Flax-based natural composites hierarchically reinforced by cast or printed carbon fibres

Plant-derived natural fibres hold great potential as renewable and sustainable reinforcing elements in structural composites. However, a broader use of natural fibre composites requires further improvements in their mechanical properties, to reach performance comparable to carbon fibre-reinforced polymers. In this study, we exploit discontinuous carbon fibres in rheologically modified inks to controllably reinforce flax-based laminates in specific directions. The carbon fibres are incorporated by tape casting or 3D printing approaches directly on the pre-aligned flax structures. With the help of quasi-static flexural tests and dynamic mechanical analysis, we show that the elastic modulus, the strength, and the damping behaviour of the flax-based composites can be significantly enhanced by controlling the relative orientation of the hierarchically structured carbon and flax fibres. The flexural stiffness of composites reinforced with carbon fibres oriented along and perpendicular to the flax fibres increases, respectively, 62% and 146% relative to the carbon-free reference. This is accompanied by a 1.6-fold increase in loss modulus, which is a performance indicator for damping. The experimentally observed stiffening of the flax-based structures can be described using simple beam theory. By combining reinforcing elements of different length scales with readily available manufacturing techniques, this work shows the potential of hierarchical structuring in improving the mechanical performance of flax-based composites.