Capacitive and piezoelectric air-coupled transducers for resonant ultrasonic inspection

Both micromachined air-coupled capacitance transducers and piezoelectric air-coupled devices have been used to form images of artificial graphite foil defects in a fibre-reinforced thermoplastic composite plate. This has been achieved using toneburst excitation at the fundamental through-thickness resonant frequency of the plate. Images are presented for both types of transducer system, and the results discussed. It is found that capacitance transducers have distinct advantages in materials characterization due to their wide bandwidth. These advantages include the ability to vary the excitation frequency in order to match to higher order resonant modes of the plate.