On the Thermal Sensitivity of CFRP friction-riveted Joints under Quasi-static Loading

Among many sources of potential damage, operational range of temperature (ORT) is critical for designing durable riveted thermoplastic composite structures. ORT may lead to composite embrittlement or undesired ductility, impairing structural integrity. This work focuses on the thermal sensitivity of carbon-fiber reinforced polyether-ether-ketone (CF-PEEK) joined with Ti6Al4V rivet by means of Friction Riveting - an alternative joining technique for similar thermoplastic/composite and dissimilar metal-thermoplastic/composite joints. We have recently demonstrated that CF-PEEK/Ti6Al4V friction riveted single lap joints have excellent quasi-static and fatigue mechanical performance, comparable to or better than lock-bolt connections. Lap shear testing under controlled temperatures of -55°C, 70°C, and 120°C was performed. Failure and fracture analyses were carried out using non-destructive techniques (X-ray micro-computed tomography, confocal and laser scanning microscopy scanning electron microscopy). The integrity of the fiber-matrix interface and matrix ductility showed to contribute the most to the joint’s mechanical strength at different temperatures. Despite a 24 % increase in joint strength at -55 °C embrittlement was observed, while at 120 °C the composite presented high plasticization and enlargement of the composite hole edges, which consequently decreased joint strength by 23 %. This work has shown that ORT modifies the composite material properties in friction riveted joints in a similar manner as for state-of-the-art mechanical fastened composite joints. The finds of this work contributes to extend the available knowledge of Friction Riveting towards certification in aircraft.