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Title:
PREPARATION OF A COMPONENT FOR USE IN A JOINT
Document Type and Number:
WIPO Patent Application WO/2008/110835
Kind Code:
A1
Abstract:
A method of preparing a component (60) to be joined to another component (55). The method comprises growing an array of projections (56, 57) on a bond region of the component (55) in a series of layers, each layer being grown by directing energy and/or material from a head to selected parts of the bond region. The joint may be used to join a pair of structural components, for instance in an aerospace application. For instance the joint may be used to join a reinforcing plate, floating rib foot, or stringer to a panel such as a wing or fuselage cover. Alternatively the joint may be used to join adjacent layers in a laminate structure.

Inventors:
MEYER, Jonathan (Airbus UK Limited, New Filton House Filton, Bristol BS99 7AR, GB)
JOHNS, Daniel (Airbus UK Limited, New Filton House Filton, Bristol BS99 7AR, GB)
HENSTRIDGE, Andrew (Airbus UK Limited, New Filton House Filton, Bristol BS99 7AR, GB)
Application Number:
GB2008/050152
Publication Date:
September 18, 2008
Filing Date:
March 04, 2008
Export Citation:
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Assignee:
AIRBUS UK LIMITED (Ip Dept, New Filton HouseFilton, Bristol BS99 7AR, GB)
MEYER, Jonathan (Airbus UK Limited, New Filton House Filton, Bristol BS99 7AR, GB)
JOHNS, Daniel (Airbus UK Limited, New Filton House Filton, Bristol BS99 7AR, GB)
HENSTRIDGE, Andrew (Airbus UK Limited, New Filton House Filton, Bristol BS99 7AR, GB)
International Classes:
B23K26/34; B23B3/30; B23K15/00; C09J5/02; F16B5/07
Domestic Patent References:
WO1996033837A11996-10-31
Foreign References:
US20060163222A12006-07-27
US20030006217A12003-01-09
EP1741952A12007-01-10
JPS5985392A1984-05-17
Attorney, Agent or Firm:
SCOTT, Alistair (Ip Dept, 09ANew Filton House, Filton, Bristol BS99 7AR, GB)
Download PDF:
Claims:
CLAIMS

1. A method of joining a pair of components, the method comprising preparing one of the components by growing an array of projections on a bond region of the component in a series of layers, each layer being grown by directing energy and/or material from a head to selected parts of the bond region, wherein the components are joined by embedding the array of projections in the other component.

2. The method of claim 1 further comprising causing relative movement between the head and the bond region during the growing step.

3. The method of claim 1 or 2 wherein during the preparation step, the head directs material to selected parts of the bond region.

4. The method of claim 1 wherein during the preparation step, the head directs energy to fuse the material as it is directed to the bond region.

5. The method of claim 3 or 4 wherein during the preparation step, material is directed to selected parts of the bond region with the component in a first orientation relative to the head; rotating the component so that it adopts a second orientation relative to the head; and directing material to selected parts of the bond region with the component in the second orientation.

6. The method of claim 1 wherein the projections are formed by depositing a series of beds of material on the bond region; and directing energy from the head to selected parts of each bed.

7. The method of any preceding claim wherein the projections are formed by fusing a powder.

8. The method of any preceding claim wherein the component is grown in a series of layers, each layer being grown by directing energy and/or material from a head to selected parts of a build surface.

9. The method of claim 1 further comprising hardening the other component after the array of projections has penetrated into it.

10. The method of claim 1 or 2 further comprising agitating the array of projections and/or the other component to assist penetration.

11. A joint formed according to the method of any one of claims 1 to 10.

12. The joint of claim 11 wherein at least one of the projections includes a part with an overhanging edge.

13. The joint of claim 11 or 12 wherein at least one of the projections is asymmetrical.

14. The joint of claim 11, 12 or 13 wherein at least one of the projections has a pointed tip.

15. The joint of any of claims 11 to 14 further comprising a third component joined to a second bond region of the first component, the second bond region having an array of projections formed by the method of claims 1 to 10.

16. The joint of claim 15 wherein the bond regions are on opposite faces of the first component.

17. The joint of any of claims 11 to 16 wherein the second component is a laminar component.

18. The joint of any of claims 11 to 17 wherein the second component is a composite component.

19. The joint of claim 18 wherein the composite component is a fibre -reinforced composite component.

20. The joint of claim 18 wherein the composite component is a fibre -reinforced laminated carbon-fibre composite component.

21. The joint of any of claims 11 to 20 wherein the projections are metallic.

22. The joint of any of claims 11 to 21 wherein the projections are formed from the same material as the first component.

23. The joint of any of claims 11 to 22 wherein the projections are formed from a different material to the second component.

24. The joint of any of claims 11 to 23 wherein the second component has a bond region with an array of projections formed by the method of any of claims 1 to 10 , and the bond region of the second component is joined to the bond region of the first component.

25. The joint of any of claims 11 to 24 wherein at least one of the projections has a height (H) perpendicular to the bond region of the component and an average width (W) parallel to the bond region, and wherein the aspect ratio H/W is greater than 1.

26. The joint of claim 25 wherein the aspect ratio H/W is greater than 2.

27. A laminate comprising a series of layers, each adjacent pair of layers being coupled together by a joint according to any of claims 11 to 27 .

28. A component prepared by the method of any of claims 1 to 10 .

Description:

PREPARATION OF A COMPONENT FOR USE IN A JOINT

FIELD OF THE INVENTION

The present invention relates to a method of preparing a component to be joined to another component; a component prepared by such a method; and a joint including such a component.

BACKGROUND OF THE INVENTION

Joining between composite and metallic or thermoplastic components is currently approached in a number of ways, each with its own limitations.

The use of fasteners is commonplace but tends to result in de-lamination around fastener holes, as well as the associated difficulties of drilling holes in composites such as Carbon Fibre and Aramids such as Kevlar.

The bearing strength of laminated composites tends to be low, as does the inter-laminar shear strength. This results in a requirement for significant reinforcement around fastener holes, leading to a large weight increase, which is particularly undesirable in aerospace applications.

Fastened joints tend to be particularly weak in the pull-through direction (that is, the direction of axial load through the fastener) and as such are not well suited to aerospace applications such as fastening ribs to covers, where air and fuel pressure loads tend to result in significant axial component of load through the fastener.

Adhesive bonds are an increasingly common means of joining metallic components to composite laminates, however these perform poorly in peel, tension and cleavage, and tend to fail with little or no warning. Their weakness in peel and in tension makes bonded joints similarly limited in their application within conventional aerospace structures. Any mitigation for the poor performance in peel or tension tends to result in large bond surface areas, with the associated weight penalties that go with this.

Existing research into the use of surface features to improve the strength of metallic / composite joints is limited.

WO 2004/028731 Al describes a method by which surface features are generated by using a 'power-beam' such as an electron beam, in order to 'flick-up' surface material on a metallic component to sculpt protruding features that are intended to increase bond surface area and improve bond strength when incorporated into the matrix of a co-cured laminate.

This process has certain limitations.

Firstly, the process displaces surface material to create the protruding features. This could lead to surface damage of the component, and is likely to generate crack initiators that will adversely affect the fatigue life of the parent part.

Secondly, the process does not provide scope for optimising the profile and shape of the protrusions, which could produce significant improvements in the performance of the joint - particularly in tension and peel.

Thirdly, the process is an additional step in the production of a component and as such has an adverse time and cost impact.

Fourthly, the method can only form protruding features with a relatively low aspect ratio.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a method of preparing a component to be joined to another component, the method comprising growing an array of projections on a bond region of the component in a series of layers, each layer being grown by directing energy and/or material from a head to selected parts of the bond region.

A second aspect of the invention provides a method of joining a pair of components, the method comprising growing an array of projections on a bond region of a first component in a series of layers, each layer being grown by directing energy and/or material from a head to selected parts of the bond region; and joining the bond region of the first

component to a second component such that the projections are embedded in the second component.

A third aspect of the invention provides a joint comprising a first component having a bond region with an array of projections; and a second component joined to the bond region of the first component, wherein the projections have been grown in a series of layers, each layer being grown by directing energy and/or material from a head to selected parts of the bond region.

The invention employs an additive fabrication technique to form the projections, instead of forming the projections by displacing surface material as in WO 2004/028731 Al . This reduces the risk of damaging the component. Also, by growing the projections in a series of layers, the shape of each layer can be selected to enable the profile of the protrusions to be optimised.

The layered fabrication method also permits the projections to be formed with a relatively high aspect ratio, which we define herein as the ratio between the height of the projection perpendicular to the bond region and its average width parallel to the bond region. Typically the aspect ratio is greater than 1, preferably it is greater than 2, and most preferably it is greater than 3. This can be contrasted with the projections described in WO 2004/028732 which have aspect ratios less than 1.

The head and the bond region may remain stationary during the growth process: for example the head may have a fixed array of lasers and/or nozzles which extend over the entire bond region and are modulated as required to directing energy and/or material to selected parts of the bond region. However more preferably the method further comprises causing relative movement between the head and the bond region. Preferably this relative movement is caused by moving the head, but it will appreciated that the relative movement may be caused by moving the component or by a combined movement of both parts.

Various additive fabrication techniques may be used, including techniques in which the head directs material to selected parts of the bond region, and techniques in which a series

of beds of material are deposited on the bond region and the head directs energy to selected parts of each bed.

Examples of the former include fused deposition modelling (in which the head extrudes hot plastic through a nozzle) and powder feed fabrication (in which a laser beam directs energy to fuse a powdered material as it is delivered to the bond region). Advantages of these methods are that:

• the amount of wastage of material in the fabrication process is minimized;

• the projections can be made from a different material to the component; and

• the component can be rotated relative to the head during the fabrication process in order to form a complex shape.

Examples of the latter include stereolithography (in which a laser is used to cure selected parts of a bed of liquid photopolymer) and powder bed fabrication (in which a series of beds of powder are deposited on the bond region and selected parts of each bed are fused by a laser). Advantages of using the head to deliver energy to the selected parts of a previously deposited bed of material are that:

• it enables the component and the array of projections to be formed together; and

• unconsolidated parts of each bed can support successive beds, enabling relatively complex shapes to be formed.

Typically the projections are formed by fusing a powder, for instance in a powder bed process or a powder feed process.

The components may be joined by penetrating the second component with the array of projections. This may be achieved by moving the first component, the second component, or both.

The joint may be secured by hardening the second component after the array of projections has penetrated into it, or by using an intermediate adhesive layer between the two

components. In the latter case the second component may also have a bond region with an array of projections formed by an additive fabrication method.

The projections may be symmetrical (for instance cylindrical or conical, and extending at right angles to the component) or at least one of the projections may be asymmetrical (for instance the projection(s) may lean to one side and/or may have a non-circular cross- section). Asymmetrical projections can be used to improve properties in a particular load direction.

The first component may be an interfacing element between two components. In this case the joint further comprises a third component joined to a second bond region of the first component, the second bond region having an array of projections formed by the method of the first aspect of the invention. The bond regions may be on opposite feces of the first component, on separate parts of a single face, or on adjacent faces.

The second component may be a laminar component formed in a series of layers. In this case, the projections reduce the risk of de-lamination compared with conventional fastener joints. Preferably the second component is a composite component such as a fibre- reinforced composite component.

The projections may be formed from a metallic material (for instance Titanium or stainless steel); a thermoplastic material such as polyetheretherketone (PEEK); or any other material which can be grown by the additive fabrication technique of the first aspect of the invention.

The projections may be formed from the same material as the first component and/or the second component, or they may be formed from a different material.

The joint may be used to join a pair of structural components, for instance in an aerospace application. For instance the joint may be used to join a reinforcing plate, floating rib foot, or stringer to a panel such as a wing or fuselage cover. Alternatively the joint may be used to join adjacent layers in a laminate structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view of a floating rib foot;

FIG. 2 is a cross sectional view showing the floating rib foot being integrated into a mould tool;

FIG. 3 is showing a composite lay-up on the mould tool;

FIG. 4 is a perspective view showing the floating rib foot connecting a rib to a cover;

FIG. 5 is a cross-sectional view showing integral resin bleed channels on the floating rib foot;

FIG. 6 is a schematic view of a powder bed fabrication system;

FIG. 7 is a schematic view of a powder feed fabrication system;

FIG. 8 is a perspective view of a first interfacing strip;

FIG. 9 is a side view of a second interfacing strip;

FIG. 10 is a cross-sectional view showing the second interfacing strip connecting a pair of composite workpieces;

FIG. 11 is a perspective exploded view showing a stringer run-out being joined to a cover by an interfacing strip;

FIG. 12 is a perspective view of the stringer run-out joined to the cover;

FIG. 13 is a cross-sectional view of a fibre -metal laminate with metal layers incorporating surface features;

FIG. 14 is an end view showing an initial step in a method of fabricating the fibre-metal laminate;

FIG. 15 is a side view showing the method of fabricating the fibre -metal laminate;

FIG. 16 is an end view showing a subsequent step in the method of fabricating the fibre- metal laminate;

FIG. 17 is a cross-sectional view of a metal-metal joint; and

FIG. 18 shows a set of alternative projection profiles.

DETAILED DESCRIPTION OF EMBODIMENTS

A metallic floating rib foot 1 shown in Figure 1 comprises a web portion 3 and a pair of flanges 2. The web portion 3 has a pair of fastener holes 4. An array of projections 5 extend from the underside of the flanges 2. As can be seen in Figure 1, the projections 5 are distributed evenly over a bond region which extends around the periphery of the flanges 2 and surrounds a central region 6 with no projections.

The floating rib foot 1 is integrated into a mould tool 10 as shown in Figure 2. The mould tool 10 has a mould surface 12 with a recess 11 which receives the flanges 2 as shown in

Figure 3. Web portion 3 extends into a channel between a pair of plates 13, 14, and is secured in place by a fastener 17 passing through a pair of holes 15, 16 in the plates 13, 14 as shown in Figure 3. In the example of Figure 3 only one fastener 17 is shown, but in alternative arrangements two or more fasteners may be used to secure the floating rib foot to the mould tool. In the case where two fasteners are used, then they may be passed through the holes 4 in the web portion 3.

After the floating rib foot 1 has been integrated into the mould tool 10, a composite lay-up 18 is laid onto the mould tool. The composite lay-up 18 comprises a series of plies of uniaxial carbon fibre, pre -impregnated with uncured epoxy resin. Each ply is conventionally known as a "prepreg". The initial prepregs are penetrated by the projections 5 as shown in Figure 3.

After the lay-up 18 has been formed as shown in Figure 3, it is cured and consolidated by a so-called "vacuum bagging" process. That is, the lay-up is covered by a vacuum membrane (and optionally various other layers such as a breather layer or peel ply); the vacuum membrane is evacuated to apply consolidation pressure and extract moisture and volatiles; and the lay-up is heated (optionally in an autoclave) to cure the epoxy resin matrix. As the epoxy resin matrix melts prior to cure, it flows into intimate contact with the projections 5. The projections 5 mechanically engage with the matrix, while also increasing the surface area of the bond.

The components are then removed from the mould and assembled with various other wing box components as shown in Figure 4. In this example the cured lay-up 18 is a wing cover, and the floating rib foot 1 secures a rib to the wing cover 18. The rib comprises a rib web 20 and a fixed rib foot 21 extending downwardly from the rib web 20. Fasteners (not shown) are passed through the fastening holes 4 in the web portion 3 of the floating rib foot 1 to secure the floating rib foot 1 to the fixed rib foot 20.

As well as carrying projections 5, the lower surface of the flanges 2 may also be formed with resin bleed channels 26 shown in Figure 5. Resin flows through the channels 26 during the curing process.

Each projection 5 is grown in a series of layers by an additive manufacturing process: either a powder bed process as shown in Figure 6, or a powder feed process as shown in Figure 7.

In the powder bed process shown in Figure 6, the array of projections is formed by scanning a laser head laterally across a powder bed and directing the laser to selected parts of the powder bed. More specifically, the system comprises a pair of feed containers 30, 31 containing powdered metallic material such as powdered Titanium. A roller 32 picks up powder from one of the feed containers (in the example of Figure 6, the roller 32 is picking up powder from the right hand feed container) and rolls a continuous bed of powder over a support member 33. A laser head 34 then scans over the powder bed, and a laser beam from the head is turned on and off to melt the powder in a desired pattern. The

support member 33 then moves down by a small distance (typically of the order of 0.1mm) to prepare for growth of the next layer. After a pause for the melted powder to solidify, the roller 32 proceeds to roll another layer of powder over support member 33 in preparation for sintering. Thus as the process proceeds, a sintered part 35 is constructed, supported by unconsolidated powder parts 36. After the part has been completed, it is removed from support member 33 and the unconsolidated powder 36 is recycled before being returned to the feed containers 30, 31.

The powder bed system of Figure 6 can be used to construct the entire floating rib foot 1 , including the web portion 3, flanges 2 and projections 5. Movement of the laser head 34 and modulation of the laser beam is determined by a Computer Aided Design (CAD) model of the desired profile and layout of the part.

The powder feed fabrication system shown in Figure 7 can be used to build up the projections 5 on a previously manufactured floating rib foot. That is, the web portion 3 and flanges 2 have been previously manufactured before being mounted in the powder feed fabrication mechanism.

A projection 5 is shown being built up on the underside of one of the flanges 2 in Figure 7. The powder feed fabrication system comprises a movable head 40 with a laser 41 and an annular channel 42 around the laser 41. Un-sintered powder flows through the channel 42 into the focus of the laser beam 43. As the powder is deposited, it melts to form a bead 44 which becomes consolidated with the existing material.

The powder feed system may be used to grow the projections in series, or in parallel. More specifically, the projections may be grown in parallel by the following sequence:

P(I)L(I), P(2)L(l),...P(n)L(l),P(l)L(2),P(2)L(2),...P(n)L(2)...etc.

or in series by the following sequence:

P(I)L(I), P(l)L(2),...P(l)L(m),P(2)L(l),P(2)L(2),...P(2)L(m)...etc.

where P(X)L(Y) represents the growth of a layer X of a projection Y.

This can be contrasted with the powder bed system which can only grow the projections in parallel.

In contrast to the powder bed system of Figure 7, the powder feed system of Figure 6 directs powder to only the selected parts of the bond region, and fuses the powder as it is delivered. Therefore the powder feed mechanism produces structures that are unsupported by powder, and so supports (not shown) may need to be built integrally into the part and machined off later, in particular where the projections have large overhanging parts.

The head 40 may be the only moving feature in the process, or the part may be rotated during fabrication. In other words, the head 40 directs powder to selected parts of the bond region with the part in a first orientation relative to the head 40; the part is rotated so that it adopts a second orientation relative to the head 40; and the head then directs material to selected parts of the bond region with the part in the second orientation. This facilitates the manufacturing of complex shapes without the need for removable supports. For instance overhanging features can be formed by rotating the part between layers in order to always ensure that the element being built is at no more than 30 degrees from the vertical. As the build area is at a temperature significantly below the melting point of the material, the area being built will only need to maintain a supportable angle for a brief time after the laser energy is removed in order for it to solidify enough to become self supporting. If the projections are built in a parallel sequence then it is possible to re-orientate the part between each layer to enable unsupported overhanging features to be built.

Figure 8 shows an interfacing strip 50 with an upper face carrying an array of projections 51 for joining the interfacing strip to an upper workpiece, and an opposite lower face with an array of projections 52 for joining the interfacing strip to a lower workpiece. The interfacing strip and projections may be manufactured by the powder bed process of Figure 6, or the projections may be built onto a prefabricated strip using the powder feed process of Figure 7.

Figure 9 shows an interfacing strip 55 with upper and lower arrays of projections 56, 57. The interfacing strip 55 is shown in Figure 10 joining an upper workpiece 58 to a lower

workpiece 59. The interfacing strip 55 is particularly useful where significant through- thickness stresses are acting to pull the workpieces 58,59 apart. The penetration of the projections into the workpieces 58,59 provides a significant increase in the strength of the joint when it is subject to tensile, peel or cleavage loads.

The joint shown in Figure 10 may be manufactured in a number of ways, including:

• pressing the interfacing strip 55 into one of the workpieces (using a vibrating hammer or roller); then pressing the other workpiece onto the exposed projections of the interfacing strip (using the vibrating hammer or roller); or

• joining the interfacing strip with a first one of the workpieces using a method similar to that shown in Figures 2 and 3; co-curing the interfacing strip and first workpiece; pressing the second (uncured) workpiece onto the exposed projections of the interfacing strip (using a vibrating hammer or roller); and then curing the second workpiece; or

• joining the interfacing strip with a first one of the workpieces using a method similar to that shown in Figures 2 and 3; co-curing the interfacing strip and first workpiece; integrating the co-cured interfacing strip and first workpiece into a second mould; laying up the second workpiece onto the second mould using a method similar to that shown in Figure 2; and curing the second workpiece.

An example of the use of the interfacing strip 55 is shown as an exploded view in Figure 11. Note that in Figure 11 the bond region carrying the projections 56, 57 is at one end of the interfacing strip 55 only. In this case, the lower workpiece is a wing cover 61 and the upper workpiece is a stringer run-out 60 comprising a pair of flanges 63, 64 and a blade portion 62. Figure 12 shows the stringer run-out 60 joined to the cover 61 by the interfacing strip (which is not visible in Figure 12).

A fibre -metal laminate 70 is shown in Figure 13 in cross-section. The laminate 70 comprises a series of layers of carbon-fibre reinforced polymer (CFRP) 71 interleaved with layers of Titanium 72. Each Titanium layer 72 carries an array of projections 73 on its

lower surface and an array of projections 78 on its upper surface, each array of projections being embedded in the adjacent CFRP layer.

The laminate 70 is fabricated using the process shown in Figures 14-16. In an initial step, a first CFRP layer 71 is laid up (for instance as a stack of prepregs) on a mould tool (not shown). A first Titanium layer 72 is then laid onto the CFRP layer 71 with its lower projections 73 engaging the upper surface of the CFRP layer 71 as shown in Figure 15. A roller 74 carrying a series of annular ridges 75 with the same spacing as the upper projections 78 is then applied to the upper surface of the layer 72. The upper projections 78 are each received in a channel between an adjacent pair of ridges 75 as shown in Figure 14. The roller 74 is then rolled over the interfacing strip, and vibrated to agitate the projections 73 as they penetrate into the uncured CFRP layer 71 as shown in Figure 14. The roller 74 may be rolled back and forth in a number of passes to fully embed the projections 73 in the CFRP layer 71.

A second CFRP layer 76 is then laid on top of the layer 72 as shown in Figure 16, and a second roller 77 (without ridges) is rolled over the second CFRP layer 76 and vibrated to press the upper projections 78 into the second CFRP layer 76. The process is then repeated to form a series of pairs of layers as shown in Figure 13. Note that the projections for each Titanium layer 72 are offset from the previous layer.

A metal-metal joint is shown in Figure 17. A first workpiece 80 is formed with a series of projections 81. A second workpiece 82 is formed with a set of complementary projections 83 which interlock with the projections 81 as shown. A thin layer of adhesive (not shown) is provided in the gap between the work pieces, sealing them together in the manner of a tongue-and-groove joint.

The workpieces 80,82 and projections 81,83 may be manufactured by the powder bed process of Figure 6, or the projections 81,83 may be built onto prefabricated workpieces using the powder feed process of Figure 7.

Various alternative projection profiles are shown in Figure 18 which can be manufactured by additive layer manufacturing and used in any of the joints described above. Projection

90 comprises a conical spike. Projection 91 comprises a frustoconical base 92 and a conical tip 93 with an overhanging edge 94. Projection 95 comprises a cone which leans by an angle of θ to the vertical. Projection 96 comprising a cone leaning at an angle of θ to the vertical, with a pair of ridges 97, 98 on its overhanging side. Projection 100 comprises a cylindrical base 101 and a conical tip 102. Projection 103 comprises a frustoconical base 104, a frustoconical part 105 with an overhanging edge, and a conical tip 106 with an overhanging edge.

Note that the aspect ratios of the projections are relatively high, giving firm mechanical engagement and a high surface area. If we define the aspect ratio as H/W, where H is the height perpendicular to the bond region of the component and W is the average width parallel to the bond region, then the aspect ratio varies between approximately 3.5 (for the projection 100) and 5 (for the projections 90 and 95). The aspect ratio of the projections may be increased or decreased to give the desired properties.

The various geometries shown in Figure 18 may be selected to maximise the performance of the joint, and tailored to the specific loading that it is subjected to. Thus for example:

• projections 91, 96 and 103 (all of which include a part with an overhanging edge) may be used in a joint (or a selected part of a joint) that requires enhanced pull-off (tensile) strength;

• an asymmetrical projection 95 or 96 may be used to improve properties in a particular load direction; and

• each of the projections have pointed tips to enable them to penetrate easily into the composite material.

Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.