Flexural Performance Degradation of <scp>3D</scp> Orthogonal Jute/Basalt Hybrid Composites Under Hygrothermal Aging

ABSTRACT This study fabricated hybrid composites via vacuum‐assisted resin transfer molding (VARTM) using basalt fibers (BF) and alkali‐treated jute fibers (JF) in a seven‐layer through‐thickness orthogonal 3D woven structure, investigating their flexural property evolution after 90‐day hygrothermal aging at 25°C/65°C. Moisture absorption followed Fick's law: a higher JF volume fraction increased the equilibrium moisture content, while elevated temperature‐accelerated moisture diffusion. Three‐point bending tests showed aging‐induced degradation; at 65°C after 90‐day aging, the flexural strength retention rates of [7B], [3B4J], [4J3B], [5J2B], and [7J] were 74.19%, 72.62%, 69.13%, 68.03%, and 67.01%, respectively. Notably, the interleaved [3B4J] outperformed the concentrated [4J3B] in flexural retention. For single‐fiber composites, hygrothermal conditions especially at 65°C exacerbated the degradation of pure jute‐based composites, which the hybrid effect effectively mitigated, though it compromised the inherent mechanical advantages of pure basalt‐based composites. The degradation mechanisms involved moisture‐induced fiber–matrix debonding where water infiltrates the interfaces, matrix cracking caused by swelling‐induced internal stress, and hydrophilic deterioration of jute fibers due to hydroxyl‐induced swelling. All these processes were aggravated by high‐temperature thermal stress. JF moisture absorption exerted a dual role: it may lead to the decline of its own mechanical properties but enhance material toughness to inhibit crack propagation. This work provides insights for optimizing hydrothermal‐resistant natural fiber hybrid composites.