Brittle fracture of adhesive joints

The effect of mode mix upon crack path is examined for a brittle epoxy adhesive. The mode mix is varied over wide limits by using asymmetrical double cantilever beam and Brazil nut specimen configurations. The observed crack paths are interfacial, in-layer, and alternating from one interface to the other in a periodic fashion. The probability of occurrence and toughness of each crack path is measured, and is compared with existing theory. Interfacial fracture is the most common path for all values of mode mix; the interfacial toughness increases dramatically with increasing mode II component of remote load. Under remote mode I loading the alternating crack path covers approximately 20 percent of the fracture surface of the specimens and has the highest macroscopic toughness. The effect of residual stress magnitude, layer thickness and mode mix upon the wavelength of the alternating crack mode is measured. Predictions based on a previous analysis by Akisanya and Fleck [1] compare favourably with the measured values.