High Strength-to-Weight Ratio Non-Woven Technical Fabrics for Aerospace Applications

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Nov 18, 2013 (3 years and 10 months ago)

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Copyright
©
2009 by Cubic Tech Corp.



1

High Strength
-
to
-
Weight Ratio Non
-
Woven Technical
Fabrics for Aerospace Applications

Keith McDaniels
1
, RJ Downs
2
, Heiner Meldner
3
, Cameron Beach
4
, Chris Adams
5

Cubic Tech Corp., Mesa, Arizona, 85205

Flexible laminates, customized for specific performance
parameters, are of interest to
aerospace programs such as lighter
-
than
-
air vehicles, balloon systems, decelerator systems,
flexible inflatable structures and pressure vessels. The material requirements for these
applications include high strength
-
to
-
weigh
t ratio and modulus, low gas permeability,
pressure retention and the capability to survive in harsh atmospheric, marine and/or
stratospheric environments for extended periods of time. Non
-
woven multidirectional
oriented composite laminates using high per
formance engineering fibers are produced by
Cubic Tech to meet these requirements. These flexible laminates achieve a significant weight
savings over woven fabrics of similar strengths by eliminating strength and modulus loss and
other structural deficienc
ies caused by crimping of yarns during the weaving process. The
absence of crimp in non
-
woven fabrics results in a linear elastic response that allows for ease
in predicting material properties and simplification of structural models. These flexible
compo
sites afford the ability to specify structural properties, oriented to meet any design
requirement. Parts can be manufactured with complex 2D and 3D geometries with
integrated structures, load patches and attachment points. Structures fabricated from thes
e
laminates can be joined using standard industry seaming techniques to produce seams
stronger than the base laminates. Properly designed seams hold structural loads for
extended periods without failure, slip or creep.

Nomenclature

CT


=

Cubic Tech Corp.

FAW


=

Fiber Areal Weight

LTA


=

Lighter
-
Than
-
Air

MDA


=

Missile Defense Agency

PBO


=

Poly(p
-
phenylene
-
2,6
-
benzobisoxazole)

PEN


=

Polyethylene Naphthalate

PET


=

Polyethylene Terephthalate

PVF


=

Polyvinyl Flouride

SBIR


=

Small Business Innovation Rese
arch

UHMWPE

=

Ultra High Molecular Weight Polyethylene

USASMDC

=

United States Army Space and Missile Defense Command

I.

Introduction

he development of advanced materials will continue to enable previously impossible aerospace applications.
Research contin
ues on novel materials that minimize weight and improve strength, while addressing other
critical properties. Flexible composites, owing their heritage to high performance, low stretch sails used in yacht
racing competitions, including around the world ra
ces, continue to be developed by
Cubic Tech Corp. (
CT
)
. These
materials take advantage of the newest in high strength
-
to
-
weight ratio fibers
,
technical films
, woven fabrics, and



1
Materials Engineer, Research and Technology

2
P
resident of Cubic Tech and Senior Engineer, Research and Technology

3
CEO of Cubic Tech

4
Materials Engineer, Research and Technology

5
Vice President of Operations, Industrial Engineer, Research and Technology

T


Copyright
©
2009 by Cubic Tech Corp.



2

other high performance materials.
Lightweight outdoor products, medical devic
es, inflatable tubes, parachutes,
parafoils, large kites, and most recently high altitude airships have been developed using this technology.

CT
received funding from the MDA
(a program transferred to the USASMDC)
for two phases of SBIR work on the
develop
ment of “Very Lightweight High Tenacity Fabrics” for high altitude lighter
-
than
-
air systems
.

This paper will introduce concepts of flexible composite technology and the potential advancement in the state
-
of
-
the
-
art of

materials for LTA
, terrestrial
inflat
ables,
space inflatable
s
, and decelerator systems.
To encompass the
spectrum of potential material properties, examples of ultra
-
light, medium, and heavyweight materials of various
constructions will be presented. The intuitive engineer may find utility
in these materials for applications requiring
similar weight materials or anything in
-
between.

II.

Material Challenges

Material challenges in aerospace applications have been well documented in previous publications. For high
altitude, LTA systems, strength
-
to
-
weight ratio of the material is critical to payload capability, the size of the
syste
m, and maximum operational altitude. Materials must be able to survive tough stratospheric environmental
conditions, which include low temperature, intense UV radiation, and high ozone concentration
; l
ow gas
transmission, through the hull and ballonet, is
necessary to maintain lift for long duration missions, high tear
resistance ensures the durability of the vehicle
(preventing catastrophic tear propagation), and
materials must have adequate bondability for durable
joints
.
Finally, l
ow temperature flexib
ility is necessary
for ballonet materials, which inflate and deflate during
operation.
1
-
5

Lockheed Martin
’s
High Altitude Airship
(HAA
TM
) program (Fig. 1) and
DARPA’s I
ntegrated
S
ensor Is
S
tructure (ISIS)

program are examples of
unmanned stratospheric air
ships
requiring high
performance ultra
-
light hull fabrics
.
In an early phase
of the ISIS program, CT developed advanced hull
material samples for an unmanned stratospheric airship.
These hull fabric samples achieved critical performance
parameters such as
: Areal density less than 100 g/m
2

for a fiber strength
-
to
-
weight ratio greater than 1000
kN*m/kg and a matrix glass transition temperature of
less than minus 90°C
as listed in Fig. 2. T
hese
laminates must also retain greater than 85% strength at
5 years
and must incorporate coatings for
environmental protection and thermal control while
passing radar transmittance specifications.

Besides
LTA applications, high
performance
materials
are used for re
-
entry airbag systems
. The
requirements for these m
aterials are similar to those for
LTA systems except the need for increased abrasion
and puncture resistance. Low mass due to launch
vehicle mass constraints and high tensile strength for
high
-
pressure impact loads is necessary. In addition,
high tear st
rength ensures survivability during the
impact with sharp surfaces, and low gas permeability
retains
the inflation gas
.
At low temperatures t
he
material must have
adequate flexibility for retraction

and maintain sufficient
strength at
the
elevated
tempera
tures
encountered during inflation and
on
impact
. Lastly
, the material
can only be minimally
affected by creasing for tight packaging.
6
For other
decelerator systems, including parachutes and parafoils,
fabrics must be flexible, unaffected by folding, an
d
resistant to shock and tearing loads.



Figure 1. HAA
TM
Conceptual Images


(courtesy of Lockheed Martin)




Figure 2
.
From DARPA Briefing:
http://www.darpa
.mil/sto/space/pdf/ISIS.pdf


Copyright
©
2009 by Cubic Tech Corp.



3

Recent advances in high performance fibers, including
Dyneema®
, Zylon, and Vectran meet and exceed many
of the material requirements in these applications. These materials maintain a significant advantage over tradi
tional
materials, such as those made from polyester or nylon, whose strength
-
to
-
weight ratios are too low for advanced
aerospace applications.

III.

Flexible Composite Overview

A.

Flexible Composite Implementations

CT produces flexible, multidirectional, non
-
woven
laminates from oriented filament layers and high
performance films or surface coatings. The resulting composite laminates are tailored and optimized for strength,
stretch properties, and minimum thickness. These materials are easily customizable to a wid
e variety of weights,
ranging from 0.3 oz/yd
2
to over 20 oz/yd
2
, and may be formed in 2 and 3
-
dimensional parts. CT’s laminates can
utilize most of the advanced engineering fiber materials available, and design is based on the required operating
condition
s and parameters of the specific application. Available fiber choices include, but are not limited to:
UHMWPE (
Dyneema®
, Spectra), liquid crystal polymer (Vectran), PBO (Zylon), para
-
aramids (Kevlar, Twaron,
Technora), carbon, glass, nylon, and polyester.
A variety of surface films, foils, or coatings can be incorporated
including, but not limited to: PET (Mylar), polyamide (Nylon), polyimide (Kapton, Upilex, LaRC
-
CP1), PVF
(Tedlar), and urethane. These surface materials add customizable properties inclu
ding toughness, low gas
permeability, low and high temperature operating capability, and visible and UV light protection, to name a few.
There are also woven backing options for improved durability and abrasion resistance, although these often add to
the
weight and thickness of the material.

B.

Internal Reinforcement / Complex Geometries

The laminated construction of
composite fabrics enables the introduction of internal reinforcement patterns that
minimize weight and are structurally sound locations for attachment points. Reinforcements may include
attachment loops, corner patches, radial and edge load point reinforc
ement, and feathered patterns. Figure 3
demonstrates an integrated 360° radial reinforcement pattern, and a large integrated structure panel.

C.

Woven vs. Non
-
Woven Construction

Non
-
woven fabrics have a number of technical and performance advantages over w
oven fabrics. Lightweight
woven products must use a multitude of low denier tows, while CT is able to produce similar, and oftentimes
-
lighter
weight, materials from a lower number of high denier tows. This eases manufacturability, while allowing for weigh
t
and thickness optimization. Additionally, woven fabrics suffer from crimp, which is caused by fibers passing over
and under each other in the
weave. Tensile loading of woven fabric induces transverse loads at fiber overlap
sections as crimped fibers at
tempt to straighten
; t
his reduces the translation of fiber strength to fabric strength and
decreases long
-
term fatigue
and creep rupture
performance.
Crimp related reduction in properties is particularly
pronounced with higher performance engineering fibe
rs where optimization of axial filaments properties weakens
transverse properties of the filaments.
Figure 4 is a magnified image of a woven polyester fabric followed by an
illustration of crimp in woven fabrics as load
is applied.
7,8

Non
-
woven oriented
composite laminates are free from



Figure 3. (Left) 3D 360° Radial Reinforced Panel & (Right) Large 40ft x 40ft Integrated
Structure Panel for Decelerator Applications


Copyright
©
2009 by Cubic Tech Corp.



4

these limitations and may be produced with an unlimited range of fiber areal weights having multiple oriented layers
positioned at any angle.
The most significant advantage
s
of
oriented laminates are
the ability to optimi
ze weight,
thickness, and strengths at particular locations or along predetermined load paths.
Non
-
woven flexible composites
constructed from high modulus fibers have predictable and linear properties for engineered designs. Figure 5
illustrates this poi
nt with a stress
-
strain graph of a non
-
woven material, with no crimp, and a woven fabric having
crimp.

A conceptual drawing of a non
-
woven fabric is also included in Fig. 5.

IV.

Materials Overview

CT has experience engineering laminates using a wide variety of high performance fibers that are chosen
depending on the specific application and technical requirements. Engineering fibers each exhibit a particular blend
of
properties and differing applications require fibers that have differing property tradeoffs. A thorough
understanding of material properties along with the use of appropriate safety factors simplifies application specific
fiber selection and minimizes ri
sk.
9,10
An overview of the material properties for the Dyneema
®
, Zylon, and
Vectran fibers is presented in the following paragraphs. CT can also produce composites from UHMWPE, para
-
aramid, carbon, glass, nylon, polyester, and most other engineering fibe
rs. The fiber descriptions are followed by a
brief overview of typical surface coating options. For aerospace applications requiring specific surface properties,
CT will work with specialized surface coating manufacturers to meet project requirements.




Figure 5. (Left) Comparative Stress
-
Strain of materials with and wi
thout creep. (Right) Conceptual
Drawing of a non
-
woven material.



Figure 4. Magnified Image of Woven Polyester and Crimp Diagram.
7,8

Tensile loading of woven fabric
induces transverse loads at thread overlap intersections due to straightening of cri
mped fibers.


Copyright
©
2009 by Cubic Tech Corp.



5

A.

High Performance Engineering Fibers

DSM and Honeywell produce similar UHMWPE fiber under the trade names of
Dyneema®
and Spectra
respectively.
Dyneema®
has the 2
nd
highest strength
-
to
-
weight ratio (second only to PBO for commercially
available fibers), an
d outperforms Zylon and Vectran in flex fatigue and resistance to moisture and UV light.
Dyneema®
products are often a preferred choice for flexible composites based on these properties. Performance
concerns with
Dyneema®
include creep, which increases w
ith temperature and load, adhesion, and its low melting
temperature.
2,5
This tradeoff of less than ideal creep but excellent UV performance may be easier to design for, and
determine life factors from, than from materials with opposite trade
-
offs. Creep m
ay be modeled from existing
parameters based on temperature and load conditions
11,12
, while the environmental degradation of PBO or Vectran is
less quantified and more difficult to monitor in situ. References 13
-
16 provide information on the mechanisms an
d
test results for the creep behavior of
Dyneema®
.

Zylon fiber is made from rigid
-
rod chain molecules of PBO. It is available with slightly higher strength
-
to
-
weight ratio than
Dyneema®
. In addition, Zylon has a high decomposition temperature and excel
lent creep
resistance. The fiber has good abrasion resistance, but performs significantly lower than
Dyneema®
. The strength
of Zylon is significantly decreased by exposure to high humidity/high temperature, and UV/visible light. As a result
it must be v
ery carefully protected from these conditions.
2,17
For additional information on environmental effects on
the performance of Zylon see references 18
-
22.

Vectran offers a unique balance of properties in comparison to other high performance fibers. Vectr
an is a liquid
crystal polymer having strength and modulus lower than
Dyneema®
or Zylon but other properties falling between
those of
Dyneema®
and Zylon including: creep, abrasion, UV performance and flex/fold resistance.
2,5,23,24
Vectran
was qualified fo
r use in the Mars Pathfinder landing system, due to its performance over a wide temperature range
and flex fatigue resistance.
6


Figure 6 compares the density, strength, and modulus for
Dyneema®
, Zylon, and Vectran. Specific strength and
specific modulus
is based on the tensile strength and modulus divided by each fiber’s density. These values provide
a better point for comparison.

B.

Surface Coat
ings

Film coatings, such as Mylar (a b
iaxially oriented

PET
),

are used for their
high tensile strength,
toughness,
chemical and dimensional stabi
lity,
gas barrier properties
, high temperature resistance and low coefficient of
friction. For space related a
pplications polyimide coatings and films, such as Kapton or Upilex, remain stable over a
wide range of temperatures and have good electric properties. For weatherability, PVF films such as Tedlar, provide
long
-
term durability and environmental stability.
Nylon and Urethane coatings offer excellent toughness and
flexibility, at a lower cost than other options,

but have lower mechanical and permeability properties.


Woven coatings are often incorporated into CT laminates when high abrasion resistance is th
e priority.

The use
of woven materials on one side of composite, or on both, creates a laminate hybrid that blends the high strength,
dimensionally stable flexible composite construction with the limited benefits of woven technology.

Woven
coatings are t
ypically heavier than other types of surface coatings but can be the best choice in areas of the part
design where abrasion resistance is the driving factor.

C.

Bondability & Seaming

Regardless of the strength of an aerospace quality material, the strength o
r dimensional stability of joints used to
construct the structure is often a limiting factor in structures made using flexible materials. Due to the high strength,
low elongation and homogenous distribution of fibers within the laminate, flexible composite
laminates can be
Fiber
Type

Density
(g/cm^3)

Tensile
Strength
(GPa)

Specific
Strength
(Strength /
Density)

Tensile
Modulus
(GPa)

Specific
Modu
lus
(Modulus /
Density)

Zylon AS

1.54

5.8

3.8

180

116.9

Zylon HM

1.56

5.8

3.7

270

173.1

Dyneema®


0.97

3.4

3.5

111

114.4

Vectran HT

1.41

3.2

2.3

75

53.2

Vectran UM

1.4

3.0

2.1

103

73.6

Figure 6. Fiber Properties Comparison Table
12,15,21


Copyright
©
2009 by Cubic Tech Corp.



6

constructed with seams that are actually stronger than the base materials and are capable of carrying high structural
loads for long periods of time without failure, seam slippage, seam distortion, or creep. Depending on the
composite’s d
esign, most standard forms of seaming are suitable for fabrication including sewing, adhesive bonding,
heat welding, ultrasonic welding, and laser enhanced bonding.

V.

Example Materials

One of the unique characteristics of the CT flexible composite manufactu
ring method is the ability to produce
uniform materials, with engineered and multidirectional tailored properties, in weights ranging from 0.3 oz/yd
2
to
over 20 oz/yd
2
, while utilizing a wide variety of engineering fibers and coatings, but only a single pr
ocess
technology. Listed below are several classes of implementations using various engineering fibers that are suitable
for a wide range of applications.

A.

Test Methods

Materials were tensile tested using the ASTM D3039M test method and also a non
-
standard
test method for
tensile testing. ASTM D3039M utilizes tabbed specimens to relieve gripping stresses caused by mechanical
gripping forces. For more rapid testing without the need for tabbed specimens, self
-
tightening Bollard grips (Fig. 7)
are often used
. The Bollard grips show utility for tests up to a few hundred pounds but may not provide optimal
gripping for tests reaching higher loads. Most of the tensile tests in this paper were conducted using the Bollard grip
test method at an approximate strain
rate of 30%/min. Instron 5567 and Instron 5568 load frames with appropriate
load cells, Instron Bluehill software, and Instron clip on extensometers were used.

The slit tear method is specified by Mil
-
C
-
21189 Method 10.2.4, and similarly FAA
-
P
-
8110
-
2, both specifically
developed as airship design criteria. The slit tear specimen is 4in by 6in having a 1.25in long cut in the center of the
s
pecimen (Fig. 8). Ref. 1 is also a good resource on appropriate tear test methods for airship materials.

B.

Lightweight Flexible Composit
es

Lightweight oriented multidirectional composite laminates may offer a higher performance alternative to nylon
or polyester woven materials in applications such as parachutes, parafoils, balloons, and other applications where
very thin, lightweight, stro
ng, and tear resistant material is required. Compared to silicone coated nylon of similar
weight and thickness, CT’s laminate is 80% stronger, has 10 times higher modulus, and 4 times higher tear strength.
For a comparative strength to the silicone coated
woven nylon, an ultra
-
light CT material (CT0.3
HB
UHMWPE
Composite), weighing 0.5 oz/yd^2 is included in the table below (Fig. 9). While woven fabrics have very little
strength in the bias directions, t
he CT0.3
HB
and CT1.5
HB
UHMWPE composites have been designed
with quasi
-
isotropic properties
. As a result, the shear properties of CT composites significantly outperform woven fabrics.



Figure 8. Slit
-
Tear Specimen (Mil
-
C
-
21189 10.2.4).
Pictured during tear testing using Pne
umatic/Hydraulic
grips.




Figure 7. Bollard Grips.

Pictured during tensile
testing with loaded specimen and attached Instron
extensometer.


Copyright
©
2009 by Cubic Tech Corp.



7

Although there is some strength contribution from coatings and films, which is not accounted for, high
conversion efficiency mean
s that the fiber strength is successfully transferred to the strength of the composite. The
theoretical strength of the CT1.5
HB
UHMWPE Composite, based on fiber density alone, is 75 lbf/in. In comparison
to the tested tensile strength of 87 lbf/in, this re
presents a conversion efficiency of 116%. The theoretical strength of
the CT0.3
HB
UHMWPE composite is 28 lbf/in, when compared to a tested tensile strength of 41 lbf/in, the
CT0.3
HB
conversion efficiency is 146%.


Product

Silicone Coated
Woven Nylon

C
T1.5
HB
UHMWPE
Composite

CT0.3
HB
UHMWPE
Composite

Weight (oz/yd^2)

1.3

1.2

0.5

Thickness (in)

0.002

0.002

0.001

Tensile Strength (lbf/in)

48

87

41

Theoretical Strength @ 23°
C
(lbf/in)

--

75

28

Conversion Efficiency @ 23°
C
(%)

--

116

146

Modulus (
(
lbf/in
)/
(in/in)
)

237

2774

1670

Strain to Failure (%)

33.0

3.2

2.9

Slit
Tear Strength (lbf)

26

108

38

Bias Tensile Strength (lbf/in)

--

87

41

Bias Modulus ((lbf/in)/(in/in))

--

2774

1670

Helium Gas Permeability (L/m
2
/24hrs)

--

<0.2

<0.2

Figure 9. Properties o
f Silicone Coated Woven Nylon and CT Lightweight Composites


C.

Medium Weight Flexible Composites

The properties of medium
-
weight laminates constructed from Vectran and Aramid fibers are included in the
table below (Fig. 10). Materials of these weights may b
e utilized for LTA or other inflatable structure applications.
In addition to
low gas permeability, these composites have excellent low temperature performance
and pressure
retention
. T
he high conversion efficiencies

reflect

the excellent transfer of
fib
er
mechanical properties to laminate
mechanical properties that allows for predictable and repeatable material elastic properties.


Product

CT35
HB
Vectran
Composite

CT35
HB
Aramid
Composite

Weight (oz/yd^2)

4.1

4.7

Thickness (in)

0.006

0.005

Tensile Strengt
h @ 23
°
C /
-
60
°
C (lbf/in)

523 / 670

587 / 650

Theoretical Strength @ 23°
C
(lbf/in)

501

546

Conversion Efficiency @ 23°
C
(%)

104

107

Modulus @ 23
°
C /
-
60
°
C (
(
lbf/in)
/(in/in))


18951 / 22285

26556 / 28645

Strain to Failure @ 23
°
C /
-
60
°
C (%)

2.6 / 2.8

2.
2 / 2.2

Slit
Tear Strength @ 23
°
C /
-
60
°
C (lbf)

205 / 229

131 / 125



Figure 10. Properties of CT Vectran and Aramid Medium Weight Composites


D.

Heavyweight Flexible Composites

CT’s heavyweight composites may be used in tension structures having high st
rength, low elongation, linear
stress
-
strain behavior, tear and damage resistant requirements, and/or application requiring multi
-
axial structural
reinforcement. Examples of potential applications include: large
-
scale heavy lift airships, inflatable struc
tures or
beams, tension structures and flexible pressure vessels. Examples of these material’s characteristics are included in
the table below (Fig 11).

Tabbed specimens were tested in addition to Bollard grip specimens when the Bollard grip method result
ed in
low strength results. It is likely that the Bollard grips are not sufficient to provide adequate gripping and may cause
stress concentration and edge effects at high loads. Using Bollard grips, the conversion efficiencies for the

Copyright
©
2009 by Cubic Tech Corp.



8

CT155
HB
UHMWPE Compo
site and CT135
HB
PBO composite are only 75% and 72% respectively. However,
tests
using the ASTM D3039M method using tabbed specimens resulted in an improvement in conversion efficiency
of 90% for the CT155
HB
UHMWPE Composite, and 85% for the CT135
HB
PBO Composi
te.



Product

CT15
5
HB
UHMWPE
Composite

CT135
HB
PBO
Composite

Weight (oz/yd^2)

13.0

10.3

Thickness (in)

0.01
6

0.011




Bollard Grip
Method



Tensile Strength @ 23
°
C /
-
60
°
C (lbf/in)

1813 / 2122

1618 / 1924

Theoretical Strength @ 23°
C
(lbf/in)

2402

225
0

Conversion Efficiency @ 23°
C
(%)

75

72

Modulus @ 23
°
C /
-
60
°
C (
(
lbf/in)
/(in/in))

85820 / 109965

72794 / 81682

Strain to Failure @ 23
°
C /
-
60
°
C (%)

2.
1
/
1.9

2.0 / 2.4




Tabbed Specimen Method (ASTM D3039)



Tensile Strength @ 23
°
C (lbf/in)

2167

19
34

Theoretical Strength @ 23°
C
(lbf/in)

2402

2250

Conversion Efficiency @ 23°
C
(%)

90

86




Slit
Tear Strength @ 23
°
C /
-
60
°
C (lbf)

313 / 504

250 / 423

Helium Gas Permeability (L/m
2
/24hrs)

<0.2

<0.2


Figure 11. Properties of CT UHMWPE and PBO Heavyw
eight Composites



VI.

Conclusion

CT is developing materials to meet the challenges of current and future aerospace applications. High
performance fibers and surface coatings are enabling this technology along with a unique manufacturing process that
produces
customizable flexible composites with optimum strengths and weights (weights ranging from 0.3 to over
20.0 oz/yd
2
). Material properties of these materials can be tailored for strength and modulus in multiple arbitrary
directions and can be produced in se
amless two
-
dimensional flat or three
-
dimensional complex curved structures
exceeding 40 ft in width and exceeding 100 ft in length. Materials can be joined with seams that are stronger than
the base laminate materials and capable of carrying structural lo
ads for extended periods without failure, slippage, or
creep. Laminates can be engineered for pressure retention, low gas permeability, environmental resistance and
abrasion resistance.

Acknowledgments

The authors thank intern John
-
Paul Deitz for testing
the hundreds of samples in support of this paper.

References

1
Maekawa, S., Shibasaki, K., Kurose, T., Maeda, T., Sasaki, Y., Yoshino, T., “Tear Propagation of a High Performance
Airship Envelope Material,”
7
th
AIAA Aviation Technology, Integration and Oper
ations Conference,
Belfast, 2007.

2
Zhai, H., Euler, A., “Material Challenges for Lighter
-
Than
-
Air Systems in High Altitude Applications,”
5
th
AIAA Aviation
Technology, Integration and Operations Conference
, 2005.

3
Komatsu, K., Sano, M., Kakuta, Y., “Develo
pment of High
-
Specific
-
Strength Envelope Materials,”
3
rd
AIAA Aviation
Technology, Integration and Operations Conference,
2003.

4
Kang, W., Suh, Y., Woo, K., Lee, I., “Mechanical Property Characterization of Film
-
Fabric Laminate for Stratospheric
Airship En
velope,”
Composite Structures
, Vol. 75, No.1
-
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