JP2023150334A - Elongation flange crack evaluation method - Google Patents

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

The present invention relates to a stretch flange crack evaluation method.

When manufacturing a press-formed product by press-forming a metal plate-shaped member, the press-forming of the plate-shaped member is simulated in advance using a press-forming analysis using the finite element method (FEM analysis). Molded products are evaluated for cracks to determine whether or not they can be press-formed.

When using a plate-shaped member such as a high-strength steel plate having a tensile strength of about 980 MPa or more, for example, the stretch flange part that is stretch-flange formed in press forming is accompanied by stretch flange deformation, so the plate is deformed during deformation. Cracks may occur from the ends of shaped members, and in order to evaluate and predict such cracks in advance, stretch flange cracks in press-formed products are evaluated using press forming analysis.

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

Patent No. 6852426

The critical strain characteristic data that indicates the relationship between the critical strain and the strain gradient of a plate-shaped member described in Patent Document 1 is obtained by calculating the fracture limit based on the crack at the end of the hole in the hole expansion test, and determining the crack occurrence. The critical strain characteristic data is calculated by considering only the edge crack located at the edge of the hole.

However, when stretch flange cracking is evaluated by press forming analysis using limit strain characteristic data that takes only edge cracking into consideration, even if the press forming analysis evaluates that stretch flange cracking does not occur, it is not edge cracking. In some cases, stretch flange cracking may occur during actual press forming.

Additionally, when evaluating stretch flange cracking through press forming analysis using critical strain characteristic data that takes into account only edge cracking, where the critical strain increases as the strain gradient increases, the critical strain may be excessive depending on the size of the strain gradient. Even if it is evaluated by press forming analysis that stretch flange cracking will not occur, there is a risk that stretch flange cracking will occur during actual press forming.

An object of the present invention is to provide a stretch flange crack evaluation method that can improve the evaluation accuracy of stretch flange cracks.

The present invention is a stretch flange crack evaluation method for evaluating cracks in the stretch flange portion of a press-formed product, in which a plurality of plate-shaped members are formed so as to generate cracks at the ends of each of the plate-shaped members to reach a limit. A stretch flange forming test was performed to calculate the strain, and a uniaxial tensile deformation test was performed to calculate the strain at ductile fracture by subjecting a plate member made of the same material as the plurality of plate members to uniaxial tensile deformation. For each of the plate-shaped members, it is determined whether the cracking state at the time of crack occurrence in the stretch-flange forming test is an edge crack or an internal crack, and the limit strain in the stretch-flange forming test and the The strain gradient in the internal direction of the plate-shaped member is calculated, and based on the limit strain and strain gradient of each plate-shaped member when the crack state in the stretch flange forming test is edge cracking, the plate-shaped member with edge cracking is calculated. Calculate the edge crack critical strain characteristic data that shows the relationship between the critical strain and strain gradient, and based on the critical strain and strain gradient of each plate member when the crack state in the stretch flange forming test is an internal crack. , Calculate internal crack limit strain characteristic data that shows the relationship between the limit strain and strain gradient of a plate-like member at internal cracking, and calculate the strain at ductile failure in the uniaxial tensile deformation test for the plate-like member regardless of the strain gradient. Calculate ductile fracture critical strain characteristic data to be the critical strain, and calculate at least the edge crack critical strain characteristic data and the A stretch flange crack evaluation method is provided that evaluates cracks in a stretch flange portion using ductile fracture limit strain characteristic data.

According to the present invention, a stretch flange forming test is performed on each of a plurality of plate-shaped members, and it is determined whether the crack state at the time of crack occurrence in the stretch flange forming test is an edge crack or an internal crack, and the stretch flange forming test is performed. The critical strain and strain gradient are calculated, and edge crack critical strain characteristic data and internal crack critical strain characteristic data are calculated. In addition, a uniaxial tensile deformation test is performed on the plate member, and ductile fracture limit strain characteristic data is calculated, with the strain at ductile fracture in the uniaxial tensile deformation test as the limit strain. Then, cracks in the stretch flange portion are 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, internal cracking limit strain characteristic data, and ductile fracture limit strain characteristic data.

As a result, the critical strain characteristic data indicating the relationship between the critical strain and the strain gradient of the plate-like member can be used as the critical strain characteristic data for edge cracking according to the cracking conditions in the actual stretch flange forming test, such as edge cracking or internal cracking. Since the internal crack limit strain characteristic data is calculated and the edge crack limit strain characteristic data and the internal crack limit strain characteristic data are used to evaluate stretch flange cracking, it is easier to evaluate stretch flange cracking than when using limit strain characteristic data that takes only edge cracks into account. As a result, the evaluation accuracy of stretch flange cracking can be improved. Furthermore, since we evaluate cracks in the stretch flange using ductile fracture critical strain characteristic data in which the critical strain is the strain at ductile fracture in a uniaxial tensile deformation test, the critical strain may be excessively large even when the strain gradient is large. Therefore, it is possible to improve the evaluation accuracy of stretch flange cracking. For a plate-shaped member in which the only cracking state at the time of cracking is edge cracking, the accuracy of evaluating stretch flange cracking can be improved by using edge cracking limit strain characteristic data and ductile fracture limit strain characteristic data.

The stretch flange crack evaluation method evaluates cracks in a 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.

With this configuration, cracks in the stretch flange portion are evaluated using edge crack limit strain characteristic data, internal crack limit strain characteristic data, and ductile fracture limit strain characteristic data, so that the crack state at the time of crack occurrence is determined to be edge crack or internal crack. For plate-shaped members that are cracked, it is possible to improve the evaluation accuracy of stretch flange cracks.

The stretch flange crack evaluation method includes the edge crack limit strain characteristic data, the internal crack limit strain characteristic data, and the ductile fracture limit strain during press forming analysis of press forming a press molded product having a stretch flange portion from a plate member. Cracks in the stretch flange portion are evaluated using at least the edge crack limit strain characteristic data and the ductile fracture limit strain characteristic data among the characteristic data.

With this configuration, at the time of press forming analysis, 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, internal cracking limit strain characteristic data, and ductile fracture limit strain characteristic data are used. To evaluate cracks in the flange part, during press forming analysis, the maximum principal strain in each element of the analysis model divided into finite elements for the end of the stretch flange part and the adjacent element in the direction away from the end of the stretch flange part are calculated. By calculating the strain gradient with respect to the elements, it is possible to accurately evaluate cracks in the stretch flange portion in press forming analysis.

The stretch flange crack evaluation method includes capturing an image of the surface of the plate-like member using a camera in the stretch-flange forming test, and determining the above-mentioned conditions for the plate-like member based on the image of the surface of the plate-like member captured by the camera. It is preferable to calculate the critical strain and strain gradient by a stretch flange forming test.

With this configuration, the critical strain and strain gradient in the stretch flange forming test for the plate-like member are calculated based on the image of the surface of the plate-like member captured by the camera, so the digital image correlation method (DIC) ) can be used to calculate the critical strain and strain gradient, and it is possible to calculate the critical strain and strain gradient with high accuracy.

The stretch flange crack evaluation method includes capturing an image of the surface of the plate-like member using a camera in the stretch-flange forming test, and determining the above-mentioned conditions for the plate-like member based on the image of the surface of the plate-like member captured by the camera. It is preferable to determine whether the crack state at the time of crack occurrence is an edge crack or an internal crack by a stretch flange forming test.

With this configuration, based on the image of the surface of the plate-shaped member taken by the camera, it is determined whether the crack state at the time of cracking in the stretch flange forming test is an edge crack or an internal crack. By finding the starting point of a crack when a crack occurs based on an image of the surface, it is possible to improve the accuracy of determining the crack state of edge cracks or internal cracks, and improve the evaluation accuracy of stretch flange cracks.

The stretch flange crack evaluation method includes capturing an image of the surface of the plate member in the uniaxial tensile deformation test using a camera, and evaluating the plate member based on the image of the surface of the plate member taken by the camera. It is preferable to calculate the strain at the time of ductile failure by the uniaxial tensile deformation test.

With this configuration, the strain at the time of ductile failure in the uniaxial tensile deformation test is calculated for the plate-like member based on the image of the surface of the plate-like member taken by the camera, so the strain at the time of ductile failure in the uniaxial tensile deformation test is calculated using the digital image correlation method. It is possible to calculate the strain at the ductile fracture limit with high accuracy. Based on the images captured by the camera, we calculated the critical strain and strain gradient of the plate-shaped member in the stretch flange forming test, and also calculated the strain at the time of ductile failure in the uniaxial tensile deformation test of the plate-shaped member. By using the digital image correlation method, it is possible to calculate the critical strain and the strain gradient as well as the critical strain for ductile fracture, and it is possible to improve the evaluation accuracy of stretch flange cracking.

It is a figure which shows the press molded product which has a stretch flange part. It is an explanatory view for explaining edge cracking and internal cracking of a stretch flange part. It is a graph showing the relationship between the critical strain and strain gradient of a plate member used for evaluation of stretch flange cracking according to an embodiment of the present invention. FIG. 2 is a schematic configuration diagram of a cylindrical hole expansion test. It is a perspective view which shows the plate-shaped member and punch after a cylindrical hole expansion test. It is a figure which shows the maximum principal strain in the vicinity of the hole part of a plate-shaped member. It is a graph showing the relationship between the distance from the hole center of the plate-shaped member and the maximum principal strain. It is a graph which shows the relationship between the distance from the end surface of a plate-shaped member, and the maximum principal strain. It is a schematic block diagram of a conical hole expansion test. It is a perspective view which shows the plate-shaped member and punch after a conical hole expansion test. It is a schematic block diagram of a uniaxial tensile test. It is a figure which shows the maximum principal strain of the longitudinal direction central part of a plate-shaped member. It is a graph which shows the relationship between the board width direction position of a plate-like member, and the maximum principal strain. 2 is a flowchart showing a method for evaluating stretch flange cracking.

Embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a diagram showing a press-formed product having a stretch flange portion. The press-formed product 1 shown in FIG. 1 is a press-formed product 1 that is 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, for example. A center pillar outer 1 is shown as a press-formed product 1, which constitutes a side surface of a vehicle body and is disposed between front and rear door openings.

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 longitudinal direction of the vehicle body and attached to the roof side rail, and a lower end portion 4 extending in the longitudinal direction of the vehicle body and attached to the side sill. ing. The center portion 2 of the center pillar outer 1 has a bottom portion 5, side portions 6 on both sides, and flange portions 7 on both sides, and has a substantially hat-shaped cross section.

The flange portion 7 of the center pillar outer 1 is provided so as to extend from the center portion 2 to the upper end portion 3 and the lower end portion 4, and is provided at the connection portion between the center portion 2, the upper end portion 3, and the lower end portion 4 during press molding. It has a stretch flange portion 8 which is stretch flange formed. When the stretch flange portion 8 is subjected to stretch flange forming, the strain becomes large and cracks are likely to occur.

FIG. 2 is an explanatory diagram for explaining edge cracks and internal cracks of the stretch flange portion. FIG. 2 shows an enlarged view of a part of the press-formed product shown in FIG. 1, and also shows cracks that may occur at the end of the stretch flange portion 8. In FIG. 2(a), the crack occurrence position indicated by the white arrow indicates an edge crack, which is the end face of the stretch flange part 8. In FIG. 2(b), the crack occurrence position indicated by the white arrow indicates an edge crack from the end face of the stretch flange part 8. It shows an internal crack. As shown in FIGS. 2(a) and 2(b), cracks such as edge cracks or internal cracks may occur in the stretch flange portion 8.

In this embodiment, when a press-formed product is press-formed from a plate-shaped member, the limit strain characteristic indicating the relationship between the limit strain and strain gradient of the plate-shaped member is used as the fracture limit characteristic of the plate-shaped member used for press forming analysis. By calculating the data according to the actual cracking state from the stretch flange forming test and calculating the ductile fracture limit strain from the uniaxial tensile deformation test, press forming analysis can be used to prevent stretch flange cracking of press-formed products. Improve evaluation accuracy. The stretch flange forming test is a stretch flange forming test in which a plate member is formed so as to generate a crack at the end of the plate member and the critical strain is calculated. The uniaxial tensile deformation test is a uniaxial tensile deformation test in which a plate member is uniaxially tensile deformed and the strain at the time of ductile failure is calculated.

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

In this embodiment, a hole expansion test is used as a stretch flange forming test, although it is not limited to this, and a uniaxial tensile test is used as a uniaxial tensile deformation test, although it is not limited to this. there was. The hole expansion test was conducted by using a plate-shaped member having a hole and pushing a punch into the hole until a crack generated at the end of the hole penetrated in the thickness direction. The uniaxial tensile test uses a plate member that is a tensile test piece made of the same material as the plate member used for the hole expansion test, and applies a tensile load to both ends of the plate member until it breaks. went. As the plate member, a high tensile strength steel plate made of material A and having a tensile strength of about 980 MPa or more was used.

FIG. 4 is a schematic diagram of the cylindrical hole expansion test. In the evaluation of stretch flange cracking, a hole expansion test is performed on each of multiple plate members using a plate member with a hole, and the state of cracking is determined by the hole expansion test, and the limit strain and strain of the plate member are determined. Obtain critical strain characteristic data that shows the relationship with the slope.

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 member 20 having a hole 21 to generate a crack near the hole. . The press tool 11 includes a die 12 and a blank holder 13 that hold the plate-shaped member 20, and a punch 14 that press-forms the plate-shaped member 20.

In the hole expansion test, a plate member 20 having a circular hole 21 is held between the die 12 and the blank holder 13 with a predetermined pressing force, and the punch 14 is moved until a crack occurs near the hole. let 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 its central axis coincides with the hole 21 of the plate-shaped member 20.

The hole expanding 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 equipped with a control unit 30, and the control unit 30 controls the operation of the moving mechanism.

The hole expansion test device 10 is also provided with a camera 35 as an imaging device that images the surface near the hole of the plate member 20, and a camera 35 is provided on the opposite side of the punch 14 so as to image the surface of the plate member 20. Two cameras 35 are arranged above the plate member 20. The two cameras 35 are arranged at symmetrical positions with respect to the central axis of the hole 21, which coincides with the central axis of the punch 14, for example.

Images of the surface of the plate member 20 captured by the two cameras 35 are input to the control unit 30. The control unit 30 stores the image captured by the camera 35 in a storage device such as a memory, and calculates the maximum principal strain for the entire surface of the plate member 20 in the vicinity of the hole by analysis using the digital image correlation method. It is supposed to be done.

The control unit 30 uses two cameras 35 to image the paint dot pattern sprayed onto the surface of the plate member 20 before the hole expansion test, and detects changes in the positional relationship of the dot patterns based on the before and after images taken. The maximum principal strain on the surface of the plate member 20 is calculated from. The control unit 30 also displays the image captured by the camera 35 and the maximum principal strain near the hole of the plate member 20 on a display device (not shown) such as a display.

In the cylindrical 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, a punch outer diameter D2 was 50 mm, and a punch shoulder radius R2 was used. A punch 14 with a diameter of 10 mm was used. The thickness t of the plate member 20 was 1.6 mm, and the initial hole diameter D3 of the hole portion 21 was various hole diameters such as 20 mm. The hole 21 of the plate member 20 was punched out using a die, a blank holder, and a punch with a clearance of 12% of the plate thickness.

FIG. 5 is a perspective view showing the plate member and punch after the cylindrical hole expansion test. FIG. 5 shows the plate member 20 and punch 14 after a hole expansion test in which the punch 14 was moved until a crack that penetrated the plate member 20 in the thickness direction occurred. As shown in FIG. 5, after the hole expansion test, the vicinity of the hole in the plate member 20 is deformed by the punch 14, and a crack 22 is generated in the vicinity of the hole in the plate member 20.

In this embodiment, an image of the surface near the hole of the plate-like member 20 is captured by the camera 35 during the hole expansion test, and based on the image of the surface near the hole of the plate-like member 20, an operator, etc. The crack state of the crack 22 occurring at the end of the plate member 20 is determined. As for the crack state, if the crack occurrence position, which is the starting point of the crack, is the crack occurrence position C1, which is the end face of the hole 21, it is determined that it is an edge crack, and the crack occurrence position, which is the starting point of the crack, is determined to be an edge crack. If the crack occurrence position C2 is inside the plate member 20 away from the end face, it is determined that the crack is an internal crack.

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

In the hole expansion test, the maximum principal strain occurs in the circumferential direction of the hole 21 in the vicinity of the hole of the plate member 20 . As shown in FIG. 6, the maximum principal strain near the hole of the plate-like member 20 is from the end face of the hole 21, which is the end face of the plate-like member 20, to the radially outer side of the hole 21, which is the inside of the plate-like member 20. The maximum principal strain becomes smaller as the distance increases. In the plate-shaped member 20 in which cracks have occurred at two locations in the circumferential direction at the end of the hole 21, as shown in FIG. It can be seen that the maximum principal strain becomes smaller as the distance from No. 21 increases in the radial direction.

FIG. 7 is a graph showing the relationship between the distance from the center of the hole in the plate member and the maximum principal strain. In FIG. 7, the maximum principal strain calculated for the entire surface near the hole of the plate 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 the maximum principal strain calculated in the circumferential direction inside the plate member 20 away from the end surface of the hole 21 is relatively large. The maximum principal strain as a whole has a maximum principal strain distribution that decreases as the distance from the hole 21 increases in the radial direction.

FIG. 8 is a graph showing the relationship between the distance from the end surface of the plate member and the maximum principal strain. In FIG. 8, the horizontal axis represents the distance from the end surface of the hole 21 as the end surface of the plate member 20, and the maximum principal strain is represented on the vertical axis, and the maximum principal strain is shown as a black square. The maximum principal strain in the hole 21 of the plate-like member 20 at a predetermined distance from the end face of the hole 21, which is the end face of the plate-like member 20, in the radial direction (the direction perpendicular to the tangent to the end face), which is the inside direction of the plate-like member 20, is The average value of all the maximum principal strains in the circumferential direction at a predetermined distance radially outward from the end face of is calculated as the maximum principal strain.

As shown in FIG. 8, the maximum principal strain of the plate member 20 in the hole expansion test becomes smaller as the distance from the end surface of the plate member 20 increases. In this embodiment, the maximum principal strain ε0 at the end face of the plate member 20 is calculated, and the maximum principal strain at a position away from the end face of the plate member 20 is calculated as the strain gradient α near the hole of the plate member 20. Calculate the slope α.

As the strain gradient α, the absolute value of the slope of the maximum principal strain between the distances d1 and d2 from the end surface of the plate-shaped member 20 was calculated as the strain gradient. As shown in FIG. 8, the slope [(ε2-ε1)/(d2-d1)] of the straight line connecting the maximum principal strains ε1 and ε2 at distances d1 and d2 from the end surface of the plate-shaped member 20 is calculated, and the straight line The absolute value of the slope was calculated as the strain gradient α. Note that the strain gradient α may be calculated from the slope of the maximum principal strain in another predetermined range apart from the end surface of the plate-like member 20.

In this way, a cylindrical hole expansion test is carried out, and the state of the crack at the time of crack occurrence is determined as edge cracking or internal cracking. The critical strain ε0 of the plate-shaped member 20 is calculated, and the strain gradient α near the hole of the plate-shaped member 20 is calculated.

A cylindrical hole expansion test is performed on a plurality of plate-shaped members 20 by changing the hole diameter, etc., and edge cracking or internal cracking is determined as the state of cracking at the time of cracking. The maximum principal strain at the end face of the portion 21 is calculated as the critical strain ε0 of the plate-like member 20, and the strain gradient α near the hole of the plate-like member 20 is calculated.

FIG. 9 is a schematic diagram of the conical hole expansion test. In this embodiment, a conical punch is used instead of a cylindrical punch in the hole expansion test device 10 described above, and a conical hole expansion test is performed in addition to a cylindrical hole expansion test. For each of the plate-shaped members 20, the state of cracking was determined by the hole expansion test, and the maximum principal strain at the end face of the hole in the plate-shaped member at the time of cracking was calculated as the limit strain of the plate-shaped member. Calculate the strain gradient near the hole in the member.

A conical hole expansion test device as a hole expansion test device has the same configuration 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 a plate member 20 having a hole 21 to generate cracks near the hole. The press tool 11 includes a die 12 and a blank holder 13 that hold 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 its central axis coincides with the hole 21 of the plate member 20.

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, a punch outer diameter D2 was 50 mm, and a punch apex angle θ1 was used. Punch 15 was used with the angle of 120 degrees. The thickness t of the plate member 20 was 1.6 mm, and the initial hole diameter D3 of the hole portion 21 was various hole diameters such as 20 mm. The hole 21 of the plate member 20 was punched out using a die, a blank holder, and a punch with a clearance of 12% of the plate thickness.

FIG. 10 is a perspective view showing the plate member and punch after the conical hole expansion test. FIG. 10 shows the plate-shaped member 20 and punch 15 after a hole expansion test in which the punch 15 was moved until a crack that penetrated the plate-shaped member 20 in the thickness direction occurred. As shown in FIG. 10, after the hole expansion test, the vicinity of the hole in the plate member 20 is deformed by the punch 15, and a crack 22 is generated in the vicinity of the hole in the plate member 20.

Regarding the conical hole expansion test, an image of the surface near the hole of the plate-like member 20 is taken by the camera 35 during the hole expansion test, and the work is performed based on the imaged image of the surface near the hole of the plate-like member 20. A person or the like determines the crack state of the crack 22 occurring at the end of the plate-shaped member 20. As for the crack state, if the crack occurrence position, which is the starting point of the crack, is the crack occurrence position C1, which is the end face of the hole 21, it is determined that it is an edge crack, and the crack occurrence position, which is the starting point of the crack, is determined to be an edge crack. If the crack occurrence position C2 is inside the plate member 20 away from the end face, it is determined that the crack is an internal crack.

Regarding the conical hole expansion test, in the same manner as the cylindrical hole expansion test, edge cracking or internal cracking is determined as the state of cracking when cracking occurs, and the plate-like As the maximum principal strain at a predetermined distance in the radial direction (direction perpendicular to the tangent of the end surface), which is the internal direction of the member 20, the maximum principal strain in all the circumferential directions at a predetermined distance radially outward from the end surface of the hole 21 of the plate-shaped member 20 Calculate the average value of the principal strains as the maximum principal strain.

Then, the maximum principal strain ε0 at the end face of the plate 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 member 20 is calculated as the strain gradient α near the hole of the plate member 20. calculate. Regarding the conical hole expansion test, the absolute value of the slope of the maximum principal strain between the distances d1 and d2 from the end surface of the plate-like member 20 was calculated as the strain gradient α. Note that the strain gradient α may be calculated from the slope of the maximum principal strain in another predetermined range apart from the end surface of the plate-like member 20.

In this way, a conical hole expansion test is carried out to determine whether the crack is in the state of cracking at the time of cracking, edge cracking or internal cracking, and the maximum principal strain ε0 at the end face of the hole 21 of the plate member 20 at the time of cracking is determined. The critical strain ε0 of the plate-shaped member 20 is calculated, and the strain gradient α near the hole of the plate-shaped member 20 is calculated.

A conical hole expansion test is performed on a plurality of plate-shaped members 20 by changing the hole diameter, etc., and edge cracking or internal cracking is determined as the state of cracking at the time of cracking. The maximum principal strain ε0 at the end face of the portion 21 is calculated as the critical strain ε0 of the plate-like member 20, and the strain gradient α near the hole of the plate-like member 20 is calculated.

In FIG. 3, the horizontal axis represents the strain gradient α, and the maximum principal strain ε0 at the end face of the plate member 20 is represented as the critical strain on the vertical axis. From the calculation results of the maximum principal strain as the critical strain and the strain gradient in the cylindrical hole expansion test and the conical hole expansion test, it is found that when the strain gradient is relatively large, edge cracking occurs in the plate member 20, and the strain gradient is relatively large. It has been found that when the difference is small, internal cracks have occurred in the plate-shaped member 20.

In this embodiment, based on the calculation results of the maximum principal strain and strain gradient from the hole expansion test, an approximation 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 using the known least squares method. Edge crack limit strain characteristic data L1a indicating the relationship between the limit strain and strain gradient of the plate member 20 at edge cracks is calculated. The edge cracking critical strain characteristic data L1a indicating the relationship between the critical strain and the strain gradient of the plate member 20 has a critical strain characteristic such that the larger the strain gradient is, the larger the critical strain is.

In addition, based on the calculation results of the maximum principal strain and strain gradient from the hole expansion test, an approximate expression of a quadratic function showing the relationship between the strain gradient and the maximum principal strain in the case of internal cracking was calculated using the known least squares method. Then, internal crack limit strain characteristic data L1b indicating the relationship between the limit strain and strain gradient of the plate-like member at internal cracks is calculated. Internal cracking critical strain characteristic data L1b indicating the relationship between the critical strain and the strain gradient of the plate member 20 has critical strain characteristics such that the greater the strain gradient, the greater the critical strain.

FIG. 11 is a schematic diagram of a uniaxial tensile test. In the evaluation of stretch flange cracking, a uniaxial tensile test was conducted on a plate member made of the same material as the plate member used in the hole expansion test, using a plate member that is a No. 5 test piece in JIS Z2241. A member is subjected to uniaxial tensile deformation to obtain ductile fracture limit strain characteristic data, where the strain at ductile fracture is the limit strain regardless of the strain gradient.

As shown in FIG. 11, the tensile test device 40 includes a lower grip part 41 and an upper grip part 42 that grip the lower end and the upper end of a plate member 50, which is a tensile test piece, respectively. The plate member 50 has a parallel portion 51 having a predetermined width and a predetermined thickness at the center 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 the plate member 50 is not limited to this, a plate having a thickness of 1.6 mm was used.

In the uniaxial tensile test, the lower gripping part 41 and the upper gripping part 42 are separated from each other in a direction (white direction) shown by the arrow. The tensile test device 40 includes a moving mechanism (not shown) that moves the lower gripping part 41 and the upper gripping part 42, and a control unit 60 that controls the operation of the moving mechanism.

The tensile testing apparatus 40 is also provided with a camera 45 as an imaging device that images the surface of the central portion in the longitudinal direction of the plate-like member 50. Two cameras 45 are disposed at. The two cameras 45 are arranged, for example, at symmetrical positions in the vertical direction with respect to a direction perpendicular to the longitudinal direction of the plate-shaped member 50.

Images of the surface of the plate member 50 captured by the two cameras 45 are input to the control unit 60. The control unit 60 stores the image captured by the camera 45 in a storage device such as a memory, and calculates the maximum principal strain on the entire surface of the plate-shaped member 50 at the longitudinal center by analysis using a digital image correlation method. It is designed to be calculated.

Regarding the uniaxial tensile test, the control unit 60 images the paint dot pattern applied by spraying on the surface of the plate member 50 before the tensile test using two cameras 45, and determines the dot pattern based on the before and after images taken. The maximum principal strain on the surface of the plate member 50 is calculated from the change in the positional relationship of the patterns. The control unit 60 is also configured to display the image captured by the camera 45 and the maximum principal strain at the longitudinal center of the plate member 50 on a display device (not shown) such as a display.

FIG. 12 is a diagram showing the maximum principal strain at the central portion in the longitudinal direction of the plate member. FIG. 12 shows the maximum principal strain at the longitudinal center of the plate member 50 immediately before fracture in the tensile test. In this embodiment, an image of the surface of the central portion in the longitudinal direction of the plate-like member 50 is captured by the camera 45 during the tensile test, and based on the image of the central portion in the longitudinal direction of the plate-shaped member 50, a digital image correlation method is used. The maximum principal strain at the central portion in the longitudinal direction of the plate member 50 is calculated using .

In the tensile test, the maximum principal strain occurs in the longitudinal direction of the plate-shaped member 50 on the center side of the central portion in the longitudinal direction of the plate-shaped member 50 . As shown in FIG. 12, the maximum principal strain at the longitudinal center of the plate-like member 50 becomes smaller as it moves away from the center of the longitudinal center of the plate-like member 50 in the longitudinal direction. It becomes smaller as it moves away from the center side in the board width direction.

FIG. 13 is a graph showing the relationship between the position of the plate-shaped member in the plate width direction and the maximum principal strain. In FIG. 13, the surface of the plate member 50 along the line VV in FIG. 12 is displayed with the horizontal axis representing the position of the plate member 50 in the width direction and the vertical axis representing the maximum principal strain of the plate member 50. ing. As shown in FIG. 13, ductile failure occurs at the central side of the plate-shaped member 50 in the width direction between positions P10 and P11 in the width direction.

As shown in FIG. 13, when a tensile test is performed until the plate member 50 undergoes uniaxial tensile deformation and undergoes ductile failure, the maximum principal strain at the time of ductile failure of the plate member 50 is in the longitudinal direction of the plate member 50. The maximum principal strain distribution has a maximum value εmax at the center side in the sheet width direction (position P10 in the sheet width direction) and decreases toward one end and the other end in the sheet width direction.

In this embodiment, the maximum value εmax of the maximum principal strain at the time of ductile failure of the plate member 50 in the uniaxial tensile test is acquired as the strain at the time of ductile failure in the uniaxial tensile test. Then, ductile fracture limit strain characteristic data L1c is calculated in which the strain at the time of ductile fracture in the uniaxial tensile test of the plate member 50 is set as the limit strain regardless of the strain gradient. 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 depending on the material of the plate member 50 regardless of the strain gradient.

In this embodiment, in evaluating stretch flange cracking, edge cracking critical strain characteristic data L1a, internal cracking critical strain characteristic data L1b, and ductile fracture critical strain characteristic data L1c are used as critical strain characteristic data L1 of the plate member 20. . If the strain gradient is less than or equal to the predetermined value P1, use 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, and set the strain gradient to a predetermined value larger than the predetermined value P1. If it is less than P2, use the edge cracking limit strain data L1a, which has a smaller limit strain than the internal cracking limit strain characteristic data L1b and the ductile fracture limit strain characteristic data L1c, and if the strain gradient is greater than the predetermined value P2, use the edge cracking limit strain data. The ductile fracture limit strain characteristic data L1c, which has a smaller limit strain than L1a and the internal crack limit strain characteristic data L1b, is used.

During press forming analysis of press-forming a press-formed product having an elongated flange portion from a 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 calculate elongation. Evaluate cracks in the flange. During press forming analysis, calculate the maximum principal strain in each element of the analysis model divided into finite elements for the end of the stretch flange and the strain gradient with the element adjacent to this element in the direction away from the end of the stretch flange. , when the maximum principal strain at a predetermined strain gradient in each element exceeds the critical strain of critical strain characteristic data L1 based on edge crack critical strain characteristic data L1a, internal crack critical strain characteristic data L1b, and ductile fracture critical strain characteristic data L1c, elongation occurs. It is evaluated that flange cracking occurs.

As the stretch flange forming test, another stretch flange forming test may be used in which a plurality of plate members are formed so as to generate cracks at the ends of each plate member and the critical strain is calculated. When using other stretch flanging tests, it is also necessary to determine whether the cracking condition at the time of cracking is an edge crack or an internal crack, and also determine the limit strain from the stretch flanging test and the end face of the plate member. By calculating the strain gradient in the internal direction (direction perpendicular to the tangent to the end face), edge crack limit strain characteristic data L1a and internal crack limit strain characteristic data L1b are calculated. It is also possible to use a plurality of stretch flanging tests as the stretch flanging test.

As a stretch flange forming test, in order to calculate characteristic data that shows the relationship between the critical strain of a plate-like member and the strain gradient when the strain gradient is very small, cracks were created at the ends of each of the plate-like members for multiple plate-like members. A one-sided punching tensile test can be used in which a plate-like member is formed so as to generate , and the critical strain is calculated.

The one-sided punching tensile test can be performed using a tensile testing device similar to the one used for the uniaxial tensile test described above. Regarding the one-sided punching tensile test, a plate-shaped member having a parallel portion having a predetermined width and a predetermined thickness at the center in the longitudinal direction can be used as the plate-shaped member which is a No. 5 test piece in JIS Z2241. In the single-sided punching tensile test, the end face on one side in the plate width direction of the parallel part is formed by punching so as to give a predetermined punched end shape, and the end face on the other side in the plate width direction of the parallel part is formed by a mechanical process different from the punching process. Shaped by etc. As a punching process, for example, a die, a blank holder, and a punch can be used to form the blank with a clearance of 12% of the plate thickness.

When using a one-sided punching tensile test as a stretch flange forming test, the tensile test is performed until a crack occurs that penetrates the plate-like member in the thickness direction. Cracks occur. During the test, a camera captures an image of the surface of the parallel part of the plate-shaped member, and based on the captured image of the surface of the parallel part of the plate-shaped member, a worker etc. can identify the cracks that occur at the end of the plate-shaped member. Determine the crack condition.

In the single-sided punching tensile test, the maximum principal strain ε0 at the end face of the plate-like member is calculated, and the strain gradient α near the end of the plate-like member is calculated from the end face of the plate-like member in the internal direction of the plate-like member (as described above). The strain gradient α of the maximum principal strain at a position away from the end surface in the direction perpendicular to the tangent to the end surface is calculated.

As a stretch flange forming test, a one-sided punching tensile test is used to determine whether the cracking state at the time of cracking in the stretch flange forming test for each of a plurality of plate members is an edge crack or an internal crack. It is possible to calculate the critical strain from the forming test and the strain gradient in the internal direction of the plate-like member (direction perpendicular to the tangent to the end face) from the end face of the plate-like member.

As critical strain characteristic data L1 showing the relationship between critical strain and strain gradient of a plate-shaped member, edge crack critical strain characteristic data L1a and internal cracking are determined according to edge cracking or internal cracking, which is the state of cracking in an actual stretch flange forming test. Since the limit strain characteristic data L1b for cracking is calculated, and the edge crack limit strain characteristic data L1a and the internal crack limit strain characteristic data L1b are used to evaluate stretch flange cracking, when using limit strain characteristic data considering only edge cracks. The evaluation accuracy of stretch flange cracking can be improved compared to the above. Furthermore, since the cracking of the stretch flange is evaluated using the ductile fracture critical strain characteristic data L1c, in which the critical strain is the strain at ductile fracture in the uniaxial tensile test, the critical strain may be excessively large even when the strain gradient is large. Therefore, it is possible to improve the evaluation accuracy of stretch flange cracking.

As shown in FIG. 3, when only the limit strain characteristic data L1a that takes into account only edge cracking is used as the limit strain characteristic data L1 that indicates the relationship between the limit strain and the strain gradient of the plate member 20, the strain gradient is a predetermined value. If it is less than P1, the critical strain is larger than the internal cracking critical strain characteristic data L1b, and even if it is evaluated by press forming analysis that stretch flange cracking will not occur, there is a risk that stretch flange cracking will occur during actual press forming. be.

In addition, when using only the limit strain characteristic data L1a that takes only edge cracking into consideration, if the strain gradient is larger than the predetermined value P2, the limit strain is larger than the ductile fracture limit strain characteristic data L1c, and the press forming analysis shows that stretch flange cracks are not detected. Even if it is evaluated that no cracking will occur, there is a risk that stretch flange cracking will occur during actual press forming.

In this embodiment, 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, so that the evaluation accuracy of stretch flange cracks can be improved.

FIG. 14 is a flowchart showing a method for evaluating stretch flange cracking. As shown in FIG. 14, when evaluating cracks in the stretch flange portion 8 of the press-formed product 1, first, a stretch flange forming test is performed on the plate member 20 having the hole portion 21 (step S1). During the stretch flange forming test, the cracking state at the time of crack occurrence in the stretch flange forming test is determined, and it is determined whether the cracking state is an edge crack or an internal crack (step S2). When performing a hole expansion test as a stretch flange forming test, it is determined whether the crack state during the hole expansion test is an edge crack or an internal crack.

During the stretch flanging test, the limit strain due to the stretch flanging test and the strain gradient in the direction from the end face of the plate member to the inside of the plate member are calculated (step S3). When performing a hole expansion test as a stretch flange forming test, the strain gradient in the radial direction of the hole is calculated. When a crack occurs at the end of the hole 21 during the hole expansion test, based on the image of the surface of the plate member 20 taken by the camera 35, the entire surface of the plate member 20 near the hole is removed. Calculate the maximum principal strain of. As described above, the critical strain and strain gradient of the plate-shaped member 20 are calculated from the maximum principal strain of the plate-shaped member 20 calculated for the entire surface of the plate-shaped member 20.

A cylindrical hole expansion test and a conical hole expansion test are performed as hole expansion tests using a plurality of plate-shaped members 20, and steps S1 to S3 are repeated for each hole expansion test to check the cracking state of each of the plurality of plate-shaped members 20. At the same time, the limit strain and strain gradient are calculated by the hole expansion test.

Next, regarding the case where the cracking state in the stretch flange forming test is an edge cracking, based on the limit strain and strain gradient of each plate member 20 when the cracking state is an edge cracking, Edge crack limit strain characteristic data L1a indicating the relationship between the limit strain and the strain gradient is calculated (step S4).

Even when the crack state in the stretch flange forming test is an internal crack, the limit of the plate member 20 in internal cracking is determined based on the limit strain and strain gradient of each plate member 20 when the crack state is an internal crack. Internal crack limit strain characteristic data L1b indicating the relationship between strain and strain gradient is calculated (step S5).

In steps S1 to S5, as a stretch flange forming test, along with a hole expansion test and the like, one-sided punching tensile You may also conduct a test.

When performing a one-sided punching tensile test, the one-sided punching tensile test is also performed (step S1), the crack state at the time of cracking is determined (step S2), and the critical strain and the internal direction of the plate-like member from the end face of the plate-like member are determined. (Step S3), and calculate edge cracking critical strain characteristic data L1a that shows the relationship between the critical strain of the plate member at edge cracking and the strain gradient, along with other stretch flange forming tests such as hole expansion tests. is calculated (step S4), and internal crack limit strain characteristic data L1b indicating the relationship between the limit strain and strain gradient of the plate-like member at internal cracks is calculated (step S5).

When evaluating cracks in the stretch flange portion 8, a uniaxial tensile test is also performed as a uniaxial tensile deformation test on the plate member 50 made of the same material as the plate member 20 used in the stretch flange forming test (step S6). . During the uniaxial tensile test, the maximum principal strain of the plate member 50 is calculated. Until the plate member 50 undergoes ductile failure during the tensile test and cracks occur, the maximum length of the longitudinal center portion of the plate member 50 is determined based on the image of the longitudinal center portion of the plate member 50 taken by the camera 45. Calculate the principal strain.

When the plate-shaped member 50 undergoes ductile fracture and cracks occur, the maximum value εmax of the maximum principal strain at the time of fracture of the plate-shaped member 50 is set as the strain at the time of ductile fracture in the tensile test, and the plate-shaped member 50 undergoes ductile fracture in the tensile test. Ductile fracture limit strain characteristic data L1c is calculated in which the strain at time is the limit strain regardless of the strain gradient (step S7).

Next, 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, limit strain characteristic data L1 indicating the relationship between the limit strain and the strain gradient of the plate-shaped member are generated. is calculated (step S8). The limit strain characteristic data L1 is formed by limit strain characteristic data having a small limit strain for a predetermined strain gradient among edge crack limit strain characteristic data L1a, internal crack limit strain characteristic data L1b, and ductile fracture limit strain characteristic data L1c.

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

Then, using the limit strain characteristic data L1 of the plate-shaped member calculated in this way, a press forming analysis is performed to form a press-formed product having a stretch flange part from the plate-shaped member, and the end part of the stretch flange part is The maximum principal strain in each element of the divided analytical model and the strain gradient of the element adjacent to the element in the direction away from the end of the stretch flange are calculated, and if the strain gradient is greater than or equal to the limit strain characteristic data L1, cracking is detected. Stretch flange cracking is evaluated by evaluating that it occurs (step S9).

In this embodiment, instead of the above-mentioned high tensile strength steel plate made of material A having a tensile strength of about 980 MPa or more, a material of a 980 MPa class high tensile strength steel plate similar to material A with different additive components is used as the plate member. For the plate-like members made of the materials B and C used, limit strain characteristic data indicating the relationship between the limit strain and the strain gradient of the plate-like member 20 was similarly calculated.

Similarly to the limit strain characteristic data L1 using material A, the limit strain characteristic data using material B and material C are edge crack limit strain data, internal crack limit strain characteristic data, and ductile fracture limit strain characteristic data. formed by. Regarding the limit strain characteristic data using material B and material C, similarly to the plate member using material A, a press molded product having a stretch flange portion is formed from the plate member using the limit strain characteristic data. It is possible to perform press forming analysis and evaluate stretch flange cracking.

In this way, as critical strain characteristic data showing the relationship between the critical strain and strain gradient of a plate-like member, the critical strain characteristic of internal cracking can be calculated according to internal cracking or edge cracking, which is the state of cracking in the actual stretch flange forming test. data and limit strain characteristic data for edge cracks are calculated, and stretch flange cracks are evaluated using limit strain characteristic data for internal cracks and edge cracks, compared to using limit strain characteristic data that takes only edge cracks into account. Therefore, it is possible to improve the evaluation accuracy of stretch flange cracking. Furthermore, since we evaluate cracks in the stretch flange using ductile fracture critical strain characteristic data in which the critical strain is the strain at ductile fracture in a uniaxial tensile deformation test, the critical strain may be excessively large even when the strain gradient is large. Therefore, it is possible to improve the evaluation accuracy of stretch flange cracking.

In this embodiment, limit strain characteristic data for internal cracks and limit strain characteristic data for edge cracks are obtained using a cylindrical hole expansion test and a conical hole expansion test as hole expansion tests. Using a spherical hole expansion test using a formed spherical punch, it is possible to determine the state of cracking and to obtain critical strain characteristic data that shows the relationship between the critical strain and strain gradient of a plate-shaped member. be.

As the internal crack limit strain characteristic data L1b, from the calculation results of the maximum principal strain and strain gradient by the hole expansion test, an approximate expression 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, the internal crack limit strain characteristic data may be calculated by calculating an approximate expression of a linear function.

In this embodiment, a 980 MPa class high tensile strength steel plate is used as the material of the plate-like member, but the present invention can be similarly applied to plate materials made of other metals such as other steel plates and aluminum alloy plates. In addition, regarding the stretch flange forming test, perform the stretch flange forming test multiple times using multiple plate members under the same conditions, and use the average value of the maximum principal strain of the multiple stretch flange forming tests. is also possible.

As limit strain characteristic data L1, if the crack state at the time of crack occurrence in the stretch flange forming test is edge cracking or internal cracking, edge crack limit strain data, internal crack limit strain characteristic data, and ductile fracture limit strain characteristic data. The critical strain characteristic data formed by used. As the limit strain characteristic data L1, at least edge crack limit strain characteristic data and ductile fracture limit strain characteristic data are used among edge crack limit strain characteristic data, internal crack limit strain characteristic data, and ductile fracture limit strain characteristic data.

In this embodiment, the maximum principal strain when cracking occurs in the stretch flange forming test is used to calculate the critical strain characteristic data that indicates the relationship between the critical strain and the strain gradient that is the strain gradient of the maximum strain. It is also possible to calculate critical strain characteristic data indicating the relationship between critical strain and strain gradient using the maximum principal strain immediately before cracking occurs in the stretch flange forming test.

In this embodiment, in the stretch flange forming test, the surface of the plate member is imaged by a camera, and the critical strain and strain gradient are calculated based on the image of the surface. Other methods may be used to calculate the critical strain and strain gradient. Regarding the uniaxial tensile deformation test, the strain at ductile failure may be calculated using other methods such as the scribed circle method. Further, in the stretch flange forming test, the crack state is determined based on the image of the surface taken by the camera, but the determination may be made using other methods.

As described above, the stretch flange crack evaluation method according to the present embodiment performs a stretch flange forming test on each of a plurality of plate members 20, and determines whether the crack state at the time of crack occurrence in the stretch flange forming test is an edge crack or an internal crack. At the same time, the limit strain and strain gradient from the stretch flange forming test are calculated, and the limit strain characteristic data L1 indicating the relationship between the limit strain and strain gradient of the plate-shaped member 20 is used as edge crack limit strain characteristic data. L1a and internal crack limit strain characteristic data L1b are calculated. Further, a uniaxial tensile deformation test is performed on the plate member 50, and ductile fracture limit strain characteristic data L1c is calculated, with the strain at the time of ductile fracture in the uniaxial tensile deformation test as the limit strain. Then, cracks in the stretch flange portion are detected 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 L1a, the internal crack limit strain characteristic data L1b, and the ductile fracture limit strain characteristic data L1c. evaluate.

As a result, edge cracking limit strain characteristic data L1, which indicates the relationship between the limit strain and strain gradient of a plate-shaped member, is obtained according to edge cracking or internal cracking, which is the state of cracking in the actual stretch flange forming test. L1a and internal crack limit strain characteristic data L1b are calculated, and stretch flange cracking is evaluated using the edge crack limit strain characteristic data L1a and internal crack limit strain characteristic data L1b, so the limit strain characteristic data takes only edge cracks into account. The evaluation accuracy of stretch flange cracking can be improved compared to the case of using the method. Furthermore, since the cracking of the stretch flange is evaluated using the ductile fracture critical strain characteristic data L1c, in which the critical strain is the strain at ductile failure in the uniaxial tensile deformation test, the critical strain is not excessive even when the strain gradient is large. It is possible to suppress the increase in size, and it is possible to improve the evaluation accuracy of stretch flange cracks. For a plate-shaped member in which the only cracking state at the time of cracking is edge cracking, the accuracy of evaluating stretch flange cracking can be improved by using edge cracking limit strain characteristic data and ductile fracture limit strain characteristic data.

Furthermore, the stretch flange crack evaluation method evaluates cracks in the stretch flange portion using edge crack limit strain characteristic data, internal crack limit strain characteristic data, and ductile fracture limit strain characteristic data. As a result, it is possible to improve the evaluation accuracy of stretch flange cracking for plate-shaped members whose cracking state at the time of cracking is edge cracking or internal cracking.

In addition, the stretch flange crack evaluation method uses edge crack limit strain characteristic data L1a, internal crack limit strain characteristic data L1b, and ductile fracture limit strain during press forming analysis in which a press molded product having a stretch flange portion is press-formed from a plate member. Of the characteristic data L1c, at least the edge crack limit strain characteristic data L1a and the ductile fracture limit strain characteristic data L1c are used to evaluate cracks in the stretch flange portion. As a result, during press forming analysis, the maximum principal strain in each element of the analysis model divided into finite elements for the end of the stretch flange and the strain gradient of the element adjacent to the element in the direction away from the end of the stretch flange can be calculated. By calculating , it is possible to accurately evaluate cracks in the stretch flange portion in press forming analysis.

In addition, the stretch flange crack evaluation method is such that the surface of the plate member 20 is imaged by the camera 35 in the stretch flange forming test, and based on the image of the surface of the plate member 20 taken by the camera 35, the Calculate the critical strain and strain gradient from the stretch flange forming test. Thereby, it is possible to calculate the critical strain and the strain gradient using the digital image correlation method, and it is possible to calculate the critical strain and the strain gradient with high accuracy.

In addition, the stretch flange crack evaluation method is such that the surface of the plate member 20 is imaged by the camera 35 in the stretch flange forming test, and based on the image of the surface of the plate member 20 taken by the camera 35, the Determine whether the crack state at the time of crack occurrence in the stretch flange forming test is an edge crack or an internal crack. As a result, by finding the crack origin when a crack occurs based on the image of the surface of the plate member 20, it is possible to improve the accuracy of determining the crack state of edge cracks or internal cracks, and improve the evaluation accuracy of flange cracks. can.

In addition, the stretch flange crack evaluation method involves imaging the surface of the plate-like member 50 with a camera 45 in a uniaxial tensile deformation test, and based on the image of the surface of the plate-like member 50 taken by the camera 45, Calculate the strain at ductile failure using a uniaxial tensile deformation test. Thereby, the strain at the time of ductile failure can be calculated using the digital image correlation method, and the ductile failure limit strain can be calculated with high accuracy. Based on the images captured by the camera, we calculated the critical strain and strain gradient of the plate-shaped member in the stretch flange forming test, and also calculated the strain at the time of ductile failure in the uniaxial tensile deformation test of the plate-shaped member. By using the digital image correlation method, it is possible to calculate the critical strain and the strain gradient as well as the critical strain for ductile fracture, and it is possible to improve the evaluation accuracy of stretch flange cracking.

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

As described above, according to the present invention, it is possible to improve the evaluation accuracy of stretch flange cracking, so it is suitable for simulating press forming in which a press-formed product having a stretch flange portion is formed from a plate-like member. Available.

1 Press-formed product 8 Stretch flange portion 10 Hole expansion test device 14, 15 Punch 20, 50 Plate member 22 Cracks 30, 60 Control unit 35, 45 Camera 40 Tensile test device L1 Critical strain characteristic data L1a Edge crack critical strain characteristic data L1b Internal cracking limit strain characteristic data L1c Ductile fracture limit strain characteristic data

Claims (6)

A stretch flange crack evaluation method for evaluating cracks in a stretch flange portion of a press-formed product,
Conducting a stretch flange forming test in which a plurality of plate-like members are formed so as to generate cracks at the ends of each of the plate-like members to calculate the critical strain,
Conducting a uniaxial tensile deformation test in which a plate member made of the same material as the plurality of plate members is subjected to uniaxial tensile deformation and the strain at the time of ductile failure is calculated,
For each of the plurality of plate-shaped members, it is determined whether the crack state at the time of crack occurrence in the stretch-flange forming test is an edge crack or an internal crack, and the critical strain in the stretch-flange forming test and the limit strain of the plate-shaped member are determined. Calculating the strain gradient in the internal direction of the plate-shaped member from the end surface,
Based on the critical strain and strain gradient of each plate-like member when the cracking state in the stretch-flange forming test is edge cracking, the edge cracking limit shows the relationship between the critical strain and strain gradient of the plate-like member in edge cracking. Calculate strain characteristic data,
Based on the critical strain and strain gradient of each plate-like member when the cracking state in the stretch-flange forming test is internal cracking, the internal cracking limit shows the relationship between the critical strain and strain gradient of the plate-like member in internal cracking. Calculate strain characteristic data,
Calculating ductile fracture limit strain characteristic data for the plate-shaped member in which the strain at ductile fracture in the uniaxial tensile deformation test is the limit strain regardless of the strain gradient,
Using at least the edge cracking limit strain characteristic data and the ductile fracture limit strain characteristic data of the edge cracking limit strain characteristic data, the internal cracking limit strain characteristic data, and the ductile fracture limit strain characteristic data, cracks in the stretch flange portion are determined. evaluate,
Stretch flange crack evaluation method. Evaluating cracks 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 stretch flange crack evaluation method according to claim 1. At the time of press forming analysis in which a press-formed product having a stretch flange portion is press-formed from a plate member, at least the edge cracking limit strain characteristic data, the internal cracking limit strain characteristic data, and the ductile fracture limit strain characteristic data are detected. Evaluating cracks in the stretch flange portion using the critical strain characteristic data and the ductile fracture critical strain characteristic data;
The stretch flange cracking evaluation method according to claim 1 or 2. In the stretch flange forming test, the surface of the plate member is imaged by a camera,
Calculating the limit strain and strain gradient in the stretch flange forming test for the plate-shaped member based on an image of the surface of the plate-shaped member captured by the camera;
The stretch flange crack evaluation method according to any one of claims 1 to 3. In the stretch flange forming test, the surface of the plate member is imaged by a camera,
Based on an image of the surface of the plate-shaped member taken by the camera, determining whether the cracking state of the plate-shaped member when a crack occurs in the stretch flange forming test is an edge crack or an internal crack.
The stretch flange crack evaluation method according to any one of claims 1 to 4. In the uniaxial tensile deformation test, the surface of the plate member is imaged by a camera,
Calculating the strain at the time of ductile failure in the uniaxial tensile deformation test for the plate-shaped member based on an image of the surface of the plate-shaped member captured by the camera;
The stretch flange crack evaluation method according to any one of claims 1 to 5.

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