Air-Coupled and Resonant Pulse-Echo Ultrasonic Technique

• Published in: Sensors (Basel, Switzerland) Vol. 19; no. 10; p. 2221
• Main Authors: Álvarez-Arenas, Tomás Gómez, Camacho, Jorge
• Format: Journal Article
• Language: English
• Published: Switzerland MDPI AG 14.05.2019; MDPI
• Subjects: Acoustic attenuation; Acoustic impedance; air-coupled pulse-echo; air-coupled transducers; air-coupled ultrasound; Attenuation coefficients; Broadband; Nondestructive testing; pipe NDT; pipe wall gauge; Q factors; Resonant frequencies; S waves; Silicone rubber; Stainless steel; Thickness; Transducers; Ultrasonic imaging; ultrasonic NDT; Ultrasonic testing; Velocity; Viscoelasticity; New York; United States > US; Iowa; air-coupled ultrasound; air-coupled pulse-echo; pipe NDT; air-coupled transducers; ultrasonic NDT; pipe wall gauge
• ISSN: 1424-8220; 1424-8220
• DOI: 10.3390/s19102221

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 ( ) and low attenuation coefficient ( ). Under these conditions, these resonances will present a very large quality factor ( ) and decay time ( ). 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 and the lower the , 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.