Thermodynamic formulation of plastic work hardening materials

Thermodynamic formulation of elastoplastic or viscoplastic models using the internal variable approach currently does not include models using the plastic work as a hardening variable. This means that many realistic material models do not fit into the current thermodynamic framework. An approach to extend the thermodynamic framework to include models where plastic work is used as a hardening variable is considered. This approach can even be extended to viscoplasticity where the change of the quasistatic yield function is controlled by the plastic work. As an application of the approach, the Lade-Kim model is considered. This nonassociated plasticity model uses plastic work as a hardening variable, and it is capable of predicting the behavior of frictional materials, in particular soils. It is shown that the model can be described in a general thermodynamic framework and that it is well defined in a thermodynamic sense.