Electromagnetic Nondestructive Testing Research Articles

Effect of bias magnetic field on transducer efficiency in electromagnetic ultrasonic non-destructive testing of Rock bolt

Non-destructive testing (NDT) of rock bolt anchoring quality is an important part of engineering quality and safety control, electromagnetic ultrasonic NDT is one of the promising detection methods, and improving the efficiency of ultrasonic transducer by optimizing the bias magnetic field is an important factor to obtain effective information. In this paper, several factors affecting the bias magnetic field are studied, and the work is divided into simulation and demonstration: COMSOL was used to simulate the electromagnetic ultrasonic transducer(EMAT) in order to analyze the influence of the number of magnetic circuit, the thickness of magnet and the thickness of yoke iron on the bias magnetic field. Then, an experimental platform was built, the magnetic circuit structure of the transducer was set with reference to the simulation results, and several important parameters of the bias magnetic field were determined by analyzing the collected signals. The results show that with the increase of the bias magnetic field magnetic circuit structure from independent to ring form, the magnetic field strength of the rock bolt increases, the magnetic field uniformity becomes better, and the efficiency of the transducer is also improved accordingly. In addition, under the same magnetic circuit condition, increasing the thickness of permanent magnet and yoke iron does not increase the magnetic flux density of the rock bolt axis proportionally, and has a limited effect on the efficiency of the transducer. In practical application, the corresponding structure is selected according to the needs, and the better bias magnetic field structure is selected while meeting the efficiency of the transducer.

Magnetic characterisation of ferromagnetic material

In magnetic and electromagnetic non-destructive testing, the physical properties of the material are often not well known or completely unknown. The design of the non-destructive testing systems that provide the magnetisation is often oversized in order to get enough magnetic saturation (e.g. in magnetic flux leakage method or in magnetoscopy method). An improvement of the device performances, expressed usually by the signal to noise ratio, can be obtained only when the designer of the device well knows the material characteristic in terms of magnetic curve, the curve that links the magnetic field strengths to the magnetic flux density. A good device must be able to saturate the material sample under test but not to overcame determinate values. In fact, when the magnetisation is low, the leakage of the flux is negligible, while when the magnetisation is too strong the material tends to be like air and the leakage flux is negligible too. The experimental characterisation is derived from the technique adopted for the testing of soft magnetic material adopted in electrical machines or electromechanical actuators. In the present paper will be presented some material characterisation useful for the non-destructive testing of railway track, metallic ropes or reinforcement iron.

Electromagnetic image recognition of a defect profile on a metal surface with a protective layer based on magnetic disturbance

In order to obtain defect information quickly and effectively and improve the accuracy and evaluation ability of traditional electromagnetic non-destructive testing (NDT), an electromagnetic image recognition method for the defect profile based on magnetic field disturbance is proposed in this paper. The excitation coil structure is designed, the excitation mode of the signal source is optimised and a three-dimensional electromagnetic transient analysis model is established for defect profile identification of a metal surface with an anti-corrosion protective layer. The research shows that the disturbed magnetic field Bz has the characteristics of high-resolution imaging and symmetry. The orientation of the defect on the surface has different effects on the clarity of image recognition. The larger the angle between the defect boundary and the induced current, the more complete and clear the image formed by the disturbed magnetic field Bz . A rectangular square wave is the best excitation signal for defect recognition. Its Bz image at t = 0 can present complete shape and position information about the defect. In addition, the excitation coil structure based on the principle of the disturbed magnetic field must provide a uniform induced current to produce a pronounced disturbed magnetic field. It is concluded that electromagnetic imaging technology based on the disturbed magnetic field Bz can better detect and characterise the shape of metal surface defects without damaging the metal protective layer and has good application potential for NDT and safety evaluation of in-service equipment.

Eddy current testing in drill-bit tools

Machining technology plays a major role in modern manufacturing industries due to the growing demand for equipment and products for aerospace, automotive, precision machinery sectors etc. A key issue associated with any type of machining operation is the accelerated tool wear which leads to shorter tool life, thus to low machining precision and decreased productivity. In this paper an electromagnetic Non-Destructive Testing method (NDT), and in particular Eddy Current Testing (ECT), is used to detect the wear of used cutting tools vs a new one.

Magnetic anisotropy quantification in steel fiber reinforced materials

Fiber reinforced composites have grown in popularity over last few decades. Among these, steel fiber reinforced materials such as ultra-high performance concrete (UHPC) have begun use in the construction industry. Such materials provide an advantage over conventional fabrication techniques due to higher compressive, flexural, and tensile strengths. Yet, one challenge with fiber reinforcement is presence of unintended alignment during fabrication, which may deviate the material’s strength from its intended design. This anisotropy needs to be nondestructively quantified to validate the material design post-fabrication. While electromagnetic nondestructive testing sensors have been demonstrated for quantifying the characteristics of ferromagnetic fibers in a material, there has not been a robust study on quantifying anisotropy with such systems. In this work, we provide the fundamental theory and process for quantifying anisotropy in steel fiber reinforced resins using electromagnetic sensors, with our data achieving a 95% goodness-of-fit with our theoretical model, on average.

High Temperature Magnetic Properties of Selected Steel Grades

In order to effectively control the microstructure of steel strip during manufacture it is vital that techniques be developed to monitor microstructural changes in the steel at high temperatures. Electromagnetic non-destructive testing is increasingly being used in these applications as it is non-contact, robust, relatively inexpensive and has the potential to provide real-time feedback to control the manufacturing process. However, if the data produced by these systems is to be fully exploited it is necessary to accurately measure the magnetic properties of steel grades of interest at temperatures up to the Curie point. This work details measurements carried out to assess the magnetic properties of selected steels grades, supplied by Tata Steel, up to the Curie temperature using a novel test rig based on a modified Epstein frame designed by the University of Manchester with high temperature trials at the University of Warwick. Several steel grades are studied with the full magnetic hysteresis (BH) loops reported during heating and cooling, along with extracted magnetic parameters including coercivity, magnetic saturation and magnetic permeability. The magnetic parameters of the selected steel grades are compared and implications of the results on electromagnetic monitoring of steel strip during manufacture considered. Results show the developed system has the potential to provide valuable data to inform online electromagnetic monitoring systems.

Numerical analysis and experimental research on detection of welding defects in pipelines based on magnetic flux leakage

Regular inspection of long-distance oil and gas pipelines plays an important role in ensuring the safe transportation of oil and gas, and inspection on welding defects is an important part of the inspection process. Magnetic flux leakage (MFL) is an electromagnetic non-destructive testing technique which has been commonly utilized to detect welding defects in pipelines. In the present study, Maxwell electromagnetic simulation software was used to carry out numerical study on the welding defects in pipelines, including incomplete penetration and undercut. The Ф406 pipeline with a wall thickness of 7 mm was selected as the study case to establish the numerical model. Setting the life-off value at 1 mm, the distribution of magnetic leakage field was investigated for pipeline without defect, pipeline with incomplete penetration defect and pipeline with undercut defect respectively, the characteristic values describing the depth and width of defects were found. Furthermore, quantified equations which can be used to describe the defect depth were proposed. Finally, experimental research was carried out to validate the effectiveness of the numerical model, and the experimental results showed good consistence with the numerical calculation results. The research results indicate that, it is technically feasible and reliable to diagnose the incomplete penetration and undercut welding defects in pipelines using MFL.

Research on detection method of concrete compressive strength based on dielectric properties

Compressive strength of concrete structures is a crucial quality parameter that determines their performance throughout lifespan. This study introduces an innovative approach of utilizing electromagnetic nondestructive testing to assess the compressive strength of mature concrete. The dielectric constant and compressive strength of standard specimens were tested by open coaxial probe and pressure testing machine, respectively, using ordinary Portland cement concrete with various water-cement ratios (0.4, 0.45, 0.5, 0.55, 0.6 and 0.65) as the research object. It has been discovered that increasing porosity reduces the number of polarized particles per unit volume of concrete, resulting in a decrease in polarization intensity. This decrease is manifested as a reduction in the dielectric constant in macro parameters. The relationship between porosity and dielectric constant was characterized using a series-parallel composite concrete dielectric model. Additionally, as porosity increases, the compressive strength of concrete decreases exponentially. By considering porosity as an intermediate variable, a concrete compressive strength model based on the dielectric constant was established. The model exhibits an average error of 3.62%, indicating the precise characterization of concrete's compressive strength using the dielectric constant. This model can serve as a theoretical basis for the implementation of electromagnetic nondestructive testing techniques in strength evaluation.

RCF crack direction assessment in moving ferromagnetic material by DC electromagnetic NDT technique

The rolling contact fatigue (RCF) cracks that initiate from the surface of ferromagnetic materials in aviation bearings and high-speed rails are typically inclined at varying angles, posing a serious threat to safety. Therefore, a rapid and quantitative characterization of inclined cracks is of great importance. This paper investigates the distribution of magnetic fields and motion-induced eddy current (MIEC) in direct current (DC) electromagnetic non-destructive testing (NDT), and proposes an approach for measuring the direction of RCF cracks in moving ferromagnetic materials. The magnetic field and MIEC distributions in the moving ferromagnetic material are initially investigated, followed by a numerical simulation exploring the impact of crack inclination angle and depth on detection signals. The resulting relationship between crack propagation angle, depth, and detection signal is then obtained. Furthermore, a DC electromagnetic NDT platform has been designed and a quantitative detection approach for inclined cracks has been proposed and validated through experiments. Finally, a real scenario detection is carried out. The investigation presented in this paper demonstrates that the DC electromagnetic NDT technique is applicable not only for assessment RCF crack direction in moving ferromagnetic materials, such as rotating aviation bearings, but also for characterizing inclined cracks under conditions of fast relative motion between the probe and the ferromagnetic material (e.g., high-speed rail, pipeline, etc.).

Stress Measurement of Ferromagnetic Materials Using Hybrid Magnetic Sensing

Stress measurement and evaluation of ferromagnetic components using electromagnetic nondestructive testing (ENDT) are affected by the microstructure properties of the material, such as inhomogeneity and anisotropy. This leads to magnetic induction sensitivity in the variety of both the stress and microstructure. To address this challenge, a hybrid magnetic sensing structure for measuring eddy current, alternating electromagnetic (EM) field, Barkhausen noise, and incremental permeability signals is designed and instrumented. Additionally, a stress detection verification test for oriented silicon steel samples is carried out, and a comparison with commercial instrument in terms of sensitivity and repeatability is performed. The results show that the proposed sensor has advantages in detection repeatability, good sensitivity as well as high signal-to-noise (SNR) for MBN features, which brings potential in stress characterization and assessment.

Study on surface crack detection of ferromagnetic materials based on remanence

Remanence detection is a technique of electromagnetic non-destructive testing (NDT). This paper studies a quantitative detection method for surface cracks of ferromagnetic materials based on remanence. The finite element analysis software COMSOL Multiphysics was used to establish the remanence detection model and the 'moving grid' function was used to realise the simulation of the remanence signal. The leakage magnetic field occurs due to the distortion of the magnetic induction lines near the surface cracks after ferromagnetic materials are magnetised. Remanence detection uses the leakage magnetic field to detect cracks. The relationship of the leakage magnetic field versus the crack depth and width was analysed using the magnetic dipole model. The relationship between the crack size and the remanence signal was verified by measuring the surface remanence signal of cracks of different sizes. The characteristic parameters related to the crack size were extracted and the regression model between the characteristic parameters and the crack size was established. For the remanence detection, the maximum error of width prediction was 16.25% and the maximum error of depth prediction was 18.48%. For the magnetic flux leakage (MFL) detection, the maximum error of width prediction was 12.1% and the maximum error of depth prediction was 12.32%. Under the same experimental conditions, the maximum error of crack width measurement of remanence detection was 4.15% larger than that of MFL detection and the maximum error of depth was 6.16% larger than that of MFL detection.

Eddy current assessment of flaw severity of cylindrical products

This paper proposes the use of specific procedures of electromagnetic nondestructive testing method for evaluation of flaw severity in cylindrical products. The phenomena of mechanical and / or thermal fatigue as well as accidental handling can generate the appearance of flaws followed by the increase of these in the metal tubes. This paper proposes the use of specific procedures of electromagnetic nondestructive testing method for evaluation of flaw severity in cylindrical products, which included in a database, makes it useful for the operation of automatic classification of the severity of discontinuities, being based on the method of neural network “learning” to remain immune to certain signal-to-noise ratio. With this, inner circumferential discontinuities with 1mm depth can be detected with probability of detection more than 89% for 90% confidence level.

Lift-off suppression based on combination of bridge and transformer signal conditionings of eddy current testing

Lift-off is a challenging issue in electromagnetic non-destructive testing and evaluation, especially in eddy current testing. It seriously affects the detection sensitivity and quantification accuracy. This paper proposes a combined signal conditionings of differential eddy current structure based on bridge and transformer for lift-off suppression. It provides both amplitude and phase information via two signal conditionings for multiple parameter measurement. Specifically, a multiple parameters fusion response model is established according to equivalent circuits between the electromagnetic sensing coupling and lift-off. This model enables to greatly reduces the lift-off ambiguities from mixed signal since lift-off parameter can be eliminated and defect information is increased. In particular, it is suitable for irregular lift-off variation suppression with non-parameters adjusting. Different defect samples have been conducted to validate the proposed method.

Analytical Solutions to Eddy-Current-Induced Field in Cylindrical Conductor Considering Velocity Effect

Electromagnetic nondestructive testing methods to inspect metal rods and pipes are widely used in industrial production. When the scanning speed of the electromagnetic coil is close to the magnitude of eddy current diffusion speed in conductive specimens, the coil motion will significantly affect the distribution of the eddy current field and detection signal. In this article, based on the basic equations and boundary conditions of electromagnetic field problems for moving conductors, the eddy-current analytical model of coaxial relative motion between the coil and the cylindrical conductor rod is solved by using Fourier transform method, and the impedance change expression of the pick-up coil is obtained. The theoretical results are in excellent agreement with the experimental results, which verify the analytical expression of impedance change. Furthermore, the effects of a conductor moving velocity and excitation frequency on impedance-change modulus values for differential and absolute pick-up coils are analyzed, respectively. The theoretical methods proposed in this article quantitatively analyze the velocity effect on the eddy current field, which can be applied to fast real-time eddy current testing for conductive specimens.

Quantitative sizing of compound location defects based on PECT-EMAT hybrid testing methods

Metal structures are widely used in modern industrial fields, in which various compound location defects are inevitably produced from manufacture to service. Facing such complex conditions, a single electromagnetic nondestructive testing method usually cannot meet all needs. For metal structure with surface crack and bottom thinning defect, a novel PECT-EMAT hybrid testing method is proposed in this study. The relevant simulation and experimental method are developed to realize the compound location defects detection. The proposed method employs only one set of equipment to realize the compound location defects detection, which greatly reduces the complexity of the experimental system and improves the detection efficiency. In addition, the correlation between the extracted signal features and the defect parameters is established through simulation, and further verified by experiment. Furthermore, in order to estimate the damage level of the defects, the inversion calculation procedure of the hybrid NDT method are developed based on genetic algorithm and neural network algorithm, respectively. The reconstruction of the compound location defects using the above two inverse algorithm are carried out using simulation data and experimental data. Finally, the effects of the two inverse methods are compared and the reasons for their advantages and disadvantages are analyzed.

Determining Feature Extraction Parameters for Pulsed Eddy Current Sensor: A Minimisation Problem Approach

Pulsed eddy current (PEC) is an electromagnetic non-destructive testing (NDT) technique mainly used to inspect corrosion in pipelines. Like many other NDT techniques, to quantify pipe wall thickness, a signal feature has to be extracted from the PEC response. The authors’ previous work has exploited the linear relationship of the time-derivative feature to propose an in-situ calibration routine. However, to extract the time-derivative feature, two configuration parameters need to be determined prior to feature extraction. To the authors’ knowledge, none of the previous works explores the determining factors of this configuration parameters, where the current technique is still limited to using brute force method or trial-and-error. This paper brings novelty by formulating the aforementioned problem as a minimisation problem, and solving it using a particle swarm optimisation (PSO) algorithm. The obtained results, compared against brute force method, demonstrate the feasibility and performance improvement of using PSO in determining the configuration parameters. The approach thus presented is validated and the results obtained are justified by analysing the underlying physical system theory.

High temperature magnetic characterisation of structural steels using Epstein frame

Electromagnetic non-destructive testing techniques provide an attractive solution to the problem of monitoring microstructural changes in steels undergoing heat treatment as they are non-contact, have a short response time and are relatively inexpensive. However, to take full advantage of these techniques it is necessary to be able accurately measure the magnetisation characteristics of the materials of interest at temperatures up to the Curie point. This paper details the development of a novel high temperature Epstein frame for installation in a furnace with the design informed and results validated by finite element modelling. Hysteresis loop characteristics are successfully measured for a dual phase steel up to the Curie point for heating and cooling. Results show the developed system has the potential to provide valuable data to inform online electromagnetic monitoring systems.

A novel electromagnetic nondestructive testing method for carbon fiber reinforced polymer laminates based on power loss

Carbon fiber reinforced polymer (CFRP) is electrically conductive, so eddy-current testing (ECT) method can be used for non-destructive testing (NDT) of CFRP. However, due to the low bulk conductivity of CFRP, the high-frequency systems are considered more suitable in conventional ECT method. But high-frequency systems are more difficult to build and implement. Concerning this issue, a novel CFRP non-destructive testing method with working frequency up to 750 kHz is proposed in this paper. The method is based on power loss of the micro-probe, which makes use of Joule heat generated by the eddy-current in carbon fibers. The method is combination of ECT and thermography principle, which not only maintains higher signal-to-noise ratio at low frequency but also avoids the problems caused by high frequency. An experimental setup using C-Scan technique was built. Scanning experiments on orthogonally woven laminates, two-directional laminates, four-directional laminates and unidirectional laminates at various working frequencies including 750 kHz were carried out. Scanning results prove that the method can detect the cracks and fiber distribution of CFRP at low frequency up to 750 kHz, and has a good signal-to-noise ratio.

An improved generative adversarial network with modified loss function for crack detection in electromagnetic nondestructive testing

In this paper, an improved generative adversarial network (GAN) is proposed for the crack detection problem in electromagnetic nondestructive testing (NDT). To enhance the contrast ratio of the generated image, two additional regulation terms are introduced in the loss function of the underlying GAN. By applying an appropriate threshold to the segmentation of the generated image, the real crack areas and the fake crack areas (which are affected by the noises) are accurately distinguished. Experiments are carried out to show the superiority of the improved GAN over the original one on crack detection tasks, where a real-world NDT dataset is exploited that consists of magnetic optical images obtained using the electromagnetic NDT technique.

A novel 3D simulation prediction model of mechanical properties of ferromagnetic materials via incremental permeability method

Ferromagnetic material is one of the basic materials in industrial production and whose mechanical properties are key indicators to evaluate quality. At present, incremental permeability (IP), one method of nondestructive testing, is used in the mechanical properties testing field of ferromagnetic materials, but its 3D simulation prediction model and method have not been established. To solve this problem, a finite element simulation method for mechanical properties of ferromagnetic materials based on Jiles-Atherton (JA) hysteresis model was proposed in this paper, which is established on the basis of the experimental method of electromagnetic nondestructive testing via incremental permeability. In this paper, the finite element 3D simulation prediction model based on IP method is proposed. Besides, the difficulty in JA hysteresis model parameter measurement is solved by combining magnetic-force method and artificial fish swarm algorithm. The 3D simulation prediction model is applied to the yield strength prediction of ferromagnetic materials for the first time. By testing the yield strength of the material, the simulation method and 3D simulation prediction model proposed in this paper can accurately reflect the incremental permeability characteristics of real materials and can provide a simulation platform for testing the mechanical properties of ferromagnetic materials via IP method.