U-shaped beam prefabricated body and preparation method thereof
Through gradient lamination and adaptive stitching technology, the problems of lossless forming and high-precision Z-direction reinforcement of complex inner cavity structures were solved, the efficient preparation of U-beam preforms was achieved, the load-bearing efficiency and damage tolerance of the structure were improved, and the lightweight and high reliability requirements of the new generation of equipment were met.
Patent Information
- Application Number
- CN202511162973.X
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-08-19
- Publication Date
- 2025-09-26
AI Technical Summary
Existing technologies make it difficult to achieve lossless forming of complex inner cavity structures, optimize internal stress distribution, and perform high-precision Z-direction reinforcement. This results in both structural performance redundancy and defects, and is unable to meet the requirements of the new generation of equipment for lightweight structures, long life, and high reliability.
By adopting gradient layup and adaptive stitching technology, by designing gradient structure fiber layup and adaptive stitching in the U-beam preform, combined with multi-dimensional tooling positioning and closed-loop feedback control, precise distribution and stress matching of fiber reinforced materials can be achieved.
It significantly improves the load-bearing efficiency and damage tolerance of the structure, eliminates stress concentration and weak interfaces, and improves the overall performance and reliability of the structure.
Smart Images
Figure CN120697151A_ABST
Abstract
Description
Technical Field
[0001] The present invention belongs to the technical field of composite material manufacturing, and in particular relates to a fiber-reinforced preform of a high-performance load-bearing structural member, in particular to a U-shaped beam preform and a preparation method thereof. Background Art
[0002] U-shaped beams or U-shaped reinforced structures are typical configurations in composite monolithic wall panels, wing spars, and other primary load-bearing structures. Traditional manufacturing methods typically use pre-cured "U"-shaped ribs for secondary adhesive bonding or mechanical connection to the skin. These interfaces often become stress concentration points and weak links that can lead to structural failure, increasing structural weight and reducing efficiency and reliability.
[0003] In order to achieve a high degree of structural integration, the use of three-dimensional reinforced fiber preform technology has become the mainstream development direction. At present, commonly used technologies such as three-dimensional weaving and needle punching can provide reinforcement in the thickness direction (Z direction), but they have some inherent defects. The three-dimensional weaving process is complex and it is difficult to form complex components with sharply changing cross-sections; when the needle punching process introduces Z-direction fibers, it will damage and disturb the continuous main load-bearing fibers in the plane, reducing its in-plane mechanical properties, and the distribution and preload of the Z-direction fibers are difficult to accurately control. In addition, for structures with complex internal cavities such as U-shaped structures, the removal of the rigid core mold is extremely difficult, which often leads to damage to the preform. At the same time, the uniform layer design and single reinforcement method cannot adapt to the complex stress state of key parts such as corners and joints of the U-beam during service, resulting in the coexistence of structural performance redundancy and defects.
[0004] Therefore, there is an urgent need to develop a method for preparing U-beam preforms that can achieve non-destructive forming of complex inner cavity structures, optimize internal stress distribution, and perform low-damage, high-precision Z-direction reinforcement to meet the stringent requirements of the new generation of equipment for lightweight structure, long life and high reliability. Summary of the Invention
[0005] The present invention aims to solve the above technical problems at least to a certain extent. To this end, the present invention aims to provide a U-beam prefabricated body with optimized structure and a method for preparing the same.
[0006] To achieve the above object, the present invention provides a method for preparing a U-shaped beam prefabricated body, comprising the following steps:
[0007] a) Gradient Lamination: Fiber reinforcement material is laid on a removable U-shaped core mold to form at least one U-shaped base element. The layup structure of the U-shaped base element is a gradient structure, which at least includes:
[0008] A continuous layer of matrix running through the U-shaped profile;
[0009] A hybrid stress transition layer located in the U-shaped corner area for dispersing shear stress;
[0010] Gradual thickening reinforcement layers located at the ends of the two arms of the U-shaped base element, used for connection or bearing;
[0011] b) integrated assembly: combining one or more of the U-shaped base elements prepared in step a) with at least one continuous layer of skin to form a preform assembly to be reinforced;
[0012] c) Multi-dimensional tooling positioning: using modular tooling to clamp, compact and three-dimensionally position the preform assembly;
[0013] d) Adaptive stitching enhancement: Using CNC stitching equipment, the preform assembly is stitched along a preset path to form an integrated U-beam preform; the stitching is adaptive stitching, and its stitching parameters are dynamically adjusted according to the stress distribution characteristics of the preform structure area;
[0014] e) Demolding and shaping: removing the combined tooling and the U-shaped core mold, and performing pre-shaping on the stitched U-shaped beam prefabricated body.
[0015] Preferably, the hybrid stress transition layer in step a) comprises main reinforcing fiber layers and toughness fiber layers alternately stacked, or is formed by a mixed stacking of at least two fiber materials with different moduli and elongations at break.
[0016] Preferably, the gradually thickening reinforcement layer in step a) is laid in a multi-stage staggered overlapping manner to achieve a smooth transition in thickness and suppress stress concentration at the overlapping edges.
[0017] Preferably, the adaptive suturing described in step d), whose dynamically adjusted suturing parameters include suturing density; in the hybrid stress transition layer area and the T-shaped connection area between the U-shaped base element and the skin continuous layer, the suturing density is higher than that in the straight web area of the base continuous layer.
[0018] Preferably, the adaptive stitching described in step d) includes dynamically adjusted stitching parameters that further include stitching stitch type; a lock stitch is used in the thickness direction of the preform, and cross-reinforced stitching is added along the diagonal direction in the cross-intersection area formed between the U-shaped base elements.
[0019] Preferably, the adaptive suturing in step d) includes closed-loop feedback control of the suturing process, the control comprising:
[0020] monitoring at least one process state parameter in real time by a sensor, wherein the process state parameter is selected from one or more of suture thread tension, preform surface morphology, suture needle temperature, or fiber breakage acoustic emission signal;
[0021] The monitored value is compared with the preset process window, and the controller drives the servo motor to adjust the motion parameters of the suture actuator in real time to keep the process state parameters within the process window.
[0022] Preferably, the removable U-shaped core mold described in step a) is an inflatable flexible core mold with a multi-chamber structure. During the laying and stitching process, local compaction of specific areas of the preform assembly is achieved by differentially inflating different chambers.
[0023] Preferably, the preforming treatment in step e) is: applying a layer of thermoplastic polymer powder on the surface of the U-beam preform by electrostatic spraying or fluidized bed process, then heating it at low temperature to melt it, and forming fixed nodes at the intersection of the fiber network after cooling.
[0024] The present invention also provides a U-shaped beam prefabricated body, which is prepared by the above method, comprising:
[0025] U-beam structure made of fiber-reinforced material;
[0026] The U-shaped beam structure has a gradient ply, including a continuous matrix layer, a hybrid stress transition layer at the corner, and a gradually thickened reinforcement layer at the arm end;
[0027] The stitching reinforcement lines that run through the thickness direction of the U-shaped beam structure have an uneven spatial distribution density in the prefabricated body, and their density in the corners and connection areas is higher than that in the straight area of the web.
[0028] Preferably, the hybrid stress transition layer comprises alternating layers of carbon fiber and aramid fiber or high-strength glass fiber; and the suture reinforcement thread is a carbon fiber tow or a ceramic fiber tow.
[0029] The beneficial effects of the present invention are:
[0030] The present invention uses a gradient ply and adaptive stitching design to highly match the distribution of fiber-reinforced materials with the structural stress field, eliminating stress concentration and weak interfaces, and significantly improving the structure's load-bearing efficiency and damage tolerance. BRIEF DESCRIPTION OF THE DRAWINGS
[0031] Figure 1 It is a cross-sectional schematic diagram of the hybrid gradient ply structure in the U-shaped base element of the present invention.
[0032] Figure 2 It is a schematic diagram of the combined tooling of the present invention.
[0033] Figure 3 Schematic diagram of multi-sensor closed-loop feedback control in the adaptive suturing process of the present invention.
[0034] Figure 4It is a schematic diagram of the suturing method in the present invention.
[0035] Markings in the figure: 1-U-shaped core mold, 2-U-shaped base element, 21-matrix continuous layer, 22-hybrid stress transition layer, 23-gradually thickened reinforcement layer, 3-skin continuous layer, 4-combined tooling, 5-suture line, 51-tension sensor, 52-controller, 53-servo motor, 6-3D scanning equipment. DETAILED DESCRIPTION
[0036] The present invention will be further explained below with reference to specific embodiments. It should be noted that the embodiments and features of the embodiments of the present invention can be combined with each other without conflict.
[0037] A method for preparing a U-shaped beam prefabricated body comprises the following steps:
[0038] a) Gradient Lamination: Select and prepare a removable U-shaped core mold 1, such as one made of a water-soluble material, a low-temperature fusible alloy, or preferably an inflatable flexible core mold with a multi-chamber structure. Perform precise gradient lamination on the U-shaped core mold 1 to form a U-shaped base element 2. Lamination structure (see Figure 1 ) is designed to change gradually to match the load-bearing requirements of the structure at different locations:
[0039] Matrix continuous layer 21: serves as the basic skeleton of the structure and is composed of multiple layers of main load-bearing fibers (such as 0° / 90° carbon fiber cloth) to ensure the overall rigidity and strength of the structure.
[0040] Hybrid stress transition layer 22: A hybrid design is employed in the U-shaped corner area, where stress is highly concentrated. For example, high-strength carbon fiber layers are alternately laid with high-toughness aramid fiber or S-glass fiber layers. The inclusion of the tough fiber layer absorbs energy when microcracks initiate through its greater deformation capacity, inhibiting crack propagation and preventing catastrophic failure.
[0041] Gradual Thickening Reinforcement Layer 23: Localized thickening is required at the tops of the U-shaped beam arms, where they connect to the skin or adjacent structures, to transfer loads and prevent connection failure. This invention utilizes a multi-level, staggered, overlapping layer approach to achieve a smooth thickness change, avoiding stress singularities caused by sudden thickness changes.
[0042] b) Integrated assembly: one or more prepared U-shaped base elements 2 are precisely placed on the flat continuous layer of skin 3 to form a rib-plate integrated prefabricated assembly.
[0043] c) Multi-dimensional tooling positioning: A set of combined tooling 4 including a bottom plate, adjustable lateral clamps and a segmented top plate is used to close the mold of the preform assembly (see Figure 2This tooling not only provides precise three-dimensional positioning but also applies uniform preload, ensuring a tight fit between fiber layers and expels interlayer air. If a multi-chamber inflatable mandrel is used, differential pressurization of chambers in areas like corners and webs can achieve superior localized densification.
[0044] d) Adaptive stitching reinforcement: Use a CNC stitching head mounted on a multi-axis robot to perform Z-direction reinforcement on the preform assembly. The stitching process is adaptive, not fixed (see Figure 3 ):
[0045] Variable-density stitching: Based on pre-input structural finite element analysis (FEA) results, the control system uses a smaller stitching spacing (high density) in areas with predicted high interlaminar stress (such as corners and T-connections); and a larger spacing (low density) in areas with lower stress (such as the center of the web).
[0046] Variable stitch sewing: Automatically switch stitches according to the connection form. For example, in the T-shaped connection area between the ribs and the skin, a lock stitch that provides high peel strength is used; and in the cross-intersection area of multiple ribs, in addition to vertical stitching, cross stitching along the 45° diagonal direction is also added (see Figure 4 ) to resist multi-directional shear loads.
[0047] Closed-loop feedback control: During the suturing process, a multi-sensor fusion monitoring system (see Figure 3 ) achieves closed-loop control. A tension sensor 51 monitors the tension of the suture thread 5 in real time; a 3D scanning device 6 or a laser profilometer monitors the preform surface for depressions or protrusions due to excessive or insufficient tension; a thermocouple integrated into the suture needle monitors frictional heat generation; and an acoustic emission probe monitors fiber breakage. All data is collected by a controller 52, which uses a control algorithm to adjust the speed and torque of the servo motor 53 in real time, dynamically adjusting parameters such as suture tension and speed to ensure consistent suture quality and minimize fiber damage.
[0048] e) Demolding and Shaping: After stitching is complete, the tooling 4 is removed. The U-shaped core mold 1 is removed by degassing, dissolving, or melting. To enhance the preform's shape stability and facilitate subsequent handling and matrix infusion, a layer of thermoplastic polymer powder (such as PEEK or PEKK) is sprayed onto its surface. This is then briefly baked at a temperature below its melting point to soften the powder and form spot welds at the fiber intersections, providing a certain degree of rigidity to the entire preform.
[0049] The U-shaped beam preform prepared by the above method has highly optimized internal fiber arrangement, Z-direction reinforcement distributed as needed, good structural integrity and excellent mechanical properties.
[0050] Example 1
[0051] This embodiment aims to prepare a carbon fiber / epoxy resin composite U-shaped stiffened wall panel preform for the leading edge of an aircraft wing.
[0052] (1) Gradient Lamination: Use a U-shaped core mold 1 made of water-soluble 3D printing material (such as BVOH). Select T700 grade carbon fiber dry fiber cloth and Kevlar-49 aramid fiber cloth. Lay the following on the U-shaped core mold 1 in sequence:
[0053] Eight layers of [0° / 90°] carbon fiber cloth serve as the base continuous layer 21 .
[0054] At the corners of the core mold, six layers of hybrid stress transition layers 22 are laid, which are composed of alternating [+45°C / -45°C] carbon fiber layers and [-45°K / +45°K] aramid fiber layers, where C represents carbon fiber and K represents aramid.
[0055] At the top of the U-shaped arm, 10 layers of [0°] carbon fiber unidirectional tape are laid in a staggered overlap manner with the width of each two layers reduced by 8 mm to form a gradually thickening reinforcement layer 23.
[0056] At this point, the preparation of a U-shaped base element 2 is completed.
[0057] (2) Integrated assembly: Lay out 6 layers of [0° / 90° / ±45°]s carbon fiber cloth on a flat mold as the skin continuous layer 3. Place the five U-shaped base elements 2 prepared in step (1) side by side and upside down on the skin continuous layer 3 according to the designed spacing.
[0058] (3) Multi-dimensional tooling positioning: Use modular aluminum alloy modular tooling 4 to position and compact the above-mentioned assembly parts, apply a pre-pressure of 0.05 MPa to ensure that the parts fit tightly.
[0059] (4) Adaptive suture enhancement: A six-axis robot suture workstation from KSL is used, and the suture line 5 is a 1K carbon fiber tow.
[0060] Stitching strategy settings: Set the stitch spacing to 3mm in the U-shaped corners and rib-to-sheet T-joints (high-density stitching areas); set the spacing to 6mm in the rib web area (low-density stitching areas). Use lockstitching that penetrates the skin in the T-joints, and add 45° cross stitching in the cross-section areas.
[0061] Closed-loop control: The baseline suture tension is set at 4 N, with an allowable fluctuation range of ±1 N. During the suture process, the suture thread tension and preform surface flatness are monitored in real time. If the 3D scanning device 6 detects that a certain area of fibers is arching due to insufficient tension, the controller 52 immediately instructs the servo motor 53 to increase the suture thread tension to 4.5 N until the area is restored to flatness.
[0062] (5) Demolding and shaping: After the stitching is completed, the modular tooling 4 is removed. The entire preform is immersed in a circulating warm water tank at 70°C. After 2 hours, the water-soluble core mold is completely dissolved and removed. After the preform is dried, a layer of polyamide (PA) micropowder with an average particle size of 40 μm is evenly attached to its surface by electrostatic spraying. It is then placed in a 150°C oven and heated for 5 minutes. After cooling, the preform is completed.
[0063] Example 2
[0064] This embodiment aims to prepare a carbon fiber / glass fiber hybrid U-shaped longitudinal beam preform for a high-speed train body chassis, which requires high stiffness, fatigue resistance and vibration reduction performance.
[0065] (1) Gradient Lamination: A U-shaped core mold 1 made of precision casting of a low melting point alloy (such as bismuth-tin alloy) is used. M40J grade high modulus carbon fiber fabric and S-2 high strength glass fiber fabric are selected.
[0066] Base continuous layer 21: 12 layers of M40J carbon fiber fabric with a main [0°] layer are laid to provide maximum longitudinal (driving direction) stiffness.
[0067] Hybrid stress transition layer 22: Eight layers of hybrid material, alternating between [+45°C / -45°C] M40J carbon fiber and [+45°G / -45°G] S-2 glass fiber, are laid in the corners. The G stands for glass fiber. The glass fiber is introduced to improve the impact toughness and damping properties of this area.
[0068] Gradual thickening reinforcement layer 23: In the area where both ends of the beam are connected to the vehicle body crossbeam, 16 layers of [0° / 90°] M40J carbon fiber fabric are laid using a pyramid overlap method to form a reinforcement area for bolt connection.
[0069] (2) Integrated assembly: Assemble two prepared U-shaped base elements 2 with a continuous layer of skin 3 also made of a carbon / glass hybrid layer.
[0070] (3) Multi-dimensional tooling positioning: A rigid steel modular tooling 4 is used, and a uniform pre-pressure of 0.1 MPa is applied through a hydraulic system to ensure the uniformity of the fiber volume content.
[0071] (4) Adaptive suture enhancement: An automated suture device equipped with an acoustic emission sensor is used, and the suture thread 5 is made of PBO (poly(p-phenylene benzobisoxazole)) fiber bundles with excellent toughness and wear resistance.
[0072] Suture strategy setting: According to the fatigue hot spot map of finite element analysis, encrypted chain stitches are used in the U-shaped corners and arm end connection areas (high-density stitching areas) with a stitching spacing of 2.5mm; ordinary lock stitches are used in the middle of the web (low-density stitching area) with a spacing of 5mm.
[0073] Closed-loop control: The baseline suture tension is set at 6N, with a fluctuation range of ±1.5N. During the suturing process, an acoustic emission sensor monitors fiber breakage caused by needle penetration. When the energy value of the monitored acoustic emission signal exceeds a preset threshold, the controller 52 determines that severe fiber damage may have occurred. It automatically reduces the suturing speed and fine-tunes the needle position to find the path with the least damage, achieving low-damage suturing.
[0074] (5) Demolding and shaping: After stitching is completed, the preform with the core mold is placed in a tunnel oven, heated to 150°C and kept warm for 30 minutes. The low-melting-point alloy core mold melts and flows out from the reserved outlet and is recovered. Subsequently, a layer of thermosetting epoxy resin powder is evenly adhered to the surface of the preform through a fluidized bed process, and heated at 120°C for 10 minutes to partially solidify it (stage B), completing the pre-shaping with good rigidity and viscosity, which is convenient for subsequent overall resin infusion.
[0075] Example 3
[0076] This embodiment aims to prepare a special-shaped U-beam preform for a high-precision six-axis industrial robot arm, which requires extreme lightness and high torsional stiffness.
[0077] (1) Gradient Lamination: A multi-chamber inflatable silicone flexible U-shaped core mold 1 is used. T800 grade high-strength carbon fiber unidirectional tape is selected.
[0078] Base continuous layer 21: Using automatic tape laying technology, 10 layers of [0° / 90° / +45° / -45°]s quasi-isotropic plies are laid to resist complex combined tensile, compressive and torsional loads.
[0079] Hybrid stress transition layer 22: In the corners, an additional layer of ultra-thin PEEK (polyetheretherketone) thermoplastic veil is placed between every two carbon fiber plies (±45°). This PEEK veil forms a tough resin-rich mass during the subsequent curing process, effectively inhibiting the initiation and propagation of microcracks.
[0080] Gradual thickening reinforcement layer 23: In the flange area where the robot arm is connected to the base and wrist, a reinforcement structure is formed by locally thickening the layer and embedding prefabricated metal inserts.
[0081] (2) Integrated assembly: This embodiment is a single U-beam structure and does not need to be assembled with the skin.
[0082] (3) Multi-dimensional tooling positioning: The prefabricated part, along with the flexible U-shaped core mold 1 inside, is placed into the closed outer mold. The core mold is inflated. By differentially inflating different chambers, the chamber pressure in the corner area (0.3 MPa) is higher than the chamber pressure in the web area (0.2 MPa), achieving focused compaction of key areas and eliminating wrinkles and voids.
[0083] (4) Adaptive suture enhancement: High-precision suture equipment is used, and the suture thread 5 is a ceramic fiber bundle with a low thermal expansion coefficient to ensure the dimensional stability of the robot arm under working temperature changes.
[0084] Stitching strategy setting: High-density stitching is used throughout the beam, with an average spacing of 2mm. In the flange connection area, the stitching forms a combination of circular and radial patterns to match the stress distribution around the bolt holes.
[0085] Closed-loop control: This system utilizes a suture tension sensor 51 in conjunction with a laser profiler. The baseline tension is set at 5N, with an allowable fluctuation of only ±0.5N. The laser profiler scans the preform's three-dimensional surface topography in real time. If it detects a surface indentation exceeding 0.1mm due to excessive tension, the controller 52 immediately fine-tunes the servo motor 53 to reduce tension, ensuring the preform's extremely high surface flatness and dimensional accuracy.
[0086] (5) Demolding and shaping: After stitching is completed, the flexible U-shaped core mold 1 is vacuumed and deflated. After shrinking, it can be easily pulled out from the preform, achieving non-destructive and reusable demolding. Then, a layer of PEKK (polyetherketoneketone) thermoplastic micropowder is sprayed on the surface of the preform, and it is quickly passed through a 280°C induction heating device to melt the micropowder surface and bond the fibers. After cooling, the pre-shaping is completed.
[0087] The present invention is not limited to the above-mentioned optional implementation modes. Anyone can derive other forms of products under the inspiration of the present invention. However, no matter what changes are made in the shape or structure, any technical solution that falls within the scope defined by the claims of the present invention falls within the scope of protection of the present invention.
Claims
1. A method for preparing a U-shaped beam preform, characterized in that: The following steps are involved: a) Gradient ply: Fiber reinforcement material is laid on a removable U-shaped core mold (1) to form at least one U-shaped base element (2), wherein the ply structure of the U-shaped base element (2) is a gradient structure, and the gradient structure at least includes: a continuous layer of substrate (21) extending through the U-shaped profile; A hybrid stress transition layer (22) located in the U-shaped corner region for dispersing shear stress; Gradual thickening reinforcement layers (23) located at the ends of the two arms of the U-shaped base element (2) for connection or support; b) integrated assembly: combining one or more of the U-shaped base elements (2) prepared in step a) with at least one continuous layer of skin (3) to form a preform assembly to be reinforced; c) Multi-dimensional tool positioning: using a combined tool (4) to clamp, compact and three-dimensionally position the preform assembly; d) Adaptive stitching enhancement: Using CNC stitching equipment, the preform assembly is stitched along a preset path to form an integrated U-beam preform; the stitching is adaptive stitching, and its stitching parameters are dynamically adjusted according to the stress distribution characteristics of the preform structure area; e) Demolding and shaping: removing the combined tooling (4) and the U-shaped core mold (1), and performing pre-shaping on the stitched U-shaped beam preform.
2. The preparation method according to claim 1, characterized in that The hybrid stress transition layer (22) described in step a) comprises main reinforcing fiber layers and toughness fiber layers alternately stacked, or is formed by mixing and laying at least two fiber materials with different moduli and elongations at break.
3. The preparation method according to claim 1 or 2, characterized in that The gradually thickened reinforcement layer (23) described in step a) adopts a multi-level staggered overlapping layering method to achieve a smooth transition of thickness and suppress stress concentration at the overlapping edge.
4. The preparation method according to claim 1, characterized in that The adaptive stitching described in step d) has dynamically adjusted stitching parameters including stitching density; in the hybrid stress transition layer (22) area and the T-shaped connection area between the U-shaped base element (2) and the skin continuous layer (3), the stitching density is higher than in the straight web area of the base continuous layer (21).
5. The preparation method according to claim 1 or 4, characterized in that The adaptive stitching described in step d) has dynamically adjusted stitching parameters further including stitching type; a lock stitch is used in the thickness direction of the preform, and cross-reinforced stitching is added along the diagonal direction in the cross-intersection area formed between the U-shaped base elements (2).
6. The preparation method according to claim 1, characterized in that The adaptive stitching described in step d) includes closed-loop feedback control of the stitching process, which includes: monitoring at least one process state parameter in real time by a sensor, wherein the process state parameter is selected from one or more of suture thread tension, preform surface morphology, suture needle temperature, or fiber breakage acoustic emission signal; The monitored value is compared with a preset process window, and the servo motor (53) is driven by a controller (52) to adjust the motion parameters of the suture actuator in real time so that the process state parameters are maintained within the process window.
7. The preparation method according to claim 1, characterized in that The removable U-shaped core mold (1) described in step a) is an inflatable flexible core mold with a multi-chamber structure. During the laying and stitching process, local compaction of specific areas of the preform assembly is achieved by differentially inflating different chambers.
8. The preparation method according to claim 1, characterized in that The preforming treatment in step e) is as follows: applying a layer of thermoplastic polymer powder on the surface of the U-beam preform by electrostatic spraying or fluidized bed process, then heating at low temperature to melt it, and forming fixed nodes at the intersection of the fiber network after cooling.
9. A U-shaped beam prefabricated body, characterized in that: Prepared by the method according to any one of claims 1 to 8, comprising: U-beam structure made of fiber-reinforced material; The U-shaped beam structure has a gradient ply, including a base continuous layer (21), a hybrid stress transition layer (22) located at the corner, and a gradually thickened reinforcement layer (23) located at the arm end; A stitching reinforcement line (5) runs through the thickness direction of the U-shaped beam structure, wherein the spatial distribution density of the stitching reinforcement line (5) in the prefabricated body is uneven, and the density in the corner and connection areas is higher than the density in the straight area of the web.
10. The preform according to claim 9, characterized in that The hybrid stress transition layer (22) comprises alternating layers of carbon fiber and aramid fiber or high-strength glass fiber; the suture reinforcement line (5) is a carbon fiber tow or a ceramic fiber tow.