Humidity-dependent fatigue damage evolution in unidirectional flax fibre composites

This study investigates unidirectional flax fibre composites with epoxy (EP) and partially bio-based unsaturated polyester (UP) matrices conditioned to three humidity conditions (20%RH, 50%RH, and 85%RH) to assess how the resin system influences mechanical performance under hygroscopic exposure. Specimens were subjected to quasi-static tension, flexion, and tension-tension fatigue. Moisture uptake was observed to strongly influence both the quasi-static and fatigue behaviour of the flax fibre composites. Increasing the relative humidity from 20% to 85% led to a reduction in both tensile and flexural modulus, with decreases of approximately 12% for EP-Flax and about 24% for UP-Flax. The modulus measured during fatigue decreased by approximately 25% for both composites with increasing humidity. The effect of humidity on fatigue life depended on the matrix system, with EP-Flax exhibiting greater fatigue resistance than UP-Flax. Increasing humidity also increased hysteresis energy, loss factor and residual strain, indicating greater irreversible deformation. Interrupted cross-sectional microscopy revealed earlier fibre-bundle cracking in UP-Flax and a humidity-dependent shift towards cracking within individual fibres. Post-fatigue fibre pull-out also increased with humidity, indicating reduced interfacial resistance. These apparently competing effects suggest that, in the fibre-dominated EP-Flax laminates, increased deformation capacity and stress redistribution can delay final failure despite partial interfacial degradation.