Preloading fixture for sheet metal and mechanical property analysis method

Abstract

The present application relates to the technical field of metal plate mechanical property analysis, and specifically discloses a preloading clamp for metal plate and a mechanical property analysis method, the preloading clamp is used for preloading deformation of a sample of the metal plate, and comprises a long concave die and a short concave die; the long concave die and the short concave die are butted and combined to form a limiting space for placing the sample, the top of the limiting space and a lateral opening; the lateral opening is provided with a limiting piece, the limiting piece is used for applying constraint to the sample in the deformation process, so that buckling of the sample in the thickness direction and the width direction is avoided; the top opening is provided with a pressing block, and the pressing block is used for connecting a testing machine and uniformly transmitting the pressure of the testing machine to the sample. The preloading clamp for metal plate and the mechanical property analysis method for metal plate of the present application embodiment innovatively design the preloading clamp and the sample, and the same set of clamp realizes four types of preloading deformation modes of two types of thinning pre-deformation and two types of thickening pre-deformation.

Description

Technical Field

[0001] This invention relates to the field of mechanical property analysis technology for sheet metal, and particularly to a preload fixture for sheet metal and a method for mechanical property analysis of sheet metal. Background Technology

[0002] In sheet metal forming, mechanical properties are the standard for evaluating the quality of materials. Testing includes tensile and compressive tests, which can determine key indicators such as yield strength and tensile strength, reflecting the material's ability to undergo plastic deformation and resist deformation. Traditional tensile and compressive tests show that the sheet metal undergoes a linear strain path, while in actual sheet metal forming, the strain history is usually non-linear. For example, the strain path of flange materials in deep-drawing of cylinders is complex.

[0003] Currently, two-stage loading experiments are commonly used to evaluate the mechanical properties of sheet metal under varying loading paths. However, due to the small thickness dimension of the sheet metal, thickening is difficult during the preloading stage. Typically, it involves uniaxial stretching or rolling within the sheet surface, resulting only in thinning deformation. This makes it impossible to effectively complete stable thickening preloading of the metal sheet and to evaluate the impact of different preloading deformation methods (especially thickening deformation) on the mechanical properties of the sheet metal. Summary of the Invention

[0004] This invention aims to at least partially solve one of the technical problems in related technologies. Therefore, the object of this invention is to provide a preloading fixture for sheet metal and a method for analyzing the mechanical properties of sheet metal, in order to improve driving safety and efficiency.

[0005] To achieve the above objectives, a first aspect of the present invention provides a preloading fixture for metal sheets, used for preloading deformation of metal sheet samples, comprising a long die and a short die, wherein the long die and the short die are joined together to form a limiting space for placing the sample, the limiting space having a top and a side opening, wherein a limiting member is installed in the side opening, the limiting member being used to constrain the sample during the deformation process to prevent buckling of the sample in the thickness and width directions, and a pressure block is installed in the top opening, the pressure block being used to connect to a testing machine and uniformly transmit the pressure of the testing machine to the sample.

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

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

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

[0009] In some embodiments of the present invention, a fastening slider and a clamping base are further included. The clamping base has a mounting space in the middle for accommodating the long die, the short die, and the fastening slider. The mounting space is a trapezoidal space. The end of the long die away from the short die abuts against the horizontal surface of the mounting space, and the end of the short die away from the long die abuts against the horizontal surface of the fastening slider. The surfaces of the fastening slider that abut against the mounting space are all inclined surfaces, and the fastening slider can slide up and down along the inclined surfaces.

[0010] To achieve the above objectives, a second aspect of the present invention provides a method for mechanical property analysis of metal sheets, including a pre-loading test of the metal sheet, tensile specimen cutting, subsequent tensile test, and data processing.

[0011] The preloading test for metal sheets includes the following steps: sample preparation, preloading, and testing machine setup, wherein:

[0012] Sample preparation: The original metal sheet is wire-cut to the corresponding size. Each group of samples is polished, rinsed, and the average size is measured and calculated. The samples are then wrapped with Teflon film and trimmed.

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

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

[0015] The tensile specimen cutting process includes: cutting the pre-loaded and deformed specimen into three pieces along the thickness direction using a slow wire cutter, taking the thickest piece towards the core and cutting out a standard tensile specimen.

[0016] Subsequent tensile tests include:

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

[0018] Testing machine and DIC system setup: The sample is mounted on the universal testing machine and the tensile speed is set. The DIC system is debugged, including aligning the camera, adjusting the distance, initializing parameters, and calibrating the center line. The DIC system test image acquisition, processing and calibration parameters are clarified.

[0019] Tensile test and test data acquisition: Tensile test is performed, the DIC system identifies and records speckle images, and the strain field is calculated after the tensile test is completed;

[0020] Data processing includes:

[0021] The above tensile test data are processed to obtain average strain and elongation data. Force-time curves are derived and interpolated to obtain force-global time curves and elongation-global time curves.

[0022] Calculate the stress-strain curve, engineering stress, and actual strain and actual stress during the tensile process of metal sheet, compare these three sets of results, and determine their validity.

[0023] In some embodiments of the present invention, when the preloading deformation type is thickening-plane strain compressive preloading deformation, the assembly method of the preloading fixture and the specimen is as follows:

[0024] Place the wrapped sample into the limiting space composed of the long and short concave molds, ensuring that the end of the sample fits against the inner wall of the limiting space.

[0025] Place the toothed buckling-resistance block inside the limiting space, ensuring that the bottom of the toothed buckling-resistance block is in full contact with the bottom of the limiting space, and that the top of the toothed buckling-resistance block is flush with the upper surface of the sample. Then, install the bridge-shaped buckling-resistance pad and the limiting block at the lateral opening of the limiting space. Check whether the pressure of the disc spring is appropriate to ensure that the sample deforms normally in the thickness direction and does not buckle in the width direction.

[0026] Press the pressure block into the top opening of the corresponding limiting space, and place the long die and the short die into the installation space of the clamping base. Check whether the fastening slider is in close contact with the short die to ensure that the long die and the short die are constrained in the length direction. After the pressure block is pressed down, the thickened pre-deformed sample is obtained.

[0027] In some embodiments of the present invention, when the preloaded deformation type is thickened-uniaxial compression preloaded deformation, the difference between it and the thickened-plane strain compression preloaded deformation is as follows:

[0028] Place the wrapped sample in the middle of the limiting space formed by the long and short concave molds, avoiding contact between the ends of the sample and the inner walls of the limiting space.

[0029] In some embodiments of the present invention, when the preloaded deformation type is thinning-plane strain compressive preloaded deformation, the assembly method of the preloaded fixture and the specimen is as follows:

[0030] Place the wrapped sample into the limiting space composed of the long and short concave molds, ensuring that the end of the sample fits against the inner wall of the limiting space.

[0031] Press the pressure block into the top opening of the corresponding limiting space, and place the long die and the short die into the installation space of the clamping base. Check whether the fastening slider is in close contact with the short die to ensure that the long die and the short die are constrained in the length direction. After the pressure block is pressed down, a thinned pre-deformed sample is obtained.

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

[0033] Place the wrapped sample in the middle of the limiting space formed by the long and short concave molds, avoiding contact between the ends of the sample and the inner walls of the limiting space.

[0034] This invention relates to a preloading fixture for metal sheets and a method for analyzing the mechanical properties of metal sheets. The innovative design of the preloading fixture and specimen allows for four types of preloading deformation—two types of thinning pre-deformation and two types of thickening pre-deformation—to be achieved using a single fixture. Furthermore, the experimental operation is simple, demonstrating the impact of thickening and thinning preloading on the mechanical properties of metal sheets. It can evaluate the influence of different preloading deformation methods on the mechanical properties of the sheets, providing a more comprehensive and effective evaluation compared to traditional methods that only evaluate the effect of thinning preloading deformation. Attached Figure Description

[0035] Figure 1 This is a schematic flowchart of a mechanical property analysis method for metal sheets according to an embodiment of the present invention;

[0036] Figure 2 This is an exploded structural diagram of a preload fixture for thickening and preloading a sample according to another embodiment of the present invention;

[0037] Figure 3 This is an exploded structural diagram of a pre-loading fixture for thinning and pre-loading a sample according to another embodiment of the present invention;

[0038] Figure 4 This is a schematic diagram of the specimens used for various preloaded deformations and the subsequent tensile specimens in this invention;

[0039] Figure 5 This is a graph showing the experimental results of strain magnitude after preloading deformation tests on various types of specimens in this invention;

[0040] Figure 6 These are experimental results of tensile stress-strain curves after preloading deformation tests on various types of specimens in this invention;

[0041] Figure 7 This is an experimental result diagram of tensile r-strain curves after preloading deformation tests of various specimens in this invention.

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

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

[0044] The following description, with reference to the accompanying drawings, describes an embodiment of the present invention of a preloading fixture for metal sheets and a method for analyzing their mechanical properties.

[0045] Figure 2 , Figure 3 This is an exploded structural diagram of a preloading fixture for metal sheets according to an embodiment of the present invention, which performs thickening-thinning preloading on a sample.

[0046] The aforementioned preloading fixture is used to preload and deform metal sheet samples. The preloading fixture includes a long die 8 and a short die 12. After the long die 8 and the short die 12 are joined together, a limiting space for placing the sample is formed. The limiting space has a top and a side opening. The side opening is equipped with a limiting component, which is used to constrain the sample during the deformation process and prevent buckling of the sample in the thickness and width directions. The top opening is equipped with a pressure block 9, which is used to connect to the testing machine and uniformly transmit the pressure of the testing machine to the sample.

[0047] The long cavity mold 8 has a specific cavity shape and size. The bottom shape of its cavity is adapted to the bottom shape of the sample, which can accurately accommodate the sample and ensure the accurate initial position of the sample during the preloading process.

[0048] As an example, the short die 12 and the long die 8 are fitted together and locked together by a locating pin 11, forming a limiting space for placing the sample.

[0049] In some embodiments of the present invention, the limiting member includes a bridge-shaped anti-buckling pad 2, a disc spring 4, a gasket 5, and an anti-buckling pressure block 6. The position of the bridge-shaped anti-buckling pad 2 corresponds to the position of the lateral opening. A cavity is formed inside the bridge-shaped anti-buckling pad 2, and the opening of the cavity faces the sample in the limiting space. The disc spring 4 and the anti-buckling pressure block 6 are disposed in the cavity, and a gasket 5 is also disposed 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 respectively connected and locked to the bridge-shaped buckling-resistance pad 2 by fastening screws 1 located at both ends. The fastening screws 1 have a threaded structure, which can be tightened or loosened by rotation. The fastening screws 1 match the threaded holes of the bridge-shaped buckling-resistance pad 2, and the threaded engagement achieves the fixation and pressure adjustment of the bridge-shaped buckling-resistance pad 2. The bridge-shaped buckling-resistance pad 2 has a certain structural shape, with space inside to accommodate the disc spring 4 and the washer 5. The shape and size of the bridge-shaped buckling-resistance pad 2 are adapted to the long die 8 to ensure that appropriate pressure can be provided by the disc spring 4 during the preloading process of the specimen thickening, especially during unidirectional compression in the width direction and plane strain compression.

[0051] As an example, the disc spring 4 has elastic deformation characteristics, and its specifications and performance are matched with the anti-buckling pressure required during the preloading of the bridge-type anti-buckling pad 2, the fastening screw 1, and the specimen thickness increase. By adjusting the fastening screw 1, the disc spring 4 can provide different magnitudes of pressure, acting on the specimen thickness direction to prevent buckling of the specimen in the width direction, while ensuring normal deformation of the specimen in the thickness direction, thus achieving precise control of the specimen thickness increase deformation process together with other components;

[0052] The gasket 5 is installed inside the bridge-shaped buckling-resistance pad 2, located between the disc springs 4, and together with the disc springs 4, the bridge-shaped buckling-resistance pad 2, and other components, forms an elastic pressure adjustment system. The material properties of the gasket 5 can buffer and evenly distribute the pressure when the disc springs 4 provide pressure, ensuring that the pressure is stably transmitted to the sample. In conjunction with the disc springs 4, the bridge-shaped buckling-resistance pad 2, and other components, it can precisely adjust the buckling-resistance pressure of the sample during the thickening preloading process, ensuring the stability and accuracy of the sample deformation.

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

[0054] The toothed buckling-resistance block 7 has a special toothed structure, which helps to increase friction and contact stability with the specimen and the bottom of the fixture. Its size matches the contact area of ​​the bottom of the limiting space of the specimen and the long die 8, providing initial buckling restraint in the width direction of the specimen. The toothed buckling-resistance block 7 cooperates with the bridge-type buckling-resistance pad 2 to further prevent buckling of the specimen 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 the thickening preloading, the long concave mold 8 works together with the toothed anti-buckling pressure block 7, the bridge-shaped anti-buckling pad 2, and other components to restrict the deformation direction of the sample; during the thinning preloading, the long concave mold 8 works with the pressure block 9 and other components to achieve thinning preloading in the thickness direction of the sample. Its overall structure is coordinated with other components to ensure that the preloading process proceeds smoothly.

[0056] In some embodiments of the present invention, a limiting block 3 is also provided at the lateral opening, and the limiting block 3 is installed above the bridge-shaped anti-buckling pad 2. The shape and size of the limiting block 3 match the bridge-shaped anti-buckling pad 2 and the entire fixture structure. The limiting block 3 is connected and locked to the long die 8 by fastening screws 1 located at both ends. The function of the limiting block 3 is to assist in limiting the movement range of other components, such as the pressure block 9, during the deformation of the sample, ensuring that the pressure block 9 remains vertically loaded during the deformation of the sample, so that the sample is uniformly compressed in the thickness direction, and ensuring 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 also included. The clamping base 15 has an installation space in the middle that accommodates the long die 8, the short die 12 and the fastening slider 14. The installation space is a trapezoidal space. The end of the long die 8 away from the short die 12 abuts against the horizontal surface of the installation space. The end of the short die 12 away from the long die 8 abuts against the horizontal surface of the fastening slider 14. The surfaces of the fastening slider 14 that abut against the installation space are all inclined surfaces. The fastening slider 14 can slide up and down along the inclined surfaces. The fastening slider 14 is connected and locked to the clamping base 15 by an ejector screw 13 that is vertically set and 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 mounting space of the clamping base 15, allowing for vertical movement via rotation. During preloading, by turning the ejector screw 13 downwards, the fastening slider 14 is displaced downwards along the inclined plane, limiting the length of the long die 8 and the short die 12 located within the mounting space. This constrains the length of the long die 8 and the short die 12, and the ejector screw 13 and the fastening slider 14 are in specific positions, without affecting the normal operation of other components. After preloading, the ejector screw 13 and the fastening slider 14 are structurally coordinated, and by turning the ejector screw 13 upwards, the fastening slider 14 is ejected, facilitating the removal of the long die 8, the short die 12, and the sample. The ejector screw 13, the fastening slider 14, the clamping base 15, and the long die 8 together constitute a stable preloading system, ensuring that the sample is not disturbed by external factors during preloading and deforms stably. Its structural design provides a reliable foundation for the entire preloading test.

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

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

[0061] S1. The preloading test for metal sheets includes the following steps: sample preparation, preloading, and testing machine setup, wherein:

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

[0063] As an example, such as Figure 4 As shown, the original 6061 aluminum alloy sheet was cut by slow wire EDM. The original sheet thickness was 3mm. The following four cuboid specimens were obtained by wire EDM, with the following dimensions: 92mm*12mm*3mm (thickening-plane strain compression specimen), 82mm*12mm*3mm (thickening-uniaxial compression specimen), 92mm*12mm*3mm (thinning-plane strain compression specimen), and 72mm*12mm*3mm (thinning-uniaxial compression specimen).

[0064] Before each set of samples is loaded into the preload fixture, the surface of the wire cut should be finely sanded with fine sandpaper (P400~P800) until smooth. Then, the surface should be rinsed with anhydrous ethanol to remove stains and metal powder from the sample surface to prevent affecting the accuracy of the preload test. After that, the actual length, width and thickness of the upper, middle and lower samples should be measured and the average value of the three dimensions should be taken.

[0065] It is important to cut a section of Teflon film that completely covers the sample to minimize the impact of friction on the sheet's compression deformation. After wrapping, any excess Teflon film should be trimmed to reduce the impact of overlapping film thickness on the pre-deformation amount.

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

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

[0068] As an example, the pressure block 9 in the preloading fixture is placed upwards onto a testing machine with flat anvils on both the top and bottom. The testing machine control system is set to a downward compression speed of 1.0–1.5 mm / min, and the testing machine is stopped after the predetermined deformation is reached. After removing the specimen, the actual dimensions after deformation are measured and the actual pre-deformation is calculated. The results of the pre-strain magnitude in different directions obtained from the preloading experiment are as follows. Figure 5 As shown.

[0069] S2. Tensile specimen cutting includes: a) placing the pre-loaded deformed specimen into a wire cutting device, using slow wire feeding, and dividing the specimen into three pieces along the specimen thickness direction, prioritizing ensuring that the thickness of the middle layer is 1±0.02mm.

[0070] b) Take a piece of the sample from the core (i.e., the middle layer) and cut a standard tensile sample using a slow wire cutter.

[0071] S3, Subsequent tensile tests include:

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

[0073] Testing machine and DIC system settings: The sample 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, and calibrating the center line. The DIC system test image acquisition, processing and calibration parameters are clarified.

[0074] Tensile test and test data acquisition: Tensile test is performed, the DIC system identifies and records speckle images, and the strain field is calculated after the tensile test is completed.

[0075] S4. Data processing includes:

[0076] The above tensile test data are processed to obtain average strain and elongation data. Force-time curves are derived and interpolated to obtain force-global time curves and elongation-global time curves.

[0077] Calculate the stress-strain curve, engineering stress, and actual strain and actual stress during the tensile process of metal sheet, compare these three sets of results, and determine their validity.

[0078] As an example, the subsequent tensile tests in S3 specifically include the following:

[0079] a) Sample preparation: After preloading, the standard tensile test specimens cut out are finely sanded with fine sandpaper (P400~P800) to make the sides of the gauge section smooth. Then, the surface is rinsed with anhydrous ethanol to remove stains and metal powder from the test specimen surface to prevent affecting the accuracy of the tensile test and the subsequent spraying effect. After that, the actual width and thickness of the upper, middle and lower gauge sections are measured and the average value of the width and thickness is taken.

[0080] DIC speckle patterns were sprayed onto the gauge length of specimens with good surface quality, and tests were conducted using the same tensile parameters to obtain strain data on the gauge length during the tensile process of each group of specimens.

[0081] b) Setting up the testing machine and DIC system: Install the sprayed standard tensile specimen on the universal testing machine, with the tensile direction being the extension direction of the gauge length. Set the tensile 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 measurement distance between the measuring head and the sample according to the DIC camera's frame parameters, initialize the settings on the computer, calibrate the camera's crosshair center line, and acquire images in real time during the stretching process.

[0083] 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).

[0084] c) Tensile test and data acquisition: Two sets of tensile tests under the same loading parameters were performed sequentially, and the speckle images on the painted surface of the two sets of specimens were identified and recorded using the DIC system during the tensile process. When the tensile process ended (the tensile test ended when the specimen broke), the image acquisition was terminated. A patch area and seed point were created in the DIC system, speckle images on the specimen were acquired by the camera, and the deformation points on the surface of the gauge length section were matched using the 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.

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

[0086] To obtain the average strain and elongation data of the gauge length: Select the strain calculation region in the speckle image of the uniaxial tensile test, and process the test data using the area averaging method in the DIC system to accurately obtain the strain field changes in the length, thickness, and width directions of the specimen during compression. Establish a 25mm long electronic extensometer at a suitable location within the gauge length, centering it at the center of the gauge length, and derive the elongation in the tensile direction throughout the entire tensile process.

[0087] Export the force-time curve from the testing machine, and insert the time data in DIC as interpolation into the time data of the testing machine to obtain the force-global time curve and the elongation-global time curve.

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

[0089] Where l1 is the gauge length and l0 is the original gauge length;

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

[0091]

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

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

[0094] σ true =(ε true +1)×σ eng ;

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

[0096] like Figure 7 As shown, the r-value is calculated by using the strain evolution of the sample in different directions during the tensile process obtained from the DIC system to calculate the anisotropy r-value of the metal sheet during the tensile process. The calculation formula is as follows:

[0097] in

[0098] Comparing these three sets of calculation results, since the r value has a certain degree of instability, the comparison is made at the stable stage before the sample necking. 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 preloading the specimen, if the deformation type of the preloading is thickening-plane strain compressive preloading deformation, the assembly method of the preloading fixture and the specimen is as follows:

[0100] Place the wrapped sample into the limiting space formed by the long concave mold 8 and the short concave mold 12, ensuring that the end of the sample fits against the inner wall of the limiting space.

[0101] Place the toothed buckling-resisting block 7 inside the limiting space, ensuring that the bottom of the toothed buckling-resisting block 7 is in full contact with the bottom of the limiting space and the top of the toothed buckling-resisting block 7 is flush with the upper surface of the sample. Then, install the bridge-shaped buckling-resisting pad 2 and the limiting block 3 at the side opening of the limiting space. Check whether the pressure of the disc spring 4 is appropriate by adjusting the fastening screw 1 to ensure that the sample deforms normally in the thickness direction and does not buckle in the width direction.

[0102] Press the pressure block 9 into the top opening of the corresponding limiting 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 in close contact with the short die 12 to ensure that the long die 8 and the short die 12 are constrained in the length direction. After the pressure block 9 is pressed down, the thickened pre-deformed sample 10 is obtained.

[0103] In some embodiments of the present invention, when the preloaded deformation type is thickened-uniaxial compression preloaded deformation, the difference between it and thickened-plane strain compression preloaded deformation is as follows:

[0104] Place the wrapped sample into the middle position of the limiting space formed by the long concave mold 8 and the short concave mold 12, avoiding contact between the end of the sample and the left and right inner walls of the limiting space.

[0105] In some embodiments of the present invention, when the preloaded deformation type is thinning-plane strain compressive preloaded deformation, the assembly method of the preload fixture and the specimen is as follows:

[0106] Place the wrapped sample into the limiting space formed by the long concave mold 8 and the short concave mold 12, ensuring that the end of the sample fits against the inner wall of the limiting space.

[0107] Press the pressure block 9 into the top opening of the corresponding limiting 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 in close contact with the short die 12 to ensure that the long die 8 and the short die 12 are constrained in the length direction. After the pressure block 9 is pressed down, the thinned pre-deformed sample 16 is obtained.

[0108] In some embodiments of the present invention, when the preloaded deformation type is thinning-uniaxial compression preloaded deformation, the difference between it and thinning-plane strain compression preloaded deformation is as follows:

[0109] Place the wrapped sample into the middle position of the limiting space formed by the long concave mold 8 and the short concave mold 12, avoiding contact between the end of the sample and the left and right inner walls of the limiting space.

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

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

[0112] 1. Different strain conditions:

[0113] During uniaxial compression, the metal sheet specimen primarily bears compressive stress in one direction, resulting in strain. For example, in the preloading of a thickening-uniaxial compression specimen, deformation along the specimen's length is unrestricted (because the length is less than the length of the limiting space cavity), with compressive deformation mainly occurring in the thickness direction. Deformation in the width direction is relatively free, but due to the action of buckling-resistant blocks, the overall deformation is still somewhat controlled, ultimately achieving thickening in the thickness direction. Similarly, in the preloading of a thinning-uniaxial compression specimen, the primary compressive strain is generated in the thickness direction, causing the sheet to thin, while deformation in other directions is relatively minor.

[0114] Plane strain compression means that during sheet deformation, strain in a certain direction is suppressed or limited to near zero. For example, during the preloading of a thickening-plane strain compression specimen, the length direction deformation is suppressed through fixture design (such as the long concave die 8, toothed buckling-resistance blocks 7, and bridge-type buckling-resistance pads 2 working together), causing the specimen to thicken primarily in the thickness direction. Simultaneously, the presence of buckling-resistance blocks 6 ensures stable thickening deformation and controlled strain in the width direction. Similarly, during the preloading of a thinning-plane strain compression specimen, length direction deformation is suppressed, resulting mainly in thinning strain in the thickness direction.

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

[0116] During uniaxial compression, because the deformation is relatively concentrated in one main direction, the grains inside the metal sheet are subjected to greater stress in that direction. The grains will undergo deformation behaviors such as rotation and slippage along the compression direction, resulting in directional changes in the internal microstructure. This directionality affects the subsequent mechanical properties of the material, such as potentially increasing the strength and hardness in the compression direction, while the changes in properties in other directions are relatively small, exhibiting significant anisotropy.

[0117] Plane strain compression: Under plane strain compression, the deformation of the grains within the metal sheet becomes more complex because the strain in both directions is restricted to different degrees. While the material experiences compressive strain in the thickness direction, the grains in the suppression direction exert a restraining and coordinating effect on the deformation, resulting in a more uniform change in the internal microstructure and a degree of anisotropy that differs from uniaxial compression. For example, in plane strain compression that suppresses strain in the length direction, the changes in the material's properties in the thickness and width directions are interconnected and influence each other. The impact on overall mechanical properties (such as yield strength, tensile strength, and anisotropy coefficient) differs from that of uniaxial compression; the changes in properties more comprehensively reflect the synergistic effect of strain constraints in both directions.

[0118] II. Impact on the use of preload fixtures: 3. Differences in sample placement; 4. Different focuses on anti-buckling components.

[0119] 3. Differences in sample placement:

[0120] Uniaxial compression: In uniaxial compression with thickening preloading, since the deformation of the specimen along its length is unrestricted, the specimen should be placed in the middle of the bottom of the die to prevent excessive deformation along its length from touching the cavity wall and affecting the preloading result. The same consideration should be given to the relationship between the specimen length and the cavity during thinning preloading uniaxial compression; the specimen should be placed in a suitable position to prevent abnormal deformation.

[0121] Plane strain compression: Whether it is a 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, so as to achieve the effect of plane strain compression.

[0122] 4. The focus of attention differs for anti-buckling components.

[0123] Uniaxial compression: In uniaxial compression with thickening preloading, the bridge-type buckling-resistance pad 2 plays a crucial role. The pressure provided by the disc spring 4 ensures that the specimen deforms normally in the thickness direction while preventing buckling in the width direction. This is because the relatively free deformation in the width direction during uniaxial compression requires a buckling-resistance device to ensure overall deformation stability. In contrast, in uniaxial compression with thinning preloading, the focus is mainly on the thinning deformation of the specimen in the thickness direction, and the requirements for the buckling-resistance components are relatively lower. However, the overall fixture still needs to ensure stable loading of the specimen during the thinning process.

[0124] Plane strain compression: During thickening preloaded plane strain compression, buckling-resistant components such as the toothed buckling-resistant block 7 and the bridge-type buckling-resistant pad 2 must not only prevent buckling in the width direction but also work together to ensure strain suppression in the length direction, ensuring stable thickening plane strain compression in the thickness direction. During thinning preloaded plane strain compression, the buckling-resistant components must also work in conjunction with the overall fixture structure to ensure accurate thinning of the specimen in the thickness direction under strain suppression in the length direction, preventing inaccurate test results due to uneven deformation or buckling.

[0125] Therefore, uniaxial compression and plane strain compression differ significantly in terms of strain state, impact on material microstructure and mechanical properties. These differences necessitate different operating methods during the use of preloading fixtures, from specimen placement to the focus of buckling-resistant components, to ensure accurate preloading for thickening and thinning of metal plates and to provide reliable specimen deformation conditions for subsequent research.

[0126] This invention innovatively designs a pre-loading fixture and specimen for the mechanical property analysis of metal sheets. The same fixture enables four types of pre-deformation: two types of thinning pre-deformation (unidirectional compression along the sheet thickness direction and plane strain compression along the thickness direction) and two types of thickening pre-deformation (unidirectional compression along the width direction and plane strain compression along the width direction). A special thickness-direction buckling-resistant device ensures stable thickening deformation of the elongated specimen under unidirectional compression and plane strain compression along the width direction. A low-friction coefficient polymer film is coated on the specimen surface to reduce the influence of friction on the compression-type pre-loading, resulting in accurate and stable specimen pre-deformation. The experiment is simple to operate, demonstrating the effects of thickening and thinning pre-loading on the mechanical properties of metal sheets, and evaluating the impact of different pre-loading deformation methods on the sheet's mechanical properties. Compared to traditional methods that only evaluate the effect of thinning pre-deformation, this approach is more comprehensive and effective.

[0127] In the description of this specification, references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0128] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0129] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for analyzing the mechanical properties of metal sheets, characterized in that, A preloading fixture for metal sheets is provided, which is used to preload and deform metal sheet samples. The fixture includes a long die and a short die, which, when joined together, form a limiting space for placing the sample. The limiting space has a top and a side opening. A limiting element is installed in the side opening to constrain the sample during deformation, preventing buckling in the thickness or width direction. A pressure block is installed in the top opening to connect to a testing machine and uniformly transmit the pressure of the testing machine to the sample. The limiting component includes a bridge-shaped buckling-restraining pad, a disc spring, a washer, and a buckling-restraining block. The position of the bridge-shaped buckling-restraining pad corresponds to the position of the lateral opening. A cavity is formed inside the bridge-shaped buckling-restraining pad, and the opening of the cavity faces the sample in the limiting space. The disc spring and the buckling-restraining block are disposed in the cavity, and a washer is also disposed between the disc spring and the buckling-restraining block. A toothed buckling-restraining block is also disposed between the buckling-restraining block and the sample. The surface of the toothed buckling-restraining block has a toothed structure. The bottom of the toothed buckling-restraining block abuts against the bottom of the limiting space, and the top of the toothed buckling-restraining block is flush with the upper surface of the sample. A limiting block is also provided at the lateral opening, and the limiting block is installed above the bridge-shaped anti-buckling pad; The preload fixture also includes a fastening slider and a clamping base. The clamping base has an installation space in the middle that accommodates the long die, the short die, and the fastening slider. The installation space is trapezoidal. The end of the long die away from the short die abuts against the horizontal surface of the installation space, and the end of the short die away from the long die abuts against the horizontal surface of the fastening slider. The surfaces of the fastening slider that abut against the installation space are all inclined surfaces, and the fastening slider can slide up and down along the inclined surfaces. The method includes a metal sheet preloading test, tensile specimen cutting, subsequent tensile test, and data processing. The preloading test for metal sheets includes the following steps: sample preparation, preloading, and testing machine setup, wherein: Sample preparation: The original metal sheet is wire-cut to the corresponding size. Each group of samples is polished, rinsed, and the average size is measured and calculated. The samples are then wrapped with Teflon film and trimmed. Preloading: Place the wrapped sample into the preloading fixture according to the required preloading deformation type and assemble it. Pay attention to the placement of the sample under different preloading deformation types and the contact points with the preloading fixture. Testing machine settings: Place the preloaded fixture on the testing machine, set the pressing speed, stop after reaching the predetermined deformation amount, and measure and calculate the actual deformation amount; The tensile specimen cutting process includes: cutting the pre-loaded and deformed specimen into three pieces along the thickness direction using a slow wire cutter, taking the thickest piece towards the core and cutting out a standard tensile specimen. Subsequent tensile tests include: Sample preparation: Grind the gauge length of the tensile specimen, rinse, measure and calculate the average width and thickness, and spray DIC speckle on the gauge length; Testing machine and DIC system setup: The specimen is mounted on the universal testing machine and the tensile speed is set. The DIC system is debugged, and the parameters for DIC system test image acquisition, processing and calibration are clarified. Tensile test and test data acquisition: Tensile test is performed, the DIC system identifies and records speckle images, and the strain field is calculated after the tensile test is completed; Data processing includes: The above tensile test data are processed to obtain average strain and elongation data. Force-time curves are derived and interpolated to obtain force-global time curves and elongation-global time curves. Calculate the stress-strain curve, engineering stress, and actual strain and actual stress during the tensile process of metal sheet, compare these three sets of results, and determine their validity.

2. The method for mechanical property analysis of metal sheets according to claim 1, characterized in that, When preloading is performed, and the deformation type of the preloading is thickening-plane strain compressive preloading deformation, the assembly method of the preloading fixture and the specimen is as follows: Place the wrapped sample into the limiting space composed of the long and short concave molds, ensuring that the end of the sample fits against the inner wall of the limiting space. Place the toothed buckling-resistance block inside the limiting space, ensuring that the bottom of the toothed buckling-resistance block is in full contact with the bottom of the limiting space, and that the top of the toothed buckling-resistance block is flush with the upper surface of the sample. Then, install the bridge-shaped buckling-resistance pad and the limiting block at the lateral opening of the limiting space. Check whether the pressure of the disc spring is appropriate to ensure that the sample deforms normally in the thickness direction and does not buckle in the width direction. Press the pressure block into the top opening of the corresponding limiting space, and place the long die and the short die into the installation space of the clamping base. Check whether the fastening slider is in close contact with the short die to ensure that the long die and the short die are constrained in the length direction. After the pressure block is pressed down, the thickened pre-deformed sample is obtained.

3. The method for mechanical property analysis of metal sheets according to claim 2, characterized in that, When the preloaded deformation type is thickened-uniaxial compressive preloaded deformation, its difference from the thickened-plane strain compressive preloaded deformation is as follows: Place the wrapped sample in the middle of the limiting space formed by the long and short concave molds, avoiding contact between the end of the sample and the left and right inner walls of the limiting space.

4. The method for mechanical property analysis of metal sheets according to claim 1, characterized in that, When the preloaded deformation type is thinning-plane strain compressive preloaded deformation, the assembly method of the preload fixture and the specimen is as follows: Place the wrapped sample into the limiting space composed of the long and short concave molds, ensuring that the end of the sample fits against the inner wall of the limiting space. Press the pressure block into the top opening of the corresponding limiting space, and place the long die and the short die into the installation space of the clamping base. Check whether the fastening slider is in close contact with the short die to ensure that the long die and the short die are constrained in the length direction. After the pressure block is pressed down, a thinned pre-deformed sample is obtained.

5. The method for mechanical property analysis of metal sheets according to claim 4, characterized in that, When the preloaded deformation type is thinning-uniaxial compression preloaded deformation, the difference between it and the thinning-plane strain compression preloaded deformation is as follows: Place the wrapped sample in the middle of the limiting space formed by the long and short concave molds, avoiding contact between the end of the sample and the left and right inner walls of the limiting space.

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