NONDESTRUCTIVE EVALUATION OF THERMALLY SHOCKED SILICON-CARBIDE BY IMPACT ACOUSTIC-RESONANCE

Monolithic ceramics are under consideration as structural components f or hot-stage section of gas-fired turbine engines. In addition to manu facturing quality control, other important aspects for this applicatio n include life prediction modeling and time between engine overhauls. One nondestructive evaluation (NDE) method that provides information a bout material condition involves an analysis of resonant vibrations. I n previous work by this general approach, changes in modal parameters have been related to bulk defect mechanisms such as microcracking due to thermal shock damage. In this work resonant vibrations from monolit hic ceramic specimens were excited by an instrumented impact hammer an d detected by a noncontact acoustic microphone over frequencies up to 100 kHz. Computer-based analysis of vibration signatures from test spe cimens allowed extraction of modal frequencies and damping constants. Downward shifts in detected resonant frequencies and increases in inte rnal friction or (specific) damping capacity measurements were obtaine d from SiC cylindrical rings, and these measurements were shown to rel ate to thermal shock severity. This NDE method not only provides measu rable parameters that could be used as accept-reject criteria for in-l ine process inspection, it also provides a means for tracking the mech anical integrity of in-service engine components to support life predi ction modeling.