,y,Fluid pressure plvar,2,4,6 finish


177

178


Index B beam-to-beam contact, 99 bolt thread modeling, 27

C contact analysis, 1 asymmetric vs. symmetric contact, 14 beam-to-beam, 2, 99 birth and death option, 70 bolt thread modeling, 27 boundary conditions for node-to-node contact, 123 boundary conditions for surface-to-surface contact, 84 chattering controls, 38 choosing surfaces, 13 conduction, 110 contact algorithm, 38, 123 contact direction, 21 contact location detection, 49 Contact Manager, 7, 11 contact pairs, 9, 12 contact status, 60, 109 contact stiffness, 38-39 contact surface elements, 3 Contact Wizard, 8 convection, 111 debonding, 161 deformable contact surface, 22 direct generation to create rigid target elements, 16 electric contact, 114 electrostatic elements, 114 emissivity, 112 explicit dynamics, 1 flexible-to-flexible, 1, 11 fluid pressure-penetration loads for surface-to-surface contact, 84 free surface convection, 110 free surface radiation, 110 free thermal surface, 110 friction model, 44 gap, 123 Gauss integration points, 49 generating node-to-node contact elements, 120 generating surface-to-surface contact elements, 24 geometry correction, 25 heat flux, 110, 114 heat generation due to electric current, 115 heat generation due to friction, 113 initial contact conditions, 52

initial interference, 123 internal MPC, 127 line-to-line, 4 line-to-surface, 105 localized contact zone, 20 magnetic contact, 116 moving contact nodes, 59 node ordering in node-to-node analysis, 121 node-to-node, 2, 5, 119 node-to-surface, 2, 4, 95 node-to-surface KEYOPTS, 96 node-to-surface real constants, 97 normals, 7, 21, 24, 122 open far-field contact status, 60 open near-field contact status, 60 overall steps, 11 overconstrained contact problem, 123 penalty stiffness, 24 penetration, 38 piezoelectric elements, 114 pinball region, 60 point-to-point, 5 point-to-surface, 4 radiation, 112 results, 92 rigid-to-flexible, 1, 11, 82 self contact, 14, 61 shells, 69 sliding contact status, 60 slippage, 49 solution options for node-to-node contact, 124 solution options for surface-to-surface contact, 90 solving, 91 spurious contact, 61 Stefan-Boltzmann constant, 112 sticking contact status, 60 summary of contact capabilities, 2 superelements, 65 surface interaction for electric contact, 114 surface interaction models, 62 surface-to-surface, 2-3, 11, 22 surface-to-surface KEYOPTS, 35 surface-to-surface material properties, 24 surface-to-surface real constants, 24, 29 symmetric/unsymmetric solver, 44 target element shapes, 16 target surface definition, 15 target surface element types and real constants, 16 target surface modeling and meshing tips, 20 target surface nodal number ordering, 21 target surface pilot node, 15, 82 target surface primitives, 15


179

Index temperature required, 110 thermal contact, 109 thickness effect, 69 time step control, 70 using meshing tools to create rigid target elements, 18 wear, 165 contact stabilization, 66 contact surface wear, 165

D debonding, 161 delamination in a contact analysis, 161

F fluid pressure-penetration loads, 84

I internal MPC, 127

L line-to-surface contact, 105

M magnetic analysis contact, 116 multipoint constraint, 127

P penalty stiffness in contact analysis, 24 pilot node in contact analysis, 15, 82

S spot welds, 149

W wear, 165

180


ANSYS Mechanical APDL Contact Technology Guide

Recommend Documents