Safety evaluation method for defective PE pipeline under point load

Abstract

The application discloses a kind of safety evaluation methods of defect PE pipeline under point load, which obtains PE pipe material constitutive model parameters and fracture toughness and other mechanical parameters, determines the defect form of PE pipe under the action of point load;And select the failure evaluation curve used in failure evaluation diagram;Based on M integral, the fracture ratio Kr is calculated, and the load ratio L r Is calculated based on elastic-plastic J integral, and the parameter is verified, corrected by experiment and simulation. According to different analysis parameters, the three-dimensional simulation model of PE pipe under point load is established;Extract the stress intensity factor K I And the maximum value J max Of J integral before crack front, calculate the failure evaluation point;Finally, according to the failure evaluation curve and the failure evaluation point, whether the pipeline is safe is judged by drawing. The method of the application can comprehensively consider the actual working condition, scientifically evaluate the safety of defect PE pipe under point load, and has reference significance for the safety evaluation of other thermoplastic polymer material pipeline.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of gas pipeline strength, in particular to a safety evaluation method for a defective PE gas pipeline under point load. BACKGROUND

[0002] Compared with traditional metal pipelines, plastic pipelines represented by polyethylene (PE) pipelines are gradually widely used in gas transportation fields due to their light weight, corrosion resistance, hygiene and safety, and convenient installation, etc. Since the strategy of "replacing steel with plastic" was proposed, PE pipelines have gradually replaced metal pipelines and become the main mode of urban gas transportation, significantly reducing the problem of corrosion damage of metal pipelines.

[0003] Most PE gas pipelines are in a buried state, and the soil environment and temperature difference in different regions are quite different, and they are also affected by different external loads, so they are easily affected by environmental factors, leading to problems such as pipeline deformation, rupture and fracture, and causing pipeline failure. Gas pipeline failure may cause losses such as local oil and gas pipeline fires, burning of railway power supply equipment, etc. Although such accidents have not caused casualties, the occurrence of gas pipeline accidents still requires reasonable and safe use of pipelines.

[0004] In PE pipe failure accidents, point load is an important cause of PE pipe failure. Point load refers to the extrusion of rigid objects such as stones and tree roots in the soil on the surface of the pipeline, which is usually caused by non-standard gas pipeline laying or movement of hard objects to the surface of the pipeline due to soil changes. This extrusion can cause stress concentration on the surface of the pipeline, and compared with the case of only internal pressure, the stress on the inner surface of the pipeline will increase significantly. The increase of the stress on the inner surface of the pipeline can cause small production defects or scratches to cause axial cracks earlier, causing major safety accidents. The population in cities and towns is dense, and pipelines are densely distributed, so in order to ensure the safety of people's lives and property, the safe use of urban gas pipelines must be ensured, but the stress of point load pipelines is complex, and existing research is less and lacks a scientific safety evaluation method. Therefore, it is necessary to study the phenomenon of point load causing gas pipeline failure, and to evaluate the safety of buried PE gas pipelines under point load to assess whether the gas pipeline can continue to be used. SUMMARY

[0005] The present application provides a safety evaluation method for a defective gas pipeline under point load, to solve the problems of limited applicability and accuracy of existing point load PE pipeline evaluation calculation methods, and the calculation difficulty of multi-factor coupling of buried pipelines, etc., to realize the safety evaluation of buried PE gas pipelines under point load, and to provide reference for the laying of PE gas pipelines in the early stage and maintenance and replacement in the later stage, etc.

[0006] The present application is realized by the following technical solutions:

[0007] A safety evaluation method for a defective PE pipeline under point load, comprising:

[0008] Obtaining a PE pipe material constitutive model parameter and a mechanical parameter based on a stratum settlement rate; the mechanical parameter includes a fracture toughness parameter;

[0009] Carrying out a first point load experiment to determine a defect form for characterizing the PE pipe under point load;

[0010] Selecting a failure assessment curve FAC used for a failure assessment diagram;

[0011] Determining a calculation method of a failure assessment point (L r , K r ), calculating a parameter fracture ratio K r based on M integral, and calculating a parameter load ratio L r based on elastic-plastic J integral;

[0012] Verifying and correcting the parameter load ratio L I , including verifying and correcting the parameter load ratio L r through a first point load experiment and simulation data, and carrying out a second point load experiment and simulation to verify and correct the parameter load ratio L r ;

[0013] According to different analysis parameters, a plurality of PE pipe three-dimensional simulation models under point load are established;

[0014] The PE pipe three-dimensional simulation models under point load with different parameters are respectively simulated and analyzed, the crack front stress intensity factor K r and the elastic-plastic J integral maximum value are extracted, and the failure assessment points are calculated according to the parameter fracture ratio K r and the parameter load ratio L r calculation method;

[0015] According to the failure assessment curve FAC and the failure assessment point (L r , K r ), a diagram is drawn to determine whether the failure assessment point is in the failure assessment curve FAC, if yes, it is proved that the pipeline is safe, and if not, it is proved that the pipeline has failed and cannot continue to serve.

[0016] In view of the problem that the prior art lacks a safety evaluation method for PE gas pipeline under point load, the application first proposes a safety evaluation method for defective PE pipeline under point load, the method first determines the mechanical parameters of the PE pipeline material used for gas transmission, such as the constitutive model parameter of stratum subsidence rate and fracture toughness, fully understands the material characteristics, then determines the defect form of the PE pipe under point load, then selects the appropriate failure assessment curve (FAC) and the calculation method of failure assessment point (L r , K r ) according to the failure assessment diagram (FAD), and verifies and corrects the parameters through experiments and simulations, establishes a three-dimensional simulation model of the PE pipe under point load according to different analysis parameters, and respectively performs simulation, finally calculates (L r , K r ) and the curve FAC to form an evaluation diagram, judges whether (L r , K r ) is in the curve FAC, if yes, it proves that the pipeline is safe, if not, it proves that the pipeline is unsafe.

[0017] In addition, the safety evaluation method of the PE gas pipeline obtained by the application is based on fracture mechanics and the constitutive parameters of PE material related to strain rate, and has reference significance in the safety evaluation of other thermoplastic pipes.

[0018] Further, the method for determining the constitutive model parameters of the PE pipe material comprises:

[0019] Cut a dumbbell-shaped sample from the PE pipeline, the sample thickness is the pipe wall thickness, perform uniaxial tensile test at different rates, and obtain the true stress-strain relationship curve related to the tensile rate according to the experimental results;

[0020] Based on the true stress-true strain relationship curve, the m and n under different tensile rates are fitted by the following formula:

[0021]

[0022] In the formula, σ t is the true stress, ε t is the true strain, and m and n are fitting constants;

[0023] Determine the stress-strain relationship of the PE material according to the rate of the stratum settlement to be evaluated, and calculate the elastic parameters and plastic parameters of the material.

[0024] The scheme takes into account that the PE material has obvious nonlinear behavior, the stress-strain curves at different rates have great differences, and the sample thickness is not convenient to process. Therefore, the sample thickness is set as the wall thickness, the parameters are fitted at different rates, and the stress-strain relationship of the PE material is calculated according to the actual stratum subsidence rate.

[0025] Those skilled in the art should understand that the elastic and plastic parameters in the scheme are the constitutive model parameters required for subsequent finite element modeling, and m and n are intermediate parameters in the calculation process. M and n are fitted at each stretching rate, and m and n fitted at different rates are not the same. Therefore, the values of m and n at different rates need to be fitted by logarithm respectively, so as to determine the specific functional relationship between the values of m and n and the stretching rate.

[0026] Further, the method for determining the fracture toughness parameter of the PE pipe material comprises:

[0027] Two kinds of CT samples are cut from the PE pipeline, which are notched and unnotched CT samples. The notched CT sample experiment is loaded to the maximum load, while the unnotched CT sample experiment is loaded to more than 10% of the load of the notched CT sample experiment, and the displacement-load relationship curves are obtained respectively;

[0028] Based on the displacement-load relationship curve, the J integral of the material and the corresponding crack propagation displacement are calculated by the following formula:

[0029]

[0030] U = U T -U I

[0031] η = 2 + 0.522 · (b0 / W)

[0032] y = 2 · σ y · (Δa-0.2)

[0033] In the formula: J represents the elastic-plastic J integral; U T is the total energy in the crack propagation process, which is obtained by integrating the area surrounded by the load and opening displacement curve of the notched CT sample; U I is the energy required for plastic deformation in the crack propagation process, which is obtained by integrating the area surrounded by the load and displacement curve of the corrected CT sample; U is the energy required for crack propagation; η is a dimensionless function; B is the thickness of the notched CT sample; W is the width from the center of the clamping hole of the notched CT sample to one side of the sample surface; a0 is the distance from the center of the clamping hole of the notched CT sample to the front edge of the prefabricated crack; b0 is the width from the sharp corner of the notched CT sample to one side of the sample surface; σ y is the yield stress; Δa is the crack propagation displacement; y is the blunting curve;

[0034] The J-R curve is calculated by the following formula

[0035] wherein C1 and C2 are experimental fitting constants;

[0036] The J-integral fracture toughness J IC .

[0037] In the present scheme, considering that the performance of PE materials produced by the same manufacturer and in the same batch is different and the manufacturer may not have experimental data related to the fracture parameters, the fracture toughness parameters need to be measured by experiments.

[0038] In addition, in the present scheme, the fracture toughness J IC of the PE pipe is obtained through the J-R curve of the material, and the J-R curve of the plastic material is determined by compact tension (CT) test of the material; the CT test needs to prepare two kinds of samples, namely notched CT sample and unnotched CT sample; the curve is fitted according to the fitting formula of the J-R curve, and (Δa, J) corresponding to each CT experiment can be obtained according to the J-integral calculation formula, and the J-R curve can be obtained by fitting all (Δa, J). The intersection of the offset blunting curve with the J-R curve is used to determine the fracture toughness J IC .

[0039] In the present scheme, the material characteristics and specific parameters of the evaluated PE pipe are obtained through the material parameter experiment, which lays a prerequisite for subsequent point load experiment and research on fracture behavior.

[0040] The measured constitutive parameters and fracture toughness parameters need to be verified through notched CT sample simulation, and the simulation results are compared with the experimental results to verify the reliability of the parameters.

[0041] Further, the method for determining the defect form of the PE pipe under the action of point load comprises:

[0042] Point load experiments are respectively conducted on the complete PE pipe and the defective PE pipe;

[0043] A defect in the axial direction is prepared inside the complete PE pipe using a craft knife to prepare the defective PE pipe;

[0044] The complete PE pipe and the defective PE pipe are fixed on the backing plate to ensure that the defect is located at the highest position;

[0045] Pads are added around the pipe to fix the pipe to prevent the pipe from rolling during loading;

[0046] A constant rate is used for loading, and the loading rate of the spherical indenter is 1 mm / min, and the maximum loading displacement is 50 mm;

[0047] By comparing the surface morphology of the complete PE pipe and the defective PE pipe after the experiment, the cross-sectional morphology of the PE pipe under the point load, the corresponding crack propagation mechanism and the morphology are analyzed.

[0048] The inventors find that the crack morphology of the PE pipe caused by different loads is different. Therefore, the fracture morphology after the experiment is observed from the macroscopic and microscopic aspects, and the crack propagation form of the PE pipe under the point load is determined.

[0049] Further, the method for selecting the failure assessment curve FAC used in the failure assessment diagram includes:

[0050] According to the establishment method of the failure assessment curve FAC in different standards, the corresponding failure assessment curve of the PE material is drawn;

[0051] The cutoff value of the parameter load ratio L r is determined;

[0052] The failure assessment curves FAC under different standards are compared;

[0053] The failure assessment curve FAC that is obviously lower than other standards is selected.

[0054] The failure assessment diagram FAD includes a failure assessment curve FAC and a failure assessment point (L r , K r ). Different failure assessment curves FAC are roughly similar, but they are different according to different situations. The conservative curve can be selected, and the result of the conservative curve evaluation is safer.

[0055] The failure assessment diagram FAD method used in the present scheme for safety evaluation of defect structure is the current mainstream method, which is suitable for the brittle failure to plastic instability range of materials.

[0056] Among them, those skilled in the art should understand that the failure assessment curve FAC in the present scheme has been revised many times by various standards, and is relatively perfect. Therefore, the present scheme only compares and selects the failure assessment curve FAC.

[0057] Further, the method for calculating the parameter fracture ratio K r based on M integral and the parameter load ratio L r based on elastic-plastic J integral includes:

[0058] The fracture ratio K r corresponds to the brittle fracture criterion, and the fracture ratio K r is calculated by the following formula:

[0059]

[0060] Among them, — the maximum value of the stress intensity factor at the crack front; K C — the fracture toughness with the stress intensity factor; J IC — the fracture toughness with the J integral; f1— the safety factor; E represents the elastic modulus; and μ represents the Poisson's ratio;

[0061] the load ratio L r Corresponding to the plastic instability criterion, the parameter load ratio L is calculated by the following formula r :

[0062]

[0063] In the formula, J max — the maximum value of the elastic-plastic J integral at the crack front; J IC — the fracture toughness with the J integral of the PE material.

[0064] In the scheme, the failure evaluation point coordinates contain two failure evaluation parameters, namely the fracture ratio K r and the load ratio L r . The fracture ratio K r represents the longitudinal coordinate of the evaluation point, and the load ratio L r represents the horizontal coordinate of the evaluation point. When the PE pipe under the action of the point load is evaluated, the corresponding fracture ratio K r and load ratio L r parameter values need to be calculated, so as to determine the failure evaluation point.

[0065] In the scheme, the working condition of the PE pipe is complex, and the internal pressure of the pipe is only one of the influencing factors. The load ratio L r corresponds to the plastic instability criterion, which is the ratio of the externally applied load to the plastic yield limit load of the structure containing a crack, or the ratio of the applied stress to the plastic instability stress. The calculation method considering only the internal pressure is not comprehensive enough, and the method of selecting the plastic limit internal pressure P0 is not suitable. When the effective net section is selected, the structure is considered to be subjected to a single stress on the crack surface. However, the defective pipe under the action of the point load is subjected to the internal pressure, the gravity of the soil, the friction of the soil on the pipe, and the point load of the hard object, so the effective net section method is also not applicable. Therefore, a new L r calculation method is proposed in the scheme.

[0066] Those skilled in the art should understand that, for ductile fracture, the plastic deformation at the crack tip is dominant, and the stress intensity factor is only applicable to the linear elastic stage and is not applicable to the ductile fracture of the material; and the elastic-plastic J integral is widely used, and is applicable to both linear elastic fracture mechanics and elastic-plastic fracture mechanics. For linear elastic material behavior, the J integral is equal to the energy release rate at the crack front, which is related to the stress intensity factor. For the elastic-plastic material behavior of PE, the J integral is a local measure that integrates the fracture energy release rate and the plastic power generated by the virtual crack propagation.

[0067] J integral has path independence, that is, its value only depends on the stress-strain state at the crack tip, and is independent of the selection of the integral path. This characteristic makes the J integral have stronger applicability and advantages of simplifying calculation in crack problems of complex geometry and loading conditions. In the calculation of the parameter load ratio L r , the specific form of the integral path does not need to be considered, thereby simplifying the calculation process.

[0068] Further, the method for verifying and correcting the parameter load ratio L r based on the elastic-plastic J integral calculation includes:

[0069] According to the first point load experiment, a 1:1 finite element simulation model of the complete PE pipe and the defective PE pipe is established;

[0070] wherein the gasket is set as a three-dimensional entity, and the gasket boundary condition is set as a fixed constraint; the spherical indenter is a three-dimensional entity, and the spherical indenter is set with a downward displacement load in the pipeline direction;

[0071] The PE pipe is set in surface-to-surface contact with the gasket and the spherical indenter; a crack with the same size as the complete PE pipe and the defective PE pipe in the first point load experiment is set on the pipeline as a pipeline defect, and the local grid of the defect is encrypted; the crack front shape is the crack shape observed in the experiment for characterizing the crack propagation of the PE pipe under the point load;

[0072] According to the simulation results, the load ratio L r is calculated, and the L r =1 in the current critical state is compared and corrected;

[0073] In order to verify the corrected load ratio calculation method based on the J integral, the same pipeline material and experimental scheme as the first point load experiment are used, the defect size of the PE pipe is changed, and the point load experiment and simulation on the defective PE pipe are performed again, which are defined as the second point load experiment; the second point load experiment and simulation data are used to verify the corrected parameter load ratio L r .

[0074] The scheme first calculates the load ratio L r1 according to the first point load experiment and simulation results, compares the L r0 =1 in the current critical state, and corrects the simulation results at the critical crack initiation to 1 by correcting the safety factor, and the corrected load ratio is calculated as follows:

[0075]

[0076] In the formula, J max is the maximum J integral value along the crack front path; JC J integral fracture toughness; J IC J integral fracture toughness, f1 is the correction safety factor.

[0077] In this scheme, the fracture ratio K r The calculation method and the load ratio L r The calculation method. At the same time, in order to verify whether the load ratio calculation method based on elastic-plastic J integral is reliable, the first point load experiment results are verified, and the plastic instability criterion is used to obtain the load ratio L r =1 at the critical state. Based on the first point load experiment, a 1:1 point load simulation model of the defect PE pipe is established, and the load ratio L r ; The load ratio L r calculated by the simulation result is compared and corrected with the current L r at the critical state.

[0078] In order to verify the corrected load ratio L r calculation method, the second point load experiment and simulation are carried out to verify the corrected load ratio L r calculation method, which ensures the reliability of the parameter L r calculation through the verification-correction-verification method.

[0079] Further, the method for establishing a number of three-dimensional simulation models of PE pipes under different point loads includes:

[0080] According to the actual working conditions, a number of pipe internal pressures, a number of pipe sizes, a number of defect sizes, a number of point load sizes, and a number of burial depths are determined.

[0081] For a number of pipe internal pressures, a number of pipe sizes, a number of defect sizes, a number of point load sizes, and a number of burial depths, change a single factor parameter and fix the values of other parameters to obtain a number of simulation models of defective pipes under point loads.

[0082] The mesh is divided into eight-node linear hexahedral elements, and the required mesh size is determined through mesh independence verification. The hard object is simplified as a spherical stone, and the defect is on the inner surface of the pipe directly above the hard object, and the hard object is completely fixed.

[0083] A 1 / 2 three-dimensional point load-pipe-soil model is established, the model 1 / 2 section and the left and right sides are set to be symmetrical constraints, and the upper surface of the model is set to be a settlement displacement and the lower surface is set to be a fixed constraint.

[0084] The simulation model established in this solution not only considers pipeline size and material properties, but also the impact of the buried environment, simulating the environmental conditions and operating parameters of the PE gas pipeline as closely as possible. Furthermore, the mesh is encrypted where the pipeline contacts hard objects, and the model mesh undergoes independence verification to ensure accurate simulation results while reducing computational time and costs.

[0085] Furthermore, the stress intensity factor K at the crack front is extracted I The methods for maximizing the elastic-plastic J integral include:

[0086] According to the results of the simulation model under the influence of different factors, the stress intensity factor K of the crack front is extracted in the simulation software. I and the maximum value of elastic-plastic J integral J max ;

[0087] According to the parameter fracture ratio K r and parameter load ratio L r Calculation method, respectively calculate the fracture ratio K r , load ratio L r , and obtain the failure assessment point (L r , K r ).

[0088] Fracture ratio K r Usually defined as the current stress intensity factor K I and material fracture toughness K IC The ratio of K r =K I / K IC . K IC It is the critical stress intensity factor of the material to resist crack growth under plane strain conditions; load ratio L r It is defined as the ratio of the current load to the critical load that causes crack propagation. The stress intensity factor K at the crack front is extracted from the simulation results. I This usually involves the accurate calculation and analysis of the stress field near the crack tip, and the extraction of the maximum value of the elastic-plastic J integral J max , J integral represents the energy required per unit area for crack extension and is an important indicator for evaluating the crack extension potential.

[0089] Furthermore, the method for determining whether the failure assessment point is within the failure assessment curve FAC includes:

[0090] According to the obtained failure assessment point (L r , K r ) and the failure assessment curve FAC, constitute the failure assessment diagram FAD, which is used to evaluate the safety of defective pipelines under point loads;

[0091] Determine the failure assessment point (Lr , K r ) whether in the failure assessment curve FAC, if in, prove that the pipeline is safe, if not in the curve, prove that the pipeline has failed, can no longer continue to serve.

[0092] According to the evaluation rules, if the evaluation point is inside the FAC, it means that the PE pipe is in a safe state and can still be used; if the evaluation point is on the FAC, it means that the PE pipe is in a failure critical state; if the evaluation point is outside the FAC curve, the failure evaluation point is in an unsafe area, and the PE pipe is in a failure state. In addition, the farther the evaluation point deviates from the FAC curve outside the FAC curve, the higher the safety risk and the greater the degree of failure.

[0093] Compared with the prior art, the present application has the following advantages and beneficial effects:

[0094] 1. The safety evaluation method of the defective PE pipeline under point load of the present application establishes a three-dimensional simulation model of the buried PE gas pipeline under the action of point load, considers the possible situations in the actual pipeline burying process, evaluates the safety and acceptability of the defective PE pipe under the action of point load, and also has reference significance for the safety evaluation of pipelines of other thermoplastic polymer materials.

[0095] 2. The safety evaluation method of the defective PE pipeline under point load of the present application is based on fracture mechanics and material nonlinearity to propose a PE gas pipeline safety evaluation scheme, which is beneficial to obtain a more scientific evaluation of the safety of the PE pipeline.

[0096] 3. The safety evaluation method of the defective PE pipeline under point load of the present application determines the constitutive parameters and fracture performance parameters of the PE material required for simulation through experiments, and verifies the reliability of the material parameters by means of the simulation model, thereby ensuring the accuracy of subsequent finite element analysis.

[0097] 4. The safety evaluation method of the defective PE pipeline under point load of the present application considers that the working conditions of the buried PE pipe are complex, the internal pressure of the pipeline is only one of the influencing factors and the stress is complex, and the load ratio L r The calculation methods have limitations, therefore, the elastic-plastic J integral representing the elastic-plastic fracture parameter of the material is used to calculate the load ratio L r ; and the improved calculation method of L r is verified and corrected through experimental and simulation results. Considering the three-dimensional stress state of the pipeline, the calculation method is more reasonable, and the reliability of the evaluation results is improved by verifying and correcting the parameters through experiments.

[0098] 5. The safety evaluation method of the defective PE pipeline under point load of the present application, in the process of establishing the finite element model of pipeline safety evaluation, the coupling of multiple factors such as material performance, environmental conditions, operating parameters is considered to simulate the possible situation of the real PE gas pipeline, the finite element model is verified by grid independence, the key part is refined unit and 1 / 2 symmetry model is set, which has calculation efficiency while ensuring calculation precision.

[0099] 6. The safety evaluation method of the defective PE pipeline under point load of the present application, considering that the current failure model is based on statistical analysis and empirical formula, for the failure mode of complex pipeline system and multiple factors, the applicability and accuracy of the model may be limited, and the safety of the gas pipeline under the action of point load can be accurately evaluated by experiment and simulation, which has important significance for the pre-laid pipeline and the damaged pipeline in the later maintenance and inspection. BRIEF DESCRIPTION OF DRAWINGS

[0100] The drawings described herein are used to provide further understanding of the embodiments of the present application, constitute a part of the present application, and do not constitute a limitation on the embodiments of the present application. In the drawings:

[0101] Figure 1 The safety evaluation method of the defective PE pipeline under point load in the specific embodiments of the present application is shown in the schematic diagram;

[0102] Figure 2 The material stress-strain curve obtained according to the stratum subsidence rate in the specific embodiments of the present application is shown in the schematic diagram;

[0103] Figure 3 The J-R curve and fracture toughness J of the PE material in the specific embodiments of the present application is shown in the schematic diagram; IC

[0104] Figure 4 The schematic diagram of the point load experiment in the specific embodiments of the present application is shown in the schematic diagram;

[0105] Figure 5 The schematic diagram of the failure assessment diagram FAD method used in the specific embodiments of the present application is shown in the schematic diagram;

[0106] Figure 6 The J integral variation curve of the PE pipe along the crack front with different defect sizes in the specific embodiments of the present application is shown in the schematic diagram;

[0107] Figure 7 The failure assessment diagram FAD of the PE pipe under different internal pressure in the specific embodiments of the present application is shown in the schematic diagram. DETAILED DESCRIPTION

[0108] ​In order to make the objects, technical solutions and advantages of the present application clearer, further detailed description will be made to the present application in combination with embodiments and drawings, the schematic embodiments and their descriptions are only used to explain the present application, and do not limit the present application.

[0109] Embodiment 1

[0110] As Figure 1 The safety evaluation method for the PE pipeline with defects under point load comprises the following steps,

[0111] The constitutive model parameters and mechanical parameters of the PE pipe material based on the stratum subsidence rate are obtained, and the mechanical parameters include the fracture toughness parameters;

[0112] The first point load experiment is carried out to determine the defect form of the PE pipe under the point load;

[0113] The failure assessment curve FAC used in the failure assessment diagram is selected;

[0114] The calculation method of the failure assessment point (L r , K r ) is determined, the parameter fracture ratio K r is calculated based on M integral, and the parameter load ratio L r is calculated based on elastic-plastic J integral;

[0115] The parameter load ratio L r is verified and corrected, including verifying and correcting the parameter load ratio L r through the data of the first point load experiment and simulation, and carrying out the second point load experiment and simulation to verify and correct the parameter load ratio L r ;

[0116] According to different analysis parameters, a plurality of PE pipe three-dimensional simulation models under point load are established;

[0117] The PE pipe three-dimensional simulation models under point load with different parameters are simulated and analyzed respectively, the crack front stress intensity factor K I and the maximum value of the elastic-plastic J integral are extracted, and the failure assessment points are calculated according to the parameter fracture ratio K r and the parameter load ratio L r ;

[0118] According to the failure assessment curve FAC and the failure assessment point (L r , K r ), a diagram is drawn, whether the failure assessment point is in the failure assessment curve FAC is judged, if yes, it is proved that the pipeline is safe, if not, it is proved that the pipeline has failed and cannot continue to serve.

[0119] The method for determining the constitutive model parameters of the PE pipe material comprises the following steps:

[0120] A dumbbell-shaped sample is cut from the PE pipe, the sample thickness is the pipe wall thickness, uniaxial tensile experiments are performed at different rates, and a real stress-strain relationship curve related to the tensile rate is obtained according to the experimental results;

[0121] Based on the real stress-real strain relationship curve, m and n at different tensile rates are fitted by the following formula:

[0122]

[0123] In the formula, σ t is the real stress, ε t is the real strain, and m and n are fitting constants.

[0124] The stress-strain relationship of the PE material is determined according to the rate of the stratum settlement to be evaluated, and the elastic parameters and plastic parameters of the material are calculated.

[0125] The stratum settlement of the town is equivalent to a quasi-static loading process, and the settlement rate is between 1x10 -6 s -1 and 1x10 -5 s -1 In this example, the settlement rate is selected as 1x10 -6 s -1 The constitutive parameters of the PE are calculated, such as Figure 2 The stratum settlement rate is 1x10 -6 s -1 The stress-strain curve of the PE is calculated.

[0126] In this example, the method for determining the fracture toughness parameters of the PE pipe material comprises the following steps:

[0127] Two types of CT samples are cut from the PE pipe, which are a notched CT sample and a non-notched CT sample, the notched CT sample is experimentally loaded to the maximum load, and the non-notched CT sample is experimentally loaded to 10% of the load of the notched CT sample, and a displacement-load relationship curve is obtained.

[0128] Based on the displacement-load relationship curve, the J integral of the material and the corresponding crack propagation displacement are calculated by the following formula:

[0129]

[0130] U=U T -U I

[0131] η=2+0.522·(b0 / W)

[0132] y = 2 σ y · (Δa - 0.2)

[0133] In the formula: J represents the elastic-plastic J integral; U T is the total energy in the process of crack propagation, which is obtained by integrating the area surrounded by the load and opening displacement curve of the notched CT specimen; U I is the energy required for plastic deformation in the process of crack propagation, which is obtained by integrating the area surrounded by the load and displacement curve of the corrected CT specimen; U is the energy required for crack propagation; η is a dimensionless function; B is the thickness of the notched CT specimen; W is the width from the center of the clamping hole of the notched CT specimen to the side surface of the specimen; a0 is the distance from the center of the clamping hole of the notched CT specimen to the front edge of the pre-crack; b0 is the width from the sharp corner of the notched CT specimen to the side surface of the specimen; σ y is the yield stress; Δa is the crack propagation displacement; y is the blunting curve;

[0134] The J-R curve is calculated by the following formula

[0135] In the formula: C1 and C2 are experimental fitting constants;

[0136] The J integral fracture toughness J IC of the material is obtained through the J-R curve of the material.

[0137] Preferably, the corresponding (Δa, J) of each CT experiment is obtained, and all (Δa, J) are fitted to obtain the J-R curve. Figure 3 The intersection of the blunting curve with an offset of 0.2 and the J-R curve is given in the table, and the intersection value is 13.23 kJ / m 2 , that is, the fracture toughness of the PE material is J IC = 13.23 kJ / m 2 .

[0138] In order to verify the measured constitutive parameters and fracture toughness parameters of the PE material in this embodiment, the applicant simulates and verifies the scheme of the notched CT experiment, and the simulation process includes:

[0139] A model with a 1:1 ratio of the specimen used in the notched CT experiment is established;

[0140] The center hole of the notched CT specimen is coupled and constrained;

[0141] A displacement along the Y direction is set at the upper center hole constraint point, and the movement freedom degree is limited at the lower center hole constraint point;

[0142] A 2mm crack is set at the sharp corner of the notched CT specimen, the grid at the pre-crack is locally encrypted during grid division, and the grid at the crack front edge is divided by 5 circles of unit ring grid.

[0143] The simulation results of the same notched CT experiment are compared with the experimental results, and the comparison results show that during the expansion of 0.2mm from the prefabricated crack of 2mm, the support reaction force of the notched CT specimen continuously increases with the increase of displacement, and the support reaction force of the CT specimen after expansion decreases. The simulation result curve and the experimental result curve have consistent change trend, and the maximum error is 6.7%, which is in the acceptable range. Therefore, the obtained PE constitutive parameters and fracture toughness parameters can better simulate the notched CT experimental results, which shows that the obtained PE constitutive parameters and fracture toughness parameters are reliable.

[0144] After fully understanding the material properties of the PE pipe, the method for characterizing the defect form of the PE pipe under the action of the point load is determined, which includes:

[0145] The point load experiment is respectively conducted on the complete PE pipe and the defective PE pipe;

[0146] The defect in the axial direction is prepared inside the complete PE pipe by using a craft knife, and the defective PE pipe is prepared;

[0147] The complete PE pipe and the defective PE pipe are fixed on the backing plate, and it is ensured that the defect is located at the highest position;

[0148] The cushion block is added around the pipeline to fix the pipeline and prevent the pipeline from rolling during loading;

[0149] The constant rate loading is used, the loading rate of the spherical indenter is 1mm / min, and the maximum loading displacement is 50mm;

[0150] The surface morphology of the complete PE pipe and the defective PE pipe after the experiment is compared and analyzed, and the cross-sectional morphology, the corresponding crack propagation mechanism and the morphology of the PE pipe under the action of the point load are analyzed.

[0151] As shown in Figure 4 the schematic diagram of the point load experiment, in the embodiment, the defect is located inside the pipeline and is distributed in the axial direction, the depth is 1mm, and the length is 44mm; the pipeline will be deformed during the continuous loading of the spherical indenter, and the pipeline surface in contact with the spherical indenter is in a state of extrusion, and the inner surface of the pipeline opposite to it is in a state of stretching.

[0152] The defect in the defective pipeline expands cracks under the action of the indenter, the defect expansion leads to the thinning of the pipe wall, and aggravates the stress concentration phenomenon of the pipeline under the action of the spherical indenter, so that the defect of the pipeline further expands.

[0153] Embodiment 2

[0154] A safety evaluation method for a defective PE pipeline under a point load, based on the embodiment 1, the method for selecting the failure evaluation curve FAC used in the failure evaluation diagram includes:

[0155] According to the failure assessment curve FAC in different standards, the corresponding failure assessment curve of PE material is drawn;

[0156] The cut-off value of the parameter load ratio L r is determined;

[0157] The failure assessment curves FAC under different standards are compared;

[0158] The failure assessment curve FAC that is significantly lower than other standards is selected.

[0159] The failure assessment diagram FAD contains a failure assessment curve FAC and a failure assessment point (L r , K r ). Different FACs are roughly similar, but also have some differences according to different situations; the failure assessment diagram FAD is a double-criterion evaluation method, which is a combination of brittle fracture criterion and plastic instability criterion, in which the fracture ratio K r represents the degree of brittle fracture of the structure containing defects, and the load ratio L r represents the degree of plastic instability of the structure containing defects, as shown in FIG. 1, which is a schematic diagram of the FAD failure assessment method. Figure 5

[0160] After comparing the FAC curves of multiple standards, the applicant found that the FAC curve of API 579 standard is significantly lower than the curves of other standards; therefore, the FAC curve of API 579 is selected, which is conservative, and the results evaluated by this curve are safer.

[0161] In this example, the parameter fracture ratio K r is calculated based on M integral, and the parameter load ratio L r is calculated based on elastic-plastic J integral, which includes the following steps:

[0162] The fracture ratio K r corresponds to the brittle fracture criterion, and the fracture ratio K r is calculated by the following formula:

[0163]

[0164] wherein, Kmax—maximum stress intensity factor at the crack front; K C —fracture toughness of stress intensity factor for evaluation; J IC —fracture toughness of J integral; f1—safety factor; E represents elastic modulus; μ represents Poisson's ratio;

[0165] The load ratio L r corresponds to the plastic instability criterion, and the parameter load ratio L r is calculated by the following formula:

[0166] ​

[0167] wherein: J max — maximum value of the elastic-plastic J-integral at the crack front; J IC — J-integral fracture toughness of the PE material.

[0168] To ensure the reliability of the calculation method, the method for verifying and correcting the parameter load ratio L r based on the elastic-plastic J-integral includes:

[0169] According to the first point load experiment, a 1:1 finite element simulation model of the complete PE pipe and the defective PE pipe is established;

[0170] wherein, the gasket is set as a three-dimensional entity, and the gasket boundary condition is set as a fixed constraint; the spherical indenter is a three-dimensional entity, and the spherical indenter is set with a downward displacement load in the pipeline direction;

[0171] the PE pipe, the gasket and the spherical indenter are set with surface-to-surface contact; a crack with the same size as the complete PE pipe and the defective PE pipe in the first point load experiment is set on the pipeline as a pipeline defect, and the local grid of the defect is densified; the crack front shape is the crack shape observed in the experiment for characterizing the crack propagation of the PE pipe under the point load;

[0172] According to the simulation result, the load ratio L r is calculated, and the L r =1 currently in the critical state is compared and corrected;

[0173] To verify the J-integral-based load ratio calculation method after correction, the same pipeline material and experimental scheme as the first point load experiment are adopted, the defect size of the PE pipe is changed, and the point load experiment on the defective PE pipe is performed again, which is defined as the second point load experiment; the second point load experiment and simulation data are used to verify the parameter load ratio L r after correction.

[0174] A crack with the same size as in the first point load experiment is set on the pipeline model as a pipeline defect, with a depth of 1mm and a length of 44mm, and the local grid of the defect is densified;

[0175] The J-integral-based load ratio calculation method can obtain that the load ratio L r =J max / J IC =0.79. However, under the same load condition, the defective PE pipe under the experimental condition is in the critical state, and the load ratio L r =1. Therefore, the simulation result is less than the experimental result, and the J-integral-based load ratio calculation method needs to be corrected.

[0176] The fracture ratio K rThe calculation method is used to calculate the J integral fracture toughness J of PE IC A correction safety factor is set to obtain the J integral fracture toughness J for evaluation C The simulation result at the critical crack initiation is corrected to 1 by the correction safety factor, and the correction safety factor f2 = 1.26 is obtained. In order to make the evaluation result more safe and reliable, the correction safety factor f2 = 1.3 is taken. Finally, the correction load ratio calculation formula shown below is obtained:

[0177]

[0178] Wherein, J max is the maximum J integral value along the crack front path; J C is the J integral fracture toughness for evaluation, J C = 10.18 kJ / m 2 ; J IC is the J integral fracture toughness; and f2 is the correction safety factor.

[0179] In the example, in order to verify the modified J integral-based load ratio, two defective pipes are prepared for the second point load test. The defect depth of the defective PE pipe 1 is 1 mm, and the length is 54 mm. The defect depth of the defective PE pipe 2 is 1 mm, and the length is 80 mm. The rest of the operation is the same as the first point load test.

[0180] According to the simulation models of the defective pipes 1 and 2, the load ratios L r are calculated and compared with the critical state value.

[0181] Figure 6 The J integral along the crack front of the PE pipe with different defect sizes is shown in the figure. The J integral along the crack front of the defective PE pipe changes in a concave manner, and the J integral values at both ends are the largest. The J integral value along the crack front increases as the defect size of the PE pipe increases. The change of J integral is related to the stress field at the crack tip, which indicates that the stress at both ends of the crack front is the largest, and it is the dangerous position for crack initiation and propagation.

[0182] The load ratio of the defective PE pipe 1 is 10.75 / 10.18 = 1.056, and the load ratio of the defective PE pipe 2 is 11.12 / 10.18 = 1.092. The load ratios obtained by using the modified calculation method are all greater than the critical value 1, but they are close to the critical value, and the maximum error is 9.2%, which is within the allowable error range. The verification-correction-reverification method ensures the reliability of the calculation method, so the load ratio L r based on the elastic-plastic J integral calculation parameters in the example is reliable. r The method is reliable.

[0183] Example 3

[0184] A safety evaluation method for a defective PE pipeline under point load, based on examples 1 or 2, the method for establishing a three-dimensional simulation model of the PE pipeline under several different point loads comprises:

[0185] According to the actual working conditions of the buried gas pipeline, the pipeline internal pressure, the pipeline size, the defect size, the point load size, and the burial depth are determined.

[0186] For the pipeline internal pressure, the pipeline size, the defect size, the point load size, and the burial depth, a single factor parameter is changed, and the values of other parameters are fixed to obtain a simulation model of the defective pipeline under several point loads.

[0187] The mesh division adopts eight-node linear hexahedral element division of the sleeve, and the required mesh size for calculation is determined through mesh independence verification. The hard object is simplified as a spherical stone block, and the defect is on the inner surface of the pipeline directly above the hard object, and the hard object is completely fixed.

[0188] A 1 / 2 three-dimensional point load-pipe-soil model is established, the model 1 / 2 section and the left and right sides are set to be symmetrical constraints, and the upper surface of the model is set to be a settlement displacement, and the lower surface is set to be a fixed constraint.

[0189] In this embodiment, the method for extracting the crack front stress intensity factor K I and the maximum value J max of J integral comprises:

[0190] According to the results obtained from the simulation model under the influence of different factors, the crack front stress intensity factor K I and the maximum value J max of elastic-plastic J integral are extracted.

[0191] According to the results obtained from the simulation model under the influence of different factors, the crack front stress intensity factor K I and the maximum value J max of elastic-plastic J integral are extracted in the simulation software.

[0192] According to the parameter fracture ratio K r and the parameter load ratio L r , the fracture ratio K r and the load ratio L r are calculated respectively to obtain the failure evaluation points (L r , K r ) under different analysis conditions.

[0193] The method for judging whether the failure evaluation points are in the failure evaluation curve FAC comprises:

[0194] According to the calculated failure evaluation points (L r , K r) and a selection failure assessment curve (FAC) to form a failure assessment diagram (FAD) for evaluating the safety of the defective pipeline under the point load.

[0195] The applicant takes a PE pipeline with a buried depth of 5 m, an outer diameter of 110 mm, a wall thickness of 10 m and a hard object with a size of 60 mm causing a point load as a simulation object, and bears an internal pressure of 0, 0.2, 0.4, 0.6, 0.8 and 1.0 MPa to explore the safety of the pipeline when the internal pressure of the pipeline changes. The following table shows the failure assessment parameter values of the defective PE pipeline when the internal pressure of the pipeline changes, which are calculated according to the finite element results, the evaluation points are calculated and the failure assessment diagram is drawn.

[0196]

[0197] Figure 7 The failure assessment diagram obtained by the simulation results of different internal pressures of the pipeline in the embodiment, the safety evaluation method of the PE pipeline in the embodiment calculates the fracture ratio K r， The value is greater than 1, and the evaluation point is outside the evaluation curve, which is due to the large plastic deformation of the crack tip, resulting in the large stress intensity factor value and the large fracture ratio K r Therefore, when the PE gas pipeline is erected in the early stage, the pipeline periphery and the backfill soil should be ensured to be free of hard objects, and the buried depth should be appropriately reduced to ensure the safety of gas transmission. After the pipeline cannot continue to serve, the damaged PE pipeline needs to be repaired or replaced in time. The embodiment not only can evaluate whether the buried PE pipeline is safe, but also can reflect the failure degree and safety degree of the pipeline.

[0198] The above-described specific embodiments have further detailed the purposes, technical solutions and beneficial effects of the present application, and it should be understood that the above-described specific embodiments are merely specific embodiments of the present application and are not used to limit the protection scope of the present application, and any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application should be included in the protection scope of the present application.

[0199] It should be noted that, in this document, the relationship terms such as first and second are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply that there is any such actual relationship or order between these entities or operations. Moreover, the terms "include", "contain" or any other variants thereof are intended to cover non-exclusive inclusion, so that the process, method, article or equipment including a series of elements not only includes those elements, but also includes other elements not explicitly listed or inherent to such process, method, article or equipment.

Claims

1. A safety evaluation method for defective PE pipelines under point load, characterized in that: include: Obtaining constitutive model parameters and mechanical parameters of the PE pipe material based on the formation settlement rate; the mechanical parameters include fracture toughness parameters; Conduct the first point load test to determine the defect form used to characterize the PE pipe under point load; Select the failure assessment curve FAC used in the failure assessment diagram; Determine the failure assessment point (L r , K r ) calculation method, based on the M integral calculation parameter fracture ratio K r , based on the elastic-plastic J integral to calculate the parameter load ratio L r ; Verify and correct the parameter load ratio L r , including verification and correction of the parameter load ratio L through the first point load test and simulation data r , and then conduct a second point load test and simulation to verify the modified parameter load ratio L r ; According to different analysis parameters, a number of PE pipe three-dimensional simulation models under different point loads are established; The three-dimensional simulation model of PE pipe under different point load parameters was simulated and analyzed respectively to extract the stress intensity factor K of the crack front. I and the maximum value of the elastic-plastic J integral, and according to the parameter fracture ratio K r and parameter load ratio L r The calculation method calculates the failure assessment point; According to the failure assessment curve FAC and failure assessment point (L r , K r ) Draw a graph to determine whether the failure assessment point is within the failure assessment curve FAC. If it is, it proves that the pipeline is safe. If it is not within the curve, it proves that the pipeline has failed and can no longer continue to serve; The verification and correction parameter load ratio L r The methods include: Based on the first point load test, a 1:1 finite element simulation model of the intact PE pipe and the defective PE pipe was established; The pad is set as a three-dimensional entity, and the boundary condition of the pad is set as a fixed constraint; the spherical pressure head is a three-dimensional entity, and the spherical pressure head is set with a downward displacement load along the pipeline direction; The PE pipe is placed in surface-to-surface contact with the backing plate and the spherical indenter. A crack of the same size as the intact PE pipe and the defective PE pipe described in the first point load test is introduced into the pipe as a pipe defect, and the local mesh density of the pipe at the defect is increased. The crack front shape is the same as the crack shape observed in the experiment for characterizing crack propagation in the PE pipe under point load. Calculate the load ratio L based on the simulation results r , will be in the critical state of L r =1 for comparison correction; In order to verify the revised load ratio calculation method based on J integral, the same pipe material and experimental scheme as the first point load experiment were used, the defect size of the PE pipe was changed, and the point load experiment and simulation were carried out on the defective PE pipe again, which was defined as the second point load experiment. The second point load experiment and simulation data were used to verify the revised parameter load ratio L r .

2. The safety evaluation method for defective PE pipes under point load according to claim 1, characterized in that: The method for determining the constitutive model parameters of the PE pipe material includes: Dumbbell-shaped specimens were cut from PE pipes. The specimen thickness was the same as the pipe wall thickness. Uniaxial tensile tests were performed at different rates. The true stress-strain curve related to the stretching rate was obtained based on the experimental results. Based on the true stress-true strain relationship curve, m and n at different stretching rates are fitted by the following formula: Where σ t is the true stress, ε t is the true strain, m and n are fitting constants; The stress-strain relationship of the PE material is determined according to the settlement rate of the stratum to be evaluated, and the elastic parameters and plastic parameters of the material are calculated.

3. The safety evaluation method for defective PE pipes under point load according to claim 1, characterized in that: Methods for determining the fracture toughness parameters of PE pipe materials include: Two types of CT specimens were cut from the PE pipe, namely notched and unnotched CT specimens. The notched CT specimens were loaded to the maximum load, while the unnotched CT specimens were loaded to a load 10% greater than the load of the notched CT specimens. Displacement-load relationship curves were obtained. Based on the displacement-load relationship curve, the J integral of the material and the corresponding crack extension displacement are calculated using the following formula: U=U T -IN I η=2+0.522·(b0 / W) y=2·σ y (Δa-0.2) Where: J represents the elastic-plastic J integral; U T is the total energy during crack propagation, which is obtained by integrating the area enclosed by the load and opening displacement curve of the notched CT specimen; U I is the energy required for plastic deformation during crack propagation, which is obtained by integrating the area enclosed by the load-displacement curve of the calibrated CT specimen; U is the energy required for crack propagation; η is a dimensionless function; B is the thickness of the notched CT specimen; W is the width from the center of the notched CT specimen clamping hole to the surface of one side of the specimen; a0 is the distance from the center of the notched CT specimen clamping hole to the front edge of the prefabricated crack; b0 is the width from the sharp corner of the notched CT specimen to the surface of one side of the specimen; σ y is the yield stress; Δa is the crack extension displacement; y is the blunting curve; The JR curve is calculated by the following formula Where: C1 and C2 are experimental fitting constants; The J-integral fracture toughness J of the material is obtained through the JR curve IC .

4. The safety evaluation method for defective PE pipes under point load according to claim 1, characterized in that: The method for determining the defect form for characterizing the PE pipe under the action of point load includes: Point load tests were conducted on intact PE pipes and defective PE pipes respectively; A defect in the axial direction is made inside the intact PE pipe using a utility knife to prepare a defective PE pipe; Fix the intact PE pipe and the defective PE pipe on the pad, making sure the defect is at the highest point; Add blocks around the pipe to secure it and prevent it from rolling when loaded; Use constant rate loading, the spherical indenter loading rate is 1mm / min, and the maximum loading displacement is 50mm; By comparing and analyzing the surface morphologies of intact PE pipes and defective PE pipes after the experiment, the cross-sectional morphology, corresponding crack propagation mechanism and morphology of PE pipes under point load are analyzed.

5. The safety evaluation method for defective PE pipes under point load according to claim 1, characterized in that: The method for selecting the failure assessment curve FAC used in the failure assessment diagram includes: According to the failure assessment curve FAC establishment method in different standards, draw the corresponding failure assessment curve of PE material; Determine the parameter load ratio L r cutoff value; Compare failure assessment curves FAC under different standards; Select the failure assessment curve FAC which is significantly lower than other standards.

6. The safety evaluation method for defective PE pipes under point load according to claim 1, characterized in that: The fracture ratio K calculated based on the M integral r , based on the elastic-plastic J integral to calculate the parameter load ratio L r The methods include: Parameter fracture ratio K r Corresponding to the brittle fracture criterion, the parameter fracture ratio K is calculated by the following formula: r : in, —maximum value of stress intensity factor at crack front; K C —Evaluation of fracture toughness using stress intensity factor; J IC —J integral fracture toughness; f1—safety factor; E represents elastic modulus; μ represents Poisson’s ratio; Load ratio L r Corresponding to the plastic instability criterion, the parameter load ratio L is calculated by the following formula: r : Where: J max —maximum value of elastic-plastic J integral at the crack front; J IC —J integrated fracture toughness.

7. The safety evaluation method for defective PE pipes under point load according to claim 1, characterized in that: The method of establishing a 3D simulation model of a PE pipe under several different point loads according to different analysis parameters includes: According to the actual working conditions of the buried gas pipeline, determine the pipeline internal pressure, pipeline dimensions, defect dimensions, point load dimensions, and burial depths; For several pipeline internal pressures, several pipeline sizes, several defect sizes, several point load sizes, and several burial depths, a single factor parameter is changed, and the values ​​of other parameters are fixed to obtain a simulation model of defective pipelines under several point loads; The casing is meshed using eight-node linear hexahedral elements, and the mesh independence verification is used to determine the mesh size required for the calculation. The hard object is simplified to a spherical stone, and the defect is located on the inner surface of the pipe directly above the hard object. The hard object is completely fixed. A 1 / 2 three-dimensional point load-pipe-soil model was established, symmetrical constraints were set on the 1 / 2 section and the left and right sides of the model, settlement displacement was set on the upper surface of the model, and fixed constraints were set on the lower surface.

8. The safety evaluation method for defective PE pipes under point load according to claim 1, characterized in that: The extracted crack front stress intensity factor K I The methods for maximizing the elastic-plastic J integral include: According to the results of the simulation model under the influence of different factors, the stress intensity factor K of the crack front is extracted in the simulation software. I and the maximum value of elastic-plastic J integral J max ; According to the parameter fracture ratio K r and parameter load ratio L r Calculation method, respectively calculate the fracture ratio K r , load ratio L r , and obtain the failure assessment point (L r , K r ).

9. The safety evaluation method for defective PE pipes under point load according to claim 1, characterized in that: Methods for determining whether a failure assessment point is within the failure assessment curve FAC include: According to the obtained failure assessment point (L r , K r ) and the failure assessment curve FAC, constitute the failure assessment diagram FAD, which is used to evaluate the safety of defective pipelines under point loads; Determine the failure assessment point (L r , K r ) is within the failure assessment curve FAC. If it is, it proves that the pipeline is safe. If it is not within the curve, it proves that the pipeline has failed and can no longer continue to serve.

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