The interaction of loading mode and humidity on the properties degradation of an epoxy adhesive subjected to strength, fracture, and fatigue tests

Abstract Hygrothermal aging has a significant impact on the performance of adhesives and bonded joints. However, despite extensive investigations into the aging effect of bonded structures, there is still limited knowledge on how aging affects joint performance as a function of the loading type and the load angle. Accordingly, in this study, an epoxy adhesive was tested under different loading types/angles including tensile and shear standard strength tests, mode I and mode II fracture tests, and fatigue and static Arcan tests. All these experiments were performed for both aged and unaged conditions. To understand the influence of loading angle on the failure response of aged bonded joints, Arcan samples were subjected to mixed mode loading conditions as well. A fractography analysis was conducted to provide a better understanding of the failure mechanism of aged samples loaded at different modes. Based on these results, it was found that mode I (tensile loading conditions) is more sensitive to the aging conditions than mode II (shearing) for all loading types (strength, fracture, and fatigue tests). The influence of moisture on fracture toughness was however less relevant than the strength. The results also showed that for the same loading mode and for the same Arcan joints, the fatigue strength is significantly more degraded by the aging conditions than the static strength.