Fatigue ANSYS

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Fatigue Example Problem Solved In Ansys

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ANSYS Fatigue Module

7207778-AnsysTutorial-1

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Fatigue Analysis Using ANSYS D. Alfred Hancq, Ansys Inc.

Contents 1) Introduction 2) Overview of Capabilities 3) Typical Use Cases 4) Additional Fatigue Resources

1. Introduction It is estimated that 50-90% of structural failure is due to fatigue, thu a need for quality fatigue design tools. However, at this time a fatigu not available which provides both flexibility and usefulness compara other types of analysis tools. This is why many designers and analyst "in-house" fatigue programs which cost much time and money to dev is hoped that these designers and analysts, given a proper library of tools could quickly and accurately conduct a fatigue analysis suited needs.

The focus of fatigue in ANSYS is to provide useful information to t design engineer when fatigue failure may be a concern. Fatigue r have a convergence attached. A stress-life approach has been ado conducting a fatigue analysis. Several options such as accounting stress and loading conditions are available.

 2. Capabilities Sign up to vote on this title A fatigue analysis can be separated into3Useful areas: analy Not useful materials, results evaluation. Each area will be discussed in more detail bel

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2.1.1 Stress-life Data Options/Features \ \

\ \ \

\

Fatigue material data stored as tabular alternatin u 2 0 2 2 The ability to define mean stress dependent or the data is available. u 2 0 2 2 Options to have log-log, semi-log, or linear interpol u 2 0 2 2 Ability to graphically view the fatigue material data u 2 0 2 2 The fatigue data is saved in XML format along with material data. u 2 0 2 2 Figure 1 is a screen shot showing a user editing u

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Figure 1: Editing SN curves in ANSYS

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2.2 Analysis Fatigue results can be added before or after a stress solution has

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Figure 2: Fatigue tool information page in ANSYS


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2.2.1 Loading  Useful  Not useful Fatigue, by definition, is caused by changing the load on a compo time. Thus, unlike the static stress safety tools, which perform ca

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only 1 set of finite element stress results along with a loading r required to calculate the alternating and mean stress. The loa defined as the ratio of the second load to the first load (LR = L Loading is proportional since only 1 set of finite element stress needed (principal stress axes do not change over time). No cu damage calculations need to be done. Common types of consta amplitude loading are fully reversed (apply a load then apply a and opposite load; a load ratio of –1) and zero-based (apply a lo remove it; a load ratio of 0). Fully reversed, zero-based, or a sp loading ratio can be defined in the details view under the “Loa section. •

Non-constant amplitude, proportional loading: In this case, again only 1 set of results are needed, however instead of using a sin ratio to calculate the alternating and mean stress, the load rati over time. Think of this as coupling an FEM analysis with strai results collected over a given time interval. Cumulative damag calculations including cycle counting and damage summation n done. A rainflow cycle counting method is used to identify stre reversals and Miner’s rule is used to perform the damage summ The load scaling comes from an external data file provided by t (such as the one in Figure 3) and is simply a list of scale factor


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Several sample load histories can be found in the “Load Histor directory under the “Engineering Data” folder. Setting the load to “History Data” in the fatigue tool details view specifies non-c amplitude loading. Several analysis options are available for n constant amplitude loading. Since rainflow counting is used, u “quick counting” technique substantially reduces runtime and In quick counting, alternating and mean stresses are sorted int before partial damage is calculated. Without quick counting, th not sorted into bins until after partial damages are found. The accuracy of quick counting is usually very good if a proper number of bins is used when counting. The default setting for the number of bins can be set in the Control Panel. Turning off quick counting is not recommended and in fact is not a documented feature. To allow quick counting to be turned off, set the variable “AllowQuickCo to 1 in the Variable Manager. Another available option when co a variable amplitude fatigue analysis is the ability to set the va for infinite life. In constant amplitude loading, if the alternatin lower than the lowest alternating stress on the fatigue curve, A use the life at the last point. This provides for an added level of because many materials do not exhibit an endurance limit. Ho  Signwith up to vote on this title alterna non-constant amplitude loading, cycles very small Not usefultoo much  Useful  stresses may be present and may incorrectly predict the number of the small stress cycles is high enough. To help c this, the user can set the infinite life value that will be used if t

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Rainflow matrix for a given load history.

Damage matrix with an infinite life of 1e6 cycles. Total damage is calculated to be .19 .


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2.2.2 Load Effects

Fatigue material tests are usually conducted in a uniaxial loading fixed or zero mean stress state. It is cost-prohibitive to conduct e that capture all mean stress, loading, and surface conditions. Thu relations are available if the fatigue data is not. • Mean Stress correction. If the loading is other than fully reve mean stress exists and should be accounted for. Methods for h mean stress effects can be found in the “Options” section. If experimental data at different mean stresses or rratio’s exist, mean stress can be accounted for directly through interpolation between material curves. If experimental data is not available, several empirical options may be chosen including Gerber, Goodman and Soderberg theories which use static material properties (yield stress, tensile strength) along with S-N data to account for any mean stress. In genera experimental data fall between the Goodman and Gerber theor the Soderberg theory usually being over conservative. The Goo theory can be a good choice for brittle materials with the Gerb  Sign up to vote this be title seen from usually a good choice for ductile materials. As on can Useful treats screen shots in Figure 5, the Gerber theory negative and  Not useful mean stresses the same whereas Goodman and Soderberg do n any correction for negative mean stresses. This is because alth

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Figure 5: The chosen mean stress theory is illustrated in the graphics window

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positive and negative mean stresses differently. Setting the “S Component” is done in the Options section in the fatigue tool d

2.2.3 Miscellaneous Analysis options Fatigue material property tests are usually conducted under very controlled conditions (eg. axial loading, polished specimens, .5 inc diameter). If the service part conditions differ from as tested, mo factors can be applied to try to account for the difference. The fat alternating stress is usually divided by this modification factor an found in design handbooks. (Dividing the alternating stress is equ multiplying the fatigue strength by Kf.) The fatigue strength redu is defined by setting “Fatigue Strength Factor (Kf)” in the details the fatigue tool. Note that this factor is applied to the alternating and does not affect the mean stress.

2.3 Results Output Several results for evaluating fatigue are available to the user. contour plots of a specific result over the model while others give information about the most damaged point in the model(or the mo damaged point in the scope of the result). Outputs include fatigue damage, factor of safety, stress biaxiality, fatigue sensitivity, rainfl matrix, and damage matrix output. Each output will now be descr detail.


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A contour plot of available life over the model. This result the whole model or scoped to a given part or surface. This resu plot shows the available life for the given fatigue analysis. If lo constant amplitude, this represents the number of cycles until will fail due to fatigue. If loading is non-constant, this represen number of loading blocks until failure. Thus if the given load hi represents one month of loading and the life was found to be 1 expected model life would be 120 months. In a constant amplit analysis, if the alternating stress is lower than the lowest altern defined in the S-N curve, the life at that point will be used. See 2.2.1 for more information about the difference between consta non-constant amplitude loading.


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A contour plot of the fatigue damage at a given design life. damage is defined as the design life divided by the available lif result may be scoped. The default design life may be set throu Control Panel.

A contour plot of the factor of safety with respect to a fatig given design life. The maximum FS reported is 15. Like damag life, this result may be scoped. This calculation is iterative for constant amplitude loading and may substantially increase solv


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A stress biaxiality contour plot over the model. As mentione previously, material properties are uniaxial but stress results a multiaxial. This result gives the user some idea of the stress st the model and how to interpret the results. Biaxiality indicatio defined as the principal stress smaller in magnitude divided by principal stress with the principal stress nearest zero ignored. biaxiality of zero corresponds to uniaxial stress, a value of –1 corresponds to pure shear, and a value of 1 corresponds to a pu state. From the sample biaxiality plot shown below, most of the under a pure shear or uniaxial stress. This is expected since a torque has been applied at the top of the model. When using th biaxiality plot along with the safety factor plot above, it can be the most damaged point occurs at a point of nearly pure shear. would be desirable to use S-N data collected through torsional available. Of course collecting experimental data under differe conditions is cost prohibitive and not often done.


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A fatigue sensitivity plot. This plot shows how the fatigue as a function of the loading at the critical location on the mode result may be scoped to parts or surfaces. Sensitivity may be f life, damage, or factory of safety. The user may set the number of fill points as well as the load variation limits. For example, the user may wish to see the sensitivity of the model’s life if the load was 50% of the current load up to if the load 150% of the current load. (The xvalue of 1 on the graph corresponds to the life at the current loading of the model; The x-value at 1.5 corresponds to the critical fatigue life if the finite element loads were 50% higher then they are c etc…). Negative variations are allowed in order to see the effe possible negative mean stress if the loading is not totally rever Linear, Log-X, Log-Y, or Log-Log scaling can be chosen for char display. Default values for the sensitivity options may be set th Control Panel.


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A plot of the rainflow matrix for the critical location. This applicable for non-constant amplitude loading where rainflow c needed. This result may be scoped. In this 3-D histogram, alte and mean stress is divided into bins and plotted. The Z-axis co to the number of counts for a given alternating and mean stres result gives the user a measure of the composition of a loading (Such as if most of the alternating stress cycles occur at a nega stress.) From the rainflow matrix below, the user can see that alternating stresses have a positive mean stress and that bulk o smaller alternating stresses have a higher mean stress then the alternating stresses.


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A plot of the damage matrix at the critical location on the result is only applicable for non-constant amplitude loading wh rainflow counting is needed. This result may be scoped. This r similar to the rainflow matrix except the %damage that each bi plotted as the Z-axis. As can be seen from the corresponding d matrix for the above rainflow matrix, in this particular case alth most of the counts occur at the lower stress amplitudes, most o damage occurs at the higher stress amplitudes.


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3. Typical Use Cases

Scenario I, Connecting Rod under fully reversed loading: Here w a connecting rod in a compressor under fully reversed loading (lo applied, removed, then applied in the opposite direction with a m of 1000 pounds). •

Import geometry and apply boundary conditions. Apply corresponding to the maximum developed load of 1000 poun


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reversed loading). Specify that Von-Mises stress will be use compare against fatigue material data. Specify a modification factor of .8 since material data rep polished specimen and the in-service component is cast. Perform stress and fatigue calculations (Solve command menu). Plot factor of safety for a design life of 1,000,000 cycles.


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Find the sensitivity of available life with respect to loadin minimum base load variation of 50% (an alternating stress o and a maximum base load variation of 200% (an alternating 2000 lbs.)

Determine multiaxial stress state (uniaxial, shear, biaxial, critical life location by inserting “biaxiality indicator” into fa tool. The stress state near the critical location is not far from (.1~.2), which gives and added measure of confidence since material properties are uniaxial. Sign up to vote on this title

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Scenario II, Connecting Rod under random loading: Here we have t same connecting rod and boundary conditions but the loading is n constant amplitude over time. Assume that we have strain gauge were collected experimentally from the component and that we k strain gauge reading of 200 corresponds to an applied load of 1,0 •

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Conduct the static stress analysis as before using a load pounds. Insert fatigue tool. Specify fatigue loading as coming from a scale history an scale history file containing strain gauge results over time(e

Files\Ansys Inc\Engineering Data\Load Histories\SAEBracketHistory.d

Define the scale factor to be .005. (We must normalize the load history so that the FEM load matches the scale factors in the lo history file).

  1 FEMload  1 FEM load ×  1000lbs  =  = neededloadscalefact  1000lbs  200straingauge 200straingauge •

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Specify a bin size of 32 (Rainflow and damage matrices dimension 32x32).

Specify Goodman theory to account for mean-stress effec chosen theory will be illustrated graphically in the graphics Specify that a signed Von-Mises stress will be used to compa  against fatigue material data. (Use signed Goodman t Sign up to votesince on this title treats negative and positive mean stresses Useful differently.) Not useful

Perform fatigue calculations (Solve command in context View rainflow and damage matrix.

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Plot life, damage, and factor of safety contours over the design life of 1000. (The fatigue damage and FS if this loadi was experienced 1000 times). Thus if the loading history corresponded to the loading experienced by the part over a time, the damage and FS will be at a design life of 1000 mon that although a life of only 88 loading blocks is calculated, t scale factor (since FS@1000=.61) is only .61 to reach a life blocks.


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Change bin size to 50, rerun analysis, and compare fatig verify that the bin size of 32 was of adequate size to get des precision for alternating and mean stress bins.

4. Additional fatigue resources •

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Hancq, D.A., Walters, A.J., Beuth, J.L., “Development of Oriented Fatigue Tool”, Engineering with Computers, Vol 16, 20 pp. 131-144. This paper gives details on both the underlying structure an engineering aspects of the fatigue tool used by the DesignSp program.

Bannantine, J., Comer, J., Handrock, J. “Fundamentals of Fatigue Analysis”, New Jersey, Prentice Hall (1990). This is an excellent book that explains the fundamentals of f novice user. Many topics such as mean stress effects and ra counting are topics in this book.

Lampman, S.R. editor, “ASM Handbook: Volume 19, Fatigu Fracture”, ASM International (1996). Good reference to have when conducting a fatigue analysis. papers on a wide variety of fatigue topics.

U.S. Dept. of Defense, “MIL-HDBK-5H: “Metallic materials Elements for Aerospace Vehicle Structures”, (1998). This publication distributed by the United States governmen fatigue material properties of several common engineering  Sign up to vote on this title is freely downloadable over the Internet from the NASA web  Useful  Not useful

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