mechanical and thermal material properties of restraint
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1© 2018 The Aerospace Corporation
Mechanical and Thermal Material
Properties of Restraint Filaments
for Use in Low Cost Satellites
Joseph Severino, Naoki Oishi
CubeSat Developer’s Workshop 2018
April 30 2018 – May 2, 2018
2
Outline
• Background/Motivation
• Materials Introduction-Fiber Types
• Mechanical Properties
• Thermal Cutting
– DMA (Table/Scatter Plot)
– Heating Coil (Air/Vac)
• Friction Coefficients
• CTE under Load
• Conclusion
3
Background/Motivation
• Complexity of CubeSats is expanding with new
deployable structures
– Antenna -- Booms
– Solar panels -- Others
• Lightweight and compact actuating mechanisms
necessary to fit into restrictive cube format.
– Shape memory alloy latches
– Paired CTE alloys
– Nichrome burn wires
• low mass | compact | simple
• Various configurations of burn wires
– all rely on melting a thermoplastic filament that restrains
components
• Goal here was to evaluate the materials properties of
various low cost filament materials (fishing line) used in
CubeSats to determine the advantages and
disadvantages of their application. Patented nichrome
cutting mechanism
Japanese
Waseda-SAT3
Filament
Filament
[2][3]
[1]
[1] Miyashita et al. Expansion and Measurement of Spiral Folded Membrane by Small Satellite. 55th AIAA Aerospace Sciences Meeting, Grapevine,
Texas, January 9 - 13 2017
[2] Thurn et al. A Nichrome Burn Wire Release Mechanism for CubeSats. Proceedings of the 41st Aerospace Mechanisms Symposium, Jet
Propulsion Laboratory, May 16-18, 2012
[3] US20150102172A1, Burn Wire Release Mechanism for Spacecraft and Terrestrial Applications
4
Materials
Trade Name Material Formula
MonofilamentPolyamide
(Nylon)
FluorocarbonPolyvinylidene fluoride
(PVDF)
BraidPolyethylene
(UHMWPE)
DacronPolyethylene terephthalate
PET
m
5
Mechanical Properties
Only UHMWPE has significantly different stiffness and strength within group
• Compared the 4 types of filaments
– UHMWPE far out performs rating
– PET underperforms but has low
scatter
• Compared filament rating of common
Nylon
– Properties similar but failure load
increases
– Congruent to increase in filament
diameter
0
5
10
15
20
25
30
35
0 5 10 15 20 25 30
Lo
ad
(lb
s)
Strain (%)
UHMWPE
Nylon
PVDF
PET
Material Rating Failure Load Failure Strain DiameterLbs Lbs % mil
UHMWPE 10 32.3 ± 3.2 2.7 ± 0.4 8PVDF 10 10.7 ± 0.5 26.5 ± 2.8 11PET 12 10.2 ± 0.1 27.0 ± 0.6 15
Nylon 4 7.2 ± 0.2 24.9 ± 1.4 8Nylon 8 11.9 ± 0.9 23.7 ± 3.2 11Nylon 10 13.4 ± 0.6 24.1 ± 2.0 12Nylon 12 16.7 ± 0.5 26.1 ± 2.8 130
5
10
15
20
0 5 10 15 20 25 30
Lo
ad
(Lb
s)
Strain (%)
Nylon 12Lb
Nylon 8lb
Nylon 4lb
6
Nichrome Coiled Wire Cutting
Filaments cut instantaneously well below incandescence of nichrome wire
V
Nichrome Coil
Filament
1.1 N
Weights DC Power Supply
0
0.5
1
1.5
2
2.5
PE PVDF Nylon PET
Wat
ts
Filament Thermal Cutting
0
100
200
300
400
500
0 0.5 1 1.5 2 2.5
Tem
per
atu
re (°C
)
Watts
Thermocouple Temperature
• Investigated hot wire filament cutting in
air and vacuum
• Voltage ramped at 5 V/min until filament break
• Wattage recorded at fiber break
– Breaks were instantaneous
– Filament could consistently withstand
load when 0.01 Watts below breaking
point
• Excessive power (wire glowing red hot) was
not necessary to cut any of the filaments
– Nichrome incandescence at ~550 °C
7
Thermal Cutting
Filament breaking temperature dominated by melting point
Liquid nitrogen feed
Furnace
(open position)
Filament
-100
102030405060708090
100
25 50 75 100 125 150 175 200 225 250
Stra
in (
%)
Temperature (°C)
PETNylonUHMWPEPVDF
0.001
0.01
0.1
1
10
25 45 65
Filament DSC Melting Point (°C)
UHMWPE 156
PVDF 164
Nylon 160
PET 254
• Directly measured breaking temperature with thermal
ramp/creep test
– 5 °C/min | 4 N tension
• Breaking temperature corresponded to melting point
• Minor inflection around 40 °C
– Glass transition of Nylon 25 to 45 °C
8
Coefficient of Thermal Expansion
CTE can be negative or positive and structure changes well before melting/break
Furnace (open position)Filament
-15
-10
-5
0
5
-100 -50 0 50 100 150 200
Mic
rostr
ain
(µ
ε)
Temperature (°C)
Nylon
PET
PVDF
UHMWPE
Filament CTE (µε/°C)
UHMWPE -22.5
PVDF 30.0
Nylon 23.3
PET -7.0
• Observed thermal expansion of filaments with 0.1 N
load using TMA system
• All filaments shrunk at elevated temperature in
contrast to measurements made at 4N
– Low loads here
– Polymer softening
– Entropy
9
Coefficient of Friction
UHMWPE good for routing but nylon easier to hold without slip
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Stainless Steel Aluminum Silicone Rubber
Coe
ffic
ien
t o
f F
riction
Nylon PET PVDF UWMWPE
𝑇𝑙𝑜𝑎𝑑 = 𝑇ℎ𝑜𝑙𝑑 × 𝑒𝑥𝑝 𝜇𝜑
𝜇 = 𝐶𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 𝑜𝑓 𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛𝜑 = 𝑎𝑛𝑔𝑙𝑒 𝑜𝑓 𝑐𝑜𝑛𝑡𝑎𝑐𝑡
Test
Arrangement
𝑇𝑙𝑜𝑎𝑑 𝑇ℎ𝑜𝑙𝑑
𝜑
• Used a capstan approach to measure sliding friction
of the fibers on 3 materials
– Stainless steel | aluminum | silicone rubber
• Consideration for complex routing or termination
approaches
10
Pinched Tensile Test
Larger diameter filaments and braids are more reliable with crush/pinch
0
5
10
15
20
25
0
100
200
300
400
500
600
700
800
900
100
0
110
0
Bre
akin
g L
oa
d, lb
f
Compressive Load, lbs
Nylon 4Lbs
0
5
10
15
20
25
0
100
200
300
400
500
600
700
800
900
100
0
110
0
Bre
akin
g L
oa
d, lb
fCompressive Load, lbs
Nylon 12Lbs
0
5
10
15
20
25
0
100
200
300
400
500
600
700
800
900
100
0
110
0
Bre
akin
g L
oa
d, lb
f
Compressive Load, lbs
PET 12Lbs
0
5
10
15
20
25
0
100
200
300
400
500
600
700
800
900
100
0
110
0
Bre
akin
g L
oa
d, lb
f
Compressive Load, lbs
UHMWPE 10Lbs
Small diameter less reliable
Braided PET has low variance
Filament rating
Filament
Average Load
Individual Load
11
Conclusions
• It is necessary to exceed the melting point of the polymer to reliably cut the
filament but it is not necessary to far exceed this temperature.
– Incandescent heating of nichrome wire unnecessary
• Cutting the filament was easier in vacuum and required less power than when
air provided convective cooling.
• UHMWPE braids appear to be the most robust filament based on strength,
cutting temperature, low friction and pinch resistance; however, unique
circumstances could make any of these filaments beneficial.
• Work still needed
– Creep
– Abrasion
– Knot retention
– Contamination and outgassing
– Space environments / radiation stability
13
Fourier Transform Infrared SpectroscopyVerification of filament materials
PET
PET Match
PVDF
PVDF Match
Nylon
Nylon Match
UHMWPE
HMPE Match
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