Method for testing large-range equivalent stress-strain curve of metal material

Abstract

The invention belongs to the technical field of material performance testing, and relates to a metal material large-range equivalent stress-strain curve testing method which comprises the following steps: 1, preparing a test piece; preparing the test piece model into a round bar test piece, wherein the middle section of the round bar test piece is provided with an axisymmetric annular notch; 2, testing; a loading device is adopted to load the round bar test piece under displacement control, meanwhile, a high-speed camera is adopted to directly face the round bar test piece to shoot, and the applied load and the change of the notch size are monitored; stopping when the round bar test piece is broken; 3, processing data after the test; sequentially calculating the size of the notch, the strain in the transverse direction of the notch, the real stress in the test process and the engineering stress; 4, correcting real stress; converting the real stress obtained by the notched sample into the equivalent stress of the material; the equivalent stress-strain curve of the material can be obtained by measuring the initial fracture size and the hardening index, and the test cost is lower.

Description

Technical Field

[0001] The present invention belongs to the technical field of material property testing, and relates to a method for testing a large-range equivalent stress-strain curve of a metal material. Background Art

[0002] When analyzing problems such as plastic forming and ductile fracture by using the finite element method, it is necessary to obtain accurate full-range material equivalent stress-strain curves or flow stress-strain curves. For heterogeneous materials such as welded joints, due to the non-uniformity of the welded parts and the unpredictable fracture positions on the tensile specimens of intersecting welds, the traditional smooth round bars or rectangular specimens cannot determine the stress-strain curves of their respective regions. Moreover, when the material enters the necking stage, a complex triaxial stress state is formed in the local area, resulting in the deviation of the true stress-strain curve obtained by the traditional smooth round bar or rectangular specimen from the equivalent stress-strain curve of the material. The existing solutions rely on the Bridgman correction method, but the Bridgman correction method needs to synchronously monitor the dynamically changing notch radius ratio during the test, and the application cost is very high; the equivalent stress-strain curve obtained by the Bridgman correction method is not accurate when the strain is large; the assumption of uniform cross-section stress distribution based on the Bridgman correction method also does not conform to the actual strain distribution in the necking zone.

[0003] Therefore, a method or device with lower cost that can determine the equivalent stress-strain curve of the material through the tensile test of an axially symmetric notched specimen is needed to solve the above technical problems. Summary of the Invention

[0004] The present invention aims to develop a new and effective testing technology to test the equivalent stress-strain curve of the material, accurately obtain the constitutive relationship of the material, and make the performance prediction of the material in simulation and practical applications more accurate.

[0005] The technical solution adopted by the present invention to solve the technical problems is: a method for testing a large-range equivalent stress-strain curve of a metal material, comprising the following steps:

[0006] Step 1, specimen preparation; preparing the specimen model as a round bar specimen, and an axially symmetric annular notch is provided in the middle section of the round bar specimen;

[0007] Step 2, conducting the test; using a loading device to load the round bar specimen under displacement control, and at the same time using a high-speed camera to photograph the round bar specimen directly to monitor the applied load and the change of the notch size; stopping when the round bar specimen deforms or breaks into two parts;

[0008] Step 3, post-test data processing; sequentially calculating the notch size a, the strain in the notch transverse direction , the true stress during the test With engineering stress ;

[0009] Step 4, correct the true stress; convert the true stress obtained from the notched specimen into the equivalent stress of the material.

[0010] Preferably, in the said Step 1, the length of the specimen model is L, the cross-sectional diameter is d0, the cross-sectional radius of the annular notch is R0, and the cross-sectional radius of the specimen model at the annular notch is a0.

[0011] More preferably, the range of the radius ratio at the annular notch is: 0.25 ≤ a0 / R0 ≤ 3.

[0012] Preferably, in the said Step 2, the frame rate of the high-speed camera is greater than 100 FPS.

[0013] Preferably, in the said Step 3, the calculation formula for the notch size a is:

[0014] (1)

[0015] The strain in the transverse direction of the notch is calculated as:

[0016] (2)

[0017] The true stress during the test and the engineering stress are:

[0018] (3)

[0019] In formulas (1) to (3), a represents the notch size at time t, a0 represents the notch size at the initial time, N t represents the number of pixel points occupied by the notch in the transverse direction in the photo at time t, N0 represents the number of pixel points occupied by the notch in the transverse direction in the photo at the initial time; P represents the load applied at both ends of the specimen during the test.

[0020] Preferably, in the said Step 4, the calculation formula for converting the true stress into the equivalent stress of the material is:

[0021] (4)

[0022] (5)

[0023] (6)

[0024] (7)

[0025] (8)

[0026] In Formulas (4) to (8), represents the true stress of the notched specimen under the corresponding strain and the equivalent stress of the material ratio, represents the strain in the transverse direction of the notch, represents a function related to the initial geometric dimensions and strain of the specimen, represents a function related to the material properties of the specimen, represents the strain value corresponding to the maximum stress in the engineering stress-strain curve, characterized as a function related to the stress concentration introduced by the notch, R0 represents the cross-sectional radius of the annular notch, and a0 represents the cross-sectional radius of the specimen model at the annular notch.

[0027] The beneficial effects of the present invention are:

[0028] 1. The present invention can obtain the equivalent stress-strain curve of the material only by measuring the initial fracture size and the hardening index, and the test cost is lower.

[0029] 2. The present invention recommends that the geometric requirement of the axisymmetric specimen is 0.25 ≤ a0 / R0 ≤ 3, and the applicable range is wide.

[0030] 3. The present invention can obtain the equivalent stress-strain curve of the material when the strain is large, and is suitable for measuring the stress-strain curve of the local material of the circumferential weld.

[0031] 4. The present invention can be used to obtain the stress-strain curve of the heterogeneous material in a large range. Description of the Drawings

[0032] Figure 1 is a schematic diagram of the geometric dimensions of the tensile specimen of a method for testing the equivalent stress-strain curve of a metal material in a large range according to the present invention;

[0033] Figure 2 is a diagram of the method for monitoring the change of the notch size according to the present invention;

[0034] Figure 3 is a true stress-strain curve diagram obtained by the test of the present invention;

[0035] Figure 4 is a corrected equivalent stress-strain curve diagram of the material according to the present invention;

[0036] Figure 5 is a true stress-strain curve diagram of the specimen with a0 / R0 = 1.5 and n = 0.11 according to the present invention;

[0037] Figure 6It is the curve of the equivalent stress-strain of the material after correction for the specimen with a0 / R0 = 1.5 and n = 0.11 of the present invention. Detailed implementation manners

[0038] The following will clearly and completely describe the related technologies in the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0039] Refer to Figures 1 to 6 As shown, a test method for the large-range equivalent stress-strain curve of a metal material in this embodiment specifically includes the following steps:

[0040] 1. Specimen preparation:

[0041] The geometric shape of the specimen model is an axially symmetric round bar specimen with a notch in the center. The length of the specimen is L, the cross-sectional diameter is d0, there is a circular notch with a radius of R0 in the center of the specimen, and the radius of the notch of the specimen is a0, as Figure 1 shown.

[0042] The recommended range of the ratio of the notch size to the radius of the notch of the specimen should be 0.25 ≤ a0 / R0 ≤ 3.

[0043] 2. Test device:

[0044] A loading device should be equipped for the test so that the applied load can be continuously measured during the test. The force sensor and the recording device should meet the requirements of GB / T 16825.1—2022.

[0045] A high-speed camera is used to photograph the specimen directly to monitor the change of the notch size during the test. The frame rate of the camera should be greater than 100 FPS, and the position of the camera relative to the specimen is fixed during the test.

[0046] 3. Conduct the test:

[0047] The overall goal of the test method is to load the specimen under displacement control while monitoring the applied load and the change of the notch size. The test stops when the specimen breaks into two parts.

[0048] 4. Data processing after the test:

[0049] Each pixel in the picture obtained by the high-speed camera occupies the same length. According to the number of pixel points occupied by the minimum cross-section of the specimen in the photo and the number of pixel points occupied by the notch in the horizontal direction in the photo at the initial moment, the notch size a can be calculated:

[0050] (1)

[0051] Wherein, a is the notch size at time t; N t is the number of pixels occupied by the notch in the horizontal direction in the photo at time t; N0 is the number of pixels occupied by the notch in the horizontal direction in the photo at the initial time, as Figure 2 shown.

[0052] Then, calculate the strain in the horizontal direction of the notch according to the formula :[[]]

[0053] (2)

[0054] From Equation (2), the strain change at the notch over time can be obtained .

[0055] After the test is completed, export the change of the applied load over time, and calculate the true stress and engineering stress during the test according to the formula:

[0056] (3)

[0057] By corresponding the true stress and strain one by one according to time, the true stress-strain curve during the test can be obtained, as Figure 3 shown.

[0058] 5. Correct the true stress:

[0059] Use the following formula to convert the true stress obtained from the notched specimen into the equivalent stress of the material:

[0060] (4)

[0061] Wherein, is the true stress of the notched specimen corresponding to the strain The ratio of the true stress of the notched specimen to the equivalent stress of the material can be calculated by Equation (5):

[0062] (5)

[0063] In Equation (5), is a function related to the initial geometric dimensions and strain of the specimen. The specific expression is as follows:

[0064] (6)

[0065] In Equation (5), is a function related to the material properties of the specimen, is the strain value corresponding to the maximum stress in the engineering stress-strain curve. The specific expression is:

[0066] (7)

[0067] In Equation (6), characterized as a function related to the stress concentration introduced by the notch:

[0068] (8)

[0069] Example

[0070] In this example, an axisymmetric notched specimen with a0 / R0 = 1.5 is taken as an example, and the equivalent stress-strain curve of the material is determined through a tensile test.

[0071] 1. Specimen preparation:

[0072] The geometric shape of the specimen model is an axisymmetric round bar specimen with a notch in the center. The length of the specimen is L = 80 mm, the cross-sectional diameter is d0 = 24 mm, there is a circular notch with a radius of R0 = 4 mm in the center of the specimen, and the radius at the notch of the specimen is a0 = 6 mm.

[0073] 2. Test device:

[0074] The test is equipped with a loading device and a test recording device, and the force sensor and the recording device record the magnitude of the applied load in real time.

[0075] A camera is used to photograph the side of the specimen directly. During the test, the change in the notch size can be monitored. The frame rate of the camera is 100 FPS, and the position of the camera relative to the specimen is fixed during the test.

[0076] 3. Conduct the test:

[0077] The overall objective of the test method is to load the specimen under displacement control while monitoring the applied load and the change in the notch size. The test stops after the notch of the specimen deforms to a certain extent.

[0078] 4. Data processing after the test:

[0079] After the test is completed, the notch photos of each photographic cycle are exported from the high-speed camera. According to the number of pixel points occupied by the notch in the horizontal direction in the photo and the number of pixel points occupied by the notch in the horizontal direction in the photo at the initial moment, the notch size a is calculated using Equation (1). Then, the strain in the horizontal direction of the notch is calculated using Equation (2) , and the strain at the notch can be obtained The change with time can be obtained. The change in the applied load with time is exported from the loading and recording device, and the true stress during the test is calculated using Equation (3). By corresponding the true stress and strain one by one according to time, the true stress-strain curve during the test can be obtained, as shown in Figure 5 shown.

[0080] 5. True stress correction:

[0081] Based on the strain corresponding to the maximum stress in the engineering stress-strain curve , the material hardening index n = = 0.11. Substitute a0 / R0 = 1.5 and n = 0.11 into formula (5-8) to calculate the correction factor , and its simplified formula is:

[0082] (9)

[0083] After obtaining the correction factor corresponding to the strain, use formula (4) to calculate the equivalent stress of the material corresponding to the strain. The equivalent stress-strain curve of the material is as Figure 6 shown.

[0084] In summary, the present invention can obtain the equivalent stress-strain curve of the material only by measuring the initial fracture size and the hardening index, and the test cost is lower. Therefore, the present invention has a wide application prospect in the field of the equivalent stress-strain curve or the flow stress-strain curve of the material.

[0085] It should be emphasized that: the above are only the preferred embodiments of the present invention, and do not constitute any form of limitation to the present invention. Any simple modification made to the above embodiments based on the technical essence of the present invention also belongs to the protection scope of the present invention. Other equivalent changes and modifications are still within the scope of the technical solution of the present invention.

Claims

1. A test method for the large-range equivalent stress-strain curve of a metal material, characterized in that, Including the following steps: Step 1, specimen preparation: Prepare the specimen model into a round bar specimen, with an axially symmetric annular notch in the middle section of the round bar specimen; Step 2, conduct the test: Use a loading device to load the round bar specimen under displacement control, and at the same time use a high-speed camera to directly face the round bar specimen for shooting, monitor the applied load and the change in the notch size; Stop when the round bar specimen deforms or breaks; Step 3: Post-test data processing; sequentially calculate the notch size a, the strain in the transverse direction of the notch , the true stress during the test and the engineering stress ; Step 4, correct the true stress; Convert the true stress obtained from the notched specimen into the equivalent stress of the material.

2. The method for testing the large-range equivalent stress-strain curve of a metal material according to claim 1, characterized in that In the said Step 1, the length of the specimen model is L, the cross-sectional diameter is d0, the cross-sectional radius of the annular notch is R0, and the cross-sectional radius of the specimen model at the annular notch is a0.

3. A method for testing the large-range equivalent stress-strain curve of a metal material according to claim 2, characterized in that, The range of the radius ratio at the annular notch is: 0.25 ≤ a0 / R0 ≤ 3.

4. A method for testing the large-range equivalent stress-strain curve of a metal material according to claim 1, characterized in that, In the said Step 2, the frame rate of the high-speed camera is greater than 100 FPS.

5. A method for testing the large-range equivalent stress-strain curve of a metal material according to claim 1, characterized in that, In the said Step 3, the calculation formula for the notch size a is: (1) The strain in the lateral direction of the notch The calculation formula is as follows: (2) The true stress of the test process and the engineering stress are as follows: (3) In formulas (1) to (3), a represents the notch size at time t, a0 represents the notch size at the initial time, N t represents the number of pixels occupied by the notch in the horizontal direction in the photo at time t, N0 represents the number of pixels occupied by the notch in the horizontal direction in the photo at the initial time; P represents the load applied to both ends of the specimen during the test.

6. A method for testing the large-range equivalent stress-strain curve of a metal material according to claim 1, characterized in that, In the said step 4, the true stress is converted into the equivalent stress of the material, and the calculation formula is: (4) (5) (6) (7) (8) In Formulas (4) to (8), represents the true stress of the notched specimen corresponding to the strain and the equivalent stress of the material ratio of, represents the strain in the transverse direction of the notch, represents a function related to the initial geometric dimensions and strain of the specimen, represents a function related to the material properties of the specimen, represents the strain value corresponding to the maximum stress in the engineering stress-strain curve, characterized as a function related to the stress concentration introduced by the notch, R0 represents the cross-sectional radius of the annular notch, and a0 represents the cross-sectional radius of the specimen model at the annular notch.

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