JEC COMPOSITES MAGAZINE - Issue #91 - August/September 2014 - 51

a few additional sensors or placing them strategically in the cavity.

Permeability profile reconstruction
Complementary to flow-front tracking, an additional algorithm has
been developed [4] to map the textile permeability by elaborating
the pressure history from the transducers through flow modelling
equations. As a result, the algorithm determines the flow-front
position and velocity in real time and concurrently reconstructs the
permeability profile along the flow direction in linear injections cases.
Figure 2 shows a system implemented to monitor injections in a
450 x 230 mm rectangular cavity. Here, the glass tooling is used only
to compare real and estimated flow-front evolutions for validation
purposes. The system can detect the flow-front position with an
accuracy better than 10 mm and correctly identifies the permeability distribution within the expected range of values (determined
through conventional permeability tests).

ment of the flow rate, so that the actual resin front velocity remains in
the defined optimal range of low void contents (Figure 4). n
More information: www.structures.ethz.ch
Contact: claudiodifratta@mavt.ethz.ch

Fig. 3: Estimated fibre volume content (FVC) and elastic modulus map for the
monitored impregnation of a preform with a patch. The map provides information
for online quality assessment; for example, the red zone indicates the position,
extent and expected properties of the patch region in the laminate

Fig. 2: Flow-front and permeability monitoring during impregnation of a preform
with a patch

Online quality assessment
and flow rate feedback control
The textile permeability is a function of the fibre arrangement and
volume content. Models such as the Kozeny-Carman formula can
be used to convert the estimated local permeability values into local
fibre volume contents [3, 4]. The final mechanical properties of the
parts under manufacturing can consequently be estimated by, e.g.,
applying the simple rule of mixture with the reconstructed fibre
volume content map (Figure 3). This approach would allow online
identification of critical areas - where defined quality requirements
are probably not met - in each manufactured part, indicating which
parts need further investigation in the post-production phase.
The structural performance of a composite part is also affected by its
void content. In RTM processes, voids are usually formed during the
preform impregnation stage, depending on capillarity effects in the
fibre bundles [1]. To minimize the void content, the resin front velocity should be kept within an optimal range of values, determined by
the specific combination of resin system and fibrous reinforcement in
the process. The above-presented monitoring system is enhanced by
a computer-controlled pressure regulator, which changes automatically the injection pressure according to the online measurements of
flow-front velocity. Such a feedback mechanism allows self-adjust-

Fig. 4: The developed system identifies the current flow-front velocity and keeps
it within a range of minimum void formation, by adjusting automatically injection
pressure and, thus, flow rate.

References
[1] P. Ermanni, C. Di Fratta, F. Trochu. Molding: Liquid Composite Molding (LCM), in Wiley Encyclopedia of Composites, 2nd Edition. L. Nicolais, A. Borzacchiello, S. M. Lee Editors 2012, John
Wiley & Sons Inc.
[2] C. Di Fratta, F. Klunker, P. Ermanni. A methodology for flow-front estimation in LCM processes
based on pressure sensors. Composites Part A: Applied Science and Manufacturing, Volume 47,
2013, pp. 1-11.
[3] C. Di Fratta, W. Xiong, C. Thurnherr, F. Klunker, P. Ermanni. Concept for on-line flow monitoring
and permeability mapping in RTM processes to evaluate laminate quality. Proceedings of SAMPE
Europe 34th International Technical Conference (SEICO 13), 2013, Paris, France.
[4] C. Di Fratta, L. Di Lillo, F. Klunker, P. Ermanni. Detection of permeability variations for early
quality assessment in Liquid Composite Molding. Proceedings of 19th International Conference
on Composite Materials (ICCM 19), 2013, Montreal, Canada.

No91 August - September 2014 /

jec composites magazine 51
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JEC COMPOSITES MAGAZINE - Issue #91 - August/September 2014

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