The effect of vacuum-like environment inside sub-surface fatigue crack on the formation of ODA fracture surface in high strength steel

Abstract

Very high cycle fatigue (VHCF) of high strength steel has become an important issue for mechanical engineers in recent years. In VHCF regime over 107 cycles, fatigue crack initiates not from surface but from sub-surface of materials. The sub-surface fractures even occur in a lower stress than surface-originating fractures; therefore, to clarify its mechanism is strongly needed for the safety use of high strength steel. In sub-surface fractures, a typical fracture surface with a fine concavo-convex pattern called “ODA” was discovered. To reveal the formation mechanism of ODA is regarded as a key point for sub-surface crack growth because it is never observed in surface-originating fractures. This study focuses on a special environment inside sub-surface crack. The sub-surface crack is not exposed to atmosphere and the adsorption of gaseous molecules on fresh-surface at crack tip seems to be negligible. With this in mind, fatigue crack growth tests in high vacuum were conducted to simulate sub-surface crack propagation, and fracture features were thoroughly investigated by SEM analyses. As a result, it was clarified that high vacuum is closely similar to the environment inside sub-surface crack, and is a necessary condition to form ODA in VHCF.