Application of Third Harmonic wave Intensity Ratio in Crack Detection for Rail Joint Screw Hole

Cracks in rail joint screw holes have a potential threat to the safe service of railway transportation, and the early identification of screw hole cracks is of great importance to ensure operational safety. However, the rail joint due to the complex cross-sectional shape, the signal obtained is unusually complex by the traditional ultrasonic nondestructive testing. The linear characteristics such as reflection, transmission, attenuation, and impedance cannot effectively detect the rail joint screw hole crack. Besides, the traditional ultrasonic detection method only detects larger cracks, but it is very insensitive to the rail joint screw hole micro-crack. Compared with traditional ultrasonic NDT methods, nonlinear ultrasonic inspection methods use the geometric nonlinearity of the rail to cause waveform distortions in the ultrasonic propagation process, resulting in nonlinear effects such as higher harmonics, which have a higher sensitivity to screw hole cracks. In this paper, it is found that the ultrasonic signal of detecting the crack of rail joint screw hole has obvious second and third harmonic characteristics by the experimental study. By constructing the intensity factors of the fundamental wave and second/third harmonic, the intensity ratio of the second/third harmonic with the fundamental wave can be calculated, we find that the intensity ratio factor will increase with the crack size, which can propose an online detection method of rail joint screw hole crack. The experimental study shows that with the gradual increase of crack size, the third harmonic wave intensity ratio has better sensitivity compared with the second harmonic wave intensity ratio. The total harmonic distortion is deduced from the wave intensity ratio factor, and the calculation shows that there is an obvious positive correlation between the total harmonic distortion and the crack size at the hole edge, which can provide a basis for the online detection of the cracks in the screw holes of rail joints.