DAC

IIW/EWF Diploma Design and Construction (Foundation) DAC1

Training & Examination Services Granta Park, Great Abington Cambridge CB21 6AL, UK Copyright © TWI Ltd

Rev 4 April 2013 Contents Copyright  TWI Ltd 2013

IIW/EWF Diploma Design and Construction (Foundation) Contents Section

Subject

Pre training briefing 1 1.1 1.2 1.3

Introduction. Designing Things. Background Course aims Course objective

2 2.1 2.2 2.3 2.4 2.5

Welded Joint Design. Introduction Types of joints Types of weld Ability of welds to transmit loads Design examples

3 3.1 3.2

Forces and Strength of Materials. Introduction Materials under load

4 4.1 4.2 4.3

Fatigue Introduction Characterisation Fatigue of welded joints

5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10

Design of Pressure Equipment. Types of pressure vessels Construction of pressure vessels Internal pressure stresses Calculation of stresses Welding pressure vessels Welded attachments High and low temperature services Standards and specifications Summary Revision questions

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IIW/EWF Diploma Design and Construction (Foundation) Contents Section

Subject

Pre training briefing 1 1.1 1.2 1.3

Introduction. Designing Things. Background Course aims Course objective

2 2.1 2.2 2.3 2.4 2.5

Welded Joint Design. Introduction Types of joints Types of weld Ability of welds to transmit loads Design examples

3 3.1 3.2

Forces and Strength of Materials. Introduction Materials under load

4 4.1 4.2 4.3

Fatigue Introduction Characterisation Fatigue of welded joints

5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10

Design of Pressure Equipment. Types of pressure vessels Construction of pressure vessels Internal pressure stresses Calculation of stresses Welding pressure vessels Welded attachments High and low temperature services Standards and specifications Summary Revision questions

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6 6.1 6.2 6.3 6.4 6.5 6.6 6.7

Stresses in The Welds Making things simple Different types of stresses in welds Butt welds Fillet welds Summary Revision questions References

7 7.1 7.2 7.3 7.4 7.5 7.6

Different Types of Loading Static strength Effect of temperature on strength Stress concentrations Modes of failure Reading fracture faces Summary

8 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9

Design Considerations for Aluminium Introduction Advantages of aluminium compared to steel Welding and joining aluminium Disadvantages of aluminium Aluminium alloys Heat affected zone softening References and further reading Summary Revision questions

9 9.1 9.2 9.3 9.4 9.5

Static Loading Introduction Allowable stress Structural details Node joints Stress reinforced concrete

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Section 1 Introduction – Designing Things

Rev 4 April 2013 Introduction - Designing Things Copyright  TWI Ltd 2013

1

Introduction - Designing Things

1.1

Background An engineering structure is one that is designed and built to withstand loads for a specified period of time. These loads may arise from a wide range of sources and include self weight (such as buildings including the pyramids), external components (eg cars traversing a bridge), internal pressure (eg pipelines and boilers), environmental loads (due to wind, waves, ice, snow etc), reaction to an acceleration (eg rotating components) and many other sources. Furthermore, engineering structures are built using materials such as steel, aluminium, fibre reinforced composites that have been specifically selected to meet the lifetime demands of the structure. These materials are used to make components which are then assembled and joined together (usually by welding but not always) to form the structure itself.

1.2

Course aim – Design and Construction Modules The overall aim of the Construction and Design Modules of the European Welding Engineer training course is to: 

Provide guidance on how to design engineering structures so that they operate safely to satisfy specified performance targets.

The training is provided at three levels: European Welding Specialist, European Welding Technologist and European Welding Engineer. The present course is the first of these levels and is intended to cover the scope appropriate for a European Welding Specialist. Two subsequent courses address the scope of the higher level qualifications.

1.3

Course objectives: European Welding Specialist (Foundation) The objectives of the European Welding Specialist course are to enable attendees to:        

Recognise the sources of loads to be withstood by engineering structures. Recognise that these loads give rise to stresses in components of the structure. Understand the fundamentals of strength of materials. Understand the principles of weld design. Recognise the different types of loading experienced by engineering structures. Understand the principles of design for static loading. Understand the principles of design for fatigue loading. Recognise the special requirements of pressure vessels.

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Rev 4 April 2013 Introduction - Designing Things Copyright  TWI Ltd 2013



Appreciate the principles of designing aluminium structures.

The course consists of eight sessions, specifically intended to address these objectives; there is a final revision session.

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Section 2 Welded Joint Design

Rev 4 April 2013 Welded Joint Design Copyright  TWI Ltd 2013

2

Welded Joint Design

2.1

Introduction This course is principally concerned with structures fabricated by welding steel plates together. Examples of such structures include bridges, ships, offshore platforms, pressure vessels and pipelines, although obviously in some cases this may involve welding curved plates together. This session provides an introduction to typical joint geometries that are involved in joining plates together and describes the types of weld that are used in these joint configurations. Typical features of butt and fillet welds are described. For the structure to function loads must be transferred from one plate to another and the features of welds that enable them to transmit loads are described. Finally, some examples of good and bad design practice are illustrated.

2.2

Welds A weld is a permanent union between materials caused by the application of heat or pressure or both. A weld made between two faces that are approximately parallel is known as a butt weld.

Figure 2.1 Butt weld.

A weld made between two faces that are approximately at right angles to each other is known as a fillet weld.

Figure 2.2 Fillet weld.

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For simplicity these diagrams show an arc welding process that deposits filler weld metal in a single weld pass. Typical features of a butt weld are shown in Figure 2.3. Typical features of a fillet weld are shown in Figure 2.4. The weld or weld metal refers to all the material that has melted and resolidified. The heat-affected zone is material that has not melted, but whose microstructure has been changed as a result of the welding. The fusion line is the interface between the weld metal and the heat affected zone. The root is the bottom of the weld, or the narrowest part and the face is the top, or the widest part. At the corners of the weld cross section where the weld metal joins the parent metal are the weld toes. Weld toes are at each corner of both the weld face and the weld root in a butt weld, but only on the weld face in a fillet weld.

a)

b) Figure 2.3 Typical features of a butt weld, shown schematically in a) and in b) for a double-sided butt weld.

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Figure 4 Typical features of a fillet weld.

The application of heat naturally causes some changes to the microstructure parent material; the region concerned is known as the heat affected zone (HAZ) and is shown diagrammatically in Figure 2.5 for a butt weld in steel. Similar HAZs are developed in the parent material of fillet welds. Close to the fusion line the temperature in the HAZ has been sufficient to cause microstructural phase changes, which will result in recrystallisation and grain growth. Further away from the fusion line the parent material has been heated to a lower maximum temperature and the effect is to temper the parent microstructure.

Figure 2.5 Heat affected zones in a butt weld.

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The distance between the weld toes is called the weld width. When the distance is between the toes at the weld cap, it is the weld cap width; the distance between the toes at the root is the weld root width. The height of the additional weld metal in the weld cap is called the excess weld metal. This used to be called reinforcement which wrongly gives the impression that increasing this dimension will strengthen the weld. If the excess weld metal is too great it old serves to increase the stress concentration at the weld toe. This extra weld metal at the weld root is called the excess root penetration.

Figure 2.6 Definitions of excess weld metal, root penetration and weld width on a butt weld.

2.3

Types of joint A joint can simply be described as a configuration of members and it can be described independently of how the joint it to be welded. Figures 2.7 and 2.8 show the most common joint types - a butt joint and a T joint. Other typical joint types are shown in Figures 2.9-2.11; a lap joint, a cruciform joint and a corner joint. When designing a lap joint the overlap between the two plates needs to be at least four times the plate thickness (D = 4t), but not less than 25mm.

Figure 2.7 Butt joint.

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Figure 2.8 T joint.

Figure 2.9 Lap joints.

Figure 2.10 Cruciform Joint

Figure 2.11 Corner joint.

An alternative to a conventional lap joint is to weld the joint using plug or slot welding. Slot and plug welds are shown in Figure 2.12 we can drastically alter the typical lap joint. The hole for a slot weld should have a width at least three times the plate thickness and not less than 25mm. In plate less than 10mm thickness, a hole of equal width to the plate thickness can be welded as a plug weld.

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a)

b)

Figure 2.12 a) Slot welded lap joint; b) Plug welded lap joint.

Corner joints can be fitted and welded in a number of ways. The unwelded pieces can be assembled either with an open corner or closed together. The weld can be placed on the external corner, the internal corner or both in a double-sided weld.

Figure 2.13 Different types of corner joints, unwelded and welded.

2.4

Fillet welds The throat and leg length of a fillet welds are shown in Figure 2.14. Throat size (a) is generally used as the design parameter of fillet welds since this part of the weld bears the stresses. It can also be related to the leg length (z) by the following relationship: a 0.7z and z 1.4a. ≈

≈

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Figure 2.14 Leg length (z) and throat size (a) in a fillet weld.

This is only valid for mitre fillet welds having similar leg lengths (see Figure 2.15), but is not valid for concave, convex or asymmetric welds. In concave fillet welds the throat thickness will be much less than 0.7 times the length. The leg length of a fillet weld is often approximately equal to the material thickness. The actual throat size is the width between the fused weld root and the segment linking the two weld toes, shown as the red line in Figure 16. Thanks to root penetration, the actual throat size of a fillet weld is often larger than its design size, but because of the unpredictability of the root penetration area, the design throat size must always be taken as the stress parameters in design calculations.

z

a

z

Figure 2.15 Mitre fillet weld

Figure 2.16 Design throat of a fillet weld.

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Convex fillet weld

Concave fillet weld

Mitre fillet weld

Fillet weld cross-sections. Figure 2.17 Fillet

Definition of design and actual throat in concave and convex fillet Figure 2.18 Definition welds.

The choice between mitre weld, concave and convex fillet weld needs to take into account the weld toe blend. A concave fillet weld gives a smooth blend profile and a low stress concentration at the fillet weld toe. Convex fillet welds can have a higher stress concentration at the weld toe. If the fluidity of the weld pool is not controlled, it is possible to obtain an asymmetrical fillet weld where the weld pool has sagged into the joint preparation and there is also a risk of undercut on the bottom weld toe (see Figure 2.19). Having a smooth toe blend is important to give better fatigue performance for fillet welds.

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Figure 2.19 Fillet weld toe blends.

2.5

Butt welds The design throat (t1) of a butt weld is the penetration depth below the parent plate surface and no account is made of the excess weld metal. The design throat is therefore less than the actual throat (t2).

Design throat (t 1 ) and the actual throat throat (t 2 Figure 2.20 Design 2 ) for butt welds.

The weld toe blend is important for butt welds as well as fillet welds. Most codes state that the weld toes shall blend smoothly. This statement is open to individual interpretation however. The higher the toe blend angle the greater the amount of stress concentration. The toe blend angle ideally should be between 20-30o (Figure 2.21).

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Figure 2.21 Toe blend in butt welds.

2.6

Dilution When filler and parent material do not not have the same composition, the resulting composition of the weld depends largely on the weld preparation before welding. The degree of dilution results from the edge preparation and process used; the percentage of dilution (D) is particularly important when welding dissimilar materials and is expressed as the ratio between the weight of parent material melted and the total weight of fused material (multiplied by 100 to be expressed as a percentage), as shown by the equation below. D

Weight of parent material melted Total weight of fused material

 100

Low dilutions are obtained with fillet welds and with butt welds with multiple runs. However, considering a single pass, better dilution is obtained with grooved welds; see Figure 2.22.

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Figure 2.22 Effect of weld preparation on dilution and weld metal composition (for a single pass only).

2.7

Welding symbols On engineering drawings a welded joint can be represented by different means. A detailed representation shows every detail and dimension of the joint preparation together with carefully written, extensive notes. It provides all the details required to produce a particular weld in a very clear manner, but requires a separate detailed sketch (time consuming and can overburden the drawing). For a special weld preparation not covered in the relevant standards (eg narrow groove welding); it is the only way to indicate the way components are to be prepared for welding or brazing.

Figure 2.23 Detailed representation of U bevel angle.

Symbolic representation using weld symbols can be used to specify joining and inspection information. The standard for welding symbols that the UK has traditionally used is BS 499 Part 2. This standard has now been

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superseded by BS EN 22553, however in many welding and fabrication organisations there will be old drawings used that make reference to out of date standards such as BS 499 Pt 2. BS EN 22553 is almost identical to the original ISO 2553 standard on which it was based. In America AWS A2.4 is followed, while symbols for brazing are given in EN 14324. Symbolic representation has the following advantages:    

Simple and quick to visualise on the drawing. Does not overburden the drawing. No need for additional views; all welding symbols can be placed on the main assembly drawing. Gives all necessary indications regarding the specific joint to be obtained.

However, symbolic representation can only be used for common joints and it requires training to understand the symbols properly. Symbolic representation of a welded joint contains an arrow line, a reference line and an elementary symbol. The elementary symbol can be complemented by a supplementary symbol. The arrow line can be at any angle (except 180 degrees) and can point up or down. The arrow head must touch the surfaces of the components to be joined and the location of the weld. Any intended edge preparation or weldment is not shown as an actual cross sectional representation, but is replaced by a line. The arrow also points to the component to be prepared with single prepared components

Figure 2.24 Symbolic representation of U bevel angle.

ISO 2553 and AWS A2.4 list all the main elementary symbols, some examples are shown in Table 1. The symbols for arc welding are often shown as cross sectional representations of either a joint design or a completed weld. Simple, single edge preparations are shown in Figure 2.25.

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Table 1 Elementary weld symbols

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Figure 2.25 Welding symbols for the most common joint types shown on a reference line.

These simple symbols can be interpreted as either the joint details alone or the completed weld, however, for a finished weld it is normal to find that an appropriate weld shape is specified. There are a number of options and methods to specify an appropriate weld shape or finish. Butt welded configurations would normally be shown as a convex profile (Figure 2.26 'a', 'd' and 'f') or as a dressed-off weld as shown in 'b' and 'c'. Fillet weld symbols are always shown as a mitre fillet weld and a convex or concave profile can be superimposed over the original symbol's mitre shape.

Figure 2.26 Welding symbols showing the weld profile for the most common joint types.

In order that the correct size of weld can be applied, it is common to find numbers to either the left or to the right of the symbol. For fillet welds, numbers to the left of the symbol indicate the design throat thickness, leg length, or both design throat thickness and leg length requirements (Figure 2.27).

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Figure 2.27 Throat and leg length dimensions given on the weld symbol for a fillet weld.

For butt joints and welds, an S with a number to the left of a symbol refers to the depth of penetration. When there are no specific dimensional requirements specified for butt welds on a drawing using weld symbols, it would normally be assumed that the requirement is for a full penetration butt weld. Numbers to the right of a symbol or symbols relate to the longitudinal dimension of welds, eg for fillets, the number of welds, weld length and weld spacing for non-continuous welds.

Figure 2.28 Weld symbols showing the weld length dimensions to the right of the weld joint symbols for an intermittent fillet weld.

Supplementary symbols can be used for special cases where additional information is required (Figure 2.29). The weld all round symbols may be used for a Rectangular Hollow Section (RHS) welded to a plate, for example. The flag symbol for ‘weld in the field or on site’ can be added to any standard symbol. A box attached to the tail of the arrow can be used to contain, or point to, other information, such as whether NDT is required or not. This information is sometimes the welding process type, given as a three number reference from ISO 4063, for example, 135 refers to MAG welding.

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Figure 2.29 Examples of supplementary symbols.

2.8

Welding positions In weld procedure documents and engineering drawings, the type and orientation of welds are often given a two letter abbreviation which defines them. These can vary depending on which standard the welds are conforming to; the abbreviations here are consistent with ISO 6947. These two letter abbreviations are summarised in Table 2.2.

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Rev 4 April 2013 Welded Joint Design Copyright  TWI Ltd 2013

Table 2.2 Welding

Welding position

positions Figure/symbol

Flat

Welding position

Abbreviation

PA

Symbol

Abbreviation

Horizontal

PB

Horizontal vertical

PC

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2.9

Vertical up, vertical down

PG/PF

Overhead

PE

Horizontal overhead

PD

Weld joint preparations The simplest kind of weld joint preparation is a square edged butt joint, either closed or open. A closed butt joint is used in thick plate for keyhole welding processes such as laser welding or electron beam welding. A square edged open butt joint is used in thinner plate up to 3mm thick for arc welding in a single pass, or in thick plate for welding processes such as electroslag welding.

Figure 2.30 Square edge butt joints.

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It is most common to use a bevel on the edges of the parent metal to be welded. This allows access to the root for the first welding pass and is then filled using fill passes. Single sided preparations are normally made on thinner materials, or when access from both sides is restricted. Double sided preparations are normally made on thicker materials, or when access from both sides is unrestricted. The design of the edge prep includes not only the bevel angle (or included angle is both sides are bevelled), but also the square edges root face and root gap. In a joint where both sides are bevelled, the prep is called a V or vee preparation (Figure 31). V preps are usually used for plate of thickness between 3-20mm. An alternative is a U prep (or J prep if only one side has the edge prep) in which the edge is machined into the shape of a U. This type of edge preparation is used in thicker plate, over 20mm thick, where it uses less filler metal than a V prep joint. J or U edge preparations also requires a bevel angle and root face and gap to be defined, but also needs a root radius and land to be specified (Figure 2.32). Single sided edge preparations are often used for thinner materials or when there is no access to the root of the weld (such as pipelines). If there is access to both sides of the material then a double-sided edge preparation is used, especially for thicker materials. Single and double edge preps are shown in Figure 2.33. Included angle

Bevel angle

Root face Gap

Figure 2.31 Single V bevel. Included angle Root radius Bevel angle

Root face Gap Land

Figure 32 U bevel.

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Figure 2.33 Range of single and double sided bevel, vee, J and U preps.

2.10

Designing welded joints The selection of the weld joint design will also be influenced by practical issues such as the welding process to be used and the access required obtaining root fusion. The bevel angle must allow good access to the root and sufficient manipulation of the electrode to ensure good sidewall fusion (Figure 2.34). If the included angle is too large then heavy distortions can result and a larger amount of filler metal is required. If the included angle is too small then there is a risk of lack of penetration or lack of side wall fusion. Typical bevel angles are 30-35° in a vee preparation (60-70° included angle). In a single bevel joint the bevel angle might be increased to 45°.

Figure 2.34 Bevel angle to t o allow electrode manipulation for sidewall fusion.

The root gap and root face are selected to ensure good root fusion (Figure 2.35). This will depend on the welding process and the heat input. If the root gap is too wide or the root face is too narrow then there is a risk of burn through. If the root gap is too narrow or the root face is too deep the there is a risk of lack of root penetration. A balance must be found and designed for and this difference in weld root size is shown in Figure 2.36. High heat input process require a larger root face, but less weld metal is required, which reduces distortions and increases productivity. Typical values for the root face are around 1.5-2.5mm and the root gap around 2-4mm.

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The importance of selecting the correct root face and root gap. Figure 2.35 The

a) b) Root size for welding processes with different heat inputs; Figure 2.36 Root a) Low heat input; b) High heat input.

If the components are going to be joined by an arc welding process, then the selected bevels need to be adequately machined to allow room for the welding tool to access the root of the weld. This consideration would not apply for a procedure such as electron beam welding as shown in Figure 2.37. If using gas-shielded processes then the size of the gas nozzle may limit the ability to use a J-prep for thick section material, as it would be difficult to ensure good root fusion if the welding head could not access the bottom of the weld groove and a single bevel may be needed instead (Figure 2.38).

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a)

b)

Figure 2.37 Preparation differences between; a) Arc; b) Electron beam welding.

a)

b)

Figure 2.38 Using gas-shielded arc welding; a) Difficulties of root access in a J-prep; b) Improved design using a bevel prep.

Choosing between a J or U-prep and a bevel or Vee-prep is also determined by the costs or producing the edge preparation. Machining a J or U preparation requires machining, which can be slow and expensive. Using this joint design also results in tighter tolerance which can be easier to setup. A bevel or Vee preparation can be flame or plasma cut fast and cheaply. This results in larger tolerances, meaning that set-up can be more difficult. Backing bar or backing strip is used to ensure consistent root fusion and avoid burn through. However, if you choose to use permanent backing strip (rather than a backing bar which is removed after welding), be aware that it gives a built-in crevice which can make the joints susceptible to corrosion (Figure 39). When using backing for aluminium welds, make sure any chemical cleaning reagents have been removed before assembling the joint. A backing strip will also give a lower fatigue life.

Figure 2.39 Using a backing strip for a butt weld.

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Outside of the design of the joint and weld, access to weld locations and the order in which welds are made are just as important. Figure 2.40 shows examples of the limitations of access in designing welded joints and gives improved designs. It is important to ensure that it is indeed possible to make welds as required by the drawing.

Figure 2.40 Examples of improved weld designs where there is limited access.

2.11

Welding standards AWS A2.4: ‘Standard symbols for welding, brazing and non-destructive examination’. This provides the standardised welding symbols on drawings. BS EN ISO 9692: Parts 1-4: ‘Welding and allied processes. Recommendations for joint preparation’. BS EN 14324: ‘Brazing. Guidance on the application of brazed joints’. BS EN ISO 13920: ‘Welding. General tolerances for welded constructions. Dimensions for lengths and angles, shape and position’. This gives accepted tolerances for welds. BS EN ISO 6947: ’Welds. Working positions. Definitions of angles of slope and rotation’. This provides the definitions of weld positions and provides the abbreviations used in the notes. ISO 2553: ‘Welded, brazed and soldered joints - Symbolic representation on drawings’.

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2.12

Summary At the end of this module you should be able to label the parts of a butt and fillet weld and to label the parts of a vee and U edge preparation. You should be able to recognise welding symbols and know what they mean.

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Revision questions 1

Draw and label the different features of a butt weld.

2

Draw and label the significant features of a single sided vee preparation butt joint.

3

Sketch the weld that would be fabricated from the weld symbols shown in this design drawing:

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Section 3 Forces and Strength of Materials

Rev 4 April 2013 Forces and Strength of Materials Copyright  TWI Ltd 2013

3

Forces and Strength of Materials

3.1

Introduction to structures This section of the course will describe common structures and consider the loads acting on structures. It will review the resulting forces and stresses and describe the materials properties that enable materials to withstand these forces. A structure is an object or part of an object which has to carry and resist loads. Loads can be due to the deadweight of the structure itself, or an external component. Loads or forces can arise through the reaction to acceleration, or environmental loads (such as winds or waves). Internal pressure or vacuum imposes loads, as does thermal expansion when a structure is heated and cooled. Industrial structural elements for carrying loads include cables, bars, beams, plates, slabs and shells. Some of these can be seen on the bridge structures in Figure 3.1.

a

b

c

d Figure 3.1 Bridge and crane structures showing cables, bars and beams as examples of load carrying components.

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Individual load carrying members are joined together to fabricate the entire structure, such as the complete bridge, crane, offshore structure, or building. A simple arrangement of structural components can form a frame, which is an assembly of bars arranged to support the loads. These are relatively easy to design and an example of a truss frame is shown in the bridge verticals in Figure 3.1a, or the crane arm in Figure 3.1c. Joining the components together is where the importance of welding comes in; although many structures are joined using rivets or bolts as well as or instead of welds (Figure 3.2).

Structural joining methods. Figure 3.2 Structural

3.2

Forces Now let’s consider the loads, or forces, that act on structural components in more detail. A force has a size (magnitude) and a direction. Two or more forces may be added together to give a single equivalent force, as shown in Figure 3. Instead of simply adding the magnitudes of the forces together, their directions must be taken into account as well. The forces are represented as arrows with a length equal to their magnitude and pointing in the direction of the force. The two (or more) force arrows are added point to tail and the single equivalent force is the arrow which points from the origin to the final arrow point. The combination of five different forces is shown in Figure 3.4.

Figure 3.3 Combination of two forces (F x and F y force, F. y ) into a single force,

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Figure 3.4 Combination of five forces (F 1 to F 5 ) into a single force, F R.

It is also possible for a single force to be represented by two forces acting at right angles, as shown in Figure 3.5. This is useful when an engineer needs to consider the forces acting parallel to and perpendicular to a weld (or other cross section) independently in a calculation.

Figure 3.5 Resolution of a single force, F, into two forces at right angles (F x and F y ).

Engineering structures have to resist loads due to a range of sources including self-weight, wind wave etc. These loads give rise to forces in the structure and Figure 3.6 illustrates some of the forces that may exist in a typical lattice frame that could represent, say, a railway bridge. As the structure may be subject to different loads, there may be different forces acting in different directions in any one member.

Figure 3.6 Typical forces in a lattice frame.

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Given that the structure does not itself move, there is no resultant force and all the loads acting on it must be in equilibrium, iethe sum of all the forces added together must equal zero. That means that any applied force is reacted by an internal reaction force inside the components. This is illustrated in Figure 7 by a free body diagram of the truss members of a bridge. All the loading on the bridge is carried as forces inside the truss members. The overall force on the bridge is reacted by the bearings at either end of the bridge too.

Figure 3.7 Forces in equilibrium in a truss member bridge.

When making calculations of the load-bearing capability of a structure, generally speaking, only one force is assessed at any one time. So for example, the process for determining whether a lattice bridge design is appropriate, such as that shown in Figure 3.8 is as follows: Step 1 - find out if frame can be statically calculated. If the design will be dominated by fatigue then an alternative design approach will be needed. Step 2 - find reactive forces in bearings, based on the loads the structure is designed to carry. Step 3 – calculate the loads in the individual members. Step 4 - calculate weld sizes for the connections, based on the forces they are required to carry (plus a safety factor)

Figure 3.8 The method to determine whether this bridge design is appropriate and the required welds.

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3.3

Materials under load To obtain information about the behaviour of a material under load, a simple tensile test is carried out. This involves taking a sample of the material and using a tensile test machine to steadily increase the applied load. The response of the material is determined by measuring the steadily increasing deflection as the load increases. This gives rise to a load-displacement curve, Figure 3.9.

Figure 3.9 Load-displacement curve.

However, the specific information from a load-displacement curve is very dependent on specimen size, as a thicker specimen would be capable of bearing larger loads (Figure 3.10). To produce information that is not geometry dependent and therefore represents materials property data, two new parameters are used; stress and strain.

Figure 3.10 Load-displacement curves for thick and thin specimens of the same material.

Stress (Figure 3.11) is defined as load (or force) divided by the cross sectional area,. If the force, F, is in newtons (N) and the cross sectional area is in millimetres squared (mm2), then the tensile stress, given the symbol , is in newtons per millimetre squared (N/mm2), which is also the same as megapascals (MPa).

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Figure 3.11 Definition of stress

The stress equation is often written as:

 

Stress can act either as a tensile stress (pulling something apart) or a compressive stress (squashing something together) and is calculated the same way for each, ie load over cross sectional area. Tensile stress is often considered to be worse, because it requires a tensile stress to propagate a crack.

Figure 3.11 Tension and compression.

Strain is defined as the change in length due to the application of a force divided by the original length, Figure 3.12. If the original length is called L, then the change in length is given as L. The symbol for strain is the Greek symbol epsilon, . Note that strain is dimensionless (has no units) and by convention is positive for tensile loads and negative for compressive loads when the length decreases.

Figure 3.12 Definition of strain.

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3.4

Stress-strain curves Converting the load and displacement data generated from a tensile test into a stress and strain data allows a stress-strain curve to be plotted which is a characteristic of the material and does not depend on the specimen size. The tensile stress-strain curve contains typical features which are specific to each material. A typical stress-strain curve is shown in Figure 3.13, which illustrates the important characteristics of tensile behaviour, which we will then look at in detail.

Figure 3.13 Characteristics of a typical stress strain curve.

As the stress is increased from zero, initially there is a linear relationship between stress and strain. Under these loads, if the stress is relaxed to zero then the strain also reduces to zero. This region is known as the elastic region and the linear relationship between stress and strain is known as Hooke’s Law. The ratio of stress to strain is constant in this region and is known as Young’s modulus, E, which is given in units of N/mm 2 or GPa. Young’s modulus gives a measure of the stiffness of the material. Stress Strain

 Young’s modulus E

As the stress is increased further, a deviation from linear behaviour occurs at the yield point. The definition of the yeld strength is the point at which plastic deformation occurs without any increase in the force ie at the yield plateau. At this point if the material is unloaded down to zero stress, a small permanent strain offset remains. This permanent deflection is known as plastic deformation and the region of the stress-strain above the yield point is known as the plastic region. The yield strength is the stress corresponding to the yield point.

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Where there is no obvious yield point, such as a yield plateau and the stress strain curve rises smoothly into the plastic region, it is necessary to define an arbitrary yield point. In such cases, the 0.2% proof strength (Rp0.2) is used as a design parameter. Rp0.2 describes the stress obtained for an elongation of 0.2% and is determined by plotting a line parallel to the elastic part of the stress-strain curve, at an offset of 0.2% along the strain axis. Where this line intersects the stress-strain curve is the 0.2% proof strength (Figure 3.14).

Figure 3.14 Definition of the 0.2% proof strength for stress-strain curves without an obvious yield point.

With increasing applied stress, the stress-strain curve reaches a maximum at the ultimate tensile strength. The UTS is the maximum load that can be tolerated by the specimen and is defined as the stress corresponding to the maximum force. After reaching the UTS the stress-strain curve declines and necking occurs, where the sample becomes thinner and develops a neck. As a result, the load drops due to the lack of resistance from the material (Figure 3.15).

Figure 3.15 Definition of the ultimate tensile strength, followed by necking.

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The design assumption of load bearing assumes that the cross-section area remains the same and this is how the Engineering stress-strain curve is produced, as shown in the stress-strain curves above. BUT necking of the material reduces the REAL cross-section area. In reality, the stress does not decrease with increasing applied loading but flattens out around the maximum stress while the cross sectional area decreases. Allowing for this reduction in cross sectional area gives the real or true stress-strain curve. After the UTS and necking, fracture occurs at the fracture stress. The strain at fracture is usually defined as a percentage elongation. In some materials fracture occurs before the stress-strain curve reaches a maximum. The ability of a material to deform plastically before fracture is known as ductility. Some examples of stress-strain curves for real materials are given in Figure 3.16.

A

b

Figure 3.16 Examples of stress strain curves in a) for 1-low carbon steel; 2medium carbon steel; 3-high carbon steel; 4-bronze; and in b) for aluminium and Duralmin.

3.5

Tensile tests In a tensile test, a sample is clamped between two jaws and pulled apart. The load and extension are measured within a narrower section parallel sided gauge length within the specimen.

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Figure 3.17 Tensile test specimen.

As the test progresses and necking and final failure occur, measurements of the original and final gauge length are taken and of the original and final diameters at the neck location. The reduction of area and the elongation are reported as percentages. The yield strength (or 0.2% proof strength) is reported along with the value of UTS. Often the data points from logging the whole stress strain curve are recorded, so the stress-strain curve can be plotted.

Figure 3.18 Tensile test experimental procedure.

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3.6

Hardness tests Hardness is the resistance of a material against penetration. There is a direct correlation between UTS and hardness, so hardness measurements are sometimes used to approximate the tensile properties. Hardness is measured by indentation under a constant load, often using a pyramid indenter in the Vickers hardness testing method, or a ball indenter in the Brinell hardness test method.

Figure 3.19 Hardness testing.

3.7

Different types of forces Four different types of force are considered; compression, tension, shear and bending as shown in Figure 3.20.

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Figure 3.20 Types of loading.

Tension and compression act perpendicular to the cross sectional area and give rise to direct axial stress, as has been discussed in Section 3.3. When the load is applied parallel but offset to the cross sectional area then a shear stress results.

a

b.

Figure 3.21 Axial stress a and shear stress b.

Shear stresses are particularly significant for calculating stresses in fillet and lap welds. The shear stress is given the Greek symbol tau,  and is also calculated as the shear force, Q, over the cross sectional area, but in this case it is the force that acts parallel to the cross sectional area, A. Likewise the shear strain, , is also the change in dimensions over the original dimensions, but it is the shear strain, , (acting parallel to the applied, Q) over the offset between the two opposite shear forces, h. These dimensions are shown in Figure 3.22.

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Figure 3.22 Dimensions for the definition of shear stress and strain.

Bending a beam, such as shown in Figure 3.23, imposes tension (tensile stress) in the outer surface and compression (compressive stress) on the inner surface. There is a line at which there is no net stress on the beam, which is called the neutral axis, shown as a dotted line in Figure 3.23.

Figure 3.23 Stresses in a beam under bending.

When bending is imposed on a beam it is called a bending moment. The bending moment, M, is the applied force multiplied by the perpendicular distance that force is applied at. It is easiest to imagine a cantilever (a horizontal beam fixed to a wall at one end), with a load on the free end (Figure 3.24). The applied force, F, results in a bending moment, M, equal to F x d where d is the length of the beam. The bending moment causes an axial reaction force inside the beam, Fx. The applied force, F, is also reacted by a shear force, Fy, acting at the fixed end.

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Figure 3.24 Stresses in a cantilever beam under a bending moment, M.

The amount of axial stress in the beam caused by the applied moment depends on how stiff the design of the beam is. This stiffness is characterised by the beam’s second moment of area (also called its moment of inertia) and given the symbol, I. A stiff beam with a large second moment of area tends to be tall and thin, with most of the beam’s mass at a vertical distance from the neutral axis (such as a tube section or an I-beam). Beams with low stiffness (low second moment of area) are wide flat beams. You can demonstrate this by bending your ruler while it’s flat and then turning it on its edge and trying to bend it that way – it’s much harder! A range of beam cross sections are shown in Figure 3.25 in order of their stiffness for the same cross sectional area.

Figure 3.25 Beam cross sections ranked in order of stiffness by their 2 nd moment of area, for an equivalent cross section area.

The engineer’s bending formula is used to calculate the maximum bending stress, , in a beam. It is equal to the applied bending moment, M, multiplied by the vertical distance from the neutral axis, y, divided by the second moment of area, I.  = My / I

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This means that for the same applied bending moment (ie the same length of cantilever beam with the same load on the end), a beam with a larger 2 nd moment of area will result in a much lower maximum stress at the tension surface. This is why structures are designed with square or round hollow section beams and I-beams, because they allow greater loads to be carried without redundant extra weight being required. 3.8

Worked example Let’s use the example of the cantilever beam to calculate the bending stress as a result of an applied load on the free end of the beam. No calculations of this kind will be on the Specialist exam, but by using an example with actual numbers it can help to show what the effect of the bending moment and second moment of area mean for a beam under load. Start by assuming the beam is square section (10mm by 10mm) and then we’ll calculate the bending stress if we use a rectangular section beam of the same cross sectional area, but 5mm by 20mm. Assume the beam is 300mm long with a load of 200N on its free end. First, calculate the bending moment, M.

Bending moment is force x distance: M = 200N x 300mm = 60,000Nmm. Let’s calculate the second moment of area of the beam. For square and rectangular beams the formula for a beam with a breadth, b and depth, d is I = bd3/12

This means that the second moment of area for the square section beam is 833mm4, whereas the rectangular beam has a second moment of area of 3333mm4. The rectangular beam is four times stiffer than the square beam. The distance from the neutral axis, y, is basically half the depth of each beam, ie 5mm for the square beam and 10mm for the rectangular beam.

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Rev 4 April 2013 Forces and Strength of Materials Copyright  TWI Ltd 2013

The engineer’s bending formula,  = MY/I can now be used to work out the bending stress in each beam. The square section beam has a stress equal to 60,000 x 5 / 833 which equals 360MPa. The rectangular section beam has stress equal to 60,000 x 10 / 3333 which equals 180MPa. Therefore by changing the square section beam to a rectangular beam the stress in the beam is halved! 3.9

Summary At the end of this module you should understand how structures carry loads and forces and that reaction forces are set up to give equilibrium conditions. You should understand how to calculate the stress that a force will impose on a structure and how to draw and interpret a stress-strain curve for a given material. You ought to know how to calculate a bending moment and to recognise the Engineer’s bending formula.

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Rev 4 April 2013 Forces and Strength of Materials Copyright  TWI Ltd 2013


Describe how to add two forces together.

2

When a structure is in equilibrium, what is the resultant force on the structure?

3

What is the formula to calculate axial stress?

4

What is the formula to calculate axial strain?

5

Draw and label a typical stress-strain curve.

6 How can you define the yield point where there is no yield plateau?

7

What is the Engineer’s Bending Formula?

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Section 4 Fatigue

Rev 4 April 2013 Fatigue Copyright  TWI Ltd 2013

4

Fatigue

4.1

Introduction Fatigue loading is the repeated application of a load. A simplified fatigue loading cycle is shown in Figure 4.1.

Figure 4.1 Simple fatigue loading.

Think of four types of structure that have to withstand fatigue loading. Identify the sources of fatigue loading on those structures. Which of your structures are of welded construction? Typical structures that have to withstand fatigue loading include ships, bridges, offshore platforms and rigs, earthmoving and off-highway vehicles, towers, axles etc. The sources of fatigue loading include fluctuating loads from a variety of sources. Acceleration forces in moving structures, pressure changes, temperature fluctuations, environmental loads (wind, current, wave etc), rotation and mechanical vibrations from machinery or shaft etc can all cause fatigue.

Figure 4.2 Earth moving equipment can suffer from fatigue.

Fatigue failures have been occurring for many years; a train returning to Paris from Versailles crashed in May 1842 at Meudon after the leading locomotive broke an axle (Figure 3). The carriages behind piled into the wrecked engines and caught fire, killing at least 55 passengers. The accident was widely reported in Britain and discussed extensively by

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engineers, who sought an explanation. An investigation suggested a crack growth mechanism through repeated stressing, but this was mainly ignored, meaning that fatigue failures kept occurring on the railways. It is only since the second world war that the causes of fatigue failures have been studied and understood scientifically.

Figure 4.3 Axle failure from 1843.

Fatigue failure occurs by the initiation and propagation of a crack, which progresses slowly and steadily across the load bearing area until final fracture occurs. This can occur even when the stress remains entirely in the elastic regime, ie well below the yield stress. In engineering applications, the fatigue crack grows at right angles to the applied stress direction. The fracture surface is relatively flat and macroscopically featureless. However, some fatigue fracture surfaces exhibit beach marks, Figure 4.4, which usually correspond to the position of the crack front when say a change of loading or environment occurred.

Figure 4.4 Typical fatigue fracture surface, showing bench marks.

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4.2

Characterisation of fatigue loading The typical characteristics of simple fatigue cyclic loading are shown in Figure 4.5.

Figure 4.5 Typical characteristics of fatigue loading.

If the minimum stress is zero then the fatigue cycle is known as a pulsating cycle. If the maximum stress is equal and opposite to the minimum stress then the fatigue spectrum is known as alternating cycles. If the minimum stress is half the maximum stress then the cycling is known as half tensile cycles.

a

b

c Figure 6 Definitions of different fatigue cycles, a pulsating cycle, b alternating cycle, c half tensile cycle.

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The most important parameters are the stress range, Sr (the difference between the maximum and minimum stress) and the stress cycle, ie the interval between equivalent points in the stress history. Other fatigue parameters include the stress ratio, R (the minimum stress divided by the maximum stress) and the stress amplitude, which is half the stress range.

4.3

S-N curve Extensive fatigue tests on simple specimens showed that for high stress ranges the fatigue life was short; as the stress range was decreased the fatigue life of the specimen increased. A graph of stress range against number of cycles to failure is a very convenient method of presenting fatigue behaviour. When a line is drawn through individual test data points, this f orm of graph is known as an S-N curve.

Figure 4.7 Graph of stress range against number of cycles to failure - the S-N curve.

Increasing the stress range increases the fatigue damage. Increasing the number of cycles also increases (Figure 8).

Figure 4.8 Effect of increasing the stress range or number of cycles on the fatigue damage.

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It is more common to see Typical SN curves are plotted on a logarithmic scale, which produces a straight line in the high cycle regime, at greater than 104 cycles to failure. Low cycle fatigue occurs at very high stress ranges which result in fewer than 104 cycles to failure. At sufficiently low stress ranges, fatigue cracks may not propagate at all and this is known as the fatigue endurance limit.

Figure 4.9 S-N curve plotted on logarithmic scale.

When discussing fatigue in structures, it is common to use a range of terms, the definitions of which need to be understood. The fatigue stress history is the variation of stress at a point with time. Constant amplitude stress history is a stress history in which successive stress fluctuations are equal. The fatigue life is the number of stress cycles sustained before failure, while fatigue strength means the stress range which causes failure at a certain specified life.

4.4

Fatigue of welded joints Fatigue is a particular concern in welded joints because nearly all welds contain inherent stress concentrations. The effect of a stress concentration can be imagined using stress contour lines and when stress is applied to a component the stress distribution inside the component is similar to the contour lines. In plain material under stress the contour lines would run through the material parallel with the principal direction of the stress. The introduction of a notch creates a concentration of the lines, as the stress cannot be carried across the notch; it has to go around the notch. The concentration of the lines indicates a concentration of the stress.

Figure 4.10 Stress concentration effect of a notch.

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Figure 4.11 Effect of a notch on the fatigue S-N curve.

The effect of a notch or stress concentration on the fatigue resistance of a structure is to lower the S-N curve, so that for the same stress range it takes fewer cycles until failure, or for an equivalent fatigue life, the stress range will be less (Figure 4.11). This is because fatigue cracks are most likely to initiate and propagate from high stress concentration areas. Stress concentrations can also occur at changes of section, such as at welds (Figure 4.12).

Figure 4.12 Stress concentration areas in structures.

Welding almost inevitably introduces stress concentrations at locations such as the weld toe or weld root. These provide sites for relatively easy fatigue crack initiation, Figure 4.13. A key feature of weld toes is the inevitable presence of sharp discontinuities. Undercut or cold laps are examples, but more important, on a much smaller scale, are small non-metallic intrusions (typically about 0.1-0.4mm in depth). The fatigue crack in the photograph has propagated from such a flaw, which extended as far as ‘A’. These nonmetallic intrusions are produced at the weld toes by arc welding processes. They are typically 0.1-0.4mm in depth and given that fatigue life is governed by the growth from this pre-existing flaw, there is usually little or no initiation stage for fatigue in welded structures.

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Factors which affect crack initiation (the formation of a microscopically sharp starter crack) can be quite different to those that affect crack growth (stress range, environmental conditions). Another consequence for welds is that design features like the weld toe can be far more severe as sources of stress concentration than welding flaws. This emphasises the need for rational criteria for assessing the significance of flaws.

Figure 4.13 Stress raisers at weld toes provide easy fatigue crack initiation sites.

Figure 4.14 High magnification image of a weld toe intrusion, which extends as far as ‘A’, initiating the rest of the fatigue cracking from that location.

The effect of these fatigue initiation sites on the S-N curve is shown in Figure 4.15, which shows fatigue data for one specific steel in three conditions - unwelded, unwelded but with a stress raiser (a hole) and welded with two plates attached to the surface. It is clear that the fatigue performance of the welded material is very much inferior to that of the unwelded material.

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Figure 4.15 Fatigue data for one type of steel in the unwelded condition, unwelded but with a stress raiser and the welded conditions.

One of the most serious consequences of the fact that the fatigue lives of welded joints are dominated by crack growth concerns the influence of material strength. Although the fatigue strength of un-notched material usually increases with tensile strength, the level of increase decreases if the material contains a notch until there is no increase at all for welded material. This is because rate of fatigue crack growth is not dependent on material strength and hence welded low and high strength materials give the same fatigue life. The benefit of material strength comes in the crack initiation stage, which is effectively absent in the welded material. Fatigue data from unwelded and welded steels of different tensile strengths are shown in Figure 4.16.

Figure 4.16 The effect of material strength on fatigue strength.

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A large number of fatigue tests have been carried out on many different joint geometries, Fatigue tests can be carried out on full-scale structures (Figure 4.17a), or on smaller-scale specimens. A common specimen is a flat strip with fillet welded attachments on either side (Figure 4.17b). A series of these specimens have been tested at a variety of stress ranges and the fatigue lives plotted on an S-N curve, shown in Figure 4.18. As is often the case with fatigue data, these results exhibit some scatter. For design purposes, the lower limit S-N curve is used.

A

b

Figure 4.17 Fatigue testing, a) full-scale beam and b) fatigue test specimen after test.

Figure 4.18 Fatigue test results from one specimen geometry.

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When the appropriate design curves obtained from fatigue tests on different geometries are compared, it is clear that fatigue performance is strongly dependent on joint geometry, shown in Figure 4.19. Fillet welds have a shorter fatigue life than butt welds under equivalent stress cycles. Welded joints that exhibit similar fatigue strengths can then be grouped into classes and this approach is used in fatigue design rules. Welds in the same fatigue class have similar stress concentration effects. The fatigue joint classifications range from A (plain material with the best fatigue resistance and longest fatigue life) down the alphabet as the fatigue resistance decreases to F, F2, G and then W, these latter few being used only for special types of weld joints.

Figure 4.19 Design S-N curves for different joint geometries.

4.5

Residual stress A further important factor in the fatigue performance of welds is the effect of the tensile residual stresses that are present in the region where the crack initiates as a result of contraction on cooling after welding. These high tensile residual stresses mean that even when subject to compressive remote stresses, the stresses near the weld remain tensile. Hence, the stress range experienced by the weld region always is at a high tensile mean stress, sometimes expressed as hanging down from yield, even for partly compressive stress cycles, Figure 4.20.

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Residual stress (tensile yield)

Stress R=0 Effective stress range

R = -1

Time 0

R = 0 (compression)

Figure 4.20 Effective stress range in the presence of high tensile residual stresses.

It is the stress range, therefore, that determines the fatigue strength of an as-welded joint, even if the applied cycle is partly compressive and fatigue cracks can propagate under these conditions in welded structures, even though compressive cyclic loading will not propagate fatigue in parent metal.

4.6

Fatigue improvement techniques It is possible to reduce the effect of the stress concentration at the weld toe of fillet welds and butt welds and improve the fatigue life of welded structures using techniques such as grinding the weld toe to remove the intrusion and to blend the toe profile and reduce the stress concentration. A low heat input autogenous TIG pass along the weld toes can re-melt and remove the toe intrusions (known as TIG dressing or TIG washing). Peening techniques such as hammer or needle peening can put the weld surface at the weld toe into compression and slow down the fatigue crack propagation. Flush grinding butt welds will also improve the fatigue performance.

Figure 4.21 Fatigue improvement technique, showing grinding of the weld toes.

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4.7

Summary At the end of this module you should understand an S-N diagram and describe the influence of notches and weld defects of fatigue performance. You need to recognise which welded joints are most susceptible to fatigue. You should also be able to describe modifications for fatigue improvement.

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What types of structures and applications are at most risk of fatigue cracking?

2

Sketch an alternating fatigue cycle and label the fatigue parameter on the diagram.

3

Why are welds more susceptible to fatigue than parent materials?

4

What effect does increasing the strength of the steel have on its fatigue performance?

5 List four fatigue improvement techniques.

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Section 5 Design of Pressure Vessels

Rev 4 April 2013 Design of Pressure Vessels Copyright  TWI Ltd 2013

5

Design of Pressure Vessels

5.1

Types of pressure vessels A pressure vessel is a storage container for liquids or gases under pressure. They can be internally pressurised, which is the most common type of vessel, or externally pressurised (ie containing a vacuum). Pressure vessels are widely used in industries such as oil and gas, chemical refineries, etc. Fired pressure vessels are heated using gas or oil burners, for example boilers. Unfired pressure vessels are all the other types of pressure vessel.

Figure 5.1 Pressure vessels in a chemical refinery.

5.2

Construction of pressure vessels The main standards for construction of pressure vessels are BS PD 5500 in the UK, AD MerkBlatt in Germany and BS EN 13445 in Europe. In other parts of the world, the American ASME Code is most widely used. A range of materials are available for construction of pressure vessels depending on the service pressure and temperature and the fluid being contained. The base requirements for a material to be used in a pressure vessel are good mechanical properties and adequate corrosion resistance for the purpose. The majority of vessels are made from steel, some from stainless steel or aluminium and some are even made from composite materials, such as wound carbon fibre held in place with a polymer. Pressure vessels can be lined with a variety of materials such as polymers, ceramics and other metals to improve corrosion resistance and carry a portion of the applied load. The relevant standard will contain a list of all the approved materials which can be used.

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A simple pressure vessel comprises the following: Shell Main body of the vessel, most often cylindrical but some pressure vessels use conical or spherical shells. Head At each end to complete the basic shape and produce a closed container, are most often dished but can be flat. Nozzles A number of openings for filling, inspection or drainage. Saddle supports Hold the pressure vessel in place. Nameplate Indicates the main working parameters of the pressure vessel including work pressure and temperature. Details may include the manufacturing company, year of manufacture, the code to which the pressure vessel has been designed and manufactured and the inspection body stamp. 5.2.1

Pressure vessel shell Cylindrical shells are usually made of a number of curved plates welded together as in Figure 5.2.

Offset

Figure 5.2 Cylindrical shell.

The shell of a pressure vessel can range in thickness from a few millimetres for a BBQ propane gas bottle to several hundred millimetres for industrial pressure vessels. The minimum design thickness is dependant on the shape of vessel, internal pressure, diameter of the vessel and material strength. A spherical shell requires a much smaller wall thickness than a cylindrical shell for the same diameter, internal pressure and construction material.

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Rev 4 April 2013 Design of Pressure Vessels Copyright  TWI Ltd 2013

5.2.2

Dished heads Dished heads are produced from a blank (usually circular) by pressing or forming it in a die using a former. Sometimes, the blank is small enough to be cut out from a single plate but in many situations the blank is so large that it cannot be obtained from a single plate and must be fabricated by welding together a central round piece called the crown with a number of petals as shown in Figure 5.3.

A

b

c

Figure 5.3 Fabricated dished head: a) Hemispherical shape; b) Ellipsoidal shape; c) Torispherical shape.

These welds are full penetration welds to produce a continuous envelope, without voids, which might affect the strength of the dished head. To fulfil this requirement, it is common practice to subject all these welds to 100% UT examination. A dished head is fabricated out of an odd number of petals to avoid the propagation of a longitudinal defect from one weld across the entire dished head and can be fabricated to different shape designs as described below. Hemispherical dished head is half of a sphere. Taking into consideration the stress that occurs in a sphere and the maximum stress that occurs in a cylinder (ie the hoop stress), a hemispherical dished head would require the smallest thickness among all types of dished heads, but is the most difficult to fabricate.

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Ellipsoidal dished head the longitudinal section of this type of dished head (ie a section along its longitudinal axis) is half an ellipse. Since the stress developed in the dished head in this case is equal to the hoop stress, the required thickness for an ellipsoidal dished head is equal to that of the cylindrical shell. Advantages are there is no need to supply different plate thickness to manufacture a pressure vessel with ellipsoidal dished heads nor for a transition between two different thicknesses, making manufacture easier. An ellipse is difficult to generate and compressive stresses occur in the ellipsoidal dished head. Under internal pressure, the shell tends to expand whilst the ellipsoidal dished head tries to contract diametrically at the headshell junction and there is also a tendency towards local buckling. Torispherical dished head a torisphere consists of a spherical central portion or crown with radius R and a toroidal knuckle of radius r, where R/r is approximately twelve and R is about 95% of the cylinder diameter as shown in Figure 5.4.

Knuckle radius, r Crown radius, R

Figure 5.4 Torispherical dished head.

The junction of the torus with a cylinder gives rise to bending stresses as the greater the deviation from a sphere, the higher these stresses would be. Torispherical dished heads are often preferred to ellipsoidal ones since the depth of drawing is less so they are cheaper to manufacture. The small axial dimension is an advantage when the longitudinal size of the pressure vessel is a critical factor, but their higher stress concentration and lower allowable pressure for a given material size may outweigh this as a result, the thickness required for a torispherical dished head is larger than that for a cylindrical shell.

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5.3

Internal pressure stresses

5.3.1

Internal pressure Consider a cylindrical closed end pressure vessel, with radius r and thickness t subjected to internal pressure p as shown in Figure 5.5.

Figure 5.5 Closed end cylinder subjected to internal pressure.

The force which acts on each closed end of the cylinder due to the internal pressure is dependent on the cross section area on which this pressure is acting. Cylinders with larger ends but the same internal pressure have a smaller force pushing against them. 5.3.2

Axial stress The internal pressure acting on the ends os the pressure vessel generates axial forces in the shell wall. In Figure 5.6, the axial stress, σx, is developed in the shell, as a result of internal pressure, p.

Figure 5.6 Axial stress in the shell due to internal pressure.

5.3.3

Hoop stress If the cylinder is cut across the diameter as in Figure 5.7, the internal pressure acting radially creates an outward circumferential stress known as the hoop stress. In Figure 5.8, p is the internal pressure acting on the shell, L is the length of the section and y is the hoop stress. L 5-5

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Figure 5.7 Hoop stress in the shell due to internal pressure.

5.4

Calculation of stresses Assuming that the wall thickness is small compared with the radius of the shell (thin-wall assumption), the stresses in the shell are calculated by the following formulae: For a cylindrical pressure vessel, the axial stress x is given by: x 

p r 2t

The hoop stress y 

y is

twice the axial stress and is calculated by:

p r t

Since the hoop stress is twice the value of the axial stress, failure of a cylindrical pressure vessel will preferentially occur along the longitudinal welds, also true for other similar pressure components such as pipelines. Consequently longitudinal welds are subjected to more stringent acceptance standards than the circular girth welds (welds around the circumference of the vessel or pipe). For a spherical pressure vessel the stress in the vessel wall is symmetric about all planes therefore there is only one membrane stress calculated: 

p r 2t

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5.5

Welding pressure vessels

5.5.1

Longitudinal welds Since the hoop stress is twice the axial stress, problems are more likely to occur along the longitudinal welds, which experience the hoop stress. To avoid the propagation of such a defect from one course to the next, it is common practice to offset longitudinal welds as shown in Figure 5.8.

Offset

Figure 5.8 Offset longitudinal welds.

In the ASME Boiler & Vessel Code, Section VIII, Division 1, vessels made of two or more courses shall have the centres of the welded longitudinal joints of adjacent courses staggered or separated by a distance of at least five times the thickness of the thicker plate. In PD 5500, the longitudinal seams of adjacent courses shall be staggered by four times the thickness of the plate or 100mm, whichever is the greater, measured from the toe of the welds. 5.5.2

Circumferential welds Circumferential welds are used to join the heads to the shell. Where the shell and head thicknesses differ, taper transitions are used as shown in Figure 9. The gradient is often specified as 1 in 4 as a minimum to avoid making the joint a stress concentrator.

Figure 5.9 Taper transition.

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Various weld joint designs exist to join the heads to the shell and the choice of joint will depend on the welding process to be used, access conditions and the material to be welded. Some examples are shown below.

a

b

C

d

Figure 5.10 Head-shell weld profiles.

The weld preparations shown in Figures 5.10a and b are the simplest joint designs and assume that access for welding can be made from the inside of the vessel and so would not be used for small pressure vessels. Figure 5.10c shows a self-jigging joint design with an integrated backing strip which can be welded entirely from the outside. This joint would be appropriate for thick section material, whereas Figure 5.10d shows a self-jigging joint that could be formed in thinner material.

Figure 5.11 Welds between shell and dished head.

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5.6

Welded attachments

5.6.1

Nozzles

Nozzles connect the pressure vessel to other components such as pipework. Depending on the vessel and application a vessel can have a large number of nozzles of varying size and design. The type of nozzle used can depend on: 

diameter/thickness ratio of the shell,



diameter/thickness ratio of the nozzle,



access (one or both sides),



type of joint required (partial/full penetration),



groove preparation methods available.

A

b

Figure 5.12 a Set-on nozzle;b Set-through nozzle.

5.6.2

Flange connections

To connect other plant to the pressure vessel via the nozzles and to join long runs of pipe together, separable joints are used to allow easy installation and repair of the vessel and its components. The simplest joint used is a screwed connection; but can only be used at relatively low pressures. The most common method is to weld a flange to both the nozzle and the end of the pipe to be connected and the flanges are then connected together using bolts as shown in Figure 5.13.

Figure 5.13 A flange connection.

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A gasket made of a relatively soft material is normally present between the two flanges to produce a tight seal. The higher the fluid pressure and temperature, the more robust the gasket and bolts need to be. Flanges are available in a variety of designs to suit every application. As with the nozzle to shell weld, the flange to nozzle weld has various different profiles to suit the type of flange and pipe used. 5.6.3

Reinforcement A hole or opening in the shell of a pressure vessel for nozzles can have a detrimental effect on the structural integrity of the shell. The hole can act like a local stress concentration. The shell thickness is a function of the operating stresses within the shell, if the shell experiences greater stress in a region due to stress concentration the shell thickness needs to be greater in this region to withstand the stresses. Welded plates of additional thickness around openings in the pressure vessel are called reinforcement plates or compensation plates. Reinforcement can be used on the shell, the nozzle or both. Figure 14 shows two types of reinforcement. Holes are made in the compensating plates to allow the weld to be tested.

Figure 5.14 Methods of reinforcement.

5.7

High and low temperature service Pressure vessels for high or low temperature service have special requirements. Often they will be insulated to maintain the internal temperature. Some are designed with a double-wall, such that the outer envelope containing a pressurised inner container. Examples of doublewalled pressure vessels include: 

Autoclaves Sterilise medical equipment by heating them to a high temperature inside a sealed container. An autoclave is a pressurised device designed to heat aqueous solutions above their boiling point without evaporation. Heating is by feeding hot steam into the outer envelope. During sterilisation, medical equipment must be protected from contamination by being in a hermetic container, ie the inner container.

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5.8

5.9



Dewar vessels For storing low temperature fluids like liquefied gases. To avoid heat transfer from the outside to the contents in the inner vessel, the jacket space between the two walls is evacuated.



Vertical storage tanks Used mainly for storing petroleum or chemical products. Although the inner tank may not be highly pressurised, if at all, the outer wall is vital to keep the tank contents cold and the inter-space is sometimes filled with thermal insulation material.

Standards and specifications 1

ASME 2007: ‘Boiler & Pressure Vessel Code’, Section VIII.

2

BS PD 5500 2009: ‘Specification for unfired fusion welded pressure vessels’, London: British Standards Institution.

3

BS EN 13445 2009: ‘Specification for unfired fusion welded pressure vessels’, London: British Standards Institution.

Summary At the end of this section you should understand the weld design details for pressure vessel construction, of the vessel shell and head, as well as attachments. You should also be able to outline how to calculate hoop stress and axial stress in a pressure vessel shell.

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Revision questions 1 Sketch a pressure vessel and label the shell, head and a nozzle connection. Show the weld joints on your sketch.

2 When welding a thicker plate dished head onto a thinner plate shell wall, what gradient of taper should be used?

3

What is the formula for hoop stress in a cylindrical pressure vessel?

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Section 6 Stresses in Welds

Rev 4 April 2013 Stresses in Welds Copyright  TWI Ltd 2013

6

Stresses in Welds

6.1

Making things Simple In the majority of situations it is very difficult, if not impossible to take into consideration all the factors that might influence the load carrying capacity of a welded joint. This is why designers use assumptions to simplify the approach and reach a practical result without over-complicated calculations.

6.1.1

Welds and HAZs It is assumed that the weld and HAZ have a higher strength than the parent material based on the use of an overmatching filler material (ie a material with a higher strength). Since the parent material is the weakest link, the weld joint should always fail in the parent material instead of the weld; this assumption is checked during the qualification of the welding procedure, when the cross-weld tensile test specimens ‘shall have a strength not less than the corresponding specified minimum value for the parent material’ (see BS EN ISO 15614-1, paragraph 7.4.2). However, there are a few situations when this assumption is not true. Aluminium alloys suffer strength loss in the HAZ. For TMCP steels or HSLA steels, cold work processing and/or microalloying is used to increase the strength of the parent metal. The heat from welding causes the HAZ to recrystallise and becomes softer than the parent material; hence this part of the joint would have the lowest strength. For undermatched welds, the implication would be that the overall strength of the joint is dictated by the strength of the HAZ (which can be assumed to be equal to the parent metal in the annealed condition).

6.1.2

Excess weld metal Excess weld metal does not bear any stress and is neglected when accounting for joint strength in weld design. Since deposition of the weld metal cannot be controlled accurately, the size of the excess weld metal is not constant over the entire length of the weld. Although ignoring it is a conservative assumption, for design calculation, the design throat is used instead of the actual throat.

a

b

Figure 6.1 Excess weld metal in a butt weld a and fillet weld b.

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a

b

Figure 6.2 The design throat in a butt weld a and fillet weld b.

6.1.3

Weld quality It is assumed that the welds are defect-free at the design stage. Joint factors are sometimes used to reduce the design stress to reflect a reduced level of non-destructive testing (NDT) and the risk of weld flaws being present. The presence of defects reduces the cross-section area through which load is carried.

6.1.4

Stress concentrations Stress concentrations due to bead shape are neglected. Ripples on the surface of the weld and weld toes are points of abrupt change in weld profile so concentrate the stress. Although the effect of these features is quite significant, especially in fatigue resistance, this effect cannot be exactly quantified and hence is neglected when designing for static joint strength.

6.1.5

Partial penetration welds In partial penetration welds, the throat is reduced by the amount of lost penetration. This assumption is essential since the load is transmitted only through the welded section (since the non-penetrated section lacks material continuity, it cannot transmit any load).

6.1.6

Residual stresses Residual stresses due to welding are ignored at the design stage. Although they can reach the yield of the material, their exact magnitude and distribution are hard to determine (they are influenced by the heat input which varies a lot especially during manual and semi-automatic welding). As a result of this uncertainty, residual stresses are not taken into consideration for design calculations.

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Rev 4 April 2013 Stresses in Welds Copyright  TWI Ltd 2013

6.2

Different types of stresses in welds

6.2.1

Stress units Welded structures are designed to carry loads so the welds themselves contribute to the support of these loads. Materials supporting a load will be subject to stress. Common units of stress measurement are Pascals, Pa, or Newtons per metre squared, N/m2. The Pascal is a very small unit so we normally use MegaPascals (MPa) or Newtons per millimetre squared (N/mm2). 1MPa = 1N/mm2

6.2.2

Nominal stress The concept of stress introduced in Section 3, was defined as the force/cross-sectional area. This is called the nominal stress (see Figure 6.3). This is the average stress over the area: Stress (MPa or N/mm2) =

Load or force (N) Cross section area (mm2)

Figure 6.3 Nominal stress.

6.2.3

Design stress or Maximum allowable stress Different application standards offer values for the maximum allowable stress depending on the specific requirements and service condition (for instance, BS 5400-2 Steel, concrete and composite bridges – Part 2: Specification for loads or the ASME Pressure Vessel codes). If you are not using a code, the maximum allowable stress can be approximated as two thirds of the parent metal yield strength. In the USA (and historically in the UK), design stress was based on UTS reduced by a factor of safety (typically 4). The designer must make calculations to ensure that the stress in the welds does not exceed the maximum allowable stress, which is also known as the design stress. Joint factors are sometimes used to reduce this initial design stress to r eflect a reduced level of NDT and the risk of weld flaws being present. For example. if only 10% of welds will be inspected, the value of 2/3 yield strength is multiplied by a joint factor of 0.8, thus reducing the value of the design stress.

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6.2.4

Hot spot and notch stress In reality the stress distribution over the material cross-section is not always uniformly distributed. Near geometrical features and stress concentrators the distribution will contain a non-linear component. This is the case for fillet welds. Traditionally, design calculations and experimental test data are generated using the nominal stress approach. However, alternative design methods estimate the strength of connections using structural stresses which are a summation of the membrane and bending stresses that occur across welds, based on a linear stress distribution and ignore local non-linear stress profiles. The concept of hot spot stress was developed in the 1970s for determining the strength of tubular joints in offshore platforms. The approach is now used to assess the resistance of fillet welds and its concept extrapolates the structural stress profile measured close to the weld toe. The hot spot stress can be experimentally determined using strain gauges positioned at very specific locations or using finite element calculations. A particular advantage of using the hot spot stress approach is that it is far easier to extract the hot spot stresses from finite element analysis, instead of trying to determine the equivalent nominal stress. When much of the current design of welded joints is done using finite element modelling, this makes it easier to extract the stresses to compare to the design stress limit. Stress Notch stress Hot spot stress Structural stress Nominal stress

Figure 6.4 Stress terminology close to the weld toe of a fillet weld.

The notch stress design approach uses the peak stress by capturing the increase in stress intensity due to the presence of the weld. It takes into account the radius of the weld toe and geometry of the joint. This calculation requires more parameters than the hot spot stress approach so is more complex.

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6.3

Butt welds

6.3.1

Stress in a full penetration butt weld In carbon and carbon-manganese steels the weld metal is at least as strong as the parent material so for most purposes the butt weld can be neglected when assessing static strength. In this case, the weld is between two identical parent materials using matching filler. The weld is full penetration and to avoid any start/stops along the weld which might impair its quality, run off tabs were used.

Figure 6.5 Full penetration butt weld under uniaxial tension load.

Remembering the simplifying assumptions from Section 6.1, the design throat would be equal to the thickness of the parent plate t. Because the load, P, is acting perpendicularly to the weld, the joint is subjected to tension. The stressed area of the weld cross-section area (CSA) is directly calculated by: Cross section area, CSA = length, L × thickness, t The cross section area is in millimetre squared (mm2) if the thickness and length are both expressed in millimetres (mm). For a flat plate in uniaxial tension with a butt weld (Figure 6.5), the stress is calculated in the same way as the stress in a flat plate under uniaxial tension with no weld (Figure 3). In a full penetration butt weld the stress is given by the following equation: Stress,  =

load, P cross section area, CSA

=

Load, P length, L x thickness, t

For the stress to be calculated in MPa (equivalent to N/mm2), the load must be converted into Newtons (N) and the length and thickness of the weld are both in millimetres. Be careful because often loads are given in kiloNewtons and will need to be multiplied by 1000 to get the equivalent number of Newtons.

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6.3.2

Stress in a partial penetration butt weld The only difference between partial and full penetration welds is that the weld does not penetrate through the entire thickness of the parent material. Therefore the cross section area of the weld is reduced and there is an unfused land down the centre of the joint (Figure 6). To avoid an asymmetrical stress distribution, the depth of penetration on both sides must be equal (ie t1 = t2). This avoids the welds causing additional bending stresses around the joint. The cross sectional area is now CSA = length, L × (throat of weld 1, t1 + throat of weld 2, t2) The tensile stress which occurs in the weld is therefore calculated by: Stress,  = load, P = CSA

Load, P length, L x total thickness, t1 + t2

Again, if the stress is to be in MPa, the load must be converted into Newtons (N) and the length and thicknesses of the welds are all in millimetres.

Figure 6.6 Partial penetration, double sided butt weld under uniaxial load.

Another feature to consider in this type of weld is the crack-like gap left between the two welds. Since defects are more likely close to the weld root, adding a sharp corner in this area is not good practice. Therefore, it is strongly advised to avoid partial penetration butt welds and to opt for fully penetrated butt welds with a prepared edge to obtain a r eliable joint.

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6.3.3

Stressed area in pipes For butt welds in pipe (often referred to as girth weld), it is necessary to know the cross section area of the pipe. A common approach is to imagine ‘unrolling’ the pipe and using the same method as for plates. The length of the plate is assumed to be the inside circumference of the pipe (along the root of the weld). The length of inner circumference is determined by multiplying the inner diameter (ID) by  (which is roughly 3.14).

Figure 6.7 Cross-section area of pipes and tubular sections.

The CSA of the weld under loading (ie the shaded area) is therefore equal to: CSA = length, L x thickness, t =  x ID x t. The second method to determine the CSA of a pipe is to imagine a larger and smaller circle and take the area of the small circle away from the area of the large one. The formula for the area of a circle is CSA = r 2, where r is the radius of the circle. The radius of the larger circle is OD/2 and the radius of the smaller circle is ID/2. 6.3.4

Shear stress in full penetration butt welds Shear stresses in welds can be calculated using a similar approach as for tension stresses. Shear stress has the symbol , compared to axial stress, which is given the symbol . In a butt weld subjected to a shear force P acting parallel to the weld, the shear stress is given by the following equation:

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Figure 6.8 Full penetration butt weld under shear loading.

Shear stress,  (MPa) = shear force, P (N) Length, L (mm) x thickness, t (mm)

6.4

Fillet welds

6.4.1

Advantages Fillet welds are widely used in welded fabrication due to these advantages:

6.4.2



Simplest design. All that has to be done is to stand one piece against the other and run the welding electrode/gun where the parent metals touch.



Cheap. Fillet welds require virtually no preparation (items can be welded straight after flame cutting if the process is set correctly).



Can be made in flat (PA) or horizontal (PB) position by semi-skilled operators. A welder qualification for fillet welds requires only macro examination or a fracture test which are less demanding than butt weld tests.



Can be made with any number of passes. The welder can either increase productivity and reduce distortion by reducing the number of passes or avoid a wide HAZ in a sensitive material (eg fine grained structural steel) by reducing the heat input and increasing the number of passes.

Disadvantages 

If not performed correctly, lack of penetration can occur. This means that even if the leg size is achieved, the weld throat which carries the load is reduced. Visual examination or other NDT techniques cannot reveal this defect, the only way is by macro examination, but since this is a destructive test it cannot be applied to a welded assembly.

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

6.4.3

The volume or weight of weld metal is proportional to the square of the leg length. Once you calculate the required throat/leg size, stick to it. Over-welding fillet welds is easy, but costly in terms of consumables used and can lead to heavy distortion and lamellar tearing.

Shape of fillet welds For design purposes, a fillet weld is assumed to be triangular in shape, the size being defined by the weld throat or leg length as shown in Figure 6.9.

Figure 6.9 Terms used to describe the features of a fillet weld.

Throat thickness is regarded as the most important dimension for design purposes but mechanical failure of fillet welds is often along the fusion line or through the parent material itself. One reason for this in carbon or low alloy steels is that the weld metal is mostly substantially stronger than the parent metal. The throat is the shortest distance from the root to the face of the weld. Fillet weld sizes should be specified by referring to the throat thickness, a, although leg length, z, is often used and can be easier to measure during weld inspection. Conventionally, leg lengths are regarded as being of equal dimension, the weld forming an isosceles triangle in cross-section. Convex, concave and deep penetration welds are illustrated in Figure 6.9 below.

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Figure 6.10 Convex, concave and deep penetration fillet welds.

Figure 6.11 Design versus actual throat thickness for fillet welds.

The convex fillet is generally undesirable for two main reasons. The junction of the weld metal with the parent metal at the weld toe can form a significant stress raiser and will adversely affect both fatigue life and brittle fracture resistance. Excess weld metal in the cap costs time and money to deposit without contributing to joint strength. The concave fillet weld can be beneficial with respect to fatigue strength but the minimum specified throat thickness MUST be achieved. Deep penetration fillet welding can give a stronger joint, but it is not possible to allow for this during design as the actual penetration depth cannot be verified by inspection techniques during production. 6.4.4

Stress in fillet welds In a fillet weld, the stress is supported by the throat, a so, it is assumed that fillet welds always fail across the throat because during the application of load, the throat is the smallest section which supports this load and thus the stress is at its maximum level in this area. The result of design calculations for a fillet weld would give the throat size.

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Figure 6.12 Single sided fillet weld under tension.

In a simplified way, the stress in the weld throat can be calculated by the following equation: Stress =

load P length, L x throat, a

Similarly, this equation also holds for fillet welds under shear such as in Figure 6.13 below.

Figure 6.13 Single-sided fillet weld under shear.

Often when fillet weld sizes are calculated, they are mainly subjected to shear. The allowable, or design, shear stresses on the weld throat area are applied. Some codes specify these values depending on the welding electrode but in the absence of such information ½ yield stress of the parent material is assumed as the design shear stress (compared to 2/3 yield for the design axial tensile stress). This value of design shear stress takes into consideration the higher sensitivity towards cold cracking shown by fillet

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welds due to the increased combined thickness (see MAB module) as well as the effect of the natural lack of penetration present at the root of the joint. In some standards such as AWS D1.1: ‘Structural Welding Code’, American Welding Society, 2008, the leg length z may be used as a design parameter. In a mitre fillet weld, the relationship between the throat and the leg is as follows:

Figure 6.12 Weld throat and leg length in a mitre fillet weld.

For a fillet weld with equal leg lengths, the cross section triangle is a rightangle triangle with angles of 45° in each corner. The relationship between weld throat, a and leg length, z, is given by the following equation. a ≈ 0.7z

and

z ≈ 1.4 a

(For the maths-minded, 0.7 is 1/√2 and 1.4 is √2)

6.5

References IIW Guidelines; Niemi E, Fricke W, Maddox S J: ‘Fatigue analysis of welded components: Designer's guide to the structural hot-spot stress approach’, Woodhead Publishing, 2009.

6.6

Summary At the end of this section you should be able to understand the cross section areas of different welds and how forces act on them. You should also know the differences between design stress, nominal stress, hot-spot stress and notch stress.

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Revision questions 1 Calculate the stress in this butt weld

Answer: 64MPa

2

What size fillet weld is needed in this joint? Steel has a yield strength of 350MPa.

8mm

Answer: 5.8mm

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Section 7 Different Types of Loading

Rev 4 April 2013 Different Types of Loading Copyright  TWI Ltd 2013

7

Different Types of Loading Structures experience loading in a wide range of conditions. Firstly designers assess a structure under static loading at ambient temperature. But when the structure is in service it may experience extremes of temperature from low to high. The loading may fluctuate and may affect the structural integrity of any welds in the structure. It is important to consider all types of loading when designing welded structures.

7.1

Static strength Two tensile specimens of the same material but different size (Figure 7.1). Which one is stronger, specimen 1 or 2?

Specimen 1

Specimen 2

Figure 7.1 Tensile specimens of different sizes.

The load-extension curves from the two specimens are shown in Figure 2 and it can be seen that specimen 2 can withstand the greater load. However, tensile strength is what is a material specific property and these specimens are of the same material so should have the same strength regardless of size. Therefore we take the size of the specimen into account, when determining the stress (as opposed to the load) and the units of stress are N/mm2.

Figure 7.2 Load-extension curves for tensile specimens 1 and 2.

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Stress takes into account the cross section area (CSA) of a specimen so is the true indicator of strength, not the maximum load applied. The CSA of the specimen in Figure 7.3 is the area being tested perpendicular to the tensile direction of loading and is often a circle in a round tensile specimen, but square or rectangular section specimens are also possible. The tensile specimen is thicker at the ends where the specimen is gripped, but the material being tested is in the central parallel section, called the gauge of the specimen. It is the CSA within the gauge length that is used to calculate stress.

Figure 3 Cross-section area of tensile specimens.

Instead of load versus displacement curves, the tensile test is represented by a stress versus strain graph. Strain is the proportional extension of the specimen (the amount it has extended divided by its original length). The stress-strain curves for Specimens 1 and 2 from Figure 7.1 become the same, as shown in Figure 7.4.

Figure 7.4 Different sized specimens of the same material give the same stressstrain curve.

Using stress-strain curves effectively normalises different specimen sizes. It allows us to test small scale samples and apply the results to larger structural components that would be impractical to test in a laboratory. Load-extension would not allow this.

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We can use the stress–strain curves for different materials to help design structures. To ensure there is no plastic deformation in structures, engineers usually design each structural member to operate at a static stress equivalent to two thirds of the yield stress, leaving a third as a safety factor so as that the structure can tolerate some further loading without plastic deformation. (Some design codes limit the applied stress to half yield strength). These other loads that can act on the structure are mostly due to adverse weather conditions such as added weight due to snow fall or additional forces due to high winds. Figure 7.5 shows this safety margin between the design stress, σdesign and yield stress, σy.

Figure 7.5 Indicated design stress at two thirds yield stress.

7.2

Effect of temperature on strength The material specific description of strength is not a set value. Factors can affect the strength of a material like they can all material properties. One of the biggest factors which affect the strength of a material is the temperature of operation as shown in Figure 7.6 for different metallic materials. Generally steels exhibit a reduction in tensile strength at elevated temperatures, however, certain steels exhibit trends showing a decrease and then an increase in strength between certain temperatures. This characteristic is also dependent on the strain rate.

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Figure 7.6 Influence of temperature on different metallic materials.

However, despite higher temperatures causing a drop in tensile strength, operating at these temperatures also increases the ductility of the material and avoids the risk of brittle fracture. Ferritic steels experience a ductile to brittle transition at a given (usually low) temperature, as shown in Figure 7 for the Charpy impact curve at different test temperatures for ferritic steel. Therefore, operating steels at lower temperatures, while giving higher strength, risk suffering a significant drop in fracture toughness.

Figure 7.7 Typical Charpy impact curve for ferritic steel.

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Choosing materials which can best suit certain applications and the method to assess a material’s suitability is termed material selection. An example where this process has been optimised to the maximum is the turbine engine, shown in Figure 8:

Figure 7.8 Turbine aero-engine.

A colour coded key describes what sort of material is used at different locations within the engine. At the back, where combustion takes place, the temperature is very high and normal materials would not be able to withstand the heat generated and would lose their integrity. Ni-superalloys have been engineered by materials scientists to perform the duties required of turbine blades without degrading. As with all aerospace components, weight saving is a big issue. The fan blades at the front need to be light but strong and resistant to creep so selecting the correct material is challenging. A titanium alloy is used due to its excellent mechanical properties and most of all its strength to weight ratio. The engine’s driveshaft running through the centre of the engine is made of steel. Steel is three times heavier than titanium so there is no real replacement for this component due to the intense levels of torque that need to be tolerated by the shaft. Titanium is as strong as steel, but cannot withstand twisting forces to the same extent. Other areas of the engine not requiring demanding mechanical properties or high temperature resistance are made of very light composite materials such as carbon fibre to save weight.

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7.3

Stress concentrations

Sudden changes in geometry in a material section (such as holes, notches, grooves, corners, fillet welds, or defects) can act as stress raisers which concentrate the stress so that the local stress increases. The concept of stress concentration can be visualised by considering flow lines. The lines of transmission of the stress are similar to the flow lines if fluid were to enter the material from one end to the other, as in Figure 9. Densely packed flow lines are representative of the concentration of stress at those points.

Figure 7.9 Stress concentration around a notch.

Figure 7.10 shows the stress concentration effect of a circular hole in a large flat plate under tension. Even this simple detail increases the stress at the edge of the hole by a factor of 3. This means that close to the edges of holes (such as bolt holes), the maximum stress in the plate is about three times the nominal applied stress. Sharper notches concentrate the stress by much more and very sharp notches, such as cracks have a very high concentration of stress at their tips.

Figure 7.10 Stress concentration due to

7.4

a circular hole.

Modes of failure

There are a number of ways that a metal structure can fail. In the MAB module you learnt a number of ways that welds can suffer cracking. But ultimately, these flaws lead to catastrophic failure in one of three main ways. By ductile failure, by fatigue and by brittle fracture.

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7.4.1

Ductile failure Ductile failure or plastic collapse occurs when yielding and deformation precede failure and is the result of overloading. Purely ductile failures are rare since most structures are designed well within their load bearing capacity. They can occur when the strength has been degraded, for example in the high temperatures during a fire. Ductile failures are most likely to occur in service as a secondary failure mode after the section thickness has been reduced as a result of fatigue crack growth, corrosion or erosion. When examining the fracture surface, a ductile failure shows evidence of gross yielding or plastic deformation, the fracture surface is rough and torn and may be highly fibrous as a result of deformation. The failure surface may show 45° shear lips or have surfaces inclined at 45° to the load direction. Two ductile failures are shown in Figures 7.11 and 7.12.

Figure 7.11 Ductile rupture of a component.

Figure 7.12 Ductile fracture surface.

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7.4.2

Fatigue failure Fatigue is discussed in detail in Section 4 of this DAC1 course. The failure surface from a fatigue failure is smooth, flat and bounded by a curve (Figure 7.13). There are sometimes bands, known as beachmarks on the fracture surface which show the progress of the crack front. The beachmarks can help identify the point of origin at the middle of the beachmark curves. A fatigue failure surface is always at 90° to the load, but final fracture will usually take the form of gross yielding, or sometimes will result in a brittle fracture.

Figure 7.13 Fatigue fracture surface.

7.4.3

Brittle fracture Brittle fracture involves little or no plastic deformation and occurs in a fast, unstable manner. The crack propagates at about the speed of sound, so it is a very fast rupture process and the results can be catastrophic. A characteristic of brittle fracture (Figure 7.14) is there is little or no plastic deformation before failure. The fracture surface may show chevron marks or river lines pointing back to the fracture initiation point. With a brittle impact fracture, the surface is rough but not torn and will sometimes have a crystalline appearance (particularly under high strain rate loading, for example in a Charpy specimen).

Figure 7.14 Brittle fracture surfaces where a fatigue crack propagated following brittle fracture.

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The likelihood of brittle fracture is caused by three main factors: Having sufficiently low toughness; the presence of a flaw; and the application of a stress. These can be remembered visually from a triangle.

Figure 7.15 The three factors for brittle fracture.

The ductile to brittle transition in steels at low temperature influences whether a failure will be ductile or brittle. At low temperatures the material has lower fracture toughness and is more prone to brittle fracture. Low toughness is also more likely in materials with a crystalline structure which i s bcc (ferritic steels) because they show the toughness transition, compared to those with fcc crystal structures, such as austenitic stainless steel or aluminium, which do not show a marked transition between ductile and brittle behaviour. Low toughness can also result from the steels’s microstructure. A fine grain size has high toughness, whereas martensite or coarse grain HAZ has low toughness. Material thickness also has an effect on the fracture toughness. Thick material has lower effective toughness than thinner plate made from the same material. Brittle fracture is more likely in the presence of high residual stresses or if the structure is highly loaded, particularly under high strain rate (impact loading). Finally, stress-concentrations (from weld toes, change of section, notches) and weld defects (such as cracks or lack-of-fusion) can also have a major effect on the likelihood of brittle f racture. In welded structures, brittle fracture is a particular concern due to the presence of weld defects (poor quality), Poor fracture toughness in parent material (wrong design choice) or in HAZ (too high or low heat input), combined with a high level of residual stress (no PWHT, wrong design) can combine to make brittle fracture a very real danger. The welding engineer needs to be very careful when designing a welded structure to make sure that brittle fracture will be avoided.

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7.5

Reading fracture faces It is rare that a fracture surface will exhibit just one kind of failure mode, so when reading a fracture face it is important to look for clues to all the modes which might play a part in the overall failure. In particular, beachmarks will be characteristic of fatigue failure. River lines will indicate that brittle fracture has occurred. Where there are failure planes at 45° to the main loading, it is evidence of ductile failure. However, the absence of any of these clues does not mean that those failure modes could not have occurred. In Figure 16 the smooth flat region with beachmarks identifies that fatigue has occurred. The final fracture is rough and torn and is at 45° to the fatigue crack, pointing to ductile overload as the final failure.

Figure 7.16 Clues to understand a fracture face.

7.6

Summary At the end of this module you should be able to identify the various types of fracture. You should understand the requirements for designing under different loading or temperature service and how to choose materials that will meet these requirements.

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What effect does high temperature have on strength?

2

Give four examples of stress concentrations.

3

What are the three main factors for brittle fracture to occur?

4

Beachmarks occur on a fracture surface from which failure mechanism?

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Section 8 Design Considerations for Aluminium

Rev 4 April 2013 Design Considerations for Aluminium Copyright  TWI Ltd 2013

8

Design Considerations for Aluminium

8.1

Introduction Aluminium is the most widely used non-ferrous metal and has a number of properties which make it very attractive to designers. Compared to steel it has a higher strength-to-weight ratio makes it very attractive to the aerospace, marine and automotive industries where weight savings have a dramatic impact on performance and fuel economy. 70% of commercial civil aircraft airframes are made from aluminium alloys and without aluminium civil aviation would not be economically viable. Ships, boats, cars, trains, all utilise the lower weight of aluminium alloys. Unlike steel, aluminium’s non-magnetic properties make it well suited for applications where electromagnetic interference is undesirable, for example the electronics industry. Its high electrical conductivity makes aluminium a popular material for welding cables and overhead transmission lines. Its corrosion resistance makes it useful for food and medicine packaging. The recyclability of aluminium makes it even more attractive as it shows no sign of degradation when recycled, which means it can be recycled indefinitely without loss of quality. Furthermore, the recycling only requires 5% of the energy required to make new aluminium.

8.2

Advantages of aluminium compared to steel 

Weight Low density (2,702g/cm3), roughly one third that of steel so the deadweight of aluminium structures is dramatically reduced which promotes its use where weight is an important factor such as the automotive, shipping or aerospace industries (Figure 8.1).

A

b

Figure 8.1 Applications of aluminium alloys in a train carriage bodies and b helicopter components.

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

Corrosion resistance Aluminium alloys have excellent corrosion resistant properties due to a thin self-healing oxide layer and can normally be used unpainted. Aluminium’s corrosion resistance makes it widely used for food packaging such as aluminium foil, drink and food cans. Other uses range from gas cylinders to ladders to ski poles. Higher strength alloys will corrode in some hostile environments and may need protection.



Magnetic properties Aluminium is non-magnetic which allows its use in applications where no electromagnetic interference is allowed, such as avionics devices. Unfortunately, this property means that magnetic particle examination cannot be used as an NDT method aiming at detecting surface/near surface defects in an aluminium weld.



Ductility for extrusion Extruding is the standard process for producing aluminium sections and is vastly more versatile than the rolling procedures used for steel. Complex cross sections can be extruded in a single operation, rather than requiring extensive welding (Figure 8.2) and exploiting this ability is a major feature in aluminium design.

Figure 8.2 Extruded complex aluminium sections frictions stir welded together. 

Machinability Milling can be an economic fabrication technique for aluminium because of the high metal removal rates possible so U or J weld preparations are easier to produce. These machined preparations can lead to better joint fit-up, reducing the amount of weld metal required to fill the preparation and avoiding possible weld defects caused by mismatch.



Low temperature performance With a face-centred cubic crystalline structure, aluminium possesses excellent strength and toughness characteristics at low temperatures so is suited for cryogenic applications (Figure 8.3). Unlike steel it does not exhibit a ductile to brittle transition at low temperature.

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Figure 8.3 Storage spheres for holding cryogenic liquids.

8.3



Electrical conductivity Aluminium possesses a high coefficient of electrical conductivity which combined with a lower price per kg compared with copper, make it the standard material for overhead transmission lines (with a central steel strand to carry the weight of the cable). Aluminium alloys have an electrical conductivity approximately 65% that of copper but because of their density can carry more than twice the electricity as an equivalent weight of copper.



Thermal conductivity Aluminium has a high coefficient of thermal conductivity (237W/m°C about four times greater than steel) so pure aluminium can be used in heat exchangers as an alternative to copper tubes. From the welding point of view, high thermal conductivity is a disadvantage since the heat tends to dissipate quickly from the heated point.

Welding and joining aluminium 

Weldability Most aluminium alloys can be arc welded using gas-shielded processes. The low melting point of aluminium (660ºC) means welding speeds can be faster compared with steel. However arc welds in aluminium can suffer from porosity, lack of fusion due to the presence of particles of high melting point oxide in the weld pool and solidification cracking for susceptible weld pool compositions. Most aluminium welds suffer from an unavoidable loss of strength in the heat-affected-zone due to grain growth (see Section 8.6). Molten aluminium has high fluidity (compared to molten steel) so the weld pool can spill out or run ahead of the joint preparation, leading to possible fusion or burn-through problems. To avoid this, a smaller (or no) root gap is used.

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Electron beam and laser welding is now used extensively for joining aluminium components with the advantages of high processing speeds, ease of automation and low heat input (low distortion). Friction welding processes (such as friction stir welding) avoid the disadvantages of fusion welds and can produce extremely strong defect-free welds in aluminium alloys. 

8.4

Adhesive bonds The use of adhesive bonding is well established as a valid method for making structural joints in aluminium and does not produce residual stresses or other defects which can occur during welding (hot cracking, porosity, etc). Unfortunately, adhesive bonded joints have limited life as most adhesive systems degrade rapidly when the joint is both highly stressed and exposed to a hot, humid environment.

Disadvantages of aluminium 

Cost The cost of aluminium (sections, sheet and plate) is typically about 1.5 times that of structural steel, volume for volume. For aircraft grade material, the differential is much more. Fabrication costs are lower because of easier handling, use of complex extrusions, easier cutting or machining, no painting and simpler erection so in terms of total cost the effect of switching to aluminium is usually much less than one could expect. Under certain circumstances, an aluminium design can even be cheaper than a steel one. Aluminium has a relatively high scrap value; a significant factor in material selection for components designed to have a limited life, but high scrap value encourages theft.



Thermal expansion Aluminium expands and contracts with temperature approximately twice as much as steel – its coefficient of thermal expansion being 24×10-6 ºC-1 compared with only 11×10-6 ºC-1 for steel. Greater thermal expansion leads to greater distortion; expect twice as much distortion in an aluminium structure compared with steel. Because of the lower Young’s modulus, thermal stresses in a restrained member are only two-thirds those in steel.

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Figure 8.4 Weld distortion of aluminium. 

Thermal conductivity Having a high coefficient of thermal conductivity, aluminium is capable of cooling the weld pool much faster than steel. Since heat is dissipated much more quickly, a larger included angle is required to prevent lack of sidewall fusion. If the included angle for a V preparation in structural steel weldment is approximately 60º, this value may need to be increased to 90º for aluminium. A high thermal conductivity means also that a larger area will be heated up by welding, thus increasing distortions and giving a wide HAZ.



Young’s modulus Aluminium exhibits a low Young’s modulus value, 0.7×105N/mm2, a third that of steel. Because of this, aluminium beams are more prone to buckling than equivalent steel ones and have lower stiffness and rigidity. Elastic deflection is therefore a key factor to consider when designing aluminium structures, which may not be a concern when using steel instead.



Fatigue resistance Aluminium alloys are more prone to fatigue than steel because of their lower Young’s modulus. When designing steel structures, potential fatigue sites should be identified. The number of fatigue cycles to failure for a given stress range is normally obtained from an endurance curve, according to the weld geometry. For a mass-produced component, the fatigue life can be found by testing. Fatigue is covered in more detail in Chapter 4.



Tensile strength Pure aluminium has modest UTS (70-150N/mm2 depending on delivery condition, annealed or cold worked). For use in structural applications it is alloyed with different elements to increase its tensile strength up to 650N/mm2. This compares to standard grades of steel which have yield strengths of 150-450MPa and tensile strengths of 300-650MPa.

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Aluminium does not present a clear yield point so to define a useable limit for the stress, proof stress is used (ie the stress at which the material undergoes a certain permanent strain, commonly 0.2%). It should be noted, however, that when designing steel structures, the limit state is usually the rigidity of the structure rather than its strength.

8.5



High temperature service Aluminium loses strength more quickly than steel with increasing temperature and some alloys begin to lose strength when operating above 100°C. As a result, aluminium structures have a limited upper service temperature and are not intended for creep resisting applications.



Corrosion Serious electrolytic corrosion of the aluminium may occur at joints with other metals unless correct precautions are taken. This can apply even when using alloys that are otherwise highly durable. Aluminium is also susceptible to stress corrosion cracking (SCC) which can occur in aqueous chloride solutions and tropical marine conditions.



Affinity to oxygen Aluminium forms a tenacious oxide film with a melting point more than three times that of aluminium. Failure to remove this oxide both before and during the welding operation results in entrapment of oxides and/or incomplete fusion giving a joint with impaired mechanical properties. To produce a sound weld the oxide layer needs to be removed by mechanical or chemical methods. The use of chemical cleaning must be considered from the design stages as since the reagents used are highly corrosive, permanent backing strips and lap joints should be assembled after chemical cleaning due to possible entrapment. Due to its high affinity to oxygen, aluminium is mainly welded using gas-shielded arc welding processes and since the shielding gas column can be affected by draughts, on-site welding of aluminium is difficult unless special measures are used to protect the weld area. This high affinity to oxygen requires larger diameter gas nozzles for TIG and MIG welding which leads to an increase in included angle and/or an increase of land in U preparations.

Aluminium alloys Alloy series

Main alloying element

Heat treatable?

1XXX

None (pure Al)

No

2XXX

Copper

Yes

3XXX

Manganese

No

4XXX

Silicon

No

5XXX

Magnesium

No

6XXX

Magnesium & silicon

Yes

7XXX

Zinc and magnesium

Yes

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Rev 4 April 2013 Design Considerations for Aluminium Copyright  TWI Ltd 2013

Non-heat treatable alloys gain strength from cold working. Heat treatable alloys gain strength from both work hardening and precipitation hardening.

8.6

Heat affected zone softening Aluminium alloys are normally used in cold worked or precipitation hardened conditions to take advantage of their high strength-to-weight ratio. The strengthening benefits of both cold work and precipitation hardening are lost when aluminium is exposed to the high temperatures from welding. The temperature in the heat-affected zone is sufficient to cause grain growth and hence softening. The lower strength of the HAZ must therefore be considered in the design by allowing for the amount of softening (loss of strength) when calculating the load carrying capacity of a weld. This may be done by locally thickening the material in the region of the weld, or by designing the locations of the welds away from the most highly stressed regions (ie on neutral axes).

Figure 8.5 The loss of strength (and hardness) in the HAZ of welds in aluminium.

8.7

References and further reading AWS D1.2: ‘Structural Welding Code – Aluminium’. American Welding Society. BS 8118-1: ‘Structural use of aluminium. Code of practice for design’, London: British Standards Institution. BS EN 1999: ‘Eurocode 9: Design of aluminium structures. General structural rules’, London: British Standards Institution. Bull J W 1994: ‘The practical design of structural elements in aluminium’. Avebury Technical, UK.

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Conserva M, Donzelli G, Trippodo R 1992: ‘Aluminium and its applications’, Edimet, Brescia, Italy.

8.8

Summary At the end of this module you should be able to name some typical applications of aluminium alloys and the advantages of this material compared to steel. You should recognise common aluminium imperfections and solutions to avoid them. You should also be able to recognise typical weld preps for aluminium alloys.

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Revision questions 1 What is the density of aluminium compared to steel?

2 What is the Young’s modulus of aluminium compared to steel?

3

What are aluminium alloys used for and why?

4

What effect does the difference in Young’s modulus have on the fatigue resistance of aluminium welds?

5

Why is distortion a problem for aluminium welds?

6 Why do aluminium welds suffer from HAZ softening?

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Section 9 Static Loading

Rev 4 April 2013 Static Loading Copyright  TWI Ltd 2013

9

Static Loading

9.1

Introduction For some structures, the main loading does not change over time, it is essentially static. A typical example of this is a building based on a steel frame where the frame supports the weight of the building (and the frame itself) and the weight of the contents. The majority of new buildings are based on steel frames, because this is also a fast and efficient construction method. Figure 9.1 is a classic image of construction workers on a 1920s skyscraper in New York, which shows the steel frame skeleton of the structure.

Figure 9.1 Construction workers take a break on a New York skyscraper in the 1920s.

9.2

Structural details There are two major kinds of structural details for static loading that this module will examine. Trusses are structures with a number of structural members joined together to form triangular units. The structure transmits loads through axial forces rather than bending. A pylon is an example of a space frame truss. Nodal joints occur in fixed offshore steel jacket structures. Tubular legs and cross members are welded together at the nodal joints. Various methods exist for linking structural members and strengthening structures, such as stiffeners, braces and steel reinforcement.

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Figure 9.2 Pylon truss joints.

a Steel reinforcement

Figure 9.3 Nodal joints in an offshore structure.

b Stiffeners

c Nodal joints

d Braces

Figure 9.4 Methods of joining and strengthening steel structure joints.

9.3

Strength of beams For a structure under stable conditions, ie static, all the forces on the structure must balance in equilibrium. An example is a truss bridge where the forces in all the members are shown in a free-body diagram in Figure 9.5.

Figure 9.5 Free body diagram of a truss bridge (not every force is shown here).

The stress, , that each force, F, imposes is calculated by dividing the force by the cross sectional area, A, of the member. The units of force or load is Newtons, N, the units of area is mm2 and the units of tensile stress is N/mm2, or megapascals, MPa.

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Figure 9.6 Definition of stress.

In Session 3 (Focus and Strength of Materials), the concept of a stressstrain curve, Figure 9.7, was introduced as method of characterising material tensile behaviour. The elastic design method bases the design stresses on the elastic limit of the structure, but ensures that the stresses in the structure do not exceed the yield stress (ie elastic deformation is designed for, but no plastic deformation occurs). However it is not normally possible to design up to the yield stress safely due to the presence of material defects, joint/weld mismatch, unforeseen loads such as wind or snow and degradation over time. For static design, the allowable stress is limited to a proportion of the specified minimum yield strength of the material. Relevant design codes provide guidance on what proportion this should be, but it is quite common to use an allowable design stress that is 2/3 of the material yield strength, although historically for safety critical structures such as pressure vessels the stress was limited to a quarter of the UTS. Generally, the welding consumable is chosen such that the weld metal strength is greater than the parent material. In these cases the parent material strength defines the load bearing capacity of the structure. However, when defining the throat dimension of load carrying welds it is the weld metal strength that is used.

Figure 9.7 Material tensile properties.

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The ratio of the yield stress (or UTS) to the design stress is known as factor of safety (FoS). The factor of safety depends on the material and the utilisation of the structural member.

A material’s load capacity depends on design method, whether it’s based on the minimum yield strength or the ultimate tensile strength. A welded joint is very complex since it is heterogeneous; it has parent, HAZ and weld metal microstructures, each with different individual strengths. To simplify the approach to calculating the load capacity of welded joints, it is assumed that the weld metal overmatches parent metal and therefore that the parent strength defines load carrying capacity. There are significant exceptions to this rule, however. High strength low alloy (HSLA) steels sometimes have weld metal that undermatches the very high strength parent metal. Welded joints in aluminium very often have strength undermatching in the weld metal and HAZ, as the static strength can be reduced by the heat of welding.

9.4

Types of loading Static axial tensile stress is just one kind of loading that can be imposed on a structure. Cyclic and dynamic loading is discussed in the section on Fatigue in this course, but even static loading can take several forms (Figure 9.8). Axial stress can be in tension or compression. Where the application of load is offset by a perpendicular distance then shear stresses are imposed. For members that are loaded in bending, one surface is stresses in tension and one surface in compression, Figure 9.9. A similar approach to design is adopted; the maximum stress allowed at the extreme fibre is limited to the same fraction of material strength as for tensile loading.

Figure 9.8 Different kinds of static loading.

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Figure 9.9 A beam loaded in bending.

Bending moments can be introduced into beams such as cranes or cantilever beams (Figure 9.10), where a load is suspended from the end of an arm. There are reaction forces at the other end of the beam to counter balance both the shear force, compressive force and the bending moment. The definition of bending moment is force multiplied by perpendicular distance, M = F x d.

Figure 9.10 The forces on a cantilever beam.

9.5

Nodal joints Fixed steel jacket structures such as those used for offshore platforms (Figure 9.3), are fabricated by welding together tubular legs and cross members at node joints (also called nodal joints). Although in these joints it is usually fatigue considerations that limit the design, the materials and joint design have to satisfy static design criteria. The parts of a simple node joint are illustrated in Figure 11. Figures 12, 13 and 14 illustrate typical node joint geometries. T nodal joints have the brace attached at roughly right angles to the chord. Where the offset from perpendicular is greater than 10 degrees then the joint becomes a Y nodal joint. With two such braces positioned in opposite directions is called a K nodal joint. Adding a perpendicular brace to two angled braces forms a T-K nodal joint.

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It is important to recognise that the geometry of a node joint introduces a stress concentrating effect. The magnitude of the stress concentration depends on the joint geometry, material thickness and position around the intersection. This stress concentration factor must be taken into account when designing node joints for both static and fatigue applications.

Figure 9.11 The parts of a node joint.

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Figure 9.12 An X joint

Figure 9.13 A K joint

Figure 9.14 A T-K joint.

Figure 9.15 Tolerances and definitions for T and Y nodal joints.

When designing a nodal joint, consideration must be made of the gap between the various braces, so that there is still sufficient access for welding all round both seams. If the braces need to be close, then a seam weld which incorporates the welding of both can be designed by allowing the two to overlap. This results in a complex weld seam and challenges for welding and inspection access. Sometimes a brace may be offset from the centre of the chord, although this design will impose additional bending on the joint. Usually in a K joint the two braces are angled so that their axes meet at the middle of the chord diameter, to give the joint its strength. It is also possible to angel the braces more deeply or more shallow and the difference is known as the eccentricity of the joint.

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Figure 9.16 Definitions of nodal joint details.

Figure 9.17 Eccentricity, e, in the attachment of braces in a K node joint.

It is possible to construct nodal joints from square box sections as well as tubular members. The way the joints are labelled is very similar to tubular node joints (Figure 9.18).

Figure 9.18 Parts of a node joint in square box section.

The stresses in nodal joints must be calculated to take into account the stress concentration that occurs at the intersection of the chord and brace. The hot spot stress approach was developed to account for the stress concentration at nodal joints in offshore platforms (Figure 9.19) and has been readily adopted since it is easy to extract from a numerical model of a joint and can be measured from strain gauges at the toes of the node joint welds.

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Figure 9.19 Hot spot stress definition.

There are some similarities between the attachment of braces to chords in a nodal joint and the attachment of nozzles onto pressure vessels. However, in nodal joints there is no hole in the chord at the brace attachment, whereas in pressure vessels there is a hole in the shell at the nozzle and therefore a need for compensation for the loss in the strength of the shell. In nodal joints there are large axial and bending loads in braces as they are structural members, whereas nozzles are subjected only to pressure and are not designed to be load-bearing. However, nozzles tend to be proof tested up to pressure beyond the maximum allowable, but for nodes there is not usually a proof loading prior to service. The complex geometry of nodal joints can make them harder to weld and inspect than the mainly circular or saddle-shaped weld seams in nozzles. Nodal joints are generally designed for ambient temperature operation, but can be subject to significant cyclic loading. Nozzles can operate at high or low temperatures but tend not to be designed for fatigue loading.

9.6

Designing structures One of the first considerations for designing structures is what kind of loading the structure will be under. Take the example of a flagpole. Its design is mainly to sustain its own deadweight, since the flag adds little to the structural loads. The additional loading is from wind loads on the pole. The loading imposes a requirement for thicker material at the base than the top. When structures are more complex, containing many members, for example a pylon (Figure 20b) and the approach is similar, but necessarily more complex.

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A

b

Figure 9.20 Designing structures from those with a) simple components to those b) more complex structures.

The various ways that a beam can be carried by a crane illustrates the different types of loading that can be imposed. Carried vertically it imposes only tensile stress, but carrying the beam horizontally from a single point hook will impose bending as well. If the horizontal beam is carried using a chain attached to each end then compression as well as bending occurs. If the beam is carried from a chain attached at a single point in the middle, the loading becomes mainly bending. This is a rather unstable way to carry a long beam though.

Pure tension

Compression and bending

Bending

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Another example of consideration of the loading modes when designing welded joints is the position and location of a lifting lug on an I-beam. The correct design would have the lifting lug oriented in the same plane as the web of the I-beam so that the lug does not impose additional bending, which would occur if the lug was oriented across the web of the flange. This poor design can be improved by adding stiffeners between the webs of the flange beam to reduce the deflection in the beam.

Correct design 9.7

Poor design

Improved design

Stress reinforced concrete

Many structures that are mainly statically loaded are fabricated from concrete, reinforced by steel. Concrete itself has good strength properties in compression but has a very low strength in tension. This limits the use of concrete in construction and makes it unsuitable for use in many structural members when used on its own. However, by introducing a high initial compressive stress such that the concrete still experiences compression stresses when loaded in tension, concrete can be used in tension members and in members loaded in bending. The compression is introduced by pre-stressing steel reinforcing bars in tension and then pouring the concrete around the bar. The concrete shrinks when it sets and grips the steel bar. The pre-stressing in the steel bar is then released and the contraction of the steel bar introduces compressive stresses in the concrete. One common application is in the tension flange of concrete members loaded in bending, Figure 9.21. The term reinforcing-steel is used to describe the use of steel to reinforce any materials, but is most often used concrete.

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Figure 9.21 Reinforcing steel bars (rebars) in the tension flange of a concrete beam loaded in bending.

A range of materials is used for the reinforcing bar and a range of welding techniques is used for joining bars together. The rebars have a textured profile (Figure 9.22) to allow them to key into the concrete and provide the pre-stressing. Reinforcing bar is available in sizes ranging from 6mm up to 50mm diameter. A whole assembly of reinforcing bars will usually be used and the frame fabricated together before the concrete is poured into a surrounding mould. The bars are joined together using one of several methods. The rebars can be welded, they can be joined using a wire joint where wire is wrapped around bars and tightened. It is also possible to use a rebar coupler which is a mechanical fixing. Reinforcing bars are available for a wide range of chemical compositions and mechanical properties. Not all reinforcing bars are weldable; the weldability is determined by the carbon equivalent value and the limitations on the content of certain elements. Rebars are usually welded using MMA or MAG welding processes. Welding rebars using butt joints (Figure 9.23) is usually used just for load bearing joints only, because they need joint preparation and possibly backing strip may be used as well. Lap welds (Figure 9.24) are used for load bearing and non-load bearing joints. It is possible to weld double sided lap joints as well. The requirement is for a minimum throat thickness, a, greater and 30% of the rebar diameter, ie a ≥ 0.3d.

Figure 9.22 surface profile of a steel rebar.

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A

b

Figure 9.23 Weld butt joint preparations in steel rebars a and b showing the stages of filling up the joint when backing is used.

Figure 9.24 Lap welding steel rebars.

9.8

Summary At the end of this module you should recognise structural designs for static loading, such as trusses and nodes. You should understand the different kinds of static loading and know how to determine the tensile stress and the bending moment on a beam. It is expected that you can explain different designs of nodal joint and how they differ from nozzles in pressure vessels. Finally, you should be able to describe welded joints for joining steel reinforcement bars for concrete structures.

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Rev 4 April 2013 Static Loading Copyright  TWI Ltd 2013


Sketch a structure which incorporates a truss frame. How is the load transmitted?

2 How do you calculate the factor of safety on the design stress?

3 How is bending moment calculated? Sketch a see-saw to illustrate your answer.

4 Sketch a Y nodal joint and label the brace, chord, heel and toe of the structure.

5 Why are steel reinforcement bars used in structures? List three ways to join them.

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Section 10 Revision Session IWS

Rev 4 April 2013 Revision session IWS Copyright  TWI Ltd 2013

Revision Session IWS Multiple choice questions 1

Charpy tests are carried out on a steels to: a b c d

2

Arc butt welds in work-hardenable aluminium alloys: a b c d

3

One third. Four times. The same. Twice.

Designers work out fatigue life from a diagram called: a b c d

7

Bolted joints. Magnetic fields. Weld distortions. Vibration.

By how much does a carbon manganese steel beam deflect, compared to an aluminium alloy beam of the same size under the same load? a b c d

6

The extension of material in relation to its original length. A filter in a gas hose. A cause of brittle fracture. None of the above.

Buckling in a column in a steel structure is made more likely by: a b c d

5

Are stronger than the parent material. Are weaker than the parent material. Can only be made in the flat position. Are always porous.

Strain is: a b c d

4

Calculate critical defect sizes. Check for corrosion. Check the moisture content of the plate. Measure notch toughness.

IIW formula. S-N curve. Schaeffler diagram. A flow chart.

Compensating plates are used on pressure vessels to: a b c d

Prevent vibration. Stop corrosion. Reduce stress concentrations at openings. Balance the weight.

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8

Fatigue cracks in otherwise sound butt welds transverse to the stress start at: a b c d

9

The root. The toe. Interrun ripples. Chipping hammer marks.

The main purpose of mid-span stiffeners in a girder is to: a b c d

Keep the flanges square. Increase the moment of inertia. Distribute the load. Stop distortion.

10 The tensile strength of mild steel weld metal: a b c d

Is greater than the tensile strength of mild steel plate. Is lower than the tensile strength of mild steel plate. Is the same as its yield strength. Cannot be measured.

11 The elastic limit of a mild steel bar is the: a b c d

Breaking stress of the bar. Greatest stress which can be applied without yielding the bar. Working stress of the bar. Strain at failure of the bar.

12 A fatigue crack in a crankshaft is most likely to appear: a b c d

Smooth. Jagged. Torn. With a chevron pattern.

13 The neutral axis of bending in a section is: a b c d

The central point of the section. The axis of maximum stress. The axis of zero strain. None of the above.

14 A 10mm fillet welds indicated with a Z to BSEN 22553 requirements has a nominal: a b c d

Throat dimension of 10mm. Area of 10mm. Leg length of 10mm. None of the above.

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15 The fatigue life of a welded joint is NOT likely to be improved by: a b c d

Changing the design. Peening the weld toes. Stress relieving. Using higher strength material.

16 The static strength used for designing a building frame is based upon the: a b c d

Percentage elongation. Hardness. Ultimate tensile strength. Yield strength.

17 Welds are classified with respect to fatigue resistance based upon: a b c d

Heat input Stress concentration effect Thickness] Current

18 A mitre fillet weld with two equal leg lengths of 12mm has a throat thickness of: a b c d

0.8mm. 17.0mm. 9.5mm. 8.5mm.

19 In a cylindrical pressure vessel a lack of fusion flow in the circumferential seams of the shell experience: a b c d

Half the hoop stress. The same as the hoop stress. Twice the hoop stress. Quarter of the hoop stress.

20 The design strength of a fillet weld is based upon: a b c d

The root penetration. The throat thickness. Face concavity. Mitre angle.

21 Distortion from welding is greatest in metals which have: a b c d

High coefficient of thermal expansion. Low coefficient of thermal expansion. Low melting temperature. Low thermal conductivity.

10-3

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22 Which of the following is characteristic of overload failure: a b c d

Fracture at 45 degrees to the load. Rough and torn appearance. Plastic deformation. All of the above.

23 A product operating at high temperature could experience: a b c d

Low fracture toughness. Creep. Brittle fracture. All of the above.

24 Welds often reach which level of residual stress? a b c d

50% yield. 80% yield. Yield stress. Twice the yield stress.

Short answer questions 1 What is the law that describes the elastic area on a stress-strain graph?

2

How does the Young’s modulus for aluminium alloys compare to that for steel? Give the value of Young’s modulus in steel.

3

What is design stress usually based on in the UK?

4

What is stress?

5

What is the effect of drilling a hole in stressed plate?

6 What is the typical level of residual stress in a welded joint, before and after PWHT?

7

Will a different grade of steel have a different fatigue life?

8 Will an unwelded component fail by fatigue when cyclically loaded in compression? What difference would welding make?

10-4

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9

List five techniques that can be used to improve fatigue life.

10 Describe the features on a brittle fracture surface.

11 Describe the features of a ductile fracture surface.

Long question A lifting lug attached by fillet welds requires a design review. Comment on 50 items which would be assessed during such a review.

10-5

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Definition

• Engineering structure: one that carries loads.

Design and Construction Introduction – Designing Things

• Activity: List four examples of engineering structures (working in pairs).

TWI Training & Examinations Services (EWS/IWS Diploma)

Copyright © TWI Ltd 2013

Stonehenge (about 4500 years old)


Giza Pyramids (about 4500 years old)


Course Aim


Engineering Structures

• Provide guidance on how to design engineering structures so that they operate safely to satisfy specified performance requirements.

• Need to withstand loads. • Made from particular materials. • Constructed in particular ways.



1-1

Course Objectives

Course Objectives

• Recognise sources and effects of loads.

• Recognise the special requirements of pressure vessels.

• Understand fundamentals of strength of materials.

• Appreciate the principles of designing aluminium structures.

• Understand principles of weld design. • Recognise different types of loading. • Understand principles of design for static loading. • Understand principles of design for fatigue loading. Copyright © TWI Ltd 2013


1-2

Outline • Weld features. • Types of welded joints.

Design and Construction Welded Joint Design

• Welding symbols. • Weld positions. • Weld bevels.


• Designing welded joints.



Types of Welds

Weld Features Butt weld on plate

Weld A permanent permanent union between between materials materials caused by heat, heat, and or pressure (BS499) Butt weld

Face

Toe Parent metal

Toe

Fillet weld

Weld

HAZ Root



Weld Features Fusion line

Weld metal

Weld toe

Weld Features Fillet weld on plate

HAZ

Face

Parent metal

Toe

Parent metal

Weld Root


HAZ


2-1

Heat Affected Zone (HAZ) Maximum Temperature

solid weld metal

Weld Zone Terminology Excess weld metal

solid-liquid Boundary grain growth zone recrystallised zone partially transformed zone tempered zone unaffected base material

Excess root penetration Copyright © TWI Ltd 2013


Weld Zone Terminology

Types of Joint • Different types of joints

Weld width

– Tee, cruciform, lap-joints, slots, plugs …

• Weld preparations – Bevels: U, V, J, double V …



Types of Joints

Types of Joints Joint: a configuration of members.

Butt joint

Cruciform joint

T joint

Lap joint Copyright © TWI Ltd 2013


2-2

Lap Joint

Corner Joints

Overlap limits for lap joints: open

closed

t

D D=4xt

but not less than 25 mm Exte Extern rnal al corn corner er join jointt

Inte Intern rnal al corn corner er join jointt

Doub Double le side side corn corner er join jointt



Slot Weld

Plug Weld

Holes can be circular or oval. Holes can be circular or oval

Weld all round. d d

t

d > 3t

t

If t < 10 mm, d = t. If t > 10 mm, slot technique should be used, in circular holes (d = 3t but minimum 25mm see BS 1011-2).

but d = minimum 25 mm!


Fillet Welds


Fillet Weld Features Shape of fillet welds:

Fillet welds

Convex fillet

throat

leg

Concave fillet leg size leg

throat size Copyright © TWI Ltd 2013

Mitre fillet Copyright © TWI Ltd 2013

2-3

Fillet Weld Geometry

Fillet Weld Toe Blend

Actual throat

Design throat

Design throat = actual throat

leg length = 1.4 x throat size Does NOT apply for concave fillets Copyright © TWI Ltd 2013


Butt Weld Features

Weld Geometry

Weld considerations for design of butt welds. • Geometry.

t2

Butt welds

t1

• Partial or full penetration. • Blend toe. t1 = design throat thickness

• Excess metal.

t2 t1

t2 = actual throat thickness t1



Weld Dilution

Toe Blend 6 mm

80

°

Poor Weld Toe Blend Angle 3 mm

20

°

Improved Weld Toe Blend Angle

D Copyright © TWI Ltd 2013

Weight of parent material melted Total weight of fused material



100 Copyright © TWI Ltd 2013

2-4

Weld Symbols

Constructing Welding Symbols

• BS EN ISO 22553 Welded, brazed and soldered joints - Symbolic representation on drawings.

Parts 1 to 4 1. The arrow line.

• AWS A2.4 Standard symbols for welding, brazing and non-destructive examination.

2. The dual reference line. 3. The elementary symbol. 4. Combined symbols for symmetrical welds.


The Reference and Identification Lines

The Arrow Line

Convention of the reference line: a. Shall touch the arrow line

Convention of the arrow line:

a. Shall touch the joint intersection.

b. Shall be parallel to the bottom of the drawing

b. Shall not be parallel to the drawing. c. Shall point towards a single plate preparation (when only one plate has preparation). Other side


c. There shall be a further broken line (identification line) above or beneath the reference line. (Not necessary where the weld is symmetrical!)

or

Arrow side



Elementary Weld Symbols Designation

Illustration of joint preparation

Weld Symbols Symbo l

Square butt weld

Single V butt weld with broad root face (only in BS EN ISO standard!)

Single V butt weld

Single bevel butt weld with broad root face (only in BS E N ISO standard!)

Single U butt weld Single bevel butt weld

Single J butt weld



2-5

Put it Together

Weld Symbols

Fillet weld

Weld symbol Reference line

Surfacing (cladding)

Arrow line

Identification line

Backing run



Symmetrical Both Sides

Arrow Side and Other Side

The dashed identification line can be omitted when symmetrical welds are made from both sides of the joint.

Arrow side

Double bevel

Double V

Fillet weld

Other side

Double U

Double J


Fillet Weld Dimensions

Supplementary Weld Symbols

Convex

Concave


Ground flush

Leg length dimension prefixed by z Design throat thickness dimension prefixed by a

Z10 a7



2-6

Complementary Indications

Intermittent Fillet Welds No. of welds

z 10

weld length length of gap

Weld all round (peripheral weld)

3 x 25 (50)

50 25

10 Copyright © TWI Ltd 2013


Complementary Indications

Weld Preparation • Root face.

Site (field) weld

• Root gap. • Bevel angle. • Impact of welding on preparation. • Practical aspects. Copyright © TWI Ltd 2013

Butt Joint Preparations


Single Sided Butt Preparations Single sided preparations are normally made on thinner materials, or when access from both sides is restricted

Square edge closed butt

Single Bevel

Single Vee

Single-J

Single-U

Square edge open butt



2-7

Joint Preparation Terminology

Double Sided Butt Preparations Double sided preparations are normally made on thicker materials, or when access form both sides is unrestricted

Included angle

Included angle Angle of bevel

Root radius

Double -Vee

Double -Bevel

Root face Double- U

Double - J

Root face

Root gap

Root gap

Single-V Butt

Single-U Butt


Weld Preparation

Joint Preparation Terminology Angle of bevel


Angle of bevel

Terminology and Typical Dimensions:V-Joints bevel angle included angle

Root radius

root face root gap

Root face Root gap

Root gap

Single Bevel Butt

Typical dimensions

Root face Land

Single-J Butt

bevel angle

30-35°

root face

~1.5 to ~2.5mm

root gap

~2 to ~4mm


Joint Design and Weld Preparation


Root Gap and Root Face

Bevel angle

Root face and root gap set to: • Allow controlled root fusion. • Reduce the risk of burn-through.

Bevel angle must allow: • Good access to the root. • Manipulation of electrode to ensure sidewall fusion.


Too shallow or too wide

Too deep or too narrow

= burn-through

= lack of root penetration Copyright © TWI Ltd 2013

2-8

Weld Preparation

Weld Preparation

Welding process impacts upon weld preparation

Welding process impacts upon weld preparation

Arc welding MMA

EBW

MAG

High heat input process allow a larger root face, less weld metal required, less distortions, higher productivity. If the gap is too big risk of possible burn-through,

X

If gap is too small risk of lack of penetration. Copyright © TWI Ltd 2013

Preparing Weld Preparations


Backing Backing bar or backing strip is used to ensure consistent root fusion and avoid burnthrough

Requires machining slow and expensive. Tight tolerance easier set-up.

Can be flame/plasma cut fast and cheap. Large tolerance set-up can be difficult.

Warning! Backing strips give a built-in crevice • Susceptible to corrosion. • Give a lower fatigue life.


Access and Weld Preparations


Weld Preparations Access impacts upon weld preparation



2-9

Welding Standards

Standards

• BS EN ISO 9692: Parts 1-4. Welding and allied processes. Recommendations for joint preparation. • BS EN 14324: Brazing. Guidance on the application of brazed joints. • BS EN ISO 6947: Welds. Working positions. Definitions of angles of slope and rotation.

• BS EN ISO 13920: Welding. General tolerances for welded constructions. Dimensions for lengths and angles, shape and position. • BS EN 1011-2: Welding. Recommendations for welding of metallic materials. Arc welding of ferritic steels. • BS EN 25817: Arc-welded joints in steel. Guidance on quality levels for imperfections.

• ISO 2553: Welded, brazed and soldered joints Symbolic representation on drawings.



2-10

Objectives • Describe common structures • Consider the loads acting on structures

Design and Construction Forces and Strength of Materials

• Review resulting forces • Describe properties that enable materials to withstand these forces



Structures


Structural Elements

Structure • Object or part of an object which has to carry and resist loads.

• Cables. • Bars.

Loads can be due to: • Deadweight of the structure itself. • An external component. • The reaction to an acceleration. • Environmental loads. • Pressure. • Thermal expansion. • Etc.

• Beams. • Plates. • Slabs. • Shells.


Types of Welded Structures


Basic Connections The separate members of a structural framework are joined together by bolting, riveting or welding.

• Examples. – Bridges, Cranes, Masts, Buildings

• Frame.

Rivets

– Assembly of bars – Arranged to support the loads

Welding

– Relatively easy to design

Bolts



3-1

Forces

Combination of Forces y

• Have a size and a direction.

Fx

Many forces can be added together

Fy

F FR = F1 + F2 + F3 + F4 + F5

y

x

• Can be combined into a single force.

F2

F3

y

F4

Fy

F

• Can be decomposed into several forces.

F1

FR

F5

x

x

Fx Copyright © TWI Ltd 2013


Latticed Beams and Frames

Combination of Forces

A frame is an assembly of bars arranged so they can support a load.

Application: - Forces in truss members of a bridge – free-body diagram

Basic assumption: Members carry axial load only. Load is acting in centre of gravity of the bar’s transverse section. All the forces are in equilibrium Note: All the forces are not represented in this diagram Copyright © TWI Ltd 2013


Load-Displacement Curve

Latticed Beams and Frames Solving the problem:

Record of the load and displacement applied during testing. Load, kN

• Step 1 - find out if frame can be statically calculated. • Step 2 - find reactive forces in bearings. • Step 3 - calculate loads in members.

Extension, mm

• Step 4 - calculate welds from connections. Copyright © TWI Ltd 2013


3-2

Load Displacement Curves

Stress • Stress definition Force divided by cross-section area

Load

Applied force

F Stress in the cross section area

Displacement Copyright © TWI Ltd 2013


Tension and Compression

Stress Calculations

Tensile stress

• Stress definition

Compressive stress

Force divided by cross-section area



F A

= tensile stress (N/mm 2 or MPa) F = load or axial force (N) A = cross section area (mm 2) Copyright © TWI Ltd 2013


Strain

Strain,  

Strain

L

L

Strain,   F

L

•

∆L

L

•

∆L

L

L

= Change in Length

• L = Original Length

= Change in Length

• L = Original Length Strain is dimensionless Strain is dimensionless, can be positive value (extended) or negative value (compressed) Copyright © TWI Ltd 2013


3-3

Stress-strain curve

Young’s Modulus

The tensile stress-strain curve contains typical features which are specific to each material. Elastic region Ultimate tensile strength

Plastic region

Extension in the elastic region is proportional to load. This relationship is given by Hooke’s Law which is valid for the elastic region only.

Fracture

, s s a e r t P S M

Young’s modulus:

E

Yield point

Yield strength

Stress Strain



 

Often given in N/mm 2 or GPa Strain, % Copyright © TWI Ltd 2013

Yield and Proof Strength


Yield and Proof Strength

Yield stress: • Stress at which permanent deformation starts to occur.

• Yield point may not always be obvious • In such cases, the 0.2% proof strength (Rp0.2) is used as a design parameter

Stress, MPa

Rp0.2

• Rp0.2 describes the stress obtained for a permanent elongation of 0.2%.

Yield Point

Strain, % Copyright © TWI Ltd 2013

Tensile Test Results


Ultimate Tensile Strength (UTS) • The UTS is the maximum load that can be tolerated by the specimen. • Necking: the sample becomes thinner and develops a neck As a result, the load drops due to the lack of resistance from the material. Ultimate Tensile Strength

, s s e a r t P S M

Necking Point

Strain, % Copyright © TWI Ltd 2013


3-4

Real and Engineering Stress-Dtrain Curve • Design assumption:

Fracture • •

– The cross-section area remains the same. – Engineering stress-strain curve.

The final failure of the component is the point of rupture Elongation of the material = Strain at fracture

Stress, MPa

• BUT necking of the material reduces the REAL crosssection area.

Fracture

• In reality, the stress does not decrease with increasing applied loading and flattens out around the maximum stress.

Strain, %


Factors Affects Stress-Strain Curve


Tensile Test In a tensile test, a sample is clamped between two jaws and pulled apart. The load and extension are measured.

The type of materials

Parallel length Gauge length

1) low carbon steel; 2) medium carbon steel; 3) high carbon steel; 4) bronze

Radius

Diameter of the reduced section

Gripped end

Reproduced by permission Westmoreland Mechanical Testing & Research Inc. Copyright © TWI Ltd 2013

Elastic and Plastic Deformation


Tensile Test: Experimental



3-5

Hardness

Hardness Test: Vickers

• Hardness is the resistance of a material against penetration. • It is measured by indentation under a constant load. • There is a direct correlation between UTS and hardness.



Hardness Test: Vickers

d

Hardness Test: Brinell

d1  d2 2

d

d1  d2 2



Types of Forces

– Compression – Tension

Types of Stress

σ

Compression

Direct Stress,  

Tension

Shear Stress,  

– Shear

Shear

F A

Q A

F

– Bending

τ

Q τ

Bending


Q Copyright © TWI Ltd 2013

3-6

Strain

Bending Stresses

Shear Strain,  

Consider a beam subjected to pure bending (no shear).



h

Before bending

δ

Tension (+)

Q M

h

M After bending

Q

Compression (-)

Neutral Axis Longitudinal stresses are zero

A Copyright © TWI Ltd 2013


Second Moment of Area

Bending Moment A cantilever beam: Force, F

Fy M

Fx

d

Bending moment:

Reaction forces:

M=Fxd

Fx

Fy

High stiffness

Low stiffness

High 2nd moment of area

Low 2nd moment of area

M



Bending Stress

Worked Example

• Calculation of the stress. -

Force, 200N

Using engineers’ beam theory

M

Bending stress, 

• Bending stress s = My/I.

D, 300mm

• M = Bending moment (Nmm). • Y = Vertical distance from neutral axis (mm). • I = Second moment of area (mm 4).

Bending moment: M=Fxd = 200 x 300 = 60,000 Nmm



3-7

Second Moment of Area

Bending Stress

10mm

y Depth, d

20mm

10mm

10mm

Breadth, b

I

For a rectangle I = bd3 /12 = d/2

=My/

20mm

I

10mm

= 5mm

5mm I

=

833mm4

= 5mm

= 833mm4

The taller beam has half the stress!

5mm I

= 3333mm4 = 10

I = 3333mm4

 = 60,000 x 5 / 833 360MPa 

= 10 Copyright © TWI Ltd 2013

 = 60,000 x 10 / 3333 180MPa 


3-8

Objectives • Introduce concept of fatigue loading. • Describe mechanism of failure. • Recognise why welded joints have relatively poor fatigue lives.

Design and Construction Fatigue - Introduction



Simplified Fatigue Loading Cycle


Activity • List four types of structure that have to withstand fatigue loading.

Stress

• Identify the sources of fatigue loading on those structures. • Which of your structures are of welded construction?

Time


Typical Structures Subjected to Fatigue


Sources of Fatigue Loading

• Bridges.

• Fluctuating loads.

• Offshore platforms and rigs.

• Acceleration forces in moving structures.

• Earthmoving/off highway vehicles.

• Pressure changes.

• Ships.

• Temperature fluctuations.

• Towers.

• Mechanical vibrations: machinery, shafts.

• Axles.

• Environmental loading (wind, currents and waves).

• Etc.



4-1

Axle Fatigue Failure 1843

Introduction • Fatigue - crack initiates and propagates under repeated fluctuating loads. • Failure thus occurs by the steady progression of the crack until a final failure mode such as fracture occurs. • Fatigue crack growth is relatively slow. • Stress range required to produce fatigue cracking may be well below yield stress.



Features of Stress Cycle

Types of Stress Cycle Stress

Maximum stress Stress

Cycle

Stress range

Mean stress

Time Minimum stress

Time

Smin= 0

Pulsating cycle



Types of Stress Cycle

Types of Stress Cycle Stress

Stress

Smax

Smax

Time Smin Smin Smin= -Smax

Time

Alternating cycle

Smin= Smax/2 Copyright © TWI Ltd 2013

Half tensile cycle Copyright © TWI Ltd 2013

4-2

Fatigue Parameters

S-N Curves

• Stress ratio (or R-ratio).

R

If applied stress range, σ, is plotted against number of applications of load required for failure (N) we obtain the S-N curve.

Smin Smax

Stress range, σ

• Stress range.

Failure

Sr  Smax  Smin

Endurance limit No failure

• Stress amplitude. - Half the stress range.

No of cycles Copyright © TWI Ltd 2013


Fatigue Strength

S-N Curve

Impact of stress and number of cycles on fatigue failure.

• Typical SN curves are plotted on a logarithmic scale. • Produces a straight line in the high cycle regime.

σ

σ σ2

Log S

Low

σ σ1

N

N

Increase stress - more damage.

N1

N

N2

Increase number of cycles - more damage.

Very High Endurance limit Log N


Fatigue - Terminology


Stress Distribution

• Stress history - variation of stress at a point with time. • Constant amplitude stress history - a stress history in which successive stress fluctuations are equal.

• If stress is applied to a component the stress distribution inside the component would be similar to contour lines. • The contour lines would run through the material parallel with the principal direction of the stress.

• Fatigue life - number of stress cycles sustained before failure. • Fatigue strength - stress range which causes failure at a certain specified life.



4-3

Influence of Notches

Effect of a Notch

• The introduction of a notch creates a concentration of the lines. • Stress cannot be carried across the notch; it has to go around the notch.

Log σ

Without notch

With notch

Log N Copyright © TWI Ltd 2013


Stress Concentrations

Influence of Welding

Stress concentrations occur at a change in section of a stressed member.

Welds introduce stress concentrations from which fatigue can propagate. Weld toe

Weld toe

Weld root toes

Weld toes and weld roots are the most critical areas in respect to stress concentration.

In welds, fatigue cracks start from toes or defects. Copyright © TWI Ltd 2013


Stress Concentrations in Welded Joints Fatigue cracks are most expected in high stress concentration areas.


Fatigue Cracking from a Weld Toe •

Production arc welding processes lead to the formation of non-metallic intrusions at the weld toe.

•

Typically 0.1-0.4mm in depth.

•

Fatigue life governed by the growth of this pre-existing flaw.

•

Little orno initiationstage.

•

Factorswhich affectcrack initiation can be quite different to those that affect crack growth.

Preexisting sharp flaw A

Fatigue crack ~ 50 m


4-4

Effect of Steel Strength on Fatigue Strength

Fatigue of Welded Joints

Fatigue strength of welded joints unaffected by parent material strength.

Fatigue strength of welded joints << parent material. 6

0 1 f o 2 e f i m l m r / o N f , e s g l e n c a r y c s s e r t S

400

Stress range,

300

N/mm2

100

200

50

Steel 350 N/mm2 yield

500 400 300 200 100

400 500 600 700 800 900

10 105

106

107

108

Cycles

Ultimate tensile strength of steel, N/mm2



Fatigue Testing

Typical Fatigue Results

Stress range

R = 0.1

Results Unbroken Mean and 95% confidence intervals

Endurance, cycles Copyright © TWI Ltd 2013


Fatigue Resistance Data - S-N Curves •

Resistance quantified in terms of constant amplitude S-N curves.

•

SmN = A (m and A are constants).

•

Curves for different design details based on statistical analysis of test data.

•

BS 7608.

Stress range, N/mm 300

Joint Classes Behaviour of a structure under cyclic loads is determined by the severity of stress concentration.

2

UK design rules: welds are grouped into Classes giving similar fatigue strength (similar stress concentration effect).

static design limit

20 0

Class of joint

100

A

B

C

D

E

F

F2

G

W

3 0

10 5

10 6

10 7

Expected life increase

Endurance, cycles Copyright © TWI Ltd 2013


4-5

Effect of Tensile Residual Stress on Fatigue •

•

• R =  min/max

Improving Fatigue Strength

Superposition of applied and residual stresses, eliminates effect of mean stress.

• Grinding.

Even when loading is compressive, local stress range cycles down from high maximum stress.

• Peening.

Fatigue life dependent on full stress range regardless of whether tensile or compressive.

– Burr. – Disc. – Hammer. – Needle. – Shot.

• Dressing. – TIG. – Plasma.


Fatigue Improvement Techniques • Grind flush excess weld metal. • Remove weld toes and edges. • Removes intrusions. Weld toe burr grinding (machining)

• Reduce stress concentration.


Summary • Fatigue loading is the repeated application stress. • Failure occurs by the initiation of a crack at a stress raiser and subsequent relatively slow growth. • Design approach by SN curves. • Fatigue performance of welded joints is poor. • Fatigue performance of welded joints is the same for steels with different strength levels.

• Impose surface compressive stress. Copyright © TWI Ltd 2013


4-6

Pressure Vessels • A pressure vessel is a container which holds fluids under pressure.

Design and Construction Design of Pressure Equipment

• Internally pressurised. • Externally pressurised.


• Fired (eg gas and oil fired boilers). • Unfired (eg gas storage vessels).



Construction of Pressure Vessels • Shell.

Shells • Cylindrical shell constructed of a number of curved plates.

– Main body, usually cylindrical.

• Head. – Used to close the cylindrical shell, usually dished.

• Longitudinal welds offset.

• Noz zle – Opening for filling, drainage or inspection.

• Saddle Supports. – Used to hold vessel in place.

• Nameplate. – Contains important information about the vessel.


Dished Heads Can be manufactured from: • One piece.


Hemispherical Dished Heads Knuckle

Advantages: • Requires smallest thickness.

• Segments.

• Sphere is the perfect shape for a pressure vessel.

Petals

• Also good for external pressure.

Disadvantages: • Difficult to manufacture.

Crown Always an odd number of petals. Copyright © TWI Ltd 2013


5-1

Ellipsoidal Dished Heads

Torispherical Dished Heads

Advantages:

Advantages:

• Thickness equal to that of the shell.

• Smallest axial dimension. • Easy to generate.

Disadvantages: • Ellipse difficult to generate.

Disadvantages:

• Big axial dimension.

• Thickness greater than that of the shell.


Pressurisation Axial stress


Welding Pressure Vessels

Hoop stress  x



• Longitudinal welds subject to higher hoop stress. • Offset long welds between different courses of the vessel.

Pr 2t

• Circumferential welds are used to weld the head to the shell. • With different thicknesses, taper transitions are used. 

x

Pr 

2t



y

Pr 

Minimum 1:4

t Copyright © TWI Ltd 2013

Shell-Head Weld Profiles


Shell-Head Welding



5-2

Nozzles

Nozzles

• To connect a pressure vessel with other components we need nozzles.

Types of nozzles:

Type of nozzle depends on: • Diameter/thickness ratio of the shell. • Diameter/thickness ratio of the nozzle. • Access (one side only or both sides). • Type of joint required (partial/full penetration). • Groove preparation methods available. Set-on

Set-through



Flange Joints

Reinforcement

• Separable joints used to connect plant to vessel nozzles. • Flange joint most commonly used.

• A hole in the shell weakens the vessel. • To compensate for loss in strength, add reinforcement to the shell or nozzle. Reinforcing ring/ Compensating plate

Long neck nozzle


High and Low Temperature Service • Vessels often insulated or double walled.


Pressure Vessel Standards • BS EN 286-1 – Simple unfired pressure vessels designed to contain air or nitrogen.

• Autoclave. • BS PD 5500 – Specification for unfired fusion welded pressure vessels.

– High temperature internal chamber. – Used to sterilise medical apparatus.

• ASME Boiler and Pressure Vessel Code, Section VIII.

• Dewar Vessel. – Low temperature storage.

• Welds subject to their own standards and specifications.

– Gap between internal and external chamber is evacuated. Copyright © TWI Ltd 2013


5-3

Making Things Simpler • For structural steels, the weld metal usually overmatches the parent metal. – Use parent metal strength for calculations.

Design and Construction Stresses in Welds

• Excess weld metal is neglected. – Does not carry load. – Use the weld throat thickness.




Making Things Simpler

Nominal Stress

• Assume defect-free welds.

Nominal stress calculation:

• Stress concentrations are neglected. – Due to the bead shape, ripples, abrupt changes, defects.

Stress (

=

N/mm2 or

MPa)

Load (N)

cross section area (mm2)

• Residual stresses are ignored. – Their magnitude and distribution are not known with certainty.

cross section area

– Vary with welding parameters. load

load


Design Stress


Non-Uniform Stress

• Design stress for a structure is often 2/3 yield stress.

• Stress distribution over the cross section is not always uniformly distributed.

• Ensure that the stress in the weld does not exceed the maximum allowable design stress.

• Near geometric features and stress concentrators distribution increase the maximum stress.

• Joint factors often used to reflect level of NDT and risk that flaws could be present.

• At fillet welds, use Hot Spot Stress.



6-1

Hot Spot Stress

Notch Stress

• Originally developed for tubular joints in offshore platforms.

• Approach uses peak stress by looking at stress intensity due to presence of weld.

Stress

• Determined by:

Notch stress

– Strain gauges.

Hot spot stress

– Finite element analysis.

Structural stress Nominal stress

• Takes into account the radius of weld toe and joint geometry. • Complex approach since requires the knowledge of microstructural parameters.


Stress in a Butt Weld


Stress in a Partial-Penetration Weld

• Weld metal overmatches parent metal.

• Partial penetration, double sided butt weld.

– Butt welds can usually be neglected.

• Full penetration butt weld under uniaxial tension.

Load, P

 

thickness, t

P Lt

Load, P t2

Load, P

t1

Load, P

Length , L

Crack-like unfused land

Length, L



Cross Section Area of Pipes Thickness (t)

Shear Stress

Outer Diameter • Method 1: Unroll the tube (OD) into a flat plate.

– Length is the inner circumference.

Shear force, P

Shear force, P

t

– Circumference is p x ID.

Inner Diameter (ID)

– Multiply this by the wall thickness.

Length, L

• Method 2: Subtract area of large circle from small one. – Area of circle = pr 2. – Large radius, r 1 is OD/2. – Small radius, r 2 is ID/2. Copyright © TWI Ltd 2013

Shear stress,  =

shear force, P length, L x thickness, t Copyright © TWI Ltd 2013

6-2

Fillet Welds

Fillet Welds • Terms.

Advantages: • Cheap. • Simple. • Can be made flat (PA) or horizontal (PB). • Can be made with any number of passes. Disadvantages: • Lack of penetration may occur.

– Toe. – Root. – Leg length. – Weld throat.

• Cheap weld design. • East to weld.

– Cannot be revealed by NDT.

• All positions and multi-pass.

• Volume (and weight) of weld increases as the square of the leg length.

• Lack of pen risk. • Easy to overweld.



Fillet Weld Throat

Shape of Fillet Welds • Convex fillet.

Actual throat

– Stress concentration at the weld toe. – Excess weld metal.

Design throat

• Concave. – Smoother transition at the weld toe.

Design throat = actual throat

– Ensure weld throat is big enough. Copyright © TWI Ltd 2013

Stresses in Fillet Welds

Section Properties

Stresses supported by weld throat.

1 _ √2

• In fillet welds, the stress is supported by the throat.

P Stress =


load, P length, L x throat, a

• Mitre fillet is assumed.

a ≈ 0.7z z ≈ 1.4a

L

√2 a

a

z Copyright © TWI Ltd 2013


6-3

Summary

Revision Question 1

• Simplifying assumptions.

800kN

• Design stresses and nominal stresses. – Butt welds. Thickness = 25mm

– Fillet welds.

800kN

• Cross-section areas of welds.

Length = 500mm

• Stress calculations.

What is the stress in this butt weld?


Revision Question 1 - Solution


Revision Question 2 What size fillet weld is needed in this joint?

800kN

200kN

Steel with a yield strength of 350MPa

800kN

25mm 500mm

200mm

Cross section area = length x thickness = 12500 mm2 Load = 800kN = 800,000N 6mm

Stress = load/CSA = 800,000 / 12500 = 64 MPa ? Copyright © TWI Ltd 2013

Revision Question 2 - Solution

Revision Question 2 -Solution

Steel with a yield strength of 350MPa

200kN

Design stress is ⅔ of 350 = 233MPa

200mm


200kN

200mm

length x throat = load stress throat throat ==

Stress = load CSA 6mm ?

6mm

CSA = load stress length x throat = load stress Copyright © TWI Ltd 2013

?

load 200,000 200,000 length 200 x stress stress 233 throat = 4.3mm

Specify a 5mm throat fillet weld


6-4

Static Strength Load, N Specimen 2

Design and Construction Different Types of Loading

Specimen 1

TWI Training & Examinations Services (EWS/IWS Diploma) Extension, mm Specimen 1 Specimen 2 Copyright © TWI Ltd 2013


Cross Section Area

Stress-Strain Curve Stress-Strain curve normalises different specimen sizes.

• Stress is the load divided by the CSA. • In a tensile specimen, use the gauge CSA. Gauge length

σ

Cross sectional area

, s s e r t

Specimens 1 and 2

S

Width/diameter Strain, ε Copyright © TWI Ltd 2013

Design Strength


Effect of Temperature on Strength

• Design components to operate at stresses less than material yield strength. • Limit static stress to 2/3 of yield. • Factor of safety.

Metals (including steel) lose tensile strength at higher temperature.

σ

y

σ

design σ

, s s e r t

σ

design

= 2/3σy

S

Strain, ε Copyright © TWI Ltd 2013


7-1

Effect of Temperature on Toughness

Materials Selection: Select the Most Suited Material For a Given Application

But, higher temperatures cause ferritic steel to increase ductility.

Key:

Lightweight

Ni Superalloy Transition range

) J (

y g r e n E

Brittle

Ti alloy

Upper shelf

Steel

Ductile

Composite

Lower shelf

High torque High temperatures Lightweight but creep resistant

Test temperature (°C) Copyright © TWI Ltd 2013

Stress Concentration


Stress Concentration at a Hole

• Sudden changes in geometry cause localised areas of high stress.

• The stress concentration at the edge of a hole is 3.

• Imagine flow lines which get close together at stress concentrations.

• Sharper notches concentrate the stress much more.

• The maximum stress is three times the applied stress.

Maximum stress


Modes of Failure

Applied stress


Ductile Fracture • It is the result of overloading.

• Ductile failure.

– Possibly poor design, or elevated temperature such as a fire reducing strength.

• Fatigue failure. • Brittle fracture.

• Evidence of gross yielding or plastic deformation. • The fracture surface is rough and torn. • The surface may show 45° shear lips or have surfaces inclined at 45° to the load direction. Copyright © TWI Ltd 2013


7-2

Ductile Fracture

Fatigue Failure

• Ductile fracture, or plastic collapse, occurs when yielding and deformation precedes failure. • Fracture surface appears torn and fibrous.

• Fatigue failure surface is smooth, flat and bounded by a curve. • Bands or beachmarks may sometimes be seen showing the progress of the crack front from the point of origin. • The surface is 90° to the load. • Final fracture will usually take the form of gross yielding.


Brittle Fracture


Features of a Brittle Fracture

• It is a fast and unstable type of fracture. • The results can be catastrophic.

• There is little or no plastic deformation before failure. • The crack surface may show chevron marks or river lines pointing back to the fracture initiation point. • With impact fracture, the surface is rough but not torn and will usually have a crystalline appearance.



Factors Causing Brittle Fracture

Brittle Fracture

Low toughness

Bang Stress Copyright © TWI Ltd 2013

Flaw Copyright © TWI Ltd 2013

7-3

Brittle Fracture – Low Toughness

Brittle Fracture – Stress and Flaws

• Low temperature. – Ductile to brittle transition in steels at low temperature.

• Residual stress from welding.

• Crystalline structure. – Ferritic materials (carbon steel) show a transition while austenitic materials (stainless steel, aluminium).

• Strain rate.

• Microstructure. – Fine grain size has high toughness. – Martensite or coarse grain HAZ has low toughness. • Material thickness. – Thick material has lower effective toughness than thinner. Copyright © TWI Ltd 2013

• Applied stress from loading. – Higher strain rate more likely to cause britt le fracture.

• Stress concentrations. – Weld toes, change of section, notches.

• Weld defects. – Cracks, lack of fusion. Copyright © TWI Ltd 2013

Reading Fracture Faces Ductile final failure – rough, 45° shear, deformed

• Not usually just one mode of failure. • Look for clues to identify each mode that plays a part. • Beachmarks. – Fatigue.

• River lines. – Brittle fracture.

Fatigue – flat, smooth, beachmarks, curve

• 45° failure planes. – Ductile. Copyright © TWI Ltd 2013

7-4

Use of Aluminium Alloys • Light-weight. • High-strength-to weight ratio.

Design and Construction Design Considerations for Aluminium

Use for its light weight in:


• Trains. • Aircraft. • Cars. • Ships and boats.


Aluminium Compared to Steel


Advantages of Aluminium

• Low density - 1/3 of steel.

• High thermal conductivity.

• Good resistance to corrosion.

• High electrical conductivity.

• Non-magnetic. • Good ductility - use extrusion processes.

• Excellent strength and notch toughness at low temperatures.

• Good machinability.

• Cryogenic applications.


Applications of Aluminium


Welding and Joining Aluminium

• Good corrosion resistance ideal for food and drink industry – foil, packaging, drinks cans. • High electrical conductivity – transmission lines, welding cables.

• Generally good weldability, but. • Fusion welds can suffer porosity, lack of fusion, solidification cracking. • Loss of strength in HAZ region.

• Building industry – roofing, windows, doors, cladding, fittings.

• Very fluid weldpool.

• Wide range of other uses – ski poles, ladders, gas cylinders.

• Adhesive bonding widely used, has no residual stress, but not for hot/humid environments.


• Consider laser welding or FSW.


8-1

Disadvantages of Aluminium

Other Properties of Aluminium

• High cost – 1.5x steel. • Coefficient of thermal expansion: 2x steel.

• Loses strength at high temperatures. • Low strength of pure aluminium.

– Causes distortion after welding.

• High affinity to oxygen. • Corrodes in some circumstances.

• Low Young’s modulus - 1/3 of steel. – Higher risk of fatigue. – Lower stiffness.

• High thermal conductivity. – Risk of lack-of-fusion defects.


Aluminium Alloys


HAZ Softening

Series

Main Alloying Element

Type

1XXX

None (Pure Al)

Non Heat Treatable1

2XXX

Copper

Heat Treatable2

3XXX

Manganese

Non Heat Treatable1

• High temperatures during welding causes grain growth the metal in the HAZ.

4XXX

Silicon

Non Heat Treatable1

• Results in lower strength.

5XXX

Magnesium

Non Heat Treatable1

6XXX

Magnesium & Silicon

Heat Treatable2

7XXX

Zinc & Magnesium

Heat Treatable2

• Partially restored for heat treatable alloys, but unavoidable strength loss is non-heat treatable alloys.

1 - Cold worked 2 - Precipitation hardened Copyright © TWI Ltd 2013


Standards and Specifications • BS 8118: Structural use of aluminium. Code of practice for design. • BS EN 1999: Eurocode 9 - Design of aluminium structures. General structural rules. • AWS D1.2: Structural Welding Code Aluminium.


8-2

Objective • Introduce principles of design for static loading. • Introduce reinforcing bars for use in concrete.

Design and Construction Static Loading TWI Training & Examinations Services (EWS/IWS Diploma)



Steel Frame Structures

Structural Details Trusses

• Majority of new buildings based on steel frame.

• Number of structural members joined together to form triangular units.

• Frame supports weight of the building and contents.

• Transmits loads through axial forces rather than bending.

• Fast and efficient construction method.

• Pylon is an example of a space frame truss. Copyright © TWI Ltd 2013


Structural Details

Structural Details

Fixed steel jacket structure • Tubular legs and cross members welded together at nodal joints.

Various methods exist for linking structural members and strengthening structures.

Stiffeners

Steel reinforcing Nodal joint

Braces on truss



9-1

Combination of Forces

Stress Calculations

Application:

Stress definition:

• Forces in truss members of a bridge – free-body diagram.

Force divided by cross-section area.



F A

 = tensile stress (N/mm2 or MPa)

F = load or axial force (N) All the forces are in equilibrium.

A = cross section area (mm2)

Note: All the forces are not represented in this diagram. Copyright © TWI Ltd 2013


Task

Tensile Test Results

Considering the material stress strain curve in the previous slide, what limit would you put on the allowable stress and therefore on the allowable load?


Elastic Design Method


Elastic Design Method

Elastic design method: • Ensure that stresses in structure do not exceed yield stress (ie elastic deformation). However we cannot design up to yield stress safely due to: • Material defects.

• Use design stress which is a fraction of the yield strength of the parent material. • For critical structures such as pressure vessels this was once set at 1/4 UTS, but later changed to 2/3 yield stress.

• Joint/weld mismatches.

• Relevant codes dictate design stresses.

• Unforeseen loads. • Degradation. Copyright © TWI Ltd 2013


9-2

Factor of Safety

Material Load Carrying Capacity

• Ratio of yield stress (or UTS) to design stress is known as factor of safety (FoS).

FoS 

• Weld metal overmatches parent metal. – Parent strength defines load carrying capacity.

• High strength low alloy steels.

Yield Stress 1 DesignStress

– Weld metal sometimes under matches parent metal.

• Welded joints in aluminium. – The static strength may be reduced by the heat of welding.

• FoS depends on: - Material. - Utilisation. Copyright © TWI Ltd 2013


Types of Forces

Bending Stresses Consider a beam subjected to pure bending (no shear).

Compression

• Compression.

Before bending

• Tension.

Tension

Tension (+)

• Shear.

Shear

M

M

After bending

• Bending.

Compression (-)

Bending



Nodal Joints

Bending Moment A cantilever beam:

Fixed steel jacket structure: Force, F

Fy M

Neutral Axis Longitudinal stresses are zero

• Tubular legs and cross members welded together at nodal joints.

Fx

d

Bending moment:

Reaction forces:

M=Fxd

Fx

Fy

M



9-3

Nodal Joint Welds Nomenclature Crown point

Parts of a Nodal Joint Circular sections:

Brace

Chord

Heel

Toe



Side  = Local dihedral angle ( usually 30-150°)

Saddle point Copyright © TWI Ltd 2013


Types of Nodal Joints

Types of Nodal Joints

Combination of nodal joints

greater than 10° max.10°

max.10°

T-K nodal joint

T-Y nodal joint

Cross nodal joint

T nodal joint

Y nodal joint


K Nodal Joints


Deviations from Concentric Joints Gap Offset

gap Eccentricity Overlap


Through member


9-4

Eccentricity

Parts of a Nodal Joint Box sections:

Heel

Heel

Corner

Corner

Corner

e>0

e=0

Side

Side

Side

Brace Corner

Corner

Corner e<0

Chord Toe

Toe Copyright © TWI Ltd 2013

Hot Spot Stress

Nodal Joints Vs. Nozzles

• Originally developed for tubular joints in offshore platforms. Stress

• Determined by: – Strain gauges. – Finite element analysis.


Notch stress Hot spot stress Structural stress Nominal stress

Nodal joints:

Nozzles:

• Have no hole in the chord at the brace.

• There is a hole in the shell at the nozzle - need compensation for loss in the strength.

• There are large axial and bending loads in braces.

• Are subjected only to pressure.

• No scope for proof loading. • Complex geometry - difficult to inspect.

• Can be proof (pressure) tested. • Simple geometry - easier to inspect.

• Work at ambient temperature. • Complex load history.

• High/low temperatures.



Tension and Compression

Tension and Compression Practical implications:

Practical implications:

A flag pole.

Lifting weight with a crane.

Due to its deadweight, the cross section area increase at the base of the pole.

Compression and bending

Bending

Pure tension Copyright © TWI Ltd 2013


9-5

Static Design

Static Design

Correct design

Poor design

Improved design

Correct design

Improved design for horizontal forces

Poor design Copyright © TWI Ltd 2013

Rebar


Purpose

• The term reinforcing-steel is used to describe the use of steel to reinforce materials, most often concrete.

• In concrete beams the steel bars are embedded in the tension fibres of the beam.

• Concrete is a brittle material which is strong in compression but weak in tension. • This limits the use of concrete in construction and makes it unsuitable for use in many structural members.



Principle

Bar Profile

• Reinforcing bar is prestressed in tension. • Concrete shrinks around the bar when it sets to grip the reinforcement. • Concrete becomes prestressed in compression, hence even the concrete remains in compression even for applied tensile loads.



9-6

DAC

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