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

Abstract

The invention provides a device for testing the circumferential tensile stress and strain curve of a pipe. The device comprises a circumferential 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 comprises the following steps: uniformly spraying speckles on the outer surface of the reducing section of the annular tensile sample before testing, and measuring the strain in the linear length and width directions of the reducing section of the annular sample by using a three-dimensional full-field strain measurement and analysis system in the annular tensile process. Adjusting the position of the circumferential tensile sample, so that the included angle between the central line of the reducing section and the tensile direction is 30 degrees, and the infrared spots of the high-resolution camera are emitted to the middle upper part of the reducing section of the sample. And finally, starting the universal testing machine to stretch at a fixed speed. The method has the advantages that the circumferential tensile stress and strain curve of the pipe can be directly and accurately tested, and the method has important theoretical and practical significance for accurately constructing an anisotropic thin-wall pipe plastic constitutive model and realizing high-precision simulation of thin-wall tubular member forming.

Description

Technical Field

[0001] The present invention belongs to the field of testing the mechanical properties of materials, and particularly relates to a device and method for testing the circumferential tensile stress-strain curve of a pipe based on DIC technology. Background Art

[0002] A device and method for testing the circumferential tensile stress-strain curve of a pipe based on DIC technology, that is, a device and method for testing the circumferential tensile stress and strain curve of a pipe based on three-dimensional full-field strain measurement technology. Thin-walled hollow connectors in a pipeline system are usually pipe-shaped components such as bent pipes, T-shaped pipes, Y-shaped pipes, and corrugated pipes, which have the characteristics of light weight, high strength, and high fatigue performance, and are widely used in fields such as aerospace, nuclear industry, petrochemical industry, and automotive transportation.

[0003] Such thin-walled hollow connectors are usually prepared by advanced precision manufacturing technologies such as hydroforming. During the manufacturing process, they are subjected to complex stress, and are prone 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 possible defects during the forming process, and then obtaining the most reasonable process parameters to greatly reduce the development cycle and experimental cost has always been a common optimization method and pursuit goal in the industrial community.

[0005] To achieve this goal, it is particularly crucial to improve the accuracy of finite element simulation calculations. Establishing a plastic constitutive model that can accurately describe the pipe during the hydroforming process is the basis for ensuring the accuracy of finite element simulation calculation results.

[0006] The tensile stress and strain curve of the pipe is important experimental data for establishing an accurate plastic constitutive model. Therefore, accurately obtaining the tensile stress and strain curve of the pipe is crucial for establishing an accurate plastic constitutive model, improving the accuracy of finite element simulation, and then obtaining the most reasonable process parameters.

[0007] Currently, for the method of obtaining the tensile stress and strain curve of the pipe, there are only relevant regulations in the national standard GB / T228.1-2021 for metal material tensile testing to obtain the axial tensile stress and strain curve of the pipe. During the hydroforming preparation process of thin-walled hollow connectors, the cracking defect is usually caused by excessive radial pressure. Using the plastic constitutive model established with the axial tensile stress and strain curve data of the pipe to simulate the hydroforming preparation process of thin-walled hollow connectors is obviously inaccurate. Therefore, current research scholars stretch the pipe radially to obtain the circumferential tensile stress and strain curve data of the pipe, and establish a plastic constitutive model for simulating the hydroforming preparation process of thin-walled hollow connectors.

[0008] Currently, the methods for obtaining the circumferential tensile stress and strain curves of pipes are usually the D-block method and other derivative methods of the D-block method. However, there are still several problems with using such D-block methods to obtain the circumferential tensile stress and strain curves of pipes. First, the size of the D-block is too small, and the curvature of the pipe changes significantly during the circumferential tensile process of the pipe. This results in a change in the stress direction of the pipe during the tensile process, making the measurement results inaccurate, such as ISO 8469-2013; second, the reduced-diameter section of the circumferential tensile specimen is parallel to the tensile direction, which also causes a significant change in the curvature of the pipe during the circumferential tensile process of the pipe, making the measurement results inaccurate, such as ASTM D2290-16; third, the circumferential tensile stress and strain curves are determined based on the recorded values of the universal testing machine. The values recorded by the universal testing machine are two-dimensional test data along the tensile direction, rather than along the circumferential direction, and there is obviously a certain difference between these two values; fourth, the friction between the circumferential tensile specimen of the pipe and the D-block is inevitable, and it is directly related to the magnitude of the circumferential stress. The current research does not consider the influence of friction on the circumferential stress. Summary of the Invention

[0009] The object of the present invention is to propose a device and method capable of directly and accurately testing the circumferential tensile stress and strain curves of pipes.

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

[0011] The device for testing the circumferential tensile stress and strain curves of pipes according to the present invention includes: a circumferential tensile specimen, an upper buckle, a lower buckle, an upper D-block, a lower D-block, and a connecting rod, as Figure 1 shown.

[0012] The upper and lower parts of the device are respectively an upper buckle 10 and a lower buckle 40. The upper D-block 50 and the lower D-block 60 are arranged in the middle position of the device and fixed by a 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 pipe does not change during the tensile process, the outer diameter of the D-block should be 0.1-0.2 mm smaller than the inner diameter of the pipe.

[0014] That is to say, the sizes of the D-blocks for testing the circumferential tensile stress and strain curves of pipes with different sizes are different.

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

[0016] Before the test, the outer surface of the reduced-diameter section of the circumferential tensile specimen 30 is evenly sprayed with speckles. During the circumferential tensile process, the digital image correlation method DIC of the three-dimensional full-field strain measurement and analysis system is used to measure the strains in the straight length and width directions of the reduced-diameter section of the annular specimen.

[0017] To reduce the influence of friction and improve the deformation uniformity of the annular specimen, the die / specimen interface should be lubricated, usually by applying a polytetrafluoroethylene lubricating coating between the interfaces.

[0018] First, the upper and lower fasteners are mechanically connected to the loading mechanism of the universal testing machine and the base respectively, and the position of the loading mechanism is adjusted to align the upper and lower fasteners.

[0019] Then, the upper D-shaped block 50 and the lower D-shaped block 60 are passed through the circumferential tensile specimen 30 with a reduced-diameter section, and the upper D-shaped block 50 and the lower D-shaped block 60 are connected to the upper and lower fasteners respectively through connecting rods.

[0020] Adjust the position of the circumferential tensile specimen 30 so that the included angle between the midline of its reduced-diameter section and the tensile direction is 30°, as Figure 2 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-shaped block 50 and the lower D-shaped block 60.

[0022] Adjust the high-resolution camera of the DIC measurement system to be placed obliquely above the specimen, so that the infrared spots of the high-resolution camera are emitted in the upper middle part of the reduced-diameter section of the specimen, to ensure that the specimen is always within the field of view of the high-resolution camera during the tensile process, as Figure 3 shown.

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

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

[0025] First, output the three-dimensional full-field strain measured by the DIC test system to the strain points 1 and 2 between two points along the length direction of the reduced-diameter section and the strain points 3 and 4 between two points in the width direction of the reduced-diameter section at the specimen fracture. The strain between points 1 and 2 is denoted as e 1-2 , and the strain between points 3 and 4 is denoted as e 3-4 , as Figure 4 shown.

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

[0027] where is the circumferential engineering strain, is the arc length after tension, is the initial arc length between point 1 and point 2.

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

[0029] Assuming that the reducing section is incompressible in the plastic stage, the strains in the three directions can be described as:

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

[0031]

[0032] The circumferential true strain can be described as:

[0033]

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

[0035] For the force-bearing unit at any position on the specimen, under the contact conditions of the axial tension P and the friction coefficient μ, the radial tension and the circumferential tension are in force balance:

[0036]

[0037] When is very small,

[0038] The circumferential engineering stress (σ0) and the true stress can be described respectively as:

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

[0040] Advantages of the present invention:

[0041] The device and method for testing the circumferential tensile stress-strain curve of a pipe based on the DIC technology according to the present invention can directly and accurately test the circumferential tensile stress and strain curves of the pipe, which has important theoretical and practical significance for accurately constructing the plastic constitutive model of anisotropic thin-walled pipes and realizing the high-precision simulation of the forming of thin-walled tubular components. Description of the drawings

[0042] Figure 1 Schematic diagram of the device for testing the circumferential tensile stress and strain curves of the pipe material;

[0043] Figure 2 Schematic diagram of the position of the circumferential tensile specimen at 30 during the test of the circumferential tensile stress and strain curves of the pipe material;

[0044] Figure 3 Schematic diagram of the placement position of the high - resolution camera;

[0045] Figure 4 Schematic diagram of the positions of Point 1, Point 2, Point 3 and Point 4 on the circumferential tensile specimen 30;

[0046] Figure 5 Schematic diagram of the stress state of the specimen during the circumferential tensile process;

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

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

[0049] Figure 8 Schematic diagram of the circumferential stress, strain curves and true stress, strain curves of the GH4169 pipe material. Specific implementation manner

[0050] Combined with the content of the present invention, the specific implementation manner of the content of the present invention will be described in detail and clearly. Taking the GH4169 pipe material as an example, the circumferential tensile stress and strain curves of the pipe material are tested.

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

[0052] The GH4169 pipe material is processed by wire cutting to prepare a circumferential tensile sample as shown in Figure 6 the figure.

[0053] Before stretching, it needs to be polished with 800# sandpaper to reduce the influence of wire - cutting marks on the circumferential mechanical properties.

[0054] The outer surface of the reduced - diameter section of the circumferential tensile sample is sprayed with a matte white background color and an irregular matte black spot pattern.

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

[0056] Align the horizontal planes of the upper D - shaped block 50 and the lower D - shaped block 60 and insert the circumferential tensile sample.

[0057] Mechanically connect the upper and lower buckles to the loading mechanism and the base of the universal testing machine respectively, and adjust the position of the loading mechanism to align the upper and lower buckles.

[0058] Connect the upper D-shaped block 50 and the lower D-shaped block 60 to the upper and lower buckles respectively using connecting rods.

[0059] Rotate the circumferentially stretched sample so that the center line of its reduced-diameter section forms a 30° angle with the center line of the D-shaped block.

[0060] Fine-tune the loading mechanism of the universal testing machine so that the inner surface of the circumferentially stretched specimen 30 is in close contact with the outer surface of the D-shaped block.

[0061] Calibrate the DIC measurement system.

[0062] Adjust the position of the high-resolution camera to the upper oblique side of the circumferentially stretched sample, ensuring that the circumferentially stretched sample is in the lower middle of the field of view of the high-resolution camera.

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

[0064] Start the universal testing machine to perform stretching at a stretching speed of 0.6 mm / min until the circumferential specimen breaks.

[0065] Output the curves of strain and axial tensile force between point one 1 and point two 2, and between point three 3 and point four 4, as Figure 7 shown.

[0066] Calculate the circumferential stress, strain curve, true stress, and strain curve of the GH4169 pipe according to the content of the present invention, as Figure 8 shown.

Claims

1. An apparatus and method for testing the circumferential tensile stress-strain curve of a pipe based on DIC technology, characterized in that: The described device and method for testing the circumferential tensile stress-strain curve of pipes based on DIC technology include: 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); Among them: 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 in the middle position of the device and fixed by the connecting rod (20). The circumferential tensile specimen (30) is arranged between the upper D-shaped block (50) and the lower D-shaped block (60).

2. The device and method for testing the circumferential tensile stress-strain curve of a pipe based on the DIC technology according to claim 1, characterized in that: The outer diameters of the upper D-shaped block (50) and the lower D-shaped block (60) are 0.1 - 0.2 mm smaller than the inner diameter of the pipe.

3. A testing method for the device and method of testing the circumferential tensile stress-strain curve of a pipe based on the DIC technology as described in claim 1, characterized in that: The experimental steps involved are as follows: Before the test, the outer surface of the reduced-diameter section of the circumferential tensile specimen (30) is evenly sprayed with speckles. During the circumferential tensile process, the three-dimensional full-field strain measurement and analysis system is used to measure the strain in the linear length and width directions of the reduced-diameter section of the circumferential tensile specimen (30). To reduce the influence of friction and improve the deformation uniformity of the annular specimen, the interface between the die and the circumferential tensile specimen (30) is lubricated. Specifically, a polytetrafluoroethylene sliding coating is applied between the interfaces. First, the upper buckle (10) and the lower buckle (40) are mechanically connected to the loading mechanism and the base of the universal testing machine respectively, 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) are passed through the circumferential tensile specimen (30) with a reduced-diameter section and connected to the upper buckle (10) and the lower buckle (40) respectively. Adjust the position of the circumferential tensile specimen (30) so that the included angle between the midline of its reduced-diameter section and the tensile direction is 30°. 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-shaped block (50) and the lower D-shaped block (60). Adjust the high-resolution camera of the DIC measurement system to be placed obliquely above the specimen, so that the infrared spots of the high-resolution camera are emitted in the upper-middle part of the reduced-diameter section of the circumferential tensile specimen (30) to ensure that the specimen is always within the field of view of the high-resolution camera during the tensile process. Finally, start the universal testing machine to perform tensile at a fixed speed. The specified tensile rate refers to GB / T 228.1-2021 until the circumferential tensile specimen (30) breaks and terminates.

4. The testing method of the device and method for testing the circumferential tensile stress-strain curve of a pipe based on the DIC technology according to claim 3, characterized in that: Since the strain measured by the DIC measurement system is the linear strain in three-dimensional space, rather than the circumferential strain of the pipe, the present invention provides a method for calculating the circumferential engineering strain and true strain of the pipe: First, take the strain between point one (1) and point two (2) along the length direction of the diameter-reducing section and the strain between point three (3) and point four (4) in the width direction of the diameter-reducing section at the fracture of the specimen output by the DIC test system. 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 is the circumferential engineering strain, is the arc length after stretching, is the initial arc length between point one (1) and point two (2); where ε r is the strain in the wall thickness direction, is the outer diameter of the pipe, and L0 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: e 1-2 +e r +e 3-4 =0 Circumferential true strain can be described as: The force-bearing unit at any position on the specimen, under the contact conditions of axial tension P and friction coefficient μ, has a radial tension and a circumferential tension in force equilibrium: When is very small, Circumferential engineering stress (σ0) and true stress can be described respectively as: Wherein, A0 and A are respectively the cross-sectional area of the initial reducing section and the cross-sectional area of the reducing section during the stretching process, A0 = W0t0, A = W0t0(1 + e3-4)(1 + er); t0 is the wall thickness of the pipe, and W0 is the width of the reducing section.

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