Crack tip plastic zone size out-of-plane displacement field measurement method

Abstract

The invention discloses a crack tip plastic zone size out-of-plane displacement field measurement method, which is characterized in that after a crack tip out-of-plane displacement field is calculated based on out-of-plane displacement field change generated by crack tip plastic deformation, rigid body displacement is removed. Taking the position of the crack tip as an original point, drawing a straight line along the crack propagation direction, calculating the out-of-plane displacement change on the straight line, marking a point with the maximum out-of-plane displacement change gradient in the crack propagation direction, and taking the distance from the crack tip to the maximum out-of-plane displacement change gradient on the straight line as the plastic zone size of the crack tip. According to the method, only the change of the out-of-plane displacement field near the crack tip needs to be measured, the yield criterion does not need to be considered, the strain field and the stress field of the crack tip do not need to be measured, mechanical property parameters (yield strength, elastic modulus and the like) of a material do not need to be measured, the measurement process of the plastic zone size is simplified, and the measurement efficiency of the crack tip plastic zone is improved; and the method is convenient and simple.

Description

Technical Field

[0001] The invention belongs to the field of fracture mechanics and its engineering application, and specifically relates to a method for measuring the off-surface displacement field of the crack tip plastic zone size. Background Art

[0002] How to quantitatively characterize the behavior of cracks has always been one of the most important issues in fracture mechanics. For elastic-plastic fracture mechanics, parameters such as the crack tip plastic zone size (PZS), crack tip opening displacement, and J integral are usually used to characterize crack growth behavior to replace the stress intensity factor theory in linear elastic fracture mechanics. Among all these parameters, the size of the plastic zone at the crack tip has a strong correlation with the fracture characteristics of the material. The crack tip plastic zone has obvious physical significance and can sometimes be directly observed in experiments. Therefore, the plastic zone size is often the preferred parameter for characterizing the crack growth behavior in elastic-plastic fracture mechanics. The research and characterization of the plastic zone size mainly include the Irwin and Dugdale models, but these models are limited to the evaluation of the plastic zone size for small-scale yield and cracks in infinitely wide plates. In addition, the quantitative evaluation of the plastic zone size is also affected by the selected yield criterion. For unfamiliar materials, it is necessary to re-measure the material's (mechanical) performance parameters (such as yield strength, elastic modulus, etc.) in order to calculate the size of the plastic zone at the crack tip. Sometimes, finite element or other data processing software is also needed. The whole process is complicated and computationally intensive, and requires a lot of time and economic costs. Therefore, a method is needed that is not affected by the yield criterion, does not require the measurement of the mechanical properties of the material, and can quickly measure the size of the plastic zone at the crack tip.

[0003] The Digital Image Correlation (DIC) method is an optical measurement method that obtains the displacement change of the specimen by processing the grayscale change of the specimen surface. The basic principle of the DIC method is digital image processing. Given a point and its identity (grayscale gradient) in the undeformed image, the correlation function is used to search / track the point with the maximum similarity identity in the deformed image, and the displacement is calculated by the position of the point before and after deformation. In recent years, with the emergence of high-resolution digital equipment and the development of image correlation algorithms, the DIC method has been most widely used to measure the crack tip field information in fracture mechanics due to its advantages such as simple operation and economy, including measuring the crack tip plastic zone size (PZS), identifying the crack tip position, stress intensity factor measurement (K), crack opening displacement (COD), J integral measurement, etc. Among the measurements of these parameters, the measurement of the crack tip plastic zone size is undoubtedly the most needed. The DIC method for measuring the crack tip field is based on displacement deformation. The traditional method of using DIC to measure the size of the crack tip plastic zone is first based on the strain field calculated by DIC, then the stress field is calculated by strain, and the plastic area is obtained by substituting the yield stress. However, the displacement system error of DIC measurement is proportional to the second-order displacement gradient, and the strain system error is proportional to the third-order displacement gradient. Therefore, DIC is difficult to measure small strains (system noise makes it difficult for DIC to distinguish small strains). In addition, since 3D DIC can only measure in-plane strain, the out-of-plane displacement will also affect the measurement of in-plane strain.

[0004] In summary, the traditional method of measuring the size of the plastic zone at the crack tip requires measuring the mechanical properties of the material and selecting the appropriate yield criterion according to the specimen standard, loading conditions, etc. The measurement results are greatly affected by subjective factors. In addition, the method of using DIC to measure strain and then obtain the size of the plastic zone will introduce a large error. Therefore, the traditional method is not conducive to the efficient, rapid and accurate measurement of the size of the plastic zone. Summary of the invention

[0005] In order to address the shortcomings of existing measurement methods, the present application proposes a method for measuring the out-of-plane displacement field of the plastic zone size at the crack tip. The method uses a dual-camera image acquisition device to collect the speckle image of the specimen before loading as a reference pattern; then a load is applied to the specimen, and the speckle changes of the crack specimen during the crack propagation process after loading are recorded. The digital image correlation method is used to calculate the out-of-plane displacement field of the crack tip after loading, and the size of the plastic zone at the crack tip is calculated based on the change in the out-of-plane displacement of the crack tip.

[0006] The technical solution adopted by the present invention is as follows:

[0007] A method for measuring the off-surface displacement field of the size of the crack tip plastic zone comprises the following steps:

[0008] Step 1: Prepare the test piece;

[0009] Step 2: Collect an image of the test piece before deformation at the crack tip as a reference image;

[0010] Step 3: Apply a load to the test piece to cause crack initiation and expansion, and collect an image of the crack tip after deformation as the target image;

[0011] Step 4: Based on the reference image and the target image, the in-plane displacement field of the area to be measured is obtained by the digital image correlation method. Based on the in-plane displacement field, three-dimensional space reconstruction is performed through the binocular vision system. The out-of-plane displacement field is calculated using the reconstruction result, and then the rigid body displacement is removed to retain the out-of-plane displacement field generated by the deformation;

[0012] Step 5: Based on the off-plane displacement field, calculate the off-plane displacement gradient along the crack propagation direction with the crack tip as the starting point, mark the point with the largest off-plane displacement gradient in the crack propagation direction, calculate the distance between the crack tip starting point and the point with the largest off-plane displacement gradient, and use this distance as the plastic zone size.

[0013] Furthermore, in step 4, the method for obtaining the off-plane displacement field of the area to be measured by the digital image correlation method is as follows: the area containing the crack tip is recorded as the area of ​​interest, the area of ​​interest of the reference image is divided into several sub-areas, and the center point P (x 0 ,y 0 ), the gray value feature of sub-area I is taken as P(x 0 ,y 0 ) point identification; search the target image corresponding to the center point P (x 0 ,y 0 ) position; search the sub-region with the highest correlation with sub-region I in the target image through the correlation function as the matching sub-region I', mark the center point P'(x' 0 ,y' 0 ), according to the center point P(x 0 ,y 0 ) matches the center point P'(x') of the target image sub-area I' 0 ,y' 0 ), and calculate the displacement of any point Q(x,y) and Q'(x',y') in the sub-area according to the displacement mapping function.

[0014] Further, the related function expression is:

[0015]

[0016] Where P is the direction vector of the displacement mapping function, f(x,y) is the grayscale intensity of the reference image at the coordinate (x,y), g(x',y') is the grayscale intensity of the target image at the coordinate (x',y'), and f m and g m is the average grayscale intensity of the reference image and the target image, and M is the sub-region size.

[0017] Furthermore, the displacement mapping relationship between any point Q(x, y) in the reference image and any point Q'(x', y') in the target image is expressed as:

[0018]

[0019] Among them, x 0 ,y 0 is the coordinate point of the center point P in the X and Y directions, u and v are the displacement components of the sub-area I in the X and Y directions, Δx and Δy are the distances from the center point P of the sub-area I to Q in the X and Y directions; u x 、u y 、v x 、v y is the displacement gradient.

[0020] Furthermore, the method for calculating the crack tip off-plane displacement field is:

[0021] Based on the in-plane displacement between the target image and the reference image, three-dimensional space reconstruction is performed using the dual-camera stereo imaging ranging principle to calculate the depth difference between each point in the target image and the reference image. The depth difference is the out-of-plane displacement, and the out-of-plane displacement of all points in the target image constitutes the out-of-plane displacement field.

[0022] Furthermore, the method for calculating the size of the plastic zone in step 5 is: taking the crack tip as the starting point, draw a straight line along the crack propagation direction, calculate the gradient of the off-surface displacement change on the straight line, mark the point with the maximum off-surface displacement gradient, calculate the distance between the starting point and the point with the maximum off-surface displacement gradient, and obtain the size of the plastic zone.

[0023] Furthermore, a uniform random speckle pattern needs to be produced on the surface of the specimen.

[0024] Furthermore, a method for producing a speckle pattern is as follows: the surface of the specimen is covered with matte white paint, and an alcohol-graphite mixture is sprayed on the surface of the specimen with a spray gun, thereby obtaining a high-contrast, uniform, and randomly distributed speckle pattern.

[0025] Furthermore, the sub-region contains 3-10 scattered spots, and the search step is set to 1 / 10-1 / 4 of the sub-region size.

[0026] Furthermore, the spot size occupies 5-10 pixels, and the spot spacing is also 5-10 pixels.

[0027] Beneficial effects of the present invention:

[0028] (1) The implementation of this method requires simple equipment and measurement methods, and has high measurement efficiency. Since only the off-plane displacement field at the crack tip and its gradient change along the crack propagation direction need to be measured, strain measurement is not required, which reduces the systematic error caused by the displacement shape function and eliminates the error caused by the off-plane displacement for the in-plane strain measured by DIC. In addition, the resolution of DIC displacement measurement is much greater than the strain resolution, and the plastic zone size obtained by the off-plane displacement is closer to the actual value.

[0029] (2) Since this method only measures the off-plane displacement field of the crack tip, it is not necessary to measure the mechanical properties of the material in advance, it is not necessary to measure the strain field at the crack tip, and it is not necessary to introduce the yield criterion. This simplifies the solution process and omits the acquisition of the mechanical properties of the new material, thus improving the measurement efficiency of the plastic zone size at the crack tip. It is convenient and simple.

[0030] (3) Since this method uses the gradient of the off-plane displacement in the crack propagation direction in front of the crack tip to solve the size of the plastic zone at the crack tip, it is easy to determine the crack propagation direction in front of the crack tip for a simple type I crack, and then quickly obtain the gradient of the off-plane displacement in the crack propagation direction, and quickly obtain the size of the plastic zone through the gradient of the off-plane displacement. For complex cracks where it is difficult to accurately determine the crack propagation direction in front of the crack tip, the plastic zone boundaries at different angles can be obtained by calculating the gradient of the off-plane displacement at different angles in front of the crack tip, and then the size of the plastic zone can be obtained.

[0031] (4) The present invention can quickly measure the size of the crack tip plastic zone, and then substitute the measured plastic zone size into the theoretical model and formula of the plastic zone parameters, and then quickly obtain other parameters related to the plastic zone size. For example, for a standard specimen under fixed load conditions, substitute the measured plastic zone size into the Irwin theoretical formula The material yield strength is obtained by reverse calculation. Therefore, this method can not only quickly measure the size of the crack tip plastic zone, but also can quickly measure the material yield strength and other parameters related to the plastic zone size in combination with theoretical models and formulas, which is of great significance for the measurement of mechanical properties of new materials and the characterization of crack extension behavior in elastic-plastic fracture mechanics. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a flow chart of the present invention.

[0033] Figure 2 Schematic diagram of the speckle pattern effect.

[0034] Figure 3 , which are the regions of interest and the divided sub-regions. The figure illustrates the reference image sub-region before deformation and the target image sub-region after deformation.

[0035] Figure 4 This is the principle diagram of dual-camera stereo imaging ranging.

[0036] Figure 5 The off-plane displacement and equivalent strain cloud diagrams of the crack tip and their curves of change with the distance from the crack tip. Among them, (a) is the DIC off-plane displacement field cloud diagram; (b) is the curve of change of the off-plane displacement field of the crack tip along the distance away from the crack tip; (c) is the Von mises equivalent strain cloud diagram; (d) is the curve of change of the Von mises equivalent strain of the crack tip along the distance away from the crack tip.

[0037] Figure 6 This is a normalized illustration of the crack tip position.

[0038] Figure 7 The figure shows the variation of the off-plane displacement in the crack extension direction in front of the crack tip with the distance from the crack tip for different crack lengths in this embodiment. Figure (a) is a fitting curve diagram of the original data to reduce the system error, and Figure (b) is a gradient curve of the maximum off-plane displacement corresponding to the fitting result.

[0039] Figure 8 Figure (a) is the off-plane displacement field caused by plastic deformation of the crack tip of different crack lengths, and Figure (b) is the equivalent strain field of the crack tip of different crack lengths.

[0040] Fig. 9 This is a comparison curve between the plastic zone size results of different crack lengths measured by the present invention and the calculation results of the Irwin theoretical formula shown in this embodiment. DETAILED DESCRIPTION

[0041] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0042] The present invention is based on the existing digital image correlation method, and uses a three-dimensional digital image correlation method to calculate the crack tip out-of-plane displacement field, and quickly measures the size of the crack tip plastic zone through a new test process and displacement gradient processing method. The method of measuring the size of the crack tip plastic zone of the present invention is based on the gradient of the out-of-plane displacement change near the crack tip. After calculating the out-of-plane displacement field of the crack tip, the rigid body displacement is removed (the displacement caused by deformation is retained). Then, taking the crack tip position as the starting point, a straight line segment is made along the crack propagation direction, and the out-of-plane displacement change on the straight line segment is calculated. The point with the largest out-of-plane displacement change gradient on the straight line segment is marked, and the distance between the crack tip starting point and the point with the largest out-of-plane displacement change gradient on the straight line segment is the size of the plastic zone in front of the crack tip. The specific test methods and analysis measures are as follows:

[0043] like Figure 1As shown, a method for measuring the off-surface displacement field of the crack tip plastic zone size is provided, and the steps are as follows:

[0044] Step 1: Prepare a standard compact tensile specimen and select a suitable coating to produce a speckle pattern. In order to facilitate the accurate measurement of the off-plane displacement field near the crack tip, it is necessary to ensure that the speckle in the crack tip area is uniform and has high contrast. Therefore, the method for producing speckles is as follows: Use matte white paint to cover the surface of the specimen. In order to obtain uniform random inverted speckles, use a spray gun to spray a mixture of alcohol and graphite on the surface of the specimen to obtain a high-contrast speckle pattern. The size of the speckle spots is 5-10 pixels, and the spacing between the speckle spots is also 5-10 pixels. The speckle pattern is as follows: Figure 2 shown.

[0045] Step 2: Collect the image of the crack tip before deformation, and use the image of the specimen with speckles collected before deformation, i.e. before the load is applied, as the reference image.

[0046] Step 3: Apply load to the specimen to cause crack initiation and expansion, determine the crack extension and crack tip area to be detected; and collect the image of the crack tip after deformation, and use the image of the plastic area of ​​the crack tip to be measured after the crack expands after loading as the target image.

[0047] Step 4: Based on the reference image and the target image, the in-plane displacement field of the area to be measured is obtained by the three-dimensional digital image correlation method; based on the in-plane displacement field, the out-of-plane displacement field is measured according to the principle of dual-camera stereo imaging ranging, and the rigid body displacement is removed (the rigid displacement in the displacement result is removed, and only the displacement caused by deformation is retained). The specific process is as follows:

[0048] (1) The method for obtaining the in-plane displacement field of the crack tip area to be measured by three-dimensional digital image correlation method is as follows:

[0049] Only the area near the crack tip is concerned, so the region of interest of the specimen is a certain range of the crack tip. The region of interest is divided into sub-areas containing 3-10 scattered spots, and the analysis step is set to 1 / 10-1 / 4 of the sub-area size. The smaller the step, the higher the accuracy, but the slower the calculation speed, so it is necessary to set a reasonable step according to actual requirements. Figure 3 As shown in the figure, the area containing the crack tip is recorded as the region of interest, the region of interest of the reference image is divided into several sub-areas, and the center point P(x 0 ,y 0 ), the gray value feature of sub-area I is taken as the center point P(x 0 ,y 0 ) mark; search the target image corresponding to the center point P (x 0 ,y 0 ) position; search the sub-region with the highest correlation with sub-region I in the target image through the correlation function as the matching sub-region I', mark the center point P'(x'0 ,y' 0 ), according to the center point P(x 0 ,y 0 ) matches the center point P'(x') of the target image sub-area I' 0 ,y' 0 ), the displacement of any point Q(x,y) and Q'(x',y') in the sub-area can be calculated according to the displacement mapping function. The related function expression is:

[0050]

[0051] Where P is the direction vector of the displacement mapping function. f(x,y) is the grayscale intensity of the reference image at the coordinate (x,y), and g(x',y') is the grayscale intensity of the target image at the coordinate (x',y'). M is the sub-region size. f m and g m are the average grayscale intensity of the reference image and the target image, respectively, expressed as follows:

[0052]

[0053] The displacement mapping relationship between any point Q(x,y) in the reference image and any point Q'(x',y') in the target image can be expressed as:

[0054]

[0055] Among them, x 0 ,y 0 is the coordinate point of the center point P in the X and Y directions, u and v are the displacement components of the sub-area I in the X and Y directions, and Δx and Δy are the distances from the center point P of the sub-area I to any point Q in the X and Y directions. x 、u y 、v x 、v y is the displacement gradient. The displacement in the plane of any point Q'(x',y') is calculated, and the displacements of all points constitute the displacement field.

[0056] (2) Based on the in-plane displacement, three-dimensional space reconstruction is performed through a binocular vision system, and the out-of-plane displacement is calculated using the reconstruction results. The out-of-plane displacement field is composed of the out-of-plane displacements of all points in the space.

[0057] like Figure 4 As shown, the principle of dual-camera stereo imaging ranging is:

[0058]

[0059] Among them, f is the focal length of the camera, L is the baseline (the distance between the two cameras), d is the binocular disparity, z is the depth, and x is the distance between the two cameras.c1 、x c2 are the horizontal coordinates of point P in the coordinate systems of the two cameras. The depth difference between the target image and the reference image is the out-of-plane displacement. The out-of-plane displacement represents the displacement of a point, which has only magnitude and direction. The displacement of all points in space constitutes the out-of-plane displacement field. In the out-of-plane displacement field, because the displacement magnitude and direction of each point are different, a displacement field gradient is generated.

[0060] Step 5: Based on the above-mentioned off-plane displacement field, calculate the off-plane displacement gradient along the crack propagation direction with the crack tip as the starting point, mark the point with the largest off-plane displacement gradient in the crack propagation direction, and calculate the distance between the crack tip starting point and the point with the largest off-plane displacement gradient as the plastic zone size. For example, the crack tip is marked as the starting point O(x), and the position on the straight line segment with the largest off-plane displacement gradient is marked as O'(x). The plastic zone size is equal to the value of O'(x)-O(x). Figure 5 As shown in the figure, elastic-plastic deformation occurs near the crack tip under load. Due to the stress concentration at the crack tip, the strain (stress) level at the crack tip exceeds the yield strain (stress) and yields. The yielded area (plastic zone) causes plastic flow and produces a large off-surface deformation displacement. The unyielded area (elastic zone) has no or a small off-surface deformation displacement due to elastic constraints. At this time, the maximum off-surface deformation displacement gradient is formed in the elastic-plastic transition area (elastic-plastic boundary) along the crack propagation direction with the crack tip as the starting point. The distance from the crack tip to the maximum off-surface deformation displacement gradient is the length of plastic yield before the crack tip, which is the plastic zone size of the type I crack. Figure 5 (a) is the cloud diagram of the off-surface displacement caused by plastic deformation near the crack tip. The distance from the crack tip to the purple area boundary is the size of the plastic zone. Figure 5 (b) shows Figure 5 (a) The off-plane displacement gradient change curve on the white solid straight line, Figure 5 (c) is the equivalent Von Mises strain cloud diagram before the crack tip. Figure 5 (d) shows Figure 5 (c) The variation curve of the equivalent Von Mises strain on the white solid straight line.

[0061] In order to verify the specific effect of the method of the present invention on measuring the size of the crack tip plastic zone in the crack growth test. According to the above steps, a standard compact tensile specimen is used to carry out a type I fatigue crack growth test as an example, and the size of the plastic zone at the load peak point is measured. The pattern collected without loading before crack growth is used as the reference image, and the pictures collected with different crack lengths are used as the target image. The size of the crack tip plastic zone with different crack lengths is measured by the method of the present invention.

[0062] like Figure 6 As shown, after calculating the off-plane displacement field according to the steps of the present invention, the crack tip position is marked as the starting point O(x). Figure 6 In the figure, O(x) = -1.6171 mm, and the right end of the straight line segment is 2.097 mm. In order to more clearly show the change in the off-plane displacement on the crack tip straight line of the embodiment, the position of the crack tip O(x) is normalized, that is, no matter how far the crack tip moves forward due to crack expansion, the position of O(x) is still recorded as 0 mm. Figure 6 The right end of the straight line segment becomes 3.7141mm.

[0063] according to Figure 6 The normalized method shown in the figure shows the variation curves of the off-plane displacement on the straight line segment in the crack tip crack extension direction with different crack lengths. Figure 7 As shown in (a), Figure 7 (b) is based on Figure 7 (a) The gradient of the out-of-plane displacement change is obtained by the out-of-plane displacement change. According to the above, Figure 7 The distance away from the crack tip corresponding to the point with the maximum off-plane displacement gradient shown in (b) is the size of the plastic zone in front of the crack tip. Figure 8 (a) and (b) show the crack tip off-plane displacement and equivalent strain cloud diagram under different crack lengths, respectively.

[0064] Due to the system error caused by the irresistible test conditions and the test DIC equipment system, the off-plane displacement change on the straight line segment will fluctuate. Therefore, when processing the off-plane displacement change data, it is necessary to remove the noise disturbance. The methods that can be adopted include curve smoothing, function fitting, etc. According to the embodiment, the off-plane displacement change curve on the straight line segment can be fitted and regressed to the Logistic function, from Figure 7 From the results of (a) and (b), it is obvious that at the maximum gradient, the fitting result is almost consistent with the change trend of the original data, so the error caused by regression fitting can be ignored. In order to verify the accuracy of the method of the present invention in measuring the size of the crack tip plastic zone, Fig. 9 The plastic zone size measured by the present invention is compared with the Irwin theoretical formula The calculation results and specific data are recorded in Table 1.

[0065] Table 1

[0066]

[0067] Compared with the value calculated by the theoretical formula, the plastic zone size measured by the present invention is higher than the theoretical result, which is in line with the actual situation. In summary, the method proposed by the present invention can accurately and quickly measure the size of the crack tip plastic zone.

[0068] The above embodiments are only used to illustrate the design ideas and features of the present invention, and their purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly. The protection scope of the present invention is not limited to the above embodiments. Therefore, any equivalent changes or modifications made based on the principles and design ideas disclosed by the present invention are within the protection scope of the present invention.

Claims

1. A method for measuring the off-surface displacement field of the plastic zone size at the crack tip. It is characterized in that The steps include: Step 1: Prepare the test piece; Step 2: Collect an image of the test piece before deformation at the crack tip as a reference image; Step 3: Apply a load to the test piece to cause crack initiation and expansion, and collect an image of the crack tip after deformation as the target image; Step 4: Based on the reference image and the target image, the in-plane displacement field of the area to be measured is obtained by the digital image correlation method. Based on the in-plane displacement field, three-dimensional space reconstruction is performed through the binocular vision system. The out-of-plane displacement field is calculated using the reconstruction result, and then the rigid body displacement is removed to retain the out-of-plane displacement field generated by the deformation; Step 5: Based on the off-plane displacement field, calculate the off-plane displacement gradient along the crack propagation direction with the crack tip as the starting point, mark the point with the largest off-plane displacement gradient in the crack propagation direction, calculate the distance between the crack tip starting point and the point with the largest off-plane displacement gradient, and use this distance as the plastic zone size.

2. A method for measuring the off-surface displacement field of the crack tip plastic zone size according to claim 1, It is characterized in that The method for obtaining the off-plane displacement field of the area to be measured by the digital image correlation method in step 4 is as follows: the area containing the crack tip is recorded as the area of ​​interest, the area of ​​interest of the reference image is divided into several sub-areas, and the center point P (x 0 ,y 0 ), the gray value feature of sub-area I is taken as P(x 0 ,y 0 ) point identification; search the target image corresponding to the center point P (x 0 ,y 0 ) position; search the sub-region with the highest correlation with sub-region I in the target image through the correlation function as the matching sub-region I', mark the center point P'(x' 0 ,y' 0 ), according to the center point P(x 0 ,y 0 ) matches the center point P'(x') of the target image sub-area I' 0 ,y' 0 ), and calculate the displacement of any point Q(x,y) and Q'(x',y') in the sub-area according to the displacement mapping function.

3. A method for measuring the off-surface displacement field of the crack tip plastic zone size according to claim 2, It is characterized in that The related function expression is: Where P is the direction vector of the displacement mapping function, f(x,y) is the grayscale intensity of the reference image at the coordinate (x,y), g(x',y') is the grayscale intensity of the target image at the coordinate (x',y'), and f m and g m is the average grayscale intensity of the reference image and the target image, and M is the sub-region size.

4. A method for measuring the off-surface displacement field of the crack tip plastic zone size according to claim 2, It is characterized in that The displacement mapping relationship between any point Q(x, y) in the reference image and any point Q'(x', y') in the target image is expressed as: Among them, x 0 ,y 0 is the coordinate point of the center point P in the X and Y directions, u and v are the displacement components of the sub-area I in the X and Y directions, Δx and Δy are the distances from the center point P of the sub-area I to Q in the X and Y directions; u x 、u y 、v x 、v y is the displacement gradient.

5. A method for measuring the off-surface displacement field of the crack tip plastic zone size according to claim 1, It is characterized in that The method for calculating the out-of-plane displacement field of the crack tip is as follows: based on the in-plane displacement of the target image and the reference image, three-dimensional space reconstruction is performed using the principle of dual-camera stereo imaging ranging, and the depth difference between each point of the target image and the reference image is calculated. The depth difference is the out-of-plane displacement, and the out-of-plane displacement of all points in the target image constitutes the out-of-plane displacement field.

6. A method for measuring the off-surface displacement field of the crack tip plastic zone size according to claim 1, It is characterized in that The method for calculating the size of the plastic zone in step 5 is: starting from the crack tip, draw a straight line along the crack propagation direction, calculate the gradient of the off-surface displacement change on the straight line, mark the point with the maximum off-surface displacement gradient, calculate the distance between the starting point and the point with the maximum off-surface displacement gradient, and obtain the size of the plastic zone.

7. A method for measuring the off-surface displacement field of the crack tip plastic zone size according to claim 1, It is characterized in that A uniform random speckle pattern needs to be produced on the surface of the specimen.

8. A method for measuring the off-surface displacement field of the crack tip plastic zone size according to claim 7, It is characterized in that The method for producing the speckle pattern is as follows: the surface of the specimen is covered with matte white paint, and an alcohol-graphite mixture is sprayed on the surface of the specimen with a spray gun, thereby obtaining a high-contrast, uniform, and randomly distributed speckle pattern.

9. A method for measuring the off-surface displacement field of the crack tip plastic zone size according to claim 8, It is characterized in that The sub-region contains 3-10 scattered spots, and the search step is set to 1 / 10-1 / 4 of the sub-region size.

10. A method for measuring the off-surface displacement field of the crack tip plastic zone size according to claim 9, It is characterized in that The spot size is 5-10 pixels, and the spot spacing is also 5-10 pixels.

Images