Remote Field Eddy Current Testing Research Articles

Frequency Characteristics Analysis of Remote Field Eddy Current Testing on Ferromagnetic Pipes

Pipes made of ferromagnetic alloys are widely installed in petrochemical plants and the steel plants. To implement non-destructive corrosion inspections on these pipes, remote field eddy current testing (ECT) which installs a search coil inside the pipe is a practical inspection method. Remote field ECT has the advantage of being able to detect external surface defects while the distance between the detection coil and the excitation coil is twice of the outer diameter of the testing pipe. Nevertheless, the inspection principle such as the frequency characteristics etc. of ECT signals have not been elucidated, inspection devices based on remote field ECT had not been employed widely. In this paper, we measured the frequency characteristics of ferromagnetic pipes with artificial outer surface defects by remote field ECT, and then detailed finite element analysis are carried out by using the nonlinear eddy current analysis to clarify the inspection principle of the remote field ECT. In both measurement and analysis, peak signal occurs directly under the defect and the optimal frequency range are the same. The phase angle of flux density due to the excitation current and flux density due to eddy current are found to greatly affect the frequency characteristics.

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.

The applicability of remote field eddy current testing to outer flaws on cooling tubes in the blanket of a fusion DEMO reactor

This study investigated the applicability of remote field eddy current testing (RFECT) to the non-destructive inspection of cooling tubes in the blanket of a fusion DEMO reactor. Pipes made of F82H steel with inner and outer diameters of 9.0 and 11.0 mm, respectively, were prepared, and artificial grooves were machined on the outer surface of the pipes. The pipes were inspected using an RFECT probe consisting of one exciter and one detector. The results confirmed that RFECT was capable of detecting a groove with a depth of 0.5 mm if the axial length of the groove was 5 mm or more. Subsequently, stress analysis was performed using the three-dimensional finite element method to evaluate the stress caused by the presence of a groove. The calculated stress was compared with the strength of the F82H steel. The stress caused by undetectable flaws was smaller than the strength.

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.

Local Defect Detection of Ferromagnetic Metal Casing Based on Pulsed Eddy Current Testing

Remote-field eddy current testing (RFECT)-based methods have been widely used in conductive pipe evaluation. However, the response signals of RFECT are affected both by the casing thicknesses around the driver coil and in the vicinity of the pick-up coil, making it difficult to invert and separate the thicknesses around the coils. Moreover, the relative permeability difference between the near-field and the remote-field zones causes a further deviation in the thickness inversion. To avoid these problems, a near-field pulsed eddy current testing method is proposed for defect evaluation. Time-domain transient features, which can be applied for defect detection, are analyzed based on the theoretical data of pulsed eddy current testing. A prototype of the testing apparatus and a defective ferromagnetic metal casing are designed to verify the feature performance for defect evaluation. The experimental results indicate that the time-related features are sensitive to circumferential defects and insensitive to the axial defects, and the magnitude-related feature is sensitive to both the circumferential and axial defects. Therefore, the magnitude-based method can be used to detect the local defects of ferromagnetic metal casings.

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.

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.

Detection of Fiber Fracture in Unidirectional CFRP by Remote Field Eddy-Current Testing

Detection of fiber fracture in unidirectional carbon-fiber-reinforced polymer (CFRP) using the conventional eddy-current method is difficult due to the low electrical conductivity of the material. This article explores the possibility of detecting fiber fracture using the remote field eddy-current method. A remote field eddy-current probe consisting of an excitation coil and a reception coil is designed and optimized. The experimental results show that the signal of scanning the probe over fiber fracture has a dual-valley characteristic and the distance between the two minima is identical to the distance between the coil axes. This characteristic of signal is analyzed by studying distributions of eddy current (EC) using a finite-element analysis. A comparison of the testing results of the remote field EC probe and the conventional EC probes demonstrates the superiority of the remote field EC method in detecting fiber fracture in unidirectional carbon-fiber-reinforced polymer.

Numerical Simulation of Non-Destructive Remote Field Eddy Current Testing of Rolled Metal Tubes

The first publications describing the physical principles of the non-destructive remote field eddy current testing method appeared about 30 years ago. This method allows to significantly expand the field of application of eddy current testing. However, due to the lack of a theoretical justification, this method did not get widespread use around the world. Domestic publications in this area are completely absent, and the descriptions given in few foreign publications often contradict each other. There are no results of full-scale simulation using numerical methods in available domestic and foreign sources. The distinctive feature of this method under consideration is the ability of detecting defects on the external (with respect to the eddy current transducer) side of the tested object, which is impossible for the classical eddy current method due to the limited eddy current penetration depth. The basics of the method were considered, the distinctive features were presented, and the advantages and disadvantages of remote field eddy current testing of metals were pointed out. A numerical simulation with the subsequent analysis of the obtained results has been carried out, the transducer design for remote field eddy current testing is given. The influence of various factors on the change in the added voltage of the signal coil of the eddy current transducer in the presence of a defect in the external wall of the tube was considered. Expressions that determine the optimal ratio of the diameters of the transducer and the tested product were obtained. The values of the test parameters and the limiting wall thickness of the tested ferromagnetic product were determined.

Numerical simulation of non-destructive remote field eddy current testing of rolled metal tubes

The distinctive feature of remote field eddy current method is the ability of detecting defects on the external (with respect to the eddy current transducer) side of the tested object, which is impossible for the classical eddy current method due to the limited eddy current penetration depth. The basics of the method were considered, the distinctive features were presented, and the advantages and disadvantages of remote field eddy current testing of metals were pointed out. A numerical simulation with the subsequent analysis of the obtained results has been carried out, the probe design for remote field eddy current testing is given. The influence of various factors on the change in the added voltage of the eddy current probe in the presence of a defect in the external wall of the tube was considered. Expressions that determine the optimal ratio of the diameters of the probe and the tested product were obtained. The values of the test parameters and the limiting wall thickness of the tested ferromagnetic product were determined.

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.

Analysis of a Defect Signal Deformations Induced by Eddy Current in RFECT System for Pipeline Inspection

For testing a conductive pipeline, remote field eddy current testing (RFECT) is used, which is composed of solenoidal exciter coil and receiving sensor coils. In RFECT, there is remote field zone where the eddy current distributions are concentrated on the external surface of the pipe wall. Therefore, it is possible to detect a defect that occurs on the external surface of the pipe wall even if the receiving coils are located inside the pipe. From the measured coil signals, it is necessary to decompose background offset and predict the pattern of defect signal in order to estimate the defect size for the maintenance of underground pipelines eventually. In the process, the amplitude variations of the induced magnetic field at sensor coils are closely related to defect depth. Therefore, it is important to analyze and eliminate distortions of measured RFECT signals. This paper analyzes the effects of the induced magnetic field on the sensing signals with respect to different depth sizes in the RFECT system. The amplitude and the distribution pattern of defect signals are computed by a 3-D finite-element method and the method to eliminate the signal distortions caused by induced magnetic fields are presented. Simulated results in this paper agreed well with measuring ones.

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.

Enhanced Detection of Defects Using GMR Sensor Based Remote Field Eddy Current Technique

Remote field eddy current testing (RFECT) can detect defects in ferromagnetic pipes without requiring a couplant and contact with the pipe wall. Because the response in the remote field zone is extremely low, testing requires high-sensitivity detection to sense low magnetic fields. Therefore, a magnetic circuit design comprising a RFECT system is necessary for excitation and reception. In this study, a RFECT system using a giant magnetoresistance (GMR) sensor is proposed, and magnetic circuits with magnets and/or yokes are designed and optimized for improved reception. To investigate the characteristics of the designed RFECT system, finite element analysis (FEA) and experiments are performed on various magnetic circuit system configurations. System performance is verified with respect to sensitivity and the detectability of defect characterizations in 4-inch ferromagnetic pipelines having different defect sizes. Consequently, the magnetic circuit system using the permalloy yoke shows the highest sensitivity, thus achieving effective RFECT detection.

Research on Electromagnetic NDE Inspection Method for Broken Wire of PCCP

预应力钢筒混凝土管(Pre-stressed Concrete Cylinder Pipe, PCCP)作为一种新型的输水管材，被广泛应用于水利工程。在PCCP运行过程中，环境腐蚀或操作不当会导致管内高强钢丝的腐蚀或断裂，进而导致泄漏、爆管等事故。目前国内外普遍采用电磁涡流检测技术，对PCCP的断丝与金属管壁健康状态进行无损检测，检测系统在管内运行并配合计程轮等工具实现断丝位置的定位。本文针对国内外两种常用的PCCP断丝电磁检测技术，即远场涡流检测方法和正交电磁检测方法，建立了简化分析模型，并给出了等效交流电路，通过仿真分析验证了模型的正确性。分析了电磁检测频率的选取原则，建议选取23 Hz作为检测频率。给出了电磁检测系统的设计与实现，并经过了管道断丝试验验证，有效提取了故障特征。 Pre-stressed Concrete Cylinder Pipe (PCCP), as a new type of pipeline for water delivery, has been widely used in water conservancy projects. During the operation of PCCP, environmental corrosion or improper operation will lead to corrosion or fracture of high-strength steel wire in the pipe, resulting in leakage, explosion and other accidents. Currently some nondestructive methods, such as electromagnetic eddy current inspection method, are widely used to test the broken wire and assess the health PCCP, and equipped with a counting-wheel or other tools to achieve the localization of broken wires. This paper presents a simple model of two famous electromagnetic detection technologies for PCCP broken wire, i.e. the remote field eddy current testing method and orthogonal electromagnetic detection method. Also the equivalent AC circuit is given. By simulation analysis the simple model is verified. The selection principle of electromagnetic detection frequency is analyzed, and 23 Hz is selected as the detection frequency. The design and implementation of the electromagnetic detection system are given, and the fault characteristics are effectively extracted through the broken wire tests.

Development of RFECT system for in-line inspection robot considering extendibility of receiving sensors based on parallel lock-in amplifier

This paper proposes a new and effective approach to design a remote-field eddy current testing (RFECT) system equipped with a large number of sensors in order to provide full circumferential coverage of pipelines larger than 6 inches. By developing a parallel digital lock-in amplifier (LIA), the extendibility of receiving sensors can be achieved, and therefore, the modification of RFECT systems, which should be accompanied by sufficiently securing the receiving sensors whose number increases with the pipeline size, can be minimized. Using the design method for an RFECT system based on a parallel digital LIA, a new non-destructive testing (NDT) platform that can be applied to RFECT systems of various sizes without modifying the system architecture is developed. It is then applied to an RFECT system that can be mated with an in-line inspection (ILI) robot and has 36 receiving sensors to inspect unpiggable gas pipelines. The performance of the RFECT system is verified with respect to the sensitivity and the accuracy of defect characterization though the pull-rig test having a number of artificial 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.