A method for testing the elongation at fracture of a uniaxial tensile specimen of a metal plate.

Abstract

This invention discloses a method for testing the elongation at fracture of a uniaxial tensile specimen of a metal plate, mainly addressing the technical problems of low testing efficiency, large errors in automatic measurement data, and lack of data traceability when using manual methods for testing the elongation at fracture of uniaxial tensile specimens of metal plates. The technical solution is as follows: a method for testing the elongation at fracture of a uniaxial tensile specimen of a metal plate includes the following steps: 1) preparing a uniaxial tensile specimen of a metal plate; 2) performing a uniaxial tensile test on the specimen; 3) cleaning the specimen after fracture; 4) measuring the distance between the dot matrix marks on the surface of the specimen remnant; 5) splicing the specimen remnant together; 6) capturing a digital image of the spliced ​​specimen remnant; 7) processing the digital image of the specimen remnant; and 8) calculating the elongation at fracture of the uniaxial tensile specimen of the metal plate. This invention provides high accuracy in measuring elongation at fracture, traceable test data, and convenient testing.

Description

Technical Field

[0001] This invention relates to tensile testing methods for metallic materials, and particularly to a method for testing the elongation after fracture of a uniaxial tensile specimen of a metal plate, belonging to the technical field of mechanical property testing of metallic materials. Background Technology

[0002] Elongation after fracture is an important mechanical property indicator reflecting the plasticity of metallic materials. The higher the value, the better the plasticity of the metallic material, and it is an important basis for the application of metallic materials. In the existing technology, elongation after fracture is mainly measured by two methods: manual and automatic measurement.

[0003] GB / T228.1-2021, "Tension Testing of Metallic Materials", specifies the requirements for determining elongation after fracture. The manual measurement method is as follows: carefully splice the fractured parts of the tensile specimen together so that the center lines of the specimen remnants on both sides of the fracture are on the same straight line, and ensure that the fractured parts of the specimen are in proper contact before measuring the gauge length after fracture. The elongation after fracture is calculated based on the gauge length after fracture and the original gauge length.

[0004] When manually measuring the elongation at fracture of tensile specimens, the marking of the original gauge length and the measurement of the gauge length after fracture are particularly important. Typically, methods such as manual marking or indentation with a marking machine punch are used, but the shape and size of the marks or indentations can vary during the process. Measuring the gauge length after fracture requires butting the fracture surfaces together; however, due to the presence of chips, fragments, and burrs, accurately butting the fracture surfaces is very difficult, and measurement errors are still unavoidable. If the fracture location is not in the middle of the gauge length, a displacement method must be used, making the operation even more complex. Manual measurement generally uses vernier calipers. Because the positioning of the vernier caliper head relative to the mark or indentation is difficult to control precisely manually, it is challenging for a single person to accurately position the vernier caliper head. Even with two people operating the instrument, significant measurement errors occur, resulting in low testing efficiency.

[0005] Automatic measurement typically employs methods such as extensometer clamping or speckle optical tracking to set the gauge length equal to the original gauge length of the specimen. During the tensile process, the displacement of the specimen and the total elongation at the fracture point are automatically measured. The deformation of the elastic part is deducted to obtain the elongation after fracture, and then the elongation rate after fracture is calculated.

[0006] Automated measurement methods reduce manual intervention. In actual testing, the fracture location of the specimen is almost always not in the middle of the extensometer gauge length. In this case, automated measurement cannot use the displacement method for correction as in manual measurement, resulting in a measured elongation that is less than the actual elongation. Furthermore, due to severe necking of the specimen at fracture, if the elastic deformation is calculated based on uniform deformation when deducting the elastic component, the deducted elastic component will be inaccurate. In addition, the strong impact generated by the tensile fracture of the specimen during extensometer clamping accelerates extensometer wear, increases assembly clearance, and reduces measurement accuracy. While setting a load mode to open the extensometer before the specimen fractures can protect the extensometer, the measured elongation after fracture can still have an error of over 10%.

[0007] Although automatic measurement of elongation after fracture reduces the workload of manual operation, it has a large error, and manual measurement is still the main method in practical applications.

[0008] Chinese patent application CN117250088A discloses a method for measuring the elongation at break of a tensile specimen. However, this method requires different length scales for different gauge lengths. Due to the obstruction of the length scale, it is difficult to observe whether the fracture joint is tightly compressed, increasing human error. Furthermore, the microscope's sample stage is small, and when the size of a conventional tensile specimen is much larger than the microscope's sample stage, placing the length scale close to the specimen surface is difficult. Because the length scale has a certain thickness, there is a height difference between the length scale plane and the specimen plane. The measurement accuracy is low, and it can only be used for micro-tensile specimens, making it almost impossible to promote its application in mass production testing.

[0009] Chinese patent application publication number CN109870355A discloses an automatic method for measuring the elongation after uniaxial tensile fracture of metal plate specimens. It provides a method to obtain the elongation after uniaxial tensile fracture of plate specimens by elastic deformation analysis under the premise of known material elastic modulus and Poisson's ratio. This method has high requirements for the processing accuracy of experimental samples and the testing and calculation process is relatively complex.

[0010] Chinese patent application publication number CN109870354A discloses an automatic method for measuring the elongation after uniaxial tensile fracture of a metal round bar specimen. It provides a method to calculate the elongation after uniaxial tensile fracture of a round bar specimen through elastic deformation analysis under the premise of known material elastic modulus and Poisson's ratio. This method requires high precision in the processing of experimental samples and the testing and calculation process is relatively complex.

[0011] Existing publicly available technologies have technical problems in the preservation of specimen remnants after tensile fracture and the traceability of measurement data. Specimen remnants after tensile fracture are prone to corrosion, and when there are many specimens, they are easily missed. Generally, specimens are only kept for a short period after testing, and after being discarded, it is impossible to trace the original specimen; only the original measurement records can be traced. Summary of the Invention

[0012] The purpose of this invention is to provide a method for testing the elongation at fracture of a uniaxial tensile specimen of a metal plate, which mainly solves the technical problems of low testing efficiency, large error in automatic measurement data, and lack of traceability of test data when the elongation at fracture of a uniaxial tensile specimen of a metal plate is tested manually.

[0013] The method of this invention can reduce the workload of test operators, reduce measurement errors, improve test accuracy, change the way the specimen remains are preserved after tensile fracture, and provide a guarantee for data traceability and arbitration.

[0014] The technical approach of this invention is as follows: Before the uniaxial tensile test of a metal plate, the clamping parts at both ends of the specimen are marked and numbered. Along the central axis of the specimen (i.e., the tensile direction), dot matrix marks (such as lines, crosses, squares, rings, and indentations) are made at equal intervals on the surface of the tensile specimen. After the tensile test, oil, metal shavings, or oxide scale on the surface of one or more broken specimens are cleaned to make the dot matrix marks clearly visible. The specimen fragments after the breakage of one or more specimens are paired and classified according to the correspondence before tensile testing. The center distance between two dot matrix marks on the surface of the specimen fragments is measured using a qualified measuring instrument such as a vernier caliper, and the measured value is recorded. The broken specimen fragments are carefully paired and spliced ​​together along the fracture surface, so that the center distance between the specimen fragments on both sides of the fracture surface is equal. The center lines are coaxial, ensuring proper contact between the fracture surfaces of the specimen remnants, maintaining the tensile fracture state, preserving the splicing posture, and laying the specimens flat on the workbench or a dedicated specimen fixing device. Digital images of the flat-laid specimens are then captured. During imaging, the lens's central axis is aligned with the center of the field of view of the specimen arrangement, perpendicular to the field of view plane. The acquired digital images are processed using drawing software. For the two dot matrix marks with the measured center distance, their corresponding pixel values ​​are further measured. Based on the proportional relationship between the measured center distance and the pixel values, the average pixel line density λ of the digital images on both sides of the tensile fracture is calculated, in pixels (px / mm). Next, the pixel value P between the centers of the dot matrix marks at the specified gauge length is measured, in pixels (px). The specified gauge length L after fracture is then calculated. u =P / λ, in mm; the elongation after fracture A of the tensile specimen can be calculated based on the specified gauge length L0.

[0015] The technical solution adopted in this invention is a method for testing the elongation after fracture of a uniaxial tensile specimen of a metal plate, comprising the following steps:

[0016] 1) Prepare a uniaxial tensile specimen of metal plate, mark and number the clamping parts at both ends of the specimen, and make equally spaced dot matrix marks on the surface of the tensile specimen along the central axis of the specimen; the dot matrix marks can be any one of lines, crosses, squares, circles and indentations.

[0017] 2) Perform uniaxial tensile tests on the specimens. Use a tensile testing machine to perform uniaxial tensile tests on the metal plate specimens.

[0018] 3) Clean the broken specimen and clean the surface of the specimen residue on both sides of the fracture surface so that the dot matrix markings on the surface of the specimen residue are clearly visible.

[0019] 4) Measure the distance between the lattice marks on the surface of the specimen remnant, and measure the center distance L between two lattice marks near the fracture surface of the specimen remnant. i , i = 1, 2;

[0020] 5) Splice the broken specimen fragments. Lay the tensile specimen flat on the workbench and butt the specimen fragments on both sides of the same specimen fracture along the fracture, keeping the center lines of the specimen fragments on both sides of the fracture coaxial.

[0021] 6) Take digital images of the spliced ​​specimen fragments. When taking pictures with a digital camera, the digital camera lens should be aimed at the center of the field of view of the spliced ​​specimen, and the central axis of the digital camera lens should be perpendicular to the field of view.

[0022] 7) Process the digital image of the specimen remnant and use drawing software to measure the center distance L between two adjacent dot matrix marks near the fracture surface of the specimen remnant. i The corresponding pixel value P i , i = 1, 2; then measure the corresponding pixel value P for the center distance of the dot matrix markers with the specified gauge length defined by the conventional method, in pixels;

[0023] 8) Calculate the elongation at fracture of the uniaxial tensile specimen of the metal plate. The elongation at fracture of the uniaxial tensile specimen of the metal plate is calculated according to Formula 1.

[0024] A = (L u Formula 1: -L0) / L0×100%;

[0025] In Formula 1, A is the elongation after fracture of the uniaxial tensile specimen of metal plate, in %; L0 is the specified gauge length, in mm; L u The specified gauge length is given after the fracture, in mm; L u Calculated according to Formula 2,

[0026] L u =P / λ Formula 2;

[0027] In Formula 2, P is the pixel value corresponding to the center distance of the dot matrix marks at the specified gauge length, in pixels (px); λ is the average pixel line density of the digital images on both sides of the tensile fracture, in pixels (px / mm); λ is calculated according to Formula 3.

[0028] Formula 3: λ = (P1 / L1 + P2 / L2) / 2

[0029] In Formula 3, P1 and P2 are the pixel values ​​corresponding to the center distance of the two dot matrix marks on the surface of the specimen residue on both sides of the fracture, respectively, in pixels (px); L1 and L2 are the center distance of the two dot matrix marks on the surface of the specimen residue on both sides of the fracture, respectively, in mm.

[0030] In this invention, the tensile direction of the specimen is the same as the direction of the central axis of the specimen.

[0031] Further preferably, the dot matrix markings are indentations, the indentations are conical indentations with a 90° included angle, the indentation depth is 0.15 to 0.5 mm, and the spacing between adjacent indentation centers is preferably 10 mm.

[0032] Further preferably, the digital image has the same horizontal and vertical resolution, with a horizontal resolution ≥ 96 dpi.

[0033] Further optimization is achieved when L1 and L2 in step 4) are the center distance between the two adjacent lattice marks closest to the fracture surface of the specimen residue on both sides of the tensile fracture surface, which yields better results.

[0034] Further optimization is to ensure that P1 / L1 and P2 / L2 are equal or approximately equal within the allowable error range; otherwise, it is necessary to remeasure to reduce the error, or retake the digital image to reduce the digital image projection distortion error.

[0035] The rationale for the measurement method employed in this invention is as follows:

[0036] Before the tensile test, the clamping parts at both ends of the specimen are marked and numbered. This is to ensure that the specimen fragments on both sides of the fracture can be spliced ​​together and to ensure accurate digital image recognition. This step is especially important for multiple tensile specimens.

[0037] The dot matrix markings are conical indentations, with the included angle of the cone set at 90°. This is because the vernier caliper's caliper head is at a 45° angle. During measurement, when the vernier caliper scale is parallel to the length of the sample gauge, the caliper head fits tightly against the 90° conical indentation, resulting in higher positioning accuracy and reduced measurement error. The indentation depth is 0.15-0.5mm. This is mainly based on the fact that a suitable indentation depth is selected for metal plate samples of different thicknesses. A depth less than 0.15mm corresponds to an indentation surface aperture that is too small, leading to inaccurate positioning of the vernier caliper head and increased measurement error. A depth greater than 0.5mm may affect the fracture location, resulting in a lower elongation after fracture of the sample.

[0038] In step 4), L1 and L2 are the center distance between the two adjacent dot matrix marks closest to the fracture surface of the specimen residue on both sides of the tensile fracture surface. When the digital image is magnified, it is convenient to measure the pixels on both sides of the fracture surface with the fracture surface as a reference.

[0039] In step 6) of this invention, the horizontal and vertical resolutions of the digital image are the same, and both resolutions must be no less than 96 dpi, which is equivalent to 96 / 25.4 = 3.78 pixel cells per millimeter. Too low a resolution will result in inaccurate identification of dot matrix marks and indentations, leading to errors.

[0040] The method of this invention involves carefully splicing together the fractured portions of a tensile specimen, ensuring that the centerlines of the specimen remnants on both sides of the fracture are coaxial, and ensuring proper contact between the specimen fracture surfaces to maintain the instantaneous tensile fracture state. If two or more tensile specimens are used, the same method is employed, maintaining the spliced ​​posture, and the specimens are laid flat on a workbench or a dedicated specimen fixing device. Digital images of the flat-laid specimens are then taken. During the photographing, the digital camera lens is aimed at the center of the field of view of the spliced ​​specimens, with the central axis of the digital camera lens perpendicular to the field of view. The purpose is to obtain digital images of the specimen surface dot matrix markings, such as indentations, that are clear and without obvious visual distortion.

[0041] Theoretically, for the same digital photograph, P1 / L1 = P2 / L2. However, due to measurement errors and projection errors in digital image acquisition, P1 / L1 and P2 / L2 are not perfectly equal. When the error is large, further assessment is needed to determine whether the measured actual physical lengths L1 and L2 meet error control requirements and whether a second measurement correction is necessary. Additionally, it's crucial to determine whether the framing distortion of the digital image meets error control requirements and whether a second shooting correction is required. The average pixel line density λ of the image is calculated using two approximately equal measurements.

[0042] The method of this invention uses vernier calipers to measure the center distance of dot matrix marks, such as indentations, on the tensile specimen remnant. It does not require splicing the fracture surface, making the detection convenient and highly accurate.

[0043] Compared with existing technologies, this invention has the following advantages: 1. The method of this invention uses vernier calipers to measure the center distance of the dot matrix marks, such as indentations, on the tensile specimen remnant, without requiring splicing of the fracture surface. Therefore, it can be easily operated by one person, effectively solving the technical problems of difficulty in single-person operation and splicing of fracture specimens, and large errors in measuring the elongation after fracture of tensile specimens. 2. The method of this invention uses digital image processing technology to measure the gauge length, and the digital image can be magnified to observe the indentation position. The pixel value of the specified gauge length in the spliced ​​state of the fracture surface is measured, and the elongation after fracture is calculated, improving the measurement accuracy. 3. The method of this invention realizes digital image archiving of the spliced ​​state of the tensile specimen fracture surface, solving technical problems such as the preservation of tensile specimen fracture remnants and the traceability and arbitration of measurement data. The test data is traceable and the detection is convenient. Attached Figure Description

[0044] Figure 1 This is a top view of the plane of the uniaxial tensile specimen of the metal plate described in an embodiment of the present invention.

[0045] Figure 2 for Figure 1 The image shows a magnified view of the area near the fracture surface of a uniaxial tensile test specimen of a metal plate.

[0046] The markings in the figure are as follows: 1-Tensile specimen, 2-Dot matrix markings, 3-Tensile fracture surface. Detailed Implementation

[0047] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0048] Example 1, refer to Appendix Figure 1 , 2 As per Appendix 1, a method for testing the elongation at fracture of a uniaxial tensile specimen of a metal plate includes the following steps:

[0049] 1) Tensile specimen 1 was prepared from BTC330R pickled enamel steel with a thickness of 1.65mm. Before the tensile test, the clamping parts at both ends of the specimen were marked and numbered. A qualified JW-F30 fiber laser marking machine was used to etch 0.8mm diameter circular dot matrix marks 2 perpendicular to the surface of the tensile specimen along the central axis (i.e. the tensile direction) of the tensile specimen 1. The dot matrix marks 2 were etched at equal intervals with a diameter of 0.05mm, the line width of the marks was 0.05mm, the depth of the marks was 0.02mm, and the center-to-center distance between adjacent circular dot matrix marks 2 was 10mm.

[0050] 2) Perform a uniaxial tensile test on the metal plate tensile specimen 1 using a tensile testing machine;

[0051] 3) Cleaning of tensile specimen 1: At the end of the tensile test, clean the oil, metal shavings or oxide scale on the surface of the specimen residue on both sides of the fracture surface 3 with anhydrous ethanol to make the indentation outline clearly visible.

[0052] 4) Measure the distance between the dot matrix marks on the surface of the specimen remnant, record the position and length of the measured circular dot matrix mark 2, and use a qualified vernier caliper to measure the center distance L1 and L2 of the two adjacent circular dot matrix marks 2 on both sides of the tensile fracture 3 of the specimen remnant on multiple tensile specimens 1.

[0053] 5) Splice the broken specimen fragments. Lay the tensile specimen 1 flat on the workbench or a special specimen fixing device. Connect the fracture surfaces 3 of the specimen fragments on both sides of the fracture surface 3 of the same tensile specimen 1, keeping the center lines of the specimen fragments on both sides coaxial. According to the number and classification of specimens, lay multiple tensile specimens flat in the same way.

[0054] 6) Take digital images of the spliced ​​specimen fragments. When taking pictures with a digital camera, the digital camera lens should be aimed at the center of the projection field of view when the specimen is laid out flat and viewed from above. The central axis of the lens should be perpendicular to the projection field of view. The purpose is to obtain digital images of the circular dot matrix marks 2 on the specimen surface that are visible to the naked eye and the tensile specimen without obvious framing distortion.

[0055] 7) Process the digital images of the specimen remnants. For the digital images of multiple specimens collected, use Photoshop software to measure the pixel values ​​P1 and P2 corresponding to the center distances L1 and L2 of the two adjacent annular dot matrix marks 2 close to the fracture 3 on both sides of the tensile fracture surface 3. Then measure the pixel value P between the centers of the annular dot matrix marks 2 with a specified gauge length. The unit is px.

[0056] 8) Calculate the elongation after fracture of the uniaxial tensile specimen of the metal plate. Calculate the average pixel line density λ of the digital images on both sides of the tensile fracture surface, which is λ = (P1 / L1 + P2 / L2) / 2, in pixels (px / mm). Based on the average pixel line density λ, calculate the specified gauge length L after fracture. u =P / λ, in mm, based on the specified gauge length L0 and the specified gauge length value L after the break. u , will L u Substituting L0 into the formula for calculating the elongation at break of a tensile specimen, we can calculate the elongation at break of the tensile specimen, A = (L0 + L0) / L0. u -L0) / L0×100%, taking a gauge length of L0 = 50mm as an example, test 3 tensile specimens, and the calculated value A of the technical solution of this invention is... 50mm The measured value A obtained by manual measurement with vernier calipers using conventional methods is different from the measured value A. 50mm The test results are shown in Table 1. The calculated deviation rate δ = (calculated value A) / (measured value A) 50mm -Measured value A 50mm ) / Measured value A 50mm The percentages are 2.4%, 2.2%, and 1.5% respectively. Calculated value A 50mm High measurement accuracy.

[0057] Example 2, Example 1, refer to Appendix Figure 1 , 2 And Appendix 2, a method for testing the elongation at fracture of a uniaxial tensile specimen of a metal plate, comprising the following steps:

[0058] 1) Tensile specimen 1 was prepared from cold-rolled enamel steel with a thickness of 1.65mm and a yield strength of 350MPa. Before the tensile test, the clamping parts at both ends of the specimen were marked and numbered. Using a qualified YD-300A manual gauge mark machine, a carbide punch was used to press conical indentation dot matrix marks 2 at equal intervals along the central axis of the specimen (i.e. the tensile direction) perpendicular to the surface of the tensile specimen. The indentation depth was 0.15mm and the center-to-center distance between adjacent indentations was 10mm.

[0059] 2) Perform a uniaxial tensile test on the metal plate tensile specimen 1 using a tensile testing machine;

[0060] 3) Cleaning of tensile specimen 1: At the end of the tensile test, clean the oil, metal shavings or oxide scale on the surface of the specimen residue on both sides of the fracture surface 3 with anhydrous ethanol to make the indentation outline clearly visible.

[0061] 4) Measure the distance between the dot matrix marks on the surface of the specimen remnant, record the position and length of the dot matrix mark 2 of the conical indentation, and use a qualified vernier caliper to measure the center distance L1 and L2 of the two adjacent conical indentation marks 2 on both sides of the tensile fracture 3 of the specimen remnant on multiple tensile specimens 1.

[0062] 5) Splice the broken specimen fragments. Lay the tensile specimen 1 flat on the workbench or a special specimen fixing device. Connect the fracture surfaces 3 of the specimen fragments on both sides of the fracture surface 3 of the same tensile specimen 1, keeping the center lines of the specimen fragments on both sides coaxial. According to the number and classification of specimens, lay multiple tensile specimens flat in the same way.

[0063] 6) Take photos of the flat-laid specimens. When taking photos, the lens should be aimed at the center of the projection field of view of the flat-laid specimens, and the central axis of the lens should be perpendicular to the projection field of view. The purpose is to obtain the visible conical indentation dot matrix mark 2 on the specimen surface and the digital image of the tensile specimen without obvious framing distortion.

[0064] 7) Process the digital images of the specimen remnants. For the digital images of multiple specimens collected, use Photoshop software to measure the pixel values ​​P1 and P2 corresponding to the center distances L1 and L2 of the two adjacent conical indentation matrix marks 2 close to the fracture surface 3 on both sides of the tensile fracture surface 3. Then measure the pixel value P between the centers of the conical indentation matrix marks 2 with a specified gauge length. The unit is px.

[0065] 8) Calculate the elongation after fracture of the uniaxial tensile specimen of the metal plate. Calculate the average pixel line density λ of the digital images on both sides of the tensile fracture surface, which is λ = (P1 / L1 + P2 / L2) / 2, in pixels (px / mm). Based on the average pixel line density λ, calculate the specified gauge length L after fracture.u =P / λ, in mm, based on the specified gauge length L0 and the specified gauge length value L after the break. u , will L u Substituting L0 into the formula for calculating the elongation at break of a tensile specimen, we can calculate the elongation at break of the tensile specimen, A = (L0 + L0) / L0. u -L0) / L0×100%, taking a gauge length of L0 = 50mm as an example, test 3 tensile specimens, and the calculated value A of the technical solution of this invention is... 50mm The measured value A obtained by manual measurement with vernier calipers using conventional methods is different from the measured value A. 50mm The test results are shown in Table 2. The calculated deviation rate δ = (calculated value A) / (measured value A) 50mm -Measured value A 50mm ) / Measured value A 50mm The percentages are 2.8%, 1.4%, and 1.9% respectively. Calculated value A 50mm High measurement accuracy.

[0066] Example 3, Example 1, refer to the appendix Figure 1 , 2 And Appendix 3, a method for testing the elongation at fracture of a uniaxial tensile specimen of a metal plate, comprising the following steps:

[0067] 1) Tensile specimen 1 was prepared from cold-rolled enamel steel with a thickness of 1.85mm and a yield strength of 210MPa. Before the tensile test, the clamping parts at both ends of the specimen were marked and numbered. Using a qualified MGM4350 electric marking machine, a carbide punch was used to punch conical indentation dot matrix marks 2 at equal intervals with an angle of 90° along the central axis of the specimen (i.e., the tensile direction) and perpendicular to the surface of the tensile specimen. The indentation depth was 0.35mm and the center-to-center distance between adjacent indentations was 10mm.

[0068] 2) Perform a uniaxial tensile test on the metal plate tensile specimen 1 using a tensile testing machine;

[0069] 3) Cleaning of tensile specimen 1: At the end of the tensile test, clean the oil, metal shavings or oxide scale on the surface of the specimen residue on both sides of the fracture surface 3 with anhydrous ethanol to make the indentation outline clearly visible.

[0070] 4) Measure the distance between the dot matrix marks on the surface of the specimen remnant, record the position and length of the dot matrix mark 2 of the conical indentation, and use a qualified vernier caliper to measure the center distance L1 and L2 of the two adjacent conical indentation marks 2 on both sides of the tensile fracture 3 of the specimen remnant on multiple tensile specimens 1.

[0071] 5) Splice the broken specimen fragments. Lay the tensile specimen 1 flat on the workbench or a special specimen fixing device. Connect the fracture surfaces 3 of the specimen fragments on both sides of the fracture surface 3 of the same tensile specimen 1, keeping the center lines of the specimen fragments on both sides coaxial. According to the number and classification of specimens, lay multiple tensile specimens flat in the same way.

[0072] 6) Take photos of the flat-laid specimens. When taking photos, the lens should be aimed at the center of the projection field of view of the flat-laid specimens, and the central axis of the lens should be perpendicular to the projection field of view. The purpose is to obtain the visible conical indentation dot matrix mark 2 on the specimen surface and the digital image of the tensile specimen without obvious framing distortion.

[0073] 7) Process the digital images of the specimen remnants. For the digital images of multiple specimens collected, use Photoshop software to measure the pixel values ​​P1 and P2 corresponding to the center distances L1 and L2 of the two adjacent conical indentation matrix marks 2 close to the fracture surface 3 on both sides of the tensile fracture surface 3. Then measure the pixel value P between the centers of the conical indentation matrix marks 2 with a specified gauge length. The unit is px.

[0074] 8) Calculate the elongation after fracture of the uniaxial tensile specimen of the metal plate. Calculate the average pixel line density λ of the digital images on both sides of the tensile fracture surface, which is λ = (P1 / L1 + P2 / L2) / 2, in pixels (px / mm). Based on the average pixel line density λ, calculate the specified gauge length L after fracture. u =P / λ, in mm, based on the specified gauge length L0 and the specified gauge length value L after the break. u , will L u Substituting L0 into the formula for calculating the elongation at break of a tensile specimen, we can calculate the elongation at break of the tensile specimen, A = (L0 + L0) / L0. u -L0) / L0×100%, taking a gauge length of L0 = 50mm as an example, test 3 tensile specimens, and the calculated value A of the technical solution of this invention is... 50mm The measured value A obtained by manual measurement with vernier calipers using conventional methods is different from the measured value A. 50mm The test results are shown in Table 3. The calculated deviation rate δ = (calculated value A) / (measured value A) 50mm -Measured value A 50mm ) / Measured value A 50mm The percentages are 1.6%, 0.4%, and 3.1% respectively. Calculated value A 50mm Higher measurement accuracy.

[0075] Table 1 Test parameters of uniaxial tensile specimens of metal plates in Example 1

[0076]

[0077] Table 2 Test parameters of uniaxial tensile specimens of metal plates in Example 2

[0078]

[0079] Table 3 Test parameters of uniaxial tensile specimens of metal plates in Example 3

[0080]

[0081] As shown in Tables 1, 2, and 3, compared with the prior art, due to the reduction of manual measurement errors, the elongation after fracture (A) of the uniaxial tensile specimens of metal plates in Examples 1-3 is significantly improved. 50 mm The calculated values ​​are more accurate. This method enables the preservation of digital images of the fracture surface residue of tensile specimens and ensures the traceability of measurement data.

[0082] In addition to the embodiments described above, the present invention may have other implementations. All technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope claimed by the present invention.

Claims

1. A method of testing the elongation at break of a uniaxially stretched test specimen of a sheet metal, characterized in that, The method includes the following steps: 1) Prepare a uniaxial tensile specimen of metal plate, mark and number the clamping parts at both ends of the specimen, and make equally spaced dot matrix marks on the surface of the tensile specimen along the central axis of the specimen; the dot matrix marks can be any one of lines, crosses, squares, circles and indentations. 2) Perform uniaxial tensile tests on the specimens. Use a tensile testing machine to perform uniaxial tensile tests on the metal plate specimens. 3) Clean the broken specimen and clean the surface of the specimen residue on both sides of the fracture to make the dot matrix markings on the surface of the specimen residue clearly visible. 4) Measure the distance between the lattice marks on the surface of the specimen residue, and measure the center-to-center distance L between the two lattice marks on the specimen residue near the fracture surface, respectively i , i = 1, 2; 5) Splice the broken specimen fragments. Lay the tensile specimen flat on the workbench and butt the specimen fragments on both sides of the same specimen fracture along the fracture, keeping the center lines of the specimen fragments on both sides of the fracture coaxial. 6) Take digital images of the spliced ​​specimen fragments. When taking pictures with a digital camera, the digital camera lens should be aimed at the center of the field of view of the spliced ​​specimen, and the central axis of the digital camera lens should be perpendicular to the field of view. 7) Process the digital image of the specimen remnant and use drawing software to measure the center distance L between two adjacent dot matrix marks near the fracture surface of the specimen remnant. i The corresponding pixel value P i , i=1,2; then measure the corresponding pixel value P for the center distance of the dot matrix markers with the specified gauge length defined by the conventional method, in pixels; 8) Calculate the elongation at fracture of the uniaxial tensile specimen of the metal plate. The elongation at fracture of the uniaxial tensile specimen of the metal plate is calculated according to Formula 1. A = (L u - L0) / L0 x 100% Equation 1 In Formula 1, A is the elongation after fracture of the uniaxial tensile specimen of metal plate, in %; L0 is the specified gauge length, in mm; L u The specified gauge length is given after the fracture, in mm; L u Calculated according to Formula 2, L u = P / λ Equation Two; In Formula 2, P is the pixel value corresponding to the center distance of the dot matrix marks at the specified gauge length, in pixels (px); λ is the average pixel line density of the digital images on both sides of the tensile fracture, in pixels (px / mm); λ is calculated according to Formula 3. λ = (P1 / L1 + P2 / L2) / 2 (Formula 3) In Formula 3, P1 and P2 are the pixel values ​​corresponding to the center distance of the two dot matrix marks on the surface of the specimen residue on both sides of the fracture, respectively, in pixels (px); L1 and L2 are the center distance of the two dot matrix marks on the surface of the specimen residue on both sides of the fracture, respectively, in mm.

2. The method of testing the elongation after fracture of a uniaxially stretched sheet metal test specimen according to claim 1, characterized in that The dot matrix markings are indentations, which are conical indentations with a 90° included angle, an indentation depth of 0.15–0.5 mm, and an adjacent spacing of 10 mm between the centers of the indentations.

3. The method of testing the elongation after fracture of a uniaxially-stretched sheet metal test specimen according to claim 1, characterized in that, The digital image has the same horizontal and vertical resolution, with a horizontal resolution ≥ 96 dpi.

4. The method of testing the elongation after fracture of a uniaxially-stretched sheet metal test specimen according to claim 1, characterized in that, In step 4), L1 and L2 are the center distances between the two adjacent lattice marks closest to the fracture surface of the specimen residue on both sides of the tensile fracture.

5. The method of testing the elongation after fracture of a uniaxially-stretched sheet metal test specimen according to claim 1, characterized in that, In step 7), P1 / L1 and P2 / L2 are equal or approximately equal within the allowable error range; otherwise, it is necessary to remeasure or retake the digital image.

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