What will have become obvious from the previous two articles on fatigue is that a welded joint behaves in a radically different way from an unwelded item, even if this item contains a significant stress raiser.
The last article, number 79, made the statement that a welded joint exhibits no clearly defined fatigue limit, the limit varying dependent upon the joint type and weld quality. It is vitally important to understand this if fatigue analysis of welded joints is to be carried out.
As mentioned earlier, rules for the design of components subject to fatigue loading were produced by TWI and these were incorporated into the design rules in BS 5400, the British bridge design code. These rules were later adopted by the offshore industry for offshore structures and adaptations of these rules now appear in many other specifications such as BS PD 5500 Unfired pressure vessels and BS 8118 Structural use of aluminium.
The basis of all the rules is a system whereby various joint designs are assigned a 'classification' related to the joint's fatigue performance. Fig.1 is an example of how this classification has been formalised in BS 7608 - the same or similar methods will be found in other application standards.
In BS 7608 each joint type is assigned a classification letter. For example, a plate butt weld with cap and root ground flush is class 'C', an undressed plate butt weld class 'D' and a fillet weld class 'F' ( Fig.2).
For each classification a fatigue curve has been developed and from these curves the design life can be predicted. This is obviously an over-simplification of what can be a very complicated task -the forces acting on a joint arising from changes in temperature, changes in internal or external pressure, vibration, externally applied fluctuating loads etc can be complex and difficult to determine.
Whilst the joint design has a major effect on design life and is the basis for calculating service performance, the weld quality also has a decisive effect - any fatigue analysis assumes that the welds are of an acceptable quality and comply with the inspection acceptance standards. However, in practice it is not always possible to guarantee a 'perfect' weld and cracks, lack of fusion, slag entrapment and other planar defects may be present, reducing the fatigue life, perhaps catastrophically.
Other less obvious features will also have an adverse effect. Excessive cap height or a poorly shaped weld bead will raise the stress locally and reduce the design life; misalignment may cause local bending with a similar effect. Good welding practices, adherence to approved procedures and competent and experienced staff will all help in mitigating these problems.
In some applications an as-welded joint will not have a sufficient design life and some method of improving the fatigue performance needs to be found. There are a number of options available. The first and perhaps simplest is to move the weld from the area of highest stress range, the next is to thicken up the component or increase the weld size. Note that, as mentioned in the earlier article, using a higher strength alloy will not improve the fatigue life.
Local spot heating to induce compressive stresses at the weld toes will also help, although this needs very accurate positioning of the heated area and very careful control of the temperature if an improvement is to be seen and the strength of the metal is not to be affected. For these reasons, spot heating for fatigue improvements has been virtually discontinued.
Hammer peening with a round nosed tool or needle gun peening gives very good results although the noise produced may prevent their use. Shot peening can also be used to introduce compressive stresses at weld toes with equally good results. Compressive stresses can be induced in a component by over stressing - a pressure test of a pressure vessel is a good example of this - where local plastic deformation at stress raisers induces a compressive stress when the load is released. This technique needs to be approached with some care as it may cause permanent deformation and/or any defects to extend in an unstable manner resulting in failure.
Although the next techniques described are not as beneficial as hammer peening of the weld toes they have the advantage of being more consistent and easier to control. The techniques rely upon dressing the weld toes to improve the shape and remove the intrusion mentioned in article 79. The dressing may be carried out using a TIG or plasma-TIG torch which melts the region of the weld toe, providing a smooth blend between the weld face and the parent metal.
Alternatively the toe may be dressed by the careful use of a disc grinder but for best results the toe should be machined with a fine rotary burr as shown in Fig.3 and 4. Great care needs to be exercised to ensure that the operator does not remove too much metal and reduce the component below its minimum design thickness and that the machining marks are parallel to the axis of the main stress. Ideally the dressing should remove no more than 0.5mm depth of material, sufficient to give a smooth blend and remove the toe intrusion. The results of these improvement techniques are summarised in Fig.5.
Whilst fatigue has resulted in some catastrophic and unexpected failures, the improvements in design life calculation methods, particularly the use of powerful software packages allowing detailed finite element analyses to be performed, has enabled engineers to approach the design of fatigue limited structures with far more confidence. This still means, however, that the designer has to recognise the effect of welds in the structure and must consider all possible sources of loading and ALL welds, even non-load carrying attachments that may be thought to be unimportant to service performance.
This article was written by Gene Mathers.