Fracture prediction method, fracture prediction device, fracture prediction program, and recording medium

Abstract

To accurately predict a bending fracture in a case where a deformation path is changed between a present step and a previous step and the bending fracture occurs when the deformation analysis is performed over a plurality of simulation steps.SOLUTION: A fracture prediction device includes: a limit line creation part 1 for creating a fracture limit line in a stress space; a simulation part 2 for performing the coupled analysis of a plurality of deformation simulations; a conversion part 3 for determining the presence or absence of bending of a steel plate member using a plate thickness after the bending deformation and a surface layer strain component during deformation of the steel plate member based on an analysis result obtained by the simulation part 2, extracting an element determined to be a bending part and converting the bending outside surface layer strain component of the bending part into the maximum principal stress and the minimum principal stress of the surface layer; and a determination part 4 for determining the presence or absence of material fracture.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method and device for predicting fracture of a member (such as a steel plate member), as well as a fracture prediction program and a recording medium. [Background technology]

[0002] It is generally known that the ductility of steel sheets decreases as their strength increases, and it is thought that the possibility of material fracture increases during collision deformation of an automobile body, for example. In particular, it is known that in steel sheets known as ultra-high tensile steel sheets with a tensile strength of over 980 MPa, in addition to fracture due to in-plane tensile force during collision deformation, material fracture due to buckling deformation, i.e., bending deformation (hereinafter referred to as bending fracture), occurs.

[0003] Furthermore, because automobile bodies are assembled by welding press-formed components, the effect of pre-deformation introduced into the press-formed components during the forming process must be taken into account when examining the possibility of material fracture during subsequent crash tests. However, the direction of deformation (strain) introduced during the forming process is not necessarily the same as the direction of deformation (strain) during the crash deformation process, resulting in a change in the so-called deformation path. It is known that when this change in deformation path occurs, the fracture limit also changes, making it difficult to predict fracture using conventional methods. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-141237 [Patent Document 2] Patent No. 4621216 Summary of the Invention [Problem to be solved by the invention]

[0005] Patent Document 1 discloses a method for predicting bending fracture of ultra-high tensile steel. This method determines fracture based on whether the strain value of the surface layer of the plate exceeds the fracture limit line in the strain space. However, this method is only a method for predicting fracture when the deformation path is constant, i.e., when the plate is continuously subjected to deformation in the same direction, and cannot be applied to cases where the deformation path changes between the forming process and the collision deformation process, as described above.

[0006] Furthermore, in Patent Document 2, experiments and analyses have verified that by using a fracture limit line expressed in stress space for in-plane tensile force, the fracture limit line can be expressed almost uniquely regardless of the deformation path. As a result, it is believed that fracture can be predicted when in-plane tensile force is applied, even if the deformation path changes between the forming process and collision deformation. On the other hand, bending fracture, which is a problem with ultra-high tensile steel, is a phenomenon in which high tensile force occurs in the surface layer of the plate thickness when the in-plane (plate thickness center) tensile force is very small, leading to fracture. Therefore, the technology in Patent Document 2 cannot be applied to predicting bending fracture.

[0007] The present invention has been made in consideration of the above-mentioned problems, and aims to provide a fracture prediction method and fracture prediction device, as well as a fracture prediction program and recording medium, that can accurately predict bending fracture when deformation analysis is performed across multiple simulation processes and the deformation path changes between this process and the previous process and bending fracture occurs. [Means for solving the problem]

[0008] In order to solve the above problems, the present inventors have conducted extensive research and have come up with the following aspects of the invention.

[0009] 1. a limit line creation step of creating a fracture limit line in stress space; A main simulation process in which the strain, stress, and thickness of the member obtained in the final step through one or more previous simulation processes are taken over and mapped as initial values, and a deformation analysis of the member is performed; The surface strain component during deformation of the member obtained in the present simulation step is a conversion step of converting the stress into the maximum principal stress and the minimum principal stress of the surface layer; a determination step of determining whether the component is fractured using the maximum principal stress and the minimum principal stress of the surface layer obtained in the conversion step; With death, The limit line creating step includes: constructing a forming limit diagram in strain space; A step of offsetting the plane strain value of the forming limit diagram until it coincides with the limit surface maximum principal strain during bending deformation to create a surface strain limit line; converting the surface layer strain limit line into a stress space to generate the surface layer stress fracture limit line; and The determining step determines whether or not the maximum principal stress and the minimum principal stress of the surface layer obtained in the converting step exceed the fracture limit line. A fracture prediction method characterized by:

[0010] 2. The fracture prediction method described in 1., characterized in that the conversion process determines whether or not the component is bent using the surface strain components during deformation of the component obtained in the simulation process, and converts the bending outer surface strain components of elements determined to be bent in the component into the maximum principal stress and minimum principal stress of the surface layer.

[0013] 3 1. The component to be judged for fracture is a steel plate with a tensile strength of 980 MPa or more. Or 2. The fracture prediction method according to claim 1.

[0014] 4 . a limit line creating unit that creates a fracture limit line in stress space; A main simulation unit that takes over the strain, stress, and plate thickness of the member obtained in the final step through one or more previous simulation processes, maps them as initial values, and performs deformation analysis of the member; a conversion unit that converts the surface layer strain component during deformation of the member obtained by the main simulation unit into a maximum principal stress and a minimum principal stress of the surface layer; a determination unit that determines whether the member is broken using the maximum principal stress and the minimum principal stress of the surface layer obtained by the conversion unit; With death, The limit line creation unit a first creating unit that creates a forming limit diagram in a strain space; A second creating unit that creates a surface strain limit line by offsetting the plane strain value of the forming limit diagram until it coincides with the limit surface maximum principal strain during bending deformation; a third creating unit that converts the surface layer strain limit line into a stress space to create the fracture limit line of the surface layer stress; and The determining unit determines whether or not the maximum principal stress and the minimum principal stress of the surface layer obtained by the converting unit exceed the fracture limit line. A fracture prediction device characterized by:

[0015] 5 The conversion unit determines whether the member is bent or not using the surface layer strain components during deformation of the member obtained by the simulation unit, and converts the bending outer surface layer strain components of the element determined to be a bent part of the member into the maximum principal stress and minimum principal stress of the surface layer. 4 . A fracture prediction device as described in.

[0018] 6 The member to be subjected to fracture judgment is a steel plate having a tensile strength of 980 MPa or more. 4. or 5. The fracture prediction device described in

[0019] 7 . a limit line creation procedure for creating a fracture limit line in stress space; This simulation procedure involves taking over the strain, stress, and thickness of the component obtained in the final step through one or more previous simulation processes, mapping them as initial values, and performing deformation analysis of the component. The surface strain component during deformation of the member obtained by the present simulation procedure is A conversion procedure for converting the maximum and minimum principal stresses of the surface layer; a determination step of determining whether the component is fractured using the maximum principal stress and the minimum principal stress of the surface layer obtained in the conversion step; A fracture prediction program characterized by causing a computer to execute the following: and The limit line creation procedure includes: constructing a forming limit diagram in strain space; A step of offsetting the plane strain value of the forming limit diagram until it coincides with the limit surface maximum principal strain during bending deformation to create a surface strain limit line; converting the surface layer strain limit line into a stress space to generate the surface layer stress fracture limit line; and the determination step determines whether or not the maximum principal stress and the minimum principal stress of the surface layer obtained in the conversion step exceed the fracture limit line. .

[0020] 8 The conversion procedure is procedure The method is characterized in that the presence or absence of bending of the member is determined using the surface layer strain components during deformation of the member obtained in step 2, and the bending outer surface layer strain components of the element determined to be a bent part of the member are converted into the maximum principal stress and minimum principal stress of the surface layer. 7 . A fracture prediction program described in.

[0023] 9 The member to be subjected to fracture judgment is a steel plate having a tensile strength of 980 MPa or more. 7. or 8. Described in Fracture prediction program .

[0024] 10 . 7 .~ 9 A computer-readable recording medium having recorded thereon the fracture prediction program described in any one of claims 1 to 4. [Effects of the Invention]

[0025] According to the present invention, when deformation analysis is performed across multiple simulation processes, if the deformation path changes between this process and the previous process and bending fracture occurs, the bending fracture can be predicted with high accuracy. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a block diagram showing a fracture prediction device according to an embodiment of the present invention. [Figure 2] FIG. 10 is a characteristic diagram showing the state of conversion from strain space to stress space. [Figure 3] FIG. 10 is a contour diagram for explaining the coupled analysis in the deformation simulation step S2. [Figure 4] FIG. 2 is a schematic diagram showing a location where deformation occurs in a steel plate member. [Figure 5] FIG. 10 is a contour diagram showing how FEM elements determined to be bent portions are extracted. [Figure 6] FIG. 10 is a characteristic diagram showing an example of when a breakage is determined. [Figure 7] FIG. 2 is a plan view showing the shape of a test piece used in the present example. [Figure 8] FIG. 10 is a schematic diagram showing a deformation simulation step S2 in the example of the present invention. [Figure 9] FIG. 10 is a characteristic diagram showing the results of fracture prediction according to a comparative example. [Figure 10] FIG. 10 is a characteristic diagram showing the results of fracture prediction according to an example of the present invention. [Figure 11] FIG. 10 is a characteristic diagram showing a comparison between an experiment, an example of the present invention, and a comparative example. [Figure 12] FIG. 2 is a schematic diagram showing the internal configuration of a personal user terminal device. DETAILED DESCRIPTION OF THE INVENTION

[0027] [Embodiment] Specific embodiments will be described in detail below with reference to the drawings. Fig. 1 is a block diagram showing a fracture prediction device according to this embodiment. Fig. 1 also shows each process and each step of a fracture prediction method executed by the fracture prediction device. In this embodiment, steel plate members having a tensile strength of 980 MPa or more are mainly targeted for fracture prediction.

[0028] 1, the fracture prediction device according to this embodiment includes a limit line creation unit 1 that creates a fracture limit line in stress space, a simulation unit 2 that performs coupled analysis of multiple deformation simulations, a conversion unit 3 that converts the analysis results obtained by the simulation unit 2 from strain space to stress space, and a determination unit 4 that determines whether or not a material fracture has occurred. The limit line creation unit 1, simulation unit 2, conversion unit 3, and determination unit 4 are configured by a central processing unit (CPU) of a personal computer that can perform analysis using the finite element method (FEM) (FEM analysis), for example.

[0029] The limit line creating unit 1 includes a first creating unit 11, a second creating unit 12, and a third creating unit 13. The simulation unit 2 performs one or more pre-simulation steps and a main simulation step in which the strain, stress, and plate thickness of the steel plate member that is the fracture judgment target obtained in the final step after the pre-simulation steps are taken over and mapped as initial values, and a deformation analysis of the steel plate member is performed. The simulation unit 2 includes a pre-simulation unit 21 that performs the pre-simulation step and a main simulation unit 22 that performs the main simulation step. In this embodiment, an example is shown in which a forming step is performed as the pre-simulation step and a collision step is performed as the main simulation step. The conversion unit 3 includes a bending determination unit 31 and a stress conversion unit 32 .

[0030] In this embodiment, the fracture database 10 shown in FIG. 1 is used to acquire the input information. The fracture database 10 stores information on material properties (tensile strength properties (stress-strain: SS curve, etc.)), plate thickness of steel plate members, FEM element size, bending deformation limit of steel plate members (α, R / t), etc. for multiple materials.

[0031] By utilizing the fracture database 10, various input information corresponding to the component to be subjected to fracture judgment can be obtained easily and efficiently in a short time. Note that instead of using the fracture database 10, various input information may be obtained by performing various tensile tests or the like.

[0032] The following describes the fracture limit line creation step S1 by the limit line creation unit 1. Figure 2 is a characteristic diagram showing the state of conversion from strain space to stress space, where (a) shows the strain space and (b) shows the stress space.

[0033] The limit line creating unit 1 acquires input information such as tensile strength characteristics from a fracture database 10 . First, the first creation unit 11 creates a forming limit diagram (FLD) limit line in strain space using a theoretical formula that extends the Storen-Rice necking occurrence criterion based on the tensile strength characteristics, which are the acquired input information (step S11).

[0034] Next, the second creating unit 12 calculates the maximum principal strain (ε ) in plane strain (minimum principal strain = 0) of the created FLD limit line based on the bending deformation limit, which is the acquired input information. max ') is the critical surface maximum principal strain during bending deformation (ε max ) to create a surface strain limit line (step S12). That is, first, the outer limit maximum principal strain (ε max Next, the FLD limit line created in step S11 is proportionally enlarged to create a surface layer strain fracture limit line. max / ε max The minimum principal strain and maximum principal strain values ​​of the FLD limit line created in step S11 are multiplied by '.

[0035] Next, the third creating unit 13 converts the surface layer strain fracture limit line created in step S12 into stress space to create a surface layer stress fracture limit line (step S13). 11 , horizontal axis: ε 22 ) to the surface stress rupture limit line (vertical axis: σ 11 , horizontal axis: σ 22 ), we assume (1) the law of constant volume, (2) the Mises yield function, (3) isotropic hardening due to the work-hardening law, (4) the normal law, and (5) plane stress.

[0036] Next, a description will be given of the deformation simulation step S2 in which coupled analysis is performed by the simulation unit 2, here, a coupled analysis of the forming process and the collision process. Fig. 3 is a contour diagram for explaining the coupled analysis in the deformation simulation step S2.

[0037] The simulation unit 2 acquires the above input information from the fracture database 10. First, the pre-simulation unit 21 executes the molding process (step S21). In step S21, the pre-simulation unit 21 first performs a deformation simulation when forming a steel plate member that is to be subjected to fracture judgment (step S101). In step S101, based on the above input information, a deformation simulation analysis is performed using an FEM model of the steel plate member when forming the steel plate member that is to be formed. An FEM model of a steel plate member 111 before forming is shown in FIG. 3(a), and an FEM model of a hat-shaped steel plate member (hat member) 112 after forming the steel plate member 111 is shown in FIG. 3(b).

[0038] Next, the pre-simulation unit 21 outputs information on the final step in the forming simulation of step S101, in this case, information on the deformation strain and plate thickness introduced into the steel plate member that has undergone the forming process (step S102).

[0039] Next, the pre-simulation unit 21 maps the information of the final step in the forming simulation output in step S102 to step 0 of the collision process following the forming process (step S103). In the collision process following the forming process, the steel plate member subjected to the FEM analysis is given information on the deformation strain (pre-deformation) introduced into the steel plate member in the forming process as its initial state.

[0040] Next, the simulation unit 22 executes the collision process (step S22). In step S22, a deformation simulation analysis is performed using an FEM model of the steel plate member (the hat member 112 in the example of FIG. 3(b)) that has inherited the information from the final step of the forming simulation as pre-deformation. Here, so-called three-point bending is performed. FIG. 3(c) shows the impactor 114 coming into contact with the FEM model of the hat member 112, which is supported at both ends by supports 113a and 113b, and FIG. 3(d) shows the FEM model of the hat member 112 after the collision.

[0041] In this embodiment, the simulation analysis of the collision process is performed using information from the final step in the forming process. The effect of pre-deformation imparted to the steel plate member by the forming process is reliably reflected in the simulation analysis of the collision process, which contributes to accurate fracture prediction.

[0042] Next, a conversion step S3 in which the conversion unit 3 converts the analysis results obtained by the simulation unit 2 from the strain space to the stress space will be described. The conversion unit 3 obtains information on the thickness of the steel plate member after bending deformation and the surface strain components (circumferential strain components on the outside of the bend and the inside of the bend) from the simulation unit 2 as analysis results obtained by the simulation unit 2.

[0043] First, the bending determination unit 31 determines whether or not the steel plate member is bent, using the plate thickness after bending deformation obtained by the simulation unit 2 and the surface strain component during deformation of the steel plate member (step S31). In step S31, the presence or absence of bending of the steel plate member is determined as follows: Fig. 4 is a schematic diagram showing a location where deformation has occurred in the steel plate member.

[0044] Here, the following conditions are assumed: (1) The neutral plane coincides with the center plane of the steel plate member (in shell elements assuming plane stress, the neutral plane does not move due to bending). (2) A plane that was perpendicular to the axis of the beam before bending deformation remains flat and perpendicular to the axis after bending deformation, i.e., shear deformation is not taken into account (Kirchhoff's assumption).

[0045] Under the above assumptions, the center plane of the deformation location of the steel plate member is defined as a radius R n When bent to the axial direction, the circumferential strain ε at a distance η from the neutral axis is θ can be approximated as follows: First, suppose that the line segment AB and the line segment CD located at a distance η from it become A'B' and C'D' after bending deformation. CD=AB=A'B'=R n Δθ C'D'=(R n +η)Δθ Therefore, the circumferential strain ε due to bending of the surface located η away from the neutral axis is θ can be approximated by the following equation: ε θ ≒(C'D'-CD) / CD=η / R n

[0046] Next, the initial thickness of the steel plate member is t0, and the circumferential strain component on the outside of the bending is ε θ i , the circumferential strain component on the inner side of the bending is ε θ 0 Then, the following equation is obtained from the above equation: ε θ 0 -ε θ i =t0 / R n twist, 1 / R n =(ε θ 0 -ε θ i ) / t0 Here, since the position of the integral point of the outermost layer changes depending on the number of integral points in the thickness direction, the calculation is performed using a value converted into the strain of the outermost layer.

[0047] The corresponding curvature components are calculated from the front and back plastic strain tensors of each component. κ xx =(ε xx 0 -ε xx i ) / t0 κ yy =(ε yy 0 -ε yy i ) / t0 κ xy =(ε xy 0 -ε xy i ) / t0

[0048] To find the curvature in the direction where the bending is greatest, the principal values ​​λ1 and λ2 of the curvature of the neutral plane are found from these equations. Furthermore, the radii of curvature R1 and R2 (= reciprocals of the curvature) are calculated. λ 1,2 =1 / 2(κ xx +κ yy )±1 / 2{(κ xx -κ yy ) 2 +4κ xy 2} 1 / 2 twist, R1=1 / λ1 R2=1 / λ2

[0049] From these equations, the radius of curvature in the direction where the radius of curvature is smallest (the direction where the radius of curvature is largest) is determined, and the radius of curvature on the inside of the bend, R i where t is the current thickness of the steel plate member. R n =min(R1,R2), R i =R n -t / 2

[0050] The bending determination unit 31 determines the radius of curvature R of the inner side of the bending at the deformation occurrence point of the steel plate member calculated based on the above formula. i is equal to or smaller than a predetermined threshold, the deformation occurrence location is determined to be a bent portion.

[0051] Next, the stress conversion unit 32 extracts FEM elements that are determined to be bent parts in the steel plate member. An example of this is shown in Fig. 5. In Fig. 5, three-point bending is performed on a hat member formed from a steel plate member, and elements that are determined to be bent parts by the bending determination unit 31 from among the deformation occurrence locations are extracted. The stress conversion unit 32 converts the bending outer surface layer strain component of the bent part (the circumferential strain component on the bending outer side) into the maximum principal stress and minimum principal stress of the surface layer, and outputs them to the determination unit 4 (step S32).

[0052] Thereafter, the judgment unit 4 judges whether or not the steel plate member has undergone bending fracture based on whether or not the maximum principal stress and minimum principal stress of the surface layer obtained by the conversion unit 3 (stress conversion unit 32) exceed the surface layer stress fracture limit line obtained by the fracture limit line creation step S1 (step S13) (fracture judgment step S4). Figure 6 shows an example of fracture determination based on the stress in the outermost layer of the plate thickness of an element that has been subjected to bending deformation after pre-deformation (forming).

[0053] As explained above, when deformation analysis is performed across multiple simulation processes, if the deformation path changes between this process and the previous process and bending fracture occurs, specifically, as shown in this embodiment, the previous process is the forming process and the current process is the collision process, and generally, even if the direction of the deformation (strain) introduced in the forming process differs from the direction of the deformation (strain) in the collision deformation process, a surface stress fracture limit line using stress that can almost uniquely express the fracture limit line regardless of the deformation path can be applied, and the presence or absence of bending fracture occurring in the steel plate member can be accurately predicted based on whether the maximum principal stress and minimum principal stress of the surface layer exceed the surface stress fracture limit line.

[0054] [Example] Examples will be described below. In these examples, the effects of the examples of the present invention according to the above-described embodiments will be described based on a comparison with comparative examples.

[0055] In this example, a tensile test and a bending test of metallic materials (VDA bending test) were conducted using a predetermined test member (test specimen) in accordance with the VDA German Association of the Automotive Industry test standard (VDA 238-100 "Plate bending test for metallic materials" Validation Rule, 01 June 2017). The shape of the test specimen used in this example is shown in FIG. 7. The length unit is mm. In this example and comparative example, a bending direction (L-axis bending) was selected in the VDA bending test, in which tension was applied in a direction perpendicular to the tensile direction (L-axis direction) of the test specimen in the tensile test, so that the deformation path of the test specimen generated in the tensile test, which corresponds to the previous process, would be significantly different from the deformation path of the test specimen generated in the VDA bending test, which corresponds to the main process.

[0056] To impart pre-strain, a tensile test was stopped midway. The tensile speed was 3 mm / min. The tensile strength of the test specimen was determined as the unloading start displacement (maximum load displacement) x 50%, 80%, or 100%, and the pre-strain was varied between three levels.

[0057] (VDA bending test) A 60(C) x 30(L) specimen was taken from the center of the specimen that had been pre-strained by the tensile test, and a bending test3) (L-axis bending) in accordance with the VDA standard was carried out. A VDA bending test (L-axis bending) was also carried out on a specimen that had not been pre-strained.

[0058] (1) Example of the present invention In the present invention example, the fracture prediction method of the present embodiment was used, and the information about the test specimen described above was used as input information, and the fracture limit line creation step S1, the deformation simulation step S2, the conversion step S3, and the fracture determination step S4 were executed in this order.

[0059] 8 is a schematic diagram showing the deformation simulation step S2 in an example of the present invention. Fig. 8(a) shows an FEM analysis of a test piece 121 simulating a tensile test, and Fig. 8(b) shows an FEM analysis of a mapped test piece 122 simulating a VDA bending test. In the deformation simulation step S2, for example, LS-DYNA was used for FEM analysis. In step S21, a deformation simulation simulating the above-mentioned tensile test was performed, and the deformation strain (pre-strain) and plate thickness information introduced into the tensile test specimen were mapped to the test specimen for the VDA bending test. In step S22, a deformation simulation simulating the above-mentioned VDA bending test was also performed. In addition, an FEM deformation simulation simulating the VDA bending test was also performed on a test specimen that was not pre-strained.

[0060] (2) Comparative Example In the comparative example, the information about the test specimen described above was used as input information, and in the fracture prediction method of this embodiment, only steps S11 and S12 were performed in the fracture limit line creation step S1 to obtain the surface strain fracture limit line, steps S21 and S22 were performed in the same manner as above in the deformation simulation step S2, and only step S31 was performed in the conversion step S3 to obtain the bending outer surface strain component of the bent portion, and the fracture determination step S4 was then executed.

[0061] (Fracture prediction in strain space) Fig. 9 is a characteristic diagram showing the fracture prediction results for the comparative example. Fig. 9(a) shows the results for the test piece of high-tensile steel with a tensile strength of 980 MPa, and Fig. 9(b) shows the results for the test piece of high-tensile steel with a tensile strength of 1470 MPa.

[0062] In the comparative example, for the test specimens that were not pre-strained (tensile deformation), the fracture limit obtained in the comparative example (the intersection of the surface strain fracture limit line and the strain history of the bending outer surface strain component) was approximately consistent with the bending limit obtained in the above experiment (indicated by an x ​​in the figure). However, for the test specimens that were pre-strained, it was confirmed that the deviation between the fracture limit obtained in the comparative example and the bending limit obtained in the above experiment increased with increasing pre-strain. Therefore, it was found that the greater the pre-strain, the greater the difference from the experimental results in the comparative example.

[0063] (Fracture prediction in stress space) Fig. 10 is a characteristic diagram showing the fracture prediction results of an example of the present invention. Fig. 10(a) shows the results for a test piece of high-tensile steel with a tensile strength of 980 MPa, and Fig. 10(b) shows the results for a test piece of high-tensile steel with a tensile strength of 1470 MPa.

[0064] In the present invention, not only for the test specimens without pre-strain (tensile deformation amount) but also for the test specimens with large pre-strain, the fracture limit obtained in the present invention example (the intersection of the surface layer stress fracture limit line and the stress history of the maximum principal stress and minimum principal stress of the surface layer converted from the bending outer surface layer strain component) was almost consistent with the bending limit obtained in the above experiment (indicated by the x mark in the figure). Therefore, it was found that the experimental results can be reproduced in the present invention example even when pre-strain is applied.

[0065] (Comparison between invention examples and comparative examples) Fig. 11 is a characteristic diagram comparing the experiment with an example of the present invention and a comparative example. Fig. 11(a) shows the relationship between the bending limit (maximum bending angle) and the magnitude of pre-strain in the experiment, Fig. 11(b) shows the relationship between the fracture limit (bending angle at which fracture is judged) and the magnitude of pre-strain in the comparative example, and Fig. 11(c) shows the relationship between the fracture limit (bending angle at which fracture is judged) and the magnitude of pre-strain in the example of the present invention.

[0066] The bending limit in the experiment tends to decrease as the pre-strain increases. In contrast, the fracture limit in the comparative example tends to increase as the pre-strain increases, i.e., the opposite trend to the experimental results. On the other hand, the fracture limit in the inventive example tends to decrease as the pre-strain increases, similar to the experimental results. From the above, it was confirmed that the fracture limit in stress space in the inventive example, unlike the fracture limit in strain space in the comparative example, accurately reproduces the trend of the bending limit in the experiment as the pre-strain increases.

[0067] [Other embodiments] 1, which are components of the fracture prediction device described above, namely, the limit line creation unit 1 (first creation unit 11, second creation unit 12, and third creation unit 13), the simulation unit 2 (pre-simulation unit 21 and main simulation unit 22), the conversion unit 3 (bending determination unit 31 and stress conversion unit 32), and the determination unit 4, may be realized by dedicated hardware. Furthermore, each of the above components may be configured by a memory and a CPU (Central Processing Unit), and the functions of each component may be realized by loading a program for realizing the functions of each component into the memory and executing the program.

[0068] Furthermore, the programs for realizing the functions of the above-described components (the fracture limit line creation step S1 (steps S11 to S13), the deformation simulation step S2 (steps S21 (steps S101 to S103) to S22), the conversion step S3 (steps S31 to S32), and the fracture determination step S4 shown in FIG. 1) may be recorded on a computer-readable recording medium, and the programs recorded on this recording medium may be read into a computer system and executed to execute the processing of the above-described components. Note that the term "computer system" as used herein includes hardware such as an OS and peripheral devices.

[0069] Furthermore, if a WWW system is used, the "computer system" may also include the homepage providing environment (or display environment). "Computer-readable recording media" refers to portable media such as flexible disks, optical magnetic disks, ROMs, and CD-ROMs, as well as storage devices such as hard disks built into computer systems. Furthermore, "computer-readable recording media" may also include devices that dynamically store programs for a short period of time, such as communication lines used when transmitting programs over networks like the Internet or communication lines like telephone lines, or devices that store programs for a fixed period of time, such as volatile memory within computer systems that serve as servers or clients. Furthermore, the above-mentioned programs may be programs that realize some of the aforementioned functions, or may be programs that can realize the aforementioned functions in combination with programs already stored in the computer system.

[0070] As a specific example, the fracture prediction method shown in this embodiment is implemented by a computer function 300 as shown in FIG. The computer function 300 includes a CPU 301, a ROM 302, and a RAM 303. It also includes a controller (CONSC) 305 for an operation unit (CONS) 309, and a display controller (DISPC) 306 for a display (DISP) 310 such as a CRT or LCD. It also includes a controller (DCONT) 307 for a hard disk (HD) 311 and a storage device (STD) 312 such as a flexible disk, and a network interface card (NIC) 308. These functional units 301, 302, 303, 305, 306, 307, and 308 are connected to each other via a system bus 304 so as to be able to communicate with each other.

[0071] The CPU 301 executes software stored in the ROM 302 or the HD 311, or software supplied from the STD 312, thereby performing overall control of each component connected to the system bus 304. That is, the CPU 301 reads out and executes a processing program (fracture prediction program) for performing the above-described operations from the ROM 302, the HD 311, or the STD 312, thereby performing control for realizing the operations of this embodiment. The RAM 303 functions as the main memory, work area, etc. of the CPU 301.

[0072] CONSC 305 controls instruction input from CONS 309. DISPC 305 controls the display of DISP 310. DCONT 307 controls access to HD 311 and STD 312, which store the boot program, various applications, user files, network management program, and the above-mentioned processing programs in this embodiment. NIC 508 exchanges data bidirectionally with other devices on network 313. Instead of using a normal computer terminal device, a specific computer specialized for the fracture prediction device may be used. [Explanation of symbols]

[0073] 1. Limit line creation section 2 Simulation Section 3. Conversion section 4 Judgment section 10 Fracture Database 11 First Creation Section 12 Second Creation Section 13 Third Creation Department 21 Pre-Simulation Section 22 Simulation Section 31 Bending judgment unit 32 Stress conversion section 111 Steel plate members 112 Hat component 121,122 Test specimens

Claims

1. A limit line creation step of creating a fracture limit line in stress space; A main simulation process in which the strain, stress, and plate thickness of the member obtained in the final step through one or more previous simulation processes are taken over and mapped as initial values, and a deformation analysis of the member is performed; The surface strain component during deformation of the member obtained in the present simulation step is a conversion step of converting the stress into the maximum principal stress and the minimum principal stress of the surface layer; a determination step of determining whether the component is fractured using the maximum principal stress and the minimum principal stress of the surface layer obtained in the conversion step; and The limit line creating step includes: constructing a forming limit diagram in strain space; A step of offsetting the plane strain value of the forming limit diagram until it coincides with the limit surface maximum principal strain during bending deformation to create a surface strain limit line; converting the surface layer strain limit line into a stress space to generate the surface layer stress fracture limit line; and The fracture prediction method is characterized in that the determination step determines fracture based on whether or not the maximum principal stress and minimum principal stress of the surface layer obtained in the conversion step exceed the fracture limit line.

2. The fracture prediction method according to claim 1, characterized in that the conversion process determines whether or not the component is bent using the surface layer strain components during deformation of the component obtained in the simulation process, and converts the bending outer surface layer strain components of elements determined to be bent in the component into maximum principal stress and minimum principal stress of the surface layer.

3. 3. The fracture prediction method according to claim 1, wherein the member to be judged for fracture is a steel plate having a tensile strength of 980 MPa or more.

4. A limit line creating unit that creates a fracture limit line in stress space; A main simulation unit that takes over the strain, stress, and plate thickness of the member obtained in the final step through one or more previous simulation processes, maps them as initial values, and performs deformation analysis of the member; a conversion unit that converts the surface layer strain component during deformation of the member obtained by the main simulation unit into a maximum principal stress and a minimum principal stress of the surface layer; a determination unit that determines whether the member is broken using the maximum principal stress and the minimum principal stress of the surface layer obtained by the conversion unit; and The limit line creation unit a first creating unit that creates a forming limit diagram in a strain space; A second creating unit that creates a surface strain limit line by offsetting the plane strain value of the forming limit diagram until it coincides with the limit surface maximum principal strain during bending deformation; a third creating unit that converts the surface layer strain limit line into a stress space to create the fracture limit line of the surface layer stress; and The fracture prediction device is characterized in that the judgment unit judges fracture based on whether or not the maximum principal stress and minimum principal stress of the surface layer obtained by the conversion unit exceed the fracture limit line.

5. The fracture prediction device described in claim 4, characterized in that the conversion unit determines whether or not the member is bent using the surface strain components during deformation of the member obtained by the simulation unit, and converts the bending outer surface strain components of elements determined to be bent in the member into maximum principal stress and minimum principal stress of the surface layer.

6. 6. The fracture prediction device according to claim 4, wherein the member to be judged for fracture is a steel plate having a tensile strength of 980 MPa or more.

7. A limit line creation step for creating a fracture limit line in stress space; This simulation procedure involves taking over the strain, stress, and thickness of the component obtained in the final step through one or more previous simulation steps, mapping them as initial values, and performing deformation analysis of the component. The surface strain component during deformation of the member obtained by the present simulation procedure is A conversion procedure for converting the maximum and minimum principal stresses of the surface layer; a determination step of determining whether the component is fractured using the maximum principal stress and the minimum principal stress of the surface layer obtained in the conversion step; A fracture prediction program that causes a computer to execute the following: The limit line creation procedure includes: constructing a forming limit diagram in strain space; A step of offsetting the plane strain value of the forming limit diagram until it coincides with the limit surface maximum principal strain during bending deformation to create a surface strain limit line; converting the surface layer strain limit line into a stress space to generate the surface layer stress fracture limit line; and The fracture prediction program is characterized in that the judgment step judges fracture based on whether or not the maximum principal stress and minimum principal stress of the surface layer obtained in the conversion step exceed the fracture limit line.

8. The fracture prediction program described in claim 7, characterized in that the conversion procedure determines whether or not the member is bent using the surface strain components during deformation of the member obtained in the present simulation procedure, and converts the bending outer surface strain components of elements determined to be bent in the member into the maximum principal stress and minimum principal stress of the surface layer.

9. 9. The fracture prediction program according to claim 7, wherein the member to be subjected to fracture judgment is a steel plate having a tensile strength of 980 MPa or more.

10. A computer-readable recording medium having the fracture prediction program according to any one of claims 7 to 9 recorded thereon.

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