Studying the Signals Produced by an Eddy Current Array Probe

Abstract

The signal produced by an eddy current array probe is analyzed. Recommendations for the probe geometric parameters and the gap between the probe and the examined object’s surface are presented. The effect of electrical conductivity and magnetic permeability is shown, and the possibility to estimate each of them individually is demonstrated. It is shown that the effect caused by corrosion is equivalent to the cumulative effects from the object’s magnetic and electrical parameters. To select the probe parameters, the effect the distance between the coils has on the signal induced by the excitation coil to the adjacent coil was investigated. The excitation current can be applied to each of the probe coils alternately, and the magnetic field generated by the excitation coil induces a signal in the adjacent coils. This signal determines the noise level. Good prospects of using transformer-type array probes, which are quite competitive with other types of probes, are demonstrated. A numerical method for calculating the test modes and for evaluating the signals corresponding to the examined and interfering parameters is selected. Practical recommendations on selecting the testing modes are given. The signals generated by the excitation coil in the adjacent measuring coil located close to exciting one the arrangement in which they influence each other to the highest extent---are investigated. The study was carried out for the probe without the tested specimen, and when the probe is placed above a flat ferromagnetic object and above non-magnetic objects with high (aluminum) and low (titanium) electrical conductivity. A comparison of the obtained loci has shown the possibility of separately determining the electrical and magnetic properties of the examined objects. The signals produced by eddy current probes for crack-like flaws at their different locations have been determined. It is noted that the signals generated in response to cracks having different positions differ significantly from each other in phase. This feature makes it possible to estimate both the position of cracks and their depth.