The role of loading mode on the property degradation of adhesives at high temperatures

Despite existing studies on the effects of temperature on the behavior of bonded joints, the role of loading mode and test type (strength, fatigue, and fracture tests) on the rate of degradation of adhesive/joint properties at high temperatures has not been adequately investigated. The aim of the current study is to determine if adhesives experience the same rate of degradation at high temperature when they are subjected to different mode mixities. To do so, an epoxy adhesive was subjected to strength, fracture, and S–N fatigue tests, for modes I and II, at different temperatures. The results show that there is a significant difference between the rate of degradation in mode I/tensile and mode II/shear loading conditions. It was also found that the rate of degradation in adhesive properties is a function of test type. Based on the results, by increasing the temperature, a less reduction in tensile strength was found in the Arcan joints (almost 50% reduction) compared with the dogbone specimen (where the strength was reduced by less than 40%). According to the results, for all the considered tests, temperature sensitivity was more noticeable in tensile loading conditions compared with the shear mode. It is shown that despite the reduction in tensile fracture energy with temperature (by around 23%), the shear fracture energy was improved (by over 20%) at 50°C. Based on these results, an S–N model was also developed to create a master S–N curve, plotted as a function of temperature. Highlights The role of loading mode on high temperature response of bonded joints is investigated. Under tensile/mode I conditions, the effect of temperature is quite significant. Under pure shear the adhesive is practically insensitive to temperatures. High temperature bulk results can overestimate the adhesive response in real joints.