BRIDGES that mend themselves as well as lighter aircraft and faster
racing cars could all spring from the biggest development since carbon fibres
were invented 30 years ago. Researchers have made the world鈥檚 first hollow
strands of carbon fibre.
The fibres were developed at the Defence Evaluation and Research Agency鈥檚
Structural Materials Centre in Farnborough, Hampshire鈥攖he same site where
the first carbon fibres were invented鈥攚ith the help of a team at the
University of Strathclyde. Paul Curtis, head of the team at DERA, says that
weight for weight the hollow fibres can carry 30 per cent more load than solid
fibres without buckling.
鈥淐arbon fibres can be likened to miniature columns that impart strength to a
material,鈥� says Curtis. 鈥淭hey must carry loads and you have to rely on them not
to buckle. But lack of compressive strength is the biggest weakness of existing,
solid fibres.鈥�
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Hollow fibres overcome this limitation by concentrating the mass of the fibre
away from centre of the filament. 鈥淭he amount of carbon is the same, but we鈥檝e
thrown mass out from the middle, and that could give us improvements in
compressive strength of up to 30 per cent,鈥� he says.
And smart fibres with exotic properties might eventually be created by
threading special materials through the core, says Curtis. If filled with fluid
such as glue that hardens on contact with air the fibres could mend themselves
if ruptured.
Curtis says that it has taken so long to introduce a hollow version because
manufacturing carbon fibres is an exacting process. Normal carbon fibres are
made in three stages from fibres of the polymer polyacrylonitrile.
First, the carbon backbone is formed by oxidising the polyacrylonitrile. The
backbone is mainly made up of rings of carbon atoms, but some rings contain
other elements, such as nitrogen and oxygen. If left in place they would
destabilise and weaken the fibres. So in the second stage, the non-carbon
atoms are burnt off at a temperature of between 800 and 900 (掳C鈥攁
process called carbonisation. Finally, the fibres are roasted at 1500 (掳C,
which turns the carbon skeleton into fibres formed from concentric sheets of
material similar to graphite.
鈥淚f you run a hollow fibre through that lot, it鈥檚 quite demanding,鈥� says
Curtis. He says that previous attempts to make hollow fibres have faltered
because the harsh processing introduces cavities into the fibre walls, weakening
the structure. Through subtle but critical modifications to the existing
process, the team at Farnborough has now overcome these problems. The 鈥渞ecipe鈥�
is secret for now, while the agency applies for patents.
So far, the researchers have made single filaments up to 10 centimetres long
and 20 micrometres across. Now they are developing ways to make multi-filament
strands like those required for most practical applications. 鈥淭hen, we hope to
make objects of carbon fibre-reinforced plastics to demonstrate the benefits of
the hollow fibres,鈥� says Curtis.
Few changes are needed to the existing process, so Curtis expects that
mass-produced hollow fibres will be almost as cheap as solid fibres.
Brian O鈥橰ourke, senior engineer responsible for composite material structures
at Williams Grand Prix Engineering of Wantage, Oxfordshire, says: 鈥淚鈥檇 heard
about it already, but it鈥檚 too early to consider how it might be applied in
Formula 1 motor racing. But I鈥檒l be watching development of hollow fibres
肠濒辞蝉别濒测.鈥�
Glyn Rogers, the chief airframe engineer at British Aerospace鈥檚 Military
Division in Warton, Lancashire, says hollow fibres could reduce the weight of
military aircraft by as much as 10 per cent.
