The effect of damage nucleation on the toughness of an adhesive joint

The intrinsic toughness of an adhesive joint has been shown to be different depending on whether the adherends remain elastic or deform in a plastic fashion. This phenomenon occurs because the different constraint imposed by the adherends results in a change in the deformation mechanisms of the adhesive layer. In the elastic geometry, damage nucleation occurs when the stresses in the adhesive layer reach a critical value before the conditions for fracture are met. Void growth then leads to large-scale bridging across the adhesive layer and an increase in the measured toughness. In contrast to this behavior, the reduced constraint associated with adherends that are thin enough to deform plastically allows the fracture criterion to be met before damage nucleation occurs. There is then no bridging contribution to the toughness. The effect of damage in an adhesive layer can be viewed either as a bridging zone behind the crack tip, or as an extended cohesive zone ahead of the crack tip. The toughness of an adhesive joint can either be increased or decreased by the nucleation of damage. The effects of a damage zone on the behavior of an adhesive joint with elastic adherends are discussed, and it is shown how numerical techniques can be used to model this behavior and to deduce the fracture parameters.