Some remarks on the recent development of the foundations of the theory of plasticity

In this paper some aspects of the recent development of the phenomenological theory of plasticity are reviewed. Deformation processes running with finite rate represent coupled thermo‐mechanical processes. Therefore the theory of plasticity has to be embedded into a consistent thermodynamical frame. How this can be done is sketched for a simple material model. The bounds of this thermodynamical frame result from the assumption that the thermodynamical state of each material element is describable by a set of external and internal state variables at any time. Concerning the rate dependence of deformation processes two different kinds of phenomena can be distinguished: Phenomena representing internal damping (viscous) effects which are responsible for short‐time creep processes among others; they are characterized by the existence of constraint thermodynamical equilibrium states. Phenomena due to thermal activation essentially; they are prevailing at higher homologous temperature leading to long‐time creep processes among others. In inelastic deformations different yield mechanisms are involved. Regarding these facts more complex material models are required exhibiting some generalizations of the classical theory of plasticity. Particularly certain deviations from the so‐called normality rule have to be taken into account. Another kind of coupling between thermal and mechanical processes arises from occuring heat conduction as soon as the temperature field becomes inhomogeneous. This kind of coupling particularly affects the localization of deformations as, for instance, in the necking problem in tension tests. Furthermore local damage and fracture have to be considered as coupled thermo‐mechanical problems.