Unified saturation and micro-macro voids method in Liquid Composite Molding
ABSTRACT The topic of this paper concerns void defects. We introduce a model to describe the macroscopic resin flow considering the
compression of residual air in the preform. A microscopic flow model is also developed to account for the microvoid formation
due to the variation of resin velocity at the flow front. According to these mathematical models, a numerical simulation code
was developed to perform a combined macro and micro flow analysis with the saturation of the medium. The results of these
simulations are compared with experimental results obtained by the laboratory device. We present the real-time measurement
of the void content evolutions during the injection, using an original sensor. Our technique is based on the electrical conductivity
of the injected liquid. After the material and method presentation, some results obtained during injection are shown. The
developed analysis technique and the numerical simulation code can be used for obtaining the optimal processing conditions
and design parameters in manufacturing of composites by Liquid Composite Molding (LCM) processes.
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ABSTRACT: The influence of different process variables on the void content in resin transfer modling (RTM) has been investigated experimentally. The moldings were made in a flat mold filled by a parallel flow from one edge of the laminate to the other. The viods were found concentrated in a narrow region close to the ventilation side of the laminate. The void volume fraction in this region was almost constant and dropped over a short distance to basically no voids in the rest of the laminate. Micrographs from cross sections in different directions revealed that the voids were of two different types, long cylinderical bubbles inside the fiber bundles. An efficient way of reducing the void content was to use vacuum assistance during mold filling. This technique was benefical both for the magnitude of the void content and for the extent of the void region. The void content with the highest level of vacuum assistance (≈ 1 kPa absolute pressure), was practically negligible. Strong indications for void generation by mechanical entrapment at the flow front was found. The lowering of the void content with vacuum assistance can be interpreted as aresult of compression of voids when the vacuum is released and a higher mobility of voids created at a lower pressure.Polymer Composites 08/2004; 15(1):25 - 33. · 1.48 Impact Factor
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ABSTRACT: Effects of the void content on the interlaminar shear strength (ILSS) of carbon-fibre-reinforced plastics (CFRP), including carbon-epoxy (C–E) and carbon-unsaturated polyester (C–P) composites, have been investigated. In order to obtain specimens containing a wide range of void contents, a small quantity of foaming agent was sprayed between each lamina prior to fabrication of the composites.A regression analysis of the relationship between ILSS and void content was performed for each sample. A void content criterion which is required for quality evaluation of composites was calculated using the regression lines and the procedures of analysis of variance. The void content criteria were for C–E and C–P specimens, respectively.In order to find effects of voids on the mechanical reliability of CFRP, the probability of survival, , was analysed using a statistical technique. It has been determined that the value of decreases with increasing void content, and features of this reduction depend considerably on the working stress and the standard error between the regression line and the measured value of ILSS.Composites Science and Technology 07/1986; 25(1):3–18. · 3.33 Impact Factor
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ABSTRACT: This paper presents the results of an experimental investigation of the effects of voids upon the hygral and mechanical properties of AS4/3502 graphite/epoxy. Simple uniaxial tension tests were employed to determine the effects of voids upon ply moduli E11, E22 and G12, and the Poisson ratio ν12. The matrix dominated moduli E 22 and G12 were found to depend significantly upon void content while little dependence was noted for the fiber dominated properties E11 and ν12. Moisture conditioning experiments showed that both the rate and the equilibrium level of moisture sorption depend upon void content. In addition, non-Fickian diffusion anomalies were observed in high void content specimens while specimens with low void content displayed classical Fickian behavior.Journal of Composite Materials 01/1987; 21(3):280-289. · 0.94 Impact Factor
Unified saturation and micro-macro voids method in Liquid Composite
C.H. Park1, J. Bréard1, A. Saouab1, W.I. Lee2
1Laboratoires d’Ondes et Milieux Complexes, FRE 3102-CNRS-Université du Havre,
53 rue de Prony, BP 540, 76058 Le Havre, France
e-mail: email@example.com; firstname.lastname@example.org; email@example.com;
2School of Mechanical and Aerospace Engineering, Seoul National University,
Shinlim-dong, Kwanak-gu, 151-742 Seoul, Korea
http://www.snu.ac.kr e-mail: firstname.lastname@example.org;
ABSTRACT: The topic of this paper concerns void defects. We introduce a model to describe the
macroscopic resin flow considering the compression of residual air in the preform. A microscopic flow model
is also developed to account for the microvoid formation due to the variation of resin velocity at the flow
front. According to these mathematical models, a numerical simulation code was developed to perform a
combined macro and micro flow analysis with the saturation of the medium. The results of these simulations
are compared with experimental results obtained by the laboratory device. We present the real-time
measurement of the void content evolutions during the injection, using an original sensor. Our technique is
based on the electrical conductivity of the injected liquid. After the material and method presentation, some
results obtained during injection are shown. The developed analysis technique and the numerical simulation
code can be used for obtaining the optimal processing conditions and design parameters in manufacturing of
composites by Liquid Composite Molding (LCM) processes.
Key words: Void defect, Air compression, Microvoid, Real-time measurement, LCM
Liquid Composite Molding (LCM) processes, such
as Resin Transfer Molding (RTM) and Vacumm
Assisted Resin Transfer Molding (VARTM) are
gaining their popularities for the manufacturing of
large and complex parts in the aeronautic and
aerospace industries, by virtue of their cost-
effectiveness. In the case of LCM processes,
however, the air can be entrapped in the part during
the process, and this residual air results in the
defects such as dry spots or microvoids that are, in
turn, responsible for the degeneration of mechanical
properties of the final product [1-8]. In general, most
fabrics consist of fiber tows that are woven or
stitched, and hence the microstructure of fabrics is
non-uniform. Although the field of resin average
velocity may be appear smooth, the local velocity
can vary significantly, from point to point, at the
micro scale. Due to the non-uniform microstructure,
the local permeability and the local capillary
pressure may differ by several orders of magnitude
between inside and outside the tows. This leads to
the non-uniform velocity field and, subsequently, the
formation of air voids at the micro scale.
Given the fiber preform, the microvoid content has
been known to largely depend on the capillary
number, which is a dimensionless number to be
defined as a ratio of viscous force to capillary force
If the global resin velocity is smaller than the
capillary suction inside the fiber tows, microvoids
are formed in the large channel between the fiber
tows. On the contrary, microvoids are entrapped
inside the fiber tows when the global resin velocity
is greater than the capillary suction between the fiber
Once the microvoids are formed, they may be
transported along the pressure gradient. In general,
the void content is measure from the final part where
the time evolutionary features of microvoid physics
cannot be observed. To deal with this problem, we
developed a special sensor that measures the
electrical conductivity of liquid which can be
correlated with the void content.
In this study, we present mathematical models for
the analysis of formation of macro and micro voids
during the filling process. Then, these models are
validated through the real-time experimental
observation using the special sensors.
2 MACROSCOPIC FLOW MODEL
2.1 Resin flow model
We can derive the governing equation for resin flow
combining Darcy’s law into the mass conservation
equation for incompressible fluid. Kempner et al.
proposed a unified governing equation for thermoset
composite manufacturing .
where Vf is the fiber volume fraction and vif is the
volume-averaged velocity of fibers.
Depending on the process characteristics, the above
equation can be further simplified. In RTM (Resin
Transfer Molding) process, the fiber volume fraction
does not change since the fiber reinforcement is
located between two rigid toolings during the mold
filling process, and the fiber velocity is negligible.
Consequently, the governing equation becomes a
In VARTM (Vacuum Assisted Resin Transfer
Molding) process, the fiber volume fraction change
can not be ignored. Due to the flexibility of vacuum
bag, the preform thickness changes as the resin
pressure changes. As a result, the mold gap changes
and fiber volume fraction also changes with time.
However, the principal fiber movement is in
transverse direction, in the case of thin structures.
Hence, the planar fiber velocity can be ignored.
Furthermore, the fiber volume fraction can be
regarded to be uniform in the thickness direction,
even though it changes with time. Consequently, the
governing equation for VARTM process needs an
additional term to consider the temporal change of
fiber volume fraction.
2.2 Air compressibility and dry spot formation
To avoid dry spots, air vents are placed at the last fill
points. In conventional mold filling processes, the
air vent pressure is assigned at the flow front as a
boundary condition. As the multiple flow fronts
merge and the air is entrapped, however, the air may
be compressed and the air pressure may increase.
This phenomenon gives rise to a discrepancy in
prediction of pressure distribution. The air bubbles
interact with the global distribution of pressure in
resin and hence the resin flow pattern as well.
(a) Resin flow with the vent pressure applied at the flow front
(b) Resin and air flow with the vent pressure applied at the air
Fig. 1 Mold filling simulations of automotive front panel
We can derive the equation relating the air flow to
the changes in fiber volume fraction and pressure
where μa is the air viscosity. The second term on the
left side is the result of the air being compressible.
The results of numerical simulation of mold filling
of automobile front panel are illustrated in figure 1.
The last fill point is indicated by a black solid circle.
If the only resin flow is considered, the actual last
fill point coincides with the air vent position. If the
air flow as well as the resin flow is considered,
however, the last fill point differs from the air vent
position. In this case, as the air is entrapped at this
region and is eventually compressed in the mold.
Consequently, the mold may not be filled with the
resin before the resin starts to gel, and there may
remain dry spots in the final part.
3 MICROSCOPIC FLOW MODEL
3.1 Formation of microvoids at the flow front
The resin flows through a complicated network of
micro pathways between fibers. This microscopic
architecture can be represented by several shape
factors. A mathematical model to predict the
formation of air voids should be able to obtain the
resin velocities within and between the fiber tows
and to determine the air void contents. Using these
velocities, the required for the resin front to advance
a given distance (the cross-sectional distance of the
fiber tows in the flow direction) can be estimated
within and between fiber tows. The ratio of these
two times is calculated as [11, 12]
where FK,C(φ), FK,T(φ),and FC,T(φ) are the shape
factors. dC is the average distance between the fiber
filaments and dC is the average distance between the
fiber tows. lT(θ) is the width of the tow cross section
in different angles. ΔtlT,T and ΔtlC,T are the times
required for the resin to travel the distance of lT(θ)
within and between the tows. In order to account for
the anisotropic effect, the permeability K is given as
the function of the orientation θ between the flow
front and the fiber tows.
Fig. 2 Flow front of resin between fiber tows
Fig. 3 Flow front of resin within a fiber tow
4 EXPERIMENTAL MEASUREMENTS
4.1 Sensors for void detection
We use the sensors made of two flat brass electrodes
to measure the electrical conductivity of liquid. If
the electrically-conductive liquid is injected into the
mold and passes by the sensor location, the increase
of voltage would be observed by the sensor. If the
liquid contains non-conducting material, the voltage
would drop. The air void can be considered as non-
conducting material. Consequently, the voltage can
be correlated with the void contents. For the given
conductive liquid, the maximum voltage value is
obtained. Then, the voltages are measured for given
volumes of non-conducting materials in the liquid.
In this work, glass beads were used as the non-
conducting material for the sensor calibration.
4.2 Experimental results
A linear injection of conductive liquid into a glass
fabric was performed. As a conductive liquid,
aqueous solution of glycerine was used. The
viscosity of the resin was 0.15 Pa⋅s and the surface
tension was 31.7 mN/m. We used a bidirectional
glass fabric of which fiber volume fraction is 0.58.
Injection pressure was kept constant at 0.175 MPa.
The length of rectangular mold was 530 mm. Four
sensors are placed at the positions from the resin
inlet port; 125 mm, 273 mm, 383 mm, 430 mm.
5 RESULTS AND DISCUSSION
The results of real time measurement of void
contents is presented in figure 4. The void content is
expressed in terms of degree of saturation. If the
degree of saturation is a unity, the composite is free
from air void.
Fig. 4 Degree of saturation as a function of position
We can see that the voids are formed at the flow
front and are reduced as the flow passes by. The
void can be compressed or collapsed as the resin
pressure around the void increases. Otherwise, the
void can be transported along the resin flow. For an
exact understanding of the physics about the
formation and the transport of voids, consequently,
the measurement of voids in the final part is not
sufficient and the real time measurement of the void
contents is essential.
We presented the mathematical models to predict the
formation of macro-micro voids during the Liquid
Composite Molding process. To validate the model,
we also developed the real time sensors that measure
the void contents. In the current study, the influence
of saturation on the macro flow is not considered. As
a future study, this effect (e.g. pressure drooping in
the constant flow rate injection) will be taken into
account in the mathematical model.
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