Two methods for the detection of structural defects in railroad tracks are discussed. The first method employs air-coupled ultrasonic testing for non-contact probing and ease of transducer positioning. It is shown that resonant standing waves can be successfully generated in the rail cross-section to alleviate the challenges associated with the large acoustic impedance mismatch between air and steel. The cross-sectional inspection is particularly well-suited for the detection of longitudinal defects. The second method uses structural vibrations of the track for long-range detection of transverse and oblique defects. In the presence of various defect types and sizes, the reflection coefficient spectra of broadband longitudinal, vertical and lateral vibrations are extracted by a joint time-frequency analysis based on the wavelet transform. These results can be useful for the development of a rail defect detection system based on automatic pattern recognition.
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Scalea et al. (2003) studied this question.
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