KR20230141555A - Stretch-flange crack evaluation method - Google Patents

Abstract

Improves the evaluation accuracy of elongation flange cracks.
A stretching flange forming test is performed on a plurality of plate-shaped members, a uniaxial tensile strain test is performed on the plate-shaped members, and it is determined whether the cracking state by the stretching flange forming test is an edge crack or an anti-cracking test. Calculate the limit strain and strain gradient, calculate edge crack limit strain characteristic data and crack resistance limit strain characteristic data that show the relationship between the limit strain and strain gradient of the plate-shaped member in edge cracking and internal cracking, respectively, and perform a uniaxial tensile strain test. Calculate ductile fracture limit strain characteristic data with the deformation at the time of ductile fracture as the limit strain, and determine at least the edge crack limit among edge crack limit deformation characteristic data L1a, crack resistance limit deformation characteristic data L1b, and ductile fracture limit deformation characteristic data L1c. Cracks in the elongated flange section are evaluated using strain property data and ductile fracture limit strain property data.

Description

STRETCH-FLANGE CRACK EVALUATION METHOD}

The present invention relates to a method for evaluating elongation flange cracks.

When manufacturing a press-formed product by press forming a metal plate-shaped member, the press-formed product is evaluated for cracks by simulating the press-molding of the plate-shaped member through press forming analysis using a finite element method (FEM analysis) in advance. , evaluation of the success or failure of press molding is being carried out.

As a plate-shaped member, for example, when using a plate-shaped member such as a high-strength steel plate having a tensile strength of about 980 MPa or more, in the stretch flange portion that is stretched flange formed during press forming, the stretch flange is deformed, so it is deformed during deformation. Cracks may occur at the ends of plate-shaped members, and in order to evaluate and predict these cracks in advance, stretch flange cracks in press-formed products are evaluated through press-molding analysis.

When evaluating elongation flange cracks, it is known to use limit strain characteristic data showing the relationship between the limit strain of the plate member and the strain gradient as the fracture limit characteristic of the plate 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 a simulation of press forming to produce an elongation flange. Evaluating cracks is disclosed.

Japanese Patent No. 6852426 Publication

The limit strain characteristic data showing the relationship between the limit strain and strain gradient of the plate-shaped member described in Patent Document 1 calculates the fracture limit based on the crack at the end of the hole portion in the hole expansion test, and the crack occurrence position is at the hole portion. Limit deformation characteristic data is calculated by considering only edge cracks at the end.

However, when elongation flange cracking was evaluated by press forming analysis using limit strain characteristic data considering only edge cracks, even in cases where it was evaluated that no elongation flange cracking occurred by press forming analysis, fracture rather than edge cracking occurred. In some cases, elongation flange cracks may occur during actual press molding.

In addition, when evaluating elongation flange cracks by press forming analysis using limit strain characteristic data considering only edge cracks where the limit strain increases as the strain gradient increases, there is a concern that the limit strain may excessively increase depending on the size of the strain gradient. Even in cases where it is evaluated that no stretching flange cracks have occurred by press forming analysis, there is a risk that stretching flange cracks may occur when actually press forming.

The object of the present invention is to provide an elongation flange crack evaluation method that can improve the evaluation accuracy of elongation flange cracks.

The present invention is an extension flange crack evaluation method for evaluating cracks in the extension flange portion of a press-formed product, and an extension flange forming test in which a plurality of plate-shaped members are formed to generate cracks at the ends of the plate-shaped members, respectively, and the limit strain is calculated. A uniaxial tensile strain test is performed to calculate the strain at ductile fracture by subjecting plate-shaped members made of the same material as the plurality of plate-shaped members to uniaxial tensile strain, and performing a uniaxial tensile strain test for each of the plurality of plate-shaped members by the stretching flange forming test. In addition to determining whether the crack state at the time of crack occurrence is an edge crack or an internal crack, the limit strain by the stretching flange forming test and the strain gradient in the inner direction of the plate-shaped member from the end surface of the plate-shaped member are determined. Calculate, based on the limit strain and strain gradient of each plate-shaped member when the crack state by the stretching flange forming test is edge crack, edge crack showing the relationship between the limit strain and strain gradient of the plate-shaped member in the edge crack. Limit strain characteristic data is calculated, and based on the limit strain and strain gradient of each plate-shaped member when the cracking state in the stretching flange forming test is internal cracking, the limit strain of the plate-shaped member in internal cracking and Calculate crack resistance limit strain characteristic data showing the relationship between strain gradients, and calculate ductile fracture limit strain property data in which the strain at ductile failure by the uniaxial tensile strain test for the plate-shaped member is the limit strain regardless of the strain gradient. Calculate, 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 crack resistance limit strain characteristic data, and the ductile fracture limit strain characteristic data, the elongated flange portion Provides a method for evaluating cracks in elongated flange.

According to the present invention, a stretching flange forming test is performed on a plurality of plate-shaped members, and it is determined whether the crack state at the time of crack occurrence by the stretching flange forming test is an edge crack or an internal crack, and the limit according to the stretching flange forming test is determined. Calculate deformation and strain gradient, and calculate edge crack limit deformation characteristic data and crack resistance limit deformation characteristic data. Additionally, a uniaxial tensile strain test is performed on the plate-shaped member, and ductile fracture limit strain characteristic data is calculated using the strain at ductile failure by the uniaxial tensile strain test as the limit strain. Then, cracks in the elongated flange portion are evaluated using at least the edge crack limit deformation characteristic data and the ductile fracture limit deformation characteristic data among the edge crack limit deformation characteristic data, crack resistance limit deformation characteristic data, and ductile fracture limit deformation characteristic data.

As a result, as limit strain characteristic data showing the relationship between the limit strain and strain gradient of the plate-shaped member, the edge crack limit strain characteristic data and the crack resistance limit are determined according to the edge crack or crack resistance limit, which is the state of cracking in the actual stretching flange forming test. Deformation characteristic data is calculated and elongation flange cracking is evaluated using edge crack limit deformation characteristic data and crack resistance limit deformation characteristic data. Therefore, compared to the case of using limit deformation characteristic data considering only edge cracks, evaluation of elongation flange cracking is possible. Precision can be improved. In addition, since cracking in the elongation flange section is evaluated using ductile fracture limit strain characteristic data, which uses the strain at ductile failure by the uniaxial tensile strain test as the limit strain, it is ensured that the limit strain becomes excessively large even in cases where the strain gradient is large. It can be suppressed and the evaluation accuracy of elongation flange cracks can be improved. For plate-shaped members whose crack state at the time of crack occurrence is only edge cracking, the evaluation accuracy of elongation flange cracking can be improved by using edge cracking limit strain characteristic data and ductile fracture limit strain characteristic data.

The elongation flange crack evaluation method evaluates cracks in the elongation flange portion using the edge crack limit deformation characteristic data, the crack resistance limit deformation characteristic data, and the ductile fracture limit deformation characteristic data.

With this configuration, cracks in the elongated flange portion are evaluated using edge crack limit strain characteristic data, crack resistance limit strain characteristic data, and ductile fracture limit strain characteristic data, so that the crack state at the time of crack occurrence is edge crack or resistant crack. For plate-shaped members, the evaluation accuracy of elongation flange cracks can be improved.

The elongation flange crack evaluation method includes the edge crack limit deformation characteristic data, the crack resistance limit deformation characteristic data, and the ductile fracture limit deformation characteristic during press forming analysis for press forming a press formed product having an elongation flange portion from a plate-shaped member. Among the data, at least the edge crack limit strain characteristic data and the ductile fracture limit strain characteristic data are used to evaluate cracking in the elongated flange portion.

According to this configuration, at the time of press forming analysis, at least edge crack limit deformation characteristic data and ductile fracture limit deformation characteristic data among edge crack limit deformation characteristic data, crack resistance limit deformation characteristic data, and ductile fracture limit deformation characteristic data are used, In order to evaluate cracks in the elongation 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 elongation flange part and the corresponding element from the end of the elongation flange part. By calculating the strain gradient with elements adjacent to the spaced apart direction, cracks in the elongated flange portion in press forming analysis can be evaluated with high accuracy.

In the stretching flange crack evaluation method, in the stretching flange forming test, the surface of the plate-shaped member is imaged by a camera, and based on the surface image of the plate-shaped member imaged by the camera, the elongation of the plate-shaped member is determined. It is desirable to calculate the limiting strain and strain gradient by flange forming tests.

With this configuration, based on the surface image of the plate-shaped member captured by the camera, the limiting strain and strain gradient by the stretch flange forming test are calculated for the plate-shaped member, so digital image correlation (DIC) is used. It is possible to calculate the limit strain and strain gradient using this method, and it is possible to calculate the limit strain and strain gradient with high precision.

In the stretching flange crack evaluation method, in the stretching flange forming test, the surface of the plate-shaped member is imaged by a camera, and based on the surface image of the plate-shaped member imaged by the camera, the elongation of the plate-shaped member is determined. It is desirable to determine whether the crack state when cracking occurs by a flange forming test is an edge crack or an internal crack.

With this configuration, based on the surface image of the plate-shaped member captured by the camera, it is determined whether the crack state at the time of crack generation by the stretching flange forming test is an edge crack or an internal crack, so based on the surface image of the plate-shaped member By finding the crack origin at the time of crack occurrence, the judgment accuracy of the crack state of edge cracks or internal cracks can be improved, and the evaluation accuracy of elongated flange cracks can be improved.

In the elongation flange crack evaluation method, in the uniaxial tensile strain test, the surface of the plate-shaped member is imaged by a camera, and based on the surface image of the plate-shaped member imaged by the camera, the uniaxial crack is evaluated for the plate-shaped member. It is desirable to calculate the strain at ductile failure by a tensile strain test.

With this configuration, the strain at ductile failure by a uniaxial tensile strain test is calculated for the plate-shaped member based on the surface image of the plate-shaped member captured by a camera, so the strain at ductile fracture can be calculated using the digital image correlation method. can be calculated, and it is possible to calculate the ductile fracture limit strain with high precision. Based on the image captured by the camera, the limit strain and strain gradient are calculated for the plate-shaped member by the stretching flange forming test, and the strain at ductile failure is calculated for the plate-shaped member by the uniaxial tensile strain test, Using the digital image correlation method, the limit strain and strain gradient can be calculated, as well as the ductile fracture limit strain, and the evaluation accuracy of elongation flange cracks can be improved.

Fig. 1 is a diagram showing a press molded product having an elongated flange portion.
Figure 2 is an explanatory diagram for explaining edge cracks and internal cracks of the elongated flange portion.
FIG. 3 is a graph showing the relationship between the limit strain and strain gradient of a plate-shaped member used for evaluation of stretching flange cracks according to an embodiment of the present invention.
Figure 4 is a schematic configuration diagram of a cylindrical hole expansion test.
Figure 5 is a perspective view showing the plate-shaped member and the punch after the cylindrical hole expansion test.
Fig. 6 is a diagram showing the maximum principal strain near the hole portion of the plate-shaped member.
Figure 7 is a graph showing the relationship between the distance from the center of the hole portion of the plate-shaped member and the maximum peripheral shape.
Figure 8 is a graph showing the relationship between the distance from the end face of the plate-shaped member and the maximum peripheral shape.
Figure 9 is a schematic diagram of the cone hole expansion test.
Figure 10 is a perspective view showing the plate-shaped member and the punch after the cone hole expansion test.
Figure 11 is a schematic diagram of a uniaxial tensile test.
Fig. 12 is a diagram showing the maximum peripheral shape of the longitudinal central portion of the plate-shaped member.
Figure 13 is a graph showing the relationship between the position of the plate-shaped member in the width direction and the maximum peripheral shape.
Figure 14 is a flowchart showing a method for evaluating elongation flange cracks.

EMBODIMENT OF THE INVENTION Hereinafter, embodiment of this invention will be described with reference to the accompanying drawings.

Fig. 1 is a diagram showing a press molded product having an elongated flange portion. The press-formed product 1 shown in FIG. 1 is a press-formed product 1 formed by, for example, press-molding a metal plate-shaped member such as a high-strength steel plate having a tensile strength of about 980 MPa or more. As a press-molded product 1, it shows a center pillar outer 1 that constitutes the side part of the vehicle body and is disposed between the front and rear door openings.

The center pillar outer (1) includes a central part (2) extending in the up and down direction of the car body, an upper part (3) extending in the front and rear direction of the car body and installed on the roof side rail, and a lower part (3) extending in the front and rear direction of the car body and installed in the side sill ( It has 4). The central 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 to extend from the central portion 2 to the upper portion 3 and the lower portion 4, respectively, and is connected to the central portion 2, the upper portion 3, and the lower portion 4. Each part has an extension flange portion 8 that is formed into an extension flange during press molding. When the extension flange portion 8 is formed into an extension flange, the strain increases and cracks are likely to occur.

Figure 2 is an explanatory diagram for explaining edge cracks and internal cracks of the elongated flange portion. FIG. 2 shows an enlarged portion of the press-formed product shown in FIG. 1 and shows cracks that may occur at the end of the elongated flange portion 8. In Fig. 2(a), the crack occurrence position indicated by a white arrow represents an edge crack that is the end surface of the elongation flange portion 8, and in Fig. 2(b), the crack occurrence location indicated by a white arrow represents an edge crack at the elongation flange portion 8. It shows an internal crack inside the end surface of (8). As shown in Fig. 2(a) and Fig. 2(b), cracks that are edge cracks or internal cracks may occur in the elongated flange portion 8.

In this embodiment, when press forming a press-formed product from a plate-shaped member, limit strain characteristic data indicating the relationship between the limit strain of the plate-shaped member and the strain gradient are used as the fracture limit characteristics of the plate-shaped member used in press forming analysis, and the extension flange In addition to calculating according to the actual cracking state in the forming test, the ductile fracture limit strain is calculated and used in the uniaxial tensile strain test to improve the evaluation accuracy of elongation flange cracks in press-formed products by press forming analysis. The stretching flange forming test is a stretching flange forming test in which a plate-shaped member is formed so as to generate a crack at the end of the plate-shaped member and the limit strain is calculated. The uniaxial tensile strain test is a uniaxial tensile strain test in which a plate-shaped member is subjected to uniaxial tensile strain and the strain at ductile fracture is calculated.

FIG. 3 is a graph showing the relationship between the limit strain and strain gradient of a plate-shaped member used for evaluation of stretching flange cracks according to an embodiment of the present invention. In this embodiment, based on the stretching flange forming test, edge cracking limit strain characteristic data showing the relationship between the limit strain and strain gradient of the plate-shaped member in edge cracking is calculated, and based on the stretching flange forming test, in crack resistance Calculate crack resistance limit strain characteristic data showing the relationship between the limit strain and strain gradient of the plate-like member, and calculate ductile fracture limit strain characteristic data with the strain at ductile failure as the limit strain based on the uniaxial tensile strain test, As shown in Figure 3, as the fracture limit characteristics of the plate-shaped member, edge crack limit deformation characteristic data L1a, crack resistance limit deformation characteristic data L1b, and ductile fracture limit deformation characteristic data L1c were used to evaluate cracks in the elongated flange portion. do.

In this embodiment, as the stretch flange forming test, but not limited to this, the hole expansion test was used, and as the uniaxial tensile strain test, but not limited to this, the uniaxial tensile test was used. The hole expansion test was performed by using a plate-shaped member having a hole and pressing a punch into the hole until a crack occurring at the end of the hole penetrated in the thickness direction. The uniaxial tensile test was conducted by using a plate member that was a tensile test piece made of the same material as the plate member used in the hole expansion test, and applying a tensile load to both ends of the plate member until the plate member fractured. 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.

Figure 4 is a schematic configuration diagram of a cylindrical hole expansion test. In the evaluation of stretching flange cracks, a hole expansion test is performed on a plurality of plate-shaped members, each using a plate-shaped member having a hole, and the state of the crack is determined by the hole expansion test, as well as the limit strain and strain gradient of the plate-shaped member. Obtain limit deformation characteristic data representing the relationship.

As shown in FIG. 4, the cylindrical hole expansion test device 10 as a hole expansion test device is a press tool ( 11) is provided. The press tool 11 is equipped with a die 12 and a blank holder 13 for holding the plate-shaped member 20, and a punch 14 for press forming the plate-shaped member 20.

In the hole expansion test, the plate-shaped member 20 having a hole 21 formed in a circular shape is held between the die 12 and the blank holder 13 with a predetermined pressing force until a crack occurs near the hole. Move the punch (14). In the cylindrical hole expansion test, a cylindrical punch 14 whose tip is formed in a circular shape is used, and the cylindrical punch 14 is arranged so that its central axis coincides with the hole portion 21 of the plate-shaped member 20.

The hole expansion test device 10 is equipped with 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 is configured to control the operation of the moving mechanism.

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

The image of the surface of the plate-shaped member 20 captured by the two cameras 35 is 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 analyzes the image captured by the camera 35 in the vicinity of the hole portion of the plate-shaped member 20 by analyzing it using a digital image correlation method. It is designed to calculate the maximum principal strain over the entire surface.

The control unit 30 images the paint dot pattern applied by spray on the surface of the plate-shaped member 20 before the hole expansion test with two cameras 35, and creates a dot pattern based on the images before and after the capture. The maximum peripheral shape of the surface of the plate-shaped member 20 is calculated from the change in positional relationship. The control unit 30 is further configured to display the image captured by the camera 35 and the maximum peripheral shape near the hole of the plate-shaped member 20 on a display device (not shown) such as a display.

In the cylindrical hole expansion test, although it is not limited to this, die 12 is used with the die inner diameter D1 set to 53.8 mm, the die shoulder radius R1 set to 5 mm, the punch outer diameter D2 set to 50 mm, and the punch shoulder A punch 14 with a radius R2 of 10 mm was used. The thickness t of the plate-shaped member 20 was set to 1.6 mm, and the initial hole diameter D3 of the hole portion 21 used various hole diameters such as 20 mm. The hole portion 21 of the plate-shaped member 20 was punched using a die, a blank holder, and a punching punch with a clearance of 12% of the plate thickness.

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

In this embodiment, during the hole expansion test, an image of the surface near the hole of the plate-shaped member 20 is captured by the camera 35, and an image of the surface near the hole of the plate-shaped member 20 captured is used. Thus, an operator or the like determines the crack state of the crack 22 occurring at the end of the plate-shaped member 20. As a crack state, if the crack occurrence position, which is the origin of the crack, is the crack occurrence position C1, which is the end surface of the hole portion 21, it is determined to be an edge crack, and the crack occurrence location, which is the origin of the crack, is the end surface of the hole portion 21. In the case of the crack occurrence position C2, which is inside the plate-shaped member 20 spaced apart from, it is determined to be crack-resistant.

Fig. 6 is a diagram showing the maximum peripheral shape near the hole portion of the plate-shaped member. In Fig. 6, the maximum peripheral shape near the hole portion of the plate-shaped member 20 after the hole expansion test is shown. In this embodiment, during the hole expansion test, an image of the surface near the hole of the plate-shaped member 20 is captured by the camera 35, and an image of the surface near the hole of the plate-shaped member 20 captured is used. Then, the maximum peripheral shape near the hole of the plate-shaped member 20 is calculated using a digital image correlation method.

In the hole expansion test, maximum peripheral strain occurs in the circumferential direction of the hole portion 21 near the hole portion of the plate-shaped member 20. As shown in FIG. 6, the maximum peripheral shape near the hole portion of the plate-shaped member 20 is the hole inside the plate-shaped member 20 from the end face of the hole portion 21, which is the end face of the plate-shaped member 20. As the portion 21 is spaced outward in the radial direction, the maximum peripheral shape becomes smaller. In the plate-shaped member 20 where cracks occur in two places in the circumferential direction at the ends of the hole portion 21, as shown in FIG. 6, there are portions with a large maximum circumferential shape at four ends of the hole portion 21. It can be seen that the maximum peripheral shape becomes smaller as it is spaced radially away from the hole portion 21.

Figure 7 is a graph showing the relationship between the distance from the center of the hole portion of the plate-shaped member and the maximum peripheral shape. In FIG. 7 , the maximum peripheral shape calculated for the entire surface near the hole portion of the plate-shaped member 20 is plotted using black circles according to the distance from the center of the hole portion 21. As shown in FIG. 7, the maximum circumferential shape calculated in the circumferential direction of the end near the hole 21 is relatively large, and inside the plate-shaped member 20 spaced from the end face of the hole 21, the circumferential shape is relatively large. The maximum peripheral shape calculated in the direction has a maximum peripheral shape distribution that becomes smaller as it moves away from the hole portion 21 in the radial direction as a whole.

Figure 8 is a graph showing the relationship between the distance from the end face of the plate-shaped member and the maximum peripheral shape. In FIG. 8, the distance from the end surface of the hole portion 21 as the end surface of the plate-shaped member 20 is indicated on the horizontal axis, the maximum peripheral shape is indicated on the vertical axis, and the maximum peripheral shape is indicated as a black square. The maximum peripheral shape at a predetermined distance from the end surface of the hole portion 21, which is the end surface of the plate-shaped member 20, in the radial direction in the inner direction of the plate-shaped member 20 (direction perpendicular to the tangent to the end surface), The average value of all maximum peripheral shapes in the circumferential direction at a predetermined distance radially outward from the end surface of the hole portion 21 of the member 20 is calculated as the maximum peripheral shape.

As shown in FIG. 8, the maximum peripheral shape of the plate-shaped member 20 according to the hole expansion test becomes smaller as the plate-shaped member 20 is spaced apart from the end surface. In this embodiment, the maximum peripheral shape ε0 at the end surface of the plate-shaped member 20 is calculated, and the strain gradient α near the hole portion of the plate-shaped member 20 is separated from the end face of the plate-shaped member 20. Calculate the maximum peripheral strain gradient α at the given position.

As the strain gradient α, the absolute value of the maximum peripheral gradient between the distance d1 and d2 from the end surface of the plate-like member 20 was calculated as the strain gradient. As shown in FIG. 8, the distance from the end surface of the plate-shaped member 20 is the slope of the straight line connecting the maximum peripheral shapes ε1 and ε2 in d1 and d2 [(ε2-ε1)/(d2-d1) ] was calculated, and the absolute value of the slope of the straight line was calculated as the deformation gradient α. Additionally, the deformation gradient α may be calculated from the gradient of the maximum peripheral shape in another predetermined range spaced apart from the end surface of the plate-shaped member 20.

In this way, a cylindrical hole expansion test is performed to determine edge cracking or internal cracking as the state of cracking at the time of crack occurrence, and the end surface of the hole portion 21 of the plate-shaped member 20 at the time of cracking is determined. The maximum peripheral shape ε0 is calculated as the limit strain ε0 of the plate-shaped member 20, and the strain gradient α near the hole portion of the plate-shaped member 20 is also 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 the crack at the time of crack occurrence, and the plate-shaped member at the time of crack occurrence is determined. The maximum peripheral shape at the end surface of the hole 21 in (20) is calculated as the limit strain ε0 of the plate-shaped member 20, and the strain gradient α near the hole of the plate-shaped member 20 is also calculated.

Figure 9 is a schematic diagram of the cone hole expansion test. In this embodiment, a cone punch is used instead of a cylindrical punch in the hole expansion test device 10 described above, and as a hole expansion test, a cone hole expansion test is performed in addition to the cylindrical hole expansion test. Also, for each of the plurality of plate-shaped members 20, the state of cracking is determined by a hole expansion test, and the maximum circumferential shape on the end surface of the hole portion of the plate-shaped member at the time of crack occurrence is determined. It is calculated as the limit strain, and the strain gradient near the hole portion of the plate-shaped member is also calculated.

The conical hole expansion test device as a hole expansion test device is configured similarly to the cylindrical hole expansion test device 10, except for the punch. As shown in FIG. 9, the cone hole expansion test device 10 is equipped with a press tool 11 for press forming a plate-shaped member 20 having a hole portion 21 to generate a crack near the hole portion, there is. The press tool 11 is equipped with a die 12 and a blank holder 13 for holding the plate-shaped member 20, and a punch 15 for press forming the plate-shaped member 20. In the cone hole expansion test, a cone punch 15 whose tip is formed in a cone shape is used, and the cone punch 15 is arranged so that its central axis coincides with the hole portion 21 of the plate-shaped member 20.

In the cone hole expansion test, but not limited to this, die 12 is used with the die inner diameter D1 set to 53.8 mm, the die shoulder radius R1 set to 5 mm, the punch outer diameter D2 set to 50 mm, and the punch apex angle The punch 15 with θ1 set to 120 degrees was used. The thickness t of the plate-shaped member 20 was set to 1.6 mm, and the initial hole diameter D3 of the hole portion 21 used various hole diameters such as 20 mm. The hole portion 21 of the plate-shaped member 20 was punched using a die, a blank holder, and a punching punch with a clearance of 12% of the plate thickness.

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

Also for the cone hole expansion test, an image of the surface near the hole of the plate-shaped member 20 is captured by the camera 35 during the hole expansion test, and the captured image of the surface near the hole of the plate-shaped member 20 is Based on this, an operator or the like determines the crack state of the crack 22 occurring at the end of the plate-shaped member 20. As a crack state, if the crack occurrence position, which is the origin of the crack, is the crack occurrence position C1, which is the end surface of the hole portion 21, it is determined to be an edge crack, and the crack occurrence location, which is the origin of the crack, is the end surface of the hole portion 21. In the case of the crack occurrence position C2, which is inside the plate-shaped member 20 spaced apart from, it is determined to be crack-resistant.

The cone hole expansion test is similar to the cylindrical hole expansion test, and edge cracking or crack resistance is determined as the state of the crack when cracking occurs, and the end of the hole portion 21, which is the end surface of the plate-shaped member 20, is determined. The maximum peripheral shape at a predetermined distance in the radial direction (the direction perpendicular to the tangent to the end surface) which is the inner direction of the plate-shaped member 20 from the minor surface, in the radial direction from the end face of the hole portion 21 of the plate-shaped member 20. The average value of all maximum peripheral shapes in the surrounding direction at a predetermined distance outward is calculated as the maximum peripheral shape.

Then, the maximum peripheral shape ε0 at the end surface of the plate-shaped member 20 is calculated, and at a position spaced apart from the end surface of the plate-shaped member 20 as the strain gradient α near the hole portion of the plate-shaped member 20. The strain gradient α of the maximum peripheral type is calculated. Also for the cone hole expansion test, as the strain gradient α, the absolute value of the slope of the maximum peripheral shape between the distance d1 and d2 from the end surface of the plate-shaped member 20 was calculated as the strain gradient. Additionally, the deformation gradient α may be calculated from the gradient of the maximum peripheral shape in another predetermined range spaced apart from the end surface of the plate-shaped member 20.

In this way, a cone hole expansion test is performed to determine edge cracking or internal cracking as the state of cracking at the time of cracking, and at the end surface of the hole portion 21 of the plate-shaped member 20 at the time of cracking. The maximum peripheral shape ε0 is calculated as the limit strain ε0 of the plate-shaped member 20, and the strain gradient α near the hole portion of the plate-shaped member 20 is also calculated.

A cone 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 the crack at the time of crack occurrence, and the plate-shaped member at the time of crack occurrence is determined. The maximum peripheral shape ε0 at the end surface of the hole 21 in (20) is calculated as the limiting strain ε0 of the plate-shaped member 20, and the strain gradient α near the hole of the plate-shaped member 20 is also calculated. .

In FIG. 3, the strain gradient α is shown on the horizontal axis, and the maximum peripheral shape ε0 at the end surface of the plate-shaped member 20 is shown on the vertical axis as the limiting strain. From the calculation results of the strain gradient with the maximum peripheral shape as the limit strain by the cylindrical hole expansion test and the cone hole expansion test, when the strain gradient is relatively large, edge cracks occur in the plate-shaped member 20, and the strain gradient is relatively large. If it is small, it can be seen that internal cracks are occurring in the plate-shaped member 20.

In this embodiment, from the calculation result of the strain gradient with the maximum peripheral shape by the hole expansion test, the linear function approximation expression showing the relationship between the strain gradient and the maximum peripheral shape in the case of edge cracking is calculated using the known least squares. Edge crack limit strain characteristic data L1a, which is calculated according to the method and shows the relationship between the limit strain of the plate-like member 20 and the strain gradient at the edge crack, is calculated. The edge crack limit strain characteristic data L1a, which shows the relationship between the limit strain and the strain gradient of the plate-like member 20, has a limit strain characteristic in which the limit strain increases as the strain gradient increases.

In addition, from the results of calculating the strain gradient from the maximum peripheral shape by the hole expansion test, the quadratic function approximation expressing the relationship between the strain gradient and the maximum peripheral shape for the case of crack resistance was approximated using the known least squares method. Calculate, and calculate crack-resistant limit strain characteristic data L1b, which shows the relationship between the limit strain of the plate-shaped member in crack resistance and the strain gradient. Crack-resistant limit strain characteristic data L1b, which represents the relationship between the limit strain and the strain gradient of the plate-like member 20, has a limit strain characteristic in which the limit strain increases as the strain gradient increases.

Figure 11 is a schematic diagram of a uniaxial tensile test. In the evaluation of elongation flange cracks, a uniaxial tensile test was performed on a plate-shaped member made of the same material as the plate-shaped member used in the hole expansion test using a plate-shaped member that is the No. 5 test piece in JIS Z2241, and a uniaxial tensile test was performed on the plate-shaped member. By deforming, ductile fracture limit deformation characteristic data that sets the deformation at ductile fracture as the limit deformation regardless of the deformation gradient is acquired.

As shown in FIG. 11, the tensile test device 40 is provided with a lower gripper 41 and an upper gripper 42 that respectively grip the lower and upper portions of the plate-shaped member 50, which is a tensile test specimen. The plate-shaped 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 is formed by the same processing, such as machining, on the end surfaces on both sides of the plate width direction. As the plate-shaped member 50, although it is not limited to this, one with a plate thickness of 1.6 mm was used.

In the uniaxial tensile test, the lower gripping portion 41 and the upper gripping portion 42 are spaced apart from each other to apply a tensile load to the longitudinal center portion of the plate-shaped member 50 until the plate-shaped member 50 fractures after plastic deformation. Move in the direction (direction indicated by the white arrow). The tensile test device 40 includes a moving mechanism (not shown) that moves the lower gripping portion 41 and the upper gripping portion 42, and a control unit 60 that controls the operation of the moving mechanism. It is available.

The tensile testing device 40 is further provided with a camera 45 as an imaging device for imaging the surface of the longitudinal central portion of the plate-shaped member 50, and a camera 45 is provided to image the surface of the plate-shaped member 50. Two cameras 45 are placed on the sides. The two cameras 45 are arranged in symmetrical positions in the vertical direction with respect to a direction perpendicular to the longitudinal direction of the plate-shaped member 50, for example.

The image of the surface of the plate-shaped member 50 captured by the two cameras 45 is 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 analyzes the surface at the longitudinal center of the plate-shaped member 50 through analysis using a digital image correlation method. It is designed to calculate the maximum marginal form for the whole.

Also for the uniaxial tensile test, the control unit 60 images the paint dot pattern applied by spray on the surface of the plate-shaped member 50 before the tensile test using two cameras 45, and based on the images before and after the captured images, Thus, the maximum peripheral shape of the surface of the plate-shaped member 50 is calculated from the change in the positional relationship of the dot pattern. The control unit 60 is further configured to display the image captured by the camera 45 and the maximum peripheral shape of the longitudinal central portion of the plate-shaped member 50 on a display device (not shown) such as a display.

Fig. 12 is a diagram showing the maximum peripheral shape of the longitudinal central portion of the plate-shaped member. In Fig. 12, the maximum peripheral shape of the longitudinal central portion of the plate-shaped member 50 just before fracture in the tensile test is shown. In this embodiment, during the tensile test, an image of the surface of the longitudinal central portion of the plate-shaped member 50 is captured by the camera 45, and based on the captured image of the longitudinal central portion of the plate-shaped member 50, a digital The maximum peripheral shape of the longitudinal central portion of the plate-shaped member 50 is calculated using the image correlation method.

In the tensile test, the maximum peripheral strain occurs in the longitudinal direction of the plate-shaped member 50 at the center side of the longitudinal central portion of the plate-shaped member 50. As shown in FIG. 12, the maximum peripheral shape of the longitudinal central portion of the plate-shaped member 50 becomes smaller as it is spaced apart in the longitudinal direction from the center side of the longitudinal central portion of the plate-shaped member 50. It becomes smaller as it is spaced apart from the center side of the longitudinal central part in the direction of the plate width.

Figure 13 is a graph showing the relationship between the position of the plate-shaped member in the width direction and the maximum peripheral shape. In FIG. 13, with respect to the surface of the plate-shaped member 50 along line V-V in FIG. 12, the position in the plate width direction of the plate-shaped member 50 is shown on the horizontal axis, and the maximum peripheral shape of the plate-shaped member 50 is shown on the vertical axis. there is. As shown in FIG. 13, ductile fracture occurs between the plate width direction positions P10 and P11 on the central side of the plate member 50 in the plate width direction, and a crack is generated.

As shown in FIG. 13, when a tensile test is performed until the plate-shaped member 50 undergoes uniaxial tensile deformation and ductile failure, the maximum peripheral shape at the time of ductile failure of the plate-shaped member 50 is The maximum value εmax is taken at the center side in the sheet width direction (position P10 in the sheet width direction) in the longitudinal central part, and has a maximum peripheral distribution that becomes smaller toward one end and the other end in the sheet width direction.

In this embodiment, the maximum value εmax of the maximum peripheral shape at the time of ductile failure of the plate-shaped member 50 according to the uniaxial tensile test is acquired as the deformation at the time of ductile failure by the uniaxial tensile test. Then, for the plate-shaped member 50, ductile fracture limit strain characteristic data L1c is calculated, which determines the strain at ductile failure by the uniaxial tensile test 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-shaped member 50, regardless of the strain gradient.

In this embodiment, in the evaluation of elongation flange cracking, edge cracking limit strain characteristic data L1a, crack resistance limit strain characteristic data L1b, and ductile fracture limit strain characteristic data L1c are used as limit strain characteristic data L1 of the plate-like member 20. use. When the strain gradient is less than or equal to the predetermined value P1, using the edge cracking limit strain data L1a and the crack resistance limit strain property data L1b, which has a limit strain smaller than the ductile fracture limit strain property data L1c, the strain gradient is greater than the predetermined value P1 and the predetermined value P2. If the strain gradient is greater than the predetermined value P2, using the edge crack limit strain data L1a whose limit strain is smaller than the crack resistance limit strain characteristic data L1b and the ductile fracture limit strain characteristic data L1c, and the edge crack limit strain data L1a and The ductile fracture limit strain characteristic data L1c, which has a smaller limit strain than the crack resistance limit strain characteristic data L1b, is used.

During press forming analysis for press forming a press formed product having an elongation flange portion from a plate-shaped member, edge crack limit deformation characteristic data L1a, crack resistance limit deformation characteristic data L1b, and ductile fracture limit deformation characteristic data L1c are used to determine the elongation flange portion. Evaluate cracks. During press forming analysis, the maximum peripheral shape of each element of the analysis model divided into finite elements for the end of the elongation flange section and the strain gradient between the elements adjacent to the element in the direction away from the end of the elongation flange section are determined. Calculate, and the maximum peripheral shape at a given strain gradient for each element is the limit strain characteristic data L1 based on the edge crack limit strain characteristic data L1a, the crack resistance limit strain characteristic data L1b, and the ductile fracture limit strain characteristic data L1c. It is assessed that elongation flange cracks occur when the strain exceeds the limit.

As the stretching flange forming test, you may use another stretching flange forming test in which a plurality of plate-shaped members are formed so as to generate cracks at the ends of each plate-shaped member and the limit strain is calculated. In the case of using other stretching flange forming tests, it is also determined whether the crack state at the time of crack occurrence is an edge crack or an internal crack, and the limit strain by the stretching flange forming test and the internal direction of the plate-shaped member from the end surface of the plate-shaped member are determined. By calculating the strain gradient in (the direction perpendicular to the tangent of the end surface), edge cracking limit strain characteristic data L1a and cracking resistance limit strain characteristic data L1b are calculated. As a stretching flange forming test, it is also possible to use multiple stretching flange forming tests.

As a stretching flange forming test, in order to calculate characteristic data showing the relationship between the limit strain and strain gradient of a plate-shaped member in the case where the strain gradient is very small, cracks are generated at the ends of each plate-shaped member for a plurality of plate-shaped members. A one-sided punching tensile test can be used to calculate the limit strain by forming a plate-like member.

The one-side 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 one-sided punching tensile test, a plate-shaped member that is the No. 5 test piece in JIS Z2241 and has a parallel portion having a predetermined plate width and a predetermined plate thickness at the longitudinal central portion can be used. In the one-sided punching tensile test, the end surface on one side of the parallel portion in the sheet width direction is formed by punching to give a predetermined punched end surface shape, and the end surface on the other side of the parallel portion in the sheet width direction is formed by machining different from the punching processing. Shape it. As a punching process, for example, a die and a blank holder and a punching punch can be used to form a clearance of 12% of the sheet thickness.

When using a one-sided punching tensile test as a stretching flange forming test, the tensile test is performed until a crack penetrates the plate-shaped member in the plate thickness direction, thereby causing a crack to occur near the end of the parallel portion of the plate-shaped member in the plate width direction. . During the test, an image of the surface of the parallel portion of the plate-shaped member is captured by a camera, and based on the captured image of the surface of the parallel portion of the plate-shaped member, an operator or the like determines the cracking state of the crack occurring at the end of the plate-shaped member.

In the one-sided punching tensile test, the maximum peripheral shape ε0 at the end face of the plate-shaped member is calculated, and the strain gradient α near the end of the plate-shaped member is calculated from the end face of the plate-shaped member in the inner direction of the plate-shaped member ( The maximum peripheral deformation gradient α at a position spaced apart (in the direction perpendicular to the tangent of the end surface) is calculated.

As an elongation flange forming test, a one-side punching tensile test is used to determine whether the crack state at the time of crack occurrence in the elongation flange forming test is an edge crack or an anti-crack for each of a plurality of plate-shaped members, and an elongation flange forming test is performed. From the limit strain and the end surface of the plate-shaped member, the strain gradient in the inner direction of the plate-shaped member (direction perpendicular to the tangent to the end surface) can be calculated.

As the limit strain characteristic data L1, which represents the relationship between the limit strain and the strain gradient of the plate-shaped member, the edge crack limit strain characteristic data L1a and the crack resistance limit strain are determined according to the edge crack or inner crack, which is the state of cracking in the actual stretching flange forming test. Since the property data L1b is calculated and the edge crack limit strain property data L1a and the crack resistance limit strain property data L1b are used to evaluate the elongation flange crack, compared to the case of using the limit strain property data considering only the edge crack, the elongation flange crack The evaluation precision can be improved. In addition, since cracking in the elongation flange section is evaluated using ductile fracture limit strain characteristic data L1c, which uses the deformation at ductile failure in a uniaxial tensile test as the limiting strain, it is ensured that the limiting strain becomes excessively large even when the strain gradient is large. It can be suppressed and the evaluation accuracy of elongation flange cracks can be improved.

As shown in FIG. 3, when only the limit strain characteristic data L1a considering only edge cracks is used as the limit strain characteristic data L1 showing the relationship between the limit strain and the strain gradient of the plate-shaped member 20, the strain gradient is a predetermined value P1. In the case below, the limit strain is larger than the crack resistance limit deformation characteristic data L1b, and even if it is evaluated by press forming analysis that no elongation flange cracking has occurred, there is a risk that elongation flange cracking may occur during actual press forming.

In addition, when only the limit strain characteristic data L1a, which considers only edge cracks, is used, if the strain gradient is greater than the predetermined value P2, the limit strain is larger than the ductile fracture limit strain characteristic data L1c, and no stretching flange cracks occur according to the press forming analysis. Even if it is evaluated that it is not, there is a risk of cracking of the elongation flange during actual press molding.

In this embodiment, since the edge cracking limit deformation characteristic data L1a, the cracking resistance limit deformation characteristic data L1b, and the ductile fracture limit deformation characteristic data L1c are used, the evaluation accuracy of elongation flange cracking can be improved.

Figure 14 is a flowchart showing a method for evaluating elongation flange cracks. As shown in FIG. 14, when evaluating cracks in the stretching flange portion 8 of the press-formed product 1, a stretching flange forming test is first performed on the plate-shaped member 20 having the hole portion 21 (Step S1 ). In the stretching flange forming test, the cracking state when cracking occurs in the stretching 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 cracking state during the hole expansion test is an edge crack or an internal crack.

During the stretching flange forming test, the limit strain by the stretching flange forming test and the strain gradient in the inner direction of the plate-shaped member from the end surface of the plate-shaped 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 portion is calculated. If a crack occurs at the end of the hole 21 during the hole expansion test, the entire surface of the plate-shaped member 20 near the hole is cracked based on the surface image of the plate-shaped member 20 captured by the camera 35. The maximum peripheral shape of the plate-shaped member 20 is calculated. As described above, the limit strain and strain gradient of the plate-shaped member 20 are calculated from the maximum peripheral shape 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, and the plurality of plate-shaped members 20 are tested. In addition to determining the crack state, the limit strain and strain gradient are calculated by the hole expansion test, respectively.

Next, for the case where the crack state according to the stretching flange forming test is edge cracking, based on the limit strain and strain gradient of each plate-shaped member 20 when the crack state is edge cracking, the plate-shaped member at the edge crack Edge cracking limit strain characteristic data L1a, which represents the relationship between the limit strain and the strain gradient in (20), is calculated (step S4).

Even in the case where the crack state according to the stretching flange forming test is crack-resistant, based on the limit strain and strain gradient of each plate-shaped member 20 when the crack state is crack-resistant, the plate-shaped member 20 in the crack-resistant Crack resistance limit strain characteristic data L1b, which represents the relationship between the limit strain and the strain gradient, is calculated (step S5).

In steps S1 to S5, in order to calculate characteristic data showing the relationship between the limit strain and the strain gradient of the plate-shaped member for the case where the strain gradient is very small, along with the hole expansion test as the stretching flange forming test, a one-side punching tensile test is performed. You may do so.

Also in the case of performing a one-side punching tensile test, a one-side punching tensile test is performed (Step S1), the cracking state at the time of crack occurrence is determined (Step S2), and the limit strain and the strain from the end surface of the plate-shaped member are measured in the inner direction of the plate-shaped member. Calculate the strain gradient (step S3), and use edge cracking limit strain characteristic data L1a, which shows the relationship between the strain gradient and the strain gradient of the plate-like member at the edge crack, along with other stretching flange forming tests such as hole expansion tests, etc. Calculate (step S4) and calculate crack-resistant limit strain characteristic data L1b, which shows the relationship between the limit strain of the plate-shaped member in crack resistance and the strain gradient (step S5).

When evaluating cracks in the stretching flange portion 8, a uniaxial tensile test is further performed as a uniaxial tensile strain test on the plate-shaped member 50 made of the same material as the plate-shaped member 20 used in the stretching flange forming test (step S6). During the uniaxial tensile test, the maximum peripheral shape of the plate-shaped member 50 is calculated. During the tensile test, until the plate-shaped member 50 undergoes ductile fracture and cracks occur, the longitudinal direction of the plate-shaped member 50 is measured based on the image of the longitudinal central portion of the plate-shaped member 50 captured by the camera 45. Calculate the maximum peripheral shape of the central part.

When the plate-shaped member 50 undergoes ductile fracture and a crack occurs, the maximum value εmax of the maximum circumferential shape at the time of fracture of the plate-shaped member 50 is referred to as the deformation at the time of ductile fracture by the tensile test, and the tensile force is applied to the plate-shaped member 50. Ductile fracture limit strain characteristic data L1c, which refers to the strain at ductile fracture by test as the limit strain regardless of the strain gradient, is calculated (step S7).

Next, based on the edge crack limit strain characteristic data L1a, the crack resistance 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 calculated ( Step S8). The limit deformation characteristic data L1 is formed by the limit deformation characteristic data in which the limit deformation for a predetermined strain gradient is small among the edge cracking limit deformation characteristic data L1a, the cracking resistance limit deformation characteristic data L1b, and the ductile fracture limit deformation characteristic data L1c.

The limit deformation characteristic data L1 is formed by the crack resistance limit deformation characteristic data L1b when the deformation gradient is less than or equal to the predetermined value P1, and by the edge cracking limit deformation characteristic data L1b when the deformation gradient is greater than the predetermined value P1 and less than or equal to the predetermined value P2. is formed, and when the strain gradient is greater than the predetermined value P2, it is formed based on the ductile fracture limit strain characteristic data L1c.

Then, using the limit deformation characteristic data L1 of the plate-shaped member calculated in this way, a press forming analysis was performed to form a press-formed product having an elongation flange portion from the plate-like member, and the end portion of the elongation flange portion was divided into finite elements. By calculating the maximum peripheral shape of each element of the model and the strain gradient between the elements adjacent to the element in the direction away from the end of the elongation flange portion, and evaluating that cracks occur when the limit strain characteristic data is L1 or more, Evaluate extension flange cracks (step S9).

In the present embodiment, as a plate-shaped member, instead of a high-strength steel plate made of the above-described material A and having a tensile strength of about 980 MPa or more, a material using a 980-MPa high-strength steel plate similar to material A with a changed additive component is used. Similarly, for the plate-shaped member made of B and material C, limit strain characteristic data showing the relationship between the limit strain of the plate-shaped member 20 and the strain gradient were calculated.

As for the limit strain characteristic data using material B and material C, similar to the limit strain characteristic data L1 using material A, they were formed by edge crack limit strain data, crack resistance limit strain characteristic data, and ductile fracture limit strain characteristic data, respectively. . As for the limit deformation characteristic data using material B and material C, similarly to the plate-shaped member using material A, a press forming analysis is performed to form a press-formed product having an elongation flange portion from the plate-shaped member using the limit deformation characteristic data. , elongation flange cracks can be evaluated.

In this way, as limit deformation characteristic data showing the relationship between the limit deformation of the plate-shaped member and the strain gradient, the limit deformation characteristic data and edge cracking are based on the state of cracking or edge cracking in the actual stretching flange forming test. Calculate limit deformation characteristic data of and evaluate elongation flange cracks using limit deformation characteristic data of internal cracks and edge cracks. Compared to the case of using limit deformation characteristic data considering only edge cracks, the evaluation precision of elongation flange cracks is higher. can be improved. In addition, since cracking in the elongation flange section is evaluated using ductile fracture limit strain characteristic data, which uses the strain at ductile failure by the uniaxial tensile strain test as the limit strain, it is ensured that the limit strain becomes excessively large even in cases where the strain gradient is large. It can be suppressed and the evaluation accuracy of elongation flange cracks can be improved.

In this embodiment, as the hole expansion test, the cylindrical hole expansion test and the conical hole expansion test are used to obtain the limit deformation characteristic data of the crack resistance and the limit deformation characteristic data of the edge crack, but the tip is formed in the shape of a round head. By using a round head hole expansion test using a punch, it is also possible to determine the state of cracking and obtain limit strain characteristic data that shows the relationship between the limit strain and strain gradient of the plate-shaped member.

As the crack resistance limit deformation characteristic data L1b, from the calculation results of the strain gradient with the maximum peripheral shape by the hole expansion test, an approximate equation of the quadratic function showing the relationship between the strain gradient and the maximum peripheral shape in the case of crack resistance is obtained. Although it is calculated, it is also possible to calculate the crack resistance limit deformation characteristic data by calculating an approximation of the linear function.

In this embodiment, a 980 MPa high-strength steel sheet is used as the material for the plate-like member, but the same can be applied to other metal sheets such as other steel sheets and aluminum alloy sheets. In addition, regarding the stretching flange forming test, it is also possible to perform multiple stretching flange forming tests using a plurality of plate-shaped members under the same conditions and use the average value of the maximum peripheral shape of the multiple stretching flange forming tests.

As the limit deformation characteristic data L1, if the crack state at the time of crack occurrence by the elongation flange forming test is edge cracking or internal cracking, the edge cracking limit deformation data, the cracking limit deformation characteristic data, and the ductile fracture limit deformation characteristic data are used. The formed limit strain characteristic data is used, but if the crack state at the time of crack generation by the stretching 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 limit deformation characteristic data L1, at least edge cracking limit deformation characteristic data and ductile fracture limit deformation characteristic data among edge cracking limit deformation characteristic data, cracking resistance limit deformation characteristic data, and ductile fracture limit deformation characteristic data are used.

In this embodiment, 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 peripheral shape when a crack occurs in the stretch flange forming test. In testing, it is also possible to calculate limit strain characteristic data showing the relationship between limit strain and strain gradient using the maximum peripheral shape just before cracking occurs.

In this embodiment, in the stretching flange forming test, the surface of the plate-shaped member is imaged with a camera, and the limiting strain and strain gradient are calculated based on the image of the captured surface. However, other methods such as the scribed circle method are used. You can also use it to calculate the limiting strain and strain gradient. Also for the uniaxial tensile strain test, the strain at ductile fracture may be calculated using another method such as the scribed circle method. In addition, in the stretch flange forming test, the crack state is determined based on the surface image captured by a camera, but other methods may be used to determine the state.

In this way, the stretching flange crack evaluation method according to the present embodiment performs a stretching flange forming test on each of the plurality of plate-shaped members 20, and determines whether the crack state at the time of crack generation by the stretching flange forming test is an edge crack or crack resistance. In addition to determining the recognition, the limit strain and strain gradient are calculated by the stretching flange forming test, and as limit strain characteristic data L1 that shows the relationship between the limit strain and strain gradient of the plate-like member 20, edge crack limit strain characteristic data L1a and calculate the crack resistance limit deformation characteristic data L1b. Additionally, a uniaxial tensile strain test is performed on the plate-shaped member 50, and ductile fracture limit strain characteristic data L1c is calculated, with the strain at ductile failure by the uniaxial tensile strain test as the limit strain. Then, cracks in the elongated flange portion were 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 L1a, the crack resistance limit strain characteristic data L1b, and the ductile fracture limit strain characteristic data L1c. do.

As a result, as the limit strain characteristic data L1 that represents the relationship between the limit strain of the plate-shaped member and the strain gradient, the edge crack limit strain characteristic data L1a and Crack limit deformation characteristic data L1b is calculated, and elongation flange cracks are evaluated using edge crack limit deformation characteristic data L1a and crack resistance limit deformation characteristic data L1b. Compared to the case of using limit deformation characteristic data considering only edge cracks, The evaluation accuracy of elongation flange cracks can be improved. In addition, since cracking in the elongation flange portion is evaluated using ductile fracture limit strain characteristic data L1c, which uses the strain at ductile failure in the uniaxial tensile strain test as the limiting strain, even in cases where the strain gradient is large, the limiting strain does not become excessively large. can be suppressed, and the evaluation accuracy of elongation flange cracks can be improved. For plate-shaped members whose crack state at the time of crack occurrence is only edge cracking, the evaluation accuracy of elongation flange cracking can be improved by using edge cracking limit strain characteristic data and ductile fracture limit strain characteristic data.

Additionally, the elongation flange crack evaluation method evaluates cracks in the elongation flange portion using edge crack limit deformation characteristic data, crack resistance limit deformation characteristic data, and ductile fracture limit deformation characteristic data. As a result, the evaluation accuracy of elongation flange cracks can be improved for plate-shaped members whose crack state at the time of crack occurrence is an edge crack or an internal crack.

In addition, the elongation flange crack evaluation method uses edge crack limit deformation characteristic data L1a, crack resistance limit deformation characteristic data L1b, and ductile fracture limit deformation characteristic during press forming analysis for press forming a press formed product having an elongation flange portion from a plate-shaped member. Among the data L1c, at least the edge crack limit deformation characteristic data L1a and the ductile fracture limit deformation characteristic data L1c are used to evaluate cracking in the elongated flange portion. As a result, during press forming analysis, the maximum peripheral shape of each element of the analysis model divided into finite elements for the end of the elongation flange portion and the elements adjacent to the element in the direction away from the end of the elongation flange portion are By calculating the strain gradient, cracks in the stretching flange portion in press forming analysis can be evaluated with high accuracy.

In addition, the stretching flange crack evaluation method is based on the surface image of the plate-shaped member 20 captured by the camera 35 in the stretching flange forming test, and the surface image of the plate-shaped member 20 captured by the camera 35. Then, the limit strain and strain gradient are calculated for the plate-shaped member 20 by the stretching flange forming test. As a result, the limit strain and strain gradient can be calculated using the digital image correlation method, and it is possible to calculate the limit strain and strain gradient with high accuracy.

In addition, the stretching flange crack evaluation method is based on the surface image of the plate-shaped member 20 captured by the camera 35 in the stretching flange forming test, and the surface image of the plate-shaped member 20 captured by the camera 35. Then, it is determined whether the crack state at the time of crack occurrence in the plate-shaped member 20 by the stretching flange forming test is an edge crack or an internal crack. Accordingly, by finding the crack origin when cracking occurs based on the surface image of the plate-shaped member 20, the determination accuracy of the crack state of edge cracking or internal cracking can be improved, and the evaluation accuracy of flange cracking can be improved.

In addition, the elongation flange crack evaluation method is based on the surface image of the plate-shaped member 50 captured by the camera 45 in the uniaxial tensile strain test, and the surface image of the plate-shaped member 50 captured by the camera 45. Then, the strain at ductile failure of the plate-shaped member 50 is calculated by a uniaxial tensile strain test. As a result, it is possible to calculate the strain at ductile fracture using a digital image correlation method, and it is possible to calculate the ductile fracture limit strain with high accuracy. Based on the image captured by the camera, the limit strain and strain gradient are calculated for the plate-shaped member by the stretching flange forming test, and the strain at ductile failure is calculated for the plate-shaped member by the uniaxial tensile strain test, Using the digital image correlation method, the limit strain and strain gradient can be calculated, as well as the ductile fracture limit strain, and the evaluation accuracy of elongation flange cracks can be improved.

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.

As described above, according to the present invention, the evaluation accuracy of stretching flange cracks can be improved, so it can be suitably used when simulating press molding to form a press molded product having an stretching flange portion from a plate-shaped member.

1: Press molded product
8: Kidney flange part
10: Hole expansion test device
14, 15: Punch
20, 50: plate-shaped member
22: crack
30, 60: Control unit
35, 45: Camera
40: tensile test device
L1: Limit strain characteristic data
L1a: Edge crack limit strain characteristic data
L1b: Crack resistance limit deformation characteristic data
L1c: Ductile fracture limit strain characteristic data

Claims (6)

It is an elongation flange crack evaluation method for evaluating cracks in the elongation flange part of press molded products,
A stretch flange forming test is performed on a plurality of plate-shaped members to calculate the limit strain by forming them to generate cracks at the ends of each plate-shaped member,
A uniaxial tensile strain test is performed to calculate the strain at ductile fracture by subjecting plate-shaped members of the same material as the plurality of plate-shaped members to uniaxial tensile strain,
For each of the plurality of plate-shaped members, it is determined whether the crack state at the time of crack occurrence by the stretch flange forming test is an edge crack or an internal crack, and from the limit strain by the stretch flange forming test and the end surface of the plate-shaped member, Calculating a strain gradient in the inner direction of the plate-shaped member,
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 crack, edge crack limit strain characteristics showing the relationship between the limit strain and strain gradient of the plate-shaped member in edge cracking. calculate data,
Based on the limit strain and strain gradient of each plate-shaped member when the crack state in the stretching flange forming test is crack-resistant, the crack-resistant limit strain characteristic shows the relationship between the limit strain and strain gradient of the plate-shaped member in crack-resistant. calculate data,
For the plate-shaped member, calculate ductile fracture limit strain characteristic data in which the strain at ductile failure by the uniaxial tensile strain test is the limit strain regardless of the strain gradient,
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 crack resistance limit strain characteristic data, and the ductile fracture limit strain characteristic data, cracking of the elongated flange portion is performed. evaluating,
How to evaluate elongation flange cracks. According to paragraph 1,
An elongation flange crack evaluation method for evaluating cracks in an elongation flange portion using the edge crack limit deformation characteristic data, the crack resistance limit deformation characteristic data, and the ductile fracture limit deformation characteristic data. According to claim 1 or 2,
When performing a press forming analysis for press forming a press formed product having an elongated flange portion from a plate-shaped member, at least the edge cracking limit strain characteristic data, the cracking resistance limit strain characteristic data, and the ductile fracture limit strain characteristic data are selected. An elongation flange crack evaluation method for evaluating cracks in an elongation flange portion using property data and the ductile fracture limit strain characteristic data. According to claim 1 or 2,
In the stretching flange forming test, the surface of the plate-shaped member is imaged with a camera,
An elongation flange crack evaluation method for calculating a limit strain and a strain gradient by the elongation flange forming test for the plate member, based on a surface image of the plate member captured by the camera. According to claim 1 or 2,
In the stretching flange forming test, the surface of the plate-shaped member is imaged with a camera,
Based on the surface image of the plate-shaped member captured by the camera, an extension flange crack evaluation method determines whether the crack state of the plate-shaped member at the time of crack generation by the extension flange forming test is an edge crack or an internal crack. . According to claim 1 or 2,
In the uniaxial tensile strain test, the surface of the plate-shaped member is imaged with a camera,
An elongation flange crack evaluation method that calculates strain at ductile fracture for the plate-shaped member by the uniaxial tensile strain test, based on a surface image of the plate-shaped member captured by the camera.

Images