1. The method for predicting the oxidation life of the ceramic matrix composite structure is characterized by comprising the following steps of:

step 1: establishing a macroscopic model of the CMCs structural member according to working conditions, and calculating the temperature and stress distribution in the CMCs structural member through finite elements;

step 2: calculating the oxygen concentration distribution inside each unit in the CMCs structural member based on a homogenization method according to the temperature and stress distribution in the step 1;

and step 3: calculating microscopic oxidation morphology parameters inside each unit in the CMCs structural member based on an oxidation kinetic model according to the calculation result of the oxygen concentration distribution in the step 2;

and 4, step 4: calculating the mesoscopic mechanical property parameters of each unit of the CMCs structural member according to the mesoscopic oxidation morphology parameters obtained in the step 3;

and 5: calculating the mechanical property parameters of each weaving unit cell according to the mesoscopic mechanical property parameters obtained in the step 4; the mechanical property parameters comprise unit residual strength;

step 6: determining the stress distribution after the oxidation of the macroscopic structure, redefining the mechanical property parameters of the weaving unit cells obtained in the step 5 as the material parameters of the oxidized units in the macroscopic model, and performing finite element calculation on the macroscopic model of the CMCs structural member under the thermal-force-oxygen coupling environment again to obtain the stress distribution and the unit stress in the oxidized structure;

and 7: and (3) predicting the oxidation life of the structural member, comparing the unit stress obtained in the step (6) with the unit residual strength obtained in the step (5), if the unit stress is greater than the unit residual strength, considering the unit to fail, if the failure unit is accumulated to form a through failure unit group, considering the structural failure, otherwise, increasing the oxidation time, continuously adjusting the diffusion-oxidation process in the structural member, repeating the steps until the structural failure occurs, wherein the oxidation time at the moment is the natural oxidation life of the CMCs structure under the working condition.

2. The method of claim 1, wherein in step 2, the relationship between diffusion and oxidation kinetics during oxidation is described by a partial differential equation according to the law of conservation of energy as follows:

where φ is the porosity of the CMCs, c_AIs the oxygen concentration, D_effEquivalent diffusion coefficient, R, for each unit braided cell_AAnd solving the above formula by using a finite difference method to obtain the oxygen concentration of each unit in the structure, wherein x is the diffusion distance and t is the time.

3. The method according to claim 1, wherein in step 3, the calculation result of step 2 is used as the oxygen concentration in the pores of the woven unit cells inside each unit; part of oxygen diffuses to the fiber interface through the crack of the matrix, and the matrix and the fiber at the crack channel are oxidized into SiO₂The oxide layer and PyC interface consumption, the oxide layer thickness of the substrate and the fiber are respectively expressed as:

wherein z is_m、z_fRespectively showing the thickness of the oxide layer of the matrix and the oxide layer of the fiber,respectively represents the parabolic rate constants of the component materials under the pure oxygen of 100KPa of the matrix and the fiber when being oxidized, c^*Represents the oxygen concentration, p, at the oxidation of the constituent material at 100kPa pure oxygen_m、p_fThe indices of the matrix and the fiber, respectively; PyC interface consumption length l_rThe following formula is used for calculation:

wherein b is the number of moles of carbon consumed per mole of oxygen,is a mixed diffusion coefficient, M_CIs C molar mass, p_CIs the density of C, and is,indicating the oxygen concentration gradient within the crack, c₀Alpha is the ratio of the mass of reactant gas to product gas in terms of oxygen concentration at the tip of the crack.

4. The method of predicting oxidation life of a ceramic matrix composite structure of claim 1, wherein in step 4, the axial residual stiffness E 'of the fiber bundle composite after oxidation is based on a mesomechanics model'₁Comprises the following steps:

wherein l_dThe length of the interface debonding is,d_eis half of the crack width of the matrix, L is the average crack spacing of the matrix, v_fIs the fiber volume content, E'_f、E′_m、E′_cRespectively representing the elastic modulus of the oxidized fiber, the matrix and the fiber bundle composite material, and calculating by using a mixing ratio formula:

E′_f＝E_fv_f(x)+E_o(1-v_f(x))

E′_m＝E_mv_m(x)+E_o(1-v_m(x))，

E′_c＝E′_mv_m+E′_fv_f

wherein E is_f、E_mInitial modulus of elasticity, E, of the fiber and matrix, respectively_oAs oxidation products SiO₂Modulus of elasticity, v_mIs the initial matrix volume content, v_f(x)、v_m(x) Respectively representing the volume contents of the fiber and the matrix at a certain position x in an oxidation interval, and calculating the residual stiffness of the fiber bundle composite material after oxidation in other directions according to an equal proportion reduction method.

5. The method for predicting the oxidation life of a ceramic matrix composite structure according to claim 4, wherein in the step 5, the mechanical property parameters of the mesoscopic fiber bundle composite material are taken as the material parameters of the yarn units in the unit cell scale of each unit of the macroscopic model, and the macroscopic stress of the weaving unit is defined by the volume average method by adopting a standard homogenization processAnd macroscopic strain

Where V is the unit cell volume, σ and ε are the microscopic stresses and strains of each cell in the unit cell model,

according to the elastic constitutive relation of the material, 6 groups of periodic boundary conditions are applied to the weaving unit, finite element analysis is carried out, stress and strain distribution in the weaving unit is calculated, the stress-strain response of a plurality of groups of materials is obtained by combining the calculation formula of macroscopic stress and macroscopic strain, and finally the flexibility matrix [ S ] is solved_ij]：

According to the definition of the compliance matrix:

wherein E represents elastic modulus, G represents shear modulus, ν represents Poisson's ratio, subscripts 1, 2, 3 respectively correspond to warp, weft, thickness direction of the weaving unit, calculate the equivalent elastic modulus of the weaving unit in each direction according to the formula, and calculate the residual strength of the weaving unit through progressive damage analysis.

6. The method of claim 5, wherein the elastic constitutive relation of the material is expressed as follows based on the equal strain assumption and the stiffness averaging method:

in the formula, subscripts 1, 2, and 3 correspond to the warp direction, weft direction, and thickness direction in the weaving unit, respectively.

Background

The ceramic matrix Composite Materials (CMCs) have excellent mechanical properties at high temperature, so that the ceramic matrix composite materials have wide application prospects in the field of high-performance aircraft engine hot end components. A large amount of oxidizing gas exists in the service environment of the hot end component, and the CMCs structure and oxygen are subjected to oxidation reaction at high temperature to lose effectiveness. Predicting the oxidation life of the structural parts of the CMCs of the engine is one of the important problems to be solved by the structural design of the CMCs.

In order to reliably apply the ceramic matrix composite material to engineering practice, many scholars at home and abroad develop researches on the service life prediction of the ceramic matrix composite material. At present, most of the existing methods are fatigue life prediction, such as: a fatigue life prediction method (CN105760605A) of a complex braided structure ceramic matrix composite material discloses a prediction method of a fatigue life curve of the complex braided structure ceramic matrix composite material, and a fatigue life prediction method (CN111507038A) of the ceramic matrix composite material discloses a fatigue life prediction method of a ceramic matrix composite material structural member under a high-temperature and variable-load environment, but the prediction research of the oxidation life of the ceramic matrix composite material structural level under the condition of considering 'heat-force-oxygen' coupling environment is not disclosed.

The conventional method is mainly used for researching the fatigue life of CMCs material level and structure level based on a microscopic model, but the CMCs structural part faces a complex service environment in an aircraft engine, and comprises factors such as high temperature, stress, oxidation and the like. The existing literature indicates that the oxidative damage of CMCs is one of the important factors influencing the strength and the service life of the CMCs. Therefore, it is necessary to provide a method for predicting the oxidation life of the ceramic matrix composite structure so as to reflect the problem of influence of oxidation damage distribution on the strength of the structural member and realize the oxidation life prediction of the CMCs structural member.

Disclosure of Invention

The invention provides a method for predicting the oxidation life of a ceramic matrix composite structure aiming at the blank in the technical field and the defects of the prior art and aims to solve the problem of predicting the oxidation life of the ceramic matrix composite structure in a high-temperature-stress-oxidation coupling environment.

In order to achieve the purpose, the invention adopts the following technical scheme:

a method for predicting the oxidation life of a ceramic matrix composite structure comprises the following steps:

step 1: establishing a macroscopic model of the CMCs structural member according to working conditions, and calculating the temperature and stress distribution in the CMCs structural member through finite elements;

step 2: calculating the oxygen concentration distribution inside each unit in the CMCs structural member based on a homogenization method according to the temperature and stress distribution in the step 1;

and step 3: calculating microscopic oxidation morphology parameters inside each unit in the CMCs structural member based on an oxidation kinetic model according to the calculation result of the oxygen concentration distribution in the step 2;

and 4, step 4: calculating the mesoscopic mechanical property parameters of each unit of the CMCs structural member according to the mesoscopic oxidation morphology parameters obtained in the step 3;

and 5: calculating the mechanical property parameters of each weaving unit cell according to the mesoscopic mechanical property parameters obtained in the step 4; the mechanical property parameters comprise unit residual strength;

step 6: determining the stress distribution after the oxidation of the macroscopic structure, redefining the mechanical property parameters of the weaving unit cells obtained in the step 5 as the material parameters of the oxidized units in the macroscopic model, and performing finite element calculation on the macroscopic model of the CMCs structural member under the thermal-force-oxygen coupling environment again to obtain the stress distribution and the unit stress in the oxidized structure;

and 7: and (3) predicting the oxidation life of the structural member, comparing the unit stress obtained in the step (6) with the unit residual strength obtained in the step (5), if the unit stress is greater than the unit residual strength, considering the unit to fail, if the failure unit is accumulated to form a through failure unit group, considering the structural failure, otherwise, increasing the oxidation time, continuously adjusting the diffusion-oxidation process in the structural member, repeating the steps until the structural failure occurs, wherein the oxidation time at the moment is the natural oxidation life of the CMCs structure under the working condition.

Further, in step 2, according to the law of conservation of energy, the relationship between diffusion and oxidation kinetics during oxidation is described by using partial differential equation as follows:

where φ is the porosity of the CMCs, c_AIs the oxygen concentration, D_effEquivalent diffusion coefficient, R, for each unit braided cell_AAnd solving the above formula by using a finite difference method to obtain the oxygen concentration of each unit in the structure, wherein x is the diffusion distance and t is the time.

Further, taking the calculation result of the step 2 as the oxygen concentration in the pores of the weaving unit cells inside each unit; part of oxygen diffuses to the fiber interface through the crack of the matrix, and the matrix and the fiber at the crack channel are oxidized into SiO₂The oxide layer and PyC interface consumption, the oxide layer thickness of the substrate and the fiber are respectively expressed as:

wherein z is_m、z_fRespectively showing the thickness of the oxide layer of the matrix and the oxide layer of the fiber,respectively represents the parabolic rate constants of the component materials under the pure oxygen of 100KPa of the matrix and the fiber when being oxidized, c^*Represents the oxygen concentration, p, at the oxidation of the constituent material at 100kPa pure oxygen_m、p_fThe indices of the matrix and the fiber, respectively; PyC interface consumption length l_rThe following formula is used for calculation:

wherein b is the number of moles of carbon consumed per mole of oxygen,is a mixed diffusion coefficient, M_CIs C molar mass, p_CIs the density of C, and is,indicating the oxygen concentration gradient within the crack, c₀Alpha is the ratio of the mass of reactant gas to product gas in terms of oxygen concentration at the tip of the crack.

Further, in the step 4, according to a mesomechanics model, the axial residual stiffness E of the fiber bundle composite material after oxidation₁' is:

wherein l_dIs the interfacial debonding length, d_eIs half of the crack width of the matrix, L is the average crack spacing of the matrix, v_fIs the fiber volume content, E'_f、E'_m、E'_cRespectively representing the elastic modulus of the oxidized fiber, the matrix and the fiber bundle composite material, and calculating by using a mixing ratio formula:

wherein E is_f、E_mInitial modulus of elasticity, E, of the fiber and matrix, respectively_oAs oxidation products SiO₂Modulus of elasticity, v_mIs the initial matrix volume content, v_f(x)、v_m(x) Respectively representing the volume contents of the fiber and the matrix at a certain position x in an oxidation interval, and calculating the residual stiffness of the fiber bundle composite material after oxidation in other directions according to an equal proportion reduction method.

Further, in the step 5, the mechanical property parameters of the microscopic fiber bundle composite material are used as the material parameters of the yarn units in the unit cell scale of each unit of the macroscopic model, a standard homogenization process is adopted, and the macroscopic stress of the weaving unit is defined by a volume average methodAnd macroscopic strain

Where V is the unit cell volume, σ and ε are the microscopic stresses and strains of each cell in the unit cell model,

according to the elastic constitutive relation of the material, 6 groups of periodic boundary conditions are applied to the weaving unit, finite element analysis is carried out, stress and strain distribution in the weaving unit is calculated, the stress-strain response of a plurality of groups of materials is obtained by combining the calculation formula of macroscopic stress and macroscopic strain, and finally the flexibility matrix [ S ] is solved_ij]：

According to the definition of the compliance matrix:

wherein E represents elastic modulus, G represents shear modulus, ν represents Poisson's ratio, subscripts 1, 2, 3 respectively correspond to warp, weft, thickness direction of the weaving unit, calculate the equivalent elastic modulus of the weaving unit in each direction according to the formula, and calculate the residual strength of the weaving unit through progressive damage analysis.

Further, based on the isostrain assumption and the rigidity averaging method, the elastic constitutive relation of the material is expressed as:

in the formula, subscripts 1, 2, and 3 correspond to the warp direction, weft direction, and thickness direction in the weaving unit, respectively.

The method for predicting the oxidation life of the ceramic matrix composite structure, provided by the invention, considers the local material performance degradation caused by the nonuniform oxidation degree inside the CMCs structure under the thermal-force-oxygen coupling environment, simulates the processes of oxidation failure and strength degradation of the internal units of the CMCs structure after reaction diffusion interaction based on the diffusion theory and the oxidation kinetics theory, and provides a reference basis for the CMCs structure design. The method simulates the evolution process of oxidation damage and unit failure of the structural member from the perspective that each region in the structural member is oxidized in different degrees under the complex environment, integrates the coupling evolution of gas concentration distribution, component material consumption oxidation and mechanical property degradation in the CMCs, and realizes the oxidation life prediction of the CMCs under different working conditions at the structural level.

Drawings

FIG. 1 is a flow chart of the method of the present invention.

FIG. 2 is a diagram of the accumulation process of failure units of a CMCs regulating sheet model under a thermal-force-oxygen coupling environment.

Detailed Description

The present invention will now be described in further detail with reference to the accompanying drawings.

Based on the initial condition of the actual working condition, the oxidation life of the CMCs is analyzed by adopting the method provided by the invention by taking the CMCs regulating sheet as an example.

Step 1: and establishing a macroscopic geometric model of the adjusting sheet, applying linearly changed temperature and pressure loads on the inner side of the adjusting sheet, applying constant loads on the outer side of the adjusting sheet, and calculating the temperature and stress distribution in the adjusting sheet through finite elements.

Step 2: and (4) determining the oxygen concentration at each point of the regulating sheet based on a homogenization method according to the temperature field and the stress field in the step (1). The relationship between diffusion and oxidation kinetics during oxidation can be described by a partial differential equation according to the law of mass conservation:

wherein phi is the porosity of the CMCs, c_AOxygen concentration, t time, D_effEquivalent diffusion coefficient, R, for each unit braided cell_AIs the reaction rate. And solving the above formula by using a finite difference method to obtain the oxygen concentration at each point inside the CMCs regulating sheet along the gas transmission direction.

And step 3: and (3) calculating the oxidation morphology of each point in the regulating sheet based on an oxidation kinetic model according to the calculation result of the concentration in the step (2). Wherein, the calculation result of the step 2 is used as the oxygen concentration in the pores of the weaving unit cells in each unit, and the yarn matrix around the pores is firstly uniformly oxidized. The other part of the oxygen is diffused to the fiber interface through the crack of the matrix, and SiO is generated by oxidation of the matrix and the fiber at the crack channel₂Oxide layer and PyC interface consumption. Wherein, the thickness of the oxide layer of the matrix and the fiber is expressed as follows:

in the formula, z_m、z_fRespectively showing the thickness of the oxide layer of the matrix and the oxide layer of the fiber,respectively represents the parabolic rate constants of the component materials under the pure oxygen of 100KPa of the matrix and the fiber when being oxidized, c^*Is the oxygen concentration, p_m、p_fThe indices of the matrix and the fiber, respectively. PyC interface consumption length l_rThe following formula can be used for calculation:

wherein b is the number of moles of carbon consumed per mole of oxygen,is a mixed diffusion coefficient, M_CIs C molar mass, p_CIs the density of C, and is,indicating the oxygen concentration gradient within the crack, c₀Alpha is the ratio of the mass of reactant gas to product gas in terms of oxygen concentration at the tip of the crack.

And 4, step 4: and (4) calculating the mesoscopic mechanical property degradation condition of each point according to the mesoscopic oxidation morphology parameters of each point obtained by calculation in the step (3). According to a mesomechanics model, the axial residual stiffness of the fiber bundle composite material after oxidation is as follows:

in the formula I_dIs the interfacial debonding length, d_eIs half of the crack width of the matrix, L is the average crack spacing of the matrix, v_fIs the fiber volume content, E_f'、E_c' respectively represents the elastic modulus of the oxidized fiber and fiber bundle composite materials, and can be calculated by using a mixing ratio formula:

the residual stiffness of the fiber bundle composite material after oxidation in the other directions can be calculated according to an equal proportion reduction method. Wherein the initial material parameters of the fiber bundle composite material are listed in the following table:

TABLE 1SiC/SiC fiber bundle composite parameters

And 5: and (4) calculating the mechanical property degradation condition of the weaving unit cell at each point of the regulating sheet model according to the mesoscopic mechanical property parameters obtained by calculation in the step (4). Taking the mechanical property calculation result of the mesoscopic fiber bundle composite material as the material parameters of yarn units in unit cell scales of a macroscopic adjusting sheet model, adopting a standard homogenization process, and defining the macroscopic stress and macroscopic strain of a weaving unit cell by a volume average method:

solving a compliance matrix according to the elastic constitutive relation of the material:

by definition of the compliance matrix:

ν₁₂＝-S₁₂·E₁,ν₁₃＝-S₁₃·E₁,ν₂₃＝-S₂₃·E₂

the equivalent elastic modulus of each weaving unit cell in each direction can be calculated, and the residual strength of each weaving unit cell can be calculated through progressive damage analysis.

Step 6: and determining the stress distribution of the CMCs after the adjustment sheet is oxidized. And (5) redefining the mechanical parameter calculation result of the woven unit cell in the step (5) as the material parameter of each oxidized unit in the macroscopic model, and performing finite element calculation on the macroscopic model of the CMCs regulating piece under the working condition of the embodiment again to obtain the stress distribution condition in the oxidized regulating piece.

And 7: and predicting the oxidation life of the CMCs regulating tablet. And (3) judging the failure of each unit in the adjusting sheet according to the maximum stress failure criterion, comparing the unit stress calculated in the step 6 with the unit residual strength calculated in the step 5, if the unit stress exceeds the residual strength of the unit, considering that the unit fails, marking a failure unit, and recording the number of the failure units. If the failure units are accumulated to form a through failure unit group, the structure is considered to be failed; otherwise, increasing the oxidation time, continuing to adjust the oxidation process in the sheet, and repeating the steps until the structure fails. Fig. 2 shows a damage accumulation process of unit failure caused by strength degradation after material oxidation in the CMCs adjusting sheet, with the increase of oxidation time, the failure units of the CMCs adjusting sheet model continuously expand along the axial direction and the thickness direction, when the oxidation time reaches 1200h, the failure units almost cover the whole plane, and after that, the increase rate of the failure units becomes very small, so that the oxidation life of the CMCs adjusting sheet after being subjected to coupling damage such as "high temperature-stress-oxidation" can be predicted to be 1200h in this example. In the embodiment, different geometric model structural members under different working conditions can be analyzed according to given temperature and load conditions, and the oxidation life of the CMCs structural member can be predicted.

The above is only a preferred embodiment of the present invention, and the protection scope of the present invention is not limited to the above-mentioned embodiments, and all technical solutions belonging to the idea of the present invention belong to the protection scope of the present invention. It should be noted that modifications and embellishments within the scope of the invention may be made by those skilled in the art without departing from the principle of the invention.