FIELD OF THE INVENTION

The invention relates to a joint made of a fibre- reinforced composite material. The composite material includes reinforcing fibres that are embedded in a binding matrix (e.g. resin) . The joint may be used to connect components together and to transfer loads between the components.

BACKGROUND TO THE INVENTION

A joint is a part that is used to connect two or more components together and transfer loads between the components. One of the most commonly used types of joint is a T-joint. A T-joint has a flange (corresponding to the stem of the "T") which is joined to a base plate (corresponding to the cross bar in the "T") . A first surface (e.g. on a first component) can be attached to the flange and a second surface (e.g. on a second component) can be connected to the base plate to connect the surfaces together and transfer loads between them. Thus, a T-joint enables surfaces that are angled relative to one another (e.g. perpendicular) to be connected together.

Many other types and shapes of joint are also known. Joints may be made of many different types of materials, including metals, plastics and composite materials. In applications where a high strength-to-mass is desired, joints may be made of fibre-reinforced composite materials, as these materials may provide high levels of strength for a relatively low mass.

Joints made of fibre-reinforced composite materials are used in a wide range of industries, including the aerospace, automotive, construction and marine industries. T-joints made of composite materials are generally used in aircrafts at the bulkhead-to-skin, rib-to-skin and spar-to-skin interfaces. For example, they are typically used to attach the stiffeners (also known as "stringers") to the skin of the aircraft. T- joints made of reinforced composite materials are also often used in ships, where, for example, the bulkhead may be connected at a right angle to the ship's hull with a T-joint.

Fibre-reinforced composite materials are typically made of a binding matrix in which reinforcing fibres are embedded. Common types of fibre used in composite materials include glass (e.g. in fibreglass) or carbon. Carbon reinforced composite materials may be extremely strong and lightweight, and so are commonly used in a wide range of industries. The fibres in a fibre-reinforced composite material are often woven into sheets (often referred to as a "fabric") which are layered on top of one another. However, other types of textile can also be used (e.g. sheets non-woven or unidirectional fibres) . The sheets may be layered such that the direction of the weave of the sheets changes from layer to layer, in order to increase the strength of the composite material in all directions. The binding matrix is a polymer which serves to bond the fibres together to form a solid component. The binding matrix is often a thermosetting resin such as epoxy. Other types of thermosetting polymers may also be used, such as polyester, vinyl ester or nylon.

There are many different manufacturing techniques for making components out of fibre-reinforced composite materials, such as the wet lay-up technique, the pre-impregnation moulding technique, and the spray lay-up technique. The wet lay-up technique involves layering sheets of fibre in a mould and impregnating the sheets with resin. This may be done by filling the mould with resin, or by applying resin to the sheets (e.g. using a brush or roller) . The resin may then be cured, either at room temperature or in a heated environment, depending on the thermosetting properties of the resin. The pre-impregnation moulding technique is similar to the wet lay up technique, except that the sheets of fibre are pre

impregnated with resin. The pre-impregnated sheets of fibre are layered in a mould, and then heated up to allow the resin to flow between the layers and cure. Unlike the wet lay-up and pre-impregnation moulding techniques, the spray lay-up technique does not involve using sheets of fibre. Instead, the spray lay-up technique involves cutting and depositing segments of fibre into a mould (e.g. using a pneumatic gun having a chopper) and spraying a resin into the mould over the deposited fibres. The resin is then cured under the required conditions to form the component.

A drawback of using composite materials for making joints is that stresses in the material which arise when a load is applied to the joint may cause delamination (i.e. separation of the layers) of the composite material. Such delamination may reduce the structural integrity of the joint, which may reduce the load that the joint is capable of bearing and increase the risk of failure of the joint. For example, in the case of a T-joint where a load is applied to the flange of the T-joint, the flange may bend relative to the base plate. This may result in large out-of-plane stresses (i.e. stresses in a direction normal to the layers) in the vicinity of the connection between the base plate and the flange, which may result in delamination of the composite material. In some cases, this may result in the flange completely breaking off from the base. Another failure mode of T-joints made of fibre- reinforced composite materials is when the flange is pulled away from the base. This may result in delamination and de bonding between the flange and the base plate.

Several approaches can be used to improve the strength of composite T-joints, as shown in Figs. 1A-1E. Fig. 1A shows a T-joint 100 where two L-shaped members 102, 104 are adhered together to form the joint. Each L-shaped member 102, 104 is formed of multiple layers of fibre material embedded in a binding matrix. The T-joint 100 is strengthened by adding a toughening material in the deltoid region 106 of the T-joint between the two L-shaped members 102, 104. The toughening material may serve to absorb shocks, and reduce deformation of the joint near bends 108, 110.

Fig. IB shows a T-joint 112 having a base plate 114 and flange 116 which are formed of layers of fibre material embedded in a binding matrix. The base plate 114 and flange 116 are secured together (e.g. with resin) . The connection between the base plate 114 and the flange 116 is reinforced with overlaminate structures 118, 120 on either side of the flange 116. Each overlaminate structure is adhered to a side of the flange 116 and to the base plate 114. The overlaminate structures 118, 120 may be pre-formed and made of a suitable material (e.g. metal, carbon fibre) . Alternatively, the overlaminate may be formed by depositing layers of fibre material at the connection between the base plate and the flange 116 and embedding them in resin (e.g. similar to the wet lay-up technique discussed above) .

Fig. 1C shows a T-joint 122 having a base plate 124 that is connected to a flange 126 via a Pi-shaped preform 128. Such a T-joint is also referred to as a "Pi- joint". The base plate 124, flange 126 and Pi preform 128 may be bonded together by infusing resin into the fibres using vacuum assisted resin transfer moulding (VARTM) . Such Pi-joints have been found to be efficient connectors for integrated composite structures.

Fig. ID illustrates another approach for strengthening a T-joint. Fig. ID shows a T-joint 130 having a base plate 132 and a flange 134 which are formed of layers of fibre material embedded in a binding matrix. The base plate 132 and flange 134 are secured together (e.g. with resin) . The connection between the base plate 132 and the flange 134 is strengthened by inserting pins 136 through a thickness of the base plate so that they pass into the flange 134. The pins serve to increase inter-layer strength in the composite material, which may prevent delamination. The pins may also serve to resist cracking of the binding matrix or adhesive at the interface between the base plate 132 and the flange 134. The pins may be made of various materials, including fibrous materials (e.g. carbon fibre) or metal. This strengthening method is known as "z-pinning", as the pins are oriented perpendicularly to the layers in the base plate.

Fig. IE shows a "bio-inspired" T-joint 138. The T-joint 138 is formed of multiple layers 140 of fibre material embedded in a binding matrix (not shown) . Portions of the layers 140 of fibre material (corresponding to the base of the T-joint) are embedded directly into a surface 142 to which the T-joint 138 is to be attached. For example, in an aircraft the base of the T-joint 138 may be embedded directly into the skin of the aircraft. Performance of such a T-joint may vary depending of the percentage of layers embedded in the surface. Embedding of the T-joint in the surface may increase the load- bearing capabilities of the T-joint, by spreading damage over a larger area (e.g. to the surface) .

SUMMARY OF THE INVENTION

At its most general, the present invention provides a joint made of a composite material, where a bend in the joint is strengthened by an integrally formed support structure that is spaced apart from a main body of the composite material in the vicinity of the bend. The support structure may act as a brace or buttress across the bend, in order to improve the strength of the joint around the bend. This may avoid stress concentration around the bend by spreading the applied loads over a larger area to avoid cracking and/or delamination of the composite material around the bend, which is typically the weakest area in a composite joint. As the support structure is formed using a portion of the composite material with which the joint is made, forming the support structure does not involve adding any extra mass to the joint. Thus, the

invention enables the strength of the joint to be improved without adding extra mass to the joint. This may be

advantageous in applications such as aerospace, where the strength-to-mass ratio of components is a primary concern. In contrast, conventional methods of strengthening joints (e.g. overlaminate, Pi-joint, z-pinning) typically involve adding material to the joint in order to strengthen the joint.

Furthermore, because the support structure is formed with a portion of the composite material, the joint may be

manufactured in one go using conventional manufacturing methods for composite materials. This may enable cheap and quick manufacture of the joints of the invention. The

inventors have found that joints according to the invention may have significantly increased strength and durability compared to conventional joints.

According to a first aspect of the invention, there is provided a joint made of a composite material, the joint comprising: a body formed from a bent first portion of composite material, the body having a first component mounting region and a second component mounting region that are joined by a bend in the body; and a buttress support formed from a second portion of composite material that spans between the first component mounting region and the second component mounting region to form a gap between the first portion of composite material and the second portion of composite material at the bend in the body.

The joint may be a unitary part formed of the composite material. Accordingly, the first component mounting region, the second component mounting region and the bend may form a continuous piece of composite material. The first and second component mounting regions may be flanges or other connection structures configured to receive or have mounted thereon components in a larger structure. In some examples the joint may be an independent component that is mountable in such a structure. However, in other examples, the joint may be directly integrated into a larger structure (e.g. it may be "built in" to the structure) . For example, the joint of the invention may be used in an I-beam (e.g. to join a web of the I-beam to a flange of the I-beam) .

The first and second component mounting regions may comprise respective first and second surfaces between which the buttress support extends. The first and second surface may be angled relative to one another (i.e. there may be a non-zero angle between them) . The bend may be sharp (e.g. it may be formed by two flat surfaces intersecting) or it may be rounded. A rounded bend may be preferred, as this may reduce stress concentration at the bend. Where the joint is an independent component, the first surface and the second surface may be formed on a first flange and a second flange of the joint, respectively. The first flange and the second flange may be connected by the bend in the composite material. The first flange may be attachable to a component, e.g. by bolting, adhering or otherwise securing the first flange to the component. Similarly, the second flange may also be attachable to a component. Thus, the joint may be used to connect two components together and transfer loads between them. Where the joint is integrated into a structure, the first surface and the second surface may correspond to surfaces of that structure. In this case, the joint serves to connect the first and second surfaces together, and to transfer loads between them.

The composite material may be a fibre-reinforced material having a plurality of fibres embedded in a binding matrix. Any known type of fibre-reinforced material may be used (e.g.

fibreglass, carbon fibre composite) . The fibres may be randomly arranged, flattened into a sheet, unidirectional or woven into a fabric. The fibres may extend along the whole length of the joint, or they may be shorter. In some examples, different lengths of fibre may be used in different parts of the joint. This may be, for example, to reduce stress

concentration at the edges of the buttress support and to increase the fibre density near the bend. The binding matrix serves to hold the fibres together to form a solid piece of material. The binding matrix may include a polymer.

Preferably, the binding matrix includes a thermosetting resin such as epoxy. Other types of thermosetting polymers such as polyester, vinyl ester or nylon or tough thermoplastic polymers such as acrylonitrile butadiene styrene,

polycarbonate, etc. may also be used. The joint may be manufactured using conventional techniques for producing composite materials, e.g. wet lay-up, spray lay-up, pre impregnation moulding.

At the bend in the composite material, the first portion and the second portion of the composite material are spaced apart to form the gap. The first portion of the composite material forms the bend, whilst the second portion of the composite material forms the buttress support. Thus, at the bend in the composite material, the composite material is split into two separate portions. The first portion may include a first subset of the plurality of fibres and the second portion may include a second subset of the plurality of fibres. Away from the bend (e.g. near or in the first and second component mounting regions) , the first and second portions of the composite material may be joined together such that there is no gap between them (e.g. they may form a continuous thickness of composite material) . In other words, the gap may only be present at the bend, and the first and second portions of the composite material may be joined together at edges of the gap. For example, where the joint is an independent component having first and second flanges, the first and second flanges may be formed by fibres from the first subset and the second subset of the plurality of fibres. The gap may be a region in the joint which is free from the composite material (e.g. it does not contain fibres) . In some examples, the gap may be a void (i.e. it does not contain any material) . In other examples, the gap may be partially or entirely filled with a filler or spacer material (e.g. resin, foam or some other suitable material) .

The buttress support may also be referred to herein as a "support structure". The buttress support may correspond to a branch of the composite material (including fibres from the first portion and the binding matrix) that joins the first component mounting region and the second component mounting region inside the bend. Thus, the buttress support may act as a brace that extends across the bend between the first and second surfaces. In this manner, the buttress support may act to increase an effective radius of the joint at the bend. The increased effective radius at the bend may serve to reduce stress concentrations around the bend in the composite material. This may reduce the risk of cracking or delamination of the composite material and spread any damage over a wider area, which may in turn increase the load bearing

capabilities, durability and damage tolerance of the joint. Furthermore, the buttress support may act as a "sacrificial" structure, such that it takes on damage before critical parts of the joint, especially the bend. The buttress support may therefore serve to delay onset of damage to the composite material around the bend. As the buttress support is

integrally formed from the composite material of the joint, the buttress support does not add any additional mass to the joint (e.g. compared to a conventional composite joint where no buttress support is formed) . Thus, the strength of the joint may be increased, without increasing the mass of the joint. Compared to a conventional joint, the amount of material in the joint of the invention may be the same, however fibres may be moved from less critical areas to more critical areas. This may be viewed as an optimisation of the topology of the joint, to reduce stress concentration and have a uniform stress distribution for the materials used.

In some embodiments, the joint may further include a spacer in the gap between the first portion of the composite material and the second portion of the composite material. The spacer may be made of any suitable material. Preferably, the spacer may be made out of a lightweight material, in order to minimise the amount of mass added to the joint. The spacer may have a surface that is in contact with the bend in the composite material (e.g. the shape of the surface may be complementary to the bend) , and another surface that is in contact with the buttress support. The spacer may entirely or partially fill the gap. For example, a cross-section of the spacer may have the same shape and size as the gap.

The spacer may serve to space the first portion of the composite material from the second portion of the composite material during manufacture of the joint, in order to form the gap. The spacer may also serve to strengthen the buttress support, e.g. by providing support between the buttress support and the bend in the composite material. The spacer may be secured in the joint by the binding matrix or with some other adhesives. The spacer may be inserted at the same time of manufacturing and assembling the other parts of the joint. Some adhesives used for securing the spacer in the joint may have properties that act to reinforce bonding surfaces of the spacer and delay debonding between the spacer and the rest of the joint. A surface of the spacer may be treated in order to improve adhesion between the composite material and the spacer. For example, some tough thermoplastic resin may be used to treat the surface of the spacer. Alternatively, a tough nano-fibre nylon mat may be used on the surface of the spacer. In some examples, a first spacer which is used to form the gap during manufacture of the joint may be removed after manufacture of the joint. This may provide space for further parts such as wires to go through. A second spacer (e.g.

having different material properties compared to the first spacer) may then be introduced into the gap in order to reinforce the joint.

In some embodiments, the spacer may include a foam. The spacer may be made entirely or partially of foam. Any known type of solid foam may be used. For example, polyurethane foam may be used. In another example, a metallic foam, e.g. made from aluminium, made be used. Foams are well-known

lightweight engineering materials. A foam is a solid material in which a plurality of cells (or pockets) is formed. The pockets may be interconnected and open to the environment ("open-cell foam") , in which case the cells may be filled with air. Alternatively, the pockets may be isolated from each other and the environment ("closed-cell foam") and filled with a specialised gas, liquid or solid. A foam spacer may be pre formed (e.g. by injection moulding) and used to form the gap during manufacture of the joint, as discussed above.

Alternatively, a foam spacer may be introduced into the gap after the joint has been made, e.g. by injecting the foam into the gap. Using foam in the spacer may present several

advantages. As a result of its cellular structure, a foam may have a very low density. Thus, including foam in the spacer may minimise the amount of mass that is added to the joint by the spacer. Additionally, using a spacer made of foam may facilitate removing the spacer from the gap (e.g. using a solvent or by heating) . Indeed, many foams may be dissolved using a solvent, such as polystyrene foam which may be dissolved using acetone. Similarly, polyurethane foam may also easily be removed (e.g. using a solvent) . This may facilitate removing a spacer which was used to form the gap during manufacture of the joint, and replace it with a different spacer. This may also facilitate replacing a damaged spacer in the joint, without having to replace the entire joint.

In some embodiments, cells in the foam may contain a strengthening material. For example, the strengthening material may be injected into the foam, to fill the cells with the strengthening material. The strengthening material may serve to increase the strength of the foam, e.g. by increasing its rigidity. The strengthening material may also increase the structural integrity of the foam, through increasing their shear and compression properties. This may reduce the risk of tears or cracks in the foam. For example, the strengthening material may be a thermosetting polymer that is injected into the foam, or the foam may be reinforced by some nano-particles to improve the properties of the foam. The thermosetting polymer may be cured at room temperature.

In some embodiments, the spacer may include an auxetic material. The spacer may be entirely or partially made of auxetic material. An auxetic material is a material having a negative Poisson ratio. In other words, when an auxetic material is stretched, it becomes thicker in a direction perpendicular to the applied force. Many different types of auxetic material are known and can be used in the spacer.

Where the spacer includes a foam, the foam may be an auxetic foam (e.g. auxetic polyurethane foam) . By including an auxetic material in the spacer, the spacer may act to resist

deformations in the composite material. For example, if a load is applied to bend the first surface away from the second surface, the spacer may become stretched. This may cause the auxetic material to expand towards the buttress support so that the spacer exerts a pressure on the buttress support.

Such a pressure on the support structure may increase the buttress support's resistance to delamination and intra laminar damage in the joint. In some embodiments, the spacer may include a self- healing material. The spacer may be entirely or partially made of self-healing material, or the self-healing material can be injected after the manufacture or during servicing

(maintenance) of the joint. A self-healing material is a material having the ability to automatically repair damage to itself, without outside intervention. Examples of self-healing materials include self-healing polymers materials and self- healing fibre-reinforced composites. By including a self- healing material in the spacer, damage that may occur in the spacer over time (e.g. cracks) may be automatically repaired. This may increase the durability of the joint.

In some embodiments, the first portion of the composite material may include fibres of a first type and the second portion of the composite material may include fibres of a second type. Thus, different types of fibre may be used to form the bend and the buttress support. For example the properties of the fibres (e.g. thickness, length, strength) in the bend may be different from the properties of the fibres in the buttress support. Where woven sheets of fibre are used, different types of weave may be used in the bend and the buttress support. In other examples, different fibre materials may be used in the bend and the buttress support. For example, the bend may include carbon fibres, whilst the buttress support may include glass or other types of carbon fibres. By using different types of fibres in the buttress support and the bend, it is possible to adjust the stiffness and strength of the composite material in the buttress support relative to the stiffness and strength of the composite material in the bend .

The configuration of the buttress support alters the shape of the cross-sectional area of the T-joint in a manner that increases its second moment of area compared with conventional T-joint shapes. The increased second moment of area provides improved bending properties, i.e. an increase in stiffness, for the joint. These improved properties may be further supplemented by including in the buttress support fibres that have a higher rigidity than fibres in the bend. This may improve the buttress support' s resistance to bending of the joint, as the buttress support may take most of the load. This may reduce stress concentrations around the bend in the composite material.

In some embodiments, the fibres in the first portion of the composite material may be arranged in a plurality of layers forming a first layered structure. The layers in the first layered structure may be stacked on top of one another and embedded in the binding matrix to form the composite material. For example, the fibres in the first portion may be woven into sheets (fabric) which are layered on top of each other. In another example, the fibres in the first portion may be arranged in unidirectional sheets that are layered on top of each other. The use of layers of fibre (e.g. woven sheets) may increase the strength of the composite material. This may also facilitate manufacture of the joint, as manufacturing techniques such as wet lay-up or pre-impregnation moulding techniques may be used.

In some embodiments, the first layered structure may include a first layer and a second layer, the first layer being shorter than the second layer, and the first layer being disposed at the bend in the composite material. The second layer may extend along the whole joint, i.e. from an edge of the first surface to an edge of the second surface along the bend. Alternatively, the second layer may extend along a portion of the joint. The first layer is shorter than the second layer, meaning that it extends along a shorter portion of the joint than the second layer. The first layer is at the bend, meaning that it forms part of the bend in the composite material. In some cases, multiple shorter layers may be provided at the bend. By providing a shorter layer at the bend, it is possible to increase the number of layers at the bend without increasing the thickness of the composite material away from the bend (e.g. at the first and second surfaces) . Thus, the strength of the composite material may be strengthened at the bend without adding mass to the joint away from the bend, in order to minimise the mass of the joint.

In some embodiments, the first layer may be on a side of the first layered structure that is closer to the gap than the second layer. Thus, the first layer may be on a side of the layered structure that is towards an inside of the bend, and the second layer may be on a side of the layered structure that is towards an outside of the bend. Placing the longer layer towards the outside of the bend may enable a smoother outer surface of the joint to be obtained. In some examples, the layers in the first layered structure may get

progressively shorter towards the inside of the bend. Such a progressive shortening of the layers towards the inside of the bend may serve to increase the amount of material present in the bend, whilst minimising the amount of material added away from the bend. This also may reduce the stress concentration in the edges of the buttress support and increase the fibre density near the bend. Indeed, the radius of curvature of the layers around the bend may be shorter towards the inside of the bend compared to the outside of the bend, such that shorter lengths of material are necessary to reinforce the bend closer to the inside of the bend.

In some embodiments, the fibres in the second portion of the composite material may be arranged in a plurality of layers forming a second layered structure. The second layer structure may be similar to the first layered structure discussed above, e.g. it may include sheets of fibres layered on top of each other.

In some embodiments, the second layered structure may include a third layer and a fourth layer, the third layer being shorter than the fourth layer, and the third layer being disposed in the buttress support. The fourth layer may extend the whole or a portion of the length from an edge of the first surface to an edge of the second surface along the buttress support. The third layer is shorter than the fourth layer, meaning that it extends along a shorter portion of the joint than the fourth layer. The third layer is in the buttress support, meaning that it forms part of the composite material in the buttress support. In some cases, multiple shorter layers may be provided in the buttress support. By providing a shorter layer in the buttress support, it is possible to increase the number of layers in the buttress support without increasing the thickness of the composite material away from the bend (e.g. at the first and second surfaces) . Thus, the strength of the composite material may be strengthened in the buttress support without adding mass to the joint away from the bend, in order to minimise the mass of the joint.

In some embodiments, the third layer may be on a side of the second layered structure that is closer to the gap than the fourth layer. Thus, the third layer may be nearer to an inner surface of the buttress support that faces towards the bend, and the fourth layer may be nearer to an outer surface of the buttress support that faces away from the bend. As the inner surface of the buttress support is closer to the bend than the outer surface (due to the thickness of the buttress support) , a length of the inner surface may be shorter than a length of the outer surface. Thus, using a shorter length of material near the inner surface of the buttress support, may reduce the amount of material away from the buttress support. Similarly to the first layered structure, the layers in the second layered structure may become progressively shorter towards the inner surface of the buttress support.

In some embodiments, the joint may further include a reinforcing element to strengthen a connection between the first portion of the composite material and the second portion of the composite material, the reinforcing element being at an edge of the gap. Herein, the reinforcing element may also be referred to as a "reinforcing structure". The edge of the gap may represent a region where the first and second portions of the composite material split apart to form the gap. In other words, on a first side of the edge, the first and second portions of the composite material may be attached together (e.g. they form a continuous thickness of composite material), whilst on a second side of the edge the first and second portions of the composite material may be separated to form the gap. The reinforcing element may be disposed near the gap, on the first side, the second, or both sides of the gap. By reinforcing the connection between the first and second portions of the composite material at the edge of the gap, separation between the first and second portions of the composite material away from the gap may be inhibited. This may serve to avoid cracks or delamination from occurring at the edge of the gap and propagating to other parts of the joint .

In some embodiments, the reinforcing element may include a strip of material disposed at the edge of the gap, the strip of material being embedded in the binding matrix. The strip of material may extend along a width of the gap. In some

examples, the strip of material may be disposed on the second side of the edge of the gap, such that it is disposed in a space between the first and the second portions of the composite material. The strip of material may be a fibre material (e.g. carbon fibre) . The strip of material may serve to avoid the presence of voids in the binding material at the edge of the gap. The strip of material may also serve to avoid a build-up of binding matrix at the edge of the gap which is not reinforced by any fibre material .

In some embodiments, the reinforcing element may include a resin having a greater toughness than the binding matrix.

The resin used in the reinforcing element may be different to a resin used as the binding matrix. For example, the resin may have a greater stiffness than the binding matrix. Using a stronger resin at the edge of the gap may prevent cracks or delamination from occurring at the edge of the gap.

In some embodiments, the reinforcing element may include a pin passing through a thickness of the first portion of the composite material and through a thickness of the second portion of the composite material. The pin may be made of any suitable material, e.g. metal, fibre or composite material. As the pin passes through a thickness of the first and second portions of the composite material, the pin may strengthen the connection between the first and second portions at the edge of the gap. In some examples, multiple pins may be used. Where the first and second portions of the composite material include layered structures, the pin may be oriented

perpendicularly to the layers. As the pin is connected to the fibres via the binding matrix, the pin may prevent

delamination of the composite material at the edge of the gap. This approach to strengthening the connection between the first and second portions of the composite material is similar to the known z-pinning method discussed above. The pin may be inserted into the joint using known techniques for z-pinning.

In some embodiments, the reinforcing element may include a first engagement feature formed in the first portion of composite material, and a second engagement feature formed in the second portion of composite material, the first engagement feature being engaged with the second engagement feature. The first and second engagement features may serve to mechanically interlock the first and second portions of composite material at the edges of the gap. This may increase the bonding strength between the first and second portions of composite material near the edges of the gap. For example, the first and second engagement features may have complementary shapes, so that they can mechanically interlock. Where the first and second portions include layered structures of fibre material, the first and second engagement features may be formed in layers near the interface between the first and second portions. For example, an undulation in a layer of the first portion may be engaged with an undulation in a layer of the second portion.

In some embodiments, the plurality fibres may include carbon fibres. Carbon fibres are known to be lightweight and extremely strong. Thus, making the joint of carbon fibre reinforced composite material may provide a lightweight joint capable of withstanding large loads.

In some embodiments, the first component mounting region may define a first surface, the second component mounting region may define a second surface, and the first surface and the second surface may be substantially perpendicular to one another .

In some embodiments, the joint may be a T-joint. A T- joint is a type of joint which is typically used to connect two surfaces that are perpendicular to one another. A T-joint may have a base plate and a stem. In this embodiment, the stem may correspond to the first surface of the joint, and the base plate may correspond to the second surface of the joint. In some embodiments, a T-joint may be made by attaching two separate joints of the invention together. For example where the joints are manufactured with an "L" shape, two L-shaped joints may be attached back-to-back to form a T-joint. The two L-shaped joints may be bonded together by any suitable means, e.g. using a resin. Many other different types and shape of joint may also be made according to the invention. For example, the joint may be C-shaped

According to a second aspect of the invention, there is provided a method of making a joint, the method including: depositing a first portion of a composite material (e.g. a fibre-reinforced material) in a mould, wherein the mould has a first surface for forming a first component mounting region of the joint, a second surface for forming a second component mounting region of the joint, and a third surface for forming a bend in the joint between the first component mounting region and the second component mounting region; placing a spacer on the first portion of the composite material adjacent to the third surface; and depositing a second portion of the composite material over the first portion of the composite material and spacer to form a buttress support spanning between the first component mounting region and the second component mounting region, wherein the spacer forms a gap between the first portion of composite material and the second portion of composite material at the bend in the joint.

Features of the first aspect of the invention discussed above may be shared with the second aspect of the invention, although these are not repeated. The method of the second aspect of the invention may be used to manufacture a joint according to the first aspect of the invention.

The method of this aspect may be used with conventional manufacturing techniques for composite materials, e.g. wet lay-up, spray lay-up, pre-impregnation moulding. For example, where a wet lay-up technique is used, the steps of depositing first and second portions of the composite material may include depositing layers of fibres in the mould and

impregnating the layers with the binding matrix. The mould may be a male mould or a female mould. The method may also include further standard processing steps that are used for making composite materials. For example, after depositing the composite material, the composite material may be sealed in a bag which is put under vacuum to consolidate the composite material. The composite material may also be placed in an autoclave to cure the binding matrix at elevated temperatures and pressures.

The method may include steps for producing any of the features of the joint discussed in relation to the first aspect of the invention. For example, where the spacer is to be removed or replaced after manufacture of the joint, the method may include the step of removing the spacer after the binding matrix has been cured. Where the joint includes a reinforcing element at the edge of the gap, the method may include steps for forming the reinforcing element. For example, where the reinforcing element includes a strip of material at the edge of the gap, the method may include a step of depositing the strip of material at an edge of the spacer, before depositing the second portion of the composite material. Where the reinforcing element includes a resin having a greater stiffness than the binding matrix, the method may include the step of depositing the resin at an edge of the spacer, before depositing the second portion of the composite material .

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention are discussed below with reference to the accompanying drawings, in which:

Figs. 1A-1E a schematic cross-sectional side views of known T-joints;

Fig. 2 is a schematic cross-sectional side view of a joint that is an embodiment of the invention;

Fig. 3 is a schematic cross-sectional side view of a joint that is another embodiment of the invention;

Fig. 4 is a schematic cross-sectional side view of a joint that is another embodiment of the invention;

Fig. 5 is a schematic cross-sectional side view of a joint that is another embodiment of the invention;

Fig. 6 is a schematic cross-sectional side view of a joint that is another embodiment of the invention;

Fig. 7 is a schematic cross-sectional side view of a T- joint that is another embodiment of the invention; and

Figs. 8A and 8B are schematic diagrams illustrating different stages in a manufacturing process of a joint according to an embodiment of the invention;

Fig. 9 shows load displacement plots measured for a conventional T-joint and a T-joint according to an embodiment of the invention.

For completeness, it is noted that none of the figures are drawn to scale.

DETAILED DESCRIPTION; FURTHER OPTIONS AND PREFERENCES

Fig. 2 shows a schematic cross-sectional side view of a joint 200 that is an embodiment of the invention. The joint 200 is made of a composite material having a plurality of fibres embedded in a binding matrix. For example, the fibres may be carbon fibre, such as TC35 carbon fibre. The binding matrix may be an epoxy such as Sicomin SR5550 epoxy resin. In the example shown, the fibres are disposed in sheets 202 which are layered on top of each other to form a layered structure. For example, the sheets 202 may be made of woven or

unidirectional fibres. The joint is formed as a unitary piece of composite material.

The joint has a first flange 204 and a second flange 206 which are joined by a bend 208 in the composite material. The first flange 204 and the second flange 206 include a first surface 210 and a second surface 212, respectively. The first and second surfaces 210 and 212 may each be attached to a separate component, in order to join the components together and transfer loads between the components. For example, the components may be attached to the first and second flanges 204, 206 by means of an adhesive, bolts, or some other suitable fastening mechanism. In the embodiment shown, the joint 200 is L-shaped, with the first and second surfaces 210, 212 being perpendicular to each other. However, in other embodiments, the surfaces may be at a different angle (e.g. more or less than 90°) .

A first portion 214 of the composite material,

corresponding to a first set of the layers of fibre material, forms the bend 208 of the joint 200. In the embodiment shown, the first portion 214 includes five layers of fibre material (e.g. five sheets of the fibre material), however in other examples more or fewer layers may be included in the first portion 214. A second portion 216 of the composite material is spaced apart from the first portion 214 in the vicinity of the bend 208, in order to form a gap 218 between the first portion 214 and the second portion 216. At the first and second flanges 204, 206, the first and second portions 214, 216 are joined together to form a continuous thickness of the

composite material. In the embodiment shown, the second portion includes four layers of fibre material, however in other examples more or fewer layers may be included. Where the second portion 216 is spaced apart from the first portion 214, it forms a support structure (or "buttress support") 220 that extends across the bend 208 and joins the first flange 204 to the second flange 206. The support structure 220 may therefore act as a brace between the first flange 204 and the second flange 206 across the bend 208, in order to strengthen the joint 200 around the bend 208. In the embodiment shown, a spacer 222 is provided in the gap 218 between the first portion 214 and the second portion 216. The spacer 222 may be made out of foam, in order to minimise the weight of the joint. In some examples, a

toughening material may be injected into the foam so that the spacer 222 provides additional support to the support

structure 220 across the bend 208. In some examples, the spacer 222 may be made out of an auxetic material, in order to counteract deformations of the joint 200 when loads are applied to the joint.

In the embodiment shown, each of the sheets 202 of fibre in the composite material extend the entire length of the joint 200, i.e. they extend from an edge 224 of the first flange 204 to an edge 226 of the second flange 206. Using sheets that extend along the whole length of the joint may increase the strength of the joint. However, in other

examples, it is possible to use sheets of different lengths. For example, a layer in the composite material may be formed using multiple sheets rather than a single sheet.

Fig. 3 shows a schematic cross-sectional view of a joint 300 that is another embodiment of the invention. The same reference numerals are used to indicate features already discussed above in relation to Fig. 2. The joint 300 is similar to joint 200, except that some of the layers in the first portion 214 of the composite material are shorter than the rest. In particular, a first sheet 302 and a second sheet 304 in the first portion 214, which are disposed in the bend 208, are shorter than the other sheets in the first portion 214. The first and second sheets 302, 304 do not extend all the way into the first and second flanges 204, 206. Thus, the first and second sheets 302, 304 do not contribute to the thickness of the composite material near the edges of the flanges 204, 206. As a result, first and second sheets 302,

304 increase the thickness of the composite material at the bend 208, without increasing the thickness of the composite material near the edges of the flanges 204, 206. Indeed, as can be seen in Fig. 3, there are seven layers in the first portion 214 of the composite material at the bend 208, whilst there are only five layers in the first portion at the edges of the flanges. This enables the bend 208 to be strengthened (by adding more layers around the bend 208), without

increasing the mass or thickness of the flanges 204, 206.

The shorter sheets 302, 304 are on a side of the first portion 214 that is closer to the gap 218, i.e. they are located towards the inside of the bend 208. This may ensure that an outer surface of the joint 300 is smooth, as it is formed by a single sheet that extends along the whole length of the joint 300. The first sheet 302 is longer than the second sheet 304, and the second sheet 304 is closer to the inside of the bend than the first sheet 304. Thus, the sheets in the first portion 214 are successively shortened towards the inside of the bend 208. Successively shortening the sheets towards the inside of the bend may serve to strengthen the bend whilst minimising the mass of the joint. In other examples, there may be more than two shortened sheets in the first portion 214, and these may be successively shortened towards the inside of the bend. For example, the sheets may be shortened at a rate of 20% towards the inside of the bend, e.g. a first shortened layer may be 80% of the maximum length, a second shortened layer may be 60% of the maximum length, and a third shortened layer may be 40% of the maximum length.

Alternatively, each shortened layer may have a length that is 80% of the immediately preceding layer.

In other embodiments (not shown) sheets in the second portion 216 of the composite material disposed in the support structure 220 may be shorter than the other sheets in the second portion 216. This may strengthen the support structure 220 (by adding additional layers to the support structure) , without adding mass to the flanges 204, 206. Similar to the discussion above, the sheets in the support structure 220 may be sequentially shortened towards the inside of the support structure 220, i.e. layers that are closer to the gap 218 may be shorter than those further away. This may further reduce the mass of the joint.

Fig. 4 shows a schematic cross-sectional view of a joint 400 that is another embodiment of the invention. The same reference numerals are used to indicate features already discussed above in relation to Fig. 2. The joint 400 is similar to joint 200, except that it includes reinforcing structures near edges of the gap 218 between the first and second portions 214, 216 of the composite material. The joint 400 includes a first strip of material 402 located near a first edge 404 of the gap 218, between the first and second portions 214, 216 of the composite material. A second strip of material 406 is located near a second edge 408 of the gap, between the first and second portions 214, 216 of the

composite material. The first and second strips of material 402, 406 extend in a width direction of the joint 400, i.e. in a direction perpendicular to the cross-section shown in Fig.

4. The first and second strips of material 402, 406 are embedded in the binding matrix, and so form an integral part of the joint 400. The first and second strips of material 402, 406 may be made of a fibre material (e.g. carbon fibre, such as TC35 carbon fibre) . For example, they may be strips of woven fibre. The purpose of the first and second strips of material 402, 406 is to avoid the presence of voids (e.g. in the binding matrix) that may occur near the edges of the gap 218. The strips of material 402, 406 may also serve to avoid the presence of resin rich areas (e.g. areas where the resin- to-fibre ratio is higher) and to improve the bonding

properties at the edges of the gap 218. This may serve to reinforce the connection between the first and second portions 214, 216 of the composite material at the edges of the gap 218, and avoid delamination of the composite material in those regions .

Fig. 5 shows a schematic cross-sectional view of a joint 500 that is another embodiment of the invention. The same reference numerals are used to indicate features already discussed above in relation to the previous figures. The joint 500 is similar to joint 400, except that it includes a different type of reinforcing structure near the edges 404,

408 of the gap 218 between the first and second portions 214, 216 of the composite material. The joint 500 includes a first set of pins 502 near the edge 404 of the gap 218. The pins 502 pass through a thickness of the first and second portions 214, 216 of the composite material. A second set of pins 504 is provided near the edge 408 of the gap 218. The pins 504 also pass through a thickness of the first and second portions 214, 216 of the composite material. In the example shown, the pins 502, 504 are oriented perpendicularly to the layers in the first portion 214 of the composite material. However, in other examples, they may be oriented at different angles. The pins 502, 504 may be made of any suitable material, e.g. metal or fibrous material. The pins 502, 504 may behave similarly to the pins used in z-pinning, and act to secure the layers in the composite material together. This may reduce the risk of delamination at the edges of the gap 218.

Fig. 6 shows a schematic cross-sectional view of a joint 600 that is another embodiment of the invention. The same reference numerals are used to indicate features already discussed above in relation to the previous figures. The joint 600 is similar to joint 400, except that it includes a different type of reinforcing structure near the edges 404,

408 of the gap 218 between the first and second portions 214, 216 of the composite material. Undulations 602 are formed in layers of the first portion 214 near the interface between the first portion 214 and the second portion 216 of the composite material. Additionally, undulations 604 are formed in layers of the second portion 216 near the interface between the first portion 214 and the second portion 216 of the composite material. Undulations 602 and 604 are located near the edges 404, 408 of the gap 218. Undulations 602 and 604 have

complementary shapes, and are engaged to form a mechanical interlocking between the layers of the composite material near the interface between the first and second portions 214, 216. The undulations 602 and 604 therefore act as engagement features between the first and second portions 214, 216, and may serve to improve the bonding strength between the first and second portions 214, 216 at the edges 404, 408 of the gap 218.

In other embodiments (not shown) , the joint may be reinforced near the edges of the gap by using a resin at the edges of the gap which is stronger than the rest of the binding matrix. This may reduce the risk of delamination at the edges of the gap. Some embodiments of the invention may combine two or more of the approaches discussed above (e.g. strips of material, pins, stronger resin) for reinforcing the joint near the edges of the gap.

Fig. 7 shows a schematic cross-sectional view of a T- joint 700 that is another embodiment of the invention. The T- joint 700 is formed of two identical L-shaped joints 702, 704 which are attached to each other back-to-back. For example, such a T-joint may be formed using two of the joints shown in Figs. 2-5, where the first flange of the first joint is attached to or integrally formed with the first flange of the second joint. Each of the L-shaped joints 702, 704 may be manufactured (e.g. moulded) separately and then assembled together. The two joints 702, 704 may be attached to each other using a resin. The resin may be the same resin which is used for the binding matrix in each of the joints.

Alternatively, a different type of resin may be used. In some examples, 24-hour cure Araldite resin may be used. However, in other examples, the T-joint 700 may be manufactured as a single piece, e.g. using a single moulding process.

As can be seen in Fig. 7, the T-joint 700 includes a base 706 which is formed by flanges of the L-shaped joints 702,

704. The T-joint 700 also includes a stem 708 which is formed by flanges of the L-shaped joints 702, 704, and which is connected to the based 706 via bends in the L-shaped joints 702, 704. The first L-shaped joint 702 includes a first portion 710 of composite material which forms a bend 711 in the joint 702, and a second portion 712 of composite material which forms a support structure 714 in the joint 702.

Similarly, the second L-shaped joint 704 includes a first portion 716 of composite material which forms a bend 717 in the joint 704, and a second portion 718 of composite material which forms a support structure 720 in the joint 704. The support structures 714, 720 are disposed on either side of the stem 708 of the T-joint 700, and act as braces between the stem 708 and the base 706 of the T-joint 700. Thus, the connection between the base 706 and the stem 708 of the T- joint 700 may be strengthened by the support structures 714, 720, in a similar manner as discussed for the joints shown in Figs. 2-5.

The sheets of fibre material in the first portions 710 of the composite material forming L-shaped joint 702 are

sequentially shortened towards the inside of bend 711. In particular, the three sheets of the first portion 710 that are closest to the inside of the bend 711 are shorter than the other sheets in the first portion 710. The other sheets in the first portion 710 extend the full length of the joint 702.

Each of the three innermost sheets is shorter by approximately 20% compared to the sheet below it. Similarly, the three sheets in the first portion of the L-shaped joint 704 are sequentially shortened towards the inside of the bend 717. As discussed above in relation to Fig. 3, shortening the sheets towards the inside of the bend enables the bend to be

strengthened, whilst minimising the mass of the joint. Of course, any of the reinforcing structures discussed above in relation to Figs. 4 and 5 may be incorporated into T-joint 700. Also the fibres in the base 706 can be embedded directly into a surface to which the joint is to be attached, similar to the "bio-inspired" design in Fig. l.E.

Figs. 8A and 8B illustrate different stages in a

manufacturing process making a joint according to an

embodiment of the invention. In Figs. 8A and 8B, an L-shaped female mould 800 is used to produce an L-shaped joint similar to those shown in Figs 2-5. The mould 800 includes a first surface 802 for forming a first surface of the joint, a second surface 804 for forming a second surface of the joint, and a third surface 806 for forming a bend in the joint connecting the first and second surfaces. In the example shown, the third surface 806 is rounded, to form a joint having a rounded bend. However, where a sharp bend in the joint is desired, the third surface 806 may have a sharp corner instead.

In the stage shown in Fig. 8A, a first portion 808 of composite material has been deposited in the mould to form the bend of the joint. In the example shown, the first portion 808 includes multiple sheets (e.g. woven sheets) of fibre material which are layered on top of each other. The sheets of fibre material may deposited in the mould and impregnated with a resin using a conventional technique (e.g. wet lay-up) .

However, in other examples, the fibres need not be arranged in sheets (e.g. where the composite material is deposited using a wet spray-up technique) . After the first portion 808 of the composite material has been deposited in the mould 800, a spacer 810 is placed on the first portion 708 of the composite material. The spacer 810 is positioned such that it is located in the bend in the composite material, i.e. adjacent to the third surface 806 of the mould 800. The spacer 810 may be made of any suitable material, e.g. foam. The spacer may be pre formed, e.g. it may be manufactured before the moulding of the composite material.

In the stage shown in Fig. 8B, a second portion 812 of composite material is deposited on top of the first portion 808 of the composite material. The second portion 812 of composite material may be deposited using the same technique as for the first portion 808. However, in some examples, a different technique may be used. The spacer 810 acts to space the second portion 812 away from the first portion 808 in the vicinity of the third surface 806. In other words, the spacer 810 creates a gap between the first portion 808 and the second portion 812 in the vicinity of the bend in the first portion 808 of composite material. Where the spacer 810 does not space the first and second portions 808, 812 apart, the first and second portions 808, 812 are in direct contact and so form a continuous thickness of composite material. As discussed above in relation to Fig. 2, creating a gap between the first portion 808 and the second portion 812 in the vicinity of the bend creates a support structure 812 that acts as a brace across the bend.

The inventors carried out measurements using real samples, and have found that joints made according to the invention may be stronger and have an improved durability compared to conventional joints with the same mass.

Conventional T-joints (having a structure similar to that shown in Fig. 1A) and T-joints according to the embodiment of Fig. 6 were produced from a composite material having TC35 carbon fibre sheets embedded in a binding matrix of Sicomin SR5550 epoxy resin. Static loading tests were carried out by holding the base of the T-joint fixed and applying a load to the stem. Fig. 9 illustrates the load-displacement results for the conventional and the new design. As can be seen in Fig. 9, there is a significant increase in the load level that can be tolerated by the new joint. The area under the load- displacement curve corresponds to mechanical energy absorbed by the joint. Thus, it can be seen that the joint of the embodiment is capable of absorbing a much larger amount of energy compared to the conventional joint.

Tables 1A and IB below summarise some of the important features of the results of the static loading tests on the conventional T-joint and the T-joint of the embodiment, respectively. The "Damage Load" in Tables 1A and IB refers to the applied load where damage (e.g. delamination) started to appear in the joints. This can be seen by an abrupt drop in a plot of applied load against displacement. As can be seen from Tables 1A and IB, and Figure 9 the Initial Damage Load and the maximum load for the T-joints of the embodiment is higher than for the conventional T-joints. This indicates that the T- joints of the embodiment may be able to withstand greater loads before experiencing damage. The T-joints of the

embodiment also experience slightly smaller displacement at the damage initiation and maximum displacement compared to the conventional T-joints.

Table 1A: Static load test for conventional T-joints

Table IB: Static load test for T-joints of the embodiment

Fatigue tests were also carried out on the T-joints, the results of which are shown in Tables 2A and 2B below. In the tests, a time-varying load was applied to the T-joints. The time-varying load had a sinusoidal form with a frequency of 3 Hz . The minimum and maximum loads of the sinusoidal waveform are shown in Tables 2A and 2B. As before, the load was applied to the stem, holding the base fixed.

Table 2A: Fatigue test for conventional T-joints

Table 2B: Fatigue test for T-joints of the embodiment

The Cycle Number refers to the number of loading cycles (i.e. number of periods of the time-varying load) applied to the T-joint before the joint broke. Where there is a "+" after the Cycle Number, the test was stopped before the sample was broken. As can be seen from Tables 2A and 2B, the T-joints of the embodiment were able to withstand a significantly larger number of loading cycles compared to conventional T-joints under equivalent conditions. In particular, for a time-varying load between 0.2-1.5 kN, the T-joint of the embodiment withstood more than 190, 000 cycles, whereas the conventional T-joints broke after around 10,000 cycles. This indicates an improved durability and resilience of the T-joints of the embodiment compared to the conventional T-joints.