Aerospace materials
Changing planes
A new breed of aircraft built from lightweight carbon composites is taking flight.
But are these materials all they’re cracked up to be, asks Hayley Birch
60 | Chemistry World | October 2011
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Weaving carbon threads
UK chemist Paul Weaver,
who specialises in developing
lightweight structures at the
University of Bristol’s Advanced
Composites Centre for Innovation
and Science (ACCIS) walked on
Boeing’s first Dreamliner three
years ago. ‘Boeing realised that
carbon fibre is 40 per cent lighter
than aluminium,’ he says. ‘So they
made the fuselage thicker and
sacrificed a bit of the weight saving
that they would have had over
aluminium. But they’ve ended up
with a lighter solution and higher
internal pressure which means
better cabin comfort.’ Not only that,
but the lighter structure means
lower fuel burn, saving 20 per cent
compared to other planes of its size.
So it sounds like it’s all over
for aluminium. Well, perhaps
not just yet. Aircraft component
manufacturers are facing what are
tactfully referred to in the industry
as ‘challenges’ in working with
composites. In addressing these
challenges, new manufacturing
processes are emerging as well as
smart structures that can change
their properties and even mimic
biological functions like healing to
repair themselves.
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According to Weaver, however,
the most urgent problem facing
industry right now is manufacturing
enough carbon fibre to meet the
demands of aerospace giants
like Boeing and Airbus. CFRP is
essentially very thin fibres of carbon
embedded in an epoxy resin matrix.
During component manufacture
for aerospace applications, many
layers of carbon fibre are built up
and then cooked under pressure
in an autoclave to cure and harden
the constituents of the matrix.
The underlying chemical reaction
is a copolymerisation between
short polymer chains of resin and
monomers of a hardening chemical.
‘Prepreg’ carbon fibre, which comes
on a roll and is already impregnated
with resin, is often used to speed up
the process.
There is no getting around the fact
that making aircraft components
from composites is slower and more
labour intensive than making them
from aluminium. A pattern and
then a mould have to be produced
before the first components emerge.
As Weaver notes, composite
components have to be designed
with a larger margin for error, owing
to the different properties associated
with each of the fibre and matrix
materials. Accurately predicting
the collective properties of the
finished material is tricky and as a
result, manufacturing tolerances for
composites tend to be larger than for
aluminium.
The laying of prepreg in carbon
fibres can be carried out by a
machine, but achieving optimum
Carbon fibre reinforced
polymers are the
most commonly used
aerospace composites
In short
 Boeing’s Dreamliner
and the Airbus A350
are the first aeroplanes
to use large amounts of
composite materials
 Carbon fibre reinforced
polymers are the most
widely used materials.
They are lighter and
stronger than aluminium
 Using composites
requires a very different
approach to component
design to make the most
of their properties
 Repair poses the
biggest problem for
composites, and has
spawned research into
self-healing materials
stiffness and resistance to buckling
means precisely tailoring the
directions in which the fibres are
laid. Weaver’s team has developed
a technology based on a modified
embroidery machine that stitches
fibres into place with threads that are
later dissolved. ‘When we dissolve
those stitches out, we’re left with the
orientation of the carbon fibre where
we want it, and then we can put resin
film on top, then another layer of
carbon fibre however we want it,’
Weaver explains. Unsurprisingly,
the approach is painfully slow and
creates defects. Plus, dissolving
the stitches adds a further layer of
complication to the already arduous
process of making CFRP. But one of
Weaver’s co-workers, Byung Chul
Kin, has recently come up with an
improved and virtually defect-free
machine that has just been granted
a patent. Weaver says he expects to
be using it to make A350s – Airbus’s
53 per cent composite jetliners – in
the next few years.
Still, industrial curing processes
are time- and energy-consuming. As,
Brian Smith, director of composites
for Boeing Research & Technology,
explains, most CFRP composites
currently produced for the aerospace
industry are cured in autoclaves at
around 180°C. ‘Recent advances
in the ability to cure outside of
the autoclave look promising,’ he
says. Companies like Quickstep in
Australia, whose customers include
Airbus and the US Department of
Defence, claim to be able to make
impressive energy savings of up to 85
per cent by using out-of-autoclave
technologies. The Quickstep process
uses a curing chamber containing
fluid that rapidly transfers heat to the
material and requires shorter heat
cycles. This has the added advantage
of reducing the curing time. Other
methods with potential include
electron beam, microwave and
infrared curing.
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Travelling on an aeroplane subjects
the body to unusual stresses.
Besides the psychological trauma
of being 10km above solid ground,
the atmosphere inside a jet plane
– although partly pressurised – is
akin to being half way up Mount
Kilimanjaro; low enough to allow
feet and legs to swell and to increase
the risk of deep vein thrombosis.
So why not make things a bit more
comfortable by raising the pressure
to something approaching sea
level? A straightforward solution,
you might think. But it’s not that
simple – such high cabin pressure
would create too much stress for a
conventional aluminium fuselage.
Using composites, however,
Boeing appears to have solved
the problem. The company’s new
Dreamliner aircraft operate at
slightly higher cabin pressures by
virtue of their marginally thicker
fuselages, made largely from carbon
fibre reinforced polymer (CFRP).
By volume, each plane is 80 per cent
composite materials – these extend
to almost every part of the plane
bar the engines. Despite the bulkier
fuselage, there’s still a weight saving
to be made, thus demonstrating
two of the major advantages of
composites: strength and lightness.
Cracks are showing
Unfortunately, owing to their
unpredictable properties, repairing
composite materials is no more
straightforward than making them.
Repairing aluminium is a cinch, by
comparison, says Weaver. ‘It’s the
same material in all directions,’ he
observes. ‘But in a composite, the
weave is different in different parts,
so repairing it is very difficult. I’m
not aware of any particularly elegant
repair methods yet.’
Currently, the lack of
straightforward repair strategies
for composites places a massive
Chemistry World | October 2011 | 61
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Aerospace materials
financial burden on the aerospace
industry, with some estimates
suggesting that as the contribution
of composite materials to the overall
structure of an aircraft increases,
the maintenance costs begin to
approach or even exceed the fuel
costs. As Duncan Wass – another of
Weaver’s colleagues – points out, by
this logic the cost must be ‘absolutely
vast’. In another department at the
University of Bristol, Wass, a chemist
with a background in petrochemicals
and catalysis, is working on a rather
intriguing solution to the problem
– composite materials that can heal
themselves.
It might sound like something
from the realms of science fiction,
but Wass’s approach to fixing
Repair and maintenance
of composite aircraft
pose new challenges
CHRIS BOWEN/UNIVERSITY OF BATH
Smart structures
Composites can stand unusual stresses that would break conventional materials, like morphing
Sensor systems already exist that are
capable of self-diagnosing and reporting
damage to aircraft materials. Also within the
bounds of possibility are energy-harvesting
materials that could turn wasted energy
from turbulence into useful electricity, as
well as bi-stable, morphing composites that
snap from one shape to another to influence
lift and drag – traditional aircraft materials
62 | Chemistry World | October 2011
could not withstand such strains. Chris
Bowen and colleagues at the University
of Bath in the UK describe using standard
carbon fibre manufacturing techniques and
piezoelectric actuators to control the shape
of wing structures.4 During manufacture,
they layered up carbon fibre sheets nonsymmetrically to produce composites that
would snap between the two stable states.
damaged aeroplane wings follows
biology’s example. ‘Our inspiration
for this is basically the human body,’
he explains. ‘If you get cut, the wound
bleeds, then it scabs and heals. We’re
trying to make aeroplane wings that
do the same thing.’ In his team’s
self-healing system, the ‘cut’ is a
small crack caused by damage to the
composite structure. This ruptures
tiny microcapsules embedded in the
material, releasing healing agents.
A forthcoming paper1 describes
how the team use a scandium-based
catalyst as a healing ‘initiator’ –
akin to the inflammatory response
in biological systems – to drive
polymerisation of epoxy monomers
contained within the capsules. The
aim is to fill the crack before it has
a chance to grow. As in the body,
the effects are not instantaneous,
but Wass claims his self-healing
materials can regain close to 80 per
cent of their original performance
within 48 hours.
At the University of Illinois in
Urbana-Champaign, US, a team
of self-healing materials experts
led by Scott White and Nancy
Sottos has been working on similar
capsule-based healing systems for
years. In fact, they have devoted
a whole series of publications to
one particular system involving
dicyclopentadiene-containing
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N R SOTTOS/UNIVERSITY OF ILLINOIS
The long term view
But according to Sottos, it could
be a long time before self-healing
materials see anything resembling
an aeroplane. ‘New materials in
the aerospace industry have a very
long life cycle,’ she says. ‘It takes a
long time to get them on something
that flies human beings because of
the safety issues, so the composites
that are in Airbus A350s or Boeing’s
Dreamliner, those are composites
that were developed 20 to 30 years
ago.’ Military planes or drones may
be the first candidates for testing
self-healing materials.
Perry Cohn, composites
engineering director at Alpha
Composites in Buckingham, UK,
says self-healing composites are
‘blue sky stuff’, although he admits
that better repair strategies are
needed – especially for repairing
materials that are damaged in the
field. ‘One of the main problems
with repairing composite structures
until now has been that if the main
component is made in an oven at
over 120°C, how do you repair that
in the field?’ Which brings us back
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N R SOTTOS/UNIVERSITY OF ILLINOIS
microcapsules and a rutheniumbased catalyst, achieving healing
efficiencies around the 80 per cent
mark.2 The trouble with these
capsule-based approaches is that
the capacity for healing agents
within the capsules is limited and
after several rounds of healing their
store becomes depleted. Thus,
self-healers at Urbana-Champaign
and Bristol are also exploring
other approaches; ones that take
bioinspired materials to a whole
other level.
In 2010, Sottos’s group reported
embedding a so-called ‘vascular’
system into an epoxy matrix to heal
cracks in the material.3 This vascular
approach incorporates channels
designed to mimic blood vessels
in the human circulatory system.
They injected epoxy resin (the
healing agent) and an amidoamine
hardening agent into alternative
layers of channels, so that the two
would only mix if a crack occurred
that severed both. ‘We found that
we could get repeated healing even
though we damaged the network,’
says Sottos. Their samples healed
through 13 rounds of damage and
healing, although by the final rounds,
healing efficiency was below 50 per
cent. They also suggest they could
extend the healing capacity of their
materials even further by actively
pumping in healing agents when
they are damaged.
to the central problem with the
autoclave process. Cohn says low
temperature resins are an area of
fervent research within composites
and resins already exist that will cure
at 50 or 60°C. Thus, future composite
manufacturing processes should
combine low temperature – and low
energy – curing processes with better
prospects for repair.
From Cohn’s point of view, there
isn’t so much a problem with the
process for making composites
as with the engineers involved
in it, many of whom still haven’t
come to terms with the different
design philosophy needed for
composites and are still taking a
‘metals approach’. It’s a more labour
intensive process, he says, but
offering opportunities unparalleled
by metals to tailor every aspect of
the material. Whereas an aluminium
component has to be machined from
Self-healing materials
will take a long time
to be accepted by
manufacturers
Attaching capsules
of healing reagents
to carbon fibre could
overcome difficulties in
repairing composites
the existing metal, the process for
producing a composite material is
part and parcel of the component
manufacturing process. And if
designed properly, argues Cohn, each
component should be fully optimised.
‘The secret to designing composites
is just to design composites and
not to make metal components out
of composites,’ he says. ‘There are
properties of composite materials
that are phenomenal, but you have
to use a different mindset when
designing them.’
Worth the effort
The benefits of using composites
far outweigh the drawbacks,
according to Cohn. And if there
were any doubts about whether
such a tortuous manufacturing
process could make the numbers
add up, Weaver looks at them from
an airline’s point of view. ‘Why
are we using composites? Because
the weight saving is so enormous.
Which means lower ongoing costs
due to lower fuel burn. So initially,
they may be more expensive than
aluminium, but you have to factor
in the lifetime cost of 20 years of use
every day.’ Because they’re lighter
materials, he adds, there’s also the
potential to make bigger planes and
put more passengers on them. One
way or another, they’ll make their
money back.
Hayley Birch is a freelance science
writer and editor based in Bristol, UK
References:
1 T S Coope et al, Adv. Func. Mat., 2011, in press.
2 B J Blaiszik et al, Annu. Rev. Mater. Res.,
2010, 40, 179 (DOI: 10.1146/annurevmatsci-070909-104532
3 A R Hamilton, N R Sottos and S R White,
Adv. Mat., 2010, 22, 5159 (DOI: 10.1002/
adma.201002561)
4 Bowen et al, J. Intell. Mater. Syst. Struct. 2007,
18, 89 (DOI: 10.1177/1045389X07064459)
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