Air-coupled ultrasonic testing of carbon-carbon composite aircraft brake disks

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Air-coupled ultrasonic testing of carbon-carbon composite aircraft brake disks

Poudel, Anish / Strycek, Jan / Chu, Tsuchin P.

This paper discusses the application of air-coupled ultrasonic testing for the inspection of commercial carbon-carbon (C/C) composite aircraft brake disks. Several tests were performed using air-coupled ultrasonic transducers in a throughtransmission mode by varying scan increments and resolutions. The test samples were fitted into polystyrene foam cutouts to reduce the amount of signal bypassing from the edge of the test specimens. A calibration was performed at 80% of the through-transmitted ultrasonic amplitude signals. This was carried out on a discontinuity-free area for each C/C disk tested to ensure accuracy in the results. It was found that a testing frequency of 125 kHz at a 2 mm step size provided the best results for C/C disks. The relative through-transmitted signal drop was approximately -18 dB in the discontinuous areas. This allowed for easy distinction between the discontinuous and nondiscontinuous regions in the C/C disks. After the inspection, the disks were cut open around these regions for visual inspection. It was revealed that regions in the C/C disks with significant attenuation of the through-transmitted ultrasonic signals had many delaminations and cracks.