Composite fuel tank and method of forming same

Abstract

The present application relates to the technical field of composite materials, and particularly relates to a composite material oil tank and a forming method thereof. The present application provides a composite material oil tank, which comprises a plurality of barrel bodies with open ends, and the barrel bodies are connected in the axial direction; the cross section of the first end of each barrel body is a ladder structure with two ladder shapes which are symmetrical along the axis; the cross section of the second end of each barrel body is a protrusion structure with two protrusion shapes which are symmetrical along the axis and extend in the axial direction; the ladder structure and the protrusion structure are matched in shape to complete splicing; the surface of the protrusion structure close to the axis and one surface of the ladder structure are sealed and bonded by sealing glue; and the protrusion surface of the protrusion structure perpendicular to the axis and the ladder structure are bonded by structural glue. The present application provides a composite material oil tank and a forming method thereof, which can realize the forming of a large-size and lightweight composite material oil tank and improve the sealing reliability of the composite material manufacturing.

Description

Technical Field

[0001] This invention relates to the field of composite material technology, and in particular to a composite material fuel tank and its molding method. Background Technology

[0002] Traditionally, larger fuel tanks are mostly made of metal. However, with the rapid development of the aerospace industry, aircraft are becoming faster and have longer ranges, leading to increasingly higher requirements for lightweight design. Compared to traditional metal materials, composite materials have higher specific modulus and specific strength, which can maximize the achievement of structural lightweighting goals.

[0003] Fuel tanks made of composite materials are relatively rare. On the one hand, it's difficult to use composite molding methods for large, complex fuel tanks. On the other hand, to leverage the lightweight advantages of composite materials, composite fuel tanks often have thin walls and poor rigidity. Under high-temperature conditions and high internal pressure, the weakly rigid structure is prone to deformation, leading to seal leaks. While increasing rigidity by adding fasteners and reinforcing ribs can improve rigidity, the large size of the fuel tank necessitates a large number of fasteners and ribs, significantly increasing structural weight and hindering the achievement of lightweight goals. Furthermore, each additional fastener penetrating the tank adds a potential leak point, increasing the risk of leaks. Therefore, achieving a completely effective seal without increasing structural weight is quite challenging.

[0004] In view of this, how to achieve the molding of large-size, lightweight composite material fuel tanks and improve sealing reliability are important technical problems that urgently need to be solved in the field of composite material manufacturing and application. Summary of the Invention

[0005] This invention provides a composite material fuel tank and its molding method, which can realize the molding of large-size, lightweight, internal pressure resistant composite material fuel tanks and improve sealing reliability in composite material manufacturing.

[0006] In a first aspect, embodiments of the present invention provide a composite material oil tank, comprising a plurality of cylindrical bodies with openings at both ends, the plurality of cylindrical bodies being connected axially; The first end of each cylinder has a cross-section of two stepped structures symmetrical along the axis, and the second end of each cylinder has a cross-section of two axially extending protruding structures symmetrical along the axis. The stepped structures and the protruding structures are shaped to match to complete the splicing. The surface of the protruding structure near the axis and one surface of the stepped structure are sealed and bonded with sealant. The protruding surface of the protruding structure perpendicular to the axis and the stepped structure are bonded with structural adhesive.

[0007] Optionally, the surface of the protruding structure away from the axis of the cylinder and the non-cylinder surface of the stepped structure away from the axis of the cylinder are coplanar, so that after the adjacent cylinders are spliced, an annular recess is formed at the splice, with a cross-section of two symmetrical grooves along the axis, and the annular recess is filled with axially extending unidirectional fiber composite material.

[0008] Optionally, the structural adhesive and / or the sealant has a thickness of no more than 0.2 mm, and the structural adhesive and / or the sealant is not completely applied to both sides of the bonding surfaces to form a space on both sides of the structural adhesive and / or the sealant.

[0009] Optionally, at least one axial recess is provided on the outer surface of the cylinder, and a longitudinal stringer is formed in the axial recess by co-bonding process.

[0010] Optionally, the protruding structure is hollow inside, and fasteners penetrate the two surfaces of the protruding structure and the stepped structure connected by sealant. The fasteners also penetrate the fixing sealant and the surfaces on both sides.

[0011] Optionally, a skin structure is provided on the outside of the plurality of cylinders connected along the axial direction. The skin structure is formed by curing fiber prepreg laid on the outside of the plurality of connected cylinders. The front and rear end caps of the oil tank are installed at both ends of the skin.

[0012] Secondly, embodiments of the present invention also provide a method for molding a composite material fuel tank, based on any of the composite material fuel tanks described above, the molding method comprising: Apply sealant and structural adhesive to the bonding surfaces of the protruding and stepped structures at both ends of the multiple cylinders. The surface of the protruding structure near the axis and one surface of the stepped structure are sealed and bonded together with sealant, while the protruding surface of the protruding structure perpendicular to the axis and the stepped structure are bonded together with structural adhesive.

[0013] Optionally, the surface of the protruding structure away from the axis of the cylinder and the non-cylindrical surface of the stepped structure away from the axis of the cylinder are coplanar, so that after the adjacent cylinders are spliced, an annular recess is formed at the splice, with a cross-section of two grooves symmetrical along the axis. The molding method includes: A unidirectional fiber prepreg is laid in the annular recess and cured to obtain a unidirectional fiber composite material within the annular recess.

[0014] Optionally, at least one axial recess is provided on the outer surface of the cylinder; The molding method includes: The longitudinal stringer prepreg is laid in the axial recess and cured by co-bonding process to obtain the longitudinal stringer.

[0015] Optionally, the molding method further includes: Using multiple cylinders bonded along the axis as molds, the outer layer prepreg is laid as a male mold, and the oil tank skin is obtained by curing through co-bonding process. The front and rear end caps of the oil tank are installed at both ends.

[0016] Compared with the prior art, the present invention has at least the following beneficial effects: This invention employs a mixed secondary bonding process (using sealant and structural adhesive) through multiple composite material cylinders with raised and stepped structures. This enables the fabrication of large-sized, pressure-resistant, sealing, and leak-proof composite fuel tanks with a length exceeding 3 meters and a diameter exceeding 1 meter. The fuel tank exhibits excellent sealing performance, molding quality, and lightweight design, solving the problems of heavy traditional metal fuel tank structures and the difficulty in molding and sealing large-sized assembled composite fuel tanks. Specifically, the molding process utilizes multiple co-bonding and secondary bonding processes, significantly reducing the number of fasteners and effectively lowering the fuel tank's structural weight, fully leveraging the lightweight advantages of composite materials. This invention uses a mixed bonding method of sealant and structural adhesive. By rationally designing the bonding area of ​​the sealant and structural adhesive, it ensures both sufficient connection strength and excellent pressure-resistant sealing performance of the fuel tank. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic diagram of the structure of a composite material fuel tank provided in an embodiment of the present invention; Figure 2 This is a cross-sectional structural diagram of a composite material fuel tank provided in an embodiment of the present invention; Figure 3 This is a partial cross-sectional schematic diagram of a composite material fuel tank provided in an embodiment of the present invention.

[0019] In the picture: 1-Cylinder body; 11-Step structure; 12-Protruding structure; 13- Annular depression; 14-Axial concavity; 2-Sealant; 3-Structural adhesive; 4-Fasteners; 5- Unidirectional fiber composite material; 6-Skin. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0021] In the description of the embodiments of the present invention, unless otherwise expressly specified and limited, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance; unless otherwise specified or stated, the term "multiple" refers to two or more; the terms "connected," "fixed," etc., should be interpreted broadly. For example, "connected" can be a fixed connection, a detachable connection, an integral connection, or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in the present invention according to the specific circumstances.

[0022] In this specification, it should be understood that the directional terms such as "upper" and "lower" used in the description of the embodiments of the present invention are used to describe the angles shown in the accompanying drawings and should not be construed as limiting the embodiments of the present invention. Furthermore, in the context, it should also be understood that when it is mentioned that one element is connected "upper" or "lower" to another element, it can be directly connected to the other element "upper" or "lower," or indirectly connected to the other element "upper" or "lower" through an intermediate element.

[0023] like Figures 1 to 3 As shown, an embodiment of the present invention provides a composite material oil tank, comprising a plurality of cylindrical bodies 1 with openings at both ends, the plurality of cylindrical bodies 1 being connected axially; The first end of each cylinder 1 has a cross-section of two stepped structures 11 symmetrical along the axis, and the second end of each cylinder 1 has a cross-section of two axially extending protruding structures 12 symmetrical along the axis. The stepped structures 11 and the protruding structures 12 are shaped to match to complete the splicing. The surface of the protruding structure 12 near the axis and one surface of the stepped structure 11 are sealed and bonded by sealant 2. The protruding surface of the protruding structure 12 perpendicular to the axis and the stepped structure 11 are bonded by structural adhesive 3.

[0024] In this embodiment, the protruding structure 12 and the stepped structure 11 are shaped to allow them to be spliced ​​together. There are multiple splicing surfaces, and the bonding surfaces are perpendicular to each other, increasing the strength of the connection. Furthermore, the sealant 2 is closer to the inside of the cylinder 1, preventing leakage at the source. The structural adhesive 3 is closer to the outside of the cylinder 1. Because the sealant 2 isolates it from the internal environment of the cylinder 1, it is not affected by changes in air pressure. At the same time, its high adhesion makes the connection of the cylinder 1 more stable, preventing significant interference from external stress, which could compromise the sealing effect of the sealant 2.

[0025] In this embodiment, the cylinder 1 can be prepared using known composite material molding processes such as autoclave, molding, and RTM.

[0026] Understandably, to ensure precise splicing of adjacent cylinders 1, positioning holes can be provided on the parting surfaces at both ends. The main and auxiliary positioning holes work together to ensure positioning accuracy during the assembly of the annular cylinder 1.

[0027] In some embodiments of the present invention, the surface of the protruding structure 12 away from the axis of the cylinder 1 and the non-cylinder 1 cylindrical surface of the stepped structure 11 away from the axis of the cylinder 1 are coplanar, so that after the adjacent cylinders 1 are spliced, an annular recess 13 is formed at the splice, with a cross-section of two symmetrical grooves along the axis, and the annular recess 13 is filled with axially extending unidirectional fiber composite material 5.

[0028] In this embodiment, by making the outermost edges of the stepped structures 11 and protruding structures 12 at both ends of the cylinder 1 thinner than the outermost edge of the cylinder 1, that is, the surface of the protruding structure 12 away from the axis of the cylinder 1 and the non-cylindrical surface of the stepped structure 11 away from the axis of the cylinder 1 are coplanar, so that after two adjacent cylinders 1 are spliced, an annular recess 13 is formed at the splice. An axially extending unidirectional fiber composite material 5 is filled into the annular recess 13. The unidirectional fiber prepreg connects the two adjacent cylinders, improving the structural connection strength; on the other hand, it serves as a barrier layer on the parting surface of the adjacent cylinders, acting as a second line of defense for interlayer sealing, improving sealing reliability.

[0029] In this embodiment, the surface of the prepreg should be sanded before it is laid, sanded until diffuse reflection, and then cleaned.

[0030] In some embodiments of the present invention, the structural adhesive 3 and / or the sealant 2 have a thickness of no more than 0.2 mm, and the structural adhesive 3 and / or the sealant 2 are not completely applied to the bonding surfaces on both sides, so as to form a space on both sides of the structural adhesive 3 and / or the sealant 2.

[0031] In this embodiment, by partially applying structural adhesive 3 and / or sealant 2, the bonding surfaces are not completely filled with adhesive, but rather a certain space is left. This space can provide an escape space for the gas generated during the curing process, preventing the gas from being trapped inside the adhesive layer and affecting the interfacial bonding strength and sealing performance.

[0032] Preferably, the sealant 2 is a fluorinated oil-resistant sealant 2, which has room temperature curing conditions or accelerated curing conditions below 100°C, and the structural adhesive 3 has room temperature curing conditions or accelerated curing conditions below 100°C.

[0033] It should be noted that the amount of gas produced is relatively small, and considering the strength of the bond, the thickness of the colloid should not exceed 0.2 mm.

[0034] In some embodiments of the present invention, at least one axial recess 14 is provided on the outer surface of the cylinder 1, and a longitudinal stringer is formed in the axial recess 14 by co-bonding process.

[0035] In this embodiment, the girder is a structural and functional integrated component, which not only provides structural strength, but also serves as a channel for oil, gas, and cables.

[0036] In this embodiment, the longitudinal stringer is fabricated using a co-bonding process, which can avoid the risk of structural deformation caused by stress assembly.

[0037] In this embodiment, the longitudinal stringers are distributed circumferentially on the inner main structure, and their number and location are determined according to the fuel pressure inside the fuel tank and the aerodynamic loads and other load-bearing conditions during flight.

[0038] In this embodiment, the cross-sectional shape of the axial groove is the cross-sectional shape of the longitudinal stringer, including but not limited to semi-circular, arc-shaped, square, etc.

[0039] In some embodiments of the present invention, the protruding structure 12 is hollow inside, and fasteners 4 penetrate through the two surfaces of the protruding structure 12 and the stepped structure 11 connected by the sealant 2. The fasteners 4 also penetrate through the sealant 2 and the surfaces on both sides.

[0040] In some embodiments of the present invention, a skin 6 structure is provided on the outside of a plurality of cylinders 1 connected along the axial direction. The skin 6 structure is formed by curing fiber prepreg laid on the outside of the plurality of connected cylinders 1. Oil tank front and rear end caps are installed at both ends of the skin 6.

[0041] In this embodiment, the outer layer structure is an integral continuous composite skin 6, which further improves the structural strength, rigidity, and sealing and seepage prevention. Specifically, the outer surface of the connected cylinder 1 serves as the male mold layup layer of the outer layer structure. The cylinder 1 forming mold ensures the contour and flatness of the outer surface of the cylinder 1. As a layup male mold, it can improve the interlayer forming quality of the outer skin 6 and further improve the sealing performance.

[0042] In this embodiment, the skin 6 is prepared by a layering method, which can avoid risks such as structural deformation caused by stress assembly.

[0043] In this embodiment, the front and rear end caps adopt a foam sandwich structure and a thermosetting molding process, which can improve the structural rigidity and reduce the structural weight.

[0044] In this embodiment, the surface of the prepreg should be sanded until diffuse reflection before laying, and then cleaned thoroughly. Before laying the prepreg, a vacuum tightness test should be performed on the internal main structure, with a vacuum pressure of not less than -90 kPa. After the pressure stabilizes, it should be maintained for 5 minutes, ensuring the pressure drop does not exceed 1 kPa. During prepreg laying, a vacuum pre-compression should be performed after each layer is laid, with a vacuum degree of not less than -90 kPa and a pre-compression time of not less than 15 minutes. After curing, the tank piping and front and rear end caps are installed to obtain the composite material tank. Before installing the front and rear end caps, the inside of the tank should be cleaned to ensure there are no visible foreign objects. When installing the front and rear end caps, the caps should be installed from the inside out; the internal compression method is more conducive to achieving a tight seal.

[0045] This invention also provides a method for molding a composite material fuel tank. Based on any of the composite material fuel tanks described above, the molding method includes: Apply sealant 2 and structural adhesive 3 to the bonding surfaces of the protruding structures 12 and stepped structures 11 at both ends of the plurality of cylinders 1. The surface of the protruding structure 12 near the axis and one surface of the stepped structure 11 are sealed and bonded by sealant 2, and the protruding surface of the protruding structure 12 perpendicular to the axis and the stepped structure 11 are bonded by structural adhesive 3.

[0046] In some embodiments of the present invention, the surface of the protruding structure 12 away from the axis of the cylinder 1 and the non-cylinder 1 cylindrical surface of the stepped structure 11 away from the axis of the cylinder 1 are coplanar, so that after the adjacent cylinders 1 are spliced, an annular recess 13 with a cross-section of two grooves symmetrical along the axis is formed at the splice. The molding method includes: A unidirectional fiber prepreg is laid in the annular recess 13 and cured to obtain a unidirectional fiber composite material 5 within the annular recess 13.

[0047] In some embodiments of the present invention, at least one axial recess 14 is provided on the outer surface of the cylinder 1; The molding method includes: The longitudinal stringer prepreg is laid in the axial recess 14 and cured and formed by co-bonding process to obtain the longitudinal stringer.

[0048] In some embodiments of the present invention, the molding method further includes: Using multiple cylinders 1 bonded along the axis as molds, the outer layer prepreg is laid as a male mold and cured through a co-bonding process to obtain the tank skin 6. The front and rear end caps of the tank are installed at both ends.

[0049] In some embodiments of the present invention, based on the structure and functional characteristics of the fuel tank, the overall structure is divided into four cylindrical bodies 1, four longitudinal girder sections, one integral outer skin 6 structure, and front and rear end caps according to the principle of minimization. The surfaces connecting adjacent cylindrical bodies 1 serve as longitudinal sealing surfaces. Each cylindrical body 1 has four pre-set semi-circular longitudinal groove structures for forming the longitudinal girder sections, and pre-set circumferential flanges and circumferential grooves for forming a sealing reinforcement layer on the bonding surfaces of adjacent cylindrical bodies 1.

[0050] In this embodiment, the prepreg material is T800 grade high-strength carbon fiber, the resin is bismaleimide resin, and the annular cylinder 1 is formed by autoclave molding process. The molding mold ensures that the outer surface contour of the cylinder 1 is within ±0.1mm.

[0051] The molded annular cylinder 1 is pre-assembled onto the frame. A bonding gap of no more than 0.2mm is reserved between adjacent cylinders 1. The half of the contact area near the inner side of the oil tank is bonded with fluorinated sealant 2, including but not limited to HM804, HM812, KH-CL-SF100, etc. The other half is bonded with high-temperature resistant epoxy structural adhesive 3, including but not limited to J-324, J-194, etc.

[0052] The skeleton assembled from the annular cylinder 1 is used as the layup male mold. The surface is polished and cleaned. The plane formed by the circumferential flanges of the adjacent cylinders 1 is successively covered with sammonia structural adhesive 3 film and sammonia composite prepreg, and then bagged and sealed. It is then put into a thermostatic jar for curing to form the sealing and reinforcing layer of the cylinder 1.

[0053] The surface of the longitudinal groove on the annular cylinder 1 is roughened and cleaned. As a layup male mold, smeared 3-layer structural adhesive film and smeared composite prepreg are sequentially laid on the longitudinal groove. The longitudinal stringer is then cured in a hot autoclave.

[0054] The entire inner layer structure is polished and cleaned. The bag is then vacuumed for negative pressure airtightness testing. The vacuum pressure is not less than -90 kPa. After the pressure stabilizes, it is held for 5 minutes. The pressure drop is not allowed to exceed 1 kPa.

[0055] Then, using this mold as the layup male mold, continue to lay up the smeared 3mm structural adhesive film and smeared composite prepreg, bag and seal, autoclave curing, and form the outer structure.

[0056] After the outer structure is formed, the inside of the fuel tank is cleaned and purified, and pipelines and front and rear end caps are installed. The caps are installed from the inside out to obtain an integral composite material fuel tank.

[0057] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A composite material fuel tank, characterized in that, It includes multiple cylindrical bodies (1) with openings at both ends, and the multiple cylindrical bodies (1) are connected axially; The first end of each cylinder (1) has a cross section of two stepped structures (11) symmetrical along the axis, and the second end of each cylinder (1) has a cross section of two axially extending protruding structures (12) symmetrical along the axis. The stepped structures (11) and the protruding structures (12) are shaped to match to complete the splicing. The surface of the protruding structure (12) near the axis and one surface of the stepped structure (11) are sealed and bonded by sealant (2). The protruding surface of the protruding structure (12) perpendicular to the axis and the stepped structure (11) are bonded by structural adhesive (3).

2. The composite material fuel tank according to claim 1, characterized in that, The surface of the protruding structure (12) away from the axis of the cylinder (1) and the non-cylinder (1) cylindrical surface of the stepped structure (11) away from the axis of the cylinder (1) are coplanar, so that after the adjacent cylinders (1) are spliced, a ring is formed at the splice, and the cross section is a convex annular recess (13) with two grooves symmetrical along the axis. The convex annular recess (13) is filled with axially extending unidirectional fiber composite material (5).

3. The composite material fuel tank according to claim 1, characterized in that, The structural adhesive (3) and / or the sealant (2) have a thickness of no more than 0.2 mm, and the structural adhesive (3) and / or the sealant (2) are not completely applied to the bonding surfaces on both sides to form a space on both sides of the structural adhesive (3) and / or the sealant (2).

4. The composite material fuel tank according to claim 1, characterized in that, At least one axial recess (14) is provided on the outer surface of the cylinder (1), and a longitudinal stringer is formed in the axial recess (14) by co-bonding process.

5. The composite material fuel tank according to claim 1, characterized in that, The protruding structure (12) is hollow inside. The two surfaces of the protruding structure (12) and the stepped structure (11) connected by the sealant (2) are penetrated by fasteners (4). The fasteners (4) also penetrate the sealant (2) and the surfaces on both sides.

6. The composite material fuel tank according to claim 1, characterized in that, Multiple cylinders (1) connected along the axial direction are provided with a skin (6) structure on the outside. The skin (6) structure is formed by curing fiber prepreg laid on the outside of the multiple connected cylinders (1). Oil tank front and rear end caps are installed at both ends of the skin (6).

7. A method for molding a composite material fuel tank, characterized in that, Based on the composite material fuel tank according to any one of claims 1-6, the molding method includes: Apply sealant (2) and structural adhesive (3) to the bonding surfaces of the protruding structures (12) and stepped structures (11) at both ends of the multiple cylinders (1). The protruding structure (12) near the axis and one surface of the stepped structure (11) are sealed and bonded by sealant (2), and the protruding surface of the protruding structure (12) perpendicular to the axis and the stepped structure (11) are bonded by structural adhesive (3).

8. The molding method according to claim 7, characterized in that, The surface of the protruding structure (12) away from the axis of the cylinder (1) and the non-cylinder (1) cylindrical surface of the stepped structure (11) away from the axis of the cylinder (1) are coplanar, so that after the adjacent cylinders (1) are spliced ​​together, a ring is formed at the splice, and the cross section is a convex annular recess (13) with two grooves symmetrical along the axis. The molding method includes: A unidirectional fiber prepreg is laid in the convex annular recess (13) and cured to obtain a unidirectional fiber composite material (5) in the convex annular recess (13).

9. The molding method according to claim 7, characterized in that, At least one axial recess (14) is provided on the outer surface of the cylinder (1). The molding method includes: The longitudinal stringer prepreg is laid in the axial recess (14) and cured by co-bonding process to obtain the longitudinal stringer.

10. The molding method according to claim 7, characterized in that, The molding method further includes: Using multiple cylinders (1) bonded along the axis as molds, the outer layer prepreg is laid as a padded male mold and cured and formed by co-bonding process to obtain the tank skin (6), and the front and rear end caps of the tank are installed at both ends.

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