Method for estimating stress-strain relation

Abstract

To provide a method for estimating a stress-strain relationship, for estimating a plurality of stress-strain relationships in performing one high-temperature tensile test using one test piece.SOLUTION: A method for estimating a stress-strain relationship (S10) includes: a strain measurement step (S11) of changing a strain rate to be measured with respect to a test piece to different rates, by applying an external force to the test piece in one high-temperature tensile test using one test piece formed of a metallic material; and an estimation step S12 of estimating a plurality of stress-strain relationships respectively corresponding to different strain rates on the basis of the external force applied in the strain measurement step and the plurality of strain rates.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a method for estimating stress-strain relationships. [Background technology]

[0002] In the past, in steel-framed structures that require fire resistance, the common method was to apply fire-resistant coating to columns and beams to reduce heat input during a fire and prevent the columns and beams from collapsing. However, in Japan, there is a serious shortage of fire-resistant coating workers. For this reason, there is hope for the practical application of new fire-resistant structures that reduce the need for fire-resistant coating by properly evaluating the actual behavior of steel members exposed to heat during a fire.

[0003] As a study to improve the fire resistance of steel frame members, a design method is known in which the thickness of the fire-resistant coating is determined based on the stress-temperature relationship at 1% strain taking into account the strain rate (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-167310 Summary of the Invention [Problem to be solved by the invention]

[0005] In the design method of Patent Document 1, when determining the stress-temperature relationship at 1% strain, a stress-strain curve is estimated using variables such as temperature. When estimating the stress-strain curve, for example, a tensile test is performed using a test piece. However, with the design method of Patent Document 1, only one stress-strain relationship can be estimated when one tensile test is performed using one test piece.

[0006] The present invention has been made in consideration of the above-mentioned problems, and an object of the present invention is to provide a method for estimating stress-strain relationships that can estimate multiple stress-strain relationships when a single high-temperature tensile test is performed using a single test specimen. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention proposes the following means. (1) Aspect 1 of the present invention is a stress-strain relationship estimation method that performs a strain measurement step in which, in a single high-temperature tensile test using a single test piece formed of a metal material, an external force is applied to the test piece to change the strain rate measured on the test piece into multiple values, and an estimation step in which, based on the external force applied in the strain measurement step and the multiple strain rates, multiple stress-strain relationships corresponding to the multiple different strain rates are estimated.

[0008] In this invention, in the strain measurement step, a single high-temperature tensile test is performed using a single test piece made of a metal material, and a plurality of strain rates measured on the test piece are changed by applying stress to the test piece. Then, in the estimation step, a plurality of stress-strain relationships corresponding to the plurality of different strain rates are estimated based on the external force applied in the strain measurement step and the plurality of strain rates. Therefore, multiple stress-strain relationships can be estimated when a single high-temperature tensile test is performed using a single test specimen.

[0009] (2) A second aspect of the present invention may be the stress-strain relationship estimation method according to (1), wherein, in the strain measurement step, the strain rate is changed from a first strain rate to a second strain rate greater than the first strain rate when the strain of the test piece reaches 10%. In general numerical analysis, a stress-strain relationship corresponding to a strain with an upper limit of about 10% is used. In the present invention, for example, a stress-strain relationship sufficient for use in numerical analysis can be estimated.

[0010] (3) A third aspect of the present invention may be the stress-strain relationship estimation method according to (1), wherein, in the strain measurement step, the strain rate is changed from a first strain rate to a second strain rate greater than the first strain rate when the strain of the test piece reaches 2%. The Eurocode uses the stress at a strain of 2% for design. In this invention, for example, it is possible to easily compare the estimated stress-strain relationship with previous test data based on the Eurocode regulations.

[0011] (4) A fourth aspect of the present invention may be the stress-strain relationship estimation method according to (1), wherein, in the strain measurement step, the strain rate is changed from a first strain rate to a second strain rate greater than the first strain rate when the strain of the test piece reaches 1%. The Architectural Institute of Japan (AIJ, hereafter simply referred to as the Architectural Institute of Japan) uses the stress at a strain of 1% for design. With this invention, it is possible to easily compare the estimated stress-strain relationship with previous test data based on the Architectural Institute of Japan's regulations, for example.

[0012] (5) A fifth aspect of the present invention may be the stress-strain relationship estimation method according to any one of (1) to (4), wherein the strain measuring step heats the test piece to 400° C. or higher. Generally, when a test piece made of a metal material is heated to 400°C or higher, the influence of strain rate becomes significant. In this invention, it is possible to estimate multiple stress-strain relationships in a state where the influence of strain rate is large.

[0013] (6) A sixth aspect of the present invention may be a stress-strain relationship estimation method according to any one of (1) to (5), wherein, in the strain measurement step, the strain rate is changed from a first strain rate to a second strain rate greater than the first strain rate after a reference time has elapsed since the external force was applied to the test piece; and, in the estimation step, a first stress-strain relationship, which is the stress-strain relationship, is estimated based on the external force and the first strain rate up to the reference time; and a second stress-strain relationship, which is the stress-strain relationship, is estimated based on the external force and the second strain rate from the time after the strain of the test piece at the reference time has increased by an amount of strain change.

[0014] In this invention, the inventors have found, as a result of extensive research, that in the strain measurement process, when the strain rate is changed from a first strain rate to a second strain rate greater than the first strain rate after a reference time has elapsed since an external force was applied to the test piece, a certain increase in strain is required for the stress of the test piece to change from the first stress corresponding to the first strain rate to the second stress corresponding to the second strain rate. As a result, in the estimation step, a first stress-strain relationship is estimated based on the external force and the first strain rate up to the reference time. Then, using the constant increase in strain as the strain change amount, a second stress-strain relationship is estimated based on the external force and the second strain rate from the time when the strain of the test specimen at the reference time has increased by the strain change amount. Therefore, the first stress-strain relationship and the second stress-strain relationship can be more accurately estimated by taking into account the strain change amount required to change from the first stress to the second stress.

[0015] (7) A seventh aspect of the present invention may be a stress-strain relationship estimation method according to any one of (1) to (6), wherein the estimation step estimates the plurality of stress-strain relationships by optimization calculation. In the present invention, by using optimization calculations, it is possible to more accurately estimate multiple stress-strain relationships.

[0016] (8) Aspect 8 of the present invention may be the stress-strain relationship estimation method according to any one of (1) to (7), wherein the metallic material is steel. In this invention, it is possible to estimate a plurality of stress-strain relationships when the metallic material is steel. [Effects of the Invention]

[0017] The stress-strain relationship estimation method of the present invention makes it possible to estimate a plurality of stress-strain relationships when a single high-temperature tensile test is performed using a single test piece. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a side view of a test piece used in a method for estimating a stress-strain relationship according to one embodiment of the present invention. [Figure 2] FIG. 2 is a diagram showing the general configuration of a high-temperature tensile testing device for carrying out the stress-strain relationship estimation method. [Figure 3] 1 is a flowchart illustrating a method for estimating a stress-strain relationship according to one embodiment of the present invention. [Figure 4] FIG. 1 is a diagram illustrating an example of a change in strain over time that is applied to a test piece by a high-temperature tensile testing device. [Figure 5] 1 is a diagram illustrating distorted waveforms A1, A2, and B1 to B7. [Figure 6] FIG. 10 is a diagram illustrating a distorted waveform C. [Figure 7] FIG. 10 is a diagram showing the temperature history of the test piece from the start of heating until 120 seconds when strain waveform A1 is used. [Figure 8] FIG. 1 is a diagram showing the stress-strain relationship of strain waveforms A1, A2, and B1 to B4. [Figure 9] FIG. 10 is a diagram showing the stress-strain relationship of strain waveforms A1, A2, and B5 to B7. [Figure 10] FIG. 10 shows the change in 0.2% offset intensity with velocity increasing strain. [Figure 11] FIG. 10 shows the change in strength at 1% strain due to rate-increasing strain. [Figure 12] FIG. 10 shows the change in strength at 2% strain due to rate-increasing strain. [Figure 13] FIG. 1 shows the change in tensile strength with increasing strain rate. [Figure 14]FIG. 1 shows the change in strain at the onset of tensile strength with increasing strain rate. [Figure 15] FIG. 10 is a diagram showing the stress-strain relationship of strain waveform C. [Figure 16] FIG. 10 is a diagram illustrating a process of estimating multiple stress-strain relationships by optimization calculation when strain waveform B2 is used. [Figure 17] FIG. 10 is a diagram illustrating a process of estimating a plurality of stress-strain relationships by optimization calculation when strain waveform B7 is used. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, one embodiment of a stress-strain relationship estimation method (stress-strain relationship estimation method, hereinafter also simply referred to as an estimation method) according to the present invention will be described with reference to FIGS.

[0020] [1. Stress-strain relationship estimation method] In this estimation method, for example, one test piece 100 shown in Fig. 1 is used to perform one high-temperature tensile test using a high-temperature tensile testing device 10 shown in Fig. 2. First, the test piece 100 will be described below.

[0021] [1.1. Test piece] 1, for example, the test piece 100 is a so-called hourglass-shaped test piece. However, the shape of the test piece is not limited to this. The test piece 100 has a parallel portion 101, two gripping portions 102A and 102B, and two shoulder portions 103A and 103B. The parallel portion 101 and the gripping portions 102A and 102B are each formed in a cylindrical shape. The diameters of the gripping portions 102A and 102B are each larger than the diameter of the parallel portion 101. The parallel portion 101 is disposed between the gripping portions 102A and 102B and coaxially with the gripping portions 102A and 102B. Two marks (not shown) are set on the parallel portion 101. The two marks are arranged in the direction of the axis O1 of the test piece 100 with a gap between them.

[0022] The shoulder portion 103A is disposed between the parallel portion 101 and the grip portion 102A, and is joined to both the parallel portion 101 and the grip portion 102A. The diameter of the shoulder portion 103A gradually increases as it approaches the grip portion 102A. The shoulder portion 103B is disposed between the parallel portion 101 and the grip portion 102B, and is joined to both the parallel portion 101 and the grip portion 102B. The diameter of the shoulder portion 103B gradually increases as it approaches the grip portion 102B.

[0023] The test piece 100 is integrally formed from a metal material. Specifically, in this example, the test piece 100 is formed from steel. The metal material forming the test piece 100 is not limited to steel, but may be aluminum, copper, or the like.

[0024] [1.2. High-Temperature Tensile Testing Equipment] The configuration of the high-temperature tensile testing apparatus 10 is not limited as long as it is an apparatus that can tensile the test piece 100 in the axial direction O1 of the test piece 100 at a high temperature (high temperature) and estimate (measure) the stress-strain relationship of the test piece 100. Here, high temperature means, for example, 400°C or higher and 1000°C or lower. Strain means, for example, logarithmic strain (strain; true strain) or engineering strain (nominal strain). As shown in FIG. 2, for example, the high-temperature tensile testing device 10 includes gripping units 11A and 11B, an external force applying unit (not shown), a heating unit 16, a measuring unit 21, and a control unit (not shown). The gripping portions 11A and 11B grip the gripping portions 102A and 102B, respectively, of the test piece 100. For example, the gripping portions 11A and 11B grip the test piece 100 so that the axis O1 is aligned in the up-down direction.

[0025] The external force applying section applies an external force to the test piece 100 by relatively moving the gripping sections 11A and 11B in the vertical direction. The heating unit 16 heats the test piece 100 to a predetermined temperature by high-frequency induction heating, etc. The heating unit may be an electric furnace or a gas furnace. The measurement unit 21 has a distance measurement unit 22, an external force measurement unit (not shown), and a temperature measurement unit (not shown). The distance measurement unit 22 measures the distance between two ratings on the test piece 100 and sends the measurement results to the control unit. The external force measurement unit measures the external force applied by the external force application unit and sends the measurement results to the control unit. The temperature measurement unit measures the temperature of the test piece 100 and sends the measurement results to the control unit.

[0026] The control unit is connected to the external force applying unit, the heating unit 16, and the measuring unit 21. The control unit controls the external force applying unit and the heating unit 16 based on the measurement results of the measuring unit 21. The control unit calculates (measures) the stress, strain, and strain rate of the test piece 100 based on the measurement results of the distance measuring unit 22 of the measuring unit 21 and the external force applying unit. Details of the control method by the control unit will be explained in [1.3].

[0027] [1.3. Specific details of the stress-strain relationship estimation method] 3 is a flowchart showing the estimation method S10 in this embodiment. The estimation method S10 includes a distortion measurement step S11 and an estimation step S12. In the strain measurement step S11, the control unit heats the test piece 100 to 400° C. or higher. In a single high-temperature tensile test using one test piece 100, the control unit applies an external force to the test piece 100 using the external force loading unit, thereby changing the strain rate measured on the test piece 100 into multiple values.

[0028] Specifically, the change in strain over time (strain waveform) that the high-temperature tensile testing apparatus 10 applies to the test piece 100 will be described using Figure 4. In Figure 4, the horizontal axis represents time t (s: seconds) and the vertical axis represents strain ε (%). The strain ε at time t (time) 0 is 0%. From time 0, the control unit applies an external force to the test piece 100 by the external force loading unit, which applies a tensile external force in the direction of the axis O1. For example, the control unit changes the strain rate from a first strain rate v1 to a second strain rate v2 that is larger (faster) than the first strain rate v1 after a reference time t1 (s) has elapsed since an external force was applied to the test piece 100 at time 0 (s). The strain of the test piece 100 at the reference time t1 is defined as ε1 (see FIG. 16).

[0029] Specifically, in the strain measurement step S11, when the strain of the test piece 100 reaches 1%, the strain rate is changed from a first strain rate v1 to a second strain rate v2. For example, the test piece 100 is pulled at the second strain rate v2 until it breaks. In the strain measurement step S11, the control unit controls the external force applied by the external force loading unit so that the strain rate of the test piece 100 becomes the first strain rate v1 and the second strain rate v2. When the distortion measurement step S11 is completed, the process proceeds to the estimation step S12.

[0030] In the estimation step S12, the control unit estimates a plurality of stress-strain relationships corresponding to a plurality of different strain rates, respectively, based on the external force applied in the strain measurement step S11 and a plurality of strain rates. Specifically, two stress-strain relationships corresponding to two different strain rates v1 and v2 are estimated based on the external force and two strain rates v1 and v2 applied in the strain measurement step S11. That is, a first stress-strain relationship R1, which is the stress-strain relationship, is estimated based on the external force and the first strain rate v1 from time 0 to the reference time t1 (see also FIG. 16). A second stress-strain relationship R2, which is the stress-strain relationship, is estimated based on the external force and the second strain rate v2 after the time when the strain of the test piece 100 increases by the strain change amount Δε from the strain of the test piece 100 at the reference time t1 (see also FIG. 16). Here, the time required for the strain of the test piece 100 to change by the strain change amount Δε is defined as the transition time t2.

[0031] 16, the second stress-strain relationship is estimated based on the external force and the second strain rate v2 when the strain of the test piece 100 is equal to or greater than (ε1+Δε) (after (t1+t2) time from the application of the external force). The strain change amount Δε and the transition time t2 are positive values. For example, in the estimation step S12, a plurality of stress-strain relationships are estimated by optimization calculations, for example, Differential Evolution can be used as the optimization calculations. When the estimation step S12 is completed, all steps of the estimation method S10 are completed, and multiple stress-strain relationships are estimated.

[0032] In the strain measurement step, the strain rate may be increased from the first strain rate to the second strain rate before the stress reaches the tensile strength (before the stress reaches its maximum value). For example, whether the stress reaches the tensile strength can be estimated from past test results. The control unit for the strain measurement step S11 and the control unit for the estimation step S12 may be configured separately.

[0033] [2. Confirmation test of estimation method] Next, the results of tests conducted to verify the estimation method will be described.

[0034] [2.1. Distorted waveform used in the test] The distorted waveforms used in the test are distorted waveforms A1, A2, B1 to B7, and C, which will be described below. Note that the distorted waveforms A1 and A2 are distorted waveforms used in the estimation method of the comparative example, and the distorted waveforms B1 to B7, and C are distorted waveforms used in the estimation method of the example. 5 and 6 are diagrams illustrating strain waveforms A1, A2, B1 to B7, and C. In Fig. 5 and Fig. 6, the horizontal axis represents strain ε (%), and the vertical axis represents strain rate dε / dt (% / min [minute]).

[0035] 5, the strain waveform A1 is a strain waveform in which the strain rate dε / dt is kept constant at 0.3% / min until the strain ε of the test piece 100 reaches 10%. The strain waveform A2 is a strain waveform in which the strain rate dε / dt is kept constant at 7.5% / min until the strain ε of the test piece 100 reaches 10%. The strain waveform B1 is a strain waveform in which the first strain rate v1, which is a strain rate dε / dt, is 0.3% / min until the strain ε of the test piece 100 reaches 1.0%, and the second strain rate v2, which is a strain rate dε / dt, is 7.5% / min until the strain ε of the test piece 100 reaches from 1.0% to 10%. The strain waveform B2 is a strain waveform in which the first strain rate v1 is 0.3% / min until the strain ε of the test piece 100 reaches 2.0%, and the second strain rate v2 is 7.5% / min until the strain ε of the test piece 100 reaches from 2.0% to 10%. The distorted waveforms B5 to B7 are similar to the distorted waveforms B1 and B2. When a high temperature tensile test is performed on the test piece 100, the stress reaches a maximum value when the strain ε is about 4.3%, for example.

[0036] As shown in Figure 6, strain waveform C is a strain waveform in which the strain rate dε / dt is increased stepwise from a first strain rate v1 of 0.3% / min to an eleventh strain rate v11 of 7.5% / min until the strain ε of test piece 100 reaches 2.0%.

[0037] [2.2. Other test conditions] As the high temperature tensile test device 10, an Axial / Torsional Test Systems model: 319.25 manufactured by MTS Systems Corporation was used. For example, the test piece 100 was taken from a 22 mm thick steel plate in the rolling direction so that the center of the steel plate thickness coincided with the center in the direction of the axis O1. The diameters of the gripping portions 102A and 102B were 18 mm, and the diameter of the parallel portion 101 was 10 mm. The steel type of the test piece 100 is SS400 (general structural rolled steel). The number of test pieces 100 was one (N=1) for each strain waveform. A contact displacement meter was used as the distance measuring unit 22. The distance between two rating points displaced so as to extend from 12 mm was measured by the distance measuring unit 22. The test piece 100 was heated to a predetermined temperature (600°C) by the heating unit 16, and after holding for 10 minutes, an external force was applied to the test piece 100 by the external force loading unit. The external force was applied to the test piece 100 until the strain ε of the test piece 100 reached 10%.

[0038] The heating temperature of the test piece 100 was set to 600° C. The temperature in the glass gap was controlled to be within (600±5)° C.

[0039] [2.3. Test Results] [2.3.1. Temperature control results] For example, Fig. 7 shows the temperature history of the test piece 100 from the start of heating to 120 seconds when strain waveform A1 is used. In Fig. 7, the horizontal axis represents the time t (s) from the start of heating, and the vertical axis represents the temperature T (°C) of the test piece 100. The solid line represents the control signal (control value) from the control unit, and the dashed line represents the actual measurement value (measurement value) from the temperature measurement unit. From the temperature history shown in Figure 7, it was confirmed that the target temperature was maintained until the end of the test.

[0040] [2.3.2. Comparison of tensile strength] Table 1 shows the tensile strength σu (N / mm 2 ), and the strain εu (%) at which the tensile strength σu is generated.

[0041] [Table 1]

[0042] As shown in Table 1 and Figure 8, when strain waveform A1 (strain rate 0.3% / min) was used, the strain εu was 5.5% and the tensile strength σu was 128 N / mm 2 When strain waveform A2 (strain rate 7.5% / min) is used, the strain εu is 8.7% and the tensile strength σu is 164 N / mm 2 showed. In FIG. 8 and FIG. 9 described later, the large jitter in the distorted waveforms other than the distorted waveform A2 is due to the influence of the measurement interval. As described above, the tensile strength σu increased as the strain rate increased.

[0043] [2.3.3. Comparison of stress-strain relationships] As shown in the results using strain waveforms B1 to B7 shown in Figures 8 and 9, when the strain rate is changed from 0.3% / min to 7.5% / min at an arbitrary strain, the strength (stress for the same strain) of test piece 100 increases under all conditions, approaching the results using strain waveform A2. As shown in Table 1, at a constant strain rate of 0.3% / min, the strain εu was approximately 5.5% and the tensile strength σu was obtained. However, as shown in Figures 8 and 9, when strain waveforms B1 to B7 were used, the strain εu changed slightly compared to the result when strain waveform A1 was used, but the tensile strength σu was greater.

[0044] [2.3.4. Comparison of various strengths] Figure 10 shows the 0.2% offset intensity σ0.2% for the results using strain waveforms B1 to B7. Note that εup in Figure 10 and Figures 11 to 14 described below refers to the strain when the strain rate is increased in strain waveforms B1 to B7 (hereinafter referred to as rate-increasing strain). Figures 10 to 14 also show the results using strain waveforms A1 and A2. Figure 11 shows the strength (effective yield strength) σ1% at a strain of 1%, Figure 12 shows the strength σ2% at a strain of 2%, Figure 13 shows the tensile strength σu, and Figure 14 shows the strain εu at the time of tensile strength. Table 2 shows the values of the material properties for each strain waveform.

[0045] [Table 2]

[0046] In Figures 10 and 11, the values are those under loading at a strain rate of 0.3% / min (values when strain waveform A1 is used), so the results for all of the strain waveforms B1 to B7 generally match the results when strain waveform A1 is used. In FIG. 12, except for the test in which the strain rate was increased at a strain of 1.0% (results using strain waveform B1), the results for all of the strain waveforms B1 to B7 generally coincide with the results using strain waveform A1. In Figure 13, the stress value is slightly lower in the test where the strain rate was changed at 7% strain (results using strain waveform B7), but although there is some variation in the other tests, the results are generally consistent with the results of the constant strain rate of 7.5% / min (results using strain waveform A2). In Figure 14, except for the test in which the strain rate was increased at 1.0% strain (results using strain waveform B1), almost all of the results showed tensile strength at a strain similar to that of the test conducted at a constant strain rate of 7.5% / min (results using strain waveform A2).

[0047] [2.4. Results of Distorted Waveform C] As shown in Figure 15, when the strain rate was gradually increased, the results were initially close to those obtained at a constant strain rate of 0.3% / min (results using strain waveform A1), but as the strain rate increased, the results approached those obtained at a constant strain rate of 7.5% / min (results using strain waveform A2).From a strain of around 4%, the results showed similar behavior to those obtained using strain waveform A2.

[0048] [2.5. Estimated results of multiple stress-strain relationships] The equation underlying the stress-strain relationship is described in Eurocode 3 Part 1-2, which defines stress as a function of temperature-dependent Young's modulus and strain. The second stress-strain relationship R2 is a relationship obtained by multiplying the stress obtained from the first stress-strain relationship R1 by a certain number. Note that the relationship between the first stress-strain relationship R1 and the second stress-strain relationship R2 is not limited to a certain number. The results of estimating the first stress-strain relationship R1 and the second stress-strain relationship R2 for the results using the strain waveform B2 shown in FIG. 16 will be described. In this example, the transition time t2 was set to the time required for the strain to increase by 2% (2% / v2 [7.5% / min] = 16 seconds). That is, in this example, the first stress-strain relationship R1 was estimated based on the external force and the first strain rate v1 from time 0 to the reference time t1 (2% / v1 [0.3% / min] = 400 seconds). The second stress-strain relationship was estimated based on the external force and the second strain rate v2 from (400 + 16 =) 416 seconds onwards.

[0049] It can be seen from FIG. 16 that the first stress-strain relationship R1 and the second stress-strain relationship R2 can be estimated with high accuracy from the results of a single high-temperature tensile test using the strain waveform B2.

[0050] The results of estimating the first stress-strain relationship R1 and the second stress-strain relationship R2 for the results using the strain waveform B7 shown in FIG. 17 will be described. In this example, the transition time t2 was the same as in Fig. 16. That is, in this example, the first stress-strain relationship R1 was estimated based on the external force and the first strain rate v1 from time 0 to the reference time t1 (7% / v1 [0.3% / min] = 1400 seconds). The second stress-strain relationship R2 was estimated based on the external force and the second strain rate v2 from (1400 + 16 =) 1416 seconds onwards.

[0051] It can be seen from FIG. 17 that the first stress-strain relationship R1 and the second stress-strain relationship R2 can be estimated with high accuracy from the results of a single high-temperature tensile test using the strain waveform B7.

[0052] 3. Effects of this embodiment As described above, in the estimation method S10 of this embodiment, in the strain measurement step S11, in a single high-temperature tensile test using one test piece 100 made of a metal material, stress is applied to the test piece 100 to change the strain rate measured on the test piece 100. Then, in the estimation step S12, a plurality of stress-strain relationships corresponding to a plurality of different strain rates are estimated based on the external force applied in the strain measurement step S11 and the plurality of strain rates. Therefore, when one high-temperature tensile test is performed using one test piece 100, multiple stress-strain relationships can be estimated.

[0053] In the strain measurement step S11, the strain rate may be changed from a first strain rate v1 to a second strain rate v2 when the strain of the test piece 100 reaches 1%. The Architectural Institute of Japan uses the stress at a strain of 1% for design. In this case, it is possible to easily compare the estimated stress-strain relationship with, for example, previous test data based on the Architectural Institute of Japan's regulations. In the strain measurement step S11, the test piece 100 is heated to 400°C or higher. Generally, when a test piece made of a metal material is heated to 400°C or higher, the influence of strain rate becomes significant. This makes it possible to estimate multiple stress-strain relationships in a state where the influence of strain rate is large.

[0054] In the strain measurement step S11, the strain rate is changed from a first strain rate v1 to a second strain rate v2 after a reference time t1, and in the estimation step S12, a first stress-strain relationship R1 and a second stress-strain relationship R2 are estimated. As a result of extensive research, the inventors have found that when the strain rate is changed from the first strain rate v1 to the second strain rate v2 after a reference time t1 has elapsed since an external force was applied to the test piece 100 in the strain measurement step S11, a certain increase in strain is required for the stress of the test piece 100 to change from a first stress corresponding to the first strain rate v1 to a second stress corresponding to the second strain rate v2. For this reason, in the estimation step S12, a first stress-strain relationship R1, which is the stress-strain relationship, is estimated based on the external force and the first strain rate v1 up to the reference time t1. Then, with the constant increase in strain being defined as the strain change amount Δε, a second stress-strain relationship R2, which is the stress-strain relationship, is estimated based on the external force and the second strain rate v2 from the time when the strain of the test piece 100 at the reference time t1 increases by the strain change amount Δε. Therefore, by taking into account the strain change amount Δε required to change from the first stress to the second stress, the first stress-strain relationship R1 and the second stress-strain relationship R2 can be estimated more accurately.

[0055] In the estimation step S12, a plurality of stress-strain relationships R1, R2 are estimated by optimization calculations. By using optimization calculations, the plurality of stress-strain relationships R1, R2 can be estimated more accurately. The metallic material is steel. Therefore, it is possible to estimate multiple stress-strain relationships when the metallic material is steel.

[0056] Although one embodiment of the present invention has been described in detail above with reference to the drawings, the specific configuration is not limited to this embodiment, and configuration changes, combinations, deletions, etc. are also included within the scope that does not deviate from the gist of the present invention. For example, in the above embodiment, in the strain measurement step, the strain rate may be changed from a first strain rate to a second strain rate greater than the first strain rate when the strain of the test piece 100 reaches 10%. In general numerical analysis, a stress-strain relationship with an upper limit corresponding to a strain of approximately 10% is used. Therefore, for example, a stress-strain relationship sufficient for use in numerical analysis can be estimated.

[0057] In the above embodiment, in the strain measurement step, the strain rate may be changed from a first strain rate to a second strain rate greater than the first strain rate when the strain of the test piece 100 reaches 2%. The Eurocode uses the stress at a strain of 2% for design. This allows, for example, easy comparison of the estimated stress-strain relationship with previous test data based on the Eurocode regulations.

[0058] In this embodiment, the optimization calculation is not limited to the differential evolution method, but may be a genetic algorithm or the like. In the strain measurement step, the strain rate may be decreased (slowed down). In the estimation step, multiple stress-strain relationships may be estimated without taking into consideration the strain change amount Δε (the strain change amount Δε is set to 0). Optimization calculations may not be used to estimate multiple stress-strain relationships. [Explanation of symbols]

[0059] 100 test specimens v1 1st strain rate v2 2nd strain rate S10 Estimation method (stress-strain relationship estimation method) S11 Strain measurement process S12 Estimation process t1 Reference time Δε Strain change amount

Claims

1. a strain measurement step in which, in a single high-temperature tensile test using a single test piece formed of a metal material, an external force is applied to the test piece to change the strain rate measured on the test piece; an estimation step of estimating a plurality of stress-strain relationships corresponding to the plurality of strain rates different from each other, based on the external force applied in the strain measurement step and the plurality of strain rates; A method for estimating stress-strain relationships.

2. 2. The stress-strain relationship estimating method according to claim 1, wherein, in the strain measuring step, the strain rate is changed from a first strain rate to a second strain rate that is greater than the first strain rate when the strain of the test piece reaches 10%.

3. 2. The stress-strain relationship estimating method according to claim 1, wherein, in the strain measuring step, the strain rate is changed from a first strain rate to a second strain rate that is greater than the first strain rate when the strain of the test piece reaches 2%.

4. 2. The stress-strain relationship estimating method according to claim 1, wherein, in the strain measuring step, the strain rate is changed from a first strain rate to a second strain rate that is greater than the first strain rate when the strain of the test piece reaches 1%.

5. 2. The stress-strain relationship estimation method according to claim 1, wherein the test piece is heated to 400° C. or higher in the strain measuring step.

6. In the strain measuring step, after a reference time has elapsed since the external force was applied to the test piece, the strain rate is changed from a first strain rate to a second strain rate that is greater than the first strain rate, In the estimation step, estimating a first stress-strain relationship, which is the stress-strain relationship, based on the external force and the first strain rate up to the reference time; 2. The stress-strain relationship estimation method according to claim 1, wherein a second stress-strain relationship, which is the stress-strain relationship, is estimated based on the external force and the second strain rate after a time when the strain of the test specimen has increased by an amount of strain change from the strain of the test specimen at the reference time.

7. 2. The stress-strain relationship estimation method according to claim 1, wherein the estimation step estimates the plurality of stress-strain relationships by optimization calculation.

8. The stress-strain relationship estimation method according to claim 1 , wherein the metallic material is steel.

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