A nonproportionality parameter and a cyclic viscoplastic constitutive model taking into account amplitude dependences and memory effects of isotropic hardening.

The present paper is concerned with the formultion of a viscoplastic constitutive model describing both cyclic hardening and cyclic softening under proportional and nonproportional loading conditions. First a nonproportionality parameter and the relevant internal structural tensor to describe the nonproportional hardening is discussed in detail. Internal variables and the related evolution equations to describe the amplitude dependence of cyclic hardening/softening are also examined. Then the history effects of cyclic hardening and softening are considered in the evolution equations of isotropic hardening variable. The proposed model is established by incorporating these equations into Chaboche model