Fatigue limit evaluation considering crack initiation for lamellar pearlitic steel

摘要

In order to evaluate the fatigue limit of lamellar pearlitic steel used for railroad rails, tensile tests and fatigue tests are performed. Although the fatigue ratio of the lamellar pearlitic steel is lower than that of general steels, the reason for this has not been clarified. The fatigue cracks of the pearlitic steel initiate at a very early stage during the fatigue test. It is speculated that the steel should be treated as a steel with initial defects. In order to determine the initial defect size of the ultra-low cycle fatigue test, tensile tests are performed. Based on the test results, it was clarified that the crack initiation size depends on the crystal structure. In order to predict the fatigue limit of the pearlitic steel, Murakami's prediction method is applied to the steel. The measured defect sizes are applied to the method, and the fatigue tests are performed. The predicted fatigue limits and the test results have good agreement. In addition, from the SEM observations, the initial crack causing the fatigue failure was found to be a pearlite block. We then concluded that the fatigue limit of the pearlitic steel can be predicted by Murakami's method and the defect size is the pearlite block size. If the pearlite block sizes then become small, the fatigue limit of the pearlitic steel will increase.