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A three-dimensional micromechanical model based on the assumptions of simplified unit cell method (SUCM) is presented to obtain the closed-form solution for overall behavior of a unidirectional polymer matrix composite. The composite system consists of nonlinear viscoelastic matrix reinforced by transversely isotropic elastic fibers. The Schapery single integral viscoelastic constitutive equation in multiaxial stress state is used to model the nonlinear viscoelastic matrix. The presented analytical formulation is able to predict the effective response of the composite in any combination of normal and shear loading conditions. In particular, the effective creep response of the material in the off-axis loading is investigated. Prediction of the presented analytical model for the creep response of glassy amorphous polymer PMMA material shows good agreement with available experimental data. Furthermore, the predicted overall creep behavior of 10 and 908 off-axis coupons of graphite/epoxy composite demonstrate close agreement with experimental and other numerical results available in the literature. The overall creep-recovery of the graphite/epoxy composite in various off-axis loading conditions for several stress levels is also presented.

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