Device and method for testing pipe hoop tensile stress-strain curve based on DIC technology

Abstract

The application provides a device for testing hoop tensile stress and strain curve of pipe material, which comprises a hoop tensile sample, an upper buckle, a lower buckle, an upper D-shaped block, a lower D-shaped block and a connecting rod. The testing method is as follows: before the test, the outer surface of the reduced diameter section of the hoop tensile sample is uniformly sprayed with speckles; during the hoop tensile process, the strain in the linear length and width direction of the reduced diameter section of the hoop tensile sample is measured by using a three-dimensional full-field strain measurement and analysis system. The position of the hoop tensile sample is adjusted so that the included angle between the center line of the reduced diameter section and the tensile direction is 30°, and the infrared speckle of the high-resolution camera is emitted on the upper middle part of the reduced diameter section of the sample. Finally, the universal testing machine is started to perform the tensile test at a fixed speed. The application has the advantages that the hoop tensile stress and strain curve of the pipe material can be directly and accurately tested, and it has important theoretical and practical significance for accurately constructing the plastic constitutive model of anisotropic thin-walled pipe material and realizing high-precision simulation of the forming of the thin-walled tubular component.

Description

Technical Field

[0001] This invention belongs to the field of material mechanical property testing, specifically relating to a device and method for testing the circumferential tensile stress-strain curve of pipes based on DIC technology. Background Technology

[0002] A device and method for testing the circumferential tensile stress-strain curves of pipes based on DIC technology, specifically a device and method for testing the circumferential tensile stress and strain curves of pipes based on three-dimensional full-field strain measurement technology. Thin-walled hollow connectors in piping systems are typically tubular components such as bent pipes, T-tubes, Y-tubes, and corrugated pipes, characterized by their lightweight, high strength, and high fatigue performance, and are widely used in aerospace, nuclear industry, petrochemical, and automotive transportation fields.

[0003] These thin-walled hollow connectors are usually manufactured using advanced precision manufacturing technologies such as hydroforming. During the manufacturing process, they are subjected to complex stresses, which can easily lead to failure defects such as wrinkling and cracking.

[0004] Therefore, using finite element simulation technology to simulate and calculate the forming process of thin-walled hollow connectors, quickly and accurately predicting the defects that may occur during the forming process, and thus obtaining the most reasonable process parameters, has always been a common optimization method and pursuit goal in the industry, which can significantly reduce the development cycle and experimental costs.

[0005] To achieve this goal, improving the accuracy of finite element simulation calculations is crucial. Establishing a plastic constitutive model that accurately describes the pipe material during the hydroforming process is fundamental to ensuring the accuracy of finite element simulation results.

[0006] The tensile stress-strain curves of pipes are crucial experimental data for establishing accurate plastic constitutive models. Therefore, accurately obtaining the tensile stress-strain curves of pipes is essential for establishing accurate plastic constitutive models, improving the accuracy of finite element simulations, and ultimately obtaining the most reasonable process parameters.

[0007] Currently, the national standard GB / T228.1-2021 for tensile testing of metallic materials only specifies methods for obtaining axial tensile stress and strain curves of pipes. During the hydroforming process of thin-walled hollow connectors, cracking defects are usually caused by excessive radial pressure. Using a plastic constitutive model established using the axial tensile stress and strain curve data of the pipe to simulate the hydroforming process of thin-walled hollow connectors is clearly inaccurate. Therefore, current researchers stretch the pipe radially to obtain circumferential tensile stress and strain curve data and establish a plastic constitutive model to simulate the hydroforming process of thin-walled hollow connectors.

[0008] Currently, the methods for obtaining circumferential tensile stress and strain curves of pipes are usually the D-block method and other derivative methods. However, this type of D-block method also has several problems. First, if the size of the D-block is too small, the curvature of the pipe changes significantly during circumferential tension. This causes a change in the direction of force on the pipe during tension, resulting in inaccurate measurement results, as in ISO 8469-2013. Second, if the diameter reduction section of the circumferential tension specimen is parallel to the tension direction, it also causes a significant change in the curvature of the pipe during circumferential tension, resulting in inaccurate measurement results, as in ASTM D2290-16. Third, the circumferential tensile stress and strain curves are determined based on the values ​​recorded by the universal testing machine. The values ​​recorded by the universal testing machine are two-dimensional test data along the tension direction, not along the circumferential direction, and there is obviously a certain difference between the two values. Fourth, friction between the circumferential tension specimen and the D-block is unavoidable and is directly related to the magnitude of the circumferential stress; current research has not considered the influence of friction on the circumferential stress. Summary of the Invention

[0009] The purpose of this invention is to provide a device and method that can directly and accurately test the circumferential tensile stress and strain curve of pipes.

[0010] To achieve the above objectives, the experimental scheme provided by this invention is as follows:

[0011] The apparatus for testing the circumferential tensile stress and strain curves of pipe materials involved in this invention includes: a circumferential tensile specimen, an upper clip, a lower clip, an upper D-block, a lower D-block, and a connecting rod, as shown below. Figure 1 As shown.

[0012] The upper and lower parts of the device are the upper buckle 10 and the lower buckle 40, respectively. The upper D-block 50 and the lower D-block 60 are set in the middle of the device and fixed by the connecting rod 20. The circumferential tensile specimen 30 is arranged between the upper D-block 50 and the lower D-block 60.

[0013] To ensure that the curvature of the circumferential specimen of the tube does not change during the tensile process, the outer diameter of the D-block should be 0.1 to 0.2 mm smaller than the inner diameter of the tube.

[0014] In other words, the size of the D-block used to test the circumferential tensile stress and strain curves of pipes of different sizes is different.

[0015] The experimental steps involved in this invention are as follows:

[0016] Before the test, speckle pattern was uniformly sprayed onto the outer surface of the reduced diameter section of the circumferential tensile specimen 30. During the circumferential tensile test, the strain in the straight length and width directions of the reduced diameter section of the annular specimen was measured using the digital image correlation method (DIC) of the three-dimensional full-field strain measurement and analysis system.

[0017] To reduce the impact of friction and improve the uniformity of deformation of the ring-shaped specimen, the mold / specimen interface should be lubricated, usually by applying a polytetrafluoroethylene (PTFE) slip coating between the interfaces.

[0018] First, mechanically connect the upper and lower latches to the loading mechanism and base of the universal testing machine, and adjust the position of the loading mechanism to align the upper and lower latches.

[0019] Then, the upper D-block 50 and the lower D-block 60 are passed through the circumferential tensile specimen 30 with the diameter reduction section, and the upper D-block 50 and the lower D-block 60 are connected to the upper and lower buckles respectively by the connecting rod.

[0020] Adjust the position of the circumferential tensile specimen at 30° so that the angle between the centerline of its diameter reduction section and the tensile direction is 30°. Figure 2 As shown.

[0021] Fine-tune the position of the loading mechanism of the universal testing machine so that the inner surface of the circumferential tensile specimen 30 is in close contact with the outer surfaces of the upper D-block 50 and the lower D-block 60.

[0022] Adjust the high-resolution camera of the DIC measurement system to be positioned diagonally above the specimen, so that the infrared spot emitted by the high-resolution camera is emitted in the upper-middle part of the specimen's diameter reduction section. This ensures that the specimen remains within the field of view of the high-resolution camera during the tensile process. Figure 3 As shown.

[0023] Finally, start the universal testing machine and perform tensile testing at a fixed speed, with the tensile rate specified in GB / T 228.1-2021, until the circumferential specimen breaks.

[0024] Since the strain measured by the DIC measurement system is linear strain in three-dimensional space, rather than the circumferential strain of the pipe, this invention provides a method for calculating the circumferential engineering strain and true strain of the pipe:

[0025] First, the strain between two points along the length of the reduction section and points 1 and 2, measured by the DIC testing system, and the strain between two points along the width of the reduction section at the fracture point and points 3 and 4, are denoted as e. 1-2 The strain between point 3 and point 4 is denoted as e. 3-4 ,like Figure 4 As shown.

[0026] The engineering strain in the circumferential direction of the pipe can be described as:

[0027] in For circumferential engineering strain, The length of the arc after stretching. Let be the initial arc length between point 1 and point 2.

[0028] Where e r For strain in the wall thickness direction, L0 is the outer diameter of the pipe, and L0 is the straight line length between point 1 and point 2.

[0029] Assuming the reduced diameter section is incompressible during the plastic stage, the strain in the three directions can be described as follows:

[0030] e 1-2 +e r +e 3-4 =0

[0031]

[0032] Circular True Strain It can be described as:

[0033]

[0034] The stress state of the specimen during circumferential tension is as follows: Figure 5 As shown.

[0035] For a force-bearing element at any position on the specimen, under the contact conditions of axial tensile force P and friction coefficient μ, the radial tensile force... and circumferential tension Force balance:

[0036]

[0037] when When it was very small,

[0038] Circumferential engineering stress (σ0) and true stress They can be described as follows:

[0039] Where A0 and A are the cross-sectional areas of the initial diameter reduction section and the diameter reduction section during the stretching process, respectively, A0 = W0t0, A = W0t0(1+e3-4)(1+er). t0 is the pipe wall thickness, and W0 is the width of the diameter reduction section.

[0040] Advantages of this invention:

[0041] The device and method for testing the circumferential tensile stress-strain curve of pipes based on DIC technology described in this invention can directly and accurately test the circumferential tensile stress and strain curve of pipes. It has important theoretical and practical significance for accurately constructing plastic constitutive models of anisotropic thin-walled pipes and realizing high-precision simulation of the forming of thin-walled tubular components. Attached Figure Description

[0042] Figure 1 A schematic diagram of the device used to test the circumferential tensile stress and strain curves of pipes;

[0043] Figure 2 A schematic diagram of position 30 of the circumferential tensile specimen during the testing of the circumferential tensile stress and strain curves of the pipe.

[0044] Figure 3 Diagram showing the placement of the high-resolution camera;

[0045] Figure 4 A schematic diagram showing the positions of points 1, 2, 3, and 4 on the circumferential tensile specimen 30;

[0046] Figure 5 A schematic diagram of the stress state of the specimen during circumferential tension.

[0047] Figure 6 Schematic diagram of circumferential tensile sample dimensions;

[0048] Figure 7 Schematic diagram of strain variation with axial tensile force between point 1 and point 2, and between point 3 and point 4;

[0049] Figure 8 Schematic diagram of circumferential stress-strain curves and true stress-strain curves for GH4169 pipe. Detailed Implementation

[0050] The following describes in detail the specific embodiments of the present invention, taking GH4169 pipe as an example, and testing the circumferential tensile stress and strain curves of the pipe.

[0051] The selected GH4169 pipe has an outer diameter (D0) of 20.92 mm and a wall thickness (t0) of 0.20 mm.

[0052] GH4169 pipe is wire-cut to prepare it into the following shape: Figure 6 The circumferentially stretched sample shown.

[0053] Before stretching, the surface needs to be sanded with 800# sandpaper to reduce the impact of wire cutting marks on circumferential mechanical properties.

[0054] The outer surface of the diameter reduction section of the circumferentially stretched sample is sprayed with a matte white base color and an irregular matte black speckled pattern.

[0055] PTFE grease is evenly sprayed onto the outer surfaces of the upper D-block 50 and the lower D-block 60 and the inner surface of the sample.

[0056] Align the horizontal surfaces of the upper D-block 50 and the lower D-block 60, and insert the circumferential stretching sample.

[0057] Connect the upper and lower latches mechanically to the loading mechanism and base of the universal testing machine, and adjust the position of the loading mechanism to align the upper and lower latches.

[0058] The upper D-block 50 and the lower D-block 60 are connected to the upper and lower clips respectively using the connecting rod.

[0059] Rotate the circumferentially stretched sample so that the centerline of its diameter reduction section is at a 30° angle to the centerline of the D-block.

[0060] Fine-tune the loading mechanism of the universal testing machine to make the inner surface of the circumferential tensile specimen 30 fit tightly against the outer surface of the D-block.

[0061] Calibrate the DIC measurement system.

[0062] Adjust the position of the high-resolution camera to be diagonally above the circumferentially stretched sample, ensuring that the circumferentially stretched sample is in the lower center of the high-resolution camera's field of view.

[0063] Adjust the angle of the high-resolution camera so that the infrared spot of the high-resolution camera is at the center of the diameter reduction section of the circumferentially stretched sample.

[0064] Start the universal testing machine and stretch it at a tensile speed of 0.6 mm / min until the circumferential specimen breaks.

[0065] Output the strain versus axial tensile force curves between points 1 and 2, and between points 3 and 4, as shown below. Figure 7 As shown.

[0066] According to the present invention, the circumferential stress-strain curves and true stress-strain curves of GH4169 pipe are calculated, as follows: Figure 8 As shown.

Claims

1. A device for testing pipe hoop tensile stress-strain curve based on DIC technology, characterized in that: The device comprises an upper buckle (10), a lower buckle (40), an upper D-shaped block (50), a lower D-shaped block (60) and a connecting rod (20); Wherein: the upper and lower parts of the device are the upper buckle (10) and the lower buckle (40) respectively, the upper D-shaped block (50) and the lower D-shaped block (60) are arranged at the middle position of the device and are fixed through the connecting rod (20), and the circumferential tensile sample (30) is arranged between the upper D-shaped block (50) and the lower D-shaped block (60); The test method of the device for testing the circumferential tensile stress-strain curve of pipe based on the DIC technology involves the following experimental steps: Before the test, the outer surface of the reduced diameter section of the circumferential tensile sample (30) is uniformly sprayed with speckles, and the strain in the straight line length and width direction of the reduced diameter section of the circumferential tensile sample (30) is measured during the circumferential tensile process by using a three-dimensional full-field strain measurement and analysis system; In order to reduce the influence of friction and improve the deformation uniformity of the ring sample, the interface between the mold and the circumferential tensile sample (30) is lubricated, specifically a polytetrafluoroethylene sliding coating layer is coated between the interface; Firstly, the upper buckle (10) and the lower buckle (40) are respectively mechanically connected with the universal testing machine loading mechanism and the base, and the position of the loading mechanism is adjusted so that the upper buckle (10) and the lower buckle (40) are aligned; Then the upper D-shaped block (50) and the lower D-shaped block (60) pass through the circumferential tensile sample (30) with a reduced diameter section and are connected with the upper buckle (10) and the lower buckle (40) respectively; The position of the circumferential tensile sample (30) is adjusted so that the included angle between the center line of the reduced diameter section and the tensile direction is 30°; The position of the universal testing machine loading mechanism is finely adjusted so that the inner surface of the circumferential tensile sample (30) is in close contact with the outer surface of the upper D-shaped block (50) and the lower D-shaped block (60); The high-resolution camera of the DIC measurement system is placed obliquely above the sample, so that the infrared speckle emission of the high-resolution camera is in the upper middle part of the reduced diameter section of the circumferential tensile sample (30), so as to ensure that the sample is always in the field of view of the high-resolution camera during the stretching process; Finally, the universal testing machine is started to stretch at a fixed speed, and the stretching rate is specified with reference to GB / T 228.1-2021, until the circumferential tensile sample (30) is broken and the stretching is terminated.

2. The apparatus for testing pipe hoop tensile stress strain curve based on DIC technique according to claim 1, characterized in that: The outer diameter of the upper D-shaped block (50) and the lower D-shaped block (60) is smaller than the inner diameter of the pipe by 0.1-0.2mm.

3. The device for testing the circumferential tensile stress-strain curve of pipe based on the DIC technology according to claim 1, wherein: The three-dimensional full-field strain output measured by the DIC test system is the strain between point one (1) and point two (2) along the length direction of the reduced diameter section and the strain between point three (3) and point four (4) in the width direction of the reduced diameter section at the fracture of the sample, the strain between point one (1) and point two (2) is denoted as e 1-2 , and the strain between point three (3) and point four (4) is denoted as e 3-4 ; The engineering strain in the circumferential direction of the pipe is described as: ； wherein e φ is the circumferential engineering strain, L φ is the arc length after stretching, L φ0 is the initial arc length between point one (1) and point two (2); , ; wherein e r D0 is the outer diameter of the pipe material, L 0 is the straight-line length between point one (1) and point two (2); Assuming that the reduced diameter section is incompressible in the plastic stage, the strains in the three directions are described as: hoop true strain (εh) The circumferential engineering stress (σ0) and the true stress (σφ) are respectively described as: φ ) is described as: The force balance of the radial tension (F(φ)) and the hoop tension (N(φ)) under the contact condition of the axial tension P and the friction coefficient μ at the force unit at an arbitrary position on the test sample is: When (φ) is very small, ; ​ , ; wherein A0 and A are the cross-sectional area of the initial reducing section and the cross-sectional area of the reducing section during stretching, respectively, A0 = W0t0, A = W0t0(1 + e 3-4 )(1 + e r ); t0 is the wall thickness of the tube, and W0 is the width of the reducing section.

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