Unifying concepts in dislocation plasticity

The thermally activated annihilation of screw dislocation dipoles provides a rationale for the endurance limit in fatigue and for the onset of stage III hardening in single crystal monotonic stress-strain curves. Such annihilation amounts to the thermal diffusion of shear, which along with the diffusion of matter (edge dipole annihilation, climb) and of orientation (recrystallization, grain and sub-grain growth) provides a framework within which a self-consistent account can be given of plastic phenomena displayed by ductile crystals. This short note shows that the work-hardening behaviour in stages III and IV can be brought into the scheme, if the exhaustion of dislocation sources is identified with splitting apart of progressively narrower edge dislocation dipoles arranged in patterns established in earlier stages of work-hardening. The scheme permits an estimate to be made of the ratio of the endurance limit in fatigue to the ultimate tensile strength.