JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 30

characterisation

Fig. 7: Effects of defects in L-angles subjected to unfolding loading: experimental vs. numerical results

Moreover, the failure pattern at the onset
of delamination was also in good agreement with the experimental results. The
use of stereo-digital image correlation to
detect delamination cracks [4] (through
the analysis of the correlation error)
during loading was essential to record
the propagation of delamination cracks
from the radius to the legs and thus to
validate the present failure approach,
as also reported in figure 7. Again, the
initial waviness induced additional outof-plane shear stresses leading to early
failure, a phenomenon well captured
by the proposed modelling approach.
Therefore, the present methodology
(combining a 3D failure criterion and
an explicit description of defects) was
applied to different test cases (different
lay-ups, different initial waviness, and
different loadings) and was able to estimate accurately the effects of defects in
laminated L-angles.

Conclusions and prospects
Due to the massive use of composite
structures and to the very large size of
"one-shot" composite components (such
as a number of wing, fuselage, vertical
and horizontal tail plane and keel-beam
components of the Airbus A350XWB),
the proper management of manufacturing defects and potential reworks is
becoming a real issue for industrials. The
acceptance criteria are currently determined through the analysis of mechanical tests on samples extracted from serial
production parts (only a few available
specimens) or samples manufactured
with controlled defects (not necessarily
representative of the real defects). Due to

these limitations, the current acceptance
criteria are often very strict, leading to
unnecessary inspections and rework.
Using simulation to estimate the effects
of defects can be a very interesting alternative to define specific acceptance rules
for a component subjected to particular
loadings.
Onera, in collaboration with Airbus,
proposed a methodology to predict the
influence of initial defects, such as initial
waviness, in laminated L-angles.
These structures are located in various
aircraft components, meaning that their
accurate design, taking into account
initial defects, is a critical point. In the
present study, several laminated L-angles, with or without defects and with
two different lay-ups, were subjected to
different loading cases, such as four-point
bending or unfolding loading. The failure
of pristine specimens being due to the
instantaneous propagation of delamination, the 3D failure criterion, already
proposed by Onera, allowed an accurate
prediction of the failure loads for all the
tested configurations.
Then, for L-angles with some initial ply
waviness, the defects were explicitly
introduced in the finite element simulations. The simulated specimens presented the real non-conformal geometry and
the local orientation of the plies was also
updated. Using an appropriate 3D failure
criterion and describing the initial defect,
it was demonstrated that it is possible to
estimate accurately the effects of initial
defects on the onset of delamination for
the different test configurations.

30 jec composites magazine / No106 July 2016

Therefore, a next step could consist
in generating a wide range of defects
virtually and estimating their influence
on the structural strength of L-angles
with different stacking sequences for a
large number of loading cases in order to
adapt the existing acceptance criteria to
such composite structures, thus reducing
conservatism, adapting quality control
plans and reducing unnecessary rework.
The use of virtual testing to estimate the
effects of defects should soon become
an absolute necessity in the aeronautical
industry. n
Acknowledgements
This work was carried out in the framework of the STRENGTH project, directed
by Airbus and funded by DGAC (Directorate General of Civil Aviation), which is
gratefully acknowledged. The authors
would like to express their sincere
gratitude to Dr. R. Valle, for valuable and
helpful discussions.
More information:
www.onera.fr
www.airbus.com
References
[1] Charrier J.-S., Laurin F., Carrere N.,
Mahdi S., Determination of the out-ofplane tensile strength using four-point
bending tests on laminated L-angle specimens with different stacking sequences
and total thicknesses, Composites Part
A, vol. 81, pp. 243-253, 2016
[2] Charrier J.-S. Développement de
méthodologies dédiées à l'analyse
robuste de la tenue de structures composites sous chargements complexes tridimensionnels, Doctorate thesis ENSAM/
Paristech, 2013
[3] Z-set 6.5, ZeBuLoN code. Non-linear
material and structure analysis suite, Users' manual. http://www.zset-software.
com, 2015
[4] Laurin F., Charrier J.-S., Lévêque D.,
Maire J.-F., Mavel A., Nuñez F., Determination of the properties of composite
materials thanks to digital image correlation measurements, Procedia of IUTAM,
vol. 4, pp. 106-115, 2012

http://www.onera.fr http://www.airbus.com http://www.zset-softwarec http://www.zset-softwarec

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