A phenomenological model of fatigue macrocrack initiation near stress concentrators

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, ai = d *, for the micro‐ to macrocrack transition. The proposed relationships, Δσ*y versus Ni , and d * versus Ni , can be used as a basis for the establishment of the materials resistance to macrocrack initiation.

Fatigue Properties of Thermally Deformed Alloys of the TI–SI System

“…2) depends on the time of the stages of initiation and growth of a system of microstructurally short cracks and stages of growth of a dominant physically small macrocrack and main macrocrack [16−18]. The initiation and The region of growth of fatigue microcracks within the process zone was recorded in microfractograms of fatigue fractures of specimens where the system of microcracks propagates predominantly by the shear mechanism, and the completion of the formation of the macrocrack, which corresponds to the moment a i ≥ d * ≥ a 0 , is accompanied by the growth of the dominant crack by the mechanism of opening-mode fracture [18,20].…”

“…The total time of the stages of initiation and growth of microcracks in the process zone determines the period N i of initiation of a macrocrack with a length a i = d * , where d * is the size of the fatigue process zone. In this case, the structural-mechanical parameter d * also determines the maximum size a 0 ≤ d * of nonpropagating fatigue microcracks[17,18].…”

On some criterial problems of fatigue crack initiation and growth in polycrystals

“…By the main crack propagation direction we mean the crack path on a scale larger than one or two grains. It should be mentioned that in addition to the grain size there are other criteria for distinguishing between fatigue cracks [20,21].…”

The effects of notch size and material microstructure on the notch sensitivity factor for notched components

“…Attempts have been made by different researchers to succinctly define the fatigue damage process zone [51][52][53]. In general, the fatigue damage process zone is a volume comprising of pre-fracture zones in the vicinity of the notch, inclusion or any other stress raisers identified in the material.…”