Precision Characterization of Reinforcement Fabrics

MEASURING AND TESTING
Precision Characterization
of Reinforcement Fabrics
Resin Injection Process. Efficient production of fiber-polymer composites requires
that the tool and process be precisely matched to the materials used. This applies
not only to the resin system, but also to the reinforcing fabrics. Innovative systems
enable the hydrodynamic impregnating performance of the fabrics to be determined. This affords a way of comprehensively describing their process behavior.
DAVID BECKER
PETER MITSCHANG
ndustrial production of fiber reinforced
plastics (FRPs) is dominated by resin
injection processes, such as resin transfer molding (RTM). The latter consists in
I
Translated from Kunststoffe 4/2014, pp. 61–64
Article as PDF-File at www.kunststoffeinternational.com; Document Number: PE111624
impregnating a fabric preform (Fig. 1) under positive or negative pressure with a
thermosetting resin system, which then
cures. By virtue of their very good
mechanical properties and extensive scope
for load-oriented design, FRPs offer huge
potential for lightweight construction in a
plethora of applications. The biggest obstacle to substituting FRPs for conventional materials, such as metals, is their high
cost. As prices of semifinished goods are
unlikely to decline significantly in the near
future, developments in the next few years
will essentially have to take place in
processes [1]. Sound mold and process design based on reliable simulations and reliable quality assurance will feature prominently here. Both require stable systems
which are capable of determining material properties, on one hand to serve as inputs for simulations and, on the other, to
safeguard constant material performance
Fig. 1. Permeability
data, determined
with the “2D-CapaPerm” system of the
IVW, formed the basis
for the flow simulation of a flap track of
FACC AG (figures: IVW)
38
© Carl Hanser Verlag, Munich
Kunststoffe international 4/2014
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0,14
1,95 mm
0,19
0,17
0,17
0,16
0,12
FVG = 48 %
1,95 mm
0,13
0,14
0,13
0,14
0,14
3
0,14
The inputs for simulating flow in FRPs are
the properties of the fiber structure and
matrix polymer. The matrix polymer, a
thermosetting resin system in this case, influences the process via its flow behavior;
this is very complex in terms of time and
temperature dependence due to the interplay of rheological properties and the viscosity dependence of the reaction kinetics.
Another significant factor is the impregnation behavior of the fabrics.Often in this
regard, only the permeability of the fabric
to the resin system is considered. And in
many cases, knowledge of the permeability is indeed sufficient for obtaining reliable simulations. Already, sophisticated
and reliable measuring systems for determining permeability exist [2]. The effects
of draping and compaction of the material in the mold can be studied. The degree
of compaction, usually quantified by the
fiber volume ratio (FVR), has a very large
influence on the permeability in this regard.By way of example, Figure 2 shows ICT
images of glass fiber fabric embedded in
epoxy resin at two different FVRs. It can
be clearly seen that the void space (black)
available for matrix flow is reduced. At the
same time, the structure of the fabric
changes, in this case at the yarn height,
which decreases from an average of about
0.17 mm to about 0.13 mm.These changes
cause a sharp drop in permeability [3]. If
changes in the FVR occur at the impreg-
0,15
Degree of Compaction
Influences Permeability
0,15
0,15
during the process. Impregnation can be
described by flow simulation if the material data are known (Fig. 1).
0,16
MEASURING AND TESTING
FVG = 56 %
Fig. 2. Compaction induces a structural change in the glassfiber fabric in the epoxy resin matrix. For
matrix flow, at a fiber volume ratio of 56 % (bottom picture), less void space (black) is available than
at a fiber volume ratio of 48 % (top). At the same time, the average yarn height falls from about
0.17 mm to about 0.13 mm
nation stage, more needs to be known
about compaction of the fabric for the purpose of providing a process description;
existing systems fail to provide this information.
Currently popular RTM variants, such
as compression RTM (CRTM), are particularly notable for their short impregnation
distances. These are achieved by impregnating the fabric through its thickness.
Since the pressures which are transmitted
to the fabric via the mold and the resin are
variable, process-induced changes also occur in the FVR. An example of this phenomenon is shown in Fig. 3 for CRTM; it
also occurs in other resin injection methods that impregnate through the thickness.
Hydrodynamic Compaction in
Resin Transfer Molding
Fig. 3. In CRTM, the resin is initially distributed over the preform. Closing the mold forces the resin
into the fabric. This creates three zones. Zone 1 contains just resin and the resin pressure is homogeneous. In Zone 2, the resin flows through the fabric. Zone 3 contains dry fabric
Kunststoffe international 4/2014
During injection, the mold is not fully
closed and so the resin is distributed over
the surface of the preform.When the mold
closes, the resin is forced into the fabric.
This creates three zones. Zone 1 contains
just resin,and its pressure is homogeneous.
In zone 2, which is bounded by the pure >
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MEASURING AND TESTING
Fig. 4. The new measuring system (left) can realistically simulate hydrodynamic impregnation. The picture on the right shows the cavity and the sensors
without measuring cell
compaction. Current approaches to numerically modeling this phenomenon often rely on permeability and compaction
values that have been determined separately. However, that is to ignore the influence
which they exert on each other. Preliminary tests have shown, for example, that
the flow exerts an influence on compaction
which the compaction tests cannot simulate on a universal testing machine.
Measuring Cell Detects the
Hydrodynamic Fabric Behavior
Fig. 5. Main components of the “HyKoPerm” measuring system
resin boundary x0 and the flow front xf, the
resin flows through the fabric. The effective pressure acting on an individual layer
on one hand is due to the pressure difference arising from the flow resistance of the
individual layer itself. On the other, there
is the difference in pressure created by the
flow resistance of all the previous layers, as
these are supporting themselves against the
individual layer. The diagram shows this
qualitatively. In Zone 3, where there is dry
fabric, the pressure difference between the
pure resin pressure in zone 1 and the atmospheric pressure in zone 3 is effectively
40
acting on the fabric. The fabric is forced
against the lower mold (xw).
In this case, knowledge of permeability
through the thickness as a function of the
FVR is insufficient. It is also necessary to
know the FVR obtaining at a specific effective pressure. The effective pressure,
however, in turn depends on the permeability, which is strongly influenced by
compaction. Permeability, effective pressure and FVR therefore exert a mutual influence on, and are in a complex relationship with,one another.These relationships
determine the degree of hydrodynamic
For this reason, at the Institut für Verbundwerkstoffe GmbH (IVW) in Kaiserslautern, Germany, a new system to realistically simulate the process conditions and
capture hydrodynamic fabric behavior in
its entirety was set up. This new system,
called HyKoPerm, can determine all the
requisite parameters simultaneously and
so identify interdependencies. To this end,
the measuring cell (Fig. 4) has a sampleholding cavity,which is surrounded by two
distribution fluids. These distribution fluids ensure two-dimensional, uniform flow
through the thickness. Compression rings
prevent leakage flow in the planar direction while pressure sensors in the upper
and lower manifolds continuously sense
the pressure drop across the fabric. Rapeseed oil serves as the measuring fluid because it has a viscosity and surface tension
at room temperature similar to that of a
© Carl Hanser Verlag, Munich
Kunststoffe international 4/2014
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MEASURING AND TESTING
First-layer displacement
0
0
50
s
Test time
LVDT-1
LVDT-2
bar
1.5
1.0
0.5
0
40
100
Thickness permeability
[×10-12 m2]
0.2
Pressure drop
0.4
Interval-5
Interval-3
0.6
Interval-4
0.8
K vs. FVR
2.5
Interval-2
Interval-1
Displacement
1.2
mm
Pressure drop vs. FVR
LVDT-3
45
%
Fiber volume ratio
Interval-1
50
Interval-2
Interval-3
6
5
4
3
2
1
0
40
42
44
%
Fiber volume ratio
Interval-4
48
Interval-5
Fig. 6. First-layer displacement (left diagram) increases the
fiber volume ratio
(center). The layer
structure was compacted in five steps.
This compaction
reduces the permeability through the
thickness from interval to interval (right)
© Kunststoffe
typical resin system at processing temperature.
A flow meter at the inlet provides information on volumetric flow, so that the
permeability through the thickness can be
calculated continuously. The calculation
is performed with the help of Darcy’s Law,
which expresses the relationship between
flow rate (q), pressure drop (Δp), viscosity (η), cross-sectional area to flow (A),
flow length (Δx) and permeability (K):
The innovative heart of the system is the
movable mounting of the lower distribution fluid. This is designed to ensure that
the distribution fluid is always in contact
with the fabric, following every movement
without affecting the actual measurement.
When the difference in pressure leads to
compaction of the fabric, the distribution
fluid follows this movement. This is detected by three linear variable differential
transformers (LVDTs). The instantaneous
FVR can therefore be calculated at any
time. Calculation of the permeability
through the thickness, which utilizes the
flow length (in this case the height of the
layer structure), can thus be corrected.
Moreover, the measured displacement enables the permeability through the thickness to be assigned to the instantaneous
FVR. Different initial cavity heights can
be set so as to simulate pre-compaction of
the fabric in the mold. The overall
“HyKoPerm” system is shown in Figure 5.
Reproducible Determination of
Parameters
A programmatic control device allows
software based on LabVIEW to specifically set various pressure differences and flow
rates on the fabric. High measuring efficiency is achieved via optional programs
that successively select different pressure
Kunststoffe international 4/2014
and volumetric flow steps and autonomously check when a steady flow state
has been reached. This allows the detected permeability through the thickness values to be averaged for each step.Thus every
trial provides a plurality of data groups and
reveals relationships between pressure difference, flow rate, FVR, viscosity, and particularly permeability through the thickness. Complete data sets are thus generated which describe the hydrodynamic impregnation and take into account all
inherent interdependencies.The step-wise
increase in pressure difference at intervals
gives rise to a continuously fluid-driven increase in compaction (Fig. 6) – represented
here by the first-layer displacement (left
diagram) and the resulting increase in FVR
(center). In this case, the layer structure
was compacted in five steps from an FVR
of about 37 % at 0 bar pressure difference
to about 47 % at 2 bar pressure difference.
As a result of this compaction, the permeability through the thickness from interval to interval has eventually fallen by
about 80 % (right diagram). This clearly
means that the impregnation process slows
down disproportionately, since compaction increases and permeability decreases.
Potential for
FRP Manufacturing
The outcome is a reproducible way of
studying materials, so as to generate data
for a simulation database. This can serve
as the basis for designing the mold and its
peripherals, as well as for the choice of
material and process parameters. Moreover, the system lends itself to quality assurance. Checks can be made quickly and
with little effort as to whether the materials used in series production offer consistent process performance. Deliveries
with excessive deviations, which would
lead to reduced quality in the process or
even gaps and scrap, could thus be rejected during the goods-in control. This allows for more accurate timing of production, thereby increasing the efficiency and
cost-effectiveness. The system also offers
far-reaching opportunities for research
into the influences exerted by processand fabric-related parameters on hydrodynamic impregnation. These results
could flow into the fabric selection stage
and process development [4]. ACKNOWLEDGMENTS
The authors would like to thank the German Research
Foundation for funding the project “Influence of Preform Technology on 3-D Permeability and Flow Front
Development in Liquid Impregnation Processes”
(Mi-647/15-2).
REFERENCES
1 Lässig, R.; Eisenhut, M.; Mathias, A.; Schulte, R.
T.; Peters, F.; Kühmann, T.; Waldmann, T.; Begemann, W.: Serienproduktion von hochfesten
Faserverbundbauteilen. Roland Berger Strategies,
VDMA, 09/2012
2 Arnold, M.; Rieber, G.; Mitschang, P.: Permeabilität als Schlüsselparameter für kurze Zykluszeiten.
Plastics International 102 (2012) 3, pp. 45-48
3 Rieber, G.: Einfluss von fabricn Parametern auf die
Permeabilität von Multifilamentgeweben für
Faserverbundkunststoffe. TU Kaiserslautern 2011
4 Becker, D.; Brzeski, M.; Linster, D.; Mitschang, P.:
Preform compaction and deformation during
through-the-thickness impregnation. ICCM19,
Montreal, 28.07.2013-02.08.2013
THE AUTHORS
DIPL.-WIRTSCH.-ING. DAVID BECKER, born in
1987, is research associate at the competence team
“Imprägnier- und Fügetechnologien” at the Institut für
Verbundwerkstoffe GmbH.
PROF. DR.-ING. PETER MITSCHANG, born in 1960,
is technical and scientific manager of the department
for processing technology at the Institut für Verbundwerkstoffe GmbH and professor of “Verarbeitungstechnik der Faser-Kunststoff-Verbunde”at the
technical university Kaiserslautern.
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