Elongation flange crack evaluation method

Abstract

To improve evaluation accuracy of an elongation flange crack.SOLUTION: A method includes: performing an elongation flange molding test on a plurality of plate-like members; performing a monopodium tensile deformation test on the plate-like members; determining whether a cracking condition from the elongation flange molding test is an edge crack or an inner crack; calculating a limit strain and a strain gradient from the elongation flange molding test; calculating edge crack limit strain characteristic data and inner crack limit strain characteristic data, showing the relationship between the limit strain and the strain gradient of the plate-like member in the edge crack and the inner crack, respectively; calculating ductile fracture limit strain characteristic data, in which the limit strain is the strain at ductile fracture by the monopodium tensile deformation test; and evaluating a cracking at an elongation flange part using at least the edge crack limit strain characteristic data and the ductile fracture limit strain characteristic data among edge crack limit strain characteristic data L1a, inner crack limit strain characteristic data L1b and ductile fracture limit strain characteristic data L1c.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a method for evaluating stretch flange cracking.

Background Art

[0002] When manufacturing a press-formed product by press-forming a metal plate-like member, press-forming analysis using finite element method analysis (FEM analysis) is performed in advance to simulate the press-forming of the plate-like member and evaluate cracking of the press-formed product, and the feasibility of press-forming is evaluated.

[0003] When using, for example, a plate-like member such as a high-tensile steel plate having a tensile strength of about 980 MPa or more as the plate-like member, in the stretch flange portion that is stretch flange formed in press-forming, since stretch flange deformation is involved, cracking may occur from the end of the plate-like member during deformation. In order to evaluate and predict this cracking in advance, evaluation of stretch flange cracking in the press-formed product is performed by press-forming analysis.

[0004] When evaluating stretch flange cracking, it is known to use limit strain characteristic data showing the relationship between the limit strain and strain gradient of the plate-like member as the fracture limit characteristic of the plate-like member. For example, in Patent Document 1, a hole expansion test of a steel plate is performed, characteristic data is determined from the relationship between the limit strain at the hole edge and the strain gradient in the hole diameter direction, and the characteristic data is applied to the simulation of press-forming to evaluate stretch flange cracking.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] The limit strain characteristic data showing the relationship between the limit strain and the strain gradient of the plate-like member described in the above Patent Document 1 calculates the fracture limit based on the cracks at the ends of the holes in the hole expansion test, and calculates the limit strain characteristic data considering only the edge cracks where the crack generation position is at the end of the hole.

[0007] However, when evaluating the elongation flange crack by press forming analysis using the limit strain characteristic data considering only the edge cracks, even when it is evaluated by press forming analysis that the elongation flange crack does not occur, there are cases where fractures other than edge cracks occur, and elongation flange cracks may occur when actually press forming.

[0008] Also, when evaluating the elongation flange crack by press forming analysis using the limit strain characteristic data considering only the edge cracks where the limit strain increases as the strain gradient increases, depending on the magnitude of the strain gradient, the limit strain may become excessively large. Even when it is evaluated by press forming analysis that the elongation flange crack does not occur, there is a possibility that the elongation flange crack may occur when actually press forming.

[0009] An object of the present invention is to provide an elongation flange crack evaluation method capable of improving the evaluation accuracy of elongation flange cracks.

Means for Solving the Problems

[0010] The present invention relates to a method for evaluating cracking of an elongation flange portion of a press-formed product, the method comprising: performing an elongation flange forming test on a plurality of plate-like members, each of which is formed so as to generate a crack at an end portion of the plate-like member to calculate a limit strain; performing a uniaxial tensile deformation test on a plate-like member made of the same material as the plurality of plate-like members to calculate the strain at ductile fracture; determining, for each of the plurality of plate-like members, whether the crack state at the time of crack generation in the elongation flange forming test is an edge crack or an internal crack, and calculating the limit strain and the strain gradient in the internal direction of the plate-like member from the end face of the plate-like member in the elongation flange forming test; calculating edge crack limit strain characteristic data indicating the relationship between the limit strain and the strain gradient of the plate-like member in an edge crack based on the limit strain and the strain gradient of each plate-like member when the crack state in the elongation flange forming test is an edge crack; calculating internal crack limit strain characteristic data indicating the relationship between the limit strain and the strain gradient of the plate-like member in an internal crack based on the limit strain and the strain gradient of each plate-like member when the crack state in the elongation flange forming test is an internal crack; calculating ductile fracture limit strain characteristic data in which the strain at ductile fracture in the uniaxial tensile deformation test for the plate-like member is set as the limit strain regardless of the strain gradient; and providing an elongation flange crack evaluation method for evaluating cracking of the elongation flange portion using at least the edge crack limit strain characteristic data and the ductile fracture limit strain characteristic data among the edge crack limit strain characteristic data, the internal crack limit strain characteristic data, and the ductile fracture limit strain characteristic data.

[0011] According to the present invention, elongation flange forming tests are performed on a plurality of plate-like members, and it is determined whether the crack state at the time of crack generation in the elongation flange forming test is an edge crack or an internal crack, and the limit strain and strain gradient in the elongation flange forming test are calculated, and edge crack limit strain characteristic data and internal crack limit strain characteristic data are calculated. Further, a uniaxial tensile deformation test is performed on the plate-like member, and ductile fracture limit strain characteristic data having the strain at the time of ductile fracture in the uniaxial tensile deformation test as the limit strain is calculated. Then, the crack in the elongation flange portion is evaluated using at least the edge crack limit strain characteristic data and the ductile fracture limit strain characteristic data among the edge crack limit strain characteristic data, the internal crack limit strain characteristic data, and the ductile fracture limit strain characteristic data.

[0012] Thereby, as the limit strain characteristic data showing the relationship between the limit strain and the strain gradient of the plate-like member, edge crack limit strain characteristic data and internal crack limit strain characteristic data are calculated according to the edge crack or the internal crack which is the crack state in the actual elongation flange forming test, and the elongation flange crack is evaluated using the edge crack limit strain characteristic data and the internal crack limit strain characteristic data. Therefore, compared with the case of using the limit strain characteristic data considering only the edge crack, the evaluation accuracy of the elongation flange crack can be improved. Further, since the crack in the elongation flange portion is evaluated using the ductile fracture limit strain characteristic data having the strain at the time of ductile fracture in the uniaxial tensile deformation test as the limit strain, it is possible to suppress the limit strain from becoming excessively large even when the strain gradient is large, and the evaluation accuracy of the elongation flange crack can be improved. For a plate-like member in which the crack state at the time of crack generation is only an edge crack, the evaluation accuracy of the elongation flange crack can be improved by using the edge crack limit strain characteristic data and the ductile fracture limit strain characteristic data.

[0013] The elongation flange crack evaluation method evaluates the crack in the elongation flange portion using the edge crack limit strain characteristic data, the internal crack limit strain characteristic data, and the ductile fracture limit strain characteristic data.

[0014] With this configuration, since the cracking of the elongation flange portion is evaluated using the edge cracking limit strain characteristic data, the internal cracking limit strain characteristic data, and the ductile fracture limit strain characteristic data, it is possible to improve the evaluation accuracy of the elongation flange cracking for a plate-shaped member in which the cracking state at the time of cracking is edge cracking or internal cracking.

[0015] In the elongation flange cracking evaluation method, at the time of press forming analysis for press forming a press-formed product having an elongation flange portion from a plate-shaped member, at least the edge cracking limit strain characteristic data and the ductile fracture limit strain characteristic data among the edge cracking limit strain characteristic data, the internal cracking limit strain characteristic data, and the ductile fracture limit strain characteristic data are used to evaluate the cracking of the elongation flange portion.

[0016] With this configuration, at the time of press forming analysis, since the cracking of the elongation flange portion is evaluated using at least the edge cracking limit strain characteristic data and the ductile fracture limit strain characteristic data among the edge cracking limit strain characteristic data, the internal cracking limit strain characteristic data, and the ductile fracture limit strain characteristic data, at the time of press forming analysis, by calculating the maximum principal strain in each element of the analysis model obtained by finite element division for the end portion of the elongation flange portion and the strain gradient between the element and the element adjacent in the direction away from the end portion of the elongation flange portion, it is possible to accurately evaluate the cracking of the elongation flange portion in the press forming analysis.

[0017] The elongation flange cracking evaluation method preferably captures an image of the surface of the plate-shaped member with a camera in the elongation flange forming test, and calculates the limit strain and the strain gradient of the plate-shaped member by the elongation flange forming test based on the image of the surface of the plate-shaped member captured by the camera.

[0018] With this configuration, based on the image of the surface of the plate-shaped member captured by the camera, the limit strain and strain gradient of the plate-shaped member in the stretch flange forming test are calculated. Therefore, the limit strain and strain gradient can be calculated using the digital image correlation method (DIC: Digital Image Correlation), and it is possible to accurately calculate the limit strain and strain gradient.

[0019] In the stretch flange cracking evaluation method, it is preferable to image the surface of the plate-shaped member with a camera in the stretch flange forming test, and based on the image of the surface of the plate-shaped member captured by the camera, determine whether the cracking state at the time of crack generation in the stretch flange forming test of the plate-shaped member is an edge crack or an internal crack.

[0020] With this configuration, based on the image of the surface of the plate-shaped member captured by the camera, it is determined whether the cracking state at the time of crack generation in the stretch flange forming test is an edge crack or an internal crack. Therefore, by finding the crack initiation point at the time of crack generation based on the image of the surface of the plate-shaped member, the determination accuracy of the cracking state of the edge crack or internal crack can be improved, and the evaluation accuracy of the stretch flange crack can be improved.

[0021] In the stretch flange cracking evaluation method, it is preferable to image the surface of the plate-shaped member with a camera in the uniaxial tensile deformation test, and based on the image of the surface of the plate-shaped member captured by the camera, calculate the strain at the time of ductile fracture of the plate-shaped member in the uniaxial tensile deformation test.

[0022] With this configuration, based on the image of the surface of the plate-shaped member captured by the camera, the strain at the time of ductile fracture in the uniaxial tensile deformation test for the plate-shaped member is calculated. Therefore, the strain at the time of ductile fracture can be calculated using the digital image correlation method, and it is possible to accurately calculate the ductile fracture limit strain. Based on the image captured by the camera, the limit strain and strain gradient in the stretch flange forming test for the plate-shaped member are calculated, and at the same time, the strain at the time of ductile fracture in the uniaxial tensile deformation test for the plate-shaped member is calculated. Thus, the limit strain and strain gradient can be calculated using the digital image correlation method, and the ductile fracture limit strain can be calculated, and the evaluation accuracy of stretch flange cracking can be improved.

Brief Description of the Drawings

[0023]

Figure 1

Figure 2

Figure 3

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Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Embodiments for Carrying Out the Invention

[0024] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0025] FIG. 1 is a diagram showing a press-formed product having an elongation flange portion. The press-formed product 1 shown in FIG. 1 is a press-formed product 1 formed by press-forming a metal plate-like member such as a high-tensile steel plate having a tensile strength of about 980 MPa or more. As the press-formed product 1, a center pillar outer 1 that constitutes a side surface portion of a vehicle body and is disposed between front and rear door openings is shown.

[0026] The center pillar outer 1 has a central portion 2 extending in the vertical direction of the vehicle body, an upper end portion 3 extending in the front-rear direction of the vehicle body and attached to the roof side rail, and a lower end portion 4 extending in the front-rear direction of the vehicle body and attached to the side sill. The central portion 2 of the center pillar outer 1 includes a bottom surface portion 5, side surface portions 6 on both sides, and flange portions 7 on both sides, and is formed in a substantially hat-shaped cross section.

[0027] The flange portion 7 of the center pillar outer 1 is provided so as to extend from the central portion 2 to the upper end portion 3 and the lower end portion 4, and each has an elongation flange portion 8 that is formed into an elongation flange during press-forming at the connection portion between the central portion 2 and the upper end portion 3 and the lower end portion 4. When the elongation flange portion 8 is formed into an elongation flange, the strain becomes large and cracking is likely to occur.

[0028] FIG. 2 is an explanatory diagram for explaining edge cracking and internal cracking of the stretch flange portion. FIG. 2 shows an enlarged view of a part of the press-formed product shown in FIG. 1 and shows cracks that may occur at the end of the stretch flange portion 8. In FIG. 2(a), the crack generation position indicated by the white arrow shows edge cracking where the end face of the stretch flange portion 8 is located, and in FIG. 2(b), the crack generation position indicated by the white arrow shows internal cracking where it is inside the end face of the stretch flange portion 8. As shown in FIGS. 2(a) and 2(b), cracks such as edge cracking or internal cracking may occur in the stretch flange portion 8.

[0029] In this embodiment, when press-forming a press-formed product from a plate-like member, as the fracture limit characteristics of the plate-like member used in the press-forming analysis, limit strain characteristic data showing the relationship between the limit strain and the strain gradient of the plate-like member is calculated according to the actual crack state by the stretch flange forming test, and the ductile fracture limit strain by the uniaxial tensile deformation test is calculated and used, thereby improving the evaluation accuracy of the stretch flange crack of the press-formed product by the press-forming analysis. The stretch flange forming test is a stretch flange forming test in which the plate-like member is formed so as to generate cracks at the end of the plate-like member to calculate the limit strain. The uniaxial tensile deformation test is a uniaxial tensile deformation test in which the plate-like member is uniaxially tensile deformed to calculate the strain at the time of ductile fracture.

[0030] FIG. 3 is a graph showing the relationship between the limit strain and the strain gradient of the plate-like member used for evaluating the stretch flange crack according to the embodiment of the present invention. In this embodiment, edge crack limit strain characteristic data showing the relationship between the limit strain and the strain gradient of the plate-like member in edge cracking is calculated based on the stretch flange forming test, and internal crack limit strain characteristic data showing the relationship between the limit strain and the strain gradient of the plate-like member in internal cracking is calculated based on the stretch flange forming test, and ductile fracture limit strain characteristic data with the strain at the time of ductile fracture as the limit strain is calculated based on the uniaxial tensile deformation test. As shown in FIG. 3, as the fracture limit characteristics of the plate-like member, the edge crack limit strain characteristic data L1a, the internal crack limit strain characteristic data L1b, and the ductile fracture limit strain characteristic data L1c are used to evaluate the crack of the stretch flange portion.

[0031] In this embodiment, although not limited thereto, a hole expansion test was used as the elongation flange forming test, and although not limited thereto, a uniaxial tensile test was used as the uniaxial tensile deformation test. The hole expansion test was performed by using a plate-shaped member having a hole portion and pushing a punch into the hole portion until a crack generated at the end of the hole portion penetrated in the thickness direction. The uniaxial tensile test was performed by using a plate-shaped member that was a tensile test piece made of the same material as the plate-shaped member used in the hole expansion test and applying a tensile load to both ends of the plate-shaped member until the plate-shaped member broke. As the plate-shaped member, a high-tensile steel plate made of Material A and having a tensile strength of about 980 MPa or more was used.

[0032] FIG. 4 is a schematic configuration diagram of a cylindrical hole expansion test. In the evaluation of elongation flange cracking, for a plurality of plate-shaped members, a hole expansion test was performed using a plate-shaped member having a hole portion, the state of cracking by the hole expansion test was determined, and limit strain characteristic data showing the relationship between the limit strain and strain gradient of the plate-shaped member was acquired.

[0033] As shown in FIG. 4, a cylindrical hole expansion test device 10 as a hole expansion test device includes a press tool 11 that press-forms a plate-shaped member 20 having a hole portion 21 to generate a crack in the vicinity of the hole portion. The press tool 11 includes a die 12 and a blank holder 13 that sandwich the plate-shaped member 20, and a punch 14 that press-forms the plate-shaped member 20.

[0034] In the hole expansion test, with the plate-shaped member 20 having a circular hole portion 21 sandwiched between the die 12 and the blank holder 13 with a predetermined pressing force, the punch 14 is moved until a crack occurs in the vicinity of the hole portion. In the cylindrical hole expansion test, a cylindrical punch 14 having a circular tip is used, and the cylindrical punch 14 is arranged so that the central axis coincides with the hole portion 21 of the plate-shaped member 20.

[0035] The hole expansion test device 10 includes a moving mechanism (not shown) that moves the blank holder 13 and the punch 14. The hole expansion test device 10 is provided with a control unit 30, and the control unit 30 is configured to control the operation of the moving mechanism.

[0036] The hole expanding test apparatus 10 is also provided with cameras 35 as imaging devices for imaging the surface near the hole of the plate-like member 20, and two cameras 35 are arranged above the plate-like member 20 on the side opposite to the punch 14 so as to image the surface of the plate-like member 20. The two cameras 35 are arranged at symmetric positions with respect to the central axis of the hole portion 21 whose central axis coincides with that of the punch 14, for example.

[0037] Images of the surface of the plate-like member 20 captured by the two cameras 35 are input to the control unit 30. The control unit 30 stores the images captured by the cameras 35 in a storage device such as a memory, and calculates the maximum principal strain for the entire surface near the hole of the plate-like member 20 by analysis using the digital image correlation method.

[0038] The control unit 30 images the paint dot pattern applied to the surface of the plate-like member 20 by spraying before the hole expanding test with the two cameras 35, and calculates the maximum principal strain of the surface of the plate-like member 20 from the change in the positional relationship of the dot pattern based on the images before and after imaging. The control unit 30 also displays the images captured by the cameras 35 and the maximum principal strain near the hole of the plate-like member 20 on a display device (not shown) such as a display.

[0039] In the cylindrical hole expanding test, although not limited thereto, a die 12 with a die inner diameter D1 of 53.8 mm and a die shoulder radius R1 of 5 mm was used, and a punch 14 with a punch outer diameter D2 of 50 mm and a punch shoulder radius R2 of 10 mm was used. The thickness t of the plate-like member 20 was 1.6 mm, and various hole diameters such as an initial hole diameter D3 of 20 mm were used for the hole portion 21. The hole portion 21 of the plate-like member 20 was punched with a clearance of 12% of the plate thickness using a die, a blank holder, and a punching punch.

[0040] FIG. 5 is a perspective view showing the plate-like member and the punch after the cylindrical hole expansion test. In FIG. 5, the plate-like member 20 and the punch 14 after the hole expansion test in which the punch 14 is moved until a crack occurs that penetrates the plate-like member 20 in the plate thickness direction are shown. As shown in FIG. 5, after the hole expansion test, the vicinity of the hole portion of the plate-like member 20 is deformed by the punch 14, and a crack 22 occurs in the vicinity of the hole portion of the plate-like member 20.

[0041] In the present embodiment, during the hole expansion test, an image of the surface in the vicinity of the hole portion of the plate-like member 20 is captured by the camera 35, and based on the captured image of the surface in the vicinity of the hole portion of the plate-like member 20, an operator or the like determines the crack state of the crack 22 generated at the end portion of the plate-like member 20. As the crack state, when the crack generation position, which is the starting point of the crack, is the crack generation position C1 that is the end face of the hole portion 21, it is determined as an edge crack, and when the crack generation position, which is the starting point of the crack, is the crack generation position C2 that is inside the plate-like member 20 away from the end face of the hole portion 21, it is determined as an internal crack.

[0042] FIG. 6 is a diagram showing the maximum principal strain in the vicinity of the hole portion of the plate-like member. FIG. 6 shows the maximum principal strain in the vicinity of the hole portion of the plate-like member 20 after the hole expansion test. In the present embodiment, during the hole expansion test, an image of the surface in the vicinity of the hole portion of the plate-like member 20 is captured by the camera 35, and based on the captured image of the surface in the vicinity of the hole portion of the plate-like member 20, the maximum principal strain in the vicinity of the hole portion of the plate-like member 20 is calculated using the digital image correlation method.

[0043] In the hole expansion test, the maximum principal strain occurs in the circumferential direction of the hole portion 21 in the vicinity of the hole portion of the plate-like member 20. As shown in FIG. 6, the maximum principal strain in the vicinity of the hole portion of the plate-like member 20 decreases as it moves radially outward from the end face of the hole portion 21, which is the end face of the plate-like member 20, to the inside of the hole portion 21. In the plate-like member 20 in which cracks have occurred at two circumferential positions at the end of the hole portion 21, as shown in FIG. 6, it can be seen that there are portions with large maximum principal strain at four positions at the end of the hole portion 21, and the maximum principal strain decreases as it moves radially away from the hole portion 21.

[0044] FIG. 7 is a graph showing the relationship between the distance from the center of the hole of the plate-like member and the maximum principal strain. In FIG. 7, the maximum principal strain calculated for the entire surface near the hole of the plate-like member 20 is plotted using black circles according to the distance from the center of the hole 21. As shown in FIG. 7, the maximum principal strain calculated in the circumferential direction at the end near the hole 21 is relatively large, and in the interior of the plate-like member 20 away from the end face of the hole 21, the maximum principal strain calculated in the circumferential direction has a maximum principal strain distribution that generally decreases as it radially departs from the hole 21.

[0045] FIG. 8 is a graph showing the relationship between the distance from the end face of the plate-like member and the maximum principal strain. In FIG. 8, the distance from the end face of the hole 21 is taken as the horizontal axis for the end face of the plate-like member 20, and the maximum principal strain is taken as the vertical axis for display, and the maximum principal strain is shown as a black square. As the maximum principal strain at a predetermined distance in the radial direction (the direction perpendicular to the tangent of the end face) from the end face of the hole 21 of the plate-like member 20, which is the interior direction of the plate-like member 20, the average value of the maximum principal strains in the entire circumferential direction at a predetermined distance radially outward from the end face of the hole 21 of the plate-like member 20 is calculated as the maximum principal strain.

[0046] As shown in FIG. 8, the maximum principal strain of the plate-like member 20 by the hole expansion test decreases as it departs from the end face of the plate-like member 20. In the present embodiment, the maximum principal strain ε0 at the end face of the plate-like member 20 is calculated, and the strain gradient α of the maximum principal strain at a position away from the end face of the plate-like member 20 is calculated as the strain gradient α in the vicinity of the hole of the plate-like member 20.

[0047] As the strain gradient α, the absolute value of the inclination of the maximum principal strain between the distances d1 and d2 from the end face of the plate-like member 20 is calculated as the strain gradient. As shown in FIG. 8, the inclination [(ε2 - ε1) / (d2 - d1)] of the straight line connecting the maximum principal strains ε1 and ε2 at the distances d1 and d2 from the end face of the plate-like member 20 is calculated, and the absolute value of the inclination of the straight line is calculated as the strain gradient α. Note that the strain gradient α may be calculated from the inclination of the maximum principal strain in other predetermined ranges away from the end face of the plate-like member 20.

[0048] In this way, a cylindrical hole expansion test is performed, and edge cracking or internal cracking is determined as the state of cracking when cracking occurs. At the same time, the maximum principal strain ε0 on the end face of the hole portion 21 of the plate-like member 20 when cracking occurs is calculated as the limit strain ε0 of the plate-like member 20, and the strain gradient α in the vicinity of the hole portion of the plate-like member 20 is calculated.

[0049] For a plurality of plate-like members 20, the cylindrical hole expansion test is performed by changing the hole diameter and the like. Edge cracking or internal cracking is determined as the state of cracking when cracking occurs. At the same time, the maximum principal strain on the end face of the hole portion 21 of the plate-like member 20 when cracking occurs is calculated as the limit strain ε0 of the plate-like member 20, and the strain gradient α in the vicinity of the hole portion of the plate-like member 20 is calculated.

[0050] FIG. 9 is a schematic configuration diagram of a conical hole expansion test. In the present embodiment, in the hole expansion test device 10 described above, a conical punch is used instead of the cylindrical punch. As the hole expansion test, in addition to the cylindrical hole expansion test, a conical hole expansion test is performed. Also for the conical hole expansion test, for each of the plurality of plate-like members 20, the state of cracking by the hole expansion test is determined, and the maximum principal strain on the end face of the hole portion of the plate-like member when cracking occurs is calculated as the limit strain of the plate-like member, and the strain gradient in the vicinity of the hole portion of the plate-like member is calculated.

[0051] The conical hole expansion test device as the hole expansion test device is configured in the same manner as the cylindrical hole expansion test device 10 except for the punch. As shown in FIG. 9, the conical hole expansion test device 10 includes a press tool 11 that press-forms the plate-like member 20 having the hole portion 21 to generate a crack in the vicinity of the hole portion. The press tool 11 includes a die 12 and a blank holder 13 that sandwich the plate-like member 20, and a punch 15 that press-forms the plate-like member 20. In the conical hole expansion test, a conical punch 15 having a conical tip is used, and the conical punch 15 is arranged so that the central axis coincides with the hole portion 21 of the plate-like member 20.

[0052] In the conical hole expansion test, although not limited thereto, a die 12 with a die inner diameter D1 of 53.8 mm and a die shoulder radius R1 of 5 mm was used, and a punch 15 with a punch outer diameter D2 of 50 mm and a punch apex angle θ1 of 120 degrees was used. The thickness t of the plate-like member 20 was 1.6 mm, and various hole diameters such as an initial hole diameter D3 of 20 mm were used for the hole portion 21. The hole portion 21 of the plate-like member 20 was punched with a clearance of 12% of the plate thickness using the die, the blank holder, and the punching punch.

[0053] FIG. 10 is a perspective view showing the plate-like member and the punch after the conical hole expansion test. In FIG. 10, the plate-like member 20 and the punch 15 after the hole expansion test in which the punch 15 was moved until a crack penetrating the plate-like member 20 in the plate thickness direction occurred are shown. As shown in FIG. 10, after the hole expansion test, the vicinity of the hole portion of the plate-like member 20 is deformed by the punch 15, and a crack 22 occurs in the vicinity of the hole portion of the plate-like member 20.

[0054] Regarding the conical hole expansion test as well, an image of the surface of the vicinity of the hole portion of the plate-like member 20 is captured by a camera 35 during the hole expansion test, and based on the captured image of the surface of the vicinity of the hole portion of the plate-like member 20, an operator or the like determines the crack state of the crack 22 occurring at the end of the plate-like member 20. As the crack state, when the crack generation position, which is the starting point of the crack, is the crack generation position C1 that is the end face of the hole portion 21, it is determined as an edge crack, and when the crack generation position, which is the starting point of the crack, is the crack generation position C2 that is inside the plate-like member 20 away from the end face of the hole portion 21, it is determined as an internal crack.

[0055] Regarding the conical hole expansion test as well, in the same manner as the cylindrical hole expansion test, whether it is an edge crack or an internal crack is determined as the crack state at the time of crack generation, and as the maximum principal strain at a predetermined distance in the radial direction (the direction perpendicular to the tangent of the end face) from the end face of the hole portion 21, which is the end face of the plate-like member 20, to the inside of the plate-like member 20, the average value of all the maximum principal strains in the circumferential direction at a predetermined distance radially outward from the end face of the hole portion 21 of the plate-like member 20 is calculated as the maximum principal strain.

[0056] Then, the maximum principal strain ε0 on the end face of the plate-like member 20 is calculated, and the strain gradient α in the vicinity of the hole of the plate-like member 20 is calculated as the strain gradient α of the maximum principal strain at a position away from the end face of the plate-like member 20. Also, for the conical hole expansion test, as the strain gradient α, the absolute value of the inclination of the maximum principal strain between the distances d1 and d2 from the end face of the plate-like member 20 was calculated as the strain gradient. Note that the strain gradient α may be calculated from the inclination of the maximum principal strain in another predetermined range away from the end face of the plate-like member 20.

[0057] In this way, the conical hole expansion test is performed, the edge crack or internal crack is determined as the state of the crack at the time of crack generation, the maximum principal strain ε0 on the end face of the hole 21 of the plate-like member 20 at the time of crack generation is calculated as the limit strain ε0 of the plate-like member 20, and the strain gradient α in the vicinity of the hole of the plate-like member 20 is calculated.

[0058] For a plurality of plate-like members 20, the conical hole expansion test is performed by changing the hole diameter and the like, the edge crack or internal crack is determined as the state of the crack at the time of crack generation, the maximum principal strain ε0 on the end face of the hole 21 of the plate-like member 20 at the time of crack generation is calculated as the limit strain ε0 of the plate-like member 20, and the strain gradient α in the vicinity of the hole of the plate-like member 20 is calculated.

[0059] In FIG. 3, the strain gradient α is taken on the horizontal axis, and the maximum principal strain ε0 on the end face of the plate-like member 20 is taken on the vertical axis as the limit strain. From the calculation results of the maximum principal strain and the strain gradient as the limit strain by the cylindrical hole expansion test and the conical hole expansion test, it was found that when the strain gradient is relatively large, an edge crack occurs in the plate-like member 20, and when the strain gradient is relatively small, an internal crack occurs in the plate-like member 20.

[0060] In this embodiment, from the calculation results of the maximum principal strain and the strain gradient obtained by the hole expansion test, an approximate formula of a linear function showing the relationship between the strain gradient and the maximum principal strain in the case of edge cracking is calculated by the known least squares method, and the edge cracking limit strain characteristic data L1a showing the relationship between the limit strain and the strain gradient of the plate member 20 in edge cracking is calculated. The edge cracking limit strain characteristic data L1a showing the relationship between the limit strain and the strain gradient of the plate member 20 has a limit strain characteristic in which the limit strain increases as the strain gradient increases.

[0061] Also, from the calculation results of the maximum principal strain and the strain gradient obtained by the hole expansion test, an approximate formula of a quadratic function showing the relationship between the strain gradient and the maximum principal strain in the case of internal cracking is calculated by the known least squares method, and the internal cracking limit strain characteristic data L1b showing the relationship between the limit strain and the strain gradient of the plate member in internal cracking is calculated. The internal cracking limit strain characteristic data L1b showing the relationship between the limit strain and the strain gradient of the plate member 20 has a limit strain characteristic in which the limit strain increases as the strain gradient increases.

[0062] FIG. 11 is a schematic configuration diagram of a uniaxial tensile test. In the evaluation of flange cracking due to elongation, a uniaxial tensile test is also performed on a plate member made of the same material as the plate member used in the hole expansion test, using a plate member which is a No. 5 test piece in JIS Z2241, and ductile fracture limit strain characteristic data is obtained by uniaxially deforming the plate member and taking the strain at the time of ductile fracture as the limit strain regardless of the strain gradient.

[0063] As shown in FIG. 11, the tensile test device 40 includes a lower gripping portion 41 and an upper gripping portion 42 that respectively grip the lower end portion and the upper end portion of a plate member 50 that is a tensile test piece. The plate member 50 has a parallel portion 51 having a predetermined plate width and a predetermined plate thickness at the central portion in the longitudinal direction. The parallel portion 51 has end faces on both sides in the plate width direction formed by the same processing such as machining. Although not limited thereto, a plate member having a plate thickness of 1.6 mm was used as the plate member 50.

[0064] In the uniaxial tensile test, the lower gripping part 41 and the upper gripping part 42 are moved in a direction away from each other (the direction indicated by the white arrow) so as to apply a tensile load to the central part in the longitudinal direction of the plate-like member 50 until the plate-like member 50 breaks after plastic deformation. The tensile test apparatus 40 includes a moving mechanism (not shown) for moving the lower gripping part 41 and the upper gripping part 42, and also includes a control unit 60 for controlling the operation of the moving mechanism.

[0065] The tensile test apparatus 40 is also provided with cameras 45 as imaging devices for imaging the surface of the central part in the longitudinal direction of the plate-like member 50, and two cameras 45 are arranged on the side of the plate-like member 50 so as to image the surface of the plate-like member 50. The two cameras 45 are arranged at symmetric positions in the vertical direction with respect to a direction orthogonal to the longitudinal direction of the plate-like member 50, for example.

[0066] Images of the surface of the plate-like member 50 captured by the two cameras 45 are input to the control unit 60. The control unit 60 stores the images captured by the cameras 45 in a storage device such as a memory, and calculates the maximum principal strain for the entire surface at the central part in the longitudinal direction of the plate-like member 50 by analysis using the digital image correlation method.

[0067] Regarding the uniaxial tensile test as well, the control unit 60 images the paint dot pattern applied to the surface of the plate-like member 50 by spraying with the two cameras 45 before the tensile test, and calculates the maximum principal strain of the surface of the plate-like member 50 from the change in the positional relationship of the dot pattern based on the images before and after imaging. The control unit 60 also displays the images captured by the cameras 45 and the maximum principal strain at the central part in the longitudinal direction of the plate-like member 50 on a display device (not shown) such as a display.

[0068] FIG. 12 is a diagram showing the maximum principal strain at the central portion in the longitudinal direction of the plate-like member. In FIG. 12, the maximum principal strain at the central portion in the longitudinal direction of the plate-like member 50 immediately before fracture in the tensile test is displayed. In the present embodiment, during the tensile test, the camera 45 captures an image of the surface of the central portion in the longitudinal direction of the plate-like member 50, and based on the image of the central portion in the longitudinal direction of the captured plate-like member 50, the maximum principal strain at the central portion in the longitudinal direction of the plate-like member 50 is calculated using the digital image correlation method.

[0069] In the tensile test, the maximum principal strain occurs in the longitudinal direction on the center side of the central portion in the longitudinal direction of the plate-like member 50. As shown in FIG. 12, the maximum principal strain at the central portion in the longitudinal direction of the plate-like member 50 decreases as it moves away from the center side in the longitudinal direction of the central portion in the longitudinal direction of the plate-like member 50 and also decreases as it moves away from the center side in the plate width direction of the central portion in the longitudinal direction of the plate-like member 50.

[0070] FIG. 13 is a graph showing the relationship between the plate width direction position of the plate-like member and the maximum principal strain. In FIG. 13, for the surface of the plate-like member 50 along the V-V line in FIG. 12, the plate width direction position of the plate-like member 50 is taken as the horizontal axis, and the maximum principal strain of the plate-like member 50 is taken as the vertical axis and displayed. As shown in FIG. 13, ductile fracture and cracking have occurred between the plate width direction positions P10 and P11 on the central side in the plate width direction of the plate-like member 50.

[0071] As shown in FIG. 13, when the tensile test is performed until the plate-like member 50 undergoes uniaxial tensile deformation and ductile fracture, the maximum principal strain at the time of ductile fracture of the plate-like member 50 takes the maximum value εmax at the central side in the plate width direction (plate width direction position P10) of the central portion in the longitudinal direction of the plate-like member 50, and has a maximum principal strain distribution that decreases toward one end side and the other end side in the plate width direction.

[0072] In this embodiment, the maximum value εmax of the maximum principal strain at the ductile fracture of the plate-shaped member 50 by the uniaxial tensile test is obtained as the strain at the ductile fracture by the uniaxial tensile test. Then, the ductile fracture limit strain characteristic data L1c, which sets the strain at the ductile fracture of the plate-shaped member 50 by the uniaxial tensile test as the limit strain regardless of the strain gradient, is calculated. As shown in FIG. 3, the ductile fracture limit strain characteristic data L1c has a limit strain characteristic in which the limit strain is constant according to the material of the plate-shaped member 50 regardless of the strain gradient.

[0073] In this embodiment, in the evaluation of the elongation flange crack, the edge crack limit strain characteristic data L1a, the internal crack limit strain characteristic data L1b, and the ductile fracture limit strain characteristic data L1c are used as the limit strain characteristic data L1 of the plate-shaped member 20. When the strain gradient is equal to or less than a predetermined value P1, the internal crack limit strain characteristic data L1b, which has a smaller limit strain than the edge crack limit strain data L1a and the ductile fracture limit strain characteristic data L1c, is used. When the strain gradient is greater than the predetermined value P1 and equal to or less than a predetermined value P2, the edge crack limit strain data L1a, which has a smaller limit strain than the internal crack limit strain characteristic data L1b and the ductile fracture limit strain characteristic data L1c, is used. When the strain gradient is greater than the predetermined value P2, the ductile fracture limit strain characteristic data L1c, which has a smaller limit strain than the edge crack limit strain data L1a and the internal crack limit strain characteristic data L1b, is used.

[0074] At the time of press forming analysis for press forming a press-formed product having an elongation flange portion from a plate-shaped member, the cracks in the elongation flange portion are evaluated using the edge crack limit strain characteristic data L1a, the internal crack limit strain characteristic data L1b, and the ductile fracture limit strain characteristic data L1c. At the time of press forming analysis, the maximum principal strain in each element of the analysis model obtained by finite element division at the end of the elongation flange portion and the strain gradient between the element and the adjacent element in the direction away from the end of the elongation flange portion are calculated. When the maximum principal strain at a predetermined strain gradient in each element becomes equal to or greater than the limit strain of the limit strain characteristic data L1 based on the edge crack limit strain characteristic data L1a, the internal crack limit strain characteristic data L1b, and the ductile fracture limit strain characteristic data L1c, it is evaluated that an elongation flange crack occurs.

[0075] As an elongation flange forming test, for a plurality of plate-like members, other elongation flange forming tests may be used to calculate the limit strain by forming each plate-like member so as to generate cracks at the end of the plate-like member. Also, for the case of using other elongation flange forming tests, it is determined whether the crack state at the time of crack generation is an edge crack or an internal crack, and by calculating the limit strain by the elongation flange forming test and the strain gradient in the internal direction of the plate-like member (the direction perpendicular to the tangent of the end face) from the end face of the plate-like member, the edge crack limit strain characteristic data L1a and the internal crack limit strain characteristic data L1b are calculated. It is also possible to use a plurality of elongation flange forming tests as the elongation flange forming test.

[0076] As an elongation flange forming test, in order to calculate characteristic data showing the relationship between the limit strain and the strain gradient of the plate-like member when the strain gradient is very small, a single-sided punching tensile test can be used to calculate the limit strain by forming each of the plurality of plate-like members so as to generate cracks at the ends of the plate-like members.

[0077] The single-sided punching tensile test can be performed using a tensile test device similar to the tensile test device used in the uniaxial tensile test described above. Also, for the single-sided punching tensile test, as the plate-like member which is the No. 5 test piece in JIS Z2241, a plate-like member having a parallel portion with a predetermined plate width and a predetermined plate thickness at the central portion in the longitudinal direction can be used. In the single-sided punching tensile test, one end face in the plate width direction of the parallel portion is formed by punching so as to give a predetermined punched end face shape, and the other end face in the plate width direction of the parallel portion is formed by machining different from punching, such as mechanical machining. As the punching, for example, a clearance of 12% of the plate thickness can be formed using a die, a blank holder, and a punching punch.

[0078] When using the one-sided punching tensile test as the elongation flange forming test, a crack occurs in the vicinity of the end in the plate width direction of the parallel portion of the plate-like member by performing the tensile test until a crack penetrating the plate-like member in the plate thickness direction occurs. During the test, an image of the surface of the parallel portion of the plate-like member is captured by a camera, and based on the captured image of the surface of the parallel portion of the plate-like member, an operator or the like determines the crack state of the crack occurring at the end of the plate-like member.

[0079] In the one-sided punching tensile test, the maximum principal strain ε0 on the end face of the plate-like member is calculated, and the strain gradient α of the maximum principal strain at a position away from the end face of the plate-like member in the internal direction of the plate-like member (perpendicular to the tangent of the end face) is calculated as the strain gradient α in the vicinity of the end of the plate-like member.

[0080] As the elongation flange forming test, the one-sided punching tensile test is used, and for each of a plurality of plate-like members, it is determined whether the crack state at the time of crack occurrence by the elongation flange forming test is an edge crack or an internal crack, and the limit strain by the elongation flange forming test and the strain gradient in the internal direction of the plate-like member from the end face of the plate-like member (perpendicular to the tangent of the end face) can be calculated.

[0081] As the limit strain characteristic data L1 showing the relationship between the limit strain and the strain gradient of the plate-like member, the edge crack limit strain characteristic data L1a and the internal crack limit strain characteristic data L1b are calculated according to whether the crack state by the actual elongation flange forming test is an edge crack or an internal crack, and the elongation flange crack is evaluated using the edge crack limit strain characteristic data L1a and the internal crack limit strain characteristic data L1b. Therefore, compared with the case of using the limit strain characteristic data considering only the edge crack, the evaluation accuracy of the elongation flange crack can be improved. Further, since the crack of the elongation flange portion is evaluated using the ductile fracture limit strain characteristic data L1c having the strain at the time of ductile fracture by the uniaxial tensile test as the limit strain, it is possible to suppress the limit strain from becoming excessively large even when the strain gradient is large, and the evaluation accuracy of the elongation flange crack can be improved.

[0082] As shown in Fig. 3, when only the limit strain characteristic data L1a considering only edge cracking is used as the limit strain characteristic data L1 showing the relationship between the limit strain and the strain gradient of the plate-like member 20, if the strain gradient is equal to or less than a predetermined value P1, the limit strain is larger than the internal cracking limit strain characteristic data L1b, and even if it is evaluated by press forming analysis that elongation flange cracking does not occur, there is a possibility that elongation flange cracking may occur when actually press forming.

[0083] Also, when only the limit strain characteristic data L1a considering only edge cracking is used, if the strain gradient is larger than a predetermined value P2, the limit strain is larger than the ductile fracture limit strain characteristic data L1c, and even if it is evaluated by press forming analysis that elongation flange cracking does not occur, there is a possibility that elongation flange cracking may occur when actually press forming.

[0084] In the present embodiment, since the edge cracking limit strain characteristic data L1a, the internal cracking limit strain characteristic data L1b, and the ductile fracture limit strain characteristic data L1c are used, the evaluation accuracy of elongation flange cracking can be improved.

[0085] Fig. 14 is a flowchart showing a method for evaluating elongation flange cracking. As shown in Fig. 14, when evaluating the cracking of the elongation flange portion 8 of the press formed product 1, first, an elongation flange forming test is performed on the plate-like member 20 having the hole portion 21 (step S1). During the elongation flange forming test, the cracking state at the time of cracking due to the elongation flange forming test is determined, and it is determined whether the cracking state is edge cracking or internal cracking (step S2). When a hole expanding test is performed as the elongation flange forming test, it is determined whether the cracking state during the hole expanding test is edge cracking or internal cracking.

[0086] During the stretch flange forming test, the limit strain and the strain gradient in the inner direction of the plate-like member from the end face of the plate-like member are also calculated by the stretch flange forming test (step S3). When a hole expansion test is performed as the stretch flange forming test, the strain gradient in the radial direction of the hole is calculated. When cracks occur at the end of the hole 21 during the hole expansion test, based on the image of the surface of the plate-like member 20 captured by the camera 35, the maximum principal strain of the plate-like member 20 is calculated for the entire surface near the hole of the plate-like member 20. As described above, the limit strain and the strain gradient of the plate-like member 20 are calculated from the maximum principal strain of the plate-like member 20 calculated for the entire surface of the plate-like member 20.

[0087] Using a plurality of plate-like members 20, a cylindrical hole expansion test and a conical hole expansion test are performed as hole expansion tests, steps S1 to S3 are repeated for each hole expansion test, the cracking state is determined for each of the plurality of plate-like members 20, and the limit strain and the strain gradient by the hole expansion test are calculated.

[0088] Next, when the cracking state by the stretch flange forming test is an edge crack, based on the limit strain and the strain gradient of each plate-like member 20 when the cracking state is an edge crack, edge crack limit strain characteristic data L1a showing the relationship between the limit strain and the strain gradient of the plate-like member 20 in the edge crack is calculated (step S4).

[0089] Even when the cracking state by the stretch flange forming test is an internal crack, based on the limit strain and the strain gradient of each plate-like member 20 when the cracking state is an internal crack, internal crack limit strain characteristic data L1b showing the relationship between the limit strain and the strain gradient of the plate-like member 20 in the internal crack is calculated (step S5).

[0090] In steps S1 to S5, as the stretch flange forming test, together with a hole expansion test or the like, in order to calculate characteristic data showing the relationship between the limit strain and the strain gradient of the plate-like member when the strain gradient is very small, a single-sided punching tension test may be performed.

[0091] Even when performing a single-sided punching tensile test, a single-sided punching tensile test is conducted (step S1), the cracking state at the time of cracking is determined (step S2), the ultimate strain and the strain gradient in the internal direction of the plate-like member from the end face of the plate-like member are calculated (step S3), together with other elongation flange forming tests such as a hole expansion test, the edge cracking limit strain characteristic data L1a showing the relationship between the ultimate strain and the strain gradient of the plate-like member in edge cracking is calculated (step S4), and the internal cracking limit strain characteristic data L1b showing the relationship between the ultimate strain and the strain gradient of the plate-like member in internal cracking is calculated (step S5).

[0092] When evaluating the cracking of the elongation flange portion 8, a uniaxial tensile test is also performed as a uniaxial tensile deformation test on a plate-like member 50 made of the same material as the plate-like member 20 used in the elongation flange forming test (step S6). During the uniaxial tensile test, the maximum principal strain of the plate-like member 50 is calculated. Until the plate-like member 50 undergoes ductile fracture and cracking occurs during the tensile test, based on the image of the central portion in the longitudinal direction of the plate-like member 50 captured by the camera 45, the maximum principal strain of the central portion in the longitudinal direction of the plate-like member 50 is calculated.

[0093] When the plate-like member 50 undergoes ductile fracture and cracking occurs, the maximum value εmax of the maximum principal strain at the time of fracture of the plate-like member 50 is taken as the strain at the time of ductile fracture by the tensile test, and the ductile fracture limit strain characteristic data L1c in which the strain at the time of ductile fracture of the plate-like member 50 by the tensile test is taken as the ultimate strain regardless of the strain gradient is calculated (step S7).

[0094] Next, based on the edge cracking limit strain characteristic data L1a, the internal cracking limit strain characteristic data L1b, and the ductile fracture limit strain characteristic data L1c, the limit strain characteristic data L1 showing the relationship between the ultimate strain and the strain gradient of the plate-like member is calculated (step S8). The limit strain characteristic data L1 is formed by the limit strain characteristic data with a smaller ultimate strain for a predetermined strain gradient among the edge cracking limit strain characteristic data L1a, the internal cracking limit strain characteristic data L1b, and the ductile fracture limit strain characteristic data L1c.

[0095] The limit strain characteristic data L1 is formed by the internal cracking limit strain characteristic data L1b when the strain gradient is less than or equal to a predetermined value P1, by the edge cracking limit strain characteristic data L1b when the strain gradient is greater than the predetermined value P1 and less than or equal to a predetermined value P2, and by the ductile fracture limit strain characteristic data L1c when the strain gradient is greater than the predetermined value P2.

[0096] Then, using the limit strain characteristic data L1 of the plate-like member calculated in this way, a press forming analysis is performed to form a press-formed product having an elongation flange portion from the plate-like member. The maximum principal strain and the strain gradient between the element and the adjacent element in the direction away from the end of the elongation flange portion are calculated for each element of the analysis model obtained by finite element division of the end of the elongation flange portion. When it is equal to or greater than the limit strain characteristic data L1, it is evaluated that cracking occurs, thereby evaluating the elongation flange cracking (step S9).

[0097] In this embodiment, also, as the plate-like member, instead of the high-tensile steel plate made of the above-described material A having a tensile strength of about 980 MPa or more, for the plate-like members made of material B and material C, which are materials obtained by changing the additive components in the 980 MPa-class high-tensile steel plate similar to material A, similarly, the limit strain characteristic data showing the relationship between the limit strain and the strain gradient of the plate-like member 20 was calculated.

[0098] Regarding the limit strain characteristic data using material B and material C, respectively, similar to the limit strain characteristic data L1 using material A, they are formed by the edge cracking limit strain data, the internal cracking limit strain characteristic data, and the ductile fracture limit strain characteristic data. Regarding the limit strain characteristic data using material B and material C, respectively, similar to the plate-like member using material A, a press forming analysis is performed to form a press-formed product having an elongation flange portion from the plate-like member using the limit strain characteristic data, and the elongation flange cracking can be evaluated.

[0099] Thus, as limit strain characteristic data indicating the relationship between the limit strain and strain gradient of the plate-like member, in accordance with the internal cracking or edge cracking which is the state of cracking in the actual stretch flange forming test, the limit strain characteristic data of internal cracking and the limit strain characteristic data of edge cracking are calculated, and since the stretch flange cracking is evaluated using the limit strain characteristic data of internal cracking and edge cracking, the evaluation accuracy of stretch flange cracking can be improved as compared with the case of using the limit strain characteristic data considering only edge cracking. Further, since the cracking of the stretch flange portion is evaluated using the ductile fracture limit strain characteristic data with the strain at the time of ductile fracture in the uniaxial tensile deformation test as the limit strain, it is possible to suppress the limit strain from becoming excessively large even when the strain gradient is large, and the evaluation accuracy of stretch flange cracking can be improved.

[0100] In the present embodiment, as the hole expansion test, the cylindrical hole expansion test and the conical hole expansion test are used to obtain the limit strain characteristic data of internal cracking and the limit strain characteristic data of edge cracking. However, it is also possible to use a spherical head hole expansion test using a spherical head punch having a spherical head-shaped tip to determine the state of cracking and obtain the limit strain characteristic data indicating the relationship between the limit strain and strain gradient of the plate-like member.

[0101] As the internal cracking limit strain characteristic data L1b, from the calculation results of the maximum principal strain and strain gradient in the hole expansion test, an approximate formula of a quadratic function showing the relationship between the strain gradient and the maximum principal strain in the case of internal cracking is calculated. However, it is also possible to calculate an approximate formula of a linear function to calculate the internal cracking limit strain characteristic data.

[0102] In the present embodiment, as the material of the plate-like member, a 980 MPa grade high-tensile steel sheet is used, but it can be similarly applied to other metal plates such as other steel sheets and aluminum alloy plates. Also, regarding the stretch flange forming test, it is also possible to perform a plurality of stretch flange forming tests using a plurality of plate-like members under the same conditions and use the average value of the maximum principal strain of the plurality of stretch flange forming tests.

[0103] As the limit strain characteristic data L1, when the crack state at the time of crack generation in the stretch flange forming test is an edge crack or an internal crack, the limit strain characteristic data formed by the edge crack limit strain data, the internal crack limit strain characteristic data, and the ductile fracture limit strain characteristic data is used. However, when the crack state at the time of crack generation in the stretch flange forming test is only an edge crack, the edge crack limit strain data and the ductile fracture limit strain characteristic data are used. As the limit strain characteristic data L1, at least the edge crack limit strain characteristic data and the ductile fracture limit strain characteristic data among the edge crack limit strain characteristic data, the internal crack limit strain characteristic data, and the ductile fracture limit strain characteristic data are used.

[0104] In this embodiment, the limit strain characteristic data showing the relationship between the limit strain and the strain gradient, which is the strain gradient of the maximum strain, is calculated using the maximum principal strain when a crack occurs in the stretch flange forming test. However, it is also possible to calculate the limit strain characteristic data showing the relationship between the limit strain and the strain gradient using the maximum principal strain immediately before a crack occurs in the stretch flange forming test.

[0105] In this embodiment, in the stretch flange forming test, the surface of the plate-like member is imaged by a camera, and the limit strain and the strain gradient are calculated based on the image of the imaged surface. However, the limit strain and the strain gradient may be calculated using other methods such as the scribed circle method. For the uniaxial tensile deformation test, the strain at the time of ductile fracture may also be calculated using other methods such as the scribed circle method. Also, in the stretch flange forming test, the crack state is determined based on the image of the surface imaged by the camera, but it may also be determined using other methods.

[0106] As described above, the method for evaluating elongation flange cracking according to this embodiment performs an elongation flange forming test on each of a plurality of plate-like members 20, determines whether the cracking state at the time of cracking in the elongation flange forming test is edge cracking or internal cracking, calculates the ultimate strain and strain gradient in the elongation flange forming test, and calculates edge cracking ultimate strain characteristic data L1a and internal cracking ultimate strain characteristic data L1b as ultimate strain characteristic data L1 indicating the relationship between the ultimate strain and strain gradient of the plate-like member 20. Further, a uniaxial tensile deformation test is performed on the plate-like member 50, and ductile fracture ultimate strain characteristic data L1c with the strain at the time of ductile fracture in the uniaxial tensile deformation test as the ultimate strain is calculated. Then, the cracking of the elongation flange portion is evaluated using at least the edge cracking ultimate strain characteristic data and the ductile fracture ultimate strain characteristic data among the edge cracking ultimate strain characteristic data L1a, the internal cracking ultimate strain characteristic data L1b, and the ductile fracture ultimate strain characteristic data L1c.

[0107] Accordingly, as the ultimate strain characteristic data L1 indicating the relationship between the ultimate strain and strain gradient of the plate-like member, edge cracking ultimate strain characteristic data L1a and internal cracking ultimate strain characteristic data L1b are calculated according to whether the cracking state in the actual elongation flange forming test is edge cracking or internal cracking, and the elongation flange cracking is evaluated using the edge cracking ultimate strain characteristic data L1a and the internal cracking ultimate strain characteristic data L1b. Therefore, compared with the case of using the ultimate strain characteristic data considering only edge cracking, the evaluation accuracy of elongation flange cracking can be improved. Further, since the cracking of the elongation flange portion is evaluated using the ductile fracture ultimate strain characteristic data L1c with the strain at the time of ductile fracture in the uniaxial tensile deformation test as the ultimate strain, it is possible to suppress the ultimate strain from becoming excessively large even when the strain gradient is large, and the evaluation accuracy of elongation flange cracking can be improved. For a plate-like member in which the cracking state at the time of cracking is only edge cracking, the evaluation accuracy of elongation flange cracking can be improved by using the edge cracking ultimate strain characteristic data and the ductile fracture ultimate strain characteristic data.

[0108] In addition, the elongation flange crack evaluation method evaluates the cracks in the elongation flange portion by using edge crack limit strain characteristic data, internal crack limit strain characteristic data, and ductile fracture limit strain characteristic data. Thereby, for a plate-like member in which the crack state at the time of crack generation is an edge crack or an internal crack, the evaluation accuracy of the elongation flange crack can be improved.

[0109] In addition, in the press forming analysis for press forming a press formed product having an elongation flange portion from a plate-like member, the elongation flange crack evaluation method uses at least the edge crack limit strain characteristic data L1a and the ductile fracture limit strain characteristic data L1c among the edge crack limit strain characteristic data L1a, the internal crack limit strain characteristic data L1b, and the ductile fracture limit strain characteristic data L1c to evaluate the crack in the elongation flange portion. Thereby, at the time of press forming analysis, by calculating the maximum principal strain in each element of the analysis model obtained by finite element division for the end portion of the elongation flange portion and the strain gradient between the element and the element adjacent in the direction away from the end portion of the elongation flange portion, the crack in the elongation flange portion in the press forming analysis can be accurately evaluated.

[0110] In addition, the elongation flange crack evaluation method images the surface of the plate-like member 20 by the camera 35 in the elongation flange forming test, and calculates the limit strain and the strain gradient of the plate-like member 20 by the elongation flange forming test based on the image of the surface of the plate-like member 20 imaged by the camera 35. Thereby, the limit strain and the strain gradient can be calculated by using the digital image correlation method, and it is possible to accurately calculate the limit strain and the strain gradient.

[0111] In addition, the method for evaluating the elongation flange crack images the surface of the plate-like member 20 by the camera 35 in the elongation flange forming test, and determines whether the crack state at the time of crack generation in the elongation flange forming test for the plate-like member 20 is an edge crack or an internal crack based on the image of the surface of the plate-like member 20 captured by the camera 35. By finding the crack initiation point at the time of crack generation based on the image of the surface of the plate-like member 20, it is possible to improve the determination accuracy of the crack state of the edge crack or the internal crack and improve the evaluation accuracy of the flange crack.

[0112] In addition, the method for evaluating the elongation flange crack images the surface of the plate-like member 50 by the camera 45 in the uniaxial tensile deformation test, and calculates the strain at the time of ductile fracture in the uniaxial tensile deformation test for the plate-like member 50 based on the image of the surface of the plate-like member 50 captured by the camera 45. Thereby, the strain at the time of ductile fracture can be calculated using the digital image correlation method, and it is possible to accurately calculate the ductile fracture limit strain. Based on the image captured by the camera, the limit strain and the strain gradient in the elongation flange forming test for the plate-like member are calculated, and the strain at the time of ductile fracture in the uniaxial tensile deformation test for the plate-like member is calculated. Thus, the limit strain and the strain gradient can be calculated using the digital image correlation method, and the ductile fracture limit strain can be calculated, and the evaluation accuracy of the elongation flange crack can be improved.

[0113] The present invention is not limited to the illustrated embodiments, and various improvements and design changes are possible without departing from the gist of the present invention.

Industrial Applicability

[0114] As described above, according to the present invention, since the evaluation accuracy of the elongation flange crack can be improved, it is suitably applicable when simulating the press forming for forming a press-formed product having an elongation flange portion from a plate-like member.

Explanation of Signs

[0115] 1 Press-formed product 8 Elongation flange part 10 Hole expansion test device 14, 15 Punches 20, 50 Plate-like members 22 Crack 30, 60 Control units 35, 45 Cameras 40 Tensile test device L1 Limit strain characteristic data L1a Edge crack limit strain characteristic data L1b Internal crack limit strain characteristic data L1c Ductile fracture limit strain characteristic data

Claims

1. A method for evaluating cracking in the stretch flange portion of a press-formed product, comprising: Performing a stretch flange forming test on a plurality of plate-like members, in which each plate-like member is formed so as to generate a crack at an end thereof to calculate a limit strain; Performing a uniaxial tensile deformation test on a plate-like member made of the same material as the plurality of plate-like members to calculate the strain at ductile fracture; For each of the plurality of plate-like members, determining whether the crack state at the time of crack generation by the stretch flange forming test is an edge crack or an internal crack, and calculating the limit strain by the stretch flange forming test and the strain gradient in the internal direction of the plate-like member from the end face of the plate-like member; Based on the limit strain and strain gradient of each plate-like member when the crack state by the stretch flange forming test is an edge crack, calculating edge crack limit strain characteristic data indicating the relationship between the limit strain and strain gradient of the plate-like member in edge cracking; Based on the limit strain and strain gradient of each plate-like member when the crack state by the stretch flange forming test is an internal crack, calculating internal crack limit strain characteristic data indicating the relationship between the limit strain and strain gradient of the plate-like member in internal cracking; Calculating ductile fracture limit strain characteristic data in which the strain at ductile fracture by the uniaxial tensile deformation test for the plate-like member is taken as the limit strain regardless of the strain gradient; Evaluating cracking in the stretch flange portion using at least the edge crack limit strain characteristic data and the ductile fracture limit strain characteristic data among the edge crack limit strain characteristic data, the internal crack limit strain characteristic data, and the ductile fracture limit strain characteristic data; A method for evaluating stretch flange cracking.

2. Evaluating cracking in the stretch flange portion using the edge crack limit strain characteristic data, the internal crack limit strain characteristic data, and the ductile fracture limit strain characteristic data; The method for evaluating stretch flange cracking according to Claim 1.

3. During press forming analysis of a press-formed product having a stretch flange portion from a plate-like member, evaluating cracking in the stretch flange portion using at least the edge crack limit strain characteristic data and the ductile fracture limit strain characteristic data among the edge crack limit strain characteristic data, the internal crack limit strain characteristic data, and the ductile fracture limit strain characteristic data; The method for evaluating stretch flange cracking according to Claim 1 or Claim 2.

4. In the elongation flange forming test, image the surface of the plate-like member with a camera, Based on the image of the surface of the plate-like member captured by the camera, calculate the ultimate strain and strain gradient of the plate-like member by the elongation flange forming test, The method for evaluating elongation flange cracking according to any one of claims 1 to 3.

5. In the elongation flange forming test, image the surface of the plate-like member with a camera, Based on the image of the surface of the plate-like member captured by the camera, determine whether the cracking state at the time of crack generation in the plate-like member by the elongation flange forming test is edge cracking or internal cracking, The method for evaluating elongation flange cracking according to any one of claims 1 to 4.

6. In the uniaxial tensile deformation test, image the surface of the plate-like member with a camera, Based on the image of the surface of the plate-like member captured by the camera, calculate the strain at the time of ductile fracture of the plate-like member by the uniaxial tensile deformation test, The method for evaluating elongation flange cracking according to any one of claims 1 to 5.

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