APPLICATION OF A HIGH-SPEED NON-CONTACT ULTRASONIC TECHNIQUE COUPLED WITH STATISTICAL DATA REDUNDANCY FOR RAIL INSPECTION

This paper presents a high-speed non-contact rail inspection technique that has been tested on the field at speeds up to 80 mph. The technique utilizes an array of capacitive air-coupled ultrasonic transducers in continuous recording mode to extract a reconstructed transfer function for a rail segment in a passive manner. The passive approach utilizes the ambient excitation of the rail induced by the wheels of the test car and eliminates the need of a controlled source. A normalized cross correlation operator with modified Welch’s periodogram technique is used to extract the transfer function which is independent of the frequency spectrum of the random excitation source (wheels). Presence of discontinuities in the rail reduces the signal-to-noise ratio of the reconstructed transfer function which is statistically tracked using an outlier analysis for multiple reconstructions along the inspected rail. Data from multiple transducer pairs are compounded in the statistical outlier analysis which ensures removal of bias from the data. An adaptive baseline model from pristine rail is used to compute a parameter called the Damage Index (DI) to determine if the probed rail segment has a discontinuity. Raw ultrasonic signals comprising of thousands of data points for a given recording time within a rail segment are therefore compressed statistically into a single DI parameter. Full-scale field tests were carried out at testing speeds of up to 80 mph. Discontinuity detection performance in terms of identifying joints, welds and known transverse defects through Receiver Operating Characteristic (ROC) curves were studied for a range of varying operational parameters such as raw signal strength, baseline length, and testing speeds. Data from multiple passes of the train over the same rail segment were compounded to further introduce redundancies and increase the rate of true detections and reduce the rate of false alarms.