ID 1
Title Quantitative profile evaluation of natural cracks in a steam generator tube from eddy current signals
Author Noritaka Yusa, Zhenmao Chen, and Kenzo Miya
Published in International Journal of Applied Electromagnetics and Mechanics, Vol. 12, pp. 139-150, (2000)
Keywords
Abstract This paper reports the progress of studies in eddy current inversion in the authorsf group and a novel inversion scheme that can deal with conductive cracks using eddy current signals. The novel scheme is based on artificial neural networks with its accuracy enhanced by using a shifting aperture method and signal processing techniques. A four sensor type probe is used in the eddy current testing. Several reconstruction results are introduced in this paper as validation of the proposed new scheme. The scheme is also applied to profile reconstruction of cracks which occurred in an actual steam generator tube of a pressurized water nuclear power plant. Remarkable agreement between reconstructed profiles and those obtained by destructive testing is observed. It can be concluded that the new inverse scheme is promising and will be a powerful tool for the practical applications.
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ID 2
Title Inversion of eddy current NDE signals using genetic algorithm
Author Noritaka Yusa, Zhenmao Chen, and Kenzo Miya
Published in International Journal of Applied Electromagnetics and Mechanics , Vol. 15, pp. 249-255, (2001/2002)
Keywords
Abstract The present paper presents an inversion scheme that reconstructs crack profile from eddy current signals. The scheme is based upon the genetic algorithm and reconstructs crack profile from the signals obtained by numerical simulations with use of an accelerated FEM-BEM coupling code. The cracks are assumed to have uniform electrical conductivity in order to model natural cracks that occur in steam generator tubes of pressurized nuclear power plants. On the basis of calculated eddy current signals in the previous generations, the inversion scheme is accelerated by eliminating individuals that are obviously different from the global optimum solution. Reconstruction results are shown in the paper, which reveal that the scheme is very promising.

ID 3
Title Profile reconstruction of simulated natural crack from eddy current signals
Author Noritaka Yusa, Weiying Cheng, Tetsuya Uchimoto, and Kenzo Miya
Published in NDT&E international, Vol. 35, pp. 9-18, (2002)
Keywords Eddy currents, inverse problem, modeling, natural cracks
Abstract Based upon the fact that internal conductivity of the crack has serious effect on the eddy current signals, two kinds of inversion schemes that can reconstruct cracks with non-vanishing conductivity from eddy current signals are proposed in this paper. One is a physics-based approach that modifies crack parameters iteratively and the other is a neuronet-based approach. Two models that are supposed to be appropriate models of natural crack are also proposed. After detailed explanation of the schemes, several reconstruction results using simulated data are presented. The results show that the schemes can reconstruct crack profile with high accuracy although not only shape of the crack but also internal conductivity are unknown. At the end of this paper, advantages and limitations of both schemes are discussed.

ID 4
Title Generalized neural network approach to eddy current inversion for real cracks
Author Noritaka Yusa, Weiying Cheng, Zhenmao Chen, and Kenzo Miya
Published in NDT&E International, Vol. 35, pp. 609-614, (2002)
Keywords Eddy currents, steam generator, inverse problem, natural cracks
Abstract This paper proposes a generalization for the solutions of an inversion method developed by the authors. The method is based upon an artificial neural network that simulates mapping between eddy current signals and crack profiles. One of the biggest advantages of the approach is that it can deal with conductive cracks, which is necessary to reconstruct natural cracks. However, it has one significant disadvantage: the reliability of reconstructed profiles was unknown. This paper also proposes a novel parameter that provides an index for assessment of the crack profile and overcomes this disadvantage. After the parameter is validated by reconstruction of simulated cracks, it is applied to reconstruction of natural cracks that occurred in steam generator tubes of a pressurized water reactor. It is shown that the parameter is applicable to not only simulated cracks but also real (natural) ones.

ID 5
Title Large-scale parallel computation for the reconstruction of natural stress corrosion cracks from eddy current testing signals
Author Noritaka Yusa, Zhenmao Chen, Kenzo Miya, Tetsuya Uchimoto, Toshiyuki Takagi
Published in NDT&E international, Vol. 36, pp. 449-459, (2003)
Keywords Eddy currents, Inverse problem, Natural crack, Parallel computing, Tabu search
Abstract An inversion algorithm for the reconstruction of cracks from eddy current signals is developed in this study and applied to the profile evaluation of natural stress corrosion cracks that were found in steam generator tubes of a nuclear power plant. A crack is modeled as an assembly of small regions having conductivities inside so that eddy currents that flow across the cracks are considered. The conductivity of each region, which is assumed to be a discrete value, is reconstructed by means of the algorithm. Since the algorithm is based upon a tabu search that usually requires a large number of evaluating solution candidates, simulations are carried out on a supercomputer with the use of parallel computation using up to 128 CPUs so as to reconstruct the crack profiles within a reasonable computational time. It is demonstrated that the algorithm can estimate the profiles of the natural cracks with sufficient accuracy. The simulations also show that the algorithm is highly compatible with parallel computation. Additional simulations using other models of natural cracks are performed. Reconstructed profiles of the natural cracks, as a notch with zero conductivity, are very different from the true profiles, even though the reconstructed signals agree well with the measured values. This reveals that it is necessary to take the internal conductivity into consideration when dealing with natural cracks.

ID 6
Title Comparison of efficiencies of metaheuristics in ECT inversion problems
Author Noritaka Yusa, Zhenmao Chen, Kenzo Miya, Tetsuya Uchimoto, and Toshiyuki Takagi
Published in Journal of Japanese Siciety for Non-destructive Inspection , Vol. 52, pp. 504-510, (2003)
Keywords Eddy current testing, Reconstruction, Parallel computation, Metaheuristics, Supercomputer, Inverse problem
Abstract This study applied a simple local search and five metaheuristic approaches : a simple genetic algorithm, a parameter-free genetic algorithm, simulated annealing, threshold accepting and tabu search, to ECT inversion problems and compared them from the viewpoint of their efficiency in reconstructing crack profile. Among them, the tabu search provided the best results. Since the approaches were very time-consuming, simulations were carried out on a supercomputer using parallel computation. It was shown that the parallel computation is very effective for those problems and can reduce computational time drastically. The advantage and disadvantage of each approach are discussed.

ID 7
Title Application of a superconductor to the shield of a remote field ECT probe
Author Noritaka Yusa, Kazuyuki Demachi, and Zhenmao Chen
Published in International Journal of Applied Electromagnetics and Mechanics, Vol. 19, pp. 41-46, (2004)
Keywords remote field eddy current testing, superconductor, magnetic shield, non-destructive testing
Abstract This paper proposes the application of a superconductor to a remote field eddy current testing probe in order to shorten the probe length and then overcome many of the disadvantages of the remote field eddy current testing, such as the small signal amplitude and the difficulty in handling the probes. Numerical simulations were carried out to confirm that a superconductor located between the exciter and the detector of a remote field eddy current testing probe inhibits the magnetic flux from coming directly from the exciters. Then, a remote field eddy current testing probe equipped with a superconducting shield was designed and fabricated. Experimental results confirmed that a superconducting shield can shorten the probe length and provide clearer signals due to artificial wall thinnings machined on the outside surface of a ferromagnetic tube.
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ID 8
Title Reconstruction of multiple cracks in an ECT round-robin test
Author Mihai Rebican, Noritaka Yusa, Zhenmao Chen, Kenzo Miya, Tetsuya Uchimoto, and Toshiyuki Takagi
Published in International Journal of Applied Electromagnetics and Mechanics, Vol. 19, pp. 399-404, (2004)
Keywords eddy current testing, reconstruction, simulated annealing, multiple cracks, parallel computing
Abstract This paper proposes an algorithm for multiple cracks reconstruction from ECT signals by means of a simulated annealing parallelized on a supercomputer. The algorithm takes not only the profiles of cracks but also the number of cracks as unknown, which is a very important feature from the practical point of view. A fast forward code using a database is upgraded for the rapid computation of the ECT signals of a differential mutual induction probe due to multiple cracks. The algorithm is applied to the reconstruction of the multiple cracks of a JSAEM round-robin test geometry. The reconstruction result using a simulated signal proves the validity of the proposed approach.

ID 9
Title Advanced MFLT for detecting far side defects in a welding part of an austenitic stainless steel plate
Author Zhenmao Chen, Noritaka Yusa, Kenzo Miya, Akira Tanahashi, Kazuo Sakai, and Naoki Chigusa
Published in International Journal of Applied Electromagnetics and Mechanics , Vol. 19, pp. 527-532, (2004)
Keywords magnetic flux leakage method, high frequency demagnetization, inverse problem, remanent magnetization
Abstract In this paper an advanced MFLT method is proposed for detecting far side defects in a welding part of a thick stainless steel plate based on the measurement of the remanent magnetic field. The method consists of a double yoke magnetizer, high frequency demagnetization procedure and an inverse analysis scheme. The performance of the proposed method is investigated through numerical simulations and experiments. The results show that a far side defect in a plate as thick as 20 mm is possibly to be detected by using the proposed strategy.

ID 10
Title Application of eddy current inspection to the Inconel weld of BWR internals
Author Eiji Machida and Noritaka Yusa
Published in The Thermal and Nuclear Power, Vol. 5, pp. 34-41, (2004)
Keywords
Abstract

ID 11
Title Inversion techniques for eddy current NDE using optimization strategies and a rapid 3D forward solver
Author Zhenmao Chen, Noritaka Yusa, Mihai Rebican, Kenzo Miya
Published in International Journal of Applied Electromagnetics and Mechanics
Keywords
Abstract In this paper, new progress on the ECT-based crack sizing techniques is presented. At first, a fast and accurate forward simulator, which is indispensable in the inversion of ECT signals, is briefly introduced. Several schemes for inversion of crack signals are presented then for EDM cracks, stress corrosion cracks (SCC) respectively. In addition, scheme and reconstruction results for multiple cracks, deep cracks and cracks in the welding part are also given. From the numerical results, it is demonstrated that sizing of single crack from ECT signals has become possible even for a natural crack or a crack as deep as 10 mm. For multiple cracks, more effort is necessary for its modelization and reconstruction.

ID 12
Title Three-dimensional inversion of volumetric defects profiles from electromagnetic nondestructive testing signals by means of stochastic methods with the aid of parallel computation
Author Noritaka Yusa, Mihai Rebican, Zhenmao Chen, Kenzo Miya, Tetsuya Uchimoto, and Toshiyuki Takagi
Published in Inverse Problem in Science and Engineering, Vol. 13, No. 1, pp. 47-63, (2005)
Keywords non-destructive testing, eddy current testing, inversion, volumetric defects, parallel computing
Abstract This study applies five stochastic inversion methods to the reconstruction of three-dimensional volumetric defect region from eddy current testing signals, and evaluates their efficiencies in this problem. The five stochastic methods considered in this study are iterative local search, tabu search, simple genetic algorithm, parameter-free genetic algorithms, and simulated annealing. Since stochastic methods require evaluation of solution candidates many times, simulations are performed on a supercomputer with the aid of parallel computation so as to avoid too long computational time. The results of this study demonstrate that stochastic methods applied to eddy current inversion problems are highly compatible with parallel computation, and computational time can be significantly shortened with the use of parallel computation. Among the five algorithms adopted in this study, the tabu search and simulated annealing provided good results whereas genetic algorithms showed very poor performance.

ID 13
Title Application of eddy current inversion technique to the sizing of defects in Inconel welds with rough surface
Author Noritaka Yusa, Eiji Machida, Ladislav Janousek, Mihai Rebican, Zhenmao Chen, Kenzo Miya
Published in Nuclear Engineering and Design, Vol. 235, 1469-1480 (2005).
Keywords eddy current, non-destructive testing, Inconel, weld inspection, inverse problem, reconstruction
Abstract This paper evaluates the applicability of eddy current inversion techniques to the sizing of defects in Inconel welds with rough surfaces. For this purpose, a plate Inconel weld specimen, which models the welding of a stub tube in a boiling water nuclear reactor, is fabricated, and artificial notches machined into the specimen. Eddy current inspections using six eddy current probes are performed and the efficiencies of the six probes in weld inspection evaluated. It is revealed that if suitable probes are applied, an Inconel weld does not provide large noise signals in eddy current inspections even though the surface of the weld is rough. Finally, reconstruction of the notches is performed using eddy current signals measured using the uniform eddy current probe that showed the best results among the six probes in the inspection. A simplified configuration is proposed in order to consider the complicated configuration of the welded specimen in numerical simulations. While reconstructed profiles of the notches are slightly larger than the true profiles, quite good agreements are obtained in spite of the simple approximation of the configuration, which reveals that eddy current testing would be an efficient non-destructive testing method for the sizing of defects in Inconel welds.

ID 14
Title Investigation of Numerical Precision of 3D RFECT Signal Simulations
Author Mihai Rebican, Zhenmao Chen, Noritaka Yusa, Kenzo Miya, Tetsuya Uchimoto, and Toshiyuki Takagi
Published in IEEE transactions on Magnetics Vol. 41 (2005), 1968-1971.
Keywords Edge elements method, remote field eddy-current technique, signal calculation, 3-D defects
Abstract This paper evaluates the numerical precision of a three dimensional Ar method with edge-element finite elements in the simulation of remote field eddy current testing of a ferromagnetic tube. The method is employed with a suitable formula for the EMF signal calculation in magnetic materials. Using a supercomputer, analyses of the mesh division effect on numerical precision are carried out to find out the proper mesh conditions for signal simulations of 3D defects. Good agreements of the simulated signals with experimental ones are obtained using this method with an acceptable computational resource. In this way, the 3D simulation of remote field eddy current testing signals for a magnetic tube has become possible in practical applications.

ID 15
Title Excitation with phase-shifted fields - enhancing deep defect evaluation using eddy currents
Author Ladislav Janousek, Zhenmao Chen, Noritaka Yusa, Kenzo Miya
Published in NDT&E international, Vol. 38, pp. 508-515, (2005).
Keywords eddy current probe, excitation with phase-shifted fields, deeper penetration of eddy currents, evaluation of deep defects
Abstract This study enhances the applicability of eddy current testing to the evaluation of deep defects in thick structures, while keeping the advantages of higher frequency inspection. The key idea is to suppress eddy currents on the surface of a test-piece, and thus realize the deeper penetration of eddy currents. Based upon this idea, a mutual induction eddy current testing probe, consisting of four coaxial rectangular tangential exciter coils and a pancake pick-up coil, has been designed. The exciters are driven by AC currents with different phases and amplitudes, which contrive to emulate the desired behavior. Numerical simulations and consecutive experimental verifications are presented in this paper. It is revealed, that when the new probe is used for inspections under a frequency of 10 kHz there is a 20 % difference between signals due to an electro-discharge machined notch of 15 mm depth and those due to one of 20 mm depth. On the other hand, a conventional probe driven with the same frequency gives only 1 % difference between the signals of the two notches.

ID 16
Title Sizing of stress corrosion cracking on austenitic stainless piping in a nuclear power plant from eddy current NDT signals
Author Noritaka Yusa, Zhenmao Chen, Kenzo Miya
Published in Nondestructive Testing and Evaluation, Vol. 20, pp. 103-104, (2005)
Keywords eddy currents, inverse problem, stress corrosion crack, nuclear power plant, tabu search
Abstract This paper proposes a novel algorithm for the reconstruction of stress corrosion cracking from eddy current testing signals, and reconstructs natural stress corrosion cracking on 8.7 mm thick austenitic stainless piping that provided coolant water for a nuclear reactor. The cracks, which were caused by vinyl chloride labels attached to the outer surface of the piping, are modeled as a conductive region, and both the boundary profile and internal conductivity are reconstructed with the algorithm. The reconstructed profiles agree well with the true ones revealed by destructive testing, which confirms the validity of the algorithm and proves that the applicability of the eddy current testing to the sizing of real cracks not only in thin structures such as steam generator tubes but also general structures.

ID 17
Title Diagnostics of stress corrosion and fatigue cracks using benchmark signals
Author Noritaka Yusa, Ladislav Janousek, Zhenmao Chen, Kenzo Miya
Published in Materials Letters, Vol. 59, pp. 3656-3659, (2005)
Keywords nondestructive testing, eddy current testing, natural crack, benchmark problem, numerical modeling, Alloy 600
Abstract On the basis of the background that most studies for quasi-static low-frequency electromagnetic nondestructive methods consider artificial notches that are, in reality, not always similar to real cracks such as stress corrosion and fatigue cracks, this paper introduces original eddy current inspection data in order to make it available to other researchers in the field. The authors expect this data to be helpful to those who are interested in the application of quasi-static low-frequency electromagnetic field simulations to practical eddy current testing problems. Stress corrosion and fatigue cracks are fabricated into nickel-chromium alloy plates, and eddy current inspections of the plates are performed using four eddy current probes. Signals of four specimens with stress corrosion cracking and another four with fatigue cracks are presented together with profiles of the cracks revealed through destructive testing.

ID 18
Title Eddy current inversions of defects in rough welds using a simplified computational model
Author Noritaka Yusa, Ladislav Janousek, Mihai Rebican, Zhenmao Chen, Kenzo Miya
Published in Nondestructive Testing and Evaluation, Vol. 20, pp. 191-199, (2005)
Keywords eddy current testing, non-destructive evaluation, inverse problem, nuclear power plants, nickel-chromium alloys
Abstract This study demonstrates that the profiles of defects in Inconel welds with a rough surface can be sufficiently evaluated by eddy current testing with the aid of computational inversion techniques. A plate specimen with Inconel welding is fabricated and artificial notches are introduced between the welding metal and base material. Eddy current inspections are performed to measure signals caused by the artificial notches, and their profiles are reconstructed from the measured signals using a computational inversion algorithm based on the conjugate gradient method. Since it is difficult to model the specimen exactly, simulations to calculate eddy current signals due to a reconstructed profile are performed in a simplified configuration. The lengths and maximum depths of the notches are well estimated, which validates the applicability of eddy current testing to not only the detection but also evaluation of defects in Inconel welds.

ID 19
Title Caution when applying eddy current inversion to stress corrosion cracking
Author Noritaka Yusa, Ladislav Janousek, Mihai Rebican, Zhenmao Chen, Kenzo Miya, Nobuki Dohi, Naoki Chigusa, Yoshihiro Matsumoto
Published in Nuclear Engineering and Design, Vol. 236, pp. 211-221, (2006).
Keywords nondestructive testing, weld inspection, reconstruction, inverse problem, stress corrosion cracking, Alloy 600
Abstract This study evaluates the applicability of computer-aided eddy current inversion techniques to the profile evaluation of stress corrosion cracking in Inconel welds. Welded plate specimens, which model head penetration welds of pressurized water reactors, are fabricated; notches and stress corrosion cracks are artificially introduced into the welding of the specimens. Eddy current inspections are performed using a uniform eddy current probe driven at frequencies of 10 and 40 kHz. Since weld noise is observed uniformly along the weld line, a simple signal processing is applied to eliminate it. First, the artificial notches are reconstructed and good agreements between reconstructed and true profiles are provided, which demonstrates that computer-aided eddy current inversion technique can deal with defects in welds. Then, numerical simulations are performed to evaluate the profiles of the stress corrosion cracks. In the numerical simulations, the stress corrosion cracks are modeled as a conductive region with a fixed width of 0.3 mm. The cross-sectional profiles of the cracks are reconstructed from measured eddy current signals directly above and along a crack. Although eddy current signals calculated from the reconstructed profiles agree well with measured ones, the true profiles revealed by destructive testing are found to be very different from the reconstructed ones. Whereas the most plausible reason for the difference is the unexpectedly volumetric profile of the stress corrosion cracks, this study has revealed that computer-aided eddy current inversion techniques that have been used to consider cracks in thin structures would not at this point be directly applicable to those in thick structures.

ID 20
Title Shape reconstruction of multiple cracks from ECT signals by means of a stochastic method
Author Mihai Rebican, Zhenmao Chen, Noritaka Yusa, Ladislav Janousek, Kenzo Miya
Published in IEEE transactions on Magnetics, Vol. 42, pp. 1079-1082 (2006)
Keywords nondestructive evaluation, eddy current testing, multiple cracks, signal inversion, tabu search
Abstract This paper presents an approach to reconstruct multiple cracks from eddy current testing signals by means of a stochastic method such as tabu search. The size, number, and locations of cracks are considered unknown in the inversion process, whereas only the orientation of cracks is supposed to be revealed in advance. Eddy current inspections of a stainless steel plate with closely located EDM slits are performed, and the inversions of measured signals are carried out. The results agree well with true ones, which validates the proposed algorithm.