In recent years, developed countries have attached great importance to the development of magnetic flux leakage detection technologies. Although the main body in many types of researches is not the seamless steel pipe, it is also crucial to the development of magnetic flux leakage detection technology for seamless steel tubes. In foreign countries, the research of these related technologies is mainly reflected in the following five aspects:
(1) Continue deepening theoretical research on magnetic flux leakage detection technology
The theoretical research of magnetic flux leakage detection technology is mainly the research of defect leakage magnetic fields. In 1966, Shcherbinin and Zatsepin began to study the calculation of the defect leakage magnetic field. They used the magnetic dipole model to calculate the infinite length of the crack, and obtained some relations between the defect leakage magnetic field and defect size. These works laid the foundation for the subsequent research on the theory of leakage magnetic fields. After their research was affirmed, Europe and the United States successively carried out research on the theory of magnetic flux leakage. In summary, the methods they used were mainly divided into magnetic dipole methods and numerical calculation methods. The magnetic dipole method was the first to have a lot of research, and its research has deepened people's understanding of the leakage magnetic field around the defect. In the study of the magnetic dipole model, Foster expressed the leakage magnetic field on the surface as a function related to the excitation source, relative permeability and defect parameters. Zhang and others extended the magnetic dipole model from external defects to internal defects, and studied the rectangular and elliptical defects inside the workpiece. Edwards and Palmer used the magnetic dipole method to calculate the infinite length of the crack, and obtained a two-dimensional expression; they extended the two-dimensional expression to the three-dimensional expression. Mandache and Clapham extended the magnetic dipole model of defects with single shapes to a multi-shape magnetic dipole model. They used the magnetic dipole method to study the leakage magnetic field of single and multiple cylindrical pits.
Although the theoretical research of the magnetic dipole method continues to make progress, the magnetic dipole method uses Maxwell's equations to solve the electromagnetic field's intensity at each point in the space near the test piece. This solution method is extremely cumbersome. Moreover, with the change of the defect model, the model has to be re-established, and the calculation efficiency is not high. Scholars began to use numerical calculation methods to study the leakage magnetic field around the defects based on these defects. Among the numerical calculation methods, the finite element method is the most used. In 1975, Hwang and Lord used the finite element method to calculate the leakage magnetic field around the defect for the first time, and obtained the relationship among the internal magnetic induction of the test piece, magnetic permeability of the material and leakage magnetic fields. Since then, the theoretical research of leakage magnetic field based on numerical calculation has been developed rapidly. Jansen and others used the finite element method to study the relationship between the magnetic induction intensity of the steel plate under DC magnetization and the leakage magnetic field of the defect. At the same time, the thickness of the steel plate was studied. The effect of sensor lift-off value on leakage magnetic fields. Katoh and others used the finite element method to study the influence of the magnetization gap and the thickness of the measured part on the leakage magnetic field of the defect. Atherton and others used the finite element method to study the leakage magnetic field around the pipeline defect, and obtained the influence of the defect's size and the material's magnetic permeability on the leakage magnetic field.
(2) Continue achievement for research on the reconstruction technology of the defect
The reconstruction of the defect is that finding the position and shape of the defect based on the leakage magnetic field information to achieve the purpose of quantitative detection of defects. Bruno used the magnetic dipole method to obtain the leakage magnetic field of simply shaped defects, and completed the two-dimensional reconstruction of these simply shaped defects through the generalized inverse matrix theory. Minkov used the magnetic dipole method to study the leakage magnetic field of complex-shaped defects, and used the direct search to minimize the root-mean-square error; reconstructed the shape of the defect. Yan used the finite element to analyze the magnetic field leakage defect model, and then used the feedback iteration to reconstruct the defect of the pipeline; obtained the approximate appearance of the defect. Ramuhalli used the neural network method to reconstruct the defect. First, the neural network was trained with training samples to make it have a memory for the defect. After training, the defective shape can be formed approximately by inputting the leakage magnetic field information into the neural network. Carvalhoa used BP network to realize the reconstruction of the welding type of the pipeline.
(3) Developing from the initial one-dimensional magnetic flux leakage detection technology to two-dimensional or even three-dimensional magnetic flux leakage detection technology
At the present stage, the developed magnetic flux leakage detection equipment adopts one-dimensional magnetic flux leakage detection technology, that is, only one component of the leakage magnetic field is picked up to judge the defect. Although the one-dimensional component can roughly reflect the defect information, the one-dimensional magnetic flux leakage detection technology must be replaced with the two-dimensional magnetic flux leakage detection technology or even the three-dimensional magnetic flux leakage detection technology to more accurately reflect the distribution of the magnetic field leakage. Some scholars have obtained some relationships between defects and the components of the three-dimensional leakage magnetic field through calculations or simulations using finite element methods. For example, YongLi, Johnwllson, GuiYunTian and others used the finite element method to study the three-dimensional component distribution of the leakage magnetic field of the defect, and obtained certain rules; after verifying the obtained rules through experiments of various models, they finally obtained that by determining the defect and quantitative analysis; the horizontal and normal components of the leakage magnetic field played a key role. At the same time, the circumferential component of the leakage magnetic field could also provide useful information for judging defects.
(4) The continuous emergence of new magnetic sensors
In magnetic flux leakage detection of seamless steel tubes, the magnetic sensor is the element that picks up the leakage magnetic field. Its performance directly determines the final test result. At present, many companies and scholars have modified the existing magnetic sensors to make them have better performance, and have also developed some magnetic sensors with different detection principles. For example, to detect small defects in thick-walled pipelines, NBSGloria and others specially developed a new type of magnetic sensor called ICS, which could detect magnetic flux leakage without being affected by the wall thickness of the pipeline. Yaofei Chen, Qun Han and others have developed an optical fiber magnetic field sensor based on single modes, multi modes to single modes. The sensor could measure the magnetic field's strength through the wavelength shift of the transmission spectrum. They also conducted research on the magnetic fluid form of the sensor.
(5) More and more ways to magnetize the DUT
In the initial magnetic flux leakage detection device, DC or permanent magnet was generally used. With the continuous deepening of research, AC magnetization technology, pulse magnetization technology and composite magnetization technology have been greatly developed. Among them, Professor Tian and others in the United Kingdom proposed pulse magnetization technology, which combined pulsed eddy current detection technology and magnetic flux leakage detection technology. Under this magnetization method, more leakage magnetic field information could be obtained than other magnetization technologies. Defects could be better judged. Yuji Gotoh, Norio Takahashi and others used the finite element method to study the leakage magnetic field of the specimen under AC magnetization, and designed the excitation signal as a sinusoidal signal with a certain frequency. Under this magnetization method, they studied the identification of multiple cracks, and analyzed the distribution of the radial component and the axial component in the alternating leakage magnetic field.
