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15. Failure Modes, Defect Characterization, and Accept/Reject Criteria

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This paper presents the results of a program sponsored by NADC for NASC to develop an ultrasonic surface wave technique for detection of small flaws, <100 11m (< 0.004 inch), in gas turbine quality ceramics. A 45 MHz ultrasonic surface wave inspection technique is described, which employs immersion scanning ndd C-scan recording. Inspection results are presented using this technique on specimens of hot pressed Silicon nitride and silicon carbide, from two sources each, and reaction bonded silicon nitride. Results are also presented of four-point-bend tests and scanning electron micrography, which were used to identify defect sizes and types and to correlate flexural strength with inspection results. The flexural strength is shown to correlate, at least qualitatively, with the extent of ultrasonic response from machining damage. The sensitivity to individual defects is shown to be limited primarily by the extent of machining damage and the spot size of the ultrasonic beam.



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9 p.