Method for establishing Chboch viscoplasticity unified constitutive model under thermal-mechanical coupling condition

Abstract

The invention discloses a method for establishing a Chaboche viscoplasticity unified constitutive model under a thermal-mechanical coupling condition, and belongs to the technical field of material thermal mechanical fatigue analysis, and the method comprises the following steps: determining a basic equation of the Chaboche viscoplasticity unified constitutive model; based on the basic equation, a temperature change item is added, and a heat-force coupling Chabolish viscoplasticity unified constitutive model is established; carrying out statistics on parameters of the Chboch viscoplasticity unified constitutive model at different temperatures; fitting the parameters with respect to the temperature to obtain a fitting curve of the constitutive model parameters with respect to the continuous change temperature, so as to calibrate the temperature-related Chboche viscoplastic constitutive model parameters; and performing simulation verification. The constitutive model for simulating the thermal mechanical fatigue cycle response is effectively provided, and the constitutive model is simple in formula, easy to understand and high in operability.

Description

Technical Field

[0001] The present invention relates to the technical field of thermo-mechanical fatigue analysis of materials, and specifically to a method for establishing a Chaboche viscoplastic unified constitutive model under thermo-mechanical coupling conditions. Background Art

[0002] With the development of aero-engine technology, in order to improve the thrust-to-weight ratio of aero-engines, the temperature before the turbine has been continuously increasing, and the temperature before the turbine of the new generation of aero-engines exceeds 2417K. Nickel-based powder metallurgy (P / M) superalloys are widely used in the manufacture of key hot-end components of aircraft engines, such as turbine disks, due to their good high-temperature strength, high tissue stability, and excellent fatigue, creep resistance, and oxidation resistance. During the actual working processes of an aircraft taking off, accelerating, and landing, the turbine disk is in a harsh environment of high temperature, high pressure, high speed, and alternating loads for a long time. The turbine disk bears huge thermal loads and centrifugal loads during actual service. At the same time, due to the asynchronous temperature and speed of the turbine disk, there is a phase lag between the thermal stress and the centrifugal stress. This lag causes the turbine disk to bear not only complex mechanical loads but also thermal cyclic loads. In addition, the oxidation effect in a high-temperature environment further accelerates the fatigue failure of the turbine disk. This thermo-mechanical fatigue (TMF) failure caused by the coupling of thermal, mechanical, and oxygen is one of the important failure modes of aircraft engine turbine disks. Therefore, how to establish a constitutive model method using finite element simulation to effectively and accurately simulate the thermo-mechanical fatigue cyclic deformation behavior of materials is of irreplaceable significance for the life assessment and damage tolerance design of turbine disks.

[0003] Currently, according to the calculation methods of plastic and viscous strains, thermo-mechanical fatigue constitutive models can be divided into two types, namely non-unified type and unified type. Among them, the viscoplastic unified constitutive model can not only consider the interaction between plastic and viscous deformations but also reduce the viscoplastic equations in the constitutive model, and is widely used to simulate the cyclic mechanical response of high-temperature materials. In particular, the Chaboche viscoplastic unified constitutive model can reasonably simulate the stress-strain response behavior of many materials under complex high-temperature loading conditions. However, most of the existing constitutive models can only simulate the cyclic response behavior under isothermal fatigue conditions, there are fewer constitutive models for simulating the thermo-mechanical fatigue cyclic response, and the constitutive model formulas are complex, difficult to understand, and have low operability. Summary of the Invention

[0004] The technical solution of the present invention addresses the technical problem of the overly single technical solution in the prior art and provides a solution significantly different from the prior art. It mainly provides a method for establishing a Chaboche viscoplastic unified constitutive model under thermo-mechanical coupling conditions to solve the technical problems in the above-mentioned background art, namely, there are few constitutive models for simulating the thermo-mechanical fatigue cycle response in the prior art, and the constitutive model formula is complex, difficult to understand, and has low operability.

[0005] The technical solution adopted by the present invention to solve the above technical problems is as follows:

[0006] A method for establishing a Chaboche viscoplastic unified constitutive model under thermo-mechanical coupling conditions, comprising the following steps:

[0007] S1. Determine the basic equation of the Chaboche viscoplastic unified constitutive model;

[0008] S2. Based on the basic equation in step S1, add a temperature change term to establish a thermo-mechanical coupling Chaboche viscoplastic unified constitutive model;

[0009] S3. Statistically analyze the parameters of the Chaboche viscoplastic unified constitutive model at different temperatures;

[0010] S4. Fit the parameters in step S3 with respect to temperature to obtain a fitting curve of the constitutive model parameters with respect to continuously varying temperature, thereby calibrating the temperature-dependent Chaboche viscoplastic constitutive model parameters;

[0011] S5. Conduct simulation verification.

[0012] Further, in step S1, the basic equation of the Chaboche viscoplastic unified constitutive model is:

[0013]

[0014] f = |σ - X| - R - σ y (3)

[0015]

[0016] σ = X + (R + k0 + σ v ) sgn(σ - X) (15)

[0017] In formula (1), represents the total strain rate of the material, represents the elastic strain rate of the material, represents the inelastic strain rate of the material;

[0018] In formula (2), represents the stress rate of the material, and E represents the elastic modulus of the material;

[0019] In Equation (3), f represents the Von Mises yield function, σ is the yield stress of the material, and σ y is the initial yield stress of the material, representing the size of the initial yield surface in the stress space. Since it is relatively difficult to calibrate in the viscoplastic stage, the material parameter k0 can be used to represent it; X represents the kinematic hardening internal variable, which is used to describe the direction-dependent characteristics of the material; R represents the isotropic hardening internal variable, which is used to describe the change in the size of the yield surface in the stress space;

[0020] In Equation (4), Z and n are material parameters; Ω represents the viscoplastic potential function; f represents the yield function;

[0021] In Equation (8), is the evolution rate of the isotropic hardening internal variable, Q is the value approaching saturation of R, and b is the rate of approaching saturation of R, is the cumulative inelastic strain rate of the material;

[0022] In Equation (9), is the evolution rate of the kinematic hardening internal variable; c is the material parameter describing the linear gradient of the function;

[0023] In Equation (10), γ is a material parameter;

[0024] In Equation (11), the first term is the Prage linear kinematic hardening term, which is a linear equation describing the inelastic strain rate of the material; a i has the stress dimension and describes the saturated stress value of the material in the inelastic stage in the kinematic hardening equation, and c i is a material parameter without the stress dimension, which describes the rate at which the material stress approaches saturation in the kinematic hardening equation;

[0025] In Equation (12), M is the number of components of the back stress X of the kinematic hardening internal variable considered;

[0026] In Equation (14), σ v represents the viscoplastic stress.

[0027] Furthermore, in step S1, the material parameters adopted in the basic equation of the Chaboche viscoplastic unified constitutive model include E, k0, Z, n, b, Q, a i , c i .

[0028] Furthermore, in step S1, M = 2, i = 1, 2;

[0029] or M = 3, i = 1, 2, 3.

[0030] Further, in step S2, the total strain rate of the Chaboche viscoplastic unified constitutive model under variable temperature conditions is:

[0031]

[0032] In Equation (16), ε to is the total strain, ε el is the elastic strain, ε in is the inelastic strain, and ε th is the thermal strain.

[0033] Further, in step S2, the expression of the viscoplastic potential function of the Chaboche viscoplastic unified constitutive model under variable temperature conditions is:

[0034]

[0035] In the formula, Ω(T) is the viscoplastic potential function of the Chaboche viscoplastic unified constitutive model under variable temperature conditions; K and n are material parameters, and T represents temperature.

[0036] Further, in step S2, under variable temperature conditions, temperature-related terms are added to the evolution equations of X i and R, which are:

[0037]

[0038] In the above formula, represents the evolution equation of X i under isothermal conditions, and represents the evolution equation of R under isothermal conditions.

[0039] Further, in step S3, the parameters of the Chaboche viscoplastic unified constitutive model at different temperatures from 500 to 800 °C are statistically analyzed.

[0040] Further, in step S5, using the finite element software Abaqus and its built-in user-defined subroutine UMAT, the thermo-mechanical coupled Chaboche viscoplastic unified constitutive model determined in step S4 is adopted, combined with the temperature-related Chaboche viscoplastic unified constitutive model parameters, to simulate the hysteresis loops of the first cycle and the half-life cycle under the thermo-mechanical fatigue IP 2% condition and the OP 2% condition.

[0041] Further, the simulation results are compared with the thermo-mechanical fatigue test results.

[0042] Compared with the prior art, the beneficial effects of the present invention are:

[0043] Based on the Chaboche viscoplastic unified constitutive model, this invention adds a temperature change term to establish a thermo-mechanical coupled Chaboche viscoplastic unified constitutive model. The parameters of the Chaboche viscoplastic unified constitutive model at different temperatures are used for fitting with respect to temperature to obtain the fitting curves of the constitutive model parameters with respect to continuously varying temperature, thereby calibrating the Chaboche viscoplastic constitutive model parameters related to temperature, and thus establishing the Chaboche viscoplastic unified constitutive model under thermo-mechanical coupling conditions, effectively providing a constitutive model for simulating the thermo-mechanical fatigue cycle response. Moreover, the Chaboche model unifies the time-independent plastic deformation and the time-dependent viscous deformation in one framework, and uses an inelastic variable to represent the sum of the two. This unified description method enables the model to simultaneously consider the deformation behavior of materials under different loading rates and temperature conditions, without the need to separately process plastic and viscous deformations, simplifying the complexity of the model. At the same time, the Chaboche viscoplastic constitutive model under thermo-mechanical coupling conditions only adds a temperature change term to the kinematic hardening equation and the isotropic evolution equation, and the formula modification is simple and highly operable. The model characterizes the mechanical behavior of materials through a series of material parameters, which can be obtained by fitting experimental data. And these parameters have clear physical meanings, making the model have good applicability under different materials and working conditions, and making the established constitutive model under thermo-mechanical coupling conditions more operable. Moreover, the model has good adaptability to numerical calculation methods (such as implicit integration algorithms). For example, the model can be implemented in finite element software through algorithms such as the backward Euler method and the radial return method. This good numerical implementation ability makes the model highly operable in engineering applications. Kinematic hardening is used to describe the cyclic hardening behavior of materials, while isotropic hardening is used to describe the overall hardening or softening of materials. This construction method based on physical mechanisms makes the model easy to understand and explain. Therefore, the constitutive model provided by this invention has a simple formula, is easy to understand, and is highly operable.

[0044] The following will combine the accompanying drawings with specific embodiments to explain the present invention in detail. Description of the Drawings

[0045] Figure 1 It is the process flow chart of the present invention.

[0046] Figure 2 It is the fitting curve graph of the Chaboche viscoplastic unified constitutive model parameters of FGH4108 alloy with respect to temperature at different temperatures; among them, Figure (a) is the fitting curve graph of the elastic modulus E and the initial yield stress k0 with respect to temperature; Figure (b) is the fitting curve graph of the kinematic hardening parameters α1 and a2 with respect to temperature; Figure (c) is the fitting curve graph of the kinematic hardening parameters c1 and c2 with respect to temperature; Figure (d) is the fitting curve graph of the isotropic evolution parameter Q with respect to temperature.

[0047] Figure 3 It is a comparison diagram of the simulated results and test results of the thermo-mechanical fatigue cyclic hysteresis loops of the FGH4108 alloy under different conditions; among them, Figure (a) is the comparison diagram of the results under the condition of 2% initial plastic strain (IP); Figure (b) is the comparison diagram of the results under the condition of 2% half-life plastic strain (IP); Figure (c) is the comparison diagram of the results under the condition of 2% initial plastic strain (OP); Figure (d) is the comparison diagram of the results under the condition of 2% half-life plastic strain (OP). Detailed implementation manners

[0048] To facilitate the understanding of the present invention, the present invention will be described more comprehensively below with reference to the relevant drawings. Several embodiments of the present invention are given in the drawings, but the present invention can be implemented in different forms and is not limited to the embodiments described in the text. On the contrary, these embodiments are provided to make the disclosure of the present invention more thorough and comprehensive.

[0049] It should be noted that when an element is referred to as being "fixedly provided on" another element, it can be directly on the other element or there can be an intermediate element. When an element is considered to be "connected" to another element, it can be directly connected to the other element or there may be an intermediate element at the same time. The terms "vertical", "horizontal", "left", "right" and similar expressions used in the present invention are only for the purpose of illustration.

[0050] Unless otherwise defined, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used in the present invention includes any and all combinations of one or more of the related listed items.

[0051] The present invention establishes a Chaboche viscoplastic unified constitutive model under thermo-mechanical coupling conditions applicable to the FGH4108 superalloy. Please pay attention to referring to the attached Figure 1 The steps are as follows: First, modify the Chaboche viscoplastic unified constitutive model in the user-defined subroutine UMAT of the Abaqus finite element software, that is, add a temperature change term to the isotropic hardening and kinematic hardening equations; subsequently, fit the Chaboche viscoplastic unified constitutive model parameters of the material at different temperatures from 500 °C to 800 °C with respect to temperature and substitute them into UMAT, thereby establishing a thermo-mechanical coupling Chaboche viscoplastic unified constitutive model, and verify by simulating the thermo-mechanical fatigue cyclic stress-strain response under the condition that the mechanical strain range is 2%. The specific steps are as follows:

[0052] 1. Determine the basic equations of the Chaboche unified viscoplastic constitutive model

[0053] The Chaboche unified viscoplastic constitutive model describes the inelastic strain rate and stress level of materials by defining a viscoplastic potential function. It is called a unified constitutive model because it uses a time-dependent inelastic strain rate to describe time-independent plastic strain and time-dependent creep.

[0054] Considering the case of low strain levels, the total strain rate of the material can be divided into elastic strain rate and inelastic strain rate, and the formula is as follows:

[0055]

[0056] In the formula, represents the total strain rate of the material, represents the elastic strain rate of the material, represents the inelastic strain rate of the material.

[0057] Based on the time-dependent expression of Hooke's law, the stress rate of the material can be expressed by the elastic strain rate. From the above equation, it can be known that the elastic strain rate can be expressed as the difference between the total strain rate and the inelastic strain rate, and the formula is as follows:

[0058]

[0059] In the formula, represents the stress rate of the material, and E represents the elastic modulus of the material.

[0060] In the Chaboche unified viscoplastic constitutive model, different from time-independent plasticity, kinematic hardening and isotropic hardening are considered in the yield function, and the Von Mises yield function is used as follows:

[0061] f = |σ - X| - R - σ y (3)

[0062] When the Von Mises yield function f < 0, the material undergoes elastic deformation. When the Von Mises yield function f ≥ 0, the material undergoes inelastic deformation. The Von Mises yield function uses the kinematic hardening internal variable X to describe the direction-dependent characteristics of the material. In the stress space, it represents the movement of the center point of the yield surface. The isotropic hardening internal variable R is used to describe the change in the size of the yield surface in the stress space, which is usually represented by the plastic strain in scalar form. σ is the yield stress of the material, and σ y is the initial yield stress of the material, representing the size of the initial yield surface in the stress space. Since it is difficult to calibrate in the viscoplastic stage, the material parameter k0 can be used to represent it.

[0063] In the Chaboche viscoplastic unified constitutive model, a viscoplastic potential function is used to describe the inelastic strain rate and stress level of the material, and its expression is as follows:

[0064]

[0065] In the formula, Z and n are material parameters; Ω represents the viscoplastic potential function; f represents the yield function. When the Von Mises yield function f ≥ 0, the inelastic strain rate of the material can be expressed by the following formula:

[0066]

[0067] Isotropic hardening is manifested as a uniform change in the size of the yield surface in all directions in the stress space, and is used to describe the stress change during the plastic flow of the material. In the cyclic stress curve of the material, with the increase of the number of cycles and the accumulation of inelastic strain, the material undergoes cyclic softening / hardening due to isotropic hardening. The evolution equation of the isotropic hardening internal variable is as follows:

[0068]

[0069] In the formula, is the evolution rate of the isotropic hardening internal variable, Q is the value close to saturation of R, b is the rate of R approaching the saturation value, p is the cumulative inelastic strain of the material, is the cumulative inelastic strain rate of the material. In the stress space, the isotropic hardening internal variable R is manifested as a uniform change in the size of the yield surface in all directions, describing the functional relationship between stress and inelastic strain.

[0070] Kinematic hardening describes the mechanical behavior of the material during cyclic deformation. It is manifested as a certain direction movement of the yield surface in the stress space, describing the functional relationship between stress and cumulative inelastic strain. During the kinematic hardening process, the size of the material yield surface remains unchanged all the time, that is, the material parameter k0 is a constant. The kinematic hardening internal variable X is called the back stress, which determines the instantaneous position of the yield surface at this moment.

[0071] In the kinematic hardening equation, the simplest one is the linear kinematic hardening that describes the linear functional relationship between stress and cumulative inelastic strain. Its evolution equation of the kinematic hardening internal variable related to time is:

[0072]

[0073] In the formula, X is the back stress of the kinematic hardening internal variable, is the evolution rate of the kinematic hardening internal variable; c is the material parameter describing the linear gradient of the function, represents the inelastic strain rate of the material.

[0074] Since it is difficult to obtain the linear function relationship between stress and cumulative inelastic strain during actual experiments, the Armstrong-Frederick (A-F) nonlinear kinematic hardening model is usually adopted to describe the nonlinear relationship between material stress and inelastic strain. Armstrong and Frederick considered the historical effect of parameters instead of using the instantaneous parameters of the material, and defined the back stress X of the kinematic hardening internal variable by introducing nonlinear dynamic recovery. The expression of the evolution equation of the time-dependent nonlinear kinematic hardening internal variable back stress X is as follows:

[0075]

[0076] In the formula, γ is a material parameter.

[0077] Chaboche and Rousselier expressed the evolution equation of the time-dependent nonlinear kinematic hardening internal variable by using different parameters as:

[0078]

[0079] The first term in the formula is the description of the Prager linear kinematic hardening term, which is a linear equation about the inelastic strain rate of the material; p is the cumulative inelastic strain of the material, a i has the dimension of stress, and describes the saturated stress value of the material in the inelastic stage in the kinematic hardening equation, c i is a material parameter without the dimension of stress, which describes the rate at which the material stress approaches saturation in the kinematic hardening equation. In the A-F nonlinear kinematic hardening model, by decomposing the kinematic hardening internal variable back stress X into the sum of multiple X i (i = 1, 2,... M), its application range is improved. Among them, each back stress X has the same kinematic hardening evolution law, and the expression is as follows:

[0080]

[0081] In the formula, M is the number of components of the kinematic hardening internal variable back stress X considered, usually 2 or 3. The more the number of components of the back stress X, the higher the accuracy of describing the kinematic hardening effect. In the present invention, M = 2, so the back stress X can be decomposed into the following formula:

[0082] X = X1 + X2 (13)

[0083] In an inelastic constitutive model that combines an isotropic hardening evolution model and a kinematic hardening evolution model, the stresses used are the initial yield stress k0, the isotropic hardening internal variable R, and the kinematic hardening internal variable back stress X. When the Von Mises yield function equals 0, the material enters the plastic stage. If the viscoplastic deformation of the material is considered, since the yield point of the material in the stress space may be outside the yield surface at this time and the ultimate yield stress cannot be used to determine the deformation stage of the material, a viscoplastic stress σ v is added to the constitutive model, and its formula is as follows:

[0084]

[0085] Therefore, the stress can be expressed as:

[0086] σ = X+(R + k0+σ v )sgn(σ - X) (15)

[0087] All the equations described above are the basic equations of the Chaboche viscoplastic unified constitutive model. Ten material parameters are adopted in the above constitutive model, namely E, k0, Z, n, b, Q, a1, c1, a2, and c2.

[0088] 2. Establishment of the thermo-mechanical coupled Chaboche viscoplastic unified constitutive model

[0089] When temperature changes are considered, basically each material constitutive parameter is related to temperature. In the case of variable temperature, the total strain rate of the Chaboche viscoplastic unified constitutive model becomes:

[0090]

[0091] At this time, in Equation (16), ε to is the total strain, ε el is the elastic strain, ε in is the inelastic strain, and ε th is the temperature strain.

[0092] Under isothermal conditions, the Chaboche viscoplastic unified constitutive model represents viscoplasticity through a viscoplastic potential function as shown in Equation (4). Under variable temperature conditions, its expression is as shown in Equation (17).

[0093]

[0094] In the formula, Ω(T) is the viscoplastic potential function of the Chaboche viscoplastic unified constitutive model under variable temperature conditions; K and n are material parameters, and T represents temperature.

[0095] Under variable temperature conditions, for X iThe temperature-related terms are added to the evolution equations of \(X\) and \(R\), which are as follows:

[0096]

[0097] In the above equations, represents the evolution equation of \(X\) under isothermal conditions, i and represents the evolution equation of \(R\) under isothermal conditions.

[0098] 3. The parameters of the Chaboche viscoplastic unified constitutive model for the FGH4108 alloy at different temperatures are as follows:

[0099]

[0100] 4. Regarding the temperature fitting:

[0101] Using the parameters of the Chaboche viscoplastic unified constitutive model at different temperatures above, fitting them with respect to temperature, the fitting curves of the constitutive model parameters with respect to continuously varying temperature are obtained, so as to calibrate the temperature-related Chaboche viscoplastic constitutive model parameters. The fitting curves are as Figure 2 shown, which are the elastic modulus \(E\), the initial yield stress \(k_0\), the kinematic hardening parameters \(a_1\), \(a_2\), \(c_1\), \(c_2\) and the isotropic evolution parameter \(Q\) in sequence.

[0102] 5. Simulation results of thermo-mechanical fatigue cyclic deformation

[0103] Using the finite element software Abaqus and its built-in user-defined subroutine UMAT, adopting the established thermo-mechanical coupled Chaboche viscoplastic unified constitutive model considering temperature changes, combined with the temperature-related Chaboche viscoplastic unified constitutive model parameters, the hysteresis loops of the first cycle and the half-life cycle under the thermo-mechanical fatigue IP 2% condition and OP 2% condition are simulated. In the Abaqus finite element software, a temperature field that cyclically changes from 500 - 800 °C is applied to the established model, the load waveform is a triangular wave, the cycle period is 60 s. Under the IP condition, the temperature load and the displacement load change synchronously, that is, the mechanical strain is the maximum value of 1% at the highest temperature of 800 °C and the minimum value of -1% at the lowest temperature of 500 °C. The OP condition is opposite to the IP condition, the temperature load and the displacement load change in antiphase, that is, the mechanical strain is the minimum value of -1% at the highest temperature of 800 °C and the maximum value of 1% at the lowest temperature of 500 °C.

[0104] The simulation is carried out using the established thermo-mechanical coupled Chaboche viscoplastic unified constitutive model, and the comparison between the simulation results and the experimental results is as Figure 3 shown.

[0105] From the simulation results of thermo-mechanical fatigue cyclic deformation (Figure 3 ) It can be seen that the established thermo-mechanical coupled Chaboche viscoplastic unified constitutive model has relatively high overall simulation accuracy for the initial cycle and half-life cycle under IP and OP conditions. In particular, the simulation accuracy for the linear elastic stage and part of the viscoplastic stage is good, and it can reflect the typical characteristics of the asymmetry of the tensile and compressive hysteresis loops in the thermo-mechanical fatigue cycle, that is, the maximum tensile stress is less than the maximum compressive stress under IP conditions, and the maximum tensile stress is greater than the maximum compressive stress under OP conditions, which is consistent with the results of the thermo-mechanical fatigue test.

[0106] The present invention has been described exemplarily in conjunction with the accompanying drawings. Obviously, the specific implementation of the present invention is not limited by the above-mentioned manner. As long as such non-substantial improvements are made by adopting the method concept and technical solution of the present invention, or the concept and technical solution of the present invention are directly applied to other occasions without improvement, they are all within the protection scope of the present invention.

Claims

1. A method for establishing a Chaboche viscoplastic unified constitutive model under thermal-mechanical coupling conditions, characterized in that: The steps include: S1. Determine the basic equations of the Chaboche viscoplastic unified constitutive model; S2, based on the basic equation in step S1, add the temperature change term to establish the thermal-mechanical coupled Chaboche viscoplastic unified constitutive model; S3. Statistical analysis of the parameters of the Chaboche viscoplastic unified constitutive model at different temperatures; S4, fitting the parameters in step S3 with respect to temperature to obtain a fitting curve of the constitutive model parameters with respect to the continuously changing temperature, thereby calibrating the temperature-dependent Chaboche viscoplastic constitutive model parameters; S5. Perform simulation verification.

2. The method for establishing a Chaboche viscoplastic unified constitutive model under thermal-mechanical coupling conditions according to claim 1, characterized in that: In step S1, the basic equation of the Chaboche viscoplastic unified constitutive model is: f=|σ-X|-R-σ y (3) σ=X+(R+k0+σ v )sgn(σ-X) (15) In formula (1), represents the total strain rate of the material, represents the elastic strain rate of the material, represents the inelastic strain rate of the material; In formula (2), represents the stress rate of the material, and E represents the elastic modulus of the material; In formula (3), f represents the Von Mises yield function, σ is the yield stress of the material, σ y is the initial yield stress of the material, which indicates the size of the initial yield surface in the stress space. Since it is difficult to calibrate in the viscoplastic stage, it can be represented by the material parameter k0; X represents the kinematic hardening internal variable, which is used to describe the direction-related characteristics of the material; R represents the isotropic hardening internal variable, which is used to describe the change in the size of the yield surface in the stress space; In formula (4), Z and n are material parameters; Ω represents the viscoplastic potential function; f represents the yield function; In formula (8), is the evolution rate of the isotropic hardening internal variable, Q is the value of R approaching saturation, b is the rate at which R approaches saturation, The accumulated inelastic strain rate for the material; In formula (9), is the evolution rate of the internal variables of kinematic hardening; c is the linear gradient of the material parameter description function; In formula (10), γ is the material parameter; In formula (11), the first term is the Prage linear motion hardening term, which is a linear equation describing the inelastic strain rate of the material; i With the stress dimension, the saturation stress value of the material in the inelastic stage is described in the kinematic hardening equation, c i It is a material parameter without stress dimension, describing the rate at which the material stress approaches saturation in the kinematic hardening equation; In formula (12), M is the number of components of the internal variable back stress X of the kinematic hardening considered; In formula (14), σ v represents viscoplastic stress.

3. The method for establishing a Chaboche viscoplastic unified constitutive model under thermal-mechanical coupling conditions according to claim 2, characterized in that: In step S1, the material parameters used in the basic equation of the Chaboche viscoplastic unified constitutive model include E, k0, Z, n, b, Q, a i 、c i .

4. The method for establishing a Chaboche viscoplastic unified constitutive model under thermal-mechanical coupling conditions according to claim 3, characterized in that: In step S1, M is 2, i=1,2; Or M is 3, i=1,2,3.

5. The method for establishing a Chaboche viscoplastic unified constitutive model under thermal-mechanical coupling conditions according to claim 1, characterized in that: In step S2, the total strain rate of the Chaboche viscoplastic unified constitutive model under variable temperature conditions is: In formula (16), ε to is the total strain, ε el is the elastic strain, ε in is the inelastic strain, ε th is the temperature strain.

6. The method for establishing a Chaboche viscoplastic unified constitutive model under thermal-mechanical coupling conditions according to claim 1, characterized in that: In step S2, the expression of the viscoplastic potential function of the Chaboche viscoplastic unified constitutive model under variable temperature conditions is: Where Ω(T) is the viscoplastic potential function of the Chaboche viscoplastic unified constitutive model under variable temperature conditions; K and n are material parameters, and T represents temperature.

7. The method for establishing a Chaboche viscoplastic unified constitutive model under thermal-mechanical coupling conditions according to claim 1, characterized in that: In step S2, under the variable temperature condition, at X i The temperature-related terms are added to the evolution equations of and R, which are: In the above formula, represents X under isothermal conditions i The evolution equation of Represents the evolution equation of R under isothermal conditions.

8. The method for establishing a Chaboche viscoplastic unified constitutive model under thermal-mechanical coupling conditions according to claim 1, characterized in that: In step S3, the parameters of the Chaboche viscoplastic unified constitutive model at different temperatures of 500-800°C are counted.

9. The method for establishing a Chaboche viscoplastic unified constitutive model under thermal-mechanical coupling conditions according to claim 1, characterized in that: In step S5, the finite element software Abaqus and its own user-defined subroutine UMAT are used to simulate the hysteresis loops of the first cycle and half-life cycle under the thermomechanical fatigue IP 2% condition and OP 2% condition by adopting the thermal-mechanical coupled Chaboche viscoplastic unified constitutive model determined in step S4 and combining the temperature-dependent Chaboche viscoplastic unified constitutive model parameters.

10. The method for establishing a Chaboche viscoplastic unified constitutive model under thermal-mechanical coupling conditions according to claim 9, characterized in that: The simulation results are compared with the results of thermomechanical fatigue tests.

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