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.