An Improvement in Sensibility of the Eddy Current Testing

概要

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Eddy current testing (ECT) is one of the most representative nondestructive methods of testing metallic materials, parts, and structures. The operating principle underlying ECT is based on the two major properties of magnetic fields. The first is that alternating a magnetic field induces eddy current in all of the conducting materials. Thereby, input impedance of the magnetic field source, i.e., the electric source, depends on the eddy current path. The second is that the magnetic field distribution not only depends on the exciting but also the reactive magnetic fields caused by the eddy currents in targets. The former and latter are impedance sensing and magnetic flux sensing types. This paper is concerned with improving the sensibility of impedance sensing. The sensibility of ECT can be improved in two steps. The first is achieved by selecting the optimum exciting frequency. We employed a natural parallel resonant frequency in the ECT coil. The second is achieved by increasing the sharpness of the resonance curve on impedance versus frequency characteristics by changing the coil connections. Thus, we succeeded in developing an ECT sensor that had up to four times greater sensibility than the conventional one.