Surface morphology effects on mixed-mode fracture toughness for CFRP/metal interfaces

Abstract

Adhesive joints have played a significant role in various engineering applications. In an attempt to improve the strength of adhesive joints, many researchers have found that the strength of joints could be enhanced by introducing roughness on the adherends and are trying to correlate the joint strength with roughness parameters, although the studies still need further investigations. Meanwhile, in terms of mixed-mode failure analysis of adhesive joints, adhesion strength should be characterized using a fracture mechanics parameter, namely interfacial fracture toughness as a function of phase angles, since the critical stress alone cannot accurately predict the failure of the adhesive joint. The single-leg bending (SLB) test offers a reliable method to measure interfacial fracture toughness over various mode mixities. Thus, we investigated surface morphology effects on mixed-mode fracture toughness for CFRP/metal interfaces by performing SLB tests. A variety of surface conditions were fabricated, including sandpaper-abraded surfaces, surfaces with a micro scale line pattern, and surfaces with nano scale pores on a micro scale line pattern. As a result, the fracture toughness for CFRP/metal interfaces with the various surface conditions was obtained over a relatively wide range of phase angles. Firstly, the experimental result of the sandpaper-abraded CFRP/steel interface conformed that the interfacial fracture toughness increases as the mode II component becomes dominant. The effect of loading mode on interfacial crack growth was discussed on the basis of crack path observation with microscopic images. The influence of interfacial roughness on adhesion strength was also examined. Secondly, the fracture toughness of the CFRP/steel interfaces with the micro scale line pattern was compared to that with sandpaper abrasion. It was found that the micro scale line pattern could significantly improve the interfacial fracture toughness over the all mixed-mode ratios tes...