Proportionally and non-proportionally perturbed fatigue of stainless steel

Computational fluid dynamics and finite element based simulation of the cyclic thermal shock loading caused by turbulent mixing of water of different temperatures in the primary cooling system of a nuclear power plant, suggest a specific kind of non-proportional multi-axial fatigue loading exists on the inner surface cooling structures. Stress controlled experiments mimicking these loading conditions were designed and run on an axial-torsional fatigue testing system. The cyclic loading observed in the finite element simulations consists of the superposition of proportional loading and a second, non-proportionality inducing component, the latter with a substantially lower amplitude and running at a higher cycling frequency. The amplitude of the non-proportionality inducing loading component is of a magnitude so small that the fatigue loading criteria, typically used in engineering standards for nuclear power plant safety, do not recognize the difference with the reference loading without this component. However, first experimental results show that the endurance limit of stainless steel of grade 316L is reduced by this additional loading, which we name non-proportional fatigue noise. The fact that the endurance limit is unexpectedly lowered is a non-conservative safety issue that challenges the currently existing criteria used to estimate the fatigue damage for this type of loading.