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Papers by Theodoros Theodoulidis
IEEE Transactions on Magnetics, 2015
The solution to the eddy-current interaction problem between a probe coil and a cylindrical boreh... more The solution to the eddy-current interaction problem between a probe coil and a cylindrical borehole in a planar conducting sheet is solved using the truncated region eigenfunction expansion (TREE) approach and decomposition into two subproblems of different parities: odd and even. The impedance variation of the probe is calculated and the theoretical results are compared with numerical ones obtained by the finite elements method as well as with measurements.
IEEE Transactions on Magnetics, 2015
ABSTRACT A semi-analytical solution to the eddy-current induction problem in a conducting, non-ma... more ABSTRACT A semi-analytical solution to the eddy-current induction problem in a conducting, non-magnetic plate near a cylindrical through-hole is derived by means of the second order vector potential and the domain truncation technique. The construction of a solution, which satisfies simultaneously the field continuity across the cylindrical surface of the hole and the planar interfaces of the plate proves to be a very challenging problem, and thus a special superposition of partial sub-solutions is proposed. The presented solution is part of a more general development aiming at the calculation of the probe interaction with material flaws located in the vicinity of the hole by means of the integral equation formalism. These tasks will be the subject of a further article. The comparison of the semi-analytical results against both finite elements simulations as well as experimental data demonstrates an excellent agreement.
We present numerical results for the World Federation of NDE Centers (WFNDEC) 2012 eddy current b... more We present numerical results for the World Federation of NDE Centers (WFNDEC) 2012 eddy current benchmark problem obtained with a commercial FEM package (Comsol Multiphysics). The measurements of the benchmark problem consist of coil impedance values acquired when an inspection probe coil is moved inside an Inconel tube along an axial through-wall notch. The simulation runs smoothly with minimal user interference (default settings used for mesh and solver) and agreement between numerical and experimental results is excellent for all five inspection frequencies. Comments are made for the pros and cons of FEM and also some good practice rules are presented when using such numerical tools.
IEEE Transactions on Magnetics, 2015
We report on further measurements and a numerical solution for a proposed benchmark problem invol... more We report on further measurements and a numerical solution for a proposed benchmark problem involving an eddy current nondestructive evaluation configuration. This is a simplified version of the inspection of fastener holes in aircraft structures, comprising combinations of plates with through holes and a crack. Previously only line (B-scan) data were provided for the coil impedance while in this follow-up work we provide precision surface (C-scan) data as well as details on the evaluation of the critical parameters of the problem. Furthermore, the experimental data are cross-validated with numerical ones by using a model that combines an integral/FEM approach.
ABSTRACT Le contrôle non destructif (CND) par Courants de Foucault (CF) de défaut de fissuration ... more ABSTRACT Le contrôle non destructif (CND) par Courants de Foucault (CF) de défaut de fissuration est d'un grand intérêt dans de nombreux secteurs industriels. L'utilisation d'outils de modélisation adaptés permet d'améliorer les procédés de contrôle et la compréhension des données expérimentales observées. Les développements présentés dans cette communication concernent l'introduction d'une nouvelle approche de modélisation des défauts fins dans un milieu plan stratifié. Ce travail a été réalisé au CEA LIST en collaboration avec l'Université de Western Macedonia et le Laboratoire des Signaux et Sytèmes. Du point de vue de la simulation, la complexité du problème à traiter est liée aux particularités des défauts de fissuration : une ouverture très fine, un profil complexe et la possibilité d'avoir des ponts de conductivité entre les deux faces latérales du défaut. Ces caractéristiques expliquent la difficulté qu'ont les méthodes de simulation classiques, semi-analytiques ou purement numériques, à traiter efficacement ce type de configuration. Pour ces raisons, une approche dédiée aux défauts fins, fondée sur la méthode des éléments de frontière (BEM), a été développée. Elle présente l'avantage majeur de ne requérir qu'une discrétisation surfacique du défaut, en traitant analytiquement le calcul dans la direction de son ouverture. Après la résolution, avec la Méthode des Moments, de l'équation intégrale décrivant les interactions entre le champ d'excitation et le défaut, la réponse de la sonde est calculée en appliquant le théorème de réciprocité. L'approche théorique, permet la simulation du CND par CF d'une pièce plane stratifiée, contenant un nombre quelconque de défauts de faible ouverture parallèles ou perpendiculaires. Par ailleurs, de récents développements ont permis de traiter le cas des défauts fin de profil complexe, ainsi que celui des défauts présentant des ponts de matière. Des comparaisons de résultats avec des données issues de la littérature et des données simulées sont ensuite détaillées et les perspectives ouvertes par ce travail sont discutées.
This benchmark proposes the investigation of Eddy Current Testing (ECT) signals obtained during i... more This benchmark proposes the investigation of Eddy Current Testing (ECT) signals obtained during inspection of planar structures affected by hole(s) and crack, and during inspection of a riveted multilayered structure. Laboratory controlled measurements of eddy current coil impedance have been carried out during inspection of a multilayered planar structure affected by hole(s) and a crack.
In both aircraft bolt-hole and steam generator tube inspections, eddy current probes are utilized... more In both aircraft bolt-hole and steam generator tube inspections, eddy current probes are utilized to scan the inner surface of the cylindrical structure for defects. A combination of integral methods and analytical approaches provide an effective means of simulating such inspections thereby quantifying their performance and providing a means of characterizing flaws. For integral methods to be successful, a knowledge of the relevant integral kernel is required but there is also a need for fast computations of the electromagnetic field. In this paper we show that by truncating the solution domain, traditional incident field calculations can be accelerated and extended to new configurations. Both the field and probe signal are expressed in the form of a double series instead of an integral and a series. The double series form provides an efficient means of evaluating the field in a finite hole or tube using analytical expressions. As an example, double-series impedance calculations for a coil whose axis is offset and parallel to that of the hole are shown to give good agreement with experiment.
A fast simulation tool of Eddy Current Testing (ECT) inspections, based on a Boundary Element Met... more A fast simulation tool of Eddy Current Testing (ECT) inspections, based on a Boundary Element Method (BEM), is proposed in this paper. Simulated signals consist of impedance variation of a coil due to multiple narrow cracks located in conductive planar multilayered structures (MLS). The cracks can have various shapes or orientations and can lie in any layer of the structure inspected. After a presentation of the method, validations results, consisting of comparison of simulated signals with experimental data and results obtained with finite element calculations, are presented. In all the problems tested, a very good accuracy and a very low computation time have been observed. Further perspective and conclusions of this work are detailed at the end of the paper.
IEEE Transactions on Magnetics, 2015
The solution to the eddy-current interaction problem between a probe coil and a cylindrical boreh... more The solution to the eddy-current interaction problem between a probe coil and a cylindrical borehole in a planar conducting sheet is solved using the truncated region eigenfunction expansion (TREE) approach and decomposition into two subproblems of different parities: odd and even. The impedance variation of the probe is calculated and the theoretical results are compared with numerical ones obtained by the finite elements method as well as with measurements.
IEEE Transactions on Magnetics, 2015
ABSTRACT A semi-analytical solution to the eddy-current induction problem in a conducting, non-ma... more ABSTRACT A semi-analytical solution to the eddy-current induction problem in a conducting, non-magnetic plate near a cylindrical through-hole is derived by means of the second order vector potential and the domain truncation technique. The construction of a solution, which satisfies simultaneously the field continuity across the cylindrical surface of the hole and the planar interfaces of the plate proves to be a very challenging problem, and thus a special superposition of partial sub-solutions is proposed. The presented solution is part of a more general development aiming at the calculation of the probe interaction with material flaws located in the vicinity of the hole by means of the integral equation formalism. These tasks will be the subject of a further article. The comparison of the semi-analytical results against both finite elements simulations as well as experimental data demonstrates an excellent agreement.
We present numerical results for the World Federation of NDE Centers (WFNDEC) 2012 eddy current b... more We present numerical results for the World Federation of NDE Centers (WFNDEC) 2012 eddy current benchmark problem obtained with a commercial FEM package (Comsol Multiphysics). The measurements of the benchmark problem consist of coil impedance values acquired when an inspection probe coil is moved inside an Inconel tube along an axial through-wall notch. The simulation runs smoothly with minimal user interference (default settings used for mesh and solver) and agreement between numerical and experimental results is excellent for all five inspection frequencies. Comments are made for the pros and cons of FEM and also some good practice rules are presented when using such numerical tools.
IEEE Transactions on Magnetics, 2015
We report on further measurements and a numerical solution for a proposed benchmark problem invol... more We report on further measurements and a numerical solution for a proposed benchmark problem involving an eddy current nondestructive evaluation configuration. This is a simplified version of the inspection of fastener holes in aircraft structures, comprising combinations of plates with through holes and a crack. Previously only line (B-scan) data were provided for the coil impedance while in this follow-up work we provide precision surface (C-scan) data as well as details on the evaluation of the critical parameters of the problem. Furthermore, the experimental data are cross-validated with numerical ones by using a model that combines an integral/FEM approach.
ABSTRACT Le contrôle non destructif (CND) par Courants de Foucault (CF) de défaut de fissuration ... more ABSTRACT Le contrôle non destructif (CND) par Courants de Foucault (CF) de défaut de fissuration est d'un grand intérêt dans de nombreux secteurs industriels. L'utilisation d'outils de modélisation adaptés permet d'améliorer les procédés de contrôle et la compréhension des données expérimentales observées. Les développements présentés dans cette communication concernent l'introduction d'une nouvelle approche de modélisation des défauts fins dans un milieu plan stratifié. Ce travail a été réalisé au CEA LIST en collaboration avec l'Université de Western Macedonia et le Laboratoire des Signaux et Sytèmes. Du point de vue de la simulation, la complexité du problème à traiter est liée aux particularités des défauts de fissuration : une ouverture très fine, un profil complexe et la possibilité d'avoir des ponts de conductivité entre les deux faces latérales du défaut. Ces caractéristiques expliquent la difficulté qu'ont les méthodes de simulation classiques, semi-analytiques ou purement numériques, à traiter efficacement ce type de configuration. Pour ces raisons, une approche dédiée aux défauts fins, fondée sur la méthode des éléments de frontière (BEM), a été développée. Elle présente l'avantage majeur de ne requérir qu'une discrétisation surfacique du défaut, en traitant analytiquement le calcul dans la direction de son ouverture. Après la résolution, avec la Méthode des Moments, de l'équation intégrale décrivant les interactions entre le champ d'excitation et le défaut, la réponse de la sonde est calculée en appliquant le théorème de réciprocité. L'approche théorique, permet la simulation du CND par CF d'une pièce plane stratifiée, contenant un nombre quelconque de défauts de faible ouverture parallèles ou perpendiculaires. Par ailleurs, de récents développements ont permis de traiter le cas des défauts fin de profil complexe, ainsi que celui des défauts présentant des ponts de matière. Des comparaisons de résultats avec des données issues de la littérature et des données simulées sont ensuite détaillées et les perspectives ouvertes par ce travail sont discutées.
This benchmark proposes the investigation of Eddy Current Testing (ECT) signals obtained during i... more This benchmark proposes the investigation of Eddy Current Testing (ECT) signals obtained during inspection of planar structures affected by hole(s) and crack, and during inspection of a riveted multilayered structure. Laboratory controlled measurements of eddy current coil impedance have been carried out during inspection of a multilayered planar structure affected by hole(s) and a crack.
In both aircraft bolt-hole and steam generator tube inspections, eddy current probes are utilized... more In both aircraft bolt-hole and steam generator tube inspections, eddy current probes are utilized to scan the inner surface of the cylindrical structure for defects. A combination of integral methods and analytical approaches provide an effective means of simulating such inspections thereby quantifying their performance and providing a means of characterizing flaws. For integral methods to be successful, a knowledge of the relevant integral kernel is required but there is also a need for fast computations of the electromagnetic field. In this paper we show that by truncating the solution domain, traditional incident field calculations can be accelerated and extended to new configurations. Both the field and probe signal are expressed in the form of a double series instead of an integral and a series. The double series form provides an efficient means of evaluating the field in a finite hole or tube using analytical expressions. As an example, double-series impedance calculations for a coil whose axis is offset and parallel to that of the hole are shown to give good agreement with experiment.
A fast simulation tool of Eddy Current Testing (ECT) inspections, based on a Boundary Element Met... more A fast simulation tool of Eddy Current Testing (ECT) inspections, based on a Boundary Element Method (BEM), is proposed in this paper. Simulated signals consist of impedance variation of a coil due to multiple narrow cracks located in conductive planar multilayered structures (MLS). The cracks can have various shapes or orientations and can lie in any layer of the structure inspected. After a presentation of the method, validations results, consisting of comparison of simulated signals with experimental data and results obtained with finite element calculations, are presented. In all the problems tested, a very good accuracy and a very low computation time have been observed. Further perspective and conclusions of this work are detailed at the end of the paper.