Elucidating links between the mechanical performance of flax fibres and their structural defects

Abstract

Single flax fibres develop morphological deformations, called kink-bands, which are considered to be areas of weakness. The correlation between the occurrence of kink-bands, fibre extraction method and fibre mechanical properties still needs to be clarified. To this end, an extensive multi-scale statistical study was conducted on 96 elementary fibres extracted from four distinct batches processed with different scutching and combing conditions. First, kink-bands were observed and quantified through polarized light microscopy (PLM), scanning electron microscopy (SEM) and synchrotron X-ray microtomography. PLM quantification demonstrated that uncombed fibres have more cortical residues and larger surface kink-bands with many small internal porosities. Then, the mechanical characteristics of each PLM-quantified single fibre were measured by tensile tests. A larger number of kink-bands was observed for the most intensively combed batch, 18.2 kink-bands/mm, as well as a surprisingly low average kink-band area, of the order of 158 µm² per kink-band. In addition, the same batch had a higher tensile strength and modulus than the other three batches with a negative Stearman correlation of − 0.64 and − 0.78 with the proportion of defects, respectively. Finally, 74% of the fibres in this intensively combed batch, had a ‘Type I’ linear elastic behaviour, highlighting a significant change of behaviour in comparison to the uncombed batch, attributed to local reorientation and pre-stretching. The results suggest that changes in kink-bands structure are principally responsible for these evolutions; due to both new distribution of pores in kink-bands regions and reduction in kink-band size possibly induced by local cellulose realignment.