Pre-loading clamp for sheet metal and mechanical property analysis method

Abstract

The invention relates to the technical field of metal plate mechanical property analysis, and particularly discloses a preloading clamp for a metal plate and a mechanical property analysis method.The preloading clamp is used for conducting preloading deformation on a sample of the metal plate and comprises a long female die and a short female die; the long female die and the short female die are in butt joint and combined to form a limiting space for placing a sample, the limiting space is provided with a top opening and a lateral opening, the lateral opening is provided with a limiting piece, and the limiting piece is used for restraining the sample in the deformation process, so that the sample is prevented from buckling in the thickness direction and the width direction; a pressing block is mounted at the top opening and is used for being connected with a testing machine and uniformly transmitting the pressure of the testing machine to the sample. According to the preloading clamp for the metal plate and the mechanical property analysis method for the metal plate, the preloading clamp and the sample are innovatively designed, and four preloading deformation modes including two types of thinning predeformation and two types of thickening predeformation are achieved through the same clamp.

Description

Technical Field

[0001] The present invention relates to the technical field of mechanical property analysis of metal plates, and in particular to a preloading fixture for metal plates and a mechanical property analysis method for metal plates. Background Art

[0002] In the plastic forming of metal sheets, mechanical properties are the standard for measuring the quality of materials. Its tests include tensile and compression experiments, which can measure key indicators such as yield strength and tensile strength, reflecting the plastic deformation and deformation resistance of materials. In traditional tensile and compression tests, the sheet undergoes a linear strain path, while the strain history of the sheet in actual plastic forming is usually nonlinear. For example, the strain path of the flange material in the deep drawing of a cylinder changes complexly.

[0003] Currently, a two-stage loading experiment is often used to evaluate the mechanical properties of plates under the condition of changing loading paths. However, in the preloading stage, due to the small thickness dimension of the plate and the difficulty in thickening, the plate is usually uniaxially stretched or rolled within the plate surface, and only thinning deformation occurs. It is impossible to effectively complete the stable thickening preloading of the metal plate, and it is difficult to evaluate the influence of different preloading deformation modes (especially thickening deformation) on the mechanical properties of the plate. Summary of the invention

[0004] The present invention aims to solve one of the technical problems in the related art to at least some extent. To this end, the purpose of the present invention is to provide a preloading fixture for metal sheets and a mechanical property analysis method for metal sheets to improve driving safety and efficiency.

[0005] To achieve the above-mentioned purpose, the first aspect of the embodiment of the present invention proposes a preloading fixture for metal plates, which is used for preloading and deforming specimens of metal plates, including a long die and a short die, wherein the long die and the short die are butt-jointed to form a limiting space for placing the specimen, the limiting space has top and side openings, wherein the side openings are installed with limiting members, and the limiting members are used to impose constraints on the specimen during the deformation process to avoid buckling of the specimen in the thickness and width directions, and the top opening is installed with a pressure block, which is used to connect to a testing machine and evenly transfer the pressure of the testing machine to the specimen.

[0006] In some embodiments of the present invention, the limiting member includes a bridge-type anti-buckling pad, a disc spring, a gasket and an anti-buckling pressure block. The position of the bridge-type anti-buckling pad corresponds to the position of the lateral opening. A cavity is formed inside the bridge-type anti-buckling pad. The opening of the cavity faces the sample in the limiting space. The disc spring and the anti-buckling pressure block are arranged in the cavity, and a gasket is also arranged between the disc spring and the anti-buckling pressure block.

[0007] In some embodiments of the present invention, a toothed anti-buckling block is further provided between the anti-buckling block and the specimen, a toothed structure is formed on the surface of the toothed anti-buckling block, the bottom of the toothed anti-buckling block abuts against the bottom of the limiting space, and the top of the toothed anti-buckling block is flush with the upper surface of the specimen.

[0008] In some embodiments of the present invention, a limit block is further provided at the lateral opening, and the limit block is installed above the bridge-type anti-buckling pad.

[0009] In some embodiments of the present invention, a fastening slider and a clamping base are further included, wherein an installation space for accommodating the long die, the short die and the fastening slider is formed in the middle portion of the clamping base, and the installation space is a trapezoidal space, wherein one end of the long die away from the short die abuts against the horizontal plane of the installation space, and one end of the short die away from the long die abuts against the horizontal plane of the fastening slider, and the surfaces where the fastening slider abuts against the installation space are both inclined surfaces, and the fastening slider can slide up and down along the inclined surfaces.

[0010] To achieve the above-mentioned purpose, the second aspect of the present invention provides a method for analyzing the mechanical properties of a metal sheet, including a metal sheet preloading test, tensile specimen cutting, subsequent tensile testing, and data processing;

[0011] The sheet metal preload test includes the following steps: specimen preparation, preload application, and test machine setup, where:

[0012] Sample preparation: Cut the original metal sheet into corresponding sizes, grind, rinse, measure and calculate the average size of each group of samples, wrap them with Teflon film and trim them;

[0013] Preload: Place the wrapped specimen into the preload fixture according to the required preload deformation type and assemble it. Pay attention to the placement of the specimen under different preload deformation types and the key points of contact with the preload fixture.

[0014] Testing machine setting: Place the preload fixture on the testing machine, set the pressing speed, stop after reaching the predetermined deformation, and measure and calculate the actual deformation;

[0015] The tensile specimen cutting includes: cutting the preloaded and deformed specimen into three pieces along the thickness direction by slow wire cutting, taking a piece of the core in the thickness direction and then cutting out the standard tensile specimen;

[0016] Subsequent tensile tests include:

[0017] Sample preparation: Grind the gauge section of the tensile specimen, rinse, measure and calculate the average width and thickness, and spray DIC speckle on the gauge section;

[0018] Testing machine and DIC system settings: Install the specimen on the universal testing machine and set the stretching speed, debug the DIC system, including aligning the camera, adjusting the distance, initializing parameters, correcting the center line, etc., and clarify the DIC system test image acquisition, processing and calibration parameters;

[0019] Tensile test and test data collection: During the tensile test, the DIC system identifies and records the speckle image, and calculates the strain field after the stretching is completed;

[0020] Data processing includes:

[0021] Processing the tensile test data to obtain average strain and elongation data, deriving a force-time curve and interpolating to obtain a force-global time curve and an elongation-global time curve;

[0022] Calculate the stress-strain curve, engineering stress, true strain and true stress of the metal sheet during stretching, compare the three sets of results and judge their validity.

[0023] In some embodiments of the present invention, when the preloading is performed and the deformation type of the preloading is thickening-plane strain compression preloading deformation, the assembly method of the preloading fixture and the sample is:

[0024] Place the wrapped specimen into the confined space formed by the long die and the short die, ensuring that the end of the specimen fits the inner wall of the confined space;

[0025] Place the toothed anti-buckling block into the limited space, ensure that the bottom of the toothed anti-buckling block is in full contact with the bottom of the limited space, and the top of the toothed anti-buckling block is flush with the upper surface of the specimen, then install the bridge anti-buckling pad and the limit block at the lateral opening of the limited space, check whether the pressure of the disc spring is appropriate, and ensure that the specimen deforms normally in the thickness direction and does not buckle in the width direction;

[0026] Press the pressing block into the top opening of the corresponding limit space, and place the long die and the short die into the installation space of the clamping base. Check whether the fastening slider is tightly abutted against the short die to ensure that the long die and the short die are constrained in the length direction. When the pressing block is pressed down, a thickened pre-deformed specimen is obtained.

[0027] In some embodiments of the present invention, when the preload deformation type is thickening-uniaxial compression preload deformation, it is different from the thickening-plane strain compression preload deformation as follows:

[0028] Place the wrapped sample in the middle of the limiting space formed by the long die and the short die to avoid contact between the sample and the inner wall of the limiting space.

[0029] In some embodiments of the present invention, when the preload deformation type is thinning-plane strain compression preload deformation, the assembly method of the preload fixture and the sample is:

[0030] Place the wrapped specimen into the confined space formed by the long die and the short die, ensuring that the end of the specimen fits the inner wall of the confined space;

[0031] Press the pressing block into the top opening of the corresponding limit space, and place the long die and the short die into the installation space of the clamping base. Check whether the fastening slider is tightly abutted against the short die to ensure that the long die and the short die are constrained in the length direction. When the pressing block is pressed down, a thinned pre-deformed specimen is obtained.

[0032] In some embodiments of the present invention, when the preload deformation type is thinning-uniaxial compression preload deformation, it is different from the thinning-plane strain compression preload deformation as follows:

[0033] Place the wrapped sample in the middle of the limiting space formed by the long die and the short die to avoid contact between the sample and the inner wall of the limiting space.

[0034] The preloading fixture for metal sheets and the mechanical properties analysis method for metal sheets of the embodiments of the present invention are innovatively designed preloading fixtures and specimens, and the same set of fixtures can realize two types of thinning pre-deformations and two types of thickening pre-deformations, a total of four types of preloading deformation modes. The experimental operation is simple, and the effects of thickening and thinning preloading on the mechanical properties of metal sheets can be demonstrated. The effects of different preloading deformation modes on the mechanical properties of the sheet can be evaluated, which is more comprehensive and effective than the traditional method of only evaluating the effects of thinning preloading deformation. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic flow chart of a method for analyzing mechanical properties of a metal plate according to an embodiment of the present invention;

[0036] Figure 2 It is a schematic diagram of an explosion structure of a preloading fixture of another embodiment of the present invention for thickening and preloading a specimen;

[0037] Figure 3 It is a schematic diagram of an exploded structure of a preloading fixture of another embodiment of the present invention for thinning and preloading a sample;

[0038] Figure 4 It is a schematic diagram of various preload deformation specimens and subsequent tensile specimens used in the present invention;

[0039] Figure 5 It is a graph showing the strain size experimental results of various types of samples after preload deformation tests in the present invention;

[0040] Figure 6It is a graph showing the experimental results of tensile stress-strain curves of various types of samples after preload deformation tests in the present invention;

[0041] Figure 7 It is a graph of the experimental results of the tensile r value-strain curve after the preload deformation test of various types of samples in the present invention.

[0042] In the figure: 1. Fastening screw; 2. Bridge-type anti-buckling pad; 3. Limit block; 4. Disc spring; 5. Gasket; 6. Anti-buckling pressure block; 7. Toothed anti-buckling pressure block; 8. Long die; 9. Pressure block; 10. Thickened pre-deformed specimen; 11. Locating pin; 12. Short die; 13. Ejector screw; 14. Fastening slider; 15. Clamping base; 16. Thinned pre-deformed specimen. DETAILED DESCRIPTION

[0043] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

[0044] The following describes a preloading fixture for metal plates and a mechanical property analysis method according to embodiments of the present invention with reference to the accompanying drawings.

[0045] Figure 2 , Figure 3 It is a schematic diagram of the exploded structure of a preloading fixture for metal plates for thickening-thinning preloading of a sample according to an embodiment of the present invention.

[0046] The above-mentioned preloading fixture is used to preload and deform the sample of the metal plate. The preloading fixture includes: a long die 8 and a short die 12. The long die 8 and the short die 12 are connected and combined to form a limiting space for placing the sample. The limiting space has top and side openings, wherein the side openings are installed with limiting members, which are used to impose constraints on the sample during the deformation process to avoid buckling of the sample in the thickness and width directions. The top opening is installed with a pressure block 9, which is used to connect to the testing machine and evenly transfer the pressure of the testing machine to the sample.

[0047] The long concave die 8 has a specific cavity shape and size, and the shape of the bottom of the cavity is adapted to the shape of the bottom of the sample, so that the sample can be accurately accommodated, ensuring that the initial position of the sample is accurate during the preloading process.

[0048] As an example, the short die 12 cooperates with the long die 8, and the two are connected and locked by a positioning pin 11, and together form a limiting space for placing the sample.

[0049] In some embodiments of the present invention, the limiting member includes a bridge-type anti-buckling pad 2, a disc spring 4, a gasket 5 and an anti-buckling pressure block 6. The position of the bridge-type anti-buckling pad 2 corresponds to the position of the lateral opening. A cavity is formed inside the bridge-type anti-buckling pad 2. The opening of the cavity faces the sample in the limiting space. The disc spring 4 and the anti-buckling pressure block 6 are arranged in the cavity. A gasket 5 is also arranged between the disc spring 4 and the anti-buckling pressure block 6.

[0050] As an example, the long die 8 and the short die 12 are connected and locked with the bridge-type anti-buckling pad 2 by the fastening screws 1 located at both ends of the two. The fastening screw 1 has a threaded structure, and the fastening or loosening operation can be achieved by rotation. The fastening screw 1 matches the threaded hole of the bridge-type anti-buckling pad 2, and the fixing and pressure adjustment of the bridge-type anti-buckling pad 2 are achieved by thread matching. The bridge-type anti-buckling pad 2 has a certain structural shape, and there is space inside to accommodate the disc spring 4 and the gasket 5. The shape and size of the bridge-type anti-buckling pad 2 are adapted to the long die 8 to ensure that during the thickening preloading process of the specimen, especially in unidirectional compression in the width direction and plane strain compression, the disc spring 4 can provide appropriate pressure.

[0051] As an example, the disc spring 4 has elastic deformation characteristics, and its specifications and performance match the bridge-type anti-buckling pad 2, the fastening screw 1, and the anti-buckling pressure required when the specimen is thickened and preloaded. By adjusting the fastening screw 1, the disc spring 4 can provide different pressures to act in the thickness direction of the specimen to prevent the specimen from buckling in the width direction, while ensuring the normal deformation of the specimen in the thickness direction, and together with other components, achieve precise control of the specimen thickening deformation process;

[0052] The gasket 5 is installed inside the bridge-type anti-buckling pad 2, between the disc springs 4, and together with the disc springs 4, the bridge-type anti-buckling pad 2 and other components, forms an elastic pressure regulating system. The material characteristics of the gasket 5 can play a role in buffering and evenly distributing the pressure when the disc spring 4 provides pressure, ensuring that the pressure is stably transmitted to the specimen, and cooperate with the disc springs 4, the bridge-type anti-buckling pad 2 and other components to accurately adjust the anti-buckling pressure of the specimen during the thickening preloading process, ensuring the stability and accuracy of the specimen deformation.

[0053] In some embodiments of the present invention, a toothed anti-buckling block 7 is further provided between the anti-buckling block 6 and the specimen, a toothed structure is formed on the surface of the toothed anti-buckling block 7, the bottom of the toothed anti-buckling block 7 abuts against the bottom of the limiting space, and the top of the toothed anti-buckling block 7 is flush with the upper surface of the specimen.

[0054] The toothed anti-buckling block 7 has a special toothed structure, which helps to increase the friction and contact stability with the specimen and the bottom of the fixture. Its size matches the contact area between the specimen and the bottom of the limiting space of the long die 8, and can provide preliminary anti-buckling restrictions in the width direction of the specimen. The toothed anti-buckling block 7 cooperates with the bridge-type anti-buckling pad 2 to further prevent the specimen from buckling in the width direction during the thickening deformation process of the specimen, ensuring stable deformation of the specimen in the thickness direction.

[0055] It should be noted that during thickening preloading, the long die 8 works together with the toothed anti-buckling pressure block 7, the bridge-type anti-buckling pad 2 and other components to limit the deformation direction of the specimen; during thinning preloading, the long die 8 cooperates with the pressure block 9 and other components to achieve thinning preloading in the thickness direction of the specimen, and its overall structure is coordinated with other components to ensure a smooth preloading process.

[0056] In some embodiments of the present invention, a limit block 3 is also provided at the lateral opening, and the limit block 3 is installed above the bridge-type anti-buckling pad 2. The shape and size of the limit block 3 match the bridge-type anti-buckling pad 2 and the entire fixture structure, and the limit block 3 is connected and locked with the long die 8 by the fastening screws 1 located at both ends thereof. The function of the limit block 3 is to assist in limiting the range of motion of other components, such as the pressure block 9, during the deformation of the specimen, to ensure that the pressure block 9 maintains vertical loading during the deformation of the specimen, so that the specimen is evenly compressed in the thickness direction, and to ensure the stability and accuracy of the thickening preloading process.

[0057] In some embodiments of the present invention, a fastening slider 14 and a clamping base 15 are further included. An installation space for accommodating the long die 8, the short die 12 and the fastening slider 14 is formed in the middle of the clamping base 15. The installation space is a trapezoidal space. The end of the long die 8 away from the short die 12 abuts against the horizontal plane of the installation space, and the end of the short die 12 away from the long die 8 abuts against the horizontal plane of the fastening slider 14. The surfaces where the fastening slider 14 abuts against the installation space are both inclined surfaces. The fastening slider 14 can slide up and down along the inclined surface. The fastening slider 14 is connected and locked to the clamping base 15 by a vertically arranged ejector screw 13 that passes through the fastening slider 14.

[0058] As an example, the threaded structure of the ejector screw 13 matches the threaded hole at the bottom of the installation space of the clamping base 15, and can be moved up and down by rotating. During the preloading process, by screwing the ejector screw 13 downward, the fastening slider 14 is displaced downward along the inclined surface, and the long die 8 and the short die 12 located in the installation space are limited, and the length direction of the long die 8 and the short die 12 is constrained, and the ejector screw 13 and the fastening slider 14 are in a specific position, which does not affect the normal operation of other components; after the preloading is completed, the ejector screw 13 cooperates with the structure of the fastening slider 14, and by screwing the ejector screw 13 upward, the fastening slider 14 is ejected, so as to facilitate the removal of the long die 8 and the short die 12 and the sample. The ejector screw 13, the fastening slider 14, the clamping base 15, the long die 8 and other components together constitute a stable preloading system, ensuring that the sample is not disturbed by the outside world during the preloading process and stably deforms. Its structural design provides a reliable basic guarantee for the entire preloading test.

[0059] The various components in the preloading fixture of the present invention work together to achieve functions such as stable clamping, precise loading and anti-buckling of metal sheet specimens through precise connection relationships, reasonable relative positions and mutually matching features, thereby ensuring the accuracy and reliability of the preloading process and providing effective experimental conditions for subsequent research on the mechanical properties of metal sheets under different pre-deformation methods.

[0060] To achieve the above object, the present invention also proposes a method for analyzing the mechanical properties of metal plates, such as Figure 1 As shown, it includes metal sheet preloading test, tensile specimen cutting, subsequent tensile test and data processing.

[0061] S1. The metal sheet preloading test includes the following steps: specimen preparation, preloading and test machine setting, wherein:

[0062] Sample preparation: The original metal sheet is wire-cut into corresponding sizes, such as 6061 aluminum alloy sheet is cut into rectangular specimens of different specifications. Each group of specimens is polished, rinsed, measured and the average size is calculated, wrapped with Teflon film and trimmed.

[0063] As an example, Figure 4 As shown, the original 6061 aluminum alloy sheet was cut by slow wire cutting. The thickness of the original sheet was 3 mm. The following four rectangular specimens were obtained by wire cutting. The length, width and thickness of the specimens were 92 mm*12 mm*3 mm (thickening-plane strain compression specimen), 82 mm*12 mm*3 mm (thickening-uniaxial compression specimen), 92 mm*12 mm*3 mm (thinning-plane strain compression specimen), and 72 mm*12 mm*3 mm (thinning-uniaxial compression specimen).

[0064] Before loading the preloading fixture, each group of samples needs to be finely polished with fine sandpaper (P400-P800) to make the wire cutting surface smooth, and then rinse the surface with anhydrous ethanol to remove stains on the sample surface and metal powder polished off to prevent affecting the accuracy of the preloading test. Then measure the actual length, width and thickness of the upper, middle and lower samples and take the average value of the three dimensions;

[0065] It should be noted that a section of Teflon film that can completely wrap the sample should be cut to minimize the effect of friction on the compression deformation of the sheet. After the wrapping operation is completed, the excess part of the Teflon film needs to be trimmed to reduce the effect of the film thickness on the pre-deformation of the overlapping part of the film.

[0066] Preload: Place the wrapped specimen into the preload fixture according to the required preload deformation type and assemble it. Pay attention to the placement of the specimen under different preload deformation types and the key points of contact with the preload fixture.

[0067] Testing machine setting: Place the preload fixture on the testing machine, set the pressing speed to 1.0-1.5mm / min, stop after reaching the predetermined deformation, and measure and calculate the actual deformation.

[0068] As an example, the pressure block 9 in the preloading fixture is placed upward on a testing machine with flat anvils on both the top and bottom. The pressing speed is set at 1.0-1.5 mm / min in the testing machine control system. The testing machine is stopped after the predetermined deformation is reached. After taking out the sample, the actual size after deformation is measured and the actual pre-deformation is calculated. The results of the pre-strain magnitude in different directions obtained by the preloading experiment are as follows: Figure 5 shown.

[0069] S2. Cutting of tensile specimens includes: a) placing the preloaded and deformed specimen into a wire cutting device, using a slow wire cutting device, and dividing the specimen into three pieces along the thickness direction of the specimen, preferably ensuring that the thickness of the middle layer is 1±0.02 mm;

[0070] b) Take a piece of the specimen that is thicker than the core (i.e., the middle layer) and use a slow wire cutter to cut out a standard tensile specimen.

[0071] S3. Subsequent tensile tests include:

[0072] Sample preparation: Grind the gauge section of the tensile specimen, rinse, measure and calculate the average width and thickness, and spray DIC speckle on the gauge section;

[0073] Testing machine and DIC system settings: The specimen is installed on the universal testing machine and the tensile speed is set to 1.0 mm / min. The DIC system is debugged, including aligning the camera, adjusting the distance, initializing parameters, correcting the center line, etc., and the DIC system test image acquisition, processing and calibration parameters are clarified;

[0074] Tensile test and test data collection: During the tensile test, the DIC system identifies and records the speckle image, and calculates the strain field after the stretching is completed.

[0075] S4. Data processing includes:

[0076] Processing the tensile test data to obtain average strain and elongation data, deriving a force-time curve and interpolating to obtain a force-global time curve and an elongation-global time curve;

[0077] Calculate the stress-strain curve, engineering stress, true strain and true stress of the metal sheet during stretching, compare the three sets of results and judge their validity.

[0078] As an example, the subsequent tensile test in S3 specifically includes the following:

[0079] a) Sample preparation: Use fine sandpaper (P400-P800) to grind the side of the gauge section of the standard tensile specimen cut out after preloading until it is smooth, then rinse the surface with anhydrous ethanol to remove stains on the surface of the specimen and the metal powder polished off to prevent affecting the accuracy of the tensile test and the subsequent spraying effect, then measure the actual width and thickness of the upper, middle and lower gauge sections, and take the average value of their width and thickness;

[0080] The speckle pattern for DIC was sprayed on the gauge length section of the specimens with better surface quality, and the tests were carried out respectively with the same test tensile parameters to obtain the strain data on the gauge length section of each group of specimens during the tensile process.

[0081] b) Testing machine and DIC system settings: Install the sprayed standard tensile specimen on the universal testing machine, with the stretching direction being the extension direction of the gauge length section, and set the stretching speed to 1.0 mm / min in the testing machine control system;

[0082] Debug the DIC system settings, aim the camera at the painted surface of the sample, adjust the measuring distance between the measuring head and the sample according to the DIC camera format parameters, initialize the setting parameters on the computer, calibrate the camera cross center line, and collect images in real time during the stretching process.

[0083] The test images in the DIC system are recorded by a pair of cameras with a resolution of 2572×2200 pixels. The test data are processed by the ARAMIS optical strain measurement system from GOM. During the test, the DIC system is 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 point distance of 8 pixels (1.8mm).

[0084] c) Tensile test and test data collection: Two groups of tensile tests under the same test loading parameters are carried out in sequence, and the DIC system is used to identify and record the speckle images on the painted surface of the two groups of specimens during the stretching process; when the stretching process is over (the tensile test ends when the specimen breaks), the image collection is terminated, and the patch area and seed points are created in the DIC system. The speckle images on the specimens are collected by the camera, and the deformation points on the surface of the gauge section are matched using the DIC algorithm. The strain field on the surface of the specimen in the width and thickness directions can be calculated through the changes in the three-dimensional coordinates of each point.

[0085] As an example, the data processing in S4 specifically includes the following:

[0086] Obtain the average strain data and elongation data of the gauge section: select the strain calculation area in the speckle image of the uniaxial tensile test, use the area averaging method in the DIC system to process the test data, and accurately obtain the strain field changes in the length, thickness and width directions of the specimen during the compression process. Establish an electronic extensometer with a length of 25mm at a suitable position of the gauge section, place the extensometer at the center of the gauge section, and derive the elongation in the tensile direction during the entire tensile process;

[0087] The force-time curve in the testing machine is exported, and the time data in the DIC is inserted into the time data of the testing machine as an interpolation to obtain the force-global time curve and the elongation-global time curve.

[0088] like Figure 6 As shown, calculate the stress-strain curve: use the interpolated elongation-global time curve to calculate the engineering strain of the metal sheet during the stretching process. The calculation formula is:

[0089] Among them, l1 is the length of the gauge section, and l0 is the original length of the gauge section;

[0090] The engineering stress of the metal sheet during the stretching process is calculated using the interpolated force-global time curve. The calculation formula is:

[0091]

[0092] Where F is the force, b is the average width of the gauge section, and h is the average thickness of the gauge section;

[0093] Finally, the true strain and true stress are calculated, and the calculation formula is ε true =ln(1+ε eng ),

[0094] σ true =(ε true +1)×σ eng ;

[0095] Compare these three sets of stress-strain curves. When the error range of the three curves is within ±10%, the results are valid.

[0096] like Figure 7 As shown, calculate the r value: through the strain evolution of the sample in different directions during the stretching process obtained in the DIC system, calculate the anisotropic r value of the metal sheet during the stretching process. The calculation formula is

[0097] in

[0098] Comparing these three sets of calculation results, because the r value has a certain instability, the stable stage before the sample necking is taken for comparison. When the error range of the three curves is within ±10%, the calculation results are valid.

[0099] like Figure 2 , Figure 3 As shown, in some embodiments of the present invention, when the sample is preloaded, when the preload deformation type is thickening-plane strain compression preload deformation, the assembly method of the preload fixture and the sample is:

[0100] Place the wrapped sample into the confined space formed by the long die 8 and the short die 12, ensuring that the end of the sample fits the inner wall of the confined space;

[0101] Place the toothed anti-buckling pressure block 7 into the limiting space, ensure that the bottom of the toothed anti-buckling pressure block 7 is in full contact with the bottom of the limiting space, and the top of the toothed anti-buckling pressure block 7 is flush with the upper surface of the specimen, then install the bridge-type anti-buckling pad 2 and the limiting block 3 at the lateral opening of the limiting space, check whether the pressure of the disc spring 4 is appropriate, and adjust it by tightening the screw 1 to ensure that the specimen deforms normally in the thickness direction and does not buckle in the width direction;

[0102] Press the pressing block 9 into the top opening of the corresponding limit space, and place the long die 8 and the short die 12 into the installation space of the clamping base 15, check whether the fastening slider 14 is tightly abutted against the short die 12, and ensure that the long die 8 and the short die 12 are constrained in the length direction. When the pressing block 9 is pressed down, a thickened pre-deformed specimen 10 is obtained.

[0103] In some embodiments of the present invention, when the preload deformation type is thickening-uniaxial compression preload deformation, it is different from thickening-plane strain compression preload deformation as follows:

[0104] The wrapped sample is placed in the middle of the limiting space formed by the long die 8 and the short die 12 to prevent the sample from contacting the inner wall of the limiting space.

[0105] In some embodiments of the present invention, when the preload deformation type is thinning-plane strain compression preload deformation, the assembly method of the preload fixture and the sample is:

[0106] Place the wrapped sample into the confined space formed by the long die 8 and the short die 12, ensuring that the end of the sample fits the inner wall of the confined space;

[0107] Press the pressing block 9 into the top opening of the corresponding limit space, and place the long die 8 and the short die 12 into the installation space of the clamping base 15, check whether the fastening slider 14 is tightly abutted against the short die 12, and ensure that the long die 8 and the short die 12 are constrained in the length direction. When the pressing block 9 is pressed down, a thinned pre-deformed specimen 16 is obtained.

[0108] In some embodiments of the present invention, when the preload deformation type is thinning-uniaxial compression preload deformation, it is different from thinning-plane strain compression preload deformation as follows:

[0109] The wrapped sample is placed in the middle of the limiting space formed by the long die 8 and the short die 12 to prevent the sample from contacting the inner wall of the limiting space.

[0110] Based on the above content of uniaxial compression and plane strain compression, their differences and the use of preload fixtures are analyzed:

[0111] 1. The differences include: 1. Different strain states; 2. Different effects on the internal microstructure and mechanical properties of metal sheets.

[0112] 1. Different strain states:

[0113] During uniaxial compression, the metal sheet specimen is mainly subjected to compressive stress in one direction, thereby generating strain. For example, in the preloading of the thickening-uniaxial compression specimen, the deformation of the specimen in the length direction is not restricted (because the length is less than the length of the limiting space cavity), and the compression deformation is mainly performed in the thickness direction. The deformation in the width direction is relatively free, but due to the effect of the anti-buckling pressure block 6, etc., the overall deformation is still subject to a certain control, and finally the thickening in the thickness direction is achieved. When the thinning-uniaxial compression specimen is preloaded, the main compressive strain is also generated in the thickness direction, which makes the sheet thinner, and the deformation in other directions is relatively minor;

[0114] Plane strain compression means that during the deformation of the sheet metal, the strain in a certain direction is suppressed or limited to near zero. For example, when the thickening-plane strain compression specimen is preloaded, the deformation in the length direction is suppressed through the fixture design (such as the synergistic effect of the long die 8, the toothed anti-buckling block 7, the bridge-type anti-buckling pad 2 and other components), so that the specimen is mainly thickened in the thickness direction. At the same time, due to the presence of the anti-buckling block 6 and other components in the width direction, the thickening deformation is stable and the strain in the width direction is controlled. When the thinning-plane strain compression specimen is preloaded, the deformation in the length direction is also suppressed, and the thinning strain is mainly generated in the thickness direction.

[0115] 2. Different effects on the internal microstructure and mechanical properties of metal sheets:

[0116] During uniaxial compression, since the deformation is relatively concentrated in one main direction, the grains inside the metal sheet are subjected to greater stress in this direction, and the grains will undergo deformation behaviors such as rotation and slip along the compression direction, resulting in directional changes in the internal microstructure. This directionality will affect the subsequent mechanical properties of the material, such as possibly increasing the strength and hardness of the material in the compression direction, but the performance changes in other directions are relatively small, and anisotropy is obvious.

[0117] Plane strain compression: Under plane strain compression, the deformation of the grains inside the metal sheet is more complicated because the strains in the two directions are restricted to different degrees. While the thickness direction is subjected to compressive strain, the grains in the suppression direction will constrain and coordinate the deformation, making the changes in the internal microstructure more uniform and the degree of anisotropy different from that of uniaxial compression. For example, in plane strain compression that suppresses the strain in the length direction, the performance changes of the material in the thickness and width directions will be interrelated and affect each other, and the impact on the overall mechanical properties (such as yield strength, tensile strength, anisotropy coefficient, etc.) is different from that of uniaxial compression, and its performance changes more comprehensively reflect the synergistic effect of strain restrictions in the two directions.

[0118] 2. Impact on the use of preload fixtures: 3. Differences in specimen placement; 4. Different focus on anti-buckling components.

[0119] 3. Differences in sample placement:

[0120] Uniaxial compression: In the uniaxial compression with thickening preloading, since the deformation of the specimen in the length direction is not restricted, the specimen should be placed in the middle of the bottom of the die to avoid the preloading result being affected by the excessive deformation in the length direction touching the cavity wall during the compression process. When performing uniaxial compression with thinning preloading, the relationship between the specimen length and the cavity should also be considered, and the specimen should be placed in a suitable position to prevent abnormal deformation.

[0121] Plane strain compression: Regardless of whether it is thickening or thinning plane strain compression preloading, the specimen must be accurately placed at the bottom of the die to ensure that the loading in the thickness direction can be accurately carried out while the strain in the length direction is effectively suppressed, thereby achieving the effect of plane strain compression.

[0122] 4. The focus of anti-buckling components is different.

[0123] Uniaxial compression: During thickening preload uniaxial compression, the bridge-type anti-buckling pad 2 plays a vital role. The pressure provided by the disc spring 4 must ensure that the specimen deforms normally in the thickness direction while preventing buckling in the width direction, because the relatively free deformation in the width direction during uniaxial compression requires an anti-buckling device to ensure the overall deformation stability. In thinning preload uniaxial compression, the main focus is on the thinning deformation of the specimen in the thickness direction, and the requirements for the anti-buckling component are relatively low, but the fixture as a whole still needs to ensure stable loading of the specimen during the thinning process.

[0124] Plane strain compression: When preloading the plane strain compression during thickening, the anti-buckling components such as the toothed anti-buckling pressure block 7 and the bridge-type anti-buckling pad 2 should not only prevent buckling in the width direction, but also coordinate to ensure the strain suppression effect in the length direction, so as to ensure stable thickening plane strain compression in the thickness direction. When preloading the plane strain compression during thinning, the anti-buckling components should also cooperate with the overall structure of the fixture to ensure that the specimen is accurately thinned in the thickness direction under the strain suppression in the length direction, so as to prevent inaccurate test results due to uneven deformation or buckling.

[0125] Therefore, there are obvious differences between uniaxial compression and plane strain compression in terms of strain state, influence on material microstructure and mechanical properties. These differences determine that different operating methods need to be adopted in the use of preloading fixtures, from the specimen placement position to the focus of anti-buckling components, so as to ensure the accurate realization of thickening and thinning preloading of metal sheets and provide reliable specimen deformation conditions for subsequent research.

[0126] The mechanical properties analysis method for metal sheets in the present invention innovatively designs preloading fixtures and specimens. The same set of fixtures can realize two types of thinning pre-deformations (unidirectional compression along the thickness direction of the sheet, and plane strain compression in the thickness direction) and two types of thickening pre-deformations (unidirectional compression along the width direction, and plane strain compression in the width direction), a total of four types of pre-deformation methods. Among them, the special thickness-direction anti-buckling device enables the long strip specimen to stably thicken and deform during unidirectional compression and plane strain compression in the width direction. The surface of the specimen is wrapped with a low-friction polymer film material to reduce the influence of friction on the compression preload of the sheet, and obtain accurate and stable specimen pre-deformation. The experiment is simple to operate, and can show the influence of thickening and thinning pre-loads on the mechanical properties of metal sheets. It can evaluate the influence of different pre-load deformation methods on the mechanical properties of the sheet. Compared with the traditional method of only evaluating the influence of thinning pre-deformation, it is more comprehensive and effective.

[0127] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0128] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0129] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A preloading fixture for metal sheets, used for preloading and deforming a sample of the metal sheet, characterized in that: It comprises a long die and a short die, wherein the long die and the short die are butt-jointed to form a limiting space for placing a sample, wherein the limiting space has top and side openings, wherein the side openings are provided with limiting members, wherein the limiting members are used to impose constraints on the sample during the deformation process to avoid buckling of the sample in the thickness and width directions, and wherein a pressure block is provided at the top opening, wherein the pressure block is used to connect to a testing machine and evenly transfer the pressure of the testing machine to the sample.

2. A preloading fixture for metal sheets according to claim 1, characterized in that: The limiting member includes a bridge-type anti-buckling pad, a disc spring, a gasket and an anti-buckling pressure block. The position of the bridge-type anti-buckling pad corresponds to the position of the lateral opening. A cavity is formed inside the bridge-type anti-buckling pad. The opening of the cavity faces the sample in the limiting space. The disc spring and the anti-buckling pressure block are arranged in the cavity, and a gasket is also arranged between the disc spring and the anti-buckling pressure block.

3. A preloading fixture for metal sheets according to claim 2, characterized in that: A toothed anti-buckling block is also provided between the anti-buckling block and the specimen, a toothed structure is formed on the surface of the toothed anti-buckling block, the bottom of the toothed anti-buckling block abuts against the bottom of the limiting space, and the top of the toothed anti-buckling block is flush with the upper surface of the specimen.

4. A preloading fixture for metal sheets according to claim 2, characterized in that: A limit block is also provided at the lateral opening, and the limit block is installed above the bridge-type anti-buckling pad.

5. A preloading fixture for metal sheets according to claim 1, characterized in that: It also includes a fastening slider and a clamping base, wherein an installation space for accommodating the long die, the short die and the fastening slider is formed in the middle of the clamping base, and the installation space is a trapezoidal space, wherein one end of the long die away from the short die abuts against the horizontal plane of the installation space, and one end of the short die away from the long die abuts against the horizontal plane of the fastening slider, and the surfaces where the fastening slider abuts against the installation space are both inclined planes, and the fastening slider can slide up and down along the inclined planes.

6. A method for analyzing the mechanical properties of metal sheets, characterized in that: Including metal sheet pre-loading test, tensile specimen cutting, subsequent tensile test and data processing; The sheet metal preload test includes the following steps: specimen preparation, preload application, and test machine setup, where: Sample preparation: Cut the original metal sheet into corresponding sizes, grind, rinse, measure and calculate the average size of each group of samples, wrap them with Teflon film and trim them; Preload: Place the wrapped specimen into the preload fixture according to the required preload deformation type and assemble it. Pay attention to the placement of the specimen under different preload deformation types and the key points of contact with the preload fixture. Testing machine setting: Place the preload fixture on the testing machine, set the pressing speed, stop after reaching the predetermined deformation, and measure and calculate the actual deformation; The tensile specimen cutting includes: cutting the preloaded and deformed specimen into three pieces along the thickness direction by slow wire cutting, taking a piece of the core in the thickness direction and then cutting out the standard tensile specimen; Subsequent tensile tests include: Sample preparation: Grind the gauge section of the tensile specimen, rinse, measure and calculate the average width and thickness, and spray DIC speckle on the gauge section; Testing machine and DIC system settings: install the specimen on the universal testing machine and set the stretching speed, debug the DIC system, and clarify the DIC system test image acquisition, processing and calibration parameters; Tensile test and test data collection: During the tensile test, the DIC system identifies and records the speckle image, and calculates the strain field after the stretching is completed; Data processing includes: Processing the tensile test data to obtain average strain and elongation data, deriving a force-time curve and interpolating to obtain a force-global time curve and an elongation-global time curve; Calculate the stress-strain curve, engineering stress, true strain and true stress during the stretching of metal sheets, compare the three sets of results and judge their validity.

7. A mechanical properties analysis method for metal plates according to claim 6, characterized in that: When the preloading deformation type is thickening-plane strain compression preloading deformation, the assembly method of the preloading fixture and the sample is as follows: Place the wrapped specimen into the confined space formed by the long die and the short die, ensuring that the end of the specimen fits the inner wall of the confined space; Place the toothed anti-buckling block into the limited space, ensure that the bottom of the toothed anti-buckling block is in full contact with the bottom of the limited space, and the top of the toothed anti-buckling block is flush with the upper surface of the specimen, then install the bridge anti-buckling pad and the limit block at the lateral opening of the limited space, check whether the pressure of the disc spring is appropriate, and ensure that the specimen deforms normally in the thickness direction and does not buckle in the width direction; Press the pressing block into the top opening of the corresponding limit space, and place the long die and the short die into the installation space of the clamping base. Check whether the fastening slider is tightly abutted against the short die to ensure that the long die and the short die are constrained in the length direction. When the pressing block is pressed down, a thickened pre-deformed specimen is obtained.

8. A mechanical properties analysis method for metal plates according to claim 7, characterized in that: When the preload deformation type is thickening-uniaxial compression preload deformation, the difference between it and the thickening-plane strain compression preload deformation is: Place the wrapped sample in the middle of the limiting space formed by the long die and the short die to avoid contact between the sample and the inner wall of the limiting space.

9. A mechanical properties analysis method for metal plates according to claim 6, characterized in that: When the preload deformation type is thinning-plane strain compression preload deformation, the assembly method of the preload fixture and the specimen is: Place the wrapped specimen into the confined space formed by the long die and the short die, ensuring that the end of the specimen fits the inner wall of the confined space; Press the pressing block into the top opening of the corresponding limit space, and place the long die and the short die into the installation space of the clamping base. Check whether the fastening slider is tightly abutted against the short die to ensure that the long die and the short die are constrained in the length direction. When the pressing block is pressed down, a thinned pre-deformed specimen is obtained.

10. A mechanical properties analysis method for metal plates according to claim 9, characterized in that: When the preload deformation type is thinning-uniaxial compression preload deformation, the difference between it and the thinning-plane strain compression preload deformation is: Place the wrapped sample in the middle of the limiting space formed by the long die and the short die to avoid contact between the sample and the inner wall of the limiting space.

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