Relationship between Fatigue Crack Propagation Originating at Inclusion and Fracture-Mode Transition of High-Strength Steel.

Abstract

A study has been conducted on the relationship between the initiation and propagation behavior of a fatigue crack originating at the non metallic inclusion and the fracture-mode transition from a surface to subsurface fracture known as fish eye fracture in high-strength steels. Special attention is paid to the propagation of microcracks originating at each inclusion both on the specimen surface and in the subsurface. The results obtained are summarized as follows. (1) Metallographic observation using SEM revealed that the initiation of microcracks from inclusions occur at numerous inclusions both on the specimen surface and in the subsurface. (2) The initiation of these microcracks from inclusions on the specimen surface occurs at a very early stage in the fatigue cycles. (3)The propagation of a crack initiating from an inclusion on the specimen surface stops at an early stage in the fatigue cycles before failure occurs as a fish eye fracture. (4) These phenomena suggest that there exists a fatigue limit in the surface fracture mode, and that the mechanism of fracture mode transition is associated with the difference in crack propagation behavior of microcracks from inclusions on the specimen surface and in the subsurface.