Composite articles with core material and method of manufacture

Abstract

Disclosed is a composite and a method of manufacturing the composite article The composite article has first and second skins and a core structure therebetween. The core structure includes a plurality of core units in a tessellating array. Each of the core units comprise a core material block, and a peripheral reinforcement formed of a fibre-reinforced composite material around a periphery of the core material block that extends between the skins.

Description

[0001] COMPOSITE ARTICLES WITH CORE MATERIAL AND METHOD OF MANUFACTURE

[0002] Field of the Invention

[0003] The invention relates to the field of composite manufacture, of lightweight composite articles with sandwich structures or a core material.

[0004] Background of the Invention

[0005] Composite materials are widely used in a number of industries, such as aerospace, automotive, civil engineering and sports goods, due to their high strength-to-weight ratio.

[0006] Composite materials such as carbon fibre composite and fibreglass composite are formed from reinforcement material (typically fibrous) impregnated with a matrix material. Typically, multiple plies or layers of a fabric reinforcement material are impregnated with and reinforce a polymer matrix. For example, a carbon fibre fabric is formed by carbonizing a synthetic polymer fabric material and may be provided in the form of woven fabric, non-woven fabric or may consist of unidirectional fibres. Similar composites may be formed using alternative fibrous materials, such as glasses or synthetic polymers (e.g. aramid), or combinations of such materials. Composites may also be formed by mixing or dispersing relatively short strands or fibrils of a fibre material, such as carbon fibre or a glass fibre, within a polymer matrix to form a mouldable or injectable composite material.

[0007] For some applications, further weight savings can be made with the use of low density core materials, which are typically sandwiched between or within a skin of fibre-reinforced composite material.

[0008] Core materials, such as honeycomb or expanded polymer foams, typically have lower strength than fibre-reinforced composite and can have other less favourable structural properties, such as low flexibility, brittleness etc. The structural properties of the composite article of which they form a part are consequently reliant on good bonding between the skin(s) and the core material. In turn, the integrity of core structures can be vulnerable to delamination between the core material and skin across the surface of an article. Moreover, the inherent structural properties of the core material can limit the size and geometries of articles in which such they can be used. One approach to address these limitations has been the provision of reinforcing ribs extending between the fibre-reinforced composite skins, such as in an orthogrid or isogrid pattern, with core material positioned between the reinforcing ribs.

[0009] The bond between the ribs and the skins both increases overall part rigidity, but can also interrupt delamination paths.

[0010] However, use of such “grid reinforcement” in composites can be difficult to implement and additional weight is added to the resulting composite article. A particular issue arises at intersections or “nodes” between reinforcing ribs. Composite ribs are normally built up from successive fibre layers or tows. The nodes will therefore tend to build up in thickness as the sum of the thickness of the ribs that terminate or extend through the node. Images of an example prior art composite structure are shown in Figs 1 (a) and 1 (b). The structure 1 has a skin 2 formed of multiple plies of carbon fibre composite fabric in a conventional manner. The skin 2 is reinforced with an isogrid reinforcement formed of intersecting ribs 3. These are laid down from reels of unidirectional pre-preg tape using an automated end effector.

[0011] As can be most clearly seen in Fig. 1 (b) the ribs 3 intersect at nodes 4 which are necessarily of a thickness that is the sum of the thicknesses of the ribs 3 that intersect at each node (in this case, three).

[0012] This can lead to additional weight, curing and / or infusion time, and inconsistent fibre-volume fraction (Vf) and thus undesirable variation in mechanical properties of the resulting composite article.

[0013] These issues can be of particular concern for larger composite articles or in high performance applications such as aerospace or satellite applications, and there remains a need for improvements in the manufacture of composite articles incorporating core materials.

[0014] Summary of the Invention

[0015] An aspect of the invention is a composite article comprising: a first skin formed of a fibre-reinforced composite material; having a first outer face and a first inner face a second skin formed of a fibre-reinforced composite material; having a second outer face and a second inner face; wherein the first inner face and the second inner face are oriented towards one another; and a core structure disposed between and bonded to first and second inner faces; wherein the core structure comprises a plurality of core units, each comprising a core material block, and a peripheral reinforcement formed of a fibre- reinforced composite material around a periphery of the core material block and extending from the first inner face to the second inner face; and wherein the core units form a tessellating array, across at least a region of the first and second inner faces.

[0016] The core units form a tessellating array across at least a region of the inner faces of the first and second skins, thereby filling the zone between the skins with core material. The fibre- reinforced composite material around each core unit extends between the skins and provides structural reinforcement to the composite article analogous to a conventional grid reinforcement, with comparatively lighter weight and improved consistency of structural parameters, such as Vf. Indeed, by avoiding material inefficiencies of convention composite ortho- or iso-grid reinforcement and / or by allowing for thinner skins to be used, it has been found to be possible to provide for weight savings of up to 20%, 30% or 40% in comparison to conventional composite articles with core structures or open (void) cores.

[0017] The peripheries of each of the tessellated core units act as a barrier to defect propagation (i.e. of delamination between any of the core units and the skins). Provision of peripheral reinforcement around the core units also provides for the layup of the fibre-reinforcement material therein to be tuned according to a particular requirement, which may not be practicable when laying down the tape to form conventional iso or orthogrids.

[0018] Each core unit may have: a first core face and a second core face, adjacent the first inner face and second inner face, respectively; and a peripheral wall extending between the first and second core faces; wherein the peripheral reinforcement is against the peripheral wall around the periphery of the core material block.

[0019] The peripheral reinforcement may be bonded to the core material block. The peripheral reinforcement may advantageously be bonded to the first and second skins.

[0020] The peripheral reinforcement may include a first flange portion extending across a part of the first core face and / or a second flange portion extending across a part of the second core face, around the all or at least a part of the periphery of the core material block. Each flange portion thus abuts the corresponding inner face and further improves bonding of the peripheral reinforcement with the first and / or second skin.

[0021] Each core unit may include a first and / or second reinforcement sheet of fibre-reinforced composite material over the first and / or second core face. When assembled in a tessellating array, the core units thereby together form a substantially continuous layer of fibre-reinforced composite material adjacent the first and / or second inner face. In turn this may provide for use of a thinner (e.g. fewer plies) first and / or second skin.

[0022] The first and / or second reinforcement sheet may be positioned on the respective core face within the corresponding first or second flange, or may overlap therewith. The first and / or second reinforcement sheet may extend partly over the peripheral wall, over or under the peripheral reinforcement.

[0023] The peripheral reinforcement and the first and / or second peripheral sheet may be continuous, for example formed from a continuous sheet of fibre-reinforced composite material.

[0024] The first and second skins are typically parallel or substantially parallel, at least in the regions thereof that are separated by and bonded to the core units.

[0025] The first and second skins may be planar, across at least region thereof. Alternatively, or in addition, first and second skins may each define a curved, convex and / or concave surface across at least region thereof.

[0026] The terms tessellating, tessellation or tessellating array refers to a repeating pattern of polygonal core units with substantially no overlaps or gaps therebetween, analogous to tiling or mosaic arrays. It will be understood that manufacturing tolerances will exist, with concomitant deviations from perfect geometrical tessellation.

[0027] The core structure comprising a tessellating array of core units disclosed herein results from individual and separate core units being brought together to form an array, as distinct from a surface formation such as an isogrid or orthogrid having a regular pattern.

[0028] The tessellating array of core units may comprise one configuration of core units (i.e. core units all having the same size and shape). The core units may be triangular, quadrilateral (square shaped, diamond shaped, rectangular, rhombohedral etc.), hexagonal or the like. The tessellating array of core units may comprise two or more than two configurations of core units.

[0029] The tessellating array may comprise a first array of a first configuration of core units and a second array of a second configuration of core units in interstices defined by the first array.

[0030] The first array may for example comprise octahedral or octagonal core units, and the second array may comprise square or quadrilateral core units.

[0031] The first array may for example comprise hexagonal or triangular core units, and the second array may comprise pentagonal units having vertices with the same length as those of the core units of the first array; whereby a concave or convex (e.g. domed) tessellating array may be formed.

[0032] In some embodiments, at least one said configuration cannot alone form a tessellating array.

[0033] The composite article may comprise more than one tessellating array. The core structure may comprise more than one tessellating array. A first tessellating array may extend across a first region between the first and second inner surfaces. A second tessellating array may extend across a second region between the first and second inner surfaces.

[0034] First and second tessellating arrays may comprise core units of different sizes and / or configurations, for example to accommodate changes in the shape, size or configuration of the first and second skins. A composite article may, for example, be tapered, or have variations in curvature, concavity etc. such that a different type or array or size of core units may be better adapted for some regions. A composite article may require additional strength or rigidity in some regions, which may be provided by the peripheral reinforcement of smaller core units.

[0035] The composite article may include one or more regions of alternative construction, lacking core units, or formed entirely of fibre reinforced material between opposite outer surfaces thereof.

[0036] The first skin, second skin, the peripheral reinforcements and / or the first and / or second reinforcement sheets (where present) may independently comprise a fabric material. The fabric material may be woven, non-woven, non-crimp, unidirectional or the like. Multiple plies of fabric material may be used, for the first and / or second skin in particular. The first and second skin may include the same number, or a different number, of plies.

[0037] The fabric material may comprise one or a blend of fibrous reinforcement material, such as carbon fibre reinforcement, or a glass fibre reinforcement, optionally interwoven with another fibrous reinforcement material, such as Kevlar, aramid fibre, flax or the like. The composite article may comprise more than one type of fabric material, such both woven and unidirectional fabric in different regions of the composite article, or fabric of a first material (e.g. carbon fibre fabric) interleaved with fabric of a second material (e.g. aramid fibre fabric).

[0038] The fabric material may be pre-impregnated with matrix material, or may be a dry fabric material that is later infused with matrix material. A dry fabric material may include a binder, to assist in manufacture as disclosed herein.

[0039] As known in the art, the fibre orientation of reinforcement fibres within a fabric material can be selected for a particular purpose, to tune the mechanical properties of the resulting composite article. Conventional grid reinforcements, such as isogrid and orthogrid patterns, are typically formed using a tape, such as unidirectional tape, and tuning fibre orientation can be disproportionately time consuming. The present invention provides more flexibility in the selection of fibre orientation of the peripheral reinforcements than has previously been possible.

[0040] Any suitable matrix material may be used, such as a curable matrix material (e.g. an epoxy) or a settable matrix material (e.g. a thermoplastic matrix material).

[0041] The composite article may comprise infilling material along interfaces between adjacent core units and the first inner face and / or the second inner face. The infilling material may be provided during manufacture as a “noodle” or “noodles”.

[0042] The infilling material comprises fibrous reinforcement material dispersed in matrix material. The infilling material may be formed from chopped fibres mixed with matrix material, and during manufacture extruded as one or more beads. The infilling material can include a braided rope, string or roll of fibrous reinforcement material (such as might be formed by twisting or otherwise rolling a reinforcement material tape along its length), optionally pre-preg material, or any other elongate form suitable to infill along said interfaces. As used herein, a string or roll of fibre reinforcement material may be referred to as a “noodle” in the technical field of the invention. The infilling material may be used to infill volumes between reinforcement material, as might otherwise remain between the core units and the skins. A noodle or bead of the infilling material may subsequently be compressed to cause the noddle material to flow and / or infill small gaps within the composite article.

[0043] The invention also extends to a pre-form comprising: a first skin formed of a reinforcement material; having a first outer face and a first inner face; and a core structure disposed against the first inner face; wherein the core structure comprises a plurality of core units, each comprising a core material block, and a peripheral reinforcement formed of a reinforcement material around a periphery of the core material block and extending across a thickness of the core structure; and wherein the core units form a tessellating array, across at least a region of the first face.

[0044] The pre-form may further comprise a second skin formed of a reinforcement material; having a second outer face and a second inner face; wherein the first inner face and the second inner face are oriented towards one another, the core structure is disposed between and against the first inner face and the second inner face; and wherein the peripheral reinforcement extends from the first inner face to the second inner face.

[0045] As known to one skilled in the art, the term pre-form refers to a semi-rigid structure in which reinforcement material (typically plies of fabric) are retained to one another following a degree of compaction and typically also via surface binder. A pre-form will have sufficient structural integrity to be moved as a single unit if required (for example to transfer the pre-form to a mould or to a different tooling surface), but will typically be sufficiently flexible to permit a degree of re-configuration and require structural support from a tooling surface or mould.

[0046] A pre-form may comprise dry reinforcement material (i.e. not including matrix material), or may comprise pre-impregnated or “pre-preg” reinforcement material. A pre-form of dry reinforcement material is intended to be infused with matrix material to form a fibre-reinforced composite material as disclosed herein, and a pre-form of pre-preg material is intended to be heated, compressed or further compressed to cure and / or cause the matrix material to flow and thereby form a fibre-reinforced composite material. In a further aspect of the invention there is provided a method of making a composite article, comprising the steps of: providing a first skin on a tooling or mould surface; the first skin being formed of a reinforcement material and having a first outer surface against the tooling or mould surface, and a first inner surface opposite to the first outer surface; providing a plurality of core units, each comprising a core material block, and a peripheral reinforcement formed of a reinforcement material around a periphery of the core material block across a thickness of the core material block; forming a core structure by positioning the core units across at least a region of the first inner surface in a tessellating array; and providing a second skin; the second skin being formed of a reinforcement material having a second inner face and a second outer face, and positioning the second inner face across and against the core structure.

[0047] The method may comprise laying up of a reinforcement material (typically fabric) on a tooling or mould surface. The method may involve laying up reinforcement material on a tooling surface and transferring the layup to a mould, optionally after consolidation via heating and / or compression. A mould may for example have a degree of curvature, such that laying up is more conveniently performed on a flat tooling surface.

[0048] In some embodiments the reinforcement material of the first skin and / or second skin are laid up on a tooling surface and subsequently transferred to a mould. The second skin in particular is positioned in use against the core structure, having been first laid up on a tooling surface and transferred.

[0049] In some embodiments, a peripheral reinforcement and / or reinforcement sheet may comprise more than one fabric ply and be laid up on a tooling surface and subsequently transferred to a forming tool as disclosed herein.

[0050] The method may comprise one or more steps of consolidating by compressing and / or heating. Compression may be achieved by sealing a vacuum bag around a layup or partial layup on the mould or tooling surface, and evacuating to reduce the pressure in the vacuum bag and cause the layup to be compressed under the action of a pressure differential. Consolidation may include heating, via external heaters or heaters embedded in or under the mould or tooling surface. Such heating may activate a binder to bind plies of reinforcement fabric material to one another. The method may comprise, for example, laying up reinforcement material of the first skin and / or the second skin on the tooling or mould surface (for example by positioning plies of a reinforcement fabric material on the surface) and consolidating by compressing and / or heating the laid up reinforcement material. The method may comprise consolidating after the core structure has been provided. The method may comprise separately consolidating by compressing and / or heating after the second skin has been provided over the core structure.

[0051] The method may comprise a plurality of the steps of providing a core material block, and positioning reinforcement material around the periphery of the core material block across a thickness thereof.

[0052] The core material blocks may be machined or cut from a larger piece of core material.

[0053] The core material will typically comprise voids, pores of channels so as to provide a degree of rigidity but at a lower density than the reinforcement material around the periphery thereof or the first or second skins. The core material of the core units will typically have a different moropholgy and / or be made from different materials to the reinforcement materials and / or matrix materials of the skins and peripheral reinforcement, flange portion etc.

[0054] Any suitable type of core material may be used, such as a honeycomb or expanded polymer foam material. Honeycomb or other porous structures can be made of, for example, aluminium or Nomex (a trademark) or other aramid sheet material. Fibreglass, thermoplastics material or the like may also be used. A core material block may include facing sheets, above and below a honeycomb or porous core.

[0055] The method may comprise covering each core material block with reinforcement material, around at least the periphery thereof.

[0056] When covering each core material block, the orientation and / or relative orientation of reinforcement fibres of at least the peripheral reinforcement may be selected for a particular purpose. For example, in use of a fabric, the fibres may be oriented to be parallel to, perpendicular to and / or at 45 degrees to the eventual orientation of the first and second skins. Successive plies may have different fibre orientation. The fibre orientation of reinforcement fibres may vary between core blocks, across the array.

[0057] The step of covering may comprise wrapping. The core units may be made using a forming tool, the forming tool having a shaped recess having the external dimensions of the desired core unit.

[0058] The method may comprise positioning reinforcement material around a periphery of the recess and placing the core material block in the recess.

[0059] The method may comprise lining the entire recess with reinforcement material.

[0060] The method may comprise positioning reinforcement material across the forming tool, at least partly over the recess and then placing the core material block into the recess. The reinforcement material is thereby forced into the recess and around the periphery of the core material block, to form a preform of the peripheral reinforcement. Advantageously, the recess may be sized such that the reinforcement material is forced against the peripheral walls of the core material block (e.g. via an interference or compression fit of the block within the recess), thereby assisting in bonding to the reinforcement material.

[0061] The method may comprise placing a reinforcement sheet in the recess.

[0062] The method may comprise placing reinforcement material entirely over the recess and then placing the core material block into the recess, such that at least a part of the reinforcement material forms a pre-form of a reinforcement sheet against a first face of the core material block. The reinforcement material may extend over the tool over and beyond the recess and be shaped so as to also form the peripheral reinforcement and optionally a said second flange, when the core material block is placed into the recess.

[0063] The method may comprise placing a reinforcement sheet over the core material block, after the core material block has been placed in the recess.

[0064] The reinforcement material used to form the core unit may include one or more sheets of material.

[0065] The reinforcement material used to form the core unit may define one or more upper and / or lower tabs. By upper and lower, reference is made to the position in relation to the recess in the forming tool.

[0066] The method may comprise folding the upper tabs across at least a part of a face of the core material block to form a flange, after the core material block has been placed in the recess. The method may comprise positioning the lower tabs on the base of the recess (by which we mean the face of the recess oriented to an outside of the recess) to thereby form a flange.

[0067] The method may comprise removing the core unit from the forming tool. The forming tool may be adapted to facilitate such removal, for example by splitting apart or otherwise varying the size or configuration of the recess to facilitate such removal.

[0068] The method may comprise extruding one or more beads of infilling material along interfaces between adjacent core units and the first inner face and / or the second inner face. The method may comprise positioning one or more noodles of infilling material along interfaces between adjacent core units and the first inner face and / or the second inner face.

[0069] The method may comprise extruding noodles in a pattern on the first inner surface reflecting the interfaces between adjacent core units, and then positioning the core units across the at least a region of the first inner surface.

[0070] The method may comprise positioning the core units across the at least a region of the first inner surface and then extruding noodles along the interfaces between adjacent core unit, prior to providing the second skin.

[0071] The provision of infilling material as disclosed herein is of particular utility with use of pre-preg fibre-reinforced material.

[0072] One or more steps of the method may be automated. For example, the cutting of the core material blocks may be automated via a suitable cutting or machining tool. As known in the art, reinforcement fabric may be cut (e.g. laser cut) and positioned on a tooling or mould surface, or in a said recess, using a pick and place machine or an end effector or the like. Noodles may be extruded using an automated end effector.

[0073] The composite article may be manufactured using autoclave or out of autoclave methods.

[0074] The composite article may be manufactured using pre-preg reinforcement material, wherein the pre-preg is heated and compressed (to render flowable and / or to cure the matrix material therein) in a vacuum bag or in an autoclave.

[0075] The composite article may be manufactured using dry reinforcement material, wherein the method comprises infusing the reinforcement material with matrix material. Any suitable infusion method may be used.

[0076] The method may include sealing the layup in a vacuum bag (i.e. a flexible barrier material sealed to a mould surface around the layup), reducing pressure in the vacuum bag and infusing the layup with flowable matrix material.

[0077] The method may include resin transfer infusion, typically within an autoclave, or resin transfer moulding, as known to one skilled in the art.

[0078] The matrix material may be curable and method may comprise curing the matrix material. Curing may be performed in an autoclave, while the layup is under pressure. Curing may be performed in a vacuum bag, in an out of autoclave method.

[0079] The matrix material may be a thermoplastic matrix material, and the method may comprising cooling the layup to allow the matrix material to solidify. Typically, thermoplastic matrix materials are used in out of autoclave methods.

[0080] The invention in a further aspect extends to a composite article obtained or obtainable by the methods as disclosed herein.

[0081] The term “fibre-reinforced composite material” herein refers to a blend of a fibre reinforcement with a matrix material. The fibre reinforcement may be in the form of a fabric or tape, woven, non-woven, non-crimp or unidirectional. The term “reinforcement material” herein refers to fibre reinforcement prior to being formed into a composite material, and thus prior to matrix material infusion, curing or cooling, or prior to melting / curing of pre-preg reinforcement material; as the case may be.

[0082] The term “plurality” includes two or more than two, for example, two or more than two core units.

[0083] Unless otherwise stated, the term “comprises” herein means consists of, or includes, i.e. that the relevant method step, apparatus or material consists of the stated feature, or includes the stated feature in addition to other optional features.

[0084] It will be understood that the various optional features set out in relation to any aspect of the invention correspond to optional features of any other aspect of the invention. Description of the Drawings

[0085] Nonlimiting example embodiments will now be described with reference to the following drawings:

[0086] Figs. 1 (a) and 1(c) show images, and Figs. 1(b) and 1(d) corresponding photographs of a prior art composite structure with iso-grid reinforcement.

[0087] Fig. 2 shows a mould for making a composite article.

[0088] Fig. 3 is an image showing core material blocks of a core structure positioned over material plies on the mould surface.

[0089] Fig. 4 is an image showing a plies of a second skin being laid up over the core structure.

[0090] Fig. 5 is an image of a comparative example of a composite article.

[0091] Fig. 6(a) is a schematic cross sectional view of an embodiment of a composite article in accordance with the invention; and Fig. 6(b) is a schematic perspective view of a core unit of the composite article of Fig. 6(a).

[0092] Figs. 7(a) and 7(b) are schematic cross sectional and perspective view of alternative embodiments of a composite article and core unit.

[0093] Fig. 8(a) is a schematic cross sectional view of another embodiment of a composite article in accordance with the invention; and Figs. 8(b) and 8(c) are schematic perspective views of a core unit of the composite article of Fig. 8(a).

[0094] Fig. 9(a) is a schematic cross sectional view of a still further embodiment of a composite article in accordance with the invention; and Figs. 9(b) and 9(c) are schematic perspective views of a core unit of the composite article of Fig. 9(a).

[0095] Figs. 10(a)-10(f) illustrate an exemplary method of making a preform of a core unit, in accordance with the invention; Figs. 11-14 are images showing steps of an embodiment of a method of manufacture a composite article. Fig. 14(a) shows a close up view and Fig 14(b) shows the corresponding photograph of an inner layer of the composite article following consolidation.

[0096] Fig. 15 shows an embodiment of a composite article in accordance with the invention.

[0097] Fig. 16 shows alternative tessellating arrays of core units.

[0098] Fig. 17 is an image showing alternative embodiments of core unit pre-forms being assembled together to form a tessellating array.

[0099] Detailed description

[0100] A mould 10 for manufacturing a composite article is shown in Fig. 2. The mould is for an aerodynamic fairing, and has a complex concave and tapered form of a general type that has conventionally presented challenges for composite articles, such as radomes, nose cones, cowlings, satellite applications, marine applications and the like; for which core structures are commonly advantageous.

[0101] The intended composite article (the aerodynamic fairing) is intended to be formed from an outer skin and an inner skin, which are sandwiched to a core material.

[0102] With reference to Figures 2 to 5, an initial demonstrator fairing 20 was constructed using the mould 10. In a first step, three layers of pre-preg carbon fibre fabric were positioned on the mould surface 12 of the mould 10 (not shown in the figures) an outer skin 22 was positioned in the mould 10, and multiple blocks 24 of a polymethacrylimide (PMI) structural foam was positioned over the plies forming the outer skin 22 in a tessellating pattern as shown in Fig. 3. Together, the foam blocks 24 form a core structure 26. The PMI foam selected was Rohacell® (manufactured by Evonik Industries AG, Essen, Germany).

[0103] The inventors have found that forming the cores structure from separate blocks 24, the foam was better able to follow the contours of the mould surface 12; particularly towards the tapered end 12a. It was also found to be beneficial to reduce the size of the blocks 24 towards the tapered end 12a. As such, comparatively larger blocks 24b were used at the wider end 12b of the mould 10, and comparatively smaller blocks 24a used at the tapered end 12a. Three further layers of pre-preg carbon fibre fabric were then positioned in the mould 10, across the core structure 26, as shown in Fig. 4, which would ultimately form the second, inner skin 28. The resulting layup was then sealed under a semi flexible vacuum bag 14 (shown in readiness for use in Fig. 2) and the layup compressed and cured in a conventional out of autoclave method, to form the composite article - the fairing 20 shown in Fig. 5. The vacuum compression and curing process bonded the skins 22, 28 to the core structure 26.

[0104] The fairing 20 weighed around 25 kg and, other than the novel core construction, can be considered to represent a comparative example in terms of weight and structural properties. As discussed above, a potential limitation of this construction is susceptibility to crack propagation across the core structure (between one or other of the skins). This limits the potential size and / or lifespan of articles having this type of construction.

[0105] A schematic cross sectional view of a region of a composite article 100 in accordance with the invention is shown in Fig. 6(a). The article 100 has a first skin 120, a second skin 140 and a core structure 150, formed of a plurality of core units 160 abutting one another in a tessellating array.

[0106] Each core unit 160 has a core material block 162 and peripheral reinforcement 164 around the hexagonal (in the embodiment shown) periphery thereof (see Fig 6(b)). The peripheral reinforcement 164 extends across a thickness T of the block 162 from a first core face 166 to a second core face 168 thereof. The core units 160 shown are hexagonal, but as disclosed herein any alternative tessellating shape or combination of shapes can be used.

[0107] The first skin 120 has a first outer face 122 and a first inner face 124. The second skin has a second outer face 142 and a second inner face 144. The inner faces 124, 144 are oriented towards one another and, in the embodiment shown, parallel.

[0108] The inner faces 124, 144 are bonded to the core material blocks 162 (the first core face and second core faces 166, 168 thereof, respectively). The peripheral reinforcements 164 extend between the first and second inner faces 124, 144.

[0109] The first and second skins 120, 140 and the peripheral reinforcement 164 are formed from fibre-reinforced composite material. Any suitable material may be used, but the fibre- reinforced composite material is conveniently carbon fibre composite material, impregnated with a matrix material, typically an epoxy resin. The core material blocks may be formed from any suitable core material as disclosed herein, such as a structural foam material. The peripheral reinforcements 164 mimic a grid reinforcement structure and in addition resist propagation of cracks between core material and the first or second skins, such that any delamination that does occur is less likely to propagate from one core unit to an adjacent core unit.

[0110] An alternative composite article 100a is shown in Fig. 7(a). Features in common with the composite article 100 are provided with like reference numerals.

[0111] The article 100a has a first skin 120, a second skin 140 and a core structure 150, formed of a plurality of core units 160a abutting one another in a tessellating array.

[0112] Each core unit 160a has a core material block 162 and peripheral reinforcement 164a around the hexagonal (in the embodiment shown) periphery thereof (see Fig 6(b)). The peripheral reinforcement 164a extends across a thickness T of the block 162 from a first core face 166 to a second core face 168 thereof, and further includes flange portions 170a extending around the periphery of the core unit 160a partly across the first and second core faces 166, 168.

[0113] The inner faces 124, 144 are bonded to the flange portions 170a and the first and second core faces 166, 168. The peripheral reinforcements 164 extend between the first and second inner faces 124, 144 and the flange portions 170a assist in bonding of the core units 160a to the skins 120, 140 - in particular, of the peripheral reinforcements 164a to the skins 120, 140. This arrangement may be advantageous in some applications, including where surface uniformity of the outer skin surfaces 122, 142 is not critical.

[0114] Further embodiments are shown in Figs. 8(a)-(d). Features in common with composite articles 100 and 100a are provided with like reference numerals. The composite article 100b has core units 160b each provided with a reinforcing sheet 172b of fibre reinforced composite material bonded to the first core faces 166 thereof. The second core faces 168, including the upper surfaces of second tabs 172b2 are against and bonded to the second skin 140. The lower, first flanges 170b1 and reinforcing sheets 172b together form a generally continuous layer adjacent to the first inner face 124. This can improve finish of the first outer face 122 (e.g. where the outer face 122 is an aerodynamic surface). Additionally, in some applications this can provide for a thinner first skin 120 to be used, reducing the weight of the composite article 100b.

[0115] Reinforcement sheets may optionally also be provided on the second core face. A composite article 100c (Fig. 8(d)) further include infilling material, provided by beads or noodles 180 of matrix material infused with reinforcement fibre, along the interfaces between composite units 160c and the first and / or second skins 120, 140.

[0116] Alternatively, as illustrated in Fig. 9(a) and (b), core units 160d which may be substituted for the core units of any embodiment disclosed herein may have a covering formed of a single sheet of reinforcement fabric that defines the reinforcement sheet 172d over the first core face 166, and the peripheral reinforcement 164d.

[0117] An exemplary method of making a core unit 160d is illustrated with reference to Figs. 10(a)- (f).

[0118] A forming tool 200 is shown in Fig. 10(a). The tool 200 has an upper face 202 into which is set a recess 220, having a shape and depth corresponding to that of the desired core unit. The upper face 202 has raised profiles 204 to assist in locating a sheet of reinforcement material blank 300 (of pre-preg carbon fibre fabric), and insets 206 whereby the blank 300 can be manipulated as discussed below.

[0119] In a first step, the blank 300 is positioned on the tool 200 across the recess 220. The blank has six tabs 320 around the outer edge thereof that extend outwardly beyond the recess. The tabs 320 locate between the correspondingly shaped raised profiles 204.

[0120] A core material block 162, typically cut or machined from a larger sheet, is placed on the tool 200, over the blank 300 and recess 220, with the apices 163 thereof located and thus aligned by the raised profiles 204 (Fig. 10(c)).

[0121] In a next step, Fig. 10(d), the core material block 162 is pressed into the recess 220. This forces the blank into the recess and around the core material block 162. A central region 330 of the blank 300 is then adjacent the lower (in the orientation of the images) face of the core material block, and first tab portions 322 are trapped between the inner walls 222 of the recess 220 and the outer peripheral faces 169 of the core material block. Second tab portions 324 can then be folded over the second core face 168 using forming levers 240 which are clipped around the tool 200, into the insets 206, as shown in Fig. 10(e) (only two such levers are installed), to form a pre-form 170d1 of flanges around a periphery of the second core face 168. The tool 200 is provided with a split line 230, such that it can be removed in halves from the resulting core unit preform 160d1 , shown in Fig. 10(f).

[0122] The recess 220 is sized to provide a close fit with the core material block 162, such that with the additional thickness of the fabric of the blank 300 results in a degree of compression to assist in binding the first tab portions 322 to the outer faces 169. In addition, the tool 200 can be warmed to activate binder and pre-form the fabric. The recess may similarly have a depth corresponding to or slightly less than that of the core material block 162, whereby force applied over the tool 200 when the core material block 162 has been inserted can similarly assist to bind the central region 330 to the first core face and the second tab portions 324 to the second core face 168. Alternatively, this final step of compression may be omitted, as the core units will be sufficiently compressed during manufacture of the composite article.

[0123] An improved fairing in accordance with the invention was manufactured using the mould 10, having mechanical properties (strength, rigidity etc. comparable to the fairing 20).

[0124] As shown in Fig. 11 , a single layer 420 of pre-preg fabric material was laid on to the mould surface 12. Each of a plurality of core unit preforms 160d1 were then positioned over the layer 420 in a tessellating pattern, until the entire layer 420 was covered across the mould surface 12 (Fig. 12). Some core unit preforms 164d1 abutting an edge of the mould 10 were cut to size (see, e.g. core unit preform 165 marked in the figure).

[0125] To accommodate the increased curvature of the mould surface 12 towards the tapered end 12a, two tessellating arrays were used. The first array (indicated generally as 430), extending from the end 12b, has larger sized hexagonal core units 160d, and the second array (indicated generally as 432) has smaller core units 160d’ of analogous construction.

[0126] Optionally the layup can be bagged and evacuated at this stage, to promote binding of the core units to the fabric layer 420. In the embodiment shown, due to the use of single layers of fabric, consolidation was not considered essential.

[0127] The plurality of core units 160d1 , 160d’ in the arrays 430, 432 together form a core structure 450.

[0128] A further layer 440 of pre-preg fabric was positioned over the core units (Fig. 13). The further layer 440 comprises multiple portions, including a shaped first portion 442 (also visible in Fig. 12) and a complimentary shaped second portion 444, to reduce or avoid wrinkling in the high curvature region towards the end 12a.

[0129] The layup was then bagged, evacuated to compress and consolidate the layers 420, 440 to the core structure 450, and the preform was then cured.

[0130] Fig. 14 shows a close up view of the inner layer 440 following consolidation, illustrating that the fabric 440 has conformed to the second surfaces 168 of the core unit preforms 160d1 .

[0131] The resulting composite article, fairing 400 (Fig. 15), in which the first skin 422 was formed from the layer 420 and the second skin 442 was formed from the layer 440 was found to be around 15kg in weight, representing a substantial (c.a. 40%) weight saving over the comparative composite article, fairing 20 (discussed above in relation to Figs 2-5). This was made possible by the novel structure. In particular, the regions 330 of fabric tessellate to provide an essentially complete layer adjacent to the inner face of the outer skin 422 and the peripheral reinforcements 164d (including flanges formed from the flange preforms 170d1) provide additional rigidity. Together, this allows for only a single layer of fabric to be used for each skin, conveying substantial weight savings.

[0132] Other tessellating patterns of core units are envisaged, such as the alternative first and second arrays 434, 436 shown in Fig. 16.

[0133] Other core units were also tested such as the diamond array 530, of the preforms of core units 160e1 shown in Fig. 17.

[0134] The example embodiments described are made using pre-preg materials in out of autoclave methods, but the invention may also be used in connection with autoclave manufacture, dry fabric and indeed other known methods in which core structures have conventionally been employed.

[0135] Whilst exemplary embodiments have been described herein, these should not be regarded as limiting to the modifications and variations possible within the scope of the invention as disclosed herein and recited in the appended claims.

Claims

CLAIMS1 . A composite article comprising: a first skin formed of a fibre-reinforced composite material; having a first outer face and a first inner face a second skin formed of a fibre-reinforced composite material; having a second outer face and a second inner face; wherein the first inner face and the second inner face are oriented towards one another; and a core structure disposed between and bonded to first and second inner faces; wherein the core structure comprises a plurality of core units, each comprising a core material block, and a peripheral reinforcement formed of a fibre-reinforced composite material around a periphery of the core material block and extending from the first inner face to the second inner face; wherein the core units form a tessellating array, across at least a region of the first and second inner faces.

2. The composite article of claim 1 , wherein peripheral reinforcement is bonded to the core material block and / or to the first and second skins.

3. The composite article of claim 1 or 2, wherein each core unit has a first core face adjacent the first inner face adjacent the first inner face and a second core face adjacent the second inner face, and a peripheral wall extending between the first and second core faces; wherein the peripheral reinforcement is against the peripheral wall around the periphery of the core material block; and wherein the peripheral reinforcement includes a first flange portion extending across a part of the first core face and / or a second flange portion extending across a part of the second core face, around the all or at least a part of the periphery of the core material block; and / or wherein each core unit includes a first and / or second reinforcement sheet of fibre-reinforced composite material over the first and / or second core face.

4. The composite article of any preceding claim, wherein the first and second skins are parallel or substantially parallel in the regions thereof that are separated by and bonded to the core units.

5. The composite article of any preceding claim, wherein the first and second skins may each define a curved, convex and / or concave surface across at least region thereof.

6. The composite article of any preceding claim, wherein the tessellating array of core units comprises two or more than two configurations of core units.

7. The composite article of any preceding claim, wherein a first tessellating array extends across a first region between the first and second inner surfaces and a second tessellating array extends across a second region between the first and second inner surfaces; wherein the first and second tessellating arrays comprise core units of different sizes and / or configurations.

8. The composite article of any preceding claim, further comprising infilling material along interfaces between adjacent core units and the first inner face and / or the second inner face, the infilling material comprising fibrous reinforcement material dispersed in matrix material.

9. A pre-form comprising: a first skin formed of a reinforcement material; having a first outer face and a first inner face; and a core structure disposed against the first inner face; wherein the core structure comprises a plurality of core units, each comprising a core material block, and a peripheral reinforcement formed of a reinforcement material around a periphery of the core material block and extending across a thickness of the core structure; and wherein the core units form a tessellating array, across at least a part of the first face.

10. The pre-form of claim 9, comprising a second skin formed of a reinforcement material; the second skin having a second outer face and a second inner face; wherein the first inner face and the second inner face are oriented towards one another, the core structure is disposed between and against the first inner face and the second inner face; and wherein the peripheral reinforcement extends from the first inner face to the second inner face.

11. The pre-form of claim 9 or 10, wherein the reinforcement material is a dry reinforcement material, or wherein the reinforcement material is a pre-impregnated” reinforcement material.

12. A method of making a composite article, comprising the steps of: providing a first skin on a tooling or mould surface; the first skin being formed of a reinforcement material and having a first outer surface against the tooling or mould surface, and a first inner surface opposite to the first outer surface; providing a plurality of core units, each comprising a core material block, and a peripheral reinforcement formed of a reinforcement material around a periphery of the core material block across a thickness of the core material block; forming a core structure by positioning the core units across at least a part of the first inner surface in a tessellating array; and providing a second skin; the second skin being formed of a reinforcement material having a second inner face and a second outer face; and positioning the second inner face across and against the core structure.

13. The method of claim 12, comprising laying up the reinforcement material of the first skin and / or second skin on a tooling surface and subsequently transferring to a mould.

14. The method of claim 12 or 13, comprising one or more of: laying up reinforcement material of the first skin on the tooling or mould surface and consolidating by compressing and / or heating the laid up reinforcement material; consolidating by compressing and / or heating after the core structure has been provided; consolidating by compressing and / or heating after the second skin has been provided over the core structure.

15. The method of any one of claims 12 to 14, comprising a plurality of the steps of providing a core material block, and positioning reinforcement material around the periphery of the core material block across a thickness thereof.

16. The method of claim 15, comprising making each core unit using a forming tool, the forming tool having a shaped recess having the external dimensions of the desired core unit.

17. The method of claim 16, comprising positioning reinforcement material around a periphery of the recess and placing the core material block in the recess.

18. The method of claim 16 or 17, comprising positioning reinforcement material across the forming tool, at least partly over the recess; andthen placing the core material block into the recess to thereby force the reinforcement material into the recess and around the periphery of the core material block to form a pre-form of the peripheral reinforcement.

19. The method of claim 18, comprising placing reinforcement material that extends over the tool over and beyond the recess and shaped so as to also form the peripheral reinforcement, when the core material block is placed into the recess.

20. The method of any one of claims 16 to 19, wherein the reinforcement material used to form the core unit defines one or more upper tabs, wherein the method comprises folding the upper tabs across at least a part of a face of the core material block to form a flange, after the core material block has been placed in the recess.21 . The method of any one of claims 16 to 20, comprising positioning one or more beads or noodles of infilling material along interfaces between adjacent core units and the first inner face and / or the second inner face.

22. The method of claim 21 , comprising positioning the core units across the at least a part of the first inner surface, and extruding noodles along the interfaces between adjacent core unit prior to providing the second skin.

23. The method of any one of claims 16 to 22, comprising manufacturing using autoclave or out of autoclave methods.

24. The method of any one of claims 16 to 23, wherein the reinforcement material is prepreg reinforcement material, wherein the layup comprising the first skin, the core units and the second skin is heated and compressed, to render flowable and / or to cure the matrix material therein, in a vacuum bag or in an autoclave.

25. The method of any one of claims 16 to 23, wherein the reinforcement material is dry reinforcement material and the method comprises infusing the reinforcement material with matrix material.

26. The method of claim 25, wherein the layup comprising the first skin, the core units and the second skin is heated and compressed, to render flowable and / or to cure the matrix material therein, in a vacuum bag or in an autoclave.

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