Non-contact ultrasonic NDE of carbon/carbon composite

Non-contact materials NonDestructive Evaluation techniques with ultrasound are useful for many industrial settings particularly where high temperatures or speeds are involved. This paper presents results on non-contact ultrasonic wave propagation through thick carbon/carbon material. Specifically, at an air distance of about 200 mm carbon/carbon aircraft brake sections about 25 mm thick were examined in through-transmission at 250 kHz with signal-to-noise ratios of about 30 dB. In regions of delaminations the amplitude decreased sufficiently to indicate the presence of a flaw. The non-contact, air-coupled data are compared with those obtained with the more traditional dry-contact measurements. Non-contact velocity measurements are also presented with time-of-flight data obtained with phase stability of a few nanoseconds. These results are discussed in terms of newly developed air-coupled transducers and digital signal processing electronics.