Analysis to stress relaxation phenomena of viscoelastic materials by means of irreversible thermodynamics (I)

Through defining a piece of viscoelastic medium as a thermodynamic system described by the generalized coordinates in the stress relaxation process, the evolution equation is derived by making use of the Is' law of thermodynamics, the 2(nd) law of thermodynamics and the Onsager's principle. Based on the general solutions of the evolution, the constitutive expressions of uniaxial stress relaxation are obtained for both ideal viscoelastic solid materials and ideal viscoelastic fluid one respectively, in terms of the situation whether the coordinates participating in the entropy production are in stable or neutrally stable equilibrium state. As the result, whether the stress relaxes to a constant or zero depends on whether the free energy in viscoelastic medium is left or not.