A theory of viscoplasticity without a yield surface

Abstract : A mathematical theory is proposed of thermo-viscoplasticity which is a synthesis of experimentally observed material behavior on one hand, and the concepts of irreversible thermodynamics on the other. The underlying principle is that the history of deformation is defined in terms of a 'time scale' which is not measured by a clock, but is in itself a property of the material at hand. The theory is unifying in the sense that theories of plasticity, viscoelasticity and elasticity can be obtained from it as special cases by imposing suitable constraints on the material parameters involved; furthermore, it does not make use of the idea of a yield surface.