Zhang, D., D. Heider, and J. W. Gillespie, Jr., “Volatile removal during out of autoclave processing of high performance thermoplastic composites,” CAMX - 2014, Orlando, FL, Oct 13-16 2014.

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VOLATILE REMOVAL DURING OUT OF AUTOCLAVE

PROCESSING OF HIGH PERFORMANCE THERMOPLASTIC

COMPOSITES

Danning Zhang1, 2

, Dirk Heider1, 3

, John W. Gillespie, Jr1, 2, 4, 5

1 Center for Composite Materials, University of Delaware, Newark, DE, 19716, USA

2Department of Materials Science and Engineering, University of Delaware, Newark, DE, 19716, USA

3Department of Electrical and Computer Engineering, University of Delaware, Newark, DE, 19716, USA

4Department of Mechanical Engineering, University of Delaware, Newark, DE, 19716, USA

5Department of Civil and Environmental Engineering, University of Delaware, Newark, DE, 19716, USA

ABSTRACT

In this work, void free thin and thick AS4/APC2 thermoplastic composites were obtained

through oven vacuum bag (OVB) processing. Entrapped air is the major volatile source

considered. Diffusion through the layer thickness and flow through the interlayer regions to the

part perimeter are two mechanisms governing void reduction. The air permeability of the

interlayer regions of thermoplastic prepreg was measured experimentally. Characteristic air

removal times for diffusion and interlayer flow were estimated, and correlated well with the

experimental consolidation time measured for an OVB process cycle (2.8°C/min to 380°C and

zero dwell time). It is found that for thin laminates (i.e. 2 layers processed using this cycle),

diffusion governs the time for air removal and void reduction. For thick laminates (e.g. 72 layers

processed using this cycle), low void content was achieved when interlayer permeability was

maintained (single ply diffusion to the interface and interlayer flow to the perimeter). Significant

void content was measured for the case where interlayer flow was restricted at the panel

perimeter. In this case, void reduction requires diffusion through the laminate thickness (i.e. 36

layers). Results for non-isothermal diffusion during the OVB process cycle show that much

greater dwell times would be required. In the cycle with short or no dwell time at 380°C, high

void content is predicted in the interior of the laminate, and the void content reduction would be

limited to only layers nearest the surfaces. The study demonstrates the feasibility of OVB

processing of thick section carbon thermoplastic composites.

Copyright 2014 by University of Delaware.

Published by the Society for the Advancement of Material and Process Engineering with permission

See paper in http://www.nasampe.org/products?combine=volatile+removal&field_conference_name_tid=All

CONFERENCE: CAMX 2014 Orlando, FL

SKU/CODE: CX1-47823

Zhang, D., D. Heider, and J. W. Gillespie, Jr., “Volatile removal during out of autoclave processing of high performance thermoplastic composites,” CAMX - 2014, Orlando, FL, Oct 13-16 2014.

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1. INTRODUCTION

Manufacturing cost is an important issue for continuous fiber reinforced thermoplastic

composites to be increasingly used for large structural applications. [1,2] Automated placement

technology is used to reduce the labor time and improve placing accuracy. However, high

pressure post consolidation with autoclave processing is required to achieve high part quality.

[3,4] Oven vacuum bag (OVB) processing can provide significant reduction of manufacturing

cost with no positive pressure involved. OVB consolidation for thermoplastic composites is not

well established. Voids are the major concern. Voids are detrimental to the performance of

thermoplastic composites [5–8], thus low void content in the final parts is required. Before

processing, a relatively high level of void content of 5% - 10% can exist in thermoplastic prepreg

materials. The feasibility of void reduction during OVB processing of thick sections of high

viscosity thermoplastic composites without positive pressure has been demonstrated in this study.

A microscopic void model was established to predict void content reduction during

processing of thermoplastic composites. [9–12] This model assumes that a void is a sphere

encapsulated in a polymer shell that is equivalent to a given void content. During processing, the

composite is heated and subjected to a consolidation pressure to reduce void content. The

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

internal pressure at temperature is greater than the applied pressure or the local pressure of

thermoplastic resin, the void will initially grow, and then air dissolves into the polymer and

diffuses to the surface. In time, the internal pressure drops below the applied pressure and void

reduction occurs. The mechanism clearly shows the importance of diffusion and the benefit of

high positive pressure on reducing the time to achieve a given final void content. For OVB

processing, only 1 atmosphere pressure is available. Our previous study [13] shows that the voids

in the AS4/APC2 prepreg do not provide continuous porous pathways for air/volatile removal;

and that through the through-thickness z-direction diffusion of air within voids is required. Other

studies on void formation in thermoplastic composites also indicate volatile gas diffusion and

nucleation can affect the dynamics of voids. [14,15] Since diffusion time is proportional to the

square of diffusion length, thick laminates may need extremely long time to reduce voids or end

up with high void content in short processing time in OVB processing. If diffusion is the only

mechanism for volatile removal, it will be a key technical challenges for thick and large

components processed with OVB.

In some studies of thermosetting prepregs, it is shown that air permeability is a crucial

element for gas venting off during curing, especially for thick laminates. [16–21] Nam et al.[19]

found that the inter-laminar voids in a plain weave carbon fiber prepreg stack initially form a

porous network due to the undulation of the fiber tow and uneven prepreg surfaces for gas to

flow through, reducing the intralayer voids. Xin et al. [16] found that the air entrapped in a

prepreg stack with higher air permeability can be vented off more easily than with lower air

Zhang, D., D. Heider, and J. W. Gillespie, Jr., “Volatile removal during out of autoclave processing of high performance thermoplastic composites,” CAMX - 2014, Orlando, FL, Oct 13-16 2014.

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permeability, and stressed the impact of air permeability on void reduction in vacuum bag only

processes. The role of in-plane permeability between layers of thermoplastic prepreg stacks can

also be very important for void reduction in OVB processing.

In this paper, thin and thick AS4/APC2 carbon fiber reinforced/poly (ether ether ketone)

laminates were manufactured through OVB processing under different conditions. Voids in the

processed laminates were inspected with 2D optical microscopy and 3D X-ray micro-CT. The air

permeability of prepreg stacks were measured experimentally. The characteristic times for

volatile removal through diffusion and flow through permeable paths were calculated. The

mechanisms for volatile removal in thermoplastic OVB processing were discussed.

2. EXPERIMENTAL PROCEDURE

2.1 Laminate preparation

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

prepreg. A thin cross-ply laminate consists of 2 plies, and the thick laminates are 72 plies with

the stacking sequence of [(04/904)404]s. In order to investigate the effect of the porous interlayer

permeability, the perimeter edges of one thick laminate were sealed with tacky tape. All

laminates are 300mm × 300mm. Oven vacuum bag processing was performed with the bag

assembly shown in Figure 1. In order to keep the edges from getting pinched, a steel bar was

placed 1 inch apart from each lateral edge of the thick laminates. Breathers were placed between

the edges and the steel bars to keep the path open to vacuum. The temperature and vacuum

profiles of the consolidation cycle are shown in Figure 2. Standard heating and cooling rates of

2.8°C/min (5°F/min) were applied. Recorded vacuum level in the bag for all laminates was

within the range of 24inHg-27inHg. Samples from the central region of the consolidated

laminates were prepared and examined with optical microscopy and micro-CT.

Figure 1. Schematic of vacuum bag assembly

Zhang, D., D. Heider, and J. W. Gillespie, Jr., “Volatile removal during out of autoclave processing of high performance thermoplastic composites,” CAMX - 2014, Orlando, FL, Oct 13-16 2014.

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Figure 2. Idealized temperature and vacuum profiles of the consolidation cycle of 0 min dwell at

380°C

2.2 Void Inspection

2.2.1 Optical Microscopy

Samples were cut from different area of prepreg tape, embedded in epoxy resin, and polished

down with Al2O3 particles of 0.3m in diameter. The cross-sections of the polished samples

were observed with Nikon ECLIPS LV100 at 200× magnification. Images were taken with a

resolution of 0.48m/pixel. Single images were processed with Image J to obtain void content.

The detailed image processing method is provided in [22].

Table 1. X – ray micro CT scanning parameters

Parameters Values

Filter None

Rotation Steps 0.25°

Frame Averaging 8

Random Movement 5

2.2.2 X-ray Micro CT

SkyScan’s 1172 High Resolution Micro-CT from Bruker is the X-ray micro CT system used

in this study. The raw x-ray tomograms were collected at 40KV and 250A with a maximum

power of 10W. The image size is 4000×2096 pixels. Considering the inspection volume,

accuracy, and practical time, 1.48m/pixel was used for the 2-layer laminates with sample width

of 4.5mm and length of 3.6mm and full thickness. In order to obtain higher resolution, thick

Zhang, D., D. Heider, and J. W. Gillespie, Jr., “Volatile removal during out of autoclave processing of high performance thermoplastic composites,” CAMX - 2014, Orlando, FL, Oct 13-16 2014.

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laminate samples were cut into 2 parts along their thickness direction, and the resolution used

was 2.6m/pixel for sample size of 6.4mm (width) × 6mm (length) × 5.8mm (thickness). Other

relevant scanning parameters are shown in Table 1. Skyscan NRecon software was used to

reconstruct the raw images of a sample into a set of parallel 2D x-ray micrographs, providing the

3D information of the entire volume of the sample.

2.3 Air permeability test of prepreg interlayer regions

The air permeability of the interlayer regions of 0°/0° (fiber direction is flow direction),

0°/90°, 90°/90° stacks were measured. Assuming laminar flow, the testing set-up was prepared

as in Figure 3. In order to reduce any specimen geometry effect and improve the reliability and

reproducibility of the test results, 4 slices of prepreg tapes with the size of 50mm (along flow

direction) × 300mm (across the flow direction) were placed together to form 3 interlayer regions

for the measurements. The flow meter (0-10 l/min) measures the total flow rate of three

interlayer regions. To maintain uniform and realistic contact between the prepreg layers, an

INSTRON machine was used to apply 1atm pressure on top of the prepreg layers with a rigid

iron bar during the tests. Based on the Darcy’s Law, the boundary conditions, and considering

the compressibility of air, the total flow rate from 3 parallel interlayer regions follows eq(1):

𝑄 = 3 ×𝐾×ℎ×𝑊×𝑃𝑎𝑡𝑚

2×𝜇×𝐿 (1)

Where Q is the total flow rate measured; K is the permeability of the interlayer regions; h is

the height of the interlayer region; W is the sample width (300mm across the flow direction); L

is the flow length/sample length (50mm); Patm is equal to the atmosphere pressure (101325Pa);

and is the air viscosity, 1.821E-5 Pa.s. Since the surfaces of the prepreg are very rough, h is

very difficult to determine accurately. However, it does not affect our calculation and

understanding of the role of permeability with unknown h. We define K×h denoted by Pe with

the unit of m3 as the permeability parameter of each prepreg interlayer region which can be

derived and calculated through:

𝑷𝒆 = 𝐾ℎ =2𝑄∗𝜇∗𝐿

3∗𝑊∗𝑃𝑎𝑡𝑚 (2)

Zhang, D., D. Heider, and J. W. Gillespie, Jr., “Volatile removal during out of autoclave processing of high performance thermoplastic composites,” CAMX - 2014, Orlando, FL, Oct 13-16 2014.

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Figure 3. Experimental set-up of air permeability test

3. Experimental Results

3.1 Voids in Consolidated Laminates

2-layer and 72-layer laminates after consolidation with perimeters open to vacuum were

found to have void content smaller than 1% with optical microscopy inspection. This confirms

that voids in thick laminates can be reduced in OVB processing. The typical cross-sectional

optical micrographs of the consolidated laminates are shown in Figure 4. It is seen that the

middle layers of the normal 72-layer laminate contain very few voids, and no significant gradient

of void content through the thickness was observed. The void contents quantitatively determined

from the 2D micrographs for the 2-layer and 72-layer laminates with the perimeter open to

vacuum was 0.69% and 0.57%, respectively.

For the thick laminate processed with lateral edges sealed, a significant amount of voids were

trapped in the laminate, mainly interlaminar voids. Very small void content (<0.5%) was found

in the layers close to the two surfaces, and the middle layers contained more voids. The void

content from the 2D micrographs for the 72-layer edge sealed laminate was 3.4%.

Vacuumpump

Flow meter

Rigid tooling plate

Prepreg stacks with vacuum grease on

both surfaces

Breather with vacuum bag on top

Tacky tape sealing

Tube

Flow direction

Zhang, D., D. Heider, and J. W. Gillespie, Jr., “Volatile removal during out of autoclave processing of high performance thermoplastic composites,” CAMX - 2014, Orlando, FL, Oct 13-16 2014.

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Zhang, D., D. Heider, and J. W. Gillespie, Jr., “Volatile removal during out of autoclave processing of high performance thermoplastic composites,” CAMX - 2014, Orlando, FL, Oct 13-16 2014.

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Figure 4. Optical micrographs of the cross-sections of consolidated laminates

The cross-sectional images of consolidated laminates from X-ray micro-CT are shown in

Figure 5. Similar cross-sectional X-ray images as those obtained via optical microscopy for 2

layer laminate were obtained, and very few voids were observed in the samples. For the thick

laminates, at least half of the thickness was included in the reconstructed images. Although small

voids are not shown due to the low scanning resolution, it is seen that the 72-layer samples with

the perimeter open to vacuum shows homogeneous cross-sections through the entire thickness.

The 72-layers with edge sealed samples contain large interlayer voids in the inside layers, as well

as some large intralayer voids. The 3D volume and the voids in the 72 layer thick laminates are

shown in Figure 6. It is seen that both the intra- and inter-laminar voids are oriented along the

fiber directions.

Zhang, D., D. Heider, and J. W. Gillespie, Jr., “Volatile removal during out of autoclave processing of high performance thermoplastic composites,” CAMX - 2014, Orlando, FL, Oct 13-16 2014.

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Figure 5. X-ray images of the cross-sections of consolidated laminates

Figure 6. 3D volume and voids in a 72-layer edge sealed thick laminate sample from X-ray

micro-CT

Zhang, D., D. Heider, and J. W. Gillespie, Jr., “Volatile removal during out of autoclave processing of high performance thermoplastic composites,” CAMX - 2014, Orlando, FL, Oct 13-16 2014.

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3.2 Interlayer Air Permeability of Prepreg Tape

The permeability parameters Pe of the AS4/APC2 prepreg with the stacking sequences of

0°/0°, 0°/90°, and 90°/90° were calculated after measuring the flow rate, and are presented in

Figure 7. It is seen that when all the fibers are along the flow direction, the permeability is the

largest. When the fiber orientation is rotated perpendicular to the flow direction, the rough tape

surfaces impede the flow, reducing the permeability. The permeability of stacking sequence of

0°/0° and 0°/90°is about an order of magnitude higher than 90°/90°. The permeability values will

be used to estimate the air flow time in the following sessions.

Figure 7. Permeability parameter of AS4/APC2 prepreg stacks

4. Discussion of Void Consolidation Mechanisms

Our previous study shows that the voids originally existing in the prepreg are sealed, and do

not form continuous pathways for volatile molecules to escape. Volatile molecules in thin

laminates can be removed through diffusion with the two surfaces exposed to the vacuum. [13]

For laminates which are 30 times thicker, the theoretical diffusion time for volatile molecules

can be 9000 times longer – resulting in significant amounts of voids remaining in the laminates

in the practical processing time. However, the experimental results in the current paper show that

72 layer thick laminates can also achieve very low void content with OVB processing. The

diffusion mechanism and the effect of porous interlayer regions formed by the rough surfaces of

prepreg are discussed below.

4.1 Air Removal through Diffusion

The volatile concentration evolution through diffusion in thin and thick laminates during the

non-isothermal processing was investigated. Based on Fick’s second law,

Zhang, D., D. Heider, and J. W. Gillespie, Jr., “Volatile removal during out of autoclave processing of high performance thermoplastic composites,” CAMX - 2014, Orlando, FL, Oct 13-16 2014.

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𝜕𝑐

𝜕𝑡= 𝐷(𝑇)

𝜕2𝑐

𝜕𝑥2 (3)

Concentration c is a function of time, t, and diffusion length, x. Diffusion coefficient D(T) is a

function of temperature, T, and temperature is changing with time during the consolidation (see

Figure 2). Therefore, the real temperature profile of the processing cycle was fitted into a 5-term

Gauss function of time with the general equation written below:

𝑇 = 𝑓(𝑡) = ∑ 𝑎𝑖 × exp(−(𝑡−𝑏𝑖

𝑐𝑖)2)5

𝑖=1 (4)

The fitting parameters to achieve R-square of 0.9996 are shown in Table 2.

Table 2 Fitting parameters of 5 term Gauss function

a b c

1 24.57 9402 981.3

2 -25.55 6303 2477

3 234.5 9040 5874

4 400.2 1.008e+04 1.987e+04

5 -1.005e+15 -4.216e+04 7525

The diffusion coefficient of air at room temperature (298K) is 10-14

m2/s taken from literature

[23, 24]. Based on the assumption of simple gas diffusion, the temperature dependent diffusion

coefficient was estimated through the methods introduced in Ref.[25]. Taking the parameters of

O2 for conservative calculations, the temperature dependent diffusion coefficient follows:

𝐷(𝑇(𝐾)) = 10−14𝐸𝑥𝑝(−5327 (1

𝑇(𝐾)−

1

298)) (5)

Both top and bottom surfaces of the laminates are open to vacuum (with peel ply) in the

experiment. Therefore, a symmetric boundary condition is applied:

𝑐(0, 𝑡) = 𝑐(𝐻, 𝑡) = 0 (6)

H is the thickness of an entire laminate.

Finite difference method was used to solve this non-isothermal time dependent problem, and the

time steps and the grids through thickness follow:

𝐷∆𝑡

∆𝑥2≤

1

2 (7)

Since the evolution of normalized concentration with time is only affected by the thickness of

the materials, the initial concentration of unity was used. The evolution of the concentration

profile for different laminate thicknesses during the consolidation cycles are presented in Figure

Zhang, D., D. Heider, and J. W. Gillespie, Jr., “Volatile removal during out of autoclave processing of high performance thermoplastic composites,” CAMX - 2014, Orlando, FL, Oct 13-16 2014.

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8. It is seen that for 1- layer and 2- layer laminates, the air molecules can be removed in 5000-

7000 seconds at approximately 300°C. However, for 72 layer laminates, 83% of the volatile

molecules remain in the laminates when the temperature is cooled below the glass transition

temperature over 16,000 seconds. This theoretical calculation is consistent with the results for

the thick laminate with sealed edges where only diffusion can occur experimentally. However,

the results do not agree with our experimental results for the thick laminates with the perimeter

open to vacuum, indicating that the diffusion mechanism cannot be the only mechanism for

volatile removal in the thermoplastic composite OVB processing.

For the edge sealed 72 layer laminates, there is less than half of the volatile concentration

(3.4% versus 7% initially in the prepreg) remaining in the laminates including the interlaminar

voids, which is smaller than these calculated in Figure 8. This may due to higher real diffusion

coefficient of the molecules, the molecules dissolving in the resin, or volatiles escaping out of

the perimeter due to imperfect sealing. This requires further investigation but the importance of

interlayer permeability mechanism for void reduction during OVB processing of thermoplastic

prepreg has been established.

Figure 8. Evolution of normalized concentration of AS4/APC2 prepreg stacks with processing

time (entire thickness)

4.2 Air Flow Through Permeable Interlayer Regions

Since each prepreg layer inside the 72-layer laminates is in between two air permeable

interlayer regions, it is possible that the volatile molecules in the prepreg diffuse out firstly from

single layers and then flow out through the interlayer regions. The characteristic time for the

volatiles to flow out could be calculated through Darcy’s law. During the processing, when the

laminate edges are not sealed with tacky tape, the permeable interlayer regions are open to

vacuum in the bag. Therefore, steady state Darcy’s flow with 1 atm pressure drop was assumed.

Zhang, D., D. Heider, and J. W. Gillespie, Jr., “Volatile removal during out of autoclave processing of high performance thermoplastic composites,” CAMX - 2014, Orlando, FL, Oct 13-16 2014.

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The total volatile volume is the void volume in the prepreg tapes, and the characteristic time for

volatiles to flow out at temperature T(K) could be calculated through:

𝒕𝒑 =𝑣

𝑄=

𝑉𝑣∗𝑉𝑃𝑒∗𝑊∗𝑃𝑎𝑡𝑚

2∗𝜇∗𝐿

=𝑉𝑣∗𝐿2∗𝑡0∗𝑇∗𝜇

596∗𝑃𝑎𝑡𝑚∗𝑃𝑒 (7)

where 𝑣 is the total volatile volume which is changing with temperature; t0 is the thickness

of a prepreg layer. It is seen that the time, tp, depends on the length (in-plane dimension) of the

laminates, L, temperature, T, and permeability parameter, Pe.

From Figure 8, it is seen that the normalized concentration is nearly 0 when the processing

temperature is around 250°C for 1- and 2- layer laminates. It is assumed that at this temperature

the intimate contact of the rough surfaces do not change significantly compared to the initial

prepreg. Therefore, the permeability measured at room temperature was used initially for the

calculation. Taking the laminates processed in this research as an example, L = 0.3m, Pe =

2.98E-15 m3, at 250°C, tp is calculated to be 0.06s. This characteristic time is extremely short. It

shows that the volatiles can be totally removed before resin gets melted at 340°C in the 72-layer

thick laminate. The combination of diffusion through a single layer to a permeable interface for

interlayer flow to the perimeter implies that thick section laminates can be OVB processed with

the same effectiveness as the 2 - layer thin laminate.

5. Conclusions

Thin and thick AS4/APC2 laminates were consolidated with OVB processing (2.8°C/min to

380°C with zero dwell time). Voids in the consolidated laminates were examined through optical

microscopy and X-ray micro-CT. It is seen that both the 2-layer and 72- layer laminates with

permeable interlayers open to perimeter vacuum achieved less than 1% void content. Significant

intra- and inter-layer voids were trapped in the 72 layer laminates with lateral edges sealed. In

order to investigate the volatile removal through diffusion mechanism, non-isothermal diffusion

analysis was conducted with the real temperature profile of the consolidation cycle. It is shown

that the volatiles can be removed through diffusion in thin laminates during this cycle. However,

thick laminate cannot achieve low void content after this cycle through diffusion only. Both

results for thin and thick laminates are consistent with experimental observations.

The feasibility of achieving low void content during OVB processing of thick laminates was

established experimentally. The mechanism considers the diffusion of volatiles through a single

layer to a permeable interlayer formed between the rough surfaces of the thermoplastic

composite prepreg, followed by the interlayer flow to the panel perimeter that is open to vacuum.

A test set-up was created, and the air permeability of the prepreg stacks was measured

experimentally. Characteristic times for diffusion through a single layer and the interlayer flow

Zhang, D., D. Heider, and J. W. Gillespie, Jr., “Volatile removal during out of autoclave processing of high performance thermoplastic composites,” CAMX - 2014, Orlando, FL, Oct 13-16 2014.

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time were calculated based on Fick’s Law and Darcy’s law for the panel size considered in this

study. These times are consistent with the experimental results for achieving low void content in

thick panels during the OVB cycle. It is concluded that volatiles diffuse out from single layers

and then flow out through interlayer regions. This is an important mechanism for void reduction

in thick thermoplastic OVB processing. Further study on the non-isothermal condition and the air

permeability reduction with intimate contact developments are needed for better understanding

of the volatile removal time and the void reduction in thick laminates.

6. Acknowledgements

The authors would like to thank EADS CTO North America and especially Marc Fraser for

funding this research. Our thanks also go to Patrice Lefebure, from EADS IW, France who

allowed this collaboration.

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