Magneto-optical imaging detection method and device for multi-dimensional magnetic field reconstruction

Abstract

The invention discloses a magneto-optical imaging detection method and device for multi-dimensional magnetic field reconstruction, and relates to the technical field of crack measurement. The method comprises the following steps: S1, selecting a to-be-detected magnetisable sample piece and constructing a magneto-optical imaging detection device; s2, performing multi-layer lift-off crack measurement detection by using a testing device; s3, processing the magneto-optical image, reducing a gray distribution error caused by a light source system, and fitting to obtain a leakage magnetic field peak-to-peak value position; and S4, realizing crack size measurement according to the crack magneto-optical image inversion analysis under the fixed step length. The magneto-optical imaging detection device comprises a magneto-optical imaging system, an infinitesimal displacement lift-off platform, excitation equipment and a magnetisable sample piece to be detected. The influence of the lift-off height on the measurement result is reduced through the multi-layer lift-off technology, finer cracks can be measured, the influence of a light source system is reduced, and meanwhile, the influence of the crack size on magnetic field measurement is reduced by adopting an alternating current excitation and time dimension-considered image processing method.

Description

Technical Field

[0001] The present invention relates to the field of crack measurement, and in particular to a magneto-optical imaging detection method and device for multi-dimensional magnetic field reconstruction. Background Technique

[0002] Nondestructive testing (NDT) technology is one of the main means for detecting metal cracks at present. It evaluates the integrity of materials without damaging them. The main methods include ultrasonic testing, electromagnetic testing, ray testing, magnetic particle testing, etc. In recent years, with the introduction of machine learning and artificial intelligence technologies, the sensitivity and detection range of NDT methods have been significantly improved, which provides a more powerful tool for the early detection of metal cracks.

[0003] Currently, the magneto-optical imaging method has many application scenarios in nondestructive testing applications. The key research direction is to analyze and characterize cracks with more complex morphologies. In this regard, the research on this method mainly includes two aspects: the excitation method and the processing of imaging results. In order to effectively detect cracks with different depth features, Ma Nvjie proposed to use alternating current excitation to enrich the magnetic leakage information; considering the difficulty of dynamic magnetization in alternating current excitation, the literature proposed to use a composite excitation method to distinguish cracks; in order to collect cracks in different directions completely, the team proposed to use a rotating excitation magnetic field method to enrich the crack magnetic leakage field signal. In the same year, the team of Teacher Cheng Yuhua proposed that it is necessary to perform more detailed characterization analysis on the crack morphology before quantifying and inverting complex cracks. Therefore, a reconstruction study was carried out on the "Z"-shaped open crack. Subsequently, for the same "Z"-shaped crack opening, Shengping Li proposed to use the Newton regularization method to perform a more refined reconstruction of this type of crack. On the other hand, using neural network technology to identify cracks in dynamic magneto-optical images is also one of the mainstream research directions. The latest research literature mentions using an image fusion technology method based on the standard deviation of gray scale to achieve the fusion of dynamic images, thereby improving crack features.

[0004] In the inversion analysis of the leakage magnetic field, it is found that there is a certain numerical deviation between the magnetic field values collected by the actual sensor and the leakage magnetic field generated by the standard crack. Part of this deviation comes from the lift-off height of the sensor. In the existing crack measurement and detection using non-destructive testing methods, it is found that when using single-layer lift-off for magnetic flux leakage collection, the problem that the collected signal cannot be normalized occurs. At the same time, limited by the spatial resolution of the sensor, dense cracks often cannot be detected effectively and truly, and there are many imperfections in the inversion process. Based on the analysis of the magnetic dipole model, it is found that the field strength of the crack leakage magnetic field shows a rapid downward trend with the change of the lift-off height. Therefore, when using single-layer lift-off collection, it is extremely difficult to control the lift-off height. When the lift-off is low, the micron-level vibration up and down of the lift-off will cause a large range of changes in the leakage magnetic field. However, when the lift-off is high, the crack leakage magnetic field is weaker and more difficult to detect. From the measurement results of crack sizes using sensors with low spatial resolution such as Hall elements, it is found that the distance between the peaks of the measurement signals often depends on the spatial resolution of the sensor itself, which brings great challenges to the measurement of characteristic sizes. Based on the actual situation in the current magneto-optical measurement and collection method, due to the existence of a large number of lenses in the designed optical path system, the problem of light source concentration is caused, which brings more trouble to the processing of magneto-optical images. In addition, during the excitation process using the direct current excitation method in the current magneto-optical measurement and collection method, the size of the crack will cause the leakage magnetic field to deviate from the detection range of the leakage magnetic field of the magneto-optical thin film, thus affecting the measurement of the magnetic field. The existence of these problems indicates that how to design and study the inversion problem of crack characteristic sizes and reduce the influence of the lift-off height to improve the crack measurement accuracy is the core challenge that urgently needs to be solved in this field. Summary of the Invention

[0005] The object of the present invention is to address the above-mentioned deficiencies of the existing technologies. The present invention proposes a magneto-optical imaging detection method and device for multi-dimensional magnetic field reconstruction, which uses the magneto-optical imaging method to accurately collect crack parameters at a higher spatial resolution; reduces the influence of the lift-off height on the measurement results through multi-layer lift-off technology; designs a collection and processing process for magneto-optical images under extremely high lift-off to reduce the influence of the light source system on the magneto-optical imaging results and improve the reliability and accuracy of the experimental results; proposes to use the alternating current excitation method to expand the crack size measurement range, introduce the time relationship in the system processing design, and achieve multi-dimensional collection and measurement.

[0006] The multi-layer lift-off technology is used to calibrate the width parameter and depth parameter of the crack more finely. Finally, using the designed magneto-optical image processing algorithm, the magneto-optical image can be effectively processed, and the measurement of crack parameters can be better realized.

[0007] A magneto-optical imaging detection method for multi-dimensional magnetic field reconstruction, characterized by including the following steps:

[0008] Step S1: Select the magnetizable sample to be detected and set up the magneto-optical imaging detection device;

[0009] Step S2: Use the magneto-optical imaging detection device to perform crack measurement detection with multi-layer lift-off;

[0010] Step S3: Process the magneto-optical image, reduce the gray-scale distribution error caused by the light source system, and obtain the peak-to-peak position of the leakage magnetic field through fitting;

[0011] Step S4: Perform inverse analysis on the crack size based on the magneto-optical images of the crack under a set of fixed step sizes to achieve parameter measurement of the crack size.

[0012] Further, the crack measurement detection with multi-layer lift-off in step S2 specifically includes the following steps:

[0013] Step S2-1: Magnetize the magnetizable sample to be detected with the excitation device;

[0014] Step S2-2: A leakage magnetic field is formed at the crack of the magnetizable sample to be detected;

[0015] Step S2-3: The leakage magnetic field magnetizes the magneto-optical thin film;

[0016] Step S2-4: Use the camera to collect the magneto-optical image at extremely high lift-off;

[0017] Step S2-5: Adjust the micro-displacement lift-off platform, reduce the lift-off height until a crack is found in the magneto-optical image, and use the camera to collect the magneto-optical image at the current lift-off height;

[0018] Step S2-6: Reduce the lift-off height according to a fixed step size, and use the camera to collect the magneto-optical image at the current lift-off height each time the lift-off height is reduced. Repeat this step until the extremely low lift-off height is reached.

[0019] Further, the processing of the magneto-optical image in step S3 to reduce the gray-scale distribution error caused by the light source system specifically includes the following steps:

[0020] Step S3-1: Select the magneto-optical image at extremely high lift-off collected in step S2;

[0021] Step S3-2: Perform gray-scale processing on the magneto-optical image at extremely high lift-off to obtain the original gray-scale image of the magneto-optical image at extremely high lift-off, and find the gray-scale value with the largest distribution as the main gray-scale image of the magneto-optical image at extremely high lift-off;

[0022] Step S3-3: Subtract the original gray-scale image of the magneto-optical image at extremely high lift-off from the main gray-scale image of the magneto-optical image at extremely high lift-off to obtain the gray-scale error distribution image of the magneto-optical image at extremely high lift-off;

[0023] Step S3-4: Subtract the gray error distribution map of the magneto-optical image at other lift-off heights from the gray map of the original magneto-optical image at extremely high lift-off to obtain the gray distribution map after reducing the gray distribution error caused by the light source system.

[0024] Further, in step S3, in the obtained gray distribution map after reducing the gray distribution error caused by the light source system, extract the waveform extreme points on each row of pixels along the magnetic field direction, record the values and positions of the gray maximum and minimum values in each row; use the position information to mark the peak-to-peak position of the leakage magnetic field obtained by fitting on the gray map of the original magneto-optical image.

[0025] Further, in step S4, perform inverse analysis on the crack size parameters according to the crack magneto-optical images under a set of fixed step sizes, and establish a magnetic dipole model based on the standard crack; in the magnetic dipole model, for any position P(x, y, z) in the leakage magnetic field space, it will interact with the crack surface point Q(x1, a, z1). Let the Q point be the differential source, and the leakage magnetic field B y1 at point P has the following differential form:

[0026]

[0027] Performing surface integral gives:

[0028]

[0029] where μ0 is the magnetic permeability in vacuum, ρ is the magnetic flux density, a is the true width of the crack, b is the true length of the crack, and c is the true depth of the crack.

[0030] Considering that there are opposite magnetic fields simultaneously on another leakage plane, the leakage magnetic field model obtained after integration is as follows:

[0031]

[0032] B = B z1 - B z2

[0033] where B z1 is the vertical magnetic field component generated by one side of the single crack, B z2 is the vertical magnetic field component generated by the other side of the crack, and B is the combined magnetic field generated by their superposition.

[0034] According to the leakage magnetic field magnetic dipole model of the standard crack, it is found that:

[0035]

[0036] Among them, a′ is the peak-to-peak position width in the magneto-optical image, and t is the lift-off height. The formula is used to fit the peak-to-peak position width of the leakage magnetic field at multiple lift-off heights to measure the true width of the crack.

[0037] In the step S4, an inversion analysis of the depth parameter of the crack size is performed based on the magneto-optical images of the crack under a set of fixed step sizes. There is a glass reflection interface in the lower layer of the magneto-optical thin film, and there is a distance between the leakage magnetic field that can magnetize the magneto-optical thin film and the crack. There is a maximum value in the magnetic field during the lift-off acquisition process.

[0038] The magnetic field extreme value B generated locally by the long crack max is as follows:

[0039]

[0040] Furthermore, in the step S2, the magneto-optical imaging system is improved. When the improved device passes over the surface of the magnetizable sample to be detected in a parallel placement direction, magneto-optical images at different lift-off heights at the same position are continuously captured. This multi-lift-off acquisition process starts from the first frame acquisition. An image with a lift-off height of Y1 is captured at position X1, an image with a lift-off height of Y2 is captured at position X2, and an image with a lift-off height of Y3 is captured at position X3. Then, the device is translated, and the second frame acquisition is entered. An image with a lift-off height of Y2 is captured at position X1, and an image with a lift-off height of Y3 is captured at position X2. The device is translated again, and the third frame acquisition is entered. An image with a lift-off height of Y3 is captured at position X1. Finally, the images captured in the above different frames are summarized to obtain a multi-lift-off image at a single position.

[0041] Furthermore, the excitation and detection methods are improved. Alternating current excitation is used instead of direct current excitation to magnetize the magnetizable sample to be detected. An image processing method of jointly processing multiple frames of images is adopted. Magneto-optical imaging is processed in different time dimensions to obtain the minimum saturation magnetization voltage. The multi-lift-off images under saturation magnetization are found using the multi-frame multi-lift-off images, and a crack feature map is obtained through data fusion for further measuring the crack size parameters.

[0042] In addition, the present invention also provides a magneto-optical imaging detection device for multi-dimensional magnetic field reconstruction. The magneto-optical imaging detection device for multi-dimensional magnetic field reconstruction includes: a magneto-optical imaging system, a micro-displacement lift-off platform, an excitation device, and a magnetizable sample to be detected. The magneto-optical imaging system is mounted on the micro-displacement lift-off platform. The micro-displacement platform realizes up and down movement with a minimum of 10 μm, so as to obtain acquisition results at different sensor lift-off heights. The excitation device is used to excite the magnetizable sample to be detected.

[0043] Further, the magneto-optical imaging system includes: an attached polarized light source, a semi-transparent and semi-reflective mirror, a magneto-optical thin film, a polarizer, and a camera; the single-wavelength light generated by the attached polarized light source forms linearly polarized light through a polarizer and is directly incident on the magneto-optical thin film through the semi-transparent and semi-reflective mirror. After the magneto-optical thin film is magnetized by the leakage magnetic field, an induced magnetic field is generated inside. The induced magnetic field causes the incident linearly polarized light to have an optical rotation effect. The deflected linearly polarized light is reflected on the bottom surface of the magneto-optical thin film to the semi-transparent and semi-reflective mirror, and then reflected again into the analyzer, and finally captured by the camera; the camera is connected to a computer system, and the collected magneto-optical images are processed and analyzed by the computer.

[0044] Further, the magneto-optical imaging system is improved by replacing the magneto-optical thin film in the original magneto-optical imaging system with three magneto-optical thin films arranged in the same row and at different lift-off heights.

[0045] Compared with the prior art, the present invention can achieve the following beneficial effects:

[0046] 1. Compared with the existing crack measurement method based on single-layer lift-off acquisition, the magneto-optical imaging detection method and device for multi-dimensional magnetic field reconstruction proposed by the present invention make the measurement result no longer affected by the lift-off height through the multi-layer lift-off method, and the measurement result is more reliable.

[0047] 2. The magneto-optical imaging detection method and device for multi-dimensional magnetic field reconstruction proposed by the present invention are based on the magneto-optical imaging method, have extremely high spatial resolution, break through the limitations of existing sensors with low spatial resolution such as Hall elements, and can measure more fine crack parameters.

[0048] 3. The magneto-optical imaging detection method and device for multi-dimensional magnetic field reconstruction proposed by the present invention design a collection and processing process for magneto-optical images under extremely high lift-off. By obtaining the gray error distribution map, the influence of the light source system on the magneto-optical imaging result can be effectively reduced.

[0049] 4. The magneto-optical imaging detection method and device for multi-dimensional magnetic field reconstruction proposed by the present invention adopt an AC excitation method and a processing method for multiple frames of images, reduce the influence on magnetic field measurement caused by the leakage magnetic field deviating from the detection range of the magneto-optical thin film due to crack size, introduce a time relationship in the processing design, and realize multi-dimensional acquisition and measurement.

[0050] 5. The magneto-optical imaging detection method and device for multi-dimensional magnetic field reconstruction proposed by the present invention have good uniformity in the gray scale of magneto-optical images obtained under repeated experimental conditions, ensure the reliability and repeatability of the detection results, and provide stable technical support for crack detection. Description of the Drawings

[0051] Figure 1Flow chart of a magneto - optical imaging detection method for multi - dimensional magnetic field reconstruction proposed by the present invention.

[0052] Figure 2 Magneto - optical image processing flow for reducing the gray - scale distribution error caused by the light source system and obtaining the peak - to - peak position of the leakage magnetic field by fitting proposed by the present invention.

[0053] Figure 3 Effect diagram of peak - to - peak fitting of the leakage magnetic field in crack magneto - optical imaging.

[0054] Figure 4 Schematic diagram of a magnetic dipole model established for a standard crack.

[0055] Figure 5 Relationship curve between crack width and lift - off height when the crack width is 0.4 mm provided by an embodiment of the present invention.

[0056] Figure 6 Relationship curve between the maximum magnetic field and crack depth when the crack width is 2 mm, the sensor lift - off height is 0.3 mm, and the magnetic field peak varies from 1 - 3 mm in depth provided by an embodiment of the present invention.

[0057] Figure 7 Schematic diagram of a magneto - optical imaging detection device for multi - dimensional magnetic field reconstruction proposed by the present invention.

[0058] Figure 8 Detail schematic diagram of the magneto - optical imaging system in the magneto - optical imaging detection device.

[0059] Figure 9 Schematic diagram of an improved multi - lift - off acquisition device proposed by the present invention.

[0060] Figure 10 Flow chart of an improved multi - lift - off acquisition proposed by the present invention.

[0061] Figure 11 Multi - lift - off acquisition scheme diagram under AC excitation conditions proposed by the present invention.

[0062] In the figure, 1. Magneto - optical imaging system; 2. Excitation device; 3. Micro - displacement lift - off platform; 4. Magnetizable sample to be detected; 5. Light source with attached polarizer; 6. Half - transparent and half - reflecting mirror; 7. Magneto - optical thin film; 8. Analyzer; 9. Camera. Detailed implementation method

[0063] The following further describes the specific embodiments of the present invention in conjunction with the accompanying drawings. The technical solutions in the embodiments of the present invention are clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0064] Combined with Figure 1 , a magneto-optical imaging detection method for multi-dimensional magnetic field reconstruction, characterized by comprising the following steps:

[0065] Step S1: Select a magnetizable sample to be detected and build a magneto-optical imaging detection device;

[0066] Step S2: Use the magneto-optical imaging detection device to perform crack measurement detection with multi-layer lift-off;

[0067] Step S3: Perform image processing on the magneto-optical image, reduce the gray-scale distribution error caused by the light source system, and obtain the peak-to-peak position of the leakage magnetic field through fitting;

[0068] Step S4: Perform inverse analysis on the crack size based on the magneto-optical images of the crack under a set of fixed step sizes to realize parameter measurement of the crack size.

[0069] Further, the crack measurement detection with multi-layer lift-off in step S2 specifically includes the following steps:

[0070] Step S2-1: Magnetize the magnetizable sample to be detected by an excitation device;

[0071] Step S2-2: A leakage magnetic field is formed at the crack of the magnetizable sample to be detected;

[0072] Step S2-3: The leakage magnetic field magnetizes the magneto-optical thin film;

[0073] Step S2-4: Use a camera to collect magneto-optical images at extremely high lift-off;

[0074] Step S2-5: Adjust the micro-displacement lift-off platform, reduce the lift-off height until a crack is found in the magneto-optical image, and use a camera to collect the magneto-optical image at the current lift-off height;

[0075] Step S2-6: Reduce the lift-off height according to a fixed step size, and use a camera to collect the magneto-optical image at the current lift-off height each time the lift-off height is reduced. Repeat this step until the extremely low lift-off height is reached.

[0076] Further, combined with Figure 2 , the image processing of the magneto-optical image in step S3 to reduce the gray-scale distribution error caused by the light source system specifically includes the following steps:

[0077] Step S3-1: Select the magneto-optical image under extremely high lift-off collected in Step S2;

[0078] Step S3-2: Perform grayscale processing on the magneto-optical image under extremely high lift-off to obtain the original grayscale image of the magneto-optical image under extremely high lift-off, and find the grayscale value with the largest distribution as the main grayscale image of the magneto-optical image under extremely high lift-off;

[0079] Step S3-3: Subtract the original grayscale image of the magneto-optical image under extremely high lift-off from the main grayscale image of the magneto-optical image under extremely high lift-off to obtain the grayscale error distribution image of the magneto-optical image under extremely high lift-off;

[0080] Step S3-4: Subtract the original grayscale image of the magneto-optical image at other lift-off heights from the grayscale error distribution image of the magneto-optical image under extremely high lift-off to obtain the grayscale distribution image after reducing the grayscale distribution error caused by the light source system.

[0081] Further, in combination with Figure 2 and Figure 3 , in Step S3, in the grayscale distribution image after reducing the grayscale distribution error caused by the light source system, extract the waveform extreme points on each row of pixels along the magnetic field direction, record the values and positions of the grayscale maximum and minimum values in each row; use the position information to mark the peak-to-peak position of the leakage magnetic field on the original grayscale image of the magneto-optical image.

[0082] Further, in Step S4, perform inverse analysis on the crack size parameters according to the crack magneto-optical images under a set of fixed step sizes, and establish a magnetic dipole model as shown in Figure 4 ; in the magnetic dipole model, for any position P(x, y, z) in the leakage magnetic space, there will be an interaction with the crack surface point Q(x1, a, z1). Let Q point be the differential source, and the differential form of the leakage magnetic field B y1 generated at point P is as follows:

[0083]

[0084] Performing surface integral gives:

[0085]

[0086] where μ0 is the magnetic permeability in vacuum, ρ is the magnetic flux density, a is the true width of the crack, b is the true length of the crack, c is the true depth of the crack,

[0087] Considering that there are opposite magnetic fields simultaneously in another leakage plane, the leakage magnetic field model obtained after integration is as follows:

[0088]

[0089] B = B z1 -B z2

[0090] wherein, B z1 is the vertical magnetic field component generated by a single crack side, B z2 is the vertical magnetic field component generated by the other side of the crack, and B is the resultant magnetic field generated by the superposition of the two;

[0091] According to the magnetic dipole model of the leakage magnetic field of a standard crack, it is found that:

[0092]

[0093] wherein, a' is the peak-to-peak position width in the magneto-optical image, and t is the lift-off height; for a crack with a width of 0.4 mm, the relationship curve between the crack width and the lift-off height Figure 7 is shown; using this formula to fit the peak-to-peak position width of the leakage magnetic field at multiple lift-off heights to achieve the measurement of the true width of the crack;

[0094] In the step S4, the depth parameter of the crack size is inversely analyzed according to the magneto-optical image of the crack under a set of fixed step sizes. During the depth inversion process, when the magneto-optical film is close to the surface of the leakage magnetic field, the value of the leakage magnetic field increases infinitely numerically. However, there is a glass reflection interface in the lower layer of the magneto-optical film, and there is at least a certain thickness between the leakage magnetic field that can magnetize the magneto-optical film and the crack. Therefore, there is a maximum value of the magnetic field during the lift-off acquisition process;

[0095] The magnetic field extreme value B max generated locally by a long crack is as follows:

[0096]

[0097] When the crack width and the lift-off height are fixed, the magnetic field peak is approximately proportional to the crack depth at a fixed lift-off, B max ∝c; when the crack width is 2 mm and the lift-off height of the sensor is 0.3 mm, the relationship curve between the maximum magnetic field and the crack depth when the magnetic field peak changes with the crack depth from 1 to 3 mm is as Figure 6 shown.

[0098] Furthermore, combined with Figure 10, in step S2, the magneto-optical imaging system is improved. When the improved device sweeps across the surface of the magnetizable sample to be detected in a parallel placement direction, magneto-optical images at different lift-off heights at the same position are continuously captured. This multi-lift-off acquisition process starts from the first frame acquisition. An image with a lift-off height of Y1 is captured at position X1, an image with a lift-off height of Y2 is captured at position X2, and an image with a lift-off height of Y3 is captured at position X3. Then, the device is translated, and the second frame acquisition is entered. An image with a lift-off height of Y2 is captured at position X1, and an image with a lift-off height of Y3 is captured at position X2. The device is translated again, and the third frame acquisition is entered. An image with a lift-off height of Y3 is captured at position X1. Finally, the images captured in the above different frame acquisitions are summarized to obtain a multi-layer lift-off image at a single position.

[0099] Further, the excitation and detection methods are improved. Alternating current excitation is used instead of direct current excitation to magnetize the magnetizable sample to be detected. An image processing method of jointly processing multiple frames of images is adopted. Figure 11 , magneto-optical imaging is processed in different time dimensions to obtain the minimum saturation magnetization voltage. The multi-layer lift-off images under saturation magnetization are found using the multi-frame multi-layer lift-off images, and a crack feature map is obtained through data fusion for further measurement of crack size parameters.

[0100] Combined with Figure 7 , a magneto-optical imaging detection device for multi-dimensional magnetic field reconstruction, the magneto-optical imaging detection device for multi-dimensional magnetic field reconstruction includes: a magneto-optical imaging system, a micro-displacement lift-off platform, an excitation device, and a magnetizable sample to be detected; the magneto-optical imaging system is mounted on the micro-displacement lift-off platform; the micro-displacement platform realizes an up and down movement of at least 10 μm, so as to achieve acquisition results at different sensor lift-off heights; the excitation device is used to excite the magnetizable sample to be detected.

[0101] Further, combined with Figure 8 , the magneto-optical imaging system includes: an attached polarized light source, a semi-transparent and semi-reflective mirror, a magneto-optical thin film, a polarizer, and a camera; the single-wavelength light generated by the attached polarized light source forms linearly polarized light through a polarizer and is directly incident on the magneto-optical thin film through the semi-transparent and semi-reflective mirror. After the magneto-optical thin film is magnetized by the leakage magnetic field, an induced magnetic field is generated inside. The induced magnetic field produces an optical rotation effect on the incident linearly polarized light. The deflected linearly polarized light is reflected on the bottom surface of the magneto-optical thin film to the semi-transparent and semi-reflective mirror and then reflected again into the analyzer and finally captured by the camera; the camera is connected to an external computer system, and the collected magneto-optical images are transmitted to the computer system for processing and analysis.

[0102] Further, combined with Figure 9 , the magneto-optical imaging system is improved by replacing the magneto-optical thin film in the original magneto-optical imaging system with three magneto-optical thin films arranged in the same row and at different lift-off heights.

[0103] The above embodiments are only the technical ideas of the present invention, and the protection scope of the present invention cannot be limited thereby. Any modification made on the basis of the technical solution according to the technical ideas proposed by the present invention falls within the protection scope of the present invention.

Claims

1. A magneto-optical imaging detection method for multi-dimensional magnetic field reconstruction, characterized in that It includes the following steps: Step S1: Select the magnetizable sample to be detected and set up the magneto-optical imaging detection device; Step S2: Use the magneto-optical imaging detection device to perform crack measurement detection with multi-layer lift-off; Step S3: Perform image processing on the magneto-optical image, reduce the gray-scale distribution error caused by the light source system, and obtain the peak-to-peak position of the leakage magnetic field through fitting; Step S4: Perform inverse analysis on the crack size based on the magneto-optical images of the crack under a set of fixed step sizes, and realize the parameter measurement of the crack size.

2. The magneto-optical imaging detection method for multi-dimensional magnetic field reconstruction according to claim 1, characterized in that: The crack measurement detection with multi-layer lift-off in step S2 specifically includes the following steps: Step S2-1: Magnetize the magnetizable sample to be detected by the excitation device; Step S2-2: A leakage magnetic field is formed at the crack of the magnetizable sample to be detected; Step S2-3: The leakage magnetic field magnetizes the magneto-optical thin film; Step S2-4: Use the camera to collect the magneto-optical image at an extremely high lift-off; Step S2-5: Adjust the micro-displacement lift-off platform, reduce the lift-off height until a crack is found in the magneto-optical image, and use the camera to collect the magneto-optical image at the current lift-off height; Step S2-6: Reduce the lift-off height according to a fixed step size, and use the camera to collect the magneto-optical image at the current lift-off height every time the lift-off height is reduced. Repeat this step until the extremely low lift-off height is reached.

3. The magneto-optical imaging detection method for multi-dimensional magnetic field reconstruction according to claim 1, characterized in that: The image processing of the magneto-optical image in step S3 to reduce the gray-scale distribution error caused by the light source system specifically includes the following steps: Step S3-1: Select the magneto-optical image at an extremely high lift-off collected in step S2; Step S3-2: Perform gray-scale processing on the magneto-optical image at an extremely high lift-off to obtain the original gray-scale image of the magneto-optical image at an extremely high lift-off, and find the gray-scale value with the largest distribution as the main gray-scale image of the magneto-optical image at an extremely high lift-off; Step S3-3: Subtract the main gray-scale image of the magneto-optical image at an extremely high lift-off from the original gray-scale image of the magneto-optical image at an extremely high lift-off to obtain the gray-scale error distribution image of the magneto-optical image at an extremely high lift-off; Step S3-4: Subtract the gray-scale error distribution image of the magneto-optical image at an extremely high lift-off from the original gray-scale images of the magneto-optical images at other lift-off heights to obtain the gray-scale distribution image after reducing the gray-scale distribution error caused by the light source system.

4. A magneto-optical imaging detection method for multi-dimensional magnetic field reconstruction according to claim 1, characterized in that: In step S3, in the gray-scale distribution image after reducing the gray-scale distribution error caused by the light source system, extract the waveform extreme points on each row of pixels along the magnetic field direction, and record the values and positions of the gray-scale maximum and minimum values in each row; use the position information to mark the peak-to-peak position of the leakage magnetic field obtained by fitting on the original gray-scale image of the magneto-optical image.

5. A magneto-optical imaging detection method for multi-dimensional magnetic field reconstruction according to claim 1, characterized in that: In the step S4, the parameters of the crack size are inversely analyzed according to the crack magneto-optical images under a set of fixed step lengths, and a magnetic dipole model is established based on the standard crack; in the magnetic dipole model, for any position P(x, y, z) in the magnetic leakage space, there will be an interaction with the crack surface point Q(x1, a, z1). Let the Q point be the differential source, and the differential form of the magnetic leakage field B y1 generated at the P point is as follows: Performing area integration gives: where μ0 is the magnetic permeability in vacuum, ρ is the magnetic flux density, a is the true width of the crack, b is the true length of the crack, and c is the true depth of the crack. Considering that there are opposite magnetic fields existing simultaneously in another leakage plane, the leakage magnetic field model is obtained after integration as follows: Among them, B z1 is the vertical magnetic field component generated by one side of the single crack, and B z2 is the vertical magnetic field component generated by the other side of the crack. B is the resultant magnetic field generated by the superposition of the two; It is found according to the leakage magnetic field magnetic dipole model of the standard crack that: where a′ is the peak-to-peak position width in the magneto-optical image, and t is the lift-off height; use this formula to fit the peak-to-peak position widths of the leakage magnetic fields at multiple lift-off heights to realize the measurement of the true width of the crack; In step S4, perform inverse analysis on the depth parameter of the crack size based on the magneto-optical images of the crack under a set of fixed step sizes. There is a glass reflection interface in the lower layer of the magneto-optical thin film, and there is a distance between the leakage magnetic field that can magnetize the magneto-optical thin film and the crack. There is a maximum value of the magnetic field during the lift-off acquisition process; The extreme value B of the magnetic field generated locally by the long crack max is as follows:

6. The magneto-optical imaging detection method for multi-dimensional magnetic field reconstruction according to claim 1, characterized in that: In step S2, the magneto-optical imaging system is improved. When the improved device passes over the surface of the magnetizable sample to be detected in a parallel placement direction, magneto-optical images at different lift-off heights at the same position are continuously captured. This multi-lift-off acquisition process starts with the first frame acquisition. An image with a lift-off height of Y1 is acquired at position X1, an image with a lift-off height of Y2 is acquired at position X2, and an image with a lift-off height of Y3 is acquired at position X3. Then, the device is translated, and the second frame acquisition is entered. An image with a lift-off height of Y2 is acquired at position X1, and an image with a lift-off height of Y3 is acquired at position X2. The device is translated again, and the third frame acquisition is entered. An image with a lift-off height of Y3 is acquired at position X1. Finally, the images acquired in different frames are summarized to obtain a multi-layer lift-off image at a single position.

7. A magneto-optical imaging detection method for multi-dimensional magnetic field reconstruction according to claim 1, characterized in that: The excitation and detection methods are improved. Alternating current excitation is used instead of direct current excitation to magnetize the magnetizable sample to be detected. An image processing method that jointly processes multiple frames of images is adopted. Magneto-optical imaging is processed in different time dimensions to obtain the minimum saturation magnetization voltage. The multi-layer lift-off images under saturation magnetization are found using the multi-frame multi-layer lift-off images, and a crack feature map is obtained through data fusion for further measurement of crack size parameters.

8. A magneto-optical imaging detection device for multi-dimensional magnetic field reconstruction, characterized in that The magneto-optical imaging detection device for multi-dimensional magnetic field reconstruction is applied to the magneto-optical imaging detection method for multi-dimensional magnetic field reconstruction as described in any one of claims 1-7. The magneto-optical imaging detection device for multi-dimensional magnetic field reconstruction includes: a magneto-optical imaging system, a micro-displacement lift-off platform, an excitation device, and a magnetizable sample to be detected. The magneto-optical imaging system is mounted on the micro-displacement lift-off platform. The micro-displacement platform enables vertical movement of at least 10 μm, thereby achieving acquisition results at different sensor lift-off heights. The excitation device is used to excite the magnetizable sample to be detected.

9. A magneto-optical imaging detection device for multi-dimensional magnetic field reconstruction according to claim 8, characterized in that, The magneto-optical imaging system includes: an attached polarized light source, a semi-transparent and semi-reflective mirror, a magneto-optical thin film, a polarizer, and a camera. The single-wavelength light generated by the attached polarized light source forms linearly polarized light through the polarizer and is directly incident on the magneto-optical thin film through the semi-transparent and semi-reflective mirror. After the magneto-optical thin film is magnetized by the leakage magnetic field, an induced magnetic field is generated inside. The induced magnetic field causes the incident linearly polarized light to have a Faraday rotation effect. The deflected linearly polarized light is reflected at the bottom surface of the magneto-optical thin film to the semi-transparent and semi-reflective mirror and is reflected again into the analyzer, and finally captured by the camera. The camera is connected to the computer system, and the acquired magneto-optical images are processed and analyzed by the computer.

10. A magneto-optical imaging detection device for multi-dimensional magnetic field reconstruction according to claim 8, characterized in that The magneto-optical imaging system is improved by replacing the magneto-optical thin film in the original magneto-optical imaging system with three magneto-optical thin films arranged in the same row and at different lift-off heights.

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