Long-term properties of the matrix of composites
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Water diffusion into epoxy can be looked upon on as a 3-stage process. The first stage is the initial reaction, where the ends and available surfaces of the molecular chains bond with the water molecules. These easily accessible reaction sites, where no existing bonds and a larger amount of free volume exists, are the first to yield place for the water. This explains the rapid mass uptake that both the epoxy and CF/epoxy specimens experienced during the first 24 hours of absorption, whereas the process then stabilized and initiated stage two of the process. This stage showed linear Fickian behaviour, which lasted for the next 600 hours. The third stage, as the rate of water absorption decreased, showed non-linear Fickian behaviour. Diffusion coefficients for the pure epoxy specimens were calculated by linear regression based on Fickian's second law, as the absorption process proved to act linearly Fickian during the first 650 hours. The specimens containing CF reached their saturation mass during the 742 hour diffusion experiment. Hence providing results that show that the diffusion process is more accelerated in the transverse direction of a UD CF/epoxy in comparison to a pure epoxy specimen. According to Fickians second law should the diffusivity eventually slow down, but in this study the curves were near linear until the CF containing specimens reached mass saturation. Accordingly have some mechanism within the specimens provided an accelerated diffusion path, which may be an indication that wicking have occurred. Desorption occurs at a much higher rate than absorption. The initial behaviour is in accordance with the initial behaviour of absorption, as a rapid mass release was experienced during the first 24 hours. After the easily available surfaces and ends of the molecular chains had yielded their H-bonds the rate of desorption decreases. It was observed that the desorption process was approximately twice as fast as the absorption process. The mechanical behaviour of the pure epoxy specimens experienced several changes during the water absorption process. During the first 120 hours of diffusion in this study the average elastic modulus decreased by 18\%, whereas the average ultimate tensile strength decreased by 21\%. Further, the maximum strain suffered a 13\% loss. After 310 hours of absorption all three properties decreased further, although only by a small degree. At this time the specimens showed a greater plastic region, as necking was observed in the tensile test results. These results show how water acts as a plasticiser, effectively increasing the material's ductility. This plastic behaviour completely disappears after 742 hours of water absorption. Both the elastic modulus and the ultimate tensile strength remain relatively unchanged, but the maximum strain have decreased. This is a clear indication that the formation of multiple H-bonds between the molecules have begun. The CF containing specimens suffered more than a 15\% loss of ultimate tensile strength in the fibre direction due to the water mass uptake. Although de-lamination may be an issue when the interface gets saturated, was the failure of the specimens due to matrix cracking. Even though the strength decreased, it was observed that the elongation properties increased with a higher water mass uptake.