Résumé
Defects in flax fibres limit the use of more sustainable load bearing composites in industry, which highlights the need for a thorough understanding of their nature. In this work, X-ray micro-tomography of flax revealed pores in the cell wall of elementary fibres: kink-bands pores and longitudinal pores, a previously unseen defect. Their morphology and organisation are examined, highlighting fibre deterioration and locally increased porosity up to 14.86 %. Finite element modelling under a 1.5 % tensile strain reveals that kink-band pores concentrate stress up to 7.15 times, while longitudinal pores reach 2.35 times compared to defect-free areas. Under tension, cracks are thus likely to initiate at kink-band defects, may propagate through longitudinal pores to other kink-bands, and lead to fibre and composite failure as these defects are favoured zones for crack initiation and propagation. In situ peeling of fibres due to knot tightening under scanning electron microscopy suggests interlaminar decohesion between cellulose macrofibrils as the origin of the longitudinal pores. The study explores hypotheses on the origin of these weak interfaces related to fibre growth and extraction processes. It provides insights for improving flax fibre properties and widening the use of more sustainable composites.