Influence of welding residual stresses on fatigue design of welded joints and components

Residual stresses, as the consequence of several production steps of welded structures, influence the fatigue strength of welded joints and components. In order to determine the relevant residual stress level, the modification of residual stresses by the effect of external loading has to be considered. This modification depends on the material, shape and dimension of the welded structure, on the welding process and the direction of loading stresses respectively on the degree of multiaxial stress state. In the recommendations and codes for fatigue design at the present state of the consideration of welding residual stresses, the basis of assumptions is that most welded components perform independently of mean stresses, because of existing high tensile residual stresses. External loadings can reduce the residual stresses, but subsequently the highest residual stress has the magnitude of the yield limit. For this reason the fatigue strength of welded components is independent of load stress ratio as given in most design codes. A profound analysis of the present knowledge shows a so significantly high residual stress decrease for many welded components and so their fatigue strength depends on mean stress. This fact results from comparable evaluations of significant fatigue test data of small-scale specimens and full-scale structural components - all welded joints with continuous welds and with plate thicknesses of 10-20 mm. For the fatigue strength of the analysed structural com ponents, a mean stress dependence can be assumed, which corresponded to an enhancement factor of 1.2... 1.3 for stress amplitude of the stress ratio R = - 1 in comparison with R = 0. Because the notch effect of welded joints also reduces the mean stress dependence, for the application of this enhancement factor a limitation at welded com ponents with the notch or stress magnification factor of k t < 2.5 is assumed. The mean stress dependence of welded components is a function of their design and loading, which can be qualitatively described within an evaluation model. This evaluation model considers all relevant effects for the combination of residual and loading stresses. It refers above all to the variation of yield behaviour depending on the degree of the multiaxial stress state after the combination of residual and loading stresses. For four typical welded components with different loading situations (evaluation cases), the residual stress decreases and an enhancement factor for the mean stress dependence is represented. This evaluation model allows a practical and economical fatigue design of welded components by the application of IIW-Recommendations or compatible design codes (e.g. Eurocode 3).