Determination of cross-sectional area of natural plant fibres and fibre failure analysis by in situ SEM observation during microtensile tests

Reported tensile mechanical properties of many natural plant fibres vary to a large extent due to very often inappropriate measurement of the fibre’s cross-sectional area by diameter estimation. Using natural ramie filament as a model testing elementary fibre, a more realistic stereological determination is presented, including microscopic imaging analysis of the fibres’ cross-sectional area. When applying the area data using this approach to calculate tensile strength, a far narrower variation in the fibres’ strength distribution according to Weibull analysis was found. The gauge length effects on the mechanical performance of the natural fibre were revealed and analysed. In addition, in situ SEM observations during microtensile measurements detected real time changes in the fibres’ structure during stress. It was found that fibre failure was mainly caused by macroscopic physical defects and associated microscopic slippage of the microfibrils. Furthermore, results of cyclic tensile tests indicated that the fibre underwent elastic deformations under progressive loading–unloading cycles, which is due to bonding restriction that the surrounding matrix presents against the slippage of the microfibrils and reorganisation of hydrogen bonds.