Thermomechanical behaviors of polyether ether ketone (PEEK) with stretch-induced anisotropy

Polyether ether ketone (PEEK) is a semi-crystalline thermoplastic polymer with excellent thermo-mechanical properties, bio-compatibility, corrosion resistance, and 3D printability. Due to these merits, it has wide applications in aeronautics and biomedical devices. However, PEEK's excellent thermo-mechanical properties come from its complicated crystalline domains, making it hard to predict and to design PEEK structures under complex service conditions. In this paper, we studied the thermomechanical behaviors of PEEK with stretch-induced anisotropy and developed a constitutive model to incorporate the influence of the complex loading history along different loading axes. From the experiments, it was found that when it is stretched, PEEK demonstrates viscoplastic behaviors with reduced transversal modulus and yield stress in the subsequent loading, due to the initiation and growth of voids during stretching. The tensile sample also shows a necking behavior at relatively low temperature. To capture these behaviors, the constitutive model consists of two main parts. The undamaged part has three branches, one hyperelastic branch for the nonlinear elastic behavior, one viscoelastic branch for glass transition and relaxation in the amorphous domains, and one plastic branch for yielding and hardening in the crystalline domains. The damaged loose-chain part with history-dependent reduced relaxation time is used to capture the microscopic interface debonding between the crystallites and the amorphous domains. Compared with the experimental results, this model captures the stretch-induced volume expansion and the anisotropic evolution of material properties. This developed model is also able to capture the temperature-dependent necking phenomenon and the corresponding nominal stress-strain behaviors in the uniaxial tensile tests at different strain rates and temperatures. The developed model can be used to facilitate the design of PEEK-based structures under complicated loading conditions.