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An air-coupled ultrasonic technology is introduced as a new tool for evaluating the interfacial quality of bonded structures in this paper. Thus, a mathematical model including the stiffness coefficient, the transmission coefficient and the frequency, etc. is proposed. Two type resonant frequencies, namely, the first type resonance frequency (RFI) and the second type resonance frequency (RFII), have been obtained by numerical solving the model. The interval of the RFI is related to the thickness of a polymethyl methacrylate (PMMA) layer. Similarly, the interval of the RFII depends on the thickness of an aluminum (Al) layer. In addition, the stiffness coefficient is associated with the RFI but not with the RFII. The RFI drifts to high-frequency with different speed from the low stiffness coefficient to the high stiffness coefficient, which no longer changes when the stiffness coefficient tends to infinity. The before-RFII moves faster than after-RFI in a &#x201C;Period&#x201D; of the RFII. With the increase of the stiffness coefficient, the transmission coefficient gradually increases, and then increases slowly when the stiffness coefficient approaches to a critical value. Several double-layer bonded samples with the different thicknesses were made to carry out air-coupled ultrasonic experiment. The change of stiffness coefficients was simulated by different bonding time. Transmission signals from ultrasonic testing were recorded for further processing. Comparing the experimental and simulation results, the stiffness coefficient of the bonded structure can be judged by the RFI and the transmission coefficient.

Measurement Interface Stiffness of Bonded Structures Using Air-Coupled Ultrasonic Transmission Technology