Influence of single or multi-factor coupling of temperature, humidity and load on the aging failure of adhesively bonded CFRP / aluminum alloy composite joints for automobile applications

Applying materials such as CFRP (Carbon Fiber Reinforced Plastics) and aluminum alloys to automobiles is an effective way to accomplish lightweight design, thereby bringing energy conservation and emission reduction. Bonding structure is the key joining technology for dissimilar materials, but it will degrade during the automotive service by environmental temperature, humidity and load. Scholars mainly carry out accelerated aging tests by increasing the temperature or humidity, but it is hard to compare the degree of influence of temperature, humidity and load owing to the aging environments and adhesive materials are various and different. In this study, to comprehensively compare the effects of various aging environments coupled with load that may be experienced during the service life of an automobile on the aging failure of composite bonded joints, seven typical aging environments were selected, namely room temperature (RT), high temperature (HT), low temperature (LT), high/low temperature cycle (TC), immersed in water at room temperature (IRT), high temperature and high humidity (HTHH), and hygrothermal cycles (HC). Aging environments were coupled with static load to form single or multi-factor aging conditions of temperature, humidity and load. Accelerated aging tests were conducted to analyze the failure strength, failure mode and SEM microscopic fracture surface of bonded joints made of aluminum alloy and CFRP. The degree of influence of single or multi-factor coupling of temperature, humidity and load on the aging failure of bonded joints was analyzed by visual analysis and ANOVA (Analysis of Variance). The results show that single temperature, humidity or load had limited effects on the degradation of the failure strength of butt joints, while coupling effects of multiple factors are more obvious. When the aging environment was coupled with the load, the aging environment played a major role, and the load accelerated the aging rate without changing the decreasing trend caused by the aging environment. Both the adhesive and CFRP underwent different degrees of aging, the effect of load on the degree of aging of the adhesive was significantly greater than the CFRP, but the effect of aging environment on the degree of aging of adhesive and CFRP was related to the aging time. Normally, aging environment, aging time and load had significant effects on the failure strength, and the degree of influence decreased successively. As for the selection of the accelerated aging environments for the car, HC and TC environments should be preferred and the influence of load should be taken into consideration in accelerated aging tests.