Remote Field Eddy Current Research Articles

Research on High Resolution Stress Corrosion Crack Detection Based on Remote Field Eddy Current Non-Destructive Testing

Although remote field eddy current had achieved successive application in pipes and tubes non-destructive testing, the poor ability and sensitivity to axial crack such as stress corrosion crack prevented its further application. A novel high resolution remote field eddy current detection method which can be suitable for axial crack especially stress corrosion crack detection was proposed and verified by simulation and experiment. First of all, conventional detection ability of remote field eddy current for crack in different directions was researched with detail. Aside from this, applicability of orthogonal magnetic filed in remote field eddy current was analyzed and simulated by finite element method, and appropriate configuration using to generating orthogonal magnetic fields for tubular structure was discussed and verified. Thirdly, design of detection system including key parameters selection and detection coil mode were researched. At last, proposed high resolution detection system were verified experimentally using various type defects, such as parallel axial crack and real stress corrosion crack. From the above research, conclusions were drawn as followed: the detection ability and sensitivity of proposed remote field eddy current detection system could be improved significantly compared with conventional system, especially for axial crack which was not easily to detect, and real stress corrosion crack could be detected and evaluated successively.

基于传感器阵列和独立分量分析的远场涡流缺陷信号盲分离技术研究

针对远场涡流检测中管道磁导率不均匀严重影响缺陷信号检测的问题，本文提出一种新的基于独立分量分析的远场涡流缺陷信号盲分离技术。首先利用有限元仿真对独立分量分析在缺陷分离中的适用性进行了详细分析，证实了磁导率不均匀和缺陷信号满足独立分量分析的应用前提。提出了基于实部和虚部信号的独立分量分析缺陷分离方法，并进行了仿真和实验验证。结果表明，基于独立分量分析技术能够有效的从磁导率不均匀区域分离和识别出缺陷。 According to influence of magnetic perturbation in remote field eddy current defect detection, a novel blind separation technique based on independent component analysis was proposed. Firstly, finite element method was adopted to declare the applicability of independent component analysis in remote field eddy current. Secondly, a new method based on real part and imaginary part rather than amplitude and phase signal was proposed, and simulation and experiment was implemented to verify it. At last, a conclusion could be deduced from simulation and experiment results as followed: the influence of magnetic perturbation could be eliminated significantly by using independent component analysis.

A novel high sensitivity sensor for remote field eddy current non-destructive testing based on orthogonal magnetic field.

Remote field eddy current is an effective non-destructive testing method for ferromagnetic tubular structures. In view of conventional sensors' disadvantages such as low signal-to-noise ratio and poor sensitivity to axial cracks, a novel high sensitivity sensor based on orthogonal magnetic field excitation is proposed. Firstly, through a three-dimensional finite element simulation, the remote field effect under orthogonal magnetic field excitation is determined, and an appropriate configuration which can generate an orthogonal magnetic field for a tubular structure is developed. Secondly, optimized selection of key parameters such as frequency, exciting currents and shielding modes is analyzed in detail, and different types of pick-up coils, including a new self-differential mode pick-up coil, are designed and analyzed. Lastly, the proposed sensor is verified experimentally by various types of defects manufactured on a section of a ferromagnetic tube. Experimental results show that the proposed novel sensor can largely improve the sensitivity of defect detection, especially for axial crack whose depth is less than 40% wall thickness, which are very difficult to detect and identify by conventional sensors. Another noteworthy advantage of the proposed sensor is that it has almost equal sensitivity to various types of defects, when a self-differential mode pick-up coil is adopted.

管材远场涡流有限元仿真及内外壁缺陷检测灵敏度分析

远场涡流作为涡流无损检测技术的一个重要分支，目前在各个行业的管道日常维护和安全保障中发挥着重要的作用。本文重点对有限元方法在远场涡流仿真中的应用进行了研究。首先通过建立有限元电磁仿真模型，对于远场涡流效应的基本现象，如“矢位峡谷”、“相位节点”等现象进行了仿真研究，并对其结果进行了分析。同时为进一步证实铁磁性管道中远场涡流相对于常规涡流技术的优势所在，选取分别位于远场区和近场区的内外壁缺陷进行有限元仿真和对比分析。Remote Field Eddy Current (RFEC) is an important branch cut of eddy current nondestructive testing (NDT) technique, and is drawing more and more attention in the testing and repairing of pipes and tubes. The finite element method used for RFEC simulation was researched in this thesis with detail. Firstly, some important phenomena of RFEC technique such as “amplitude canyon” and “phase knot” were simulated and analyzed. At the same time, to clarify the superiority of RFEC technology used for ferromagnetic tube compared with conventional eddy current, differential defects in “near field” and “remote field” were chosen to simulate and analyzed; Finally, advantages of remote field eddy current for ferromagnetic tube NDT were drawn compared with conventional eddy current.

Pulsed remote field eddy current technique applied to non-magnetic flat conductive plates

Non-magnetic metal plates are widely used in aviation and industrial applications. The detection of cracks in thick plate structures, such as multilayered structures of aircraft fuselage, has been challenging in nondestructive evaluation societies. The remote field eddy current (RFEC) technique has shown advantages of deep penetration and high sensitivity to deeply buried anomalies. However, the RFEC technique is mainly used to evaluate ferromagnetic tubes. There are many problems that should be fixed before the expansion and application of this technique for the inspection of non-magnetic conductive plates. In this article, the pulsed remote field eddy current (PRFEC) technique for the detection of defects in non-magnetic conducting plates was investigated. First, the principle of the PRFEC technique was analysed, followed by the analysis of the differences between the detection of defects in ferromagnetic and non-magnetic plain structures. Three different models of the PRFEC probe were simulated using ANSYS. The location of the transition zone, defect detection sensitivity and the ability to detect defects in thick plates using three probes were analysed and compared. The simulation results showed that the probe with a ferrite core had the highest detecting ability. The conclusions derived from the simulation study were also validated by conducting experiments.

3-D Simulation of Remote Field Eddy Current Detection for Stress Corrosion Cracks in Pipeline

Three-dimensional (3-D)finite element simulation models are conducted to simulate the in-serviceinspection of the axial stress corrosion cracking (SCC) in ferromagneticpipeline using remote field eddy current (RFEC) probes. In this paper, atypical remote field eddy current probe which is internal to and coaxial withthe pipe is firstly introduced. Then a 3-D finite element simulation model isestablished in ANSYS to analyze the detection of RFEC technology. Finally theRFEC signals are observed when changing the geometric parameters, and thedefinite influences of crack size on the amplitude and phase lag of REFCsignals are evaluated. The results of the investigations are beneficial to thedetection and sizing of SCC in pipelines

A QUASI-ANALYTICAL MODEL FOR REMOTE FIELD EDDY CURRENT INSPECTION

The Remote Field Eddy Current (RFEC) method is widely used to inspect both ferrous and nonferrous metal tubes when internal access is the only possible way of inspection. An axisymmetric quasi-analytical model is presented in this paper in order to simulate the behavior of a RFEC system during its operation. The proposed model, based on the application of the Fourier Transform in space, is able to take into account the finite length of the excitation coil, fed by an AC current, and the relative movement between the RFEC system and the wall tube. Numerical simulations based on integral formulations and experimental measurements were used to validate the proposed quasi-analytical model.

IMAGING OF CORROSION PITS ON METALLIC PIPES USING A LASER SCANNER

This paper describes the hardware and software of a laser scanning system that is used to produce 3D images of external surfaces of pipes. The images are produced in the form of 3D raster images with a resolution of up to 0.1×0.1 mm and an accuracy of 3-10 microns, depending on the desired scanning speed, to portray the corrosion pits on the pipes. The main application of the scanner is to establish patterns for calibrating nondestructive testing techniques (e.g., Remote Field Eddy Current (RFEC) and ultrasound testing that are commonly used to measure the remaining wall thickness of ductile and cast iron pipes), and also identifying the statistical characteristics of the measuring instruments utilized in those methods. The images may also be useful to scrutinize corrosion and failure mechanisms, especially when estimates of average or maximum pit depth are insufficient. The scanning system consists of a 2-DOF robot that can move a laser displacement sensor along a pipe and an instrumented rig that rotates the pipe about its axis. Rotating the pipe and moving the rangefinder along the pipe’s axis, the scanning system acquires laser measurements into a host computer to produce the image. The paper also presents the images and statistical analysis of corrosion pits of pipe samples exhumed and sandblasted for scanning.

Methodology for non‐destructive assessment of integrity of steam generator shell welds

PurposeDevelopment of non‐destructive methodology for detection of arc strike, spatter and fusion type of welding defects which may form on steam generator (SG) tubes that are in close proximity to the circumferential shell welds. Such defects, especially fusion‐type defects, are detrimental to the structural integrity of the SG. This paper aims to focus on this problem.Design/methodology/approachThis paper presents a new methodology for non‐destructive detection of arc strike, spatter and fusion type of welding defects. This methodology uses remote field eddy current (RFEC) ultrasonic non‐destructive techniques and K‐means clustering.FindingsDistinctly different RFEC signals have been observed for the three types of defects and this information has been effectively utilized for automated identification of weld fusion which produces two back‐wall echoes in ultrasonic A‐scan signals. The methodology can readily distinguish fusion‐type defect from arc strike and spatter type of defects.Originality/valueThe methodology is unique as there is no standard guideline for non‐destructive evaluation of peripheral tubes after shell welding to detect arc strike, spatter and fusion type of welding defects.

Techniques for processing remote field eddy current signals from bend regions of steam generator tubes of prototype fast breeder reactor

Steam generator (SG) is one of the most critical components of sodium cooled fast breeder reactor. Remote field eddy current (RFEC) technique has been chosen for in-service inspection (ISI) of these ferromagnetic SG tubes made of modified 9Cr–1Mo steel (Grade 91). Expansion bends are provided in the SGs to accommodate differential thermal expansion. During ISI using RFEC technique, in expansion bend regions, exciter–receiver coil misalignment, bending stresses, probe wobble and magnetic permeability variations produce disturbing noise hindering detection of defects. Fourier filtering, cross-correlation and wavelet transform techniques have been studied for noise reduction as well as enhancement of RFEC signals of defects in bend regions, having machined grooves and localized defects. Performance of these three techniques has been compared using signal-to-noise ratio (SNR). Fourier filtering technique has shown better performance for noise reduction while cross-correlation technique has resulted in significant enhancement of signals. Wavelet transform technique has shown the combined capability of noise reduction and signal enhancement and resulted in unambiguous detection of 10% of wall loss grooves and localized defects in the bend regions with SNR better than 7 dB.

Performance Estimation of a Remote Field Eddy Current Method for the Inspection of Water Distribution Pipes

Remote Field Eddy Current (RFEC) technology allows the in situ inspection of metallic water distribution pipes. RFEC tools provide the location and magnitude of corrosion defects on the inspected pipes. The capacity of an RFEC tool to detect corrosion defects is evaluated in this paper by comparing its results with those obtained from the analysis of computed tomography (CT) scan images of the inspected pipes. Localization and characteristics of defects identified with the RFEC tool and from the CT scan images were compared for six cast iron pipes. An original method is proposed for the analysis of the CT scan images from which wall thickness losses were estimated by using the basic principle that the attenuation coefficient of X-rays in a homogenous material is a linear function of its density. The results show that the RFEC tool is able to localize most of the defects identified from the analysis of CT scan images. These findings reveal that the tested RFEC probe provided reliable information on the main corrosion defects, and thus on the general structural integrity of the inspected pipes.

Three-Dimensional Finite Element Modeling of Remote Field Eddy Current Technique for Detection of Localized Defects

Three-dimensional finite element (FE) modeling of remote field eddy current (RFEC) technique has been performed to detect localized defects in ferromagnetic steam generator tubes. The excitation frequency, intercoil spacing, and the design of localized receiver coil have been optimized using this model. Amplitude and phase of RFEC signals of radial and axial receiver coils have been predicted for a through hole and two flat bottom holes (FBH). The detection sensitivity of the receiver coils has been compared, and the model has been validated for a few cases.

Pulsed remote field technique used for nondestructive inspection of ferromagnetic tube

Remote field eddy current (RFEC) technique is an effective method for measurement of ferromagnetic tube. However, traditional RFEC is unable to differentiate the internal and external defect and the probe has a long length. Pulsed eddy current techniques excite the induction coil with a pulsed waveform and have the richness of frequency harmonics. The wideband excitation is thought to be a potential in providing more information about the flaw. In this paper, pulsed RFEC technique is used to inspect ferromagnetic tube. The finite element analysis and experiment method is used to give a thorough analysis of the influence effect with the variations of the system parameters. Results show that this technique effectively combines the advantages of RFEC and pulsed excitation, which not only acquires more inspection information, including measurements of inner diameter of tubes, internal and external defects, but also reduces the length of probe and power consumption. The agreement between simulation and experiment shows that the present method is correct.

Remote Field Eddy Current Control Using Rotating Magnetic Field Transducer: Application to Pressure Tubes Examination

The remote field eddy current (RFEC) technique is used to investigate the possibility of detecting the discontinuities practiced on pressure tubes samples from nuclear reactors, pressurized heavy water reactors (PHWR) type. In this article, we propose to develop the RFEC using the technique of rotating magnetic field (RMF). A method for calculating the field generated by the eddy current transducer with RMF using propagator matrix was developed. The experimental measurements are realized for artificial discontinuities practiced in pressure tubes samples.

Influence of sodium deposits in steam generator tubes on remote field eddy current signals

The presence of sodium deposits in defective regions of steam generator (SG) tubes of fast-breeder reactors is expected to influence the remote field eddy current (RFEC) signals. By exposing five SG tubes having uniform wall loss grooves to a sodium environment in a specially designed test vessel, changes in the shape of RFEC signals were observed and it was possible to approximate the volume of sodium deposited in defects. An invariant signal parameter was determined for quantitative characterization of defects despite the presence of sodium in the defects.

Markov Chain Monte Carlo Posterior Density Approximation for a Groove-Dimensioning Purpose

The purpose of this paper is to present a new approach for measurand uncertainty characterization. The Markov chain Monte Carlo (MCMC) is applied to measurand probability density function (pdf) estimation, which is considered as an inverse problem. The measurement characterization is driven by the pdf estimation in a nonlinear Gaussian framework with unknown variance and with limited observed data. These techniques are applied to a realistic measurand problem of groove dimensioning using remote field eddy current (RFEC) inspection. The application of resampling methods such as bootstrap and the perfect sampling for convergence diagnostics purposes gives large improvements in the accuracy of the MCMC estimates.

Using Mathematical Methods to Compensate for Problems Resulting from Differences in Material Properties for Remote-Field Eddy Current Testing in Tubes

Remote-field eddy current (RFEC) testing is a nondestructive testing method. It has been comprehensively applied to detect wall loss in ferromagnetic tubes. According to our experience, the problem of variations in a material's electromagnetic characteristics often occurred in practice in carbon steel tubes. Therefore, if we fail to compensate for changes in electromagnetic characteristics during inspection, an error of evaluation will be generated. This study applied the skin-depth theory of RFEC and geometric relationships on the voltage plane to derive a compensatory model using a mathematical methodology. The new evaluation curve established on the basis of this mathematical methodology compensates for the error contributed by changing electromagnetic characteristics in the tube. The method offered by this study has proved to be reasonable, feasible and acceptable in terms of its mathematical derivation and in comparison with experimental result.

Small Diameter Remote Field Eddy Current Inspection for Unpiggable Pipelines

In-line inspection tools cannot inspect most of the natural gas transmission pipelines and distribution mains due to restrictions in the pipelines that will not allow a tool equipped with current inspection technologies to pass. Remote field eddy current (RFEC) inspection is an excellent candidate for inspecting a pipeline with multiple diameters, valve and bore restrictions and tight or miter bends. The results of this paper show that the RFEC technique can inspect pipeline materials, and that all of the components needed for RFEC inspection can be made much smaller that the pipe diameter. RFEC inspection is commercially available for inspecting small diameter piping without restrictions, several hundred feet at a time. The prototype design described in this paper shows this technology will work in a free-swimming tool that can inspect miles of pipeline at time and bypass restrictions.

Pulsed Eddy-Current Nondestructive Testing of Ferromagnetic Tubes

In sinusoidal eddy-current (EC) nondestructive testing (NDT) of thick ferromagnetic tubes, such as oil-well casings, average wall thickness is measured with an exciter coil and a detector coil displaced by more than two tube diameters [remote field eddy current (RFEC) technique]. Since RFEC cannot differentiate outer from inner defects, the average tube inner diameter is determined at higher frequency with another pair of coils displaced by around one tube diameter (electrical caliper). It is expected that the key drawbacks of such EC tools (several coils, complex transducer electronics, relatively high power consumption, and length) can be overcome with the application of pulse excitation. In this paper, we give a thorough analysis of the effect of the excitation frequency and the distance between the coils on the detected signal sensitivity to the tube properties, as a basis for usage of pulsed eddy current (PEC). We show that pulse excitation, thanks to its continuous frequency spectrum and time separation of direct and remote zone signals, allows simultaneous measurement of average wall thickness and inner diameter with one detector coil separated from the exciter by one to two tube diameters. The results of our experimental work confirm that PEC technique provides a significant improvement on present EC tools for NDT of ferromagnetic tubes.

Design of encircling remote field eddy-current probe

This paper explains the progress in designing a shielded encircling remote-field eddy-current (RFEC) probe to inspect nuclear fuel rods and investigates resulting signal characteristics. To force electromagnetic energy from the encircling exciter coil to penetrate into the rod, the exciter coil is shielded outside. The effects of the shielding and operating frequency are studied by finite-element analysis and the location for the sensor coil is decided. The sensor coil is also shielded to prevent direct influence from exciter fields. The totally shielded probe dramatically improves signal characteristics. Numerical modeling with the designed probe shows that the phase signal is almost equally sensitive to inner- and outer-diameter defects and there is a linear relationship between phase signal strength and defect depth, which are typical characteristics of RFEC signal.