15. Failure Modes, Defect Characterization, and Accept/Reject Criteria
The brittleness of ceramic materials like silicon nitride and silicon carbide makes it necessary to fabricate homogeneous structures and to detect small defects in the region of 10 to 100 microns diameter. In the German program on NDE for the gas turbine therefore a study was made to compare different NDE methods and to develop new techniques. Tests were made with ultrasonics, microradiography, vibration analysis, acoustic emission and optical-holographical interferometry on test samples and real components of the gas turbine (rotor, stator, combustor). The results show that especially
- microradiography with projection technique and X-ray focus of ≈ 10 µm diameter,
- ultrasonics with different kind of transducers, equipment and wave modes in the frequency range until about 150 MHz
are well suited to detect the small defects and to characterize structure heterogeneities.
- Vibration analysis seems to be a good method to compare many samples of the same kind and to detect matter of the fabrication process data.
The comparison between UT, vibration analysis, acoustic emission and destructive tests (fracture strength) indicates that there are more or less correlations between NDE and the destructive analysis.
Goebbels, K. and Reiter, H., "Characterization of Defects and Heterogeneities in Silicon Nitride and Silicon Carbide by Different NDE Methods" (1980). Proceedings of the DARPA/AFML Review of Progress in Quantitative NDE, July 1978–September 1979. 99.