Effect of alternating load on the residual strength of environmentally aged adhesively bonded CFRP-aluminum alloy joints

Abstract

Due to their good mechanical performance and lightweight advantages, adhesively bonded CFRP-aluminum alloy joints have seen increasingly widespread use in the automotive industry. However, the mechanical properties of adhesive joints may degrade when exposed to a hygrothermal environment and to alternating loads. In this study, the influence of alternating load on the residual strength of hygrothermally aged joints was investigated. CFRP-aluminum alloy joints bonded with epoxy adhesive were aged in constant and cyclic hygrothermal (Con-H\\Cyc-H) environments, after which the fatigue life and residual strength of the joints were measured. A differential scanning calorimetry (DSC) analysis of Araldite® 2015 and CFRP was performed to evaluate the effect of hygrothermal aging. The failure modes were also investigated to explain the failure mechanism. The results show that the adhesive, CFRP and joints all degenerate after hygrothermal aging. For the materials, the decrease in the glass transition temperature (Tg) is more significant under Con-H environmental conditions, while the static failure strength and fatigue performance of the joint after Cyc-H aging are the lowest. As the number of cycles of alternating load increases, the damage in the adhesive system accumulates continuously, resulting in the strength of the aged and unaged joints declining, first slowly and then quickly. The decrease in strength is more apparent at higher load. After 15 days of hygrothermal exposure, nearly complete cohesive failure is more likely to be observed relative to CFRP fiber tear for initial joints, and the interfacial failure tends to occur at the boundary of the CFRP-adhesive interface under the influence of alternating load conditions. Finally, environmental and fatigue degradation factors were defined based on the test data and incorporated into the numerical simulation to predict the residual strength of CFRP-Al alloy bonding joints exposed to alternating load.