A method for evaluating the strength of ceramic matrix composites in a reuse environment

Abstract

The application discloses a strength evaluation method for ceramic matrix composites in a reuse environment, which is based on a molecular dynamics simulation method and a mesoscopic finite element simulation method, and uses a multi-scale method to transit a fatigue damage mechanism to a failure criterion, quantitatively describes fatigue damage of the ceramic matrix composites in the reuse environment through a damage variable, and can accurately predict strength changes of the ceramic matrix composites in various reuse environments (force / heat / oxygen). The method is suitable for various ceramic matrix composites and has universality.

Description

TECHNICAL FIELD

[0001] The application relates to a strength evaluation method of a ceramic matrix composite material under a repeated use environment and belongs to the field of composite material mechanics. BACKGROUND

[0002] The ceramic matrix composite material is widely applied to the design of heat-proof and load-carrying integrated structures. The heat-proof and load-carrying integrated structures have good mechanical properties and thermophysical properties, can ensure the aerodynamic shape of the structures and ensure that the structures do not fail under the aerodynamic pressure, thermal stress, vibration, noise and impact environment, and play the double roles of heat-proof and load-carrying. In recent years, a large part of the reason for the continuous failure of hypersonic vehicle flight tests is the strength problem of the heat-proof and load-carrying integrated structures, and the importance of the strength evaluation method of the ceramic matrix composite material as a key material for developing the heat-proof and load-carrying integrated structures is self-evident. Under the repeated use condition, the strength degradation of the ceramic matrix composite material under the coupling of force, heat and oxygen has appeared in engineering, and how to evaluate the strength of the ceramic matrix composite material under the repeated use environment is an important research content for the development of the hypersonic vehicle.

[0003] The heat-proof and load-carrying integrated structures prepared from the ceramic matrix composite material have realized high-temperature service and have obtained many basic achievements, but generally adopt relatively constant high-temperature test conditions. Although the fatigue load-carrying performance of the ceramic matrix composite material under the thermal cycle condition is given in the research based on the hot end part of the aero-engine, the rapid cycle change in the working range of 700 DEG C to 1200 DEG C of the engine is considered, which is very different from the service condition simulated in the reusability evaluation of the space-air vehicle, that is, the cycle change process of normal temperature to high temperature and then to normal temperature under the oxygen environment, and the material properties are also different. The foreign public research data has not reported the test examination simulating the repeated use, and the given is still the force-thermal coupling structure test satisfying the single use requirement.

[0004] There are many studies on the mechanical properties of ceramic matrix composites under the force / thermal / oxygen coupling environment for single use. Li et al. observed the compression properties and failure mechanism of needled C / C composites in the presence of oxygen at room temperature to 950℃. They found that the failure mode of needled C / C composites under compression load was mainly shear failure, and when the temperature exceeded 600℃, the material showed obvious nonlinear failure characteristics due to oxidation. NASA Glenn Research Center supported by UEET (Ultra Efficient Engine Technology) and NGLTQ (Next Generation Launch Technology) programs, carried out high-temperature performance research on SiC / SiC composites mainly by CVI and MI processes, and obtained the material properties under different temperature, environment and time factors. Li et al. of Xiamen University studied the microstructure evolution of C / SiC composites with SiC coating after annealing at different temperatures in a wet oxygen environment. The characterization results showed that high-temperature annealing (1000℃) would react to form molten silicon oxide, which would bridge the microcracks on the surface and inside of the composite, hindering the rapid inward diffusion of oxygen molecules along geometric defects, but would cause the volatilization of Si(OH)4 and produce new defects when the temperature was raised again (1200℃), resulting in a decrease in residual strength.

[0005] There are relatively few studies on the mechanical behavior of ceramic matrix composites under repeated use of force / thermal / oxygen coupling environment. The US Air Force supported by the IHPTET (High Performance Turbine Engine, Technology) project conducted in-depth research on the fatigue properties of 2D woven SiC / SiC air and steam composites in 1200-1300℃ air and steam environment, obtained key data such as fatigue limit strength of SiC / SiC composites prepared by different processes, and discussed the damage mechanism of SiC / SiC composites fatigue. Northwest Industrial University prepared 3D woven Hi-Nicalon SiC / SiC composites by CVI process, and studied the damage evolution of fatigue oxidation in 1300℃ water-oxygen environment. Zhang Lixiang's group studied the thermal cycle damage mechanism of C / SiC composites under strain constraint and oxidation atmosphere, and characterized the transverse cracks in the matrix, fiber bridging, debonding and fracture, and oxidation path of fiber bundles after thermal cycling. Lv Qihui carried out repetitive thermal loading tests on the inner and outer structures of integrated materials, namely ceramic heat insulation tiles and ablation-resistant materials, respectively, to explore the changes in the heat insulation performance of the materials, focusing on the macro / microstructure evolution of the inner and outer single-layer structure materials under different thermal loading times, revealing the evolution mechanism, determining the heat insulation performance evaluation variables and building a life prediction model, and realizing the life prediction of the heat protection and insulation performance of integrated thermal protection materials.

[0006] In summary, the current research on the mechanical properties of ceramic matrix composites under single-use requirements has made good progress, but there are still few reports on the strength prediction and evaluation of ceramic matrix composites under repeated use environment. The related research is more focused on the damage mechanism or fatigue life of ceramic matrix composites under repeated use environment. SUMMARY

[0007] The technical problem solved by the present application is to overcome the shortcomings of the prior art and provide a ceramic matrix composite strength evaluation method under repeated use environment, which solves the problem of lack of method for predicting the strength change of ceramic matrix composites under repeated use environment in the prior art.

[0008] The technical solution of the present application is:

[0009] A ceramic matrix composite strength evaluation method under repeated use environment, comprising:

[0010] (1) A molecular dynamics model is established according to the microcharacterization of fibers and matrix, load simulation is performed to obtain the load stress-strain curve of fibers and matrix, and the constitutive relation and mechanical properties are extracted;

[0011] (2) A fiber bundle representative volume element finite element model is established according to the microcharacterization of the fiber bundle, the mechanical properties extracted in step (1) are set to the material mechanical property parameters of the finite element model, and the constitutive relation extracted in step (1) is programmed into the UMAT subroutine to realize the customization of the material constitutive relation, load simulation is performed to obtain the load stress-strain curve of the fiber bundle, and the constitutive relation and mechanical properties of the fiber bundle are extracted;

[0012] (3) A ceramic matrix composite representative volume element finite element model is established according to the microcharacterization of the ceramic matrix composite, the mechanical properties extracted in step (2) are set to the material mechanical property parameters of the finite element model, and the constitutive relation extracted in step (2) is programmed into the UMAT subroutine to realize the customization of the material constitutive relation, load simulation is performed to obtain the load stress-strain curve of the ceramic matrix composite, and the constitutive relation and mechanical properties of the ceramic matrix composite are extracted;

[0013] (4) Microscopic damage characterization is performed on the ceramic matrix composite under different number of load simulations, and compared with the damage form of the finite element model in step (3), the constitutive relation in step (3) is corrected until the damage form of the finite element model simulation result is consistent with the characterization result of the microscopic damage characterization.

[0014] (5) using the finite element model established in step (3) and defining the constitutive relation obtained in step (4), load simulation is performed to obtain the load stress-strain curve of the ceramic matrix composite, and the mechanical properties of the ceramic matrix composite are extracted;

[0015] (6) setting different load times, repeating step (5) to obtain the finite element model after different load times are applied;

[0016] (7) performing load simulation on the finite element model in step (6) to obtain the residual strength of each model, and processing the data to establish a mathematical model.

[0017] In the step (1), the micro characterization should obtain the composition of the fiber and the matrix.

[0018] In the step (2), the fiber bundle is composed of the fiber and the matrix in the step (1), and the micro characterization should obtain the proportion of the fiber in the fiber bundle, i.e. the volume fraction.

[0019] In the step (2), the selected failure criterion for the constitutive relation in the UMAT subroutine is the maximum strain criterion: when the ratio of the nominal strain to the maximum nominal strain is 1, i.e. the damage starts when the failure coefficient f is 1;

[0020] The calculation formula of the failure strain ε0 and the failure coefficient f is as follows:

[0021]

[0022] In the formula, S is the material strength, E is the material elastic modulus, if the tensile strength is calculated, S is set as the tensile strength, and E is the elastic modulus in the tensile direction; ε1 is the strain at the end of the current analysis step, which is obtained according to the load simulation in step (2).

[0023] After the representative volume element finite element model of the fiber bundle is damaged, the stiffness is reduced, and the stiffness matrix changes as follows:

[0024]

[0025] In the formula, C 11 , C 12 , C 13 , C 21 , C 22 , C 23 , C 31 , C 32 , C 33 , C 44 , C 55 , C 66 are stiffness matrix parameters; d is a damage variable, and the calculation formula is as follows: d = 1 - e a(1-f) , (f > 1)

[0026] a is a constant, which is set in relation to the material stiffness degradation rate, and is determined by the stress-strain curve in step (1).

[0027] In step (3), the ceramic matrix composite is composed of the fiber bundle in step (2) and the matrix in step (1), and the micro-characterization should obtain the arrangement of the fiber bundle and the cross-sectional image of the fiber bundle.

[0028] In step (3), the failure criterion selected in the constitutive relationship in the UMAT subroutine is the Tsai-Wu criterion, and when the failure coefficient F TW reaches 1, the corresponding element in the finite element model of the ceramic matrix composite is damaged;

[0029] F TW is calculated as follows:

[0030]

[0031] where σ1 is the stress obtained by load simulation in step (3);

[0032] In the formula, each coefficient is calculated as follows:

[0033]

[0034]

[0035]

[0036]

[0037]

[0038]

[0039] In the formula, X T , X C are the tensile and compressive strengths in the x direction, Y T , Y C are the tensile and compressive strengths in the y direction, Z T , Z C are the tensile and compressive strengths in the z direction, S 12 is the shear strength in the xy direction, S 13 is the shear strength in the xz direction, and S 23 is the shear strength in the yz direction.

[0040] After the representative volume element finite element model of the ceramic matrix composite in step (3) is damaged, the stiffness is reduced, and the stiffness matrix changes as follows:

[0041]

[0042] In the formula, D1, D2, D3, D4, D5, D6 are damage parameters, which are obtained from the constitutive relation and mechanical properties of the fiber bundle simulated in step (2); C 11 、C 12 、C 13 、C 21 、C 22 、C 23 、C 31 、C 32 、C 33 、C 44 、C 55 、C 66 are stiffness matrix parameters.

[0043] In the step (4), the modified part of the constitutive relation is mainly the damage parameters corresponding to different failure modes.

[0044] In the step (6), the setting of different load times is determined according to the maximum load times when the finite element model is completely destroyed in the step (5).

[0045] In summary, the present application at least includes the following beneficial technical effects:

[0046] (1) The strength evaluation method of the ceramic matrix composite material in the repeated use environment provided by the present application obtains the constitutive relation and mechanical properties of the ceramic matrix composite material through molecular dynamics simulation and finite element simulation, combines with the characterization research on the damage mechanism of the ceramic matrix composite material in the repeated use environment, and modifies the failure criterion to better describe the damage evolution of the ceramic matrix composite material, so that the mechanical properties of the ceramic matrix composite material composed of different ceramic materials are predicted, and the performance degradation of the ceramic matrix composite material in the repeated use environment is quantitatively described.

[0047] (2) The present application describes the damage mechanism of the ceramic matrix composite material at different scales from microscale to mesoscale, and then from mesoscale to macroscale, considers the damage evolution trend of the ceramic matrix composite material under different damage modes, predicts the strength change of the ceramic matrix composite material under various repeated use environments (force / heat / oxygen) based on this, and is suitable for the strength evaluation of various ceramic matrix composite materials, and has universality. BRIEF DESCRIPTION OF DRAWINGS

[0048] Figure 1 is a flowchart of the method of the present application;

[0049] Figure 2 is a stress-strain diagram extracted by the method of the present application;

[0050] Figure 3 is a residual strength curve diagram extracted by the method of the present application. Detailed Implementation

[0051] The present application will now be described in further detail with reference to the accompanying drawings and specific embodiments:

[0052] This application discloses a method for evaluating the strength of ceramic matrix composites in a reusable environment, such as... Figure 1 As shown, the steps are as follows:

[0053] (1) Based on the microscopic characterization of the fiber and matrix, the components of the fiber and matrix are obtained. The potential energy function is selected based on the relevant components to describe the interatomic interactions. A thermodynamic system is set up, and molecular models of the fiber and matrix are established. Appropriate loads, oxidation environments, and temperature environments are set according to requirements. Molecular dynamics simulations are performed to obtain stress-strain curves, such as... Figure 2 Based on the slope of the elastic stage and the peak of the stress-strain curve, the elastic modulus and strength are extracted respectively. The entire stress-strain curve is then fitted to obtain the material constitutive relation.

[0054] (2) The proportion of fibers in the fiber bundle, i.e., the volume fraction, is obtained based on the microscopic characterization of the fiber bundle. Representative volume elements of the fiber bundle are then established based on this volume fraction. Periodic boundary conditions are established by setting linear constraint equations at the corresponding mesh nodes on the parallel surfaces of the representative volume elements. The representative volume element has a length of Wx, a width of Wy, and a height of h, with the origin at point D. Six typical strain loads are applied. Under the following conditions, periodic boundary conditions can be achieved by the following set of linear constraint equations to realize the corresponding load cases:

[0055] On the opposite plane perpendicular to the x-axis

[0056]

[0057] On the opposite plane perpendicular to the y-axis

[0058]

[0059] On the opposite plane perpendicular to the z-axis

[0060]

[0061] In the formula, the three planes x = Wx, y = Wy, and z = h are called the principal planes, and the planes parallel to or opposite to the principal planes are called secondary planes.

[0062] The model material mechanical property parameters are set to the elastic modulus and strength extracted in step (1), the corresponding environment and load are set according to the requirements, the constitutive relation (such as the trend of the elastic stage and the yield stage) extracted in step (1) is programmed into the UMAT subroutine, the related parameters of UMAT (such as the related parameters for describing the damage variable) are modified, the UMAT subroutine is called, the maximum strain criterion is used as the damage criterion to describe the judgment basis for the damage of the element, the load simulation is performed to obtain the load stress-strain curve of the fiber bundle, and the constitutive relation and mechanical properties of the fiber bundle are generally extracted in step (1).

[0063] Maximum strain criterion: when the ratio of the nominal strain to the maximum nominal strain is 1, i.e. the failure coefficient f is 1, the damage starts. The calculation formula of the failure strain ε0 and the failure coefficient f is as follows:

[0064]

[0065] In the formula, S is the material strength, E is the material elastic modulus, if the tensile strength is calculated, S is set to the tensile strength, and E is the elastic modulus in the tensile direction; ε1 is the strain at the end of the current analysis step, which is obtained according to the load simulation in step (2).

[0066] After damage, the stiffness is reduced, and the stiffness matrix changes as follows:

[0067]

[0068] In the formula, d is the damage variable, and the calculation formula is as follows:

[0069] d = 1-e a(1-f) , (f>1)

[0070] In the formula, α is a constant, which is related to the stiffness degradation speed of the material, and is determined according to the stress-strain curve in step (1).

[0071] (3) According to the micro-characterization of the ceramic matrix composite, the arrangement mode of the fiber bundle, such as the weaving mode (five-satin weaving / flat weaving, 2D weaving mode / 3D weaving mode), and the cross-sectional image of the fiber bundle are obtained, a representative volume element finite element model of the ceramic matrix composite is established according to the characterization results, the material mechanical property parameters of the fiber bundle part in the model are set as the elastic modulus and strength extracted in step (2), the material mechanical property parameters of the matrix part in the model are set as the matrix elastic modulus and strength extracted in step (1), the periodic boundary conditions are set as in step (2), the constitutive relation (such as the elastic stage and the yield stage trend) extracted in step (2) is programmed into the UMAT subroutine, the corresponding environment and load are set according to the requirements, the UMAT subroutine is called, the Tsai-Wu criterion is used as the failure criterion to describe the judgment basis for the element failure, and the load simulation is performed to obtain the load stress-strain curve of the ceramic matrix composite, such as shown in FIG. 1, and the constitutive relation and mechanical properties of the ceramic matrix composite are generally extracted in step (1) simultaneously. Figure 2

[0072] Tsai-Wu criterion, when the failure coefficient F TW reaches 1, the corresponding element in the finite element model of the ceramic matrix composite is damaged.

[0073] The calculation formula of F TW is as follows:

[0074]

[0075] In the formula, each coefficient is calculated as follows:

[0076]

[0077]

[0078]

[0079]

[0080]

[0081]

[0082] In the formula, X T , X C are the tensile and compressive strengths in the x direction, Y T , Y C are the tensile and compressive strengths in the y direction, Z T , Z C are the tensile and compressive strengths in the z direction, S 12 is the shear strength in the xy direction, and S 13 ​Sxz is the shear strength in xz direction 23 Syz is the shear strength in yz direction.

[0083] After damage, the stiffness is reduced, and the stiffness matrix changes as follows:

[0084]

[0085] In the formula, D1, D2, D3, D4, D5, D6 are damage parameters, which are obtained from the constitutive relation and mechanical properties of the fiber bundle simulated in step (2).

[0086] (4) For typical failure modes, micro-damage characterization is performed on ceramic matrix composites, such as fiber pull-out, fiber breakage, oxidation damage, etc. Different damage parameters are set for different failure modes to obtain the damage caused by single load in the repeated use environment. The damage process after load application in step (3) is retrieved, the damage process of micro-damage characterization is compared, and the constitutive relation in step (3) is modified until the damage mode and subsequent crack propagation mode of the finite element damage simulation result are consistent with the characterization result of the micro-damage characterization;

[0087] (5) Continue to use the finite element model established in step (3), and use the constitutive relation modified in step (4), call the modified UMAT subroutine, modify the load amplitude according to the load spectrum to simulate the real repeated load environment, and apply the load for repeated load simulation. Call the stress-strain curve of ceramic matrix composite under repeated load, and extract the mechanical properties (strength) of ceramic matrix composite under repeated load in step (1);

[0088] (6) According to the stress-strain curve and mechanical properties obtained in step (5), analyze the fatigue performance of ceramic matrix composite, obtain the life (load cycle number) of ceramic matrix composite under repeated load environment simulation, normalize the life of ceramic matrix composite, and set different load cycle numbers according to a certain proportion. The setting of different load numbers is determined according to the maximum load number of the finite element model in step (5) when it is completely damaged. Repeat step (5) according to the set load number to obtain the finite element model after applying different load numbers;

[0089] (7) For several finite element models in step (6), apply the repeated load in step (5) to obtain the residual strength (normalized) of each model, such as Figure 3 , process the data and fit it into a mathematical model of residual strength in the failure load.

[0090] Although the present application is disclosed with reference to the preferred embodiments above, it is not intended to limit the present application, and any person skilled in the art can make possible variations and modifications without departing from the spirit and scope of the present application. Therefore, the scope of protection of the present application should be defined by the scope of the claims.

Claims

1. A method for evaluating the strength of a ceramic matrix composite material in a reuse environment, characterized by, The method comprises the following steps: (1) a molecular dynamics model is established according to the micro-characterization of the fiber and the matrix, load simulation is performed to obtain the load stress-strain curve of the fiber and the matrix, and the constitutive relation and the mechanical property are extracted; (2) a fiber bundle representative volume element finite element model is established according to the micro-characterization of the fiber bundle, the material mechanical property parameters of the finite element model are set through the mechanical property extracted in step (1), the constitutive relation extracted in step (1) is programmed into a UMAT subroutine to realize the self-definition of the material constitutive relation, load simulation is performed to obtain the load stress-strain curve of the fiber bundle, and the constitutive relation and the mechanical property of the fiber bundle are extracted; (3) a ceramic matrix composite representative volume element finite element model is established according to the micro-characterization of the ceramic matrix composite, the material mechanical property parameters of the finite element model are set through the mechanical property extracted in step (2), the constitutive relation extracted in step (2) is programmed into a UMAT subroutine to realize the self-definition of the material constitutive relation, load simulation is performed to obtain the load stress-strain curve of the ceramic matrix composite, and the constitutive relation and the mechanical property of the ceramic matrix composite are extracted; (4) the micro-damage characterization of the ceramic matrix composite is performed for different times of load simulation, and the damage form of the finite element model in step (3) is compared, the constitutive relation in step (3) is modified until the damage form of the finite element model simulation result is consistent with the characterization result of the micro-damage characterization; (5) the finite element model established in step (3) is used, the constitutive relation obtained in step (4) is defined, the load stress-strain curve of the ceramic matrix composite obtained by load simulation is obtained, and the mechanical property of the ceramic matrix composite is extracted; (6) different load times are set, and step (5) is repeated to obtain the finite element model after different load times are applied; (7) the residual strength of each model is obtained by performing load simulation on the finite element model in step (6), the data is processed, and a mathematical model is established.

2. The method of claim 1, wherein the method is used for evaluating the strength of a ceramic matrix composite material in a repeated use environment. In step (1), the micro-characterization obtains the components of the fiber and the matrix.

3. The method of claim 1, wherein the method is used for evaluating the strength of a ceramic matrix composite material in a repeated use environment. In step (2), the fiber bundle is composed of the fiber and the matrix in step (1), and the micro-characterization obtains the proportion of the fiber in the fiber bundle, i.e. the volume fraction.

4. The method of claim 1 or 3, wherein the method is used for evaluating the strength of a ceramic matrix composite material in a repeated use environment. In step (2), the damage criterion selected in the constitutive relation in the UMAT subroutine is the maximum strain criterion: when the ratio of the nominal strain to the maximum nominal strain reaches 1, i.e. when the damage starts, the failure coefficient f is 1. failure strain The formula for calculating the failure strain and the failure coefficient f is as follows: In the formula, S is the material strength, E is the material modulus of elasticity, and if the tensile strength is to be calculated, S is set as the tensile strength and E is set as the modulus of elasticity in the tensile direction; is the strain at the end of the current analysis step, is obtained from the load simulation according to step (2).

5. The method of claim 4, wherein: After the fiber bundle representative volume element finite element model is damaged, the stiffness is reduced, and the stiffness matrix changes as follows: where C 11 , C 12 , C 13 , C 21 , C 22 , C 23 , C 31 , C 32 , C 33 , C 44 , C 55 , C 66 are stiffness matrix parameters; d is a damage variable, and the calculation formula is as follows: d = f > 1, is a constant, and the setting of the number is related to the material stiffness degradation speed and is determined according to the stress-strain curve in step (1).

6. The method of claim 1, wherein: In step (3), the ceramic matrix composite is composed of the fiber bundle in step (2) and the matrix in step (1), and the micro-characterization obtains the arrangement mode of the fiber bundle and the cross-sectional image of the fiber bundle.

7. The method of claim 1 or 6, wherein: In the step (3), the selected failure criterion in the constitutive relation in the UMAT subroutine is Tsai-Wu criterion, and when the failure coefficient F TW When the failure coefficient F reaches 1, the corresponding element in the ceramic matrix composite finite element model is damaged. F TW The calculation formula is as follows: Wherein, σ1 is the stress, which is obtained by load simulation in step (3); In the formula, each coefficient is calculated as follows: In the formula, , are the tensile and compressive strengths in the x direction, respectively, , are the tensile and compressive strengths in the y direction, respectively, , are the tensile and compressive strengths in the z direction, respectively, is the shear strength in the xy direction, is the shear strength in the xz direction, is the shear strength in the yz direction.

8. The method of claim 5, wherein the ceramic matrix composite material is evaluated for strength in a re-use environment. After the ceramic matrix composite representative volume element finite element model in step (3) is damaged, the stiffness is reduced, and the stiffness matrix changes as follows: where D1, D2, D3, D4, D5, D6 are damage parameters, which are obtained from the constitutive relation and mechanical properties of the fiber bundle simulated in step (2).

9. The method of claim 8, wherein the ceramic matrix composite material is a ceramic matrix composite material used in a repeated use environment. In step (4), the modified part of the constitutive relation is the damage parameter corresponding to different damage modes.

10. The method of claim 9, wherein the ceramic matrix composite material is a ceramic matrix composite material for a reusable environment. The step (6) is determined by the maximum load number of the finite element model in the step (5).

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