Understanding the lodging stability of green flax stems; The importance of morphology and fibre stiffness

•An original comparison of the lodging resistance of two varieties of flax is shown.•A detailed analysis of stem structure and fibres mechanical properties was conducted.•Interestingly, Eden fibres exhibit higher mechanical properties than TDL25 ones.•Innovating bending tests on dried and green flax stems were developed.•The risk lodging was estimated thanks to a new mechanical safety factor.

Flax fibres (Linum usitatissimum) with good mechanical properties are required to reinforce polymers. Usually, their characterisation is made on retted and scutched fibres. This work provides a new and original method to both determine the stiffness of green fibres contained inside the plant and to estimate the crop lodging stability.We studied two recent flax varieties (Eden and Terre de Lin (TDL) 25) with a distinct lodging resistance (respectively high and low). Both varieties, grown under the same conditions, exhibit a similar fibre yield. The analysis conducted is based on the correlation between the bending stiffness of the stems, the distribution of fibres in a cross section and the properties of elementary flax fibres. The results of the mechanical characterisation indicate that the Eden variety has a superiority concerning fibre stiffness (68 GPa versus 55 GPa). The analysis of the bending stiffness of a dried plant at different localisations along the stem and of its fibre distribution in the corresponding cross section allowed us to estimate the fibres average modulus. Results obtained by this method were very close to the tensile tests values. The same procedure was used on green stems to approach the living state of the plant and to determine the Young's modulus of green fibres. The results highlighted a variation of fibre stiffness between the green state and the dried state (around +25%). The results enabled the use of a simplified buckling model, which confirmed the superiority of the Eden variety in lodging resistance. Thus, the analysis of the mechanical properties of flax stems and their structure could be a selection criterion.