Thierry Bore

Semi-analytical modeling of an eddy current imaging system for the characterization of defects in metallic structures

In this paper, the authors report on the implementation of a simplified and computationally efficient electromagnetic modeling of the interactions that take place between an eddy current imaging device and a metallic assembly, in the context of the non destructive evaluation of defects in metallic parts. The model is implemented in the case of the evaluation of a millimetric defect buried in a three plate aluminum assembly, for eddy current frequencies ranging from 300Hz to 3kHz. The model is validated against experimental data in terms of two-dimensional distributions of the computed magnetic field, as well as in terms of modulus and phase of the magnetic extrema observed in the images, in the whole frequency range. The obtained result provides promising prospects for the use of such model for the automatic characterization of defects in the context of the eddy current imaging of metallic parts.

CAPACITIVE PROBE FOR NON DESTRUCTIVE INSPECTION OF EXTERNAL POST‐TENSIONED DUCTS: MODELLING BY DPSM TECHNIQUE

This paper presents both theoretical and experimental aspects of a measurement problem in the field of non‐destructive evaluation. The purpose of our work is to develop a capacitive probe devoted to post‐tensioned cable inspection, mainly for bridge monitoring applications. In this paper will successively be presented the industrial problem, the current probe in use, and an original modeling achieved for sensor design and signal processing. In a first step, we will compare the experimental and theoretical data obtained with different ’devices under test’ configurations. Then we will discuss about both capabilities of our direct model, on one hand to provide some help in sensor design improvements and on other hand to be inserted in an inverse problem scheme, to get an estimate of some interesting data of our problem from the measured signals.

3D-FEM modeling of F/TDR sensors for clay-rock water content measurement in combination with broadband dielectric spectroscopy

The use of electromagnetic sensors such as Time Domain Reflectometry (TDR) probes has gained increasing importance for long term monitoring of the water content in radioactive waste repositories. TDR probes are sensitive to changes in electromagnetic properties of the surrounding material, a clay rock in our case. Prior to the in situ application, it is mandatory to have an accurate relationship between the electromagnetic properties of the intact host clay rock and the water content. For this purpose, the dielectric properties of intact clay rock samples were systematically studied at frequencies from 1 MHz to 10 GHz with network analyzer technique in combination with coaxial transmission line cells. Samples were conditioned to achieve a water saturation range from 16 % to nearly saturation. The relaxation behavior was quantified based on a generalized fractional relaxation model under consideration of an apparent direct current conductivity assuming three relaxation processes: a high-frequency water process and two interfacial processes which are related to interactions between the aqueous pore solution and mineral particles (adsorbed/hydrated water relaxation, counter ion relaxation and Maxwell-Wagner effects). In a second step, these data are introduced in 3-D numerical frequency domain finite element field calculations to model the one port broadband frequency or time domain transfer function for a three rode based TDR-probe embedded in the clay rock. The results are analyzed with classical travel time analysis (onset/inflection) which under/overestimates the value of the permittivity compared to effective permittivity at 1 GHz. Indeed, apparent permittivity contains not only the water-content contribution but also effects due to water-mineral interaction processes. The results demonstrate the capabilities of a combined TD/FD analysis procedure for the monitoring of physical and chemical properties of materials with high frequency electromagnetic sensor techniques.

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

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.

Imaging and detection of cracks in metallic structures with eddy current sensors

In this paper we propose an original analytical modeling applied to electromagnetic (EM) systems, in the aim of performing an inversion scheme. The purpose consists in imaging cracks through eddy current sensors, the image being constituted by an estimation of the conductivity of the piece of metal under test, voxel by voxel. There are plenty of industrial applications in Non Destructive Evaluations for monitoring of cracks in structures: Aeronautics, Metallurgy, ships building and so on. For modeling, a relatively new developed technique, called ‘Distributed Points Source Method’ (DPSM) has been used. This original modeling was invented at Ecole Normale Superieure of Cachan in 2000, and since developed and patented in many kind of applications (Ultrasonics, Electrostatics…).

Surface crack reconstruction from eddy current images using a direct semi-analytic model

A method relying on a direct semi-analytic model is proposed for reconstructing cracks from eddy current images. We consider systems featuring a uniform excitation flow which can be modelled by means of fictitious current sources distributed in the crack volume. Thanks to the relative simplicity of the model a reconstruction method based the comparison of EC images is considered. The sensitivity of different images comparison criteria is studied for 2D surface cracks, leading to a reconstruction method based on a genetic algorithm. Numerical experiments are carried out to examine the performances of the algorithm in terms of convergence and accuracy.