Damage characterisation of flax fibre fabric reinforced epoxy composites during low velocity impacts using high-speed imaging and Stereo Image Correlation

This work aims at studying the low velocity impact and post-impact behaviour of flax/epoxy composites. The samples were composed of 8 plies of 2/2 twill weave fabric. The experimental device used for impact tests was a drop tower with impact energies varying from 5 to 34 J (perforation). Pictures of the sample were captured during the test with two high-speed cameras in order to perform high-speed imaging and Stereo Image Correlation (SIC) analysis. Residual properties have been assessed by bending after impact tests. High-speed imaging easily detected the back face cracks at maximum deflection while some were almost invisible after impact. The results also highlighted the material's behaviour in three-stages. At low energy, no damage was observed. The cracks and the recovered energy evolved proportionally with the impact energy between 5 and 25 J, and then reached a threshold. The post-impact tests showed that even a 5 J impact resulted in a degradation of composite performance, with a loss of approximately 10% in stiffness and 21% in strength.