EDDY-CURRENT PROBE IMPEDANCE DUE TO A FLAW - THE INFLUENCE OF PHASE-SHIFTED FIELDS

An approximate formula for the change in impedance of an eddy current two-coil probe situated above a conducting medium with a volumetric fl aw is derived in this paper. The primary field is generated by the sup erposition of two phase-shifted alternating currents of the same frequ ency. The Lorentz reciprocity theorem and a layer approximation are us ed to obtain the change in impedance. Numerical results for different values of the phase difference are presented. These results can be use d for developing more selective eddy current testing methods.