Residual strength results are presented on four composite material systems that were exposed for three years at locations on the North American Continent. The exposure locations are near the areas where Bell Model 206L Helicopters, that are in a NSA/U.S. Army sponsored flight service program, are flying in daily commercial service. The composite systems are: (1) Kevlar-49 fabric/F-185 epoxy; (2) Kevlar-49 fabric/LRF-277 epoxy; (3) Kevlar-49 fabric/CE-306 epoxy; and (4) T-300 Graphite/E-788 epoxy. All material systems exhibited good strength retention in compression and short beam shear. The Kevlar-49/LRF-277 epoxy retained 88 to 93 percent of the baseline strength while the other material systems exceeded 95 percent of baseline strength. Residual tensile strength of all materials did not show a significant reduction. The available moisture absorption data is also presented

Baker, D. J.

English

NASA Technical Reports Server

NASA Technical Me morandum
AVSCOM Technical Me morandum
GROUND EXPOSURE OF COMPOSITE MATERIALS
FOR HELICOPTERS
DONALD J. BAKER
OCTOBER 1984
NI\5I\
National Aeronautics and
Space Administration
Langley Research Center
Hampton, Virginia 23665
86311
TM-84-B-2
NASA-TM-86311 19840026506
liBRARY COpy
LANGLEY RESEARCH CENTER
LIBRARY, NASA
HAMPTON, VIRGINIA
https://ntrs.nasa.gov/search.jsp?R=19840026506 2020-03-20T21:34:00+00:00Z

GROUND EXPOSURE OF COMPOSITE
MATERIALS FOR HELICOPTERS
Donald J. Baker
U.S. Army
Structures Laboratory
Research and Technology Laboratory
Langley Research Center
Hampton, VA. 23665
(AVSCOM)
Abstract
Residual strength results are presented
on four composite mat~rial systems that
have been exposed for three years at loca-
tions on the North American Continent.
The exposure locations are near the areas
where Bell Model 206L Helicopters, that
are in a NASA/U.S. Army sponsored flight
service program, are flying in daily com-
mercial service. The composite systems
are 1.) Kevlar-49 fabric/F-185 epoxy,
2.) Kevlar-49 fabric/ LRF-277 epoxy,
3.} Kevlar-49 fabric/CE-306 epoxy, 4.}
T-300 graphite/E-788 epoxy. All material
systems exhibited good strength retention
in compression and short beam shear. The
Kevlar-49/LRF-277 epoxy retained 88 to 93
percent of the baseline strength while
the other material systems exceeded 95
percent of baseline strength. Residual
tensile strength of all materials did not
show a significant reduction. The avail-
able moisture absorption data are also
presented.
Introduction
During the past ten years, NASA has
sponsored programs to build a data base
and establish confidence in the long-term
durability of advanced composite materials
(reference I). Fl ight service experience
is being obtained on primary and secondary
structural compbnents installed on commer-
cial aircraft and from material specimens
exposed at different locations. Although
commercial aircraft and helicopters may
fly in the same environment the behavior
of composite materials on each vehicle
may differ substantially. Most bf the
projected usage for composites in heli-
copter fuselage is Kev1ar/epoxy with
selective reinforcement of graphite/
1 epoxy using 250°F curing epoxies. Most
'commercial aircraft are using 350°F cure
graphite/epoxy systems with very little
use of Kev1ar/epoxy. Considering only the
effects of moisture, materials in the
minimum gage structure in most helicopter
fuselage structure would reach the
equilibrium moisture content in a short
time whereas the heavier gage structure on
a commercial aircraft could take months to
reach an equilibrium moisture condition.
Therefore, in 1978, NASA and the U.S.
Army Research and Technology Laboratory
initiated the first major program to
evaluate composite helicopter components
in flight service. The flight service
program includes four components per
aircraft. There are three secondary
structural components fabricated from
Kevlar-49/epoxy and one primary structural
component fabricated from graphite/epoxy
per aircraft. Concurrent with the flight
program, specimens from materials used to
fabricate the components are being exposed
in ground racks and are being returned for
testing at prescribed intervals.
This paper describes the results of
tests on specimens that have been exposed
for the first three years of a planned
ten (10) year ground exposure program.
Use of commercial products or names of
manufacturers in this report does not
constitute official endorsement of such
products or manufacturers, either expressed
or implied, by the National Aeronautics
and Space Administration or the U.S. Army
Research and Technology Laboratories.
Exposure Specimens
Composite material systems are being
exposed on the North American Continent in
areas that have varyi ng envi ronmental
conditions. The composite systems are:
I.} Kevlar-49 fabric (style 281}/F-185
epoxy, 2.) Kevlar-49 fabric(style
120}/LRF-277 epoxy, 3.} Kevlar-49
fabric (style 281)/CE-306 epoxy, and 4.)
T-300 Graphite tape/E-788 epoxy. The
F-185 epoxy is a 250°F cure system
manufactured by Hexcel Corp. The LRF-277
is a 250°F cure proprietary resin
manufactured by Brunswick Corp. The
CE-306, manufactured by Ferro Corp., is
cured at 200°F for five (5) hours. The
E-788 is a 350°F cure epoxy manufactured
by U.S. Polymeric. Style 281 Kevlar-49
fabric is a plain weave with 17 ends/inch
of 1140 denier yarn in each di rection and
has a weight of 5.0 ounces per square
yard. Style 120 fabric is a plain weave
~ith 34 ends/inch of 195 denier yarn in
'each direction and has a weight of 1.8
ounces per square yard.
1
r
I The materials are exposed in racks
(figure 1) at five (5) locations on the
North American Continent as shown in fig-
ure 2. The racks at Toronto, Canada and
Hampton. VA are installed on the roofs of
buildings. Racks at Cameron, LA and Ft.
Greely, AK are installed on stands
approximately 18 inches above the ground.
The remaining rack is on a working oil
platform in the Gulf of Mexico. The racks
were installed in 1980 and contain five (5)
panels each for removal after 1, 3, 5, 7,
and 10 years of exposure. A panel contains
24 each of tension, short-beam-shear (SBS),
IITRI compression specimens and four (4)
2.0 inch wide specimens to observe the
weathering characteristics of each mate-
r i a1 sy stem. The s peci me n des i gn i s
shown in figure 3and is also given in
reference 2. The tension, compression and
SBS specimens are painted with a polyure-
thane paint (DuPont IMIRON) that is used
on the flight service helicopters. The
remaining specimens were left unpainted
to determine the weathering effect on
on bare composites.
The specimens used for moisture
determination were cut from the tested
tension specimens. A 0.5 inch long
section was cut from the undamaged area of
the tension specimens as soon as possible
after completion of testing. The paint
was removed by sanding, using caution not
to remove an excessive amount of the outer
ply. Each specimen was weighed after the
paint removal. A 0.5 inch long specimen
was also removed from the unpainted
exposure specimen and weighed prior to
being used for moisture determination.
All specimens were stored in sealed
plastic bags between different operations.
Test Methods
Each panel was received at Langley
Research Center sealed in a plastic bag.
The panel remained in the sealed bag until
testing was initiated. All tests were
performed at room temperature on six (6)
repl i cates fo reach s pecime n ty pe. The
tests were performed in accordance with
the following ASTM standards; 1.)
Tension-D3039,2.) SB5-D2349,3.)
Compression-D3410 using the IITRI test
fixture.
The specimens used for moisture
determination were placed in a vacuum oven
at 140°F. Each specimen was weighed
periodically to determine weight loss as a
function of drying time.
2
Results and Discussion
Average baseline strengths for the
as-fabricated exposure specimens are given
in Table 1. Residual compression strength
data for painted specimens after three
years of exposure are shown in figure 4.
The data points represent a comparison of
the average baseline compression strength
with the average compression strength data
of specimens exposed at the five different
locations shown in figure 1. All material
systems exhibit good strength retention in
compression. The Kev1ar-49/LRF-277 has
the lowest strength retention of 90
percent and 88 percent after one and three
years of exposure respectively. The other
materials have a residual compression
strength greater than 95 percent of the
baseline strength. Effect of environment
on the residual compression strength for
each materi al is shown in fi gures 5
through 8. The data points represent a
comparison of the average compression
strength at each exposure site with the
average baseline compression strength
value for that material system. The
Kevlar-49/LRF-277 material(figure 6) has
lower compression strength retention than
the other materials. The Kevlar-49/
LRF-277 varies from 84 percent to 94 per-
cent after one year exposure and 85 per-
cent to 91 percent after three years of
exposure while the other material systems
exceeded 90 percent after one year of
exposure and 94 percent after three years
of exposure. The strength retention
trends compare well with pUblished data
for other graphite/epoxy and Kevlar/epoxy
systems used in NASA's worldwide exposure
pro gram (r e fer ence 1).
,
I Residual SBS strength data for the
painted specimens with three years of
exposure are shown in figure 9. The
Kevlar-49/LRF-277 material has the lowest
SBS strength retention of 93 percent and
91 percent after one and three years of
exposure, respectively. The other
materials have a residual SBS strength
greater than 97 percent of the baseline
strength. Effect of environment on the
residual SBS strength for each material is
shown in figures 10 through 13. In
general the Kevlar-49/LRF-277 material has
lower SBS strength than the other material
systems.
The tension specimens did not show any
significant reduction in strength from
exposure.
Moisture specimens from panels removed
from the Gulf of Mexico, Cameron, LA and
Hampton, VA have been drying for approxi-
mately six months. Specimen weight loss as
a function of drying time is shown in fig-
ure 14 for Kevlar-49/LRF-277 material that
'Was exposed at Cameron, LA. The triangle
symbol is the weight change of one specimen
that was weighed at regular intervals. The
other 5 specimens are shown with a square
symbol. The trend of data shown in figure
14 is typical of all painted specimens
removed from the exposure sites at Cameron,
LA, Gulf of Mexico, and Hampton, VA. Most
specimens are approaching equilibrium. A
curve has been faired through the data
points. A summary of weight loss at 6
months drying time is shown in Table 2.
Kevlar-49/epoxy absorbs approximately
three times more moisture than graphite/
epoxy because Kevlar fibers absorb mois-
ture. The average values shown in Table
2 compare well with published values for
other Kevlar/epoxy and graphite/epoxy
systems (reference 1). The dryi ng of speci-
imens removed from the unpainted material
is approximately six weeks behind the first
three panels tested. Weight loss as a
function of drying time for unpainted
Kevlar-49/LRF-277 is shown in figure 15
where a curve has been faired through the
data points. Comparing the data in
figures 14 and 15 at a drying time of 4
months indicates the unpainted Kevlar-49/
LRF-277 specimens had approximately 0.1
percent greater weight loss. A summary
of weight loss on unpainted specimens at
4 1/2 months drying time is given in Table
3. All values in Table 3 appear reason-
able except for the 0.75 percent weight
loss for the T-300/E-788 material exposed
at the Gulf of Mexico. This specimen had
a higher weight loss during the drying
time as shown in figure 16 which shows
data for specimens from all three sites.
Because there is only one specimen avail
able it is not possible at this time to
determine if this is a valid data point.
Comparing the average weight losses for
Kevlar-49/F-185 and Kevlar-49/CE-306 in
Tables 2 and 3 indicates that the painted
specimens retain more moisture than the
unpainted materials. The average weight
loss for both painted and unpainted
Kevlar-49/LRF-277 material were approxi-
mately equal. The T-300/E-788 materi al
exposed at Cameron, LA and Hampton, VA
,also retained more moisture when painted.
Concluding Remarks
Results of three years of ground expo-
sure indicates that all material systems
exhibited good strength retention in com-
pression and short beam shear. The
Kevlar-49/LRF-277 epoxy retained 88 to 93
percent of the baseline strength while the
other material systems exceeded 95 percent
of baseline strength. Residual tensile
strength of all materials did not show a
significant reduction.
3
The available data on moisture absorp-
tion indicates the Kevlar-49/LRF-277 mate-
erial absorbs the same moisture whether
the material is painted or unpainted. The
other material systems (Kev1ar and graphite
absorb more moisture when painted.
References
1. Dext er, H. Benson; and Bake r, Donal d J.:
Worldwide Flight and Ground-Based
Exposure of Composite Materials,
NASA CP 2321, p.17-49, August 1984.
2. Zinberg, Herbert: Flight Service
Evaluation of Composite Components
on the Bell Model 206L: Design,
Fabrication and Testing,
NASA CR-166002, November 1982.
Table I.-Baseline Strengths of
as-fabricated Material Specimens
Strength (psU
Material Short Beam Shear Compression TensionSystems
Mean* S.D.· Mean* S.D.· Mean* S. D.·
Kevlar-49/
F-185 6018 197 20176 489 57363 2448
Kevlar-49/ 3873 119 22363 909 83658 2198LRF-277
Kevlar-49/ 5277 258 18265 337 61090 2917CE-306
1-300/ 11222 285 126343 4025 126478 4209E-788
*) Mean of 6 Specimens
.) S. D. - Standard Deviation
Table 2.-Weight loss of Painted Specimens
after 3 years Exposure Time
Weight Loss (percent)
Material Drying Exposure SiteTimeSystems (Days) Cameron Gulf of Hampton, Avg.
lao Mexico Va.
Kevlar-49/ 182 2.53 2.51 2.13 2.39F-185
Kevlar-49/ 182 2.17 2.05 1.98 2.07LRF-277
Kevlar-49/ 182 2.05 1.92 1.81 1. 93CE-306
T-3OO/ 182 0.77 0.73 0.76 0.75E-788
Table 3.-Weight loss of Unpainted Specimens-
after 3 years Exposure Time
Weight loss (percent)
Material Drying Exposure SiteTimeSystems Ways) Cameron Gulf of Hampton, Avg.
La. Mexico Va.
Kevlar--491 147 1. 63 1.83 1. 47 1.64H85
Kevlar-491 147 2.00 2.30 2.05 2.12lRF-277
Kevlar--491 147 1.82 1.93 1.47 1.74CE -306
T-3001 147 0.46 0.75 0.43 0.55E-788
T --0.015-0.10
7.00 T
1 4 50 - - 0.075 - O. 10T 1· ~ .60 -11- .075 - 0.10~ -.L TO 0
1. 00H -11-. 05 .25 -11--11-. 050 -11-.25
Tension Specimen Compression Short Beam
Specimen Shear Specimen
All Dimensions Shown in Inches
Figure 3.-Specimen geometry
40
100
Residual 80
Strength,
Percent
Baseline 60
o Kevlar Fabric (281)/CE -306 Epoxy
6 Kevlar Fabric (l2Q)!lRF-277 Epoxy
t::. Kevlar Fabric (281)/F-IB5 Epoxy
o T-300 GraphitelE -788 Epoxy
L-L_.~~
024 6 8 W
Exposure Time, Years
4.-Average Residual Compressive
Strength of Composite Materials
after Exposure at five dif-
ferent locations
Figure
, Figure I.-Environmental Exposure Rack
with Specimens Installed
40
o
Figure
100 ~ 0R
~
Residual 80 o Cameron, La.
strength, o Gulf of MexicoPercent o Ha mpton, Va.Baseline 60 /:). Ontario, Canada
~ Ft. Greely, Alaska
2 4 6
Exposure Time, Years
Figure 5.-Residual Compression Strength
of Kevlar-49/F-185
8
4
100 100 8 ~
§ ~ ~<> ~~
Residual 80 Residual 80
strength, o ca meron, La. Strength, o cameron, La.
Percent o Gulf of Mexico Percent o Gulf of Mexico
Baseline 60 o Hampton, Va. Baseline 60 o Hampton, Va./)" Ontario, Canada /)" Ontario, Canada
~ Fl Greely, Alaska ~ Ft. Greely, Alaska
40 40
L.___J J I L_-.J_~
0 2 4 6 8 0 2 4 6 8
Exposure Time, Years Exposure TI me, Years
Figure 6.-Residual Compression Strength Figure 8.-Residual Compression Strength
of Kevlar-49/LRF-277 of T-300/E-788
<> <>
, 100
Residual 80
strength,
Percent
Baseline 60
o Cameron, La.
o Gulf of Mexico
o Hampton, Va.
/)" Ontario, canada
~ Fl Greely, Alaska
lOll ~ &
A A
Residual 80
Strength, o Kevlar Fabric 128U/cE-306 Epoxy
Percent A Kevlar Fabric U201/LRF-277 Epoxy
Baseline 60 t:.. Kevlar Fabric 128U/F-185 Epoxy¢ BOll Graphlte/E-788 Epoxy
40 40
102 4 6 8
Exposure Time, Years
L,_--.L-_---l..._-...l._----JL.------I
o82 4 6
Exposure Time, Years
o
Figure 7.-Residual Compression Strength
of Kevlar-49/CE-306
Figure 9.-Average Residual Short Beam
Shear Strength of Composite
Materials after Exposure at
five different locations
5
100 0 100 ~ ~~ 0~ 0
Residual 80 80Residual
strength, o Cameron, lao Strength, o Cameron, lao
Percent o Gulf of Mexico Percent o Gulf of Mexico
Baseline 60 o Hampton, Va. Baseline 60
o Hampton, Va.
I:::. Ontario, Canada I:::. Ontario, Canada
~ Ft. Greely. Alaska ~Fl Greely, Alaska
40
o 246
Exposure Time, Years
8
40
o 246
Exposure Time, Years
8
Figure 10.-Residual Short Beam Shear
Strength of Kevlar-49/F-185
100
Figure 12.-Residual Short Beam Shear
Strength of Kevlar-49/CE-30'
100
Residual 80
Strength,
Percent
Baseline 60
40
o Cameron, La.
o Gulf of Mexico
o Hampton, Va.
I:::. Ontario, Canada
~ Ft. Greely, Alaska
Residual 80
Strength,
Percent
Baseline 60
40
o Ca meron, La
o Gulf of Mexico
o Hampton, Va.
I:::. Ontario, Canada
~ Ft. Greely, Alaska
o-L_--'-__.1...-1__-.L_--.J
02468
Exposure Time, Years
o 2 4 . 6
Exposure Time, Years
8
Figure ll.-Residual Short Beam Shear
Strength of Kevlar-49/LRF-277
6
Figure l3.-Residual Short Beam Shear
St r e ngt h 0 f T- 300/ E- 788
1.0
o
2.5
2.0
Weight 1.5
Loss,
Percent 1.0
0,5
o
o 0 0 __ -{:;"
t{fr-&1"-&-i1--C.--~----~--- ~
" 0I,
I
I
I C. Speci men Weighed Periodically
o Remaining 5 Specimens
.8
.6Weight
Loss,
Percent .4
.2
o
D. Ha mpton, Va.
OGulf of Mexico
+ Cameron, La.
0.0 1 2 3 4
Drying Time, Months
5 6 o 1 2 3 4
Drying Time, Months
5
Figure 14.- Weight loss for painted
Kevlar-49/LRF-277 Specimens
from Cameron,LA.
Figure 16.-Weight loss for unpainted
T-300/E-788 Specimens.
2.5
2.0
. 1.5Weight
loss,
Percent 1.0
0.5
o 234
Drying Time, Months
5 6
Figure IS.-Weight loss for Unpainted
Kevlar-49/LRF-277 Specimens
from Cameron,LA.
7
.--~~~;.TM_~~~~;TM;:86311 r2-G~~7,:;;.~-~_'-t:t:'_-S._':_~-_'N_O_' -1f-:3_'-=-R_llC_IP_,e-::nl~'S_Ca__tAI._log_N_O. --1
4. Tille dnd SulJIIlIl! 5, Report Dilte
Ground Exposure of Composite Materials for Helicopters October 1984
6. Performing OrganizAtion Code
505-42-23-03
7. AUlhorls1
Donald J. Baker
8. Performing Organization Report No.
..
Technical Memorandum
11. Contract or Grant No.
14. Sponsoring Agency Code
(AVSCOM)
12 5l)On$orlOg A!JIlncy Nallle dnd AddrMS
National Aeronautics and Space Administration
Washington. DC 20546 and
U.S. Army Aviation Systems Command. St. Louis. MO 631661--.-~:..:.:~...::..:.:..::~ ...:..:.:...:..:.::..:..:::-:::!.::..::::.:.....:..::::::.:.:::..:.:.:::.!-,.::.:...:..-:::..::..:....:.!....:.:.::..-::.=..=..:::.::.....1_ _,
15. s..pplllllllmldry Notes
The author is employed by the Structures Laboratory. USARTL (AVSCOM). This paper
was presented at the AHS National Specialist's Meeting-Helicopter·Testing Technology.
October 29-November 1. 1984. Williamsburg. VA.
1------------------------------1 10. Work Unit No.
9. PllrfollOing Org.mllatlon IIIdme dnd Addresa
Structures Laboratory. USARTL
NASA-Langley Research Center
Mail Stop 188B
...----.:.H;.::a;;.:.m:c...::.D1t.=.0:.,:.nz.-.V:...:.A..::...--=2:.;:3-=6..:;6..:;5 .. --1 13. Type of Report and Period Covered
16 AbslldCI
Residual strength results are presented on four composite material systems that have been
exposed for three years at locations on the North American Continent. The exposure loca-
tions are near the areas where Bell Model 206L Helicopters. that are in a NASA/U.S. Army
sponsored flight service program. are flying in daily commercial service. The composite
systems are; 1.) Kev1ar-49 fabric/F-185 epoxy. 2.) Kev1ar-49 fabric/LRF-277 epoxy. 3.'
· Kevldr-49 fabric/CE-306 epoxy. 4.) T-300 Graphite/E-788 epoxy. All material systems
! exhibited good strength retention in compression and short beam shear. The Kevlar-49/
LRF··27J epoxy retained 88 to 93 percent of the baseline strength while the other material
· sy~t~~s exceeded 95 percent of baseline strength. Residual tensile strength of all
· '1dt~"'als did not show a signifkant reduction. The available moisture absorption data
i ':S a! so presented •
...--.-.---------------------r-:~~--:--:::-:---_:_----------"_"1
11 Key Words (Su'J9Mtlld bV Authorlsll 18. Distribution Statement
Unclassified-Unlimited
Subject Category-24
Composite Materials
Environmental Evaluation
I,. s«"~., a.n,l. '"..h........'
Unclassified
20. Securitv C1.ds.f lef .this pagel
Unclassified
21. No. of Pages
8
22. Pl'ice
A02
For sa~ ~v the NatIOnal Technical InfOllnatlon Sp.rvlce. Spflilgfleld, VlIgllllit 22161




Ground exposure of composite materials for helicopters

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