Observations of Decreased Fracture Toughness for Mixed Mode Fracture Testing of Adhesively Bonded Joints

In contrast to fracture in monolithic materials, where cracks naturally grow in a mode I manner, fracture in laminated or adhesively bonded joints often involves cracks growing under mixed mode conditions. In many cases, the mixed mode fracture energy increases as the fracture mode changes from mode I to mode II, but exceptions have been noted for several practical engineering adhesives. In some cases, G IIc may be less than G Ic, while in other situations, failure may occur at total energy release rate (G T = G I + G II) values that are smaller than either of the pure mode fracture energies. Several examples of this behavior are reported along with possible explanations for the behavior, which often involves the propagation of the growing debond into regions where less energy is dissipated by the fracture process. These examples showing that mixed mode fracture energies of adhesive joints may be lower than pure mode fracture energies remind us of the importance of developing fracture envelopes over a wide range of mode mixities for engineering design.