A STATIC-DYNAMIC MODEL FOR THE PROCESS OF CYCLIC SATURATION IN FATIGUE OF METALS

Recent experimental observations of the effects of temperature and cum ulative strain on the dislocation structure and cyclic stress-strain b ehaviour of Cu single crystals show that new ideas are needed for a di slocation theory of cyclic saturation. The observations have enabled a static-dynamic model to be comprehensively checked. The model provide s quantitative accounts of the available observations and it shows ver y clearly that cyclic saturation is not a steady state. It is a dynami c process of continued matrix hardening and persistent slip band (PSB) formation. The model includes a partial static model based on the ide a that edge dipole walls in fatigue are metastable but impenetrable to uniform slip. Earlier criticism against this idea has been dealt with in detail on the basis of a simple dislocation account of slip unifor mity, which correlates the wall spacings of the static structure with the density of the dynamic structure of gliding dislocations. In addit ion the present static-dynamic model includes a line tension model for the observed motion of primary matrix and PSB walls. By invoking only conservative dislocation motion the model has the crucial advantage t hat its validity remains possible even at very low temperatures, where point defects are immobile. The line tension model allows the observe d fragmentation of primary matrix wails to be understood, and it accou nts for the recently discovered relation between the dislocation densi ty in and the spacing of PSB walls. The model also shows that irrevers ible matrix hardening drives the continued condensation, whereby matri x walls reduce their volume and cause localization of cyclic strain wi th slip instabilities and hence fatigue damage.