Inelastic Constitutive Equation of Polymer-Matrix Composites by Taking into Account Transient Recovery Surface

In the previous paper, it was found that a constitutive equation based on the kinematic hardening creep theory of Malinin-Khadjinsky and the nonlinear kinematic hardening rule of Armstrong-Frederick cannot describe strain recovery of polymer-matrix composites in the process of unloading; this was found to be attributable to rapid development of back stress just after the start of unloading. Thus, in order to describe the intrinsic strain recovery of the composites, the Armstrong-Frederick model of kinematic hardening is modified by taking into account a new concept of transient recovery surface proposed by the authors. In order to modify the model, a transient recovery term is introduced into the evolution equation of back stress, and the term is active under unloading to suppress strain hardening of back stress when viscous strain state is inside the transient recovery surface defined in viscous strain space. By using the modified model incorporated into the kinematic hardening creep theory of Malinin-Khadjinsky, hysteresis loops in stress-strain relation including the processes of loading, unloading and reloading are simulated, and the validity of the modification is discussed.