Magnetic Flux Leakage Testing
As early as 1868, the British Naval Architects Association began to use magnetic flux leakage technology and found defects on the steel pipe through the compass. In 1918, it was found that powder was absorbed near defects due to the changes of surface mag
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Magnetic Flux Leakage Testing
6.1
Introduction
As early as 1868, the British Naval Architects Association began to use magnetic flux leakage technology and found defects on steel pipe through the compass. In 1918, it was found that powder was absorbed near defects due to the changes in surface magnetic field; thus, magnetic particle detection method was invented. After continuous development, the magnetic flux leakage testing technology not only can detect the defects, but also can analyze the characteristics of the defects [1, 2]. In the theoretical analysis of magnetic leakage field, it can be divided into analytical method and numerical method. In 1966, Zatsepin and Shcherbinin were the first to put forward the analytical equation of the magnetic flux leakage field of infinite long rectangular crack, as shown in Fig. 6.1 [3]. Based on the theoretical model, the magnetic flux leakage field can be generated by a dipole with two opposite polars. Taking the hole defect in the two-dimensional space as an example, the two magnetic poles are assigned to the two sides of the defect. The magnetic charge densities of the two poles are −Q and +Q, respectively. So, the magnetic flux leakage of the defect in point P(x, y) is given as follows: Q ! Q ! ! ! ! B ¼ B1 þ B2 ¼ r þ r2 3 1 4pl0 r1 4pl0 r23
ð6:1Þ
Among them, μ0 is the vacuum permeability. The holes, pits, and other defects on the surface of the workpiece can be simulated by the equivalent point dipole model. For the crack defect, it should be equivalent to the rectangular groove, whose length is infinite, and then, the equivalent surface dipole model is used to simulate the crack. Using the dipole model, the authors calculated the magnetic leakage field of infinite long cracks in the surface [3]. After that, the magnetic field distribution of the finite length cracks in the surface was calculated by Shcherbinin and Pashagin [4]. © Tsinghua University Press and Springer Science+Business Media Singapore 2016 S. Huang and S. Wang, New Technologies in Electromagnetic Non-destructive Testing, Springer Series in Measurement Science and Technology, DOI 10.1007/978-981-10-0578-7_6
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6 Magnetic Flux Leakage Testing
Fig. 6.1 Magnetic dipole model
However, they did not connect the strength of the magnetic leakage field to the strength of the magnetizing field and the permeability. They used two-dimensional models. The main progress of the analytical method of magnetic leakage field analysis was completed by Edwards and Palmer in 1986 [5]. They perfected the magnetic dipole model. The shape parameters of the defect, the magnetizing field strength, the permeability, and the magnetic flux leakage of the material were firstly connected. The limitation of Edwards and Palmer’s model was that the permeability of the material was supposed as a linear parameter in the derivation process. So there were some errors between the theoretical results and the experimental results. Therefore, numerical calculation method is usually adopted. The numerical calculation method overcomes the limi
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