This paper discusses the application of an air-coupled ultrasonic non-destructive evaluation (NDE) method for aircraft braking materials. The main objective of this research work was to identify and characterize flaws such as delaminations, cracks, porosity (resin pockets), and disbonds in commercial Carbon/Carbon (C/C) composite aircraft brake disks.
Air-coupled ultrasonic testing (ACUT) method was applied for the inspection of commercial C/C brake disks. Several tests were performed by using various air-coupled ultrasonic transducers at center frequencies 50, 120, 125, 225, 400, and 436 kHz in a through-transmission mode by varying scan increments and resolutions. It was found that a testing frequency of 125 kHz provided the best results for commercial C/C composite aircraft brake disks. The relative through-transmitted ultrasonic signal drop in the defect areas was around −18 dB which allowed for easy distinction of abnormal regions within the C/C brake disks. In addition, material inhomogeneity within the C/C brake disks was also revealed by the ACUT C-scan results.