Macromechanical Modeling and Analysis of the Viscoplastic Behavior of Unidirectional Fiber-Reinforced Composites

Abstract

A combined isotropic and kinematic hardening model for the viscoplastic behavior of unidirectional fiber-reinforced composites is presented. Unidirectional composites are treated as transversely isotropic homogeneous media that harden with viscoplastic deformation. The formulation is based on the irreversible thermodynamics with internal variables, and the constitutive equations are derived from free energy and dissipation function. The combined isotropic and kinematic hardening model for transversely isotropic media can be reduced to pure isotropic hardening and kinematic hardening versions. Although the isotropic hardening model is similar to special cases of existing macromechanics models, it is furnished with an enhanced capability to describe the nonlinearity of the off-axis viscoplastic deformation of unidirectional composites. Validity of the proposed model is evaluated through comparisons between the predictions and observations for the rate-dependent nonlinear behavior of the unidirectional carbon fiber-reinforced composite AS4/PEEK under off-axis monotonic loading conditions.