Characterizing fracture performance and the interaction of propagating cracks with locally weakened interfaces in adhesive joints

This paper experimentally investigates adhesive fracture resistance and crack path selection in adhesive joints containing well-defined localized interfacial defects. Several systematic patterns of localized interfacial defects were created on base-acid treated aluminum adherends by physical vapor deposition of copper through a mask. Adhesive joints were prepared using a commercially available, structural epoxy adhesive and the effect of localized interface defects on the performance of adhesive joints was studied. Under mode-I loading conditions, the presence of localized weak interfaces influenced the fracture energy of a propagating debond over a considerable distance. For a crack tip approaching a given weak interface pattern, a falling or reverse R-curve type trend was observed. Within the same DCB specimen, as the crack tip advanced beyond the patterned region, a rising R-curve type trend was observed as fracture energy increased with increasing crack lengths, which were recorded visually and were also inferred using compliance and crack length relationship. In addition, the mode-I fracture energy was found to scale with the area fraction of the weak interfaces according to a rule of mixtures. For the adhesive system and the joint geometry used in this study, it was observed that cohesive failure in the adhesive layer can be obtained even in the presence of exceptionally weak interfaces (similar in size to those detected in mode-I loading) when loaded under mixed mode conditions (achieved through asymmetric loading of DCB-like specimens), that tended to steer the crack away from the interface. When loaded with the opposite mode mixity direction, the crack tip propagated through the defects, though the fracture energy did not exhibit similar R-curve type trends as observed in mode-I tests. The results offer insights into the interaction of propagating cracks in adhesive layers and their interactions with discrete, localized defects, which could lead to improvements in surface preparations and bond integrity or even to joint designs having intentionally placed defects useful in controlled disassembly or for other purposes.