Eddy Current Probe Research Articles

Simple Giant Magnetoresistance Probe Based Eddy Current System of Defect Characterization for Non-Destructive Testing

The purpose of this paper is to present a new giant magnetoresistance (GMR) sensor, in eddy current testing technique for surface defect detection, in conducting materials, we show that the GMR based eddy currents probe is more sensitive than the inductive probe. A flat coil mounted on ferrite pot used to produce an alternate magnetic field, which gives rise to eddy currents in the material under test. Aluminum plates use with defects have nominal depths, widths, and lengths. The defects scanned with the sensing axis perpendicular to the defect length. Two parameters extracted from the GMR output voltage signal obtained, and a simple correlation between the defect’s dimensions and the GMR output voltage proposed.

Eddy Current Microsensor and RBF Neural Networks for Detection and Characterization of Small Surface Defects

Abstract The growing complexity of industrial processes and manufactured parts, the growing need for safety in service and the desire to optimize the life of parts, require the implementation of increasingly complex quality assessments. Among the various anomalies to consider, sub-millimeter surface defects must be the subject of particular care. These defects are extremely dangerous as they are often the starting point for larger defects such as fatigue cracks, which can lead to the destruction of the parts. Penetrant testing is now widely used for this type of defect, due to its good performance. Nevertheless, it should be abandoned eventually due to environmental standards. Among the possible alternatives, the use of eddy currents (EC) for conductive materials is a reliable, fast, and inexpensive alternative. The study concerns the design and modeling of eddy current probe structures comprising micro-sensors for non-destructive testing. The moving band finite element method is implemented for this purpose to take into account the movement of the sensor, experimental validations were conducted on a nickel-based alloy specimen. The real and imaginary parts of the impedance at every position of the sensor computed by experiments and simulations were in good agreement. The crack detection quality was quantified and the geometric characteristics of the defects were estimated using RBF NN (Radial Basis Function Neural Networks) that were designed and implemented on the acquired signals.

Detection of Surface and Subsurface Flaws with Miniature GMR-Based Gradiometer.

The eddy-current (EC) testing method is frequently utilized in the nondestructive inspection of conductive materials. To detect the minor and complex-shaped defects on the surface and in the underlying layers of a metallic sample, a miniature eddy-current probe with high sensitivity is preferred for enhancing the signal quality and spatial resolution of the obtained eddy-current images. In this work, we propose a novel design of a miniature eddy-current probe using a giant magnetoresistance (GMR) sensor fabricated on a silicon chip. The in-house-made GMR sensor comprises two cascaded spin-valve elements in parallel with an external variable resistor to form a Wheatstone bridge. The two elements on the chip are excited by the alternating magnetic field generated by a tiny coil aligned to the position that balances the background output of the bridge sensor. In this way, the two GMR elements behave effectively as an axial gradiometer with the bottom element sensitive to the surface and near-surface defects on a conductive specimen. The performance of the EC probe is verified by the numerical simulation and the corresponding experiments with machined defects on metallic samples. With this design, the geometric characteristics of the defects are clearly visualized with a spatial resolution of about 1 mm. The results demonstrate the feasibility and superiority of the proposed miniature GMR EC probe for characterizing the small and complex-shaped defects in multilayer conductive samples.

Optimization of the design of eddy current probe of parametric type to detect surface cracks

The Paton Welding Journal, 2022, №03. International Scientific-Technical and Production Journal «The Paton Welding Journal» «The Paton Welding Journal» has been published monthly since 2000 in English, ISSN 0957-798X. «The Paton Welding Journal» is a cover-to-cover English translation of the «Avtomaticheskaya Svarka» (Automatic Welding) journal. The «Avtomaticheskaya Svarka» journal has been published monthly since 1948 in Russian, ISSN 005-111X.

A novel planar differential excitation eddy current probe based on the fractal Koch curve

Planar eddy current (EC) probe is one of the most important EC probe for detecting the defects in conductive parts. A novel planar differential excitation EC probe are proposed and its excitation coil is obtained by the topological transformation combining the fractal Koch curve. To explore the right combination of the excitation coil and pickup coil and compare the performance of the probes, ten different configurations of the EC probe are studied by the simulations and experiment. The results show that the best pickup coil of each excitation is with the same shape and size of the excitation; The Koch excitation and Koch pickup probe have higher sensitivity for 3mm and 5mm length defects in the special direction than other configurations.

Inspection of Defect Under Thick Insulation Based on Magnetic Imaging With TMR Array Sensors

A lot of industrial structures are covered by insulation. Defects under insulation (DUI) frequently distress the structure maintenance and may lead to serious safety concerns. In this article, a probe with tunneling magnetoresistance (TMR) array sensors is proposed to inspect defects under thick insulation. The probe contains 64 TMR sensors with the size of each sensor length <inline-formula> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> width &#x003D; 0.45 mm <inline-formula> <tex-math notation="LaTeX">$\times0.45$ </tex-math></inline-formula> mm, which can generate a magnetic field image with high spatial resolution. Excitation current with extremely low frequency (in the order of a few hundred Hertz) is utilized for inspection of defects with large liftoff distances. In this low-frequency range, the TMR sensors have much better sensitivity than conventional induction coils. A data processing method based on discrete wavelet transform (DWT) is utilized to reduce noise and highlight defect indication. Experimental results demonstrate that the probe is capable of detecting a machined rectangular defect with dimensions of 6 mm (length), 3 mm (width), and 1 mm (depth) under 10 mm thick insulation. The minimum dimensions of the machined defect under 20 mm insulation that can be detected by the probe are 9 mm (length), 3 mm (width), and 1 mm (depth). As a comparison, for a commercial pulsed eddy current probe (PECA-HR-SM), the length of minimum detectable detect is 19 mm under 18 mm thick insulation. If the thickness of the insulation is 30 mm, a machined rectangular defect with dimensions 21 mm (length), 3 mm (width), and 7 mm (depth) can be detected by the proposed probe. The experimental results demonstrate that the proposed probe is a promising alternation with excellent sensitivity for inspection of defects with large liftoff distances.

Superminiature Eddy Current Probe for Measuring the Electrical Conductivity of Copper Thin Films

The article considers the possibility of applying the eddy current method of non-destructive testing for measuring the electrical conductivity of new material - thin metal films. Copper films of various thickness obtained by physical vapour deposition were used as the measurement object. The deposition method and the hardware and software complex for measuring the electrical conductivity of the film were briefly described. A calibration curve that makes it possible to restore the values of the electrical conductivity of the material by the signal value of the eddy current probe was presented. The test results of films with different characteristics were given, and the distribution of the electrical conductivity of the films depending on the batch was shown. Based on different values of the electrical conductivity in a batch, the difference in deposition quality of various films was found.

Microstructure and Eddy-Current Analysis of Aluminum 01570 Welded Joints Obtained by Friction Stir Welding

The article presents the results of studies of aluminum alloys connected by a welded joint obtained by friction stir welding. During this kind of welding a recrystallized fine-grained microstructure is formed in the joint. The pictures of the weld area microstructure are presented, and the eddy current probe signal variations when scanning the joint are shown. The resulting parameter of the eddy current probe was the local electrical conductivity of the weld area. It is demonstrated that the average grain size of the welded joint microstructure varies in the area of weld defects, which was determined by the signal variation of the eddy current probe. The microstructure peculiarities formed in the weld area where the defects are located, and their influence on the mechanical properties of welded joints are discussed. The results of a series of experiments allow concluding about the qualitative characteristics of the obtained welded joints.

Simultaneous Measurement of Radius and Coating Thickness of Finite Length Metal Rods by Eddy Current Surface Probes

The paper proposes an efficient inverse method for simultaneous measurement of radius and coating thickness of a finite length metal rod, using the output signal of a surface eddy current probe. The proposed method adopts a physical model for iterative solution of the inverse problem. It utilizes a hybrid conjugate gradient optimization algorithm in the inversion process, and adopts an efficient forward problem solver to obtain the predicted probe output signal in each iteration. The solution of the forward problem is based on the truncated region eigenfunction expansion method and uses the even/odd parity solution decomposition to treat the governing boundary value problem. The main feature of the proposed inversion method is its robustness in the presence of noise regardless of the choice of the initial values. Experimental and simulation results are provided to demonstrate the validity of the proposed methods for solving the forward and inverse problems. It has been shown that the rod radius and coating thickness can be measured with an error less than 5% of the true value when the probe is located at the half distance from the edge of the rod.

Specific Features of Testing of Anisotropic Nonmagnetic Materials by Eddy-Current Probes with Circular Windings

Specific Features of Testing of Anisotropic Nonmagnetic Materials by Eddy-Current Probes with Circular Windings

A New Detection Method of the Surface Broken Wires of the Steel Wire Rope Using an Eddy Current Differential Probe

This paper describes a new detection method of the surface broken wires of the steel wire rope based on the conducting property of the rope. The surface of the steel wire rope is spiral. This spiral characteristic has influence on the eddy current (EC) response signal of the broken wires. Therefore, an EC differential probe is designed to improve the detection accuracy. First, a finite element analysis (FEA) of detecting the broken wires with the EC differential probe is conducted. When the probe scans above the intact part of the steel wire rope, the distribution of the EC in the rope just under the probe is symmetrical about the axis of the rope. Then the output voltage of the differential probe is almost zero. Accordingly, the output voltage is out of balance when the surface broken wires appear. The simulation results show that the effect of the surface alternating peaks and valleys of the rope on the EC response signal of the broken wires can be eliminated by the proposed EC differential probe. Then an experimental platform is built to carry out the scanning detection of different degrees of the surface broken wires. It can be observed that when the probe moves above the broken wires, the output voltage of the differential probe is significantly larger. Finally, the obtained scanning signal is post processed using the wavelet-based denoising method to improve the signal-to-noise rate. The experimental results show that the proposed method can effectively identify the damage degrees of the surface broken steel wires.

Jensen-Shannon Divergence of Two Eddy Current Distributions Induced by Circular and Fractal Koch Excitation Coils

Eddy current distribution is important to the performance of planar eddy current probes. In this paper, the Jensen-Shannon divergences of tangential intersection angle spectrum and radial direction energy spectrum were proposed to evaluate the difference between eddy current distributions generated by circular and fractal Koch excitation coils. By the simulation for the circular and Koch shape excitation coils, it works out that the difference of the eddy current distributions between the two kinds of coils becomes larger and larger with an increase in the values of the two Jensen-Shannon divergences. At the same time, the correlation between the change of Jensen-Shannon divergence and the detectability of the short crack in the special direction was discussed through simulation and experiment results. It is found that, relative to the crack in 0° direction, the detectability of the Koch and circular differential pickup probes to the crack in 90° direction has a correlation with the Jensen-Shannon divergence of tangential intersection angle spectrum. The width of each signal generated by the two probes has a correlation with the Jensen-Shannon divergence of radial direction energy spectrum.

Comprehensive study on radial coil influence in designing segmented remote field eddy current probe for defect detection in ferromagnetic tubes

ABSTRACT In the present study, two types of receiver coils, viz axial and radial type receivers, in the Remote field eddy current technique are considered to detect defects in ferromagnetic tubes. The appropriate distance between the exciter and the receiver coils and the excitation frequency are estimated using the finite element model and validated experimentally. Model-assisted probability of detection studies are performed to examine the capability of radial and axial type receiver coils. Experimental studies on various artificial defects reveal the excellent performance of the radial type receiver coil in terms of the higher number of defect detections, sensitivity and imaging. This paper provides a comprehensive study on radial and axial type receiver coils for developing array probes. Radial coil performs better and also preserves localisation of defect, which is more desirable for developing array probe.

Development of an Eddy Current Test Configuration for Welded Carbon Steel Pipes under the Change in Physical Properties

Carbon steel pipe is used in various industries, including nuclear power plants. Due to the daily cyclic operation of the pipe over time, environmental influences, and extreme working conditions, the probability of developing small fine cracks in the welded areas of the pipes increases. For that reason, it requires earlier assessment, and providing adequate inspection and evaluation of the weld area of the pipes used in such an installation is crucial to increase the safety level. In this paper, two different probe configurations were used to assess the integrity of the girth weld of the SA106 carbon steel pipe welded by gas tungsten arc welding. The conventional eddy current probe was initially used, but as it had some limitations, a new probe configuration was proposed to overcome these constraints. Numerical simulations using the finite element method were performed, based on the real measurement of the physical properties of the specimen, to complement the experimental data. In addition, the experimental results were successfully reproduced by the simulations. Simulation and experimental results show that the proposed probe configuration allows adequate inspection.

Research of oscillation mode in automated pulsed eddy current testing systems

The formation and analysis of eddy current probe signals obtained in pulsed excitation mode is considered. The proposed method of implementing pulsed eddy current testing with the formation of attenuating harmonic oscillations is more resistant to the effects of noise and interference that accompany the process of inspected object parameters evaluation. The equivalent scheme of the system “test object–eddy current probe” is developed and analyzed. The obtained mathematical model of the eddy current probe signals allowed proposing the natural frequency and the attenuation as informative signals parameters, which are determined from signals phase and amplitude characteristics. Developed algorithm and the proposed methodology was implemented for evaluation of eddy current signals parameters and related characteristics of testing objects. This method was experimentally verified on a series of different test specimens. The obtained results confirm the possibility to apply the proposed informative signals to solve some problems concerned with automated eddy current testing. The formation and analysis of eddy current probe signals obtained in pulsed excitation mode are considered. The proposed method of implementing pulsed eddy current testing with the formation of attenuating harmonic oscillations is more resistant to the effects of noise and interference that accompany the process of automated eddy current testing. The equivalent scheme of the system “test object–eddy current probe” is developed and analyzed. The obtained mathematical model of the eddy current probe signals allows proposing the natural frequency and the attenuation as informative signals parameters, which are determined from signal phase and amplitude characteristics. Methods of increasing the accuracy of determining the eddy current probe signals attenuation and frequency using trends of signals phase and amplitude characteristics are considered. The proposed signal processing method was verified by modeling the process of determining the eddy current probe signals attenuation and the frequency from the signal with Gaussian noise. Algorithmic and software were developed based on the simulation results and the proposed improved methodology was implemented for determining signals parameters and related parameters and characteristics of testing objects.

MATHEMATICAL MODELING OF MULTISTRAND STAY CABLES EDDY CURRENT NON-DESTRUCTIVE TESTING

Some aspects of mathematical simulation of the eddy current non-destructive testing for ferromagnetic components are considered that allow simplifying the mathematical model. The influence of the motion of an inhomogeneous test object in an inhomogeneous field of an eddy current probe of finite dimensions is considered. The influence of the inhomogeneous magnetization of the object, the nonlinearity and ambiguity of the dependence of the magnetic flux density on the magnetic field intensity is described. An example of a mathematical model and simulation results is presented.

A comparison of non-linear optimisation algorithms for recovering the conductivity depth profile of an electrically conductive block using eddy current inspection

This paper compares 15 different non-linear optimisation algorithms for reconstructing the depth profile of electrical conductivity for a conductive block of material using multi-frequency measurements from an eddy current probe. Synthetic data was generated using a finite element model, where the underlying conductivity profile was randomly generated and smoothed, noise was then superimposed on the synthetic data. The compared algorithms were designed such that optimal performance was achieved for each algorithm by using the knowledge of the true underlying conductivity profile used to generate the synthetic data. The most competitive algorithm after ten iterations was determined to be dependent on the a priori estimate and level of noise, where the Broyden–Fletcher–Goldfarb–Shanno, augmented Levenberg-Marquardt and Levenberg-Marquardt are each top of different case studies. The augmented Levenberg-Marquardt and Levenberg-Marquardt were then employed on measured data, where the algorithms used a novel update procedure (in the nuclear graphite application) for the regularisation parameter. The study considers the conductivity profile of graphite blocks of the type used in the nuclear industry.

Monitoring of LISS Nozzle Proximity to CANDU® Fuel Channels Using the Eddy Current Gap Probe

Fuel channels in CANDU® (CANadian Deuterium Uranium) nuclear reactors consist of two non-concentric tubes; an inner pressure tube (PT) and a larger diameter calandria tube (CT). The gas annulus space between the 6 m long tubes, known as the gap, is monitored from within the PT using a transmit-receive eddy current probe that can detect changes in the environment using electromagnetic induction according to Faraday’s Law. Accurate measurements are required to ensure that contact between PT and CT is not imminent. Fuel channels may also pass over perpendicularly oriented tubes (nozzles) that are part of the liquid injection shutdown system (LISS). The proximity of LISS nozzles not only compromises gap measurement, but flow-induced vibrations from within the moderator could lead to CT deformation and/or fretting at LISS-CT contact. Investigation of the eddy current based gap probe as a tool to measure proximity of LISS nozzles was carried out experimentally. Eddy current response as a function of LISS-PT proximity, the distance between the PT outer diameter and LISS nozzle outer diameter, was determined. When PT wall thickness, PT resistivity, probe lift-off, and PT-CT gap variations spanned typical values found in-reactor this dependence could be used to determine the LISS-PT proximity up to 20 mm with sub-millimeter accuracy. This method has the potential to provide LISS-CT proximity using existing gap measurement data. Obtaining LISS nozzle proximity at multiple inspection intervals could be used to provide an estimate of the time to LISS-CT contact, and thereby, provide a means of optimizing maintenance schedules.

Surrogate synthesis of excitation systems for frame tangential eddy current probes

Surrogate synthesis of excitation systems for frame tangential eddy current probes

Detection of fiber waviness in carbon fiber prepreg using Eddy current method

In order to control the quality of carbon fiber reinforced polymer in the production process, a method of detecting fiber waviness in carbon fiber prepreg using eddy current (EC) probe is proposed. The EC probe includes orthogonal rectangular coils. The excitation coil is parallel to the designed fiber direction, so that the output voltage of the reception coil is only related to the bending of EC caused by fiber waviness. The phase of the output voltage is used for characterizing fiber direction. The results show that the probe is able to distinguish fiber direction as small as 0.5° and fiber waviness in prepreg can be detected. Moreover, algorithm of reconstructing fiber distribution is proposed and fiber waviness is visualized.