Flush-patch aluminum–FRP repair joints for construction and retrofitting: effects of laminate type, hybrid bonding and PAN nanofiber interleaves under pre-damage and hygrothermal ageing

Flush-patch repairs are used to restore local structural integrity in aluminum–FRP panels, but the joint layout and hot–wet exposure strongly affect the capacity retained after damage and ageing. We designed a coupon-scale flush-patch-like joint in which a surface patch bridges a central gap in Elium-matrix laminates reinforced with glass, basalt or carbon fibers (GFRP, BFRP and CFRP), and the patch is supported by a 6061-T6 aluminum backing. Five joint concepts were compared: neat bonded (NE-B), PAN nanofiber-reinforced bonded (NFR-B), bolt-only (BLT), hybrid bonded/bolted (HB/B), and nanofiber hybrid (NFR-HB/B). Monotonic tensile tests were conducted in three conditions: unaged, pre-damaged by three-point bending to 70% of the peak bending load and hydrothermally aged in water at 70 °C for 30 days. In the unaged condition, nanofiber interleaving increased peak load by 21–24% for bonded joints and by 10–22% for hybrid joints compared with neat counterparts. After pre-damage, the same interleaving raised peak load by 29–37% in bonded joints and improved hybrids by 20–33% relative to neat hybrids, while the post-peak bearing plateau was preserved. Hydrothermal ageing reduced peak loads by 21–40% depending on the configuration, yet nanofiber hybrids still carried 8–24% higher loads than neat hybrids in the aged condition. CFRP reached the highest absolute capacities, and BFRP retained higher capacity than GFRP after damage and ageing. Among the designs tested, the nanofiber-reinforced hybrid joint offered the best balance between peak strength and retention.