In situ measurement of fatigue crack growth rates in a silicon carbide ceramic at elevated temperatures using a DC potential system

The understanding of the mechanisms of fatigue-crack propagation in advanced ceramics at elevated temperatures (>800 degrees C) has in part been hampered by the experimental difficulty in directly measuring crack lengths, and hence crack growth rates, at such high temperatures. In this study, we show how the direct-current (DC) electrical-potential technique, which has been used for such measurements in metallic materials for over 30 years, can be successfully utilized to monitor fatigue crack growth rates in situ in a silicon carbide ceramic at temperatures between 850 and 1300 degrees C, because of the electrical conductivity in SiC at these temperatures. In addition to providing a highly efficient means of collecting such data, this approach offers several significant advantages over the techniques that have been used to date for advanced ceramics, particularly in avoiding artifacts due to thermal fatigue and oxidation from repeated exposure to air and/or lower temperatures while making measurements. Effects of parameters such as load ratio and loading frequency are examined, both on crack growth behavior and the accuracy of measurement. With appropriate considerations, electrical-potential calibrations determined at ambient temperatures in metallic materials can be applied readily to elevated temperature measurements in silicon carbide.