Electricity: conductors and insulators – Lightning protection
Reexamination Certificate
1999-09-17
2001-11-20
Reichard, Dean A. (Department: 2831)
Electricity: conductors and insulators
Lightning protection
C361S216000, C361S218000, C244S00100R
Reexamination Certificate
active
06320118
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to adhesively bonded joints in carbon fibre composite structures or hybrids thereof.
Carbon fibre composites or hybrids thereof comprising one or more laminates or plies are being used increasingly for aircraft structures in place of the aluminum or metallic compounds formerly used. Although carbon fibre composite structures are generally stronger and stiffer than their metallic counterparts, nevertheless they lack the lightning strike tolerance of the metallic components.
2. Discussion of Prior Art
A lightning strike to an aircraft causes a high electric current, which may typically be of the order of a hundred thousand amps, to flow through the aircraft frame. In a carbon fibre composite structure, the carbon fibre plies act as very high resistance conductors and the resin in the composite acts as a highly capacitive dielectric so that lightning striking carbon fibre composite results in an increasing potential difference across the ply structure but no readily available electrically conductive path for discharging the current. The current therefore tends to concentrate at the structural joints making them particularly vulnerable to damage.
Methods of joining carbon fibre composites using metal fasteners are known, and indeed our UK patent application 9411006.1 describes the problems caused by lightning strike associated with the use of metal fasteners in composite structures and discloses means to overcome these problems.
Often it is preferable to make permanent joints in structural members by adhesively bonding carbon fibre composites. The use of adhesives in these joints can lead to failure when lightning strikes the airframe because known adhesives have very poor conductivity and cannot safely transfer lightning current.
Lightning current flowing in an airframe can produce voltage stresses across adhesively bonded joints. If the stresses exceed the joint's dielectric strength, the insulation will break down. This may lead to arcing and sparking which can cause extensive damage to the joint and explosive fuel ignition if in contact with fuel or fuel vapour.
For the above reasons critical joints in primary structure likely to be subjected to a lightning strike are generally not adhesively bonded. Where bonded joints are at risk of lightning strike damage, one known approach is to protect the joint by additionally installing fasteners usually in the form of solid metal rivets. Such protection of joints from lightning strike is discussed in Aircraft Lightning Protection Handbook by F. A. Fisher, J. A. Plumer, R. A. Perala DOT/FAA/CT-89/22. This solution is unsatisfactory when used near fuel tanks because the lightning current tends to concentrate at the highly conductive fasteners and is unable to dissipate at a fast enough rate so that arcing and dangerous sparks are likely to occur, carrying the high risk of injecting sparks, molten material and hot gases into the fuel tank. Additionally even in areas away from fuel tanks, the installation of a large number of fasteners compromises the benefits of the adhesive bonding by adding time to the manufacturing process and weight to the airframe.
SUMMARY OF THE INVENTION
The present invention seeks to overcome the problems of high electric currents flowing across adhesively bonded joints in carbon fibre composite structures, including those close to fuel, without compromising structural integrity.
According to the invention in one aspect thereof, there is provided a structural joint for the transmission and control of high current flows in a carbon fibre or carbon fibre hybrid composite structure, said joint comprising:
at least two composite structures comprising a multiplicity of resin bonded carbon fibre plies;
an adhesive applied there between and capable of conducting high currents through and away from the joint;
said adhesive including a conductive carrier film comprising carbon fibres,
wherein the adhesive has an electrical conductivity comparable with that of the adjacent composite structures.
Preferably the carbon fibres in the adhesive are in the form of a lightweight scrim.
Alternatively the carbon fibres in the adhesive may be in the form of chopped fibres.
An electrically conductive layer may be incorporated in or be applied to an outer surface of at least one of said composite structures. This electrically conductive layer is preferably a copper strip. Conductive fasteners may be in contact with the electrically conductive layer.
According to the invention in a further aspect thereof an adhesive for bonding carbon fibre or carbon fibre hybrid composite structures and capable of conducting high current comprises a conductive carrier film comprising carbon fibres.
Advantageously, the electrical conductivity of the adhesive is comparable with that of the carbon fibre or carbon fibre hybrid composite structures to be bonded.
The carbon fibres in the adhesive are preferably in the form of a lightweight scrim. Alternatively the carbon fibres may be in the form of chopped fibres.
According to the invention in a further aspect thereof a method of forming a structural joint includes the steps of providing two or more carbon fibre or carbon fibre hybrid composite structures, applying an adhesive therebetween and curing the joint, wherein the adhesive comprises a conductive carrier film including carbon fibres and has an electrical conductivity comparable with that of the adjacent composite structures. Preferably, the adhesive comprises few, if any, metallic fillers.
Structural joints according to the present invention allow composite structures to be adhesively bonded together without the need for mechanical fasteners whilst displaying similar strength characteristics as traditionally fastened joints, making them ideal for applications requiring high performance, lightning resistant, joints.
We have found that structural joints according to the present invention having a conductivity comparable to that of the carbon fibre composite are advantageous in aircraft lightning protection arrangements, particularly in bonding aircraft skin panels to an aircraft substructure, which may for example be the wall of the fuel tank, where the panels and substructure are made from carbon fibre composites.
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Duffy Roger P
Jones Christopher C. R.
Pridham Barry J
BAE Systems plc
Nixon & Vanderhye P.C.
Reichard Dean A.
Walkenhorst W. David
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