CREEP HARDENING RULE UNDER MULTIAXIAL REPEATED STRESS CHANGES

Anisotropic creep behavior of polycrystalline metals under multiaxial nonproportional repeated loading conditions is modeled from phenomenological points of view. The creep model consists of basic constitutive equations (BCEs) and an auxiliary hardening rule (AUX) to enhance the predictive capability of the BCEs. The BCEs are specified on the basis of a modification of the conventional kinematic hardening model, and they are characterized by a new kinematic hardening variable which is defined as the sum of two component variables; one represents the back stress in the conventional sense and the other a flow resistance in the opposite direction of the deviatoric stress. The AUX is governed by a memory region in which only the evolution of the back stress takes place. Two different formulations of the AUX are presented. The applicability of the creep model is discussed on the basis of simulations for multiaxial nonproportional repeated creep of type 304 stainless steel at high temperature.