A method and system for predicting the compressive strength of a woven fiber composite

Abstract

This invention provides a method and system for predicting the compressive strength of braided fiber composite materials, belonging to the field of composite material mechanical property analysis technology. The method includes: determining the critical deflection angle of the fiber bundle when compressive damage begins to occur in the matrix, based on the initial deflection angle of the fiber bundle and the critical load state parameters when the matrix begins to show compressive damage; determining that the braided fiber composite material has entered the matrix damage propagation stage when the real-time deflection angle of the fiber bundle is greater than or equal to the critical deflection angle; determining the equivalent critical local shear strength based on the critical deflection angle and the critical load state parameters; and determining the local matrix stress of the fiber folded surface through stress coordinate transformation. This invention eliminates the need for randomly searching for the most dangerous folded surface in three-dimensional space, achieving continuous simulation from microscopic damage initiation to macroscopic stiffness degradation.

Description

Technical Field

[0001] This invention relates to the field of mechanical property analysis technology for composite materials, and more specifically, to a method and system for predicting the compressive strength of braided fiber composite materials. Background Technology

[0002] With the widespread application of high-strength, high-stiffness fiber-reinforced composites (FRPs) in aerospace, shipbuilding, and other fields, the safety assessment of this type of structure under complex compressive conditions is becoming increasingly critical. Numerous experimental evidence reveals that the final compression failure mode of braided composite structures is fiber compression buckling. When the material is subjected to compressive stress, the fibers undergo micro-buckling due to initial micro-geometric deflection, triggering shear deformation of the matrix. After the matrix fractures, it loses lateral support, leading to overall instability and breakage of the fiber bands. Therefore, accurately predicting this process is crucial for lightweight equipment design.

[0003] In related technologies, the theoretical initial fiber deflection angle of unidirectional composite materials is first set. Then, under compressive load, a transcendental equation is constructed by combining the matrix nonlinear shear constitutive model. The real-time fiber deflection angle is solved through numerical iteration. Then, the most dangerous folded surface that maximizes the frictional coupling effect is found and the global stress is transformed into this three-dimensional deflection coordinate system. Finally, the Mohr-Coulomb criterion is applied in this coordinate system to determine whether the local matrix has failed. Once failure is determined, the material stiffness is instantly reduced to zero.

[0004] The above-mentioned technologies equate local damage to the matrix with the final bending failure of the structure, ignore the compression deformation buffering effect between the interlaced gaps of the woven structure, and cannot characterize the long-term nonlinear stiffness degradation process that the structure exhibits before the arrival of the ultimate load; forcibly applying the three-dimensional most dangerous surface optimization algorithm ignores the geometric rigidity constraint of the woven fabric on the fiber deflection direction, dissipates a lot of computing power in finite element analysis, and makes numerical convergence difficult. Summary of the Invention

[0005] The problem to be solved by this invention is at least one of the following technical issues: equating local damage to the matrix with the final failure of the structure fails to characterize the long-term nonlinear stiffness degradation process that the structure exhibits before the arrival of the ultimate load; forcibly applying the three-dimensional most dangerous surface optimization algorithm results in large computational dissipation and difficulty in numerical convergence.

[0006] To address the aforementioned problems, in a first aspect, the present invention provides a method for predicting the compressive strength of a braided fiber composite material, wherein the braided fiber composite material includes fiber bundles and a matrix; the method for predicting the compressive strength of the braided fiber composite material includes: Based on the initial deflection angle of the fiber bundle and the critical load state parameters at which the matrix begins to show compressive damage, the critical deflection angle of the fiber bundle at which the matrix begins to show compressive damage is determined. When the real-time deflection angle of the fiber bundle is greater than or equal to the critical deflection angle, it is determined that the braided fiber composite material has entered the matrix damage propagation stage. Based on the critical deflection angle and the critical load state parameters, the equivalent critical local shear strength is determined, and the matrix local stress of the fiber folded surface is determined through stress coordinate transformation. When the local stress of the matrix on the fiber folded surface satisfies the failure criterion of using the equivalent critical local shear strength as a strength parameter, the matrix is ​​determined to have entered a crack propagation state. Based on the crack propagation state, the equivalent shear modulus of the braided fiber composite material is updated by the shear stiffness reduction factor. Based on the updated equivalent shear modulus, the critical deflection angle of the fiber bundle for flexural instability is determined. When the real-time deflection angle is greater than or equal to the critical deflection angle for flexural instability, it is determined whether the woven composite material has entered the flexural instability failure stage.

[0007] The compressive strength prediction method for braided fiber composite materials provided by this invention first determines the critical deflection angle based on the initial deflection angle of the fiber bundle and the critical load state parameters. The solution of the critical deflection angle is limited to the braided plane by the geometric rigidity constraint of the braided structure on the fiber deflection direction. This eliminates the need to randomly search for the most dangerous fold surface in three-dimensional space, thus avoiding the large computational dissipation and numerical convergence difficulties caused by three-dimensional search in traditional algorithms. When the local matrix stress of the fiber folded surface satisfies the failure criterion using the equivalent critical local shear strength as a strength parameter, the braided fiber composite material is determined to have entered the matrix damage propagation stage. At this point, only the entry into the matrix damage propagation stage is determined, without directly determining structural failure, thus separating the local matrix damage from the final structural failure in time. The material can continue to bear load after entering the damage stage, thereby avoiding the defect of equating local matrix damage with instantaneous structural failure in traditional criteria. After entering the matrix damage propagation stage, the equivalent critical local shear strength is determined based on the critical deflection angle and critical load state parameters. At the same time, the local matrix stress of the fiber folded surface is determined through stress coordinate transformation. When the local matrix stress of the fiber folded surface satisfies the failure criterion using the equivalent critical local shear strength as a strength parameter, the matrix is ​​determined to have entered the crack propagation state, and the equivalent shear modulus of the braided fiber composite material is updated through the shear stiffness reduction coefficient. At this point, only the matrix is ​​judged to have entered the crack propagation state, without structural failure. The equivalent shear modulus is continuously updated by the shear stiffness reduction factor, allowing the stiffness to gradually degrade rather than abruptly return to zero. This achieves continuous simulation from microscopic damage initiation to macroscopic stiffness degradation, solving the problem that traditional criteria cannot characterize the long-term nonlinear stiffness degradation process before the ultimate load. As the equivalent shear modulus continues to decrease, the critical deflection angle for fiber bundle flexural instability is determined based on the updated equivalent shear modulus. When the real-time deflection angle is greater than or equal to the critical deflection angle for flexural instability, it is determined whether the braided composite material has entered the flexural instability failure stage. Final failure requires the additional independent condition of "real-time deflection angle greater than or equal to the critical deflection angle for flexural instability," rather than being triggered solely by matrix damage. This ensures that even if the matrix has suffered damage and stiffness degradation, the structure is not completely failed. Only when the fiber bundle deflection angle accumulates to the instability critical value will final collapse be triggered. Thus, the result of stiffness degradation is used as the input for the final instability determination, completing the entire evolution path from continuous stiffness degradation to transient instability failure.

[0008] Secondly, the present invention also provides a compressive strength prediction system for braided fiber composite materials, which applies the compressive strength prediction method for braided fiber composite materials as described above, including: The elastic stage discrimination module is used to: determine the critical deflection angle of the fiber bundle when the matrix begins to show compressive damage, based on the initial deflection angle of the fiber bundle and the critical load state parameters when the matrix begins to show compressive damage. The matrix damage stage determination module is used to: determine that the braided fiber composite material has entered the matrix damage propagation stage when the real-time deflection angle of the fiber bundle is greater than or equal to the critical deflection angle; The matrix damage evolution module is used to: determine the equivalent critical local shear strength based on the critical deflection angle and the critical load state parameters, and determine the matrix local stress of the fiber folded surface through stress coordinate transformation; The crack propagation update module is used to: determine that the matrix has entered a crack propagation state when the local stress of the matrix on the fiber folded surface meets the failure criterion of using the equivalent critical local shear strength as a strength parameter; and update the equivalent shear modulus of the braided fiber composite material based on the crack propagation state by using the shear stiffness reduction coefficient. The instability determination module is used to: determine the critical deflection angle of the fiber bundle for bending instability based on the updated equivalent shear modulus; and determine whether the woven composite material has entered the bending instability failure stage when the real-time deflection angle is greater than or equal to the critical deflection angle for bending instability.

[0009] Thirdly, the present invention provides an electronic device, including a memory and a processor; The memory is used to store computer programs; The processor is configured to, when executing the computer program, implement the method for predicting the compressive strength of braided fiber composite materials as described in the first aspect.

[0010] Fourthly, the present invention provides a computer-readable storage medium storing a computer program that, when executed by a processor, implements the method for predicting the compressive strength of braided fiber composite materials as described in the first aspect.

[0011] The compressive strength prediction system, electronic device, and computer-readable storage medium for braided fiber composite materials provided by this invention have the same beneficial effects as the compressive strength prediction method for braided fiber composite materials compared to the prior art, and will not be repeated here. Attached Figure Description

[0012] Figure 1 A flowchart illustrating a method for predicting the compressive strength of a braided fiber composite material according to an embodiment of the present invention is shown. Figure 2 This diagram illustrates the fiber deflection angle and damage failure of the braided composite material in an embodiment of the present invention. Figure 3 A schematic diagram of the structure of the compressive strength prediction system for braided fiber composite materials in an embodiment of the present invention is shown; Figure 4 A schematic diagram of the structure of an electronic device according to an embodiment of the present invention is shown. Detailed Implementation

[0013] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0014] It should be noted that relational terms such as "first" and "second" in this invention are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0015] In the description of this specification, references to terms such as "embodiment," "one embodiment," and "one implementation" indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or implementation is included in at least one embodiment or illustrative implementation of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or implementation. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or implementations.

[0016] Reference Figure 1 and Figure 2 As shown, this invention proposes a method for predicting the compressive strength of braided fiber composites based on a stepped damage model, which discretizes the compression failure process into three progressive physical stages: Phase I (Elastic Undamaged Phase): From the start of loading until the matrix begins to show compressive damage (i.e., from the start of loading until the real-time deflection angle of the fiber bundle is less than the critical deflection angle). During this stage, the fiber bundle undergoes elastic deflection, the material's principal stiffness matrix remains intact, and there is no damage evolution.

[0017] Stage II (Matrix Damage Propagation and Continuous Stiffness Degradation Stage): From the onset of compressive damage in the matrix (when the real-time deflection angle of the fiber bundle reaches the critical deflection angle) to just before the fiber bundle becomes unstable, the matrix first enters a damage propagation state (while maintaining its stiffness). Subsequently, when the local stress on the fiber folded surface satisfies the failure criterion using the equivalent critical local shear strength as the strength parameter, microcracks begin to appear and propagate in the matrix, activating the continuous damage model. The damage variable increases from zero, the shear stiffness reduction factor decreases, and the equivalent shear modulus continuously decreases, simulating the progressive stiffness loss of the braided structure caused by the extrusion deformation between fiber bundles.

[0018] Stage III (Fiber Compression and Bending Failure Stage): When the real-time deflection angle of the fiber bundle reaches or exceeds the critical deflection angle for bending instability, and the failure judgment index reaches or exceeds the set critical value, the fiber bundle loses the lateral support of the matrix, and instantaneous bending instability occurs. The material stiffness decreases sharply to near zero, and the structure completely loses its load-bearing capacity.

[0019] When the fiber bundle loses lateral support from the matrix, it undergoes instantaneous bending and instability, and the material stiffness decreases sharply to near zero, causing the structure to completely lose its load-bearing capacity.

[0020] for Figure 2 A complete explanation can be found in the following embodiments.

[0021] This invention achieves complete prediction from microscopic matrix damage to macroscopic fiber instability through a continuous-transient hybrid stiffness degradation mechanism. The following provides a detailed description of each step.

[0022] This invention proposes a method for predicting the compressive strength of braided fiber composite materials. The braided fiber composite material includes fiber bundles and a matrix. The fiber bundles are yarns composed of thousands of continuous fibers (such as carbon fiber or glass fiber) bundled together, bearing the main compressive load. The matrix is ​​a resin material (such as epoxy resin) that wraps the fiber bundles, serving to fix the fibers, transfer loads, and provide lateral support. During the braiding process, the fiber bundles naturally form a wavy geometric shape, giving the material an inherent initial deflection angle.

[0023] The method for predicting the compressive strength of the braided fiber composite material includes: S100: Determine the critical deflection angle of the fiber bundle when the matrix begins to show compressive damage based on the initial deflection angle of the fiber bundle and the critical load state parameters when the matrix begins to show compressive damage.

[0024] Specifically, this step corresponds to the critical state determination in Stage I. Because the fiber bundles of the braided fiber composite material have an initial deflection angle (e.g., naturally formed by the braiding process), this deflection angle increases with increasing load under axial compressive load. This step uses the initial deflection angle, the axial compressive strength included in the critical load state, and the material's shear modulus to establish an analytical relationship to solve for the critical deflection angle, for example, through iterative solutions. This solution process utilizes the geometric constraints of the braided structure on the deflection direction (e.g., the deflection is confined to a plane determined by the braided texture), eliminating the need for a three-dimensional spatial search.

[0025] S200: When the real-time deflection angle of the fiber bundle is greater than or equal to the critical deflection angle, it is determined that the braided fiber composite material has entered the matrix damage propagation stage.

[0026] Specifically, this step determines the state transition from Stage I to Stage II. In each analysis step, the real-time deflection angle is compared with the critical deflection angle. When the real-time deflection angle is less than the critical deflection angle, the material is in the elastic, damage-free stage. When the real-time deflection angle reaches or exceeds the critical deflection angle, the material is determined to have entered the matrix damage propagation stage, triggering subsequent continuous damage determination and stiffness degradation processes.

[0027] S300: Based on the critical deflection angle and the critical load state parameters, determine the equivalent critical local shear strength, and determine the matrix local stress of the fiber folded surface through stress coordinate transformation.

[0028] Specifically, this step comprises two parallel branches. Branch 1: Using the critical deflection angle and critical load state parameters, the equivalent critical local shear strength is determined. This equivalent strength replaces the traditional macroscopic interlaminar shear strength as the damage assessment threshold. Branch 2: Through stress coordinate transformation, the matrix local stress of the fiber folded surface is determined. Those skilled in the art will understand that stress coordinate transformation generally requires inputting a rotation angle and the stress tensor being transformed. In this step, the rotation angle is the real-time deflection angle, and the stress tensor being transformed is the current macroscopic compressive load stress component. The macroscopic stress is decomposed to the fiber folded surface direction through stress coordinate transformation, obtaining the normal and shear stress components on this surface. The above transformation utilizes the geometric constraints of the braided structure on the deflection direction, for example, limiting the transformation to within the braided plane.

[0029] S400: When the local stress of the matrix on the fiber folded surface satisfies the failure criterion of using the equivalent critical local shear strength as a strength parameter, the matrix is ​​determined to have entered a crack propagation state. Based on the crack propagation state, the equivalent shear modulus of the braided fiber composite material is updated by the shear stiffness reduction coefficient.

[0030] Specifically, this step corresponds to the damage evolution mechanism of Stage II. When the failure criterion calculated based on the local stress on the fiber folded surface and using the equivalent critical local shear strength as the strength parameter (the equivalent critical local shear strength replaces the macroscopic interlaminar shear strength in the Pinho failure criterion, and the criterion after parameter replacement is used to determine whether the matrix has entered the continuous compression damage stage) reaches or exceeds the set critical value, the matrix is ​​determined to have entered the crack propagation state (that is, the matrix has entered the continuous compression damage stage, rather than the complete failure of the unit determined by the original criterion in related technologies), and the continuous damage model is activated, with the damage variable increasing from zero. At the same time, the shear stiffness reduction coefficient (e.g., using the geometric mean) is determined according to the damage variable, and the original shear modulus is reduced to obtain the updated equivalent shear modulus, thereby simulating the stiffness degradation phenomenon during the compression process of the braided composite material.

[0031] S500: Based on the updated equivalent shear modulus, determine the critical deflection angle of the fiber bundle for bending instability. When the real-time deflection angle is greater than or equal to the critical deflection angle for bending instability, determine whether the braided composite material has entered the bending instability failure stage.

[0032] Specifically, this step corresponds to the final instability determination in Stage III, where the fiber bundle loses lateral support as stiffness continues to decrease. Using the updated equivalent shear modulus and relevant strength parameters, the critical deflection angle for flexural instability is determined through fiber compression-flexural failure criteria (e.g., by solving implicit equations). When the real-time deflection angle reaches or exceeds this critical deflection angle, the corresponding failure criterion is selected based on the sign of the normal stress on the fiber flexural surface. The failure criterion is compared with a set critical value; when the critical value is reached or exceeded, the material is determined to have entered the flexural instability failure stage.

[0033] It should be noted that the key parameters in each step of this invention can be obtained through various means, such as extraction based on microscopic simulation, measurement through macroscopic experiments, reference to material handbooks, or assignment based on experience.

[0034] In practical applications, this embodiment first determines the critical deflection angle based on the initial deflection angle of the fiber bundle and the critical load state parameters. The solution for the critical deflection angle is then constrained within the braided plane by the geometric rigidity of the braided structure on the fiber deflection direction. This eliminates the need to randomly search for the most dangerous folded surface in three-dimensional space, thus avoiding the large computational dissipation and numerical convergence difficulties caused by three-dimensional search in traditional algorithms. When the local matrix stress of the fiber folded surface satisfies the failure criterion using the equivalent critical local shear strength as a strength parameter, the braided fiber composite material is determined to have entered the matrix damage propagation stage. At this point, only the entry into the matrix damage propagation stage is determined, without directly determining structural failure, thus separating the local matrix damage from the final structural failure in time. The material can continue to bear load after entering the damage stage, thereby avoiding the defect of equating local matrix damage with instantaneous structural failure in traditional criteria. After entering the matrix damage propagation stage, the equivalent critical local shear strength is determined based on the critical deflection angle and critical load state parameters. At the same time, the local matrix stress of the fiber folded surface is determined through stress coordinate transformation. When the local matrix stress of the fiber folded surface satisfies the failure criterion using the equivalent critical local shear strength as a strength parameter, the matrix is ​​determined to have entered the crack propagation state, and the equivalent shear modulus of the braided fiber composite material is updated through the shear stiffness reduction coefficient. At this point, only the matrix is ​​judged to have entered the crack propagation state, without structural failure. The equivalent shear modulus is continuously updated by the shear stiffness reduction factor, allowing the stiffness to gradually degrade rather than abruptly return to zero. This achieves continuous simulation from microscopic damage initiation to macroscopic stiffness degradation, solving the problem that traditional criteria cannot characterize the long-term nonlinear stiffness degradation process before the ultimate load. As the equivalent shear modulus continues to decrease, the critical deflection angle for fiber bundle flexural instability is determined based on the updated equivalent shear modulus. When the real-time deflection angle is greater than or equal to the critical deflection angle for flexural instability, it is determined whether the braided composite material has entered the flexural instability failure stage. Final failure requires the additional independent condition of "real-time deflection angle greater than or equal to the critical deflection angle for flexural instability," rather than being triggered solely by matrix damage. This ensures that even if the matrix has suffered damage and stiffness degradation, the structure is not completely failed. Only when the fiber bundle deflection angle accumulates to the instability critical value will final collapse be triggered. Thus, the result of stiffness degradation is used as the input for the final instability determination, completing the entire evolution path from continuous stiffness degradation to transient instability failure.

[0035] This invention (main research results originate from: Research on Characterization Theory Methods and Adaptation Technology of Lightweight Materials and Structures 2022YFB4300102) discretizes compression failure into three stages: elastic non-damage compression, fiber-matrix compression damage, and fiber compression bending instability. It overcomes the shortcomings of existing technologies through a progressive logical step: "determining the critical deflection angle → determining entry into the matrix damage propagation stage → determining the equivalent critical local shear strength and matrix local stress → determining the matrix entering crack propagation and updating the equivalent shear modulus → determining the critical deflection angle for bending instability and determining whether it has entered the bending instability failure stage." In finite element analysis, this invention utilizes the fixed initial deflection angle direction of the woven fiber bundles, establishing the deflection direction without three-dimensional search, significantly improving computational efficiency and numerical stability. Simultaneously, by introducing the equivalent critical local shear strength to replace macroscopic parameters, it achieves a progressive determination of matrix compression damage and a smooth transition of stiffness, filling the theoretical gap in predicting early stiffness degradation of woven structures. This invention can be embedded in the user material subroutine of finite element software for strength prediction of woven fiber composite structures under axial compressive load.

[0036] As an optional embodiment of the present invention, determining the critical deflection angle of the fiber bundle when the matrix begins to show compressive damage, based on the initial deflection angle of the fiber bundle and the critical load state parameters when the matrix begins to show compressive damage, includes: The critical deflection angle is determined by transcendental equations based on the initial deflection angle, the axial compressive strength included in the critical load state, and the shear modulus of the braided fiber composite material.

[0037] Specifically, this step corresponds to the critical state determination in Stage I. Because the fiber bundles in the braided fiber composite material naturally possess a wavy geometric shape due to the interlacing process, this shape imparts an inherent initial deflection angle to the fiber bundles. When an axial compressive load is applied, the fiber bundle will slightly buckle in accordance with the initial deflection direction, with a real-time deflection angle. It increases with increasing load. When the real-time deflection angle reaches a critical value... At this point, compressive damage begins to appear in the matrix. This step establishes the initial deflection angle based on the linear elastic deformation compatibility relationship. Axial compressive strength (Macroscopic stress at the moment damage to the matrix first occurs) and shear modulus The transcendental equations between them are solved numerically through iteration to obtain the critical deflection angle. Because the braided structure imposes geometric constraints on the fiber bundle deflection direction, the solution to this transcendental equation is limited to the braided plane (such as the xz plane or yz plane), eliminating the need for a three-dimensional spatial search.

[0038] In practical applications, this embodiment utilizes the physical property that the deflection direction of the fiber bundles in a braided structure is geometrically constrained to a specific plane. Traditional Pinho's three-dimensional criterion requires searching for the most dangerous fracture surface in three-dimensional space that maximizes the Mohr-Coulomb equation, performing multiple iterations at each integration point and increment step. This results in extremely high computational complexity and often convergence failure due to the non-positive definiteness of the stiffness matrix. This invention directly determines the deflection plane using prior geometric knowledge of the braided structure, reducing the three-dimensional search to a single-plane solution, significantly reducing computational complexity and fundamentally solving the numerical curse problem caused by dimensional redundancy in finite element simulations. Simultaneously, this transcendental equation establishes macroscopically measurable parameters (axial compressive strength). shear modulus ) and critical deflection angle The analytical bridge between (also the micro-damage initiation angle) enables the model to have true predictive ability.

[0039] As an optional embodiment of the present invention, the shear modulus is obtained according to a first method, and the axial compressive strength is determined according to the stress value when the matrix of the braided fiber composite material begins to show microscopic damage under pure compression; the first method includes at least one of macroscopic experiments, theoretical derivation, material property test results, and empirical assignment.

[0040] Specifically, this embodiment describes the methods for obtaining key parameters to ensure the feasibility of the technical solution in different engineering scenarios.

[0041] shear modulus The yield can be determined through macroscopic tests, such as the ±45° tensile test (ASTM D3518) or the V-notch track shear test (ASTM D7078), by measuring the slope of the initial linear elastic segment of the shear stress-shear strain curve; it can be derived theoretically, such as by the mixed law formula based on the elastic constants of the fiber and the matrix or the Halpin-Tsai semi-empirical equation; it can be referenced from the manual data provided by the material supplier or the test results in publicly available publications; or it can be reasonably estimated based on engineering experience.

[0042] Axial compressive strength Determination: The stress-strain curve of the braided fiber composite material is obtained through a pure axial compression test (such as the ASTM D6641 combined loading compression test). The stress value corresponding to the end of the linear elastic segment or the starting point where the tangent modulus begins to decrease significantly on the stress-strain curve is the axial compressive strength at which microscopic damage begins to appear in the matrix. The damage initiation point can be accurately identified using auxiliary methods such as strain gauge monitoring of microcrack initiation on the specimen surface, a significant increase in the acoustic emission event count, or deviations in the linearity of the stress-strain curve.

[0043] Numerical solution of transcendental equations: critical deflection angle Satisfies the following transcendental equations: (1) because When trigonometric function terms appear simultaneously on both the left and right sides of an equation, the equation cannot be solved directly and explicitly. In practical finite element implementations (such as the Abaqus VUMAT / UMAT subroutine), a numerical iterative algorithm can be used to solve it: [Setting...] The initial guess value (e.g., take) Substitute into the right side of the equation to calculate the new Iterate repeatedly until the difference between two adjacent calculated values ​​is less than the preset tolerance (e.g., 1×10). -6 (radians). This solution process only needs to be executed once during material initialization, without increasing the computational burden of incremental steps.

[0044] In practical applications, this embodiment significantly enhances the engineering adaptability of the method by providing diverse parameter acquisition methods. When dedicated testing conditions are lacking in the early stages of research and development, material handbook values ​​or empirical assignments can be used for rapid scheme verification. When high-precision structural evaluation is required, specialized experiments can be employed to obtain accurate parameters. This flexibility in parameter acquisition methods makes the invention suitable for both precise predictions in academic research and rapid iterative design scenarios in engineering practice. More importantly, these parameters all have clear physical meanings and standard measurement / calibration procedures.

[0045] As an optional embodiment of the present invention, determining the equivalent critical local shear strength based on the critical deflection angle and the critical load state parameters includes: Obtain the critical deflection angle, the axial compressive strength included in the critical load state, and the static friction coefficient in the fiber folded surface; Specifically, this step clarifies the three basic parameters required to derive the equivalent critical local shear strength: critical deflection angle. (Obtained by solving the transcendental equations of Stage I), Axial compressive strength (Macroscopic stress value when the matrix begins to show compressive damage), and the static friction coefficient within the fiber folded surface. The static friction coefficient can be obtained from the fracture surface angle measured by a compression test. calculate: (2) Typical composite materials ≈53°, corresponding to ≈0.3. It is the static friction coefficient in the plane formed by the first direction (fiber direction) and the third direction (thickness direction).

[0046] Using the axial compressive strength and critical deflection angle included in the critical load state as input, the local shear stress and local normal compressive stress on the fiber folded surface are determined by coordinate rotation transformation. Specifically, at the critical state (the moment when damage just occurs in the matrix), the macroscopic stress is under uniaxial compression: , =0, =0, = = =0. Rotate the macroscopic stress about the y-axis by an angle θ. i (The deflection occurs in the xz plane), yielding the local stress components on the fiber folded surface: Local shear stress: (3) Local normal stress: (4) (This value is negative, indicating that the matrix on the folded surface is subjected to compressive stress.)

[0047] in, , , The three normal stresses under macroscopic compressive load ( This refers to the axial compressive stress along the principal direction of the fiber. It is a transverse normal stress. (Normal stress in the thickness direction) , For macroscopic shear stress components ( The shear stress is in the plane 1-2. (This refers to the in-plane shear stress of 2-3).

[0048] Based on the frictional coupling shear equilibrium relationship, the effective shear stress is defined as the local shear stress minus the product of the static friction coefficient and the absolute value of the local normal compressive stress. Specifically, based on the Mohr-Coulomb frictional coupling theory, normal compressive stress generates frictional constraint, improving the material's resistance to shear failure. The effective shear stress that truly drives matrix shear failure... for: (5) Substituting the local stress components, (6) That is, at the critical starting point of stage II, "effective shear stress ( The value is exactly equal to the local shear strength under the current condition (i.e., the equivalent critical local shear strength). Substituting the stress components obtained in step 2 into the equation, we get: (7) Taking the onset of compressive damage in the matrix as the critical boundary condition, the effective shear stress is set to be equal to the equivalent critical local shear strength to be determined. The functional relationship between the equivalent critical local shear strength and the axial compressive strength, the critical deflection angle and the static friction coefficient is solved inversely, thereby obtaining the equivalent critical local shear strength.

[0049] Specifically, at the critical point where the matrix just begins to show compressive damage, the effective shear stress It is exactly equal to the local shear strength in that state, i.e., the equivalent critical local shear strength. Therefore, an equation can be established, and the solution can be obtained by inverse equation. and , , The functional relationship between them.

[0050] In practical applications, the derivation process in this embodiment achieves a physical mapping from macroscopic compressive strength to microscopic local shear strength, which is the core manifestation of the "parameter replacement" strategy of this invention. By replacing the macroscopic shear strength in the same friction coupling criterion framework with the derived equivalent local shear strength, it can be used to replace the macroscopic interlaminar shear strength in the Pinho failure criterion to determine the initiation of microscopic damage in the matrix, thereby achieving accurate determination of stage II continuous damage.

[0051] As an optional embodiment of the present invention, the following calculations are performed: Will Simplifying, we can obtain the following functional relationship: (8-1) in, This represents the equivalent critical local shear strength (within the plane formed by the horizontal and vertical directions, i.e., plane 1-3, or the fiber folded surface). This indicates the axial compressive strength included in the critical load state; Indicates the critical deflection angle; The static friction coefficient is represented by: direction 1 being the horizontal direction (main fiber direction), direction 2 being the Y direction (front-back direction in the figure), and direction 3 being the vertical direction (thickness direction). Similarly, it can be calculated... , = (8-2) in, This represents the equivalent critical local shear strength (within the plane formed by the horizontal and vertical directions, i.e., plane 2-3, or another fiber folded surface). Indicates the compressive strength in the Y direction (lateral direction); This indicates the angle of deflection of the fiber in the vertical direction during lateral compression; Indicates the coefficient of friction in the Y-vertical plane; It needs to be explained that, When used for compression in the horizontal direction (direction 1), the fiber bends in the horizontal-vertical plane (plane 1-3), the local shear strength required for the matrix to begin to be damaged in this plane; When used for transverse compression in the Y direction (2 directions), the fiber bends in the Y-vertical plane (2-3 plane), which is the local shear strength required for the matrix to begin to be damaged.

[0052] Specifically, this formula is the core mathematical expression for determining the initiation of damage in Stage II of this invention. The first term within the parentheses on the right side of the formula... The second term corresponds to the contribution of local shear stress. The corresponding normal compressive stress contributes to the resistance through friction. When the fiber deflection angle θ... i When the stress is small, local shear stress dominates; as the stress decreases... As the frictional stress increases, the normal compressive stress increases, the frictional resistance increases, and the equivalent shear strength decreases accordingly. The derivation of this formula is based on the Mohr-Coulomb frictional coupling theory, has no empirical fitting parameters, and possesses a clear physical meaning.

[0053] In practical finite element implementation, this formula is executed before damage determination at each integration point: first, it reads the pre-calculated values ​​from the material initialization module. and Substitute into the stress update of the current increment step Calculations yielded Then, this value is compared with the local shear stress obtained through coordinate transformation. Comparison: If | |≥ If the matrix is ​​found to have entered a crack propagation state, the continuous damage model is activated.

[0054] In practical applications, this embodiment uses a closed-form analytical approach to link microscopic local shear strength with macroscopic compressive strength, giving the model clear physical interpretability without introducing any empirical fitting parameters. Compared to the traditional Hashin criterion, which requires fitting multiple empirical coefficients, and the Tsai-Wu tensor criterion, which requires extensive biaxial experimental data calibration, the method of this invention significantly reduces the workload of material characterization and improves the practicality and generalizability of the prediction method. Furthermore, the simple form of the formula results in extremely high computational efficiency in the finite element subroutine, requiring only a few floating-point operations, without affecting the overall computational speed of large-scale structural models.

[0055] As an optional embodiment of the present invention, determining the local matrix stress of the fiber folded surface through stress coordinate transformation includes: Obtain the real-time deflection angle and the current macroscopic compressive load stress components; Specifically, real-time deflection angle This represents the total deflection angle of the fiber bundle in the current increment step, which is the superposition of the initial deflection angle and the additional deflection angle caused by the load. The current macroscopic compressive load stress components include three normal stresses. (Axial compressive stress along the principal fiber direction) (Transverse normal stress) (Thickness direction normal stress) and three shear stresses (1-2 Plane Shear Stress) (1-3 Plane Shear Stress) (2-3 Plane Shear Stress), calculated by the finite element solver in each increment step, serves as the input for stress coordinate transformation, and is controlled by real-time deflection angle. The local stress components on the fiber folded surface are obtained after rotational transformation and used for subsequent matrix damage assessment and final instability assessment.

[0056] Based on the real-time deflection angle, the macroscopic compressive load stress components are decomposed into a local coordinate system parallel to the fiber folded surface through rotational transformation, thereby determining the normal stress, the first folded surface shear stress, and the second folded surface shear stress on the fiber folded surface.

[0057] Specifically, due to the geometric constraints of the braided structure on the deflection direction of the fiber bundles, the deflection is confined within the braiding plane (e.g., plane 1-3). The macroscopic stress components are rotated about the normal to this plane by an angle. The stress components in the local coordinate system are obtained, and the calculation formula is as follows: Normal stress (perpendicular to the fiber bending surface) along the fiber direction: (9) First folded surface shear stress (acting along the fiber direction on the fiber folded surface): (10) Second folded surface shear stress (perpendicular to the fiber direction but parallel to the fiber folded surface): (11) Fiber orientation normal stress and another stress component It can be calculated using the following formula: Second directional stress: (12) Normal stress in fiber direction: (13) in, , , The three normal stresses under macroscopic compressive load ( This refers to the axial compressive stress along the principal direction of the fiber. It is a transverse normal stress. (Normal stress in the thickness direction). , For macroscopic shear stress components ( The shear stress is in the plane 1-2. (Shear stress in plane 2-3) For real-time deflection angle, Normal stress in the fiber direction, For the stress in the second direction, σ3 fm It is the normal stress (perpendicular to the fiber bending surface). This is the shear stress on the first folded surface (acting along the fiber direction on the fiber folded surface). This is the shear stress on the second folded surface (perpendicular to the fiber direction but parallel to the fiber folded surface).

[0058] When the local stress of the matrix on the fiber folded surface satisfies the failure criterion of using the equivalent critical local shear strength as a strength parameter, the matrix is ​​determined to have entered a crack propagation state.

[0059] Specifically, the "failure criterion using the equivalent critical local shear strength as a strength parameter" refers to the macroscopic interlaminar shear strength in the original fiber compression and bending failure criterion (i.e., the sum of squares criterion used to determine final instability in Stage III). , Replace them with equivalent critical local shear strength respectively , The mathematical form of the criterion (sum of squares, tension / compression branch, friction coupling term) remains unchanged. At this point, based on the normal stress on the fiber folded surface... The positive and negative choices correspond to the following branches: like (Stretch-dominant), calculation: (14) like (Compression-dominated), calculation: (15) When When the value is ≥1, the local stress of the matrix is ​​considered to meet the failure criterion, thus determining that the matrix has entered the crack propagation state (i.e., the continuous compressive damage stage). The meaning of the relevant letters can be found in the explanations of formulas (21) and (22).

[0060] In practical applications, this embodiment utilizes the geometric constraints of the braided structure on the fiber bundle deflection direction to limit the rotational transformation to the braided plane. This eliminates the need to search for the most dangerous folded surface in three-dimensional space, significantly reducing computational complexity and avoiding the numerical convergence difficulties caused by three-dimensional searches in traditional algorithms. The transformed normal stress σ3 fm The positive and negative values ​​are used to distinguish the dominant state of the fiber folded surface in stage III (positive values ​​correspond to tensile dominance, negative values ​​correspond to compressive dominance); shear stress of the first folded surface Second fold surface shear stress Combining the normal stress, the failure criterion (i.e. the judgment criterion after parameter replacement) with the equivalent critical local shear strength as the strength parameter is used to determine whether the matrix has entered the crack propagation state in stage II; in stage III, the above three local stress components jointly participate in the determination of the final bending instability.

[0061] As an optional embodiment of the present invention, updating the equivalent shear modulus of the braided fiber composite material based on the crack propagation state using a shear stiffness reduction factor includes: The equivalent critical local shear strength, the local matrix stress on the fiber folded surface, and the fracture energy of the braided fiber composite material are obtained. Specifically, the input parameters required for continuous damage evolution are defined as: equivalent critical local shear strength. Obtained through the aforementioned parameter replacement strategy. The local matrix stress on the fiber folded surface is obtained through stress coordinate transformation. Fracture energy. This represents the integral area under the stress-displacement softening curve.

[0062] The damage variable is determined according to the continuous damage model, wherein the damage variable increases from zero to a preset threshold according to the linear softening law based on the fracture energy; Specifically, after entering the damage propagation stage, a linear continuous damage model based on the energy dissipation law is used to reduce the structural stiffness. The model relies on two key parameters: the damage parameter threshold. (The damage limit when the matrix is ​​completely damaged but the fibers are not broken) and damage fracture energy (Represents the integral area under the stress-displacement softening curve, used to eliminate mesh size sensitivity). The damage variable increases from zero to a preset threshold according to a linear softening law based on fracture energy (corresponding to...). At the same time, a small residual truncation limit is set to prevent singularity collapse of the stiffness matrix.

[0063] The shear stiffness reduction factor is determined based on the damage variables of the braided fiber composite material in the first and second directions; To ensure the robustness of the algorithm, a non-intrusive weakening scheme is adopted for shear stiffness. According to the formula in the image, the shear stiffness reduction factor is... ,in , For the damage variables in the first and second directions.

[0064] The original shear modulus of the braided fiber composite material is reduced according to the shear stiffness reduction factor to obtain the updated equivalent shear modulus.

[0065] Specifically, assuming the material's stiffness matrix in the undamaged initial state is as follows: Based on the above method, the stiffness matrix after damage can be constructed. , (16) in, Stiffness matrix under damage-free conditions Element; , , ; , , (17) for The components describing shear stiffness ( ), that is, to use The coefficients of the form are reduced for stiffness to obtain the updated equivalent shear modulus.

[0066] in, This represents the initial stiffness matrix under the undamaged state. Damage variable in the first direction (main fiber direction); For damage variables in the second direction (lateral); For third-party damage variables (thickness direction); This is the stiffness reduction factor in the first direction; The stiffness reduction factor in the second direction is 2; This is the third-party stiffness reduction factor; The harmonic average reduction factor for the 1-2 planar shear components; The harmonic average reduction factor for the 2-3 planar shear components; The harmonic average reduction factor for the 1-3 planar shear components; It is the geometric mean reduction factor for shear stiffness, used to reduce the original shear modulus to obtain the updated equivalent shear modulus.

[0067] In practical applications, the aforementioned continuous damage evolution mechanism can be directly embedded into the user material subroutine VUMAT / UMAT of finite element software (such as Abaqus). When the local matrix stress of the fiber folded surface satisfies the failure criterion using the equivalent critical local shear strength as the strength parameter, the linear continuous damage model is activated. The damage variable increases from zero to a preset threshold according to the linear softening law controlled by the fracture energy G_fmc. Simultaneously, a small residual cutoff threshold is set to prevent singularity in the stiffness matrix, wherein the shear stiffness reduction factor is adopted. A non-invasive weakening scheme, based on the post-damage stiffness matrix. ,make , , The original shear modulus is reduced to obtain the updated equivalent shear modulus. This mechanism realizes a smooth transition from microscopic matrix damage to macroscopic stiffness degradation, and can simulate the early stiffness degradation phenomenon in the compression process of braided composite materials, while ensuring the numerical robustness of finite element calculation and mesh objectivity.

[0068] As an optional embodiment of the present invention, determining the critical deflection angle of the fiber bundle for flexural instability based on the updated equivalent shear modulus, and determining whether the woven composite material has entered the flexural instability failure stage when the real-time deflection angle is greater than or equal to the critical deflection angle for flexural instability includes: Based on the updated equivalent shear modulus, the axial ultimate compressive strength, macroscopic shear strength, and static friction coefficient of the braided fiber composite material, the critical deflection angle for flexural instability is determined by the fiber compression flexural failure criterion. Specifically, this step corresponds to the calculation of the critical deflection angle in stage III; when the material approaches final instability, the critical state of the fiber bundle is determined by the axial ultimate compressive strength. Macroscopic shear strength and static friction coefficient This is a joint decision. This embodiment utilizes the geometric constraints of the braided structure to simplify the search for the most dangerous folded surface in traditional three-dimensional space to a single-degree-of-freedom solution within the braided plane. The critical deflection angle for folding instability is determined using a fiber compression folding failure criterion based on friction coupling. This criterion is expressed by the following implicit equation: (18) Among them, It is the axial ultimate compressive strength (the macroscopic ultimate stress when the fiber is bent and unstable). Macroscopic shear strength, The static friction coefficient is Let be the critical deflection angle for bending instability to be determined. Because When both trigonometric function terms and product terms appear simultaneously, the equation cannot be solved directly and explicitly; a numerical iterative method (such as the Newton-Raphson method or the bisection method) is required. In finite element programs, this solution is typically executed only once at each integration point (material initialization stage), and the initial value for iteration can be set to the value obtained in stage I. It can converge to engineering accuracy after a finite number of iterations.

[0069] It needs to be explained that, It is the stress at which the matrix begins to damage in stage II. It is the ultimate stress at which the fiber bundle in stage III finally becomes unstable, and > .

[0070] When the real-time deflection angle is greater than or equal to the critical deflection angle for bending instability, the failure judgment index corresponding to the dominant state of the fiber bending surface is determined according to the sign of the normal stress on the bending surface. The corresponding failure judgment index is compared with the set critical value to determine whether the woven composite material has entered the bending instability failure stage.

[0071] Specifically, this step performs the final instability determination. First, it determines whether the real-time deflection angle θ has reached or exceeded the critical deflection angle for bend instability. .like < If the material is still in the continuous damage process of Stage II and has not yet triggered final instability, then incremental step iterations continue. ≥ Then proceed to the final failure determination process: based on the normal stress of the fiber folded surface obtained by coordinate transformation. The sign of the value determines the corresponding failure criterion calculation formula. The calculated failure criterion is compared with the set critical value (e.g., 1). When the failure criterion is greater than or equal to the set critical value, the braided composite material is determined to have entered the bending instability failure stage.

[0072] Fiber bundle deflection angle The relevant calculation methods are as follows:

[0073] in, is the real-time deflection angle of the fiber bundle, where The shear stress is in the plane 1-2. It is the in-plane shear modulus. The axial ultimate compressive strength, and For normal stress in two directions; The critical deflection angle for bending instability, where For shear strength parameters, The axial ultimate compressive strength, The static friction coefficient. Real-time deflection angle. It increases with increasing load and is used for stress coordinate transformation and damage assessment; critical deflection angle for bending instability. A fixed threshold is used in Phase III to determine whether the final failure determination process is triggered.

[0074] Instructions for using the real-time deflection angle θ at each stage: Stage I (Elastic No-Damage Stage): Using the simple linear elastic formula θ = + / During this stage, the load is relatively small, the fiber bundle is in a state of small deformation, and the axial stress is low. and transverse stress The coupling effect is negligible; only the elastic deflection caused by shear stress needs to be considered.

[0075] Stage II (Matrix Damage Propagation and Stiffness Degradation Stage): After entering the damage stage, simple formulas can still be used for initial assessment. However, when the local stress in the matrix approaches the equivalent critical local shear strength, a switch to a complex coupling formula should be considered to capture the nonlinear effect of the normal stress coupling effect on the deflection angle. In other words, to simplify, when judging fiber bending failure (Stage III), since damage has already occurred, a complex formula should be used. That's all.

[0076] Stage III (Bending Instability Determination Stage): Complex coupling formulas must be used. During this stage, the deflection angle is large, and the axial stress is high. and transverse stress The coupling effect is significant, so a real-time deflection angle calculation formula that considers normal stress coupling and nonlinear correction must be used for stress coordinate transformation and final instability determination.

[0077] In practical applications, this embodiment fully utilizes the geometric constraints of the braided structure on the deflection direction of the fiber bundles. Traditional Pinho's three-dimensional criterion requires searching for the most dangerous fracture surface in three-dimensional space that maximizes the Mohr-Coulomb equation, performing multiple iterations at each integration point and increment step. This results in extremely high computational complexity and often convergence failure due to the non-positive definiteness of the stiffness matrix. This invention directly determines the deflection plane using the geometric prior knowledge of the braided structure, reducing the three-dimensional search to a single-plane solution, significantly decreasing computational complexity. Simultaneously, this implicit equation establishes macroscopically measurable parameters (X_fc, S...). 13 The analytical bridge between the micro instability angle θ_c and the micro instability angle enables the model to have true predictive ability.

[0078] As an optional embodiment of the present invention, determining the failure criterion corresponding to the dominant state of the fiber folded surface based on the sign of the normal stress on the folded surface, and comparing the corresponding failure criterion with a set critical value to determine whether the woven composite material has entered the folding instability failure stage includes: When the normal stress is negative, the dominant state of the fiber folded surface is determined to be compressive stress. Based on the ratio of shear stress to shear strength on the folded surface, and combined with the weighting of the normal stress by the static friction coefficient, the first failure criterion is determined. Specifically, this branch corresponds to the failure criterion dominated by local matrix compression (σ2). fm <0). When the normal stress on the fiber folded surface is compressive stress, this compressive stress will cause the potential fracture surface to tend to close, enhancing the material's resistance to shear failure through the friction effect. Therefore, the first failure criterion is composed of the sum of the squares of the ratios of the effective shear stress weighted by the friction coefficient to the corresponding shear strength. If the failure is dominated by local matrix compression ( ): ;(twenty one) in, The primary failure criterion is compressive stress; among which... This is the shear stress on the first folded surface (acting along the fiber direction). This is the shear stress on the second folded surface (perpendicular to the fiber direction but parallel to the fiber folded surface). To and The corresponding static friction coefficient, To and The corresponding static friction coefficient, To and The corresponding macroscopic interlaminar shear strength, To and The corresponding macroscopic interlaminar shear strength.

[0079] When the normal stress is positive, the dominant state of the fiber folded surface is determined to be tensile stress dominant state. Based on the ratio of normal stress to tensile strength and the ratio of shear stress to shear strength on the folded surface, the second failure criterion is determined. Specifically, this branch corresponds to the failure determination dominated by local matrix tension. ≥0). When the normal stress on the fiber folded surface is tensile stress, this tensile stress will cause the potential fracture surface to open, and the material's resistance to shear will no longer have the gain from the frictional effect. Therefore, a more direct quadratic interaction criterion is adopted. The second failure criterion consists of the sum of the squares of the normal stress contribution and the two shear stress contributions. If the failure is dominated by local normal tension ( Then we have: ;(twenty two) in, The second failure criterion (tensile stress dominant); This represents the interlaminar tensile strength; for the explanation of the other letters, please refer to the previous formula.

[0080] The failure determination index is compared with a set critical value. When the failure determination index reaches or exceeds the set critical value, the braided composite material is determined to have entered the bending instability failure stage. The failure determination index is either the first failure determination index or the second failure determination index.

[0081] Specifically, this step performs the final numerical determination; for example, the critical value is set to 1. The physical meaning of this value comes from the classic definition of the secondary failure criterion: when the sum of the squares of all contributing terms is 1, it indicates that the material has just reached the failure boundary under this combined stress state. If Φ < 1, the material has not failed, continues to bear load, and enters the next incremental step; if Φ ≥ 1, the braided composite material is determined to have entered the flexural instability failure stage. In finite element implementation, the element state is usually marked as "failure," and the stress tensor is multiplied by a very small coefficient (such as 1 × 10⁻⁶). -6 Alternatively, the value can be directly reset to zero. At the same time, the cell deletion technique can be used to remove the failed cells to prevent numerical singularity from affecting the overall calculation.

[0082] In conjunction with the foregoing embodiments, Figure 2 The failure evolution of braided fiber composites under axial compressive load is fully demonstrated: from the initial fiber bundle with natural deflection defects (Figure a), to the increase of fiber deflection angle under compressive load, when the local stress on the fiber bending surface satisfies the equivalent critical local shear strength of the 1-3 plane. Equivalent critical local shear strength in plane 2-3 When the failure criterion is used as a strength parameter, the matrix undergoes initial damage and crack formation (Stage II, Figure b), activating the continuous damage model. The shear stiffness gradually decreases as the crack propagates. As the load continues to increase, the matrix damage intensifies, and the fiber deflection angle further increases. When the real-time deflection angle reaches the critical deflection angle for bending instability and the failure criteria are met... When the value is ≥1, the fiber bundle undergoes bending instability (Stage III, Figure c); Figure d shows the spatial interweaving of the fiber bundles and the matrix filling relationship in the braided structure, illustrating the geometric constraint of the braided structure on the fiber deflection direction, eliminating the need for three-dimensional search. The entire process fully corresponds to the complete technical solution of "real-time deflection angle update - matrix stress calculation - damage assessment - stiffness reduction - bending instability assessment".

[0083] In practical application of this embodiment, among the above criteria, the localized normal tension is dominant ( The criterion for ) directly refers to the corresponding form in Pinho[3]'s compression bending criterion. Local compression is dominated by normal compression ( The criterion for the second time is slightly modified compared to Pinho[3]. The physical logic of Pinho's "effective strength" method (denominator modification) is that compressive stress makes the potential failure surface tend to close, so the apparent strength of the material against shear failure increases (i.e., the denominator becomes (in the form of...); the physical logic of this method is that the inherent shear strength S of the material is constant, but the frictional resistance generated by the compressive stress offsets part of the actual shear stress driving failure (i.e., the molecules become...). The two are essentially the same in physical logic (in the form of variables); however, in engineering applications, the advantage of moving the variables to the molecule is that, on the one hand, if the local compressive stress changes drastically with the load... When placed in the denominator, the criterion number fluctuates as the apparent strength of the denominator changes. It will exhibit strong nonlinear abrupt changes, which can easily lead to failure in the differentiation of the stiffness matrix (Jacobi matrix), thus causing calculation divergence; on the other hand, the molecular form (effective stress) can guarantee the criterion number. It maintains a good linear positive correlation with the external load, making the changes in the damage variable smoother and more stable, and increasing the robustness of the calculation program.

[0084] like Figure 3 As shown, the present invention also provides a braided fiber composite material compressive strength prediction system 200, which applies the braided fiber composite material compressive strength prediction method described in the above embodiments, including: The elastic stage discrimination module 210 is used to: determine the critical deflection angle of the fiber bundle when the matrix begins to show compressive damage, based on the initial deflection angle of the fiber bundle and the critical load state parameters when the matrix begins to show compressive damage. The matrix damage stage determination module 220 is used to: determine that the braided fiber composite material has entered the matrix damage propagation stage when the real-time deflection angle of the fiber bundle is greater than or equal to the critical deflection angle. The matrix damage evolution module 230 is used to: determine the equivalent critical local shear strength based on the critical deflection angle and the critical load state parameters, and determine the matrix local stress of the fiber folded surface through stress coordinate transformation; The crack propagation update module 240 is used to: determine that the matrix has entered a crack propagation state when the local stress of the matrix of the fiber folded surface meets the failure criterion of using the equivalent critical local shear strength as a strength parameter; and update the equivalent shear modulus of the braided fiber composite material based on the crack propagation state by using the shear stiffness reduction coefficient. The instability determination module 250 is used to: determine the critical deflection angle of the fiber bundle for bending instability based on the updated equivalent shear modulus; and determine whether the woven composite material has entered the bending instability failure stage when the real-time deflection angle is greater than or equal to the critical deflection angle for bending instability.

[0085] The specific implementation method of this embodiment can be referred to the corresponding implementation method described above, and will not be described again here.

[0086] like Figure 4 As shown, an electronic device 300 provided in this embodiment of the invention includes a memory 310 and a processor 320; the memory 310 is used to store a computer program; the processor 320 is used to implement the method for predicting the compressive strength of braided fiber composite materials as described above when the computer program is executed.

[0087] Alternatively, an electronic device 300 includes a memory 310 and a processor 320 coupled to the memory 310; the memory 310 is configured to store a computer program; and the processor 320 is configured to perform the following operations when the computer program is executed: Based on the initial deflection angle of the fiber bundle and the critical load state parameters at which the matrix begins to show compressive damage, the critical deflection angle of the fiber bundle at which the matrix begins to show compressive damage is determined. When the real-time deflection angle of the fiber bundle is greater than or equal to the critical deflection angle, it is determined that the braided fiber composite material has entered the matrix damage propagation stage. Based on the critical deflection angle and the critical load state parameters, the equivalent critical local shear strength is determined, and the matrix local stress of the fiber folded surface is determined through stress coordinate transformation. When the local stress of the matrix on the fiber folded surface satisfies the failure criterion of using the equivalent critical local shear strength as a strength parameter, the matrix is ​​determined to have entered a crack propagation state. Based on the crack propagation state, the equivalent shear modulus of the braided fiber composite material is updated by the shear stiffness reduction factor. Based on the updated equivalent shear modulus, the critical deflection angle of the fiber bundle for flexural instability is determined. When the real-time deflection angle is greater than or equal to the critical deflection angle for flexural instability, it is determined whether the woven composite material has entered the flexural instability failure stage; the critical deflection angle for flexural instability is greater than the critical deflection angle.

[0088] This invention provides a computer-readable storage medium storing a computer program. When the computer program is executed by a processor, it implements the method for predicting the compressive strength of braided fiber composite materials as described above.

[0089] Alternatively, a non-volatile computer-readable storage medium storing a computer program that, when executed by a processor, causes the processor to perform the following operations: Based on the initial deflection angle of the fiber bundle and the critical load state parameters at which the matrix begins to show compressive damage, the critical deflection angle of the fiber bundle at which the matrix begins to show compressive damage is determined. When the real-time deflection angle of the fiber bundle is greater than or equal to the critical deflection angle, it is determined that the braided fiber composite material has entered the matrix damage propagation stage. Based on the critical deflection angle and the critical load state parameters, the equivalent critical local shear strength is determined, and the matrix local stress of the fiber folded surface is determined through stress coordinate transformation. When the local stress of the matrix on the fiber folded surface satisfies the failure criterion of using the equivalent critical local shear strength as a strength parameter, the matrix is ​​determined to have entered a crack propagation state. Based on the crack propagation state, the equivalent shear modulus of the braided fiber composite material is updated by the shear stiffness reduction factor. Based on the updated equivalent shear modulus, the critical deflection angle of the fiber bundle for flexural instability is determined. When the real-time deflection angle is greater than or equal to the critical deflection angle for flexural instability, it is determined whether the woven composite material has entered the flexural instability failure stage; the critical deflection angle for flexural instability is greater than the critical deflection angle.

[0090] Those skilled in the art will understand that all or part of the processes in the above embodiments can be implemented by a computer program instructing related hardware. The program can be stored in a non-volatile computer-readable storage medium, and when executed, it can include the processes of the embodiments of the above methods. Any references to memory, storage, databases, or other media used in the embodiments provided by this invention can include non-volatile and / or volatile memory. Non-volatile memory can include read-only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM), or flash memory. Volatile memory can include random access memory (RAM) or external cache memory. By way of illustration and not limitation, RAM is available in various forms, such as static RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), dual data rate SDRAM (DDRSDRAM), enhanced SDRAM (ESDRAM), synchronous link DRAM (SLDRAM), Rambus direct RAM (RDRAM), direct memory bus dynamic RAM (DRDRAM), and memory bus dynamic RAM (RDRAM), etc.

[0091] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features of the invention herein.

[0092] While the present invention has been disclosed above, its scope of protection is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention, and all such changes and modifications will fall within the scope of protection of the present invention.

Claims

1. A method for predicting the compressive strength of braided fiber composite materials, characterized in that, Braided fiber composite materials include fiber bundles and a matrix; the method for predicting the compressive strength of the braided fiber composite material includes: Based on the initial deflection angle of the fiber bundle and the critical load state parameters at which the matrix begins to show compressive damage, the critical deflection angle of the fiber bundle at which the matrix begins to show compressive damage is determined. When the real-time deflection angle of the fiber bundle is greater than or equal to the critical deflection angle, it is determined that the braided fiber composite material has entered the matrix damage propagation stage. Based on the critical deflection angle and the critical load state parameters, the equivalent critical local shear strength is determined, and the matrix local stress of the fiber folded surface is determined through stress coordinate transformation. When the local stress of the matrix on the fiber folded surface satisfies the failure criterion of using the equivalent critical local shear strength as a strength parameter, the matrix is ​​determined to have entered a crack propagation state. Based on the crack propagation state, the equivalent shear modulus of the braided fiber composite material is updated by the shear stiffness reduction factor. Based on the updated equivalent shear modulus, the critical deflection angle of the fiber bundle for flexural instability is determined. When the real-time deflection angle is greater than or equal to the critical deflection angle for flexural instability, it is determined whether the woven composite material has entered the flexural instability failure stage; the critical deflection angle for flexural instability is greater than the critical deflection angle.

2. The method for predicting the compressive strength of braided fiber composite materials according to claim 1, characterized in that, The step of determining the critical deflection angle of the fiber bundle when the matrix begins to show compressive damage, based on the initial deflection angle of the fiber bundle and the critical load state parameters when the matrix begins to show compressive damage, includes: The critical deflection angle is determined by transcendental equations based on the initial deflection angle, the axial compressive strength included in the critical load state, and the shear modulus of the braided fiber composite material.

3. The method for predicting the compressive strength of braided fiber composite materials according to claim 2, characterized in that, The shear modulus is obtained according to the first method, and the axial compressive strength is determined according to the stress value when the matrix of the braided fiber composite material begins to show microscopic damage under pure compression; the first method includes at least one of macroscopic experiments, theoretical derivation, material property test results, and empirical assignment.

4. The method for predicting the compressive strength of braided fiber composite materials according to claim 2, characterized in that, The step of determining the equivalent critical local shear strength based on the critical deflection angle and the critical load state parameters includes: Obtain the critical deflection angle, the axial compressive strength included in the critical load state, and the static friction coefficient in the fiber folded surface; Using the axial compressive strength and critical deflection angle included in the critical load state as input, the local shear stress and local normal compressive stress on the fiber folded surface are determined by coordinate rotation transformation. Based on the frictional coupling shear equilibrium relationship, the effective shear stress is defined as the local shear stress minus the product of the static friction coefficient and the absolute value of the local normal compressive stress. Taking the onset of compressive damage in the matrix as the critical boundary condition, the effective shear stress is set to be equal to the equivalent critical local shear strength to be determined. The functional relationship between the equivalent critical local shear strength and the axial compressive strength, the critical deflection angle and the static friction coefficient is solved inversely, thereby obtaining the equivalent critical local shear strength.

5. The method for predicting the compressive strength of braided fiber composite materials according to claim 4, characterized in that, The functional relationship is expressed by the following formula: ； in, This represents the equivalent critical local shear strength within the fiber's folded surface. This indicates the axial compressive strength included in the critical load state; This indicates the critical deflection angle; This represents the static friction coefficient within the folded surface of the fiber.

6. The method for predicting the compressive strength of braided fiber composite materials according to claim 1, characterized in that, The determination of the local matrix stress on the fiber folded surface through stress coordinate transformation includes: Obtain the real-time deflection angle and the current macroscopic compressive load stress components; Based on the real-time deflection angle, the macroscopic compressive load stress components are decomposed into a local coordinate system parallel to the fiber folded surface through rotational transformation, thereby determining the normal stress, the first folded surface shear stress, and the second folded surface shear stress on the fiber folded surface; the first folded surface shear stress is the shear stress component acting on the fiber folded surface along the fiber direction; the second folded surface shear stress is the shear stress component perpendicular to the fiber direction but parallel to the fiber folded surface.

7. The method for predicting the compressive strength of braided fiber composite materials according to claim 4 or 5, characterized in that, The step of updating the equivalent shear modulus of the braided fiber composite material based on the crack propagation state using a shear stiffness reduction factor includes: The equivalent critical local shear strength, the local matrix stress on the fiber folded surface, and the fracture energy of the braided fiber composite material are obtained. Damage variables are determined according to a continuous damage model, wherein the damage variables increase from zero to a preset threshold according to a linear softening law based on the fracture energy. The shear stiffness reduction factor is determined based on the damage variables of the braided fiber composite material in the first and second directions; The original shear modulus of the braided fiber composite material is reduced according to the shear stiffness reduction factor to obtain the updated equivalent shear modulus.

8. The method for predicting the compressive strength of braided fiber composite materials according to claim 4 or 5, characterized in that, The step of determining the critical deflection angle of the fiber bundle for flexural instability based on the updated equivalent shear modulus, and determining whether the braided composite material has entered the flexural instability failure stage when the real-time deflection angle is greater than or equal to the critical deflection angle for flexural instability, includes: Based on the updated equivalent shear modulus, the axial ultimate compressive strength, macroscopic shear strength, and static friction coefficient of the braided fiber composite material, the critical deflection angle for flexural instability is determined by the fiber compression flexural failure criterion. When the real-time deflection angle is greater than or equal to the critical deflection angle for bending instability, the failure judgment index corresponding to the dominant state of the fiber bending surface is determined according to the sign of the normal stress on the bending surface. The corresponding failure judgment index is compared with the set critical value to determine whether the woven composite material has entered the bending instability failure stage.

9. The method for predicting the compressive strength of braided fiber composite materials according to claim 8, characterized in that, The step of determining the failure criterion corresponding to the dominant state of the fiber folded surface based on the sign of the normal stress on the folded surface, and comparing the corresponding failure criterion with a set critical value to determine whether the woven composite material has entered the folding instability failure stage includes: When the normal stress is negative, the dominant state of the fiber folded surface is determined to be compressive stress. Based on the ratio of shear stress to shear strength on the folded surface, and combined with the weighting of the normal stress by the static friction coefficient, the first failure criterion is determined. When the normal stress is positive, the dominant state of the fiber folded surface is determined to be tensile stress dominant state. Based on the ratio of normal stress to tensile strength and the ratio of shear stress to shear strength on the folded surface, the second failure criterion is determined. The failure determination index is compared with a set critical value. When the failure determination index reaches or exceeds the set critical value, the braided composite material is determined to have entered the bending instability failure stage. The failure determination index is either the first failure determination index or the second failure determination index.

10. A system for predicting the compressive strength of braided fiber composite materials, characterized in that, include: The elastic stage discrimination module is used to: determine the critical deflection angle of the fiber bundle when the matrix begins to show compressive damage, based on the initial deflection angle of the fiber bundle and the critical load state parameters when the matrix begins to show compressive damage. The matrix damage stage determination module is used to: determine that the braided fiber composite material has entered the matrix damage propagation stage when the real-time deflection angle of the fiber bundle is greater than or equal to the critical deflection angle; The matrix damage evolution module is used to: determine the equivalent critical local shear strength based on the critical deflection angle and the critical load state parameters, and determine the matrix local stress of the fiber folded surface through stress coordinate transformation; The crack propagation update module is used to: determine that the matrix has entered a crack propagation state when the local stress of the matrix on the fiber folded surface meets the failure criterion of using the equivalent critical local shear strength as a strength parameter; and update the equivalent shear modulus of the braided fiber composite material based on the crack propagation state by using the shear stiffness reduction coefficient. The instability determination module is used to: determine the critical deflection angle of the fiber bundle for bending instability based on the updated equivalent shear modulus; and determine whether the woven composite material has entered the bending instability failure stage when the real-time deflection angle is greater than or equal to the critical deflection angle for bending instability.

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