Unit cell based discrete equivalent method for predicting nonlinear creep behavior of unidirectional fiber-reinforced epoxy resin composites

The inherent viscoelasticity of polymer matrices causes pronounced nonlinear behavior in fiber-reinforced polymer composites, potentially leading to unexpected catastrophic failure of composite structures. A unit cell based discrete equivalent method is proposed in this study to predict the nonlinear creep response of unidirectional glass fiber-reinforced epoxy composites. The method integrates a user-defined material subroutine (UMAT) with a Python script to discretize the continuous nonlinear viscoelastic response of the epoxy matrix into a series of time-dependent equivalent elastic parameters. These parameters, together with the elastic properties of fibers, are assigned to a three-dimensional fiber-matrix representative volume element (RVE), enabling direct prediction of the composite’s nonlinear creep response in the time domain. The method requires only a limited short-term creep tests on the pure resin and eliminates the need for Laplace and inverse Laplace transforms throughout the analysis. The predicted results show deviations of less than 5% from the experimental measurements, demonstrating the accuracy and effectiveness of the proposed approach.