An impact energy absorber for a vehicle

Abstract

An impact energy absorber 200 for a vehicle, for example a floor, roof or door panel, has a lattice structure with a plurality of hollow, elongate cells 206 with cell walls 202 having an elongatiion a

Description

FIELD OF THE INVENTION The present invention relates to impact energy absorbers for vehicles. BACKGROUND Commercially available vehicles, such as cars, typically comprise crumple zones to absorb impact energies in collision events and protect passengers. Impact energy absorbers can additionally be provided within commercial vehicles to further protect passengers. Such impact energy absorbers may be provided in the floor of a vehicle, in door panels or in the roof, for example. Known impact energy absorbers have recently been formed using injection moulding (IM) techniques and additive manufacturing techniques. The need to provide shapes that can be formed by these techniques has led to the production of impact energy absorbers formed of a lattice of cells, which cells may be layered up in additive manufacture or removed from an injection moulding tool. One problem with impact energy absorbers of this shape is that, in the event of an impact, as the impact progresses, the material forming the lattice of cells tends to concertina, causing a build-up of material within the cells, eventually packing into a solid that can abruptly increase the g-force response and reduce the usable length of the impact energy absorber. It is desirable to provide impact energy absorbers that do not suffer from the above problem. SUMMARY OF INVENTION In accordance with a first aspect of the invention, there is provided an impact energy absorber for a vehicle, comprising: a lattice structure comprising a plurality of hollow elongate cells defined by one or more cell walls formed of a material having an elongation at break of no more than 20%. Elongation at break, also referred to as elongation at fracture or elongation after fracture, is determined for metallic materials in accordance with international standard ISO 6892-1:2019, and for plastics and plastic composites in accordance with the general principles of international standard ISO 527-1:2019, and for moulding and extrusion plastics in accordance with ISO 527-2:2012, for isotropic and orthotropic fibre-reinforced plastic composites in accordance with ISO 527-4:2023 and for unidirectional fibre-reinforced plastic composites in accordance with ISO 527-5:2021. It will be appreciated that tensile properties are well understood and for any other suitable material, the elongation at break may be determined in accordance with the relevant standards. In accordance with the above standards, determination of elongation at break of a material for the impact energy absorber may be performed by machining a test specimen from the impact energy absorber, where dimensions and material allow, but is preferably performed by manufacturing a test specimen using the same material and manufacturing process that is used to form the impact energy absorber and testing the specimen in near-net-shape condition. Manufacturing processes that result in anisotropic mechanical behaviour, such as additive manufacture or using unidirectional fibre-reinforcements, should have the test specimen machined or formed so that orientation of the tensile testing measures the tensile properties along the elongate direction of the hollow elongate cells, and a material should be selected based on its elongation at break along this direction. A preferred test piece may have Form C or Form D as set out in DIN 50125, and may preferably have dimensions C6x30 or D6x30. It has been found that by forming the lattice structure of a relatively brittle material, i.e. having an elongation at break of no more than 20%, then in the event of an impact, the cell walls tend to break and disperse away from the remaining lattice structure, rather than concertinaing and compacting. This has been found to improve the g-force response and increase the usable length of the impact energy absorber. Preferably, the one or more cell walls are formed of a material having an elongation at break of no more than 15%, preferably no more than 12%, more preferably no more than 10%, more preferably no more than 9%, more preferably no more than 8%, most preferably no more than 7%. A value of about 7% has been found to be particularly suitable for impact energy absorbers. However, it has also been found that the material should not be too brittle, or else the ability of the material to absorb an impact is reduced. Preferably, the one or more cell walls are formed of a material having an elongation at break in the range 20% to 1%, preferably in the range 15% to 2%, more preferably in the range 12% to 3%, more preferably in the range 10% to 4%, more preferably in the range 9% to 5%, most preferably about 7%. Another important tensile property of the material is the ultimate tensile strength, and preferably the one or more cell walls are formed of a material having an ultimate tensile strength of at least 100 MPa, preferably at least 150 MPa, more preferably at least 200 MPa, more preferably at least 250 MPa, most preferably at least 300 MPa. Again, these values may be measured in accordance with the same standards referenced above. By using a brittle material with relatively high ultimate tensile strength to form hollow elongate cells, the structure is able to provide a good combination of deformation under impact that effectively absorbs energy, whilst allowing the deformed material to escape the impact energy absorber to avoid interfering with the remaining portion of the impact energy absorber. The one or more cell walls of the impact energy absorber should preferably be formed of a metallic material (e.g. formed of a metal or alloy thereof), a plastic, or a plastic composite, preferably a moulding or extrusion plastic, an isotropic or orthotropic fibre-reinforced plastic composite, or a unidirectional fibre-reinforced plastic composite. Whereas many known impact energy absorbers formed of hollow elongate cells have been formed of plastics, it has been found that the required brittleness, in combination with other preferred tensile properties, can be preferably provided by one or more cell walls formed of a metallic material, preferably an aluminium alloy, most preferably an alloy of aluminium, silicon and magnesium. In particular, AISiWMg has been found to have especially good tensile properties for operating in the manner described above. Each hollow elongate cell will typically comprise a plurality of cell walls, e.g. connected at a vertex. However, in some embodiments, each hollow elongate cell may have only one wall, e.g. cylindrical cells, and in some embodiments, the lattice may comprise only one cell wall, e.g. a continuous sinusoidal wall that snakes back and forth across the lattice structure. Preferably, the one or more cell walls have a minimum thickness of no more than 2 mm, preferably no more than 1.5 mm, more preferably no more than 1 mm, more preferably no more than 0.5 mm, most preferably no more than 0.3 mm at least at one end of the hollow elongate cells. Preferably, each of the plurality of hollow elongate cells has at least one cell wall with a minimum thickness as stated above at one end of the hollow elongate cells (i.e. the same end for each hollow elongate cell). Thin-walled cells, at least at one end of the lattice structure, allow for small pieces of the brittle material to break away during an impact, which are more readily able to move away from the impact point and minimising a compacting of material at the impact point. Preferably, the mean wall thickness of the lattice structure, measured in cross-section in at least one plane perpendicular to the elongate direction of the hollow elongate cells, is no more than 2 mm, preferably no more than 1.5 mm, more preferably no more than 1 mm, more preferably no more than 0.5 mm, most preferably no more than 0.3 mm. Preferably, the mean wall thickness is maintained within the above ranges over the full length of the lattice structure along the elongate direction of the hollow elongate cells. However, alternatively, the above mean wall thickness may be maintained along at least 50%, or 60%, or 70%, or 80%, or 90%, of the elongate direction of the hollow elongate cells, preferably along a continuous at least 50%, or 60%, or 70%, or 80%, or 90%. Providing that a large proportion of the lattice structure has thin walls along the elongate direction of the cells ensures good performance of the crash structure. In most embodiments, the cell walls have constant wall thickness, such that the mean and minimum wall thickness are the same. It has been found that the high brittleness of the material achieves especially good performance with low wall thicknesses. Particularly good combinations include: an elongation at break of no more than 15% and minimum or mean cell wall thickness of no more than 2 mm; an elongation at break of no more than 15% and minimum or mean cell wall thickness of no more than 1.5 mm; an elongation at break of no more than 12% and minimum or mean cell wall thickness of no more than 1.5 mm; an elongation at break of no more than 12% and minimum or mean cell wall thickness of no more than 1 mm; an elongation at break of no more than 10% and minimum or mean cell wall thickness of no more than 1 mm; an elongation at break of no more than 10% and minimum or mean cell wall thickness of no more than 0.5 mm; and an elongation at break of no more than 8% and minimum or mean cell wall thickness of no more than 0.5 mm. While it is preferred for the cell walls to be thin, at least at one end, the one or more cell walls may increase in thickness from a first end of the hollow elongate cells to a second end of the hollow elongate cells. This allows for control of the g-force response of the impact energy absorber. Preferably, the plurality of hollow elongate cells form part of one or more hollows extending through the full length of the impact energy absorber along the elongate direction. Preferably, one or more, preferably each, of the plurality of hollow elongate cells defines a respective hollow extending along its full length between opposing open ends of the hollow elongate cell. By providing these hollows through the impact energy absorber, this more readily allows material that breaks away from the lattice structure to escape the lattice structure during an impact, improving the performance of the impact energy absorber. The hollows of the elongate cells may open into other hollows of the impact energy absorber, i.e. the elongate cells themselves do not need to extend along the full length of the impact energy absorber. Preferably, the impact energy absorber comprises at least 10, preferably at least 20, preferably at least 50 hollow elongate cells. The number of hollow elongate cells may be determined in accordance with the necessary shape and size of the impact energy absorber for a particular region of a vehicle. Preferably, the hollow elongate cells define a regular lattice structure. In most embodiments, one or more of the hollow elongate cells, preferably each hollow elongate cell, has a largest dimension perpendicular to its elongate direction of between 1 mm and 50 mm, preferably between 2 mm and 20 mm, preferably between 3 mm and 10 mm. Using relatively small hollow elongate cells allows for a relatively high density of material to be achieved with thin cell walls, which produce small pieces of material that may more easily escape the point of impact, rather than contributing to material build up. It is preferred that the hollow of one or more hollow elongate cells, preferably each hollow elongate cell, makes up at least 50% of the volume of the hollow elongate cell, preferably at least 60%, more preferably at least 70%, more preferably at least 80%, most preferably at least 90%. In this way, the relatively large hollow provides adequate space for broken pieces of the impact absorber to escape in the event of an impact. Preferably, the lattice structure has an areal density in the range 25% to 1%, preferably in the range 20% to 2%, more preferably in the range 15% to 3%, more preferably in the range 12% to 4%, most preferably in the range 10% to 5%, typically about 7%. The areal density will be understood to be the proportion of the area of the lattice structure comprising cell walls as opposed to hollows, measured in cross-section in a plane perpendicular to the elongate direction of the hollow elongate cells. This value must be balanced, as a density that is too high will cause the material to offer too much resistance to a crash and compact too quickly, worsening the g-force response, while a density that is too low will provide insufficient material to resist the impact. Preferably the above areal density is maintained over the full length of the lattice structure along the elongate direction of the hollow elongate cells. However, alternatively, the above areal density may be maintained along at least 50%, or 60%, or 70%, or 80%, or 90%, of the elongate direction of the hollow elongate cells, preferably along a continuous at least 50%, or 60%, or 70%, or 80%, or 90%. For example, the areal density may start below this range on an impact receiving side of the crash structure and may end above this range on the opposite side. In some embodiments, it may be preferred for the areal density to increase along the elongate direction of the hollow elongate cells, preferably while remaining within the above ranges along a continuous at least 50%, or 60%, or 70%, or 80%, or 90%, of the elongate direction of the hollow elongate cells. It will be appreciated that areal density may be controlled independently of cell wall thickness by setting a suitable cell width, i.e. smaller cells for a fixed cell wall thickness will increase the areal density. The areal densities specified above are particularly preferred in combination with small cell wall thicknesses. For example, good combinations include: an elongation at break of no more than 15%, minimum or mean cell wall thickness of no more than 2 mm and areal density in the range 20% to 2%; an elongation at break of no more than 15%, minimum or mean cell wall thickness of no more than 1.5 mm and areal density in the range 15% to 3%; an elongation at break of no more than 12%, minimum or mean cell wall thickness of no more than 1.5 mm and areal density in the range 15% to 3%; an elongation at break of no more than 12%, minimum or mean cell wall thickness of no more than 1 mm and areal density in the range 12% to 4%; an elongation at break of no more than 10%, minimum or mean cell wall thickness of no more than 1 mm and areal density in the range 12% to 4%; an elongation at break of no more than 10%, minimum or mean cell wall thickness of no more than 0.5 mm and areal density in the range 10% to 5%; and an elongation at break of no more than 8%, minimum or mean cell wall thickness of no more than 0.5 mm and an areal density in the range 10% to 5%. The g-force response of the impact energy absorber may also be tuned by controlling the density. Therefore, preferably, the lattice structure has a density that varies along the elongate direction of the hollow elongate cells. While it would be possible to form each hollow elongate cell individually and arrange them into a lattice structure, preferably, the one or more cell walls are integrally formed. For example, integral cell walls may be formed when the hollow elongate cells are formed together by additive manufacturing, or by moulding. The integral arrangement further improves the strength and predictability of the structure. Preferably, each hollow elongate cell shares at least one cell wall or vertex with one or more adjacent hollow elongate cells. While preferred, it would also be possible to have each cell spaced from its neighbours, e.g. held together by some other retaining element. Particularly preferably, the hollow elongate cells have a hexagonal cross-sectional shape. Such a shape makes efficient use of the material forming the cells and helps maximise the space within the cells through which the broken pieces of the cells may escape. While preferred, other arrangements are also envisaged, such as triangular or square cells. Many embodiments further comprise one or more external walls surrounding the lattice structure. This external wall may contain, support and / or protect the cell walls defining the lattice structure. Preferably, the one or more external walls are thicker than the one or more cell walls. This may protect the cell walls from damage outside of an impact. Like the cell walls, the one or more external walls may increase in thickness from a first end of the hollow elongate cells to a second end of the hollow elongate cells, typically in the same direction as an increase in the thickness of the cell walls, as noted above. The one or more external walls are preferably formed of the same material as the one or more cell walls, as described above. In particular, the external wall is preferable the same metallic material (e.g. metal or alloy thereof), plastic, plastic composite, preferably a moulding or extrusion plastic, an isotropic or orthotropic fibre-reinforced plastic composite, or a unidirectional fibre-reinforced plastic composite. In particular, the external wall is preferably an aluminium alloy, most preferably an alloy of aluminium, silicon and magnesium, especially AISiWMg. Typically, the one or more external walls are integral with the one or more cell walls. Again, this may improve the strength of the impact energy absorber. The hollow elongate cells and the one or more external walls may be formed together by additive manufacturing, or by moulding. Whereas the shape of the hollow elongate cells is typically dictated mainly by the impact response, the shape of the external wall may be dependent on the required shape of the impact energy absorber, e.g. for packaging the impact energy absorber within a particular volume of a vehicle, or on aerodynamic requirements if a surface of the impact energy absorber will be on an exterior of the vehicle, or on a required appearance. Therefore, typically, the one or more external walls have a different shape to the one or more cell walls. For example, whereas the cell walls may define a regular hexagonal lattice structure, the external wall may define a shape not contained within this regular hexagonal lattice structure. In some examples, the one or more cell walls are substantially straight along the elongate direction of the cells, whereas the one or more of the external walls are shaped along the elongate direction of the cells. This may provide a variation in the shape or cross-sectional area of the impact energy absorber along the elongate direction of the cells, while the lattice structure remains substantially homogeneous across the impact energy absorber. It may be preferred that the cross-sectional area of the lattice structure varies along the elongate direction of the cells. This may tune an impact response of the impact energy absorber as an impactor gradually meets more lattice structure through the course of the impact. In these embodiments, the cross-sectional area of the lattice structure is varied (e.g. without varying the shape of the hollow elongate cells) by providing one or more hollow elongate cells that start or terminate part way along the lattice structure along the elongate direction of the cells. In this way, the cross-sectional shape and size of each hollow elongate cell may remain substantially constant along the elongate direction of the cells, i.e. ignoring any variation in shape due to changing cell wall thickness and ignoring any tapering off of cells that may start or terminate part way along the lattice structure. In other embodiments, the shape of the lattice structure varies along the elongate direction of the cells, which may allow the lattice structure to be packaged within a specified area of a vehicle. For further improved strength, preferably, the impact energy absorber comprises one or more fixing points formed integrally with the one or more cell walls. A fixing point may be a point configured to receive a mechanical fastener, such as a blind or through hole. Integrated fixing points may be formed conveniently when the cell walls are formed by additive manufacturing, for example. Preferably, the impact energy absorber further comprises a hollow backing structure located at one end of the lattice structure along the elongate direction of the hollow elongate cells, wherein the hollow backing structure comprises internal and / or external walls that are thicker than one or more cell walls, measured along the same direction as the thickness of the cell walls. In this way, the hollow backing structure may have improved strength and can withstand crash loads without failing. Preferably, the hollow backing structure defines one or more hollows extending between an end of one or more of the hollow elongate cells and an end of the impact energy absorber. The hollow backing structure may therefore define openings in one end of the impact energy absorber. Preferably, the hollow backing structure has a length that is at least 10% of the length of the longest hollow elongate cell, preferably at least 20%, preferably at least 30%. Preferably, one or more fixing points are located in the hollow backing structure, spaced from the lattice structure along the elongate direction of the hollow elongate cells. The hollow backing structure due to its improved strength, represents a good location for fixing points, and this prevents failure of the fixing point in an impact. As noted above, preferably, the one or more cell walls are formed by additive manufacturing, although they may also be formed by moulding processes, such as injection moulding. In accordance with a second aspect of the invention, there is provided a vehicle comprising one or more impact energy absorbers according to the first aspect of the invention. All of the optional features described above may be implemented in this aspect. In accordance with a third aspect of the invention, there is provided a method of manufacturing an impact energy absorber for a vehicle, comprising: forming a lattice structure comprising a plurality of hollow elongate cells defined by one or more cell walls formed of a material having an elongation at break of no more than 20%. This corresponds to a method of manufacturing the impact energy absorber according to the first aspect of the invention, and so all of the comments above and preferred features apply equally to this aspect. Preferably, the present method comprises comprising forming the lattice structure by additive manufacturing, preferably by laser powder bed fusion. In particular, the one or more cell walls may be formed by additive manufacturing, allowing the hollow elongate cells to be conveniently formed. BRIEF DESCRIPTION OF DRAWINGS Embodiments of the invention are now described, by way of example, by reference to the drawings, in which: Figure 1 depicts the implementation of an impact energy absorber in a vehicle in an embodiment of the invention; Figure 2A is a perspective view of an impact energy absorber in an embodiment of the invention; Figure 2B is a plan view of the impact energy absorber in Figure 2A; Figure 3 is a perspective plan view of an impact energy absorber in an embodiment of the invention; Figure 4 is a cutaway perspective view of the impact energy absorber in Figure 3; Figure 5 is a rear view of the impact energy absorber in Figure 3; Figure 6A is a cutaway perspective view of an impact energy absorber in another embodiment of the invention; and Figure 6B is a plan view of the impact energy absorber in Figure 6A; DETAILED DESCRIPTION Figure 1 is a perspective view of a portion of a vehicle 1 showing an example implementation of an impact energy absorber 100. In this scenario the impact energy absorber 100 is provided beneath a floor panel 2 of the vehicle 1 and above the chassis of the vehicle. The impact energy absorber 100 extends along the length of the vehicle 1 from the front wheel arches 3 to the rear wheel arches 4 and also extends across the width of the vehicle 1 from the left set door frames 5 to the right set of door frames 6. The impact energy absorber 100 is generally elongate and cuboidal in shape. In this example, the impact energy absorber 100 is provided within the floor of the vehicle 1; however, the impact energy absorber 100 may be provided within the roof 7 of the vehicle body and / or in door panels (not shown) and / or in other areas of the vehicle 1 as may be desired by a vehicle designer to absorb impacts from crashes and collisions. Figure 2A is a perspective view of an impact energy absorber 200 in an embodiment of the invention, and Figure 2B is a plan view. The impact energy absorber 200 comprises an array of cell walls 202 which defines a lattice structure 204. The lattice structure 204 comprises a plurality of hollow elongate cells 206. The elongate cells 206 in this example are hollow hexagonal prisms and are arranged in a hexagonal lattice pattern, thereby forming the lattice structure 204. The cell walls 202 and the hollow elongate cells 206 extend from a backplate 208 which is integral with the rest of the structure to form a base. The hollow of the hollow elongate cells 206 extends through the backplate 210, such that the hollow extends right through the impact energy absorber along the elongate direction of the cells 206. The impact energy absorber 200 also comprises a plurality of tapered portions 210 that are tapered from the backplate 208 to the opposite ends of the hollow elongate cells 206. The tapered portion 210, the backplate 208 and the cells 206 defined by the cell walls 202 form a single, strong, unitary component in this particular embodiment. The impact energy absorber 200 has a generally elongate cuboidal shape. In this embodiment, the cell walls have a uniform thickness, at least at a certain level of resolution. In this particular embodiment the hexagonal elongate cells 206 are arranged hexagonally in the lattice structure 204, forming a hexagonal lattice similar to a honeycomb, which is an inherently mechanically strong structure. In alternative embodiments, the repeating pattern in the lattice structure 204 may be circular, square, rectangular or irregular. The elongate cells 206 have an interior cross-sectional shape that is hexagonal. The elongate cells 206 extend perpendicularly outwardly from the surface of the backplate 208. In this way, the longitudinal axis of the elongate cells 206 is provided in an orientation that is normal to the plane defined by the surface of the backplate 208. In alternative embodiments, the longitudinal axis of the elongate cells 206 may be provided in an orientation offset from the normal to the plane defined by the surface of the backplate 208. For example, the longitudinal axis of the elongate cells 206 may be orientated at 85 degrees from the plane defined by the surface of the backplate 208. The impact energy absorber 200 may be particularly effective at absorbing collision energy in a particular direction that is aligned with the longitudinal axis of the elongate cells 206. By facilitating a design with which cells can be orientated at different angles to the backplate 208 the flexibility for integrating the impact energy absorber into a vehicle at different positions is improved. The backplate 208 defines a base of the impact energy absorber 200. In this particular embodiment the backplate extends outwards away from the base of the lattice structure 204. The backplate 208 provides a surface that is suitable for attachment to corresponding surfaces in a vehicle where the impact energy absorber 200 is to be installed. The plurality of tapered portions 210 are tapered ribs provided at the exterior of the lattice structure. More specifically, the plurality of tapered portions 210 are arranged in parallel and adjacent one another along one side lattice structure. In this embodiment, the tapered portions are provided along one of the shorter sides of the lattice structure. The tapered portions 210 are formed integral with the cell walls 202 and the backplate 208 to provide structural support between the elongate cells 206 and the backplate 208 when the impact energy absorber 200 is in use. In this embodiment, one tapered portion 210 is provided for each elongate cell 206 provided at the edge of the lattice structure 204 along the one side of the wall 202. In alternative embodiments, greater or fewer tapered portions 210 may be provided, and tapered portions may be provided in different positions around the lattice structure. The impact energy absorber 200 can be formed by additive manufacture using laser powder bed fusion with a powder of AISilOMg. This material has an elongation at break along the elongate direction of the cells of approximately 7%. The absorber 200 may be built up by additive manufacturing starting with the base plate 208 and the base of the cell walls 202 defining the lattice structure, successively depositing layers of material to build up the lattice structure along the elongate direction of the hollow elongate cells. Figures 3 to 5 show another impact energy absorber 300. This energy absorber includes a lattice structure 304 at a front end of the impact energy absorber and a hollow backing structure 314 at a rear end of the impact energy absorber. The lattice structure again comprises an array of hollow hexagonal cells 306 formed by cell walls 302. The cell walls 302 are approximately 0.3 mm thick in the direction perpendicular to the elongate direction, as can be seen in Figure 3, where the cell wall thickness is revealed at the front end of the impact energy absorber. This cell wall thickness remains constant along the elongate direction of the cells As can be seen in Figures 4 and 5, the cell walls meet an angled supporting wall, which extends away from the internal wall 320. The angled supporting wall has a series of circular openings therethrough, opening into the rear end of each hollow elongate cell, such that each cell is in fluid communication with the rear end of the crash structure Each cell is approximately 8.5mm across, measured from flat wall to parallel flat wall of the hexagonal cell structure. The areal density of this lattice structure is approximately 7%. In this embodiment, the impact energy absorber 300 further comprises an external wall 312 surrounding the lattice structure 304. The shape of this external wall 312 is independent of the shape of the lattice structure. As a result, as shown in Figure 3, some of the cells are only partial cells where they are directly adjacent to the external wall 312. Figure 4 shows most clearly that individual cells start at different positions along the elongate direction of the lattice structure 304, with the cells tapering off as the external wall defines the outer shape of the impact energy absorber. The outer wall has a shape that includes a curved surface, which represent packaging or aerodynamic requirements of the vehicle to which this impact energy absorber will be fitted. The outer wall 312 has a greater thickness than the cell walls, being approximately 1 mm thick at the front end of the impact energy absorber 300. The hollow backing structure 314 is most clearly shown in Figures 4 and 5. This comprises a region of the impact energy absorber that does not include the lattice structure 304. Instead, this region is characterised by a much thicker section of the external wall 312, e.g. approximately 2.5 mm thick at the rear end of the impact energy absorber. The hollow backing structure 314 also comprises internal walls 320 defining four internal hollows 318 inside of the hollow backing structure. These four internal hollows 318 each open through the rear surface of the impact energy absorber. As a result, the impact energy absorber comprises a number of hollows extending all the way from the front of the impact energy absorber, where one end of the hollow elongate cells 306 opens through the front, to the rear of the impact energy absorber, where the four internal hollows 318 open through the rear surface. As a result of this arrangement, in the event of an impact, broken pieces of material can travel right along the impact energy absorber and out of the rear surface. This also relieves air pressure in the event of an impact, which further improves performance. The hollow backing structure 314 further comprises in the external wall 312 four fixing points 316. Each of these fixing points is a through hole extending substantially along the elongate direction, configured to receive a bolt for fixing the impact energy absorber 300 in place in a vehicle. Each through hole is formed in a recess into the external wall 312 in order to keep the footprint of the impact energy absorber 300 compact. As with the preceding embodiment, this impact energy absorber 300 is formed by additive manufacture using laser powder bed fusion with a powder of AISilOMg. The impact energy absorber 300 may be built up, starting with the rear surface. As shown in Figure 4, the rear end of the lattice structure tapers to a line, which meets the internal wall 320 in the hollow backing structure 314. This allows the the internal wall 320 to be printed first, from which the supporting wall behind the lattice structure may be printed, and then the lattice structure may be built up from this supporting wall, without forming any temporary internal supports. Figure 6A is a perspective view of an impact energy absorber 600 in an embodiment of the invention and Figure 6B is a plan view. As shown in Figure 6B, the energy absorber 600 comprises a single continuous cell wall 602 having a sinusoidal profile. The single cell wall 602 extends back and forth along the impact energy absorber 600 so as to form a plurality of rows of the sinusoidal cell wall defining a lattice structure 604. The lattice structure 604 comprises a plurality of hollow elongate cells 606 which each comprise a cross-sectional shape that is eye-shaped, bounded by two sinusoidal curves, above and below. Each pair of rows of the cell wall 602 comprises a row with a sine-wave profile and a row with a cosine-wave profile provided adjacent one another, thereby defining the eye-shaped hollow elongate cells 606 therebetween. The cell wall 602 and hollow elongate cells 606 extend from a backplate 608 which forms a base of the energy absorber 600. Due to the continuous pattern of the lattice structure 604, the production of the impact energy absorber 600 can be carried out with high efficiency as the material can be deposited continuously using additive manufacture. In other words, there is no need to stop and start the deposition process, which improves the manufacturing efficiency. Various features of the embodiments described herein may combined in various ways to produce an impact energy absorber falling within the scope of the invention. The embodiments described herein are exemplary and many features of which are compatible with each other.

Claims

1. An impact energy absorber for a vehicle, comprising:a lattice structure comprising a plurality of hollow elongate cells defined by one or more cell walls formed of a material having an elongation at break of no more than 20%.

2. An impact energy absorber according to claim 1, wherein the one or more cell walls are formed of a material having an elongation at break of no more than 15%, preferably no more than 12%, more preferably no more than 10%, more preferably no more than 9%, more preferably no more than 8%, most preferably no more than 7%.

3. An impact energy absorber according to claim 1 or claim 2, wherein the one or more cell walls are formed of a material having an elongation at break in the range 20% to 1%, preferably in the range 15% to 2%, more preferably in the range 12% to 3%, more preferably in the range 10% to 4%, more preferably in the range 9% to 5%4. An impact energy absorber according to any of the preceding claims, wherein the wherein the one or more cell walls are formed of a material having an ultimate tensile strength of at least 100 MPa, preferably at least 150 MPa, more preferably at least 200 MPa, more preferably at least 250 MPa, most preferably at least 300 MPa.

5. An impact energy absorber according to any of the preceding claims, wherein the one or more cell walls are formed of a metallic material, preferably an aluminium alloy, most preferably an alloy of aluminium, silicon and magnesium.

6. An impact energy absorber according to any of the preceding claims, wherein the one or more cell walls have a minimum thickness of no more than 2 mm, preferably no more than 1.5 mm, more preferably no more than 1 mm, more preferably no more than 0.5 mm, most preferably no more than 0.2 mm at least at one end of the hollow elongate cells.

7. An impact energy absorber according to any of the preceding claims, wherein the one or more cell walls increase in thickness from a first end of the hollow elongate cells to a second end of the hollow elongate cells.

8. An impact energy absorber according to any of the preceding claims, wherein the plurality of hollow elongate cells form part of one or more hollows extending through the full length of the impact energy absorber along the elongate direction.

9. An impact energy absorber according to any of the preceding claims, wherein one or more of the hollow elongate cells has a largest dimension perpendicular to its elongate direction of between 1 mm and 50 mm, preferably between 2 mm and 20 mm, preferably between 3 mm and 10 mm.

10. An impact energy absorber according to any of the preceding claims, thehollow of one or more hollow elongate cells makes up at least 50% of the volume of the hollow elongate cell, preferably at least 60%, more preferably at least 70%, more preferably at least 80%, most preferably at least 90%.

11. An impact energy absorber according to any of the preceding claims, wherein the lattice structure has an areal density in the range 25% to 1%, preferably in the range 20% to 2%, more preferably in the range 15% to 3%, more preferably in the range 12% to 4%, most preferably in the range 10% to 5%.

12. An impact energy absorber according to any of the preceding claims, wherein the lattice structure has a density that varies along the elongate direction of the hollow elongate cells.

13. An impact energy absorber according to any of the preceding claims, wherein the one or more cell walls are integrally formed.

14. An impact energy absorber according to any of the preceding claims, wherein each hollow elongate cell shares at least one cell wall or vertex with one or more adjacent hollow elongate cells.

15. An impact energy absorber according to any of the preceding claims, wherein the hollow elongate cells have a hexagonal cross-sectional shape.

16. An impact energy absorber according to any of the preceding claims, comprising one or more external walls surrounding the lattice structure.

17. An impact energy absorber according to claim 16, wherein the one or more external walls are thicker than the one or more cell walls.

18. An impact energy absorber according to claim 16 or claim 17, wherein the one or more external walls increase in thickness from a first end of the hollow elongate cells to a second end of the hollow elongate cells.

19. An impact energy absorber according to any of claims 16 to 18, wherein the one or more external walls are formed of the same material as the one or more cell walls.

20. An impact energy absorber according to claim 19, wherein the one or more external walls are integral with the one or more cell walls.

21. An impact energy absorber according to any of claims 16 to 20, wherein the one or more external walls have a different shape to the one or more cell walls.

22. An impact energy absorber according to any of claims 16 to 21, wherein the one or more cell walls are substantially straight along the elongate direction of the cells and one or more of the external walls are shaped along the elongate direction of the cells.

23. An impact energy absorber according to any of the preceding claims, wherein the cross-sectional area of the lattice structure varies along the elongate direction of the cells.

24. An impact energy absorber according to claim 23, wherein the cross-sectional area of the lattice structure is varied by providing one or more hollow elongate cells that start or terminate part way along the lattice structure along the elongate direction of the cells.

25. An impact energy absorber according to claim 23 or claim 24, wherein the cross-sectional shape and size of each hollow elongate cell remains substantially constant along the elongate direction of the cells.

26. An impact energy absorber according to any of the preceding claims, further comprising one or more fixing points formed integrally with the one or more cell walls.

27. An impact energy absorber according to any of the preceding claims, comprising a hollow backing structure located at one end of the lattice structure along the elongate direction of the hollow elongate cells, wherein the hollow backing structure comprises internal and / or external walls that are thicker than one or more cell walls, measured along the same direction as the thickness of the cell walls.

28. An impact energy absorber according to claim 27, wherein the hollow backing structure defines one or more hollows extending between an end of one or more of the hollow elongate cells and an end of the impact energy absorber.

29. An impact energy absorber according to any of the preceding claims, wherein the one or more cell walls are formed by additive manufacturing.

30. A vehicle comprising one or more impact energy absorbers according to any of the preceding claims.

31. A vehicle according to claim 29, wherein the hollow elongate cells extend along a substantially horizontal direction, preferably substantially perpendicular to an adjacent external surface of the vehicle.

32. A method of manufacturing an impact energy absorber for a vehicle, comprising:forming a lattice structure comprising a plurality of hollow elongate cells defined by one or more cell walls formed of a material having an elongation at break of no more than 20%.

33. A method according to claim 32, comprising forming the lattice structure by additive manufacturing, preferably by laser powder bed fusion.A

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