Remote Field Eddy Current Technique 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.

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.

Comprehensive study on radial coil influence in designing segmented remote field eddy current probe for defect detection in ferromagnetic tubes

ABSTRACT In the present study, two types of receiver coils, viz axial and radial type receivers, in the Remote field eddy current technique are considered to detect defects in ferromagnetic tubes. The appropriate distance between the exciter and the receiver coils and the excitation frequency are estimated using the finite element model and validated experimentally. Model-assisted probability of detection studies are performed to examine the capability of radial and axial type receiver coils. Experimental studies on various artificial defects reveal the excellent performance of the radial type receiver coil in terms of the higher number of defect detections, sensitivity and imaging. This paper provides a comprehensive study on radial and axial type receiver coils for developing array probes. Radial coil performs better and also preserves localisation of defect, which is more desirable for developing array probe.

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.

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.

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.

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.

Development of magnetic flux leakage technique for examination of steam generator tubes of prototype fast breeder reactor

For non-destructive examination of small diameter (outer diameter, OD 17.2mm) and thick walled (wall thickness, 2.3mm) ferromagnetic Modified 9Cr–1Mo steel steam generator (SG) tubes of Prototype Fast Breeder Reactor (PFBR), this paper proposes magnetic flux leakage (MFL) technique. Three dimensional finite element (3D-FE) modeling has been performed to optimize the magnetizing unit and inter-coil spacing of bobbin coils used for axial magnetization of the tube. The performance of the technique has been evaluated experimentally by measuring the axial (Ba) component of the leakage fields from localized machined defects in SG tubes. The MFL technique has shown capability to detect and image tube outside defects with a signal-to-noise ratio (SNR) better than 6dB. Study reveals that Inconel support plates surrounding the SG tubes do not influence the MFL signals. As the MFL technique can detect localized defects in the presence of support plates as well as sodium and the remote field eddy current technique is sensitive to distributed wall thinning, their combined use will ensure comprehensive inspection of the SG tubes.

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.

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.

Pulsed remote eddy current field array technique for nondestructive inspection of ferromagnetic tube

One pick-up coil with a large inner diameter is usually used in pulsed remote field eddy current technique, which decreases the identification ability to defect. With the purpose of overcoming this problem, array pulsed remote field eddy current technique is proposed to enhance the precision in quantification of defect. The finite element method is used to optimise the structure of probe and analyse of the influence effect of response signal with the variation of the defect depths. The results of experimental work confirm that the array pulsed remote field technique has the advantages of high precision and sensitivity, which can be used as an effective method for quantification of defect in tube.

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.

Robust estimation of flaw dimensions using remote field eddy current inspection

The remote field eddy current technique is used to inspect conductive pipes and to estimate the dimensions of flaws liable to exist in the conductive material. A data set which contains observations for calibrated flaws is used to learn the processing. This learning problem is addressed in the context of a small size data set in which the overfitting problem is often present. To obtain a robust estimation of flaw size, this problem is minimized as follows: the estimation of flaw size uses parameters whose number is chosen the smallest possible. To obtain this set of parameters, three approaches are proposed. A reduction of the data space dimension by means of principal component analysis and parametric modelling is carried out. Then, for both cases a bilinear regression is performed to estimate the flaw size. The third approach uses a neural network to learn the processing and to directly calculate an estimate of flaw size. An MDL (minimum description length) criterion is used in the learning step to choose the smallest number of required parameters and thus to avoid the overfitting risk. The three approaches are compared in terms of accuracy and robustness. A cross-validation test is carried out on noisy data.

Frontiers in NDE research nearing maturity for exploitation to ensure structural integrity of pressure retaining components

In this paper, research and developmental efforts that demonstrate high sensitivity detection and characterization of defects and assessment of microstructural degradation, residual stresses and fatigue damage in materials using different non-destructive evaluation (NDE) techniques, have been discussed. Applications of eddy current techniques for quantitative defect characterization and for generalized applications, and remote field eddy current technique for inspection of steam generator and heat exchanger tubes have been discussed. Advanced ultrasonic methods such as time of flight diffraction, synthetic aperture focusing technique, phased array and signal processing for detection, characterization and imaging of defects have been discussed. Applications of ultrasonics and magnetic Barkhausen emission techniques for characterization of microstructures and residual stresses have been discussed. Applications of acoustic emission and infrared thermography techniques for weld quality evaluation of critical nuclear components as part of intelligent processing of materials (IPM) work have been discussed. Application of acoustic emission technique for integrity assessment of pressurized components has been discussed. Development of a software called assets and infrastructure management system (AIMS), for storing and retrieving information for various materials, components and systems, has also been highlighted. The techniques and applications discussed are result of systematic and innovative R&D efforts in the multidisciplinary areas of physics, materials, instrumentation, sensors and softwares for providing solutions to various challenging problems.

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.

Remote field eddy current technique applied to non-magnetic steam generator tubes

Unlike the impedance plane analysis form of common eddy current testing (ECT), the remote field eddy current (RFEC) technique is a through-transmission effect that reduces problems such as lift-off normally associated with ECT. In the inspection of steam generator (SG) tubes, the real issue is to detect the minute cracks growing up from the outside. However, using ECT, it is considered infeasible to accurately find them from the inside because of the limitations of penetration of eddy currents. This paper describes a finite-element approach to the solution of time-harmonic electromagnetic fields for the RFEC technique based on a magnetic vector potential and an electric scalar potential. A comparison is made of experimental and finite-element predictions of electromagnetic phenomena under the inspection of non-magnetic tubes. For the cracks outside demanding high sensitive and precise measurements in the SG tube inspection, numerical results are given for parameters to design a RFEC probe.