An implicit bamboo-reinforced composite pipe and a forming process thereof

Abstract

This application provides an internally concealed bamboo-joint reinforced composite pipe and its molding process, belonging to the field of composite material technology. It includes a pultruded layer, an inner braided layer, a first thermoplastic mesh layer, a winding layer, an internally concealed bamboo joint, a second thermoplastic mesh layer, an outer braided layer, and an epoxy resin layer arranged sequentially. The pultruded layer is located on the innermost side, and the epoxy resin layer is located on the outermost side. This not only gives the composite pipe a smooth outer surface but also effectively solves the problems of interlayer separation and energy absorption.

Description

Technical Field

[0001] This application relates to an internally concealed bamboo-joint reinforced composite pipe and its molding process, belonging to the field of composite material technology. Background Technology

[0002] Carbon fiber composite tubes possess excellent specific strength and energy absorption properties, making them widely used in aerospace and rail transportation collision avoidance applications. Currently, the fabrication of carbon fiber composite tubes mainly involves three processes: pultrusion, winding, and braiding. However, each single process has limitations in axial or circumferential performance. The emerging "internal pultrusion + intermediate winding + external braiding" integrated continuous molding process achieves complementary properties through multi-layer composites and solves some strength issues. However, in applications as high-performance materials, this integrated continuous molding process still has the following significant drawbacks: First, the interlayer interface is fragile. Because the inner layer pultrusion angle is 0° and the middle layer winding angle is close to 90°, there is a significant abrupt change in stiffness between the two. When the material is subjected to impact, it is very easy to induce shear failure and brittle delamination.

[0003] Secondly, the pipe fittings have a uniform overall structure. When the material is impacted, it cannot be guided to produce orderly layer-by-layer folding, which easily leads to uncontrollable brittle fracture and poor energy absorption.

[0004] Therefore, to address the aforementioned defects, introducing a biomimetic bamboo-like structure into composite materials to form bamboo-joint tubes is an effective way to solve the problems of interlayer separation and energy absorption. However, existing bamboo-joint tube manufacturing processes result in wavy protrusions on the outer surface of the tube, which damages the flatness of the tube and makes it impossible to precisely assemble with standard joints. Furthermore, constructing localized thickening structures on continuous pultrusion production lines can easily lead to mold blockage. Summary of the Invention

[0005] In view of this, this application provides an internally concealed bamboo-joint reinforced composite pipe, which introduces an internally concealed bamboo-joint structure into the composite pipe, not only giving the composite pipe a smooth outer surface, but also effectively solving the problems of interlayer separation and energy absorption.

[0006] Specifically, this application is implemented through the following scheme: An internally concealed bamboo-joint reinforced composite pipe includes a pultruded layer, an inner braided layer, a first thermoplastic mesh layer, a winding layer, an internally concealed bamboo joint, a second thermoplastic mesh layer, an outer braided layer, and an epoxy resin layer arranged in sequence, with the pultruded layer located on the innermost side and the epoxy resin layer located on the outermost side.

[0007] Furthermore, as a preferred option: The pultruded layer is composed of carbon fibers and a resin matrix. More preferably, the carbon fibers are T700 grade or 12K carbon fibers, arranged axially (0°). The resin matrix is ​​E-51 epoxy resin.

[0008] The first thermoplastic web layer is a copolynylon (CoPA, melting point about 125-135°C) web or a low-melting-point polyester web.

[0009] The second thermoplastic web layer can be made of copolynylon (CoPA) web or low-melting-point polyester web.

[0010] To address the technical problems of existing composite pipes, such as weak interfacial bonding and insufficient shear resistance due to interlaminar stiffness mismatch, and the lack of controlled induced failure mechanisms in homogeneous, uniform cross-section structures, which easily lead to uncontrollable longitudinal splitting and brittle fracture under impact, resulting in low specific energy absorption efficiency, the aforementioned multi-layer composite pipe employs a "internal pultrusion-internal large-angle braiding-mesh toughening-variable-density winding-mesh toughening-external small-angle braiding" structure. This aims to solve the aforementioned problems of uncontrollable interlaminar separation and energy absorption modes, while overcoming the manufacturing bottleneck of existing biomimetic reinforcement structures that struggle to achieve both high performance and high surface quality, thus achieving a synergistic improvement in interlaminar shear strength and specific energy absorption of the pipe.

[0011] In addition, the applicant also provided the molding process for the aforementioned concealed bamboo-joint reinforced composite pipe, the steps of which are as follows: Step 1: Using a rigid mandrel as a carrier, carbon fiber bundles are drawn out from the yarn rack, sorted by a yarn separating plate, and then sent to the wet preforming mold of the pultrusion layer. Preforming is carried out on the carrier, and the carbon fibers are impregnated with resin and bundled into unidirectional uncured wet pultrusion layers.

[0012] Step two: The carrier moves to the inner braiding machine, where it is braided and anchored at a large angle on the surface of the uncured wet pultruded layer, forming an inner constraint braided layer with high friction texture. The large angle is preferably 50-60°.

[0013] Step 3: The carrier continues to move to the first mesh supply mechanism, where the inner constraint braided layer surface is covered with a thermoplastic mesh online.

[0014] Step four: The carrier moves to the variable-speed winding mechanism, where it periodically winds the inner constraint braided layer to form a tube blank. The winding speeds of the conventional segments and the bamboo-joint segments are different, with the bamboo-joint segments winding at a higher speed than the conventional segments. Prefabricated bamboo joints with localized protrusions are constructed between adjacent conventional segments by stacking fibers, with a winding angle of 85–88°. The winding speed of the bamboo-joint segments is 3–5 times that of the conventional segments.

[0015] Step 5: The carrier moves to the second mesh supply mechanism, where a thermoplastic mesh is applied online to the outer surface of the tube blank with pre-formed bamboo joints.

[0016] Step six: The carrier moves to the outer braiding machine, where the outer braiding machine braids and binds the tube blank at a small angle, forming an outer constraint braided layer with self-locking properties. The small angle is 27-29° (nearly 30°).

[0017] Step 7: The carrier moves to the flared end thermosetting mold, which includes a tapering section and a curing section. The inner diameter of the tapering section decreases. The tube blank enters the flared end thermosetting mold along with the carrier. Under the forced inward compression of the inner wall of the tapering section, the pre-formed bamboo joints are embedded in the uncured tube blank, transforming it into an internally concealed structure. The carrier continues to move, and the tube blank undergoes three-stage variable temperature gradient curing in the curing section of the mold. The thermoplastic mesh is cured and undergoes in-situ melting and diffusion to complete toughening. After being output by the traction wheel and cut, the internally concealed bamboo joint reinforced composite tube is obtained. The three-stage variable temperature gradient curing includes three gradients: a preheating section, a gelling section, and a curing section. The temperature of the preheating section is 100–135℃, the temperature of the gelling section is 145–155℃, and the temperature of the curing section is 170–180℃.

[0018] In the structure of the internally concealed bamboo-joint reinforced composite tube, the uncured wet pultruded layer in step one, the inner constrained braided layer in step two, the thermoplastic mesh in step three, the variable speed winding in step four, the thermoplastic mesh in step five, and the outer constrained braided layer in step six sequentially form the pultruded layer, the inner braided layer, the first thermoplastic mesh layer, the winding layer, the internally concealed bamboo joint, the second thermoplastic mesh layer, and the outer braided layer. The pultruded layer is located on the innermost side of the internally concealed bamboo-joint reinforced composite tube, and the epoxy resin matrix forms an epoxy resin layer on the outside of the outer braided layer.

[0019] In the above process: 1) The carrier adopts a rigid core mold to ensure that the inner diameter of the composite tube is constant and has bending stiffness, while the resin body gives the tube blank an uncured wet state before entering the flared mouth thermosetting mold. This state not only gives the inner core of the tube blank compressibility, but also provides a space for the subsequent acceptance of the bamboo joints that are squeezed inward.

[0020] 2) During the construction of the internal constraint braided layer, the braiding nodes of the large-angle braiding process form physical uneven textures. These physical uneven textures, together with the smooth pultruded layer surface, form the anchoring base for the subsequent bamboo joints to attach to.

[0021] 3) The thermoplastic mesh is a thermoplastic nonwoven mesh. During the curing stage, the mesh melts and penetrates into the adjacent carbon fibers to form a tough intercalation structure, which significantly blocks the interlayer propagation of conflict cracks and solves the problem of easy breakage at the bamboo joint.

[0022] 4) The variable speed winding mechanism uses two winding speeds, which are alternated periodically. When constructing the conventional section, the winding speed is matched with the traction speed to form a uniform wall thickness. When constructing the bamboo section, the winding speed is increased to a high winding speed. The speed ratio difference is used to stack an excessive amount of fiber in a very short axial distance to form an outwardly convex spindle-shaped prefabricated bamboo section.

[0023] 5) During the construction of the outer constraint braided layer, the small-angle braided structure initially binds the loose thermoplastic mesh and the raised prefabricated bamboo joints to ensure that the surface of the finished composite pipe is flat and smooth.

[0024] 6) The molding and curing at the flared end thermosetting mold is crucial and key to achieving the concealed bamboo-like structure. Its tapered section serves as the inlet for the tube blank, employing a flared, conical structure. The inlet diameter of the tapered section is larger than the outer diameter of the pre-made bamboo-like structure, while the outlet diameter matches the designed outer diameter of the composite tube. When the tube blank enters the flared end thermosetting mold, its outer diameter is restricted by the inner wall of the tapered section, preventing outward expansion. Under radial pressure, the loose fibers in the pre-made bamboo-like structure are forced to collapse inward, squeeze, and embed themselves within the inner constraint braided layer. Simultaneously, the two layers of thermoplastic mesh deform and fill the gaps, further preventing interlayer slippage and completing the transformation from an outwardly convex and loose structure to an inwardly concealed and dense structure.

[0025] This application overcomes the contradiction between biomimetic structure manufacturing and surface precision control, obtaining a continuous molding method that can integrate "hidden bamboo joints" to enhance interlayer bonding and energy absorption efficiency while ensuring a highly flat outer surface. This method has significant practical implications for improving the overall performance of composite material pipe fittings and expanding their engineering applications. Compared with existing technologies, it has the following beneficial effects: First, it achieves superior failure mode control. Compared to the "end blooming" and intermediate layer loosening and collapse that are prone to occur in existing braided-wound-pultruded pipe fittings, the implicit bamboo joints in this application provide stronger radial support for the intermediate winding layer. This structure effectively suppresses brittle delamination between layers, forcing the pipe fitting to change from unstable splitting failure to a more controllable and tighter controlled step-by-step folding mode, significantly improving the stability of the load platform.

[0026] Secondly, higher specific energy absorption efficiency was achieved. Compared with traditional homogeneous pipe fittings that lack crack-prevention mechanisms, the "bamboo ring" built into this application can more effectively block the rapid propagation of long longitudinal cracks. This mechanism induces more complete plastic deformation and fiber fracture in the pipe fitting, thereby achieving a significant increase in plateau stress and specific energy absorption for the same weight.

[0027] Finally, the contradiction between structural reinforcement and surface precision was resolved. Compared to the inherent surface bulges or fiber damage caused by machining in traditional bamboo-joint tubes, the "inner convex, outer flat" structure of this application ensures a more stringent geometric flatness of the outer surface while maintaining higher fiber continuity and tension retention. This not only overcomes the difficulty of precision assembly in biomimetic tube fittings but also achieves superior engineering applicability compared to traditional irregular-shaped tube fittings. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application.

[0029] Figure 1 This is a schematic diagram of the processing flow of this application.

[0030] Figure 2 This is a cross-sectional schematic diagram of the flared end thermosetting mold in this application.

[0031] Figure 3 This is a schematic diagram of the internally concealed bamboo-joint reinforced composite pipe in this application.

[0032] Labels in the diagram: 1. Yarn frame; 2. Yarn guide plate; 3. Pultruded layer wet preforming mold; 4. Inner braiding machine; 5. First web film supply mechanism; 6. Variable speed winding mechanism; 7. Second web film supply mechanism; 8. Outer braiding machine; 9. Trumpet-shaped thermosetting mold; 901. Tapered section; 902. Curing section; 10. Traction wheel; 11. Cutting unit; 12. Inner concealed slub reinforced composite tube; 121. Pultruded layer; 122. Inner braided layer; 123. First thermoplastic web film layer; 124. Inner concealed slub; 125. Winding layer; 126. Second thermoplastic web film layer; 127. Outer braided layer; 128. Epoxy resin layer. Detailed Implementation

[0033] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer, the technical solutions in the embodiments of this application will be further described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit the technical solutions of this application. All other embodiments obtained by those skilled in the art based on the embodiments in this application without creative effort are within the scope of protection of this application.

[0034] It should be noted that when a component is referred to as "fixed to" or "set on" another component, it can be located directly or indirectly on that other component. When a component is referred to as "connected to" another component, it can be directly or indirectly connected to that other component. The terms "upper," "lower," "left," "right," "front," "rear," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate the orientation or position based on the orientation or position shown in the accompanying drawings, and are only for ease of description and should not be construed as limiting the present technical solution.

[0035] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or specifying the number of technical features. "A plurality of" means two or more, unless otherwise explicitly defined.

[0036] This embodiment provides an internally concealed bamboo-joint reinforced composite pipe. The embodiment of this application is described below with reference to the accompanying drawings.

[0037] See Figure 1 , Figure 1 The molding process of the concealed bamboo-joint reinforced composite pipe in this embodiment is shown, and the specific steps are as follows: S1, wet preforming of the pultruded layer under rigid mandrel support. T700 grade 12K carbon fiber is selected as the reinforcing material, and E-51 epoxy resin is used as the matrix. 120 bundles of carbon fiber are drawn from the yarn rack 1, impregnated with resin, and then evenly coated onto the surface of a long cantilevered rigid mandrel with a diameter of 36mm under the guidance of the yarn separating plate 2. This is then preformed in the wet preforming mold 3 of the pultruded layer, forming a unidirectional (0°) uncured wet pultruded layer. The uncured wet pultruded layer serves as the axial skeleton of the pipe fitting, and is in a softened and compressible state at this time. The rigid mandrel inside provides sizing and bending support.

[0038] S2, Inner layer large-angle braiding anchoring. Supported by a rigid core mold, the uncured wet pultruded layer enters the first braiding device, namely the inner braiding machine 4, and is braided at a large angle of 50-60° to form an inner constraint braided layer. In this step, the high circumferential force generated by the large-angle braiding tightly wraps the wet inner core, and the physical texture of the braided nodes "bites" into the smooth pultruded layer, preventing axial slippage of the subsequent bamboo joints during molding deformation.

[0039] S3, First layer interface toughening treatment. Using the first web feeding mechanism 5, a layer of low melting point copolymer nylon (CoPA) thermoplastic nonwoven web (melting point about 125℃~135℃) is wrapped online to cover the surface of the inner woven layer.

[0040] S4, Bamboo Joint Online Construction (Variable Speed ​​Winding). The tube blank enters the variable speed winding mechanism 6, with the basic winding angle set to 88°. The winding machine executes a "periodic variable density" program: Regular section: The winding spindle maintains a constant speed, forming a winding layer of uniform thickness; Bamboo joint section: Every 100mm (preset pitch), the winding spindle performs "high-frequency rotation," instantly increasing the speed to 4 times the regular speed. Utilizing the speed ratio difference, excess fibers are locally stacked on the surface of the tube blank, forming an outwardly convex spindle-shaped prefabricated bamboo joint.

[0041] S5, Second layer interface toughening treatment. Using the second web feeding mechanism 7, another layer of CoPA thermoplastic nonwoven web is wrapped online on the outer surface of the winding layer with pre-formed bamboo joints.

[0042] S6, outer layer small-angle braiding restraint. Entering the second braiding device, the outer braiding machine 8, a small-angle braiding of 27~29° (nearly 30°) is performed on the outermost layer, forming an outer restraint braiding layer. Utilizing the self-locking characteristic of the small-angle braiding structure, similar to a "buckle," a strong radial contraction force is generated under axial traction, acting like a "high-strength bodysuit" to pre-bind the loose mesh and protruding bamboo joints, ensuring a smooth and flat surface on the finished product.

[0043] S7, flared end tapering molding and bamboo joint concealment transformation. Four layers of wet tube preform are introduced into the flared end thermosetting mold 9. Combined Figure 2 The flared-mouth thermosetting mold 9 includes a tapered section 901 and a curing section 902. The tapered section 901 is located at the entrance of the flared-mouth thermosetting mold 9 and has a tapered tapering structure. The entrance diameter (45mm) is larger than the maximum outer diameter of the pre-made bamboo joint. The middle section and the exit of the mold are both the curing section 902, and the diameter of the curing section 902 is equal to the designed outer diameter of the finished product (40mm). When the tube blank passes through the tapered section 901, the outer diameter of the tube blank is restricted by the inner wall of the tapered section 901 and cannot expand outward. The loose bamboo joint fibers are forced to collapse inward under radial pressure, squeezed and deeply embedded in the softened inner constraint braided layer and pultruded layer, completing the structural transformation from "outwardly convex and loose" to "inwardly concealed and dense".

[0044] S8, Three-stage variable temperature gradient curing and fixed-length cutting. The tube preform sequentially passes through a preheating zone (110℃), a gelation zone (150℃), and a curing zone (175℃) within the curing section 902 to complete gradient curing. In the preheating and gelation zones: the resin viscosity decreases, the CoPA mesh (melting point <140℃) begins to soften and melt, and penetrates into the adjacent carbon fiber layer with the help of molding pressure. In the curing zone: the temperature (175℃) is much higher than the mesh melting point, and the mesh completely melts and diffuses to form a "thermoplastic / thermosetting interpenetrating network structure," preventing crack propagation. The cured rigid tube is pulled out of the mold by the traction wheel 10 and then cut online to a fixed length by the cutting unit 11 according to the bamboo joint distribution signal to obtain the finished product, the internally implicit bamboo joint reinforced composite tube 12.

[0045] The cross-sectional view of the resulting product is as follows Figure 3 As shown: the uncured wet pultruded layer of step one, the inner constraint braided layer of step two, the thermoplastic mesh of step three, the variable speed winding of step four, the thermoplastic mesh of step five, and the outer constraint braided layer of step six sequentially form the pultruded layer 121, the inner braided layer 122, the first thermoplastic mesh layer 123, the inner hidden bamboo joint 124 and the winding layer 125, the second thermoplastic mesh layer 126, and the outer braided layer 127. The pultruded layer 121 is located on the innermost side of the inner hidden bamboo joint reinforced composite tube 12, and the resin body 128 forms a resin layer on the outside of the outer braided layer.

[0046] The performance of the obtained implicit bamboo-joint reinforced composite pipe was tested, and the results are as follows: the load platform of the composite pipe was stable without violent fluctuations during the loading process, and no interlayer separation, longitudinal splitting or end cracking occurred, and the load structure stability was significantly improved.

[0047] Compared with traditional homogeneous composite pipes, the internally concealed bamboo-joint reinforced composite pipe of this application has a specific energy absorption value that is more than 30% higher, a more stable peak load, higher energy absorption efficiency, and significantly improved interlaminar shear strength.

[0048] The above-described embodiments are merely illustrative of several feasible implementations of the present invention, and their descriptions are relatively specific and detailed. However, they should not be construed as limiting the scope of the present invention, nor are the embodiments intended to limit the scope of protection in the claims of the present invention. For those skilled in the art, various modifications and improvements can be made without departing from the concept of the present invention. All equivalent implementations or changes that do not depart from the present invention should be included in the technology of the present invention.

Claims

1. A concealed bamboo-joint reinforced composite pipe, characterized in that: It includes a pultruded layer, an inner braided layer, a first thermoplastic web layer, a winding layer and an inner hidden slub layer, a second thermoplastic web layer, an outer braided layer and an epoxy resin layer arranged in sequence, with the pultruded layer located on the innermost side and the epoxy resin layer located on the outermost side.

2. The concealed bamboo-joint reinforced composite pipe according to claim 1, characterized in that: The pultruded layer is composed of carbon fiber and resin.

3. The concealed bamboo-joint reinforced composite pipe according to claim 2, characterized in that: The carbon fiber is T700 grade or 12K carbon fiber, and the carbon fiber is arranged axially in the pultruded layer; the resin body is E-51 epoxy resin.

4. The concealed bamboo-joint reinforced composite pipe according to claim 1, characterized in that: The first thermoplastic web layer is a copolynylon web or a low-melting-point polyester web.

5. The concealed bamboo-joint reinforced composite pipe according to claim 1, characterized in that: The second thermoplastic web layer is a copolynylon web or a low-melting-point polyester web.

6. A molding process for the concealed bamboo-joint reinforced composite pipe as described in claim 1, characterized in that, The steps are as follows: Step 1: Using a rigid mandrel as a carrier, carbon fiber bundles are drawn out from the yarn rack, sorted by a yarn separating plate, and then sent to the wet preforming mold of the pultrusion layer. The carbon fibers are impregnated with resin and bundled into an uncured wet pultrusion layer with unidirectional arrangement. Step 2: Perform large-angle braiding anchoring on the surface of the uncured wet pultruded layer to form an internal constraint braided layer, wherein the large angle is 50-60°; Step 3: Online coating of thermoplastic mesh film onto the surface of the inner constraint braided layer; Step 4: Periodically wind the inner constraint braided layer to form a tube blank containing conventional segments and prefabricated bamboo segments. The winding speed of the bamboo segments constructed online is higher than that of the conventional segments, and the winding angle is 85-88°. Step 5: Online wrapping of thermoplastic mesh film on the outer surface of the tube blank with pre-made bamboo joints; Step six: The tube blank processed in step five is braided and bound at a small angle to form an outer constraint braided layer, wherein the small angle is 27-29°. Step 7: The tube blank processed in Step 6 is fed into the flared-mouth thermosetting mold. The flared-mouth thermosetting mold includes a tapering section and a curing section. The inner diameter of the tapering section decreases. The tube blank enters the flared-mouth thermosetting mold along with the carrier. Under the forced inward compression of the inner wall of the tapering section, the pre-made bamboo joint is transformed into an internally concealed structure. The tube blank undergoes three-stage variable temperature gradient curing in the curing section. The thermoplastic mesh is cured and melted and diffused in situ to complete the toughening. After being output by the traction wheel and cut, the internally concealed bamboo joint reinforced composite tube is obtained.

7. The molding process of an internally concealed bamboo-joint reinforced composite pipe according to claim 6, characterized in that: The thermoplastic web is a thermoplastic nonwoven web.

8. The molding process of an internally concealed bamboo-joint reinforced composite pipe according to claim 6, characterized in that: In step four, the winding speed of the prefabricated bamboo segment is 3 to 5 times that of the conventional segment winding speed.

9. The molding process of an internally concealed bamboo-joint reinforced composite pipe according to claim 6, characterized in that: In step seven, the three-stage variable temperature gradient curing includes three gradients: a preheating stage, a gel stage, and a curing stage. The temperature of the preheating stage is 100–135°C, the temperature of the gel stage is 145–155°C, and the temperature of the curing stage is 170–180°C.

Images