A model for the nonlinear viscoelastic response in polymers at finite strains

New constitutive equations are derived for viscoelastic media which do not possess the separability property. The model extends the concept of transient polymeric networks by assuming the rates of breakage and reformation of adaptive links to depend on strain energy density. This permits mechanically induced aging/rejuvenation of polymers to be described. To verify the constitutive relations, we use experimental data obtained in tensile relaxation tests for poly(methyl methacrylate) and polycarbonale, in shear relaxation tests for poly(methyl methacrylate) and an epoxy glass, and in tensile tests with constant rate of strains for ethylene-butene and ethylene-octene copolymers, poly(ethylene terephthalate), and polycarbonate at strains up to 300 per cent. The model provides fair agreement between observations and their predictions. As an example of inhomogeneous deformations, we analyze inflation of a thick-walled viscoelastic spherical vessel under the action of internal pressure. The effect of material parameters on stresses and displacements is studied numerically. (C) 1998 Elsevier Science Ltd. All rights reserved.