Fracture toughness of synthesised high-performance epoxies subject to accelerated water

The effects of water uptake on the fracture toughness of epoxy systems with high glass-transition temperatures (Tg above 170 degrees C) are investigated. Aging conditioning has been conducted in hot water, followed by a desorption conditioning in a room temperature dry-airborne. Water aging determines plasticisation effects and crosslink-density modifications, revealed by reductions of the Tg. It is less known how such modifications influence the material fracture toughness. In this study, Single Edge Notched Bending samples have been tested according to standards, to evaluate the Krc fracture toughness at different stages of water absorption-desorption. The characterisation has been supported by Photoelastic Stress Analysis, allowing to evaluate stresses induced by swelling during aging. Results indicate that swelling stresses activate a crack closure mechanism that might contribute to the observed increase of the measured fracture toughness during water absorption, but are ineffective at saturation. Two epoxy systems, one based on DGEBA and the other on DGEBF, having different crosslink densities, have been studied. The plasticisation achieved at saturation translated into a significant increase of fracture toughness only in DGEBA, while fracture toughness measured at other conditioning stages was likely influenced by internal swelling stresses and sample pre-cracking strategy.