Inside the kink-bands of archaeological flax artefacts via sub-micrometer resolution micro-CT: A comprehensive microstructural analysis to better understand degradation mechanisms of fibres

This work explores how the morphology of kink-band zones in flax fibres impacts the mechanical properties of the elementary fibres. Kink-bands are structural defects and are particularly sensitive to physical and biological stresses, on isolated fibres or in bio-based composite materials. To this end, a panel of archaeological samples from different time periods and preserved under different environmental conditions were selected and studied using synchrotron micro-tomography. It is demonstrated that although kink-bands are generally more numerous in ancient fibres, their degree of severity is sometimes less. This underlines the importance of fibre extraction methods, which are principally responsible for kink-band formation. The results also show that kink-band are weaknesses points, allowing rapid development of internal porosity (up to 25 %) when the fibres are used or stored in extreme environments, and that this porosity can also extend to healthy areas of the fibres. However, in some cases, even after millennia of conservation, it appears that the fibres can present morphologies comparable to modern samples, probably due to their good initial quality. Thanks to the findings of the present work, simplified schemes of degradation in kink-band zones, useable on single fibres but also in composite materials, are proposed. These results confirm the importance of fibre extraction processes on fibre quality and durability, and subsequent use for sustainable and high-performance composite materials and textiles.