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AUTOMATIC LASER ULTRASONICS FOR RAIL INSPECTION S. A. Nielsen1, A. L. Bardenshtein1, A. M. Thommesen1, and B. Stenum2 Sensor & NDE Innovation Division, FORCE Technology, 2605 Brøndby, Denmark Optics and Plasma Research Dept., Risø National Laboratory, 4000 Roskilde, DK 1 2

Abstract: This paper presents the state of the current activities to develop a high-speed inspection system for evaluating rail defects. Since conventional contact ultrasonic techniques are speed limited, the aim was to build an automatic inspection system, which applies non-contact ultrasonic methods. An automatic Laser Ultrasonic system for Rail Inspection (LURI) has been developed and tested on a railroad line containing man-made structural defects. LURI is mounted on a specially designed railroad vehicle and allows detection of defects on the running surface of the rail, as well as, horizontal and vertical flaws in the railhead. LURI is tested in the field up to 40 km/h (25 mph). In order to support LURI, a battery-operated ultrasonic rail flaw detector based on the P-scan Lite system was developed and tested.

Introduction: Efforts to improve the safety and efficiency of railways have generated new NDT techniques to verify defects in rails. Especially ultrasonic methods have been developed for many years. In these methods ultrasonic beams are sent into the rail and scattered from defects and the rail itself. These measurements can detect the presence of a variety of defects but as the inspection speed increases there are practical problems with the technology. They are concerned with the traditional contact ultrasonic technology employed by the majority of rail flaw detectors [1]. Rail inspection is traditionally based on either rolling-contact or sliding transducers, which require maintaining a liquid- column acoustic contact. Acoustic contact is difficult to maintain during high inspection speeds and may result in loss of data. Furthermore, such transducers can only send and receive ultrasound on the running surface of the rail. This makes contact ultrasonic technology almost blind to critical flaws as transverse and longitudinal vertical cracks in the railhead. Moreover, surface-breaking defects are difficult to find by contact ultrasonic interrogation of a rail. Laser-based ultrasonics provides a very attractive solution to these problems. Since in this technique, a pulsed laser is used to generate ultrasonic modes in the rail by ablation, and a continuous-wave or a long-pulse laser, coupled to an optical interferometer, is used to detect ultrasound after it has propagated through the material, the following significant advantages are special for the laser-ultrasonic rail inspection system [2]:

  • LURI is a non-contact method, where only laser light interacts with the running surface of the rail.

  • LURI implies high-speed inspection up to traffic speed, reducing the need for blocking the rail traffic.

  • LURI identifies surface breaking defects.

  • LURI can detect transverse and longitudinal vertical cracks in the railhead – the most common defects found in rails – by impinging ultrasound from the side rather than from the top of the head.

  • LURI reveals smaller defects compared with traditional ultrasonic methods for rail inspection.

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