A method for obtaining the crack growth resistance curve of high-pressure natural gas pipelines

Abstract

This application belongs to the technical field of pipeline strength assessment, and discloses a method for obtaining the crack growth resistance curve of high-pressure natural gas pipelines, including: S1, preparing a circumferential weld pipeline tensile specimen to characterize the mechanical properties of the weld and the pipe body; S2, constructing a fully coupled constitutive model of plasticity and ductility damage applicable to the pipe body and the weld to obtain the material parameters of the weld and the pipe body; S3, establishing a finite element model of the circumferential weld pipeline and prefabricating a crack at the circumferential weld position; S4, respectively simulating the tearing process of the circumferential weld pipeline under the action of loads in different directions to obtain the crack growth resistance curve of the circumferential weld pipeline. By establishing a finite element model of the circumferential weld pipeline with a prefabricated crack based on the material parameters of small-sized specimens and simulating its tearing process to obtain the crack growth resistance curve, the strain capacity of the circumferential weld pipeline can be evaluated quickly and accurately, thereby guiding the design of long-distance high-pressure natural gas pipelines, reducing the test cost, saving the test time, and improving the pipeline design efficiency.

Description

Technical Field

[0001] This application belongs to the technical field of pipeline strength assessment, and particularly relates to a method for obtaining the crack propagation resistance curve of high-pressure natural gas pipelines. Background Art

[0002] Since long-distance high-pressure natural gas pipelines often need to pass through areas with complex geological movements such as earthquakes and debris flows, such complex service conditions cause large deformations of the pipelines. How to ensure that high-pressure natural gas pipelines do not fracture under large deformation conditions is of great significance for disaster prevention and mitigation in the natural gas transportation process.

[0003] However, most long-distance pipelines are connected on-site by welding. Due to the complexity of on-site operations, the quality of girth welds is relatively low, and girth weld cracks inevitably exist, which may become the weak links of long-distance pipelines. Therefore, it is necessary to obtain an accurate crack propagation resistance curve to accurately evaluate the strain capacity of girth weld pipelines, and then guide the design of long-distance high-pressure natural gas pipelines.

[0004] The crack propagation resistance curve is the corresponding relationship between the crack tip opening displacement and the crack propagation path length, which reflects the ability of the structure itself to resist crack propagation. At present, most of the under-construction long-distance natural gas pipelines are designed based on strain. This design method requires accurately and quickly obtaining the crack propagation resistance curves at different positions of the pipeline. Generally speaking, the methods for obtaining the pipeline resistance curve in engineering are mainly the full-scale test method and the numerical simulation method. The full-scale test method refers to using a full-scale pipeline to conduct axial tension or bending tests, and at the same time using special pressure loading equipment to achieve high-pressure conditions, and then recording the crack propagation amount and elongation amount through an extensometer to obtain the crack propagation resistance curve. This method is time-consuming and expensive, and is suitable for low-pressure pipelines with short-distance laying; while long-distance natural gas pipelines have many welded joints and need to serve under high-pressure conditions, resulting in high test difficulty and long cycle, which seriously affect the progress and overall cost of pipeline laying. The numerical simulation method is a numerical simulation method based on commercial finite element software. By constructing a finite element model of the whole pipeline, the fracture process of the pipeline under axial tension or bending loads is simulated to obtain the crack propagation resistance curve; this method has a faster calculation speed and lower cost, but the current numerical simulation does not consider the true constitutive relationship of the pipeline steel and weld materials, lacks a true description of the large plastic deformation and damage accumulation at the crack tip, and the obtained crack propagation resistance curve is not accurate. Summary of the Invention

[0005] This application provides a method for obtaining the crack propagation resistance curve of high-pressure natural gas pipelines to solve at least one of the above technical problems.

[0006] The technical solution adopted by this application is as follows:

[0007] A method for obtaining the crack growth resistance curve of a high-pressure natural gas pipeline, comprising the following steps:

[0008] S1. Prepare a circumferential weld pipe tensile specimen to characterize the mechanical properties of the weld and the pipe body;

[0009] S2. Respectively construct a fully coupled constitutive model of plasticity and ductile damage applicable to the pipe body and the weld, and use the material parameters of the weld and the pipe body obtained in step S1;

[0010] S3. Establish a finite element model of the circumferential weld pipe, and prefabricate a crack at the circumferential weld position;

[0011] S4. Use the finite element model to respectively simulate the tearing process of the circumferential weld pipe under the action of loads in different directions, and obtain the crack growth resistance curve of the circumferential weld pipe.

[0012] The material parameters include elastic parameters, plastic parameters, and damage parameters.

[0013] Furthermore, step S1 includes:

[0014] S11. Obtain a pipe body uniaxial specimen and a weld uniaxial specimen from the circumferential weld pipe tensile specimen;

[0015] Respectively conduct uniaxial tensile tests on the pipe body uniaxial specimen and the weld uniaxial specimen, and obtain the pipe body stress-strain curve and the weld stress-strain curve;

[0016] Obtain a pipe body notched specimen and a weld notched specimen from the circumferential weld pipe tensile specimen;

[0017] Respectively conduct notched tensile tests on the pipe body notched specimen and the weld notched specimen, and obtain the pipe body load-displacement curve and the weld load-displacement curve.

[0018] Furthermore, step S2 further includes:

[0019] S21. The established constitutive model introduces isotropic hardening and kinematic hardening, and the stress strain, isotropic hardening, and kinematic hardening are coupled with the damage variable;

[0020] S22. Calculate the pipe body elastic parameters through the elastic stage of the pipe body stress-strain curve, and calculate the pipe body plastic parameters through the strengthening stage of the pipe body stress-strain curve;

[0021] Calculate the weld elastic parameters through the elastic stage of the weld stress-strain curve, and calculate the weld plastic parameters through the strengthening stage of the weld stress-strain curve;

[0022] S23. Calculate the pipe body damage parameter based on the pipe body load-displacement curve;

[0023] Calculate the weld damage parameter based on the weld load-displacement curve.

[0024] Furthermore, step S23 further includes:

[0025] Establish a finite element model of the pipe body for the pipe body notch specimen, simulate the notch tensile test through the pipe body finite element model, and obtain the pipe body simulated load-displacement curve;

[0026] Calculate the pipe body damage parameter by fitting the pipe body load-displacement curve and the pipe body simulated load-displacement curve;

[0027] Establish a finite element model of the weld for the weld notch specimen, simulate the notch tensile test through the weld finite element model, and obtain the weld simulated load-displacement curve;

[0028] Calculate the weld damage parameter by fitting the weld load-displacement curve and the weld simulated load-displacement curve.

[0029] The pipe body uniaxial specimen, the pipe body notch specimen, the weld uniaxial specimen, and the weld notch specimen are all obtained along the axial direction of the specimen; the weld uniaxial specimen and the weld notch specimen are obtained multiple times at different positions of the circumferential weld.

[0030] Furthermore, step 3 further includes:

[0031] S31. Compile program code in Fortran language in the ABAQUS user subroutine and embed it into ABAQUS for numerical calculation:

[0032] S32. The shape of the prefabricated crack defect is arc-shaped, and the tip of the crack is semi-circular;

[0033] S33. Only draw refined meshes in the prefabricated crack area, and the meshes in other parts are coarser to improve the calculation efficiency of the model.

[0034] The prefabrication direction of the crack is the same as the extension direction of the circumferential weld, and the prefabrication of the crack includes plastic changes at the crack tip.

[0035] The depth of the crack does not exceed two-thirds of the wall thickness of the specimen.

[0036] Furthermore, step 4 includes:

[0037] S41. Simulate the tearing process of the circumferential weld pipe under tensile load to obtain the tensile crack propagation resistance curve;

[0038] S42. Simulate the tearing process of the girth-welded pipeline under the combined action of internal high-pressure load and tensile load to obtain the high-pressure crack propagation resistance curve.

[0039] Due to the adoption of the above technical solutions, the beneficial effects achieved by this application are as follows:

[0040] 1. Sample and test the pipe body and the weld respectively. Rapidly characterize the mechanical properties of the pipe body and the weld through uniaxial tension and notched tension tests, and obtain the elastic parameters, plastic parameters, and damage parameters of the two materials in the constitutive model respectively. Both the uniaxial tension test and the notched tension test are conventional small-scale tests with simple test conditions and short test time requirements. The simulated tearing process of the girth-welded pipeline by the finite element model includes the elastic deformation of the pipeline from the beginning to complete tearing, accurately predicting the opening angle of the crack tip and the depth of crack propagation, and then obtaining the crack propagation resistance curve, which can quickly and accurately evaluate the strain capacity of the girth-welded pipeline, guide the design of long-distance high-pressure natural gas pipelines, and overcome the disadvantages of the full-scale pipeline test method, such as high cost, long cycle, and high difficulty.

[0041] 2. Establish a constitutive model coupling plasticity and ductile damage and apply it to the finite element model to reflect the true constitutive relationship of the material and improve the authenticity of the finite element model. Both the pipe body constitutive model and the weld constitutive model introduce isotropic hardening and kinematic hardening, and the stress-strain, isotropic hardening, and kinematic hardening are all coupled with the damage variable, more accurately describing the plastic deformation and damage accumulation of the material, and further improving the accuracy of the constitutive model.

[0042] 3. Considering that in complex geological movements, the stress direction of the pipeline is complex and variable, and according to the load condition that the long-distance high-pressure natural gas pipeline always bears internal high pressure, the tearing process of the girth-welded pipeline under internal high pressure can be simulated; the plastic deformation at the crack tip is also considered, avoiding the problem of deviation in the results due to the lack of description of plastic deformation in the conventional fracture mechanics method, improving the accuracy of the crack propagation resistance curve, and quickly and accurately obtaining the crack propagation resistance curves of the pipeline under different load conditions and different defect forms, and then guiding the strain-based pipeline design under complex geological movement conditions, saving the construction time and economic cost of long-distance high-pressure natural gas pipelines, and improving the reliability of the safe operation of the pipeline. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The drawings described herein are used to provide a further understanding of this application and form a part of this application. The schematic embodiments of this application and their descriptions are used to explain this application and do not constitute an improper limitation to this application. In the drawings:

[0044] Figure 1 is the flowchart of the present invention;

[0045] Figure 2Schematic diagram of the sampling position of the pipeline specimen to be tested in the embodiment of the present invention;

[0046] Figure 3 Comparison of the test force-displacement curve and the simulated force-displacement curve of the notched tensile specimen in the embodiment of the present invention;

[0047] Figure 4 Process diagram of the simulated pipeline crack propagation to the overall fracture of the pipeline in the embodiment of the present invention;

[0048] Figure 5 Crack propagation resistance curve of the circumferential weld pipeline in the embodiment of the present invention.

[0049] Wherein:

[0050] 1 pipe body; 2 weld; 3 sampling position of pipe body material; 4 sampling position of weld material. Specific implementation mode

[0051] In order to more clearly explain the overall concept of the present application, the following will be described in detail by way of examples in conjunction with the drawings of the specification.

[0052] In the following description, many specific details are set forth in order to fully understand the present application. However, the present application may also be implemented in other ways different from those described herein. Therefore, the protection scope of the present application is not limited by the specific embodiments disclosed below.

[0053] In addition, in the description of the present application, it should be understood that the orientation or positional relationship indicated by the terms "axial direction", "radial direction", "circumferential direction", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present application 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 construed as a limitation of the present invention.

[0054] In the description of this specification, the description referring to terms such as "implementation mode", "embodiment", "an embodiment", "example" or "specific example" means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic descriptions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0055] Embodiment 1:

[0056] A method for obtaining the crack propagation resistance curve of a high-pressure natural gas pipeline according to the present invention includes the following steps:

[0057] S1. Prepare a tensile specimen of the circumferential weld pipe to characterize the mechanical properties of the weld and the pipe body.

[0058] S11. Obtain a uniaxial specimen of the pipe body and a uniaxial specimen of the weld from the X80 circumferential weld pipe.

[0059] Conduct uniaxial tensile tests on the uniaxial specimen of the pipe body and the uniaxial specimen of the weld respectively, and obtain the stress-strain curve of the pipe body and the stress-strain curve of the weld.

[0060] As Figure 2 shown, specifically, a round-bar-shaped uniaxial specimen of the pipe body is taken from the pipe body part of the X80 circumferential weld pipe tensile specimen by wire cutting. The length of the uniaxial specimen of the pipe body is 125 mm, the gauge length is 65 mm, and the diameter is 8.9 mm. A round-bar-shaped uniaxial specimen of the weld is taken from the weld part of the X80 circumferential weld pipe tensile specimen. The length of the uniaxial specimen of the weld is 90 mm, the gauge length is 65 mm, and the diameter is 6.25 mm. The obtained uniaxial specimen of the pipe body and the uniaxial specimen of the weld are respectively subjected to uniaxial tensile tests on a universal testing machine to obtain the stress-strain curve of the pipe body and the stress-strain curve of the weld.

[0061] Obtain a notched specimen of the pipe body and a notched specimen of the weld from the specimens.

[0062] Conduct notched tensile tests on the notched specimen of the pipe body and the notched specimen of the weld respectively, and obtain the load-displacement curve of the pipe body and the load-displacement curve of the weld.

[0063] As Figure 2 shown, specifically, a round-bar-shaped notched specimen of the pipe body is taken from the pipe body part of the X80 circumferential weld pipe tensile specimen by wire cutting. The length of the notched specimen of the pipe body is 125 mm, the gauge length is 65 mm, the diameter is 8.9 mm, the notch radius is 5 mm, and the minimum diameter at the notch is 5 mm. A round-bar-shaped notched specimen of the weld is taken from the weld part of the X80 circumferential weld pipe tensile specimen. The length of the notched specimen of the weld is 90 mm, the gauge length is 65 mm, the diameter is 6.25 mm, the notch radius is 5 mm, and the minimum diameter at the notch is 3.5 mm. The obtained notched specimen of the pipe body and the notched specimen of the weld are respectively subjected to notched tensile tests on a universal testing machine to obtain the load-displacement curve of the pipe body and the load-displacement curve of the weld.

[0064] It should be noted that the uniaxial test of the weld and the notched specimen of the weld need to be obtained multiple times at different positions of the circumferential weld. Since welding cannot ensure that the welds at all positions of the circumferential weld are exactly the same and there must be certain differences, the weld positions need to be sampled and tested multiple times along the circumference of the circumferential weld pipe to reduce the deviation of the test results.

[0065] The tensile specimen in the shape of a round bar is a commonly used choice for tensile tests. There are many influencing factors for the test results of the mechanical properties of metallic materials. In the product standards, regardless of the diameter of the round bar-shaped specimen in the tensile test, the requirements for mechanical property indexes are basically the same.

[0066] The "universal testing machine" is a material testing machine integrating functions such as tension, bending, compression, and shear. It is mainly used for the mechanical property tests of metallic and non-metallic materials and is an ideal testing device for departments such as industrial and mining enterprises, scientific research institutions, universities, and engineering quality supervision stations.

[0067] S2. Based on the continuum damage theory, a fully coupled constitutive model of plasticity and ductility damage applicable to pipeline steel and weld materials is constructed; the material parameters of pipeline steel and welds are calibrated using the stress-strain curve and force-displacement curve of the tensile specimen in step S1.

[0068] S21. Under the framework of continuum mechanics, define the damage evolution factor in the constitutive model of X80 high-pressure natural gas pipeline materials. Among them, the failure area A d and the effective area A bearing the external force. The damage variable d = 0 indicates that the material is undamaged, and d = 1 indicates that the material has completely failed. By fully coupling the stress-strain, kinematic hardening, and isotropic hardening with the damage variable, its expression is:

[0069]

[0070]

[0071]

[0072] The effective stress part of the Cauchy stress σ after considering damage is In the model, a mixed method of kinematic hardening and isotropic hardening is adopted to describe the plastic deformation behavior in the strengthening stage of the material. Among them, the isotropic hardening is R = (1 - d)Qr, and the kinematic hardening is A new yield equation and potential energy equation are established using the effective stress part of the state variable, thereby realizing the coupling of plasticity and ductility damage. Taking the partial derivative of the damage potential energy function, the damage accumulation rate can be obtained

[0073] In the above formula, α is the strain of kinematic hardening, and r is the strain of isotropic hardening is the damage accumulation rate, is the plastic multiplier, Y represents the damage energy release rate, C and Q are parameters controlling kinematic hardening and isotropic hardening, and k and S are damage parameters.

[0074] S22. To obtain the elastic parameters, plastic parameters, and damage parameters of the pipe body material and the weld material in the constitutive model, tests need to be conducted on the two materials separately. Tensile test is the most basic and widely used test to study the mechanical properties of materials. Due to its simple test method and easy access to relatively reliable test data, tensile test is widely used in engineering and laboratories to measure the mechanical properties of materials.

[0075] Calculate the elastic parameters of the pipe body through the elastic stage of the stress-strain curve of the pipe body, and calibrate the plastic parameters of the pipe body through the strengthening stage of the stress-strain curve of the pipe body;

[0076] Calculate the elastic parameters of the weld through the elastic stage of the stress-strain curve of the weld, and calibrate the plastic parameters of the weld through the strengthening stage of the stress-strain curve of the weld;

[0077] Among them, the plastic parameters are calculated only based on the length of 30%-50% of the strengthening stage of the stress-strain curve.

[0078] The "stress-strain curve" is a curve that reflects various deformation processes such as brittleness, plasticity, yield, and fracture of materials under the action of external forces. The abscissa of this curve is strain, and the ordinate is the applied stress. Its process is generally divided into four stages: elastic stage, yield stage, strengthening stage, and local deformation stage;

[0079] In the "elastic stage", the stress and strain are linearly related and satisfy Hooke's law. The elastic modulus, that is, the elastic parameter, can be obtained through calculation;

[0080] In the "yield stage", the linear relationship between stress and strain is broken, the strain increases significantly, and the stress first decreases and then fluctuates slightly. There are small serrated line segments close to the horizontal line on the curve, and the yield strength of the material can be directly obtained.

[0081] In the "strengthening stage", obvious and uniform plastic deformation occurs in the specimen. If the strain of the specimen is increased, the stress value must be increased. In this embodiment, the plastic parameters are obtained only based on the length of 30%-50% of the strengthening stage of the stress-strain curve because the cumulative amount of damage in the length greater than 50% of the strengthening stage is very large, which affects the accuracy of the plastic parameters. In this embodiment, the kinematic hardening parameter C of the pipe body is 35000 MPa, the isotropic hardening parameter Q of the pipe body is 1950 MPa, the kinematic hardening parameter C of the weld is 1000 MPa, and the isotropic hardening parameter Q of the weld is 598 MPa;

[0082] S23. Establish a finite element model of the pipe body for the pipe body notched specimen, and simulate the notched tensile test through the finite element model of the pipe body to obtain the simulated load-displacement curve of the pipe body;

[0083] Such as Figure 3As shown, the damage parameter of the pipe body is calculated by fitting the load-displacement curve of the pipe body and the simulated load-displacement curve of the pipe body.

[0084] A finite element model of the weld of the weld notch specimen is established, and the notch tensile test is simulated through the finite element model of the weld to obtain the simulated load-displacement curve of the weld.

[0085] As Figure 3 shown, the damage parameter of the weld is calculated by fitting the load-displacement curve of the weld and the simulated load-displacement curve of the weld.

[0086] Specifically, since the notch tensile specimen is an axisymmetric model, to save calculation time, only a 1 / 4 model needs to be established for calculation and simulation; the notch tensile experiment is simulated under normal temperature (20°C) and quasi-static conditions. In this embodiment, the damage parameter of the pipe body S = 18 MPa, k = 1, and the damage parameter of the weld S = 17.1 MPa, k = 2.

[0087] The "load-displacement curve" obtained through the notch tensile test can quickly obtain the damage parameters of the material; in this embodiment, a finite element model is established to simulate the notch tensile test, and the simulated load-displacement curve is fitted with the real load-displacement curve, greatly improving the accuracy and authenticity of the damage parameters of the pipe body and the weld.

[0088] S3. Solve the above constitutive model, compile a user subroutine to implement the numerical algorithm, establish a finite element model of the circumferential weld pipe, and prefabricate cracks at the circumferential weld position.

[0089] Specifically, decouple the plastic deformation and damage in the established model, construct a numerical algorithm framework for elastic prediction and plastic correction, and use the return mapping algorithm to keep the updated generalized stress, etc. on the yield surface. Use the Newton-Raphson algorithm to obtain the nonlinear increments of all state variables, thereby updating the values of all internal variables. Write an ABAQUS user subroutine using Fortran language, and call the subroutine in the ABAQUS finite element software for finite element structural analysis.

[0090] Call the constitutive model and material parameters obtained in step S2, establish a finite element model of the circumferential weld pipe with an inner diameter of 783.6 mm, an outer diameter of 813 mm, and a length of 5000 mm. Prefabricate crack defects in the weld area, the tip of the prefabricated crack is circular and the plastic deformation at the crack tip is considered, and establish a finite element model with a crack depth of 15 mm.

[0091] Plastic deformation is likely to occur at the crack tip of engineering materials with even a little plastic deformation ability. The occurrence of plastic deformation at the crack tip will relieve the stress concentration at the crack tip. Considering the influence of plastic deformation at the crack tip on the stress concentration, the authenticity of the entire test results can be ensured and the accuracy of the crack propagation resistance curve can be improved. By prefabricating cracks with different depths and conducting multiple simulation experiments, the accuracy of the crack propagation resistance curve can be improved.

[0092] S4. Using the finite element model of the girth weld pipeline, simulate the tearing process of the girth weld pipeline under different loadings respectively to obtain the crack propagation resistance curve.

[0093] In this embodiment, two pipeline stress conditions are selected to simulate the tearing process of the girth weld pipeline, namely the no-internal-pressure process and the internal pressure of 12 MPa.

[0094] S41. Simulate the tearing process of the girth weld pipeline from crack opening to pipeline fracture under the tensile load. Derive the crack opening amount and crack propagation amount from the simulation results, as Figure 5 shown, to obtain the tensile crack propagation resistance curve without internal pressure;

[0095] S42. Simulate the tearing process of the girth weld pipeline from crack opening to pipeline fracture under the internal high-pressure load of 12 MPa. As Figure 4 shown, the prefabricated crack gradually opens, and at the same time the crack tip extends downward; as Figure 5 shown, derive the crack opening amount and crack propagation amount from the simulation results to obtain the crack propagation resistance curve under high-pressure conditions.

[0096] It should be noted that in complex geological movements, the stress direction of the pipeline is complex and changeable. Among them, the tensile stress is the direction that makes the crack propagate most easily and quickly. Simulating the tearing process of the girth weld pipeline under tension can obtain the crack propagation resistance curve for the minimum force of the girth weld pipeline tearing; according to the load condition that the long-distance high-pressure natural gas pipeline always bears internal high pressure, simulating the tearing process of the girth weld pipeline under internal high pressure can obtain the crack propagation resistance curve for the minimum internal gas pressure of the girth weld pipeline tearing.

[0097] In the embodiments of the present invention, small-sized tensile specimens are used to characterize the mechanical behaviors of the pipe body material and the weld material. A constitutive model coupling plasticity and damage is constructed. Based on the stress-strain curves and load-displacement curves of the characterization tests, the material parameters of the elastoplastic damage model are calibrated. A finite element model of a circumferential weld pipe with defects is established, and the tearing process of cracks in the circumferential weld pipe under different loads is simulated. By recording the opening displacement of the crack tip and the crack propagation amount, the crack propagation resistance curves of the pipe under different load conditions and different defect forms can be obtained quickly and accurately. The calculation process is more accurate and easier to understand, overcoming the disadvantages of high cost and long cycle of the full-scale pipe test method. All the tests required by this method are carried out on standard small-sized specimens using general test equipment. This method performs numerical simulation based on a macroscopic constitutive model, ensuring the calculation speed and accuracy, and can quickly obtain the crack propagation resistance curve of the pipe, thereby guiding the strain-based pipeline design, saving the construction time and economic cost of long-distance high-pressure natural gas pipelines, and improving the reliability of the safe operation of the pipeline.

[0098] What is not described in this application can be achieved by adopting or referring to the existing technologies.

[0099] Each embodiment in this specification is described in a progressive manner. The same or similar parts among the embodiments can be referred to each other, and the differences between each embodiment and other embodiments are emphasized.

[0100] The above description is only for the embodiments of this application and is not intended to limit this application. For those skilled in the art, various changes and modifications can be made to this application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of this application shall be included within the scope of the claims of this application.

Claims

1. A method for obtaining the crack growth resistance curve of a high-pressure natural gas pipeline, characterized in that, It includes the following steps: S1. Prepare a tensile specimen of the circumferential weld pipe to characterize the mechanical properties of the weld and the pipe body; S2. Respectively construct a fully coupled constitutive model of plasticity and ductility damage applicable to the pipe body and the weld, and use the material parameters of the weld and the pipe body obtained in step S1; S3. Establish a finite element model of the circumferential weld pipe and prefabricate a crack at the circumferential weld position; S4. Use the finite element model to respectively simulate the tearing process of the circumferential weld pipe under the action of loads in different directions, and obtain the crack propagation resistance curve of the circumferential weld pipe; Step S2 further includes: S21. The established constitutive model introduces isotropic hardening and kinematic hardening, and the stress-strain, isotropic hardening and kinematic hardening are coupled with the damage variable; S22. Calculate the elastic parameters of the pipe body through the elastic stage of the stress-strain curve of the pipe body, and calculate the plastic parameters of the pipe body through the strengthening stage of the stress-strain curve of the pipe body; Calculate the elastic parameters of the weld through the elastic stage of the stress-strain curve of the weld, and calculate the plastic parameters of the weld through the strengthening stage of the stress-strain curve of the weld; S23. Calculate the damage parameter of the pipe body through the load-displacement curve of the pipe body; Calculate the damage parameter of the weld through the load-displacement curve of the weld.

2. The method for obtaining a crack growth resistance curve of a high-pressure natural gas pipeline according to claim 1, characterized in that, The material parameters include elastic parameters, plastic parameters and damage parameters.

3. A method for obtaining a crack growth resistance curve of a high-pressure natural gas pipeline according to claim 2, characterized in that, Step S1 includes: S11. Obtain a uniaxial specimen of the pipe body and a uniaxial specimen of the weld from the tensile specimen of the circumferential weld pipe; Respectively conduct uniaxial tensile tests on the uniaxial specimen of the pipe body and the uniaxial specimen of the weld, and obtain the stress-strain curve of the pipe body and the stress-strain curve of the weld; Obtain a notched specimen of the pipe body and a notched specimen of the weld from the tensile specimen of the circumferential weld pipe; Respectively conduct notched tensile tests on the notched specimen of the pipe body and the notched specimen of the weld, and obtain the load-displacement curve of the pipe body and the load-displacement curve of the weld.

4. A method for obtaining a crack growth resistance curve of a high-pressure natural gas pipeline according to claim 3, characterized in that, Step S23 further includes: Establish a finite element model of the pipe body of the notched specimen of the pipe body, simulate the notched tensile test through the finite element model of the pipe body, and obtain the simulated load-displacement curve of the pipe body; Calculate the damage parameter of the pipe body through the fitting of the load-displacement curve of the pipe body and the simulated load-displacement curve of the pipe body; Establish a finite element model of the weld of the notched specimen of the weld, simulate the notched tensile test through the finite element model of the weld, and obtain the simulated load-displacement curve of the weld; Calculate the damage parameter of the weld through the fitting of the load-displacement curve of the weld and the simulated load-displacement curve of the weld.

5. A method for obtaining a crack growth resistance curve of a high-pressure natural gas pipeline according to claim 3, characterized in that, The uniaxial specimen of the pipe body, the notched specimen of the pipe body, the uniaxial specimen of the weld and the notched specimen of the weld are all obtained along the axial direction of the specimen; the uniaxial specimen of the weld and the notched specimen of the weld are obtained multiple times at different positions of the circumferential weld.

6. A method for obtaining a crack growth resistance curve of a high-pressure natural gas pipeline according to claim 1, characterized in that, Step 3 further includes: S31. Compile program code in Fortran language in the ABAQUS user subroutine and embed it into ABAQUS for numerical calculation; S32. The shape of the prefabricated crack defect is circular arc, and the tip of the crack is semi-circular; S33. Only draw refined meshes in the prefabricated crack area, and the meshes in other parts are coarser to improve the calculation efficiency of the model.

7. A method for obtaining a crack growth resistance curve of a high-pressure natural gas pipeline according to claim 6, characterized in that, The prefabrication direction of the crack is the same as the extension direction of the circumferential weld, and the prefabrication of the crack includes the plastic change at the crack tip.

8. A method for obtaining a crack growth resistance curve of a high-pressure natural gas pipeline according to claim 7, characterized in that, The depth of the crack does not exceed two-thirds of the wall thickness of the specimen.

9. A method for obtaining a crack growth resistance curve of a high-pressure natural gas pipeline according to claim 1, characterized in that, Step 4 includes: S41, simulating the tearing process of the circumferential weld pipe under the action of a tensile load to obtain a tensile crack propagation resistance curve; S42, simulating the tearing process of the circumferential weld pipe under the combined action of an internal high-pressure load and a tensile load to obtain a high-pressure crack propagation resistance curve.

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