# Inductive Thermography as Non-Destructive Testing for Railway Rails
###### Abstract  
Inductive thermography is a non-destructive testing method, whereby the specimen is slightly heated with a short heating pulse (0.1-1 s) and the temperature change on the surface is recorded with an infrared (IR) camera. Eddy current is induced by means of high frequency (HF) magnetic field in the surface'skin' of the specimen. Since surface cracks disturb the eddy current distribution and the heat diffusion, they become visible in the IR images. Head checks and squats are specific types of damage in railway rails related to rolling contact fatigue (RCF). Inductive thermography can be excellently used to detect head checks and squats on rails, and the method is also applicable for characterizing individual cracks as well as crack networks. Several rail pieces with head checks, with artificial electrical discharge-machining (EDM)-cuts and with a squat defect were inspected using inductive thermography. Aiming towards rail inspection of the track, 1 m long rail pieces were inspected in two different ways: first via a'stop-and-go' technique, through which their subsequent images are merged together into a panorama image, and secondly via scanning during a continuous movement of the rail. The advantages and disadvantages of both methods are compared and analyzed. Special image processing tools were developed to automatically fully characterize the rail defects (average crack angle, distance between cracks and average crack length) in the recorded IR images. Additionally, finite element simulations were used to investigate the effect of the measurement setup and of the crack parameters, in order to optimize the experiments. inductive thermography; nondestructive testing; rail defects; head checks; squats; crack detection; crack characterization; scanning thermography; rectification; edge detection +
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specific defect phenomenon is thought to initiate via small bits of hard material (such as pieces of track ballast) that are caught between rail and wheel and cause one crack to grow rather than another. The crack then preferentially grows in the direction of the heavily sheared surface layer; this generates the characteristic round shape of the squat. A striking description of the rail damage pattern \"squats\" can be found in Reference [6]. For a detailed investigation [PERSON] et al. [2] partitioned the rail head around a squat defect in several strips in longitudinal and transversal direction and investigated them by optical and by scanning electron microscopy. They summarized their observations in a schematic diagram that allows a better understanding of the layout of the rail surface with the directions of the microstructure deformation and the location of the squat initiation. The relevant literature on squats allows the conclusion that the residual stress state below the rail surface after operations is a main key to understanding the problem. However, currently only hypotheses are available to explain how squats are initiated or are able to grow from initial damages. Non-destructive testing methods able to detect squats and to elucidate the development of subsurface cracks around the squats are highly needed to understand this damage phenomenon and its evolution over load cycles, and to subsequently define adequate rail maintenance measures, e.g., the ones proposed by [PERSON] et al. [4]. Nowadays railway rails are usually tested by ultrasound or by eddy current testing methods in order to detect rail defects [7]. Additionally, visual inspection, at least in laboratory conditions, can be very helpful to characterize the rail damage [1]. The traditional ultrasound testing requires a coupling medium such as water in order to induce the ultrasonic waves in the work-piece to be tested. Novel techniques were developed to generate ultrasonic waves without a coupling medium, as e.g., it has been demonstrated that electromagnetic acoustic transducers (EMAT) can be used to induce Rayleigh surface waves in the rail for detecting defects [8].