GB2212580A

GB2212580A - Anti-lightning fastener

Info

Publication number: GB2212580A
Application number: GB8726711A
Authority: GB
Inventors: Christopher Charles Rawl Jones
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1987-11-14
Filing date: 1987-11-14
Publication date: 1989-07-26
Anticipated expiration: 2007-11-14
Also published as: GB2212580B; GB8726711D0

Abstract

An anti-lightning fastener for joining an aircraft skin panel 5 made of composite material to a substructural member 6, comprising a bevel headed countersunk bolt 1 and captive nut 4, wherein the bolt 1 has a head surface region 8 of an electrically conductive material 10 for contact with the outer plies 11 of the skin panel 5, and a shank surface region coated with a layer 12 of electrically insulating material for contact with the inner plies of the skin panel and substructural member, whereby lightning is conducted and dissipated controllably and without arcing into the aircraft structure. <IMAGE>

Description

LIGHTING PROTECTIVE FASTENEPE FOR COMPOSITE AIRCRAFT SKINS This invention relates to aircraft lightning protection systems and more particularly to fasteners for fastening panels to an aircraft substructure especially panels made from composite materials.

Aircraft in flight are subjected on occasion to natural lightning strike discharges. Aircraft made from composite structural components are no exception and if unprotected will be damaged more severely than aircraft with aluminium structures. Lightning strikes occur initially at aircraft extremities and protuberances, e.g.

radome, wing tips, fin tips, etc. The first attachment strike is accompanied by a fast rising electrical current, typically 2x105 amps, and a large energy transfer to the aircraft. After the first strike a conductive path will have been established between the charged atmosphere and the aircraft leading to a succession of subsequent strikes of lesser magnitude. These later strikes sweep progressively aft of the initial strike area due to movement of the aircraft. It has been found that the fasteners used to couple outer composite skin material to inner composite or metal supporting structure in an airframe are preferred attachment points for lightning and often generate dangerous sparks because the lightning energy is unable to dissipate into the surrounding low thermal/electrical conductivity, composite material at a fast enough rate.

Present designs of fastener are made of highly conductive alloys for strength. Thus they act as electrically conductive paths in the event of a lightning strike and, as described above, are often the cause of arcing. Such arcing if in the vicinity of fuel tanks and other sensitive parts of an aircraft, may result in fires and even explosions.

Composite materials, particularly carbon fibre reinforced composites, are prone to de-laminate when struck by lightning. The carbon fibre plies act as high resistance conductors and the resin between the plies acts as highly capacitive dielectric layers. When lightning strikes the carbon fibre composite, an increasing potential difference is produced across the ply structure. When the potential difference is great enough arcing may occur across the resin layers with consequent de-lamination of the composite material.

One known approach, to reduce the effect of lightning strikes on non-metallic structures, is to apply, by flame spraying, woven screen, foil or plating, a layer of aluminium to the surface of the structure.

Bowever, this is clearly expensive and does not work satisfactorily with every type of composite material. Moreover the fastener heads must then be insulated from the conductive surface layer.

There are generally two different approaches to fastener design to reduce damage from lightning strikes. One approach is to completely insulate the fastener head and shank, to prevent lightning from being conducted from the surface through the composite to the supporting structure. The other approach is to ensure that the fastener is in intimate electrical contact with the composite material so that no large potential difference can occur between the different plies of the laminate. Conduction is thereby controlled in such a manner that arcing will not occur.

A known example of the former approach is the use of bushings, made from an insulating material, around the shanks of conventional fasteners and such a method is described in Boeing's US Patent 4,502,092. However, although the bushings prevent arcing a problem with their use is that larger holes have to be drilled into the panels of composite material than would be needed for the fasteners alone.

These larger holes reduce the strength of the aircraft structure.

Moreover, a disadvantage of this approach is that if, by chance, the lightning should find its way through a weak spot, arcing will be highly localised and the damage will still be considerable.

An example of the latter approach is described in our earlier UK Patent Application 8628555 in which a fastener is disclosed for improving electrical conduction through a composite material. The physical/electrical contact between the fastener and plies of composite material is enhanced by use of two bevelled portions at the ends of the fastener. Because each ply makes electrical contact with each of the others, via the fastener, no potential differences occur between different plies and no arcing results when the composite is struck by lightning and ideally the lightning should then rapidly discharge harmlessly throughout the aircraft structure. However, there remains the possibility that residual regions deep within the material remain partially insulated from each other.If a sufficiently high potential difference is produced across these regions of the composite material extensive structural damage and de-lamination will result. The conductive fastener will then have contributed to the damage by conducting the lightning deep into the composite where it can cause maximum damage.

It is an object of the present invention to provide a fastener which has the advantages of both the approaches described and yet which does not suffer the drawbacks of either to any great extent.

It is a further object of the invention to provide fasteners with improved electrical characteristics which minimise damage due to lightning strikes.

According to one aspect of > .lis invention there is provided a fastener which comprises a nut and a bolt having at least two separate surface regions each made of a separate one of two different types of material having different electrical properties from the other.

One material may be substantially non-conducting and the other material may be substantially electrically conducting. Different regions of the nut and bolt are thus made conductive and insulating respectively and electrical currents e.g. caused by lightning may then be conducted to and from localised regions of the fastener in a predetermined way only.

The nut and bolt may be coated, at least at one of said surface regions with the material. Coating is preferable to providing an encasing bushing because then the diameters of holes drilled through the composite can be kept to the minimum necessary to accommodate the fastener.

A specific embodiment of the invention will now be described by way of example only and with reference to the accompanying drawing namely: Figure 1, which is a cross sectional view through a fastener installed in an aircraft structure and subject to a lightning strike.

Referring to Figure 1 the fastener comprises a bolt I which has a bevelled head portion 2 at one end and a threaded portion 3 at the other end for receiving a nut 4. The bolt passes through an aircraft skin panel 5 of composite material and substructural member 6 and is fastened by nut 4 which is held captive on the underside of member 6 by means of rivets 7. The rivets 7 are made of a metallic conductive material other than aluminium which would corrode by galvanic action if used with carbon fibre composites. The rivets are of the expanding type which compensate for expansion due to heating.

The bevelled head portion 2 of bolt 1 is countersunk into the surface of panel 5 so that an outer sloping surface 8 of the bevelled portion makes all round intimate contact with sloping surfaces 9 of the countersink in the panel 5. The outer face 8 of the bevelled port ion of the nut 4 is coated with a soft metallic coating 10 which beds into the panel 5 and improves physical/electrical contact with the top three or so plies thereof in surface region 11. The shank of the bolt 1 is coated with a layer of electrically insulating material 12 so that the bolt is insulated from all plies below the region 11. The insulating material is deposited on the bolt as a layer which is typically 0.5 to 1 thousandth of an inch thick. Suitable insulating materials are ceramic or the fluorocarbon material manufactured by Du Pont and known by the trade name TEFLON. If ceramic materials are used they can be grown onto the shank of the bolt. If TEFLON is used it can be spray painted onto the bolt. Alternatively the bolt can be dipped into liquid TEFLON. If the latter is done the head of the bolt may be first covered in a releasing agent to prevent it from becoming coated.

Layers of insulating material 15 are arranged between the substructural member 6 and composite material 5 and between the nut 4 and substructural member 6 to further reduce electrical conductivity.

The material of the insulating layers 15 may be quartz, glass fibre or PTFE sheets.

Nut 4 has a chamfered inner edge 13 to prevent dielectric breakdown of the insulating material 15 thus preventing arcing between the nut 4 and adjacent surfaces of member 6. The chamfered edge enables a thicker layer of insulator 15 to be introduced between the nut and bolt and member 6 at critical sharp pointed areas.

A liquid shim 14 is also interposed between the composite material 5 and substructural member 6, the shim is loaded with aluminium particles to increase its strength and conductivity. The liquid shim is used as a filler to make up for differences in thickness of the composite outer skin 5. These thickness variations occur because, for instance, when a sheet of composite material is to be curved into a complex shape, for example a wing camber, it is established practice to cut away successive plies of the material at selected positions. This reduces its thickness and stiffness and makes it easier to bend the sheet to the required shape. The process of cutting away successive plies of the composite gives rise to an uneven stepped surface.Liquid shim material is applied to the stepped edges to smooth them over and to restore a uniform thickness to the area of the sheet.

Figure 1 also shows schematically the effect of lightning striking the fastener.

The lightning 16 strikes the preferred attachment point, namely the head 2 of the fastener and 50% to 90% of the lightning current is conducted through sides 8 of the non-insulated bevelled head portion of the bolt. A large proportion 16' of the lightning is thus conducted from the bolt and dissipated through the top three or so plies of the aircraft skin in region 11. Because the lightning is dissipated through more than one layer of plies there is likely to be less damage than if the lightning was allowed to strike through say the surface only. If for any reason the potential differences, generated by the lightning, should exceed the current handling capacity of the plies in region 11 and de-lamination occurs, damage will be limited to the top three surface plies in region 11, to a surface depth which is typically in the order of 15 to 30 thousands of an inch.More serious structural damage will be avoided because the plies below region 11 are electrically isolated. In effect, the plies in region 11 are sacrificed to the advantage of the plies below.

By comparison if lightning were to strike an unprotected conventional fastener it would be conducted along the shank of the bolt and would disperse through plies within the body of the composite material 5 in an uncontrolled manner and would cause arcing and delamination of the plies and this damage would not be restricted to the top few plies.

The remaining 50% to 10% of the lightning 16" passes down through the insulated shank of the bolt 1 and is conducted through the nut 4 and rivets 7 into the substructure 6 where it is dissipated. Because the lightning 16'" passes into the material 6 in a controlled manner, very little damage is caused in comparison to a conventional, uninsulated, fastener arrangement.

The nut and bolt of the present invention are fastened together in the same way as a conventional arrangement i.e. bolt 1 is screwed into the nut 4 by inserting a tool such as a screwdriver into a slot (not shown) in bevelled portion 2 of the bolt and by rotating the bolt until it screws into the nut.

Although only one embodiment of the invention has been described other arrangements are possible without departing from the scope of the invention for example the conductive coating 10 could be deposited in any required combination with the insulating layers 12 on the fastener designed to dissipate the lightning in other pre-determined ways. For example, the bolt could be coated over a larger area with conductive material 10 to increase conductivity or else it could be coated over a sraller area, than that shown, to make it less conductive. In this way the depth of the conductive region 11 can be changed to suit different requirements. Alternatively, the conductive coating 10 could be omitted completely and reliance placed on the conductive properties of the bolt 1 and its head 2 to restrict and direct the path of the lightning into the region 11.

The bevelled edges of the bolt head may comprise one or more flat surfaces or alternatively the bolt head may be substantially frusto-conical in shape.

Claims

1. A fastener comprising a nut and bolt having at least two separate surface regions each made of a separate one of two different types of material having different electrical properties from the other.

Amendments to the claims have been filed as follows 1 A fastener comprising a nut and bolt having at least two separate surface regions each made of a separate one of two different types of material having different electrical properties from the other.

2 A fastener as claimed in Claim 1 and wherein one material is substantially electrically non-conducting and the other is substantially electrically conducting.

3 A fastener as claimed in Claim 1 or Claim 2 and wherein at least one of said surface regions is coated with the separate one of two different types of material.

4 A fastener as claimed in any preceding claim and wherein one of said materials is a ceramic.

5 A fastener as claimed in any of claims 1 tp 3 inclusive and wherein one of said materials is a fluorocarbon.

6 A fastener as claimed in Claim 4 and wherein the ceramic- is grown onto the shank of the bolt.

7 A fastener as claimed in Claim 5 and wherein the fluorocarbonois spray painted onto the shank of the bolt.

8 A fastener as claimed in Claim 5 and wherein the bolt is partially coated with fluorocarbon by dipping the bolt into the fluorocarbon in liquid form.

9 A fastener substantially as hereinbefore described and with reference to Figure 1 of the accompanying drawings.

10 An anti-lightning assembly of a composite structural skin member and a substructural member for an aircraft substantially as hereinbefore described and with reference to the accompanying drawings.