Abstract
Polymer composite materials are widely used in marine applications where an understanding of the long-term performance is essential for economic, safety and durability requirements. Although moisture absorption in composites has been studied for many years, the relationship between the mechanisms of moisture absorption and consequent changes in material behaviour has received much less attention. In this work, long-term exposure of three unidirectional composite materials in water has been studied in relation to mechanical property changes and moisture uptake. Elevated temperature was used to accelerate ageing, and the effects of pressure and mechanical load on moisture uptake were investigated. The resultant material property changes were characterised using four-point bend flexure tests, dynamic mechanical analysis, Fourier transform infra-red spectroscopy, and both optical and scanning electron microscopy. Fickian style diffusion is shown to be inadequate at describing moisture uptake and instead a multi-mechanism Langmuir style diffusion model was used. Using this model, it is possible to correlate water absorption to changes in flexural properties (modulus and strength) and in the glass transition temperature, T. Flexural modulus has been shown to be most affected by water interaction with the fibres; T by water interaction within the resin; and flexural strength equally by water interaction with both fibres and resin. Developing this relationship between the effects of the moisture diffusion mechanism on material property changes is essential for understanding long term performance of composite materials.