Non-linearities in tensile behaviour of flax fibre bundles

This work focused on the mechanical behaviour of flax fibre bundles to understand their response to tensile testing. Flax bundles exhibit the same non-linear behaviour under mechanical stress as elementary fibres. Video recorded tests revealed that during tensile loading, fibre bundles undergo a coupling between tension and rotation. Three different scenarios were identified: pure tension, tension-torsion coupling to failure, and partial tension-torsion coupling ending before specimen failure. Elementary fibre decohesion was also observed during fibre bundle tensile tests. Overall, the tensile behaviour of fibre bundles is influenced by their initial morphology, especially the degree of twist, and the mechanical efficiency of the lamella between the elementary fibres (middle lamella). In addition, the influence of the geometric model used to represent the cross-section on the mechanical properties was investigated using circular and elliptical models. The results show a significant dependence on the geometrical model considered. Therefore, a standardised method for measuring the cross-sectional area of plant fibres is required to enable comparison of results from the literature.