Low-frequency ultrasonic imaging of composite structures with squirter-based water-coupled and air-coupled transducers

Emergent requirements for lower frequency, higher penetration ultrasound for inspection of ever-lower weight and larger cross-sectional area composite material components are successfully being addressed through research and development efforts on improved transducers, coupling, and instrumentation. Newly developed squirter and air-based ultrasonic transducers and associated instrumentation are described. A 250 kHz squirter-based transducer pair is designed to favor increased loop sensitivity in through-transmission mode. Enhanced loop sensitivity, or voltage gain, increases the signal amplitude in proximity to the transducer before noise is impressed upon it in cable paths or via amplifiers. The transducers afforded a minimum 6 dB increase in amplitude compared to conventional devices when testing difficult to penetrate foam and honeycomb core composite materials, essentially doubling the signal to baseline noise level and enabling inspection of several components heretofore not adequately penetrated with ultrasound. Results are also presented from several applications employing enhanced air-coupled ultrasonic transducers. Novel horn collimators are discussed that enabled improved imaging of internal structure and near-edge defects. These collimators with integral acoustic shielding also minimized wrap-around of the signal, thereby enabling an increased pulse repetition rate that supported scan speeds up to 0.8 m/s at a 3 mm pixel increment.