A metal pipe circumferential fatigue test fixture and test method

Abstract

The present invention relates to a metal pipe circumferential fatigue test fixture and test method, comprising an upper D-shaped part, a lower D-shaped part, an upper fixing part, and a lower fixing part, wherein the upper fixing part is connected to the upper D-shaped part, and the lower fixing part is connected to the lower D-shaped part, an upper positioning groove is arranged on the arc surface of the upper D-shaped part, and a lower positioning groove is arranged on the arc surface of the lower D-shaped part, and a metal pipe sample is arranged in the upper positioning groove and the lower positioning groove. A plurality of metal pipe samples are subjected to tensile tests and corresponding formulas are calculated, and then fatigue tests are performed on formal samples. The problem that only axial arc samples can be cut for traditional pipe fatigue tests, resulting in inaccurate results due to work hardening or deformation of the samples, is avoided; the relationship between the test machine load and the stress of the working section of the metal pipe sample is obtained, and the complexity of load calculation using simulation software is avoided; it is suitable for conducting circumferential stress controlled tensile fatigue tests on pipes of different diameters.

Description

Technical Field

[0001] The invention relates to the technical field of metal pipe performance testing, and in particular to a metal pipe circumferential fatigue test fixture and a test method, which are aimed at a tension-tension fatigue performance test of a metal pipe circumferential stress control. Background Art

[0002] Metal pipes include pipeline pipes, boiler pipes, mechanical structure pipes, pipes for hydraulic and pneumatic systems, pipes for oil and natural gas transportation, chemical pipes, etc. Due to different usage occasions and purposes, there are usually different requirements for pipe materials and mechanical properties.

[0003] At present, there are two methods for testing the mechanical properties of pipes: using axial mechanical properties testing instead of circumferential mechanical properties testing, that is, cutting a section of pipe or cutting an arc-shaped specimen along the axial direction for mechanical properties testing; or cutting the pipe along the axial direction, then flattening it and then testing the mechanical properties. However, metal pipes are generally prone to anisotropy of material mechanical properties due to texture reasons, especially for seamless pipes, such as extruded pipes or cold-drawn pipes, the mechanical properties of the pipes along the axial and circumferential directions are very different. The process of cutting the pipe along the axial direction and then flattening it will cause work hardening to cause changes in the pipe properties, which will affect the accuracy of the test results of the mechanical properties of the pipe.

[0004] Fatigue performance test is an important test to evaluate the comprehensive mechanical properties of metal pipes. Existing patents, such as the patent application publication number CN105388061A, propose a circumferential fatigue test fixture for small-diameter metal pipes, but the diameter of the metal pipe that can be tested does not exceed 50mm. Due to its structure, one end of the fixture used to position the pipe is suspended in the air, and the load is transmitted to the fixture through the other end of the fixture, which will cause the fixture to swing in the vertical direction. It is not suitable for fatigue testing of large-diameter pipes. Existing patents for circumferential tensile performance testing of pipes, such as authorization announcement numbers CN100487416C, CN102445384B, and CN202693413U, introduce circumferential tensile test fixtures or methods for pipes, but the specimen is not fixed, that is, the specimen can still slide axially or circumferentially, resulting in the fatigue testing machine may not be able to start vibration or the vibration frequency is not high, so this type of fixture cannot be used for circumferential fatigue testing of pipes. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a circumferential fatigue test fixture and a test method for metal pipes with larger pipe diameters, which will not cause axial and circumferential sliding.

[0006] The technical solution adopted by the present invention to solve its technical problems is: a metal pipe circumferential fatigue test fixture is installed on a testing machine for stretching, including an upper D-shaped part, a lower D-shaped part, an upper fixing part connected to the testing machine, and a lower fixing part connected to the testing machine, the upper fixing part is connected to the upper D-shaped part, the lower fixing part is connected to the lower D-shaped part, the upper D-shaped part and the lower D-shaped part form a circular block for fixing the metal pipe sample, an upper positioning groove is arranged on the arc surface of the upper D-shaped part, and a lower positioning groove is arranged on the arc surface of the lower D-shaped part, and the metal pipe sample is arranged in the upper positioning groove and the lower positioning groove.

[0007] More specifically, the force axes of the upper fixing member and the lower fixing member coincide with each other and pass through the center of the circular block.

[0008] More specifically, the upper fixing member and the lower fixing member have the same structure and both include a U-shaped block and a round rod arranged on the U-shaped block. The U-shaped block includes a fixed end and extended ends extending to the same side at both ends of the fixed end, and the round rod is fixed on the fixed end.

[0009] To be more specific, an upper connecting through hole is provided on the upper D-shaped part, and upper connecting holes are respectively provided on the two protruding ends of the upper fixing part, and an upper pin is inserted into the upper connecting through hole and the upper connecting hole to connect the upper fixing part and the upper D-shaped part; a lower connecting through hole is provided on the lower D-shaped part, and lower connecting holes are respectively provided on the two protruding ends of the lower fixing part, and a lower pin is inserted into the lower connecting through hole and the lower connecting hole to connect the lower fixing part and the lower D-shaped part.

[0010] More specifically, a threaded hole is provided on the fixed end, and threads are provided on the round rod, and the threads match the threaded hole.

[0011] More specifically, one end of the round rod of the lower fixing member passes through the fixing end and abuts against the metal pipe sample in the lower positioning groove.

[0012] A circumferential fatigue test method for metal pipes, the test method is:

[0013] S1. Cut a number of metal pipe samples and a formal sample from the metal pipe for standby use;

[0014] S2. Fix the metal pipe sample by a fatigue test fixture and fix the fatigue test fixture on the testing machine, and ensure that the force axis of the fatigue test fixture passes through the center of the metal pipe sample;

[0015] S3. Stretch the metal pipe sample at a constant speed until it breaks, and record the maximum load;

[0016] S4, select the next metal pipe sample, return to step S2 and ensure that the metal pipe sample is rotated to another angle, which is different from the angle of the previously broken metal pipe sample, until all metal pipe samples are tested;

[0017] S5. By performing stress analysis on multiple metal pipe specimens, the friction coefficient between the metal pipe specimen and the fatigue test fixture is obtained by processing the above test results, and the relationship between the load applied by the testing machine and the stress on the working section of the metal pipe specimen is obtained;

[0018] S6, according to the relationship between the load applied by the testing machine and the stress on the working section of the metal pipe sample obtained in step S5, and then according to the peak stress and valley stress set in the fatigue test, calculate the maximum load and minimum load required to be set for the testing machine;

[0019] S7, select a formal sample, install and fix it according to step S2, and ensure that the center of the working section of the formal sample is located on the force axis, and perform fatigue test;

[0020] S8. After the fatigue test is completed, remove the formal sample.

[0021] More specifically, there are at least two metal pipe samples.

[0022] More specifically, in step S7, after the formal sample is clamped, a fixing member is used to ensure that the formal sample does not rotate.

[0023] More specifically, the relationship between the load applied by the testing machine in step S5 and the stress on the working section of the metal pipe sample is:

[0024]

[0025] Among them, F(θ) is the hoop tension, P is the tension on the circular block, μ is the friction coefficient between the metal pipe sample and the circular block, and θ is the angle between the working section of the metal pipe sample and the horizontal direction.

[0026] The beneficial effects of the present invention are: 1. Through the circumferential fatigue test, the problem that only axial arc specimens (or plate specimens) can be cut in the traditional pipe fatigue test, resulting in inaccurate results due to work hardening or deformation of the specimens, is avoided; 2. Through the force analysis of the specimens and the processing of the tensile test results before the fatigue test, the relationship between the test machine load and the stress of the working section of the metal pipe specimen is obtained, avoiding the complexity of load calculation using simulation software; 3. The test fixture is simple and easy to process, and the test method is easy to operate, which is suitable for tension-tension fatigue tests with circumferential stress control on pipes of different diameters. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram of the assembly structure of the circumferential fatigue test fixture of the present invention;

[0028] FIG2 is a schematic diagram of the structure of an upper D-shaped block and a lower D-shaped block of the present invention;

[0029] Figure 3 It is a schematic diagram of the structure of the upper fixing member and the lower fixing member of the present invention;

[0030] Figure 4 It is a schematic diagram of the structure of the metal pipe sample and the formal sample of the present invention;

[0031] Figure 5 It is a schematic flow diagram of the test method of the present invention;

[0032] Figure 6 It is a schematic diagram of the angle between the radial direction and the horizontal direction of the working section center of the sample of the present invention;

[0033] Figure 7 It is a schematic diagram of the force analysis of the sample of the present invention;

[0034] Figure 8 It is a schematic diagram of the force on the circumferential fatigue specimen when θ=0° according to the present invention.

[0035] In the figure: 1, upper fixing member; 2, lower fixing member; 3, upper D-shaped member; 4, lower D-shaped member; 5, metal pipe sample; 11, upper U-shaped block; 12, upper round rod; 21, lower U-shaped block; 22, lower round rod; 31, upper connecting through hole; 32, upper positioning groove; 41, lower connecting through hole; 42, lower positioning groove; 51, working section; 111, fixed end; 112, protruding end. DETAILED DESCRIPTION

[0036] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. 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.

[0037] In the description of the present invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying 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", "second", and "third" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance.

[0038] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected" and "connected" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal connection of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances. In addition, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0039] like Figure 1 The metal pipe circumferential fatigue test fixture shown is installed on a testing machine for tension, comprising an upper D-shaped piece 3, a lower D-shaped piece 4, an upper fixing piece 1 connected to the testing machine, and a lower fixing piece 2 connected to the testing machine. The upper fixing piece 1 is connected to the upper D-shaped piece 3, and the testing machine transmits a part of the force to the metal pipe sample 5 through the upper fixing piece 1 and the upper D-shaped piece 3. The lower fixing piece 2 is connected to the lower D-shaped piece 4, and the testing machine transmits a part of the force to the metal pipe sample 5 through the lower fixing piece 2 and the lower D-shaped piece 4. On the metal pipe sample 5, the upper D-shaped part 3 and the lower D-shaped part 4 form a circular block for fixing the metal pipe sample 5, an upper positioning groove 32 is arranged on the arc surface of the upper D-shaped part 3, and a lower positioning groove 42 is arranged on the arc surface of the lower D-shaped part 4, and the metal pipe sample 5 is arranged in the upper positioning groove 32 and the lower positioning groove 42, and the upper D-shaped part 3 and the lower D-shaped part 4 are not connected, and the metal pipe sample 5 is stretched by pulling the upper D-shaped part 3 and the lower D-shaped part 4 in opposite directions.

[0040] Based on the above structure, in order to ensure the stability and balance of the stretching, when the testing machine applies load to the upper fixing member 1 and the lower fixing member 2, it is necessary to ensure that the load passes through the axis of the upper fixing member 1 and the lower fixing member 2, and at the same time, the force axis of the upper fixing member 1 and the lower fixing member 2 coincides and passes through the center of the circular block, that is, the force axis needs to pass through the center of the metal pipe sample 5.

[0041] like Figure 2a , Figure 2b and Figure 3The upper fixing member 1 and the lower fixing member 2 described above are consistent in structure, both of which include a U-shaped block and a round rod arranged on the U-shaped block, and the U-shaped block includes a transversely arranged fixed end 111 and an extended end 112 located at both ends of the fixed end 111 and extending to the same side, and the round rod is fixed at the center position of the fixed end 111, that is, the upper fixing member 1 includes an upper U-shaped block 11 and an upper round rod 12, and the lower fixing member 2 includes a lower U-shaped block 21 and a lower round rod 22; the upper D-shaped member 3 and the lower D-shaped member 4 are both semi-arc-shaped, and a connecting through hole is arranged at the center position of the semi-arc, a pin is arranged in the connecting through hole, and a connecting hole is arranged on the corresponding extended end, that is, the upper D-shaped member 3 There are corresponding upper connecting through holes 31, upper connecting holes and upper pin shafts, and there are corresponding lower connecting through holes 41, lower connecting holes and lower pin shafts on the lower D-shaped piece 4; the upper D-shaped piece 3 is inserted between the two protruding ends 112 of the upper U-shaped block 11, and the upper pin shaft is fixed on the two protruding ends 112 after passing through the upper connecting hole and the upper connecting through hole 31, at which time the upper D-shaped piece 3 and the upper U-shaped block 11 can rotate relative to each other around the axis of the upper pin shaft; the lower D-shaped piece 4 is inserted between the two protruding ends 112 of the lower U-shaped block 21, and the lower pin shaft is fixed on the two protruding ends 112 after passing through the lower connecting hole and the lower connecting through hole 41, at which time the lower D-shaped piece 4 and the lower U-shaped block 21 can rotate relative to each other around the axis of the lower pin shaft.

[0042] The installation process of the metal pipe fitting sample 5 is as follows: first, the upper D-shaped part 3 and the lower D-shaped part 4 are inserted into the metal pipe material sample 5 to ensure that the metal pipe material sample 5 is inserted into the upper positioning groove 32 and the lower positioning groove 42. At this time, the upper D-shaped part 3 and the lower D-shaped part 4 form a circular block, and the upper connecting shaft is inserted into the upper connecting through hole 31, and the upper connecting shaft is fixed to the two protruding ends 112 of the upper U-shaped block 12 at the same time. The lower connecting shaft is inserted into the lower connecting through hole 41, and the lower connecting shaft is fixed to the two protruding ends 112 of the lower U-shaped block 21 at the same time. At this time, the installation of the metal pipe fitting sample 5 is completed, and then the upper round rod 12 and the lower round rod 22 are fixed on the testing machine to start the test.

[0043] There can be a variety of connection methods between the round rod and the fixed end 111, such as welding, threaded connection, etc., wherein the round rod, the fixed end 111 and the protruding end 112 can also be formed in one piece; in this embodiment, the round rod and the fixed end 111 are connected by threaded connection, that is, a threaded hole is set at the center position of the fixed end 111, and a thread is set on the round rod, and the thread cooperates with the threaded hole to achieve a fixed connection; after the metal pipe sample 5 is fixed, the lower round rod 22 can pass through the fixed end 111 through the cooperation of the thread and the threaded hole and then rest on the metal pipe sample 5 located in the lower positioning groove 42.

[0044] The upper positioning groove 32 and the lower positioning groove 42 have the same structure, and the groove widths for fixing the metal pipe sample 5 are consistent. Meanwhile, the width of the groove is equal to or slightly larger than the width of the metal pipe sample 5, which can ensure its positioning in the axial direction.

[0045] Based on the structure of the above-mentioned fixture, a new circumferential fatigue test method for metal pipes is provided. The following fatigue test is performed on a certain brand of pipe with a specification of ψ90mm (diameter) × 3.0mm (wall thickness) with a stress ratio of R = 0.1. Figure 5 The steps are as follows:

[0046] S1, cut a plurality of metal pipe samples 5 and a formal sample from the metal pipe for standby use, and at the same time, process a working section 51 (such as Figure 4 As shown); wherein, there are at least 2 metal pipe samples 5, and more metal pipe samples 5 can also be selected. In this scheme, 3 metal pipe samples 5 can be selected; the metal pipe sample 5 has the same structure as the formal sample, its width is 25mm, the length of the working section 51 is 25mm, the transition arc radius between the working section 51 and the non-working section is 12.5mm, and the width of the working section 51 is 12.5mm.

[0047] S2. First, fix the first metal pipe sample 5 by a fatigue test fixture and fix the fatigue test fixture on the testing machine, ensure that the working section 51 of the first metal pipe sample 5 is located at a first angle of 30°, and ensure that the force axis of the fatigue test fixture passes through the center of the first metal pipe sample 5; the subsequent metal pipe samples 5 and the formal samples are installed by the above-mentioned installation method of the metal pipe sample 5, and it is necessary to apply lubricating oil or pad a polytetrafluoroethylene film between the annular block and the sample to reduce the friction between the sample and the annular block.

[0048] S3. Start the testing machine, stretch the first metal pipe sample 5 at a crossbeam displacement speed of 2 mm / min until it breaks, and record the maximum load borne by the first metal pipe sample 5 as 66700 N.

[0049] S4. Select a second metal pipe sample 5, clamp the second metal pipe sample 5 in the manner of step S2, ensure that the second metal pipe sample 5 is rotated to a second angle of 60°, perform a tensile test on the second metal pipe sample 5 in the manner of step S3, and record the maximum load borne by the second metal pipe sample 5 as 68975N.

[0050] Select the third metal pipe sample 5, clamp the third metal pipe sample 5 in the manner of step S2, ensure that the third metal pipe sample 5 is rotated to a third angle of 90°, perform a tensile test on the third metal pipe sample 5 in the manner of step S3, and record the maximum load borne by the third metal pipe sample 5 as 71799N.

[0051] S5, by analyzing the stress of the three metal pipe samples 5, the different angle values ​​of the three tensile samples are compared with The linear fitting is performed with the values, and the slope of the straight line is 0.0703. Then the friction coefficient μ between the metal pipe sample 5 and the fatigue test fixture (ring block) is 0.0703. Substitute the friction coefficient μ into formula (8), and the relationship between the load applied by the testing machine and the stress on the working section 51 of the metal pipe sample 5 is obtained.

[0052] S6. Based on the relationship between the load applied by the testing machine and the stress on the working section 51 of the metal pipe sample 5 obtained in step S5, and then based on the peak stress and valley stress set in the fatigue test, calculate the maximum load and minimum load required to be set for the testing machine.

[0053] S7. Select the formal sample, and install and fix it according to step S2 while ensuring that the center of the working section 51 of the formal sample is located on the force axis. At the same time, after the formal sample is clamped, the lower round rod 22 is used to support the formal sample to complete the fixation to ensure that the formal sample will not rotate axially. Then, perform fatigue test and read the corresponding results.

[0054] S8. After the fatigue test is completed, remove the formal sample.

[0055] The stress analysis of the metal pipe sample in the above step S5 is as follows:

[0056] Cut a stress unit at any position on the specimen, and its stress state is as shown in the attached figure. Figure 7 As shown, according to the radial and annular force balance, we can get:

[0057]

[0058]

[0059] Where: F(θ) is the annular tensile force, N(θ) is the supporting force of the annular block, is the friction coefficient between the metal pipe sample 5 and the circular block, and θ is the angle between the working section 51 of the metal pipe sample 5 and the horizontal direction (such as Figure 6 shown).

[0060] When θ is very small, At this time, equations (1) and (2) can be simplified to:

[0061]

[0062] μN(θ)+dF(θ)=0 (4);

[0063] In formula (3), the higher-order infinitesimals are ignored. Substituting into equation (4), we can obtain the following differential equation:

[0064]

[0065] From the differential equation (5), it can be obtained that the tensile force on the sample satisfies:

[0066] F(θ)=exp(-μθ-c) (6);

[0067] By the attached Figure 8 The balance of the forces in the center shows that the boundary conditions are: when θ = 0, Where P is the tensile force on the circular block, which is equal to the axial load applied by the testing machine.

[0068] Substituting the above boundary conditions into equation (6) we can determine

[0069] Therefore, the hoop tensile force on the specimen is:

[0070]

[0071] In the circumferential fatigue test of the present invention, the center of the working section of the specimen is located at the upper vertex of the upper D-shaped piece, that is, (angles are expressed in radians), substituting into equation (7) yields:

[0072]

[0073] Then calculate the friction coefficient μ value based on the results of three tensile tests before the formal fatigue test:

[0074] The force analysis of the specimen during the tensile test is the same as the above force analysis. Taking the logarithm of both sides of formula (7), we get:

[0075] Right now

[0076] In formula (9), μθ is very small relative to P and F(θ), so it can be basically assumed that when θ is constant, P and F(θ) are in direct proportion. The maximum load of the testing machine before the sample breaks is P max At this time, the hoop tension on the working section of the sample is the largest, which is F(θ) max , and (θ) max / S0 is equal to the circumferential tensile strength of the specimen (S0 is the cross-sectional area of ​​the specimen working section). Theoretically, the circumferential tensile strength of the specimen is a certain value, so F(θ) max Set P max and F(θ) max Substituting into formula (9) we get:

[0077]

[0078] In the three tensile tests, the angles between the radial direction at the center of the working section of the specimen and the horizontal direction are different, that is, three different values ​​are used.

[0079] In formula (10), let θ=x,lnF(θ) max =, μ = b, then equation (10) can be written as:

[0080] y=a+bx (11);

[0081] According to three tensile tests, different value, and then three different θ values ​​with different The slope of the straight line obtained by linear fitting is the value of the friction coefficient μ.

[0082] Substituting the obtained μ value into formula (8) can obtain the relationship between the test machine load and the stress on the working section of the metal pipe sample during the fatigue test.

[0083] It should be emphasized that the above are only preferred embodiments of the present invention and do not limit the present invention in any form. Any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A method for annular fatigue testing of metal pipes, wherein a metal pipe sample is fixed by a metal pipe reversing fatigue testing fixture, the fatigue testing fixture is installed on a testing machine for tension, the fatigue testing fixture comprises an upper D-shaped piece, a lower D-shaped piece, an upper fixing piece connected to the testing machine, and a lower fixing piece connected to the testing machine, the upper fixing piece is connected to the upper D-shaped piece, the lower fixing piece is connected to the lower D-shaped piece, the upper D-shaped piece and the lower D-shaped piece form a circular block for fixing the metal pipe sample, characterized in that: An upper positioning groove is provided on the arc surface of the upper D-shaped piece, and a lower positioning groove is provided on the arc surface of the lower D-shaped piece, and the metal pipe sample is provided in the upper positioning groove and the lower positioning groove; The circumferential fatigue test method for metal pipes is: S1. Cut a number of metal pipe samples and a formal sample from the metal pipe for standby use; S2. Fix the metal pipe sample by a fatigue test fixture and fix the fatigue test fixture on the testing machine, and ensure that the force axis of the fatigue test fixture passes through the center of the metal pipe sample; S3. Stretch the metal pipe sample at a constant speed until it breaks, and record the maximum load; S4, select the next metal pipe sample, return to step S2 and ensure that the metal pipe sample is rotated to another angle, which is different from the angle of the previously broken metal pipe sample, until all metal pipe samples are tested; S5. By performing stress analysis on multiple metal pipe specimens, the friction coefficient between the metal pipe specimen and the fatigue test fixture is obtained by processing the above test results, and the relationship between the load applied by the testing machine and the stress on the working section of the metal pipe specimen is obtained; S6, according to the relationship between the load applied by the testing machine and the stress on the working section of the metal pipe sample obtained in step S5, and then according to the peak stress and valley stress set in the fatigue test, calculate the maximum load and minimum load required to be set for the testing machine; S7, select a formal sample, install and fix it according to step S2, and ensure that the center of the working section of the formal sample is located on the force axis, and perform fatigue test; S8. After the fatigue test is completed, remove the formal sample.

2. The metal pipe circumferential fatigue test method according to claim 1, characterized in that: The force-bearing axes of the upper fixing member and the lower fixing member coincide with each other and pass through the center of the circular block.

3. The metal pipe circumferential fatigue test method according to claim 1, characterized in that: The upper fixing member and the lower fixing member have the same structure and both include a U-shaped block and a round rod arranged on the U-shaped block. The U-shaped block includes a fixed end and extended ends extending to the same side at both ends of the fixed end. The round rod is fixed on the fixed end.

4. The metal pipe circumferential fatigue test method according to claim 3, characterized in that: An upper connecting through hole is arranged on the upper D-shaped part, and upper connecting holes are respectively arranged on the two protruding ends of the upper fixing part. An upper pin is inserted into the upper connecting through hole and the upper connecting hole to connect the upper fixing part and the upper D-shaped part; a lower connecting through hole is arranged on the lower D-shaped part, and lower connecting holes are respectively arranged on the two protruding ends of the lower fixing part. A lower pin is inserted into the lower connecting through hole and the lower connecting hole to connect the lower fixing part and the lower D-shaped part.

5. The metal pipe circumferential fatigue test method according to claim 3, characterized in that: A threaded hole is arranged on the fixed end, and a thread is arranged on the round rod, and the thread matches the threaded hole.

6. The metal pipe circumferential fatigue test method according to claim 3, characterized in that: One end of the round rod of the lower fixing piece passes through the fixing end and abuts against the metal pipe sample in the lower positioning groove.

7. The metal pipe circumferential fatigue test method according to claim 1, characterized in that: There are at least two metal pipe samples.

8. The metal pipe circumferential fatigue test method according to claim 1, characterized in that: In the step S7, after the formal sample is clamped, a fixing member is used to ensure that the formal sample will not rotate.

9. The metal pipe circumferential fatigue test method according to claim 1, characterized in that: The relationship between the load applied by the testing machine in step S5 and the stress on the working section of the metal pipe sample is: Among them, F(θ) is the hoop tension, P is the tension on the circular block, μ is the friction coefficient between the metal pipe sample and the circular block, and θ is the angle between the working section of the metal pipe sample and the horizontal direction.

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