# Inductive Thermography as Non-Destructive Testing for Railway Rails
###### Abstract
It is also possible to generate ultrasonic waves by laser pulses, which were presented for defect detection in railway axles and wheels [9; 10]. In the last few years the new technique of inductive thermography has been developed, which has a high potential for automated rail inspection [11; 12; 13; 14]. In the case of inductive thermography, the specimen to be tested is heated with a short inductive heating pulse, usually in the range of 0.1-1 s. The applied excitation frequency is in the range of 100-200 kHz, whereby in ferro-magnetic steel the induced eddy current penetrates only up to about 0.03 mm below the surface. Surface cracks, as e.g., head checks, disturb the eddy current distribution and also the heat diffusion, therefore they become visible in the infrared (IR) images, recorded by an IR camera. After evaluating this IR sequence, the cracks can be easily detected. In the last few years several publications have proved [15; 16; 17; 18; 19] that inductive thermography is able to localize surface cracks in metals. For industrial applications it is important that an inspection technique is standardized, and this was recently done by the German Institute for Standardization (DIN) [20]. In this paper we present the results of an inductive thermography inspection and we show that it is an excellent tool for characterization of the rail. Defects such as head checks and squats can be localized in a quick, contact-free and non-destructive way. Additionally, the defects can be characterized in a fully automated way by determining e.g., the number of head checks, their inclination angle and their length. The big advantage of this method, compared to traditional inspections as ultrasonic and eddy current testing, is that it delivers an image of the rail, showing the information from the surface along with additional information from a thin, 1-2 mm thick layer below the surface, which is relevant for these kinds of defects. The technique is quick (to inspect a 30 cm long rail piece needs less than 0.5 s) and the resulting phase image is easily understandable. Several different rail pieces were tested in the laboratory, having a length in the range of 10 cm up to 1 m. To inspect them two different kinds of measurement techniques have been developed. The first technique is a stop-and-go technique where the images of static, consecutive measurements are merged to form a panorama image. In the case of the second'scanning' technique the rail is inspected during a continuous movement. These two techniques are compared and their advantages and disadvantages explained. Finite element simulations were carried out using the multi-physics package ANSYS. The goal of the simulations was, on the one hand, to achieve a better understanding of the physical processes, how the eddy current distribution and the heat diffusion are disturbed by the cracks. On the other hand, the simulations were used to investigate the influence of the cracks' properties, i.e., crack depth and inclination angle, on the results. Additionally, the influence of the measurement parameters, i.e., the excitation frequency and heating pulse duration, were investigated in order to optimize the experimental setup and the crack detectability.