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autoclave, hot press or oven and then
deposited, burned or milled.
Significant advantages with regard to the
recycling of production scrap are offered by
manufacturing technologies based on textile
preforms and resin injection processes. By
using near net shaped preform technologies
(e.g. braiding), together with optimised resin
injection processes, waste fibres and resin
can be reduced to a minimum. Another
advantage of preform technologies is that
the scrap is already separated into fibres
and resin, and not mixed as with prepregs.
Modelling
Modelling is, in combination with design and
simulation (see page 11), an important
factor for improving the performance,
affordability, safety and development costs
of composite aerospace structures.
The modelling of composites is a highly
sophisticated and complex task due to the
inhomogeneity and anisotropy of the
materials. Models for layered twodimensional
reinforced structures have been
developed successfully over recent
decades. Stiffness, strength and fracture
properties are generally understood and are
applied in finite element programs and
analytical models.
Much more challenging is the modelling of
new material and manufacturing concepts
based on textile preforms and liquid
moulding technologies. The fibre structure
can be very complex, the degree of
integration is much higher, and the
performance is significantly influenced by
the manufacturing process. Issues such as
three-dimensional fibre reinforcement, fibre
distortion caused by draping, and the
variation of resin content due to local
permeability changes all have to be
addressed. In particular, the following
influential factors have to be taken into
consideration:
• Fibre and matrix material.
• Fibre architecture (two-dimensional or
three-dimensional, fibre curvature, etc.).
• Fibre-matrix interface.
• Fibre volume fraction.
15
Consequently, the following manufacturing
processes have to be understood and
modelled:
• Textile technologies (non-crimp fabrics,
braiding, embroidery, weaving).
• Resin processing technologies (resin
transfer moulding - RTM, resin film
infusion - RFI, vacuum assisted nonautoclave
processes).
• Integration / bonding process (e.g.
stitching).
Clearly, therefore, the modelling of
mechanical performance must be combined
with the modelling of the manufacturing
process. Integrated tools are required taking
all influential factors into account.
Furthermore, these models must be
accessible for development engineers. If
input data is required that is very difficult to
determine, or deeply specialist processing
knowledge is necessary (e.g. of advanced
textile processes), then the models will
never make the transfer from academia to
the aerospace industry. It is therefore
necessary to integrate the fundamental
understanding within professional software
packages and to prepare associated design
guidelines.
Further research is also required into the
modelling of new material and design
principles such as advanced sandwich
structures (e.g. employing folded cores), or
adaptive structures with integrated
actuators.
Fire Safety
The requirements regarding fire, smoke and
toxicity are defined by the Federal Aviation
Administration (FAA). They are dominated
by a post-crash scenario that dictates that
passengers must be able to leave a crashed
aircraft within five minutes without being
injured by toxic gases or heat, or hindered
by smoke. This scenario is quite different
from the approaches of the rail, marine or
bus industries. Furthermore, the technical
regulations relating to aerospace fire safety
are very different from other transport
sectors.
There is a goal of the FAA to further improve
fire safety by cutting the allowable peak heat
release rate of interior component materials
by a factor of two. For composites, this goal
cannot be reached by current phenolic resin
technologies. However, up until now, no
other resin material has been able to fulfil
these requirements either.
Current development work is largely
concentrating on the development of new
fire-safe core materials to replace Nomex
honeycombs. For example, phenolic foams
or folded honeycombs could improve
processing and affordability.
Other recent requirements and research
topics that have arisen in the field are
hidden fires (e.g. due to cable burning or
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