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.
Highlights
Head checks and squats are common defects on rails stemming from rolling contact fatigue (RCF) [1,2,3,4]
As e.g., head checks, disturb the eddy current distribution and the heat diffusion, they become visible in the infrared (IR) images, recorded by an IR camera
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
Summary
Head checks and squats are common defects on rails stemming from rolling contact fatigue (RCF) [1,2,3,4]. Squats are rolling contact fatigue RCF defects occurring in straight or (gently) curved track within the running band on the rail head where the rail is heavily sheared This 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. As e.g., head checks, disturb the eddy current distribution and the heat diffusion, they become visible in the infrared (IR) images, recorded by an IR camera. 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. 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
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