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Fatigue testing - Part 2

Job Knowledge

Part 1
Part 3

The article in the September/October issue of Connect established some basic facts about fatigue and the statement was made that a welded joint exhibited no clearly established fatigue limit as in an unwelded component. In this article we will be looking at some of the reasons for this behaviour.

It should be mentioned that, in service, few structures experience purely static loads and that most will be subjected to some fluctuations in applied stresses and may therefore be regarded as being fatigue loaded. Motorway gantries, for example, are buffeted by the slipstream from large lorries and offshore oil rigs by wave action. Process pressure vessels will experience pressure fluctuations and may also be thermally cycled.

If these loads are not accounted for in the design, fatigue failure may occur in as few as a couple of tens of cycles or several million and the result may be catastrophic when it does.

Fatigue failures can occur in both welded and unwelded components, the failure usually initiating at any changes in cross section - a machined groove, a ring machined onto a bar or at a weld. The sharper the notch the greater will be its effect on fatigue life.

The effect of a change in section is illustrated in Fig.1, where it can be seen that the stress is locally raised at the weld toe. The illustration shows a bead-on-plate run but a full penetration weld will show the same behaviour.

Fig.1. Stress concentrating effect of a change in thickness
Fig.1. Stress concentrating effect of a change in thickness

In addition, misalignment and/or distortion of the joint will cause the applied stress to be further increased, perhaps by introducing bending in the component, further reducing the expected fatigue life. A poorly shaped weld cap with a sharp transition between the weld and the parent metal will also have an adverse effect on fatigue performance.

In addition to these geometrical features affecting fatigue life there is also the small intrusion at the weld toe, mentioned in the last article and illustrated in Fig.2. In an unwelded component the bulk of the fatigue life is spent in initiating the fatigue crack with a smaller proportion spent in the crack propagating through the structure. In a welded component the bulk of the fatigue life is spent in propagating a crack. The consequences of this difference in behaviour are illustrated in Fig.3.

Fig.2. Weld toe intrusion
Fig.2. Weld toe intrusion
Fig.3. Effect of stress concentration on fatigue life
Fig.3. Effect of stress concentration on fatigue life

This shows that this small intrusion reduces the fatigue life of a fillet welded joint by a factor of perhaps 10 compared with that of an unwelded item and some eight times that of a sample with a machined hole. The other consequence is that fatigue cracks in welded joints almost always initiate at the toe of a weld, either face or root.

It may be thought that the use of a higher strength material will be of benefit in increasing fatigue life. The rate of crack propagation, however, is determined by Young's Modulus - a measure of the elastic behaviour of the metal - and not simply by tensile strength.

Alloying or heat treatment to increase the strength of a metal has very little effect on Young's Modulus and therefore very little effect on crack propagation rates. Since the bulk of a welded component's life is spent in propagating a crack, strength has little or no influence on the fatigue life of a welded item. There is thus no benefit to be gained by using high strength alloys if the design is fatigue limited. This is illustrated in Fig.4 which shows the benefits of increasing the ultimate tensile strength of a steel if the component is unwelded or only machined but how little effect this has on the life of a welded item.

Fig.4. Effect of increase in tensile strength on fatigue life
Fig.4. Effect of increase in tensile strength on fatigue life

One additional feature in welded joints that set them apart from unwelded or machined items is the presence of residual tensile stress.

In a welded component there will be stresses introduced into the structure by, for example, assembly stress. These stresses are long range reaction stresses and from a fatigue point of view have little effect on fatigue life.

Of far greater significance with respect to fatigue are the short range stresses introduced into the structure by the expansion and contraction of material close to and within the welded joint. Whilst the actual level of residual stress will be affected by such factors as tensile strength, joint type and size and by run size and sequence, the peak residual stress may be regarded as being of yield point magnitude. The implications of this are that it is the stress range that determines fatigue life and not the magnitude of the nominal applied stress.

Even if the applied stress range is wholly compressive and there is apparently no fluctuating tensile stress to cause a crack to form and grow, the effect of welding residual stress is to make the structure susceptible to fatigue failure. This is illustrated in Fig.5, where it can be seen that, irrespective of the applied stress, the effective stress range is up to the level of residual stress at the welded joint.

Fig.5. Effect of residual stress on stress range
Fig.5. Effect of residual stress on stress range
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It would seem reasonable, therefore, that a post-weld stress relief treatment would be of benefit to the fatigue life by reducing the residual stresses to low levels. This is only true, however, where the applied stress range is partly or wholly compressive. If the applied stress range is all tensile, research has shown that as-welded and stress relieved components have almost identical fatigue performances with only a marginal improvement in the stress relieved joints.

This is the result of the bulk of the fatigue life of a welded joint being spent in crack propagation where propagation rates are only marginally affected by mean stress. It may be difficult therefore to justify the cost of stress relief if the only criterion is that of improving fatigue life.

The methods of determining fatigue performance of welded joints, as detailed in BS 7608, and how fatigue performance can be improved will be dealt with in the next Connect article.

This article was written by Gene Mathers.