Method for correcting and checking data when necking is considered in metal tensile experiment

Abstract

The invention provides a method for correcting and checking data when necking is considered in a metal tensile experiment, and relates to the technical field of metal deformation, the method comprises the following steps: carrying out an isothermal tensile experiment to obtain a pseudo-true stress-strain curve, and substituting the pseudo-true stress-strain curve into a strain-strain database of a finite element model; simulation software with a drawing function is selected to establish a workpiece three-dimensional model, an isothermal tensile experiment simulation test is carried out, the displacement distribution condition of each node in the compression process of the workpiece is obtained through finite element simulation, and then the change condition of the cross sectional area is obtained; the method comprises the following steps: calculating true stress by using a ratio of a load obtained by a test to a central cross section area, further obtaining a corrected rheological curve, substituting the true stress-strain curve into a strain-strain database of a finite element model, and comparing simulation results of the necking size, the true stress-strain curve and a load-displacement curve with experimental data to obtain a final result. And if the comparison result meets a preset condition, ending the calculation, and if the comparison result does not meet the preset condition, repeating the above steps.

Description

Technical Field

[0001] The present application relates to the technical field of metal deformation, and in particular to a method for correcting and verifying data when considering necking in a metal tensile test. Background Art

[0002] Metal tensile testing is a commonly used test method in materials science and engineering, used to determine the mechanical properties of metals under external forces, including tensile strength, yield strength, and elongation. However, during the actual tensile process, metal workpieces (specimens) often undergo localized deformation, known as necking. This phenomenon is particularly pronounced near fracture, leading to uneven stress and strain distribution, thus affecting the accuracy and reliability of experimental data.

[0003] However, traditional tensile testing methods typically assume uniform stress and strain distribution across the workpiece cross-section, ignoring the necking effect. However, the presence of necking can cause the actual stress in the workpiece cross-section to be significantly greater than the average stress, thereby affecting the evaluation of the material's mechanical properties. This error is particularly significant in high-precision material research and engineering applications, potentially leading to inaccurate material property data and impacting engineering design and safety assessments. Summary of the Invention

[0004] The embodiment of the present application proposes a method for data correction and verification when considering necking in metal tensile experiments. Through a simplified mathematical model and reasonable experimental steps, the experimental data errors caused by the necking effect can be effectively corrected without significantly increasing the complexity and cost of the experiment, thereby providing more accurate material mechanical property parameters.

[0005] The embodiment of the present application proposes a method for correcting and verifying data when considering necking in a metal tensile test, including: step S1, performing an isothermal tensile test, calculating the values ​​of pseudo-true strain and pseudo-true stress by traditional calculation methods, obtaining a pseudo-true stress-strain curve, and bringing it into the strain-strain database of a finite element model; step S2, selecting the drawing function of the simulation software to establish a three-dimensional model of the workpiece; step S3, performing an isothermal tensile test simulation test; step S4, obtaining the displacement distribution of each node of the necking part of the workpiece during the stretching process through finite element simulation, and then calculating the change in cross-sectional area; step S5, using the ratio of the simulated load to the cross-sectional area of ​​the necking part to calculate the true stress, further obtaining a corrected rheological curve, and obtaining a true stress-strain curve; step S6, using the true stress-strain curve to bring it into the strain-strain database of the finite element model, comparing the simulation results of the necking area length, true stress-strain curve, and load-displacement curve with the experimental data, if the relative error of any comparison item meets the preset condition, the calculation is terminated, if the relative error of any comparison item does not meet the preset condition, repeating steps S1-S6 until the preset condition is met.

[0006] According to the method of data correction and verification during necking in the metal tensile experiment according to the embodiment of the present application, it is possible to effectively correct the experimental data errors caused by the necking effect through simplified mathematical models and reasonable experimental steps without significantly increasing the complexity and cost of the experiment, thereby providing more accurate material mechanical property parameters.

[0007] In addition, the method for data correction and verification considering necking in a metal tensile test according to the above embodiment of the present application may also have the following additional technical features:

[0008] In one embodiment of the present application, the preset condition is that the relative error of any comparison item is less than or equal to a preset threshold.

[0009] In one embodiment of the present application, the conventional calculation method described in step S1 is specifically as follows: using a data system to collect data from the displacement sensor and the pressure sensor during the isothermal stretching experiment, and calculating the collected data to obtain the values ​​of the pseudo-true strain ε and the pseudo-true stress σ. The calculation formulas are shown in the following equations (1) and (2):

[0010]

[0011] Where F is the loading force, N, S is the cross-sectional area, mm 2 , l0 is the initial workpiece length, mm; l is the length of the workpiece after deformation, mm.

[0012] In one embodiment of the present application, the isothermal tensile test simulation test in step S3 adopts an elastic-plastic body during the simulation process, and the initial temperature is the same as the deformation temperature designed in the isothermal tensile test. The displacement is used to control the movement of the workpiece during the simulated tensile process. Since the workpiece is set with a left-right symmetrical surface, the strain rate The conversion formula of displacement is shown in the following formula (3), which is the functional relationship between displacement and deformation:

[0013]

[0014] in, is the strain rate, s-1, l0 is the height of the workpiece before stretching, mm, l is the length of the workpiece after deformation, and mm, t is the time of the workpiece stretching process.

[0015] In one embodiment of the present application, in step S4, the displacement distribution of each node of the workpiece during the stretching process is obtained by finite element simulation; based on the modified geometric equation Nodal displacement gradient is used to calculate the local strain components (∈ ij ), and then through the volume conservation relationship A0l0=A(i) l (i) Inverse the cross-sectional area.

[0016] If the displacement of a node is u x (x, y), the corresponding local strain is:

[0017]

[0018] Among them, ∈ xx is the component of the strain tensor, the tensile positive strain in the x direction; ∈ xy is the shear strain in the xy plane; u x is the displacement in the x direction; is the component of the displacement gradient tensor in the x direction;

[0019] For the volume conservation formula, since the deformation in the necking region is highly localized, the formula becomes:

[0020]

[0021] Among them, l (i) is the local length increment at a certain point i along the length direction of the workpiece (calculated as a gradient); A (i) is the real-time cross-sectional area at the corresponding position;

[0022] Calculate the local length change, which can be obtained from formula (6):

[0023]

[0024] in, is the axial normal strain component, and the cross-sectional area can be inferred in reverse.

[0025] In one embodiment of the present application, in step S5, the true stress is calculated using the ratio of the simulated load to the cross-sectional area of ​​the necking portion, and a corrected true stress-strain curve is further obtained; based on the corrected stress equilibrium equation and the geometric equation, the true stress σ true By the instantaneous load F m The actual cross-sectional area A of the necking part necking The ratio of is determined by:

[0026]

[0027] Among them, F m is the instantaneous load obtained during the simulation; A necking Calculated by nodal displacement distribution in finite element simulation.

[0028] In one embodiment of the present application, the preset threshold in step S6 is 5%, and the relative error includes the relative error between the simulated value and the experimental value of the necking zone length, the relative error between the simulated value and the experimental value of the peak stress of the true stress-strain curve, and the relative error between the simulated value and the experimental value of the maximum load value of the load-displacement curve, wherein the calculation formula (8) of the relative error is:

[0029]

[0030] Where M is the relative error, σ exp is the experimental value, σ sim is the analog value.

[0031] Additional aspects and advantages of the present application will be given in part in the description below, and in part will become apparent from the description below, or will be learned through practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the following description of the embodiments in conjunction with the accompanying drawings, in which:

[0033] Figure 1 A schematic flow chart of a method for data correction and verification considering necking in a metal tensile test according to one embodiment of the present application;

[0034] Figure 2 This is a diagram of the necking part of the workpiece (specimen) after the experiment in this application;

[0035] Figure 3 This is a schematic diagram of the three-dimensional model for numerical simulation of this application;

[0036] Figure 4 This is a schematic diagram of the simulation results of this application;

[0037] Figure 5 This is a process diagram for the isothermal hot tensile test of 20MnTiB steel provided in an embodiment of the present application;

[0038] Figure 6 True stress-true strain curves of 20MnTiB steel under different deformation conditions provided in the examples of this application;

[0039] Figure 6 (a)- conditions, Figure 6 (b) - Under T = 1000 °C;

[0040] Figure 7 A comparison diagram of the simulated and experimental necking position of 20MnTiB steel after secondary iteration under different deformation conditions provided in the embodiment of the present application;

[0041] Figure 7(a) Under the condition of T=900℃, Figure 7 (b) Under the condition of T=1000℃, Figure 7 (c) Under the condition of T=1100℃;

[0042] Figure 8 A comparison chart of the original rheological curve and the finite element corrected rheological curve provided in the embodiment of the present application;

[0043] Figure 8 (a) Under the condition of T=900℃, Figure 8 (b) Under the condition of T=1000℃, Figure 8 (c) Under the condition of T=1100℃;

[0044] Figure 9 A load-displacement comparison diagram of the test and finite element method provided in the embodiment of the present application;

[0045] Figure 9 (a) Under the condition of T=900℃, Figure 9 (b) Under the condition of T=1000℃, Figure 9 (c) Under the condition of T=1100℃. DETAILED DESCRIPTION

[0046] The following describes in detail embodiments of the present application, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present application, and should not be construed as limiting the present application.

[0047] The following describes, with reference to the accompanying drawings, a method for data correction and verification when considering necking in a metal tensile test according to an embodiment of the present application.

[0048] Figure 1 Schematic diagram of a flow chart of a method for data correction and verification considering necking in a metal tensile test according to one embodiment of the present application.

[0049] like Figure 1 As shown in FIG, the method for correcting and verifying the data when considering necking in the metal tensile test may include:

[0050] Step S1: perform an isothermal tensile test, calculate the pseudo-true strain and pseudo-true stress using a traditional calculation method, obtain a pseudo-true stress-strain curve, and bring it into the strain-strain database of the finite element model.

[0051] In an embodiment of the present application, the strain-strain database of the finite element model can be pre-stored in the storage space of an electronic device for easy access. The electronic device may include a personal computer, a server (cloud server), a tablet computer, etc., and the storage space is not limited to physical storage space, such as a hard drive. The storage space can also be the storage space of a network hard drive connected to the electronic device (cloud storage space).

[0052] To clearly illustrate the above embodiment, in one embodiment of the present application, the traditional calculation method in step S1 is specifically:

[0053] The data system is used to collect the values ​​of the displacement sensor and pressure sensor during the isothermal tensile test, and the collected data are calculated to obtain the values ​​of the pseudo-true strain ε and pseudo-true stress σ. The calculation formulas are shown in the following equations (1) and (2):

[0054]

[0055] Where F is the loading force, N, S is the cross-sectional area, mm 2 , l0 is the initial workpiece length, mm; l is the length of the workpiece after deformation, mm.

[0056] It should be noted that if the above formula assumes that the material (for example, the metal material used in the experiment) is uniformly deformed and does not take into account the necking effect, in actual application, it is necessary to obtain the instantaneous cross-sectional area through finite element simulation and calculate the true stress. The true strain needs to be corrected in combination with the local strain analysis in the geometric equation. In the embodiment of the present application, assuming that the workpiece (specimen) is isotropic, the strain after necking is l / l0=A0 / A, where A0 and A can be calculated from the cross-sectional area of ​​the workpiece, mm 2 .

[0057] The traditional calculation method described in this embodiment can be calculated by deriving displacement-load data, or it can be automatically calculated by experimental equipment such as a thermal simulation test machine. The results of the calculations do not accurately consider the effect of necking. The necking photos of the workpiece after the experiment are as follows: Figure 2 Especially when considering necking, the error is even greater, so the curve obtained by the traditional calculation method is a pseudo-true stress-strain curve.

[0058] Step S2: Use the simulation software's built-in drawing function to create a three-dimensional model of the workpiece. It should be noted that the simulation software described in this embodiment can be specified by the user, and the following experimental steps all need to be performed based on the three-dimensional model (i.e., the three-dimensional model needs to be applied).

[0059] like Figure 3As shown, in the embodiment of the present application, in order to reduce the simulation calculation time, the calculation can be performed by taking 1 / 4 of the stretched workpiece.

[0060] Step S3: performing an isothermal stretching simulation test.

[0061] In order to clearly explain the above embodiment, in one embodiment of the present application, in the isothermal tensile test simulation test in step S3, the workpiece adopts an elastic-plastic body during the simulation process, and the initial temperature is the same as the deformation temperature designed in the isothermal tensile test. The displacement is used to control the movement of the workpiece during the simulated tensile process. Since the workpiece is set with a left-right symmetrical surface, the strain rate The conversion formula of displacement l is shown in the following formula (3), which is the functional relationship between displacement and deformation:

[0062]

[0063] in, is the strain rate, s-1, l0 is the height of the workpiece before stretching, mm; l is the length of the workpiece after deformation, mm, and t is the time of the workpiece stretching process.

[0064] Step S4, obtain the displacement distribution of each node of the necking part of the workpiece during the stretching process through finite element simulation, and then calculate the change in cross-sectional area. Among them, the displacement distribution may include the size of the support reaction force at the upper and lower ends of the workpiece after the stretching fracture, and its size is approximately zero. This is because there is a large friction between the workpiece and the fixture. During the stretching process, the upper and lower ends of the workpiece are clamped by the fixture, and it is difficult to break here due to uneven force. However, the center part of the workpiece is stretched by the external force in the vertical direction to deform and neck until it breaks. When the deformation increases, it reaches the necking stage, causing the stretched workpiece to form obvious necking. The schematic diagram of the simulated necking result is shown as follows Figure 4 shown.

[0065] In order to clearly illustrate the above embodiment, in one embodiment of the present application, in step S4, the displacement distribution of each node of the workpiece during the stretching process is obtained by finite element simulation; based on the modified geometric equation Nodal displacement gradient is used to calculate the local strain components (∈ ij ), and then through the volume conservation relationship A0l0=A (i) l (i) Inverse the cross-sectional area.

[0066] If the displacement of a node is u x (x, y), the corresponding local strain is:

[0067]

[0068] Among them, ∈xx is the component of the strain tensor, the tensile positive strain in the x direction; ∈ xy is the shear strain in the xy plane; u x is the displacement in the x direction; is the component of the displacement gradient tensor in the x direction;

[0069] For the volume conservation formula, since the deformation in the necking region is highly localized, the formula becomes:

[0070]

[0071] Among them, l (i) is the local length increment at a certain point i along the length direction of the workpiece (calculated as a gradient); A (i) is the real-time cross-sectional area at the corresponding position;

[0072] Calculate the local length change, which can be obtained from formula (6):

[0073]

[0074] in, is the axial normal strain component, and the cross-sectional area can be inferred in reverse.

[0075] Specifically, the data corresponding to the displacement distribution of each node of the workpiece during the stretching process are extracted from the finite element simulation results, and then the cross-sectional area (i.e., the cross-sectional area of ​​the necking region of the workpiece) is calculated based on the data and the above formulas (4)-(6).

[0076] Step S5: Calculate the true stress using the ratio of the simulated load to the cross-sectional area of ​​the necking portion, further obtain a corrected rheological curve, and obtain a true stress-strain curve.

[0077] In order to clearly illustrate the above embodiment, in one embodiment of the present application, in step S5, the true stress is calculated using the ratio of the simulated load to the cross-sectional area of ​​the necking portion, and a corrected true stress-strain curve is further obtained; based on the corrected stress equilibrium equation and the geometric equation, the true stress σ true By the instantaneous load F m The actual cross-sectional area A of the necking part necking The ratio is determined by:

[0078]

[0079] Among them, F m is the instantaneous load obtained during the simulation; A necking Calculated by nodal displacement distribution in finite element simulation.

[0080] It should be noted that the instantaneous load F described in this embodimentm It can be obtained through finite element simulation.

[0081] In step S6, the true stress-strain curve is entered into the strain-strain database of the finite element model. The simulation results of the necking zone length, true stress-strain curve, and load-displacement curve are compared with the experimental data. If the relative error of any comparison item meets the preset conditions, the calculation ends. If the relative error of any comparison item does not meet the preset conditions, steps S1-S6 are repeated until the preset conditions are met. The preset conditions can be calibrated according to actual conditions.

[0082] In one embodiment of the present application, the preset condition may be that the relative error of any comparison item is less than or equal to a preset threshold value, wherein the preset threshold value may be calibrated according to actual conditions, and the preset threshold value may be 5%.

[0083] To clearly illustrate the previous embodiment, in one embodiment of the present application, the relative error in step S6 may include the relative error between the simulated value and the experimental value of the necking region length, the relative error between the simulated value and the experimental value of the peak stress of the true stress-strain curve, and the relative error between the simulated value and the experimental value of the maximum load value of the load-displacement curve, wherein the calculation formula (8) of the relative error is:

[0084]

[0085] Where M is the relative error, σ exp is the experimental value, σ sim is the analog value.

[0086] Specifically, the true stress-strain curve is brought into the strain-strain database of the finite element model, and then the relative error between the simulated value and the experimental value of the necking zone length, the relative error between the simulated value and the experimental value of the peak stress of the true stress-strain curve, and the relative error between the simulated value and the experimental value of the maximum load value of the load-displacement curve are calculated by the above formula (8) and the data in the strain-strain database based on the finite element model. Then, the above relative errors are judged one by one. If the relative error of any comparison item is less than or equal to 5%, the calculation ends. If the relative error of any comparison item is greater than 5%, steps S1-S6 are repeated until the preset conditions are met.

[0087] In the embodiment of the present application, after the above calculation is completed, it is not necessary to continue the iterative simulation calculation, the simulation is stopped, and the last result is output.

[0088] It should be noted that the material used for the workpiece (workpiece) described in the above embodiment can be 20MnTiB steel after industrial hot rolling, and the specification of the stretched workpiece is Φ8mm×95mm. The process diagram of the single-pass stretching process is as follows Figure 5As shown in the figure, the workpiece (workpiece) was heated by resistance heating at a controlled heating rate of 5°C / s to 1200°C. The temperature was then held for 180 seconds to ensure complete austenitization of the workpiece. The workpiece was then cooled at a controlled cooling rate of 5°C / s to the deformation parameter temperature. The temperature was then held for 120 seconds to ensure uniform temperature distribution, completing the tensile test. The deformation parameters for the tensile process were set at temperatures of 900, 1000, and 1100°C, and strain rates of 0.1, 1, and 10 s⁻¹. The workpiece was water quenched after stretching.

[0089] The traditional calculation method obtains the pseudo-true stress-strain curve:

[0090] Figure 6 The pseudo-true stress-true strain curve of 20MnTiB steel under different deformation conditions can be calculated using the above formulas (1) and (2) after the experiment. It should be noted that, assuming that the workpiece (specimen) is isotropic, the strain after necking is l / l0=A0 / A, where A0 and A can be calculated from the cross-sectional area of ​​the workpiece, mm 2 . Figure 6 (a) shows the rheological curves at various deformation temperatures when the strain rate is 1s-1. It can be seen that as the deformation temperature increases, the peak stress reached becomes lower. Figure 6 (b) shows the rheological curves at various strain rates when the deformation temperature is 1000℃. It can be seen that with the increase of strain rate, the peak stress required for stretching gradually increases.

[0091] If the streamline curve data calculated using formula (1) is used directly, the peak stress will be used to determine whether DRX has occurred, which will lead to an erroneous conclusion. The subsequent constitutive model and DRX-related model will also deviate from the true value, resulting in greater errors. Therefore, it is very necessary to eliminate the error caused by necking. It should be noted that the DRX described in this embodiment refers to the dynamic recrystallization process, a phenomenon that occurs in metal materials during thermal deformation.

[0092] This study will use finite element simulation software to simulate the experimental process, and use point tracking to obtain the actual cross-sectional area of ​​the workpiece necking part during the stretching process, and then correct the rheological curve.

[0093] Establishment of finite element simulation model:

[0094] Use the simulation software with its own drawing function to build a three-dimensional model, such as Figure 3 As shown, the workpiece material is 20MnTiB steel, and the stress-strain curve of the material is the experimental data obtained from the experiment. Other material constants are calculated by Jmatpro software. In order to reduce the simulation calculation time, 1 / 4 of the tensile workpiece is taken for calculation.

[0095] During the simulation, the time step was set to 1, the workpiece was an elastic-plastic body, the initial temperature was the same as the deformation temperature of the experimental design, and the thermal radiation coefficient between the workpiece and the environment was 28.35×10-6 (W / mm2·K). The ambient temperature was 20°C, the friction coefficient between the workpiece and the fixture was set to 0.7, the thermal conductivity was 30,000 (W / mm\cdotpK), and the specific heat was 0.46 (J / g\cdotpK). The simulated tensile process used displacement to control the movement of the fixture, with a displacement load of 20 mm. Since the workpiece was set with a bilaterally symmetrical surface, the conversion formula between strain rate and displacement is shown in Equation (3), which is a functional relationship between displacement and deformation.

[0096] Comparison of simulation and test results (necking size comparison):

[0097] The accuracy of the adopted model in predicting the high-temperature plastic behavior with the effect of necking was further verified by comparing the necking length measured in the finite element simulation and the actual tensile test. Figure 7 The data analysis shows that at a strain rate of 1 s⁻¹ and 900°C, the simulated necking zone length is 26.82 mm, while the experimental value is 25.5 mm, resulting in a calculated relative deviation of 4.92%. Similarly, at 1000°C, the simulated necking zone length is 27.6 mm, with a relative deviation of 3.99% compared to the experimental value of 26.5 mm. At 1100°C, the simulated necking zone length is 28.55 mm, with a relative deviation of 5.43% from the experimental value of 27 mm. Combining the data from all three temperatures, the average relative error in the necking zone is 4.78%. This demonstrates that iterative correction of the data during the simulation process is appropriate and that the cross-sectional area of ​​the neck obtained using point tracking during the simulation is reliable.

[0098] Comparison of simulation and test results (comparison of true stress-strain curves):

[0099] The true stress can be calculated using the ratio of the load to the central cross-sectional area obtained from the test, and the corrected rheological curve can be drawn. Figure 8 Simulated and experimental necking from tensile loading to fracture at a strain rate of 1 s-1 and temperatures of 900°C, 1000°C, and 1100°C, showing a comparison of the original rheological curve and the finite element corrected rheological curve. Figure 8As can be seen from the figure, the rheological curves after the first finite element iteration are lower than the original ones. At the initial stage of deformation, the error between the two is 10.62%, but as the deformation increases, the difference gradually increases. Following the second iteration, the simulated true stress and strain data of the workpiece match the true stress and strain data after accounting for necking in the actual situation, with an error of 4.69%. Table 1 shows the relative errors before and after the iterative correction of the true stress and strain. The calculated average error for the overall iteration is 4.67%.

[0100] Table 1 Relative error of true stress and strain before and after iterative correction

[0101]

[0102] Table 1Relative errorbefore and after correction of true stress andstrain

[0103] Comparison of simulation and test results (comparison of load-displacement curves):

[0104] The force-displacement results obtained from the simulation were analyzed by applying the modified true stress-strain curve to the finite element analysis. These results were then compared with the experimentally observed force-displacement curve to evaluate the accuracy of the modified true stress-strain curve and the effectiveness of the simulation method. Figure 9 As shown, the finite element simulation results without iterations show that the maximum deviation occurs at the fracture point, with a deviation of 39.47% at 1100°C and a strain rate of 1 s⁻¹. After the first iteration, the deviation is reduced to 25.61%, and after the second iteration, it is further reduced to 8.57%. As another example, at 900°C and a strain rate of 1 s⁻¹, the deviation is minimized at the onset of necking, at 7.12%. After the first iteration, the deviation is reduced to 2.81%, and after the second iteration, it is further reduced to 0.39%. Overall, the finite element simulation at 900°C without iterations shows an overall relative deviation of 18.33%, which is reduced to 6.9% after the first iteration. After the second iteration, the overall relative deviation of the displacement-load curve between the simulation and experimental results is 3.33%. At 1000°C, the overall relative deviation without iterations is 19.82%. After the first iteration, this deviation is reduced to 13.45%. In the second iteration, the maximum deviation at fracture was 13.27%, while the minimum deviation at necking during the yield phase was 0.12%. Table 2 shows the relative error between the load and displacement before and after iterative correction. The overall deviation calculated after the second iteration was 4.84%. The load values ​​obtained in the re-simulation after correction were essentially identical to the load values ​​in the test, indicating that the corrected rheological curve closely matches the material's true flow-force curve.

[0105] Table 2 Relative error of load displacement before and after iterative correction

[0106]

[0107] Conclusion of this example:

[0108] In this example, the flow curve of 20MnTiB steel was obtained. The influence of true stress and strain during necking during the stretching process was considered and the flow curve was corrected using finite element method. The results can be summarized as follows:

[0109] (1) Under different temperature conditions, the comparison between the necking area length obtained by finite element simulation and the experimental simulation results shows that the average deviation of the necking length under all temperature conditions under the second iteration is 4.78%, indicating that the area value after necking obtained by point tracking in the simulation process is reliable.

[0110] (2) By comparing the true stress-strain results after iterative correction, the overall relative error without iterative correction is 12.45%, the average relative error of the three temperatures after the first iterative correction is 8.11%, and the overall average relative error after the second iterative correction is 4.67%. This shows that the iterative correction method is feasible for correcting the true stress-strain curve under the influence of necking.

[0111] (3) During the simulated tensile process, without iterative correction, the overall displacement-load relative error was 22.59%. After one iteration, the relative error was reduced to 12.06%. By reapplying the true stress and strain corrected by the second iteration to the finite element model, the relative error between the calculated load value and the experimental measurement value was 4.84%. This verifies that the corrected true stress and strain can accurately reflect the actual flow characteristics of the material and confirms the effectiveness of the adopted method.

[0112] In summary, the method of data correction and verification during necking in the metal tensile test according to the embodiment of the present application can effectively correct the experimental data errors caused by the necking effect through simplified mathematical models and reasonable experimental steps without significantly increasing the complexity and cost of the experiment, thereby providing more accurate material mechanical properties parameters.

[0113] In the description of this specification, the terms "first" and "second" are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly specify the number of the technical features indicated. Therefore, a feature specified as "first" or "second" may explicitly or implicitly include at least one of such features. In the description of this application, "plurality" means at least two, for example, two, three, etc., unless otherwise specifically defined.

[0114] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification and features of different embodiments or examples without contradiction.

[0115] Although the embodiments of the present application have been shown and described above, it can be understood that the above embodiments are exemplary and cannot be understood as limitations on the present application. Ordinary technicians in this field can change, modify, replace and modify the above embodiments within the scope of the present application.

Claims

1. A method for data correction and verification in metal tensile testing considering necking, characterized in that: include: Step S1, performing an isothermal tensile test, calculating the pseudo-true strain and pseudo-true stress using traditional calculation methods, obtaining a pseudo-true stress-strain curve, and inserting it into the strain-strain database of the finite element model; Step S2, using the simulation software's built-in drawing function to create a three-dimensional model of the workpiece; Step S3, performing an isothermal stretching simulation test; Step S4, obtaining the displacement distribution of each node at the necking portion of the workpiece during the stretching process through finite element simulation, and then calculating the change in cross-sectional area; Step S5, calculating the true stress using the ratio of the simulated load to the cross-sectional area of ​​the necking portion, further obtaining a corrected rheological curve, and obtaining a true stress-strain curve; In step S6, the true stress-strain curve is brought into the strain-strain database of the finite element model, and the simulation results of the necking zone length, the true stress-strain curve, and the load-displacement curve are compared with the experimental data. If the relative error of any comparison item meets the preset conditions, the calculation ends. If the relative error of any comparison item does not meet the preset conditions, steps S1-S6 are repeated until the preset conditions are met.

2. The method for data correction and verification in metal tensile testing considering necking according to claim 1, characterized in that: in, The preset condition is that the relative error of any comparison item is less than or equal to a preset threshold.

3. The method for data correction and verification in metal tensile testing considering necking according to claim 1, characterized in that: The traditional calculation method in step S1 is specifically: The data system is used to collect the values ​​of the displacement sensor and pressure sensor during the isothermal tensile test, and the collected data are calculated to obtain the values ​​of the pseudo-true strain ε and pseudo-true stress σ. The calculation formulas are shown in the following equations (1) and (2): Where F is the loading force, N, S is the cross-sectional area, mm 2 , l0 is the initial workpiece length, mm; l is the length of the workpiece after deformation, mm.

4. The method for data correction and verification in metal tensile testing considering necking according to claim 1, characterized in that: In the isothermal tensile test simulation test in step S3, the workpiece adopts an elastic-plastic body during the simulation process, and the initial temperature is the same as the deformation temperature designed in the isothermal tensile test. The displacement is used to control the movement of the workpiece during the simulated tensile process. Since the workpiece is set with a left-right symmetrical surface, the strain rate The conversion formula of displacement is shown in the following formula (3), which is the functional relationship between displacement and deformation: in, is the strain rate, s -1 , l0 is the height of the workpiece before stretching, mm, l is the length of the workpiece after deformation, mm, t is the time of the workpiece stretching process.

5. The method for data correction and verification in metal tensile testing considering necking according to claim 1, characterized in that: In step S4, the displacement distribution of each node of the workpiece during the stretching process is obtained by finite element simulation; based on the modified geometric equation Nodal displacement gradient is used to calculate the local strain components (∈ ij ), and then through the volume conservation relationship A0l0=A (i) l (i) Inverse the cross-sectional area. If the displacement of a node is u x (x, y), the corresponding local strain is: Among them, ∈ xx is the component of the strain tensor, the tensile positive strain in the x direction; ∈ xy is the shear strain in the xy plane; u x is the displacement in the x direction; is the component of the displacement gradient tensor in the x direction; For the volume conservation formula, since the deformation in the necking region is highly localized, the formula becomes: Among them, l (i) is the local length increment at a certain point i along the length direction of the workpiece (calculated as a gradient); A (i) is the real-time cross-sectional area at the corresponding position; Calculate the local length change, which can be obtained from formula (6): in, is the axial normal strain component, and the cross-sectional area can be inferred in reverse.

6. The method for data correction and verification in metal tensile testing considering necking according to claim 1, characterized in that: In step S5, the true stress is calculated using the ratio of the simulated load to the cross-sectional area of ​​the necking part, and the corrected true stress-strain curve is further obtained; based on the corrected stress equilibrium equation and geometric equation, the true stress σ true By the instantaneous load F m The actual cross-sectional area A of the necking part necking The ratio is determined by: Among them, F m is the instantaneous load obtained during the simulation; A necking Calculated by nodal displacement distribution in finite element simulation.

7. The method for data correction and verification in metal tensile testing considering necking according to claim 2, characterized in that: The preset threshold in step S6 is 5%, and the relative error includes the relative error between the simulated value and the experimental value of the necking zone length, the relative error between the simulated value and the experimental value of the peak stress of the true stress-strain curve, and the relative error between the simulated value and the experimental value of the maximum load value of the load-displacement curve. The calculation formula (8) of the relative error is: Where M is the relative error, σ exp is the experimental value, σ sim is the analog value.

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