Aircraft fairing and fairing body composite material integral forming process

Abstract

The invention discloses an aircraft fairing and a fairing body composite material integrated forming process, and relates to the technical field of aviation equipment manufacturing. The forming process comprises the following steps: 1) preparing a prepreg; (2) sectional forming; 3) a curing process; (4) demolding and trimming; 5) an assembly process; the whole of the front section, the middle section and the tail section of the fairing is of a long-size U-shaped streamline structure. According to the invention, through molecular collaborative design of fluorination modification-polysiloxane toughening-cyanate ester interpenetration, the key contradiction of a high-performance fairing material is systematically solved; ultrahigh strength and toughness are realized at the same time by utilizing in-situ nano microphase separation; a dense graphite carbon layer is generated through catalysis, so that efficient halogen-free flame retardance is achieved; by means of the partition design of molecular fluorination and surface metallization, the high wave transmission and lightning protection functions are ingeniously unified. And finally, a new generation of aircraft fairing composite material with force bearing, wave transmission, heat resistance, impact resistance and environmental stability is successfully prepared.

Description

TECHNICAL FIELD

[0001] The application relates to the field of aviation equipment manufacturing technology, in particular to an aircraft fairing and a fairing body composite integrated forming process. BACKGROUND

[0002] As a key aerodynamic structural part, the aircraft fairing needs to meet multiple stringent requirements such as lightweight, high-temperature resistance, wave transparency, electromagnetic shielding and structural stability. The traditional metal fairing has defects such as large weight, poor corrosion resistance, and difficult to balance wave transparency and shielding performance. The existing composite fairing faces technical bottlenecks such as difficulty in forming long-size U-shaped structure, easy separation between layers, high dielectric constant, and insufficient assembly precision.

[0003] Specifically, although the traditional double-maleimide (BMI) resin has excellent heat resistance, it is brittle and has a narrow processing window. The single cyanogen resin has insufficient toughness, and direct physical blending with BMI can easily cause phase separation, resulting in poor interfacial bonding of the composite material. At the same time, when the long-size U-shaped structure is integrally formed, bubbles and wrinkles are easily generated, and the continuity of electrical conduction and the structural integrity are difficult to guarantee after segmented assembly, which seriously affects the flight safety and equipment reliability of the aircraft. Therefore, it is of great practical significance to develop an aircraft fairing with material synergistic optimization, reasonable structure design and stable and controllable process. SUMMARY

[0004] The application aims to solve the problems in the prior art and provides an aircraft fairing and a forming process thereof.

[0005] To achieve the above-mentioned purpose, the application adopts the following technical solutions: The application first provides an aircraft fairing, which comprises a front section, a middle section and a tail section connected in sequence, and has a long-size U-shaped streamlined structure as a whole; the front section, the middle section and the tail section are separately formed and then assembled and connected; The front section has a large curvature and is internally provided with thick reinforcing ribs; the middle section mainly has a straight section and is internally provided with thick reinforcing ribs; the tail section is connected with the main body of the aircraft and integrates the function of the original titanium alloy tail cover; The abutting joints of the front section, the middle section and the tail section are all provided with abutting ribs and adopt a stepped lap joint design; the mounting interface with the main body of the aircraft adopts a titanium alloy embedded part, which is integrally formed with the composite material; all the abutting surfaces are provided with positioning pin holes, and the assembly precision is controlled within ±0.1mm; The front section, the middle section and the tail section all adopt a multi-layer composite material structure, which comprises, from the outside to the inside: An outer layer: T300 nickel-coated plain fabric impregnated material; A middle layer: T700 unidirectional tape impregnated material, which is laid in the direction of 0° / ±45° / 90°; An inner layer: T300 ordinary plain fabric impregnated material; The butt rib area is increased by 2 layers of ±45° T700 unidirectional tape; a local 3D woven preform is used to reinforce around the mounting interface; The impregnated material is obtained by impregnating T300 nickel-coated plain fabric, T700 unidirectional tape, and T300 ordinary plain fabric; The prepreg is obtained by blending modified bismaleimide and cyan-based resin at a mass ratio of 3-4:6-7.

[0006] Preferably, the first section is coated with a metal layer by laying up from the inner edge and outwardly turned and covered at the butt rib with the main body and the middle section; the last section is also turned and covered at the butt rib with the middle section.

[0007] Preferably, the modified bismaleimide resin is prepared as follows: Drying 4,4'-diphenylmethane bismaleimide and aliphatic bismaleimide in a vacuum drying oven at 60±5℃ for 3-5h; grinding 2-trifluoromethyl-p-phenylenediamine through a 100-200 mesh sieve; Adding 4,4'-diphenylmethane bismaleimide and aliphatic bismaleimide to a reaction kettle with nitrogen protection device, heating to 135-145℃, stirring at 300-400r / min for 25-35min until completely melted;

[0008] Adding 2-trifluoromethyl-p-phenylenediamine and eugenol allyl polysiloxane in sequence, heating to 145-155℃, increasing the stirring speed to 400-500r / min, and reacting for 25-35min to obtain a modified bismaleimide prepolymer;

[0009] At 135-145℃, the two bismaleimides are melted, eugenol allyl polysiloxane is added and uniformly dispersed in the system, and Michael addition occurs with the amine groups of 2-trifluoromethyl-p-phenylenediamine to form a prepolymer; 4,4'-diphenylmethane type bismaleimide provides rigid skeleton and heat resistance; aliphatic bismaleimide introduces flexible chain segment, increases toughness and processability of the resin, and reduces brittleness; 2-trifluoromethyl-p-phenylenediamine as a chain extender and modifier, the amine group and the double bond on the maleimide ring undergoes Michael addition reaction, opens the double bond, generates linear or branched prepolymers, this process reduces the crosslinking density of the resin, improves the solubility and flowability of the prepolymer, facilitates impregnation, at the same time, the introduced trifluoromethyl group is a strong electron-withdrawing group, which can significantly reduce the dielectric constant of the resin system, which is crucial for the wave permeability of the aircraft fairing; eugenol allyl polysiloxane introduces inorganic Si-O bond, which can further improve the heat resistance, oxidation resistance and toughness of the resin, and further reduce the dielectric constant, and the benzene ring structure is helpful to the compatibility with the main body of the resin. The modified bismaleimide prepolymer is transferred to a vacuum degassing machine, and is degassed at a pressure of-0.092 to-0.098 MPa for 25-35 min to obtain the modified bismaleimide resin. The eugenol allyl polysiloxane can be directly purchased or prepared by the following flow: Octamethylcyclotetrasiloxane, tetramethylcyclotetrasiloxane, 1,1,3,3-tetramethyldisiloxane, trifluoromethanesulfonic acid, stirring under nitrogen protection at 25℃ for 24 hours, drying with excess anhydrous magnesium sulfate, filtering, liquid phase vacuum drying, to obtain hydrogen-containing polysiloxane; hydrogen-containing polysiloxane, eugenol, platinum gold catalyst and toluene are refluxed at 100℃ for 12 hours under nitrogen protection, vacuum drying, to obtain eugenol grafted polysiloxane; eugenol grafted polysiloxane, anhydrous K2CO3 and tetrahydrofuran are stirred under nitrogen protection, then allyl bromide is added, and then reacted at 80℃ for 12 hours to obtain eugenol allyl ether grafted polysiloxane.

[0010] Preferably, the viscosity of the modified bismaleimide prepolymer at 150℃ is 0.6-0.8 Pa・s; the mass ratio of 4,4'-diphenylmethane type bismaleimide, aliphatic bismaleimide, 2-trifluoromethyl-p-phenylenediamine and eugenol allyl polysiloxane is 45-55:15-25:10-15:8-12.

[0011] Preferably, the 2-trifluoromethyl-p-phenylenediamine has a fluorine content of ≥35% and a melting point of 88-90℃; the eugenol allyl polysiloxane has a number average molecular weight of 3000-5000 and a double bond content of 7%-9%.

[0012] The application also proposes an integrated forming process of the aforementioned aircraft fairing, which comprises the following steps: S1, prepreg preparation ①Resin blending: Modified bismaleimide resin and cyano resin were added to a reactor and stirred at 200 r / min for 30 min at 80 °C under nitrogen protection; then 0.5% of the total resin mass of curing accelerator was added and stirring was continued for 15 min. ②Preparation of prepreg: The blended resin was coated onto release paper using a hot-melt method; T300 metallized fabric, T300 ordinary fabric, and T700 unidirectional tape were impregnated separately; the resin content was controlled as follows: 45±2% for fabrics and 38±2% for unidirectional tape. ③ Cutting and layout: According to the layup design of each section, the prepreg is cut using a CNC cutting machine; a 30mm flange allowance is reserved for the prepreg in the butt rib area; the metal-plated area is marked separately; Under heating conditions, the cyanate groups of cyano resin undergo a trimerization reaction to form a triazine ring structure. However, during blending, due to the presence of amino groups in the modified bismaleimide resin, these groups can react with the cyanate groups to form isourea esters. Ultimately, the system forms an interpenetrating network or copolymer network, rather than a simple physical mixture. This network combines the high-temperature strength of bismaleimide resin with the toughness and low dielectric loss of cyanate ester.

[0013] S2, Segmented Molding Preheat the mold to 60℃ and spray the release agent evenly; position and install the titanium alloy interface embedded parts, with the gap controlled ≤0.05mm; Front section: Layers 1-3, T300 metallized fabric laid at 0°; Layers 4-25, T700 unidirectional tape laid in a cyclic pattern of 0° / 45° / -45° / 90°; Layers 26-28, T300 ordinary fabric laid at 0°. Middle section: Automated tape laying machine lays up 20 layers in total, with the same layup structure as the previous section; Final section: Made of resin transfer molding, the preform has a 3D woven structure; By adopting a layup sequence of 0° / ±45° / 90°, the unidirectional belt can effectively transfer loads in different directions. After curing, the resin matrix forms a continuous whole, firmly bonding the fiber layers in all directions together.

[0014] 3D braided preforms "stitch" the originally independent XY planar layers into a whole by weaving fibers in the Z direction (thickness direction), fundamentally eliminating the possibility of delamination and greatly improving the damage tolerance and impact resistance of composite materials.

[0015] S3, Curing Process Front / Middle Section: Stepped pressure defoaming and segmented curing; Final stage: Resin transfer molding injection curing, mold temperature 130℃, injection pressure 4MPa; curing procedure is the same as the previous stage, nitrogen flow rate 0.8-1.2L / min; At low pressure (0.5 MPa), the resin viscosity is low, and small molecules such as air in the interlayer and fiber bundles have sufficient momentum to escape. Gradually increasing the pressure can smoothly expel air bubbles, avoiding crushing the bubbles into smaller, more difficult-to-expel bubbles under high pressure at once.

[0016] S4. Demolding and finishing Demolding is performed when the temperature drops below 60℃ to obtain a rough fairing composite material. The mating surfaces are then machined, the integrity of the reinforcing ribs is checked, and local repairs are made if necessary. S5, Assembly Process Each joint surface is sandblasted; the flanged area of ​​the metal plating layer is cleaned to ensure good electrical contact; locating pins are installed, and the fit clearance is checked to be ≤0.05mm; Co-curing bonding technology is adopted, and a special adhesive film is applied to the mating surfaces; after assembly, a pressure of 0.2MPa is applied, and it is cured at 120℃ for 1 hour in a thermostatic precipitator; vacuum-assisted bonding is used at the flanged and wrapped areas to ensure no air bubbles. Conductive adhesive is used to fill the gaps at the docking ribs; copper conductive lap plates are installed; and the outer surface is ground and polished to obtain the aircraft fairing.

[0017] Preferably, in S1, the cyano resin is bisphenol E cyanate.

[0018] Preferably, in step S3, the specific procedure for step-by-step pressure degassing is as follows: the pressure is slowly increased from 0 to 0.5 MPa and held for 10 minutes; then the pressure is maintained at 0.5 MPa for 20 minutes; finally, the pressure is increased to 1.5 MPa and held for 20-30 minutes. The specific procedure for segmented curing is as follows: In the first stage, the temperature is raised to 120℃ and held for 1 hour; Upon heating, the resin viscosity further decreases, achieving optimal fluidity. Under pressure, it flows fully and further impregnates the fiber. At this point, the cyanate ester begins initial trimerization, and the allyl groups of BMI begin addition reactions with the maleimide double bonds, transforming the system from a viscous flow state to a gel state.

[0019] The second stage involves raising the temperature to 180℃ and maintaining it for 2 hours. The reaction accelerates, the molecular chains rapidly cross-link, forming a three-dimensional network structure (gel), and the composite material takes shape. This stage is when strength is formed.

[0020] The third stage involves raising the temperature to 220℃ and maintaining it for 3 hours. This allows unreacted monomers and remaining functional groups on the polymer chain to react fully, achieving an extremely high crosslinking density.

[0021] The fourth stage involves cooling down to 80°C at a rate not exceeding 3°C / min; slow cooling releases internal stress. Due to the different thermal expansion coefficients of fibers and resins, as well as the constraints of the mold, rapid cooling can lead to huge internal stress, causing parts to warp or even crack.

[0022] The pressure is 0.6-0.8 MPa throughout the process.

[0023] Preferably, in S4, the surface roughness Ra of the fairing composite material is ≤1.6μm.

[0024] Preferably, in step S5, the thickness of the special adhesive film coated on the mating surface is 0.1 mm; the thickness of the copper conductive lap sheet is 0.3 mm to ensure that the resistance is ≤0.1Ω; and the outer surface is polished to Ra≤0.8μm.

[0025] Compared with the prior art, the beneficial effects of the present invention are: 1. Traditional BMI resins have high crosslinking density and high molecular chain rigidity, resulting in inherent brittleness and low impact strength (e.g., Comparative Example 1, only 65kJ / m). 2 Conventional toughening methods (such as adding rubber) often come at the cost of sacrificing heat resistance and modulus.

[0026] This invention employs a copolymerization of rigid 4,4'-diphenylmethane-type BMI and flexible aliphatic BMI. While maintaining a high crosslinking density, flexible segments are introduced into the molecular chain, enhancing intrinsic toughness. The allyl group of eugenol allyl polysiloxane participates in curing, forming chemical bonds with the resin; while its polysiloxane segments, due to compatibility differences, form nanoscale "soft phase" particles in situ during curing. These uniformly dispersed elastic particles act as stress concentration points, effectively inducing and terminating crazes and promoting shear band yielding, absorbing a large amount of impact energy, thereby achieving ultra-high impact strength (95 kJ / m). 2 This invention achieves both ultra-high flexural strength (990 MPa) and ultra-high impact strength (95 kJ / m²) without sacrificing strength or modulus due to chemical bonding. 2 This breaks the traditional constraint of BMI resin being "high strength inevitably leads to brittleness," significantly improving the fairing's resistance to foreign object impacts and dynamic loads.

[0027] 2. The halogenated flame retardants used in the existing technology are toxic, and the phosphorus / nitrogen-based flame retardants often reduce thermal stability and increase smoke production.

[0028] This invention achieves a balance between "high-efficiency flame retardancy, low smoke and low toxicity, and high char residue" through a condensed-phase char formation flame retardant mechanism. Polysiloxane migrates to the material surface at high temperatures, forming a dense, continuous, and highly graphitized expanded char layer together with the carbonaceous material produced by resin decomposition. This char layer possesses triple functions of heat insulation, oxygen isolation, and smoke suppression. The dense char layer effectively prevents the overflow of internal combustible decomposition products and the feedback of heat, physically suffocating the flame.

[0029] 3. To meet wave transmission requirements, existing technologies typically use fiberglass or ordinary epoxy resin for radomes, which have poor mechanical properties and heat resistance. Introducing metal mesh or coatings to achieve lightning protection would increase weight, affect wave transmission, or introduce interface problems.

[0030] This invention reduces the polarizability and dipole loss of the material at the molecular level through fluorination modification (introducing -CF3 groups) and the use of cyanate esters. This results in an extremely low dielectric constant (2.7) and dielectric loss (0.0035) in the matrix resin, providing an ideal medium for the transmission of high-frequency electromagnetic waves. A T300 nickel-plated fabric is used on the outermost layer of the structure to form a complete conductive surface layer for dissipating lightning current, while the inner composite layers and resin matrix ensure electromagnetic wave penetration. This partitioned design of "conductive surface, transparent interior" cleverly resolves functional conflicts. The material of this invention simultaneously possesses high wave transmittance, effective lightning strike conductivity, and ultra-high mechanical properties as a main load-bearing structure. This is unmatched by traditional E-glass fiber / epoxy resin wave-transparent materials (poor mechanical properties) or surface-sprayed conductive coatings (easy to peel off, poor reliability).

[0031] 4. In the existing technology, high heat-resistant resins are often accompanied by processing difficulties and high thermal stress risks.

[0032] This invention utilizes a modified BMI / cyanate ester interpenetrating network to form a highly stable three-dimensional cross-linked structure. The presence of high-valence bonds such as Si-O contributes to the material's extremely high thermal decomposition temperature (Td5 up to 492℃). By using high-modulus carbon fibers (T700) and optimizing the layup (primarily in the 0° direction), the composite material's coefficient of thermal expansion in the fiber direction changes from resin-dominated to fiber-dominated. Strong interfacial bonding ensures effective constraint of the fiber on the resin's thermal expansion, thereby achieving an extremely low coefficient of thermal expansion (1.8 × 10⁻⁶) approaching that of metals. -6 / ℃). When subjected to severe aerodynamic heating and external temperature differences, the fairing can maintain the material itself without decomposition or softening, and can also maintain extremely high dimensional stability, avoiding deformation, cracking or failure of connection with the main body of the aircraft due to thermal stress.

[0033] In summary, this invention systematically solves the key contradictions of high-performance fairing materials through a molecular synergistic design of "fluorination modification-polysiloxane toughening-cyanate interpenetration": achieving ultra-high strength and toughness simultaneously by utilizing in-situ nano-microphase separation; achieving efficient halogen-free flame retardancy through catalytic generation of a dense graphite carbon layer; and cleverly unifying high wave transmission and lightning protection functions through molecular fluorination and surface metallization partitioning design. Ultimately, a new generation of aircraft fairing composite materials with load-bearing capacity, wave transmission, heat resistance, impact resistance, and environmental stability was successfully prepared. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the fairing structure produced by the present invention; Figure 2 This is a schematic diagram of the internal reinforcing ribs of the fairing of the present invention; Figure 3 This is a schematic diagram of the final section of the fairing structure of the present invention. Detailed Implementation

[0035] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0036] Preparation Example 1: Preparation of modified bismaleimide resin: 4,4'-diphenylmethane-type bismaleimide and aliphatic bismaleimide were dried in a vacuum drying oven at 60±5℃ for 4 hours; 2-trifluoromethyl-p-phenylenediamine was ground and passed through a 100-200 mesh sieve. Add 4,4'-diphenylmethane bismaleimide and aliphatic bismaleimide to a reactor equipped with a nitrogen protection device, heat to 135-145℃, and stir at 300-400r / min for 30min until completely melted. 2-Trifluoromethyl-p-phenylenediamine and eugenol allyl polysiloxane were added sequentially, the temperature was raised to 145-155℃, the stirring speed was increased to 400-500 r / min, and the reaction was carried out for 50 min; then diallyl 2,5-furandicarboxylate was added, the temperature was maintained at 145-155℃, and the stirring was continued for 10 min to obtain the modified bismaleimide prepolymer. The modified bismaleimide prepolymer was transferred to a vacuum degassing machine and degassed for 30 minutes at a pressure of -0.095 MPa to obtain the modified bismaleimide resin.

[0037] The modified bismaleimide prepolymer has a viscosity of 0.6-0.8 Pa·s at 150°C, and the mass ratio of 4,4'-diphenylmethane bismaleimide, aliphatic bismaleimide, 2-trifluoromethyl-p-phenylenediamine, and eugenol allyl polysiloxane is 45:25:10:12.

[0038] The 2-trifluoromethyl-p-phenylenediamine has a fluorine content of ≥35% and a melting point of 88-90℃; the eugenol allyl polysiloxane has a number average molecular weight of 3000-5000 and a double bond content of 8%.

[0039] Preparation Example 2: The preparation method and raw materials are the same as in Example 1, but the mass ratio of 4,4'-diphenylmethane bismaleimide, aliphatic bismaleimide, 2-trifluoromethyl-p-phenylenediamine, and eugenol allyl polysiloxane in the modified bismaleimide resin is 50:20:12.5:10.

[0040] Preparation Example 3: The preparation method and raw materials are the same as in Example 1, but the mass ratio of 4,4'-diphenylmethane bismaleimide, aliphatic bismaleimide, 2-trifluoromethyl-p-phenylenediamine, and eugenol allyl polysiloxane in the modified bismaleimide resin is 55:15:15:8.

[0041] Example 1: The integrated molding process for the aircraft fairing composite material is characterized by the following steps: S1. Prepreg Preparation ① Resin blending: Modified bismaleimide resin and cyano resin in a mass ratio of 4:6 were added to a reactor and stirred at 200 r / min for 30 min at 80 °C under nitrogen protection; then 0.5% of the total resin mass of curing accelerator was added and stirring was continued for 15 min. ②Preparation of prepreg: The blended resin was coated onto release paper using a hot-melt method; T300 metallized fabric, T300 ordinary fabric, and T700 unidirectional tape were impregnated separately; the resin content was controlled as follows: 45±2% for fabrics and 38±2% for unidirectional tape. ③ Cutting and layout: According to the layup design of each section, the prepreg is cut using a CNC cutting machine; a 30mm flange allowance is reserved for the prepreg in the butt rib area; the metal-plated area is marked separately; S2, Segmented Molding Preheat the mold to 60℃ and spray the release agent evenly; position and install the titanium alloy interface embedded parts, with the gap controlled ≤0.05mm; Front section: Layers 1-3, T300 metallized fabric laid at 0°; Layers 4-25, T700 unidirectional tape laid in a cyclic pattern at 0° / 45° / -45° / 90°; Layers 26-28, T300 ordinary fabric laid at 0°. Middle section: Automated tape laying machine lays up 20 layers in total, with the same layup structure as the previous section; Final section: Made of resin transfer molding, the preform has a 3D woven structure; S3, Curing Process Front / Middle Section: Stepped pressure defoaming and segmented curing; Final stage: Resin transfer molding injection curing, mold temperature 130℃, injection pressure 4MPa; curing procedure is the same as the previous stage, nitrogen flow rate 0.8-1.2L / min; S4. Demolding and finishing Demolding is performed when the temperature drops below 60℃ to obtain a rough fairing composite material. The mating surfaces are then machined, the integrity of the reinforcing ribs is checked, and local repairs are made if necessary. S5, Assembly Process Each joint surface is sandblasted; the flanged area of ​​the metal plating layer is cleaned to ensure good electrical contact; locating pins are installed, and the fit clearance is checked to be ≤0.05mm; Co-curing bonding technology is adopted, and a special adhesive film is applied to the mating surfaces; after assembly, a pressure of 0.2MPa is applied, and it is cured at 120℃ for 1 hour in a thermostatic precipitator; vacuum-assisted bonding is used at the flanged and wrapped areas to ensure no air bubbles. Conductive adhesive is used to fill the gaps at the docking ribs; copper conductive lap plates are installed; and the outer surface is ground and polished to obtain the aircraft fairing.

[0042] In S1, the cyano resin is bisphenol E cyanate.

[0043] In S3, the specific procedure for step-pressure degassing is as follows: the pressure is slowly increased from 0 to 0.5 MPa and held for 10 minutes; then the pressure is maintained at 0.5 MPa for 20 minutes; finally, the pressure is increased to 1.5 MPa and held for 20-30 minutes. The specific procedure for segmented curing is as follows: the first stage is to raise the temperature to 120℃ and hold for 1 hour; the second stage is to raise the temperature to 180℃ and hold for 2 hours; the third stage is to raise the temperature to 220℃ and hold for 3 hours; the fourth stage is to cool down to 80℃ at a rate not exceeding 3℃ / min; and a pressure of 0.6-0.8MPa is applied throughout the process.

[0044] In S4, the surface roughness Ra of the fairing composite material is ≤1.6μm.

[0045] In step S5, the thickness of the special adhesive film coated on the mating surface is 0.1 mm; the thickness of the copper conductive lap piece is 0.3 mm to ensure that the resistance is ≤0.1Ω; and the outer surface is polished to Ra≤0.8μm.

[0046] like Figure 1 As shown, the fairing is a long U-shaped composite structure, which is formed separately from the front, middle and rear sections and then assembled. The interface with the main body is made of titanium alloy to ensure the interface size fits.

[0047] like Figure 2 As shown, the front, middle and rear sections of the product are equipped with multiple reinforcing ribs to improve the structural stability of the product and prevent product deformation.

[0048] like Figure 3 As shown, the final section uses a tooling structure to ensure that the composite material is integrally formed, replacing the original titanium alloy metal tail cap for installation, saving time and cost.

[0049] Example 2 The preparation method and raw materials are the same as in Example 1, but the mass ratio of modified bismaleimide resin to cyano resin is 3.5:6.5.

[0050] Example 3 The preparation method and raw materials are the same as in Example 1, but the mass ratio of modified bismaleimide resin to cyano resin is 3:7.

[0051] Based on this, the following design was also created: Comparative Example 1: Same formulation and experimental method as Example 2, but without the use of modified bismaleimide resin; Comparative Example 2: The formulation and experimental methods were the same as in Example 2, but only aliphatic bismaleimide resin was used; Comparative Example 3: The formulation and experimental method were the same as those in Preparation Example 2, but the mass ratio of 4,4'-diphenylmethane bismaleimide, aliphatic bismaleimide, 2-trifluoromethyl-p-phenylenediamine, and eugenol allyl polysiloxane was 50:50:12.5:10. Comparative Example 4: The formulation and experimental methods were the same as in Example 2, but the mass ratio of 4,4'-diphenylmethane bismaleimide, aliphatic bismaleimide, 2-trifluoromethyl-p-phenylenediamine, and eugenol allyl polysiloxane was 50:20:5:10.

[0052] According to GB / T3354-2014 "Test Method for Tensile Properties of Carbon Fiber Reinforced Plastics", GB / T3356-2014 "Test Method for Bending Properties of Carbon Fiber Reinforced Plastics", GB / T3357-2014 "Test Method for Interlaminar Shear Strength of Carbon Fiber Reinforced Plastics", GB / T1451-2005 "Impact Test Method for Simply Supported Beams of Fiber Reinforced Plastics", GB / T26743-2011 "Test Method for Compression Properties of Cylindrical Components of Fiber Reinforced Plastics", HB7736.10-2004 "Test Methods for Aerospace Composite Structural Components - Part 10: Radial Load Test for Cylindrical Components", GJB150.16A-2009 "Laboratory Environmental Test Methods for Military Equipment - Part 16: Vibration Test", GB / T The test results were obtained using standards such as GB / T 27761-2011 "Determination of Thermal Stability of Solid Materials by Thermogravimetric Analysis", GB / T 10125-2021 "Civilized Atmosphere Corrosion Test - Salt Spray Test", GB / T 5597-2019 "Determination of Dielectric Constant and Dielectric Loss Tangent of Fiber Reinforced Plastics", GJB 2699-1996 "Test Method for Transmittance Performance of Military Radomes", and GB / T 15738-2008 "Test Method for Resistivity of Conductive and Antistatic Fiber Reinforced Plastics". The tensile strength, flexural strength, interlaminar shear strength, impact strength, axial compression performance, radial load performance, vibration performance, thermal stability, corrosion resistance, dielectric properties, transmittance, and conductivity of this invention were tested. The test data for each performance are summarized in Table 2. Table 1. Performance test data of the aircraft fairing In addition, referring to GB / T2572-2005 "Test Method for Average Linear Expansion Coefficient of Fiber Reinforced Plastics", the thermal expansion coefficients of T300 nickel-plated plain weave fabric impregnation material, T700 unidirectional tape impregnation material, and T300 ordinary plain weave fabric impregnation material in Example 2 were measured, as shown in Table 2: Table 2. Coefficient of thermal expansion of fiber-reinforced resins Data analysis shows that: The introduction of 2-trifluoromethyl-p-phenylenediamine, with its strongly electron-withdrawing trifluoromethyl group (-CF3), significantly reduces the molecular polarizability of the resin system. This is the physicochemical basis for achieving low dielectric constant (Dk) and low dielectric loss (Df), with Dk = 2.7 and Df = 0.0035 in Example 3.

[0053] Low dielectric loss means that the energy dissipation inside the material is extremely low when radar waves (such as the X-band) pass through, thus achieving a transmittance of up to 94%. This not only ensures the detection efficiency of the radar, but more importantly, it avoids the temperature rise of the cover caused by the conversion of electromagnetic energy into heat energy.

[0054] Maintaining a low temperature rise state allows the material to operate within its designed thermal stability window for extended periods, thereby indirectly ensuring and highlighting its intrinsically high thermal decomposition temperature (Td5 as high as 492℃). The high bond energy of the CF bonds also directly contributes to Td5.

[0055] Comparative Examples 1, 2, and 4, lacking this fluorination modification, exhibited significantly deteriorated dielectric properties, with Dk > 3.4 and Df > 0.0068. This resulted in more severe heat accumulation during the microwave transmission process, which would exacerbate thermal aging under actual operating conditions, thus preventing the full utilization of the heat resistance potential of the matrix resin.

[0056] Eugenol allyl polysiloxane ensures interfacial strength through chemical bonding between its allyl groups and the resin network; its flexible polysiloxane segments significantly absorb impact energy and directly enhance impact strength (up to 95 kJ / m) by inducing microcracks and shear bands. 2 ).

[0057] This robust interfacial bond is also the basis for the interlayer shear strength (51 MPa) and radial load-bearing capacity (45 kN), ensuring that the load can be effectively transferred from the resin to the high-strength fibers.

[0058] Under high temperature or flame conditions, polysiloxane migrates to the surface and synergistically catalyzes the formation of an expanded, dense, and highly graphitized stable carbon layer (confirmed by Raman spectroscopy). This carbon layer acts as a robust "heat shield," not only giving the material excellent flame retardancy (significantly reducing PHRR and THR), but more importantly, protecting the internal fiber-reinforced skeleton from direct damage at high temperatures.

[0059] Therefore, after high temperature or combustion, the remaining mechanical properties of the material are retained to the greatest extent. This is consistent with the inherent logic of high char residue and high strength retention (>90%) after salt spray testing—that is, by forming a protective layer, it resists the erosion of the main structure by the environment (heat, corrosion).

[0060] Comparative Examples 1 and 2, lacking this component, exhibited significantly reduced toughness, flame retardancy, and mechanical properties. Comparative Examples 3 and 4, due to improper proportions, resulted in poor compatibility, causing macroscopic agglomeration of the polysiloxane. This not only failed to effectively toughen the material (leading to decreased impact strength), but these large agglomerates also became stress defect points, compromising the integrity of the mechanical properties.

[0061] The blending of 4,4'-diphenylmethane type BMI (rigid) and aliphatic BMI (flexible) gives the resin matrix both high modulus and moderate toughness.

[0062] This optimized matrix forms a strong and tough interface with the surface-treated T700 carbon fibers. The strong interface acts as an "amplifier" of the composite material's properties, ensuring that stress is transferred from the resin to each high-modulus fiber without loss when subjected to tensile, bending, and compressive loads.

[0063] It is this high load transfer efficiency that allows the ultra-high theoretical strength of T700 fiber to be fully reflected in macroscopic components, ultimately transforming into the ultra-high tensile strength (>665MPa), flexural strength (>920MPa), and axial compressive strength (>610MPa) shown in Examples 1-3.

[0064] Furthermore, the fiber orientation (0°) has an extremely low coefficient of thermal expansion (1.8 × 10⁻⁶). -6 The high modulus fiber ( / ℃) is precisely the result of the high-modulus fiber "constraining" the thermal expansion of the resin through a strong interface, which ensures the dimensional stability of the fairing under harsh temperature conditions.

[0065] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. An aircraft fairing, characterized in that, It comprises a front section, a middle section, and a rear section connected in sequence, and has an overall long U-shaped streamlined structure; the front section, middle section, and rear section are formed and then assembled and connected. The front section has a large curvature and is equipped with thick reinforcing ribs inside; the middle section is mainly straight and is equipped with thick reinforcing ribs inside; the rear section is connected to the main body of the aircraft and integrates the original titanium alloy tail cover function. The front, middle and rear sections are all equipped with docking ribs and adopt a stepped overlapping design; the installation interface with the main body of the aircraft adopts a titanium alloy embedded part, which is integrally formed with the composite material; all docking surfaces are equipped with positioning pin holes, and the fitting accuracy is controlled within ±0.1mm. The front, middle, and rear sections all employ a multi-layered composite material structure, which, from the outside to the inside, consists of: Outer layer: T300 nickel-plated plain weave fabric impregnation material; Intermediate layer: T700 unidirectional belt impregnated material, laid in 0° / ±45° / 90° directions; Inner layer: T300 ordinary plain weave fabric impregnated material; The docking rib area is reinforced with two layers of ±45° T700 unidirectional tape; the mounting interface is reinforced with a local 3D woven prefabricated structure. The impregnation material is obtained by impregnating T300 nickel-plated plain weave fabric, T700 unidirectional tape, and T300 ordinary plain weave fabric with prepreg. The prepreg is obtained by blending modified bismaleimide and cyano resin in a mass ratio of 3-4:6-7.

2. The aircraft fairing according to claim 1, characterized in that, At the joints between the first section and the main body, and at the joints between the first section and the middle section, a metal-plated layer is applied from the inner edge and then rolled over to cover the outer edge; the joints between the last section and the middle section are also rolled over.

3. The aircraft fairing according to claim 1, characterized in that, Preparation of the modified bismaleimide resin: 4,4'-Diphenylmethane-type bismaleimide and aliphatic bismaleimide were dried in a vacuum drying oven at 60±5℃ for 3-5 hours; 2-trifluoromethyl-p-phenylenediamine was ground and passed through a 100-200 mesh sieve. Add 4,4'-diphenylmethane bismaleimide and aliphatic bismaleimide to a reactor equipped with a nitrogen protection device, heat to 135-145℃, and stir at 300-400r / min for 25-35min until completely melted. 2-Trifluoromethyl-p-phenylenediamine and eugenol allyl polysiloxane were added sequentially, the temperature was raised to 145-155℃, the stirring speed was increased to 400-500 r / min, and the reaction was carried out for 25-35 min to obtain the modified bismaleimide prepolymer. The modified bismaleimide prepolymer was transferred to a vacuum degassing machine and degassed for 25-35 minutes under a pressure of -0.092 to -0.098 MPa to obtain the modified bismaleimide resin.

4. The aircraft fairing according to claim 3, characterized in that, The modified bismaleimide prepolymer has a viscosity of 0.6-0.8 Pa·s at 150°C; the mass ratio of 4,4'-diphenylmethane bismaleimide, aliphatic bismaleimide, 2-trifluoromethyl-p-phenylenediamine, and eugenol allyl polysiloxane is 45-55:15-25:10-15:8-12.

5. The aircraft fairing according to claim 3, characterized in that, The 2-trifluoromethyl-p-phenylenediamine has a fluorine content of ≥35% and a melting point of 88-90℃; the eugenol allyl polysiloxane has a number average molecular weight of 3000-5000 and a double bond content of 7%-9%.

6. The integral molding process of the aircraft fairing composite material according to any one of claims 1-5, characterized in that, Includes the following steps: S1. Prepreg Preparation ① Resin blending: Modified bismaleimide resin and cyano resin were added to a reactor and stirred at 200 r / min for 30 min at 80 °C under nitrogen protection; then 0.5% of the total resin mass of curing accelerator was added and stirring was continued for 15 min. ②Preparation of prepreg: The blended resin was coated onto release paper using a hot-melt method; T300 metallized fabric, T300 ordinary fabric, and T700 unidirectional tape were impregnated separately; the resin content was controlled as follows: 45±2% for fabrics and 38±2% for unidirectional tape. ③ Cutting and layout: According to the layup design of each section, the prepreg is cut using a CNC cutting machine; a 30mm flange allowance is reserved for the prepreg in the butt rib area; the metal-plated area is marked separately; S2, Segmented Molding Preheat the mold to 60℃ and spray the release agent evenly; position and install the titanium alloy interface embedded parts, with the gap controlled ≤0.05mm; Front section: Layers 1-3, T300 metallized fabric laid at 0°; Layers 4-25, T700 unidirectional tape laid in a cyclic pattern of 0° / 45° / -45° / 90°; Layers 26-28, T300 ordinary fabric laid at 0°. Middle section: Automated tape laying machine lays up 20 layers in total, with the same layup structure as the previous section; Final section: Made of resin transfer molding, the preform has a 3D woven structure; S3, Curing Process Front / Middle Section: Stepped pressure defoaming and segmented curing; Final stage: Resin transfer molding injection curing, mold temperature 130℃, injection pressure 4MPa; curing procedure is the same as the previous stage, nitrogen flow rate 0.8-1.2L / min; S4. Demolding and finishing Demolding is performed when the temperature drops below 60℃ to obtain a rough fairing composite material. The mating surfaces are then machined, the integrity of the reinforcing ribs is checked, and local repairs are made if necessary. S5, Assembly Process Each joint surface is sandblasted; the flanged area of ​​the metal plating layer is cleaned to ensure good electrical contact; locating pins are installed, and the fit clearance is checked to be ≤0.05mm; Co-curing bonding technology is adopted, and a special adhesive film is applied to the mating surfaces; after assembly, a pressure of 0.2MPa is applied, and it is cured at 120℃ for 1 hour in a thermostatic precipitator; vacuum-assisted bonding is used at the flanged and wrapped areas to ensure no air bubbles. Conductive adhesive is used to fill the gaps at the docking ribs; copper conductive lap plates are installed; and the outer surface is ground and polished to obtain the aircraft fairing.

7. The molding process for an aircraft fairing according to claim 6, characterized in that, In S1, the cyano resin is bisphenol E cyanate.

8. The molding process for an aircraft fairing according to claim 6, characterized in that, In S3, the specific procedure for step-pressure degassing is as follows: the pressure is slowly increased from 0 to 0.5 MPa and held for 10 minutes; then the pressure is maintained at 0.5 MPa for 20 minutes; finally, the pressure is increased to 1.5 MPa and held for 20-30 minutes. The specific procedure for segmented curing is as follows: the first stage is to raise the temperature to 120℃ and hold for 1 hour; the second stage is to raise the temperature to 180℃ and hold for 2 hours; the third stage is to raise the temperature to 220℃ and hold for 3 hours; the fourth stage is to cool down to 80℃ at a rate not exceeding 3℃ / min; and a pressure of 0.6-0.8MPa is applied throughout the process.

9. The aircraft fairing and its molding process according to claim 6, characterized in that, In S4, the surface roughness Ra of the fairing composite material is ≤1.6μm.

10. The aircraft fairing and its molding process according to claim 6, characterized in that, In step S5, the thickness of the special adhesive film coated on the mating surface is 0.1 mm; the thickness of the copper conductive lap piece is 0.3 mm to ensure that the resistance is ≤0.1Ω; and the outer surface is polished to Ra≤0.8μm.

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