Adhesive bonding of glass-fiber-reinforced polymer (GFRP) to steel is a promising approach for lightweight hybrid structures because it eliminates mechanical fasteners while improving interfacial load transfer. Nevertheless, reliable joint performance remains difficult to achieve, as failure is often governed by the steel/adhesive interface, where surface topography, contamination, and chemical reactivity strongly affect adhesion. In this study, steel surface pretreatments designed to modify surface roughness mechanically and surface activity chemically, including sanding, sandblasting, and plasma treatment, were systematically evaluated to clarify their effects on the bonding performance of GFRP–steel joints. Abrasive sanding produced lap-shear strengths in the range of 13.43-14.78 MPa, whereas sandblasting increased the strength to 16.33 MPa. The improved bonding performance after sandblasting is considered to result from the combined effects of modified surface topography and the effective removal of surface contaminants. Plasma treatment applied after sandblasting further increased the joint strength to 19.84 MPa. Contact-angle and surface-energy measurements supported this improvement by indicating enhanced wettability and surface activity after plasma treatment. Fracture-surface observations revealed a transition from adhesive-dominated interfacial failure under weaker surface conditions to predominantly mixed and substrate-related failure after sandblasting and plasma treatment. These results demonstrate that the bonding performance of GFRP–steel joints is governed by the coupled effects of surface topography and chemical activation, and provide a practical process–surface–performance framework for selecting steel pretreatments for reliable, high-strength bonding.

Keywords: GFRP–steel, Adhesive bonding, Surface treatment, Sandblasting, Plasma activation, Roughness, Wettability, Failure mode