Squirter, Roller Probe, and Air-Coupled Ultrasonic Transducer Techniques for Low Frequency Inspection of Advanced Composites Materials

摘要

Research and development efforts on lower frequency, higher penetration ultrasonic transducers, coupling methods, and instrumentation are meeting the requirements for nondestructive evaluation of advanced ultra-low weight and high strength composite materials. Herein, three newly developed methods are described along with their associated instrumentation. These methods are squirter-based 250 kHz transducers, dry-coupled 250 kHz roller probe composite transducers, and air-coupled transducers of 100 kHz and 250 kHz. A 250 kHz squirter-based transducer pair is designed to favor increased loop sensitivity, or voltage gain, in through transmission mode. The transducers afforded a minimum 6 dB increase in amplitude 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. Recently developed 250 kHz composite transducer roller probes provided a greater than 8 dB increase in through transmission amplitude compared to conventional PZT (lead zirconate titanate). Mixed coupling mode inspection employing a dry-coupled roller probe transmitter with a non-contact air-coupled receiver is also addressed. The large signal loss from one air-to-component interface is eliminated, transmit side destructive interference is reduced, and susceptibility of the high gain receiver channel to roller vibration is alleviated. Results are presented from several applications employing enhanced air-coupled ultrasonic transducers. Novel horn collimators with integral acoustic shielding are discussed that enable improved imaging of internal structural detail 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. The versatility of choosing 100 kHz or 250 kHz transducer frequencies and utility of selecting the wave mode for through transmission imaging to overcome standing wave or resonance problems commonly encountered in air-coupled ultrasound inspection are addressed.