Non-destructive evaluation of carbon/carbon brakes using air-coupled ultrasonic inspection systems

This thesis is focused on non-contact air-coupled ultrasonic Non-Destructive Evaluation (NDE) of Carbon-Carbon (C/C) disc brake materials. The minimum detectable defect size in the C/C composite disc brakes up to the maximum thickness of 1 7/16" (36.33 mm) using 120, 225 and 400 kHz transducers was investigated in the experimental section of this thesis. The effect of scanning increment step size on resolution of the final C-scan image was also investigated.;The results indicated that the 12.7 mm diameter flat bottom drilled holes were reliably detectable using 225 kHz transducers. The flat bottom drilled holes and the side drilled holes of 6.35 mm in diameter were detectable on the final C-scan images of 225 kHz testing mainly due to the known locations of the defects. Results showed that testing frequency of 120 kHz provides very transparent C-scans. Testing frequency of 225 kHz provides also good transparency and better resolution. Testing frequency of 400 kHz did not provide satisfactory results. No advanced signal filtering techniques were utilized during the experiments. The relationship between the speed of sound in C/C material and the carbon fiber orientation in the carbon matrix needs to be investigated.;The air-coupled ultrasonic testing of the C/C composite disc brake samples was conducted at the Center for Non-Destructive Evaluation at Iowa State University.