Hygro-mechanical coupling and multiscale swelling coefficients assessment of flax yarns and flax / epoxy composites

Abstract

This paper proposes to assess, in a multi-scale experimental approach, the hygroscopic expansion coefficients of flax yarns in radial direction and of flax fibre reinforced composites in the three directions (longitudinal, transverse and out of plane). Results show a significant flax yarn radial swelling coefficient (βr=1.06) and a composite highly anisotropic swelling behaviour (βL=-0.02;βT=0.13;βT′=0.85). An analysis of conditioning until saturation at different relative humidities shows the dramatic effect of water content on the composite tensile behaviour. The evolutions of tangent modulus with respect to stress indicate the existence of two types of behaviour depending on water content within the material. As water content increases, the behaviour becomes predominantly plastic and a rigidification phenomenon is highlighted.