Interfacial Mechanisms of Moisture Degradation in Graphite-Epoxy Composites

Abstract

Abstract A detailed study of the kinetics of environmental degradation in graphite-epoxy composites shows a close correlation between analytical predictions and experimentally observed changes in interlaminar shear and fracture energy response under high moisture exposure conditions. Unaged composite specimens exhibit high interlaminar shear strength λb > 850 kg/cm2 (12,000 psi) and relatively low fracture energy Wb/A ≃ 10–20 kg cm/cm2 (56 to 1121b. in./in.2). Exposure to 95% relative humidity or water immersion at 100°C for times t > 200 hours produces a 30 to 50% reduction in λb accompanied by a concurrent two to fivefold increase in Wb/A and acoustic energy absorption. These property changes are shown to be irreversible and directly related to cumulative moisture degradation of the fiber-matrix interfacial bond. The magnitudes of these property changes are consistent with surface energy analysis and micro-mechanics predictions which show that fracture energy response optimizes at intermediate values of interlaminar shear strength.