Plastic strain evaluation method for excessive deformation of nuclear-grade pipeline

Abstract

The invention discloses a plastic strain evaluation method for excessive deformation of a nuclear-grade pipeline, which comprises the following steps of: 1, carrying out a monotonic tensile test on a nuclear-grade pipeline material to obtain a material stress-strain curve at a test temperature; 2, establishing a cyclic plasticity constitutive model of the material, introducing a nonlinear follow-up hardening law, and determining material constants of the cyclic plasticity constitutive model based on test data; 3, establishing a nuclear-grade pipeline structure analysis model in finite element analysis software, endowing material attributes, inputting a displacement boundary and applying a cyclic load; 4, solving to obtain the maximum cumulative plastic strain of the nuclear-grade pipeline structure under the action of the cyclic load; and 5, judging whether the maximum cumulative plastic strain of the nuclear-grade pipeline structure meets the plastic evaluation criterion requirement of excessive deformation or not. The method has the technical effects that the conservative property of the elastic design of the nuclear-grade pipeline can be reduced, more accurate cyclic deformation is obtained, the structural safety is guaranteed, and the economical efficiency is improved.

Description

Technical Field

[0001] The invention relates to a nuclear reactor component material performance evaluation technology, and in particular to a nuclear-grade pipeline excessive deformation plastic strain evaluation method. Background Art

[0002] Nuclear-grade piping is an important barrier to contain radioactive materials in nuclear reactor systems, and is also the main artery connecting various main equipment and transporting coolants. Due to transient fluctuations such as starting and stopping reactors and power regulation in power plants, nuclear-grade piping is subjected to high-temperature and high-pressure cyclic reciprocating loads, and local plastic deformation continues to accumulate, which may lead to excessive deformation failure, thus threatening the safe operation of nuclear reactors. Therefore, the risk of excessive deformation failure of nuclear-grade piping should be fully considered during design.

[0003] Elastic analysis and design method is widely used in engineering. This method is based on the idea of ​​"stress classification". It prevents excessive deformation by limiting the primary and secondary stress strength of nuclear-grade pipeline structures to meet the limit requirements. The calculation process is relatively simple. However, for some discontinuous areas of nuclear-grade pipeline structures with stress concentration or facing a more severe load environment, the stress classification method of elastic analysis exposes limitations. The elastic calculation results are highly conservative and are prone to exceeding the design limit. At this time, the engineering can prevent excessive deformation and failure of nuclear-grade pipeline structures by modifying the nuclear-grade pipeline structure design or improving the load environment, but it will lead to reduced economic efficiency. On the other hand, there are many pipelines in the nuclear reactor system and the layout is complex. For some complex structural positions of nuclear-grade pipelines, such as the connection between nuclear-grade pipelines and supports or nozzles, stress classification may be ambiguous. Elastic analysis using stress as a parameter to evaluate excessive deformation is not very accurate. Summary of the invention

[0004] In view of the problems existing in the prior art, the technical problem to be solved by the present invention is to provide a plastic strain assessment method for excessive deformation of nuclear-grade pipelines, which effectively reduces the conservatism of the elastic design of nuclear-grade pipelines while ensuring the safety of nuclear-grade pipelines, obtains more accurate cyclic deformation, and improves the economic efficiency of the nuclear-grade pipeline structure.

[0005] The technical problem to be solved by the present invention is achieved through such a technical solution, which comprises the following steps:

[0006] Step 1: Conduct a monotonic tensile test on nuclear-grade pipeline materials to obtain a stress-strain curve of the material at the test temperature;

[0007] Step 2: Establish a cyclic plastic constitutive model of the material, introduce a nonlinear kinematic hardening law, and determine the material constants of the cyclic plastic constitutive model based on test data;

[0008] Step 3: Establish a nuclear-grade pipeline structural analysis model in the finite element analysis software, assign material properties, input displacement boundaries and apply cyclic loads;

[0009] Step 4, solving and obtaining the maximum accumulated plastic strain of the nuclear-grade pipeline structure under cyclic load;

[0010] Step 5: Determine whether the maximum accumulated plastic strain of the nuclear-grade pipeline structure meets the requirements of the plasticity assessment criteria for excessive deformation; if so, the nuclear-grade pipeline structure meets the excessive deformation assessment requirements and will not fail due to excessive deformation.

[0011] The present invention establishes a cyclic plastic constitutive model of the material, introduces a nonlinear kinematic hardening law, and obtains a more accurate cumulative plastic deformation of the nuclear-grade pipeline structure under cyclic loads through finite element analysis. The technical effects of the present invention are:

[0012] 1. The cyclic plastic constitutive model, parameter determination method and calculation process of the present invention are simple and easy to operate, have high calculation accuracy, and have good practicality and operability.

[0013] 2. When the calculation results of the primary and secondary stress strength of the nuclear-grade pipeline structure using the traditional elastic analysis design method exceed the design limit, the present method is used to ensure that the nuclear-grade pipeline structure will not fail due to excessive deformation, reduce the conservatism of the elastic analysis, and do not need to modify the nuclear-grade pipeline structure design or reduce the operating load. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings of the present invention are as follows:

[0015] Figure 1 A flow chart invented for the method;

[0016] Figure 2 Schematic diagram of key points for stress-plastic strain curve;

[0017] Figure 3 Hexahedral element model for simulating the uniaxial tensile behavior of materials;

[0018] Figure 4 This is a comparison chart between the material tensile stress-strain curve test results and the simulation results;

[0019] Figure 5 Schematic diagram of displacement and load boundary conditions for the analysis model;

[0020] Figure 6 is the temperature and pressure transient load under a single cycle;

[0021] Figure 7 It is the plastic strain cloud diagram. DETAILED DESCRIPTION

[0022] The present invention will be further described below in conjunction with the accompanying drawings and embodiments:

[0023] like Figure 1 As shown, the method invention comprises the following steps:

[0024] Step 1: Conduct a monotonic tensile test on nuclear-grade pipeline materials to obtain the material stress-strain curve at the test temperature.

[0025] In the embodiment, austenitic stainless steel, a nuclear-grade pipeline material, is used as the test object, and austenitic stainless steel is processed into a standard high-temperature specimen according to GB / T 4338-2006 standard. The single-pull test is performed using an MTS electronic universal testing machine. The test temperature should be able to envelop the transient temperature load (referring to the maximum value of the transient temperature). The test temperature of this embodiment is 300°C. The stress-strain curve of austenitic stainless steel is recorded and output by the testing machine.

[0026] Step 2: Establish a cyclic plastic constitutive model of the material, introduce nonlinear kinematic hardening law, and determine the material constants.

[0027] The cyclic plastic constitutive model of the material is established as follows:

[0028] According to the classical plasticity theory recorded in the book "Inelastic Constitutive Theory and Its Finite Element Implementation", Kang Guozheng, pages 57-62, Southwest Jiaotong University Press, August 2010, the main governing equations of the cyclic plasticity constitutive model are composed of strain decomposition, elastic strain relationship, plastic flow law and yield function equation:

[0029] ε=ε e +ε p (1)

[0030] ε e =D -1 :σ (2)

[0031]

[0032] In equations (1)-(4), ε is the total strain tensor; ε e is the elastic strain tensor; ε p is the plastic strain tensor; p is the accumulated plastic strain; σ is the stress tensor; D is the elastic tensor; λ is the plastic multiplier; n represents the direction of plastic flow; F y is the yield function; s is the deviatoric stress tensor; α is the back stress tensor; S y is the yield strength.

[0033] Based on the theoretical framework of the nonlinear kinematic hardening model proposed by Chaboche (the literature "Constitutive equations for cyclic plasticity and cyclic viscoplasticity", Chaboche, JL, International Journal of Plasticity, 1989, 5: 149-188.), a nonlinear kinematic hardening law is introduced into the cyclic plasticity constitutive model. The present invention divides the back stress tensor α into three superpositions, and the equation of the nonlinear kinematic hardening law is composed of the following equations:

[0034]

[0035] In formula (5) and (6), C i , γ i is the material constant, The dots on the letters represent derivatives.

[0036] like Figure 2 As shown in the figure, on the stress-strain curve obtained from the test data in step 1, three key points are selected according to the slope change, and the material constant C i , γ i The calculation formula is as follows:

[0037]

[0038] In formula (7), σ i and ε i p are the stress value and plastic strain corresponding to the i-th point respectively.

[0039] The cyclic plastic constitutive model with determined parameters is used to simulate the monotonic tensile behavior of austenitic stainless steel at 300°C. The simulation process is as follows: a hexahedral unit model simulating the uniaxial tensile behavior of uniform materials is established in the ANSYS software, the cyclic plastic constitutive model with determined parameters is input into the ANSYS material module, and the normal displacement of the three mutually perpendicular surfaces of the unit model is constrained. A tensile load is applied to one of the other three surfaces that are not constrained in normal displacement to simulate the uniaxial tensile behavior of the material. Figure 3 As shown, the hexahedral element model that simulates the uniaxial tension behavior of the material.

[0040] Comparison of simulation results with test results Figure 4 As shown, from Figure 4 It can be seen that the test value is consistent with the simulation curve and the agreement is good.

[0041] Step 3: Establish a nuclear-grade pipeline structural analysis model in the finite element analysis software, assign material properties, constrain displacement boundaries and apply cyclic loads.

[0042] A typical nozzle structure of a nuclear reactor system was selected for analysis. According to the axisymmetric characteristics of the structure, a two-dimensional axisymmetric analysis model was established in the finite element analysis software ANSYS. The material properties were assigned to the material cyclic plastic constitutive model and its material constant (C i , γ i ), displacement constraints and cyclic loads are applied to the analysis model, wherein the cyclic loads include temperature cycle transient loads and pressure cycle transient loads.

[0043] According to the actual stress characteristics of nuclear-grade pipeline structures, the displacement and load boundary condition diagram of the analysis model can be found in Figure 5 , for the assumed pressure and temperature transient loads, see Figure 6 , the transient cycle number is 100 times.

[0044] Step 4: Solve and obtain the maximum accumulated plastic strain of the nuclear-grade pipeline structure under cyclic load.

[0045] The nuclear-grade pipeline structure analysis model is run on the ANSYS software platform to obtain the strain cloud diagram, such as Figure 7 As shown, the maximum plastic strain under cyclic load is 3.33%, and the maximum plastic strain occurs at the inner corner where the pipeline is connected to the nozzle.

[0046] Step 5: Determine whether the maximum cumulative plastic strain of the nuclear-grade pipeline structure meets the plasticity assessment criteria for excessive deformation; if so, the nuclear-grade pipeline structure meets the excessive deformation assessment requirements and will not fail due to excessive deformation. In this embodiment, the plasticity assessment criteria for excessive deformation are: the maximum cumulative plastic strain of the nuclear-grade pipeline structure under cyclic load is less than 5%.

[0047] In the embodiment, the maximum cumulative plastic strain of the nuclear-grade pipeline structure is 3.33%, which is less than the strain limit of 5%. Therefore, the nuclear-grade pipeline structure involved in the analysis model of this embodiment meets the excessive deformation assessment requirements, and the structural design under the analyzed load conditions is reasonable.

Claims

1. A method for evaluating plastic strain of excessive deformation of a nuclear-grade pipeline, characterized in that: The following steps are involved: Step 1: Conduct a monotonic tensile test on nuclear-grade pipeline materials to obtain a stress-strain curve of the material at the test temperature; Step 2: Establish a cyclic plastic constitutive model of the material, introduce a nonlinear kinematic hardening law, and determine the material constants of the cyclic plastic constitutive model based on test data; Step 3: Establish a nuclear-grade pipeline structural analysis model in the finite element analysis software, assign material properties, input displacement boundaries and apply cyclic loads; Step 4, solving and obtaining the maximum accumulated plastic strain of the nuclear-grade pipeline structure under cyclic load; Step 5: Determine whether the maximum accumulated plastic strain of the nuclear-grade pipeline structure meets the requirements of the plasticity assessment criteria for excessive deformation; If so, the nuclear-grade piping structure meets the excessive deformation assessment requirements and will not fail due to excessive deformation.

2. The method for evaluating plastic strain of excessive deformation of nuclear-grade pipelines according to claim 1 is characterized in that: In step 1, the monotonic tensile test is performed as follows: the metal material is processed into a standard high-temperature specimen in accordance with GB / T 4338-2006 standard, the test temperature envelopes the transient temperature load, and a stress-strain curve is obtained after completing the uniaxial tensile test.

3. The method for evaluating plastic strain of excessive deformation of nuclear-grade pipelines according to claim 1 is characterized in that: In step 2, the cyclic plastic constitutive model of the material is: e=e e +e p e e =D -1 :s In the above formula, ε is the total strain tensor; ε e is the elastic strain tensor; ε p is the plastic strain tensor; p is the accumulated plastic strain; σ is the stress tensor; D is the elastic tensor; λ is the plastic multiplier; n represents the plastic flow direction; F y is the yield function; s is the deviatoric stress tensor; α is the back stress tensor; S y is the yield strength; The back stress tensor α is divided into three terms, and the nonlinear kinematic hardening law equation is introduced as follows: Where, i = 1, 2, 3; C i , γ i is the material constant; 4. The method for evaluating plastic strain of excessive deformation of a nuclear-grade pipeline according to claim 3 is characterized in that: The material constant C of the kinematic hardening law equation i , γ i The method for determining is: obtain the stress-strain curve based on the test data in step 1, select three key points on the curve according to the change of slope, C i , γ i The calculation formula for the material constant is as follows: In the formula, σ i and are the stress value and plastic strain corresponding to the i-th point respectively.

5. The method for evaluating plastic strain of excessive deformation of a nuclear-grade pipeline according to claim 4 is characterized in that: In step 3, a two-dimensional axisymmetric analysis model is established in the finite element analysis software ANSYS, the material cyclic plastic constitutive model and its parameters obtained in step 2 are input into the material module of ANSYS, and displacement constraints and cyclic loads are applied to the analysis model, wherein the cyclic loads include temperature cycle transient loads and pressure cycle transient loads.

6. The method for evaluating plastic strain of excessive deformation of a nuclear-grade pipeline according to claim 5, characterized in that: In step 5, the plasticity assessment criterion for excessive deformation is: the maximum accumulated plastic strain of the nuclear-grade pipeline structure under cyclic load is less than 5%.

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