A method for measuring the micro-mechanical constitutive relationship of a welded joint

Abstract

The application provides a kind of measurement method of welded joint micro zone mechanics constitutive relation, respectively carry out micro zone instrumented indentation test and tensile test under digital image correlation method on aluminum alloy welded joint sample, obtain the stress-strain information of micro zone in complete plastic deformation stage from instrumented indentation test, obtain the stress-strain information of micro zone in elastic deformation stage and elastic-plastic deformation stage from tensile test under digital image correlation method, the stress-strain information of micro zone in elastic deformation stage, elastic-plastic deformation stage and complete plastic deformation stage is fitted and constructed according to a kind of strain hardening index changes with strain variation mechanics constitutive relation formula, finally obtain complete micro zone mechanics constitutive relation, can completely describe the mechanical deformation behavior of material micro zone.This method can solve the problem that only the mechanical constitutive relation of the weakest region of the material can be obtained during the mechanical property test of non-heat treatable strengthened aluminum alloy.

Description

Technical Field

[0001] This invention relates to the field of material mechanical property testing technology, and more specifically, to a method for measuring the mechanical constitutive relationship of a micro-region of a welded joint. Background Technology

[0002] Aluminum alloys possess advantages such as high specific strength, good corrosion resistance, and good weldability, making them widely used in shipbuilding, aerospace, and automotive industries. Welding is a common method for forming aluminum alloy components. However, many common aluminum alloy series, such as 3-series, 5-series, and 6-series, contain alloys that cannot be heat-treatable for strengthening. In these alloys, the heat input during welding causes grain coarsening, and the weld seam and heat-affected zone of the weld joint are typically weak phases, resulting in lower mechanical strength properties than the base metal. Therefore, in the process design of aluminum alloy welded structures, the mechanical properties of the base metal cannot be directly used. Characterizing the mechanical properties of localized areas of the weld joint is crucial for aluminum alloy structural design. Conventional aluminum alloy welding process qualification tests only perform overall tensile tests on the weld joint to obtain the tensile strength of the weakest area, failing to characterize the mechanical properties of other parts of the joint. This leads to inaccurate calculations of structural strength when using methods such as weld reinforcement or stiffeners to compensate for strength deficiencies. Consequently, designs with high safety redundancy are commonly adopted, increasing production costs and component weight. Yield strength is also a key indicator in structural design. In the overall tensile testing method for weld procedure qualification, the measurement of yield strength is affected by the position and gauge length of the extensometer, with the minimum measurement length exceeding 5 mm. This makes it difficult to accurately characterize the yield strength of localized areas of the joint.

[0003] Currently, methods for characterizing the micro-area mechanical properties of metallic materials include microhardness testing, digital image correlation (DIR), and instrumented indentation. Microhardness testing measures the hardness of micro-areas of the cross-sectional microstructure of a welded joint and, combined with the metallographic structure of the corresponding regions, derives tensile mechanical properties from the hardness results of different areas. This method is a semi-quantitative empirical method and cannot guarantee accurate experimental results. DIR involves spraying markings onto the surface of a tensile specimen and recording the changes in these markings during the tensile process to obtain the overall strain of the joint. However, for non-heat-treatable aluminum alloys, only the weakest region can obtain a complete stress-strain constitutive relationship; for other regions, only a partial constitutive relationship can be obtained. While it can accurately characterize yield strength, it cannot characterize the tensile strength of micro-areas. Instrumented indentation involves pressing a small indenter into the surface of a micro-area of ​​the material under test to obtain the indentation load-depth curve. The mechanical constitutive relationship of the material is constructed by fitting the load-depth curve, thereby obtaining the corresponding mechanical property parameters. The instrumented indentation method assumes that the stress-strain information of the material obtained from the indentation load-depth curve is in the stage of complete plastic deformation of the material, while the yield strength of the material is in the stage of elastic-plastic deformation. Therefore, the instrumented indentation method has poor accuracy in characterizing the yield strength of the micro-area, with a deviation of more than 15%.

[0004] In summary, existing testing methods cannot accurately measure the micro-regional mechanical constitutive relationship of non-heat-treatable reinforced aluminum alloy welded joints. Summary of the Invention

[0005] In view of this, the present invention aims to propose a method for measuring the micro-regional mechanical constitutive relationship of non-heat-treatable strengthened aluminum alloy welded joints. This addresses the problem that existing measurement methods cannot accurately measure the micro-regional mechanical constitutive relationship of non-heat-treatable strengthened aluminum alloy welded joints.

[0006] To achieve the above objectives, the technical solution of the present invention is implemented as follows:

[0007] A method for measuring the micro-regional mechanical constitutive relationship of a welded joint, wherein the welded joint is a non-heat-treatable aluminum alloy welded joint, comprising the following steps:

[0008] S1. Process the aluminum alloy welded test plate into a standard plate-shaped tensile specimen;

[0009] S2. The weld sections on both sides of the sample are designated as section A and section B, respectively. Section A is subjected to corrosion treatment to make the weld area visible to the naked eye, while section B is left untreated. The micro-area location of the sample is determined based on the weld center.

[0010] S3. Based on the micro-region location of the sample in step S2, conduct an instrumented ball indentation test on section A at the micro-region location to obtain the load-depth curve.

[0011] S4. Spray the pattern marks required for the digital image correlation test onto section B of the specimen, and then carry out a tensile test under digital image correlation monitoring on a tensile testing machine.

[0012] S5. Calculate the true stress-true strain information of the fully plastic deformation stage obtained by the instrumented ball indentation test in step S3.

[0013] S6. Calculate the stress-strain information of the elastic and elastoplastic stages obtained from the digital image correlation test in step S4;

[0014] S7. Fit the results obtained in steps (5) and (6) to obtain the complete stress-strain mechanical constitutive relation of the micro-region location.

[0015] The method of this invention obtains complete micro-region mechanical constitutive relations, which can fully describe the mechanical deformation behavior of the material's micro-regions, thereby solving the problem that only the mechanical constitutive relations of the weakest region of mechanical properties can be obtained in the process of testing the mechanical properties of non-heat-treatable aluminum alloys.

[0016] Furthermore, step S5 also includes the following steps:

[0017] S51. Calculate the true strain ε p :

[0018]

[0019] In the formula: R is the radius of the ball indenter, and a is the actual contact radius of the ball indentation.

[0020] S52, Calculate the true stress σ t :

[0021]

[0022] In the formula: F is the applied load, The actual indentation contact depth is Ψ, which is the plastic constraint factor, Ψ = 3.0 - n, where n is the strain hardening index of the material being tested. An initial value of n can be obtained based on the curvature ratio of the loading curve.

[0023] Furthermore, in step S52, the actual indentation contact depth The calculation formula is:

[0024]

[0025] Where h is the maximum contact depth under loading, s is the curvature of the tangent line of the unloading curve at load P, and hpile This indicates the change in indentation contact depth caused by accumulation or settlement; h pile with h p The relationship is expressed as:

[0026]

[0027] Further, in step S51, the actual contact radius 'a' of the ball indentation is calculated using the following formula:

[0028]

[0029] Further, in step S7, the stress-strain mechanical constitutive relation is as follows:

[0030]

[0031] In the formula, σ is the true stress, ε is the true strain, B is the strength coefficient, N is the hardening index, and C1 and C2 are constants.

[0032] 6. The measurement method according to claim 1, wherein in step S1, the micro-area position includes the welding center micro-area point, micro-area point one, micro-area point two, micro-area point three... micro-area point L, where L is an even number.

[0033] Furthermore, in step S1, the original thickness of the sample must be ≥2mm, and the weld section of the sample is polished with 1000-grit sandpaper to remove the oxide layer on the surface.

[0034] Furthermore, in step S2, the corrosion treatment of section A uses a mixture of hydrofluoric acid, hydrochloric acid, nitric acid and water in a volume ratio of 2:3:5:190, wherein the concentration of hydrofluoric acid is 1.15 g / mL, the concentration of hydrochloric acid is 1.19 g / mL and the concentration of nitric acid is 1.40 g / mL.

[0035] Furthermore, in step S3, a carbide ball head with a diameter of 0.5 mm to 1.0 mm is used for pressing. The pressing test is controlled by displacement and is carried out for 10 to 15 loading and unloading cycles. The maximum pressing depth of each cycle is 0.12 to 0.16 mm, and the unloading rate of each loading and unloading cycle is not less than 40% of the maximum load of that cycle.

[0036] Furthermore, in step S4, the test is conducted using beam displacement control at a speed of 2 mm / min until the specimen breaks.

[0037] Compared with existing technologies, the method for measuring the micro-region mechanical constitutive relationship of welded joints described in this invention has the following advantages:

[0038] 1) This invention involves conducting instrumented indentation tests and tensile tests using digital image correlation (DIR) on the same non-heat-treatable aluminum alloy welded joint specimen. The instrumented indentation test yields stress-strain information for the fully plastic deformation stage of the micro-region, while the tensile test using DIR yields stress-strain information for the elastic and elasto-plastic deformation stages. This stress-strain information for the elastic, elasto-plastic, and fully plastic deformation stages is then fitted and constructed using a mechanical constitutive relation where the strain hardening exponent changes with strain. This results in a complete mechanical constitutive relation for the micro-region, capable of fully describing the mechanical deformation behavior of the material's micro-regions. This method solves the problem that only the mechanical constitutive relation of the weakest region can be obtained during the mechanical property testing of non-heat-treatable aluminum alloys.

[0039] 2) The method of this invention can accurately characterize the micro-region mechanical properties and mechanical constitutive relations of aluminum alloy welded joints. The characterization results can be applied to finite element simulation in the design process of aluminum alloy structures, accurately simulate the mechanical behavior of aluminum alloy structures under load, change the current status of high safety redundancy design, effectively reduce the size of the excess height or stiffeners of aluminum alloy welded structures, reduce material and process costs, and improve equipment lightweighting. It has broad application prospects in the design and mechanical performance characterization of aluminum alloy welded structures in the fields of shipbuilding, aerospace, and automobiles. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of the test of section A of the specimen according to the present invention;

[0041] Figure 2 This is a schematic diagram of the test of section B of the specimen according to the present invention;

[0042] Figure 3 This is a schematic diagram of the micro-area mechanical property testing location for welded joint samples in Embodiments 5083-5383 of the present invention;

[0043] Figure 4 The graphs show the load and depth at different micro-area locations during instrumented indentation of the welded joint samples in Examples 5083-5383 of this invention.

[0044] Figure 5 The strain distribution diagrams of the welded joint specimens in Examples 5083-5383 of this invention were obtained by digital image correlation method.

[0045] Figure 6 The constitutive relation diagrams of micro-area mechanics obtained by digital image correlation and instrumented indentation of welded joint specimens 5083 (left)-5383 (right) in Embodiments of the present invention are shown.

[0046] Figure 7This is a schematic diagram of the welded joint area simulated by Abaqus software in an embodiment of the present invention;

[0047] Figure 8 This is a comparison chart of the experimentally measured stress-strain curve and the finite element simulation stress-strain curve in an embodiment of the present invention.

[0048] Explanation of reference numerals in the attached figures:

[0049] 1-Instrumented pressing head, 2-Specimen A section, 3-Micro-zone test position, 31-Weld zone, 311-First micro-zone point, 32-5083 heat-affected zone, 321-Second micro-zone point, 33-5383 heat-affected zone, 331-Third micro-zone point, 34-5083 base material zone, 341-Fourth micro-zone point, 35-5383 base material zone, 351-Fifth micro-zone point, 4-Specimen weld fusion line, 5-Tensile testing machine clamp, 6-Stain mark, 7-Specimen B section, 8-Digital image correlation equipment. Detailed Implementation

[0050] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0051] A method for measuring the constitutive mechanical relationship of micro-regions in non-heat-treatable strengthened aluminum alloy welded joints is disclosed. The method involves preparing a transverse specimen on the welded test plate or structure to be tested. On one side of the specimen's weld cross-section, the absolute position of the weld center on the specimen cross-section is determined by dimensional measurement. Then, the relative position of the micro-region to be measured and the weld center is determined. Next, an instrumented continuous ball indentation test is conducted on the micro-region to obtain the load-depth curve and fit the stress-strain point information of the fully plastic deformation stage. Subsequently, a tensile test under digital image correlation monitoring is conducted on the other side of the specimen to obtain the incomplete stress-strain curve of the same micro-region. Finally, according to the constitutive model function relationship, the stress-strain information of the fully plastic deformation stage obtained by the instrumented continuous ball indentation test and the stress-strain information of the elastic deformation and elastoplastic deformation stages obtained by the digital image correlation method are input and fitted to calculate the complete constitutive mechanical model parameters of the corresponding welded joint micro-region.

[0052] The specific implementation steps are as follows:

[0053] (1) The aluminum alloy welded test plate is processed into a standard plate tensile specimen according to the requirements of GB / T 228.1 "Metallic materials, tensile testing - Part 1: Room temperature test method". The original thickness of the specimen must be ≥2mm. The weld section of the specimen is polished with 1000-grit sandpaper to remove the oxide layer on the surface.

[0054] (2) A mixed solution of hydrofluoric acid (ρ1.15 g / mL), hydrochloric acid (ρ1.19 g / mL), nitric acid (ρ1.40 g / mL), and water in a volume ratio of (2+3+5+190) was used to corrode one side of the weld section of the sample, making the weld area visible to the naked eye. The corroded section was designated as section A, and the other side as section B. The thickness of the sample and the width of the weld center were measured using vernier calipers to determine the absolute position of the weld center on the sample, designated as P0. Subsequently, the relative position of the test position with respect to the weld center P0 was determined according to the micro-area to be tested, designated as Pi (i = 1, 2, 3...). For example, the intersection of the weld center axis and the fusion line was designated as P1 and P2, the heat-affected zone position 3 mm outside the intersection of the weld center axis and the fusion line was designated as P3 and P4, and the base material position 8 mm outside the intersection of the weld center axis and the fusion line was designated as P5 and P6.

[0055] (3) Conduct an instrumented ball indentation test on section A of the specimen. It is recommended to use a carbide ball head with a diameter of 0.5 mm to 1.0 mm for indentation. The indentation test is controlled by displacement and is performed for 10 to 15 loading and unloading cycles. The maximum indentation depth of each cycle is between 0.12 and 0.16 mm. The unloading rate of each loading and unloading cycle is not less than 40% of the maximum load of that cycle. Perform the instrumented indentation test at the specimen position selected in step (2). During the test, the center of the indenter and the selected micro-area point are on the same axis. Record the load-depth curve during the test.

[0056] (4) Spray the pattern mark required for digital image correlation test on section B of the specimen, and then carry out tensile test under digital image correlation monitoring on the tensile specimen machine. The test adopts beam displacement control and the test speed is 2 mm / min until the specimen breaks.

[0057] (5) Calculate the true stress-true strain information for the fully plastic deformation stage obtained from the instrumented indentation test. True plastic strain ε p The calculation is performed using formula (1), and the expression is as follows:

[0058]

[0059] In the formula: R is the radius of the ball indenter, and a is the actual contact radius of the ball indentation.

[0060] True stress σ t The calculation is performed using formula (2), and the expression is as follows:

[0061]

[0062] In the formula: F is the applied load, The actual indentation contact depth is given by Ψ, where Ψ = 3.0 - n, and n is the strain hardening exponent of the tested material. An initial value for n can be obtained from the curvature ratio of the loading curve. Actual indentation contact depth The calculation is performed using formula (3), and the expression is as follows:

[0063]

[0064] Where h is the maximum contact depth under loading, s is the curvature of the tangent line of the unloading curve at load P, and h pile h represents the change in indentation contact depth caused by accumulation or settlement. pile The calculation is performed using formula (4), and the expression is as follows:

[0065]

[0066] The true contact radius 'a' of the ball indentation is calculated according to formula (5), and the expression is as follows:

[0067]

[0068] At this point, the true stress and true strain information for the fully plastic deformation stage corresponding to each loading and unloading cycle can be calculated.

[0069] (6) Calculate the stress-strain information of the elastic and elastoplastic stages obtained by digital image correlation methods. In the digital image calculation software, the center position of the area to be calculated is determined according to the absolute position of the weld center and the relative position of the instrumented indentation test point. According to the indentation depth, the stress-strain information of the area is calculated by selecting a length of twice the maximum indentation depth in the tensile direction.

[0070] (7) Constitutive relation calculation. Step (6) obtained the tensile mechanical stress-strain relation of the selected region. Except for the fracture location of the weld, the stress-strain relation of the other regions is incomplete, but it has covered all constitutive information of the elastic deformation and elastoplastic deformation stages. The incomplete stress-strain information obtained in step (6) and the stress-strain information points of the complete plastic deformation stage obtained in step (5) are fitted by least squares according to the mechanical constitutive relation of strain hardening index with strain change shown in formula (6). The values ​​of B, N, C1 and C2 are fitted, and finally the complete stress-strain mechanical constitutive relation of the region is obtained. The relation is shown in formula (6).

[0071]

[0072] Where σ is the true stress, ε is the true strain, B is the strength coefficient, N is the hardening index, and C1 and C2 are constants.

[0073] A method for measuring the micro-regional mechanical constitutive relation of welded joints can be applied to the micro-regional mechanical property testing of welded joints made of various aluminum alloys. The following example uses a 5083-5383MIG welded joint material to demonstrate the process and results of its micro-regional constitutive relation measurement, and the results are verified.

[0074] Test materials: 10mm thick 5083-5383 welded joints, welding method: MIG. Welding material: 1.2mm diameter ER5183, welding current: 170A, argon flow rate: 20-25L / min.

[0075] Test temperature: 23–25℃, relative humidity: 40–50%

[0076] Experimental procedure:

[0077] (1) The aluminum alloy welded test plate is processed into a standard plate tensile specimen according to the requirements of GB / T 228.1 "Metallic materials, tensile testing - Part 1: Room temperature test method". The weld section on both sides of the specimen is polished with 1000 grit sandpaper to remove the oxide layer on the surface.

[0078] (2) A mixed solution of hydrofluoric acid (ρ1.15g / mL), hydrochloric acid (ρ1.19g / mL), nitric acid (ρ1.40g / mL) and water in a volume of (2+3+5+190) was used to etch one side of the weld section of the sample, so that the weld area was visible to the naked eye. The etched section was recorded as section A. The thickness of the specimen and the width of the weld center were measured using vernier calipers to determine the absolute position of the weld center. Weld zone 31, heat-affected zone 32 (5083), heat-affected zone 33 (5383), base metal zone 34 (5083), and base metal zone 35 were selected along the central axis of the weld joint section of the specimen. Specifically, a first micro-area point 311 was set in weld zone 31, a second micro-area point 321 in heat-affected zone 32 (5083), a third micro-area point 331 in heat-affected zone 33 (5383), a fourth micro-area point 341 in base metal zone 34 (5083), and a fifth micro-area point 351 in base metal zone 35 (5383). These five micro-area points served as the locations for testing the micro-area mechanical properties. Figure 3 As shown, the second and third micro-area points are equidistant from the first micro-area point, and the fourth and fifth micro-area points are equidistant from the first micro-area point. The five micro-area points are on the same straight line, and this straight line is parallel to the long side of the welded joint.

[0079] (3) Instrumented ball indentation tests were conducted at five micro-points on section A of the specimen. A 1.0 mm diameter carbide ball head was used for indentation. Displacement control was employed in the indentation tests, with 15 loading and unloading cycles performed. The maximum indentation depth was 0.12 mm, and the unloading rate for each cycle was 40% of the maximum load for that cycle. The load-depth curves during the tests were recorded, such as... Figure 4 As shown.

[0080] (4) Spray the required pattern markings for the digital image correlation test onto section B of the specimen. Then, conduct a tensile test under digital image correlation monitoring on an INSTRON 5985 250kN tensile testing machine. The test uses beam displacement control, with a test speed of 2 mm / min, until the specimen fractures. The obtained strain distribution diagram is shown below. Figure 5 As shown.

[0081] (5) Calculate the true stress-true strain information of the full plastic deformation stage obtained by instrumented indentation test.

[0082] (6) Calculate the stress-strain information of the elastic and elastoplastic stages obtained by digital image correlation methods. In the digital image calculation software, the center position of the area to be calculated is determined according to the absolute position of the weld center and the relative position of the instrumented indentation test point. According to the indentation depth, a length of 2 mm is selected according to the tensile direction to calculate the stress-strain relationship of the area.

[0083] (7) Constitutive relation calculation. The stress-strain information of the elastic deformation stage, elastoplastic deformation stage, and fully plastic deformation stage of the micro-region was fitted using the least squares method according to the mechanical constitutive relation of strain hardening exponent versus strain to obtain the values ​​of B, N, C1, and C2, as shown in Table 1. Based on the calculated constitutive relation, strain-strain curves were plotted as follows: Figure 6 As shown.

[0084] Table 1. Constitutive relation calculation results

[0085]

[0086]

[0087] (8) Result Verification. The constitutive relation results obtained using this invention will be verified by performing finite element simulations and comparing them with actual test results. To obtain more refined performance elements in the finite element simulation, more densely distributed micro-region mechanical constitutive relation tests will be conducted on the same type of weld plate. Three regions will be taken at the center of the weld, two regions will be taken on each side of the heat-affected zone, and two regions will be taken on each side of the base material. The obtained constitutive relation results will be input into Abaqus software, such as... Figure 7As shown in Table 2, the tensile test specimen was simulated. Based on the assumption that the volume remains constant during the tensile process, it was assumed that the tensile strength was reached when the true strain equaled the strain hardening exponent. The tensile strength and yield strength were calculated, and the results are shown in Table 2. The simulated test curves and the actual test curves are shown in Table 2. Figure 8 The mechanical property results obtained by finite element simulation based on the constitutive relation obtained by this invention deviate from the experimentally measured mechanical property results by less than 3%, proving that the method described in this invention has high accuracy in measuring the constitutive relation of micro-area mechanical properties of non-heat-treatable reinforced aluminum alloy welded joints.

[0088] Table 2 compares the results of finite element simulation and measured results using the constitutive relation measured according to the present invention.

[0089]

[0090] While the present invention has been disclosed above, it is not limited thereto. Any person skilled in the art can make various modifications and alterations without departing from the spirit and scope of the invention; therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

Claims

1. A method for measuring the mechanical constitutive relationship of a micro-region of a welded joint, characterized in that, The welded joint is a non-heat-treatable aluminum alloy welded joint, and includes the following steps: S1. Process the aluminum alloy welded test plate into a standard plate-shaped tensile specimen; S2. The weld sections on both sides of the sample are designated as section A and section B, respectively. Section A is subjected to corrosion treatment to make the weld area visible to the naked eye, while section B is left untreated. The micro-area location of the sample is determined based on the weld center. S3. Based on the micro-region location of the sample in step S2, conduct an instrumented ball indentation test on section A at the micro-region location to obtain the load-depth curve. S4. Spray the pattern marks required for the digital image correlation test onto section B of the specimen, and then carry out a tensile test under digital image correlation monitoring on a tensile testing machine. S5. Calculate the true stress-true strain information of the fully plastic deformation stage obtained by the instrumented ball indentation test in step S3. S6. Calculate the stress-strain information of the elastic and elastoplastic stages obtained from the digital image correlation test in step S4; S7. Fit the results obtained in steps (5) and (6) to obtain the complete stress-strain mechanical constitutive relationship of the micro-region location; Step S5 also includes the following steps: S51. Calculate True Strain : ； In the formula: R is the radius of the ball indenter, and a is the actual contact radius of the ball indentation; S52, Calculate True Stress : ； In the formula: F is the applied load, The actual indentation contact depth is Ψ, which is the plastic constraint factor, Ψ=3.0-n, and n is the strain hardening index of the material being tested. In step S52, the actual indentation contact depth The calculation formula is: Where h is the maximum contact depth under loading, and s is the curvature of the tangent line of the unloading curve at load P. This indicates the change in the indentation contact depth caused by accumulation or settlement. with h p The relationship is expressed as: ； In step S51, the actual contact radius 'a' of the ball indentation is calculated using the following formula: ； In step S7, the stress-strain mechanical constitutive relation is as follows: ； In the formula, True stress, For true strain, B is the strength coefficient, N is the hardening exponent, and C1 and C2 are constants; In step S2, the corrosion treatment of section A uses a mixture of hydrofluoric acid, hydrochloric acid, nitric acid and water in a volume ratio of 2:3:5:190, wherein the concentration of hydrofluoric acid is 1.15 g / mL, the concentration of hydrochloric acid is 1.19 g / mL and the concentration of nitric acid is 1.40 g / mL. In step S3, a carbide ball with a diameter of 0.5mm to 1.0mm is used for pressing.

2. The measurement method according to claim 1, characterized in that, In step S1, the micro-area locations include the welding center micro-area point, micro-area point one, micro-area point two, micro-area point three... micro-area point L, where L is an even number.

3. The measurement method according to claim 1, characterized in that, In step S1, the original thickness of the sample must be ≥2mm. The weld section of the sample is polished with 1000-grit sandpaper to remove the oxide layer on the surface.

4. The measurement method according to claim 1, characterized in that, In step S3, the indentation test adopts displacement control and performs 10 to 15 loading and unloading cycles. The maximum indentation depth of each cycle is 0.12 to 0.16 mm, and the unloading rate of each loading and unloading cycle is not less than 40% of the maximum load of that cycle.

5. The measurement method according to claim 1, characterized in that, In step S4, the test is conducted using beam displacement control at a speed of 2 mm / min until the specimen breaks.

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