Mechanical behaviour of thick flax and carbon-flax hybrid laminates under multiple load cases

The potential of natural fibres as a replacement reinforcement in traditional fibre reinforced polymer composite applications has seen increasing investigation due to their cost and environmental impact. Thick composites are also being investigated for both structural and tooling applications during manufacturing. This work assessed the structure-property relationships governing failure in thick (∼10 mm) flax fibre reinforced polymer (FFRP) laminates and 1-8-1 hybrid laminates with Carbon Fiber Reinforced Polymer (CFRP) facesheets, across different loading modes, using unnotched tensile, combined loading compression (CLC), open-hole tensile (with Digital Image Correlation), flexural and interlaminar shear strength (ILSS) tests. Post-mortem visual and optical observations were made using a microscope and scanning electron microscopy (SEM). The results provide insight into the mechanical behaviour of these laminates and the influence of carbon face-ply hybridisation. Hybridisation selectively enhances stiffness- and interface-dominated properties (flexure, ILSS), while exerting minimal influence on net-section tensile and OHT strength. Results were consistent among tests, with carbon-flax and flax-only laminates exhibiting comparable net-section tensile strengths. The unnotched tensile strengths differed by less than 1%, indicating that hybridisation does not significantly alter tensile strength in this laminate configuration. The addition of carbon-fibre facing plies had a modest effect on strengths except for ILSS and flexural strength which increased by 29% and 66%, respectively.