Role of persistent slip bands in fatigue

Perhaps the most thoroughly studied feature of cyclic plasticity has been persistent slip bands (PSBs) formed in some metals during cyclic loading. The aim of this paper is to delimit the conditions for the occurrence of the PSBs and to discuss their role in the fatigue process in all its stages, that is in cyclic hardening resulting in a saturated stress-strain response, in initiation of fatigue microcracks and in propagation of fatigue cracks. The PSBs are zones of high cyclic slip activity. Therefore the cyclic stress-strain response depends on their volume fraction and on the ratio of cyclic slip activities within the PSBs to those outside the PSB. The cyclic plastic deformation within the PSBs leads to the formation of surface extrusions and intrusions along the traces of the active slip planes; fatigue microcracks start from the surface intrusions. Thus the PSBs play a crucial role in the process of crack initiation. Stage I fatigue cracks usually run along the PSBs. At very early stages of crack propagation these PSBs are formed before the cracks are initiated; later the PSBs are formed in the plastic zones ahead of the cracks. Stage II fatigue cracks often follow the PSBs on the microscopic level in the threshold region.