A method and device for measuring the circumferential r value of a pipe

Abstract

The present invention discloses a method and device for determining the annular r value of a pipe, and relates to the field of mechanical property testing of pipes; the method comprises: determining the size of an annular sample and cutting the sample from the pipe to be tested; making a speckle or a circular grid on the outer surface of the sample; sleeve the annular sample in the middle position outside the block rigid mold; filling the fluid medium into the liquid bag and controlling the internal pressure, using the uniform load generated by the pressurization of the liquid bag to drive the block rigid mold to move uniformly outward in the radial direction, so that the annular sample undergoes equal-diameter bulging; measuring the width and annular strains of the speckle or grid area on the annular sample; and determining the thickness-wise profile coefficient of the annular pipe according to the width and annular strains. The present invention realizes the direct determination of the annular r value of the pipe by uniformly pressurized equal-diameter bulging, and significantly reduces the influence of friction and greatly improves the measurement accuracy by reasonably designing the width of the sample and the number of rigid mold petals, and designing a lubricating layer between the sample / rigid mold or prefabricating a microstructure on the surface of the rigid mold.

Description

Technical Field

[0001] The invention relates to the field of pipe mechanical property testing, in particular to a method and a device for measuring the circumferential r value of a pipe. Background Art

[0002] Complex curved thin-walled tubular components are widely used in aerospace and automotive fields due to their lightweight and high performance. Such components are generally manufactured by advanced technologies such as hydraulic forming. They are subject to complex stresses during the manufacturing process and are prone to wrinkling and cracking defects. Therefore, using the finite element method to accurately simulate the forming process of complex curved thin-walled structural parts has always been the goal pursued by scientific research and industry. This can not only quickly and accurately predict the defects that may occur during the forming process, thereby determining reasonable process parameters, but also greatly shorten the product development cycle and reduce mold manufacturing costs. However, in order to improve the accuracy of finite element simulation, it is necessary to accurately obtain the plastic constitutive relationship of the tube. During the preparation (extrusion, drawing, rolling or coil welding) of the initial tube blank, the macroscopic mechanical properties in different directions show obvious anisotropic characteristics due to the texture orientation. The thickness anisotropy coefficient r value of the tube in the axial and circumferential directions is an important parameter for constructing the plastic constitutive relationship model. Whether the tube r value can be accurately obtained is crucial for accurately constructing the plastic constitutive relationship of the tube, describing the deformation behavior of the tube, improving the simulation accuracy and formulating reasonable forming process parameters.

[0003] However, due to the special geometric structure of the tube, it is impossible to conduct uniaxial tensile tests in any direction to test its r value like the plate. At present, only the axial thickness anisotropy coefficient of the tube (0°) can be obtained by a method similar to the uniaxial tensile test of the plate (GB / T 5027-2016), and the determination of r values ​​in other directions is very difficult. In the paper entitled "On constitutive modeling of aluminum alloys for tube hydroforming application" published in the top international journal International Journal of Plasticity, the tube was unfolded into a plate and then the specimens were prepared in different directions for testing, referring to the existing standard uniaxial tensile test of the plate to test the thickness anisotropy coefficient r value in any direction within the surface. However, the tube undergoes obvious work hardening during the flattening process, which changes its mechanical properties. The test results cannot accurately reflect the true anisotropic performance parameters of the original tube. For the circumferential direction (90°) of the tube, the D-block die, hydraulic bulging tube blank or three-jaw chuck is currently used to drive the sample to perform circumferential stretching to test the r value (patents ZL200610010154.4, ZL201310722088.3 and ZL201510267394.1). Although this method can ensure that the sample is expanded in equal diameter, the r value obtained is more direct and reliable than single pulling after flattening, and it also avoids the error caused by approximating the axial r value, but it cannot overcome the influence of friction, resulting in a large error in the test result. The researchers proposed a theoretical prediction method for the r value of the tube in different directions, established the relationship between the thickness anisotropy coefficient and the yield criterion in any direction of the tube, and then determined the undetermined coefficient of the yield criterion through the test data of the hydraulic bulging of the tube, and finally determined the thickness anisotropy coefficient r value through the yield criterion calculation (patents ZL201911186112.X, ZL201911186245.7 and ZL201911186251.2). The anisotropy coefficient r value can also be obtained by substituting the stress ratio and strain increment ratio obtained from the axial single pull test and two sets of bidirectional loading tests into the above relationship (patent number ZL201911186271.X).

[0004] On the one hand, the above theoretical method is affected by the non-equal diameter deformation caused by hydraulic bulging, and on the other hand, the measurement accuracy of the r value is affected by the accuracy of the selected yield criterion theoretical model, and the yield criterion parameters themselves are not easy to measure accurately. In the process of determining the yield criterion parameters, the thickness anisotropy coefficient r value of the tube blank is required. There is a paradox in the principle of this method, which will also lead to uncertainty in the test results. Therefore, it is urgent to establish a method and device that can directly and accurately measure the thickness anisotropy coefficient r value of the tube in different directions (especially the circumferential direction), which has important theoretical significance for accurately constructing the plastic constitutive relationship of anisotropic thin-walled tube blanks and realizing high-precision simulation of tubular thin-walled component forming. Summary of the invention

[0005] The purpose of the present invention is to provide a method and device for determining the circumferential r value of a pipe, so as to solve the problems existing in the above-mentioned prior art, accurately construct an anisotropic plastic constitutive model of the pipe, and realize high-precision numerical simulation of the forming of tubular thin-walled components.

[0006] To achieve the above object, the present invention provides the following solutions:

[0007] The present invention provides a method for measuring the circumferential r value of a pipe, which uses a uniformly distributed load generated by pressurizing a liquid bag to drive a segmented rigid mold to move uniformly outward in the radial direction, so that the pipe ring sample undergoes equal-diameter bulging. During the bulging process, the strain in the width direction and circumferential direction of a certain area on the ring sample is measured, and the thickness strain is calculated according to the principle of constant volume of plastic deformation to determine the circumferential r value of the metal pipe; the method specifically includes the following steps:

[0008] Step S1: determining the size of the annular sample according to the diameter and wall thickness of the pipe to be tested, and cutting the annular sample from the pipe;

[0009] Step S2: creating speckles or printing a circular grid on the outer surface of the annular sample;

[0010] Step S3: putting the annular sample on the middle position outside the block rigid mold;

[0011] Step S4: Fill the fluid bag with a fluid medium and control the internal pressure, and use the uniformly distributed load generated by the pressurization of the fluid bag to drive the segmented rigid mold to move uniformly outward in the radial direction, so that the annular sample undergoes equal-diameter bulging;

[0012] Step S5: measuring the strains in the width direction and the circumferential direction on the annular specimen in a certain rigid mold area of ​​a certain block;

[0013] Step S6: According to the width direction and the circumferential direction strains, combined with the volume invariance condition of plastic deformation, the value of the thickness-wise profile coefficient r of the tube along the circumferential direction is determined.

[0014] Optionally, the pipe to be tested is a metal pipe or a non-metal pipe, and the thickness of the pipe to be tested is 0.1 to 20 mm.

[0015] Optionally, the width B0 and the diameter D0 of the annular specimen satisfy B0 / D0≤1 / 10.

[0016] Optionally, in step S3, a lubricant is sprayed between the segmented rigid mold and the annular sample, a lubricating layer such as polytetrafluoroethylene (PTFE) is provided, or a microstructure is prefabricated on the outer surface of the segmented rigid mold to minimize the influence of friction and improve deformation uniformity.

[0017] Optionally, in step S2, white matte paint is sprayed on the outer surface of the annular sample within a range of at least 60° to completely cover the white base on the surface, and then black matte paint is used to evenly spray speckles on the white base. In step S5, during the deformation of the annular sample, a three-dimensional full-field strain measurement and analysis system (digital image correlation method DIC) is used to test the strains of the sample surface in the width direction and the annular direction in real time.

[0018] Optionally, in step S2, a circular grid with a diameter of 1 to 5 mm is printed on the surface of the sample by electrolytic corrosion, and in step S5, a grid strain analysis test system is used to obtain the strains in the width direction and the circumferential direction of the surface of the annular sample.

[0019] The present invention also provides a device for determining the circumferential r value of a pipe, comprising a strain measurement and data acquisition system and a block rigid mold for sleeved annular specimens, wherein a liquid bag is placed inside the block rigid mold, and the initial outer diameter and length of the liquid bag are the same as the inner diameter and length of the block rigid mold; an upper limit plate and a lower limit plate are symmetrically arranged at both ends of the block rigid mold, and the positions of the upper limit plate and the lower limit plate are limited by a moving crossbeam of a universal testing machine or other limiting mechanisms to prevent the liquid bag from bulging outward at both ends during pressurization; the block rigid mold and the liquid bag are clamped between the upper limit plate and the lower limit plate; a pressurization and control system is externally connected to the liquid bag; the pressurization and control system can apply a fluid medium to the inside of the liquid bag, and use the uniformly distributed load generated by the pressurization of the liquid bag to drive the block rigid mold to move uniformly outward in the radial direction, so that the annular specimen undergoes equal-diameter bulging during the outward movement of the block rigid mold; the strain measurement and data acquisition system can measure the strain on the surface of the annular specimen to calculate the circumferential r value of the pipe.

[0020] Optionally, the liquid bag is a cylindrical elastic sac with one end closed and the other end provided with a liquid filling port, and the thickness of the liquid bag needs to ensure that it will not be squeezed out from the gap between the block rigid molds during the expansion process; the pressurization and control system includes a booster, a pressure control system and a fluid pressure sensor; the liquid filling port of the liquid bag is connected to the booster through a pipeline.

[0021] Optionally, the number of petals N of the segmented rigid mold is at least 8, and N ≥ 12 is usually recommended.

[0022] Compared with the prior art, the present invention has achieved the following technical effects:

[0023] The present invention achieves accurate determination of the anisotropic coefficient r-value of the circumferential thickness of the pipe by driving the block rigid die equal-diameter bulging with the uniformly distributed load generated by the liquid bag pressurization. Compared with the existing methods, the present invention can directly experimentally determine the circumferential r-value of the pipe, is not affected by the yield criterion, and has a simple testing process; in addition, during the equal-diameter bulging process, the circumferential and axial radii of curvature of the measuring area of ​​the pipe ring sample do not change, eliminating the influence of non-equal-diameter deformation on the r-value measurement. The unidirectional tensile deformation of the pipe ring sample along the circumferential direction is achieved by using uniformly distributed pressurized equal-diameter bulging. By reasonably designing the width of the ring sample and the number of block rigid die petals, and designing a lubricating layer between the ring sample and the block rigid die or prefabricating a microstructure on the surface of the block rigid die, the friction effect between the sample / rigid die is significantly reduced, and the measurement accuracy of the circumferential r-value of the pipe blank is greatly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0025] Figure 1 It is a schematic diagram of the initial stage of testing the circumferential r value of a pipe by uniformly distributed pressurized equal-diameter bulging according to an embodiment of the present invention;

[0026] Figure 2-1 It is a schematic diagram of the intermediate deformation stage of the circumferential r value of a pipe tested by uniformly distributed pressurized equal-diameter bulging according to an embodiment of the present invention;

[0027] Figure 2-2 The stress state of the pipe during the uniformly distributed pressurized equal-diameter bulging process according to the embodiment of the present invention;

[0028] Figure 3 For the embodiment of the present invention Figure 2-1 Middle AA section view;

[0029] Figure 4 This is a flow chart of a method for directly measuring the circumferential r value of a pipe by uniformly distributed pressurized equal-diameter bulging according to an embodiment of the present invention;

[0030] Figure 5 A ring-shaped sample with speckles is produced for the embodiment of the present invention;

[0031] Figure 6 A ring-shaped sample with a circular grid printed thereon for an embodiment of the present invention;

[0032] Figure 7 This is a schematic diagram of the principle of an experimental device for directly measuring the circumferential r value of a pipe by uniformly distributed pressurized equal-diameter bulging according to an embodiment of the present invention;

[0033] Figure 8 A schematic diagram of providing a lubricating layer between an annular specimen and a segmented rigid mold according to an embodiment of the present invention;

[0034] Fig. 9 A schematic diagram of prefabricating a microstructure on the outer surface of a segmented rigid mold according to an embodiment of the present invention;

[0035] Fig.10 The width direction strain ε of the 5A02 aluminum alloy tube ring sample measured in the embodiment of the present invention is z -Thickness strain ε t path.

[0036] In the figure, 1-upper limit plate; 2-block rigid mold; 3-annular specimen; 4-pressurized fluid; 5-fluid bag; 6-lower limit plate; 7-booster; 8-pressure control system; 9-fluid pressure sensor; 10-strain measurement and data acquisition system; 11-lubricating layer; 12-surface microstructure. DETAILED DESCRIPTION

[0037] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] The purpose of the present invention is to provide a method and device for determining the circumferential r value of a pipe, so as to solve the problems existing in the above-mentioned prior art, accurately construct an anisotropic plastic constitutive model of the pipe, and realize high-precision numerical simulation of the forming of tubular thin-walled components.

[0039] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0040] The present invention provides a method and device for directly measuring the anisotropic coefficient r value of the circumferential thickness of a pipe by uniformly distributed pressurized equal-diameter bulging, which can directly and accurately measure the circumferential r value of a pipe blank, and provide accurate parameters for constructing an anisotropic plastic constitutive model of the pipe and realizing high-precision numerical simulation of the forming of tubular thin-walled components.

[0041] Specific plan 1

[0042] like Figure 1 , Figure 2-1 , Figure 2-2 and Figure 3As shown, the uniformly distributed pressurized equal-diameter bulging is used to test the anisotropic coefficient r of the thickness of the pipe in the circumferential direction. The basic idea is to apply a fluid medium in the liquid bag 5, the fluid medium is a pressurized fluid 4, and the block rigid mold 2 is driven to move uniformly outward in the radial direction by controlling the fluid pressure p in the liquid bag 5, and a contact pressure p' is generated between the block rigid mold 2 and the tube blank annular sample 3, thereby driving the sample 3 to undergo equal-diameter bulging (circumferential single-pulling deformation). By measuring the strain of the M point area on the annular sample 3 at the corresponding position of a certain rigid module, the anisotropic coefficient r of the thickness in the circumferential direction can be obtained.

[0043] Since there is inevitably a certain amount of friction between the tube billet annular specimen and the block rigid die, the friction will cause uneven distribution of stress and strain. Friction will not only cause the annular stress σ at different positions on the annular specimen to θ Different (such as the hoop stress at the module edge L, R point and the center point M is different), and it will cause axial tensile stress σ z , which will cause the deformation of the specimen to deviate from the circumferential single-pulling state. The magnitude of the friction effect is mainly related to three factors: first, it is affected by the size of the friction coefficient; second, it is affected by the contact angle α between the annular specimen and the block rigid mold, that is, it is related to the number of petals of the block rigid mold; third, it is affected by the width B0 of the annular specimen. The following specifically analyzes the conditions that need to be met when the influence of friction on the uniformity of stress and strain distribution can be ignored.

[0044] Assuming the number of petals of the block rigid module is N, the contact angle of the single rigid module is α = 2π / N; the annular specimen is at the edge of the block rigid module (such as Figure 3 The hoop stress at point R is In the middle of the split rigid module ( Figure 3 The hoop stress at point M is but Must be greater than Assuming that the friction coefficient between the annular specimen and the petal rigid module is μ, the mechanical equilibrium conditions in the radial and annular directions of the region between M and R on the annular specimen are:

[0045]

[0046]

[0047] Where σ θ is the hoop stress on the microelement with an angle of dθ. Since the angle dθ is very small, sindθ / 2 = dθ / 2, cosdθ / 2 = 1, and by rearranging formulas (1) and (2), we can obtain:

[0048]

[0049] Integrate both sides of the above equation and combine the boundary conditions: θ=α / 2=π / N, The integral constant is Therefore, the hoop stress at any point in the region between M and R can be calculated by the following formula:

[0050]

[0051] When θ=0, Substituting into formula (4) we can get

[0052]

[0053] According to formula (5), the ratio of the stress at the edge point R of the rigid module to the stress at the middle point M is The change of the number of petals N of the block rigid mold is shown in Table 1. It can be seen from Table 1 that with the increase of the number of petals N, the distribution of the hoop stress gradually tends to be uniform. When the number of petals N reaches 8, even if the friction coefficient reaches 0.20, the maximum difference in the hoop stress is 8.2%. When the number of petals N reaches 12, the maximum difference in the hoop stress is only about 5%. Continuing to increase N does not significantly change The influence of friction on hoop stress can be ignored. Therefore, in actual testing, at least N≥8 is required, and N≥12 is recommended.

[0054] Table 1

[0055]

[0056] like Figure 2-1 and Figure 2-2 As shown in the figure, the hoop stress σ generated at point M when the tube billet ring specimen undergoes hoop tensile deformation θ and axial stress σ z They can be expressed as:

[0057]

[0058] Then, the ratio of axial to hoop stress can be obtained:

[0059]

[0060] According to formula (7), the axial stress σ z and hoop stress σ θ The ratio σ z / σ θ The changes of friction coefficient μ and ring specimen width / diameter ratio B0 / D0 are shown in Table 2. It can be seen from Table 2 that when B0 / D0 reaches 0.10, even if the friction coefficient reaches 0.20, σ z / σ θThe value of B0 / D0 is 0.02, which means that the axial stress is only 2% of the hoop stress. It can be considered that the specimen is in a uniaxial tensile stress state. Therefore, it is recommended to use B0 / D0≤0.10 in actual testing.

[0061] Table 2

[0062]

[0063] In addition, when using uniformly distributed pressurized equal-diameter bulging to determine the circumferential r value of the pipe, the block rigid die and the inner surface of the annular specimen can be fully lubricated to reduce the friction coefficient μ as much as possible, thereby further improving the uniformity of stress and strain and making the stress state on the pipe annular specimen during the uniformly distributed pressurized equal-diameter bulging process almost close to that of circumferential single pulling.

[0064] During the uniform pressure equal diameter bulging process, the mesh with size a0×b0 at point M on the tube ring specimen is deformed into a mesh with size a×b. The hoop strain generated here is ε θ =ln(a / a0), the strain in the width direction is ε z =ln(b / b0). According to the volume invariance condition, the anisotropy coefficient of the circumferential thickness is:

[0065]

[0066] Combination Figure 1-4 To explain this embodiment, Figure 1 It is a schematic diagram of the initial stage of testing the circumferential r value of a pipe by uniformly distributed pressurized equal-diameter bulging according to an embodiment of the present invention; Figure 2-1 It is a schematic diagram of the intermediate deformation stage of the circumferential r value of a pipe tested by uniformly distributed pressurized equal-diameter bulging according to an embodiment of the present invention; Figure 3 For the embodiment of the present invention Figure 2-1 Middle AA section view; Figure 4 This is a flow chart of a method for directly measuring the circumferential r value of a pipe by uniformly distributed pressurized equal-diameter bulging according to an embodiment of the present invention. The measuring method specifically includes:

[0067] Step S1: determining the size of the annular sample according to the diameter and wall thickness of the pipe to be tested, and cutting the annular sample from the pipe;

[0068] Step S2: creating speckles or printing a circular grid on the outer surface of the annular sample;

[0069] Step S3: putting the annular sample on the middle position outside the block rigid mold;

[0070] Step S4: Fill the fluid bag with a fluid medium and control the internal pressure, and use the uniformly distributed load generated by the pressurization of the fluid bag to drive the segmented rigid mold to move uniformly outward in the radial direction, so that the annular sample undergoes equal-diameter bulging;

[0071] Step S5: measuring the strains in the width direction and the circumferential direction on the annular specimen in a certain rigid mold area of ​​a certain block;

[0072] Step S6: According to the width direction and hoop direction strain, combined with the plastic deformation volume invariance condition, determine the thickness-wise profile coefficient r of the tube along the hoop direction θ value.

[0073] The following is a detailed discussion of each step:

[0074] Step S1: Determine the size of the annular sample according to the diameter and wall thickness of the pipe to be tested, and cut the annular sample from the pipe. The pipe to be tested can be a metal pipe or a non-metallic pipe, and its diameter is D0, which generally needs to be greater than 10 mm, otherwise it cannot be processed due to the small size of the block rigid mold and the liquid bag; the thickness is t0, which is generally in the range of 0.1 to 20 mm. The width B0 and diameter D0 of the annular sample 3 are designed to satisfy B0 / D0≤1 / 10. Use electric spark wire cutting or laser cutting to cut the annular sample 3 perpendicular to the axis of the pipe, and then use sandpaper or a grinding head to grind the cut section to eliminate the influence of the oxide layer formed during the cutting process on the r value determination result.

[0075] Step S2: creating speckles or printing a circular grid on the outer surface of the annular sample, specifically comprising: determining whether to create speckles or print a circular grid on the outer surface of the annular sample according to the selected strain test method.

[0076] When the three-dimensional full-field strain measurement and analysis system (digital image correlation method DIC) is used to test the strain distribution on the surface of the annular specimen during deformation, white matte paint is sprayed on the outer surface of the annular specimen within a range of at least 60° to completely cover the surface before the test, and then black matte paint is used to evenly spray speckles on the white background, such as Figure 5 shown.

[0077] When using traditional grid strain analysis test systems such as ASAME to test the strain distribution on the surface of the specimen during deformation, a circular grid with a diameter of 1 to 5 mm is printed on the outer surface of the circumferential specimen within a range of at least 60° using the electrolytic corrosion method before testing. The number of grids is 3 to 10. Figure 6 shown.

[0078] Step S3: putting the annular specimen on the middle position outside the block rigid mold, specifically including:

[0079] Step S31: Press the block rigid mold 2, the liquid bag 5, and the lower limit plate 6 Figure 1 Assembled in the position shown.

[0080] Step S32: Put the annular sample 3 on the middle position outside the block rigid mold 2 from the top. In order to minimize the influence of friction, spray molybdenum disulfide lubricating coating on the inner surface of the annular sample 3 and the outer surface of the block rigid mold 2. The thickness of the lubricating coating is 10 to 20 μm.

[0081] Step S33: Finally, the upper limit plate 1 is installed on the upper side of the liquid bag 5 and the divided rigid mold 2.

[0082] Step S4: Filling the fluid bag with a fluid medium and controlling the internal pressure, using the uniformly distributed load generated by the pressurization of the fluid bag to drive the segmented rigid mold to move uniformly outward in the radial direction, so as to cause the annular sample to undergo isodiametric bulging, specifically comprising:

[0083] Step S41: filling the fluid bag 5 with a fluid medium 4 through the inlet thereof, wherein the fluid medium 4 is a liquid such as water, emulsion, hydraulic oil, or a gas such as air and nitrogen;

[0084] Step S42: The fluid medium 4 inside the liquid bag is pressurized by a pressurizing device, and its pressure p is controlled. The liquid bag 5 generates a uniformly distributed load under the action of the internal pressure, driving the segmented rigid mold 2 to move uniformly outward in the radial direction, and the segmented rigid mold 2 will drive the annular sample 3 to undergo equal-diameter bulging.

[0085] Step S5: measuring the strain in the width direction and the circumferential direction of the annular specimen in a certain rigid mold area, specifically including:

[0086] During the deformation process, the circumferential strain ε in the speckle area on the surface of the annular specimen 3 is measured in real time using a three-dimensional full-field strain measurement and analysis system (digital image correlation method DIC). θ and the width direction strain ε z .

[0087] After the deformation, the circumferential strain ε of the printed grid on the surface of the annular specimen 3 was tested using a conventional grid strain analysis test system such as ASAME. θ and the width direction strain ε z .

[0088] Step S6: According to the width direction and hoop direction strain, combined with the plastic deformation volume invariance condition, determine the thickness-wise profile coefficient r of the tube along the hoop direction θ Values, including:

[0089] Step S61: When the strain is tested using a three-dimensional full-field strain measurement and analysis system (digital image correlation method DIC)

[0090] Step S611: Select 3-5 typical points in the measured speckle area and extract the hoop strain ε of each typical point during the deformation process. θ - Width direction strain ε z path;

[0091] Step S612: Combine the plastic deformation volume invariance condition ε t =-(ε z +ε θ ), and plot the deformation process of each typical point ε z -ε t The path of each typical point is z -ε t The path is linearly fitted, and the slope of the fitting line is the ratio of the width strain to the thickness strain ε z / ε θ , that is, the thickness anisotropy coefficient r at a typical point θ .

[0092] Step S613: The thickness anisotropy coefficients at the selected typical points are averaged to determine the thickness anisotropy coefficient of the pipe to be tested along the circumferential direction.

[0093] Step S62: When using a conventional grid strain analysis test system such as ASAME to test strain

[0094] Step S621: After the deformation is completed, the annular strain ε of each circular mesh is calculated. θ and the width direction strain ε z Substitute the data into formula (8) to calculate the thickness anisotropy coefficient of a single grid position;

[0095] Step S622: averaging the thickness anisotropy coefficients at each circular grid, thereby determining the thickness anisotropy coefficient of the pipe to be tested along the circumferential direction.

[0096] like Figure 7 As shown, the present invention also provides a device for directly measuring the circumferential r value of a pipe by uniformly distributed pressurized equal-diameter bulging, the device comprising:

[0097] The segmented rigid mold 2 moves outward uniformly in the radial direction under the uniformly distributed load generated by the pressurized liquid bag 5, and is used to apply a normal pressure p′ to the inner surface of the annular sample 3, so that the annular sample 3 undergoes isodiametric bulging (annular single tensile stress state). The number of petals N of the segmented rigid mold 2 is at least 8, and N≥12 is recommended;

[0098] The liquid bag 5 is composed of an elastic body such as rubber or polyurethane, and is a cylindrical elastic bladder with one end completely closed and the other end provided with a liquid filling port. The thickness thereof is 0.5 to 20 mm, and it is necessary to ensure that the liquid will not be squeezed out from the gap between the segmented rigid molds during the expansion process. The liquid bag 5 contains a pressurized fluid 4, which generates a uniformly distributed load under the action of the pressurized fluid 4, and is used to drive the segmented rigid molds 2 to move uniformly outward.

[0099] The upper limit plate 1 and the lower limit plate 6 are used to limit the axial movement and deformation of the liquid bag 5 to prevent the liquid bag 5 from expanding and cracking in both axial directions under the action of the pressurized fluid medium 4.

[0100] The pressurization and control system includes a booster 7, a pressure control system 8 and a fluid pressure sensor 9, which are used to fill the pressurized fluid medium 4 into the liquid bag 5 and control its pressure, wherein the booster 7 is used to fill the fluid medium and pressurize it, the pressure control system 8 is used to control the pressure inside the liquid bag 5, and the fluid pressure sensor 9 is used to measure and display the pressure and provide real-time feedback to the pressure control system 8.

[0101] The strain measurement and data acquisition system 10 is used to measure the strain on the surface of the annular sample using a three-dimensional full-field strain measurement and analysis system (DIC digital correlation technology) or a grid strain analysis system.

[0102] The liquid bag 5 is placed inside the block rigid mold 2, and the initial outer diameter and length of the liquid bag 5 are the same as the inner diameter and length of the block rigid mold 2; the block rigid mold 2 and the liquid bag 5 are clamped between the upper limit plate 1 and the lower limit plate 6, and the positions of the upper limit plate 1 and the lower limit plate 6 are limited by the movable crossbeam of the universal testing machine or a special limit device; the liquid bag 5 is connected to the pressurization and control system, including a booster 7, a pressure control system 8 and a fluid pressure sensor 9; a pressurized fluid medium 4 is applied to the inside of the liquid bag 5 through the booster 8, and the uniformly distributed load generated by the pressurization of the liquid bag 5 is used to drive the block rigid mold 2 to move uniformly outward in the radial direction, so that the annular specimen 3 undergoes equal-diameter expansion during the outward movement of the block rigid mold 2; the strain measurement and data acquisition system 10 is used to measure the strain on the surface of the annular specimen 3, which is used to calculate the annular r value of the pipe.

[0103] Specific plan 2:

[0104] The difference between this scheme and scheme 1 is that in order to further reduce the influence of friction on the uniformity of stress and strain, a lubricating layer 11 (such as polytetrafluoroethylene PTFE) is provided between the annular sample 3 and the block rigid mold 2. Figure 8 The lubricating layer 11 is an annular structure, with a thickness of 0.1-5 mm and a width at least 20% greater than the width B0 of the annular sample. Others are the same as the first solution.

[0105] Specific plan three:

[0106] The difference between this scheme and scheme 1 is that in order to further reduce the influence of friction on the uniformity of stress and strain, a microstructure is prefabricated on the surface of the block rigid mold 2, such as Fig. 9 As shown. The surface microstructure can be a corrugated shape along the annular or axial direction or a cross-corrugated shape along the annular and axial direction, or can be other surface lattice structures that can reduce the contact area and friction. Other structures are the same as those in Scheme 1.

[0107] The present invention is further described below in conjunction with specific embodiments

[0108] Example 1: The anisotropy coefficient of the circumferential thickness of the 5A02 aluminum alloy thin-walled tube is r θ Taking the measurement as an example, the outer diameter of the tube is D0=63mm and the wall thickness is t0=2.0mm.

[0109] Step 1: According to the outer diameter of the pipe to be tested D0 = 63mm, the width B0 of the ring sample is designed to meet B0 / D0 = 1 / 10, and the width of the ring sample is determined to be B0 = 6.3mm. Cut the ring sample perpendicular to the axis of the pipe using electric spark wire cutting, and then use sandpaper to grind the cut section to eliminate the oxide layer formed during the cutting process. θ The impact of the value determination results.

[0110] Step 2: Before testing, spray white matte paint on the outer surface of the circumferential specimen within a range of at least 60° to completely cover the surface, and then use black matte paint to evenly spray a speckle pattern on the white background.

[0111] Step 3: Press the block rigid mold, liquid bag, and lower limit plate Figure 1 Assemble in the position shown, spray Molykote D-321R quick-drying molybdenum disulfide on the inner surface of the annular specimen and the outer surface of the block rigid mold respectively, with a coating thickness of at least 10μm, put the annular specimen on the middle position outside the block rigid mold from the top, install the upper limit plate on the upper side of the liquid bag and the block rigid mold, and use the crossbeam of the universal testing machine to fix and constrain its position.

[0112] Step 4: Use a supercharger to fill the liquid bag with emulsion and control the internal pressure p to increase linearly through a pressure control system. The pressurized liquid bag generates a uniformly distributed load to drive the segmented rigid mold to move uniformly outward in the radial direction, thereby causing the annular specimen to bulge in equal diameters until the annular specimen breaks.

[0113] Step 5: Use the three-dimensional full-field strain measurement and analysis system (digital image correlation method DIC) to measure the annular strain ε in the speckle area on the surface of the 5A02 annular sample in real time θ and the width direction strain ε z .

[0114] Step 6: Select five typical points A1, A2, A3, A4, A5 in the measured speckle area ( Fig.10 The hoop strain ε of each typical point during deformation is obtained. θ - Width direction strain ε z Path. Combined with the plastic deformation volume invariance condition ε t =-(ε z +ε θ), draw the path ε of each typical point A1-A5 during the deformation process z -ε t ,like Fig.10 As shown. For the path ε at 5 typical points z -ε t Perform a linear fit, and the slope of the fitted line is the ratio of the width strain to the thickness strain ε z / ε θ , that is, the thickness anisotropy coefficient r at a typical point θ The thickness anisotropy coefficients measured at five typical points are averaged to determine the thickness anisotropy coefficient of the pipe to be tested along the circumferential direction, as shown in Table 3.

[0115] Table 3

[0116]

[0117] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0118] In the description of the present invention, it should be noted that the terms "center", "top", "bottom", "left", "right", "vertical", "horizontal", "inside", "outside" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, which are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first" and "second" are used for descriptive purposes only and cannot be understood as indicating or implying relative importance.

[0119] The present invention uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only used to help understand the method and core ideas of the present invention. At the same time, for those skilled in the art, according to the ideas of the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A method for measuring the circumferential r value of a pipe, characterized in that: The steps include: Step S1: determining the size of the annular sample according to the diameter and wall thickness of the pipe to be tested, and cutting the annular sample from the pipe; Step S2: making speckles or printing circular grids on the outer surface of the annular sample, spraying a white matte paint on the outer surface of the annular sample within a range of at least 60° to completely cover the white background, and then spraying speckles evenly on the white background with black matte paint; Step S3: putting the annular sample on the middle position outside the block rigid mold, the number of petals N of the block rigid mold is ≥ 12; Step S4: filling the fluid bag with a fluid medium and controlling the internal pressure, and using the uniformly distributed load generated by pressurizing the fluid bag to drive the segmented rigid mold to move uniformly outward in the radial direction, so that the annular sample undergoes isodiametric bulging; the fluid bag is a cylindrical elastic bladder with one end closed and the other end provided with a liquid filling port, and the initial outer diameter and length of the fluid bag are the same as the inner diameter and length of the segmented rigid mold; Step S5: measuring the strains in the width direction and the circumferential direction on an annular specimen in a certain rigid mold area of ​​a certain block, wherein the width B0 and the diameter D0 of the annular specimen satisfy B0 / D0≤1 / 10; Step S6: According to the width direction and the circumferential direction strains, combined with the volume invariance condition of plastic deformation, the value of the thickness-wise profile coefficient r of the tube along the circumferential direction is determined.

2. The method for measuring the circumferential r value of a pipe according to claim 1, characterized in that: The pipe to be tested is a metal pipe or a non-metal pipe, and the thickness of the pipe to be tested is 0.1-20 mm.

3. The method for measuring the circumferential r value of a pipe according to claim 1, characterized in that: In step S3, a lubricant is sprayed between the segmented rigid mold and the annular sample, a lubricating layer is provided, or a microstructure is prefabricated on the outer surface of the segmented rigid mold.

4. The method for measuring the circumferential r-value of a pipe according to claim 1, characterized in that: In step S5, a three-dimensional full-field strain measurement and analysis system is used to test the strains of the sample surface in the width direction and the circumferential direction in real time.

5. The method for measuring the circumferential r-value of a pipe according to claim 1, characterized in that: In step S2, a circular grid with a diameter of 1 to 5 mm is printed on the surface of the sample by electrolytic corrosion, and in step S5, a grid strain analysis test system is used to obtain the strains in the width direction and the circumferential direction of the surface of the annular sample.

6. A device for measuring the circumferential r value of a pipe based on the method for measuring the circumferential r value of a pipe according to any one of claims 1 to 5, characterized in that: It includes a strain measurement and data acquisition system and an external block rigid mold for sleeve annular specimens, wherein a liquid bag is placed inside the block rigid mold, and the initial outer diameter and length of the liquid bag are the same as the inner diameter and length of the block rigid mold; upper limit plates and lower limit plates are symmetrically arranged at both ends of the block rigid mold, and the block rigid mold and the liquid bag are clamped between the upper limit plates and the lower limit plates; the liquid bag is externally connected to a pressurization and control system; the pressurization and control system can apply a fluid medium to the inside of the liquid bag; the strain measurement and data acquisition system can measure the strain on the surface of the annular specimen.

7. The device for measuring the circumferential r value of a pipe according to claim 6, characterized in that: The pressurization and control system includes a pressure booster, a pressure control system and a fluid pressure sensor; the filling port of the fluid bag is connected to the pressure booster through a pipeline.

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