Air-Coupled and Resonant Pulse-Echo Ultrasonic Technique.

Tomás Gómez Álvarez-Arenas,Jorge Camacho

doi:10.3390/s19102221

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.

Highlights

Air-coupled ultrasound is a convenient method for material characterization and nondestructive testing (NDT) when conventional techniques based on water immersion, local immersion, gel coupling, or dry coupling cannot be used
The main output of this technique is the resonant frequency of the specimen that can be used to determine specimen dimensions or to determine velocities
The main output of this technique is the resonant frequency of the specimen that can be used to determine specimen dimensions or to determine velocities if the dimensions are known

Summary

Introduction

Air-coupled ultrasound is a convenient method for material characterization and nondestructive testing (NDT) when conventional techniques based on water immersion, local immersion, gel coupling, or dry coupling cannot be used. In the case of material characterization, examples are commonly found in the determination of elastic and viscoelastic constants. Examples are found in determining the microstructural properties of porous, open-pore materials or soluble materials, where the use of coupling fluids must be avoided. In other cases, coupling fluids cannot be used because they can potentially contaminate or modify the material. Examples correspond to cases where the potential penetration of fluids within the testing piece is to be avoided. Air-coupled techniques can be thoroughly used to generate and receive guided waves in solid structures. This may present some advantages in order to test large specimens or those that present difficult access points.

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