Thermographic detection of fatigue damage of pressure vessel steels at 1,000 Hz and 20 Hz

A thermographic infrared detection system has been used to measure the temperature profiles during fatigue testing of specimens of reactor pressure vessel steels at 1000 Hz and 20 Hz. Four stages of temperature profiles were observed during fatigue testing: an initial increase of the average specimen temperature, an equilibrium-temperature region, an abrupt increase of the temperature, and a drop of the temperature following specimen failure. Temperature oscillation was observed within each fatigue cycle. An image subtraction technique in the infrared detection system can be used to monitor crack initiation and propagation behavior. At 1000 Hz, the equilibrium temperature can reach > 95 deg C, depending on the applied maximum stress level, while at 20 Hz, it approachs only approx23-24 deg C. The much greater temperature generated at 1000 Hz than 20 Hz could contribute to the shorter fatigue life at 1000 Hz.