A multiscale viscoelastic constitutive model of unidirectional carbon fiber reinforced PEEK over a wide temperature range

Due to super toughness and good repairability, the fiber reinforced polyether ether ketone (PEEK) composites are promising to replace metals in the primary-load bearing structures for the next generation aircraft. However, at the high service temperature close to or even above the glass transition temperature of PEEK, composite properties strongly depend on the loading temperature and time, bringing great challenges to their applications. How to evaluate their temperature/time dependent properties over a wide temperature range is the key issue in both the structure design and the process design. This study proposes a multiscale viscoelastic constitutive model for the composites to separately consider the individual contribution of the intrinsic matrix properties and the fiber induced strain concentration. The generalized Maxwell model for the PEEK viscoelastic properties was extended to a three-dimensional case by the genetic integration method. Based on the bottom-up approach, the isotropic viscoelastic matrix and the anisotropic linear elastic reinforcement were considered to construct a multiscale model of the fiber reinforced PEEK composites. To verify the developed model, the unidirectional carbon fiber reinforced PEEK composite samples were fabricated and tested. By the PEEK matrix relaxation tests, the composite transverse tensile tests and the representative volume element (RVE) simulations, the model parameters were calibrated step by step. Good agreements among the experimental results, the RVE simulations, and the model predictions under different tensile states over a wide temperature range (25–200 °C), proved that this model could be used to predict and to design the relaxation behaviors of the fiber reinforced PEEK composites.