A method and device for testing the circumferential tensile stress-strain curve of a pipe

Abstract

The present invention discloses a method and device for testing the annular tensile stress-strain curve of a pipe, including: determining a theoretical model of the radial force applied by a block rigid die to the inner surface of an annular specimen, thereby obtaining the normal pressure applied to the inner surface of the annular specimen; determining the theoretical model of the annular stress of the annular specimen; a testing machine pushes a right pyramid-shaped pressure head to move downward at a uniform speed, thereby pushing the block rigid die to expand uniformly in the radial direction, causing the annular specimen to undergo equal-diameter bulging; calculating the annular stress according to the load and specimen deformation data, and drawing the annular tensile stress-strain curve of the pipe in combination with the measured annular strain data. The present invention utilizes the surface-to-surface cooperation between the right pyramid-shaped pressure head and the block rigid die, and can quantitatively convert the vertical load applied by the testing machine to the right pyramid-shaped pressure head into the radial force applied by the block rigid die to the inner surface of the annular specimen, thereby achieving the accurate calculation of the normal pressure applied to the annular specimen, thereby directly and accurately testing the annular tensile stress-strain curve of the pipe.

Description

Technical Field

[0001] The invention relates to the technical field of pipe mechanical property testing, and in particular to a method and a device for testing a circumferential tensile stress-strain curve of a pipe. Background Art

[0002] Complex curved thin-walled tubular components are widely used in aerospace, aviation and automotive fields due to their lightweight and high performance. Such components are generally manufactured by advanced technologies such as hydroforming. During the manufacturing process, they are subject to complex stresses and are prone to wrinkling and cracking defects. Therefore, accurate prediction of the forming process of complex curved thin-walled tubular components using finite element simulation has always been the goal pursued by scientific research and industry. By quickly and accurately predicting the defects that may occur during the forming process, and thus determining reasonable process parameters, the product development cycle can be greatly shortened and the mold manufacturing cost can be reduced. However, in order to improve the accuracy of finite element simulation, the plastic constitutive model of the tube must be accurately constructed. During the preparation (extrusion, drawing, rolling, etc.) of the initial tube blank, the macroscopic mechanical properties in different directions show obvious anisotropy due to the texture orientation. The tensile stress-strain relationship (curve) of the tube is an important data for constructing the plastic constitutive model. Whether the tensile stress-strain relationship of the tube can be accurately obtained is crucial for accurately constructing the plastic constitutive model 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 perform tensile tests in any direction like the plate to obtain the stress-strain curve in that direction. At present, only the axial stress-strain curve of the tube (0°) can be obtained by a method similar to the plate tensile test (GB / T 228.1-2021), while tensile tests in other directions are very difficult. Some studies have tested the stress-strain curve in any direction within the surface of the existing uniaxial tensile standard test surface of the plate after the tube billet is unfolded into a plate billet. However, the tube billet undergoes obvious work hardening during the flattening process, which changes its mechanical properties, and the test results cannot accurately reflect the true anisotropic mechanical properties of the original tube billet. For the circumferential direction of the tube (90°), a D-block mold is currently commonly used to perform circumferential stretching on the annular specimen to test the stress-strain curve. Although this method can ensure that the specimen undergoes approximate circumferential single-pulling deformation, and the obtained stress-strain curve is more direct and reliable than single-pulling after flattening, and it also avoids the error caused by approximating the axial stress-strain curve, the specimen produces obvious bending deformation at the gap position of the two D-block molds, which will lead to calculation errors of the normal pressure and circumferential stress of the specimen, and the friction at the specimen / mold interface has a great influence on the uniformity of the normal pressure and circumferential stress distribution. Some scholars combined the D-block mold circumferential tensile experiment and used finite element simulation to reversely obtain the circumferential tensile stress-strain curve, but this cannot fundamentally solve the influence of friction, and the accuracy of the results obtained by the inverse method is limited by the accuracy of the finite element model.

[0004] Researchers have also proposed a theoretical prediction method for the circumferential tensile stress-strain curve of pipes. The main principle is to substitute the deformation data of the pipe under biaxial stress state (such as hydraulic bulging of pipes) into the equivalent stress and equivalent strain calculation formula to obtain the circumferential tensile stress-strain curve. The prediction accuracy of the above theoretical method is affected by the measurement accuracy of the deformation data of hydraulic bulging of pipes on the one hand, and by the accuracy of the selected yield criterion model on the other hand. In fact, the calibration of the yield criterion model parameters requires the circumferential stress-strain curve of the pipe. This method has a paradox in principle, and the reliability of the prediction results is questionable.

[0005] Therefore, there is an urgent need to establish methods and devices that can directly and accurately test the tensile stress-strain curves of pipes in different directions (especially circumferential direction), which has important theoretical significance for accurately constructing the plastic constitutive model of anisotropic thin-walled pipes and realizing high-precision simulation of thin-walled tubular component forming. Summary of the invention

[0006] The purpose of the present invention is to provide a method and device for testing the circumferential tensile stress-strain curve 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 simulation of the forming of thin-walled tubular components.

[0007] To achieve the above object, the present invention provides the following solutions:

[0008] The present invention provides a method for testing the circumferential tensile stress-strain curve of a pipe. By utilizing the surface-to-surface cooperation between a right pyramid indenter and a block rigid die, the vertical load applied by a testing machine to the right pyramid indenter can be quantitatively converted into the radial force applied by the block rigid die to the inner surface of an annular sample, thereby realizing the accurate calculation of the normal pressure of the block rigid die on the annular sample, thereby determining the circumferential stress theoretical model, and drawing the circumferential tensile stress-strain curve of the pipe according to the measured circumferential stress and circumferential strain data. The method specifically comprises the following steps:

[0009] Step S1: determining a theoretical model for converting the vertical load applied by the testing machine to the regular pyramid indenter into the radial force of the block rigid die acting on the inner surface of the annular specimen, and then obtaining a calculation formula for the normal pressure of the block rigid die on the inner surface of the annular specimen;

[0010] Step S2: determining a theoretical model of hoop stress of the annular specimen according to a calculation formula of the normal pressure of the block rigidity module on the inner surface of the annular specimen;

[0011] Step S3: determining the semi-cone angle of the right pyramid indenter, the number of segmented rigid mold halves, the size of the annular specimen, and the friction coefficients between the molds and between the annular specimen and the mold;

[0012] Step S4: installing the annular specimen and the mold on the testing machine, using the crossbeam to push the regular pyramid-shaped indenter downward at a uniform speed, and the inclined surface of the regular pyramid-shaped indenter pushes the divided rigid mold to expand uniformly outward in the radial direction, so that the annular specimen undergoes equal-diameter bulging;

[0013] Step S5: During the isodiametric bulging process, the width, widthwise strain and hoopwise strain of the annular specimen are measured in real time, and the wall thickness reduction rate of the specimen is calculated according to the volume invariance principle;

[0014] Step S6: Substitute the test machine load and the deformation data of the annular specimen into the hoop stress theoretical model, calculate the hoop stress of the annular specimen, and draw the hoop tensile stress-strain curve of the pipe according to the hoop stress and hoop strain data.

[0015] Optionally, the regular pyramid indenter and the block rigid mold are matched face-to-face; the vertical load F applied by the testing machine to the regular pyramid indenter z Transformed into the radial force F acting on the inner surface of the annular specimen by the block rigid module r The theoretical model is

[0016]

[0017] Among them, β is the semi-cone angle of the right pyramid indenter, and μ′ is the friction coefficient between the dies.

[0018] Optionally, the calculation formula for the normal pressure p on the inner surface of the annular specimen subjected to the block rigidity module is:

[0019]

[0020] Wherein, D0 is the initial outer diameter of the annular sample, t0 is the initial thickness of the annular sample, and B is the real-time width of the annular sample; the normal pressure p is evenly distributed on the inner surface of the annular sample.

[0021] Optionally, according to the calculation formula of the normal pressure on the inner surface of the block rigid mold under the action of the block rigid mold, the hoop stress theoretical model of the annular specimen is determined, which specifically includes:

[0022] The hoop stress of the annular specimen in the suspended area is

[0023]

[0024] The hoop stress of the annular specimen in the contact area with the block rigid mold is

[0025]

[0026] Wherein, t is the thickness of the annular specimen, θ is the angle between the specimen cross section and the side of the block mold, and μ is the friction coefficient between the specimen and the mold; the hoop stress theoretical model takes into account the friction shear stress between the annular specimen and the mold.

[0027] Optionally, the stress theoretical model of the annular specimen should be combined when determining the semi-cone angle of the right pyramid indenter, the number of segmented rigid mold halves, and the size of the annular specimen to ensure that the annular specimen is in an approximate annular uniaxial tensile stress state (axial / annular stress ratio is not higher than 2%) during the equal-diameter bulging process, and has a high uniformity of annular stress distribution (not less than 98%). It is recommended that the semi-cone angle of the right pyramid indenter be 10°≤β≤15°, the number of segmented rigid mold halves N≥8, and the ratio of the initial width B0 of the annular specimen to the initial outer diameter D0 B0 / D0≤1 / 8.

[0028] Optionally, in order to reduce the influence of friction and improve the deformation uniformity of the annular specimen, the mold / mold interface and the mold / specimen interface should be lubricated, and a molybdenum disulfide lubricating coating is usually prepared between the interfaces.

[0029] Optionally, the friction coefficient between the molds and between the annular specimen and the mold is determined, specifically including: selecting a specimen with the same material and surface roughness as the mold and the annular specimen, performing the same lubrication treatment on the interface, and then performing a sliding friction test to obtain the friction coefficient.

[0030] Optionally, a crossbeam is used to push the right pyramid indenter downward at a uniform speed, and the downward moving speed of the right pyramid indenter is determined in the following manner: the circumference of the annular specimen is used as the length of the parallel segment, the annular tensile velocity is estimated based on the strain rate of the annular specimen, and then the radial expansion velocity of the block rigid mold is calculated, and finally the downward moving speed of the right pyramid indenter is reversed.

[0031] Optionally, before the test, the outer surface of the annular specimen is sprayed with speckles, and during the equal-diameter bulging process, a three-dimensional full-field strain measurement and analysis system (digital image correlation method DIC) is used to measure the width, widthwise strain, and hoopwise strain deformation data of the outer surface of the annular specimen.

[0032] Optionally, the method of drawing a hoop tensile stress-strain curve of the pipe based on the hoop stress and hoop strain data specifically includes: taking the hoop strain of a section of the annular specimen in the contact area with the block rigid mold as the horizontal axis, and taking the calculated hoop stress of the section as the vertical axis to obtain the hoop tensile stress-strain curve of the section, averaging the hoop tensile stress-strain curves of multiple sections to obtain the hoop tensile stress-strain curve of the pipe, and recommending that the number of sections is not less than 5, and they are symmetrically distributed along the center line of the block rigid mold.

[0033] The present invention also provides a device for testing the circumferential tensile stress-strain curve of a pipe, comprising a right pyramid-shaped pressure head for transmitting the load of a testing machine, a block rigid mold for performing equal-diameter expansion on an annular specimen to achieve circumferential single pulling, a testing machine and a control system for pushing the right pyramid-shaped pressure head downward, and a three-dimensional full-field strain measurement and analysis system; the testing machine and the control system comprise a testing machine crossbeam, a testing machine control system and a force sensor; the testing machine crossbeam is used to push the right pyramid-shaped pressure head downward, the testing machine control system is used to control the moving speed of the testing machine crossbeam, and the force sensor can measure the load of the testing machine in real time; the three-dimensional full-field strain measurement and analysis system can perform real-time measurement of deformation data of the width, widthwise strain and circumferential strain of the annular specimen.

[0034] Compared with the prior art, the present invention has achieved the following technical effects:

[0035] The present invention utilizes the surface-to-surface fit between the right pyramid-shaped pressure head and the block rigid mold, and can quantitatively convert the vertical load applied by the testing machine to the right pyramid-shaped pressure head into the radial force of the block rigid mold acting on the inner surface of the annular sample, thereby achieving accurate calculation of the normal pressure of the block rigid mold on the annular sample, thereby determining the hoop stress theoretical model, and drawing the hoop tensile stress-strain curve of the pipe according to the measured hoop stress and hoop strain data. The present invention can directly obtain the hoop tensile stress-strain curve of the pipe through experimental testing, and the testing process is simple; the friction shear stress between the sample and the mold is considered in the hoop stress theoretical model, which improves the calculation accuracy of the hoop stress. In addition, by reasonably designing the width of the annular sample and the number of block rigid mold petals, and lubricating the mold / sample interface, the influence of friction is significantly reduced, and the uniformity of sample deformation is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] 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.

[0037] Figure 1 This is a schematic diagram of the initial stage of the circumferential tensile stress-strain curve of the pipe tested in an embodiment of the present invention;

[0038] Figure 2 This is a schematic diagram of the intermediate stage of the circumferential tensile stress-strain curve of the pipe tested in an embodiment of the present invention;

[0039] Figure 3 For the embodiment of the present invention Figure 2 Middle AA section view;

[0040] Figure 4 This is a schematic diagram of the force applied to the mold when testing the circumferential tensile stress-strain curve of the pipe according to an embodiment of the present invention;

[0041] Figure 5 This is a schematic diagram of the force on the annular specimen when testing the annular tensile stress-strain curve of the pipe according to an embodiment of the present invention;

[0042] Figure 6 This is a flow chart of a method for testing the circumferential tensile stress-strain curve of a pipe according to an embodiment of the present invention;

[0043] Figure 7 The annular sample sprayed with speckles is an embodiment of the present invention;

[0044] Figure 8 A three-dimensional schematic diagram of a tube hoop tensile stress-strain curve test mold according to an embodiment of the present invention;

[0045] Fig. 9 This is a schematic diagram of a device for testing the circumferential tensile stress-strain curve of a pipe according to an embodiment of the present invention;

[0046] Fig.10 The circumferential tensile true stress-strain curves at different cross sections of a 6061 aluminum alloy tube annular specimen measured in an embodiment of the present invention;

[0047] Fig.11 This is the true stress-strain curve of the circumferential tension of the 6061 aluminum alloy pipe measured in the embodiment of the present invention.

[0048] In the figure: 1- regular pyramid indenter; 2- block rigid mold; 3- annular specimen; 4- base; 5- suspended area of ​​annular specimen; 6- contact area of ​​annular specimen; 7- testing machine crossbeam; 8- force sensor; 9- testing machine control system; 10- three-dimensional full-field strain measurement and analysis system. DETAILED DESCRIPTION

[0049] 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.

[0050] The purpose of the present invention is to provide a method and device for testing the circumferential tensile stress-strain curve of a pipe, so as to solve the problems existing in the prior art, accurately construct an anisotropic plastic constitutive model of the pipe, and realize high-precision simulation of the forming of thin-walled tubular components.

[0051] 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.

[0052] like Figure 1-Figure 5 As shown, the circumferential tensile stress-strain curve of the pipe is directly tested by using a block rigid die equal-diameter bulging experiment. The basic idea is: the testing machine crossbeam 7 pushes the right pyramid-shaped pressure head 1 to move downward at a uniform speed, and the inclined surface of the right pyramid-shaped pressure head 1 pushes the block rigid die 2 to expand radially and uniformly outward. The block rigid die 2 generates normal pressure on the inner surface of the annular sample 3, causing the annular sample 3 to undergo equal-diameter bulging (approximately circumferential single-tension deformation). By establishing a theoretical model of the circumferential stress of the annular sample 3, the circumferential stress of the annular sample 3 can be calculated, and combined with the measured circumferential strain of the annular sample 3, the circumferential tensile stress-strain curve of the pipe is finally obtained. The accurate acquisition of the circumferential stress of the annular sample 3 is the core content of this test method, and the calculation process will be described in detail below.

[0053] Before establishing the theoretical model of the hoop stress of the annular specimen 3, the calculation formula of the normal pressure p of the block rigidity module 2 on the inner surface of the annular specimen 3 should be solved. Before solving the calculation formula of p, the vertical load F applied by the testing machine to the right pyramid indenter should be established. z Transformed into the radial force F acting on the inner surface of the annular specimen 3 by the block rigid mold 2 r Theoretical model. Figure 4 According to the force balance of the mold in the vertical and radial directions, we can get

[0054]

[0055] Wherein, F′ is the normal force of the inclined surface of the right pyramid indenter 1 acting on the inner surface of the block rigid mold 2, μ′ is the friction coefficient between the molds, and β is the semi-cone angle of the right pyramid indenter.

[0056] Rearranging the above formula, we can get

[0057]

[0058] Therefore, the calculation formula for the normal pressure p of the block rigid module 2 on the inner surface of the annular sample 3 is:

[0059]

[0060] Wherein, D0 is the initial outer diameter of the annular sample, t0 is the initial thickness of the annular sample, and B is the real-time width of the annular sample; the normal pressure p is evenly distributed on the inner surface of the annular sample.

[0061] The friction between the annular specimen 3 and the block rigid mold 2 will cause the annular stress distribution of the annular specimen 3 to be uneven, and there are two areas: the annular specimen hanging area 5 and the annular specimen contact area 6, such as Figure 5 In the suspended area 5 of the annular specimen, the material is in the gap position and there is no friction. The hoop stress of the annular specimen 3 in this area can be obtained by the shell force balance equation:

[0062]

[0063] In the annular sample contact area 6, the annular sample 3 is in contact with the segmented rigid mold 2. Due to the friction, the closer the annular sample 3 is to the center of the area along the annular direction, the smaller the annular stress is. Figure 5 In the equation, angle θ is the angle between the cross section of the annular specimen 3 and the side of the block rigid mold 2, F(θ) and T / 2 are the cross-sectional tensile forces, and τ is the friction shear stress. Figure 5 The circular segment (shaded part) corresponding to the angle θ is the research object. It is assumed that it has unit length in the width direction. Figure 5 In the local coordinate system, the force equilibrium equation of the ring segment along the x direction is

[0064]

[0065] Rearranging the above formula, we can get

[0066]

[0067] Therefore, in the contact area 6 of the annular specimen, the hoop stress on the cross section of the annular specimen 3 with an angle of θ (0≤θ≤α / 2, α is the central angle corresponding to a single block rigid mold) with the side of the block rigid mold 2 is

[0068]

[0069] Among them, t is the thickness of the annular specimen, θ is the angle between the cross section of the annular specimen and the side of the block rigid mold, μ is the friction coefficient between the annular specimen and the mold,

[0070] When μ=0, k=1, and the hoop stresses at all locations of the annular specimen 3 are equal; when μ is constant, the k value decreases with the increase of the angle θ, and the larger the μ, the more significant the decreasing trend; when the angle θ is constant, the k value decreases with the increase of μ, and the larger the angle θ, the more significant the decreasing trend. It can be seen that increasing the number of petals N of the block rigid mold 2 and reducing the friction coefficient μ can both improve the uniformity of the hoop stress distribution of the annular specimen 3, thereby improving the deformation uniformity. In other words, when the number of petals of the block rigid mold 2 is large and good lubrication treatment is adopted, friction has little effect on the hoop stress of the annular specimen 3, that is, k≈1. Taking the 12-petal block rigid mold 2 and μ=0.15 as an example, at this time k min =0.980, that is, the maximum hoop stress non-uniformity is 2.0%.

[0071] During the circumferential tensile deformation process, the annular specimen 3 shrinks and deforms in the width direction. According to the force balance, the widthwise stress on the annular cross section at a distance B1 (0≤B1≤B0 / 2, B0 is the initial width of the annular specimen) from the end face of the annular specimen 3 along the width direction is obtained as follows:

[0072]

[0073] Among them, u z is the displacement of the crossbeam of the testing machine;

[0074] From the above formula, we can get: the narrower the width of the annular specimen 3, the smaller the widthwise stress and its variation gradient, and reducing the friction coefficient μ can reduce the widthwise stress. We can further get: in the contact area 6 of the annular specimen, the widthwise / circumferential stress ratio of the annular specimen 3 is

[0075]

[0076] In order to achieve the stress state of the annular specimen 3 approximating the annular tensile stress, the width of the annular specimen 3 should be reduced as much as possible. Taking the 12-petal block die and μ = 0.15 as an example, when B0 / D0 = 1 / 8 (D0 is the initial outer diameter of the annular specimen), (σ w / σ θ ) max =0.019, that is, the axial stress is only 1.9% of the hoop stress, and the annular specimen 3 can be approximately considered to be in a uniaxial tensile stress state. Therefore, in actual testing, it is recommended to use B0 / D0≤1 / 8, and it is necessary to ensure that the annular specimen 3 has a sufficient width-to-thickness ratio.

[0077] Combination Figure 1-Figure 6 To explain this embodiment, Figure 1 This is a schematic diagram of the initial stage of the circumferential tensile stress-strain curve of the pipe tested in an embodiment of the present invention; Figure 2 This is a schematic diagram of the intermediate stage of the circumferential tensile stress-strain curve of the pipe tested in an embodiment of the present invention; Figure 3 For the embodiment of the present invention Figure 2 Middle AA section view; Figure 4 This is a schematic diagram of the force applied to the mold when testing the circumferential tensile stress-strain curve of the pipe according to an embodiment of the present invention; Figure 5 This is a schematic diagram of the force on the annular specimen when testing the annular tensile stress-strain curve of the pipe according to an embodiment of the present invention; Figure 6 This is a flow chart of a method for testing the circumferential tensile stress-strain curve of a pipe according to an embodiment of the present invention. The measuring method specifically includes:

[0078] Step S1: determining a theoretical model for converting the vertical load applied by the testing machine to the regular pyramid-shaped indenter 1 into the radial force of the segmented rigid mold 2 acting on the inner surface of the annular specimen 3, and then obtaining a calculation formula for the normal pressure of the segmented rigid mold 2 on the inner surface of the annular specimen 3;

[0079] Step S2: determining a theoretical model of hoop stress of the annular specimen 3 according to a calculation formula of the normal pressure of the block rigid mold 2 on the inner surface of the annular specimen 3;

[0080] Step S3: determining the semi-cone angle of the regular pyramid indenter 1, the number of petals of the block rigid mold 2, the size of the annular sample 3, and the friction coefficients between the molds and between the annular sample 3 and the mold;

[0081] Step S4: the annular specimen 3 and the mold are mounted on a testing machine, and the testing machine crossbeam 7 is used to push the regular pyramid-shaped indenter 1 downward at a uniform speed, and the inclined surface of the regular pyramid-shaped indenter 1 pushes the segmented rigid mold 2 to expand uniformly outward in the radial direction, so that the annular specimen 3 undergoes equal-diameter bulging;

[0082] Step S5: During the isodiametric bulging process, deformation data such as the width, widthwise strain and hoopwise strain of the annular specimen 3 are measured in real time, and the wall thickness reduction rate of the specimen is calculated according to the volume invariance principle;

[0083] Step S6: Substitute the test machine load and the deformation data of the annular sample 3 into the hoop stress theoretical model, calculate the hoop stress of the annular sample 3, and draw the hoop tensile stress-strain curve of the pipe according to the hoop stress and hoop strain data.

[0084] The following is a detailed discussion of each step:

[0085] Step S1: Determine a theoretical model for converting the vertical load applied by the testing machine to the regular pyramid indenter into the radial force of the block rigid mold 2 acting on the inner surface of the annular specimen 3, and then obtain a calculation formula for the normal pressure of the block rigid mold 2 on the inner surface of the annular specimen 3, which specifically includes:

[0086] Step S11: Obtain the vertical load F applied by the testing machine to the regular pyramid-shaped indenter 1 according to the force balance of the experimental mold in the vertical direction and the radial direction. z The radial force F acting on the inner surface of the annular specimen 3 by the block rigid mold 2 is r The expression of F is as follows, as shown in formula (1). By rearranging the expression, we can get z Convert to F r The theoretical model is as follows:

[0087] Step S12: According to F z Convert to F r The theoretical model of the method is used to obtain the calculation formula for the normal pressure p of the block rigid module 2 on the inner surface of the annular specimen 3, as shown in formula (3).

[0088] Step S2: According to the calculation formula of the normal pressure of the block rigid mold 2 on the inner surface of the annular sample 3, the hoop stress theoretical model of the annular sample 3 is determined, which specifically includes:

[0089] Step S21: In the suspended area 5 of the annular specimen, the material is in the gap position and there is no friction. The theoretical model of the hoop stress of the specimen in this area can be obtained by the shell force balance equation, such as formula (4);

[0090] Step S22: In the annular specimen contact area 6, the annular specimen 3 contacts the block rigid mold 2. Due to the friction, the closer the annular specimen 3 is to the center of the contact area along the annular direction, the smaller the annular stress is. First, a force balance equation for the annular specimen 3 is established, such as formula (5). The equation is rearranged to obtain: in this area, the theoretical model of the annular stress on the specimen cross section with an angle of θ (0≤θ≤α / 2, α is the central angle corresponding to a single block rigid mold) with the side of the block rigid mold 2 is, such as formula (7).

[0091] Step S3: determining the half cone angle of the regular pyramid indenter 1, the number of petals of the block rigid mold 2, the size of the annular sample 3, and the friction coefficient between the molds and between the annular sample 3 and the mold, specifically including:

[0092] Step S31: In combination with the stress theoretical model of the annular specimen 3, under the conditions of ensuring that the annular specimen 3 is in an approximate annular uniaxial tensile stress state (axial / annular stress ratio is not higher than 2%) during the equal-diameter bulging process and having a high uniformity of annular stress distribution (not lower than 98%), determining that the semi-cone angle of the right pyramid indenter 1 is 10°≤β≤15°, the number of petals N of the block rigid mold 2 is ≥8, and the ratio of the initial width B0 to the initial outer diameter D0 of the annular specimen 3 is B0 / D0≤1 / 8;

[0093] Step S32: Select a sample with the same material and surface roughness as the mold and the annular sample 3, and after lubricating the interface (the same as step S41), perform a sliding friction test to obtain the friction coefficient.

[0094] Step S4: The annular specimen 3 and the mold are mounted on a testing machine, and the testing machine crossbeam 7 is used to push the regular pyramid-shaped indenter 1 downward at a uniform speed. The inclined surface of the regular pyramid-shaped indenter 1 pushes the segmented rigid mold 2 to expand uniformly radially outward, so that the annular specimen 3 undergoes equal-diameter bulging, specifically including:

[0095] Step S41: in order to reduce the influence of friction, a molybdenum disulfide lubricating coating is prepared on the inner surface of the annular sample 3, the outer surface of the block rigid mold 2, and the surfaces of the regular pyramid-shaped indenter 1 and the base 4;

[0096] Step S42: After lubrication, the annular sample 3 is placed in the middle of the outer surface of the segmented rigid mold 2, and then the regular pyramid indenter 1 is placed in the pyramid hole inside the segmented rigid mold 2, and finally the segmented rigid mold 2 is placed in the center of the base 4;

[0097] Step S43: Place the annular specimen 3 and the mold assembled in step S42 on the testing machine platform, move the testing machine crossbeam 7 until it contacts the upper surface of the right pyramid-shaped indenter 1, and then the testing machine control system 9 controls the testing machine crossbeam 7 to push the right pyramid-shaped indenter 1 to move downward at a uniform speed. The inclined surface of the right pyramid-shaped indenter 1 pushes the divided rigid mold 2 to expand uniformly outward in the radial direction, so that the annular specimen 3 undergoes isodiametric bulging.

[0098] Step S5: Before the test, the outer surface of the annular sample 3 is sprayed with speckles. During the equal-diameter bulging process, the width, widthwise strain, and hoopwise strain of the outer surface of the annular sample 3 are measured using a three-dimensional full-field strain measurement and analysis system (digital image correlation method DIC), and the wall thickness thinning rate of the sample is calculated based on the volume invariance principle.

[0099] Step S6: Substituting the test machine load and the deformation data of the annular sample 3 into the hoop stress theoretical model, calculating the hoop stress of the annular sample 3, and drawing the hoop tensile stress-strain curve of the pipe according to the hoop stress and hoop strain data, specifically including:

[0100] Step S61: Set the test machine load F z Substitute the measured real-time width B of the annular specimen 3 into formula (3) to calculate the normal pressure p of the block rigid mold 2 on the inner surface of the annular specimen 3;

[0101] Step S62: Substitute the normal pressure p, the friction coefficient μ, the angle θ between the sample section and the side surface of the block rigid mold 2, and the sample wall thickness t into formula (7) to calculate the hoop stress at different sections of the annular sample contact area 6, and combine the measured hoop strain of the annular sample 3 to obtain the hoop tensile stress-strain curves at different sections;

[0102] Step S63: The hoop tensile stress-strain curves at different cross sections are averaged to obtain the hoop tensile stress-strain curve of the pipe to be tested.

[0103] like Figure 8-Figure 9 As shown, the present invention also provides a device for testing the circumferential tensile stress-strain curve of a pipe, the device comprising:

[0104] The right pyramid-shaped indenter 1 is made of die steel and is face-to-face matched with the block rigid die 2. It moves downward at a uniform speed under the push of the testing machine crossbeam to transfer the test machine load F z The energy is transferred along the inclined surface to the segmented rigid mold 2. The semi-cone angle β of the regular pyramid-shaped indenter 1 is recommended to be between 10° and 15°.

[0105] The block rigid mold 2 is made of mold steel and expands outward uniformly in the radial direction under the push of the inclined surface of the regular pyramid-shaped pressure head 1, and is used to apply a normal pressure p to the inner surface of the annular sample 3 to cause the annular sample 3 to undergo equal-diameter bulging. The number of petals of the block rigid mold 2 is recommended to be N ≥ 8;

[0106] The base 4 is used to support the segmented rigid mold 2 , and an inner hole of a suitable size is processed in the center to increase the maximum moving distance of the regular pyramid-shaped indenter 1 and the maximum bulging amount of the annular sample 3 .

[0107] The testing machine and control system include a testing machine beam 7, a force sensor 8 and a testing machine control system 9, which are used to push the regular pyramid-shaped pressure head 1 downward at a uniform speed and control its moving speed, wherein the testing machine beam 7 is used to push the regular pyramid-shaped pressure head, the force sensor 8 is used to measure the testing machine load in real time, and the testing machine control system 9 is used to control the moving speed of the testing machine beam 7.

[0108] The three-dimensional full-field strain measurement and analysis system 10 uses a three-dimensional full-field strain measurement and analysis system (DIC digital correlation technology) to measure the widthwise strain and the hoopwise strain of the outer surface of the annular sample 3, and calculates the sample wall thickness thinning rate based on the volume invariance principle.

[0109] Under the control of the testing machine control system 9, the testing machine crossbeam 7 pushes the right pyramid pressure head 1 to move downward at a uniform speed, and the inclined surface of the right pyramid pressure head 1 pushes the block rigid mold 2 to expand radially and uniformly outward, and the block rigid mold 2 generates normal pressure on the inner surface of the annular sample 3, causing the annular sample 3 to undergo equal-diameter expansion (approximately circumferential single-tension deformation). The force sensor 8 measures the testing machine load in real time. By establishing a theoretical model of the annular stress of the annular sample 3, the annular stress of the annular sample 3 can be calculated, and the annular strain of the annular sample 3 measured by the three-dimensional full-field strain measurement and analysis system 10 can be combined to finally obtain the annular tensile stress-strain curve of the pipe. The present invention is further described below in conjunction with specific embodiments.

[0110] Example 1

[0111] Taking the circumferential tensile stress-strain curve test of 6061 aluminum alloy thin-walled tube as an example, the outer diameter of the tube blank is D0=60mm, the wall thickness is t0=1.8mm, and it is in annealed state.

[0112] Step 1: Combined with the stress theoretical model of the annular specimen 3, while ensuring that the annular specimen 3 is in an approximate annular uniaxial tensile stress state during the equal-diameter bulging process (the axial / annular stress ratio is not higher than 2%), and has a high uniformity of annular stress distribution (not less than 98%), determine that the semi-cone angle β of the right pyramid indenter 1 is 15°, the number of petals N of the block rigid mold 2 is 12, and the initial width B0 of the annular specimen 3 is 7.5 mm.

[0113] Step 2: Prepare Molykote D-321R quick-drying molybdenum disulfide lubricating coating on the inner surface of the annular sample 3, the outer surface of the block rigid mold 2, and the surfaces of the regular pyramid indenter 1 and the base 4, respectively. Figure 1 The position shown is installed and placed on the testing machine platform, and the testing machine crossbeam 7 is moved to contact the upper surface of the regular pyramid-shaped indenter 1.

[0114] Step 3: The testing machine crossbeam 7 pushes the right pyramid-shaped indenter 1 to move downward at a uniform speed, the testing machine control system 9 controls the moving speed (20 mm / min), the force sensor 8 measures the testing machine load, and the inclined surface of the right pyramid-shaped indenter 1 pushes the divided rigid mold 2 to expand outward uniformly in the radial direction, causing the annular specimen 3 to undergo equal-diameter bulging until it breaks.

[0115] Step 4: Spray the outer surface of the ring sample 3 with speckle ( Figure 7As shown in FIG. 1 , during the equal-diameter bulging process, a three-dimensional full-field strain measurement and analysis system 10 (digital image correlation method DIC) is used to measure the deformation data such as the width, widthwise strain and hoopwise strain of the outer surface of the annular sample 3, and the wall thickness thinning rate of the sample is calculated based on the volume invariance principle.

[0116] Step 5: Select five sections S1, S2, S3, S4, S5 ( Fig.10 The illustration in the figure shows S1-S5), and the θ angles corresponding to the sections S1 to S5 are 9.5°, 12.3°, 15°, 12.3° and 9.5° respectively. The test machine load F z The measured real-time width B of the annular specimen 3 is substituted into formula (3) to calculate the normal pressure p of the block rigidity module 2 on the inner surface of the annular specimen 3. Then, the normal pressure p, friction coefficient μ, angle θ, and specimen wall thickness t are substituted into formula (7) to calculate the annular stress of different sections in the contact area 6 of the annular specimen. Combined with the measured annular strain, the annular tensile stress-strain curves of sections S1 to S5 are plotted. Fig.10 shown.

[0117] Step 6: Average the circumferential tensile stress-strain curves of sections S1 to S5, and finally draw the circumferential tensile stress-strain curve of the 6061 aluminum alloy pipe to be tested, such as Fig.11 shown.

[0118] 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.

[0119] 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.

[0120] 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 testing the circumferential tensile stress-strain curve of a pipe, characterized in that: The following steps are involved: Step S1: determining a theoretical model for converting the vertical load applied by the testing machine to the regular pyramid indenter into the radial force of the block rigid die acting on the inner surface of the annular specimen, and then obtaining a calculation formula for the normal pressure of the block rigid die on the inner surface of the annular specimen; The right pyramid indenter and the block rigid mold are matched face to face; the vertical load F applied by the testing machine to the right pyramid indenter z Transformed into the radial force F acting on the inner surface of the annular specimen by the block rigid module r The theoretical model is: Among them, β is the semi-cone angle of the right pyramid indenter, μ′ is the friction coefficient between the dies; The calculation formula for the normal pressure p of the block rigidity module on the inner surface of the annular specimen is: Wherein, D0 is the initial outer diameter of the annular specimen, t0 is the initial thickness of the annular specimen, and B is the real-time width of the annular specimen; the normal pressure p is evenly distributed on the inner surface of the annular specimen; Step S2: Determine the hoop stress theoretical model of the annular specimen according to the calculation formula of the normal pressure of the block rigidity module on the inner surface of the annular specimen; specifically including: The hoop stress of the annular specimen in the suspended area is: The hoop stress of the annular specimen in the contact area with the block rigid mold is: Where t is the thickness of the annular specimen, θ is the angle between the cross section of the annular specimen and the side of the block rigid mold, μ is the friction coefficient between the annular specimen and the mold, The hoop stress theoretical model takes into account the friction shear stress between the annular specimen and the die; Step S3: determining the semi-cone angle of the right pyramid indenter, the number of segmented rigid mold halves, the size of the annular specimen, and the friction coefficients between the molds and between the annular specimen and the mold; Step S4: installing the annular specimen and the mold on the testing machine, using the crossbeam to push the regular pyramid-shaped indenter downward at a uniform speed, and the inclined surface of the regular pyramid-shaped indenter pushes the segmented rigid mold to expand uniformly outward in the radial direction, so that the annular specimen undergoes equal-diameter bulging; Step S5: During the isodiametric bulging process, the width, widthwise strain and hoopwise strain of the annular specimen are measured in real time, and the wall thickness reduction rate of the specimen is calculated according to the volume invariance principle; Step S6: Substitute the test machine load and the deformation data of the annular specimen into the hoop stress theoretical model, calculate the hoop stress of the annular specimen, and draw the hoop tensile stress-strain curve of the pipe according to the hoop stress and hoop strain data.

2. The method for testing the circumferential tensile stress-strain curve of a pipe according to claim 1, characterized in that: When determining the semi-cone angle of the right pyramid indenter, the number of rigid mold halves and the size of the annular specimen, the stress theoretical model of the annular specimen is combined to ensure that the axial / annular stress ratio of the annular specimen during the equal-diameter bulging process is not higher than 2%, the uniformity of the annular stress distribution is not lower than 98%, the semi-cone angle of the right pyramid indenter is 10°≤β≤15°, the number of rigid mold halves N≥8, and the ratio of the initial width B0 to the initial outer diameter D0 of the annular specimen is B0 / D0≤1 / 8.

3. The method for testing the circumferential tensile stress-strain curve of a pipe according to claim 1, characterized in that: To reduce the influence of friction, the mold / mold interface and the mold / specimen interface are lubricated; The friction coefficient between the molds and between the annular specimen and the mold is determined, specifically including: selecting a specimen with the same material and surface roughness as the mold and the annular specimen, performing the same lubrication treatment on the interface, and then performing a sliding friction test to obtain the friction coefficient.

4. The method for testing the circumferential tensile stress-strain curve of a pipe according to claim 1, characterized in that: A crossbeam is used to push the right pyramid indenter downward at a uniform speed. The downward moving speed of the right pyramid indenter is determined in the following way: the circumference of the annular specimen is used as the length of the parallel segment, the annular tensile velocity is estimated according to the strain rate of the annular specimen, and then the radial expansion velocity of the block rigid mold is calculated. Finally, the downward moving speed of the right pyramid indenter is inversely deduced.

5. The method for testing the circumferential tensile stress-strain curve of a pipe according to claim 1, characterized in that: The real-time measurement of the deformation data of the width, widthwise strain and hoop strain of the annular specimen specifically includes: spraying speckles on the outer surface of the annular specimen before the test, and using a three-dimensional full-field strain measurement and analysis system to measure the deformation data of the width, widthwise strain and hoop strain of the outer surface of the annular specimen during the equal-diameter bulging process.

6. The method for testing the circumferential tensile stress-strain curve of a pipe according to claim 1, characterized in that: The method of drawing a hoop tensile stress-strain curve of the pipe according to the hoop stress and hoop strain data specifically includes: taking the hoop strain of a certain section of the annular specimen in the contact area with the block rigid mold as the horizontal axis, taking the calculated hoop stress of the section as the vertical axis, obtaining the hoop tensile stress-strain curve of the section, averaging the hoop tensile stress-strain curves of multiple sections to obtain the hoop tensile stress-strain curve of the pipe, the number of sections is not less than 5, and they are symmetrically distributed along the center line of the block rigid mold.

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