Magnetic Flux Leakage Testing Research Articles

Solenoid Model for the Magnetic Flux Leakage Testing Based on the Molecular Current

Magnetic flux leakage (MFL) testing is a widely applied method for the ferromagnetic specimen defect detection. In this paper, we present a solenoid analytical model to MFL field based on the molecular current model of a magnetic medium. This model applies $B$ – $H$ characteristics of the material to confirm the homogeneous magnetization of the specimen and to obtain the magnetic moment of solenoids, and then it uses a semi-infinite solenoid as the source to simulate the leakage field. Furthermore, this model proves that the edge effect and defect geometry can be represented properly by setting ellipsoid defects at different positions in the specimen. The results of the proposed model are influenced not only by defect shape as that of the magnetic dipole model but also by the relationship between the magnetization and the normal direction of the defect surface, which leads to the more accurate reconstruction of defects. Based on these findings, the error from the assumption of homogeneous magnetic dipole distribution or magnetization can be explained by the solenoid interaction. This explanation provides a new way to eliminate the computation error, and the solenoid model shows higher computational efficiency compared with the numerical model.

Small Eddy Current Testing Sensor Probe Using a Tunneling Magnetoresistance Sensor to Detect Cracks in Steel Structures

Small Eddy Current Testing Sensor Probe Using a Tunneling Magnetoresistance Sensor to Detect Cracks in Steel Structures

Development of Magnetic Flux Leakage Robot for Large-Sized Tanks Bottom Floor

This study develops a wheeled robot with magnetic flux leakage testing device. This testing robot is designed to detect the corrosion-oriented defects on the bottom plates of large-sized tanks at petrochemical plants. A Hall-effect encoder is used to achieve the function of location detection. Besides, the magnetization device consists of NdFeB permanent magnets and silicon steel sheets, with the detection probe adopting Hall magnetic sensor. Experimental results show that the magnetic flux leakage testing device is highly reliable as it can accurately detect defects sized 2 mm and being 4 mm deep on the surface of a S20C steel plate being 10 mm thick. Besides, with a S20C steel plate being 5 mm thick, the device can detect defects sized 2 mm and being 2 mm deep on the front and reverse sides.

Solenoid model for visualizing magnetic flux leakage testing of complex defects

Magnetic flux leakage (MFL) techniques are widely used for nondestructive testing of ferromagnetic materials. An analytical model of the MFL field is essential for precise determination and reconstruction of defects. In this paper, we proposed a solenoid model based on magnetization mechanisms of the magnetic medium to explain the MFL principle and simulate the MFL field. By introducing the interaction of solenoids and the Jiles–Atherton model, this model can accurately calculate the MFL field of complex defects, particularly the field distortion caused by the coupling of the defect's components. The solenoid model was experimentally demonstrated with a Z-shaped defect in the specimen and proved to be valid for precise calculation of the magneto-optical image. This work reveals the MFL testing principle from the viewpoint of the magnetic medium and thus helps elucidate and eradicate the simulation errors. The more accurate simulation than the common magnetic dipole model facilitates the inverse calculation of defects, even if field distortion is involved.

Numerical Analysis of Magnetic Flux Leakage of Transverse Defects of Sucker Rod

Abstract Sucker rods are required to operate in a very aggressive environment in the oil well industry. The combined effect of significant mechanical forces and a corrosive environment lead to frequent failures during operation. Furthermore, the detection of the defects of a sucker rod is a difficult, urgent problem to be solved in the oil industry. A solution to this situation would be quite helpful for oil field management as well as in regulating the safety management system of the sucker rod. In this paper, a nondestructive testing model of transverse defect, which is the complex crack of the sucker rod, was established using magnetic flux leakage (MFL) technique and a finite element simulation model for transverse defect was achieved using ANSYS software. Firstly, the model of single-coil magnetization and double-coil magnetization were discussed; the results demonstrated the superior performance of double-coil magnetization in comparison with single-coil magnetization. Secondly, two-dimensional MFL modeling was performed and the influence of magnetization and defect size on MFL testing were analyzed. Finally, a three-dimensional MFL model was proposed, and then the magnetic induction intensity of different liftoff values was analyzed for enhancing the detection reliability. The results showed that the MFL technique can be used to detect transverse defects in a sucker rod.

A New Micro Magnetic Bridge Probe in Magnetic Flux Leakage for Detecting Micro-cracks

Magnetic flux leakage (MFL) testing has been widely used as an efficient non-destructive testing method to detect damage in ferromagnetic materials. It’s of great importance to improve the testing capability of MFL sensors. In this paper, a micro magnetic bridge method in MFL of high sensitivity is proposed to detect micro-cracks. This method consists of a micro magnetic bridge core and an induction coil. Furthermore, a novel micro magnetic bridge probe (MMBP) of higher spatial resolution is designed and developed with $$10~\upmu \hbox {m}$$ width between the two sides of this MMBP in the testing magnetic bridge. The lift-off effect of this new MMBP is studied via finite element method and experimental verification. The results show this MMBP can achieve high sensitivity only when working with a micro-lift-off value. To examine the detecting capability of this MMBP, micro-cracks in magnetic particle inspection sensitivity testing pieces are all inspected, and the lowest depth value is only $$7~\upmu \hbox {m}$$ . The MMBP in this paper improves the testing capability of MFL to the micrometre scale and can be widely used to detect grinding micro-cracks in bearing rings.

De-noising of Magnetic Flux Leakage Signals Based on Wavelet Filtering Method

ABSTRACTTo improve the accuracy of the magnetic flux leakage (MFL) nondestructive testing in practical applications, it is very significant and key to deal with the detected MFL signals. As for the de-noising process of the MFL signals, a multilevel filtering approach based on wavelet de-noising combined with median filtering is proposed. By analyzing and comparing the de-noising properties of three wavelet families, i.e., Daubechies wavelet, Coiflets wavelet, and Symlets wavelet, two wavelet bases with the best de-noising performance are recognized and selected, namely sym6 and sym8 (the Symlets wavelet functions of order 6 and 8). Then, a new cascaded filter is constructed by combining sym6 and sym8 wavelets and cascading the median filtering method. An experimental platform is established to carry out the MFL testing, through the de-noising process for the measured MFL signals, and the results indicate that the proposed improved algorithm integrates with the merits of wavelet de-noising and median filtering. Compared with the traditional wavelet de-noising, the improved algorithm can not only improve the signal-to-noise ratio (SNR), but also reduce the de-noising error, resulting in enhancing signal quality to facilitate subsequent defect recognition.

Assessment of the Stray Flux, Losses, and Temperature Rise in the End Region of a High-Power Turbogenerator Based on a Novel Frequency-Domain Model

The electromagnetic and heat issue is the core of the key problems for large synchronous generators. This paper presents a new method to account for the rotary excitation field of a large synchronous generator using a new 3-D static frequency-domain finite element to solve the end magnetic field and electromagnetic losses via phase transformation. The solution time is significantly reduced. Both magnetic flux leakage and temperature testing of the end structures are conducted. There is good consistency between the theoretical simulation and the experimental results for both magnetic and thermal fields. The regularity of the magnetic and thermal field distributions is studied drawing on the electromagnetic and heat transfer theories.

A High-Sensitivity MFL Method for Tiny Cracks in Bearing Rings

In order to detect tiny cracks in bearing rings, a high-sensitivity magnetic flux leakage (MFL) method based on magnetic induction head is proposed and investigated in this paper. First, the MFL sensing method based on magnetic induction head is theoretically illustrated. A magnetic core with an open gap is applied to guide leaked magnetic flux into a detection magnetic circuit, where an induction coil is placed to detect the change of the magnetic flux. To obtain an optimal gap width for higher sensitivity, its influence on MFL sensitivity is analyzed by building magnetic circuits. Then, to realize automatic inspection for bearing rings, a U-shape magnetizer is proposed and 3-D numerical simulations are conducted to investigate the signal characteristics of the proposed method and the sensitivity affected by gap width. Furthermore, MFL experiments for bearing rings were performed and it was found that the proposed method has high sensitivity for tiny cracks and when the gap width is approximately equal to the width of the crack, the greatest sensitivity will be obtained. Finally, the automatic MFL testing system for bearing rings is designed and employed in the production line. In-line tests show that the novel MFL system can fulfill the inspection requirement for bearing rings.

Magnetization Time Lag Caused by Eddy Currents and Its Influence on High-Speed Magnetic Flux Leakage Testing

ABSTRACTIn high-speed magnetic flux leakage (MFL) testing, the tested workpieces pass rapidly through magnetizers. Thus, the magnetization time for workpieces is short. Because of the eddy current effect, the magnetic field inside the workpieces cannot instantly reach equilibrium, and if the magnetizing time is insufficient for the field to reach equilibrium, the MFL signals will be changed because of incomplete magnetization. In this article, the magnetization time lag caused by eddy currents and the influence of this lag on high-speed MFL testing is investigated. The time required for magnetic field to reach equilibrium in specimens, including steel bars and pipes, is obtained by theoretical calculations, finite element simulations, and experiments. The results indicate that the time required for a magnetic field inside a specimen to reach equilibrium is in the range of 50–100 ms. Using conventional magnetizers, the defect signals at testing speed of 10 m/s change because the workpiece reaches the detection zone before the magnetic field inside reaches the stable state. A simple solution is to increase the axial length of the magnetizing coil. After this procedure, signals obtained at 0.1 m/s and 10 m/s are almost identical.

An element-scaling-revising method (ESRM) for magnetic flux leakage signal analysis

Magnetic flux leakage (MFL) testing is a commonly used nondestructive testing (NDT) technique for signal analysis and defect detection. MFL signal prediction is essential to build forward model and solve defect inverse problem. This paper innovatively proposed an element-scaling-revising method (ES RM) for MFL signal analysis. A forward model for predicting MFL signal is established by ESRM. It doesn’t need to solve the forward finite element method (FEM) model in the calculation process; thus can greatly reduce computation cost, while maintaining a high precision. In this method, the MFL signals of element defects is obtained from FEM model in advance. Then detection plane of defect is scaled and the lift-off value is revised according to scaling factor k. This method is verified both numerically and experimentally to demonstrate its feasibility.

Study on the adaptive wavelet threshold denoising method for coal mine hoisting wire rope signals based on novel thresholding function

The preprocessing of wire rope online testing signals is important for quantifying any wire rope defects. As existing signal denoising methods are inadequate, the present study proposes an adaptive wavelet threshold denoising method based on a novel thresholding function. First, the adaptive denoising method for a coal mine hoisting wire rope magnetic flux leakage testing signal is analysed. Then, the adaptive denoising method for the wire rope magnetic flux leakage testing signal is studied based on Stein's unbiased risk estimate (SURE). On this basis, a new threshold function adapted to the testing signal features of the wire rope magnetic flux leakage is designed. The new function not only has a high-order continuous derivative but also no constant bias, so that it can be run in adaptive iterative form to dynamically find the best threshold. Finally, this work experimentally verifies the new method and the results show that it can effectively identify seven defects that totally conform to the real situation with a signal-to-noise ratio that is 25% higher than that of the traditional method, which also missed two defects. The new method can be more effectively used for denoising.

Window Feature-Based Two-Stage Defect Identification Using Magnetic Flux Leakage Measurements

Magnetic flux leakage (MFL) testing, one of the nondestructive testing methods, is widely adapted by approximately 90% of in-service pipelines. It is very important to identify defects in MFL testing. This paper presents a method to detect defects and determine precise defect regions using window features generated from MFL measurements. The main contributions are: 1) four novel window features of defects, namely, saliency, contrast, center point (CP), and fingerprint with parameters, are presented, which provide sufficient information for defect identification. In particular, a Bayesian method is developed to estimate the near-optimal parameters of these features and 2) a novel two-stage identification process is proposed, which can not only detect defects, but also segment the complicated defect regions precisely. The performance of the proposed method is demonstrated by real MFL measurements collected from experimental pipelines and in-service pipelines, respectively. The results show that the proposed method has satisfied accuracy for defect identification of experimental and engineering application.

Detection of a Rectangular Crack in Martensitic Stainless Steel Using a Magnetoreactance Sensing System

A magnetic flux leakage testing system based on the magnetoreactance effect was used to detect a rectangular crack in martensitic stainless steel. Detection of the transverse rectangular crack was achieved by probing the magnetic leakage fields from the crack using a magnetoreactance sensing system. The system was able to detect a small crack with a width of 1 mm and a depth of 1.5 mm even at a low frequency of 100 kHz. The influences of exciting-current intensity, sensing direction, and testing distance on the detection sensitivity were measured. As expected, the magnetic leakage signal strength increased with increasing exciting-current intensity. The magnetoreactance sensor was more sensitive to the magnetic leakage fields when it was perpendicular to the crack.

An Online MFL Sensing Method for Steel Pipe Based on the Magnetic Guiding Effect.

In order to improve the sensitivity of online magnetic flux leakage (MFL) testing for steel pipe, a sensing method based on the magnetic guiding effect is proposed and investigated in this paper. Compared to the conventional contact sensing method using a non-ferromagnetic support, the proposed method creatively utilizes a ferromagnetic one to guide more magnetic flux to leak out. Based on Hopkinson’s law, the principle of the magnetic guiding effect of the ferromagnetic support is theoretically illustrated. Then, numerical simulations are conducted to investigate the MFL changes influenced by the ferromagnetic support. Finally, the probe based on the proposed method is designed and developed, and online MFL experiments are performed to validate the feasibility of the proposed method. Online tests show that the proposed sensing method can greatly improve the MFL sensitivity.

Design of a Comprehensive Pressure Vessel Experimental Device

The equipment used in stress- and pressure-related non-destructive testing (NDT) experiments tends to have single functionality and a low degree of automation. There are also safety risks inherent in existing pressure testing methods. This paper develops a remote online NDT comprehensive experimental device. It uses a high degree of automation to remotely monitor changes in a pressure vessel’s prefabricated defects during pressure tests. Safety is guaranteed, as the operator is located remotely. The experimental pressure vessel employs a split structure connected by a quick-release system using flange-type U-slot bolt holes. Pressure vessel defects can be prefabricated in different forms, and the vessels can be disassembled and changed easily. This process reduces the experiment materials needed, lowers costs, and saves experimental time. The device can be used for various experiments, such as acoustic emission monitoring of the pressure-bearing and load-holding capacities of pressure vessels, measurement of pressurized vessel strain, and pressure vessel NDT (magnetic powder inspection, ultrasonic flaw detection, magnetic flux leakage testing, magnetic memory testing). Operators can obtain experimental data and conduct analyses simply by controlling the computer. The device can enrich the content of undergraduate experiments and solve the single-functionality problem of existing experimental equipment. With this device, the acoustic emission online monitoring experiment of pressure vessel and the stress measurement of pressure vessel are carried out. Which verify the diversity of function and reliability of the test.

Magnetic Flux Leakage Model of a Rectangular Parallelepiped Ferro-Medium

Based on the Biot-Savart law, a magnetic model on the metal magnetic memory (MMM) of a rectangular parallelepiped ferro-medium is proposed, by which inner magnetization can be determined in the magnetic flux leakage (MFL). The predicted curve of the model is identical with the MFL test of a metal specimen in simple tension. The model will play a role in the MFL technology in nondestructive test on metal specimens.

Detection of Inner Cracks in Thick Steel Plates Using Unsaturated AC Magnetic Flux Leakage Testing With a Magnetic Resistance Gradiometer

In order to ensure the safety of infrastructure, it is important to be able to detect both surface and inner cracks in the thick steel plates used. However, it is difficult to use conventional magnetic measurement techniques for inspecting inner cracks in steel because of its high permeability as well as owing to the variations in it. To solve this problem, we developed a method called unsaturated ac magnetic flux leakage (MFL) testing based on a magnetic resistance gradiometer for analyzing the inner cracks in steel. The proposed method uses a sensor probe consisting of a semicircular yoke with induction coils at each end, and a gradiometer with two anisotropic magnetic resistance sensors for detecting the components perpendicular to the steel surface. The conventional MFL testing method requires a strong power source for observing the MFL in the magnetic saturation region. In contrast, in this paper, a gradiometer-based method for detecting the differential intensity and phase is used to detect low levels of magnetic leakage using a weak power source when the test steel sample is unsaturated. During the measurements, a deep inner crack could be detected by decreasing the frequency. The line-scanned differential signal exhibited a peak just above the crack position and was observed to be depth dependent. Based on these results, the method could be used to determine the position of the crack as well as its depth.

A real time high sensitivity high spatial resolution quantum well hall effect magnetovision camera

A real time magnetovision camera with a refresh rate of 600 frames per second (fps) was developed based on small size, high sensitivity Quantum Well Hall Effect (QWHE) sensors. This handheld system consists of a 16×16 QWHE sensor array covering an 80mm×80mm area. This system has a spatial resolution of 5.08 pixels per inch (ppi). By using a superheterodyne technique to reduce the impact of (1/f) noise, magnetic fields down to 2μT can be detected for both direct and alternating magnetic field operations. Experimental results of five case studies demonstrate that magnetic flux leakage (MFL) and magnetovision imaging in DC and AC magnetic field resulting from defects and shape, can be successfully measured. The major applications of this new system are: (1) in MFL testing equipment with permanent magnets or electromagnets, (2) to obtain 2D graphical imaging similar to magnetic particle inspection (MPI) without its inherent drawbacks, (3) for examination of non-ferromagnetic materials such as aluminium, copper, and stainless steels for deep defects below the surface using low frequency alternating magnetic fields, and (4) for material identification, which has a considerable potential in several applications such as in security, product protection, bio-imaging, and medical.

Effects of excitation system on the performance of magnetic-flux-leakage-type non-destructive testing

The effects of excitation system on the performance of magnetic flux leakage (MFL)-type non-destructive testing were investigated. The investigation employed parameterized finite element analysis of the MFL testing of ferromagnetic samples to reveal the effects of the structural design and parameters of the excitation system, including the geometry and dimensions of the yoke, lengths and positions of coils and thickness of the shielding layer. Moreover, a parameter optimization method, combining parameterized finite element analysis with a genetic algorithm, was proposed to optimize the dimensions of the yoke and thus obtain maximum magnetic flux leakage. Simulation and optimization results were used to propose an MFL sensor with a preferred structural design and preferred parameters of the excitation system. Comparative experiments were conducted to validate the numerical simulation results. The experimental results agreed well with the results of numerical simulation, and the superior performance of the MFL sensor with the preferred excitation system was validated.