Thermomechanical model for monotonic and cyclic loading of PEEK

An internal state variable (ISV) model for polyether ether ketone (PEEK) is developed to capture experimental measurements conducted below its glass transition temperature. The model is based on the response of two relaxing components put in parallel, one having a back stress response that softens with plastic flow. This softening allows reproducing both monotonic and cyclic loading, and is possibly associated with detangling, melting of network junctions, and/or change in crystallinity. The resulting model, which is thermodynamic and multidimensional, was used to capture the response of PEEK 450G using thermal expansion (23 °C to 120 °C), heat capacity (−40 °C to 140 °C), monotonic extension and compression (−85 °C to 150 °C; 0.0001 s⁻¹ to 3000 s⁻¹; strains up to 40–80%), equilibrium stress measurements in compression (23 °C to 120 °C; strains up to 60%), and ultrasonic longitudinal and shear wave speed measurements along and transverse to the directions of compression (23 °C to 120 °C; up to 50% plastic compression). The model is compared to the response under monotonic and cyclic shear and internal dissipation is assessed using the equivalent adiabatic temperature rise.