Relaxation of welding residual stresses – Part II: under cyclic loading

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In this paper the variation of the initial residual stresses at the surface of butt–welded specimens made out of S690QL under cyclic loading was investigated. In low cycle fatigue and within the finite life, relaxation takes place during the very first loadings. The major portion of the relaxation occurs in the first half of the first loading cycle and could be treated as relaxation under quasi–static loading. The von Mises failure criterion with the local yield strength as the material resistance against plastic deformation is able to describe this behaviour. In high cycle fatigue loading, with or without mean stresses, except for a small reduction at the beginning, no considerable residual stress relaxation as a function of number of cycles was observed. That is, in the range of high cycle fatigue, high tensile residual stresses in butt welds with low stress concentration at the weld toe, accompany the weld until the failure initiation. Hereafter, the relaxation because of the crack initiation and propagation begins.