Research on the Aging Effects of Adhesive and Composites on the Degradation Mechanism of Composite Adhesive Joints in Thermal-Humidity-Cycles for Automobiles

ABSTRACT The existing studies on the degradation mechanism of composite adhesive joints mostly focus on the aging of adhesives, but the aging effect of composites is often ignored. In this paper, the degradation mechanism of adhesive, CFRP, as well as shear joints (SJ) and butt joints (BJ) in thermal-humidity-cycles (T-H-C) for automobiles were investigated by chemical detection and mechanical testing. The aging effects of adhesives and composites are compared and analyzed by the variation in chemical and mechanical properties to reveal the degradation mechanism of composite adhesive joints. Experimental testing show that the adhesive and CFRP underwent hydrolysis and oxidative decomposition respectively, and there was a reduction in Tg (glass transition temperature) and thermal stability. The joint strength of composite adhesive joints fell by around 40% after 30d (days), and interfacial failure appeared. The variation in the composition of adhesive and CFRP, interfacial failure and thermal stress contributed to the degradation of composite adhesive joints. Degradation mechanism analysis shows that SJ under shear stress state were less affected by CFRP aging, while BJ under normal stress state were more affected by CFRP aging and the effect became more obvious with time growth.