Viscoplasticity based on an additive split of the conjugated forces

A generalized form of viscoplasticity is proposed, within a thermodynamic concept. The theory developed is based on an expansion of the dissipation inequality, where additional quantities are introduced. These quantities are obtained by an additive split of the conjugated thermodynamic forces. Additional potential functions that depend on these quantities can be introduced, which enables one to achieve a generalized form of non-associative viscoplastic theory. Within this concept, the Duvaut-Lions formulation follows naturally as a special case and other important possibilities are also discussed. It is shown that the proposed concept can be generalized to the case where corners exist on the yield and potential functions. Finally, some specific models that all take the Tresca criterion as the yield surface are discussed and used to illustrate some of the findings.