Advances in magnetic flux leakage testing technology

Magnetic flux leakage(MFL)testing technology has the advantages of simple principle,easy engineering implementation and low requirements on the surface of the detected workpiece.Therefore,it has been one of the research hotspots in the field of non-destructive testing(NDT)and widely used for testing long distance pipelines.This paper presents the development of MFL tesing technology from the aspects of basic theory,influencing factors,magnetization technology,signal processing,etc.The problems to be solved and the future development are summarized,which can provide reference for the research and system development of MFL testing technology.

Three-axis magnetic flux leakage in-line inspection simulation based on finite-element analysis

With the increase of pipelines, corrosion leakage accidents happen frequently. Therefore, nondestructive testing technology is important for ensuring the safe operation of the pipelines and energy mining. In this paper, the structure and principle of magnetic flux leakage （MFL） in-line inspection system is introduced first. Besides, a mathematic model of the system according to the ampere circuit rule, flux continuity theorem, and column coordinate transform is built, and the magnetic flux density in every point of space is calculated based on the theory of finite element analysis. Then we analyze and design the disposition of measurement section probes and sensors combining both three-axis MFL in-line inspection and multi-sensor fusion technology. Its advantage is that the three-axis changes of magnetic flux leakage field are measured by the multi-probes at the same time, so we can determine various defects accurately. Finally, the theory of finite element analysis is used to build a finite element simulation model, and the relationship between defects and MFL inspection signals is studied. Simulation and experiment results verify that the method not only enhances the detection ability to different types of defects but also improves the precision and reliability of the inspection system.

Magnetic Flux Leakage Course of Inner Defects and Its Detectable Depth

As a promising non-destructive testing(NDT)method,magnetic flux leakage(MFL)testing has been widely used for steel structure inspection.However,MFL testing still faces a great challenge to detect inner defects.Existing MFL course researches mainly focus on surface-breaking defects while that of inner defects is overlooked.In the paper,MFL course of inner defects is investigated by building magnetic circuit models,performing numerical simulations,and conducting MFL experiments.It is found that the near-surface wall has an enhancing effect on the MFL course due to higher permeability of steel than that of air.Further,a high-sensitivity MFL testing method consisting of Helmholtz coil magnetization and induction coil with a high permeability core is proposed to increase the detectable depth of inner defects.Experimental results show that inner defects with buried depth up to 80.0 mm can be detected,suggesting that the proposed MFL method has the potential to detect deeply-buried defects and has a promising future in the field of NDT.

Evaluation of Surface Cracks Using Magnetic Flux LeakageTesting

The magnetic field distribution characteristics of surface cracks with various widths are discussed based on finite element (FEM) results. The crack depth was 0.20 mm, the width range was from 0.02 to 1.00 mm. The results showed that crack width and lift-off (the distance between surface and sensor) will influence signals. Discussed in this paper is the influence of various lift-off parameters on the peak to peak values of the normal component in magnetic flux leakage testing. The effects can be applied to evaluate surface breaking cracks of different widths and depths. An idea is presented to smooth narrow, sharp crack tips using alternating current (AC) field magnetization.

Online Magnetic Flux Leakage Detection System for Sucker Rod Defects Based on LabVIEW Programming

Aiming at the detection of the sucker rod defects,a real-time detection system is designed using the non-destructive testing technology of magnetic flux leakage(MFL).An MFL measurement system consists of many parts,and this study focuses on the signal acquisition and processing system.First of all,this paper introduces the hardware part of the acquisition system in detail,including the selection of the Hall-effect sensor,the design of the signal conditioning circuit,and the working process of the single chip computer(SCM)control serial port.Based on LabVIEW,a graphical programming software,the software part of the acquisition system is written,including serial port parameter configuration,detection signal recognition,original signal filtering,real-time display,data storage and playback.Finally,an experimental platform for the MFL detection is set up,and the MFL measurement is carried out on the transverse and longitudinal defects of the sucker rod surface.The experimental result shows that the designed acquisition and processing system has good detection performance,simple design and high flexibility.

Composite excitation multi-extension direction defect magnetic flux leakage detection technology

In the traditional pipeline magnetic flux leakage(MFL)detection technology,circumferential or axial excitation is mainly used to excite the magnetic field of defects.However,the domestic and foreign pipeline detection devices currently in operation are mainly axial excitation MFL detection tools,in which circumferential cracks can be clearly identified,but the detection sensitivity of axial cracks is not high,thus forming a detection blind zone.Therefore,a composite excitation multi-extension direction defect MFL detection method is proposed,which can realize the simultaneous detection of axial and circumferential defects.On the basis of the electromagnetic theory Maxwell equation and Biot Savart law,a mathematical model of circumferential and axial magnetization is firstly established.Then finite element simulation software is used to establish a model of a new type of magnetic flux leakage detection device,and a simulation analysis of crack detection in multiple extension directions is carried out.Finally,under the conditions of the relationship model between the change rate of leakage magnetic field and external excitation intensity under unsaturated magnetization and the multi-stage coil magnetization model,the sample vehicle towing experiment is carried out.The paper aims to analyze the feasibility and effectiveness of the new magnetic flux leakage detection device for detecting defects in different extension directions.Based on the final experimental results,the new composite excitation multi extension direction leakage magnetic field detector has a good detection effect for defects in the axial and circumferential extension directions.

Wavelet domain adaptive filtering algorithm for removing the seamless pipe noise contained in the magnetic flux leakage data

With the widespread application and fast development of gas and oil pipeline network in China, the pipeline inspection technology has been used more extensively. The magnetic flux leakage （MFL） method has established itself as the most widely used in-line inspection technique for the evaluation of gas and oil pipelines. The MFL data obtained from seamless pipeline inspection is usually contaminated by the seamless pipe noise （SPN）. SPN can in some cases completely mask MFL signals from certain type of defects, and therefore considerably reduces the detectability of the defect signals. In this paper, a new de-noising algorithm called wavelet domain adaptive filtering is proposed for removing the SPN contained in the MFL data. The new algorithm results from combining the wavelet transform with the adaptive filtering technique. Results from application of the proposed algorithm to the MFL data from field tests show that the proposed algorithm has good performance and considerably improves the detectability of the defect signals in the MFL data.

Research on Magnetic Flux Leakage Detection Method for Subsea Pipeline

The intelligent pig based on the (MFL) is frequently used for in-line inspection of transportation pipelines. The article discusses the key technology of an MFL tool that includes the sensors structure, the constitution of tool hardware, software and the analysis method of MFL signal.

A Dynamics Study of a Magnetic Flux Leakage(MFL) Signal and its Application to Defect Location Discrimination in Steel Pipes MFL Testing Equipment

Difference processing was used to the direct current magnetic flux leakage （DC-MFL） signal, emanating from the defects machined artificially on the internal and external surfaces of a steel pipe. Consequently, the loea-tion discriminating index 8 was provided to identify the defect whether it is on the internal surface or the external one. Three characteristics, shape, depth and orientation of the defect, were discussed through a series of experiments on the artificial defects, such as transverse notches, oblique notches and pits on the steel pipe. The approach has been verified effective to address the defect location identifying problem, albeit the limits on the accuracy assessment to those natural defects on steel pipes in service.

Influence of Slot Defect Length on Magnetic Flux Leakage

A key issue, which influences the applications of magnetic flux leakage testing, is defect quantification. There have been many research on the relationship between width, depth and magnetic flux leakage of slot defect. However, the length factor is often ignored. The relationship between characteristics of defect leakage field and defect length was investigated. The magnetic flux leakages of a series of plate specimens with the same width, same depth and different length slot defects were tested under the same magnetizing conditions. Testing results show that defect length is an important parameter needed to consider in quantifying defects.

Coefficients de-noising with wavelet transform for magnetic flux leakage data obtained from oil pipeline

This paper considers the problem of noise cancellation for the magnetic flux leakage (MFL) data obtained from the inspection of oil pipelines. MFL data is contaminated by various sources of noise, and the noise can considerably reduce the detectability of flaw signals in MFL data. This paper presents a new de-noising approach for removing the system noise contained in the MFL data by using the coefficients de-noising with wavelet transform. Experimental results are presented to demonstrate the advantages of this de-noising approach over the conventional wavelet de-noising method.

Magnetic flux leakage during longitudinal magnetic flux induction heating

A three-dimensional electromagnetic thermal coupled model of a 45#steel strip during longitudinal magnetic flux induction heating was established in this study.The influence of different power levels,frequencies,and electromagnetic shielding on the magnetic induction intensity and the heating of parts of different materials around the inductor was analyzed by calculating the magnetic induction intensity around the longitudinal flux induction heater under different conditions.The results show that leakage flux can be reduced up to 49%by electromagnetic shielding.The higher the frequency,the greater the electromagnetic shielding effect,and the lower the magnetic leakage around the inductor.Conversely,the higher the power,the greater the magnetic induction around the inductor.By adding electromagnetic shielding to reduce magnetic leakage of the inductor and replacing the metal part material around the inductor with stainless steel,the heat generation of the parts around the inductor can be greatly reduced.

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 electro-magnetic simulation software was used to carry out numerical study on the welding defects in pipelines,including incomplete penetration and undercut.TheФ406 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.

Design and Analysis of Wide Speed Regulation of Variable Leakage Flux Reverse Salient-pole Motor

In this article, a new variable leakage flux reverse salient-pole motor(VLF-RSPM) is raised to widen the speed range. The innovation is to realize both reverse salient-pole characteristics and variable leakage flux characteristics by using a method of adding magnetic bridges and magnetic barriers. Firstly, the evolution of the topological structure and working principle of the motor are introduced. Secondly, based on 2D Finite Element Analysis(FEA), the electromagnetic properties and noise of the motor are analyzed in detail, and the electromagnetic properties are contrasted with that of the conventional V-type synchronous motor(CVTSM). The results show that VLF-RSPM has the advantages of small torque ripple, strong magnetic weakening ability, low noise, high efficiency, and low risk of permanent magnet demagnetization under different conditions. In addition, it is verified that the proposed motor extends the speed range.

Reduction of Unipolar Leakage Flux and Torque Ripple in Consequent-pole PM Vernier Machine

This paper proposes a new consequent-pole permanent magnet vernier machine(CPMVM),which can be regarded as a combination of two conventional CPMVM with opposite polarities.Based on the simplified axial magnetic circuit model,it is verified that the proposed CPMVM can reduce the unipolar leakage flux.In order to reduce the torque ripple of machine and improve the output torque of machine,the flux barrier is placed on the rotor of the proposed machine.Then,the parameters of the proposed CPMVM are optimized and determined.Moreover,the electromagnetic performance,including no-load air-gap flux density,average torque and torque ripple,flux linkage,back-electromotive force,cogging torque,average torque,torque ripple,power factor and loss,is compared with conventional surface-mounted permanent magnet vernier machine(SPMVM)and CPMVM.Finally,it is demonstrated that proposed CPMVM with flux barrier can effectively reduce the unipolar leakage flux and greatly reduce the torque ripple of machine.Also,compared with the SPMVM,the proposed CPMVM with flux barrier saves more than 45%of the permanent magnet material without reducing output torque.

Magnetic Flux Leakage Signal Inversion of Corrosive Flaws Based on Modified Genetic Local Search Algorithm

In this paper,a modified genetic local search algorithm(MGLSA) is proposed.The proposed algorithm is resulted from employing the simulated annealing technique to regulate the variance of the Gaussian mutation of the genetic local search algorithm(GLSA).Then,an MGLSA-based inverse algorithm is proposed for magnetic flux leakage(MFL) signal inversion of corrosive flaws,in which the MGLSA is used to solve the optimization problem in the MFL inverse problem.Experimental results demonstrate that the MGLSA-based inverse algorithm is more robust than GLSA-based inverse algorithm in the presence of noise in the measured MFL signals.

Design and Optimization of Reverse Salient Permanent Magnet Synchronous Motor Based on Controllable Leakage Flux

In this paper,a controllable leakage flux reverse salient permanent magnet synchronous motor(CLF-RSPMSM)is designed,which has the advantages of wide speed range and low irreversible demagnetization risk.Firstly,the principle of controllable leakage flux and reverse saliency effect is introduced,and the design of the rotor flux barrier is emphatically discussed.Secondly,multiple design variables are stratified by the comprehensive sensitivity method,and the main variables are screened out.Then the relationship between the main variables and the optimization goal is discussed according to the response surface diagram.Thirdly,a sequential nonlinear programming algorithm(SNP)is used to optimize the three optimization objectives comprehensively.Finally,the electromagnetic performance of the proposed motor is compared with the initial IPM motor,the mechanical strength of the proposed rotor is analyzed,and the results verify the effectiveness of the design and optimization method of the proposed motor.

Analysis and Modeling of Tooth-Tip Leakage Fluxes in a Radial-Flux Dual-Stator Machine with Diametrically Magnetized Cylindrical Permanent Magnets

Tooth-tip Leakage flux(TLF)has a major effect on the prediction of air-gap flux distribution and electromagnetic torque in the permanent magnet(PM)machines.Therefore,deriving a model for TLF is necessary for machine design and optimization.Accurate modeling of TLF can lead to fast and precise solutions,which ease the analysis of electromagnetic devices.It also provides the opportunity to increase torque density by more efficient utilization of PM’s volume and prevent saturation in machine optimization.This paper presents a method for modeling and analyzing TLFs in a radial-flux dual-stator permanent magnet(DSPM)machine with diametrically magnetized cylindrical permanent magnets(DMCPM)in series and parallel magnetic circuit structures.In this model,some expressions in terms of machine dimensions are derived for the TLF analysis.Finite element method(FEM)is applied to validate the proposed model.Results indicate that the maximum error between the proposed model and FEM is insignificant(less than 6%).Finally,by a prototyped machine the validity of the proposed model was investigated with the experimental tests.

Torque Characteristics of High Torque Density Partitioned PM Consequent Pole Flux Switching Machines With Flux Barriers

Unique double salient structure of Permanent Magnet Flux Switching Machines(PMFSM)with both Concentrated Armature inding(CAW)and Permanent Magnet(PM)on stator attract researcher's interest for high speed brushless application when high torque density(T den)and power density(P den)are the primal requirements.However,despite of stator leakage flux,high rare-earth PM usage,PMFSM is subjected to slot effects due to presence of both PM and CAW in stator and partial saturation due to double salient structure which generates cogging torque(T cog),torque ripples(Trip)and lower average torque(T avg).To overcomne aforesaid demerits,this paper presents Partitioned PM(PPM)Consequent Pole Flux Switching Machine(PPM-CPFSM)with flux barriers to enhance flux mnodulation,curtail PM usage and diminish stator leakage flux which reduces slotting effects and partial saturation to ultimately reduces T cog and Trip In comparison with the existing state of the art,proposed PPM-CPFSM reduces 46.5390 of the total PM volumne and offer Tavg higher up to 88.8%,suppress Trip naximun up to 24.8%,diminish Tcog up to 22.74%and offer 2.45 times Tden and Pden.Furthermore,torque characteristics of proposed PPM-CPFSM is investigated utilizing space harmonics injection i.e.inverse cosine,inverse cosine with 3rd harmonics and rotor pole shaping techniques i.e.,ecce ntric circle,chanfering and notching.Detailed electromagnetic perfornance analysis reveals that harmonics injection suppressed Tcog maximun up to 83.5%,Trip up to 40.72%at the cost of 4.71%Tavg.Finally,rotor mnechanical stress analysis is utilized for rotor withstand capability and 3D-FEA based Coupled Elctromagnetic Thermal Analysis(CETA)for thermal behavior of the developed PPM CPFSM.CETA reveals that open space along PPM act as cooling duct that inprove heat dissipation.

Quantitative Detection of Corrosion State of Concrete Internal Reinforcement Based on Metal Magnetic Memory

Corrosion can be very harmful to the service life and several properties of reinforced concrete structures.The metal magnetic memory(MMM)method,as a newly developed spontaneous magnetic flux leakage(SMFL)non-destructive testing(NDT)technique,is considered a potentially viable method for detecting corrosion damage in reinforced concrete members.To this end,in this paper,the indoor electrochemical method was employed to accelerate the corrosion of outsourced concrete specimens with different steel bar diameters,and the normal components Bz and its gradient of the SMFL fields on the specimen surfaces were investigated based on the metal magnetic memory(MMM)method.The experimental results showed that the SMFL experimental Bz curves are consistent with the analytical results of the theoretical model.Furthermore,the crest-to-trough behavior on the Bz signal curve and its zero-point gradient spacing can more accurately indicate the corroded area’s extent.Then,a magnetic characteristic parameter W based on a large amount of experimental data was established to characterize the degree of corrosion of the steel bars.The magnetic characteristic parameter W is linearly related to the maximum cross-sectional area loss rate
of the corroded reinforcement.This paper will lay the foundation for future research on corrosion detection of reinforced concrete structures based on the MMM method and provide a feasible way for non-destructive detection of corrosion independent of the influence of reinforcement diameter and magnetization history.