Method of bonding a chrome steel to a fibre composite

Abstract

A method of bonding a high chrome steel to a fiber composite. An epoxy primer is applied to the high chrome steel, the primer is dried, and thereafter the high chrome steel and the fiber composite are adhered together by using an epoxy adhesive. The invention also includes bonded joints that are made by the method and bonded assemblies having the bonded joint. A joint made using this bonded method has a high torque-to-failure, even after exposure to extreme ambient conditions of high temperature and high relative humidity. The bonding method is particularly suitable for use in bonding high chrome steel and fiber composite components for use in the aerospace industry.

Description

[0001] The present invention relates to a method of bonding, and has particular reference to a method of bonding high chrome steel to a fibre reinforced composite material. The invention also comprehends a bonded joint formed using the method of the present invention, and a bonded assembly comprising a high chrome steel component and a fibre composite component that are bonded together using the bonding method of the present invention.

[0002] In a particular aspect, the present invention provides a rotary drive shaft comprising a carbon fibre composite tube and a high chrome steel flange that are bonded together using a bonding method of the present invention.

[0003] Composite rotary drive shafts are widely used throughout the aerospace industry, and are employed, for instance, in the wings of aircraft. WO-A-98/20263, the contents of which are included herein by reference, discloses such a rotary drive shaft and a method for manufacturing it. The shaft of WO-A-98/20263 comprises a carbon fibre composite tube that is fitted at one end with a titanium flange. Said titanium flange is bonded to the composite tube, and the joint is also riveted for reinforcement. Said titanium flange constitutes an expensive component, and at the time of writing there is a general desire within the aerospace industry to reduce the unit cost of components for aircraft.

[0004] Those skilled in the art will know that aerospace components are subjected to extreme ambient conditions in use. In particular a special problem in the aircraft industry is the high temperature and high relative humidity of the operating conditions. In order to be qualified for use on aircraft, components are therefore subjected to rigorous testing under extreme conditions of temperature and relative humidity.

[0005] There is a general requirement to bond components together, rather than to use mechanical fixings such as rivets or bolts, because the latter tend to work loose and/or fail through repeated stress-relax cycles. However, it will be appreciated that adhesive bonding may become vulnerable to attack by moisture. Furthermore, some adhesively bonded joints may fail through differences in the coefficients of thermal expansion of the adherends. Accordingly, there is an ongoing need to find new methods and materials for bonding components for use in the aerospace industry which are subject to high temperature and high relative humidity conditions.

[0006] The use of carbon fibre component materials is preferred within the aerospace industry because of the lightness of such components and their high strength and rigidity. However, the bonding of composite components to other components represents an area of particular difficulty, because moisture is able penetrate a joint not only along the bond line, but also through the composite itself.

[0007] According to one aspect of the present invention there is provided a method of bonding a first high chrome steel adherend to a second fibre composite adherend, which method comprises the steps of applying an epoxy primer to said first adherend, causing or allowing said primer to dry and thereafter adhering the first and second adherends together using an epoxy adhesive.

[0008] In another aspect of the present invention there is provided a bonded joint comprising a first high chrome steel adherend and a second fibre composite adherend, which first and second adherends are bonded together using the method of bonding of the present invention.

[0009] In yet another aspect of the present invention there is provided a bonded assembly comprising a first high chrome steel component and a second fibre composite component, which first and second components are bonded together using the method of bonding of the present invention.

[0010] In a preferred aspect of the present invention, said bonded assembly comprises a rotary drive shaft comprising a fibre composite tube and a high chrome steel flange, which tube and flange are bonded together using the bonding method of the present invention.

[0011] Preferably said tube is formed with at least one, and preferably two, flared ends, each of which is deformed to form an integrant flange portion having an annular bonding face as described in WO-A-98/20263. Said bonding face is bonded to said steel flange using the bonding method of the present invention.

[0012] Said high chrome steel flange represents a significantly cheaper component than the titanium flange of the prior art. Optionally, the joint between the tube and flange may be reinforced by rivets, bolts or any other suitable mechanical fixings.

[0013] Surprisingly, it has been found that the bonding method of the present invention allows a high chrome steel adherend to be bonded to a fibre composite adherend to provide a joint which has a high torque-to-failure, even after exposure to extreme ambient conditions of high temperature and high relative humidity.

[0014] Said high chrome steel adherend may comprise a aerospace grade steel. Specifically, the steel may comprise 12% to 20% by weight chromium, preferably 15% to 19% wt. Said steel may further comprise 0% to 8% wt nickel, preferably 2% to 6% wt. Typical additions of chromium and nickel are 17% wt and 4% wt respectively. Thus, said first high chrome steel adherend may comprise a 17/4 grade steel.

[0015] Said second fibre composite adherend may comprise an aerospace-grade fibre-reinforced composite material comprising au epoxy resin. Preferably, said composite will comprise a carbon fibre reinforced composite material, although it is envisaged that the composite may be reinforced with any suitable fibres known in the art. A specific carbon fibre composite material suitable for use as the second fibre composite adherend is the material 6376cHTA(12K-5-35) which is commercially available from Hexcal Corporation.

[0016] Said first adherend is preferably pre-treated by abrading or etching before the primer is applied. Such pre-treatment may serve to remove contaminants or weak boundary layers and/or alter the surface chemistry or surface morphology of the metal or any metal oxide surface layer(s).

[0017] Said first high chrome steel adherend may be decreased to remove contaminants before abrading or etching is carried out. A suitable abrasion agent is particulate aluminium oxide, for example 60 mesh alumina in a water vehicle, which may be wet-blasted onto the first adherend. Preferably, the water used for such wet-blasting is free of anti-corrosion agents.

[0018] In some instances, etching ray be preferable to abrading, and a suitable etching solution comprises iron (III) chloride and hydrochloric acid. Such an etching solution will be familiar to those skilled in the arts Alternative etching methods comprise an oxalic acid/sulfuric acid etch, or etching using hydrochloric acid.

[0019] Said second adherend may be degreased before application of the adhesive. In some embodiments the second adherend will be protected by a strippatte film of the kind well known to those skilled in the art.

[0020] Said second adherend may also be pre-treated if and where necessary by abrading, particularly light abrading, to remove contaminants and/or loose or weakly bound material. Light abrasion may be effected using, for example, a suitable grit paper, followed by decreasing. A suitable grit paper is 120 mesh carborundum grit paper.

[0021] Said epoxy primer will advantageously have a rheology and chemical composition that is selected to match the morphology and surface chemistry of the first adherend, to provide optimum protection of the pre-treated surface, and to be compatible with the adhesive. Said primer may be solvent based, but is preferably water based, and may have a solids content of 5-15% wt. typically about 10% wt. Said primer may comprise a simple surface protection solution. Advantageously however, the primer comprises a hardening agent. In some embodiments of the invention, the primer comprises an anti-corrosion agent. Said anti-corrosion agent may be any suitable such agent known to those skilled in the art such, for example, as a zinc salt anti-corrosion agent. Preferably however the anti-corrosion agent comprises a chromate salt such, for example, as strontium chromate.

[0022] In one embodiment, the primer comprises a dispersion of an epoxy resin or mixture of epoxy resins in water. Said dispersion may further incorporate an epoxy curing agent. In particular, said curing agent may be provided as a distinct phase. Said epoxy resin or mixture of epoxy resins may comprise one or more chain-extended, solid glycidyl ethers of phenols, such as resorcinol and the bisphenols, such as bisphenol A, bisphenol F, and others familiar to the man skilled in the art. Alternatively, said epoxy resin or mixture of epoxy resins may comprise one or more of the solid glycidyl derivatives or aromatic amines and aminophenols, such as N,N,N′N′-solid DGEBA resins.

[0023] Preferably, said curing agent is substantially water insoluble, and is solid at room temperature. Said curing agent may comprise an aromatic amine curing agent such as 4,4′-diaminodiphenylmethane; 3,3′- or 4,4′-diaminodiphenyloxide; 3,3′- or 4,4′-diaminodiphenylsulfide; or 3,3′- or 4,4′-diaminodiphenylketone. In particular, said curing agent may comprise 4,4′-[1,4-phenylene(1-methylethylidene)]-bis(benzeneamine). However, various other solid diamine curing agents of the type well known to the man skilled in the art may also be used.

[0024] Said primer may further comprise a toughening agent, such as, for example, an elastomer. Advantageously, said primer may further comprise an anti-corrosion additive or mixture of anti-corrosion additives. Said anti-corrosion additive may comprise a chromate salt, for example strontium chromate, barium chromate, zinc chromate or lead chromate. Alternatively, said anti-corrosion additive may comprise a non-chromate corrosion inhibitor, such as zinc phosphate, zinc molybdate or SICORIN RZ available from BASF AG, Ludwigshafen, Germany. In some embodiments the primer may be free of any anti-corrosion additives.

[0025] The preparation of a suitable primer is described in U.S. Pat. No. 5,461,090, the contents of which are incorporated herein by reference.

[0026] Specifically, said primer may be BR®6747-1-A which is commercially available from Cytec Industries, Inc. The CAS Registry number for this substance is 192390-61-7. BR®6747-1-A is a 100% water based corrosion inhibiting primer comprising a one part modified epoxy chromate primer which contains substantially no volatile organic compounds. A chromate-free equivalent of BR®6747-1-A may alternatively be used.

[0027] Said primer may be air-dried or dried in an oven. In some embodiments the primer may be pre-cured, usually in an oven, before application of the adhesive.

[0028] Pre-curing of the primer may be advantageous because it may serve to localise functional ingredients of the primer (such as the anti-corrosion agent) juxtaposed the first adherend.

[0029] Said epoxy adhesive may be a film adhesive, a paste adhesive or any other suitable form of adhesive. Where a film adhesive is used, it may comprise a suitable reticulated carrier (US: “support”) or it may be unsupported.

[0030] Said carrier is preferably formed from a polymeric plastics material such as nylon or polyester, with polyester being especially preferred. Said carrier may comprise a woven, knitted or needled fabric. Preferably, said fabric is formed from monofilamentis fibres, so as to avoid wicking of water into the bond.

[0031] Preferably the adhesive is a modified epoxy adhesive. A suitable adhesive is that which is available commercially from Cytec Engineering Materials Inc under the trade name FM300K, particularly FM300K.05 (250 g/m2) . FM 300K is an epoxy resin adhesive film which may include 0-0.5% titanium dioxide. The CAS Registry number for FM 300K is 71210-48-5.

[0032] Said adhesive may be applied to the first adherend or the second adherend, or both. The adhesive is then cured in an oven or autoclave.

[0033] The second composite adherend may be used in a pre-cured or uncured state. Advantageously, the second adherend is used uncured, and after application of the primer to the first adherend (and optionally the second adherend) and application of the adhesive, the composite material is co-cured with the primer and adhesive.

[0034] The present invention thus provides a method of bonding a high chrome steel component to a carbon fibre composite component which has sufficient torque-to-failure under conditions of high moisture and high temperature to make it suitable for use in the aerospace industry.

[0035] Following is a description by way of example only with reference to the accompanying drawings of embodiments of the present invention.

[0036] In the drawings:

[0037] FIG. 1 is a side view, partly in cross-section, of one end of a rotary power transmission shaft in accordance with the present invention.

[0038] FIG. 2 is an end view of the shaft of FIG. 1.

EXAMPLES

Adherends

[0039] Stainless steel (AMS 5643 (H1025), 17/4, 54 mm dia) and fibre reinforced epoxy composite (Fibredux 6376, 75×753 mm) components were used. The epoxy composite was supplied with a protective strip ply that was removed before use.

Adhesive

[0040] Cytec FM300K—this is a modified epoxy film adhesive with a polyester carrier and supplied by Cytec Engineering Materials Inc. CAS Registry no. 71210-48-5. The grade used was FM300K.05 (250 g/m2)

Primer

[0041] BR®6747-1-A, 100% water based corrosion inhibiting primer. This is a 100% water based corrosion inhibiting primer comprising a one part modified epoxy chromate primer which contains substantially no volatile organic compounds. CAS Registry no. 192390-61-7. This primer was sprayed on stainless steel adherends after pre-treatment.

Surface Pre-Treatment (After Removing the Strip Ply)

[0042] The epoxy composite components were first degreased using Lotoxane® wipes then abraded with 120 alumina grit and degreased. A number of different pre-treatments (See Table 1 below) were used to prepare the surfaces of the stainless steel adherend before bonding. Details of each pre-treatment are as follows:

Ferric Chloride

Hydrochloric Acid Etch

[0043] The following etch solution was used to treat stainless steel components.

[0044] a) 50/50 (w/w) of Iron (III) chloride and 35-37% HCl

[0045] b) Temperature of solution was maintained at 20-23° C.

[0046] c) Etched for 10 minutes

[0047] d) washed with deionised water

[0048] e) Dried at room temperature,

Wet Blasting

[0049] Wet blasting was carried out using 60 mesh aluminium oxide at an air pressure of 401 bf/in2 for a duration of approximately 2-3 seconds.

Primer

[0050] The primer was used as follows:

[0051] BR®6747-1-A thin coating (3-40 m) of this primer was applied using a spray gun. The coated specimen was first allowed to dry at room temperature and then the primer was cured at 120° C. for 60 minutes. A gravity feed air pressure spray gun (M21G) supplied by Kremlin) was used to spray the Cytec primer. This gun works on the principle of conventional spraying which mixes the primer with air as it leaves the gun. The primer comes out of the nozzle under low pressure and is sprayed by a fan or compressed air coming from the aircap at around 3-6 bar. On pulling the trigger, firstly the air is liberated by the air valve, and then the product is emitted by the needle. The whole assembly of the aircap, the needle and the nozzle is called the projector. The primer was sprayed at a pressure of 4 bar. The specimens were coated by a single pass or spraying to give the required thickness as described earlier.

Joint Preparation

[0052] The joints were prepared using the stainless steel and fibre reinforced epoxy composite components after various pre-treatments or primer/adhesive combinations. The preparation conditions and number of specimens required to assess the joint strength before and after ageing are shown in Tables 1 and 2 below. A jig was specifically designed and constructed for assembly and bonding of the components. The base of the jig consisted of a large mild steel washer (65 mm diameter) with an M6 clearance hole. A length of 6 mm studding was pet through the hole and a M6 nut as well as a locking nut were placed on the underside of the washer. The composite part of the bonding component was positioned in place and this was followed by placement of the film adhesive and stainless steel component. A calibrated music wire spring with a rating of 20 N/mm was placed over the studding and this was held compressed by a washer and a nut. This type of spring allowed pressure of 2 bar to be applied to the joint during cure. The whole assembly of each joint was then placed in an oven to cure.

Joint Assessment

[0053] The strength of each set of joints was determined by measuring the torque-to-failure using a torque wrench as supplied by MHH Engineering. The torque wrench was equipped with 200-2000 Nm range and 1 inch drive. A one to three quarter inch adapter was used to attach the torque wrench to the specimen which also had a ¾ inch shaft. In order to hold the bonded components during testing a jig was constructed. The jig mainly consisted of a top and a bottom plate, with a steel joint location plate in between.

Durability Testing

[0054] The durability of the bonded joint in a hot and humid environment was studied, at a temperature of 70° C. and a relative humidity of 95%. Eighty-four epoxy composite/stainless steel bonded joints, as well as thirty epoxy composite components (75×75×3 mm plates), were placed in the chamber. The epoxy composite test pieces were removed from the chamber and weighed three comes during the first week, and then once a week for a period of 8 weeks. The percentage moisture uptake for each individual composite specimen was measured and recorded. Different specimens were exposed to four and eight weeks ageing at 70° C. and 85% RH as shown in Table 2 and then removed for testing. The specimens (see Table 3 below) were also exposed at 70° C. and 100%RH after reaching 1.2% moisture (10 days exposure) uptake. The torque-to-failure of these joints was also determined as shown in Table 3.

Assessment of Joints after Ageing

[0055] The results of four and eight weeks ageing of the joints at 70° C. and 85% RH are shown in Table 2.

[0056] The samples prepared after grit blasting and aged for eight weeks resulted in mean failure torque similar to those aged for four weeks. However, the result of four weeks ageing is only based on two joints and therefore can not be statistically compared. Nevertheless, it should be mentioned that these results shown there is little or no loss in failure torque on additional four weeks ageing

[0057] A similar trend in failure torque was observed with specimens treated with FeCl3+HCl etch during four and eight weeks ageing. The mean failure torque was 1025 Nm after eight weeks compared with 1150 Nm after four weeks ageing.

[0058] Compared with control values (Table 1) both pre-treatments show reductions in failure torque; 20% for grit blast, 26% for etch pre-treatment.

[0059] The grit blast pre-treatment shows little change in failure mode, with mixed cohesive failure in the adhesive and composite ply failure, indicating exposure effects may be limited to the adhesive and not interfaces.

[0060] The etch pre-treated specimens show a reduction in composite failure after exposure, with an increase in cohesive failure within the adhesive layer. This may be the result of water ingress into the adhesives either along the carrier/adhesive interface, or through the bulk of the adhesive or both. It is noteworthy that one specimen shows a high level of adhesion failure to the stainless steel interface.

[0061] The higher coefficient of variation value for surface exposed etch specimens (15%) compared with exposed grit blast specimens (12%) follows the trend in control specimens results (grit blast 10%, etch 19%).

[0062] The differences in mean strength after exposure are probably not significant (grit blast 1085 Nm, etch 1025 Nm).

Ageing After 1.2% Moisture Uptake

[0063] The results of ageing of joints at 70° C. and 100% RH to reach 1.2% moisture uptake after 10 days is shown in Table 3.

[0064] With only one sample for the two treatments (grit blast and etch) used with Cytec EM300k/6747-1 definitive conclusions cannot be drawn. However, from inspection of the results both failure torque and failure mode are similar to the results of both 4 and 8 weeks high humidity exposure. It should be noted however, that for the 1.2% moisture uptake equilibrium test, exposure conditions (70° C./100%RH) differed slightly from the 4 and 8 week exposure conditions (70° C./85%RH). 1 TABLE 1 Steel Pre- Failure Specimen No treatment Adhesive Primer Torque (Nm) Failure Mode Comments 1 a Grit Blast Cytec Cytec 1500  80% Co.Adh 20% 1/2 ply fail  b FM300K BR ®6747-1 1250  50% Co.Adh to Steel/Comp 50% 1/2/3 ply fail  c  700 Visible crack in glue-line edge - discounted  d 1400  50% Co.Adh to Steel/Comp 50% 1/2/3 ply fail  e 1250  75% Co.Adh to Steel/Comp 25% 1/2 ply fail 2 a FeCl3 + HCl Etch Cytec Cytec 1750  90% Co.Adh to Steel/Comp 10% 1/2 ply fail  b FM300K BR ®6747-1 1550 100% co.Adh to Steel/Comp <2.5% 1/2 ply fail  c 1250  75% co.Adh to Steel/Comp 25% 1/2/3 ply fail  d 1100  20% Co.Adh to Steel/Comp 80% 1/2/3 ply fail  e 1250  10% Co.Adh  0% 1/2/3 ply fail V = Coefficient of variation on sample = Co.Adh = Cohesive failure in adhesive at carrier interfaces Co.Adh to Steel/Comp = As Co.Adh. with visible apparent adhesion failure of adhesive to primer and to composite surface in carrier interslices 1/2 ply fail = Failure in 1st and 2nd plies 1/2/3 ply fail = Failure in 1st, 2nd and 3rd plies

[0065] 2 TABLE 2 Ageing Specimen Time Steel Pre- Failure No (weeks) treatment Adhesive Primer Torque (Nm) Failure Mode Comments 1 g 4 Grit Blast Cytec Cytec  700  70% Co.Adh to S/C 30% Comp. Fail  o FM300K BR ®6747-1 1150  90% Co.Adh to S/C 10% Comp. Fail 2 g 4 FeCl3 + HCl Cytec Cytec 1150  40% Co.Adh in S/C 60% Comp. Fail  o Etch FM300K BR ®6747-1 1150  50% Co.Adh to S/C 50% Comp. Fail 1 f 8 Grit blast Cytec Cytec  800 100% Co.Adh to S/C  h FM300K BR ®6747-1 1150  80% Co.Adh to S/C 20% Comp. fail  i 1200  30% Co.Adh to S/C 70% Comp. fail  j 1150  70% Co.Adh to S/C 30% Comp. fail  k 1100  75% Co.Adh to S/C 25% Comp. fail  l 1100  75% Co.Adh to S/C 25% Comp. fail  m  950  40% Co.Adh to S/C 60% Comp. fail  n 1200  90% Co.Adh to S/C 10% Comp. fail  p 1050  80% Co.Adh to S/C 20% Comp. fail  q 1150  80% Co.Adh to S/C 20% Comp. fail 2 f 8 FeCl3 + HCl Cytec Cytec 1200  80% Co.Adh. to S/C 20% Comp. fail  h Etch FM300K BR ®6747-1 1150  80% Co.Adh to S/C 20% Comp. fail  i 1050 100% Adh to S/C Slight “snicking” noise daring test  j 1150  60% Co.Adh to S/C 40% Comp. fail  k 1000 100% Co.Adh to S/C Slight “snicking” noise during test  l 1200  90% Co.Adh to S/C 10% Comp. fail  m  850  60% Co.Adh to S/C 40% Comp. fail  n 1000 100% Co.Adh to S/C Sample slipped in text fixture  p  800 100% Co.Adh to S/C Sample slipped in text fixture  q  850 100% Co.Adh to S/C Co.Adh to S/C = Cohesive failure in adhesive at carrier interfaces, with visible apparent adhesion failure of adhesion to primer and composite surface carrier interslices Comp. Fail = Localised failure in surface plies of composite

[0066] 3 Speci- Steel Failure men Pre- Torque Failure No treatment Adhesive Primer (Nm) Mode Comments 1r Grit Cytec Cytec 1000  97% 3% Comp. Blast FM300K BR ® Co.Adh fail 6747-1 to S/C 2r FeCl3 + Cytec Cytec 1000 100% HCl Etch FM300K BR ® Co.Adh 6747-1 to S/C Co.Adh to S/C = Cohesive failure in adhesive at carrier interfaces, with visible apparent adhesion failure of adhesive to primer and composite surface in carrier interslices Comp. Fail = Localised failure in surface plies of composite

[0067] FIG. 1 shows one end of a rotary power transmission shaft that is suitable for use in driving the flaps in aircraft wings. The other end of the shaft may, in some embodiments, be substantially the same as the end shown, or it may be different. Said one end of the shaft comprises an end plate (10) that is made as one piece from a high chrome steel such, for example, as AMS 5643 (H1025)17/4. Said end plate (10) comprises a generally annular portion (12) having a central aperture (14), and a tubular spigot portion (16) that is positioned concentrically with the aperture (14) and meets the annular portion (12) around the edge of the said aperture (14). As can be seen from FIG. 1, the end plate (10) is chamfered on its inner surface at (18) where the annular portion meets the spigot portion around the aperture (14). The opposing outer surface of the end plate is curved where the annular portion (12) and spigot portion (16) meet to provide a smooth transition therebetween. The outer surface of the spigot portion (16) is rebated at (20).

[0068] Said spigot portion (16) of the end plate (10) is spigotted in one end of a tubular mandrel (30) that is made of carbon fibre reinforced plastics material. The spigot portion (16) forms a snug fit within the end of the mandrel (30), and the two are mutually located by the rebate (20).

[0069] Said tubular mandrel (30) lines the inner surface of a tube (32) that has an integrant annular flange (34) at the said one end. Said flange (34) protrudes beyond the end of the mandrel (30) and lies contiguous the outer surface of the end plate (10) as shown in the figure. The flange (34) of the tube (32) is radially coterminous with the annular portion (12) of the end plate (10) and is fixedly secured thereto using an adhesive bond in accordance with the invention.

[0070] The rotary transmission shaft shown in FIGS. 1 and 2 can be assembled substantially in the same way as described in WO-A-98/20263 by winding carbon fibre tape impregnated with thermosetting resin onto the mandrel (30) using a conventional tape winding machine fitted with suitable shaped centres for holding the mandrel. The outer surface of the end plate (10) that is to mate with the flange (34) of the tube (32) is pre-treated by abrasion in the manner described above, and a layer of an epoxy primer such as BR® 6747-1-A available from Cytec Engineering Materials Inc is applied. An epoxy film adhesive such as Cytec FM300K® is then applied to the outer surface of the end plate (10), and said surface is then offered up to the annular flange (34) of the tube (32) such that the end plate (10) is spigotted within the mandrel (30). The assembly is then fitted with a suitably shaped mould and cured within an autoclave as described in WO-A-98/20263 to cure the primer, adhesive and tube (32). The bonded assembly is then removed from the autoclave and the edges finished.

[0071] Optionally, the joint between the end plate (10) and the tube (32) may he reinforced by rivets or other suitable fixings. For instance, in the embodiment shown in FIGS. 1 and 2 the bonded flange portion (12) of the end plate and flange (34) of the tube (32) are drilled at a plurality of circumferentially spaced locations, in this case four, to form holes (22). In the embodiment shown, each of the holes (22) is formed in a respective mesa (24) which protrudes from the face of the end plate (10) that is opposite to the face bonded to the tube (32). Each of the holes (22) accommodates a tubular bush (not shown) which constitutes a rivet for holding the composite and steel components together should the adhesive bond fail.

Claims

1. A method of bonding a first high chrome steel adherend to a second fibre composite adherend, which method comprises the steps of applying an epoxy primer to said first adherend, causing or allowing said primer to dry and thereafter adhering the first and second two adherends together using an epoxy adhesive.

2. A method as claimed in

claim 1, wherein said high chrome steel comprises an aerospace grade steel.

3. A method as claimed in

claim 1, wherein said high chrome steel comprises 12 to 20% by weight of chromium.

4. A method as claimed in

claim 1, wherein said steel comprises 0 to 8% by weight nickel.

5. A method as claimed in

claim 1, wherein said first high chrome steel adherend comprises a 17/4 grade steel.

6. A method as claimed in

claim 1, wherein second fibre composite adherend comprises an aerospace-grade fibre-reinforced composite material comprising an epoxy resin.

7. A method as claimed in

claim 1, wherein said composite comprises a carbon fibre-reinforced composite material.

8. A method as claimed in

claim 1, wherein said second fibre composite adherend comprises the material 6378cHTA(12K-5-35) which is commercially available from Hexcel Corporation.

9. A method as claimed in

claim 1, wherein said first adherend is pre-treated by abrading or etching before the primer is applied.

10. A method as claimed in

claim 9, wherein said first high chrome steel adherend is degreased to remove contaminants before abrading or etching.

11. A method as claimed in

claim 1, wherein said second adherend is degreased before application of said adhesive.

12. A method as claimed in

claim 1, wherein said second adherend is pre-treated by abrading to remove contaminants and/or loose or weakly bound material.

13. A method as claimed in any

claim 1, wherein said epoxy primer has a rheology and chemical composition that is selected to match the morphology and surface chemistry of the first adherend.

14. A method as claimed in

claim 1, wherein said primer is water based, having a solids content of 5-15% weight.

15. A method as claimed in

claim 1, wherein said primer comprises a hardening agent.

16. A method as claimed in

claim 1, wherein said primer comprises an anti-corrosion agent.

17. A method as claimed in

claim 16 wherein said anti-corrosion agent comprises a zinc salt and/or a chromate salt.

18. A method as claimed in

claim 1, wherein said primer comprises an aqueous epoxy resin dispersion which contains as a distinct phase a solid epoxy curing agent.

19. A method as claimed in

claim 18, wherein said aqueous epoxy resin dispersion comprises one or more solid epoxy resins.

20. A method as claimed in

claim 19, wherein one or more of said solid epoxy resins comprises a glycidyl ether of a phenol or a glycidyl derivative of an aromatic amine or aminophenol.

21. A method as claimed in

claim 18, wherein said curing agent is substantially water insoluble and solid at room temperature.

22. A method as claimed in

claim 18, wherein said curing agent comprises an aromatic amine or diamine curing agent.

23. A method as claimed in

claim 18, wherein said primer further comprises a chromate or non-chromate corrosion inhibitor.

24. A method as claimed in

claim 1, wherein said primer is BR®6747-1-A which is commercially available from Cytec Industries Inc.

25. A method as claimed in

claim 1, wherein said primer consists of a chromate-free equivalent to BR®6747-1-A.

26. A method as claimed in

claim 12 wherein said primer is pre-cured.

27. A method as claimed in

claim 1, wherein said epoxy adhesive comprises a film adhesive or paste adhesive.

28. A method as claimed in

claim 1, wherein said adhesive comprises a carrier that is formed from a polymeric plastics material.

29. A method as claimed in

claim 1, wherein said adhesive is a modified epoxy adhesive.

30. A method as claimed in

claim 1, wherein said adhesive is FM300K.05 which is available from Cytec Engineering Materials Inc.

31. A method as claimed in

claim 1, wherein the second adherend is used uncured and, after application of said primer to the first adherend and application of the adhesive, the composite material is co-cured with the primer and adhesive.

32. A bonded joint comprising a first-chrome steel adherend and a second fibre composite adherend, which first and second adherend bonded together using a method of bonding as claimed in

claim 1.

33. A bonded assembly comprising a first high chrome component and a second fibre composite component, which first and second components are bonded together using a method of bonding as claimed in

claim 1.

34. A bonded assembly as claimed in

claim 33, which assembly comprises a rotary drive shaft comprising a fibre composite tube and high chrome steel flange, which tube and flange are bonded together using said bonding method.

35. An assembly as claimed in

claim 34, wherein said tube is formed with at least one flared end, and the or each tube is deformed to form an integrant flange portion having an annular bonding face.

36. An assembly as claimed in

claim 35, wherein the joint between the tube and the flange is reinforced by rivets, bolts or any other suitable mechanical fixings.