A method for forming a structural element for a vehicle

Abstract

Disclosed herein is a method (100) for forming a structural element (200) for a vehicle, comprising: arranging (102) a plurality of thin-walled tubes (2) into a cluster (3), the thin-walled tubes having internal surfaces (21) and external surfaces (22); providing (104) support to the internal surfaces of the thin-walled tubes; and overmoulding (106) a material (4) to at least a portion of the external surfaces of the thin-walled tubes thereby to bond the cluster together.

Description

[0001] A METHOD FOR FORMING A STRUCTURAL ELEMENT FOR A VEHICLE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a method for forming a structural element for a vehicle. More specifically, the present invention relates to a method for forming a structural element for a vehicle such as an impact energy absorber.

[0004] BACKGROUND

[0005] Thin-walled tubes, typically comprising carbon fibre-reinforced plastics, are efficient structures for providing general structural support for a vehicle and as impact energy absorbers in the event of a vehicle collision. Thin-walled tubes such as these can be manufactured to a high level of quality and consistency, which is required for safety features in vehicles.

[0006] One issue with thin-walled tubes is that they often have a low geometrical aspect ratio. In other words, the lengths of the thin-walled tubes are often much greater than their diameters. This can mean that thin-walled tubes have reduced structural stability when the direction of an impact is off-axis.

[0007] It is an object of the invention to attempt to address this issue.

[0008] SUMMARY OF INVENTION

[0009] In an aspect of the invention there is provided a method for forming a structural element for a vehicle, comprising: arranging a plurality of thin-walled tubes (or tubes) into a cluster, the thin-walled tubes having internal surfaces and external surfaces; providing support to the internal surfaces of the thin-walled tubes; and overmoulding a material to at least a portion of the external surfaces of the thinwalled tubes thereby to bond the cluster together.

[0010] It has been found that forming a cluster of thin-walled tubes by connecting the tubes together means that the structural stability of the tubes can be improved thereby forming a strong structural element for a vehicle. One such means of connecting the thin-walled tubes together is to overmould a material to the external surfaces of the tubes. By providing support to the internal surfaces of the thinwalled tubes during the overmoulding process it is possible to prevent damage to the tubes, since the forces experienced in overmoulding could otherwise crush them or deform them or displace them relative to their intended position.

[0011] As used herein, the term “overmoulding” may be defined as a process in which a substrate (such as a cluster of tubes) can be placed into a cavity mould, or overmoulding mould. A material can then be moulded to the substrate by injecting molten material into the cavity mould. In this way, the substrate and the injected material can be bonded together.

[0012] As such, in various embodiments, the present invention may involve: arranging a plurality of tubes into a cluster, the tubes having internal surfaces and external surfaces; placing the plurality of tubes into a cavity mould; providing support to the internal surfaces of the tubes; and injecting a material into the cavity mould to overmould the material to at least a portion of the external surfaces of the tubes, thereby to bond the cluster together.

[0013] Preferably, providing support to the internal surfaces of the thin-walled tubes comprises providing a mandrel within each of the thin-walled tubes. In this way, the walls of the thin-walled tubes can be physically supported from within during the overmoulding process to prevent the walls of the tubes from buckling or collapsing. The mandrels may be made of a strong material such as metal, which is resistant to crushing during the overmoulding process.

[0014] Preferably, providing support to the internal surfaces of the thin-walled tubes comprises providing a fluid within each of the thin-walled tubes. In this way, the walls of the thin-walled tubes can be pneumatically or hydrostatically supported from within during the overmoulding process to prevent the walls of the tubes from buckling or collapsing. Preferably, the fluid is a high-pressure gas. In this way, the support can be easily removed from within the thin-walled tubes once the overmoulding process has been completed and the structural element has been formed.

[0015] Providing support to the internal surfaces of the thin-walled tubes may also comprise providing a foam within each of the thin-walled tubes. In this case, the overmoulding process could be executed and the foam inside the thin-walled tubes will prevent the thin-walled tubes from being crushed by the in-mould pressure arising from the overmoulding process.

[0016] Preferably, the longitudinal axes of the thin-walled tubes are arranged in parallel to one another in the cluster. In this way, the geometrical aspect ratio of the structural element is increased. Therefore, the structural stability of the structural element is greater than that of a single tube, particularly when the direction of an impact is off-axis.

[0017] In one configuration the longitudinal axes of two or more thin-walled tubes are arranged in different orientations to one another in the cluster. In this way, when employed as an impact energy absorber for a vehicle, the structural element is able to efficiently absorb impact energies from multiple potential directions.

[0018] The method may further comprise arranging a plurality of thin-walled tubes into a first cluster and a second cluster, wherein the longitudinal axes of the first cluster and the second cluster are arranged in different orientations to one another. In this way, when employed as an impact energy absorber for a vehicle, the structural element is able to efficiently absorb impact energies from multiple potential directions. In this way, it is also possible to incorporate the structural element within various regions of the interior of a vehicle, such as the roof arches. In certain embodiments, two or more cluster may be provided in a structural element.

[0019] The thin-walled tubes may be arranged in a lattice pattern in the cluster. Such a lattice pattern may be a hexagonal lattice, for example. Structures comprising lattice patterns such as hexagonal lattices are typically strong and have high structural stability. In one configuration, the thin-walled tubes may be arranged in such a way that a gap is provided between two or more of the thin-walled tubes between their respective external surfaces. This gap may be left void, or it may be filled with the overmoulding material, in order to achieve the preferred structural response from the cluster.

[0020] In another configuration, the thin-walled tubes may be arranged in such a way that two or more of the thin-walled tubes are in contact with each other along at least a portion of their respective external surfaces. This point of contact may be further reinforced with an adhesive in order to achieve the preferred structural response from the cluster.

[0021] One or more of the thin-walled tubes in the cluster may comprise different lengths, as measured in the direction of the longitudinal axes of the thin-walled tubes. In this way, the structural stability of the structural element will be further increased during collision events. The impact force pulse will be spread out over the structural element as different thin-walled tubes will be impacted by the force at different times in the collision due to their varying lengths. The impulse of the collision on the structural element will be less extreme, therefore.

[0022] One or more of the thin-walled tubes in the cluster may comprise different cross- sectional shapes, as viewed in the direction of the longitudinal axes of the thinwalled tubes. For example, cross-sectional shapes could be round or oval. They could also be square or rectangular, or polygonal shapes, regular or irregular. The cross-sectional shapes could have sharp comers or preferably round comers.

[0023] Within a cluster of thin-walled tubes, the cluster may comprise different cross- sectional shapes or sizes. This can create an overall shape or size of the cluster that is more space-efficient and therefore fit in a more optimal configuration within the vehicle that it mounts to.

[0024] Overmoulding a material to at least a portion of the external surfaces of the thinwalled tubes may comprise overmoulding an attachment feature to enable assembly of the structural element to a corresponding attachment feature on the vehicle. In this way, the structural element can be securely connected to the relevant region of the vehicle. The attachment feature may comprise a flange. Alternatively, or additionally, ducts or fixing points may be included on the structural element. The attachment feature may be configured to enable multiple similar structural elements to be connected to one another to form a modular of structural elements.

[0025] In one arrangement, the external surface of at least one of the thin-walled tubes may comprise a bonding feature. Overmoulding a material to at least a portion of the external surfaces of the thin-walled tubes may therefore comprise overmoulding the material to a bonding feature of the external surface of at least one of the thin-walled tubes in the cluster. The bonding feature may be an indentation, the indentation could be of any geometry, for example a hole or a cut or a small deformation in the external surface of the thin-walled tube. The bonding feature may comprise a protrusion or protrusions from the external surface of the thin-walled tube. Additionally, or alternatively, the bonding feature may comprise grooves, channels, or crenelations in the external surface of the thin-walled tube.

[0026] In this way, the surface area of contact between the external surfaces of the thinwalled tubes and the overmoulded material is increased, thereby strengthening the bond between the thin-walled tubes and the overmoulded material and thus strengthening the structural element overall. Overmoulding material over the bonding feature will cause mechanical interlocking between the tube structure and the overmoulded structure, thus creating an additional load transfer capability for the structural system. This mechanical interlock can be designed to transfer loads between the thin-walled tubes and the overmoulded material in the axial direction or in the transverse direction or in the tangential direction or a combination of all directions.

[0027] Additionally, such bonding features, if designed with appropriate geometry and location, can conveniently function as a crash initiator. Crash initiators are notoriously useful design features applied to crash structures in order to provide repeatable initiation of the crash response and also to reduce any unwanted peak loads that may be necessary to initiate the crash response and could cause structural damage elsewhere in the system. It is understood that such bonding features could also conveniently correspond to at least one of the end cuts of the at least one thin-walled tube.

[0028] Preferably, the plurality of thin-walled tubes are arranged into a cluster and the cluster is then inserted into an overmoulding mould. In this way, the thin-walled tubes in the cluster are located accurately with respect to one another, thereby improving the structural stability of the structural element.

[0029] Preferably, the plurality of thin-walled tubes are arranged into a cluster within the overmoulding mould. Preferably, the plurality of thin-walled tubes are arranged into a cluster using locating features provided within the overmoulding mould. In this way, the thin-walled tubes in the cluster are located accurately with respect to one another, thereby improving the structural stability of the structural element.

[0030] The thin-walled tubes and the overmoulding material may comprise different materials. In one arrangement the thin-walled tubes may comprise fibre- reinforced plastic. Alternatively, the thin-walled tubes may comprise aluminium, fibre-reinforced plastics or steel.

[0031] Overmoulding a material to at least a portion of the external surfaces of the thinwalled tubes may comprise injection moulding fibre-reinforced or unreinforced resins, or compressions moulding fibre-reinforced compounds. In this way, the strength of the structural element is further increased.

[0032] According to another aspect of the invention there is provided a structural element for a vehicle formed using the disclosed method. The disclosed method steps may be provided as apparatus features.

[0033] According to another aspect of the invention there is provided a structural element for a vehicle, comprising: a plurality of thin-walled tubes arranged into a cluster, the thin-walled tubes having internal surfaces and external surfaces; and a material overmoulded to at least a portion of the external surfaces of the thinwalled tubes thereby to bond the cluster together. BRIEF DESCRIPTION OF DRAWINGS

[0034] Embodiments of the invention are now described, byway of example, by reference to the drawings, in which:

[0035] Figure 1A is a flow diagram of a method for forming a structural element for a vehicle in an embodiment of the invention;

[0036] Figure 1 B is a flow diagram of a method for forming a structural element for a vehicle in embodiment of the invention;

[0037] Figure 2A is a perspective view of a structural element for a vehicle in an embodiment of the invention;

[0038] Figure 2B is a top view of a structural element for a vehicle in an embodiment of the invention;

[0039] Figure 3A is a perspective view of a structural element for a vehicle in an embodiment of the invention;

[0040] Figure 3B is a top view of a structural element for a vehicle in an embodiment of the invention;

[0041] Figure 4 is a perspective view of a structural element for a vehicle in an embodiment of the invention; and

[0042] Figure 5 is a perspective view of a structural element for a vehicle in an embodiment of the invention.

[0043] DETAILED DESCRIPTION

[0044] Figure 1 A is a flow diagram of a method 100 for forming a structural element for a vehicle in embodiment of the invention. Step 102 comprises arranging a plurality of thin-walled tubes into a cluster; step 104 comprises providing support to the internal surfaces of the thin-walled tubes; and step 106 comprises overmoulding a material to the external surfaces of the thin-walled tubes. In this example, the support is provided to the internal surfaces of the thin-walled tubes at step 104 prior to the overmoulding step 106 so as to protect the thinwalled tubes from damage during the overmoulding step 106. The support is provided to the internal surfaces of the thin-walled tubes at step 104 after the arrangement of the plurality of thin-walled tubes into a cluster at step 102. In alternative embodiments, the support may be provided to the internal surfaces of the thin-walled tubes at step 104 prior to the arrangement of the plurality of thinwalled tubes into a cluster at step 102. The plurality of thin-walled tubes are arranged accurately into a cluster at step 102 by connecting them to one another prior to the overmoulding step 106.

[0045] The overmoulding step 106 involves placing the cluster of thin-walled tubes into a cavity mould and subsequently injecting a molten overmoulding material into the cavity mould to bond the overmoulding material to the cluster. Once the overmoulding material has cooled, the formed structural element can be moved from the cavity mould.

[0046] Figure 1 B is a flow diagram of a method 150 for forming a structural element for a vehicle in embodiment of the invention.

[0047] At steps 152 and 153, a plurality of thin-walled tubes are arranged into first and second clusters respectively. In each of the clusters each of the thin-walled tubes are located adjacent one another and are connected to maintain the shape of the cluster. In embodiments, adjacent thin-walled tubes are not in direct contact with each other in their respective clusters. This is to provide a gap between each of the tubes for material to be overmoulded within the gaps.

[0048] In embodiments, the thin-walled tubes may be arranged into a uniform repeating pattern to form a cluster. The longitudinal axes of the thin-walled tubes may be arranged in parallel to one another. Alternatively, the longitudinal axes of the thinwalled tubes may also be arranged in different orientations to each other in a single cluster. A high-pressure gas is provided within each of the thin-walled tubes at step 154. The high-pressure gas is provided within each of the thin-walled tubes to pneumatically support the internal surfaces of the thin-walled tubes during the overmoulding process at step 156. To provide the high-pressure gas within each of the thin-walled tubes, the plurality of tubes may be provided within a pressurised chamber and then sealed at each end, for example. The support to each of the thin-walled tubes may be removed from each of the tubes after the overmoulding process at step 156.

[0049] In alternative embodiments, a mandrel or a group of mandrels may be provided within each of the thin-walled tubes to mechanically support the internal surfaces of the thin-walled tubes. A mandrel or mandrels may be provided with or without a high-pressure gas in various embodiments. In further alternative embodiments, a fluid such as a gel or liquid may instead be provided within each of the thinwalled tubes to hydraulically support the internal surfaces of the thin-walled tubes.

[0050] At step 155, the first and second clusters of thin-walled tubes, formed at steps 152 and 153 respectively, are each inserted into an overmoulding mould. In alternative embodiments, the first and second clusters of thin-walled tubes, or a single cluster of thin-walled tubes, may instead be arranged within the overmoulding mould. The overmoulding mould may be provided with locating features for each of the thinwalled tubes to be arranged into the cluster in these cases. In other words, the plurality of thin-walled tubes may be inserted into the overmoulding mould prior to being arranged into a cluster. In further alternative embodiments, the thin-walled tubes may be connected to each other before being inserted into the overmoulding mould.

[0051] A material is overmoulded to the external surfaces of the thin-walled tubes in the first and second clusters at step 156. The material is overmoulded using known overmoulding techniques. The overmoulding process 156 involves injecting a molten overmoulding material into the overmoulding mould into which the first and second clusters have been inserted to bond the first and second clusters together. Two separate structural elements are formed, each comprising a cluster of thinwalled tubes. In the repeated formation of structural elements using the method 151 , the material is added precisely to the mould in a consistent dose quantity.

[0052] In embodiments, the material is overmoulded to the entirety of the external surfaces of the thin-walled tubes, thereby encasing the sides of each of the thinwalled tubes in the clusters. In alternative embodiments, the material is overmoulded to a portion of the external surfaces of the thin-walled tubes in the clusters.

[0053] At step 157, an attachment feature is overmoulded. The attachment feature is overmoulded such that it is provided on the exterior of the structural element. Multiple attachment features may be overmoulded to the structural element in various embodiments. In embodiments, steps 156 and 157 may be carried out as part of the same overmoulding step, with the mould comprising an attachment feature mould.

[0054] Figure 2A is a perspective view of a structural element 200 for a vehicle in an embodiment of the invention and Figure 2B is a top view. The structural element 200 comprises 19 thin-walled tubes 202 arranged in parallel in a cluster 203. The thin-walled tubes 202 are arranged in the cluster 203 in a hexagonal lattice pattern. Each of the thin-walled tubes 202 comprises an internal surface 221 and an external surface 222. An overmoulded material 204 is overmoulded to the cluster 203 of thin-walled tubes 202. The structural element 200 has the shape of a hexagonal prism.

[0055] The thin-walled tubes 202 are straight elongate cylinders. Each of the thin-walled tubes 202 in the structural element 200 have the same dimensions in the axial, radial and azimuthal directions. Each of the thin-walled tubes 202 comprise an internal surface 221 , which is the internal surface of the wall of a tube, and an external surface 222, which is the external surface of the wall of the tube.

[0056] In this embodiment, the thin-walled tubes 202 each comprise a carbon fibre- reinforced polymer. In alternative embodiments, the thin-walled tubes 202 may comprise a different material such as aluminium, other fibre-reinforced polymers or steel. In further alternative embodiments, some of the thin-walled tubes 202 in the structural element 200 may comprise a different material to the other thinwalled tubes 202 in the structural element 200. For example, half of the total number of tubes may comprise aluminium and the other half may comprise a carbon fibre-reinforced polymer.

[0057] In alternative embodiments, the external surfaces 222 of thin-walled tubes 202 may comprise bonding features (not shown). The bonding features may comprise protrusions from the external surfaces 222 of the thin-walled tubes 202. Additionally, or alternatively, the bonding features may comprise indentations or holes in the walls of the thin-walled tubes 202. The bonding features may be provided halfway along the lengths of the thin-walled tubes 202 and / or at the ends of the thin-walled tubes. Alternatively, the bonding features may be provided over the entirety of the external surfaces 222 of the thin-walled tubes 202.

[0058] The longitudinal axes of thin-walled tubes 202 are arranged parallel in the cluster 203. The longitudinal axes of the thin-walled tubes 202 are also arranged in a hexagonal lattice pattern in the cluster 203. The thin-walled tubes 202 do not make direct contact with each other in the cluster 203. In alternative embodiments, some of the thin-walled tubes 202 may make contact with each other in the cluster 203.

[0059] In alternative embodiments, a number of the thin-walled tubes 202 may be arranged in various orientations in the cluster 203. More specifically, the longitudinal axes of a number of the thin-walled tubes 202 may be arranged in different orientations to each other in the cluster 203. The cluster 203 may comprise thin-walled tubes 202 arranged in a lattice pattern such as a square, triangular, or circular lattice, for example.

[0060] The overmoulded material 204 is overmoulded to the external surfaces 222 of each of the thin-walled tubes 202 over the entirety of their lengths and is provided in the gaps between each of the thin-walled tubes in the cluster 203. The overmoulded material 204 encases the walls of thin-walled tubes 202, leaving openings at both ends of each of the tubes exposed. The length of the overmoulded material 204 in the axial direction is equal to that as each of the thinwalled tubes 202 in the cluster 203. As shown in Figure 2B, the overmoulded material 204 has an external profile that is hexagonal in shape. The structural element 200 therefore has the shape of a hexagonal prism.

[0061] In this embodiment, the overmoulded material 204 comprises a resin. In embodiments, the overmoulded material 204 may comprise a fibre-reinforced resin. The overmoulded material 204 may be different to the material of which the thin-walled tubes 202 are comprised. Alternatively, the overmoulded material 204 and the thin-walled tubes 202 may comprise the same material.

[0062] Figure 3A is a perspective view of a structural element 300 for a vehicle in an embodiment of the invention and Figure 3B is a top view. The structural element 300 comprises thin-walled tubes 302 arranged in parallel into a cluster 303. The thin-walled tubes 302 are arranged in the cluster 303 in a straight pattern side-to- side. Each of the thin-walled tubes comprises an internal surface 321 and an external surface 322. An overmoulded material 304 is overmoulded to the cluster 303 of thin-walled tubes 302. The structural element 300 also comprises attachment features 305 that are overmoulded to the exterior of the structural element 300.

[0063] The embodiment of Figures 3A and 3B is similar to that of Figures 2A and 2B, except that the thin-walled tubes 302 are arranged in the cluster 303 in a straight pattern and that the overmoulded material 304 has an external profile that is elongate and rectangular in shape. The structural element 300 therefore has the shape of an elongate rectangular prism. The structural element 300 also further comprises attachment features 305.

[0064] The thin-walled tubes 302 are straight elongate cylinders. Each of the thin-walled tubes 302 in the structural element 200 have the same dimensions in the axial, radial and azimuthal directions. Each of the thin-walled tubes 302 comprise an internal surface 321 , which is the internal surface of the wall of a tube, and an external surface 322, which is the external surface of the wall of the tube. In this embodiment, the thin-walled tubes 302 each comprise a carbon fibre-reinforced polymer.

[0065] The longitudinal axes of thin-walled tubes 302 are arranged in parallel in the cluster 303. The longitudinal axes of the thin-walled tubes 302 are also arranged in a straight pattern in the cluster 303 such that the tubes are placed side-to-side. The thin-walled tubes 302 do not make direct contact with each other in the cluster 303.

[0066] The overmoulded material 304 is overmoulded to the external surfaces 322 of each of the thin-walled tubes over the entirety of their lengths and is provided in the gaps between each of the thin-walled tubes in the cluster 303. The overmoulded material 304 encases the walls of thin-walled tubes 302, leaving both ends of each of the tubes exposed. The length of the overmoulded material 304 in the axial direction is the same as that as each of the thin-walled tubes 302 in the cluster 303. As shown in Figure 3B, the overmoulded material 304 has an external profile that is elongate and rectangular in shape. The structural element 200 therefore has the shape of an elongate rectangular prism. In this embodiment, the overmoulded material 304 comprises a resin.

[0067] The structural element 300 also comprises attachment features 305 that are overmoulded with the overmoulded material 304. The attachment features 305 enable the structural element 300 to be connected to attachment features located on similar structural elements and / or to attachment features in a vehicle. The attachment features 305 in this embodiment are flanges that may be fastened to other components. In alternative embodiments, the attachment features 305 may instead comprise fixing points and / or ducts. The attachment features 305 may comprise a number of different attachment features, as discussed herein.

[0068] Figure 4 is a perspective view of a structural element 400 for a vehicle in an embodiment of the invention. The structural element 400 comprises a plurality of thin-walled tubes 402. The plurality of thin-walled tubes 402 are arranged into a first cluster 431 , a second cluster 432 and a third cluster 433. Each of the thinwalled tubes comprises an internal surface 421 and an external surface 422. The longitudinal axes of the first, second and third 431 , 432, 433 clusters are each provided in different orientations. An overmoulded material 404 is overmoulded to the first, second and third 431 , 432, 433 clusters of thin-walled tubes 402 to form three structural sub-elements which each have a hexagonal prism shape. The three structural elements are connected to form the modular structural element 400.

[0069] The thin-walled tubes 402 in the structural element 400 are straight elongate cylinders. Each of the thin-walled tubes 402 comprise an internal surface 421 , which is the internal surface of the wall of a tube, and an external surface 422, which is the external surface of the wall of the tube. Each of the thin-walled tubes 402 in the structural element 400 have the same dimensions in the radial and azimuthal directions. The thin-walled tubes 402 in the first cluster 431 have the same length, or the same dimensions in the axial direction. The thin-walled tubes 402 in the second and third 432, 433 clusters have different lengths, however. In this embodiment, the thin-walled tubes 402 each comprise a carbon fibre- reinforced polymer.

[0070] Within each of the first, second and third clusters 431 , 432 and 433 the longitudinal axes of thin-walled tubes 402 are arranged in parallel. The longitudinal axes of each of the clusters 431 , 432 and 433 are not in parallel with each other in the structural element 400, however. The longitudinal axis of the first cluster 431 is approximately offset by 30 degrees to the longitudinal axes of the second and third clusters 432, 433. The longitudinal axes of the second and third clusters are approximately offset by 60 degrees. In alternative embodiments, the longitudinal axis of one cluster may be offset by 15 to 90 degrees to the longitudinal axis of another cluster. In the first, second and third clusters 431 , 432 and 433 the longitudinal axes of the thin-walled tubes 402 are arranged in a hexagonal lattice pattern in the cluster 403 and the thin-walled tubes 402 do not make direct contact with each other in each of the clusters 403.

[0071] In this embodiment, the overmoulded material 404 is overmoulded to the external surfaces 422 of each of the thin-walled tubes in the first, second and third clusters 431 , 432 and 433 over the entirety of their lengths and is provided within the gaps between each of the thin-walled tubes 402. The overmoulded material 404 encases the walls of the thin-walled tubes in the first, second and third clusters 431 , 432 and 433. The length of the overmoulded material 404 is the same as that as each of the thin-walled tubes in the first cluster 431 .

[0072] The longitudinal axis of the overmoulded material 404 is provided in a different orientation to that of the longitudinal axes of the second and third 432, 433 clusters. Some of the thin-walled tubes 402 in the second and third 432, 433 clusters extend from one of the ends of the overmoulded material 404 to the other end and some of the thin-walled tubes 402 in the second and third 432, 433 clusters extend from one of the ends of the overmoulded material 404 to a side of the overmoulded material.

[0073] As shown in Figure 4, the overmoulded material 404 encasing each of the first, second and third clusters 431 , 432 and 433 has an external profile that is hexagonal in shape. The structural element 400 is therefore a modular structure that comprises three sub-elements, each having a hexagonal prism shape, arranged in a honeycomb-like pattern.

[0074] In this embodiment, the modular structural element 400 is formed by connecting the individual structural elements, corresponding to the first, second and third clusters 431 , 432 and 433, together. This is achieved using attachment (not shown) features similar to that as shown in the embodiments of Figures 3A and 3B. In alternative embodiments, the modular structural element 400 is formed by overmoulding the overmoulded material 404 around the first, second and third clusters 431 , 432 and 433, wherein the first, second and third clusters 431 , 432 and 433 are all provided within an overmoulded mould in different orientations.

[0075] Figure 5 is a perspective view of a structural element 500 for a vehicle in an embodiment of the invention. The structural element 500 comprises a plurality of thin-walled tubes 502 arranged into a cluster 503. The longitudinal axes of the thin-walled tubes 502 are provided in different orientations in the cluster 503. Each of the thin-walled tubes comprises an internal surface 521 and an external surface 522. An overmoulded material 504 is overmoulded to the cluster 503 of thin-walled tubes 502.

[0076] The thin-walled tubes 502 in the structural element 500 are straight elongate cylinders. Each of the thin-walled tubes 502 comprise an internal surface 521 , which is the internal surface of the wall of a tube, and an external surface 522, which is the external surface of the wall of the tube. Each of the thin-walled tubes 502 in the structural element 500 have the same dimensions in the radial and azimuthal directions. The lengths of the thin-walled tubes in the cluster 503 vary, however. In this embodiment, the thin-walled tubes 502 each comprise a carbon fibre-reinforced polymer.

[0077] In the cluster 503, the longitudinal axes of the thin-walled tubes 502 are not all provided in parallel with one another. Some of the longitudinal axes of the tubes 502 are provided in different orientations to the others. However, a proportion of the total number of tubes 502 do have their longitudinal axes provided in parallel. At an end of the structural element 500 the thin-walled tubes are arranged in a circular pattern. The thin-walled tubes 502 do not make direct contact with each other in the cluster 503.

[0078] The overmoulded material 504 is overmoulded to the external surfaces 522 of each of the thin-walled tubes over the entirety of their lengths and is provided in the gaps between each of the thin-walled tubes in the cluster 503. The overmoulded material 504 encases the walls of thin-walled tubes 502, leaving both ends of each of the tubes exposed. Some of the thin-walled tubes 502 in the cluster 503 extend from one of the ends of the overmoulded material 504 to the other end and some of the thin-walled tubes 502 in the cluster 503 extend from one of the ends of the overmoulded material 504 to a side of the overmoulded material 504. The overmoulded material 504 has an external profile that is hexagonal in shape. The structural element 500 therefore has the shape of a hexagonal prism.

[0079] In the embodiments described herein, the thin-walled tubes each comprise a carbon fibre-reinforced polymer. In alternative embodiments, the thin-walled tubes may comprise a different material such as aluminium, other fibre-reinforced polymers or steel. In further alternative embodiments, some of the thin-walled tubes in a structural element may comprise a different material to the other thinwalled tubes in the structural element. For example, half of the total number of tubes may comprise aluminium and the other half may comprise a carbon fibre- reinforced polymer.

[0080] In the embodiments described herein, the thin-walled tubes are elongate and cylindrical in shape. In alternative embodiments, the thin-walled tubes may comprise an ovular prism, a square prism, a rectangular prism, or any regular or irregular polygonal prism. The edges of the thin-walled tubes may be sharp or rounded in embodiments. In further alternative embodiments, some of the thinwalled tubes in a structural element may comprise a different shape to the other thin-walled tubes in the structural element. For example, half of the total number of tubes may comprise a cylindrical shape and the other half may comprise a square prism shape.

[0081] In alternative embodiments to those described herein, the thin-walled tubes may comprise bonding features to increase the surface area of contact between the external surfaces of the thin-walled tubes and the overmoulded material. As the surface area of contact between the external surfaces of the thin-walled tubes and the overmoulded material is increased, the strength of the bond between the two is also increased, thereby increasing the strength of the structural element. In the structural element, the thin-walled tubes and the overmoulded material interlock with each other. In certain alternative embodiments, the bonding features may comprise protrusions, such as a lug or flange, from the external surfaces of the thin-walled tubes. In other alternative embodiments, the bonding features may comprise an indentation or hole in the wall of the thin-walled tubes. In other alternative embodiments, the external surfaces of the thin-walled tubes may be textured thereby providing bonding features. The texturing of the external surfaces may comprise grooves, channels or crenelations. The bonding features may be provided halfway along the lengths of the thin-walled tubes and / or at the ends of the thin-walled tubes. Alternatively, the bonding features may be provided over the entirety of the external surfaces of the thin-walled tubes.

[0082] In the embodiments described herein embodiment, the overmoulded material comprises a resin. In alternative embodiments, the overmoulded material may comprise a fibre-reinforced resin. The overmoulded material may be different to the material of which the thin-walled tubes are comprised. Alternatively, the overmoulded material and the thin-walled tubes may comprise the same material.

Claims

CLAIMS1 . A method for forming a structural element for a vehicle, comprising: arranging a plurality of thin-walled tubes into a cluster, the thin-walled tubes having internal surfaces and external surfaces; providing support to the internal surfaces of the thin-walled tubes; and overmoulding a material to at least a portion of the external surfaces of the thin-walled tubes thereby to bond the cluster together.

2. A method according to claim 1 , wherein providing support to the internal surfaces of the thin-walled tubes comprises providing a mandrel within each of the thin-walled tubes.

3. A method according to claim 1 or claim 2, wherein providing support to the internal surfaces of the thin-walled tubes comprises providing a fluid within each of the thin-walled tubes.

4. A method according to claim 3, wherein the fluid is a high-pressure gas.

5. A method according to any preceding claim, wherein the longitudinal axes of the thin-walled tubes are arranged in parallel to one another in the cluster.

6. A method according to any of claims 1 to 4, wherein the longitudinal axes of two or more thin-walled tubes are arranged in different orientations to one another in the cluster.

7. A method according to any preceding claim, further comprising: arranging a plurality of thin-walled tubes into a first cluster and a second cluster, wherein the longitudinal axes of the first cluster and the second cluster are arranged in different orientations to one another.

8. A method according to any preceding claim, wherein the thin-walled tubes are arranged in a lattice pattern in the cluster.

9. A method according to any preceding claim, wherein the plurality of thinwalled tubes are arranged in the cluster such that a gap is provided between two or more of the thin-walled tubes between their respective external surfaces.

10. A method according to any of claims 1 to 8, wherein the plurality of thinwalled tubes are arranged in the cluster such that two or more of the thin-walled tubes are in contact with each other along at least a portion of their respective external surfaces.

11. A method according to any preceding claims, wherein one or more of the thin-walled tubes in the cluster comprise different lengths, as measured in the direction of the longitudinal axes of the thin-walled tubes.

12. A method according to any preceding claim, wherein overmoulding a material to at least a portion of the external surfaces of the thin-walled tubes comprises overmoulding an attachment feature to enable assembly of the structural element to a corresponding attachment feature on the vehicle.

13. A method according to any preceding claim, wherein overmoulding a material to at least a portion of the external surfaces of the thin-walled tubes comprises overmoulding the material to a bonding feature of the external surface of at least one of the thin-walled tubes in the cluster.

14. A method according to any preceding claim, wherein the plurality of thinwalled tubes are arranged into a cluster and the cluster is then inserted into an overmoulding mould.

15. A method according to any of claims 1 to 13, wherein the plurality of thinwalled tubes are arranged into a cluster within the overmoulding mould.

16. A method according to claim 15, wherein the plurality of thin-walled tubes are arranged into a cluster using locating features provided within the overmoulding mould.

17. A method according to any preceding claim, wherein the thin-walled tubes and the overmoulding material comprise different materials.

18. A method according to any preceding claim, wherein the thin-walled tubes comprise fibre-reinforced plastic.

19. A method according to any preceding claim, wherein overmoulding a material to at least a portion of the external surfaces of the thin-walled tubes comprises injection moulding fibre-reinforced or unreinforced resins, or compression moulding fibre-reinforced compounds.

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