The exceptional mechanical properties of Polyether-ether-ketone(PEEK) polymers make them ideal candidates for interlayer toughening of carbon fibre/epoxy composites. Herein, ultra-thin PEEK films with a thickness of 8μm, 18μm and 25μm were used for interlayer toughening of an aerospace-grade carbon fibre/epoxy composite. The mode-I and mode-II fracture behaviour of the interleaved laminates were investigated, with the fracture mechanisms being investigated. The surfaces of the PEEK films were treated by a UV-irradiation technique to enhance their intrinsically low surface activities. This significantly increased the adhesion at the interface between the PEEK interlayers and the composite matrix. A topography analysis on the fracture surfaces revealed extensive damage of the PEEK interlayers during the fracture process of the laminates. Owing to the exceptional properties of the PEEK films, significant enhancements in the mode-I and mode-II fracture properties of the laminates were obtained, i.e. the mode-I and mode-II fracture energies were significantly increased by 227% and 441%, respectively. Overall, the UV-treated PEEK films proved superior effectivenesses for laminate toughening when compared to the other state-of-the-art interlayer materials.

The inherently low surface energy of carbon fibre reinforced Polyether-ether-ketone (CF/PEEK) composites results in an extremely low compatibility with adhesives. This subsequently causes significant challenges in the adhesive joining of them to other dissimilar materials. Herein, the bonding surfaces of the CF/PEEK composites were treated by a high-power UV-irradiation technique prior to the adhesive bonding, with an attempt to develop hybrid composite-to-aluminium joints with excellent fracture resistance. The mode-I, mode-II and mix-mode fracture behaviour of CF/PEEK-to-aluminium joints bonded by two commercial aerospace adhesives was evaluated. Cohesive failure within the adhesive layers or substrate damage to the CF/PEEK composites were observed in all the cases. This indicated that the adhesion between the CF/PEEK composites and the adhesives was sufficient to prevent an adhesive failure at the composite/adhesive interfaces under different fracture modes. This study explored an effective route to develop strong and tough CF/PEEK-to-aluminium joints for aerospace applications. Additionally, it revealed that the form of the adhesive supporting carrier was a key factor affecting the fracture behaviour and fracture energies of the adhesive joints.

The development of effective methods for the bonding of Poly-etherether-ketone (PEEK) and Polyphenylene-sulphide (PPS) composites to thermoset composites is appealing to expand their applications in aerospace industry. Herein, the surfaces of PEEK and PPS composites were treated by a high-power UV-irradiation technique for 6 s, that proved to significantly improve their intrinsically low surface activities. Carbon fibre reinforced epoxy composites were then directly cured onto the PEEK and PPS composites with or without an aerospace film adhesive at the joining interfaces. The mode-II and mix mode-I/II fracture behaviour of the hybrid joints were studied using an end notched flexural test and a fixed-ratio mixed-mode test, respectively. It was observed that the failure of the hybrid joints without adhesives mainly took place at the joining interfaces. In this case, the lack of resins at the fracture plane resulted in relatively low fracture toughness. Encouragingly, a cohesive failure was observed for the hybrid joints with adhesives in all the cases, owing to the enhanced adhesion between the adhesive and the PEEK/PPS composites upon the UV-treatment. This phenomenon indicated that optimal fracture resistance of the hybrid adhesive joints was obtained for the given material systems.