Application of the critical plane approach to the torsional fatigue assessment of welds considering the effect of residual stresses
This study aims to investigate how the welding residual stresses affect the fatigue life of steel tubular joints. A single pass weld with filler material is created on the tubular specimens made of structural steel S355J2H. At first, welding residual stresses are calculated numerically by means of FE simulations. The validity of the results is verified experimentally by means of diffraction measurement techniques. Then, an uncoupled damage model based on the critical plane approach is utilized in order to calculate the fatigue life and the initial crack orientation. The as-welded specimens were subjected to cyclic torsional loading with constant stress amplitudes. The Fatemi-Socie (FS) damage parameter is used which is interpreted as the cyclic shear strain damage modified by the normal stress to include the crack closure effect. The stable portion of welding residual stresses is accounted for by the normal stress term of the model. It was observed that despite the common understandings about the critical weld notches, the crack initiation sites under torsional loading have been shifted from the weld toe and from the surface to the heat affected zone and beneath the surface where high tensile residual stresses exist. Then the initial cracks propagated toward the surface whose orientations were influenced by the presence of compressive residual stresses on the surface. It was generally found that the Fatemi-Socie parameter is capable of torsional fatigue damage assessment of welded tubular specimens which contain residual stresses since this critical plane parameter is based on the maximum shear strain plane.