Showing papers on "Eddy-current testing published in 1998"

Numerical evaluation of correlation between crack size and eddy current testing signal by a very fast simulator

Abstract: This paper describes work on the development of a very fast eddy current testing (ECT) signal simulator, and its application to the evaluation of the correlation between cracks and signals. This simulator is developed here based on a reduced magnetic vector potential, edge based finite elements, and the pre-computed unflawed database approach. Using this simulator, three kinds of probes are tested in terms of their linearity and signal to noise ratio.

Evaluation of probe impedance due to thin-skin eddy-current interaction with surface cracks

Abstract: Crack detection using eddy-current nondestructive testing is often carried out at frequencies such that the skin depth of the induced current is much smaller than the crack dimensions. The induced current then flows in a thin skin at the conductor surface and at the faces of a surface crack. In the case of a crack that acts as an impenetrable barrier to electric current, the electromagnetic field at the crack surface can be represented, at an arbitrary frequency, in terms of a potential which satisfies a two-dimensional Laplace equation. The boundary conditions required in the solution of the Laplace equation have not yet been determined for the general case, but we have derived approximate boundary conditions which are applicable in the thin-skin regime. The conditions derived are valid for cracks in materials of arbitrary permeability. From the harmonic solutions of the Laplace equation, the impedance change of the excitation coil due to the defect has been calculated for cracks in aluminum and ferromagnetic steel. Comparisons between predictions and experimental measurements on rectangular slots show good agreement, thus corroborating the theory and the numerical calculations.

Eddy-current testing by flexible microloop magnetic sensor array

Abstract: A flexible microloop magnetic sensor array has been developed for nondestructive evaluation in nuclear power plants and its fundamental characteristics have been investigated A flexible micro-eddy current testing (micro-ECT) probe consists of a pancake-type exciting coil of the diameter of 32 /spl mu/m and a flexible microloop magnetic sensor array with an area of 14/spl times/14 mm/sup 2/ and a thickness of 125 /spl mu/m The array has 16 microloop sensors made of copper formed on a flexible polyimide film by sophisticated photolithography Each coil has 40 turns (8 /spl mu/m line and 8 /spl mu/m space) within an area of 2/spl times/2 mm/sup 2/ The flexible sensor array enables the micro-ECT probe to be applied to an arbitrary geometric configuration The purpose of this work is to detect and reconstruct flaws in conductive tubing of a steam generator in a pressurized nuclear power reactor by using the flexible micro-ECT probe with higher spatial resolution than that of a conventional ECT probe The spatial resolution of flaw detection was 3 mm and the detectability of an outer 20% flaw made by electric discharge machining was confirmed The reconstruction of circumferential and axial flaws using the electromotive force and phase measured with the sensor array is described Three-dimensional eddy current and magnetic field analysis was also performed for comparison with measured data Finally, a design study of a practical and robust micro-ECT for a realistic tube configuration with respect to detectability was carried out

Eddy Current, Ultrasonic C-Scan and Scanning Acoustic Microscopy Testing of Delaminated Quasi-Isotropic CFRP Materials: A Case Study

Abstract: This paper presents recent non-destructive testing results obtained from the inspection of delaminated quasi-isotropic carbon fibre reinforced plastic materials. Inspections were carried out using ...

A new approach for optimal design of Eddy Current Testing probes

Abstract: The optimal design of ECT probes for advanced nondestructive inspection of steam generator tubing in nuclear power plants is studied in this paper. A new approach for probe design is proposed, on basis of both a simplified detectability analysis method and a ring current model newly developed by the authors. The new approach is incorporated in the optimization of pick-up arrangements as well as the corresponding excitation coils. Two designs of a potential ECT probe which may show high performance in practical applications are proposed finally. In addition, the ring current model and the simplified detectability analysis method are validated further for a conducting tube by comparing the magnetic field perturbation due to a crack and the S/N ratios evaluated by the present method and an FEM-BEM hybrid code respectively. Consequently, the validity of the new design approach and the high performance of the new probe designs are assured.

Identification of crack depths from eddy current testing signal

Abstract: This paper demonstrates the identification of crack depths using signals obtained from eddy current testing (ECT). The identification method is based on finite elements with the pre-computed unflawed database approach and a meshless crack representation technique, and parameter estimation in nonlinear problems. Four different cracks are estimated by using laboratory data.

Quantitative imaging of corrosion in plates by eddy current methods

Abstract: We consider the problem of determining material loss due to corrosion in a plate. Eddy current data are collected on one side of the plate with corrosion damage occurring on the opposite side. Our work is based on linearizing the relationship between eddy current loop impedance change with the loss profile. In this context, we show that eddy current data are sufficient to uniquely determine the material loss profile. Because of the ill posedness of the reconstruction problem, we propose three regularization strategies in devising computational methods. Numerical results using consistent data, and with added noise, indicate that the computational methods are quite capable of obtaining accurate quantitative estimates of the loss profile.

Anisotropic grain noise in eddy current inspection of noncubic polycrystalline metals

Abstract: This letter discusses the role electrical anisotropy plays in the structural integrity assessment of polycrystalline titanium alloys from the standpoint of fatigue crack detection and the related issue of microstructural noise. In eddy current inspection of noncubic crystallographic classes of polycrystalline metals the electric anisotropy of individual grains produces an inherent microstructural variation or noise that is very similar to the well-known acoustic noise produced by the elastic anisotropy of both cubic and noncubic materials in ultrasonic characterization. The presented results demonstrate that although the electrical grain noise is clearly detrimental in eddy current nondestructive testing for small flaws, it can be also exploited for characterization of the microstructure in noncubic polycrystalline materials such as titanium alloys in the same way acoustic grain noise is used for ultrasonic characterization of the microstructure in different materials.

Fast flaw reconstruction from 3D eddy current data

Abstract: An eddy current flaw reconstruction strategy based on the minimization of nonlinear least-squares error functionals is developed for problems with arbitrary specimen, probe and defect shapes. A fast 3D forward solver is created to rapidly predict eddy current signals in the inversion shell. The high speed of the signal evaluation comes by utilizing a reaction data set constructed before performing the inversion by a finite element electromagnetic field simulator. The same pre-calculated reaction data set supports the quick evaluation of sensitivity information, thereby ensuring the efficient implementation of an optimization algorithm. This optimization algorithm combines first-order and stochastic optimization strategies and improves the reliability of the reconstruction significantly if the observed data contain large noise components. Examples with tube specimens are presented for different 3D flaw shapes.

Uniform Eddy Current Probe with Little Disrupting Noise

Abstract: The conventional eddy current testing using a pancake coil probe is prone to suffer from various noises generated by variations of many factors such as probe lift-off, material electromagnetic characteristics and material configurations. Thus it is very difficult for the conventional eddy current testing to detect flaws in variable areas such as weld zone and material edge. In order to detect those flaws, it is indispensable to develop new eddy current probes with little disrupting noise[l–3].

Quantitative Assessment of Corrosion in Aircraft Structures Using Scanning Pulsed Eddy Current

Abstract: Eddy current nondestructive testing techniques are used extensively in industry for detection of hidden cracks and corrosion in multi-layer conductive structures such as those found in aircraft. Most conventional eddy current techniques employ a probe coil excited with a continuous sine wave. The changes in the probe’s impedance are detected (usually with an electronic bridge circuit) and displayed in real time on an impedance plane CRT display. Interpretation of the impedance-plane trajectories that result from scanning the probe over a defect is difficult; it involves a considerable amount of operator skill and knowledge and the equipment is sometimes difficult to set up and operate. We have developed an eddy current inspection system using the pulsed eddy current technique, which greatly reduces the possibility of operator errors and subjectivity in the quantitative interpretation of test results.

Flexible eddy current test probe

Abstract: An eddy current test probe having a flexible sensor assembly for non-destructive testing of conductive parts. The flexible eddy current sensor assembly includes a coil assembly having a drive coil and a receive coil positioned in close proximity to each other to maximize inductive coupling. The drive coil receives an alternating voltage from an alternating voltage source. The receive coil is coupled to a visual display which displays the eddy current signal strength in appropriate units. The coil assembly is formed on a flexible membrane. The flexible membrane allows the coil assembly to contour to the PUT surface. By doing so, surface coupling is not only maintained but maximized between the PUT and the coil assembly which, in turn, improves the induced electromagnetic field. A wear resistant film is formed around the outside of the compliance membrane enclosing the coil assembly. The wear resistant film prevents damage to the coils from the inevitable surface irregularities and abrasive particles found in PUTs. The flexible membrane is layered on a flexible compliance membrane. The compliance membrane provides a degree of stiffness to the flexible membrane for maintaining close surface coupling with the PUT. An electromagnetic shield is optionally formed on the flexible membrane on each side of the coil assembly and underneath the wear resistant film. The electromagnetic shield contains the energy exchange between the PUT and the coil assembly by preventing the electromagnetic field induced in the PUT from spreading out on the PUT. The electromagnetic shield is of particular importance when the sensor assembly approaches a PUT edge because the field tends to distort around PUT edges causing the test probe to saturate and miss a surface flaw or anomaly located near the PUT edge.

Simulation and design of a superconducting eddy current probe for nondestructive testing

Abstract: Numerical modeling studies were carried out for the design and performance evaluation of an eddy current probe for superconducting quantum interference device (SQUID) based noncontacting electromagnetic nondestructive testing. The probe utilizes high-Tc SQUID from electromagnetic interference and to focus magnetic flux due to the eddy current excitation coil into the object under test (OUT). This is very important in order to achieve “self-nulling,” where the probe responds only to the defects in the OUT. Simulations are carried out with the eddy current probe located a finite distance above a defect free conducting half-space of infinite thickness. Eddy current densities in the OUT are then calculated for various probe designs. These eddy current densities give an indication of the flux focusing, and hence increased spatial resolution for combinations of HTS and μ-metal shielding. Shielding factors are also calculated for the screening of the primary excitation field and the “defect-free” eddy current fi...

Eddy current analysis by integral equation method utilizing loop electric and surface magnetic currents as unknowns

Abstract: The surface integral equations whose unknowns are the surface electric and magnetic currents are widely used in eddy current analysis. However, when the skin depth is thick, computational error is increased especially in obtaining electromagnetic fields near the edge of the conductor. In order to obtain the electromagnetic field accurately, we propose an approach to solve surface integral equations utilizing loop electric and surface magnetic currents as unknowns.

Non-destructive testing of weld zone with a uniform eddy current probe

Abstract: In this article, a new eddy current probe, inducing uniform eddy currents in the test material, is presented. The features of the probe are self-differential, lift-off noise-free, and self-nulling in principle. Experimental results presented here show that the probe has a promising possibility of applying eddy current testing to weld zones.

One-dimensional eddy current multi-frequency data fusion : a multi-resolution analysis approach

Abstract: The application ofdata fusion in the field of non-destructive testing (NDT) has become attractive in recent years. It is believed that a single measurement method or condition is not enough to discover all the characters of the objects examined. Researchers try to fuse the information from different resources. In eddy current testing (ET), the multi-frequency method is often applied for its 'skin effect'. In this paper, we introduce the multi-resolution analysis method to fuse the information obtained at different frequencies, which enhances the one-dimension eddy current Signal-to-Noise Ratio (SNR). The information on cracks on both sides of the specimen can be emphasised by the fused signal. The fusion process is achieved in the transform domain. The maximum selection rule is applied as the basic fusion algorithm. The application of a revised maximum method is also discussed.

Characterization of Electric and Magnetic Material Properties with Eddy-Current Measurements

Abstract: The electrical conductivity σ and the magnetic permeability μ are the basic material properties that describe the interaction between an eddy-current sensor and a sample, They are connected strongl...

Numerical simulation and design of a fluxset sensor by finite element method

Abstract: A 3D model of a fluxset sensor serving to measure the magnetic fields arising in eddy current nondestructive testing applications is analyzed by the finite element method. The voltage induced in the pick-up coil is obtained by computing the flux of the core of the sensor for several values of the exciting current at various external fields. It is shown that the time shift of the ensuing voltage impulse depends linearly on the external field in a wide range. The behavior of the sensor is furthermore simulated in a real nondestructive testing arrangement consisting of an exciting coil located above a conducting plate with a crack.

Investigation of steel thickness measurement by an electromagnetic method using a step exciting current

Abstract: The electromagnetic measurement of steel thickness was noted as a new method for inspecting corroded surfaces of ship structures. Experiments to measure the electromagnetic response of steel and simulations using finite element methods (FEM) were carried out. Two peaks appeared in the induced voltage waveforms obtained from the sensing coil (situated on the opposite side of the steel plate from the exciting coil) as the response to a step exciting current. It was found that the peak that appeared first was the response of the magnetic flux that came around the steel plate, while the second peak was the response of the magnetic flux that penetrated the steel plate. Three values, amplitude, delay time, and half value breadth (HVB), were defined in both peaks. All the values from the second peak were correlated to the thickness of steel. Delay time was expected to be the most useful value, which was most stable for the effect of exciting and sensing coil lift-off and amplitude of the step exciting current. The permeability of steel had great effect on the values, but the slits in the specimens had only a minimal effect.

HTS SQUID System for Eddy Current Testing of Airplane Wheels and Rivets

Abstract: Nondestructive testing (NDT) of new and aging aircraft structures is essential for flight safety. Inspection costs can be reduced by using an inspection technique with high sensitivity for small flaws. Of the many NDT methods being used in aircraft maintenance, eddy-current testing is well established, especially for layered structures. Nevertheless, some test tasks cannot be assured with conventional eddy current systems with sufficient sensitivity and dynamic range. Superconducting Quantum Interference Devices (SQUIDs) are the most sensitive magnetic field sensors known to date. With the discovery of High Temperature Superconductors (HTS) ten years ago and the subsequent development of HTS SQUIDs requiring only cooling down to liquid nitrogen temperature, the greatest application barrier appears solvable. SQUID systems offer a high sensitivity at low excitation frequencies, permitting the detection of deeper flaws, and a high linearity, allowing quantitative evaluation of magnetic field maps from the investigated structure [1–3]. The potential of eddy current testing with HTS SQUIDs has previously been demonstrated for up to 5 cm deep-lying defects in stacks of aluminum sheets using a stationary axial SQUID gradiometer [4]. Kreutzbruck et al. [5] performed a direct comparison between a SQUID magnetometer system and a conventional eddy current testing unit (Elotest Bl of Rohmann GmbH), with a well defined saw cut in a plate of aircraft aluminum alloy hidden under a stack of flawless aluminum plates. They demonstrated an improvement in signal-to-noise ratio of approximately 150, when comparing the SQUID signature of the slot with the conventional system.

On the use of the 3D H-/spl psi/-formulation for the forward solution for eddy current nondestructive testing

Abstract: Eddy current nondestructive testing has the potential for estimating the shape and size of defects in conducting materials. In the testing process an artificial defect is varied in size and position until the measured and estimated fields are in tolerable agreement. An accurate and fast three-dimensional field computation is therefore required which can resolve very small field perturbations generated by, for example, a subsurface crack. In this paper an implementation of a boundary element formulation with four variables on linear or quadratic isoparametric elements is presented, which is able to fulfil these requirements. The accuracy of the proposed method is validated with the TEAM benchmark problem No.6. Tests were also carried out for the TEAM Benchmark Problem No.15.

Eddy current imaging using multi-frequency mixing methods for aircraft structural integrity verification

Abstract: Automated eddy current scanning of military or civilian aircraft structures is rarely performed in the field. Manual tests usually are performed by simply placing or sliding a probe across the interrogated surface, while an eddy current instrument’s impedance plane is observed for flaw responses. Scanning and imaging technologies, however, have sparked considerable interest and investigations in the feasibility of using automated eddy current nondestructive testing (NDT) methods in the aircraft community. The scanning system’s intent is to provide the inspector with a rapid and sensitive method to identify locations of potential or immediate concern. Automated imaging techniques offer several advantages over conventional methods, including better reproducibility, reportability and detectability. Recent strides in these areas have dealt mostly with the detection of corrosion in thin skin structures between the fasteners. Wing structures, however, typically consist of much thicker material, where sub-surface corrosion in overlapping joints or fatigue cracks can occur under the fasteners. These flaws may not extend to the surface nor beyond the fastener before they become critical, thus making visual (including enhanced) techniques ineffective.

A moving magnetic flux type sensor with shading coils for eddy current testing

Abstract: This paper presents a moving magnetic flux type sensor with shading coils for eddy current testing. The eddy current testing is useful for detecting cracks and defects on the surface of the conductive materials. However, it is much difficult to detect them on the reverse side surface otherwise the cracks and defects occur on this surface. Therefore we have developed a new sensor for detecting cracks and defects on the reverse side surface. This sensor consists of shading coils which can yield the moving magnetic field, pick-up coils and an exciting coil. In this paper, the results measured by this new sensor are shown and the availability of our new sensors is discussed.

Contrôle non destructif de matériaux composites par la technique des courants de foucault : une nouvelle approche

Abstract: Quality testing of composite materials is regularly carried out using non-destructive testing (NDT) techniques such as ultrasound, radiography or infrared thermography. However, these techniques present some limitations for the NDT of composites in terms of accuracy, sensitivity, repeatability of the signal and sometimes difficulty of calibration. In this article, it is demonstrated that eddy current testing, once solely dedicated for NDT of metallic materials, can also be adapted for the examination of composite materials. Indeed, this technique appears to be adequate for the detection and the characterisation of major flaws such as delamination and impact. A good reproducibility of the signals has been observed and this technique may become an efficient and low cost alternative compared to more complex and expensive techniques. Experimental results are shown and analysed and the advantages and limitations of eddy current testing of composite materials are discussed.

Identification of flaws using eddy current testing

Abstract: The eddy current method of non‐destructive testing uses an alternating current excitation to induce secondary currents in a specimen under test. Flaws within the specimen affect the induced currents, causing changes in the impedance of a test coil. In this paper we present a method for obtaining a solution of inverse problems, in which the parameters of defects are unknown and the excitation function and the eddy current system response are given. The method is based on the use of artificial neural networks, which are trained using measurements. Illustrative examples are given.

The Application of Wavelets and Fuzzy Logic to Eddy Current Flaw Detection in Steam Generator Tubes

Abstract: Eddy current testing is a widely used nondestructive testing method, especially for inspecting steam generator tubes in nuclear power plants. Due to the complex nature of this technique, the analysis of inspection data is a difficult task requiring a great deal of work by experienced human analysts. This is time consuming, expensive, and can be inconsistent due to human nature. Also, the presence in eddy current signals of interference from the tube support plates and deposits can make the data very difficult to analyze. To help overcome these obstacles, an automatic eddy-current analysis system is needed to aid the analysts.

Eddy current testing, an emerging NDT method for ferritic weld inspection

Abstract: This article discusses the emergence and acceptance of manual eddy current weld inspection as a high quality and cost saving NDT method for in-service inspections. In a recent American Welding Society journal article on NDT, eddy current testing was not mentioned as a basic weld inspection method. In fact, it has become common in the European sector, and American oil and gas companies are now beginning to use eddy current. In this month`s column, the author compares the use of eddy current testing and magnetic particle testing (MT) for ferritic weld inspection.