Eddy Current Probe Research Articles

Towards Advancing Real-Time Railroad Inspection Using a Directional Eddy Current Probe

In the field of railroad safety, the effective detection of surface cracks is critical, necessitating reliable, high-speed, non-destructive testing (NDT) methods. This study introduces a hybrid Eddy Current Testing (ECT) probe, specifically engineered for railroad inspection, to address the common issue of “lift-off noise” due to varying distances between the probe and the test material. Unlike traditional ECT methods, this probe integrates transmit and differential receiver (Tx-dRx) coils, aiming to enhance detection sensitivity and minimise the lift-off impact. The study optimises ECT probes employing different transmitter coils, emphasising three main objectives: (a) quantitatively evaluating each probe using signal-to-noise ratio (SNR) and outlining a real-time data-processing algorithm based on SNR methodology; (b) exploring the frequency range proximal to the electrical resonance of the receiver coil; and (c) examining sensitivity variations across varying lift-off distances. The experimental outcomes indicate that the newly designed probe with a figure-8 shaped transmitter coil significantly improves sensitivity in detecting surface cracks on railroads. It achieves an impressive SNR exceeding 100 for defects with minimal dimensions of 1 mm in width and depth. The simulation results closely align with experimental findings, validating the investigation of the optimal operational frequency and lift-off distance for selected probe performance, which are determined to be 0.3 MHz and 1 mm, respectively. The realisation of this project would lead to notable advancements in enhancing railroad safety by improving the efficiency of crack detection.

Measurement of Hydraulic Fracture Aperture by Electromagnetic Induction.

We present a new method for accurately measuring the aperture of a fluid-driven fracture. This method uses an eddy current probe located within a completion tool specifically designed to obtain the fracture aperture in the wellbore at the location where the fluid is injected into the fracture. The probe induces an eddy current in a target object, producing a magnetic field that affects the overall magnetic field. It does not have any limitations with respect to fluid pressure and temperature within a large range, making it unlike other methods. We demonstrate the accuracy and performance of the sensor under laboratory conditions. A hydraulic fracture experiment in a porous sandstone is conducted and discussed. The obtained measurement of the evolution of the fracture inlet aperture by the eddy current probe during the multiple injection cycles performed provided robust information. The residual fracture aperture (after the test) measured by the probe is in line with estimations from image processing of X-ray CT scan images as well as a thin-section analysis of sub-parts of the fractured specimen. The robustness and accuracy of this electromagnetic induction probe demonstrated herein under laboratory conditions indicate an interesting potential for field deployment.

Performance Investigation of Multi-Turn Pickup for Circular Fractal Flexible Planar Eddy Current Probe

ABSTRACT This paper aims to compare and analyze the performance differences between a single-coil and multi-coil pickup coil layout in a flexible circular fractal planar eddy current probe. Flexible planar eddy current probes offer flexibility and adaptability, making them widely used in the detection of complex structures. The circular fractal flexible planar eddy current probe is a new type of probe that has a lower miss rate in detecting cracks at specific angles compared to traditional circular and rectangular coil eddy current probes. However, the single-coil structure of the probe’s pickup coil has the drawback of being less sensitive to weak signals. This paper investigates the detection performance of the flexible circular fractal probe by varying the number of turns in the original pickup coil. Experimental comparisons were conducted by preparing and installing single-coil and multi-coil circular fractal planar eddy current probes. The performance differences between single-coil and multi-coil configurations were compared and analyzed under various excitation frequencies, liftoff distances, and internal crack depths. The results showed that, compared to the single-coil probe, the multi-coil probe exhibited significantly improved detection signal values and sensitivity.

Circumferential Crack Detection in Ultra-High-Pressure Tubular Reactors with Pulsed Eddy Current Testing.

Ultra-high-pressure tubular reactors are crucial pieces of equipment for polyethylene production. Long-term operation under high temperature, high pressure, and other extremely harsh conditions can lead to various defects, with circumferential cracks posing a major safety risk. Detecting cracks is challenging, particularly when they are under a protective layer of a certain thickness. This study designed a pulsed eddy current differential probe to detect circumferential cracks in ultra-high-pressure tubular reactors, with the lift-off distance acting as a protective layer. Detection models for traditional cylindrical and semi-circular excitation differential probes were established using finite element simulations. Corresponding experiments under different lift-off conditions were carried out, and the model's accuracy was verified by the consistency between the simulation results and experimental data. The distribution of the eddy current field under different conditions and the disturbances caused by cracks at various positions to the detection signal were then calculated in the simulations. The simulation results showed that the cracks significantly disturbed the eddy current field of the semi-circular excitation differential probe compared with that of the traditional cylindrical probe. The designed differential probe effectively detected circumferential cracks of specific lengths and depths using the difference in the voltage signals. The experimental results were in agreement with the simulation results, showing that the designed probe could effectively detect 20 mm-long circumferential cracks at a lift-off of 60 mm. The experimental results also show that the probe's detection coverage area in the axial direction varied with the lift-off height. The probe design and findings are valuable for detecting cracks in ultra-high-pressure tubular reactors with protective layers.

Research of EDDY current probes for inspection of aluminum alloy structure welds using smartphone-based flaw detector

Research of EDDY current probes for inspection of aluminum alloy structure welds using smartphone-based flaw detector

Modeling of two-element tangential eddy current probe with active shielding for soldered joints inspection

The paper is devoted to designing a finite-element model of the eddy current probe for testing the degree of soldering of lap soldered joints of conductive busbars. The method for finite-element model designing is developed, which parameters provide calculations error of signals of finite-element model of eddy current probe below 1.5%. The test results confirm the performance of the eddy current probe, the measurement range for soldered joints defect size from 0 to 100 % and provision of the declared basic absolute error of the degree of soldering measurement of 5 %.

Eddy Currents Probe Design for NDT Applications: A Review.

Eddy current testing (ECT) is a crucial non-destructive testing (NDT) technique extensively used across various industries to detect surface and sub-surface defects in conductive materials. This review explores the latest advancements and methodologies in the design of eddy current probes, emphasizing their application in diverse industrial contexts such as aerospace, automotive, energy, and electronics. It explores the fundamental principles of ECT, examining how eddy currents interact with material defects to provide valuable insights into material integrity. The integration of numerical simulations, particularly through the Finite Element Method (FEM), has emerged as a transformative approach, enabling the precise modeling of electromagnetic interactions and optimizing probe configurations. Innovative probe designs, including multiple coil configurations, have significantly enhanced defect detection capabilities. Despite these advancements, challenges remain, particularly in calibration and sensitivity to environmental conditions. This comprehensive overview highlights the evolving landscape of ECT probe design, aiming to provide researchers and practitioners with a detailed understanding of current trends in this dynamic field.

Non-Contact Eddy Current Conductivity Measurements as an Effective Tool for Evaluating Aluminum Alloys in Aircraft

ABSTRACT Aluminum alloys (AAs) are pivotal materials in modern aircraft due to their superior mechanical properties and low weight. The structural integrity of these alloys, crucial for aircraft safety, heavily depends on heat treatment processes that alter their mechanical characteristics. Nondestructive evaluation (NDE) techniques, such as eddy current (EC) conductivity measurements, play a vital role in assessing these alloys throughout their lifecycle. EC methods enable the measurement of electrical conductivity, a structure-sensitive parameter that correlates with mechanical properties affected by heat treatments and operational stresses. This paper reviews the application of EC conductivity measurements in the aerospace industry, focusing on their role in assessing AA structural integrity. It discusses how EC methods can penetrate non-conductive coatings, crucial for in-service measurements without surface removal. Recent developments include a novel small-size EC probe and signal processing algorithms aimed at enhancing sensitivity to conductivity changes through dielectric coatings, up to 0.5 mm thick, commonly found in aircraft structures. Key findings include analyses of specific electrical conductivity (SEC) changes in AAs due to heat treatment deviations and long-term operational stresses, crucial for predicting residual life and maintaining safety standards. Case studies on aircraft wing skins and helicopter rotor blades demonstrate the practical application of EC conductivity meters in identifying critical damage zones. The methodology proves effective in evaluating localized degradation based on SEC distributions, thereby enhancing maintenance efficiency and aircraft safety. Overall, this research underscores the significance of EC conductivity measurements in advancing NDE practices for AAs in aircraft applications. The methodologies and findings presented aim to improve safety, durability assessment, and maintenance efficiency in the aerospace industry.

Robotic Technology Enables Automated Nondestructive Weld Inspection in Subsea Zone

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 215588, “Automated Nondestructive Weld Inspection in Splash/Subsea Zone,” by Karsten Husby, Magnus Bjerkeng, and Jens T. Thieleman, SINTEF, et al. The paper has not been peer reviewed. _ Jacket structures all over the world are now reaching the limit of their estimated service life, but new discoveries and improved production methods require lifetime extension of the assets. The complete paper presents an automated underwater robot and a nondestructive testing (NDT) eddy-current probe for alternating-current field-measurement analysis of jacket main welds. This robot makes integrity testing, which often is required for lifetime extensions, possible. Introduction Jacket lifetime calculations are based on long service intervals. Some welded braces in the splash zone often are considered unserviceable because of the hostile environment. Thus, divers or remotely operated vehicles cannot be used under normal weather conditions and wave heights. Structural integrity is ensured with long service intervals, often long enough for the lifetime of the jacket. However, multiple jacket platforms are approaching their certified lifespan limit. The question, then, is whether they should be decommissioned, reused, or have their lifespan extended. New discoveries and improved production methods make extended operation desirable for certain fields. For other platforms, repurposing for offshore wind, aquaculture, or geothermal energy may be a cost-effective alternative. Regardless, before any action can be taken, the integrity of the jackets must be tested. There is, therefore, a growing need for subsea condition monitoring of aging offshore jacket steel platforms. To speed up operations and to handle complicated health, safety, and environment (HSE) issues in the splash zone, robots can be used. A robot for NDT inspection in the hostile splash zone is pictured in Fig. 1. The robot clamps itself to the circular braces and is equipped with 3D vision camera, kinematics algorithms, and an NDT probe. Integrity Testing Under Rough Conditions Testing for integrity of critical infrastructure often is performed by different NDT methods. In this study, eddy-current testing was chosen as the preferred method because it is fast and can penetrate layers of marine growth on the surface. The normal procedure is to use lightweight hand-held equipment. The splash zone, however, is often inaccessible under modern HSE standards. In this project, an automated NDT robot helped conduct efficient NDT testing for longer periods in challenging conditions. This robot offloads the NDT operator and makes testing more efficient.

Eddy Current Sensor Probe Design for Subsurface Defect Detection in Additive Manufacturing.

Pore and crack formation in parts produced by additive manufacturing (AM) processes, such as laser powder bed fusion, is one of the issues associated with AM technology. Surface and subsurface cracks and pores are induced during the printing process, undermining the printed part durability. In-situ detection of defects will enable the real-time or intermittent control of the process, resulting in higher product quality. In this paper, a new eddy current-based probe design is proposed to detect these defects in parts with various defects that mimic pores and cracks in additively manufactured parts. Electromagnetic finite element analyses were carried out to optimize the probe geometry, followed by fabricating a prototype. Artificial defects were seeded in stainless steel plates to assess the feasibility of detecting various flaws with different widths and lengths. The smallest defect detected had a 0.17 mm radius for blind holes and a 0.43 mm notch with a 5 mm length. All the defects were 0.5 mm from the surface, and the probe was placed on the back surface of the defects. The surface roughness of the tested samples was less than 2 µm. The results show promise for detecting defects, indicating a potential application in AM.

Optimizing Surface Type Eddy Current Probe Design with Dual Induction for Wobble Noise Suppression

Optimizing Surface Type Eddy Current Probe Design with Dual Induction for Wobble Noise Suppression

Design of petal-shaped probe for pulse eddy current testing based on finite element simulation

Abstract This paper employs finite element simulation techniques to devise an innovative array-type petal-shaped pulsed eddy current testing probe and investigates its distinctions related to conventional cylindrical probes. Within the simulation framework, a thorough comparative analysis of the detection capabilities of the two probe types is conducted, encompassing an examination of the eddy current regions formed, the impact of lift-off effects and sensitivity to circular corrosion defects. The simulation outcomes reveal that the probe designed in this study exhibits a more extensive detection range and superior detection performance in comparison to its counterparts. This observation holds significance for defect detection in coated materials and the refinement of pulsed eddy current testing probe designs.

Detecting Near-Surface Sub-Millimeter Voids in Additively Manufactured Ti-5V-5Al-5Mo-3Cr Alloy Using a Transmit-Receive Eddy Current Probe.

Additive Manufacturing (AM) Direct Laser Fabrication (DLF) of Ti-5Al-5V-5Mo-3Cr (Ti5553) is being developed as a method for producing aircraft components. The additive manufacturing process can produce flaws near the surface, such as porosity and material voids, which act as stress raisers, leading to potential component failure. Eddy current testing was investigated to detect flaws on or near the surface of DLF Ti5553 bar samples. For this application, the objective was to develop an eddy current probe capable of detecting flaws 500 µm in diameter, located 1 mm below the component's surface. Two initial sets of coil parameters were chosen: The first, based on successful experiments that demonstrated detection of a near surface flaw in Ti5553 using a transmit-receive array probe, and the second, derived from simulation by Finite Element Method (FEM). An optimized transmit receive coil design, based on the FEM simulations, was constructed. The probe was evaluated on Ti5553 samples containing sub-surface voids of the target size, as well as samples with side-drilled holes and samples with holes drilled from the opposing inspection surface. The probe was able to effectively detect 80% of the sub-surface voids. Limitations included the probe's inability to detect sub-surface voids near sample edges and a sensitivity to surface roughness, which produces local changes in lift-off. Multifrequency mixing improved signal-to-noise ratio when surface roughness was present on average by 22%. A probe based on that described in this paper could benefit quality assurance of additively manufactured aircraft components.

Magnetic Field-Based Eddy-Current Probe Design, Modeling, and Computing Methods for Edge Defect Detection

Magnetic Field-Based Eddy-Current Probe Design, Modeling, and Computing Methods for Edge Defect Detection

THE INFLUENCE OF SURFACE EDDY CURRENT PROBE SIGNAL SMOOTHING ON ASSESSING STRESS-CORROSION CRACKS DEPTH IN MAIN GAS PIPELINE

The paper presents the results on the influence of signals smoothing of absolute and differential surface eddy current probes (ECP) above an individual crack and a group of stress-corrosion cracks on the accuracy of estimating the depths of stress-corrosion cracks during eddy current flaw detection of a main gas pipeline (MGP). Mathematical models demonstrate that the degree of signal smoothing of the surface ECP over an individual crack is higher than the degree of smoothing of the ECP signal over a group of stress-corrosion cracks in the MGP. Smoothing results in a decreased resolution of the surface ECP signal. The differential surface ECP is characterized by a higher resolution than the similar absolute surface ECP. The study demonstrates the reduction in amplitude difference between the actual signals of the absolute surface ECP over an individual crack and a group of stress-corrosion cracks, compared to the smoothed signal of the surface ECP. In the estimates of depths in the group of stress-corrosion cracks in MGP, obtained from the amplitude of the actual signal of the absolute surface ECP, where smoothing of the ECP signal is excluded, a significant reduction in the relative error is observed.

IMPROVING THE ACCURACY OF FINITE ELEMENT MODELING OF A HIGH-FREQUENCY EDDY CURRENT PROBE

The paper considers the issues of finite element modeling of an eddy current probe designed to measure the thickness of a dielectric coating on an electrically conductive non-magnetic base metal. The quality criteria of the model are formulated and the requirements for them are presented: the precision and error of calculating the signals of the finite element model of an eddy current probe should not exceed ±(0.001h + 0.1) microns when the thickness h of the coating changes. To achieve this task, an analysis of the parameters affecting precision and error was carried out, the criteria for their assessment were justified and an approach to the problem of increasing the frequency of the finite element grid in critical places of the finite element model was formulated. To confirm the achieved characteristics, a real eddy current probe was manufactured, its characteristics were evaluated using a coating thickness simulator machine and measures of specific electrical conductivity. An algorithm for calibrating the signals of a real eddy current probe is presented, which is used to validate a finite element model.

Detecting and Estimating Local Corrosion Damages in Long-Service Aircraft Structures by the Eddy Current Method with Double-Differential Probes

ABSTRACT Monitoring corrosion in aircraft structures through nondestructive testing is crucial for maintaining long-term aircraft serviceability. Corrosion monitoring is particularly challenging when corrosion damage is situated on internal surfaces of multilayer aircraft structures. The eddy current method is one of the most promising techniques for detecting and measuring such subsurface corrosion damage without direct contact or disassembly. However, due to their low sensitivity traditional eddy current probes with coaxial coils are not well suited for detecting corrosion damages of the local type, such as pitting or corrosion pits, in multilayer aircraft structures. This study tested the use of low-frequency eddy current probes of the double-differential type, characterized by 8 and 10 mm operational diameters, in detecting and measuring hidden corrosion damages of this local type. Such corrosion damages were simulated by means of flat-bottomed drilled holes of differing diameters and depths (or different diameters and residual thicknesses of the inspected sheet in the damaged area). The signals from the eddy current probes were evaluated in the complex plane using a universal eddy current flaw detector. The correlations between the amplitude and phase of the eddy current signal and depth of location of the local corrosion damages were analyzed. Results indicate that it is possible to estimate the residual thickness of the skin in locally corroded areas by measuring the eddy current signal phase, independently of the local corrosion damage diameter (size), providing useful information for residual service life determination.

Radial Field Detector to Improve Sensitivity of Localized Defects in Remote Field Eddy Current Technique

ABSTRACT This paper presents the development of a cone-shaped radial magnetic-field-detector–based remote field eddy current (RFEC) probe for enhanced detection of localized defects in ferritic steel tubes. The performance of the radial coil detector has been evaluated for the detection of different types of defects such as through holes, flat bottom holes and notches of different sizes and orientations. The RFEC signals are observed to be corrupted by heavy noise due to spatial variations in material permeability. To remove the random noise from the nonstationary time series RFEC signals, empirical mode decomposition (EMD) processing has been adopted and has been found to significantly improve the signal-to-noise (SNR) ratio. Furthermore, the effect of the circumferential extent of defects on the signal amplitude is analyzed through experimental and finite-element-model–based studies. The linear resolution of the designed radial coil detector and its repeatability behavior are also scrutinized. Studies demonstrate that the developed radial field detector is a promising configuration in remote field eddy current technique for enhanced defect detection and also in designing an array probe.

An improved multi-frequency eddy current array sensor for detecting micro-defects of welding seam in metal plates

This study proposes an improved multi-frequency eddy current array sensor for the accurate detection of micro-defects (under-welding, over-welding, partial welding breakage, and minute penetration holes) of welding seam in metal plates. The primary goal is to further reduce the external and internal noise of the induced coil by optimizing the composition structure of the eddy current probe, thereby improving its sensitivity and signal-to-noise ratio in micro-defect detection. In response to the issue of external noise, this research introduces a gradient differential coil model, which can compensate for initial errors based on a geometrically balanced structure. By establishing a 3D simulation model and a physical mechanical structure, the suppression effect of this structure on excitation magnetic field and external environmental interference is verified. Furthermore, for the internal noise, a mathematical model of the direct stranded litz wire is established. Theoretical calculations and experimental validation confirm that the litz wire exhibits lower eddy current loss and higher Q value compared to traditional copper conductors, thus mitigating internal noise and improving signal-to-noise ratio. On this basis, the excitation mode of the eddy current coil probe array is set to a multi-frequency excitation method based on a center frequency, greatly reducing the electromagnetic interference between channels. Experimental results demonstrate that the improved eddy current sensor can effectively recognize as low as Φ1 mm weld penetration defect at a lift-off value of 0.5 mm, achieving a remarkably high signal-to-noise ratio of approximately 17.7 dB.

Lift-Off Effect of Koch and Circular Differential Pickup Eddy Current Probes

A flexible or planar eddy current probe with a differential structure can suppress the lift-off noise during the inspection of defects. However, the extent of the lift-off effect on differential probes, including different coil structures, varies. In this study, two planar eddy current probes with differential pickup structures and the same size, Koch and circular probes, were used to compare lift-off effects. The eddy current distributions of the probes perturbed by 0° and 90° cracks were obtained by finite element analysis. The analysis results show that the 90° crack can impede the eddy current induced by the Koch probe even further at relatively low lift-off distance. The peak-to-peak values of the signal output from the two probes were compared at different lift-off distances using finite element analysis and experimental methods. In addition, the effects of different frequencies on the lift-off were studied experimentally. The results show that the signal peak-to-peak value of the Koch probe for the inspection of cracks in 90° orientation is larger than that of the circular probe when the lift-off distance is smaller than 1.2 mm. In addition, the influence of the lift-off distance on the peak-to-peak signal value of the two probes was studied via normalization. This indicates that the influence becomes more evident with an increase in excitation frequency. This research discloses the lift-off effect of differential planar eddy current probes with different coil shapes and proves the detection merit of the Koch probe for 90° cracks at low lift-off distances.