中文摘要：

在U71Mn钢轨截面上不同位置取9个带有燕尾状缺口的试件进行三点弯曲疲劳试验和超声波非线性检测,提取试件在不同疲劳加载循环次数下超声波的基波幅值和二次谐波幅值,据此计算钢轨疲劳过程的超声非线性系数,并用扫描电镜和金相显微镜观察对应试件表面的微观形貌,研究超声非线性系数与钢轨疲劳裂纹萌生和扩展的关系。结果表明：在钢轨疲劳过程中,超声非线性系数呈规律性变化;在试件疲劳的初期,超声非线性系数缓慢升高,当疲劳加载循环次数达到7万次以后超声非线性系数开始快速增长,对应的试件开始萌生裂纹,到9万次以后试件的裂纹开始生长,当疲劳加载循环次数达到11万次时超声非线性系数达到峰值,然后随着疲劳加载循环次数的继续增加,超声非线性系数开始呈降低趋势,并在13万次时裂纹已经生长为3mm长的宏观裂纹,这说明超声非线性系数对钢轨疲劳过程中裂纹的萌生和扩展的变化非常敏感,验证了超声非线性系数用于表征U71Mn钢轨疲劳损伤过程的有效性。

英文摘要：

Nine specimens with swallow-tailed notch were taken from different points on the cross section of U71Mn rail to carry through three-point bending fatigue test and nonlinear ultrasonic testing. The amplitudes of ultrasonic fundamental and second harmonic were extracted from the specimens under different fatigue loading cycles. Then, the nonlinear ultrasonic coefficient during rail fatigue process was calculated accordingly. The relationship between nonlinear ultrasonic coefficient and the fatigue crack initiation and growth of rail was research by observing the micro-morphology of corresponding specimen surface with scanning electron microscope and metallographic microscope. The results showed that the nonlinear ultrasonic coefficient changed regularly in fatigue process. The nonlinear ultrasonic coefficient went up slowly at the early stage of fatigue test and then started to increase rapidly after fatigue loading cycles reached 70 000 and crack initiated in the corresponding specimen. After fatigue loading cycles reached 90 000, crack started to grow. The nonlinear ultrasonic coefficient reached the peak when fatigue loading cycles was 110 000. Thereafter, with the increase of fatigue loading cycles, the nonlinear ultrasonic coefficient tended to decrease. The crack grew into macroscopic crack with the length of about 3 mm when fatigue loading cycles reached 130 000. It indicates that linear ultrasonic coefficient is very sensitive to the change in the initiation and growth of crack during rail fatigue process, which has verified the validity of nonlinear ultrasonic coefficient to characterize the fatigue damage process of U71Mn rail.