Evolution of physico-chemical and mechanical properties of bonded epoxy assemblies during hygrothermal cyclic aging

This work investigates the hygrothermal cyclic aging of epoxy-based bonded assemblies. The focus is given here to severe long-term environmental conditions. The aging procedure is performed between 70 degrees C with a relative humidity of 90% and -40 degrees C without humidity, over a cyclic period of 12 h and for a total exposure time of 2 months. The aim of this study is to highlight the consequences of such hygrothermal aging on physico-chemical and mechanical parameters at both the material scale (adhesive) and the structural scale (bonded assemblies). Evolution of water absorption and glass transition temperature with aging cycles are monitored on adhesive bulk specimens by gravimetric analysis and DSC analysis respectively. The evolution of the quasi-static mechanical behavior, strength and elongation at failure, is characterized on adhesive massive specimens and on Single Lap Joint assemblies. Results show a progressive and important degradation of physico-chemical and mechanical properties with the number of cycles, of greater amount than that observed during static aging (loss of SLJ strength by 30% after 56 aging days; loss of the adhesive Tg by 45% for around 5% water uptake after aging 21 days). This behavior is related with an embrittling effect at the adhesive scale very specific to water change of state involved in these aging conditions. This combined analysis at different scales ensures a deep understanding of physico-chemical phenomena induced by hygrothermal aging and provides significant information for the further modeling of bonded joints assemblies.