ASSESSMENT OF DAMAGE IN CERAMICS AND CERAMIC-MATRIX COMPOSITES USING ULTRASONIC TECHNIQUES

This paper addresses the application of ultrasonic methods to damage a ssessment in ceramics and ceramic matrix composites. It focuses on dam age caused by thermal shock and oxidation at elevated temperatures. Th e damage-induced changes in elastic constants and elastic anisotropy a re determined by measuring the velocities of ultrasonic waves in diffe rent propagation directions within the sample. Thermal shock damage me asurement is performed in ceramic samples of reaction bonded silicon n itride (RBSN) and aluminum oxide. Thermal shock treatment from differe nt temperatures up to 1000 degrees C is applied to produce the microcr acks. Both surface and bulk ultrasonic wave methods are used to correl ate the change of elastic constants to microstructural degradation and to determine the change in elastic anisotropy induced by microcrack d amage. Oxidation damage is studied in silicon carbide fiber/reaction b onded silicon nitride matrix (SCS-6/RBSN) composites. The oxidation is done by exposing the samples in a flowing oxygen environment at eleva ted temperatures, up to 1400 degrees C, for 100 hours. Significant cha nges of ultrasonic velocities were observed for composites before and after oxidation. The elastic constants of the composites were determin ed from the measured velocity data. The Young's modulus in the fiber d irection as obtained from ultrasonic measurements decreases significan tly at 600 degrees C but retains its original value at temperatures ab ove 1200 degrees C. This agrees well with the results of destructive t ests by other authors. The transverse longitudinal and shear moduli ob tained from ultrasonic measurements decrease continually until 1200 de grees C. The results of this work show that the damage-induced anisotr opy in both ceramics and ceramic matrix composites can be determined s uccessfully by ultrasonic methods. This suggests the possibility of as sessing damage severity using ultrasonic techniques.