Crack tip deformation in-situ measurement method based on sub-reference image splicing matching DIC

Abstract

A crack tip deformation in-situ measurement method based on sub-reference image splicing matching DIC includes the steps that firstly, the two sides of a compact tensile CT test piece are processed, one side is polished through abrasive paper, speckle spraying is conducted on the other side, a microscopic camera moves in the crack growth direction at the same step length, and multiple speckle images are shot; obtaining a whole-field reference image by using an image splicing technology, and obtaining a reference image corresponding to any position on a crack propagation path in cooperation with a template matching technology; speckle images collected in the hysteresis influence interval are matched with a global reference image, a corresponding reference image is captured, data of a crack tip displacement field and a smooth surface image shot by a camera on the other side are analyzed through a DIC technology to obtain a crack tip morphology change trend, and a result is further calculated through the data. According to the method, the Qamp is obtained at low cost; in the P steel crack propagation process, the deformation field of the crack tip and the crack morphology change under any crack length in an area are affected by overload hysteresis.

Description

Technical Field

[0001] The present invention belongs to the field of metal fatigue testing, and provides an in-situ measurement method for fatigue crack tip deformation based on sub-reference image stitching and matching DIC, which is suitable for accurate detection of fatigue crack deformation at any crack length in an overload hysteresis influence zone during Q&P steel fatigue testing. Background Art

[0002] Energy saving, safety and environmental protection are the core issues in the development of the modern automobile industry. Quenched ductile steel is the third generation of advanced high-strength steel, referred to as Q&P steel. While meeting these requirements, it achieves a good match between cost and performance and is a representative new material for future automobile manufacturing steel. During the operation of the car, the main components are continuously subjected to alternating loads, especially after a collision accident in the car body or other parts or microcracks are generated during the manufacturing and installation of materials. The variable amplitude alternating load caused by road conditions and other reasons during driving will further expand the cracks and seriously affect the fatigue service life of automobile materials. In particular, for engineering structures that bear complex loads, the study of material behavior under overload conditions is particularly important.

[0003] Overload hysteresis behavior refers to the significant changes in the mechanical properties and microstructure of the specimen after it has been overloaded, such as changes in hardness, residual stress, and slowed crack growth rate. At present, with the rapid development of the manufacturing industry, the requirements for the strength, toughness, and fatigue resistance of steel are increasing. Therefore, in-depth research on overload hysteresis behavior is of great significance for understanding the deformation and fracture mechanism of steel under overload conditions, optimizing material design, improving the reliability of engineering structures, and promoting green manufacturing and sustainable development.

[0004] However, the measurement of the deformation field in the tiny area of ​​the crack tip of Q&P steel has the characteristics of complex material microstructure, stress and strain distribution in the crack tip area, large gradient, rapid evolution over time, and high measurement accuracy requirements. In addition, the overload hysteresis influence area is a large area. If you want to further understand the fatigue crack growth behavior in the overload plastic zone under different overload ratios, you need to use a high magnification camera to shoot the crack tip to obtain more accurate crack tip displacement field and strain field. In addition, it is necessary to collect multiple cycles within the overload hysteresis influence area to study the fatigue crack growth behavior in the overload hysteresis influence area, and conduct in-depth research on the crack tip passivation, closure, and deformation-induced martensitic transformation (DIMT) behavior, revealing the overload hysteresis mechanism of Q&P steel. Therefore, under high magnification and large overload ratio conditions, the field of view of the lens is limited and it is impossible to accurately obtain specific information such as the relevant deformation of the crack tip in the measurement cycle. DIC technology is required to measure the relevant deformation information of the crack tip, and the problem of how to accurately obtain the reference image and target image required for DIC calculation is particularly important. Summary of the invention

[0005] In order to overcome the shortcomings of the existing technology, the present invention provides an in-situ measurement method for crack tip deformation based on sub-reference image stitching and matching DIC. By using DIC and sub-reference image stitching technology, a method for accurately locating the crack position in the overload hysteresis influence zone and collecting speckle images for DIC calculation is provided, which solves the difficulties existing in the measurement and acquisition links, facilitates the subsequent study of the crack growth rate, crack growth path and morphology, crack tip deformation and other behaviors in the overload hysteresis influence zone of Q&P steel after a single overload, and can further conduct in-depth research on the crack tip passivation, closure and other behaviors, revealing the overload hysteresis mechanism of Q&P steel.

[0006] The technical solution adopted by the present invention to solve its technical problem is:

[0007] A method for in-situ measurement of crack tip deformation based on sub-reference image stitching and matching DIC, the method comprising the following steps:

[0008] Step 1: Preparation work before fatigue crack growth test: polishing and speckle spraying treatment are performed on both sides of the compact tensile CT specimen. The polished surface is used to locate the crack length, and the speckle surface is used to collect speckle images for DIC calculation.

[0009] Step 2: Install the two microscope cameras used in the experiment on the micro-motion platform, and then calibrate the cameras to determine the distortion coefficient;

[0010] Step 3: Fix the CT specimen on the fatigue test bench, and then aim the camera lens for capturing speckles at the crack notch, and then move the camera several times with a set step length. Each movement collects an image, and the step length must not be larger than the camera's lateral field of view;

[0011] Step 4: The image collected in step 3 is corrected according to the distortion coefficient determined in step 2, and the stitching area is determined using a matching algorithm, and then stitched in sequence to obtain a full-field reference image. The size of the global reference image includes the size range of the overload hysteresis influence area under the experimental overload condition;

[0012] Step 5: Use the micro-motion platform to move the camera to the initial position and aim at the prefabricated crack of the specimen to take a picture;

[0013] Step 6: Start the fatigue crack growth test. During the test, the camera tracks the crack tip and takes pictures. When the specimen is subjected to a single overload, the camera collects several crack tip images and corresponding speckle images of different crack lengths in the overload hysteresis influence area, and records the force value corresponding to each image. These images are called target images.

[0014] Step 7: Based on the crack image collected in step 6, the length data of the crack under different cycles can be obtained, and the fatigue crack growth rate can be calculated;

[0015] Step 8: For the target image under the same crack length, select the image at the moment of minimum loading force value and match it with the full-field reference image obtained in step 4, cut out the corresponding reference image, and use this method to obtain the corresponding reference images under other crack lengths;

[0016] Step 9: Use DIC technology to calculate each group of images obtained in step 8 to obtain the displacement field of the crack tip within one cycle under different crack lengths in the overload hysteresis influence zone. Use the cross-correlation function to find the displacement before and after the change of any point, so as to obtain the displacement field data of the crack tip; and further obtain the strain field data according to the relevant algorithm;

[0017] Step 10: In step 9, a virtual extensometer is arranged at a set position behind the crack tip, and the fatigue crack opening displacement at different distances behind the crack tip is measured in combination with the displacement field data to obtain the crack opening displacement curve under different loads in the load cycle, which can further analyze the changing trend of the crack closure behavior in the overload hysteresis influence zone.

[0018] Furthermore, in step 1, the preparatory work before the test includes system debugging, camera calibration, and specimen preparation; the camera is calibrated using Zhang's calibration method. Due to the confidence requirement of the ROI in the speckle image, it is necessary to prepare microscopic speckles; one side of the CT specimen is polished into a glossy surface with a diffuse reflection effect, and the other side is prepared for microscopic speckles; the specimen is installed on the fatigue testing machine, and dual microscopic cameras are installed on both sides, each camera is equipped with a lens.

[0019] Further, in step 2, the camera used to take the microscopic crack image has a resolution of 5472×3648 pixels, and the lateral field of view should be controlled at about 2 mm, which should be slightly larger than the length of the fatigue crack. The captured image is corrected and the image spatial resolution is recorded. Note: The image spatial resolution is required to be about 0.35 μm / pixel. The camera used to take the microscopic speckle image has a resolution of 4096×2168 pixels, and the field of view is controlled at about 4 mm×2 mm, which is convenient for subsequent DIC analysis. The captured image is corrected and the image spatial resolution is recorded. Note: The image spatial resolution is required to be about 1 μm / pixel. Further, in step 4, a microscopic camera is used to collect and correct multiple speckle images, and the multiple collected images are spliced ​​according to the common feature points between each other; after splicing, a complete full-field reference image is obtained, which is used to match and intercept the speckle image taken at any time, so as to serve as a reference image when no load is applied at this cycle moment. The advantage of this method is that it can enable DIC to calculate the crack tip displacement field information more accurately.

[0020] In step 4, the process of using the matching algorithm to determine the stitching area is as follows: cut out the middle part of the image in image 1, use the template matching technology to search for the matching area in image 2, expand these two areas horizontally to the entire image and cut out, then the two images are strictly matched, and finally the two images are stitched together.

[0021] The common feature points in the three speckle images are stitched and matched using a matching algorithm to obtain a full-field reference image with a lateral field of view of about 8.6 mm. This field of view size includes the hysteresis influence area under the overload ratio of 2.0.

[0022] Furthermore, in step 6, the test load and frequency are set to perform fatigue extension test, and the collected images are saved; 52 micro crack images and speckle images within a load cycle of different crack lengths are synchronously collected by dual micro cameras; the collected images are saved; the micro crack image under the maximum load is observed, and when the crack image is in the field of view, the test is continued; otherwise, the test is stopped if the crack length exceeds the field of view.

[0023] Furthermore, in step 6, the acquisition software receives the force value fed back by the control unit of the fatigue testing machine, and triggers the camera acquisition when the force value is equal to the set acquisition load value.

[0024] Furthermore, in step 6, based on the fatigue crack identification and extension length measurement algorithm, the crack length is calculated and the crack extension rate is analyzed using the microscopic crack image under the maximum load.

[0025] Furthermore, in step 6, the fatigue crack identification and extension length measurement algorithm mainly utilizes high-pass filtering, adaptive threshold filtering and crack skeleton extraction to obtain the crack length.

[0026] In step 7, based on the fatigue crack identification and extension length measurement algorithm, the microcrack image under maximum load is used to calculate the crack length and analyze the crack extension rate; the fatigue crack identification and extension length measurement algorithm uses high-pass filtering, adaptive threshold filtering and crack skeleton extraction to obtain the crack length.

[0027] Furthermore, in step 8, the captured speckle image is corrected, and displacement field data is obtained through DIC analysis. The process is to select two speckle images, one is a reference image collected before the specimen is deformed, and the other is an image after the specimen is deformed, and the displacement of any point before and after the change is found by using a cross-correlation function, so as to obtain the displacement field data of the crack tip; the reference image before deformation is obtained by matching and intercepting with the spliced ​​global reference image.

[0028] Furthermore, in step 10, according to the crack closure situation obtained, combined with the collected microscopic crack image, the evolution of fatigue crack morphology under different loads in the same cycle when the crack is opened and closed is analyzed; according to the crack tip displacement field data combined with the evolution law of the crack closure effect, the strain field analysis of the speckle image is performed to obtain the evolution law of the fatigue crack tip deformation field during the crack closure process of the Q&P steel.

[0029] Preferably, in step 10, the strain field is directly calculated by differentiating the displacement field data; in order to reduce noise, the Green Lagrangian strain tensor is used to describe the strain, and the local least squares transformation technology is used to solve the strain.

[0030] The beneficial effects of the present invention are mainly manifested in:

[0031] A dual-microscope test platform was built to solve many measurement problems. The fatigue crack morphology of Q&P steel can be identified and the length can be measured based on the microscopic camera and crack identification and length measurement algorithms. In particular, before the specimen is loaded, the microscopic camera moves in the crack growth direction with the same step length and takes multiple speckle images. The common feature points in these speckle images are spliced ​​and matched to obtain a full-field reference image with a larger lateral field of view. The field size includes the hysteresis influence zone under large overload ratio conditions. In terms of data analysis, the target speckle image intercepts a suitable reference image from the global speckle image according to the feature points, and the data of the crack tip displacement field are analyzed by DIC technology. Based on the virtual extensometer technology, the evolution law of the crack morphology during the crack closure process of Q&P steel is analyzed. According to the fatigue crack growth rate, crack growth path and morphology, crack tip deformation, and combined with crack tip passivation, closure, and DIMT behavior, the crack growth behavior law of the fatigue crack tip in the overload hysteresis influence zone of Q&P steel is obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a flow chart of the in-situ measurement method of crack tip deformation based on sub-reference image stitching and matching DIC.

[0033] Figure 2 This is the specific operation process of the in-situ measurement method of crack tip deformation based on sub-reference image stitching and matching DIC.

[0034] Figure 3 It is a method to further explain the matching and interception of the corresponding reference image from the global reference image. DETAILED DESCRIPTION

[0035] The present invention will be further described below in conjunction with the accompanying drawings.

[0036] Reference Figure 1 to Figure 3, an in-situ crack tip deformation measurement method based on sub-reference image stitching and matching DIC, comprising the following steps:

[0037] Step 1: Preparation before fatigue crack growth test. Before the experiment, one side of the CT specimen was finely ground with sandpaper along the Y axis (vertical direction of fatigue crack growth) to better capture the position of the fatigue crack tip; the other side was prepared with a professional speckle airbrush to prepare uniform microscopic speckles for analyzing the deformation field at the fatigue crack tip. The speckle is required to be uniform and random to meet the DIC calculation requirements.

[0038] In step 1, the preparatory work before the test includes system debugging, camera calibration and specimen preparation; the camera is calibrated using Zhang's calibration method, and microscopic speckles need to be prepared due to the confidence requirement of ROI in the speckle image; one side of the CT specimen is polished into a glossy surface with a diffuse reflection effect, and microscopic speckles are prepared on the other side; the specimen is installed on a fatigue testing machine, and dual microscopic cameras are installed on both sides, each camera is equipped with a lens.

[0039] Step 2: Pre-test preparation includes system debugging, camera calibration, and specimen preparation. The camera is calibrated using Zhang's calibration method.

[0040] In step 2, the camera used to take microscopic crack images has a resolution of 5472×3648 pixels, and the lateral field of view should be controlled at about 2 mm. The lateral field of view needs to be slightly larger than the length of the fatigue crack, and the captured image is corrected to record the image spatial resolution. Note: The image spatial resolution is required to be around 0.35 μm / pixel. The camera used to take microscopic speckle images has a resolution of 4096×2168 pixels, and the field of view is controlled at about 4 mm×2 mm, which is convenient for subsequent DIC analysis. The captured image is corrected to record the image spatial resolution. Note: The image spatial resolution is required to be around 1 μm / pixel.

[0041] Step 3: The camera takes a picture of the specimen to obtain a reference image. First, the camera is aimed at the notch of the specimen, and then the camera is moved several times with a set step length. An image is collected for each movement. The step length must not be larger than the camera's lateral field of view.

[0042] In step 3, before the fatigue test begins, the microscopic camera moves in the crack growth direction with a moving step length of 2 mm and collects three speckle images according to the specific size of the camera's lateral field of view of 4.6 mm.

[0043] Step 4: Perform image correction on the image acquired in step 3 according to the distortion coefficient determined in step 2, use the matching algorithm to determine the stitching area, and then stitch them in sequence to obtain a full-field reference image. The size of this global reference image includes the size range of the overload hysteresis influence area under the experimental overload condition.

[0044] In step 4, a plurality of speckle images are collected and corrected by a microscope camera, and the plurality of collected images are spliced ​​according to the common feature points. After splicing, a complete full-field reference image is obtained, and the reference image is used to match and intercept the speckle image taken at any time, so as to serve as the reference image when no load is applied at the cycle time.

[0045] In step 4, the process of using the matching algorithm to determine the stitching area is as follows: cut out the middle part of the image in image 1, use the template matching technology to search for the matching area in image 2, expand these two areas horizontally to the entire image and cut out, then the two images are strictly matched, and finally the two images are stitched together.

[0046] In step 4, the common feature points in the three speckle images are spliced ​​and matched to obtain a full-field reference image with a lateral field of view of about 8.6 mm. This field of view size includes the fatigue crack overload hysteresis influence area, which is convenient for subsequent observation and analysis.

[0047] Step 5: Use the micro-motion platform to move the two microscope cameras to the initial position, that is, the pre-crack of the specimen, and make the pre-crack notch at the center of the camera field of view.

[0048] Step 6: Perform fatigue crack growth test, using two microscopic cameras to synchronously collect 52 microscopic crack images and speckle images of both sides of the Q&P steel compact tension CT specimen at different crack lengths in the overload hysteresis influence zone.

[0049] In step 6, the camera tracks the crack tip and takes pictures during the test. When the specimen is subjected to a single overload, the camera collects several crack tip images and corresponding speckle images of different crack lengths in the overload hysteresis influence area, and records the force value corresponding to each image. These images are called target images.

[0050] In step 6, the test load and frequency are set to perform fatigue extension test, and the collected images are saved; 52 micro crack images and speckle images within a load cycle of different crack lengths are synchronously collected by dual micro cameras; the collected images are saved; the micro crack image under the maximum load is observed, and when the crack image is in the field of view, the test is continued; otherwise, the test is stopped if the crack length exceeds the field of view.

[0051] In step 6, the acquisition software receives the force value fed back by the control unit of the fatigue testing machine, and triggers the camera to acquire when the force value is equal to the set acquisition load value.

[0052] Step 7: Based on the crack image collected in step 6, the length data of the crack under different cycles can be obtained, and the fatigue crack growth rate can be calculated.

[0053] In step 7, based on the fatigue crack identification and extension length measurement algorithm, the microcrack image under the maximum load is used to calculate the crack length and analyze the crack extension rate; the fatigue crack identification and extension length measurement algorithm mainly uses high-pass filtering, adaptive threshold filtering and crack skeleton extraction to obtain the crack length.

[0054] In step 7, the microscope camera captures the crack image of the smooth surface, and based on the fatigue crack recognition and extension length measurement algorithm, the fatigue crack extension morphology and crack length of the QP steel are identified and measured to obtain the fatigue crack extension rate.

[0055] In step 7, the fatigue crack identification and extension length measurement algorithm mainly utilizes high-pass filtering, adaptive threshold filtering and crack skeleton extraction to obtain the crack length.

[0056] Step 8: For the target image at the same crack length, select the image at the minimum loading force value and match it with the full-field reference image obtained in step 4, cut out the corresponding reference image, and use this method to obtain the corresponding reference images at other crack lengths.

[0057] In step 8, the captured speckle image is corrected, and displacement field data is obtained through DIC analysis. The process is to select two speckle images, one is a reference image collected before the specimen is deformed, and the other is an image of the specimen after deformation, and use the cross-correlation function to find the displacement of any point before and after the change, so as to obtain the displacement field data of the crack tip; the reference image before deformation is obtained by matching and intercepting with the spliced ​​global reference image.

[0058] Step 9: Use DIC technology to calculate each group of images obtained in step 8 to obtain the displacement field of the crack tip within one cycle under different crack lengths in the overload hysteresis influence zone. Use the cross-correlation function to find the displacement before and after the change of any point, so as to obtain the displacement field data of the crack tip; and further obtain the strain field data according to the relevant algorithm.

[0059] In step 9, based on the crack tip displacement field data combined with the crack closure effect evolution law, the strain field analysis of the speckle image is performed to obtain the evolution law of the fatigue crack tip deformation field during the crack closure process of the Q&P steel.

[0060] In step 9, the main process is to directly calculate the strain field by differentiating the displacement field data. In order to reduce noise, the Green Lagrangian strain tensor is used to describe the strain, and the local least squares transformation technology is used to solve the strain.

[0061] Step 10: Based on the data obtained in step 8, the virtual extensometer technology is combined to analyze and obtain the evolution law of the closure effect of fatigue cracks of different lengths in the overload hysteresis influence zone.

[0062] In step 10, multiple pairs of virtual extensometers are arranged at different positions behind the crack tip, and the fatigue crack opening displacement at different distances behind the crack tip is measured in combination with the displacement field data to obtain the crack opening displacement under different loads in a load cycle, and further analyze the changing trend of the crack closure behavior in the overload hysteresis influence zone.

[0063] According to the result of step 10 and combined with the corresponding micro crack image, the evolution law of the crack morphology at different lengths in the overload hysteresis influence zone is analyzed and obtained.

[0064] In step 10, based on the crack closure status obtained, combined with the collected microscopic crack images, the evolution of the fatigue crack morphology of the Q&P steel at different crack lengths in the overload hysteresis influence zone when the crack is opened and closed is analyzed.

[0065] The contents described in the embodiments of this specification are merely an enumeration of the implementation forms of the inventive concept and are for illustrative purposes only. The protection scope of the present invention should not be considered to be limited to the specific forms described in this embodiment, and the protection scope of the present invention also extends to equivalent technical means that can be thought of by ordinary technicians in this field based on the inventive concept.

Claims

1. A method for in-situ measurement of crack tip deformation based on sub-reference image stitching and matching DIC, characterized in that: The method comprises the following steps: Step 1: Spray the micro-speckle on one side of the specimen and polish the other side with sandpaper. After processing the specimen, place it on the fatigue testing machine; Step 2: Install the camera on the micro-motion platform, calibrate the camera, and determine the distortion coefficient; Step 3: The camera shoots the specimen to obtain a reference image. First, the camera is aimed at the notch of the specimen, and then the camera is moved several times with a set step length. One image is collected for each movement. The step length must not be larger than the camera's lateral field of view. Step 4: The image collected in step 3 is corrected according to the distortion coefficient determined in step 2, and the stitching area is determined using a matching algorithm, and then stitched in sequence to obtain a full-field reference image. The size of the global reference image includes the size range of the overload hysteresis influence area under the experimental overload condition; Step 5: Use the micro-motion platform to move the camera to the initial position and aim at the prefabricated crack of the specimen to take a picture; Step 6: Start the fatigue crack growth test. During the test, the camera tracks the crack tip and takes pictures. When the specimen is subjected to a single overload, the camera collects several crack tip images and corresponding speckle images of different crack lengths in the overload hysteresis influence area, and records the force value corresponding to each image. These images are called target images. Step 7: Based on the crack image collected in step 6, the length data of the crack under different cycles can be obtained, and the fatigue crack growth rate can be calculated; Step 8: For the target image under the same crack length, select the image at the moment of minimum loading force value and match it with the full-field reference image obtained in step 4, cut out the corresponding reference image, and use this method to obtain the corresponding reference images under other crack lengths; Step 9: Use DIC technology to calculate each group of images obtained in step 8 to obtain the displacement field of the crack tip within one cycle under different crack lengths in the overload hysteresis influence zone. Use the cross-correlation function to find the displacement before and after the change of any point, so as to obtain the displacement field data of the crack tip; and further obtain the strain field data according to the relevant algorithm; Step 10: In step 9, a virtual extensometer is arranged at a set position behind the crack tip, and the fatigue crack opening displacement at different distances behind the crack tip is measured in combination with the displacement field data to obtain the crack opening displacement curve under different loads in the load cycle, which can further analyze the changing trend of the crack closure behavior in the overload hysteresis influence zone.

2. The in-situ measurement method for crack tip deformation based on sub-reference image stitching and matching DIC as claimed in claim 1, characterized in that: In step 1, the preparatory work before the test includes system debugging, camera calibration and specimen preparation; the camera is calibrated using Zhang's calibration method, and microscopic speckles need to be prepared due to the confidence requirement of ROI in the speckle image; one side of the CT specimen is polished into a glossy surface with a diffuse reflection effect, and microscopic speckles are prepared on the other side; the specimen is installed on a fatigue testing machine, and dual microscopic cameras are installed on both sides, each camera is equipped with a lens.

3. The in-situ measurement method for crack tip deformation based on sub-reference image stitching and matching DIC as claimed in claim 1 or 2, characterized in that: In the step 2, the camera used to take the microscopic crack image has a resolution of 5472×3648 pixels, and the lateral field of view should be controlled at 2 mm, which is larger than the length of the fatigue crack. The captured image is corrected and the image spatial resolution is recorded. The camera used to take the microscopic speckle image has a resolution of 4096×2168 pixels, and the field of view is controlled at 4 mm×2 mm, which is convenient for subsequent DIC analysis. The captured image is corrected and the image spatial resolution is recorded.

4. The in-situ measurement method of crack tip deformation based on sub-reference image stitching and matching DIC as claimed in claim 1 or 2, characterized in that: In step 4, a plurality of speckle images are collected and corrected by a microscope camera, and the plurality of collected images are spliced ​​according to common feature points. After splicing, a complete full-field reference image is obtained, and the reference image is used to match and intercept the speckle image taken at any time, so as to serve as a reference image when no load is applied at the cycle time.

5. The in-situ measurement method for crack tip deformation based on sub-reference image stitching and matching DIC as claimed in claim 4, characterized in that: In step 4, the process of using the matching algorithm to determine the stitching area is as follows: cut out the middle part of the image in image 1, use the template matching technology to search for the matching area in image 2, expand these two areas horizontally to the entire image and cut out, then the two images are strictly matched, and finally the two images are stitched together.

6. The in-situ measurement method for crack tip deformation based on sub-reference image stitching and matching DIC as claimed in claim 1 or 2, characterized in that: In step 6, the test load and frequency are set to perform fatigue extension test, and the collected images are saved; 52 micro crack images and speckle images within a load cycle of different crack lengths are synchronously collected by dual micro cameras; the collected images are saved; the micro crack image under the maximum load is observed, and when the crack image is in the field of view, the test is continued; otherwise, the test is stopped if the crack length exceeds the field of view.

7. The in-situ measurement method for crack tip deformation based on sub-reference image stitching and matching DIC as claimed in claim 6, characterized in that: In step 6, the acquisition software receives the force value fed back by the control unit of the fatigue testing machine, and triggers the camera acquisition when the force value is equal to the set acquisition load value; in step 7, based on the fatigue crack recognition and extension length measurement algorithm, the microscopic crack image under the maximum load is used to calculate the crack length and analyze the crack extension rate; the fatigue crack recognition and extension length measurement algorithm uses high-pass filtering, adaptive threshold filtering and crack skeleton extraction to obtain the crack length.

8. The in-situ measurement method for crack tip deformation based on sub-reference image stitching and matching DIC as claimed in claim 1 or 2, characterized in that: In step 8, the captured speckle image is corrected, and displacement field data is obtained through DIC analysis. The process is to select two speckle images, one is a reference image collected before the specimen is deformed, and the other is an image of the specimen after deformation, and use the cross-correlation function to find the displacement of any point before and after the change, so as to obtain the displacement field data of the crack tip; the reference image before deformation is obtained by matching and intercepting with the spliced ​​global reference image.

9. The in-situ measurement method for crack tip deformation based on sub-reference image stitching and matching DIC as claimed in claim 1 or 2, characterized in that: In the step 10, the evolution of fatigue crack morphology under different loads in the same cycle when the crack is opened and closed is analyzed according to the crack closure situation obtained and combined with the collected microscopic crack image; the evolution law of the fatigue crack tip deformation field during the crack closure process of Q&P steel is obtained by performing strain field analysis on the speckle image according to the crack tip displacement field data combined with the evolution law of the crack closure effect.

10. The in-situ measurement method of crack tip deformation based on sub-reference image stitching and matching DIC as claimed in claim 9, characterized in that: In step 10, the strain field is directly calculated by differentiating the displacement field data; the Green Lagrange strain tensor is used to describe the strain, and the local least squares transformation technology is used to solve the strain.

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