In natural fibre based-composites, the interfaces within the fibre bundles are not as well described as those between fibres and classical polymer matrices. To compensate for this lack of information, which is however of great importance when trying to model the composite behaviour, morphological and mechanical characterization of the middle lamella present between flax fibres has been carried out. The interfacial strength of the pectic cement was found to be very low compared with that of the fibre/matrix interface. Assuming this fibre/fibre interface to be a cohesive zone material and using the experimental results obtained on pairs of fibres, a model has been developed at the scale of the fibre and applied to study numerically the tensile properties of bundles. The results of the simulations were in agreement with the experimental data obtained on bundles, both leading to a mean bundle strength of 500 MPa.
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