A correlation between fluid-induced damage and anomalous fluid sorption in polymeric composites

A mechanism is presented to explain the experimental observations that, under exposure to ambient fluid or humidity, desorption and resorption behaviors of polymeric composite samples do not replicate the initial absorption process. It is shown that the above disparities can be attributed to damage, in the form of distributed fiber/matrix interfacial microcracks, which is caused by residual hygrothermal and mechanically induced stresses. Such damage occurs in the interior of composite samples during the early stages of initial fluid absorption, and subsequently in regions adjacent to exterior boundaries during the early stages of desorption. The extents of the foregoing damage regions can be assessed by stress analysis. A certain portion of the fluid remains entrapped within the damaged regions, or attached by adsorption to the faces of the microcracks, which act like capillary channels. Analytical diffusion models, which account in part for the presence of the abovementioned damage and based upon equivalent diffusivity and capillary action, respectively, are presented and shown to yield results that exhibit all the experimentally recorded characteristics.