This paper proposes a crack detection method for rail joint screw holes based on the ultrasonic nonlinearity. Experimental and numerical studies are conducted to validate the performance of the proposed approach. The ultrasonic signals excited and received at the rail head are processed by spectral analysis and Wavelet Packet Decomposition (WPD). The third harmonic can be clearly observed from the spectrum analysis, which is supposed to be the nonlinearity of structures. Further, WPD is used to decompose the frequency components of the received signals. A damage index (DI) is defined based on the energy percentage of the energy in the WPD node containing the third harmonic with respect to the total energy of the received signal. It is found that the DI relating to the third harmonic increases with an approximately linear relationship when the crack grows bigger. It can provide a basis for the crack detection and quantification for the rail joint screw holes. A finite element model of the experimentally tested rail is also developed and numerical studies are conducted to verify the ultrasonic nonlinearity in the rail structure. Numerical results have the same trend with the experimental results, which verifies the effectiveness of the proposed method for crack detection in rail joints based on the high order ultrasonic nonlinear harmonics.
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