Enhancing fracture toughness of Al/CFRP joints bonded with CNT-modified epoxy adhesive using ultrasonic adhesive-impact bonding process

Incorporating carbon nanotubes (CNTs) into epoxy adhesives can enhance the fracture toughness of adhesively bonded joints, but the effectiveness is limited by the challenge of embedding CNTs into the micropores on adherend surface that prevents the formation of adequate interfacial fiber bridging, as well as the insufficient interfacial adhesion between the CNTs and adhesive. This study introduced a novel ultrasonic adhesive-impact bonding process in which ultrasonic vibrations are applied to a CNT-modified epoxy adhesive on the adherend surfaces before joint fabrication to enhance the mode-I fracture toughness of Al/CFRP joints. The results showed that this process increased the mode-I energy release rate ( G Ic ) reached 2.91 N/mm, representing an increase of 36.62 % compared to the joints bonded using conventional bonding process. The toughening mechanisms were analyzed from both physical and chemical perspectives using SEM, XPS, and FTIR techniques. On one hand, the ultrasonic adhesive-impact bonding process enhanced the infiltration of CNT-modified epoxy adhesive into the micropores on the Al surface through the synergistic effects of reduced adhesive viscosity and micro-liquid jet impacts, thereby improving mechanical interlocking and strengthening the fiber bridging of the CNTs at the interface. On the other hand, the ultrasonic adhesive-impact bonding process accelerated the chemical reactions of the adhesive with the aminated CNTs and the chemical groups of Al surface, which enhanced the interfacial adhesion between the CNTs and adhesive, and increased the number of C–O–Al bonds at the interface, forming an efficient stress transfer.