Modeling of disturbances of the magnetic field of a rail around a transverse crack
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MODELING OF DISTURBANCES OF THE MAGNETIC FIELD OF A RAIL AROUND A TRANSVERSE CRACK V. O. Nichoha
UDC 656.259.21
A generalized model for finding magnetic field and signals from the objects of the railroad is proposed. A 3D model of the primary field of a defect-free rail excited by a pilot alternating current is developed. The mathematical models of the secondary field of the rail disturbed around a transverse wedge-shaped crack and a rectangular crack are developed. We compare the images of signals from a transverse wedge-shaped defect obtained in the course of modeling with actual defects, such as, e.g., transverse cracks. Keywords: magnetic field, defectoscopy of rails, transverse crack, model of a signal, engineering diagnostics, rail pair.
Contemporary science proposes a broad spectrum of the procedures of nondestructive testing (NT) but only a limited part of them can be used for the diagnostics of the technical state of railroad rails. Even a smaller part of these methods is suitable for the rapid defectoscopy of rails in real time. In the present-day practice, the procedure of rapid flaw detection, especially on the Ukrainian railroads, is most often performed by using magnetic and ultrasonic methods. At present, the problem of timely detection of contact fatigue cracks in railroad rails is usually solved by using mobile means of NT and engineering diagnostics (ED). These are railway cars and flaw-detector cars in which ultrasonic channels (USC) and magnetic channels (MC) operate simultaneously [1, 2]. In most cases, in combined flaw-detector cars in which both USC and MC are realized, the preference is given to the USC guaranteeing (despite the existing disadvantages) the possibility of diagnostics of almost the entire rail, including the lower part of the railhead, the web, and the foot. Nevertheless, it does not provide the reliable acoustic contact between the sensor and the railhead, which decreases its efficiency, especially in the case of using mobile means of testing. Moreover, the USC fail to study the upper part of the railhead (the dead zone of this method). At the same time, the presence of the magnetic channel in the flaw-detection complex makes it possible not only to realize accurate referencing of the defective zones to the investigated part of the railroad but also to substantially increase the probability of detection of dangerous defects in the upper part of the railhead. It was shown [3] that, in extreme cases, the MC is the only channel capable of timely detection of the defects of critical sizes and, first of all, of subsurface defects, such as transverse cracks. These defects are classified as contact fatigue defects of rails caused by plastic deformation in the zone of its contact with wheels and the accumulation of in-service residual stresses in the upper part of the rail. This is one of the main causes of initiation of transverse cracks (closed and appearing the surface) in this zone. These defects are most dangerous and have special digital code 21OD. They represent a serious h
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