抄録
An eddy current testing (ECT) coil system including a double-D coil as a pickup coil and a ring coil as an axisymmetric excitation coil was modeled. (Despite the frequent use of a double-D coil in ECT probes as an excitation coil, we employed it as a pickup coil). A double-D coil made from two semicircular wires connected in counter-series was placed within a ring coil in such a way that the coils were coaxial and co-planar. The structure of the coil system makes it insensitive, in principle, to a magnetic field with an axisymmetric distribution and sensitive to any unbalanced field. The method presented here is based on the numerical calculation of the well-known expression for the vector potential due to a ring coil, developed by Dodd and Deeds. Using this method, the induced voltage in the double-D coil due to flaw-eddy current interaction was numerically evaluated on numerous grids placed over the surface of an aluminum plate. In this way we plotted the measured voltage versus the radial distance between the centers of the probe and the flaw. By investigating the plot we can characterize the flaw. The experimental results agree very well with the results of numerical calculation.
