Angular dependence of hygroelasticity in unidirectional glass-epoxy composites

Swelling and hygroelasticity of unidirectional glass fibre-reinforced epoxy composites in boiling water is studied with respect to the angle θ between the direction of orientation and the direction of measured dimensional change. The results are expressed in terms of the dependence of θ of a hygroelasticity coefficient. A very good agreement is observed between the experimental results and the theoretical equation, already verified in the case of the angular dependence of the coefficient of thermal expansion. This is taken as additional support for the analogy between thermoelastic and hygroelastic behaviour in composites. The experimental longitudinal and transverse hygroelasticity coefficients are found to be different from predictions based on Schapery's equation. However, it is shown that these equations can produce possible valid estimates of μ_L and μ_T provided the mechanical properties of the swollen constituent materials are used.