Magnetic Flux Leakage Testing Method Research Articles

Principle of the electromagnetic testing based on the varied frequency domain and its realization

With the development of modern industry, the types and quantities of industrial equipment are increasing. In the long-term production process, various failure modes such as corrosion and cracks will occur, therefore the workload of inspection will increase dramatically. It is extremely urgent to study the combined non-destructive testing (NDT) technology and develop corresponding multi-functional instruments to achieve rapid inspection. In this paper, an electromagnetic testing principle based on the varied frequency domain is proposed. In the first place, spatial uniformity and spatial periodic static fields are generated by the arrangement of the permanent magnetizers, these static fields are fields with different spatial frequencies. In the second place, dynamic fields with different frequencies are generated by different frequencies excitations. Under the time-space electromagnetic field coupling, a new combined electromagnetic testing principle of magnetic flux leakage (MFL), guided wave (GW) and electromagnetic acoustic transducer (EMAT) is proposed. The MFL testing method can be realized based on the uniform static field, GW based on the interaction of spatial periodic static field and low frequency dynamic field, while EMAT based on the combination of uniform static field and high frequency dynamic field. According to the above principle, a multi-functional instrument is developed, which includes a medium-low frequency excitation unit for GW, a high frequency excitation unit for EMAT, corresponding receiving amplifiers for GW/EMAT/MFL, and USB-based data transmitters. Finally, the relevant performance tests are carried out, which laid the foundation for field application.

Magnetic Flux Leakage Testing Method for Pipelines with Stress Corrosion Defects Based on Improved Kernel Extreme Learning Machine

This study aims to study the safety of oil and gas pipelines under stress corrosion conditions and grasp the corrosion damage situation timely and accurately. Consequently, a non-destructive testing method combining magnetic flux leakage testing technology and a kernel function extreme learning machine improved by genetic algorithm (GA-KELM) is proposed. Firstly, the variation of the corrosion defect dimension and profile with time is obtained by numerical simulation. At the same time, the distribution of the magnetic flux leakage signal under different defect conditions is analyzed and studied. Finally, feature selection is carried out on the magnetic flux leakage signal distribution curve, and GA-KELM is used to predict the depth and length of corrosion defects so as to realize the non-destructive testing of the pipeline defects. The results show that different geometric features result in different magnetic flux leakage signal distributions. There is a corresponding relationship between the defect dimension and extreme value, area, and peak width of the magnetic flux leakage signal distribution curve. The GA-KELM prediction model can effectively predict the depth and length of corrosion defects, and the prediction accuracy is better than the traditional extreme learning machine prediction model.

Investigation of evaluation method for wire rope breakages by magnetic flux leakage testing

Inspection using magnetic sensors based on the magnetic flux leakage testing method as soundness evaluation of wire ropes is being performed. Recently, it has been studied as detailed soundness evaluation method of the wire rope by analyzing the measurement data of multi magnetic sensors as an image grayscale. However, this is the position identification of breakages rope axis direction. For this reason, if it is not the position identification of breakages on the rope circumferential direction, it is difficult to determine soundness evaluation of wire ropes. In this study, there are three characteristics (types (crown and valley), number of wire breakages, the distance between detection plane and wire rope surface) of the breakage. In this paper, the magnetic flux density of the wire rope caused by the number of breakages is examined using numerical analysis.

A Novel Magnetic Flux Leakage Testing Method Based on AC and DC Composite Magnetization

The flexible printed coil array (FPC) is employed extensively because of its flexibility in non-destructive testing (NDT). To make the inductive coil detect both external and internal defects under static conditions in magnetic flux leakage testing (MFL), this paper proposes a method based on the AC and DC composite magnetization. The AC magnetization changes the magnetic field strength in the vicinity of external defects. The DC magnetization makes the defects generate static leakage magnetic field. The permeability in the skin layer is also altered by the AC magnetization which affects the transmission of leakage magnetic field caused by internal defects. Therefore, external and internal defects can generate a time-varying leakage magnetic field. Simulations and experiments are carried out to validate the feasibility of this method. The results indicate that the method based on the AC and DC composite magnetization can be applied to detect both external and internal defects.

A novel method of omnidirectional defects detection by MFL testing under single axial magnetization at the production stage of lathy ferromagnetic materials

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.

A Lift-Off-Tolerant Magnetic Flux Leakage Testing Method for Drill Pipes at Wellhead.

To meet the great needs for MFL (magnetic flux leakage) inspection of drill pipes at wellheads, a lift-off-tolerant MFL testing method is proposed and investigated in this paper. Firstly, a Helmholtz coil magnetization method and the whole MFL testing scheme are proposed. Then, based on the magnetic field focusing effect of ferrite cores, a lift-off-tolerant MFL sensor is developed and tested. It shows high sensitivity at a lift-off distance of 5.0 mm. Further, the follow-up high repeatability MFL probing system is designed and manufactured, which was embedded with the developed sensors. It can track the swing movement of drill pipes and allow the pipe ends to pass smoothly. Finally, the developed system is employed in a drilling field for drill pipe inspection. Test results show that the proposed method can fulfill the requirements for drill pipe inspection at wellheads, which is of great importance in drill pipe safety.

漏洩磁束法による鋼板凹凸表面欠陥の検出法の開発

Concave and convex defects on steel sheets cause problems in the steel production line. These defects are usually invisible since their height or depth is as small as the normal surface roughness of steel sheets. Therefore, they can only be detected by human inspection after grinding the steel surface with a grindstone.We found that Magnetic Flux Leakage Testing (MFLT) is effective for detecting these kinds of defects. The MFLT method can detect not only magnetic flux caused by the shape of defects, but also that caused by the strain of defects. This paper describes the results of experiments to verify the principle of the detection method and in-line experiments. As a result of the in-line experiments, it was found that some defects cause MFLT signals which are too small to detect automatically. To solve this problem, we proposed a new periodical method to improve detectability. Based on these results, we developed an in-line concave and convex defect inspection technique.

MFL testing of omni-directional cracks in steel strip using strong longitudinal magnetization

A variety of defects in continuous steel strip may result in a ccidents. Traditional MFL (Magnetic flux leakage) testing method can lead to large blind area because steel str ip is too broad to be rotated. A new MFL testing method that can inspect omni-directional cracks in steel strip is introduc ed in this paper. This method is characterized basically by t he deep saturation state of steel strip generated. The property of i nspection is investigated using 3-D finite element method. W aveform configurations of the experimental results agree with that o f the theoretical predictions. Automatic inspecting equip ment has been developed based on the new MFL method principle.

High-speed MFL Testing Method for Steel Pipe under Unidirectional Axial Magnetization

The existing magnetic flux leakage(MFL) method and apparatus by combined magnetization is presented and its limitations are also analyzed.The helical scanning approach limits the testing speed and does not suit the inspection of pipes unsuitable to rotate or with straight line weld.Afterwards,for the deficiencies,the feasibility of omni-directional defects MFL inspection under a unidirectional axial magnetization for steel pipe is found and verified by simulation of finite element model and experiments,particularly the feasibility of longitudinal cracks parallel to the axial magnetization,thus it is possible to realize testing by linear scanning.A method for ensuring the consistency of evaluation and testing for the defects in a same volume but different directions is also put forward.A MFL method and apparatus without needing a rotation of detection probes or steel pipe are developed and designed,thus resolving the problems of high-speed or ultra-high-speed inspection of seamless steel pipe and adapting to the inspection of pipes unsuitable to move helically or with straight line weld.The proposed method simplifies the conventional MFL method and reduces the cost of the testing apparatus.

Development of an inspection robot for long‐distance transmission pipeline on‐site overhaul

PurposeDamage of oil and gas long‐distance transmission pipelines caused by corrosion, stresses, geological change, and other factors can result in catastrophic failures. The purpose of this paper is to describe a mobile out‐pipe inspection robot for the pipeline inspection.Design/methodology/approachThis paper adapts to the requirements of long‐distance transmission pipelines on‐site overhaul, the robot developed in this paper utilizes a local magnetic flux leakage testing method for the pipeline inspection. Detection, walking, attachment, steering, and control units are designed for the robot.FindingsThe detection apparatus of the robot plays dual roles as detection and attachment unit. So, the structure of the robot is relatively simple and it is relatively small in size and relatively light.Practical implicationsA prototype of the robot has been manufactured according to the design. Results of the test in lab show the robot can be used to inspect pipe.Originality/valueThe robot is able to scan and inspect the pipeline along the planned route automatically.

Nonlinear finite element simulation for magnetic flux leakage tester

The paper reports about the building up a computer controlled measurement system based on National Instruments Data Acquisition card and National Instruments LabVIEW software package. The aim of this research is to detect manufactured surface cracks on steels by applying a magnetic flux leakage testing method. First of all, a finite element method based on the procedure to analyze the behavior has been implemented of the measurement system under construction. One important question is the position of the sensors or of the detecting coils. The finite element procedure contains a two dimensional arrangement. First, eddy currents have been neglected, but the nonlinear characteristic of the specimen has been taken into account by a single valued function. In this paper, the developed measurement system, which is partially ready to work, and the nonlinear finite element procedures are presented as well.

Tubing thread inspection by magnetic flux leakage

The failure of the oil well tubing thread is a very serious problem which may result in accidents and repairs during well servicing. Routine inspections on the threads are thus necessary. This paper proposes a local magnetic flux leakage testing method to inspect the end of the pieces and introduces a certain kind of tubing thread testing equipment based on the magnetic flux leakage theory. The equipment consists of four key components—magnetizer, magnetic field inducer, linear scanner and synchronous sampling controller. The equipment can inspect and prevent tubing thread failures occurring in practice, such as rupture, thread abrasion, crack in the thread root, variations of the taper angle in the thread area, etc. It can meet the testing requirements of the tubing thread and has very wide application perspective.