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 penetrationultrasonic transducers, coupling methods, and instrumentation are meetingthe requirements for nondestructive evaluation of advanced ultra-low weightand high strength composite materials. Herein, three newly developed methodsare described along with their associated instrumentation. These methods aresquirter-based 250 kHz transducers, dry-coupled 250 kHz roller probecomposite transducers, and air-coupled transducers of 100 kHz and 250 kHz.A 250 kHz squirter-based transducer pair is designed to favor increased loopsensitivity, or voltage gain, in through transmission mode. The transducersafforded a minimum 6 dB increase in amplitude when testing difficult topenetrate foam and honeycomb core composite materials, essentially doublingthe signal to baseline noise level and enabling inspection of severalcomponents heretofore not adequately penetrated with ultrasound.Recently developed 250 kHz composite transducer roller probes provided agreater than 8 dB increase in through transmission amplitude compared toconventional PZT (lead zirconate titanate). Mixed coupling mode inspectionemploying a dry-coupled roller probe transmitter with a non-contact aircoupledreceiver is also addressed. The large signal loss from one air-tocomponentinterface is eliminated, transmit side destructive interference isreduced, and susceptibility of the high gain receiver channel to rollervibration is alleviated.Results are presented from several applications employing enhanced aircoupledultrasonic transducers. Novel horn collimators with integralacoustic shielding are discussed that enable improved imaging of internalstructural detail and near-edge defects. These collimators with integralacoustic shielding also minimized wrap-around of the signal, therebyenabling an increased pulse repetition rate that supported scan speeds up to0.8 m/s at a 3 mm pixel increment. The versatility of choosing 100 kHz or250 kHz transducer frequencies and utility of selecting the wave mode forthrough transmission imaging to overcome standing wave or resonance problemscommonly encountered in air-coupled ultrasound inspection are addressed.