Magnetic Flux Leakage

Songling Huang, Wei Zhao, Tsinghua University Press

• Publisher: De Gruyter
• Year: 2016
• ISBN:
  9783110479713
• Edition: 1
• Topic: Naturwissenschaften
• Subtopic: Elektromagnetismus

This book systematically introduces magnetic flux leakage testing principles and their applications in practice. Signal detection, collection, processing, the inversion of magnetic flux leakage defect and 3D imaging are discussed in detail with extensive project experiences, making the book an essential reference for researchers, developers and engineers in nondestructive testing.

1Introduction

1.1Outline

The principle of magnetic flux leakage (MFL) detection is shown in Figure 1.1.

The MFL testing method is based on the high permeability of ferromagnetic materials [1]. When ferromagnetic materials are magnetized by the external magnetic field, and if the ferromagnetic material is continuous and uniform, then the magnetic flux in the material will be constrained in the interior of the materials, thus almost no flux leaks from the material surface. However, when there are defects in the surface or inside of the ferromagnetic materials, due to the small permeability of the defects and the large magnetic resistance, the magnetic lines of force will change the way. When the magnetic induction intensity in the material or the defect is large enough, the magnetic flux will overflow from the workpiece near the defect, pass through the air above the defect and then enter into the workpiece; thus, the MFL of defects is created. Here, the magnitude of MFL is directly related to the magnetic field strength of ferromagnetic material. In the external magnetizing field, the relationship between magnetic induction

and magnetic field strength

of the ferromagnetic materials is

Because the permeability of the material varies with the magnetic field intensity

, the relationship between

is not linear, and the magnetization curve exhibits a nonlinear variation. The magnetic characteristics of the ferromagnetic material are in accordance with the law of the magnetization if the tested material is magnetized by a permanent magnet or an exciting coil.

A steel plate with a surface defect is taken as an example to illustrate the MFL phenomenon [2]. Figure 1.2 shows the profile of the steel plate defect. Figure 1.3 shows the magnetic characteristic curve of the steel plate.

The sectional area of a defect in a steel plate is Sa. The sectional area of the steel plate is S. The remaining sectional area of the steel plate in the defect area is S – Sa. If the magnetizing field is uniform with a magnetic field strength H, the magnetic induction intensity in the plate with defects is B1, which is corresponding to the point Q on the B–H curve and to the point P on the permeability curve.

Due to the existence of defects, the magnetic induction intensity of the remaining section increases. So the operating point moves from Q to Q′ on the magnetization curve. However, the permeability of point Q′ is smaller than that of point Q, which changes from point P to point P′ in curve 2. That is to say, due to the existence of defects, the magnetic induction intensity increases with the decreasing cross-sectional parts; at the same time, the permeability change is smaller, which shows that the plate with defects cannot pass through the original magnetic flux, so a portion of the magnetic field scatters and leaks into the surrounding medium, and forms the leakage of magnetic field.

Figure 1.1: The principle of magnetic flux leakage detection.

Figure 1.2: The sketch map of a defect in steel plate.

Figure 1.3: The magnetizing characteristic curve of the steel plate.

1.2Influence Factors of Magnetic Flux Leakage Testing

Magnetic leakage fields of defects will be affected by many factors. These factors include the magnetizing magnitude, remanence, electrical conductivity, magnetic permeability, magnetic coupling loop, the distance between the magnetic poles, the moving speed of the detector, internal stress, liftoff of the probe and so on [3, 4].

1.2.1The Influence of Magnetizing

The function of the magnetizing system of the detector is to make the tested part to be magnetized, and then the MFL is generated around the defect. So the magnetizing field strength is the most important influencing factor on the magnetizing level. External magnetizing field strength is determined by the relationship between the magnetizing system and the tested material. Thickness of the tested material, components of material, scanning speed, remanence and other factors will affect the magnetizing system.

The relationship between the applied magnetizing field intensity and the magnetic induction intensity in the tested part is nonlinear, as shown in Figure 1.4.

After the tested material is magnetized by the permanent magnet, the MFL field can be generated near the defect, which is mainly determined by the magnetizing intensity. If the magnetizing intensity is not enough, then the decreasing wall thickness induced by the defects still can carry all of the magnetic flux, so there will be no MFL to the surface of the tested piece. Therefore, only when the two conditions of the defect and permeability decrease with the increase of the applied magnetizing field strength are both satisfied, and the MFL field can be produced.

Figure 1.4: The B–H curve of a steel pipe wall.

Figure 1.5: Effect of magnetization on the magnetic flux leakage signal of the same defect.

The strength of the applied magnetic field is mainly determined by the magnetic properties of the magnet. Figure 1.5 shows the magnetic leakage field of a defect calculated by the finite element with different magnetizing field strengths, which are 1.8, 1.6, 1.2 and 0.6 T, respectively. The length, width and depth of the defects are 14.6, 14.6 and 7.3 mm (50% wall thickness), respectively. A value of 0.6 T corresponds to a relatively low magnetization; 1.2 and 1.6 T correspond to moderate magnetization; and 1.8 T corresponds to a higher magnetization.

As shown in Figure 1.5, the signal tested by the probe actually contains two parts: the background magnetic field signal and the defect MFL signal. When the magnetic induction intensity of the tested material is 1.8 T, the amplitude of the background magnetic induction intensity is 32 mT and the total testing magnetic induction intensity amplitude of the probe is 58.5 mT. So the magnetic induction intensity amplitude of the defect is 58.5–32 = 26.5 mT. The ratio of the MFL signal of the defect to the background magnetic field signal is about 0.8. When the magnetic induction intensity of the tested material is 0.6 T, the magnetic induction intensity amplitude of the magnetic leakage signal is only 2 mT, and the ratio of the MFL signal of the defect to the background magnetic field signal is 0.3. Here, the background magnetic field is the magnetic induction intensity direct current signal tested by the probe when there are no defects, and this is the air-coupled magnetic field mainly from the magnetic poles (abbreviating as “air-coupled magnetic field”) as shown in Figure 1.6.

In this way, the effect of external magnetic field intensity on the MFL testing can be seen by comparing the ratio of the MFL signal to the background magnetic field under different external magnetic field strength. As shown in Figure 1.6, the magnetic induction intensity of the tested part is 1.8 T, and this ratio is the highest. And with the decrease of the magnetic field intensity, the proportion is gradually reduced. When the applied magnetic field strength is lower than 1.2 T, this ratio will be reduced to about fixed value. However, if the magnetization is improved on the basis of 1.8 T, the ratio of the magnetic leakage signal to the background magnetic field shows a downward trend. The main reason is that MFL signal amplitude will not change when the magnetic induction intensity of the tested material reaches saturation; at the same time, the background magnetic field will continue to improve with the increasing external magnetic field, which leads to lower the ratio of the two.

Figure 1.6: Schematic diagram of the air-coupled magnetic field.

To sum up, the applied magnetizing field that can reach or moderately exceed the saturation magnetization will generate large enough leakage flux, which can ensure the effective detection of corrosion defects. Therefore, in order to improve the accuracy of corrosion defect magnetic leakage detection and quantification, a higher external magnetizing field strength should be chosen, to ensure that the tested material is magnetized to moderate saturation and to get enough high ratio of the magnetic leakage field of defects to the background magnetic field. It should be noted that the external magnetizing field strength cannot be too high, because the high magnetizing field intensity will lead to reduction of the signal-to-noise ratio (SNR).

Because the magnetizing field intensity will affect the detection and quantification, the influencing factors of the magnetizing field intensity also affect the detection and quantification. The main factors that affect the detection and quantification are as follows.

1.2.1.1The Thickness of Magnetized Material

The greater the thickness of the material is, the stronger the external magnetizing field is required to reach saturation magnetization. In the case of constant external magnetizing field, the change of the thickness is inversely proportional to the magnetic field intensity and the magnetic induction intensity of the tested material. Let the thickness T of the tested materials be 9.5, 12.7, 14.6, 17.5, 21, 26.2, 30.4 and 32 mm, then the magnetizing field strength in the same situation, using finite element method, and the change trend of the magnetic induction intensity above the surface 4 mm are shown in Figure 1.7.

Figure 1.7: Relationship between the thickness and the magnetic induction intensity.

As shown in Figure 1.7, the thickness of the tested material and the magnetic induction intensity show a negative linear relationship, that is, the thicker the tested piece, the smaller the magnetic induction intensity. Therefore, when designing the magnetic parameters of the permanent magnet, the thickness of the specimen must be considered.

1.2.1.2The Component of Material

Because of the change of carbon content, alloy component and impurities, the magnetic permeability of the material is changed, and the magnetization of the material is affected. By using a strong external magnetic field, the influence can be eliminated by saturation magnetization. At this time, the change of the magnetic permeability will lead to inconsistency of the leakage magnetic signal. So it is difficult to effectively evaluate and analyze the testing signal.

1.2.1.3The Coupling Loop

In a magnetizing system, steel brushes are used to couple the magnetic flux to the measured material. The coupling efficiency between the magnetizer and the measured material will have a certain effect on the magnetic induction intensity in the material. Short steel brush can provide higher coupling efficiency and higher magnetic induction strength, but longer steel brush can improve the pass-through ability of the inner inspector of pipes. If the applied magnetic field strength is only slightly higher than the saturation level, then the reducing coupling efficiency may decrease the magnetization of the material under saturation.

1.2.1.4The Space between Magnetic Poles

The shorter distance between the magnetizing poles can produce a higher magnetization, but this will lead to a decrease in the uniformity of the magnetic field. A longer distance between the poles can provide a relatively uniform magnetic field, but it will reduce the magnetization effect because the magnetic reluctance between the two magnetizing poles will increase with the increase of the magnetic pole spacing, so stronger magnets are needed to make the material to reach saturation. Therefore, it is necessary to select the appropriate magnetic pole spacing aiming at the properties and the thickness of the material.

1.2.1.5The Remanence

Remanence refers to the residual magnetic field in the material that has been tested by MFL detection in the past. The remanence will affect the current level of magnetization, especially when the magnetizing level is low or moderate.

The remanence will reduce the magnetic induction intensity of the material being tested, thus affecting the detection and quantification of defects. Ferromaterial in magnetizing will show a hysteresis effect. That is, after the magnetic field is removed, the material will remain within a certain magnetic induction intensity. When the material is magnetized again, the magnetization process will start from the remanence, thus a new magnetization curve is produced. The magnetization curve is not only nonlinear but also different from the magne...