Showing papers on "Eddy-current testing published in 2013"

Physical interpretation and separation of eddy current pulsed thermography

Abstract: Eddy current pulsed thermography (ECPT) applies induction heating and a thermal camera for non-destructive testing and evaluation (NDT&E). Because of the variation in resultant surface heat distribution, the physical mechanism that corresponds to the general behavior of ECPT can be divided into an accumulation of Joule heating via eddy current and heat diffusion. However, throughout the literature, the heating mechanisms of ECPT are not given in detail in the above two thermal phenomena and they are difficult to be separated. Nevertheless, once these two physical parameters are separated, they can be directly used to detect anomalies and predict the variation in material properties such as electrical conductivity, magnetic permeability and microstructure. This paper reports physical interpretation of these two physical phenomena that can be found in different time responses given the ECPT image sequences. Based on the phenomenon and their behaviors, the paper proposes a statistical method based on single channel blind source separation to decompose the two physical phenomena using different stages of eddy current and thermal propagation from the ECPT images. Links between mathematical models and physical models have been discussed and verified. This fundamental understanding of transient eddy current distribution and heating propagation can be applied to the development of feature extraction and pattern recognition for the quantitative analysis of ECPT measurement images and defect characterization.

Defect Characterization With Eddy Current Testing Using Nonlinear-Regression Feature Extraction and Artificial Neural Networks

Abstract: The estimation of the parameters of defects from eddy current nondestructive testing data is an important tool to evaluate the structural integrity of critical metallic parts. In recent years, several works have reported the use of artificial neural networks (ANNs) to deal with the complex relation between the testing data and the defect properties. To extract relevant features used by the ANN, principal component analysis, wavelet decomposition, and the discrete Fourier transform have been proposed. In this paper, a method to estimate dimensional parameters from eddy current testing data is reported. Feature extraction is based on the modeling of the testing data by a template of additive Gaussian functions and nonlinear regressions to estimate their parameters. An ANN was trained using features extracted from a synthetic data set obtained with finite-element modeling of the eddy current probe. The proposed method was applied to both simulated and measured data, providing good estimates.

Crack Depth Estimation by Using a Multi-Frequency ECT Method

Abstract: In many industrial application fields as manufacturing, quality control, and so on, it is very important to highlight, to locate, and to characterize the presence of thin defects (cracks) in conductive materials. The characterization phase tries to determine the geometrical characteristics of the thin defect namely the length, the width, the height, and the depth. The analysis of these characteristics allows the user in accepting or discarding realized components and in tuning and improving the production chain. The authors have engaged this line of research with particular reference to non-destructive testing applied to the conductive material through the use of eddy currents. They realized methods and instruments able to detect, locate, and characterize thin defects. In this paper, a novel measurement method able to improve the characterization of the crack depth is proposed. It is based on the use of a suitable multi-frequency excitation signals and of digital signal processing algorithms. Tests carried out in an emulation environment have shown the applicability of the method and have allowed the tuning of the measurement algorithm. Tests carried out in a real environment confirm the goodness of the proposal.

Decoupling the Influence of Permeability and Conductivity in Pulsed Eddy-Current Measurements

Abstract: Eddy-current (EC) sensors are widely used in the industry for nondestructive evaluation (NDE) and structural health monitoring (SHM). It has this wide acceptance mainly because it has high tolerance to harsh environments, low cost, and high bandwidth. Its variant pulsed eddy current (PEC) provides even more depth information of test materials, which can be applied in liftoff measurement (displacement and coating thickness measurement), defect measurement, and material characterization. However, PEC sensors are prone to measurement errors due to a phenomenon called electrical runout (ERO), which is attributed to the inhomogeneity of the test material. The main thrust of this paper is to investigate, therefore, the contributions of the electromagnetic properties (permeability and conductivity) of the sample to ERO with a view to separate the influence of these two properties. Both time-domain and frequency-domain analyses are carried out in this investigation viz-a-viz: transient response, differential normalized response, magnitude spectrum, and normalized magnitude spectrum. This paper reveals that conductivity effects are prominent in the rising edge of the transient response, hence, changing the spectral pattern in the frequency domain while permeability effects dominate in the stable phase of the transient response, thus this effect can be suppressed or reduced by normalization showing that it is only an amplitude change.

Eddy current pulsed phase thermography and feature extraction

Abstract: This letter proposed an eddy current pulsed phase thermography technique combing eddy current excitation, infrared imaging, and phase analysis. One steel sample is selected as the material under test to avoid the influence of skin depth, which provides subsurface defects with different depths. The experimental results show that this proposed method can eliminate non-uniform heating and improve defect detectability. Several features are extracted from differential phase spectra and the preliminary linear relationships are built to measure these subsurface defects' depth.

Rotating field eddy current probe with bobbin pickup coil for steam generator tubes inspection

Abstract: Eddy current probes are used extensively to evaluate the integrity of steam generator tubes in nuclear power plants and to detect cracks in tube walls. The probe design has evolved from simple bobbin probes to mechanically rotating multi-coil probes providing high resolution images of tube integrity. This paper presents a novel rotating field eddy current probe with bobbin pickup coil that generates a rotating magnetic field, thereby avoiding mechanical rotation of probe. Three balanced identical rectangular windings carrying excitation currents with 120° phase difference generate an electrically rotating magnetic field that is sensitive to defects of all orientations. A bobbin coil located in the center is used as a pick up sensor. The phase of induced voltage in the bobbin coil is interpreted in terms of the circumferential location of the defect. The properties of the probe are investigated using a three dimensional finite element model, and validated experimentally using measurements.

Magnetic mechanisms of magnetic flux leakage nondestructive testing

Abstract: The widely applied magnetic flux leakage (MFL) nondestructive testing for ferrous materials has been simply explained by “MFL escaping from defects and being induced is exactly the MFL testing principle,” with a rough application for evaluation. Hence, we analyze the “escape” course of MFL generation using magnetic refraction and explain the reason why the MFL technology really exists in practice. Furthermore, we also point out that the presently so-called MFL testing technique essentially hides a different signal component arising from bump-shaped defects features unlike the MFL caused by concave-shaped ones, explained by another forming mechanism.

Research on spectral response of pulsed eddy current and NDE applications

Abstract: Using pulsed excitation, pulsed eddy current (PEC) technique generates responses over a wide range of frequencies, containing more information than traditional eddy current inspection in the frequency domain. This paper investigates the spectral response of PEC under varying probe lift-off, material properties (electrical conductivity and magnetic permeability) and directional tensile stress. It is shown that material properties and lift-off exhibit different spectra patterns. Using normalisation in frequency domain, the lift-off effect can be reduced. Spectra for both electrical conductivity and magnetic permeability show a complex pattern. Tensile stress measurement in aluminium 2024 and 5083 is investigated, which show similar spectra pattern to electrical conductivity variation. Applications for corrosion characterisation, displacement measurement and profile inspection can be developed in the future in conjunction with time-domain features.

Spatial and Time Patterns Extraction of Eddy Current Pulsed Thermography Using Blind Source Separation

Abstract: Eddy current pulsed thermography (ECPT), a new emerging nondestructive testing and evaluation (NDT&E) technique, has been applied for a wide range of conductive materials. The acquired image sequences contain valuable information in both spatial and time domain. ECPT techniques mainly use a specific frame to detect and quantify the defects. However, selection of specific frame from transient thermal image video to maximize the contrast of thermal variation and defect pattern from complex geometrical samples remain a challenge. In order to accurately find anomalous patterns from the transient thermal pattern for defect detection and further quantitative NDE, this paper employs a single channel blind source separation algorithm. This method enables spatial and time patterns to be extracted according to the whole transient response behavior without any training knowledge. In this paper, both mathematical and physical models are discussed, and the basis of the proper selection of contrast image is given. In addition, the artificial slot and thermal fatigue natural crack are applied to validate the proposed method.

Finite Element Analysis of Nondestructive Testing Eddy Current Problems With Moving Parts

Abstract: We present the logical expressions (LE) approach that allows fast computation of three-dimensional eddy current problems, including parts in motion. The approach applies time-dependent logical expressions to describe moving parts of the model on a fixed computational grid. The study is motivated by a novel nondestructive testing technique called Lorentz force eddy current testing (LET), which enables the detection of defects lying deep inside a conducting material. Depending on the definition of the frame of reference, we present two different implementations of the LE approach referred to as 1) moving magnet approach, and 2) moving defect approach. In order to demonstrate the advantages of the LE approach, we compare its results with the sliding mesh technique. The validation of the obtained results with experiments is also presented.

Eddy current step heating thermography for quantitatively evaluation

Abstract: This Letter proposed eddy current step heating thermography (ECSHT) combing eddy current excitation with SHT. It has been verified through numerical and experimental studies that the temperature-time1/2 curve can be used to detect the subsurface defects. Separation time was defined and extracted from temperature responses as characteristic feature. Experiment studies with mild steel sample were conducted, and the experimental results showed that two features representing separation time can be used to measure the defect's depth based on their linear relationships.

Analytical solutions to eddy current field excited by a probe coil near a conductive pipe

Abstract: Eddy current field excited by a probe coil near a conductive pipe is studied analytically by means of a second-order vector potential formalism in this paper. Closed-form expressions of the second-order vector potentials are deduced directly in the cylindrical coordinate system by using the Biot–Savart law and the reciprocal of distance described by the modified Bessel functions and exponential functions. Both the impedance change of an excitation coil and the induced voltage in a pick-up solenoid coil are obtained. The theoretical results are in excellent agreement with the experimental results, which verifies the analytical solutions.

A comparative study of principal component analysis and independent component analysis in eddy current pulsed thermography data processing.

Abstract: Eddy Current Pulsed Thermography (ECPT), an emerging Non-Destructive Testing and Evaluation technique, has been applied for a wide range of materials. The lateral heat diffusion leads to decreasing of temperature contrast between defect and defect-free area. To enhance the flaw contrast, different statistical methods, such as Principal Component Analysis and Independent Component Analysis, have been proposed for thermography image sequences processing in recent years. However, there is lack of direct and detailed independent comparisons in both algorithm implementations. The aim of this article is to compare the two methods and to determine the optimized technique for flaw contrast enhancement in ECPT data. Verification experiments are conducted on artificial and thermal fatigue nature crack detection.

Magnetic Nondestructive Test for Resistance Spot Welds Using Magnetic Flux Penetration and Eddy Current Methods

Abstract: Resistance spot welding technologies are widely used in industry. A highly reliable monitoring method is needed to effectively weld and create a robust structure. We developed a combined technique using magnetic flux penetration and an eddy current test (ECT). The magnetic measuring system consists of a pair of magnetic probes having an induction coil and detection coil, a lock-in amplifier, a current source, and a personal computer. The magnetic flux penetration through both surfaces at the weld was measured at low frequency. The ECT was performed at each surface with multiple frequencies. The magnetic flux penetration method showed good correlation with the destructive shear test because of the change in permeability due to the formation of the nugget. The ECT method reflected the depth profile of the nugget and was effective for determining a defective product.

Analytical modeling of the transient response of a coil encircling a ferromagnetic conducting rod in pulsed eddy current testing

Abstract: Analytical solutions are obtained in the time domain for the electromagnetic response to the leading edge of a square pulse excitation of a multiple turn current coil, encircling a long ferromagnetic rod, in the linear permeability regime. The resulting equations, obtained using a magnetic vector potential formalism, describe the time-dependent progression of flux into and along the rod. Results are in agreement with finite element solutions obtained for the same geometry. The work is motivated by the requirement for rapid analytical solutions and insight into pulsed eddy current response during inspection of multilayer aluminum aircraft structures in the vicinity of ferrous fasteners.

Analysis of eddy current interaction between a parallel coil and a ferromagnetic pipe with remanence

Abstract: Eddy current field excited by a parallel coil placed next to a ferromagnetic pipe with remanence is studied in this paper. The influence of the remanence is analyzed at first. The analytical solutions to the ferromagnetic pipe eddy current field are deduced by means of a second-order vector potential formalism, and the analytical expression for the impedance change of the excitation coil is constructed. Theoretical analysis of the influence for the remanence and the analytical solutions to the eddy current field are verified by experiments.

Velocity induced eddy currents technique to inspect cracks in moving conducting media

Abstract: This paper presents a new nondestructive testing technique based on velocity induced eddy currents. When a time constant magnetic field moves along a conductive material velocity induced eddy currents are generated in the media. The technique developed within this work uses these eddy currents in the same way as any eddy current testing method where the flow pattern disturbed by the existence of defects creates variations of the secondary magnetic field that can be assessed by a magnetic field sensor. This magnetic field provides the information about the defects. Within this paper the magnetic field parameters are measured using “giant” magnetoresistors (GMR). As far as the authors know this is an original method, that has not yet been reported in the literature, and presents undoubted advantages when the material to be tested is in motion relative to the test sensor (like when inspecting a rail track with a bogie) because the sensitivity of the method increases with speed. To validate the results simulations of the actual testing conditions are also presented using a commercial finite element model.

Discussion on modeling of thermal fatigue cracks in numerical simulation based on eddy current signals

Abstract: This study evaluates modeling of thermal fatigue cracks by the finite element method from the view point of eddy current testing. Five artificial thermal fatigue cracks introduced into type 304 stainless steel plates were prepared for the research. Eddy current signals were gathered by a differential type plus point probe and subsequent destructive tests were performed to confirm the true profiles of the cracks. Numerical simulation based on the results of eddy current testing and destructive tests were carried out to show how the thermal fatigue cracks should be modeled in numerical simulations. The results of the numerical simulations revealed that thermal fatigue cracks tend to be much less conductive than stress corrosion cracks if they are assumed to have uniform conductivity inside. The results also imply that taking consideration of magnetization induced by the thermal fatigue process enables eddy current signals to be analyzed more quantitatively.

Theoretical Simulations and Quantitative Magnetic Field Measurements for Eddy-Current Testing with an HTS SQUID System

Abstract: The use of a Superconducting QUantum Interference Device (SQUID) magnetometer for quantitative measurements in eddy-current nondestructive evaluation is presented. The SQUID is used in absolute mode, and the magnetic field measurements are directly compared to theoretical results from both analytical and semi-analytical models. The measurements are taken in an ambient noise environment, and the excitation has the form of a double-rectangle printed-circuit-board coil. The configurations examined involve the coil in air and above aluminum plates with or without defects in the form of notches. Agreement between experimental and theoretical results is very good in all cases, showing that SQUID magnetometers can be used for quantitative magnetic field measurements and that modeling can be readily applied for parametric analysis and optimizations of the measurement configuration including excitation coils' design and frequency selection.

Remote field eddy current inspection of metallic tubes using GMR sensors

Abstract: This article describes the inspection of metallic tubes using the remote field eddy current method. The excellent performance of giant magneto-resistance sensors in what concerns sensitivity, linearity, large bandwidth and direct assess to the magnetic field makes it a good choice for defect inspection in the applications where the measurement of low amplitude and low frequency magnetic fields is necessary. Experimental results with an artificial machined defect were included in the paper.

Method for measuring non-uniform conductive material surface coating thickness according to eddy currents

Abstract: The invention discloses a method for measuring non-uniform conductive material surface coating thickness according to eddy currents Based on the frequency sweep eddy current testing technology, continuous frequency within a certain range is used for stimulating one test point of a tested non-uniform conductive material provided with a coating and a substrate standard test block without the coating; the optimal measuring frequency at the test point is found, therefore, the effect of the nonuniformity of the conductivity of a substrate on the measurement of the surface coating thickness can be effectively avoided, and the accuracy of the measurement of the surface coating thickness is effectively improved

Detection capabilities evaluation of the advanced sensor types in Eddy Current Testing

Abstract: The purpose of this paper is to compare performances of various sensing elements in eddy current non-destructive inspection. A new eddy current testing probe is designed to compare detection and resolution capabilities of different sensors. Four magnetic sensors, specifically GMR (1D and 3D- sensor), AMR, Fluxgate, and a standard induction coil are used as a sensing element. For comparison of these sensors the numerical simulations and experimental measurements are performed under the same conditions. The results are presented and discussed in the paper. Streszczenie. W artykule umieszczono wyniki porownania czujnikow wykorzystywanych w defektoskopii wiroprądowej. Autorzy dokonują porownania przy pomocy probnika wlasnego projektu, badając czujniki: GMR, AMR, czujnik pola magnetycznego i standardową cewke indukcyjną. Dodatkowo zostają wykonane symulacje numeryczne odzwierciedlające wykonane pomiary. (Ocena zdolności detekcyjnych czujnikow uzywanych w defektoskopii wiroprądowej)

Inspection of laser welds with array eddy current technique

Abstract: Three groups of laser weld specimens made of steel 316L for nuclear applications were fabricated - acceptable, missed seam (MS) and lack of penetration (LOP). An array eddy current technique was developed and investigated. The specimens top and bottom surfaces were scanned parallel and transverse to the weld line. The MS and LOP were easily imaged, detected and sized. Additional flaw indications were found in visually acceptable weld areas. Limited weld destructive testing was performed validating characteristic eddy current flaw indications.

Electromagnetic system for nondestructive evaluation of train hollow axles

Abstract: The structural integrity of train axles is one of the important factors for safety precautions in a railway industry. Most common techniques used nowadays in the railway industry for manual inspection are visual and magnetic particle techniques as well as ultrasound testing. In this paper, a novel system for electromagnetic inspection is proposed to meet the aforementioned requirements. The system consists of two subsystems and transducers dedicated for flux leakage and eddy current method. Results of measurements and evaluation of the transducers are provided.

A novel E-shaped coil for eddy current testing

Abstract: Purpose – Eddy current testing (ECT) is widely used in the non-destructive evaluation of materials in different industries. In this paper, ECT has been used to detect the presence of cracks in boiler tubes. The most important feature in ECT is the way in which the eddy currents are induced and detected in the sample. The authors have tried to design a new sensor that is effective in detecting cracks in boiler tubes. The purpose of this paper is to study the response of this sensor to cracks of different depths and dimensions. Design/methodology/approach – The designed eddy current sensor is equipped with an exciting and a sensing coil. An alternating current is passed through the exciting coil thus producing eddy currents. The sensing coil scans the outer surface of the boiler tube and looks for abrupt changes in output signals resulting from sharp discontinuities in structure. Findings – The sensor designed can detect the position of the crack. The presence of crack is indicated by a reduction in the ind...

Research on the device of differential excitation type eddy current testing for metal defect detection

Abstract: A differential excitation type sensor is introduced based on the eddy current principle for detecting metal defects, to improve the detection performance of the sensor system by optimizing the circuit of detection system. Using Q235 specimen as the detection object, aiming to detect and analyze the cracks of different depth and width and then to study the influence of excitation frequency, excitation amplitude and lift-off on the results. The test results showed that the differential excitation type eddy current testing device can be used for detecting defects on metal with high sensitivity.

Development of eddy current testing system for inspection of combustion chambers of liquid rocket engines

Abstract: An eddy current testing (ECT) system using a high sensitive anisotropic magnetoresistive (AMR) sensor was developed. In this system, a 20 turn circular coil with a diameter of 3 mm was used to produce the excitation field. A high sensitivity AMR sensor was used to measure the magnetic field produced by the induced eddy currents. A specimen made of copper alloy was prepared to simulate the combustion chamber of liquid rocket. Scanning was realized by rotating the chamber with a motor. To reduce the influence of liftoff variance during scanning, a dual frequency excitation method was used. The experimental results proved that ECT system with an AMR sensor could be used to check liquid rocket combustion chamber.