I-II composite crack tip position identification method based on DIC

Abstract

The invention discloses a DIC-based I-II composite crack tip position identification method, which is characterized in that the change of vertical displacement near a crack tip is calculated based on crack tip plastic deformation and crack opening displacement, and rigid body displacement is removed. According to the general expansion direction of the I-II composite crack, line segments which are parallel to the expansion surface of the crack and are equidistant are arranged on the two sides of the crack respectively, and the position, with the maximum vertical displacement difference change, near the crack tip is the line segment area where the transverse coordinate of the crack tip is located; the area is set as a line segment, the vertical displacement change of the area is solved, the position, with the maximum vertical displacement change, of the line segment is the longitudinal coordinate point of the crack tip, and therefore the position of the crack tip is determined. Correcting the crack surface by using the crack tip coordinates, then introducing parallel correction line segments which are equidistant from the crack surface to the two sides of the crack surface, and determining the coordinates again; the coordinates can be converted according to the propagation path of the composite crack. According to the method, only changes of a vertical displacement field near the crack tip need to be measured, elastic-plastic parameters of a material do not need to be determined, the detection process is simple and convenient, high equipment requirements do not exist, the detection efficiency of the tip position of the I-II composite crack is improved, and convenience and simplicity are achieved.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of fracture mechanics, and relates to a method for identifying the position of an I-II composite crack tip based on a digital image correlation technique. BACKGROUND

[0002] In a fatigue crack propagation experiment, accurately positioning the tip position of an I-II composite crack is crucial for studying the crack propagation behavior. Precise measurement of the crack tip position helps to accurately measure the crack propagation length, precisely calculate the crack propagation rate, and improve the reliability of the experimental results. Compared with an I-type crack, an I-II composite crack has the action of shear stress, and the crack propagation direction needs to be considered. If the tip position of the I-II composite crack is not accurately detected, it may lead to misjudgment of the crack propagation behavior in the experiment, affect the analysis of the mechanical behavior of crack propagation, and reduce the reliability of the experimental results. Therefore, the crack tip of the I-II composite crack needs to be accurately positioned.

[0003] In experiments, the microscope observation method is a direct method for identifying the crack tip position, but it is limited by the resolution of the microscope, requires a high degree of flatness and cleanliness of the sample surface, and may need to move the microscope field of view during the experiment due to the limitation of the field of view, resulting in complicated operation. Moreover, the results have different degrees of accuracy due to the high subjectivity. Therefore, a method is needed to accurately and conveniently measure the tip position of an I-II composite crack.

[0004] Digital image correlation (DIC) technology is a non-contact modern optical measurement experimental technique, and its core principle is to track the change of gray value patterns in images to calculate the displacement and strain of the object surface during deformation. DIC technology applies a random speckle pattern to the object surface, and uses a high-resolution camera to capture images before and after deformation. The algorithm divides the image into multiple small regions (subsets), and tracks the displacement changes of these subsets through correlation calculation to obtain the deformation information of the object. The DIC method is most widely used in measuring crack tip field information in fracture mechanics due to its simple operation, economy, etc., including identifying the crack tip position, measuring the crack length, measuring the stress intensity factor (K), measuring the crack tip plastic zone size (r p ), and measuring the crack opening displacement (COD), etc. The DIC method measures the crack tip field based on displacement deformation, and the traditional measurement method is similar in that the physical coordinates of the crack tip are regarded as unknowns, the full-field displacement of the DIC experimental data is fitted to the theoretical analytical solution of the crack tip, and the crack tip position is determined by establishing an error function between the two and taking the extreme value.

[0005] However, the existing method for determining the position of the crack tip still has the following shortcomings:

[0006] (1) The displacement field obtained by fitting in the prior art is a theoretical result, and there is always artificial error with the actual test result. Different researchers select different fitting functions, resulting in uncertain results.

[0007] (2) The existing method usually needs to process a large amount of data, which is not conducive to rapid detection of the crack tip position.

[0008] (3) The existing crack tip detection method using the DIC method and the gradient change of the crack tip displacement field is mainly aimed at identifying the displacement field of the crack tip position of the I-type crack. The calculation method of the longitudinal coordinate of the crack given in the method is also based on the I-type crack. The calculation of the longitudinal coordinate is not involved in the corresponding examples.

[0009] However, the propagation path of the I-II composite crack is very different from that of the I-type crack. The current DIC method is not completely applicable to the I-II composite crack. Therefore, how to propose a crack tip detection method for the I-II composite crack by means of the DIC method is a technical problem to be solved at present. SUMMARY

[0010] In view of the shortcomings in the prior art, the present application provides a DIC-based I-II composite crack tip position identification method. After the crack tip displacement field is calculated by DIC, a line segment parallel and equidistant to the crack surface is set on both sides of the crack surface according to the approximate propagation path of the I-II composite crack, and the vertical displacement difference between the two lines is calculated. The gradient change of the vertical displacement difference obtains the line segment region where the transverse coordinate of the crack tip is located, and a vertical line segment is drawn between the two parallel lines. The vertical displacement of the vertical line is calculated to determine the longitudinal coordinate of the crack tip, thereby realizing the identification of the crack tip position based on DIC and the gradient change of the crack tip displacement.

[0011] In order to achieve the goal of identifying the position of the I-II composite crack tip, the present application adopts the following technical scheme:

[0012] A DIC-based I-II composite crack tip position identification method, which has the following specific steps:

[0013] Step 1: For the sample to be identified, prepare a speckle on the surface of the sample;

[0014] Step 2: Collect an undeformed image of the sample as a reference image, apply an off-axis load to the sample to be identified to generate an I-II composite crack, and collect a crack propagation image of the sample;

[0015] Step 3: Calculate the crack propagation image by DIC to obtain the displacement field information of the crack propagation image;

[0016] Step 4: Based on the displacement field information of the crack propagation image, analyze the crack tip displacement field information to obtain the position coordinates of the I-II composite crack tip; including the following steps:

[0017] Step 4.1: Determine the crack propagation plane direction based on the crack path. Draw straight lines L1 and L2 parallel to the crack propagation plane on both sides. Use DIC to obtain the changes in the vertical displacement fields of L1 and L2 along the crack propagation direction. Find the point X where the vertical displacement change is maximum. i 'This is denoted as the region where the crack tip's transverse coordinates lie;

[0018] Step 4.2: via X i Draw a vertical line segment L3 between L1 and L2, and use DIC to obtain the change of vertical displacement on L3 along the longitudinal coordinate. Find the point where the vertical displacement change is the largest. i ' is denoted as the longitudinal coordinate of the crack tip.

[0019] Step 4.3: After obtaining the transverse and longitudinal coordinates of the crack tip from Steps 4.1 and 4.2, correct the crack surface direction; based on the crack surface direction, reintroduce the correction segments L1', L2', and L3', and repeat the analysis of the segment displacement field in Steps 4.1 and 4.2 to correct the crack tip position.

[0020] Furthermore, the steps in step 4.1 are as follows:

[0021] Step 4.1.1: Draw straight lines L1 and L2 parallel to the crack surface on both sides of the crack propagation surface, and divide L1 and L2 into n equally spaced points; denot the vertical displacement difference between corresponding points on L1 and L2 as W. i , i = 1, 2, 3...n, and record the horizontal coordinates corresponding to the vertical displacement differences;

[0022] Step 4.1.2: Calculate the vertical displacement difference W i The lateral coordinate corresponding to the point of greatest change is the lateral coordinate X of the crack tip. c '.

[0023] Furthermore, determine the maximum vertical displacement difference W. i The method for determining the change value is as follows: after obtaining the vertical displacement difference W... i Then, the difference between adjacent vertical displacements is calculated and denoted as ΔW. i The difference ΔW between the maximum displacements i The corresponding lateral coordinate is the lateral coordinate X of the crack tip. c '.

[0024] Furthermore, determine the maximum vertical displacement difference W. i The method for determining the change value is as follows: after obtaining the vertical displacement difference W...i Afterwards, the derivative of W i is taken, and the transverse coordinate corresponding to the extreme value of the derivative is denoted as the transverse coordinate X c ’ of the crack tip.

[0025] Further, the steps of step 4.2 are as follows:

[0026] Step 4.2.1: A line segment L3 is drawn perpendicular to L1 and L2 and between L1 and L2, L3 is divided into m equidistant points, and the vertical displacement of the points is denoted as V i ’. j The corresponding longitudinal coordinate is denoted as Y j ’. i

[0027] Step 4.2.2: The longitudinal coordinate corresponding to the maximum change in V j is taken as the longitudinal coordinate of the crack tip.

[0028] Further, the method for determining the maximum vertical displacement V j change value is as follows: the adjacent data of the vertical displacement V j is differenced to obtain the vertical displacement difference AV j ; the longitudinal coordinate corresponding to the maximum vertical displacement difference is denoted as the longitudinal coordinate Y c ’ of the crack tip.

[0029] Further, the method for determining the maximum vertical displacement V j change value is as follows: the derivative of the vertical displacement V j is taken to obtain dV i ; the longitudinal coordinate corresponding to the extreme value of the displacement derivative is denoted as the longitudinal coordinate Y c ’ of the crack tip.

[0030] Further, the steps of step 4.3 are as follows:

[0031] Step 4.3.1: After the crack tip position detection in step 4.1 and step 4.2, the crack propagation surface is further corrected based on the crack tip position, and correction lines L1’ and L2’ are drawn on both sides of the crack propagation surface, the displacement field of L1’ and L2’ is reprocessed according to step 4.1 to find the corrected transverse coordinate X c ’.

[0032] Step 4.3.2: The corrected transverse coordinate X c ’ is drawn as a correction vertical line segment L3’ between L1’ and L2’, and the displacement field of L3’ is reprocessed according to step 4.2 to find the corrected longitudinal coordinate Y c ’.

[0033] Further, step 3 includes the following steps:​

[0034] Step 3.1: first, the crack propagation image is divided into a plurality of sub-regions, and the sub-regions are divided into a plurality of sub-regions;

[0035] Step 3.2: the crack propagation image is calculated by using DIC to obtain the displacement field information of the crack propagation image.

[0036] Further, each sub-region contains 3 to 10 speckle points, and the analysis step is set to 1 / 10 to 1 / 4 of the size of the sub-region.

[0037] Further, the coordinate system of the DIC processing is denoted as X'-Y', the direction of the I type prefabricated crack is taken as the X axis, the vertical direction is taken as the Y axis, and the zero point is at the coordinate system of the crack tip of the specimen notch. Denoted as X-Y; according to the expansion information of the I-II composite crack (including the crack path and the included angle in the horizontal direction, the deflection angle of the crack path, etc.), the coordinate system is converted to each other.

[0038] The beneficial effects of the present application are:

[0039] (1) The detection process is simple, does not require high equipment, and does not need to determine the elastic-plastic parameters of the material. Only the expansion path of the I-II composite crack is roughly determined according to the experiment, and the displacement field change around the crack expansion surface is calculated according to the expansion path, so that the accurate crack tip position of the I-II composite crack can be determined, which is efficient and simple to operate.

[0040] (2) The analysis of the displacement field change contains the displacement caused by plastic deformation and crack opening, so whether it is small range deformation or large range, this method is applicable. Many theoretical formulas are only applicable to small range deformation, and cannot meet the identification of the tip position of the I-II composite crack under large range deformation.

[0041] (3) This method only needs to obtain the vertical displacement difference by DIC analysis, so the tip position of the crack can be identified relatively quickly during the crack propagation process, so that the crack length and the expansion rate can be quickly and accurately determined. BRIEF DESCRIPTION OF DRAWINGS

[0042] Figure 1 The flowchart of the present application.

[0043] Figure 2 The schematic diagram of the specimen placed off-axis.

[0044] Figure 3 The speckle pattern on the surface of the specimen.

[0045] Figure 4 The schematic diagram of the crack tip position detection. Wherein, (a) is a DIC analysis example schematic diagram, and (b) is a coordinate conversion schematic diagram.

[0046] Figure 5 is the curve of the vertical displacement difference as a function of the horizontal distance from the crack tip before correction. (b) is the curve of the derivative of the vertical displacement difference.

[0047] Figure 6 is the curve of the vertical position of the crack tip as a function of the vertical displacement before correction. (b) is the curve of the derivative of the vertical displacement.

[0048] Figure 7 is the curve of the vertical position of the crack tip as a function of the vertical displacement after correction. (b) is the curve of the derivative of the vertical displacement.

[0049] Figure 8 is the curve of the vertical position of the crack tip as a function of the vertical displacement after correction. (b) is the curve of the derivative of the vertical displacement. DETAILED DESCRIPTION

[0050] In order to make the objects, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely intended to explain the present application and are not intended to limit the present application.

[0051] The application utilizes the digital image correlation (DIC) technology, analyzes the change gradient of the displacement amount to detect the position of the I-II composite crack tip by calculating the displacement field near the I-II composite crack tip. After calculating the displacement field, the influence of the rigid body displacement needs to be removed, which can be directly realized in the post-processing software. Then, the approximate propagation path of the I-II composite crack is determined through the experiment, and each of the two sides of the crack propagation surface is provided with a line segment which is parallel to and equal in length to the crack surface. The approximate area of the transverse coordinate of the crack is determined by calculating the gradient change of the vertical displacement difference of the line segment along the crack propagation direction, and the area is a vertical line segment area between the two equal-length parallel line segments. The longitudinal coordinate of the crack is determined by the gradient change of the vertical displacement difference of the line segment area, so as to determine the transverse coordinate of the crack. Since the transverse coordinate and the longitudinal coordinate of the I-II composite crack are difficult to achieve the effect of setting the line segments which are parallel to and equal in length to the crack surface on the two sides of the actual crack surface when the displacement field method is used in the I-type crack, the crack surface needs to be corrected after the first determination of the coordinates, and the corrected line segment is introduced to re-detect the coordinate position, so as to further improve the detection accuracy and complete the position detection and identification of the I-II composite crack tip.

[0052] As shown in Figure 1 , a DIC-based I-II composite crack tip position identification method includes the following steps:

[0053] Step 1: Prepare a compact tensile shear specimen and make a speckle pattern on the surface of the specimen. In order to facilitate the accurate measurement of the out-of-plane displacement field near the crack tip, the speckle is uniformly and highly contrasted, so the speckle is made by: using matte white paint to cover the surface of the specimen, in order to obtain a uniform random anti-color speckle, using a spray gun to spray the alcohol graphite mixture on the surface of the specimen, and then obtaining a high-contrast speckle pattern, the size of the speckle point occupies 5-10 pixel values, and the distance between the speckle points is also 5-10 pixel values, as shown in Figure 2 .

[0054] Step 2: Collect the image of the undeformed specimen as the reference image, and perform off-axis loading on the specimen to make the specimen expand the I-II composite crack. In the experiment, the crack propagation image is collected as the analysis image, and the propagation direction of the crack is determined through the propagation surface of the crack.

[0055] Step 3: Calculate the crack propagation image by using DIC to obtain the displacement field information of the crack propagation image, as follows:

[0056] Step 3.1: First, the crack propagation image is divided into a region of interest, and the region of interest is divided into a number of sub-regions, and each sub-region contains 3 to 10 speckle points. The analysis step can be set to 1 / 10 to 1 / 4 of the sub-region size. Smaller step size can improve the accuracy of the analysis, but it will take longer to analyze and increase the cost of calculation. Therefore, when setting the step size, the accuracy and time cost need to be considered.

[0057] Step 3.2: Calculate the crack propagation image using DIC to obtain the displacement field information of the crack propagation image.

[0058] Step 4: Based on the displacement field information of the crack propagation image, analyze the crack tip displacement field information to obtain the I-II composite crack tip position coordinates.

[0059] Step 4.1: Determination of the region where the crack tip transverse coordinates are located

[0060] Step 4.1.1: Since the crack path of the I-II composite crack is related to the angle of the off-axis loading, it is not usually expanded along the horizontal direction like the I-type crack. Therefore, the overall expansion direction of the crack needs to be determined according to the crack path, so as to determine the approximate expansion surface direction of the crack. Step 4.1.1: Draw a straight line on both sides of the crack propagation surface, which is as parallel to the crack surface as possible and is approximately equidistant from the crack surface, and is denoted as L1 and L2 respectively. Calculate and analyze the change of the vertical displacement field of L1 and L2 in the crack propagation direction using DIC post-processing software, as shown in Figure 4 The specific operation is as follows:

[0061] Divide L1 and L2 into a number of equidistant points, and the number of points is denoted as n. The vertical displacement difference of each corresponding point in the two lines is denoted as W i (i = 1, 2, 3…n), and record the transverse coordinate position corresponding to the vertical displacement difference. Since the transverse coordinate position corresponding to the vertical displacement difference of L1 and L2 is the coordinate axis in the horizontal direction by default, the transverse coordinate position corresponding to the vertical displacement difference is obtained based on the horizontal coordinate axis. Although the position of the I-II composite crack tip will not affect the calculation result of the crack propagation length under the reference of different coordinate systems after being determined, in order to describe the crack tip position of the specimen with different off-axis loading angles in the same coordinate system, the final crack tip position calculation result will further process the coordinate result calculated by the DIC post-processing software, and convert it into the coordinate result with the I-type pre-crack direction as the X axis, the vertical direction as the Y axis and the zero point at the specimen notch tip. Therefore, the coordinate system of the DIC post-processing software is denoted as X'-Y' coordinate system, and the coordinate system with the I-type pre-crack direction as the X axis is denoted as X-Y coordinate system. The transverse coordinate corresponding to the vertical displacement difference is denoted as X i(i = 1, 2, 3…n), and the lateral coordinate of the X-Y coordinate system is denoted as X i The vertical displacement difference of the corresponding points on L1 and L2 can be transformed according to the analysis results.

[0062] Step 4.1.2: After obtaining the vertical displacement difference of each corresponding point in L1 and L2, the adjacent vertical displacement difference is further differentiated, denoted as ΔW i (ΔW i = W i-1 -W i ). In the case where the lateral coordinate of the crack is unknown, the coordinate value of the lateral coordinate of the crack tip is assumed to be X c ', and when the crack opens, the vertical displacement difference W c of the region behind the crack tip X i ' where the crack has expanded contains both the crack opening displacement and the displacement amount of plastic deformation perpendicular to the crack surface; while the vertical displacement difference W c of the region in front of the crack tip X i ' only contains the displacement amount of plastic deformation perpendicular to the crack surface. Therefore, there is a maximum displacement difference ΔW i in the expansion direction of the crack. In addition, the derivative of W i is taken, and the X i ' corresponding to the extreme value of the derivative is also the X i ' corresponding to the maximum displacement difference ΔW c . The specific steps include:

[0063] (1) Differentiate the adjacent data of the vertical displacement difference W i obtained in the second point to obtain ΔW i (or directly take the derivative of W i ).

[0064] (2) Match ΔW i and the corresponding X i ', that is, start matching from ΔW2 and X2' of i = 2, and so on, to complete the matching of the remaining ΔW i and X c '(or match the derivative of W i and X i ', that is, match dW1 and X i ' of i = 1, and so on, to match dW i and X i ').

[0065] Find the lateral coordinate region corresponding to the maximum ΔW i or dW i extreme value. For a type I crack, the corresponding coordinate position Xi This represents the transverse coordinate of the crack tip. However, for I-II composite cracks, only the corresponding coordinate position X is found. i The transverse coordinates of the crack tip cannot be determined yet; only the region containing the transverse coordinates of the crack tip can be determined to be located after passing through X. i 'The region of line segments perpendicular to L1 and L2 and located between L1 and L2.' Specific steps include:

[0066] (1) For W i ΔW can be obtained by differentiation or calculation. i As the dependent variable, X i 'Plot as an independent variable'.

[0067] (2) Locate the dW near the crack tip in the diagram. i extreme points or ΔW i Find the peak point and its corresponding X coordinate. i ', then the region where the crack tip's lateral coordinates are located is determined to be after passing through X i ', on the line segment region perpendicular to L1 and L2 and between L1 and L2.

[0068] Step 4.2: Determining the transverse and longitudinal coordinates of the crack tip

[0069] Step 4.2.1: The region where the crack tip's transverse coordinates are located has been obtained, passing through X... i 'A line segment perpendicular to L1 and L2 and located between L1 and L2, denoted as L3, and the vertical displacement of L3 as a function of the longitudinal coordinate is calculated as follows:'

[0070] (1) Divide L3 into m equally spaced points, and denote the vertical displacement of each point as V. j (j = 1, 2, 3...m).

[0071] (2) The vertical displacement V j The corresponding vertical coordinate is Y i (j = 1, 2, 3...m).

[0072] Step 4.2.2: Using V j The trend of change determines the longitudinal coordinate of the crack. Assuming the longitudinal coordinate value at the tip of the I-II composite crack is Y... c When a crack opens, the two sides of the crack propagation surface behind the crack tip are not constrained by the material's toughness, so at the crack tip position Y... c At point '', the displacement perpendicular to the crack surface is the largest, and the vertical displacement calculated by DIC is also the largest. The vertical displacement difference is obtained by subtracting adjacent vertical displacements, denoted as ΔV. j (ΔV j =V j-1 -V j ), or directly to Vj Derivation, the steps are as follows:

[0073] (1) The difference between the adjacent data of the vertical displacement V j is obtained, and ΔV j is obtained (or the derivative of V j is directly derived).

[0074] (2) ΔV j is matched with the corresponding Y j ', that is, the corresponding ΔV2 and Y2' from j = 2 are matched, and the remaining ΔV j and Y j ' are matched (or the derivative of V j is matched with Y j ', that is, the corresponding dV1 and Y1' from j = 1 are matched, and dV i and Y j ' are matched).

[0075] More specifically, the longitudinal coordinate corresponding to the maximum vertical displacement difference or displacement derivative extremum is found. The specific steps include:

[0076] (1) Derivation of V j or calculation of ΔV j as the dependent variable, Y j as the independent variable is plotted.

[0077] (2) In the graph, find the extremum point of dV j near the crack tip or the peak point of ΔV j , find the corresponding coordinate Y j ', and the longitudinal coordinate of the crack tip in the X'-Y' coordinate system can be determined.

[0078] Since the lateral coordinate region of the crack tip is determined to be located on L3, after the longitudinal coordinate of the crack tip in the X'-Y' coordinate system is determined, (X c ', Y c ') is obtained. However, since the I-type crack expands along the horizontal direction before the displacement field measurement, as long as the displacement field of the horizontal line segment is detected, and as long as the line segments are symmetric to the sample tip, the symmetry degree of the crack surface on both sides can be ensured, and the I-II composite crack cannot be determined before detection. Therefore, before the displacement field measurement, the parallel and equidistant line segments L1 and L2 are made on both sides of the approximate expansion surface of the crack, and there will be a certain parallel and equidistant error between the actual crack surface. Therefore, (X c ', Y c ') obtained can be used as a correction of the true crack surface, thereby improving the calculation accuracy.

[0079] Step 4.3: Correction of the transverse coordinate and the longitudinal coordinate of the crack tip

[0080] Step 4.3.1: After the position of the crack tip is determined, the propagation surface is re-corrected according to the position of the crack tip, and the correction line segments L1' and L2' parallel and equidistant to the propagation surface are introduced and corrected, so as to further optimize the detection accuracy. The displacement field of L1' and L2' is re-processed according to step 4.1 to find the corrected transverse coordinate X c ’.

[0081] Step 4.3.2: The corrected transverse coordinate X c ’ is corrected as a vertical line segment L3' between L1' and L2', and the displacement field of L3' is re-processed according to step 4.2 to find the corrected longitudinal coordinate Y c ’.

[0082] According to the crack propagation path (including the angle between the crack path and the horizontal direction, the deflection angle of the crack path, and other information), the tip position (X c , Y c ) of the I-II composite crack can be further converted according to the corrected (X c ’, Y c ), and the tip position of the I-II composite crack is thus determined.

[0083] In order to verify the specific effect of the method for measuring the tip position of the I-II composite crack according to the present application, a 30° off-axis loading I-II composite fatigue crack propagation test is carried out on a compact tension shear specimen according to the foregoing steps, the image of the specimen before loading is taken as the reference image, the images at different cycle times after the experiment starts are taken as the target images, the crack tip positions at different cycle times are identified by the method according to the present application, and then the crack propagation length is obtained.

[0084] As shown in Figure 4 a, a Cartesian coordinate system is established with the first point at the left end of L1 and L2 as the coordinate origin. In order to unify the description of the crack tip position, the geometric shape of the specimen is used as a reference to describe the final position result, and the horizontal direction of the DIC post-processing is used as the X' axis and the vertical direction as the Y' axis in the default coordinate system during the intermediate process. As shown in the figure, the propagation direction of the I-type pre-crack is taken as the X axis, and the direction perpendicular thereto is taken as the Y axis. The zero point is set at the deflection point position from the pre-crack to the I-II composite crack in the analysis process, so after the transverse coordinate of the crack tip is converted, the pre-crack length also needs to be added to ensure that the description of the tip position of the I-II composite crack is consistent with that of the I-type crack. As shown in Figure 5 , after the horizontal distance from the crack tip to the crack deflection point is obtained, it needs to be converted into the X cThe horizontal coordinate of the I-II mixed mode crack obtained by the DIC post-processing is the projection L' of the real length of the I-II mixed mode crack stage expansion in the horizontal direction, the included angle between the crack expansion path and the prefabricated crack is α, and the included angle of the horizontal positive direction is β, then L' is converted into the length of the I type prefabricated crack in the expansion direction, and the conversion formula is:

[0085]

[0086] Wherein, in Figure 4 The sum of α and β in the crack expansion direction should be the degree of off-axis loading.

[0087] Since the I-II mixed mode crack path does not know the exact crack tip position in the approximation, after determining the length of L', the specific position of the crack tip on L3 cannot be determined, therefore, α and β cannot be accurately determined, the specific position of the crack tip on L3 needs to be obtained, and then the coordinates of the crack tip can be further determined.

[0088] As shown in Figure 4 , after determining the crack tip region, a line segment L3 is set. As shown in Figure 6 , the Y c ' coordinate of the crack tip is determined by deriving the vertical displacement difference of L3, then the coordinates (X c ',Y c ') of the crack tip in the X'-Y' coordinate system are obtained, and (X c ,Y c ) can be further solved by the value of Y c . In the DIC post-processing, the coordinates of the deflection point of the prefabricated crack after off-axis loading are obtained, and the coordinates are marked as (X1',Y1'), then β can be solved by the following formula:

[0089]

[0090] Then the solving formula of the coordinates (X c ,Y c ) of the crack tip in the X-Y coordinate system is:

[0091]

[0092] Wherein, a0 is the length of the prefabricated crack, and the length of the prefabricated crack in the experiment is 2mm.

[0093] Table 1, Table 2 and Table 3 are respectively the measurement results of X c , Y c and the crack length before the precision correction in the method of the application, and Table 4, Table 5 and Table 6 are respectively the measurement results of X c , Y cand the measurement result of the crack length, wherein the crack length does not consider the pre-crack length, is approximately calculated by (X c ,Y c ) to the linear distance of the crack deflection point.

[0094] Table 1

[0095]

[0096] Table 2

[0097]

[0098] Table 3

[0099]

[0100] Table 4

[0101]

[0102] Table 5

[0103]

[0104] Table 6

[0105]

[0106] From the data in the table, it can be seen that, whether the coordinate result or the crack length result, the relative error with the microscope observation result before the correction is not more than 5%, and the relative error with the microscope observation result after the correction is basically optimized in accuracy, which shows that the proposed method can accurately identify the crack tip position.

[0107] The above examples are only used to illustrate the design idea and characteristics of the present application, and the purpose is to enable those skilled in the art to understand the content of the present application and implement it, and the protection scope of the present application is not limited to the above examples. Therefore, any equivalent changes or modifications made according to the principles and design ideas disclosed by the present application are within the protection scope of the present application.

Claims

1. A method for identifying the location of the tip of a type I-II composite crack based on DIC, characterized in that, The specific steps of this method are as follows: Step 1: Prepare speckle patterns on the surface of the sample to be identified; Step 2: Acquire an undeformed image of the specimen as a reference image, apply an off-axis load to the specimen to be identified to generate a type I-II composite crack, and acquire an image of the crack propagation of the specimen. Step 3: Calculate the displacement field information of the crack propagation image using DIC; Step 4: Analyze the crack tip displacement field information based on the crack propagation image to obtain the position coordinates of the I-II composite crack tip; including the following steps: Step 4.1: Determine the crack propagation plane direction based on the crack path. Draw straight lines L1 and L2 parallel to the crack propagation plane on both sides. Use DIC to obtain the changes in the vertical displacement fields of L1 and L2 along the crack propagation direction. Find the point X where the vertical displacement change is maximum. i 'This is denoted as the region where the crack tip's transverse coordinates lie; Step 4.2: via X i Draw a line segment L3 perpendicular to L1 and L2 and located between L1 and L2. Use DIC to obtain the change of vertical displacement on L3 along the longitudinal axis. Find the point where the vertical displacement change is the largest. i ' is denoted as the longitudinal coordinate of the crack tip; Step 4.3: After obtaining the transverse and longitudinal coordinates of the crack tip from Steps 4.1 and 4.2, correct the crack surface direction; based on the crack surface direction, reintroduce the correction segments L1', L2', and L3', and repeat the analysis of the segment displacement field in Steps 4.1 and 4.2 to correct the crack tip position.

2. The method for identifying the tip position of a type I-II composite crack based on DIC according to claim 1, characterized in that, Step 4.1 is as follows: Step 4.1.1: Draw straight lines L1 and L2 parallel to the crack surface on both sides of the crack propagation surface, and divide L1 and L2 into n equally spaced points; denot the vertical displacement difference between corresponding points on L1 and L2 as W. i , i = 1, 2, 3...n, and record the horizontal coordinates corresponding to the vertical displacement differences; Step 4.1.2: Calculate the vertical displacement difference W i The lateral coordinate corresponding to the point of greatest change is the lateral coordinate X of the crack tip. c '.

3. The method for identifying the location of the I-II composite crack tip based on DIC according to claim 2, characterized in that, Determine the maximum vertical displacement difference W i The method for determining the change value is as follows: after obtaining the vertical displacement difference W... i Then, the difference between adjacent vertical displacements is calculated and denoted as ΔW. i The difference ΔW between the maximum displacements i The corresponding lateral coordinate is the lateral coordinate X of the crack tip. c '.

4. The method for identifying the tip position of a type I-II composite crack based on DIC according to claim 2, characterized in that, Determine the maximum vertical displacement difference W i The method for determining the change value is as follows: after obtaining the vertical displacement difference W... i Afterwards, regarding W i Taking the derivative, the lateral coordinates corresponding to the extreme values ​​of the derivative are marked as the lateral coordinates X of the crack tip. c '.

5. The method for identifying the location of the I-II composite crack tip based on DIC according to claim 1, characterized in that, Step 4.2 is as follows: Step 4.2.1: via X i Draw a line segment L3 perpendicular to L1 and L2, and between L1 and L2. Divide L3 into m equally spaced points, and denote the vertical displacement of each point as V. j V j The corresponding vertical coordinate is Y i '; Step 4.2.2: V j The longitudinal coordinate corresponding to the point of greatest change is taken as the longitudinal coordinate of the crack tip.

6. The method for identifying the location of the I-II composite crack tip based on DIC according to claim 5, characterized in that, Determine the maximum vertical displacement V j The method for changing the value is as follows: for the vertical displacement V j The vertical displacement difference ΔV is obtained by taking the difference between adjacent data. j The longitudinal coordinates corresponding to the difference in maximum vertical displacement are marked as the longitudinal coordinate Y of the crack tip. c '.

7. The method for identifying the location of the I-II composite crack tip based on DIC according to claim 5, characterized in that, Determine the maximum vertical displacement V j The method for changing the value is as follows: for the vertical displacement V j Taking the derivative, we get dV i The longitudinal coordinates corresponding to the extreme values ​​of the displacement derivative are marked as the longitudinal coordinates Y of the crack tip. c '.

8. The method for identifying the tip position of a type I-II composite crack based on DIC according to claim 1, characterized in that, Step 3 includes the following steps: Step 3.1: First, divide the crack propagation image into regions of interest, and divide the regions of interest into several sub-regions; Step 3.2: Use DIC to calculate the displacement field information of the crack propagation image.

9. The method for identifying the location of the I-II composite crack tip based on DIC according to claim 8, characterized in that, Each subregion contains 3 to 10 speckles, and the analysis step size is set to 1 / 10 to 1 / 4 of the subregion size.

10. The method for identifying the tip position of a type I-II composite crack based on DIC according to claim 1, characterized in that, The coordinate system for DIC treatment is denoted as X'-Y', and the coordinate system with the direction of the pre-crack type I as the X-axis, the vertical direction as the Y-axis, and the zero point at the tip of the notch of the specimen is denoted as XY; Based on the actual analysis needs, the coordinate systems are transformed.

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