Fatigue testing method, elastoplastic characteristic evaluation method, and elastoplastic characteristic evaluation system

Abstract

To provide a fatigue testing method with which accurate repeated elastoplastic characteristics can be acquired in consideration of the influence of the stress / strain distribution in a load axial direction unique to a sandglass type test piece.SOLUTION: A fatigue testing method includes: a determination step S1 of determining the maximum diameter of a shoulder 21 of a sandglass type round bar test piece 29 that can be processed from a target thin plate material; an estimation step S2 of estimating stress generated in the shoulder 21 assumed in a fatigue test; an acquisition step S3 of processing a smooth round bar test piece from the thin plate material, and executing a tension testing to acquire stress-strain characteristics; a shape determination step S4 of examining the minimum diameter of a grading part 20 of the sandglass type round bar test piece 29 with reference to a result of the tension testing to determine the shape of the test piece; a processing step S5 of processing the sandglass type round bar test piece 29 from the thin plate material on the basis of the determined shape; and an execution step S6 of executing the fatigue test according to a stress-strain range condition set with a fatigue life of several to several hundred times as a target, by using the processed sandglass type round bar test piece 29.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a fatigue testing method, an elastic-plastic property evaluation method, and an elastic-plastic property evaluation system. [Background technology]

[0002] In power plants, where the ageing of equipment continues to progress, it is important to improve the accuracy of the soundness assessment technology for aged equipment in order to improve safety and extend the life of the plant. With conventional integrity assessment technology, if a defect is detected by non-destructive testing during a regular inspection, a crack propagation assessment is performed taking into account the aging deterioration phenomena expected from the operating conditions of the equipment, and it is confirmed that the equipment's integrity can be maintained until the next inspection (Non-Patent Document 1).

[0003] To improve the prediction accuracy of fatigue life in actual structures, it is important to take into account elastic-plastic properties. In particular, since it is believed that a large local strain exceeding several percent occurs at the tip of a crack in a highly ductile material, it is important to obtain cyclic elastic-plastic properties in a high strain range. As a method for obtaining cyclic elastic-plastic properties, an efficient test method using an hourglass-shaped fatigue test piece (Patent Document 1) has been proposed. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2006-194287 A [Non-patent literature]

[0005] [Non-Patent Document 1] Japan Nuclear Technology Association, "Guidelines for Inspection and Evaluation of Reactor Internal Structures," [online], [Retrieved July 28, 2023], Internet<URL:http: / / www.gengikyo.jp / archive / assessmentguiideline.html> Summary of the Invention [Problem to be solved by the invention]

[0006] In the fatigue crack growth evaluation of high ductility materials, in order to accurately evaluate the stress distribution around the crack tip, it is important to understand the cyclic elastic-plastic properties under the high strain range conditions expected around the crack tip. Even the test method described in the above-mentioned Patent Document 1 has problems such as being inapplicable to tests in a high strain range that expects the crack tip.

[0007] On the other hand, thin plates with a thickness of less than 10 mm are often used for structural components such as piping in power plants. When fatigue test specimens are cut from such thin plates for fatigue testing, the specimens become elongated, increasing the possibility of buckling. Furthermore, fatigue testing by radial strain control is often difficult because the radial displacement of the evaluation point becomes small. In addition, when strain measurement of the evaluation point of the fatigue test specimen is performed using an extensometer attached to the shoulder, a stress-strain distribution occurs in the axial direction of the evaluation point of the fatigue test specimen. Therefore, it is difficult to obtain the correct cyclic elastic-plastic properties by simply performing fatigue testing.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a fatigue testing method, an elastic-plastic property evaluation method, and an elastic-plastic property evaluation system that are capable of acquiring highly accurate repeated elastic-plastic properties by taking into account the effect of the stress-strain distribution in the load axis direction that is specific to hourglass-shaped test specimens. [Means for solving the problem]

[0009] The fatigue testing method according to the present invention, which has solved the above-mentioned problems, comprises a determination step of determining the maximum diameter of the shoulder of an hourglass-shaped round bar test piece that can be machined from a target thin plate material having a thickness of less than 10 mm; an estimation step of estimating the stress generated at the shoulder part expected in a fatigue test from at least one of literature and past implementation data; an acquisition step of obtaining a smooth round bar test piece by machining the thin plate material, and then performing a tensile test using the obtained smooth round bar test piece to obtain stress-strain characteristics; a shape determination step of determining the shape of the hourglass-shaped round bar test piece by considering the minimum radius of the rating part of the hourglass-shaped round bar test piece based on the results of the tensile test; a processing step of processing an hourglass-shaped round bar test piece from the thin plate material based on the determined shape; and an implementation step of performing a fatigue test using the machined hourglass-shaped round bar test piece under stress-strain range conditions set with a fatigue life of several to several hundred cycles. Effect of the Invention

[0010] The present invention provides a fatigue testing method, an elastic-plastic property evaluation method, and an elastic-plastic property evaluation system that can obtain highly accurate repeated elastic-plastic properties by taking into account the effect of the stress-strain distribution in the load axis direction that is unique to hourglass-shaped test specimens. [Brief description of the drawings]

[0011] [Figure 1] FIG. 2 is a flow chart showing the content of a fatigue testing method according to the first embodiment. [Diagram 2] FIG. 1 is a plan view showing an example of an hourglass-shaped round bar test piece. [Diagram 3] FIG. 1 is a plan view showing another example of an hourglass-shaped round bar test piece. [Figure 4] FIG. 11 is a flow chart showing the details of an elastic-plastic property evaluation method according to the second embodiment. [Diagram 5] FIG. 11 is a flow diagram showing a specific example of an elastic-plastic property evaluation method according to the second embodiment. [Figure 6] 13 is a graph showing an example of evaluation of the occurrence of buckling in a fatigue test using an hourglass-shaped fatigue test piece according to the second embodiment. [Figure 7]FIG. 11 is a flow diagram showing another specific example of the elastic-plastic property evaluation method according to the third embodiment. [Figure 8] FIG. 11 is a block diagram showing the configuration of an elastic-plastic property evaluation system according to a fourth embodiment. [Figure 9] FIG. 13 is a block diagram showing another specific example of the elastic-plastic property evaluation system according to the fifth embodiment. [Figure 10] FIG. 13 is a block diagram showing a system in which a means for performing elastic-plastic analysis using finite element analysis and parameter surveying material parameters of elastic-plastic properties is automated by a program in embodiment 4. [Figure 11] 13 is a graph in which the elastic-plastic property in finite element analysis is defined as a repeated elastic-plastic property by using the composite hardening law in the elastic-plastic property evaluation system according to the fourth embodiment. [Figure 12] FIG. 1 is a block diagram of a computer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, a fatigue testing method, an elastic-plastic property evaluation method, and an elastic-plastic property evaluation system according to one embodiment of the present invention will be described with reference to the drawings as appropriate. The fatigue testing method, the elastic-plastic property evaluation method, and the elastic-plastic property evaluation system are for evaluating the repeated elastic-plastic property of a thin plate material used in power plant equipment. Hereinafter, in all the drawings for explaining the present invention, parts having the same function are given the same reference numerals, and repeated explanations thereof may be omitted.

[0013] (Embodiment 1) Fig. 1 is a flow diagram showing the contents of the fatigue testing method according to embodiment 1. As shown in Fig. 1, the fatigue testing method includes a determination step S1, an estimation step S2, an acquisition step S3, a shape determination step S4, a processing step S5, and an implementation step S6.

[0014] In the determination step S1, the maximum diameter of the shoulder 21 of the hourglass-shaped round bar test piece 29 (see FIG. 2) that can be processed from a target thin plate material having a plate thickness of less than 10 mm is determined. The target thin plate material may be, for example, a thin plate structure such as a piping for a power plant facility. The hourglass-shaped round bar test piece 29 is a test piece (fatigue test piece) in a substantially round bar shape used when performing a fatigue test. As shown in FIG. 2, the hourglass-shaped round bar test piece 29 has an hourglass-like shape in which the diameter of the cross section of the approximately central part in the longitudinal direction, which is the evaluation part 20 (test part), is gradually reduced. Note that FIG. 2 is a plan view showing an example of the hourglass-shaped round bar test piece. The shoulder 21 is a parallel part of the round bar part just before it is narrowed. In the determination step S1, the maximum diameter of the shoulder 21 is determined. Note that in this embodiment, an hourglass-shaped round bar test piece 30 is also shown in the plan view of FIG. 3 as an example of the hourglass-shaped round bar test piece.

[0015] In the estimation step S2, the stress expected to occur in the shoulder portion 21 in the fatigue test is estimated from at least one of literature and past implementation data. In the acquisition step S3, a smooth round bar test piece is obtained by machining the thin plate material, and then a tensile test is performed using the obtained smooth round bar test piece to obtain the stress-strain characteristics. The smooth round bar test piece is different from the above-mentioned hourglass-shaped round bar test piece 29, and is a test piece that does not have a portion with a narrowed diameter in the evaluation part 20. In other words, the smooth round bar test piece is a test piece in which the cross-sectional diameter of the evaluation part 20 is constant (not shown). The tensile test can be performed using a commercially available tensile tester (universal testing machine).

[0016] In the shape determination step S4, the minimum radius of the cross section of the evaluation portion 20 of the hourglass-shaped round bar test piece 29 is examined based on the results of the tensile test described above, and the shape of the hourglass-shaped round bar test piece 29 is determined. As an example, in this shape determination step S4, the minimum radius of the evaluation part 20 is considered so that the nominal stress range assumed to occur in the shoulder part 21 under the strain range conditions assumed in the evaluation part 20 is 1.6 times or less of the yield strength obtained by the above-mentioned tensile test. In this way, the nominal stress range of the hourglass-shaped round bar test piece 29 processed in the next processing step S5 becomes appropriate, so that buckling during the test can be prevented. Note that the nominal stress refers to the stress value obtained by dividing the external force by the original cross-sectional area during material testing, without considering the change in the cross-sectional area of ​​the sample due to deformation.

[0017] In the machining step S5, the thin plate material is machined into an hourglass-shaped round bar test piece 29 based on the determined shape. This machining can be performed by using any machining device commercially available for making test pieces.

[0018] In step S6, a fatigue test is carried out using the machined hourglass-shaped round bar test piece 29 in a stress-strain range condition set with a fatigue life of several to several hundred cycles as a target. This fatigue test is also called a very low cycle fatigue test. The fatigue test can be carried out using any commercially available fatigue testing machine.

[0019] In this way, the fatigue test method can avoid failures due to buckling in high strain range tests by performing the determination step S1 to the implementation step S6. Therefore, the fatigue test method can obtain highly accurate cyclic elastic-plastic properties by taking into account the effect of the stress-strain distribution in the load axis direction specific to the hourglass-shaped round bar test piece 29.

[0020] (Embodiment 2) Next, the elastic-plastic property evaluation method according to this embodiment will be described. This elastic-plastic property evaluation method can be broadly divided into a pre-process in which the shape of an hourglass-shaped fatigue test specimen is considered and processed from the target thin plate material, and a post-process in which the cyclic elastic-plastic properties are evaluated by elastic-plastic analysis using finite element analysis (post-process).

[0021] Fig. 4 is a flow diagram showing the contents of the elasto-plastic property evaluation method according to embodiment 2. As shown in Fig. 4, the elasto-plastic property evaluation method includes a determination step S1, an estimation step S2, an acquisition step S3, a shape determination step S4, a processing step S5, an implementation step S6, an analysis step S7, and a review step S8.

[0022] The above-mentioned determining step S1 to implementing step S6 are the same as the fatigue testing method described in embodiment 1. That is, in this elasto-plastic property evaluation method, after carrying out the above-mentioned fatigue testing method (the above-mentioned pre-process), an analyzing step S7 and an examining step S8 are carried out (the above-mentioned post-process). In describing this elasto-plastic property evaluation method, the description of the determining step S1 to the implementing step S6 will be omitted, and only the analyzing step S7 and the examining step S8 will be described.

[0023] In the analysis step S7, the range including the extensometer gap of the hourglass-shaped round bar test piece 29 is modeled, and the material parameters of the elastic-plastic properties are changed to perform an elastic-plastic analysis by the finite element method for multiple conditions to obtain the nominal stress-nominal strain hysteresis. The extensometer (not shown) is attached to the shoulder 21 at a predetermined distance (extensometer attachment position distance 26) from the cutout start portion 28, ensuring the extensometer gap 22. Here, in the analysis step S7, it is preferable to define the cyclic elastic-plastic characteristics by using the composite hardening rule for the elastic-plastic characteristics of the elastic-plastic analysis by the finite element method described above. In this way, it is easy to obtain good approximation results when defining the cyclic elastic-plastic characteristics.

[0024] In the analysis step S7, when the composite hardening law is used, the material parameters of the isotropic hardening component and the kinematic hardening component are adjusted based on the stress-strain characteristics obtained in the tensile test described above, and in this adjustment, it is preferable to change the material parameters until they match the hysteresis loop obtained in the very low cycle fatigue test, perform the elastoplastic analysis, and determine the material parameters. In this way, better approximation results are easily obtained.

[0025] Next, in the examination process S8, the material parameters of the cyclic elastic-plastic properties are examined and obtained by parameter survey (optimization) by comparing the obtained nominal stress-nominal strain hysteresis with the experimental results of the fatigue test obtained in the implementation process S6.

[0026] In this manner, in this elastic-plastic property evaluation method, after performing the determination process S1 to the implementation process S6, the analysis process S7 and the examination process S8 are performed, whereby highly accurate cyclic elastic-plastic properties can be obtained more reliably by taking into account the influence of the stress-strain distribution in the load axis direction specific to the hourglass-shaped round bar test specimen 29.

[0027] A specific example of the above is shown in Fig. 5. Fig. 5 is a flow chart showing a specific example of the elastic-plastic property evaluation method according to the second embodiment. 5, first, an hourglass-shaped fatigue test piece 29 is machined from a plate material as a test material. At this time, the shoulder diameter 25 is set as large as possible within a range that can be machined based on the plate thickness (determining step S1). Next, the stress (load) range in the shoulder portion 21 (evaluation portion 20) expected in the fatigue test, i.e., the generated stress, is estimated from literature, past implementation data, etc. (estimation step S2). Then, the nominal stress range Δσ generated in the shoulder portion 21 is n2 However, the stress-strain characteristics (yield strength σ y The minimum diameter (minimum cross-sectional diameter 24) of the evaluation part 20 is considered so that the stress (for example, twice the yield strength) is not more than an arbitrary stress based on the standard, and the shape of the hourglass-shaped round bar test piece 29 is determined (obtaining process S3, shape determining process S4).

[0028] Then, based on the determined shape, an hourglass-shaped round bar test piece 29 is machined from the thin plate material, and a fatigue test is carried out using the machined hourglass-shaped round bar test piece 29 under stress-strain range conditions set with a fatigue life of several to several hundred cycles as a target (machining process S5, implementation process S6). Here, Fig. 6 shows the presence or absence of buckling as a result of performing an extremely low cycle fatigue test using the test piece B (hourglass-type fatigue test piece 29) in Fig. 2 and the test piece A (hourglass-type fatigue test piece 30) in Fig. 3. Fig. 6 is a graph showing an example of evaluation of the presence or absence of buckling in a fatigue test using the hourglass-type fatigue test pieces 29 and 30 (test pieces B and A) according to the above-mentioned embodiment 2.

[0029] As shown in Fig. 6, the results of tests performed on test pieces A and B (TP-A and TP-B) show that the yield strength σ y Based on the standard, the nominal stress range Δσ generated at the shoulder (parallel portion) 21 is n2 1.6 times or less, more specifically, 1.62 times or less (as shown in Figure 6, Δσ n2 / 2σ y ≦0.81, i.e., Δσ n2 ≦1.62σ y It was found that buckling of the test specimens could be prevented by using the

[0030] As described above, the strain range in the very low cycle fatigue test is set so that the fracture life is several to several hundred times. For general materials, the nominal strain range Δε n It is recommended to carry out the test under multiple conditions, i.e., 1, 2, 4, and 8%. However, when the notch radii 23 and 27 of test pieces A and B are small, the strain or stress concentration cannot be ignored, so the nominal strain range Δε n needs to be set.

[0031] In the part relating to the evaluation of the test results, the elastic-plastic properties are evaluated by reproducing the behavior of stress and strain in the evaluation section 20 using finite element analysis. Here, the range including the extensometer gap 22 of the hourglass-shaped fatigue test piece 29 is modeled, and elastic-plastic analysis is performed by the finite element method for multiple conditions by changing the material parameters of the elastic-plastic properties, to obtain the hysteresis of the nominal stress-nominal strain (analysis step S7).

[0032] This involves, for example, outputting a hysteresis loop and maximum-maximum stress history for cycles of 10 or less (first analysis step S71). Also, a composite hardening law is used in the finite element method to fit the stress-strain characteristics of the tensile test results. The fitting is performed by (a) assuming the kinematic hardening component in the following equation (1), and (b) approximating the difference including the yield point behavior with an isotropic hardening law (such as a multi-linear approximation) (second analysis step S72). Note that equation (1) indicates the back stress of the kinematic hardening law. In equation (1), k indicates the number of terms in the kinematic hardening component. C and γ are material constants. εpl is the equivalent plastic strain.

[0033]

number

[0034] Then, an analytical model of the hourglass-shaped fatigue test piece 29 is created, and a repetitive analysis is performed using the composite hardening law assumed above (third analysis step S73).

[0035] The elastic-plastic analysis is performed with a number of cycles of less than 10, and the material parameters of the cyclic elastic-plastic properties (e.g., kinematic hardening components) are examined and obtained by a parameter survey by comparing the obtained nominal stress-nominal strain hysteresis with the above-mentioned experimental results (fatigue test results) (examination step S8). For example, it is confirmed whether the hysteresis loops of the fatigue test results and the analysis results match (confirmation step S81). If they match (Yes in confirmation step S81), the analysis results are deemed to be highly accurate, and the elastic-plastic property evaluation method is terminated. On the other hand, if they do not match (No in confirmation step S81), the kinematic hardening components Ck and γk are corrected and the process returns to the second analysis step S72 (correction step S82), and the second analysis step S72, the third analysis step S73, and the examination step S8 are performed again. In this context, "matching" means that a perfect match is most desirable, but an error of at least ±5% is sufficient, ±4% is desirable, ±3% is more desirable, ±2% is even more desirable, and ±1% is even more desirable.

[0036] (Embodiment 3) FIG. 7 is a flow diagram showing another specific example of the elastic-plastic property evaluation method according to the third embodiment. The second and third embodiments differ from each other in the following respects, but are otherwise similar, and a description of the similar parts will be omitted.

[0037] As shown in FIG. 7, in the third embodiment, processing step S51 differs from processing step S5 in the second embodiment described above in that two or more types of hourglass-shaped fatigue test pieces 29, 30 (for example, test piece B in FIG. 2 and test piece A in FIG. 3) having different notch radii or minimum cross-sectional diameters of the evaluation portion 20 are used. In the third embodiment, in the implementation step S6, an extremely low cycle fatigue test is carried out using the two or more types of hourglass-shaped round bar test pieces 29, 30 described above, and a hysteresis loop and a maximum-maximum stress history for a number of cycles of 10 or less are output. Next, in the third embodiment, in the analysis step S7 (first analysis step S71 to third analysis step S73), the cyclic elastic-plastic properties in the extremely low cycle fatigue life region are predicted by elastic-plastic analysis using the finite element method, taking into account the influence of the stress-strain distribution in the two or more types of hourglass-shaped round bar test pieces 29, 30 described above. As a result of the above-described configuration, in embodiment 3, evaluation can be performed taking into account the axial stress-strain distribution specific to the hourglass-shaped fatigue test pieces 29 and 30, thereby improving the evaluation accuracy of the cyclic elastic-plastic properties.

[0038] (Embodiment 4) FIG. 8 is a block diagram showing the configuration of an elastic-plastic property evaluation system according to the fourth embodiment. As shown in FIG. 8, the system S includes a determining means 1, an estimating means 2, an acquiring means 3, a shape determining means 4, a processing means 5, an implementing means 6, an analyzing means 7, and an examining means 8.

[0039] The determination means 1 determines the maximum diameter of the shoulder 21 of an hourglass-shaped round bar test piece 29 that can be machined from a target thin plate material having a plate thickness of less than 10 mm. The estimation means 2 estimates the stress expected to occur in the shoulder portion 21 in the fatigue test from at least one of literature and past implementation data. In the obtaining means 3, a smooth round bar test piece is obtained by machining the thin plate material, and then a tensile test is carried out using the obtained smooth round bar test piece to obtain the stress-strain characteristics. In the shape determining means 4, the minimum radius of the evaluation portion 20 of the hourglass-shaped round bar test piece 29 is examined based on the results of the tensile test described above, and the shape of the hourglass-shaped round bar test piece 29 is determined. The machining means 5 machines the thin plate material into an hourglass-shaped round bar test piece 29 based on the determined shape. In the implementation means 6, a fatigue test is carried out using the machined hourglass-shaped round bar test piece 29 under stress-strain range conditions set with a fatigue life of several to several hundred cycles as a target. The analysis means 7 performs an elastic-plastic analysis for repeated loads using a finite element analysis model that models the range including the extensometer gap 22 of the hourglass-shaped round bar test piece 29, and obtains the nominal stress-nominal strain hysteresis. In the examination means 8, the obtained nominal stress-nominal strain hysteresis is compared with the experimental results of the fatigue test obtained in the implementation means 6, and the material parameters of the cyclic elastic-plastic properties are examined and obtained by a parameter survey.

[0040] That is, the decision means 1 to the examination means 8 in this system S correspond to the decision step S1 to the examination step S8 described above. Therefore, for the detailed explanation of each means, the contents explained for the corresponding steps can be used. Therefore, the detailed explanation of each means is omitted.

[0041] (Embodiment 5) In the present system S, the processing means 5, the implementation means 6, and the analysis means 7 (first analysis means 71, second analysis means 72, and third analysis means 73) can be configured as follows. Fig. 9 is a block diagram showing another specific example of the elasto-plastic property evaluation system according to the embodiment 5. The embodiments 4 and 5 differ in the following respects, but are otherwise similar. A description of the similar parts will be omitted.

[0042] As shown in Figure 9, in this system S according to another specific example, the processing means 51 differs from the processing means 5 of the previously described embodiment 4 in that it uses two or more types of hourglass-shaped fatigue test pieces 29, 30 (for example, test piece B in Figure 2 and test piece A in Figure 3) which have different notch radii or minimum cross-sectional diameters of the evaluation portion 20. In the fifth embodiment, in the implementation means 6, an extremely low cycle fatigue test is carried out using the two or more types of hourglass-shaped round bar test pieces 29, 30 described above, and a hysteresis loop and a maximum-maximum stress history for a number of cycles of 10 or less are output. Next, in embodiment 5, in the analysis means 7 (first analysis means 71 to third analysis means 73), the cyclic elastic-plastic properties in the extremely low cycle fatigue life region are predicted by elastic-plastic analysis using the finite element method, taking into account the influence of stress-strain distribution in the two or more types of hourglass-shaped round bar test pieces 29, 30 described above. As a result of the above-described configuration, the present system S makes it possible to perform evaluations that take into account the axial stress and strain distribution that is unique to the hourglass-shaped fatigue test pieces 29 and 30, thereby improving the evaluation accuracy of the cyclic elastic-plastic properties.

[0043] The above-described determining means 1 to examining means 8 in the present system S can be made to function by installing a program for executing these means in a personal computer and executing the program on the personal computer. The above-described determining means 1 to examining means 8 in the present system S can also be made to function by installing a program for executing these means in a server computer or the like and executing the program on a client computer or the like connected to the server computer.

[0044] (Embodiment 6) FIG. 10 is a block diagram showing an automation system 10 in which the means for performing elastic-plastic analysis using finite element analysis and parameter surveying material parameters of elastic-plastic properties, i.e., the analysis means 7 and the examination means 8, are automated by a program in the fourth embodiment.

[0045] As shown in FIG. 10, in this embodiment, the automation system 10 has an analysis input means 11 that takes basic information (analysis model, material properties) used in finite element analysis as input, reads the information using an automation program, sets elastic-plastic properties and evaluations, and creates input (analysis input) for an analysis solver from analysis conditions including these. The automation system 10 also includes an analysis execution means 12 that reads input for the analysis solver into the analysis solver, executes the analysis, and outputs the analysis results (nominal stress and nominal strain history). Furthermore, the automation system 10 has an evaluation means 13 that uses a program to evaluate whether the elastoplastic behavior can be reproduced by comparing the analysis results obtained by the analysis execution means 12 with the experimental results. The evaluation means 13 compares the hysteresis loops of the experimental results and the analysis results obtained by the analysis execution means 12 to confirm the evaluation accuracy. If the agreement of the hysteresis loops is high and the evaluation accuracy is judged to be high (OK), the elastoplastic property data obtained as the analysis result is output. On the other hand, if the agreement of the hysteresis loops is low and the evaluation accuracy is judged to be low (NG), the settings and evaluation of the elastoplastic properties described above are reset, and the subsequent means and functions are executed.

[0046] In this embodiment, the analytical model used in the finite element analysis may be one that has been created in advance, but may also be generated by the evaluation means 13 that performs evaluation by a program. In addition, from the viewpoint of improving the efficiency of evaluation, the relationship between the evaluation results obtained by analysis and the material parameters of the elastic-plastic properties may be learned by machine learning or the like, thereby automating the next resetting of the elastic-plastic properties.

[0047] (Embodiment 7) 11 is a graph showing elastic-plastic properties in finite element analysis defined by a composite hardening rule (e.g., Abaqus) in the elastic-plastic property evaluation system according to embodiment 4. In the figure, the horizontal axis is plastic strain (MPa) and the vertical axis is stress (MPa).

[0048] In Abaqus, the kinematic hardening component 53 in the composite hardening law 51 is defined by the above-mentioned equation (1). It is easy to obtain good approximation results by setting the number of terms in the kinematic hardening component 53 to about 3 or more. This is combined with the isotropic hardening component 52 and set so as to match the stress-strain relationship 50 obtained in the tensile test. Here, the isotropic hardening component 52 can reproduce the yield point behavior 54 in the experimental stress-strain relationship 50 by using a multi-linear approximation. When evaluating the elastoplastic behavior of an actual structure, whether or not to include this yield point behavior 54 has a large effect on the reproducibility of deformation, so the application of this embodiment is important for improving the accuracy of deformation and destruction evaluation of an actual machine. In other words, in this embodiment, since this yield point behavior 54 can be included when evaluating the elastoplastic behavior of an actual structure, an hourglass-shaped fatigue test piece having a shape determined thereby can acquire highly accurate repeated elastoplastic properties.

[0049] (Embodiment 8) 12 is a block diagram of a computer 900. The above-mentioned system S includes this computer 900. 12, a computer 900 includes a CPU 901, a RAM 902, a ROM 903, a HDD 904, a communication I / F 905, an input / output I / F 906, and a media I / F 907. The communication I / F 905 is connected to a communication circuit 915. The input / output I / F 906 is connected to an input / output device 916. The media I / F 907 reads and writes data from a recording medium 917.

[0050] A control program executed by the CPU, various data, etc. are stored in the ROM 903. The CPU 901 realizes various functions by executing application programs loaded into the RAM 902. The elastoplastic property evaluation system may be configured using one computer 900, or may be configured using multiple computers 900 connected via a communication circuit 915.

[0051] The fatigue testing method, elastoplastic property evaluation method, and elastoplastic property evaluation system S according to this embodiment have the configurations described above, and therefore can obtain highly accurate cyclic elastoplastic properties by taking into account the effect of the stress-strain distribution in the load axis direction specific to the hourglass-shaped test piece. Note that, since the fatigue testing method according to this embodiment has the configurations described above, the test piece does not buckle in a high strain range test, and failures due to buckling can be avoided.

[0052] The fatigue testing method, the elastic-plastic property evaluation method, and the elastic-plastic property evaluation system S according to the present invention have been described in detail above using the embodiments. However, the present invention is not limited to the above-described embodiments, and various modified examples are included. For example, the above-described embodiments have been described in detail to easily explain the present invention, and the present invention is not necessarily limited to those having all of the configurations described. In addition, it is possible to replace a part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. In addition, it is possible to add, delete, or replace a part of the configuration of each embodiment with another configuration. [Explanation of symbols]

[0053] S1 Decision process S2 estimation process S3 acquisition process S4 Shape determination process S5, S51 Machining process S6 Implementation process S7 Analysis process S71 Analysis 1st step S72 Analysis 2nd step S73 Analysis 3rd step S8 Consideration process S81 Confirmation process S82 Correction process S This system (elastic-plastic property evaluation system) 1. Decision-making procedure 2 Estimation means 3 Acquisition method 4 Shape determining means 5 Processing means 6. Means of Implementation 7 Analysis methods 8. Methods of investigation 10. Automation Systems 11. Input methods for analysis 12 Analysis execution method 13. Evaluation methods 20. Evaluation Section 21 Shoulder part (parallel part) 22 Between extensometers 23 Notch radius 24 Minimum cross-sectional diameter 25 Shoulder diameter 26 Extensometer installation distance 27 Notch radius 28 Cutout start part 29, 30 Hourglass-shaped fatigue test specimen 50 Stress-strain relationship 51 Processing means 51 Complex Hardening Law 52 Isotropic hardening component 53 Mobile hardening component 54 Yield point behavior 71 First means of analysis 72 Second method of analysis 73 Third means of analysis A and B test pieces

Claims

1. A determination step of determining the maximum diameter of a shoulder portion of an hourglass-shaped round bar test piece that can be machined from a target thin plate material having a plate thickness of less than 10 mm; an estimation step of estimating a stress generated in the shoulder portion expected in a fatigue test from at least one of literature and past implementation data; A process of obtaining a smooth round bar test piece from the thin plate material, and then performing a tensile test using the obtained smooth round bar test piece to obtain stress-strain characteristics; A shape determination process for determining the shape of the hourglass-shaped round bar test piece by considering the minimum radius of the evaluation portion of the hourglass-shaped round bar test piece based on the result of the tensile test; A processing step of processing an hourglass-shaped round bar test piece from the thin plate material based on the determined shape; A fatigue test is carried out using the processed hourglass-shaped round bar test piece under a stress-strain range condition set to achieve a fatigue life of several to several hundred cycles; A fatigue testing method comprising the steps of:

2. 2. The fatigue testing method according to claim 1, The shape determination process is characterized in that the minimum radius of the cross section of the evaluation part is considered so that the nominal stress range assumed to occur in the shoulder part under the strain range conditions assumed in the evaluation part is 1.6 times or less the yield strength obtained by the tensile test.

3. After carrying out the fatigue test method according to claim 1, An analysis step of modeling the range including the extensometer interval of the hourglass-shaped round bar test piece, changing the material parameters of the elastic-plastic properties to perform an elastic-plastic analysis by the finite element method for a plurality of conditions, and obtaining a hysteresis of the nominal stress-nominal strain; A process of comparing the obtained hysteresis of the nominal stress-nominal strain with the experimental results of the fatigue test obtained in the implementation process to investigate and obtain material parameters of cyclic elastic-plastic properties by a parameter survey; The elastic-plastic property evaluation method according to the present invention is characterized by comprising the steps of:

4. The elastic-plastic property evaluation method according to claim 3, In the processing step, two or more types of hourglass-shaped round bar test pieces having different notch radii or minimum cross-sectional diameters of the evaluation points are prepared, In the implementation step, a very low cycle fatigue test is carried out using the two or more types of hourglass-shaped round bar test pieces, In the analysis step, the cyclic elastic-plastic properties in the very low cycle fatigue life region are predicted by elastic-plastic analysis using the finite element method, taking into account the effects of stress and strain distribution in the test piece. The elastic-plastic property evaluation method is characterized by:

5. The elastic-plastic property evaluation method according to claim 4, An elastic-plastic property evaluation method, characterized in that repeated elastic-plastic properties are defined using a composite hardening rule for the elastic-plastic properties of the elastic-plastic analysis using the finite element method.

6. The elastic-plastic property evaluation method according to claim 5, When the composite hardening law is used, the material parameters of the isotropic hardening component and the kinematic hardening component are adjusted based on the stress-strain characteristics obtained in the tensile test, and in this adjustment, the material parameters are changed until they match the hysteresis loop obtained in the very low cycle fatigue test, and the elastic-plastic analysis is performed to determine the material parameters. The elastic-plastic property evaluation method is characterized by:

7. A determination means for determining a maximum diameter of a shoulder portion of an hourglass-shaped round bar test piece that can be machined from a target thin plate material having a plate thickness of less than 10 mm; an estimation means for estimating a stress generated in the shoulder portion expected in a fatigue test from at least one of literature and past implementation data; A means for processing the thin plate material into a smooth round bar test piece, and then performing a tensile test using the obtained smooth round bar test piece to obtain stress-strain characteristics; A shape determining means for determining the shape of the hourglass-shaped round bar test piece by examining the minimum radius of the evaluation portion of the hourglass-shaped round bar test piece based on the result of the tensile test; A processing means for processing an hourglass-shaped round bar test piece from the thin plate material based on the determined shape; A means for carrying out a fatigue test using the processed hourglass-shaped round bar test piece under a stress-strain range condition set to achieve a fatigue life of several to several hundred cycles; an analysis means for performing an elastic-plastic analysis for repeated loads using a finite element analysis model that models the range including the extensometer interval of the hourglass-shaped round bar test piece, and obtaining a hysteresis of nominal stress-nominal strain; A means for comparing the obtained hysteresis of the nominal stress-nominal strain with the experimental results of the fatigue test obtained by the implementation means to investigate and obtain material parameters of the cyclic elastic-plastic properties by a parameter survey; An elastic-plastic property evaluation system comprising:

8. The elastic-plastic property evaluation system according to claim 7, two or more types of hourglass-shaped round bar test pieces having different notch radii or minimum cross-sectional diameters of the evaluation points are prepared by the processing means; The fatigue test is carried out using the two or more types of hourglass-shaped round bar test pieces in the implementation means, The influence of stress and strain distribution in the test piece is taken into consideration through numerical analysis using the finite element analysis model of the analysis means, and then the repeated elastic-plastic properties in the very low cycle fatigue life region are obtained by the examination means. An elastic-plastic property evaluation system comprising:

Images