Research On Rail Flaw Detection Technology Based On Broadband Frequency Modulation Ultrasonic Signals

The friction and impact on the rails are likely to cause the rail internally broken, likecracks and abrasion. The qualities of rails on operation are getting worse in our country,because of the heavy rail transportation tasks and overdue service of the rails. Somepotentially dangerous situation, such as further damage on the rail, or even accidents causedby broken rail would probably happen. The development and implementation of improved railmaintenance programs are expected to decrease rail flaws. Through increased reliability offlaw detection technologies, especially the ultrasonic Non Destructive Testing(NDT), adecrease in rail failures and accidents is expected.Fresnel zone exists while using conventional pulsed ultrasound detection method.Ultrasonic detection in near-field region increases the chances of false positives and falsenegatives of rail flaw, while detection in far-field region increases the difficulty of couplingand equipment design as the sound pressure attenuation. Broadband Chirp signal withanti-multi-path and anti-noises characteristics are feasible to overcome the difficulties ofdetection in near-field region, and improving SNR. Quantitative testing cannot achieve highprecision through this signal. The discussion would be related on ultrasonic rail flaw detectionbased on broadband linear frequency modulation (LFM) ultrasonic signal.1. A discussion on the acoustic pressure distribution of ultrasonic near-field acoustic fieldis launched. The acoustic field is approximately homogeneous, and the defect echo quality isimproved using broadband LFM ultrasonic acoustic near-field for detection, so that the railflaw detection rate and accuracy are improved. The calculation method of LFM signalbandwidth and initial frequency through thickness is given.2. A time delay estimation method based on Fractional Fourier Transform (FRFT) isproposed. Chirp signal has the characteristics of focusing property and appearing impulsefunction in FRFT domain. Echo parameter estimation based on the peak parameter of Chirpsignal in FRFT domain is available. Simulation results show that the delay estimation basedon FRFT method, in the case of low SNR works very stable. The vertical position estimationerror of high accuracy and reliability is less than1mm, consistent with the rail flaw detectionstandard.