Method and device for testing in-plane tensile mechanical property of pipe in any direction

Abstract

The invention relates to the technical field of pipe mechanical property testing, in particular to a method and a device for testing the tensile mechanical property in any direction in a pipe surface. The method comprises the following steps: designing geometric configurations of an asymmetric annular sample and a matched module; determining the sizes of a sample and a module by combining finite elements and multi-objective optimization; after assembly, the testing machine pulls the module to enable the central section of the sample to generate pure elongation deformation; the strain in the width direction and the tensile direction is measured in real time through a DIC system, and the strain in the thickness direction is calculated according to the volume invariant principle; obtaining a thickness anisotropy coefficient r value by fitting a width-thickness strain path; a central cross section tensile stress calculation formula considering friction force is established, real stress is calculated through testing machine load and deformation data, and a stress-strain curve is obtained in combination with strain data. According to the invention, the r value, the yield strength and the stress-strain curve in any direction in the pipe surface can be directly and accurately tested, the straightening deformation and friction influence of the sample can be effectively avoided, and the test process is simple and reliable.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of pipe mechanical property testing, and more particularly to a method and device for testing the in-plane tensile mechanical properties of pipes in any direction. BACKGROUND

[0002] In the prior art, complex curved thin-walled tubular components are widely used in aerospace, automobiles and other fields due to their lightweight and high performance. Such components are usually manufactured by advanced technologies such as hydroforming, and are prone to wrinkling and cracking defects during the manufacturing process due to complex stress. To accurately predict defects during forming, the accuracy of finite element simulation needs to be improved, which depends on the accurate construction of the pipe plastic constitutive model. The texture orientation formed in the preparation process (such as extrusion, drawing, rolling, etc.) of the initial pipe blank leads to obvious anisotropy of the macroscopic mechanical properties in different directions in the plane, and the thickness anisotropy coefficient r value (plastic strain ratio) and stress-strain curve are important parameters for constructing the plastic constitutive model. Therefore, determining the r value and stress-strain curve of pipes in different directions in the plane is crucial for studying the deformation behavior of pipes, constructing advanced plastic constitutive models, and improving the accuracy of finite element simulation.

[0003] Due to the semi-closed curved surface structure of the pipe, the in-plane tensile mechanical property testing is much more difficult than that of the plate. Currently, for the axial direction (0° direction) of the pipe, a method similar to the plate tensile test (GB / T228.1-2021) can be used to obtain the mechanical properties in this direction; for the hoop direction (90° direction) of the pipe, a block rigid die can be used to obtain the mechanical properties in this direction. However, using the axial and hoop mechanical properties can only draw the predicted trajectory of the constitutive model in the biaxial stress region, and cannot establish a complete yield ellipse in the plane stress region containing shear stress, because the undetermined parameters in the shear stress term of the calibrated constitutive model require mechanical properties in non-axial / hoop directions. Therefore, only by obtaining the r value and stress-strain curve of the pipe in any direction in the plane, the anisotropy characteristics of the pipe can be fully revealed, the prediction range of the constitutive model can be expanded from the biaxial stress state to the general plane stress state, and high-precision simulation of thin-walled tubular components can be achieved.

[0004] There are many difficulties in obtaining the mechanical properties of pipe in non-axis / circular direction. Some studies flatten the pipe blank into a plate blank and then perform plate tensile test. However, the pipe blank will be significantly work-hardened during the flattening process, which will change the mechanical properties, and the test results cannot accurately reflect the anisotropic properties of the original pipe blank. Some scholars use finite element simulation or inverse constitutive relationship to obtain the mechanical properties of pipe in any in-plane direction, but the accuracy of the results is limited by the accuracy of the finite element model and the constitutive relationship. Some literatures use D-shaped block mold to stretch the 45° inclined ring-shaped sample, but the parallel section of the sample is in a suspended state, which will produce straightening deformation during the stretching process, resulting in large test error of stress and strain, and the mechanical properties in this direction cannot be accurately obtained.

[0005] In summary, there is an urgent need to establish a method and device that can directly and accurately test the tensile mechanical properties (r value and stress-strain curve) of pipe in any in-plane direction, which is of great significance for the comprehensive study of the anisotropic deformation behavior of pipe, the accurate construction of pipe plastic constitutive model, and the high-precision simulation of thin-walled tubular component forming. SUMMARY

[0006] Therefore, the present application provides a method and device for testing the tensile mechanical properties of pipe in any in-plane direction, aiming to solve the problem that the existing technology cannot directly and accurately test the tensile mechanical properties (such as the thickness anisotropy coefficient r value, yield strength and stress-strain curve) of pipe in any in-plane direction. Specifically, the existing technology has the following problems in testing the mechanical properties of pipe in non-axis / circular direction: the flattening of pipe blank changes the mechanical properties, the accuracy of the results obtained by finite element simulation or inverse constitutive relationship is limited, and the parallel section of the sample is in a suspended state, which will produce straightening deformation and result in large test error.

[0007] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0008] A method for testing the tensile mechanical properties of pipe in any in-plane direction, the mechanical properties including the thickness anisotropy coefficient r value, yield strength and stress-strain curve, characterized in that: by designing an asymmetric ring-shaped tensile sample and a matching module, the central section of the sample only undergoes elongation deformation under the support of the module (the central section of the sample is supported by the lower module to avoid straightening deformation) and is in a uniaxial tensile stress state, and by establishing a mechanical model considering friction to calculate the true tensile stress and eliminate the influence of friction, the accurate determination of the mechanical properties is realized; the method comprises the following steps:

[0009] Step S1: design the geometric configuration of the asymmetric ring-shaped tensile sample along a certain tensile direction of the pipe and the upper and lower tensile modules;

[0010] Step S2: combine finite element simulation and optimization algorithm to perform multi-objective optimization on the size of the ring-shaped tensile sample and the module;

[0011] Step S3: Process the sample and the module according to the optimized size, assemble the sample and the module together and install them on the testing machine, the cross beam of the testing machine pulls the upper module to make the sample stretch;

[0012] Step S4: Measure the size and strain data of the sample in the width direction and the stretching direction in real time during the stretching process, and calculate the strain in the thickness direction according to the principle of invariable volume;

[0013] Step S5: Draw the strain-thickness strain path of the sample in the width direction, and obtain the r value in the stretching direction by function fitting on the path;

[0014] Step S6: Build a calculation formula of the tensile stress of the center section of the sample, substitute the testing machine load, sample deformation data and friction coefficient into the formula to calculate the tensile stress, and obtain the stress-strain curve in the direction combined with the strain data;

[0015] Step S7: Change the stretching direction, repeat steps S1 to S6, and finally obtain the r value and the stress-strain curve in any direction on the surface of the pipe.

[0016] In a specific implementable scheme, in the step S1, the whole stretching sample is an inclined elliptical ring cut from the pipe to be tested, the inclination angle of which is the same as the stretching direction, and parallel sections are designed on both sides of the sample, the width of the parallel section is less than the width of the sample, and the transition is made through a round corner.

[0017] In a specific implementable scheme, in the step S1, the upper and lower modules are inclined cylinders as a whole, the inclination angle of which is the same as the stretching direction, and the outer surface cooperates with the inner surface of the sample; the module is divided into upper and lower parts at the position corresponding to the round corner section of the sample, wherein the lower module is used to provide support for the parallel section of the sample; the division surface of the module is approximately U-shaped or inverted trapezoidal.

[0018] In a specific implementable scheme, the specific steps of multi-objective optimization in the step S2 include:

[0019] (1) Set the optimization variables, including the initial projected length L0 of the parallel section of the sample, the projected width W0, and the height H of the edge of the division surface in contact with the sample relative to the plane of the center section of the sample;

[0020] (2) Set the objective function, requiring that the stress ratio a of the center section in the width direction and the stretching direction be ≤0.05, the uniformity η of the tensile stress distribution of the center section be ≥95%, and the relative difference δ of the tensile force of the center section and one-half of the testing machine load be ≤5%;

[0021] (3) Set the constraint condition, requiring that the tensile strain ε generated by the center section under the condition of maintaining contact with the lower module be ≥0.15;

[0022] (4) The tensile process of the sample is simulated by a finite element explicit algorithm, and an optimization algorithm is used for iterative analysis to obtain the optimized size of the ring-shaped tensile sample and the module.

[0023] In a specific implementable scheme, in the step S3, the sample is cut from the pipe to be tested by using a wire cutting process; the overall module blank is first processed, and then cut into an upper module and a lower module; the outer surface of the module is lubricated during assembly, and then the sample is sleeved outside the module, and the upper module and the lower module are fixed on the upper and lower jigs respectively by using connecting pieces.

[0024] In a specific implementable scheme, in the step S4, the width of the central section of the sample, the width strain and the tensile direction strain are measured in real time by a three-dimensional digital image correlation (DIC) measurement system.

[0025] In a specific implementable scheme, in the step S5, the width strain-thickness strain path of the central section before the maximum load is reached is intercepted, a linear function is fitted thereon, and the slope obtained by the fitting is taken as the r value along the tensile direction, and the definition formula of the r value is:

[0026]

[0027] wherein dε w is the width strain increment, and dε t is the thickness strain increment.

[0028] In a specific implementable scheme, in the step S6, the calculation formula of the tensile stress σ of the central section of the parallel section of the sample is:

[0029]

[0030] wherein F is the tensile force on the central section, A is the area of the central section, W is the width of the central section during deformation, and t is the thickness of the central section during deformation.

[0031] The calculation formula of the tensile force F of the central section is:

[0032]

[0033] wherein T is the load of the testing machine, μ is the friction coefficient between the sample and the module, and θ is the central angle corresponding to the contact section of the sample above the central section.

[0034] In a specific implementable scheme, the calculation formula of the central angle θ is:

[0035]

[0036] Wherein, h is the height of the intersection of the sample straightening section and the contact section relative to the plane of the sample center section, and h≤H, H is the height of the split surface edge in contact with the sample relative to the plane of the sample center section.

[0037] The application also provides a device for testing the tensile mechanical properties of a pipe in any direction in the plane, characterized in that it comprises:

[0038] An upper module 5 and a lower module 6, the outer surface of which is configured to match the inner surface of the sample 1 to be tested, for supporting the sample and causing the central section 4 thereof to be elongated and deformed;

[0039] An upper mold frame 9 and a lower mold frame 10 for fixing the upper module 5 and the lower module 6, respectively;

[0040] A connecting piece 11 for detachably fixing the upper and lower modules 5, 6 to the upper and lower mold frames 9, 10, respectively;

[0041] Wherein, the split surface 7 of the upper and lower modules 5, 6 is approximately U-shaped or inverted trapezoidal.

[0042] In a specific implementation, the upper and lower modules 5, 6 are made of mold steel.

[0043] In a specific implementation, the connecting piece 11 is a square plug.

[0044] In a specific implementation, the upper module can apply a normal pressure to the inner surface of the sample, and the lower module can apply a normal pressure to the inner surface of the sample and provide support to the parallel section of the sample.

[0045] Compared with the prior art, the method and device for testing the tensile mechanical properties of a pipe in any direction in the plane can be used to determine the tensile mechanical properties of a pipe in any direction in the plane, including the thickness anisotropy coefficient r value, yield strength, and stress-strain curve, etc., effectively improving the accuracy and reliability of the test of the tensile mechanical properties of a pipe in any direction in the plane, overcoming the problems in the prior art such as performance change caused by flattening of the pipe blank, limited accuracy of the results of reverse solving by simulation or constitutive relation, and test error caused by straightening deformation of the sample, and has the following beneficial effects:

[0046] Through the design of the asymmetric ring-shaped sample and the asymmetric module configuration, it is ensured that the central section of the sample is tightly attached to the surface of the module to be elongated and deformed, avoiding straightening deformation and always being in a single tensile stress state;

[0047] The sample and the module size are determined by means of finite element and multi-objective optimization method to ensure uniform stress distribution of the central section;

[0048] A stress calculation formula considering friction is established to eliminate the influence of friction on the test results;

[0049] The pipe material can be directly tested without flattening treatment, and the anisotropy performance of the original pipe blank can be accurately reflected;

[0050] The test process is simple, the device requirement is low, and the application is facilitated, and reliable technical support is provided for comprehensively researching the anisotropy deformation behavior of the pipe material, accurately constructing a plastic constitutive model, and realizing high-precision simulation of thin-walled tubular component forming.

[0051] The present application can realize accurate determination of the tensile mechanical properties of the pipe material in any direction in the plane, thereby providing reliable data support for comprehensively revealing the anisotropy characteristics of the pipe material and accurately constructing a plastic constitutive model of the pipe material, and further extending the prediction range of the constitutive model from a double tensile stress state to a general plane stress state, and finally improving the finite element simulation precision of thin-walled tubular component forming, laying a foundation for accurate prediction of defects in the manufacturing process of complex curved thin-walled tubular components, determination of reasonable process parameters, shortening of product development cycle, and reduction of mold manufacturing cost. BRIEF DESCRIPTION OF DRAWINGS

[0052] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of the provided drawings.

[0053] Figure 1 The shape and size diagram of the pipe material in-plane arbitrary direction tensile mechanical property sample for the embodiment of the present application is shown in the figure;

[0054] Figure 2 The shape and size diagram of the pipe material in-plane arbitrary direction tensile mechanical property module for the embodiment of the present application is shown in the figure;

[0055] Figure 3 The finite element model diagram of the pipe material in-plane arbitrary direction tensile mechanical property for the embodiment of the present application is shown in the figure;

[0056] Figure 4 The sample and module size optimization flowchart of the pipe material in-plane arbitrary direction tensile mechanical property for the embodiment of the present application is shown in the figure;

[0057] Figure 5 The initial stage schematic diagram of the pipe material in-plane arbitrary direction tensile mechanical property for the embodiment of the present application is shown in the figure;

[0058] Figure 6 The intermediate stage schematic diagram of the pipe material in-plane arbitrary direction tensile mechanical property for the embodiment of the present application is shown in the figure;

[0059] Figure 7A-A cross-sectional view of the embodiment of the present application Figure 6 A-A cross-sectional view of the embodiment of the present application

[0060] Figure 8 A-A cross-sectional view of the embodiment of the present application Figure 6 A-A cross-sectional view of the embodiment of the present application

[0061] Figure 9 Flow chart of the method for testing the tensile mechanical properties of pipe material in any direction in the plane of the embodiment of the present application

[0062] Figure 10 A-A cross-sectional view of the embodiment of the present application

[0063] Figure 11 Wide-directional strain-thickness-directional strain path and the fitted straight line of the edge point of the center section of the 6061 aluminum alloy pipe sample measured in the embodiment of the present application

[0064] Figure 12 The r value of the 6061 aluminum alloy pipe in different directions in the plane measured in the embodiment of the present application

[0065] Figure 13 The stress-strain curve of the 6061 aluminum alloy pipe in different directions in the plane measured in the embodiment of the present application

[0066] Figure 14 Wide-directional strain-thickness-directional strain path and the fitted straight line of the edge point of the center section of the GH3625 high-temperature alloy pipe sample measured in the embodiment of the present application

[0067] Figure 15 The r value of the GH3625 high-temperature alloy pipe in different directions in the plane measured in the embodiment of the present application

[0068] Figure 16 The stress-strain curve of the GH3625 high-temperature alloy pipe in different directions in the plane measured in the embodiment of the present application

[0069] In the figure: 1-asymmetric ring-shaped tensile sample; 2-sample parallel section; 3-sample fillet section; 4-center section of the sample parallel section; 5-upper module; 6-lower module; 7-upper and lower module split surface; 8-split surface edge in contact with the sample; 9-upper die holder; 10-lower die holder; 11-square-shaped plug; 12-contact section of the sample above the center section; 13-straightened section of the sample; 14-test machine beam; 15-test machine base; 16-force sensor; 17-test machine control system; 18-three-dimensional full-field strain measurement and analysis system. DETAILED DESCRIPTION

[0070] The technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work belong to the protection scope of the present application.

[0071] The present application designs an asymmetric annular tensile specimen in a certain direction of the pipe and the geometric configuration of the upper and lower tensile modules, wherein the specimen is an inclined elliptical ring segmented from the pipe to be tested, the inclination angle is the same as the tensile direction, the two sides are designed with parallel sections and are connected through a rounded corner, the module is an inclined cylinder as a whole, the outer surface is matched with the inner surface of the specimen, the lower module provides support for the parallel section of the specimen to avoid straightening deformation, and the segmentation surface is designed as a nearly U-shaped or inverted trapezoidal shape to ensure the connecting space of the upper module and the upper module frame; the sizes of the annular tensile specimen and the module are multi-objectively optimized by combining finite element and optimization methods, the initial projection length, projection width and segmentation surface edge height of the parallel section of the specimen are set as the optimization variables, the stress ratio of the central cross-sectional width direction to the tensile direction, the uniformity of the tensile stress distribution, and the relative difference between the tensile force and one-half load are set as the objective functions, and the iterative analysis is carried out under the constraint condition that the central cross section remains in contact and the tensile strain reaches a certain value, so as to determine the optimized size; the accurate determination of the tensile mechanical properties of the pipe in any in-plane direction is realized, specifically, the specimen parallel section is elongated and deformed by pulling the module by the testing machine, the related size and strain data are measured in real time by using the DIC three-dimensional full-field strain measurement and analysis system, the thickness strain is calculated according to the principle of constant volume, the width strain-thickness strain path is drawn and the r value is obtained by fitting, the central cross-sectional tensile stress calculation formula considering the friction force is established, the tensile stress is calculated combined with the load and deformation data, and then the stress-strain curve is obtained, and the mechanical properties in any direction can be obtained by replacing the direction and repeating the operation.

[0072] Firstly, combined with Figures 1-8 As shown in the figure, the basic idea of the method for testing the tensile mechanical properties of the pipe in any in-plane direction is as follows:

[0073] Firstly, the geometry of the tensile specimen 1 and the modules 5 and 6 is designed, and then the dimensions of the tensile specimen 1 and the modules 5 and 6 are determined by using finite element and optimization methods to achieve an approximately uniform and single-pull stress state of the center section 4 of the parallel section 2 of the specimen; before testing, the specimen 1 is sleeved outside the modules 5 and 6, and the upper and lower modules 5 and 6 are fixed on the upper and lower chucks 9 and 10 by using the square pins 11; during testing, the upper module 5 is pulled upward by the upper chuck 9, and the lower module 6 is fixed and kept stationary by the lower chuck 10, so that the parallel section 2 of the specimen is deformed by elongation, the r value of the pipe is obtained according to the measured deformation data of the center section 4, the tensile stress is obtained by establishing a tensile stress calculation formula of the center section 4, and the stress-strain curve of the pipe is obtained in combination with the measured tensile strain. The tensile stress calculation formula of the center section 4 is an important content of the testing method, and the establishment process will be described in detail below.

[0074] Before establishing the tensile stress calculation formula of the center section 4, the normal tension of the center section 4 needs to be solved. In order to facilitate the establishment of the force balance equation, the specimen 1 and the module 6 are vertically flipped in Figure 8 , so that the cross-sectional tension of the straight section 13 of the specimen is vertically downward.

[0075] The force balance equation of the contact section 12 (approximately a circular ring section) of the specimen above the center section in the x direction is:

[0076]

[0077] Wherein, F is the normal tension of the center section 4, N is the normal pressure on the specimen 1, f is the tangential friction force on the specimen 1, and θ is the central angle corresponding to the contact section 12 of the specimen above the center section.

[0078] Assuming that the Coulomb friction law is satisfied, the tangential friction force f is:

[0079] f = μN (2)

[0080] Wherein, μ is the interface friction coefficient of the specimen 1 and the module 6.

[0081] Through analysis, it can be known that the normal pressure N on the specimen 1 is not uniformly distributed along the ring direction, and it is assumed to be linearly distributed:

[0082] N = kθ + N0 (3)

[0083] When θ = 90°, the force balance equation of the contact section 12 in the y direction is:

[0084]

[0085] Wherein, T is the vertical load of the testing machine beam 14.

[0086] Substituting formula (2) and formula (3) into formula (4) can obtain:

[0087]

[0088] Thus, the following equation is obtained:

[0089]

[0090] Thus, the following equation is obtained:

[0091]

[0092] Substituting equation (2) and equation (3) into equation (1) and rearranging, the following equation is obtained:

[0093]

[0094] Integrating the right side of the above equation, the following equation is obtained:

[0095]

[0096] Substituting equation (7) into equation (9) and according to the boundary condition (when θ = 0°, F = (T cos θ) / 2), the following equation is obtained: -1

[0097]

[0098] Substituting the above equation into equation (7), the calculation formula of the positive pressure is as follows:

[0099]

[0100] According to the above, the calculation formula of the normal force F of the center section 4 is as follows:

[0101]

[0102] In addition, the θ angle can be calculated according to the following equation:

[0103]

[0104] Wherein, h is the height of the intersection of the straightened section 13 and the contact section 12 relative to the plane where the center section 4 is located, and h ≤ H.

[0105] According to the above, substituting equation (12) and equation (13) into equation (14), the tensile stress of the center section 4 can be calculated:

[0106]

[0107] Wherein, A is the area of the center section 4, W is the width of the center section 4 in the deformation process, and t is the thickness of the center section 4 in the deformation process.

[0108] ​In order to make the above-mentioned purposes, features and advantages of the present application more apparent and easily understood, the following will be combined with the accompanying drawings to make a detailed description. Figures 1-9 The steps of the method for testing the tensile mechanical properties of a pipe in any direction in the plane of the pipe according to the present application are described in detail as follows:

[0109] Step S1: design the geometry of the asymmetric annular tensile specimen 1 along a certain tensile direction of the pipe, as well as the upper and lower tensile modules 5 and 6, which specifically includes:

[0110] Step S11: design the tensile specimen 1 as an inclined elliptical ring segmented from the pipe to be tested, with the inclination angle being the same as the tensile direction The same, a parallel section 2 is designed on both sides of the specimen 1, with the width of the parallel section 2 being smaller than the width of the specimen 1, and the transition being made through a round corner;

[0111] Step S12: design the tensile modules 5 and 6 as an inclined cylinder, with the inclination angle being the same as the tensile direction The same, the outer surface of the modules cooperates with the inner surface of the specimen 1, and the modules are further segmented into the upper and lower modules 5 and 6 at the corresponding round corner section 3 of the specimen, wherein the lower module 6 provides support for the parallel section 2 of the specimen to prevent it from being straightened and deformed, and the segmentation surface 7 is designed as a nearly U-shaped or inverted trapezoidal shape to ensure that the upper module 5 has enough space to be connected with the upper module frame 9.

[0112] Step S2: combine finite element and optimization methods to perform multi-objective optimization on the size of the annular tensile specimen 1 and the modules 5 and 6, which specifically includes:

[0113] Step S21: use the code to parameterize the tensile specimen 1 and the modules 5 and 6 designed in step S1, create a finite element model in Abaqus / explicit and perform simulation, and output the required simulation data;

[0114] Step S22: set the optimization variables (the initial projected length L0 and the projected width W0 of the specimen parallel section 2, and the height H of the segmentation surface edge 8 relative to the plane of the specimen center section 4) in the Optimization module of the Isight optimization software, the objective function (the width / tensile direction stress ratio α of the center section 4 ≤ 0.05, the tensile stress distribution uniformity η of the center section 4 ≥ 95%, and the relative difference δ between the tensile force of the center section 4 and one-half of the load ≤ 5%), and the constraint conditions (the tensile strain ε of the center section 4 is greater than 0.15 under the condition that it maintains contact with the lower module 6);

[0115] Step S23: obtain the single optimization result according to the output simulation data, judge whether the result converges, if it converges, output the optimized size of the specimen 1 and the modules 5 and 6, if it does not converge, change the optimization variables and continue the above steps until the result converges.

[0116] Step S3: cutting the sample 1 according to the optimized size, machining the modules 5 and 6, assembling the sample 1 and the modules 5 and 6 together and installing on the testing machine, the testing machine crossbeam 15 pulls the upper module 5 to make the sample 1 stretch and deform, specifically including:

[0117] Step S31: cutting the sample 1 from the pipe to be tested by using the wire cutting process; first, turning a metal cylindrical rod, then cutting the whole module, and finally cutting the upper and lower modules 5 and 6;

[0118] Step S32: lubricating the outer surfaces of the modules 5 and 6, then putting the sample 1 outside the modules 5 and 6, and using the square bolt 11 to fix the upper and lower modules 5 and 6 on the upper and lower chucks 9 and 10 respectively, wherein the upper chuck 9 is fixed on the testing machine crossbeam 15, and the lower chuck 10 is fixed on the testing machine base 16;

[0119] Step S33: the testing machine crossbeam 14 pulls the upper chuck 9 to move upwards at a constant speed, thereby driving the upper module 5 to move upwards at a constant speed, while the lower module 6 remains stationary due to the fixed lower chuck 10 on the testing machine base 15, and the sample parallel section 2 deforms under the action of the modules 5 and 6.

[0120] Step S4: measuring the width and tensile direction size and strain data of the sample 1 in real time during the stretching process, and calculating the thickness strain according to the principle of constant volume, specifically including:

[0121] Step S41: measuring the width, width strain and tensile direction strain of the center section 4 of the sample in real time during the stretching process by using the DIC three-dimensional full-field strain measurement and analysis system 18;

[0122] Step S42: calculating the thickness strain of the center section 4 according to the principle of constant volume, and calculating the thickness of the center section 4 according to the initial wall thickness t0 of the sample.

[0123] Step S5: drawing the width strain-thickness strain path of the sample 1, and obtaining the r value in the tensile direction by function fitting of the path, specifically including:

[0124] Step S51: determining the moment when the load reaches the maximum value, and drawing the width strain-thickness strain path of the edge points of the center section 4 before that moment;

[0125] Step S52: function fitting of the width strain-thickness strain path, and taking the slope of the fitted curve as the r value in the tensile direction.

[0126] Step S6: building the calculation formula of the tensile stress of the center section 4 of the sample, substituting the testing machine load, sample deformation data and friction coefficient into the formula to calculate the tensile stress, and obtaining the stress-strain curve in this direction combined with the strain data, specifically including:

[0127] Step S61: Obtain the height h from the surface profile of the sample 1 measured by the DIC three-dimensional full-field strain measurement analysis system 18, and substitute it into formula (13) to calculate the central angle θ;

[0128] Step S62: Substitute the central angle θ and the vertical load T of the testing machine beam 14 measured by the force sensor 16 into formula (12) to calculate the tensile force F of the central section 4;

[0129] Step S63: Substitute the tensile force F of the central section 4 and the width W and thickness t of the central section 4 in the deformation process measured by the DIC three-dimensional full-field strain measurement analysis system 18 into formula (14) to calculate the tensile stress of the central section 4;

[0130] Step S64: Combine the tensile strain of the central section 4 measured by the DIC three-dimensional full-field strain measurement analysis system 18 to finally obtain the stress-strain curve along the tensile direction.

[0131] Step S7: Change the tensile direction and repeat steps S1 to S6 to finally obtain the r value and stress-strain curve of the pipe in any direction in the plane.

[0132] As shown in Figure 10 , the present application also provides a device for testing the tensile mechanical properties of a pipe in any direction in the plane, which comprises:

[0133] The upper and lower modules 5 and 6 are made of die steel, wherein the upper module 5 is uniformly moved upward under the driving of the upper mold frame 9, the lower module 6 is fixed by the lower mold frame 10, and the sample 1 wrapped outside the modules 5 and 6 is elongated and deformed; the upper module is used to apply a normal pressure to the inner surface of the sample, and the lower module is used to apply a normal pressure to the inner surface of the sample and provide support for the parallel section of the sample.

[0134] The upper and lower mold frames 9 and 10, wherein the upper mold frame 9 is used to pull the upper module 5 upward, the lower mold frame 10 is used to fix the lower module 6 to keep it stationary, and the upper and lower mold frames 9 and 10 are fixed with the upper and lower modules 5 and 6 by square dowels 11 (two); the square dowels are used to fix the upper and lower modules on the upper and lower mold frames respectively; the upper mold frame is used to fix the upper module and transfer the load of the testing machine, and the lower mold frame is used to fix the lower module.

[0135] The testing machine and the control system mainly comprise a testing machine beam 14, a testing machine base 15, a force sensor 16, and a testing machine control system 17, which are used for pulling the upper die carrier 9 to move upward at a uniform speed and controlling the moving speed of the upper die carrier 9, wherein the testing machine beam 14 is used for pulling the upper die carrier 9, the testing machine base 15 is used for fixing the lower die carrier 10 to keep it stationary, the force sensor 16 is used for measuring the load of the testing machine in real time, and the testing machine control system 17 is used for controlling the moving speed of the testing machine beam 14; the testing machine beam is used for pulling the upper die carrier to move upward, the testing machine base is used for fixing the lower die carrier, the testing machine control system is used for controlling the moving speed of the testing machine beam, and the force sensor is used for measuring the load of the testing machine in real time.

[0136] A DIC three-dimensional full-field strain measurement and analysis system 18 is used to measure the width, width-direction strain and hoop-direction strain of the center section 4 of the sample, and to calculate the thickness-direction strain according to the principle of volume constancy; the DIC three-dimensional full-field strain measurement and analysis system is used for measuring deformation data such as the width, width-direction strain and tensile direction strain of the sample in real time.

[0137] Under the control of the testing machine control system 17, the testing machine beam 14 pulls the upper die carrier 9 to move upward at a uniform speed, thereby driving the upper die block 5 to move upward at a uniform speed, while the lower die carrier 10 fixed on the testing machine base 15 keeps the lower die block 6 stationary, the sample 1 is deformed by elongation under the action of the blocks 5 and 6, the strain data of the center section 4 of the sample are measured by the DIC three-dimensional full-field strain measurement and analysis system 18, the r value of the pipe is obtained, the tensile stress is calculated by the sample tensile stress calculation formula, and the stress-strain curve of the pipe is obtained in combination with the strain data. The specific embodiments of the present application are further described below in combination with specific examples.

[0138] The following are specific examples.

[0139] Example 1

[0140] The specific embodiments are described by taking the tensile mechanical property test of 6061 aluminum alloy thin-walled pipe in 15°, 30°, 45°, 60°, 75° and 90° directions as an example, the pipe blank outer diameter D0 is 60.0 mm, and the wall thickness t0 is 1.8 mm.

[0141] Step one: a finite element model for stretching in the 45° direction is established in the Abaqus software, the initial optimization variables L0=10.0 mm, W0=10.0 mm and H=10.0 mm are set, multi-objective optimization is performed by using the Isight optimization software, and the optimized sizes L0=15.1 mm, W0=7.4 mm and H=10.2 mm are obtained.

[0142] Step two: cut the sample 1 according to the size determined in step one and process the modules 5 and 6, install the upper die holder 9 on the testing machine crossbeam 14 and the lower die holder 10 on the testing machine base 15, prepare the Molykote D-321R fast-drying molybdenum disulfide lubricating coating on the outer surface of the modules 5 and 6, and put the sample 1 on the outside of the modules 5 and 6, and fix the upper module 5 and the lower module 6 on the upper die holder 9 and the lower die holder 10 respectively by using the square bolt 11.

[0143] Step three: the testing machine crossbeam 14 pulls the upper die holder 9 to move upwards at a constant speed, and the testing machine control system 17 controls the moving speed to be 5 mm / min, and the force sensor 16 measures the testing machine load, the upper die holder 9 pulls the upper module 5 to move upwards at a constant speed, and the lower die holder 10 fixes the lower module 6 to keep still, so that the parallel section 3 of the sample is elongated until it breaks.

[0144] Step four: the three-dimensional full-field strain measurement and analysis system 18 (digital image correlation method DIC) is used to measure the width of the outer surface of the central section 4, the width strain and the tensile strain during the stretching process, and the thickness strain is calculated according to the principle of constant volume, and then the thickness of the central section 4 is calculated.

[0145] Step five: the moment when the load reaches the maximum value is determined, the width strain-thickness strain path of the edge points of the central section 4 before this moment is drawn, and a linear function fitting is performed on the strain path, as shown in formula (5), the slope of the fitting straight line is the determined r value along the stretching direction, as shown in formula (6). Figure 11 Figure 12

[0146] Step six: the height h is obtained from the surface profile of the sample 1 measured by the DIC three-dimensional full-field strain measurement and analysis system 18, which is substituted into formula (13) to calculate the central angle θ, and then the central angle θ and the vertical load T of the testing machine crossbeam 14 measured by the force sensor 16 are substituted into formula (12) to calculate the tensile force F of the central section 4, and then the tensile force F of the central section 4 and the width W and the thickness t of the central section 4 in the deformation process measured by the DIC three-dimensional full-field strain measurement and analysis system 18 are substituted into formula (14) to calculate the tensile stress of the central section 4, and finally the stress-strain curve along the stretching direction is obtained in combination with the tensile strain of the central section 4 measured by the DIC three-dimensional full-field strain measurement and analysis system 18, as shown in formula (15). Figure 13

[0147] Step seven: the stretching direction is changed, and steps one to six are repeated, and finally the r values and the stress-strain curves of the 6061 aluminum alloy pipe in different directions in the plane are obtained, as shown in formulas (16) and (17). Figure 12 Figure 13

[0148] Example 2

[0149] ​​​​​The specific embodiments are further illustrated by taking the tensile mechanical property test of GH3625 high-temperature alloy thin-walled pipe 15°, 30°, 45°, 60°, 75° and 90° direction as an example, the pipe blank outer diameter D0 = 30.0mm, wall thickness t0 = 1.4mm.

[0150] Step one: a finite element model along the 15° direction is established in the Abaqus software, the initial optimization variables L0 = 7.5mm, W0 = 7.5mm, H = 7.5mm are set, multi-objective optimization is carried out by using Isight optimization software, and the optimized size is L0 = 10.2mm, W0 = 5.1mm, H = 7.4mm.

[0151] Step two: the sample 1 is cut according to the size determined in step one, and the modules 5 and 6 are processed, the upper die holder 9 is installed on the test machine crossbeam 14, the lower die holder 10 is installed on the test machine base 15, the Molykote D-321R fast-drying type molybdenum disulfide lubricating coating is prepared on the outer surface of the modules 5 and 6, the sample 1 is sleeved outside the modules 5 and 6, and the square plug 11 is used to fix the upper module 5 and the lower module 6 on the upper die holder 9 and the lower die holder 10 respectively.

[0152] Step three: the test machine crossbeam 14 pulls the upper die holder 9 to move upwards at a constant speed, the test machine control system 17 controls the moving speed to be 2mm / min, the force sensor 16 measures the test machine load, the upper die holder 9 pulls the upper module 5 to move upwards at a constant speed, the lower die holder 10 fixes the lower module 6 to keep still, and the sample parallel section 3 is deformed to elongate until it breaks.

[0153] Step four: the three-dimensional full-field strain measurement and analysis system 18 (digital image correlation method DIC) is used to measure the width, width strain and tensile strain of the outer surface of the center section 4 during the stretching process, and the thickness strain is calculated according to the volume invariance principle, and then the thickness of the center section 4 is calculated.

[0154] Step five: the moment when the load reaches the maximum value is determined, the width strain-thickness strain path of the edge point of the center section 4 before this moment is drawn, the strain path is linearly fitted, as shown in Figure 14 , the fitting straight line slope is the determined r value along the stretching direction, as shown in Figure 15 .

[0155] Step six: according to the height h obtained from the sample 1 surface profile measured by the DIC three-dimensional full-field strain measurement analysis system 18, the formula (13) is substituted to calculate the central angle θ, the central angle θ and the vertical load T of the testing machine beam 14 measured by the force sensor 16 are substituted into the formula (12) to calculate the central section 4 tension F, and then the central section 4 tension F and the deformation process central section 4 width W and thickness t measured by the DIC three-dimensional full-field strain measurement analysis system 18 are substituted into the formula (14) to calculate the central section 4 tensile stress, combined with the central section 4 tensile strain measured by the DIC three-dimensional full-field strain measurement analysis system 18, and finally the stress-strain curve along the tensile direction is obtained, as shown in Figure 16

[0156] Step seven: replace the tensile direction, repeat steps one to six, and finally obtain the r value and stress-strain curve of the GH3625 high-temperature alloy pipe in different directions in the plane, as shown in Figure 15 Figure 16

[0157] The present application has the following technical effects relative to the prior art: the sample is designed as an inclined annular sample along any direction of the pipe, and the tensile module is designed as an asymmetric geometric configuration, so that the central section of the parallel segment of the inclined annular sample is tightly attached to the surface of the module to elongate and deform, avoiding straightening deformation; the sample and module sizes are determined by combining finite element and multi-objective optimization methods to realize an approximately uniform and single-pull stress state of the central section; the friction force is considered to establish a central section tensile stress calculation formula to eliminate the influence of friction on the test results, and finally the accurate determination of the tensile mechanical properties (r value and stress-strain curve) in any direction in the plane of the pipe is realized. The present application can directly test the tensile mechanical properties in any direction in the plane of the pipe, and the results are accurate and reliable, the device requirements are low, the testing process is simple, and the application is convenient.

[0158] The embodiments in the specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts of each embodiment can be referred to each other.

[0159] In the description of the present application, it should be noted that the terms "center", "top", "bottom", "left", "right", "vertical", "horizontal", "inner", "outer" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, therefore cannot be understood as a limitation on the present application. In addition, the terms "first", "second" are only for descriptive purposes, and cannot be understood as indicating or implying relative importance.

[0160] ​​​The principles and implementation manners of the present application are described by using specific examples in the present application, and the above examples are only used for helping to understand the method of the present application and its core idea; meanwhile, for the general technical personnel in the art, according to the idea of the present application, the specific implementation manners and application ranges will be changed. In conclusion, the content of the present specification should not be understood as the limitation of the present application.

Claims

1. A method for testing the tensile mechanical properties of a pipe in any in-plane direction, wherein the mechanical properties include the thickness anisotropy coefficient r, yield strength, and stress-strain curve, characterized in that, By designing an asymmetric annular specimen and a matching module, the central section of the specimen undergoes only elongation deformation and is under uniaxial tensile stress under the support of the module. At the same time, the actual tensile stress is calculated by establishing a mechanical model that considers friction to eliminate the influence of friction, thereby achieving accurate determination of the mechanical properties. The method includes the following steps: Step S1: Design the geometry of the asymmetric annular tensile specimen along a certain tensile direction of the tube and the upper and lower tensile modules. Step S2: Perform multi-objective optimization of the dimensions of the annular tensile specimen and the module by combining finite element simulation and optimization algorithm; Step S3: Process the specimen and module according to the optimized dimensions, assemble the specimen and module together and install them on the testing machine. The crossbeam of the testing machine pulls the upper module, causing the specimen to undergo tensile deformation. Step S4: During the tensile process, measure the dimensions and strain data of the specimen in the width and tensile directions in real time, and calculate the thickness strain based on the principle of constant volume. Step S5: Draw the strain path from the width direction to the thickness direction of the specimen, and obtain the r value in the tensile direction by fitting the path to a function; Step S6: Establish the calculation formula for tensile stress at the center section of the specimen. Substitute the testing machine load, specimen deformation data and friction coefficient into the formula to calculate the tensile stress. Combine the strain data to obtain the stress-strain curve in this direction. Step S7: Change the stretching direction and repeat steps S1 to S6 to finally obtain the r value and stress-strain curve in any direction within the pipe surface.

2. The method for testing the tensile mechanical properties of a pipe in any in-plane direction according to claim 1, characterized in that, In step S1, the tensile specimen is an inclined elliptical ring cut from the pipe to be tested. Its inclination angle is the same as the tensile direction. Parallel segments are designed on both sides of the specimen. The width of the parallel segments is smaller than the width of the specimen and is transitioned by rounded corners.

3. The method for testing the tensile mechanical properties of a pipe in any in-plane direction according to claim 1, characterized in that, In step S1, the upper and lower modules are inclined cylinders with the same inclination angle as the stretching direction, and their outer surfaces cooperate with the inner surfaces of the sample. The modules are divided into upper and lower parts at the corresponding rounded corners of the sample, with the lower module providing support for the parallel sections of the sample. The dividing surfaces of the modules are approximately U-shaped or inverted trapezoidal.

4. The method for testing the tensile mechanical properties of a pipe in any in-plane direction according to claim 1, characterized in that, The specific steps for multi-objective optimization in step S2 include: (1) Set optimization variables, including the initial projection length L0 of the parallel section of the sample, the projection width W0, and the height H of the edge of the dividing surface in contact with the sample relative to the plane of the center section of the sample. (2) Set the objective function, requiring the stress ratio α in the width direction and the tensile direction of the central section to be ≤0.05, the uniformity of tensile stress distribution in the central section to be ≥95%, and the relative difference between the tensile force in the central section and the load of half the testing machine to be ≤5%. (3) Set constraints to require that the tensile strain ε generated by the center section is ≥0.15 while maintaining contact with the lower module; (4) The tensile process of the specimen is simulated by the finite element explicit algorithm, and the optimization algorithm is used for iterative analysis to obtain the optimized dimensions of the annular tensile specimen and the module.

5. The method for testing the tensile mechanical properties of a pipe in any in-plane direction according to claim 1, characterized in that, In step S3, the sample is cut from the pipe to be tested using wire cutting technology; the overall module blank is first processed, and then cut into upper and lower modules; the outer surface of the module is lubricated during assembly, and then the sample is put on the outside of the module, and the upper and lower modules are fixed on the upper and lower mold frames respectively using connectors.

6. The method for testing the tensile mechanical properties of a pipe in any in-plane direction according to claim 1, characterized in that, In step S4, the width, transverse strain and tensile strain at the center section of the sample are measured in real time using a three-dimensional digital image correlation (DIC) measurement system.

7. The method for testing the tensile mechanical properties of a pipe in any in-plane direction according to claim 1, characterized in that, In step S5, the strain path from the width to the thickness of the central section before the load reaches its maximum value is selected, and a linear function is fitted onto it. The slope obtained from the fitting is taken as the r value along the tensile direction. The definition of the r value is: Where, dε w For the transverse strain increment, dε t This represents the thickness strain increment.

8. The method for testing the tensile mechanical properties of a pipe in any in-plane direction according to claim 1, characterized in that, In step S6, the formula for calculating the tensile stress σ at the center section of the parallel segment of the specimen is: Where F is the tensile force on the central section, A is the area of ​​the central section, W is the width of the central section during deformation, and t is the thickness of the central section during deformation; The formula for calculating the tensile force F at the center section is: Where T is the load of the testing machine, μ is the friction coefficient between the specimen and the module, and θ is the central angle corresponding to the contact section of the specimen above the central section.

9. A method for testing the tensile mechanical properties of a pipe in any in-plane direction according to claim 8, characterized in that, The formula for calculating the central angle θ is: Where h is the height of the junction between the straightened section and the contact section of the specimen relative to the plane containing the central section of the specimen, and h≤H, where H is the height of the edge of the dividing surface in contact with the specimen relative to the plane containing the central section of the specimen.

10. An apparatus for testing the in-plane tensile mechanical properties of a pipe in any direction using the method described in any of claims 1 to 9, characterized in that, include: The upper module (5) and the lower module (6) have outer surface configurations that match the inner surface of the test sample (1) to support the sample and cause its central section (4) to elongate and deform. The upper mold frame (9) and the lower mold frame (10) are used to fix the upper module (5) and the lower module (6) respectively; Connector (11) is used to detachably fix the upper and lower modules (5,6) to the upper and lower mold frames (9,10); The dividing surface (7) of the upper and lower modules (6) is approximately U-shaped or inverted trapezoidal.

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