Micromachined ultrasonic transducers for air-coupled non-destructive evaluation

Conventional methods of ultrasonic non-destructive evaluation (NDE) use liquids to couple sound waves into the test sample. This either requires immersion of the parts to be examined or the use of complex and bulky water squirting systems that must be scanned over the structure. Air-coupled ultrasonic systems eliminate these requirements if the losses at air-solid interfaces are tolerable. Micromachined capacitive ultrasonic transducers (cMUTs) have been shown to have more than 100 dB dynamic range when used in the bistatic transmission mode. In this paper, we present results of a pitch-catch transmission system using cMUTs that achieves a 103 dB dynamic range. Each transducer consists of 10,000 silicon nitride membranes of 100 mu m diameter connected in parallel. This geometry results in transducers with a resonant frequency around 2.3 MHz. These transducers can be used in through transmission experiments at normal incidence to the sample or to excite and detect guided waves in aluminum and composite plates. In this paper we present ultrasonic defect detection results from both through transmission and guided Lamb wave experiments in aluminum and composite plates, such as those used in aircraft.