Eddy Current Array Research Articles

Synthesized Magnetic Field Focusing for the Non-Destructive Testing of Oil and Gas Well Casing Pipes Using Pulsed Eddy-Current Array

In this article, we address the problem of synthesized magnetic field focusing (SMF) of the pulsed eddy-current (PEC) technology for the non-destructive testing (NDT) of oil and gas well casing pipes. The electromagnetic response excited by multiple transmitters was derived using the signal model of downhole PEC system. The magnetic field was shown to be adjustable by modifying the currents of the multiple transmitting array. Based on this property, magnetic fields were synthesized with the mean squared error (MMSE) criterion to be focused on the center of the transmitting array, and a weighted MMSE (WMMSE)-based SMF was also proposed to achieve an optimal synthetic magnetic field. Furthermore, the primal-dual interior-point method was used to solve the WMMSE optimization problem with the total and single transmit power constraints for the practical application. Simulations and experimental results showed that the SMF with the PEC array effectively focuses the eddy-current field on the center of the transmitting array.

Effects of curvature radius on flexible eddy current array sensors for the curved surface of metal components

The flexible eddy current array sensor owns the advantages of high sensitivity and strong adaptability, but the results are always affected by the curvature radius of complex curved surfaces. The relationship between the curvature radius of the curved surface and detection signals for surface-breaking cracks is mainly discussed. The change of magnetic field caused by the curved surface in the present eddy current testing is specially pointed out, which manifest themselves in the compression or enhancement of the testing signal in its peak value and the baseline drifts. Simulation and experimental results indicate that the concave surface weakens the signal, while the convex surface enhances the signal. The signal amplitude decreases with the decrease in the curvature radius for the concave surface, while it is the opposite for the convex surface. Meanwhile, coil spacing significantly affects the amplitude–curvature radius curve. Furthermore, the fluctuation characteristic affected by the curvature radius under different coil spacing is analyzed. The discovery and results will benefit the quantitative evaluation of flexible eddy current array testing.

Automated Real-Time Eddy Current Array Inspection of Nuclear Assets.

Inspection of components with surface discontinuities is an area that volumetric Non-Destructive Testing (NDT) methods, such as ultrasonic and radiographic, struggle in detection and characterisation. This coupled with the industrial desire to detect surface-breaking defects of components at the point of manufacture and/or maintenance, to increase design lifetime and further embed sustainability in their business models, is driving the increased adoption of Eddy Current Testing (ECT). Moreover, as businesses move toward Industry 4.0, demand for robotic delivery of NDT has grown. In this work, the authors present the novel implementation and use of a flexible robotic cell to deliver an eddy current array to inspect stress corrosion cracking on a nuclear canister made from 1.4404 stainless steel. Three 180-degree scans at different heights on one side of the canister were performed, and the acquired impedance data were vertically stitched together to show the full extent of the cracking. Axial and transversal datasets, corresponding to the transmit/receive coil configurations of the array elements, were simultaneously acquired at transmission frequencies 250, 300, 400, and 450 kHz and allowed for the generation of several impedance C-scan images. The variation in the lift-off of the eddy current array was innovatively minimised through the use of a force–torque sensor, a padded flexible ECT array and a PI control system. Through the use of bespoke software, the impedance data were logged in real-time (7 ms), displayed to the user, saved to a binary file, and flexibly post-processed via phase-rotation and mixing of the impedance data of different frequency and coil configuration channels. Phase rotation alone demonstrated an average increase in Signal to Noise Ratio (SNR) of 4.53 decibels across all datasets acquired, while a selective sum and average mixing technique was shown to increase the SNR by an average of 1.19 decibels. The results show how robotic delivery of eddy current arrays, and innovative post-processing, can allow for repeatable and flexible surface inspection, suitable for the challenges faced in many quality-focused industries.

Analytical solutions for flexible circular spiral eddy current array coils inside a conductive tube under different operation modes

The eddy current induction of flexible eddy current array probe composed of thin curved circular spiral coils is studied using the second-order vector potential formalism. The analytical expressions of self-and mutual impedance change of coil array element inside a conductive tube under three standard eddy current array operation modes are derived. They are self-transmitting and receiving, absolute and differential transmit-receive, respectively. Further derivation shows that the coefficient matrices of characteristic equations in these modes are the same regardless of whether the element is placed inside or outside a tube, but the non-homogeneous terms are different. Therefore, the solution to another case can be obtained quickly through the transformation relationship. The experimental results show that the theoretical predictions are consistent with the experiment over the 5 kHz–1 MHz excitation frequency range. At the same time, the real and imaginary part errors of impedance change are analyzed, and the application of small inductance flexible coil in three operation modes is discussed. Finally, these expressions provide academic support for the eddy current testing of pipeline defects and their significance in the inversion of tube electromagnetic properties.

Series and interactive sensing coil layouts for reducing sensing channels of flexible eddy current array sensor

PurposeFlexible eddy current array (FECA) sensor is flexible and light in weight, which has broad application prospects in structural health monitoring. But, the sensor’s sensing channel number is more, increasing the added mass of sensor networks. This paper aims to reduce the sensing channel number by changing the sensing coil layout.Design/methodology/approachIn this paper, FECA sensors with series sensing coil (SSC) layout and interactive sensing coil (ISC) layout are proposed, which reduce the number of sensor’s channels by half. Then, the variation of the output signal of the sensor when the crack expands along both sides of the hole is analyzed by simulation model. Finally, the fatigue crack monitoring experiment is carried out.FindingsFor the SSC layout, the simulation results show that the amplitude of each SSC group of the sensor increases when the crack propagates to the left or right. For the ISC layout, when the crack propagates on the right side of bolt hole, the induced voltage of each ISC group decreases. When the crack propagates on the left side of bolt hole, the induced voltage of each ISC group increases. The experiment results are consistent with simulation results, which verifies the correctness of simulation model. Compared with SSC layout, the ISC layout can judge the crack propagation direction. And the crack monitoring accuracy is 1 mm.Originality/valueThe research results provide a certain reference for reducing the number of sensor’s sensing channels. Results of the simulation and experiment show that the ISC layout can judge the crack propagation direction, and the crack monitoring accuracy is 1 mm.

Assessment and detection of stress corrosion cracking by advanced eddy current array nondestructive testing and material characterization

Oil and natural gas pipelines are under a combination of loading (stresses) due to their pressurized content and some kind of corrosive environmental operational condition including the content materials as well as soil side which is called Stress Corrosion Cracking (SCC). Since the SCC occurs at stresses within the range of typical design stresses, it can cause catastrophic failures without a significant indication in pipeline integrity beforehand. Efficient Nondestructive Inspection (NDI) during the operational life of the structures can prevent such failures by providing accurate information regarding size and characteristics of the SCC flaws. Since majority of SCC defects occur in surface and near surface regions of metallic pipeline, electromagnetic NDI (Eddy Current) is proved to be a promising inspection technique. In this study, Eddy Current Array (ECA) methodology, as a recent advancement and developed version of traditional Eddy Current (EC) NDI technique, was used for assessment of SCC generated in steel samples representing steel pipeline's material in a laboratory setup. ECA as an advanced eddy current NDI uses an array of eddy current coils to provide the scanning image of the inspection area. Prefabricated notches and SCC after a period of load-corrosion application to the samples were detected and characterized. Accuracy and repeatability of the results showed the efficiency of ECA for SCC characterization.

Research on crack monitoring technology of flexible eddy current array sensor based on TMR sensors

The TMR (Tunnel MagnetoResistance) sensors are used to replace the original induction coils, and the TMR sensors are arrayed along the x-axis direction, which quantitatively monitor crack propagation through the response sequence of TMR sensors. Firstly, a current dipole model is proposed to characterize the influence of disturbed magnetic induction intensity at the test point, and the method of quantitative monitoring of subsurface crack is proposed. Then, the finite element model of the sensor is established to analyze the disturbed effect of crack propagation on eddy current with different excitation frequencies. Finally, the experiments on the surface and subsurface crack monitoring with different depths are carried out. The experimental results show that the crack propagation can be monitored quantitatively through the response sequence of the TMR sensors. Also, the effect of subsurface crack monitoring can be improved by increasing the longitudinal distance between the test point and the crack (LDTC).

A new method for measuring 3D rotation angle of spherical joint

A new method for measuring 3D rotation angle of precision spherical joint based on eddy current sensors is proposed in this study. A pseudorandom code is used to generate a 2D plane absolute code and then mapped to the sphere. The 3D absolute code is formed on the ball head through precision cutting, and an eddy current sensor array is arranged in the ball socket to identify the code. The experimental tests were conducted on the parameter-optimized prototype, and the experimental results showed that the mean squared error of the 3D rotation angle measurement was approximately 25′ in the range. The proposed new measurement principle provides a new approach to the precision measurement of spherical rotary motion, which is expected to produce a breakthrough in the theory and technology of spherical-angle sensor measurement. The proposed new principle also provides a theoretical basis and technical experience for the integration and development of intelligent devices.

Finite-Element Study of Motion-Induced Eddy Current Array Method for High-Speed Rail Defects Detection

Nondestructive testing (NDT) methods are widely used in the rail industry to detect and characterize rolling contact fatigue (RCF) defects in railroads, which is very important for railway inspection and maintenance to prevent catastrophic accidents. Existing NDT methods, e.g., ultrasonic testing (UT), magnetic flux leakage (MFL), and eddy current testing (ECT) have been successfully applied in the rail industry, and the state-of-the-art UT method reported recently achieved a high speed of 40 mi/h with a probability of detection (POD) over 80% under 30% false alarms. However, NDT methods still suffer from a bottleneck in that a higher inspection speed causes lower detection sensitivity due to their physical limits, such as negative velocity effect and long sensing time. One of the leading challenges to the rail NDT community is to develop a high-speed high-sensitivity (HSHS) capability that can provide an improved POD of rail defects in high-speed inspection scenarios over 60 mi/h. This article proposes a horizontal U-shaped magnets-based motion-induced eddy current array (MIECA) method to detect rail surface defects with the HSHS capability. The MIECA method deploys a three-axis magnetic sensor array along the rail transverse direction at the middle of the magnets to measure the MIECA signals, which utilizes the wake effect of the diffused motion-induced eddy current (MIEC) caused by the relative high-speed motion between the magnets and the rail track. Finite-element method (FEM) simulations with a wide speed range from 0 to 62.5 mi/h are carried out to investigate the relationships between the MIEC generation, diffusion and magnitude, and the three-axis MIECA signals. The simulation results show that the higher the speed, the greater the magnitude of diffused MIEC, and the greater the peak-to-peak values of three-axis MIECA signals caused by rail surface defects, which shows a great promise for detecting rail surface defects in high-speed scenarios and is superior inspection relative to existing NDT methods in terms of inspection speed, detection sensitivity, and defect characterization capability.

Understanding Eddy Current Array for High-Performance Inspections

Eddy current testing (ECT) has been used for quite a while now and has been proven a reliable surface inspection technique for conductive materials. In the last 15 to 20 years, this technique has evolved toward the use of eddy current arrays (ECAs), and many applications can now benefit from this configuration to improve data quality, inspection speed, and ease of deployment, and considerably reduce operator dependency. The physics principle behind ECT and ECA is the same and was addressed in a previous issue of Materials Evaluation (Wassink et al. 2021). In this paper, we will discuss the main differences between ECT and ECA as well as how the arrangement of coils in an array can allow for optimized detection capabilities on different materials or types of defects. Common applications where ECA has demonstrated its strength will also be discussed.

Flexible Differential Butterfly-Shape Eddy Current Array Sensor for Defect Detection of Screw Thread

Eddy current testing (ECT), is a non-destructive method widely applied in detecting defects of metal plates based on the principle of electromagnetic induction. The flexible eddy current testing (FECT) has been developed from ECT. What's more, it's also an effective method that is given a higher priority in defect detection of metal with complex surface structure due to its flexibility and small size. An arrayed differential flexible butterfly-shape eddy current sensor is proposed to detect the surface defect of iron screw thread in this paper, which has one butterfly-shape coil, four sets of differential receiver (RX) coils and one top RX coil, and the proposed sensor is designed as butterfly-shape with a certain folding angle to match with the curvature of the internal and external screw thread, which can solve the lift-off problem of screw thread defect detection at the bottom. The theoretical analysis illustrates the design principle for the proposed sensors. The simulation based on COMSOL Multiphysics is applied to illustrate the probability of the proposed sensor, and the experimental testing system is adopted to verify the high performance of the proposed sensors. The results of simulation and experiment are identical, which show the flexible differential butterfly-shape array eddy current sensor can not only improve the sensitivity and reduce the error rate, but also orientate the defect on the surface of the screw thread.

Research on crack monitoring technology of multilayer structure based on TMR-eddy current sensor

The flexible eddy current array sensor has the characteristics of flexibility, bendability and light weight, which has a great application prospect in the field of structural health monitoring. The traditional flexible eddy current array sensor is affected by the skin effect and the crack monitoring depth is relatively low. This paper proposes an eddy current array sensor which uses TMR (tunnel magneto-resistance) sensor as sensing channels. It reduces the excitation frequency and increases the eddy current skin depth. The TMR sensor realizes the quantitative monitoring of cracks by the array arrangement. First, the finite element model of the sensor is established, and the influence of the cracks on the different layers of the multilayer structure on the magnetic induction intensity at the test point is analyzed. The simulation and results show that when the crack passes through the sensor, the magnetic induction intensity at the three test points changes in sequence and gradually increases to the maximum value. The direct relative position of the maximum value of the three test points is approximately the same as the distance among the test points. Then, the simulated crack growth experiment of the corresponding sensor is carried out. The experimental results show that as the crack grows, the signals of the three TMR sensors on the eddy current sensor change in sequence, and the distance between the characteristic points is consistent with the distance between the TMR sensors. The output voltage of the sensor decrease with the crack depth increases. The experimental results are consistent with the simulation results, which further verifies the correctness of the simulation model.

AI-Enabled Robotic NDE for Structural Damage Assessment and Repair

The aim of this paper is to develop the concept and a prototype of an intelligent mobile robotic platform that is integrated with nondestructive evaluation (NDE) capabilities for autonomous live inspection and repair. In many industrial environments, such as the application of power plant boiler inspection, human inspectors often have to perform hazardous and challenging tasks. There is a significant chance of injury, considering the confined spaces and limited visibility of the inspection environment and hazards such as pressurization and improper water levels. In order to provide a solution to eliminate these dangers, the concept of a new robotic system was developed and prototyped that is capable of autonomously sweeping the region to be inspected. The robot design contains systematic integration of components from robotics, NDE, and artificial intelligence (AI). A magnetic track system is used to navigate over the vertical steel structures required for examination. While moving across the inspection area, the robot uses an NDE sensor to acquire data for inspection and repair. This paper presents a design of a portable NDE scanning system based on eddy current array probes, which can be customized and installed on various mobile robot platforms. Machine learning methods are applied for semantic segmentation that will simultaneously localize and recognize defects without the need of human intervention. Experiments were conducted that show the NDE and repair capabilities of the system. Improvements in human safety and structural damage prevention, as well as lowering the overall costs of maintenance, are possible through the implementation of this robotic NDE system.

The durability of flexible eddy current array (FECA) sensors in harsh service environments

Sensors for structural health monitoring (SHM) need to be permanently integrated on structures and withstand the harsh service environments, which has been a big challenge for the application of SHM in aircrafts. This paper focuses on the durability of flexible eddy current array (FECA) sensors in harsh service environments of aircrafts, including vibration environment and several typical exposed environments. First, a kind of FECA sensor is illustrated and its integration method is proposed. Moreover, in order to study the durability of the sensor in vibration environment, the modal analysis is performed by the finite element method. According to the simulation results, the durability experiment in vibration environment is carried out under the fourth order vibration mode, which makes the sensor suffer the harshest vibration loads. During the vibration experiment, output signals of the sensor keep stable and the sensor is well bonded to the structure, which shows the integrated sensor has high durability in vibration environment. Finally, the durability of integrated sensors is separately tested in three exposed environments, including salt fog corrosion environment, fluid immersion environment, as well as hygrothermal and ultraviolet-radiation environment. After these environmental exposure experiments, all sensors are well bonded to structures and can effectively monitor fatigue cracks, which shows great durability. Therefore, FECA sensors can survive in harsh service environments of aircrafts, which provides important support for the engineering applications of FECA sensors.

Pulsed eddy current array design and electromagnetic imaging for defects detection in metallic materials

ABSTRACT Defects caused by a harsh working environment and cyclic loading may seriously lead to catastrophic industrial accidents. Therefore, an efficient and accurate detection method is needed to quantitatively characterise the multiple defects of large-area metal components. In this paper, a pulsed eddy current (PEC) array is designed by considering mutual electromagnetic influence between the PEC array; the differential peak values at different scan positions are abstracted as feature signals for defect characterisation, and the relationship between defect width and detection signal is discovered; the detection capability of sensor array for adjacent defects is analysed. Subsequently, the electromagnetic imaging is applied to visualise and quantitatively evaluate the multiple defects, and then, the quantitative detection approaches for the localisation and width of multiple defects in metal components are proposed. An experimental setup is built and the proposed approach is validated by experiment. The results show that the proposed method can effectively determine the locations and widths of defects in metal components within the absolute error of 1 scan step.

Quantifying Hole-Edge Crack of Bolt Joints by Using an Embedding Triangle Eddy Current Sensing Film

Hole-edge crack quantification of bolt joints is critical for monitoring and estimating structural integrity of aircraft. The paper proposes a new triangle eddy current sensor array for the purpose of increasing the level of quantifying hole-edge crack parameters, especially, the crack angle. The new senor array consists of triangular coils instead of planar rectangular coils. The configuration of the novel sensor array, including the excitation current directions and the excitation winding shape, is optimized by simulation. The ability of the proposed sensing film to identify the crack parameters has been verified by finite element simulations and experiments. Results shows that triangular coils with same current directions in circumferentially adjacent coils and opposite current directions in axially adjacent coils achieve better performance in sensor linearity and resolution compared to rectangular coils. In addition, it has also been proved that the sensing film has a good potential to identify the crack depth and length.

New ways to inspect railway axles: From simulation to experimental validation

The railway industry is faced with the challenge to find adequate means to ensure absence of defects during manufacturing of axles. The use of modelling techniques has seen a rise in industry for the design of new inspection systems that can increase the capacity of complete or even substitute current inspection protocols. This study proposes two new alternative strategies based on Ultrasonic (UT) and Eddy Current Array (ECA) techniques to inspect railway axles during the manufacturing phase. Simulation models have been developed by CIVA to design and understand these new inspection procedures that increase the probability of detection (POD) and reliability of the axle. The simulations have been validated with experimental tests that have demonstrated the advantages of the use of the present approach. As a result, new inspection system concepts are presented for the inspection of railway axles.

Track non-contact detection method based on pulsed eddy current array and line structured light

Non-destructive testing methods are used to monitor the rails in operation to ensure the safety of train operation. Compared with traditional manual testing methods, non-destructive testing technology has higher detection efficiency and detection accuracy, and can also reduce the manual operation cost of railway detection. Based on the above advantages, this article combines two modules of linear structured light and pulsed eddy current array. Linear structured light can detect rail surface defects, pulsed eddy current array can detect defects inside and deep within the rail, defect information can be obtained through image processing, and the location of the defect can be accurately located by GPS locator, thereby improving the efficiency and Accuracy, and to ensure the safety of the rail. Railway transportation enables railway maintenance personnel to find and replace damaged rails in time.

A new interleaving eddy current array-based sensing film for fatigue crack quantification of bolted joints

Fatigue crack quantification for bolted joints attracts much attention to determine the structural integrity for aircraft structures. In this paper, a new sensing film with interleaving eddy current array is proposed to evaluate the parameters of the hole-edge crack for single- or multiple-lap bolted joints. In this sensing film, the exciting layer is one coil throughout the hole wall which consists of a one-dimension rectangle coil array in the axial direction connecting in series, while the sensing layer is made up of two interleaving layers with a two-dimensional sensing coil array distributed in the circumferential and axial directions for each layer. Finite element simulation is conducted to study the effect of different crack parameters on the induced voltage of sensing coils. Simulation results reveal that the sensing film has no blind angle of crack detection but a strong capability of quantifying the crack angle, and the arctangent value of the ratio between interleaving coils can be used for estimating the crack angle. After the crack angle is estimated, induced voltages of sensing coils at this angle may be used to track the crack propagation in the axial or radial directions of the bolt hole. Experiments are conducted to verify the feasibility and effectiveness of the new sensing film to monitor the angle, depth, and length of the hole-edge crack of bolted joints.

Simulation Study and Application of Eddy Current Array C-scan Imaging Technique to Examine Austenite Stainless Steel

Simulation Study and Application of Eddy Current Array C-scan Imaging Technique to Examine Austenite Stainless Steel