Remote Field Eddy Current Research Articles

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.

Investigation on magnetic shield thickness of remote field eddy current probes for inspection of ferromagnetic and non-ferromagnetic plates

Since Remote field eddy current (RFEC) detection technology is barely limited by the skin effect, it can realize the defect detection of large-thickness components. Whereas, the structure of the RFEC probe for the planar conductor is complicated, resulting in an oversized probe. This hinders the application and development of the RFEC testing technology. In this paper, finite element simulations and experimental verification are used to optimize the RFEC probe structure for flat conductors. The results have shown that the thickness of the magnetic shield is the dominant factor in achieving high performance of RFEC inspection of flat conductors. When inspecting both ferromagnetic and non-ferromagnetic flat conductors, cylindrical magnetic shields with appropriate thickness and appropriate excitation frequencies are preferred for improvement of RFEC testing.

Finite element study of remote field eddy current methods for inner diameter and outer diameter pipeline defect classification

Remote field eddy current (RFEC) methods are widely applied for detecting pipeline defects. However, current RFEC methods cannot distinguish inner diameter (ID) defects from outer diameter (OD) defects. In addition, existing RFEC probes are usually driven by an extremely low-frequency signal, which reduces the detection efficiency. To address these issues, a novel external RFEC probe is designed to improve the detection performance. First, the probe structure is designed using a simulation tool. Second, the excitation and structural parameters are optimally selected. Finally, the relationships between the signal features and the defect dimensions are analysed. The results show that the probe can realise the RFEC effect without shield cages, the exciting frequency is significantly improved and the phase angle can be used to classify ID and OD defects.

Effect of flaw orientation on magnetic flux leakage and remote field eddy current inspection of small diameter steel tubes

ABSTRACT This paper presents effect of flaw orientation on magnetic flux leakage (MFL) and remote field eddy current (RFEC) inspection of small diameter (19.05-mm outer diameter, OD and 2.77-mm wall thickness, WT) seamless low carbon steel tubes. Artificial electro-discharge machined (EDM) notches in three carbon steel Tubes A (orientations from 0° to 90° in steps of 10° and 50% WT depth), B (70° orientation and depths from 10% WT to 100% WT in steps of 10% WT) and C (50° orientation and depths from 10% WT to 60% WT in steps of 10% WT) were used to investigate the detectability of notches with different orientations and depths. Studies on different notch orientations reveal that RFEC technique detected all the notches with good signal-to-noise ratio (SNR), while MFL technique detected the notches with orientations from 40° to 90°. The effect of notch orientation is observed more in MFL technique as compared to RFEC technique. Further, the effects of scanning speed and notch depth for the detection of oriented notches were studied. Studies reveal that the effect of scanning speed is also more (40 times) dominant in MFL technique as compared to RFEC technique. Studies also reveal that the scanning effect is again dependent upon the notch depth. For 70° oriented notches, MFL technique is found to be more sensitive to notch depth as compared to RFEC technique. For 50° oriented notches, RFEC technique is found to be more sensitive to notch depth as compared to MFL technique.

Study on Remote Field Eddy Current Testing Technology for Crack-like Defects in Long Truss Structure of Aircraft.

Detection of hidden defects of aircraft long truss structures (aluminum alloy) is a challenging problem. The shape of the aircraft truss structure is complex, and the crack defects are buried in a large depth. Without the restriction of skin effect, remote field eddy current (RFEC) has great advantages in detecting buried depth defects. In this paper, in order to detect the hidden defects of the aluminum alloy aircraft long truss structure, the remote field eddy current probe is improved from two aspects of magnetic field enhancement and near-field signal suppression using the finite element method. The results show that indirect coupling energy is greatly enhanced when the connected magnetic circuit is added to the excitation coil. By adding a composite shielding structure outside the excitation coil and the detection coil, respectively, the direct coupling energy is effectively restrained. As a result, the size of the probe is reduced. By optimizing the coil spacing and probe placement position, the detection sensitivity of the probe is improved. The simulation is verified by experiments, and the experimental results are consistent with the simulation conclusions.

Simulation and experimental study of remote field current testing for hidden defects of aluminum alloy plate with damping coating

PurposeDetection of hidden defects of aluminum alloy plate with damping coating is a challenging problem. At present, only a few non-destructive testing methods exist to address this engineering problem. Without the restriction of skin effect, remote field eddy current (RFEC) overcomes the interference caused by the damping coating. The RFEC, which has potential advantages for detecting the hidden defects of aluminum plate with damping coating, can penetrate the metal plate to detect buried depth defects. This study aims to test how thick the RFEC sensor can penetrate the metal plate to detect the buried defects.Design/methodology/approachThe magnetic field distribution characteristics are analyzed, the magnetic field intensity distribution is calculated, and the structure and parameters of the coil, magnetic circuit and shielding damping are determined through the two- and three-dimensional finite element simulation methods. Optimal excitation frequency is obtained, and the distance between the excitation coil and detection coil is determined by analyzing the relationship between excitation frequency and remote field points.FindingsSimulation and experimental results verify the feasibility of applying the RFEC detection technology in detecting the hidden defects of aluminum alloy plate with damping coating.Originality/valueIn this paper, the RFEC testing model of hidden defects in aluminum plate sample with damping coating is established by using the finite element method.

Accurate Identification for 3-D Position of Hybrid Defects in Ferromagnetic Pipe Using External Remote Field Eddy Current Testing

Remote field eddy current (RFEC) testing is extensively used for the defect evaluation of ferromagnetic pipes, which depends on accurate measurement and complete information. In practice, the misalignment between the probe and defect can affect the accuracy of response signals. In addition to defect size, the 3-D position of a defect is critical information for pipe defect evaluation as well. However, existing RFEC methods find it difficult to identify the radial position of defects, especially the hybrid defect. In this article, an external RFEC (ERFEC) method under sine and pulse excitation is proposed to address these issues. By studying the influences of the axial offset distances and circumferential deflection angles between the probe and defect against RFEC signals, the strategy for position correction of the probe placement is provided to reduce signal error. A pulse signal with optimal excitation parameters is utilized to drive the probe. The time-domain characteristics of the pulsed RFEC signal in shape and magnitude are used for the radial position identification of pipe defects. The results indicate that the probe misalignment effects can be removed using RFEC signal features. Besides, the axial and circumferential positions of hybrid defects can be identified using the probe scanning method, while the radial position identification of the hybrid defect needs to use the valley feature of pulsed RFEC signals combined with the RFEC signal phase. In general, this work makes it possible for depth quantification and accurate profile inversion of the hybrid defect.

Optimisation of helical geometry of inspection probe for steam generator tubes of the PFBR

The steam generator (SG) tubes of the prototype fast breeder reactor (PFBR) located in Kalpakkam, India, need to be periodically inspected using the remote field eddy current (RFEC) technique. During the pre-service inspection of the SG tubes, it was found that the RFEC probes experienced frequent mechanical breakages. To avoid these failures, changes in the existing structural design of the RFEC probe were required. A helical groove design was proposed to obtain a smooth transition in the variation of stress across the probe during the inspection. It was difficult to calculate the flexural stiffness of the proposed helical geometry probe due to the varying cross-section along its length. In this paper, the smearing approach adopted to calculate the stiffness of the RFEC probe and the sensitivity analysis carried out to determine the optimal design of the probe are discussed. A probe was fabricated based on the helical groove design and tested to qualify its suitability for the SG inspection. The RFEC probe with helical grooves was employed for the pre-service inspection of the SG tubes of the PFBR. More than 200 tubes have been inspected using the proposed design and no mechanical failure of the probe has been observed.

RFEC Based Oil Downhole Metal Pipe Thickness Measurement

Oil downhole metal pipe wall thickness measurement is of great importance for pipe evaluation and maintenance. Regular pipe inspection can be used to prevent pipe failure, which could otherwise lead to major failure and financial loss. In this paper, a low-frequency remote field eddy current (RFEC) based internal testing approach is presented, and the analytical solutions of the RFEC electromagnetic equation are obtained based on Dodd and Cheng’s method. Meanwhile, induced voltage under different low frequencies is analyzed, and proper frequencies are chosen for follow-up calculations. For reducing the inspection time of pipe thickness measurement, quasi-Newton’s method (QNM) is implemented based on the deduced solutions. Experimental results indicate that the RFEC and QNM based method is feasible for metal pipe thickness measurement. Pipe thickness is obtained accurately, and the lift-off effect, which has a considerable influence on eddy current testing results, is effectively overcome. The proposed method achieves pipe thickness measurement with proper accuracy and reduction in inspection time.

Receiver Signal Analysis on Geometry and Excitation Parameters of Remote Field Eddy Current Probe

The traditional remote field eddy current (RFEC) testing is a method widely applied for detecting the corrosion and wall thinning in a ferromagnetic pipe system and shows intrinsic deficiencies in axial defects quantification due to the distinct probe structure. A circumferential alignment probe structure is used to improve the sensitivity to axial defects of pipes. Meanwhile, current approaches typically are incapable of classifying inner diameter (ID) and outer diameter (OD) defects of the pipe, which brings difficulties to the formulation of strategies in pipe repair. In this article, an external pulsed RFEC method is proposed to address these issues. First, we give a thorough analysis of the effects of probe geometry and excitation parameters on the output signal, targeting parameters optimization. Second, the finite-element model is built and used to analyze the relationships between the defect size and signal features under different excitation types. Finally, the improvements in probe performance and the ability to classify ID and OD defects are validated by measuring the defect depths of the pipe. Results showed that the zero-crossing time feature can be used to distinguish ID from OD defects. Thus, the proposed method is an effective complementary measure for the existing RFEC tools.

Detection of localised flaws in small-diameter carbon steel tubes using multi-NDE techniques

This paper proposes multi-NDE techniques for enhanced and reliable detection of localised defects or flaws in small-diameter (19.05 mm outer diameter (OD) and 2.77 mm wall thickness (WT)) low-carbon steel tubes. Multi-NDE techniques comprising magnetic flux leakage (MFL), remote field eddy current (RFEC) and the ultrasonic internal rotary inspection system (IRIS) are used to investigate the detectability and spatial resolution for the detection of localised flaws of different types and sizes, which simulate flaws generally formed in carbon steel tubes. The studies reveal that the MFL and IRIS techniques detect 0.55 mm-deep holes and notches while the RFEC technique detects 1.11 mm-deep holes and 0.55 mm-deep notches. The IRIS technique is capable of resolving axial and circumferential holes (1 mm diameter and 1.39 mm depth) of 2 mm hole-to-hole separation, while the MFL technique can resolve axial holes of 2 mm hole-to-hole separation but not circumferential holes. The RFEC technique can detect axial holes as a single hole but is unable to detect circumferential holes, resulting in no spatial resolution for the axial and circumferential holes. Compared to the MFL and RFEC signals, IRIS readily discerns the spatial extent of the flaws with the possibility of sizing from the images. The study also reveals that the resolution capability of the IRIS technique is influenced by the speed of the turbine rotation inside the tube.

Optimal design of remote field eddy current testing probe for ferromagnetic pipeline inspection

Remote field eddy current (RFEC) nondestructive testing has its unique advantages in defect detection of metal pipelines, such as being unaffected by skin effect and material properties, but the size of RFEC probe is large and the signal received by the detection coil is weak. In this paper, a shielding plate is introduced between excitation and detection coil, and a ferromagnetic ring is introduced outside the ferromagnetic pipeline. Both theoretical analysis and finite element method (FEM) simulation are carried out. The result of FEM simulation shows that the distance between the excitation and the detection coil can be shortened after adding a shielding plate; particularly, setting a 1 mm-thick shielding plate 5 mm away from the excitation coil has an optimum optimization effect, which could shorten the distance between the excitation coil and the detection coil by 2 times. The ferromagnetic ring can strengthen the detected signal, and the closer it gets to the detection coil, the stronger magnetic flux density will be detected in the latter. Therefore, it provides a direction for the structural optimization of RFEC testing device.

Defect Detection of Oil and Gas Pipeline using Remote Field Eddy Current Technology

To solve the problem of wall thickness reduction and defects of oil and gas pipeline caused by corrosion or erosion, a remote field eddy current (RFEC) testing method with coaxial double coil structure is proposed to detect corrosion residual wall thickness of oil and gas gathering pipelines. Based on electromagnetic field theories, RFEC technology is theoretically analyzed. The theoretical model of RFEC detection for coaxial double coil structure is established. The relationship between the voltage phase of detection signal and the wall thickness of pipeline is derived, and an evaluation method of the residual wall thickness of the pipeline based on the phase trough time of the RFEC detection signal is proposed. The parameters of RFEC probe are optimized by Finite Element Method, and the detection system is designed on the basis of it. The practicability and the correctness of the theoretical model of the detection system are verified by experiments. It can be used to detect the residual wall thickness of pipelines in real time under different media conditions.

Multi-Layer Magnetic Focusing Sensor Structure for Pulsed Remote Field Eddy Current

Pulsed remote field eddy current (PRFEC) technique has been utilized broadly in the detection of tubular material because of its abundant frequency spectrums with lower power consumption. A novel multi-layer magnetic focusing sensor structure is proposed for PRFEC. The new sensing structure consists of several promising characteristics. It significantly enhances the detectability of the deeper depth of ferromagnetic material, and provides detection capability without covering U-type component. Both driver and receiver coils are integrated in the same sensing structure while the traditional methods are put them separated. The theoretical derivation based on electromagnetic principles has been developed for analyzing and interpreting the results. In addition, number of experiments and simulations on different defects have been studied. In experiments, both surface and subsurface defects with different width, depth, inclined angle, and circular type have been carried out. The results have confirmed that all types of defects can be detected and it has shown the relatively preferable linear relationships and reliabilities.

Dual sensing coils used for RFEC testing of joint casings in oil wells

In the detecting of joint casings in oil wells based on remote field eddy current (RFEC) testing, the two penetrations of RFEC on casings (near the transmitter and near the sensing coils) enhance the complicacy of detecting joint casings. The proposed method in this paper is an efficient way to overcome this problem by removing the signal caused by wall’s proximity to the transmitter from the testing signal. In the method, the secondary peaks caused by the transmitter passing over defects need to be removed from the testing signal by employing dual sensing coils firstly. Then the iterations of the testing signals from the dual sensing coils are deduced for the further application of the Wiener deconvolution filter to identify the signal that is used to indicate anomalies. Thirdly, the acquisition of the signal caused by normal wall’s proximity to the transmitter is obtained according to the indicator signal. Lastly, after the both removal of secondary peaks and the signal caused by normal wall’s proximity to the transmitter from the testing signal, the remaining signal can be used to monitor the present joint casings in a well adaptively. The method proposed in this paper is validated via a logging tool, the Kesuo #1 well and ANSYS simulation.

Development of a High-Sensitive Dual Frequency Remote Field Eddy Current Instrument for Inspection of Ferromagnetic Steam Generator Tubes

ABSTRACTThis paper presents the development of a high-sensitive remote field eddy current (RFEC) instrument for detection of flaws in ferromagnetic steel tubes. The instrument is capable of simultaneously driving the exciter coil of the RFEC probe with two different frequencies and measuring the phase lag of the receiver coil using software-implemented lock-in amplifier. The performance of this high-sensitive RFEC instrument has been demonstrated by detecting a variety of localised flaws in a modified 9Cr–1Mo steel tube used in steam generators. The instrument is sensitive to detect localised flaws of depth, 20% of the wall thickness of 17.2-mm diameter, 2.3-mm thick ferromagnetic steel tubes.

Defect Detection and Segmentation Framework for Remote Field Eddy Current Sensor Data.

Remote-Field Eddy-Current (RFEC) technology is often used as a Non-Destructive Evaluation (NDE) method to prevent water pipe failures. By analyzing the RFEC data, it is possible to quantify the corrosion present in pipes. Quantifying the corrosion involves detecting defects and extracting their depth and shape. For large sections of pipelines, this can be extremely time-consuming if performed manually. Automated approaches are therefore well motivated. In this article, we propose an automated framework to locate and segment defects in individual pipe segments, starting from raw RFEC measurements taken over large pipelines. The framework relies on a novel feature to robustly detect these defects and a segmentation algorithm applied to the deconvolved RFEC signal. The framework is evaluated using both simulated and real datasets, demonstrating its ability to efficiently segment the shape of corrosion defects.

Magnetic field shielding technique for pulsed remote field eddy current inspection of planar conductors

Pulsed remote field eddy current (PRFEC) can be used for detection of deeply buried flaws in non-magnetic flat plates. However, detection sensitivity for buried defect is low due to the great depth and the weak detecting signal. To improve the defect detection sensitivity, the PRFEC probe is optimized with the magnetic field shielding techniques in this work. On one hand, the exciting coil with shielding structure can focus the indirect magnetic field to penetrate through the plate. On the other hand, the detecting coil and the region between these two coils with shielding material can block and minimize the magnetic field energy diffusion along the direct coupling path. In order to investigate the shielding technique which blocks the direct coupling magnetic field, simulation studies using finite element method (FEM) has been first conducted, where three different PRFEC probes have been simulated and compared. Both detection abilities to defects in different directions and to thickness variation have been analyzed by using the second probe with shielding structure. The conclusions derived from the simulation study have been validated through experimental studies. Both simulation and experimental studies have indicated that the shielding technique can improve the detection sensitivity of subsurface defects.

Method for Removing Secondary Peaks in Remote Field Eddy Current Testing of Pipes

In the remote field eddy current (RFEC) testing of pipes, because the remote eddy current penetrates the pipe’s wall twice, the testing results exhibit two peaks (primary peak and secondary peak) that originate from both the transmitter and receiver passing by the same place in the pipe. The secondary peaks have the same features as the primary peaks that are used to assess defects, and if there is no separation between primary peaks and secondary peaks, incorrect evaluations of defects will be obtained. Considering the benefits of removing secondary peaks in RFEC testing, dual receivers are taken into account. Dual receivers are set in remote fields and are set coaxially to the transmitter to obtain differential signals at the same time. In the proposed method, position dependent response of the differential signals from the dual receivers is calibrated, a Wiener deconvolution filter is used to identify secondary peaks and filter testing noise, and the factors that affect results of removing secondary peaks are also analyzed. To validate the feasibility of the proposed method of RFEC testing, ANSYS is made use of when setting up the analysis model, and an experimental pipe is designed to be identical to ANSYS model. The results of the analysis of ANSYS and experiments both validate the practicality of the proposed method and show the benefits of simplifying the analysis of RFEC signals.

Approach for removing ghost-images in remote field eddy current testing of ferromagnetic pipes.

In the non-destructive testing of ferromagnetic pipes based on remote field eddy currents, an array of sensing coils is often used to detect local defects. While testing, the image that is obtained by sensing coils exhibits a ghost-image, which originates from both the transmitter and sensing coils passing over the same defects in pipes. Ghost-images are caused by transmitters and lead to undesirable assessments of defects. In order to remove ghost-images, two pickup coils are coaxially set to each other in remote field. Due to the time delay between differential signals tested by the two pickup coils, a Wiener deconvolution filter is used to identify the artificial peaks that lead to ghost-images. Because the sensing coils and two pickup coils all receive the same signal from one transmitter, they all contain the same artificial peaks. By subtracting the artificial peak values obtained by the two pickup coils from the imaging data, the ghost-image caused by the transmitter is eliminated. Finally, a relatively highly accurate image of local defects is obtained by these sensing coils. With proposed method, there is no need to subtract the average value of the sensing coils, and it is sensitive to ringed defects.