Is the low shear modulus of flax fibres an advantage for polymer reinforcement?

Flax fibres are increasingly used as polymer reinforcement as an alternative to glass fibres. Indeed, flax has attractive assets (low density, low environmental footprint, high tensile mechanical properties, etc.) but also some limits that need to be investigated. This study gives a comparison between the in-plane shear behaviour of flax/epoxy and glass/epoxy unidirectional laminates. For equivalent volume fractions, the shear stiffness of biocomposites is significantly lower than glass ones. This result is explained by the microstructure and anisotropic properties of flax fibres wall components. However, the micromechanical analytical model of in plane shear strength demonstrates that the low shear modulus of flax fibres (Gf(LT)=2500 and 29,700 MPa for flax and glass fibre, respectively) is an advantage since it limits strain concentration within plies by reducing potential cracking. Finally, due to the low failure strain of flax composites, our study show that a 2% strain limit must be chosen for a suitable comparison of flax and glass composites shear strength.