Elementary flax fibre tensile properties: Correlation between stress–strain behaviour and fibre composition

• The stress–strain behaviour of six flax fibre samples was analysed.
• The cell-wall composition was put in relation with stress–strain behaviours.
• Three main types of stress–strain behaviour were observed.
• The non-linear behaviour was found to be in favour of high tensile properties.
• The higher amount of hemicellulosic encrusting, the larger the non-linear section.

Flax fibres represent an eco-friendly alternative to glass fibres in composite materials. Natural fibres do not have a unique standard response to tensile testing. To increase understanding of the response to tensile testing, six batches of flax fibres (Linum usitatissimum) were selected for their differences in tensile properties: four batches consisted of Marylin-variety cultures grown in the same area but in different years, and two were reference-samples from Oliver and Hermes varieties. Their tensile values were either moderate (E ≈ 45–55 GPa, σ ≈ 800–1000 MPa) or high (E > 55 GPa, σ > 1000 MPa). Three major types of stress–strain behaviour were observed, but in different proportions in each sample. The first one consisted of a linear tensile behaviour; the second one was composed of two linear distinct sections, and the third one displayed a non-linear section at the beginning of the loading stage up to a threshold point, followed by a section where the tangent modulus increased up to failure. The samples exhibiting a large proportion of the third type of behaviour were characterised by high tensile properties. The extent of the non-linear section highly depended on the variety. Within the Marylin variety, the tensile properties were higher when the non-linear section was smaller. Considering the fibre as a composite per se, reinforced by cellulose microfibrils coated with hemicelluloses embedded in a matrix of incrusting pectins, we found some correlation between tensile behaviours and the cell-wall composition that highlighted the importance of the hemicelluloses and hemicelluloses/pectins ratio.

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