Effect of hygroscopy on non-impregnated quasi-unidirectional flax reinforcement behaviour

In order to analyse the influence of conditioning at relative humidity on the behaviour of flax fibre reinforced composites, a preliminary study on flax fabrics needs to be conducted. Therefore, this paper investigates the effect of conditioning on the tensile properties of non-impregnated flax fabrics at different humidities. These fabrics are used as reinforcement materials for semi-structural composite materials. The major drawback of natural fibre reinforced composites is moisture regain. Understanding how the moisture affects the behaviour of non-impregnated flax reinforcement is then crucial. The conditioning of the fabrics in different humid atmospheres made it possible to study the influence of the water content on the hygro-mechanical behaviour of flax reinforcement through tensile tests. The water content of samples is linked to the relative humidity of conditioning atmosphere through the isothermal sorption curves. This paper brings up the significant influence of water content on the stiffness and elongation at break of the fabric, while water content does not seem to have an influence on the breaking load which remains practically constant. Non-linear decrease of flax fabric rigidity and increase of its elongation at maximal force with the increase of water content are observed. It is concluded that water molecules act as a softening agent on flax fabrics. Two kinds of typical tensile curves are observed. Most of the tensile curves of samples show one unique linear region where the behaviour appears to be linear prior to the first yarn failure. For some samples in which water content is between 2.7% and 3.7%, the emergence of a two-linear region behaviour is pointed out. This two-linear region behaviour could be attributed to local heterogeneities. Indeed, this study shows that, at saturation, the amount of absorbed water is not uniform through flax fabric samples. The moisture uptake heterogeneities did not resorb with time after saturation. These local heterogeneities would induce a premature local plasticity in some regions of the flax fabric sample. These regions would present local weakness areas that could induce an exhibition of the two-linear region behaviour.