Crack path selection in adhesively-bonded joints: The role of material properties

This paper investigates the role of material properties on crack path selec tion in adhesively bonded joints, First, a parametric study of directionall y unstable crack propagation in adhesively-bonded double cantilever beam sp ecimens (DCB) is presented. The results indicate that the characteristic le ngth of directionally unstable cracks varies with the Dundurs' parameters c haracterizing the material mismatch. Second, the effect of interface proper ties on crack path selection is investigated. DCB specimens with substrates treated using various surface preparation methods are tested under mixed m ode fracture loading to determine the effect of interface properties on the locus of failure. As indicated by the post-failure analyses, debonding ten ds to be more interfacial as the mode II fracture component in the loading increases. On the other hand, failures in specimens prepared with more adva nced surface preparation techniques appear more cohesive for given loading conditions. Using a high-speed camera to monitor the fracture sequence, DCB specimens are tested quasi-statically and the XPS analyses conducted on th e failure surfaces indicate that the effect of crack propagation rate on th e locus of failure is less significant when more advanced surface preparati on techniques are used. The effect of asymmetric interface property on the behavior of directionally unstable crack propagation in adhesive bonds is a lso investigated. Geometrically-symmetric DCB specimens with asymmetric sur face pretreatments are prepared and tested under low-speed impact. As indic ated by Auger depth profile results, the centerline of the crack trajectory shifts slightly toward the interface with poor adhesion due to the asymmet ric interface properties. Third, through varying the rubber content in the adhesive, DCB specimens with various fracture toughnesses are prepared and tested. An examination of the failure surfaces reveals that directionally u nstable crack propagation is more unlikely to occur as the toughness of the adhesive increases, which is consistent with the analytical predictions th at were discussed using an energy balance model.