Eddy Current Nondestructive Evaluation Research Articles

A model-based method for the characterisation of stress in magnetic materials using eddy current non-destructive evaluation

A precise knowledge of the distribution of internal stresses in materials is key to the prediction of magnetic and mechanical performance and lifetime of many industrial devices. This is the reason why many efforts have been made to develop and enhance the techniques for the non-destructive evaluation of stress. In the case of magnetic materials, the use of eddy current (EC) techniques is a promising pathway to stress evaluation. The principle is based on the significant changes in magnetic permeability of magnetic materials subjected to mechanical stress. These modifications of magnetic permeability affect in turn the signal obtained from an EC probe inspecting the material. From this principle, a numerical tool is proposed in this paper to predict the EC signal obtained from a material subjected to stress. This numerical tool is a combination of a 3D finite element approach with a magneto-mechanical constitutive law describing the effect of stress on the magnetic permeability. The model provides the variations of impedance of an EC probe as a function of stress. An experimental setup in which a magnetic material subjected to a tension stress is inspected using EC techniques is tailored in order to validate the model. A very good agreement is found between experimental and modelling results. For the Iron-Cobalt alloy tested in this study, it is shown that a uniaxial tensile stress can be detected with an error lower than 3 MPa in the range from 0 to 100 MPa.

Pulsed Eddy Current NDE of Groove Dimensions by Inversion With Simplex Method Associated With Coupled Electric Circuits Method

This paper describes a pulsed eddy current nondestructive evaluation of an annular groove dimensions using a simplex inversion method associated with a semianalytical direct model, in axisymmetric case. The proposed semianalytical formulation, used for direct calculations, is developed from coupled electric circuits modeling. It uses the current excitation allowing the determination of the sensor voltage. The feature of the variation of voltage provides information on the electromagnetic and geometrical properties of the device. The transient sensor voltage is then calculated, allowing the evaluation of the groove dimensions of the considered work piece. The peak's amplitude and the zero and peak crossing times are used to determine these properties.

Effective de-noising and characterisation of defects in eddy current-NDE images

Non Destructive Evaluation (NDE) plays an important role in assessment of structural integrity of engineering components. Among various NDE techniques, eddy current NDE is widely used for detection of defects in installed components because of its ease of operation, non-contact and versatility. However, in the case of welded components, the presence of microstructural variations affects the material parameters such as conductivity and permeability. This in-turn produces large amplitude disturbing signals (weld noise) that mask small amplitude signals from defects. This demands the development of de-noising methodology, which enhances the characterisation of defects using Artificial Neural Network (ANN). This paper discusses the discrete wavelet transform-based de-noising method for elimination of weld noise and enhancement of defect information. The paper also briefs the characterisation of defects from the enhanced defect region using Multilayer Perceptron (MLP) based back propagation ANN.

Development of Non-destructive Evaluation System Using an HTS-SQUID Gradiometer with an External Pickup Coil

We are developing a new eddy-current non-destructive evaluation (NDE) system using a high-temperature superconducting quantum interference device (HTS-SQUID) gradiometer with the aim of applying it to power plants. Electric power facilities such as ducts and vessels are generally untransportable because of their size, and thus it is difficult to apply a conventional SQUID NDE system. The new NDE system employs an external Cu pickup coil which is supposed to be driven flexibly by a robot arm at room temperature and an HTS-SQUID chip which is placed in a magnetically shielded vessel. In the present research, we investigated the performance of an HTS-SQUID sensor connected with external pickup coils before mounting them to a robot arm. By varying the Cu coil conditions such as their sizes, the number of turns, and the diameter of wire, we qualitatively evaluated the frequency dependence of the effective area and the cutoff frequency.

A Benchmark Problem for Eddy Current Nondestructive Evaluation

Currently, there are three TEAM Workshop benchmark problems involving eddy current nondestructive evaluation configurations, Nos. 8, 15, and 27. Regarding the first two, they have ceased to constitute challenging problems since they can be routinely solved with existing commercial software. In this paper, we present precision impedance measurements in a configuration involving a system of plates with through holes and a crack and propose them for benchmarking numerical codes. The benchmark represents a simplified version of eddy current inspection of fastener holes in aircraft structures, provides a lot of experimental data, and overall constitutes a more challenging problem.

Eddy Current Inversion Models for Estimating Dimensions of Defects in Multilayered Structures

In eddy current nondestructive evaluation, one of the principal challenges is to determine the dimensions of defects in multilayered structures from the measured signals. It is a typical inverse problem which is generally considered to be nonlinear and ill-posed. In the paper, two effective approaches have been proposed to estimate the defect dimensions. The first one is a partial least squares (PLS) regression method. The second one is a kernel partial least squares (KPLS) regression method. The experimental research is carried out. In experiments, the eddy current signals responding to magnetic field changes are detected by a giant magnetoresistive (GMR) sensor and preprocessed for noise elimination using a wavelet packet analysis (WPA) method. Then, the proposed two approaches are used to construct the inversion models of defect dimension estimation. Finally, the estimation results are analyzed. The performance comparison between the proposed two approaches and the artificial neural network (ANN) method is presented. The comparison results demonstrate the feasibility and validity of the proposed two methods. Between them, the KPLS regression method gives a better prediction performance than the PLS regression method at present.

Development of HTS-SQUID Gradiometer with an External HTS Multiturn Thin Film Pickup Coil

Highly-sensitive HTS-SQUIDs are applied to various systems such as eddy-current non-destructive evaluation (NDE) and compact magnetometer systems. To avoid unstable operation of the SQUID due to an excitation field, a flux transformer made of normal Cu wire is used to separate the pickup coil and SQUID. However, the resistance of the flux transformer generates thermal noise and reduces the signal bandwidth at low frequencies. In this study, we investigate use of a superconducting flux transformer with resistive connections (SFTR). The SFTR consists of pickup and input coils made of HTS thin films. The two coils are connected by using an HTS coated conductor. The input coil is stacked on an HTS thin film gradiometer. From the measured results of resistances of the connections between each coil and the coated conductor, it was found that the resistance of bonding wire was a dominant component. The estimated resistance of the SFTR was 7.2 mΩ, corresponding to a lower cut-off frequency of 11.7Hz.

Study on Non-destructive Evaluation of Flaws in Multilayer Duct Using an HTS-SQUID Gradiometer

We have investigated the performance of an eddy-current non-destructive evaluation (NDE) system using an HTS Superconducting quantum interference device (SQUID) gradiometer with the aim of applying it to the equipment in power plants. Use of the SQUID-NDE technique has an advantage, because SQUID has a high sensitivity or low noise characteristics at low frequencies for detecting magnetic signals. In the present research, we have tried to detect flaws in test samples simulating a multilayer duct in a plant, which consists of multilayer aluminum and resin plates. In the test samples, resin plates were sandwiched by aluminum plates. The top aluminum plate and the bottom multilayer aluminum plates were assumed as outer and inner duct layers, and the resin plates were assumed as an insulation layer in a multilayer duct of the plant. A slit-shaped through hole was opened in the bottom layer, and was assumed as a flaw in the duct layer. By using a SQUID-ECT system, we could detect signals from flaws located at more than 10mm deep positions in the test samples.

Integral equation method for evaluation of eddy-current impedance of a tilted, surface-breaking crack

An integral equation method for solving the eddy-current nondestructive evaluation problem for a flat, tilted, and surface-breaking crack in a conducting half-space is presented. The method involves use of a half-space Green's tensor and the Bowler potential. This potential describes the jump in the electric field over the crack and is expanded in basis functions related to the Chebyshev polynomials, being a more analytical approach than the commonly used boundary element method. In the method, the scatterer defines a transformation operator to be applied on the incoming field. This is practical in simulations of the eddy-current inspection where this operator is independent of the position of the probe. The numerical calculations of the change in impedance due to the crack are compared to a Finite Element model of the problem and good agreement is found.

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

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.

Recent Innovative Solutions in Eddy Current Non-Destructive Diagnosis

Recent innovative solutions in eddy current non-destructive evaluation of conductive materials are reported in the paper. Two main areas influencing reliability and preciseness of real cracks diagnosis are discussed. Advances in sensor technology are concerned at first. Innovative sensor design allowing sensing all the three spatial components of the perturbation electromagnetic field is presented. A novel approach for automatic diagnosis of real cracks from two-dimensional eddy current response signals is then introduced. The tabu search is applied for the three-dimensional reconstruction of partially conductive cracks.

Statistical Assessment of Probability of Detection for Automated Eddy Current Nondestructive Evaluation Inspection

Eddy current inspection is widely used in nondestructive evaluation to detect cracks in metal structures. The impedance plane measurement response, collected using our motion controlled eddy current inspection system, is used in the analysis. A scalar reduction from the impedance plane response is used to minimize human-factor variation. We apply a noise interference model to the data from a large-scale experiment taking measurements on fastener rivet holes in multilayer structures with fatigue cracks, and estimate the probability of detection (POD). The estimates of POD as a function of crack size will be valuable for future model-assisted POD studies.

Eddy current nondestructive evaluation of dual phase steel

This study proposes eddy current testing (ECT) as a tool for nondestructive material characterization. In the present study, different dual phase steels (DPSs) with different martensite phase percentages were manufactured using various intercritical annealing temperatures (from 745 to 890°C). The aim of the present study is to correlate ECT outputs with the different percentages of martensite and find a relation for determination of martensite percentage of any unknown DPS sample. Increasing the intercritical annealing temperature leads to an increase in the percentage of martensite in DPS, which it caused to a decrease in magnetic permeability of DPS and subsequent decreasing of ECT outputs. Mechanical properties of DPS specimens can also be predicted by the means of ECT. The study revealed that a good correlation exists between ECT outputs and microstructural and mechanical changes in the DPS specimens (R2>0.85) and it shows potential of using ECT for prediction of material properties in the DPS.

A Bayesian Network Method for Automatic Classification of Eddy Current NDE Signals

Eddy current nondestructive evaluation (ECNDE) techniques are widely used in structural integrity and health monitoring. A novel algorithm was proposed for characterizing eddy current (EC) signals. In scanning inspection, the EC signals responding to impedance change were pre-processed for noise elimination and feature extraction. After feature extraction, Bayesian networks (BNs) were carried out to classify EC signals. It is shown by extensive experiments that kernel principal component analysis (KPCA) is better than principal component analysis (PCA) for feature extraction. The methods using BNs by KPCA feature extraction can perform better than the other classification methods.

Analytical approach to pulsed eddy current nondestructive evaluation of multilayered conductive structures

This paper proposes an analytical approach that can describe the Pulsed Eddy Current Testing (PECT) of stratified conductive structures by a single air-coil used for detecting and sizing of metal loss due to interlayer corrosion. Specifically for a single air-cored coil above a multilayered conductive structure, the coil impendance response of the cylindrical eddy-current probe is calculated based on the truncated region eigenfunction expansion (TREE) method using the generalized matrix coefficients in series form. Analytical modeling of pulsed eddy current is developed using the Fourier series for multilayered conductive structures. A simulation study is carried out for demonstrating the capability of the proposed approach for sizing the metal loss in a two-layer specimen. Furthermore, the experiment is set up to confirm the simulating results of the analytical model for multilayered steels. Good agreement between the theoretical results and measured signals is observed.

Sensor-tilt invariance analysis for eddy current signals

In the application of electromagnetic nondestructive evaluation (NDE), the measured signals received by sensors are decided by the flaw associated with operational parameters during inspection. The techniques of invariant pattern recognition have been studied to render NDE signals insensitive to operational variations and preserve or recover crack information. The invariant scheme and algorithms have been addressed to facilitate magnetostatic flux leakage and eddy current NDE, which eliminated operational parameters and lift-off changes corrupt from signal measurements. A novel invariance analysis of eddy current (EC) signals in the inspection of deeply embedded cracks under layered fastener heads has been presented in this paper. A detection system based on uniform EC excitation and giant-magnetoresistive (GMR) pick-up sensors has been developed and shown improved detectability of 2nd and 3rd layer defects around fastener sites in multilayer structures. However, the sensor-tilt due to variation of probe lift-off can generate redundant response as noise effect and obscure the flaw inspection. The variations of GMR sensor-tilt with crack inspection are investigated using an efficient numerical model that simulates the used EC-GMR system. The scheme of invariance transformation is proposed to exclude the sensor-tilt noise and keep the defect inspection identical. The statistical features insensitive to tilt effects are extracted after the invariance processing, which has ensured the probability of flaw detection.

Time and Frequency Domain Feature Fusion for Defect Classification Based on Pulsed Eddy Current NDT

Pulsed Eddy current (PEC) is an emerging nondestructive evaluation (NDT) technique excited with a broadband pulse that consists of rich frequency information. This technique generally uses the response peak value and peak arrival time to detect and quantify defects, which is not sufficient to achieve enough information associated with flaw quantification. This article introduces the application of empirical mode decomposition and Hilbert transform in extracting a new frequency domain feature from PEC responses. Time and frequency features are combined to enhance the accuracy of defect classification. The validation and robustness of the new feature and the improved classification by experimental studies are presented.

Development of non-destructive evaluation system using an HTS-SQUID gradiometer for magnetized materials

We have developed a new eddy-current non-destructive evaluation (NDE) system using an HTS SQUID gradiometer with the aim of applying it to practical materials with magnetization. The new NDE system employs a LN2-cooled external Cu pickup coil and an HTS SQUID chip placed in a magnetic shield made of HTS material. The HTS SQUID chip consists of an HTS planar gradiometer manufactured by using a ramp-edge junction technology and a multi-turn HTS thin film input coil coupled with the flip-chip configuration. The first-order coaxial gradiometric Cu pickup coil with a diameter of 16 mm and the baseline of 5.6 mm was used in the present NDE experiments. By using this NDE system, we could observe defect-induced magnetic signals without an appreciable influence of magnetization up to 10 mT. We also examined the ability of detecting deep-lying defects and compared with the results obtained using our previous NDE system.

Compensated high temperature SQUID gradiometer for mobile NDE in magnetically noisy environments

SQUID gradiometers can measure small localised magnetic fields in the presence of much larger background interference. Their high sensitivity at low frequencies means they are especially suited for use in eddy-current nondestructive evaluation (NDE) of aircraft and other conducting structures. However it is very challenging to rotate or tilt SQUID-based devices in the Earth's field without degradation in their sensitivity. Consequently many demonstrations of SQUID NDE have involved the SQUID remaining stationary or only being moved in a straight line. To overcome this issue, we report on the development of a simple, portable NDE scanning system which uses field cancellation techniques to allow the device orientation to change during scanning. We demonstrate that its signal-to-noise performance for defect detection in curved wing structures matches that obtained for flat samples using a stationary SQUID.

Pulsed eddy current non-destructive evaluation based on coupled electromagnetic quantities method

This paper proposes a semi-analytical model for Pulsed Eddy Current Non-Destructive Evaluation (PEC-NDE) based on the Coupled Electromagnetic Quantities Method (CEQM). The proposed formulation is developed from coupled electric circuit’s approach in which the Crank Nicholson formula is used to derive the transient behavior. The computational model makes use of current excitation allowing the determination of the sensor voltage in the case of an axisymmetric device. The feature of the variation of voltage provides information on the electromagnetic and geometrical properties of the device. So, the second peak of voltage variation (SPVV) and second ratio between the minimum and maximum (RMM2) are used to determine these properties. The proposed model is validated by comparison with Fourier reconstitution obtained from measurements on one hand and with finite element calculations on the other hand. The developed model, associated to an inversion technique, is applied to evaluate the lift-off and the work piece thickness.