Ebook: Electromagnetic Nondestructive Evaluation (XI)

The 12th International Workshop on Electromagnetic Nondestructive Evaluation (ENDE'07) was held from 19th–21st June 2007. The Workshop was hosted by the Wolfson Centre for Magnetics at Cardiff University, Cardiff, Wales, UK with sponsorship from Cedrat SA, Serco Assurance, Rolls-Royce plc, Welsh Assembly Government, Computer Simulation Technology, The Japan Society of Applied Electromagnetics and Mechanics, Engineering and Physical Sciences Research Council. The organizers gratefully acknowledge their support.

The aim of this annual workshop is to bring together engineers and scientists from universities, research institutions and industry to discuss and exchange the latest ideas and findings in basic research and development as well as industrial applications of Electromagnetic Nondestructive Evaluation.

After the introductory welcoming remarks from Dr. David Grant (Vice Chancellor of the Cardiff University), Prof. Hywel Thomas (Head of the School of Engineering, Cardiff University) and Prof. David Jiles (Chairman of the Workshop), the technical program of the Workshop commenced with a plenary talk “NDE Research Makes a Difference” by Prof. Chris Scruby, Director, UK Research Centre in NDE, Imperial College London, U.K. Four distinguished invited speakers discussed the challenges and achievements in various fields of ENDE. Prof. J. Bowler (Iowa State University, USA), gave a talk titled “Integral methods for calculating the interaction of eddy currents with cracks”. Prof. K. Miya (Keio University, Japan), presented the second invited talk on “The Start of a New Field of Electromagnetic and Mechanical Maintenance Engineering”. Prof. G. Dobmann (Fraunhofer-Institute for Non-destructive Testing, Germany), was invited to present “Industrial Applications of 3MA – Micromagnetic Multiparameter Microstructure and Stress Analysis”. Finally Prof. N. Takahashi (Okayama University, Japan) presented an invited talk on “3D Nonlinear Eddy Current Analysis of Electromagnetic Inspection of Defects in Steel”. A total of 75 technical papers were divided into 30 oral and 45 poster presentations. The oral presentations were organized into 7 sessions covering a variety of topics on both theoretical and experimental aspects of NDE in eddy currents, magnetic measurements, magnetic flux leakage, Barkhausen methods, new methods and inverse problems for crack detection. These sessions were chaired by experts in the field including Profs. S. Udpa, L. Udpa, D. Jiles, C. Scruby, P. Nagy, T. Moses, G. Dobmann, K. Miya, S. Takahashi, A. Tamburrino and others. During closing remarks it was announced that the next ENDE Workshop (ENDE2008) will be held June 10–12, 2008 in Seoul, Korea. The ENDE Workshop 2009 will be held in Dayton, Ohio, U.S.A. The conference concluded with remarks from the chairman Prof. David Jiles.

A total of 73 participants from 16 countries were registered for the Workshop. The short versions of the papers were published in the Workshop digest and 39 reviewed full papers were accepted for publication in this proceeding. The organizers would like to thank all the participants for their contribution and all the referees for their role in reviewing the full papers. Lastly, the editors gratefully acknowledge the help and hard work of Ms. Linda Clifford in putting this volume together.

Maintenance to ensure the integrity of structures is one of the most important issues in modern industry, but more so in the nuclear power industry. This is primarily because complete prevention of all degradation of industrial materials is not possible, at least not in a realistic way. Consequently it is extremely important to detect degradation before machines start to loose their ability to function which may lead to harmful failures. Usually there is some kind of significant interval between the start of material degradation and failure; in other words, degradation usually progresses slowly with time. The problem is whether we can detect precursors of failure in the interval or not. If it is possible, we can have economic as well as safety related benefits because we are able to stop the machine before a functional failure and prevent an accident initiated by the failure of the machine. The benefits are not restricted to the nuclear power industry because it can be applied to any other heavy industry. There are many reasons why many conferences, such as ENDE, have been established to discuss and promote the progress of nondestructive inspection techniques. Whereas nondestructive inspection plays an important role in maintaining structural integrity and the performance of nondestructive inspection techniques should be enhanced in that sense, we need to regard it as one of several components composing “maintenance engineering.” Moreover, although nondestructive inspection is a proactive measure of maintenance, it is not a predictive tool. In fact, it is effective for detecting existing defects and does not say anything about the temporal evolution of defects. On the other hand, a condition monitoring system can offer significantly more useful information as mentioned above. In this paper I would like to introduce the concept of maintenology as a new science and technology in contrast to conventional maintenance engineering and to present several important results on electromagnetic maintenance for nuclear power plants as condition monitoring techniques (CMT). In particular, the introduction of electromagnetic maintenance is expected to play a very promising role in abnormality predictions of many dynamic machines that are required to be inspected regularly by law. Application of the technique would change conventional wisdom in thinking that machines should be taken apart and inspected regularly based on regulations. In many cases, this TBM (time based maintenance) is not too conservative in achieving an optimal maintenance approach.

This paper discusses the development of research in NDE and its impact on industry. Examples will be given of past research projects that have been translated into solutions to industrial inspection problems, and present day challenges to industry and the NDE research community. Recurring themes include the need for quantification, and physical models to give scientific understanding and hence improved confidence for product quality and safety-critical application. Timely technology transfer is a continuing challenge and lessons from past experience will be discussed. Finally, the author will discuss future research strategy, including opportunities for interdisciplinary collaboration in order to integrate NDE more effectively into the engineering life cycle.

Micromagnetic NDT techniques like the measurement of the magnetic Barkhausen noise, the incremental permeability and the harmonic analysis of the tangential magnetic field allow deriving inspection procedures to online monitoring and control machinery parts and components in production processes in order to characterize mechanical properties like hardness, hardening depth, yield and tensile strength. These types of inspection procedures continuously were further developed in the last two decades so that today the second generation of system hard and software is in industrial use. The application is in steel industry where steel sheets in hot-dip-galvanizing lines were annealed after cold rolling but also in heavy plate rolling mills where after thermo-mechanical rolling special textures and texture gradients can occur. An increasing number of applications are also to find in the machinery building industry and here especially in case of machinery parts of the car supplying industry. Besides mechanical hardness determination the measurement of residual stresses and the detection of inhomogeneities in the surface of machined parts is an inspection task. In different case studies the advantage to implement a micromagnetic NDE technique into the industrial processes is discussed.

The electromagnetic inspection technology is commonly used to evaluate defects in steel in various power generation plants because of its ability to detect quickly. In this paper, the electromagnetic inspection methods using an ac magnetic field, or a biased ac field are proposed. The behavior of the flux in steel is examined using 3-D FEM that takes into account the initial magnetization curve, hysteresis (minor loop) and eddy currents in order to improve the inspection method.

Magnetic minor hysteresis loops of neutron-irradiated Fe-Cu-Ni-Mn model alloys varying Cu and Ni contents have been measured. For almost alloys minor-loop coefficients which are in proportion to internal stress decrease after neutron irradiation to a maximum fluence of 0.44×10¹⁹ n cm⁻². The decrease of the coefficients is strongly enhanced for alloys with high Cu and high Ni contents. Both magnetic and mechanical properties are correlated with each other and the coefficients are roughly in inverse proportion to yield strength.

Low carbon steel specimens cold rolled at ratios of 0–40 % have been examined comprehensively by magnetic Barkhausen noise (MBN) method, and their microstructure were studied by transmission electron microscope. In order to correlate MBN parameters with those mechanical properties, Vickers hardness and ductile-brittleness transition temperature (DBTT) were also evaluated. MBN energy and rms voltage rise rapidly with cold rolling below 10 %, and saturate at higher rolling ratio. This phenomenon is attributed to the combined effects of cell texture and dislocation density. It is also found that good correlation between MBN parameters and Vickers hardness, DBTT.

Charpy impact test is explained from the viewpoint of the transition of hammer's energy to the kinetic energy of dislocations. Ductile-brittle transition temperature is &theta; is represented as a simple function of the nucleation energy of dislocations ε_n, the dislocation density ρ, the maximum density of mobile dislocations ρ₀ and the interaction energy of obstacles with dislocations U_ob; when ρ<ρ₀, while when ρ>ρ₀, where k is Boltzmann constant and ρ_m is the density of mobile dislocations. The relationship is compared with the experimental results for cold rolled S15C steels and neutron irradiated low and high copper A533B steels of nuclear reactor pressure vessel materials. The experimental results are qualitatively explained by the present model.

Depending on the neutron fluence and the special design of the pressure vessel of nuclear power plants (NPP) the microstructure of the steels change by neutron induced embrittlement. Embrittlement is on the basis of vacancies and Curich precipitates which in the size range of 1–3 nm contribute with coherent residual stresses of the 3^rd kind to an increase in hardness and strength (yield strength and tensile strength) as well as with a reduction of the upper shelf value of Charpy energy and a shift in the brittle-to-ductile transition temperature to higher temperatures. Micromagnetic investigations sponsored by the German minister of economics were performed at full Charpy specimen and material of the last generation of German NPP in order to characterize the material degradation. The contribution reports to the results obtained by the application of the Micromagnetic-, Multiparameter-, Microstructure-, and stress-Analysis (3MA) and the magnetostrictive excitation of ultrasound using an EMAT. Both technologies document potential to be further developed to an in-service inspection technique.

In this study, feasibility of evaluation for chill contents in flake graphite cast iron is investigated based on AC magnetization method, which is one of the electromagnetic nondestructive evaluation methods. Magnetic properties of flake graphite cast iron samples with different chill contents were measured by a B-H loop analyzer in order to discuss their relation to chill contents. It was found that some magnetic parameters of the flake graphite cast iron depend on the contents of matrices and graphite. Especially, the hardness, which reflects chill contents, has good correlation with the area of hysteresis loop at relatively high frequencies. Focusing on the finding, AC magnetization method was applied to evaluation for the chill contents. Through the experiment, it was found that there is a correlation between hardness of flake graphite cast irons with chill and signals. In consequence, AC magnetization method has a capability of evaluating chill contents in flake graphite cast irons.

The effects of ageing induced microstructural changes in M250 Maraging steel widely used in aerospace industries are characterized using electromagnetic nondestructive evaluation (NDE) methods. Eddy current and magnetic Barkhausen emission parameters have been studied and the results are compared with hardness, XRD and transmission electron microscopy (TEM).

The analysis of magnetic Barkhausen noise (MBN) has been used to provide information about the stress state and microstructural properties of ferromagnetic materials. Recent work has shown that a technique using acoustic Barkhausen noise (ABN) detection can provide the similar capabilities as traditional MBN along with additional information for defect characterisation and thickness measurement in a single system. Because the detection of ABN using a piezoelectric sensor can be carried out at any point on the material surface, as well as analysing ABN for microstructural characterisation, the interaction of the surface propagating waves with defects can also be analysed and used for defect characterisation, along with frequency analysis for thickness measurement. As with electromagnetic acoustic transducer (EMAT) systems, the ABN technique applies totally different physical principles to traditional ultrasonic methods and couplant is not needed for excitation. The work is carried out through experimental investigations of calibrated steel samples with machined defects using the ABN system, in comparison to readings taken using an EMAT system. Test results show that the ABN technique has potential applications in providing a comprehensive system for material and stress characterisation along with the additional capabilities of defect characterisation and material thickness measurement.

Decarburisation of high carbon steel has been simulated, using composite samples comprised of a 316 stainless steel (paramagnetic) core and a surrounding tube of ferritic steel (ferromagnetic) with thicknesses between 100 and 600 μm, for determining the potential for on-line measurement during steel processing. Decarburization samples have also been generated, for off-line measurements, by heat treatment of an Fe-0.8 wt% C steel for various times in air at 1000–1200°C. A multi-frequency (10–10⁶ Hz) electromagnetic sensor was used to determine variations in inductance (due to differences in permeability) as a function of decarburisation depth. The relationship between sensor output and decarburised layer type/thickness has been modelled using finite element software.

The marine Gears made with Case-carburised En36 steel were subjected to grinding damage during manufacturing. The MBN measurements have been made on different Gear teeth using three different methods, namely, High Frequency, Medium Frequency and Low frequency MBN measurements with the optimised measurement device and parameters so that the MBN signal from different depth ranges can be detected and analysed. The MBN measurements on these Gear teeth have shown that the grinding damage near the surface (<10μm depth) can be detected using High frequency MBN, any sub-surface damage (within ~40μm depth) can be detected using Medium frequency MBN and the damage in the deeper layers (> 40μm depth) can be detected using Low frequency MBN measurements. The thermal damage caused by Grinding Burn is also clearly revealed by the shifting of the Low frequency MBN peak to lower magnetic field.

Gears made from Case-carburised SAE8620H steel were subjected to contact fatigue testing at different contact stress levels in the Gear Test Rig facility at Newcastle University. The High frequency Magnetic Barkhausen Noise (MBN) measurements were made at different intervals of fatigue cycles. The MBN signal level gradually increases with progressive fatigue cycles and the rate of change in MBN level systematically increases with the increase in contact stress level. However, at higher contact stress levels, the MBN level quickly reaches a maximum value and then starts to decrease with further fatigue cycles. This maximum MBN level may be considered as an indication of the critical stage of fatigue life beyond which a gear may be expected to fail soon. With increasing numbers of fatigue cycles, the large MBN activity occurs at lower magnetic field strength which clearly indicates the formation of soft magnetisation regions near the surface. This may be attributed to the deformation induced transformation of paramagnetic retained austenite into ferromagnetic martensite.

This work is focused on the image reconstruction of defects in metallic materials using time-harmonic eddy currents testing measurements. The proposed approach combines an efficient numerical model of the probe-defect interaction and a least square error regularized procedure for solving the inverse problem. The method has been successfully tested to detect multiple volumetric defects on multiple layers of a planar conductive material. The regularization term is the total variation, selected to “encode” the a priori information that defects are supposed to be “blocky”. In this manner, the method is potentially capable of treating on the same foot several defects type such as EDM, fatigue crack, stress corrosion crack etc.

The authors present a signal processing method dedicated to the detection of defects buried next to rivets in aeronautical lap joints. The method is based on a multi-frequency principal component analysis and is applied to the images provided by an original eddy current imager. The optimization of the method is carried out thanks to an experimental approach, and validated with the detection of buried defects, ranging from 2mm to 8mm long and 2mm to 8mm deep. An extension of the method to a classification scheme is also considered.

The authors present a multi-coil EC sensor dedicated to the rapid inspection of aeronautical riveted lap joints, and compare the efficiency of two signal processing methods based on principal component analysis and independent component analysis, to enhance the detection of buried defects appearing next to the rivets.

Features are investigated in eddy current data that are sensitive to corrosion and fatigue cracks in airframe structures while invariant to other NDE noise factors. To investigate subsurface corrosion characterization at the faying surface, a series of eddy current studies were performed using an analytical model for varying total subsurface thickness loss and percentage of the thickness loss occurring in each layer. Results for the simulated studies are presented demonstrating a novel feature for corrosion characterization using first and second order derivatives of the impedance response with respect to frequency. For characterization of subsurface cracks around fastener holes in structures, numerical simulations and experimental studies are presented. Unique features in the measurement response in circumferential direction were found to be sensitive to subsurface cracks around fastener holes and invariant to irregular geometric factors such as fastener fit and probe tilt. Multifrequency eddy current data combined with circumferential (spatial) measurement features were found to be promising for characterizing subsurface cracks in terms of length and depth.

This paper considers the inverse problem of estimating the depth profile of an unknown defect from measurements of transfer resistance. The results both finite-element analyses and experimental tests on specimens with notches of various shapes and sizes were used to develop a simple algorithm that allows a good reconstruction of the depth profile. A synthetic focusing technique is applied which improves the quality of the reconstruction.

A novel method for the automatic classification of defects using magnetic flux leakage inspection is presented. A technique based on geometric measures to distinguish between different defects due to petro-chemical tank corrosion is presented. In order to characterize a defect, a process of feature extraction is proposed. Principal component analysis is then used to select the most powerful set of features.

The performance is compared using two different methods: k-nearest neighbor and support vector machine. The results show an accuracy of 91% with which automatic classification is possible on unseen test examples on steel plates.

A circular electromagnetic TE11-mode is used for detection of longitudinal crack in a stainless steel pipe. To show the capability of microwave nondestructive testing for detection of cracks in large and complex piping systems such as steam generator (SG) tube, inspected pipe including U–bend are examined in this study. The crack location is determined through time of flight (TOF) of the reflected wave and group velocity of electromagnetic signal. The TOF is evaluated in the time domain via Inverse Fast Fourier Transform (IFFT) of the wave spectrum. Then, two different methods of signal processing are applied to obtain TOF more accurately.