Method for measuring stiffness coefficient of tensile testing machine

Abstract

The invention belongs to the field of mechanical property detection of metal materials, and particularly relates to a method for measuring a stiffness coefficient of a tensile testing machine, which comprises the following steps of: 1) room-temperature tensile test: measuring the value of elastic modulus E1 of a tensile sample according to the scale length by using an extensometer; stopping the test at the proportional elasticity stage of the tensile sample; unloading the tensile sample; (2) the tensile testing machine adopts the cross beam platform to carry out the room-temperature tensile test again by using the tensile sample according to the scale distance length, and the elastic modulus E2 is measured; and 3) calculating the stiffness coefficient K of the tensile testing machine according to the elasticity modulus values measured twice. The method has the advantages that the stiffness coefficient of the high-rigidity tensile testing machine can be accurately, simply and conveniently measured, and quantitative separation and measurement of linear deformation of the testing machine are realized through twice elastic tensile tests of the same sample.

Description

Technical Field

[0001] This invention belongs to the field of mechanical property testing of metallic materials, and particularly relates to a method for determining the tensile strength coefficient of a tensile testing machine. Background Technology

[0002] When performing tensile tests according to GB / T228.1 (Metallic materials, tensile testing - Part 1: Test method at room temperature), traditional room temperature tensile testing machines generally use hydraulic cylinders with steel bars to apply loads. It is a flexible manufacturing unit, and the deformation of the tensile testing machine itself is a non-linear deformation during the entire tensile test.

[0003] With the development and introduction of hydraulic double-sided flat-push electronic tensile testing machines, which are gradually replacing traditional hydraulic tensile testing machines, the tensile testing process at room temperature is no longer nonlinear, but exhibits sufficient rigidity and linearity.

[0004] Tensile testing of metallic materials at room temperature is one of the methods for evaluating the mechanical properties of materials, and its test results are widely used in materials research and development, quality control, and engineering selection. my country's current national standard GB / T228.1, "Metallic materials—Tensive testing—Part 1: Test at room temperature," provides detailed specifications for the equipment, methods, and procedures for this type of test. During the test, obtaining accurate stress-strain curves, especially precise data in the elastic phase, is crucial for determining key parameters such as the material's elastic modulus and yield strength.

[0005] Traditional tensile testing machines, especially those employing hydraulic cylinder loading and a force-transmitting steel bar, essentially constitute a "flexible" force transmission system. During load application, the machine's frame, cylinders, and connecting components undergo non-negligible elastic and even minute nonlinear deformations. This deformation is superimposed on the specimen's deformation and is collected by the measurement system. When measuring the elastic modulus of a material, directly measuring the specimen's deformation using an extensometer mounted on the specimen can effectively isolate the influence of the testing machine's deformation. However, in certain specific testing scenarios or when using certain automated measuring devices (such as beam displacement), the machine's own deformation can be introduced, resulting in significant measurement errors and causing the measured "apparent" elastic modulus to be lower than the material's true value. This systematic error introduced by the machine's structural flexibility is difficult to correct simply on traditional equipment because its deformation behavior is often nonlinear.

[0006] In recent years, with the development of testing machine technology, new types of electronic tensile testing machines, such as hydraulic double-sided push-type and fully digital closed-loop servo control machines, have been widely used. These machines employ symmetrical loading and a high-rigidity frame design, significantly improving the rigidity of their force transmission system. Within the elastic deformation range of the material, the deformation of the testing machine itself exhibits good linear characteristics. This advancement provides new possibilities for the precise separation and quantification of the deformation of the testing machine system. If the linear deformation characteristics of the testing machine system under force can be accurately measured, i.e., its "stiffness coefficient" or "frame stiffness," high-precision material deformation data can be obtained through calculation correction, even when using indirect measurement methods such as beam displacement. This expands the flexibility of equipment use and improves the accuracy of some automated testing processes. Summary of the Invention

[0007] To overcome the shortcomings of the prior art, the purpose of this invention is to provide a method for determining the tensile strength coefficient of a tensile testing machine, which can determine the tensile strength coefficient of a tensile testing machine with linear deformation, and facilitates room temperature tensile testing of metallic materials.

[0008] To achieve the above objectives, the present invention provides the following technical solution: A method for determining the tensile strength coefficient using a tensile testing machine includes the following steps: 1) Room temperature tensile test: Measure the elastic modulus E1 of the tensile specimen using an extensometer at gauge length; stop the test at the proportional elastic stage of the tensile specimen; unload the tensile specimen. 2) The tensile testing machine uses a beam platform to perform a room temperature tensile test on the same tensile specimen again at the gauge length, and the elastic modulus E2 is measured; the tensile testing machine itself should have sufficient rigidity; 3) Based on the two measured elastic modulus values, the tensile testing machine stiffness coefficient K2 is calculated: (1) In equation (1), K is the tensile strength coefficient of the tensile testing machine, in N / mm; S is the original cross-sectional area of ​​the tensile specimen, in mm; E1 is the elastic modulus measured by the extensometer, in MPa; E2 is the elastic modulus measured by the crossbeam platform, in MPa; and L is the gauge length, in mm.

[0009] During room temperature tensile testing, both the elastic moduli E1 and E2 satisfy the following conditions: (2) In equation (2), E is the elastic modulus in MPa; F is the force value corresponding to the elongation ΔL of the tensile specimen in N; L is the gauge length in mm; ΔL is the elongation of the specimen in the proportional elastic stage in mm; and S is the original cross-sectional area of ​​the tensile specimen in mm². 2 .

[0010] Compared with the prior art, the beneficial effects of the present invention are: This invention enables precise and convenient determination of the stiffness coefficient of a high-rigidity tensile testing machine. Through two elastic tensile tests on the same specimen, the linear deformation of the testing machine itself is quantitatively separated and measured. Based on the measurement results, systematic errors generated by indirect deformation measurement methods such as beam displacement can be directly corrected. This significantly improves the measurement accuracy of key parameters such as the material's elastic modulus without relying on an extensometer, ensuring the consistency and traceability of data across different measurement modes. This method is simple to operate, low in cost, and the entire process is completed within the specimen's elastic range, constituting a non-destructive measurement, and is applicable to various types of high-rigidity electronic tensile testing machines. Attached Figure Description

[0011] Figure 1 This is a graph showing the elastic modulus E1 of a tensile specimen measured with an extensometer.

[0012] Figure 2 This is a graph showing the elastic modulus E2 of the tensile specimen measured using a beam platform.

[0013] Figure 3 This is a graph showing the elastic modulus E2 of the tensile specimen measured using a beam platform. Detailed Implementation

[0014] The present invention will now be described in detail, but it should be noted that the implementation of the present invention is not limited to the following embodiments.

[0015] A method for determining the tensile strength coefficient using a tensile testing machine includes the following steps: 1) Room temperature tensile test: Measure the elastic modulus E1 of the tensile specimen using an extensometer at gauge length; stop the test at the proportional elastic stage of the tensile specimen; unload the tensile specimen. 2) The tensile testing machine uses a crossbeam platform to perform a room temperature tensile test on the same tensile specimen again at the gauge length, and the elastic modulus E2 is measured. 3) Based on the two measured elastic modulus values, the tensile testing machine stiffness coefficient K2 is calculated: (1) In formula (1), K is the tensile strength coefficient of the tensile testing machine, in N / mm; S is the cross-sectional area of ​​the tensile specimen, in mm; E1 is the elastic modulus measured by the extensometer, in MPa; E2 is the elastic modulus measured by the crossbeam platform, in MPa; and L is the gauge length, in mm.

[0016] Example: A method for determining the tensile strength coefficient using a tensile testing machine includes the following steps: 1) The elastic modulus E1 of the specially made tensile specimen was measured using an extensometer with a gauge length of 50 mm. The elongation of the tensile specimen was ΔL. The test was then stopped at this point, ensuring the specially made tensile specimen was in its proportional elastic phase, and the specimen was unloaded. The tensile specimen was restored to its initial state, and its dimensions matched the original measured dimensions. Figure 1 It can be seen that the elastic modulus E1 measured by the extensometer is 201 GPa.

[0017] The tensile specimen is made of high-strength steel (such as AHSS steel, HSLA steel, AHSS steel), and its dimensions are such that the force value of the yield strength is compatible with the range of the tensile testing machine. The tensile specimen must first be annealed to eliminate internal stress. It is then processed into a strip without shoulders, and the shape tolerance is less than 0.1 mm.

[0018] 2) Remove the extensometer and, using a beam platform with a gauge length of 50 mm, conduct another room temperature tensile test on the same specimen, measuring the elastic modulus E2; the elongation of the specimen is ΔL1. At this point, ensure the specimen is in its proportional elastic phase, then stop the test and unload the specimen. The specimen should be restored to its initial state, with dimensions consistent with the original measured dimensions. Figure 2 , Figure 3 It can be seen that the elastic modulus E of the beam platform is 39 GPa. Figure 2 Samples 3 and 4 have values ​​of 38.7 and 39.0 respectively. Multiple measurements of different samples yielded consistent results. 3) At this time, under the same force value F (the same force value as in step 2), the elongation of the tensile specimen is still ΔL1 (ΔL=ΔL1), but the testing machine itself elongates to ΔL2 under the applied load, i.e., F / K.

[0019] At this point, the elastic modulus E2 = F × L / [S × (F / K + ΔL)], combined with E1 = (F × L) / (S × ΔL), we can obtain the tensile strength coefficient K = E1 × E2 × S / [L (E1 - E2)], where E1 and E2 can be obtained from the testing software of the testing machine after two tests, S is the cross-sectional area of ​​the tensile specimen, and L is 50 mm.

[0020] The tensile strength coefficient K of the tensile testing machine can be calculated, and the following can be obtained: K=E1×E2×S / [L×(E1-E2)]=201.4*10 3 *38.7*10 3 *18.17*25.13 / [50*(201.4*10 3- 38.7*10 3 )]=437482N / mm.

[0021] This invention enables precise and convenient determination of the stiffness coefficient of a high-rigidity tensile testing machine. Through two elastic tensile tests on the same specimen, the linear deformation of the testing machine itself is quantitatively separated and measured. Based on the measurement results, systematic errors generated by indirect deformation measurement methods such as beam displacement can be directly corrected. This significantly improves the measurement accuracy of key parameters such as the material's elastic modulus without relying on an extensometer, ensuring the consistency and traceability of data across different measurement modes. This method is simple to operate, low in cost, and the entire process is completed within the specimen's elastic range, constituting a non-destructive measurement, and is applicable to various types of high-rigidity electronic tensile testing machines.

Claims

1. A method for determining the tensile strength coefficient using a tensile testing machine, characterized in that, Includes the following steps: 1) Room temperature tensile test: Measure the elastic modulus E1 of the tensile specimen using an extensometer at gauge length; stop the test at the proportional elastic stage of the tensile specimen; unload the tensile specimen. 2) The tensile testing machine uses a crossbeam platform to perform a room temperature tensile test on the same tensile specimen again at the gauge length, and the elastic modulus E2 is measured. 3) Based on the two measured elastic modulus values, the tensile strength coefficient K of the tensile testing machine is calculated: （1） In equation (1), K is the tensile strength coefficient, in N / mm, and S is the original cross-sectional area of ​​the tensile specimen, in mm. 2 E1 is the elastic modulus measured by an extensometer, in MPa; E2 is the elastic modulus measured by a beam platform, in MPa; L is the original gauge length, in mm.

2. The method for determining the tensile strength coefficient using a tensile testing machine according to claim 1, characterized in that, During room temperature tensile testing, both the elastic moduli E1 and E2 satisfy the following conditions: （2） In equation (2), E is the elastic modulus in MPa; F is the force value corresponding to the elongation ΔL of the tensile specimen in N; L is the gauge length in mm; ΔL is the elongation of the specimen in the proportional elastic stage in mm; and S is the original cross-sectional area of ​​the tensile specimen in mm². 2 .

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