Distinction between adhesive and cohesive defects in a structural bond

The ultrasonic guided modes in an aluminum/epoxy/aluminum tri-layer are used to characterize the adhesion and the cohesion of this structure. Different models are compared. The exact model (I) takes into account the adhesive layer; the two aluminum/adhesive interfaces are assumed to be perfect. The rheological model of the interfaces (II) consists to describe the two contact zones by a distribution of springs; the boundary conditions at the two aluminum/adhesive interfaces are given by Jones. The rheological model of the adhesive (III) consists to replace the whole adhesive layer by a distribution of springs with inertia; the Jones conditions are then written at the aluminum/aluminum interface. The theoretical comparison between the models (I) and (II) [respectively, (III)] gives the stiffnesses of the interface (respectively of the adhesive layer). For the model (II), the theory/experiment comparison allows us to define interface damage, i.e., adhesive defects. For the model (III), we define global damage in the adhesive layer, i.e., cohesive defects. Nondestructive tests are realized on samples with different levels of adhesion and any cohesive defect. The rheological models confirm an interface damage and any global damage. The same conclusions are obtained by destructive tests.