Vacuum forming process of carbon fiber composite square tube

Abstract

The invention discloses a vacuum forming process of a carbon fiber composite square tube, and belongs to the technical field of composite materials, and the vacuum forming process comprises the following steps: S1, filling an elastic material with a foaming material to obtain a core layer mold; s2, coating the surface of the carbon fiber with thermosetting resin to obtain carbon fiber prepreg; s3, uniformly winding the carbon fiber prepreg cloth on the surface of the core layer mold, placing the carbon fiber prepreg cloth and the core layer mold in a vacuum bag, and vacuumizing to obtain a carbon fiber preformed pipe; s4, after the inner wall of the square steel pipe and the outer surface of the carbon fiber preformed pipe are coated with thermosetting resin, the outer surface of the carbon fiber preformed pipe is sleeved with the square steel pipe in a penetrating mode, and a preformed composite square pipe is obtained; s5, heating and curing the preformed composite square tube to obtain a carbon fiber composite square tube; the coupling strength between the square steel pipe and the carbon fiber inner pipe in the carbon fiber composite square pipe can be improved, and the shock resistance and mechanical property of the carbon fiber composite square pipe are improved.

Description

Technical Field

[0001] The invention relates to the technical field of composite materials, and in particular to a vacuum forming process of a carbon fiber composite square tube. Background Art

[0002] In the transportation industry, the energy-absorbing structure in the front area of ​​the train is the most important structure for absorbing energy and transmitting impact loads when the train collides. The performance of the materials used in the energy-absorbing structure will affect the safety of the train. Therefore, the energy-absorbing structure needs to have functions such as improving energy absorption during collision and reducing deformation after collision to ensure the survival space of the driver and passengers. At the same time, in the context of the current global energy crisis and increasingly prominent environmental problems, energy conservation and environmental protection in the transportation industry have also become the focus of social attention. Reducing the overall mass of the train by lightweighting can reduce the traction energy consumption during train operation, and reducing the overall mass of the train is conducive to reducing the inertia of the train during operation. When a train collides during operation, the impact force and counter-impact force generated by the collision will be reduced, thereby improving the safety of the train in a collision.

[0003] As a lightweight, high-specific strength and high-specific stiffness material, carbon fiber composite materials can significantly reduce the overall mass of the train while maintaining high strength, which is beneficial to improving the safety of the train while reducing the train's carbon emissions and energy consumption. However, the production cost of carbon fiber materials is much higher than that of traditional metal materials such as aluminum alloys and steel, and carbon fiber may experience unstable deformations such as local buckling and brittle fracture failure when subjected to transient impact loads, resulting in a significant decrease in its crash resistance. Therefore, carbon fiber materials are usually combined with metal materials to make composite materials so that the composite materials can have the advantages of both. However, some composite materials currently only combine carbon fiber materials with metal materials through a simple nesting method. There are inevitably dimensional errors in the production process of these two materials, which leads to gaps between the carbon fiber materials and the metal materials in the composite materials and cannot be fully coupled, which makes it difficult for the mechanical properties of the composite materials to be fully exhibited.

[0004] A Chinese patent with publication number CN108204482A discloses a novel carbon fiber composite material product, including a pipeline body, the pipeline body including a carbon fiber tube 1, a hard steel tube, a carbon fiber tube 2, a carbon fiber tube 3 and a carbon fiber tube 4, the inner wall of the carbon fiber tube 1 is provided with a plurality of grooves 1, the hard steel tube is nested inside the carbon fiber tube 1 and is provided with a plurality of protrusions 1 on the periphery, the inner wall of the pipeline is provided with a plurality of grooves 2, the hard steel tube is provided with a carbon fiber tube 2, the outer side of the carbon fiber tube 2 is provided with a plurality of protrusions and the inner wall of the pipeline is provided with a plurality of grooves 3, the carbon fiber tube 2 is provided with a carbon fiber tube 3, the outer side of the carbon fiber tube 3 is provided with a plurality of protrusions 2 and the carbon fiber tube 4 is provided inside, A number of reinforcing ribs are fixed between the carbon fiber tube four and the carbon fiber tube three, and a number of carbon fiber connecting rings are connected between the reinforcing ribs. The above-mentioned pipe body realizes the card-engagement connection of the several carbon fiber tubes and the hard steel tube through a number of protrusions and slots. However, it is difficult to avoid the existence of gaps between the carbon fiber tubes and the hard steel tubes and between adjacent carbon fiber tubes, which makes it difficult to form a completely coupled structure inside the pipe body. When the pipe body is deformed by external impact, the internal gap positions are prone to stratification, peeling and other phenomena, and the carbon fiber will produce slight cracks, which will further evolve into local cracks and produce larger deformation under the action of external force. Therefore, the carbon fiber composite material product still has room for improvement. Summary of the invention

[0005] In view of the technical defects existing in the background technology, the present invention proposes a vacuum forming process of a carbon fiber composite square tube, which solves the above technical problems and meets the actual needs. The specific technical solution is as follows: A vacuum forming process for a carbon fiber composite square tube comprises the following steps: S1, filling the elastic material with a foaming material to obtain a core layer mold, wherein a sealed space is formed inside the elastic material and the foaming material is completely wrapped; S2, coating the carbon fiber surface with a thermosetting resin, and then performing hot pressing, cooling, and laminating to obtain a carbon fiber prepreg; S3, evenly winding the carbon fiber prepreg on the surface of the core layer mold and placing them together in a vacuum bag, evacuating the vacuum bag and sealing it, so that the vacuum bag wraps the carbon fiber prepreg and the core layer mold inside to form a carbon fiber preformed tube; S4, after the inner wall of the square steel tube and the outer surface of the carbon fiber preformed tube are coated with thermosetting resin, the square steel tube is inserted into the outer surface of the carbon fiber preformed tube to obtain a preformed composite square tube; S5. The preformed composite square tube is heated and cured. During the heating process, the foaming material inside the core layer mold decomposes to generate gas to expand the core layer mold and make the outer surface of the carbon fiber preformed tube close to the inner wall of the square steel tube. After the thermosetting resin is completely cured and cooled, the core layer mold is taken out, and the carbon fiber prepreg is cured on the inner wall of the square steel tube to form a carbon fiber inner tube. The carbon fiber interior and the square steel tube together form a carbon fiber composite square tube.

[0006] As a further technical solution of the present invention, the elastic material is selected from one of silicone, fluororubber and butyl rubber.

[0007] As a further technical solution of the present invention, the foaming temperature of the foaming material is 80~120℃; in step S5, the vacuum bag is heated to soften while the foaming material decomposes, and the surface of the carbon fiber preformed tube expands simultaneously during the expansion of the core layer mold, making the outer surface of the carbon fiber preformed tube close to the inner wall of the square steel tube.

[0008] As a further technical solution of the present invention, the carbon fiber prepreg includes the following components by mass percentage: 50% to 80% carbon fiber and 20% to 50% thermosetting resin; the carbon fiber is selected from carbon fiber cloth or a mixture of carbon fiber cloth and carbon fiber filaments, chopped carbon fiber, and carbon fiber powder.

[0009] As a further technical solution of the present invention, the thermosetting resin is selected from one of epoxy resin, unsaturated polyester resin, phenolic resin, bismaleimide resin and polyimide resin.

[0010] As a further technical solution of the present invention, the carbon fiber prepreg is wound on the surface of the core layer mold for 5 to 15 turns.

[0011] As a further technical solution of the present invention, the vacuum bag is made of one of polyethylene and ethylene-vinyl acetate copolymer; in step S5, during the heating and curing process of the preformed composite square tube, the vacuum bag is heated to a molten state and has fluidity, and the preformed composite square tube is heated and cured while being tightly attached to the inner wall of the square steel tube under the action of the expansion extrusion force of the core layer mold.

[0012] As a further technical solution of the present invention, before performing step S4, the square steel pipe is subjected to the following treatment: the square steel pipe is immersed in a sodium hydroxide solution to remove oil, then washed and dried, and then the inner wall of the square steel pipe is polished, then washed and dried.

[0013] As a further technical solution of the present invention, in step S5, the preformed composite square tube is heated and cured at a temperature of 120-150° C. and a time of 50-70 min.

[0014] The beneficial effects of the present invention are: The present invention winds a carbon fiber prepreg on the surface of a heat-expandable core layer mold and then places it in a vacuum bag for vacuuming. Vacuuming can make each layer of carbon fiber prepreg and the carbon fiber prepreg and the core layer mold fit tightly together, which is conducive to forming a carbon fiber inner tube with a more stable shape and a tighter structure. During the heating and curing process, the core layer mold expands to make the surface of the carbon fiber prepreg close to the inner wall of the square steel tube, thereby obtaining a carbon fiber inner tube that is highly coupled with the square steel tube. When the carbon fiber composite square tube is impacted by an external force, the carbon fiber inner tube and the square steel tube can be deformed synchronously to avoid internal delamination, peeling and the like, thereby improving the mechanical properties of the carbon fiber composite square tube. In addition, compared with the commonly used square steel tube, the carbon fiber composite square tube has an energy absorption increase of 14%, a maximum intrusion reduction of 20.2%, and an intrusion area reduction of 17.4%. The carbon fiber composite square tube can meet the lightweight requirements of the train and the protection requirements of the energy-absorbing structure for drivers and passengers. DETAILED DESCRIPTION

[0015] The implementation mode of the present invention is described below in conjunction with relevant embodiments. The implementation mode of the present invention is not limited to the following embodiments, and the present invention relates to relevant necessary components in the technical field, which should be regarded as the known technology in the technical field and can be known and mastered by the technical personnel in the technical field.

[0016] A vacuum forming process for a carbon fiber composite square tube comprises the following steps: S1, filling the elastic material with a foaming material to obtain a core layer mold, wherein a sealed space is formed inside the elastic material and the foaming material is completely wrapped; S2, coating the carbon fiber surface with a thermosetting resin, and then performing hot pressing, cooling, and laminating to obtain a carbon fiber prepreg; S3, evenly winding the carbon fiber prepreg on the surface of the core layer mold and placing them together in a vacuum bag, evacuating the vacuum bag and sealing it, so that the vacuum bag wraps the carbon fiber prepreg and the core layer mold inside to form a carbon fiber preformed tube; S4, after the inner wall of the square steel tube and the outer surface of the carbon fiber preformed tube are coated with thermosetting resin, the square steel tube is inserted into the outer surface of the carbon fiber preformed tube to obtain a preformed composite square tube; S5. The preformed composite square tube is heated and cured. During the heating process, the foaming material inside the core layer mold decomposes to generate gas to expand the core layer mold and make the outer surface of the carbon fiber preformed tube close to the inner wall of the square steel tube. After the thermosetting resin is completely cured and cooled, the core layer mold is taken out, and the carbon fiber prepreg is cured on the inner wall of the square steel tube to form a carbon fiber inner tube. The carbon fiber interior and the square steel tube together form a carbon fiber composite square tube.

[0017] The carbon fiber composite square tube of the present invention is composed of a carbon fiber inner tube made of carbon fiber material and a directional steel tube made of steel material, and is mainly used to make an anti-collision structure in the front area of ​​a train. During the production process of the carbon fiber composite square tube, a core layer mold is used to assist in the molding of the carbon fiber inner tube. The shape of the core layer mold is a rectangular parallelepiped or a cylinder, preferably a rectangular parallelepiped. The elastic material forming the surface of the mold forms a sealed structure that completely wraps the foaming material inside. The length of the core layer mold is equal to or slightly greater than the length of the square steel tube. The length-to-width ratio of the rectangular cross-section of the core layer mold is the same as the rectangular cross-section of the square steel tube. After the core layer mold adopts the above structure, the carbon fiber prepreg is wound on the surface of the core layer mold and wrapped by a vacuum bag to form a carbon fiber preformed tube. The surface contour of the carbon fiber preformed tube is closer to the internal shape of the square steel tube, reducing the gap between the carbon fiber preformed tube and the square steel tube. In the subsequent heating and curing process, when the core layer mold expands, the surface of the carbon fiber preformed tube is better attached to the inner wall of the square steel tube.

[0018] It should be noted that the specific process of making carbon fiber prepreg is as follows: after coating the surface of the carbon fiber cloth with thermosetting resin, the thermosetting resin is evenly distributed in the carbon fiber cloth by hot pressing, and then cooled to increase the viscosity of the thermosetting resin and firmly adhere to the surface of the carbon fiber cloth, and then a layer of polyethylene film is covered on both the upper and lower surfaces of the carbon fiber cloth with the thermosetting resin attached to obtain the carbon fiber prepreg; in the carbon fiber preformed tube, after the carbon fiber prepreg is wound on the surface of the core layer mold, there are gaps between each layer of carbon fiber prepreg and between the carbon fiber prepreg and the core layer mold, and the carbon fiber prepreg and the core layer mold are placed in a vacuum bag for vacuuming. The air in the vacuum bag will be evacuated, so that the air between each layer of carbon fiber prepreg and between the carbon fiber prepreg and the core layer mold is evacuated, so that each layer of carbon fiber prepreg and between the carbon fiber prepreg and the core layer mold are tightly fitted, which is conducive to forming a carbon fiber inner tube with a more stable shape and a tighter structure, and can make the surface contour of the carbon fiber preformed tube closer to the internal shape of the square steel tube, reducing the gap between the carbon fiber preformed tube and the square steel tube.

[0019] After steps S1-S3 complete the production of the carbon fiber preformed tube, in step S4, if the square steel tube is inserted into the outer surface of the carbon fiber preformed tube, there will be an obvious gap between the carbon fiber preformed tube and the square steel tube, and the closer the surface contour of the carbon fiber preformed tube is to the internal shape of the square steel tube, the more difficult it is for the square steel tube to be inserted into the outer surface of the carbon fiber preformed tube. At this time, thermosetting resin is coated on the inner wall of the square steel tube and the outer surface of the carbon fiber preformed tube. The thermosetting resin can play a certain lubricating role, making it easier for the square steel tube to be inserted into the outer surface of the carbon fiber preformed tube to obtain a preformed composite square tube, and the thermosetting resin can be filled in the gap between part of the carbon fiber preformed tube and the square steel tube. The thermosetting resin plays a role of filling the gap and bonding in the subsequent curing process.

[0020] In step S5, the preformed composite square tube is placed in a curing furnace for heating and curing. During the heating and curing process, the foaming material inside the core layer mold will be heated to the foaming temperature, and the foaming material will decompose to produce gas to increase the internal air pressure of the elastic material and expand the core layer mold. Since the carbon fiber prepreg cloth wound on the surface of the core layer mold is not yet cured and has plasticity, the surface of the carbon fiber preformed tube will gradually cling to the inner wall of the square steel tube as the core layer mold expands, so that there is almost no gap between the carbon fiber preformed tube and the square steel tube. When the temperature inside the curing furnace reaches the curing temperature of the thermosetting resin, the carbon fiber prepreg cloth will gradually cure and form a carbon fiber inner tube on the inner wall of the square steel tube, and the carbon fiber inner tube is bonded and fixed to the square steel tube by the thermosetting resin coated on the inner wall of the square steel tube and the outer surface of the carbon fiber preformed tube in step S4. After all the thermosetting resins are cured, the curing furnace stops heating and cools down. After the square steel tube and other materials are cooled, they are taken out of the curing furnace, and the gas in the core layer mold is discharged and the core layer mold is taken out, thereby obtaining a carbon fiber composite square tube composed of a square steel tube and a carbon fiber inner tube.

[0021] It should be noted that during the curing process of the carbon fiber prepreg, the core layer mold is always in an expanded state. The core layer mold can continuously extrude the carbon fiber prepreg so that the surface of the carbon fiber prepreg is always in close contact with the inside of the square steel tube. After the carbon fiber prepreg is completely cured, a carbon fiber inner tube with a surface contour that is highly matched with the internal shape of the directional steel tube can be obtained, so that there is almost no gap between the carbon fiber inner tube and the square steel tube, and the two are highly coupled. When the carbon fiber composite square tube is hit by external force, the carbon fiber inner tube and the square steel tube can be deformed synchronously to avoid internal stratification, peeling and other phenomena, thereby improving the mechanical properties of the carbon fiber composite square tube.

[0022] The present invention winds a carbon fiber prepreg cloth on the surface of a heat-expandable core layer mold and then places it in a vacuum bag for vacuuming. Vacuuming can make each layer of carbon fiber prepreg cloth and the carbon fiber prepreg cloth and the core layer mold fit tightly together, which is conducive to forming a carbon fiber inner tube with a more stable shape and a tighter structure. During the heating and curing process, the core layer mold expands to make the surface of the carbon fiber prepreg cloth close to the inner wall of the square steel tube, thereby obtaining a carbon fiber inner tube that is highly coupled with the square steel tube. When the carbon fiber composite square tube is impacted by external force, the carbon fiber inner tube and the square steel tube can be deformed synchronously to avoid internal delamination, peeling and the like, thereby improving the mechanical properties of the carbon fiber composite square tube.

[0023] As one of the preferred embodiments of the present invention, the elastic material is selected from one of silicone, fluororubber, and butyl rubber; the elastic material needs to have good elasticity, thermal stability, and pressure resistance, which enable the core layer mold to expand through the decomposition of the foaming material in the high temperature environment of the curing furnace and maintain structural stability after expansion. Among them, silicone is preferably high-temperature silicone with good air tightness, which can ensure the normal expansion of the core layer mold, and fluororubber and butyl rubber both have excellent high temperature resistance and good elasticity and strength and other properties, which meet the needs of the core layer mold expanding in a high temperature environment.

[0024] As one of the preferred embodiments of the present invention, the foaming temperature of the foaming material is 80~120℃; in step S5, the vacuum bag is heated to soften while the foaming material decomposes, and the surface of the carbon fiber preformed tube expands simultaneously during the expansion of the core layer mold, and the outer surface of the carbon fiber preformed tube is close to the inner wall of the square steel tube; the foaming material is selected from one or more of sodium bicarbonate, modified sodium bicarbonate, and a composite foaming agent. When the carbon fiber preformed tube is heated to 80~120℃ in a curing furnace, the foaming material in the core layer mold will gradually begin to decompose and generate gas. At the same time, the film on the surface of the carbon fiber prepreg cloth and the vacuum bag will also be heated to soften or even melt, and the carbon fiber prepreg cloth and the thermosetting resin between the carbon fiber prepreg tube and the square steel tube are still uncured. The gas generated by the decomposition of the foaming material causes the core layer mold to expand while squeezing the carbon fiber prepreg cloth to a surface close to the inner wall of the square steel tube, which helps to obtain a carbon fiber inner tube that is highly coupled with the square steel tube.

[0025] As one of the preferred embodiments of the present invention, the carbon fiber prepreg includes the following components by mass percentage: 50% to 80% carbon fiber and 20% to 50% thermosetting resin; carbon fiber is the main load-bearing component in the carbon fiber prepreg, which can significantly improve the mechanical properties of the carbon fiber prepreg, making the carbon fiber prepreg more durable when subjected to external forces, and the thermosetting resin is mainly used to firmly bond the carbon fibers together to form a whole. During the heating and curing process, the thermosetting resin will form a three-dimensional cross-linked structure, which can ensure that the carbon fiber prepreg maintains stable performance during use. The thermosetting resin can also effectively and evenly transfer external forces to various parts of the carbon fiber, thereby improving the overall load-bearing capacity of the carbon fiber prepreg.

[0026] Furthermore, the carbon fiber is selected from carbon fiber cloth or a mixture of carbon fiber cloth and carbon fiber filaments, chopped carbon fibers, and carbon fiber powder; the carbon fiber cloth is used as the main load-bearing material in the carbon fiber prepreg cloth, and the carbon fiber cloth is woven with carbon fibers, which can improve the integrity of the carbon fiber prepreg cloth structure and improve the mechanical properties of the carbon fiber cloth after curing. If a carbon fiber cloth with a lower weaving density is used, an appropriate amount of carbon fiber filaments can be laid on the surface of the carbon fiber cloth or an appropriate amount of chopped carbon fibers or carbon fiber powder can be added to the thermosetting resin to ensure the mechanical properties of the carbon fiber cloth after curing.

[0027] As one of the preferred embodiments of the present invention, the thermosetting resin is selected from one of epoxy resin, unsaturated polyester resin, phenolic resin, bismaleimide resin and polyimide resin; all of the above-mentioned resins are cured by heating, and these thermosetting resins contain necessary curing agents and other additives, so that the carbon fiber prepreg can be cured by heating in a curing furnace.

[0028] As one of the preferred embodiments of the present invention, the carbon fiber prepreg is wound on the surface of the core layer mold for 5 to 15 turns; after winding the carbon fiber prepreg on the surface of the core layer mold for multiple turns, the cured carbon fiber inner tube can have a certain thickness, thereby ensuring the mechanical properties of the carbon fiber inner tube, and the interior of the cured carbon fiber inner tube is composed of multiple layers of carbon fibers. When the carbon fiber composite square tube is impacted by external force, even if the carbon fiber inner tube is slightly or partially cracked, the cracked carbon fiber layer is difficult to spread to other carbon fiber layers, thereby improving the tear resistance of the carbon fiber composite square tube.

[0029] As one of the preferred embodiments of the present invention, the vacuum bag is made of one of polyethylene and ethylene-vinyl acetate copolymer; in step S5, during the heating and curing process of the preformed composite square tube, the vacuum bag is heated to a molten state and has fluidity, and the preformed composite square tube is heated and cured while being tightly attached to the inner wall of the square steel tube under the action of the expansion and extrusion force of the core layer mold; both polyethylene and ethylene-vinyl acetate copolymer have good softness and air tightness, so that the vacuum bag can be better wrapped on the surface of the carbon fiber prepreg cloth and improve the air tightness inside the carbon fiber preformed tube, so that each layer of carbon fiber prepreg cloth is tightly fitted, and the vacuum bag will be heated to melt and have fluidity during the heating and curing process, and the core layer mold can better make the surface of the carbon fiber prepreg cloth close to the inner wall of the square steel tube through expansion, thereby improving the coupling strength between the carbon fiber inner tube and the square steel tube.

[0030] As one of the preferred embodiments of the present invention, before performing step S4, the square steel pipe is subjected to the following treatment: the square steel pipe is immersed in a sodium hydroxide solution for degreasing, then washed and dried, and then the inner wall of the square steel pipe is polished, then washed and dried; soaking the square steel pipe in a sodium hydroxide solution can remove rust-proof oil, lubricating oil and other oil stains attached to the surface of the square steel pipe during processing, so that the surface of the square steel pipe remains clean, and it is difficult to bond and fix the square steel pipe and the carbon fiber inner tube through thermosetting resin, and polishing the inner wall of the square steel pipe can increase the roughness of the inner wall of the square steel pipe, so that the square steel pipe and the carbon fiber inner tube can be firmly bonded and fixed through thermosetting resin, thereby improving the coupling strength between the square steel pipe and the carbon fiber inner tube.

[0031] As one of the preferred embodiments of the present invention, in step S5, the temperature of heating and curing the preformed composite square tube is 120~150℃ and the time is 50~70min; heating the preformed composite square tube to 120~150℃ can ensure that the thermosetting resin is thermally cured, and in the environment of this temperature, the foaming material in the core layer mold can be decomposed to expand the core layer mold, and the film on the surface of the vacuum bag and the carbon fiber prepreg cloth is heated to melt, thereby improving the coupling strength between the carbon fiber inner tube and the square steel tube and between each layer of carbon fiber prepreg cloth, and improving the mechanical properties of the carbon fiber composite square tube.

[0032] The present invention is further described below by way of examples and comparative examples.

[0033] Example 1

[0034] S1. Filling a square silicone rod with modified sodium bicarbonate having a foaming temperature of 100° C. to obtain a core layer mold, and plugging the hole of the square silicone rod filled with modified sodium bicarbonate to form a sealed space inside the square silicone rod; S2, coating the surface of the carbon fiber cloth with epoxy resin, hot pressing, cooling, and covering with a polyethylene film to obtain a carbon fiber prepreg cloth; S3, evenly winding the carbon fiber prepreg on the surface of the core layer mold and placing them together in a polyethylene vacuum bag, evacuating the vacuum bag and sealing it, so that the vacuum bag wraps the carbon fiber prepreg and the core layer mold inside to form a carbon fiber preformed tube, wherein the number of turns of the carbon fiber prepreg is 10; S4, immersing the square steel pipe in a sodium hydroxide solution to remove oil, then washing and drying, and then grinding the inner wall of the square steel pipe, then washing and drying; S5. After the inner wall of the square steel tube and the outer surface of the carbon fiber preformed tube are coated with epoxy resin, the square steel tube is inserted into the outer surface of the carbon fiber preformed tube to obtain a preformed composite square tube; S6. Place the preformed composite square tube in a curing furnace and heat it to 130°C for curing for 60 minutes. During the heating process, the modified sodium bicarbonate inside the core layer mold decomposes to produce gas, which causes the core layer mold to expand and makes the outer surface of the carbon fiber preformed tube close to the inner wall of the square steel tube. After the epoxy resin is completely cured and cooled, the gas inside the core layer mold is discharged and the core layer mold is taken out from the carbon fiber inner tube. The carbon fiber prepreg is cured on the inner wall of the square steel tube to form a carbon fiber inner tube. The carbon fiber interior and the square steel tube together form a carbon fiber composite square tube.

[0035] Comparative Example 1 S1. Winding the carbon fiber prepreg on a mold surface having the same shape as the core layer mold and placing it in a polyethylene vacuum bag, evacuating the vacuum bag and sealing it, the vacuum bag wraps the carbon fiber prepreg and the mold inside to form a carbon fiber preformed tube, wherein the number of turns of the carbon fiber prepreg is 10; S2, immersing the square steel pipe in a sodium hydroxide solution to remove oil, washing and drying, and then grinding the inner wall of the square steel pipe, washing and drying; S3, after the inner wall of the square steel tube and the outer surface of the carbon fiber preformed tube are coated with epoxy resin, the square steel tube is inserted into the outer surface of the carbon fiber preformed tube to obtain a preformed composite square tube; S4. Place the preformed composite square tube in a curing furnace and heat to 130°C for curing for 60 minutes. After the epoxy resin is completely cured and cooled, take out the mold. The carbon fiber prepreg is cured on the inner wall of the square steel tube to form a carbon fiber inner tube. The carbon fiber interior and the square steel tube together form a carbon fiber composite square tube.

[0036] Comparative Example 2 S1. Winding the carbon fiber prepreg on a mold surface with the same shape as the core layer mold and placing it in a polyethylene vacuum bag, evacuating the vacuum bag and sealing it, so that the vacuum bag wraps the carbon fiber prepreg and the mold inside to form a carbon fiber preformed tube; S2. Place the carbon fiber preformed tube in a curing furnace and heat it to 130°C for curing for 60 minutes. After the carbon fiber prepreg cloth is completely cured and cooled, take out the mold to obtain a carbon fiber square tube, wherein the size of the carbon fiber square tube is the same as the size of the carbon fiber composite square tube obtained in the above Example 1.

[0037] Comparative Example 3 This comparative example is a square steel tube having the same size as the carbon fiber composite square tube obtained in Example 1 above.

[0038] The pipes obtained from all the above embodiments and comparative examples were subjected to crash resistance test: the two ends of the pipe were fixed and placed horizontally in the test area, and pressure was applied to the center of the pipe by hydraulic equipment and pressure head. The main test indicators included effective absorption energy, specific energy absorption, peak force, crushing force efficiency, maximum intrusion amount, and intrusion area. The test results are shown in Table 1:

[0039] Table 1 It should be noted that the effective absorbed energy represents the total amount of energy absorbed by the pipe during the test; the specific energy absorption represents the energy absorbed by the unit mass of the pipe during the collision, and is an important evaluation indicator for structural lightweighting. The larger the specific energy absorption, the higher the energy absorption efficiency of the structure; the peak force represents the maximum crushing force generated during the deformation of the pipe. After the pipe is applied to the train energy absorption structure, excessive peak force may lead to a decrease in the survival rate of the driver and passengers; the crushing force efficiency is usually used to evaluate the stability of the pipe during the crushing process. The ideal energy absorption structure always pursues high crushing force efficiency; the maximum intrusion amount represents the final deformation of the pipe after elastic recovery after the hydraulic equipment and pressure head and other devices have completed the pressure on the pipe. The intrusion amount after the pipe is crushed should be minimized to avoid excessive intrusion; the intrusion area represents the area swept by the pipe before and after the bending and crushing. The smaller the intrusion area, the better the elastic recovery performance of the pipe.

[0040] It can be seen that the pipe used as the train energy absorption structure needs to have high specific energy absorption, low peak force, high crush force efficiency, low maximum intrusion volume, and low intrusion area; analysis of the data in Table 1 shows that although the carbon fiber composite square tubes of Example 1 and Comparative Example 1 adopt similar structures, the manufacturing process of Comparative Example 1 cannot make the square steel tube and the carbon fiber inner tube highly coupled, so the performance of the carbon fiber composite square tube of Comparative Example 1 is poor; and although the carbon fiber square tube of Comparative Example 2 performs well in many indicators, its crush force efficiency is low, and it requires a large amount of carbon fiber to be made and the cost is high, making it difficult to be used as an ideal raw material for energy absorption structure; the square steel tube of Comparative Example 3 is a common steel pipe, which can be used as the raw material of some current train energy absorption structures. After comprehensively considering the lightweight requirements of the train and the collision resistance of the pipe, the carbon fiber composite square tube of Example 1 can be used as a substitute for conventional steel pipes and used to make train energy absorption structures.

[0041] At present, the energy-absorbing structure of a train is usually made of square steel tubes such as those in Comparative Example 3. The carbon fiber composite square tube obtained in Example 1 based on the technical solution of the present invention is compared with the square steel tube. The energy absorption is increased by 14%, the maximum intrusion amount is reduced by 20.2%, and the intrusion area is reduced by 17.4%. Although the crushing efficiency is reduced by 1.4%, based on the improvement of the above-mentioned performance parameters of the carbon fiber composite square tube, the carbon fiber composite square tube can meet the lightweight requirements of the train and the protection requirements of the energy-absorbing structure for drivers and passengers.

[0042] The above is only a preferred embodiment of the present invention. It should be pointed out that ordinary technicians in this technical field can make several improvements and modifications without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A vacuum forming process for a carbon fiber composite square tube, characterized in that: The following steps are involved: S1, filling the elastic material with a foaming material to obtain a core layer mold, wherein a sealed space is formed inside the elastic material and the foaming material is completely wrapped; S2, coating the carbon fiber surface with a thermosetting resin, and then performing hot pressing, cooling, and laminating to obtain a carbon fiber prepreg; S3, evenly winding the carbon fiber prepreg on the surface of the core layer mold and placing them together in a vacuum bag, evacuating the vacuum bag and sealing it, so that the vacuum bag wraps the carbon fiber prepreg and the core layer mold inside to form a carbon fiber preformed tube; S4, after the inner wall of the square steel tube and the outer surface of the carbon fiber preformed tube are coated with thermosetting resin, the square steel tube is inserted into the outer surface of the carbon fiber preformed tube to obtain a preformed composite square tube; S5. The preformed composite square tube is heated and cured. During the heating process, the foaming material inside the core layer mold decomposes to generate gas to expand the core layer mold and make the outer surface of the carbon fiber preformed tube close to the inner wall of the square steel tube. After the thermosetting resin is completely cured and cooled, the core layer mold is taken out, and the carbon fiber prepreg is cured on the inner wall of the square steel tube to form a carbon fiber inner tube. The carbon fiber interior and the square steel tube together form a carbon fiber composite square tube.

2. The vacuum forming process of carbon fiber composite square tube according to claim 1, characterized in that: The elastic material is selected from silicone, fluororubber and butyl rubber.

3. The vacuum forming process of carbon fiber composite square tube according to claim 1, characterized in that: The foaming temperature of the foaming material is 80-120° C.; in step S5, the vacuum bag is heated to soften while the foaming material decomposes, and the surface of the carbon fiber preformed tube expands simultaneously during the expansion of the core layer mold, making the outer surface of the carbon fiber preformed tube close to the inner wall of the square steel tube.

4. The vacuum forming process of carbon fiber composite square tube according to claim 1, characterized in that: The carbon fiber prepreg comprises the following components by mass percentage: 50% to 80% carbon fiber and 20% to 50% thermosetting resin; the carbon fiber is selected from carbon fiber cloth or a mixture of carbon fiber cloth and carbon fiber filaments, chopped carbon fiber, and carbon fiber powder.

5. The vacuum forming process of carbon fiber composite square tube according to claim 1, characterized in that: The thermosetting resin is selected from one of epoxy resin, unsaturated polyester resin, phenolic resin, bismaleimide resin and polyimide resin.

6. The vacuum forming process of carbon fiber composite square tube according to claim 1, characterized in that: The carbon fiber prepreg is wound on the surface of the core layer mold for 5 to 15 turns.

7. The vacuum forming process of carbon fiber composite square tube according to claim 1, characterized in that: The vacuum bag is made of one of polyethylene and ethylene-vinyl acetate copolymer; in step S5, during the heating and curing process of the preformed composite square tube, the vacuum bag is heated to a molten state and has fluidity, and the preformed composite square tube is heated and cured while being tightly attached to the inner wall of the square steel tube under the action of the expansion extrusion force of the core layer mold.

8. The vacuum forming process of carbon fiber composite square tube according to claim 1, characterized in that: Before performing step S4, the square steel pipe is treated as follows: the square steel pipe is immersed in a sodium hydroxide solution to remove oil, then washed and dried, and then the inner wall of the square steel pipe is polished, then washed and dried.

9. The vacuum forming process of carbon fiber composite square tube according to claim 1, characterized in that: In step S5, the preformed composite square tube is heated and cured at a temperature of 120-150° C. for 50-70 minutes.

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