WO1991019866A1

WO1991019866A1 - Composite materials

Info

Publication number: WO1991019866A1
Application number: PCT/GB1991/000981
Authority: WO
Inventors: Andrew Robert Fry
Original assignee: Team Consulting Limited
Priority date: 1990-06-19
Filing date: 1991-06-18
Publication date: 1991-12-26
Also published as: GB2245523A; GB9013629D0; AU8094291A

Abstract

The difficulty of producing composite materials in profiled form is overcome by the provision of a composite material in which a relatively thick core layer (9) comprises a continuous array of open polyhedral frames (10), which frames are bonded at their outermost points to a relatively thin outer layer or layers (7, 8) of a durable, high strength material. The array of frames may be produced by distortion of a suitable expanded metal sheet comprising a two-dimensional network of polygons. The core layer is of relatively high strength and low weight, and can be performed into desired shapes, allowing for ease of production of profiled composite materials. The invention also provides a method of manufacture of such a material, and a structure and a composite panel formed from the material. The material is of use for purposes such as cladding, flooring and panelling, for instance in the building industry.

Description

Title; Composite Materials

Field of the invention

This invention relates to composite materials and to methods for their manufacture.

Background to the Invention

Composite materials, in one form or another, have been in use for centuries. From Phonecian bows, which were laminated from wood and animal hide, through the timber and wrought iron structures used in the 18th and 19th Centuries, to the reinforced concrete of the 20th Century the principles are the same. These are to optimise the performance, weight and cost of a component or structure by the juxtaposition of materials of differing properties.

Composite panels represent one particular way in which composite materials can be manufactured and used. They typically consist of two relatively thin outer layers ("skins") of a durable, high strength material having a high elastic modulus, which skins are separated by a thicker "core" layer of a lower bulk performance material. This core layer must be able to transmit stresses throughout the panel, and must provide adequate support t the outer skins .to allow them to resist applied loads. Usually, the core material is of a low density, so that the resulting composite panel is not only high in strengt but also of a relatively light weight.

In recent years, advanced performance composite panels have become available. These typically make use of aluminium sheeting or reinforced plastics materials for the outer skins, and an aluminium core which may be, for instance, a corrugated sheet or a honeycomb-like material The resulting panel is lightweight but high in strength, and is of great use in the aerospace, marine, automotive and building industries for purposes such as cladding, flooring and panelling.

One limitation of such composite materials, however, is that they must generally be produced in flat sheet form. Any complex profile required in a structure to be produce from these materials must therefore be built up by a time consuming process of cutting the flat sheets to the required shape and size and subsequently bonding the cut sheets together using a suitable adhesive. The disadvantages of this process result both from the added labour and cost involved and from risks associated with the loss of strength and integrity of the structure that is possible when jointing planar sheets of composite material. Futhermore, structures having large, smooth curves are very difficult to produce in this way.

It is therefore an aim of the present invention to provid a composite material, and a method for its manufacture, which overcomes or at least mitigates the disadvantages associated with currently available materials.

_*

Statement of the Invention

SUBS According to the present invention there is provided a composite material comprising a relatively thin outer layer or layers of a durable , high strength material and thicker core layer bonded to the outer layer or layers , characterised in that the core layer comprises a continuous array of open polyhedral frames, which frames are bonded at their outermost points to the outer layer layers of the composite material.

The array may be formed from a two-dimensional "net" of polygons which has been distorted so as to produce a three-dimensional network of polyhedral frames. The net is conveniently an expanded metal sheet of the type commonly available , which may comprise a lattice network of squares, triangles, hexagons or any other tessellatin polygons. Preferably, an expanded metal sheet comprisin a lattice network of squares or rhombi is used , which on suitable distortion will produce a three-dimensional arr of square-based pyramidal frames.

The polyhedral frames may be formed from lengths of wire interconnected in such a way as to produce an array of such frames. The array may in this case be formed by th distortion of a two- imensional sheet of wire mesh or netting in which the interconnected wires def ine an appropriate net shape.

The polyhedral frames are preferably square-based pyramidal frames. The frames are then bonded to the out layer or layers of the composite material at their apexes.

SUBSTITUTE SHEET Thus , use is made , in the core layer of the composite material of the present invention, of the well-known technique of utilising lattice beam arrangements to support applied forces.

The structure and use of lattice beam arrangements are well known. By separating the upper ( "compression" ) and lower ( "tension" ) faces of a supporting beam with welded , or pin jointed diagonal members , the beam structure is able to resolve imposed vertical loads as pure tension and compression forces within all members. The mass per unit run is then far lower than for an equivalent rolled or drawn section. Of course , the penalty when using a single plane beam is a lack of lateral stability. Crane jibs and structural members for some buildings are therefore sometimes produced as three integrated lattice beams forming the sides of a triangular prism. This has greatl improved lateral strength compared to the sing le plane structure.

The log ical extension of this is the lattice roof , where loads across the entire horizontal plane of a roof panel are borne by diagonal lattice members radiating from the inner roof face . These lattice members are attached to horizontal "stringers" (which take the tension forces ) lying in a horizontal plane beneath the roof plane itself . Such "layer grid structures" have been widely used in conference halls, theatres , exhibition centres , etc .

Whilst the structural advantages of such arrangements are well known and widely used in the larger-scale applications described above , their use in the field of composite materials has apparently never been appreciated The present invention , however , makes use of the techniqu on a much smaller scale, in order to produce a lightweig but high strength core layer for use in composite materials. As a result, the composite material of the present invention can be greater in strength, lighter in weight and lower in cost than currently available alternatives.

In addition, the lattice-type core layer of the composit material of the present invention is more suitable than conventional core materials for pre-forming into desired shapes. This allows for easier production of complex profiles in structures formed from the composite materia One or both of the outer layers could, for instance, be pre-formed to the desired shape, and the core layer easi deformed to match that shape.

Both the outer laye (s) and the core layer can be formed of any suitable material such as aluminium, steel (including galvanised or stainless steel), certain types of plastics materials or GRP (glass reinforced plastics) sheets. Conveniently, the core layer is formed from stainless steel preferably in the form of an expanded steel sheet. However, the core layer need not necessari be made of metal; the use of a more malleable material would, for instance, allow the core layer to be easily moulded to a desired shape and hence enable a wide varie of different geometries to be explored.

Outer layers are preferably formed from materials having high elastic modulus (Young's modulus of elasticity).

Outer layers may also have a decorative or functional finish applied to their outer surfaces, for example, vitreous enabel, decorative plastics, wood or stone venee paint or a protective coating of some form.

The thickness of the composite material and of the outer and core layers will depend on the materials of which the composite material is made, the intended use and the desired weight of the final product, among other considerations. Typically, the composite material is approximately 22mm thick, and the outer layers each approximately 1mm thick.

The core layer is conveniently bonded to the outer layers by means of a suitable adhesive. The adhesive may be applied as a coating on the inner surface of one of the outer layers, prior to contacting that outer layer with the core layer to which it is to be bonded. In this way, the effect of surface tension would be to form reinforced fillets at each node, ie. at each point of contact betwee the core layer and the outer layer. However, more preferably the adhesive is applied to the nodes of the core layer (for instance, by passing the core layer between rollers which apply the adhesive). Again, surfac tension would cause the adhesive to remain at the nodes, however, the total weight and cost of the composite panel would be reduced by avoiding the need to apply adhesive t areas of the outer layers not in contact with the core layer.

The composite material of the present invention can be formed into structures of any desired shape and size. Accordingly, the present invention also provides a structure formed from the composite material of the invention, and a composite panel formed from the composite material of the invention.

SUBSTITUTE SHEET The present invention also provides a method of manufacture of a composite material, comprising the step of distorting a two-dimensional net of polygons so as to produce a three-dimensional continuous array of open polyhedral frames; and bonding the frames at their outermost points to an outer layer or layers of a durabl high strength material. In this way, a composite materi is formed which comprises a core layer (the array of polyhedral frames) and an outer "skin" layer attached to it. The array may be bonded to two outer layers, one above and one below it, so as to form a "sandwich" composite material in which the core layer separates two outer layers.

The two- imensional net is conveniently an expanded metal sheet, preferably comprising a lattice network of squares or rhombi. The net is then conveniently distorted by corrugation, such that the alternate "peak" and "trough" lines of the corrugations lie along the minor axes of the rhombi of the network.

The polyhedral frames of the array may be bonded to the outer layer or layers by applying a coating of a suitable adhesive to the inner surface of one of the outer layers, prior to contacting that outer layer with the core layer to which it is to be bonded. Alternatively, the adhesive coating may be applied to the outermost points of the core layer polyhedral frames, prior to contacting the cor layer to the outer layer or layers to which it is to be bonded. Either or both of the outer layers may be pre¬ formed to a desired profile prior to the bonding step, as may the core layer.

The present invention will now be described by means of the following examples and with reference to the accompanying drawings, of which:

Figures 1 to 3 show examples of known composite materials;

Figure 4 shows a composite material in accordance with the present invention;

Figure 5 shows a portion of the expanded metal sheet used in the manufacture of the material of Figure 4;

Figure 6 and 7 show further examples of expanded metal sheets of use in the manufacture of composite materials i accordance with the present invention; and

Figure 8 shows an alternative example of a composite material in accordance with the present invention.

Detailed Description of the Drawings

In Figure 1, a sample of a known composite material is shown in perspective view. The material comprises an inner core layer 1 and two outer "skin" layers 2 and 3. The upper layer 2 is shown cut away in part so as to reveal the nature of the core layer 1. This core layer has a honeycomb-like structure, comprising a close-packed array of hollow hexagonal cylinders 4. The core layer is formed of a lightweight metal such as aluminium and therefore has a fairly low density. The outer layers 2 and 3 are usually made of either aluminium or a reinforce (eg. by means of woven glass-fibres) plastics material.

The composite material shown in Figure 1 is generally expensive and time-consuming to produce, due to the processes involved in manufacturing the honeycomb-like core layer.

Figures 2 and 3 show further examples of known composite materials, again in perspective view. Each comprises two outer layers 5, of aluminium or galvanised steel, and a core layer 6 of corrugated metal separating the two outer layers. The core layers of Figure 2 and 3 (shown schematically) illustrate different shapes of corrugation for the core layer.

The composite material, a sample of which is shown in perspective in Figure 4, has been manufactured in accordance with the present invention. The material comprises an upper layer 7 (shown partly cut away) and a lower layer 8, separated by the core layer 9. The core layer comprises a continuous array of open frames 10, eac of which is square-based pyramidal in shape. The frames are bonded at each apex to the outer layers 7 and 8.

The core layer is formed from an expanded metal sheet 11, a portion of which is shown in Figure 5. This sheet comprises a two-dimensional "net" of rhombi. In order to produce the core layer 9 of the material shown in Figure 4, the sheet 11 is corrugated, such that the alternate "peak" and "trough" lines of the corrugations lie along the minor axes of the rhombi of sheet 11. In this way, a three-dimensional network of pyramidal frames is formed.

Expanded metal sheets such as that shown in Figure 5 are well known and widely available. They are produced by semi-shearing sheet metal, and are usually used as walkways, stair treads and the like. They are generally available in a wide variety of different patterns, two further examples of which are shown in Figure 6 (network of hexagons) and Figure 7 (a more irregularly shaped network, based loosely on a hexagonal structure).

Many differently shaped networks of the general form show in Figures 5 to 7 may be of use in the manufacture of composite materials in accordance with the present invention, so long as they can be corrugated or otherwise deformed to produce an array of open polyhedral frames having the desired strength and density. In particular, metal sheets based on a network of triangles or of hexagons are envisaged to be of use.

The upper and lower layers of the material shown in Figur 4 may be made of aluminium, GRP sheeting or a suitable laminate. They are 1 mm thick, the total material thickness (distance x) being 22 mm. They are bonded to the core layer 9 by applying a coating (of approximately 0.5 mm) of a suitable adhesive to the outermost points of the polyhedral frames of the core layer, and then contacting the core layer with the outer layers 7 and 8. This causes the formation of reinforced fillets at the apexes or nodes of the core layer, due to surface tension effects. Once both upper and lower layers have been bonded to the core layer, a composite material of considerable strength has been produced. The resultant material is also low in weight and can be manufactured at relatively low costs, especially since the core layer is so easily manufactured.

The process of bonding together the outer and core layers may be a continuous or semi-continuous automated process, whereby the three layers are brought together in

SUBSTITUTE SHEET continuous lengths with the adhesive coatings already applied.

A variety of configurations of the composite material o the present invention are possible, depending on the ultimate properties of the final structure required. F example, for increased point impact resistance a thicker skin would be appropriate, whereas for greater core compressive strength both the core layer thickness and included angle or polyhedral frame size/geometry may be appropriately selected. Similarly, as already noted, materials selected for the skins and for the core need n necessarily be the same and may be chosen to suit the application and the environment in which the final structure will be used.

As an example. Table 1 shows typical (projected) densiti for various configurations of composite panel in accordance with the invention.

The outer layers 7 and 8 may of course have a decorative or functional finish applied to their outer surfaces, su as vitreous enamel, decorative plastics, wood or stone veneer or paint, to improve aesthetic appearance, or som form of protective coating.

An alternative type of composite material, also in accordance with the present invention, is illustrated in Figure 8. This material comprises an upper and a lower outer layer (12, 13) as in the material shown in Figure However, in this case, the core layer 14 is formed from lengths of interconnected wire which together define an array of open pyramidal frames similar in structure and function of those ^"of Figure 4. The principal advantage of composite materials such as those shown in Figures 4 and 8 is the ease with which the can be formed into structures having complex profiles. The core layers 9 and 14 are readily deformed into desire shapes and can be sandwiched between outer layers which have been pre-formed (eg. by conventional pressing techniques) to the desired profile to result in the required shape of composite structure.

Both outer layers may be pre-formed, adhesive applied to their inner surfaces, and then brought together with the core layer between them. In this case, the core layer will readily deform to produce the required moulded profile. Alternatively, the core layer and one of the outer layers may be preformed and bonded together, prior to application of the second outer layer.

The composite materials of Figures 4 and 8 can also be produced, if required, with only one outer skin bonded t the "core" layer.

Since neither the outer nor the core layers need be made of metal, the composite material of the invention may comprise a moulded core layer, thus allowing for even easier shaping of the material. This is in contrast to currently available composite materials which, as described above, require complex and time-consuming fabrication before the required profile can be built up from the flat sheets available.

It will be seen from the above that the present inventio combines the advantages of currently known composite materials with the structural advantages achievable usin

SUBSTITUTE SHEET a lattice arrangement for the core layer.

Typical uses of the composite material provided by the invention may include the production of structural or semi-structural members in ships, aircraft, vehicles or buildings, or of cladding materials.

Table 1

2

Conf ig ur a t ion Thickness (mm ) Mass ( kg/m— )

( typical range ) Aluminium Stainless

Core Steel

Core

0.55mm skin 12 - 15 4.3 12.3 0.5mm core @ 60° ( inc luded ang le )

0.55mm skin 12 - 15 5.4 15.4 lmm core @ 60°

0.55mm skin 25 - 30 5.5 15.7 0.5mm core @ 30°

1.0mm skin 25 - 30 7.8 22.3 1.0mm core @ 60°

0.55 mm skin 50 - 60 7.6 21.7 1.0mm core @ 30°

1.0mm skin 50 - 60 12.5 35. 7 1.45mm core @ 30°

Mass per unit area is the total for the composite material , including outer layers, core layer (of either aluminium or stainless steel ) and bonding adhesive .

SUBSTITUTE SHEET

Claims

Cla ims

1. A composite material comprising a relatively thin out layer or layers of a durable , high strength material and thicker core layer bonded to the outer layer or layers , characterised in that the core layer comprises a continuous array of open polyhedral frames , which frames are bonded at their outermost points to the outer layer layers of the composite material.

2. A composite material according to cla im 1 , wherein the array is formed from a two-dimensional "net" of polygons which has been distorted so as to produce a three- imensional network of polyhedral frames.

3. A composite material according to claim 2 , wherein the net is an expanded metal sheet comprising a lattice network of tessellating polygons.

4. A composite material according to claim 3 , wherein the expanded metal sheet comprises a lattice network of squares or rhombi, which on suitable distortion will produce a three-dimensional array of square-based pyramidal frames.

5. A composite material according to claim 1 or claim 2 , wherein the polyhedral frames are formed from lengths of wire , interconnected in such a way as to produce the arra of frames.

6. A composite material according to claim 5 , wherein the array is formed by the distortion of a two-dimensional sheet of wire mesh or netting in which the interconnected wires define an appropriate net shape.

7. A composite material according to any one of the preceding claims, wherein the polyhedral frames are square-based pyramidal frames bonded to the outer layer o layers of the composite material at their apexes.

8. A composite material according to any one of the preceding claims, wherein the core layer is formed from stainless steel.

9. A composite material according to any one of the preceding claims, wherein the outer layer or layers are formed from a material having a high elastic modulus (Young's modulus of elasticity).

10. A composite material according to any one of the preceding claims, wherein the composite material is approximately 22 mm thick.

11. A composite material according to any one of the preceding claims, wherein the outer layer or layers are each approximately 1 mm thick.

12. A composite material according to any one of the preceding claims, wherein the core layer is bonded to the outer layer or layers by means of an adhesive.

13. A structure formed from a composite material accordin to any one of the preceding claims.

14. A composite panel formed from a composite material accord ing to any one of cla ims 1-12.

15. A method of manufacture of a composite material , comprising the steps of distorting a two-dimensional net of polygons so as to produce a three-dimensional continuous array of open polyhedral frames; and bonding the frames at the ir outermost points to an outer layer o layers of a durable , high strength material.

16. A method according to claim 15 , wherein the array of polyhedral frames is bonded to two outer layers , one abo and one below it, so as to form a "sandwich" composite material in which the array separates two outer layers.

17. A method according to claim 15 or claim 16 , wherein the two-dimensional net is an expanded metal sheet.

18. A method according to claim 17 , wherein the expanded metal sheet comprises a lattice network of squares or rhomb i .

19. A method according to any one of cla ims 15-18 , wherein the net is distorted by corrugation.

20. A method according to any one of cla ims 15-19 , wherein the polyhedral frames of the array are bonded to the outer layer or layers by applying a coating of a suitable adhesive to the inner surface of one of the oute layers , prior to contacting that outer layer with the array of frames to which it is to be bonded.

21. A method according to any one of claims 15-19 , wherein the polyhedral frames of the array are bonded to the outer layer or^'layers by applying a coating of a suitable adhesive to the outermost points of the polyhedral frames , prior to contacting the array of frame to the outer layer or layers to which it is to be bonded .

22. A method according to any one of claims 15-21 , wherein either or both of the outer layers is pre-formed to a desired profile prior to the bonding step.

23. A method according to any one of claims 15-22 , wherein the array of polyhedral frames is pre-formed to a desired profile prior to the bonding step.

SUBSTITUTE SHEET