Fatigue Crack Initiation by Cyclic Slip Irreversibilities in High-Cycle Fatigue

Abstract

Dislocation mechanisms of fatigue crack initiation in high-cycle fatigue are formulated, with special consideration given to those material properties which determine the cyclic slip mode. The models developed are related to pertinent experimental observations, referring mainly to copper mono- and polycrystals fatigued at room temperature. In particular, the following topics are considered: (1) the origin of cyclic slip irreversibilities in the bulk and near the surface, (2) computer simulations of surface roughening by random irreversible slip processes in planar- and wavy-slip materials, and (3) fatigue crack initiation by cyclic strain localization in persistent slip bands (PSBs) in mono- and polycrystalline wavy-slip materials. The evolution of the surface profile at emerging PSBs is described by a new semiquantitative model which is compared with other models. The model distinguishes between the rather rapid formation of extrusions and the more gradual development of surface roughness. It is shown that specific predictions regarding differences between PSB surface-profiles in mono- and polycrystals are borne out fully by the observations. In the case of copper polycrystals, the experiments show that PSBs are not only responsible for slip-band cracking in PSBs but also induce intergranular cracks. A model is proposed according to which cracks initiate as a result of the piling-up of PSB-matrix interface dislocations against the grain boundaries. Furthermore, the supporting role of environmental interaction and/or deformation-induced diffusion and grain boundary sliding is discussed.