A model for the amplitude and phase of ultrasonic tone-bursts incident on adherend-adhesive interfaces is developed for both reflected and transmitted waves. The model parameters include the interfacial stiffness constants, which characterize the elastic properties of idealized adherend-adhesive interfaces having a continuum of bonds. The ultrasonic model is linked to the more realistic physico-chemical model of adhesive bonding via a scaling equation that establishes the relationship between the interfacial stiffness constants of the ultrasonic model and the fraction of actual bonds in the physico-chemical model. The link to the physico-chemical model enables a quantitative assessment of the absolute bond strength. The ultrasonic model and scaling equation are applied to the simulation assessment of the absolute bond strength of two aluminum alloy adherends joined by an epoxy adhesive. Model input is obtained from the calculated phase of tone-bursts reflected from the adherend-adhesive interfaces as a function of the interfacial stiffness constants. The simulation shows that the reflected phase is dominated by the first interface encountered by the incident tone-burst with little contribution from the second interface. The simulation also shows that the accuracy in assessing the adhesive bond strength depends on the sensitivity of the reflected phase to variations in the interfacial stiffness constants, reflecting in part the nonlinearity of the scaling relationship.
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