Hicks Welded Joint Design

3. Auflage 1999
ISBN: 978-1-85573-898-0
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 160 Seiten

Based on the European Welding Engineer (EWF) syllabus Part 3 - Construction and Design, this book provides a clear, highly illustrated and concise explanation of how welded joints and structures are designed and of the constraints which welding may impose on the design. It is therefore of value both to the welding engineer and the design engineerMany engineers coming into the profession of welding engineering lack a background in design and construction of welded structures and plant. This book has been written with such engineers very much in mind.The safe performance of a structure relies on materials and methods of fabrication which can respond to the explicit or implicit design requirements. It is essential that the welding engineer has the opportunity of making his specialist input to the design process, and an understanding of the basis of the design will help that contribution to be most effective. It is also important that the practising design engineer acquires a basic knowledge of the relevant aspects of welding to be able to execute satisfactory designs and, equally important, to know when to seek the input of a qualified welding engineer.Designed for both students and practising engineers in welding and design, the book will also be of great value to civil, structural, mechanical and plant engineers. There is also much that will interest test houses, welding equipment and consumable manufacturers, classification societies and steel companies.

Autoren/Hrsg.

Hicks, J.

Inhaltsverzeichnis

1;Front Cover;1
2;Welded Joint Design;4
3;Copyright Page;5
4;Table of Contents;6
5;Foreword;10
6;Preface;12
7;Chapter 1. Fundamentals of the strength of materials;15
7.1;Normal stress;15
7.2;Shear stress;17
7.3;Shear strain;18
7.4;Three dimensional stress;21
7.5;Stress concentrations;23
7.6;Material failure;25
7.7;Summary;29
7.8;Problems;30
8;Chapter 2. Stresses in some common types of structures;31
8.1;Frames;31
8.2;Pressurised shells;33
8.3;Tubular structures;36
8.4;Effect of deviations from design shape;38
8.5;Summary;40
8.6;Problems;41
9;Chapter 3. Elementary theories of bending and torsion;42
9.1;Bending;42
9.2;Calculating / for other sections;50
9.3;Shear stresses in beams;51
9.4;Torsion;54
9.5;Vibration;55
9.6;Summary;58
10;Chapter 4. Basis of design of welded structures;59
10.1;Allowable stress design;59
10.2;Limit state design;61
10.3;Plastic design;62
10.4;Summary;65
11;Chapter 5. Weld design;66
11.1;Joint types;66
11.2;Weld types;67
11.3;Weld preparations;68
11.4;Partial penetration butt welds;77
11.5;The edge weld;80
11.6;Spot, plug and slot welds;81
11.7;Fabrication tolerances;83
11.8;Residual stresses;86
11.9;Access for welding;87
11.10;Welding information on drawings;90
11.11;Summary;93
12;Chapter 6. Calculating weld size;94
12.1;Butt welds;94
12.2;Fillet welds;96
12.3;Calculating the strength of welded joints;100
12.4;Methods of measuring stress;102
12.5;Summary;103
12.6;Problems;104
13;Chapter 7. Fatigue cracking;105
13.1;Load histories;105
13.2;Weld features;106
13.3;Fatigue crack growth;106
13.4;The practical design approach;109
13.5;Complex load histories;113
13.6;Improving on the performance of the as-welded joint;116
13.7;Using design data;119
13.8;Summary;128
13.9;Problems;129
14;Chapter 8. Brittle fracture;130
14.1;Conventional approaches to design against brittle fracture;130
14.2;Fracture toughness testing and specification;132
14.3;Fracture mechanics and other tests;134
14.4;Summary;136
15;Chapter 9. Assessment of structural integrity;137
15.1;Defect assessment;137
15.2;Fracture assessment;140
15.3;Fatigue assessment;142
15.4;Summary;143
16;Answers to problems;144
17;Bibliography;149
18;Index;153

Leseproben

1 Fundamentals of the strength of materials
Normal stress
Materials change in length when they are put under normal (or direct) stress which can be either tensile or compressive stress. An elastic material is one in which the change in length is proportional to the stress developed in it and also one in which the material will return to its original length after the stress is removed. Many metals behave in an elastic manner up to a certain level of stress beyond which they will behave in a non-linear manner. The most commonly used weldable structural materials in use are the carbon or carbon-manganese steels and these are the materials generally being considered in this book unless it is stated otherwise. This is a very large family of steels available in a multiplicity of compositions, mechanical properties, grades, qualities, etc, but in some industries these steels may just be called structural steel, perhaps divided into mild steel and high yield steel or, in industries which use only one type, just steel. Where they are introduced in this book other structural materials such as the higher alloy steels, austenitic and ferritic stainless steels and aluminium alloys are referred to specifically. The magnitude of the stress set up in a wire or a bar under a load, P, is P divided by the cross section area, A, of the bar or wire, see Fig. 1.1. 1.1 Bar under axial load. If the street is s =PA   [1.1] In the preferred units of the international system (SI) the load will bemeasured in Newtons (N) and the cross sectional area in square millimetres (mm2). So the stress is conventionally measured or calculated in units of N/mm2. If we measure the length of a bar of structural steel under an increasing tensile load and plot the stress against strain (strain = increase in length/original length) we produce a chart such as in Fig. 1.2. 1.2 Typical stress-strain curve for steel. The steel shows elastic behaviour up to a certain stress but then begins to extend without the stress having to be significantly increased. This behaviour is called yielding, or plastic deformation, and the stress at which it commences is the yield stress. This yielding does not continue indefinitely and the steel begins to offer more resistance to extension until it reaches its tensile strength at which point it fractures. Once the steel has yielded it will not recover the plastic strain but will recover the elastic strain. This yielding property gives mild steel the ability to be made into products by the cold bending or forming of bar, wire, sheet and plate. The slope of the elastic part of the stress-strain curve is called the elastic modulus or Young’s modulus, s/e, and has the units N/mm2, typically a figure of 205000 N/mm2 is used for structural steels. (Note that strain being a length divided by a length has no dimensions, it is just a ratio, a number.) In the tensile test it is the load and extension which are measured and not the strain and stress. Beyond the yield point the specimen cross section is reduced by necking and the load starts to reduce although the actual stress may be increasing. The tensile strength is then an imprecise measurement because it represents the load divided by the original cross section of the test specimen which after much yielding is reduced; the calculated tensile strength is therefore an artificial figure. In structural alloys other than the carbon steels there is no sudden departure from elastic behaviour with increasing stress; a typical stress-strain curve is as shown in Fig. 1.3. As examples, austenitic stainless steels and aluminium alloys display this type of non-linear change of strain with stress after the elastic limit is reached. To define a useable limit to the stress, the concept of the proof stress is used. This is a stress at which the material had undergone a certain permanent set, commonly 0.1% or 0.2% strain. The elastic modulus is again the ratio of stress to strain. Beyond the elastic limit the tangent to the stress-strain curve can be used to postulate a tangent modulus for analysing the post elastic behaviour of structures made of these materials. 1.3 Typical stress-strain curve for austenitic steels and aluminium alloys. These stress-strain curves, which can also be produced for compressive loading, show that a material may be used quite safely after it has been subjected to plastic or non-linear strain. Indeed this is the basis of work hardening which is used to produce materials of a higher yield or proof strength than the originally produced material. It should be noted that although the yield and proof strengths will be raised the tensile strength will not have been increased; it is important to be aware that the effect of the work hardening can be reduced or removed altogether by heating, including the heating involved in welding. Other means of enhancing the strength of engineering alloys involve metallurgical changes induced by thermal treatment or combinations of thermal and mechanical treatment and these strengthening effects can also be reversed or at least diminished by the heating from welding. Shear stress
The stresses set up in a tensile test are stresses which are in a direction normal to the cross section; they tend to separate or squash the material. Shear stress resists the tendency of a material to slide over itself as if in layers. Figure 1.4 is a simple example of shear, a pin in a tow bar. The load tries to slide the section of the pin across itself as in Fig. 1.5. 1.4 Pin in double shear. 1.5 Shearing action. Shear stress has the same units as normal stress, N/mm2. In this example we calculate the shear stress as the load divided by twice the cross sectional area of the pin. (Twice because it is in double shear.) =P2A   [1.2] This assumes that the stress is uniformly distributed over the section which is not true but good enough for most applications. Shear strain
We saw that normal stress does not change the shape of the object. However, shear strain does involve a change of shape. If we take a square element and apply shear to it, it changes its shape into a rhombus, see Fig. 1.6. 1.6 Shear strain. The shear strain is the angle, ?, through which a side of the square moves and the relationship between shear strain and stress is shown in Chapter 3 in discussing the torsion of thin walled tubes. In the simple example of a bar in tension a shear stress will be developed in any plane at an angle to the normal plane. For example, if we make an oblique cut in a bar under tension one part will tend to slide over the other as well as move away from it, see Fig. 1.7. 1.7 Resolved direct and shear stresses in a bar in tension. We can resolve the stresses across the cut into the local normal (s) and shear (t) stresses respectively. There is a simple way of calculating the shear and normal stresses using a diagram called Mohr’s circle which plots the shear and normal stresses on a circle. The size and position of the circle on the diagram are fixed by the known stresses. In this simple case of a bar in tension we know that along the bar is a tensile stress and at right angles to this there is no (applied) stress. For this simple case of a bar in tension, a is the angle from the longitudinal axis of the bar. When a is 0° the normal stress is at a maximum (P/A) and when a is 90°, 2a is 180° and the normal stress is zero. These points then fix the diameter and centre of the Mohr’s circle for this particular stress system, see Fig. 1.8. We can then see that when 2a is 90° the shear stress is at a maximum in two directions, i.e. at ± 45° to the axis of the bar. We can measure off the maximum shear stress from the diagram. Clearly in this case it will be equal to s/2. The stresses in the directions in which the shear stress is zero are called the principal stresses. 1.8 Mohr’s circle for direct stress. In a membrane such as a cylindrical pressure vessel shell normal stresses are applied in two directions (see Chapter 2). The hoop stress, sh, is twice the longitudinal stress, s1, and so the Mohr’s circle is as shown in Fig. 1.9. 1.9 Mohr’s circle for hoop and longitudinal stress in a cylinder. It can be seen that the maximum shear stress occurs at 45° to the principal planes max=sh-s12   [1.3] Note that for a sphere the membrane stresses are equal in all directions and so the plot reduces to one point (equivalent to s1) and there is no shear stress anywhere. The normal stress axis can be extended to the left of the shear stress axis to allow us to plot a compressive stress. Figure 1.10 is for material with equal magnitudes of tension and compression in orthogonal directions. 1.10 Mohr’s circle for equal tension and...