In situ observation of interfacial crack propagation in GF/epoxy model composite using bifiber specimens in mode I and mode II loading

Interfacial fracture behavior for GF/epoxy model composites was investigated under static loading using the bifiber shear (BFS) and the bifiber open (BFO) methods. Two E-glass fiber filaments were bonded with bisphenol A type epoxy. The specimens were specially designed for the in situ observation of the crack propagation in a scanning electron microscope. Three dimensional finite element analyses were carried out to obtain the complex stress intensity factors at interface, K1 and K2, and the energy release rate. The initial values of the fracture toughness were defined as the initiation of crack growth after pop in type crack onset at meniscus region, and the rising R-curve behavior was observed for both specimens. The difference in the shape of the R  -curves and the toughness values in each specimen indicated the existence of the difference in toughness along a single filament. The comparison of the results of the BS and BO tests depicts that the fracture criterion under different fracture mode ratio is roughly expressed as K12+K22=const. The mechanism of interfacial fracture under different fracture modes were discussed in conjunction with fractrographic observation.