Models of surface eddy-current transducers with tangential exciting field
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MODELS OF SURFACE EDDY-CURRENT TRANSDUCERS WITH TANGENTIAL EXCITING FIELD V. H. Rybachuk
UDC 620.179.14
We propose computational models of the surface eddy-current transducers with tangential exciting field. Analytic expressions are deduced for the description of the signals from these transducers placed over a multilayer conducting half space. Various versions of design of these devices aimed measuring both tangential and normal components of the secondary field of eddy currents are analyzed.
The development of new highly efficient eddy-current procedures of nondestructive testing of structural materials requires the profound investigation of the specific features of excitation of eddy currents in layered conducting media, in particular, for the tangential orientation of the primary exciting field untypical for the major part of contemporary surface eddy-current transducers. The available types of design of the surface eddy-current transducers can most often be described as collections of round or rectangular induction coils in which the planes of turns are parallel to the surface of the conducting tested object. For the indicated orientation of the exciting coil, the magnetic component of the primary field is perpendicular to the surface of the object and the eddy currents induced in the object flow along closed circular paths parallel to this surface [1–3]. Therefore, the indicated surface eddy-current transducers are called transducers with normal exciting field. This group of transducers includes, in fact, all known types of design of the surface eddy-current transducers applied in the eddycurrent defectoscopy, structuroscopy, and thickness measurements [3–5]. However, their efficiency is sometimes insufficient. Indeed, if a defect has a plane character (e.g., exfoliation) and is oriented parallel to the surface of the tested object, then the trajectories of eddy currents are parallel to this defect and, hence, the perturbations introduced by the defect are insignificant. Therefore, the sensitivity of classical surface eddy-current transducers to these defects is low. In order to increase it, it is necessary to change the flow of eddy currents in the conducting object with an aim to get stronger distortions of their trajectories by the defects parallel to the surface of the object. As a possible solution of this problem, we can offer the application of surface eddy-current transducers with tangential exciting field in which the magnetic component of the primary exciting field at the output of the exciting coil is parallel to the surface of the tested object. These transducers are somewhat similar to the electromagnetic search systems with excitation from horizontal magnetic dipoles applied for the detection of natural anomalies and artificial objects in Earth’s crust and aqueous media [6]. The surface eddy-current transducers of this type can also be efficient in the solution of the problems of determination of thickness of conducting coatings on conducting substrates in the case where the values of specifi
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