Subsurface crack initiation and propagation mechanism in high-strength steel in a very high cycle fatigue regime

The mechanism of subsurface crack initiation and propagation in high strength steel in a very high-cycle fatigue region was studied by a computational simulation using a fracture surface topographic analysis (FRASTA) method for specimens of a high carbon chromium bearing steel with data obtained from rotary bending fatigue testing in air. Distinctive features of the fracture surface formed in the vicinity of a non-metallic inclusion at the fracture origin inside the fish-eye zone, the GBF area, was observed in detail using a scanning probe microscope and a three-dimensional SEM for comparing the microstructures of the materials. The GBF area, in which a rich carbon distribution was detected by electron probe microanalysis, revealed a rough granular morphology compared with the area inside the fish eye. It was clearly simulated by the FRASTA method in which during the fatigue process multiple microcracks are initiated dispersively by decohesion of spherical carbide from the matrix around a non-metallic inclusion, and coalesce with each other into the GBF area. In the study, the mechanism for the formation of the GBF area was proposed as the ‘dispersive decohesion of spherical carbide’ model.