A Viscoelastic–Viscoplastic constitutive model for fiber-reinforced composites with application to variable angle tow composites

Fiber-reinforced polymer matrix composites exhibit rate-dependent and nonlinear mechanical behaviors. In this study, a novel viscoelastic–viscoplastic constitutive model is proposed to characterize these behaviors, based on a decomposition of the material response into elastoplastic and viscous components. An anisotropic plasticity framework is employed to describe the elastoplastic behavior, while an extended generalized Maxwell model is developed to capture the anisotropic viscous behavior within both elastic and plastic deformation regimes. The proposed model is validated using CF/PAEK-L unidirectional fiber-reinforced composites. Uniaxial compression tests are performed on specimens with fiber orientations of 0°, 45°, 60°, 75°, and 90° at quasi-static strain rates of 10⁻⁴/s and 10⁻³/s. The stress–strain relationships predicted by the constitutive model show excellent agreement with experimental results, demonstrating the model's capability to accurately predict the rate-dependent and nonlinear behaviors of fiber-reinforced composites. Furthermore, the validated model is applied to variable angle tow composites to investigate their mechanical behaviors. The strong correlation between numerical predictions and experimental data highlights the model's applicability to composites with complex microstructures.