Evaluation method for fatigue performance of pipe joint

Abstract

The invention discloses a method for evaluating the fatigue performance of a pipe joint, which comprises the following steps of: establishing an equivalent calculation model of a pipe joint forming process and an equivalent calculation model of a rotating bending fatigue process, simulating the rotating bending fatigue process, and identifying to obtain the stress level Ai of a guide pipe and the pipe joint at 0 moment; and identifying the position of the maximum stress change point of the conduit and the pipe joint and the stress amplitude Bi of the point. According to the material equal-life curve and the S-N curve, obtaining a relation curve of stress levels and stress amplitudes of the guide pipe and the pipe joint with the fatigue life of the pipe joint within 10 million times, and extracting a stress amplitude Ci corresponding to the stress level Ai in the relation curve; if so; ci, evaluating that the pipe joint meets the fatigue life requirement; and if Bi is greater than or equal to Ci, evaluating that the pipe joint does not meet the fatigue life requirement. The bending fatigue performance can be simulated and predicted, the verification period of the rotating bending fatigue test is remarkably shortened, and the method has good practicability.

Description

Technical Field

[0001] The invention belongs to the technical field of pipeline connection, and particularly relates to a method for evaluating fatigue performance of a pipe joint. Background Art

[0002] Hydraulic piping systems are a crucial component of aircraft hydraulic systems, and their fatigue life directly impacts aircraft safety during flight. However, many factors influence the fatigue life of pipe and fitting connections, particularly bending fatigue caused by multi-directional cyclic stresses on the ends of the fittings. Therefore, the design and manufacture of pipe and fittings must undergo rotary bending fatigue testing before they can be used on aircraft.

[0003] The rotary bending fatigue test requires that 10 million rotational bending tests be completed at a specified minimum stress level without leakage, fracture, or other failures, and the test must be performed continuously. Therefore, the key control indicator of the rotary bending fatigue test is the rotary bending fatigue stress level. However, the following problems still exist in the rotary bending fatigue test: 1. The rotary bending fatigue test requires the test piece to complete 10 million rotary bending cycles under a specified stress level, and the entire test cycle is very long; 2. The fatigue stress amplitude assessed by the rotary bending fatigue test is single, which makes it difficult to simulate complex on-board conditions, resulting in large deviations in the test results. Summary of the Invention

[0004] The object of the present invention is to provide a method for evaluating the fatigue performance of a pipe joint, aiming to solve the above-mentioned problem.

[0005] The present invention is mainly achieved through the following technical solutions: A method for evaluating the fatigue performance of a pipe joint comprises the following steps: Step S1: Establishing an equivalent calculation model for the pipe joint forming process: Based on the material and size of the pipe joint, a three-dimensional finite element solid model of the pipe assembly connection is established to obtain the stress field of the pipe assembly connection forming process; Step S2: Establishing an equivalent calculation model of the rotational bending fatigue process: Based on the stress field of the pipeline assembly connection, a three-dimensional finite element solid model of the rotational bending fatigue process is constructed; Step S3: Simulate the rotational bending fatigue process and identify the stress level A of the conduit and pipe joint at time 0. i , identify the location of the maximum stress change point of the conduit and pipe joint and the stress amplitude B at this point i ; Step S4: Evaluate the fatigue performance of the pipe joint: According to the material iso-life curve and SN curve, obtain the relationship curve between the stress level and stress amplitude of the pipe and pipe joint with a fatigue life of 10 million times, and extract the stress level A in the relationship curve. i The corresponding stress amplitude C i If B i <C i , then the pipe joint meets the fatigue life requirements; if B i ≥C i , then the pipe joint does not meet the fatigue life requirements.

[0006] In order to better implement the present invention, further, step S1 includes the following steps: Step S11: establishing a three-dimensional finite element solid model of the inner ring, outer ring, conduit, and extrusion tool, and setting the mutual contact between the inner ring, outer ring, conduit, and extrusion tool according to the assembly connection relationship; Step S12: Setting boundary conditions: Apply symmetry constraints to the inner ring, outer ring and conduit, with the symmetry plane perpendicular to the Z axis of the coordinate system; Apply a symmetry constraint to the end of the inner ring flange so that the symmetry plane is perpendicular to the Y axis of the coordinate system; Apply displacement constraint L to the extrusion tool; Step S13: Obtaining the stress field of the pipeline assembly connection formation.

[0007] In order to better implement the present invention, further, step S2 includes the following steps: Step S21: setting the interaction between models in the rotational bending fatigue process: setting the mutual contact between the components according to the assembly connection relationship; Step S22: Setting boundary conditions: a) Apply symmetry constraints to the inner ring, outer ring, and guide tube, with the symmetry plane perpendicular to the Z axis of the coordinate system; b) Apply a hinge constraint to the end of the inner ring flange to constrain translation in the X, Y, and Z directions; c) Apply a unidirectional displacement load Q to the distal end B of the conduit and a fixed displacement constraint to the front end A of the joint flange; d) Inherit the stress field of the pipe assembly connection formed in step S1.

[0008] In order to better implement the present invention, further, step S3 includes the following steps: Step S31: reading the stress level of the conduit at a position 5 mm ± 1 mm away from the pipe joint, and outputting a curve of the maximum principal stress versus time during the rotational bending process to determine the time t corresponding to when the conduit at that position reaches the target bending stress S; Step S32: reading the surface stress level at time 0 and the surface stress level at time t along the inner ring upper surface path, the inner ring lower surface path, the catheter upper surface path, and the catheter lower surface path respectively; Step S33: Identify the stress level A of the conduit and the pipe joint at time 0 i , identify the location of the maximum stress change point of the conduit and pipe joint and the stress amplitude B at this point i .

[0009] In order to better implement the present invention, the present invention further includes step S5: designing a rotational bending fatigue test of the pipe joint under different lifespans for verification; the following steps are included: Step S51: Obtain fatigue life indicators of the pipe joint and the conduit under the same stress level and different stress amplitudes according to the material isolife curve and SN curve of the pipe joint; Step S52: Calculating a reference value of a bending fatigue test offset; a) Select the stress amplitude of the catheter under the fatigue life index, and determine the time t when the catheter reaches this stress amplitude based on the maximum principal stress versus time curve in step S31 i ; b) Calculate the time at t i The deviation of the catheter at this time is δ = Q × t i ; Where: Q is the unidirectional displacement load applied to the end A of the catheter; c) Carry out a rotating bending fatigue test, and the test is divided into n groups for assessment, n ≥ 3; Step S53: Evaluate the fatigue life of the pipe joint according to the number of bending cycles tested; If the number of bending cycles in the test is greater than or equal to the fatigue life corresponding to the offset δ, it is considered to meet the identification requirement of 10 million bending cycles under the corresponding stress amplitude.

[0010] In order to better implement the present invention, further, in the step S52, the stress amplitude of the pipe under the fatigue life index of 500,000 times to 10 million times is selected, and the fatigue life index interval is greater than or equal to 1 million times; in the step S53, the assessment result must be above the curve of pipe joint life and pipe stress amplitude.

[0011] The beneficial effects of the present invention are as follows: The calculation method for simulating the rotary bending fatigue test proposed in the present invention is a method that inherits the stress field; the assessment and evaluation index for the fatigue performance of the pipe joint proposed in the present invention combines the material isolife curve and SN curve as well as the calculated stress level and stress amplitude of the pipe joint and the conduit to give the fatigue life of the pipe joint under assembly conditions. The present invention quantifies the fatigue performance of the pipe joint and provides an evaluation method for the assessment of the fatigue performance of the pipe joint. The present invention adopts a semi-virtual and semi-test method to assess and evaluate the fatigue performance of the pipe joint. Through the assessment of the equivalent stress working condition, it can shorten the verification cycle of the rotary bending fatigue test and predict the bending fatigue performance. Compared with the existing standards, the research object of the present invention is more focused, the operation process is more specific, the duration of the rotary bending fatigue test is effectively shortened, and the fatigue performance of the pipe joint can be predicted and evaluated, which has good practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 is a flow chart of a method for evaluating fatigue performance of a pipe joint according to the present invention; Figure 2 It is a structural diagram of a three-dimensional finite element solid model of a pipeline assembly connection; Figure 3 is a schematic diagram of the structure of the contact pairs between the inner ring, the outer ring, the guide tube and the extrusion tool; Figure 4 It is a schematic diagram of the structure of the three-dimensional finite element solid model of the rotational bending fatigue process; Figure 5 It is a schematic diagram of the stress field formed by the pipeline assembly connection; Figure 6 is a schematic diagram of the target test area of ​​the catheter where stress is read; Figure 7 It is a schematic diagram for reading stress path; Figure 8 It is a graph showing the relationship between stress level and stress amplitude of the conduit and pipe joint within the fatigue life of 10 million bends; Figure 9 It is a graph showing the relationship between the pipe joint life and the stress amplitude of the conduit; Figure 10 It is a curve diagram showing the relationship between the maximum principal stress at the target position and time.

[0013] Among them: 1-inner ring, 2-outer ring, 3-catheter, 4-extrusion tool, 11-contact pair of inner ring and outer ring, 22-contact pair of inner ring and catheter, 33-contact pair of extrusion tool and outer ring, 111-inner ring upper surface path, 222-inner ring lower surface path, 333-catheter upper surface path, 444-catheter lower surface path. DETAILED DESCRIPTION

[0014] Example 1: A method for evaluating the fatigue performance of pipe joints, such as Figure 1 As shown, the specific steps include: Step 1: Establish an equivalent calculation model for the pipe joint forming process.

[0015] According to the material and size of the pipe joint, a three-dimensional finite element solid model of the pipe assembly connection is established, such as Figure 2 As shown, calculate the stress field during the assembly connection process and after springback. The specific steps are as follows: Step 1-1, set up the interaction between models.

[0016] like Figure 3 As shown, the mutual contact between the components is set according to the assembly connection relationship.

[0017] Step 1-2, set the boundary conditions.

[0018] a) The inner ring 1, outer ring 2 and conduit 3 are subjected to symmetry constraints with the symmetry plane perpendicular to the Z axis of the coordinate system; b) A symmetry constraint is imposed on the end of the inner ring 1 flange, with the symmetry plane perpendicular to the Y axis of the coordinate system; c) The extrusion tool 4 applies a displacement constraint L.

[0019] Steps 1-3: Calculate the equivalent model to obtain the pipeline assembly connection forming stress field.

[0020] Step 2: Establish an equivalent calculation model for the rotational bending fatigue process.

[0021] like Figure 4 As shown, based on the forming stress field calculated in step 1, a three-dimensional solid model of the rotational bending fatigue process is constructed, and the length of the catheter 3 is set according to the length required by the catheter 3 identification test. The specific steps are as follows: Step 2-1, set up the interaction between models.

[0022] According to the assembly connection relationship, the mutual contact between the components is set.

[0023] Step 2-2, set boundary conditions.

[0024] a) The inner ring 1, outer ring 2 and conduit 3 are subjected to symmetry constraints with the symmetry plane perpendicular to the Z axis of the coordinate system; b) Apply a hinge constraint to the end of the inner ring 1 flange, that is, constrain the translation in the X, Y, and Z directions; c) Apply a unidirectional displacement load Q to the distal end B of the conduit and a fixed displacement constraint to the front end A of the joint flange; d) If Figure 5 As shown, the stress field of the pipe assembly connection formed by the calculation in step 1 is inherited.

[0025] Step 3: simulate and calculate the rotational bending fatigue process, read the maximum principal stress, and determine the stress amplitude of the conduit 3 and the pipe joint.

[0026] a) Read the stress level of the target test area of ​​duct 3 at a position 5 mm ± 1 mm away from the pipe joint. The specific location is as follows: Figure 6 For the red box area marked in the figure, output the curve of the maximum principal stress changing with time in the rotational bending process, and determine the time t corresponding to when the area reaches the target bending stress S.

[0027] b) along the inner ring upper surface path 111, the inner ring lower surface path 222, the catheter upper surface path 333 and the catheter lower surface path 444, as shown Figure 7 As shown, each path reads the stress level of the surface at time 0 and the stress level of the surface at time t.

[0028] c) Identify the stress level A of the conduit 3 and the pipe joint at time 0 i , identify the location of the maximum stress change point of the conduit 3 and the pipe joint and the stress amplitude B at this point i .

[0029] Step 4: Evaluate the fatigue life of the pipe joint.

[0030] Based on the material iso-life curve and SN curve, the relationship curve between stress level and stress amplitude corresponding to the fatigue life of the pipe joint at 10 million times is obtained, and the evaluation index of the fatigue life of the pipe joint is obtained.

[0031] a) If Figure 8 As shown, the relationship curve between stress level and stress amplitude is obtained; b) Stress level A obtained in step 3 i and stress amplitude B i , compare the corresponding stress level and stress amplitude in the stress level and stress amplitude relationship curve to evaluate whether the fatigue life requirements are met.

[0032] Specifically, in the stress level and stress amplitude relationship curve, find A i The corresponding stress amplitude is C i .

[0033] If B i <C i , then the pipe joint meets the fatigue life requirements; if B i ≥C i , the pipe joint does not meet the fatigue life requirements.

[0034] According to the above evaluation method, before waiting for the results of the rotary bending fatigue test, an equivalent model calculation can be performed in advance. Based on the calculation results, it is predicted whether the pipe joint can pass 10 million fatigue cycles under the corresponding stress level and stress amplitude.

[0035] Step 5: Design a rotational bending fatigue test for the pipe joint at different lifespans.

[0036] Step 4-1, as shown in Table 1, propose fatigue life indicators under the same stress level and different stress amplitudes based on the material isolife curve and SN curve.

[0037] Table 1 Step 4-2, calculation of reference value of bending fatigue test offset.

[0038] a) According to Table 1, select the stress amplitude of the conduit 3 under the corresponding life span, and determine the time t when the conduit 3 reaches this stress amplitude according to the maximum principal stress versus time curve in step 3. i ; b) Calculate the deflection of the lower conduit 3 at this moment δ = Q × t i .

[0039] c) Perform a rotational bending fatigue test, with an applied offset of δ, and divide the test into n groups for assessment, where n ≥ 3.

[0040] In step 4-3, the fatigue life of the pipe joint is evaluated based on the number of bending cycles tested.

[0041] a) If the number of bending cycles tested is not less than the fatigue life corresponding to the offset δ, it is considered that the qualification requirement of 10 million bending cycles under the corresponding stress amplitude can be passed; b) During the assessment, the stress amplitude of the conduit 3 is selected between 500,000 and 10 million times of the pipe joint life, and the life interval must be greater than or equal to 1 million times. Figure 9 As shown, the assessment results must be above the curve.

[0042] Example 2: A method for evaluating the fatigue performance of a pipe joint includes establishing an equivalent calculation model for the pipe joint forming process to obtain the stress field of the pipe joint after assembly and connection; establishing an equivalent calculation model for the rotational bending fatigue process to determine the stress level and stress amplitude of the pipe joint and the conduit 3; and evaluating and experimentally verifying the fatigue life of the pipe joint.

[0043] Preferably, the present invention specifically comprises the following steps: Step 1: Establish an equivalent calculation model for the pipe joint forming process.

[0044] like Figure 2 As shown, in this embodiment, the material of the conduit 3 is TA18, the material of the pipe joint is TC4, the outer diameter of the conduit 3 is 8 mm, and the end is a non-flared type. Straight pipes are directly used for assembly and connection to establish a three-dimensional finite element solid model.

[0045] Step 1-1, set up the interaction between models.

[0046] The inner ring 1 and the outer ring 2 , the inner ring 1 and the guide tube 3 , and the extrusion tool 4 and the outer ring 2 are arranged to contact each other.

[0047] Step 1-2, set the boundary conditions.

[0048] Set symmetry constraints for the inner and outer rings 2 and the catheter 3, and set the displacement constraint L=4.35 mm for the extrusion tool 4.

[0049] Calculate the pipe joint assembly and connection process to obtain the stress field after assembly and connection.

[0050] Step 2: Establish an equivalent calculation model for the rotational bending fatigue process.

[0051] A three-dimensional solid model was established for the rotational bending fatigue process, and the length of the catheter 3 was 190 mm.

[0052] Step 2-1, set up the interaction between models.

[0053] An inner ring and outer ring contact pair 11, an inner ring and catheter contact pair 22, and an extrusion tool and outer ring contact pair 33 are provided.

[0054] Step 2-2, set boundary conditions.

[0055] a) Set symmetry constraints for the inner and outer rings 2 and the conduit 3; b) Set the hinge constraint at the end of the inner ring 1 flange; c) A unidirectional displacement load of 9 mm is applied to the end of the catheter 3, in a direction perpendicular to the axial direction of the catheter 3.

[0056] d) The stress field after the pipe joint and conduit 3 are assembled and connected, which is inherited from step 1.

[0057] Steps 2-3, such as Figure 6 As shown, the maximum principal stress at the target test area on the conduit 3 is read to determine the stress amplitude of the conduit 3 and the pipe joint.

[0058] a) If Figure 6 and Figure 10 As shown, the stress level at a position 5 mm ± 1 mm away from the pipe joint (target test area) is read, and a curve of the maximum principal stress at this position changing with time is output. Based on the curve, the time corresponding to the target bending stress of 134 MPa is determined to be 0.5.

[0059] b) If Figure 7 As shown, four paths are taken along the upper and lower surfaces of the inner ring 1 and the upper and lower surfaces of the conduit 3, and the stress level at time 0 and the stress level at time 0.5 are read for each path.

[0060] c) The stress level of inner ring 1 is 756.6 MPa, and the stress level of conduit 3 is 731.8 MPa. The stress amplitude at the chamfer and groove of the inner ring 1 flange and the stress concentration point of conduit 3 is 110 MPa.

[0061] Step 3: Fatigue life evaluation of pipe joints.

[0062] According to step 2, the stress level of the inner ring 1 is 756.6 MPa, the stress level of the conduit 3 is 731.8 MPa, and the stress amplitude at the stress concentration position is 110 MPa.

[0063] contrast Figure 8 When the stress level is 756.6 MPa, the corresponding stress amplitude is 116 MPa, which is greater than the stress amplitude at the stress concentration position and meets the fatigue life requirements. It is predicted that the pipe joint under this size specification can meet 10 million bending cycles.

[0064] Step 4: Design a rotational bending fatigue test for the pipe joint at different lifespans.

[0065] As shown in Table 2, by consulting the aviation material handbook, the TC4 iso-life curve and SN curve were obtained, and the fatigue life index under the same stress level and different stress amplitudes was proposed.

[0066] Table 2 Step 4-1, calculation of reference value of offset for bending fatigue test.

[0067] a) According to Table 1, the stress amplitude of the conduit 3 under the life of 5 million times is selected as 150MPa, such as Figure 10 As shown in FIG, according to the curve of maximum principal stress variation with time, the time when the stress amplitude reaches 150 MPa is determined to be 0.56.

[0068] b) Calculate the deflection of catheter 3 at time 0.56 to be 5.04 mm.

[0069] c) Rotational bending fatigue tests were conducted with an applied offset of 5.04 mm. The tests were divided into three groups for assessment.

[0070] In step 4-2, the pipe joints of the three groups of tests all passed 5 million rotation and bending cycles successfully. It is believed that under the stress amplitude of conduit 3 of 110 MPa, they can pass the identification requirement of 10 million rotation and bending cycles.

[0071] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any simple modification or equivalent change made to the above embodiment based on the technical essence of the present invention shall fall within the scope of protection of the present invention.

Claims

1. A method for evaluating fatigue performance of a pipe joint, characterized in that: The following steps are involved: Step S1: Establishing an equivalent calculation model for the pipe joint forming process: Based on the material and size of the pipe joint, a three-dimensional finite element solid model of the pipe assembly connection is established to obtain the stress field of the pipe assembly connection forming process; Step S2: Establishing an equivalent calculation model of the rotational bending fatigue process: Based on the stress field of the pipeline assembly connection, construct a three-dimensional finite element solid model of the rotational bending fatigue process; Step S3: Simulate the rotational bending fatigue process and identify the stress level A of the conduit and pipe joint at time 0. i , identify the location of the maximum stress change point of the conduit and pipe joint and the stress amplitude B at this point i ; Step S4: Evaluate the fatigue performance of the pipe joint: According to the material iso-life curve and SN curve, obtain the relationship curve between the stress level and stress amplitude of the pipe and pipe joint with a fatigue life of 10 million times, and extract the stress level A in the relationship curve. i The corresponding stress amplitude C i If B i <C i , then the pipe joint meets the fatigue life requirements; if B i ≥C i , then the pipe joint does not meet the fatigue life requirements.

2. The method for evaluating fatigue performance of a pipe joint according to claim 1, characterized in that: The step S1 comprises the following steps: Step S11: establishing a three-dimensional finite element solid model of the inner ring, outer ring, conduit, and extrusion tool, and setting the mutual contact between the inner ring, outer ring, conduit, and extrusion tool according to the assembly connection relationship; Step S12: Setting boundary conditions: Apply symmetry constraints to the inner ring, outer ring and conduit, with the symmetry plane perpendicular to the Z axis of the coordinate system; Apply a symmetry constraint to the end of the inner ring flange so that the symmetry plane is perpendicular to the Y axis of the coordinate system; Apply displacement constraint L to the extrusion tool; Step S13: Obtaining the stress field of the pipeline assembly connection formation.

3. The method for evaluating fatigue performance of a pipe joint according to claim 2, characterized in that: The step S2 comprises the following steps: Step S21: setting the interaction between models in the rotational bending fatigue process: setting the mutual contact between the components according to the assembly connection relationship; Step S22: Setting boundary conditions: a) Apply symmetry constraints to the inner ring, outer ring, and guide tube, with the symmetry plane perpendicular to the Z axis of the coordinate system; b) Apply a hinge constraint to the end of the inner ring flange to constrain translation in the X, Y, and Z directions; c) Apply a unidirectional displacement load Q to the distal end B of the catheter and a fixed displacement constraint to the front end A of the joint flange; d) Inherit the stress field of the pipe assembly connection formed in step S1.

4. A method for evaluating fatigue performance of a pipe joint according to any one of claims 1 to 3, characterized in that: The step S3 comprises the following steps: Step S31: reading the stress level of the conduit at a position 5 mm ± 1 mm away from the pipe joint, and outputting a curve of the maximum principal stress versus time during the rotational bending process to determine the time t corresponding to when the conduit at that position reaches the target bending stress S; Step S32: reading the surface stress level at time 0 and the surface stress level at time t along the inner ring upper surface path, the inner ring lower surface path, the catheter upper surface path, and the catheter lower surface path respectively; Step S33: Identify the stress level A of the conduit and the pipe joint at time 0 i , identify the location of the maximum stress change point of the conduit and pipe joint and the stress amplitude B at this point i .

5. The method for evaluating fatigue performance of a pipe joint according to claim 4, characterized in that: The method also includes step S5: designing a rotational bending fatigue test of the pipe joint under different lifespans for verification; the method includes the following steps: Step S51: Obtain fatigue life indicators of the pipe joint and the conduit under the same stress level and different stress amplitudes according to the material isolife curve and SN curve of the pipe joint; Step S52: Calculating a reference value of a bending fatigue test offset; a) Select the stress amplitude of the catheter under the fatigue life index, and determine the time t when the catheter reaches this stress amplitude based on the maximum principal stress versus time curve in step S31 i ; b) Calculate the time at t i The deviation of the catheter at this time is δ = Q × t i ; Where: Q is the unidirectional displacement load applied to the end A of the catheter; c) Carry out a rotating bending fatigue test, and the test is divided into n groups for assessment, n ≥ 3; Step S53: Evaluate the fatigue life of the pipe joint according to the number of bending cycles tested; If the number of bending cycles in the test is greater than or equal to the fatigue life corresponding to the offset δ, it is considered to meet the identification requirement of 10 million bending cycles under the corresponding stress amplitude.

6. The method for evaluating fatigue performance of a pipe joint according to claim 5, characterized in that: In step S52, the stress amplitude of the pipe under the fatigue life index of 500,000 to 10 million times is selected, and the fatigue life index interval is greater than or equal to 1 million times; in step S53, the assessment result must be above the pipe joint life and pipe stress amplitude curve.

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