Method for obtaining the right-hand part of the forming limit diagram of an automotive sheet and corresponding die

Abstract

A method and corresponding mold for obtaining the right half of the forming limit diagram of an automotive sheet metal is disclosed. The method uses the linear deformation strain path at the fracture site as a constraint condition, spanning from a predetermined time before fracture to the fracture time. The objective is to achieve a uniform distribution of the secondary strain at the time of sheet metal fracture within at least three intervals between 0-10%. Based on this setting and combined with measurements from a three-dimensional digital speckle strain measurement and analysis system, the strain, stress, and displacement of the test points are obtained through a single stamping operation, covering the entire time from the start of pressurization to fracture. This method and mold for obtaining the right half of the forming limit diagram of an automotive sheet metal can obtain the forming limit strain under multiple biaxial tensile deformation states of a metal sheet through a single test, thereby quickly obtaining the right half of the sheet metal forming limit diagram (FLD).

Description

Technical Field

[0001] This invention belongs to the field of metal plastic processing and mold technology, specifically relating to a method for obtaining the right half of an automotive sheet forming limit diagram and a corresponding mold. Background Technology

[0002] Stamping offers advantages such as stability, high efficiency, and low cost, enabling the production of lightweight, thin-walled, high-precision, and complex-shaped parts, making it a crucial processing method in the automotive manufacturing industry. Body panels, as vital components of automotive products, are characterized by large deformation, complex spatial surfaces, and high requirements for surface quality and dimensional accuracy. During production, cracking defects often occur due to mismatches between material properties and stamping processes, significantly impacting production efficiency and manufacturing costs. Therefore, in engineering, forming limit diagrams are commonly used to comprehensively evaluate the forming performance of sheet metal. This plays a vital role in assessing the forming quality of sheet metal and is widely applied to guide the design of stamping dies and production processes.

[0003] The concept of sheet metal forming limit diagrams was first proposed by Keeler in 1965 and was improved, developed, and perfected by Goodwin, Marciniak, Ghosh, Hecker, and Nakazima, among others. Finally, in 2008, a group of automotive companies, steel companies, and research institutions jointly published the ISO 12004 FLD test standard, which established the current method for obtaining sheet metal forming limit diagrams.

[0004] The current FLD testing method first processes the sheet material into the following shape: Figure 1 The dumbbell-shaped specimen shown (the arrows in the figure define the length, width, and transition arc radius of the parallel portion, respectively) has a selected, precisely sized grid or random dot pattern printed on the sheet metal surface. Then, using Nakajima (such as...) Figure 2 (as shown) or the Marciniak method (such as Figure 3 (As shown) The sheet metal is deformed until it fractures, at which point the test is stopped. The strain of the deformed specimen is measured, and then interpolation is used to determine the maximum strain that the material can withstand without failure, thus obtaining the forming limit strain value under a given strain state. By changing the width of the parallel portion of the specimen, the range of the measured strain path is from uniaxial tension to biaxial tension. Connecting the individual forming limit data points collected under different strain states yields the forming limit curve.

[0005] Existing technologies for obtaining limit diagrams of forming performance are based on multiple bulging tests, all of which require complex machining of the sample sheet. During machining, unavoidable characteristic notch defects may be introduced at the edges. On the one hand, this requires a long testing process, and on the other hand, it requires the machining of complex sample sheets, and the inherent defects in the machining process will inevitably affect the test accuracy. At the same time, due to the use of notched samples to form different stress characteristics, narrower samples are more prone to edge cracking during the test, affecting the test accuracy.

[0006] The invention application with application number CN 2014108463726 discloses "an apparatus and test method for determining the left half of a strain limit diagram of a metal sheet," comprising two sample fixing mechanisms and a sample lifting mechanism. The two sample fixing mechanisms are arranged on both sides of the sample lifting mechanism. The sample lifting mechanism includes a cylindrical roller and a lifting device. The cylindrical roller is mounted above the lifting device, and the lifting device drives the cylindrical roller to move up and down. By gradually raising the cylindrical roller, the two ends of the sample break or break directly. Since the sample material usually breaks or breaks at one end first, the other end is just at the critical state of just necking or just breaking, thus solving the problem of the difficulty in controlling the critical state of just necking or breaking of the sample. Then, measuring the strain in this area can more accurately draw the left half of the strain limit diagram.

[0007] Utility model application CN 201920392531.8 discloses "a forming limit curve test mold and device", including: a die, a blank holder, a punch, and a punch pad; the die has a plurality of first through holes penetrating the die, the first end of the first through hole has an observation window, and the second end of the first through hole has a radially expanding groove; the blank holder has a plurality of second through holes penetrating the blank holder, the number of which is the same as the number of the first through holes, and the arrangement of the plurality of second through holes is the same as the arrangement of the plurality of first through holes, with the first through holes and second through holes at the same location being coaxial; the number of punches is the same as the number of second through holes, and the punches are axially movable and corresponding to each other in the second through holes; the punches are fixed on the punch pad.

[0008] The invention application with application number CN201910850955.9 discloses "a test method for the edge forming limit of a metal sheet". It establishes a spherical punch bulging test device including a pressure ring, a punch and die, and a sheet center positioning mechanism. A thin sheet sample with a circular arc notch in the middle of one side is prepared, and the sample edges are processed with different edge qualities as needed. The sample is placed on one side of the spherical punch bulging mold, aligning the center of the sample's arc with the center of the spherical punch. During the punch's ascent, the edge cracks at the center of the arc under the action of tensile expansion and lateral force. Tests are conducted on samples with good edge quality, samples with poor edge quality, and materials with the same edge quality but different microstructures. The displacement and principal strain changes at the crack location are measured using a digital image measurement system. By comparing the displacement or principal strain, the edge forming limit of the material under specific edge conditions and the material's sensitivity to edge quality are determined.

[0009] Invention application CN 2021109899751 discloses "a method for constructing a non-contact forming limit diagram," which employs a tensile test method and utilizes DIC technology to accurately obtain the principal and secondary strains of the material in the frame before fracture, thus precisely constructing a forming limit diagram of the metallic material. Throughout the entire test, the main deformation areas do not need to contact any mold, effectively solving the problems of traditional forming limit tests where strain at the fracture location cannot be collected and the frictional force between the punch and the sheet metal affects the test results. Summary of the Invention

[0010] To address the above problems, this invention provides a method for obtaining the right half of an automotive sheet forming limit diagram and a corresponding mold, the specific technical solution of which is as follows:

[0011] A method for obtaining the right half of an automotive sheet forming limit diagram, characterized in that:

[0012] The mold is set up with the constraint that the deformation strain path of the fracture site is linear during the period from the set time before fracture to the time of fracture, and with the goal of the secondary strain of the sheet metal at fracture having a uniform distribution in at least three intervals between 0-10%.

[0013] Based on this setup, combined with measurements from a three-dimensional digital speckle strain measurement and analysis system, the strain, stress, and displacement of the test point are obtained through a single stamping operation, covering the entire time from the start of pressurization to fracture.

[0014] A method for obtaining the right half of an automotive sheet forming limit diagram according to the present invention is characterized in that:

[0015] The aforementioned "the secondary strain at the time of sheet metal fracture is uniformly distributed in at least three intervals between 0-10%" is achieved by setting at least three independent deformation zones on the mold, and the secondary strain of the sheet metal at the time of fracture under pressure in each deformation zone corresponds to a target secondary strain interval.

[0016] A method for obtaining the right half of a vehicle body shape limit diagram according to the present invention is characterized in that:

[0017] The deformation zone consists of an ellipsoidal punch on the bottom plate of the punch and a matching ellipse on the corresponding position of the die.

[0018] Each punch has a different aspect ratio of its ellipsoidal shape, and the various deformation zones are separated by draw beads on the die.

[0019] A method for obtaining the right half of an automotive sheet forming limit diagram according to the present invention is characterized in that:

[0020] The aspect ratio of each ellipsoidal punch is determined based on the target secondary strain corresponding to each deformation zone, using finite element simulation.

[0021] A method for obtaining the right half of an automotive sheet forming limit diagram according to the present invention is characterized in that:

[0022] At least one of the three intervals is an interval that covers plane strain.

[0023] A method for obtaining the right half of an automotive sheet forming limit diagram according to the present invention is characterized in that:

[0024] The intervals are set into 5, and the corresponding secondary strains of each interval are 6%–8%, 4%–6%, 3%–5%, 1%–3%, and 0%–2%, respectively.

[0025] A method for obtaining the right half of an automotive sheet forming limit diagram according to the present invention is characterized in that:

[0026] The intervals are set to 5, and the corresponding secondary strains of each interval are 6%~8%, 4%~6%, 3%~5%, 1%~3%, and 0~2%, respectively; the length-to-minor axis ratios of the ellipsoidal punch are 6:6, 6:5, 6:4, 6:3, and 6:2, respectively.

[0027] A method for obtaining the right half of an automotive sheet forming limit diagram according to the present invention is characterized in that:

[0028] The single stamping process is repeated at least three times, and all phase images acquired in the three processes are used as the phase image set for analysis.

[0029] A mold for obtaining the right half of a forming limit diagram of an automotive sheet, characterized in that:

[0030] The mold, through the setting of different characteristic deformation zones, forms different strains of the sheet metal at the fracture time of each deformation zone during a one-time stamping, and the deformation strain path of the sheet metal at the fracture site is linear during the period from the set time before fracture to the fracture time.

[0031] According to the present invention, a mold for obtaining the right half of a forming limit diagram of an automotive sheet metal is characterized in that:

[0032] Five different deformation zones were set up, and the different strains of the sheet metal at fracture in each deformation zone were as follows: 6%–8%, 4%–6%, 3%–5%, 1%–3%, and 0–2%.

[0033] According to the present invention, a mold for obtaining the right half of a forming limit diagram of an automotive sheet metal is characterized in that:

[0034] By setting multiple ellipsoidal punches on the punch base plate and setting adaptive ellipses at corresponding positions on the die, a linear response is established in the deformation strain path of the sheet metal at the fracture point during the period from the set time before fracture to the time of fracture.

[0035] By setting draw beads on the die, each ellipsoidal punch forms a different deformation zone;

[0036] By setting different aspect ratios of the ellipsoidal punch, different secondary strains are generated at the moment of fracture in each deformation zone of the sheet metal during the one-time stamping.

[0037] According to the present invention, a mold for obtaining the right half of a forming limit diagram of an automotive sheet metal is characterized in that:

[0038] The different aspect ratios are determined by finite element simulation based on the target fracture secondary strain corresponding to each deformation zone.

[0039] According to the present invention, a mold for obtaining the right half of a forming limit diagram of an automotive sheet metal is characterized in that:

[0040] Five different deformation zones were set up, and the different strains of the sheet metal at the fracture in each deformation zone were: 6%~8%, 4%~6%, 3%~5%, 1%~3%, and 0~2% respectively.

[0041] The corresponding major and minor axis ratios are: 6:6, 6:5, 6:4, 6:3, and 6:2.

[0042] This invention discloses a method and corresponding mold for obtaining the right half of the forming limit diagram of an automotive sheet metal. This method enables the sheet metal to undergo stress and deformation processes consistent with the actual stamping process, providing valuable reference for the development and process design of actual automotive sheet metal stamping dies. Based on a three-dimensional digital speckle dynamic strain measurement and analysis system, it can accurately measure the fracture strain of automotive sheet metal under multiple characteristic deformation states. The forming limit strain of the metal sheet under multiple biaxial tensile deformation states can be obtained through a single stamping process, thereby quickly obtaining the right half of the sheet metal forming limit diagram (FLD). The required sheet metal sample has a simple shape, no characteristic notches, requires no complex machining, and the sample processing quality has minimal impact on testing accuracy.

[0043] In summary, the present invention provides a method and corresponding mold for obtaining the right half of the forming limit diagram of an automotive sheet metal. This method can obtain the forming limit strain of a metal sheet under multiple biaxial tensile deformation states through a single test, thereby quickly obtaining the right half of the sheet metal forming limit diagram (FLD). This provides effective experimental support for guiding the development of automotive sheet metal stamping forming dies and process design. Furthermore, it is well applicable to metal sheets of different materials and various thicknesses. The designed mold is simple to manufacture, easy to operate, and has low testing costs. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of a dumbbell-shaped specimen with parallel portions in the prior art of this invention;

[0045] Figure 2 This is a schematic cross-sectional view of the Nakajima test mold in the prior art of this invention;

[0046] Figure 3 This is a schematic cross-sectional view of the Nakajima test mold in the prior art of this invention;

[0047] Figure 4 This is a schematic diagram of the experimental mold used in this invention;

[0048] Figure 5 This is a schematic diagram of the punch structure in this invention;

[0049] Figure 6 This is an exploded front view of the test mold in an embodiment of the present invention;

[0050] Figure 7 for Figure 6 AA-direction cross section;

[0051] Figure 8 for Figure 6 BB-direction cross-section;

[0052] Figure 9 This is a schematic diagram of the die structure in an embodiment of the present invention;

[0053] Figure 10 for Figure 9 CC-direction schematic diagram;

[0054] Figure 11 This is a schematic diagram of the pressure plate structure in an embodiment of the present invention;

[0055] Figure 12 for Figure 11 DD-direction cross-section;

[0056] Figure 13 This is a forming limit diagram of the sheet metal measured based on an embodiment of the present invention.

[0057] In the picture,

[0058] 1-Groove;

[0059] 2-Pressure plate;

[0060] 3-Groove;

[0061] 4- Punch base plate;

[0062] 1a - Bolt hole;

[0063] 1b - Drawbeam;

[0064] 2a-Drawing groove;

[0065] 3b - Long strip positioning block. Detailed Implementation

[0066] The following describes in more detail a method for obtaining the right half of an automotive sheet forming limit diagram and a corresponding mold, based on the accompanying drawings and specific embodiments of the present invention.

[0067] A method for obtaining the right half of the forming limit diagram of an automotive sheet metal is provided, which takes the linear deformation strain path of the fracture site as a constraint condition from the set time before fracture to the time of fracture, and aims to set the mold so that the secondary strain corresponding to the sheet metal fracture has a uniform distribution in at least three intervals between 0-10%.

[0068] Based on this setup, combined with measurements from a three-dimensional digital speckle strain measurement and analysis system, the strain, stress, and displacement of the test point are obtained through a single stamping operation, covering the entire time from the start of pressurization to fracture.

[0069] in,

[0070] The aforementioned "the secondary strain at the time of sheet metal fracture is uniformly distributed in at least three intervals between 0-10%" is achieved by setting at least three independent deformation zones on the mold, and the secondary strain of the sheet metal at the time of fracture under pressure in each deformation zone corresponds to a target secondary strain interval.

[0071] in,

[0072] The deformation zone consists of an ellipsoidal punch on the bottom plate of the punch and a matching ellipse on the corresponding position of the die.

[0073] Each punch has a different aspect ratio of its ellipsoidal shape, and the various deformation zones are separated by draw beads on the die.

[0074] in,

[0075] The aspect ratio of each ellipsoidal punch is determined based on the target secondary strain corresponding to each deformation zone, using finite element simulation.

[0076] in,

[0077] At least one of the three intervals is an interval that covers plane strain.

[0078] in,

[0079] The intervals are set to 5, and the corresponding secondary strains of each interval are 6%~8%, 4%~6%, 3%~5%, 1%~3%, and 0~2%, respectively; the length-to-minor axis ratios of the ellipsoidal punch are 6:6, 6:5, 6:4, 6:3, and 6:2, respectively.

[0080] in,

[0081] The single stamping process is repeated at least three times, and all phase images acquired in the three processes are used as the phase image set for analysis.

[0082] A mold for obtaining the right half of a forming limit diagram of an automotive sheet metal.

[0083] The mold, through the setting of different characteristic deformation zones, forms different strains of the sheet metal at the fracture time of each deformation zone during a one-time stamping, and the deformation strain path of the sheet metal at the fracture site is linear during the period from the set time before fracture to the fracture time.

[0084] in,

[0085] Five different deformation zones were set up, and the different strains of the sheet metal at fracture in each deformation zone were as follows: 6%–8%, 4%–6%, 3%–5%, 1%–3%, and 0–2%.

[0086] in,

[0087] By setting multiple ellipsoidal punches on the punch base plate and setting adaptive ellipses at corresponding positions on the die, a linear response is established in the deformation strain path of the sheet metal at the fracture point during the period from the set time before fracture to the time of fracture.

[0088] By setting draw beads on the die, each ellipsoidal punch forms a different deformation zone;

[0089] By setting different aspect ratios of the ellipsoidal punch, different secondary strains are generated at the moment of fracture in each deformation zone of the sheet metal during the one-time stamping.

[0090] in,

[0091] The different aspect ratios are determined by finite element simulation based on the target fracture secondary strain corresponding to each deformation zone.

[0092] in,

[0093] Five different deformation zones were set up, and the different strains of the sheet metal at the fracture in each deformation zone were: 6%~8%, 4%~6%, 3%~5%, 1%~3%, and 0~2% respectively.

[0094] The corresponding major and minor axis ratios are: 6:6, 6:5, 6:4, 6:3, and 6:2.

[0095] Working principle

[0096] 1 Design Concept

[0097] In the stamping process of automotive sheet metal, bulging deformation is more common, corresponding to the right half of FLD (Flat Die). Furthermore, different stamping die shapes will produce different deformation effects on sheet metal of the same shape. Based on these two points, this patent obtains the sheet metal forming limit under specific strain conditions by changing the shape and contour of the bulging die, while ensuring that the stress state of the metal sheet during forming is similar to that during stamping.

[0098] 2. Composition of the test mold

[0099] The test mold consists of three parts: a concave mold, a pressure plate, and a convex mold. Figure 4 As shown, the punch is equipped with five ellipsoidal punches with different aspect ratios (6:6, 6:5, 6:4, 6:3, and 6:2, respectively). Each punch, arranged from largest to smallest ellipsoidal minor axis, induces secondary strains of 6%–8%, 4%–6%, 3%–5%, 1%–3%, and 0–2% in the sheet metal upon fracture, respectively. The secondary strain settings cover a range specific to plane strain, and the ellipsoidal shape effectively characterizes the plane strain state while ensuring a linear strain path from the point of impact to fracture, thus guaranteeing testing accuracy. The aspect ratios are calculated iteratively using the finite element method based on the respective secondary strain ranges. The secondary strain settings are designed with a primary objective of 0-10% and a secondary strain with continuous distribution within this range as a secondary objective, and measuring fracture under plane strain as a third objective.

[0100] Both the die and the pressure plate are equipped with hollow structures that match the shape of the punch. By setting draw beads around the forming area of ​​each punch, the die is divided into five independent bulging forming areas, thereby avoiding the occurrence of sheet metal flow between areas and improving the stability of the test.

[0101] 3. Method for obtaining the right half of the automotive sheet forming limit diagram

[0102] The specific evaluation process for obtaining the right half of the automotive sheet forming limit diagram is as follows:

[0103] (1) Process the tested sheet material into three rectangular samples of 400mm×400mm;

[0104] (2) Spray paint for speckle measurement onto the sample surface;

[0105] (3) Install the test mold on the press and lubricate its surface sufficiently;

[0106] (4) Place the sample on the pressure plate, turn on the press to deform the plate, and measure the strain state of the plate surface through the three-dimensional digital speckle dynamic strain measurement and analysis system.

[0107] (5) Obtain strain data of the first fracture location in each region, and continue the test until the last region fractures. Repeat the test three times.

[0108] (6) Connect the measured data points to obtain the right half of the forming limit diagram of the measured sheet metal.

[0109] Example

[0110] Figure 6 , 7 Figure 8 shows an exploded view of the test mold involved in this invention patent, including: a die 1, a pressure plate 2, a punch 3, and a punch base plate 4. During installation, the die is fixedly connected to the press die fixing plate by bolts, the pressure plate is fixedly connected to the press lower die fixing plate by bolts, the punch is fixedly connected to the punch base plate by bolts, and the punch base plate is fixedly connected to the press ejection device by bolts. During testing, the sheet metal is placed on the upper surface of the pressure plate, and the die moves downward to press the sheet metal; at this time, the punch moves upward, causing deformation in multiple areas of the sheet metal; until the last area cracks, the punch stops operating.

[0111] The structure of each component of the test mold is as follows:

[0112] (1) Die

[0113] Die such as Figure 9 , 10As shown, there are five elliptical hollow structures in the middle, and the mold is divided into five independent areas by the drawing ribs 1b set on the periphery (labeled as area one to area five in descending order of the minor axis of the ellipse); bolt holes 1a are provided at the four top corners of the mold for connecting with the press die fixing plate.

[0114] (2) Pressure plate

[0115] Pressure plate, such as Figure 11 , 12 As shown, there are also five elliptical hollow structures in the middle, and each hollow area is provided with a drawing groove 2b corresponding to the drawing bead of the die; bolt holes are provided at the four apex positions for connecting with the lower die fixing plate of the press.

[0116] (3) Punch

[0117] punch, such as Figure 5 As shown, it consists of a punch base plate and five ellipsoidal punches with different aspect ratios (6:6, 6:5, 6:4, 6:3, and 6:2, respectively). Each punch has two bolt holes and a long strip-shaped positioning block 3b; the four corners of the punch base plate have bolt holes for connecting to the ejector device of the press.

[0118] To first verify the feasibility and working accuracy of the method and mold of this invention, a simulation test was conducted on a 1mm thick 5182 aluminum alloy automotive sheet based on the Autoform simulation calculation platform. The simulation test process is as follows:

[0119] (1) Establish discretized models of the mold and sheet metal according to the aforementioned geometric parameters;

[0120] (2) Set the motion trajectory of the mold according to the aforementioned working method of the mold;

[0121] (3) Set the Coulomb friction coefficient between the sheet metal and the die, as well as the blank holder force;

[0122] (4) Submit the algorithm to perform calculations and obtain a simulation file of the sheet metal testing process;

[0123] (5) Open the simulation file and read the strain data of the first fracture location in each region;

[0124] (6) Connect the measured data points to obtain the right half of the forming limit diagram of the measured sheet metal, such as... Figure 13 As shown in the figure (ε1 is the principal strain and ε2 is the secondary strain, and the selected time period is a certain time before fracture until the moment of fracture), the strain trajectory at each fracture location is also marked. The results show that the strain path of the sheet deformation at each fracture location is close to linear, and the effectiveness of the forming limit is high; thus, the accuracy of the mold of the present invention can be simulated and verified.

[0125] This invention discloses a method and corresponding mold for obtaining the right half of the forming limit diagram of an automotive sheet metal. This method enables the sheet metal to undergo stress and deformation processes consistent with the actual stamping process, providing valuable reference for the development and process design of actual automotive sheet metal stamping dies. Based on a three-dimensional digital speckle dynamic strain measurement and analysis system, it can accurately measure the fracture strain of automotive sheet metal under multiple characteristic deformation states. The forming limit strain of the metal sheet under multiple biaxial tensile deformation states can be obtained through a single stamping process, thereby quickly obtaining the right half of the sheet metal forming limit diagram (FLD). The required sheet metal sample has a simple shape, no characteristic notches, requires no complex machining, and the sample processing quality has minimal impact on testing accuracy.

[0126] In summary, the present invention provides a method and corresponding mold for obtaining the right half of the forming limit diagram of an automotive sheet metal. This method can obtain the forming limit strain of a metal sheet under multiple biaxial tensile deformation states through a single test, thereby quickly obtaining the right half of the sheet metal forming limit diagram (FLD). This provides effective experimental support for guiding the development of automotive sheet metal stamping forming dies and process design. Furthermore, it is well applicable to metal sheets of different materials and various thicknesses. The designed mold is simple to manufacture, easy to operate, and has low testing costs.

Claims

1. A method for obtaining the right half of a forming limit diagram for automotive sheet metal, characterized in that: The mold is set up with the constraint that the deformation strain path of the fracture site is linear during the period from the set time before fracture to the time of fracture, and the objective is that the secondary strain corresponding to the sheet metal fractures is evenly distributed in 5 intervals between 0-10%. Based on this setup, combined with measurements from a three-dimensional digital speckle strain measurement and analysis system, the strain, stress, and displacement of the test point can be obtained in a single pressurization operation, covering the entire time from the start of pressurization to fracture. The aforementioned "uniform distribution of secondary strain within five intervals between 0-10% at the time of sheet metal fracture" is achieved by setting five independent deformation zones on the mold, with the secondary strain of the sheet metal at fracture in each deformation zone corresponding to a target secondary strain interval. The deformation zone consists of an ellipsoidal punch on the bottom plate of the punch and a matching elliptical hollow structure on the corresponding position of the die. Each punch has a different aspect ratio (length to minor diameter) and the deformation zones are separated by draw beads on the die. The intervals are set into 5, and the corresponding secondary strains for each interval are 6%~8%, 4%~6%, 3%~5%, 1%~3%, and 0~2%, respectively. Correspondingly, the aspect ratios of the ellipsoidal punches are 6:6, 6:5, 6:4, 6:3, and 6:2, respectively.

2. The method for obtaining the right half of an automotive sheet forming limit diagram according to claim 1, characterized in that: The aspect ratio of each ellipsoidal punch is determined based on the target secondary strain corresponding to each deformation zone, using finite element simulation.

3. The method for obtaining the right half of an automotive sheet forming limit diagram according to claim 1, characterized in that: At least one of the five intervals is an interval that covers plane strain.

4. The method for obtaining the right half of an automotive sheet forming limit diagram according to claim 1, characterized in that: The single stamping process is repeated at least three times, and all phase images acquired in the three processes are used as the phase image set for analysis.

5. A mold for obtaining the right half of the forming limit diagram of an automotive sheet metal as described in claim 1, characterized in that: The mold, through the setting of different characteristic deformation zones, forms different strains of the sheet metal at the fracture time of each deformation zone during a one-time stamping, and the deformation strain path of the sheet metal at the fracture site is linear during the period from the set time before fracture to the fracture time.

6. The mold according to claim 5, characterized in that: By setting five ellipsoidal punches on the punch base plate and setting matching ellipses at corresponding positions on the die, a linear response of the deformation strain path at the fracture point is established during the period from the set time before fracture to the time of fracture. By setting draw beads on the die, each ellipsoidal punch forms a different deformation zone; By setting different aspect ratios of the ellipsoidal punch, different strains are generated at the moment of fracture in each deformation zone during a single stamping.

7. The mold according to claim 6, characterized in that: The different aspect ratios are determined by finite element simulation based on the target fracture secondary strain corresponding to each deformation zone.

Images