Performance prediction method of aramid pulp reinforced composite material for aircraft structure

Abstract

The invention provides a performance prediction method of an aramid pulp reinforced composite material for an aircraft structure, and belongs to the technical field of composite materials. The macroscopic connection structure of the aramid pulp and the carbon fiber layer is divided into three microcosmic interface layers, namely an aramid pulp interface layer, an aramid pulp-carbon fiber interaction interface layer and a carbon fiber interface layer; respectively establishing rigidity micromechanical models of the three microcosmic interface layers, and constructing a rigidity micromechanical model of the composite material by taking the volume fraction of each interface layer in the composite material as a weight and taking the weighted sum of the moduli of all the interface layers as the modulus of the composite material; analyzing the deformation conditions of the three microcosmic interface layers in a micro-buckling mode and a fiber breaking failure mode, and constructing a strength micromechanical model of the composite material; and predicting the modulus and the compression strength of the composite material by adopting a rigidity micro-mechanical model and a strength micro-mechanical model. According to the method, the performance of the aramid pulp reinforced composite material can be effectively predicted and analyzed.

Description

Technical Field

[0001] This invention belongs to the field of composite material technology, specifically relating to a method for predicting the performance of aramid paste-reinforced composite materials that can be used in aircraft structures. Background Technology

[0002] Thin-shell or plate-like structures made of carbon fiber reinforced polymer composites have been widely used in various composite structures such as aircraft wings, fuselages, and wind turbine blades due to their high specific strength and specific stiffness. However, prepreg carbon fiber reinforced polymer composite plates suffer from significant interlayer interface problems. On the one hand, they lack fiber reinforcement in the z-direction; on the other hand, brittle epoxy resin aggregation regions exist at the carbon fiber layer interfaces, directly leading to interlayer interface cracks or delamination, causing premature failure of the carbon fiber structure.

[0003] As delamination microcracks propagate, the longitudinal properties of unidirectional carbon fiber reinforced polymers are significantly affected. Therefore, several toughening methods have been proposed, such as splicing, Z-pinning, short fiber or CNT interleaving, to suppress the initiation and propagation of these delamination microcracks. While these toughening methods effectively inhibit delamination cracking, they alter the arrangement of continuous carbon fibers, leading to a decrease in stiffness and compressive strength. Recent studies have employed sparsely distributed aramid fibers as ultrathin interlayers between unidirectional carbon fiber reinforced polymers, achieving promising results. Aramid fibers with diameters ranging from several hundred nanometers to 10 μm are randomly distributed on the carbon fiber layer during internal pre-fabrication, penetrating into the Z-direction to form a toughening cross-layer interface. Results showed that aramid fibers significantly enhanced the strength, toughness, and modulus of unidirectional carbon fiber reinforced polymers with almost no increase in cost. Therefore, a cost-effective and efficient aramid fiber interleaving method can be used to improve composite material properties. However, the aramid fiber toughening method has not yet been analyzed and validated through micromechanical modeling. Therefore, it is necessary to theoretically predict the enhancement observed in the experiment and to predict the optimal limit for the existence of aramid fiber cross-layers. Summary of the Invention

[0004] This invention provides a method for predicting the performance of aramid paste-reinforced composite materials that can be used in aircraft structures. A micromechanical model of the interlaminar toughening of unidirectional carbon fibers by aramid fibers is established, which can effectively predict and analyze the performance of aramid paste-reinforced composite materials, providing theoretical guidance for the design of composite materials, thereby solving at least one of the technical problems involved in the background art.

[0005] To solve the above-mentioned technical problems, the present invention is implemented as follows: A method for predicting the performance of aramid paste-reinforced composite materials applicable to aircraft structures, the composite material comprising a resin matrix at the top and bottom layers and multiple carbon fiber layers between the two resin matrix layers, wherein aramid paste is used as a binder between adjacent carbon fiber layers, and the performance prediction method includes the following steps: Step S1: Based on the different penetration depths of aramid paste in the carbon fiber layer, the macroscopic connection structure between aramid paste and carbon fiber layer is divided into three microscopic interface layers: aramid paste interface layer, aramid paste-carbon fiber interactive interface layer, and carbon fiber interface layer. Step S2: Establish stiffness micromechanical models for three types of micro-interface layers respectively. Based on the hierarchical interface principle, the volume fraction of each interface layer in the composite material is used as the weight, and the weighted sum of the moduli of all interface layers is used as the modulus of the composite material to construct the stiffness micromechanical model of the composite material. Step S3: Analyze the deformation of the three micro-interface layers under the micro-buckling mode and fiber breakage failure mode, and construct a strength micromechanical model of the composite material. Step S4: The stiffness micromechanical model and the strength micromechanical model of the composite material are used to predict the modulus and compressive strength of the composite material, and the results are compared with the experimentally measured modulus and compressive strength to verify the effectiveness of the stiffness micromechanical model and the strength micromechanical model.

[0006] As a preferred improvement, the aramid paste includes a resin carrier and aramid fibers randomly distributed in two dimensions on the resin carrier.

[0007] As a preferred improvement, the aramid paste interface layer is located on the surface of the carbon fiber layer, forming an ultrathin interface of 5-15 μm formed by the aramid paste that has not penetrated into the carbon fiber layer, and the volume fraction of carbon fiber in the aramid paste interface layer is 0; the aramid paste-carbon fiber interactive interface layer is formed by the interaction between the aramid paste and the carbon fiber layer after the aramid paste has penetrated into the carbon fiber layer, and is located on the surface and subsurface of the carbon fiber layer, and the volume fraction of carbon fiber in the aramid paste-carbon fiber interactive interface layer is less than 65%; the carbon fiber interface layer is located deep within the carbon fiber layer, and the volume fraction of carbon fiber in the carbon fiber interface layer is not less than 65%.

[0008] As a preferred improvement, the stiffness micromechanical model of the aramid slurry interface layer is expressed as: ; ; In the formula, , These represent the Young's modulus and shear modulus of the aramid paste interface layer, respectively. , These represent the longitudinal and transverse moduli of the aramid paste interface layer, respectively; where: ; ; In the formula, This indicates the volume fraction of aramid fibers in the aramid pulp; Indicates the modulus of the resin carrier in the aramid paste; , These represent the length and diameter of the aramid fiber, respectively. , These represent the correction factors for the longitudinal modulus (fiber direction) and transverse modulus (perpendicular to the fiber direction) of aramid fibers, respectively, where: ; ; In the formula, This indicates the modulus of aramid fibers in aramid paste.

[0009] As a preferred improvement, the stiffness micromechanical model of the aramid paste-carbon fiber interface layer is expressed as follows: ; ; ; In the formula, , , These represent the longitudinal modulus, transverse modulus, and shear modulus of the aramid paste-carbon fiber interface layer, respectively. , These represent the volume fractions of carbon fiber and aramid paste in the aramid paste-carbon fiber interface layer, respectively. , , These represent the longitudinal modulus, transverse modulus, and shear modulus of carbon fiber, respectively. , These represent the Young's modulus and shear modulus of the aramid paste, respectively.

[0010] As a preferred improvement, the micromechanical model of the stiffness of the carbon fiber interface layer is expressed as: ; ; ; In the formula, , , These represent the longitudinal modulus, transverse modulus, and shear modulus of the carbon fiber interface layer, respectively. , These represent the volume fractions of carbon fiber and aramid paste in the carbon fiber interface layer, respectively.

[0011] As a preferred improvement, the micromechanical model of the composite material is expressed as follows: ; ; ; ; ; ; In the formula, This represents the equivalent stiffness micromechanical properties of composite materials. , , , The equivalent stiffness micromechanical properties of the carbon fiber interface layer, the aramid paste-carbon fiber interactive interface layer, the aramid paste interface layer, and the resin matrix are respectively represented. , , and These represent the volume fractions of the carbon fiber interface layer, the aramid paste-carbon fiber interfacial layer, the aramid paste interface layer, and the resin matrix, respectively. express The total thickness of each carbon fiber interface layer; express The total thickness of the aramid paste-carbon fiber interface layer. ; express The total thickness of the aramid paste interface layer. ; This indicates the thickness of a single resin matrix layer, where there are two resin matrix layers. This represents the total thickness of the composite material; the equivalent stiffness micromechanical properties include Young's modulus and shear modulus.

[0012] As a preferred improvement, the thickness of each interface layer is determined as follows: the longitudinal section of the composite material is observed using microscopy, and the thickness of each interface layer in the longitudinal section is determined by combining image analysis technology.

[0013] As a preferred improvement, the strength micromechanical model of the composite material under the micro-buckling mode is expressed as follows: ; ; In the formula, This indicates the failure strength of the composite material under axial compressive load; This represents the modified resin matrix shear strength, taking into account the influence of transverse compressive strength on matrix shear failure. Indicates the total fiber misalignment angle; Indicates the shear strength of the resin matrix; This indicates the compressive strength of the composite material perpendicular to the fiber direction; This indicates the orientation angle of the kink band.

[0014] Assuming the direction of the kink band is perpendicular to the fiber axis, then Taking a value of 0 simplifies the above strength micromechanical model, which can be expressed as: ; ; In the formula, This indicates the initial fiber misalignment angle, caused by manufacturing defects, and is typically 1° to 4°. This indicates the additional fiber rotation angle, which refers to the additional rotation angle of the fibers within the kink under axial compressive load.

[0015] As a preferred improvement, the strength micromechanical model of the composite material under the fiber breakage failure mode is expressed as follows: ; In the formula, Indicates the ultimate strength of a composite material; Indicates fiber angle Strength of the composite material; fiber angle Indicates the angle between the fiber and the loading direction or the principal direction of the material in a composite material; Based on the maximum stress criterion, the above equation can also be expressed as: ; In the formula, This indicates the ultimate strength of the composite material in the longitudinal direction; It represents the ultimate strength of a composite material in the transverse direction. and It can be calculated using the following formula: ; ; In the formula, and These represent the strength parameters of the resin matrix and the aramid paste, respectively. Indicates the volume fraction of aramid paste in the composite material; Indicates the modulus of the resin matrix; Indicates the transverse modulus of aramid paste; ; ; In the formula, This represents the compressive strength of the i-th interface layer in the longitudinal direction; This represents the compressive strength of the i-th interface layer in the lateral direction; Indicates the compressive strength of carbon fiber; Let represent the compressive strength of the aramid paste in the i-th interface layer.

[0016] The beneficial effects of this invention are as follows: (1) Compared with traditional macroscopic scale modeling methods, this invention provides a set of three gradient interface microstructure mechanical modeling methods. Through the analysis of compressive strength and stiffness models, the axial compressive strength and elastic modulus of composite materials can be effectively predicted, and the correlation between macro and micro scales is established. (2) Through the new microbuckling and fiber breakage failure model, the interface toughening mechanism between composite materials and the influence mechanism of failure mode on interface performance are revealed. The model effectively solves the problems of experimental error and economic cost, and provides a solution for improving the interface performance of unidirectional carbon fiber reinforced polymer. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort, wherein: Figure 1 A flowchart illustrating a method for predicting the performance of aramid paste-reinforced composite materials applicable to aircraft structures, provided by the present invention; Figure 2 A diagram showing the microstructure of the composite material; Figure 3 A schematic diagram illustrating the failure mechanisms of composite materials in micro-buckling mode and fiber breakage failure mode; Figure 4 This is a comparison chart of the longitudinal modulus predicted by the model in Example 1 and the experimental results; Figure 5 This is a comparison chart of the lateral modulus predicted by the model in Example 1 and the experimental results. Figure 6 This figure shows a comparison between the compressive strength predicted by the micro-buckling model in Example 1 and the results of the direct axial compression test. Figure 7 The figure shows a comparison between the compressive strength predicted by the micro-buckling model and the fiber breakage failure model in Example 1 and the results of the three-point bending test. Detailed Implementation

[0018] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] like Figures 1-7 As shown, this embodiment provides a method for predicting the performance of aramid paste-reinforced composite materials that can be used in aircraft structures. The composite material includes a resin matrix located at the top and bottom layers and multiple carbon fiber layers located between the two resin matrix layers. Aramid paste is used as an adhesive between adjacent carbon fiber layers.

[0020] Aramid paste comprises a resin carrier and aramid fibers randomly distributed two-dimensionally on the resin carrier (epoxy resin). In traditional carbon fiber layer bonding technology, epoxy resin is usually used directly as a binder. However, epoxy resin itself generally has low strength, making the interface between two carbon fiber layers a weak point. In this application, short-cut or nanoscale aramid fibers are added to the epoxy resin to reinforce it, resulting in a composite material with a gradient reinforcement structure.

[0021] The performance prediction method includes the following steps: Step S1: Based on the different penetration depths of aramid paste in the carbon fiber layer, the macroscopic connection structure between aramid paste and carbon fiber layer is divided into three microscopic interface layers: aramid paste interface layer, aramid paste-carbon fiber interactive interface layer, and carbon fiber interface layer. like Figure 2 As shown, aramid paste, used as a binder for carbon fiber layers, is applied to the surface of the carbon fiber layers during application. The internal structure of the carbon fiber layers is loose, containing many voids. Therefore, the aramid paste can gradually penetrate into the carbon fiber layers. The degree of interaction between the aramid paste and the carbon fiber layers varies depending on the depth of penetration. Based on this, three types of interfaces are defined: (1) Aramid slurry interface layer The aramid paste interface layer is located on the surface of the carbon fiber layer, forming an ultrathin interface of 5-15 μm formed by aramid paste that has not penetrated into the carbon fiber layer. The aramid paste interface layer is mainly composed of aramid paste, with no continuous carbon fibers, and the volume fraction of carbon fibers is... =0. The aramid paste interface layer can form a separation effect, keeping the upper and lower aramid paste-carbon fiber interfacial layers spaced apart.

[0022] (2) Aramid paste-carbon fiber interface layer The aramid-carbon fiber interface layer is formed by the infiltration of aramid pulp into the carbon fiber layer and its interaction with the carbon fiber layer. It is mainly located on the surface and subsurface of the carbon fiber layer. After infiltration, the aramid pulp fills the voids in the carbon fiber layer, and the aramid fibers and carbon fibers are linked to form a bridging structure. Therefore, the aramid pulp content is relatively high, while the carbon fiber content is relatively low in the aramid-carbon fiber interface layer. Experiments have shown that the volume fraction of carbon fiber in the aramid-carbon fiber interface layer... <65%.

[0023] In the aramid slurry, aramid fibers and carbon fibers are linked to form a bridging structure, which strengthens the Z-direction (thickness direction) and significantly improves the bonding strength between carbon fiber layers. The linking method and principle are prior art in this field and will not be described in detail here.

[0024] (3) Carbon fiber interface layer Because aramid paste acts as a binder between carbon fiber layers, the amount of aramid paste applied to the surface of the carbon fiber layers is generally limited. Therefore, the penetration of aramid paste into the carbon fiber layers is limited, with only a small amount or no aramid paste penetrating into the deeper layers. Consequently, the content of aramid paste is low, while the content of carbon fiber is high within the carbon fiber interface layer. Experiments have shown that the volume fraction of carbon fiber in the carbon fiber interface layer... ≥65%.

[0025] Step S2: Establish stiffness micromechanical models for three types of micro-interface layers respectively. Based on the hierarchical interface principle, the volume fraction of each interface layer in the composite material is used as the weight, and the weighted sum of the moduli of all interface layers is used as the modulus of the composite material to construct the stiffness micromechanical model of the composite material.

[0026] The strength of the aramid paste interface layer mainly comes from the aramid fibers in the aramid paste and is closely related to the volume fraction of aramid fibers in the aramid paste. Therefore, the micromechanical model of the stiffness of the aramid paste interface layer can be expressed as follows: ; ; In the formula, , These represent the Young's modulus and shear modulus of the aramid paste interface layer, respectively. , These represent the longitudinal and transverse moduli of the aramid paste interface layer, respectively; where: ; ; In the formula, This indicates the volume fraction of aramid fibers in the aramid pulp; Indicates the modulus of the resin carrier in the aramid paste; , These represent the length and diameter of the aramid fiber, respectively. , These represent the correction factors for the longitudinal modulus (fiber direction) and transverse modulus (perpendicular to the fiber direction) of aramid fibers, respectively, where: ; ; In the formula, This indicates the modulus of aramid fibers in aramid paste.

[0027] The strength of the aramid paste-carbon fiber interface layer originates from the bridging structure formed by the bonding of aramid and carbon fibers, and is related to the content of aramid and carbon fibers in the aramid paste-carbon fiber interface layer. Therefore, the micromechanical model of the stiffness of the aramid paste-carbon fiber interface layer can be expressed as follows: ; ; ; In the formula, , , These represent the longitudinal modulus, transverse modulus, and shear modulus of the aramid paste-carbon fiber interface layer, respectively. , These represent the volume fractions of carbon fiber and aramid paste in the aramid paste-carbon fiber interface layer, respectively. , , These represent the longitudinal modulus, transverse modulus, and shear modulus of carbon fiber, respectively. , These represent the Young's modulus and shear modulus of the aramid paste, respectively.

[0028] The strength of the carbon fiber interface layer mainly comes from carbon fibers, as well as a small amount of aramid fibers and the bridging structure of carbon fibers. It is related to the content of aramid fibers and carbon fibers in the aramid paste-carbon fiber interface layer. Therefore, the micromechanical model of the stiffness of the carbon fiber interface layer is similar to that of the aramid paste-carbon fiber interface layer, and is expressed as follows: ; ; ; In the formula, , , These represent the longitudinal modulus, transverse modulus, and shear modulus of the carbon fiber interface layer, respectively. , These represent the volume fractions of carbon fiber and aramid paste in the carbon fiber interface layer, respectively.

[0029] It should be noted that before constructing the micromechanical model of the stiffness of composite materials, the composite materials need to be treated as equivalent homogenization, that is, the fibers and resins in each interface layer of the composite material are regarded as uniformly distributed.

[0030] Therefore, the micromechanical model of the stiffness of composite materials can be expressed as: ; ; ; ; ; ; In the formula, This represents the equivalent stiffness micromechanical properties of composite materials. , , , The equivalent stiffness micromechanical properties of the carbon fiber interface layer, the aramid paste-carbon fiber interactive interface layer, the aramid paste interface layer, and the resin matrix are respectively represented. , , and These represent the volume fractions of the carbon fiber interface layer, the aramid paste-carbon fiber interfacial layer, the aramid paste interface layer, and the resin matrix, respectively. express The total thickness of each carbon fiber interface layer; express The total thickness of the aramid paste-carbon fiber interface layer. ; express The total thickness of the aramid paste interface layer. ; This indicates the thickness of a single resin matrix layer, where there are two resin matrix layers. This represents the total thickness of the composite material; the equivalent stiffness micromechanical properties include Young's modulus and shear modulus.

[0031] This application constructs a micromechanical model of the composite material based on the micromechanical models of each interface layer and using the volume fraction of each interface layer in the composite material as a medium. This model can be effectively used to predict the micromechanical properties of the composite material. Furthermore, the volume fraction of each interface layer is converted into its relative thickness in the composite material, simplifying the computation. The thickness of each interface layer is determined as follows: the longitudinal section of the composite material is observed using microscopic techniques (such as SEM and TEM), and the thickness of each interface layer in the longitudinal section image is determined by combining image analysis techniques.

[0032] Step S3: Analyze the deformation of the three micro-interface layers under the micro-buckling mode and fiber breakage failure mode, and construct a strength micromechanical model of the composite material.

[0033] like Figure 3 As shown, under the micro-buckling mode, the deformation of each interface layer in the composite material is as follows: Carbon fiber layer: Initial fiber misalignment (1°~4°) leads to stress concentration, reduces the axial load-bearing capacity of the fiber, and the force is mainly axial compression, accompanied by fiber rotation and kinking caused by matrix shear deformation; Aramid pulp-carbon fiber interface: It is susceptible to matrix shear failure, and fiber kinking is more significant. The bridging effect of aramid fibers inhibits delamination, but the fibers themselves undergo micro-buckling with matrix deformation. Aramid slurry interface layer: Bears shear stress, transforming delamination into a matrix shear mode through fiber bridging. Shear deformation is dominant, and modulus improvement depends on the aramid volume fraction.

[0034] Under the micro-buckling mode, the strength micromechanical model of the composite material is expressed as: ; ; In the formula, This indicates the failure strength of the composite material under axial compressive load; This represents the modified resin matrix shear strength, taking into account the influence of transverse compressive strength on matrix shear failure. Indicates the total fiber misalignment angle; Indicates the shear strength of the resin matrix; This indicates the compressive strength of the composite material perpendicular to the fiber direction; This indicates the orientation angle of the kink band.

[0035] Assuming the direction of the kink band is perpendicular to the fiber axis, then Taking a value of 0 simplifies the above strength micromechanical model, which can be expressed as: ; ; In the formula, This indicates the initial fiber misalignment angle, caused by manufacturing defects, and is typically 1° to 4°. This indicates the additional fiber rotation angle, which refers to the additional rotation angle of the fibers within the kink under axial compressive load.

[0036] Under the fiber breakage mode, the deformation of each interface layer of the composite material is as follows: Carbon fiber layer: Deformation is mainly characterized by uniform compression of the fibers along the axial direction until fracture; Aramid paste-carbon fiber interface: The mixed phase of aramid fiber and resin carrier shares part of the stress and delays overall failure; Aramid slurry interface layer: It only transmits compressive stress and does not break fibers itself. It coordinates the deformation of adjacent layers through elastic deformation.

[0037] Under the fiber breakage failure mode, the strength micromechanical model of the composite material is expressed as: ; In the formula, Indicates the ultimate strength of a composite material; Indicates fiber angle Strength of the composite material; fiber angle This indicates the angle between the fiber (including carbon fiber and aramid fiber) in the composite material and the loading direction or the principal direction of the material. Based on the maximum stress criterion, the above equation can also be expressed as: ; In the formula, This indicates the ultimate strength of the composite material in the longitudinal direction (fiber axial direction); This indicates the ultimate strength of the composite material in the transverse direction (perpendicular to the fiber axis). and It can be calculated using the following formula: ; ; In the formula, and These represent the strength parameters of the resin matrix and the aramid paste, respectively. Indicates the volume fraction of aramid paste in the composite material; Indicates the modulus of the resin matrix; Indicates the transverse modulus of aramid paste; ; ; In the formula, This represents the compressive strength of the i-th interface layer in the longitudinal direction (fiber axis direction); This represents the compressive strength of the i-th interface layer in the transverse direction (perpendicular to the fiber axis). Indicates the compressive strength of carbon fiber; Let represent the compressive strength of the aramid paste in the i-th interface layer.

[0038] Step S4: The stiffness micromechanical model and the strength micromechanical model of the composite material are used to predict the modulus and compressive strength of the composite material, and the results are compared with the experimentally measured modulus and compressive strength to verify the effectiveness of the stiffness micromechanical model and the strength micromechanical model.

[0039] Example 1 This embodiment is used to verify the effectiveness of the micromechanical modeling method provided by the present invention. In this embodiment, the performance parameters of the carbon fiber, resin matrix and aramid paste are shown in Table 1.

[0040] Table 1. Performance parameters of carbon fiber, resin matrix, and aramid paste In this embodiment, the composite material comprises 12 layers, wherein the top and bottom layers are resin matrices, and 10 carbon fiber layers are sandwiched in the middle. Adjacent carbon fiber layers are bonded together with 0gsm-8gsm aramid paste, which serves to reinforce the carbon fibers.

[0041] A comparative experiment was set up to compare the performance (lateral modulus and longitudinal modulus) predicted by the modeling method of this invention with the performance measured experimentally. The results are as follows: Figures 4-5 As shown: Regarding the longitudinal modulus, it can be seen that the longitudinal modulus of the composite material is significantly improved, and the longitudinal modulus increases with the increase of aramid pulp content. However, when the aramid pulp content increases to a certain level, it leads to a significant increase in the thickness of the aramid pulp layer, which in turn leads to a decrease in the longitudinal modulus of the composite material. Therefore, the micromechanical modeling method provided by this invention is mainly applicable to cases where the aramid fiber area density in the aramid pulp is less than 8 gsm. Through comparison, it was found that the E without aramid pulp fibers... 11 The results were predicted to be within the experimental error range. Meanwhile, the E... (likely referring to a specific value or parameter) of aramid pulp fibers with a content of 10%-50% vol... 11 The predicted value is higher than the experimental minimum (2 gsm) and lower than the experimental maximum (6 gsm).

[0042] Regarding the transverse modulus, it can be seen that the transverse modulus of the composite material has also been significantly improved, and the transverse modulus increases with the increase of aramid pulp content. The predicted maximum transverse modulus is 11.92 GPa, which is close to the experimental value of 12.04 GPa, and the predicted minimum transverse modulus is 9.72 GPa, which is close to the experimental value of 9.13 GPa, indicating that the prediction error is small.

[0043] Based on the above comparison results, it can be seen that the predicted composite material properties by the prediction method provided by the present invention are close to the experimental data, which proves the effectiveness of the prediction method of the present invention.

[0044] The aramid pulp fibers of the unidirectional carbon fiber reinforced polymer range from 0 gsm to 8 gsm, serving as reference data for micromechanical strength model validation. Direct axial compression tests were conducted according to ASTM D6641. Figure 6 The compressive strength of 0 gsm to 8 gsm aramid pulp fibers was compared between calculated results (micro-buckling model) and experimental results from direct axial compression. It can be seen that unidirectional carbon fiber reinforced polymers with higher aramid pulp fiber content exhibit higher compressive strength under direct axial compression, exceeding that of unidirectional carbon fiber reinforced polymers without aramid pulp fibers. The micromechanical strength model also reproduces the experimental results demonstrating the positive impact of aramid pulp fiber content on compressive strength. Comparative results show that the lowest strength (197.35 MPa) and highest strength (289.27 MPa) are close to the experimental values ​​of 198.63 MPa and 272.85 MPa, respectively. Estimates of the compressive strength range between the minimum and maximum experimental values ​​(e.g., 2 gsm, 4 gsm, 6 gsm) are reasonable.

[0045] The micro-buckling model and fiber breakage failure model were compared with the three-point bending experiment, such as... Figure 7 As shown. The maximum compressive strength and range of aramid pulp fibers were obtained through a three-point bending test according to ASTM standard 7462. Table 2 lists detailed results from different models and tests: Table 2 Comparison of compressive strength of unidirectional carbon fiber reinforced polymers with different aramid pulp fiber contents like Figure 7 As shown, the compressive strength of the micro-buckling model correlates well with the compressive strength of the three-point bending test. This is due to the different failure mechanisms of unidirectional carbon fiber reinforced polymer compression failure tested by direct axial compression and three-point bending tests. Figure 7It can be seen that the compression failure in the direct axial compression test is mainly caused by the micro-buckling mode, which may increase fiber strength failure. This micro-buckling is caused by matrix shear cracking and fiber kinking, and can be established by the micro-buckling model. Due to the presence of fiber kinking, the carbon fiber cannot operate at its maximum axial load capacity. Therefore, the lowest compressive strength can also be obtained through the direct axial compression test and the micro-buckling model. However, for the three-point bending test, the unidirectional carbon fiber reinforced polymer with and without aramid pulp fibers has a moderate compressive strength, which is higher than that of the direct axial compression test. In the three-point bending test, the unidirectional carbon fiber reinforced polymer is mainly affected by the combination of micro-buckling and more fiber strength failure. Compared with the direct axial compression test and micro-buckling, more fiber strength failure helps to greatly improve the compressive strength. According to the fiber breakage failure model, fiber strength failure will occur when the fiber stress reaches a high compressive strength. Under the three-point bending test load, some carbon fibers can exert their greater axial load capacity. A key difference to note is that the compressive strength of the fiber breakage failure model is much higher than that of the two tests involving micro-buckling. The reason for this is that the micromechanical modeling method only considers the compressive strength failure of pure fibers, cannot reproduce the micro-buckling failure mechanism, and requires very high stress (e.g., >2000 MPa) to trigger the compressive strength failure of each fiber. This means that under these conditions, all carbon fibers exert their maximum axial load-bearing capacity. Therefore, fiber breakage failure models are mainly used to predict the high compressive strength of certain composite material systems where fiber strength failure is dominant. Therefore, the micromechanical model of this invention is expected to develop into a superior modeling method for composite materials at the microscale, used to evaluate the compressive strength of unidirectional carbon fiber reinforced polymers toughened with aramid pulp fibers with different failure mechanisms. In this regard, it is crucial to confirm the variation of key input parameters (e.g., initial and additional fiber dislocation angles, volume fraction variations, gradient thickness, etc.) with aramid pulp fiber content.

[0046] The embodiments of the present invention have been described above with reference to the accompanying drawings. However, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of the present invention without departing from the spirit and scope of the claims, and all of these forms are within the protection scope of the present invention.

Claims

1. A method for predicting the performance of aramid paste-reinforced composite materials applicable to aircraft structures, characterized in that, The composite material comprises a resin matrix at the top and bottom layers and multiple carbon fiber layers between the two resin matrix layers. Aramid paste is used as a binder between adjacent carbon fiber layers. The performance prediction method includes the following steps: Step S1: Based on the different penetration depths of aramid paste in the carbon fiber layer, the macroscopic connection structure between aramid paste and carbon fiber layer is divided into three microscopic interface layers: aramid paste interface layer, aramid paste-carbon fiber interactive interface layer, and carbon fiber interface layer. Step S2: Establish stiffness micromechanical models for three types of micro-interface layers respectively. Based on the hierarchical interface principle, the volume fraction of each interface layer in the composite material is used as the weight, and the weighted sum of the moduli of all interface layers is used as the modulus of the composite material to construct the stiffness micromechanical model of the composite material. Step S3: Analyze the deformation of the three micro-interface layers under the micro-buckling mode and fiber breakage failure mode, and construct a strength micromechanical model of the composite material. Step S4: The stiffness micromechanical model and the strength micromechanical model of the composite material are used to predict the modulus and compressive strength of the composite material, and the results are compared with the experimentally measured modulus and compressive strength to verify the effectiveness of the stiffness micromechanical model and the strength micromechanical model.

2. The method for predicting the performance of aramid paste-reinforced composite materials applicable to aircraft structures according to claim 1, characterized in that, Aramid slurry comprises a resin carrier and aramid fibers randomly distributed in two dimensions on the resin carrier.

3. The method for predicting the performance of aramid paste-reinforced composite materials applicable to aircraft structures according to claim 2, characterized in that, The aramid paste interface layer is located on the surface of the carbon fiber layer, forming an ultrathin interface of 5-15 μm formed by the aramid paste that has not penetrated into the carbon fiber layer. The volume fraction of carbon fiber in the aramid paste interface layer is 0. The aramid paste-carbon fiber interaction interface layer is formed by the interaction between the aramid paste and the carbon fiber layer after the aramid paste has penetrated into the carbon fiber layer. It is located on the surface and subsurface of the carbon fiber layer. The volume fraction of carbon fiber in the aramid paste-carbon fiber interaction interface layer is less than 65%. The carbon fiber interface layer is located deep within the carbon fiber layer. The volume fraction of carbon fiber in the carbon fiber interface layer is not less than 65%.

4. The method for predicting the performance of aramid paste-reinforced composite materials applicable to aircraft structures according to claim 3, characterized in that, The micromechanical model of the stiffness of the aramid paste interface layer is expressed as follows: ； ； In the formula, , These represent the Young's modulus and shear modulus of the aramid paste interface layer, respectively. , These represent the longitudinal and transverse moduli of the aramid paste interface layer, respectively; where: ； ； In the formula, This indicates the volume fraction of aramid fibers in the aramid pulp. Indicates the modulus of the resin carrier in the aramid paste; , These represent the length and diameter of the aramid fiber, respectively. , These represent the correction factors for the longitudinal modulus (fiber direction) and transverse modulus (perpendicular to the fiber direction) of aramid fibers, respectively, where: ； ； In the formula, This indicates the modulus of aramid fibers in aramid paste.

5. The method for predicting the performance of aramid paste-reinforced composite materials applicable to aircraft structures according to claim 4, characterized in that, The stiffness micromechanical model of the aramid paste-carbon fiber interface layer is expressed as follows: ； ； ； In the formula, , , These represent the longitudinal modulus, transverse modulus, and shear modulus of the aramid paste-carbon fiber interface layer, respectively. , These represent the volume fractions of carbon fiber and aramid paste in the aramid paste-carbon fiber interface layer, respectively. , , These represent the longitudinal modulus, transverse modulus, and shear modulus of carbon fiber, respectively. , These represent the Young's modulus and shear modulus of the aramid paste, respectively.

6. The method for predicting the performance of aramid paste-reinforced composite materials applicable to aircraft structures according to claim 5, characterized in that, The micromechanical model of the stiffness of the carbon fiber interface layer is expressed as follows: ； ； ； In the formula, , , These represent the longitudinal modulus, transverse modulus, and shear modulus of the carbon fiber interface layer, respectively. , These represent the volume fractions of carbon fiber and aramid paste in the carbon fiber interface layer, respectively.

7. The method for predicting the performance of aramid paste-reinforced composite materials applicable to aircraft structures according to claim 6, characterized in that, The micromechanical model of composite materials is represented as follows: ； ； ； ； ； ； In the formula, This represents the equivalent stiffness micromechanical properties of composite materials. , , , The equivalent stiffness micromechanical properties of the carbon fiber interface layer, the aramid paste-carbon fiber interactive interface layer, the aramid paste interface layer, and the resin matrix are respectively represented. , , and These represent the volume fractions of the carbon fiber interface layer, the aramid paste-carbon fiber interfacial layer, the aramid paste interface layer, and the resin matrix, respectively. express The total thickness of each carbon fiber interface layer; express The total thickness of the aramid paste-carbon fiber interface layer. ; express The total thickness of the aramid paste interface layer. ; This indicates the thickness of a single resin matrix layer, where there are two resin matrix layers. This represents the total thickness of the composite material; the equivalent stiffness micromechanical properties include Young's modulus and shear modulus.

8. The method for predicting the performance of aramid paste-reinforced composite materials applicable to aircraft structures according to claim 7, characterized in that, The thickness of each interface layer is determined as follows: the longitudinal section of the composite material is observed using microscopy, and the thickness of each interface layer in the longitudinal section is determined by combining image analysis technology.

9. The method for predicting the performance of aramid paste-reinforced composite materials applicable to aircraft structures according to claim 7, characterized in that, Under the micro-buckling mode, the strength micromechanical model of the composite material is expressed as: ； ； In the formula, This indicates the failure strength of the composite material under axial compressive load; This represents the modified resin matrix shear strength, taking into account the influence of transverse compressive strength on matrix shear failure. Indicates the total fiber misalignment angle; Indicates the shear strength of the resin matrix; This indicates the compressive strength of the composite material perpendicular to the fiber direction; This indicates the orientation angle of the kink band. Assuming the direction of the kink band is perpendicular to the fiber axis, then Taking a value of 0 simplifies the above strength micromechanical model, which can be expressed as: ； ； In the formula, This indicates the initial fiber misalignment angle, caused by manufacturing defects, and is typically 1° to 4°. This indicates the additional fiber rotation angle, which refers to the additional rotation angle of the fibers within the kink under axial compressive load.

10. The method for predicting the performance of aramid paste-reinforced composite materials applicable to aircraft structures according to claim 9, characterized in that, Under the fiber breakage failure mode, the strength micromechanical model of the composite material is expressed as: ； In the formula, Indicates the ultimate strength of a composite material; Indicates fiber angle Strength of the composite material; fiber angle Indicates the angle between the fiber and the loading direction or the principal direction of the material in a composite material; Based on the maximum stress criterion, the above equation can also be expressed as: ； In the formula, This indicates the ultimate strength of the composite material in the longitudinal direction; It represents the ultimate strength of a composite material in the transverse direction. and It can be calculated using the following formula: ； ； In the formula, and These represent the strength parameters of the resin matrix and the aramid paste, respectively. Indicates the volume fraction of aramid paste in the composite material; Indicates the modulus of the resin matrix; Indicates the transverse modulus of aramid paste; ； ； In the formula, This represents the compressive strength of the i-th interface layer in the longitudinal direction; This represents the compressive strength of the i-th interface layer in the lateral direction; Indicates the compressive strength of carbon fiber; Let represent the compressive strength of the aramid paste in the i-th interface layer.

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