Co-cured fiber reinforced composite material for wing girder joint

Abstract

The invention belongs to the field of aviation composite materials, and provides a co-curing fiber reinforced composite material for a wing girder joint. According to the invention, the design of a 1, 4-butanediol diglycidyl ether synergistic toughening flame-retardant resin system, the construction of a conductive thin felt layer double-modified by polydopamine coating and a silane coupling agent, and the design of a sandwich structure assembled by gradient regions are adopted; high Tg and low porosity of the resin matrix under a low-viscosity infiltration condition, low surface sheet resistance and strong interface combination of the modified conductive thin felt layer, and high lap joint bearing capacity and delamination damage tolerance of the gradient sandwich structure are realized; the problems that sufficient infiltration and conductive network continuity are difficult to consider in the co-curing forming process, interlayer weak plane and interface embrittlement risks are introduced by conductive flame-retardant modification, and residual stress concentration and insufficient dimensional stability are caused by high-temperature curing are solved, and the method has wide aerospace structure application value.

Description

Technical Field

[0001] This invention relates to the field of aerospace composite materials, and more specifically to a co-cured fiber-reinforced composite material for wing main spars joints. Background Technology

[0002] As a critical load-bearing component in the wing-fuselage connection of large civil aircraft, the wing main spars bear the long-term effects of complex multi-directional loads and fatigue cyclic stresses, placing extremely high demands on the composite material's lap load-bearing capacity, delamination damage tolerance, electrical conductivity protection performance, and thermal dimensional stability. In lightning protection applications, the joint area needs to construct a continuous and complete conductive network to effectively conduct and dissipate current, avoiding localized arc erosion and delamination damage. Simultaneously, the lap structure in the joint area needs to withstand shear stresses and peel loads reaching tens of megapascals, requiring the material to possess excellent interlaminar shear strength, Mode I and Mode II interlaminar fracture toughness, and long-term load-bearing stability under high temperature and humidity environments. Meeting and developing these comprehensive performance requirements is of great significance for improving the structural safety margin of aircraft, extending service life, and reducing structural weight, and represents a key technological breakthrough in promoting the application of aerospace composite materials from secondary load-bearing structures to primary load-bearing structures.

[0003] Regarding the current state of development of fiber-reinforced composite materials for wing main spars, research currently faces three main shortcomings. First, traditional epoxy resins, while pursuing low-viscosity wetting and degassing, exhibit low glass transition temperatures and insufficient crosslinking density after curing, making it difficult to meet high-temperature service requirements and prone to resin loss and porosity defects. This stems from the formulation contradiction between low-molecular-weight epoxy and fast-curing agents. Second, while conductive interlayers such as carbon nanotube felts can construct conductive networks, their surface inertness leads to weak interfacial bonding with the resin, easily causing interfacial debonding and microcracks during curing, forming weak interlayer surfaces and significantly reducing delamination tolerance. This is due to the lack of active functional groups on the surface of carbonaceous materials. Third, uniformly laid conductive interlayers in the joint overlap area can lead to stress concentration. While gradient structures can alleviate this, assembly precision is difficult to control. The introduction of phosphorus-containing flame-retardant components may weaken the interface and exacerbate residual stress and dimensional instability caused by curing shrinkage and thermal stress coupling. For example, Chinese patent CN116355357B discloses a long-short carbon nanotube reinforced and toughened fiber composite material and its preparation method, but it has problems of uneven dispersion and insufficient interfacial bonding; Chinese patent CN113896746B discloses a method for preparing flame-retardant epoxy resin using phosphorus and nitrogen long-chain flame retardant compounds, but it has the disadvantages of decreased toughness and narrowed curing process window. Summary of the Invention

[0004] The purpose of this invention is to provide a co-cured fiber-reinforced composite material for wing main spars joints, which solves the problems of difficulty in achieving both low viscosity wetting and continuity of conductive network in the current co-curing process, interlayer weakness and interface embrittlement risks introduced by conductive flame-retardant modification, and residual stress concentration and insufficient dimensional stability caused by high-temperature curing.

[0005] This invention employs a synergistic toughening and flame-retardant approach using phosphorus-containing epoxy adducts and 1,4-butanediol diglycidyl ether. The phosphorus-containing epoxy adduct provides flame retardant properties and crosslinking density, while the 1,4-butanediol diglycidyl ether contributes flexible segments and toughness. The synergistic effect of the two achieves a comprehensive improvement in the overall performance of the resin matrix, which possesses both a high glass transition temperature and excellent toughness under low viscosity conditions. Simultaneously, a strong interfacial chemical bond is constructed between the conductive thin felt layer and the resin matrix through dual surface modification using polydopamine coating and silane coupling agent, avoiding the insufficient interfacial bonding of single modification methods. Furthermore, a gradient-layered sandwich structure design achieves a gradual transition and dispersion of stress.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A co-cured fiber-reinforced composite material for wing main spars joints, comprising: A continuous fiber reinforcement comprising carbon fiber layups; A thermosetting resin matrix formed by curing a co-curing epoxy resin composition; Modified conductive felt layer located in the joint area of ​​the wing main spars; The co-cured epoxy resin composition comprises an epoxy resin component, a curing agent, and an accelerator. Based on 100 parts by weight of the total epoxy resin component, the epoxy resin component comprises 5-30 parts by weight of a phosphorus-containing epoxy adduct, 2-15 parts by weight of 1,4-butanediol diglycidyl ether, and bisphenol A diglycidyl ether, with the weight of bisphenol A diglycidyl ether being rounded up to the nearest 100 parts by weight. Based on 100 parts by weight of the total epoxy resin component, the curing agent is 4,4′-diaminodiphenyl sulfone, used in an amount of 15-40 parts by weight. Based on 100 parts by weight of the total epoxy resin component, the accelerator is 2-methylimidazole, used in an amount of 0.05-1.00 parts by weight. The modified conductive felt layer has a surface density of 5-30 g / m² and a thickness of 10-80 µm, and is disposed between two adjacent carbon fiber lay-ups. The wing main sparsity joint area includes a gradient zone along the overlap direction. The gradient zone has a length of 10-80mm and includes a first sub-zone and a second sub-zone. The first sub-zone and the second sub-zone are arranged sequentially along the overlap direction. The modified conductive felt layer in the first sub-zone has 1 layer, and the modified conductive felt layer in the second sub-zone has 2-3 layers.

[0007] Furthermore, the modified conductive thin felt layer is prepared through the following steps: A1) Solution preparation: Using deionized water as solvent, add dopamine hydrochloride at a concentration of 0.5-5.0 g / L and tris(hydroxymethyl)aminomethane at a concentration of 1.0-20.0 g / L to prepare a solution. Adjust the pH of the solution to 8.5-9.0 with a 0.1-1.0 mol / L hydrochloric acid solution or sodium hydroxide solution. A2) Coating reaction: The conductive thin felt substrate is immersed in the solution obtained in step A1), wherein the bath ratio of the conductive thin felt substrate to the solution is 1g of conductive thin felt substrate to 50-500mL of solution, and the reaction is carried out at a temperature of 15-35℃ and in an air atmosphere for 0.5-6.0h to obtain the coated conductive thin felt, wherein the conductive thin felt substrate is selected from multi-walled carbon nanotube thin felt or carbon fiber thin felt. A3) Washing and drying: Wash with deionized water 1-5 times, then wash with anhydrous ethanol 1-5 times. In step A3), the bath ratio for each wash is 1g of conductive felt to 20-200mL of washing solution. Then dry at a temperature of 40-80℃ for 2-12h. A4) Coupling treatment: The dried product obtained in step A3) is immersed in a coupling solution. The bath ratio of the dried product to the coupling solution is 1g of conductive felt to 30-300mL of coupling solution. The reaction is carried out at a temperature of 20-60℃ for 0.5-4.0h to obtain the coupled conductive felt. The coupling agent in the coupling solution is selected from one or two of 3-aminopropyltriethoxysilane and 3-glycidoxypropyltrimethoxysilane. The total mass fraction of the coupling agent in the coupling solution is 0.1-5.0wt%, and the mass fraction is based on the total mass of the coupling solution. The solvent of the coupling solution is a mixture of ethanol and deionized water, and the mass ratio of ethanol to deionized water is 90:10 to 50:50. A5) Drying and Quality Control: The modified conductive thin felt layer is obtained by drying at a temperature of 40-80℃ for 2-12 hours. The coating weight gain of the modified conductive thin felt layer is 0.5-5.0 wt%, based on the mass of the conductive thin felt substrate. The surface sheet resistance of the modified conductive thin felt layer is 1-10. 4 Ω / sq.

[0008] Furthermore, the gradient-assembled conductive sandwich sheet used in the gradient region is prepared through the following steps: B1) Sheet preparation: Provide a modified conductive thin felt layer and cut it into strip sheets, the strip sheets having a dimension of 10-80mm along the overlapping direction; B2) Gradient assembly: The strip sheet is stacked in a stepped manner along the overlapping direction, so that the first sub-region has 1 layer and the second sub-region has 2-3 layers, and the total length of the gradient region is 10-80mm. B3) Shaping: Hot pressing for 1-30 minutes at a temperature of 60-120℃ and a pressure of 0.05-0.50MPa to obtain a gradient-assembled conductive sandwich sheet; B4) Quality control: The positional tolerance of the gradient region of the gradient-assembled conductive interlayer sheet is ±2mm.

[0009] Furthermore, the phosphorus-containing epoxy adduct is prepared by the following steps: C1) Raw material mixing: 100 parts by weight of bisphenol A diglycidyl ether are mixed with 8-26 parts by weight of 9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide; C2) Addition reaction: under an industrial nitrogen atmosphere, the reaction is carried out at a temperature of 110-130℃ for 1-6 hours; C3) Endpoint criterion: The reaction is stopped when the phosphorus content of the resulting adduct is 1.0-3.0 wt% and the epoxy equivalent is 250-500 g / equivalent. C4) Cooling: Cool to a temperature of 60-90℃ to obtain the phosphorus-containing epoxy adduct.

[0010] Furthermore, the co-cured epoxy resin composition is prepared by the following steps: D1) Premixing: Bisphenol A diglycidyl ether, phosphorus-containing epoxy adduct and 1,4-butanediol diglycidyl ether are mixed and kept at a temperature of 60-90℃ for 0.5-2.0h. D2) Add curing agent and accelerator: Add 4,4′-diaminodiphenyl sulfone and 2-methylimidazole to the mixture obtained in step D1) and maintain it at a temperature of 80-120℃ for 0.5-3.0h; D3) Degassing: Degas under an absolute pressure of 0.1-10 kPa for 0.5-2.0 h to obtain the co-cured epoxy resin composition; D4) Viscosity control: The apparent viscosity of the co-cured epoxy resin composition, measured by a rotational viscometer or rheometer, is 5-30 Pa·s at a temperature of 80°C and a shear rate of 1-100 / s.

[0011] Furthermore, the conductive thin felt substrate is selected from one of multi-walled carbon nanotube thin felt and composite thin felt. The composite thin felt is a carbon fiber thin felt formed by loading multi-walled carbon nanotubes on a carbon fiber thin felt. The mass fraction of the multi-walled carbon nanotubes in the composite thin felt is 10-60 wt%, and the mass fraction is based on the total mass of the composite thin felt. The areal density of the carbon fiber layup is 120-300 g / m².

[0012] As a concept of this invention, an epoxy resin component designed with the synergistic formulation of a phosphorus-containing epoxy adduct, 1,4-butanediol diglycidyl ether, and bisphenol A diglycidyl ether is mainly used to enhance the comprehensive performance of co-cured fiber-reinforced composite materials. The phosphorus-containing epoxy adduct is prepared by the addition reaction of bisphenol A diglycidyl ether with 9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide, introducing phosphaphenanthrene structural units into the resin matrix. This structure can decompose at high temperatures to form phosphoric acid substances, catalyzing carbonization and forming a dense carbon layer on the material surface, blocking heat and oxygen transfer, and achieving a flame-retardant effect. At the same time, the phosphorus-containing epoxy adduct retains the reactivity of the epoxy groups, participating in the construction of the curing crosslinking network, and improving the crosslinking density and glass transition temperature. 1,4-Butanediol diglycidyl ether, as a long-chain aliphatic epoxy resin, introduces freedom of movement into the cured network through its flexible methylene segments, reducing the rigidity of the crosslinking network and improving the toughness and impact resistance of the resin matrix. Its low viscosity also facilitates the wetting and vacuum degassing of carbon fiber prepregs, reducing porosity. Bisphenol A diglycidyl ether, as the main epoxy resin, provides the skeletal structure of the cured network, balancing mechanical properties and processability. The three epoxy components work synergistically, maintaining low-viscosity wetting characteristics while also achieving a high glass transition temperature, excellent toughness, and flame retardancy, avoiding the contradictions between high crosslinking density and low viscosity, and between rigidity and toughness inherent in single epoxy systems.

[0013] This invention also discloses a method for co-curing fiber-reinforced composite materials for wing main spars joints, comprising the following steps: S1) Provides a modified conductive thin felt layer; S2) Provides a gradient-assembled conductive sandwich sheet; S3) provides phosphorus-containing epoxy adducts; S4) Bisphenol A diglycidyl ether, the phosphorus-containing epoxy adduct, 1,4-butanediol diglycidyl ether, 4,4′-diaminodiphenyl sulfone and 2-methylimidazole are mixed and degassed to obtain a co-cured epoxy resin composition. S5) In the wing main spars joint area, carbon fiber layup, the co-cured epoxy resin composition and the gradient assembly conductive sandwich sheet are stacked to form a preform, such that the gradient assembly conductive sandwich sheet is located between two adjacent carbon fiber layup layers. S6) The preform is vacuum-sealed, and then a curing process is performed under a pressure of 0.30-0.70 MPa: the temperature is increased from 20-30℃ to 90-130℃ at a heating rate of 1-5℃ / min and held for 1.0-3.0h, and then increased to 160-190℃ at a heating rate of 1-5℃ / min and held for 2.0-4.0h to obtain a co-cured fiber-reinforced composite material for the wing main spars joint.

[0014] Furthermore, the absolute pressure of the vacuum sealing described in step S6) is 0.1-10 kPa.

[0015] Furthermore, the apparent viscosity of the co-cured epoxy resin composition obtained in step S4) is 5-30 Pa·s when measured by a rotational viscometer or rheometer at a temperature of 80°C and a shear rate of 1-100 / s.

[0016] Furthermore, the modified conductive thin felt layer provided in step S1) has a coating weight gain of 0.5-5.0 wt% and a surface sheet resistance of 1-10. 4 Ω / sq.

[0017] Furthermore, in step A1), the pH value of the solution is adjusted to 8.5-9.0 by using a hydrochloric acid solution or sodium hydroxide solution with a concentration of 0.1-1.0 mol / L to adjust the pH value.

[0018] Furthermore, in step A2), the bath ratio of the conductive thin felt substrate to the solution is 1g of conductive thin felt substrate to 50-500mL of solution.

[0019] Furthermore, in step A3), the bath ratio for each wash is 1g of conductive felt corresponding to 20-200mL of washing solution.

[0020] Furthermore, in step A3), the washing method is immersion washing or ultrasonic-assisted washing.

[0021] Furthermore, in step A3), the drying method is vacuum drying or oven drying.

[0022] Furthermore, in step A3), drying to constant weight means that under the same drying conditions, the mass change does not exceed 0.5% between two consecutive weighings with an interval of 1 hour.

[0023] Furthermore, in step A4), the bath ratio of the conductive felt immersed in the coupling solution is 1g of conductive felt to 30-300mL of coupling solution.

[0024] Further, the coupling solution in step A4) is prepared by: first adding the coupling agent to ethanol and mixing it, then adding deionized water and mixing it, and then letting it stand at 20-30℃ for 0.5-2.0h to obtain the coupling solution.

[0025] Further, in step A5), the coating weight gain is (m1-m0) / m0×100%, where m0 is the mass of the conductive thin felt substrate after drying to constant weight, and m1 is the mass after coating and drying to constant weight.

[0026] Furthermore, the surface sheet resistance described in step A5) is determined using the four-probe method.

[0027] Furthermore, the gradient region position tolerance mentioned in step B4) refers to the deviation of the starting position of the gradient region from one end of the gradient assembly conductive interlayer sheet along the overlapping direction.

[0028] Furthermore, the reaction process in step C2) is carried out using magnetic stirring or mechanical stirring.

[0029] Furthermore, in step C2), the stirring speed is 100-500 rpm.

[0030] Furthermore, in step C2), the nitrogen atmosphere is established by purging the reaction vessel with nitrogen 2-5 times and continuously introducing nitrogen at a flow rate of 0.1-2.0 L / min.

[0031] Furthermore, in step D1), the mixing method is magnetic stirring or mechanical stirring, and the stirring speed is 100-500 rpm.

[0032] Furthermore, in step D2), the mixing method is mechanical stirring, with a stirring speed of 50-300 rpm.

[0033] Furthermore, the degassing temperature in step D3) is 60-90℃.

[0034] Furthermore, the test conditions for apparent viscosity in step D4) are as follows: under a shear rate of 1-100 s⁻¹, the viscosity is measured using a rotational viscometer or rheometer.

[0035] Further, the specific operation of mixing and degassing in step S4) is as follows: Bisphenol A diglycidyl ether, phosphorus-containing epoxy adduct and 1,4-butanediol diglycidyl ether are stirred and mixed at a temperature of 60-90℃ for 0.5-2.0h, then 4,4′-diaminodiphenyl sulfone and 2-methylimidazole are added and stirred at a temperature of 80-120℃ for 0.5-3.0h, and finally degassing is performed at an absolute pressure of 0.1-10kPa for 0.5-2.0h.

[0036] Furthermore, after the curing process in step S6) is completed, the temperature is cooled to 20-30°C at a cooling rate of 0.5-3.0°C / min while maintaining a pressure of 0.30-0.70 MPa.

[0037] As another aspect of this invention, a method for preparing a conductive thin felt layer with dual surface modification using polydopamine coating and silane coupling agent is employed. This method primarily enhances the interfacial bonding strength between the modified conductive thin felt layer and the resin matrix, as well as the continuity and stability of the conductive network. The polydopamine coating process utilizes the self-polymerization property of dopamine hydrochloride in a weakly alkaline aqueous solution to deposit a nanoscale polydopamine coating in situ on the surface of the conductive thin felt substrate. This coating is rich in active groups such as catechols and amino groups, significantly improving the chemical activity and wettability of the carbonaceous material surface, providing reaction sites for subsequent coupling treatment. In the silane coupling agent treatment process, the amino group of 3-aminopropyltriethoxysilane can undergo Schiff base reaction or Michael addition reaction with the quinone group in the polydopamine coating. After hydrolysis of the siloxane group, it cures and crosslinks with the epoxy resin, forming a chemically bonded interface. The epoxy group of 3-glycidoxypropyltrimethoxysilane can directly co-cur with the epoxy resin matrix, achieving strong interfacial bonding. The dual modification works synergistically. The polydopamine coating provides an active interface and uniform coating, while the silane coupling agent constructs a chemical bridge, avoiding insufficient interfacial bonding in a single coating or coupling method. At the same time, the nanoscale thickness and uniformity of the polydopamine coating ensure the low surface sheet resistance and continuity of the conductive network in the modified conductive felt layer. The coating weight gain is controlled within the range of 0.5-5.0 wt%, achieving surface modification without significantly increasing thickness and weight.

[0038] The mechanism and synergistic effect of phosphorus-containing epoxy adducts and 1,4-butanediol diglycidyl ether in the co-cured fiber-reinforced composite material of this invention are manifested in multiple synergies of rigidity-toughness balance and flame retardant toughening. Phosphorus-containing epoxy adducts focus on improving the flame retardant and high-temperature mechanical properties of the resin matrix. The introduction of phosphorus-phenanthroline structures increases the crosslinking density and glass transition temperature of the cured network, decomposing into char at high temperatures to provide flame retardant protection. However, its rigid structure may lead to decreased toughness and increased processing viscosity. 1,4-Butanediol diglycidyl ether focuses on improving the toughness and processing fluidity of the resin matrix. Long-chain aliphatic flexible segments reduce the rigidity of the cured network, improving elongation at break and impact resistance. Simultaneously, its low viscosity improves the wetting and degassing effect of carbon fibers. The two epoxy components work synergistically in the composite material system. The rigid cross-linked network of the phosphorus-containing epoxy adduct and the flexible segments of 1,4-butanediol diglycidyl ether interpenetrate at the microscale, forming a rigid-flexible cured network structure. This ensures both high glass transition temperature and flame retardant properties while avoiding brittleness caused by excessive cross-linking. At the same time, the viscosity of the two components is complementary during processing, which broadens the process window for low-temperature, low-viscosity wetting and high-temperature, high-cross-linking curing. Furthermore, the charring and flame retardancy of the phosphorus-containing epoxy adduct and the flexible energy absorption of 1,4-butanediol diglycidyl ether synergistically improve the material's damage tolerance and anti-delamination properties.

[0039] Beneficial technical effects Significantly improves the wetting quality and cured network performance of the co-curing molding process: Through the synergistic formulation of phosphorus-containing epoxy adduct and 1,4-butanediol diglycidyl ether, the carbon fiber prepreg is fully wetted and vacuum degassed while maintaining a low viscosity of 5-30 Pa·s at 80°C for the co-cured epoxy resin composition. The porosity is controlled at 0.8-1.5% (with some embodiments ≤1.0%). At the same time, the glass transition temperature of the cured resin matrix reaches 175-191°C, meeting the requirements for high-temperature service and avoiding the contradiction between low viscosity and high Tg in traditional single epoxy systems.

[0040] Constructing a high-performance conductive network and achieving strong interfacial bonding: Through dual surface modification of polydopamine coating and silane coupling agent, a nano-coating rich in active groups is formed on the surface of the modified conductive thin felt layer, establishing a chemical bonding interface with the epoxy resin matrix. The interlayer shear strength is increased by more than 30%, while the coating weight gain is controlled at 0.5-5.0 wt%, and the surface sheet resistance is maintained at 1-10. 4 Within the Ω / sq range, it achieves the dual performance of low sheet resistance conductive network and strong interface bonding, avoiding the risk of interlayer weak surfaces introduced by conductive modification.

[0041] Achieving stress optimization and load-bearing capacity enhancement of gradient structures: Through the design of a sandwich structure with gradient-divided layered assembly, the first sub-region has a 1-layer modified conductive felt layer that gradually transitions to the second sub-region with 2-3 layers. The gradient zone length is adjustable from 10-80mm, achieving gradual dispersion of stress at the lap joint ends. The single lap shear strength is increased by more than 25% compared to the uniform laying method, and the interlayer fracture toughness (GIC) of Mode I is increased by more than 40%. At the same time, the positional tolerance is controlled by ±2mm through the shaping process of the gradient-assembled conductive sandwich sheet, ensuring the accuracy and repeatability of the gradient structure.

[0042] Balancing flame retardancy and dimensional stability: The phosphorus-phenanthrene structure introduced by the phosphorus-containing epoxy adduct catalyzes char formation at high temperatures to form a dense char layer, achieving a limiting oxygen index (LOI) of over 32% and a vertical combustion rating of V-0. Simultaneously, the segmented curing process (holding at 90-130℃ for 1.0-3.0h followed by heating to 160-190℃ and holding for 2.0-4.0h) achieves a dynamic balance between curing reaction and stress release, reducing curing shrinkage to below 1.5% and residual stress by 30%, thereby improving dimensional stability and long-term service reliability.

[0043] Expanding the application scope of aerospace composite materials: The co-cured fiber-reinforced composite material of this invention simultaneously meets the comprehensive requirements of high lap load-bearing capacity, delamination damage tolerance, electrical conductivity, flame retardancy, safety, and heat resistance dimensional stability of wing main sparsity joints. It can be applied to key load-bearing parts such as wing-body connections, vertical tail connections, and landing gear joints of large civil aircraft, promoting the breakthrough of composite materials from secondary load-bearing structures to primary load-bearing structures, and has significant value in weight reduction, efficiency improvement, and enhanced safety and reliability. Attached Figure Description

[0044] Figure 1 The X-ray photoelectron spectroscopy (C1s) high-resolution spectra of Examples 1, 6, and 7 are shown.

[0045] Figure 2 The X-ray photoelectron spectroscopy N1s high-resolution spectra of Examples 1, 6, and 7 are shown.

[0046] Figure 3 The X-ray photoelectron spectroscopy (O1s) high-resolution spectra of Examples 1, 6, and 7 are shown.

[0047] Figure 4 The X-ray photoelectron spectroscopy (XPS) high-resolution Si2p spectra of Examples 1, 6, and 7 are shown.

[0048] Figure 5 The Fourier transform infrared spectra of Example 1, Comparative Example 6, and Comparative Example 7 are shown.

[0049] Figure 6 The differential scanning calorimetry curing exothermic curves are for Example 1, Comparative Example 1, and Comparative Example 2.

[0050] Figure 7 The differential scanning calorimetry (DSC) glass transition temperature curves for Example 1, Comparative Example 1, and Comparative Example 2 are shown.

[0051] Figure 8 The graphs show the dynamic mechanical analysis of loss modulus versus temperature for Example 1 and Comparative Example 3.

[0052] Figure 9 The graphs show the dynamic mechanical analysis loss factor versus temperature for Example 1 and Comparative Example 3.

[0053] Figure 10 The diagram shows the interface morphology and thickness distribution of laser confocal microscopy in Examples 1, 5, and 8, with thickness varying with position.

[0054] Figure 11 The diagram shows the interface morphology and thickness distribution roughness of Example 1, Comparative Example 5, and Comparative Example 8 under laser confocal microscopy, and the changes in roughness with position. Detailed Implementation

[0055] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Example

[0056] This embodiment provides a co-cured fiber-reinforced composite material for wing main spars joints, comprising a continuous fiber reinforcement, a thermosetting resin matrix, and a modified conductive felt layer. The continuous fiber reinforcement in this embodiment includes carbon fiber layups with an areal density of 200 g / m². The thermosetting resin matrix in this embodiment is formed by curing a co-cured epoxy resin composition. The modified conductive felt layer in this embodiment is located in the wing main spars joint area and disposed between adjacent carbon fiber layups.

[0057] The co-curing epoxy resin composition of this embodiment includes an epoxy resin component, a curing agent, and an accelerator. Based on 100 parts by weight of the total epoxy resin component, the epoxy resin component of this embodiment includes 15 parts by weight of a phosphorus-containing epoxy adduct, 8 parts by weight of 1,4-butanediol diglycidyl ether, and 77 parts by weight of bisphenol A diglycidyl ether. Based on 100 parts by weight of the total epoxy resin component, the curing agent of this embodiment is 4,4'-diaminodiphenyl sulfone, used in an amount of 27 parts by weight. Based on 100 parts by weight of the total epoxy resin component, the accelerator of this embodiment is 2-methylimidazole, used in an amount of 0.50 parts by weight.

[0058] The modified conductive felt layer in this embodiment has an areal density of 15 g / m² and a thickness of 40 µm. The wing main sparsity joint area in this embodiment includes a gradient region along the overlap direction, and the length of this gradient region is 40 mm. This gradient region includes a first sub-region and a second sub-region, which are sequentially arranged along the overlap direction. The first sub-region contains one layer of the modified conductive felt layer, and the second sub-region contains two layers of the modified conductive felt layer.

[0059] The modified conductive thin felt layer in this embodiment is prepared through the following steps: Step A1: Using deionized water as a solvent, add dopamine hydrochloride at a concentration of 2.5 g / L and tris(hydroxymethyl)aminomethane at a concentration of 10.0 g / L to prepare a solution. Adjust the pH of the solution to 8.7 using a 0.5 mol / L sodium hydroxide solution.

[0060] Step A2: Immerse the conductive thin felt substrate in the solution obtained in step A1. In this embodiment, the conductive thin felt substrate is a multi-walled carbon nanotube thin felt. The bath ratio of the conductive thin felt substrate to the solution in this embodiment is 1g of conductive thin felt substrate to 250mL of solution. The reaction is carried out at a temperature of 25℃ and in an air atmosphere for 3.0h to obtain the coated conductive thin felt.

[0061] Step A3: Wash with deionized water 3 times, then wash with anhydrous ethanol 3 times. In this embodiment, the bath ratio for each wash is 1g of conductive felt to 100mL of washing solution. The washing method in this embodiment is soaking. Then, dry in an oven at 60℃ for 6 hours until constant weight. Constant weight means that the mass change does not exceed 0.5% when weighed twice consecutively with an interval of 1 hour under the same drying conditions.

[0062] Step A4: Immerse the dried product obtained in Step A3 into the coupling solution. In this embodiment, the bath ratio of the dried product to the coupling solution is 1g of conductive felt to 150mL of coupling solution. React at 40℃ for 2.0h to obtain the coupled conductive felt. The coupling agent in the coupling solution of this embodiment is 3-aminopropyltriethoxysilane. The total mass fraction of the coupling agent in the coupling solution of this embodiment is 2.5wt%, based on the total mass of the coupling solution. The solvent of the coupling solution of this embodiment is a mixture of ethanol and deionized water, with a mass ratio of ethanol to deionized water of 70:30. The coupling solution of this embodiment is prepared as follows: first, add the coupling agent to ethanol and mix well, then add deionized water and mix well, and let stand at 25℃ for 1.0h to obtain the coupling solution.

[0063] Step A5: The modified conductive felt layer was obtained by drying in an oven at 60℃ for 6 hours and then dried to constant weight. In this embodiment, the coating weight gain of the modified conductive felt layer was 2.5 wt%. The coating weight gain was calculated based on the mass of the conductive felt substrate: Coating weight gain = (m1 - m0) / m0 × 100%, where m0 is the mass of the conductive felt substrate after drying to constant weight, and m1 is the mass after coating and drying to constant weight. The surface sheet resistance of the modified conductive felt layer in this embodiment was 500 Ω / sq, measured using the four-probe method.

[0064] The gradient-assembled conductive sandwich layer used in the gradient region of this embodiment is prepared through the following steps: Step B1: Provide a modified conductive thin felt layer and cut it into strips. In this embodiment, the strips are 40mm in size along the overlap direction.

[0065] Step B2: Stack the strip sheet in a stepped manner along the overlapping direction, so that the first sub-area has 1 layer and the second sub-area has 2 layers. The total length of the gradient area in this embodiment is 40mm.

[0066] Step B3: Hot-press for 15 minutes at a temperature of 90℃ and a pressure of 0.25MPa to obtain a gradient-assembled conductive sandwich sheet.

[0067] Step B4: The gradient region position tolerance of the gradient assembly conductive sandwich sheet in this embodiment is ±2mm. The gradient region position tolerance refers to the deviation of the starting position of the gradient region from the distance of one end of the gradient assembly conductive sandwich sheet along the overlapping direction.

[0068] The phosphorus-containing epoxy adduct of this embodiment is prepared by the following steps: Step C1: Mix 100 parts by weight of bisphenol A diglycidyl ether with 15 parts by weight of 9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide.

[0069] Step C2: Under an industrial nitrogen atmosphere, the reaction was carried out with mechanical stirring at a temperature of 120°C for 3 hours. In this embodiment, the stirring speed was 300 rpm. The nitrogen atmosphere in this embodiment was established by purging the reaction vessel with nitrogen three times and continuously introducing nitrogen at a flow rate of 0.5 L / min.

[0070] Step C3: Stop the reaction when the phosphorus mass fraction of the obtained adduct is 2.0 wt% and the epoxy equivalent is 375 g / equivalent.

[0071] Step C4: Cool to 75°C to obtain a phosphorus-containing epoxy adduct.

[0072] The co-curing epoxy resin composition of this embodiment is prepared by the following steps: Step D1: Mix 77 parts by weight of bisphenol A diglycidyl ether, 15 parts by weight of phosphorus-containing epoxy adduct and 8 parts by weight of 1,4-butanediol diglycidyl ether. In this embodiment, the mixing method is mechanical stirring at a speed of 300 rpm and maintained at a temperature of 75°C for 1.0 h.

[0073] Step D2: Add 27 parts by weight of 4,4'-diaminodiphenyl sulfone and 0.50 parts by weight of 2-methylimidazole to the mixture obtained in step D1. In this embodiment, the mixing method is mechanical stirring at a speed of 150 rpm and maintained at a temperature of 100°C for 1.5 h.

[0074] Step D3: Degas for 1.0 h under an absolute pressure of 5 kPa and a temperature of 75 °C to obtain a co-cured epoxy resin composition.

[0075] Step D4: The apparent viscosity of the co-cured epoxy resin composition in this embodiment, measured by a rotational viscometer, is 15 Pa·s at a temperature of 80°C and a shear rate of 50 / s.

[0076] The preparation method of the co-cured fiber-reinforced composite material for the wing main sparsity joint in this embodiment includes the following steps: Step S1: Provide a modified conductive thin felt layer. In this embodiment, the coating weight gain of the modified conductive thin felt layer is 2.5wt%, and the surface sheet resistance is 500Ω / sq.

[0077] Step S2: Provide gradient-assembled conductive sandwich sheets.

[0078] Step S3: Provide a phosphorus-containing epoxy adduct.

[0079] Step S4: 77 parts by weight of bisphenol A diglycidyl ether, 15 parts by weight of phosphorus-containing epoxy adduct, 8 parts by weight of 1,4-butanediol diglycidyl ether, 27 parts by weight of 4,4'-diaminodiphenyl sulfone, and 0.50 parts by weight of 2-methylimidazole were mixed and degassed to obtain a co-cured epoxy resin composition. The apparent viscosity of the co-cured epoxy resin composition obtained in this embodiment, measured by a rotational viscometer, was 15 Pa·s at a temperature of 80°C and a shear rate of 50 / s. The specific mixing and degassed operations in this embodiment were as follows: bisphenol A diglycidyl ether, phosphorus-containing epoxy adduct, and 1,4-butanediol diglycidyl ether were stirred and mixed at a temperature of 75°C for 1.0 h, then 4,4'-diaminodiphenyl sulfone and 2-methylimidazole were added and stirred at a temperature of 100°C for 1.5 h, and finally degassed at an absolute pressure of 5 kPa for 1.0 h.

[0080] Step S5: In the wing main sparsity joint area, carbon fiber layup, co-cured epoxy resin composition and gradient assembly conductive sandwich sheet are stacked to form a preform, so that the gradient assembly conductive sandwich sheet is located between two adjacent carbon fiber layup layers.

[0081] Step S6: Vacuum seal the preform. In this embodiment, the absolute pressure of vacuum sealing is 5 kPa. Then, a curing process is performed under a pressure of 0.50 MPa: the temperature is increased from 25°C to 110°C at a heating rate of 3°C / min and held for 2.0 h, then increased to 175°C at a heating rate of 3°C / min and held for 3.0 h. After the curing process is completed, the temperature is cooled to 25°C at a cooling rate of 1.5°C / min while maintaining a pressure of 0.50 MPa, to obtain the co-cured fiber-reinforced composite material for the wing main spars joint.

[0082] Features of Example 1: This embodiment employs moderately stable parameter configurations. The epoxy resin component contains 15 parts by weight of phosphorus-containing epoxy adduct, 8 parts by weight of 1,4-butanediol diglycidyl ether, 27 parts by weight of 4,4'-diaminodiphenyl sulfone as curing agent, and 0.50 parts by weight of 2-methylimidazole as accelerator. The proportions of each component are within a moderate range, ensuring the curing stability and process reproducibility of the resin system. The modified conductive felt layer has a density of 15 g / m², a thickness of 40 µm, a gradient region length of 40 mm, and two layers in the second sub-region. This configuration ensures both conductivity and interlayer interfacial strength. In the coating modification process, the concentration of dopamine hydrochloride was 2.5 g / L, the concentration of tris(hydroxymethyl)aminomethane was 10.0 g / L, the reaction temperature was 25℃, and the reaction time was 3.0 h. The coupling agent mass fraction was 2.5 wt%, the coupling temperature was 40℃, and the coupling time was 2.0 h. These process conditions ensured the uniformity of the polydopamine coating and the effective grafting of the coupling agent. In the preparation of the phosphorus-containing epoxy adduct, the amount of 9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide was 15 parts by weight, the reaction temperature was 120℃, and the reaction time was 3 h. The resulting adduct had a phosphorus mass fraction of 2.0 wt% and an epoxy equivalent of 375 g / equivalent. This parameter configuration balanced flame retardancy and curing reactivity. The apparent viscosity of the co-cured epoxy resin composition at 80℃ and a shear rate of 50 / s was 15 Pa·s, ensuring good wettability and workability. The curing process employs a two-stage curing method: 110℃ for 2.0 hours and 175℃ for 3.0 hours, with a curing pressure of 0.50 MPa. These process parameters ensure complete resin curing and the density of the composite material. This embodiment is suitable for medium-sized wing main beam joints where high process stability and overall performance are required. It is particularly suitable for applications requiring consistent quality in mass production and can be applied to fields with high reliability requirements, such as civil aircraft main beam joints and UAV load-bearing structures. Example

[0083] This embodiment provides a co-cured fiber-reinforced composite material for wing main spars joints, comprising a continuous fiber reinforcement, a thermosetting resin matrix, and a modified conductive felt layer. The continuous fiber reinforcement in this embodiment includes carbon fiber layups with an areal density of 250 g / m². The thermosetting resin matrix in this embodiment is formed by curing a co-cured epoxy resin composition. The modified conductive felt layer in this embodiment is located in the wing main spars joint area and disposed between adjacent carbon fiber layups.

[0084] The co-curing epoxy resin composition of this embodiment includes an epoxy resin component, a curing agent, and an accelerator. Based on 100 parts by weight of the total epoxy resin component, the epoxy resin component of this embodiment includes 22 parts by weight of a phosphorus-containing epoxy adduct, 5 parts by weight of 1,4-butanediol diglycidyl ether, and 73 parts by weight of bisphenol A diglycidyl ether. Based on 100 parts by weight of the total epoxy resin component, the curing agent of this embodiment is 4,4'-diaminodiphenyl sulfone, used in an amount of 32 parts by weight. Based on 100 parts by weight of the total epoxy resin component, the accelerator of this embodiment is 2-methylimidazole, used in an amount of 0.30 parts by weight.

[0085] The modified conductive felt layer in this embodiment has an areal density of 22 g / m² and a thickness of 60 µm. The wing main sparsity joint area in this embodiment includes a gradient region along the overlap direction, and the length of this gradient region is 60 mm. This gradient region includes a first sub-region and a second sub-region, which are sequentially arranged along the overlap direction. The first sub-region contains one layer of the modified conductive felt layer, and the second sub-region contains three layers of the modified conductive felt layer.

[0086] The modified conductive thin felt layer in this embodiment is prepared through the following steps: Step A1: Using deionized water as a solvent, add dopamine hydrochloride at a concentration of 3.5 g / L and tris(hydroxymethyl)aminomethane at a concentration of 15.0 g / L to prepare a solution. Adjust the pH of the solution to 8.8 using a 0.8 mol / L sodium hydroxide solution.

[0087] Step A2: Immerse the conductive felt substrate in the solution obtained in step A1. In this embodiment, the conductive felt substrate is a composite felt, specifically a carbon fiber felt formed by loading multi-walled carbon nanotubes onto a carbon fiber felt. The mass fraction of the multi-walled carbon nanotubes in the composite felt is 35 wt%, based on the total mass of the composite felt. The bath ratio for immersing the conductive felt substrate in the solution is 1 g of conductive felt substrate to 350 mL of solution. The reaction is carried out at 30°C in air for 4.5 hours to obtain the coated conductive felt.

[0088] Step A3: Wash with deionized water 4 times, then wash with anhydrous ethanol 4 times. In this embodiment, the bath ratio for each wash is 1g of conductive felt to 150mL of washing solution. In this embodiment, the washing method is ultrasonic-assisted washing. Then, vacuum dry at 70℃ for 8 hours until constant weight. Constant weight means that the mass change does not exceed 0.5% when weighed twice consecutively with an interval of 1 hour under the same drying conditions.

[0089] Step A4: Immerse the dried product obtained in Step A3 into the coupling solution. In this embodiment, the bath ratio of the dried product to the coupling solution is 1g of conductive felt to 220mL of coupling solution. React at 50℃ for 3.0h to obtain the coupled conductive felt. The coupling agent in the coupling solution of this embodiment is a mixture of 3-aminopropyltriethoxysilane and 3-glycidoxypropyltrimethoxysilane, with a mass ratio of 1:1. The total mass fraction of the coupling agent in the coupling solution of this embodiment is 3.5wt%, based on the total mass of the coupling solution. The solvent of the coupling solution of this embodiment is a mixture of ethanol and deionized water, with a mass ratio of 60:40. The coupling solution of this embodiment is prepared as follows: first, add the coupling agent to ethanol and mix well, then add deionized water and mix well, and let stand at 25℃ for 1.5h to obtain the coupling solution.

[0090] Step A5: The modified conductive thin felt layer was obtained by vacuum drying at 70℃ for 8 hours and dried to constant weight. In this embodiment, the coating weight gain of the modified conductive thin felt layer is 3.5 wt%. The coating weight gain is based on the mass of the conductive thin felt substrate and is calculated as: Coating weight gain = (m1 - m0) / m0 × 100%, where m0 is the mass of the conductive thin felt substrate after drying to constant weight, and m1 is the mass after coating and drying to constant weight. The surface sheet resistance of the modified conductive thin felt layer in this embodiment is 50 Ω / sq, and the surface sheet resistance was measured using the four-probe method.

[0091] The gradient-assembled conductive sandwich layer used in the gradient region of this embodiment is prepared through the following steps: Step B1: Provide a modified conductive thin felt layer and cut it into strips. In this embodiment, the strips are 60mm in size along the overlap direction.

[0092] Step B2: Stack the strip sheet in a stepped manner along the overlapping direction, so that the first sub-area has 1 layer and the second sub-area has 3 layers. The total length of the gradient area in this embodiment is 60mm.

[0093] Step B3: Hot-press at 105℃ and 0.40MPa for 22 minutes to obtain a gradient-assembled conductive sandwich sheet.

[0094] Step B4: The gradient region position tolerance of the gradient assembly conductive sandwich sheet in this embodiment is ±2mm. The gradient region position tolerance refers to the deviation of the starting position of the gradient region from the distance of one end of the gradient assembly conductive sandwich sheet along the overlapping direction.

[0095] The phosphorus-containing epoxy adduct of this embodiment is prepared by the following steps: Step C1: Mix 100 parts by weight of bisphenol A diglycidyl ether with 22 parts by weight of 9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide.

[0096] Step C2: Under an industrial nitrogen atmosphere, the reaction was carried out with mechanical stirring at a temperature of 125°C for 4.5 hours. In this embodiment, the stirring speed was 400 rpm. The nitrogen atmosphere in this embodiment was established by purging the reaction vessel with nitrogen four times and continuously introducing nitrogen at a flow rate of 1.2 L / min.

[0097] Step C3: Stop the reaction when the phosphorus mass fraction of the obtained adduct is 2.6 wt% and the epoxy equivalent is 420 g / equivalent.

[0098] Step C4: Cool to 80°C to obtain phosphorus-containing epoxy adduct.

[0099] The co-curing epoxy resin composition of this embodiment is prepared by the following steps: Step D1: Mix 73 parts by weight of bisphenol A diglycidyl ether, 22 parts by weight of phosphorus-containing epoxy adduct and 5 parts by weight of 1,4-butanediol diglycidyl ether. In this embodiment, the mixing method is mechanical stirring at a speed of 400 rpm and maintained at a temperature of 85°C for 1.5 hours.

[0100] Step D2: Add 32 parts by weight of 4,4'-diaminodiphenyl sulfone and 0.30 parts by weight of 2-methylimidazole to the mixture obtained in step D1. In this embodiment, the mixing method is mechanical stirring at a speed of 200 rpm and maintained at a temperature of 110°C for 2.0 h.

[0101] Step D3: Degas for 1.5 hours under an absolute pressure of 2 kPa and a temperature of 85°C to obtain a co-cured epoxy resin composition.

[0102] Step D4: The apparent viscosity of the co-cured epoxy resin composition in this embodiment, measured by a rheometer, is 22 Pa·s at a temperature of 80°C and a shear rate of 80 / s.

[0103] The preparation method of the co-cured fiber-reinforced composite material for the wing main sparsity joint in this embodiment includes the following steps: Step S1: Provide a modified conductive thin felt layer. In this embodiment, the coating weight gain of the modified conductive thin felt layer is 3.5 wt%, and the surface sheet resistance is 50 Ω / sq.

[0104] Step S2: Provide gradient-assembled conductive sandwich sheets.

[0105] Step S3: Provide a phosphorus-containing epoxy adduct.

[0106] Step S4: 73 parts by weight of bisphenol A diglycidyl ether, 22 parts by weight of phosphorus-containing epoxy adduct, 5 parts by weight of 1,4-butanediol diglycidyl ether, 32 parts by weight of 4,4'-diaminodiphenyl sulfone, and 0.30 parts by weight of 2-methylimidazole were mixed and degassed to obtain a co-cured epoxy resin composition. The apparent viscosity of the co-cured epoxy resin composition obtained in this embodiment, measured by a rheometer, was 22 Pa·s at a temperature of 80°C and a shear rate of 80 / s. The specific mixing and degassed operations in this embodiment were as follows: bisphenol A diglycidyl ether, phosphorus-containing epoxy adduct, and 1,4-butanediol diglycidyl ether were stirred and mixed at a temperature of 85°C for 1.5 h; then 4,4'-diaminodiphenyl sulfone and 2-methylimidazole were added and stirred at a temperature of 110°C for 2.0 h; finally, degassed for 1.5 h under an absolute pressure of 2 kPa.

[0107] Step S5: In the wing main sparsity joint area, carbon fiber layup, co-cured epoxy resin composition and gradient assembly conductive sandwich sheet are stacked to form a preform, so that the gradient assembly conductive sandwich sheet is located between two adjacent carbon fiber layup layers.

[0108] Step S6: Vacuum seal the preform. In this embodiment, the absolute pressure of vacuum sealing is 2 kPa. Then, a curing process is performed under a pressure of 0.60 MPa: the temperature is increased from 28°C to 120°C at a heating rate of 4°C / min and held for 2.5 h, then increased to 185°C at a heating rate of 4°C / min and held for 3.5 h. After the curing process is completed, the temperature is cooled to 28°C at a cooling rate of 2.0°C / min while maintaining a pressure of 0.60 MPa, to obtain the co-cured fiber-reinforced composite material for the wing main spars joint.

[0109] Features of Example 2: This embodiment employs parameter configurations biased towards high flame retardancy and high crosslinking density. The amount of phosphorus-containing epoxy adduct in the epoxy resin component is 22 parts by weight, significantly higher than in Example 1. Simultaneously, the amount of 1,4-butanediol diglycidyl ether is reduced to 5 parts by weight, the amount of curing agent 4,4'-diaminodiphenyl sulfone is increased to 32 parts by weight, and the amount of accelerator 2-methylimidazole is reduced to 0.30 parts by weight. This formulation is beneficial for improving the flame retardancy and heat resistance of the material. The modified conductive felt layer has a density of 22 g / m², a thickness of 60 µm, a gradient zone length of 60 mm, and three layers in the second sub-region. This configuration enhances the conductivity and lightning protection capability of the joint area. In the coating modification process, the concentration of dopamine hydrochloride was 3.5 g / L, the concentration of tris(hydroxymethyl)aminomethane was 15.0 g / L, the reaction temperature was 30℃, and the reaction time was 4.5 h. The coupling agent was a mixture of 3-aminopropyltriethoxysilane and 3-glycidoxypropyltrimethoxysilane with a total mass fraction of 3.5 wt%, the coupling temperature was 50℃, and the coupling time was 3.0 h. These process conditions ensured a thicker polydopamine coating and a higher coupling density. The surface sheet resistance was reduced to 50 Ω / sq, indicating a significant improvement in conductivity. In the preparation of the phosphorus-containing epoxy adduct, the amount of 9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide was increased to 22 parts by weight, the reaction temperature was 125℃, and the reaction time was 4.5 h. The resulting adduct had a phosphorus mass fraction of 2.6 wt% and an epoxy equivalent of 420 g / equivalent. This parameter configuration significantly improved the flame retardant properties. The apparent viscosity of the co-cured epoxy resin composition at 80°C and a shear rate of 80 / s is 22 Pa·s, which is relatively high but still within the workable range. The curing process employs a two-stage curing procedure: 2.5 hours at 120°C and 3.5 hours at 185°C, with a curing pressure of 0.60 MPa. The relatively high curing temperature and pressure are beneficial for obtaining composite materials with high crosslinking density and low porosity. This embodiment is suitable for large wing main spars with high requirements for flame retardancy, heat resistance, and lightning protection, and is particularly suitable for high-performance applications such as commercial and military aircraft. It can be applied to aerospace load-bearing structures that require stringent fire protection and lightning protection standards. Example

[0110] This embodiment provides a co-cured fiber-reinforced composite material for wing main spars joints, comprising a continuous fiber reinforcement, a thermosetting resin matrix, and a modified conductive felt layer. The continuous fiber reinforcement in this embodiment includes carbon fiber layups with an areal density of 150 g / m². The thermosetting resin matrix in this embodiment is formed by curing a co-cured epoxy resin composition. The modified conductive felt layer in this embodiment is located in the wing main spars joint region and disposed between adjacent carbon fiber layups.

[0111] The co-curing epoxy resin composition of this embodiment includes an epoxy resin component, a curing agent, and an accelerator. Based on 100 parts by weight of the total epoxy resin component, the epoxy resin component of this embodiment includes 8 parts by weight of a phosphorus-containing epoxy adduct, 12 parts by weight of 1,4-butanediol diglycidyl ether, and 80 parts by weight of bisphenol A diglycidyl ether. Based on 100 parts by weight of the total epoxy resin component, the curing agent of this embodiment is 4,4'-diaminodiphenyl sulfone, used in an amount of 20 parts by weight. Based on 100 parts by weight of the total epoxy resin component, the accelerator of this embodiment is 2-methylimidazole, used in an amount of 0.75 parts by weight.

[0112] The modified conductive felt layer in this embodiment has an areal density of 8 g / m² and a thickness of 20 µm. The wing main sparsity joint area in this embodiment includes a gradient region along the overlap direction, and the length of this gradient region is 25 mm. This gradient region includes a first sub-region and a second sub-region, which are sequentially arranged along the overlap direction. The first sub-region contains one layer of the modified conductive felt layer, and the second sub-region contains two layers of the modified conductive felt layer.

[0113] The modified conductive thin felt layer in this embodiment is prepared through the following steps: Step A1: Using deionized water as a solvent, add dopamine hydrochloride at a concentration of 1.5 g / L and tris(hydroxymethyl)aminomethane at a concentration of 5.0 g / L to prepare a solution. Adjust the pH of the solution to 8.6 with a 0.3 mol / L hydrochloric acid solution.

[0114] Step A2: Immerse the conductive thin felt substrate in the solution obtained in step A1. In this embodiment, the conductive thin felt substrate is carbon fiber thin felt. The bath ratio of the conductive thin felt substrate to the solution is 1g of conductive thin felt substrate to 150mL of solution. The reaction is carried out at 20℃ in an air atmosphere for 1.5h to obtain the coated conductive thin felt.

[0115] Step A3: Wash twice with deionized water, then twice with anhydrous ethanol. In this embodiment, the bath ratio for each wash is 1g of conductive felt to 60mL of washing solution. The washing method in this embodiment is immersion washing. Then, dry in an oven at 50℃ for 4 hours until constant weight is reached. Constant weight means that the mass change does not exceed 0.5% between two consecutive weighings under the same drying conditions with an interval of 1 hour.

[0116] Step A4: Immerse the dried product obtained in Step A3 into the coupling solution. In this embodiment, the bath ratio of the dried product to the coupling solution is 1g of conductive felt to 80mL of coupling solution. React at 30℃ for 1.0h to obtain the coupled conductive felt. The coupling agent in the coupling solution of this embodiment is 3-glycidyl etheroxypropyltrimethoxysilane. The total mass fraction of the coupling agent in the coupling solution of this embodiment is 1.2wt%, based on the total mass of the coupling solution. The solvent of the coupling solution of this embodiment is a mixture of ethanol and deionized water, with a mass ratio of ethanol to deionized water of 80:20. The coupling solution of this embodiment is prepared as follows: first, add the coupling agent to ethanol and mix well, then add deionized water and mix well, and let stand at 25℃ for 0.8h to obtain the coupling solution.

[0117] Step A5: The modified conductive felt layer was obtained by drying in an oven at 50℃ for 4 hours and then dried to constant weight. In this embodiment, the coating weight gain of the modified conductive felt layer was 1.2 wt%. The coating weight gain was calculated based on the mass of the conductive felt substrate: Coating weight gain = (m1 - m0) / m0 × 100%, where m0 is the mass of the conductive felt substrate after drying to constant weight, and m1 is the mass after coating and drying to constant weight. The surface sheet resistance of the modified conductive felt layer in this embodiment was 5000 Ω / sq, measured using the four-probe method.

[0118] The gradient-assembled conductive sandwich layer used in the gradient region of this embodiment is prepared through the following steps: Step B1: Provide a modified conductive thin felt layer and cut it into strips. In this embodiment, the strips are 25mm in size along the overlap direction.

[0119] Step B2: Stack the strip sheet in a stepped manner along the overlapping direction, so that the first sub-area has 1 layer and the second sub-area has 2 layers. The total length of the gradient area in this embodiment is 25mm.

[0120] Step B3: Hot press for 8 minutes at a temperature of 75℃ and a pressure of 0.15MPa to obtain a gradient-assembled conductive sandwich sheet.

[0121] Step B4: The gradient region position tolerance of the gradient assembly conductive sandwich sheet in this embodiment is ±2mm. The gradient region position tolerance refers to the deviation of the starting position of the gradient region from the distance of one end of the gradient assembly conductive sandwich sheet along the overlapping direction.

[0122] The phosphorus-containing epoxy adduct of this embodiment is prepared by the following steps: Step C1: Mix 100 parts by weight of bisphenol A diglycidyl ether with 10 parts by weight of 9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide.

[0123] Step C2: Under an industrial nitrogen atmosphere, the reaction was carried out with magnetic stirring at a temperature of 115°C for 2.0 h. In this embodiment, the stirring speed was 200 rpm. The nitrogen atmosphere in this embodiment was established by purging the reaction vessel with nitrogen twice and continuously introducing nitrogen at a flow rate of 0.3 L / min.

[0124] Step C3: Stop the reaction when the phosphorus mass fraction of the obtained adduct is 1.4 wt% and the epoxy equivalent is 300 g / equivalent.

[0125] Step C4: Cool to 65°C to obtain a phosphorus-containing epoxy adduct.

[0126] The co-curing epoxy resin composition of this embodiment is prepared by the following steps: Step D1: Mix 80 parts by weight of bisphenol A diglycidyl ether, 8 parts by weight of phosphorus-containing epoxy adduct and 12 parts by weight of 1,4-butanediol diglycidyl ether. In this embodiment, the mixing method is magnetic stirring at a speed of 200 rpm, and the mixture is kept at a temperature of 65°C for 0.8 h.

[0127] Step D2: Add 20 parts by weight of 4,4'-diaminodiphenyl sulfone and 0.75 parts by weight of 2-methylimidazole to the mixture obtained in step D1. In this embodiment, the mixing method is mechanical stirring at a speed of 100 rpm and maintained at a temperature of 90°C for 1.0 h.

[0128] Step D3: Degas for 0.8 h under an absolute pressure of 8 kPa and a temperature of 65 °C to obtain a co-cured epoxy resin composition.

[0129] Step D4: The apparent viscosity of the co-cured epoxy resin composition in this embodiment, measured by a rotational viscometer, is 8 Pa·s at a temperature of 80°C and a shear rate of 20 / s.

[0130] The preparation method of the co-cured fiber-reinforced composite material for the wing main sparsity joint in this embodiment includes the following steps: Step S1: Provide a modified conductive thin felt layer. In this embodiment, the coating weight gain of the modified conductive thin felt layer is 1.2wt%, and the surface sheet resistance is 5000Ω / sq.

[0131] Step S2: Provide gradient-assembled conductive sandwich sheets.

[0132] Step S3: Provide a phosphorus-containing epoxy adduct.

[0133] Step S4: Mix 80 parts by weight of bisphenol A diglycidyl ether, 8 parts by weight of phosphorus-containing epoxy adduct, 12 parts by weight of 1,4-butanediol diglycidyl ether, 20 parts by weight of 4,4'-diaminodiphenyl sulfone, and 0.75 parts by weight of 2-methylimidazole and degas to obtain a co-cured epoxy resin composition. The apparent viscosity of the co-cured epoxy resin composition obtained in this embodiment, measured by a rotational viscometer, is 8 Pa·s at a temperature of 80°C and a shear rate of 20 / s. The specific mixing and degassing operations in this embodiment are as follows: mix bisphenol A diglycidyl ether, phosphorus-containing epoxy adduct, and 1,4-butanediol diglycidyl ether at a temperature of 65°C for 0.8 h, then add 4,4'-diaminodiphenyl sulfone and 2-methylimidazole and stir at a temperature of 90°C for 1.0 h, and finally degas at an absolute pressure of 8 kPa for 0.8 h.

[0134] Step S5: In the wing main sparsity joint area, carbon fiber layup, co-cured epoxy resin composition and gradient assembly conductive sandwich sheet are stacked to form a preform, so that the gradient assembly conductive sandwich sheet is located between two adjacent carbon fiber layup layers.

[0135] Step S6: Vacuum seal the preform. In this embodiment, the absolute pressure of vacuum sealing is 8 kPa. Then, a curing process is performed under a pressure of 0.35 MPa: the temperature is increased from 22°C to 100°C at a heating rate of 2°C / min and held for 1.5 h, then increased to 170°C at a heating rate of 2°C / min and held for 2.5 h. After the curing process is completed, the temperature is cooled to 22°C at a cooling rate of 1.0°C / min while maintaining a pressure of 0.35 MPa, to obtain the co-cured fiber-reinforced composite material for the wing main spars joint.

[0136] Features of Example 3: This embodiment employs parameter configurations biased towards low viscosity, high toughness, and rapid curing. The amount of phosphorus-containing epoxy adduct in the epoxy resin component is reduced to 8 parts by weight, while the amount of 1,4-butanediol diglycidyl ether is increased to 12 parts by weight. The amount of curing agent 4,4'-diaminodiphenyl sulfone is reduced to 20 parts by weight, and the amount of accelerator 2-methylimidazole is increased to 0.75 parts by weight. This ratio is beneficial for reducing resin viscosity, improving toughness, and accelerating curing speed. The modified conductive felt layer has a density of 8 g / m², a thickness of 20 µm, a gradient zone length of 25 mm, and two layers in the second sub-region. This configuration minimizes the interlayer thickness and weight while meeting basic conductivity requirements. In the coating modification process, the concentration of dopamine hydrochloride was reduced to 1.5 g / L, the concentration of tris(hydroxymethyl)aminomethane was reduced to 5.0 g / L, the reaction temperature was 20℃, and the reaction time was 1.5 h. The coupling agent was a single 3-glycidoxypropyltrimethoxysilane with a mass fraction reduced to 1.2 wt%, the coupling temperature was 30℃, and the coupling time was 1.0 h. This process condition aimed for a thin and uniform functional coating. The surface sheet resistance of 5000 Ω / sq indicated that the conductivity was moderate but sufficient to meet basic protection requirements. In the preparation of the phosphorus-containing epoxy adduct, the amount of 9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide was reduced to 10 parts by weight, the reaction temperature was 115℃, and the reaction time was 2.0 h. The resulting adduct had a phosphorus mass fraction of 1.4 wt% and an epoxy equivalent of 300 g / equivalent. This parameter configuration optimized the resin flowability while ensuring certain flame retardant properties. The co-cured epoxy resin composition exhibits an apparent viscosity of only 8 Pa·s at 80°C and a shear rate of 20 / s, significantly reducing viscosity and facilitating resin impregnation of fibers and air bubble removal. The curing process employs a two-stage curing procedure: 1.5 hours at 100°C and 2.5 hours at 170°C, with a curing pressure of 0.35 MPa. The relatively low curing temperature and pressure reduce process complexity and energy consumption, while the high accelerator dosage ensures complete curing. This embodiment is suitable for lightweight wing main spars with weight sensitivity, high process efficiency requirements, and moderate performance requirements. It is particularly suitable for applications such as small civil aircraft, general aviation, and lightweight UAVs, where weight reduction and cost control are critical. It can be applied to lightweight structural components requiring rapid curing processes and low-energy production. Example

[0137] This embodiment provides a co-cured fiber-reinforced composite material for wing main spars joints, comprising a continuous fiber reinforcement, a thermosetting resin matrix, and a modified conductive felt layer. The continuous fiber reinforcement in this embodiment includes carbon fiber layups with an areal density of 275 g / m². The thermosetting resin matrix in this embodiment is formed by curing a co-cured epoxy resin composition. The modified conductive felt layer in this embodiment is located in the wing main spars joint region and disposed between adjacent carbon fiber layups.

[0138] The co-curing epoxy resin composition of this embodiment includes an epoxy resin component, a curing agent, and an accelerator. Based on 100 parts by weight of the total epoxy resin component, the epoxy resin component of this embodiment includes 6 parts by weight of a phosphorus-containing epoxy adduct, 14 parts by weight of 1,4-butanediol diglycidyl ether, and 80 parts by weight of bisphenol A diglycidyl ether. Based on 100 parts by weight of the total epoxy resin component, the curing agent of this embodiment is 4,4'-diaminodiphenyl sulfone, used in an amount of 37 parts by weight. Based on 100 parts by weight of the total epoxy resin component, the accelerator of this embodiment is 2-methylimidazole, used in an amount of 0.15 parts by weight.

[0139] The modified conductive felt layer in this embodiment has an areal density of 27 g / m² and a thickness of 72 µm. The wing main sparsity joint area in this embodiment includes a gradient region along the overlap direction, and the length of this gradient region is 70 mm. This gradient region includes a first sub-region and a second sub-region, which are sequentially arranged along the overlap direction. The first sub-region contains one layer of the modified conductive felt layer, and the second sub-region contains three layers of the modified conductive felt layer.

[0140] The modified conductive thin felt layer in this embodiment is prepared through the following steps: Step A1: Using deionized water as a solvent, add dopamine hydrochloride at a concentration of 4.5 g / L and tris(hydroxymethyl)aminomethane at a concentration of 18.0 g / L to prepare a solution. Adjust the pH of the solution to 8.9 using a 0.9 mol / L sodium hydroxide solution.

[0141] Step A2: Immerse the conductive felt substrate in the solution obtained in step A1. In this embodiment, the conductive felt substrate is a composite felt, which is a carbon fiber felt formed by loading multi-walled carbon nanotubes onto a carbon fiber felt. The mass fraction of multi-walled carbon nanotubes in the composite felt is 55 wt%, based on the total mass of the composite felt. The bath ratio of the conductive felt substrate to the solution in this embodiment is 1 g of conductive felt substrate to 450 mL of solution. The reaction is carried out at 33°C in an air atmosphere for 5.5 h to obtain the coated conductive felt.

[0142] Step A3: Wash with deionized water 5 times, then wash with anhydrous ethanol 5 times. In this embodiment, the bath ratio for each wash is 1g of conductive felt to 180mL of washing solution. In this embodiment, the washing method is ultrasonic-assisted washing. Then, vacuum dry at 75℃ for 10h until constant weight is achieved. Constant weight means that the mass change does not exceed 0.5% when weighed twice consecutively with an interval of 1h under the same drying conditions.

[0143] Step A4: Immerse the dried product obtained in Step A3 into the coupling solution. In this embodiment, the bath ratio of the dried product to the coupling solution is 1g of conductive felt to 270mL of coupling solution. React at 55℃ for 3.5h to obtain the coupled conductive felt. The coupling agent in the coupling solution of this embodiment is a mixture of 3-aminopropyltriethoxysilane and 3-glycidoxypropyltrimethoxysilane, with a mass ratio of 2:1. The total mass fraction of the coupling agent in the coupling solution of this embodiment is 4.5wt%, based on the total mass of the coupling solution. The solvent of the coupling solution of this embodiment is a mixture of ethanol and deionized water, with a mass ratio of 55:45. The coupling solution of this embodiment is prepared as follows: first, add the coupling agent to ethanol and mix well, then add deionized water and mix well, and let stand at 28℃ for 1.8h to obtain the coupling solution.

[0144] Step A5: The modified conductive felt layer was obtained by vacuum drying at 75℃ for 10 hours and then dried to constant weight. In this embodiment, the coating weight gain of the modified conductive felt layer was 4.5 wt%. The coating weight gain was calculated based on the mass of the conductive felt substrate: Coating weight gain = (m1 - m0) / m0 × 100%, where m0 is the mass of the conductive felt substrate after drying to constant weight, and m1 is the mass after coating and drying to constant weight. The surface sheet resistance of the modified conductive felt layer in this embodiment was 10 Ω / sq, measured using the four-probe method.

[0145] The gradient-assembled conductive sandwich layer used in the gradient region of this embodiment is prepared through the following steps: Step B1: Provide a modified conductive thin felt layer and cut it into strips. In this embodiment, the strips are 70mm in size along the overlap direction.

[0146] Step B2: Stack the strip sheet in a stepped manner along the overlapping direction, so that the first sub-area has 1 layer and the second sub-area has 3 layers. The total length of the gradient area in this embodiment is 70mm.

[0147] Step B3: Hot-press at 110℃ and 0.45MPa for 27 minutes to obtain a gradient-assembled conductive sandwich sheet.

[0148] Step B4: The gradient region position tolerance of the gradient assembly conductive sandwich sheet in this embodiment is ±2mm. The gradient region position tolerance refers to the deviation of the starting position of the gradient region from the distance of one end of the gradient assembly conductive sandwich sheet along the overlapping direction.

[0149] The phosphorus-containing epoxy adduct of this embodiment is prepared by the following steps: Step C1: Mix 100 parts by weight of bisphenol A diglycidyl ether with 9 parts by weight of 9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide.

[0150] Step C2: Under an industrial nitrogen atmosphere, the reaction was carried out with mechanical stirring at a temperature of 128°C for 5.5 hours. In this embodiment, the stirring speed was 450 rpm. The nitrogen atmosphere in this embodiment was established by purging the reaction vessel with nitrogen 5 times and continuously introducing nitrogen at a flow rate of 1.8 L / min.

[0151] Step C3: Stop the reaction when the phosphorus mass fraction of the obtained adduct is 1.2 wt% and the epoxy equivalent is 460 g / equivalent.

[0152] Step C4: Cool to 85°C to obtain a phosphorus-containing epoxy adduct.

[0153] The co-curing epoxy resin composition of this embodiment is prepared by the following steps: Step D1: Mix 80 parts by weight of bisphenol A diglycidyl ether, 6 parts by weight of phosphorus-containing epoxy adduct and 14 parts by weight of 1,4-butanediol diglycidyl ether. In this embodiment, the mixing method is mechanical stirring at a speed of 450 rpm and maintained at a temperature of 88°C for 1.8 hours.

[0154] Step D2: Add 37 parts by weight of 4,4'-diaminodiphenyl sulfone and 0.15 parts by weight of 2-methylimidazole to the mixture obtained in step D1. In this embodiment, the mixing method is mechanical stirring at a speed of 250 rpm and maintained at a temperature of 115°C for 2.5 h.

[0155] Step D3: Degas for 1.8 hours under an absolute pressure of 1 kPa and a temperature of 88°C to obtain a co-cured epoxy resin composition.

[0156] Step D4: The apparent viscosity of the co-cured epoxy resin composition in this embodiment, measured by a rheometer, is 27 Pa·s at a temperature of 80°C and a shear rate of 90 / s.

[0157] The preparation method of the co-cured fiber-reinforced composite material for the wing main sparsity joint in this embodiment includes the following steps: Step S1: Provide a modified conductive thin felt layer. In this embodiment, the coating weight gain of the modified conductive thin felt layer is 4.5 wt%, and the surface sheet resistance is 10 Ω / sq.

[0158] Step S2: Provide gradient-assembled conductive sandwich sheets.

[0159] Step S3: Provide a phosphorus-containing epoxy adduct.

[0160] Step S4: Mix 80 parts by weight of bisphenol A diglycidyl ether, 6 parts by weight of phosphorus-containing epoxy adduct, 14 parts by weight of 1,4-butanediol diglycidyl ether, 37 parts by weight of 4,4'-diaminodiphenyl sulfone, and 0.15 parts by weight of 2-methylimidazole and degas to obtain a co-cured epoxy resin composition. The apparent viscosity of the co-cured epoxy resin composition obtained in this embodiment, measured by a rheometer, is 27 Pa·s at a temperature of 80°C and a shear rate of 90 / s. The specific mixing and degassing operations in this embodiment are as follows: mix bisphenol A diglycidyl ether, phosphorus-containing epoxy adduct, and 1,4-butanediol diglycidyl ether at 88°C for 1.8 h, then add 4,4'-diaminodiphenyl sulfone and 2-methylimidazole and stir at 115°C for 2.5 h, and finally degas at an absolute pressure of 1 kPa for 1.8 h.

[0161] Step S5: In the wing main sparsity joint area, carbon fiber layup, co-cured epoxy resin composition and gradient assembly conductive sandwich sheet are stacked to form a preform, so that the gradient assembly conductive sandwich sheet is located between two adjacent carbon fiber layup layers.

[0162] Step S6: Vacuum seal the preform. In this embodiment, the absolute pressure of vacuum sealing is 1 kPa. Then, a curing process is performed under a pressure of 0.65 MPa: the temperature is increased from 29°C to 128°C at a heating rate of 4.5°C / min and held for 2.8 hours, then increased to 188°C at a heating rate of 4.5°C / min and held for 3.8 hours. After the curing process is completed, the temperature is cooled to 29°C at a cooling rate of 2.5°C / min while maintaining a pressure of 0.65 MPa, to obtain the co-cured fiber-reinforced composite material for the wing main spars joint.

[0163] Features of Example 4: This embodiment employs a configuration to verify the feasibility of parameter range boundaries. Several key parameters are close to, but do not exceed, the upper or lower limits of the specified range, to fully demonstrate the reasonableness of the technical solution's scope. In the epoxy resin component, the amount of phosphorus-containing epoxy adduct is 6 parts by weight (close to the lower limit), the amount of 1,4-butanediol diglycidyl ether is 14 parts by weight (close to the upper limit), the amount of curing agent 4,4'-diaminodiphenyl sulfone is 37 parts by weight (close to the upper limit), and the amount of accelerator 2-methylimidazole is 0.15 parts by weight (close to the lower limit). This ratio optimizes toughness while ensuring complete curing. The modified conductive felt layer has a density of 27 g / m², a thickness of 72 µm, a gradient region length of 70 mm, and three layers in the second sub-region. These parameters are all close to the upper limit of their respective ranges. This configuration verifies the manufacturability of the thick interlayer while meeting the requirements for high conductivity. In the coating modification process, the concentrations of dopamine hydrochloride (4.5 g / L), tris(hydroxymethyl)aminomethane (18.0 g / L), reaction temperature (33℃), and reaction time (5.5 h) were all close to their upper limits. The total mass fraction of the coupling agent was 4.5 wt%, the coupling temperature was 55℃, and the coupling time was 3.5 h. These process parameters ensured a thick and dense functional coating, and the surface sheet resistance decreased to 10 Ω / sq, indicating excellent electrical conductivity. In the preparation of the phosphorus-containing epoxy adduct, the amount of 9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide was 9 parts by weight, close to the lower limit; the reaction temperature was 128℃, close to the upper limit; and the reaction time was 5.5 h, close to the upper limit. The resulting adduct had a phosphorus mass fraction of 1.2 wt%, close to the lower limit, and an epoxy equivalent of 460 g / equivalent, close to the upper limit. This combination of parameters optimized the resin flowability while ensuring basic flame retardant properties. The apparent viscosity of the co-cured epoxy resin composition at 80°C and a shear rate of 90°C is 27 Pa·s, close to the upper limit. While the viscosity is high, processing is still possible even with a high shear rate. The curing process employs a two-stage curing procedure: 2.8 hours at 128°C and 3.8 hours at 188°C, with a curing pressure of 0.65 MPa. These parameters are all close to the upper limit of their respective ranges, ensuring a dense composite material structure. This embodiment is suitable for heavy-duty wing main spars with extremely high requirements for conductivity, lightning protection, and structural strength. It is particularly suitable for applications with stringent performance limits, such as large commercial passenger aircraft and heavy military transport aircraft. It can be applied to critical aerospace load-bearing structures that need to withstand extreme loads and harsh environments. Furthermore, this embodiment, by verifying near-limit parameter combinations within a safe range, fully demonstrates the feasibility and stability of the technical solution across a wide parameter range, providing strong support for subsequent process optimization and parameter adjustment.

[0164] Comparative Example 1: Basically the same as Example 1, except that the amount of phosphorus-containing epoxy adduct in the epoxy resin component is 3 parts by weight, the amount of bisphenol A diglycidyl ether is adjusted to 89 parts by weight, and the amounts of other components and preparation conditions remain unchanged.

[0165] Comparative Example 2: It is basically the same as Example 1, except that the amount of phosphorus-containing epoxy adduct in the epoxy resin component is 35 parts by weight, the amount of bisphenol A diglycidyl ether is adjusted to 57 parts by weight, and the amounts of other components and preparation conditions remain unchanged.

[0166] Comparative Example 3: It is basically the same as Example 1, except that the amount of 1,4-butanediol diglycidyl ether in the epoxy resin component is 1 part by weight, and the amount of bisphenol A diglycidyl ether is adjusted to 84 parts by weight accordingly. The amounts of other components and the preparation conditions remain unchanged.

[0167] Comparative Example 4: It is basically the same as Example 1, except that the areal density of the modified conductive thin felt layer is 3 g / m² and the thickness is 8 µm, while the amount of other components and the preparation conditions remain unchanged.

[0168] Comparative Example 5: It is basically the same as Example 1, except that the total length of the gradient region is 5 mm, the size of the strip sheet along the overlapping direction is 5 mm, and the amount of other components and preparation conditions remain unchanged.

[0169] Comparative Example 6: It is basically the same as Example 1, except that polydopamine coating modification treatment was not performed during the preparation of the modified conductive thin felt layer. The conductive thin felt substrate was directly subjected to silane coupling treatment, skipping steps A1 to A3. The dosage of other components and preparation conditions remained unchanged.

[0170] Comparative Example 7: It is basically the same as Example 1, except that silane coupling treatment was not performed during the preparation of the modified conductive thin felt layer, and it was directly dried after the polydopamine coating modification was completed, skipping step A4. The amount of other components and preparation conditions remained unchanged.

[0171] Comparative Example 8: It is basically the same as Example 1, except that a gradient assembly design was not used, and the number of modified conductive felt layers in the first and second sub-regions is 1 layer. The amount of other components and preparation conditions remain unchanged.

[0172] Performance testing: Interlaminar shear strength test Test Object: Co-cured fiber-reinforced composite material specimens in the joint area of ​​the wing main sparsity. Test Objective: To evaluate the interfacial bonding strength between the gradient conductive interlayer and the carbon fiber layup, and to verify the improvement effect of polydopamine coating and silane coupling treatment on interlaminar properties. Test Principle: The specimens are subjected to three-point bending load on a short beam to induce interlaminar shear failure. The interlaminar shear strength is calculated based on the maximum load. Experimental Method: Rectangular specimens with dimensions of 20mm × 10mm × 3mm and a span of 15mm are prepared. The loading rate is 1mm / min. Three-point bending load is applied on a universal testing machine until the specimen fails, and the maximum load is recorded. Key Parameters: Test temperature 23±2℃, relative humidity 50±10%RH, and at least 5 specimens per test group. Data Processing: Interlaminar shear strength τ = 0.75 × Pmax / (b × h), where Pmax is the maximum load, b is the specimen width, and h is the specimen thickness. The results are taken as the average ± standard deviation.

[0173] Surface sheet resistance test Test Objects: Modified conductive thin felt layer finished product and conductive interlayer in composite materials. Test Objective: To evaluate the conductivity of the modified conductive thin felt layer and verify the influence of polydopamine coating and silane coupling on the continuity of the conductive network. Test Principle: A four-probe method is used. A constant current is passed through the two outer probes, and the voltage drop is measured by the two inner probes. The surface sheet resistance is calculated according to Ohm's law. Experimental Method: The modified conductive thin felt layer is cut into 50mm × 50mm samples. A four-probe tester is used to measure the sheet resistance at five different locations on the sample surface. The probe spacing is 2mm, and the test current is 10mA. Key Parameters: Test temperature 23±2℃, relative humidity 50±10%RH. Samples need to be placed in a standard environment for 24 hours. Data Processing: Surface sheet resistance Rs = π / ln2 × (V / I) × k, where V is voltage, I is current, and k is the probe correction factor. The result is taken as the average of 5 points ± standard deviation.

[0174] Porosity test Test Object: The finished product of co-cured fiber-reinforced composite material after curing. Test Objective: To evaluate the wettability and degassing effect of the co-cured epoxy resin composition and verify the effectiveness of low-viscosity design in controlling porosity. Test Principle: The pore volume fraction is calculated by measuring the actual density and theoretical density of the composite material. Experimental Method: Samples with dimensions of 30mm × 30mm × 2mm are prepared. The actual density is determined using the Archimedes' displacement method. The theoretical density is calculated based on the component density and mass fraction. Porosity is calculated using the density difference. Key Parameters: The impregnation medium is distilled water; the impregnation temperature is 20±2℃; the absolute pressure of the vacuum is 0.1kPa; the impregnation time is 30min; each test group contains no fewer than 3 samples. Data Processing: Porosity Vv = (1-ρa / ρt)×100%, where ρa is the actual density and ρt is the theoretical density. The result is taken as the average ± standard deviation.

[0175] Glass transition temperature test Test Object: Pure resin samples after co-curing epoxy resin has cured. Test Objective: To evaluate the degree of curing and heat resistance of the resin system, and to verify the rationality of the curing process parameters. Test Principle: Differential scanning calorimetry (DSC) is used to monitor the heat flow change of the resin during the heating process. The inflection point temperature corresponding to the transition from the glassy to the rubbery state is the glass transition temperature. Experimental Method: 5-10 mg of cured resin sample is placed in an aluminum crucible and heated from 30℃ to 250℃ at a rate of 10℃ / min under nitrogen protection. The DSC curve is recorded. Key Parameters: Nitrogen flow rate 50 mL / min, sample mass accuracy ±0.01 mg, temperature correction using indium standard material, and at least 3 samples per test group. Data Processing: Tg is taken as the temperature corresponding to the peak value of the second derivative of the DSC curve, or the temperature at the intersection of the tangents of the inflection point of the heat flow curve. The results are taken as the average ± standard deviation.

[0176] Tensile strength test Test Object: Co-cured fiber-reinforced composite material specimens in the joint area of ​​the wing main sparsity. Test Objective: To evaluate the overall load-bearing capacity of the composite material and verify the effects of phosphorus-containing flame-retardant modification and flexibility modification on mechanical properties. Test Principle: An axial tensile load is applied to a dumbbell-shaped specimen until fracture. The tensile strength is calculated based on the maximum load and cross-sectional area. Experimental Method: Standard type I dumbbell-shaped specimens are prepared with a gauge length of 50 mm. The tensile rate is 2 mm / min. Uniaxial tensile tests are performed on a universal testing machine, and the load-displacement curves are recorded. Key Parameters: Test temperature 23±2℃, relative humidity 50±10%RH, strain is measured using an extensometer, and each test group contains no fewer than 5 specimens. Data Processing: Tensile strength σt = Pmax / A0, where Pmax is the maximum load and A0 is the original cross-sectional area. The result is taken as the average ± standard deviation.

[0177] Impact toughness test Test Object: Notched specimens of co-cured fiber-reinforced composite materials. Test Objective: To evaluate the material's impact resistance and toughness, and to verify the improvement effect of 1,4-butanediol diglycidyl ether flexible modification on resistance to delamination damage tolerance. Test Principle: A pendulum impact test was used to induce rapid fracture of the notched specimen. Impact toughness was calculated based on the absorbed energy and specimen cross-sectional area. Experimental Method: V-notched specimens with dimensions of 80mm × 10mm × 4mm and a notch depth of 2mm were prepared. A simply supported beam impact testing machine was used, with a pendulum energy of 5J and an impact velocity of 3.5m / s. The absorbed energy at fracture was recorded. Key Parameters: Test temperature 23±2℃, relative humidity 50±10%RH, notch bottom radius 0.25mm, and at least 5 specimens per test group. Data Processing: Impact toughness ak = Ec / (b×h), where Ec is the impact absorbed energy, b is the specimen width, and h is the remaining thickness at the notch. The results are taken as the average ± standard deviation.

[0178] Figure 1 The high-resolution C1s X-ray photoelectron spectroscopy (XPS) spectra of Examples 1, 6, and 7 are shown. The XPS testing conditions and energy calibration methods were kept constant, and the same acquisition step size and pass energy settings were used for all three samples. The varying parameters were the different surface modification paths: Example 1 simultaneously featured polydopamine coating and silane coupling; Example 6 lacked polydopamine coating; and Example 7 lacked silane coupling. In Example 1, besides the main CC peak, the CO or CN-related peak intensities in C1s were relatively higher, and the peak shapes better matched the splitting characteristics after the introduction of oxygen- and nitrogen-containing functional groups. The CN contribution in Comparative Example 6 was significantly weakened, while the indirect contribution of silicon-related effects to C1s in Comparative Example 7 was weakened. This indicates that dual modification can simultaneously introduce reactive functional groups and enhance surface chemical complexity, providing a foundation for subsequent interfacial chemical bonding.

[0179] Figure 2 The high-resolution N1s spectra of X-ray photoelectron spectroscopy (XPS) for Examples 1, 6, and 7 are shown. The XPS testing conditions were kept consistent, and the same background subtraction and peak correction strategies were used. The variable parameters were the presence or absence of polydopamine coating and silane coupling. Example 1 showed a clear N1s characteristic signal around 400 eV, with peak shapes recognizable as amine or imine-related chemical states. Comparative Example 6, lacking polydopamine coating, showed a disappearance or significant reduction in the N1s signal, while Comparative Example 7 retained N1s but exhibited insufficient overall interfacial chemical bridging ability. This indicates that the introduction of nitrogen element originates from the polydopamine coating rather than simple physical adsorption, proving the effective grafting of the polydopamine coating.

[0180] Figure 3 The images show the high-resolution O1s X-ray photoelectron spectroscopy (XPS) spectra of Examples 1, 6, and 7. The parameters were fixed to ensure consistent XPS acquisition range and resolution, and charging correction was performed under the same conditions. The parameters varied were whether silicon-oxygen bond-related structures were formed on the sample surface and the level of oxygen-containing functional groups introduced by polydopamine. Example 1 exhibited a stronger contribution of O1s components related to silicon-oxygen bonds and ether oxygen around approximately 532 eV, with peak shapes closer to the composite characteristics after chemical bonding. Comparative Example 7 showed a significant weakening of related components in this region due to the absence of silane coupling, while Comparative Example 6, although potentially showing some oxygen-containing contribution, lacked sufficient interfacial bridging. This indicates that Example 1 formed a more stable synergistic interface between silicon-oxygen-related chemical states and organic oxygen-containing structures, which is beneficial for improving interfacial bonding strength and durability.

[0181] Figure 4The images show the high-resolution Si2p X-ray photoelectron spectroscopy (XPS) spectra of Examples 1, 6, and 7. The parameters were fixed to ensure consistent XPS test pass energy and step size, and comparable signal-to-noise ratios. The variable parameter was whether or not silane coupling was performed. Example 1 showed a distinct Si2p characteristic peak around 102 eV with a significantly higher intensity than Comparative Example 7. Comparative Example 7 showed a reduced Si2p signal due to the lack of silane coupling, while Comparative Example 6 showed a Si signal but lacked active sites provided by polydopamine, limiting the effective chemical bridging. This indicates that the introduction of silicon mainly comes from silane coupling, and in Example 1, it can more effectively construct the silicon-oxygen phase key bonding interface, thereby supporting the formation of strong interfacial bonding.

[0182] Figure 5 The infrared absorption spectra of Fourier transform infrared spectra for Example 1, Comparative Example 6, and Comparative Example 7 are shown. The parameters were fixed to ensure the Fourier transform infrared spectroscopy test wavenumber range and resolution were consistent, and the same baseline correction method was used. The variable parameters were the presence or absence of polydopamine coating and silane coupling. In Example 1, the NH stretching vibration near 3350 cm⁻¹ and the C=C skeleton vibration near 1510 cm⁻¹ were more pronounced, while the corresponding peaks in Comparative Example 6 were significantly weakened or absent. Simultaneously, the Si-O-Si related absorption near approximately 1100 cm⁻¹ was enhanced in Example 1, while this absorption was weakened in Comparative Example 7. This indicates that the coupling of polydopamine and silane provides identifiable functional group evidence, and both are valid in Example 1, demonstrating that dual modification achieves detectable chemical grafting and enhances interfacial reactivity.

[0183] Figure 6 The graphs show the curing exothermic curves of Example 1, Comparative Example 1, and Comparative Example 2 using differential scanning calorimetry (DSC). The parameters were fixed: the DSC heating rate and nitrogen protection conditions were consistent, and the samples used were of the same mass and crucible type. The variable parameters were the different amounts of phosphorus-containing epoxy adduct used: Example 1 had a moderate amount, Comparative Example 1 had a relatively low amount, and Comparative Example 2 had a relatively high amount. Example 1 showed a more reasonable combination of exothermic peak temperature and exothermic area, indicating that the curing reaction proceeded fully and the crosslinking network formed more evenly. The decreased exothermic peak in Comparative Example 1 indicated insufficient reactivity and crosslinking contribution, which could easily lead to a decrease in heat resistance. The broadened peak shape or shifted peak position in Comparative Example 2 showed that excessive phosphorus-containing structure might introduce reaction restriction and diffusion control, affecting curing efficiency. This indicates that a moderate amount of phosphorus-containing adduct is beneficial for achieving a more reasonable balance between curing kinetics and network structure.

[0184] Figure 7The graphs show the glass transition temperature (Tg) curves of Example 1, Comparative Example 1, and Comparative Example 2 obtained by differential scanning calorimetry (DSC). The parameters were fixed so that the second DSC heating program was consistent with the baseline treatment, while the parameters varied the amount of phosphorus-containing epoxy adduct. Example 1 showed a higher Tg step position than Comparative Example 1 and exhibited a more balanced thermal performance compared to Comparative Example 2. This indicates that a moderate phosphorus content structure improves network rigidity and thermal stability through crosslinking without excessively sacrificing curing integrity. Comparative Example 1 had a lower Tg due to insufficient phosphorus content. While Comparative Example 2 showed improved heat resistance, it may have been accompanied by curing resistance and increased brittleness risk. This demonstrates that a synergistic formulation can achieve improved heat resistance while maintaining the processability and curability of the system.

[0185] Figure 8 The graphs show the loss modulus versus temperature curves for dynamic mechanical analysis of Example 1 and Comparative Example 3. The parameters were fixed at the same temperature program and test frequency for dynamic mechanical analysis, while the varying parameters were the different amounts of flexible modifier. Example 1 exhibited a higher or more reasonable loss modulus peak shape in the glass transition region, indicating a stronger energy dissipation and internal friction mechanism in the transition temperature region. Comparative Example 3 showed a lower loss modulus peak, reflecting insufficient energy dissipation and a greater susceptibility to brittle damage. This demonstrates that the flexible modifier can improve the energy dissipation capacity in the transition region and provide microscopic support for delamination and impact resistance.

[0186] Figure 9 The graph shows the loss factor versus temperature curves for the dynamic mechanical analysis of Example 1 and Comparative Example 3. The parameters were fixed, with the dynamic mechanical analysis measurement system and data processing consistent with the standard. The variable parameter was the amount of flexible modifier used. The more pronounced tanδ peak and shape in Example 1, which better reflects enhanced energy dissipation, indicates increased contributions from chain segment motion and interfacial friction. The decreased tanδ peak in Comparative Example 3 suggests insufficient energy absorption in the critical transition region and a greater susceptibility to rapid crack propagation. This demonstrates that the flexible modifier can improve energy dissipation and work in conjunction with the heat-resistant crosslinking network to achieve a balance between rigidity and toughness.

[0187] Figure 10 The figures show the thickness variation with position for the interface morphology and thickness distribution of Examples 1, 5, and 8 using laser confocal microscopy. The fixed parameters are the laser confocal microscopy scanning step distance and the overlap direction of the measurement area, all covered by the same algorithm to extract the thickness profile. The varying parameters are the different gradient assembly designs: Example 1 shows a reasonable gradient region length, Example 5 shows an excessively short gradient region, and Example 8 shows no gradient assembly. The thickness in Example 1 exhibits a continuous and smooth transition along the overlap direction, reflecting a stepped design that achieves gradual variation. The steeper transition region in Example 5 indicates insufficient stress transition space, while Example 8 shows an approximately abrupt change, indicating that the interface geometry is discontinuous and more prone to stress concentration. This demonstrates that gradient assembly can effectively reduce geometric abrupt changes and provide structural evidence for stress dispersion in the joint area.

[0188] Figure 11 The figures show the roughness variations as a function of location for the interface morphology and thickness distribution under laser confocal microscopy in Examples 1, 5, and 8 (Comparative Example 8). The fixed parameters are the same as the 3D morphology reconstruction parameters and roughness calculation window of the laser confocal microscopy. The varying parameters are the different gradient assembly designs. Example 1 shows a smoother roughness change and more dispersed peaks in the gradient region, indicating better interface morphology continuity and facilitating interlayer load transfer. Comparative Example 5 shows more concentrated roughness peaks in the short transition region. Comparative Example 8 shows greater roughness fluctuations at the interface abrupt change, reflecting potential local strain concentration and increased defect sensitivity. This demonstrates that the gradient structure can improve the uniformity of the interface morphology and is consistent with a higher tolerance for delamination damage, thus supporting the rationality of the design.

[0189] As can be seen from the performance of the examples and comparative examples in Table 1, the four examples are significantly superior to all comparative examples in terms of interlaminar shear strength, tensile strength, and overall performance. The interlaminar shear strength of Examples 1-4 is all above 65 MPa, while that of the comparative examples is only 38.5-61.2 MPa. This is mainly attributed to the significant enhancement of the interfacial bonding between the conductive felt and the resin matrix by polydopamine coating and silane coupling modification. In Comparative Examples 1 and 2, the amount of phosphorus-containing epoxy adduct deviates from the optimal range, resulting in an imbalance between flame retardancy and mechanical properties, high porosity (2.8-3.2%), and a decrease in glass transition temperature. In Comparative Example 3, the amount of 1,4-butanediol diglycidyl ether is too low, resulting in insufficient flexibility modification and an impact toughness of only 41.2 kJ / m², far lower than the 58.6 kJ / m² of Example 3. In Comparative Example 4, the modified conductive felt has too low a layer density, a surface sheet resistance as high as 8500 Ω / sq, and insufficient continuity of the conductive network. Comparative Example 5 had an excessively short gradient region length, resulting in insufficient stress transition and an interlaminar shear strength of only 42.8 MPa. Comparative Example 6, without polydopamine coating modification, exhibited a surface sheet resistance as high as 12000 Ω / sq and an interlaminar shear strength of only 38.5 MPa, demonstrating the crucial role of the polydopamine coating in interface modification. Comparative Example 7, without silane coupling treatment, lacked chemical bonding despite having a polydopamine coating, resulting in a surface sheet resistance of 2800 Ω / sq and an interlaminar shear strength of only 44.2 MPa. Comparative Example 8 did not employ a gradient assembly design; the second sub-region consisted of only one layer of conductive felt, leading to stress concentration in the joint area and lower interlaminar shear strength and impact toughness compared to the examples. Examples 2 and 4 used higher amounts of phosphorus-containing epoxy adduct and curing agent, reducing the surface sheet resistance to 10-50 Ω / sq and increasing the glass transition temperature to 189-191°C, demonstrating excellent conductivity and heat resistance, but with a slight decrease in impact toughness. Example 3, employing higher amounts of 1,4-butanediol diglycidyl ether and accelerator, achieved an impact toughness as high as 58.6 kJ / m², demonstrating a significant improvement in tolerance to delamination damage due to flexible modification, making it suitable for applications requiring high toughness. In summary, the parameter configurations of Examples 1-4 achieved a good balance between flame retardancy, conductivity, mechanical properties, and processability, proving the superiority of the technical solution and the rationality of the parameter range.

[0190] Table 1 Performance Comparison Summary Table Sample number Interlaminar shear strength (MPa) Surface sheet resistance (Ω / sq) Porosity (%) Glass transition temperature (°C) Tensile strength (MPa) Impact toughness (kJ / m²) Example 1 68.5±2.3 500±45 1.2±0.3 182±3 1250±55 52.8±3.2 Example 2 72.3±2.8 50±8 0.9±0.2 189±4 1320±62 48.5±2.9 Example 3 65.2±2.1 5000±420 1.5±0.4 175±3 1180±48 58.6±3.8 Example 4 71.8±2.6 10±2 0.8±0.2 191±4 1340±58 46.2±2.7 Comparative Example 1 52.3±2.8 520±62 2.8±0.6 168±5 980±72 35.8±4.2 Comparative Example 2 58.6±3.2 480±55 3.2±0.7 178±4 1050±68 28.5±3.5 Comparative Example 3 61.2±2.5 510±48 2.5±0.5 176±4 1120±58 41.2±3.8 Comparative Example 4 48.5±3.5 8500±980 1.8±0.4 180±3 1160±65 44.5±3.2 Comparative Example 5 42.8±4.2 505±58 1.4±0.3 181±3 1100±72 38.2±4.5 Comparative Example 6 38.5±3.8 12000±1500 1.6±0.4 179±4 1140±68 42.8±3.6 Comparative Example 7 44.2±3.2 2800±320 1.5±0.3 180±3 1130±62 43.5±3.4 Comparative Example 8 51.8±2.9 550±62 1.3±0.3 182±3 1190±58 40.8±3.2 Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that any equivalent structural transformations made under the concept of the present invention and using the contents of the specification and drawings of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A co-cured fiber-reinforced composite material for wing main sparse joints, characterized in that, include: A continuous fiber reinforcement comprising carbon fiber layups; A thermosetting resin matrix formed by curing a co-curing epoxy resin composition; Modified conductive felt layer located in the joint area of ​​the wing main spars; The co-cured epoxy resin composition comprises an epoxy resin component, a curing agent, and an accelerator. Based on 100 parts by weight of the total epoxy resin component, the epoxy resin component comprises 5-30 parts by weight of a phosphorus-containing epoxy adduct, 2-15 parts by weight of 1,4-butanediol diglycidyl ether, and bisphenol A diglycidyl ether, with the weight of bisphenol A diglycidyl ether being rounded up to the nearest 100 parts by weight. Based on 100 parts by weight of the total epoxy resin component, the curing agent is 4,4′-diaminodiphenyl sulfone, used in an amount of 15-40 parts by weight. Based on 100 parts by weight of the total epoxy resin component, the accelerator is 2-methylimidazole, used in an amount of 0.05-1.00 parts by weight. The modified conductive felt layer has a surface density of 5-30 g / m² and a thickness of 10-80 µm, and is disposed between two adjacent carbon fiber lay-ups. The wing main sparsity joint area includes a gradient zone along the overlap direction. The gradient zone has a length of 10-80mm and includes a first sub-zone and a second sub-zone. The first sub-zone and the second sub-zone are arranged sequentially along the overlap direction. The modified conductive felt layer in the first sub-zone has 1 layer, and the modified conductive felt layer in the second sub-zone has 2-3 layers.

2. The co-cured fiber-reinforced composite material according to claim 1, characterized in that, The modified conductive thin felt layer is prepared by the following steps: A1) Solution preparation: Using deionized water as solvent, add dopamine hydrochloride at a concentration of 0.5-5.0 g / L and tris(hydroxymethyl)aminomethane at a concentration of 1.0-20.0 g / L to prepare a solution. Adjust the pH of the solution to 8.5-9.0 with a 0.1-1.0 mol / L hydrochloric acid solution or sodium hydroxide solution. A2) Coating reaction: The conductive thin felt substrate is immersed in the solution obtained in step A1), wherein the bath ratio of the conductive thin felt substrate to the solution is 1g of conductive thin felt substrate to 50-500mL of solution, and the reaction is carried out at a temperature of 15-35℃ and in an air atmosphere for 0.5-6.0h to obtain the coated conductive thin felt, wherein the conductive thin felt substrate is selected from multi-walled carbon nanotube thin felt or carbon fiber thin felt. A3) Washing and drying: Wash with deionized water 1-5 times, then wash with anhydrous ethanol 1-5 times. In step A3), the bath ratio for each wash is 1g of conductive felt to 20-200mL of washing solution. Then dry at a temperature of 40-80℃ for 2-12h. A4) Coupling treatment: The dried product obtained in step A3) is immersed in a coupling solution. The bath ratio of the dried product to the coupling solution is 1g of conductive felt to 30-300mL of coupling solution. The reaction is carried out at a temperature of 20-60℃ for 0.5-4.0h to obtain the coupled conductive felt. The coupling agent in the coupling solution is selected from one or two of 3-aminopropyltriethoxysilane and 3-glycidoxypropyltrimethoxysilane. The total mass fraction of the coupling agent in the coupling solution is 0.1-5.0wt%, and the mass fraction is based on the total mass of the coupling solution. The solvent of the coupling solution is a mixture of ethanol and deionized water, and the mass ratio of ethanol to deionized water is 90:10 to 50:

50. A5) Drying and Quality Control: The modified conductive thin felt layer is obtained by drying at a temperature of 40-80℃ for 2-12 hours. The coating weight gain of the modified conductive thin felt layer is 0.5-5.0 wt%, based on the mass of the conductive thin felt substrate. The surface sheet resistance of the modified conductive thin felt layer is 1-10. 4 Ω / sq.

3. The co-cured fiber-reinforced composite material according to claim 1, characterized in that, The gradient-assembled conductive sandwich sheet used in the gradient region is prepared through the following steps: B1) Sheet preparation: Provide a modified conductive thin felt layer and cut it into strip sheets, the strip sheets having a dimension of 10-80mm along the overlapping direction; B2) Gradient assembly: The strip sheet is stacked in a stepped manner along the overlapping direction, so that the first sub-region has 1 layer and the second sub-region has 2-3 layers, and the total length of the gradient region is 10-80mm. B3) Shaping: Hot pressing for 1-30 minutes at a temperature of 60-120℃ and a pressure of 0.05-0.50MPa to obtain a gradient-assembled conductive sandwich sheet; B4) Quality control: The positional tolerance of the gradient region of the gradient-assembled conductive interlayer sheet is ±2mm.

4. The co-cured fiber-reinforced composite material according to claim 1, characterized in that, The phosphorus-containing epoxy adduct is prepared by the following steps: C1) Raw material mixing: 100 parts by weight of bisphenol A diglycidyl ether are mixed with 8-26 parts by weight of 9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide; C2) Addition reaction: under an industrial nitrogen atmosphere, the reaction is carried out at a temperature of 110-130℃ for 1-6 hours; C3) Endpoint criterion: The reaction is stopped when the phosphorus content of the resulting adduct is 1.0-3.0 wt% and the epoxy equivalent is 250-500 g / equivalent. C4) Cooling: Cool to a temperature of 60-90℃ to obtain the phosphorus-containing epoxy adduct.

5. The co-cured fiber-reinforced composite material according to claim 1, characterized in that, The co-cured epoxy resin composition is prepared by the following steps: D1) Premixing: Bisphenol A diglycidyl ether, phosphorus-containing epoxy adduct and 1,4-butanediol diglycidyl ether are mixed and kept at a temperature of 60-90℃ for 0.5-2.0h. D2) Add curing agent and accelerator: Add 4,4′-diaminodiphenyl sulfone and 2-methylimidazole to the mixture obtained in step D1) and maintain it at a temperature of 80-120℃ for 0.5-3.0h; D3) Degassing: Degas under an absolute pressure of 0.1-10 kPa for 0.5-2.0 h to obtain the co-cured epoxy resin composition; D4) Viscosity control: The apparent viscosity of the co-cured epoxy resin composition, measured by a rotational viscometer or rheometer, is 5-30 Pa·s at a temperature of 80°C and a shear rate of 1-100 / s.

6. The co-cured fiber-reinforced composite material according to claim 2, characterized in that, The conductive thin felt substrate is selected from one of multi-walled carbon nanotube thin felt and composite thin felt. The composite thin felt is a carbon fiber thin felt formed by loading multi-walled carbon nanotubes on carbon fiber thin felt. The mass fraction of multi-walled carbon nanotubes in the composite thin felt is 10-60 wt%, and the mass fraction is based on the total mass of the composite thin felt. The areal density of the carbon fiber layup is 120-300 g / m².

7. The method for co-curing fiber-reinforced composite materials for wing main sparse joints as described in claim 1, characterized in that, Includes the following steps: S1. Provides a modified conductive thin felt layer; S2. Provides gradient-assembled conductive sandwich sheets; S3. Provides phosphorus-containing epoxy adducts; S4. Bisphenol A diglycidyl ether, the phosphorus-containing epoxy adduct, 1,4-butanediol diglycidyl ether, 4,4′-diaminodiphenyl sulfone and 2-methylimidazole are mixed and degassed to obtain a co-cured epoxy resin composition. S5. In the wing main sparsity joint area, carbon fiber layup, the co-cured epoxy resin composition and the gradient assembly conductive interlayer sheet are stacked to form a preform, such that the gradient assembly conductive interlayer sheet is located between two adjacent carbon fiber layup layers. S6. Vacuum seal the preform, and then perform a curing process under a pressure of 0.30-0.70 MPa: heat from 20-30℃ to 90-130℃ at a heating rate of 1-5℃ / min and hold for 1.0-3.0h, then heat to 160-190℃ at a heating rate of 1-5℃ / min and hold for 2.0-4.0h to obtain a co-cured fiber-reinforced composite material for the wing main spars joint.

8. The method according to claim 7, characterized in that, The absolute pressure of the vacuum sealing in step S6 is 0.1-10 kPa.

9. The method according to claim 7, characterized in that, The apparent viscosity of the co-cured epoxy resin composition obtained in step S4, measured by a rotational viscometer or rheometer, is 5-30 Pa·s at a temperature of 80°C and a shear rate of 1-100 / s.

10. The method according to claim 7, characterized in that, The modified conductive thin felt layer provided in step S1 has a coating weight gain of 0.5-5.0 wt% and a surface sheet resistance of 1-10. 4 Ω / sq.

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