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panel (source: Eurocopter)
11
TECHNICAL ISSUES ASSOCIATED WITH THE USE OF
COMPOSITES IN AEROSPACE APPLICATIONS
The COMPOSIT thematic network focussed
on ten key issues relating to the use of
composites in transport. During the course
of the project, a workshop was dedicated to
each of these ten issues. Leading
international experts in the relevant fields
were invited to present and participate at the
workshops. Here, the findings are presented
in terms of their relation to the aerospace
industry.
Repair
Repair topics are very important for the
aerospace industry. They have an influence
on design philosophies, maintenance
procedures and material selection.
Several approaches to repair have been
developed and these are applied for
different cases:
• If the damage is small and does not
affect the structural behaviour of the
aircraft (e.g. because of damage
tolerant design), then no structural
repair is necessary. The damage is
simply filled for aesthetic or
aerodynamic reasons.
• If the damage is sufficiently significant to
affect the load carrying function, and the
part can be removed from the plane
(often possible in civil aircraft; not
applicable for very big, highly integrated
structures), then the repair can be
performed in an autoclave using the
original material and process
parameters. The procedure is described
precisely in handbooks and leads to a
performance close to the original part.
• If the damage is significant and the part
cannot be removed from the aircraft
(typical for military aircraft structures
with high integration), then an in-situ
repair method has to be employed. Two
processes have been developed. The
first involves working with a repair patch
that has been specially developed for
use in conjunction with relatively low
temperatures (up to 130oC) and
pressures (1 bar). This results in the
repaired structure having a lower glass
transition temperature than the original
and reduced mechanical performance
compared to autoclave-repaired parts. A
further complication is that moisture in
the structure can lead to voids in the
bond-line area during cure. The second
option is to manufacture a separate
replacement part in an autoclave using
the original material and process
parameters, and to integrate it within the
damaged structure by adhesive
bonding. In this case the structural
performance of the repaired part is
equivalent to the original structure, but
the bonding can again suffer from
moisture leading to weakness in the
bond-line.
In summary, it can be stated that repairmethods
for composite structures are
available and established within the
aerospace industry. Nevertheless, further
improvements are necessary to simplify the
processes and come closer to the
performance of non-damaged structures.
Design and Structural Simulation
The industry’s capabilities with respect to
the design and simulation of composite
structures has a major influence on: (i)
performance (utilisation of material
properties and reduction of safety factors),
(ii) time to market and development costs
(the number of experimental tests that must
be performed), and (iii) development risk
(identification of problems at an early stage).
Historically, it can be seen that advances in
composite design capability have tracked
the development of computer hardware and
software. Before the widespread availability
of computers, the effort associated with
laminate calculations involving more than six
plies was considerable. However, as
calculation programs running under
Assembler or FORTRAN became available,
so global laminate calculations became
increasingly feasible.
Nevertheless, in the early days, the reserve
factors associated with composite structures
were rather high and the maximum
permitted stresses were rather low.
Consequently, the full potential of composite
materials was not being realised. The next
major breakthrough was the advent of finite
element analysis (FEA) programs with the
capability to perform analyses using
laminated shell elements. The design and
calculation of local phenomena now became
feasible, and it was possible to reduce the
reserve factors for composite parts. This
allowed a big push towards lightweight
design.
Nowadays, composite FEA is a common
structural design tool within the aerospace
industry. Almost every software package
　
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