Interfacial and Mechanical Performance of a PU ‐ PMMA Copolymer Adhesive Modified With Recycled Carbon Fiber Powder for Bonded CFRP Laminates

ABSTRACT This study investigates the interfacial and mechanical performance of a PU‐PMMA copolymer adhesive modified with recycled offcut carbon fiber powder (oCFP). The effects of oCFP content (0–10 wt%) on tensile, flexural, and lap shear behavior were systematically evaluated to elucidate filler‐induced changes in stiffness, strength, and ductility. Incorporation of 1 wt% oCFP reduced tensile modulus and strength but increased fracture strain compared with the neat PU‐PMMA, while higher loadings progressively enhanced stiffness. Tensile strength decreased at elevated filler contents, whereas flexural strength partially recovered at 10 wt%, indicating a loading‐mode‐dependent stiffness‐strength–ductility trade‐off. Lap shear strength increased with 1 wt% oCFP, reaching 19.09 MPa and exhibiting fiber‐tearing failure, suggesting improved interfacial adhesion and stress transfer. To assess the practical effectiveness of the modified adhesive, impact‐damaged CFRP laminates (30 and 50 J) were bonded and subjected to compression‐after‐impact testing. All repaired specimens recovered more than 80% of their original compressive strength, with the 5 wt% formulation achieving the highest average repair effectiveness. The results provide experimental insight into the influence of recycled carbon fiber powder on the interfacial and mechanical behavior of PU‐PMMA adhesive.