Exploring the link between flexural behaviour of hemp and flax stems and fibre stiffness

Hemp (Cannabis Sativa) and flax (Liman Usitatissitruon L) are the most commonly cultivated fibre plants in Europe. The use of their fibres covers a wide range of applications and requires a precise knowledge of their mechanical properties. The aim of this study is to investigate the contribution of fibres to the stiffness of flax and hemp dry stems subjected to bending during a simple three-point flexural test. The analysis is based on measuring the difference of bending stiffness between stems with fibres and stems without fibres after removal by manual peeling. Mechanical characterization shows that hemp and flax fibres contribute 54% and 71%, respectively, on average, to the flexural stiffness of a stem. Then, a microstructural analysis of stems is used to determine fibre distribution and geometry, which leads us to obtain longitudinal moduli of 33.9 +/- 13.0 GPa and 55.8 +/- 14.5 GPa for hemp and flax, respectively. These values are close to those obtained by tensile tests on elementary fibres (30.0 +/- 13.3 GPa and 54.7 +/- 13.2 GPa). These results encourage to propose the three-point bending test on stems as a standard test for the estimation of bast fibre stiffness.