Prediction of eddy current probe signal trajectories

Development of computer based defect characterization schemes in quantitative eddy current nondestructive testing (NDT) requires an adequate mathematical model capable of describing the complicated interactions between the impressed and induced currents, primary and secondary fields and the flaws in materials. This paper describes a finite element model which predicts the apparent changes in the normalized complex impedance of a differential eddy current probe in axisymmetric NDT configurations. Using this 'numerical model' differential eddy current probe, signal trajectories (as obtained on an eddy-scope screen in actual testing) are predicted for three axisymmetric geometries associated with nuclear power plant steam generators - circumferential I.D. and O.D. slots in an INCONEL tube, and a carbon steel support plate encircling a defect-free tube. Comparison is made between the theoretical and experimental results at a probe excitation frequency of 100kHz. Finite element analysis techniques appear to be well suited to the modeling of eddy current NDT phenomena and for predicting the eddy current probe signal trajectories.