Experimental study on CF/PEEK thermoplastic fastener: Effects of fastener matrix crystallinity and fibre content on the strength of single-lap joint

Composite materials are widely used in the aerospace structures due to their characteristics of light weight and high strength, while composite material components are currently joined by heavy titanium fasteners. We propose here a novel joining method using unidirectional CF/PEEK thermoplastic composites fastener (TPCF). CF/PEEK TPCFs are prepared via mechanically machining unidirectional CF/PEEK composites laminates. The joining of TPCF is through a hot-press and cooling process. Single-lap joints reinforced by CF/PEEK TPCFs are subjected to tensile tests and compared with titanium riveted joint. Experiments results show that the failure of the TPCF joint is shear off in fastener, while the composite laminate of titanium riveted joint undergoes a bearing failure and the rivet is pulled out from the laminate. The TPCF joint strength is lower than that of titanium riveted joint, but the TPCF joint has a higher specific joint strength. The TPCF joint strength increases with increasing carbon fibre mass fraction of TPCF. Also, we find that the TPCF joint strength increases with decreasing cooling rate. The DSC tests of TPCF show that the low cooling rate leads to an increase in crystallinity, which enhances the mechanical property of TPCF and then improves the joint strength. This provides a new process to control the TPCF joint strength by changing its cooling rate. The fracture face of TPCF is observed. It is found that the TPCF has a 'U-shaped' stepped fracture face after structure failure, which is caused by shear force and impact load.

Automated summary

A novel joining method using unidirectional CF/PEEK thermoplastic composites fastener (TPCF) was proposed in this study, showing higher specific joint strength compared to traditional titanium riveted joints. The strength of TPCF joints increases with increasing carbon fibre mass fraction and decreasing cooling rate, as confirmed by experiments. Additionally, the fracture face of TPCF after structure failure revealed a 'U-shaped' stepped pattern caused by shear force and impact load.