Free energy functionals time-dependent component

Basically, the time dependence of the nonlinear behavior is considered to be in a separable form, where the viscoelasticity is accounted for by a relaxation function that is independent of stress or strain, while the effects of large deformations are incorporated in a reference potential. Simo [51] developed a nonlinear viscoelastic model based on a free energy with uncoupled volumetric and deviatoric parts. The time-dependent effects are contained in the deviatoric stress component, while volumetric stress response is assumed to be elastic. 

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