Method for testing compression yield strength of hull steel through variable cross-section sample

Abstract

According to the method for testing the compression yield strength of the ship body steel through the variable cross-section samples, the variable cross-section samples with different sizes can be designed for structural steel with different yield strengths through numerical simulation design, so that main deformation occurs in parallel sections, uniform deformation of the parallel sections is ensured, local shear deformation is avoided, and the compression yield strength of the ship body steel is tested. The influence of a barrel-shaped effect caused by friction of a traditional round bar sample on the compression yield strength is fundamentally avoided, the compression yield strength of the ship body steel can be effectively measured, and the data reliability is high; meanwhile, the measuring method is simple, the requirements for tools and equipment are low, measurement can be conducted by means of a universal testing machine, and tools do not need to be independently designed and manufactured.

Description

Technical Field

[0001] The present invention relates to the technical field of compressive yield strength testing, and in particular to a method for testing the compressive yield strength of hull steel by using a variable-section specimen. Background Art

[0002] The yield strength of hull structural steel is an important parameter in ship design, directly determining several core design criteria, including steel grade, location of use, and stress levels in the hull structure. For ideal elastic-plastic steel, its tensile yield strength and compressive yield strength are theoretically consistent, but hull structural steel is not an ideal elastic-plastic material. Due to rolling direction control, its internal grain orientations are different, and there are more or less inclusions, intercrystalline defects, and other phenomena, which can lead to slight differences in the tensile and compressive yield strengths of hull steel. The shipbuilding industry typically uses tensile tests to obtain the yield strength of hull steel, which can meet the design requirements of general surface ships, because surface ship structures are usually tensile stresses. However, for submersibles and large underwater structures, their pressure-resistant structures are under pressure under the action of water pressure. Using the yield strength obtained from tensile tests for structural design will inevitably pose a safety hazard. Compression tests are required to measure the yield strength of steel during design and material selection.

[0003] The compression strength test of steel refers to GB-T7314-2017 "Metallic Materials Room Temperature Compression Test Method", and usually a 10mm diameter round bar specimen is used for compression test ( Figure 1 (as shown), specimen length 20-30mm. Due to the small specimen size, separate tooling is required for testing. When radially deforming the upper and lower ends of uniform cross-section compression specimens, they are subject to friction from the tooling, resulting in smaller radial deformation at the ends than in the middle of the specimen, creating a "barrel-shaped" effect. This "barrel-shaped" effect caused by friction at the specimen ends can result in an inflated measured yield strength. Furthermore, due to the small size of standard specimens, accurate deformation measurement using an extensometer is difficult, and measurement using only a displacement meter or other instrumentation further reduces the reliability of the measured compressive strength data.

[0004] Patent CN110595889A - A material compressive yield strength test method uses a specimen structure with a smooth cylindrical end and a thickened diameter. Although the thickening prolongs the bending time, it does not fundamentally solve the stress concentration problem. Its uniform cross-section design causes end friction during loading, triggering a "barrel effect."

[0005] In order to meet the safety design requirements of various submersibles and large underwater structures, a more complete compression strength testing method is needed, but current research does not have the relevant technology to measure compressive yield strength through simple and easy methods. Summary of the Invention

[0006] In view of this, the present invention aims to propose a method for testing the compressive yield strength of hull steel using a variable-section specimen to solve the problems in the prior art of complex compressive yield strength testing methods and low data accuracy for various submersibles and large underwater structures.

[0007] To achieve the above object, the technical solution of the present invention is achieved as follows:

[0008] A method for testing the compressive yield strength of hull steel using a variable-section specimen comprises the following steps: S1: in simulation software, selecting hull steel to create a simulation model of a variable-section specimen, wherein the variable-section specimen comprises a clamping section, a transition section, and a parallel section connected in sequence, wherein the clamping section is used for being fixed in conjunction with a universal testing machine, the transition section is used for connecting the clamping section and the parallel section, and the parallel section is used for installing an extensometer, wherein one end of the variable-section specimen is constrained while the other end remains free, and the Euler critical load A of the parallel section is calculated using the simulation model, and then proceeding to S2;

[0009] S2: According to the formula Calculate the Euler critical load B of the parallel segment and proceed to S3;

[0010] S3: Judgment Is it true? If so, go to S4. If not, go to S1. S4: According to the formula Calculate the yield load C of the parallel segment and proceed to S5;

[0011] S5: Determine whether C < A is true. If so, go to S6; if not, go to S1.

[0012] S6: taking a variable-section specimen from a test plate according to the size of the variable-section specimen simulation model, and proceeding to S7; S7: performing a compression test on the variable-section specimen using a universal testing machine, and obtaining the compressive yield strength of the parallel section according to the stress-strain curve;

[0013] Where E is the elastic modulus, L is the total length of the variable cross-section specimen, d is the diameter of the parallel section, and σ s is the nominal yield strength of the material.

[0014] Furthermore, in step S2, when the variable cross-section specimen 100 is subjected to a compression test, one end of the variable cross-section specimen 100 is fixed and the other end is free. According to the Euler compression rod instability critical load formula:

[0015]

[0016] The formula for the moment of inertia of a cylindrical section is:

[0017]

[0018] According to formula (1) and (2), we can get the formula:

[0019]

[0020] Among them, π is the mathematical constant pi, E is the elastic modulus, I is the area moment of inertia, and μ is the length factor, μ=2.

[0021] Furthermore, the length of the clamping section is X=50 mm, and the length of the parallel section is 20 mm<Y≤30 mm.

[0022] Furthermore, the diameter of the clamping section is D, the diameter of the parallel section is d, and D=2d.

[0023] Furthermore, the transition section is an arc, the radius of the arc is R, and 36mm≤R≤64mm.

[0024] Furthermore, variable cross-section specimens with parallel section diameters of 8 mm, 9 mm, and 10 mm were taken from the transverse and longitudinal directions of the test plate, respectively, and compression tests were performed on them. The coefficients of variation of the transverse and longitudinal yield strengths were all less than 1%.

[0025] Furthermore, d=10 mm, Y=30 mm, D=20 mm, R=40 mm, and L=169 mm.

[0026] Furthermore, the simulation software is ANSYS Workbench.

[0027] Compared with the prior art, the method of testing the compressive yield strength of hull steel using a variable-section specimen of the present invention has the following advantages:

[0028] 1) It can effectively measure the compressive yield strength of hull steel with high data reliability. Through numerical simulation design, it is possible to design variable-section specimens of different sizes for structural steels with different yield strengths, so that the main deformation occurs in the parallel section, ensuring uniform deformation of the parallel section and avoiding local shear deformation. This fundamentally avoids the influence of the "barrel-shaped" effect caused by friction on the compressive yield strength of traditional round bar specimens;

[0029] 2) The variable cross-section specimen designed in this application is compatible with extensometers, with high measurement accuracy and low data dispersion;

[0030] 3) The measurement method is simple, and the requirements for tooling and equipment are low. It can be done with the help of a commonly used universal testing machine, without the need to design and manufacture tooling separately;

[0031] 4) It has strong adaptability and can measure specimens with different yield strengths. This can be done in a laboratory environment without entrusting relevant institutions to conduct residual stress tests, saving a lot of money. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a schematic structural diagram of a uniform cross-section specimen according to an embodiment of the present invention;

[0033] Figure 2 Schematic diagram of the structure of the variable cross-section specimen according to an embodiment of the present invention;

[0034] Figure 3-4 Schematic diagram of sampling positions of a variable cross-section specimen according to an embodiment of the present invention;

[0035] Figure 5 Schematic diagram of sampling positions for uniform cross-section specimens according to an embodiment of the present invention.

[0036] Description of reference numerals:

[0037] 1. Clamping section; 2. Transition section; 3. Parallel section; 100. Variable cross-section specimen. DETAILED DESCRIPTION

[0038] To make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be further described in detail below with reference to the embodiments. It should be understood that the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. The specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention. It should be noted that, unless there is a conflict, the features in the embodiments and embodiments of the present invention may be combined with each other.

[0039] Example 1

[0040] Standard small-scale compression tests suffer from reduced measurement accuracy and increased data dispersion due to friction caused by tooling. Therefore, it is necessary to develop a larger, variable-section specimen to avoid the drawbacks of small-scale tests. This application addresses this need and proposes a method for testing the compressive yield strength of hull steel using a variable-section specimen. This method is validated through numerical simulation and testing.

[0041] The purpose of this application is achieved as follows: with reference to the round bar tensile test, a variable cross-section specimen 100 is designed with a clamping section 1, a transition section 2 and a parallel section 3, as shown in FIG. Figure 2As shown, the diameters of the clamping section 1, transition section 2, and parallel section 3 are different. This is to verify the universality and data stability of the method. The design of the clamping section 1 effectively matches the widely used universal testing machine, improving the adaptability of the method. The design of the transition section 2 enables smooth transmission of compressive stress. The design of the longer parallel section 3 fully matches the extensometer used to measure the deformation of the parallel section 3 during compression, improving deformation measurement accuracy. The diameter of the clamping section 1 is larger than that of the parallel section 3 to ensure that the stress level in the parallel section 3 is significantly greater than that in the clamping section 1, thereby ensuring that deformation occurs primarily in the parallel section 3. The specimen design must be able to measure the yield strength while avoiding instability in the parallel section 3. Therefore, the variable-section specimen 100 was designed with the help of theoretical analysis and numerical simulation tools.

[0042] Specifically, the compression strength testing method of the variable cross-section specimen 100 includes the following steps:

[0043] S1: Verify the accuracy of the numerical simulation of the compression test through theoretical calculations. Select hull steel to create a simulation model of variable-section specimen 100. The model has a diameter of 30 mm and a length of 400 mm. One end is fixed and the other end remains free. The elastic modulus of the hull steel is 2×1011 Pa, and the Poisson's ratio is 0.3. The Euler critical load of parallel section 3 obtained by simulation calculation is A, A = 122.8 kN, and enter S2.

[0044] S2: When the variable cross-section specimen 100 is subjected to a compression test, one end of the variable cross-section specimen 100 is fixed and the other end is free. According to the Euler compression rod instability critical load formula:

[0045]

[0046] The formula for the moment of inertia of a cylindrical section is:

[0047]

[0048] According to formulas (1) and (2), we can get:

[0049]

[0050] According to formula (3), the Euler critical load of parallel section 3 is B, B = 122.4KN, and enter S3;

[0051] S3: Judgment Is it established? If so, it indicates that the specimen simulation model has high accuracy and can be used to predict the critical load of the specimen, then go to S4; if not, go to S1;

[0052] S4: Variable cross-section specimen design is carried out through numerical simulation. The design principle is that the yield load of the variable cross-section specimen 100 is less than the Euler critical load. According to the yield load, according to the formula:

[0053]

[0054] The yield load of the parallel section 3 is calculated to be C, and the process proceeds to S5;

[0055] S5: Determine whether C < A is true. If so, go to S6. If not, go to S1.

[0056] S7: According to the size of the simulation model of the variable cross-section specimen 100, the variable cross-section specimen 100 is taken from the test plate and tested, and then the process proceeds to S7;

[0057] S7: Performing a compression test on the variable cross-section specimen 100 using a universal testing machine, and obtaining the compressive yield strength of the parallel section 3 according to the stress-strain curve;

[0058] S8: Using simulation software, adjust the length and diameter of the parallel section 3 to produce multiple variable-section specimens 100 of different sizes. Compare the test results of the different variable-section specimens 100 through compression tests to verify the adaptability and stability of the present application. Preferably, samples can also be taken at different locations on the steel plate.

[0059] Where, E is the elastic modulus, I is the moment of inertia of the area, π is the mathematical constant pi, L is the total length of the variable cross-section specimen 100, d is the diameter of the parallel section 3, σ s is the nominal yield strength of the material, μ is the length factor, μ = 2.

[0060] Preferably, the test plate is hull steel, and the hull steel can be AH32, DH36 or EH40;

[0061] Preferably, the simulation software uses ANSYS Workbench, which belongs to the existing technology and will not be described in detail.

[0062] Preferably, in order to ensure that the variable cross-section specimen 100 is compatible with most existing universal testing machines, the diameters of the clamping section 1 and the parallel section 3 of the variable cross-section specimen 100 are limited: Figure 2 As shown, ① the length of the clamping section 1 is X=50 mm, so that it is suitable for most current universal testing machines; ② the diameter of the clamping section 1 is D, the diameter of the parallel section 3 is d, and D=2d, so that the deformation of the variable-section specimen 100 is concentrated in the parallel section 3 when loaded; ③ the diameter of the parallel section 3 is designed to be 10 mm, so that most equipment with a loading capacity of 10 tons can measure the yield strength; ④ the length of the parallel section 3 is 20 mm<Y≤30 mm, so that it can match most extensometers.

[0063] Preferably, the transition section 2 is an arc with a radius R of 36 mm ≤ R ≤ 64 mm. The arc transition transforms the cross-sectional abrupt change between the clamping section 1 and the parallel section 3 into a smooth geometric transition through continuous curvature, thereby avoiding stress concentration during right-angle or sharp-angle transitions.

[0064] The Euler critical load of the variable cross-section specimen 100 is obtained through simulation. This is because the Euler formula is only applicable to models with simple boundary conditions and cross-section forms, such as the attached Figure 1 For specimens with equal cross-section, the Euler formula is no longer applicable for specimens with variable cross-section, and the verified simulation method in this application must be used. In the simulation model, the boundary conditions of the model are consistent with the test, that is, the clamping section 1 is constrained by the chuck, the clamping section 1 at one end is kept fixed, and compression loading is performed by applying a downward displacement to the other end. Different Euler critical loads will be obtained for different model designs. By continuously adjusting the length of the parallel section 3 and the arc length of the transition section 2, a variable cross-section specimen that meets the requirements can be obtained. According to this method, the dimensions of the variable cross-section specimen designed in this embodiment are as follows: d = 10 mm, Y = 30 mm, D = 20 mm, R = 40 mm, L = 169 mm. The Euler critical load obtained by simulation calculation is 1200 kN. By the formula The calculated yield load is about 61.6 kN, which is much smaller than the Euler critical load, indicating that the design of the variable-section specimen 100 can obtain data such as compressive yield strength.

[0065] Example 2

[0066] Based on the compression strength test method of the variable cross-section specimen in Example 1, the hull structural steel was selected and tested according to Figure 3-Figure 5 The sample is sampled according to the design. The variable cross-section specimen is divided into transverse and longitudinal sections. The specimen whose axis is parallel to the rolling direction of the test plate is called the longitudinal specimen. Figure 4 As shown; the specimen perpendicular to the rolling direction of the test plate is called a transverse specimen, such as Figure 3 As shown. The uniform cross-section round bar specimen 1 is processed according to GB-T7314-2017 "Metallic Materials Room Temperature Compression Test Method". The sampling positions are divided into upper surface sample A1, upper t / 4 sample B1, center samples C11 and C12, lower t / 4 sample D1 and lower surface sample E1 along the thickness direction of the test plate. Figure 5 In order to further verify the adaptability of the variable cross-section specimens, variable cross-section horizontal and vertical specimens with parallel sections 3 and diameters of 10 mm, 9 mm and 8 mm respectively were taken from the test plate.

[0067] According to GB-T7314-2017 "Room Temperature Compression Test Method for Metallic Materials", a uniform cross-section compression test was carried out to obtain the compressive yield strength of the uniform cross-section specimen. At the same time, with the help of a universal testing machine, a variable cross-section compression test was carried out to obtain the compressive yield strength of the variable cross-section specimen.

[0068] The compression results of the variable-section specimens and the uniform-section specimens are compared. The compression results of the 10 mm variable-section specimens and the uniform-section specimens are summarized in Table 1. The discreteness of the transverse and longitudinal yield strength data obtained by the two categories is shown in Table 2. The standard deviation of the yield strength obtained by the uniform-section specimen is 39.3, which is greater than the 22.8 of the variable-section specimen. The coefficient of variation is 4.205%, which is greater than the 2.537% of the variable-section specimen. It can be seen that the test results of the variable-section specimens are superior to those of the uniform-section specimens in terms of stability, accuracy and universality.

[0069] In order to verify the universality of the compression test on variable-section specimens, 100 variable-section specimens with parallel sections 3 of 8 mm, 9 mm, and 10 mm in diameter were taken from the transverse and longitudinal directions of the test plate, and compression tests were performed on them. The compression test results are shown in Table 3, and the compressive yield strength results are shown in Table 4. It can be seen that the coefficients of variation of the transverse and longitudinal yield strengths are both less than 1%. Since different sampling positions will cause differences in the results, this shows the stability and effectiveness of the variable-section specimen in measuring the compressive yield strength.

[0070] Table 1 Comparison of yield strength of different specimen types

[0071]

[0072] Table 2 Analysis of compression test results for three sizes

[0073] Serial number category Data Points average value Standard deviation Coefficient of variation 1 Equal cross-section 36 934 39.3 4.205％ 2 variable cross-section 24 898 22.8 2.537％

[0074] Table 3 Yield of compression specimens with different variable cross-sections

[0075]

[0076] Table 4 Analysis of compression test results for three sizes

[0077] Serial number direction Data Points average value Standard deviation Coefficient of variation 1 Horizontal 6 820 6.9 0.848％ 2 Vertical 6 813 6.9 0.857％

[0078] The method of testing the compressive yield strength of hull steel by using a variable cross-section specimen described in this application has the following advantages over the prior art: 1) It can effectively measure the compressive yield strength of hull steel with high data reliability. Through numerical simulation design, it is possible to design variable cross-section specimens 100 of different sizes for structural steels with different yield strengths, so that the main deformation occurs in the parallel section 3, ensuring uniform deformation of the parallel section 3 and avoiding local shear deformation, thereby fundamentally avoiding the influence of the "barrel-shaped" effect of friction on the compressive yield strength of traditional round bar specimens; 2) The data is highly accurate and has a small degree of discreteness. Conventional standard compression specimens cannot be directly used for extensometers due to size limitations. 1) The variable cross-section specimen 100 designed in this application is compatible with an extensometer, and has high measurement accuracy and low data discreteness; 2) The measurement method is simple, and the requirements for tooling and equipment are low. The variable cross-section specimen 100 designed in this application is close to a standard tensile specimen, and the principle is clear, the method is simple and easy to obtain, and can be assisted by a commonly used universal testing machine. Many laboratories have the test conditions, and there is no need to design and manufacture tooling separately; 3) It has strong adaptability and can measure specimens with different yield strengths. It can be completed in a laboratory environment, and there is no need to entrust relevant institutions to conduct residual stress tests, saving a lot of money.

[0079] Although the present invention is disclosed as above, the present invention is not limited thereto. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be based on the scope defined by the claims.

Claims

1. A method for testing the compressive yield strength of hull steel using a variable cross-section specimen, characterized in that: The following steps are involved: S1: In the simulation software, a hull steel is selected to create a simulation model of a variable cross-section specimen (100), wherein the variable cross-section specimen (100) comprises a clamping section (1), a transition section (2) and a parallel section (3) connected in sequence, wherein the clamping section (1) is used for fixing with a universal testing machine, the transition section (2) is used for connecting the clamping section (1) and the parallel section (3), and the parallel section (3) is used for installing an extensometer. One end of the variable cross-section specimen (100) is constrained, and the other end remains free. The Euler critical load A of the parallel section (3) is calculated by the simulation model, and then the process proceeds to S2; S2: According to the formula Calculate the Euler critical load B of the parallel section (3) and proceed to S3; S3: Judgment Is it true? If so, go to S4; if not, go to S1; S4: According to the formula The yield load of the parallel section (3) is calculated to be C, and the process proceeds to S5; S5: Determine whether C < A is true. If so, go to S6. If not, go to S1. S6: taking the variable cross-section specimen (100) from the test plate according to the size of the simulation model of the variable cross-section specimen (100), and proceeding to S7; S7: performing a compression test on the variable cross-section specimen (100) using a universal testing machine, and obtaining the compressive yield strength of the parallel section (3) based on a stress-strain curve; Where, E is the elastic modulus, L is the total length of the variable cross-section specimen (100), d is the diameter of the parallel section (3), σ s is the nominal yield strength of the material.

2. The method for testing the compressive yield strength of hull steel using a variable cross-section specimen according to claim 1, characterized in that: In step S2, when the variable cross-section specimen (100) is subjected to a compression test, one end of the variable cross-section specimen (100) is fixed and the other end is free, and the Euler compression rod instability critical load formula is: The formula for the moment of inertia of a cylindrical section is: According to formula (1) and (2), we can get the formula: Among them, π is the mathematical constant pi, I is the area moment of inertia, and μ is the length factor, μ=2.

3. The method for testing the compressive yield strength of hull steel using a variable cross-section specimen according to claim 2, characterized in that: The length of the clamping section (1) is X=50 mm, and the length of the parallel section (3) is 20 mm < Y ≤ 30 mm.

4. The method for testing the compressive yield strength of hull steel using a variable cross-section specimen according to claim 3, characterized in that: The diameter of the clamping section (1) is D, the diameter of the parallel section (3) is d, and D=2d.

5. The method for testing the compressive yield strength of hull steel by using a variable cross-section specimen according to claim 4, characterized in that: The transition section (2) is an arc, the radius of the arc is R, and 36mm≤R≤64mm.

6. The method for testing the compressive yield strength of hull steel using a variable cross-section specimen according to claim 2, characterized in that: From the transverse and longitudinal directions of the test plate, parallel sections (3) with variable cross-section specimens (100) having diameters of 8 mm, 9 mm and 10 mm are taken respectively and compression tests are carried out respectively. The coefficients of variation of the transverse and longitudinal yield strengths are both less than 1%.

7. The method for testing the compressive yield strength of hull steel by using a variable cross-section specimen according to claim 5, characterized in that: Said d=10mm, Y=30mm, D=20mm, R=40mm, and L=169mm.

8. The method for testing the compressive yield strength of hull steel using a variable cross-section specimen according to claim 1, characterized in that: The simulation software is ANSYS Workbench.

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