Air-Coupled Ultrasonic Beam Transmission Applied to Material Characterization
Lamb waves have been widely used in ultrasonic NDE to characterize material properties or assess material quality , Of the previous work on materials using phase-matched fluid-loaded coupling, most has been performed in water-coupled testing . With the development of efficient non-contacting ultrasonic air-coupled transducers , it has become feasible to apply air-coupled ultrasonic methods to NDE. Because of the low signal noise ratio resulting from the large impedance mismatch between the air and the solid object, most work of air-coupled (AC) ultrasound is qualitative, with defects in plates and C-scan imaging being the principal objectives. As demonstrated by Safaeinili, et al. , however, it is possible to characterize elastic plates, both isotropic and anisotropic, by using AC ultrasound, despite the signal-to-noise ratio (SNR) penalty.