The Computation of Fields and Signals due to Ferromagnetic Anomalies
In this paper we develop a model for the computation of electromagnetic fields in anomalous regions of ferromagnetic cylinders. The role of electric and magnetic current densities as sources for these fields is explicitly presented. The starting point for the development is the computation of a three-dimensional Green’s function for the cylinder, from which the appropriate integral relations between the field and its sources can be derived. The rigorous calculation of the anomalous current source within the anomalous region requires the solution of an integral equation that has the Green’s function as its kernel. We do not carry out this calculation, but approximate the anomalous current by the applied field due to the exciting coil (which, for our examples is an infinite solenoid that is coaxial to the cylinder). Once the field within the anomalies is determined, the field external to the wall of the tube may be computed, and this provides the signal that is sensed by a coil, or other means.
KeywordsTransverse Electric Rigorous Calculation Anomalous Region Exciting Coil Quantitative Nondestructive Evaluation
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