GB2058661A

GB2058661A - Honeycomb Structure

Info

Publication number: GB2058661A
Application number: GB8023737A
Authority: GB
Original assignee: SECR DEFENCE; UK Secretary of State for Defence
Current assignee: SECR DEFENCE; UK Secretary of State for Defence
Priority date: 1979-07-19
Filing date: 1980-07-21
Publication date: 1981-04-15
Also published as: DE3027307A1; GB2058661B; JPS5653058A; FR2486874A1

Abstract

A method of producing a honeycomb structure includes the steps of: (a) providing a plurality of flexible sheets 5, 11 made from fibres including carbon fibres (e.g. woven sheets having 3000 filaments per tow); (b) depositing on each sheet a series of discrete regions 3,7 e.g. parallel stripes of a tacky adhesive (e.g. PVA) which adheres to the fibres of the sheet but does not substantially spread from the regions across the sheet; (c) stacking the sheets together so that the series of adhesive regions on alternate sheets have a mutual lateral displacement and so that the adhesive regions on each sheet adhere to corresponding regions on the adjacent sheet; (Fig. 1). (d) stretching the sheets apart at regions which ore not adhered together by the adhesive material so that a honeycomb structure is formed by the sheets, the adhesive regions forming joints between adjacent cells of the structure: (Fig. 3). (e) applying to the structure with the sheets stretched apart a material capable of being set to bond to the sheets and stiffen the structure to retain the honeycomb form (e.g. a conventional resin and hardener); (f) allowing the material applied in step (e) to set so that the structure retains its honeycomb form. <IMAGE>

Description

SPECIFICATION Honeycomb Structures The present invention relates to honeycomb structures.

Sheets of material joined together to form an array of adjoining hexagonal cells perpendicular to the surfaces of the sheets are known as honeycomb structures. Such structures made for example from sheet of paper, cotton or glass fibres have been well known for many years in the field of constructions materials. Metal, e.g. aluminium, and Nomex (Trade Mark) have also been used widely for the formation of honeycomb structures. These structures offer a reduction in density without an excessive reduction in strength or stiffness compared with similarly dimensioned solid pieces of the same material.

For high-strength, high-stiffness, applications such as certain applications in the aerospace industry, the best material available for the formation of honeycomb structures is aluminium. However in certain uses, e.g. glider wings and moving aircraft control surfaces such as rudders, brakes and ailerons, it would be beneficial to provide a honeycomb structure which has a greater stiffness and strength but a lower density than an aluminium honeycomb structure having the same cell dimensions.

According to the present invention in a first aspect a method of producing a honeycomb structure includes the steps of: a. providing a plurality of flexible sheets made from fibres including carbon fibres; b. depositing on each sheet a series of discrete regions of a tacky adhesive which adheres to the fibres of the sheet but does not substantially spread from the regions across the sheet; c. stacking the sheets together so that the series of adhesive regions on alternate sheets have a mutual lateral displacement and so that the adhesive regions on each sheet adhere to corresponding regions on the adjacent sheet; d. stretching the sheets apart at regions which are not adhered together by the adhesive material so that a honeycomb structure is formed by the sheets, the adhesive regions forming joints between adjacent cells of the structure;; e. applying to the structure with the sheets stretched apart a material capable of being set to bond to the sheets and stiffen the structure to retain the honeycomb form; f. allowing the material applied in step (e) to set so that the structure retains its honeycomb form.

In step (a) of the method according to the first aspect of the invention the fibres of the sheets may be entirely of carbon fibres or a hybrid of carbon fibres and other fibres, e.g. glass fibres. Preferably the fibres are in woven tows in which case each tow of carbon fibres should preferably have 5000 fibres per tow or less, advantageously about 3000 fibres per tow.

If the sheets are woven the number of tows per cm running in one weaving direction (e.g. the warp direction) may or may not be equal to the number of tows per cm running in the other weaving direction (e.g. the weft direction).

Where the warp and weft fibres are not equal, it is preferred to have the greater amount of fibres (usually warps) running vertically within the honeycomb (that is from top face to bottom) so as to give the maximum strength and stiffness in this direction.

The fibre sheets may be impregnated with materials known as sizes which are conventionally used to clump together carbon fibres to prevent fraying of the tows. Such materials may contain small amounts of resinous materials. But the sheets should not be treated with resinous material in such quantities that they are significantly stiffened by the material otherwise they will not be suitably flexible for stretching in steps (d) and (e). An unsuitable degree of stiffening would, for example, be that obtained by impregnating carbon fibres in a partly-cured resin as in the formation of 'pre-preg' sheets, then curing these sheets before expansion is attempted.

In step (b) the adhesive regions are preferably stripes. 'Tacky' adhesive indicates that the adhesive adheres readily to the sheets. The adhesive regions need only be deposited on one surface only of each sheet.

The adhesive may be one of the following: polyvinyl acetate, polyvinyl alcohol, polyvinyl ether, starch solution, dextrose solution, neoprene or resorcinol/formaldehyde or a thickened liquid epoxy resin.

If the adhesive regions are stripes then in step (c) the sheets are stacked with the stripes on different sheets all running parallel to one another.

In step (d) the shape of the cells of the honeycomb structure may or may not be a regular hexagonal shape, but such a shape is preferred.

In step (e) the applied material may include any suitable cold-setting or hot-setting resin or a thermoplastic polymer, such as any of the resins or polymers known in the formation of carbon fibre reinforced plastics, for example polyester resins, epoxy resins, Friedel-Crafts resins, polyethersulphones or polyimides, together with any suitable known hardener, for example, any of the boron trifluoride complex salts such as are well-known for use with epoxy resins, or any known thixotropic or other filler, as necessary.

If the material applied in step (e) is a solution containing a thermoplastic material, it will be necessary during the application to cause the material to flow to contact the sheets intimately, followed by drying off of the solvent by heat.

If the material applied in step (e) includes a hot-setting resin then step (f) will follow by means of the application of heat to cure the resin, while the honeycomb remains throughout in the stretched condition.

According to the invention in a second aspect there is provided a honeycomb structure which is the product of the method according to the first aspect above.

The method according to the first aspect of the invention provides a practical way of making a honeycomb structure from carbon fibres. The product of the method can be obtained with a greater stiffness and strength yet a smaller density than an aluminium honeycomb structure of similar dimensions.

The product according to the second aspect may be used as a lightweight filling for sandwich structures, e.g. aerofoil structures or flooring panels. The outer layer of such sandwich structures may be made of any suitable material, e.g. wood, metal, plastics, or reinforced plastics, depending on the application. The outer layers may be bonded to the filling by a suitable known adhesive, e.g. a coldsetting liquid epoxy resin, or a hot-setting film adhesive such as a nitrile/phenolic blend.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a cross-sectional view of a stack of carbon fibre fabric sheets; Figure 2 is a cross-sectional view of one of the sheets shown in Figure 1; Figure 3 is a cross-sectional view of the stack shown in Figure 1 stretched into a honeycomb structure.

In Figure 1 a stack 9 is formed, e.g. as described below, from a series of sheets 1 of carbon fibre woven cloth alternating with a series of sheets 5 of the same cloth. The cloth may be a 5-shaft satin, which contains about 3000 fibres per tow, the fibres having a thickness of about 7=8 2 microns.

One of the sheets 1 is shown separately in Figure 2. A series of parallel stripes 3 of adhesive material is deposited, e.g. by silk screen printing, on the upper surface of each sheet 1. The material of the stripes 3 may be one of the following: polyvinyl acetate, polyvinyl alcohol, polyvinyl ether, starch solution, dextrose solution, neoprene or resorcinol/formaldehyde or thickened epoxy resin. These are all tacky materials, i.e. will readily adhere to the sheet 1. Also they will not become brittle after adhering to the sheet 1. The spacing between adjacent stripes 3 is about three times the width of the stripes 3.

The sheets 5 have on their upper surface a series of parallel stripes 7 which are of the same material and are deposited by the same deposition technique as the stripes 3 as described below. In fact the stripes 7 are the same in all respects as the stripes 3 except that there is a lateral displacement between the two series and consequently there are more stripes 7 per sheet 5 than stripes 3 per sheet 1. The lateral displacement is half of the lateral periodicity of the stripes 3 so that the stripes 3 are laterally mid-way between adjacent stripes 7 and so that the stripes 7 are laterally mid-way between adjacent stripes 3.

The stack 9 is built up soon after deposition of the stripes 3 and 7 on the sheets 1 and 5 respectively so that the adhesive material will adhere to the lower surface of the next sheet 1 or 5 as appropriate whilst still tacky.

An example of a method of forming the stack 9 is as follows. Equal lengths of carbon fibre cloth from a single roll are out to form the sheets 1 and 5 which are passed alternately in a single procession by rollers and guides (not shown) to a deposition bed (not shown) where the adhesive stripes 3 and 7 are deposited by a known technique, e.g. silk screen printing, onto the sheets 1 and 5 respectively. The bed has a known displacement mechanism which reciprocates the bed laterally between receipt of consecutive sheets 1 or 5 so that the stripes 3 are displaced laterally relative to the stripes 7. The sheets 1 and 3 are then passed by further rollers and guides (not shown) into a collection tray (not shown) where the stack 9 builds up.

The uppermost sheet of the stack 9, indicated by reference numeral 11, in Figure 1 does not need any adhesive stripes on its upper surface.

After the stack 9 has been formed a weight (not shown) is placed on top of the sheet 11 to assist adhesion between the sheets 1 and 5 by the adhesive of the stripes 3 and 7, particularly at the upper end of the stack 9. The weight is left for 30 to 45 minutes.

After removal of the weight the stack is stretched vertically in a known way, e.g. by bonding a plywood sheet (not shown) to the lowest sheet (one of the sheets 5 in Figure 1), then gripping the lowest sheet and by lifting the uppermost sheet 11 by flat fingers inserted in the gaps between the sheet 11 and the next sheet (one of the sheets 5 in Figure 1 ) and between the stripes 3. Such stretching is conveniently done in a rigid curing jig (not shown) in order to achieve constant degrees of expansion.

By this stretching a honeycomb structure is formed from the stack 9 as shown in Figure 3. The stripes 3 and 7 form vertical links between adjacent cells in the structure. The stack 9 is transferred in this honeycomb structure form to a tank (not shown) containing a solution of an impregnating material suitable for setting the honeycomb structure, e.g. a hot setting epoxy resin solution, such as Shell Epikote DX2 10 (Trade Mark) with BF3 400 hardened, which after air-drying is cured at a temperature of 1500--1600C for a period of 60 minutes.

After removal from the curing jig and subsequent cooling the resultant rigid carbon fibre reinforced honeycomb structure may be cut into vertical slices which may be used in any known application for a lightweight filling for a sandwich structure, e.g. the lightweight core of a sandwich structure for a flooring panel. The outside layers of the sandwich may be of carbon fibre reinforced plastics or carbon/glass hybrid, and may be bonded to the honeycomb filling by any suitable bonding agent, e.g. a cold-setting epoxy resin, thickened with a thixotropic silica filler to prevent dripping into the cells of the honeycomb.

A suitable composition for the adhesive material of the stripes 3 and 7 is as follows: a. Liquid epoxy resin Ciba-Geigy XD927 (Trade Mark) 100 parts (g) b. Liquid amine (hardener XD927 (Trade Mark)) 36 parts (g) c. Finely divided silica. "Cab-O-Sil" Grade EH5 (Trade Mark) from Cabot Corporation 6 parts (9) d. Colouring matter (e.g. eosin dye) compatible with epoxy resin 0.25 gr The amount of filler can be adjusted for use with thinner or thicker fabrics, between four and 8 parts per hundred of resin.

Claims

1. A method of producing a honeycomb structure including the steps of: (a) providing a plurality of flexible sheets made from fibres including carbon fibres; (b) depositing on each sheet a series of discrete regions of a tacky adhesive which adheres to the fibres of the sheet but does not substantially spread from the regions across the sheet; (c) stacking the sheets together so that the series of adhesive regions on alternate sheets have a mutual lateral displacement and so that the adhesive regions on each sheet adhere to corresponding regions on the adjacent sheet; (d) stretching the sheets apart at regions which are not adhered together by the adhesive material so that a honeycomb structure is formed by the sheets, the adhesive regions forming joints between adjacent cells of the structure;; (e) applying to the structure with the sheets stretched apart a material capable of being set to bond to the sheets and stiffen the structure to retain the honeycomb form; (f) allowing the material applied in step (e) to set so that the structure retains its honeycomb form.

2. A method as claimed in Claim 1 and wherein the fibres of the flexible sheets are entirely carbon fibres.

3. A method as claimed in Claim 1 and wherein the fibres of the flexible sheets are a hybrid of carbon and other fibres.

4. A method as claimed in Claim 3 and wherein the fibres of the flexible sheet are a hybrid of carbon and glass fibres.

5. A method as claimed in any one of the preceding claims and wherein the fibres of the flexible sheet are in woven tows, each tow of carbon fibres having 5000 fibres per tow or less.

6. A method as claimed in Claim 5 and wherein each tow of carbon fibres has about 3000 fibres per tow.

7. A method as claimed in any one of the preceding claims and wherein the adhesive regions are stripes, the sheets being stacked in step (c) with the stripes on alternative sheets running parallel to one another.

8. A method as claimed in any one of the preceding claims and wherein the adhesive regions deposited in step (b) are of material seiected from the following: polyvinyl acetate, polyvinyl alcohol, polyvinyl ether, starch solution, dextrose solution, neoprene or resorcinol/formaldehyde or a thickened liquid epoxy resin.

9. A method as claimed in Claim 8 and wherein the adhesive regions are deposited in step (b) by silk screen printing.

1 0. A method as claimed in any one of the preceding claims and wherein the material applied in step (e) is a coid-setting resin.

11. A method as claimed in any one of Claims 1 to 9 and wherein the material applied in step (e) is a hot-setting resin, step (f) including the application of heat to cure the resin.

1 2. A method as claimed in any one of Claims 1 to 9 and wherein the material applied in step (e) is a thermoplastic polymer material.

13. A method as claimed in Claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.

14. A honeycomb structure which is the product of the method claimed in any one of the preceding claims.

1 5. A structure as claimed in Claim 14 and which forms the lightweight filling of a sandwich structure, the filling being bonded to the outer layers of the structure.