Sampe paper Danning 2012

ON THE VOID CONSOLIDATION MECHANISMS OF

CONTINOUS FIBER REINFORCED THERMOPLASTIC

COMPOSITES

Danning Zhang, Arthur Levy, John W. Gillespie, Jr

Department of Materials Science and Engineering

Center for Composite Materials

University of Delaware

Newark DE 19716

ABSTRACT

Void consolidation mechanisms of oven vacuum bag consolidation of unidirectional carbon fiber

thermoplastic matrix composites were investigated. The statistical distribution of void content

(size, spatial location and number density) of AS4 carbon/PEEK prepreg was quantified using

optical microscopy and image analysis. The kinetics of void content reduction were studied

through a series of oven vacuum bag consolidation experiments for unidirectional and cross-ply

laminates processed for 2, 4 and 5 hours at 380C°. Results show a monotonic reduction in

average void content achieving essentially void free laminates after 5 hours of consolidation at

this process temperature. Data reduction provides details on the evolution of statistical

distributions of void content, void size and spatial location of voids versus process time.

Mechanisms of void content are discussed and results are correlated to model estimates.

1. INTRODUCTION

Continuous fiber reinforced thermoplastic composites have been increasingly used as

engineering materials in aerospace and automotive industries [1-2]. Studies have shown that the

voids can severely degrade the mechanical properties of thermoplastic composite parts. [2-5]

Therefore, it is important to keep the void content low in the final part. Aerospace industry

requires less than 1% void content of the composite parts.[3, 6]. For thermosetting composites,

studies showed that entrapped air and volatiles generated during curing are the major source of

the voids, and the voids can be eliminated with resin flow introduced by high pressure, and

vacuum in various processing methods [7-9]. Thermoplastic prepregs also have inherent voids in

the tapes (Figure 1) that have to be eliminated; due to the high viscosity, there is significantly

less resin flow compared to thermosetting composites. It has been shown that high pressure and

vacuum in an autoclave can significantly reduce the void content of thermoplastic parts [10-11].

Non-autoclave processing method such as in situ filament winding and automated tow placement

have been studied and voids are difficult to eliminate in these short duration processes. In many

cases, post processing with an autoclave is required to achieve a high quality part with low void

content [2, 12-14]. There is a strong demand on finding low cost alternative to autoclave

processing for thermoplastic composites, especially in the aerospace industry. In this study, we

consider a vacuum bag oven consolidation process of thermoplastic composites

Figure 1 Voids in the composite tape (voids are the black area), Cytec AS4/APC2 prepreg

A fundamental understanding of void dynamics and consolidation mechanisms is needed to

optimize process temperature and dwell times. Given that only vacuum is used for consolidation,

it is expected that the kinetic aspects of these mechanisms will be important and that much

longer dwell times will be required compared to autoclave process where high positive pressure

is applied. Void consolidation and void dynamics were modeled and applied on the automated

tape placement process. [15-18]. This model considered void size reduction due to the

compression by the high roller pressure of the placement head applied to the tapes. In this model,

the internal void pressure is an important factor that governs the kinetics of void evolution. This

model also shows the importance of void size and spatial distribution of voids within the prepreg

tape. This mechanism clearly shows the benefit of high positive pressure on reducing the time to

achieve a given final void content.

Some recent investigations on curing of thermosetting composite with vacuum bag only process

showed the possibility of voids reduction [9, 19-20]. In this process, gaseous species diffusion

was considered. A study on voids formation and elimination during the stamping process of

PP/glass fiber composite with a labscale experiment indicated that the gas spreads out from the

voids as individual molecules and dissolves into the polymer network [21]. Nucleation and

diffusion in thermoplastic composites were found to play an important role on voids formation,

growth and void dynamics for some composites [22]. Void reduction mechanism was also

studied by considering the solubility, local diffusion, dissolution, and void growth and escape by

means of buoyancy [23]. With these proposed mechanisms, one missing part in these studies is

the experimental confirmation proving which void reduction mechanisms were active during the

consolidation process.

In this paper, optical microscopy and image analysis is used to obtain statistical distribution of

void size, spatial location and number density. The voids behavior is investigated with the

laminates prepared through a series of oven vacuum bag consolidation experiments.

The paper first introduces the experimental details of oven vacuum bag consolidation process in

this study, the method to obtain the microscopic images, the image analysis, and the statistical

approach to obtain the experimental data of the voids. After that, the results of the statistical

analysis are presented. Based on the results, the void consolidation mechanisms are then

discussed.

2. EXPERIMENTAL DETAILS

2.1 Laminate Preparation

The thermoplastic prepreg tape used throughout this study is Cytec AS4 carbon/PEEK prepreg.

Each laminate consists of 2 plies and is 300mm by 300mm. The laminates were consolidated

with vacuum bag in an oven. The vacuum bag assembly is shown in Figure 2. There are three

sets of consolidation cycles with 2, 4, and 5 hours dwell at 380°C. For each set of consolidation

cycle, both unidirectional (UD) and cross-ply laminates (CP) were manufactured. The heating

and cooling rate is 2.5°C/min. Figure 3 shows the consolidation program for 2hrs dwell at 380°C.

Figure 2 Schematic of vacuum bag assembly

Figure 3 Consolidation program for the set of 2hrs dwell at 380 °C

After consolidation, samples were cut from the center of the laminates for microscopy. The peel

plies on the top surfaces of the laminates were not always removed, preventing damage from

handling. The size of the samples is approximately 6.35×12.7 and about 0.28 mm thick. To

obtain a relatively large observation area, 4 samples were cut from unidirectional laminates, and

Base Plate

2-layer Laminate on Release Coating

Peel PlyBreather

Vacuum Bag

Vacuum Source

Sealant Tape

0

60

120

180

240

300

360

420

0 100 200 300 400

Tem

pera

ture

( C

)

Time (min)

Vacuum: 1atm

8 samples from cross-ply laminates. Void content was quantified in both upper and lower layers

for CP laminates. Besides the laminates, 8 samples of the original as-received prepreg tape from

the same lot were also prepared as our control group. All the samples were embedded in epoxy

resin and polished down to 3m for high quality microscopic observation.

2.2 Optical Microscopy

Sample cross-sections were observed under an optical microscope. The images acquired using a

mono color camera at 20×magnification along the sample cross-sections. At this magnification,

the length of cross-section framed in each image is 0.64mm. Consequently, there were around 20

images obtained from each sample. Example images of original tape, unidirectional laminates

and cross-ply (CP) used for void analysis is shown in Figure 4. Then, the images were analyzed

with Image J software for voids.

Figure 4 Examples of optical micrograph of the cross-sections of original tape and laminates

2.3 Statistical Image Analysis

Through image analysis, information about voids including void content, spatial position, size

and number density can be obtained from each image. Several steps have to be followed to get

useful quantitative information (shown in Figure 5). A micrograph was first rotated to align the

cross section along the horizontal reference line. Then a rectangular area was selected to trim off

the rough surface while covering as much composite area as possible, forming a preprocessed

image. In Image J, the histogram of 256 grey levels (0=black, 255=white) of a preprocessed

image can be plotted, from which an appropriate range of image threshold is set to distinguish

the voids from the polymer and fibers. After setting the threshold, a binary image consisting of

black voids area and white composite area is created.

The number of pixels of the black region can be counted and gives the void fraction. The

through-thickness distribution of voids was quantified using void content at each pixel position

along the thickness direction. The resolution is 2 pixels perm. An example of void through

thickness distribution can be seen in Figure 6. In the binary images, one continuous region is

considered as a void. The areas of continuous black regions were also recorded and converted

into actual area. The voids were approximated as circles in the 2D images, and then the

corresponding diameters were calculated.

(c) CP laminate(b) UD laminate(a) Original tape

Figure 5 Basic procedures for image processing

Figure 6 Example of generating voids through thickness distribution

There are a total of 4 sets of consolidation conditions: unconsolidated, 2hrs, 4hrs and 5hrs at

380°C dwell temperatures. The total number of data sets is 7: original tape, 2hrs UD and CP

laminates 4hrs UD and CP laminates, and 5hrs UD and CP laminates. The numerous images in

each study group can provide average void content, overall void spatial distribution, and the

number density, reflecting the actual evolution of voids under different consolidation conditions.

The void content of prepreg tape and UD laminates are the average of the void content values

from all the images in each group. The void content of the CP laminates are the average of the

void fractions of upper and lower layers. The overall void through thickness distributions were

obtained by averaging the void content at each through thickness positions of 10 images which

have void content close to the average void content of this group. The voids examined from

these10 images in this group were gathered and counted for the number density and size

distribution.

Rotating

Trimming

Setting

threshold

Converting into

binary image

0 255

0Through ThicknessPositions

100

0

2

4

6

8

10

12

14

16

0 20 40 60 80 100

Vo

id C

on

ten

t (%

)

Through Thickness Positions ( m)

3. RESULTS OF VOID ANALYSIS

3.1 Average Void Content of Prepreg Tape and Laminates

Since a specific size of composite cross-section area was framed in the microscope, the void

content values acquired from the images form distributions. The frequency distributions of void

fraction of the prepreg tape and UD laminates from the microscopy images are shown in Figure 7.

The void content range reduces as the consolidation time increases, indicating the void content of

the laminates decreased. The distribution of void content in prepreg tape tends to be a normal

distribution. As more voids disappear with longer consolidation time, the distribution tends to

concentrate in the low void content range of 0~0.5%.

Figure 7 Distribution of void fraction values of prepreg tape and UD

Cumulative distributions of void contents of both UD and CP laminates in Figure 8 were

converted from the frequency distribution for clear comparison. The cumulative percentage is the

probability that a sample has void content lower than a given value. For example, on the curve of

prepreg tape in Figure 8 (a), the probability to have void content lower than 17% is 100%. It is

seen that the curves of UD laminates with longer consolidation time reach 100% probability at

smaller void content values (1%, 4.5%, and 8% for 5hrs, 4hrs and 2hrs UD laminates,

respectively). For CP laminates, the distributions were presented for upper and lower layers

separately. Figure 8 (b) shows the lower layer curves of 2hrs and 4hrs consolidation CP

laminates reach 100% at smaller void content values than upper layers. The void content range of

the upper layer curve of 5hrs CP laminate is slightly larger than that of the lower layer. But the

difference is not as obvious as 2hrs and 4hrs. This is because that after 5hrs consolidation, the

laminates are almost void free, and the void content values measured are extremely small

compared to the prepreg tape and other laminates.

0

2

4

6

8

10

12

00.

5 11.

5 22.

5 33.

5 44.

5 55.

5 66.

5 77.

5 88.

5 99.

5 1010

.5 1111

.5 1212

.5 1313

.5 1414

.5 1515

.5 1616

.5 1717

.5 18M

ore

Nu

mb

er

of

Ima

ge

s

Void Content (%)

Prepreg Tape

0

1

2

3

4

5

6

00.

5 11.

5 22.

5 33.

5 44.

5 55.

5 66.

5 77.

5 88.

5 99.

5 1010

.5 1111

.5 1212

.5 1313

.5 1414

.5 1515

.5 1616

.5 1717

.5 18M

ore

Nu

mb

er o

f Im

ages

Void Content (%)

2hrs UD

0

2

4

6

8

10

12

14

00.

5 11.

5 22.

5 33.

5 44.

5 55.

5 66.

5 77.

5 88.

5 99.

5 1010

.5 1111

.5 1212

.5 1313

.5 1414

.5 1515

.5 1616

.5 1717

.5 18M

ore

Nu

mb

er o

f Im

ages

Void Content (%)

4hrs UD

0

10

20

30

40

50

60

70

80

90

100

00.

5 11.

5 22.

5 33.

5 44.

5 55.

5 66.

5 77.

5 88.

5 99.

5 1010

.5 1111

.5 1212

.5 1313

.5 1414

.5 1515

.5 1616

.5 1717

.5 18M

ore

Nu

mb

er

of

Ima

ge

s

Void Content (%)

5hrs UD

Figure 8 Cumulative distributions of void content (a) UD laminates, (b) CP laminates

Figure 9 Average void content (a) UD laminates, (b) CP laminates

The average void content values of the prepreg tape and laminates are presented in Figure 9. In

line with the features of void content distributions, the average void content values of laminates

decrease monotonically with the increasing consolidation time. Upper layers contain more voids

than lower layers. Representative original micrographs containing void content close to the

average void content are presented in Figure 10 and Figure 11.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

00.

5 11.

5 22.

5 33.

5 44.

5 55.

5 66.

5 77.

5 88.

5 99.

5 1010

.5 1111

.5 1212

.5 1313

.5 1414

.5 1515

.5 1616

.5 1717

.5 18M

ore

Cu

mu

lati

ve

Pe

rce

nta

ge

(%

)

Void Content (%)

Prepreg Tape

2hrs UD

4hrs UD

5hrs UD

(a) (b)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

00.

5 11.

5 22.

5 33.

5 44.

5 55.

5 66.

5 77.

5 88.

5 99.

5 1010

.5 1111

.5 1212

.5 1313

.5 1414

.5 1515

.5 1616

.5 17

Cu

mu

lati

ve

Pe

rce

nta

ge

(%

)

Void Content (%)

Prepreg Tape

2hrs CP Upper

2hrs CP Lower

4hrs CP Upper

4hrs CP Lower

5hrs CP Upper

5hrs CP Lower

PrepregTape6.79

2hrs UD3.82

4hrs UD1.08

5hrs UD0.07

0.00

2.00

4.00

6.00

8.00

10.00

12.00

Av

era

ge

Vo

id C

on

ten

t (%

)

Prepreg Tape6.79

2hrs CP2.01

2hrs CP Lower

2hrs CP Upper

4hrs CP1.47 4hrs CP

Lower

4hrs CP Upper

5hrs CP0.20

5hrs CP Lower0.19

5hrs CP Upper

0.20

0.00

2.00

4.00

6.00

8.00

10.00

12.00A

ve

rag

e V

oid

Co

nte

nt

[%]

(a) (b)

Prepreg, Vv=6.85% 2hrs UD, Vv=3.54%

5hrs UD, Vv=0.09%4hrs UD, Vv=1.08%

Figure 10 Typical microscopic images of prepreg and UD laminates (Peel ply is seen at the top

surface)

Figure 11 Typical microscopic images of CP laminates (some with peel ply on top surface)

Prepreg, Vv=6.85% 2hrs UD, Vv=3.54%

5hrs UD, Vv=0.09%4hrs UD, Vv=1.08%

2hrs CP Upper, Vv=2.84% 2hrs CP Lower, Vv=1.68%



3.2 Voids Spatial Distribution

In order to fully describe the behavior of voids under different consolidation conditions, void

spatial distributions were studied and obtained with the method introduced above, as shown in

Figure 12 and Figure 13. The spatial distribution in prepreg was studied within a single layer.

The distribution curve was repeated for 2 layers when comparing results with the distribution of

the 2-layers UD laminates after consolidation. Because of the rough tape surfaces, large

interlayer void area is expected for the 2-layer prepreg curve. The spatial distribution of prepreg

tape shows that more voids exist in thickness center than close to the surfaces. As the

consolidation time gets longer, the peak void content through thickness gets lower, and

distribution curve gets flattened. The interlayer voids in 2hrs UD laminate disappeared in 4hrs

UD laminate, and only a tiny amount of voids was captured in the 5hrs UD curve. The peaks due

to interlayer voids of CP laminates can also be seen in Figure 13. It is seen that the interlayer

voids still exist in 5hrs CP laminate but almost disappear in 5hrs UD. This may be due to the

better intimate contact developed within UD laminates that can nest during consolidation

compared to CP laminates. It is noticed that the curves for both UD and CP laminates are moving

toward the 0mm position, indicating the upwards movement of voids. This movement could

result in void content difference of upper and lower layer in the laminates. The possible reasons

of this movement are discussed later in this paper.

Figure 12 Voids through thickness distribution of (a) prepreg tape and (b) UD laminates

Figure 13 Voids through thickness distribution of CP laminates (a) upper layer, (b) lower layer

(a)

0

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Vo

id F

ract

ion

(%)

Through thickness Position (m)

1-layer PrepregInterlayer Region

(b)

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0 50 100 150 200 250 300

Vo

id C

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Through Thickness Position from Top to bottom (m)

2-layer Prepreg

2hrs UD

4hrs UD

5hrs UD

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Vo

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1-Layer Prepreg

2hrs CP_Upper

4hrs CP_Upper

5hrs CP_Upper

Interlayer Region

(a) (b)

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0 20 40 60 80 100 120 140

Vo

id F

ract

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(%

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1-Layer Prepreg

2hrs CP_Lower

4hrs CP_Lower

5hrs CP_Lower

From the through thickness distribution of voids, it is also seen that more voids exist in upper

layers than bottom layers of both UD and CP laminates. This is in line with the values of average

void content presented the previous section.

3.3 Void Sizes and Number Density

The equivalent void diameters and the corresponding number of voids in 10 images of each

study group are shown in Figure 14. Micro voids with equivalent diameter smaller than 3.5 m

are not shown in the figures. It is seen that most voids have equivalent diameter of 3.5m to

21m which is 0.5 to 3 times the carbon fiber diameter. Generally, the number of voids at the

same diameter range is largely reduced after consolidation, and keeps decreasing with increasing

consolidation time. Although it shows more voids within 3.5~7m range in 4hrs UD laminate

than 2hrs UD laminate, there are more large voids in the 2hrs UD laminates. Large voids

disappeared in the 5hrs laminates. The voids diameter and number density decreasing with

consolidation time also agree with the trend of average void content results.

Figure 14 Void Size Distribution (a) UD laminates, (b) CP laminates

4. DISCUSSION OF VOID CONSOLIDATION MECHANISMS

The above results show that low void content can be achieved by oven consolidation with

vacuum bag only. The high pressure in autoclave is not necessary although the process times are

increased an order of magnitude. The final void content in the laminates depends on

consolidation time under the same conditions of temperature and vacuum level. This indicates

the voids reducing mechanisms are time dependent. Some existing model and mechanisms are

introduced hereafter and compared to the trend in the experimental results obtained.

The model of microscopic void consolidation used for thermoplastic tow placement process

considers a void in a resin shell (Figure 15). The rate of void size change is governed by the

voids internal pressure and the total stress outside of the void. Since the prepreg tape was kept in

ambient condition, the initial internal pressure of voids can be assumed atmosphere pressure.

(a)

0

100

200

300

400

500

600

700

800

7 10.5 14 17.5 21 24.5 28 31.5 35 More

Nu

mb

er o

f Vo

ids

Equivalent Void Diameter (m)

2-Layer Prepreg

2hrs UD

4hrs UD

5hrs UD

3.5

(b)

0

100

200

300

400

500

600

700

800

7 10.5 14 17.5 21 24.5 28 31.5 35 More

Nu

mb

er o

f Vo

ids

Equivalent Void Diameter (m)

2-Layer Prepreg

2hrs CP

4hrs CP

5hrs CP

3.5

Based on the ideal gas low, the internal pressure of a void increases significantly when heated

from room temperature to 380 °C. Due to the relative large surface tension and high viscosity of

PEEK resin, as well as the applied pressure from vacuum, the larger voids could be only

compressed to a limited extent. And the voids stop reducing in size when the pressure within the

void is in equilibrium with the total stress from resin pressure and surface tension. Therefore, this

model cannot be the only mechanism of void reduction as achieving near zero void content

cannot be achieved even with infinite time under vacuum consolidation conditions.

Figure 15 Microscopic Void Consolidation Model [16]

Since the internal pressure of the voids is not at vacuum conditions, there are gas molecules

within the voids. The concentration of the gas molecules at the interface between the void and

surrounding resin follows Henry’s law. If one assumes the concentration of dissolved gas in the

surrounding polymer shell is lower than the gas within the void, a concentration gradient exists

and is a driving force for the gas in the voids to diffuse out into the resin. Together with the

dissolving of the gas molecules, the pressure within the void drops and the voids are compressed

by the surrounding composites as in the microscopic void model. Roychowdhury et al.

developed a model of a void in a polymer shell and considering the stress equilibrium and the

molecule diffusion through the boundary [22]. The momentum equation for the void

compression, the diffusion equation for the gas molecules, and the conservation of mass are the

coupled governing equations in this problem. The equilibrium is controlled by both the

concentration gradient and the stress. In the meantime, the top surface of the composite is

exposed to vacuum, and the gas molecules keep diffusing out from the layers during the

consolidation, which can be modeled with 1-D form of Fick’s second Law, as in [24]. The

concentration of the gas molecules in the resin that surrounds the voids is reducing, leading to

the gas in the voids diffusing out continuously with time. When the composite is void free, all

the gas molecules are already dissolved in the composite and then diffuse out of the composite

based on the concentration equilibrium. Since the diffusion kinetics is slow, the time needed for

the dissolved gas to escape is long. A simplified estimation of the time required at dwell

temperature is:

𝑡 =𝑥2

𝐷 [1]

g

Where x is the mean free path of the diffusion, D is the diffusion coefficient of the gas in PEEK

resin. Although the specific diffusion coefficient is unknown, a value of 10-7

cm2/s corresponding

to the diffusion coefficient of water for a range of uncured epoxy resins can be used [9, 25], as a

rough order of magnitude estimate. If x is 0.3mm (about the thickness of the two layers), t is

2.5hrs. This simple relationship indicates the time to achieve void free laminates is very

dependent size and distribution of the voids. This implies that each individual void of a different

size and location within the laminate thickness will take different amounts of time for void

reduction based on this mechanism. Overall, this model would predict that the overall void

content, void size and number density is reducing. Due to these complex interactions, the time

required to get void free part can be longer than 2.5hrs, and 5hrs dwell at processing temperature

to obtain void free part is reasonable. The mechanism of void compression coupled with

dissolving and diffusion could, therefore, be an important mechanism for void consolidation.

Based on the above mechanisms, gas solubility in the amorphous phase of PEEK resin could be

another important factor that affects the void reducing. At the high consolidation temperature,

the activity of the gas molecules increases significantly. As the gas molecules diffuse into the

polymers, the polymer is getting saturated. The dissolving process stops when the local gas

concentration reaches the solubility of the gas in the resin. An important question to answer is

whether the kinetics of gas solubility is faster or slower than the diffusion of gas in the polymer

to the surface. As laminate thickness increases, one would conclude that diffusion would become

the rate limiting mechanism. At this point, further study is needed to quantify the effect of

thickness on the kinetics of void reduction.

From the void analysis, the initial average void content in the prepreg is 6.79%. Assuming air is

in the voids with the density of 1.18Kg/m3, PEEK content is 45% by volume, and PEEK density

is 1320Kg/m3, the weight concentration of air in unit volume of resin is 1.4×10

-4Kg/Kg.

Assuming the solubility of air in PEEK is similar to that in oils, the solubility is on the order of

10-4

Kg/Kg [23]. Therefore, a large amount of air can be dissolved in the resin. However, the gas

species and the solubility and associated kinetics at the consolidation temperature are not well

understood. The consolidation time would still depend on the diffusion of dissolved gas to the

laminate surface.

A model of void filling by resin flow was recently developed at the University of Delaware [26].

This model considers that the distributed voids with different sizes are filled by resin flow

through the fiber bed. If the voids are gathered very close, relatively large amount of local resin

and long time are needed for voids reduction. This is in agreement with the results of void

through thickness distribution which shows that the voids in the thickness center need long time

to be reduced. However, similarly as in the microscopic void model, the driving force for voids

filling in this model is the difference between the applied pressure in the resin and void internal

pressure, and it cannot predict void free laminates manufactured with oven vacuum bag

consolidation. As discussed before, gas dissolving and diffusion can occur. Then void internal

pressure drops, and the voids can be further filled with resin. Therefore, voids filling by resin

flow coupled with gas dissolving and diffusion could be another possible mechanism.

According to Muric –Nesic et al [23], buoyancy effect could also help the voids escape. Since

the upward movement of voids was observed from the spatial distribution and different average

void content values of layers in void analysis results, buoyancy effect might be one reason to

drive the voids moving upwards. There are two forces acting on a void: buoyancy force acting

upward, and drag force acting downwards. The buoyancy force has to equal to the drag force to

make the voids float up. Based on Archimedes principle, Stokes law and Newton’s law, the

velocity of the rising void relative to the resin can be expressed as

9

2 2

Bgrv l

[2]

Where l is the density of PEEK resin, rB is the voids radius, equal to 370 Pa.s [27], is the

viscosity of PEEK resin at processing temperature. The significance of this effect largely

depends on the voids size and resin viscosity. For a 40m diameter void in the middle of the

thickness (0.15mm to the surface), the time for it to float up to the surface is 13.4hrs. During 5hrs

consolidation, the void can move 60m. In reality, the velocity of the void migration will be

further impeded by the presence of fibers, further increasing the time for this mechanism to

contribute to void reduction. Furthermore, smaller voids have even lower migration velocity.

From the void spatial distribution results, the apparent waves of the void migration curves move

towards the top surface, and the curves get flatter with local voids reducing in size is seen.

Therefore, the effect of buoyancy does not explain the experimental observations or the time

scales measured for void reduction.

5. CONCLUSIONS

Void consolidation was studied with a series of oven vacuum bag consolidation of UD carbon

fiber thermoplastic prepreg. Through the microscopy images of the cross-sections of the

manufactured laminates and the statistical approach, the detailed information of the average void

content, voids through thickness distribution, and the void sizes and the number distribution were

obtained. As the consolidation time increases at 380°C, the average void content of the laminates

decreases monotonically. More voids distribute in the thickness center of the prepreg tape than

close to the surfaces. After consolidation, the profile of the through thickness void content

distribution move towards the top surface. The upper layer contains more voids than the lower

layer. Most voids have equivalent diameter of 3.5 to 21m. The voids diameter and the number

of voids reduce with consolidation time. Voids free laminates can be eventually achieved

through oven vacuum bag consolidation within 5hrs and the high pressure in autoclave is not

necessary. After relating the results with some relevant mechanisms, voids compression coupled

with gas molecules dissolving and diffusion through the laminate thickness is identified as an

important mechanism for the voids elimination during the oven vacuum bag consolidation. The

solubility of the gas molecules in the polymer and the state of gas in the void are important factor

requiring additional study. Voids filling by resin flow coupled with gas dissolving and diffusion

is also possible. The buoyancy effect may affect the voids movement but is not a dominating

mechanism.

The detailed information of voids helped understand the kinetics of the voids evolution with

consolidation time. The voids information of prepreg tape can also be used as future model input,

and the results after consolidation can be used to validate model predictions and ultimately to

optimize the vacuum consolidation process. However, more work is needed to characterize

important material properties and to develop and validate submodels focused on specific

mechanisms.

6. REFERENCE

[1] F. N. Cogswell, Thermoplastic aromatic polymer composites : a study of the structure, processing, and properties of carbon fibre reinforced polyetheretherketone and related materials. Oxford [England]; Boston: Butterworth-Heinemann, 1992.

[2] U. K. Vaidya and K. K. Chawla, "Processing of fibre reinforced thermoplastic composites," International Materials Reviews, vol. 53, pp. 185-218, Jul 2008.

[3] A. R. Chambers, et al., "The effect of voids on the flexural fatigue performance of unidirectional carbon fibre composites developed for wind turbine applications," International Journal of Fatigue, vol. 28, pp. 1389-1398, Oct 2006.

[4] L. Ye, et al., "De-consolidation and re-consolidation in CF/PPS thermoplastic matrix composites," Composites Part a-Applied Science and Manufacturing, vol. 36, pp. 915-922, 2005.

[5] M. A. Lamontia, B.D.Olson, J.W. Gillespie, T.A. Bogetti, "The Effects and Non-Destructive Evaluation of Defects in Thermoplastic Compression-Loaded Composite Cylinders," Journal of Thermoplastic Composite Materials, vol. 8, p. 27, 1995.

[6] L. Liu, et al., "Effects of cure cycles on void content and mechanical properties of composite laminates," Composite Structures, vol. 73, pp. 303-309, Jun 2006.

[7] Y. Z. Gu, et al., "Void Formation Model and Measuring Method of Void Formation Condition During Hot Pressing Process," Polymer Composites, vol. 31, pp. 1562-1571, Sep 2010.

[8] F. Y. C. Boey and S. W. Lye, "Void Reduction in Autoclave Processing of Thermoset Compsites .1. High-Pressure Effects on Void Reduction," Composites, vol. 23, pp. 261-265, Jul 1992.

[9] L. K. Grunenfelder and S. R. Nutt, "Void formation in composite prepregs - Effect of dissolved moisture," Composites Science and Technology, vol. 70, pp. 2304-2309, Dec 2010.

[10] I. Fernandez, et al., "Autoclave forming of thermoplastic composite parts," Journal of Materials Processing Technology, vol. 143, pp. 266-269, Dec 2003.

[11] A. Lystrup and T. L. Andersen, "Autoclave consolidation of fibre composites with a high temperature thermoplastic matrix," Journal of Materials Processing Technology, vol. 77, pp. 80-85, May 1998.

[12] M. A. Lamontia, et al., "Manufacturing flat and cylindrical laminates and built up structure using automated thermoplastic tape laying, fiber placement, and filament winding," Sampe Journal, vol. 39, pp. 30-38, Mar-Apr 2003.

[13] R. Gennaro, et al., "On-line Consolidation of Commingled Polypropylene/Glass Roving During Filament Winding," Journal of Thermoplastic Composite Materials, vol. 24, pp. 789-804, Nov 2011.

[14] M. A. Lamontia and M. B. Gruber, "REMAINING DEVELOPMENTS REQUIRED FOR COMMERCIALIZING IN SITU THERMOPLASTIC ATP," SAMPE 2007 - Baltimore, MD, p. 15, 2007.

[15] R. Pitchumani, et al., "Analysis of transport phenomena governing interfacial bonding and void dynamics during thermoplastic tow-placement," International Journal of Heat and Mass Transfer, vol. 39, pp. 1883-1897, Jun 1996.

[16] J. Tierney and J. W. Gillespie, "Modeling of heat transfer and void dynamics for the thermoplastic composite tow-placement process," Journal of Composite Materials, vol. 37, pp. 1745-1768, 2003.

[17] S. Ranganathan, et al., "A Nonisothermal Process Model for Consolidation and Void Reduction During In-Situ Tow Placement of Thermoplastic Composites," Journal of Composite Materials, vol. 29, pp. 1040-1062, 1995.

[18] R. Pitchumani, et al., "Design and optimization of a thermoplastic tow-placement process with in-situ consolidation," Journal of Composite Materials, vol. 31, pp. 244-275, 1997.

[19] L. R. J. Boyd, "Vacuum-Bag-Only-Curable Prepregs That Produce Void-Free Parts," SAMPE 2002 - Long Beach, CA, p. 13, 2002.

[20] T. D. Juska, et al., "Oven Vacuum Bag (OVB) Prepreg Fabrication Infusion Processing," Sampe Journal, vol. 45, pp. 22-36, Jul-Aug 2009.

[21] Y. Leterrier and C. Gsell, "Formation and Elimination of Void During the Prcessing of Thermoplastic Matrix Composites," Polymer Composites, vol. 15, pp. 101-105, Apr 1994.

[22] S. Roychowdhury, et al., "Volatile-induced void formation in amorphous thermoplastic polymeric materials: I. Modeling and parametric studies," Journal of Composite Materials, vol. 35, pp. 340-366, 2001.

[23] J. Muric-Nesic, et al., "On the void reduction mechanisms in vibration assisted consolidation of fibre reinforced polymer composites," Composites Part a-Applied Science and Manufacturing, vol. 42, pp. 320-327, Mar 2011.

[24] J. L. Kardos, et al., "Void Growth and Resin Transport During Prcessing of Thermosetting Matrix Composites," Advances in Polymer Science, vol. 80, pp. 102-123, 1986.

[25] Y. Kobayashi, et al., "Evaluation of Polymer Free-Volume by Positron-annihilation and Gas Diffusivity Measurements," Polymer, vol. 35, pp. 925-928, 1994.

[26] M.B. Gruber, et al.," Thermoplastic In-situ Placement Requires Better Impregnated Tapes and Tows," Sample-Baltimore, 2012

[27] D. D. L. Chung, "Carbon fiber composites." 1994

D. D. L. Chung, "Carbon fiber composites." 1994

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