Ageing of out-of-autoclaved hybrid flax/carbon fibre composites: effects on water absorption and flexural behaviour

The growing demand for high-performance yet sustainable composites highlights the need to replace synthetic composites and energy-intensive processing with more efficient alternatives. However, limited research has been conducted on how out-of-autoclave (OOA) fabrication affects the durability of the natural/synthetic hybrid composites under hygrothermal ageing. This study examines the ageing behaviour of flax/carbon fibre hybrids fabricated using a vacuum-assisted oven-curing process, focusing on moisture diffusion, swelling, and flexural response. Hybridisation reduced diffusion coefficients under hot-wet conditions by 76–96 % and swelling by up to 80 % compared with flax composites. It also enhanced flexural strength and stiffness up to 200 %. The hybrids retained 60–80 % of their flexural strength and 70–90 % of their stiffness after 23, 45, 70 °C water exposure, confirming the effectiveness of the carbon layers in limiting water-induced deformation and preserving interfacial integrity. • Hybrid Flax/carbon fibre composites were developed using a vacuum-assisted, out-of-autoclave curing process. • Diffusion rates decreased by 76–96 % in the hybrid composites. • Hybrid composites retained up to ~ 70 % strength and ~80 % stiffness after conditioning in hot–wet and frozen environments. • Moisture uptake and swelling rates were significantly reduced in the hybrid composite laminates.