Pipe expansion force testing method

Abstract

The application discloses a pipe expanding force testing method, and relates to the field of pipe processing.The method comprises the following steps: selecting a testing pipe section with a length equal to an expanding step along the axial direction of a pipe; selecting at least one cross section of the pipe as a testing cross section along the axial direction of the testing pipe section; selecting at least two strain measuring points on the outer wall of the pipe at the positions of the testing cross sections; obtaining the effective contact area between an expanding die and the inner wall of the testing pipe section during the expanding process; obtaining the circumferential strain of each strain measuring point during the expanding process of the testing pipe section; obtaining the circumferential stress of each strain measuring point through the circumferential strain of each strain measuring point; obtaining the expanding pressure of each strain measuring point through the circumferential stress of each strain measuring point; and obtaining the average expanding force of all the strain measuring points through the effective contact area and the expanding pressure of each strain measuring point, wherein the average expanding force is the pipe expanding force of the expanding die on the testing pipe section.The application can improve the accuracy of pipe expanding force testing.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of pipe machining, in particular to a pipe expanding force testing method. BACKGROUND

[0002] JCO forming process and UO forming process are the main forming processes of large-diameter straight seam submerged arc welded pipe, which have good forming effect. However, in pipe machining, no matter which forming process is used, the inhomogeneous elastic-plastic deformation introduced in the forming process will lead to low cross-sectional size accuracy of the pipe, mechanical property decline and uneven distribution of residual stress. Therefore, mechanical expanding treatment is needed before delivery for use. According to the difference in deformation characteristics, the mechanical expanding deformation process of the formed pipe blank can be divided into two deformation stages of rounding and expanding. In the rounding stage, the protruding part of the inner wall of the pipe is flattened, and local compression strain is even generated. Only the protruding part is deformed, and the rest is even not in contact with the expanding die. With the movement of the piston, the pipe wall will be bent and deformed under the action of the expanding die. The long axis of the pipe is shortened, the short axis is elongated, the contact area between the expanding die and the inner wall of the pipe gradually increases, and finally the expanding die and the inner wall of the pipe are completely attached. That is, the cross section of the pipe can be changed from an ellipse to a circle in the rounding stage. The deformation of the pipe in this stage is mainly the change of the cross-sectional geometry. When the pipe enters the complete contact deformation stage, elastic-plastic deformation occurs at the same time. With the continuous expansion of the expanding die, the mechanical expanding enters the expanding deformation stage, and the elastic-plastic deformation of the pipe is completely converted into plastic deformation. The deformation characteristics of this stage are the increase of the cross-sectional geometry.

[0003] That is, the inhomogeneous contact between the expanding die and the inner wall of the pipe in the rounding stage will lead to inhomogeneous elastic-plastic deformation of the pipe. Even if the expanding die and the inner wall of the pipe are completely attached to enter the expanding deformation stage, due to the difference in the distribution of the mechanical properties of the pipe in the cross section caused by the previous bending and welding process, the deformation of each part of the pipe is not the same. At present, for the calculation of the expanding force in the expanding deformation stage of the pipe, the pipe is usually regarded as being in a uniform stress state. However, the mechanical expanding equipment is similar to a cantilever beam mechanism. Under the action of its own gravity, the expanding equipment will inevitably produce eccentric loading, which will cause differences in the expanding pressure of the expanding die on the inner wall of the pipe. The calculated expanding force based on the assumption of uniform deformation will have deviations.

[0004] When the friction coefficient between the expanding die and the outer side wall of the expanding head is known, the expanding force of the pipe can also be indirectly calculated through the change of the oil pressure of the hydraulic cylinder. However, it is difficult to measure the friction coefficient between the expanding die and the outer side wall of the expanding head under high pressure and heavy load conditions. SUMMARY

[0005] The purpose of the present application is to provide a pipe expanding force testing method to solve the problems existing in the prior art and improve the accuracy of pipe expanding force testing.

[0006] To achieve the above object, the present application provides the following scheme:

[0007] The present application provides a pipe expanding force testing method, comprising the following steps:

[0008] S1, selecting a test pipe section along the axial direction of the pipe, the axial length of the test pipe section being equal to the expanding step length, selecting at least one test section along the axial direction of the test pipe section, each test section being a cross section of the pipe, and selecting at least two test points on the outer wall of the pipe at the position of each test section as the strain measuring points;

[0009] obtaining the effective contact area between the expanding die and the inner wall of the test pipe section during the expanding process of the test pipe section;

[0010] obtaining the circumferential strain at each strain measuring point of the test pipe section during the expanding process; obtaining the circumferential stress at each strain measuring point corresponding to the circumferential strain at each strain measuring point; and obtaining the expanding pressure at each strain measuring point corresponding to the circumferential stress at each strain measuring point;

[0011] S1 further comprises: obtaining the expanding pressure at each strain measuring point by formula 1, formula 1 being:

[0012] Formula 1

[0013] wherein, ; p is the expanding pressure at each strain measuring point; is the outer radius of the expanding die; is the outer radius of the test pipe section;

[0014] S2, obtaining the average expanding force at all strain measuring points by the effective contact area and the expanding pressure at each strain measuring point, and taking the average expanding force as the pipe expanding force of the expanding die on the test pipe section.

[0015] Preferably, S2 further comprises: obtaining the average expanding pressure by the expanding pressure at all strain measuring points, and obtaining the pipe expanding force of the expanding die on the test pipe section by the effective contact area and the average expanding pressure.

[0016] Preferably, S1 further comprises: obtaining the circumferential stress at each strain measuring point by formula 2, formula 2 being:

[0017] Formula 2

[0018] wherein, a circumferential stress at each of the strain measuring points; a circumferential strain at each of the strain measuring points; an elastic modulus of the pipe base material; a yield strength of the pipe base material; a ratio of a plastic modulus of the pipe base material to the elastic modulus of the pipe base material, which is a reinforcement coefficient; a yield strain of the pipe base material.

[0019] Preferably,

[0020] Preferably, S1 further comprises: obtaining an effective contact area of the expander die and the inner wall of the test pipe segment by Formula 3, Formula 3 being:

[0021] Formula 3

[0022] wherein, B an expander step length of the expander die; D a diameter of the expander die.

[0023] Preferably, S2 further comprises: obtaining the average expander pressure at all the strain measuring points by Formula 4; and obtaining the pipe expander force by Formula 5, Formula 4 and Formula 5 being respectively:

[0024] Formula 4

[0025] Formula 5

[0026] wherein, P the average expander pressure; n a total number of the strain measuring points.

[0027] Preferably, S1 further comprises: selecting a middle cross section of each of the test pipe segments as the test cross section, and selecting a plurality of test points on the outer wall of the pipe at the position of each of the test cross sections as the strain measuring points.

[0028] Preferably, S1 further comprises: disposing a strain gauge at each of the strain measuring points, and measuring the circumferential strain at each of the strain measuring points by the strain gauge.

[0029] Preferably, S1 further comprises: determining an expander ratio of the test pipe segment, and selecting the strain gauge according to the expander ratio of the test pipe segment.

[0030] Preferably, S1 further comprises: the selection method of the strain gauge comprises: a range of the strain gauge is greater than a theoretical circumferential tensile strain that can be generated by the test pipe segment in the expander process, the theoretical circumferential tensile strain being equal to the expander ratio of the test pipe segment.

[0031] The present application has the following technical effects relative to the prior art:

[0032] The present application provides a pipe expansion force testing method, comprising the following steps:

[0033] S1, the axial length of the test pipe section is equal to the expansion step length, at least one test section is selected along the axial direction of the test pipe section, each test section is a cross section of the pipe, and at least two test points on the outer wall of the pipe at the position of each test section are selected as strain measurement points;

[0034] The effective contact area between the expansion die and the inner wall of the test pipe section during the expansion process is obtained;

[0035] The circumferential strain at each strain measurement point of the test pipe section during the expansion process is obtained, the circumferential stress at each strain measurement point is obtained through the circumferential strain at each strain measurement point, and the expansion pressure at each strain measurement point is obtained through the circumferential stress at each strain measurement point;

[0036] S2, the average expansion force at all strain measurement points is obtained through the effective contact area and the expansion pressure at each strain measurement point, and the average expansion force is taken as the pipe expansion force of the expansion die on the test pipe section.

[0037] The present application sets multiple strain measurement points in the circumferential direction of the test pipe section for monitoring, so that the strain stress data of the multiple strain measurement points can be obtained, the circumferential strain information of the test pipe section can be obtained as comprehensively as possible, the average expansion force at the multiple strain measurement points can be obtained, the average expansion force is taken as the pipe expansion force of the test pipe section, and the accuracy of the pipe expansion force testing can be improved. BRIEF DESCRIPTION OF DRAWINGS

[0038] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0039] Figure 1 The flowchart of the pipe expansion force testing method provided for embodiment 1 is shown in the figure;

[0040] Figure 2 The structure diagram of the pipe expansion device provided for embodiment 2 is shown in the figure Figure 1 ;

[0041] Figure 3 The structure diagram of the pipe expansion device provided for embodiment 2 is shown in the figure Figure 2 ;

[0042] Figure 4 A flowchart of the pipe expansion force test method provided in Example 3;

[0043] In the figure: 100, pipe expansion device; 1, pipe fitting; 2, expansion head; 3, expansion mold; 301, mold flap; 4, tie rod sleeve; 5, tie rod; 501, limiting shoulder; 6, clamp; 7, piston; 8, cylinder; 9, nut; 10, intermediate ring; 11, strain gauge. Detailed Implementation

[0044] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0045] The purpose of this invention is to provide a method for testing the expansion force of pipe fittings, so as to solve the problems existing in the prior art and improve the accuracy of pipe fitting expansion force testing.

[0046] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0047] Example 1

[0048] like Figure 1 As shown, this embodiment provides a method for testing the expansion force of pipe fittings, including the following steps:

[0049] S1. Select a test pipe section along the axial direction of pipe fitting 1. The axial length of the test pipe section is equal to the diameter expansion step. Select at least one test section along the axial direction of the test pipe section. Each test section is the cross section of the pipe fitting. Select at least two test points as strain measurement points on the outer wall of pipe fitting 1 at the location of each test section. Preferably, select multiple test points as strain measurement points on the outer wall of pipe fitting 1 at the location of each test section.

[0050] Obtain the effective contact area between the expansion mold and the inner wall of the test pipe section during the expansion process;

[0051] The circumferential strain at each strain measuring point of the test pipe section during the expansion process is obtained; the circumferential stress at each strain measuring point is obtained through the circumferential strain at each strain measuring point; and the expansion pressure at each strain measuring point is obtained through the circumferential stress at each strain measuring point.

[0052] S2. Obtain the average expansion force at all strain measurement points by using the effective contact area and the expansion pressure at each strain measurement point. Use the average expansion force as the expansion force of the expansion mold on the test pipe section.

[0053] The embodiment monitors at least two strain measuring points circumferentially arranged at the test section, so as to obtain circumferential strain stress data of the plurality of strain measuring points, to obtain circumferential strain information of the test pipe section as comprehensively as possible, and to obtain average expansion force at the plurality of strain measuring points, so as to take the pipe expansion force of the expansion die on the test pipe section as the average expansion force, and to improve the accuracy of the pipe expansion force test.

[0054] In the embodiment, S2 further comprises: obtaining average expansion pressure by expansion pressure at all strain measuring points, and obtaining pipe expansion force of the expansion die on the test pipe section by effective contact area and average expansion pressure.

[0055] In the embodiment, S1 further comprises: obtaining expansion pressure at each strain measuring point by formula 1, and formula 1 is:

[0056] Formula 1

[0057] Wherein, ; p is the expansion pressure at each strain measuring point; is the outer radius of the expansion die; is the outer radius of the test pipe section.

[0058] In the embodiment, S1 further comprises: obtaining circumferential stress at each strain measuring point by formula 2, and formula 2 is:

[0059] Formula 2

[0060] Wherein, is the circumferential stress at each strain measuring point; is the circumferential strain at each strain measuring point; is the elastic modulus of the pipe base material; is the yield strength of the pipe base material; is the ratio of the material plastic modulus of the pipe base material to the elastic modulus of the pipe base material, and is the strengthening coefficient; is the yield strain of the pipe base material.

[0061] In the embodiment, the circumferential stress is obtained according to the pipe base material constitutive relation model, the pipe base material constitutive relation model is regarded as a linear hardening model, when the circumferential strain of the pipe 1 caused by expansion is less than the yield stress of the pipe 1, the circumferential stress is calculated according to elastic deformation; when the circumferential strain of the pipe 1 caused by expansion is greater than the yield strain of the pipe 1, the circumferential stress is calculated according to plastic hardening stage deformation; therefore, the embodiment adopts formula 2 to obtain the circumferential stress at each strain measuring point.

[0062] In the embodiment, S1 further comprises: obtaining the effective contact area between the expander die and the inner wall of the pipe 1 by formula 3, formula 3 is:

[0063] Formula 3

[0064] wherein, B is the expansion step length of the expander die; D is the diameter of the expander die.

[0065] In the embodiment, S2 further comprises: obtaining the average expansion pressure at each strain measurement point by formula 4; obtaining the pipe expansion force by formula 5; formula 4 and formula 5 are respectively:

[0066] Formula 4

[0067] Formula 5

[0068] wherein, P is the average expansion pressure; n is the total number of strain measurement points.

[0069] In the embodiment, S1 further comprises: selecting the middle cross section of each test pipe section as the test section, and selecting a plurality of test points on the outer wall of the pipe 1 at the position of each test section as strain measurement points.

[0070] In the embodiment, S1 further comprises: setting a strain gauge at each strain measurement point, and measuring the circumferential strain at each strain measurement point by the strain gauge.

[0071] In the embodiment, S1 further comprises: determining the expansion rate of the test pipe section, and selecting the strain gauge according to the expansion rate of the test pipe section.

[0072] In the embodiment, S1 further comprises: the selection method of the strain gauge comprises: the range of the strain gauge is greater than the theoretical circumferential tensile strain that the test pipe section can generate in the expansion process, and the theoretical circumferential tensile strain is equal to the expansion rate of the test pipe section.

[0073] In the embodiment, S1 further comprises: obtaining the yield strain of the pipe base material by formula 6, formula 6 is:

[0074] Formula 6

[0075] The pipe expanding force testing method provided by the embodiment is based on the elastic-plastic mechanics theory of metal materials, an analytical formula for calculating the expanding force of the inner wall of the pipe 1 on the expanding die is established, the calculation method is simple, the influence of the uneven elastic-plastic deformation generated in the expanding process is considered, the pipe expanding force can be accurately obtained, on the one hand, the strain measurement value of the strain measurement point can reflect the deformation degree of different positions on the steel pipe cross section, and can provide a reference basis for the selection of the structural parameters of the expanding die; on the other hand, the axial tension of the pull rod of the expanding device can be obtained by combining the structural parameters of the expanding cylinder and the oil pressure data, and the friction coefficient of the outer side wall of the expanding die and the conical expanding head can be indirectly obtained, which is used for guiding the selection of the contact surface material of the expanding die and the design of the microstructure of the contact surface.

[0076] Embodiment 2

[0077] As shown in Figure 2 , 3 , the embodiment provides a pipe expanding device 100 for testing the pipe expanding force in embodiment 1, which comprises an expanding head 2, an expanding die 3, a pull rod sleeve 4, a pull rod 5, a clamp 6, a power cylinder and a nut 9. The power cylinder comprises a piston 7 and a cylinder body 8, and the cylinder body 8 is connected with the pull rod sleeve 4 through the clamp 6. The piston 7 is arranged in the cylinder body 8, and the pull rod 5 is arranged in the pull rod sleeve 4. One end of the pull rod 5 close to the cylinder body 8 is provided with a limiting shaft shoulder. The end of the limiting shaft shoulder away from the cylinder body 8 can be in contact with the inner ring of the end face of the pull rod sleeve 4 close to the cylinder body 8, and the end of the limiting shaft shoulder close to the cylinder body 8 can be in contact with the end face of the piston 7 close to the pull rod sleeve 4. The other end of the piston 7 is connected with the nut 9, and the nut 9 can lock the piston 7 and the limiting shaft shoulder on the end face of the pull rod sleeve 4 close to the cylinder body 8, so as to realize the fixed connection of the piston 7 and the pull rod 5. The expanding head 2 is fixed on the end of the pull rod 5 away from the cylinder body 8 through the nut. The expanding die 3 comprises a plurality of die petals and an elastic locking ring. The plurality of die petals are arranged along the circumference of the expanding head 2, each die petal is arranged between the expanding head 2 and the inner side wall of the pipe 1, and the elastic locking ring is arranged outside all the die petals and can provide radial holding force for each die petal, so that the inner side wall of the die petal can keep in contact with the outer side wall of the expanding head 2. The outer side wall of the expanding head 2 is provided with a sliding groove, the end of the pull rod sleeve 4 close to the expanding head is fixedly installed with an intermediate ring 10, the end face of the intermediate ring 10 close to the expanding die 3 is provided with a radial sliding groove, the expanding die 3 is connected with the expanding head 2 and the intermediate ring 10 through a long T-shaped guide key and a short T-shaped guide key respectively, the expanding die 3 and the intermediate ring 10 can move relatively in the radial direction, and the expanding die 3 and the expanding head 2 can move relatively in the axial direction. The expanding head 2 is conical, the diameter of the end of the expanding head 2 close to the piston 7 is smaller than the diameter of the end of the expanding head 2 away from the piston 7. The axial section of each die petal is trapezoidal, and the small end of the trapezoid is located at the end away from the piston 7.

[0078] The working principle of the pipe expanding device 100 provided by the embodiment is as follows: when the piston 7 moves to the right, the piston 7 drives the pull rod 5 to move to the right, and then drives the expanding head 2 to move to the right through the pull rod 5, the expanding head 2 acts on the expanding die 3 through the inclined outer side wall, so that the expanding die 3 moves radially along the pipe 1, and at the same time, the expanding force is transmitted to the inner side wall of the pipe 1 through the expanding head 2 and the expanding die 3, so that the pipe 1 is expanded in a distance. Then, the piston 7 drives the expanding head 2 to move to the left through the pull rod 5, the extrusion of the expanding head 2 on the expanding die 3 gradually decreases until it disappears, and the expanding die 3 is separated from the pipe 1, at this time, the pipe 1 is not subjected to the action force. After the pipe body 1 moves a step, the expanding of the next step is carried out in the same way, and the expanding of the whole pipe body is completed through reciprocating and stepping.

[0079] Embodiment 3

[0080] As shown in the drawings, the embodiment provides a pipe expanding force test method, which comprises the following steps: Figure 4

[0081] Step one, the expanding rate of the pipe 1 is determined in advance, and a suitable strain gauge is selected;

[0082] Step two, the expanding step is determined in advance, a test pipe section is selected along the axial direction of the pipe 1, the axial length of the test pipe section is equal to the expanding step, at least one test section is selected along the axial direction of the test pipe section, each test section is a cross section of the pipe, and at least two test points are selected on the outer wall of the pipe 1 at the position of each test section as strain measurement points;

[0083] Step three, the strain measurement points are polished and cleaned, at least one strain gauge is pasted at the selected strain measurement points, the pasting direction of the strain gauge is the circumferential direction of the pipe 1, and the strain gauge is used for measuring the circumferential strain of the pipe, epoxy resin protective glue is coated on the surface of the pasted strain gauge, and the strain gauge is connected to the data acquisition channel of the strain acquisition instrument through shielding wires;

[0084] Step four, the strain acquisition instrument collects the data measured by each strain gauge during the expanding of the pipe 1, and the circumferential stress of each strain measurement point is calculated according to formula 2 respectively;

[0085] Step five, the expanding pressure corresponding to each strain measurement point is calculated based on the circumferential stress data of each strain measurement point through formula 1;

[0086] Step six, the contact area between the expanding die and the inner side wall of the pipe 1 is calculated through formula 3 according to the axial length of the test pipe section and the outer radius of the expanding die, and the average expanding pressure of the expanding die on the pipe is calculated through formula 4;

[0087] Step seven, the pipe expanding force is calculated according to the average expanding pressure, the contact area and formula 4.

[0088] ​Further, the strain gauges selected in step one are resistance strain gauges, and the range of each strain gauge is greater than the theoretical maximum circumferential tensile strain of the pipe 1 caused by the expansion , the theoretical circumferential tensile strain of the pipe 1 is equal to the expansion rate of the pipe 1.

[0089] Further, the strain acquisition frequency in step four is 1000 Hz.

[0090] The principles and implementation manners of the present application are described by using specific examples in the present application, and the above example is only used to help understand the method of the present application and its core idea; meanwhile, for the general technical personnel in the field, the specific implementation manners and application ranges will be changed according to the idea of the present application. 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 expansion force of pipe fittings, characterized in that: Includes the following steps: S1. Select a test pipe section along the axial direction of the pipe fitting. The axial length of the test pipe section is equal to the diameter expansion step. Select at least one test section along the axial direction of the test pipe section. Each test section is the cross section of the pipe fitting. Select at least two test points as strain measurement points on the outer wall of the pipe fitting at the location of each test section. Obtain the effective contact area between the expansion mold and the inner wall of the test pipe segment during the expansion process; The circumferential strain at each strain measuring point of the test pipe section is obtained during the diameter expansion process; the circumferential stress at each strain measuring point is obtained through the circumferential strain at each strain measuring point. The expansion pressure at each strain measuring point is obtained by measuring the circumferential stress at each strain measuring point. The expansion pressure at each strain measurement point is obtained using Formula 1, which is: Formula 1 in, ; p The expansion pressure at each of the aforementioned strain measurement points; The outer radius of the expanding die; The outer radius of the test pipe section; The circumferential stress at each of the aforementioned strain measurement points; The circumferential strain at each of the aforementioned strain measurement points; The elastic modulus of the base material of the pipe fitting; The yield strength of the base material of the pipe fitting; The ratio of the plastic modulus to the elastic modulus of the base material of the pipe fitting is given by the strengthening coefficient. The yield strain of the base material of the pipe fitting; S2. Obtain the average expansion force at all strain measurement points by using the effective contact area and the expansion pressure at each strain measurement point, and use the average expansion force as the expansion force of the expansion mold on the test pipe section.

2. The method for testing the expansion force of pipe fittings according to claim 1, characterized in that: S2 further includes: obtaining the average expansion pressure through the expansion pressure at all the strain measurement points, and obtaining the expansion force of the expansion mold on the test pipe section through the effective contact area and the average expansion pressure.

3. The method for testing the expansion force of pipe fittings according to claim 1, characterized in that: S1 further includes: obtaining the circumferential stress at each strain measurement point using Formula 2, where Formula 2 is: Formula 2.

4. The method for testing the expansion force of pipe fittings according to claim 1, characterized in that: S1 further includes: obtaining the effective contact area between the expanding mold and the inner wall of the test pipe section using Formula 3, where Formula 3 is: Formula 3 in, B The diameter expansion step size is the diameter expansion step size of the diameter expansion die.

5. The method for testing the expansion force of pipe fittings according to claim 4, characterized in that: S2 further includes: obtaining the average expansion pressure at all strain measurement points using Formula 4; and obtaining the expansion force of the pipe fitting using Formula 5; Formula 4 and Formula 5 are respectively: Formula 4 Formula 5 in, P The average expansion pressure; n The total number of strain measurement points.

6. The method for testing the expansion force of pipe fittings according to claim 1, characterized in that: S1 further includes: selecting the middle cross section of each of the test pipe segments as the test section, and selecting multiple test points on the outer wall of the pipe at the location of each test section as the strain measurement points.

7. The method for testing the expansion force of pipe fittings according to claim 6, characterized in that: S1 further includes: setting a strain gauge at each of the strain measurement points, and measuring the circumferential strain at each of the strain measurement points through the strain gauge.

8. The method for testing the expansion force of pipe fittings according to claim 7, characterized in that: S1 further includes: determining the expansion ratio of the test pipe section, and selecting the strain gauge according to the expansion ratio of the test pipe section.

9. The method for testing the expansion force of pipe fittings according to claim 8, characterized in that: S1 further includes: the method for selecting the strain gauge includes: the range of the strain gauge is greater than the theoretical circumferential tensile strain that the test pipe section can generate during the expansion process, and the theoretical circumferential tensile strain is equal to the expansion rate of the test pipe section.

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