Ebook: Electromagnetic Nondestructive Evaluation (XVIII)

This volume contains selected papers from the 19th International Workshop on Electromagnetic Non-Destructive Evaluation (ENDE), which was held in Xi'an Jiaotong University, China on June 25–28, 2014. ENDE workshop is an important event for the researchers with interest in non-destructive evaluation. The first workshop was organized in 1995 in London, UK and then followed 17 workshops held in various countries in different parts of the world. The last one – ENDE 2014 workshop was organized in China, for the first time.

The aim of the workshop is to bring together engineers and scientists from universities, research institutions and industry, who are active in research, development and industrial applications of electromagnetic nondestructive evaluation. The 19th workshop focuses on the theoretical and application researches on ENDE methods and provides a forum for exchanging ideas and discussing recent developments.

The workshop was organized by the State Key Laboratory for Strength and Vibration of Mechanical Structures at Xi'an Jiaotong University, with support of the Chinese Society of Nondestructive Testing, the National Natural Science Foundation of China, the Japan Society of Maintenology and the Japan Society of Applied Electromagnetics and Mechanics. 116 participants from 10 countries (China, Japan, Slovakia, Poland, USA, UK, France, Italy, Brazil, Canada – listed according to the number of participants) were officially registered. There were presented 103 contributions: 3 plenary lectures, 8 invited talks, 25 oral and 67 poster presentations.

Plenary Lectures:

1. Norbert Meyendorf: Heinrich Barkhausen in Dresden – New Applications of His Ideas for Electromagnetic NDE.

2. Tetsuya Uchimoto: Evaluation of Residual Strain in Structural Materials of Nuclear Power Plants.

3. Minglong Xu: Development of Measurement Techniques for Monitoring and Diagnosis of Mechanical Structures.

Invited Talks:

1. Guglielmo Rubinacci: 3D Efficient Simulation of a Magnetic Probe for Material Characterization of Ferromagnetic Specimens with Hysteresis and Eddy Currents.

2. Fumio Kojima: Numerical Simulation of Ultrasonic Source Mechanism for EMAT Based NDE System.

3. Luming Li: Stress Concentration Caused Magnetic Field Abnormality.

4. Dominique Lesselier: Fast Calculation of Electromagnetic Scattering in Anisotropic Multilayers and Its Inverse Problem.

5. Lalita Udpa: Use of Asymmetry in Three Phase Voltages in the Rotating Field Eddy Current Probe.

6. Songling Huang: Reconstruction of Arbitrary Defect Profiles from Three-axial MFL Signals Based on Metaheuristic Optimization Method.

7. Gerd Dobmann: Probability of Detection – A Concept to Combine Nondestructive Testing and Evaluation with Probabilistic Fracture Mechanic Approaches.

8. Tomasz Chady: Electromagnetic Nondestructive Evaluation – from Hertz to Exahertz.

Short version papers of all the contributions have been reviewed and published in the ENDE2014 Proceedings. 42 revised and peer reviewed full papers have been accepted and included in the volume “Electromagnetic Nondestructive Evaluation (XVIII)” published by IOS Press in the book series “Studies in Applied Electromagnetics and Mechanics”. Another 12 revised and peer reviewed full papers have been recommended and accepted for publication in International Journal of Aerospace and Lightweight Structures (IJALS) as 2 special issues.

We would like to express our sincere thanks to all workshop participants for their presentations and manuscripts submissions and to the reviewers for their effort to produce a high quality of volume works. We thank also to the session chairmen for their helpful work during the workshop.

Thanks are due to the members of the ENDE Standing Committee, especially to the chairperson of STC Professor Fumio Kojima, for their helpful supports during the workshop organization.

Last we would like to thank to all members of the workshop organization committee for their hard work and excellent assistance during the preparation of the ENDE2014.

We hope that the readership of this book will find the included papers interesting and inspiring.

Zhenmao Chen, Shejuan Xie and Yong Li

Editors

Residual strain measurements are required for structural integrity evaluation of components in nuclear power plants after seismic loading. In this study, various electromagnetic nondestructive testing methods are applied to estimate small residual strain in austenitic stainless steels and carbon steels that are representative structural materials of nuclear power plants. In view of practical application performance of nondestructive testing methods are evaluated and compared using identical set of specimens with residual strain.

The material microstructure affects both the mechanical and the magnetic properties of ferromagnetic materials. This behavior can be conveniently used for designing systems for electromagnetic nondestructive material characterization (MC) of specimens. In this paper, we present an efficient numerical model of a MC system, which measures the incremental permeability of a specimen along a hysteresis loop in the presence of the eddy currents induced by a suitable coil. The numerical model solves in an efficient way two coupled 3D volume integral equations in the presence of periodic sources, eddy currents and vector hysteretic media. The results highlight the relationship between the incremental permeability tensor and the measured quantity related to the impedance of the excitation coil

The effect of magnetic yoke properties on the results of the magnetic NDE using impedance measurement was investigated. The measurement impedance depends on the yoke materials, thus, an estimation of initial permeability of specimen evaluated from the obtained impedance profile is required, because the initial permeability of the specimen has good correlation with mechanical properties and microstructures. We can calculate the impedance profile on the basis of equivalent magnetic circuit analysis, and therefore, we can estimate an initial permeability of specimens from the experimental results with comparing calculation with experimental results.

Stress concentration is an important cause of metal structure failures. In ferromagnetic materials, the nearby magnetic field changes when there is stress concentration. This phenomenon has been used to test stress concentrations. However, the mechanism that magnetic field changed by stress concentration is complicated, obtaining detailed stress information or even evaluating the impact of stress is difficult. This paper presents a method based on genetic algorithm to help analyze the stress distribution using the magnetic field. The ferromagnetic specimens are firstly discretized into finite elements and the initial stress value of elements is set as the “genes” of the initial parent chromosome. Then, the group “evolved”, and their descendants are selected with the law that the surface magnetic field shall close to the magnetic field. The results show that during the evolution, the stress distribution can be closer and closer to the real situation.

The article presents the procedure for diagnosis of real materials, which are often used in medicine praxis – stainless steel specimens, after mechanical deformation. Mechanical but also the magnetic properties are changed due to the previous deformation. This leads to phase transitions which are measurable by means of magnetic properties. Three types of steel grades are inspected under the same conditions. There are considered two types of advanced magnetic field sensors, namely fluxgate and GMR sensors. The gained results are presented and discussed in the paper.

Plastic deformation, as a type of micro-damage caused by external loads such as earthquake, is necessary to be evaluated using an efficient non-destructive evaluation technique in order to guarantee the structural safety. In this study, the feasibility of pulsed eddy current testing (PECT) method for evaluating the biaxial plastic deformation in austenitic stainless steel has been studied. Moreover, the optimal coil distance of TR probe for PECT method has also been investigated. Finally, the comparison of the evaluation effect for the uniaxial and biaxial plastic deformation is carried out.

Eddy current testing is commonly applied on austenitic stainless steel. 304 austenitic stainless steel always deforms during the process of manufacture or in service. The undesired deformation creates ferrite and martensite which causes a change of permeability. In that case, the non-magnetic austenitic stainless steel turns to be ferromagnetic. Thus, the needed eddy current testing frequency needs to be changed too. Experimental results show that when the deformation of 304 stainless steel specimens is less than 20%, the permeability of specimens increased and shown magnetic properties of ferromagnetic materials. Therefore, the skin depth of eddy current testing decreases and the optimal excitation frequency is no longer the best. The eddy current signal characteristics with different frequency had been studied which got from austenitic stainless steel specimen with a groove. The result shows that the best testing frequency varies with content of martensite. This paper discusses the algorithm of acquiring optimal testing frequency and demonstrates the frequency range can be achieved in the limited deformed range.

Metal magnetic memory (MMM) method is a novel, passive magnetic method for inspecting mechanical degradation of ferromagnetic components. In this paper, the correlation between large deformation and the deformation-induced magnetic field was experimentally investigated for the X80 steel. Various tensile plastic deformations are introduced into test specimens with different initial defects. The surface strain distributions and normal natural magnetic fields component are measured after the tensile testing. The results show that an abnormal wave occurred in the stress concentration zone and its height value was sensitive to deformation levels, the relationship between the intensity of damage-induced magnetization and the maximum plastic strain follows a simple formula proposed in this paper. The magnetic field signal depend on the deformation in accordance with the simulation results based on the mechanical-magnetic coupling FEM model. Different magnetic signal characteristics in the plastic region indicate that the magnetic memory technique can identify macro yielding and early damage.

To explore the influences of plastic stress and residual stress on the magnetization of ferromagnetic steel in geomagnetic field, series of experiments and analyses are conducted in this paper. Two different types of specimens made of Q195 carbon steel with/without initial geometric discontinuities are manufactured. Various plastic tensile stresses are introduced into the specimens and the corresponding applied stresses, deformation distributions and residual magnetic fields of specimens are measured. The gradients of the residual magnetic field of the specimens are computed and the relationship between the residual magnetic field and the plastic loading stress is summarized. It is concluded that the intensity of magnetization of ferromagnetic steel in geomagnetic field increases with the plastic stress, but scattered by the dislocation. The influences of residual stress on the magnetization of ferromagnetic steel are not significant in the geomagnetic field.

Fatigue widely exists in the industrial structures and is a common reason for structural failure. Monitoring the fatigue process of some key parts has significant meaning for avoiding material fracture. The Magnetic Barkhausen Noise (MBN) method for ferromagnetic material fatigue state evaluation is presented in this paper. Q345 carbon steel used in pressure vessel is studied with stress control in high cycle fatigue process. The MBN signal response to different exciting frequencies during the fatigue process is specially investigated. The waveform, frequency spectral, RMS and peak amplitude of MBE signals were analyzed. The RMS vs fatigue cycle curves show a stepladder characteristic. It illustrated that MBE signals have great potential to indicate the fatigue state of ferromagnetic material.

An analytical model is developed for evaluating the eddy current signal from an inspection coil inside a tube under the area of a support plate. The electromagnetic field problem is formulated in terms of the magnetic vector potential and is solved using domain truncation and eigenfunction expansions. Closed-form expressions are derived for the coil impedance and the electromagnetic field. The support plate is effectively replaced by its own surface onto which the Leontovich impedance boundary condition is enforced. The model is quite efficient for the usual configuration that involves a non-ferromagnetic tube and a ferromagnetic support plate and its validity is verified with a commercial FEM package.

An analytical modeling is presented for the axially symmetric pulsed eddy currents produced by the driver coil in a ferromagnetic plate with a flat-bottom hole (FBH) defect. The induced voltage in the pickup coil calculated by the analytical model has been compared with signal calculated by numerical model. The results show good agreement.

Magnetic dipole model is a kind of important method for studying magnetic leakage of ferromagnetic components with defects. In this paper, an elliptic cylinder magnetic dipole model is represented. Some magnetic leakage features of typical defects, such as crack, hole, pit and slot, which are normal to surface of components are studied by the using model. In addition, the coordinate conversion method is combined with the model to study magnetic leakage features in case that defects slope an angle to the component surface. Theoretical results are in good agreement with experimental ones. It provides a method and model to process magnetic leakage signal in order to identify and evaluate defects for ferromagnetic components.

To clarify the influence of the electromagneto-mechanical coupling effect due to the additional electric field induced by the velocity effect on the simulation of EMAT signals, a numerical scheme and corresponding code are developed for structures of ferromagnetic material with consideration of coupling effect. Through introducing the term to the governing equation of eddy current problem, the reduced vector potential (Ar) method and a step by step time integration algorithm were adopted for simulation. The simulation results demonstrated that the proposed numerical algorithm and the corresponding code are efficient. To investigate the influence of the coupling effect, EMAT signals of different condition were simulated by using both the codes with and without the coupling effect. It is demonstrated that the coupling effect does not give significant influence on the EMAT signals, which could be ignored in practice.

By applying a periodic pulsed current, the minor hysteresis curves of ferromagnetic material are measured. Because the minor hysteresis curve obtained has the characteristics of multiple values, the traditional solving method of nonlinear equations can't be directly applied. So considering a piecewise method, the multi-value minor hysteresis curve is converted to a series of single-value nonlinear curves according to the turning points, and then uses the solving method of nonlinear equations to solve every single-value curve. The paper uses finite element software for simulation to obtain the pulsed eddy current field distribution of a ferromagnetic pipeline and the transient induced voltage curve of pick-up coil. The experimental results show that the simulation results are effective. In the case of a constant pipeline outer radius, the wall thickness is changed, and the curves of the relative change of the transient induced voltage with pipelines and without pipelines are obtained. Then effective characteristics are extracted, the mathematical expressions are established between wall thickness and characteristics. The final experimental results which document the accuracy of the mathematical expressions can be used to detect the pipeline wall thickness. At the same time, it is proved that the characteristics extracted can effectively reflect the change of pipeline wall thickness.

Prediction and characterization of irregular crack geometrical characters from detection signals are major challenges for electromagnetic non-destructive evaluation (ENDE). In this paper, a new approach using spatial leakage magnetic field vectors in three mutually perpendicular coordinate planes is proposed for characterization of crack geometrical features considering defect surface profile, orientation and propagation angles. New signal features of B_α, B_β and B_γ extracted from leakage magnetic field in three coordinate planes of X-Y, X-Z and Y-Z are provided to improve detection sensitivity and capability for crack geometrical features evaluation. Sample including four irregular surface-breaking cracks is employed and finite element method (FEM) simulations of magnetic flux leakage (MFL) in 3D are carried out to analyze leakage magnetic field distribution and verify feasibility of the proposed approach. This approach shows competitive ability for crack shape, orientation and propagation features recognition compared with three axis components in 3D MFL. The validation of this approach for natural RCF cracks evaluation and crack dimension reconstruction is scheduled to be done in a near future.

Differential excitation coils and differential pick-up probes for EC-NDT are qualitatively analyzed by using 3D FEA models. Eddy current density and magnetic flux density induced by the two types of probes in a conducting plate with/without defects are simulated and analyzed. The signals of the two types of probes for different lift-off heights are also simulated. Experiments are carried out using an aluminum plate with artificial defects. It is found that the sensitivity of the differential excitation probe is higher than the differential pick-up probe from both the simulation and experimental results.

The dynamic mechanical-electric coupled finite element models of a piezoelectric balance is built as SOLID5 (piezoelectric couple element type) and SOLID45 (3-D structural element type) by ANSYS software. In the simulating process of calibration of the piezoelectric balance, the first six natural frequencies and mode shapes were calculated by the element modal analysis. And the static analysis of the piezoelectric balance was developed under three loads, which includes axial force, yaw force and pitch force. To analyze the influence of other components by principal components, the output voltages of all twelve piezoelectric pieces were observed. Furthermore, we changed thickness of the piezoelectric pieces, and investigated the relationship between piezoelectric pieces thickness and output voltages. Finally, to reduce the impact on other components of the principal components and to improve the calibration accuracy, the piezoelectric balance can be optimized by modifying all kinds of parameters of the piezoelectric balance.

A rigorous and robust algorithm is presented for solving the electromagnetic scattering problems in the planar multilayered anisotropic and composite media. The Lippman-Schwinger integral formulation is exploited and integrated with the wave iterative technique in the derivation of the spectral-domain state equation. The presented algorithm relies on the accurate expressions of the dyadic Green's function and the introduction of polarization tensor. The Padua-Domínguez interpolation-integration technique is used for improving the computational efficiency. Then, the MUltiple SIgnal Classification (MUSIC) method is utilized for dealing with the inverse imaging problem for the planar multilayered composite structure. The validity of the presented algorithm and the effectiveness of the inverse imaging are verified through numerical examples.

This paper is focused on the non-iterative imaging of defects in a conductive material using pulsed eddy current testing. The approach for imaging is based on the monotonicity of the time constants of the natural modes of eddy current induction phenomenon. The hypothesis is proved numerically with the help of a finite element model. This result makes it possible to apply a real-time non-iterative imaging algorithm, initially developed for elliptic problems such as electrical resistance tomography, to a parabolic problem such as eddy current testing.