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

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We have developed a low frequency theory for scattering from surface cracks. For the case of halfpenny shaped surface cracks, we are able to relate the reflection coefficient of a Rayleigh wave incident on the crack with the crack size as well as the fracture stress of a sample with the crack in it. Comparisons of the theoretical predictions for the fracture stress with the actual fracture stresses for silicon nitride samples containing cracks with estimated radii ranging from 51 μm to 274 μm show excellent agreement, with less than 16% error. A qualitative study in the high frequency regime of the reflected echos from surface cracks in silicon nitride turbine blades has also been made.



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7 p.