Enhance the Sensibility of the Eddy Current Testing

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.

Enhance the sensibility of the film infinite eddy current sensors

“…The eddy currents on a no defect metallic surface yield the opposing Figure 1 shows a typical planar type conventional ∞ coil which is composed of the two planar exciting coils and one sensing coil wound around the ferrite bar. According to the shape of the two exacting coils, we had named this sensor as 'the ∞ coil' [3,4].…”

A Study on Backside Defect Searching by Low Frequency Excitation of the ∞ Coil

“…Among these methods, ECT does not need complex electronic circuits and direct contact to the targets. Also, most of the targets, whose major frame parts are composed of conductive metallic materials, can be selectively inspected by ECT [1][2][3].…”

Backside defect searching by means of the low frequency ∞ coil excitation

“…Among these methods, ECT needs not complex electronic circuits and direct contact to the targets. And also, most of the targets whose major frame parts are composed of conductive metallic materials can be selectively inspected by ECT [1][2][3].…”

“…One detects the defect in the target as a change of input impedance of the exciting coil. This is because the magnetic field distribution is changed by the detour eddy currents flowing around the defect in the target which corresponds to the secondary circuit of a single phase transformer [2][3]. The other type ETC sensor equips a sensing coil to detect the magnetic field change caused by the detour eddy currents flowing around the defect.…”

“…A key idea of our ∞ coil is that the sensing coil wound around a ferrite bar is installed at the lowest magnetic field intensity region between the north and south poles of exciting coils [1].…”