Crack Path Selection in Adhesively Bonded Joints: the Role of Material Properties

This paper investigates the role of material properties on crack path selection in adhesively bonded joints. First, a parametric study of directionally unstable crack propagation in adhesively bonded double cantilever beam specimens (DCB) is presented. The alternating crack trajectories in specimens of various material systems are simulated using the finite element method and the results indicate that the characteristic length of the crack varies with the Dundurs’ parameters characterizing the material mismatch. Second, the effect of interface properties on crack path selection is investigated. DCB specimens with substrates treated using various surface preparation methods are tested under mixed mode fracture loading to determine the effect of interface properties on the locus of failure. As indicated by the post-failure analyses of the failure surfaces, debonding tends to be more interfacial as the mode II fracture component in the loading increases. On the other hand, failures in specimens prepared with more advanced surface preparation techniques appear more cohesive for given loading conditions. Using a high-speed camera to monitor the fracture sequence in quasi-static DCB tests, the effect of interface properties on the rate dependence of the locus of failure in adhesive bonds is studied. XPS analyses of the failure surfaces indicate that the effect of crack propagation rate on the locus of failure is less significant when more advanced surface preparation techniques were used. The effect of asymmetric interface property on the behavior of directionally unstable crack propagation in adhesive bonds is also investigated. Geometrically symmetric DCB specimens with asymmetric surface pretreatments are prepared and tested under low-speed impact. As indicated by Auger depth profile results, the centerline of the crack trajectory shifts slightly toward the interface with poor adhesion due to the asymmetric interface property. Third, through varying the rubber content in the adhesive, DCB specimens with various fracture toughnesses are prepared and the effect of adhesive toughness on the directional stability of cracks is then investigated. An examination of the failure surfaces reveals that directionally unstable crack propagation is more unlikely to occur as the toughness of the adhesive increased, which is consistent with the analytical predictions that were discussed using an energy balance model.