JEC COMPOSITES MAGAZINE - Issue #87 - March 2014 Special JEC Europe - 60

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high-temperature forming, where the
resin is in the melt state.
The laminates used, usually consist of a
composite blank with constant thickness
and near-isotropic properties. However,
for optimum performance, the blank may
consist of patches of unidirectional (UD)
fibre architecture that are oriented along
the load path of the final part. Since the
forming process is highly non-linear and
easily leads to defects such as tearing
or wrinkling, the development of an
optimum solution for lay-up, blank shape
and processing conditions still involves
a trial and error approach, which is time
and cost intensive.
As an alternative, process simulations
make an early optimization of these parameters possible; however, they rely on
accurate material models and parameters.
Therefore, new material characterization
methods have been developed at TPRC.

The shear mechanism in the ply, often
referred to as intra-ply shear, can be
isolated and measured by a so-called bias
extension test. This test is well-suited for
woven fabric, but fails for unidirectional
materials due to the poor consistency
perpendicular to the fibres. A new test
proposed by Haanappel [1] uses unidirectional bars that are torsion-loaded
under well-controlled conditions in a
rheometer (see Figure 2).
Bending is tested using the rheometer
with a dedicated fixture and is performed
on small specimens taken from the laminate. Typically specimens with different
numbers of layers are applied in order to
investigate scaling effects (see Figure 2).
Fig. 4: Predicted wrinkles (using AniForm) of G/PPS
hemispheres with cross-ply (top) and quasi-isotropic (middle) lay-up and of a C/PEEK UD hemisphere
with quasi-isotropic lay-up (bottom), and experimental results (right)

Characterization for forming
During stamp forming, the stiff fibres are
hardly able to accumulate deformation.
Instead, deformation is allowed by less
energy-consuming mechanisms such as
shear within the ply, friction between
plies and between ply and tool, and ply
bending. Since the formability of the
material is a delicate interaction of these
mechanisms, it is important that each
of these mechanisms is well understood
and characterized. Over the past years,
a lot of effort has been invested in the
development of new measurement technologies for high-temperature testing up
to 450°C.

Fig. 2: Torsion bar test for intra-play shear testing
of UD materials (left) and bending fixture (right)
mounted in a rheometer

Fig. 3: Inter-ply and tool-ply friction tester

Friction behaviour is characterized using a friction testing device developed
in-house (see Figure 3). A composite
specimen is pulled through two counter
surfaces consisting of either composite
or tool steel. The whole package is heated
and loaded in the normal direction. The
load needed to pull out the specimens
is a measure for the friction coefficient
which depends on temperature, pressure
and sliding velocity..
The parameters that determine the influence of the different mechanisms on the
forming behaviour are incorporated in
material cards for subsequent process
simulations with AniForm (www.aniform.com).

Hemispherical tools
As an example, hemispherical tools are
used to show the difference in forming

60 jec composites magazine / No87 March 2014

behaviour depending on the lay-up. A
composite control arm serves as a demonstrator for automotive applications.
The hemispherical tools are suited a
method to characterize the formability of
thermoplastic composites (see Figure 1).
Since the shape is doubly curved, all of the
previously discussed mechanisms are triggered. In this example, a G/PPS woven
fabric with different lay-ups was used to
study formability.
The simulations with AniForm show that,
in the case of a quasi-isotropic lay-up,
the wrinkles are very well predicted. The
cross-ply lay-up does not give reason
for wrinkling since all deformations can
be accumulated by intra-ply shearing. A
comparison with a quasi-isotropic C/
PEEK UD shows that severe wrinkling
occurs for this material, which is perfectly
predicted by simulation (see Figure 4).
Note that also the final contour compares
very well.

Control arm
The composite control arm (see Figure 7)
was developed in-house as a demonstrator part. The arm was designed as a single

http://www.aniform.com http://www.aniform.com

JEC COMPOSITES MAGAZINE - Issue #87 - March 2014 Special JEC Europe

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