Effect of thermal cycling on the degradation of adhesively bonded CFRP/aluminum alloy joints for automobiles

Thermal cycling is one of the representative service environments of automobiles. As epoxy adhesive and resin matrix of CFRP (Carbon Fiber Reinforced Plastics) composites are polymer materials, their properties may change during long-term thermal cycling and thus affect the joint strength of adhesively bonded composites joints. To investigate the degradation mechanism of adhesively bonded CFRP/aluminum alloy joints subjected to thermal cycling, firstly, the chemical transformations of adhesive and CFRP were analyzed by FTIR (Fourier Transform Infrared Spectroscopy), DSC (Differential Scanning Calorimetry) and TG/DTG (Thermal Gravimetric/Differential Thermal Gravimetric), and changes in mechanical properties of adhesive and CFRP were also tested by quasi-static tests. Then the variations of failure strength and failure modes of adhesively bonded CFRP/aluminum alloy shear joints and butt joints after different aging time were investigated. Results show that the T-g (glass transition temperature), thermal stability, failure strength and Young's modulus of adhesive improved after thermal cycling because of post-curing. The T-g and thermal stability of the CFRP decreased due to surface oxidation, resulting in the decline of surface adhesiveness of CFRP and the appearance of interface failure. Besides, the mechanical performance of the fiber/matrix interface also declined after thermal cycling, which was verified by SEM (Scanning Electron Microscope). The failure strength of shear and butt joints dropped by more than 40% after thermal cycling for 30 days. The degradation of shear joints was mainly caused by the combined effect of thermal stress and post-curing of adhesive as well as interface failure, while the butt joints were also affected by the fiber tear of CFRP.