GB2463446A

GB2463446A - Honeycomb construction system

Info

Publication number: GB2463446A
Application number: GB0816826A
Authority: GB
Inventors: Andrew Alderson; Kim Alderson; Khaled Zied
Original assignee: Auxetic Technologies Ltd
Current assignee: Auxetic Technologies Ltd
Priority date: 2008-09-15
Filing date: 2008-09-15
Publication date: 2010-03-17
Also published as: GB0816826D0

Abstract

A honeycomb is formed of a plurality of node and ligament elements, the node and ligament elements being discrete components which are connected to form the honeycomb. A kit of nodes and ligaments for constructing a honeycomb is also disclosed. The ligaments may be flexible to allow construction of flexible honeycombs. Conventional and chiral honeycombs may be constructed.

Description

I

HONEYCOMB CONSTRUCTION SYSTEM

Background

This invention relates to a system for the production of honeycomb structures.

Honeycomb structures are porous structures formed of a plurality of interconnected ligaments. Figure 1 shows a diagram of a conventional hexagonal honeycomb. Honeycomb structures have a high strength to mass ratio in the through-plane direction, but have a low strength in in-plane directions. Honeycomb structures have found particular application in sandwich panels where they are employed as a core between the two skins.

Such sandwich panels have two continuous surfaces, low mass and high through-plane strength. Composite panels are commonly employed in automotive and aeronautical applications, but such panels naturally form anticlastic surfaces rather synclastic surfaces. Their use to form synclastic surfaces (domes) is therefore limited by the need for complex manufacturing techniques to form those shapes.

Chiral honeycombs are a particular form of honeycomb structure in which the ligaments are joined at chiral nodes. A chiral node is one which cannot be superimposed on its mirror image. Figure 2 shows an example of the two forms of a chiral node having three ligaments.

Honeycombs are conventionally manufactured by bonding flat pieces or strips of material together or by cutting the honeycomb from solid material. Such techniques limit the materials and dimensions in which honeycombs can be produced.

There is therefore a requirement for an improved construction technique for honeycomb structures.

Summary

The following presents a simplified summary of the disclosure in order to provide a basic understanding to the reader. This summary is not an extensive overview of the disclosure and it does not identify key/critical elements of the invention or delineate the scope of the invention. Its sole purpose is to present some concepts disclosed herein in a simplified form as a prelude to the more detailed description that is presented later.

There is provided a honeycomb formed of a plurality of node and ligament elements, the node and ligament elements being discrete components which are connected to form the honeycomb.

At least some of the ligament elements may have a section that is flexible in the plane of the honeycomb.

At least some of the ligament elements may have a section that is flexible out of the plane of the honeycomb.

In one aspect the ligament elements and/or the node elements may be completely or substantially rigid.

The flexible ligaments may be only flexible in a discrete section or may be flexible along their full length.

The ligament elements may be joined to the node elements by a hinging joint which hinges in the plane of the honeycomb.

The hinging joint may be provided by a flexible material or by a rotating hinge.

A portion of the node elements may be only rotationally constrained by ligaments attached to the node.

The node elements may be moveable relative to one another.

The ligament and node elements may be joined by a removable joint or by a permanent joint.

The honeycomb may be flexible in the plane of the honeycomb or out of the plane of the honeycomb.

The honeycomb may have a positive in-plane Poisson's ratio or a negative in-plane Poisson's ratio.

The honeycomb may have a region with a positive in-plane Poisson's ratio and a region with a negative in-plane Poisson's ratio.

The honeycomb may be a chiral honeycomb.

The honeycomb may further comprise at least one sensor.

The sensor may be located at a node or a ligament of the honeycomb.

The sensor may be a strain sensor.

At least some of the nodes may further comprise electrical connections for connecting to electrical connections on ligaments joined to that node.

At least some of the ligaments may further comprise electrical connections for connecting to electrical connections on nodes joined to that ligament.

There is also provided a honeycomb construction kit, comprising at least one ligament element, and at least one node element, each node element having a plurality of connection points to which an end of a ligament element can be connected such that a plurality of nodes and ligaments can be interconnected to form a honeycomb.

At least some of the ligament elements may have a flexible section.

The flexible ligaments may only be flexible in a discrete section or may be flexible along their full length.

The connection points may be configured to form a hinged connection between a node and a ligament.

The hinged connection may be provided by a flexible material or by a rotating hinge.

The connection point may form a removable joint between a ligament and a node.

The ligaments may be configured to lie radially or tangentially from a node when connected to that node.

At least one of the nodes or ligaments may comprise a sensor.

The at least one sensor may be strain sensor.

At least some of the nodes further may comprise electrical connections for connecting to electrical connections on ligaments joined to that node.

At least one of the node elements may have an opening within the node.

Description of the drawings

Embodiments of the present invention will now be further described, by way of example, with reference to the drawings, wherein:-Figure 1 shows a hexagonal honeycomb design; Figure 2 shows examples of a chiral node; Figure 3 shows nodes and ligaments according to embodiments of the invention; Figures 4a and 4b show unit cells of an anti-tri-chiral honeycomb and a hexagonal conventional honeycomb constructed according to an embodiment of the current invention; Figure 5a, 5b and 5c show schematic examples of chiral honeycomb structures; Figure 6 shows charts of Poisson's ratio against honeycomb parameters for tri-chiral honeycombs; Figure 7 shows charts of Poisson's ratio against honeycomb parameters for anit-tri-chiral honeycombs; Figure 8 shows a hollow node according to an embodiment of the current invention; Figure 9 shows an example of a ligament incorporating a sensor; Figure 10 shows a ligament and node having connections; and Figure 11 shows a honeycomb having regions of positive and negative Poisson's ratio.

Detailed description

The detailed description provided below in connection with the appended drawings is intended as a description of the present examples and is not intended to represent the only forms in which the present example may be constructed or utilized. The description sets forth the functions of the example and the sequence of steps for constructing and operating the example.

However, the same or equivalent functions and sequences may be accomplished by different examples.

The current invention provides a system for the manufacture of honeycomb structures. The structures are formed of a plurality of connectable node and ligament pieces and thus are formed in a modular manner to the required design and shape. This system allows the convenient manufacture of a range of sizes and layouts of honeycombs and allows the performance and design to be tailored to particular requirements. By the use of a range of compatible components different features can be provided across a structure formed using the system.

Figure 3 shows a node 12 of an embodiment of the current invention. The node 12 is a three-ligament design and can therefore be connected to up to three ligaments. The node can be utilised to form both regular and chiral honeycombs depending on the ligaments utilised to interconnect them.

Ligament 10 is a design utilised to manufacture a chiral honeycomb since when attached to a node the ligament runs at a tangent to a circumference of the node. Ligament 11 is a design utilised to manufacture a conventional honeycomb as when attached to a node the ligament lies radially from the centre of that node.

The node 12 has a series of three slots 13 around its circumference. Each slot 13 has a straight region 14 adjacent the surface leading to an expanded region 15 at the inner end of the slot 13. A recess 16 is also provided adjacent to each slot 13 to allow chiral ligaments 10 to lie against the node surface when fitted. The node has a uniform cross-section through its thickness.

Ligaments 10 and 11 have an expanding region 17 at each end which matches the shape, and fits within, the expanded region 15 of the node piece.

The width 18 of the ligaments adjacent the expanded region 17 is of a suitable size to fit within the straight region 14 of the slots 13 of a node 12.

The expanded regions 15, 17 of the example node and ligaments shown in Figure 1 are circular, but as will be appreciated by the skilled person any suitable shape which performs the function of retaining the ligament in the slot may be utilised.

The cross-section of the slots, and of the ligaments adjacent the ends, is constant through the depth of the pieces and thus the end of the ligament pieces can slide into the slots in the node pieces to join a ligament to a node.

The pieces are formed with suitable dimensions to provide the required tightness of fit to retain to the ligament in the node. A locking arrangement may also be provided to lock the ligament into place in the node. For example, a bump may be provided on the ligament's surface to engage with a notch in the slot of the node. Adhesives may be utilised in conjunction with, or instead of, locking mechanisms. As will be appreciated by the skilled person, any suitable locking arrangement may be utilised.

Figure 4a shows an anti-tri-chiral unit-cell formed of the nodes 12 and ligaments 10 shown in Figure 1. Figure 4b shows a conventional hexagonal honeycomb unit cell gormed of the nodes 12 and ligaments 11 shown in Figure 1.

The principles described in relation to the nodes and ligaments of Figure 1 can be utilised to provide nodes for other than three ligaments by the provision of the relevant number of slots around the circumference of the node. Nodes with any required number of slots, subject to physical restrictions on manufacturing sizes, may be provided according to the invention. Figures 5a, 5b and 5c show various chiral honeycomb configurations, which may be manufactured utilising embodiments of the invention. Figure 5a shows examples of chiral and anti-chiral structures having 3, 4 and 6 ligaments per node. An anti-chiral structure having 6 ligaments cannot be constructed. Figure 5b shows re-entrant tri-chiral and anti-tri-chiral honeycombs. Figure 5c shows a double-arrowhead honeycomb, including cylindrical nodes.

Honeycombs, and hence sandwich panels utilising those honeycombs, constructed using the nodes and ligaments described above have an improved through-plane compressive strength as the cylindrical nodes provide additional strength when compared to conventional honeycombs formed of only flat sections. The ligaments also provide resistance to through-plane shear loading, which is important in relation to the flexural stiffness of sandwich panels.

In-plane behaviour of honeycombs constructed according to the current invention can be defined by selection of the properties of materials. For example, a rigid honeycomb can be manufactured using rigid materials, or a flexible honeycomb can be manufactured using flexible ligaments.

Chiral honeycombs, when constructed in a suitable manner, may display a negative in-plane Poisson's ratio. The Poisson's ratio of honeycombs is dependent on the layout of the honeycomb and the relative sizes of the nodes and ligaments. Table 1 summarises the Poisson's ratios for selected configurations.

Negative Positive Negative or Positive Tetra chiral Tri-chiral Anti-trichiral Anti-tetra-ch iral Hexa chiral Re-entrant tn chiral Re-entrant anti-tri-chiral Cylinder ligament double-arrowhead

Table 1

The relative sizes of the nodes and ligaments affects the Poisson's ratio of a honeycomb. Figure 6 shows charts of the Poisson's ratio of a tri-chiral honeycomb, where x = (Ligament length)/(Node radius) and = (Ligament thickness)/(Node radius). The Poisson's ratio is seen to be strongly dependent on c', and also to a lesser degree on.

Figure 7 shows the same charts as Figure 6, but for anti-tri-chiral structures.

For these structures the Poisson's ratio is strongly dependent on both cx and 13 and is either negative or positive depending on the values.

Materials or structures having a negative Poisson's ratio are known as auxetic'. The effect of a negative Poisson's ratio is that when a stress is applied in a first direction, the material expands in a direction perpendicular to that stress.

Auxetic chiral honeycombs may be constructed using nodes and ligaments according to the current invention if the ligament/node joint is formed as a hinge such that nodes are able to rotate. As a node rotates the ligaments attached to that node are pulled or pushed around the circumference of the node. Since each ligament is connected at its opposite end to a second node, the distance between the two nodes is varied by the pulling or pushing of the ligament (provided the ligament is rigid lengthwise). The rotation of the first node may be caused by a force being applied via another ligament connected to that node. Stretching the structure in a first direction may therefore cause rotation of nodes and hence expansion in the perpendicular direction. The structure therefore displays in-plane auxetic behaviour.

Planar non-auxetic materials (for example a non-auxetic honeycomb) cannot adopt a synclastic (doubly-curved) shape without machining or forcing into shape. This limits the use of conventional sandwich panels to largely planar or singly-curved structures. In contrast, in-plane auxetic materials naturally form synclastic shapes. Sandwich panels can therefore be constructed with a synclastic curve without forcing or machining of the honeycomb core.

Furthermore, the through-plane strength and stiffness of the core is maximised as the ligaments are always aligned normal to the curved surface.

In order for an auxetic honeycomb to adopt a synclastic curve differential expansion occurs between the inner and outer surfaces of the honeycomb.

This differential expansion may be accommodated by utilising ligaments having flexibility under out of plane bending.

It has been observed that for certain chiral honeycombs the materials behaviour is different for in-plane and out-of-plane deformation. For example an anti-tri-chiral auxetic structure naturally undergoes anticlastic curvature associated with conventional honeycombs, not synclastic curvature as would be expected of an auxetic honeycomb. This effect is due to out-of-plane bending causing the nodes on the outer surface to rotate in one direction (for expansion) and in the other direction on the inner surface (for contraction).

The overall rotation effect (which drives the auxetic effect for in-plane behaviour) is thus cancelled and the honeycomb behaves in a non-auxetic manner. This effect may also occur in other chiral honeycomb structures.

As described above, in order to be auxetic, the nodes and ligaments must be free to move in relation to each other, and the nodes must be able to rotate to allow extension/retraction of the ligaments to the adjacent nodes. To allow this movement the ends of the ligaments adjacent to the nodes may be formed of a flexible material such that the ligament can wrap/unwrap around the node. Alternatively, or in addition, the connection of the ligament to the node may allow the ligament to hinge about its connection point.

As a chiral honeycomb expands and contracts the ligaments may wrap and unwrap from around the nodes. In the absence of compression/extension of the ligaments, the amount of rotation that is possible in the nodes of the honeycomb thus defines the degree of in-plane compression/extension that the honeycomb can display. The amount of possible rotation is defined at least in part by the ability of the ligaments to wrap-around the circumference of the nodes. The ligaments may therefore be produced in a material that can bend around that circumference, whilst being sufficiently rigid to have the required mechanical strength. The central regions of the ligaments do not need to wrap around nodes and therefore the ligaments may have mechanical properties that vary along their length such that each point is optimised for its role.

Figure 8 shows a node 60 having an internal cavity 61. The cavity may pass all of the way through the node, or may be closed at one end. The cavity may be utilised to mount sensors, or other components, within the honeycomb.

For example the honeycomb may be utilised to form an array receiver with receivers spread across the honeycomb. The node 6 provides a convenient method of mounting the receivers at the required location by utilising that node in place of a solid node 13 where a receiver is required. The receiver, or other item of equipment, may be permanently mounted in the node, or may be detachable and exchangeable.

Nodes utilised in a honeycomb may be manufactured in a range of different materials and having different properties. Since each node is manufactured independently the manufacturing techniques can be optimised for that particular node. For example if a node is to incorporate a sensor, the materials and techniques used may be different to a node which is simply structural. Such variation is not possible in prior art construction techniques where the entire honeycomb is formed in a single process. The ability to tailor techniques and materials for each node may allow an increase in the versatility of honeycombs that may be constructed.

The ligaments may be used in addition to, or instead of, the nodes to mount sensors. Figure 9 shows a schematic diagram of a ligament 70 incorporating a sensor 71, for example a strain sensor may be formed in, or on, the ligament such that strains throughout the honeycomb can be measured. Such a ligament can be utilised in the honeycomb in place of a standard ligament at any point where a measurement is required.

Electrical connections between sensors in ligaments and nodes may be provided integrally within a constructed honeycomb by the provision of contacts and connections on the ligaments and nodes. As shown in Figure 10 nodes 80 and ligaments 81 may be provided with contact points 82 which make contact when the nodes and ligaments are assembled. Nodes may be provided with contacts in each of the slots such that all suitably equipped ligaments connected to that node make contact. Contacts in each slot of the nodes and at each end of the ligaments may be connected 83 within the node and ligament such that as a honeycomb is constructed a network is formed.

The ligament shown in Figure 8 is an example of a connecting ligament which does not have a sensor or other itself, but is provided with connections such that signals can pass between the nodes joined by the ligament. Each node may thus be connected to each other node in the honeycomb allowing the transfer of signals throughout the honeycomb. Nodes or ligaments with communications hardware or software may be provided to control and coordinate communication in the honeycomb. Any suitable communication format can be utilised for communication between devices in a honeycomb.

One or more of the nodes or ligaments may provide an external connection point to allow connection to the network for monitoring or extraction of data, or the provision of power. A modular smart structure is thus enabled by embodiments of the current invention.

Ligaments and nodes incorporating actuators may also be provided such that an active structure can be constructed which can vary its shape or properties.

Actuators may cause a change in the length of ligaments or rotation of nodes to effect shape or property changes.

The honeycomb pattern and number of ligaments per node are not necessarily the same across the honeycomb structure but may be varied to vary the characteristics of the honeycomb across its area. Figure 11 shows a honeycomb having both a hexa-chiral region with negative Poisson's ratio and a tri-chiral region with positive Poisson's ratio.

Embodiments of the current invention allow a honeycomb to be formed of discrete elements and thus a honeycomb can be maintained by the replacement of particular nodes or ligaments without requiring replacement of the entire honeycomb as would be required with prior-art honeycombs.

Honeycombs according to embodiments of the current invention may be constructed in a wide range of sizes and for a wide range of applications.

Possible applications for honeycombs manufactured according to the techniques and systems described herein include microwave absorbers and cores for sandwich panels.

The nodes described hereinbefore have had a circular cross-section, but other cross-sections are also applicable, provided the ligaments are joined at a point radially distant from the centre of the node. For example, polygonal nodes may be utilised with any appropriate number of sides. Furthermore, non-regular cross-sections may be utilised.

It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. It will further be understood that reference to an' item refers to one or more of those items.

It will be understood that the above description of a preferred embodiment is given by way of example only and that various modifications may be made by those skilled in the art. The above specification, examples and data provide a complete description of the structure and use of exemplary embodiments of the invention. Although various embodiments of the invention have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this invention.

Claims

Claims 1. A honeycomb formed of a plurality of node and ligament elements, the node and ligament elements being discrete components which are connected to form the honeycomb.
2. A honeycomb according to claim 1, wherein at least some of the ligament elements have a section that is flexible in the plane of the honeycomb.
3. A honeycomb according to claim 1, wherein at least some of the ligament elements have a section that is flexible in the plane of the honeycomb.
4. A honeycomb according to claim 2 or claim 3 wherein the flexible ligaments are only flexible in a discrete section.
5. A honeycomb according to claim 2 or claim 3 wherein the flexible ligaments are flexible along their full length.
6. A honeycomb according to any preceding claim, wherein the ligament elements are joined to the node elements by a hinging joint which hinges in the plane of the honeycomb.
7. A honeycomb according to claim 6 wherein the hinging joint is provided by a flexible material.
8. A honeycomb according to claim 6 wherein the hinging joint is provided by a rotating hinge.
9. A honeycomb according to any preceding claim, wherein at least a portion of the node elements are only rotationally constrained by ligaments attached to the node.
10. A honeycomb according to any preceding claim, wherein the nodes are moveable relative to one another.
11. A honeycomb according to any preceding claim, wherein the ligament and node elements are joined by a removable joint.
12. A honeycomb according to any preceding claim, wherein the ligament and node elements are joined by a permanent joint.
13. A honeycomb according to any preceding claim wherein the honeycomb is flexible in the plane of the honeycomb.
14. A honeycomb according to any preceding claim wherein the honeycomb is flexible out of the plane of the honeycomb.
15. A honeycomb according to any preceding claim wherein the honeycomb has a positive in-plane Poisson's ratio.
16. A honeycomb according to any preceding claim wherein the honeycomb has a negative in-plane Poisson's ratio.
17. A honeycomb according to any preceding claim wherein the honeycomb has a region with a positive in-plane Poisson's ratio and a region with a negative in-plane Poisson's ratio.
18. A honeycomb according to any preceding claim wherein the honeycomb is a chiral honeycomb.
19. A honeycomb according to any preceding claim wherein the honeycomb further comprises at least one sensor.
20. A honeycomb according to claim 19 wherein the sensor is located at a node of the honeycomb.
21. A honeycomb according to claim 19 wherein the sensor is located at a ligament of the honeycomb.
22. A honeycomb according to claim 19 wherein the sensor is a strain sensor.
23. A honeycomb according to any preceding claim wherein at least some of the nodes further comprise electrical connections for connecting to electrical connections on ligaments joined to that node.
24. A honeycomb according to any preceding claim wherein at least some of the ligaments further comprise electrical connections for connecting to electrical connections on nodes joined to that ligament.
25. A honeycomb construction kit, comprising at least one ligament element, and at least one node element, each node element having a plurality of connection points to which an end of a ligament element can be connected such that a plurality of nodes and ligaments can be interconnected to form a honeycomb.
26. A kit according to claim 25, wherein at least some of the ligament elements have a flexible section.
27. A kit according to claim 26 wherein the flexible ligaments are only flexible in a discrete section.
28. A kit according to claim 26 wherein the flexible ligaments are flexible along their full length.
29. A kit according to any of claims 25 to 28, wherein the connection points are configured to form a hinged connection between a node and a ligament.
30. A kit according to claim 29 wherein the hinged connection is provided by a flexible material.
31. A kit according to claim 29 wherein the hinged connection is provided by a rotating hinge.
32. A kit according to any of claims 25 to 31, wherein the connection point forms a removable joint between a ligament and a node.
33. A kit according to any of claims 25 to 32 wherein the ligaments are configured to lie radially from a node when connected to that node.
34. A kit according to any of claims 25 to 32 wherein the ligaments are configured to lie tangentially from a node when connected to that node.
35. A kit according to any of claims 25 to 34 wherein at least one of the nodes comprises a sensor.
36. A kit according to any of claims 25 to 35 wherein at least one of the ligaments comprises a sensor.
37. A kit according to claim 35 or 36 the at least one sensor is a strain sensor.
38. A kit according to any of claims 25 to 37 wherein at least some of the nodes further comprise electrical connections for connecting to electrical connections on ligaments joined to that node.
39. A kit according to any of claims 25 to 38 wherein at least some of the ligaments further comprise electrical connections for connecting to electrical connections on nodes joined to that ligament.
40. A kit according to any of claims 25 to 39 wherein at least one of the node elements has an opening within the node.