Room-temperature tensile test method for replacing extensometer

Abstract

The invention belongs to the field of metal material testing, and particularly relates to a room-temperature tensile testing method for replacing an extensometer, the position of a cross beam platform is changed, so that the distance between clamps is larger than the actual distance between the clamps, and the rigid strain of a testing machine in the loading process is counteracted by using the difference between the two. The method has the advantages that the dependence on the extensometer is completely eliminated through mathematical modeling and rigidity compensation of the testing machine; firstly, a shoulder-free strip-shaped sample is combined with high-precision milling or grinding, so that the size consistency of the sample is ensured; secondly, setting the distance between clamps to be equal to the gauge length of the sample and introducing a beam platform position correction formula to accurately offset the elastic deformation of the testing machine in the loading process, so that the real strain of the sample is reversely deduced by directly utilizing beam displacement data, and a stress-strain curve is drawn according to the real strain; therefore, various parameters related to linear deformation and overall deformation can be accurately measured.

Description

Technical Field

[0001] This invention belongs to the field of metal material testing, and in particular relates to a room temperature tensile testing method that replaces an extensometer. Background Technology

[0002] Room temperature tensile testing of metallic materials is the most fundamental and standard method for evaluating their basic mechanical properties, with yield strength being one of the most critical parameters in engineering design and material selection. According to the national standard GB / T228.1 (Metallic materials, tensile testing—Part 1: Room temperature test method), for materials exhibiting a clear physical yield phenomenon, their upper and lower yield strengths can be directly measured. However, with the continuous development and application of high-strength steel, advanced high-strength steel (AHSS), ultra-high-strength steel, and various high-value-added alloy steels, these materials often do not exhibit a clear yield plateau during tensile testing, and their stress-strain curves show continuous yield characteristics. In such cases, yield strength can only be determined by measuring R... p0.2 R t0.5 Parameters such as these are used to scientifically characterize its "yielding" behavior.

[0003] There are many types of extensometers, including manual, fully automatic mechanical, laser, and video extensometers. However, existing extensometer technologies generally suffer from the following inherent defects and bottlenecks: 1. Accuracy Decline and Long-Term Reliability Issues: As precision electromechanical devices, contact extensometers (strain gauge type, LVDT type, etc.) inevitably experience wear, loosening, or drift in their blades, bearings, and sensors under long-term, high-frequency fatigue use. This leads to a gradual decrease in measurement accuracy, requiring frequent and expensive calibration and maintenance, increasing laboratory operating costs and data risks. While non-contact extensometers (such as video extensometers) avoid contact wear, they are extremely sensitive to sample surface markings, lighting conditions, and lens cleanliness, requiring high environmental stability. In actual industrial testing environments, they are difficult to maintain optimal performance at all times, and similarly face the risk of accuracy fluctuations.

[0004] 2. Technological Dependence and Cost Monopoly Issues: High-precision, high-reliability extensometers, especially models suitable for complex testing environments (high and low temperatures, high speeds), have long been subject to patent restrictions and control over their core sensing technologies, control algorithms, and advanced manufacturing processes by companies in developed countries. Domestic testing institutions and enterprises mostly have to purchase complete products directly at high prices or pay technology licensing fees, which not only significantly increases the cost of materials research and development and quality control but also creates technological dependence on key testing equipment, posing supply chain security risks.

[0005] 3. The problem of separation from the testing machine system: As an external measurement system independent of the universal testing machine's main unit (beam, load sensor), the extensometer requires additional manual operation and parameter settings for installation, calibration, and use. This "main unit + external device" model suffers from low system integration and complex operation procedures when acquiring data on the overall deformation of the specimen (beam displacement) and the precise local deformation (extensometer). More importantly, the testing machine's main frame, fixtures, and transmission system also undergo elastic deformation (system flexibility) under stress. This deformation is mixed in with the beam displacement signal, making the directly read beam displacement much larger than the actual deformation within the specimen's gauge length, thus making it impossible to directly use it to calculate strain. This is precisely why the extensometer is essential.

[0006] Therefore, the industry urgently needs a new testing method that can overcome the above limitations, abandon the reliance on independent extensometers, and directly and accurately calculate the true strain within the gauge length of the specimen using only the signals from the testing machine itself (load, beam displacement) through a rigorous mathematical model and correction program, thereby achieving R... p0.2 R t0.5 Independent, high-precision, and low-cost measurement of key parameters. Summary of the Invention

[0007] To overcome the shortcomings of existing technologies, the purpose of this invention is to provide a room temperature tensile testing method that replaces an extensometer, facilitating room temperature tensile testing of metallic materials and accurately determining the R0 of metallic materials without relying on an extensometer. p0.2 R t0.5 Parameters such as these.

[0008] To achieve the above objectives, the present invention provides the following technical solution: A room temperature tensile testing method that replaces the extensometer involves changing the position of the crossbeam platform so that the displayed value of the clamp spacing on the testing machine software is larger than the actual clamp spacing. The difference between the two is used to offset the rigid strain of the testing machine itself during loading.

[0009] Specifically, the following steps are included: 1) Sample preparation: The tensile test specimens are machined into strip-shaped specimens without milling out the shoulders; the shape, dimensional tolerances and surface finish of the tensile test specimens shall meet the requirements of GB / T228.1; 2) The clamp spacing is set to the original gauge length L0 of the specimen, and a room temperature tensile test is performed using a beam platform method; 3) Change the position of the crossbeam platform in the testing machine software to ensure the fixture spacing meets the requirements: Fixture spacing = L0 + E1 × S / K (1) In equation (1), K is the tensile strength coefficient when the testing machine applies the load, in N / mm; L0 is the original gauge length of the specimen, in mm; and S is the original cross-sectional area of ​​the specimen, in mm.2 E1 is the elastic modulus measured by the extensometer, in MPa; Equation (1) is the mathematical model established after compensating for the rigidity of the testing machine.

[0010] That is, after the position of the crossbeam platform is modified, the clamp spacing is larger than the actual clamp spacing value. The rigid strain of the testing machine itself during the loading process is offset by mathematical calculation. The specific intermediate calculation can be obtained. 4) The tensile testing machine itself has sufficient rigidity. After the test, all the measured performance parameters are based on the values ​​corrected by the testing machine according to the elastic modulus E.

[0011] The tensile specimen is a specimen with a rectangular or circular cross-section, and the clamping end of the tensile specimen is rectangular or circular.

[0012] Compared with the prior art, the beneficial effects of the present invention are: This invention completely eliminates the reliance on extensometers through mathematical modeling and rigidity compensation of the testing machine. First, the use of shoulderless strip-shaped specimens combined with high-precision grinding ensures consistent specimen dimensions. Second, by setting the fixture spacing equal to the specimen gauge length and introducing a correction formula for the crossbeam platform position, the elastic deformation of the testing machine during loading is precisely offset, allowing direct deduction of the specimen's true strain from the crossbeam displacement data. This method not only avoids the accuracy degradation caused by long-term use of extensometers and reduces testing costs, but more importantly, its corrected measurement results meet the requirements of GB / T228.1 standard for R... p0.2 R t0.5 The high-precision measurement of key parameters provides a reliable and independent alternative for evaluating the mechanical properties of high-value-added metallic materials. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the tensile specimen.

[0014] Figure 2 This is a schematic diagram of the location of the crossbeam platform.

[0015] Figure 3 This is a display diagram of the Rt0.5 value tester obtained by the method of this invention.

[0016] Figure 4 This is a graph showing the Rt0.5 value measured using an extensometer.

[0017] In the diagram: 1. Scale; 2. Position of the crossbeam platform; 3. Crossbeam platform; 4. Upper clamp; 5. Lower clamp; 6. Sample. Detailed Implementation

[0018] The present invention will now be described in detail with reference to the accompanying drawings, but it should be noted that the implementation of the present invention is not limited to the following embodiments.

[0019] A room temperature tensile testing method that replaces an extensometer includes the following steps: 1) Sample Preparation: The tensile specimen is machined into a strip with a rectangular cross-section. The clamping end of the tensile specimen is also rectangular, without milling out the shoulder. The shape, dimensional tolerances, and surface finish of the tensile specimen shall conform to the requirements of GB / T228.1 to ensure accuracy. See [link to relevant documentation]. Figure 1 .

[0020] 2) The clamp spacing is set to the original gauge length L0 of the specimen (e.g., 50 mm), and a room temperature tensile test is performed using a beam platform method; see Figure 2 The upper clamp 4 and the lower clamp 5 are respectively clamped at the upper and lower ends of the tensile specimen 6. The distance between the non-clamped parts clamped between the clamps is measured. This distance is equal to the original gauge length L0 of the specimen (e.g., 50 mm), and the position 2 of the crossbeam platform is recorded at this time.

[0021] 3) Change the position of the crossbeam platform, see Figure 2 This ensures that the clamp spacing satisfies: Fixture spacing = L0 + E1 × S / K (1) In equation (1), K is the tensile strength coefficient when the testing machine applies the load, in N / mm; L0 is the original gauge length of the specimen, in mm; and S is the original cross-sectional area of ​​the specimen, in mm. 2 E1 is the elastic modulus measured by the extensometer, in MPa. In reality, the clamp spacing is equal to the gauge length, but the position of the crossbeam platform needs to be modified so that the clamp spacing is larger than the actual clamp spacing. The reason for this is that the rigid strain of the testing machine itself during the loading process needs to be offset through mathematical calculations. The specific intermediate calculations can be obtained.

[0022] The method for determining the K value includes the following steps: 1) Room temperature tensile test: The elastic modulus E of the tensile specimen is measured using an extensometer at the gauge length. 11 The value of the tensile specimen; stop the test during the proportional elastic phase 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 under the applied load by the tensile testing machine is calculated: (2) In equation (2), K is the tensile strength coefficient when the tensile testing machine applies the load, in N / mm; S is the cross-sectional area of ​​the tensile specimen, in mm; E 11 E1 is the elastic modulus measured using an extensometer, in MPa; E2 is the elastic modulus measured using a beam platform, in MPa; L is the gauge length, in mm.

[0023] 4) The crossbeam platform 3 moves upward at the set distance and speed. After stopping, the stress-strain curve is fully displayed on the tensile testing machine software.

[0024] 5) After the test, all measured performance parameters shall be based on the corrected values ​​(the computer software attached to the testing machine corrects itself according to the elastic modulus E).

[0025] The performance parameters are similar to those measured with an extensometer. Taking the measured Rt0.5 value as an example, from... Figure 3 , Figure 4 It can be seen that the Rt0.5 value measured by the beam platform is 515.18, which is comparable to the Rt0.5 value of 509.88 measured by the extensometer. This shows that the method of the present invention can replace the extensometer for tensile testing.

Claims

1. A room temperature tensile testing method that replaces an extensometer, characterized in that, The position of the crossbeam platform is changed so that the displayed value of the clamp spacing on the testing machine software is larger than the actual clamp spacing. The difference between the two is used to offset the rigid strain of the testing machine itself during the loading process.

2. The room temperature tensile testing method for replacing an extensometer according to claim 1, characterized in that, Includes the following steps: 1) Sample preparation: The tensile test specimens are machined into strip-shaped specimens without milling out the shoulders; the shape, dimensional tolerances and surface finish of the tensile test specimens shall meet the requirements of GB / T228.1; 2) The clamp spacing is set to the original gauge length L0 of the specimen, and a room temperature tensile test is performed using a beam platform method; 3) Change the position of the crossbeam platform in the testing machine software to ensure the fixture spacing meets the requirements: Fixture spacing = L0 + E1 × S / K (1) In equation (1), K is the tensile strength coefficient when the testing machine applies the load, in N / mm; L0 is the original gauge length of the specimen, in mm; and S is the original cross-sectional area of ​​the specimen, in mm. 2 E1 is the elastic modulus measured by the extensometer, in MPa. 4) The tensile testing machine itself has sufficient rigidity. After the test, all the measured performance parameters are based on the values ​​corrected by the testing machine according to the elastic modulus E.

3. The room temperature tensile testing method for replacing an extensometer according to claim 2, characterized in that, The tensile specimen is a specimen with a rectangular or circular cross-section, and the clamping end of the tensile specimen is rectangular or circular.

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