A Phenomenological Formulation and Simulation for Off-Axis Viscoplastic Behavior of Unidirectional Fiber-Reinforced Composites

An extension of the macromechanical constitutive model to describe the viscoplastic behavior of unidirectional polymer-matrix composites is presented. A particular emphasis is placed on a more systematic incorporation of isotropic and kinematic hardening variables into modeling. Three-dimensional constitutive relatiops are derived from free energy and dissipation functions; these thermodynamic potentials are defined by using a transversely isotropic tensor of the fourth rank. The proposed model can be reduced to the existing macromechanical models for unidirectional composites as special cases. Consideration of the superposition of internal variables with different nonlinearities and introduction of the elastic region enhance the predictive capability of the proposed model. Nonlinear stress-strain behavior of a unidirectional carbon fiber-reinforced composite AS 4/PEEK under various off-axis loading conditions at different rates is simulated using the proposed model, and it is demonstrated that the model can favorably reproduce the rate-dependent off-axis nonlinear behavior of this composite system.