Eddy Current Imaging Research Articles

Pipe Crack Recognition Based on Eddy Current NDT and 2D Impedance Characteristics

Girth weld cracking of long-distance oil and gas pipelines yields substantial harm to pipeline safety and may cause serious accidents. As of today, non-destructive testing has been one of the most common methods for predicting potential faults and ensuring safe operation. Classical pipeline non-destructive testing methods include magnetic flux leakage testing and the use of ultrasonic testing by electromagnetic acoustic transducers. However, they are incapable of identifying the defects in complex surfaces like girth welds. Magnetic flux leakage testing exhibits poor anti-interference abilities and low space resolution. Ultrasonic testing by electromagnetic acoustic transducers suffer from low conversion efficiency and poor signal quality. In order to overcome the disadvantages of conventional pipeline non-destructive testing methods, we propose an embedded eddy current testing system by leveraging image processing and neural networks. Hough transform and the contour extraction technique are employed to extract the characteristic features from the two-dimensional (2D) eddy current impedance image. Experiment results show that the system can effectively identify the girth weld defects, featuring an accuracy of up to 92%. The low power consumption and compactness of the proposed system makes it a great candidate for pipeline inner inspection.

Eddy-Current Imaging with Nitrogen-Vacancy Centers in Diamond

We demonstrate microwave-free eddy-current imaging using nitrogen-vacancy centers in diamond. By detecting the eddy-current induced magnetic field of conductive samples, we can distinguish between different materials and shapes and identify structural defects. Our technique allows for the discrimination of different materials according to their conductivity. The sensitivity of the measurements is calculated as 8$\times 10 ^{5}$\,S/m\,$\sqrt[]{\textrm{Hz}}$ at 3.5\,MHz, for a cylindrical sample with radius $r_0$\,=\,1\,mm and height $h$\,=\,0.1\,mm (volume $\sim$\,0.3\,mm$^3$), at a distance of 0.5\,mm. In comparison with existing technologies, the diamond-based device exhibits a superior bandwidth and spatial resolution. In particular, we demonstrate a flat frequency response from DC to 3.5 MHz and a spatial resolution of 348\,$\pm$\,2\,$\mu$m.

Magnetoelastic Effect Detection with the Usage of Eddy Current Tomography.

The possibility of application of the eddy current tomography setup to measure the small permeability variations caused by magnetoelastic effect was presented. A ferromagnetic steel sample was prepared for applying wall stresses and measured for 30 MPa stresses. The Finite Element Method (FEM) was utilized to conduct numerical forward tomography transformation for samples of known permeability. Developed forward tomography transformation was applied for single variable inverse tomography transformation, utilized for determining magnetic permeability. This confirmed the possibility of the application of eddy current tomography for quantitative measurements of magnetoelastic effect in samples of known geometry.

Epitaxial films of garnet ferrite with anisotropy “easy plane” for magneto-optical eddy current flaw detection

The possibilities of magnetooptical sensors based on ferrite garnet films with various types of magnetic anisotropy are studied in the visualization of eddy current images of linear defects in the test object from aluminum alloys. Potentially high efficiency of sensors based on an easy-plane anisotropy films is shown.

Research on Eddy Current Imaging Detection of Surface Defects of Metal Plates Based on Compressive Sensing

Accurate detection and quantitative evaluation of defects and damage in metal plates is a crucial task in a range of technological applications, such as maintaining the integrity, enhancing the safety, and assuring the reliability of structures. There is scope for improving eddy current testing methods by incorporating compressive sensing (CS) in the inspection process. The key scientific problems in eddy current imaging of defects of metal plates are sparse representations and transform domain mapping, sparse testing constraints, and sparse image reconstruction. The main research content of this paper is as follows. We first provide basic theory based on research of sparse representations, transform domain mapping, sparse matrices, sparse transform matrices, and signal recovery a priori errors. We then propose information-recovery methods for completing compressive sensing. Third, we establish an experimental system for validating theories and methods. Finally, we establish theories and methods for eddy current imaging of metal plates.

A model-based approach for estimation of the crack depth on a massive metal structure

This paper deals with the eddy current technique for nondestructive evaluation of the crack depth on a massive specimen used in aeronautical industry. A set of C-scan eddy current images is analyzed to reduce the noise and to select suitable features, which can be used to estimate the crack depth. Based on this study, a method relying on polynomial forward models of the relationship between crack depth and the maximum value of the sensor impedance is proposed. The least square and the non-negative least square techniques are applied to analyze the usability of proposed models. The error of obtained estimations is smaller than 10%, for almost used experimental data.

Magnetic Field of Conductive Objects as Superposition of Elementary Eddy Currents and Eddy Current Tomography

The paper represents induced currents in an electrically conductive object as a totality of elementary eddy currents. The proposed scanning method includes measurements of only one component of the secondary magnetic field. Reconstruction of the current distribution is performed by deconvolution with regularization. Numerical modeling supported by the field experiments show that this approach is of direct practical relevance.

Pulsed Eddy Current Imaging of Inclined Surface Cracks

Inclined fatigue cracks can potentially cause severe damage to metallic structures as they affect larger region in the tested structure compared to crack perpendicular to the sample surface. The abilitiy to detect and characterize such cracks is paramount in non-destructive testing (NDT). Pulsed eddy current testing (PEC) is known to offer a broadband of excitation frequencies, which in conjuction with C-scan imaging, may offer discrimination of inclination angles of cracks. Finite element modelling (FEM) was carried out to study the effects of different crack inclination angles, while experimental results were used to verify the FEM results. Selection of both time and frequency domain features for C-scan image construction was also presented, where C-scan images of peak value and amplitude at 200 Hz were shown to be potentially capable in determining different inclination angles. Nevertheless, between these two signal teatures, the amplitude at 200 Hz was found to be more effective in the discriminataion of inclined cracks.

Pulsed Eddy Current Imaging of Inclined Surface Cracks

Inclined fatigue cracks can potentially cause severe damage to metallic structures as they affect larger region in the tested structure compared to crack perpendicular to the sample surface. The abilitiy to detect and characterize such cracks is paramount in non-destructive testing (NDT). Pulsed eddy current testing (PEC) is known to offer a broadband of excitation frequencies, which in conjuction with C-scan imaging, may offer discrimination of inclination angles of cracks. Finite element modelling (FEM) was carried out to study the effects of different crack inclination angles, while experimental results were used to verify the FEM results. Selection of both time and frequency domain features for C-scan image construction was also presented, where C-scan images of peak value and amplitude at 200 Hz were shown to be potentially capable in determining different inclination angles. Nevertheless, between these two signal teatures, the amplitude at 200 Hz was found to be more effective in the discriminataion of inclined cracks.

Monotonicity based imaging method for time-domain eddy current problems**This paper is dedicated to Professor Guglielmo Rubinacci on the occasion of his 65th Birthday.

Eddy current imaging is an example of inverse problem in nondestructive evaluation for detecting anomalies in conducting materials. This paper introduces the concept of time constants and associated natural modes in eddy current imaging. The monotonicity of time constants is then described and applied to develop a non-iterative imaging method. The proposed imaging method has a low computational cost which makes it suitable for real-time operations. Full 3D numerical examples prove the effectiveness of the method in realistic scenarios.

3D reconstruction of surface cracks using bi-frequency eddy current images and a direct semi-analytic model

We propose a method for reconstructing 3D surface cracks in metallic parts form eddy current (EC) images. To do so, we use a semi-analytic direct model for representing the interactions between EC and a crack. In order to cope with the inverse problem cons isting in reconstructing 3D cracks, we propose the use of a genetic algorithm based on the computation of bi-frequency EC images. Numerical experiments are carried out in order to analyze the performance of the reconstruction method for different signal to noise ratios. Moreover, the results obtained with a bi-frequency genetic algorithm are compared to those obtained with a mono-frequency algorithm.

Analysis of the 3D draping behavior of carbon fiber non-crimp fabrics with eddy current technique

Abstract Assessing and controlling the complex deformation behavior of textile reinforcements fabrics remains one of the major challenges in the production of fiber-reinforced plastics. In this paper, the draping of +45°/-45° biaxial non-crimp fabrics to a hemispherical shape is investigated with an eddy current imaging technique. After an automated draping process, the textiles are scanned with a robot-guided eddy current measurement system. From the resulting conductivity maps of the samples, the local yarn directions are extracted by image analysis and the paths of individual yarns are reconstructed for both the upper and the lower layer. Experiments are carried out for different forming speeds, blank holder forces and different non-crimp fabric parameters (stitch length, stitch type, stitch tension and yarn count). The influences of these parameters are compared and discussed, with the conclusion that blank holder forces have by far the most significant influence on the draping result. Sample experimental results are compared to results from an FEM draping simulation.

Image processing algorithms for crack detection in welded structures via pulsed eddy current thermal imaging

As the common metal structure type, welded structures are widely used in civil infrastructure, bridges, ships and engineering machinery. Fatigue cracks often originate in welds enduring intense stress concentration or in welding defects under long-term continuous alternating loading [1]. Once these cracks propagate to a critical length, the structural integrity will be challenged. As a result, welding cracks detection is significant in engineering applications.

Utilization of Eddy Current Tomography in Automotive Industry

Utilization of Eddy Current Tomography in Automotive Industry

Eddy current thermography imaging for condition-based maintenance of overlay welded components under multi-degradation

This paper presents a new multi-wave imaging method for the condition-based maintenance of offshore hydraulic systems containing overlay welded components under multi-degradation. These components suffer from combined tribocorrosion interacting with static and cyclic loading, for which effective mathematical models have not yet been established. In this paper, we propose eddy current thermography imaging for real-time structural health monitoring to enable condition-based maintenance of these critical components. The main challenge for the structural health monitoring of these components is that defects are often embedded at the substrate-coating interfacial regions that have irregular shapes in anisotropic and heterogeneous material as a result of the welding process. The eddy current thermography imaging method utilizes coupled electromagnetic–thermal fields and diffusion waves for robust detection of these hidden defects. A mathematical algorithm and software based on the discrete Fourier transform of thermal videos registered by an infrared detector was developed to characterize and visualize defects. The detectability of this method was investigated using a real cobalt superalloy overlay welded onto a carbon steel piston rod, in which an artificial volumetric defect was made at the interfacial region. The probability of detection and condition-based maintenance using the detected defect data is discussed based on the P-F curve and the probability of failure graphs.

Reconstruction de fissures 2D à partir d’images courants de Foucault utilisant un modèle direct semi-analytique

We propose a method for reconstructing 2D surface cracks in electrically conducting parts from Eddy-current images. The proposed method relies on the use of a direct semi-analytic model suitable for Eddy-current systems featuring a uniform current excitation. The surface crack reconstruction approach is based on the comparison of eddy current images computed by the model with the eddy current image of the crack to reconstruct. The method is implemented by means of a genetic algorithm and accurate reconstruction is carried out with synthetic data.

Evaluation of the lift-off robustness of eddy current imaging techniques

The development of lift-off invariant strategies is one of the main goals in Eddy Current Non-Destructive Testing research. In the present work, from the analysis of amplitude and phase signals of magnetic field sensors under Multi-Frequency Eddy Current excitation, two imaging procedures are analyzed and compared with respect to their ability to retrieve reliable results even in presence of huge changes of lift-off. A figure of merit based on the Signal to Noise Ratio evaluated on the 2D reconstructed images allows the comparison of the different strategies in terms of the quality of the image to show the defect. The numerical and the experimental tests realized show that the imaging procedure relying on the analysis of the phase-lag is quite insensitive to changes in the lift-off with respect to that based on the amplitude analysis. In detail the former guarantees good results even when the probe lift-off is randomly changed during the test with variations up to 3mm, while the latter is able to tolerate only lift-off fluctuation lower than 1mm.

Visualization and size estimation of fiber waviness in multidirectional CFRP laminates using eddy current imaging

This paper presents an eddy current method to visualize fiber waviness in multidirectional carbon fiber reinforced plastics. Since eddy currents induced by a driver coil flow along carbon fibers, waviness can be visualized if the eddy current path is visualized. We proposed a new complex plane analysis method to visualize an eddy current path from magnetic field measurements. The validity of the method was verified by finite element method analyses. Experiments were performed for multidirectional carbon fiber reinforced plastic specimens. In-plane waviness with misalignment angle from 6.9° to 24.9° were artificially induced in the specimens. An eddy current path was visualized from magnetic field data and it corresponded to the shape of induced waviness. The sizes of the wavy eddy current path were compared with waviness sizes measured by X-ray computed tomography and from optical images. Experimental results indicate that the surface waviness size can be estimated accurately, whereas the subsurface waviness size is underestimated.

Eddy current imaging with an atomic radio-frequency magnetometer

We use a radio-frequency 85Rb alkali-vapor cell magnetometer based on a paraffin-coated cell with long spin-coherence time and a small, low-inductance driving coil to create highly resolved conductivity maps of different objects. We resolve sub-mm features in conductive objects, we characterize the frequency response of our technique, and by operating at frequencies up to 250 kHz we are able to discriminate between differently conductive materials based on the induced response. The method is suited to cover a wide range of driving frequencies and can potentially be used for detecting non-metallic objects with low DC conductivity.

Automated detection of yarn orientation in 3D-draped carbon fiber fabrics and preforms from eddy current data

Abstract Ensuring the correct fiber orientation in draped textiles and 3D preforms is one of the current challenges in the production of carbon-fiber reinforced plastics (CFRP), especially in resin transfer molding (RTM). Small deviations in fiber angle during preforming have a considerable effect on the mechanical properties of the final composite. Therefore, this paper presents an automated method for determining local yarn orientation in three-dimensionally draped, multi-layered fabrics. The draped fabric is scanned with a robot-guided high-frequency eddy current sensor to obtain an image of the sample's local conductivity and permittivity. From this image, the fiber orientation not only of the upper, but also of the lower, optically non-visible layers can be analyzed. A 2D Fast Fourier Transform is applied to local segments of the eddy current image to determine the local yarn orientation. Guidelines for processing the eddy current data, including phase rotation, filtering and evaluation segment size, are derived. For an intuitive visualization and analysis of the determined yarn orientation, reference yarn paths are reconstructed from the determined yarn angles. The developed process can be applied to quality inspection, process development and the validation of forming simulation results.