Evaluation of multiaxial low cycle fatigue life under non-proportional loading considering effect of mean strain (Loading simulated thermal fatigue due to temperature fluctuation in junction piping)

Abstract

In a pressure vessel, a heat exchanger and a junction piping where cold and hot water are mixed, temperature fluctuation yields multiaxial stress and random amplitude loading conditions. In most cases, the loading becomes non-proportional loading in which directions of principal stress and strain are changed in a cycle. Fatigue lives under the non-proportional loading are reduced depending on non-proportionality of loading path. In this paper, cyclic deformation under multiaxial loading simulated thermal fatigue is discussed based on the test results and a method of fatigue life evaluation under complex multiaxial random loading is presented. T-shaped loading, in which reversed torsion loading with static tension (tension T-shape loading) or compression (compression T-shape loading) followed push-pull in a cycle, is used as the simulated loading in the junction piping. Fatigue lives under T-shaped loading depend on the number of the reversed torsion loading. The fatigue life is increased due to increase in the number of the reversed torsion loading which leads to relaxation of loading non-proportionality. The fatigue lives under compression T-shape loading is shorter than that under tension T-shape loading. The reason is effect of mean stress, which the mean stress in push-pull under compression T-shape loading is shifted to tension due to static compression stress relaxation by reversed torsion loading. Modified IS-method, which takes into account of mean stress by using centroid of loading path, is proposed to represent the reduction in non-proportionality and the stress relaxation in T-shape loading.