Moisture diffusion in a fiber-reinforced composite: Part I - Non-Fickian transport and the effect of fiber spatial distribution

The results of an investigation into the effects of non-Fickian sorption and fiber spatial distribution on moisture diffusion in laminated composite materials are presented as part of a study to develop a predictive model for fiber-reinforced composite moisture transport mechanics. Experimental diffusion rate data are obtained from gravimetric analysis of T800/924 carbon fiber epoxy composites using Fickian absorption models adapted to account for hygro-elastic relaxation and polymer-penetrant interaction mechanisms. The transverse diffusion coefficient of the composite (D-z) is determined relative to that of the polymer matrix (D-p) and the ratio D-z/D-p is compared with experimental data provided in other published research. The experimental data, both from this investigation and that obtained from other sources, exhibit significant scatter and only limited correlation with a traditional flux-continuity model. The use of the adapted sorption models reduces experimental scatter by 60% through improved representation of the true sorption mechanics of polymer-based systems. The effects of fiber spatial distribution on the predictions of the flux-continuity model are also considered. There is found to be negligible difference between fully random and, the more commonly assumed and analyzed, regular-packed distributions. A novel finding of the investigation is an assessment of the effect layered high and low fiber volume fraction regions have on the transverse diffusivity of a composite. In such configurations, D-z/D-p is significantly reduced from that predicted using bulk composite properties, providing a possible explanation for the variation between some experimental data and predicted behavior.