Dual Microscopic Fatigue Crack and Tip Deformation Field Measurement Method for Hybrid Image Processing

Abstract

A method for measuring double microscopic fatigue cracks and tip deformation fields in hybrid image processing, comprising the following steps: Step 1: Preparation before the fatigue crack propagation test; Step 2: Collect microscopic crack images and speckle images on both sides of the Q&P steel compact tension CT specimen with a set quantity; Step 3: Obtain the fatigue short crack propagation rate; Step 4: Obtain the data of the crack tip displacement field; Step 5: Analyze the evolution law of the closure effect of fatigue short cracks at different lengths in combination with the virtual extensometer technology; Step 6: Analyze the evolution law of the crack morphology during the crack closure process within one load cycle at different lengths in combination with the microscopic crack images; Step 7: Analyze the evolution law of the crack tip strain field during the crack closure process within one load cycle at different lengths in combination with the collected speckle images. The present invention solves the measurement problems such as small tip area, large deformation gradient, and strong material anisotropy in the crack closure process of QP steel.

Description

Technical Field

[0001] The present invention belongs to the technical field of batch crack detection, and relates to a method for measuring double-microscopic fatigue cracks and tip deformation fields based on hybrid image processing technology, which is applicable to the research on the fatigue short crack closure effect and tip deformation field of Q&P steel. Background Art

[0002] Energy conservation, safety, and environmental protection are the core issues in the current development of the modern automotive industry. Quenched and partitioned steel is the third-generation advanced high-strength steel, abbreviated 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 automotive manufacturing steels. During the operation of the main components of an automobile, they are constantly subjected to the excitation of alternating loads. Therefore, in the design, it is necessary to analyze the crack initiation and propagation characteristics under alternating loads.

[0003] Small cracks with a length usually of 1 - 2 mm are called physical short cracks. The crack propagation of these cracks is no longer affected by the material microstructure. In addition to the applied load level, the crack tip closure effect caused by the plastic zone and the deformation field distribution are the main factors affecting the fatigue crack propagation.

[0004] However, the measurement of the deformation field in the tiny area at the crack tip of Q&P steel has the characteristics of complex microstructure, stress, and strain distribution in the crack tip area, large change gradient, rapid evolution over time, and high measurement accuracy requirements. The present invention uses DIC, DIP, and virtual extensometer technologies to provide a method for measuring double-microscopic fatigue cracks and tip deformation fields, solving the difficulties existing in the measurement process and facilitating the subsequent research on the closure effect on the evolution of fatigue short cracks and tip deformation fields during the crack closure process of Q&P steel. Summary of the Invention

[0005] In order to overcome the deficiencies of the prior art, the present invention provides a method for measuring double-microscopic fatigue cracks and tip deformation fields by hybrid image processing, solving the measurement problems such as small area at the tip of fatigue short cracks, large deformation gradient, and strong material anisotropy during the crack closure process of QP steel, and being applicable to the research on the fatigue short crack closure effect and tip deformation field of Q&P steel.

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

[0007] A method for measuring double-microscopic fatigue cracks and tip deformation fields by hybrid image processing, the method comprising the following steps:

[0008] Step 1: Preparation before the fatigue crack propagation test;

[0009] Step 2: Conduct a fatigue crack growth test. Use a dual-microscope camera to synchronously collect 50 microscopic crack images and 50 microscopic speckle images on both sides of the Q&P steel compact tension CT specimen at different crack lengths.

[0010] Step 3: Use DIP technology to identify and measure the fatigue short crack growth morphology and crack length of the captured microscopic crack images of QP steel, and obtain the fatigue short crack growth rate.

[0011] Step 4: Use microscopic DIC technology to analyze the displacement field at the tip of the fatigue short crack of the captured microscopic speckle images, and obtain the data of the displacement field at the crack tip.

[0012] Step 5: Analyze the evolution law of the closure effect of fatigue short cracks at different lengths based on the data obtained in Step 4 combined with virtual extensometer technology.

[0013] Step 6: Based on the results of Step 5 combined with the captured microscopic crack images, analyze the evolution law of the crack morphology during the crack closure process within one load cycle at different lengths.

[0014] Step 7: Based on the results of Step 5 combined with the captured microscopic speckle images, analyze the evolution law of the strain field at the crack tip during the crack closure process within one load cycle at different lengths.

[0015] Furthermore, in Step 1, the pre-test preparation work includes system debugging, camera calibration, specimen preparation. Calibrate the camera using Zhang's calibration method. Due to the confidence requirement of the ROI in the microscopic speckle image, it is necessary to prepare microscopic speckles. Grind one side of the CT specimen into a smooth surface with a diffuse reflection effect and prepare microscopic speckles on the other side. Install the specimen on the fatigue testing machine and install a dual-microscope camera on both sides, with each camera equipped with a lens.

[0016] Still further, in Step 1, the camera for taking microscopic crack images has a resolution of 5472×3648 pixel, and the horizontal field of view should be controlled within 2 mm. The horizontal field of view needs to be slightly larger than the length of the fatigue short crack, and the captured images should be corrected, and the image spatial resolution should be recorded. The image spatial resolution requirement is 0.35 μm / pixel. The camera for taking microscopic speckle images has a resolution of 4096×3000 pixel, and the field of view is controlled within 4 mm×2 mm for subsequent DIC analysis, and the captured images should be corrected, and the image spatial resolution should be recorded. The image spatial resolution requirement is 1 μm / pixel.

[0017] Further, in step 2, a microscopic camera is used to collect and correct microscopic speckle images as reference images. Set the test load, frequency, etc. to conduct a fatigue crack growth test. Save the collected images; synchronously collect 50 microscopic crack images and 50 microscopic speckle images within one load cycle at different crack lengths through a double microscopic camera. Save the collected images; observe the microscopic crack images at the maximum force value. If the crack image is within the field of view, continue the test; otherwise, if the crack length exceeds the field of view, stop the test.

[0018] Preferably, in step 2, the acquisition software receives the force value fed back by the control unit of the fatigue testing machine. When the force value is equal to the set acquisition load value, the camera is triggered to collect images.

[0019] Further, in step 3, based on the fatigue short crack identification and crack growth length measurement algorithm, the maximum force value microscopic crack images are used to calculate the crack length and analyze the short crack growth rate.

[0020] In step 3, the fatigue short crack identification and crack growth length measurement algorithm mainly uses high-pass filtering, adaptive threshold filtering, and crack skeleton extraction to obtain the crack length.

[0021] Further, in step 4, the captured microscopic speckle images are corrected, and displacement field data is obtained through DIC analysis. The process is as follows: Select two microscopic speckle images, one is the reference image collected before the specimen deformation, and the other is the image after the specimen deformation. 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 at the crack tip.

[0022] In step 5, virtual extensometers are arranged at 20μm, 40μm, and 80μm behind the crack tip. Combining with the displacement field data, the fatigue crack opening displacement at different distances behind the crack tip is measured, and the crack opening displacement under different loads in one load cycle is obtained, and further analyze the influence of crack closure on fatigue crack growth.

[0023] In step 6, according to the obtained crack closure situation, combined with the collected microscopic crack images, analyze the evolution of the fatigue short crack morphology under different loads within the same cycle when the crack opens and closes.

[0024] In step 7, according to the short crack tip displacement field data combined with the evolution law of the crack closure effect, through the strain field analysis of the microscopic speckle images, the evolution law of the deformation field at the fatigue short crack tip during the crack closure process of Q&P steel is obtained.

[0025] In step 7, the main process is to directly calculate the strain field through the differentiation of displacement field data. To reduce noise, the Green-Lagrange strain tensor is used to describe the strain, and the local least squares transformation technique is used to solve for the strain.

[0026] The beneficial effects of the present invention are mainly manifested in: building a dual-microscope test platform, solving many measurement problems, being able to identify the morphology and measure the length of fatigue short cracks in Q&P steel according to the DIP technology. Analyzing the data of the displacement field at the crack tip according to the DIC technology. Based on the virtual extensometer technology, analyzing the evolution law of the crack morphology during the crack closure process of Q&P steel. According to the displacement field data at the short crack tip and the evolution law of the crack closure effect, obtaining the evolution law of the deformation field at the fatigue short crack tip during the crack closure process of Q&P steel. Description of the Drawings

[0027] Figure 1 is a flowchart of a dual-microscope fatigue crack and tip deformation field measurement method for hybrid image processing. Detailed Embodiments

[0028] The present invention will be further described below with reference to the accompanying drawings.

[0029] Refer to Figure 1 , a dual-microscope fatigue crack and tip deformation field measurement method for hybrid image processing, comprising the following steps:

[0030] Step 1: Preparation before the fatigue crack propagation test.

[0031] In step 1, the preparation before the test includes system debugging, camera calibration, and specimen preparation; the Zhang's calibration method is used to calibrate the camera. Due to the confidence requirement of the ROI in the microscopic speckle image, it is necessary to prepare microscopic speckles. One side of the CT specimen is polished into a smooth surface with a diffuse reflection effect, and microscopic speckles are prepared on one side. The specimen is installed on the fatigue testing machine, and dual-microscopes are installed on both sides. Each camera is equipped with a lens;

[0032] In step 1, the camera for taking microscopic crack images has a resolution of 5472×3648 pixel, and the horizontal field of view should be controlled at about 2 mm. The horizontal field of view needs to be slightly larger than the length of the fatigue short crack, and the captured images are corrected, and the image spatial resolution is recorded. The image spatial resolution requirement is about 0.35 μm / pixel. The camera for taking microscopic speckle images has a resolution of 4096×3000 pixel, and the field of view is controlled at about 4 mm×2 mm for subsequent DIC analysis, and the captured images are corrected, and the image spatial resolution is recorded. The image spatial resolution requirement is about 1 μm / pixel;

[0033] Step 2: Conduct a fatigue crack growth test. Use a dual-microscope camera to synchronously collect 50 microscopic crack images and 50 microscopic speckle images on both sides of the Q&P steel compact tension CT specimen at different crack lengths.

[0034] In step 2, use a microscope camera to collect and correct the microscopic speckle images as reference images. Set the test load, frequency, etc. to conduct the fatigue growth test. Save the collected images. Synchronously collect 50 microscopic crack images and 50 microscopic speckle images within one load cycle at different crack lengths through a dual-microscope camera. Save the collected images. Observe the microscopic crack images at the maximum force value. If the crack image is within the field of view, continue the test. Otherwise, if the crack length exceeds the field of view, stop the test.

[0035] In step 2, the acquisition software receives the force value feedback from the control unit of the fatigue testing machine. When the force value is equal to the set acquisition load value, trigger the camera to acquire images.

[0036] Step 3: Use DIP technology to identify and measure the fatigue short crack growth morphology and crack length of the captured microscopic crack images of QP steel, and obtain the fatigue short crack growth rate.

[0037] In step 3, based on the fatigue short crack identification and growth length measurement algorithm, use the microscopic crack image at the maximum force value to calculate the crack length and analyze the short crack growth rate.

[0038] In step 3, the fatigue short crack identification and growth length measurement algorithm mainly uses high-pass filtering, adaptive threshold filtering, and crack skeleton extraction to obtain the crack length.

[0039] Step 4: Use microscopic DIC technology to analyze the displacement field at the tip of the fatigue short crack in the captured microscopic speckle images, and obtain the data of the displacement field at the crack tip.

[0040] In step 4, correct the captured microscopic speckle images. Obtain the displacement field data through DIC analysis. The main process is to select two microscopic speckle images, one is the reference image collected before the specimen deformation, and the other is the image after the specimen deformation. Use the cross-correlation function to find the displacement of any point before and after the change, so as to obtain the data of the displacement field at the crack tip.

[0041] Step 5: Analyze and obtain the evolution law of the closure effect of fatigue short cracks at different lengths based on the data obtained in step 4 in combination with the virtual extensometer technology.

[0042] In step 5, 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 influence of crack closure on fatigue crack propagation.

[0043] Step 6: Based on the results of step 5 and the microcrack image, analyze and obtain the evolution law of crack morphology during crack closure at different lengths within a load cycle.

[0044] In step 6, according to the crack closure status obtained, combined with the collected microscopic crack images, the evolution of the Q&P steel fatigue short crack morphology under different loads in the same cycle when the crack is opened and closed is analyzed.

[0045] Step 7: Based on the results of step 5 and the displacement field data, the evolution law of the strain field at the crack tip during the crack closure process within a load cycle at different lengths is analyzed and obtained.

[0046] In step 7, based on the short crack tip displacement field data combined with the crack closure effect evolution law, the strain field analysis of the microscopic speckle image is performed to obtain the evolution law of the fatigue short crack tip deformation field during the crack closure process of the Q&P steel.

[0047] In step 7, 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.

[0048] The contents described in the embodiments of this specification are merely enumerations of 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 measuring double microscopic fatigue cracks and tip deformation fields in hybrid image processing, characterized in that, the method comprises the following steps: Step 1: Preparation before the fatigue crack growth test; Step 2: Conduct a fatigue crack growth test, and use a double microscopic camera to synchronously collect 50 microscopic crack images and 50 speckle images on both sides of a Q&P steel compact tension CT specimen at different crack lengths; Step 3: Use DIP technology to identify and measure the growth morphology and crack length of QP steel fatigue short cracks in the captured microscopic crack images, and obtain the fatigue short crack growth rate; Step 4: Use microscopic DIC technology to analyze the displacement field at the tip of the fatigue short crack in the captured speckle images, and obtain the data of the crack tip displacement field; Step 5: Analyze the evolution law of the closure effect of fatigue short cracks at different lengths in combination with the virtual extensometer technology according to the data obtained in Step 4; Step 6: Analyze the evolution law of the crack morphology during crack closure within one load cycle at different lengths in combination with the captured microscopic crack images according to the results of Step 5; Step 7: Analyze the evolution law of the crack tip strain field during crack closure within one load cycle at different lengths in combination with the captured speckle images according to the results of Step 5.

2. The method for measuring double microscopic fatigue cracks and tip deformation fields in hybrid image processing according to claim 1, characterized in that, in the said Step 1, the preparation before the test includes system debugging, camera calibration, specimen preparation. The camera is calibrated using the 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 smooth surface with a diffuse reflection effect, and microscopic speckles are prepared on one side. The specimen is installed on the fatigue testing machine, and double microscopic cameras are installed on both sides, and each camera is equipped with a lens.

3. The method for measuring double microscopic fatigue cracks and tip deformation fields in hybrid image processing according to claim 2, characterized in that, in the said Step 1, the camera for taking microscopic crack images has a resolution of 5472×3648 pixel, the horizontal field of view should be controlled within 2 mm, the horizontal field of view needs to be slightly larger than the length of the fatigue short crack, and the captured images are corrected, and the image spatial resolution is recorded. The image spatial resolution requirement is 0.35 μm / pixel. The camera for taking microscopic speckle images has a resolution of 4096×3000 pixel, and the field of view is controlled within 4 mm×2 mm for subsequent DIC analysis, and the captured images are corrected, and the image spatial resolution is recorded. The image spatial resolution requirement is 1 μm / pixel.

4. The method for measuring double microscopic fatigue cracks and tip deformation fields in hybrid image processing according to any one of claims 1 to 3, characterized in that, In step 2, a microscopic camera is used to collect and correct the speckle images, which are used as reference images. The test load and frequency are set to conduct a fatigue propagation test, and the collected images are saved. 50 microscopic crack images and 50 speckle images within one load cycle at different crack lengths are synchronously collected by a double microscopic camera, and the collected images are saved. Observe the microscopic crack images at the maximum force value. If the crack image is within the field of view, continue the test; otherwise, if the crack length exceeds the field of view, stop the test.

5. The method for measuring double microscopic fatigue cracks and tip deformation fields by hybrid image processing according to claim 4, characterized in that in step 2, the acquisition software receives the force value fed back by the control unit of the fatigue testing machine. When the force value is equal to the set acquisition load value, the camera is triggered to acquire images.

6. The method for measuring double microscopic fatigue cracks and tip deformation fields by hybrid image processing according to any one of claims 1 to 3, characterized in that in step 3, based on the fatigue short crack identification and propagation length measurement algorithm, the maximum force value microscopic crack image is used to calculate the crack length and analyze the short crack propagation rate.

7. The method for measuring double microscopic fatigue cracks and tip deformation fields by hybrid image processing according to any one of claims 1 to 3, characterized in that in step 3, the fatigue short crack identification and propagation length measurement algorithm uses high-pass filtering, adaptive threshold filtering, and crack skeleton extraction to obtain the crack length.

8. The method for measuring double microscopic fatigue cracks and tip deformation fields by hybrid image processing according to any one of claims 1 to 3, characterized in that in step 4, the captured speckle images are corrected, and the displacement field data is obtained through DIC analysis. The process is as follows: Two speckle images are selected, one is the reference image collected before the specimen is deformed, and the other is the image after the specimen is deformed. The cross-correlation function is used to find the displacement of any point before and after the change, so as to obtain the crack tip displacement field data.

9. The method for measuring double microscopic fatigue cracks and tip deformation fields by hybrid image processing according to any one of claims 1 to 3, characterized in that in step 5, virtual extensometers are arranged at 20μm, 40μm, and 80μm behind the crack tip. Combining the displacement field data, the fatigue crack opening displacement at different distances behind the crack tip is measured, and the crack opening displacement under different loads within one load cycle is obtained. Further analyze the influence of crack closure on fatigue crack propagation; in step 6, according to the obtained crack closure situation, combined with the collected microscopic crack images, analyze the evolution of the fatigue short crack morphology under different loads within the same cycle when the crack opens and closes.

10. The method for measuring double microscopic fatigue cracks and tip deformation fields by hybrid image processing according to any one of claims 1 to 3, characterized in that in step 7, according to the short crack tip displacement field data combined with the evolution law of the crack closure effect, through strain field analysis of the speckle images, the evolution law of the fatigue short crack tip deformation field during the crack closure process of Q&P steel is obtained; In step 7, the process directly calculates the strain field through the differentiation of displacement field data. To reduce noise, the Green-Lagrange strain tensor is used to describe the strain, and the local least squares transformation technique is used to solve for the strain.

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