Numerical analysis of residual stresses and crack closure during cyclic loading of a longitudinal gusset

The fatigue strength of welded joints is determined by crack initiation and propagation, the latter forming in several cases the major part of the fatigue life. Failure is usually assumed when the fatigue crack has penetrated the adjacent plate or as specimen fracture in case of fatigue tests. Crack propagation is influenced by crack closure effects, slowing down crack propagation mainly in case of load cycles partly in compression. However, it is well-known that tensile residual stresses occurring particularly in welded joints can decrease crack closure and lead to fatigue behaviour independent of the applied stress ratio, i. e. mean stress. This has been observed for example with welded longitudinal gussets. On the other hand, recent measurements have shown that tensile residual stresses at the weld toe are smaller than further away and that they are relaxed or even become compressive after the first load cycle. Insofar, the role of residual stresses is still unclear. For this reason, numerical investigations have been performed in addition to fatigue tests to clarify the matter further. After describing a numerical model to investigate the crack closure behaviour by the example of a center crack in a plate originally studied by Newman, the crack closure behaviour of a semi-elliptical crack in front of a longitudinal steel gusset is analysed for depths between 5 and 50% of the plate thickness. In addition to the stress-relieved state, also welding-induced residual stresses were generated with a simplified model, calibrated by measurements, and used for the analysis of crack closure. It is shown that the residual stresses strongly affect the crack closure although these are compressive at the weld toe after the first load cycle. The simulations are performed for different load levels and stress ratios.