An improved algorithm of loading/unloading judgment for extended subloading surface plasticity model: Application to non-proportional cyclic loading

Abstract

The extended subloading surface model is endowed with the capability for predicting the elastoplastic deformation behavior in engineering problems involving cyclic loadings. In the conventional plasticity theory, the interior of the yield surface is always considered as a pure elastic domain. In contrast to this, the extended subloading surface model is capable of predicting the evolution of plastic strains even under a stress state below the yield stress. In this model, the elastic domain bounded by the subloading surface shrinks during elastic unloading, and then expands towards the normal-yield surface during plastic loading. The standard loading/unloading criterion for conventional plasticity models is not applicable to the extended subloading surface model, since it is not able to capture the transition from elastic unloading to plastic reverse loading during the above-mentioned process. Hence, a suitable scheme for loading/unloading judgment is required for the extended subloading surface model. We thus developed a novel stress-update algorithm for the subloading surface model equipped with an improved loading/unloading criterion. Furthermore, a series of numerical examples with regard to cyclic loadings along proportional and non-proportional strain paths were performed to demonstrate that the proposed algorithm enables highly accurate stress calculation even in arbitrarily large strain increments.