An investigation of the influence of matrix properties and fibre–matrix interface behaviour on the mechanical performance of carbon fibre-reinforced PEKK and PEEK composites

Poly-ether-ketone-ketone (PEKK) is an emerging alternative to poly-ether-ether-ketone (PEEK) as a matrix for high-performance carbon fibre (CF) reinforced composites. Herein, the results of an experimental investigation to examine the influence of matrix properties and fibre–matrix interface behaviour on the mechanical performances of CF/PEKK and CF/PEEK composites are presented. CF/PEKK presents superior strength under longitudinal tension, longitudinal and transverse compression, as well as in-plane shear. It also exhibits better interfacial shear strength (IFSS) than CF/PEEK, which contributes to its superior strength, as damage typically initiates at the fibre–matrix interface under in-plane loading. Predictions of different analytical models adopted from the literature, which assess the influence of fibre–matrix adhesion on the in-plane strength, compare favourably to the experiments. Under cyclic shear tests, CF/PEKK exhibited more gradual stiffness reductions and low shear plasticity until 5% shear strain, indicating a more damage tolerant matrix. Relative to CF/PEEK, CF/PEKK presents superior interlaminar shear strength (ILSS) and mode I fracture toughness (G_IC), but similar mode II fracture toughness (G_IIC). High G_IC is due to a synergistic interaction between its inherently ductile matrix and high IFSS, while ILSS strongly correlates with IFSS. Overall, CF/PEKK offers a better combination of strength and toughness, exceeding CF/PEEK.