A phenomenological model of fatigue macrocrack initiation near stress concentrators

Abstract

ABSTRACT Fatigue macrocrack initiation is considered to be a two‐parameter process. It is governed by the local or strain amplitude, and a certain linear parameter of the material. Corresponding parameters have been proposed, i.e. the local stress range Δσ*y and a characteristic distance d *, the prefracture zone size. The formation of this zone is conditioned by a decrease in yield strength within the material’s surface layers, microstructure, loading amplitude, cyclic strain hardening and environment. The value of d * is estimated experimentally by several methods and is assumed to be a certain material constant, independent of both notch and specimen geometry. At the prefracture zone boundary, a major barrier exists that retards the growth of a physically small fatigue crack. The moment when the physically small crack overcomes the prefracture zone boundary is assumed to be a quantitative criterion, a i = d * , for the micro‐ to macrocrack transition. The proposed relationships, Δσ*y versus N i , and d * versus N i , can be used as a basis for the establishment of the materials resistance to macrocrack initiation.