Application of magnetic flux leakage method in the crimping inspection of carbon fiber conductor

Compared with steel core aluminum strand wire, carbon fiber composite core wire has many excellent characteristics such as large flow capacity, good heat resistance, light weight, but its mechanical properties are poor, and the unqualified crimping quality is easy to lead to wire breakage accidents, which greatly affects its popularization. For this reason, a scheme for the detection of crimping defects of carbon fiber composite mandrel based on magnetic leakage method is proposed. Firstly, the finite element analysis model of the clamping fixture was established, and then the magnetic pole distance k and radial air gap n of the magnetic leakage testing device were explored. Based on the signal-to-noise ratio under the two typical defect conditions of fracture and low pressure, the most suitable excitation structure parameters of the magnetic leakage device were obtained. The results show that when k=55mm and n=3.5mm, the signal-to-noise ratio of MFL detection device reaches the best effect. The research results are of great significance to the quality inspection of compacts of carbon fiber composite mandrel.


Introduction
Under the national "double carbon" goal and the new power system construction goal, the use of new carbon fiber composite conductor to transform the original transmission corridor has significant advantages in the power grid [1].However, carbon fiber conductor lacks sufficient experience in the process of wire crimping and network operation, and there is no effective defect detection means.It is easy to bring considerable security risks to the safety and reliability of transmission lines.Therefore, it is urgent to explore the non-destructive testing method of the defects in the crimping part.
At present, the non-destructive testing methods for the crimping position of overhead transmission lines mainly include infrared detection, ultrasonic detection, X-ray detection and Magnetic flux leakage (MFL).Literature [2], aiming at the traditional steel-core aluminum stranded wire, tried to use X-ray method to verify the effectiveness of the detection of tension-resistant wire clips and splinters.However, due to the large size of the X-ray device and the difficulty of protecting radioactive materials, it is difficult to apply in the field.With the improvement of lightweight X-ray equipment, portable X-ray equipment has been widely used at home and abroad.Literature [3] combined with X-ray detection technology to simulate various defects in the crimping process of fittings, providing a reliable basis for the crimping quality detection of practical engineering applications.Compared with steel core, carbon fiber composite core has a lower density, so it is difficult to obtain a clear image of the inner core.Literature [4] proposed to add special developer to epoxy resin system to improve the development effect of mandrel.Although the imaging effect of the core rod is significant, it will affect the mechanical properties of the carbon fiber core rod and increase the production cost.Literature [5] developed new X-ray equipment and improved image processing algorithm, so as to improve the imaging effect of mandrel.However, it mainly focuses on the crimping quality inspection of the mandrel in the unpressed part of the aluminum tube and the inner mandrel of the aluminum stranded wire, and the crimping quality of the inner mandrel of the steel anchor is not studied.
In summary of the above literature, most scholars adopt the X-ray method to detect the crimping quality of carbon fiber composite core wire fittings, but often ignore the research on the crimping quality of the mandrel at the steel anchor.This is because the density of the steel anchor is higher than that of the carbon fiber mandrel, and the imaging effect of the carbon fiber mandrel here is poor, and the crimping quality cannot be judged.ACSR and CFCC are very similar in structure, and MFL can be used for steel core in addition to X-ray method [6].MFL can not only accurately identify a variety of metal defects, but also has excellent characteristics such as strong anti-interference ability, fast detection speed and high detection accuracy [7].The research of MFL technology has been very mature in three aspects: magnetic leakage theoretical model [8], excitation structure device [9] and magnetic leakage signal analysis [10].
Therefore, a new scheme based on magnetic flux leakage (MFL) detection technology is proposed for crimping detection of steel anchors with carbon fiber composite mandrel.Firstly, the surface of the carbon fiber mandrel is magnetized by the magnetic film coating.Then, a numerical model for crimping of carbon fiber mandrel fixtures with different defects was constructed, and the influence of excitation structure parameters on MFL signal was calculated and analyzed, and MFL detection parameters suitable for this crimping model were obtained.The research results of this paper provide a new idea for the crimping quality inspection of carbon fiber composite mandrel steel anchors, which is of great significance for ensuring the safe operation of transmission lines.

Principle of magnetic leakage detection
Carbon fiber mandrel crimping magnetic leakage detection principle, first of all, the flexible film material with high permeability is coated on the surface of carbon core, and then the magnetic film is magnetized to the saturation state along the axis through the excitation device.Because the magnetic film attached to the carbon core has good followability, when the carbon core is defective when crimped, the magnetic film will also change in this part.Finally, the leakage magnetic field signal is collected through the magnetic sensor to determine whether there is damage.
Its principle is shown in Figure 1.When there is no defect and no structural change in the ferromagnetic material, the magnetic field line density of the same section is evenly distributed under the magnetization of the magnetizing device.If there is a defect cutting the direction of the magnetic field line inside or on the surface of the material, it will cause the magnetic field line to bend, resulting in magnetic field distortion, and the magnetic field line will leak out of the material surface at the defect position and enter the air to form a leakage magnetic field.

Governing equation
The magnetic circuit of this simulation model is composed of armature, permanent magnet, air, steel anchor, aluminum liner and magnetic mandrel.The intensity of magnetic circuit and magnetic field in air under transient magnetic field can be expressed by Maxwell equations and constitutive equations.When the displacement current is ignored, the governing equation of the transient magnetic field can be written as: Where: Jn represents the current density vector,  represents the material conductivity,  represents the magnetic permeability of the magnetic medium material, v represents the motion speed of the crimped fixture; Since the magnetic field is A passive field, magnetic vector A is introduced to solve the field.make According to the Coulomb gauge: The constitutive equation of permanent magnet can be expressed as: 0 () Where: m0 represents the vacuum permeability, M represents the magnetization vector of the permanent magnet, B represents the magnetic flux density, H represents the magnetic field strength.
In the finite element solution, the magnetizer is a permanent magnet and there is no change of magnetic field.At the same time, the magnetic field generated by permanent magnets is usually treated by equivalent surface current method.Ignoring its eddy current effect, the governing equation of the permanent magnet region is as follows: The excitation device adopts axial magnetization, and the axial magnetic flux density Bx of the magnetic film at the detection site is much larger than the radial magnetic flux density By.Because the crimping fixture model moves in the same direction as Bx, eddy current effects can be ignored.For static magnetic fields, the governing equation for the permanent magnet region is: The governing equation of the air domain outside the permanent magnet is:

Simulation model and parameters
In this paper, the carbon fiber composite mandrel in the crimping fittings is mainly twisted structure, the mandrel model is JLRX1/JF1B-400/40, and the crimping part mainly includes stainless steel anchor, aluminum liner and carbon fiber mandrel.The excitation device mainly consists of two parts: permanent magnet and armature.In the overall MFL inspection model, the magnetic film is attached to the surface of the carbon fiber mandrel, the aluminum liner is located between the magnetic film and the stainless steel anchor, and the external "ring" stainless steel anchor is pressed into a regular hexagon by a hydraulic press.In order to ensure that the magnetic film can be uniformly magnetized in the magnetic leakage detection area, the excitation device adopts a double-circuit structure.The excitation device is composed of permanent magnet and armature, wherein the parameters of permanent magnet are the same, its outer diameter is 58mm, the thickness is 15mm; The armature has an outer diameter of 61mm and a thickness of 18mm.As shown in Figure 2, the radial distance between the permanent magnet and the outer circle of the steel anchor is expressed as n; The distance between permanent magnets is expressed as k; Fracture and low pressure models are shown in Figure 3.

Permanent magnet
Crimp fittings

Silicon steel foil
Carbon fiber core armature n In practical engineering applications, the portability and reliability of MFL detection devices are the key factors for their application in different complex environments.The permanent magnet is the excitation of this simulation model because of its high coercive force and no power supply.Set the magnetization direction of the permanent magnet to the negative direction of the X axis, then the magnetization direction of the magnetic film is along the positive direction of the X axis.The boundary conditions of the magnetic field automatically meet the boundary conditions of the infinitely distant field, and no additional setting is required.The air region is an air region with a radius of 80mm and a length of 400mm.The parameters of the simulation model are shown in Table 1.

Simulation analysis
Considering the actual detection accuracy and effect of magnetic leakage device, the size of armature in exciter should not be too large, otherwise it is impossible to detect the head and end of the crimping fixture.Therefore, the magnetic pole distance k and radial air gap n were selected to vary between 30 and 60mm, and the radial air gap n was selected to vary between 1~6mm.The optimal magnetic pole distance k and radial air gap N were investigated using the magnetic induction intensity amplitude Bmag of the magnetic sensor part under non-destructive conditions and the axial and radial signal-to-noise ratio under low pressure and fracture defects as indicators.
As shown in Figure 4 and 5, axial and radial signal-to-noise ratio change surfaces under different magnetic pole spacing and radial air gap at 2mm fracture.The axial and radial signal-to-noise ratio of the fracture defect refers to the ratio of the fracture defect to the normal peak amplitude.As can be seen from Figure 4, the Bx signal-to-noise ratio varies greatly under different k and n, and is positively correlated with the increase of k and n.When k=50~55mm and n= 3~5mm, the accuracy of Bx SNR is improved by 28.53% compared with k=30~45mm and n=1~6mm.It can be seen from FIG. 5 that with the increase of k and n, SNR is also positively correlated.However, when k=52~55mm and n=2~4mm, the accuracy of By is improved by 26.67% compared with k=30~45mm and n=1~6mm.Therefore, when the fracture defect is selected as k=52~55mm, n =3~4mm, the accuracy change of Bx and By signal-tonoise ratio does not exceed 3%.
As shown in Figure 6 and 7, axial and radial signal-to-noise ratio change surfaces under different magnetic pole spacing and radial air gap at a lower pressure of 65mm.The axial and radial signal-tonoise ratio (SNR) of low pressure defect refers to the ratio of low pressure defect to normal trough amplitude.As can be seen from Figure 6, with the increasing of k and n, the Bx signal-to-noise ratio is negatively correlated.This is because under the low pressure defect, the magnetic field line first propagates in the air, and then enters the magnetic film, because the air reluctance is large, not only weakens the magnetization effect, but also distorts the air to the magnetic film.However, for the axial component, with the continuous increase of k and n, the normal trough value is positive, and the trough value changes from positive to negative under low pressure defects.For axial components, the smaller the SNR, the better.When k=53~55mm and n= 2~4mm, the accuracy of Bx SNR is improved by 73.14% compared with k=45~52mm and n=1~6mm.As can be seen from Figure 7, as k and n continue to increase, the SNR By presents a positive correlation.The trough value of radial component is negative both for low-pressure defects and under normal conditions, indicating that the larger the signal-to-noise ratio of radial component, the better.When k=52~55mm and n= 1.5~3.5mm, the accuracy of Bx SNR is increased by 55.74% compared with k=45~50mm and n=1~6mm.Therefore, when the low voltage defect is selected as k=53~55mm, n =1.5~3.5mm, the accuracy change of Bx and By signal-to-noise ratio does not exceed 5%.
Based on the above analysis, the selection of k and n also needs to be combined with the actual magnetic flux leakage detection.Due to the local magnetization of the silicon steel foil during the motion of the excitation device, and the hysteresis characteristic of the silicon steel foil, the magnetization effect will be weakened.If the magnetic pole spacing is properly increased, the negative effect caused by speed can be weakened.At the same time, considering the cost and portability of the magnetic leakage device, k is selected as 55mm.The outermost part of the crimping fixture model is a regular hexagonal steel anchor.In order to avoid collision with the permanent magnet during movement, sufficient distance between the permanent magnet and the surface of the steel anchor should be ensured in the radial direction.Therefore, choose n to be 3.5mm.

Conclusion
In this paper, magnetic leakage method based on carbon fiber core wire crimping is proposed to detect.The excitation structure parameters k and n of MFL testing device are explored.Based on the SNR of two typical defects, fracture and low voltage, it is concluded that when k=55mm and n=3.5mm, the SNR of MFL testing device can achieve the best effect.

Table 1 .
Simulation model parameter table.