A viscoplasticity model for characterizing loading and unloading behavior of polymeric composites

An overstress viscoplasticity model is proposed to describe the rate-depend ent behavior of a polymeric composite during loading and unloading. In the model, a three-parameter function is used to describe viscoplastic strain r ate. in the loading stage, the equilibrium; stress is determined using a mu lti-step relaxation test performed during loading. During the initial unloa ding stage, owing to the fact that the viscoplastic strain rate is :still p ositive, the material still experiences "loading," and the corresponding eq uilibrium stress is the equilibrium stress-strain curve for Loading. In the second unloading stage, the viscoplastic strain rate becomes negative, and the material is in a true unloading mode for which the equilibrium stress is determined again using the multi-step "relaxation" test. The viscoplasti city model is found to be capable of capturing the characteristics of the r ate-dependent loading and unloading behavior.