Electrical potential technique for monitoring subcritical crack growth at elevated temperatures

The electrical potential technique has been successfully used to monitor crack extension under fatigue as well as sustained loading at elevated temperatures in the presence of gross creep deformation. Calibration curves for actual crack extension vs change in electrical potential were determined for two specimen geometries, namely the compact type (CT) and the center crack tension (CCT) type, for an ASTM, grade A470 class 8 steel at 538°C (1000°F) and for a type 304 stainless steel at 594°C (1100°F).

A normalizing factor for expressing crack extension has been derived for the CT specimen. This factor accounts for changes in calibration due to small differences in initial crack length and, it also makes the calibration curve independent of the test temperature and material. Hence, the calibration curves presented herein are applicable to other materials and temperatures provided the specimen geometry and size is the same and the current input and potential leads are also located at the same position.