Study of the moisture/stress effects on glass fibre/epoxy composite and the impact of the interphase area

This paper shows the influence of interphase areas in a UD glass fibre/epoxy composite on water diffusion process and mechanical properties thanks to experimental, analytical and numerical analysis. The interphase is first characterized in terms of molecular mobility by micro-thermal analysis and in terms of mechanical properties by force measurements with atomic force microscopy. The measurements show an increase in elastic modulus and in glass transition temperature around each fibre. Optical microscopy reveals an important heterogeneity of fibre distribution inside the matrix with many contacts between fibres. A realistic microstructure including the interphase size (defined by μTA) and fibre distribution has then been considered to model the composite accurately. Diffusion parameters resulting from water absorption measurements on both the resin and the composite were also used in the finite element analysis. The comparison between experimental data, analytical and finite element models shows that the water diffusion coefficient is about five times higher in the interphase area than in the matrix whereas the gradient in modulus around the fibres barely modify the composite mechanical response.