Rail crack detection: an infrared approach to in-service track monitoring

The growth of fatigue cracks in rail is a cause of ongoing concern to the railway industry, and much research and development work has been conducted to develop both trackside and train mounted analysis systems which are able to monitor the structural health of railway networks. To this end a new infrared-based method for surface crack quantification at conventional train speeds is being developed which delivers crack detection capability over the full depth of the rail section, with a crack resolution capability comparable to competing detection techniques. Experimental results are presented for a laboratory three-point bend notched specimen, the geometry of which is representative of surface-connected cracks with lengths below 2 mm in sections of rail. Two analyses are considered: a simulated trackside system, where the observation point is fixed and the repeated loading event experienced by a section of rail during the passage of multi-carriage rolling stock is considered; and a simulated train-based system, where the observation point moves along the rail and the loading event produced by the passage of a single train wheel is considered. Data from the trackside simulation system clearly identifies the precise location and severity of an artificially introduced notch on the upper surface of the specimen. Initial data from the train-based simulation system identifies the notch location precisely, but is unable to quantify the magnitude of the flaw using the current processing method. The paper then describes modifications to the testing and data processing methods required to improve the performance of both systems, and outlines the future on-site testing work planned to validate the methods presented. INTRODUCTION Significant energy has been expended in recent years to develop sensitive and repeatable crack detection systems for monitoring the structural integrity of rail in service. Much of this effort has focused on the use of ultrasonic monitoring systems, both train-mounted and handheld, which are used to detect and precisely locate flaws respectively. Portable handheld ultrasonic inspection systems are used for two broad applications: to inspect the quality of welds between sections of rail shortly after the welds have been created, but not necessarily before the first rolling stock has used the track; and to provide a high quality flaw detection system to support train-based methods, which often lack precision in determining both the precise location and magnitude of the flaw [1]. This method is effective when there is no change in material properties through an analysis region, and observes the ultrasound wave reflected from discontinuities in the rail structure. However, in cases where there is a change of material properties, this approach is ineffective due to a reflection of ultrasound at the material interfaces. For example, this occurs in welds where a filler material is used which is different from the parent material of the two rail sections being joined. In these situations the complementary method of guided wave ultrasonic detection may be employed [2]. Train mounted ultrasonic inspection systems are most commonly designed to provide a large quantity of comparatively low precision data, allowing the structural integrity of many miles of track to be assessed during the normal operation of the railway. Ultrasound waves are introduced into the rail using a specially constructed train wheel which contains probes for both the emission and detection of waves, and as with the handheld systems reflections occur at discontinuities, enabling a database of possible flaws to be developed for a length of track [3]. This data is then used to seed the handheld inspection process, which subsequently provides the precision required for accurate flaw location and quantification. The principal weakness of this method, the need for a consistent and contaminant-free contact between the rail and the instrumented wheel, can be eliminated by using air-coupled ultrasonic methods which use the rail head as a resonant chamber [4]. However, the losses experienced at the air-rail boundaries are so significant that, despite the use of high energy ultrasonic excitation, the signal to noise ratio of the data generated using this approach is lower than that for the contacting method and there is a corresponding reduction in the maximum achievable train speed during analysis [5]. It is clear that the development of a train-based system which could provide greater precision, and a trackside based system which would require less operator skill, would be of considerable attraction. Ultrasonic techniques do not require a change in loading condition of the rail in order to detect flaws. In contrast, the non-contacting technique of thermoelastic stress analysis is typically performed using repeated loading of a structure whilst recording the observed surface temperature, most commonly using an infrared imaging device [6]. If a sufficient number of infrared data frames can be recorded whilst cyclic loading occurs, and if the cyclic change in surface stress is sufficiently large, for most materials a correlation can be made between the loading cycle and the observed surface temperature, yielding the cyclic change in surface stress condition. More precisely, commercially available thermoelastic stress analysis systems [7] output a voltage from the load vs. temperature change correlation. The magnitude of this voltage S is the thermoelastic response of the observed component and is related to the cyclic change in the sum of the principal surface stresses by