Flexural behavior of epoxy resin-based carbon fiber/halloysite nanotube hybrid reinforced composites under pre-bending load and hygroscopic conditions

Abstract Carbon fiber reinforced epoxy (CF/EP) composites are widely used in marine environments, facing challenges from water exposure and pre-bending (PB) during construction and service. While many studies have focused on the residual impact strength after pre-compression loading and the residual flexural strength of thermoplastic composites under various temperature conditions, nevertheless, analysis of the residual flexural behavior after PB in situations such as water absorption environments with thermosetting composites remains limited. In this study, residual flexural behavior under water absorption conditions was analyzed to evaluate the influence of PB and water absorption in residual flexural strength with CF/EP. In particular, the research examines CF/EP composites reinforced with halloysite nanotubes (HNTs), evaluating their residual flexural strength after PB under various conditions: room temperature, water immersion, and post-immersion. Results show that HNTs enhance flexural strength and modulus by improving fiber–matrix interfacial bonding and reducing water absorption due to their non-polar surface. Under 80% PB, HNT-reinforced composites exhibited significantly lower reductions in flexural strength (5%) and modulus (14%) compared to unreinforced CF/EP (15% and 24%, respectively). These reductions are attributed to fiber kinking and misalignment induced by compressive loads. Water absorption further deteriorates flexural properties, leading to non-linear stress–strain behavior. Specimens pre-bent after water immersion showed more pronounced property degradation compared to those pre-bent before immersion. This combined effect arises from weakened interfacial bonding due to water ingress and the formation of kink bands under compressive loads. This study emphasizes the critical importance of considering both water absorption and PB during the design and application of CF/EP composites in marine environments. The incorporation of HNTs offers a promising approach to enhance the durability and performance of these composites in such challenging conditions.