Constitutive equations for nonlinear viscoelastic composites are discussed. The effects of time-independent inelastic behavior, microcracking and time-dependent residual strains are considered along with the viscoelastic effects that are traditionally associated with the behavior of monolithic and reinforced polymeric materials. Time-independent behavior is discussed first, in which the experimentally observed insensitivity of mechanical work to deformation or load paths is used as the basis for a simplified constitutive model. This representation is then modified to account for time- or rate-effects due to microcrack-like evolution laws. Effects due to broad spectrum nonlinear, viscoelastic behavior of the polymer matrix are reviewed and then used in a generalized constitutive equation with both time-independent and time-dependent effects. Emphasis of this paper is on a thermodynamically-based phenomenological description of deformation response and the use of simplifications based on experimental observations. However, there is a limited discussion of physical mechanisms for nonlinear time-dependent behavior.

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