Constitutive Model and High Temperature Rheological Behavior of PEEK

Accurately characterizing the mechanical behavior of polyether ether ketone (PEEK) under low strain rate conditions and over a wide temperature range is essential for predicting the mechanical performance of PEEK and PEEK-based composites. In this study, PEEK grade ZYPEEK®-330G was used. Specimens were prepared by injection molding. Quasi-static uniaxial tensile tests were conducted at temperatures of 20, 50, 80, 110, and 140 °C and strain rates from 10−4 to 10−1 s−1, with three replicate specimens for each condition. The high-temperature rheological behavior and flow stress below glass transition temperature (Tg ≈ 143 °C) were studied based on the free volume theory. Results indicate that with the increasing of temperature and the decreasing of strain rate, the yield strength, flow stress and elastic modulus of PEEK decrease, while the ductility and toughness are enhanced. The strain rate sensitivity index demonstrates a transition trend from non-Newtonian flow to Newtonian flow below Tg. A seven-parameter modified constitutive model considering both strain rate and temperature effects is proposed to accurately characterize the tensile mechanical response of PEEK before yielding. Model parameters were determined from the true stress–strain data at 20, 80, 110, and 140 °C, and the model was independently validated against the 50 °C data not used for fitting, with a prediction error of less than 3%. The model is limited to the pre-yield regime, the quasi-static strain rate range of 10−4 to 10−1 s−1, and temperatures below Tg. These findings demonstrate that the tensile response of PEEK is sensitive to both temperature and strain rate, and this constitutive model provides a foundation for predicting the tensile behavior of PEEK under complex environmental conditions.