高强度钢的超高周疲劳裂纹扩展模型研究

当疲劳寿命在106或107周时，Wöhler S-N 曲线被看作渐近于水平轴，107的疲劳强度被看成是疲劳极限。现代应用要求延长零件的工作寿命，实际齿轮部件应用超过107循环的疲劳失效。本文应用压电超声疲劳试验机对经过热处理和渗碳处理后的低铬合金钢材料进行研究，采用红外摄像仪观测试件表面的温度场随疲劳裂纹萌生和扩展的过程。试验条件是室温，应力比为0.1（R=0.1），频率为20KHz。通过对表面渗碳处理后试件的断口分析，探讨表面渗碳处理、微观结构和与杂质有关的断裂机理，根据Paris公式建立超高周疲劳裂纹扩展模型。对裂纹扩展过程中裂纹尖端的塑性区的分析结果，结合传热学原理，建立热耗散模型，有限元方法的数值解结果较好地符合红外摄像仪的观测的试验结果。

THE MODEL OF VHCF CRACK PROPAGATION FOR HIGH STRENGTH STEEL

When fatigue life is beyond 106, the Wöhler S-N curve was always considered to be asymptotic in horizontal axis, but the fatigue behaviour over 106 cycles can not be neglected. Carburized process hardens surface of structure parts in order to improve wear and fatigue resistance. A piezoelectric gigacycle fatigue machine is employed to the tests in VHCF regime with 20KHz frequency and at stress ratio R=0.1, room temperature. The effects of heat and Carburized treatment on VHCF fatigue strength are investigated by test method. The infrared camera is applied to study the energy dissipation during the tests after calibration by blackbody. The cycle of crack initiation can be determined by the temperature increasing within several cycles near end of test. Through Scanning Electron Microscopy (SEM) analysis, the mainly parameters have been obtained to model the crack propagation (CP) based on Paris law, which permits estimating energy dissipation whose power and position are variable with crack propagation. The temperature distribution and evolution of specimen surface can be calculated by the numerical method FEM. The comparison of test and simulation results shows that the model is good agreement with test in the period of crack propagation.