Distributed point source method for the modeling of a three-dimensional eddy current NDE problem

Abstract

This paper deals with modeling in electromagnetism in the field of eddy current for Non Destructive Evaluation. Several techniques could be used to diagnose structural damages. In eddy current application, a magnetic field generates by an excitation coil (or primary coil), interacts with a conductive target and generates eddy current. Variations in the phase and the magnitude of these eddy currents can be monitored using a second “receiver” coil. Variations in the physical properties (electrical conductivity, magnetic permeability,..) or the presence of any flaw in the target will cause a change in eddy current and a corresponding change in the phase and amplitude of measured signal. The interpretation of the signals requires a good understanding of the interaction between eddy current and structure. Therefore, researchers need analytical or numerical techniques to obtain a clear understanding of wave propagation behaviors. However, modeling of wave scattering phenomenon by conventional numerical techniques such as finite elements requires very fine mesh and heavy computational power. To go further, an innovative implementation of a semi-analytical modeling method, called the Distributed Points Source Method (DPSM), has been developed and used. The DPSM has already shown great potentialities for the versatile and computationally efficient modeling of complex electrostatic, electromagnetic or ultrasounic problems. In this paper, we report on a new implementation of the DPSM, called differential DPSM, which shows interesting prospects for the modeling of complex eddy current problems. In parallel, an Eddy Current Imager (ECI) has been recently developed in our laboratory in the aim of imaging cracks in metallic structures. In this paper, a simplified modeling of the ECI is presented using DPSM technique, the basics of DPSM formalism being firstly developed. A comparison between experimental and computed data obtained for a millimetric surface defect is presented in the form of complex magnetic cartographies. The obtained results show good agreement. Then, imaging in the case of a buried object in a metallic target is discussed. The effect of 2 parameters (the conductivity and the depth of the buried object) on the magnetic field which is computed at the surface of the material through our DPSM modeling is presented. The objective is to predict the sensor behavior for different values of these parameters, and to plot some arrays of curves, which can be used as calibration curves for the sensor’s user.