Influence of processing temperature on mechanical performance of unidirectional polyamide 11–flax fibre composites

• Innovating UD biocomposites made up from PA11 and flax fibres were processed.
• We investigated the impact of the thermal cycle on single fibre mechanical properties.
• Heat-treated fibre properties were used to estimate composite stiffness and strength.
• We pointed that fibres does not have same mechanical behaviour inside or outside the matrix.
• An interesting correlation between mixture rule and composite stiffness was evidenced.

This paper reports the mechanical behaviour of novel biocomposites made up from PA11 and flax fibres. We find that, using various flax fibre loadings (i.e. volume fractions), it is possible to obtain bio-based composites with good and reliable tensile properties compared to other biocomposites such as PLA/Flax. Our investigations reveal that the thermal cycle process used for composite moulding has a significant impact on the fibre mechanical properties. Indeed, a 210 °C thermal cycle for 8 min induces a decrease in fibre stiffness and strength of 32.8 and 12.9%, respectively. A simple micromechanical model, i.e. mixing rule, is applied to estimate composite stiffness and strength based on heat-treated fibre properties. In the case of composite stiffness, our results indicate an underestimation of the Young’s modulus compared to experimental data, especially when the composite stiffness is taken in the first part of the stress–strain curve where the stiffness exhibit unstable values. This implies that the mechanical behaviour of flax fibre bundles compared to fibre bundles constrained within the polymer matrix is not the same, probably due to rotational locking, which prevents the micro fibrillar realignment of the cellulose leading to higher stiffness values.