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

摘要

A study of the long-term exposure to mosture 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 prperties such as fracture (Mode I, Mode II, and Edge Delamination) and diffusin phenomena. Unidirectional laminate coupons were exposed to up to several thousand hours at 38 deg C (100 deg 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, the levle of pH had little influence on the diffusion behavior of the laminates. Some physical insight into the resons for the observed behavior were given. Fickian models were used to characterize the moisture. Comparisons were made between carbon- and glass-fiber fracture specimens in both dry and wet conditions to defferentiate the failure mechanisms such as resin property variations, fiber-pull-out, and fiber-matrix interface. Fiber / matrix interfacial propeties 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 G_Ic and G_IIc. Edge delamination results along with SEM observations suggest that such long-term exposure has not degraded the fiber-mitrix interface for the systems studies.