Crack tip position displacement field identification method based on digital image correlation technology

Abstract

The invention discloses a crack tip position displacement field identification method based on a digital image correlation technology, and the method comprises the steps: calculating a crack tip displacement field based on a crack tip displacement change gradient, and removing rigid body displacement. Two equal-length horizontal lines are arranged on the two sides of the crack surface, and the horizontal coordinates of the crack tip are determined by calculating the change of the vertical displacement difference between the two horizontal lines in the crack propagation direction. Setting a vertical line to connect the horizontal lines on the two sides of the crack through the horizontal coordinates of the crack tip, and determining the vertical coordinates of the crack tip by calculating the change of the vertical displacement of the vertical line along the vertical line. The method only needs to consider the displacement difference change of the two sides of the crack expansion surface and the crack tip vertical line, does not need to use a theoretical formula to fit the crack tip displacement field, simplifies the calculation process, improves the crack tip identification efficiency, and is convenient and simple.

Description

Technical Field

[0001] The present invention belongs to the field of fracture mechanics and its engineering application, and specifically relates to a method for identifying the displacement field of a crack tip position based on digital image correlation technology. Background Art

[0002] In fracture mechanics, the detection and identification of fatigue cracks is crucial for the safety assessment of engineering structures. Crack length and crack tip position are parameters that must be accurately measured in many fracture mechanics experiments. In fatigue crack growth tests, the crack tip position can be identified, the crack extension amount can be output in real time, and the crack growth rate can be detected. In fatigue crack growth tests, the crack tip can be identified and the crack length can be measured by observation under a microscope, but this method is limited by the microscopic field of view, measurement efficiency, extreme environment, etc., and is not conducive to tracking and monitoring cracks. Due to the high gradient deformation at the crack tip, the measurement becomes complicated and there is a large accuracy problem. Therefore, a crack tip identification method that can both ensure measurement accuracy and improve measurement efficiency is needed.

[0003] Digital Image Correlation (DIC) technology 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 identifier (sub-area) in the undeformed image, the point with the identifier that maximizes the similarity function is searched / tracked 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 crack tip field information in fracture mechanics due to its advantages such as simple operation and economy, including identification of crack tip position, crack length measurement, stress intensity factor measurement (K), crack tip plastic zone size (rp), crack opening displacement (COD), J integral, etc. Among the measurements of these parameters, the identification and measurement of the crack tip is undoubtedly the most needed. The DIC method for measuring the crack tip field is based on displacement deformation. The similarity of traditional measurement methods is that they all regard the physical coordinates of the crack tip as unknowns, fit the full-field displacement of the DIC experimental data to the theoretical analytical solution of the crack tip, and determine the crack tip position by establishing an error function between the two to obtain the extreme value. However, for the above methods, the displacement field obtained by fitting is a theoretical result, and there is always a human error compared with the actual test results. The results obtained by different researchers for the choice of fitting function are often uncertain. In addition, these methods need to process extremely large data, which is not conducive to the rapid detection of the crack tip position. Summary of the invention

[0004] In order to address the deficiencies in the prior art, the present application proposes a method for identifying the displacement field of the crack tip position based on digital image correlation technology. The method uses an 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 specimen with different crack lengths or extended cracks under different loads are recorded. The digital image correlation method is used to calculate the crack tip displacement field of different crack lengths or different loads.

[0005] The technical solution adopted by the present invention is as follows:

[0006] A method for identifying a crack tip position displacement field based on digital image correlation technology comprises the following steps:

[0007] Step 1: Prepare a tensile specimen and create a speckle pattern on the surface of the specimen;

[0008] Step 2: Apply load to the specimen to cause crack initiation and expansion, and determine the crack extension and crack tip area to be tested;

[0009] Step 3: Collect images of the crack tip before and after deformation respectively;

[0010] Step 4: Based on the reference image and the target image, the displacement field of the area to be measured is obtained by digital image correlation method, and the rigid body displacement is removed;

[0011] Step 5: Determine the crack tip position based on the crack tip displacement field, where the crack tip position includes a horizontal coordinate and a vertical coordinate of the crack tip.

[0012] Further, in step 4, the method for obtaining the 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 sub-areas, the center point O(x, y) of the sub-area I is marked, and the gray value feature of the sub-area I is used as the identifier of the O(x, y) point; the position of the center point O(x, y) of the corresponding sub-area I of the target image is searched through the identifier; the sub-area with the highest correlation with the sub-area I is searched in the target image through the correlation function as the matching sub-area I', the center point O'(x+u, y+v) of I' is marked, and the pixel displacement is calculated according to the center point O(x, y) of the sub-area I of the reference image and the center point O'(x+u, y+v) of the matching sub-area I' of the target image, wherein x, y are pixel coordinates, and u, v are pixel displacement.

[0013] Further, the related function expression is:

[0014]

[0015] Where x, y are pixel coordinates, u, v are pixel displacements, n is the sub-region pixel size, I is the reference image, and I' is the target image.

[0016] Furthermore, the method for determining the horizontal coordinates of the crack tip is:

[0017] S1. Draw two equal and parallel horizontal lines L1 and L2 on both sides of the crack extension surface, and align the two ends of L1 and L2;

[0018] S2. Assume that L1 and L2 are composed of n equally spaced points. The points on L1 and L2 correspond to each other one by one. The vertical displacement difference corresponding to each point is recorded as the array D n :

[0019] D n = {D1, D2…D i-1 , D i , D i+1 …D n , (2≤i≤n-1)}

[0020] And array D n Each vertical displacement in corresponds to a horizontal coordinate position, so the corresponding horizontal coordinate position is recorded as:

[0021] X n = {X1, X2…X i-1 , X i , X i+1 …X n , (2≤i≤n-1)}

[0022] Among them, D i is the vertical displacement difference at the i-th point, X i is the horizontal coordinate of the i-th point;

[0023] S3. Set array D n Subtract the previous term from the next term, and the vertical displacement difference is recorded as array ΔD n , expressed as:

[0024] ΔD n ={ΔD2, ΔD3…ΔD i-1 , ΔD i , ΔD i+1 …ΔD n , (3≤i≤n-1)}

[0025] And the array ΔD n The horizontal coordinate position corresponding to each number in is:

[0026] X n-1 = {X2, X3…X i-1 , X i , X i+1 …X n , (3≤i≤n-1)}

[0027] Where, ΔDi Expressed as ΔD i =D i-1 -D i , that is, the difference between the i-1th vertical displacement difference and the i-th vertical displacement difference;

[0028] S4. Set the horizontal coordinate corresponding to the maximum vertical displacement difference as the horizontal coordinate of the crack tip.

[0029] Furthermore, the method for determining the vertical coordinate of the crack tip is:

[0030] S1, based on the determined horizontal coordinate of the crack tip, draw a vertical line L3 connecting L1 and L2 through the horizontal coordinate of the crack tip;

[0031] S2. Assume that m points are arranged at equal intervals on L3. The vertical displacement formed by the crack tip deformation at each point is recorded as array E. m :

[0032] E m ={E1, E2…E j-1 、E j 、E j+1 …E m , (2≤j≤m-1)}

[0033] And the vertical coordinates corresponding to each vertical displacement are:

[0034] Y m = {Y1, Y2…Y j-1 , Y j , Y j+1 …Y m , (2≤j≤m-1)}

[0035] Among them, E j is the vertical displacement corresponding to the jth point on L3, Y j is the vertical coordinate corresponding to the jth point;

[0036] S3, array E m Subtract the previous term from the next term, and the vertical displacement difference is recorded as array ΔE m , expressed as:

[0037] ΔE m ={ΔE2, ΔE3…ΔE j-1 , ΔE j , ΔE j+1 …ΔE m , (3≤j≤m-1)}

[0038] And the array ΔE m The vertical coordinate position corresponding to each number is:

[0039] Y m-1= {Y2, Y3...Y j-1 , Y j , Y j+1 …Y m , (3≤j≤m-1)}

[0040] Where, ΔE j Expressed as ΔE j =E j-1 -E j .

[0041] S4. The maximum vertical displacement difference is formed on both sides of the crack surface, and the corresponding coordinate position is the vertical coordinate of the crack tip.

[0042] Furthermore, an XY graph was drawn with the help of Origin drawing software. In the XY graph, the vertical coordinate and horizontal coordinate of the crack tip were obtained according to the peak point.

[0043] 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 speckle pattern, in which the size of the speckle spots occupies 5-10 pixels and the spacing between the speckle spots is also 5-10 pixels.

[0044] 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.

[0045] Furthermore, the image of the specimen with speckles collected before the load is applied is used as a reference image, and the image of the crack tip position to be detected after the crack propagates after loading is used as a target image.

[0046] Beneficial effects of the present invention:

[0047] (1) The implementation of this method has simple equipment requirements and detection methods, and has high detection efficiency. Since only the displacement difference changes on both sides of the crack extension surface and the vertical line of the crack tip need to be considered, there is no need to use theoretical formulas to fit the crack tip displacement field, which is equivalent to simplifying the calculation process and improving the efficiency of crack tip identification. It is convenient and simple.

[0048] (2) This method expands the scope of application and supplements the detection of crack tip position under large deformation conditions in fracture mechanics. The existing theoretical formulas used to describe the crack tip displacement field are mostly linear elastic and small-scale yield solutions, which are not applicable to the crack tip displacement field under large deformation conditions, and therefore cannot determine the crack tip position under large deformation conditions. The vertical displacement difference considered by this method includes the displacement caused by plastic deformation and the crack opening displacement, so it is not only applicable to linear elasticity, but also meets the requirements for crack tip position identification under large deformation conditions.

[0049] (3) Since this method uses the vertical displacement difference between two horizontal lines on both sides of the crack surface to determine the horizontal position of the crack tip, for a simple I-type crack, it is only necessary to set two horizontal lines parallel to the initial crack and extending along the crack propagation direction on both sides of the initial crack. When the crack propagates, the vertical displacement difference between the two lines due to the forward propagation of the crack can be output in real time to obtain the crack propagation length in real time, and then the crack propagation amount can be obtained, and the crack propagation rate can be measured quickly. This is of great significance for the determination of crack length in fatigue crack propagation tests. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 It is a flow chart of the present invention.

[0051] Figure 2 Schematic diagram of the speckle pattern effect.

[0052] Figure 3 Schematic diagram of the reference image sub-region before deformation and the target image sub-region after deformation.

[0053] Figure 4 Schematic diagram of the positions of points in the displacement field of Example 1 and the analysis results, wherein (a) is a schematic diagram of the positions of L1, L2, and L3 in the DIC displacement field of Example 1; (b) is the analysis result of Example 1: the vertical displacements of n points on L1 and L2 are distributed along the X direction.

[0054] Figure 5 It is a vertical displacement change diagram, where (a) is the change of the vertical displacement difference along the horizontal direction; (b) is the horizontal coordinate of the vertical displacement difference; (c) is the change of the vertical displacement of L3 along the Y direction; (d) is the change of the vertical displacement of L3 along the Y direction.

[0055] Figure 6 Schematic diagrams of the embodiments of the present invention, where (a) is a schematic diagram of the coordinate origin. (b) is the result of the vertical displacement difference analysis of different crack lengths. (c) is a comparison of the results of three measurement methods. (d) is a crack pattern photographed by a microscope.

[0056] Figure 7 This is a diagram showing the effect of different horizontal spacings d on the identification of crack tip positions during error analysis.

[0057] Figure 8 The results of the analysis of the influence of L1 and L2 on the identification of the crack tip position, where (a) and (b) are the results of the analysis of the influence of the distance e of L1 and L2 on the identification of the crack tip position. (c) and (d) are the results of the analysis of the influence of the misalignment of the horizontal lines L1 and L2 on the identification of the crack tip position. DETAILED DESCRIPTION

[0058] 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.

[0059] The present invention is based on the existing digital image correlation method, and uses the digital image correlation method to calculate the crack tip displacement field, and detects and identifies the crack tip position through a new test process and displacement gradient processing method. The method for identifying the crack tip position of the present invention is based on the crack tip displacement change gradient. After calculating the crack tip displacement field, the rigid body displacement is removed. Thereafter, two equal-length horizontal lines are set on both sides of the crack surface, and the horizontal coordinates of the crack tip are determined by calculating the change in the vertical displacement difference between the two horizontal lines along the crack propagation direction. A vertical line is set through the crack tip horizontal coordinate to connect the horizontal lines on both sides of the crack, and the vertical coordinates of the crack tip are determined by calculating the change in the vertical displacement of the vertical line along the vertical line. The specific test methods and analysis measures are as follows:

[0060] like Figure 1 As shown in FIG. 1 , a method for identifying crack tips using displacement fields based on digital image correlation technology is shown in FIG. 1 . The steps are as follows:

[0061] Step 1: Prepare a standard compact tensile specimen and select a suitable coating to produce a speckle pattern. In order to facilitate accurate identification of 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 speckle is as follows: Use matte white paint to cover the surface of the specimen. In order to obtain uniform random inverted speckle, 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 is 5-10 pixels, and the spacing between the speckles is also 5-10 pixels. The speckle pattern is as follows: Figure 2 shown.

[0062] Step 2: Apply load according to the test conditions to cause crack initiation and expansion, and determine the crack extension and crack tip areas to be tested.

[0063] Step 3: Collect images of the crack tip before deformation) and after deformation, respectively, wherein the image of the specimen with speckles collected before deformation, i.e. before the load is applied, is used as the reference image, and the image of the crack tip position to be detected after deformation, i.e. after the crack is extended after loading, is used as the target image.

[0064] Step 4: Obtain the displacement field of the area to be tested by digital image correlation method and remove the rigid body displacement. We only focus on the specific position of the crack tip, so the region of interest of the specimen is a certain range of areas including the crack tip. The region of interest is divided into sub-areas containing 3-10 scattered spots, and the analysis step size is set to 1 / 10-1 / 4 of the sub-area size. The smaller the step size, the higher the accuracy, but the slower the calculation speed, so it is necessary to set a reasonable step size according to actual requirements. Figure 3As shown, the reference image is divided into sub-regions according to the crack tip region of interest, the center point O(x, y) of sub-region I is marked, and the gray value feature of sub-region I is used as the identifier of point O(x, y). The center point O(x, y) position of the corresponding sub-region I of the target image is searched through the identifier. The sub-region with the highest correlation with sub-region I is searched in the target image through the correlation function as the matching sub-region I', and the center point O'(x+u, y+v) of I' is marked. The pixel displacement is calculated based on the center point O(x, y) of sub-region I of the reference image and the center point O'(x+u, y+v) of the matching sub-region I' of the target image. The correlation function expression is:

[0065]

[0066] Where x, y are pixel coordinates, u, v are pixel displacements, n is the sub-area pixel size, I is the reference image, and I' is the target image. The smaller the value of C(x, y, u, v), the higher the correlation. The rigid body displacement is the displacement caused by the gap between the specimen and the fixture, the rotation of the specimen, etc. during loading. Removing the rigid body displacement can obtain higher precision measurements.

[0067] Step 5: Identify the crack tip position through the calculated displacement field, including the horizontal and vertical coordinates of the crack tip.

[0068] Part I: Determination of the horizontal coordinates of the crack tip

[0069] The first step, such as Figure 4 As shown in (a), two equal-length horizontal lines L1 and L2 are drawn on both sides of the crack extension surface. The two horizontal lines are located on both sides of the crack surface, and the crack tip is located within the range of the two horizontal lines. The vertical displacement distribution of the n points that make up the two horizontal lines is shown in Figure 4 As shown in (b). Figure 4 (b) Calculate the vertical displacement difference between the two horizontal lines as the crack propagation direction changes. Figure 5 As shown in (a) in the figure, the specific steps of calculating the vertical displacement difference between two horizontal lines include:

[0070] (1) Figure 4 As shown in (a), the two lines are composed of n points with equal horizontal spacing. The vertical displacement differences between the n points forming the two lines due to crack extension are D1, D2, ... D i-1 , D i , D i+1 …D n .

[0071] (2) Record the vertical displacement difference as array D n :

[0072] D n = {D1, D2…D i-1 , Di , D i+1 …D n , (2≤i≤n-1)}

[0073] (3) Record array D n The coordinate position corresponding to each number in is X n :

[0074] X n = {X1, X2…X i-1 , X i , X i+1 …X n , (2≤i≤n-1)}

[0075] In the second step, the vertical displacement difference along the crack propagation direction obtained in the first step is used to further obtain the vertical displacement difference along the crack propagation direction. Assume that the horizontal coordinate of the crack tip is X i When the load is applied to open the crack, the crack tip X i The vertical displacement difference D of the rear side (crack extension has occurred) i-1 The crack opening displacement COD(δ i-1 ) and the displacement caused by plastic deformation in the vertical direction (deformation caused by yield stress in the plastic zone) V i-1 Composition (D i-1 =δ i-1 +V i-1 ), while at the crack tip X i The vertical displacement difference D i Displacement V caused by plastic deformation only i (D i =V i ). Because V i-1 With V i are the displacements caused by plastic deformation very close to the crack tip. Due to the existence of toughness constraints, it can be considered that V i-1 ≈V i , so the crack tip X i The maximum displacement difference ΔD is formed before and after i (ΔD i =D i-1 -D i =δ i-1 ). The specific steps include:

[0076] (1) For the array D in step (2) n Execute D in sequence i-1 -D i ;

[0077] (2) The obtained vertical displacement difference is recorded as array ΔD n :

[0078] ΔD n ={ΔD2, ΔD3…ΔD i-1 , ΔD i , ΔD i+1 …ΔD n , (3≤i≤n-1)}

[0079] (3) Record array ΔD n The coordinate position corresponding to each number in X n-1 :

[0080] X n-1 = {X2, X3…X i-1 , X i , X i+1 …X n , (3≤i≤n-1)}

[0081] The third step is to determine the horizontal coordinate of the crack tip using the maximum vertical displacement difference. The maximum vertical displacement difference ΔD will be formed before and after the crack tip. i , the corresponding coordinate position X i The specific steps include:

[0082] (1) Figure 5 As shown in (b), the array ΔD n As the dependent variable, X n-1 Import the data as independent variables into Origin graphing software to draw XY graphs.

[0083] (2) In the XY graph, the peak point corresponds to the maximum displacement difference. Find the coordinate X corresponding to the maximum displacement difference. i , X i as the horizontal coordinate of the crack tip.

[0084] Part II: Determination of the vertical coordinates of the crack tip

[0085] In the first step, the horizontal coordinates of the crack tip have been obtained in the first part, such as Figure 4 As shown in (a), draw a vertical line L3 connecting L1 and L2 through the horizontal coordinate of the crack tip, and calculate the change of the vertical displacement on L3 along the Y direction. The specific steps include:

[0086] (1) L3 consists of m points with equal vertical spacing. The vertical displacement formed by the crack tip deformation at m points is denoted as array E m :

[0087] E m ={E1, E2…E j-1 、E j 、E j+1 …E m, (2≤j≤m-1)}

[0088] (2) Figure 5 As shown in (c), the Y coordinates of the m points correspond to the array E m The coordinates of the numbers inside are denoted by Y m :

[0089] Y m = {Y1, Y2…Y j-1 , Y j , Y j+1 …Y m , (2≤j≤m-1)}

[0090] The second step is to determine the vertical coordinate of the crack tip using the change in vertical displacement on L3. j is the vertical coordinate of the crack tip. When the crack opens, only the toughness constraint of the material no longer exists between the two sides of the crack extension surface. At this time, the crack tip Y j The vertical displacement at the position is the largest, resulting in the maximum displacement difference ΔE j , (ΔE j =E j-1 -E j ). The specific steps include:

[0091] (1) For the array E in step (1) m Execute E in sequence j-1 -E j ;

[0092] (2) The obtained vertical displacement difference is recorded as array ΔE m :

[0093] ΔE m ={ΔE2, ΔE3…ΔE j-1 , ΔE j , ΔE j+1 …ΔE m , (3≤j≤m-1)}

[0094] (3) Record array ΔE m The coordinate position corresponding to each number in Y m-1 :

[0095] Y m-1 = {Y2, Y3...Y j-1 , Y j , Y j+1 …Y m , (3≤j≤m-1)}

[0096] The third step is to determine the vertical coordinate of the crack tip using the maximum vertical displacement difference. The maximum vertical displacement difference ΔE is formed on both sides of the crack surface. j, the corresponding coordinate position Y j The specific steps include:

[0097] (1) Figure 5 As shown in (d), the array ΔE m As the dependent variable, Y m-1 Import the data as independent variables into Origin graphing software to draw XY graphs.

[0098] (2) In the XY graph, the peak point corresponds to the maximum displacement difference. Find the coordinate Y corresponding to the maximum displacement difference. j , Y j

[0099] as the vertical coordinate of the crack tip.

[0100] In order to verify the specific effect of the method of the present invention on identifying the crack tip position of fatigue crack growth test. According to the above steps, a type I fatigue crack growth test is carried out using a standard compact tensile specimen as an example, and the pattern collected without loading before crack growth is used as a reference image, and the pictures collected at different cycles and different crack lengths are used as target images. The method of the present invention is used to identify the crack tip position at different cycles, and then the crack growth length is obtained.

[0101] like Figure 6 As shown in (a), a Cartesian coordinate system is established with the first point on the leftmost end of L2 as the origin of the coordinates, and the origin position of the coordinates read out in the DIC displacement field is X0 = 648.96 pixels. According to the steps, the vertical displacement difference array D is obtained. n After that, execute ΔD i =D i-1 -D i Get the vertical displacement difference array ΔD n ,like Figure 6 As shown in (b), find the coordinate X corresponding to the maximum displacement difference when the crack lengths (a1, a2, a3) are different. i is the horizontal position of the crack tip. This embodiment 1 is a type I crack, so X i The value of -X0 is the length of the crack.

[0102] like Figure 6 As shown in (c), the method of the present invention simplifies the crack tip identification process and improves the measurement efficiency. The measurement results (all the results are converted into pixel values ​​of DIC measurement, such as for a1, the resolution of the microscope photo is 1.923μm / pixel, the resolution of the DIC target image is 31.590μm / pixel, the actual microscope crack pixel length is 1745.187pixel, which is converted to micrometer length of 3355.995μm, which is converted to DIC pixel length of 106.236pixel) are consistent with the microscope observation results ( Figure 6 The results obtained by (d)) and the traditional method (calculated by fitting the actual crack tip displacement field to the analytical solution of the Westergaard displacement field theory) are consistent. The crack length results obtained by measuring the crack tip position by different methods are shown in Table 1. From the data in the table, it can be seen that the error of the crack length measured by microscope observation does not exceed 1%, indicating that the proposed method can accurately identify the crack tip position.

[0103] Table 1 Comparison of data from three measurement methods

[0104]

[0105] Finally, an error analysis is performed through implementation example 1. The crack tip identification method involved in the present invention identifies the crack tip position through the difference of equally spaced vertical displacement differences. In theory, the smaller the spacing, the closer the measured crack tip is to the actual crack tip position. However, due to the performance limitations of the DIC equipment, such as displacement resolution (the resolution of DIC displacement measurement in this embodiment 1 is 0.06 pixel), errors such as system noise, and the influence of excessive spacing, there will be errors in identifying the crack tip position, so it is necessary to consider the range of the spacing, that is, the size of n and m. To this end, the actual crack length is measured using an optical microscope. Taking the actual crack length a1 as a reference, the influence of different values ​​of n on the DIC measurement results is studied. Figure 7 Shows the vertical displacement difference ΔD i Follow D i-1 With D i The change of the spacing d (pixel) between them. The smaller the spacing d, the smaller the array D n With ΔD n The larger the number of terms n contained in . Table 2 records the X-axis position of the crack tip (i.e. the coordinate corresponding to the maximum vertical displacement difference) and the crack length corresponding to different spacings d. Figure 7 From the results in Table 2, it can be seen that when the spacing d>2.13pixel, the error increases with the increase of the spacing d. When 0.11pixel≤d≤2.13pixel, the measured crack length error is less than 1pixel. When d=0.5pixel, the minimum error is 0.007pixel. In fact, although the measurement error is still less than 1pixel when d=0.11pixel, the X coordinate corresponding to the maximum vertical displacement difference has fluctuated around 756pixels, so it should be avoided in actual measurement. Combined with the displacement resolution of DIC in this paper is about 0.06pixel, it is recommended that the spacing d be 10-20 times the displacement resolution of DIC when using the proposed method.

[0106] Table 2 Crack tip position, crack length and error at different horizontal spacings

[0107]

[0108] Generally speaking, the displacement field theory solution in the elastic zone is only applicable to linear elastic models and small-scale yield (SSY) cases. If the order of the fitting function is smaller than the order of the displacement field, it will lead to undermatching function and cause systematic errors. The displacement system error is proportional to the second-order displacement gradient, and the strain system error is proportional to the third-order displacement gradient. The method proposed in this invention is not affected by the high gradient deformation in the crack tip plastic zone, but it is necessary to consider whether it is also applicable in the elastic region. Still taking the crack length a1 observed by microscope as the reference object, consider two cases, such as Figure 8 As shown in (a) to (d), the distance e between the two horizontal lines L1 and L2 (that is, the length of the vertical line L3) affects the identification of the crack tip position; the influence of the degree of asymmetry between L1 and L2 on the results, with the position of L2 fixed, the larger the distance e1 between L1 and L2, the lower the degree of symmetry. Table 3 shows the influence of distance e and asymmetry on the measurement results.

[0109] Table 3 Crack tip position, crack length and error measured at different distances e of L1 and L2, and the degree of asymmetry e1 of the crack surface

[0110]

[0111]

[0112] It can be seen from Table 3 that the distance e between L1 and L2 has almost no effect on the accurate identification of the crack tip; the higher the asymmetry e1 of L1 and L2 with respect to the crack surface, the greater the measurement error. Therefore, when identifying the crack tip, try to make L1 and L2 parallel and symmetrical to the crack surface, which will greatly reduce the measurement error.

[0113] 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 identifying the displacement field of crack tip position based on digital image correlation technology, characterized in that: The steps include: Step 1: Prepare a tensile specimen and create a speckle pattern on the surface of the specimen; Step 2: Apply load to the specimen to cause crack initiation and expansion, and determine the crack extension and crack tip area to be tested; Step 3: Collect images of the crack tip before and after deformation respectively; Step 4: Based on the reference image and the target image, the displacement field of the area to be measured is obtained by digital image correlation method, and the rigid body displacement is removed; Step 5: Determine the crack tip position based on the crack tip displacement field, where the crack tip position includes a horizontal coordinate and a vertical coordinate of the crack tip.

2. According to the method for identifying the displacement field of crack tip position based on digital image correlation technology in claim 1, it is characterized in that: The method for obtaining the 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 sub-areas, the center point O(x, y) of the sub-area I is marked, and the gray value feature of the sub-area I is used as the identifier of the O(x, y) point; the center point O(x, y) position of the corresponding sub-area of ​​the target image is searched by the identifier; the sub-area with the highest correlation with the sub-area I in the target image is searched by the correlation function as the matching sub-area I′, and the center point O′(x+u, y+v) of I′ is marked; the pixel displacement is calculated according to the center point O(x, y) of the sub-area I of the reference image and the center point O′(x+u, y+v) of the matching sub-area I′ of the target image, where x, y are pixel coordinates, and u, v are pixel displacement.

3. The method for identifying the crack tip position displacement field based on digital image correlation technology according to claim 2 is characterized in that: The related function expression is: Where x, y are pixel coordinates, u, v are pixel displacements, n is the sub-region pixel size, I is the reference image, and I′ is the target image.

4. The method for identifying the crack tip position displacement field based on digital image correlation technology according to claim 1, characterized in that: The method to determine the horizontal coordinates of the crack tip is: S1. Draw two equal and parallel horizontal lines L1 and L2 on both sides of the crack extension surface, and align the two ends of L1 and L2; S2. Assume that L1 and L2 are composed of n equally spaced points. The points on L1 and L2 correspond to each other one by one. The vertical displacement difference corresponding to each point is recorded as the array D n : D n ={D1、D2...D i -1、D i 、D i+1 ...D n ,(2≤i≤n-1)} And array D n Each vertical displacement in corresponds to a horizontal coordinate position, so the corresponding horizontal coordinate position is recorded as: X n {X1、X2...X i-1 、X i 、X i+1 ...X n ,(2≤i≤n-1)} Among them, D i is the vertical displacement difference at the i-th point, X i is the horizontal coordinate of the i-th point; S3. Set array D n Subtract the previous term from the next term, and the vertical displacement difference is recorded as array ΔD n , expressed as: ΔD n ={ΔD2、ΔD3...ΔD i-1 、ΔD i 、ΔD i+1 ...ΔD n ,(3≤i≤n-1)} And the array ΔD n The horizontal coordinate position corresponding to each number in is: X n-1 ={X2、X3...X i-1 、X i 、X i+1 ...X n ,(3≤i≤n-1)} Where, ΔD i Expressed as ΔD i =D i-1 -D i , that is, the difference between the i-1th vertical displacement difference and the i-th vertical displacement difference; S4. Set the horizontal coordinate corresponding to the maximum vertical displacement difference as the horizontal coordinate of the crack tip.

5. The method for identifying the crack tip position displacement field based on digital image correlation technology according to claim 4 is characterized in that: The method to determine the vertical coordinate of the crack tip is: S1, based on the determined horizontal coordinate of the crack tip, draw a vertical line L3 connecting L1 and L2 through the horizontal coordinate of the crack tip; S2. Assume that m points are arranged at equal intervals on L3. The vertical displacement formed by the crack tip deformation at each point is recorded as array E. m : THE m }{E1、E2...E j-1 ,THE j ,THE j+1 ...THE m ,(2≤j≤m-1)} And the vertical coordinates corresponding to each vertical displacement are: Y m ={Y1、Y2...Y j-1 、Y j 、Y j+1 ...Y m ,(2≤h≤m-1)} Among them, E j is the vertical displacement corresponding to the jth point on L3, Y j is the vertical coordinate corresponding to the jth point; S3, array E m Subtract the previous term from the next term, and the vertical displacement difference is recorded as array ΔE m , expressed as: ΔE m ={ΔE2、ΔE3...ΔE j-1 、ΔE j 、ΔE j+1 ...ΔE m ,(3≤j≤m-1)} And the array ΔE m The vertical coordinate position corresponding to each number is: AND m-1 ={Y2,Y3...Y j-1 ,AND j ,AND j+1 ...AND m ,(3≤j≤m-1)} Where, ΔE j Expressed as ΔE j =E j-1 -E j . S4. The maximum vertical displacement difference is formed on both sides of the crack surface, and the corresponding coordinate position is the vertical coordinate of the crack tip.

6. The method for identifying the crack tip position displacement field based on digital image correlation technology according to claim 1 is characterized in that: Method for producing speckle pattern: Cover the surface of the specimen with matte white paint, spray the alcohol-graphite mixture on the surface of the specimen with a spray gun, and then obtain a high-contrast speckle pattern.

7. The method for identifying the crack tip position displacement field based on digital image correlation technology according to claim 1 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.

8. The method for identifying the crack tip position displacement field based on digital image correlation technology according to claim 6 is characterized in that: The spot size is 5-10 pixels, and the spot spacing is also 5-10 pixels.

9. The method for identifying the crack tip position displacement field based on digital image correlation technology according to claim 1, characterized in that: The specimen image with speckles collected before the load is applied is used as the reference image, and the image of the crack tip position to be detected after the crack propagates after loading is used as the target image.

10. The method for identifying the displacement field of crack tip position based on digital image correlation technology according to claim 1, characterized in that: The XY graph was drawn with the help of Origin drawing software. In the XY graph, the vertical and horizontal coordinates of the crack tip were obtained according to the peak point.

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