Showing papers on "Eddy-current testing published in 1992"

Eddy currents: theory and applications

Abstract: The theory and applications of eddy currents induced in conducting materials by time-varying magnetic fields are reviewed. The mathematical methods employed in solving the relevant problems are presented. Both analytical and numerical methods are described. Applications based on effects arising from eddy currents are discussed in detail. These applications are to magnetic levitation, electromagnetic launching, hyperthermia treatment of cancer, and nondestructive testing. >

Eddy current inversion for layered conductors

Abstract: By making multifrequency eddy current measurements on a layered conductor, it is possible to acquire information on the depth dependence of the conductivity. We consider an inversion problem in which coil impedance data are used to determine either the layer thicknesses or layer conductivities. The algorithm is based on a well known forward model which gives the impedance of an air cored coil above a stratified conductor from a closed form expression. In the forward calculation, estimates of the unknown material parameters are used to get tentative predictions of the measurements. Differences between these predictions and measured impedances are expressed in terms of a global error that is minimized iteratively with the aid of a descent algorithm by varying the parameters of the structure. Examples of minimization searches for layer parameters are given.

Numerical and experimental analysis of eddy current testing for a tube with cracks

Abstract: A novel method for calculating the impedance signal in eddy current testing (ECT) for a tube with surface cracks is developed using the two-dimensional current vector potential method. Three-dimensional current flow around a surface crack is approximated by introducing quasi-conductivity in the crack region. A final matrix relating the current vector potentials is obtained by decreasing the width of the crack to zero, and then the elements in the crack region are not necessary any longer. Numerical results show good agreement with the experimental ones, which indicates the validity of the proposed method and the possibility of designing a more efficient probe of ECT using the code. >

Eddy-current imaging of buried cracks, by inverting field data

Abstract: A novel method for imaging flaws, based on the inversion of eddy-current data, has been demonstrated. A thin subsurface slot was manufactured in an aluminum plate. Oscillating current was injected into the plate, and the magnetic field was sampled with a Hall transducer scanned over the blind face. An integral operator defines an analytical relationship between the distribution of electric dipoles excited on the slot faces and the scattered field. The relationship was inverted using singular value decomposition to obtain the dipole density as a representation of the slot in the form of an image. Enhanced spatial information on the flow may be obtained by performing the inversion at several individual frequencies, and constructing a composite image. >

FEM calculation of eddy current losses and forces in thin conducting sheets of test facilities for fusion reactor components

Abstract: An efficient finite element formulation of the eddy current field in thin conducting sheets has been developed coupled to a description of the magnetic field by a gauged magnetic vector potential The numerical problems associated with the thin conductors have thereby been eliminated Results of computations on a model of the TOSKA Upgrade test facility are presented The aluminium radiation shields are not in danger; the stresses acting upon them are negligible This is due to their being located relatively far from the superconducting coil Only parts of the supply lines near the connecting flange of the vacuum vessel experience considerable stress Their reinforcement under rapid changes may be necessary The vacuum vessel is not endangered, since its wall of 20 mm thickness easily withstands the stresses due to the induced eddy currents Normal pressures of up to about 50 t/m/sup 2/ occur at the LN2 shield This makes modifications of its structure necessary >

Numerical model based approach for estimating probability of detection in NDE applications

Abstract: A method for using a numerical model in a probabilistic framework for estimating the probability of detecting a flaw in the presence of measurement variabilities is presented. The approach used centers around a measurement model which is based on the finite-element analysis of the eddy current nondestructive testing (NDT) phenomenon. The probability of detection (POD) model consists of modeling the different sources of measurement variabilities by a multivariate Gaussian distribution about a mean vector and a diagonal covariance matrix. The effects of various sources of noise in a practical eddy current NDT situation have been studied. Results obtained show that the probability of detecting a flaw is decreased as a consequence of the measurement variabilities. >

Eddy current inspection with stationary magnetic fields and scanning sensor arrays

Abstract: Defects such as corrosion located on a covered metal component are detected through Eddy current inspection using stationary magnetic fields and scanning sensors arrays. A magnetizing yoke is held fixed to the cover of the component and a magnetic flux sensor is used to scan the area between the legs of the yoke over the cover of the component. Also, a fixed array of sensors can be provided between the legs of the yoke for automatic multiplexing of the sensors and sampling in sequence of signal responses picked up by sensors in the sensor array. Additionally, a stationary magnetic field arrangement is provided by a plurality of probes aligned adjacent and parallel with each other. A fixed array of magnetic flux sensors are arranged between the legs of the aligned probes for scanning for defects. This arrangement of fixed sesnsors can be used for underwater use and sealed by a sealing compound. An attachment means can be used for holding the arrangement stationary with respect to the surface of a submerged component during sampling of the signal responses.

Inspection unit, e.g for non=destructive testing of aerospace vehicle structural parts, - contains x=ray, ultrasonic eddy current and safety test systems controlled by portable computer

Abstract: The inspection unit has a mobile housing contg. a portable computer (r) which controls an x-ray test system (X), an ultrasonic test system (US) integrated into the computer, an eddy current test system and a safety test system (VIS) with video camera. Test results in the form of images and test data are placed in memory with related explanatory text and printed out in facsimile compatible form or written onto diskette for data transfer. USE/ADVANTAGE - E.g. for non-destructive testing of aerospace and motor vehicle components. Rapid testing is achieved with well-documented, comparable test results.

Application of 3D eddy current analysis on magnetically levitated vehicles

Abstract: The eddy currents induced on the superconducting magnet (SCM) vessels of magnetically levitated vehicles (MAGLEV) have been analyzed. A 3D eddy current analysis code, based on a finite-element method (FEM) with thin shell approximation, is developed and verified through a mock-up SCM experiment. Through a coupled electromagnetic and mechanical analysis under SCM vibration, a SCM structure with low-resistivity material coating on the inner vessel of SCM is found to be suitable for the significant reduction of helium evaporation due to eddy current loss. >

Remote Field Eddy Current Testing of Ferromagnetic Double Wall Tube

Abstract: Ferromagnetic double wall tube has been under development to ensure the high reliability of the steam generator in fast breeder reactor. The airgap between the tube two walls should prevent the conventional nondestructive test methods from detecting defects especially in the outer tube wall. Thus the development of the in-service inspection technique of the double wall tube has become a necessity in the field. The authors have investigated the feasibility of applying the remote field eddy current technique (RFECT)to the in-service inspection of the ferromagnetic double wall tube. The energy flow and the distribution of low frequency electromagnetic field in the RFECf have been derived through finite element analysis. It has been shown that defects in both inner and outer tube walls can be detected with almost the same defect indication by RFECT if the air gap between the ube walls is as small as 0.04 mm.

Three dimensional hybrid finite-boundary element model for eddy current NDE

Abstract: A three-dimensional hybrid finite-boundary element formulation of eddy current nondestructive evaluation (NDE) is presented. The vector potential formulation considered is suitable for combining the boundary-element method (BEM) and finite-element method (FEM), because the linking conditions are expressed by the tangential component of the magnetic field for both methods. Even though there are four unknowns in the FEM formulation, it is attractive because it is easily implemented. The uniqueness of the vector potential is guaranteed and regions with different conductivities or nonlinear permeability can be modeled. The results indicate the feasibility of using the formulation for 3-D eddy current NDE applications. >

Eddy current testing of carbon-carbon composites

Abstract: The objective of this project is the evaluation of eddy current methods for monitoring the electrical conductivity of carbon-carbon composite materials during high-temperature pyrolytic processing. During the processing changes in conductivity of the order of 100% have been reported [1]. Changes in conductivity due to temperature are also of this order of magnitude. The principal results of this investigation will be briefly outlined now.

Skin Depth Considerations in Eddy Current NDT

Abstract: Eddy current nondestructive testing depends upon the interaction of time-varying electromagnetic fields with the material under test. The electromagnetic fields are applied to the material under test via some finite sized transducer, usually an inductive coil or set of coils. If the material is conducting the fields will penetrate the conductor, but will attenuate to negligible levels after some distance. The classical skin depth, \({\delta _s} = \sqrt {2/\omega \mu \sigma } = \sqrt {1/\pi f\mu \sigma }\) is the standard assumption for the characteristic distance of field penetration in conductors. This paper reports the results of a study which examined the decay of sinusoidal steady-state (AC) fields in conductors induced by finite sized coils. Comparisons are made among the classic Dodd and Deeds formulations [1], the 3D-axisymmetric finite element method (FEM) [2], and published results [3]. Where possible experimental observations were compared to the computed and published results.

Influence of speed effect in eddy current nondestructive testing

Abstract: Eddy current testing has been used for high speed nondestructive testing of materials with simple shapes such as bars, tubes, and wires. It is known that the high relative speed between the test coil and tested material has some influence on the test indications. There have been many reports on the speed effect in eddy current testing till now. Those reports have mainly clarified the quantitative speed effects for a certain test condition and the fundamental characteristics of the speed effect has remained obscure. The authors have investigated the effect by combining Doppler effect and the phase velocity of electromagnetic field to qualitatively elucidate the speed effect in eddy current testing. The investigation leads to the following facts. The speed effect is determined by the ratio of the test speed to the phase velocity of electromagnetic field diffusion in the tested material. The higher the frequency of eddy current testing is, the smaller the speed effect is.

Calculation of the alternating magnetic field near a metal surface

Abstract: A medium-addition method and its recent development are recommended for calculating the 2-D alternating magnetic field in the vicinity of a defect. A fictitious medium with negative conductivity is suggested to simulate the defect. The prominent advantages of this method are that it requires much less computer capacity and is time saving. It is concluded that a medium-addition method can be used to solve certain electromagnetic field problems in nondestructive testing if the media are linear and the defect is a tiny crack or a small hole. >

Three-dimensional analysis on eddy current testing for S/G tubes

Abstract: The authors describe the application of a three-dimensional eddy current analysis to the eddy current testing for a PWR steam generator tube. A sinusoidal steady-state eddy current analysis code has been developed using an FEM-BEM (finite element method-boundary element method) coupling technique. A comparison is made of experimental and theoretically predicted probe impedance trajectories for a major problem in steam generator testing. The results show the feasibility of the analysis code in a practical situation. >

Analysis of a problem in non destructive testing

Abstract: The TEAM Workshop Problem 'Coil above a Crack, a Problem in Non Destructive Testing' is solved analytically by using the principle of superposition for the linear system and the Fourier transform method. The solution variables are the magnetic vector potential and the electric scalar potential. Computed results agree well with the experimental results given by T. Takagi et al. (1990). >

Application of Automated Eddy Current Techniques for Off-Shore Inspection

Abstract: Publisher Summary This chapter presents the application of automated eddy current techniques for offshore inspection. The nondestructive in-service inspections of the weld zones are necessary to detect fatigue cracks in an early state before they increase to a critical size. These cracks are resultant of high stress concentration factor and changes of the structural condition in weld area. The most applied techniques for the detection of cracks in weld seams of underwater constructions are the magnetic particle test methods and the ultrasonic technique. The advantage of the magnetic particle testing technique is the high detection capability in the indication of cracks in the weld seam and the simple evaluation of the results, but the disadvantage is the fact that the area to be inspected has to be cleaned accurately. The ultrasonic technique is applied easily in remote controlled manipulators and is often used in such systems. The main disadvantage of the ultrasonic technique is the complicated and permanent changing geometries of the three dimensional structure, leading serious problems in the evaluation of the signals. The chapter also presents the eddy current testing method. It is a traditional technique for the detection of surface cracks that is applied in offshore for a very short time, because the signals of weld seam roughness and the changes of magnetic permeability variation, and electric conductivity caused by the welding process superpose the crack signal that reduces the detectability of cracks. Experiments showed that an inspection of weld seams of offshore constructions by means of eddy current testing methods detects fatigue cracks with the same grade of reliability as the magnetic particle technique.

Measuring the homogeneity of ceramic superconductors by eddy current probing

Abstract: The homogeneity of a bulk YBa2Cu3Ox sample prepared by standard methods has been determined. Measurements of the conductivity, Tc (by induction), and x‐ray analysis showed no deviation from bulk material previously characterized as ‘‘homogeneous’’ in the literature; the sample was shown to be inhomogeneous by the eddy current method. We use the sample’s response to multifrequency eddy current probing as a measure of homogeneity. This response is calculated from measurements of the complex impedance of a coil, which contains the sample, as a function of frequency. The homogeneity of the sample after being quenched in air from three successively higher temperatures (650, 700, 800 °C) has also been determined. A rough conductivity profile in one dimension is obtained for all four sample conditions.

Electromagnetic forces and applications : proceedings of the Third International ISEM Symposium on the Application of Electromagnetic Forces, Sendai, Japan, 28-30 January 1991

Abstract: Abbreviated Micro- and Piezoelectric Actuators. Microactuators in LIGA technique (W. Menz). Characteristics and vibrational behaviors of a rod type ultrasonic motor using longitudinal and torsional vibrations (M. Aoyagi et al.). A card sending linear ultrasonic motor using multi-beam piezoelectric vibrators (T. Kosawada et al.). Magnetic Fluids and Melted Steel. Recent progress of electromagnetic processing of materials (S. Asai). Influence of concentration on field-induced phase transition in magnetic fluids (R.E. Rosensweig, J. Popplewell). Effect of magnetic fields on gas-liquid two-phase pipe flow of magnetic fluid (S. Kamiyama et al.). Oscillatory flow of magnetic fluid in a vertical pipe in a transverse and longitudinal magnetic field (K. Shimada et al.). Magnetic Bearings. The father of magnetic bearings: Professor Jesse W. Beams (P.E. Allaire et al.). Robust control of magnetic bearing systems by means of sliding mode control (K. Nonami, H. Yamaguchi). Nondestructive Testing Using Electromagnetic Phenomena. Electromagnetic field modeling for nondestructive testing of composite materials (N. Ida). Numerical and experimental analysis of eddy current testing based on T-method (Y. Yamada et al.). Advanced Computational Electromagnetic Phenomena. Electromagnetic field computation from DC to daylight (C.R.I. Emson, C.W. Trowbridge). Terminal boundary impedances for short and long slots in finite element electromagnetic field computation (S.R.H. Hoole, S. Sukumar). Magnetic Levitation and Application of Superconductivity. Electromagnetic levitation of elastic plates (T. Shimogo, Y. Oshinoya). Study of electromagnetic force on superconducting magnets of the magnetically levitated train (T. Kobayashi et al.). Electromagnetic Materials and Wave Propagation. Pseudomomentum and computation of material forces in electromagnetic materials (G.A. Maugin). Dynamics of the elastic multi layered sheet with a superconductive electrical current (K.B. Kazarian, R.A. Kazarian). Electromagnetomechanical Interaction. A calculating method of the deflection and eddy current of conductors in a time varying electromagnetic field (Y. Tanaka). Linearized equations for SAW in stressed media. Force sensor (D. Gafka). Various Problems of Electromagnetic Forces. Dynamics of dislocations in ferroelectric crystals (J.P. Nowacki, Z. Pretczynski). Quick stopping system of high-speed movement by electromagnetic force and its applications (I. Aoki, T. Higuchi). Author index.

Eddy Current Test of Fin Tubes for a Heat Exchanger

Abstract: Eddy current probes were designed to be suitable for the test of fin tubes. Fin tubes, often used for heat exchangers, have uneven outer and inner surfaces to enhance the heat emission. The surface roughness makes it difficult to detect flaws employing eddy current test (ECT). In order to overcome the difficulties we performed two kinds of works, one is the development of ECT probes, and the other is the signal processing including fast Fourier transform and digital filtering. In the development of ECT probes, we adopted empirical design method. Our ECT probes for the fin tubes are inside type. And we are specially concerned about geometric features such as the widths of the coils composing an ECT probe. We fabricated four probes with various coil widths. Eddy current test was performed using those ECT probes on artificially flawed specimens. After analyzing the output signals, we found that, for the effective testing, the width of a coil should be determined considering the pitch of the fins of a tube. And we also learned that the frequency filtering could improve the s/n.

Method for measuring surface shape of conductor

Abstract: PURPOSE:To measure the surface shape of a conductor with high accuracy even under a bad condition where lift-off variation or a disturbed magnetic filed exists. CONSTITUTION:By respectively connecting two coils 3a and 3b for measuring eddy currents to the opposite sides of a bridge circuit in a differential type, the detecting sensitivity of eddy current test equipment is corrected against lift-off variation after obtaining the average moving amount of an impedance difference at scanning time detected by the eddy current test equipment. Therefore, the surface shape of a conductor can be measured with high accuracy even under a bad condition where lift-off variation or a disturbed magnetic field exists by sharply suppressing the influence of noise caused by the disturbed magnetic field and correcting the variation of detection sensitivity caused by the lift-off variation.

Eddy current impedance imaging in butt weld joints of AISI stainless steel plates

Abstract: Eddy current impedance imaging (ECII) is a recently emerging trend in the area of non-destructive testing. Compared to conventional eddy current testing (ECT), ECII gives a global perspective and allows a balanced interpretation. Imaging techniques have the potential of automating the measurement process and providing estimates of defect size from the image data. This paper describes the work carried out at the authors' laboratory with respect to development of the ECII technique as a tool for in-service inspection of welds. Details of the ECII system, implications of probe characteristics and some relevant applications to weld joints are presented.

Modelling of flaw responses in eddy current testing

Abstract: This paper presents the practical configuration for detecting cracks in material, by applying an electromagnetic field along the largest dimension of the crack. An electromagnetic field formulation is proposed using Helmholtz's equation and Biot‐ Savart's law. The system equation is solved by using the finite element method (FEM). The exemplary results of calculation ‐ eddy currents lines in material and relative resistance versus probe position are presented.

Fast Pattern Recognition Method for Eddy Current Testing

Abstract: One of the benefits of eddy current (EC) testing is the attainability of high testing speeds while maintaining high sensitivity and requiring little with regard to material preparation. For this reason it is commonly automated and integrated in production lines of semi-finished products such as bars, tubes and wires. Because of the requirements of high throughput on-line digital analysis of EC signals is rarely applied, in contrast to ultrasonic testing. However, the usual methods of analog filtering and phase-selective or phase-insensitive threshold evaluation of EC signals are limited in regard to the suppression of false or pseudo-defect indications, classification of defect types, quantitative assessment of defect features and the suppression of signals originating from manmade structures.