System and method for measuring fatigue life of pipe joint

Abstract

According to the system and the method for measuring the fatigue life of the pipe joint, firstly, a plug filler neck, a jacket nut, a ball head and a counterweight connecting plug are assembled into a test piece according to a design torque requirement, and then two unidirectional strain gauges are axially adhered to the root of the ball head. After the vibration test bench is started, small-magnitude and broadband sine frequency sweeping is firstly carried out, and the resonance frequency is determined according to the maximum value of strain measurement. And then carrying out a resonance test under the conditions of the resonance frequency and the working condition vibration magnitude, stopping the test when the vibration response begins to rapidly drop, and recording the vibration time. The fatigue life can be obtained by multiplying the resonance frequency by the vibration time. According to the method, the fatigue life and the design margin of the pipe joint can be accurately described through phenomena and data, the result can guide the design and use of the engine pipe joint and evaluate the reliability of the pipe joint, meanwhile, the test verification frequency of an engine level is reduced, and the research and development cost is greatly reduced.

Description

Technical Field

[0001] The invention relates to a measuring system and method for measuring the fatigue life of a pipe joint, which are mainly used for measuring and judging the fatigue life of aerospace engine pipeline connectors. Background Art

[0002] Rocket engines are the core components of launch vehicles. The sealing reliability of their pipeline systems will directly determine the reliability of launch vehicles and play a decisive role in the success or failure of flight missions. Pipe joints, as the main sealing structure in the pipeline system, are widely used in engines. According to the experience of rocket engine development, pipe joints, especially the root of the pipe joint ball head, are the weakest link in the safety and stability of the entire engine pipeline system and the part with the highest probability of failure. The quality problems of some key pipe joints will directly affect the success or failure of rocket flight. Obtaining the fatigue life and design margin of pipe joints, and then controlling their use time and number of uses, has become an effective way to reduce the probability of engine failure and improve the safety factor of the engine.

[0003] At present, the fatigue life of aerospace engine pipeline connectors is mainly measured through computer finite element simulation, high design margin and engine thermal test. The boundary condition setting of finite element simulation is complex and the error is large; the high design margin leads to large mass and bulky structure of pipeline connectors; the engine thermal test results are the most realistic, but the cost is huge and the iteration speed is slow. Summary of the invention

[0004] The technical problem solved by the invention is: to overcome the deficiencies of the prior art and to provide a test method for measuring the fatigue life of a pipe joint, by which the fatigue life and design margin of the pipe joint can be measured.

[0005] The technical solution of the present invention is: a measurement system for fatigue life of a pipe joint, comprising a control system, a vibration test bench, a vibration measurement system, a strain measurement system, a test fixture, a ball head, a sleeve nut, a plug nozzle, a counterweight connection plug and a counterweight block; the plug nozzle, the sleeve nut, the ball head and the counterweight connection plug constitute a test piece; wherein,

[0006] The bell mouth side of the plug nozzle is connected to the ball head and fixed by the outer nut, and anti-loosening measures are added at the connection part; the other end of the plug nozzle is connected to the test fixture; the other end of the ball head is welded to the non-threaded end of the counterweight connection plug; the threaded end of the counterweight connection plug is connected to the counterweight block;

[0007] The test fixture is an I-shaped metal block with holes punched on the facade. After the test piece is assembled, it is fixed to the test fixture through the threads on the plug nozzle, and the test fixture is fixed to the vibration test bench.

[0008] The counterweight block is a solid columnar metal block with internal threads on the end surface. It is connected to the ball head during the test to simulate the load generated by the conduit during vibration.

[0009] The vibration test bench is connected to a control system, and the control system controls the vibration level and frequency of the vibration test bench;

[0010] A vibration sensor is attached to the tail of the counterweight and connected to the vibration measurement system to measure the actual vibration on the vibration test bench and the test piece during the test;

[0011] Two unidirectional strain gauges are axially attached to the root of the ball head. The two strain gauges are connected to the strain measurement system to record the strain value of the root of the ball head in real time during the test.

[0012] The fatigue life of the test piece is obtained by inputting the vibration frequency, vibration time and the change in the strain gauge reading.

[0013] The test fixture has a locking function to ensure that the test piece and the fixture are always in a rigid connection, thereby preventing the test piece from rotating and shaking relative to the fixture during the test.

[0014] The sampling frequencies of the vibration measurement system and the strain measurement system are both more than 5 times higher than the maximum vibration frequency.

[0015] The two strain gauges are distributed at an angle of 90° in the circumferential direction of the ball head.

[0016] A test method for measuring the fatigue life of a pipe joint using the system comprises:

[0017] 1) Two strain measuring points are attached to the ball head, close to the root of the ball head, with the two strain measuring points forming a 90° angle, and both are connected to the strain measurement system; vibration sensors are attached to the tail end surface of the counterweight block, and both are connected to the vibration measurement system;

[0018] 2) Perform a small vibration magnitude and wide-band sine sweep to obtain the response result;

[0019] 3) According to the response results under the frequency sweep state, the frequency band with the maximum response is selected as the setting range of the fatigue test frequency. During the test, any value in the range is selected as the resonant frequency;

[0020] 4) During the test, the vibration response and strain size are monitored in real time, and the test time is recorded. When the strain slowly decreases by more than 10%, the resonance time and resonance frequency at that moment are recorded;

[0021] 5) Repeat steps 2)-4), record each resonance frequency and resonance time; stop the test when the vibration response or strain value changes suddenly, and record the Nth resonance frequency and resonance time at the time of stopping the test;

[0022] 6) Calculate the fatigue life under vibration level conditions = the resonance frequency recorded for the first time × the resonance time + the resonance frequency recorded for the second time × the resonance time + ... + the resonance frequency recorded for the Nth time × the resonance time.

[0023] The small vibration magnitude is specifically a vibration less than <1g, and the vibration magnitude remains unchanged during each test.

[0024] The wide-band sinusoidal frequency sweep refers to a sinusoidal frequency sweep with a frequency of 20 to 2000 Hz.

[0025] The mutation specifically refers to a mutation exceeding 20% ​​within 10 seconds.

[0026] Each strain measuring point is no more than 3 mm away from the root of the ball head, and the strain measuring point is unidirectional.

[0027] The frequency band when the response is maximum is a frequency band formed by ±5 Hz of the frequency corresponding to the maximum strain value.

[0028] The advantages of the present invention compared with the prior art are:

[0029] The present invention provides a test method for effectively measuring the fatigue life of a pipe joint. The method can accurately describe the fatigue life and design margin of the pipe joint through phenomena and data. The results can guide the design and use of engine pipe joints and evaluate the reliability of the pipe joints. At the same time, the number of test verifications at the engine level is reduced, and the research and development costs are greatly reduced. The test system and method have higher credibility and authenticity than finite element calculations. The test system and method can measure the fatigue life of pipeline connectors close to reality, reduce over-design, simplify the structure, reduce the mass of pipeline connectors, and effectively improve the thrust-to-weight ratio of the engine. Compared with engine thermal testing, it can greatly reduce research and development costs and increase the speed of design iterations. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Schematic diagram of the test piece and test fixture, where 1a is the test piece and 1b is the test fixture.

[0031] Figure 2 Schematic diagram of the test piece and measuring point installation.

[0032] Figure 3 Schematic diagram of the test system.

[0033] Figure 4 Schematic diagram of the test process. DETAILED DESCRIPTION

[0034] The invention relates to a measuring system for measuring the fatigue life of a pipe joint, which comprises a control system, a vibration test bench, a vibration measuring system, a strain measuring system, a test fixture, a ball head, a sleeve nut, a plug pipe nozzle, a counterweight connecting plug and a counterweight block.

[0035] The bell mouth side of the plug nozzle is connected to the ball head and fixed by the outer nut, and anti-loosening measures are added at the connection part; the other end of the plug nozzle is connected to the test fixture; the other end of the ball head is welded to the non-threaded end of the counterweight connection plug; the threaded end of the counterweight connection plug is connected to the counterweight block;

[0036] The test fixture is an I-shaped metal block with holes punched on the facade. After the test piece is assembled, it is fixed to the test fixture through the threads on the plug nozzle, and the test fixture is fixed to the vibration test bench.

[0037] The counterweight block is a solid columnar metal block with internal threads on the end surface. It is connected to the ball head during the test to simulate the load generated by the conduit during vibration.

[0038] The vibration test bench is connected to a control system, and the control system controls the vibration level and frequency of the vibration test bench;

[0039] A vibration sensor is attached to the tail of the counterweight and connected to the vibration measurement system to measure the actual vibration on the vibration test bench and the test piece during the test;

[0040] Two unidirectional strain gauges are axially attached to the root of the ball head. The two strain gauges are connected to the strain measurement system to record the strain value of the root of the ball head in real time during the test.

[0041] The fatigue life of the test piece is obtained by inputting the vibration frequency, vibration time and the change in the strain gauge reading.

[0042] Before the test, finite element simulation calculation is performed on the test piece to determine the modal frequency of the test piece.

[0043] Before the test, a strain measuring point should be attached to the ball head of the pipe joint, no more than 3mm away from the root of the ball head. The strain measuring point is unidirectional and as close to the root of the ball head as possible. A vibration sensor should be attached to the counterweight block and connected to the vibration measurement system and strain measurement system respectively.

[0044] The test fixture should have a locking function to ensure that the test piece and the fixture are always in a rigid connection and prevent the test piece from rotating and shaking relative to the fixture during the test.

[0045] In order to ensure the accuracy and precision of the data, the sampling frequency of the measuring equipment should be more than 5 times higher than the maximum vibration frequency.

[0046] Before the formal test, a sine sweep with a frequency of 20 to 2000 Hz and a magnitude of <1g is performed to determine the resonant frequency of the test piece based on the strain and vibration response. The vibration magnitude is set according to the actual demand value.

[0047] During the test, the strain and vibration response should be observed at all times. When the strain decreases significantly, the vibration time should be recorded, the frequency should be re-scanned, and a new test frequency should be determined.

[0048] When designing the test piece, the connection part with the counterweight block should be strengthened to avoid the weak point of the test piece being other than the root of the ball head.

[0049] The present invention will be further described in detail below in conjunction with the accompanying drawings. Figure 1 and Figure 2 The structures and tools shown are only used to explain the present invention, but not to limit the present invention. Figure 3 Only the parts related to the present invention are shown, not all structures.

[0050] Figure 1 The figure is a schematic diagram of the test piece and the tooling. First, connect the plug nozzle 1, the outer nut 2, and the ball head 3, and take anti-loosening measures. Then weld the counterweight connection plug 4 and the ball head 3.

[0051] Figure 2 The diagram is a schematic diagram of the test piece and the measuring point installation. The first strain measuring point 10 and the second strain measuring point 11 are pasted at the root of the ball head 3 of the test piece 6. The two strain measuring points are at a 90° angle. The vibration measuring point 7 is pasted on the counterweight block 8. After completion, one end of the test piece 6 is connected to the counterweight block 8, and the other end is fixed to the test fixture 5 with a self-locking nut 9. The test fixture 5 can assemble 2 to more test pieces 6 at the same time. Anti-loosening measures include but are not limited to self-locking nuts 9.

[0052] Figure 3 The test fixture is fixed on the vibration table 12 , the vibration measuring point 7 is connected to the vibration measuring system 15 , and the first strain measuring point 10 and the second strain measuring point 11 are both connected to the strain measuring system 14 .

[0053] Figure 4 The following is a schematic diagram of the test process. Start the control system, vibration table and measurement system, and first perform a small-scale, wide-band sine sweep with a frequency range of 20 to 2000 Hz and a vibration level of no more than 1g. According to the response results under the sweep state, select the frequency or frequency band with the maximum response (bandwidth of about 5 Hz). Use the selected frequency or frequency band as the fatigue test frequency setting value, and perform fixed-frequency vibration or narrow-band random vibration. The vibration level can be set by yourself or selected according to the actual working conditions.

[0054] During the test, the vibration response and strain size are monitored in real time, and the test time is recorded. When the strain slowly decreases by more than 10%, the resonance time and resonance frequency at that moment are recorded.

[0055] Repeat the above steps and record each resonance frequency and resonance time; when the vibration response or strain value changes by more than 20% within 10 seconds, stop the test and record the Nth resonance frequency and resonance time at the time of stopping the test;

[0056] The fatigue life under the vibration level condition is calculated as follows: the resonance frequency recorded for the first time × the resonance time + the resonance frequency recorded for the second time × the resonance time + ... + the resonance frequency recorded for the Nth time × the resonance time.

[0057] Although the present invention has been disclosed as above in the form of a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art can make possible changes and modifications to the technical solution of the present invention by using the technical contents disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the protection scope of the technical solution of the present invention.

Claims

1. A measurement system for fatigue life of a pipe joint, characterized in that: It includes a control system, a vibration test bench, a vibration measurement system, a strain measurement system, a test fixture, a ball head, a sleeve nut, a plug nozzle, a counterweight connection plug and a counterweight block; the plug nozzle, the sleeve nut, the ball head and the counterweight connection plug constitute a test piece; wherein, The bell mouth side of the plug nozzle is connected to the ball head and fixed by the outer nut, and anti-loosening measures are added at the connection part; the other end of the plug nozzle is connected to the test fixture; the other end of the ball head is welded to the non-threaded end of the counterweight connection plug; the threaded end of the counterweight connection plug is connected to the counterweight block; The test fixture is an I-shaped metal block with holes punched on the facade. After the test piece is assembled, it is fixed to the test fixture through the threads on the plug nozzle, and the test fixture is fixed to the vibration test bench. The counterweight block is a solid columnar metal block with internal threads on the end surface. It is connected to the ball head during the test to simulate the load generated by the conduit during vibration. The vibration test bench is connected to a control system, and the control system controls the vibration level and frequency of the vibration test bench; A vibration sensor is attached to the tail of the counterweight and connected to the vibration measurement system to measure the actual vibration on the vibration test bench and the test piece during the test; Two unidirectional strain gauges are axially attached to the root of the ball head. The two strain gauges are connected to the strain measurement system to record the strain value of the root of the ball head in real time during the test. The fatigue life of the test piece is obtained by inputting the vibration frequency, vibration time and the change in the strain gauge reading.

2. A pipe joint fatigue life measurement system according to claim 1, characterized in that: The test fixture has a locking function to ensure that the test piece and the fixture are always in a rigid connection, thereby preventing the test piece from rotating and shaking relative to the fixture during the test.

3. A pipe joint fatigue life measurement system according to claim 1, characterized in that: The sampling frequencies of the vibration measurement system and the strain measurement system are both more than 5 times higher than the maximum vibration frequency.

4. A pipe joint fatigue life measurement system according to claim 1, characterized in that: The two strain gauges are distributed at an angle of 90° in the circumferential direction of the ball head.

5. A test method for measuring the fatigue life of a pipe joint using the system of claim 1, characterized in that: include: 1) Two strain measuring points are attached to the ball head, close to the root of the ball head, with the two strain measuring points forming a 90° angle, and both are connected to the strain measurement system; vibration sensors are attached to the tail end surface of the counterweight block, and both are connected to the vibration measurement system; 2) Perform a small vibration magnitude and wide-band sine sweep to obtain the response result; 3) According to the response results under the frequency sweep state, the frequency band with the maximum response is selected as the setting range of the fatigue test frequency. During the test, any value in the range is selected as the resonant frequency; 4) During the test, the vibration response and strain size are monitored in real time, and the test time is recorded. When the strain slowly decreases by more than 10%, the resonance time and resonance frequency at that moment are recorded; 5) Repeat steps 2)-4), record each resonance frequency and resonance time; stop the test when the vibration response or strain value changes suddenly, and record the Nth resonance frequency and resonance time at the time of stopping the test; 6) Calculate the fatigue life under vibration level conditions = the resonance frequency recorded for the first time × the resonance time + the resonance frequency recorded for the second time × the resonance time + ... + the resonance frequency recorded for the Nth time × the resonance time.

6. The method according to claim 5, characterized in that The small vibration magnitude is specifically a vibration less than <1g, and the vibration magnitude remains unchanged during each test.

7. The method according to claim 5, characterized in that The wide-band sinusoidal frequency sweep refers to a sinusoidal frequency sweep with a frequency of 20 to 2000 Hz.

8. The method according to claim 5, characterized in that The mutation specifically refers to a mutation exceeding 20% ​​within 10 seconds.

9. The method according to claim 5, characterized in that Each strain measuring point is no more than 3 mm away from the root of the ball head, and the strain measuring point is unidirectional.

10. The method according to claim 5, characterized in that The frequency band when the response is maximum is a frequency band formed by ±5 Hz of the frequency corresponding to the maximum strain value.

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