Flange structure stress relaxation prediction method based on short-time test data

Abstract

The invention provides a flange structure stress relaxation prediction method based on short-time test data, and the method comprises the following steps: S1, a material high-temperature short-time stress relaxation test: preparing one or more material sample blocks, carrying out the high-temperature short-time stress relaxation test on the material sample blocks, and obtaining the stress relaxation data of the material sample blocks under different high-temperature conditions; s2, establishing a high-temperature creep constitutive model: on the basis of performing curve fitting on the stress relaxation data in the step S1, establishing the high-temperature creep constitutive model of the material according to a logarithmic creep constitutive equation; s3, temperature field distribution calculation of the flange structure: constructing a finite element model of the flange structure, and calculating temperature field distribution at a specified temperature to obtain a temperature field of the flange structure; and S4, predicting the high-temperature long-time stress relaxation behavior of the flange structure: performing thermal-solid finite element calculation of the flange structure based on the temperature field of the flange structure in the step S3, and predicting the stress relaxation of the flange structure under the high-temperature long-time service condition.

Description

Technical Field

[0001] The invention relates to the field of thermo-solid finite element analysis, and in particular to a method for predicting high-temperature long-term creep stress relaxation based on short-term stress relaxation test data. Background Art

[0002] The creep behavior of metal materials is an important mechanism for plastic deformation when they are in long-term service under high temperature conditions. Creep is the plastic deformation of metal materials that occurs over time under constant stress, and is the deformation mechanism of materials at high temperatures. Creep behavior has a significant impact on the service performance of materials. In industrial fields such as power stations, steelmaking, and oil refining, the creep behavior of metal materials will cause their working stress to decrease over time, thereby affecting their performance and safety. Therefore, understanding the creep behavior of metal materials is of great significance for predicting their service performance under high temperature conditions.

[0003] In order to predict the creep behavior of materials, the main methods currently used are theoretical models, experimental methods, numerical simulation methods and combined methods. The theoretical model method is mainly based on the microscopic creep mechanism, using molecular dynamics or thermodynamic models to construct the creep model of the material. The creep model constructed by the theoretical model is usually complex and has a large amount of calculation, which is not suitable for occasions with large amounts of calculations. The experimental method mainly uses standard material specimens for creep tests and establishes a creep model based on creep test data, but the creep model of this method can often only be based on specific materials and can only be used to predict specific life. The numerical simulation method mainly uses the finite element method and the creep constitutive equation to construct the creep model of the material, and predicts the creep behavior of the material through numerical simulation calculations. This method can predict different materials based on the creep constitutive equation and has a wide range of applicability, but it usually requires a large amount of creep test data. The combined method is mainly a combination of experimental methods and numerical simulation methods. Based on creep test data and creep constitutive equations, a creep model is constructed to predict the creep behavior of the material. This method can predict the creep behavior of the material, but it requires a large amount of creep test data.

[0004] However, the above methods usually require a large amount of creep test data, and when building a creep model for a specific material, a large amount of creep test data needs to be obtained. For complex structures commonly used in industry, such as flange structures, it is usually impossible to obtain their creep test data. In addition, for specific materials commonly used in industry, such as 316H alloy and GH4169 alloy, the creep activation energy of the materials is low, which makes it difficult to fit their creep model parameters through creep test data, affecting the accuracy and reliability of the prediction.

[0005] In view of the above technical problems, the present invention is specially introduced. Summary of the invention

[0006] The main purpose of the present invention is to provide a flange structure stress relaxation prediction method based on short-time test data, which solves the problem of long research cycle and huge capital consumption in high-temperature stress relaxation test of flange structure.

[0007] In order to achieve the above object, the present invention provides a method for predicting flange structure stress relaxation based on short-term test data, comprising the following steps:

[0008] Step S1, high-temperature short-time stress relaxation test of materials: preparing one or more material samples, performing a high-temperature short-time stress relaxation test on the material samples, and obtaining stress relaxation data of the material samples under different high-temperature conditions;

[0009] Step S2, establishing a high temperature creep constitutive model: on the basis of curve fitting of the stress relaxation data in step S1, a high temperature creep constitutive model of the material is established according to the logarithmic creep constitutive equation;

[0010] Step S3, calculation of the temperature field distribution of the flange structure: constructing a finite element model of the flange structure, and calculating the temperature field distribution at a specified temperature to obtain the temperature field of the flange structure;

[0011] Step S4, prediction of the long-term high-temperature stress relaxation behavior of the flange structure: based on the temperature field of the flange structure in step S3, a thermal-solid finite element calculation of the flange structure is performed to predict the stress relaxation of the flange structure under long-term high-temperature service conditions.

[0012] The following is a further optimization of the above scheme by the present invention:

[0013] Furthermore, the stress relaxation prediction method also includes a computational model establishment step: based on the high temperature creep constitutive model in step S2, the creep properties of the material sample are defined, thereby establishing a computational model. The computational model establishment step is between step S2 and step S3.

[0014] Furthermore, in step S4, the finite element model obtained in step S3 may be imported into simulation software, and the computational model may be loaded to predict stress relaxation of the flange structure under high temperature and long-term service conditions.

[0015] Further, in step S2, the curve fitting of the stress relaxation data is the curve fitting of the cubic delay function.

[0016] Further, the specified temperature load in step S3 includes a periodic temperature load and / or a constant temperature load.

[0017] Furthermore, the manner of applying the periodic temperature load includes the frequency and amplitude of the temperature change.

[0018] Furthermore, in step S4, when the finite element model is imported into the simulation software, it also includes presetting the stress relaxation time and the bolt preload force in the simulation software.

[0019] Furthermore, the material of the material sample is 316H alloy and / or GH4169 alloy.

[0020] Furthermore, the cubic delay function expression in step S2 is as follows: Among them, σ is the instantaneous stress, σ r is the residual stress, t is the stress relaxation time, B1, τ1, B2, τ2, B3, τ3 are all material constants of the stress relaxation process.

[0021] Furthermore, the logarithmic creep constitutive equation in step S2 is: in, is the creep strain rate, E is Young's modulus, σ is the instantaneous stress, σ0 is the initial load, and the parameters parameter t is the stress relaxation time.

[0022] Furthermore, when the material sample is made of 316H alloy, the parameter a is set to have an exponential relationship with the temperature T, and the parameter b is set to have a linear relationship with the temperature T; the expressions of the parameters a, b and temperature T of the 316H alloy are as follows:

[0023]

[0024] Furthermore, when the material sample is made of GH4169 alloy, the parameter a is set to have a linear relationship with the temperature T, and the parameter b is set to have an exponential relationship with the temperature T; the expressions of the parameters a, b and temperature T of the GH4169 alloy are as follows:

[0025]

[0026] Furthermore, in step S3, first material parameters of the flange structure may be input, wherein the first material parameters include one or more of material density, Poisson's ratio, thermal conductivity and convection heat transfer coefficient.

[0027] Furthermore, in step S3, an upper limit temperature and a lower limit temperature may be applied to the inner wall of the flange structure.

[0028] Furthermore, the upper limit temperature is 500°C and the lower limit temperature is 420°C.

[0029] Furthermore, in step S4, second material parameters of the flange structure may be input, and the second material parameters include one or more of material density, elastic modulus, Poisson's ratio, plastic section stress-strain curve, and linear expansion coefficient.

[0030] By applying the technical solution of the present invention, at least the following beneficial effects are achieved:

[0031] 1. The present invention adopts a cubic delay function to supplement the data points of the initial stage of stress relaxation, which has obvious abrupt changes due to rapid relaxation, thereby solving the sharp corner phenomenon and making the curve smoother.

[0032] 2. The present invention establishes a logarithmic creep constitutive model and determines corresponding parameters, which can better describe the creep properties of 316H alloy and GH4169 alloy.

[0033] 3. The present invention establishes a 1 / 40 flange structure finite element model and calculates the temperature field finite element model of the flange structure under periodic temperature load, which has high authenticity and calculation efficiency. At the same time, it proposes a method of using upper and lower limit constant temperature loads instead of periodic temperature loads to calculate the temperature field, which can quickly predict the high-temperature stress relaxation behavior of the flange structure under longer test time, further improving the calculation efficiency.

[0034] 4. Based on short-term test data, the present invention constructs a high-temperature creep constitutive model and uses finite element software to perform thermal-solid finite element numerical calculations of the flange structure, which better predicts the stress relaxation behavior of the flange structure during long-term service at high temperature, and solves the problems of insufficient high-temperature stress relaxation test data of the main materials of the flange structure and long research cycle and huge capital consumption of high-temperature stress relaxation tests of the flange structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0036] Figure 1 It is a flow chart of a method for measuring high temperature and long-term stress relaxation of a pre-flange structure based on short-term test data of the present invention;

[0037] Figure 2 A schematic diagram of the structure of the stress relaxation standard specimen provided by the present invention;

[0038] Figure 3 The stress relaxation curves and cubic delay function curves of the 316H alloy and GH4169 alloy provided by the present invention are compared, wherein: Figure 3 (a) is 316H alloy, Figure 3 (b) GH4169 alloy;

[0039] Figure 4 It is a fitting diagram of parameter a in the logarithmic creep constitutive model of 316H alloy and GH4169 alloy provided by the present invention, wherein, Figure 4 (a) is 316H alloy, Figure 4 (b) GH4169 alloy;

[0040] Figure 5 It is a fitting diagram of parameter b in the logarithmic creep constitutive model of 316H alloy and GH4169 alloy provided by the present invention, wherein, Figure 5 (a) is 316H alloy, Figure 5 (b) GH4169 alloy;

[0041] Figure 6 The structural schematic diagram and 1 / 40 finite element model diagram of the flange provided by the present invention, wherein: Figure 6 (a) is a schematic diagram of the flange structure. Figure 6 (b) is the 1 / 40 finite element model diagram of the flange structure;

[0042] Figure 7 The temperature load schematic diagram of two cycles and the temperature field distribution diagram of the flange structure provided by the present invention, wherein: Figure 7 (a) is a schematic diagram of temperature loads for two cycles. Figure 7 (b) is the temperature field distribution diagram of the flange structure;

[0043] Figure 8 The stress relaxation cloud diagrams of the upper flange and the lower flange at different stress relaxation times provided by the present invention;

[0044] Fig. 9 Creep strain cloud diagrams of the upper flange and the lower flange at different stress relaxation times provided by the present invention;

[0045] Fig.10 The stress relaxation cloud diagram of the bolt at different stress relaxation times provided by the present invention;

[0046] Fig.11 The creep strain cloud diagram of the bolt under different stress relaxation times provided by the present invention;

[0047] Fig.12 A curve diagram showing the longitudinal displacement of the inner and outer sealing rings provided by the present invention as a function of stress relaxation time;

[0048] Fig.13 Compressive stress cloud diagrams of the inner and outer sealing grooves provided by the present invention at different stress relaxation times;

[0049] Fig.14 The stress relaxation curve of the flange structure under the periodic and constant temperature load provided by the present invention;

[0050] Fig.15 The curve of the linear load and bolt preload of the inner sealing ring provided by the present invention as a function of the rebound amount of the inner sealing ring is shown in FIG. Fig.15(a) is the curve of the linear load of the inner sealing ring changing with the rebound amount of the inner sealing ring. Fig.15 (b) is the curve of the change of bolt preload force with the rebound of the inner sealing ring;

[0051] Fig.16 A curve diagram showing the change of line load with stress relaxation time under different preload forces provided by the present invention;

[0052] Fig.17 The present invention provides a curve diagram showing the change of the bolt pre-tightening force with the stress relaxation time.

[0053] The above drawings include the following reference numerals:

[0054] 1. Upper flange; 2. Lower flange; 3. Bolts; 4. Sealing groove; 5. Sealing ring; 6. Nuts; 7. Washers. DETAILED DESCRIPTION

[0055] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0056] The present invention is further described in detail below in conjunction with specific embodiments, which should not be construed as limiting the scope of protection claimed by the present invention. The term "comprising" when used indicates the existence of features or steps, but does not exclude the existence or addition of one or more other features or steps; "first", "second", etc. are used to distinguish different objects, rather than to describe a specific order.

[0057] The purpose of the present invention is to provide a method for predicting the high-temperature and long-term stress relaxation behavior of a flange structure based on short-term test data. By conducting high-temperature stress relaxation tests on the main materials of the flange structure, a high-temperature creep constitutive model of the material is established, and then finite element software is used to numerically predict the stress relaxation behavior of the flange structure under high temperature and for a long time, thereby solving the technical problems of low test temperature and short test time in the existing test data of the main materials of the flange structure. The prediction method of the present invention has the characteristics of high calculation efficiency, accurate prediction results and low cost.

[0058] The method for predicting the long-term stress relaxation of flange structures at high temperature based on short-term test data includes the following steps.

[0059] Step S1, high temperature short-time stress relaxation test of materials: prepare one or more material samples, conduct high temperature short-time stress relaxation test on the material samples, and obtain stress relaxation data of the material samples under different high temperature conditions. Figure 2 The present invention is a schematic structural diagram of a stress relaxation standard specimen provided by the present invention, wherein the stress relaxation standard specimen is composed of an integrally formed thread clamping section, a transition section, a boss and a parallel section.

[0060] Step S2, establishing a high-temperature creep constitutive model: Based on the curve fitting of the stress relaxation data in step S1, a high-temperature creep constitutive model of the material is established according to the logarithmic creep constitutive equation, wherein the creep constitutive equation is a functional relationship between four parameters: stress, time, temperature and damage.

[0061] Step S3, calculation of the temperature field distribution of the flange structure: construct a finite element model of the flange structure, and calculate the temperature field distribution under the specified temperature load to obtain the temperature field of the flange structure. In this step, the finite element model of the flange structure can be constructed using simulation software, the mesh is divided, the material parameters of the flange structure are input, a periodic temperature load is applied to the inner wall of the flange, and the heat transfer finite element calculation of the flange structure is performed to obtain the temperature field distribution of the flange structure.

[0062] Step S4, prediction of long-term high-temperature stress relaxation behavior of flange structure: based on the temperature field of the flange structure as the predefined field in step S3, thermal-solid finite element calculation of the flange structure is performed to predict the stress relaxation of the flange structure under high-temperature long-term service conditions.

[0063] Based on short-term test data, the present invention constructs a high-temperature creep constitutive model and applies it to the numerical simulation of high-temperature long-term stress relaxation of flange structures. It can predict the stress relaxation behavior of flange structures during long-term service at high temperatures, and solves the technical problems of low test temperature and short test duration in the existing test data of main materials of flange structures. This has important guiding significance for studying the stress relaxation behavior and sealing performance of flange systems under long-term service at high temperatures. The present invention makes up for the shortcomings of low test temperature and short test duration in the existing test data of main materials of flange structures by conducting high-temperature stress relaxation tests on the main materials of flange structures.

[0064] The stress relaxation prediction method also includes a computational model establishment step, thereby establishing the computational model. The computational model establishment step is between step S2 and step S3. The present invention establishes a logarithmic creep constitutive model and determines corresponding parameters, which can better describe the creep properties of 316H alloy and GH4169 alloy. Creep properties are the basic properties of materials, which can be expressed by the creep constitutive equation of creep materials. Among them, the computational model can be presented in a subroutine manner, such as Figure 1 shown.

[0065] In step S4, the finite element model obtained in step S3 may be imported into the simulation software, and the computational model may be loaded to predict the stress relaxation of the flange structure under high temperature and long-term service conditions.

[0066] The present invention is based on short-term test data, by constructing a high-temperature creep constitutive model, and using finite element software to perform thermal-solid finite element numerical calculations of the flange structure, so as to better predict the stress relaxation behavior of the flange structure during long-term service at high temperature, thereby overcoming the difficulties of insufficient high-temperature stress relaxation test data of the main materials of the flange structure, as well as the long research cycle and huge capital expenditure of the high-temperature stress relaxation test of the flange structure.

[0067] The following is a detailed description of each step:

[0068] In step S1, the material of the material sample block can be selected as 316H alloy and / or GH4169 alloy with reference to GB / T10120-2013 "High Temperature Tensile Stress Relaxation Test Method". A microelectronic creep endurance tester and a heating furnace are used to perform high temperature stress relaxation tests on stress relaxation standard specimens of 316H alloy and GH4169 alloy, respectively, to determine stress relaxation test data of 316H alloy and GH4169 alloy in which load changes with stress relaxation time at different temperatures, and the stress relaxation test data includes stress relaxation time and a drop in load corresponding to the stress relaxation time.

[0069] Among them, the initial load of 316H alloy is 125MPa, the test time is 1000h, and the test temperature is 500℃ and 550℃; the initial load of GH4169 alloy is 800MPa, the test time is 600h, and the test temperature is 450℃ and 500℃; the heating rate of 316H alloy and GH4169 alloy is 4℃ / min, the temperature deviation is within 0.1℃, the extensometer resolution is 0.0001mm, and the tensile speed is 72kN / min.

[0070] In step S2, the origin software can be used to perform curve fitting of the cubic delay function on the stress relaxation test data of 316H alloy and GH4169 alloy at different temperatures obtained in step 1, which can supplement the data points of the initial stage of stress relaxation, which has obvious abrupt changes due to rapid relaxation, solve the sharp corner phenomenon, and make the curve smoother. The cubic delay function expression in step S2 is as follows: Among them, σ is the instantaneous stress, σ r is the residual stress, t is the stress relaxation time, B1, τ1, B2, τ2, B3, τ3 are all material constants of the stress relaxation process. These parameters are obtained by curve fitting of stress relaxation data in step S2. Each parameter is only applicable to fitting complex curves and has no practical significance. The logarithmic creep constitutive equation in step S2 is as follows: in, is the creep strain rate, E is Young's modulus, σ is the instantaneous stress, σ0 is the initial load, and the parameters parameter t is the stress relaxation time. The cubic delay function is used to supplement the data points of the initial stage of stress relaxation, which has obvious abrupt changes due to rapid relaxation, to solve the sharp corner phenomenon and make the curve smoother.

[0071] The values ​​of the relevant fitting parameters are shown in Table 1. R2 is the correlation coefficient, and its value is between 0 and 1. The larger the value, the higher the degree of fitting of the equation. The cubic delay function curve is compared with the stress relaxation test curve. Figure 3 As shown, Figure 3 (a) is 316H alloy material, Figure 3 (b) is GH4169 alloy material.

[0072] Table 1 Fitting parameters and correlation coefficients of cubic retardation function of GH4169 alloy and 316H alloy

[0073]

[0074] In the computational model building step, the logarithmic creep constitutive equation and the cubic delay function in step 2 determine the corresponding parameters a and b of the logarithmic creep constitutive equation.

[0075] During the stress relaxation process, the following relationship exists:

[0076] ε t =ε e +ε c (2)

[0077] Among them, ε t is the total strain, ε e is the elastic strain, ε c is the creep strain; the total strain remains unchanged during the stress relaxation process, and the strain rate relationship is expressed as follows:

[0078]

[0079] in, is the total strain rate, is the elastic strain rate, is the creep strain rate;

[0080] Considering the generalized Hooke's law, the creep strain rate can be obtained:

[0081]

[0082] Where, dσ / dt is the stress relaxation rate, E is Young's modulus;

[0083] The logarithmic formula requires fewer parameters to be identified during fitting, and the formula itself is a monotonically decreasing function, which is more consistent with the nature of the continuous attenuation of stress during stress relaxation.

[0084] The explicit expression of the logarithmic creep constitutive model is as follows:

[0085] σ=σ0-aln(bt+1)(5)

[0086] Among them, σ is the instantaneous stress, σ0 is the initial load, t is the stress relaxation time, and the parameters parameter

[0087] Substituting formula (5) into formula (4) yields:

[0088]

[0089] By transforming formula (4), we can obtain the relationship between stress relaxation time t and instantaneous stress σ, which is expressed as follows:

[0090]

[0091] The implicit expression of the logarithmic creep constitutive model is as follows:

[0092]

[0093] Figure 4 is the fitting diagram of parameter a in the logarithmic creep constitutive model of 316H alloy and GH4169 alloy, where: Figure 4 (a) is 316H alloy, Figure 4 (b) is the GH4169 alloy, where the stress change is linearly related to the logarithm of the stress relaxation time, and its slope is the value of parameter a.

[0094] Figure 5 is the fitting diagram of parameter b in the logarithmic creep constitutive model of 316H alloy and GH4169 alloy, where Figure 5 (a) is 316H alloy, Figure 5 (b) is the GH4169 alloy, where the stress term is linearly related to the stress relaxation time, and its slope is the value of parameter b.

[0095] In this embodiment, the relevant parameter values ​​of the logarithmic creep constitutive model are shown in Table 2.

[0096] Table 2 Related parameter values ​​of logarithmic creep constitutive model

[0097]

[0098] Considering the influence of temperature T on parameters a and b, for 316H alloy, parameter a is set to have an exponential relationship with temperature T, and parameter b is set to have a linear relationship with temperature T; the expressions of parameters a, b and temperature T of 316H alloy are as follows:

[0099]

[0100] For GH4169 alloy, the parameter a is set to have a linear relationship with the temperature T, and the parameter b is set to have an exponential relationship with the temperature T; the expressions of the parameters a, b and temperature T of GH4169 alloy are as follows:

[0101]

[0102] In the computational model establishment step, the computational model establishment can be presented in the form of a subroutine, and the subroutine can be a CREEP creep subroutine. In this step, the 316H alloy and the GH4169 alloy are defined respectively, the expressions of the parameters a, b and the temperature T and the logarithmic creep constitutive model are defined, the CREEP creep subroutine is compiled, and the creep properties of the 316H alloy and the GH4169 alloy are defined.

[0103] In step S3, the specified temperature load includes a periodic temperature load and / or a constant temperature load. The manner of applying the periodic temperature load includes the frequency and amplitude of the temperature change. In step S3, the first material parameter of the flange structure can be input, wherein the first material parameter includes one or more of the material density, Poisson's ratio, thermal conductivity and convection heat transfer coefficient. In this step, the upper limit temperature and the lower limit temperature can be applied to the inner wall of the flange structure, and the heat transfer finite element calculation of the flange structure can be performed to obtain the temperature field distribution of the flange structure, which is conducive to the subsequent rapid prediction of the stress relaxation behavior of the flange structure for a longer time (30000h); wherein, the upper limit temperature load is 500℃ and the lower limit temperature load is 420℃.

[0104] The test temperature of 316H alloy is 500°C and 550°C; the test temperature of GH4169 alloy is 450°C and 500°C. The simulation software can use finite element software such as ABAQUS.

[0105] The present invention establishes a 1 / 40 flange structure finite element model, such as Figure 6 As shown, Figure 6 (a) is a schematic diagram of the flange structure. Figure 6 (b) is the 1 / 40 finite element model diagram of the flange structure. Figure 6As shown in (a), the flange structure includes an upper flange 1, which is connected to the lower flange 2 by multiple bolts 3. Two sealing grooves 4 are processed between the upper flange 1 and the lower flange 2, namely, an inner sealing groove and an outer sealing groove, and corresponding sealing rings 5 ​​are arranged in the sealing grooves 4, namely, an inner sealing ring and an outer sealing ring; the outer wall of the bolt 3 is threadedly connected with a nut 6, and a gasket 7 is arranged between the bolt 3 and the nut 6; in the flange structure, the materials of the upper flange 1 and the lower flange 2 are 316H alloy, and the materials of the bolt 3, the nut 6 and the gasket 7 are GH4169 alloy; the sealing ring 5 is composed of a spring layer, a coating layer and an outer plating layer, and the materials thereof are Inconel750X, Inconel600 and N6 respectively.

[0106] Use 1 / 40 of the overall flange structure finite element model, such as Figure 6 (b) As shown. A periodic temperature load is applied to the inner wall of the flange, with the highest temperature being 500°C and the lowest temperature being 420°C, the ambient temperature being 200°C, the total time being 1128h, and the single cycle being 24h. The heat transfer finite element calculation of the flange structure is performed to obtain the temperature field distribution of the flange structure at different time points; Figure 7 (a) is a schematic diagram of temperature loads for two cycles. Figure 7 The temperature field distribution of the flange structure corresponding to the three time points a, b, and c in (a) is shown in the figure. Figure 7 As shown in (b), it can be clearly seen that the temperature gradually decreases along the inner wall of the flange to the outer wall, with the maximum value on the inner wall and the minimum value near the two ends of the nut.

[0107] The finite element model for calculating the temperature field of the flange structure under periodic temperature load has high authenticity and calculation efficiency. Figure 7 The temperature load diagram of two cycles and the temperature field distribution diagram of the flange structure are shown. Figure 7 (a) is a schematic diagram of temperature loads for two cycles. Figure 7 (b) is the temperature field distribution diagram of the flange structure. At the same time, a method of using upper and lower limit constant temperature loads to replace periodic temperature loads to calculate the temperature field is proposed, which can quickly predict the high temperature stress relaxation behavior of the flange structure under longer test time, further improving the calculation efficiency.

[0108] In step S4, when the finite element model is imported into the simulation software, it also includes presetting the stress relaxation time and the bolt preload in the simulation software, inputting the force boundary conditions, and inputting the material parameters of the flange structure. In this step, the second material parameters of the flange structure can be input, and the second material parameters include one or more of the material density, elastic modulus, Poisson's ratio, plastic section stress-strain curve and linear expansion coefficient. Among them, the force boundary conditions include equivalent tensile stress, internal pressure, fixed constraints and symmetry constraints. In step S4, a thermal-solid finite element calculation of the flange structure is performed to predict the high-temperature and long-term stress relaxation behavior of the flange structure. The high-temperature and long-term stress relaxation behavior of the flange structure includes stress relaxation and creep strain of the flange and bolts, stress relaxation of the sealing groove, and the rebound amount and linear load change of the sealing ring, such as Figure 8-Figure 17 shown.

[0109] Force boundary conditions include equivalent tensile stress, internal pressure, fixed constraints and symmetry constraints, such as Figure 6 As shown in (b), internal pressure is applied to the inner wall of the flange, equivalent tensile stress is applied to the top of the upper flange 1, a fixed constraint is applied to the bottom of the lower flange 2, and a symmetric constraint is applied to the side wall of the flange; the material parameters include material density, elastic modulus, Poisson's ratio, plastic section stress-strain curve, and linear expansion coefficient;

[0110] In this step, the preset stress relaxation time is 1128 h, the equivalent stress is 23.16 MPa, the internal pressure is 3 MPa, and the bolt preload is 0.2σs (σs is the yield strength of GH4169 alloy, 1125 MPa).

[0111] Figure 8 The stress relaxation cloud diagrams of the upper flange and the lower flange at different stress relaxation times provided by the present invention, S, Mises represents stress; Figure 8 It can be seen that the stress relaxation on the flange does not change significantly with time.

[0112] Fig. 9 The creep strain cloud diagram of the upper flange and the lower flange under different stress relaxation times provided by the present invention, CEEQ represents the creep strain; Fig. 9 It can be seen that the flange produced a large creep strain from the beginning of stress relaxation (0h) to 264h, and the creep strain from 264h to 576h still increased significantly. However, compared with the early stage of stress relaxation (0h), the creep strain from 576h to 1128h was not obvious, indicating that the stress relaxation of the flange structure has entered a relatively gentle stage.

[0113] Fig.10 The stress relaxation cloud diagram of the bolt under different stress relaxation times provided by the present invention is Fig.10 It can be seen that the bolt stress decays significantly from the beginning of stress relaxation (0h) to 264h, and then there is no obvious change in the bolt stress until 1128h.

[0114] Fig.11 The creep strain cloud diagram of the bolt under different stress relaxation times provided by the present invention is Fig.11 It can be seen that the creep strain is mainly concentrated in the lower half of the bolt bearing section and increases with time, especially from 576h to 1128h, where a significant increase can still be observed, indicating that the stress relaxation limit has not yet been reached. Fig. 9 It is found that the creep strain of the bolt is much smaller than that of the flange in magnitude.

[0115] Fig.12 The longitudinal displacement of the inner and outer sealing rings provided by the present invention varies with the stress relaxation time. Fig.12 It can be seen that both the inner and outer sealing rings have obvious rebound, and the rebound amounts are 0.036mm and 0.022mm respectively.

[0116] Fig.13 The compressive stress cloud diagram of the inner and outer sealing grooves provided by the present invention at different stress relaxation times is shown in FIG. Fig.13 It can be seen that the compressive stress of the inner sealing groove gradually decreases with the stress relaxation time. There is obvious stress relaxation in the first 264 hours, and then the stress changes slightly; the compressive stress of the outer sealing groove has no obvious change.

[0117] Fig.14 The stress relaxation curve of the flange structure under the periodic and constant temperature load provided by the present invention is Fig.14 It can be seen that within the 1128h temperature cycle, the isothermal stress relaxation curve coincides with the upper and lower limits of the stress relaxation curve at the periodic temperature, and the maximum error does not exceed 0.5%, indicating that the results of predicting stress relaxation behavior using isothermal loads can better reflect the upper and lower limits of periodic temperature loads. Therefore, isothermal loads can be used to quickly predict stress relaxation behavior at periodic temperatures for a longer period of time (30,000h).

[0118] Fig.15 The curve of the linear load and bolt preload of the inner sealing ring provided by the present invention as a function of the rebound amount of the inner sealing ring is shown in FIG. Fig.15 (a) is the curve of the linear load of the inner sealing ring changing with the rebound amount of the inner sealing ring. Fig.15 (b) is the curve of the change of bolt preload force with the rebound of the inner sealing ring; Fig.15 It can be seen that with the increase of stress relaxation time, the rebound of the inner sealing ring increases, and the bolt preload and the linear load of the inner sealing ring change linearly with the rebound of the inner sealing ring. Therefore, the bolt preload and linear load can be predicted according to the rebound of the sealing ring.

[0119] Fig.16 The curve diagram of the change of line load with stress relaxation time under different bolt preload provided by the present invention is Fig.16 It can be seen that the linear load of the inner sealing groove decays significantly with the stress relaxation time, and the linear load of the outer sealing groove decays slightly with the stress relaxation time. The greater the bolt preload, the smaller the decay.

[0120] Fig.17 The curve diagram of the bolt preload under the present invention with respect to the stress relaxation time is shown in FIG. Fig.17 It can be seen that the bolt preload gradually decays with the increase of stress relaxation time, and the relaxation amount of the bolt preload increases with the increase of the bolt preload.

[0121] In summary, the present invention provides a method for predicting long-term stress relaxation of flange structures under high temperature based on short-term test data, including but not limited to the prediction of relaxation behavior of key factors such as flange structure stress, creep strain, bolt preload, sealing ring line load and rebound amount under complex loads.

[0122] In summary, from the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0123] 1. The present invention adopts a cubic delay function to supplement the data points of the initial stage of stress relaxation, which has obvious abrupt changes due to rapid relaxation, thereby solving the sharp corner phenomenon and making the curve smoother.

[0124] 2. The present invention establishes a logarithmic creep constitutive model and determines corresponding parameters, which can better describe the creep properties of 316H alloy and GH4169 alloy.

[0125] 3. The present invention establishes a 1 / 40 flange structure finite element model and calculates the temperature field finite element model of the flange structure under periodic temperature load, which has high authenticity and calculation efficiency. At the same time, it proposes a method of using upper and lower limit constant temperature loads instead of periodic temperature loads to calculate the temperature field, which can quickly predict the high-temperature stress relaxation behavior of the flange structure under longer test time, further improving the calculation efficiency.

[0126] 4. Based on short-term test data, the present invention constructs a high-temperature creep constitutive model and uses finite element software to perform thermal-solid finite element numerical calculations of the flange structure, which better predicts the stress relaxation behavior of the flange structure during long-term service at high temperature, and solves the problems of insufficient high-temperature stress relaxation test data of the main materials of the flange structure and long research cycle and huge capital consumption of high-temperature stress relaxation tests of the flange structure.

[0127] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for predicting flange structure stress relaxation based on short-term test data, characterized in that: The steps include: Step S1, high-temperature short-time stress relaxation test of materials: preparing one or more material samples, performing a high-temperature short-time stress relaxation test on the material samples, and obtaining stress relaxation data of the material samples under different high-temperature conditions; Step S2, establishing a high-temperature creep constitutive model: on the basis of curve fitting of the stress relaxation data in step S1, establishing a high-temperature creep constitutive model of the material according to a logarithmic creep constitutive equation; Step S3, calculation of the temperature field distribution of the flange structure: constructing a finite element model of the flange structure, and calculating the temperature field distribution at a specified temperature to obtain the temperature field of the flange structure; Step S4, prediction of long-term high-temperature stress relaxation behavior of flange structure: based on the temperature field of the flange structure in step S3, thermal-solid finite element calculation of the flange structure is performed to predict the stress relaxation of the flange structure under long-term high-temperature service conditions.

2. The stress relaxation prediction method according to claim 1, characterized in that: The stress relaxation prediction method also includes a computational model establishment step: based on the high-temperature creep constitutive model in step S2, the creep performance of the material sample is defined to establish the computational model, and the computational model establishment step is between step S2 and step S3.

3. The stress relaxation prediction method according to claim 2, characterized in that: In the step S4, the finite element model obtained in the step S3 can be imported into the simulation software, and the calculation model can be loaded to predict the stress relaxation of the flange structure under high temperature and long-term service conditions.

4. The stress relaxation prediction method according to claim 1, characterized in that: In the step S2, the curve fitting of the stress relaxation data is the curve fitting of a cubic delay function.

5. The stress relaxation prediction method according to claim 1, characterized in that: The specified temperature load in step S3 includes a periodic temperature load and / or a constant temperature load.

6. The stress relaxation prediction method according to claim 5, characterized in that: The application method of the periodic temperature load includes the frequency and amplitude of the temperature change.

7. The stress relaxation prediction method according to claim 6, characterized in that: In the step S4, when the finite element model is imported into the simulation software, it also includes presetting the stress relaxation time and the bolt preload force in the simulation software.

8. The stress relaxation prediction method according to claim 1, characterized in that: The material of the material sample block is 316H alloy and / or GH4169 alloy.

9. The stress relaxation prediction method according to claim 4, characterized in that: The three-time delay function expression in step S2 is as follows: Among them, σ is the instantaneous stress, σ r is the residual stress, t is the stress relaxation time, and B1, τ1, B2, τ2, B3, τ3 are all material constants of the stress relaxation process, which are obtained by curve fitting the stress relaxation data in step S2.

10. The stress relaxation prediction method according to claim 1, characterized in that: The logarithmic creep constitutive equation in step S2 is: in, is the creep strain rate, E is Young's modulus, σ is the instantaneous stress, σ0 is the initial load, and the parameters parameter t is the stress relaxation time.

11. The stress relaxation prediction method according to claim 10, characterized in that: When the material sample is 316H alloy, the parameter a is set to have an exponential relationship with the temperature T, and the parameter b is set to have a linear relationship with the temperature T; the expressions of the parameters a, b and temperature T of the 316H alloy are as follows:

12. The stress relaxation prediction method according to claim 11, characterized in that: When the material sample adopts GH4169 alloy, it is set that parameter a has a linear relationship with temperature T, and parameter b has an exponential relationship with temperature T; the expressions of parameters a, b and temperature T of GH4169 alloy are as follows:

13. The stress relaxation prediction method according to claim 1, characterized in that: In the step S3, the first material parameters of the flange structure may be input, wherein the first material parameters include one or more of material density, Poisson's ratio, thermal conductivity and convection heat transfer coefficient.

14. The stress relaxation prediction method according to claim 1, characterized in that: In the step S3, an upper limit temperature and a lower limit temperature may be applied to the inner wall of the flange structure.

15. The stress relaxation prediction method according to claim 14, characterized in that: The upper limit temperature is 500°C and the lower limit temperature is 420°C.

16. The stress relaxation prediction method according to claim 1, characterized in that: In the step S4, second material parameters of the flange structure may be input, wherein the second material parameters include one or more of material density, elastic modulus, Poisson's ratio, plastic section stress-strain curve and linear expansion coefficient.

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