Thermodynamic formulation of constitutive equations and application to the viscoplasticity and viscoelasticity of metals and polymers

Elastoviscoplasticity for metallic materials and viscoelasticity for polymers are generally treated as completely independent processes. In the present paper we intend to discuss some possible bridges between the two kind of constitutive theories. Two aspects are more specifically addressed: (i) the classical thermodynamics of irreversible processes, using material slate variables, is further extended in order to incorporate more easily dynamic and static recovery effects in the kinematic hardening evolutionary equations; (ii) the qualitative and quantitative equivalence between elastoviscoplasticity and viscoelasticity is discussed both in terms of the simplest linear model and for a more complicated non-linear situation. In fact, it is demonstrated that superposing several nonlinear kinematic hardening and static recovery rules allows to model a viscoelastic behavior, One of the advantages of the approach is to describe easily the cyclic loading situations, as shown by comparison with experiments made on a glass-epoxy composite. (C) 1997 Elsevier Science Ltd.