Ebook: Electromagnetic Nondestructive Evaluation (XIX)

This volume contains selected papers from the 20th International Workshop on Electromagnetic Non-Destructive Evaluation (ENDE) held at Katahira Sakura Hall, Tohoku University, Sendai, Japan, from September 21–23, 2015. The first ENDE Workshop was held in London, UK exactly 20 years ago, beginning September 21st, 1995. The following 18 workshops were held in various countries around the world including the UK, Japan, France, USA, Hungary, Germany, South Korea, Poland, India, Brazil, Slovakia, and China. ENDE Workshops aim to provide an international forum for discussion on the state of art and perspectives in the field of electromagnetic non-destructive methods from the view of science, technology and their applications in engineering, which has contributed to the development of non-destructive testing and evaluation techniques using electromagnetic fields.

The 20th ENDE (ENDE2015) was organized by the ENDE2015 Organizing Committee, and co-organized by the School of Engineering, Tohoku University, the Institute of Fluid Science, Tohoku University, and the Japan Society of Maintenology. ENDE2015 was sponsored by the Intelligent Cosmos Research Institute, Sendai Tourism, Convention and International Association, and The Kajima Foundation.

There were 108 participants, including 33 students and 12 accompanying persons. The participants were from 17 countries: China, Czech Republic, Finland, France, Germany, Greece, Hungary, India, Italy, Japan, Korea, Portugal, Republic of China, the Slovak Republic, Thailand, the UK, and the USA. There were 82 contributions, including two keynote lectures and one invited lecture, 33 presentations over seven oral sessions, 26 presentations in the poster session, and 20 presentations in a special ‘Student Session’. The Student Session consisted of short oral presentations without Q&A and subsequent poster presentations. The presentations in the Student Session were evaluated by the participants. One student from Germany and three students from Japan were awarded the ‘ENDE ISC Award’ and ‘ENDE2015 Award’, respectively. Forty full papers were ultimately accepted and included in this volume.

As this is the commemorative twentieth workshop, several special sessions were arranged within the theme of ‘Origin, Legacy, and Future Directions of ENDE’. The workshop started with a keynote lecture by Professor Kenzo Miya, the founding father of ENDE Workshops, titled ‘Why we established ENDE Workshops 20 years ago’. The keynote lecture was followed by an invited lecture, ‘History and development of ENDE Workshops’, by Professor Toshiyuki Takagi. The second day of the workshop included a keynote lecture by Professor Gerd Dobmann titled ‘Global trend of ENDE studies’ to review the development of the study of electromagnetic non-destructive evaluations in the last couple of decades. The recent trends of the studies were then discussed in a subsequent panel session titled ‘ET&ENDE – Where we are and where we want to go?’ where three distinguished professors, Professor Antonello Tamburrino, Professor Gui Yui Tian, and Professor Lalita Udpa, were invited as panelists. On the last day of the workshop before the closing ceremony, another panel session was organized, inviting ENDE ISC members to discuss the future directions of ENDE Workshops. The panelists of the session were Professor Zhenmao Chen, the chairman of ENDE ISC, Professor Toshiyuki Takagi, Dr. Christophe Reboud, and Dr. Noritaka Yusa, who facilitated the discussion.

In addition to these special sessions, special events such as the pre-workshop meeting and farewell party were organized for the students. We believe that ENDE Workshops should promote friendships, especially between young researchers. It is our great pleasure if these friendships continue after this workshop.

ENDE2015 Organizing Committee would like to express sincere gratitude to all the members of the international standing committee members, participants, and the sponsors for their contributions. It was our great pleasure to hold ENDE2015 in Sendai where the huge earthquake and the resultant tsunami hit during an earlier ENDE Workshop. We deeply appreciate support from all over the world.

Noritaka Yusa

Tetsuya Uchimoto

Hiroaki Kikuchi

Editors

This article presents the monotonicity property of time constants in pulsed eddy current testing problems. A fast non-iterative imaging method based on this monotonicity property is also presented. The focus of this paper is to achieve 3d reconstructions and address the problem of isometry encountered in a real world structure.

New dynamic window multi-frequency algorithms are investigated to be used with signals from multi-coils remote-field eddy current (ECT) system in order to suppress tube support plate signal in steam generator tubes of fast breeder reactors (FBR) and enhance outer tube defect signal/noise ratio. The applicability of the method is confirmed using three dimensional FEM simulations to assess the ECT signal variations due to sodium accumulated inside of defects volume.

Carbon-fiber-reinforced plastic (CFRP) is a structural material with excellent mechanical properties. However, CFRP is a brittle material, so defects must be detected early. Moreover, it is important to precisely detect any misalignments of carbon fibers in the CFRP to maintain its reliable mechanical properties. In this study, we used eddy current testing with a mutual induction-differential probe to detect various orientations of carbon fibers in CFRP specimens. We could identify the fiber orientations in three types of CFRPs—unidirectional, cross, and quasi-isotropic orientations—with stable signals.

This manuscript presents a numerical model for Electromagnetic Nondestructive Evaluation of composite materials. In this framework, the challenge is devising a single numerical model that is capable of handling different operational regimes: (i) from static to almost microwave frequencies, (ii) from dielectric to conducting anisotropic composite materials. Numerical cross-validation proves the effectiveness of the numerical model and makes it suitable for benchmarks.

We developed eddy current testing (ECT) devices with saw-wave excitation. The saw wave has many harmonics, which can be used to get more information about defects. Multi-frequency digital lock-in amplifier for the harmonics was developed based on labview program. Using the saw-wave ECT, we did the experiments of crack detection for CFRP cable. An excitation coil and differential receiver coil were wrapped around a tube. The CFRP cable was moved through the tube for crack detection. The diameter of the CFRP cable was 1.3 mm and the diameter was 2.5 mm with the plastic coating. Three artificial defects with the interval of 50 mm were made. The depths of the cracks were about 1/4, 2/4, and 3/4 of the diameter of the cable respectively. Using the saw-wave ECT with the fundamental frequency of 200 kHz and the excitation current of 20 mA, the artificial defects in the CFRP cable was successfully detected.

We extend a procedure to determine the optimal magnet design in the framework of Lorentz force eddy current testing. We consider classical cylinders and Halbach magnet systems for specimens made by metal injection molding. Evaluating as objective function the absolute defect response signal of the Lorentz force measurements using finite element method leads to an optimal and specific magnet design for each defect scenario. The proposed Halbach magnet enables defect responses up to four times higher outperforming cylinder.

In this paper the inversion process is related to the eddy current evaluation of aluminum plates. A special probe that applies a spatially uniform excitation field is used. The excitation field is invariant under the probe translations and permits to simplify the resolution of the inverse problem. The inversion process consists on the approximate determination of the current density on the plate that permits the characterization of the defects geometry. The inversion is regularized by the Tikhonov method and the optimum regularization parameter is determined using the L-curve.

ECT technique was applied to evaluate the fatigue damage progression of Cu-Cr-Zr alloy during low-cycle fatigue testing at room temperature. Subtracted ECT-Y signal from the applied strain signal was indicated low-cycle fatigue damage such as work-hardening, micro-crack generation and macro-crack propagation. The damage propagation of the low-cycle fatigue testing in Cu-alloy could be depended on work-hardening inside of the specimen and micro-crack nucleation on the surface.

U-bend sections are critical parts of steam generator tubes in nuclear power plants. Efficient and accurate numerical modeling of eddy current inspection of U-bend section is non-trivial, time consuming and computationally intensive. The co-variant nature of Maxwell equations can be exploited to circumvent these issues by efficient change of spatial coordinates. In this contribution, co-ordinate transformation is introduced for simulating eddy current testing of U-bend tube sections. Cross-validation between the numerical computations carried out using the standard Cartesian coordinates and the transformed coordinates are presented.

In this study a new nondestructive testing (NDT) method called transient eddy current oscillations (TECO) is used to detect a change in the thickness of stainless steel plates. The TECO method uses a probe that includes a capacitor and an induction coil, which are connected in parallel, to generate a transient oscillatory regime in the excitation current to induce eddy currents in the test object. A Hall-effect sensor is used to measure the resultant magnetic field in the vicinity of the test material. The experimental results were analyzed to conclude about the thickness variation of the test object using the absolute integral value of the decay of oscillations that are obtained from the probe at different thicknesses of test material in the time domain and their frequency was calculated using fast Fourier transform (FFT). If there is a decrease in the thickness of the test specimen, experimental data shows an increase in the integral value of the sensor output signal and a decrease in the frequency. A differential FFT method is being proposed in this paper to separate any unwanted lift-off variation from the local wall thinning. In fact, with the thickness change in specimen, the differential spectrum show two differential FFT amplitude peaks in which one is positive and other is negative. However, if there is a change in lift-off on the reference thickness of specimen, the differential FFT shows three differential FFT amplitude peaks in which two peaks appeared in the positive side of the differential FFT. This enables an easy identification of any lift-off change, which is very useful in real time applications.

As one of the advanced Electromagnetic Non-destructive Evaluation (ENDE) techniques, Pulsed Remote Field Eddy Current technique (PRFEC) has been found advantageous over the other ENDE methods particularly in evaluation of ferromagnetic multi-layered tubular structures. In this paper, PRFEC in conjunction with magnetic field measurement using solid-state magnetic sensors for detection and classification of subsurface corrosion in ferromagnetic multi-layered tubular structures is intensively investigated through simulations and experiments. The characteristics and features of magnetic field signals against different subsurface corrosion are analysed, whilst a noise-suppression method integrating principle component analysis and independent component analysis is proposed in a bid to mitigate the non-stochastic noise in defect-scanning curves. Following this, the feasibility of most likelihood method for classification of subsurface corrosion via PRFEC is investigated.

This paper presents a nonlinear parametric approach for simultaneous estimation of depth and length of uniform wall loss grooves from the RFEC phase signals. The nonlinear parametric equation has been arrived after detailed analysis of the model predicted RFEC phase signals of grooves of different dimensions. The performance of the approach has been validated and accurate sizing of depth of the grooves within a maximum absolute error of ±0.03 mm has been achieved.

In this paper, we described the changes of two nonlinear acoustic characterizations; resonant frequency shift and three-wave interaction, with electromagnetic acoustic resonance (EMAR) throughout the creep life in the welded joints of ASME Grade 91, one of high Cr ferritic heat resisting steels. EMAR was a combination of the resonant acoustic technique with a non-contact electromagnetic acoustic transducer (EMAT). Two nonlinear acoustic parameters and ultrasonic attenuation decreased from the start to 20% of creep life. They gradually increased from 50% of creep life to rupture. We interpreted these phenomena in terms of dislocation recovery, recrystallization, and restructuring related to the initiation and growth of creep void, with support from the SEM and TEM observation.

A bearing steel of a 62 HRC hardness was milled using the cutting tools with different flank wears. The low-frequency measurements of the magnetic Barkhausen noise were performed under a controllable waveform of the surface magnetic field measured by the Hall sensors. The high-frequency measurements with a smaller magnetization depth were performed by a commercial Rollscan device. The measurements were conducted in two perpendicular magnetization directions: along the feeding and the rotation directions of the cutting shaft. Suitable magnetic parameters for detection of the induced surface damage were proposed.

A recently developed nondestructive method, called Magnetic Adaptive Testing (MAT) was applied for investigation of flake graphite cast iron samples having various metallic matrices and graphite structures. MAT is typical by its low required magnetization of samples, because it is based on measurement of families of minor magnetic hysteresis loops. The flat samples were magnetized by an attached yoke and sensitive descriptors of their magnetic/structural state were obtained from evaluation of the measured data. Conductivity measurements were performed and results of the non-destructive magnetic tests were compared with these data. A very good correlation was found between the magnetic descriptors and conductivity. Based on these results, MAT is recognized as a highly promising non-destructive alternative of destructive tests for monitoring structural changes in cast iron.

A 2D numerical model for the solution of the eddy-current induction problem in non-linear hysteretic media is presented. The convergence of the iterative algorithm for the inversion of the non-linear operator is discussed. The numerical results are compared with simulations carried our using a commercial finite elements solver. Finally, an experimental procedure for obtaining the input material curves is presented.

As a further study on the low-level magnetized magnetic flux leakage testing, numerical simulations and measurements were conducted to investigate the influence of liftoff variation, applicability of the approach to local thinning and material characterization. The results showed that by magnetizing a test object to a low strength level and acquiring the weak magnetic flux leakage signals by a highly sensitive magneto-impedance sensor, local thinning and the change of magnetic property can be detected.

This paper introduces a new monitoring technique for detecting the presence of wear-out in friction bearings with an electromagnetic sensor. This new method detects the presence of wear-out indirectly, through the detection of airgap eccentricity. Firstly, the electromagnetic sensor and the airgap eccentricity are introduced. Then, the effectiveness of the method is illustrated by experimental results on a pump with a 3-phase induction motor.

The paper introduces multimodality non-destructive testing and evaluation. Multiphysics integrated NDT&E approaches e.g. dual probe of EMAT and PEC, integrative MFL system, are reviewed. Different strengths and weakness are reported. Recent work on eddy current pulsed thermography as a new integrative ENDE system, is discussed. Their multiphysics and their interaction and quantitative analysis including simulation and data processing are investigated and evaluated. Case studies and future work are also reported.

Our goal is to detect defects in composite materials composed by multi-layer planar plates with a periodic set of circular cylindrical fibers embedded within each layer. As a starter, the work presented is electromagnetic (EM) modeling and imaging of missing fibers in a fiber array standing in air. The multiple scattering method is utilized to analyze the electromagnetic behavior, and the corresponding imaging model is established directly from Lippman-Schwinger integral formulation. With the imaging model, standard MUltiple SIgnal Classification (MUSIC), and the proposed joint sparsity which borrows the idea from sparsity theory, are applied to retrieve the locations of missing fibers. Various numerical results are provided to illustrate availability and accuracy of the modeling and imaging.

This paper examines Hall Effect measurements of stainless steel 316 (SS316) under applied stress. In free carrier gas model, the Hall coefficient is inverse proportional to charge carrier density, and thus expected to vary under stress. This relationship is simpler than that of the electrical conductivity, which depends not only on carrier density but also on the mean-free path. Thanks to the simpler underlying physics, it is anticipated that Hall Effect measurements can show more straightforward stress dependency than the conductivity-based measurements such as eddy current. In contrast, at least in the free carrier approximation with single type of carriers, the Hall coefficient has no dependence on the mean free path, while depending on elastic stress. We measured the Hall coefficient of SS316, in comparison with that of copper. First, the expected stress dependence of the Hall coefficient due to the simple volume effect under the stress was estimated in free carrier gas model. Second, Hall coefficients of these metals were measured under varying magnetic fields. It was observed that SS316 exhibited magnetic field dependency of the Hall coefficient outside measurement errors, while no such dependence was observed for copper. Third, the relationship between tensile stress and the Hall coefficient in SS316 was measured. The Hall coefficient increases in magnitude linearly under tension. The stress sensitivity is higher than predicted from the simple volume effect.

In this work, a phased-array (PA) electromagnetic acoustic transducer (EMAT) is proposed for detection of delamination defect in multilayer tubes of small inner diameter. In order to generate focused ultrasonic waves for the small tube and small defects, a solenoid EM magnet setting outside the tube is adopted for the bias magnetic field generation and an array of bobbin coils setting inside the tube is used to induce eddy current and Lorentz force. Numerical simulations are performed to investigate the validity of the proposed PA EMAT method for monoblock of Tokamak divertor structures. The numerical results show that the detectability can be significantly improved, which demonstrated the validity of the proposed PA EMAT for multilayer tube of small diameter.

Reduced Activation Ferritic/Martensitic (RAFM) steel is considered to be a candidate structural material for future Tokamak fusion nuclear reactors. The plastic deformation may occur during the operation of the reactor for reason of unexpected loads. Therefore, a reliable non-destructive evaluation (NDE) of RAFM steel is needed. In this study, magnetic NDE methods such as magnetic Barkhausen noise (MBN), magnetic incremental permeability (MIP) and magnetic flux leakage (MFL) method have been applied to evaluate the residual plastic strains in RAFM steel to ensure the structural integrity after an unexpected giant loading process. The study shows that the MBN signals and the amplitude of the high harmonic components of the MFL signals decrease regularly with the increasing residual strain. The shape of MIP butterfly trajectory also changes evidently in range of small plastic deformation. The experimental results reveal that the MFL method is the most suitable way in the three magnetic NDE methods for measurement of plastic strain in the RAFM steels.

In this paper, a numerical scheme for simulation of DC potential drop (DCPD) signal is updated at first to deal with multiple defects in order to reconstruct multiple cavities in metallic foam (MF) from measured DCPD signals. A hybrid inversion strategy combining the genetic algorithm and the steepest descent optimization method is proposed then to evaluate the size and position parameters of the multiple defects. As example, simulated signals with artificial noise are adopted to reconstruct the profile of double cavity defects. The numerical results validated the hybrid inversion strategy and the updated forward scheme for sizing the multiple defects in metallic foam.