Extensive investigation of the ultrastructure of kink-bands in flax fibres

Abstract

Fibres from annual plants are a sustainable alternative to glass fibres in composite manufacturing. However, both synthetic fibres, such as carbon fibres, and cellulosic fibres exhibit heterogeneities along their lengths, which appear as localised morphological misorientations known as kink-bands. In plant fibres, kink-bands occur due to growing conditions under abiotic stress during the retting stage and the fibre extraction process. Many studies have been conducted to identify the origin of such kink-bands and their impact on the mechanical behaviour of composites, but their characteristics and fine ultrastructure remain unclear. The presence of cavities in transition zones was assessed by scanning electron microscopy (SEM) and atomic force microscopy (AFM) and confirmed by low-intensity second harmonic generation microscopy (SHG) signals, especially when large kink-bands were considered. Moreover, the transverse indentation modulus was obtained by AFM in peak force mode; no substantial differences were observed between the kink-band and defect-free regions, with average values ranging from 6.2–7.3 GPa. Additionally, important MFA changes were measured through SHG imaging, especially in large kink-bands with local misorientation up to 47°. Thus, this in-depth investigation of kink-band areas reveals ultrastructure heterogeneities and the presence of local defects.