Fibres from annual plants are a sustainable alternative for glass fibres in composite manufacturing. However, both synthetic fibres, such as carbon fibres, and cellulosic fibres exhibit heterogeneities along their lengths, which appeared as localized morphological disorientations 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. Presence of cavities in transition zones was assessed by SEM and AFM and confirmed by low intensity SHG signal, especially when large size kink-bands are considered. Moreover, transverse indentation modulus is 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 to 7.3 GPa. Also, important MFA changes are 2 measured through SHG imaging, especially in large kink-bands with local misorientation up to 47°. Thus, this intense investigation of kink-band areas reveals ultrastructure heterogeneities and the presence of local defects.
Plant fibres and especially flax can be distinguished from most synthetic fibres by their intricate shape and intrinsic porosity called lumen, which is usually assumed to be tubular. However, the real shape appears more complex and thus might induce stress concentrations influencing the fibre performance. This study proposes a novel representation of flax fibre lumen and its variations along the fibre, an interpretation of its origin and effect on flax fibre tensile properties. This investigation was conducted at the crossroads of complementary characterization techniques: optical and scanning electron microscopy (SEM), high-resolution X-ray microtomography (µCT) and mechanical tests at the cell-wall and fibre scale by atomic force microscopy (AFM) in Peak-Force Quantitative Nano-Mechanical property mapping (PF-QNM) mode and micromechanical tensile testing. Converging results highlight the difficulty of drawing a single geometric reference for the lumen. AFM and optical microscopy depict central cavities of different sizes and shapes. Porosity contents, varying from 0.4 to 7.2%, are estimated by high-resolution µCT. Furthermore, variations of lumen size are reported along the fibres. This intricate lumen shape might originate from the cell wall thickening and cell death but particular attention should also be paid to the effects of post mortem processes such as drying, retting and mechanical extraction of the fibre as well as sample preparation. Finally, SEM observation following tensile testing demonstrates the combined effect of geometrical inhomogeneities such as defects and intricate lumen porosity to drive the failure of the fibre.
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