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offers sophisticated linear or quadratic
elements with laminate capabilities, often
with a selection of different failure criteria.
The ability to visualise local effects and
stress concentrations makes FEA an
essential tool in the early and middle design
phases.
The widespread use of FEA for composites
has reduced time to market enormously. For
example, parametric FEA models can
provide a broad range of stress evaluations
under different conditions within a short
time. FEA models also deliver more results
than experimental techniques because
“conditions” can be determined at all
element integration points and not only at
specific (e.g. strain-gauged) test locations.
Furthermore, FEA analysis is much cheaper
than performing experimental tests. It
therefore facilitates a more extensive study
of structural behaviour, thus reducing the
risk of poor design.
Still, the maturity of FEA codes for
composite design is insufficient at present.
As the drive towards lightweighting produces
increasingly complex structures, so the
standard laminated shell elements become
increasingly restrictive. New calculation
capabilities are required by the aerospace
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industry, including the ability to
accommodate dynamic behaviour (crash
and impact), post-buckling, sandwich
materials with anisotropic cores, and multiply
or last-ply failure.
Often, user subroutines for advanced failure
criteria evaluation are implemented within
FEA codes. Recently, considerable effort
has been directed towards the development
of new delamination behaviour laws for FEA,
both in implicit static and explicit dynamic
software codes.
In summary, it can be stated that the design
and simulation of composite structures has
improved dramatically over the last decades.
This is largely due to the improvement of
hardware, software, and fundamental
material understanding. Nevertheless, a lot
of research work is still required, with some
of the most important topics being:
• Advanced material models for stiffness,
strength and failure mechanisms
(especially for three-dimensionally
reinforced composites and new
sandwich materials).
• Integrated tools covering design,
manufacturing (e.g. draping or
injection), and mechanical performance.
• Advanced design concepts (e.g. those
based on bionic approaches).
• Adapted certification methods for new
design philosophies (e.g. highly
integrated structures).
• Improved design tools and methods for
adaptive composites (e.g. with
integrated actuators).
• Improved simulation of highly dynamic
failure (e.g. crash and impact).
• Improved understanding of in-service
and long-term behaviour.
Crashworthiness
With the assumed wider use of composites
(e.g. CFRP fuselages), crashworthiness is
becoming an increasingly important topic for
aerospace applications. This is because of
concerns over passenger safety and the
associated legal requirements.
Composite materials are of particular
interest for use in energy absorbing
structures because of their high mass
specific energy absorption capability. Energy
absorption capacities of more than a 100
kJ/kg can be achieved with careful design.
This is realised by the complex failure
mechanisms of composites, involving fibre
breakage, matrix cracking, and fibre-matrix
interface cracking.
To realise composite structures with high
energy absorption capabilities, special
designs are required. Stable, progressive
failure must be initiated by appropriate
trigger mechanisms. Furthermore, careful
consideration needs to be given to the
design of the fibre reinforcement and the
structural geometry.
The first successful aerospace applications
to employ energy absorbing composites
have been helicopter sub-floors. Recently, a
complete passenger cell of the NH90
helicopter was successfully crash tested. It
was demonstrated that all the requirements
relating to passenger decelerations and
structural integrity could be fulfilled by a
composite structure.
In the future, civil aircraft will also be
designed to meet more stringent crash
requirements. New design concepts are
needed to allow, for example, the sub-floor
to absorb energy. One interesting approach
is the “Gondel-Concept” of DLR
Braunschweig.
Most importantly, a good compromise needs
to be found between composite material
energy absorption and wider requirements
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