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Ultrasonic Detection of Surface Flaws in Gas Turbine Ceramics.

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A 45 MHz ultrasonic surface wave technique was developed and evaluated for detection of small surface flaws of 100 microns 0.004 inches in gas turbine quality ceramics. The surface wave technique, which employs conventional immersion C-scan recording, was evaluated on hot pressed silicon nitride and silicon carbide of two sources each, and also on reaction bonded silicon nitride and sintered silicon carbide. These evaluations included tests on specimens run in a burner rig to simulate the effects on NDE of a gas turbine operating environment. Conventional mechanical tests were also run to verify defect sizes and types and to correlate flexural strength with ultrasonic inspection results. The technique was found to be quite sensitive to surface conditions such as grinding damage, as well as to defects. Flexural strength was correlated qualitatively with the extent of ultrasonic response from machining damage. The sensitivity to defects was found to be limited by the depth of machining damage and the focal spot size of the ultrasonic beam. For the 580 microns 0.023 inch diameter focal spot size used, the smallest defect verified metallurgically was equivalent to a 30 microns 0.0012 inch deep semi-circular crack. Specimens run in the burner rig were found to become uninspectable because of fused silica on the surface. Author

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Final technaical rept. 13 Sep 77-12 Dec 78,



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