为了实现对金属材料中微纳米级裂纹的超声检测，建立了非线性超声检测系统，研究了超声波与金属材料中裂纹的相互作用以及超声波的畸变效应。介绍了固体中普遍存在的超声非线性现象；以金属材料中的微纳米级裂纹为例研究了裂纹与超声波相互作用产生的畸变效应；在分析超声波产生畸变的基础上，描述了超声高次谐波振幅的测量方法。最后，以金属材料疲劳裂纹为例，利用常规超声检测和非线性超声检测两种方法，证明了非线性超声检测在微纳米级裂纹检测中的有效性。实验结果表明：疲劳裂纹扩展长度与二次谐波激发效率关系密切；利用二次谐波，非线性超声检测方法检测出的裂纹长度比常规超声检测方法检测出的裂纹长度增加了30%，实现了对金属材料微纳米级裂纹的检测。

A self-built nonlinear ultrasonic test system was established and the interaction between ultrasonic waves and cracks in metal materials and the distortion effects of ultrasonic waves were researched to implement the ultrasonic test of micro-nano cracks in metal materials. On the basis of the presumption that nonlinearity of ultrasonic waves in solid materials is universal, the distortion effect caused by the interaction between ultrasonic waves and the micro-nano cracks in metal materials was explored. Then, according to the analysis on the distortion effect, the measurement of ultrasonic high-order harmonic amplitude was illustrated. Finally,a regular ultrasonic test and a nonlinear ultrasonic test were conducted respectively on fatigue cracks in metal materials, which proves that the nonlinear ultrasonic test is more effective for the micro-nano crack detection. Experiments show that the length of a fatigue crack is relative closely to the exciting efficiency of the second harmonics and the length of the crack detected by the nonlinear ultrasonic method based on the second harmonics is 30% longer than that by the regular ultrasonic test, which realizes the test for micro-nano cracks in metal materials.