This paper is concerned with the characterization of residual adhesive strength. The deterioration of adhesive strength depends on damage mechanisms in thin layers at the interfaces of adherends and adhesives. In this work adhesive-bond damage mechanisms, particularly distributions of disbonds, are modeled and related to nonlinear changes of the gross stiffness of the adhesive layer. In turn, the changes in adhesive-layer stiffness affect coefficients of reflection and transmission for reflection and transmission of ultrasonic signals by the adhesive-bond layer. Hence the reflection and transmission measurements can be used to characterize adhesive bond damage.