Test method for bulging of metal sheets using double elliptical dies

Abstract

The present invention relates to a method for testing the properties of thin metal plates, and proposes a double-elliptical die bulging test method for thin metal plates. The experimental apparatus employed primarily comprises a double-elliptical die, a sealed pressure plate, a CCD camera, a control system, a pressure medium output system, and a temperature regulation system. The asymmetry of the double-elliptical die allows for different boundary conditions and loading conditions to be established at different locations on the plate surface, resulting in different stress ratios at various points within the plate surface during bulging. By monitoring the stress, strain, and dimensional parameters of each point with different stress ratios during the bulging deformation process, a single bulging test can obtain sufficient plate performance parameters under a sufficient range of stress states and deformation degrees. Compared to bulging tests using circular or elliptical dies, the double-elliptical die bulging test significantly reduces the number of tests, lowers testing costs, shortens the testing cycle, and improves experimental efficiency.

Description

Technical Field

[0001] The invention relates to a method for testing the properties of a metal sheet, in particular to a method for testing the bulging of a metal sheet using a double elliptical die. Background Art

[0002] Sheet metal is the primary raw material for manufacturing complex thin-walled metal components. During the process of deforming sheet metal into complex components using methods such as bulging and deep drawing, the sheet metal deforms under different stress states. It is well known that sheet metal, especially sheet metal with significant anisotropy, exhibits different deformation behaviors and properties under different stress states. To theoretically analyze or simulate the process of forming sheet metal into complex metal components, it is necessary to experimentally deform the sheet metal under different stress states to obtain stress, strain, and other data. This allows for the characterization and evaluation of sheet metal properties, the determination of its constitutive model, and the establishment of its forming limits.

[0003] Currently, testing methods for thin metal sheet properties can be categorized into uniaxial loading and biaxial loading based on the stress state during deformation. Uniaxial loading tests generally refer to uniaxial tension or compression tests, while biaxial loading tests are primarily conducted through cross-specimen tensioning or sheet bulging. When using cross-specimens for biaxial loading, the shape and dimensions of the specimen's intermediate deformation zone and the biaxial tension load must be monitored in real time during the test to achieve continuous deformation with a constant ratio of principal stresses in mutually perpendicular directions within the sheet's plane. Therefore, this method requires complex and expensive specialized equipment, and maintaining a constant stress ratio in the later stages of deformation is difficult, resulting in a relatively small effective strain range. Another commonly used biaxial loading test is the sheet bulging test. To achieve biaxial isotropic deformation with equal principal stresses in mutually perpendicular directions within the sheet's plane, a circular die (referred to as circular die bulging) can be used for bulging. To achieve general biaxial tensile deformation with unequal principal stresses in mutually perpendicular directions within the sheet's plane, an elliptical die (referred to as elliptical die bulging) is often used for bulging. Using circular or elliptical dies for bulging, the center of a metal sheet can be continuously deformed until fracture occurs while maintaining a constant stress ratio. This is the primary advantage of this type of testing. However, this also presents a limitation. To obtain material properties under various stress ratios, multiple bulging tests must be conducted using dies of different shapes, resulting in a significant number of tests. Further testing is required to obtain specimens at different deformation stages or degrees under varying stress ratios, requiring even more tests.

[0004] It should be noted that both unidirectional and bidirectional loading tests achieve a constant stress ratio during loading and deformation, meaning the stress ratio in the central deformation zone of the sheet metal remains essentially constant throughout the entire loading and deformation process. To examine the impact of changes in stress ratio during deformation on material properties, an invention patent (patent number 201611240021.6) proposes a stress-strain measurement device and method for thin sheet metal deformation under complex stress paths. This test method utilizes a stepped die with a continuously changing cross-sectional shape along the bulging height. This causes the stress ratio in the central deformation zone of the sheet metal to continuously change throughout the bulging process, resulting in a non-constant stress ratio loading and deformation. This test method provides the potential for more comprehensive characterization and evaluation of the properties of thin metal sheets. However, this method also requires a dedicated test apparatus, and the variation in stress ratio achieved by each stepped die bulging is fixed. Furthermore, each test can only produce specimens with a specific strain.

[0005] In summary, the aforementioned uniaxial tension / compression tests, cross-specimen tension tests, and circular / elliptical / stepped die bulging tests can cause sheet metal to undergo continuous deformation under uniaxial stress states, biaxial isotensive stress states, biaxial stress states with a fixed stress ratio, and stress states with a continuously varying stress ratio. However, these testing methods can only achieve simple or single loading conditions, or only achieve loading conditions with a gradually varying stress ratio at a relatively small degree of deformation. To obtain test results under a sufficient variety of stress states and deformation levels, numerous tests must be conducted, resulting in a large number of tests, high testing costs, and long testing cycles. More importantly, since each specimen inevitably exhibits varying degrees of variability, the test results obtained from testing multiple specimens will inevitably include unknown effects caused by these specimen variations, making the test results lack comparability.

[0006] Therefore, in order to accurately describe the deformation behavior of metal sheets under various complex loading conditions and deformation degrees, it is necessary to establish a new test method that can obtain sufficient, stable and consistent test data using as few metal sheet specimens as possible. Summary of the Invention

[0007] The purpose of the present invention is to solve the problem that the existing metal sheet performance test method can only obtain specimens under a specific stress state or deformation degree. In order to obtain enough test results, it will face the problems of large test volume, high test cost and long test cycle, which leads to the current situation that it is difficult to accurately and comprehensively describe the deformation characteristics of metal sheets under more stress states and more deformation degrees. Therefore, a double elliptical die bulging test method for metal sheets is proposed, which uses a double elliptical die to bulge the metal sheet to obtain its performance.

[0008] The technical solutions of the present invention are as follows:

[0009] A double elliptical die bulging test method for a metal sheet, wherein the experimental device mainly comprises a double elliptical die 4, a sealing plate 6, a CCD camera 1, a control system 7, a pressure medium output system 8, and a temperature control system 12;

[0010] The lower surface of the double-elliptical die 4 is machined with a pressure sealing groove 4-2, which cooperates with the raised pressure sealing rib 6-2 at the corresponding position on the upper surface of the sealing plate 6 to clamp the metal sheet sample 3 between the two to achieve a good seal. The sealing plate 6 is machined inside the channel, with a pressure medium outlet 6-1 and a pressure medium inlet 6-3 at the two ends of the channel respectively. The sealing plate 6 is connected to the medium output end of the pressure medium output system 8 through a high-pressure pipe. The outer surfaces of the double-elliptical die 4 and the sealing plate 6 are wrapped with an induction heating coil 11 connected to a temperature control system 12.

[0011] A CCD camera 1 is positioned above the double-elliptical die 4, with the side of the metal sheet sample 3 sprayed with random speckles facing the CCD camera 1. The deformation and displacement information of the metal sheet sample 3 during the bulging process is recorded in real time by the CCD camera 1 and connected to the image signal input terminal of the control system 7. The control system 7 is connected to the pressure medium output system 8 and the temperature control system 12, respectively, to receive and adjust the data in real time.

[0012] During the bulging deformation process of the double elliptical die 4, the shape of the double elliptical die 4, the selected position of point K, and the degree of deformation are changed to achieve the length δx, δy of the micro unit at point K in the x and y directions and the main curvature radius R x and R y By adjusting the parameters and changing the stress ratio at point K in Equations 3 and 4, the purpose of obtaining sufficient test results under various stress states and deformation degrees in a single bulging test is achieved.

[0013] The specific steps of the double elliptical die bulging test method for metal sheets are as follows:

[0014] Step 1: Based on the stress state, stress ratio, or stress ratio range to be achieved during the bulging test of the metal sheet specimen 3, the shape of the double elliptical die 4 used in the bulging test is determined through theoretical model calculation and simulation model analysis. The cross-sectional profile of the double elliptical die 4 is formed by splicing two ellipses with the same semi-axis length along the semi-axis direction. The parameters to be determined for its shape are the lengths of the major and minor semi-axes of the two semi-ellipses.

[0015] Step 2: Design and manufacture the corresponding double elliptical die 4 according to the scheme determined in Step 1, and cut the metal sheet sample 3 according to the outer dimensions of the double elliptical die 4 and the outer dimensions of the sealing plate 6; the metal sheet sample is circular or has a special shape similar to the cross-sectional outer contour of the double elliptical die 4, ensuring that the entire edge of the metal sheet sample 3 is reliably sealed;

[0016] Step 3: Place the metal sheet sample 3 cut in Step 2 between the double elliptical die 4 and the sealing platen 6. The enclosed space between the metal sheet sample 3 and the sealing platen 6 serves as a pressure medium chamber. Continuously fill the space between the metal sheet sample 3 and the pressure medium chamber with pressure medium. The control system 7 monitors and adjusts the pressure value and pressurization rate to ensure that the metal sheet sample 3 undergoes stable and continuous bulging deformation at a specific deformation rate.

[0017] Step 4: During the bulging process, the deformation data of the selected area on the metal sheet specimen 3 is measured in real time, and the three-dimensional surface shapes of the different areas are directly measured or fitted;

[0018] Step 5: Save the experimental data and remove the metal sheet sample 3 after the bulging experiment;

[0019] Step 6: Based on the experimental data, the stress, strain, principal curvature radius and secondary curvature radius of the sheet metal during bulging are calculated using a theoretical model to obtain the stress state, stress ratio, Rod coefficient, wall thickness and principal strain of the selected area.

[0020] Step 7: Using bulging time, bulging pressure, and bulging height as independent variables, output deformation data on different areas and contour lines in the form of curves or surfaces to provide raw data for subsequent further analysis;

[0021] The present invention adopts a double elliptical die bulging method to accurately and comprehensively describe the deformation characteristics of metal sheets under more stress states and more deformation degrees. The technical principle is described as follows:

[0022] When the double elliptical die 4 is bulging, any point K on the metal sheet specimen 3 can be regarded as being on a certain ellipsoidal surface, and the force balance analysis of the small unit body corresponding to any point K is performed:

[0023] 2R y δytσ x sin(δx / 2)+2R x δxtσ y sin(δy / 2)=p(R x δx)(R y δy)(1)

[0024] Among them, δx and δy are the lengths of the micro-unit body in the x and y directions corresponding to point K respectively; t is the thickness of the micro-unit body corresponding to point K; the two perpendicular directions have the main curvature radius R x and R y, are the two principal curvature radii along the x-axis and y-axis at K, respectively. Their values ​​are determined according to the shape parameters a, b, and c of the double elliptical die 4 and the geometric shape of the metal sheet specimen 3 at the deformation moment. The ratio of any two of the shape parameters a, b, and c of the double elliptical die 4 should be between 0.2 and 1.0. p is the pressure exerted on the metal sheet specimen 3 during bulging of the double elliptical die 4.

[0025] Simplifying formula (1) yields the Laplace equation:

[0026] σ x / R x +σ y / R y =p / t (2)

[0027] Combining the geometric characteristics of point K, we can get the stress expressions in two directions:

[0028]

[0029]

[0030] Where, stress ratio α = σ x / σ y ;

[0031] Based on the assumption of constant volume, the expression of the wall thickness at point K is obtained:

[0032] t=t0exp[-(ε x +ε y )] (5)

[0033] The true strain ε in two directions at point K x and ε y The data is collected by the CCD camera 1 and input to the control system 7 for processing.

[0034] The pressure medium is a liquid pressure medium 5 or a gas pressure medium 10. When performing normal temperature bulging, the liquid pressure medium 5 is used, and the speckle pattern is normal temperature speckle 2; when performing high temperature bulging, the gas pressure medium 10 is used, and the speckle pattern is high temperature speckle 9.

[0035] The deformation data of step 4 is the bulging height and the true strain in two directions at any point.

[0036] The beneficial effects of the present invention are:

[0037] First, the double-elliptical die bulging test method for metal sheet proposed in this invention leverages the asymmetry of the double-elliptical die to establish different boundary conditions and loading conditions at different locations on the sheet surface. This results in different stress ratios at various points within the sheet surface during bulging. By monitoring the stress, strain, and dimensional parameters of each point with different stress ratios during the bulging deformation process, a single bulging test can obtain sufficient sheet performance parameters under various stress states and deformation degrees. Compared with circular or elliptical die bulging tests, the double-elliptical die bulging test can significantly reduce the number of tests, lower testing costs, shorten testing cycles, and improve experimental efficiency.

[0038] Second, the double elliptical die bulging test method for metal sheets proposed in this invention can obtain experimental results under a sufficient number of stress states and deformation degrees in a single test, effectively avoiding the unknown influence caused by the test results containing sample differences when multiple samples are tested during the test process, making the experimental results under different stress states more comparable;

[0039] 3. The double elliptical die bulging test method for metal sheets proposed in the present invention can be used to perform bulging tests on slabs of different materials, different temperatures and different thicknesses, which broadens the application scope of bulging experiments and can better understand the material properties under different conditions.

[0040] Fourth, the double elliptical die bulging test method for metal sheets proposed in this invention, based on the bulging results of the sample in the double elliptical die, can obtain the deformation characteristics of the material under different stress conditions by simply selecting specific points, paths or areas on the bulging surface of the sample; BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1(a) shows a schematic diagram of the circles and their relationships;

[0042] Figure 1(b) shows the schematic diagram of the ellipse and its relationship;

[0043] Figure 1(c) shows the schematic diagram of the double ellipse and their relationship.

[0044] Figure 2 Schematic diagram of the principle of room temperature bulging of metal sheets using double elliptical dies.

[0045] Figure 3 Schematic diagram of the principle of high-temperature bulging of metal sheets using double elliptical dies.

[0046] Figure 4 Schematic diagram of different stages of double elliptical die bulging of metal sheet.

[0047] Figure 5 Schematic diagram of the sample after bulging using a double elliptical die for metal sheet.

[0048] Figure 6Schematic diagram of the bulging area of ​​the metal sheet using a double elliptical die.

[0049] Figure 7 Schematic diagram of the typical contour line of double elliptical die bulging of metal sheet.

[0050] Figure 8 Schematic diagram of typical points of double elliptical die bulging of metal sheet.

[0051] Among them, 1 is a CCD camera, 2 is room temperature speckle pattern, 3 is a metal sheet sample, 3-1, 3-2, and 3-3 are the cross-sectional profiles of the metal sheet sample cut along the symmetry axis of the die in the successive stages of bulging, respectively, 3-4 is the symmetric contour line of the bulging area of ​​the metal sheet sample, 3-5 is the bulging area of ​​the metal sheet sample, 3-6 is the typical contour line of the metal sheet sample, 3-7 is a typical point of the metal sheet sample, 4 is a double elliptical die, 4-1 is the radius r of the double elliptical die corner, 4-2 is the pressure edge sealing groove, 5 is the liquid pressure medium, 6 is the sealing pressure plate, 6-1 is the pressure medium outlet, 6-2 is the pressure edge sealing rib, 6-3 is the pressure medium inlet, 7 is the control system, 8 is the pressure medium output system, 9 is high temperature speckle pattern, 10 is the gas pressure medium, 11 is the induction heating coil, and 12 is the temperature regulation system. DETAILED DESCRIPTION

[0052] The technical solution of the present invention is further described below with reference to the accompanying drawings and through specific embodiments:

[0053] Implementation Plan 1: Combined with Figures 1-2, Figure 4-8 To illustrate, the metal sheet double elliptical die bulging test method proposed by the present invention is implemented according to the following steps:

[0054] Step 1: According to the stress state, stress ratio and deformation degree required for the metal sheet bulging test, reasonable theoretical model calculation or simulation model analysis can be used. It is recommended to use ABAQUS simulation software to determine the shape of the double elliptical die 4 used in the bulging test and the blank holding force required for sealing; the double elliptical die 4 adopts a double elliptical die form, and its cross-sectional contour line is composed of two ellipses with the same semi-axis length spliced ​​along the semi-axis. In order to reduce the influence of the bulging boundary on the free bulging area of ​​the metal sheet during the bulging process, Figure The ratio of any two of the geometric parameters a, b, and c of the double ellipse in 1 should be between 0.2 and 1.0; when a=b=c, it is a circular die, when a=c≠b, it is an elliptical die, and when a≠b≠c, it is a double elliptical die; the dimensional parameters a, b, and c of the cross section of the double elliptical die and the original thickness t0 of the metal sheet specimen should satisfy: min{a; b; c} / t0≥33; the radius r4-1 of the double elliptical die corner and the original thickness t0 of the metal sheet specimen should satisfy: 5×t0≤r≤15×t0.

[0055] Step 2: Design and manufacture a corresponding double-elliptical die 4 according to the scheme determined in Step 1, and cut a metal sheet sample 3 according to the outer dimensions of the double-elliptical die 4 and the outer dimensions of the sealing plate 6; spray a randomly distributed normal temperature speckle pattern 2 on one side of the cut metal sheet sample 3. The normal temperature speckle pattern 2 should be uniform in size and not too sparse or too dense, so that the CCD camera 1 can record the deformation information of the metal sheet in real time during the subsequent bulging process and feed it back to the control system 7;

[0056] Step 3: Place the metal sheet sample 3, which has been cut intact in Step 2 and sprayed with the room-temperature speckle pattern 2, between the double elliptical die 4 and the sealing plate 6. Apply a uniform pressure force below the sealing plate 6 to achieve a sealing effect. A CCD camera 1 is rationally arranged above the double elliptical die 4 to monitor the subsequent bulging information of the metal sheet sample 3. The control system 7 controls the pressure medium output system 8 to continuously fill the closed space formed by the metal sheet sample 3 and the pressure medium chamber with liquid pressure medium 5, causing the metal sheet sample to bulge until the set bulging pressure is reached.

[0057] Step 4: During the bulging process, the CCD camera 1 is used to measure in real time the deformation data of different areas on the metal sheet sample 3 under different bulging pressures, such as bulging height and strain, and the data is fed back to the control system 7. The control system 7 matches the pressure information of the liquid pressure medium 5 output in real time by the pressure medium output system 8 with the bulging information measured by the CCD camera 1. The displacement and strain at different positions on the bulging surface of the metal sheet sample 3 during the bulging process and the pressure information of the liquid pressure medium 5 obtained by the control system 7 are analyzed and processed.

[0058] Step 5: Save the experimental data and remove the metal sheet specimen 3 after the bulging test;

[0059] Step 6: Determine the symmetrical contour line 3-4 of the bulging area of ​​the metal sheet sample based on the experimental results, extract the strain and displacement information on the bulging area 3-5 of the metal sheet sample, and import the displacement information on the bulging area 3-5 of the metal sheet sample into MATLAB to fit the three-dimensional contour of the bulging surface; select the following on the bulging area 3-5 of the metal sheet sample: Figure 6 The typical contour line shown in FIG. 3 is a graph showing the strain information on the typical contour line 3-6 of the metal sheet specimen according to the bulging test data. According to the strain information on the typical contour line 3-6 of the metal sheet specimen, the typical points 3-7 of the metal sheet specimen are reasonably arranged at equal intervals on the typical contour line of the metal sheet specimen, as shown in FIG. Figure 8 As shown, take any typical point and perform stress analysis according to formulas (3)-(5);

[0060] Step 7: Based on the double elliptical die bulging test results, with bulging time, bulging pressure, bulging height, etc. as independent variables, the strain data of different areas, different contour lines and different points on the bulging surface are output in the form of curves or surfaces to provide raw data for subsequent further analysis.

[0061] In this embodiment, the double elliptical die has more adjustable dimensional parameters than the circular die and the elliptical die. Reasonable setting of the double elliptical dimensional parameters a, b, and c during the bulging process can achieve a wider range of stress ratio changes, which can well realize the loading of metal sheet specimens under complex stress states. Since the bulging information of the metal sheet specimen is recorded in real time in the control system during the experiment, sufficient stress and strain information can be obtained through a single bulging experiment, which can significantly reduce the test volume and shorten the test cycle. In addition, selecting typical contour lines and typical points on the bulging surface of the metal sheet specimen for analysis can make better use of the experimental results. By comparing the stress and strain at different points on the same bulging surface, we can better understand the deformation characteristics of the plate under complex loading conditions, and make the deformation results of the metal sheet specimens under different stress states more comparable, which effectively avoids the unknown influence caused by the use of multiple specimens for testing during the test process, resulting in the test results containing specimen differences.

[0062] Implementation Plan 2: Combination Figure 2 、 Figure 4 Note that 3-1, 3-2, and 3-3 represent the cross-sectional profiles of the metal sheet specimen, obtained by sectioning along the die's symmetric axis during the subsequent stages of bulging. In step one, the dimensional parameters of the double-elliptical die 4 can be appropriately adjusted based on the material differences of the metal sheet specimen 3. For materials with poor plasticity, such as 6061 aluminum alloy, the dimensional parameters a, b, and c of the double-elliptical concave cross section and the original thickness t0 of the metal sheet specimen should satisfy the following relationship: min{a; b; c} / t0 ≥ 43. The double-elliptical die corner radius 4-1 and the original thickness t0 of the metal sheet specimen should satisfy the following relationship: 10×t0 ≤ r ≤ 15×t0. For materials with high plasticity, such as 1Cr18Ni9Ti stainless steel, the double-elliptical die corner radius r and the original thickness t0 of the metal sheet specimen should satisfy the following relationship: 5×t0 ≤ r ≤ 10×t0. Other steps and parameters are the same as those in Specific Implementation Plan 1.

[0063] In this embodiment, in view of the differences of the metal sheet sample 3, the dimensional parameters a, b, and c of the cross-section of the double elliptical die are adjusted to reduce the influence of the bulging boundary on the free bulging zone of the metal sheet sample; for test materials with poor plasticity, the radius r4-1 of the double elliptical die corner is reasonably increased to reduce the stress concentration generated at the corner of the double elliptical die during the bulging process of the metal sheet sample 3, thereby avoiding damage to the corner of the double elliptical die during the bulging process of the metal sheet sample 3, thereby affecting the smooth progress of the test.

[0064] Implementation Plan 3: Combination Figure 3 Description: When conducting a double elliptical die bulging test under high temperature conditions (less than 1000 degrees Celsius), in step 2, the corresponding double elliptical die 4 is designed and manufactured according to the plan determined in step 1, and the metal sheet sample 3 is cut according to the outer dimensions of the double elliptical die 4 and the outer dimensions of the sealing pressure plate 6; one side of the cut metal sheet sample 3 is sprayed with randomly distributed high-temperature specks 9, and the size of the high-temperature specks 9 should be uniform and should not be too sparse or too dense, so that the CCD camera 1 can record the deformation information of the metal sheet in real time during the subsequent bulging process. In step 3, before the bulging test, a layer of high-temperature resistant coating is evenly applied to the contact surfaces of the double elliptical die 4 and the sealing plate 6 with the metal sheet sample 3 to prevent adhesion between the double elliptical die 4, the metal sheet sample 3, and the sealing plate 6 during the bulging process. The metal sheet sample 3 that has been cut intact and sprayed with high-temperature speckles 9 in step 2 is then placed between the double elliptical die 4 and the sealing plate 6, and a uniform pressing force is applied under the sealing plate 6 to achieve a sealing effect. A CCD camera 1 is then reasonably arranged above the double elliptical die 4 to monitor the subsequent bulging information of the metal sheet sample 3. The temperature control system 12 is started, and the metal sheet sample 3 is heated to a predetermined temperature by the induction heating coil 11. The temperature control system 12 feeds back the temperature of the metal sheet sample 3 to the control system 7 in real time. The control system 7 controls the pressure medium output system 8 to continuously fill the closed space formed by the metal sheet sample 3 and the pressure medium chamber with gas pressure medium 10, causing the metal sheet sample to bulge until the set bulging pressure is reached.

[0065] In this embodiment, a double elliptical die bulging test under high temperature can be carried out by simply adding a temperature control system 12 and an induction heating coil 11 for heating. Since the side of the metal sheet sample 3 exposed to the outside is sprayed with high-temperature speckles 9, oxidation of the metal sheet sample 3 at high temperature is avoided, thereby enabling the performance test of the metal sheet sample 3 to be achieved simply and quickly.

[0066] Implementation Plan 4: In step 2, the ordinary metal sheet blank is replaced with a blank having uneven size and performance, such as a tailor-welded blank or a differential thickness plate. The other steps and parameters are the same as those in Specific Implementation Plan 1.

[0067] In this embodiment, multiple groups of tests are carried out on the same non-uniform slab, which can avoid the different degrees of differences on each sample caused by factors such as unstable welding parameters and unstable thickness difference control, thereby eliminating the unknown influence caused by sample differences and greatly improving the comparability of test results.

Claims

1. A double elliptical die bulging test method for metal sheets, characterized in that: The experimental device used in the method mainly consists of a double elliptical die (4), a sealing platen (6), a CCD camera (1), a control system (7), a pressure medium output system (8) and a temperature control system (12); The lower surface of the double elliptical die (4) is processed with a pressure edge sealing groove (4-2), which cooperates with the pressure edge sealing rib (6-2) protruding at the corresponding position on the upper surface of the sealing pressure plate (6), and clamps the metal sheet sample (3) between the two to achieve a good seal; the sealing pressure plate is processed inside a channel, and the two ends of the channel are respectively a pressure medium outlet (6-1) and a pressure medium inlet (6-3), and the sealing pressure plate (6) is connected to the medium output end of the pressure medium output system (8) through a high-pressure pipe; the outer surfaces of the double elliptical die (4) and the sealing pressure plate (6) are wound with an induction heating coil (11) connected to a temperature regulation system (12); A CCD camera (1) is arranged above the double elliptical die (4), and the side of the metal sheet sample (3) sprayed with random speckles faces the CCD camera (1); deformation and displacement information of the metal sheet sample (3) during the bulging process is recorded in real time by the CCD camera (1) and connected to the image signal input end of the control system (7); the control system (7) is respectively connected to the pressure medium output system (8) and the temperature adjustment system (12), and receives and adjusts the data in real time; During the bulging deformation process of the double elliptical die (4), the shape of the double elliptical die (4), the selected position of the K point, and the degree of deformation are changed to achieve the lengths δx and δy of the micro unit at K in the x and y directions and the main curvature radius R x and R y By adjusting the parameters, the stress ratio at point K in equations (3) and (4) is changed to achieve the goal of obtaining sufficient test results under a variety of stress states and deformation degrees in a single bulging test. The specific steps of the double elliptical die bulging test method for metal sheets are as follows: Step 1: According to the stress state, stress ratio or stress ratio range to be achieved during the bulging test of the metal sheet specimen (3), the shape of the double elliptical die (4) used in the bulging test is determined through theoretical model calculation and simulation model analysis; the cross-sectional contour line of the double elliptical die (4) is formed by splicing two ellipses with the same semi-axis length along the semi-axis direction, and the parameters to be determined for its shape are the lengths of the major and minor semi-axes of the two semi-ellipses; Step 2: Design and manufacture a corresponding double elliptical die (4) according to the scheme determined in step 1, and cut the metal sheet sample (3) according to the outer dimensions of the double elliptical die (4) and the outer dimensions of the sealing pressure plate (6); the metal sheet sample is circular or has a special shape similar to the outer contour of the cross section of the double elliptical die (4), ensuring that the entire edge of the metal sheet sample (3) is reliably sealed; Step 3: Place the metal sheet sample (3) cut in step 2 between the double elliptical die (4) and the sealing pressure plate (6); the closed space formed between the metal sheet sample (3) and the sealing pressure plate (6) is a pressure medium cavity; continuously fill the space between the metal sheet sample (3) and the pressure medium cavity with pressure medium; monitor and adjust the pressure value and the pressurization rate through the control system (7) so that the metal sheet sample (3) undergoes stable and continuous bulging deformation at a specific deformation rate; Step 4: During the bulging process, the deformation data of the selected area on the metal sheet sample (3) is measured in real time, and the three-dimensional surface shapes of the different areas are directly measured or fitted; Step 5: Save the experimental data and remove the metal sheet sample after the bulging experiment (3); Step 6: Based on the experimental data, the stress, strain, principal curvature radius and secondary curvature radius of the sheet metal during bulging are calculated using a theoretical model to obtain the stress state, stress ratio, Rod coefficient, wall thickness and principal strain of the selected area. Step 7: Using bulging time, bulging pressure, and bulging height as independent variables, output deformation data on different areas and contour lines in the form of curves or surfaces to provide raw data for subsequent further analysis; When the double elliptical die (4) is used for bulging, any point K on the metal sheet specimen (3) can be regarded as being on a certain ellipsoidal surface, and the force balance analysis of the small unit body corresponding to any point K is performed: 2R y two x sin(δx / 2)+2R x right y sin(δy / 2)=p(R x δx)(R y δy)(1) among them, δx and δy are the lengths of the micro-unit in the x and y directions corresponding to point K respectively; t is the thickness of the micro-unit corresponding to point K; the two perpendicular directions have the main curvature radius R x and R y , are the two principal curvature radii at K along the x-axis and y-axis directions, respectively. Their values ​​are determined according to the shape parameters a, b and c of the double elliptical die (4) and the geometrical shape of the metal sheet sample (3) at the deformation moment, wherein the ratio of any two of the shape parameters a, b and c of the double elliptical die (4) should be between 0.2 and 1.0, and p is the pressure exerted on the metal sheet sample (3) during bulging by the double elliptical die (4); wherein a≠b≠c; Simplifying formula (1) yields the Laplace equation: s x / R x +s y / R y =p / t(2) Combining the geometric characteristics of point K, we can get the stress expressions in two directions: Where, stress ratio α=σ x / σ y ; Based on the assumption of constant volume, the expression of the wall thickness at point K is obtained: t=t0exp[-(ε x +ε y )](5) The true strain ε in two directions at point K x and ε y The data is collected by a CCD camera (1) and input to a control system (7) for processing.

2. The metal sheet double elliptical die bulging test method according to claim 1, characterized in that: The pressure medium is a liquid pressure medium (5) or a gas pressure medium (10); when performing normal temperature bulging, the liquid pressure medium (5) is used, and the speckle pattern is the normal temperature speckle pattern (2); when performing high temperature bulging, the gas pressure medium (10) is used, and the speckle pattern is the high temperature speckle pattern (9).

3. The metal sheet double elliptical die bulging test method according to claim 1 or 2, characterized in that: The deformation data of step 4 is the bulging height and the true strain in two directions at any point.

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