Quantitatively determining the local effect of hygrothermal aging on the creep behaviour of epoxy adhesives

The adhesive creep properties significantly affect the overall performance of bonded structures, which suffer from severe service conditions. However, quantitative determination of the effect of environmental aging remains a challenging task since the aging process is time- and spatial-dependent process. In this paper, a novel method via nanoindentation was used to investigate the local effect of hygrothermal aging on the creep behaviour of epoxy adhesives. The adhesive was aged for different durations, and nanoindentation creep tests were subsequently conducted on various local positions along the thickness direction. It was shown that the creep resistance decreased after hygrothermal exposure and varied with aging time and positions due to the existence of moisture concentration gradients. In addition, the correlation between the creep displacement and local moisture concentration was explored. Microscopic morphology observation was conducted to reveal the local degradation effects. Furthermore, a semi-empirical creep model was developed by combining the Burgers model with regression analysis, which could quantitatively predict the creep displacement as a function of the local moisture concentration and was corroborated through experimental validation. The strategy proposed in this study is believed to be promising for studies on the environmental durability of adhesives and bonded structures.