The length of the fatigue cracks is always underestimated by linear ultrasonic testing, for example that the phased array method based on linear ultrasonic technique can be used for detecting the opened crack rather than closed crack. In the paper, a novel nonlinear ultrasonic modulation method is proposed for detecting the closed crack. Firstly, the transducers’ arrangement is optimized to excite the nonlinear ultrasonic response of closed crack. Secondly, the hypothesis of quantitative measurement is given for the closed crack. Finally, the hypothesis is proved to be correct by comparing with the metallographic measurement. The results indicate that the big modulation coefficients can be extracted from the closed crack covered by the superimposed area of sound field, and the length of closed crack which cannot be detected by linear ultrasonic method can be determined by the nonlinear ultrasonic modulation method.

线性超声检测总是低估疲劳裂纹的长度，例如基于线性超声技术的相控阵方法可用于检测开放裂纹而不是闭合裂纹。本文提出了一种新的非线性超声调制方法来检测闭合裂纹。首先，优化换能器的布置以激发闭合裂纹的非线性超声响应。其次，给出了闭合裂纹定量测量的假设。最后，通过与金相测量的对比，证明了该假设的正确性。结果表明，可以从声场叠加区域覆盖的闭合裂纹中提取出较大的调制系数，