JEC COMPOSITES MAGAZINE - Issue #112 - April/May 2017 - 53

Optimisation of sandwich composite structures for space applications

Fig.3a and 3b: View of a stacking sequence table

while the second step (lay-up optimisation)
is performed using an in-house code.
Optimisation of a dual launch system
The application case described in this paper
is derived from a typical aerospace structure: a Dual Launch System.
However, this work does not intend to
reproduce or elaborate on the actual design
process of this part. Rather, it uses its
mechanical specifications to define an optimization problem that will be solved using
a global design strategy. Thus, the results
should not be interpreted as potential gains
for the real structure, since a significant
number of constraints and specifications
are not considered here, but as a proof of
concept of this strategy.

- minimizing the overall stiffness of the
structure,
- preserving the initial performance with
respect to buckling,
- ensuring that no failure occurs in the
composite skins.
The optimization variables are the thickness
of the skins and core, as well as a set of
stiffness parameters - the lamination parameters [3] - that fully characterize the behaviour of the sandwich shell. A dedicated
GUI is used to enter the parameters of the
optimization problem, such as the materials
for the elementary layer and the core, or
the admissible range for the variables. The

user is also allowed to subdivide the part as
needed in order to design separate composite
lay-ups. Here for instance, the upper and
lower cones were separated from the centre
cylindrical body of the structure, as shown in
Figure 4.
Promising results
In this case, the resulting design exhibits a
significant saving on the total weight of the
structure, while the mechanical constraints
remain valid. The weight saving was mainly
achieved through the reduced number of
plies in the skin. The proposed design is not
expected to be compared with the actual
current design of the part, if only because the
specifications are incomplete. In particular,
due to the lack of technological and process
constraints, this example resulted in excessively thin cores of the sandwich shells, and
thus would not be technologically feasible.
Nevertheless, this work still shows that the
global design strategy presented here is able
to significantly improve a design based on a
given bill of specifications. Additionally, as
far as computation times are concerned, the
whole process lasted less than fifteen minutes, which is compatible with design office
requirements.
In light of these promising results, further
work is currently in progress to improve
the tool and expand its capabilities. For instance, the analysis described above is being
performed again but with a much broader
set of mechanical constraints in order to
validate the strategy on a realistic test case.
Additionally, this optimization tool will soon
be applied to stiffened panels, and is expected
to be able to define the number, shape, and

lay-ups of optimum composite stiffeners
for any given structure.
Conclusion
This paper introduced a novel global design
strategy for composite structures. The goal
of this strategy is to improve the design of
a composite structure based on its specifications, while taking into account many
specific features of these materials. The prototype version of this design tool showed
very promising results and the development of an advanced version is underway.
This advanced version will introduce new
functionalities, such as taking into account
damage initiation or failure prediction, or
optimizing a structure with respect to cost/
process constraints. Ultimately, the tool
would also be able to guide the designer
in finding the optimum shape and process
for the part, in order to, finally, fully exploit
the potential of composite materials in
aerospace structures.
Contact:
cedric.julien@onera.fr
florian.lavelle@cnes.fr
References
[1] F.X. Irisarri, M.M. Abdalla, Z. Gurdal.
Improved Shepard's Method for the
Optimization of Composite Structures,
AIAA Journal (49), 2726-2736, 2011
[2] F.X. Irisarri, A. Lasseigne, F.H. Leroy, R.
Le Riche. Optimisation des empilements
dans les structures composites stratifiées
avec arrêts de plis, Comptes rendus des
JNC 18, Ecole Centrale Nantes, 2013
[3] Z. Gurdal, R.T. Haftka, P. Hajela. Design
and optimization of laminated composite
materials, Wiley, 1999

Feature Aeronautics

The objective of the overall design problem
is to minimize the weight of the structure
while enforcing several mechanical constraints:

Fig.4: Geometry of the A5 Sylda

/ jec composites magazine

No112 April - May 2017

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