A probabilistic model on crack initiation modes of metallic materials in very high cycle fatigue

Abstract

In the very high cycle regime, the duplex S-N property consisting of S-N curve for surface-initiated fracture and that for interior-initiated fracture was often reported for high strength steels. However, recent studies show us the fact that the surface-initiated fracture can be sometimes observed even at low stress levels in the very high cycle regime. In the case of interior fracture mode, a non-metallic inclusion is usually found at the center of the fish-eye. If we suppose that such inclusions are distributed at random inside the material space, a certain number of inclusions would be located within the thin surface layer of the specimen. In such cases, the fatigue crack can take place within the surface layer giving the surface-initiated fracture even in the very high cycle regime. In the present work, the authors have attempted to construct a probabilistic model on the overall feature of these crack initiation modes in the very high cycle regime under the loading type of rotating bending. Thus, based on the distribution property of the inclusions inside the material, the appearing probability of the surface-initiated fracture and the distribution characteristics of the fatigue strength at N=109 cycles were well explained by the present model.