Air-coupled ultrasonic measurements of adhesively bonded multi-layer structures

An air-coupled ultrasonic system employing wideband micromachined capacitan ce transducers has been used for non-contact measurements of material prope rties in adhesively bonded multi-layer aluminum structures. By sweeping the frequency of the ultrasonic toneburst applied to the air-coupled source, w hile measuring the air-coupled receiver response, through-transmission spec tra for normally incident ultrasound were obtained for various multi-layer structures at frequencies below 2 MHz. Resonant transmission peaks, which a ppeared in the resultant spectra, agree well with the output of a theoretic al algorithm that predicts ultrasonic transmission through multi-layer stru ctures. A separate theoretical model is also developed analytically to pred ict the modal shapes and resonance frequencies of the allowed longitudinal resonances of the structures. Comparison of the results of this model with the experimental data shows that the resonant transmission peaks observed i n measured spectra resulted from the excitation of certain longitudinal nor mal modes of vibration within the layered structures. As such resonant norm al modes provide a significant increase in transmission coefficient, they w ere employed for non-destructive testing purposes by accurately imaging spa tial variations in the adhesive layers of the structures (e.g. variations i n adhesive thickness, Young's modulus, density, etc.). In particular, the a bility of the air-coupled system to detect and image similar to 30 mu m thi ckness variations within the bond-line of an aluminum lap-joint is demonstr ated, thus showing the high-accuracy that can result from air-coupled measu rements due to the reduced damping of sample vibrations by the air. Such ac curacy may prove of interest in developing a non-contact air-coupled ultras onic bond-tester for multi-layer adhesively bonded structures. (C) 1999 Els evier Science B.V. All rights reserved.