Investigation of the effect of grain clusters on fatigue crack initiation in polycrystals

Fatigue crack initiation in ductile alloys like austenitic stainless steels is mainly due to the occurrence of localized deformation in persistent slip bands (PSB). The presence of PSB is classically related to the orientation of the surface grains. In fact, the local fields in a grain does not depend on the local orientation only. The aim of the present paper is to investigate the consequences of this observation, and to propose an analysis, where the neighborhood of the grain also plays a significant role. The study is made on a 316 stainless steel. Finite element computations using a crystal plasticity model are performed to simulate an aggregate submitted to a cyclic tension–compression loading. Various configurations of grain orientations (“clusters”) are studied at the free surface of the aggregate. A statistical analysis of the results is carried out to extract significant information concerning the local strain and stress fields, including the most critical arrangements of grain orientations. The introduction of local fields in classical fatigue life prediction models provides an explanation of the experimental scatter.