A special forming process for helicopter composite hat-shaped fairings

Abstract

The present application relates to the field of aviation manufacturing technology, and discloses a special forming process for a helicopter composite material cap-shaped fairing, which comprises the following steps: carbon fibers and a toughened bisphenol A epoxy resin system are used to prepare a prepreg sheet; a layer structure is laid based on a warping prevention criterion, wherein the layer structure is symmetrical about the thickness of the component and the fiber angle is gradiently distributed; and vacuum packaging is performed; the layer structure component is placed on a rigid core mold for medium-temperature low-pressure pre-compaction to discharge the gas between the fiber layers; then the component is transferred into a hot press tank for temperature rising and pressure increasing for curing; during the process, a synergistic calculation model combining a basic reaction and heat conduction compensation is used to determine the holding time of the constant temperature and constant pressure stage; after the component is demolded, edge micro-grinding and combined finishing are performed under the control of a water cooling and closed-loop force feedback system. The present application improves the fracture toughness of the resin matrix, reduces the internal porosity and the warping deformation after demolding, and simultaneously inhibits the surface thermal damage and micro-crack defects caused by subsequent mechanical processing.

Description

Technical Field

[0001] This invention relates to the field of aerospace manufacturing technology, specifically to a special molding process for a helicopter composite material cap-shaped fairing. Background Technology

[0002] Composite materials, due to their high specific strength and strong designability, have been widely used in the manufacture of helicopter airframe structures. As a typical irregularly shaped component, the dome-shaped fairing needs to meet the requirements of high-precision external aerodynamic shape while withstanding complex aerodynamic and vibration loads. The quality of the molding process directly affects the final service performance of the component.

[0003] Currently, the mainstream manufacturing method for such composite material parts mostly adopts the traditional process of prepreg manual lay-up combined with autoclave curing. The process involves laying prepreg layer by layer and sealing it in vacuum bags, applying temperature and pressure in the autoclave to promote resin flow, cross-linking and curing, and expelling volatiles to obtain composite material products with a certain strength.

[0004] As helicopter platforms place increasing demands on the curvature complexity and mechanical performance of fairings, the limitations of traditional processes are becoming increasingly apparent. Hat-shaped fairings feature continuous double-curvature surfaces and localized sharp-angle transitions, making it difficult for fibers to fully adhere to the mold surface during conventional layup processes. Fiber bridging or wrinkling can occur at bends and corners. Stress transmission in anisotropic materials in non-uniform curvature regions can be distorted, increasing the potential risk of interlaminar delamination. During the hot-pressing curing stage, if there are deviations in the initial layup, the flowing resin cannot effectively fill the microcavities formed by fiber misalignment. Furthermore, mismatched vacuum and pressurization timing can lead to resin loss or localized resin-rich or resin-poor conditions, resulting in high internal porosity and weakening overall interlaminar shear strength and fatigue life.

[0005] Traditional linear operation modes are difficult to meet the comprehensive requirements of manufacturing precision and performance for complex irregular-shaped components. How to solve the technical problems such as uneven ply stress, high internal porosity and insufficient dimensional stability in irregular areas while ensuring the mechanical properties and aerodynamic precision of the components has become an urgent technical problem to be solved in this field. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a special molding process for helicopter composite material cap-shaped fairings, which solves the problems of uneven ply stress, high internal porosity, insufficient dimensional accuracy, and low production efficiency that exist when existing molding processes are adapted to the complex curved surfaces of cap-shaped fairings.

[0007] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a special molding process for helicopter composite material cap-shaped fairings, comprising the following steps: A unidirectional prepreg was prepared using a carbon fiber and toughened bisphenol A epoxy resin system, and then cut according to the three-dimensional geometric model of the fairing using a CNC cutting equipment to obtain prepreg sheets; In a controlled temperature and humidity environment, a functional gradient structure symmetrical about the center plane of the component thickness is laid based on the thermosetting anti-warping principle. Along the thickness direction of the component, from both sides to the middle, it is divided into an outer layer area composed of fibers in the 90° direction, a middle layer area composed of alternating +45° and -45° direction fibers, and an inner layer area composed of fibers in the 0° direction. After the layup is completed, it is sealed in a vacuum bag and vacuumed. The encapsulated plywood component is placed on a rigid aluminum alloy mandrel, and after purging, a medium-temperature and low-pressure pre-compaction process is adopted. The pre-compacted components are transferred into a horizontal autoclave and cured by heating and pressurizing using a three-stage temperature and pressure control strategy. The heat preservation time in the constant temperature and pressure stage is determined by a collaborative calculation model that combines the basic chemical reaction time with the heat conduction compensation time. The vacuum degree is maintained and protective gas is introduced throughout the process. After demolding, the fairing components undergo micro-grinding and final assembly. The micro-grinding is carried out under water cooling and closed-loop force feedback control to suppress surface thermal damage and machining damage.

[0008] By adopting the above technical solution, due to the use of toughened bisphenol A epoxy resin system, the resin matrix will form a micro-phase separation structure during the curing process, which helps to improve the interlaminar shear strength and fracture toughness of the composite material; the gradient symmetrical layup with different fiber angles along the thickness direction can achieve a certain degree of balance in the internal stress field of the material, alleviate the residual stress caused by anisotropic shrinkage during the curing process, and thus reduce the amount of warping deformation after demolding of the component. A medium-temperature, low-pressure pre-compaction process is introduced before formal curing to give the resin a certain degree of fluidity before gelation. This allows for the expulsion of air and volatiles trapped between prepreg layers, reducing the porosity of the product. Simultaneously, a collaborative computational model is used to control temperature-pressure curing and force-feedback water-cooled grinding, ensuring good component molding quality in terms of both curing degree and dimensional accuracy, and suppressing heat diffusion and mechanical peeling defects. Therefore, this solution aims to obtain a cap-shaped fairing component with good dimensional stability, relatively few internal defects, and the required mechanical load-bearing capacity.

[0009] Preferably, the step of preparing a unidirectional prepreg using a carbon fiber and toughened bisphenol A epoxy resin system, and then cutting it according to the three-dimensional geometric model of the fairing using a CNC cutting device to obtain a prepreg sheet includes: preparing a unidirectional prepreg using carbon fiber and the toughened bisphenol A epoxy resin system, controlling the fiber areal density to be 150 to 200 g / m³. 2 The density deviation is -5 to 5 g / m³2 The resin content is 38% to 42%; the fairing is automatically cut according to the three-dimensional geometric model by a five-axis linkage CNC cutting equipment, so that the fiber direction deviation of the material sheet is -1° to 1°, the dimensional tolerance is -0.1 to 0.1mm, and a trimming allowance of 0.5 to 1.0mm is reserved at the edge.

[0010] By adopting the above technical solution, limiting the areal density and resin content of the prepreg within a certain range helps to ensure the thickness uniformity of the laminated structure; while the CNC five-axis automatic cutting limits the geometric accuracy of the fiber orientation, which largely prevents the angular deviation caused by manual cutting, thereby avoiding the resulting local stress concentration and weakening of mechanical properties.

[0011] Preferably, the toughened bisphenol A epoxy resin system is made from raw materials comprising the following parts by weight: bisphenol A epoxy resin: 90 to 110 parts; polyethersulfone powder: 15 to 25 parts; dicyandiamide: 6 to 10 parts; organic urea accelerator: 1 to 3 parts.

[0012] By adopting the above technical solution, the curing, crosslinking, and phase separation process of the resin system under thermal action is roughly as follows: During the heating stage, the organic urea accelerator undergoes thermal decomposition, activating the active hydrogen in the dicyandiamide molecule structure to produce nucleophilicity, which then attacks the epoxy groups at the ends of the bisphenol A type epoxy resin molecular chains, undergoing a ring-opening addition reaction to generate secondary amines and hydroxyl groups; as the ambient temperature increases, the generated secondary amine further reacts with the residual epoxy groups in the system to generate tertiary amines, initially forming a cross-linked network, while some of the generated hydroxyl groups undergo etherification reactions with epoxy groups to form a three-dimensional polymer cross-linked structure; As this cross-linked network develops, the molecular weight of the system increases with the increase in the degree of cross-linking of the epoxy resin. The solubility of the polyethersulfone component initially uniformly dissolved in the epoxy resin decreases, thereby triggering a reaction-induced phase separation process. Finally, polyethersulfone precipitates from the epoxy resin-enriched phase, forming a phase separation morphology of bicontinuous phase or island structure at the microscopic level. When the composite material is subjected to external load and cracks are generated, the crack tip often deflects or bifurcates when it encounters the high-toughness polyethersulfone microphase region, so as to consume the elastic strain energy required for crack propagation and play a role in improving the impact toughness of the resin matrix.

[0013] Preferably, the steps for preparing the toughened bisphenol A epoxy resin system include: adding the bisphenol A epoxy resin to a reactor equipped with mechanical stirring and vacuum functions, and heating it to 120 to 130°C; slowly adding polyethersulfone powder, and stirring at a speed of 300 to 400 r / min for 1.5 to 2.5 h at a constant temperature until the polyethersulfone is completely dissolved, forming a transparent and homogeneous mixture; cooling the reactor system to 60 to 70°C, and adding dicyandiamide and organic urea accelerators; dispersing and grinding the mixture using a three-roll mill, controlling the roller heating temperature at 50 to 70°C, and grinding 3 to 5 times until the fineness of the solid particles in the resin system is no greater than 5 μm.

[0014] By adopting the above technical solution, the physical entanglement between polyethersulfone molecular chains is destroyed by high temperature environment, and mechanical shear force is used to promote dispersion and dissolution in epoxy resin. Considering the safety of the reaction, the curing agent and accelerator are added after cooling, which can avoid premature polymerization and excessive reaction in the resin system. In addition, the use of three-roll milling can appropriately reduce the fineness of solid particles and increase the specific surface area of ​​solid components such as dicyandiamide, which to a certain extent helps to improve the reactivity and dispersion uniformity of the curing agent in the resin system.

[0015] Preferably, the total number of layers in the functionally graded structure is 40 to 80; the specific layer ratio of the inner, middle and outer regions in the graded layer structure is customized according to the fairing service load spectrum, and the inner region accounts for 12% to 15% of the total number of layers; the middle region accounts for 50% to 60% of the total number of layers; and the outer region accounts for 25% to 30% of the total number of layers.

[0016] By adopting the above technical solution, based on the aerodynamic and vibration load distribution characteristics during the service of the helicopter fairing, the fiber ratio in each direction is specifically allocated. Specifically, the middle layer ±45° fibers are mainly used to bear in-plane shear loads and have the highest proportion to improve the torsional stiffness of the component; the outer layer 90° fibers tend to bear spanwise bending stress, while the inner layer 0° fibers are responsible for coping with axial tensile and compressive stress. This customized functional layup improves the specific strength of the component in a specific direction while also enabling the overall mechanical response to achieve a relatively balanced match.

[0017] Preferably, the step of laying a functional gradient structure symmetrical about the thickness of the component based on the thermosetting anti-warping principle in a controlled temperature and humidity environment, and sealing it in a vacuum bag after the layup is completed includes: laying in an environment with a temperature of 20 to 24°C and a relative humidity of 45% to 55%; using laser projection for positioning and applying constant tension during the layup process, with a constant tension control accuracy of -1 to 1N; after every 3 to 5 layers are laid, using a 55 to 65°C hot air gun for grid-like spot fixing, with a spot fixing area diameter of 5 to 8 mm, a spot fixing spacing of 110 to 130 mm × 90 to 110 mm in the planar area, and a density of 70 to 90 mm × 70 to 90 mm in the corner area; after all the layers are laid, sealing it in a vacuum bag with a double-layer fluororubber film, wherein the inner film directly contacts the surface of the prepreg, the outer film provides a mechanical seal, and a breathable felt and a guide net are set between the two films to optimize the gas exhaust path; vacuuming is performed until the gauge pressure is no higher than -0.098 MPa and maintained for 20 to 30 minutes.

[0018] By adopting the above technical solutions, a constant temperature and humidity environment is used to maintain the resin viscosity of the prepreg; applying constant tension during laying can reduce the probability of prepreg loosening and wrinkling; hot air spot curing softens the interlayer resin locally to generate initial bonding force, fixes the fiber position and prevents interlayer slippage and misalignment during subsequent curing; in addition, with the combination of double-layer fluororubber film encapsulation and the setting of breathable felt and guide net, relatively smooth interlayer and surface gas channels are established, so that the volatiles generated by the curing reaction and trapped air can be discharged into the vacuum pipeline more quickly.

[0019] Preferably, the step of placing the encapsulated plywood on a rigid mandrel, purging it with air, and then pre-compacting it with a medium-temperature, low-pressure process to drive the resin to undergo micro-rheology to expel inter-fiber gas, followed by cooling, includes: placing the encapsulated plywood on an aluminum alloy rigid mandrel with a surface roughness value of no more than 0.8 μm, purging it with nitrogen for 5 to 10 minutes; heating it to 60 to 80°C at a heating rate of 2 to 3°C / min, holding it at that temperature for 10 to 15 minutes, then applying a pressure of 0.1 to 0.2 MPa through a servo electro-hydraulic system, controlling the pressure response delay time to be no more than 0.5 seconds, so that the pressure curve and temperature curve are synchronized in the time domain; holding it at that temperature for 30 to 60 minutes, and then allowing it to cool naturally to a temperature no higher than 50°C before removing it.

[0020] By adopting the above technical solution, when the component is heated to the range of 60 to 80°C, the resin viscosity decreases and enters a micro-rheological state. With the low pressure applied by the servo system, the interlayer gas can be squeezed out without causing excessive resin loss. The low-delay pressure response control ensures that the temperature-induced resin rheology and the pressure application are matched at the right time, so as to avoid the adverse situation of applying pressure too early, which would cause the resin viscosity to be too high and produce local dry spots, or applying pressure too late, which would cause the resin to cross-link prematurely and lose its fluidity.

[0021] Preferably, the step of transferring the pre-compacted component into a horizontal autoclave and using a three-stage temperature and pressure control strategy for heating and pressurizing curing, while maintaining vacuum and introducing protective gas throughout the process, includes: slowly heating to 140 to 160°C at a rate of 2 to 3°C / min, simultaneously applying pressure of 0.4 to 0.6 MPa through a servo electro-hydraulic system and entering the constant temperature and pressure stage, controlling the pressure response delay time to be no more than 0.5 s, so that the pressure curve and temperature curve are synchronized in the time domain; after curing, cooling at a rate of 1 to 2°C / min; maintaining the gauge pressure no higher than -0.095 MPa throughout the process and introducing nitrogen for protection.

[0022] By adopting the above technical solution, controlling the heating rate helps to avoid excessive temperature gradients that could cause internal residual thermal stress. During this period, the rated working pressure is applied to improve the interlayer bonding force, so that the resin matrix can fully impregnate the carbon fiber and fill the micropores. The nitrogen protective atmosphere used can reduce the risk of oxidative degradation reaction on the resin surface under high temperature conditions.

[0023] Preferably, the step of determining the heat preservation time of the constant temperature and pressure stage using a collaborative calculation model combining the basic chemical reaction time and the heat conduction compensation time includes: setting the basic curing heat preservation time corresponding to the basic chemical reaction time to 90 to 120 minutes; calculating the heat conduction compensation time by multiplying the maximum effective thickness of the component by a compensation coefficient of 8 to 10 minutes / mm; and adding the basic curing heat preservation time and the heat conduction compensation time to obtain the total heat preservation time of the constant temperature and pressure stage.

[0024] By adopting the above technical solution, considering that the increase in the thickness of the composite component will change the internal heat conduction path and heat transfer rate, resulting in a time difference in temperature distribution between the core and the surface, the collaborative calculation model supplements the heat conduction time on top of the basic curing reaction time, thereby compensating for the heat conduction lag effect in the thickness direction, so that the resin in the region with the maximum cross-sectional thickness can also reach the preset degree of cross-linking, thereby reducing the problem of local mechanical property deterioration caused by incomplete curing or uneven curing degree distribution.

[0025] Preferably, the steps of performing micro-grinding and combined finishing on the demolded fairing component include: using a diamond grinding wheel under water cooling and closed-loop force feedback control of an integrated force feedback control unit to perform micro-grinding on the edge, monitoring the contact pressure between the grinding wheel and the workpiece in real time, ensuring that the grinding force fluctuation range is -0.5 to 0.5N, the amount of grinding removed is controlled between 0.1 and 0.2mm, and the cooling water temperature is maintained between 20 and 25℃; after grinding, the surface is finished by a combination of 800 to 1000 grit dry grinding and 1200 to 1500 grit wet grinding, controlling the final surface roughness value to be no greater than 0.4μm.

[0026] By adopting the above technical solution, the closed-loop force feedback system dynamically adjusts the grinding pressure to ensure that the amount of material removed is relatively constant and the force is uniform, thus avoiding overcutting and fiber pull-out defects caused by human operation or rigid feed. The water cooling system removes the cutting heat generated by the friction of the diamond grinding wheel, keeping the temperature of the processing area below the glass transition temperature of the resin. This can suppress the substrate coating and thermal degradation caused by thermal softening. Finally, with multi-grade sandpaper polishing, a smooth and clean aerodynamic outer surface can be obtained.

[0027] This invention provides a special molding process for helicopter composite material cap-shaped fairings. It has the following beneficial effects: 1. This invention uses a toughened bisphenol A epoxy resin system to prepare prepreg. By initiating a reaction during the curing process to induce phase separation, polyethersulfone precipitates from the epoxy resin matrix to form microphase regions. When the component is subjected to external loads and internal cracks are generated, these microstructures can cause the crack path to deflect and bifurcate, consuming the strain energy during crack propagation, thereby improving the interlaminar shear strength and fracture toughness of the composite material and improving the problem of the traditional fairing resin matrix being too brittle and cracking.

[0028] 2. Based on the load distribution characteristics of the fairing during service, the present invention lays a functionally gradient symmetrical structure along the thickness direction. At the same time, a medium-temperature and low-pressure pre-compaction process is introduced before hot-press curing, and the hot-press curing process is controlled by a heat conduction compensation time model. This multi-stage molding method balances the residual thermal stress inside the component, alleviates the stress concentration caused by anisotropic shrinkage during curing, and reduces the amount of warpage deformation after demolding of asymmetric geometric components by using pre-compaction to expel interlayer air and volatiles to reduce porosity.

[0029] 3. After demolding, the present invention uses a closed-loop force feedback system combined with water cooling for micro-grinding. By monitoring and dynamically adjusting the contact pressure between the grinding wheel and the workpiece in real time, the cutting force is kept uniform. The water cooling circulation removes the frictional heat from the process. The grinding process avoids fiber pull-out and overcutting defects caused by feed force fluctuations. The temperature of the processing area is controlled below the glass transition temperature of the resin, which inhibits the thermal degradation of the matrix and reduces mechanical damage and surface thermal damage. Attached Figure Description

[0030] Figure 1 A schematic diagram of the three-dimensional structure of a helicopter composite material cap-shaped fairing; Figure 2 This is a schematic diagram of the gradient layering sequence; Figure 3 This is a schematic diagram of the overall process for the special molding technology of helicopter composite material hat-shaped fairing described in this invention; Figure 4The rheological properties of the toughened bisphenol A epoxy resin system of the present invention are shown in the figure, wherein (a) is the viscosity-temperature curve of each resin system and (b) is the viscosity-time curve of each resin system under constant temperature of 70°C. Figure 5 The graphs show the curing reaction kinetics and degree of curing of the present invention, wherein (a) is the differential scanning calorimetry exothermic curve and (b) is the dynamic thermodynamic analysis loss factor temperature curve. Figure 6 This is a curve showing the normal deviation distribution of the main axis centerline of the fairing component of the present invention. Figure 7 This is a graph showing the amplitude distribution of the bottom wave reflection signal from the ultrasonic scan of the fairing component of the present invention. Figure 8 The diagram shows the microscopic characterization curves of the processing quality of the fairing component of the present invention, wherein (a) is the microscopic surface contour distribution curve and (b) is the splitting depth distribution curve along the length direction of the processing edge. Detailed Implementation

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] Please see Figures 1-3 ; The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0033] Unidirectional carbon fiber is selected from T700 grade polyacrylonitrile-based carbon fiber, with tensile strength ≥4900MPa and tensile modulus ≥230GPa. Bisphenol A type epoxy resin uses liquid base resin with an epoxy equivalent of 184 to 194 g / eq and a dynamic viscosity of 11,000 to 15,000 mPa·s at 25°C. The polyethersulfone is a thermoplastic toughening resin with CAS number 9002-88-4, a weight-average molecular weight (Mw) of 45,000 to 55,000 g / mol, and a glass transition temperature of 220 to 230 °C. The dicyandiamide used is a micronized latent curing agent with CAS number 461-58-5 and an average particle size D50 ≤ 5 μm. The organic urea accelerator selected is 1,1'-(4-methyl-1,3-phenylene)bis(3,3-dimethylurea), CAS number 17526-94-2, with an average particle size D50 ≤ 10 μm.

[0034] In this invention, the units of measurement for all materials and solvents are uniformly referred to as parts by weight.

[0035] Preparation Example 1: This preparation example provides a method for preparing a toughened bisphenol A epoxy resin system, including the following steps: 100 parts of bisphenol A epoxy resin were added to a reactor equipped with mechanical stirring and vacuuming functions, and the temperature was raised to 120°C. 15 parts of polyethersulfone powder were slowly added, and the mixture was stirred at 300 r / min for 1.5 h at a constant temperature until the polyethersulfone was completely dissolved, forming a transparent and homogeneous mixture. The reactor system was cooled to 60°C, and 6 parts of dicyandiamide and 1 part of organic urea accelerator were added. The mixture was dispersed and ground using a three-roll mill, with the roller heating temperature controlled at 50°C, and milled 3 times until the fineness of the solid particles in the resin system was no greater than 5 μm. The prepared resin system was frozen and stored at -18°C for later use, and is called toughened bisphenol A epoxy resin system A.

[0036] Preparation Example 2: This preparation example provides a method for preparing a toughened bisphenol A epoxy resin system, including the following steps: 100 parts of bisphenol A epoxy resin were added to a reactor equipped with mechanical stirring and vacuum function, and the temperature was raised to 125°C. 20 parts of polyethersulfone powder were slowly added, and the mixture was stirred at 350 r / min for 2 hours at a constant temperature until the polyethersulfone was completely dissolved, forming a transparent and homogeneous mixture. The reactor system was cooled to 65°C, and 8 parts of dicyandiamide and 2 parts of organic urea accelerator were added. The mixture was dispersed and ground using a three-roll mill, with the roller heating temperature controlled at 60°C, and milled 4 times until the fineness of the solid particles in the resin system was no greater than 5 μm. The prepared resin system was frozen and stored at -18°C for later use, and is called toughened bisphenol A epoxy resin system B.

[0037] Preparation Example 3: This preparation example provides a method for preparing a toughened bisphenol A epoxy resin system, including the following steps: 100 parts of bisphenol A epoxy resin were added to a reactor equipped with mechanical stirring and vacuuming functions, and the temperature was raised to 130°C. 25 parts of polyethersulfone powder were slowly added, and the mixture was stirred at a speed of 400 r / min for 2.5 h at a constant temperature until the polyethersulfone was completely dissolved, forming a transparent and homogeneous mixture. The reactor system was cooled to 70°C, and 10 parts of dicyandiamide and 3 parts of organic urea accelerator were added. The mixture was dispersed and ground using a three-roll mill, with the roller heating temperature controlled at 70°C, and milled 5 times until the fineness of the solid particles in the resin system was no greater than 5 μm. The prepared resin system was frozen and stored at -18°C for later use, and is called toughened bisphenol A epoxy resin system C. Example 1

[0038] This embodiment provides a special molding process for helicopter composite material cap-shaped fairings, including the following steps: S1. A unidirectional prepreg was prepared using T700 grade unidirectional carbon fiber and toughened bisphenol A epoxy resin system A, with the fiber areal density controlled at 150 g / m². 2 And the fluctuation is within ±5g / m 2 The resin content is 38%; the material is automatically cut according to the three-dimensional geometric model of the fairing using a five-axis linkage CNC cutting equipment, so that the fiber direction deviation of the material does not exceed ±1°, the dimensional tolerance is controlled within ±0.1mm, and a 0.5mm trimming allowance is reserved at the edge.

[0039] S2. In an environment with a temperature of 20℃ and a relative humidity of 45%, based on the thermosetting anti-warping criterion, the specific layer ratio is customized according to the service load spectrum of the fairing, and a functional gradient structure symmetrical about the thickness of the component is laid. The total number of layers is 50, and the effective thickness is 7.5mm. Along the thickness direction of the component, it is divided into an outer layer area, a middle layer area and an inner layer area from both sides to the middle surface. In the overall structure, the inner layer area is composed of 0° direction fibers and accounts for 12% of the total number of layers, i.e., 6 layers. The middle layer area is composed of +45° and -45° direction fibers alternately laid and accounts for 60% of the total number of layers, i.e., 30 layers. The outer layer area is composed of 90° direction fibers and accounts for 28% of the total number of layers, i.e., 14 layers. During the layup process, laser projection positioning is used and constant tension is applied to control the accuracy to ±1N. After every four layers are laid up, a 55℃ hot air gun is used for grid-like spot fixing. The diameter of the spot fixing area is 5mm, and the spot fixing spacing is 120mm×100mm in the planar area and 80mm×80mm in the corner area. After all layers are laid up, a double-layer fluororubber film is used to vacuum bag the prepreg. The inner film directly contacts the surface of the prepreg, and the outer film provides a mechanical seal. A breathable felt and a guide net are placed between the two films to optimize the gas exhaust path. The vacuum is evacuated to a gauge pressure of no more than -0.098MPa and maintained for 20 minutes.

[0040] S3. Place the encapsulated layered component on a rigid aluminum alloy core mold with a surface roughness value of no more than 0.8μm. After purging with nitrogen for 5 minutes, heat the mold to 60℃ at a heating rate of 2℃ / min. Hold the temperature for 10 minutes and then apply a pressure of 0.1MPa using a servo electro-hydraulic system with a pressure response delay of no more than 0.5 seconds to ensure that the pressure curve and temperature curve are strictly synchronized in the time domain. Hold the temperature for 30 minutes and then allow it to cool naturally to below 50℃ before removing it.

[0041] S4. A three-stage temperature and pressure control strategy is adopted in the horizontal autoclave. The temperature is slowly increased to 140℃ at a rate of 2℃ / min, and a servo-electric hydraulic system applies a pressure of 0.4MPa simultaneously. The pressure response delay time is no more than 0.5 seconds to ensure that the pressure curve and temperature curve are strictly synchronized in the time domain. Then, the autoclave enters the constant temperature and pressure heat preservation stage. The heat preservation time adopts a collaborative calculation model that combines the basic chemical reaction time with the heat conduction compensation time. The basic curing heat preservation time is set to 90min, and an additional 8min of heat conduction time is added for every 1mm increase in the maximum effective thickness of the component (7.5mm). The total heat preservation time is 150min. Then, the temperature is reduced at a rate of 1℃ / min. Throughout the process, the vacuum gauge pressure is maintained at no less than -0.095MPa and nitrogen protection is introduced.

[0042] S5. After initial visual inspection of the demolded components, diamond grinding wheels are used for micro-grinding of the edges under water cooling and closed-loop force feedback system control. The water-cooled grinding system integrates a force feedback control unit to monitor the contact pressure between the grinding wheel and the workpiece in real time, ensuring that the grinding force fluctuation range does not exceed ±0.5N, the grinding removal amount is controlled at 0.1mm, and the cooling water temperature is maintained at 20℃. Then, a combination of 1000-grit dry grinding and 1500-grit wet grinding is used for finishing, so that the surface roughness value is not greater than 0.4μm. Example 2

[0043] This embodiment provides a special molding process for helicopter composite material cap-shaped fairings, including the following steps: S1. A unidirectional prepreg was prepared using T700 grade unidirectional carbon fiber and toughened bisphenol A epoxy resin system B, with the fiber areal density controlled at 180 g / m². 2 And the fluctuation is within ±5g / m 2 Within this range, the resin content is 40%; the fairing is automatically cut using a five-axis linkage CNC cutting device based on the three-dimensional geometric model, so that the fiber direction deviation of the material sheet does not exceed ±1°, the dimensional tolerance is controlled within ±0.1mm, and a 0.8mm trimming allowance is reserved at the edge.

[0044] S2. In an environment with a temperature of 22℃ and a relative humidity of 50%, based on the thermosetting anti-warping criterion, the specific layer ratio is customized according to the service load spectrum of the fairing, and a functional gradient structure symmetrical about the thickness of the component is laid. The total number of layers is 40, and the effective thickness is 7.2mm. Along the thickness direction of the component, it is divided into an outer layer area, a middle layer area and an inner layer area from both sides to the middle surface. In the overall structure, the inner layer area is composed of 0° direction fibers and accounts for 15% of the total number of layers, i.e., 6 layers. The middle layer area is composed of +45° and -45° direction fibers alternately laid and accounts for 55% of the total number of layers, i.e., 22 layers. The outer layer area is composed of 90° direction fibers and accounts for 30% of the total number of layers, i.e., 12 layers. During the layup process, laser projection positioning is used and constant tension is applied to control the accuracy to ±1N. After every four layers are laid up, a 60℃ hot air gun is used for grid-like spot fixing. The diameter of the spot fixing area is 6mm, and the spot fixing spacing is 120mm×100mm in the planar area and 80mm×80mm in the corner area. After all layers are laid up, a double-layer fluororubber film is used to vacuum bag the prepreg. The inner film directly contacts the surface of the prepreg, and the outer film provides a mechanical seal. A breathable felt and a guide net are placed between the two films to optimize the gas exhaust path. The vacuum is evacuated to a gauge pressure of no more than -0.098MPa and maintained for 20 minutes.

[0045] S3. Place the encapsulated layered component on a rigid aluminum alloy core mold with a surface roughness of no more than 0.8 μm. After purging with nitrogen for 5 min, heat the mold to 70 °C at a rate of 2 °C / min. Hold the temperature for 10 min and then apply a pressure of 0.15 MPa using a servo electro-hydraulic system with a pressure response delay of no more than 0.5 seconds to ensure that the pressure curve and temperature curve are strictly synchronized in the time domain. Hold the temperature for 45 min and then allow it to cool naturally to below 50 °C before removing it.

[0046] S4. A three-stage temperature and pressure control strategy is adopted in the horizontal autoclave. The temperature is slowly increased to 150℃ at a rate of 2.5℃ / min. At the same time, a servo-electric hydraulic system applies a pressure of 0.5MPa with a pressure response delay of no more than 0.5 seconds to ensure that the pressure curve and temperature curve are strictly synchronized in the time domain. Then, the autoclave enters the constant temperature and pressure heat preservation stage. The heat preservation time adopts a collaborative calculation model that combines the basic chemical reaction time with the heat conduction compensation time. The basic curing heat preservation time is set to 105min. For every 1mm increase in the maximum effective thickness of the component (7.2mm), an additional 9min of heat conduction time is added, resulting in a total heat preservation time of 170min. Then, the temperature is reduced at a rate of 1.5℃ / min. Throughout the process, the vacuum degree is maintained at no less than -0.095MPa and nitrogen protection is introduced.

[0047] S5. After initial visual inspection of the demolded components, diamond grinding wheels are used for minor edge grinding under water cooling and closed-loop force feedback system control. The water-cooled grinding system integrates a force feedback control unit to monitor the contact pressure between the grinding wheel and the workpiece in real time, ensuring that the grinding force fluctuation range does not exceed ±0.5N, the grinding removal amount is controlled at 0.15mm, and the cooling water temperature is maintained at 22℃. Then, a combination of 1000-grit dry grinding and 1500-grit wet grinding is used for finishing, so that the surface roughness value is not greater than 0.4μm. Example 3

[0048] This embodiment provides a special molding process for helicopter composite material cap-shaped fairings, including the following steps: S1. A unidirectional prepreg was prepared using T700 grade unidirectional carbon fiber and toughened bisphenol A epoxy resin system C, with the fiber areal density controlled at 200 g / m².2 And the fluctuation is within ±5g / m 2 The resin content is 42%; the material is automatically cut according to the three-dimensional geometric model of the fairing using a five-axis linkage CNC cutting equipment, so that the fiber direction deviation of the material does not exceed ±1°, the dimensional tolerance is controlled within ±0.1mm, and a 1.0mm trimming allowance is reserved at the edge.

[0049] S2. In an environment with a temperature of 24℃ and a relative humidity of 55%, based on the thermosetting anti-warping criterion, the specific layer ratio is customized and adjusted according to the service load spectrum of the fairing. A functional gradient structure symmetrical about the mid-plane of the component thickness is laid, with a total of 60 layers and an effective thickness of 12.0mm. Along the thickness direction of the component, from both sides to the mid-plane, it is divided into an outer layer region, a middle layer region, and an inner layer region. In the overall structure, the inner layer region is composed of 0° direction fibers and accounts for 13.3% of the total number of layers, i.e., 8 layers. The middle layer region is composed of alternating +45° and -45° direction fibers and accounts for 60% of the total number of layers, i.e., 36 layers. The outer layer region is composed of 90° direction fibers and accounts for 26.7% of the total number of layers, i.e., 16 layers. During the layup process, laser projection positioning is used and constant tension is applied to control the accuracy to ±1N. After every four layers are laid up, a 65℃ hot air gun is used to perform grid-like spot fixing. The diameter of the spot fixing area is 8mm, and the spot fixing spacing is 120mm×100mm in the planar area and 80mm×80mm in the corner area. After all layers are laid up, a double-layer fluororubber film is used to vacuum-seal the bag. The inner film directly contacts the surface of the prepreg, and the outer film provides a mechanical seal. A breathable felt and a guide net are placed between the two films to optimize the gas exhaust path. The vacuum is evacuated to a gauge pressure of no more than -0.098MPa and maintained for 20 minutes.

[0050] S3. Place the encapsulated layered component on a rigid aluminum alloy core mold with a surface roughness value of no more than 0.8μm. After purging with nitrogen for 5 minutes, heat the mold to 80℃ at a heating rate of 2℃ / min. Hold the temperature for 10 minutes and then apply a pressure of 0.2MPa using a servo electro-hydraulic system. The pressure response delay time should be no more than 0.5 seconds to ensure that the pressure curve and temperature curve are strictly synchronized in the time domain. Hold the temperature for 60 minutes and then allow it to cool naturally to below 50℃ before removing it.

[0051] S4. A three-stage temperature and pressure control strategy is adopted in the horizontal autoclave. The temperature is slowly increased to 160℃ at a rate of 3℃ / min. At the same time, a servo-electric hydraulic system applies a pressure of 0.6MPa with a pressure response delay of no more than 0.5 seconds to ensure that the pressure curve and temperature curve are strictly synchronized in the time domain. Then, the autoclave enters the constant temperature and pressure heat preservation stage. The heat preservation time adopts a collaborative calculation model that combines the basic chemical reaction time with the heat conduction compensation time. The basic curing heat preservation time is set to 120min. For every 1mm increase in the maximum effective thickness of the component (12.0mm), an additional 10min of heat conduction time is added, resulting in a total heat preservation time of 240min. Then, the temperature is reduced at a rate of 2℃ / min. Throughout the process, the vacuum degree is maintained at no less than -0.095MPa and nitrogen protection is introduced.

[0052] S5. After initial visual inspection of the demolded components, diamond grinding wheels are used for minor edge grinding under water cooling and closed-loop force feedback system control. The water-cooled grinding system integrates a force feedback control unit to monitor the contact pressure between the grinding wheel and the workpiece in real time, ensuring that the grinding force fluctuation range does not exceed ±0.5N, the grinding removal amount is controlled at 0.2mm, and the cooling water temperature is maintained at 25℃. Then, a combination of 1000-grit dry grinding and 1500-grit wet grinding is used for finishing, so that the surface roughness value is not greater than 0.4μm. Example 4

[0053] This embodiment provides a special molding process for helicopter composite material cap-shaped fairings, including the following steps: S1. A unidirectional prepreg was prepared using T700 grade unidirectional carbon fiber and toughened bisphenol A epoxy resin system B, with the fiber areal density controlled at 150 g / m². 2 And the fluctuation is within ±5g / m 2 Within this range, the resin content is 40%; the fairing is automatically cut using a five-axis linkage CNC cutting device based on the three-dimensional geometric model, so that the fiber direction deviation of the material sheet does not exceed ±1°, the dimensional tolerance is controlled within ±0.1mm, and a 0.8mm trimming allowance is reserved at the edge.

[0054] S2. In an environment with a temperature of 22℃ and a relative humidity of 50%, based on the thermosetting anti-warping criterion, the specific layer ratio is customized according to the service load spectrum of the fairing, and a functional gradient structure symmetrical about the thickness of the component is laid. The total number of layers is 80, and the effective thickness is 12.0mm. Along the thickness direction of the component, from the two side surfaces to the middle surface, it is divided into an outer layer area, a middle layer area and an inner layer area. In the overall structure, the inner layer area is composed of 0° direction fibers and accounts for 12.5% ​​of the total number of layers, i.e., 10 layers. The middle layer area is composed of +45° and -45° direction fibers alternately laid and accounts for 57.5% of the total number of layers, i.e., 46 layers. The outer layer area is composed of 90° direction fibers and accounts for 30% of the total number of layers, i.e., 24 layers. During the layup process, laser projection positioning is used and constant tension is applied to control the accuracy to ±1N. After every four layers are laid up, a 60℃ hot air gun is used for grid-like spot fixing. The diameter of the spot fixing area is 6mm, and the spot fixing spacing is 120mm×100mm in the planar area and 80mm×80mm in the corner area. After all layers are laid up, a double-layer fluororubber film is used to vacuum bag the prepreg. The inner film directly contacts the surface of the prepreg, and the outer film provides a mechanical seal. A breathable felt and a guide net are placed between the two films to optimize the gas exhaust path. The vacuum is evacuated to a gauge pressure of no more than -0.098MPa and maintained for 20 minutes.

[0055] S3. Place the encapsulated layered component on a rigid aluminum alloy core mold with a surface roughness of no more than 0.8 μm. After purging with nitrogen for 5 min, heat the mold to 70 °C at a rate of 2 °C / min. Hold the temperature for 10 min and then apply a pressure of 0.15 MPa using a servo electro-hydraulic system with a pressure response delay of no more than 0.5 seconds to ensure that the pressure curve and temperature curve are strictly synchronized in the time domain. Hold the temperature for 45 min and then allow it to cool naturally to below 50 °C before removing it.

[0056] S4. A three-stage temperature and pressure control strategy is adopted in the horizontal autoclave. The temperature is slowly increased to 150℃ at a rate of 2.5℃ / min. At the same time, a servo-electric hydraulic system applies a pressure of 0.5MPa with a pressure response delay of no more than 0.5 seconds to ensure that the pressure curve and temperature curve are strictly synchronized in the time domain. Then, the autoclave enters the constant temperature and pressure heat preservation stage. The heat preservation time adopts a collaborative calculation model that combines the basic chemical reaction time with the heat conduction compensation time. The basic curing heat preservation time is set to 120min. For every 1mm increase in the maximum effective thickness of the component (12.0mm), an additional 10min of heat conduction time is added, resulting in a total heat preservation time of 240min. Then, the temperature is reduced at a rate of 1.5℃ / min. Throughout the process, the vacuum degree is maintained at no less than -0.095MPa and nitrogen protection is introduced.

[0057] S5. After initial visual inspection of the demolded components, diamond grinding wheels are used for minor edge grinding under water cooling and closed-loop force feedback system control. The water-cooled grinding system integrates a force feedback control unit to monitor the contact pressure between the grinding wheel and the workpiece in real time, ensuring that the grinding force fluctuation range does not exceed ±0.5N, the grinding removal amount is controlled at 0.15mm, and the cooling water temperature is maintained at 22℃. Then, a combination of 1000-grit dry grinding and 1500-grit wet grinding is used for finishing, so that the surface roughness value is not greater than 0.4μm.

[0058] Comparative Example 1: Compared with Example 2, the difference is that S2 does not use symmetrical laying based on thickness mid-plane, but adopts unidirectional absolute gradient layup, that is, all 0° fiber layers, ±45° fiber layers and 90° fiber layers are laid in sequence from the inside to the outside along the thickness direction of the component, and the rest are the same.

[0059] Comparative Example 2: Compared with Example 2, the difference is that the medium-temperature and low-pressure pre-compaction process in S3 is cancelled. After vacuum bag sealing, it directly enters the horizontal autoclave to perform the heating and pressurization curing in S4. All other steps are the same.

[0060] Comparative Example 3: Compared with Example 2, the difference is that the constant temperature and pressure insulation stage of S4 does not use the collaborative calculation model of basic chemical reaction time combined with heat conduction compensation time. Instead, the total insulation time is set only by a linear calculation method of 9 minutes per millimeter of the maximum effective thickness of the component. All other aspects are the same.

[0061] Comparative Example 4: Compared with Example 2, the difference is that the post-processing stage of S5 does not use water cooling and force feedback control, but directly uses conventional dry grinding process for edge trimming and surface finishing. All other aspects are the same.

[0062] Test Example 1: The toughened bisphenol A epoxy resin system A, toughened bisphenol A epoxy resin system B, and toughened bisphenol A epoxy resin system C obtained from Preparation Examples 1 to 3 were taken out from the -18°C frozen storage conditions and allowed to thaw naturally at room temperature of 25°C for 30 minutes until each resin system returned to a weighable state.

[0063] The temperature-dependent rheological properties of the above resin system were tested using a rotational rheometer. The test mode was set to oscillating shear mode, and a parallel plate fixture with a diameter of 25 mm was selected, with the gap between the plates controlled at 1.0 mm. The thawed toughened bisphenol A epoxy resin system sample was placed in the center of the lower fixture, and the upper fixture was lowered to the set gap. Excess resin overflowing from the edge of the fixture was scraped off to ensure that the sample filled the gap between the fixtures.

[0064] Set the variable temperature test parameters: heating rate of 2℃ / min, initial test temperature of 30℃, and final test temperature of 130℃. Set the fixed oscillation frequency of 1Hz and control the strain amplitude of 1%. Turn on the rheometer real-time acquisition system to record the change data of complex viscosity with temperature, which is used to determine the process rheological window of the resin system.

[0065] Another sample of the same toughened bisphenol A epoxy resin system, after thawing, was loaded into the rotational rheometer fixture for isothermal rheodynamic testing. The rheometer fixture was preheated to the set temperature of 70°C. After loading the sample, the temperature was quickly equilibrated. The oscillation frequency was set to 1 Hz and the strain amplitude was 1%. The change in complex viscosity over time was continuously monitored for 120 minutes under isothermal conditions to evaluate the stabilization time of the low viscosity window.

[0066] Table 1: Rheological property test data of toughened bisphenol A epoxy resin system

[0067] According to Table 1 and Figure 4 It is known that toughened bisphenol A epoxy resin systems A, B, and C exhibit thermosetting rheological characteristics of initial viscosity decrease followed by viscosity increase during temperature changes. In the initial stage of temperature increase from 30°C to 60°C, the complex viscosity of the resin system decreases as the thermal motion of the macromolecular chain segments increases. When the temperature enters the range of 60°C to 80°C, the molecular chain segments of the thermoplastic toughening polymer polyethersulfone in the epoxy resin undergo swelling and partial relaxation behavior. Combined with the thermal motion of the bisphenol A epoxy resin matrix, this leads to... The complex viscosity of the resin system remained in the low range suitable for macroscopic flow. Test data showed that the initial viscosity of toughened bisphenol A epoxy resin system A at 60℃ and 80℃ was 18450 mPa·s and 5820 mPa·s, respectively; the initial viscosity of toughened bisphenol A epoxy resin system B at 60℃ and 80℃ was 26800 mPa·s and 8940 mPa·s, respectively; and the initial viscosity of toughened bisphenol A epoxy resin system C at 60℃ and 80℃ was 39150 mPa·s and 13200 mPa·s, respectively.

[0068] Within this temperature range, the reactivity of the latent curing agent dicyandiamide and the organic urea accelerator is at a low level, and the system has not yet formed a large-scale chemical crosslinking network. Under constant temperature conditions of 70°C, the viscosity of resin systems A, B, and C remains below 50,000 mPa·s for 98 min, 84 min, and 71 min, respectively, indicating that the system has a wide low viscosity time window. This window can provide a physical basis for the medium-temperature and low-pressure pre-compaction process in step S3 of the subsequent special molding process, allowing the liquid resin to undergo micro-rheological behavior under low pressure of 0.1 to 0.2 MPa. While assisting in the removal of trapped gases and small volatile molecules between the fiber laminate layers, it also fills local microcavities. When the temperature continues to rise above 90°C, the chemical crosslinking reaction rate exceeds the viscosity reduction effect caused by thermal motion. The comprehensive test results show that although increasing the proportion of polyethersulfone increases the overall viscosity base of the system, it still retains the process rheological characteristics of 60°C to 80°C. This phenomenon can provide basic data support for the rationality of the process parameter limits of this invention.

[0069] Test Example 2: Approximately 10 mg each of uncured toughened bisphenol A epoxy resin system A, toughened bisphenol A epoxy resin system B, and toughened bisphenol A epoxy resin system C were placed in an aluminum crucible. A non-isothermal dynamic scan was performed using a differential scanning calorimeter. The heating rate was set to 10 °C / min, and the test temperature range was from room temperature to 250 °C. During the test, a constant nitrogen gas flow rate of 50 mL / min was introduced. The exothermic peak area was calculated using an integral program to obtain the initial total heat of reaction for each resin system.

[0070] Destructive sampling was performed on the fairing components after demolding in Examples 1, 2, 3 and Comparative Example 3, respectively. The sampling points were set along the thickness direction of the components in the surface area, the middle layer area of ​​the thickness, and the thick-walled corner area, respectively, to cover the characteristic parts with different heat transfer boundary conditions.

[0071] The cured composite material samples obtained from the above locations were finely ground, and 5.2 mg to 8.7 mg of powder samples were weighed and placed in aluminum crucibles. The residual exothermic peak area was tested using a differential scanning calorimeter at the same heating rate of 10 °C / min to obtain the residual heat of reaction data of each region. The final degree of curing of each sampling region was calculated based on the initial total heat of reaction of the corresponding resin system.

[0072] The composite materials at the corresponding sampling points in the above steps were cut and processed into rectangular standard test strips with dimensions of 35mm×10mm×2mm. Dynamic mechanical properties were tested using a dynamic thermodynamic analyzer in single cantilever beam mode. The oscillation frequency of the equipment was set to 1Hz, the heating rate was 3℃ / min, and the test temperature range was 50℃ to 250℃. The evolution curve of the system loss factor with temperature was recorded, and the temperature corresponding to the peak value of the loss factor was taken as the glass transition temperature of the sampling area.

[0073] Table 2: Test data on the degree of curing and glass transition temperature of different regions of the fairing component under different molding processes

[0074] According to Table 2 and Figure 5 It can be seen that the fairing components of Examples 1 to 3 all achieved a high degree of final curing in the surface area, the thick surface area, and the thick-walled corner area, and the glass transition temperature distribution in each area was relatively concentrated, reflecting that the cross-linking state inside the component was relatively uniform. The curing process of the thermosetting resin system is affected by the chemical kinetic law. The resin needs to be maintained at a certain temperature for a certain period of time to promote the construction of the cross-linking network. In the constant temperature and pressure heat preservation stage, the present invention adopts a synergistic control method that combines the basic chemical reaction time and the additional heat conduction time, which can compensate for the heat conduction lag in the thickness direction of the composite material to a certain extent. In contrast, in Comparative Example 3, without the aforementioned synergistic control method, the curing degree of the component's surface area was 92.2%, while the curing degree of the thick surface area and the thick-walled corner area decreased to 86.6% and 84.5%, respectively. The residual heat of reaction increased in these areas, and the glass transition temperature decreased to 127.5℃ and 123.1℃, respectively. Simultaneously, combined with... Figure 5 The trend reflected is that the surface layer sample in Comparative Example 3 exhibits post-curing exothermic characteristics during the heating process. The peak position of the loss factor shifts to the low temperature region and the peak shape shows a broadening trend, indicating the non-uniformity of the cross-linked network structure. The above test results show that the curing temperature parameter control model provided by the present invention has a reference technical basis for promoting the uniformity of curing of the entire thickness of the component.

[0075] Test Example 3: The fairing components of Examples 1 to 4 and Comparative Example 1, after molding, demolding, and machining, were placed in a standard test environment of 23°C and 50% relative humidity for 24 hours to release residual stress on the machined surface.

[0076] A high-precision 3D laser scanner was used to perform full-size surface scanning and inspection. An aluminum alloy positioning reference fixture was used to fix the stationary fairing components onto the scanning platform, and a global measurement coordinate system consistent with the original 3D theoretical model of the fairing was established.

[0077] Turn on the laser scanner and collect non-contact full-field point cloud data of the inner and outer surfaces of the component according to the set scanning path. During the scanning process, the control point spacing is 0.05mm. After the collection is completed, the software is used to perform noise filtering and surface smoothing preprocessing on the original point cloud.

[0078] The preprocessed measured point cloud data is imported into the 3D detection software. The best fitting alignment algorithm is used to spatially match the theoretical 3D model. The normal deviation of each measured node of the component relative to the theoretical model is calculated. The 3D form and position deviation color cloud map is output. The extreme value range of the positive and negative deviation of the maximum warping deformation and the root mean square error of the surface are extracted in the whole domain. The measured dimensions of the characteristic lengths in the 0° direction of the main axis and the 90° direction of the radial direction of the component are extracted and the absolute anisotropic shrinkage rate is calculated.

[0079] Table 3: Geometric Dimension Inspection Data of Fairing Components under Different Ply Structure Designs

[0080] According to Table 3 and Figure 6It can be seen that the fairing components formed in Examples 1 to 4 exhibited small macroscopic deformation during dimensional inspection, with the maximum warpage deformation controlled between 0.28 mm and 0.44 mm, and the root mean square error of the profile less than 0.14 mm. Due to the orthotropic characteristics of carbon fiber composite materials, the axial and radial coefficients of thermal expansion and chemical shrinkage rates have certain differences. In this application, Examples 1 to 4 adopt the functional gradient layup criterion based on thickness mid-plane symmetry. During the heating and subsequent cooling processes, the interlaminar stress generated by the shrinkage of each unidirectional fiber layer tends to form a moment balance under the layup structure with mid-plane symmetry. This reduces the driving force of bending and torsional deformation caused by the mismatch of thermal expansion coefficients to a certain extent. At the same time, the improvement in the dimensional accuracy of the components can also be attributed to the high-gloss core mold, closed-loop temperature control, and slow heating and cooling strategies adopted in the molding stage. The synergistic control of the above process parameters can reduce the thermochemical shrinkage amplitude of the resin during the curing cycle and reduce the springback effect caused by stress concentration after demolding. In contrast, Comparative Example 1 did not use thickness mid-plane symmetry constraint, and the test data showed that the maximum warpage deformation reached 3.87 mm, and the root mean square error of the profile was 1.62 mm. Figure 6 As can be seen from the distribution characteristics, Comparative Example 1 shows obvious asymmetric normal displacement at both ends of the centerline. This is mainly due to the asymmetric ply breaking the local mechanical equilibrium of the laminate during temperature change, resulting in unequal thermal strain on the inner and outer surfaces of the component. When the rigid core mold is removed and the geometric constraint is lost, the residual internal stress is transformed into macroscopic structural deformation. The above comparative data shows that the symmetric gradient ply criterion combined with the temperature and pressure synergistic control strategy adopted in this application has technical feasibility reference in improving the dimensional accuracy of composite material structures and suppressing warping deformation.

[0081] Test Example 4: The surfaces of the fairing components molded and demolded in Examples 1 to 4, Comparative Examples 2 and 3 were cleaned and dried. They were then fixed on the water tank bracket of the ultrasonic immersion focused C-scan non-destructive testing system. Degassed pure water was injected until the components were completely submerged. The robotic arm was adjusted so that the ultrasonic probe was vertically aligned with the surface of the components.

[0082] A wideband focusing probe with a center frequency of 5MHz was selected, and the scanning step interval was set to 0.5mm. The entire morphology of the fairing components was automatically scanned. The acquisition system recorded the amplitude of the bottom wave reflection signal of the ultrasonic pulse in real time, and the quantitative acoustic attenuation coefficient of each node was calculated by acoustic analysis software.

[0083] After the non-destructive testing was completed, destructive sampling was carried out along the typical characteristic parts of the component. Test blocks with a size of 20mm×20mm were cut from the planar area and the thick-walled corner area respectively. After ultrasonic cleaning of the surface debris of the test blocks with anhydrous ethanol, they were placed in a vacuum drying oven at 60℃ for 2 hours.

[0084] The apparent mass of each sample in air and the suspended mass in constant-temperature deionized water were determined using a hydrostatic weighing method with an analytical balance of 0.1 mg accuracy. Combined with the previously measured density data of the cured resin matrix and carbon fiber, the absolute porosity percentage of each region of the sample was calculated by converting the density equation, and the global average porosity of the component was statistically analyzed.

[0085] Table 4: Sound Attenuation and Porosity Detection Data of Fairing Components under Different Molding Processes

[0086] According to Table 4 and Figure 7 It can be seen that the overall average porosity of the fairing components formed in Examples 1 to 4 is controlled within 1.0%, and the ultrasonic attenuation value is in a low range, indicating that the internal structure of the component has good compactness. The low porosity of the molded component in this application can be attributed to the coupling effect of medium-temperature low-pressure pre-compaction and temperature-pressure synergistic curing. According to the rheological characteristics of the resin matrix, in the range of 60°C to 80°C, the bisphenol A type epoxy resin is in a low viscosity state and has not yet been extensively cross-linked. The low pressure applied in the stage drives the liquid resin to undergo micro-rheology, so that the trapped gas and small molecule volatiles between the fiber layers can be discharged along the gaps between the fiber bundles. The coupling effect inhibits the formation of microcavities to some extent. In contrast, Comparative Example 2, without pre-compaction, directly enters the autoclave for heating and pressurization curing. Test data shows that the overall average porosity increases to 2.58%, and the local porosity in the thick-walled corner region reaches 3.96%. Figure 7 As can be seen from the curve distribution, Comparative Example 2 shows a significant downward trend in the bottom wave reflection amplitude when crossing the thick-walled corner area, reflecting that there are pore defects inside that cause sound waves to scatter. Since the low viscosity stage of degassing treatment was not carried out, the resin rapidly cross-linked and cured under subsequent high temperature and high pressure, resulting in the gas trapped in the initial layup being sealed in the interlayer and inside the matrix. Especially in the corner area with relatively complex geometry, the gas exhaust obstruction phenomenon is more prominent. Although Comparative Example 3 was pre-compacted, the parameters of the curing time calculation model were simplified. Due to the sluggish heat conduction in the thick-walled corner area, the corresponding venting time could not be obtained in the low viscosity stage. The shrinkage stress caused by the difference in the internal cross-linking state would induce micropores, and the local porosity reached 2.81%. The above test data comparison shows that the medium-temperature rheological venting pre-compacting mechanism and the temperature-pressure synergistic curing model adopted in this application have technical reference value in improving the internal pore distribution of composite materials.

[0087] Test Example 5: Take the fairing components of Examples 1 to 4 and Comparative Example 4 that have undergone demolding and corresponding post-processing. Wipe the test surface and the machined edges with a lint-free cloth soaked in anhydrous ethanol. Place them in a constant temperature test chamber at 23°C and 50% relative humidity for 4 hours to dry before use.

[0088] A contact stylus surface roughness tester was used to perform microscopic profile scanning on the surface of the component. The probe sampling length was set to 0.8 mm, the evaluation length to 4.0 mm, and the probe moving speed to 0.5 mm / s. Data was collected from five independent test areas randomly selected on the outer surface of the component along two directions: parallel to the main axis of the component and perpendicular to the main axis. The measured values ​​of surface roughness were recorded, and the average value was taken as the surface roughness characterization data of the component.

[0089] The microscopic morphology of the machined edge of the component was observed using an industrial-grade stereomicroscope. An observation window was set every 50 mm along the entire length of the trimmed edge to record the carbon fiber tearing and resin matrix delamination in the edge area. Using image processing measurement software, the vertical physical distance of the machining edge defect extending into the component was measured and recorded as the maximum splitting depth of the edge.

[0090] The layering factor was calculated by combining the morphological measurement data under the microscope to evaluate the overall degree of machining damage in the trimmed edge area. At the same time, the presence of burn characteristics such as localized yellowing and carbonization of resin caused by overheating during processing was recorded on the surface of the observation area.

[0091] Table 5: Test data on surface roughness and edge damage of fairing components under different post-processing techniques

[0092] According to Table 5 and Figure 8 It can be seen that the surface roughness test value of the fairing components formed in Examples 1 to 4 is controlled below 0.4 μm, the maximum splitting depth at the edge is within 0.2 mm, the delamination factor is close to 1, and no obvious local burning phenomenon of resin is observed under microscopic observation. Combined with the analysis of the process strategy adopted in the post-processing stage of this application, the introduction of water cooling medium can provide a certain heat dissipation channel at the processing interface, which helps to control the local temperature of the cutting area below the glass transition temperature of bisphenol A epoxy resin, thereby preventing the resin matrix from softening, spreading or carbonizing and burning due to heat accumulation. At the same time, the dynamic adjustment of the feed pressure by the relevant force feedback control system helps to maintain the relative stability of the cutting force, thereby reducing the carbon fiber debonding and interlayer splitting phenomenon caused by processing stress overload to a certain extent. In contrast, Comparative Example 4 used a conventional belt sander for dry grinding and edge trimming. Test data showed that the surface roughness increased to 3.54 μm, the maximum edge splitting depth reached 1.82 mm, the delamination factor increased to 1.462, and multiple resin burn spots were found during microscopic observation. Figure 8 As can be seen from the wide oscillations and local abrupt peaks presented by the curves in Comparative Example 4, under the combined effect of non-uniform cutting stress and continuously accumulated cutting heat, the machining edge of the composite material will produce obvious matrix damage and interlayer delamination propagation. The above comparative data shows that the water cooling and feed control dressing process adopted in this application has certain process optimization reference value in suppressing surface thermal damage and mechanical stress damage.

[0093] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A special molding process for a helicopter composite material cap-shaped fairing, characterized in that, Includes the following steps: A unidirectional prepreg was prepared using a carbon fiber and toughened bisphenol A epoxy resin system, and the prepreg sheets were cut according to the geometric model of the fairing using a CNC cutting equipment. In a controlled temperature and humidity environment, a functional gradient structure symmetrical about the thickness of the component is laid. Along the thickness direction, it is divided from both sides to the middle into an outer layer area composed of fibers in the 90° direction, a middle layer area composed of alternating +45° and -45° direction fibers, and an inner layer area composed of fibers in the 0° direction. After completion, it is sealed in a vacuum bag and vacuumed. The encapsulated plywood component is placed on a rigid aluminum alloy mandrel with a surface roughness of no more than 0.8 μm. After purging with nitrogen for 5 to 10 minutes, a medium-temperature and low-pressure pre-compaction process is adopted: the temperature is increased to 60 to 80°C at a rate of 2 to 3°C / min, held for 10 to 15 minutes, and then 0.1 to 0.2 MPa pressure is applied, with the pressure response delay controlled to be no more than 0.5 seconds. The temperature is held for 30 to 60 minutes, and then the component is naturally cooled to no more than 50°C before being removed. The removed components are transferred to an autoclave for heating and pressurization curing, maintaining a vacuum throughout the process and introducing protective gas. The total insulation time during the constant temperature and pressure stage is determined by a collaborative calculation model: the basic curing insulation time is set to 90 to 120 minutes, and the heat conduction compensation time is calculated by multiplying the maximum effective thickness of the component by a compensation coefficient of 8 to 10 minutes / mm. The total insulation time is obtained by adding the two together. After demolding, the components undergo micro-grinding and assembly finishing. The micro-grinding is carried out under water cooling conditions and closed-loop force feedback system control.

2. The special molding process for helicopter composite material cap-shaped fairings according to claim 1, characterized in that, The step of preparing unidirectional prepreg using a carbon fiber and toughened bisphenol A epoxy resin system, and cutting the prepreg sheet according to the fairing geometry using a CNC cutting device includes: Unidirectional prepreg was prepared using carbon fiber and the toughened bisphenol A epoxy resin system, with the fiber areal density controlled to be 150 to 200 g / m³. 2 The density deviation is -5 to 5 g / m³ 2 The resin content is 38% to 42%; the fairing is automatically cut according to the three-dimensional geometric model by a five-axis linkage CNC cutting equipment, so that the fiber direction deviation of the material sheet is -1° to 1°, the dimensional tolerance is -0.1 to 0.1mm, and a trimming allowance of 0.5 to 1.0mm is reserved at the edge.

3. The special molding process for helicopter composite material cap-shaped fairings according to claim 1, characterized in that, The toughened bisphenol A epoxy resin system is made from raw materials comprising the following parts by weight: Bisphenol A type epoxy resin: 90 to 110 parts; Polyethersulfone powder: 15 to 25 parts; Dicyandiamide: 6 to 10 parts; Organic urea accelerator: 1 to 3 parts.

4. The special molding process for helicopter composite material cap-shaped fairings according to claim 1, characterized in that, The steps for preparing the toughened bisphenol A epoxy resin system include: The bisphenol A epoxy resin is added to a reaction vessel equipped with mechanical stirring and vacuuming functions, and the temperature is raised to 120 to 130°C. Slowly add polyethersulfone powder and stir at a speed of 300 to 400 r / min for 1.5 to 2.5 h at a constant temperature until the polyethersulfone is completely dissolved and a transparent and homogeneous mixture is formed. Cool the reaction vessel system to 60 to 70°C, and add dicyandiamide and organic urea accelerators; The mixture is dispersed and ground using a three-roll mill. The roller heating temperature is controlled at 50 to 70°C, and the milling is performed 3 to 5 times until the fineness of the solid particles in the resin system is no greater than 5μm.

5. The special molding process for helicopter composite material cap-shaped fairings according to claim 1, characterized in that, The total number of layers in the functionally graded structure is 40 to 80. The specific layer ratio of the inner, middle and outer layers in the gradient layer structure is customized according to the fairing service load spectrum, and the inner layer accounts for 12% to 15% of the total number of layers; the middle layer accounts for 50% to 60% of the total number of layers; and the outer layer accounts for 25% to 30% of the total number of layers.

6. The special molding process for helicopter composite material cap-shaped fairings according to claim 1, characterized in that, The steps of laying a functional gradient structure symmetrical about the thickness of the component in a controlled temperature and humidity environment, dividing the structure from both sides to the middle along the thickness direction into an outer layer region composed of fibers in the 90° direction, a middle layer region composed of alternating +45° and -45° direction fibers, and an inner layer region composed of fibers in the 0° direction, followed by vacuum bag sealing and vacuuming, include: Laying is carried out in an environment with a temperature of 20 to 24°C and a relative humidity of 45% to 55%; during the laying process, laser projection is used for positioning and constant tension is applied, with a constant tension control accuracy of -1 to 1N; After completing 3 to 5 layers, use a 55 to 65°C hot air gun to perform grid-like spot fixation. The diameter of the spot fixation area is 5 to 8 mm. The spot fixation spacing in the planar area is 110 to 130 mm × 90 to 110 mm, and the spacing in the corner area is increased to 70 to 90 mm × 70 to 90 mm. After all layers are laid up, a double-layer fluororubber membrane is used for vacuum bag sealing. The inner membrane directly contacts the surface of the prepreg, while the outer membrane provides a mechanical seal. A breathable felt and a flow guide net are placed between the two membranes to optimize the gas exhaust path. The vacuum is evacuated to a gauge pressure not higher than -0.098 MPa and maintained for 20 to 30 minutes.

7. The special molding process for helicopter composite material cap-shaped fairings according to claim 1, characterized in that, The step of transferring the removed component into an autoclave for heating and pressurizing curing, maintaining a vacuum throughout the process and introducing a protective gas, includes: The temperature is slowly increased to 140 to 160°C at a rate of 2 to 3°C / min, and a pressure of 0.4 to 0.6 MPa is applied simultaneously through a servo electro-hydraulic system to enter the constant temperature and pressure stage. The pressure response delay time is controlled to be no more than 0.5 s, so that the pressure curve and the temperature curve are synchronized in the time domain. After curing, the temperature is reduced at a rate of 1 to 2 °C / min; the gauge pressure is maintained at no higher than -0.095 MPa throughout the process and nitrogen gas is introduced for protection.

8. The special molding process for helicopter composite material cap-shaped fairings according to claim 1, characterized in that, The steps of performing micro-grinding and assembly finishing on the demolded components, with micro-grinding performed under water cooling conditions and closed-loop force feedback system control, include: The edge is micro-grinding using a diamond grinding wheel under water cooling and closed-loop force feedback control system with integrated force feedback control unit. The contact pressure between the grinding wheel and the workpiece is monitored in real time to ensure that the grinding force fluctuation range is -0.5 to 0.5N, the grinding removal amount is controlled between 0.1 and 0.2mm, and the cooling water temperature is maintained between 20 and 25℃. After grinding, the surface is finished by a combination of dry grinding with 800 to 1000 grit and wet grinding with 1200 to 1500 grit, controlling the final surface roughness value to be no greater than 0.4 μm.

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