Short-term moisture effects on the flexural performance of sustainable flax–epoxy and carbon-flax–carbon (CFC) composites

Natural fibers provide a sustainable alternative to synthetic reinforcements, offering environmental and economic benefits. However, their hydrophilic nature weakens fiber–matrix adhesion in the presence of moisture, leading to performance degradation. Hybridizing with carbon fibers offers an effective means of enhancing durability and reducing moisture uptake. This study examines the short-term (4 h and 72 h) effects of moisture on the flexural behavior of flax–epoxy and carbon–flax–carbon (CFC) epoxy laminates. Standard specimens were immersed in plain water, natural rainwater, and saline solutions (10% and 22% NaCl) to emulate storm splash or brief flooding. Unlike most prior works centered on long-term immersion, we quantify short-duration responses across distinct environments. Results show that carbon outer plies reduced water uptake by ~45–55% relative to flax-only laminates. After wetting, flax laminates lost ~13–15% of flexural load, while CFC laminates showed a similar reduction (≤15%). Critically, wet CFC specimens maintained a load capacity comparable to that of dry flax, demonstrating the protective efficiency of carbon skins. Increased salinity further curtailed moisture ingress and performance loss. These findings demonstrate that strategic carbon/flax hybridization effectively limits short-term moisture damage and preserves flexural performance, providing practical guidance for lightweight, moisture-resistant laminates in transiently wet outdoor service.