Under the action of repeated stress on the components such as turbine blades, springs, crankshafts, biomedical transplantation, and load components, the strength of the material is reduced or the material is damaged. These components continue to be subjected to different types of tension, compression, bending, vibration, thermal expansion, and contraction or other repetitive stress effects. These stresses are often lower than the yield strength of the material, but must be greater than the fatigue limit of the material. The stress can cause the dislocation substructure of the material, thereby changing fatigue failure. In this study, we mainly used non-linear ultrasonic measurement to generate high-energy waves, made it incident on the steel and received its double-frequency signal at the other end. Then we calculated the non-linear factor of steel based on the ratio of the double frequency amplitude to the square of the fundamental frequency amplitude. Besides, the non-linear factor of steel was measured under different fatigue cycles. Then, the curve of the material non-linear factor was determined and the cycle stress number was measured by the relationship between different fatigue cycles and their non-linear factors. The final results show that, with the increase in the number of applied cyclic stresses, the non-linear factor also tends to increase. The non-linear factor of the test piece with no damage at all and the test piece with one million fatigue damages can be up to 6.5%. Non-linear ultrasonic technique was successfully implemented to analyse the fatigue damages.

涡轮叶片、弹簧、曲轴、生物医学移植、负载部件等部件在反复应力作用下，材料强度降低或材料损坏。这些部件继续受到不同类型的拉伸、压缩、弯曲、振动、热膨胀和收缩或其他重复应力的影响。这些应力通常低于材料的屈服强度，但必须大于材料的疲劳极限。应力会引起材料亚结构的位错，从而改变疲劳失效。在本研究中，我们主要利用非线性超声波测量产生高能波，使其入射到钢材上，并在另一端接收其双频信号。然后根据双频幅值与基频幅值平方的比值计算钢材的非线性系数。此外，还测量了钢在不同疲劳循环下的非线性系数。然后，确定材料非线性因素的曲线，并通过不同疲劳循环与其非线性因素之间的关系来测量循环应力数。最终结果表明，随着施加循环应力次数的增加，非线性因子也趋于增加。无损伤试件和疲劳损伤100万次的试件非线性系数可达6.5%。成功地采用非线性超声技术来分析疲劳损伤。