两种加载频率下LZ50车轴钢疲劳短裂纹行为对比

在加载频率为180、15Hz条件下, 分别利用高频疲劳试验机和电液伺服疲劳试验机完成了各6根光滑漏斗形圆棒试样的短裂纹复型试验。试验结果表明: 在微观短裂纹(MSC) 阶段, 主导短裂纹的扩展均经历2次显著的减速过程, 在加载频率为180Hz, 扩展率降至最小值时, 裂纹统计平均尺度分别为11.49、106.32μm, 在15Hz下分别为14.14、122.29μm; 考虑试样个体间不可避免地存在微观结构细节差异, 从统计角度出发, 可以认为2次减速完成时的裂纹尺度分别对应铁素体晶粒平均直径14.26μm和富珠光体带状结构间距111.53μm这2种特征尺度; 进入物理短裂纹(PSC) 阶段后, 扩展率随主导短裂纹尺度增加持续上升; 2种加载频率下主导短裂纹扩展率曲线和密度曲线在很大程度上相互重合, 变化趋势一致, 整体来看无显著差异; 在MSC阶段, 低加载频率下的短裂纹扩展率略高于高加载频率下的结果, 但差异并不明显, 最大速率差未超过一个数量级; 加载频率15Hz下短裂纹突破微观组织结构障碍消耗的寿命占总寿命比例较小, 2次降速对应的平均寿命分数分别为0.027和0.525, 而180Hz下对应的寿命分数分别为0.071和0.688;通过统计分析, 对比了7种常用统计分布, 确定了主导短裂纹尺度服从极大值分布, 疲劳寿命分数和有效短裂纹密度服从极小值分布。

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.