Experimental determination of interfacial toughness curves using Brazil-nut-sandwiches

A Brazil-nut-sandwich with a crack on a substrate/interlayer interface is developed for fracture testing. The fracture loading phase is controlled by the angle of diametral compression. Interfacial fracture mechanics is summarized and adopted in reporting data. Experiments are conducted with aluminum, brass, steel and plexiglass as substrates and epoxy as interlayer. Interfacial toughness curves are measured for large range of loading phase. Effects of the roughness of the surfaces prior to bonding on the interfacial toughness are demonstrated. Failure patterns for the adhesive structure under different loading modes are observed with a scanning electron microscope. For the metal/epoxy systems, when the remote loading is predominantly mode I, cracks tend to kink out of interfaces and run within the epoxy layer, although the bulk epoxy fracture energy is much higher than the interfacial toughness. At large loading phases, abnormally high apparent toughness is measured. These observations are discussed in the light of crack path selection criteria in adhesive joints and large scale contact zone of crack faces.