Impact of flax fibre micro-structural features on composite damage observed through micro-CT characterisation

To exploit the potential of flax fibres in reinforcing polymers, the performance of flax fibres must first be understood and then optimized. In this context, this paper aims to provide a visual and comprehensive description of the impact flax fibre micro-structural features, such as kink bands, porosity and cortical residues, have on damage evolution during tensile loading of Polylactic acid (PLA) matrix composites reinforced by flax fibres. Insitu synchrotron radiation computed tomography (SRCT) has been used for 3D visualisation of microstructural evolution at stress levels between 10% and 90% of the ultimate failure stress. First, the main defects of the overall microstructure are described, including a quantitative analysis of porosities. Then, novel visual insights, highlighting the main role of kink-bands in fibre failure and subsequent composite breakage, are described. Interestingly, it appears that for the flax/PLA composite studied, kink-band inter-distances are consistently smaller than critical fibre lengths, explaining the likelihood of rupture in kink-band regions. These findings demonstrate that fibre extraction and subsequent textile reinforcement manufacturing are critical steps and should be optimised to increase the performances of natural fibre composites.