Effect of adhesive type on mechanical properties of galvanized steel/SMC adhesive-bonded joints

Abstract Adhesive bonding provides a solution for joining sheet molding compound (SMC) and metals. In this study, three typical automotive structural adhesives (i.e. polyurethane adhesive A236, epoxy adhesive 120HP and acrylic adhesive M10) were applied to joining galvanized steel and SMC to investigate the compatibility of adhesive type and adherend materials in terms of the lap-shear tensile strength of as-fabricated joints. In addition, adhesive-bonded joints after immersion in 63 °C deionized water for 0, 7, 14 and 21 days were mechanically tested. Results show that the joint strength gradually decreased with increasing exposure time. Among the adhesive-bonded joints fabricated from three adhesives, the lap-shear strength of A236 joints is the highest before and after water soak. It is also observed that the fracture mode of A236 joints switched from SMC substrate failure to the interfacial failure between galvanized steel and adhesive after exposure, while 120HP joints and M10 joints always fractured at the galvanized steel/adhesive interface during lap-shear tensile testing. The fracture surface observations indicate that the corrosion of galvanized steel extends inward the bonding area with an increase of exposure time, which is verified with an energy dispersive spectrometry (EDS) and can be explained by the moisture diffusion along the galvanized steel/adhesive bonding interface.