Method for measuring anisotropy r-value of sheet metal compression process
By combining uniaxial compression tests and the DIC system with finite element simulation, the problem of large measurement error of r-value in traditional methods was solved, realizing efficient and accurate anisotropic r-value determination of metal sheet during compression process and reducing the influence of deformation non-uniformity.
Patent Information
- Application Number
- CN202310051529.5
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2023-02-02
- Publication Date
- 2026-02-10
- Estimated Expiration
- 2043-02-02
AI Technical Summary
Traditional uniaxial tensile testing methods are insufficient to accurately describe the anisotropy of metal sheets during complex forming processes, and the uneven deformation and friction during compression lead to large measurement errors in the r-value.
A uniaxial compression test was combined with a digital image correlation (DIC) system and finite element simulation. By identifying and recording speckle images during the compression process, the test data were processed using the area averaging method to determine the optimal calculation area and calculate the anisotropy r value of the metal sheet.
It effectively reduces the influence of deformation non-uniformity during compression on the calculation of r-value, realizes efficient and accurate anisotropic r-value measurement, and can show the strain evolution law of materials during compression.
Smart Images
Figure CN115876593B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the characterization of the mechanical properties of sheet metal, and particularly to a method for determining the anisotropy r-value of sheet metal during compression. Background Technology
[0002] In sheet metal forming, anisotropy is one of the important parameters characterizing the mechanical behavior of materials. In the field of sheet metal stamping, the Lankford coefficient (r-value) is often used to measure the anisotropy of metals. The r-value is a key indicator for evaluating the sheet metal's resistance to thinning, lug forming, and other material parameters. In traditional r-value testing methods, material parameters are generally calibrated using a uniaxial tensile test. The change in gauge length of the specimen before and after tension is measured manually or automatically using an extensometer, and the material's r-value is calculated based on the principle of constant volume. The calculation formula is: r = ε l / ε t , where ε t ε represents the true plastic strain in the thickness direction. l It is the actual plastic strain in the width direction.
[0003] As product geometries become increasingly complex, sheet metal forming processes have expanded from traditional sheet stamping to sheet volume forming. Sheet volume forming utilizes molds and process design to control material flow, enabling localized active thickening or thinning of the sheet to form complex product features. During this process, the material undergoes a complex loading history of tensile thinning, compressive thickening, or both simultaneously. Considering the different mechanical behaviors exhibited by sheet metal during tension and compression—a phenomenon known as tension-compression asymmetry—traditional methods using uniaxial tensile measurements of the sheet's r-value are insufficient to accurately describe the sheet's anisotropy.
[0004] Uniaxial compression and uniaxial tension both represent the uniaxial stress state of sheet metal, reflecting the material's fundamental mechanical response and deformation characteristics. However, uniaxial compression tests are rarely used to characterize the r-value of sheet metal. Compared to uniaxial tensile tests, to avoid buckling instability during uniaxial compression, specimens are typically designed based on the sheet thickness, resulting in small specimen volumes. This leads to significant errors when measuring gauge length changes using similar tensile testing methods. Furthermore, the friction between the die and material during compression introduces non-uniform deformation into the specimen, further complicating accurate r-value measurement. Summary of the Invention
[0005] The purpose of this invention is to provide a method for determining the anisotropic r-value during the compression process of metal sheets, which can eliminate calculation errors caused by uneven deformation and efficiently and accurately determine the anisotropic r-value of material compression.
[0006] The above-mentioned technical objective of the present invention is achieved through the following technical solution:
[0007] A method for determining the anisotropy r-value during the compression process of a metal sheet includes the following steps:
[0008] Sample preparation: Cut the original metal sheet to obtain two sets of samples, and spray DIC speckle on different surfaces of the samples.
[0009] Uniaxial compression tests were conducted using the same test loading parameters, and speckle images of different sprayed surfaces of the two sets of samples were identified and recorded using the DIC measurement system during the compression process.
[0010] Based on the experimental conditions, a compression test model was established in the finite element software, a reference r value was set, and a finite element simulation of the quasi-static uniaxial compression process was performed.
[0011] Based on the strain data of the sample surface in the simulation results, the average strain r value of different area regions is selected to calculate the value. By comparing with the reference r value, the optimal calculation area to be selected by the area averaging method is determined as the experimental strain calculation area.
[0012] In the DIC system, the corresponding strain calculation region is selected, and the test data is processed using the area averaging method to obtain the strain field changes on different surfaces of the specimen during compression.
[0013] The anisotropy r-value of the metal sheet during compression was calculated by using the strain evolution of the sample in different directions obtained from the DIC system.
[0014] Preferably, the sample preparation is as follows:
[0015] Two sets of samples were cut based on the thickness direction of the original metal sheet.
[0016] Spraying was performed on the width direction surface of one set of samples and the thickness direction surface of the other set of samples.
[0017] As a preferred option, the compression test process is as follows:
[0018] The coated sample is mounted on the testing machine, and the compression direction is along the long axis of the sample. The pressing speed of the testing machine control system is set.
[0019] Debug the DIC system, aim the camera at the sample spraying surface, adjust the measurement distance between the measuring head and the sample according to the DIC camera frame parameters, initialize the setting parameters, correct the camera crosshair center line, and acquire images in real time during the compression process;
[0020] Compression tests were conducted on samples with different surfaces using the same test loading parameters. The DIC system was used to identify and record the speckle images on the sprayed surfaces in the width and thickness directions during the compression process of the two sets of samples.
[0021] Preferably, the testing machine uses a mirror-finish compression mold.
[0022] As a preferred embodiment, the anisotropy r value for obtaining simulation result data from finite element simulation is specifically as follows:
[0023] Establish a compression test model and input the specimen size, material parameters, and test loading parameters into the model;
[0024] An anisotropy criterion is introduced, and a reference r value is set to provide a reference for the optimization selection of the strain calculation region in the subsequent process.
[0025] Input the model parameters corresponding to the reference r value;
[0026] Quasi-static uniaxial compression finite element simulation was performed to obtain simulation results data. Different sizes of calculation regions were selected in the width and thickness directions, and the plastic strain data in the length, width, and thickness directions of the calculation region obtained by the simulation software were exported.
[0027] Two r values in the compression simulation are calculated based on the exported data: r′ is calculated from the average strain in the length and width directions within a given region on the surface of a set of specimens. w The value, and r′ calculated based on the average strain in the length and thickness directions within a given region on the surface of another set of specimens. t value.
[0028] As a preferred method, the optimal strain calculation region is determined by finite element simulation as follows:
[0029] Based on the calculation results, the values of r′ under different strain calculation region methods are compared. w 、r′ t Convergence performance compared to the reference r value;
[0030] The selected strain calculation region yields r′ w 、r′ t The closer the value is to the reference r value of the model input, the more suitable the calculation region of the r value is. The calculation region that is closest to the reference r value is determined as the optimal calculation region and applied to the calculation of subsequent experimental data results.
[0031] Preferably, the anisotropy r-value of the metal sheet sample during compression is obtained as follows:
[0032] Based on the finite element simulation results, the strain calculation region was selected in the speckle image of the uniaxial compression test, and the area averaging method in the DIC system was used to process the test data to obtain accurate data on the strain field changes in the length, thickness and width directions of the specimen during compression.
[0033] By obtaining strain evolution of the sample in different directions during compression using the DIC system, the anisotropy r-values of the metal sheet during compression are calculated. These r-values are obtained from the average strain in the length and width directions of a selected area on the surface of a set of samples. w The value, r, is calculated based on the average strain in the length and thickness directions within a selected area on the surface of another set of specimens. t value;
[0034] Compare r w and r t The calculation result is valid if both converge to the same value.
[0035] As a preferred option, the anisotropy r value is calculated as follows:
[0036]
[0037]
[0038] in, In the formula ε l ε w ε t The average strains in the length, width, and thickness directions are obtained by using the area averaging method on a selected area of the sample surface.
[0039] Preferably, in the finite element simulation compression test model, the reference r values are set to 0.8 and 1.2 for materials with r values less than and greater than 1, respectively.
[0040] As a preferred option, the experimentally calculated r-value exhibits a certain degree of fluctuation with increasing strain. Referring to the tensile r-value calculation method in GB / T5027-2016, an r-value within the strain range of 0.08 to 0.12 in the length direction is selected. w and r t Perform averaging separately, and then average the results for each r. w r t Alternatively, the average of the two values can be used as the compression value (r) of the material.
[0041] In summary, the present invention has the following beneficial effects:
[0042] The compression r-value testing method based on the DIC system extends the testing of sheet metal r-values to uniaxial compressive stress states. It uses finite element simulation to determine the optimal calculation area in the DIC image and employs the area averaging method to minimize the impact of calculation errors caused by non-uniform deformation during compression. The method is highly accurate and easy to operate, minimizing the influence of non-uniform deformation on the calculated r-value and demonstrating the evolution of the material's r-value with deformation during compression. By comprehensively calculating the strain relationships in different in-plane directions, it can efficiently and accurately determine the material's compressive anisotropy r-value. Attached Figure Description
[0043] Figure 1 A flowchart for the method of determining the compression r-value of sheet metal;
[0044] Figure 2 A schematic diagram of a C35R low-carbon steel specimen for compression testing.
[0045] Figure 3 For the finite element simulation results of compression deformation, the evolution diagram of r value under different strain calculation domain methods is selected;
[0046] Figure 4 The graph shows the results of the compression r-value determination of C35R low carbon steel obtained by using DIC to measure the strain on the surface of the specimen for compression testing. Detailed Implementation
[0047] The present invention will be further described in detail below with reference to the accompanying drawings.
[0048] According to one or more embodiments, a method for determining the anisotropy r-value during the compression process of a metal sheet is disclosed, such as... Figure 1 As shown, the process includes the following steps: uniaxial compression test of sheet metal, finite element simulation of the compression process, and calculation of the r-value. Specifically:
[0049] Sample preparation: Cut the original metal sheet to obtain two sets of samples, and spray DIC speckle on different surfaces of the samples.
[0050] Uniaxial compression tests were conducted using the same test loading parameters, and speckle images of different sprayed surfaces of the two sets of samples were identified and recorded using the DIC measurement system during the compression process.
[0051] A compression test model was established in the finite element software according to the experimental conditions, a reference r value was set, and a quasi-static uniaxial compression simulation was performed on the finite element model.
[0052] Based on the strain data of the sample surface in the simulation results, the average strain r value of different area regions is selected to calculate the value. By comparing with the reference r value, the optimal calculation area to be selected by the area averaging method is determined as the experimental strain calculation area.
[0053] In the DIC system, the corresponding strain calculation region is selected, and the test data is processed using the area averaging method to obtain the strain field changes on different surfaces of the specimen during compression.
[0054] The anisotropy r-value of the metal sheet during compression was calculated by using the strain evolution of the sample in different directions obtained from the DIC system.
[0055] The uniaxial compression test procedure for sheet metal includes:
[0056] a) Sample preparation: The original metal sheet is cut. Due to the thickness limitation of the sheet, in order to prevent instability during the compression process, the standard sample for the compression test needs to be cut into small volume samples with the thickness direction as the reference. A cuboid sample with a length-width-thickness ratio of 2:1:1 is obtained by wire cutting. Two sets of samples are required to determine one compression r value.
[0057] DIC speckle patterns were sprayed onto the surfaces of the two sets of samples. Spraying was performed on the width direction surface of one set of samples and the thickness direction surface of the other set of samples. Compression tests were conducted on the samples with different sprayed surfaces using the same test loading parameters to obtain strain data on different surfaces during the compression process of the two sets of samples.
[0058] b) Setting up the testing machine and DIC system: Install the sprayed compression test specimen on the universal testing machine, with the compression direction along the long axis of the compression test specimen, and set the pressing speed in the testing machine control system; use a mirror-finish compression mold to minimize the influence of friction on the compression deformation of the sheet material.
[0059] Debug the DIC system settings, aim the camera at the sample coating surface, adjust the measurement distance between the measuring head and the sample according to the DIC camera frame parameters, initialize the setting parameters on the computer, correct the camera crosshair center line, and acquire images in real time during the compression process.
[0060] c) Compression test and data acquisition: Two sets of compression tests under the same loading parameters were performed in sequence, and the speckle images on the sprayed surface of the two sets of specimens were identified and recorded using the DIC system during the compression process; after the compression process was completed, the strain field of the specimen surface in the width and thickness directions was calculated using the digital image correlation (DIC) algorithm.
[0061] In DIC data processing, the traditional method of calculating strain using virtual extensometers is significantly affected by the non-uniformity of deformation during compression. This paper proposes an area-averaged method, which calculates the average strain using a given in-plane region, effectively reducing the impact of deformation non-uniformity. To determine the optimal calculation region for the area-averaged method, a finite element simulation of the compression process is required. The simulation steps are as follows:
[0062] a) Finite element setup: In the finite element software, establish a compression test model, input the specimen size, material parameters, test loading parameters, etc. into the model, and introduce anisotropy criteria, set a reference r value. For materials with r values less than 1 or greater than 1, set the reference r values to 0.8 and 1.2 respectively, to provide a reference for the optimization selection of the strain calculation region in the later stage, and input the model parameters corresponding to the reference r value.
[0063] b) Calculate the anisotropy r-values of the simulation results: Perform a quasi-static uniaxial compression simulation based on the finite element model file to obtain simulation result data. Select calculation regions of different sizes in the width and thickness directions, and export the plastic strain data in the length, width, and thickness directions within the calculation region obtained from the simulation software. Calculate the two r-values in the compression simulation based on the exported data, namely, the r-values calculated from the average strain in the length and width directions within a given region on the specimen surface. in And the r value calculated based on the average strain in the length and thickness directions within a given area on the sample surface, i.e. in In the formula ε l ε w ε t These represent the average strains extracted from a given region on the sample surface in the length, width, and thickness directions, respectively.
[0064] c) Determine the optimal computational region on the width and thickness planes: Based on the calculation results, compare r′ under different strain computational region selection methods. w 、r′ t Compared with the convergence effect of the reference r value, the r′ obtained from the selected strain calculation region w 、r′ t The closer the value is to the reference r value of the model input, the more suitable the calculation region of the r value is; the calculation region closest to the reference r value is determined as the optimal region and applied to the calculation of subsequent experimental data results.
[0065] The steps for calculating the sheet metal compression r-value include:
[0066] a) Obtain the average strain data of the optimal calculation area: Based on the finite element simulation results, select the strain calculation area in the speckle image of the uniaxial compression test, and use the area averaging method in the DIC system to process the test data to accurately obtain the strain field changes in the length, thickness and width directions of the specimen during compression.
[0067] b) Calculate the r-value: Using the strain evolution of the sample in different directions during compression obtained from the DIC system, calculate the anisotropy r-value of the metal sheet during compression. The r-values are calculated based on the average strain in the length and width directions within a selected area on the surface of a set of samples. wThe value, r, is calculated based on the average strain in the length and thickness directions within a selected area on the surface of another set of specimens. t The value is calculated using the following formula: in Compare r w and r t The calculation results are valid when both converge to the same value. Since the experimentally calculated r-value often fluctuates with increasing strain, the tensile r-value calculation method in GB / T 5027-2016 can be referenced, taking an r-value within the range of 0.08 to 0.12 in the length direction. w and r t By averaging them separately, the final average r can be obtained. w r t Alternatively, the average of the two values can be used as the compression value (r) of the material.
[0068] Compared to traditional methods that calculate strain by measuring the change in gauge length before and after a tensile test and determine the anisotropy r-value of the material using the principle of constant volume, this invention proposes a compression r-value testing method based on a DIC system. This extends the testing of sheet metal r-values to uniaxial compressive stress states. A mirror-finish mold is used to reduce contact friction during compression, and the optimal calculation area in the DIC image is determined using finite element simulation. The area averaging method is then employed to minimize the impact of deformation inhomogeneity during compression. This method is highly accurate and easy to operate, minimizing the influence of deformation inhomogeneity on the calculated r-value and demonstrating the evolution of the material's r-value with deformation during compression.
[0069] To illustrate this clearly, here is an example: a test for the anisotropy r-value of C35R low carbon steel sheet during compression.
[0070] Uniaxial compression test of sheet metal:
[0071] a) Specimen preparation: The original C35R low-carbon steel sheet was cut to a rectangular specimen with dimensions of 10×5×5mm using wire cutting. The original sheet thickness was 5mm. Figure 2 As shown.
[0072] DIC speckle coating was applied to the surfaces of two sets of samples. The coating was applied twice, once on the width direction of one set of samples and once on the thickness direction of the other set of samples. Two compression tests were then conducted to obtain the strain on different surfaces of the two sets of samples during compression.
[0073] b) Testing machine and DIC system settings: Mount the coated compression specimen onto the universal testing machine, with the compression direction along the long axis of the specimen. Set the compression speed in the testing machine control system to achieve a compression strain rate of 0.8 × 10⁻⁶.-3 A mirror-finish compression mold is used to minimize the impact of friction on the compression deformation of the sheet metal.
[0074] Debug the DIC system settings, aim the camera at the sample coating surface, adjust the measurement distance between the measuring head and the sample according to the DIC camera frame parameters, initialize the setting parameters on the computer, correct the camera crosshair center line, and acquire images in real time during the compression process.
[0075] The test images in the DIC system were recorded by a pair of cameras with a resolution of 2572×2200 pixels. The test data were processed by the ARAMIS optical strain measurement system from GOM. During the test, the DIC system was calibrated with a reference area of 60×50mm, corresponding to a spatial resolution of 0.023mm per pixel, a surface size of 10 pixels (0.23mm), and a dot distance of 8 pixels (1.8mm).
[0076] c) Compression test and data acquisition: Two sets of compression tests under the same loading parameters were performed sequentially, and the speckle images on the sprayed surface of the two sets of specimens were identified and recorded using the DIC system during the compression process. After the compression process ended, the image acquisition was terminated, and a patch area and seed point were created in the DIC system. The speckle images on the specimen were acquired by the camera, and the deformation points on the surface were matched using the Digital Image Correlation (DIC) algorithm. The strain field of the specimen surface in the width and thickness directions could be calculated by the change of the three-dimensional coordinates of each point.
[0077] The following steps are taken to determine the selection of the strain calculation region in DIC data processing through finite element simulation:
[0078] a) Finite element setup: Establish a compression test model in the finite element software, input the specimen size, material parameters, test loading parameters, etc. into the model, consider the symmetrical specimen geometry and loading conditions, establish a 1 / 8 model of the specimen, set symmetrical boundary conditions, divide the mesh, and the number of elements along the length edge, width edge, and thickness edge are 20, 10, and 10, respectively. Since the friction of the mirror mold is small, the Coulomb friction coefficient of the contact surface is taken as 0.01.
[0079] To determine the impact of the selected range for subsequent strain calculations on the accuracy of r-value calculation, the Hill 48 yield criterion was adopted, and the reference value of r-value in the simulation process was taken as 0.8. The corresponding parameters were input into the model to provide a reference for the optimization selection of the strain calculation region in the subsequent process. The closer the r-value obtained by the strain calculation method used in the simulation results is to the reference value of 0.8, the more accurately the method can calculate the r-value in the compression process.
[0080] b) Calculate the anisotropy r value of the simulation results: Perform a quasi-static uniaxial compression simulation based on the finite element model file to obtain the simulation result data. Select strain calculation regions of different sizes in the width direction and thickness direction, including rectangular regions that occupy 15%, 50%, and 85% of the total area respectively from the center point in the plane. Export the equivalent plastic strain data in the two directions in the plane obtained by the simulation software.
[0081] The anisotropy r-values obtained from the simulation results are calculated, including: r-values calculated based on the strain in the length and width directions within a given region on the specimen surface, i.e.: in And the r value calculated based on the strain in the length and thickness directions within a given area on the sample surface, i.e.: in In the formula ε l ε w ε t These represent the average strains extracted from a given region on the sample surface in the length, width, and thickness directions, respectively.
[0082] c) Determine the optimal computational region on the width and thickness planes: such as Figure 3 As shown, according to r′ w and r′ t The calculation results show that when r′ w 、r′ t When the selected computational region is small, the difference between r′ and the theoretical input value of 0.8 will continuously increase as compression deformation progresses, only decreasing when the selected computational region occupies 85% of the in-plane area. w 、r′ t After the value stabilizes, it approaches the theoretical r value of 0.8 from the simulation input. Therefore, the optimal strain calculation area is a rectangular area that occupies 85% of the total area from the center point outwards. This area will be used for processing the DIC data results of the compression test in the next step.
[0083] The steps for calculating the sheet metal compression value (r) are as follows:
[0084] a) Obtain the average strain data of the optimal calculation area: Based on the finite element simulation results, select the strain calculation area in the speckle image of the uniaxial compression test, process the data of the speckle image changes, and select a rectangular area that occupies 85% of the total area from the center point in the width direction and the thickness direction to accurately obtain the strain field changes in the thickness and width directions of the specimen during compression.
[0085] b) Calculate the r-value: The anisotropy r-value of the metal sheet is calculated by obtaining the strain evolution of the sample during compression in different directions using the DIC system. The calculation formula is as follows: in like Figure 4 As shown, r w r t After undergoing a certain deformation, the curve gradually approaches and converges. However, due to measurement errors, the calculated r values corresponding to different strains exhibit a certain degree of instability. Therefore, similar to the calculation method for tensile test r values in the national standard GB / T 5027-2016, a r value within the strain range of 0.08 to 0.12 in the length direction is taken. w r t The average value is used as the compression r value of the material. The compression anisotropy r value of C35R low carbon steel plate in this embodiment is calculated to be about 0.8.
[0086] This specific embodiment is merely an explanation of the present invention and is not intended to limit the invention. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they are within the scope of the claims of the present invention.
Claims
1. A method for determining the anisotropy r-value during the compression process of a metal sheet, characterized in that, It includes the following steps: Sample preparation: Cut the original metal sheet to obtain two sets of samples, and spray DIC speckle on different surfaces of the samples. Uniaxial compression tests were conducted using the same test loading parameters, and speckle images of different sprayed surfaces of the two sets of samples were identified and recorded using the DIC measurement system during the compression process. Based on the experimental conditions, a compression test model was established in the finite element software, a reference r value was set, and a finite element simulation of the quasi-static uniaxial compression process was performed. Based on the strain data of the sample surface in the simulation results, the average strain r value of different area regions is selected to calculate the value. By comparing with the reference r value, the optimal calculation area to be selected by the area averaging method is determined as the experimental strain calculation area. In the DIC system, the corresponding strain calculation region is selected, and the test data is processed using the area averaging method to obtain the strain field changes on different surfaces of the specimen during compression. The anisotropy r value of the metal sheet compression process was calculated by using the strain evolution of the sample in different directions obtained from the DIC system. Specifically, the anisotropy r value obtained from the finite element simulation results is as follows: Establish a compression test model and input the specimen size, material parameters, and test loading parameters into the model; An anisotropy criterion is introduced, and a reference r value is set to provide a reference for the optimization selection of the strain calculation region in the subsequent process. Input the model parameters corresponding to the reference r value; Quasi-static uniaxial compression finite element simulation was performed to obtain simulation results data. Different sizes of calculation regions were selected in the width and thickness directions, and the plastic strain data in the length, width, and thickness directions of the calculation region obtained by the simulation software were exported. Two r values in the compression simulation are calculated based on the exported data: r' is obtained from the average strain in the length and width directions within a given region on the surface of a set of specimens. w The value, and r' calculated based on the average strain in the length and thickness directions within a given region on the surface of another set of specimens. t value; The optimal strain calculation region is determined by finite element simulation as follows: Based on the calculation results, the values of r are compared under different strain calculation region methods. ' w 、r' t Convergence performance compared to the reference r value; r' obtained from the selected strain calculation region w 、r' t The closer the value is to the reference r value of the model input, the more suitable the calculation region of the r value is. The calculation region that is closest to the reference r value is determined as the optimal calculation region and applied to the calculation of subsequent experimental data results.
2. The method for determining the anisotropy r-value during the compression process of metal sheets according to claim 1, characterized in that, The specific preparation of the sample is as follows: Two sets of samples were cut based on the thickness direction of the original metal sheet. Spraying was performed on the width direction surface of one set of samples and the thickness direction surface of the other set of samples.
3. The method for determining the anisotropy r-value during the compression process of metal sheets according to claim 2, characterized in that, The compression test process is as follows: The coated sample is mounted on the testing machine, and the compression direction is along the long axis of the sample. The pressing speed of the testing machine control system is set. Debug the DIC system, aim the camera at the sample spraying surface, adjust the measurement distance between the measuring head and the sample according to the DIC camera frame parameters, initialize the setting parameters, correct the camera crosshair center line, and acquire images in real time during the compression process; Compression tests were conducted on samples with different surfaces using the same test loading parameters. The DIC system was used to identify and record the speckle images on the sprayed surfaces in the width and thickness directions during the compression process of the two sets of samples.
4. The method for determining the anisotropy r-value during the compression process of metal sheets according to claim 3, characterized in that: The testing machine uses a mirror-finish compression mold.
5. The method for determining the anisotropy r-value during the compression process of metal sheets according to claim 1, characterized in that, The anisotropy r-value of the metal sheet sample during compression is obtained as follows: Based on the finite element simulation results, the strain calculation region was selected in the speckle image of the uniaxial compression test, and the area averaging method in the DIC system was used to process the test data to obtain accurate data on the strain field changes in the length, thickness and width directions of the specimen during compression. By obtaining strain evolution of the sample in different directions during compression using the DIC system, the anisotropy r-values of the metal sheet during compression are calculated. These r-values are obtained from the average strain in the length and width directions of a selected area on the surface of a set of samples. w The value, r, is calculated based on the average strain in the length and thickness directions within a selected area on the surface of another set of specimens. t value; Compare r w and r t The calculation result is valid if both converge to the same value.
6. The method for determining the anisotropy r-value during the compression process of metal sheets according to claim 5, characterized in that, The anisotropy r value is calculated as follows: in, In the formula ε l ε w ε t The average strains in the length, width, and thickness directions are obtained by using the area averaging method on a selected area of the sample surface.
7. The method for determining the anisotropy r-value during the compression process of metal sheets according to claim 6, characterized in that: In the finite element simulation compression test model, the reference r values are set to 0.8 and 1.2 for materials with r values less than and greater than 1, respectively.
8. The method for determining the anisotropy r-value during the compression process of metal sheets according to claim 6, characterized in that: The experimentally calculated r-value shows some fluctuation with increasing strain. Referring to the tensile r-value calculation method in GB / T 5027-2016, an r-value within the strain range of 0.08 to 0.12 in the length direction is taken. w and r t Perform averaging separately, and then average the results for each r. w r t Alternatively, the average of the two values can be used as the compression value (r) of the material.