Local fatigue assessment of welded and high frequency mechanical impact treated joints

In general, the fatigue life of welded steel structures is independent of the base material strength. In the case of high-strength steels, however, a significant improvement in the fatigue behaviour can be achieved through post treatment processes. This thesis deals with the effect of high frequency mechanical impact treatment (HFMI) on the fatigue behaviour involving a wide range of steels, starting from mild construction steel (S355) up to high-strength steel (S960). The experiments cover fatigue tests on five millimetres, thin-walled specimens based on butt welds, T-joints, and longitudinal attachments. The fatigue assessment is performed in accordance to the nominal and the notch stress approach, taking the HFMI condition into account. The applicability of a strength magnification adjustment, considering yield strength for HFMI treated joints, is depicted in the case of nominal stress assessment. Finally, a modification of the existing IIW-recommended notch stress approach to consider the stress concentration factor and the base material yield strength on the local fatigue behaviour of HFMI treated welds is introduced. By evaluation of extensive fatigue data sets, a HFMI local stress master S/N-curve is assessed. This approach involves a material strength related base point, a material strength and notch stress concentration factor dependent slope, and a fixed transition knee point up to the endurance limit. Applicability of this new HFMI local stress approach is proven by fatigue assessment of over three hundred HFMI post treated specimens. It is shown that this method is capable to unify the examined HFMI fatigue test data into a local master S/N-curve which implies both material and joint type dependency.