Strip structure for manufacturing strip of composite structure, automatic fiber placement (AFP) device, automatic fiber placement (AFP) method, and composite structure

Abstract

The present invention relates to a strip structure, an automatic fiber placement (AFP) device, an automatic fiber placement (AFP) method and a composite structure for manufacturing a strip of a composite structure. The strip structure comprises a layer of unidirectionally pre-impregnated dry fibers (105) embedded in a polymeric matrix (104) wherein the polymeric matrix (104) comprises, by weight thereof: 20%-50%, preferably 25%-40%, of at least one first monomer or oligomer; 30-60%, preferably 35-40%, of at least one second monomer or oligomer; 4%-25%, preferably 4%-7%, of at least one curing agent; 0.5%-5%, preferably 0.8%-1.5%, of at least one reactive chain extender, and at least one additive, preferably configured as a toughening agent, in which the polymer matrix (104) is configured to be solid or semi-solid in a first temperature range and to soften in a second temperature range different from the first temperature range. The invention also relates to an automatic fiber placement apparatus, an automatic fiber placement method (102) and a composite structure (102) prepared in the method.

Description

Technical Field

[0001] This invention relates to a strip structure for manufacturing composite structures, an automated fiber placement (AFP) apparatus, an automated fiber placement (AFP) method, and a composite structure. Background Technology

[0002] Although it can be used in many applications, the invention and its potential problems will be explained in more detail regarding aircraft. However, the described apparatus and method can also be used in vehicles in all sectors of the transportation industry, such as for space launch vehicles, road vehicles, rail vehicles, or ships.

[0003] Hydrogen-propelled aircraft are a key technology for reducing in-flight emissions in aviation, requiring technological modifications to the systems used in such aircraft. One such system is the energy storage system or tank. Existing liquid hydrogen (LH2) storage systems are metallic, such as aluminum-based. Lightweight structures, particularly carbon fiber reinforced plastic (CFRP) tanks, are superior to metal tanks like aluminum tanks in terms of performance and tank weight-to-storage ratio in the cryogenic environments required for storing liquid hydrogen (LH2), as hydrogen must be stored as a liquid at -253°C to achieve a viable volumetric energy density, especially in aircraft. CFRP can significantly reduce the weight of LH2 tank systems, but the airtightness of the H2 must be ensured. To ensure the airtightness of H2, and combined with the mechanical properties of the thinnest possible laminate, improvements are needed for techniques used for, for example, cylindrical sections and curved tank domes, such as fiber patching (FPP) processes and automated fiber placement (AFP) treatments. Methods for manufacturing large composite structures, such as liquid hydrogen composite tank systems, but not limited to these, utilize the winding of composite materials to form the entire structure. Such structures can also be manufactured using automated fiber placement (AFP) methods. Slit strips, also known as cut strips, are used to construct laminated structures comprising several layers of prepreg containing a fiber matrix. Strips of a predetermined width can be obtained from wider prepreg sheets. Slitting ensures edge and width quality and prevents gaps or overlaps in strips laid parallel during processing.

[0004] Automated fiber placement is commonly used to manufacture lightweight structures, particularly carbon fiber reinforced plastic (CFRP) cans, which are superior to aluminum cans for enhanced performance in the cryogenic environments required for storing liquid hydrogen (LH2). The materials used for such can structures preferably have a lower layer thickness of less than 100 μm, rather than the standard 125 to 250 μm of aerospace prepregs. Since hydrogen must be stored as a liquid at -253°C to achieve a viable volumetric energy density, especially in aircraft, cans with more distinct layers achieve multiple different orientations within the fibers of a particular layer, preferably preventing the formation of microcracks in the structure at cryogenic temperatures.

[0005] Thinner layers or sheets are highly sensitive during processing because contact with the strip edges impairs edge quality and thus the overall strip width. Uniform lamination without corrugations or gaps is crucial for achieving optimal composite performance, especially in the aforementioned low-temperature environments associated with LH2 storage. Summary of the Invention

[0006] In this context, the object of the present invention is to find a strip configuration and manufacturing method that can achieve high quality when processing processing-sensitive materials such as carbon fiber reinforced plastics (CFRP), and ensure high quality of laminates and structures made from these materials.

[0007] This objective is achieved by a strip structure for manufacturing a composite structure having the features of claim 1, a fiber placement device having the features of claim 11, an automated fiber placement method according to claim 12, and a composite structure according to claim 15.

[0008] According to a first aspect of the invention, a strip structure for manufacturing a composite structure is provided, comprising unidirectional fibers embedded in a thermosetting polymer matrix. The polymer matrix comprises: at least one first monomer or oligomer, weighing between 20% and 50%, preferably between 25% and 40% by weight of the polymer matrix; at least one second monomer or oligomer, weighing between 30% and 60%, preferably between 35% and 40% by weight of the polymer matrix; at least one curing agent, weighing between 4% and 25%, preferably between 4% and 7% by weight of the polymer matrix; at least one reactive chain extender, weighing between 0.5% and 5%, preferably between 0.8% and 1.5% by weight of the polymer matrix; and at least one additive, preferably configured as a toughening agent, wherein the polymer matrix is ​​configured to be solid or semi-solid in a first temperature range and configured to soften in a second temperature range different from the first temperature range. Therefore, the strip structure comprises a polymer matrix consisting of uncured thermosetting monomers or oligomers, optional additives, and a plurality of fibers, wherein the uncured thermosetting monomers or oligomers are a mixture of resin and curing agent, and the plurality of fibers are preferably carbon fibers embedded therein. In the composition of the polymer matrix, at least one additive (also called a toughening agent or toughening reagent) has a modulus much lower or much higher than that of the polymer matrix, and is preferably based on rubber or siloxane, preferably having a shell that partially separates the low-modulus rubber or polysiloxane from the matrix to ensure compatibility with the matrix. At least one reactive chain extender included in the composition catalyzes the reaction between the matrix resin and the curing agent with respect to time and curing temperature. The chain extender is particularly configured or selected to react with the epoxy groups or other groups of the monomers or oligomers, and preferably extends the chains of the monomers or oligomers without the accumulation or crosslinking of a three-dimensional network. The composition of the polymer matrix in the strip structure of the present invention particularly allows for the diffusion of thermally softenable monomers or oligomers that are soluble in the mixture but phase-separated and chemically compatible. A solid core and a spontaneously fusible liquefied shell can be formed from a compactable composition to achieve a composite structure. The monomers or oligomers are selected based on their miscibility and chemical structure, i.e., layered, co-continuous, or spherical structures. Additives provide specific properties, such as toughening agents or toughening reagents, to provide a softened shell, for example, forming co-continuous, spherical, or other phase-separated core-shell particles. Additives can be configured as low-activity or inactive catalysts to prevent thermal stress, or configured in molecules to avoid crosslinking rather than chain extension effects. The advantage of this strip structure is that the modified polymer matrix used for the slit strip material, during processing, such as in the slitting of larger prestructures to form slit strips or in laying strips in or on composite structures or molds, uses monomers or oligomers in thermosetting polymers to form softenable polymers, resulting in strip stability, but without affecting or reducing the adhesiveness of the strip during or after laying.This allows for automated processing of strips with significantly reduced layer thickness. The composition of the polymer matrix of the strip structure is semi-solid or solid in a first temperature range and softenable in a second temperature range. This composition hardens the strip structure or slit strip and prevents the strip from curling, U-forming, and flipping during processing, particularly when processing and handling strips on various reels in an automated fiber placement (AFP) apparatus. The composition of the polymer matrix of the strip structure also prevents width variations due to tension applied during unfolding and processing and ensures high edge quality in the strip structure. Therefore, the advantage of this invention is that it allows for the manufacture of high-quality laminates and composite structures from thin sheets of individual layers with significantly reduced thickness and high edge quality, particularly for cryogenic environments. More strip layers with alternating fiber orientations can be laid in each layer while reducing weight and layer thickness. Crack formation is prevented in the manufactured composite structures, especially in cryogenic environments or environments requiring extended load-bearing capacity (therefore, where cracks are prone to form and propagate).

[0009] As used herein, the terms “softenable,” “soften,” “softened,” and “softened” should be understood as altering the polymer matrix in a manner such that the strip structure is changed by raising the temperature during processing to a range above a defined temperature, below the curing temperature of the thermosetting polymer, particularly above room temperature, and exhibiting tackiness or increased tackiness. In some embodiments of the polymer matrix composition of the structure and strip structure according to the invention, the increased tackiness during processing and / or laying of the strip may be limited to the strip surface.

[0010] As used herein, the term “room temperature” refers to a temperature range between 15°C and 28°C, particularly between 18°C ​​and 24°C, and preferably between 18°C ​​and 21°C.

[0011] As used herein, the term "adhesiveness" or "adhesive" refers to the adhesive properties of the resin or matrix material used in the strip structure of the present invention. "Adhesiveness" or "adhesiveness" is defined as the degree to which the resin or matrix material of the strip structure exhibits properties related to the bonding and placement of fibers, i.e., during automated fiber placement (AFP), such as adhesive or bonding properties and interdiffusion or entanglement at the molecular level.

[0012] Another aspect of the invention relates to a fiber placement apparatus having a movable placement head for fiber deposition in an automated fiber placement (AFP) method. The fiber placement apparatus includes a feeding device for feeding a strip having a strip structure according to the invention into a placement area; a temperature control unit configured to increase the processing temperature of the strip structure, particularly to a temperature range above 15°C to 28°C, preferably above 18°C ​​to 24°C, and especially above 18°C ​​to 21°C, to soften the strip structure before or during placement; a deposition device for placing and / or compacting the strip onto a composite structure or a previously formed lamination on the composite structure; and a compaction device for establishing a permanent connection between the strip and the composite structure or a previously formed lamination on the composite structure, the compaction device being configured to apply at least one of pressure, heat, and current to the strip structure during or after placement. In the fiber placement apparatus, the feeding device, temperature control unit, deposition device, and compaction device are positioned adjacent to the placement head and configured to move together with the placement head. The advantage of this device is that it can process strips made of thin sheet materials, which is particularly advantageous for cryogenic composite tanks. Furthermore, using this device, strip structures according to the invention can be laid with enhanced precision and performance during layup. Heating the strip structure, which hardens due to the composition of the polymer matrix, softens the strip structure or at least its surface only before or during layup, thus ensuring the dimensional stability of the strip structure with the composition of the invention in the polymer matrix, which is harder during feeding but softens before or during layup. Warping and damage to the strip edges are eliminated in automated fiber layup processes and devices, particularly during feeding and / or processing of strips along various paths. This significantly reduces gaps and / or overlays in the laminate, which are known to cause ripples or resin-rich areas in the composite structure or laminate that are prone to microcracks. Furthermore, it is possible to improve the precision of laying multiple layers of thin sheet strips in alternating orientations within the fiber layers.

[0013] Another aspect of the invention relates to a method for forming a composite structure using an automated fiber layup (AFP) with a strip having a strip structure according to the invention, wherein the method comprises the following steps: feeding the strip into an automated fiber layup apparatus; heating the strip to a temperature above room temperature before or during layup on the composite structure or on a previous laminate formed on the composite structure until the strip structure softens and thus reaches an adhesive state or increases strip adhesiveness; laying the strip on the composite structure or on the previous laminate formed on the composite structure by moving a layup head over the composite structure or the previous laminate formed on the composite structure; pressing the strip to establish a permanent bond between the strip and the composite structure or the previous laminate; and subsequently curing the composite structure. This has the advantage that the strip is safely and precisely transferred to the layup position and becomes adhesive or exhibits increased adhesiveness before or during layup. The strip structure of the invention prevents the strip from having corrugations or width variations during or after layup and can be laid with reduced gaps, thereby improving the overall structure of the formed composite structure. Furthermore, this method can handle toughened and other matrix systems that are typically highly adhesive or sticky and prone to clogging machines. This allows for high-quality processing of gapless and overlapping strips during layup and ensures that the resulting composite structure can withstand low temperatures. In addition, this method allows for a high degree of automation and eliminates layup errors. It also covers the fixed positioning of the layup head and the movement of the composite structure relative to the layup head.

[0014] Another aspect of the invention lies in the composite structure manufactured in the method of the invention, wherein the composite structure is an aircraft component, particularly a tank structure containing a liquid, preferably a liquid stored under cryogenic conditions, such as liquid hydrogen (LH2). This has the advantage that, due to the high strength-to-weight ratio, laminates made of aerospace composite materials can be manufactured, which have reduced susceptibility to microcracks in the composite structure or laminate. Since the strip structure of the invention advantageously allows for the processing of very thin strips, preferably with a thickness of less than 100 μm, particularly between 40 μm and 75 μm, more layers can be laid up with alternating fiber orientations.

[0015] Advantageous embodiments and further developments become clear from the description in the further dependent claims and with reference to the accompanying drawings.

[0016] According to another aspect of the invention, a first temperature range is below 18°C ​​to 28°C, preferably below 18°C ​​to 24°C, particularly below 18°C ​​to 21°C, and a second temperature range is above 15°C to 28°C, preferably above 18°C ​​to 24°C, particularly above 18°C ​​to 21°C. This has the advantage of providing at least a temperature-dependent softening layer in the prepreg tow or strip structure. Due to the composition of the polymer matrix, the strip structure of the invention is solid but “flexible” at room temperature, without increased tack or adhesiveness, and can be processed with greater precision in the layup method by raising the temperature above room temperature, thereby achieving or improving the adhesiveness of the matrix polymer shortly before or during layup, without compromising the integrity and geometry of the strip during feeding. Therefore, the overall structural quality of laminates and / or composite structures produced using strips having the strip structure of the invention is improved, which is beneficial in manufacturing composites with high strength-to-weight ratios for low-temperature applications, as the number of alternately oriented strip layers can be high.

[0017] According to embodiments of the invention, the polymer matrix further comprises vitrimeric monomers or oligomers. Depending on the bonding or breaking of the vitrimeric monomers or oligomers, this has several advantages. On the one hand, the monomers or oligomers forming the polymer matrix backbone can be modified or replaced by cleaving or bonding the vitrimeric monomers or oligomers in the molecular structure, for example, by bonding with disulfonic acid, imine, or acrylic acid in the backbone, to enhance the reactivity of the polymer matrix and stabilize the strips before and during layup. On the other hand, the vitrimeric monomers or oligomers can be bonded, particularly by secondary covalent bonds, to the monomers or oligomers forming the polymer, thereby providing or enhancing adhesion in a second temperature range due to the properties of the corresponding vitrimeric monomers or oligomers, thus providing a softenable strip structure or slit strips.

[0018] According to another embodiment of the invention, the strip structure further comprises at least one layer consisting of or including at least one of a plurality of thermoplastic particles, wool, or yarn, preferably selected to be non-reactive or reactive in a second temperature range. This has several advantages. On the one hand, the thermoplastic particles, wool, or yarn that are non-reactive in the second temperature range can be selected, thereby stabilizing the strip structure or allowing modification and control of the adhesiveness of the strip structure or surface, but without exhibiting increased adhesiveness in the second temperature range. On the other hand, due to the inherent properties and molecular structure of the thermoplastic particles, wool, or yarn, the thermoplastic particles, wool, or yarn that enhance or achieve adhesiveness of the strip structure in the second temperature range can be selected. This further contributes to the advantageous effects achieved by the strip structure of the present invention, particularly regarding gap elimination and layup accuracy.

[0019] According to another embodiment of the invention, the strip having the strip structure of the invention has a thickness between 50 μm and 300 μm, preferably less than 100 μm, particularly between 40 μm and 75 μm. The advantage of this strip is that, during the processing up to layup, the matrix polymer composition prevents curling, U-forming, and flipping of the thin strip, as well as width variations, by eliminating tensile stresses applied during the feed to the layup position on the composite structure. Therefore, high lamination uniformity without corrugations or gaps is achieved, which is important for achieving the highest composite performance even when using very thin strips. Furthermore, by being able to use very thin strips, the number of strip layers can be increased to further improve the mass and strength / weight ratio of the composite structure without excessively increasing the thickness of the structure.

[0020] According to another embodiment of the invention, at least one first monomer or oligomer has a molecular weight greater than 400 g / mol and / or is preferably selected from reactive diglycidyl ether-based epoxy resins, diglycidyl ether bisphenol A resins, epoxy phenolic varnish resins, or combinations thereof. Preferably, at least one first monomer or oligomer is configured as a base monomer or oligomer having a banded structure weight of 20% to 60%, more preferably 25% to 40%. Due to the selected first monomer or oligomer, the polymer matrix formed therefrom or containing the first monomer or oligomer is semi-solid and non-adhesive at room temperature, i.e., between 15°C and 28°C, but can soften by raising the temperature above room temperature, i.e., above the temperature between 15°C and 28°C but below the curing temperature, i.e., transforming into a softened, low-viscosity state with induced or increased adhesion. The advantage of this is that the strip structure of the present invention, which is hard or rigid due to the composition of the polymer matrix, can be transferred to the laying apparatus in a semi-solid state without compromising the quality of the strip during the transfer, and softens shortly before or during laying to ensure high quality and / or automated laying, while reducing width variation. As a non-limiting example, epoxy monomers such as diglycidyl ether bisphenol F (DGEBF), triglycidyl-m-aminophenol (TGMAP), tetraglycidylmethylenediphenylamine (TGMDA), or amines such as (poly)diglycidyl ether bisphenol F (DGEBF), 4,4'-diaminodiphenyl sulfone (4,4'DDS), diethyltoluenediamine (DETDA), 4,4'-methylenebis(2,6-diethylaniline) (MDEA), 4,4'-methylenebis(2-isopropyl-6-methylaniline) (MIPA), or dicyandiamide (DICY) can be used as the first or base monomer or oligomer. Monomers possessing the aforementioned properties are commercially available, such as DER337, DER660 (Olin Epoxy), Epikote 834, Epikote 836 (Westlake Epoxy), or Araldite GY280 (Huntsman), but the invention is not limited thereto. Furthermore, the aforementioned monomers can be replaced by their respective vitreous monomers without limiting the invention.

[0021] According to another embodiment of the strip structure of the invention, the molecular weight of the at least one second monomer or oligomer is at least 1000 g / mol, preferably 1200-1300 g / mol, and / or particularly selected from medium molecular weight epoxy resins and low viscosity epoxy resins or combinations thereof, preferably containing a glassy monomer or oligomer. Due to the selected second monomer or oligomer, the resulting polymer matrix having the second monomer or oligomer contains at least one rigid, softenable reactive monomer or oligomer having a strip structure of 30-60% by weight, preferably 35-40% by weight. The strip structure of the invention further improves the advantages of enhanced workability, strip integrity, and improved strip layup quality. Polymers having the above properties are commercially available, for example, as DER661, DER662E, DER663E, DER664E, EPON1002F (Westlake Epoxy) or Araldite GT7072 (Huntsman), but the invention is not limited thereto.

[0022] The strip structure of the present invention also has the advantage that, due to the formed shell, several layers of strip are prevented from sticking together once the strip is wound onto a spool. This is particularly advantageous in strips with a thickness of less than 100 μm and generally high adhesion in toughened or other matrix systems. The matrix composition also hardens the strip structure, thereby supporting the unfolding of the strip before or during layup and preventing the strip from curling, U-forming, and / or flipping and sticking during unfolding. During strip layup, before curing, the composition of the polymer matrix in the sheet prepreg maintains the quality of the slit strip edges due to the high bonding stiffness of the strip, thanks to the remeltable polymer matrix of the strip structure of the present invention. The polymer matrix further creates contact or interdiffusion zones between the laid strip layers, exhibiting entanglement particularly at the molecular level, and improving the composite structure and layup method. If the polymer matrix in the strip structure has a particular layered or shell-forming configuration, wherein the components of the polymer matrix form the core and shell structures in the strip structure, thereby hardening the strip to prevent edge damage, and providing a meltable or softenable matrix or shell to enhance adhesion and / or bonding between strips during layup and before curing the composite structure, then contact and / or interdiffusion are further enhanced.

[0023] According to another embodiment of the invention, the at least one curing agent is selected from the group comprising dicyandiamide and diaminodiphenyl sulfone. This has the advantage that the properties of the strip structure regarding curing performance, particularly the curing temperature and curing time / speed, can be adapted by selecting curing agents according to the specific requirements of the process forming the composite structure and the structure itself.

[0024] According to another embodiment of the strip structure of the present invention, the polymer matrix is ​​configured as a thermosetting polymer matrix that can be cured at a temperature between 100°C and 180°C, preferably between 110°C and 140°C. This has the advantage that the properties of the polymer matrix can be selected relative to the curing temperature and curing time / speed, depending on the method of forming the composite structure and the specific requirements of the structure itself. The selection of additives and reagents in the composition reduces internal stress by lowering the curing temperature, enabling slow crosslinking, and utilizing chain extension effects.

[0025] According to another embodiment of the invention, the strip includes a support layer. The support layer may be provided specifically as a removable pad or foil layer attached to the strip, or as an additional surface layer forming an integral part of the strip, wherein only this layer possesses the properties of the strip structure of the invention. This has the advantage that the support layer (i.e., the pad or foil) is peeled off before the strip is laid and compressed in the composite structure or previous laminate, or remains as an additional layer of the strip structure supporting the strip until it is laid in its final position, and thus avoids adhesion to the compaction and laying devices and rollers.

[0026] According to another embodiment, the method further includes moving the fiber placement head above the composite structure or previous laminate in a direction forming an angle with the edge of the composite structure or previous laminate. An advantage is that the second strip layer can be placed at different angles to achieve quasi-isotropic fiber orientation properties in the manufactured composite structure by utilizing the advantages of the placement method and the strip structure. Since the strip structure of the present invention allows the use of very thin strips, the effective total number of layers can be increased, further improving the strength / weight ratio and lamination uniformity of the composite structure.

[0027] According to another embodiment, the method further includes a composite structure comprising at least first and second substructures, each having a first region and a second region, wherein curing the composite structure comprises: independently curing at least one first substructure and at least one second substructure's first or second region, bringing the first or second region of at least one first substructure into contact with the first or second region of at least one second substructure, and co-curing the first or second region of at least one first substructure and the first or second region of at least one second substructure after contact to form the composite structure. This has the advantage that the composite material manufactured from strips having the strip structure of the present invention can be provided with a partially uncured / less cured matrix. The first region can be cured in a first curing step, while the second region can be finally cured in a second curing step during or after the bonding process, contacting the paired composite material and establishing chemical or covalent bonds between the regions, thereby forming a highly reliable and tight bond, particularly in critical areas of the manufactured composite structure, also utilizing the advantages of the strip structure of the present invention during feeding and layup.

[0028] The invention will be explained in more detail with reference to the exemplary embodiments shown in the accompanying drawings. Attached Figure Description

[0029] Figure 1a Views of strips without the structure according to embodiments of the present invention are schematically depicted in images a, b, and c.

[0030] Figure 2 Another view schematically depicts a strip structure without the structure according to an embodiment of the present invention;

[0031] Figure 3 An embodiment of a composite structure formed by a strip structure according to an embodiment of the present invention is schematically depicted;

[0032] Figure 4a b describes an embodiment of a composite structure formed by a strip structure according to another embodiment of the present invention.

[0033] Figure 5 An embodiment of the composite structure according to an embodiment of the present invention is described; and

[0034] Figure 6 The steps of an automated fiber placement (AFP) method according to an embodiment of the present invention are schematically depicted. Detailed Implementation

[0035] These accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. Other embodiments of the invention and many anticipated advantages of the invention will be readily understood as they become clearer with reference to the detailed description. Elements in the drawings are not necessarily to scale relative to each other. In the drawings, unless otherwise indicated, the same reference numerals denote the same or functionally equivalent parts.

[0036] Although specific embodiments have been shown and described herein, those skilled in the art will understand that various alternatives and / or equivalent embodiments may be substituted for the specific embodiments shown and described without departing from the scope of the invention. Generally, this application is intended to cover any modifications or variations to the specific embodiments discussed herein.

[0037] In the accompanying drawings, the same reference numerals are used to refer to the same elements, features, and parts that have the same function and effect, unless otherwise specified.

[0038] Figure 1aFigures b and c depict cut portions of strips 100 not provided with a polymer matrix according to the invention, used in automated fiber placement methods. As shown, if strips 100 are processed and handled on various reels or over long distances in an automated fiber placement (AFP) device, the sheet or strip material configured as a sheet with a layer thickness preferably less than 100 μm tends to curl and tumble during processing, such as... Figure 1a As shown.

[0039] Strips 100 with thinner laminations are very sensitive during processing and are prone to damage to the lamination edges 101, which significantly affects the quality of the edges 101 and thus the overall quality of the strip 100. For example... Figure 1b and 1c As shown, edge 101 may have variations in size or width, which can cause corrugations or gaps when the strip 100 is laid. This will reduce the uniformity of the overall laminate. However, the uniformity of the laminate in the composite structure 102 (see...) Figure 5 This is important for achieving the highest composite performance, especially in cryogenic environments such as the storage and handling of liquid hydrogen (LH2). The strips 100 (also known as slit strips), particularly used for manufacturing LH2 tank structures, have a preferred underlayer thickness of less than 100 μm rather than 125 to 250 μm compared to standard prepreg materials. Multiple layers 103 laid in an interleaved configuration (see...) Figure 5 It prevents microcracks in low-temperature environments and increases the strength-to-weight ratio.

[0040] Figure 2 Another view depicts a strip 100 without the modified polymer matrix 104 according to an embodiment of the invention. The strip 100 is configured as a thin sheet material that is flexible and viscous, and exhibits significant rounded or bent edges 101 due to processing damage to the strip structure. Fibers 105 are concentrated in the edge regions 106a, b. The bending or rounding results in a reduction in the initial width of the strip 100 that must be taken into account during layup. During processing, i.e., when laying the strip 100 in the AFP method, the strip 100 can be stacked to achieve complete coverage in the final composite structure 102. However, this results in locally high fiber 105 volume content in the stacked areas, which is particularly disadvantageous in low-temperature applications because the accumulation of fibers 105 in critical areas leads to an uneven surface prone to microcracks. On the other hand, if damaged strips 100 are used to maintain bandwidth, the resulting composite structure 102 includes gaps with a low fiber content 105, which is particularly disadvantageous in low-temperature applications and also due to the risk of microcracks and crack propagation.

[0041] Figure 3An embodiment of a composite structure 102 formed from strips 100 having a strip structure according to an embodiment of the invention is schematically depicted. Due to the composition of the polymer matrix 104 in the strip structure, the hardened strip 100 retains the properties of its edges 106a, b and allows the strip 100 to be processed using the full nominal width W. The strips 100 can be joined flush during layup, i.e., without compromising the overlays or gaps of the final composite structure 102, thus allowing the full nominal width W to be used even in slit strips with a thickness of less than 100 μm. During or before layup, the softenable and adhesive polymer matrix composition maintains the precision of the edges 106a, b of the slit strips and prevents curling or flipping, thereby greatly improving layup and composite quality. Fibers 105 are uniformly distributed within the laid strips 100, and the composite structure 102 is not prone to cracking.

[0042] Figure 4a Sections b and c describe an embodiment of a composite structure 102 formed by layers of first and second strips 100 having a strip structure according to a further embodiment of the invention, wherein the layers of strips 100 are stacked during a layup process, for example in an AFP method or an RTM method. Figure 4a The image depicts strips 100 prior to bonding, specifically slit strips, each with a thickness of less than 100 μm. Figure 4b The same strip 100 is shown after layup but before curing. During the layup of the strip 100, the composition of the polymer matrix 104 in the sheet strip maintains the slit strip edge quality in the cured strip 100 due to the composition of the softenable polymer matrix 104 of the strip structure of the present invention. When the strips 100 are stacked, the polymer matrix 104 also additionally creates contact or interdiffusion regions 107. The contact or interdiffusion regions 107 are formed in the bonding region by the softenable polymer matrix 104 and further improve the layup accuracy before curing.

[0043] Figure 5 An embodiment of a composite structure 102 according to an embodiment of the present invention is depicted. The composite structure 102 can be manufactured using an automated fiber placement (AFP) method to form a multilayer composite structure 102. The composite structure 102 includes at least a first layer 103a and at least a second layer 103b, which are formed from strips 100 having a strip structure according to the present invention, wherein the second layer 103b is laid on the first layer 103a. Figure 5The strip 100 shown has a strip width of, for example, 1 / 2" (12.7 mm). Laying follows an accuracy of, for example, ±1%, thus achieving the highest quality of the stack. Due to the softenable composition of the polymer matrix 104 used in the strip structure, the strips of the first and second layers 103a, b adhere to each other before curing the composite structure 102. The composition of the polymer matrix 104 achieves a relatively rigid strip structure, which allows for the laying of strips 100 with smaller width variations during processing or feeding. Due to the enhanced edge quality of the strips 100, gaps 1080 between the strips 10 are reduced or prevented, resulting in resin-rich fibers with aggregated fibers 105 in the stacked strips 100 within specific layers 103a, b. A rigid but softenable strip 100, or a strip 100 softened by applying heat, for example, in the layup head of an AFP device, can be laid in the layup area without gaps 108, thus producing a higher quality composite structure 102, for example, manufactured by an automated fiber layup (AFP) method. This strip structure allows for automated processing of thin sheet slitting strips with a thickness of less than 100 μm. The polymer matrix 104 composition that hardens the strip structure prevents the strip 100 from curling, U-forming, and flipping during processing, i.e., during manufacturing, when the strip 100 is fed into the layup area. Warping of the strip 100 is prevented, especially when handled on various reels in an automated fiber layup (AFP) device. During operation, the strip structure of the present invention also eliminates width variations and damage to the strip edges 101 caused by tension applied during unwinding of the slit strip from the reel, thus supporting the processing of slit strips produced from adhesive resins. The composite structure 102 of the present invention can be specifically configured as a tank structure for cryogenic storage of liquids such as LH2, and can also be used in piping and pumping systems. The composite structure 102 meets the highest quality requirements and can be used in aerospace applications due to its high strength-to-weight ratio. The strip structure of the present invention allows for high-quality processing without gaps 108 and overlays, and is therefore resistant to low temperatures. The strip 100 having the strip structure according to the present invention allows for a high degree of automation, even when forming the thinnest possible slit strip layer. 103a and b are also less susceptible to laying errors. This is achieved through the polymer matrix 104 in the strip structure of the present invention, which is made of, for example, reactive monomers such as BMI, cyanate esters, epoxy resins, and polyurethanes, and cured with acid anhydrides, amines, isocyanates, and imidazoles, and has a softening layer in the prepreg tow that is at least partially temperature-dependent. This provides a matrix that is solid but flexible at room temperature without increased tackiness or adhesiveness. Particularly in the manufacture of cans, preferably for cryogenic storage of liquids such as LH2, the strip structure of the present invention is robust for handling in the double-bent dome region. Furthermore, the strip structure allows for increased manufacturing speed in thin-sheet laminates with fewer errors, stops, and maintenance.The composite structure 102 exhibits enhanced resistance to low-temperature microcracks in the composite laminate while maintaining excellent processability. The composition of the polymer matrix 104 also reduces internal stress through low curing temperature, slow crosslinking of reactive groups, and chain extension by additives.

[0044] Figure 6 The steps of an automated fiber placement (AFP) method according to an embodiment of the present invention are schematically depicted. The AFP method uses multiple strips 100 having a strip structure according to the present invention to form a composite structure 102. In a first step 201, individual strips 100, each configured to have a thickness of less than 100 μm, are fed into an automated fiber placement apparatus. In a further step 202, the strip structure is heated to a temperature above room temperature, preferably above 15°C and 25°C, preferably between 18°C ​​and 21°C, but below the curing temperature of the composite structure 102 in step 203. Heating is applied to soften the strip structure, i.e., increase adhesion. In step 204, the strips 100 are placed on the composite structure 102 or the previously stacked layers 103a, b formed on the composite structure 102 by moving a placement head over the composite structure 102 or the previously stacked layers 103a, b formed on the composite structure 102. As the layup head moves over the composite structure 102, particularly in container manufacturing, gaps 108 in the strips and overlapping layers that could interfere with the uniformity of the laminate are avoided. The composite structure 102, especially the can manufactured as described above, has the uniformity of the laminate (no corrugations or gaps 108), which is important for achieving the highest composite performance, especially in low-temperature environments, such as in the storage of LH2. The strips 100 are then compacted in step 205 to establish a permanent bond between the strips 100 and the composite structure 102 or the previously laminated layers 103a, b. Once the composition of the polymer matrix 104 of the strip structure softens, interdiffusion zones 107 are formed between the laminated strips 100, preventing curling or warping, thereby eliminating damage to the edge regions and allowing for gapless or reduced-gap strip layup. In another step 206, the composite structure 102 is cured, particularly under autoclave conditions, at a temperature between 100°C and 180°C, preferably between 110°C and 140°C, to form the final structure.

[0045] To prevent the composite structure 102 from cracking in low-temperature environments, the strip 100 used in this method preferably has a thickness of less than 100 μm. This method can also be used to manufacture alternative composite structures 102, such as pipes and pumps.

[0046] In the above detailed description, various features have been grouped together in one or more instances or embodiments for the purpose of brevity. It should be understood that the above description is illustrative and not restrictive. It is intended to cover all substitutions, modifications, and equivalents. Many other embodiments will become apparent to those skilled in the art after reading the foregoing specification. These embodiments were chosen and described to best explain the principles of the invention and its practical application, thereby enabling those skilled in the art to best utilize the invention and its various embodiments with various modifications to suit a particular intended use.

[0047] List of reference numerals

[0048] 100 strips

[0049] 101 Edge

[0050] 102 composite structure

[0051] 103a,b layers

[0052] 104 polymer matrix

[0053] 105 fiber

[0054] 106a,b edge regions

[0055] 107 Interdiffusion Zone

[0056] 108 gaps

[0057] 201 steps

[0058] 202 steps

[0059] 203 steps

[0060] 204 steps

[0061] 205 steps

[0062] 206 steps

[0063] W width

Claims

1. A strip structure for manufacturing a strip (100) of a composite structure (102), the strip structure comprising a layer of unidirectional prepreg dry fibers (105) embedded in a thermosetting polymer matrix (104), the thermosetting polymer matrix (104) having a composition comprising: -At least one first monomer or oligomer, based on the weight of the polymer matrix (104), comprises between 20% and 50%, preferably between 25% and 40%. -At least one second monomer or oligomer, based on the weight of the polymer matrix (104), comprises between 30% and 60%, preferably between 35% and 40%. -At least one curing agent, based on the weight of the polymer matrix (104), at a concentration between 4% and 25%, preferably between 4% and 7%. -At least one reactive chain extender, based on the weight of the polymer matrix (104), is present in amounts between 0.5% and 5%, preferably between 0.8% and 1.5%. - At least one additive, preferably formulated as a toughening agent. in, The polymer matrix (104) is configured to be solid or semi-solid in a first temperature range and to soften in a second temperature range different from the first temperature range.

2. The strip structure according to claim 1, wherein the first temperature range is below 15°C to 28°C, preferably below 18°C ​​to 24°C, particularly below 18°C ​​to 21°C, and the second temperature range is above 15°C to 28°C, preferably above 18°C ​​to 24°C, particularly above 18°C ​​to 21°C.

3. The strip structure according to claim 1 or 2, wherein the polymer matrix (104) further comprises a glassy monomer.

4. The strip structure according to any one of claims 1 to 3 further comprises at least one layer, said at least one layer being composed of or containing at least one of a plurality of thermoplastic particles, wool, or yarn.

5. The strip structure according to any one of claims 1 to 4, wherein the strip (100) having the strip structure has a thickness between 50 μm and 300 μm, preferably less than 100 μm, particularly between 40 μm and 75 μm.

6. The strip structure according to any one of claims 1 to 5, wherein the at least one first monomer or oligomer has a molecular weight greater than 400 g / mol, and / or is preferably selected from the group consisting of reactive diglycidyl ether-based epoxy resin, diglycidyl ether bisphenol A resin, epoxy phenolic varnish resin or combinations thereof.

7. The strip structure according to any one of claims 1 to 6, wherein the at least one second monomer or oligomer has a molecular weight of at least 1000 g / mol, preferably between 1200 g / mol and 1300 g / mol, and / or is preferably selected from the group consisting of medium molecular weight epoxy resins and low viscosity epoxy resins or combinations thereof.

8. The strip structure according to any one of claims 1 to 7, wherein the at least one curing agent is selected from dicyandiamide and diaminodiphenyl sulfone.

9. The strip structure according to any one of claims 1 to 8, wherein the polymer matrix (104) is configured as a thermosetting polymer matrix (104) that can be cured at a temperature between 100°C and 180°C, preferably between 110°C and 140°C.

10. The strip structure according to any one of claims 1 to 9, wherein a support layer is provided that can be removed from the strip during or after the automatic fiber placement (AFP) process.

11. An automated fiber placement apparatus (130) having a movable placement head for laying strips (100) in an automated fiber placement (AFP) method, the automated fiber placement apparatus comprising: - A feeding device for feeding the strip (100) having the strip structure according to any one of claims 1 to 10 into the laying area. - A temperature control unit configured to increase the processing temperature of the strip structure, particularly before or during laying, to a temperature range above 15°C to 25°C, preferably between 18°C ​​and 21°C, to soften the strip structure. - A deposition apparatus for laying and / or pressing the strip structure onto the composite structure (102) or onto previously stacked layers (103a, b) formed on the composite structure (102), - A compaction device for establishing a permanent connection between the strip (100) and the composite structure (102) or a previously stacked layer (103a, b) formed on the composite structure (102), the compaction device being configured to apply at least one of pressure, heat, and electric current to the strip (100) during or after laying. The feeding device, the temperature control unit, the laying device, and the compaction device are positioned adjacent to the laying head and configured to move together with the laying head.

12. An automated fiber placement (AFP) method for forming a composite structure (102) using a strip (100) having a strip structure according to any one of claims 1 to 10, the method comprising: feeding the strip (100) into an automated fiber placement apparatus according to any one of claims 10 to 11, and heating the strip (100) to a temperature above room temperature before or during laying the strip (100) on the composite structure (102) or on a previously stacked layer (103a, b) formed on the composite structure (102). The strip structure is softened until it is softened, and the strip (100) is laid on the composite structure (103) or the previously stacked layers (103a, b) formed on the composite structure (102) by moving the layup head over the composite structure (102) or the previously stacked layers (103a, b), the strip (100) is compacted to establish a permanent connection between the strip (100) and the composite structure (102) or the previously stacked layers (103a, b), and the composite structure (102) is cured.

13. The method of claim 12, wherein moving the layup head comprises moving the layup head over the composite structure (102) or the previously stacked layers (103a, b) in a direction forming an angle with the edge region of the composite structure (102) or the previously stacked layers (103a, b).

14. The method according to claim 12 or 13, wherein the composite structure (102) comprises at least first and second substructures, each having a first region and a second region, wherein curing the composite structure (102) comprises independently curing the first or second region of the at least one first substructure and the at least one second substructure, bringing the first or second region of the at least one first substructure into contact with the first or second region of the at least one second substructure, and co-curing the first or second region of the at least one first substructure and the first or second region of the at least one second substructure after contact to form the composite structure (102).

15. A composite structure (102) manufactured by the method according to any one of claims 11 to 14, wherein the composite structure (102) is an aircraft component, particularly a tank structure for containing liquid, the liquid preferably being a cryogenically stored liquid, such as liquid hydrogen (LH2).

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