GB2041824A - Composite materials - Google Patents

Abstract

A composite material includes alternating zones, eg sheets of fibre reinforced polymeric material, 1, eg a thermosetting or thermoplastic resin containing carbon or carbon/glass fibres, and metallic material, 2, eg sheets of titanium foil, the materials of the alternating zones having a Young's modulus which is equal or roughly equal in at least one direction. <IMAGE>

Description

SPECIFICATION Composite materials The present invention relates to composite materials.

Fibre reinforced polymeric materials are finding increasing use in engineering applications where some of their specific mechanical properties can be used to advantage. For example, carbon fibre reinforced plastics have relatively high specific properties, eg specific strength and modulus, and may be used in a number of aerospace applications, eg as certain primary and secondary structures in aircraft such as wings, stabilisers, doors, floors, bulkheads, fairings, fences, luggage racks and air-ducts.

Fibre reinforced polymeric materials have certain limitations which could affect their wider use, however. For example their ductility and impact strength are too low for certain applications.

According to the present invention a composite material includesalternating zones of fibre reinforced polymeric material and metallic material the materials of the alternating zones having a Young's modulus which is equal or roughly equal in at least one direction.

In its simplest form the invention has three alternating zones although it may have any number of zones greater than three.

The alternating zones may be formed by bonding together by the action of heat interleaved sheets of fibre reinforced polymeric material and metallic material. Alternatively, at least one of the zones may be formed from a unidirectional array or a mesh or woven gauze of metal wires instead of a metal sheet.

The said Young's modulus is preferably not less than 50 GPa and suitably not less than 90 GPa.

By producing a composite according to the invention it is possible to provide material having improved ductility and impact strength (compared with the fibre reinforced polymeric material) but still possessing a relatively low density (compared, for example, with the metal) and reasonably high tensile strength, and modulus.

It is known to reinforce fibre reinforced polymeric material with metal but this is done essentially to add body and/or strength to the fibre reinforced material. There is essentially no matching of modulus of the two materials. In contrast, matching of the Young's modulus of two materials in accordance with the invention allows the two materials to be subject to uniform strains, for a given stress along the said direction. This means that the two materials may together serve as a true structural composite.

The composite according to the invention may be formed by bonding together the constituent materials by the action of heat in a press or mould. The composite may be moulded into a desired shape during the bonding process.

The polymeric material may be a thermoplastic material eg cellulose acetate, ethyl cellulose, polystyrene, polyvinyl chloride or acetate, polypropylene, polyethylene or polysulphone. Alternatively it may be a thermosetting resin, eg a cured epoxy, phenoiic, polyester, Friedel Crafts, polyimide or fluoropolyimide resin. The fibres, which may be carbon fibres, a carbon/glass fibre, or a carbon-Kevlar fibre blend, may be pre-impregnated with resin (in an uncured form) before formation of the composite.

Alternatively they may be impregnated into material, eg thermoplastics material, by the action of heat during the formation of the composite.

The metallic material is preferably titanium or an alloy containing titanium as a major component, although it may also be beryllium or a beryllium alloy. If the metal is introduced into the composite in sheet form the sheet is preferably a foil having a thickness of from 0.01 to 0.5 mm.

The outer surfaces of the composite are preferably of metal, in which case the composite may have improved resistance to weathering (compared with the reinforced polymeric material).

The composite may have other desirable properties, eg improved electrical and thermal conductivity.

The Young's modulus of the two component materials of the composite, ie metallic material and fibre reinforced polymeric material, may be matched together in a number of ways. For example, for a given fibre reinforced polymeric material of known modulus the metallic material may be specially selected, its modulus being determined by its specific crystallographic orientation or by a special heat treatment applied to it. Alternatively, for a given metal of known modulus the fibre reinforced polymeric material may be specially selected, its modulus being determined for example by the fibre type and concentration within the polymeric material.

As an example, the longitudinal Young's modulus of Type 2 and T > pe 3 carbon fibre sheets containing 60% by volume of fibre are known to be 1 32 and 1 5 GPa respectively. The modulus of the hexagonal alpha phase of titanium is known to range from about 100 GPa with the stressing direction parallel to the basal plane to about 145 GPa with the stressing direction normal to the basal plane.

Intermediate values of modulus to match those of the carbon fibre sheets may be obtained by selecting the stressing direction in relation to the basal plane in the alpha phase in strongly textured material, or by using material in which there is a degree of randomness in the orientation of the alpha grains.

As another example, the modulus of the beta titanium alloy Ti-1 5 Mo (85 weight per cent titanium plus 1 5 weight per cent molybdenum) is known to range from 104 to 123 GPa depending on the ageing treatment applied to the alloy. By selecting a specific known ageing treatment, which may be determined by experiment, the alloy may be obtained having a modulus anywhere in this range, eg 11 5 GPa.

In an aspect of the invention, if the composite is made from interleaved sheets of hexagonal alpha phase titanium or titanium alloy foil and a fibre reinforced polymeric material the basal plane of the alpha phase (which is the plane normal to the direction of greatest Young's modulus) may be in a pre-determined relationship with the surface of the foil and the alignment of the fibres. For example, for applications where unidirectional stressing is important, eg frames and stringers, the basal plane may be arranged normal to the surface of the foil and to the fibres. For applications where biaxial stressing is important, eg pressure vessels, the basal plane may be arranged parallel to the surface of the foil and to the fibres.

In such examples the moduli of the foil and of the fibre reinforced material may be matched by selecting the properties required of the fibre reinforced material as described above.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which Figure 1 is an end view of a stack of sheets ready for the production of a composite embodying the invention; Figure 2 is a plan view of sheets used to form a composite embodying the invention.

Sheets 1 of Type AS carbon fibres (60% nominal volume fraction) pre-impregnated with Carboform Code 69 (Trade Mark) thermosetting resin (an epoxy resin) are cut to size. Sheets 3 of Ti-6Al-4V alloy (a titanium alloy containing 6 weight % aluminium and 4 weight % vanadium) in the form of a foil are cut to the same size. The foil surfaces have been chemically treated by the process described below. The sheets 1 are stacked alternately with the sheets 3 in a conventional compression mould (not shown). The stack is then cured to form the desired composite in the following way.

The top face of the mould is arranged to rest against the top face of the stack and the mould is heated to about 1 200C and maintained at this temperature for about 30 minutes. The top face of the mould is then closed as far as its stops will allow to give maximum compression. The contents of the mould are then heated for a further hour at about 1 20cm followed by an hour at about 1 700C. The mould is then allowed to cool to room temperature. After cooling the cured composite is removed from the mould.

A A test piece TP 1 was produced by this method using six sheets of the pre-impregnated carbon sheet sheet approximately 3 mm thick interleaved with five sheets of the titanium alloy foil having a thickness of about 0.30 mm. The sheets each had a shape as shown in Figure 2 with notches cut into their sides (along their length), the original size ot the sheets being 100 mm x 13 mm. This shape is a conventional one characteristic of tensile test pieces. It is determined by the requirement for fracture to occur in the parallel gauge length rather than at their ends when gripped.

A test piece TP2 was also produced by the above method consisting of eight sheets of the preimpregnated carbon fibres interleaved with six sheets of the titanium alloyfoil 0.18 mm thick, two of the carbon fibre sheets forming a single outer layer. The shape and size of the sheets were otherwise as forTP1.

TENSILE STRENGTH AND YOUNG'S MODULUS The tensile strength and Young's modulus of test pieces TP 1 and TP2 were measured. For this purpose the outer surface (ie the carbon fibre reinforced plastic layers obtained in the composite) were loaded in wedge grips with aluminium alloy plates bonded to the faces of the specimens in the region of the grips. The results of the measurements are given in Table 1 as follows: TABLE 1: Tensile Strength (TS) and Young's Modulus (E) of composites

Test Piece TS (MPa) E (GPa) TPl 905 112 TP2 922 96 Both test pieces TP 1 and TP2 ultimately failed by a combination of two processes during the test, namely inelastic rupture of the carbon fibre reinforced material and elastic/plastic rupture of the titanium alloy foil.

These results may be compared with those for other high strength materials. This is done in Table 2 as follows (where specific strength and modulus are respectively the tensile strength and Young's modulus divided by specific gravity; CFRP is the same carbon fibre reinforced plastic as in the composite TP 1, TP2).

TABLE 2: Comparison of properties of various high strength materials

Tensile Specific Young's Specific Material Specific Strength Strength Modulus Modulus Gravity (MPa) (MPa) (GPa) (GPa) Steel 300 M 7.86 1900 241 206 26.2 Ti Alloy IMI 550 4.54 1250 275 110 24.2 Al Alloy DTD 5104 2.8 460 164 76 27.1 Experimental Al Alloy RAE 72 2.9 750 258 90 31.0 Mg Alloy DTD 721 1.8 230 127 48- 26.6 CFRP 1.5 1350 900 135 90.0 TiiCFRP Composite 2.81 905 322 112 39.8 TP1 Ti/CFRP Composite TP2 2.16 922 426 96 44.4 Table 2 shows that although the values of tensile strength and Young's modulus for the composites embodying the invention, ie TP 1 and TP2, are lower than the corresponding values for the CFRP nevertheless the values are still high compared with those for other lightweight high strength materials, eg aluminium and magnesium alloys, and the values of specific strength and modulus obtained for TP 1 and TP2 are higher than for all of the materials except CFRP.

IMPACT STRENGTH Unnotched test pieces TP3 50 x 6 x 3 mm in size were cut from a composite strip 100 x 1 3 x 3 mm made by the method described above using 7 layers of the Carboform pre-impregnated carbon fibres interleaved with 6 sheets of the Ti alloy foil 0.25 mm thick. One of the outer layers of the preimpregnated carbon fibres had a double thickness, being formed from two sheets.

The impact strengths of the test pieces TP3 were measured by impacting their 6 x 50 mm face.

These were conventional "Charpy-type" tests carried out using a Zwick (Trade Mark) impact testing machine.

The corresponding impact strenyths of CFRP sheets having the same composition as those in the composite were also measured for reference. The results of the measurements are given in Table 3 as follows: TABLE 3: Impact strength of composites and of CFRP

Width Thickness Impact Strength Material mm mm KJ/m2 Type of Failure Composite 5.7 3.29 225 Delamination & Bending Test Pieces TP3 5.93 3.27 197 = 217 + 17 ,, ,, 6.18 3.17 229 ,, ,, CFRP 5.87 3.11 67 Tensile 8 fine deiamination Sheets 6.04 3.10 65=74+14 " .. 5.52 3.21 91 ,, ..

Table 3 shows that in face incident impact (CFRP face) the impact strength of the composite is approximately three times greater than that of the corresponding CFRP of similar size.

Thus, although the composites embodying the invention have a lower tensile strength and Young's modulus than the corresponding CFRP (although these properties are still relatively high when compared with those of other high strength materials) the composites have the advantage of a higher face incident impact strength.

If the composites are fabricated so that the Ti alloy foil instead of the CFRP forms its outside layers they may have increased resistance to weathering. In this case the sheets 1 in Figure 1 may be considered as being of Ti alloy foil and the sheets 3 or carbon fibre reinforced material.

The composites may be fabricated with a gauze or mesh or array of wires of Ti alloy in place of the Ti alloy foil. in this case the gauze etc may be considered as replacing the sheets 3 in Figure 1 (the sheets 1 being considered as of carbon fibre reinforced material).

In order to improve the bond between the resin oi the CFRP and the titanium alloy foil, gauze etc as used in the above embodiments the latter may be treated by the following known alkaline peroxide treatment before it is placed in the mould to form the desired composite.

The alloy is degreased with trichloroethane or trichloroethylene. It may then be treated with ethyl methyl ketone or iso-butyl methyl ketone to remove other marks. This treatment is followed by the application of abrasion, eg by blasting with grit alumina or by rubbing with scouring pad charged with a scouring powder, followed by washing in hot water. Immersion in an alkali based cleaner follows, eg in an aqueous 3% solution of Stripalene 532 (Trade Mark) at 700C followed by further hot water washing.

The alloy is then immersed for about 20 minutes at 65-700C in a peroxide solution having the following composition: sodium hydroxide (20g); hydrogen peroxide 30% (100 vol) 22.5 ml; and water (to give a total volume of 1 000 ml) until it appears black. The alloy is then washed in hot water again for 1 5-20 minutes followed by drying in warm air. It is then ready for use to form a composite.

Claims (13)

1. A composite material including alternating zones of fibre reinforced polymeric material and metallic material, the materials of the alternating zones having a Young's modulus which is equal or roughly equal in at least one direction.

2. A composite material as claimed in claim 1 and wherein the alternating zones have been formed by bonding together by the action of heat interleaved sheets of fibre reinforced polymeric material and metallic material.

3. A composite material as claimed in claim 1 and wherein at least one of the zones is formed from a unidirectional array or a mesh of woven gauze of metal wires.

4. A composite material as claimed in any one preceding claim and wherein the said Young's modulus is not less than 50 GPa.

5. A composite material as claimed in claim 4 and wherein the said Young's modulus is not less than 90 GPa.

6. A composite material as claimed in any one of the preceding claims and wherein the polymeric material is a thermoplastic material.

7. A composite material as claimed in any one of claims 1 to 5 and wherein the polymeric material is a thermosetting resin which has been bonded to the metallic material during setting.

8. A composite material as claimed in any one of the preceding claims and wherein the fibre reinforced polymeric material incorporates carbon fibres.

9. A composite material as claimed in any one of the preceding claims excluding claim 3 and wherein the metallic material is titanium or an alloy containing titanium as a major component.

10. A composite material as claimed in claim 9 and wherein the metallic material is in the form of a foil sheet having a thickness of from 0.01 to 0.5 mm.

11. A composite material as claimed in any one of claims 1 to 8 and wherein the metallic material is beryllium or an alloy containing beryllium as a major component.

12. A composite material as claimed in claim 1 and substantially the same as one of the composite materials described hereinbefore.

13. A method of forming a composite material as claimed in claim 1, the method being substantially as described with reference to Figure 1 of the accompanying drawings.