Evaluation methods and procedures for delayed failure characteristics

Abstract

The purpose of this invention is to further improve the accuracy of delayed failure evaluation. Considering that the calculated stress value, which serves as the benchmark for delayed failure, varies depending on the analysis conditions of the molding analysis, the value obtained by changing the stress value, which serves as the benchmark for delayed failure, according to the analysis conditions used to analyze the molded part (practical component) as the target, is used as the benchmark for delayed failure evaluation. For example, the analysis conditions of the molding analysis in the delayed failure evaluation test are made consistent with the analysis conditions of the molding analysis of a practical component represented by an actual automotive part.

Description

Technical Field

[0001] This invention relates to a method for evaluating the delayed failure characteristics of molded articles produced by pressing and forming sheet metal, and to a technique for using the same. This invention is particularly suitable for the technology of automotive structural components formed from sheet metal of high-tensile steel. For example, this invention is suitable for evaluating the delayed failure characteristics of flange ends in pressed and formed body structural components such as center pillars and lower A-pillars, which are formed from high-tensile steel. Background Technology

[0002] Currently, automobiles require improved fuel efficiency and enhanced crash safety through lightweighting. To balance lightweighting and occupant protection in a collision, high-strength steel is used in the car body. In recent years, ultra-high-strength steel, with a tensile strength exceeding 980 MPa, has been widely applied to car bodies. One challenge in using ultra-high-strength steel in car bodies is delayed failure. Delayed failure is a failure phenomenon caused by residual stress and plastic strain after pressing, as well as hydrogen intrusion from the hydrogen environment during use. Therefore, to apply high-tensile steel to car bodies, it is necessary to evaluate the delayed failure characteristics corresponding to the pressing conditions and predict the occurrence of delayed failure.

[0003] As a conventional evaluation method related to high-tensile steel sheets for compression molding in automobiles, there are, for example, the methods described in Patent Documents 1 to 3.

[0004] Patent Document 1 describes a method for evaluating delayed failure based on the condition of bending stress caused by further tightening after V-bending a test piece. Patent Documents 2 and 3 describe methods for evaluating delayed failure based on the condition of residual tensile stress after compression deformation caused by deep drawing, stamping, or stamping-drawing forming.

[0005] These previous insights all involved calculating the stress generated on test pieces placed in a hydrogen environment. For example, the stress generated on the aforementioned test pieces due to deformation caused by molding was calculated using computer simulation analysis (molding analysis).

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2017-142086

[0009] Patent Document 2: Japanese Patent Application Publication No. 2018-185184

[0010] Patent Document 3: Japanese Patent Application Publication No. 2018-185183 Summary of the Invention

[0011] The problem that the invention aims to solve

[0012] The inventors of this application, in their research on the delayed failure characteristics of high-tensile steel sheets after pressing and forming, have obtained the following insight: the calculated stress values ​​in computer simulations during delayed failure evaluation tests depend on the analysis conditions. Computer simulations include, for example, CAE-based forming analysis. The analysis conditions referred to here include the types of elements, element dimensions, stress output locations within the formed product, and the location of the integration point of the stress output within the shell element, all included in the forming analysis based on the finite element method.

[0013] Therefore, even when evaluating delayed failure characteristics using evaluation methods described in various patent documents and calculating a benchmark value for the stress (limit stress) that limits delayed failure, the following problem exists: When the analytical conditions for calculating the benchmark value differ from the analytical conditions for the molding analysis of the actual molded article, the calculated value differs due to the difference in analytical conditions. Therefore, the accuracy of the delayed failure evaluation may correspondingly decrease. Here, practical parts are often large and complex in shape compared to the shape of the pressed molded article of the test piece. Therefore, for practical parts, molding analysis is performed under analytical conditions with fewer elements and lower computational load. On the other hand, test pieces are small and have simple shapes after deformation. Therefore, in the analytical conditions for calculating the benchmark value using the test piece, there is a tendency to relatively increase the number of elements and relatively increase the computational load. That is, there is a tendency to calculate the benchmark stress with high accuracy.

[0014] The baseline stress (baseline stress) is a value determined through various delayed failure evaluation methods and serves as the limit for delayed failure. Furthermore, this baseline stress is applied to the calculation results of forming analysis in actual automotive parts, allowing for the prediction of delayed failure solely based on the forming analysis. This prediction is crucial in the design of car bodies using high-tensile steel sheets.

[0015] The present invention was made in view of the above-mentioned problems, and its purpose is to further improve the evaluation accuracy of delayed destruction.

[0016] Methods for solving problems

[0017] This invention focuses on the fact that the calculated stress value, which serves as the benchmark for delayed failure, varies depending on the analytical conditions of the molding analysis. This invention modifies the stress value used as the benchmark for delayed failure based on the analytical conditions applied when analyzing the molded part (practical component) as the target. Furthermore, using the modified value as the benchmark for evaluating delayed failure is one of the characteristics of this invention.

[0018] Furthermore, to address the aforementioned issues, one aspect of the present invention is to ensure that the analysis conditions for molding analysis in the delayed failure evaluation test are consistent with at least one of the analysis conditions for practical molding analysis representing actual automotive parts. For example, aligning the analysis conditions for molding analysis in the delayed failure evaluation test with the analysis conditions for practical molding analysis representing actual automotive parts provides an analysis condition with a smaller computational load than previously possible.

[0019] Therefore, the discrepancy between the calculated stress values ​​caused by the analytical conditions used in the delayed failure evaluation test and the actual automotive component molding analysis can be eliminated or reduced. As a result, the accuracy of delayed failure evaluation is improved by using the reference stress for delayed failure obtained from the delayed failure evaluation test when comparing the calculated results of the molding analysis in the actual automotive component.

[0020] In another aspect of the invention, based on the analytical conditions of practical molding analysis representing actual automotive parts, the stress calculated in the molding analysis during the delayed failure evaluation test, which serves as the benchmark for delayed failure, is converted to a value close to that calculated under those analytical conditions. The converted stress is then used as the benchmark for delayed failure evaluation.

[0021] Therefore, the computational effort required to align the molding analysis conditions in the aforementioned delayed failure evaluation tests with the practical molding analysis conditions represented by actual automotive parts can be reduced. Thus, other aspects of the present invention, by multiplying the stress value, which serves as the benchmark for delayed failure, by different conversion factors for correcting the analysis conditions, can easily correspond the calculated ultimate stress, which differs due to different analysis conditions, to the actual automotive parts.

[0022] The effects of the invention

[0023] According to the method of the present invention, delayed failure characteristics are evaluated based on the differences in analytical conditions caused by molding analysis between experimental and actual evaluation. Therefore, the method of the present invention can further improve the evaluation accuracy.

[0024] Therefore, the benchmark stress value that determines the limit of delayed failure, determined through various delayed failure evaluation methods, can be applied with high precision to the calculation results of forming analysis of actual automotive parts. This allows for the high-precision prediction of delayed failure in advance based on forming analysis. This facilitates the design of car bodies using high-tensile steel sheets, making the application of high-tensile steel sheets in automobiles easier. Attached Figure Description

[0025] [ Figure 1 [This is an explanatory diagram of the types of elements used in the V-shaped bending forming test piece as an example.]

[0026] [ Figure 2 [This is an explanatory diagram of the stress distribution in the thickness direction of a plate, taking V-shaped bending and fastening of both ends after V-shaped bending as examples. It is also an explanatory diagram of how the calculated stress values ​​vary significantly depending on the output area, taking V-shaped bending and fastening of both ends after V-shaped bending as examples.]

[0027] [ Figure 3 [ ] is a diagram illustrating the processing steps in the first embodiment.

[0028] [ Figure 4 [ ] is a diagram illustrating the process of calculating the reference stress.

[0029] [ Figure 5 [ ] is a diagram illustrating the evaluation process for practical components.

[0030] [ Figure 6 [Illustration 1] is a diagram showing the software configuration in the first embodiment.

[0031] [ Figure 7 [ ] is a diagram illustrating the processing steps in the second embodiment.

[0032] [ Figure 8 [Illustration 1] is a diagram showing the software configuration in the second embodiment.

[0033] [ Figure 9 This is an illustration of a fixture used in experiments involving V-shaped bending and fastening.

[0034] [ Figure 10 [A] is a diagram showing the appearance of 2D solid elements, 3D solid elements, and shell elements on a CAE.

[0035] [ Figure 11 [A diagram showing the area where the calculated stress is output within the plate thickness in the embodiment, namely ABCD.]

[0036] [ Figure 12The graph shows the calculated stresses of three different elements in the first molded article according to the calculation area, and shows the above stresses in the second molded article together with the presence or absence of delayed failure, and compares them.

[0037] [ Figure 13 ] is Figure 12 The diagram shows the calculation stress (reference stress) of three different elements in the first molded part when the conversion coefficient based on the type of element is applied for correction. Detailed Implementation

[0038] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0039] Imagine using computer-aided forming analysis to calculate the residual stress generated in a pressed product when a metal sheet is formed. In this case, the following problem exists: even if the forming conditions are the same, the calculated stress value depends on the analysis conditions, and the calculated stress value varies depending on the analysis conditions used.

[0040] Here, for the molding analysis of compression molding, molding analysis based on the finite element method is typically used. In this analysis, there are conditions that significantly affect the calculated values ​​of the corresponding forces. These conditions, in the molding analysis based on the finite element method, include the type of element, element size, the location of stress output within the molded part, and the location of the integration point of the output stress within the shell element.

[0041] Regarding the types and dimensions of elements in forming analysis based on the finite element method, for example, an embodiment described in Patent Document 1. This embodiment describes assuming the bending forming analysis as a two-dimensional solid element (or plane strain element) under plane strain conditions. Furthermore, it describes the possibility of using three-dimensional solid elements for analysis when computational power is sufficient.

[0042] On the other hand, in actual automotive component forming analysis, when the component is large, it is difficult to select three-dimensional solid elements with an excessive number of elements due to limitations in computing power. Furthermore, unlike simple bending forming as in Patent Document 1, complex overall component forming analysis is required. Therefore, in actual automotive component forming analysis, simplified analysis using two-dimensional solid elements is not preferred.

[0043] Therefore, in the actual forming analysis of automotive parts, there is a tendency to use shell elements, which have a small number of elements, low computational load, and no constraints.

[0044] As such, there is a tendency to use shell elements in the actual forming analysis of automotive parts. Furthermore, since delayed failure tends to occur at the parts' ends, the evaluation of the parts' ends is important.

[0045] Furthermore, predictions of delayed failure using calculated values ​​from two-dimensional solid elements are less accurate than predictions based on residual stress values ​​at the end faces of shell elements. The reasons are as follows: Two-dimensional solid elements assume a plane strain state. On the other hand, since the deformation state at the end faces of shell elements is not constrained by the surrounding material, it is a uniaxial deformation state. Therefore, the stress values ​​for the same strain differ significantly within shell elements. Consequently, using two-dimensional solid elements results in decreased accuracy.

[0046] Taking the V-shaped bending of the test piece as an example, Figure 1 The above three elements are shown.

[0047] At this point, the location of stress output in the two-dimensional solid element and the location of the integration point of the output stress within the shell element also have a significant impact on the calculated stress value. For example, in the embodiment of Patent Document 1, the calculated value of residual stress is evaluated for the outermost mesh of the curved outer side in the two-dimensional solid element.

[0048] On the other hand, to predict delayed failure originating from the end face, it is preferable to consider the residual stress in all regions (all output points) along the thickness direction of the end face. Therefore, the integration point for calculating stress in the shell element also needs to be selected from this perspective. However, the residual stress may not reach its maximum on the outer surface of the bend. In reality, the end face exhibits a complex stress distribution along the thickness direction, depending on the forming process, springback, and other forming conditions. Therefore, the stress used as a benchmark for delayed failure varies significantly depending on which integration point is used for the calculated stress value.

[0049] This applies to any type of element. The calculated stress values ​​vary significantly depending on how the region from which the same calculated value is output is selected. Figure 2 The diagram illustrates the stress distribution along the thickness of the plate, using V-shaped bending and the tightening deformation at both ends after V-shaped bending as examples.

[0050] As stated above, even when evaluating delayed failure characteristics using methods found in existing literature, the accuracy decreases accordingly when using the benchmark stress value that becomes the limit for delayed failure calculated through forming analysis. That is, when the analytical conditions for forming analysis differ, the accuracy decreases when using the benchmark stress value that becomes the limit for delayed failure to predict the occurrence of delayed failure based on the calculation results of forming analysis of actual automotive parts.

[0051] In contrast, in the first embodiment of the present invention, the first analysis condition of the forming analysis used to determine the reference value of stress is the same as at least one of the second analysis conditions. The second analysis condition is the analysis condition of the forming analysis used to calculate the stress generated in the forming of the practical part shape.

[0052] Here, it is also considered to change the second analysis condition to be consistent with the first analysis condition. However, the setting of the first analysis condition has a large degree of freedom. Therefore, it is preferable to make the first analysis condition consistent with the second analysis condition. In the first embodiment, at least the conditions for the type and size of the elements in the first analysis condition are set to be the same as the conditions for the type and size of the elements in the second analysis condition described above.

[0053] Furthermore, in the second embodiment of the present invention, a conversion factor is determined to make the stress value calculated under the first analysis conditions close to the stress value calculated under the second analysis conditions. Then, the reference stress calculated using the first analysis conditions is converted using the conversion factor, and the converted stress is used as the reference stress for evaluation.

[0054] The following provides a more detailed description of this embodiment.

[0055] "First Embodiment"

[0056] (constitute)

[0057] like Figure 3 As shown, the delayed failure method of this embodiment includes at least the following steps: test step 9, reference stress calculation step 10, evaluation stress calculation step 11, and evaluation step 12.

[0058] The reference stress calculation process 10 constitutes the first process, and the evaluation stress calculation process 11 constitutes the second process.

[0059] Test step 9 involves deforming the test piece 1, which is formed from a metal plate. Test step 9 then places the deformed test piece 2 in a hydrogen immersion environment to evaluate the formation of cracks on the test piece 2.

[0060] The reference stress calculation step 10, based on the evaluation in the test step 9, inputs information related to the deformation determined to have delayed failure. Based on this input, the reference stress calculation step 10 calculates the maximum residual stress generated in the deformed test piece by applying deformation to the test piece using molding analysis under the first analysis condition of a computer. The reference stress calculation step 10 calculates the reference stress based on the calculated maximum residual stress. The reference stress is used to determine whether delayed failure occurs in the molded metal sheet under a hydrogen environment.

[0061] In addition, the stress evaluation calculation step 11 performs a molding analysis under the second analysis conditions using a computer on the shape of the molded article (pressed part) designed as a practical component. Then, the stress evaluation calculation step 11 calculates the residual stress generated in the molded article by molding the metal sheet into the target molded article. The residual stress is calculated at multiple locations on the molded article. The maximum residual stress calculated in this way can also be used to represent the evaluated residual stress.

[0062] Evaluation step 12 evaluates the delayed failure characteristics of the target molded article by comparing the residual stress obtained in step 2 with the reference stress.

[0063] In the first embodiment described above, the first analysis condition used in the reference stress calculation step 10 is set to the same analysis condition as the second analysis condition used in the evaluation stress calculation step 11. In this embodiment, at least the type and size of the elements in the first analysis condition are set to be the same as the type and size of the elements in the second analysis condition. Alternatively, all analysis conditions set as the first analysis condition may be used as the same as the second analysis condition.

[0064] <Test Procedure 9 and Reference Stress Calculation Procedure 10>

[0065] Test procedure 9 and reference stress calculation procedure 10, for example, through Figure 4 The process shown is executed.

[0066] First, in step S10, the deformed test piece is made.

[0067] In this process, a metal sheet (e.g., a high-tensile steel sheet) of the same material and thickness as the product being manufactured is prepared. That is, the same metal sheet used in the product is prepared. Then, the metal sheet is sheared to produce a test piece 1 of a predetermined shape. Next, the test piece is deformed by a predetermined amount using a pre-set forming method.

[0068] Examples of forming methods include V-bending and deep drawing as described in Patent Documents 1-3. In this embodiment, any known forming method can be used. Thus, for example, in the case of V-bending, the bending portion 2A is subjected to strain and residual stress simulating compression forming. Similarly, in the case of deep drawing, the ends such as the flange are subjected to strain and residual stress simulating compression forming.

[0069] Here, it is preferable to produce multiple test pieces with different deformation amounts together.

[0070] Next, in step S20, the test piece 1 prepared in step S10 is placed in a hydrogen intrusion environment (hydrogen intrusion atmosphere).

[0071] The setup for introducing the test piece into a hydrogen immersion environment is carried out, for example, by immersing the molded test piece in a bath containing acidic solutions such as hydrochloric acid or NH4SCN aqueous solution.

[0072] Next, in step S30, the delayed destructiveness is evaluated based on the occurrence of cracks at the strain-imposed location of the test piece (e.g., the time until crack formation).

[0073] For example, in step S30, the delayed destructiveness of the metal plate is evaluated based on the crack formation at the strain-imposed location in a test piece that has been subjected to a predetermined time in a hydrogen intrusion environment (e.g., the time until crack formation).

[0074] Next, in step S40, if it is determined that the test piece evaluated in step S30 has delayed failure, the process proceeds to step S50. If no test piece has delayed failure, the deformation amount is increased, and the process returns to step S10, executing steps S10 to S40. Test pieces with different deformation amounts can also be evaluated simultaneously.

[0075] Here, the processing steps S10 to S40 correspond to test procedure 9.

[0076] In step S50, among the test pieces determined to have delayed failure, based on the deformation of the test piece with the smallest deformation, a forming analysis is performed using a computer under the first analysis condition to calculate the maximum tensile residual stress.

[0077] Here, the molding conditions in the molding analysis are set to the processing method used in step S10, and the deformation amount in the processing is set to the deformation amount of the above-mentioned test piece.

[0078] In this embodiment, the first analysis condition is determined based on the second analysis condition. Furthermore, in this embodiment, the first analysis condition is set to be the same as the second analysis condition.

[0079] In step S60, the maximum tensile residual stress calculated in step S50 is set as the reference stress. The reference stress may not necessarily be the same as the maximum tensile residual stress. For example, the reference stress may be the value obtained by reducing the calculated maximum tensile residual stress by a predetermined margin.

[0080] Steps S50 and S60 correspond to the first process (reference stress calculation process 10) performed by the computer.

[0081] This reference stress serves as a benchmark for determining whether delayed failure occurs in the pressed product during the pressing and forming of metal sheets, and as a benchmark for determining how much leeway there is before delayed failure occurs.

[0082] The above describes the process of determining the reference stress used for delayed failure evaluation.

[0083] <Evaluation and Processing>

[0084] Next, the evaluation of the practical components of the molded product as the target will be handled in accordance with... Figure 5 Please provide an explanation.

[0085] First, based on the specifications, determine the shapes of candidate molded parts to serve as practical components.

[0086] Then, in step S100, a molding analysis is performed on the molded product shape (the shape of the practical part) under the second analysis condition using a computer. Then, when the metal sheet is molded into the above-mentioned molded product shape, the residual stress generated at each position at the end of the target molded product is calculated.

[0087] It should be noted that the pressing processing conditions when molding practical parts are set as the molding conditions for molding analysis.

[0088] This step S100 corresponds to the stress calculation process 11 (the second process).

[0089] Next, proceed to step S110, compare the residual stresses obtained in step S100 with the above-mentioned reference stresses, and evaluate whether each residual stress is less than the reference stress.

[0090] This step S110 corresponds to the processing and evaluation process 12.

[0091] Then, in step S120, if any residual stress is above the reference stress, it is evaluated (predicted) that delayed failure will occur, and the process proceeds to step S140. On the other hand, if all residual stresses are below the reference stress, it is determined that no delayed failure will occur, and the process proceeds to step S130. It should be noted that the evaluation can also be performed using the maximum residual stress among the residual stresses at each location as a representative.

[0092] In step S130, an evaluation of no delayed destruction is output within the set molded shape.

[0093] On the other hand, in step S140, as a delayed destruction, the process proceeds to step S150.

[0094] In step S150, the specifications are changed as needed. Then, based on the changed specifications, the shape of the candidate molded part as a practical component is determined again and transferred to step S100, and the above process is repeated.

[0095] (Software Configuration Example)

[0096] In this embodiment, the processes of the reference stress calculation step 10 constituting the first process, the evaluation stress calculation step 11 constituting the second process, and the evaluation step 12 are performed using a computer. The evaluation step 12 can also be performed without using a computer.

[0097] In this embodiment, for example, such as Figure 6 As shown, the program 23A for the first process, the program 23B for the second process, and the program 23C for the evaluation process are stored in the program storage unit 22. Symbol 21 represents a computer with hardware such as a CPU, which executes the programs stored in the program storage unit 22.

[0098] The first process program 23A is a program that performs the processing described in the first process. The first process program 23A, for example, has a first molding analysis unit based on CAE to calculate the maximum residual stress, and a reference stress setting unit to calculate the reference stress based on the maximum residual stress calculated by the first molding analysis unit.

[0099] The second process program 23B is a program that performs the processing described in the second process. The second process program 23B, for example, has a second molding analysis unit based on CAE calculation of residual stress.

[0100] Evaluation procedure 23C is a procedure that performs the processing of evaluation procedure 12 described above. Evaluation procedure 23C performs a process of comparing the reference stress obtained in the first procedure 23A and the residual stress obtained in the second procedure 23B.

[0101] In this embodiment, the conditions for the type and size of elements in the analysis conditions of the molding analysis used in the first molding analysis unit are set to be the same as those for the type and size of elements in the analysis conditions of the molding analysis used in the second molding analysis unit.

[0102] (Actions and others)

[0103] This embodiment is suitable for processing when the metal sheet being evaluated is a high-tensile steel sheet with a tensile strength of 980 MPa or higher. In addition, it is suitable for evaluating automotive parts.

[0104] This embodiment evaluates delayed failure characteristics by determining whether cracking due to delayed failure occurs in molded articles formed from metal sheets such as high-tensile steel sheets in a hydrogen environment. At this time, a reference stress, serving as the benchmark for evaluating delayed failure characteristics, is determined through experiments on test pieces and molding analysis. In this embodiment, the first analysis condition for molding analysis is set to the same analysis condition as the second analysis condition used in the molding analysis during the design of practical components.

[0105] Here, the calculated stress value based on the forming analysis varies depending on the analysis conditions used. Therefore, the stress that serves as the reference for delayed failure varies depending on the analysis conditions. Therefore, in this embodiment, the stress value that serves as the reference for delayed failure is treated as a value that varies according to the second analysis conditions.

[0106] Furthermore, the stress used as the reference for delayed failure is preferably treated as a stress that varies with the strain calculated under the same analytical conditions. As described in Patent Documents 1-3, processing-induced strain sometimes affects the occurrence of delayed failure. Therefore, it is sometimes assumed that the stress used as the reference for delayed failure changes. The strain referred to here means, for example, equivalent plastic strain.

[0107] All forming analyses used in this embodiment are performed using the finite element method.

[0108] Furthermore, in the first and second analysis conditions, to ensure consistency, it is preferable to include the types and dimensions of elements in the molding analysis based on the finite element method. Moreover, the analysis conditions for the stress output locations within the molded article and the locations of stress output integration points within the shell element can also be made consistent.

[0109] Here, delayed failure can occur at any point within the sheet thickness on the end face of the molded article. Therefore, for the calculation location of residual stress in the second analysis condition, it is preferable to output the maximum stress of all elements or integration points within the sheet thickness. The calculation location of residual stress is the output location of stress within the analyzed molded article, or the integration point location of the output stress within the shell element.

[0110] Furthermore, in this embodiment, the discrepancy between the calculated stress values ​​due to analysis conditions and the processing for determining the reference stress used to evaluate delayed failure can be reduced between the molding analysis of a molded article representing an actual automotive part. Based on this, this embodiment compares the highly accurate reference stress for delayed failure with the calculation results of the molding analysis of the target molded article representing an actual automotive part. The occurrence of delayed failure is predicted through this comparison. Then, based on the predicted occurrence of delayed failure, the design and molding specifications of the target molded article are appropriately modified. As a result, delayed failure can be avoided in the actual manufactured press-molded article.

[0111] "Second Implementation Method"

[0112] Next, the second embodiment will be described.

[0113] The basic structure of the second embodiment is the same as that of the first embodiment.

[0114] like Figure 7 As shown, the difference between the second embodiment and the first embodiment is that the second embodiment additionally includes a conversion coefficient setting step 13, and the conversion coefficient set in the conversion coefficient setting step 13 is used in the evaluation step 12. Everything else is the same as the first embodiment, so descriptions are omitted.

[0115] (constitute)

[0116] <Conversion Coefficient Setting Procedure 13>

[0117] The conversion coefficient setting process 13 corresponds to the 3rd process.

[0118] The conversion factor setting step 13, for example, determines the correlation between the first stress calculated through molding analysis under the first analysis condition and the second stress calculated through molding analysis under the second analysis condition when the metal sheet is pressed under the same molding conditions. Based on the determined correlation, the conversion factor setting step 13 calculates a conversion factor used to make the stress calculated through molding analysis under the first analysis condition close to the stress calculated through molding analysis under the aforementioned second analysis condition. It should be noted that the metal sheet used to determine the conversion factor may be different from the metal sheet used for evaluation.

[0119] For example, pressure calculations are performed multiple times under the same molding conditions, for both the first and second analysis conditions. Then, through statistical processing based on these multiple executions, the correlation between the pressures calculated under the first analysis condition and the pressures calculated under the second analysis condition is determined. A conversion factor is then set based on this correlation.

[0120] At this point, analytical conditions considered to have a significant impact on the calculated stress values ​​can be treated as variables to determine the correlation. For example, general conversion coefficients can also be determined using the type of feature and the size of the mesh as variables.

[0121] It should be noted that the types of elements are 2D solid elements, 3D solid elements, or shell elements.

[0122] [The case where the element type in the second analysis condition is a shell element]

[0123] Next, we will illustrate an example of the transformation coefficient for the case where the element type in the second analysis condition is a shell element.

[0124] In this case, the conversion factor K that converts the stress σ0 calculated by the forming analysis under the first analysis condition into the stress σ calculated by the forming analysis under the second analysis condition can be obtained by the following equation (1).

[0125] K=α[β(m / t)+1]…(1)

[0126] Here, t[mm] is the thickness of the metal sheet before forming. m[mm] is the mesh size of the element used for forming analysis under the second analysis condition and the first analysis condition.

[0127] In addition, α and β are pre-set coefficients.

[0128] Specifically, when the type of element in the first analysis condition is a 2D solid element, the coefficient α is set to a constant selected from the range of 0.7 to 0.9. Furthermore, when the type of element in the first analysis condition is a 3D solid element or a shell element, the coefficient α is set to 1.

[0129] In addition, the coefficient β is set as a constant selected from the range of 0.05 to 0.15.

[0130] [The case where the element type in the first analysis condition is a shell element]

[0131] Next, we will illustrate an example of the transformation coefficient for the case where the element type in the first analysis condition is a shell element.

[0132] In this case, the conversion factor K that converts the stress σ0 calculated by the forming analysis under the first analysis condition into the stress σ calculated by the forming analysis under the second analysis condition can be obtained by the following equation (2).

[0133] K=1 / (α[β(m / t)+1])…(2)

[0134] Here, t[mm] is the thickness of the metal sheet before forming. m[mm] is the mesh size of the element used for forming analysis under the second analysis condition and the first analysis condition.

[0135] In addition, α and β are pre-set coefficients.

[0136] Specifically, when the type of element in the second analysis condition is a 2D solid element, the coefficient α is set to a constant selected from the range of 0.7 to 0.9. When the type of element in the second analysis condition is a 3D solid element or a shell element, the coefficient α is set to 1.

[0137] In addition, the coefficient β is set as a constant selected from the range of 0.05 to 0.15.

[0138] <Evaluation Process 12>

[0139] Furthermore, the evaluation step 12 of the second embodiment evaluates the delayed failure characteristics of the target molded article. The evaluation is performed by comparing the residual stress obtained in the evaluation stress calculation step 11 (the second step) with the stress converted by a conversion factor from the reference stress obtained in the reference stress calculation step 10.

[0140] The conversion factor can be calculated using the following formula, for example.

[0141] σ=K·σ0

[0142] Here, the calculation of the stress after converting the reference stress with the conversion factor K can also be performed in the reference stress calculation step 10.

[0143] (Software Configuration Example)

[0144] In this embodiment, a computer is used to perform the following processes: the reference stress calculation process 10 constituting the first process, the evaluation stress calculation process 11 constituting the second process, the conversion coefficient setting process 13 constituting the third process, and the evaluation process 12. The evaluation process 12 can also be performed using a computer.

[0145] In this embodiment, for example, such as Figure 8 As shown, the program storage unit 22 stores the program 23A for the first process, the program 23B for the second process, the program 23E for the third process, and the program 23C for the evaluation process. Symbol 21 represents a computer with hardware such as a CPU, which executes the program stored in the program storage unit 22.

[0146] The first process program 23A is a program that performs the processing described in the first process. The first process program 23A, for example, has a first molding analysis unit based on CAE to calculate the maximum residual stress, and a reference stress setting unit to calculate the reference stress based on the maximum residual stress calculated by the first molding analysis unit.

[0147] The second process program 23B is a program that performs the processing described in the second process. The second process program 23B, for example, has a second molding analysis unit based on CAE calculation of residual stress.

[0148] In the third step, program 23E performs the following processing: Based on the correlation between the first stress calculated through molding analysis under the first analysis condition and the second stress calculated through molding analysis under the second analysis condition when pressing is performed under the same molding conditions, a conversion factor is determined. The conversion factor is used to make the stress calculated through molding analysis under the first analysis condition close to the stress calculated through molding analysis under the second analysis condition.

[0149] The third step, using program 23E, can also perform the calculation of conversion coefficients each time the program starts. Alternatively, the following processing structure can be used: For example, the first and second analysis conditions are used as variables, and the pre-calculated conversion coefficients 24 are stored in the database for each analysis condition. Then, the third step, using program 23E, performs the process of retrieving the conversion coefficients corresponding to the first and second analysis conditions used from the database.

[0150] Evaluation procedure 23C is a procedure that performs the processing described in evaluation procedure 12. Evaluation procedure 23C converts the reference stress obtained in the first procedure 23A by multiplying it by the conversion factor obtained in the third procedure 23E. Furthermore, evaluation procedure 23C performs a process of comparing the converted stress with the residual stress obtained in the second procedure 23B.

[0151] (Actions and others)

[0152] In this embodiment, one objective is to reduce the effort required to align the first and second analytical conditions. Therefore, in this embodiment, as a simple solution, the reference stress for delayed failure obtained under the first analytical condition is corrected using a conversion factor.

[0153] The analytical conditions that significantly influence the calculated reference stress for delayed failure include the type of element used in the forming analysis and the element's mesh size. Therefore, at least one of the elements from both the first and second analytical conditions—the type of element and the mesh size—can be used as variables to determine the conversion factor. The correlation between the calculation results of the first and second analytical conditions is almost independent of the type of metal sheet; therefore, it only needs to be determined beforehand and is unrelated to the type of metal sheet used in the pressing process.

[0154] Here, for the aforementioned coefficients, as optimal values, coefficient α is set to 0.8 in 2D solid features and 1 in 3D solid and shell features. Additionally, coefficient β is set to 0.1. For coefficient α, a value between 0.7 and 0.9 is sufficient for simple calculations in 2D solid features. For coefficient β, a value between 0.05 and 0.15 is sufficient for simple calculations. Compared to the uniaxial tensile deformation state at the end faces of 3D solid and shell features, coefficient α represents the plane strain deformation state in 2D solid features. Therefore, coefficient α in 2D solid features provides a correction for higher calculated stresses under the same deformation state. For coefficient β, a finer mesh allows for more accurate calculation of stress concentration. Therefore, the correction for coefficient β addresses the situation where stresses are calculated higher even under the same deformation state.

[0155] (other)

[0156] This published text can also be structured as follows.

[0157] (1) In this embodiment, the following steps are included: a first step in which deformation is applied to a test piece formed of a metal plate, and the generation of cracks on the test piece generated by placing the deformed test piece in a hydrogen immersion environment is evaluated: a first step in which, for the deformation determined to have delayed failure based on the above evaluation, the maximum residual stress generated on the deformed test piece by applying deformation to the test piece is calculated by molding analysis under the first analysis condition of a computer, and a reference stress for determining whether delayed failure occurs in the molded article of the metal plate in a hydrogen environment is obtained based on the calculated maximum residual stress; a second step in which the residual stress generated on the molded article by molding the metal plate into a target molded article is obtained by molding analysis under the second analysis condition of a computer; and an evaluation step 12 in which the delayed failure characteristics of the target molded article are evaluated by comparing the residual stress obtained in the second step with the reference stress, and the conditions of the type and size of the elements in the first analysis condition are set to be the same as the conditions of the type and size of the elements in the second analysis condition.

[0158] Based on this configuration, delayed failure characteristics are evaluated considering the differences in analytical conditions during testing and actual evaluation, as well as the conditions under which molding analysis is performed. Therefore, the accuracy of the evaluation can be further improved.

[0159] (2) In this embodiment, the deformation of a test piece formed from a metal plate is applied, and the generation of cracks on the test piece generated by placing the deformed test piece in a hydrogen immersion environment is evaluated. The following steps are included: First step, for the deformation determined to have delayed failure based on the above evaluation, the maximum residual stress generated on the deformed test piece by applying deformation is calculated using molding analysis under the first analysis condition of a computer, and the reference stress for determining whether delayed failure occurs in the molded product of the metal plate in a hydrogen environment is obtained based on the calculated maximum residual stress; Second step, the reference stress for determining whether delayed failure occurs in the molded product of the metal plate in a hydrogen environment is obtained by applying molding analysis under the second analysis condition of a computer. The process includes: a third step, which determines a conversion factor that makes the stress calculated by the molding analysis under the first analysis condition close to the stress calculated by the molding analysis under the second analysis condition, based on the correlation between the first stress calculated by the molding analysis under the first analysis condition and the second stress calculated by the molding analysis under the second analysis condition when the metal sheet is pressed under the same molding conditions; and an evaluation step 12, which evaluates the delayed failure characteristics of the target molded article by comparing the residual stress obtained in the second step with the stress converted by the conversion factor.

[0160] Based on this configuration, delayed failure characteristics are evaluated considering the differences in analytical conditions during testing and actual evaluation, as well as the conditions under which molding analysis is performed. Therefore, the accuracy of the evaluation can be further improved.

[0161] (3) The above forming analysis is based on the forming analysis of the finite element method. The first and second analysis conditions mentioned above include one or more conditions selected from the type of element, the size of the element, the stress output location in the molded article, and the location of the integration point of the output stress in the shell element.

[0162] (4) As the types of elements in the first and second analysis conditions mentioned above, the conversion coefficients are set based on the differences in the types of elements in the first and second analysis conditions mentioned above.

[0163] (5) Set the thickness of the metal plate before forming to t [mm], set the type of element to 2D solid element, 3D solid element, or shell element. For the second analysis condition, the type of element is shell element, the mesh size of the element used for its forming analysis is m [mm], the mesh size of the element under the first analysis condition is m [mm], and the conversion coefficient K that converts the stress calculated by the forming analysis under the first analysis condition to the stress calculated by the forming analysis under the second analysis condition is expressed by the following formula (1).

[0164] K=α[β(m / t)+1]…(1)

[0165] in,

[0166] When the type of element in the first analysis condition is a 2D solid element, the coefficient α is set to a constant selected from the range of 0.7 to 0.9. When the type of element in the first analysis condition is a 3D solid element or a shell element, the coefficient α is set to 1.

[0167] The coefficient β is set to a constant selected from the range of 0.05 to 0.15.

[0168] Based on this structure, conversion can be performed easily.

[0169] (6) Set the thickness of the metal plate before forming to t [mm], and set the type of element to 2D solid element, 3D solid element, or shell element. For the first analysis condition, the type of element is shell element, and the mesh size of the element used for its forming analysis is m [mm]. The mesh size of the element under the second analysis condition is m [mm]. The conversion coefficient K that converts the stress calculated by the forming analysis under the first analysis condition to the stress calculated by the forming analysis under the second analysis condition is expressed by the following formula (2).

[0170] K=1 / (α[β(m / t)+1])…(2)

[0171] In the case where the type of element in the second analysis condition is a 2D solid element, the coefficient α is set to a constant selected from the range of 0.7 to 0.9. In the case where the type of element in the second analysis condition is a 3D solid element or a shell element, the coefficient α is set to 1.

[0172] The coefficient β is set to a constant selected from the range of 0.05 to 0.15.

[0173] Based on this structure, conversion can be performed easily.

[0174] (7) The above forming analysis is based on the forming analysis of the finite element method. For the stress output location in the formed shape or the integration point location of the shell element in the forming analysis under the first and second analysis conditions, the maximum stress of all elements or integration points within the plate thickness is output.

[0175] Based on this configuration, by considering the stress values ​​at all output points in the thickness direction of the plate, the accuracy of the maximum residual stress obtained through forming analysis is improved.

[0176] (8) The metal plate mentioned above is a high-tensile steel plate with a tensile strength of 980 MPa or more.

[0177] (9) The above-mentioned target molded product is a component of an automobile.

[0178] (10) A program for a delayed failure characteristic evaluation method, wherein the delayed failure characteristic evaluation method applies deformation to a test piece formed of a metal plate and evaluates the generation of cracks on the test piece generated by placing the deformed test piece in a hydrogen immersion environment, comprising the following steps: a first step, wherein information related to the deformation determined by the above evaluation to have delayed failure is input, and the maximum residual stress generated on the deformed test piece by applying deformation to the test piece is calculated by using a forming analysis under the first analysis condition of a computer, and the maximum residual stress is calculated based on the calculated maximum residual stress to determine the crack generation under hydrogen immersion environment. The first step involves determining whether a reference stress for delayed failure occurs in the molded product of the metal sheet; a second step, which uses a computer to perform a molding analysis under a second analysis condition to determine the residual stress generated in the molded product by molding the metal sheet into a target molded product; and an evaluation step, which evaluates the delayed failure characteristics of the target molded product based on a comparison between the residual stress determined in the second step and the reference stress, and sets the conditions for the type and size of the elements in the first analysis condition to be the same as the conditions for the type and size of the elements in the second analysis condition. The above program is used to enable the computer to perform the first step.

[0179] (11) A program for a delayed failure characteristic evaluation method, wherein the delayed failure characteristic evaluation method applies deformation to a test piece formed of a metal plate and evaluates the generation of cracks on the test piece generated by placing the deformed test piece in a hydrogen immersion environment, comprising the following steps: a first step, wherein information related to the deformation determined to have delayed failure by the above evaluation is input, and for the deformation determined to have delayed failure, the maximum residual stress generated on the deformed test piece by applying deformation to the test piece is calculated by using molding analysis under the first analysis condition of a computer, and a reference stress for determining whether delayed failure occurs in the molded product of the metal plate in a hydrogen environment is obtained based on the calculated maximum residual stress; a second step, wherein the second analysis is performed by using the second analysis condition of a computer. The process includes a forming analysis under the above-mentioned conditions, which determines the residual stress generated in the above-mentioned molded article by forming the above-mentioned metal sheet into the target molded article; a third step, which determines a conversion factor that makes the stress calculated by the forming analysis under the above-mentioned conditions close to the stress calculated by the forming analysis under the above-mentioned conditions when pressing is performed under the same forming conditions; and an evaluation step, which evaluates the delayed failure characteristics of the above-mentioned target molded article by comparing the residual stress obtained in the above-mentioned second step with the stress converted by the above-mentioned reference stress using the above-mentioned conversion factor. The above program is used to enable a computer to implement the above-mentioned first step and the above-mentioned third step.

[0180] Example

[0181] Next, an embodiment of this implementation will be described.

[0182] Here, an example is described using a test material (metal plate) with a thickness of 1.4 mm. A high-tensile steel plate with a tensile strength of 1520 MPa was used as the test material.

[0183] First, execute Figure 9 The procedure for the bending test is shown. That is, a V-shaped punch with a bending radius of 7mm is used.

[0184] The test material, formed from a rectangular plate 1 of 110 mm × 30 mm, is subjected to a V-shaped bend. Then, at a position 22 mm from both ends of the plate at the bending processing section 2A that holds the test piece, the test piece is fastened and shaped using a stress-load fixture. Thus, a test piece 2 with residual stress is produced.

[0185] Here, as Figure 9As shown in (d), the change in the distance between two points where a load is applied by tightening is defined as the tightening amount. Symbol 6 represents the fastener. The end face of the test piece is fabricated by shearing. In the shearing process, the fracture surface on the side where burrs are generated is taken as the outer side of the initial V-bend. The inner side of the initial V-bend is taken as the plate surface, and the outer side of the bend is taken as the plate inside to define the plate's surface and inner side.

[0186] For the aforementioned molded articles, multiple articles were produced by varying the tightening amount during molding. These articles were then immersed in a bath (pH 3, hydrochloric acid 8) for 100 hours, and the occurrence of cracking was investigated. The limit of the tightening amount that induces delayed failure was then investigated. The results showed that a tightening amount of 22 mm or more could induce delayed failure from the shear face.

[0187] Therefore, the test piece with a fastening amount of 22 mm is defined as a test piece with the stress that produces the limit of delayed failure, and is the first molded article used to determine the reference stress that produces delayed failure.

[0188] Next, the same V-bending process and fastening molding based on the same amount of fastening as the first molded product were used as molding conditions, and computer simulation analysis was conducted. The finite element method software used in the analysis was LS-DYNA ver.971.

[0189] At this point, in order to confirm the differences in the calculated stress values ​​based on the analysis conditions regarding the types of elements, the following forming analysis is performed. That is, based on... Figure 10 The three types of elements shown are 2D solid elements, 3D solid elements, and shell elements, and their forming characteristics are analyzed.

[0190] Set the mesh size for 2D solid features and 3D solid features to 0.1mm. Set the mesh size for shell features to 1.4mm.

[0191] Then, in order to represent the difference in the calculated stress values ​​due to the location of the stress output point within the first molded article, or the location of the integration point of the output stress within the shell element, the following operation is performed. For example... Figure 11 As shown in (d), three regions are defined in the plate thickness direction: region A (within 0.3 mm from the surface of the plate), region B (the area near the center of the plate thickness, between the portion 0.3 mm from the surface and the portion 0.3 mm from the inner side of the plate), and region C (within 0.3 mm from the inner side of the plate). Then, the maximum stress in each of the three regions is output as the calculated value. It should be noted that... Figure 11 (a) to (c) are recorded for specific surfaces and interiors.

[0192] In addition, the overall plate thickness formed by merging all regions A, B, and C is set as region D (overall plate thickness region), and the maximum stress in region D is also output as the calculated value.

[0193] However, in 2D solid features, the mesh portion encompassing the aforementioned regions along the plate thickness direction is used for outputting the calculated stress value. Furthermore, in 3D solid features, the maximum stress value of the mesh portion encompassing regions A, B, C, and D along the plate thickness direction in the mesh located on the plate end face surface is used for outputting the calculated stress value. Similarly, for shell features, the maximum stress value is outputting the maximum stress value at the integration points within the portion of regions A, B, C, and D along the plate thickness direction in the mesh located on the plate end face surface.

[0194] Table 1 shows the calculated stress values ​​for each element type when the tightening amount in regions A, B, C, and D is 22 mm.

[0195] Table 1

[0196]

[0197] Calculated stress value / MPa

[0198] As shown in Table 1, in this case, region A is compressed due to the fastening process, therefore the calculated stress value is negative. Region B applies a small amount of tensile stress due to the fastening process to the residual stress after bending, therefore the calculated stress value is positive. Region C is subjected to the strongest tension due to the fastening process, therefore the calculated stress value is the largest and positive. As a difference based on the type of element, a plane strain state is assumed in 2D solid elements. Therefore, in 2D solid elements, the stress is higher than that of uniaxial tension near the end face, and thus the absolute value of the calculated stress value is relatively high. In 3D solid and shell elements, the regions with the highest stress are different, indicating the difference in stress distribution within the plate thickness based on the calculation method.

[0199] Next, considering components actually used in automobiles, a prototype part is made from the material blank through compression molding. This prototype part is defined as the second molded product.

[0200] The second molded product uses the same material as the first molded product. The outer periphery of the material blank is formed by shearing. In the shearing process, the fracture surface on the side where burrs are generated in the thickness direction is defined as the surface of the plate, and the opposite side is defined as the inside of the plate.

[0201] The second molded article was also investigated, in the same manner as the first molded article, by immersing it in hydrochloric acid at pH 3 for 100 hours and examining whether cracking occurred, thereby investigating whether delayed damage occurred at various points on the molded article.

[0202] Next, a molding analysis is performed on the second molded article to calculate the stress in all areas of the component. This molding analysis uses shell elements, as used in the molding analysis of actual automotive parts. In this embodiment, the mesh size of the shell elements is set to 1.4 mm.

[0203] Next, 14 representative locations, including those where delayed failure occurs, are selected from the second molded article. These 14 locations are designated as delayed failure prediction locations 1 to 14. For each delayed failure prediction location, the calculated stress value is output based on the above calculation results. Similar to the first test piece, using the grid located on the end face surface of the plate as the object, the maximum stress value at the integration point of the portion in the plate thickness direction that includes the regions A, B, C, and D mentioned above is used for the output of the calculated value.

[0204] Table 2 shows the calculated stress values ​​for regions A, B, C, and D in the delayed failure prediction regions 1 to 14 of the second molded article, as well as the presence or absence of delayed failure in those regions.

[0205] Table 2

[0206]

[0207] Calculated stress value / MPa

[0208] Each delayed failure prediction location exhibits various stress distributions along the plate thickness direction, depending on the forming mode during forming, the strain during bending, the direction of bending, and the degree of springback. Therefore, the calculated stress values ​​vary significantly depending on the selected area of ​​output stress.

[0209] Finally, as Figure 12 As shown, the reference stress that causes delayed failure is represented by the region used in the calculation. In the first molded article, the reference stress that causes delayed failure is calculated through molding analysis of three elements: 2D solid elements, 3D solid elements, and shell elements, and is determined by the output of the calculated stress values ​​for each of regions A, B, C, and D.

[0210] in addition, Figure 12 The output of the calculated stress values ​​for regions A, B, C, and D, derived from the molding analysis of the shell elements in the second molded article, is also shown. This output, along with the presence or absence of delayed failure, is represented by the region used in the calculation.

[0211] Depend on Figure 12As can be seen, the stress used to determine delayed failure varies significantly depending on the region from which the calculated stress value is output. By using the maximum stress value across all regions, the difference in stress distribution between the second molded product and the first molded product does not affect the outcome. Therefore, delayed failure can be predicted with optimal accuracy.

[0212] Next, we focus on the types of elements used in the forming analysis. In this case, when using 2D solid elements in the forming analysis of the first molded article, the stress is overestimated due to the assumption of a plane strain state. As a result, the stress that causes delayed failure cannot be accurately predicted in the second molded article. On the other hand, when using shell elements in the forming analysis of the first molded article, delayed failure can be predicted with the best accuracy. Furthermore, when using 3D solid elements in the forming analysis of the first molded article, the accuracy of the prediction of delayed failure is lower compared to the case of using a 3D shell. The reason is as follows: Calculation based on a fine mesh of 3D solids can calculate the stress of the 3D shell with high accuracy. On the other hand, the increased calculation accuracy deviates from the calculated value of the 3D shell, which is used as a practical condition.

[0213] In this way, it is desirable for the analysis conditions of the first and second molded products to be as similar as possible. Preferably, the types of elements should be completely consistent with the mesh size.

[0214] Furthermore, as an example of matching simple analytical conditions using the conversion factor, the effectiveness of matching the delayed failure reference stress based on the conversion factor was confirmed.

[0215] Table 3 shows the conversion of the calculated stress values ​​of the 2D and 3D solids in Table 1 into the stress of the shell elements using the above equation (1).

[0216] Table 3

[0217]

[0218] Calculated stress value / MPa

[0219] In equation (1), σ is the converted pressure value, which is the calculated stress when converted to the shell element used as the reference. m is the mesh size. t is the plate thickness of 1.4 mm.

[0220] Here, the coefficient α is set to 0.8 for 2D solid features and 1 for 3D solid and shell features. The coefficient β is set to 0.1. σ0 is the reference stress before delayed failure calculated for each feature.

[0221] Figure 13 This involves replacing the reference stress σ0, which causes delayed failure, with the converted (corrected) value σ using the conversion factors listed in Table 3. The reference stress σ0 is obtained through... Figure 12 The values ​​are calculated by performing molding analysis on the three elements of the first molded article: 2D solid elements, 3D solid elements, and shell elements, and determined by outputting the calculated stress values ​​of regions A, B, C, and D respectively.

[0222] Depend on Figure 13 It can be seen that by using the simple conversion coefficient correction of equation (1), the difference in calculated stress based on the type of element is reduced. Furthermore, it can be seen that even without calculating according to different forming analysis conditions, the occurrence of delayed failure can be predicted simply and with high accuracy.

[0223] As described above, according to the present invention, delayed failure characteristics can be evaluated while taking into account differences in analytical conditions based on molding analysis. Therefore, benchmark values ​​of stresses that constitute the limit for delayed failure, determined through various delayed failure evaluation methods, can be applied to the calculation results of molding analysis of actual automotive parts. As a result, the occurrence of delayed failure can be predicted in advance based on molding analysis.

[0224] Explanation of reference numerals in the attached figures

[0225] 1 Test piece

[0226] 9. Test Procedures

[0227] Step 10: Calculation of reference stress (Step 1)

[0228] 11. Evaluation of stress calculation process (Step 2)

[0229] 12 Evaluation Process

[0230] 13. Conversion coefficient setting process (Step 3)

[0231] 21 Computer

[0232] 22 Program Storage Unit

[0233] 23A First Process Procedure

[0234] 23B Process 2 Procedure

[0235] 23C Evaluation Process Procedure

[0236] 23E Process 3 Procedure

[0237] 24 conversion factor

Claims

1. A method for evaluating delayed failure characteristics, characterized in that, The following processes are required: The test procedure involves deforming a test piece made of a metal plate and evaluating the crack formation on the test piece caused by placing the deformed test piece in a hydrogen immersion environment. In the first step, for the deformation determined to have delayed failure based on the evaluation, based on the deformation of the test piece with the smallest deformation among the test pieces determined to have delayed failure, the maximum residual stress generated in the deformed test piece by applying deformation to the test piece is calculated by using molding analysis under the first analysis condition of the computer, and the reference stress used to determine whether delayed failure occurs in the molded article of the metal sheet in a hydrogen environment is determined based on the calculated maximum residual stress. The second step involves using a computer to perform a molding analysis under the second analysis conditions to determine the residual stress generated in the target molded article by molding the metal sheet into the target molded article. and The evaluation process assesses the delayed failure characteristics of the target molded article by comparing the residual stress obtained in the second process with the reference stress. The second analysis condition is the analysis condition used to calculate the stress generated during the molding of the target molded article shape. The conditions for the type and size of the elements in the first analysis condition are set to be the same as the conditions for the type and size of the elements in the second analysis condition.

2. A method for evaluating delayed failure characteristics, characterized in that, The following processes are required: The test procedure involves deforming a test piece made of a metal plate and evaluating the crack formation on the test piece caused by placing the deformed test piece in a hydrogen immersion environment. In the first step, for the deformation determined to have delayed failure based on the evaluation, based on the deformation of the test piece with the smallest deformation among the test pieces determined to have delayed failure, the maximum residual stress generated in the deformed test piece by applying deformation to the test piece is calculated by using molding analysis under the first analysis condition of the computer, and the reference stress used to determine whether delayed failure occurs in the molded article of the metal sheet in a hydrogen environment is determined based on the calculated maximum residual stress. The second step involves using a computer to perform a molding analysis under the second analysis conditions to determine the residual stress generated in the target molded article by molding the metal sheet into the target molded article. The third step involves determining a conversion coefficient that converts the stress calculated under the first analysis condition into the stress calculated under the second analysis condition, based on the correlation between the first stress calculated under the first analysis condition and the second stress calculated under the second analysis condition when the pressing is performed under the same molding conditions. and The evaluation process assesses the delayed failure characteristics of the target molded article by comparing the residual stress obtained in the second process with the stress calculated by converting the reference stress using the conversion factor. The second analysis condition is the analysis condition used for molding analysis to calculate the stress generated during the molding of the target molded article shape.

3. The method for evaluating delayed damage characteristics as described in claim 2, characterized in that, The forming analysis is based on the finite element method. The first and second analysis conditions include one or more conditions selected from the following: the type of element, the size of the element, the location of stress output in the molded article, and the location of the integration point of the output stress in the shell element.

4. The method for evaluating delayed damage characteristics as described in claim 3, characterized in that, The conversion coefficient is set based on the difference in the types of elements in the first and second analysis conditions, which are the types of elements in the first and second analysis conditions.

5. The method for evaluating delayed damage characteristics as described in claim 4, characterized in that, Set the thickness of the metal plate before forming to t [mm], and set the type of element to 2D solid element, 3D solid element, or shell element. For the second analysis condition, the feature type is a shell feature, and the mesh size of the feature used for its shaping analysis is m [mm]. The mesh size of the feature under the first analysis condition is m [mm]. The conversion factor K, which converts the stress calculated by the molding analysis under the first analysis condition into the stress calculated by the molding analysis under the second analysis condition, is expressed by the following equation (1). K = α[β(m / t) +1]···(1) in, When the element type in the first analysis condition is a 2D solid element, the coefficient α is set to a constant selected from the range of 0.7 to 0.

9. When the element type in the first analysis condition is a 3D solid element or a shell element, the coefficient α is set to 1. The coefficient β is set to a constant selected from the range of 0.05 to 0.

15.

6. The method for evaluating delayed damage characteristics as described in claim 4, characterized in that, Set the thickness of the metal plate before forming to t [mm], and set the type of element to 2D solid element, 3D solid element, or shell element. For the first analysis condition, the feature type is a shell feature, and the mesh size of the feature used for its shaping analysis is m [mm]. The mesh size of the feature under the second analysis condition is m [mm]. The conversion factor K, which converts the stress calculated by the molding analysis under the first analysis condition into the stress calculated by the molding analysis under the second analysis condition, is expressed by the following equation (2). K = 1 / (α[β(m / t) +1]) ···(2) in, When the element type in the second analysis condition is a 2D solid element, the coefficient α is set to a constant selected from the range of 0.7 to 0.

9. When the element type in the second analysis condition is a 3D solid element or a shell element, the coefficient α is set to 1. The coefficient β is set to a constant selected from the range of 0.05 to 0.

15.

7. The method for evaluating delayed damage characteristics as described in any one of claims 1 to 6, characterized in that, The forming analysis is based on the finite element method. For the stress output location in the formed shape under the first and second analysis conditions, or the location of the integration point of the output stress in the case of shell elements, output the maximum stress of all elements or integration points within the plate thickness.

8. The method for evaluating delayed damage characteristics as described in any one of claims 1 to 6, characterized in that, The metal plate is a high-tensile steel plate with a tensile strength of 980 MPa or higher.

9. The method for evaluating delayed damage characteristics as described in any one of claims 1 to 6, wherein, The target molded product is a component of an automobile.

10. A program product comprising a program for a delayed failure characteristic evaluation method, the delayed failure characteristic evaluation method comprising the following steps: The test procedure involves deforming a test piece made of a metal plate and evaluating the crack formation on the test piece caused by placing the deformed test piece in a hydrogen immersion environment. In the first step, the input is information related to the deformation that is determined to have delayed failure through the evaluation. Based on the deformation of the test piece with the smallest deformation among the test pieces determined to have delayed failure, the maximum residual stress generated in the deformed test piece by applying deformation to the test piece is calculated by molding analysis under the first analysis condition of the computer. The reference stress used to determine whether delayed failure occurs in the molded article of the metal sheet in a hydrogen environment is obtained based on the calculated maximum residual stress. The second step involves using a computer-aided forming analysis under second analysis conditions to determine the residual stress generated in the target molded article by forming the metal sheet into the target molded article; and The evaluation process assesses the delayed failure characteristics of the target molded article by comparing the residual stress obtained in the second process with the reference stress. In the aforementioned delayed failure characteristic evaluation method, the second analysis condition is the analysis condition used for molding analysis to calculate the stress generated during the molding of the target molded article shape. The conditions regarding the types and dimensions of elements in the first analysis condition are set to be the same as those regarding the types and dimensions of elements in the second analysis condition. The program is used to enable the computer to perform the first step.

11. A program product comprising a program for a delayed failure characteristic evaluation method, the delayed failure characteristic evaluation method comprising the following steps: The test procedure involves deforming a test piece made of a metal plate and evaluating the crack formation on the test piece caused by placing the deformed test piece in a hydrogen immersion environment. In the first step, the input is information related to the deformation that is determined to have delayed failure through the evaluation. For the deformation that is determined to have delayed failure, based on the deformation of the test piece with the smallest deformation among the test pieces that are determined to have delayed failure, the maximum residual stress generated in the deformed test piece by applying deformation to the test piece is calculated by molding analysis under the first analysis condition of the computer. The reference stress used to determine whether delayed failure occurs in the molded article of the metal sheet in a hydrogen environment is obtained based on the calculated maximum residual stress. The second step involves using a computer to perform a molding analysis under the second analysis conditions to determine the residual stress generated in the target molded article by molding the metal sheet into the target molded article. The third step involves determining a conversion factor, based on the correlation between the first stress calculated through molding analysis under the first analysis condition and the second stress calculated through molding analysis under the second analysis condition during pressing under the same molding conditions, to convert the stress calculated through molding analysis under the first analysis condition into the stress calculated through molding analysis under the second analysis condition; and The evaluation process assesses the delayed failure characteristics of the target molded article by comparing the residual stress obtained in the second process with the stress calculated by converting the reference stress using the conversion factor. The second analysis condition is the analysis condition used for molding analysis to calculate the stress generated during the molding of the target molded article shape. The program is used to enable the computer to perform the first and third steps.

12. The program product as described in claim 11, characterized in that, The forming analysis is based on the finite element method. The first and second analysis conditions include one or more conditions selected from the following: the type of element, the size of the element, the location of stress output in the molded article, and the location of the integration point of the output stress in the shell element.

13. The program product as described in claim 12, characterized in that, The conversion coefficient is set based on the difference in the types of elements in the first and second analysis conditions, which are the types of elements in the first and second analysis conditions.

14. The program product as described in claim 13, characterized in that, Set the thickness of the metal plate before forming to t [mm], and set the type of element to 2D solid element, 3D solid element, or shell element. For the second analysis condition, the feature type is a shell feature, and the mesh size of the feature used for its shaping analysis is m [mm]. The mesh size of the feature under the first analysis condition is m [mm]. The conversion factor K, which converts the stress calculated by the molding analysis under the first analysis condition into the stress calculated by the molding analysis under the second analysis condition, is expressed by the following equation (1). K = α[β(m / t) +1]···(1) in, When the element type in the first analysis condition is a 2D solid element, the coefficient α is set to a constant selected from the range of 0.7 to 0.

9. When the element type in the first analysis condition is a 3D solid element or a shell element, the coefficient α is set to 1. The coefficient β is set to a constant selected from the range of 0.05 to 0.

15.

15. The program product as described in claim 13, characterized in that, Set the thickness of the metal plate before forming to t [mm], and set the type of element to 2D solid element, 3D solid element, or shell element. For the first analysis condition, the feature type is a shell feature, and the mesh size of the feature used for its shaping analysis is m [mm]. The mesh size of the feature under the second analysis condition is m [mm]. The conversion factor K, which converts the stress calculated by the molding analysis under the first analysis condition into the stress calculated by the molding analysis under the second analysis condition, is expressed by the following equation (2). K = 1 / (α[β(m / t) +1]) ···(2) in, When the element type in the second analysis condition is a 2D solid element, the coefficient α is set to a constant selected from the range of 0.7 to 0.

9. When the element type in the second analysis condition is a 3D solid element or a shell element, the coefficient α is set to 1. The coefficient β is set to a constant selected from the range of 0.05 to 0.

15.

16. The program product as described in any one of claims 10 to 15, characterized in that, The forming analysis is based on the finite element method. For the stress output location in the forming analysis under the first and second analysis conditions, or the location of the integration point of the output stress in the case of shell elements, output the maximum stress of all elements or integration points within the plate thickness.

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