Rapid Moisture-Characterization of Composites and Possible Screening Applications

Abstract

Moisture diffusion in fiber reinforced plastics is characterized by two observables: the saturation level Pe, which is the weight percent moisture uptake at equilibrium, and the susceptibility parameter Pe√D which is determined by the initial absorption rate, D being the diffusion coefficient for a direction normal to the exposed surfaces. We describe a relatively quick method for determining these parameters and discuss the possibility of using the results to screen composite materials for consistency of mechanical properties that depend on resin composition and cure quality. We designate this approach as the incremental grinding method. Experi mental tests on composites made from Narmco 5208 (epoxy) and Hexcel F178 (polyimide) resins show that this method can be used, at least in these cases, to distinguish between different compositions and postcures. Aside from the matter of sample screening, the experimental data indicate that simple diffusion theory gives good predictions of moisture distribu tion, as well as gross moisture uptake, in composites exposed to humid environments.