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

Abstract

An air-coupled ultrasonic system employing wideband micromachined capacitance transducers has been used for non-contact measurements of material properties in adhesively bonded multi-layer aluminum structures. By sweeping the frequency of the ultrasonic toneburst applied to the air-coupled source, while measuring the air-coupled receiver response, through-transmission spectra for normally incident ultrasound were obtained for various multi-layer structures at frequencies below 2 MHz. Resonant transmission peaks, which appeared in the resultant spectra, agree well with the output of a theoretical algorithm that predicts ultrasonic transmission through multi-layer structures. A separate theoretical model is also developed analytically to predict 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 in measured spectra resulted from the excitation of certain longitudinal normal modes of vibration within the layered structures. As such resonant normal modes provide a significant increase in transmission coefficient, they were employed for non-destructive testing purposes by accurately imaging spatial variations in the adhesive layers of the structures (e.g. variations in adhesive thickness, Young’s modulus, density, etc.). In particular, the ability of the air-coupled system to detect and image ∼30 μm thickness variations within the bond-line of an aluminum lap-joint is demonstrated, thus showing the high-accuracy that can result from air-coupled measurements due to the reduced damping of sample vibrations by the air. Such accuracy may prove of interest in developing a non-contact air-coupled ultrasonic bond-tester for multi-layer adhesively bonded structures.