AIAA 2002-1372
Shape Memory Composite Development for
Use in Gossamer Space Inflatable Structures
David P. Cadogan, Stephen E. Scarborough, John K. Lin,
George H. Sapna III
ILC Dover, Inc.
Frederica, DE
43rd AIAA/ASME/ASCE/AHS/ASC
Structures, Structural Dynamics, and Materials
Conference & Exhibit
AIAA Gossamer Spacecraft Forum
April 22-25, 2002 / Denver, CO
For permission to copy or republish, contact the copyright owner named on the first page.
For AIAA-held copyright, write to AIAA Permissions Department,
1801 Alexander Bell Drive, Suite 500, Reston, VA 20191-4344
2002-1372
SHAPE MEMORY COMPOSITE DEVELOPMENT FOR USE IN
GOSSAMER SPACE INFLATABLE STRUCTURES
David P. Cadogan*, Stephen E. Scarborough, John K. Lin, George H. Sapna III
ILC Dover, Inc.
Frederica, DE
inflate the structure numerous times on the ground for
testing prior to launch, and deployment in space. The
materials are initially consolidated at a highly elevated
temperature (Ts) to set the
material’s geometric shape.
This is the shape the structure
will naturally return to when
heated above its glass
transition temperature (Tg) in
subsequent heating events.
The material is heated above
its Tg, but not above its set
temperature (Ts), to soften it
for packing into small
volumes.
The packed
structure is heated above its
Tg prior to deployment in
space (or during ground
operations), to make it
flexible enough to be
deployed by inflation. The
shape
restoration
stress
Figure 1. ILC Dover
SMP Inflatable
memory of the composite is a
Space Frame 3,4,10
weak function relative to the
(Patent Pending)
stress
realized
from
1,2,7
inflation.
Stress is required for component
deployment, control, and tensioning the wall for optimal
shape accuracy. Therefore, the shape memory function
is utilized as a microscopic shape restoration feature in
most applications. The shape memory function also
allows the fabrication of structures that consist of small
diameter tubes where it would be inefficient to inflate all
elements individually (Figure 1). The individual tubes
return to shape by the shape memory of the resin, while
an outer polymeric film shell is inflated to deploy the
structure.3,4
ABSTRACT
Several new shape memory composite materials have
been developed that allow the requirements of
gossamer space structures (high packing efficiency, low
mass, high stiffness, etc.) to be met. A detailed analysis
and test program has been conducted on several
different materials at the coupon level, as well as at the
component level in the form of inflatable deployable
columns. Materials have been tested to determine their
degradation from folding and packaging, storage life
and aging characteristics, vacuum stability, outgassing
characteristics, and ability to return to shape when
heated after packing. Shape memory composites have
also been tested at the sub-component and system level
in several applications. Isogrid beam columns have
been designed, manufactured, and structurally tested to
verify materials performance parameters. The columns
were repeatedly packed and deployed to assess the
degradation of the materials in actual use and the
resultant strength and stiffness loss. Compression, and
torsion strength and stiffness were assessed in the test
program.
INTRODUCTION
One of the most important components of the gossamer
inflatable structure is the material of which it is
composed. A leading candidate among the field of
potential materials is shape memory composite
material. The shape memory composite consists of a
fibrous reinforcement, such as carbon, and a polymeric
matrix resin such as polyurethane or epoxy. The matrix
resin component provides the shape memory behavior
to the composite. The reinforcement can be utilized in
several forms such as individual tow elements or fabrics
of various weave styles. The fibers are coated with the
resin and formed into various structural shapes.
Geometric shapes such as monocoque, isogrid, and
IsoTruss™ columns can be manufactured from a shape
memory composite material.
This paper will examine system requirements for
rigidizable materials, performance characteristics of
several shape memory materials, and relevant test data.
Most of the data presented herein is from an isogrid
Shape memory composite materials utilize a reversible
heating process to provide the ability to collapse and re*
[email protected] – Associate Fellow AIAA
“Copyright  2002 by ILC Dover, Inc. Published
by the American Institute of Aeronautics and
Astronautics, Inc., with permission.”
IsoTruss is a registered Trademark of Brigham Young University
and is licensed to ILC Dover for space applications.
1
American Institute of Aeronautics and Astronautics
development program under the Inflatable Solar Array
Experiment (ISAE-II) program.
bundles (tows) during the packing of a completed
structure. When the tows are folded 180o for packing,
the fibers on the inside of the bend radius will be in
compression, while the fibers towards the outside of the
bend will be in tension. To minimize structural damage
in a tightly packed structure, the fibers must either be able
to move, which is a function of the resin, or they must be
able to withstand high strain rates, a function of both the
resin and the fiber. IM9 carbon fiber was selected for
further study because of its high modulus, high strength,
and high strain capability.
MATERIAL REQUIREMENTS
ILC generated an in-depth requirement list for the
development of potential rigidizable materials. Using
these requirements, ILC has developed a battery of tests
that are conducted in series. This provides a “gated”
approach to the development of advanced materials,
one that screens out materials efficiently and reduces
development cost.
The driving requirements are
identified as:
Ø
Ø
Ø
Ø
Ø
Table 1. Select Carbon Fiber Properties1
Brand & Type
o
Tight Packing Without Damage (180 Folds)
Low Outgassing (<1.0% TML, <0.1% CVCM)
Long Shelf Life (3+ years)
Near Zero CTE
Low Transient Deformation (Creep)
Tensile Tensile Tensile
Modulus Strength Strain
(%)
(MPa)
(GPa)
Density Avail. Tow
3
(g/cm )
Counts
Hexcel IM9
Hexcel IM7(6000
spec)
Besfight IM600
304
292
6120
5760
2
2
1.8
1.79
12K
12K
285
5790
2
1.8
A detailed selection process, covering all system
requirements, is conducted for candidate resins and
additional tests are performed to characterize the best
materials with respect to the following requirements:
Hexcel IM6
Hexcel AS4D
Grafil Pyrofil MR50
Hexcel IM7
Hexcel AS4C
279
241
290
276
231
5510
4550
5400
5080
4205
2
1.9
1.8
1.8
1.8
1.76
1.79
1.8
1.78
1.78
•
BP Amoco Thornel
T650/35
255
4280
1.7
1.77
6K,12K,
24K
12K
12K
12k
6K
3K,6K,
12K
3K, 6K,
12K
BP Amoco Thornel
T300
Grafil Pyrofil MS40
Besfight UM46
Besfight UM68
231
3750
1.4
1.76
345
435
650
4610
4705
3330
1.3
1.1
0.5
1.77
1.82
1.97
•
•
•
•
Radiation Resistance
(10 yr. life LEO/GEO)
Mechanical Properties
Thermal Conductivity
Water Absorption
Thermal Cycling Resistance
1K, 3K,
6K, 12K
12K
12K
12K
RESIN SELECTION AND DEVELOPMENT
Several candidate resins were identified from ILC internal
research and development started in 1996, commercially
available sources, and through ILC sponsored work at the
University of Delaware Center for Composite Materials.
Material properties for each of the candidate resins were
gathered for comparison (Table 2). Data was gathered
from available material data sheets, through rule of
mixtures analysis, or through testing. Assumptions made
about properties at the early stages of materials
development were deemed acceptable to reduce the
overall quantity of testing that was to be performed.
These values were used to make general trades and
identify the candidate materials that were later put
through a complete battery of tests.
FIBER SELECTION
The selection of fibers used in constructing a rigidizable
inflatable gossamer structure is driven primarily by
specific stiffness, strain to failure (to allow folding
without damage), environmental resistance, thermal
conductivity, and coefficient of thermal expansion (CTE).
CTE is very important in developing a structure that will
not suffer shape changes from variable thermal inputs.
Four fiber types are acceptable from this standpoint:
carbon, KevlarTM, PBO, and VectranTM. Of these, only
carbon exhibits the thermal conductivity and
environmental resistance desired, with a known interface
capability to the resin types being investigated. Further
study of the PBO and VectranTM reinforcements is
warranted in the future.
The individual resin candidates were also evaluated in a
trade study to determine the top three candidate
materials that would be fully characterized. Some
relative composite properties were given for
comparison. Weighting factors were applied to each
requirement to highlight the importance of certain
factors, such as those that would affect folding
characteristics or vacuum stability.
Several carbon fiber candidates were identified for
comparison (Table 1). Factors that affected structural
performance, as well as sizing and manufacturing, were
evaluated. It was important to select a fiber with a sizing
that was compatible with the resin candidates being
considered. ILC considers tensile strain to be the most
important fiber property for structures fabricated from
shape memory polymer (SMP) resins. This is because of
the high strains that will occur in the carbon filament
2
American Institute of Aeronautics and Astronautics
Table 2. Candidate Resin Properties 5,6,8
ILC Designation
Matrix
3
Density (g/cm )
Water absorption (after 24 h)
Radiation resistance
UV resistance
Liquid @ room temperature
Fiber Compatibility
Tensile Strength @ Yield (MPa)
Tensile Modulus (GPa)
Modulus vs. Temp. Transition
o
Deflection Temperature ( C)
o
Melt Temperature Tm ( C)
o
CTE (ppm/ C)
o
o
Specific Heat Cp (J/g C) (0 C-Tm)
o
Specific Heat Cp (J/g C) (Tm-Tf)
Thermal cycling
o
Thermal conductivity (W/m C)
Ability to return to shape
Packing ability
TP 264
Polyester
1.27
0.20%
Poor
Excellent
No
Good
53.1
TP275
Epoxy
1.14
0.70%
Excellent
Excellent
Yes
Excellent
62.0
2.21
Quick
o
80 C
o
81-91 C
70
2.76
Slow
o
53 C
N/A
51
2
2
Excellent
0.205
2.1
N/A
Good
0.17-0.25
Very good
Very Good
Excellent
Good
TP405
Polyurethane
1.20
0.7% *after 72 h
Good
Good
No
Fair
o
60.0 (Tg-20 C)
o
30.0 (Tg+20 C)
1.0
Quick
o
75 C
o
190 C
o
27.5 (Tg-20 C)
o
217 (Tg+20 C)
1.7 +
1.7+
Excellent
o
0.35 (Tg-20 C)
0.58 (Tg+20oC)
Very good
Very Good
The results of the trade study indicated that the epoxybased system was the leading candidate, followed by the
polyester and polyurethane materials. Out of these three
resins, epoxies and polyurethanes were the only resins
available as a liquid at room temperature. All of the other
candidate resins would have to be heated above their melt
temperatures for fiber impregnation. Because a wetwinding manufacturing technique provides the greatest
flexibility to the processing parameters, epoxy and
polyurethane resins become more attractive for use in
development projects. Epoxy is also a good candidate for
further study because it can be easily chemically
modified. Polyurethane was also identified as a candidate
that had a wide range of variables that could be adjusted
to vary performance properties. Within 4-5 months, ILC
developed several variants to the TP275 epoxy-based
material in an attempt to advance the material’s
performance. Each of these formulations is chemically
unique and provides a wide range of performance
capabilities. A summary of the material properties of
each of the epoxy variants, including the leading
polyurethane candidate (TP406) is shown in Table 3.
Tg (oC)
Laminate Folding Ability
Shape Memory
Pot life @ RT
Tensile Strength (MPa)
TP 406
TP 277 TP 279B
Urethane Epoxy
Epoxy
55
65
37
Excellent Excellent Very
Good
Yes
Yes
Yes
<10 min <16 hrs <24 hrs
60.0
69.0*
No Data
Yes
48 hrs
62.0
1.0
3.1
No Data
2.76
Possibility of Tg increase
over time
Modulus vs. Temp.
Transition
Fiber Compatability
Initial Mix Viscosity
No
Yes
Yes
Yes
Quick
Slow
Slow
Slow
Fair
Low
Excellent Excellent
Low
Low
TP230
Polyethylene
0.967
<0.01%
Excellent
Good
No
Good
30.5
1.34
Slow
o
108 C
o
157 C
68-105
1.34
Slow
o
95 C
o
165 C
68-105
1.0
Slow
o
80 C
o
133 C
108
2.1
2.7
Excellent
0.17-0.20
2.1
2.7
Excellent
0.17-0.20
2
2.7
Excellent
0.33
Good
Very Good
Fair
Good
Very Good
Very Good
FOLDING TEMPERATURE
The folding temperature of the resin is an important
parameter that has an impact on the overall spacecraft
system design and also has a major impact on ground
testing. The folding temperature is defined as the Tg of
the resin, plus an added factor of safety, which is typically
around 20oC. At these temperatures the resins exhibit a
large decrease in stiffness (i.e. they become flexible). It
would be ideal to use a resin in the structure that had a Tg
that was far above the highest anticipated temperature of
the spacecraft. However, since the resin must be heated
above the Tg for deployment, a high Tg will require
greater spacecraft power. Therefore, a balance between
thermal loading and folding temperature must be
achieved.
It is also of importance to note that
formulations with glass transition temperatures below
ambient temperatures could be developed if low
temperature rigidization is considered as an option for
future applications. However this too will affect the
ability of the completed structure to be ground tested
because these types of materials will require cold
chambers for testing. It will also affect the layers of MLI
that will be required to maintain the temperature of the
structure below its Tg. These additional layers of MLI
will lead in increased system mass.
TP 283E
Epoxy
48
Excellent
Tensile Modulus (GPa)
TP 221
Polypropylene
0.90
<0.01%-0.03%
Fair
Good
No
Good
31.7
The epoxy formulation that shows the greatest overall
performance is TP283E. This resin demonstrated a
particularly high degree of folding performance and good
shape memory. This work greatly advanced the state of
the art of SMP resin technology for use in gossamer
structures. Further advancement of these resins is
ongoing at ILC to improve performance properties and
enhance manufacturing characteristics.
Table 3. Enhanced Resin Properties1
Property
TP220
Polypropylene
0.90
<0.01%-0.03%
Fair
Good
No
Good
35.2
Excellent
High
3
American Institute of Aeronautics and Astronautics
Another area where the folding temperature is significant
is in ground handling. The fact that the structure will be
packed in a factory ambient environment causes some
potential difficulties for structures made with SMP
composites. The presence of an atmosphere around the
structure will create large convective losses and eliminate
the performance benefit of a MLI blanket. Therefore,
large amounts of power are required for ground heating of
the structures which often necessitate the need for special
heating and handling equipment that is radically different
than that which will be used in space. It is desirable to
maintain a Tg as low as possible to minimize the delta in
temperature during ground packing and testing to reduce
the potential for structural damage due to premature
cooling. Again, a balance in performance must be sought
and specific ground handling equipment manufactured.
120
108
Tg (C) By DSC
100
95
80
Tg (C)
80
80
75
65
60
48
40
37
37
40
53
55
43
24
20
Ep
ox
TP
y
28
3B
Ep
ox
TP
y
28
3C
Ep
ox
TP
y
28
3
A
Ep
ox
TP
y
28
3E
Ep
ox
y
TP
27
TP
5
Ep
40
ox
6
Po
y
lyu
re
tha
ne
TP
27
TP
7
Ep
40
5
ox
Po
y
lyu
re
tha
TP
ne
26
4P
TP
oly
es
23
ter
0P
oly
TP
eth
22
yle
1P
ne
oly
pr
TP
op
22
yle
0P
ne
oly
pr
op
yle
ne
27
9B
TP
TP
28
3F
Ep
ox
y
0
ILC Resin Designation
Figure 3. Tg of ILC Dover Resins
COMPONENT FOLDING TESTS
The most critical function of SMP/Carbon fiber
composites is to fold without significant degradation in
performance. If degradation does occur, it must be
predictable and well understood. To this end, ILC
developed a procedure for rapidly testing the material’s
response to folding. This test was used to obtain first
order qualitative test results to generalize folding
response. The folding test also included an assessment of
the material’s shape memory function, another important
parameter in material function.
The Tg of the resins studied was determined using
Differential Scanning Calorimetry (DSC). The resins
tested under ISAE-II exhibited glass transition
temperatures from 24oC to 54oC. A sample DSC curve
for the TP275 resin (ILC’s base SMP epoxy) can be seen
in Figure 2. Resins with Tg’s of 43oC and below were not
advanced because of the anticipated thermal environment.
However, further testing was performed on resins such as
TP279B because of the possibility of using them on
future programs where resins with lower Tg’s may be
required.
In this test, a T300, 1K tow, tri-axial woven fabric (0.18
mm thick) was pre-impregnated with the various resins to
a mass fraction of near 50% and consolidated into its
finished composite form. This particular fabric was
chosen because of its close resemblance to the isogrid
beam design (i.e. open weave). In the test, the sample
was heated well above its Tg (generally 120oC) and folded
180o over a 0.25mm thick aluminum sheet. Two gauge
lines marked on the sample identified the fold line. The
sample was then allowed to cool and solidify to its folded
shape.
At this point, the fold location was
microscopically inspected to identify any fracturing to the
fibers or resin. If successful, the sample was reheated in
an oven to assess its shape memory function. Notations
were made regarding the speed and accuracy of return.
The process was then repeated multiple times, always
folding along the same line, to gather information
regarding fatigue resistance. The entire process is shown
in Figure 4.
Tg=53.06oC
Figure 2. TP275 DSC Tg Analysis1
Most of the resins that were selected for advancement
exhibited the ability to slightly modify their Tg. This
ability was exercised in resin design such that the Tg
values could be kept in a range close to the anticipated
thermal challenge. Other ILC thermoplastic resins have
glass transition temperatures in the range of 60-100oC, but
at this time more intensive manufacturing processes
would be required for their use because they are not
available as liquids at room temperature. The Tg of ILC
Dover’s current thermoplastic, polyurethane, and epoxy
polymer materials are shown in Figure 3.
Some samples were immediately destroyed when folded
180o. Other samples had the ability to be folded over this
difficult bend radius, but when they returned to shape,
they were extremely brittle at the fold line. Resin systems
that achieved an excellent rating for foldability and
displayed good shape memory function were advanced to
the next level of testing, as tows. This test identified four
resins that achieved an excellent rating (TP277, TP279B,
4
American Institute of Aeronautics and Astronautics
Traditional composite structures generally have a Vf of
60%. In the case of the SMP/Carbon fiber composite, the
results of these tests showed that the test samples and
beams should have a Vf of 50% or less for folding. Also,
tows sizes from 12K-48K were able to be folded 180o
with little or no apparent visual damage. Generally, the
only difference under the microscope between pristine
and folded tows was a slight loss of sheen at the fold line.
TP283E, and TP406). The shape memory effect appeared
strongest in TP277 and TP406, while TP283E and TP406
were the most flexible resin systems above their
respective Tg’s.
Returned to Shape Inspection
Repeat
As Manufactured
The second tow test, the tow tensile test, was devised to
gather data on the structural performance of tows when
challenged by individual factors that could degrade their
performance, and these factors in combination. The
factors included lab oven heat exposure, hard vacuum
exposure, and folding cycles. The lab oven exposure was
evaluated to determine the degradation of the composite
when exposed to elevated temperatures that represented a
pre-flight bake-out to reduce the volatile content of the
system. The hard vacuum exposure was conducted in an
ILC vacuum chamber and represented the exposure of the
material to space prior to deployment (Figure 6).
Thermal exposures were
also conducted in the
vacuum
chamber
to
simulate the effects on the
system from heating during
deployment
in
space.
Repeated folding trials
were conducted that cycled
the tow from a straight to
180o bent condition, for 10
and 20 cycles, to assess
cycle
fatigue
effects.
Tensile testing was selected
for test simplicity and
because it was an accurate
Figure 6. ILC Thermal
assessment
of
fiber
Vacuum Chamber1
damage.
After Folding
Folded, Perpendicular to
Fold - Inspection
Figure 4. Laminate Folding Test Procedure1
ILC developed two tow related tests for further screening
resin candidates in a form closer to the isogrid elements,
and to obtain design data to use in the design of isogrid
booms. One was centered on microscopically studying
tow folding, and the other focused on studying the
environmental and packing effects more closely
associated with the tow elements. The tow-folding test
was developed to provide more advanced screening data
for resin selection. This test was also used to evaluate the
performance of various tow sizes (12K, 24K, 36K, and
48K) to determine if tow size had a significant effect on
degradation with folding. This was determined to be
important in developing a scalable isogrid or IsoTrussTM
structure.
The
purpose of these tow
tests was to assess the
damage that may be
caused
during
packing. The test
procedure
was
identical
to
the
Figure 5. 24K TP277 Tow
Folded 180o 1
laminate folding test
procedure. Figure 5
shows the results of one test.
ILC Dover mechanically characterized shape memory
thermoplastics in a specific configuration that was
consistent with the manufacture of isogrid booms.
Prior to testing the tow samples, each one was
measured using dial calipers to determine its diameter.
This value was then used in the calculation of the
tensile modulus for each tow. Four different tow sizes
were considered: 12k, 24k, 36k, and 48k. Five data
points were taken for each test (5 samples per test type)
to develop some statistical significance. A total of 275
individual samples were tested in this effort. Data from
the test was used to determine degradation factors for
use in analytical models and to help correlate boom test
data from the as manufactured state to the after packed
& deployed state. ILC considered the worst case
heating scenario to be eight hours at 125oC. This
represents the maximum time and temperature that the
The samples that were tested had fiber volume fractions
(Vf) of 41-60%. It was found that the samples that had
lower fiber volume fractions were more flexible and less
likely to sustain serious fiber damage than samples with
higher fiber volume fractions. For this reason, the tows
used in actual isogrid manufacturing were purposely
coated with more resin than typical composites.
5
American Institute of Aeronautics and Astronautics
that vacuum alone for 8 hours at room temperature
degrades the material (25.6% decrease in peak load). In
the case of the TP277 resin, some epoxy is volatized
during vacuum exposure. The reduction in tensile
strength can be explained by this loss of resin during
vacuum exposure. This material loss may reduce the
ability for the tow to transfer load through the matrix.
This also appears to occur with other SMP epoxies such
as TP283E. This may occur with other thermoplastic
resins as well, but at a much lower level because of
their reduced volatility.
structure would be exposed to
while in the packed state
prior to deployment. Figure 7
shows the tow tensile fixture
used
for
mechanical
characterization.
The first tow samples tested
were made from TP277
impregnated 12K and 24K
IM9 carbon fiber tows. Only
TP277 was tested at the 12Ktow level. From these tests,
as
well
as
other
manufacturing trials, ILC
Figure 7. Tow Tensile
determined that 12K tows
Test at ILC Dover1
would be too small for most
applications. The TP277 results indicate that both
tensile strength and tensile modulus decrease
substantially from various factors.
Figures 8 and 9 summarize the 24K IM9 SMP resin tow
tensile test results for the three resins tested. As with
the 12K IM9 TP277 samples, the 24K IM9 TP277 tows
also show substantial degradation when subjected to the
worst case scenarios. The peak tensile load of these
fibers decreases 60.5% and there is reduction in tensile
modulus of 70.8%. Vacuum at room temperature
causes a reduction in peak tensile load of 21.6%.
Flexing the material 180o ten and twenty times results
in similar degradation. This indicates that significant
reduction in properties is occurring before 10 folds, but
no further reduction occurs between 10 and 20 folds.
The 12K TP277 data showed that folding the tows ten
times resulted in a 41% reduction in peak tensile load.
Folding the tows twenty times after vacuum and heat
exposure reduces their peak load values another 11%
(for a total of 52% reduction from the baseline values).
At this point, the tows seem to reach a maximum
amount of degradation and will no longer experience a
reduction in performance due to vacuum, heat, or
additional 180o folds. The 12K TP277 tows also
experienced a 63.7% reduction in tensile modulus when
subjected to worst case conditions. The data showed
The 24K IM9 TP283E showed much more promising
results than the TP277 testing. However, these tows
still experienced reductions in peak tensile load (42.1%
reduction) and tensile modulus (30% reduction) when
subjected to worst case conditions. Flexing the material
10 and 20 times without any vacuum or heat also
reduces the peak tensile load (31.8% reduction after 10
2000
SMP Resins vs. Average Tensile Peak Load
1880
1800
Average Peak Load (N)
1600
1699
1739
TP 277
TP 283E
TP 406
24k IM9 Carbon Fiber
1644
1571
1528
1529
1449
1422
1358
1400
1262
1257
1159
1200
1159
1155
1154
1186
1056
1000
1003
984
885
865
803
800
726
709
659
600
500
400
200
0
No Bake,
Bake @
No Bake,
Bake @
No Bake, No Bake, No Bake, No Bake,
Bake @
No Bake,
No Vac, No 85C for 46 Vac @ RT 85C for 46
Vac @
No Vac, 10 No Vac, 20
Vac @
85C for 46
Vac @
Flex
hrs, No
for 8 hrs, hrs, Vac @ 125C for 8
Flex*
Flex*
125C for 8 hrs, Vac @ 125C for 8
Vac, No
No Flex
RT for 7
hrs, No
hrs, 10
125C for 8
hrs, 20
Flex
hrs, @
Flex
Flex*
hrs, 20
Flex*
125C for 1
Flex*
hr, No Flex
Tow Environmenal Exposure Characteristics
Figure 8. SMP Resins 24K Peak Tensile Load Results1
6
American Institute of Aeronautics and Astronautics
*Folded over 0.25mm radius
200
SMP Resins vs. Average Tensile Modulus
184
180
24k IM9 Carbon Fiber
Average Tensile Modulus (GPa)
160
160
156
147
143
140
129
120
100
TP 277
TP 283E
TP 406
154
147
141
112
119
117
110
94
88
94
93
93
93
85
85
81
80
60
41
36
40
35
37
35
20
0
No Bake,
Bake @
No Bake, No Bake, No Bake, No Bake,
Bake @
No Bake,
Bake @
No Bake,
Vac @
85C for 46
Vac @
No Vac, 10 No Vac, 20
Vac @
No Vac, No 85C for 46 Vac @ RT 85C for 46
125C for 8 hrs, Vac @ 125C for 8
Flex*
Flex*
for 8 hrs, hrs, Vac @ 125C for 8
hrs, No
Flex
hrs, 20
125C for 8
hrs, 10
hrs, No
RT for 7
No Flex
Vac, No
Flex*
hrs, 20
Flex*
Flex
hrs, @
Flex
Flex*
125C for 1
hr, No Flex
*Folded over 0.25mm radius
Tow Environmenal Exposure Characteristics
1
Figure 9. SMP Resins 24K Tensile Modulus Results
TP406 has lower baseline properties than the other two
resins, but comparing the resins after 20 flexes without
heat or vacuum exposure, it is clear that the TP406
resin is the most robust resin in terms of packing.
However, it does degrade substantially with the
combination of hard vacuum, high temperature, and
repeated 180o flexing. Further study is ongoing at ILC
with different fiber types to determine their 180o
folding damage resistance with SMP resins.
folds and 41% reduction after 20 folds). From this data,
just as with the TP277 results, there appears to be a
point at which the material properties will no longer
degrade no matter what influence the material is
subjected to. Vacuum exposure at room temperature
alone degrades the peak tensile load values by 26.0%.
However, the 180o flexing appears to contribute to the
degradation more than any other factor.
TP406 resin appears to be significantly degraded by
hard vacuum at high temperature followed by 10 and 20
cycles of 180o flex. This resulted in a 71.2 % reduction
in peak tensile load. Vacuum at room temperature does
not affect the resin. In fact, those samples held the
highest load relative to all of the other TP406 samples
tested. Flexing the material 10 and 20 times without
any vacuum or heat reduces the tensile strength only
slightly (18.2% reduction). This is the lowest reduction
in strength from 180o flexing out of the three resins
tested (TP277 had a 71% reduction and TP283E had a
41% reduction). High temperature exposure appears to
be the main cause in reduction in physical properties for
the TP406 resin. This is most likely because urethanes
are known to absorb water, and when heated, this
additional water can cause material degradation.
However, further research is required to understand and
further quantify these effects.
Figure 10 is a comparison of calculated, baseline, and
worst case values for composite tow tensile modulus for
TP283E and TP406. The calculated values in the chart
were determined using the rule of mixtures
(Ec=Vf*Ef+Vm*Em)9. Where Ec is the tensile modulus of
the composite tow, Vf is the fiber volume fraction, Ef is
the tensile modulus of the IM9 fibers, Vm is the matrix
volume fraction, and Em is the tensile modulus of the
resin. These calculated values are based on the average
fiber volume fraction of the experimental samples. The
baseline values in the chart were obtained experimentally
from composite tow samples that were not subjected to
any environmental exposure. Baseline values for every
resin and every tow size are not available at this time.
The experimental values may sometimes be higher than
the calculated values because of the diameter variations
on each of the tows, which was used to calculate
experimental modulus. This data is important for future
applications where larger tow sizes may be needed to
meet system level load requirements. From this initial
data, it can be concluded that larger tow sizes of up to
48K will experience similar degradation in tensile
properties as do the smaller (12K and 24K) tows.
From Figures 8 and 9, it becomes clear that the TP277
resin is more susceptible to environmental exposure
damage than the other resins. TP283E appears to be the
best resin in terms of tensile strength and tensile
modulus, but it does experience a drop in performance
properties from certain environmental exposures. The
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American Institute of Aeronautics and Astronautics
TP283E and TP406 Calculated Vs. Avg. Experimental Tensile Modulus
Bake out @ 85C for 46 Hrs, Vacuum 8 hrs @ 125C, 20 Flex*
200
184
180
163
Average Tensile Modulus (GPa)
160
167
156
159
157
155
140
129
119
120
TP 283E Calculated
118
112
110
114
108
TP 283E Baseline
109
TP 283E Experimental
99
100
93
89
TP 406 Calculated
TP 406 Baseline
80
TP 406 Experimental
60
IM9 Carbon Fiber
40
20
0
12 k
24 k
36 k
48 k
Tow Size
*Folded over 0.25mm radius
Figure 10. TP283E and TP406 Experimental and Calculated Tensile Modulus1
beam that had been fabricated more than seven months
prior to the test and stored at ambient temperature and
humidity. The results of this test are provided in Figure
11. Comparing this curve to Figure 2, it is evident that
the Tg has risen only 6oC (from 53.0oC to 59.4oC). This
shows that the rate of continued cross-linking is very low
at room temperature.
A gauge length comparison test was also performed
during the tow tensile testing to verify that the 20.32cm
gauge length used for fabricating all of the tow tensile
samples in the study was valid. ILC theorized that the
sample gauge length might negatively affect the results
of the testing. From the results of the gauge length
comparison test it was apparent that the 20.32cm gauge
length was valid because of the close proximity of the
peak load and modulus values obtained from the
10.16cm tow tensile samples that were tested.
Residual
Enthalpy
STORAGE LIFE / AGING
Accelerated aging is an important test that was performed
on two of the SMP resins. In actual use, space inflatable
systems may be stored at ambient conditions awaiting
launch for up to several years. Over that time period, it is
imperative that the material properties be well understood.
The main concern with some SMP resins is an increase in
Tg during the time period when they await launch and
deployment. If the Tg rises over this time period, then it is
possible that the spacecraft may not have enough power
to heat the structure above this higher Tg. If the structure
is not heated above its Tg prior to inflation, it will either
not deploy (i.e., it will be frozen in the packed position) or
deploying the structure will cause damage to the fibers.
This could greatly reduce the structural properties of the
composite and thus the overall structure.
Tg=59.4oC
Figure 11. DSC Curve of a TP275 Beam After 7
Months of Room Temperature Storage1
In order to determine what may happen to the resins
during long-term room temperature storage, TP283E and
TP406 composite samples were placed in ovens set at
80oC and 125oC for up to 168 hours. After their
prescribed aging times, the samples were then removed
from the oven and DSC testing was performed on three
samples of each material. This provided three samples
for an average Tg and enthalpy calculation. SMP epoxies,
In the case of SMP epoxies, it is possible that they will
continue to cross-link over time, especially if they are
exposed to high temperatures. The first test that was
conducted to address this issue was a DSC on a TP275
8
American Institute of Aeronautics and Astronautics
both of these factors assist in the determination of the
degree of cross-linking.
E1559,
Method
for
Measuring
Material
Outgassing/Deposition Kinetics, allows for the
exposure temperature and time to be varied over the
duration of the test. This provided a more realistic
evaluation of how outgassing might occur at the
elevated temperatures that structures would witness
during pre-heating and deployment on orbit.
Based on radiation tests results on TP283E’s predecessor,
TP275, and the fact that the TP283E resin is an SMP
epoxy, it was expected that the TP283E resin would
experience an increase in Tg after elevated temperature
exposure. It appears that high temperature (125oC) aging
does increase the Tg of the resin. TP283E samples aged
at 125oC for 1 week had Tg’s of 96oC, while the 80oC
samples had Tg’s of close to 74oC after 1 week of aging.
Furthermore, the higher the aging temperature, the higher
the rate of increase in Tg. Also, based on initial enthalpy
testing, it can also be concluded that the degree of crosslinking for TP283E will increase with thermal aging time
at high temperature, but the degree of cross-linking will
not increase significantly at ambient conditions. TP406
did not experience the same effects of temperature
exposure, as did the lightly cross-linked formulations.
However, there appeared to be a mild increase in Tg with
time at 80oC and 125oC. From the data, the Tg of the
resin actually appeared to increase more (from 55oC to
73oC) from the 80oC aging than the 125oC aging. The
80oC thermal aging test was repeated with similar results.
Further study is needed to understand the possible
chemical reactions between water and the constituents of
the polyurethane that lead to these results.
The test was performed at 125oC for 24 hours under
hard vacuum, to simulate worst case conditions for the
material. The heating duration will be much shorter (on
the order of hours) in practical application. The
temperature selected was based on the glass transition
temperature plus a safety factor. These parameters
were selected to represent the temperatures and times
that a very large structure might see while it is being
heated above its Tg to soften it prior to deployment.
All of the ASTM E595 and ASTM E1559 samples did
not have protective films as they would in actual
application. Therefore these outgassing values are
higher than what would actually be seen at the system
level. Data from each of the 4 materials tested in
ASTM E1559, TP406, TP283E, TP277, and TP279B, is
summarized below in Table 5.
Resin
TP220
TP221
TP230
TP264*
TP275
TP405
TP415
TP277
Epoxy
Resin
VACUUM STABILITY
Outgassing has been investigated for several
thermoplastic resins because of its importance in
spacecraft performance. ILC Dover conducted ASTM
E595 testing that identified the general performance of
SMP resins. The ASTM E595 test was seen as a good
screening test to gauge general resin qualities such as
Total Mass Loss (TML), Collected Volatile
Condensable Materials (CVCM), and Water Vapor
Recovery (WVR). Typical NASA acceptance values
for TML and CVCM are not to exceed 1.0% and 0.1%
respectively. Some values of the preliminary testing
can be seen in Table 4.
Table 4. ASTM E595 Results1
Matrix
TML
CVCM
(%)
(%)
Polypropylene
0.26
0.08
Polypropylene
0.25
0.06
Polyethylene
0.13
0.03
Polyester
0.23
0.02
Epoxy
1.95
1.35
Polyurethane
0.36
0.01
Styrene-Acrylic Latex
1.45
0.02
TP283E
Table 5. ASTM E1559 Results1
Matrix TML(%) CVCM(%) TML(%) CVCM(%)
(4hrs @
(4hrs @ (24hrs @ (24hrs @
125oC)
125oC)
125oC)
125oC)
Epoxy
0.29
0.25
0.75
0.71
TP279B
TP406
0.33
0.28
1.23
1.19
Epoxy
0.28
0.22
0.81
0.77
Urethane
0.33
0.31
0.37
0.35
*All samples baked out for 48 hours at 85oC
ISOGRID BEAM TESTING
From the results of the
material testing, TP283E
SMP lightly cross-linked
epoxy resin was selected
and 0.394-meter and 3.0meter long, 0.1778-meter
diameter isogrid beams
were fabricated and tested
after
packing
and
deployment. These results
were compared to beams
that had not been packed
and deployed. One of
each tow size (24K, 36K,
and 48K) of the 0.4m long
Figure 12. Rolled
isogrid
beams
were
Isogrid Beam1
flattened and rolled in an
oven three times around a
7.62cm mandrel (as in Figure 12). After each folding
trial, the beams were placed back into the oven
unconstrained and allowed to begin their return to their
WVR
(%)
0.03
0.04
0.03
0.16
0.10
0.29
0.13
*No bake out. All other resins were baked out for 60 hours at 60oC.
The ASTM E595 test provided good general
information, but the test conditions did not match those
anticipated in actual use. To better characterize the
outgassing performance of these resins, another
outgassing test, ASTM E1559, was selected. ASTM
9
American Institute of Aeronautics and Astronautics
Table 6. 0.4m Isogrid Compression Load Results1
Tow
Baseline
Packed and Deployed % Change
Size
Compression
3 Times Failure Load (N) Compression Failure
Load (N)
original tubular shape. However, it was discovered that
the shape memory effect of the resin alone was not
enough to fully return the 10.2 cm diameter beams to
their original shape. To assist in shape recovery, an
inflatable bladder had to be inserted into each of the
beams and then pressurized to 1.5psi to bring the beams
back to their original shape. This cycle was repeated
three times. Following the packing trials, the beams
were bonded into aluminum test end caps (Figure 13).
and tested for their compressive and torsion properties
(Figure 14).
24k
509.8
309.6
-39.3%
36k
1016.5
866.1
-14.8%
48k
1832.3
1570.7
-14.3%
Table 7. 0.4m Isogrid Compressive Stiffness Results1
Tow Baseline EA (N) Packed and Deployed % Change
Size
3 Times - EA (N)
24k
1.022E+06
7.249E+05
-29.1%
36k
1.600E+06
1.379E+06
-13.8%
48k
1.980E+06
1.998E+06
0.9%
Table 8. 0.4m Isogrid Torsional Stiffness Results1
Tow Baseline JG (N- Packed and Deployed % Change
Size
m2)
3 Times – JG (N-m2)
Figure 13. Isogrid Beam Prepared for Testing1
Although the packing did degrade the structural
performance of the
beams slightly, the
results are very
promising. The test
results from the
baseline and packed
beams
are
summarized
in
Tables 6-8.
The
peak
compressive
load data from Table
6 is charted in
Figure 15. The load
versus
deflection
curve for the 24K
Figure 14. 0.4m TP283E
Isogrid Beam During Test1
TP283E
0.4m
packed and deployed
isogrid tube is provided in Figure 16.
Compression Failure Load (N)
2000
1400
-7.8%
1218.8
15.6%
48k
1535.3
1802.3
17.4%
TP283E 24K Packed & Deployed Isogrid
Compression Data
Load (N)
250
200
150
100
50
0
0
1
2
3
4
5
6
7
Deflection (mm)
Figure 16. 24K Packed & Deployed Isogrid
Compression Data1
As can be seen from these initial results, larger (36K
and 48K) tow sizes are less affected by packing and
deployment than smaller (24K) tow sizes. In fact, from
the data it can be seen that the torsional stiffness
increases with the larger tow sizes. This data is based
on the average of three baseline isogrid beams and one
packed and deployed isogrid beam. Therefore, the
packed and deployed data is not statistical, but there is a
trend in the data with respect to larger tow sizes.
Further study is required to obtain statistical data and
verify these initial results.
Baseline Compression Failure
Load (N)
Packed and Deployed 3 Times Compression Failure Load (N)
1200
1000
800
600
400
The 3m isogrid beams were packed slightly differently
than the 0.4m beams. After manufacturing the 3m
beams, they were permanently bonded into 0.0508m
aluminum test end caps. The 3m beams were uniformly
200
0
36k
542.1
1054.6
300
Effect of Packing and Deploying on 0.4m
Isogrid Tubes
24k
588.3
36k
350
1800
1600
24k
48k
Tow Size
Figure 15. Effect of Packing on Compressive Load1
10
American Institute of Aeronautics and Astronautics
heated about their Tg and then they were flattened and
rolled around the aluminum test end cap starting at the
tip. Three of the 3m beams (including one beam that
was successfully deployed in a thermal-vac chamber at
–80oC) were packed in this way, while one of the 3m
beams was Z-folded. The beams were allowed to cool
below their Tg in the packed position (Figure 17) and
then they were deployed by inflation at 75oC and
mechanically characterized.
BIBLIOGRAPHY
1. Cadogan, D.P., Lin, J.K, Sapna, G.H.,
Scarborough, S.E., “Space Inflatable Technology
Development for Solar Sails and Other Gossamer
Applications: GR/SMP Isogrid Boom Development
Final Report,” NASA Task Order 10442, ILC Dover,
Inc., October, 2001
2. Cadogan, D. P. and S. E. Scarborough.
“Rigidizable Materials for Use in Gossamer Space
Inflatable Structures,” AIAA-2001-1417, 42nd
AIAA/ASME/
ASCE/AHS/ASC
Structures,
Structural Dynamics, and Materials Conference and
Exhibit AIAA Gossamer Spacecraft Forum, April 1619, 2001.
3. Darooka, D.K., S.E. Scarborough, and D.P
Cadogan, “An Evaluation of Inflatable Truss Frame
For Space Applications,” AIAA-2001-1614, 42nd
AIAA/ASME/
ASCE/AHS/ASC
Structures,
Structural Dynamics, and Materials Conference and
Exhibit AIAA Gossamer Spacecraft Forum, April 1619, 2001.
4. Darooka, D.K., S. Scarborough, S. Malghan, D.
Cadogan, C. Knoll, “Inflatable Space Frame,” Final
Report, NASA Prime Contract Number: NAS199154, July 2000.
5. Hunt, M., Editor, “1992 Materials Selector,” in
Materials Engineering, December 1991.
6. Malghan, S., “Thermoplastic Folding Trials
Report, Phase I” ILC Dover IRAD Report 5500-10-A,
July 28, 2000.
7. Otsuka, K. and Wayman, C.M., Shape Memory
Materials. Cambridge: Cambridge University Press,
pp. 203-219, 1998.
8. Paesano, A. and Palmese, G.R., “Carbon-Fiber
Reinforced Thermoplastic Materials for Rigidizable
Space Systems,” ILC IRAD Supported Work
performed by the University of Delaware Center for
Composite Materials, March 28, 2000.
9. Vinson, Jack R. and Sierakowski, R. L., The
Behavior of Structure Composed of Composite
Materials, Martinus Nijhoff Publishers, Dordrecht,
Netherlands, 1987.
10. U.S. Patent Application, “Deployable Space Frame
and Method of Deployment Therefor,” ILC Dover,
Patent Application No. US00/07706.
Figure 17. 3m Isogrid Tube Packed With End cap1
SUMMARY
Recent shape memory composite developmental work
performed at ILC Dover has greatly advanced the state of
the art of materials that can be used to construct
rigidizable gossamer space inflatable structures. New
materials have been developed and tested at both the
component and system level proving the viability of this
technology. Resins have been developed that meet
NASA spacecraft outgassing requirements, that can be
stored at room temperature for years, and can be
integrated into composite satellite structures that can be
tightly packed for launch into space. Isogrid beams
fabricated from shape memory polymer resins have been
repeatedly packed and deployed to assess damage from
folding.
Reductions caused by packing in peak
compressive load were recorded at levels as low as
14.3%, while compressive stiffness reductions were as
low as 0.9%. A 3-meter long, 0.1778-meter diameter,
shape memory composite, isogrid beam was also
successfully deployed in a thermal vacuum chamber at –
80oC and hard vacuum. Work is continuing at ILC Dover
to further improve resin and composite performance and
manufacturing characteristics.
ACKNOWLEDGEMENTS
Much of this work was in support of the ISAE-II program
funded by NASA and JPL. Special thanks go to Ms.
Jessica Woods-Vedeler and Ms. Judith Watson from
NASA Langley Research Center for their program and
technical support. The authors also thank technical
consultants Dr. Martin Mikulas and Dr. Paul McElroy for
their technical advice.
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American Institute of Aeronautics and Astronautics