Observations on the theoretical and experimental foundations of thermoplasticity

The thermodynamics of materials with internal state variables has been employed to study the properties of a class of thermoplastic materials in which the evolution equation for the internal variables is given by equation k⁽ⁱ⁾ = g⁽ⁱ⁾(σ_kl, '_kl, k⁽ⁱ⁾, θ, · '_kl) where g⁽ⁱ⁾ is homogeneous of order one in · '_kl. The most general form of the Helmholtz potential consistent with the assumption of insensitivity of the elastic relations to inelastic deformation has been derived and a geometric interpretation of the Clausius-Duhem restriction has been made employing the concept of a thermodynamic reference stress. Experimental results of one of the authors have been correlated with the theory.