TWI Industrial Member Report Summary 637/1998
In the majority of cyclically loaded welded structures, fatigue design is limited by poor strength details such as fillet welded and T butt welds, which normally fail from the weld toe. The poor fatigue strength of such joints is attributable to the conjoint effect of the following factors: the geometrical discontinuity of the weld toe which gives rise to a local concentration of stress, the presence of pre-existing weld flaws at the weld toe, and the presence of large tensile residual stresses in the weld metal and the surrounding HAZ.
Under the influence of these factors, fatigue life of a welded joint is spent almost entirely in crack propagation. The crack initiation phase, which occupies a major part of the life of unwelded components, is insignificant. An important consequence is that the fatigue strength of welded joints is largely independent of the static tensile strength of the parent steel, since steels vary little in terms of their crack propagation characteristics.
A range of post-weld treatments to improve the fatigue strength of welded joints is available. The techniques rely on addressing one or more of the three factors mentioned above. Three such techniques: hammer peening, burr-grinding and TIG dressing, were selected for detailed study to build on the existing knowledge obtained from previous work at TWI and elsewhere.
The work was conducted within a joint industry project sponsored by Amoco (UK) Exploration Co, Conoco (UK) Limited, Svenskt Stäl AB and UK Department of Trade and Industry. Additional funds were made available by TWI through the Core Research Programme which is funded by Industrial Member Companies. The other sponsors have kindly agreed for the project findings to be published.
- Quantify the benefits of applying improvement techniques to joints in high strength steels.
- Optimise TIG dressing procedure to improve fatigue life.
- Examine the benefits of hammer peening, burr grinding and TIG dressing under variable amplitude loading.
- Provide practical specifications and design recommendations for the improvement techniques.