Investigation of the diffusive and mechanical behavior of epoxy composites upon water sorption

The diffusive and mechanical behavior of the tetraglycidyl diaminodipheny methane (TGDDM) resin-based Hercules AS4/3501-6 composite and the diglycidyl ether of bisphenol-A (DGEBA) resin-based Fiberite ANC3K/948A1 graphite/epoxy composites were investigated during water sorption at different temperatures. Several water sorption kinetic models and a water uptake-stress relaxational model were developed to predict dynamic water concentration and water-induced internal stress distribution in two-dimensional as well as one-dimensional model composites. The absorption kinetics of water in both composites at 50, 70, 90 and 100$\sp\circ$C were fitted well by the Fickian model. However, Langmuir-type two step sorption was observed in Fiberite ANC3K/948A1 composite samples at 50 and 70$\sp\circ$C. Using scanning electron microscopy, internal cracks due to the water absorption were found in Fiberite ANC3K/948A1 samples conditioned in 90 and 100$\sp\circ$C water, while no cracks were detected in Hercules AS4/3501-6 samples conditioned in 100$\sp\circ$C water. Ultrasonic testing did not show significant change of modulus or density of Hercules AS4/3501-6 samples conditioned in 100$\sp\circ$C water. No significant change of dynamic modulus or damping were observed for the Hercules AS4/3501-6 samples redried after immersion in 100$\sp\circ$C water, while a slight change was observed above 120$\sp\circ$C for the samples containing absorbed water. However, both water-containing and redried Fiberite ANC3K/948A1 samples showed a decrease of dynamic modulus and an $\omega$-transition around 120$\sp\circ$C. The single fiber fragment test revealed that the absorbed water at 80$\sp\circ$C significantly reduced the interfacial shear strength of DGEBA/DDA resin-AS4 fiber samples and DGEBA/DDA resin-AU4 fiber samples. This reduction of interfacial shear strength comes mainly from the reduction of physical strength, as ATR-FTIR spectroscopy tests showed that at 100$\sp\circ$C water did not attack the interfacial bonds