Mechanical behaviour of hemp fibre composites in relation to their microstructure by micro strain mapping, computed tomography, and biochemical analysis

This manuscript describes the effects of alterations in biochemical composition on structural morphology and the mechanical behaviour of elementary and technical fibres of hemp used for composite applications. First, the strength and apparent Young's modulus distribution of technical fibres of hemp of 96 hemp samples, corresponding to 32 different hemp accessions cultivated in 3 locations, were analysed using Weibull distribution. From these, 2 samples (one with high and one with low fibre strength) were selected for further analysis. Next, full-field strain measurement at the micro-scale during tensile loading was used for evaluating both, the stress-strain behaviour at a global scale and the local mechanical behaviour heterogeneity at a micro-scale, along elementary and technical fibres of hemp. At the composite level, the local behaviour of each phase of the composite (fibre and matrix) and of the fibre/matrix interphase during a transversal 3 point bending test were characterized. Results show that the strength of technical fibres of hemp is highly dependent on the shear strength between elementary fibres, which itself is related to the biochemical composition of the middle lamellae. A correlation between the strength of a technical fibre and their elementary fibres was also observed. At the composite level, the relation of the composite mechanical behaviour (Young's modulus and strength) with the technical or elementary fibre mechanical behaviour is complex and might depend on the combination of multiple factors such as the matrix (thermoset or thermoplastic), or the technical fibre sample employed (weak or strong) and the level of fibre-matrix wetting (impregnation) and adhesion