Modeling of water wicking along fiber/matrix interface voids in unidirectional carbon/vinyl ester composites

Void formation at the fiber/matrix (F/M) interface is known to be a primary structural defect in a composite material. It is a major factor that contributes to the water uptake in composite materials for underwater applications. We develop a mathematical model to describe the kinetics of water uptake in unidirectional fiber reinforced resin composites containing voids. By using a one-dimensional two-phase fluid flow model with parameters derived from Microcomputed X-ray tomography (mu CT), we predict the water wicking process in carbon/vinylester (CF/VE) panels containing capillary voids at the F/M interphase. The capillary driven flow is impeded by viscous forces and the compressed air bubble, trapped between the two flow fronts. In particular, our calculation indicates that the effective contact angle at the F/M interface in CF/VE composite is 29.7 +/- 0.1 degrees for the equivalent capillary radius of 1.2 mu m. The results are validated by comparing the simulated water absorption to the experimental data for CF/VE composite specimens of three different sizes immersed in sea water.