D. Placko

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

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Publications (80)28.71 Total impact

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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).
    Congrès Diagnobéton 2014; 03/2014
  • 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.
    02/2014;
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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.
    NDT & E International 12/2013; 60:110–120. · 1.74 Impact Factor
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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.
    SPIE Smart Structures and Materials + Nondestructive Evaluation and Health Monitoring; 04/2013
  • 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…).
    SPIE Smart Structures and Materials + Nondestructive Evaluation and Health Monitoring; 04/2013
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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.
    IEEE Sensors Journal 01/2013; 13(2):457-465. · 1.48 Impact Factor
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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.
    Quantitative InfraRed Thermography Journal 04/2012; June 2006(Vol. 3):53-70. · 0.34 Impact Factor
  • 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.
    Proc SPIE 03/2011;
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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.
    IEEE transactions on ultrasonics, ferroelectrics, and frequency control 12/2010; 57(12):2795-807. · 1.80 Impact Factor
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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.
    IEEE transactions on ultrasonics, ferroelectrics, and frequency control 06/2010; 57(6):1396-404. · 1.80 Impact Factor
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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.
    Ultrasonics 05/2010; 50(6):583-91. · 2.03 Impact Factor
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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.
    Sensors Applications Symposium (SAS), 2010 IEEE; 03/2010
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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
    02/2010;
  • Rais Ahmad, Tribikram Kundu, Dominique Placko
    Advanced Ultrasonic Methods for Material and Structure Inspection, 01/2010: pages 159 - 187; , ISBN: 9780470612248
  • Tamaki Yanagita, Tribikram Kundu, Dominique Placko
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    ABSTRACT: Several investigators have modeled ultrasonic fields in front of transducers by Huygens-Fresnel superposition principle that integrates the contributions of a number of point sources distributed on the transducer face. This integral solution, also known as the Rayleigh integral or Rayleigh-Sommerfeld Integral solution, assumes the strengths of the point sources distributed over the transducer face. A newly developed technique called distributed point source method (DPSM) offers an alternative approach for modeling ultrasonic fields. DPSM is capable of modeling the field for prescribed source strength distribution as well as for prescribed interface conditions with unknown source strengths. It is investigated how the ultrasonic field in front of the transducer varies in different situations: (1) when the point source strengths are known, (2) when the point source strengths are unknown but obtained from the interface condition that only the normal component of the transducer velocity is continuous across the fluid-solid interface, (3) when all three components of velocity are assumed to be continuous across the interface for the no-slip condition, and (4) when the pressure instead of the velocity is prescribed on the transducer face. Results for these different interface conditions are compared with the analytical solutions along the central axis.
    The Journal of the Acoustical Society of America 11/2009; 126(5):2331-9. · 1.65 Impact Factor
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    ABSTRACT: Numerical modelling of the ultrasonic wave propagation is important for Structural Heath Monitoring and System Prognosis problems. In order to develop intelligent and adaptive structures with embedded damage detector and classifier mechanisms, detailed understanding of scattered wave fields due to anomaly in the structure is inevitably required. A detailed understanding of the problem demands a good modelling of the wave propagation in the problem geometry in virtual form. Therefore, efficient analytical, semi-analytical or numerical modelling techniques are required. In recent years a semi-analytical mesh-free technique called Distributed Point Source Method (DPSM) is being used for modelling various ultrasonic, electrostatic and electromagnetic wave field problems. In the conventional DPSM approach point sources are placed along the transducer faces, problem boundaries and interfaces to model incident and scattered fields. Every point source emits energy in all directions uniformly. Source strengths of these 360 degrees radiation sources are obtained by satisfying interface and boundary conditions of the problem. In conventional DPSM modelling approach it is assumed that the shadow zone does not require any special consideration. 360 degrees Radiation point sources should be capable of properly modelling shadow zones because all boundary and interface conditions are satisfied. In this paper it is investigated how good this assumption is by introducing the 'shadow zone' concept at the point source level and comparing the results generated by the conventional DPSM and by this modified approach where the conventional 360 degrees radiation point sources are replaced by the Controlled Space Radiation (CSR) sources.
    Ultrasonics 04/2009; 49(8):615-22. · 2.03 Impact Factor
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    ABSTRACT: The authors present an original eddy current imager (ECI) designed for the fast and accurate non-destructive evaluation of defects buried next to rivets in aeronautical lap-joints. The ECI is associated to a signal processing method based on a principal component analysis (PCA) followed by a maximum likelihood (ML) approach. The PCA was implemented using EC images obtained with selected excitation frequencies. These images are considered as resulting from a linear mixing of different sources including the presence of rivets and defects, and the PCA is used to separate these sources thanks to an eigen decomposition of the EC data covariance matrix. As a result, the defect signatures are enhanced and used to implement an automatic defect characterization. This characterization is carried out by the means of an ML approach which allows the length and depth of the defects to be estimated. The method was implemented for the evaluation of a laboratory made riveted lap joint mock-up featuring buried defects. It was experimentally optimized and successfully implemented for the characterization of calibrated defects ranging from 2 to 10 mm in length and 2 to 8 mm in depth.
    NDT & E International. 03/2009;
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    ABSTRACT: The ultrasonic field generated by a Micro Intereferometric Acoustic Lens used for high precision Rayleigh wave velocity measurements is modeled by the recently developed mesh-free technique called Distributed Point Source Method (DPSM). The field generated by the three individual ultrasonic transducer elements forming the micro intereferometric acoustic lens are computed and compared with experimental measurements. Qualitative agreement between the theoretical and experimental results is observed; both results show converging beams up to the focal point and then the beams diverge. However, some of the minute detailed features in the generated ultrasonic field could only be observed in the computed results. Effects of non-uniform surface of the transducer and its contribution to the non-uniform ultrasonic source strength are investigated to understand and optimize the acoustic lens for localized quantitative elastic property measurements.
    Proc SPIE 03/2009;
  • 7th International Workshop on Structural Health Monitoring, Stanford, CA, USA; 01/2009
  • Tamaki Yanagita, Dominique Placko, Tribikram Kundu
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    ABSTRACT: Several investigators have modeled ultrasonic fields in front of finite sized transducers. Most of these models are based on Huygens principle. Following Huygens-Fresnel superposition principle one can assume that the total field of a finite size transducer is obtained by simply superimposing the contributions of a number of point sources uniformly distributed on the transducer face. If the point source solution, also known as the Green's function, is known then integrating that point source solution over the transducer face one can obtain the total ultrasonic field generated by a finite transducer. This integral is known as Rayleigh-Sommerfield integral. It is investigated here how the ultrasonic field in front of the transducer varies for different interface conditions at the transducer face-fluid interface such as 1) when only the normal component of the transducer velocity is assumed to be uniform on the transducer face and continuous across the fluid-solid interface, or 2) when all three components of velocity are assumed to be uniform on the transducer face and continuous across the interface, 3) when the pressure instead of velocity is assumed to be uniform on the transducer face and continuous across the interface. All these different boundary and interface conditions can be modeled by the newly developed Distributed Point Source Method (DPSM). These results are compared with the Rayleigh-Sommerfield integral representation that gives the fluid pressure in front of the transducer when the transducer-fluid interface is subjected to uniform normal velocity.
    Proc SPIE 05/2008;