Air-Coupled and Resonant Pulse-Echo Ultrasonic Technique

Abstract

An ultrasonic, resonant, pulse-echo, and air-coupled nondestructive testing (NDT) technique is presented. It is intended for components, with regular geometries where it is possible to excite resonant modes, made of materials that have a high acoustic impedance (Z) and low attenuation coefficient (α). Under these conditions, these resonances will present a very large quality factor (Q) and decay time (τ). This feature is used to avoid the dead zone, produced by the echo coming from the first wall, by receiving the resonant echo from the whole specimen over a longer period of time. This echo is analyzed in the frequency domain to determine specimen resonant frequency, which can be further used to determine either velocity or thickness. Using wideband air-coupled transducers, we tested the technique on plates (steel, aluminum, and silicone rubber) by exciting the mode of the first thickness. As expected, the higher the Z and the lower the α, the better the technique performed. Sensitivity to deviations of the angle of incidence away from normal (±2°) and the possibility to generate shear waves were also studied. Then, it was tested on steel cylindrical pipes that had different wall thicknesses and diameters. Finally, the use of this technique to generate C-Scan images of steel plates with different thicknesses was demonstrated.

Machine Learning‐Based Optoacoustic Tissue Classification Method for Laser Osteotomes Using an Air‐Coupled Transducer

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Development of non‐contact foreign body imaging base on photoacoustic signal intensity measurement

“…On the other hand, the use of ultrasonic frequencies in PA measurement presents new challenges. Ultrasonic sensors generally require contact with the object surface through a coupling medium to improve the impedance between the object and the sensor 19,20 . Consequently, this measurement faces some challenges on certain objects such as small object areas, 21 uneven surfaces 22 and infections 23 .…”

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