Comparison of short fatigue crack behaviors for LZ50 axle steel under two loading frequencies

Under the loading frequencies of 180 and 15 Hz, short fatigue crack replica tests of six hourglass shaped specimens were carried out by using the high-frequency fatigue testing machine and the electrohydraulic servo fatigue testing machine, respectively.Test result shows that, at the microstructural short crack (MSC) stage, the growth of dominant short crack experiences two significant reduction processes.According to the two lowest growth rates, the statistical average crack sizes under the loading frequency of 180 Hz are 11.49 and 106.32μm, respectively, while the sizes under 15 Hz are 14.14 and 122.29μm, respectively.In consideration of the inevitable microstructural differences among the specimens, it is believed that the two main dominant crack growth rates decrease when the crack sizes are close to the average diameter 14.26μm of ferrite grains and to the average interval 111.53 μm of rich pearlite banded structures, respectively.At the physical short crack (PSC) stage, the growth rate of dominant short crack accelerates continuously with the increase of its size.The growth rate curves and density curves of dominant short crack under two loading frequencies overlap to a great extent and have the same change trends.In general, no significant differences exist.The growth rate of dominant short crack is slightly higher under low loading frequency than under high loading frequency at the MSC stage, but the difference is unobvious and less than a magnitude.The consumption life accounts for a smaller proportion of total life when the dominant crack breaks through the obstacles of microstructures, the fatigue life fractions are 0.027 and 0.525 under 15 Hz, respectively, and are0.071 and 0.688 under 180 Hz in the two significant reduction processes of dominant short crack growth.Based on 7 commonly used statistical distributions and comparative analysis, the distributions of size, fatigue life fraction and effective density of dominant short crack are determined, the size obeys the maximum distribution, and the fatigue life fraction and effective short crack density obey the minimum distribution.