Fatigue behavior of welded joints under combined multiaxial variable amplitude loading

: Sonsino, C.M.; Küppers, M.

Abstract
Flange-tube as well as tube-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(exp 3) to 5 x 10(exp 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 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 hypothesis of the effective equivalent stress leads to fairly well predictions. Herefore, the knowledge of local strains or stresses is necessary. They are determined by boundary-element analyses in dependency of 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.