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Expressions are presented for the inductance of a curved circular spiral coil in free space and the change in impedance when the coil is wrapped around the surface of a magnetic, conductive cylinder. Inductance and impedance measurements of a thin conformable coil on a cylindrical Al-alloy rod were used to test the validity of these expressions. The theoretical predictions are in very good agreement with the experimental measurements over the frequency range 1 kHz–10 MHz. The findings are compared with previous work using rectangular spiral coils and it was found that the change in normalized impedance at high frequencies for a circular coil was almost identical to that obtained for a similarly-sized square coil. The implications for use in eddy-current nondestructive testing are discussed.

Calculations are carried out to evaluate pulsed eddy-current interaction with a crack in a planar conductor. The field is excited by a coil carrying current pulses and crack signals are detected by measuring changes in the magnetic field above the conductor. The model is based on the Fourier superposition concept, where the current pulses are decomposed into distinct frequencies. An existing boundary element method model for eddy current inspection of narrow cracks is used for each frequency and the final transient crack signal is synthesized by the individual frequency responses. The model is verified by a 3D finite element commercial package with a time-stepping technique. Computation times for the presented approach are orders of magnitudes shorter and allow rapid parametric studies of the transient signals for identification of principal component characteristics as well as for inversion purposes.

Further developments in a Boundary Element Method model for eddy current inspection of ideal or narrow cracks are presented. First, the required Green’s function is rapidly computed in the spatial domain using the concept of discrete complex images. The approach is based on the analytical evaluation of the Sommerfeld type Integrals through the use of the Generalized Pencil of Function method. Computation times are recorded in order to show the remarkable reduction in the moment matrix fill time as well as in the total computational time. Second, the model is extended to arbitrary crack depth profiles. Theoretical results are compared to published benchmark measurements of the coil impedance change due to a semi-elliptic and an epicyclic crack and excellent agreement is achieved.

An existing boundary element method model for eddy current inspection of ideal or narrow cracks is revised. Using some lately devised concepts on efficiently and rapidly simulating canonical eddy current geometries, we show how this model can be made easier to implement and faster to calculate. The approach is largely based on a novel method for rapidly calculating the Green's function and reducing the matrix fill-time, and also on a similar method for rapidly computing the incident electric field. As a result, the model has been made an order of magnitude faster than the existing one without sacrificing accuracy. We pay attention to numerical analysis details and analyze issues that so far have not been clarified. The validity of our approach is also verified by the experiment. Although we have tested the model against published data, we have also produced our own precision measurements for surface and through-the-thickness slots in plates with the coils performing scans along and across the slots at various frequencies.

As for any other eddy current probe coil, the analysis of rectangular ones plays a vital role towards understanding their characteristics and performance. In this paper, closed-form expressions are provided both for the impedance of rectangular coils of rectangular cross-section located above a conducting half-space and for the eddy current density induced therein. The formulation is general and can be easily used for any coil shape.

The inversion of eddy current probe impedance measurements is widely recognized as a complex theoretical problem whose solution is likely to have a significant impact on the characterization of materials. In this paper the evaluation of the conductivity profile of a layered planar structure is performed after inverting the impedance of a circular air-cored probe coil, of rectangular cross-section, using multilayer perceptron neural networks, trained via the back propagation learning algorithm. The merits of the method are illustrated in the light of two engineering examples.