Method for predicting fatigue life of composite structure after impact based on micromechanics

Abstract

The application discloses a composite material structure fatigue life prediction method after impact based on micromechanics, adopts a bridging model based on micromechanics to predict the transverse isotropic mechanical properties of a composite material laminated structure, assigns the mechanical properties to a finite element model, performs dynamics simulation to obtain a fatigue analysis finite element model, adopts static force calculation instead of fatigue calculation, simulates material performance degradation phenomena in a cyclic loading process based on a material performance sudden degradation model and a material performance gradual degradation model, and obtains a composite material fatigue life value.

Description

Technical Field

[0001] This invention relates to a method for predicting the fatigue life of composite material structures after impact based on micromechanics, belonging to the field of fatigue performance analysis and calculation of fiber-reinforced composite materials. Background Technology

[0002] Composite materials possess excellent mechanical properties, good designability, and the ability to be integrally molded, making them widely used in the aerospace field. However, composite laminate structures are sensitive to impact; impacted composite structures can develop internal defects or cracks invisible to the naked eye. Studies have shown that the compressive strength of impact-damaged laminated composite sheets decreases by 40%-60% compared to undamaged structures. This significantly reduces aircraft safety, damage tolerance levels, and service life. Therefore, developing high-precision fatigue life prediction methods for composite structures with impact damage is of great value for damage tolerance design and maintenance strategy formulation. Summary of the Invention

[0003] The technical problem solved by this invention is to overcome the shortcomings of the prior art and propose a method for predicting the fatigue life of composite material structures after impact based on micromechanics.

[0004] The technical solution of this invention is:

[0005] A method for predicting the fatigue life of composite material structures after impact based on micromechanics includes:

[0006] A bridging model based on micromechanics is used to predict the transverse isotropic mechanical properties of composite laminates using the mechanical parameters of fibers and the matrix.

[0007] Based on the finite element model of composite laminate structure, mechanical properties are assigned to the finite element model, and explicit dynamic simulation of impact process is performed to obtain the stress distribution of each element. According to the Hashin failure criterion, the fiber tensile and compressive damage factors and matrix tensile and compressive damage factors in each element are obtained. Based on the above factors, it is determined whether each element reaches the failure standard, and the above factors are substituted into the finite element model.

[0008] In the finite element model, all elements that meet the failure criteria are deleted. A constant amplitude load is applied to the obtained fatigue analysis finite element model, and static analysis is used instead of fatigue analysis. At each time step, fatigue failure judgment and failure reduction are performed once based on the composite material performance drop model. After performing the above failure judgment and failure reduction process N times, the strength and stiffness degradation calculation is performed based on the composite material performance gradual decline model, and the fatigue analysis finite element model is updated. According to the updated fatigue analysis finite element model, fatigue failure judgment and failure reduction are performed again at each time step. The static analysis ends when the number of executions reaches the set maximum number or the fiber damage extends to the edge of the structure.

[0009] If the number of executions reaches the set maximum and the fiber damage does not extend to the edge of the cross section, the composite material structure is considered to have an infinite lifespan; if the fiber damage extends to the edge of the structure, the total number of executions will be used as the fatigue life value of the composite material structure.

[0010] Preferably, a fatigue failure assessment and failure reduction are performed based on a composite material performance drop model, including:

[0011] Based on the current finite element model for fatigue analysis, and using a composite material performance drop model, different types of fatigue failure are identified. If failure occurs, corresponding failure reductions are applied.

[0012] 1) If the longitudinal stress σ of the element 11 >0 indicates whether the fiber has failed due to tensile fatigue.

[0013]

[0014] In the formula, X k (n,σ,R),Y k (n,σ,R),Z k (n,σ,R),S ij (n, σ, R) represent the residual strength of the monolayer composite material in the three principal directions during the nth fatigue loading. Under tension, k is T; under compression, k is C; σ is the stress; and R is the stress ratio. 12 (n,σ,R),S 13 (n,σ,R) and S 23 (n,σ,R) represent the shear strength of a single-layer plate under shear fatigue load;

[0015] If the above formula is satisfied, then the fiber is considered to have suffered tensile failure.

[0016] 2) If the longitudinal stress σ of the element 11 <0 indicates whether the fiber has failed due to compression fatigue.

[0017]

[0018] If the above formula is satisfied, then the fiber is considered to have experienced compression failure.

[0019] If the conditions are not met, then continue to determine whether fiber-matrix shear fatigue has caused failure:

[0020]

[0021] If the above formula is satisfied, then the fiber-matrix shear failure is considered to be present.

[0022] 3) If the transverse stress σ of the element 22 >0 indicates whether the matrix has failed due to tensile fatigue.

[0023]

[0024] If the above formula is satisfied, the matrix is ​​considered to have suffered tensile failure.

[0025] If the transverse stress σ of the element 22 <0 indicates whether the matrix has failed due to compressive fatigue.

[0026]

[0027] If the above formula is satisfied, the matrix is ​​considered to have undergone compressive failure;

[0028] 4) If the normal stress σ of the element 33 >0 indicates whether interlaminar tensile fatigue failure has occurred.

[0029]

[0030] If the above formula is satisfied, then interlaminar tensile failure is considered to have occurred;

[0031] If the normal stress σ of the element 33 <0 indicates whether interlaminar compressive fatigue has caused failure.

[0032]

[0033] If the above formula is satisfied, then interlayer compressive failure is considered to have occurred.

[0034] Preferably, different types of fatigue failure are assessed, and if failure occurs, a corresponding failure reduction is applied. The failure reduction method is as follows:

[0035] If the fiber is determined to have suffered tensile or compressive failure, the element completely loses its load-bearing capacity, and all stiffness and strength parameters of the element are reduced: among the stiffness parameters, the longitudinal modulus E 11 Transverse modulus E 22 Normal modulus E 33 Longitudinal shear modulus G 12 Normal shear modulus G 23 Transverse shear modulus G 13 Longitudinal Poisson's ratio ν 12 Normal Poisson ratio ν 23 , transverse Poisson's ratio ν 13 When reduced to 0, the longitudinal tensile strength X in the strength parameters is... T Transverse tensile strength Y T Normal tensile strength Z T Longitudinal compressive strength X C Transverse compressive strength Y C Normal compressive strength Z C Longitudinal shear strength S 12Transverse shear strength S 13 Normal shear strength S 23 Reduced to 0;

[0036] If the matrix is ​​determined to have suffered tensile or compressive failure, the stiffness and strength of the element are reduced: the stiffness parameter E is reduced only. 22 ν 12 ν 23 Degenerates to 0, and the strength parameter Y is reduced during tension. T Degenerates to 0, compressive strength parameter Y C Degenerate to 0;

[0037] If interlaminar tensile or compressive failure is determined, the stiffness and strength of the element are reduced: the stiffness parameter E is reduced only. 33 ν 13 ν 23 The strength parameter Z degenerates to 0 when interlaminar tensile failure occurs. T Degenerates to 0, parameter Z during interlayer compression C Degenerate to 0;

[0038] If fiber-matrix shear failure is identified, the stiffness and strength of the element are reduced: stiffness parameter G 12 ν 12 Reduced to 0, strength parameter S 12 Reduced to 0.

[0039] Preferably, based on the Hashin failure criterion, the fiber tensile and compressive damage factors and matrix tensile and compressive damage factors in each unit are obtained, including:

[0040] When σ 11 ≥0

[0041] When σ 11 <0

[0042] When σ 22 +σ 33 ≥0

[0043] When σ 22 +σ 33 <0

[0044] Where, σ 11 σ 22 σ 33 These represent the stresses in the three principal directions of the element, σ. 12 σ 13 σ 23 Shear stress in each direction of the element; X t Y t Xc Y c S 12 S 13 S 23 These are the longitudinal tensile strength, transverse tensile strength, longitudinal compressive strength, transverse compressive strength, longitudinal shear strength, transverse shear strength, and normal shear strength of the element, respectively. These are fiber tensile and compressive damage factors and matrix tensile and compressive damage factors, respectively.

[0045] Preferably, the failure criteria for each unit are determined based on the fiber tensile and compressive damage factors and the matrix tensile and compressive damage factors in each unit. The specific judgment criteria are as follows: if the fiber tensile damage factor or compressive damage factor is equal to 1, the entire unit is considered to have reached the failure criteria; otherwise, the entire unit is considered not to have reached the failure criteria.

[0046] Preferably, elements that meet the failure criteria are deleted from the finite element model. For elements where the fiber is not damaged but the matrix is ​​damaged, the transverse modulus E is set to... 22 Longitudinal Poisson's ratio ν 12 Normal Poisson ratio ν 23 It degenerates to 0, forming a finite element model for fatigue analysis.

[0047] Preferably, the degradation calculation of strength and stiffness is performed based on a composite material performance degradation model, including:

[0048]

[0049]

[0050] In the formula, E(0) refers to the initial stiffness of the one-way plate, σ0 refers to the initial strength of the one-way plate, and a1, a2, b1, b2, c1, and c2 are six fitting coefficients; N f The fatigue life of a single-layer plate under the maximum fatigue stress level σ and stress ratio R.

[0051] Preferably, N f The following was obtained using the equal-life fatigue model:

[0052]

[0053] In the formula, u is a term related to logarithmic lifetime. σ t σ represents the tensile strength of a one-way composite plate. c σ represents the compressive strength of a unidirectional composite plate. a σ represents the fatigue load amplitude. m is the average stress under fatigue load; m, A, and B are fitting coefficients.

[0054] Preferably, a bridging model based on micromechanics is used to predict the transverse isotropic mechanical properties of composite laminates using the mechanical parameters of the fiber and matrix, including:

[0055]

[0056]

[0057]

[0058]

[0059]

[0060]

[0061] In the formula, E 11 E 22 E 33 G represents the longitudinal, transverse, and normal moduli of the composite material structure. 12 G 13 G 23 Let ν be the shear modulus. 12 ν 13 ν 23 is Poisson's ratio; a is an element of the bridging matrix; V f and V m These are the volume contents of the fiber and the matrix, respectively. and These are the longitudinal and transverse elastic moduli of the fiber, respectively. The Poisson's ratio of the fiber. E represents the shear modulus of the fiber. m ν is the elastic modulus of the matrix. m G is the Poisson's ratio of the matrix. m The shear modulus of the matrix. and These are the compliance matrices for the two materials, respectively.

[0062] Preferably, the method for determining whether damage extends to the edge of the structure is as follows: if the fiber breakage damage of any angle layup of the composite material structure extends to the entire width of the plate, then the structure is considered to have suffered fatigue failure.

[0063] The advantages of this invention compared to the prior art are:

[0064] (1) This invention starts from the microscopic material properties, and can predict the fatigue life of the structure even without the macroscopic modulus properties of the composite laminate.

[0065] (2) This invention characterizes the performance degradation of composite material structures under fatigue load from two dimensions: sudden drop and gradual drop in material properties. It takes into account seven types of fatigue damage in fibers, matrix and interlayer, and the evaluation of fatigue damage modes is more accurate.

[0066] (3) The present invention uses a fatigue analysis model with initial impact damage to evaluate the fatigue life of composite material structures after impact. The finite element model is closer to the actual state of the structure and has high calculation accuracy. Attached Figure Description

[0067] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0068] Figure 1 This is a flowchart illustrating the fatigue life analysis of composite laminates after impact according to an embodiment of the present invention. Detailed Implementation

[0069] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0070] This invention addresses the challenges of obtaining anisotropic parameters in composite materials, the large workload of progressive damage analysis, and the high time cost of fatigue analysis under cyclic loading. It proposes a method for predicting the fatigue life of composite structures after impact based on micromechanics. Figure 1 As shown, it specifically includes:

[0071] 1) Prediction of mechanical properties of composite laminate structures based on bridging model

[0072] Fiber-reinforced composites typically consist of two components: fibers and a matrix. Taking a representative volume element of a cuboid shape as an example, the volume contents of the fibers and the matrix are V0 and V1, respectively. f and V m The longitudinal and transverse elastic moduli of the fiber are respectively and The Poisson's ratio of the fiber is The shear modulus of the fiber is The elastic modulus of the matrix is ​​E m The Poisson's ratio of the matrix is ​​ν m The shear modulus of the matrix is ​​G. mThe compliance matrices of the two materials are respectively and It can be represented as:

[0073]

[0074]

[0075] Based on micromechanics theory and bridging model, the engineering elastic constants of transversely isotropic composite laminates are derived as follows:

[0076]

[0077] Among them, E 11 E 22 E 33 G represents the modulus of the laminate in the longitudinal, transverse, and normal directions. 12 G 13 G 23 Let ν be the shear modulus. 12 ν 13 ν 23 Let be the Poisson ratio. 'a' represents the elements of the bridging matrix, and the components used are expressed as follows:

[0078]

[0079] 2) Progressive damage analysis of composite laminates under impact

[0080] The impact damage simulation of composite materials adopts the Hashin failure criterion, one of the most commonly used failure criteria in composite engineering simulation. This criterion classifies the damage of fiber-reinforced composites into four failure modes: fiber tensile failure, fiber compressive failure, matrix tensile failure, and matrix compressive failure. These four failure modes can be described as follows:

[0081]

[0082] Where, σ 11 , σ 22 , σ 33 These represent the stresses in the three principal directions of the element, σ. 12 , σ 13 , σ 23 Shear stress in each direction of the element; X T Y T X C Y C S 12 S 13 S 23These are the longitudinal tensile strength, transverse tensile strength, longitudinal compressive strength, transverse compressive strength, longitudinal shear strength, transverse shear strength, and normal shear strength of the element, respectively. These are fiber tensile and compressive damage factors and matrix tensile and compressive damage factors, respectively.

[0083] A three-dimensional geometric model of the composite laminate structure was established using Abaqus software. First, a single-layer plate was created and meshed using C3D8R (8-node reduced integral solid element). Then, the single-layer plate mesh was used to generate meshes for other layers using an offset method. Nodes were shared between the layers, and the element stacking direction was set to match the actual layup. Material properties were assigned to each layer and the interface layer using a custom method. These properties included density, elastic modulus, damage failure and propagation criteria, with the elastic modulus being the predicted value from the previous section and the other properties being experimental values. Material orientations were assigned to each single layer from bottom to top according to the actual layup. A rigid sphere was created to simulate an impact, with mass and initial velocity assigned, and all degrees of freedom except the impact direction constrained. Dynamic analysis was selected, and state and damage variables were output. Boundary conditions (simply supported or fixed on all four sides) were applied to the laminate. The path to the Vumat subroutine required for damage calculation was set, and the analysis was submitted. After calculation, the fiber tensile and compressive damage factors of all elements and the matrix tensile and compressive damage factors were extracted and recorded in an Excel file.

[0084] 3) Composite material sudden drop and gradual damage fatigue model

[0085] The damage states of all elements are statistically categorized. A damage factor value of 1 indicates that the element has failed; a damage factor value of 0 indicates no damage; and a damage factor value between 0 and 1 indicates that the element, while damaged, still possesses a certain load-bearing capacity and has not yet failed. In the original finite element model, elements with fiber failure are deleted. For elements where the fiber is intact but the matrix has failed, E... 22 υ 12 υ 23The model is degraded to 0, removing rigid spheres and boundary settings from the impact simulation, and the new model is used for fatigue analysis. The dynamic analysis is changed to static analysis, with r×N equally divided increment steps, assuming each step represents one fatigue cycle. The value of r×N can be set to a large number, such as the material's fatigue limit. Assuming the maximum cyclic load contributes to fatigue damage in each fatigue cycle, the fatigue load spectrum is simplified to a constant amplitude load spectrum. Boundary conditions are modified according to the structural operating conditions. A Umat subroutine is written where all elements are degraded using both a sudden and gradual material property degradation model during calculation. After each cycle, the damage state of the elements is determined using the sudden degradation model; if an element experiences a certain type of damage, its properties are correspondingly degraded. Every N cycles, the overall stiffness and strength of the material are degraded using the gradual material property degradation model to simulate the continuous decrease in the overall stiffness and strength of the laminate structure during fatigue cycles. Submit the finite element model for calculation. When the calculation reaches the maximum number of iterations or is interrupted, view the calculation results in the post-processing module and statistically analyze the fatigue life cycles. The statistical method is as follows:

[0086] If the maximum number of calculations is reached and the fiber damage in each layer of the laminate does not extend to the edge of the cross-section, the laminate is considered to have an infinite lifespan, which is calculated as r×N. If the maximum number of calculations is not reached and the fiber damage in any layer of the laminate extends to the edge of the cross-section at a certain moment, the number of calculation steps at that moment is considered to be the fatigue life of the laminate.

[0087] By predicting the fatigue life of composite laminate structures under different stress levels and stress ratios, and statistically analyzing the relationship curves between stress amplitude and stress ratio fatigue life, a fatigue life database corresponding to the structure can be obtained, which can facilitate the design of fatigue life or fatigue load spectrum for engineering composite structures.

[0088] (1) Fatigue failure model of composite material properties with sudden performance degradation

[0089] For element-level fatigue, a fatigue failure criterion derived from the Hashin three-dimensional static load failure criterion is used to determine damage and failure. Its specific form is as follows:

[0090] a) When the fiber fails due to tensile fatigue, σ 11 >0

[0091]

[0092] b) When the fiber fails due to compressive fatigue, σ 11 <0

[0093]

[0094] c) When fiber-matrix shear fatigue failure occurs, σ11 <0

[0095]

[0096] d) When the matrix fails due to tensile fatigue, σ 22 >0

[0097]

[0098] e) When the matrix fails due to compressive fatigue, σ 22 <0

[0099]

[0100] f) When interlaminar tensile fatigue failure occurs, σ 33 >0

[0101]

[0102] g) When interlaminar compressive fatigue failure occurs, σ 33 <0

[0103]

[0104] In the formula, σ ii σ ij These represent the principal directions and corresponding in-plane shear stresses of the monolayer composite material during the nth fatigue loading cycle; X k (n,σ,R),Y k (n,σ,R),Z k (n,σ,R),S ij (n, σ, R) represent the residual strength of the monolayer composite material in the three principal directions during the nth fatigue loading. Under tension, k is T; under compression, k is C; σ is the stress; and R is the stress ratio. 12 (n,σ,R),S 13 (n,σ,R) and S 23 (n, σ, R) represent the shear strength of a single-layer plate under shear fatigue load. In the predictive analysis process, stress analysis, failure determination, and material degradation analysis are performed on an element-by-element basis for each loading process. For each ply, a series of damage accumulation states that vary with the load are obtained until the final joint structure fails as a whole.

[0105] When the above three-dimensional failure criteria are used to determine element failure, the program will cause a sudden drop in the performance of the damaged composite material element. In different damage modes, the following degradation methods can be used to perform performance degradation:

[0106] 1) Fiber failure is catastrophic. Once the fiber fails in tension or compression, the element completely loses its load-bearing capacity, and all its stiffness and strength parameters are reduced: Stiffness E11 E 22 E 33 G 12 G 23 G 13 ν 12 ν 23 ν 13 Reduced to 0; Strength X T Y T Z T X C Y C Z C S 12 S 13 S 23 Reduced to 0.

[0107] 2) Matrix failure only affects material properties in the matrix direction; other material properties remain unchanged. When the matrix undergoes tensile or compressive failure, the stiffness and strength are reduced as follows: Stiffness only E 22 ν 12 ν 23 Degenerates to 0; Strength Y will decrease during stretching. T Degenerates to 0, compressive strength Y C It degenerates to 0.

[0108] 3) Interlaminar failure will affect the performance parameters of the laminate in the thickness direction. When tensile or compressive failure occurs between laminations, the material stiffness and strength are reduced as follows: Stiffness only E 33 ν 13 ν 23 Degrades to 0; Strength Z at interlaminar tensile failure T Degenerates to 0, Z during interlayer compression C It degenerates to 0.

[0109] 4) When fiber-matrix shear failure occurs, the fiber, matrix, and interlaminar layers still possess a certain load-bearing capacity, but the plane shear stress is 0. The material stiffness and strength are reduced as follows: Stiffness G 12 ν 12 Reduced to 0; Strength S 12 Reduced to 0.

[0110] (2) Composite material performance degradation model

[0111] In composite material structures under fatigue cyclic loading, the performance gradually decreases with the increase of the number of cycles, a phenomenon known as progressive degradation of material properties. Under stress level σ and stress ratio R, after n fatigue cycles, internal damage occurs in the structure, and the structural strength after material degradation is S(n,σ,R), called residual strength; the tangential stiffness under this cycle is E(n,σ,R), called residual stiffness.

[0112] For elements that do not fail, the program will degrade the material properties. The degradation models for stiffness and strength are as follows:

[0113]

[0114]

[0115] Where E0 refers to the initial stiffness of the one-way plate, σ0 refers to the initial strength of the one-way plate, and a1, a2, b1, b2, c1, and c2 are six fitting coefficients that are independent of the stress state and are determined through material property tests. N f The fatigue life of a single-layer plate (material grade) under the maximum fatigue stress level σ and stress ratio R is given by the following constant-life fatigue model:

[0116]

[0117] Here, u is a term related to logarithmic lifetime, which has no practical significance.

[0118] σ t Tensile strength of a one-way composite plate; σ c σ refers to the compressive strength of a one-way composite plate. a σ represents the fatigue load amplitude. m The average stress under fatigue load; m, A, and B are fitting coefficients, which are independent of the material stress state and are obtained through one-way plate fatigue tests. It is generally recommended that m be 1.06.

[0119] The fatigue life test of the one-way plate can be carried out according to the following steps: First, obtain the logarithmic fatigue life value lgN under different stress ratios by conducting fatigue tests in each principal direction of the one-way plate; then, use the least squares method to fit the u-lgN curve using the above formula; finally, the fatigue life value N under any stress ratio can be obtained by fitting the curve. f .

[0120] Starting from the more readily available microscopic mechanical properties of materials, this invention derives macroscopic mechanical properties through bridging theory, and uses static calculations instead of fatigue calculations. It proposes material property progressive degradation and sudden degradation models applicable to fatigue problems, which greatly reduces the workload of fatigue analysis calculations.

[0121] The embodiments described above are merely preferred embodiments of the present invention. Ordinary variations and substitutions made by those skilled in the art within the scope of the technical solution of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for predicting the fatigue life of composite material structures after impact based on micromechanics, characterized in that, include: A bridging model based on micromechanics is used to predict the transverse isotropic mechanical properties of composite laminates using the mechanical parameters of fibers and the matrix. Based on the finite element model of composite laminate structure, mechanical properties are assigned to the finite element model, and explicit dynamic simulation of impact process is performed to obtain the stress distribution of each element. According to the Hashin failure criterion, the fiber tensile and compressive damage factors and matrix tensile and compressive damage factors in each element are obtained. Based on the above factors, it is determined whether each element reaches the failure standard, and the above factors are substituted into the finite element model. In the finite element model, all elements that meet the failure criteria are deleted. A constant amplitude load is applied to the obtained fatigue analysis finite element model, and static analysis is used instead of fatigue analysis. At each time step, fatigue failure judgment and failure reduction are performed once based on the composite material performance drop model. After performing the above failure judgment and failure reduction process N times, the strength and stiffness degradation calculation is performed based on the composite material performance gradual decline model, and the fatigue analysis finite element model is updated. According to the updated fatigue analysis finite element model, fatigue failure judgment and failure reduction are performed again at each time step. The static analysis ends when the number of executions reaches the set maximum number or the fiber damage extends to the edge of the structure. If the number of executions reaches the set maximum and the fiber damage does not extend to the edge of the cross section, the composite material structure is considered to have an infinite lifespan. If fiber damage extends to the edge of the structure, the current total number of execution steps will be used as the fatigue life value of the composite structure. Based on the composite material performance sudden drop model, a single fatigue failure judgment and failure reduction are performed, including: Based on the current finite element model for fatigue analysis, and using a composite material performance drop model, different types of fatigue failure are identified. If failure occurs, corresponding failure reductions are applied. 1) If the longitudinal stress of the element To determine whether the fiber has failed due to tensile fatigue: If the above formula is satisfied, then the fiber is considered to have suffered tensile failure. 2) If the longitudinal stress of the element To determine whether the fiber has failed due to compression fatigue: If the above formula is satisfied, then the fiber is considered to have experienced compression failure. If the conditions are not met, then continue to determine whether fiber-matrix shear fatigue has caused failure: If the above formula is satisfied, then the fiber-matrix shear failure is considered to be present. 3) If the transverse stress of the element Determine whether the matrix has failed due to tensile fatigue: If the above formula is satisfied, the matrix is ​​considered to have suffered tensile failure. If the transverse stress of the element Determine whether the matrix has failed due to compressive fatigue: If the above formula is satisfied, the matrix is ​​considered to have undergone compressive failure; 4) If the normal stress of the element Determine whether interlaminar tensile fatigue has caused failure: If the above formula is satisfied, then interlaminar tensile failure is considered to have occurred; If the normal stress of the element Determine whether interlaminar compressive fatigue has caused failure: If the above formula is satisfied, then interlayer compressive failure is considered to have occurred; In the formula, , , , The first The residual strength of a single-layer composite material in the three principal directions under fatigue loading, and under tension. for T When compressed, for C , For stress, R Stress ratio; , and These represent the shear strength of a single-layer plate under shear fatigue load; Different types of fatigue failure are assessed, and if failure occurs, corresponding failure reduction is applied. The failure reduction method is as follows: If the fiber is determined to have failed in tension or compression, the element completely loses its load-bearing capacity, and all stiffness and strength parameters of the element are reduced: among the stiffness parameters, the longitudinal modulus... lateral modulus Normal modulus Longitudinal shear modulus Normal shear modulus Transverse shear modulus Longitudinal Poisson's ratio Normal Poisson ratio lateral Poisson's ratio Reduced to 0, among the strength parameters, longitudinal tensile strength Transverse tensile strength Normal tensile strength Longitudinal compressive strength Lateral compressive strength Normal compressive strength Longitudinal shear strength Transverse shear strength Normal shear strength Reduced to 0; If the matrix is ​​determined to have failed under tension or compression, the stiffness and strength of the element are reduced: the stiffness parameter is reduced only... , , Degenerates to 0, and the strength parameter is reduced during tension. Degenerates to 0, compressive strength parameter Degenerate to 0; If interlaminar tensile or compressive failure is determined, the stiffness and strength of the element are reduced: the stiffness parameter is reduced only. , , The strength parameter degrades to 0 when interlaminar tensile failure occurs. Degenerates to 0, parameters during interlayer compression Degenerate to 0; If fiber-matrix shear failure is identified, the stiffness and strength of the element are reduced: stiffness parameter , Reduced to 0, strength parameter Reduced to 0.

2. The method for predicting the fatigue life of a composite material structure after impact according to claim 1, characterized in that, Based on the Hashin failure criterion, the fiber tensile and compressive damage factors and the matrix tensile and compressive damage factors in each element were obtained, including: in, , , These represent the stresses in the three principal directions of the element. , , Shear stress in each direction of the element; , , , , , , These are the longitudinal tensile strength, transverse tensile strength, longitudinal compressive strength, transverse compressive strength, longitudinal shear strength, transverse shear strength, and normal shear strength of the element, respectively. , , , These are fiber tensile and compressive damage factors and matrix tensile and compressive damage factors, respectively.

3. The method for predicting the fatigue life of a composite material structure after impact according to claim 1, characterized in that, Based on the fiber tensile and compressive damage factors and matrix tensile and compressive damage factors in each unit, it is determined whether each unit has reached the failure standard. The specific judgment criteria are as follows: if the fiber tensile damage factor or compressive damage factor is equal to 1, the entire unit is considered to have reached the failure standard; otherwise, the entire unit is considered not to have reached the failure standard.

4. The method for predicting the fatigue life of a composite material structure after impact according to claim 1, characterized in that, In the finite element model, delete all elements that meet the failure criteria. For elements where the fiber is not damaged but the matrix is ​​damaged, change the transverse modulus. Longitudinal Poisson's ratio Normal Poisson ratio It degenerates to 0, forming a finite element model for fatigue analysis.

5. The method for predicting the fatigue life of a composite material structure after impact according to claim 1, characterized in that, The degradation calculations of strength and stiffness based on the gradual decline model of composite material properties include: In the formula, The initial stiffness of a one-way plate. The initial strength of a one-way slab. , , , , , There are six fitting coefficients; For a single-layer plate at the maximum fatigue stress level and stress ratio R The fatigue life is below.

6. The method for predicting the fatigue life of a composite material structure after impact according to claim 5, characterized in that, The following was obtained using the equal-life fatigue model: In the formula, u For terms related to logarithmic lifetime, , , , Tensile strength of a composite unidirectional plate; The compressive strength of a unidirectional composite plate; This represents the fatigue load amplitude. The average stress under fatigue load; m , A , B The fitting coefficients are denoted as .

7. The method for predicting the fatigue life of a composite material structure after impact according to claim 1, characterized in that, A bridging model based on micromechanics is used to predict the transverse isotropic mechanical properties of composite laminates using the mechanical parameters of fibers and the matrix, including: In the formula, , , These are the longitudinal, transverse, and normal moduli of the composite material structure. , , Shear modulus , , Poisson's ratio; For bridging matrix elements; and These are the volume contents of the fiber and the matrix, respectively. and These are the longitudinal and transverse elastic moduli of the fiber, respectively. , The Poisson's ratio of the fiber. This represents the shear modulus of the fiber. The elastic modulus of the matrix. Poisson's ratio of the matrix The shear modulus of the matrix. and These are the compliance matrices for the two materials, respectively.

8. The method for predicting the fatigue life of a composite material structure after impact according to claim 1, characterized in that, The method for determining whether damage extends to the edge of the structure is as follows: if the fiber breakage damage of any angle layup of the composite material structure extends to the entire width of the plate, then the structure is considered to have suffered fatigue failure.

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