MULTIAXIAL FATIGUE OF WELDED-JOINTS UNDER IN-PHASE AND OUT-OF-PHASE LOCAL STRAINS AND STRESSES

Welded cruciform-type specimens from fine-grained steel StE 290 and flange-tube as well as tube-tube joints from fine-grained steel StE 460 with unmachined and machined welds were investigated under biaxial constant-amplitude loading in the range 10(3) to 5 x 10(6) cycles to crack initiation and breakthrough respectively. In order not to interfere with residual stresses they were relieved by 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 introduced hypotheses of the effective equivalent stress leads to fairly good predictions. For this, knowledge of local strains or stresses is necessary. They are determined by boundary-element analyses dependent on weld geometry. This hypothesis considers the 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 are included.