Method for testing I-type fracture toughness of fiber reinforced resin-based composite material

Abstract

The invention discloses a method for testing the I-type fracture toughness of a fiber reinforced resin matrix composite, and relates to the field of mechanical property testing of composites.A camera is used for shooting image data of crack propagation of a sample in real time, a series of digital image processing methods are adopted, and in combination with the propagation direction, the I-type fracture toughness of the sample at the t moment in stress loading is calculated; crack tip coordinate positions of layered expansion are determined; calculating the equivalent crack length Lt at the moment t; and after flexibility correction, large displacement correction and loading block influence correction are sequentially carried out by utilizing the equivalent crack length Lt, the GIC value of the sample material is calculated, and an R curve is drawn. According to the method, the threshold range of the standard deviation of the transverse displacement and the longitudinal displacement is set as the criterion of the crack tip position, so that the problems that the crack tip position is difficult to identify and the crack length data is inaccurate in the I-type interlayer fracture toughness test are solved.

Description

Technical Field

[0001] The present invention relates to the field of composite material mechanical property testing, in particular to a method for testing the mode I fracture toughness of a fiber reinforced resin-based composite material. Background Art

[0002] High-performance fiber-reinforced composites are highly promising engineering materials in fields such as aerospace due to their lightweight and high strength characteristics. Interlaminar fracture is the most common failure mode of composite materials and poses a serious threat to the structural safety of spacecraft. Among them, opening interlaminar fracture (Type I) is the most common delamination damage mode. Type I interlaminar fracture toughness is an important performance parameter in the structural design of composite materials and an important indicator for predicting the delamination expansion and failure damage mechanism of composite materials. Therefore, a reliable and efficient test and evaluation method for Type I interlaminar fracture toughness of composite materials can better meet the market demand in the field of composite material testing and provide data support for the research and development, design, and application of composite materials.

[0003] The ASTM D5528 / D5528M test method is generally used to evaluate Mode I (i.e., crack propagation) interlaminar fracture toughness. This method calculates the critical energy release rate (GIC) of the material by recording the crack length during delamination in real time, along with the applied force and displacement. However, due to interference from factors such as bridging and unsteady growth during delamination, identifying the crack tip location is a practical challenge. Furthermore, the calculation of the critical energy release rate requires extensive data fitting of parameters related to compliance and crack length, further complicating the test method and limiting its widespread adoption.

[0004] Currently, the most common method for measuring crack length is to observe and record it manually using an optical microscope. This method is cumbersome and requires constant pauses during the experiment to observe and record the crack length. This causes discontinuity in the composite material's crack growth process, interferes with the propagation of steady-state crack growth, and results in low experimental efficiency. Furthermore, crack growth may be accompanied by metastable growth or jump-like growth. Manual offline measurement cannot record these dynamic behaviors, making it impossible to simultaneously obtain the real-time load and displacement during crack growth, and to analyze the correlation between crack growth length and the driving force of crack growth. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for testing the mode I fracture toughness of fiber reinforced resin-based composite materials to solve the problems raised in the above background technology.

[0006] To solve the above technical problems, the present invention provides the following technical solution: a method for testing the mode I fracture toughness of a fiber-reinforced resin-based composite material, comprising the following steps: The image of the crack propagation of the specimen is captured in real time by a camera, and the position of the crack tip is determined by a digital image processing method, which specifically includes: pre-processing the image to eliminate the influence of noise; The set of pixels in the image is defined as a subset, the shape function is selected and the displacement field is calculated to achieve accurate matching of the subsets before and after deformation, which serves as the basis for calculating the displacement standard deviation. The standard deviation of transverse and longitudinal displacements is calculated to locate the crack tip and a subset of regions exceeding a set threshold is selected; Multi-frame tracking verification: Using the continuity and speed of crack propagation, a crack tip motion prediction model is established. The coordinates of the center point of the selected area subset are input into the crack tip motion prediction model to obtain the coordinates of the crack tip of the specimen under load at time t. Calculate the equivalent crack length L at time t t ; Using the equivalent crack length L t， After performing compliance correction, large displacement correction, and loading block effect correction in sequence, the G of the sample material is calculated. ⅠC value and draw the R curve.

[0007] Preferably, one or more of grayscale normalization, CLAHE contrast enhancement, non-local mean denoising, and speckle sharpening methods are used for image preprocessing.

[0008] Preferably, the calculation formulas for the standard deviations of the lateral and longitudinal displacements are as follows: ; ; in, is the standard deviation of the lateral displacement of pixels in the subset area, is the standard deviation of the longitudinal displacement of the pixels in the subset area, and the thresholds of the lateral displacement and the longitudinal displacement are set respectively. or When the threshold is exceeded, the center point of the subset area is defined as the crack tip candidate point; is the lateral displacement of pixel i, is the average value of the horizontal displacement of the pixel points in this column, is the longitudinal displacement of pixel j, is the average longitudinal displacement of the pixels in the row, and n represents the number of pixels in the column or row in the subset area.

[0009] Preferably, the crack tip prediction motion model is:

[0010] Among them, x t 、y tis the candidate crack tip position coordinate at time t, x t-1 ,y t-1 is the crack tip position coordinate at time t-1, v x 、v y It is determined based on historical data. The historical data is the average rate calculated from the change of the crack tip coordinates in the previous frame. Δt is the time interval between two frames. , is a random error; There is also a physical constraint: 0.1 pixel / frame <v x <10 pixels / frame, 0.1 pixels / frame <v y <10 pixels / frame, displacement angle change <30°; when physical constraints are met: =1, otherwise =0; The initial confidence is set to 1, and the indicator function of the current frame And the confidence weight of the previous frame, calculate the confidence weight C of the current frame t :

[0011] For consecutive frames, when C t When it is greater than or equal to the set threshold, the point is directly accepted as the crack tip position at time t; when C t When it is less than the set threshold, the crack tip position predicted by the crack tip motion model is used, and the local crack tip position search is performed again until an acceptable crack tip position coordinate is obtained.

[0012] Preferably, the camera includes a first camera and a second camera, the first camera is fixed on the front side of the sample loading position, and the second camera is fixed on the back side of the sample loading position; In each loading and unloading step, when loading stops and during unloading, the first camera captures the front crack tip position, and the second camera captures the back crack tip position. The deviation between the two is compared to verify whether the loading is symmetrical.

[0013] Preferably, the equivalent crack length L at time t is t The calculation formula is as follows: ; Among them, L t is the equivalent crack length at time t, ( , ) is the crack tip coordinate at time t, ( , ) is the coordinate of the crack tip at the initial moment.

[0014] Preferably, the specimen flexibility correction adopts the following formula: ; ; in, is the crack length after specimen flexibility correction, is the opening angle during the sample delamination process, is the displacement at time t, L t is the equivalent crack length at time t.

[0015] Preferably, the fracture toughness of the test specimen is evaluated Using the formula: ; in, is the critical energy release rate, B is the width of the sample, is the force load at time t. F and N are the correction factors for large displacement and loading block, respectively.

[0016] Correction for large displacements and loading block effects is done using the following formula; ; ; Where F is the large displacement correction factor, t is the distance from the center of the loading block to 1 / 4 of the specimen thickness; N is the loading block correction factor, is the distance from the center of the loading block to its edge.

[0017] Preferably, white coating is randomly sprayed on both sides of the sample, and the speckle diameter formed by the white coating is calculated using the following formula: PX; Where R is the speckle diameter, FOV is the field of view, PPI is the camera resolution, and PX is the number of target pixels.

[0018] Compared with the prior art, the present invention has the following beneficial effects: In the field of composite material mode I fracture toughness testing, the present invention applies DIC technology to dynamic crack propagation identification and monitoring. By setting the threshold range of the standard deviation of the lateral displacement and the longitudinal displacement as the criterion for the crack tip position, the present invention solves the problems of low efficiency and susceptibility to human operation errors in manual visual observation of the dynamic crack tip position and manual measurement of the dynamic crack length in mode I interlaminar fracture toughness testing, thereby achieving high-precision automatic measurement of the dynamic crack length in the crack propagation of composite material mode I interlaminar fracture toughness. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a schematic diagram of the layout of the test-related devices of the present invention; Figure 2Schematic diagram of crack propagation during the delamination and cracking process of the sample of the present invention; Figure 3 This is a schematic diagram of the sample structure after the loading block is attached to the sample; Figure 4 This is a schematic diagram of the test structure parameters of the sample of the present invention; Figure 5 is the force-displacement-crack tip coordinate correlation curve; Figure 6 Comparison of the R curve of critical energy release rate GIC (sample 1: T700 carbon fiber / bisphenol A epoxy resin); Figure 7 Comparison of R curves for critical energy release rate GIC (sample 2: T700 carbon fiber / PEEK resin); Figure 8 Comparison of R curves of critical energy release rate GIC (sample #3: glass fiber / bisphenol A epoxy resin).

[0020] In the figure: 1. Second camera; 2. Background plate; 3. Electronic universal testing machine; 4. Loading fixture; 5. First camera; 6. Loading block; 7. Specimen. DETAILED DESCRIPTION

[0021] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0022] See also Figure 1 As shown in FIG, the test method for mode I fracture toughness of fiber reinforced resin matrix composite materials includes the following steps: (1) Bond the loading block 6 to the surface of the sample 7, as shown in Figure 3 , lightly polish the surface of the loading block 6 and the specimen 7, select appropriate adhesive to stick them together, ensure that they will not become debonded during the loading process, and meet the requirements of rigid connection with the testing machine; In step (1), the length of the sample 7 is ≥140 mm, the width B is between 20-25 mm, the thickness is between 3-5 mm, and the initial delamination length L0 from the tip of the embedded film at the center of the thickness of the sample 7 to the load application point is 50 mm; Specifically, the sample 7 used in this example is a DCB sample 7, which is a 0° unidirectional 20-ply sample #1 (T700 carbon fiber / bisphenol A epoxy resin composite material). A vernier caliper and micrometer are used to measure the initial crack length L0 = 50.28 mm from the tip of the embedded film at the center of the thickness of the DCB sample 7 to the load application point. The length of the sample 7 is measured as a = 150.28 mm, and the thickness is recorded as h = 4.14 mm. The width of the sample 7 is measured and recorded as B = 24.96 mm. The width of the loading block 6 is 24.88 mm. The width of the loading block 6 is between 20-25 mm. Use 500-grit sandpaper to lightly polish the surface of the loading block 6 and the specimen 7 and wipe them with 95% ethanol solvent. Use a two-component epoxy resin adhesive to stick them together to ensure that they will not debond during the loading process and meet the requirements of rigid connection with the testing machine.

[0023] (2) White coating was randomly sprayed on both sides of the sample 7 to form speckles, and scale lines were marked with a laser, with thin vertical lines marking every 1 mm. The purpose of the speckles was to enhance the accuracy of camera recognition, so the size of the speckles was also calculated and specified.

[0024] In step (2), the coating should have low gloss and no reflection characteristics, and the coating thickness should be less than 50μm; In the embodiment, the coating thickness is controlled at 30 μm. Based on the FOV field of view of 220 mm, the camera resolution of 3000 pixels, the single pixel size of 0.0733 mm, and the target number of pixels of 5, the speckle diameter should be controlled at approximately 0.3665 mm according to formula (1). After the speckle is formed on the side of sample 7, the scale line is marked with a laser, and each 1 mm is marked with a thin vertical line; (1) Where R is the speckle diameter, FOV is the field of view, PPI is the camera resolution, and PX is the number of target pixels; The camera includes a first camera and a second camera, the first camera is fixed on the front side of the sample 7 loading position, and the second camera is fixed on the back side of the sample 7 loading position; The first camera uses a high-speed camera with a resolution of 6 million pixels and a frame rate of 1000 frames per second.

[0025] The second camera is a common camera with a resolution of 3 million pixels.

[0026] The specific layout is as follows: (3) Place the first camera 5 and a 50W fill light in front of the electronic universal testing machine 3. When the horizontal distance between the sample 7 and the first camera 5 is 380 mm, adjust the camera focus so that the crack propagation area can be clearly and completely displayed in the software display area. A black matte background plate and a second camera 1 are placed directly behind the camera. A loading fixture 4 is used to clamp the sample 7 on the electronic universal testing machine 3 at the center. The data transmission and signal communication between the electronic universal testing machine 3, the first camera 5, and the second camera 1 are completed through the computer. Figure 1 Show; The first camera 5 is used to accurately identify the crack tip position of the interlayer crack and realize the dynamic crack tip coordinate (x t ,y t ) real-time capture, in the selection, the resolution of the first camera 5 is ≥ 5 million pixels and the frame rate is ≥ 1000 frames / second; The second camera 1 is used to assist in determining the crack tip position and crack propagation status and verify the validity of the result data. Preferably, the resolution is ≥ 2 million pixels. If the deviation of the crack tip position recorded by the cameras on the front and rear sides does not exceed 2 mm, it means that the loading is symmetrical and the test can continue. Otherwise, the test fixture needs to be adjusted and the test restarted.

[0027] (4) After assembly is completed, the initial and secondary force loading and unloading tests are carried out. The initial and secondary loading rates are controlled at 1-5 mm / min, and the unloading rate is 1-25 mm / min. The crack tip position coordinates (x t ,y t ), load P t , displacement δ t Data. The initial loading and unloading process propagates the crack from the tip of the embedded film to a depth of 53-55mm. This initial cracking typically exhibits unstable, jumpy expansion, failing to form a stable crack tip. After the initial pre-cracking, the second cracking process continues beyond 55mm, resulting in stable crack propagation and reflecting the material's true interlaminar fracture toughness. Therefore, the crack propagation during the secondary loading and unloading process serves as the primary detection and identification scenario.

[0028] Initial loading was performed at a rate of 1 mm / min. When the crack tip expanded 3–5 mm from the initial point, loading was stopped and unloading began at a rate of 1 mm / min. When unloading reached 50%, the positions of the crack tips on both sides of specimen 7 (Lfront = 55.08 mm, Lback = 55.47 mm) were recorded using a front camera and a conventional back camera. The deviation did not exceed 2 mm, indicating symmetrical loading and continued testing. Unloading and data recording continued until the load reached zero.

[0029] A secondary load was performed at a loading rate of 1 mm / min. The crack tip position coordinates (xt, yt), load Pt, and displacement δt were recorded during the delamination growth process. When the crack tip had extended more than 30 mm from the initial point, loading was stopped and unloading began at a rate of 1 mm / min. The crack tip position coordinates (xt, yt), load P, and displacement δ were recorded during the unloading process. When unloading reached 50%, the crack tip positions on both sides of specimen 7 were recorded (Lpositive = 85.96 mm, Lback = 85.11 mm). The deviation did not exceed 2 mm, indicating symmetrical loading and continued testing. Unloading was continued and data recorded until the displacement reached zero.

[0030] (5) Both loadings in step (4) involve dynamic crack monitoring, which is achieved using the method described in step (5).

[0031] After the first camera 5 on the front side captures the image in step (3), the crack tip position is determined by image preprocessing → displacement field calculation → crack tip location → multi-frame tracking; Image preprocessing may adopt one or more of grayscale normalization, CLAHE contrast enhancement, non-local mean denoising, and speckle sharpening methods.

[0032] The grayscale normalization formula used in this embodiment to eliminate the influence of uneven illumination is as follows:

[0033] Among them, I norm is the grayscale value after normalization, I is the original grayscale value, α and β are adjustable parameters, α>0 (typically 0.05-0.5), β=0-255 (typically 50-200), representing the steepness of the curve and the position of the center point respectively. Adjusting α and β can change the contrast and brightness of the curve. Speckle sharpening uses the Laplace kernel:

[0034] The shape function is an affine transformation model, and the Inverse Composite Gauss-Newton method (IC-GN) is used as the core implementation algorithm for sub-pixel image registration. The shape function and matching algorithm are selected to achieve sub-pixel precision displacement field calculation.

[0035] Next, the standard deviation of the transverse displacement in the length direction and the standard deviation of the longitudinal displacement in the thickness direction of specimen 7 are used as the criterion for the crack tip position, as calculated by formula (2) and formula (3); (2) (3) The set of pixels in the image is defined as a subset. , represents the lateral displacement, represents the longitudinal displacement, where is the standard deviation of the lateral displacement of pixels in the subset area, is the standard deviation of the longitudinal displacement of the pixels in the subset area, is the lateral displacement of the pixel, is the average value of the horizontal displacement of the pixel points in this column, is the longitudinal displacement of the pixel, is the average longitudinal displacement of the pixels in the row, and n is the number of pixels in the column or row in the subset area; Furthermore, in step (5), The threshold range is 0.01~0.30, The threshold range is 0.005 to 0.015. Preferably, the threshold of the standard deviation of the horizontal displacement is set to 0.02, and the threshold of the standard deviation of the vertical displacement is set to 0.005. >0.02 or For the subset with a value greater than 0.005, the center of the most advanced subset is taken as the candidate crack tip point.

[0036] Specifically, at time t, there are k subsets in all subsets that satisfy >0.02 or > 0.005, obtain the center point coordinates of the k subsets, recorded as (x m ,y m ), m = 1 ~ k, the coordinates of the crack starting point are (x0, y0), then the coordinates of the candidate crack tip point at time t (x t ,y t ) candidate for: (x t ,y t ) candidate =(x p ,y p ) {p|(x p -x0) 2 +(y p -y0) 2 =max(x m -x0) 2 +(y m -y0) 2 ,m=1~k} Specifically, the continuity and limited speed of crack propagation are used to filter out noise and false matches, and a confidence mechanism is used to evaluate and maintain the overall quality of the tracking process. A crack tip motion prediction model is established:

[0037] Among them, x t 、yt is the candidate crack tip position coordinate at time t, x t-1 ,y t-1 is the crack tip position coordinate at time t-1, v x 、v y The historical data is determined based on the average rate calculated from the change in the crack tip coordinates of the previous frame. In this embodiment, preferably, Δt is 0.01s. , 0.5 pixels; another physical constraint: 0.1 pixels / frame <v x <10 pixels / frame, 0.1 pixels / frame <v y <10 pixels / frame, displacement angle change <30°; when physical constraints are met: =1, otherwise =0.

[0038] The initial confidence is set to 1, and the indicator function of the current frame And the confidence weight of the previous frame, calculate the confidence weight C of the current frame t :

[0039] For consecutive frames, when C t ≥0.7 (the threshold value in this embodiment), the point is directly accepted as the crack tip position at time t; when C t When the velocity is less than 0.7 (the threshold in this example), the crack tip position predicted by the crack tip motion model is used, and the local crack tip position search is repeated until an acceptable crack tip position coordinate is obtained. The updated velocity is used to predict the next frame, allowing the model to adapt to changes in crack growth behavior while limiting unreasonable changes through constraints.

[0040] At each moment t during crack propagation, the above step (5) is repeated until an acceptable crack tip position coordinate (x t ,y t ).

[0041] The above motion model prediction and physical constraint verification are used to achieve the stability and robustness of the crack tip in multiple frame images.

[0042] (6) Through the above steps, the coordinates of the crack tip at time t are obtained. The equivalent crack length Lt at time t is calculated using formula (4). In the computer application test software, mark the elastic starting and end points, the maximum force point, the force point where the compliance increases by 5% P1, P2, ... Pn, the corresponding displacement points δ1, δ2, ... δn, and the position coordinates of the delamination propagation crack tip at the same time (x1, y1), (x2, y2) ... (xn, yn). After correcting for the compliance of specimen 7, large displacement, and the influence of loading block 6, the GIC value of the material is calculated and the R curve is plotted.

[0043] (4) Among them, L t is the equivalent crack length at time t, (x t ,y t ) is the coordinate of the crack tip at time t, and (x0, y0) is the coordinate of the crack tip at the initial moment.

[0044] Precision tests were conducted on samples 1# to 3#. The DIC method was used to monitor the equivalent crack length Lt during the crack propagation process, and the data were compared with the visual inspection method. The results are shown in Tables 1 to 3.

[0045] Table 1 Equivalent crack length Lt during crack propagation (sample 1: T700 carbon fiber / bisphenol A epoxy resin)

[0046] Table 2 Equivalent crack length Lt during crack propagation (sample 2: T700 carbon fiber / PEEK resin)

[0047] Table 3 Equivalent crack length Lt during crack propagation (sample 3: glass fiber / bisphenol A epoxy resin)

[0048] For the calculation of fracture toughness, the simplified flexibility correction calculation method is adopted, and the flexibility correction of sample 7 is carried out using formulas (5) and (6); (5) (6) It is worth mentioning that, compared with the traditional test method (which establishes a relationship between the flexibility factor and the layer length by plotting a function coordinate graph, and then obtains a correction factor by data fitting, extrapolation, or slope), the flexibility correction using the formula of this embodiment simplifies the data processing process, meets the requirements of data processing precision and accuracy, improves data processing efficiency, and reduces the impact of individual test data deviations on test results, thus serving as a useful supplement to the data processing methods in existing standards.

[0049] in, is the crack length of specimen 7 after flexibility correction, is the opening angle during the delamination process of sample 7, is the displacement at time t, such as Figure 2 As shown; Correction for large displacement and the effect of loading block 6 is done using formulas (7) and (8); (7) (8) Where F is the large displacement correction factor, h' is the distance from the center of the loading block 6 to 1 / 4 of the thickness of the sample 7; N is the correction factor of the loading block 6, is the distance from the center of the loading block 6 to its edge; Furthermore, in step (6), the fracture toughness of sample 7 is evaluated. Calculated using formula (9); (9) in, is the critical energy release rate, B is the width of sample 7, is the force load at time t.

[0050] The above method is used to calculate the sampling points during the crack propagation process of samples 1#~3# The values ​​were compared with the standard calculation method (ASTM D5528 MBT), and the results are shown in Tables 4 to 6.

[0051] Table 4 Comparison of fracture toughness data calculated based on the standard calculation method and the calculation based on this embodiment (Sample 1: T700 carbon fiber / bisphenol A epoxy resin)

[0052] Table 5 Comparison of fracture toughness data calculated based on the standard calculation method and the calculation based on this embodiment (Sample 2: T700 carbon fiber / PEEK resin)

[0053] Table 6 Comparison of fracture toughness data based on the standard calculation method and the calculation based on this embodiment (sample 3: glass fiber / bisphenol A epoxy resin)

[0054] From the above data, we can see that the R curve reveals that the interlaminar fracture toughness of composite materials changes dynamically, which is crucial for a deep understanding of the fracture behavior of materials. Figure 6-8 Nine fracture toughness values ​​corresponding to crack initiation and propagation, calculated using both the standard calculation method and this example, are presented, along with corresponding resistance curves (R curves). For three different fiber-reinforced resin-based composite materials, the method in this example (DIC crack monitoring + compliance correction) achieved a relative error of less than 1.5% compared to the standard method, demonstrating that this method not only improves testing efficiency but also ensures accuracy. The R curves generated using this example's method are capable of accurately assessing the fracture toughness of different fiber-reinforced resin-based composite materials.

[0055] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for testing the mode I fracture toughness of fiber-reinforced resin-based composite materials, characterized by: The steps include: The image of the crack propagation of the specimen is captured in real time by a camera, and the position of the crack tip is determined by a digital image processing method, which specifically includes: pre-processing the image to eliminate the influence of noise; The set of pixels in the image is defined as a subset, the shape function is selected and the displacement field is calculated to achieve accurate matching of the subsets before and after deformation, which serves as the basis for calculating the displacement standard deviation. The standard deviation of transverse and longitudinal displacements is calculated to locate the crack tip and a subset of regions exceeding a set threshold is selected; Multi-frame tracking verification: Using the continuity and speed of crack propagation, a crack tip motion prediction model is established. The coordinates of the center point of the selected area subset are input into the crack tip motion prediction model to obtain the coordinates of the crack tip of the specimen under load at time t. Calculate the equivalent crack length L at time t t ; Using the equivalent crack length L t， After performing compliance correction, large displacement correction, and loading block effect correction in sequence, the G of the sample material is calculated. ⅠC value and draw the R curve.

2. The method for testing the mode I fracture toughness of a fiber-reinforced resin-based composite material according to claim 1, wherein: One or more of grayscale normalization, CLAHE contrast enhancement, non-local mean denoising, and speckle sharpening methods are used for image preprocessing.

3. The method for testing the mode I fracture toughness of a fiber-reinforced resin-based composite material according to claim 1, wherein: The calculation formulas for the standard deviation of lateral and longitudinal displacements are as follows: ； ； in, is the standard deviation of the lateral displacement of pixels in the subset area, is the standard deviation of the longitudinal displacement of the pixels in the subset area, and the thresholds of the lateral displacement and the longitudinal displacement are set respectively. or When the threshold is exceeded, the center point of the subset area is defined as the crack tip candidate point; is the lateral displacement of the pixel, is the average value of the horizontal displacement of the pixel points in this column, is the longitudinal displacement of the pixel, is the average longitudinal displacement of the pixels in the row, and n represents the number of pixels in the column or row in the subset area.

4. The method for testing the mode I fracture toughness of a fiber-reinforced resin-based composite material according to claim 3, wherein: Crack tip prediction motion model: Among them, x t 、y t is the candidate crack tip position coordinate at time t, x t-1 ,y t-1 is the crack tip position coordinate at time t-1, v x 、v y It is determined based on historical data. The historical data is the average rate calculated from the change of the crack tip coordinates in the previous frame. Δt is the time interval between two frames. , is a random error; There is also a physical constraint: 0.1 pixel / frame <v x <10 pixels / frame, 0.1 pixels / frame <v y <10 pixels / frame, displacement angle change <30°; when physical constraints are met: =1, otherwise =0; The initial confidence is set to 1, and the indicator function of the current frame And the confidence weight of the previous frame, calculate the confidence weight C of the current frame t : For consecutive frames, when C t When it is greater than or equal to the set threshold, the point is directly accepted as the crack tip position at time t; when C t When it is less than the set threshold, the crack tip position predicted by the crack tip motion model is used, and the local crack tip position search is performed again until an acceptable crack tip position coordinate is obtained.

5. The method for testing the mode I fracture toughness of a fiber-reinforced resin-based composite material according to claim 1, wherein: The camera includes a first camera and a second camera, the first camera is fixed on the front side of the sample loading position, and the second camera is fixed on the back side of the sample loading position; In each loading and unloading step, when loading stops and during unloading, the first camera captures the front crack tip position, and the second camera captures the back crack tip position. The deviation between the two is compared to verify whether the loading is symmetrical.

6. The method for testing the mode I fracture toughness of a fiber-reinforced resin-based composite material according to claim 1, wherein: Equivalent crack length L at time t t The calculation formula is as follows: ； Among them, L t is the equivalent crack length at time t, ( , ) is the crack tip coordinate at time t, ( , ) is the coordinate of the crack tip at the initial moment.

7. The method for testing the mode I fracture toughness of a fiber-reinforced resin-based composite material according to claim 2, wherein: White coating is randomly sprayed on both sides of the sample. The speckle diameter formed by the white coating is calculated using the following formula: PX; Where R is the speckle diameter, FOV is the field of view, PPI is the camera resolution, and PX is the number of target pixels.

8. The method for testing the mode I fracture toughness of a fiber-reinforced resin-based composite material according to claim 1, wherein: The specimen flexibility correction uses the following formula: ； ； in, is the crack length after specimen flexibility correction, is the opening angle during the sample delamination process, is the displacement at time t, L t is the equivalent crack length at time t.

9. The method for testing the mode I fracture toughness of a fiber-reinforced resin-based composite material according to claim 1, wherein: Evaluation of fracture toughness of specimens Using the formula: ； in, is the critical energy release rate, B is the width of the sample, is the force load at time t, F and N are the correction factors for large displacement and loading block respectively; Correction for large displacements and loading block effects is done using the following formula; ； ； Where F is the large displacement correction factor, t is the distance from the center of the loading block to the 1 / 4 of the upper layer of the specimen thickness; N is the loading block correction factor, is the distance from the center of the loading block to its vertical side.

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