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Figure 15 – examples of complex stitched fibre
preforms
• Durability.
• Comfort.
• Maintenance.
• Repair.
Therefore, the real challenge with
lightweighting is to find the optimum
compromise through a multidisciplinary
approach. Starting from the basic design
concept and ending with in-service aspects
such as maintenance and repair, optimised
lightweight design requires trained and
experienced engineers.
In terms of conceptual lightweight design,
many new ideas or approaches can be
found in other areas. The COMPOSIT
workshop on lightweighting included two
fascinating presentations – one on
lightweight structures in nature, and another
on architectural approaches to
lightweighting.
The perceived cost benefits of lightweighting
are very different for the various aerospace
fields. Whilst civil aircraft applications can
normally justify an additional cost of €100
per kilogram of weight saved, space
applications are sometimes prepared to
∅about 3 m
Bulkhead (Airbus A-300, study)
Knot Element
0,2 m
Automobile
Floor Pan
0,8 m
0,8 m
0,65 m
1,7 m
T-Stiffened Panel
(Airbus Wing Section)
J-Shaped Stiffener
for EFA Side Skin
1,8 m
0,85 m
Reinforcement for
Seat Back
0,9 m
14
accept costs of more than €10,000 per
kilogram of weight saved.
It has been demonstrated in many
aerospace applications that the use of
composites can yield weight savings of
around 20% compared to aluminium.
Joining
The dominant joining technique in the
manufacturing of civil aircraft has been, and
still is, riveting. However, structural adhesive
bonding also has a long tradition in the
aerospace industry for some aluminium
applications and nowadays also for
composite structures.
One approach for the joining of composites
is adhesive film bonding. This requires high
temperatures and pressures and
consequently has to be performed in an
autoclave. It is also limited in its design
freedom as only constant bond line
thicknesses can be achieved. Therefore, it is
not possible to compensate for large
tolerances.
Recent research efforts have focused on the
development of adhesives and processes
requiring low temperatures and pressures
for non-autoclave curing. An interesting
candidate technology is two component
paste adhesives. Their application would
simplify processing and allow a
compensation of tolerances due to the high
flexibility in bond line thickness.
An additional development goal is the
lowering of curing temperature to below
100oC to prevent the out-gassing of moisture
causing voids in the bond line
Another important topic is the preparation of
the composite parts for adhesive bonding.
The state of the art is to use a peel ply with
additional mechanical treatment after
removing the ply to ensure no residual
material is left. This procedure is both time
consuming and expensive in terms of quality
assurance. New developments in bond
preparation are examining the use of grit
blasting, or atmospheric plasma or laser
treatments.
Other research and development topics
relating to the joining of composites for
aerospace applications include:
• Nano-fillers for improved performance.
• Advanced adhesive curing based on
laser or microwave heating.
• Debonding on demand for repair
purposes.
• Adhesives with improved fire, smoke
and toxicity properties.
A final issue relating to adhesive
technologies is GLARE, in which aluminium
sheets are bonded to unidirectional glass
prepregs with an adhesive film. GLARE
panels will be used in the upper fuselage of
the A380.
Recycling
The recycling of composites is currently not
a primary topic in the aerospace industry
due to the long life cycle of aircraft. Because
of this, there are no legal requirements as in
the automotive sector.
Presently, the principle possibilities for the
“recycling” of carbon fibre composites are
burning (energy recovery) or grinding. The
resulting particles can then be used as filler
in secondary structures.
A bigger aerospace interest surrounds the
recycling of production scrap. The first
priority is of course to avoid scrap by, for
example, using optimised nesting or prepreg
tapes with reduced width. The remaining
prepreg scrap is then usually hardened in an
　
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