Long-Term Durability of Carbon and Glass-Epoxy Composite Materials in Wet Environments

Abstract

A study of the long-term exposure to moisture and pH was performed for modified epoxy composites reinforced with either AS4D carbon or S2-glass. The emphasis of this work was on resin-dominated and interfacial properties such as fracture (Mode I, Mode II, and Edge Delamination) and diffusion phenomena. Unidirectional laminate coupons were exposed to up to several thousand hours at 38°C (1 00°F) in solutions with pH levels of 7, 8.5, and 10.5; fracture coupons were conditioned at a pH of 7 only. The increased alkaline levels of the solutions used is related to the fact that glass fibers may be subject to alkaline attack. Diffusion phenomena were examined by measuring moisture uptake as a function of various pH levels. Surprisingly, changes in pH had little influence on the diffusion behavior of the laminates. Some physical insight into the reasons for the observed behavior were given. Fickian models were used to characterize the moisture. Comparisons were made between carbonand glass-fiber fracture specimens in both dry and wet conditions to differentiate the failure mechanisms such as resin property variations, fiber pull-out, and fiber-matrix interface. Fiber/matrix interfacial properties were further examined via microscopic analyses of edge delamination specimens whose fracture behavior is strongly dependent on the interface. Neat resin coupons were examined for their changes in properties due to moisture: resin compression modulus, yield strength, and fracture toughness were measured and compared to composite results. Results to date indicate that even long-term exposure has little effect on resin-dominated fracture properties such as GiC and GIC. Edge delamination results along with SEM observations suggest that such longterm exposure has not degraded the fiber-matrix interface for the systems studied.