CONSTITUTIVE MODELING OF METALS UNDER NONPROPORTIONAL CYCLIC LOADING

A two-surface kinematic hardening model for the stress-strain response of metals under nonproportional tension-torsion cyclic loading is developed and verified with critical experiments. In this model, both the yield and limit surfaces are assumed to be ellipses in the two-dimensional stress plane to account for anisotropic cyclic hardening. Areas of the yield and limit surfaces are changed in order to model the overall isotropic cyclic hardening (or softening) behavior. The strength anisotropy is modeled by changing the ellipticity and orientation of the elliptical surfaces with respect to the stress axes. A nonproportionality parameter based on the plastic strain history is developed to estimate the cyclic hardening level under nonproportional loading. It is shown that this model is able to model a number of important deformation features of metals under complex nonproportional cyclic loading.