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Ultrasonic rail inspection is the most commonly implemented method for detecting internal rail defects. While the conventional ultrasound wheel probe has gained its popularity within rail maintenance community, it suffers from the limited test speeds (25 mph at most). This paper presents the state-of-the-art developments in ultrasonic rail inspection technique that utilizes non-contact receivers and no active transmitters. The transfer function between two points of the rail is reconstructed by deconvolutions of multiple pairs of receivers that sense the acoustics naturally excited in the rail by the running wheels. The deconvolution process eliminates the random effect of the excitation to reconstruct a stable acoustic transfer function of the rail. A fixed bulk delay and frequency selection technique are introduced to facilitate the power spectral density estimation for robust transfer function reconstruction. A multivariate analysis based on selected features extracted from various frequency bands is conducted on the signals recorded by multiple sensor pairs respectively. Furthermore, damage index traces based on data from different sensor pairs provide system redundancy for improved reliability with the voting logic for damage detection. The proposed approach lends itself to extremely high testing speeds (as fast as the service train speed, e.g. 60 mph and above), that would enable the real-time evaluation of rail track integrity at train operational speeds. A prototype based on this passive-only inspection idea has been constructed and tested with the DOTX216 testing vehicle of the Federal Railroad Administration at the Transportation Technology Center (TTC) in Pueblo, CO in September 2016. Test runs were made at various speeds from 25 mph to 80 mph (the maximum speed allowed on the test track). The results show the feasibility of stable reconstruction of the transfer function from the random wheel excitation, as well as the detection of joints and welds present in the track. Some tests were also conducted on TTC Defect Farm showing the potential for defect defection.
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