Cyclic creep behaviour described in terms of a one-parameter kinetic model

A previously proposed model for describing strain-controlled cyclic plastic deformation is employed to describe cyclic creep behaviour. The model, derived via the evolutionary behaviour of mobile dislocation density, is directly applicable to the case where flow is governed by the glide motion of dislocation through an obstacle field, such as forest dislocations. For the case of stress-controlled loading, the model is modified to account for kinematic hardening and mean-stress shift that occur during cyclic creep deformation. Cyclic creep is modeled as due to the coupled behaviour of the breakdown of dislocation cell structures on the one hand, and the formation and development of new cell structures on the other hand. These two processes oppose each other in that while the former softens the material and enhances flow, the latter hardens the material and tends to decrease flow. The competition between these processes and the relative rate of one to the other during loading determines the overall shape of the strain-cycle (or strain–time) curve. The predictive capability of the present creep model is compared with experimental cyclic creep data of polycrystalline copper subjected to combined cyclic and mean stresses, and agreement between the two is very good.