The influence of surface treatment on the tensile properties of carbon fiber-reinforced epoxy composites-bonded joints

The surface characteristics of carbon fiber-reinforced composites significantly influence the tensile properties of adhesive-bonded joints. The main objective of this study was to research the effect of surface treatment by sandpaper rubbing on the strength of adhesive joints. In the present research, surfaces were prepared using four grades of sandpaper with grit sizes of up to 60, 220, 400, 800, and polish grit. Further, the two types of joints (single lap joint and scarf adhesive joint) were tested to investigate the effects of the grit sizes and sanding direction of the sandpaper (parallel to the fiber direction, perpendicular to the fiber direction, and random direction) on the tensile properties. The experimental results showed that the highest bonding strength could be achieved when the surface was sanded in the random direction. Moreover, the grit sizes of the sandpaper significantly influenced the surface characteristics, including the surface profile, surface roughness, surface contact angle, and surface free energy of the specimens, and then affected the bonding strength of the joints by mechanical interlocking and adsorption effect. Furthermore, for both single lap and scarf adhesive joints, the failure mode and the force of the adhesive layer under tensile load were different; therefore, the dominant factors affecting the bonding enhancement mechanism were consequently different.