Effect of Temperature on the Rate of Fatigue Crack Propagation in Some Steels during Low Cycle Fatigue

Abstract

Temperature dependence of the rate of fatigue crack propagation was examined, and compared with the temperature dependence of tensile ductility. As numerous microcracks initiate and affect the propagation behavior of the main crack at elevated temperatures, the following factors were found to be elucidated; initiation rate of microcracks, reduction of ductility of the material in the vicinity of the main crack tip, and relaxation of concentrated strain by multi-cracks. The results obtained are summarized as follows:
(1) During a strain controlled low cycle fatigue test at 1cpm, the rate of crack propagation is largest at the blue-brittleness temperature range (200-300°C) in a low carbon steel. On the other hand, it is largest at above 700°C in austenite stainless steels. The temperature dependence of the rate of fatigue crack propagation is opposite to that of tensile ductility.
(2) Microcracks formed in the vicinity of the main crack tip were calculated, by considering the strain concentration and strain cycles imposed. Then, the local fracture strain was evaluated. There found a good correlation between the rate of crack propagation and the local fracture strain.