Abstract
Eddy current testing (ECT) is one of the most representative nondestructive testing methods for metallic materials, parts, structures and so on. Operating principle of ECT is based on the two major properties of magnetic field. One is that alternating magnetic field induces eddy current in all of the conducting materials. Thereby, an input impedance of the magnetic field source, i.e., electric source, depends on the eddy current path. Second is that the magnetic field distribution depends only on the exciting but also the reactive magnetic fields caused by the eddy currents in targets. Former and latter are the impedance sensing and magnetic flux sensing types, respectively.
This paper concerns with an improvement of sensibility of the impedance sensing method. Sensibility of the ECT is improved by means of two steps. One is an optimum exciting frequency selection. We employ the natural parallel resonant frequency of ECT coil. The other is to increase the sharpness of the resonance curve on impedance versus frequency characteristic by changing the coil connection. As a result, we have succeeded in developing the ECT sensor having up to 4 times higher sensibility compared with those of conventional one.
