JEC COMPOSITES MAGAZINE - Issue #122 - July 2018 - 44

FEATURE AEROSPACE research

Fig. 7: Chip formation analysis (left) during CFRP/Ti multi-material drilling (simulation on the right)

Structures and
modelling strategies
Short fibre injection
The ICA has been working on the development and validation of solutions
for injected polymer materials, with
their properties induced by the injection
process. A behaviour law with damage
(CDM) was proposed, which takes the
microstructural parameters related to
the process into account and is "implementable" in an industrial finite element
code.
Its applications are dedicated to the dimensioning of composite parts injected
with PEEK thermoplastic matrices reinforced with short carbon fibres, e.g., the
calculation of a helicopter door hinge.
The ICA has also been working on the
injection of PEEK resin reinforced
with carbon fibres as part of industrial
projects and/or PhD theses. Some outcomes of these projects are already in
use.
As an extension of these studies, work
is underway on the recycling of carbon
fibres to re-use them in various forms
(mats, short fibre granules) in thermoplastic matrices.
This work contributes to the creation of
an industrial recycling sector in the Occitanie Region.
Modelling strategies
For the past twenty years, the research of
ICA professors has specifically focused on
the development of strategies to model the
damage and fracture scenarios of composite structures. Three main strategies were
developed [3].
The first one is the Diffuse Damage Model
(DDM), which combines the continuum
damage mechanics laws for mechanical

damage with cohesive zones. Due to the
general nature of implementable laws, the
DDM can be applied to many standard
materials and problems, such as impact for
example. It was recently used to size CFRP
or SiC-SiC junctions. It is also interesting
for bearing issues.
The second strategy is the Discrete Ply
Model (DPM), which is based on a mesh
that follows the orientations of the unidirectional plies, on cohesive zones, on a
fragile rupture in matrix cracking driven
by the adjacent volume elements and on
a fibre break taking strain energy release
rates into account. It has the advantage of
being based on only 13 mechanical parameters, which can be obtained easily. It was
successfully applied first to out-of-plane
problems such as low-velocity/low-energy impact, edge impact, compression after
impact or pull-through. Initially developed
for out-of-plane problems, the discrete
character of the model proved relevant to
model splitting, thus making it possible
to deal with problems such as hole failure,
scale effects and large cuts under complex
stress.
The third strategy is based on semi-continuous modelling used for frontal or razing
impact problems on helicopter blades.
Bars are used to model the fibres and volumes (for solid glass/epoxy UD) or plates
(for woven plies) to model the resin. Specific elements are developed (i) to find
the static characteristics and (ii) to model
the damage observed at the strand scale.
Specific cohesive elements were recently
developed, together with approaches to
recover the bending stiffness of a fabric
considering the actual weaving geometry.
These models have reached a high level of
confidence and are now validated for certification purposes.

jec composites magazine / N°122 July 2018

Fig. 8: Simulation of short fibre injection in a
complex part (aircraft fitting)

In general, these modelling strategies and
other, more specific ones were developed
and used to respond to many application
problems covering the whole field of composite structures:
- Impact and residual behaviour after
impact, damage tolerance, large cuts,
shields;
- Crash modelling, energy absorption
mechanisms;
- Fatigue;
- Sandwich structures: nonlinear analysis, impact, inserts, modelling of foam
or honeycomb cores;
- Junctions, Z-pinning, stitching;
- Structural details: unfolding, stiffener
debonding, post-buckling;
- Repairs;
- Optimization of composite structures.
Wood-based eco-structures
An emerging topic in the group is the
study of new wood-based structures.
Sandwich structures using birch/poplar/
okoumé plywood cores and aluminium,
fibreglass, flax or carbon skins were manufactured and studied experimentally and
numerically under static loading, impact
and compression after impact. These structures showed very interesting specific and
economic characteristics and studies will
now focus on assembly and crash issues.
Experimental methods and means
Complex loading
The Institute uses a modular multiaxial

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #122 - July 2018