JP6828476B2 - Edge fracture prediction method, program and recording medium - Google Patents

Description

The present invention relates to an edge fracture prediction method, a program, and a recording medium.

In recent years, the application of high-strength steel sheets to automobile bodies has been rapidly advancing due to the demand for collision safety and weight reduction. These high-strength steel sheets can increase the absorbed energy and strength at the time of collision without increasing the plate thickness. However, the decrease in ductility due to the increase in strength of the steel sheet increases the risk of fracture during press forming and collision deformation, so there is an increasing need for fracture prediction of materials by the finite element method and its high accuracy. ..

The margin for fracture during molding or collision deformation is generally determined by using the plate thickness reduction rate or the molding limit diagram (FLD). The FLD is a diagram showing the maximum principal strain that gives the fracture limit for each minimum principal strain, and is used for fracture evaluation in molding analysis and collision analysis. Generally, the method of measuring FLD by an experiment is to draw a circle-shaped or lattice-shaped pattern on the surface of a metal plate in advance by etching or the like, break it by hydraulic molding or overhanging with a rigid body tool, and then deform the circle. Measure the breaking limit strain from the quantity. The fracture limit line is obtained by proportionally loading a metal plate for various in-plane strain ratios, plotting the fracture limit strain at each strain ratio on the main strain axis, and connecting them with a line (Fig. 1).

On the other hand, there are various theoretical predictions of FLD, such as the combined use of Hill's local constriction model and Swift's diffusion constriction model, the Marciniak-Kuczynski method, and the Storen-Rice model. Ductile fracture of the material occurs at the location where the deformation is localized due to the local constriction. When this local constriction occurs, it breaks in an extremely short time. Therefore, in practice, the fracture limit is often considered to be the local constriction occurrence limit, and the fracture limit prediction is often handled in the framework of plastic instability. The risk of fracture is evaluated by comparing the positional relationship between the fracture limit line obtained in this way and the strain state of each part obtained from the results of numerical simulation by the finite element method, and the strain in the deformation process is this. When the limit strain is reached, it is judged that there is a high risk of breakage or its breakage.

Japanese Unexamined Patent Publication No. 2014-137185 Japanese Unexamined Patent Publication No. 2011-140046

FLDs obtained from experiments and theoretical predictions are intended for when materials separate under uniform stress conditions or when local constrictions occur (Fig. 2). However, in stretch flange molding in which cracks occur from the end of the steel sheet, the strain decreases inward from the flange end, so that the material end is constrained inside and the occurrence of constriction is suppressed (Fig. 3). .. That is, even if the end of the stretch flange satisfies the fracture condition in the uniform distribution, the condition is not yet reached on the inside, so that the plastic instability state cannot be achieved as a whole due to the support effect on the inside, and the fracture occurs. Not reachable. This point is different from the occurrence of local constriction in a uniform stress field such as uniaxial tension, overhang, and deep drawing, and when there is a strain gradient from the flange end to the inside like stretch flange fracture. The conditions under which the unstable neck is generated have not yet been clarified. Further, the fracture mechanism is complicated due to the influence of microscopic damage introduced to the end of the steel sheet during shearing, and the prediction accuracy cannot be ensured by the conventional fracture prediction by FLD due to the influence of the strain gradient described above.

Furthermore, the following problems have become apparent in edge fracture during collision deformation, which makes it difficult to ensure the accuracy of fracture prediction.
As shown in the fracture limit line of FIG. 4, it is known that the fracture limit line changes greatly depending on the strain path. For example, (a) the fracture limit line when a linear path change is applied without a change in the edge path, and (b) the fracture limit in the case of a path change in which equibiaxial tensile deformation is applied after uniaxial tensile prestrain. The line increases significantly. In many cases, the fracture limit line decreases in the case of a path change in which uniaxial tension is applied after biaxial tensile prestrain such as (c) or a path change in which planar strain tensile deformation is applied after biaxial tensile prestrain such as (d). It has become clear from experiments and numerical analysis. In the process of collision deformation of automobile body parts that have undergone press molding or pre-deformation in press molding, the deformation path often changes significantly, and when evaluating fracture using the fracture limit line obtained from experiments, it depends on the deformation path. There is no choice but to prepare innumerable limit lines. Therefore, in practice, the fracture evaluation uses the fracture limit line for the proportional load path, and high prediction accuracy cannot be expected.

The present invention has been made in view of the above problems, and when predicting the fracture of a material that undergoes a process in which the deformation path changes significantly, such as collision deformation after molding, cracks occur particularly at the plate edge. , An object of the present invention is to provide an edge fracture prediction method, a program and a recording medium capable of accurately quantitatively evaluating the risk of fracture of an edge having a strain gradient from the end to the inside by a finite element method. And.

In order to solve the above problems, the present inventors have come up with the following aspects of the invention as a result of diligent studies. The gist of the present invention is as follows.

1. 1. This is a method for evaluating the breakage of the edge of the material during the collision deformation process of automobile body parts.
The first step of inputting two or more different hole expansion rates obtained from the hole expansion test into the computer ,
The computer has a second step of calculating the two or more different breaking limit stresses and the two or more different radial stress gradients from the two or more different hole expansion ratio data.
The computer calculates the fracture criteria from the relationship between the two or more fracture limit stresses and the two or more stress gradients, and the maximum principal stress obtained from the numerical analysis by the finite element method and the stress gradient between adjacent elements. Includes a third step of assessing a fracture when and reaches the fracture criteria.
An edge fracture prediction method, characterized in that the computer evaluates the moldability of the material based on the analysis results obtained by executing the first step to the third step as a series of steps. ..

2. 2. 1. The first step is characterized in that the respective hole expansion rates obtained from the two or more different apex angle conical hole expansion tests are input to the computer . The edge fracture prediction method according to.

3. 3. In the first step, inputting the hole expansion rate obtained from a hole expansion test using a single-shaped apex angle conical tool using the materials having the two or more different initial hole diameters to the computer. Features 1. The edge fracture prediction method according to.

4. In the first step, inputting the hole expansion rate obtained from a hole expansion test using a cylindrical tool having a single shape with an apex angle using the materials having the two or more different initial hole diameters into the computer. Features 1. The edge fracture prediction method according to.

5. In the second step, the computer converts the circumferential strain distribution ε _θ and the radial strain distribution ε _r expressed in the strain space measured from the experiment, or the theoretically estimated strain distribution into the stress space. It is characterized by calculating the circumferential stress distribution σ _θ and the radial stress distribution σ _r . ~ 4. The edge fracture prediction method according to any one of the above items.

6. 1. When the computer calculates the stress gradient dσ _θ / dr, the element size used for the analysis of the finite element method is set to the reference length dr. ~ 5. The edge fracture prediction method according to any one of the above items.

7. The computer calculates the breaking criterion σ ^cr = f (dσ _θ / dr) from the two or more breaking limit stresses and the two or more stress gradients, and from numerical analysis by the finite element method between adjacent elements. The stress gradient ds ₁₁ / dr and the maximum principal stress σ ₁₁ are obtained, and an index of whether or not the stress gradient ds ₁₁ / dr and the maximum principal stress σ ₁₁ reach the fracture criteria. It is characterized in that σ ₁₁ / σ ^cr is calculated as, and the result is contour-displayed. ~ 6. The edge fracture prediction method according to any one of the above items.

8. When the dynamic solution method of the finite element method is used as a means of numerical analysis, the computer is characterized in that the plastic strain obtained from the numerical analysis is converted into stress and compared with the fracture criteria. 1. ~ 4. , 7. The edge fracture prediction method according to any one of the above items.

9. When evaluating fracture using a numerical analysis considering the rate dependence of the deformation stress of the material, the computer converts the plastic strain obtained from the numerical analysis to obtain the stress at the reference strain rate, and obtains the stress at the reference strain rate. It is characterized in comparison with the above-mentioned fracture criteria in 1. ~ 4. , 7. , 8. The edge fracture prediction method according to any one of the above items.

10. It is a program for evaluating the fracture of the edge part of the material in the process of collision deformation of automobile body parts.
The first step of entering two or more different drilling rates obtained from the drilling test,
A second step of calculating the two or more different breaking limit stresses and the two or more different radial stress gradients from the two or more different hole expansion rate data.
The fracture criterion is calculated from the relationship between the two or more fracture limit stresses and the two or more stress gradients, and the maximum principal stress obtained from the numerical analysis by the finite element method and the stress gradient between adjacent elements are the fractures. Including a third step of assessing a rupture when the criteria are reached, including
An edge fracture prediction program for evaluating the formability of the material based on the analysis results obtained by executing the first step to the third step as a series of steps.

11. 10. The first step is to enter the respective hole expansion rates obtained from the two or more different apex angle conical hole expansion tests. The edge fracture prediction program described in.

12. The first step is characterized in that the hole expansion rate obtained from a hole expansion test using a single-shaped apex angle conical tool using the materials having the two or more different initial hole diameters is input. 10. The edge fracture prediction program described in.

13. In the first step, the hole expansion rate obtained from a hole expansion test using a cylindrical tool having a single shape and an apex angle using the materials having the two or more different initial hole diameters is input. 10. The edge fracture prediction program described in.

14. In the second step, the circumferential stress is converted into the stress space by converting the circumferential strain distribution ε _θ and the radial strain distribution ε _r expressed in the strain space measured from the experiment, or the theoretically estimated strain distribution into the stress space. 10. The distribution σ _θ and the radial stress distribution σ _r are calculated. ~ 13. The edge fracture prediction program according to any one of the above items.

15. When calculating the stress gradient dσ _θ / dr, the element size used in the analysis of the finite element method is set to the reference length dr. ~ 14. The edge fracture prediction program according to any one of the above items.

16. The fracture criterion σ ^cr = f (dσ _θ / dr) is calculated from the two or more breaking limit stresses and the two or more stress gradients, and the stress gradient between adjacent elements is calculated from the numerical analysis by the finite element method. A certain ds ₁₁ / dr and the maximum principal stress σ ₁₁ are obtained, and σ ₁₁ / is used as an index of whether or not the stress gradient ds ₁₁ / dr and the maximum principal stress σ ₁₁ reach the fracture criteria. 10. The feature is that σ ^cr is calculated and the result is contour-displayed. ~ 15. The edge fracture prediction program according to any one of the above items.

17. When the dynamic solution method of the finite element method is used as a means of numerical analysis, the plastic strain obtained from the numerical analysis is converted into stress and compared with the fracture criteria. ~ 13. , 16. The edge fracture prediction program according to any one of the above items.

18. When evaluating fracture using a numerical analysis that considers the rate dependence of the deformation stress of the material, the plastic strain obtained from the numerical analysis is converted to obtain the stress at the reference strain rate, and the fracture criteria at the reference strain rate. It is characterized by comparing with 10. ~ 13. , 16. , 17. The edge fracture prediction program according to any one of the above items.

19.10. ~ 18. A computer-readable recording medium comprising recording the edge fracture prediction program according to any one of the above items.

According to the present invention, it is possible to estimate the fracture criteria of the edge portion in consideration of the influence of the strain gradient from the plate end portion and the punching state from the hole expansion ratio of the conical hole expansion test having two or more different apex angles. By applying this to molding and collision analysis, the risk of fracture of the edge portion can be quantitatively evaluated. As a result, it is possible to avoid breakage of the edge portion, which is one of the applications of high-strength steel sheets, and to realize efficient and highly accurate development of an automobile body in consideration of materials, construction methods, and structures at the same time.

It is a molding limit diagram (FLD) used for the explanation of the prior art. It is a characteristic diagram for demonstrating the local constriction in a uniform stress state. It is a characteristic figure for demonstrating the strain gradient from the plate end portion of the extension flange portion toward the inside. It is a characteristic diagram for demonstrating the distortion path dependence of FLD. It is a schematic diagram for demonstrating the hole expansion test. It is a schematic diagram for solving the distortion and the strain gradient of a hole edge in a hole expansion test. It is a characteristic diagram which compares the calculation result of the strain and strain gradient of a hole edge in a hole expansion test with an experiment. It is a characteristic diagram which shows the calculation result of the radial stress and the stress distribution of a hole edge. It is a characteristic diagram which shows the relationship between the stress gradient which is a fracture criterion and the fracture limit stress. This is one of the present embodiments, and is a characteristic for explaining the result of calculating the stress and stress gradient of the hole edge from the hole expansion ratio when materials having different hole diameters are tested by a conical tool, and the fracture criterion using the result. It is a figure. This is one of the present embodiments, and is a characteristic for explaining the result of calculating the stress and stress gradient of the hole edge from the hole expansion ratio when materials having different hole diameters are tested by a conical tool, and the fracture criterion using the result. It is a figure. It is a flowchart which shows the edge part fracture prediction method of this embodiment. It is a schematic diagram which shows the result of having evaluated the breakage risk by this embodiment. It is a schematic diagram which shows the internal structure of the personal user terminal apparatus.

(Basic outline of fracture prediction method)
In stretch flange molding in which cracks occur from the end of the steel sheet, the strain decreases inward from the end of the flange, so that the end of the material is constrained inside and the occurrence of constriction is suppressed. Further, the fracture mechanism is complicated due to the influence of microscopic damage introduced to the end of the steel sheet during shearing, and the prediction accuracy cannot be ensured by the conventional fracture prediction by FLD. Therefore, we came up with a technique for predicting fracture of the edge portion by utilizing the result of the hole expansion test considering the strain gradient from the plate edge portion and the punched state of the end portion.

Since the forming limit of the edge portion occurs when the circumferential elongation strain of the flange end reaches the limit value, the circumferential elongation strain of the flange end is considered as an index indicating the molding difficulty, and usually, the conical punched hole is widened. It is evaluated by the hole expansion rate λ obtained by the test. In the hole expansion test, a circular hole with a diameter of 10 mm is generally made in the material, the hole is expanded by a conical punch with an apex angle of 60 °, and the fracture limit is when a crack penetrates the plate thickness at the edge of the hole. To do. It can be obtained by calculating the hole expansion ratio λ = (d−d ₀ ) / d ₀ × 100 from the diameter d and the initial diameter d ₀ at that time (FIG. 5). The holes at this time are generally sheared by setting the clearance between the punch and the die to be 12% of the plate thickness. Therefore, microscopic damage such as dimples, cracks, and voids is observed on the fracture surface at the flange end. Therefore, the hole expansion test is also a test method that takes into account the microscopic damage introduced during shearing.

It is known that this hole expansion ratio greatly changes depending on the test dimensions such as the hole diameter and the punch diameter and the shape of the punch bottom. This is because the stretch flange forming limit is strongly affected by the strain gradient that decreases inward from the flange end. Therefore, it is necessary to evaluate the fracture limit by an evaluation test that combines the strain state (strain gradient from the plate end to the inside) introduced in the stretch flange molding of the actual part and the strain gradient in the hole expansion test. However, in stretch flange fracture, which is a problem in actual parts, the strain distribution often changes greatly depending on the shape of the part and the shape of the blank. Therefore, when evaluating fracture using the hole expansion rate obtained from the experiment, it is necessary to prepare the fracture limit by changing the hole diameter, punch diameter, etc. innumerably according to the strain gradient.

On the other hand, the so-called side bend test method is used as a method for examining the relationship between the strain state (strain gradient from the plate edge to the inside, the strain gradient along the edge) and the fracture limit introduced in the stretch flange molding of the actual part. It has been proposed (see, for example, Patent Documents 1 and 2). Its feature is that it introduces an image processing system for measuring distortion and is equipped with a camera for observing the behavior of cracking on the end face. Based on the relationship between the fracture limit strain and the strain distribution obtained from this method, a method has been proposed in which a fracture limit curved surface is defined and used for prediction of elongation flange fracture.

However, in order to obtain a strain gradient in the radial direction from the end by this method, a circle-shaped or grid-like pattern is drawn in advance on the surface of the material by etching or the like, and the strain gradient is measured from the deformation amount of the circle after deformation. There is a need to. There is also a method of obtaining the fracture limit of various strain gradients by numerical simulation, but it is complicated to obtain the fracture limit for innumerable test piece shapes corresponding to the strain distribution which is a problem in actual parts. Practically difficult.

Therefore, in the present invention, a method is investigated in which the fracture limit strain at the plate end and the strain gradient in the radial direction can be easily obtained from the test value of the hole expansion rate obtained from the hole expansion test, specifically, the conical hole expansion test. did.
FIG. 5 shows an outline of the hole expansion test using a conical punch. Here, the angle formed by the normal of the conical surface of the conical punch and the z-axis is φ (the half apex angle of the conical punch is 90 ° −φ). Then, the equilibrium equation of the elements of the material being deformed axisymmetrically is given below.

Here, r is the radial coordinates indicating the position of the element after deformation, t is the plate thickness after deformation, σ _θ and σ _φ are the stresses in the circumferential direction and the radial direction, respectively, and μ is the friction coefficient.

Subsequently, the circumferential strain ε _θ and the radial strain ε _φ are given as follows.

However, s represents the radial coordinates of the plate elements before deformation. From these, the conforming condition equation for the strain of the following equation can be obtained.

Further, it is assumed that the deformation is based on the total strain theory, and the work hardening characteristics of the material are approximated by the nth power hardening law.

Then, the strain distributions in the circumferential direction and the radial direction are given by the following equations, respectively.

Here, the first-order fine coefficient (strain gradient) and the second-order fine coefficient are given by the following equations, respectively.

Here, for example, when a base plate having an initial hole diameter d _{0 has} a diameter d by a hole expansion test, the strain at the end is obtained by the following equation.

From these equations and equations (6) to (11), it is possible to calculate the radial strain distribution of the circumferential strain ε _θ and the radial strain ε _φ .

Hereinafter, a method of calculating the failure limit criteria will be described by taking a 1.4 mm thick 980 MPa class high-strength steel sheet as an example. First, a conical hole expansion test with different apex angles is performed offline. Here, 30 ° conical and 60 ° conical punches were used as examples. At this time, the clearance between the die and the punch was set to 12% of the plate thickness, and a hole having a diameter of 10 mm was punched in the center of the base plate. This base plate is subjected to a hole expanding test, and the breaking limit is set when a crack penetrates the plate thickness at the edge of the hole, and the hole expanding rate λ = (d−d ₀ ) / from the diameter d and the initial diameter d ₀ at that time. d ₀ × 100 was calculated. As a result, the hole expansion rates of the 30 ° cone and the 60 ° cone were 42% and 35%, respectively.

From this result and equations (6) to (13), the strain distribution of the radial method can be calculated from the plate edges of the circumferential strain ε _θ and the radial strain ε _r when the respective conical punches are used. The result is shown in FIG. The experimental value was obtained from the amount of deformation of the circle after the test by etching a concentric scribed circle with a distance between the scores of 0.5 mm on the material. As a result, it was confirmed that the calculation result reproduces the experiment with good accuracy. That is, in both the results of the 30 ° cone and the 60 ° cone, the maximum value of the circumferential strain ε _θ was observed at the hole edge, and it decreased monotonically from the hole edge to the inside. In the hole edge, the 30 ° cone has a larger circumferential strain ε _θ than the 60 ° cone, but the tendency is reversed from the hole edge to the inside, and the 60 ° cone is sufficiently far from the hole edge. Showed a large value. On the other hand, the radial strain ε _r shows the minimum value at the hole edge and increases as the distance from the hole edge increases.

However, it is also known that the fracture limit strain changes greatly depending on the strain path, and it is difficult to use the fracture limit strain in collision analysis in which it is not always necessary to follow the same strain path in press working and collision analysis. Therefore, the inventors focused on the rupture limit of the stress space as a rupture limit criterion in which the influence of the strain path is relatively small, added a device to consider the dependence on the strain rate at the time of collision deformation, and performed collision analysis on the edge part. I came up with a method to predict breakage.

Conversion from strain to stress is performed by assuming (1) constant volume law, (2) Mises yield function, (3) isotropic hardening by work hardening rule, (4) vertical rule, and (5) plane stress. can do. First, the plate thickness strain ε _t can be obtained from the constant volume law ε _t = − (ε _θ + ε _r ). Then, the equivalent plastic strain ε _eq can be obtained by the following equation.

In this way, by using the equivalent plastic strain ε _eq obtained by integrating the equivalent plastic strain increment dε _eq with the strain path and the n-th power hardening law, the equivalent plastic stress σ _eq considering the deformation path change can be obtained. Next, the deviation stress component σ _{ij'is the} isotropic hardening of the yield surface and the perpendicular rule.

Obtained by Finally, by assuming plane stress (σ _t = 0, the stress component is

More obtained. The radial distribution of the circumferential stress σ _θ calculated in this way is shown in FIG.

Subsequently, from this calculation result, the circumferential breaking limit stress corresponding to the stress gradient is calculated. Since the circumferential stress σ _θ gradually decreases as it moves away from the hole edge, the stress gradient changes depending on the reference magnitude dr. Therefore, in calculating the stress gradient _dσ θ / dr, by the element size used for the analysis of the finite element method and the reference length dr, increased accuracy of the fracture prediction.

From the relationship between the two or more stress gradients | dσ _θ / dr | obtained in this way and the breaking limit stress σ ^cr = (σ _θ ) _{r = 0} at the hole edge, a linear approximation or a multi-linear connecting them Let the data be fracture criteria σ ^cr = f (dσ _θ / dr) (Fig. 9).

Next, collision analysis of parts to evaluate the risk of fracture is performed by numerical analysis by the finite element method, and from the resulting maximum principal stress distribution, the stress gradient dσ ₁₁ / dr and maximum principal stress σ between adjacent elements ₁₁ is obtained, and if these reach the rupture criteria, it is judged that the risk of rupture is high. Furthermore, σ ₁₁ / σ ^cr (the magnitude of OR / OA in FIG. 9) is calculated as an index for quantitatively evaluating the risk of breakage, and the result is contoured.

(Other examples of fracture prediction method)
As a hole expanding test when determining a fracture criterion, in addition to conical hole expanding with two or more different apex angles, a hole expanding test is performed in which materials having different hole diameters are expanded by a single-shaped conical punch or a cylindrical punch. You may do so. In the case of different hole diameters, the initial hole diameter d ₀ of the equation (13) may be set as each hole diameter.

[1] Hole expansion test using a conical punch As an example of a hole expansion test using a conical punch, a material obtained by punching a hole with a diameter of 10 mm and a hole with a diameter of 50 mm in the center of the base plate using a 60 ° conical punch is used. It was subjected to a hole expansion test. As a result, the hole expansion rates using the hole having a diameter of 10 mm and the material having a diameter of 50 mm were 42% and 18%, respectively. From this result and equations (6), (9) to (15), the radial strain distribution when each initial hole diameter is used can be calculated. The result is shown in FIG. 10 (a). FIG. 10 (b) shows a fracture criterion determined from the relationship between the two or more strain gradients obtained here and the fracture limit strain at the hole edge.

[2] Hole expansion test using a cylindrical punch When a cylindrical punch is used, the fracture criteria can be calculated in the same manner by setting 2φ = 180 ° with respect to equations (1) to (12). Here, a single-shaped cylindrical punch (punch shoulder radius 10 mm, die shoulder radius 10 mm) is used for materials having different hole diameters, and a hole (d = 10 mm) with a diameter of 10 mm and a diameter of 50 mm (d = 50 mm) are used in the center of the base plate. The material punched out from the hole was used for the hole expansion test. As a result, the relationship between the fracture limit stress of the hole edge calculated with φ = 180 ° for equations (6) and (9) to (15) and the distance from the hole edge is shown in FIG. 11 (a). The relationship between the critical stress and the stress gradient is shown in FIG. 11 (b).

However, when evaluating rupture using the stress obtained from collision analysis, innumerable dynamic rupture limit lines are required according to the strain rate, and it is practically difficult to evaluate rupture. Furthermore, when the dynamic explicit method is used for the numerical simulation, the stress obtained increases while vibrating greatly because the stress wave propagation is solved in minute time increments without performing the iterative calculation within the time step. The method of determining the rupture by comparing the relationship between the stress and the rupture criteria cannot predict with sufficient accuracy. The problem of dependence on strain rate and the problem of stress vibration during collision analysis should be avoided. Therefore, if the plastic strain rate tensor obtained from the collision analysis is converted into a stress tensor at a quasi-static strain rate and the obtained stress is compared with the rupture criteria at the quasi-static strain rate, the prediction result is stable. I thought it was easy to do.

(Specific configuration of this embodiment)
A specific configuration of the present embodiment will be described as an example of evaluating the risk of breakage in collision analysis of automobile parts with reference to FIG. 12.

In evaluating the moldability of an automobile part, first, the structure of the automobile is set (step S1). Subsequently, the shape of the automobile part is set using CAD (step S2), and the three-dimensional part shape is recorded on the computer (step S3). Here, in order to evaluate the collision performance using collision analysis, the parts are divided into finite elements and then subjected to processing such as spot welding to construct an analysis model of the structure (step S4), and software is used according to the purpose. Select to record on your computer. Subsequently, the risk of fracture is evaluated by collision analysis (step S8). For that purpose, first, the material parameters, plate thickness, and boundary conditions of the parts to be analyzed are set (step S5).

Subsequently, the results of the conical hole expansion test of two or more different apex angles tested offline are input (step S6). Here, in the case of performing a hole expanding test with a single-shaped apex angle conical tool using two or more materials having different initial hole diameters instead of the conical hole expanding test of two or more different apex angles. , The hole expansion rate obtained from the test is input in step S6. Also, when performing a hole expansion test with a single-shaped apex angle cylindrical tool using two or more materials with different initial hole diameters instead of the conical hole expansion test with two or more different apex angles, The hole expansion rate obtained from the test is input in step S6.

Subsequently, the fracture criteria σ ^cr = f (dσ _θ / dr) is calculated by the method described above using the two or more hole expansion ratio values (step S7). Furthermore, in the evaluation of fracture, the positional relationship between the fracture criteria obtained in this way and the maximum principal stress of each element and the stress gradient of adjacent elements obtained from the results of simulation by the finite element method of the deformation process is compared. Evaluate by that. When this stress reaches the fracture criteria, it is determined that the fracture or the risk thereof is high (steps S8 to S10). Specifically, in the relationship between the stress gradient and the breaking limit stress in FIG. 9, the strain state of the element obtained by the finite element method converted into stress is R, and the straight line connecting the states O and R with zero stress is used. When the intersection with the criteria is A, the risk of breakage can be quantified as OR / OA.

After that, the fracture risk rate is contoured (step S11), and the fracture risk is determined. As a specific implementation, a channel member having a cross-sectional shape of a hat having a size of 50 mm × 50 mm was cut out in a circular shape having a diameter of 30 mm and subjected to collision analysis. The material is a 1.4 mm thick 980 MPa class high-strength steel plate, an analysis model with a member length of 900 mm is attached to a support with a bending span of 700 mm, and a flat-bottom loader (width 50 mm, R5 mm) with a mass of 500 kg collides at an initial speed of 27 km / h. I let you. The result is shown in FIG. As the deformation progressed, stress concentrated on the hole edge at the bottom of the notch cut out in a semicircle, reaching the rupture criteria 9.6 ms after the collision.

As described above, according to the present embodiment, the influence of the strain gradient from the plate edge and the punching state are taken into consideration from the hole expansion rate of the conical hole expansion test having two or more different apex angles. The fracture criteria can be estimated, and by applying this to molding and collision analysis, the risk of fracture of the edge portion can be quantitatively evaluated. As a result, it is possible to avoid breakage of the edge portion, which is one of the applications of high-strength steel sheets, and to realize efficient and highly accurate development of an automobile body in consideration of materials, construction methods, and structures at the same time.

(Other embodiments)
Each step of the moldability prediction evaluation method according to the above-described embodiment (steps S1 to S11 in FIG. 12 and the like) can be realized by operating a program recorded in the RAM, ROM, or the like of the computer. This program and a computer-readable recording medium on which the program is recorded are included in this embodiment.

Specifically, the above program is recorded on a recording medium such as a CD-ROM, or provided to a computer via various transmission media. As a recording medium for recording the above program, a flexible disk, a hard disk, a magnetic tape, a magneto-optical disk, a non-volatile memory card, or the like can be used in addition to the CD-ROM. On the other hand, as the transmission medium of the above program, a communication medium in a computer network system for propagating and supplying program information as a carrier wave can be used. Here, the computer network is a WAN such as a LAN or the Internet, a wireless communication network, or the like, and the communication medium is a wired line such as an optical fiber, a wireless line, or the like.

Further, the programs included in the present embodiment are not limited to those in which the functions of the present embodiment are realized by the computer executing the supplied program. For example, such a program is also included in the present embodiment when the function of the present embodiment is realized in collaboration with an OS (operating system) running on a computer or other application software. Further, such a program is also included in the present embodiment when all or a part of the processing of the supplied program is performed by the function expansion board or the function expansion unit of the computer to realize the functions of the present embodiment.

For example, FIG. 14 is a schematic diagram showing an internal configuration of a personal user terminal device. In FIG. 14, 1200 is a personal computer (PC) equipped with a CPU 1201. The PC 1200 executes device control software stored in the ROM 1202 or the hard disk (HD) 1211 or supplied by the flexible disk drive (FD) 1212. The PC 1200 comprehensively controls each device connected to the system bus 1204.

The program stored in the CPU 1201, ROM 1202 or the hard disk (HD) 1211 of the PC 1200 realizes, for example, the procedures of steps S1 to S11 in FIG. 12 of the present embodiment.

The 1203 is a RAM, and functions as a main memory, a work area, and the like of the CPU 1201. Reference numeral 1205 is a keyboard controller (KBC), which controls instruction input from a keyboard (KB) 1209, a device (not shown), or the like.

Reference numeral 1206 is a CRT controller (CRTC), which controls the display of the CRT display (CRT) 1210. 1207 is a disk controller (DKC). The DKC1207 controls access to a hard disk (HD) 1211 and a flexible disk (FD) 1212 that store a boot program, a plurality of applications, edit files, user files, network management programs, and the like. Here, the boot program is a boot program that starts the execution (operation) of the hardware and software of the personal computer.

Reference numeral 1208 is a network interface card (NIC), which exchanges bidirectional data with a network printer, another network device, or another PC via LAN1220.
Instead of using the personal user terminal device, a predetermined calculator or the like specialized in the edge fracture prediction method may be used.

Claims (19)

This is a method for evaluating the breakage of the edge of the material during the collision deformation process of automobile body parts.
The first step of inputting two or more different hole expansion rates obtained from the hole expansion test into the computer ,
The computer has a second step of calculating the two or more different breaking limit stresses and the two or more different radial stress gradients from the two or more different hole expansion ratio data.
The computer calculates the fracture criteria from the relationship between the two or more fracture limit stresses and the two or more stress gradients, and the maximum principal stress obtained from the numerical analysis by the finite element method and the stress gradient between adjacent elements. Includes a third step of assessing a fracture when and reaches the fracture criteria.
An edge fracture prediction method, characterized in that the computer evaluates the moldability of the material based on the analysis results obtained by executing the first step to the third step as a series of steps. .. The edge fracture prediction according to claim 1, wherein in the first step, the respective hole expansion rates obtained from the two or more different apex angle conical hole expansion tests are input to the computer. Method. In the first step, inputting the hole expansion rate obtained from a hole expansion test using a single-shaped apex angle conical tool using the materials having the two or more different initial hole diameters to the computer. The edge fracture prediction method according to claim 1. In the first step, inputting the hole expansion rate obtained from a hole expansion test using a cylindrical tool having a single shape with an apex angle using the materials having the two or more different initial hole diameters into the computer. The edge fracture prediction method according to claim 1. In the second step, the computer converts the circumferential strain distribution ε _θ and the radial strain distribution ε _r expressed in the strain space measured from the experiment, or the theoretically estimated strain distribution into the stress space. The edge fracture prediction method according to any one of claims 1 to 4, wherein the circumferential stress distribution σ _θ and the radial stress distribution σ _r are calculated. According to any one of claims 1 to 5, the element size used in the analysis of the finite element method is set to the reference length dr when the computer calculates the stress gradient dσ _θ / dr. The described method for predicting edge breakage. The computer calculates the breaking criterion σ ^cr = f (dσ _θ / dr) from the two or more breaking limit stresses and the two or more stress gradients, and from numerical analysis by the finite element method between adjacent elements. The stress gradient ds ₁₁ / dr and the maximum principal stress σ ₁₁ are obtained, and an index of whether or not the stress gradient ds ₁₁ / dr and the maximum principal stress σ ₁₁ reach the fracture criteria. The edge fracture prediction method according to any one of claims 1 to 6, wherein σ ₁₁ / σ ^cr is calculated as sigma, and the result is contour-displayed. When the dynamic solution method of the finite element method is used as a means of numerical analysis, the computer is characterized in that the plastic strain obtained from the numerical analysis is converted into stress and compared with the fracture criteria. The edge fracture prediction method according to any one of claims 1 to 4 and 7. When evaluating fracture using a numerical analysis considering the rate dependence of the deformation stress of the material, the computer converts the plastic strain obtained from the numerical analysis to obtain the stress at the reference strain rate, and obtains the stress at the reference strain rate. The edge fracture prediction method according to any one of claims 1 to 4, 7, and 8, characterized in that the fracture criteria are compared with the fracture criteria in the above. It is a program for evaluating the fracture of the edge part of the material in the process of collision deformation of automobile body parts.
The first step of entering two or more different drilling rates obtained from the drilling test,
A second step of calculating the two or more different breaking limit stresses and the two or more different radial stress gradients from the two or more different hole expansion rate data.
The fracture criterion is calculated from the relationship between the two or more fracture limit stresses and the two or more stress gradients, and the maximum principal stress obtained from the numerical analysis by the finite element method and the stress gradient between adjacent elements are the fractures. Including a third step of assessing a rupture when the criteria are reached, including
An edge fracture prediction program for evaluating the formability of the material based on the analysis results obtained by executing the first step to the third step as a series of steps. The edge fracture prediction program according to claim 10, wherein in the first step, the respective hole expansion rates obtained from the two or more different apex angle conical hole expansion tests are input. The first step is characterized in that the hole expansion rate obtained from a hole expansion test using a single-shaped apex angle conical tool using the materials having the two or more different initial hole diameters is input. The edge fracture prediction program according to claim 10. In the first step, the hole expansion rate obtained from a hole expansion test using a cylindrical tool having a single shape and an apex angle using the materials having the two or more different initial hole diameters is input. The edge fracture prediction program according to claim 10. In the second step, the circumferential stress is converted into the stress space by converting the circumferential strain distribution ε _θ and the radial strain distribution ε _r expressed in the strain space measured from the experiment, or the theoretically estimated strain distribution into the stress space. The edge fracture prediction program according to any one of claims 10 to 13, wherein the distribution σ _θ and the radial stress distribution σ _r are calculated. The edge according to any one of claims 10 to 14, wherein the element size used in the analysis of the finite element method is a reference length dr when calculating the stress gradient dσ _θ / dr. Part breakage prediction program. The fracture criterion σ ^cr = f (dσ _θ / dr) is calculated from the two or more breaking limit stresses and the two or more stress gradients, and the stress gradient between adjacent elements is calculated from the numerical analysis by the finite element method. A certain ds ₁₁ / dr and the maximum principal stress σ ₁₁ are obtained, and σ ₁₁ / is used as an index of whether or not the stress gradient ds ₁₁ / dr and the maximum principal stress σ ₁₁ reach the fracture criteria. The edge break prediction program according to any one of claims 10 to 15, wherein σ ^cr is calculated and the result is contour-displayed. A claim characterized in that when the dynamic solution method of the finite element method is used as a means of numerical analysis, the plastic strain obtained from the numerical analysis is converted into stress and compared with the fracture criteria. The edge fracture prediction program according to any one of 10 to 13 and 16. When evaluating fracture using a numerical analysis considering the velocity dependence of the deformation stress of the material, the plastic strain obtained from the numerical analysis is converted to obtain the stress at the reference strain rate, and the fracture criteria at the reference strain rate is obtained. The edge fracture prediction program according to any one of claims 10 to 13, 16 and 17, characterized in that A computer-readable recording medium comprising recording the edge fracture prediction program according to any one of claims 10 to 18.