flow through fabric

7/30/2019 Flow Through Fabric

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Flow Through Fabrics

Douglas J. Gardner

Professor

Wood-Polymer Hybrid Composites

Fabric Preform Issues

Permeability

Compressibility

Ease of handling

Drapeability

Strength, stiffness andfiber volume fraction

http://www.umaine.edu/adhesion/gardner/5502002/fabric%20flow.pdf


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Darcys Law

L

pk

A

qv

==

''

v = velocity cm/s

q = flow rate cm3/sec

A = cross section cm2

= viscosity cps

k = permeability cm3 flow/sec(cps)(cm length)/(cm2(area))(atm

pressure drop)

p = pressure a tm

L = length cm

Darcy Units

K, cm2cps/secatm = 1 Darcy

For a porous medium having a permeability

of 1 Darcy, a fluid with a viscosity of 1 cps

will flow at 1 cm3/sec per 1 cm2 cross

section with a pressure drop of 1 atm per cmlength.


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Permeability as a function of weave

type

Open weave results in higher

porosity and higher

permeability.

Closed weave mats are difficult

to penetrate, but offer higher

strength and moduli.

Effect ofpreformstructure on

flow is represented by

permeability, K.

The more porous or openpreform structures have higher

values of K and are easier to

fill.

Permeability may have adirectional character

For anisotropic

preforms, three values

for the permeability

may be required

Kxx, Kyy, Kzz

For isotropic

preforms, one value issufficient.

K = Kxx = Kyy = Kzz


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Permeability Issues

Permeability, K, is

dependent upon

Preform architecture

Resin viscosity

Permeability

determines key

processing parameters Fill time

Injection pressure

Compressibility


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Compressibility

The closing of the mold compresses the

fabric and changes its architecture. A

number of changes may occur:

Changes in the amount of nesting between

layers

Changes in fiber packing within individual

bundles--the aspect ratio of the fiber bundlescan change

Fiber re-orientation

Relationship between compaction

pressure and the resulting fiber

volume fraction in a preform


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Anisotropic Flow

The ratio of in plane

permeability of a fiber mat

to its transverse

permeability is defined as

the anisotropy ratio.

Kxx and Kyy denote the in -

plane permeabilities of the

preform.

Kzz denotes the out -of-plane permeability of the

preform.

When Kxx=Kyy, the

preform is said to exhibit

in-plane isotropy.


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Permeability(Pressure Drop Across

Mat)

The relationship between the

pressure drop and the flow rate

is linear and indicates the

applicability of Darcy's law in

each case.

Knowing fluid viscosity and

the length L and cross-sectional

area A of the preform,

permeability can be calculated

from the slope. For higher volume fraction the

preform permeability is much

less.

Permeability (Fiber VolumeFraction)

Once again,

permeability drops off

much more steeply at

higher fiber volume

fractions.

Note that no

transverse flow canoccur at the maximum

packing fiber volume

fraction.


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Permeability (Anisotropy)

For the same fiber mat and for

similar volume fractions,

transverse permeability is much

less than in -plane permeability.

Permeabilities are one to two

orders of magnitude higher than

the out-of-plane or transverse

permeabilities

Anisotropy ratio is a strong

function of fiber volumefraction. This explain why

preform impregnation is done

largely by in-plane permeation.

NIST Permeability Data Base

Note that at high fiber volume

fractions in the range useful for

structural applications, the

permeability can vary by more

than an order of magnitude,

depending on the type of

reinforcement.

Even among similar fabric

types, such as woven or

unidirectional fabrics, the

variation can be considerable

and depends on details of the

weave or crossing thread

patterns. Figure 1: In-plane unidirectional saturated flow measurementsconducted with random, woven, unidirectional, and stitched fabrics

http://srdata.nist.gov/permeability/intro.htm


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In most cases, the permeability

measured by the radial flow

method is considerably larger

than the permeability measured

by the saturated flow method.

This discrepancy is consistent

and results primarily because of

the neglect of capillary forces in

the analysis of the unsaturated

radial flow experiments.

Figure 2: In-plane radial unsaturated flow data currently in the database.


In all cases, the thru-thickness

permeability values are a factor

of 6 to 8 smaller than the in-

plane permeability values.

A lower thru-thickness

permeability is expected,

because most fibers lie in the

fabric plane of the tested

fabrics.

However, the consistency of the

ratio of the thru-thickness to the

in-plane permeability is notable.

Figure 3: Thru-thickness unidirectional saturated flow

and corresponding in-plane unidirectional flow measurements.


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RTM Mold

The resin is injected under pressure through one or

more "gates". The air is expelled through vents.

The vents should be located in the region that fills

last in order to avoid entrapped air that can cause

"dry spots".


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Weave type


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Viscosity

Viscosity is an important factor

in selecting resins. If the

viscosity is too

high, the resin will not flow

easily and may not penetrate

fiber bundles. If

the viscosity is too low, the

resin may follow a path of least

resistance and

leave voids or dry spots. As a

rule of thumb, the resin

viscosity should bein the range

of 100 to 1000 cps.

Isotropic flow

If a preform is isotropic in

the in-plane directions

such that Kxx = Kyy , then

the flow fronts progress as

circles.

Random fiber mats and

symmetric bi-directional

fabrics produce isotropicpreforms.


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Anisotropic flow

If the preform is

anisotropic in the plane

withKxx different from

Kyy, the flow fronts

become elliptic and

remain elliptic through out

filling.

This situation can be

encountered withunidirectional stitched

mats.

Mutlilayer FlowFor multiple layers of plies, the overall effective permeabilitys can be

estimated by the relationships shown below.


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Racetracking

Race tracking can develop

when permeability

gradients exist in the

preform due to preform

deformation or "wash".

Race tracking can lead to

voids and resin rich areas.

However, in some cases

race tracking can enhancethe uniformity of flow by

designing specific

channels in the preform

Mold Filling with Fiber Preforms

Flow resistances

The complicated problem of mold filling

Permeability issues

Flow modeling

Flow and fabric deformation

Non-uniform filling


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Flow Resistances

In a woven preform, the flowing resin encounters two scales of

resistance: resistance in the fiber tows (which are called throats) andresistance between the tows (called junctions). The first scale of

resistance, in the tows, dominates the resistance. Each tow may contain

up to 3000 fibers. The tows are very densely packed, so they res ist

flow much more than do the junctions.

Measurement of Permeability

The measurement of

permeability can be

done by:

1.Plotting flow rate vs.

pressure drop for a

one-dimensional flow

experiment, and

2.Measuring the slope

of the linear portion of

this graph.


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Flow Modeling

Macroscale Flow

Microscale Flow

Transverse Flow

Macroscale Flow (Darcys Law)

Darcy's law works fairly

well in composites

processing. Although

permeability varies

somewhat for preforms, in

general, Darcy's law

provides a very good

approximation of flowbehavior on a macro scale.

It is especially good for

random fabrics.


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Microscale Flow

There are two scales of

resistance to flow through

preforms: resistance

between the tows and

resistance in the tows.

Even when a part appears

completely filled,

microvoids may remain if

the resin does not wet theentire tow. This is more

prone to happen at higher

fiber volume fractions.

Void Formation

1. Shows the flow just reaching

a tow containing many fibers.

The flow front comes around it

and actually passes it.

2. The fluid continues to

impregnate the tow after the

flow front has passed.

3. Air is trapped in the tow and

so a microvoid is formed at the

center of the tow.

4. When the resin bypasses low

permeability areas of the

preform, dry spots are created.


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Transverse Flow

Since most RTM parts are

thin compared to their

length and width,

variations in flow through

the thickness can be

neglected and the parts

can be modeled as two-

dimensional. The

thickness permeability formulti- layer preforms can

be obtained via an

averaging scheme.

Flow and Fabric Deformation

The cone geometry, with

its double curve, shears

and deforms the fabric.

Placing the fabric on the

tool also changes the fiber

volume fraction. As the

radial geometry showed,

shearing changes the

fabric permeability. Thechanges in the fabric will

change the direction as

well as the magnitude of

the flow resistance.


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Non-Uniform Filling

Non-uniform filling

occurs due to spatial

variations in both

permeability and resin

viscosity

As the resin polymerizes,

the viscosity increases.

Gelation sets the time

available for filling largeparts .

The resin formulation may

be changed to suit

processing conditions.

flow through fabric

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