Mechanical properties of flax-fibre-reinforced preforms and composites: Influence of the type of yarns on multi-scale characterisations

As the strongest natural fibres, flax fibres are being used increasingly for technical advanced products such as composites with a view to reducing the impact of the material on the environment throughout its life cycle. In order to improve the understanding of the mechanical properties of flax fibres and their reinforced preforms and composites, multi-scale characterisations are carried out to analyse the tensile properties of flax fibre yarns, fabrics and composites. From three types of yarns, quasi-UD fabrics and composites are manufactured with the same process parameters. Tensile tests are performed at each scale to study the effect on the properties of yarn such as the twist level. At the yarn scale a significant variability of the tensile behaviour is pointed out, as described in the literature. The tensile behaviour of un-impregnated fabrics shows the effect of the weaving process. The tensile results at the composite scale indicate the same trend compared to yarn after the weaving and fabric scales. From the experimental results at different scales an analysis is conducted to explain the tensile behaviour, taking into account the different steps of the manufacturing process. In addition, micro-observations through SEM images will confirm the variability of the tensile behaviour of a single flax yarn and emphasize the strong influence of the twist level on the tensile characterisation at different scales.