The steady-state fracture toughness of a glass/epoxy interface was experimentally determined to be an order of magnitude greater than the thermodynamic work of adhesion. An asymmetric increase in the values of the apparent interfacial fracture toughness with increasing mode II component of load has been observed by several authors. In this study, cracks were grown under steady-state conditions along the glass/epoxy interface in order to determine the mechanisms responsible for these phenom ena and to quantify their contributions to the total apparent fracture toughness.

Finite element analysis using a viscoplasticity model for the epoxy and a cohesive zone model for interfacial fracture showed that inelastic dissipation in the epoxy accounts for the asymmetric shielding seen in these experiments over a wide range of in-plane mode mix. Numerical predictions of the normal crack opening displacements yielded results that were consistent (within the 10 nm resolution) with values measured using crack opening interferometry as close as 300 nm from the crack tip.

Subtraction of the inelastic dissipation from the total fracture toughness resulted in an apparent intrinsic toughness which remained constant with mode mix within the experim ental uncertainty and which was equal to the mode I interfacial toughness. Mechanisms that contributed to the intrinsic toughness were found to include the thermodynamic work of adhesion, local inelastic deformations and polymer chain pullout, but their combined energy was only approximately 15% of the intrinsic toughness. Angular dependent x-ray photoelectron spectroscopy of the glass surfaces after fracture revealed epoxy adsorbed to a depth of approxim ately 3 nm. Cleavage of epoxy strands was found to be the most important intrinsic toughness mechanism contributing approximately 40% to the apparent intrinsic toughness of the interface. The rem ainder of the toughness appears to come from an additional dissipative mechanism in the fracture process zone and to depend on crack speed.