PLASTIC INSTABILITIES - PHENOMENOLOGY AND THEORY

A simple classification of plastic instabilities and the instability conditions are given in terms of phenomenological parameters, such as the strain-hardening rate h, the strain rate sensitivity S of flow stress and the derivative phi-of stress with respect to temperature. The type S instabilities associated with negative values of the strain rate sensitivity resulting from dynamic strain aging are discussed in some detail. The emphasis is on recent results pertaining to critical strains for the occurrence of the Portevin-Le Chatelier effect. A successful theoretical description of the criticl strains in terms of a model based on the evolutionary kinetics of mobile and forest dislocation densities is presented. A delay of the onset of discontinuous yielding to strains where the relaxed strain rate sensitivity, as introduced by McCormick, is sufficiently negative is also discussed. Finally, the consequences of the interaction of material elements by dislocation cross-slip for stability conditions are dealt with. A numerical exercise illustrating propagative deformation behaviour is demonstrated.