Forming limit acquisition method for metal sheet, forming limit surface creation method for metal sheet, forming limit surface creation system for metal sheet, and forming limit surface creation program for metal sheet, and press forming crack determination method, press forming crack determination system, and press forming crack determination program

Abstract

Provided are a forming limit acquisition method for a metal sheet, a forming limit surface creation method for a metal sheet, a forming limit surface creation system for a metal sheet, and a forming limit surface creation program for a metal sheet for obtaining force and strain in a sheet thickness direction as indices of a forming limit of a metal sheet, and a press forming crack determination method, a press forming crack determination system, and a press forming crack determination program for determining the presence or absence of occurrence of a crack in a press formed product using force and strain in a sheet thickness direction as crack determination parameters.SOLUTION: In the forming limit acquisition method for a metal sheet according to the present invention, forming is performed such that various forces in the sheet thickness direction are generated in the test piece 100 of the metal sheet using the plurality of forming dies 11 having different shapes, and the force in the sheet thickness direction generated in the formed test piece 100 and the maximum principal strain and the minimum principal strain of the test piece 100 formed such that the force in the sheet thickness direction is generated are obtained as indices of the forming limit of the metal sheet.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for obtaining the forming limit of a metal sheet, a method for creating a forming limit surface of a metal sheet, a system for creating a forming limit surface of a metal sheet, and a program for creating a forming limit surface of a metal sheet.Furthermore, the present invention relates to a press-forming crack determination method, a press-forming crack determination system, and a press-forming crack determination program for determining whether or not cracks have occurred in a press-formed product obtained by press-forming a metal sheet. [Background technology]

[0002] Metal sheets (e.g., thin steel sheets) used as materials for automobile bodies are mostly processed into body parts by press forming. The press formability of body parts varies depending on the shape of the body part and is also greatly affected by the material properties, such as the ductility of the metal sheet used as the material. In recent years, in response to demands for lighter car bodies, efforts have been made to increase the strength of metal sheets used in body parts. However, as the strength of metal sheets increases, their ductility decreases, making them more susceptible to cracking during press forming and reducing their press formability.

[0003] To avoid problems such as cracks during the manufacture of auto body parts by press forming, it is becoming increasingly important to design dies based on advance predictions of press formability using CAE (Computer Aided Engineering).As a result, technology that can accurately determine the forming limits of metal sheets is becoming increasingly important.

[0004] Forming limit diagrams (FLDs) are usually used to determine the forming limits of metal sheets. Forming limit diagrams are created by measuring the forming limits of various deformation modes of metal sheets in press forming, such as equibiaxial deformation, non-uniform biaxial deformation, plane strain deformation, and uniaxial deformation, through laboratory-scale forming tests. To create a forming limit diagram, the width of the test specimen is varied at several levels, and the strain ratio in the major and minor axes of the test specimen is changed, thereby measuring the strain in both the major and minor axes at the time of fracture.

[0005] Generally, in metal sheet forming tests, the metal sheet specimen undergoes uniform deformation, followed by a process in which strain is concentrated at specific locations on the specimen. During this process, a reduction in thickness, called necking, occurs at the location where strain is concentrated. After this reduction in thickness has progressed, the metal sheet fractures. In press forming, the occurrence of necking can result in a defective product even if it does not lead to fracture. Therefore, the forming limit of metal sheets must be defined using the strain just before necking occurs. In particular, in press forming of high-strength steel sheets with tensile strengths exceeding 980 MPa, necking occurs at a low strain of about 10%, and fracture occurs immediately thereafter. Therefore, the forming limit of high-strength steel sheets must be determined accurately, and several techniques for this purpose have been proposed.

[0006] Non-Patent Document 1 standardizes a method for identifying forming limit curves. In this method, first, as shown by plot 1 in Fig. 12, the strain distribution in the direction intersecting the fractured portion (X direction) of a test piece formed until fracture is measured, and the measured strain distribution is approximated to a curve expressed by the following formula (1), as shown by solid line 2 in Fig. 12. Then, from the strain distribution approximated by formula (1), the maximum value of strain, indicated by arrow 6 in Fig. 12, is calculated, and this maximum value is taken as the forming limit strain.

number

[0007] [Non-Patent Document 1] ISO 12004-2:2008, Metallic materials - Sheet and strip - Determination of forming-limit curves, 2008. Summary of the Invention [Problem to be solved by the invention]

[0008] Non-Patent Document 1 describes the Nakajima method and the Marciniak method as methods for determining the forming limit by a stretch test of a test piece 100, as shown in FIG. As shown in Figure 13(a), the Nakajima method involves bulging a test piece 100 using a molding die 201 equipped with a spherical punch 203 having a spherical-shaped tip 203a, an upper die 205, and a blank holder 207. On the other hand, the Marciniak method, as shown in Figure 13(b), uses a molding die 211 equipped with a flat-head punch 213 with a flat tip 213a, an upper die 205, and a blank holder 207, and sandwiches a driving sheet 215 between the flat-head punch 213 and the test piece 100 to bulge-mold the test piece 100.

[0009] Comparing the Nakajima method with the Marciniak method, the Nakajima method tests in a state where the ball-head punch 203 conforms to the shape of the punch, so the fractured portion of the test piece 100 where the apex of the tip 203a comes into contact receives a large force in the thickness direction. In contrast, the Marciniak method uses a flat-head punch 213, so the contact area with the test piece 100 is large, and the force in the thickness direction generated at the fractured portion of the test piece 100 is relatively small.

[0010] Furthermore, as shown in FIG. 13(c), the forming limit line obtained by the Nakajima method generally has a strain amount just before necking occurs that is about 1 to 2% larger.

[0011] In press forming of high-strength steel sheets with low ductility, slight differences in strain can lead to differences in the occurrence of necking. Therefore, until now, the forming limit of metal sheets has been evaluated by comparing the forming limit diagrams of the Nakajima method and the Marciniak method based on the strain measured in actual press-formed products and the strain obtained from press forming analysis using CAE.

[0012] However, even under press forming conditions predicted by the Nakajima method as no cracking would occur based on the strain obtained from press forming analysis, cracks sometimes occurred in actual press-formed products, especially press-formed products made of high-strength steel sheets of 980 MPa or higher. Also, even under press forming conditions predicted as likely to cause cracking using the Marciniak method, cracks sometimes did not occur in the actual press-formed products. Thus, when the presence or absence of cracking was predicted by press forming analysis based on the forming limit line created by the conventional method, there were many cases where the predicted presence or absence of cracking in the actual press-formed products was significantly different, which was problematic.

[0013] It is believed that the force in the thickness direction generated during the press forming process varies depending on the part of the actual press-formed product. However, as mentioned above, the conventional forming limit line could not determine the forming limit taking into account the degree of force in the thickness direction. Therefore, there was a need for a technology that could determine the forming limits of metal sheets while taking into account the influence of force in the thickness direction and detect cracks in actual press-formed products.

[0014] The present invention has been made to solve the above-mentioned problems, and aims to provide a method for obtaining the forming limit of a metal plate, which can determine the forming limit of a metal plate taking into account the influence of forces in the plate thickness direction, a method for creating a forming limit surface of a metal plate, a system for creating a forming limit surface of a metal plate, and a program for creating a forming limit surface of a metal plate. Furthermore, the present invention aims to provide a press-molding crack determination method, a press-molding crack determination system, and a press-molding crack determination program that determine the presence or absence of cracks in actual press-molded products, taking into account the influence of forces in the plate thickness direction. [Means for solving the problem]

[0015] (1) The method for obtaining the forming limit of a metal plate according to the present invention is characterized in that a metal plate test piece is formed using a plurality of tools having different shapes so that various forces in the plate thickness direction are generated, and the force in the plate thickness direction generated in the formed test piece, as well as the maximum principal strain and the minimum principal strain, are obtained as indicators of the forming limit of the metal plate.

[0016] (2) The method for creating a forming limit surface of a metal plate according to the present invention involves forming a test piece of the metal plate using a plurality of tools having different shapes so that various forces in the plate thickness direction are generated, and creating a forming limit surface of the metal plate that is expressed by the relationship between the force in the plate thickness direction generated in the formed test piece and the maximum principal strain and the minimum principal strain, The method includes a forming test step, a forming limit analysis step, and a forming limit surface creation step, The molding test step includes: a test piece preparation step of preparing a plurality of test pieces having different shapes and having a predetermined lattice or strain analysis pattern applied to the surface; a test piece molding step of molding each of the test pieces using each of the plurality of tools while photographing the surface of the test piece; a strain measurement step of analyzing the photographed images of the surfaces of the test pieces and measuring strains occurring in the test pieces; a thickness direction force acquisition step of determining a thickness direction force at a fracture portion occurring in each of the molded test pieces; a strain database construction step of storing the strains measured in the strain measurement step for each of the formed test pieces in chronological order from the start of forming to the occurrence of fracture, and constructing a strain database; The forming limit analysis step includes: an evaluation point sequence setting step of setting an evaluation point sequence in each of the molded test pieces to acquire a strain distribution in the vicinity of the fracture portion generated in each of the molded test pieces; a strain distribution acquisition step of extracting the strain of the evaluation point sequence set for each of the test pieces from the strain database and acquiring a strain distribution in the vicinity of the fractured portion; and a forming limit acquisition step of determining a forming limit based on the acquired strain distribution and determining a maximum principal strain and a minimum principal strain at the forming limit, The forming limit surface creation step includes: a forming limit plotting step of plotting the force in the plate thickness direction at the fracture portion and the maximum principal strain and minimum principal strain at the forming limit, which are obtained for each of the formed test pieces, in a three-dimensional coordinate space; and a forming limit surface creation process for creating a forming limit surface of the metal plate based on a group of plot points of the force in the plate thickness direction at the fracture portion and the maximum principal strain and minimum principal strain at the forming limit plotted in the three-dimensional coordinate space.

[0017] (3) In the above (2), The step of obtaining the force in the plate thickness direction is characterized in that the force in the plate thickness direction is obtained by obtaining the surface pressure generated at the fracture portion of each of the molded test pieces.

[0018] (4) In the above (3), The surface pressure is measured by placing pressure-sensitive paper between the tool and each of the test pieces during the test piece molding process, or is calculated by finite element method analysis that reproduces the molding of each of the test pieces during the test piece molding process.

[0019] (5) In the above (2), The thickness direction force acquisition process is characterized in that the thickness direction stress occurring at the fracture portion of each of the test pieces formed in the test piece forming process is obtained as the thickness direction force.

[0020] (6) In the above (5), The stress in the thickness direction is calculated by a finite element method analysis that reproduces the forming of each of the test pieces in the test piece forming step.

[0021] (7) In any one of (2) to (6) above, In the forming limit surface creation step, a polygonal surface is created that is made up of a plurality of triangular planes that connect two adjacent plot points among the plot point groups for a plurality of the test pieces formed using the tool having the same shape, and one plot point that is closest to the line segment connecting the two plot points among the plot point groups for a plurality of the test pieces formed using the tool having a shape different from that of the two plot points.

[0022] (8) In any one of (2) to (6) above, In the forming limit surface creation step, a forming limit plane or a forming limit curved surface is assumed, the sum of squares of the perpendicular distances between the assumed forming limit plane or the forming limit curved surface and each plot point of the group of plot points in the three-dimensional coordinate space is calculated, and the forming limit plane or the forming limit curved surface is determined so that the calculated sum of squares is minimum.

[0023] (9) In any one of (2) to (6) above, In the forming limit surface creation step, a forming limit plane or a forming limit curved surface is assumed, a sum of squares weighted on the perpendicular distance between the assumed forming limit plane or the formed limit curved surface and each plot point of the group of plot points in the three-dimensional coordinate space is calculated, and the forming limit plane or the formed limit curved surface is determined so that the calculated sum of squares is minimized.

[0024] (10) In the above (8), In the forming limit surface creation step, the forming limit surface is created by combining a plurality of forming limit planes and / or forming limit curved surfaces.

[0025] (11) In the above (9), In the forming limit surface creation step, the forming limit surface is created by combining a plurality of forming limit planes and / or forming limit curved surfaces.

[0026] (12) The forming limit surface creation system for a metal plate according to the present invention creates a forming limit surface for the metal plate, which is expressed by the relationship between the force in the thickness direction generated in a test piece of the metal plate formed so as to generate various forces in the thickness direction, and the maximum principal strain and the minimum principal strain, The apparatus includes a forming test section, a forming limit analysis section, and a forming limit surface creation section, The molding test section a plurality of tools having different shapes for forming a plurality of test pieces having different shapes on the surface of which a predetermined lattice or strain analysis pattern is provided so that various forces in the thickness direction are generated; an imaging device that images the surface during the process of molding each of the test pieces using each of the tools; a strain measuring device that analyzes the captured image of the surface of each of the test pieces and measures strain occurring in each of the test pieces; a thickness direction force acquisition device for determining a thickness direction force at a fracture portion generated in each of the molded test pieces; a strain database construction device that stores the strains measured by the strain measuring device for each of the formed test pieces in chronological order from the start of forming to the occurrence of fracture, and constructs a strain database; The forming limit analysis unit an evaluation point sequence setting device that sets an evaluation point sequence in each of the molded test pieces to acquire strain distribution in the vicinity of the fracture portion that occurs in each of the molded test pieces; a strain distribution acquisition device that extracts the strain of the evaluation point sequence set for each of the test pieces from the strain database and acquires the strain distribution in the vicinity of the fractured portion; a forming limit acquisition device that determines a forming limit based on the acquired strain distribution and obtains a maximum principal strain and a minimum principal strain at the forming limit, The forming limit surface creation unit a forming limit plotting device that plots the force in the plate thickness direction at the fracture portion and the maximum principal strain and minimum principal strain at the forming limit, which are obtained for each of the formed test pieces, in a three-dimensional coordinate space; and a forming limit surface creation device that creates a forming limit surface of the metal plate based on a group of plot points of the force in the plate thickness direction at the fracture portion and the maximum principal strain and minimum principal strain at the forming limit plotted in the three-dimensional coordinate space.

[0027] (13) A forming limit surface creation program for a metal plate according to the present invention creates a forming limit surface for the metal plate expressed by the relationship between a force in the thickness direction generated in a test piece of the metal plate formed so as to generate various forces in the thickness direction, and a maximum principal strain and a minimum principal strain, The present invention is characterized by having a function of causing a computer to function as the forming limit analysis unit and the forming limit surface creation unit of the metal plate forming limit surface creation system described in (12) above.

[0028] (14) A press-forming crack detection method according to the present invention is a method for determining whether or not a crack has occurred in a press-formed product obtained by press-forming a metal plate, comprising: The method includes a forming limit surface acquisition process and a press-molded product crack determination process, The forming limit surface acquisition process includes: Obtain a forming limit surface of the metal plate, which is expressed by the relationship between the force in the thickness direction of a test piece of the metal plate formed using a plurality of tools having different shapes so as to generate various forces in the thickness direction, and the maximum principal strain and the minimum principal strain; The press-molded product crack determination process includes: The force in the plate thickness direction, the maximum principal strain, and the minimum principal strain in the press-formed product are acquired as crack determination parameters in the press-formed product; The method is characterized in that it determines whether or not cracks have occurred in the press-formed product based on the acquired crack determination parameters and the forming limit surface acquired in the forming limit surface acquisition process.

[0029] (15) In the above (14), The force in the plate thickness direction in the forming limit surface acquisition process is the surface pressure at the fracture portion generated by forming each of the test pieces, In the process for determining cracks in a press-molded product, the force in the plate thickness direction obtained as a parameter for determining cracks in the press-molded product is the surface pressure in the press-molded product.

[0030] (16) In the above (15), The surface pressure in the forming limit surface acquisition process is measured by placing pressure-sensitive paper between the tool and each of the test pieces and forming them, or is calculated by finite element method analysis that reproduces the forming of each of the test pieces using the tool; The surface pressure in the process for determining cracks in the press-molded product is measured by placing pressure-sensitive paper between the molding die that forms the press-molded product and the blank and then press-molding the product, or is calculated by finite element method analysis that reproduces the press-molding of the press-molded product.

[0031] (17) In the above (14), The force in the thickness direction in the forming limit surface acquisition process is the stress in the thickness direction at the fracture portion generated by forming each of the test pieces, In the process for determining cracks in press-formed products, the force in the thickness direction obtained as a parameter for determining cracks in the press-formed product is the stress in the thickness direction of the press-formed product.

[0032] (18) In the above (17), The thickness direction stress in the forming limit surface acquisition process is calculated by a finite element method analysis that reproduces the forming of each test piece using the tool; The stress in the thickness direction in the process for determining cracks in the press-formed product is calculated by a finite element method analysis that reproduces the press forming of the press-formed product.

[0033] (19) The press-forming crack determination system according to the present invention determines whether or not a crack has occurred in a press-formed product obtained by press-forming a metal plate, and A forming limit surface acquisition unit and a press-molded product crack determination unit are included, The forming limit surface acquisition unit Obtain a forming limit surface of the metal plate, which is expressed by the relationship between the force in the thickness direction of a test piece of the metal plate formed using a plurality of tools having different shapes so as to generate various forces in the thickness direction, and the maximum principal strain and the minimum principal strain; The press-molded product crack determination unit includes: The force in the plate thickness direction, the maximum principal strain, and the minimum principal strain in the press-formed product are acquired as crack determination parameters in the press-formed product; The method is characterized in that it determines whether or not cracks have occurred in the press-formed product based on the acquired crack determination parameters and the forming limit surface acquired by the forming limit surface acquisition unit.

[0034] (20) A press-molding crack determination program according to the present invention determines whether or not a crack has occurred in a press-molded product obtained by press-molding a metal plate, The present invention is characterized in that it has a function of causing a computer to function as a forming limit surface acquisition unit and a press-molded product crack determination unit of the press-molded crack determination system for metal sheets described in (19) above. [Effects of the Invention]

[0035] According to the present invention, by forming a metal plate test piece using a plurality of tools with different shapes so that various forces in the plate thickness direction are generated, it is possible to obtain the force in the plate thickness direction in addition to the maximum principal strain and the minimum principal strain as an index of the forming limit. Then, according to the present invention, by plotting the maximum principal strain, the minimum principal strain, and the force in the plate thickness direction obtained as an index of the forming limit in a three-dimensional coordinate space, it is possible to create a forming limit surface of the metal plate represented by the relationship between the maximum principal strain, the minimum principal strain, and the force in the plate thickness direction.

[0036] Furthermore, in the present invention, the force in the plate thickness direction, and the maximum principal strain and the minimum principal strain in a press-formed product of a metal plate are obtained as crack determination parameters in the press-formed product, and the presence or absence of cracking in the press-formed product is determined based on the crack determination parameters and the forming limit surface of the metal plate expressed by the relationship between the force in the plate thickness direction and the maximum principal strain and the minimum principal strain. This makes it possible to determine the presence or absence of cracking in the press-formed product while taking into account the influence of the force in the plate thickness direction, which is not taken into account in conventional forming limit lines that use the maximum principal strain and the minimum principal strain as indicators of the forming limit. [Brief explanation of the drawings]

[0037] [Figure 1] 1 is a flowchart showing a process flow of a method for creating a forming limit surface of a metal plate according to a first embodiment of the present invention. [Figure 2] FIG. 1 is a diagram showing a forming die, which is an example of a tool used to form a metal plate test piece in a method for acquiring the forming limit of a metal plate, a method for creating a forming limit surface of a metal plate, and a system for creating a forming limit surface of a metal plate according to embodiment 1 of the present invention. [Figure 3] FIG. 2 is a diagram showing a specific shape of the tip of a punch used to form a test piece from a metal plate in the first embodiment and the first example of the present invention. [Figure 4] 1 is a diagram showing a specific example of the shape of a test piece used in a forming test for creating a forming limit surface of a metal plate in the first embodiment and the first example. FIG. [Figure 5] 1 is a diagram illustrating a configuration of a forming limit surface creation system for a metal plate according to a first embodiment of the present invention. [Figure 6] FIG. 10 is a flowchart showing the flow of processing in a press-forming crack detection method according to a second embodiment of the present invention. [Figure 7] FIG. 6 is a diagram illustrating the configuration of a press-forming crack determination system according to a second embodiment of the present invention. [Figure 8] FIG. 3 is a diagram showing a process flow for creating a forming limit surface of a metal plate in the first embodiment. [Figure 9]1 is a diagram showing a forming limit surface created in Example 1, which is expressed by the relationship between the force in the plate thickness direction (surface pressure) and the maximum principal strain and the minimum principal strain. [Figure 10] FIG. 10 is a diagram showing a press molding die used for V-bending a blank in Example 2. [Figure 11] 10 is a diagram showing the shape of the tip of a V-bending punch used to V-bend a blank in Example 2. FIG. [Figure 12] 1 is a graph illustrating a conventional method for determining the forming limit strain from the strain distribution measured by a forming test of a metal plate. [Figure 13] FIG. 1 shows the Nakajima method and the Marciniak method, which are conventional methods for determining the forming limit of metal sheets, and examples of forming limit diagrams determined by these methods ((a) Nakajima method, (b) Marciniak method, (c) forming limit diagrams determined by both the Nakajima method and the Marciniak method). DETAILED DESCRIPTION OF THE INVENTION

[0038] Before describing the first and second embodiments of the present invention, the background to the invention will be explained. Note that the dimensions and other specific numerical values ​​shown in the following description and drawings are merely examples to facilitate understanding of the present invention and are not intended to limit the present invention. Furthermore, in this specification and drawings, elements having substantially the same functions and configurations are designated by the same reference numerals, and redundant explanations are omitted or simplified.

[0039] <Background to the invention> In examining a method for determining the forming limit of a metal sheet, the inventors have closely examined the differences between the Nakajima method and the Marciniak method. In the examination, the inventors have noticed that, as mentioned above, there is a large difference in the shape of the punch used in the forming test between the Nakajima method and the Marciniak method.

[0040] In the Nakajima method, as shown in Fig. 13(a), a test piece 100 is formed using a spherical head punch 203 whose tip 203a has a spherical head shape. In the test piece 100 formed by such a spherical head punch 203, a fracture occurs at the position where the apex of the tip 203a makes contact. At this time, it is considered that the fractured portion of the test piece 100 is subjected to a large force in the plate thickness direction by the tip 203a of the spherical head punch 203.

[0041] In contrast, the Marciniak method, as shown in Fig. 13(b), uses a flat-head punch 213 with a flat tip 213a to form the test piece 100. In the test piece 100 formed by the flat-head punch 213, the contact area with the tip 213a is large, and therefore the force in the plate thickness direction that the fracture part of the test piece 100 receives is considered to be relatively small. As described above, there is a difference in the force in the thickness direction that the test piece 100 receives in the forming tests by the Nakajima method and the Marciniak method. However, the conventional method for determining the forming limit of a metal sheet does not take into account the difference in the force in the thickness direction.

[0042] Furthermore, it is believed that in actual press-formed products, the force in the thickness direction changes depending on the shape of the forming surface of the punch that contacts the blank. Therefore, it was discovered that by using the forming limit surface of a metal plate, which is expressed as the relationship between the forming limit strain and the force in the thickness direction, it is possible to accurately determine whether cracks will occur in a press-formed product, taking into account the force in the thickness direction. The present invention has been completed based on the above findings, and its specific configuration is as follows.

[0043] [Embodiment 1] <Method for obtaining forming limits for metal sheets> The method for obtaining the forming limit of a metal sheet according to the first embodiment forms a metal sheet test piece using a plurality of tools with different shapes so that various forces are generated in the thickness direction. The method for obtaining the forming limit of a metal sheet then obtains the force in the thickness direction, the maximum principal strain, and the minimum principal strain generated in the formed test piece as indexes of the forming limit of the metal sheet.

[0044] FIG. 2 shows, as an example of a tool for forming the test piece 100, a forming die 11 equipped with a punch 11a, an upper die 11b, and a blank holder 11c.

[0045] As shown in FIG. 2, the punch 11a has a spherical surface with a radius of curvature R, that is, a spherical-head shaped tip portion 11a1 with a curvature ρ (=1 / R) greater than zero. Figure 3 shows an example of a specific shape of the tip 11a1 of the punch 11a. The punch 11a is not limited to having a spherical tip 11a1 as shown in Figures 3(a) to 3(c), but may have a flat tip 11a1 (with a radius of curvature R = ∞) as shown in Figure 3(d). In other words, the punch 11a has a tip 11a1 with a curvature ρ (= 1 / R) of 0 or more.

[0046] Examples of the metal plate test piece 100 include test pieces 101 and 103 having the shapes shown in Figure 4, in order to determine the forming limit in each deformation mode (equal biaxial deformation, non-equal biaxial deformation, plane strain deformation, and uniaxial deformation).

[0047] FIG. 4(a) shows a disk-shaped test piece 101, which is suitable for determining the forming limit of equibiaxial deformation. 4(b) and (c) show test piece 103 in which arc-shaped cutout portions 103a are formed at diametrically opposing positions on the disc-shaped peripheral edge, and the width W of the central portion 103b is different. As the width W of the central portion 103b of test piece 103 is narrowed, the deformation of test piece 103 changes from equibiaxial deformation to non-equilibrium biaxial deformation and plane strain deformation, gradually approaching uniaxial tension, and is therefore suitable for determining the forming limit in these deformation modes.

[0048] The method for obtaining the forming limit of a metal sheet according to the first embodiment does not particularly limit the method for obtaining the maximum principal strain and the minimum principal strain that are used as indicators of the forming limit, as long as the maximum principal strain and the minimum principal strain at the forming limit are obtained using the same standard for test pieces 100 formed using tools with different shapes (for example, punches with different tip curvatures). For example, a test piece 100 with a marking (a grid or a pattern for strain analysis) on its surface may be molded until it breaks, and the maximum principal strain and minimum principal strain at the time of breakage may be determined from the shape of the marking near the break, or they may be determined by the method described below.

[0049] The force in the thickness direction, which is used as an index of the forming limit, is the force in the thickness direction at the fractured portion of the test piece 100 formed until the fractured portion occurs. By forming the test piece 100 using a plurality of tools with different shapes (for example, a forming die 11 provided with punches 11a having tip portions 11a1 with different curvatures), it is possible to generate various forces in the thickness direction in the test piece 100.

[0050] As described above, according to the method for obtaining the forming limit of a metal plate according to the first embodiment, by using a plurality of forming dies 11 of different shapes to form the test piece 100 so that various forces in the plate thickness direction are generated, the force in the plate thickness direction, the maximum principal strain, and the minimum principal strain can be obtained as indicators of the forming limit.

[0051] In the method for obtaining the forming limit of a metal sheet according to the first embodiment, the force in the thickness direction may be obtained by determining the surface pressure or the stress in the thickness direction generated at the fractured portion of the formed test piece 100. The surface pressure and the stress in the thickness direction can be obtained by the method described later.

[0052] <Method for creating the forming limit surface of a metal sheet> A method for creating a forming limit surface of a metal plate will be described as a specific example of the method for obtaining the forming limit of a metal plate according to the present embodiment 1. In the following description, it is assumed that the forming die 11 shown in FIG. 2 is used as a tool for forming the test piece 100.

[0053] The method for creating a forming limit surface of a metal plate according to the first embodiment involves forming a metal plate test piece 100 using a plurality of forming dies 11 with different shapes so that various forces are generated in the plate thickness direction. The method for creating a forming limit surface of a metal plate creates a forming limit surface of the metal plate that is expressed by the relationship between the force in the plate thickness direction generated in the formed test piece 100 and the maximum principal strain and the minimum principal strain. As shown in FIG. 1, the method for creating a forming limit surface of a metal sheet according to the first embodiment includes a forming test step S10, a forming limit analysis step S20, and a forming limit surface creation step S30.

[0054] <Molding test step> As shown in FIG. 1, the forming test step S10 includes a test piece preparation step S11, a test piece forming step S13, a strain measurement step S15, a thickness direction force acquisition step S17, and a strain database construction step S19.

[0055] (Test piece preparation process) The test piece preparation step S11 is a step of preparing a plurality of test pieces 100 having different shapes, each of which has a predetermined lattice or strain analysis pattern applied to the surface of a metal plate, as shown in FIG.

[0056] The strain analysis pattern applied to the surface of the test piece 100 can be, for example, an arrangement of circles or dots. The grid or strain analysis pattern may be a regular repeating pattern or an irregular random pattern. To apply the grid or strain analysis pattern, for example, a sample grid may be transferred onto the surface of the test piece 100.

[0057] In the test piece preparation step S11, a plurality of test pieces 100 with different shapes are prepared. The shape of the test piece 100 may be appropriately determined so that the forming limit can be determined in each deformation mode (equiaxial deformation, non-uniform biaxial deformation, plane strain deformation, and uniaxial deformation). Specific examples of the shape of the test piece 100 include a circular test piece 101 and a test piece 103 having a notch 103a formed in the outer edge of the circular shape, as shown in Fig. 4. For the test piece 103 having the notch 103a, it is preferable to prepare test pieces in which the width W of the central portion 103b is changed at multiple levels.

[0058] (Test piece molding process) The test piece molding step S13 is a step in which a plurality of molding dies 11 having different shapes are used to mold each test piece 100 while photographing the surface of the test piece 100. In the test piece molding step S13, a test piece 100 is molded for each combination of one molding die 11 out of the plurality of molding dies 11 having different shapes and one test piece 100 out of the plurality of test pieces 100 having different shapes.

[0059] In the test piece molding step S13, the surface of the test piece 100 may be photographed by placing a camera (photographing device) above the molding die 11 and photographing the surface of the test piece 100 during the molding process at predetermined time intervals.

[0060] (Strain measurement process) The strain measuring step S15 is a step of analyzing the image of the surface of each test piece 100 taken in the test piece forming step S13, and measuring the strain occurring in each test piece 100.

[0061] In the first embodiment, digital image correlation (DIC) is used to measure the strain on the surface of the test piece 100. In DIC, it is possible to measure the strain in two in-plane directions on the surface of the test piece 100 from the deformation state of a lattice or strain analysis pattern attached to the surface of the test piece 100. Furthermore, in DIC, it is preferable to measure the maximum principal strain and the minimum principal strain as the strain in two in-plane directions on the surface of the test piece 100.

[0062] (Process for obtaining force in the plate thickness direction) The thickness direction force acquisition step S17 is a step of obtaining the force in the thickness direction at the fracture portion generated in each test piece 100 formed in the test piece forming step S13.

[0063] In the first embodiment, the plate thickness direction force obtaining step S17 obtains, as the plate thickness direction force, the surface pressure generated at the fractured portion of the test piece 100 formed in the test piece forming step S13. The surface pressure generated at the fractured portion can be measured by placing pressure-sensitive paper 105 between the molding die 11 and each test piece 100 and molding it in the test piece molding step S13, as shown in Fig. 2. The surface pressure generated at the fractured portion can also be calculated by finite element method analysis that reproduces the molding of each test piece 100 in the test piece molding step S13.

[0064] In the thickness direction force acquisition step S17, the thickness direction stress occurring at the fractured portion of each formed test piece 100 may be obtained as the force in the thickness direction. In this case, in the thickness direction force acquisition step S17, the thickness direction stress can be calculated by a finite element method analysis that reproduces the formation of each test piece 100 in the test piece formation step S13. In such a finite element method analysis, it is preferable to divide the test piece into elements using solid elements.

[0065] (Strain database construction process) The strain database construction step S19 is a step of storing the strains measured in the strain measurement step S15 for each formed test piece 100 in chronological order from the start of forming to the occurrence of fracture, and constructing a strain database.

[0066] <Forming limit analysis step> As shown in FIG. 1, the forming limit analysis step S20 includes an evaluation point sequence setting step S21, a strain distribution acquisition step S23, and a forming limit acquisition step S25.

[0067] (Evaluation point sequence setting process) The evaluation point sequence setting step S21 is a step of setting an evaluation point sequence in each test piece 100 for acquiring the strain distribution in the vicinity of the fracture portion that occurs in each test piece 100 molded in the test piece molding step S13.

[0068] The evaluation point sequence may be set at predetermined intervals along the direction perpendicular to the fractured portion that occurs in the test piece 100, for example, so as to straddle the fractured portion.

[0069] (Strain distribution acquisition process) The strain distribution acquisition step S23 is a step of extracting the strains in the evaluation point sequence set for each test piece 100 in the evaluation point sequence setting step S21 from the strain database, and acquiring the strain distribution in the vicinity of the fractured portion.

[0070] The strain distribution acquisition step S23 may extract, for example, strains at times (time steps) before and after the occurrence of a fracture in the test piece 100 from the strains stored in chronological order at predetermined time intervals from the start of forming to the occurrence of fracture.

[0071] (Forming limit acquisition process) The forming limit acquisition process S25 is a process of determining the forming limit for each test piece 100 formed using forming molds 11 of different shapes based on the acquired strain distribution, and calculating the maximum principal strain and minimum principal strain at the forming limit.

[0072] The forming limit can be determined, for example, by the method described in the publicly known document "JP 2023-35533 A." When the forming limit is determined based on the maximum principal strain and the minimum principal strain measured at a predetermined time interval, the maximum principal strain and the minimum principal strain at the time step determined to be the forming limit or the time step immediately before that may be obtained.

[0073] The method for determining the forming limit in the forming limit acquisition step S25 is not limited to the above method, as long as the forming limit is determined using the same criteria for the strain distributions obtained for each combination of the test piece 100 and the tool (forming die 11) having different shapes, and the maximum principal strain and the minimum principal strain at the forming limit are obtained.

[0074] <Steps for creating the forming limit surface> As shown in FIG. 1, the forming limit surface creating step S30 includes a forming limit plotting step S31 and a forming limit surface creating step S33.

[0075] (Forming limit plot process) The forming limit plotting step S31 is a step of plotting in a three-dimensional coordinate space the force in the plate thickness direction at the fracture part and the maximum principal strain and minimum principal strain at the forming limit, which are obtained for each formed test piece 100. Here, the three-dimensional coordinate space has three axes, which are the force in the plate thickness direction, the maximum principal strain, and the minimum principal strain.

[0076] (Forming limit surface creation process) The forming limit surface creation process S33 is a process of creating a forming limit surface of the metal plate based on a group of plot points of the force in the plate thickness direction at the fracture part and the maximum principal strain and minimum principal strain at the forming limit plotted in three-dimensional coordinate space.

[0077] The forming limit surface can be created in the forming limit surface creation step S33 by, for example, the following three methods: In the following description, the force in the thickness direction at the fracture part and the maximum principal strain and minimum principal strain at the forming limit are referred to as forming limit data.

[0078] The first method is to first select two adjacent plot points from the group of plot points of the forming limit data obtained for each of multiple test pieces 100 formed using a forming mold 11 of the same shape in a three-dimensional coordinate space in which the forming limit data is plotted.

[0079] Next, from the group of plot points of the forming limit data obtained for each of a plurality of test pieces 100 molded using a molding die 11 having a different shape from the two plot points, one plot point is selected that is closest to the line segment connecting the two selected plot points.

[0080] Then, a triangular plane is created connecting the two selected plot points with one other plot point. This process of creating triangular planes is carried out for all plot points of the forming limit data plotted in the three-dimensional coordinate space. The polygonal surface created by combining the created triangular planes is then used as the forming limit surface.

[0081] The second method for creating a forming limit surface is to first assume a forming limit plane or a forming limit curved surface in a three-dimensional coordinate space, calculate the perpendicular distance between this assumed forming limit plane or curved surface and each plot point of the plot point group of forming limit data plotted in the three-dimensional coordinate space, and then determine the forming limit plane or curved surface so that the sum of squares of the calculated perpendicular distances is minimized.

[0082] The third method for creating a forming limit surface is to determine the forming limit plane or forming limit curved surface in the second method described above so that the sum of squares weighted on the perpendicular distance between the assumed forming limit plane or forming limit curved surface and each plot point of the plot point group of forming limit data is minimized.

[0083] One possible way of weighting is to increase the weight of the vertical distance between the plot points of the forming limit data for the shape or forming path of the forming die 11 for which error is particularly desired to be reduced and the forming limit plane or forming limit curved surface.

[0084] The second or third method may be a combination of a plurality of forming limit planes and / or forming limit curved surfaces. For example, a forming limit plane or a forming limit curved surface may be assumed for each of the regions where the minimum principal strain is negative and positive, and the forming limit plane or the forming limit curved surface may be determined so that the sum of squares of the perpendicular distances between each plot point of the plot point group of forming limit data for each region is minimized. The same applies when creating a forming limit surface so that the sum of squares of weighted perpendicular distances between each plot point of the forming limit data is minimized.

[0085] As a method other than the above, three adjacent plot points may be selected from the group of plot points of the forming limit data in the three-dimensional coordinate space, and a triangular plane may be generated by connecting the selected three plot points with straight lines. This process may be repeated to generate a polygonal surface consisting of a plurality of triangular planes, which may be used as the forming limit surface.

[0086] <Metal sheet forming limit surface creation system> The metal plate forming limit surface creation system according to the first embodiment forms a metal plate test piece using a plurality of tools with different shapes so that various forces in the plate thickness direction are generated, and creates a metal plate forming limit surface expressed by the relationship between the force in the plate thickness direction generated in the formed metal plate test piece and the maximum principal strain and the minimum principal strain. The forming limit surface creation system 1 for metal plates according to this embodiment 1 (hereinafter simply referred to as the "forming limit surface creation system 1") includes a forming test section 10, a forming limit analysis section 20, and a forming limit surface creation section 30, as shown in FIG. 5.

[0087] <Forming Test Section> The forming test section 10 includes a forming die 11 , a photographing device 13 , a strain measuring device 15 , a force acquiring device 17 in the plate thickness direction, and a strain database creating device 19 .

[0088] (Molding mold) The forming die 11 is used to form a test piece 100 having a predetermined lattice or strain analysis pattern on its surface, and is a plurality of tools with different shapes that can generate various forces in the thickness direction of the formed test piece 100.

[0089] 2, the molding die 11 has a punch 11a with a spherical tip 11a1, an upper die 11b, and a blank holder 11c. The molding die 11 has a plurality of punches 11a with tip 11a1 having different radii of curvature R (curvature ρ=1 / R), thereby producing a plurality of tools with different shapes.

[0090] In this way, the forming die 11 has punches 11a with tip portions 11a1 of different shapes, which makes it possible to generate various forces in the thickness direction on the test piece 100. As described above, the tip portion 11a1 of the punch 11a is not limited to one with a spherical head shape, but may also be flat (with a radius of curvature R=∞, see FIG. 2(b)).

[0091] (imaging device) The photographing device 13 photographs the surface of the test piece 100 in the process of molding the test piece 100 using molding dies 11 of different shapes. The photographing device 13 also photographs the deformation of the lattice or strain analysis pattern attached to the surface of the test piece 100. In the first embodiment, the photographing device 13 photographs the surface of the test piece 100 at predetermined time intervals from the start of forming to the occurrence of fracture.

[0092] (Strain measurement device) The strain measuring device 15 analyzes images of the surface of each test piece 100 photographed by the photographing device 13 for each test piece 100 molded using molding dies 11 of different shapes, and measures the strain occurring in the test piece 100.

[0093] In this embodiment 1, the strain measuring device 15 uses a digital image correlation method to measure the maximum principal strain and the minimum principal strain as the strain in two in-plane directions that occurs in the test piece 100 during the molding process from the deformation state of the grid or strain analysis pattern on the surface of the test piece 100.

[0094] (Device for acquiring force in the plate thickness direction) The thickness direction force acquisition device 17 obtains the force in the thickness direction at the fracture portion of each test piece 100 formed using the forming dies 11 of different shapes.

[0095] In the first embodiment, the thickness direction force acquisition device 17 obtains the surface pressure generated at the fractured portion of the test piece 100 as the force in the thickness direction. In this case, the thickness direction force acquisition device 17 measures the surface pressure by placing pressure-sensitive paper 105 between the molding die 11 and the test piece 100 and molding it (see Figure 2), or calculates the surface pressure by finite element method analysis that reproduces the molding of the test piece 100 by the molding die 11.

[0096] The thickness direction force acquisition device 17 may be configured to obtain the thickness direction stress generated at the fractured portion of the test piece 100 as the force in the thickness direction. In this case, the thickness direction force acquisition device 17 can calculate the thickness direction stress generated at the fractured portion by a finite element method analysis that reproduces the molding of the test piece 100 by the molding die 11. In such a finite element method analysis, it is preferable to divide the test piece into elements using solid elements.

[0097] (Strain database construction device) The strain database construction device 19 stores the strain measured by the strain measuring device 15 for each test piece 100 formed using forming molds 11 of different shapes in chronological order from the start of forming to the occurrence of fracture, and constructs a strain database.

[0098] <Forming Limit Analysis> As shown in FIG. 5, the forming limit analysis unit 20 includes an evaluation point sequence setting device 21, a strain distribution acquisition device 23, and a forming limit acquisition device 25. The forming limit analysis unit 20 can be, for example, configured by a computer (such as a PC). In this case, each unit of the forming limit analysis unit 20 functions when the CPU of the computer executes a predetermined program.

[0099] (Evaluation point sequence setting device) The evaluation point sequence setting device 21 sets an evaluation point sequence for each test piece 100 in order to obtain the strain distribution near the fracture portion that occurs in each test piece 100 molded using molding dies 11 of different shapes.

[0100] (Strain distribution acquisition device) The strain distribution acquisition device 23 extracts the strain at a series of evaluation points set on each test piece 100 molded using multiple molding dies 11 of different shapes from a strain database, and acquires the strain distribution near the fractured portion.

[0101] (Forming limit acquisition device) The forming limit acquisition device 25 determines the forming limit for each test piece 100 formed using forming molds 11 of different shapes based on the strain distribution acquired by the strain distribution acquisition device 23, and calculates the maximum principal strain and minimum principal strain at the forming limit.

[0102] The forming limit acquisition device 25 may determine the forming limit by the same method as the forming limit acquisition step S25 of the method for creating a forming limit surface of a metal plate according to the first embodiment described above.

[0103] <Forming limit surface creation section> As shown in FIG. 5, the forming limit surface creation unit 30 includes a forming limit plotting device 31 and a forming limit surface creation device 33.

[0104] (Forming limit plotting device) The forming limit plotting device 31 plots in three-dimensional coordinate space the force in the plate thickness direction at the fracture point and the maximum principal strain and minimum principal strain at the forming limit, which are obtained for each test piece 100 formed using a plurality of forming dies 11 of different shapes.

[0105] Here, the force in the thickness direction at the fractured portion was obtained by the force acquisition device 17 in the thickness direction, and the maximum principal strain and minimum principal strain at the forming limit were obtained by the forming limit acquisition device.

[0106] (Forming limit surface creation device) The forming limit surface creation device 33 creates a forming limit surface of the metal plate based on a group of plot points of the force in the plate thickness direction at the fracture part and the maximum principal strain and minimum principal strain at the forming limit, which are plotted in three-dimensional coordinate space by the forming limit plotting device 31.

[0107] The forming limit surface may be created by the forming limit surface creating device 33 by any one of the three methods described in the forming limit surface creating step S33 of the forming limit surface creating method according to the first embodiment.

[0108] <Program for creating forming limit surfaces for metal sheets> The metal plate forming limit surface creation system according to the first embodiment of the present invention can be configured as a metal plate forming limit surface creation program that causes the forming limit analysis unit 20 and the forming limit surface creation unit 30, which are configured by a computer, to function.

[0109] That is, the metal plate forming limit surface creation program according to the first embodiment has a function of causing a computer to function as the forming limit analysis unit 20 and the forming limit surface creation unit 30 shown in FIG.

[0110] The metal plate forming limit surface creation program according to the first embodiment causes the forming limit analysis unit 20 to function as an evaluation point sequence setting device 21, a strain distribution acquisition device 23, and a forming limit acquisition device 25.

[0111] Furthermore, the metal plate forming limit surface creation program according to the first embodiment causes the forming limit surface creation unit 30 to function as a forming limit plotting device 31 and a forming limit surface creation device 33.

[0112] As described above, in the method, system, and program for creating a forming limit surface of a metal sheet according to the first embodiment, the force in the thickness direction at the fractured part and the maximum principal strain and minimum principal strain at the forming limit are obtained as indexes of the forming limit of a test piece of a metal sheet formed so that various forces in the thickness direction are generated. Then, by plotting the force in the thickness direction, the maximum principal strain, and the minimum principal strain obtained as indexes of the forming limit in a three-dimensional coordinate space, it is possible to create a forming limit surface of the metal sheet expressed by the relationship between the force in the thickness direction and the maximum principal strain and the minimum principal strain.

[0113] [Embodiment 2] <Method for determining cracks in press-molded products> The press-molding crack detection method according to the second embodiment is for determining whether or not cracks have occurred in a press-molded product obtained by press-molding a metal plate, and includes a forming limit surface acquisition process P1 and a press-molded product crack detection process P3, as shown in FIG. 6.

[0114] <Forming limit surface acquisition process> The forming limit surface acquisition process P1 acquires the forming limit surface of a metal plate, which is expressed by the relationship between the force in the thickness direction generated in a test piece formed using multiple tools of different shapes to generate various forces in the thickness direction, and the maximum principal strain and minimum principal strain.

[0115] A specific example of the forming limit surface acquisition process P1 is the method for creating a forming limit surface for a metal plate according to the first embodiment described above (see FIG. 1). That is, the forming limit surface acquisition process P1 includes the forming test step S10, the forming limit analysis step S20, and the forming limit surface creation step S30 of the method for creating a forming limit surface for a metal plate according to the first embodiment. As a result, the forming limit surface acquisition process P1 creates a forming limit surface for the metal plate that is expressed by the relationship between the force in the plate thickness direction at the fracture part generated in the test piece and the maximum principal strain and minimum principal strain at the forming limit.

[0116] The force in the thickness direction, which is an index representing the forming limit surface acquired in the forming limit surface acquisition process P1, may be the surface pressure or thickness direction stress generated at the fractured part of the test piece formed in the forming test step S10. The surface pressure or thickness direction stress can be obtained by the method described in the thickness direction force acquisition step S5 of the first embodiment.

[0117] <Press-molded product crack detection process> The press-formed product crack determination process P3 acquires the force in the plate thickness direction of the press-formed product, the maximum principal strain, and the minimum principal strain as crack determination parameters for the press-formed product. Then, the press-formed product crack determination process P3 determines whether or not a crack has occurred in the press-formed product based on the acquired crack determination parameters and the forming limit surface acquired in the forming limit surface acquisition process P1.

[0118] A specific example of the press-molded product crack determination process P3 includes a press-molded FEM analysis step S51, a press-molded crack determination parameter calculation step S53, and a press-molded crack occurrence determination step S55, as shown in Figure 6.

[0119] (Press molding FEM analysis step) The press-forming FEM analysis step S51 is a step of performing an FEM analysis (finite element method analysis) of the process of press-forming a metal plate into a press-formed product.

[0120] Press-forming FEM analysis: By the FEM analysis in step S51, changes in strain, stress, plate thickness, etc. that occur in a press-formed product due to press-forming of a metal plate can be found for each element and node used in the FEM analysis. In addition, when calculating the thickness direction stress as the force in the thickness direction in the subsequent press forming crack judgment parameter calculation step S53, it is recommended to perform FEM analysis using a blank (metal plate) divided into elements using solid elements in the press forming FEM analysis step S51.

[0121] (Press forming crack judgment parameter calculation step) The press-forming crack determination parameter calculation step S53 is a step of calculating the force in the thickness direction of the press-formed product, and the maximum principal strain and minimum principal strain as crack determination parameters for the press-formed product. Here, the force in the thickness direction of the press-formed product, and the maximum principal strain and minimum principal strain are calculated based on the FEM analysis results in the press-forming FEM analysis step S51.

[0122] The force in the plate thickness direction of the press-formed product is obtained according to the force in the plate thickness direction that is used as an index of the forming limit surface of the metal plate acquired in the forming limit surface acquisition process P1. That is, when surface pressure is used as the force in the thickness direction at the forming limit surface, the surface pressure generated during the press forming process of the press-formed product is calculated. On the other hand, when thickness direction stress is used as the force in the thickness direction at the forming limit surface, the thickness direction stress generated during the press forming process of the press-formed product is calculated. The surface pressure or stress in the thickness direction generated in the press-formed product should be calculated for each element used in the FEM analysis.

[0123] (Step to determine whether or not cracks have occurred during press molding) The press-molding crack occurrence determination step S55 is a step for determining whether or not a crack has occurred in the press-molded product. The presence or absence of a crack in the press-molded product is determined based on the crack determination parameter calculated in the press-molding crack determination parameter calculation step S53 and the forming limit surface acquired in the forming limit surface acquisition process P1.

[0124] The specific procedure for determining whether or not cracks have occurred in the press-molded product in the press-molding crack occurrence determination step S55 is as follows.

[0125] First, the crack determination parameters for the press-formed product calculated in the press-forming crack determination parameter calculation step S53 are plotted on the three-dimensional coordinate space in which the forming limit surface created in the forming limit surface creation step S45 of the forming limit surface acquisition process P1 is drawn.

[0126] Then, if the plot of the crack determination parameter is not located below the forming limit surface, for example, if the maximum principal strain of the crack determination parameter is greater than or equal to the minimum principal strain of the crack determination parameter and the maximum principal strain of the forming limit surface corresponding to the force in the plate thickness direction, it is determined that a crack has occurred.

[0127] In contrast, if the plot of the crack determination parameter is located below the forming limit surface, for example, if the maximum principal strain of the crack determination parameter is less than the minimum principal strain of the crack determination parameter and the maximum principal strain of the forming limit surface corresponding to the force in the plate thickness direction, it is determined that no cracks have occurred.

[0128] As described above, in the press-forming crack detection method according to the second embodiment, the force in the thickness direction of a press-formed metal sheet product and the maximum principal strain and minimum principal strain are found as crack detection parameters for the press-formed product, and the presence or absence of cracks in the press-formed product is determined based on the crack detection parameters and the forming limit surface of the metal sheet expressed by the relationship between the force in the thickness direction and the maximum principal strain and minimum principal strain. This makes it possible to determine the presence or absence of cracks in the press-formed product while taking into account the influence of the force in the thickness direction, which is not taken into account in conventional forming limit lines that use the maximum principal strain and minimum principal strain as indicators of the forming limit.

[0129] In the second embodiment, the forming limit surface acquisition process P1 creates the forming limit surface of the metal plate by performing the forming test step S10, the forming limit analysis step S20, and the forming limit surface creation step S30. However, in the present invention, the forming limit surface acquisition process may also acquire a forming limit surface of the metal plate that has been created in advance.

[0130] In the press-forming crack determination method according to the second embodiment, the press-forming crack occurrence determination step S55 performs a press-forming analysis of the press-formed product. The press-forming analysis calculates the force in the plate thickness direction (surface pressure or plate thickness direction stress), the maximum principal strain, and the minimum principal strain as crack determination parameters. However, in the present invention, the method for determining the crack determination parameters for press-formed products is not limited to this.

[0131] The force in the plate thickness direction of a press-formed product can be measured by placing pressure-sensitive paper between the press forming die that forms the press-formed product and the metal plate and then press-forming the product, or it can be calculated by finite element method analysis that reproduces the press forming of the press-formed product. Furthermore, the maximum principal strain and minimum principal strain in a press-formed product may be determined from the dimensional change of the scribed circle or grid in the press-formed product obtained by press-forming a metal plate (blank) onto which a scribed circle or grid has been transferred.

[0132] Furthermore, the determination of whether or not cracks have occurred in a press-formed product has been carried out for each portion of the press-formed product for which a crack determination parameter has been calculated.

[0133] However, it is also possible to calculate the crack determination parameter only for a portion of the press-formed product where cracking is likely to occur, and determine whether or not a crack will occur in that portion. In this case, the crack determination parameter is calculated for each element used in the press-formed analysis, and the presence or absence of a crack can be determined for each element of the portion of the press-formed product where cracking is likely to occur.

[0134] <Crack detection system for press-molded products> The press-forming crack determination system according to the second embodiment of the present invention determines whether or not cracks have occurred in a press-formed product obtained by press-forming a metal plate. The press-forming crack determination system 3 includes a forming limit surface acquisition unit 40 and a press-formed product crack determination unit 50, as shown in FIG. 7 as an example.

[0135] The press-forming crack determination system 3 may be configured by a CPU (Central Processing Unit) of a computer (such as a PC). In this case, each of the above units functions when the CPU of the computer executes a predetermined program.

[0136] <Forming limit surface acquisition unit> The forming limit surface acquisition unit 40 acquires the forming limit surface of a metal plate, which is expressed by the relationship between the force in the thickness direction and the maximum principal strain and the minimum principal strain in a test piece of metal plate formed using multiple tools of different shapes so that various forces in the thickness direction are generated. In the press-forming crack judgment system according to the second embodiment, the forming limit surface acquisition unit 40 has a forming test result import unit 41, a forming limit analysis unit 43, and a forming limit surface creation unit 45, as shown in FIG.

[0137] (Forming test result input section) The forming test result input unit 41 inputs the forming test results obtained by measuring the force in the thickness direction, the maximum principal strain, and the minimum principal strain generated in a metal plate test piece 100 formed using multiple tools of different shapes to generate various thickness directions.

[0138] A specific example of the forming test result import unit 41 is one that imports the force in the plate thickness direction previously obtained by the plate thickness direction force acquisition device 17 according to the above-mentioned embodiment 1, and a strain database previously constructed by the strain database construction device 19.

[0139] (Forming Limit Analysis Department) The forming limit analysis unit 43 determines the maximum principal strain and the minimum principal strain at the forming limit of the test piece 100 based on the forming test results taken in by the forming test result taking-in unit 41 . The forming limit analysis unit 43 may have the same configuration as the forming limit analysis unit 20 of the forming limit surface creation system 1 according to the first embodiment described above (see FIG. 5).

[0140] (Forming limit surface creation section) The forming limit surface creation unit 45 creates a forming limit surface of a metal plate, which is expressed by the relationship between the force in the thickness direction generated in a test piece of metal plate formed so that various forces in the thickness direction are generated, and the maximum principal strain and the minimum principal strain. The forming limit surface creating section 45 may have the same configuration as the forming limit surface creating section 30 of the forming limit surface creating system 1 according to the first embodiment described above (see FIG. 5).

[0141] <Press-molded product crack detection unit> The press-formed product crack determination unit 50 acquires the force in the plate thickness direction, the maximum principal strain, and the minimum principal strain in the press-formed product as crack determination parameters for the press-formed product. Then, the press-formed product crack determination unit 50 determines whether or not a crack has occurred in the press-formed product based on the acquired crack determination parameters and the forming limit surface acquired by the forming limit surface acquisition unit 40.

[0142] A specific configuration of the press-molded product crack determination unit 50 can be exemplified by one having a press-molding FEM analysis unit 51, a press-molding crack determination parameter calculation unit 53, and a press-molding crack occurrence determination unit 55, as shown in Figure 7.

[0143] (Press molding FEM analysis department) The press-forming FEM analysis unit 51 performs FEM analysis of the process of press-forming a metal plate into a press-formed product.

[0144] By the FEM analysis by the press forming FEM analysis unit 51, changes in strain, stress, plate thickness, etc. that occur in a press-formed product due to press forming of a metal plate can be found for each element and node used in the FEM analysis.

[0145] (Press forming crack determination parameter calculation section) The press-forming crack determination parameter calculation unit 53 calculates the force in the plate thickness direction, the maximum principal strain, and the minimum principal strain in the press-formed product as crack determination parameters in the press-formed product.

[0146] The force in the thickness direction, the maximum principal strain, and the minimum principal strain in the press-formed product are calculated based on the FEM analysis results obtained by the press-forming FEM analysis unit 51.

[0147] (Press molding crack detection section) The press molding crack occurrence determination unit 55 determines whether or not cracks have occurred in the press molded product based on the crack determination parameters calculated by the press molding crack determination parameter calculation unit 53 and the forming limit surface acquired by the forming limit surface acquisition unit 40.

[0148] The determination of whether or not cracks have occurred in the press-molded product by the press-molding crack occurrence determination unit 55 may be performed in the same manner as the press-molding crack occurrence determination step S55 of the press-molding crack determination method according to the first embodiment described above.

[0149] <Press molding crack detection program> The second embodiment of the present invention can be configured as a press-molding crack determination program that causes a press-molding crack determination system configured by a computer to function. That is, the press-forming crack determination program according to the second embodiment determines whether or not cracks occur in a press-formed product obtained by press-forming a metal plate. The press-forming crack determination program has a function of causing a computer to execute the program as a forming limit surface acquisition unit 40 and a press-formed product crack determination unit 50, as shown in FIG. 7 as an example.

[0150] As described above, in the press-forming crack determination system and press-forming crack determination program according to the second embodiment, the force in the thickness direction of a press-formed product of a metal plate, and the maximum principal strain and minimum principal strain are calculated as crack determination parameters for the press-formed product, and the presence or absence of cracks in the press-formed product is determined based on the crack determination parameters and the forming limit surface of the metal plate, which is expressed by the relationship between the force in the thickness direction and the maximum principal strain and the minimum principal strain. This makes it possible to determine the presence or absence of cracks in the press-formed product while taking into account the influence of the force in the thickness direction, which is not taken into account in conventional forming limit lines that use the maximum principal strain and the minimum principal strain as indicators of the forming limit.

[0151] In the above description, the forming limit surface acquisition unit 40 imports the forming test results using the forming test result import unit 41, and creates the forming limit surface using the forming limit analysis unit 43 and the forming limit surface creation unit 45. However, in the present invention, the forming limit surface acquisition unit may include a forming mold 11, a photographing device 13, a strain measuring device 15, a plate thickness direction force acquisition device 17, and a strain database construction device 19, similar to the forming test section 10 of the forming limit surface creation system 1 according to the first embodiment described above (see FIG. 5).

[0152] Alternatively, if a forming limit surface expressed by the relationship between the force in the plate thickness direction and the maximum principal strain and the minimum principal strain is created in advance, the forming limit surface acquisition unit may acquire the forming limit surface created in advance.

[0153] In addition, in the press-molded crack determination system and program according to the second embodiment, the press-molded product crack determination unit 50 performs press-molding analysis of the press-molded product and obtains the force in the thickness direction of the press-molded product (surface pressure or stress in the thickness direction), the maximum principal strain, and the minimum principal strain as crack determination parameters.

[0154] However, the acquisition of the force in the plate thickness direction, the maximum principal strain, and the minimum principal strain in the press-formed product by the press-forming crack determination unit 50 is not limited to this. For example, the force in the plate thickness direction of a press-molded product can be measured by placing pressure-sensitive paper between the press molding die that forms the press-molded product and the metal plate and then press-molding the product, or it can be calculated by finite element method analysis that reproduces the press molding of the press-molded product.

[0155] Furthermore, the maximum principal strain and the minimum principal strain in a press-formed product may be determined from the dimensional change of the scribed circle or grid in a press-formed product obtained by press-forming using a metal plate to which a scribed circle or grid has been transferred.

[0156] In addition, in the above explanation, the press-molded product crack judgment unit 50 calculates crack judgment parameters for each part of the press-molded product using the press-molded crack judgment parameter calculation unit 53, and judges whether or not cracks have occurred in each part of the press-molded product using the press-molded crack occurrence judgment unit 55.

[0157] However, the press-formed product crack determination unit 50 may calculate the crack determination parameters only for the portions of the press-formed product where cracks are likely to occur, and determine whether or not cracks will occur in those portions. In this case, the crack determination parameters may be calculated for each element used in the press forming analysis, and the presence or absence of cracks may be determined for each element of the portions of the press-formed product where cracks are likely to occur. [Example]

[0158] An experiment was conducted to verify the effects of the present invention, and the results will be described below.

[0159] Example 1 In Example 1, the forming limit surface of the metal plate was determined according to the method for creating the forming limit surface of the metal plate according to the first embodiment, following the flow shown in FIG.

[0160] First, in the forming test step S10, the shape of the forming die 11 used as a tool for forming the test piece 100 was determined, as shown in Fig. 2. The forming die 11 was equipped with a punch 11a having a spherical head tip 11a1, an upper die 11b, and a blank holder 11c, as shown in Fig. 2. In order to generate various forces in the thickness direction on the test piece 100, the radius of curvature R of the spherical head tip 11a1 of the punch 11a was set to four types: R25 mm, R50 mm, R100 mm, and R=∞, as shown in Fig. 3.

[0161] Next, a high-strength steel plate with a tensile strength of 1470 MPa and a thickness of 1.4 mm was used as a test material for the test piece 100, and the shape of the test piece 100 was determined. As shown in Fig. 4, the shape of the test piece 100 was a circular test piece 101 with a diameter of φ200 mm, and a test piece 103 with a shape obtained by cutting out this in the width direction. The test piece 103 had a shape in which the width W of the central portion 103b was changed to five levels.

[0162] Next, a lattice pattern (grid) for strain measurement was transferred onto the surface of the test piece 100 whose shape had been determined.

[0163] Next, the forming die 11 was set in a hydraulic deep drawing test machine, and a test piece 100 was formed. In this Example 1, in order to prevent the test piece 100 from flowing in, a blank holder force of 50 tonf was applied by the blank holder 11c, and the punching speed of the punch 11a was set to 5 mm / min. Then, while forming, the surface of the test piece 100 was photographed by an image analysis camera, which is a photographing device (not shown), installed above the forming die 11. Furthermore, as shown in FIG. 2, pressure-sensitive paper 105 was installed between the forming die 11 (punch 11a) and the test piece 100 to measure the surface pressure generated on the test piece 100.

[0164] Subsequently, images of the surface of the test piece 100 taken at time intervals of once per second during the forming process were analyzed to measure the amount of strain (maximum principal strain and minimum principal strain) occurring on the surface of the test piece 100. The amounts of strain measured during the forming process were then stored in a strain database in chronological order from the start of forming to the occurrence of fracture.

[0165] Next, it was visually determined whether or not a fracture occurred in the test piece 100 during the molding process. If it was determined that no fracture occurred, molding was continued, and the surface of the test piece 100 was photographed, the amount of strain was measured by image analysis, and the results were stored in a strain database. In this way, in this Example 1, the surface of the test piece 100 was photographed from the start of molding until fracture occurred, and the amount of strain was measured for each photographed image, and this was continued until it was determined that fracture occurred. When it was determined that a break had occurred, the pressure-sensitive paper 105 placed between the molding die 11 and the test piece 100 was removed, and the surface pressure at the break was measured.

[0166] Subsequently, forming tests were carried out on all test piece shapes, and strain and surface pressure measurements were carried out to determine whether they were acceptable. When it was determined that the forming test had been performed for all the shapes of the test piece 100, it was determined whether or not the forming test had been performed for all the shapes of the prepared punch 11a. In this way, the above forming test was performed for all the shapes of the test piece and the tool (shape of the punch 11a).

[0167] After the forming test step S10 was completed, in the forming limit analysis step S20, a forming limit analysis was performed using the strain database constructed in the forming test step S10.

[0168] First, a series of evaluation points for acquiring strain distribution was set in the vicinity of a fractured portion that occurred in the molded test piece 100. Next, for the set evaluation point sequence, time series data of maximum principal strain and minimum principal strain were extracted from the strain database.

[0169] Then, the forming limit was determined for the time series data of the maximum principal strain and the minimum principal strain obtained for each shape of the test piece 100 and the shape of the forming mold 11, and the maximum principal strain and the minimum principal strain at the bending point where the deformation transitions from uniform to non-uniform was determined as the forming limit strain. Furthermore, this operation was carried out for all punches 11a having different shapes (radius of curvature) of the tip end portion 11a1 used in the forming test, and the forming limit strain (maximum principal strain, minimum principal strain) was determined.

[0170] Table 1 shows the results of the surface pressure (force in the thickness direction) generated on the test piece 103 and the forming limit strain (maximum principal strain and minimum principal strain) obtained for each shape of the test piece 103 and the shape of the forming die 11.

[0171] [Table 1]

[0172] After the forming limit analysis step S20 was completed, a forming limit surface was created in the forming limit surface creation step S30.

[0173] In the forming limit surface creation step S30, first, the forming limit data (force in the plate thickness direction at the fracture part, maximum principal strain and minimum principal strain at the forming limit) obtained for each shape of the test piece 100 and the shape of the forming mold 11 were plotted in three-dimensional coordinate space.

[0174] Next, a forming limit surface was determined that approximated the group of plotted points in the three-dimensional coordinate space. In this Example 1, forming limit planes expressed by the following formula (1) were assumed for the negative and positive regions of the minimum principal strain.

[0175]

number

[0176] Then, the coefficients in formula (1) were determined for the forming limit plane A in the positive region and the forming limit plane B in the negative region so that the sum of squares of the perpendicular distances between the assumed forming limit plane and each plot point of the forming limit data was minimized. In this way, the forming limit plane A and the forming limit plane B whose coefficients in formula (1) were determined were obtained as the forming limit surfaces.

[0177] Table 2 shows the coefficients in equation (1) for the determined forming limit plane A and forming limit plane B. Furthermore, Fig. 9 shows a group of plots of the measured forming limit strain (maximum principal strain and minimum principal strain) and force in the thickness direction (surface pressure), as well as forming limit plane A and forming limit plane B created based on the determined coefficients.

[0178] [Table 2]

[0179] As described above, according to the present invention, it was possible to create a forming limit surface expressed by the relationship between the maximum principal strain, the minimum principal strain, and the force (surface pressure) in the thickness direction, as shown in FIG.

[0180] <Example 2> In Example 2, an experiment was conducted to verify whether or not cracks occurred due to the forming limit surface created in Example 1.

[0181] In this Example 2, the forming limit surface was obtained as the forming limit surface expressed by the formula (1) obtained in Example 1. The values ​​of coefficients a to d in the formula (1) were the values ​​shown in Table 2 above.

[0182] Next, as shown in FIG. 10, press-forming was performed by V-bending the blank 110 using a press-forming die 120, and the presence or absence of cracks in the press-formed product (V-bent blank 110) was determined. The blank 110 was made of a high-strength steel plate having a tensile strength of 1470 MPa and a thickness of 1.4 mm. 10, the press molding die 120 includes a V-bending punch 121 having a tip 121a with a mountain-shaped cross section, an upper die 123, and a blank holder 125. The V-bending punch 121 has a diameter of φ50 mm, a radius of curvature of the tip 121a of R50 mm, and a radius of curvature of the die shoulder 123a of the upper die 123 of R5 mm.

[0183] Furthermore, in Example 2, in order to change the force in the thickness direction generated in the blank 110, V-bending was performed using a V-bending punch 121A having a gap forming portion 121b at the apex of the tip portion 121a so that a gap was formed between the blank 110 and the punch 121A, as shown in Figure 11(b), and the occurrence of cracks was determined. During the V-bending process, a blank holding force of 50 tons was applied and the punch speed was 5 mm / min to prevent the blank 110 from flowing in. An image analysis camera, which is an imaging device (not shown), was installed above the press-forming die 120, and images of the surface of the blank 110 were taken while the V-bending process was being carried out. Furthermore, pressure-sensitive paper was installed between the press-forming die 120 (V-bending punch 121) and the blank 110 to measure the surface pressure generated on the blank 110.

[0184] After V-bending, the maximum and minimum principal strains at the forming limit of the formed blank were determined. Furthermore, the surface pressure at the fractured portion was determined as the force in the thickness direction at the fractured portion using pressure-sensitive paper placed between the press forming die 120 and the blank 110.

[0185] When a V-bending punch 121A having a gap-forming portion 121b at its tip 121a was used, the forming limit strain of the V-bent blank 110 was a maximum principal strain of 0.13 and a minimum principal strain of -0.04, and the contact pressure was 0 MPa. When these values ​​were plotted in a three-dimensional space (FIG. 9) showing the forming limit surface, the plot points were located above the forming limit surface. From this, it was determined that cracks would occur in blanks made using a V-bending punch 121A having a gap-forming portion 121b.

[0186] On the other hand, when a V-bending punch 121 without a gap-forming portion at its tip was used, the forming limit strain of the V-bent blank 110 was a maximum principal strain of 0.15, a minimum principal strain of -0.06, and a contact pressure of 70 MPa. When these values ​​were plotted in a three-dimensional space (FIG. 9) showing the forming limit surface, the plot points were located below the forming limit surface. From this, it was determined that no cracks occurred in the blank when a V-bending punch 121 without a gap-forming portion was used.

[0187] As described above, it has been shown that the press-forming crack determination method according to the present invention makes it possible to determine whether or not cracks have occurred in a press-formed product by taking into account the influence of force in the plate thickness direction, which was not taken into account in the conventional forming limit line. [Explanation of symbols]

[0188] 1. Forming limit surface creation system 10. Forming Test Section 11 Molding mold 11a Punch 11a1 Tip 11b Upper die 11c Wrinkle holder 13 Imaging equipment 15 Strain measurement device 17. Plate thickness direction force acquisition device 19 Strain database construction device 20 Forming Limit Analysis Section 21 Evaluation point sequence setting device 23 Strain distribution acquisition device 25 Forming limit acquisition device 30 Forming limit surface creation section 31 Forming limit plotting device 33 Forming limit surface creation device 41 Molding test result input section 43 Forming Limit Analysis Section 45 Forming limit surface creation section 100 test specimens 101 Test piece 103 Test Pieces 103a Notch 103b Central part 105 Pressure-sensitive Paper 110 test specimens 120 Press molding die 121 V-bending punch 121A V-bending punch 121a Tip 121b Gap forming part 123 Upper die 125 Wrinkle Holder 201 Molding mold 203 Ball Head Punch 203a Tip 205 Upper die 207 Wrinkle holder 211 Molding mold 213 Flat Head Punch 213a Tip 215 driving seat

Claims

1. A method for obtaining the forming limit of a metal plate, comprising: forming a metal plate test piece using a plurality of tools having different shapes so that various forces in the plate thickness direction are generated; and determining the force in the plate thickness direction, maximum principal strain, and minimum principal strain generated in the formed test piece as indicators of the forming limit of the metal plate.

2. A method for creating a forming limit surface of a metal plate, which involves forming a metal plate test piece using a plurality of tools having different shapes so that various forces in the plate thickness direction are generated, and creating a forming limit surface of the metal plate represented by the relationship between the force in the plate thickness direction generated in the formed test piece and the maximum principal strain and the minimum principal strain, The method includes a forming test step, a forming limit analysis step, and a forming limit surface creation step, The molding test step includes: a test piece preparation step of preparing a plurality of test pieces having different shapes and having a predetermined lattice or strain analysis pattern applied to the surface; a test piece molding step of molding each of the test pieces using each of the plurality of tools while photographing the surface of the test piece; a strain measurement step of analyzing the photographed images of the surfaces of the test pieces and measuring strains occurring in the test pieces; a thickness direction force acquisition step of determining a thickness direction force at a fracture portion occurring in each of the molded test pieces; a strain database construction step of storing the strains measured in the strain measurement step for each of the formed test pieces in chronological order from the start of forming to the occurrence of fracture, and constructing a strain database; The forming limit analysis step includes: an evaluation point sequence setting step of setting an evaluation point sequence in each of the molded test pieces to acquire a strain distribution in the vicinity of the fracture portion generated in each of the molded test pieces; a strain distribution acquisition step of extracting the strain of the evaluation point sequence set for each of the test pieces from the strain database and acquiring a strain distribution in the vicinity of the fractured portion; and a forming limit acquisition step of determining a forming limit based on the acquired strain distribution and determining a maximum principal strain and a minimum principal strain at the forming limit, The forming limit surface creation step includes: a forming limit plotting step of plotting the force in the plate thickness direction at the fracture portion and the maximum principal strain and minimum principal strain at the forming limit, which are obtained for each of the formed test pieces, in a three-dimensional coordinate space; and a forming limit surface creation step of creating a forming limit surface of the metal plate based on a group of plot points of the force in the plate thickness direction at the fracture part and the maximum principal strain and minimum principal strain at the forming limit plotted in the three-dimensional coordinate space.

3. 3. The method for creating a forming limit surface of a metal plate according to claim 2, wherein in the plate thickness direction force acquisition step, the force in the plate thickness direction is obtained as the surface pressure generated at the fracture portion of each of the formed test pieces.

4. 4. The method for creating a forming limit surface of a metal plate according to claim 3, wherein the surface pressure is measured by placing pressure-sensitive paper between the tool and each of the test pieces during the test piece forming process, or is calculated by finite element method analysis that reproduces the forming of each of the test pieces during the test piece forming process.

5. 3. The method for creating a forming limit surface of a metal plate according to claim 2, wherein in the plate thickness direction force acquisition step, the plate thickness direction stress generated at the fracture portion of each of the test pieces formed in the test piece forming step is obtained as the plate thickness direction force.

6. 6. The method for creating a forming limit surface of a metal plate according to claim 5, wherein the stress in the plate thickness direction is calculated by a finite element method analysis that reproduces the forming of each of the test pieces in the test piece forming step.

7. 7. The method for creating a forming limit surface of a metal plate according to claim 2, wherein in the forming limit surface creation step, a polygonal surface is created that is made up of a plurality of triangular planes that connect two adjacent plot points among the plot point groups for the plurality of test pieces formed using the tool having the same shape, and one plot point that is closest to the line segment connecting the two plot points among the plot point groups for the plurality of test pieces formed using the tool having a shape different from that of the two plot points.

8. 7. The method for creating a forming limit surface of a metal plate according to claim 2, wherein in the forming limit surface creation step, a forming limit plane or a forming limit curved surface is assumed, the sum of squares of the perpendicular distances between the assumed forming limit plane or the forming limit curved surface and each plot point of the plot point group in the three-dimensional coordinate space is calculated, and the forming limit plane or the forming limit curved surface is determined so that the calculated sum of squares is minimum.

9. 7. The method for creating a forming limit surface of a metal plate according to claim 2, wherein in the forming limit surface creation step, a forming limit plane or a forming limit curved surface is assumed, a sum of squares weighted on a perpendicular distance between the assumed forming limit plane or the forming limit curved surface and each plot point of the group of plot points in the three-dimensional coordinate space is calculated, and the forming limit plane or the forming limit curved surface is determined so that the calculated sum of squares is minimized.

10. 9. The method for creating a forming limit surface of a metal plate according to claim 8, wherein in the forming limit surface creation step, the forming limit surface is created by combining a plurality of the forming limit planes and / or the forming limit curved surfaces.

11. 10. The method for creating a forming limit surface of a metal plate according to claim 9, wherein in the forming limit surface creation step, the forming limit surface is created by combining a plurality of the forming limit planes and / or the forming limit curved surfaces.

12. A forming limit surface creation system for a metal plate that creates a forming limit surface of the metal plate represented by the relationship between a force in the thickness direction generated in a test piece of the metal plate formed so as to generate various forces in the thickness direction, and a maximum principal strain and a minimum principal strain, The apparatus includes a forming test section, a forming limit analysis section, and a forming limit surface creation section, The molding test section a plurality of tools having different shapes for forming a plurality of test pieces having different shapes on the surface of which a predetermined lattice or strain analysis pattern is provided so that various forces in the thickness direction are generated; an imaging device that images the surface during the process of molding each of the test pieces using each of the tools; a strain measuring device that analyzes the captured image of the surface of each of the test pieces and measures strain occurring in each of the test pieces; a thickness direction force acquisition device for determining a thickness direction force at a fracture portion generated in each of the molded test pieces; a strain database construction device that stores the strains measured by the strain measuring device for each of the formed test pieces in chronological order from the start of forming to the occurrence of fracture, and constructs a strain database; The forming limit analysis unit an evaluation point sequence setting device that sets an evaluation point sequence in each of the molded test pieces to acquire strain distribution in the vicinity of the fracture portion that occurs in each of the molded test pieces; a strain distribution acquisition device that extracts the strain of the evaluation point sequence set for each of the test pieces from the strain database and acquires the strain distribution in the vicinity of the fractured portion; a forming limit acquisition device that determines a forming limit based on the acquired strain distribution and obtains a maximum principal strain and a minimum principal strain at the forming limit, The forming limit surface creation unit a forming limit plotting device that plots the force in the plate thickness direction at the fracture portion and the maximum principal strain and minimum principal strain at the forming limit, which are obtained for each of the formed test pieces, in a three-dimensional coordinate space; and a forming limit surface creation device that creates a forming limit surface of the metal plate based on a group of plot points of the force in the plate thickness direction at the fracture part and the maximum principal strain and minimum principal strain at the forming limit plotted in the three-dimensional coordinate space.

13. A forming limit surface creation program for a metal plate that creates a forming limit surface of the metal plate represented by the relationship between a force in the thickness direction generated in a test piece of the metal plate formed so as to generate various forces in the thickness direction, and a maximum principal strain and a minimum principal strain, A forming limit surface creation program for a metal plate, comprising a function of causing a computer to execute the forming limit analysis unit and the forming limit surface creation unit of the metal plate forming limit surface creation system according to claim 12.

14. A press-molding crack determination method for determining whether or not cracks occur in a press-molded product obtained by press-molding a metal plate, The method includes a forming limit surface acquisition process and a press-molded product crack determination process, The forming limit surface acquisition process includes: Obtain a forming limit surface of the metal plate, which is expressed by the relationship between the force in the thickness direction of a test piece of the metal plate formed using a plurality of tools having different shapes so as to generate various forces in the thickness direction, and the maximum principal strain and the minimum principal strain; The press-molded product crack determination process includes: The force in the plate thickness direction, the maximum principal strain, and the minimum principal strain in the press-formed product are acquired as crack determination parameters in the press-formed product; A press-molding crack determination method characterized by determining whether or not a crack has occurred in the press-molded product based on the acquired crack determination parameters and the forming limit surface acquired in the forming limit surface acquisition process.

15. The force in the plate thickness direction in the forming limit surface acquisition process is the surface pressure at the fracture portion generated by forming each of the test pieces, The press-molded crack determination method according to claim 14, characterized in that, in the press-molded product crack determination process, the force in the plate thickness direction acquired as a crack determination parameter in the press-molded product is the surface pressure in the press-molded product.

16. The surface pressure in the forming limit surface acquisition process is measured by placing pressure-sensitive paper between the tool and each of the test pieces and forming them, or is calculated by finite element method analysis that reproduces the forming of each of the test pieces using the tool; The press-molded crack determination method according to claim 15, characterized in that the surface pressure in the press-molded product crack determination process is measured by setting pressure-sensitive paper between a forming die for forming the press-molded product and a blank and press-molding the press-molded product, or is calculated by finite element method analysis that reproduces the press molding of the press-molded product.

17. The force in the thickness direction in the forming limit surface acquisition process is the stress in the thickness direction at the fracture portion generated by forming each of the test pieces, 15. The press-molded crack determination method according to claim 14, wherein in the press-molded product crack determination process, the force in the thickness direction obtained as a crack determination parameter in the press-molded product is the stress in the thickness direction in the press-molded product.

18. The thickness direction stress in the forming limit surface acquisition process is calculated by a finite element method analysis that reproduces the forming of each test piece using the tool; The press-molding crack detection method according to claim 17, characterized in that the plate thickness direction stress in the press-molded product crack detection process is calculated by a finite element method analysis that reproduces the press molding of the press-molded product.

19. A press-molding crack determination system for determining whether or not cracks occur in a press-molded product obtained by press-molding a metal plate, A forming limit surface acquisition unit and a press-molded product crack determination unit are included, The forming limit surface acquisition unit Obtain a forming limit surface of the metal plate, which is expressed by the relationship between the force in the thickness direction of a test piece of the metal plate formed using a plurality of tools having different shapes so as to generate various forces in the thickness direction, and the maximum principal strain and the minimum principal strain; The press-molded product crack determination unit includes: The force in the plate thickness direction, the maximum principal strain, and the minimum principal strain in the press-formed product are acquired as crack determination parameters in the press-formed product; A press-molding crack determination system characterized by determining whether or not a crack has occurred in the press-molded product based on the acquired crack determination parameters and the forming limit surface acquired by the forming limit surface acquisition unit.

20. A press-molding crack determination program for determining whether or not cracks occur in a press-molded product obtained by press-molding a metal plate, A press-forming crack determination program having a function of causing a computer to execute the press-forming crack determination system for a metal plate according to claim 19 as a forming limit surface acquisition unit and a press-formed product crack determination unit.

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