Capillary filling of water in structural defects at fiber-matrix interface of unidirectional composite materials

Presence of internal structural defects and damages in a polymeric composite can affect its moisture absorption behavior. This study developed a mathematical model to evaluate the behavior of water uptake in the voids at fiber/matrix interface of unidirectional composite materials. The model considers moisture absorption via diffusion by the Fick’s law and water filling driven by capillary action in the voids at the fiber/matrix interface. The water wicking in the voids is modeled using a one-dimensional two-phase fluid flow, where capillary filling is influenced by the initially trapped air bubbles in the voids, permeation of air is enhanced by the pressure from water front of capillary flow. A material parameter, contact angle was introduced to measure the strength of capillary action in the voids. The model was verified experimentally with a carbon fiber reinforced vinyl ester material. The predicted water filling behavior agreed well with the experimentally measured content of absorbed water. Dependences of water absorption behavior on contact angle and void size were discussed.