Internal stresses and dislocation dynamics in cyclic plasticity and fatigue of metals

Theories of the role of internal stresses in cyclic plasticity and in the origin of fatigue cracks are discussed in the light of a quantitative phenomenological characterization of the shapes of hysteresis loops and recent advances in atomistic modelling of cross slip and annihilation of screw dislocations. Part of the phenomenological ‘back stress’ derived from hysteresis loop shapes by the Cottrell procedure can be substantiated as a real stress associated with lattice strains. According to the present ‘static-dynamic’ theory this stress can destabilize accumulated edge dipole arrays. Subsequent recovery by annihilation of screw dipoles with atomistically calculated heights then accounts quantitatively for the observed slip heterogeneity. Brown’s theory of homogeneously slipping PSBs with a surface stress singularity makes no indications of the observed loop shapes or PSB slip heterogeneity. The implications for fatigue of industrial materials are discussed.