Showing papers on "Eddy-current testing published in 1999"

Review of Advances in Quantitative Eddy Current Nondestructive Evaluation

Abstract: A comprehensive review of advancements in eddy current (EC) modeling is presented. This paper contains three main sections: a general treatise of EC theory, the thin skin EC forward modeling, and the EC inverse problem. (1) The general treatise of eddy current theory begins with an exposition of the reciprocity formulas for evaluating probe impedance changes, which are derivable from first principles. Two versions of the reciprocity formulas, one with a surface integral and the other with a volume integral, are given. Any particular type of defect, as well as both one-port and two-port probes, can be treated. Second, a brief account of analytical and numerical methods for calculating the field distributions is presented. Third, theory of probe/material interactions with various defect types is described. (2) The paper then proceeds to the forward modeling section, which contains a detailed treatment of the eddy current forward problem for surface breaking cracks and EDM notches in the thin skin approximation. (3) The inverse problem section begins with a general review of commonly used inversion methods, exemplified by selected references from the literature, followed by more detailed examinations of EC inversions for surface breaking cracks and slots. The last part of this section is devoted to the inverse problem for layered structures. Although being a review in nature, the paper contains a number of new accounts for time-domain eddy current interactions. In particular, a modification is proposed to the reciprocity formula in order to take a better account of pulsed eddy current signals.

NDT data fusion at pixel level

Abstract: No single non-destructive testing (NDT) technique or method is satisfactory to fully assess the structural integrity of a material. Indeed, each method presents some limitations in terms of defect detection and characterisation. Additionally, poor signal-to-noise ratio may make signal interpretation complex or unreliable. For these reasons, the concept of NDT data fusion — based on the synergistic use of information from multiple sources in order to facilitate signal interpretation and increase defect detection and characterisation — is expanding rapidly. A carbon fibre reinforced composite material previously subject to low energy impact damage has been non-destructively inspected using eddy current and infrared thermography. Following these non-destructive examinations, the information gathered from these two NDT methods has been fused at pixel level. Several well established and recognised data fusion processes based on statistical and probabilistic algorithms were used to combine eddy current and infrared thermographic data. The results obtained with these processes are presented in this paper and their efficiency discussed. It is shown that, in certain cases, NDT data fusion at pixel level may be adequate to increase knowledge about defect location and characterisation and to reduce ambiguity.

Rapid prediction of eddy current testing signals using A−φ method and database

Abstract: This paper describes a new scheme for the fast evaluation of eddy current testing (ECT) signals by using databases and formulation of the A−φ method. As the calculation of flaw signals is localized around the flaw region, substantial computational work can be reduced comparing with the conventional FEM–BEM code even for a conductor with complex geometry. Unlike the approach proposed by Badics et al. (Rapid flaw reconstruction scheme for 3-d inverse in eddy current NDE. In Studies in Applied Electromagnetics and Mechanics, Vol. 12, eds T. Takagi et al., IOS Press, 1997, pp. 303–309), the new scheme can predict ECT signals due to a large and complex-shaped crack rapidly because of its higher order interpolation and the newly introduced special element, say, an element with different media. The efficiency of the new method is verified by comparing its numerical results with the measured impedance for several bench-mark problems. The fast and accurate features make this new forward approach especially feasible in the reconstruction of flaw shapes by combining with the conventional optimization method.

Crack detection and recognition using an eddy current differential probe

Abstract: This paper proposes a new eddy current differential sensor and a system for multi-frequency testing of conducting plates. Precise crack imaging was achieved by the use of spectrograms obtained from an eddy-current probe multi-frequency response and application of a neural network. Results of experiments with test specimens made of SUS304 showing very good sensitivity and spatial resolution are presented. The possibility of detection of an opposite side 20% crack was also confirmed.

Flaws characterization with pulsed eddy currents NDT

Abstract: In this paper, we present a method for characterizing flaws in metallic structures with pulsed eddy current nondestructive testing (NDT). In a first step, we explain why we have chosen a deconvolution method based on conjugate gradient algorithm (CG). After a description of this method and the way it is used for characterization, a validation is performed by coupling it with a 2D FEM (2 dimensional finite elements method) package. The obtained results are compared with experimental measurements. In the last step, a parametric study on parameters obtained with the impulse response gives an estimation of the dimensions of the flaw.

Eddy current probe with multi-use coils and compact configuration

Abstract: In an eddy current testing probe, efficiency is improved by multiplexing, at the probe head, signals to and from a large number of coils on the probe head onto a reduced number of signal conductors, and by reusing coils in different measurement configurations in order to provide a higher density of inspection coverage.

Inspection of aircraft parts with high remanent magnetization by eddy current SQUID NDE

Abstract: We have developed an eddy current NDE system based on HTS rf SQUIDs as magnetic field sensors. Due to their high field sensitivity even at very low frequencies, SQUIDs are especially suitable for applications where a large eddy current penetration depth is required. We have used 3 GHz rf SQUIDs in our system made from YBCO thin films with a field resolution of about 1 pT//spl radic/(Hz) and a high dynamic range, more than 140 dB//spl radic/(Hz) in unshielded environment. With this system we could detect deep lying (up to 26 mm) cracks in test samples and original aircraft parts. Measurements made on the same sample showed an improvement in the signal to noise ratio of up to 3 orders of magnitude for cracks deeper than 13 mm, when comparing a conventional eddy current NDE unit with our SQUID system. Test objects containing ferromagnetic structures with a high remanent magnetization (often more than 1 mT), like aircraft wheels or steel bolts in part of the aircraft wing, very often cause instabilities of the flux locked loop operation of the SQUID. To prevent unlocking, we have developed a new background field compensation scheme. Special compensation electronics takes care of slowly varying magnetic fields of up to 1 mT/s and enabled us to perform eddy current measurements in presence of slow (<30 Hz) background field variations of up to 5 mT.

AC fields around short cracks in metals induced by rectangular coils

Abstract: In the alternating current field measurement method, information about the magnetic field distribution above the metal surface is used to detect and size surface-breaking cracks in metals. In this measurement, the incident field is produced by a set of wires carrying a high-frequency current. This paper presents a mathematical technique for the theoretical prediction of the distribution of the magnetic field around a short crack due to a current-carrying rectangular coil located horizontally above the metal surface. The crack is assumed to have a profile that can be approximated by a rectangular boundary. The mathematical technique is developed and its experimental verification is discussed. The technique, an extension of one recently reported by the authors, is accurate and computationally efficient. The effects of crack length and depth on the crack signal, obtained theoretically, are illustrated and discussed.

Eddy current offers a powerful tool for investigating residual stress and other metallurgical properties

Abstract: Residual stress alters the natural conductivity of a metal only slightly; typical values of {Delta}{sigma}/{sigma} range from 5 to 100 parts per million (ppm). To detect such small effects, an eddy current bridge must be extremely stable and free of thermal or noise effects. By analyzing the conductivity as a function of frequency the residual stress distribution in the surface layers can be compared to standard samples whose stress distribution is known from X-ray analysis. It is not yet determined what the relationship between conductivity and stress is, only that it is repeatable. Many different shot peened metals have been measured successfully. Changes in conductivity which produce changes in the eddy current signatures have also been observed in samples of welded aluminum, heat treated steel, and others.

Two-Dimensional Eddy Current Signal Enhancement via Multifrequency Data Fusion

Abstract: Eddy current testing is one of the most popular nondestructive testing methods; a multifrequency testing is often applied to cancel the unwanted signals to improve the signal-to-noise ratio. This is usually accomplished by combining the results obtained at different frequencies in the spatial domain. In this paper a fusion strategy to integrate two-dimensional, multifrequency signals is introduced, which is based on the multiresolution analysis method. A signal-to-noise ratio criteria is adopted to evaluate the fusion results. Finally, the effectiveness of the pyramid method is discussed.

A Parametric Estimation Approach for Groove Dimensioning Using Remote Field Eddy Current Inspection

Abstract: The remote field eddy current technique is used for dimensioning grooves that may occur in ferromagnetic pipes. We propose a method to estimate the depth and the length of corrosion grooves from the measurement of a pick-up coil signal phase at different positions close to the defect. Groove dimensioning needs the knowledge of the physical relation between measurements and defect dimensions; therefore, finite-element calculations are performed to obtain a parametric algebraic function of the physical phenomena. The parameters of this model function are obtained by an optimization technique. By means of this model and a previously defined general approach, an estimate of groove size may be given. In this approach, algebraic function parameters and groove dimensions are linked through a polynomial function. To validate this estimation procedure, a statistical study has been performed. The approach is proved to be suitable for real measurements.

Reconstruction of Cracks with Multiple Eddy Current Coils Using a Database Approach

Abstract: This paper presents some recent studies dealing with flaw reconstruction using eddy current testing. The locations of parallel planar cracks in an Inconel 600 plate specimen are determined and the shapes of them are reconstructed at the same time using the signals from a multi-pancake coil probe. The reconstruction strategy, consisting of a data base used fast forward solver and a first order optimization algorithm, relies on the minimization of the nonlinear least square residual function. Validity of the inverse analysis method is investigated with respect to the influence of the arrangement of scanning paths. The efficiency of the computation is also discussed.

Artificial neural network application for material evaluation by electromagnetic methods

Abstract: The eddy current nondestructive testing of conductive materials is well known problem. For some eddy current transducers for flaw detection the mathematical models were constructed and the inverse problem (IP) were formulated In order to solve those problems the artificial neural network approach, relatively new, was adopted. The public domain software was used to teach and test the network. It was proved that, a standard feedforward multilayered neural network is sufficient to identify the parameters of different kind defects with a satisfying accuracy. Neural network approach could be extinguished to impedance tomography and to eddy current tomography. Several examples illustrated abilities of proposed approach will be presented in the paper. Two artificial neural network reconstruction methods for electrical impedance tomography have been presented in this paper. The problem under study concerns the reconstruction of the conductivity distribution inside the investigated area using the information collected from the boundary. The first approach consists in ANN learning using electrical potential vectors, which were obtained from numerical solution of the forward problems. The second method using a standard feedforward multilayered neural network applies the circuit representation for the finite element discretization. Using the quadrilateral finite element, the neural network structure for EIT problem has been proposed. The advantages and disadvantages of both methods with respect to the classical approach have been discussed in details.

Fast signal predictions of noised signals in eddy current testing

Abstract: This paper describes a new method for the simulations of signals noised by the presence of other materials except for test articles in eddy current testing (ECT). In the inspection of steam generator tubes in nuclear power plants, the sensor detects structures, for example support plates to fix the tubes, and deposits adhering to their outside, as well as tube defects. The signals from the structures are thought to be noises when finding the defects. The method developed here is based on a set of governing equations constructed with magnetic vector potentials, introducing a precondition that a small part of the inverse matrix of the coefficient matrix in an algebraic equation from finite elements and the potentials for the unflawed conductor model are stored. Following the reciprocity theorem, an impedance change due to a flaw, as an eddy current signal, can be computed through integration in the flaw region, and unknowns corresponding to the flaw region only are considered. It results in much smaller degrees of freedom than typical finite element approaches require, and the method provides a fast forward simulator. The fast forward simulator is tested with experimental measurements of ECT benchmark problems with noise factors mentioned above, and is compared with a conventional approach from the point of view of computational costs.

High-Tc SQUID systems for practical use

Abstract: Planar thin film DC-SQUID gradiometers with small base length (4...6 mm) were optimized for application in different measurement systems. The field gradient noise of these planar DC-SQUID gradiometers in unshielded environment is better than 5 pT/cm/spl radic/(Hz) (at 1 Hz, 77 K). Other components of these systems like electronics, cryostats, scanning equipment, and data analysis software are discussed. An industrial system for nondestructive evaluation and a clinical system for bedside investigations on cardiac infarction are demonstrated as examples of starting cooperation between university and small companies interested in application of superconductivity.

Experimental and numerical results of electromagnetic nondestructive testing with HTc SQUIDS

Abstract: We present here recent results on detection of surface and subsurface artificial features in Al-Ti planar structures, to show current performance of our eddy-current nondestructive evaluation system based on HTc SQUIDs. The anomalous magnetic fields generated by flaws with known electromagnetic characteristics have been modeled by three-dimensional codes based on finite element method and volume integral formulation and developed for the investigated problem. Both numerical solutions have correctly predicted the shape of the complicated magnetic field response which is mainly the result of the shape of the defect, the geometry of the inducing coil and the characteristics of the SQUID gradiometer.

Numerical calibration of fluxset probe for quantitative eddy current testing

Abstract: Fluxset type eddy current probes are used for detecting discontinuities in conducting materials. The measurements obtained by such probes can be used for the reconstruction of the parameters of the detected discontinuity if the output signal of the measurement is uniquely related to the measured field. In this paper a calibration method is presented for the establishment of the relation between the measured magnetic field and the output signal of the probe. The relation is obtained by the optimisation of the parameters of the mapping between the calculated magnetic field distribution and the measured output signal. The magnetic field distribution due to the interaction of the probe and an infinitesimally thin crack located in a conducting plate is calculated numerically by the solution of a boundary integral equation.

Eddy Current Testing

Abstract: The sections in this article are 1 Principles of Eddy Current Testing 2 Applications of Eddy Current Testing 3 Prospective Development of Eddy Current Testing

Compensation electronics for larger dynamic range of a SQUID based nondestructive evaluation system

Abstract: We have developed a compensation system for any given SQUID sensor which allows sensitive eddy current measurements above 100 Hz in the presence of strong and slowly varying background fields. High Tc SQUIDs have been used successfully in nondestructive evaluation (NDE) systems based on eddy current excitation when searching for defects in conductive samples such as aircraft parts. Due to their high and frequency independent field resolution and their excellent spatial resolution, SQUIDs provide in the case of deep lying defects—compared to other conventional electromagnetic NDE systems—a more reliable crack detection. Fast readout electronics having an unsurpassed dynamic range of up to eight orders of magnitude enabled us to perform measurements in an environment polluted with electromagnetic noise, e.g., an aircraft hangar. Nevertheless, test objects containing ferromagnetic structures with a high remanent magnetization, such as aircraft wheels or steel bolts in an aircraft wing, very often cause insta...

A measurement system based on magnetic sensors for nondestructive testing

Abstract: The paper deals with a measurement system based on a low-cost eddy current probe for nondestructive testing on conducting materials aimed at reconstructing the shape and position of thin cracks. The magnetic probe is characterized, highlighting good repeatability, linearity, and overall accuracy. A number of different measurement approaches are investigated, in order to choose the more appropriate for NDT applications. A numerical method was then illustrated; it proves to be able to reconstruct cracks also starting from noisy measurement data.

Rotating Field Eddy Current Probe for Characterization of Cracking in Non-Magnetic Tubing

Abstract: A rotating field eddy current probe was built and tested for use in small diameter, non-magnetic tubing. The rotating field probe is a driver/pickup style with two orthogonally wound drive coils and a pancake pickup coil. The driver coils are excited by two sine waves 90{degree} out of phase with each other. The physical arrangement of the drive coils and the 90{degree} phase shift of the excitation waveforms creates a field which rotates in the test piece under the drive coils. Preliminary tests on electrical discharge machined (EDM) notches show that phased based estimates of notch depth are possible. Probes currently used for detection of cracks in tubing produce responses that have proven unreliable for estimating defect depths. This recently developed version of the rotating field eddy current probe produces a bipolar response in the presence of a crack or a notch. Typically, the phase angle of a bipolar eddy current response is easily identified and measured and is used extensively for estimating depths of volumetric defects. Data are shown relating the phase angle of the rotating field probe`s bipolar response to the depth of circumferential EDM notches.

A family of matrix type sensors for detection of slight flaws in conducting plates

Abstract: The purpose of this paper is to experimentally demonstrate advantages of the new matrix type sensors useful for testing of planar specimens made of Inconel 600. A computer controlled ECT system used for multi-frequency testing is also described. The extended experiments with the sensors were performed and their results are presented. Results of the comparison of the sensors are given.

Finite element analysis of a moving magnetic flux type sensor developed for nondestructive testing

Abstract: This paper presents a magnetic field analysis of a moving magnetic field type sensor with shading coils developed for eddy current testing. The cracks and defects that are very difficult to detect with the conventional method, can be detected by using this sensor. In this paper, we have carried out the magnetic field analysis with the 2-D and 3-D finite element methods to show the effectiveness of this sensor.

Pulsed Eddy Current Method: an Overview

Abstract: As the name implies, the pulsed eddy current (PEC) method uses a transient excitation of an eddy current probe, as opposed to the continuous-wave (CW) excitation used in conventional eddy current inspection The PEC approach offers several advantages over the conventional method First, because the transient field is related, by means of a Fourier transform, to the CW field at multiple frequencies, a single PEC measurement is equivalent to multiple CW measurements as a function of frequency PEC instrumentation is, however, much simpler and less expensive, and PEC data acquisition is much faster than multiple CW measurements The transient method also offers the possibility of time-gating, which enables the user to distinguish signals originating from deep within a specimen from signals originating nearer the surface

Eddy current distribution using parameters normalized by standard penetration depth

Abstract: Conventional eddy current testing has traditionally used circular pancake coils. The recent needs of high sensitivity testing have prompted the researchers to develop various kinds of test probes. The flaw detection characteristics in eddy current testing depend theoretically on the eddy current distribution induced in the test material by the probe. Thus it is quite important to know the eddy current distribution in order to detect flaws in the material on a given test condition. However, the eddy current distribution is influenced complicatedly by many factors such as the test frequency, the electromagnetic characteristics and the configurations of the test material, and the exciting coil configurations. As a result, it is not easy to find out the appropriate probe design for a given test material.

Recent advances in remote field eddy current NDE techniques and their applications in detection, characterization, and monitoring of deeply hidden corrosion in aircraft structures

Abstract: The remote field eddy current (RFEC) technique is based on the RFEC phenomenon which is characterized by differences in the energy flow patterns in the near and remote field regions. The energy released by the probe excitation coil traverses the pipe wall twice before reaching the pickup coil. The RFEC technique, currently used in metallic tube inspection, is therefore characterized by its equal sensitivity to a flaw irrespective to its location in the tube wall. It can be used for detecting defects located several skin-depths away from the excitation source, since its inspection capacity is not limited by the skin-depth of the specimen, but by the signal-to-noise ratio for a particular measurement condition. The paper presents two new developments of the RFEC technique. The first one is to extend the technique to the inspection of flat geometry objects. Experimental measurements have shown that anomalies located in the second and third layers in multi-layered aluminum structures up to total thickness of 25.4 mm can be detected. The second development is related to the extension of the technique to pulsed excitation. Finite element modeling results show that the pulsed RFEC signals contain additional information about the object under inspection. A proper analysis of the pulsed RFEC signals can yeild information about the surface, subsurface and the rear wall condition of a conducting object when an appropriate probe is used., A demonstration of the potential applications of both techniques in detecting deeply hidden cracks in aircraft structure samples is presented.

Solving 3-D eddy current problem containing thin cracks using dual formulations and shell elements

Abstract: The thin cracks in three-dimensional eddy current problems, frequently encountered in nondestructive testing applications, are modelled by surface domains of zero thickness. The double layer nodal and edge shell elements are employed in the dual eddy current formulations for the interpolation of scalar and vector potentials in the cracks. With the help of shell elements, the volume integrals corresponding to the magnetic energy and joule losses in the cracks are transformed to surface integrals. This modelling allows consideration of the field jump across the thin cracks provided that the field continuity on the edge of the crack is correctly taken into account. An eddy current problem in presence of thin cracks is solved using the model and results are compared with those of conventional finite element modelling.

Eddy current flaw detector of steel sphere

Abstract: (57) [Summary] [Problem] To inspect the entire surface of a steel ball regardless of whether it is in the air or oil and without being affected by oil stains on the surface, and a thin cracked surface that cannot be detected by optical inspection. Detect defects and improve the accuracy of steel ball inspection. A drive roller and a support roller are provided. And a steel ball rotating device consisting of two control rollers 3 connected to an eccentric gear 5 via a holder 4 at the rear end, an inspection cycle control device and an eddy current inspection device, and the detection sensor 6 is positioned. It is configured to be fixed to the rotating device by a jig and connected to an eddy current inspection instrument via a connector.

Experimental Results in Eddy Current Non Destructive Testing Based on Superconductive and Conventional Electromagnetic Probes

Abstract: This paper deals with the electromagentic nondestructive testing performed by SQUID magnetometry on stratified aluminum alloy plates as those commonly encountered in the aircraft industry. The anomalous magnetic fields generated by flaws with know electromagnetic characteristics have been modeled by a three-dimensional specific code based on a finite element formulation. The numerical solution has correctly predicted the shape of the complicated magnetic field response due to the defect. Once accuracy and reliability of experimental data taken by superconductive probe have been tested, a benchmark-like problem has been faced. Measurements performed by conventional probes like fluxgate and inductive coil have been compared with the ones taken by innovative device based on superconductive materials.