Multiaxial fatigue of welded joints under constant and variable amplitude loadings

Flange-tube joints from fine grained steel StE 460 with unmachined welds were investigated under biaxial constant and variable amplitude loading (bending and torsion) in the range of 10(3) to 5 x 10(6) cycles to crack initiation and break-through, respectively. In order not to interfere with residual stresses they were relieved by a heat treatment. In-phase loading can be treated fairly well using the conventional hypotheses (von Mises or Tresca) on the basis of nominal, structural or local strains or stresses. But the influence of out-of-phase loading on fatigue life is severely overestimated if conventional hypotheses are used. However, the hypothesis of the effective equivalent stress that is introduced leads to fairly good predictions for constant as well as for random variable amplitude loads. Therefore, the knowledge of local strains or stresses is necessary. They are determined by boundary element analyses that are dependent on weld geometry. This hypothesis considers the fatigue-life-reducing influence of out-of-phase loading by taking into account the interaction of local shear stresses acting in different surface planes of the material. Further, size effects resulting from weld geometry and loading mode were included. Damage accumulation under a Gaussian spectrum can be assessed for in- and out-of-phase combined bending and torsion using an allowable damage sum of 0.35.