A method for joining a first composite structure to at least a second structure and a mechanical and/or electrical joint Technical field

The present invention relates to a method for joining a first composite structure to at least one second structure. The present invention also relates to a mechanical and/or electrical joint and to a fastening element used in said mechanical and/or electrical joint.

Background of the Invention

Today, research and development efforts in the aircraft industry are focusing on design and manufacturing technologies for producing more environmentally friendly aircraft. One solution is to save weight of the structural parts of the aircraft whereby the fuel consumption can be reduced. There are also other industrial fields such as wind-, land- and sea based turbines etc., where weight and strength of the structure used is essential.

A composite material is a light strong material which is used more and more when building structures in aeroplanes or other objects where a low weight and high strength is essential.

A structural part, such as an airplane wing, or a wind mill blade or any other structure, may be made from a composite structure. A composite structure may be made from multiple sheets of pre-impregnated fibers (prepreg) joined together. The sheets of pre-impregnated fibers are usually 0,1-0,5 mm thick and the number of sheets of pre-impregnated fibers used to form the composite structure varies depending on what structural part the composite structure will form. One example of sheets of pre-impregnated fibers is carbon fiber reinforced plastic (CFRP). However, other fibers such as aramid, aluminium oxide, ceramic, quartz, silicon carbide or glass fibers, as well as carbon fibers etc may be used to form a composite structure.

One alternative way of forming a composite structure is resin transfer molding where fibers are placed in a mold and resin is added to the mold. The resin is cured and a composite structure with fibers evenly distributed through the resin is formed.

By adding a thermoplastic component to the composite material used to build a structure, the fracture toughness of inherently brittle CFRP materials based on thermoset resin can be improved. The thermoplastic material can either be placed in between the sheets of pre-impregnated fibers layers as mentioned above, or be distributed uniformly in the composite matrix phase.

In order to build structural parts with various shapes and dimensions, the composite structure is formed into a desired shape. The composite structure may be joined to another composite structure with a mechanical joint. Alternatively, a composite structure can be joined to another structure, comprising metal, metal alloy, metal matrix composite or a metal bonded structure, or a ceramic, for forming structural parts. The composite structure may alternatively be joined to a fitting for load introduction or other purposes. A mechanical joint can comprise at least two structures joined to each other. Holes are arranged in the structures and fastening elements are placed in the holes of the structures, holding the structures together. The bolt bearing strength of a composite structure is relatively low which means that the composite structure needs to be sufficiently thick around the holes of the mechanical joints in order to fulfill the required bolt bearing strength. This causes the structural part, made of a composite structure and at least one more structure, which was meant to be a light construction to weigh more, and also the cost for material for the structural part and for the manufacturing of the structural part increases.

When arranging the holes in the composite structure, especially when mechanically arranging the holes, for example when drilling the holes, cracks or damages created by chips from another structure are often created in the composite structure around the hole surfaces. The damages will weaken the composite structure at the mechanical joint and could result in a low endurance of the mechanical joint. The cracks, the damages created by the chips in the hole surface and also possibly remaining chips in the holes may cause the load on each separate fastening element in the mechanical joint to vary. Also, the load on each separate fastening element, transverse the length of the fastening element, may be distributed unevenly due to the cracks and chips in the hole surfaces. Contrary to other materials, such as metal, for example, when a composite structure is to be joined to another material, the dimensions of the hole of the composite structure needs to be larger (clearance fit or close fit) than the dimensions of the fastening element due to the nature of the composite structure. The composite structure is typically relatively brittle, and hence, pressing a fastening element with a larger dimension than the dimensions of a hole into a brittle composite structure will damage the hole surface and also the composite structure surrounding the hole. The smaller dimension of the fastening element in relation to the holes may cause movements of the composite structure in relation to the other structures in a mechanical joint. The composite structure may also move in relation to the fastening elements. The movements of the composite structure may cause wearing on the fastening elements and on the joined structures and lower the expected mechanical joint strength.

The smaller dimension of the fastening element in relation to the dimension of the hole in the composite structure also creates a smaller contact area between the fastening element and the hole surface where the load to be transferred between the fastening element and the hole occurs. This reduces the load transfer capability between fastening element and the composite structure which creates an uneven load distribution in the mechanical joint. Further, a tilting of the fastening element might occur due to the different dimensions. This tilting may also contribute to an uneven load distribution in the mechanical joint. Another contribution to uneven load transfer in a mechanical joint with clearance fit of close fit between fasteners and structure is that all fastening elements will not be loaded simultaneously. In order to prevent galvanic corrosion in mechanical joints and in order to provide sealed, leak-tight mechanical joints in for example an aircraft wing tank or similar structures made of a composite structure joined to another structure, a sealant material is applied in the space between the hole surface in the composite structure and the fastening element. The sealant serves to fill the space between the hole surface of the structure and the fastening element in order to make the mechanical joint of the composite structure and another structure leak tight, prevent galvanic corrosion, increase the bolt bearing strength and to increase the endurance of the joint. However, the commonly used polysulfide based sealants are softer than the composite structure and the fastening element and, hence, the sealant in the mechanical joint may be deformed or worn whereby the composite structure will be able to move in relation to the other structure and to the fastening element, liquid might leak through the mechanical joint and galvanic corrosion may occur. Another problem that can occur with polysulfide based sealants is that due to the high viscosity of the polysulfide based sealants, the sealant may not fill out all the cracks and chips in the hole surface of the composite structure and hence, moisture may migrate into the structure and liquid or gas could leak through the mechanical joint even when the sealant has not been damaged by wearing.

The fibers in a composite structure may be electrically conductive depending of the material of the fibers in the structure, whereas the thermoplastics and resin are not electrically conductive. If a composite material comprising layers of electrically conductive fibers and layers of thermoplastics is hit by lightning, the induced current may flow through the layers of fibers to the edges of the composite structure, such as the hole surface in a mechanical joint of the structure. Sparks or glitching can then be created, either between the layers of fibers in the composite structure or between the layer of fiber and the fastening element. A spark can cause catastrophic failure since, for example in airplanes, there are fuel tanks with fuel vapor that could ignite by such a spark. In order to prevent the occurrence of sparks, it is desirable to have sufficient electrical conductivity between each layer of fibers in the composite structure, and also between the fastening element and the composite structure in a mechanical joint of a composite structure and another structure. Objective of the Invention

It is an objective of the present invention to provide a mechanical and/or electrical joint of at least two structures, wherein at least one of the structures is a composite structure. It is also an objective of the invention to provide a method for joining a composite structure to at least one other structure where at least some of the above mentioned problems are solved. It is also an objective of the present invention to provide fastening elements which can be used in said mechanical joints. Summary of the Invention

The objective of the invention has been achieved by a method for joining a first composite structure to at least a second structure comprising the steps of:

- providing a hole in the first composite structure

- providing a hole in the at least one second structure - arranging a resin comprising fiber-like electrically conducting and

mechanically reinforcing nano elements in the hole of said first and/or said second structure and/or on a fastening element,

- arranging the fastening element in said holes of said first and second

structure curing of the resin.

The joint comprises at least two structures which are to be joined to each other. At least one hole is arranged in the structures and at least one fastening element is placed in the at least one hole of the structures, holding the structures together. In one example, the holes in the first and at least second structures go through the structures. In an alternative example the hole in at least one of the first structure and second structure(s) is not a go through hole. The fastening element can extend through the structures. Alternatively, the fastening element at least at one of its ends extends into but not through the corresponding structure.

The joint hereby also comprises at least two structures and at least one fastening element, where the structures are joined to each other by for example another mechanical joint. In this embodiment, the at least one fastening element is provided in a hole in the structures, but the fastening element may not be necessary for holding the structures together. The fastening element and the matrix material comprising fiber-like electrically conducting and mechanically reinforcing nano elements could also have other purposes, such as conducting electricity from an external electricity source to the at least one structure. This embodiment is hereby included in the definition of the word joint. Thus, the joint may function as a mechanical and/or electrical joint.

Throughout this description it is understood that a structure joined together by cocuring composite material or by secondary bonding of composite parts is defined as a joint if the fastening elements are part of the critical load path. The composite structure is for example a carbon-fiber-reinforced polymer or carbon-fiber-reinforced plastic (CFRP or CRP). The material is an extremely strong and light fiber-reinforced polymer which contains carbon fibers. See for example HYPERLINK "http://en.wikipedia.org/wiki/Fiber-reinforced_polymer" \o "Fiber- reinforced polymer". The polymer is most often epoxy, but other polymers, such as polyester, vinyl ester, polyimide, bismaleimide or nylon, are sometimes used. The composite may contain other fibers, such as aramid, aluminium oxide, ceramic, quartz, silicon carbide or glass fibers, as well as carbon. The composite structure may be constructed from sheets of pre-impregnated fibers or from fibers, which are not pre-impregnated, using resin transfer molding (RTM) or other infusing methods. The composite may according to another example have fibers distributed in a plastic.

The resin comprising fiber-like nano elements comprises a resin and fiber-like electrically conducting and mechanically reinforcing nano elements. The resin may comprise a base resin such as epoxy, cyanatester, vinyl ester, polysulfide or plastic. Additionally, the resin may comprise a curing agent. The curing agent can be chosen from conventional curing agents such as cold curing materials or curing materials that need to be heated in order to cure. The heating can be through conventional methods such as oven, autoclave, IR or similar methods, as well as from an electrical field that heats the embedded nano-sized elements in the resin. There are other examples of curing agents which require other parameters, such as UV light, a certain time period etc, to be fulfilled in order to cure the resin. All curing methods are hereby incorporated in the invention.

The resin may additionally comprise a catalyst and possibly other components such as an agent which causes the resin to expand when the material is cured.

The cured resin with fiber-like electrically conducting and mechanically reinforcing nano elements forms a matrix material comprising fiber-like elements, i.e. a reinforced plastic. Throughout this description, it is understood that where applicable, the terms "matrix material comprising fiber-like electrically conducting and mechanically reinforcing nano elements" and "resin comprising electrically conducting and mechanically reinforcing nano fiber-like elements" can be exchangeable. The different components of the resin comprising fiber-like nano elements could be provided as a mixture where all the components have been mixed together before using the resin comprising fiber-like nano elements, or alternatively, the components can be added at different points of time via different components The curing agent could for example be added to the resin via the fastening element. The fastening element could be treated with curing agent prior to inserting the fastening element in the hole. One alternative is that the supplier treats the fastening element with curing agent. Another alternative is that the curing agent could be added to the resin from the hole surface. By adding the curing agent to the resin via the fastening element or via the hole surface, the amount of curing agent added could be controlled in a more accurate way. Other components of the resin comprising fiber-like nano elements could be added to the resin as described above. In order to create a robust and strong mechanical joint, as much as possible of the space between the fastening element and the hole surface should be filled with the matrix material comprising fiber-like nano elements. In order to fill the space between the fastening element and the hole surface with the resin comprising fiber-like nano elements to a high degree, the viscosity of the resin is controlled. The viscosity can, in addition to the generally known ways of controlling the viscosity such as by temperature etc also be controlled by the amount of fiber-like nano elements in the resin, or by choosing degree of precuring in the resin (B- staging). According to one alternative, the resin could comprise an agent, such as a foaming agent, which causes the resin to expand when the material is cured. This additive could for example create gas bubbles in the resin which makes the volume of the resin increase. The resin will then fill out the space between the hole surface and the fastening element to a high degree which makes the mechanical joint robust and strong. Since the resin comprising fiber-like elements will fill the space between the fastening element and the composite structure to a high degree, each fastening element will have support transverse to the elongation of the fastening element and, also, all of the fastening elements will have an approximately equal amount of load capability. Additionally, each fastening element will have the largest possible load carrying surface. These features make the mechanical joint robust, strong and increase the load carrying capability of the joint. Further, certain deviations of the hole quality could be accepted when using this method. In case of a hole being larger than what was intended, the matrix material comprising fiber-like nano elements can fill up the extra space Thereby rework of the hole and/or an oversized fastening element could be avoided.

The matrix material comprising fiber-like electrically conducting and mechanically reinforcing nano elements is a conductive medium which can conduct an induced current between the composite structures and the fastening element and also between the conductive fibers within the composite structure. Thereby sparks are prevented from being created.

By arranging the resin comprising fiber-like electrically conducting and mechanically reinforcing nano elements in the hole and/or on the fastening element prior to inserting the fastening element in the hole, the resin will be subjected to a pressure increase when inserting the fastening element in the holes. The resin comprising fiber-like nano elements could alternatively be arranged on the fastening element in a precured form. In an example wherein the fastening element comprises a pin and a thread or just a pin, the resin could be arranged on the pin of the fastening element. The resin may be arranged under the head of the fastening element, if the fastening element has a head. Also in this case a pressure increase will occur which makes the resin comprising the fiber-like electrically conducting and mechanically reinforcing nano elements to fill the space to a high degree. One advantage with these embodiments is that the resin will fill out the potential cracks and damages from chips in the hole to a high extent by the pressure increase when placing the fastening element in the hole. Another advantage with this embodiment is that the resin can be applied quickly and the application can be performed away from the composite structure which minimizes the risk of polluting the structure with resin. The mounting of the fastening element in the holes can either be performed by a robot or manually. The application of the resin comprising fiber-like electrically conducting and mechanically reinforcing nano elements in the holes and or on the fastening element can be performed by a robot or manually.

The method for joining a first composite structure to at least a second structure creates a robust, strong and leak tight mechanical joint. The matrix material comprising fiber-like electrically conducting and mechanically reinforcing nano elements forms a strong material which increases the load strength of the composite structure in a mechanical joint. The matrix material comprising fiberlike electrically conducting and mechanically reinforcing nano elements is also electrically conductive so as to form an electrical joint, which can be beneficial for certain embodiments.

In one embodiment the fastening element is tightened before the step of curing the resin. In one embodiment, said first composite structure is joined to a second structure and to at least a third structure. The second or third structures could comprise a composite structure, a metal, a metal alloy, a metal matrix composite or a ceramic.

In one embodiment, the step of arranging a resin comprising fiber-like electrically conducting and mechanically reinforcing nano elements in the hole and/or on a fastening element comprises arranging or growing the fiber-like nano elements on the fastening element and adding the resin in the holes. When the fastening element is inserted in the hole, the resin comprising fiber-like nano elements is formed

The fiber-like nano elements could be added to the fastening element when producing the fastening element, or at a later stage.

In one embodiment the fiber-like nano elements are arranged on the fastening element and the resin is added in the hole of the at least first structure to form the resin comprising fiber-like nano elements when joining the first structure and the second structure with a fastening element.

At least part of the surface of the fastening element could be coated with a curing agent. The curing agent could be added to the fastening element when the fastening element is produced, or at a later stage. The resin is placed in the holes, and when the fastening element is inserted into the hole, the curing agent is mixed into the resin. In one embodiment a curing material is added to the resin comprising fiber-like nano elements via the fastening element. In one embodiment, an agent which causes the resin comprising fiber-like elements to expand when curing is added to the resin.

In one embodiment the step of arranging the resin comprising fiber-like electrically conducting and mechanically reinforcing nano elements is performed after the fastening element has been arranged in the hole. The capillary force, in some case supported by vibration or ultrasound, could be used to fill the resin in the space. Additionally, vacuum could be used in order to fill the resin in said space. This is an alternative method which is preferred in some cases. In one embodiment, the step of arranging the resin comprising fiber-like electrically conducting and mechanically reinforcing nano elements is performed after the fastening element has been arranged in the hole of said first and said second structure. The objective of the invention has been solved by a joint comprising a first composite structure, at least a second structure and a fastening element extending in a hole in the first composite structure and the at least second structure wherein a matrix material comprising fiber-like electrically conducting and mechanically reinforcing nano elements is arranged in a space between the fastening element and a hole surface of at least the first composite structure.

The fiber-like electrically conducting and mechanically reinforcing nano elements are preferably carbon nano tubes (CNT), carbon nano fibers, graphite nano wires etc. Cured resin, plastic, comprising nano elements has a selectable thermal conductivity, electrical conductivity and strength depending on the amount and type of fiber-like nano elements in the material. The nano elements preferably have a length of 0,125 mm or less. The definition of nano means that a nano element has at least one dimension not more than 200 nm. 1 nm is defined as 10 ^"9 meter. Preferably the diameter of a multiwall carbon nano tube is 5-50 nm, suitably 10-40 nm. Suitably, the diameter of a single wall nano tube is 1,2-1,7 nm, preferably 1,35-1,45 nm. The matrix material comprising fiber-like nano elements forms a hard strong material which is conductive. Thereby, an induced current can be conducted through the matrix material from the composite structure to the fastening element or vice versa. Also, due to the strength of the matrix material, and due to the ability to distribute the matrix material to fill the space between the fastening element and the hole surface to a large extent, a very strong mechanical joint is created

In one embodiment at least one of the holes of the first composite structure or the at least second structure is a through hole. By using a through hole, the joint created is strong due to the large contact area of the fastening element in the material. A through hole makes it easy to see where the fastening element is to be placed if the holes are pre-made and it will be easy to assemble the structures since it will be visible when they are placed in a correct place in relation to each other.

The fastening element can for example comprise three parts such as a bolt, a washer and a nut or a bolt, a washer and a collar, or a shear pin comprising two parts or other similar constructions. The fastening element could also comprise one element. The fastening element can be made of different materials such as a non-metallic composite, a metal, a metal matrix composite, a ceramic etc. The fastening element can have a special purpose head, e.g. with function as a bending washer to fit into a radius to strengthen the structure. A multitude of such fastening elements can for example reinforce co-cured T-joints in CFRP structures In one embodiment, the first composite structure is a carbon composite structure.

The first structure may alternatively or in addition thereto contain other fibers, such as aramid and/or aluminum oxide and/or ceramic and/or quartz and/or silicon carbide and/or glass fibers. For example, see HYPERLINK "http://en.wikipedia.org/wiki/Composite_material" \o "Composite material"

In one embodiment, at least one of the at least second structure comprises a metal, and/or a metal alloy and/or a metal matrix composite and/or a metal bonded structure and/or a ceramic.

In one embodiment, at least one of the at least second structure is a composite structure. The second structure may alternatively or in addition thereto contain other fibers, such as aramid and/or aluminium oxide and/or ceramic and/or quartz and/or silicon carbide and/or glass fibers. For example, see HYPERLINK "http://en.wikipedia.org/wiki/Composite_material" \o "Composite material" The mechanical and/or electrical joint can also have more structures of different materials joined to each other.

In one example, one of the at least one second structure of the joint forms an electrically connecting element.

In one embodiment a fastening element for use in the mechanical and/or electrical joint according to the invention comprises fiber-like electrically conducting and mechanically reinforcing nano elements attached to at least part of the surface of the fastening element.

In one embodiment a fastening element for use in the mechanical and/or electrical joint according to the invention has at least part of its surface coated with a curing agent.

In one embodiment a fastening element for use in the mechanical and/or electrical joint according to the invention has at least part of its surface coated with a resin comprising fiber-like electrically conducting and mechanically reinforcing nano elements. The resin could be precured.

In one embodiment a fastening element for use in the mechanical and/or electrical joint according to the invention has at least part of its surface coated with an agent, such as a foaming agent, which causes the resin comprising fiber-like electrically conducting and mechanically reinforcing nano elements to expand when curing

Brief Description of the Drawings

The present invention will now be described by way of examples with references to the accompanying schematic drawings.

Fig. 1 illustrates a mechanical joint comprising a first composite structure and a second structure according to a first example.

Fig. 2 illustrates a mechanical joint comprising a first composite structure and a second composite structure according to a second example. Fig. 3 illustrates a mechanical joint comprising a first composite structure, a second composite structure and a third structure according to a third example.

Fig. 4 illustrates a mechanical joint in a first composite structure and a second structure according to a forth example.

Fig. 5 illustrates a mechanical joint comprising a first composite structure, a second composite structure and a third structure according to a fifth example. Fig. 6 illustrates flow charts of the different steps in an example of a method for joining a first composite structure to at least a second composite structure.

Detailed Description

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings, wherein for the sake of clarity and understanding of the invention some details of no importance are deleted from the drawings. In figure 1-5, only one fastening element is illustrated. However, the mechanical joint can comprise a plurality of fastening elements.

The mechanical joint illustrated in figures 1-5 can be a joint between two or more structures and at least one fastening element. The structures comprise holes, either through holes or blind holes in which a fastening element, such as a bolt, rivet or other similar fastening elements is connecting the structures. The holes can be premade or made when the structures are placed in a predetermined position in relation to each other. The holes can either be mechanically made, for example by drilling, orbital drilling, nc-machined or they can be moulded.

Figure 1 schematically illustrates a mechanical joint 10 between a first structure la and a second structure 2b. The first structure is a composite structure la. The second structure 2b comprises another material such as ceramics, a metal, a metal alloy or a metal matrix composite. A fastening element 4, 9, 11 is placed in a hole of the structures. In a space between the hole surface in the composite structure la and the fastening element 4, 9, 11 a material comprising fiber-like nano elements 6 is arranged. The composite structure la can for example be a carbon composite structure or any other suitable composite structure.

The hole in the composite structure la and the other structure 2b in figure 1 is illustrated as a through hole, but the hole could also be a blind hole at least in one of the structures.

The dimension of the part of the fastening element extending in the hole 4 is smaller than then dimension of the hole in the composite structure la. The space defined by the volume between the hole surface and the fastening element 4 in the composite structure la is illustrated. The fastening element in figure 1 comprises three parts 4, 9, 11. Alternatively the fastening element can comprise two parts such as a bolt, a washer and a nut or a bolt, a washer and a collar, or a shear pin comprising two parts or other similar constructions. The fastening element could alternatively comprise one part.

The second structure 2b comprises a material such as a ceramic a metal, a metal alloy or a metal matrix composite. The material of the second structure may be elastic and hence, there is no space created between the hole surface of the second material and the part of the fastening element extending in the hole 4 (interference fit). The hole in the second structure 2b may have been arranged to have smaller dimensions than the part of the fastening element extending in the hole 4. Alternatively, the hole of the second material has been made with the dimensions needed to precisely fit the fastening element 4 (close fit). Clearance fit can also be used.

The matrix material 6 comprises fiber-like nano elements which makes the matrix material strong. The strong matrix material in the mechanical joint creates a strong and robust mechanical joint.

The matrix material comprising fiber-like nano elements placed in the space between the fastening element 4 and the hole surface in the composite structure la improves the conductivity between the fastening element 4, 9, 11 and the composite structure la. If the composite structure la comprises layers of fibers with layers of thermo plastics dissolved or undissolved in between the layers of fibers, the matrix material comprising fiber-like nano elements 6 also improves the conductivity between the layers of fibers at the hole surfaces of the mechanical joint in the composite material. Hence, if the composite structure la is for example hit by lightning, an induced current in the composite structure la can be conducted between the layers of the composite structure la in the holes and between composite structure la and the fastening element 4, 9, 11 via the conductive matrix material 6 without sparks or glitches being created. This may improve the safety of the construction.

The composite structure la is for example a carbon-fiber-reinforced polymer or carbon-fiber-reinforced plastic (CFRP or CRP). The structure la may contain other fibers, such as aramid, aluminium oxide, ceramic, quartz, silicon carbide or glass fibers, as well as carbon. See for example HYPERLINK "http://en.wikipedia.org/wiki/Composite_material" \o "Composite material"

The above general description of the mechanical joint 10 and the components creating the mechanical joint is also valid for the examples below.

Figure 2 schematically illustrates a mechanical joint where a first composite structure la and a second composite structure 2a are joined to each other with a fastening element 4, 11, 12. In the space between the hole surfaces and the part of the fastening element 12 extending in the hole, as described in relation to fig 1, a matrix material with fiber-like nano elements 6 is disposed. The holes in the structures in fig 2 are shown as through holes, but they could also be blind holes. Both structures la, 2a in figure 2 are composite structures. The material comprising fiber-like nano elements 6 is disposed in the space between the fastening element 12 and the hole surfaces in both the first and the second composite structures la, 2a

The material comprising fiber-like nano elements 6 can also be disposed in the space between the fastening element 12 and the fastening element 4 and the hole surfaces in both the first and the second composite structures la, 2a

The fastening element illustrated in figure 2 comprises three parts 4,11, 12 but could alternatively be a different fastening element comprising one or three parts.

Figure 3 schematically illustrates three structures being joined in a mechanical joint 10 as described in relation to fig. 1. In the example illustrated in fig. 3, two composite structures la, 2a are joined to a structure 3b comprising a different material such as a metal, metal alloy, metal matrix composite or a metal bonded structure or a ceramic. In the space between the hole surfaces in the composite structures la, 2b and the part of the fastening element 4 extending in the hole, a matrix material comprising fiber-like nano elements 6 is arranged.

Figure 4 schematically illustrates two composite structures la, 2a joined together with a mechanical joint 10 as described in relation to fig. 1. In the example of fig. 4 both holes are blind holes. A matrix material comprising fiber-like nano elements 6 is arranged in between in the space between the hole surface and a fastening element 4. At least one of the structures in this example could alternatively comprise a different material such as metal, metal alloy such as metal matrix composite or a metal bonded structure or a ceramic (not illustrated in this fig 4).

Figure 5 schematically illustrates three composite structures la, 2a, 3a being joined together with a fastening element 4 in a mechanical joint 10 as described in relation to fig. 1 above. The space between the hole surfaces and the part of the fastening element extending in the hole 4 is filled with a matrix material comprising fiber-like nano elements 6. The hole of the third structure 3a is a blind hole, but it is evident that this hole could also be a through hole. Structures la, 2a and 3a are composite structures. Figure 6 schematically illustrates a flow diagram of a method according to another example of the present invention. This example relates to creating the mechanical joint joining at least one composite structure to at least one other structure.

In a first method step S100 a hole in at least one of the structures which are to be joined with a mechanical joint is arranged. This hole could be made by a mechanical method such as drilling, orbital drilling, or nc machining or, the hole could alternatively be made for example when constructing the material, for example by moulding. Alternatively, one of the holes could be made when constructing the material and the other hole could be made by a mechanical method. After the method step S100 a subsequent method step S110 is performed. In the method step S110 the resin in arranged in the hole of the structure and/or on the fastening element. It is evident that there can be more than one holes and more than one fastening elements creating the mechanical joint. After the method step S110 a subsequent method step S120 is performed. In the method step S120 the fastening element is inserted in the hole of the first composite structure and in the hole of the at least one second structure. When using a bolt and nut, also the nut may be attached to the bolt during this step. Method step S120 may include the step of tightening the fastening element. After the method step S120 a subsequent method step S130 is performed

In the method step S130 the resin is cured. This could be done by heating the resin to a certain temperature or by waiting a certain time or by any other generally known curing methods, depending on the curing material in the resin. Many modifications and variations will be apparent to practitioners skilled in the art without departing from the scope of the invention as defined in the appended claims. The examples were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various examples and with various modifications as suited to the particular use contemplated.