Polymers in the glassy state exhibit complex, nonlinear and time dependent relaxation in volume, enthalpy and stress. For engineering design and recognition of potential failure modes, we need a constitutive theory that is capable of predicting accurately the full range of relaxation behavior under arbitrary time/temperature/deformation histories. To achieve this goal, a thermodynamically consistent nonlinear viscoelastic theory has been developed. Starting from the Helmholtz free energy for an isotropic, thermorheologically simple, visoelastic material, equations for the stress and entropy have been derived using Rational Mechanics. The constitutive equations employ the Hencky finite strain measure and a material clock that is constructed from the potential part of the internal energy. This produces relaxation rates that depend on the thermal and deformation history of the material through the thermodynamic state. The resulting constitutive equations surprisingly can be populated by linear viscoelastic properties with no free-fitting parameters. The model has been implemented in a fully three-dimensional finite element code for model validation through the analysis of a wide variety of different material tests. The results obtained from thermosets and a thermoplastic have demonstrated the ability to make engineering predictions for an extremely broad range of behavior and to do so from a single set of model inputs. This leads us to believe that the underlying postulates are physically reasonable. A summary of the theory, material characterization and validation will discussed.
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