Hysteresis, yield, and energy dissipation during thermo-mechanical fatigue of a ferritic steel

Thermo-mechanical fatigue (TMF) tests were conducted at a total strain range of 0.6% on a 9Cr2Mo alloy. The minimum and maximum temperature limits were 270 and 570 °C, respectively, and the cycle time was 5 min. Diamond cycles of various phase angles together with a complex ‘industrial’ cycle were explored. These were defined (using the conventional plot of positive and negative mechanical strain on a vertical axis and temperature on a horizontal axis) as ‘clockwise’ and ‘counter-clockwise’, respectively. The physical arguments for such loops are presented and loop involving the minimum reversed yield and energy dissipation is sought. An attempt is made to predict the actual shape of each loop type from stress–strain data determined from supporting isothermal tests.