SAMPE Journal - March/April 2022 - 18

FEATURE / AERONAUTICAL FUSELAGE PARTS
Figure 2. Welding thermal cycle, from theoretical approach (left) to real bench measurements (right
curves).
Figure 3. General concept of part positioning for continuous induction welding.
difficulty arises from copper which has an electric
conductivity 1000 higher than of carbon fibres.
Therefore, focusing the energy at the interface and
not in the copper part, for structures with these two
types of material (stringer and skin made of carbon
with an external surface with a copper mesh) is
particularly challenging.
Process modelling can be a viable tool to
improve the process design of induction welding,
especially for the LSP technical challenge. IREENA
(Nantes University), an academic partner of the
project, has developed its own numerical solution
over the years3
and used it to design inductors,
select sets of parameters for welding and evaluate
the effect of specific tooling on thermal stability
and homogeneity.
MATERIALS AND PROCESS
The composite parts in this work were composed
of unidirectional CF/PEKK prepregs. Curved skins
and plate panels were made by AFP (from 7 to
19 plies) then consolidated in autoclave (Stelia
Aerospace). LSP was brought onto the outer face
of the panels after AFP, for the autoclave stage. Two
kinds of stringers were produced, either pultruded
(10 plies stacking - Hutchinson) or stamped (11
plies - AVIACOMP).
Welding cycle of thermoplastic should respect
18 | SAMPE JOURNAL | MARCH/APRIL 2022
pressure, time & temperature ranges to reach
optimum mechanical performance. Intimate
contact & healing are the two physical phenomena
taking place. We firstly defined a theoretical
thermal cycle (left diagram in Figure 2, Cender
20184
), which constitutes a target for each point at
the interface and so for the numerical model.
The challenge is to create the most homogenous
thermal field along the stringer width, from the
radius to the free edge. The coil with its surrounding
EM field is moved along the stringer above it. The
composite skin is placed under the composite L
stringer in contact with the main tooling support
(Figure 3). A jig for L stringer support is installed
to hold the radius in place. Due to asymmetric
stringer design and its effect on the shape of the EM
field, thermal homogeneity is difficult to obtain.
An important requirement is to correctly
design an effector in order to create intimate
contact between parts but also to continuously
heat and cool specific areas. This welding device
was implemented on a 6-axis robot (Figure 4).
Compaction rollers apply pressure before and
after the inductor allowing for intimate contact
and good composite compaction during the hot
stage. Specific design improvement on compaction
roller has been done (Stelia Aerospace, Europe
Technologies, IRT Jules Verne) to increase the
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SAMPE Journal - March/April 2022

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