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The magnetic flux leakage (MFL) testing method has great application in the detection of ferromagnetic materials because of its capability of detecting internal defects. Compared with external defects, the leakage magnetic field (LMF) caused by internal defects is influenced by the ferromagnetic material above it. To analyze the mechanism of the LMF generated by internal defects, this paper proposes a method that combines the magnetic dipole theory and the theoretical calculation of permeability perturbation above the internal defect. Then, the finite element method is adopted to simulate the change of the leakage magnetic field with magnetizing currents and the distribution of the relative permeability above the internal defect. And the influence of magnetizing currents on the relative permeability is also analyzed. Finally, experiments are carried out to verify the change of leakage magnetic field with magnetizing currents. The results show that the leakage magnetic field initially increases with the increase of magnetizing currents, then it decreases at a higher magnetizing current, which proves that the LMF originates from the internal defect and the permeability perturbation area above the defect. [ABSTRACT FROM AUTHOR]

Magnetic permeability perturbation testing (MPPT) owns the advantages of high sensitivity and stability for buried defect, but the signals are always affected by the externally applied magnetic field. The mechanism of MPPT method for buried defect under unsaturated magnetization is mainly discussed in detail and verified by simulations. A series of experimental results show that the multi-source effect exists in the MPPT detection. The MPPT signal for buried defects under unsaturated magnetization depends on both of the local magnetic permeability perturbation and magnetic leakage field of the defects. The detection results of different plate thickness show that the external magnetization field significantly affected the MPPT signal, presenting a significant "convex-concave" nonlinear feature, which is helpful for the selection of magnetization current in practical detection. Clarifying the dominant mechanism of the MPPT signal for buried defect is of great significance to precision evaluation of cracks and related applications. [ABSTRACT FROM AUTHOR]

Magnetic flux leakage (MFL) testing is a widely used nondestructive testing (NDT) method for the inspection of ferromagnetic materials. This review paper presents the basic principles of MFL testing and summarizes the recent advances in MFL. An analytical expression for the leakage magnetic field based on the 3D magnetic dipole model is provided. Based on the model, the effects of defect size, defect orientation, and liftoff distance have been analyzed. Other influencing factors, such as magnetization strength, testing speed, surface roughness, and stress, have also been introduced. As the most important steps of MFL, the excitation method (a permanent magnet, DC, AC, pulsed) and sensing methods (Hall element, GMR, TMR, etc.), have been introduced in detail. Finally, the algorithms for the quantification of defects and the applications of MFL have been introduced. [ABSTRACT FROM AUTHOR]

This paper proposes a method of injected alternating current-field measurement (IAC-FM) for detecting orthogonal cracks in the piston rod of a hydraulic cylinder in a gate hoist. Using this method, both longitudinal and transverse cracks can be detected at the same time. An alternating magnetic field is produced inside the steel rod by axially injecting an alternating current into the rod. The longitudinal crack perturbs the circumferential magnetic field, whereas the transverse crack perturbs the current in the axial direction. Analyses of the behaviors of the magnetic field in the vicinity of the cracks were proposed, using a three-dimensional finite element software. An experimental setup was built and validation experiments were performed. The effects of the operating frequency and scan path were also studied. The results verified the feasibility of the IAC-FM method and showed great potential for the inspection of in-service hydraulic cylinders of gate hoists. [ABSTRACT FROM AUTHOR]

Magnetic flux leakage (MFL) testing is widely used in non-destructive testing of ferromagnetic components. In view of the serious attenuation of the leakage magnetic field (LMF) caused by the transmission of LMF in the lift-off layer between the measuring point and the workpiece, this paper introduces an MFL detection method based on the slotted ferromagnetic lift-off layer (SFLL). The conventional non-ferromagnetic lift-off layer is changed to a ferromagnetic lift-off layer with a rectangular slot. The magnetic sensor is fixed above the slot and scans the workpiece together with the lift-off layer. First, the detection mechanism of the new method was studied by an equivalent LMF coil model. The permeability perturbation effect and the magnetization enhancement effect were analyzed in the new method. Based on the detection mechanism, the lift-off tolerance of the new method was investigated. Then, the LMF enhancement and lift-off tolerance of the new method in the steel plate detection model were studied. Finally, experiments were conducted to compare the new method with the conventional method. The simulation and experimental results show that the slotted ferromagnetic lift-off layer enhances the amplitude of the MFL signal and is tolerant to the lift-off value. This method provides a new idea for optimizing the design of the MFL sensor and improving the sensitivity of MFL detection at a large lift-off value. [ABSTRACT FROM AUTHOR]

DC magnetization is generally considered to suppress the usual local magnetic permeability variation and increase the penetration depth for magnetizing-based eddy current testing (MB-ECT) of ferromagnetic materials. In fact, such simple explanations lead to rough nondestructive evaluation and cause new neglected non-uniform magnetic characteristics. Hence, the "perturbation" of the internal magnetic field variation is analyzed using a magnetic dipole model and the mechanism of magnetic permeability perturbation in MB-ECT is revealed. The theoretical analysis and simulations show that a significant permeability perturbation always appears around a defect and presents opposite features with strong and weak magnetization. Furthermore, experimental results indicate that the hidden signal component arising from the local permeability perturbation is critical for both far-side surface and near-side surface defects in the MB-ECT method. [ABSTRACT FROM AUTHOR]

In oil mining system, stress cracks on thread usually lead to drill pipe failure. To maintain normal operation of drill pipes, the thread needs to be nondestructively tested. However, due to complicated structure, it is challenging to identify and quantify the cracks the using existing nondestructive testing (NDT) methods. In this article, a scanning induction thermography (SIT) system for automatic detection of drill pipe thread (DPT) is presented. Specifically, a profiling heating method (PHM) is proposed to produce uniform heating on the root part of thread tooth where most of the stress cracks occur. A weighted least squares fitting (WLSF)-based difference algorithm is used to eliminate the background temperature of scanning data. The algorithm can extract crack thermal response with the influence of lift-off distance change and marginal effect of electric field in SIT. Experiments show that the proposed system can obtain 3k thermal contrast between crack and defective-free area when testing the crack with only 0.3 mm depth. Three SIT systems with other heating methods are also used for comparison. Better heating uniformity, signal intensity, robustness of structural change, linear property with depth information, and signal consistency when testing similar cracks in different areas of the root part can be demonstrated in the proposed system. [ABSTRACT FROM AUTHOR]

Tubular solenoids have been widely used without any change since an electrical wire was discovered to create magnetic fields by Hans Christian Oersted in 1820 and thereby the wire was first coiled as a helix into a solenoid coil by William Sturgeon in 1823 and was improved by Joseph Henry in 1829 [see http://www.myetymology.com/encyclopedia/History_of_the_electricity.html; J. M. D. Coey, Magnetism and Magnetic Materials (Cambridge University Press, New York, 2010); and F. Winterberg, Plasma Phys. 8, 541553 (1996)]. A magnetic control method of C-shaped carrying-current wire is proposed, and thereby a new opening electromagnetic transducer evidently differing from the traditional tubular solenoid is created, capable of directly encircling and centering the acted objects in it, bringing about convenient and innovative electromagnetic energy conversion for electromagnetic heating, electromagnetic excitation, physical information capture, and electro-mechanical motion used in science research, industry, and even biomedical activities. [ABSTRACT FROM AUTHOR]

Featured Application: The formation mechanism of crack indications and the influences of related parameters in magnetic particle inspection were investigated by theoretical analysis and an experiment, both of which provide a valuable reference for the detecting reliability improvement and the quantitative detection of cracks in automatic magnetic particle inspection. The recent rapid development of industrial cameras and machine learning has brought new vitality to the very traditional flaw detection method, namely, magnetic particle inspection (MPI). To fully develop automatic fluorescent MPI technology, two main issues need to be solved urgently—the lack of theoretical analysis on the formation of the crack indications, and quantitative characterization methods to determine the crack indications. Here, we carry out a theoretical analysis and an experimental approach to address these issues. Theoretical models of the acting force of the leakage magnetic field were established. Subsequently, the impacts of different magnetic field strengths (1000–9000 A/m) and magnetic particle concentrations (0.5–30 mL/L) on the adsorption critical distance were analyzed. The models were solved by numerical calculations in MATLAB. In addition, a single variable control experiment was conducted to study the effects of crack images. In order to determine the quality of the crack image, three characteristic parameters were investigated, such as indication gray scale, background gray scale, and contrast ratio, were provided. The theoretical magnetic particle concentration range provided a guidance value for automated fluorescent MPI. Experimental results revealed that the optimal magnetic particle concentration was 3–4 mL/L, and, under this condition, the contrast between the crack indications and the background of crack images was obvious. [ABSTRACT FROM AUTHOR]

Magnetic flux leakage (MFL) testing is widely applied in the online detection of steel pipes. Different magnetizing directions are required for the detection of defects in different directions. As the speed of online MFL testing increases, the motion induced eddy current (MIEC) effect becomes significant, and the direction of the MIEC is perpendicular to defects in the same direction as the magnetization. Therefore, the magnetic field signal generated by the MIEC perturbation is analyzed by simulation and compared with MFL signal. It is found that the amplitude of the magnetic field signal generated by the MIEC perturbation increases with the rise of the rotational speed and magnetization. In high rotational speed and strong magnetization, the amplitude of the magnetic field signal caused by MIEC perturbation is greater relative to the amplitude of the MFL signal, providing a possibility for detecting defects that are parallel to the direction of magnetization. [ABSTRACT FROM AUTHOR]

The traditional magnetic flux leakage (MFL) method with a single magnetic excitation source has been widely used in engineering practice but fails in long-range (long liftoff distance of the sensor from the surface of the tested object) defect detection, especially when the liftoff distances increase. Thus, we propose a novel electromagnetic testing method based on the magnetic field interaction between the primary magnetic induction field and the secondary field of MFL. The principles of the new method are first introduced, and the method is compared with the common MFL methods; this comparison indicates that the new method has the capabilities of long-range defect detection and signal strength enhancement. Furthermore, a series of simulations and analyses demonstrates that the proposed method is more efficient in defect detection, particularly when the induction magnet is a diamagnetic material, which is independent of the coercive force. Finally, experiments and comparisons were conducted to verify the feasibility and reliability of our proposed method in the long-range defect detection. All results show that the novel method has great promise in widely practical applications in the future. [ABSTRACT FROM AUTHOR]

In 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. [ABSTRACT FROM AUTHOR]

In high-speed scanning MFL inspection for a steel pipe, according to Lenz's Law, motion-induced eddy current (MIEC) is induced by relative movement between axial magnetizer and the inspected pipe. On the one hand, magnetic field generated by MIEC will change the magnetization intensity of the pipe and thereafter change the signal amplitude; on the other hand, it will also cause signal baseline shift by altering background magnetic field. In this paper, the effect of MIEC on axial MFL inspection is investigated by theoretical analysis, finite element simulations, and high-speed experiments. It is found that at different locations MIEC causes different change trends for both the background magnetic field and magnetization intensity, leading to sensitivity difference, which should be taken into consideration in the high-speed MFL inspection for a steel pipe. [ABSTRACT FROM AUTHOR]

The mechanism of widely applied magnetizing-based eddy current testing (MB-ECT) sensor for ferromagnetic materials is simply emphasized by “the magnetization eliminates the non-uniformity of the permeability and increases the penetration depth”, resulting in rough application for non-destructive testing (NDT). Hence, the neglected permeability distortion around the surface crack is pointed out and analyzed at different magnetization states. In this paper, the multi-source effect of MB-ECT method is analyzed using the equivalent source method. The disturbed magnetic field source is actually composed of the primary magnetic leakage field, secondary disturbed magnetic field caused by the crack itself and the permeability distortion around the crack. Furthermore, the influence of the magnetizing current and the probe lift-off are investigated by finite element analysis (FEA). The simulation result shows “concave” feature as a consequence of the permeability distortion. A series of experiments are designed to analyze the dominant signal components under different conditions. The magnetizing current selection in MB-ECT method corresponds to a unique shape while plotting the signal amplitude as a function of the magnetizing current and lift-off of the probe. This study reveals the multi-source effect in the mechanism of MB-ECT sensors and provides the basic theory or analysis principles for the precision crack evaluation. [ABSTRACT FROM AUTHOR]

In magnetic flux leakage (MFL) testing technology, the MFL signals are thought to result from all defects and are used in their evaluation. The tested defects include two types of defects, concave and bump-shaped features, and recently described mechanisms in the MFL method indicate that the former defects produce positive MFL because of magnetic refraction and the latter ones produce negative magnetic fields because of self-magnetization regulation; consequently, these defects result in raised test signal waves and sunken test signal waves, respectively. Thereby, a new methodology for accurately identifying the defect type based on the mapping relation between the signal features and defect types is proposed. Both simulations and experiments with three representative defects (i.e., notch, protrusion and combination) were conducted to confirm their identification using this new methodology. Combined with MFL standards such as American Society for Testing and Materials (ASTM) E570-09 (Standard Practice for Flux Leakage Examination of Ferromagnetic Steel Tubular Products, ) and British Standards (BS) EN 10246-4 (Non-destructive Testing of Steel Tubes-Part 4: Automatic Full Peripheral Magnetic Transducer/Flux Leakage Testing of Seamless Ferromagnetic Steel Tubes for the Detection of Transverse Imperfections, ), suggestions for standard specimens with reference defects that consist of both types of defects are provided. [ABSTRACT FROM AUTHOR]

The widely applied magnetic flux leakage (MFL) nondestructive testing for ferrous materials has been simply explained by 'MFL escaping from defects and being induced is exactly the MFL testing principle,' with a rough application for evaluation. Hence, we analyze the 'escape' course of MFL generation using magnetic refraction and explain the reason why the MFL technology really exists in practice. Furthermore, we also point out that the presently so-called MFL testing technique essentially hides a different signal component arising from bump-shaped defects features unlike the MFL caused by concave-shaped ones, explained by another forming mechanism. [ABSTRACT FROM AUTHOR]

Abstract: On the basis of permanent magnet, perpendicular oriented to the surface of the component under inspection and surrounded by a pick-up coil, the so-called PMFP-effect (permanent magnetic field perturbation-effect) can be observed when the magnet is moved in this perpendicular position with a well defined lift-off along the surface and a surface-breaking or near-surface – but hidden – defect is coming into the influence range of the magnetic field. A new NDT (nondestructive testing) technique by using PMFP so can be proposed for the first time, and its detection mechanism is presented and analyzed by simulations. Afterwards, its testing characteristics for defects are given and its feasibility is further confirmed by experiments. Meantime and particularly, its inspection depth and the effect of the position and attitude of PMFP sensors on inspection signals are analyzed. Accordingly, a multi-differential method used for improving signal-to-noise rate is presented and some testing apparatus using PMFP method is developed. Finally, the proposed PMFP method is discussed specially compared to MFL in detail. [ABSTRACT FROM AUTHOR]

Abstract: On the basis of the analysis of the MCE (magnetic compression effect) in present MFL (magnetic flux leakage) applications, a new MFL principle and technique based on the establishment of a near-zero background magnetic field is proposed for the first time, resulting in an increase of MFL allowing a non-contact MFL signal pick-up at larger probe lift-off. Afterwards, the feasibility of the proposed principle is justified by simulations and experiments. Finally, their advantages are presented. By applying the proposed MFL principle, the severe wear of contact probes can be avoided and inspection at high-temperature heat pipes can be performed. The contributions of the technique to reduce magnetic noise, distortion and the saturation of hall sensors are also discussed. [Copyright &y& Elsevier]

Abstract: On the basis of the analysis of present MFL (magnetic flux leakage) testing sensors, the existence of MCE (magnetic compression effect) in present MFL testing sensors was specially pointed out and its relevant MCE theory was also analyzed, which manifest themselves in the compression of MFL in its volume caused by the background magnetic fields which have been overlooked. Afterwards, the presence of the proposed MCE was verified by simulations and experiments, which further indicates that the stronger background magnetic field is and the less MFL of defects forms. Furthermore, the MFL mechanism for MFL testing sensors were further discussed combining with MCE. The find of MCE and its relevant avoidance can increase the MFL in its volume, resulting in a new MFL testing sensor with a good sensitivity and a long distance and non-contact inspection method. Additionally, the work is of significant importance to the theory of MFL testing sensors by clearing and richening the MFL mechanism. [Copyright &y& Elsevier]

Magnetic Permeability Perturbation testing (MPPT) has the advantage of high sensitivity for deeply buried defects in steel pipes. However, in complex working conditions, variations in the partial wall thickness of the pipe can affect the defect detection results. The paper discusses the influence mechanism of uneven wall thickness on the MPPT of internal defects on steel pipes and analyzes the magnetic field distribution caused by uneven wall thickness which influences on the magnetic permeability perturbation (MPP) of internal defects. The MPP is enhanced at the thinning wall and weakened at the thickening wall. The effect of uneven wall thickness on MPPT signals is confirmed through simulation and experiment. The signal of defects in thinning wall is enhanced, while the signal of defects in thickening wall is weakened. The greater the wall thickness variation, the degree of signal distortion is more significant. Experimental results show that uneven wall thickness has an effect on the detection of the defects with different depths. Finally, a method using deep saturation magnetisation is proposed to mitigate the inconsistent signals caused by uneven wall thickness. The findings will contribute to quantitative evaluation of defects in MPPT. [ABSTRACT FROM AUTHOR]

In the field of non-destructive testing (NDT), the detection of surface cracks using permanent magnetic field perturbation (PMFP) testing has emerged as a novel approach. It is particularly well-suited for detecting ferromagnetic materials due to its capability to identify open cracks, facilitated by a compact and straightforward probe structure. However, the existing rigid PMFP probes pose challenges in terms of shape-conforming, resulting in large lift-off fluctuations during sweeping, and are prone to wear and tear. Furthermore, their suitability is limited when inspecting specimens with various specifications and irregular shapes. To address these issues, this paper proposes a novel PMFP detection method based on the magnetization of a Flexible Magnetic Sheet (FMS). For further optimization, the machining of a through-slot in the FMS, with the magnetic sensor embedded within, serves to capture the magnetic field perturbation. This configuration is employed for the detection of surface defects. Compared to the conventional PMFP method that relies on rigid permanent magnetization, its flexibility makes the proposed method offer notable advantages such as streamlined processing, lightweight construction, and inherent adaptability to various shapes. Moreover, compared to the traditional PMFP structure where the sensor is placed beneath the permanent magnet, the through-plot FMS design exhibits a larger and more uniform magnetic field. Consequently, this feature significantly improves the signal-to-noise ratio by approximately 30%. The flexible testing method has a high application value and can effectively detect rectangular slots of size 1 mm * 1 mm* 12.5 mm and flat-bottom holes of size Φ2mm* 0.5 mm in pipeline testing. [Display omitted] • Based on FMS magnetization, a flexible probe is proposed to optimize the adaptability of conventional PMFP testing. • Enhanced magnetization field and signal-to-noise ratio by machining through-slots in the FMS. • The proposed flexible probe structure effectively addresses the issues of fit and array scanning in pipeline testing. [ABSTRACT FROM AUTHOR]

This paper introduces a magnetic displacement sensor comprising of a double-layer parallel cables (PCs) magnetizer, tunnel magnetoresistance (TMR) sensors and magnetic flux concentrating plates (MFCPs). By utilizing a double-layer PCs magnetizer, the background magnetic field in the TMR sensor's location can be eliminated, while enhancing the magnetization field of the workpiece. This ensures that the highly sensitive TMR sensors operates within its measurement range. Additionally, the MFCPs concentrate the magnetic field at the TMR sensor location to further improve the displacement measurement sensitivity. After optimizing the geometry and excitation frequency of the probe with FEM, the probe can achieve displacement measurements with 0.1 mm accuracy in the range of 0.5–6.0 mm for ferromagnetic workpieces with linearity of 4.5 % and average relative error under 2.0 %. [Display omitted] • Parallel cables, magnetic displacement probe, ferromagnetic material, sensitivity enhancement, corrosion detection. • A magnetic displacement probe design replaces permanent magnets with lightweight energized parallel cables (PC). • This probe has minimal background noise and excellent sensitivity. • The probe offers a measuring range spanning 0.5 to 6.0mm, with a precise measuring accuracy of 0.1mm and a relative error of under 2.0%. • This probe holds promise for array displacement sensing and is well-suited for detecting deformation and corrosion in extensive ferromagnetic workpieces. [ABSTRACT FROM AUTHOR]

Aiming to the current situation that single axial magnetization can hardly qualified for omnidirectional defects detection, meanwhile, conventional complex magnetization method features with low speed and low efficiency. Thus, a novel method of magnetic flux leakage (MFL) testing for omnidirectional defects based on super-strong single axial magnetization as well as the corresponding detection apparatus were proposed. First, principles of single axial magnetization method were introduced, where a through-coil with high direct current was mainly used to generate the super-strong magnetic field. Then, methods of omnidirectional defects detection through magneto sensor arrays were concretely presented. What’s more, details of equation sets for lateral and longitudinal defects detection and classification were given. Additionally, a summation and comparison combination method was put forward and different defect signal amplitudes were compensated. Naturally, a unified judgment for omnidirectional defects inspection with same sensitivity or accuracy was formed. Most importantly, series of simulations and experiments were conducted, and finally, the feasibility and reliability of the new MFL testing method were demonstrated by those results. The obvious advantages for the suggested method are that it not only makes the detection for omnidirectional defects under single axial magnetization possible, but also the low economic cost, high speed and high efficiency detection for both lateral and longitudinal defects adaptable for various shaped ferromagnetic objects with great application promise in engineering is ultimately accomplished. [ABSTRACT FROM AUTHOR]

• The significant attenuation of the leakage magnetic field in the lift-off layer is considered. • The ferromagnetic lift-off layer with through groove can enhance the leakage magnetic field of the crack. • The principle relies on the mutual magnetization enhancement effect between the crack and the through groove. [Display omitted] In conventional magnetic flux leakage (MFL) detection, a lift-off layer based on air or non-ferromagnetic wear-resistant material is employed to prevent magnetic sensors from wearing. However, the increase of the lift-off will lower the detection sensitivity and reduce the MFL signal amplitude. To address the issue, theoretical analysis was used to investigate the effect of the ferromagnetic lift-off layer on MFL detection in this study. Then, we put forward a new MFL detection method based on the ferromagnetic lift-off layer. A through groove is machined in the ferromagnetic lift-off layer, which creates an air gap below the sensor. The leakage magnetic field (LMF) generated by the through groove increases the local magnetization intensity around the crack on the sample, while the LMF generated by the crack increases the magnetization intensity surrounding the through groove of the lift-off layer. As a result, the magnetic sensor above the through groove detects a greater MFL signal. In addition, the effect of applied magnetization intensity, groove width, and lift-off layer thickness on the detection signal were explored by experiments. The practicality of the proposed method of detecting different size cracks of nearside and farside were studied. In summary, experiments show that the ferromagnetic steel sheet with a through groove strengthens the MFL signal of the nearside and farside cracks. [ABSTRACT FROM AUTHOR]