Non-destructive Evaluation of Materials Using CMUTs
Nondestructive evaluation techniques, which use conventional piezoelectric transducers typically require liquid coupling fluids to improve the impedance mismatch between piezoelectric materials and air. Air-coupled ultrasonic systems can eliminate this requirement if the dynamic range of the system is large enough such that the losses at the air-solid interfaces are tolerable. Capacitive micromachined ultrasonic transducers (cMUTs) have been shown to have more than 100 dB dynamic range when used in bistatic transmission mode. This dynamic range, along with the ability to transmit ultrasound efficiently into air, makes cMUTs ideally suited for air-coupled nondestructive evaluation applications. These transducers can be used either in through transmission experiments at normal incidence to the sample or to excite and detect guided waves in aluminum and composite plates (Fig. 1).
In our experiments, we demonstrated results of a pitch-catch dynamic range in excess of 100 dB (Fig. 2). The pair of transducers is modeled with an equivalent electrical circuit which predicts the transmission system's insertion loss and dynamic range. An example for transmission defect imaging is shown in Fig. 3. We also demonstrated the feasibility of Lamb wave defect detection for one-sided nondestructive evaluation applications. A pair of cMUTs excites and detects the So mode in a 1.2 mm-thick aluminum plate with a received signal-to-noise ratio of 28 dB without signal averaging [1].
FIGURE 1. Defect detection methods using air-coupled ultrasound.
FIGURE 2. System dynamic range.
FIGURE 3. Transmission image of a defect in an aluminum plate.
References
[1] S. Hansen, F. L. Degertekin, B. T. Khuri-Yakub, “Air-Coupled Non-Destructive Evaluation Using Micromachined Ultrasonic Transducers,” Proc. Ultrasonics Symposium, pp. 1037 – 1040, 1999.
Acknowledgements
This work was supported by USAF Wright-Patterson Air Force Base and the U.S Office of Naval Research.
