Fatigue experiments on steels were conducted using an advanced electrohydraulic machine, which has a frequency range from approximately 1 Hz to 1000 Hz. Increasing the test frequency from 10 Hz to 1000 Hz will increase the specimen temperature, which, in turn, will decrease the fatigue life in air. However, in mercury, due to the cooling effect by mercury, little change in fatigue life was observed at different frequencies.
A high-speed and high-sensitivity thermographic infrared (IR) imaging system has been used for nondestructive evaluation of temperature evolutions during fatigue testing of steels. The temperature sensitivity of the camera is 0.015°C at 23°C. High-speed data acquisition capabilities are available at 150 Hz with a full frame, and 6100 Hz with a narrow window. Thus, the IR camera can be used to monitor in situ temperature evolutions resulting from fatigue.
Five stages of specimen temperature evolutions were observed during fatigue testing: an initial increase of the mean temperature of the test sample, a followed decrease of the temperature, an equilibrium (steady-state) temperature region, an abrupt increase of temperature before final failure, and a temperature after specimen failure. The measurements of temperature oscillations within each fatigue cycle at 20 Hz have been attempted. During each fatigue cycle, the specimen temperature was detected to oscillate within approximately 0.5°C depending on the loading conditions and test materials. When the applied stress reached the minimum, the temperature typically approached the maximum. However, the applied maximum stress did not necessarily correspond to the minimum temperature.
A theoretical framework was attempted to predict temperature evolutions based on thermoelastic and inelastic effects, and heat-conduction models. Temperature oscillation during fatigue resulted from the thermoelastic effects, while the increase in the mean temperature derived from the inelastic behavior of the materials. The predicted temperature evolutions during fatigue were found to be in good agreement with the thermographic results measured by the advanced high-speed and high-sensitivity IR camera. Furthermore, the back calculation from the observed temperature was conducted to obtain inelastic deformation and stress-strain curves during fatigue.