Modeling Long-Term Creep and Physical Aging Behavior of Carbon/Epoxy Composites Based on Coupling Model

Our previous study observed a non-monotonic time-dependent deformation from the long-term isothermal tensile creep tests, indicating the significant influence of physical aging on viscoelastic behavior. In this work, a new physical mechanism-based thermo-rheological complex constitutive relation that combines Ngai's coupling model and Schapery's nonlinear viscoelastic framework is established. A piecewise function was used to represent the non-monotonic time-varying compliance, in which molecular cooperativity and the interaction between different relaxing units were considered. The strength of the interaction is presented by a coupling strength parameter. The effects of stress and temperature on the deformation are represented by not only two shift factors but also are associated with the coupling strength parameter, which is modeled as a function of the fictive temperature. The physical aging effect is also incorporated into the evolution of the fictive temperature. The proposed model was verified by a comparison of the experimental and finite element simulated results. Finally, the physical meanings of the model parameters and their correlation with the macroscopic deformation of the laminates were discussed.