Prediction of Moisture Diffusion Parameters in Composite Materials Under Stress

Moisture absorption by polymeric composite materials is greatly dependent on the state of the applied stress. The present study deals with the most common diffusion parameters, namely, the rate, the maximum absorption capacity, and the Fickian diffusion coefficient, and proposes a model which predicts their changes with the stress level. The model is based on the considerations that the absorption capacity is determined by stress-dependent free volume changes, and that the diffusion coefficient in the unstressed as well as in the stressed state is expressed by a Doolittle-type equation. The free volume changes of the matrix are calculated through laminate theory equations using the elastic properties and the volume contents of the constituents. Experimental work with carbon fibre-reinforced epoxies of different fibre contents and orientations, loaded to various stress levels, exhibit excellent agreement with the theoretical model.