Investigating the failure behavior of hygrothermally aged adhesively bonded CFRP-aluminum alloy joints using modified Arcan fixture

Composite adhesively bonded structures used in automobiles usually withstand the harsh environment and complex stress during long service. To further popularize the carbon fiber reinforced polymer (CFRP), it is necessary to study the failure behavior of environmentally aged CFRP-aluminum alloy joints under complex stress state and establish a mechanical performance prediction method. In this work, quasi-static tensile tests on hygrothermally (80 °C and 95% relative humidity) aged adhesively bonded CFRP-aluminum alloy butt joints were carried out using a modified Arcan fixture. The stress distribution in the adhesive of Arcan tests was analyzed by the FEM, which proved the rationality of the designed fixture. After exposure to hygrothermal aging for a maximum of 30 days, the mechanical performance of the joints under different stress states decreased remarkably at a rapid degradation rate during the early stage. For unaged joints, mixed adhesive–cohesive​ failure and CFRP fiber tearing were observed when normal stress was predominant, whereas complete cohesive failure in the adhesive occurred when shear stress was more significant. For the hygrothermal aged joints, the failure modes under different stress states were adhesive–cohesive failure because of the significant degradation in adhesive performance. The q-stress failure criteria and environmental degradation factors of the CFRP-aluminum alloy bonding structure before and after aging were established, and both were embedded into the constitutive laws of the cohesive zone model (CZM) by the user-defined material subroutine. The failure prediction method was verified by the single lap joint experiment and numerical analysis before and after aging.