JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 26

characterisation

Experimental and numerical investigations :
effects of defects in laminated L-angles

By
FRÉDÉRIC LAURIN, RESEARCH SCIENTIST, ONERA
ANNE MAVEL, RESEARCH ENGINEER
ERIC DUPUY, COMPOSITE STRESS ENGINEER, AIRBUS

O

nera, in collaboration with Airbus,
is working in this direction and
managed to demonstrate the predictive capabilities of an advanced material
approach to estimate the effects of initial
defects in composite structures.
In the present project, the influence of initial
defects encountered in laminated L-angles
was studied experimentally and numerically.
Indeed, it is crucial that these structures
are designed accurately because L-angles
form the junction between perpendicular
stiffened panels used in different components, such as the centre wing box, the wings
or the vertical or horizontal tail plane. These
composite L-angles have to carry part of the
membrane loading applied to the different
panels and are subjected to bending moments leading to unfold/fold L-angles. The
failure of such components is mainly due to
delamination resulting from the out-of-plane
stresses induced by the applied moment.
Due to the complex geometry of L-angles,

Considering the widespread use of composites in aerostructures
and the sector's high production rates, it is now essential to
define standard quality and associated acceptance criteria for
composite components. Standard quality is the maximum acceptable defect size that has no detrimental effect on a part's
performance. Therefore, the characterisation of manufacturing
defects and a deep understanding of their potential consequences
on performance is a key issue. Simulations using physically-based
damage and failure approaches are a crucial tool to estimate the
influence of initial defects on mechanical properties and, therefore,
to establish acceptance criteria adapted to each composite component, thus avoiding conservatisms and "over-quality".

defects such as porosity, over-thickness or
initial ply waviness can occur in the radius.
Initial ply waviness is one of the most critical
defects for composite L-angles and its
detection, using traditional non-destructive
techniques such as C-Scan, remains very
difficult.
Therefore, the influence of initial ply
waviness on the onset of delamination
has to be taken into account in the design
of laminated L-angles. Today, design
takes into account the relevant manufacturing quality for L-angles, relying
on the results of the few available tests
performed on specimens extracted from
full-scale parts (such as manufacturing
trials or demonstrators). It is worth
mentioning that artificial defects can
be obtained by modifying the manufacturing process, but these defects are
not necessarily representative of those
encountered on full-scale parts in serial
production conditions, and thus they do
not have the same effects on mechanical
properties.

26 jec composites magazine / No106 July 2016

The objective of this study is to evaluate the predictive capabilities of the
proposed numerical methodology in
order to estimate the influence of initial
ply waviness on the strength of laminated L-angles and to lay emphasis on the
possibilities offered when estimating the
effects of defects through virtual testing.

Fig. 1: Photographs of laminated L-angles with (a)
or without (b) initial waviness defect
Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #106 - July 2016
