Ultrasonic Guided Waves for Structural Health Monitoring and Application to Rail Inspection Prototype for the Federal Railroad Administration

Abstract

Recent train accidents, associated direct and indirect costs, as well as safety concerns, have reaffirmed the need for developing rail defect detection systems more effective than those used today. Current methods for detecting internal flaws in rails rely primarily on ultrasonic pulse-echo technology operated in a water-filled wheel or sled. Presence and loss of echoes along each tested direction are analyzed in parallel to map surface and internal cracks in the rail. While this technology has served the industry well, several inherent weaknesses exist. Contact heads and cross-sectional inspection limit the speed of the measurement; liquid couplant is required to maintain efficient wave/echo transmission through the contact patch. Despite the acoustic couplant, significant transmission loss results from the pulse/echo passing through the contact patch twice. More importantly, ultrasonic beams launched vertically from the top of the rail head can miss internal defects located under horizontal shelling; this was the case, in the disastrous train derailment at Superior, WI in 1991. As a proposal to address these issues, the use of ultrasonic guide waves appears promising. One objective of this work is extending the fundamental knowledge of the guided v wave propagation in rails by predicting modal and forced solutions in the high frequency (<500 kHz) range. The selection of guided wave features sensitive to the presence of the different type of defects is essential for a successful defect detection performance. Another accomplishment of this work is the development of a rail defect detection prototype based on a laser/air-coupled ultrasonic technique. The prototype has been successfully tested twice in the field for the Federal Railroad Administration in the United States of America and it shows promise for implementation in rail inspection cars.