Cyclic Deformation and Fatigue: Some Current Problems

Abstract

Some current work on cyclic deformation and fatigue is discussed in the light of the contributions made to the understanding of 1) basic dislocation mechanisms in crystal plasticity and 2) fatigue damage mechanisms. It is noted that the cyclic saturation regime provides almost ideal conditions for experiments at “constant” microstructure. Examples are given of the application of cyclic strain-rate changes involving both instantaneous stress changes and stress transients to the study of dislocation mechanisms in bcc and fcc metals. Gradual departures from cyclic steady-state deformation during extended cycling are discussed in terms of the microstructural evolution. With regard to fatigue damage mechanisms, a detailed account is given of cyclic strain localization in persistent slip bands (PSBs) in mono- and polycrystalline fcc metals (and precipitation-hardened alloys). The different types of surface profiles at emerging PSBs are classified for mono- and polycrystalline material, and the influence of testing temperature and environment on details of the PSB surface-profiles is discussed. It is shown that PSBs are responsible not only for trans- but also for intercrystalline crack initiation, even in the high-cycle range, and that this is probably the reason why the fatigue lives of copper polycrystals are shorter than those of monocrystals. Finally, examples are given of quantitative fractographic characterizations of fatigue striation patterns which indicate their potential usefulness in advancing the understanding of mechanisms of fatigue crack-propagation.