Dominique Placko

Ecole normale supérieure de Cachan, Cachon, Île-de-France, France

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Publications (97)39.63 Total impact

  • Dominique Placko · Thierry Bore · Alain Rivollet · Pierre-Yves Joubert
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    ABSTRACT: This paper deals with the problem of imaging defects in metallic structures through eddy current (EC) inspections, and proposes an original process for a possible tomographical crack evaluation. This process is based on a semi analytical modeling, called "distributed point source method" (DPSM) which is used to describe and equate the interactions between the implemented EC probes and the structure under test. Several steps will be successively described, illustrating the feasibility of this new imaging process dedicated to the quantitative evaluation of defects. The basic principles of this imaging process firstly consist in creating a 3D grid by meshing the volume potentially inspected by the sensor. As a result, a given number of elemental volumes (called voxels) are obtained. Secondly, the DPSM modeling is used to compute an image for all occurrences in which only one of the voxels has a different conductivity among all the other ones. The assumption consists to consider that a real defect may be truly represented by a superimposition of elemental voxels: the resulting accuracy will naturally depend on the density of space sampling. On other hand, the excitation device of the EC imager has the capability to be oriented in several directions, and driven by an excitation current at variable frequency. So, the simulation will be performed for several frequencies and directions of the eddy currents induced in the structure, which increases the signal entropy. All these results are merged in a so-called "observation matrix" containing all the probe/structure interaction configurations. This matrix is then used in an inversion scheme in order to perform the evaluation of the defect location and geometry. The modeled EC data provided by the DPSM are compared to the experimental images provided by an eddy current imager (ECI), implemented on aluminum plates containing some buried defects. In order to validate the proposed inversion process, we feed it with computed images of various acquisition configurations. Additive noise was added to the images so that they are more representative of actual EC data. In the case of simple notch type defects, for which the relative conductivity may only take two extreme values (1 or 0), a threshold was introduced on the inverted images, in a post processing step, taking advantage of a priori knowledge of the statistical properties of the restored images. This threshold allowed to enhance the image contrast and has contributed to eliminate both the residual noise and the pixels showing non-realistic values.
    No preview · Article · Jul 2015 · The European Physical Journal Applied Physics
  • T. Bore · D. Placko · P.-Y. Joubert
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    ABSTRACT: 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.
    No preview · Article · Jun 2015
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    ABSTRACT: 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 techni- ues.
    No preview · Conference Paper · Apr 2015
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    ABSTRACT: This paper presents an electromagnetic method of diagnosis based on frequency domain reflectometry (FDR) associated with an inversion algorithm developed by INRIA, ISTL™ (Inverse Scattering for Transmission Lines). ISTL™ allows estimating the spatial profile of the electrical impedance of the line from the FDR measurements. Experimental results on two mockups of external post-tensioned ducts with filling defects show the feasibility of the method. We will try to show the similarities between auscultation external post-tensioned ducts and measurement of water content by TDR probes (Time Domain Reflectometry).
    Full-text · Conference Paper · Mar 2014
  • T. Bore · P. -Y. Joubert · D. Placko
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    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.
    No preview · Conference Paper · Feb 2014
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    Thierry Bore · Pierre-Yves Joubert · Dominique Placko
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    ABSTRACT: This paper deals with an innovative implementation of a semi-analytical modeling method, called the Distributed Points Source Method (DPSM), in the case of an eddy current problem. The DPSM has already shown great potentialities for the versatile and computationally efficient modeling of complex electrostatic, electromagnetic or ultrasonic problems. In this paper, we report a new implementation of the DPSM, called differential DPSM, which shows interesting prospects for the modeling of complex eddy current problems such as met in the non-destructive imaging of metallic parts. In this paper, the used eddy current imaging device is firstly presented. It is composed of an eddy current (EC) inducer and a magneto optical set-up used to translate the magnetic field distribution appearing at the surface of the imaged part, into a recordable optical image. In this study, the device is implemented for the time-harmonics (900 Hz) imaging of a two-layer aluminum based assembly, featuring surface-breaking and buried defects. Then, the basics of the time-harmonics DPSM modeling are recalled, and the differential approach is presented. It is implemented for the modeling of the interactions of the eddy current imaging device with the considered flawed assembly in the same operating conditions as the experimental implementation. The comparison between experimental and computed data obtained for millimetric surface and buried defects is presented in the form of complex magnetic cartographies and Lissajous plots. The obtained results show good agreement and open the way to the modeling of complex EC problems. Furthermore, the low computational complexity of the differential DPSM modelings makes it promising to consider for the solving of EC inverse problems.
    Preview · Article · Jan 2014 · Progress In Electromagnetics Research
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    ABSTRACT: The use of a capacitive probe as a non-destructive investigative technique for controlling the post tensioned ducts of bridges has gained increasing acceptance in France since several years. A field campaign measurement made in 2006 has shown the performance of the capacitive probe developed by the IFSTTAR. Nevertheless, some results are not understood, and in particular the behaviour of the probe when cement exudation product are present in the duct. Without an accurate knowledge of the electromagnetic properties of those products it is impossible to quantitatively assess the results of our capacitive probe. This paper reports the development of a coaxial transmission line feature. It was designed to allow the evaluation of a large type of material (liquid, paste or granular) over a large frequency range (50 MHz–4 GHz at maximum). A calibration scheme developed before at the Fresnel Institute was used. Using a two port S parameter instrument, the complex permittivity and magnetic permeability were evaluated by frequency domain measurement. The electromagnetic characterization of cement paste, cement exudation products and injection wax has brought us some key results in the interpretation of the capacitive probe signal. The results of this characterization were then used in a 3D semi analytical modelling of the problem. The studies of configurations with exudation products are presented and compared to experimental results obtained with our capacitive probe on laboratory duct.
    Full-text · Article · Dec 2013 · NDT & E International
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    ABSTRACT: The water content in concrete (nuclear structures and nuclear waste repositories) is a major topic to understand and predict the behaviour at the end of the operating period. That is the reason why ANDRA and EDF are involved in research programs dedicated to concrete Thermo-Hydro-Mechanical (THM) modeling and to in situ water content assessment technologies. Another example concerns the bridges which include “external” post-tensioned cables to reinforce these structures. These cables are not into the concrete material, hence potentially accessible for measurement. They are generally placed in High Density Poly-Ethylene (HDPE) ducts, where the residual space is filled under high pressure with a cement grout intended to prevent corrosion. Nevertheless, in some cases, the cables breaking occur in non-protected zones because of the presence of a “white paste” or grout voids. To remote diagnosis anywhere and in real time of post tensioned ducts or to measure the water content in concrete, we propose a structural health monitoring method based on Frequency Domain Reflectometry (FDR). Today’s, advanced reflectometry methods provide an efficient solution for the fault-detection and for their diagnosis in electric transmission lines. This paper presents a direct model of the FDR method based on telegrapher’s equations. An analysis of these signals, based on scattering theory, enables one to retrieve the impedance distribution of the electric line. The impedance distribution depends on damage into the duct or the water content in concrete. An inversion algorithm is realized with software 'ISTL' provided by INRIA. FDR method has been applied to two real cases: measurement of the water content in concrete and the diagnostic of the external post tensioned duct.
    Full-text · Article · Oct 2013
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    ABSTRACT: The use of electromagnetic sensors such as Time Domain Reflectometry (TDR) probes has gained increasing importance for water content measurements since several years, for long term monitoring of structures, among which radioactive waste repositories. TDR probes are basically sensitive to electromagnetic properties of the host material, clayrock in our case Prior to perform in-situ experiments with TDR probes, it is mandatory to have an accurate knowledge of the electromagnetic properties of clayrock as a function of their water content. We developed a new laboratory dielectric measurement device, consisting of a coaxial transmission line, enabling characterization of intact clayrock permittivity over the 50 MHz - 1 GHz frequency range. The study has shown a large variation of complex permittivity with (i) water content, the parameter of interest and (ii) frequency. The frequency dependence is induced by different relaxation processes. In a second step, these data are introduced in an original semi analytical model (Distributed Points Sources Method) in order to obtain a reliable modeling of the TDR probe. Taking into account some experimental aspects of the TDR probe, we propose to introduce a in this paper the effect of an air gap between the TDR antennas and the surrounding media. The effect of this influent parameter is evaluated owing to our DPSM modeling, and some solutions are proposed to overcome the problem.
    No preview · Conference Paper · Apr 2013
  • D. Placko · T. Bore · A. Rivollet
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    ABSTRACT: 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…).
    No preview · Conference Paper · Apr 2013
  • Thierry Bore · Dominique Placko · Frederic Taillade · Marc Himbert
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    ABSTRACT: This paper presents a newly developed system for bridges post-tensioned devices inspection. These devices are generally composed of cables placed inside a duct, the residual space being filled with a particular cement to avoid corrosion. The nondestructive experimental setup uses a capacitive sensor that gives some relevant information about the electrical properties of the materials located inside the duct. Then, using an original modeling of the interactions between the sensor and the post-tensioned device, a signal processing extracts useful information from the sensors data, like the thickness of an eventual air gap between the cement and the duct. Experimental and theoretical derivations and comparisons are presented in the different parts of this paper.
    No preview · Article · Feb 2013 · IEEE Sensors Journal

  • No preview · Article · Jan 2013
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    ABSTRACT: This paper considers the images provided by a photothermal camera (flying-spot camera) dedicated to open-crack detection. In this type of active thermography, both thermal and optical effects contribute to the elaboration of photothermal images. Here the thermal effect is relative to the presence of open-cracks and the optical effects are due to surface conditions. In the case of open-cracks detection, the optical effects induce high magnitude perturbation signals, possibly masking the presence of open-cracks. In this contribution a signal processing method is proposed in order to identify both thermal and optical effects separately. The method lies uses multiple principal component analysis combined with a continuous wavelet transform. It is used to enhance the open-crack detection for the inspection of an industrial mock-up showing open-cracks and various surface conditions. The enhancement of the detection performance is characterized thanks to Receiver Operating Characteristic curves. The proposed method shows high detection performances and could be extended to a classification scheme.
    No preview · Article · Apr 2012 · Quantitative InfraRed Thermography Journal
  • Alain Rivollet · Dominique Placko · Tribikram Kundu
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    ABSTRACT: This paper discusses the reasons why we need to change our habit of developing quick software to validate our theoretical results. It is explained here how to create computer programs, to use high level functions and to have a team's common strategy. To achieve this goal, an innovative concept of software development is presented, called AIS (Application Interface Software). This concept is illustrated by developing DPSM (Distributed Point Source Method) programs which is used for 3D ultrasonic field modeling.
    No preview · Article · Mar 2011 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: Modeling ultrasonic fields in front of a transducer in the presence and absence of a scatterer is a fundamental problem that has been attempted by different techniques: analytical, semi-analytical, and numerical. However, a comprehensive comparison study among these techniques is currently missing in the literature. The objective of this paper is to make this comparison for different ultrasonic field modeling problems with various degrees of difficulty. Four fundamental problems are considered: a flat circular transducer, a flat square transducer, a circular concave transducer, and a point focused transducer (concave lens) in the presence of a cavity. The ultrasonic field in front of a finite-sized transducer can be obtained by Huygens-Fresnel superposition principle that integrates the contributions of several point sources distributed on the transducer face. This integral which is also known as the Rayleigh integral or Rayleigh-Sommerfeld integral (RSI) can be evaluated analytically for obtaining the pressure field variation along the central axis of the transducer for simple geometries, such as a flat circular transducer. The semi-analytical solution is a newly developed mesh-free technique called the distributed point source method (DPSM). The numerical solution is obtained from finite element analysis. Note that the first three problems study the effect of the transducer size and shape, whereas the fourth problem computes the field in presence of a scatterer.
    Full-text · Article · Dec 2010 · IEEE transactions on ultrasonics, ferroelectrics, and frequency control
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    ABSTRACT: Interaction between a cavity or void in a liquid and a converging ultrasonic beam generated by a point-focused acoustic lens is investigated. A semi-analytical technique called the distributed point source method (DPSM) is adopted because no analytical solution is available for this problem involving cavities of different size and the finite element method is not very efficient for modeling high-frequency ultrasonic problems. The solution shows that if the cavity is placed very close to the focal point of the lens then it can be detected by the acoustic lens. The detectability of the cavity at the off-focus position depends on the distance of the cavity from the focal point. The variation of this distance as the cavity moves in horizontal and vertical directions from the focal point is also investigated.
    Full-text · Article · Jun 2010 · IEEE transactions on ultrasonics, ferroelectrics, and frequency control
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    ABSTRACT: In spite of many advances in analytical and numerical modeling techniques for solving different engineering problems, an efficient solution technique for wave propagation modeling of an electromagnetic acoustic transducer (EMAT) system is still missing. Distributed point source method (DPSM) is a newly developed semi-analytical technique developed since 2000 by Placko and Kundu (2007) [12] that is very powerful and straightforward for solving various engineering problems, including acoustic and electromagnetic modeling problems. In this study DPSM has been employed to model the Lorentz type EMAT with a meander line and flat spiral type coil. The problem of wave propagation has been solved and eddy currents and Lorentz forces have been calculated. The displacement field has been obtained as well. While modeling the Lorentz force the effect of dynamic magnetic field has been considered that most current analyses ignore. Results from this analysis have been compared with the finite element method (FEM) based predictions. It should be noted that with the current state of knowledge this problem can be solved only by FEM.
    Full-text · Article · May 2010 · Ultrasonics
  • Vincent Thomas · P.-Y. Joubert · Eric Vourc'h · Dominique Placko
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    ABSTRACT: A simplified electromagnetic modeling is proposed for the eddy current imaging of surface breaking defects. The model assumes that the interactions between the eddy currents and a defect are equivalent to current sources placed inside the defect. The use of the distributed point source method is thus suitable for implementing the model. An example is provided considering the case of aeronautical fastener holes. The results given by the proposed model are confronted with experimental data obtained with an eddy currents imaging probe as well as with finite elements modeling simulations and good agreement is shown.
    No preview · Conference Paper · Mar 2010
  • Thierry Bore · Frédéric Taillade · Dominique Placko · Marc Himbert
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    ABSTRACT: 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. Bibtex entry for this abstract Preferred format for this abstract (see Preferences) Find Similar Abstracts: Use: Authors Title Keywords (in text query field) Abstract Text Return: Query Results Return items starting with number Query Form Database: Astronomy Physics arXiv e-prints
    No preview · Article · Feb 2010
  • Rais Ahmad · Tribikram Kundu · Dominique Placko

    No preview · Chapter · Jan 2010