Apparent interfacial failure in mixed-mode adhesive fracture

Aluminium-epoxy adhesive specimens constructed with the bond at 45? to the direction of loading appear to fail very close to the interface. The actual locus of failure was investigated by¹⁴C labelling of the epoxy polymer and also by Auger spectroscopy profile analysis. Both techniques indicated a residual film of polymer a few hundred angstroms thick on the aluminium surface. The fracture energy of these specimens was determined and found to be affected by the surface roughness of the aluminium. The mixed-mode fracture energy (G _{I,II) C} ^45° of these specimens in the absence of any surface roughness effect (polished surfaces) was 140 J m^?2 compared to 136 J m^?2 for the same polymer in simple opening-modeG _{I C} adhesive fracture. The “interfacial” failure and the effect of surface finish on fracture are discussed in terms of the applied stress directing the failure toward the interface but the approach of the crack to the boundary being limited by the size of the crack tip deformation zone.