On the damage evolution and residual strength of CFRP friction-riveted joints impacted with low-velocities

The assessment of impact damage in composite structures is a requirement for aircraft industry owing to risk of visually undetectable internal defects, which may lead to catastrophic failure. Friction Riveting was recently introduced as an alternative joining technique for aircraft woven-reinforced polymer composites. The joining technique uses frictional heat and pressure to plastically deform and anchor a rotating metallic rivet into composite parts. This work focuses on the influence of impact energy on the damage size and the mechanical behavior of friction-riveted joints under quasi-static and cyclic loading. Carbon-fiber woven reinforced polyether-ether-ketone single lap joints joined with Ti-6Al-4V rivets were investigated. The drop-weight impact test was carried out using energies between 5 and 30 J at room temperature. The damage sizes were measured using ultrasonic C-scan and confocal laser scanning microscopy, while the joint sensitivity to impact damage evolution was assessed through lap shear testing and fatigue under tension-tension regime. Upon non-destructive analyzes, 5 J induced barely-visible impact damage through fiber breakage and matrix cracking of plies in the composite surface, with no indication of shear-induced delamination through coupon thickness. Impact energies up to 30 J were mainly dissipated through delamination, which propagated toward the rivet tip leading to premature adhesive failure of metal-composite interface. The nucleation and propagation of delamination through the composite thickness decreased in 35% the quasi-static strength of friction-riveted joints while barely-visible damage led to 9% decrease of joint strength. Owing to the premature adhesive failure of visible-damaged joints, the fatigue resistance decreased in 29% in comparison to undamaged joints, while no significate decrease was observed for barely-visible damaged joints. The sensibility of the friction-riveted composite joints to critical impact damage evolution was assessed for the first time over different loading scenarios, and therefore can contribute to future design of energy-absorbing aircraft CFRP structures.