Unidirectional tensile test loading device and loading method for metal sheet in complex strain path

Abstract

The invention relates to a loading device which comprises a tensile testing machine and a fixing device, and the fixing device comprises an upper plate, a middle plate and a lower plate. And a male die, a nitrogen spring and a discharging bolt are fixed on the upper plate. The nitrogen spring can push the middle plate to move, the limit range is limited through the discharging bolt, and the safety effect is achieved. And a female die and a guide post are fixed on the lower plate. The upper plate, the middle plate and the lower plate are guided by the guide columns to move in parallel. The male die and the female die are positioned through the keys, and the male die and the female die are fastened through the bolts. After bending and reverse bending pre-forming are completed under the action of a press machine, the upper plate and the lower plate are connected through bolts and nuts to achieve fixing of the device. During testing, the device is turned over by 90 degrees and moved to a tensile testing machine, an upper chuck and a lower chuck of the tensile testing machine clamp metal sheet samples reserved at the two ends of the device respectively, and a unidirectional tensile test in a complex strain path can be completed. The unidirectional tensile test loading device under the complex strain path is reasonable in structure and convenient to use.

Description

Technical Field

[0001] The present invention relates to a loading device, and particularly to a unidirectional tensile test loading device and a loading method for a metal sheet under a complex strain path. Background Art

[0002] The background of the invention of the complex strain loading device for metal sheets mainly stems from the urgent need for high-precision forming processes and material property evaluation in the fields of industrial manufacturing and materials science. Metal sheets (such as aluminum alloys and high-strength steels) are widely used in fields such as automobiles and aerospace. However, in processes such as stamping, deep drawing, and bulging, materials often undergo multiaxial non-proportional loading (such as combined bending-unbending strain). For example, the forming of automotive body panels requires simulating the coupled tensile and bending conditions, which cannot be satisfied by traditional equipment. Because existing unidirectional tensile testing machines can only achieve simple stress states and cannot reproduce complex conditions such as biaxial tension, shear, or bending-unbending cyclic loading. There is a lack of effective means for the dynamic coordinated control of strain paths (such as parameters like rate and deformation amount), resulting in a large deviation between the test results and the actual working conditions and affecting the accuracy of the construction of the material constitutive model. In a conventional bending device, the pressure applied after the specimen is bent will be unloaded. At this time, if the specimen is used for a tensile test, it is still a single axial tension. Moreover, in a traditional unidirectional tensile device, the strain path is a fixed linear path, and the same tensile device can only be used for workpieces with relatively fixed dimensions and cannot be flexibly adjusted according to the size of the workpiece to be stretched.

[0003] Therefore, there is a need for a complex strain path loading device that can couple tension and bending-unbending complex strain paths, has a simple structure and can be used in conjunction with a tensile machine, and can quickly test various strain paths to support the synchronous development of materials and processes. Summary of the Invention

[0004] Aiming at the technical bottleneck that traditional unidirectional loading equipment cannot simulate multiaxial composite strain paths, the present invention proposes a multi-adjustable complex strain path loading device for thin plates, which realizes high-precision mechanical response testing of the tension of metal sheets under a bending-unbending cyclic strain path through a bidirectional coupling loading mechanism. A bending load can be synchronously superimposed on the main direction of uniaxial tension, breaking through the limitation of unidirectional loading of traditional equipment and accurately reproducing the tensile-bending composite stress state in the sheet metal forming process. Through a modular designed replaceable convex / die tool set, a quantitative relationship model between the geometric parameters of the tool and the local strain is established. By adjusting the sizes of the die and the punch, continuous adjustment of the bending-unbending complex strain can be achieved, thereby meeting the test requirements for different metal sheets (such as aluminum alloys and high-strength steels), different thicknesses, and different magnitudes of strain.

[0005] The technical solution adopted by the present invention is: a unidirectional tensile test loading device for a metal sheet under a complex strain path, comprising: a tensile testing machine and a fixing device, the fixing device comprising an upper plate and a lower plate, the upper plate being provided with four first through holes, four first guide sleeve mounting holes and four threaded holes, the four first guide sleeves being mounted in the first guide sleeve mounting holes of the upper plate, the punch being fixedly mounted on the lower surface of the upper plate by means of a first fixing member and a second fixing member, the lower surface of the upper plate being provided with a plurality of nitrogen spring mounting grooves, and nitrogen springs being mounted in the nitrogen spring mounting grooves.

[0006] The output end of the nitrogen spring is fixedly installed with a middle plate, which is located between the upper plate and the lower plate. The middle plate is provided with a square hole, four second guide sleeve mounting holes, four unloading bolt mounting holes and four second through holes. The square hole is matched with the punch clearance, and a second guide sleeve is installed in each of the four second guide sleeve mounting holes. The unloading bolt passes through the unloading bolt mounting hole and is fixedly installed in the threaded hole of the upper plate.

[0007] The die is mounted on the upper surface of the lower plate through a third fixing member, four guide posts are fixedly mounted on the upper surface of the lower plate, four third through holes are opened on the lower plate, the four guide posts and the first guide sleeve and the second guide sleeve are slidably matched; the fixing bolts can pass through the first through hole, the second through hole and the third through hole to lock the fixing device.

[0008] The upper plate, middle plate and lower plate are arranged in parallel.

[0009] The male tool has two protrusions and the female tool has two corresponding grooves.

[0010] The length of the upper plate is 200mm, the width is 160mm, the thickness is 20mm, and the depth of the nitrogen spring mounting groove in the upper plate is 15mm; the length of the middle plate is 200mm, the width is 160mm, and the thickness is 15mm; the length of the square hole on the middle plate is 84mm, the width is 64mm, the length of the lower plate is 200mm, the width is 160mm, and the thickness is 20mm; the length of the die is 160mm, the width is 60mm; the length of the punch is 76mm, and the width is 60mm.

[0011] A loading method based on a unidirectional tensile test loading device under complex strain paths of metal sheets, comprising the following steps:

[0012] S1: Calculate the size of the fixing bolts in the fixing device;

[0013] S2: Pre-deform the metal sheet by bending and reverse bending, and fix it by a fixing device after bending;

[0014] S3: Perform tensile test on metal sheet specimens.

[0015] Furthermore, step S1 is specifically as follows:

[0016] S11: Calculate the bending and reverse bending preforming force;

[0017] The shape of the metal sheet specimen is W-shaped, and the bending and reverse bending preforming force is calculated according to the bending force formula in formula (1):

[0018]

[0019] In the formula, F Z —Forming force (N); k—Safety factor, taking 1.3; b—Width of the metal sheet specimen (mm); t—Thickness of the specimen (mm); R m —Tensile strength of the specimen material (MPa); r—Bending inner corner radius (inner corner radius, mm);

[0020] In this embodiment, the safety factor k is taken as 1.3, the width b of the metal sheet specimen is 20 mm, the maximum thickness t of the metal sheet specimen does not exceed 2 mm, and the maximum tensile strength R of the metal sheet specimen m does not exceed 1500 Mpa at most, and the minimum bending radius r is 15 mm; the bending force of free bending calculated by formula (1) is 6423 N; there are two semi-circular bends, so the total preforming force is 12846 N;

[0021] S12: Calculate the blank holding force;

[0022] During the bending and reverse bending preforming process, a blank holding force needs to be applied to the middle plate, and it can be calculated according to formula (2):

[0023] F Q =(0.3 - 0.8)F z (2) In the formula, F Q —Middle plate blank holding force (N); F z —Preforming force calculated in the previous step;

[0024] Take F Q =0.3F Z , and the calculated blank holding force is F Q =3853.8 N;

[0025] S13: Calculate the total pressure of the bending and reverse bending preforming press;

[0026] The specific calculation process is as formula (3):

[0027] F = F Z +F Q (3)

[0028] In the formula, F z —Preforming force calculated in the previous step; F Q —Middle plate blank holding force (N);

[0029] It is calculated that F = 12846N + 3853.8N = 16699.8N;

[0030] S14: Calculate the load of the nitrogen spring;

[0031] According to the moving distance of the punch during the bending and reverse bending preforming process, the stroke of the nitrogen spring can be calculated to be 19mm; the blank holding force F Q is 3853.8N, and there are 4 nitrogen springs. The minimum initial load of the nitrogen spring is one-fourth of the blank holding force F Q , that is, 963.45N; the model of the nitrogen spring is standard type, and the minimum initial load at a stroke of 19mm is 1700N, and the maximum load is 2800N;

[0032] S15: Determine the diameter of the fixing bolt;

[0033] The tensile strength condition of the dangerous section of the ordinary bolt is shown in formula (4), where δ ca —The calculated stress of the bolt (MPa); F2—The total tensile force of the bolt (N); d1—The minor diameter of the bolt (mm); [δ] is the allowable stress of the bolt; according to formula (4), the minimum diameter calculation formula of the bolt can be further calculated, as shown in formula (5);

[0034]

[0035] The total tensile force F2 of the bolt is equal to the sum of the residual pre-tightening force F1 and the working tensile force F, that is, F2 = F1 + F; for general bolt connections, when the working load is stable, F1 = (0.2 - 0.6)F; in this embodiment, F1 = 0.6F and F2 = 1.6F; the total pressure of the press obtained above is the total working tensile force F of the bolt 总 = 16699.8N; a total of 4 bolts are required to fix this device, that is, the working tensile force F of a single bolt = F 总 ×0.25 = 16699.8×0.25 = 4174.95N; it is obtained that F2 of a single bolt = 1.6F = 1.6×4174.95 = 6679.92N;

[0036] The allowable stress of the bolt is calculated according to formula (6), where δ s —The yield strength of the bolt (MPa); S—The safety factor of the bolt connection. In this device, the bolt is subjected to static load and is a tight connection, so S is taken as 1.2; the performance grade of the bolt is taken as 4.6, and its yield strength δ s = 240MPa; thus, the allowable shear stress [δ] of the bolt is calculated to be 200Mpa;

[0037]

[0038] Substituting F2 and [δ] into formula (5), we get d1≥7.435mm. Since the bolt is a standard part, the bolt specification adopted in this embodiment is M10, which can meet the tensile strength condition of the dangerous section of the bolt.

[0039] Further, step S2 is specifically as follows:

[0040] Place the tested metal thin plate specimen at the middle position of the female die. Then, use the press to push the upper part of the fixing device downward. At this time, the middle plate contacts the specimen. The upper part of the device continues to descend, and the nitrogen spring will be compressed to generate pressure and apply it to the middle plate to press the metal thin plate specimen tightly. The upper part of the device continues to descend, and the punch presses the sheet material downward into the female die until the bottom dead center position. When the pre-bending deformation is completed, the press continues to maintain the pressing state. Use four fixing bolts to pass through the first through hole, the second through hole, and the third through hole, and realize the fixation of the fixing device through bolt-nut connection to completely lock the deformation force applied to the metal thin plate specimen. At this time, the press can be opened, and the device can be taken out of the press.

[0041] Further, step S3 is specifically as follows:

[0042] Flip the fixing device by 90 degrees to make the upper plate, the middle plate, and the lower plate in a vertical state. Move the fixing device with the pre-bent metal thin plate specimen to the tensile testing machine. The upper and lower chucks of the tensile testing machine clamp both ends of the metal thin plate specimen, and then conduct a unidirectional tensile test. During the tensile process, the specimen will undergo bending and reverse bending deformations and couple with the tensile deformation until tensile fracture. Through the fixing device, a unidirectional tensile test under complex strain paths is realized in cooperation with the tensile testing machine.

[0043] Compared with the prior art, this patent application has the following advantages:

[0044] 1. In the present invention, the fixing device is locked by passing the fixing bolts through the first through hole, the second through hole, and the third through hole, so as to realize the continuous loading of bending and reverse bending strains on the metal thin plate specimen. Without unloading the strain, the fixing device is flipped by 90 degrees to make the upper plate, the middle plate, and the lower plate in a vertical state. The fixing device with the pre-bent metal thin plate specimen is moved to the tensile testing machine for a unidirectional continuous tensile test, thereby realizing the combined loading of bending - reverse bending + tension, which is beneficial to the study of the damage evolution of metal thin plate materials.

[0045] 2. Compared with the traditional fixed linear path tension, the present invention can customize the bending curvature and strain by changing the sizes of the punch and the die, realize the loading of complex paths with different strain degrees. The punch and the die are connected by fixing parts and can be quickly disassembled and replaced, so that the strain amount and the size of the strain path can be changed more quickly.

[0046] 3. The complex strain path loading device provided by the present invention has a simple fixture structure, can be freely installed on a universal testing machine, and is simple and convenient to disassemble and assemble. Description of the Drawings

[0047] Figure 1 is a schematic structural diagram of the loading device;

[0048] Figure 2 is an isometric view of the upper die;

[0049] Figure 3 is an isometric view of the lower die;

[0050] Figure 4 is a working schematic diagram of the punch and die;

[0051] Figure 5 is Figure 1 the right view of;

[0052] Figure 6 is Figure 5 the E-E sectional view of;

[0053] Figure 7 is Figure 1 the bottom view of;

[0054] Figure 8 is a schematic diagram when the loading device works on a tensile testing machine. Detailed Embodiment

[0055] The present invention will be further described below in conjunction with the drawings and specific embodiments.

[0056] As shown in the figure, a unidirectional tensile test loading device for a metal thin plate under a complex strain path includes: a tensile testing machine and a fixing device. The fixing device includes an upper plate 1 and a lower plate 2. Four first through holes, four first guide sleeve mounting holes, and four threaded holes are provided on the upper plate 1. Four first guide sleeves 8 are mounted in the first guide sleeve mounting holes of the upper plate 1. The punch 4 is fixedly installed on the lower surface of the upper plate 1 through a first fixing member 12 and a second fixing member 14. A plurality of nitrogen spring mounting grooves 15 are also provided on the lower surface of the upper plate 1, and nitrogen springs 10 are installed in the nitrogen spring mounting grooves 15.

[0057] The output end of the nitrogen spring 10 is fixedly installed with a middle plate 3. The middle plate 3 is located between the upper plate 1 and the lower plate 2. A square hole, four second guide sleeve mounting holes, four ejector bolt mounting holes, and four second through holes are provided on the middle plate 3. The square hole is in clearance fit with the punch 4. A second guide sleeve 7 is installed in each of the four second guide sleeve mounting holes. The ejector bolt 9 passes through the ejector bolt mounting hole and is fixedly installed in the threaded hole of the upper plate 1. When the fixing device is in the mold closing process, the middle plate 3 first falls downward under the action of gravity, and the middle plate 3 will be hung by four ejector bolts 6, playing a safety role.

[0058] The female die 5 is installed on the upper surface of the lower plate 3 through the third fixing member 13. Four guide pillars 6 are fixedly installed on the upper surface of the lower plate 3. Four third through holes are formed in the lower plate 3. The four guide pillars are slidably matched with the first guide sleeve 8 and the second guide sleeve 7. The fixing bolt 11 can pass through the first through hole, the second through hole and the third through hole to lock the fixing device.

[0059] Further, the upper plate 1, the middle plate 3 and the lower plate 2 are arranged in parallel.

[0060] Further, the male die 4 has two protrusions, and the female die 5 has two corresponding grooves.

[0061] Further, the length of the upper plate 1 is 200 mm, the width is 160 mm, and the thickness is 20 mm. The depth of the nitrogen spring installation groove 15 in the upper plate 1 is 15 mm. The length of the middle plate 3 is 200 mm, the width is 160 mm, and the thickness is 15 mm. The length of the square hole above the middle plate 3 is 84 mm, and the width is 64 mm. The length of the lower plate 2 is 200 mm, the width is 160 mm, and the thickness is 20 mm. The length of the female die 5 is 160 mm, and the width is 60 mm. The length of the male die 4 is 76 mm, and the width is 60 mm.

[0062] As Figure 1 shown, the working principle of the present invention is:

[0063] During operation, the test metal thin plate specimen is placed at the middle position of the female die 5; then the upper half of the fixing device is pushed downward by a press. At this time, the middle plate 3 contacts the specimen; the upper half of the device continues to move downward, and the nitrogen spring 10 will be compressed to generate pressure and apply it to the middle plate 3 to press the specimen tightly; the upper half of the device continues to move downward, and the male die 4 presses the sheet material downward into the female die 5 until the bottom dead center position. When the pre-bending deformation is completed, the press continues to maintain the pressing state. The four fixing bolts 11 pass through the first through hole, the second through hole and the third through hole, and the fixing of the fixing device is realized through bolt-nut connection to completely lock the deformation force applied to the thin plate specimen; at this time, the press can be opened and the device can be taken out of the press.

[0064] A loading method for a loading device for a unidirectional tensile test under a complex strain path of a metal thin plate includes the following steps:

[0065] S1: Calculate the size of the fixing bolt in the fixing device

[0066] S11: Calculate the pre-forming force of bending and reverse bending

[0067] The shape of the metal thin plate specimen is W-shaped, and the pre-forming force of bending and reverse bending is calculated according to the bending force formula of formula (1):

[0068]

[0069] In the formula, F Z — Forming force (N); k — Safety factor, taking 1.3; b — Width of the metal sheet specimen (mm); t — Thickness of the specimen (mm); R m — Tensile strength of the specimen material (MPa); r — Inner bend radius (inner corner radius, mm);

[0070] In this embodiment, the safety factor k is taken as 1.3, the width b of the metal sheet specimen is 20 mm, the maximum thickness t of the metal sheet specimen does not exceed 2 mm, and the tensile strength R of the metal sheet specimen m does not exceed 1500 Mpa at most, and the minimum bend radius r is 15 mm. The bending force of free bending calculated by formula (1) is 6423 N. There are two semi-circular bends in this embodiment, so the total pre-forming force is 12846 N.

[0071] S12: Calculate the blank holding force

[0072] During the bending reverse bending pre-forming process, a blank holding force needs to be applied to the middle plate, which can be calculated according to formula (2):

[0073] F Q = (0.3 - 0.8)F z (2)

[0074] In the formula, F Q — Middle plate blank holding force (N); F z — Pre-forming force calculated in the previous step.

[0075] In this embodiment, take F Q = 0.3F Z , and the calculated blank holding force is F Q = 3853.8 N.

[0076] S13: Calculate the total pressure of the bending reverse bending pre-forming press

[0077] The specific calculation process is as formula (3):

[0078] F = F Z + F Q (3)

[0079] In the formula, F z — Pre-forming force calculated in the previous step; F Q — Middle plate blank holding force (N).

[0080] In this embodiment, it is calculated that F = 12846 N + 3853.8 N = 16699.8 N

[0081] S14: Calculate the nitrogen spring load

[0082] According to the moving distance of the punch during the bending and reverse bending preforming process, the stroke of the nitrogen spring can be calculated to be 19 mm. In this embodiment, the blank holding force F Q is 3853.8 N. There are 4 nitrogen springs in this embodiment. The minimum initial load of the nitrogen spring is one-fourth of the blank holding force F Q , that is, 963.45 N. The model of the nitrogen spring is the standard type. The minimum initial load at a stroke of 19 mm is 1700 N, and the maximum load is 2800 N.

[0083] S15: Determine the diameter of the fixing bolt

[0084] The tensile strength condition of the dangerous section of the ordinary bolt is shown in formula (4), where δ ca —The calculated stress of the bolt (MPa); F2—The total tensile force of the bolt (N); d1—The minor diameter of the bolt (mm); [δ] is the allowable stress of the bolt. According to formula (4), the minimum diameter calculation formula of the bolt can be further calculated, as shown in formula (5).

[0085]

[0086] The total tensile force F2 of the bolt is equal to the sum of the residual pre-tightening force F1 and the working tensile force F, that is, F2 = F1 + F. For general bolt connections, when the working load is stable, F1 = (0.2 - 0.6)F. In this embodiment, F1 = 0.6F and F2 = 1.6F. The total pressure of the press obtained above is the total working tensile force F 总 = 16699.8 N. A total of 4 bolts are required to fix this device, that is, the working tensile force F of a single bolt = F 总 ×0.25 = 16699.8×0.25 = 4174.95 N. It is obtained that F2 of a single bolt = 1.6F = 1.6×4174.95 = 6679.92 N.

[0087] The allowable stress of the bolt is calculated according to formula (6), where δ s —The yield strength of the bolt (MPa); S—The safety factor of the bolt connection. In this device, the bolt is subjected to static load and is a tight connection, so S is taken as 1.2. The performance grade of the bolt is taken as 4.6, and its yield strength δ s = 240 MPa. Thus, the allowable shear stress [δ] of the bolt is calculated to be 200 Mpa.

[0088]

[0089] Substitute F2 and [δ] into formula (5) to get d1 ≥ 7.435 mm. Since the bolt is a standard part, the bolt specification adopted in this embodiment is M10, which can meet the tensile strength condition of the dangerous section of the bolt.

[0090] S2: Pre-deform the metal sheet by bending and reverse bending, and fix it with a fixing device after bending.

[0091] Place the tested metal sheet specimen at the middle position of the female die 5; then use a press to push the upper part of the fixing device downward. At this time, the middle plate 3 contacts the specimen; the upper part of the device continues to descend, and the nitrogen spring 10 will be compressed to generate pressure and apply it to the middle plate 3 to press the metal sheet specimen tightly; the upper part of the device continues to descend, and the punch 4 presses the sheet material downward into the female die 5 until the bottom dead center position. When the pre-bending deformation is completed, the press continues to maintain the pressing state, and four fixing bolts 11 pass through the first through hole, the second through hole and the third through hole, and the fixing of the fixing device is realized through bolt-nut connection, locking the deformation force applied to the metal sheet specimen completely; at this time, the press can be opened and the device can be taken out of the press.

[0092] S3: Conduct a tensile test on the metal sheet specimen.

[0093] Flip the fixing device by 90 degrees to make the upper plate 1, the middle plate 3 and the lower plate 2 in a vertical state. Move the fixing device with the pre-bent metal sheet specimen to the tensile testing machine. As Figure 8 shown, the upper and lower chucks of the tensile testing machine clamp both ends of the metal sheet specimen, and then conduct a unidirectional tensile test. During the tensile process, the specimen will experience bending and reverse bending deformations and couple with the tensile deformation until tensile fracture. Through the fixing device, a unidirectional tensile test under a complex strain path is realized in cooperation with the tensile testing machine.

[0094] In summary, these are only the preferred examples of the present invention. Those skilled in the art should understand that making some simple modifications, equivalent changes or decorations using the disclosed technical content all fall within the protection scope of the present invention.

Claims

1. A unidirectional tensile test loading device under complex strain paths for metal sheets, comprising: Tensile testing machine and fixing device, characterized in that: the fixing device includes an upper plate (1) and a lower plate (2). Four first through holes, four first guide sleeve mounting holes and four threaded holes are provided on the upper plate (1). Four first guide sleeves (8) are installed in the first guide sleeve mounting holes of the upper plate (1). The punch (4) is fixedly installed on the lower surface of the upper plate (1) through a first fixing member (12) and a second fixing member (14). A plurality of nitrogen spring mounting grooves (15) are also provided on the lower surface of the upper plate (1), and nitrogen springs (10) are installed in the nitrogen spring mounting grooves (15).

2. The unidirectional tensile test loading device under complex strain paths for metal sheets according to claim 1, characterized in that: The output end of the nitrogen spring (10) is fixedly installed with a middle plate (3). The middle plate (3) is located between the upper plate (1) and the lower plate (2). A square hole, four second guide sleeve mounting holes, four stripper bolt mounting holes and four second through holes are provided on the middle plate (3). The square hole is in clearance fit with the punch (4). A second guide sleeve (7) is installed in each of the four second guide sleeve mounting holes. The stripper bolt (9) passes through the stripper bolt mounting hole and is fixedly installed in the threaded hole of the upper plate (1).

3. The unidirectional tensile test loading device under complex strain paths for metal sheets according to claim 1, characterized in that: The die (5) is installed on the upper surface of the lower plate (3) through a third fixing member (13). Four guide columns (6) are fixedly installed on the upper surface of the lower plate (3). Four third through holes are provided on the lower plate (3). The four guide columns are in sliding fit with the first guide sleeve (8) and the second guide sleeve (7). The fixing bolt (11) can lock the fixing device through the first through hole, the second through hole and the third through hole.

4. The unidirectional tensile test loading device under complex strain paths for metal thin plates according to claim 1, characterized in that: The upper plate (1), the middle plate (3) and the lower plate (2) are arranged in parallel.

5. A unidirectional tensile test loading device under complex strain paths for metal sheets according to claim 1, characterized in that: The punch (4) has two protrusions, and the die (5) has corresponding two grooves.

6. A unidirectional tensile test loading device under complex strain paths for metal thin plates according to claim 1, characterized in that: The length of the upper plate (1) is 200 mm, the width is 160 mm, and the thickness is 20 mm. The depth of the nitrogen spring mounting groove (15) in the upper plate (1) is 15 mm; the length of the middle plate (3) is 200 mm, the width is 160 mm, and the thickness is 15 mm; the length of the square hole above the middle plate (3) is 84 mm and the width is 64 mm; the length of the lower plate (2) is 200 mm, the width is 160 mm, and the thickness is 20 mm; the length of the die (5) is 160 mm and the width is 60 mm; the length of the punch (4) is 76 mm and the width is 60 mm.

7. A loading method for a unidirectional tensile test loading device under complex strain paths of metal sheets according to any one of claims 1-6, comprising the following steps: S1: Calculate the size of the fixing bolt in the fixing device; S2: Pre-deform the metal sheet by bending and reverse bending, and fix it through the fixing device after bending; S3: Conduct a tensile test on the metal sheet specimen.

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