JEC COMPOSITES MAGAZINE - Issue #103 - March 2016 - 62

performing

Fig. 5: Process steps for 3D fibre angle measurement
Fig. 9: Influence of fibre angle modelling on the
displacement under loading

Fig. 6: Robot-assisted optical fibre angle measurement on two different preforms

subsequent detection of fibre angles,
waviness or gaps based on image analysis
(figure 4).
The process chain for 3D preform
analysis covering the whole part surface
consists in a virtual image planning and
robot programming step, the optical
capture of the part and the subsequent
mapping of measured fibre angles to a
customer-specific FE format for visualization and processing (figure 5). This
means that the measuring results are
available in the same format as the draping simulation results, so that they fit perfectly inside existing CAE process chains.
Preforms A and B of the L-shape are
captured and compared using the system
(figure 6). The influences of process
handling on the resulting shear angle of
the formed fabric become visible and

Fig. 7: Static load case for the numerical assessment of draping influences on the structural
behaviour

assessable. Therefore, the system helps
improve the development processes
for CFRP parts as it enables accurate
evaluation and documentation of fibre
orientations. Furthermore, it is suitable for quality assurance within series
production and scatter analysis during
preform manufacturing.

Quality assessment
with numerical simulation
Using numerical simulation methods,
the influence of preform quality on
mechanical performance can be evaluated at an early stage without extensive
experimental testing. Therefore, the
measurement results of the 3D preform
analysis are mapped directly into the
FE model. The boundary conditions of
a numerical test case using the example

of the L-shaped form are shown in
figure 7, using the fibre angles from the
preform measurement. As a reference,
an additional model with idealized
fibre orientations is considered, where
process influences are neglected. This
simplification is typically used during
part dimensioning today.
The evaluation of maximum ply stresses
in the transverse direction (figure 8)
clearly shows the influence of draping.
While the model with idealized fibre angles predicts very low transverse stresses,
the fabric shearing caused by draping
in preforms A and B leads to stresses of
about 25 MPa in the transverse direction, which is already a critical level for
the appearance of inter-fibre failure.
The improved quality of preform B
leads to lower ply stresses and less
displacement under loading conditions.
The latter is about 20% lower compared
to preform A and about 10% lower
compared to the idealized model with
projected fibre angles (figure 9).
This example demonstrates the necessity to consider real fibre angles in the
structural simulation of composite
parts, as they strongly influence the
overall structural behaviour and the anisotropic ply stresses. However, an idealized approach can lead to serious risks
during part dimensioning. Using failure
models, the real fibre orientations can
be applied in a similar way to improve
strength and damage prediction in crash
simulations [7].

Summary and outlook
Fig. 8: Numerical results for ply transverse stresses based on real fibre angles

62 jec composites magazine / No103 March 2016

Holistic approaches for part and process
development are required to establish

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #103 - March 2016