This paper presents the first experimental results relative to the study of the hygrothermal effect on the mode II fracture toughness of two commercial epoxy resins suitable for civil engineering applications. To this scope, adhesive joints (with adherents in Glass Fiber Reinforced Polymer - GFRP) for End Notch Flexure (ENF) test were produced. In particular, three different kinds of sample were considered: aging samples immersed in sea-water and tap-water at a constant temperature of about 30°C and unaging specimens. The results here presented and discussed are the first ones (relative to an immersion time of six months) of a huge experimental program still ongoing consisting of 200 samples in total for an immersion time of fifteen months. The temperature of the water (about 30°C) was selected to be lower than the resins glass transition temperature evaluated through the variation of the specific heat capacity of the samples measured with Differential Scanning Calorimetry (DSC) analysis following ASTM E1356 Standard. Experimentally, it was observed an increase of the fracture energy in the first months followed by a decrement. Parallel to the evaluation of the fracture energy in mode II, the water absorption of each kind of resin and GFRP adhesive sample was also investigated according to EN ISO 62:2008. The experimental results show that the equilibrium value of water absorption of both resins is reached in about one month, while that of GFRP samples depends on the type of liquid: three months for tap water and about 5 months for sea water.
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