Water transport in graphite/epoxy composites

The diffusive and mechanical behavior of tetraglycidyl diaminodiphenyl methane (TGDDM) resin‐based composites and diglycidyl ether of bisphenol‐A (DGEBA) resin‐based graphite/epoxy composites were investigated during water sorption at different temperatures. The water‐absorption kinetics in both systems at 50, 70, 90, and 100°C were fitted by a Fickian diffusion model. However, a Langmuir‐type, two‐step sorption behavior was observed for water transport in DGEBA‐based systems at 50 and 70°C. Using scanning electron microscopy, internal cracks due to water absorption were found in the DGEBA‐based samples after conditioning at 90 and 100°C in water, whereas no cracks were detected in TGDDM‐based samples conditioned in water at 100°C. Ultrasonic testing did not show significant modulus or density change of the TGDDM‐based samples conditioned in water at 100°C. No significant changes of dynamic modulus or damping factor were observed for the TGDDM‐based samples redried after immersion in 100°C water, whereas slight changes were observed above 120°C for the samples containing absorbed water. However, both water‐containing and redried DGEBA‐based samples showed a decrease of dynamic modulus and an ω‐transition around 120°C. A single‐fiber fragment test revealed that the absorbed water at 80°C reduced significantly the interfacial shear strength of DGEBA/DDA resin‐AS4 fiber samples and DGEBA/DDA resin‐AU4 fiber samples. © 1993 John Wiley & Sons, Inc.