Alternative formulations of isotropic hardening for Mises materials, and associated variational inequalities

This work provides insight into aspects of classical Mises-Hill plasticity, its extension to the Aifantis theory of gradient plasticity, and the formulations of both theories as variational inequalities. Firstly, it is shown that the classical isotropic hardening rule, which is dissipative in nature, may equally well be characterized via a defect energy-and, what is striking, this energetically based hardening rule mimics dissipative behavior by describing loading processes that are irreversible. A second aspect concerns the equivalence between the conventional form of the flow rule and its formulation in terms of dissipation. This equivalence has been previously established using the tools of convex analysis (cf., e.g., Han and Reddy, Plasticity: mathematical theory and numerical analysis, Springer, New York, 1999)-in the current work this equivalence is derived directly from the constitutive equations and the specific form of the dissipation, without recourse to such machinery. Variational inequalities corresponding to the dissipative and energetic forms of the flow rule are derived; these inequalities involve only the displacement and plastic strain and are well suited to computational studies. Finally, it is shown that the framework developed for the classical theory is easily extended to incorporate the gradient-plasticity theory of Aifantis (Trans ASME J Eng Mater Technol 106: 326-330, 1984).