A finite strain viscoelastic–viscoplastic damage model for thermoplastic polymer

In this study, a novel finite strain constitutive model that integrates viscoelasticity, viscoplasticity, and damage is proposed to characterize the mechanical behavior of thermoplastic polymers. The comprehensive constitutive framework of the proposed model employs hypoelastic theory to characterize elastoplastic responses under finite deformation as well as a continuum damage model to capture damage evolution, and it incorporates an extended generalized Maxwell model to account for viscous effects in both the elastic and plastic states. To isolate the elastic, plastic, viscous, and damage effects during experiments, a cyclic loading–relaxation–unloading–recovery test, which is closely aligned with the constitutive model, is utilized to determine model parameters for each decoupled mechanical response more physically, facilitating a deeper understanding of individual material properties from elasticity, plasticity, viscosity and damage. Polyether ether ketone is selected as a representative thermoplastic polymer, and the material parameters for the proposed constitutive model are systematically identified. The stress–strain relations predicted from the constitutive model for different mechanical responses show good correlation to the corresponding experimental results, thus validating the efficacy of the proposed model.