The Improvement of Flaw Detection by the Configuration of Uniform Eddy Current Probes

Abstract

In this review, the principles to detect flaws with uniform eddy currents were presented based on the shape and orientation of the excitation coils and detection coils of the probe. Techniques are applied to detect flaws like cracks, especially on the weld zone surface, of test pieces of non-magnetic and ferromagnetic materials, and have unique features which are immune to the effects of lift-off. In the technique of interest, almost all the probe models developed are the type with tangential rectangular excitation coils. The induction condition and the flaw signal for each probe were discussed based on the shape and orientation of the excitation coils and detection coils of the probe. Finally, the challenge of increasing sensitivity to detect flaws with a uniform eddy current was also presented.

A Uniform Eddy Current Probe with a Double-Excitation Coil for Flaw Detection on Aluminium Plates

“…In this configuration, not all parts of the detection coil contribute to induce the signal; only parts that are close and parallel to the test piece are actively involved in this regard. To overcome these relatively small signals, several trials using excitation currents with high frequencies and amplifier circuits have been conducted [11,12,18,19].…”

“…A unique model of the STR probe is the uniform eddy current (UEC) probe. This probe has been developed to resist the effects of lift-off caused by the uneven surface of test materials, such as those found in weld zones [7][8][9][10][11][12][13][14]. UEC probes are for detecting flaws on the surface of the test piece with high frequency of excitation current and are specially designed to have resistance to lift-off variation in the measurement [1,7,8].…”

“…The electromotive forces (emfs) generated on opposite parts of a detection coil are the same but have opposite polarities; accordingly, they cancel out each other. As a result, the output voltage, V O of the coil is zero; this nature is called self-nulling [7,11,13,15]. Because the UEC probe reduces the Metals 2019, 9,1116; doi:10.3390/met9101116 www.mdpi.com/journal/metals Metals 2019, 9,1116 2 of 11 effect of lift-off on the measurement signal, the signal can be regarded as a reliable sizing indicator of the flaw [16,17].…”

“…To overcome these relatively small signals, several trials using excitation currents with high frequencies and amplifier circuits have been conducted [11,12,18,19].…”

Flaw Detection in Aluminum Plates Using a Rotating Uniform Eddy Current Probe with Two Pairs of Excitation Coils

“…The first involves the increase in the intensity of the generated eddy current by modifying the position and shape of the excitation coils. The second is to improve the electromotive force of the detection coil by modifying its position and shape; this improvement in the electromotive force is a function of the interaction zone, the nature of self-differential, and self-nulling [7][8][9][10].…”