An in-situ measurement method of cyclic plastic zone at crack tip of Q&P steel based on micro-DIC

Abstract

An in-situ measurement method of the cyclic plastic zone at the crack tip of Q&P steel based on microscopic DIC uses image stitching technology to obtain a full-field reference image, and cooperates with template matching technology to obtain a reference image corresponding to any position on the crack extension path, solving the problem that the field of view of the microscope camera is small and DIC cannot be used to continuously collect speckle images of long crack tips online. Then the in-situ strain field data is coupled with the corresponding load information to obtain the force-strain curve of each point in the crack tip area, and the curve feature judgment algorithm is used to judge the area to which each point belongs (cyclic plastic zone, monotonic plastic zone or elastic zone), and finally the in-situ measurement of the cyclic plastic zone is realized. The present invention is a low-cost in-situ measurement method of the cyclic plastic zone, which can obtain the cyclic plastic zone of the crack tip at any crack length during the crack extension process of Q&P steel.

Description

Technical Field

[0001] The invention belongs to the field of metal fatigue testing, and provides a method for in-situ measurement of a cyclic plastic zone at a fatigue crack tip, which is suitable for accurate detection of the cyclic plastic zone at a crack tip during a Q&P steel fatigue test. Background Art

[0002] Energy saving, environmental protection and lightweight are the current development directions of automotive materials. Advanced high-strength steel has become a new material for automobile manufacturing in recent years. Among them, quenching-partitioning steel is the third generation of advanced high-strength steel with phase transformation induced plasticity, referred to as Q&P steel. It takes into account both cost and performance and has gradually become a representative new material. Fatigue damage is the main failure form of parts serving under alternating loads. Under the action of alternating loads, Q&P steel will form a tiny plastic zone in the crack tip area, and there is an even smaller cyclic plastic zone inside it. The morphology and internal response characteristics of the cyclic plastic zone are the main factors affecting the initiation and propagation of fatigue cracks. Therefore, studying the behavior of the cyclic plastic zone of Q&P steel during fatigue crack propagation is of great significance to revealing its fatigue crack propagation characteristics. Determining the size of the cyclic plastic zone is the basis for research, but the existing methods are complex to operate, the instruments are expensive, and it is difficult to achieve in-situ measurement. They are unable to characterize the dynamic response characteristics of the cyclic plastic zone during crack propagation. Therefore, there is still a lack of a low-cost in-situ measurement method for the cyclic plastic zone to study the dynamic evolution behavior of the cyclic plastic zone. Summary of the invention

[0003] In order to overcome the shortcomings of the existing technology, the present invention provides an in-situ measurement method of the cyclic plastic zone of the crack tip of Q&P steel based on microscopic DIC, which uses image stitching technology to obtain a full-field reference image, and cooperates with template matching technology to obtain a reference image corresponding to any position on the crack extension path, solving the problem that the field of view of the microscope camera is small and DIC cannot be used to continuously collect speckle images of the long crack tip online, thereby obtaining the microscopic in-situ strain field data of the crack tip during the crack extension process. The in-situ strain field data is coupled with the corresponding load information to obtain the force-strain curve of each point in the crack tip area, and the curve feature judgment algorithm is used to judge the area (cyclic plastic zone, monotonic plastic zone or elastic zone) to which each point belongs, and finally the in-situ measurement of the cyclic plastic zone is realized.

[0004] The technical solution adopted by the present invention to solve its technical problem is:

[0005] A method for in-situ measurement of the cyclic plastic zone at the crack tip of Q&P steel based on micro-DIC, the method comprising the following steps:

[0006] Step 1: Spray the surface of the specimen with microscopic speckles and place it on a fatigue testing machine;

[0007] Step 2: Install the camera on the micro-motion platform, calibrate the camera, and determine the distortion coefficient;

[0008] Step 3: The camera takes a picture of the specimen and moves the camera several times with a set step length. One image is collected for each movement. The step length must not be larger than the horizontal field of view of the camera.

[0009] 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;

[0010] 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;

[0011] Step 6: Start the fatigue crack growth test. During the test, the camera tracks the crack tip and takes pictures. Several crack tip images in the next cycle with different crack lengths are collected, and the force value corresponding to each image is recorded. These images are called target images.

[0012] Step 7: For the target image under the same crack length, select the image with the smallest force value to match with the full-field reference image obtained in step 4, cut out the corresponding reference image, and use this method to obtain the reference images corresponding to the remaining cycle numbers;

[0013] Step 8: Use DIC technology to calculate each group of images obtained in step 7 to obtain the strain field within one cycle of the crack tip under different crack lengths, that is, the in-situ strain field;

[0014] Step 9: Couple the strain field with the corresponding force value to obtain the force-strain curve of each pixel point within one cycle;

[0015] Step 10: Traverse all pixel points, use the curve feature judgment algorithm to analyze the force-strain curve characteristics, and judge the region to which the pixel point belongs according to the curve characteristics, namely, the elastic region, the monotonic plastic region, and the cyclic plastic region;

[0016] Step 11: Count the number of pixels located in the cyclic plastic zone under different crack lengths, and finally obtain the size of the cyclic plastic zone at the crack tip under different crack lengths.

[0017] Furthermore, in step 4, a matching algorithm is used to determine the stitching area to solve the image inaccuracy caused by the up and down offset of the micro-motion platform during the horizontal movement of the camera. The process is as follows: the middle part of the image is captured in image 1, and the template matching technology is used to search for the matching area in image 2. The two areas are horizontally expanded to the full image and captured respectively, so that the two images are strictly matched, and finally the two images are stitched together.

[0018] The beneficial effects of the present invention are mainly manifested in: using image stitching and template matching technology to achieve the measurement of the microscopic strain field of the crack tip at any position during the crack propagation process, and coupling the strain field with the load information to obtain the force-strain curve of each pixel point, dividing the elastic zone, monotonic plastic zone, and cyclic plastic zone through the curve feature judgment algorithm, and achieving in-situ measurement of the cyclic plastic zone. The cyclic plastic zone at the crack tip under any crack length can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is the flow chart of in-situ strain field measurement.

[0020] Figure 2 This is the flow chart of obtaining the full-field reference image.

[0021] Figure 3 It is a schematic diagram of image stitching area alignment.

[0022] Figure 4 It is the flow chart of strain field calculation based on matching algorithm.

[0023] Figure 5 It is a flow chart of cyclic plastic zone measurement. DETAILED DESCRIPTION

[0024] The present invention will be further described below in conjunction with the accompanying drawings.

[0025] Reference Figure 1 to Figure 5 , a method for in-situ measurement of the cyclic plastic zone at the crack tip of Q&P steel based on micro-DIC, comprising the following steps:

[0026] Pre-test preparation part:

[0027] Step 1: Spray the surface of the specimen with microscopic speckles and place it on a fatigue testing machine;

[0028] Step 2: Install the camera on the micro-motion platform, calibrate the camera, and determine the distortion coefficient;

[0029] Step 3: If Figure 2 As shown, the camera takes pictures of the test piece, and moves the camera several times with a set step length, and collects an image once for each movement. The step length shall not be larger than the horizontal field of view of the camera;

[0030] Step 4: Correct the image acquired in step 3 according to the distortion coefficient determined in step 2, such as Figure 3 As shown, a matching algorithm is used to determine the stitching area, and then stitching is performed in sequence to obtain a full-field reference image; fatigue crack growth test:

[0031] The matching algorithm is used to determine the stitching area to solve the image misalignment caused by the up and down offset of the micro-motion platform during the horizontal movement of the camera. The process is as follows: the middle part of the image is intercepted in image 1, and the template matching technology is used to search for the matching area in image 2. The two areas are horizontally expanded to the full image and intercepted respectively, so that the two images are strictly matched, and finally the two images are stitched;

[0032] 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;

[0033] Step 6: Start the fatigue crack growth test. During the test, the camera tracks the crack tip and takes pictures. Several crack tip images in the next cycle with different crack lengths are collected, and the force value corresponding to each image is recorded. These images are called target images.

[0034] Data processing part:

[0035] Step 7: For the target image under the same crack length, select the image with the smallest force value and match it with the full-field reference image obtained in step 4, such as Figure 4 As shown, the corresponding reference image is cut out, and the reference images corresponding to the remaining cycle numbers are obtained using this method;

[0036] Step 8: Use DIC technology to calculate each group of images obtained in step 7 to obtain the strain field within one cycle of the crack tip under different crack lengths, that is, the in-situ strain field;

[0037] Step 9: Couple the strain field with the corresponding force value to obtain the force-strain curve of each pixel point within one cycle;

[0038] Step 10: Figure 5 As shown, all pixel points are traversed, and the force-strain curve characteristics are analyzed using the curve feature judgment algorithm. The region to which the pixel point belongs is determined according to the curve characteristics, namely, the elastic region, the monotonic plastic region, and the cyclic plastic region;

[0039] Step 11: Count the number of pixels located in the cyclic plastic zone under different crack lengths, and finally obtain the size of the cyclic plastic zone at the crack tip under different crack lengths.

[0040] This embodiment uses image stitching and template matching technology to achieve microscopic strain field measurement of crack tip at any position during crack propagation, and couples the strain field with load information to obtain the force-strain curve of each pixel point. The elastic zone, monotonic plastic zone, and cyclic plastic zone are divided by the curve feature judgment algorithm to achieve in-situ measurement of the cyclic plastic zone. The cyclic plastic zone at the crack tip under any crack length can be obtained.

Claims

1. A method for in-situ measurement of the cyclic plastic zone at the crack tip of Q&P steel based on micro-DIC, characterized in that: The method comprises the following steps: Step 1: Spray the surface of the specimen with microscopic speckles and place it on a 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 takes a picture of the specimen and moves the camera several times with a set step length. One image is collected for each movement. The step length must not be larger than the horizontal field of view of the camera. 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; Fatigue crack growth test: 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. Several crack tip images in the next cycle with different crack lengths are collected, and the force value corresponding to each image is recorded. These images are called target images. Data processing: Step 7: For the target image under the same crack length, select the image with the smallest force value to match with the full-field reference image obtained in step 4, cut out the corresponding reference image, and use this method to obtain the reference images corresponding to the remaining cycle numbers; Step 8: Use DIC technology to calculate each group of images obtained in step 7 to obtain the strain field within one cycle of the crack tip under different crack lengths, that is, the in-situ strain field; Step 9: Couple the strain field with the corresponding force value to obtain the force-strain curve of each pixel point within one cycle; Step 10: Traverse all pixel points, use the curve feature judgment algorithm to analyze the force-strain curve characteristics, and judge the area to which the pixel point belongs according to the curve characteristics, that is, the elastic area, the monotonic plastic area, and the cyclic plastic area; Step 11: Count the number of pixels located in the cyclic plastic zone under different crack lengths, and finally obtain the size of the cyclic plastic zone at the crack tip under different crack lengths.

2. The in-situ measurement method of the cyclic plastic zone at the crack tip of Q&P steel based on micro-DIC as claimed in claim 1, characterized in that: In step 4, a matching algorithm is used to determine the stitching area to solve the image misalignment caused by the up and down offset of the micro-motion platform during the horizontal movement of the camera. The process is as follows: the middle part of the image is captured in image 1, and the template matching technology is used to search for the matching area in image 2. The two areas are horizontally expanded to the full image and captured respectively, so that the two images are strictly matched, and finally the two images are stitched together.

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