Air-coupled Non-destructive Testing Research Articles

Air-Coupled and Resonant Pulse-Echo Ultrasonic Technique.

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.

Comparative analysis of laser ultrasonic propagation imaging system with capacitance and piezoelectric air-coupled transducers

Air-coupled nondestructive testing is an attractive technique that allows easy scanning inspection capability due to its noncontact nature with the specimen. A piezoelectric air-coupled transducer has been combined with a laser ultrasonic propagation imaging system as a fixed noncontact sensor. However, the large impedance mismatch between air and a specimen and its narrowband frequency range has prevented it from being efficiently used in real-world applications as a laser-air-coupled transducer ultrasonic propagation imaging system. Therefore, instead of combination with a piezoelectric air-coupled transducer, a combination between a laser and a capacitance air-coupled transducer is proposed through a comparative analysis between the capacitance and piezoelectric air-coupled transducers as the ultrasonic detector of the ultrasonic propagation imaging system. Under the same conditions, a series of experiments was conducted to examine the damage sensitivity in metal and composite specimens that have a hidden crack and barely visible impact damages, respectively. This noncontact system was further improved using repeat-scanning technique to enhance the signal-to-noise ratio and to overcome high attenuation problems, especially in composite specimens.