Short-term compressive creep tests were conducted on three titanium alloy Ti-6Al-4V coupons at three different stress levels at a temperature of 714 K (825 F). The test data were compared to several creep laws developed from tensile creep tests of available literature. The short-term creep test data did not correlate well with any of the creep laws obtained from available literature. The creep laws themselves did not correlate well with each other. Short-term creep does not appear to be very predictable for titanium alloy Ti-6Al-4V. Aircraft events that result in extreme, but short-term temperature and stress excursions for this alloy should be approached cautiously. Extrapolations of test data and creep laws suggest a convergence toward predictability in the longer-term situation

Jenkins, J. M.

English

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NASA Technical Memorandum 86033 NASA-TM-8603319840010617 
Effect of Creep in Titanium Alloy 
'-i .. 6AI .. 4V at Elevated Temperature 
on Aircraft Design and Flight Test 
Jerald M. Jenkins 
February 1984 
NI\SI\ 
National Aeronautics and 
Space Administration 1111 IIII 11111 UIII IIIII IU" IIII 1111 
NF00873 
https://ntrs.nasa.gov/search.jsp?R=19840010617 2020-03-20T23:21:56+00:00Z
NASA Technical Memorandum 86033 
IEffect of Creep in Titanium Alloy 
Dri .. 6 1 .. 4V at Elevated Temperature 
on Aircraft Design and Flight Test 
Jerald M. Jenkins 
NASA Ames Research Center, Dryden Flight Research Facility, Edwards, California 93523 
1984 
National Aeronautics and 
Space Administration 
Ames Research Center 
Dryden Flight Research Facility 
Edwards, California 93523 
EFFECT OF CREEP IN TITANIUM ALLOY Ti-6Al-4V AT 
ELEVATED TEMPERATURE ON AIRCRAFT DESIGN 
AND FLIGHT TEST 
Jerald M. Jenkins 
INTRODUCTION 
Studies which are directed toward pr~dicting creep strains 
in built-up structures are critically dependent on adequate material 
characterization. Available literature (references 1 through 3) 
indicates large excursions in material behavior in coupon creep 
testing of titanium alloy Ti-6Al-4V. It is the purpose of this 
report to add to the data base by reporting the results of elevated 
temperature compressive creep tests on three coupons of this alloy 
and to relate this information to aircraft design and flight test. 
The three coupons were compressed at a temperature of 714K (825FO) 
and at2three different stress levels 2ranging from 60.12 MPa (8720 Ibs/in) to 104.45 MPa (15150 Ibs/in). The creep strains resulting 
from the tests are compared to several known creep laws and the 
implications on aircraft design and flight .testing are identified. 
e 
t 
T 
SYMBOLS 
natural logarithm base of value 2.7183 
strain due to creep, ~m/m (~in/in) 
stress, MPa (lbs/in2 ) 
time, hours 
temperature, K (FQ) 
TEST PROCEDURE 
Titanium Coupons 
A photograph showing two of the titanium alloy Ti-6Al-4V 
coupons is presented in figure 1. The specimens were machined 
from .635 cm (.25 in) thick annealed sheet stock. The long axis 
of the specimen corresponds to the longitudinal axis of the sheet. 
The test section has the dimensions of .635 cm (.25 in) by 1.905 
cm (.75 in). The overall length of the specimen is 13.970 cm 
(5.50 in). In order to maintain optimum buckling strength of the 
specimens, special end fixtures were machined to provide a fixed 
(zero rotation) end condition. One of the specimens is shown in 
the special fixture in the background of figure 1. 
Instrumentation 
The coupon instnmentation consisted of strain gages to measure 
the creep strain and thermocouples to determine specimen tempera-
tures. Two high temperature weldable strain gages were mounted 
opposite each other on the two 1.905 cm (.75 in) faces near the 
center of the specimen. Chromel-alumel thermocouples were located 
on the surface of the specimen to control the radiant heating and 
to monitor the uniformity of the heating. The instrumentation can 
be seen in figure 1. 
Several corrections were required to obtain valid data from 
the weldable strain gages utilized at elevated temperatures during 
the tests. After the strain gages were installed on the specimens, 
the specimens were heated slowly in an oven up to the test temp-
erature several times so that apparent strain information was 
available for each of the strain gages. This information was 
used to correct the test data for apparent strain. The test data 
were also corrected for temperature induced gage factor changes 
for each of the strain gages as defined by the manufacturer. 
2 
Heating and Loading 
A sketch of the general test set-up is shown in figure 2. The 
loading system consisted of a loading bar on which dead weights were 
hung& The basic mechanical advantage of the lever was approximately 
four. The system was carefully constructed, fabricated, and posi-
tioned so that bending moments in the specimen were minimized. The 
special end fixtures shown in figure 1 were also required to provide 
fixed end conditions. This allowed a suitable specimen length to be 
used without danger of column buckling during the tests. 
A radiant heating system was constructed around the test spec-
imen as shown in figure 3. The temperatures were controlled with 
surface thermocouples associated with a closed loop system. The 
specimen was enclosed in a chamber with insulation so that an oven 
situation could be maintained as much as possible. 
The basic test procedure consisted of slowly (approximately 
twenty minutes) heating the specimen from room temperature up to the 
test temperature of 714K (825FO). Once the test temperature was 
reached, it was held at that point for approximately thirty minutes 
to reach a near steady state temperature situation. At that time 
dead weights were added to the end of the loading bar to achieve 
at the specimen location a compressive force which would result in 
the desired stress in the specimen. The force and the heat were 
applied to the specimen for an excess of four hours. Three different 
specimens were tested at three different stress levels, but at 
identical temper~tures. The first specimen was compressed at 60.12 
MPa (8720 lbs/in2 ), the second at 86.18 r~a (12500 Ibs/in2), and the 
third at 104.45 MFa (15150 lbs/in2). All three specimens were 
compressed while at a test temperature of 7l4K (825FO). 
RESULTS AND DISCUSSION 
The results of the creep test are-presented in figure 4. Cum-
ulative creep strains one, two, three, and four hours after the onset 
of heating and loading are plotted on logarithmic scales for the 
three test specimens. The short duration of the test clearly limits 
the scope of the information to short term or primary creep. Although 
the creep test is of limited time duration, several creep laws were 
selected from available literature for comparison purposes. 
Four different creep laws are presented in figure 4 for the 
titanium alloy Ti-·6Al-4V tested in tension. Equation 1 was obtained 
from reference 1 and resulted from hand faired curves derived from 
test data: 
3 
Ln€ = -24.09 + 22.54T + .00000602 + .905Lno + .433Lnt 
c 
(1) 
Equations 2, 3, and 4 were obtained from curve fitting 
exercises in reference 2. Equations 2, 3, and 4 are, respectively: 
€ = 
C 
E: = 
C 
C3045 ) 
1.14100562 t o162 e T 
Ln€ = -24.9 + 21.4T + 1.16Lno + .634Lnt + .0062(Lnt)2 + 
c 
.00000702 - .0332 Ln°TLnt 
(2) 
(3) 
(4) 
It is apparent from examination of references 1 and 2 that many 
factors affect the rate of creep at elevated temperatures. This 
observation is further substantiated by examining reference 3. In 
addition to the three primary parameters, stress, temperature, and 
time, it is obvious that parameters such as material thickness, 
discrete lot, processing direction, and other factors clearly impact 
the results of characterization tests. The difficulties with curve 
fitting the inconsistent creep data of reference 2 is well substan-
tiated when the standard errors of equations 2, 3, and 4 are examined. 
The standard error of the estimate for equations 2, 3, and 4, based 
on the natural logarithm of the strain are .6009, .6234, and .4360 
respectively. 
The lack of correlation of the test data of figure 4 with the 
four creep laws is not totally unexpected when the large variations 
among the creep laws is considered. It should be noted that 
extrapolations of the test data and the creep laws appears to result 
in a convergence at an estimated value of 1000 hours. It must also 
be remembered that the three coupons were tested in compression 
while all of the creep laws were developed from tensile tests. 
Nothing could be found in the literature to relate whether a given 
material creeps similarly in compression and tension at elevated 
temperature. 
The most important single observation in this report must lie 
in the large excursions in short term (primary) creep implied by both 
the test data and the empirical creep laws of figure 4. This strongly 
4 
implies that short term creep is somewhat unpredictable. Aircraft 
events that result in extreme, but short term stress and temperature 
excursions must be approached cautiously. Deformations and/or 
residual stresses could quickly accumulate to intolerable values 
if predictive or monitorial capabilities are deficient for the 
short term effects. The apparent convergence of the creep data 
with the creep laws in the longer term situation (secondary creep) 
implies a much more predictable situation. 
CONCLUDING REMARKS 
Short-term compressive creep tests were conducted on three 
titanium alloy Ti-6AI-4V coupons at three different stress levels 
at a temperature of 7l4K (825FO). The test data were compared to 
several creep laws developed from tensile creep tests of available 
literature. The short term creep test data did not correlate well 
with any of the creep laws obtained from available literature. The 
creep laws obtained from available literature did not correlate 
well with each other either. Short-term creep does not appear to 
be very predictable for titanium alloy Ti-6Al-4V. Aircraft events 
that result in extreme, but short term temperature and stress 
excursions for this alloy should be approached cautiously. Extrap-
olations of test data and creep laws suggest a convergence toward 
predictability in the longer-term situation. 
NASA Ames Research Center 
Dryden FZight Research FaciZity 
January 18~ 1984 
REFERENCES 
1. Davis, John W.i and Cramer, Bruce A.: Prediction and Verification 
of Creep Behavior in Metallic Materials and Components for the 
Space Shuttle Thermal Protection System. NASA CR-2685, July 1976. 
2. Davis, J. W.i and Cramer, B. A.: Prediction and Verification of 
Creep Behavior in Metallic Materials and Components for the 
Space Shuttle Thermal Protection System. NASA-CR-132605-l, 
November 1974. 
3. Blackburn, Linda Be: Creep Testing on Foil-Gage Metals at Elevated 
Temperature Using An Automated Data Acquisition System. NASA 
TM-84634, March 1983. 
5 
6 
E 39729 
Figure 1. Creep Specimens and Special End Fixtures. 
Concentrated 
Load 
Inading Bar 
Test 
caufOn 
Figure 2. Sketch of General Test 
Set-Up. 
E 39730 
Figure 3. Creep Specimen in the Test Fixture. 
3000 
2000 
1000 
500 
Creep 
Strain, 
~m/m 
(lbs/in2) 
100 
50 
o 
o = 60.12 MPa (8720 lbs/in2) 
T = 7l4K (825"P) 
---
----
--
---
{ 
C3.4S) 
£c = 1~1410·562 t· 162 e-T -
Equation 2 £ 
. { (-4.21 EquatlOn 3 _ £ = .64870.738 t"299 e-T -) 
Equation 1 ;\ L C 
-""", . \ 
,....,......... 
= e-24.09 + 22.S4T + .00000602 + .905Lno + .433Lnt 
o 
o 
,....,......... 2 2 LnOLnt 
-24.9 + 21.4T + 1.16LnO + .634Lnt + .0062(Lnt) +.0000070 - .0332--
f.:c = e T 
o Test Data 
10 L-___ _ 
1 10 50 100 
Time, hours 
(a) Stress Equals 60.12 MPa (8720 lbs/in2) 
Figure 4. Comparison of Measured Creep Strains with Four 
Creep Laws. 
7 
8 
3000 
2000 
1000 
500 
Creep 
Strain, 
pm/m 
(lbs/in2) 
100 
50 
{ 
(3.45 
./ Be = 1.1410. 562 t"162 e -T-) 
_ _ Equation 2 { (~ 
___ - - _____ Equation 3 --- "'e = .64870. 738 t"299 e T ) 
___ .- - _________ Equation 1 
- - _..------ / Equation 4 ~ 
..----- / 
. / 
~-~ ~ 
/",- 0 B = e-24 • 09 + 22.54T + .00000602 + .905Lno + .433Lnt 
/"_ 0 e 
o 
a = 86.18 MPa (12500 1bs/in2) 
~ 
E == e -24.9 +21.4T + 1.16Lno + .634Lnt + .0062 (Lnt)
2 + .00000702 _ .0332
LnO 
Lnt 
C T 
o Test Data 
10 '-____ --'-__ ..L.. _____ ..L.._-' 
3000 
2000 
1000 
500 
Creep 
Strain, 
pm/m 
(lbs/in2) 
100 
50 
10 
10 50 100 
T.in'e, hours 
(b) Stress Equals 86.18 MPa (12500 lbs/in2) 
[ 
Equation 2/[ 
_---~ ~=~~.~~ 13 ~-
-- - ------____. -' Equat10n 4 
o = 104.45 MPa (15150 lbs/in2) 
T = 7l4K (8250 F) 
--
----------- ~ ~.-
~ ~#' ~ 
-3.45 
Be = 1.1410. 562 t"162 e ( -or-) 
-4.21 
"e = .64870. 738 t"299 e (-T-) 
~ "'e = e -24.09 + 22.54T + .00000602 + .905Lno + .433Lnt 
- /"'/0 
/'" 0 
o 
2 2 Lna Lnt 
= e-24.9 + 2l.4T + 1.16Lno + .634Lnt + .0062(Lnt) + .0000070 - .0332--~ T 
o Test Data 
10 50 100 
Time, hours 
(c) Stress Equals 104.45 MPa (15150 lbs/in2) 
Figure 4. Concluded. 
1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. 
NASA TM-86033 
4. Title and Subtitle 5. Report Date 
Effect of Creep in Titanium Alloy Ti-6Al-4V at Elevated February 1984 
Temperature on Aircraft Design and Flight Test 6. Performing Organization Code 
7. Author(s) 8. Performing Organization Report No. 
Jeral.d M. Jenkins H-1228 
10. Work Unit No. 
9. Performing Organization Name and Address 
NASA Ames Research Center 
Dryden Flight Research Facility 11. Contract or Grant No. 
P.O. Box 273 
Edwards, CA 93523 
13. Type of Report and Period Covered 
12. Sponsoring Agency Name and Address Technical Memorandum 
National Aeronautics and Space Administration 14. Sponsoring Agency Code 
Washington, D.C. 20546 
RTOP 506-53-64 
15. Su pplementary Notes 
16. Abstract 
Short-term compressive creep tests were conducted on three titanium 
alloy Ti-6Al-4V coupons at three different stress levels at a temperature of 
714 K (825 0 F) • The test data were compared to several creep laws developed 
from tensile creep tests of available literature. The short-term creep test 
data did not correlate well with any of the creep laws obtained from avail-
able literature. The creep laws themselves did not correlate well with each 
other. Short-term creep does not appear to be very predictable for titanium 
alloy Ti-6Al-4V: Aircraft events that result in extreme, but short-term 
temperature and stress excursions for this alloy should be approached 
cautiously. Extrapolations of test data and creep laws suggest a convergence 
toward predictability in the longer-term situation. 
17. Key Words (Suggested by Author(s)) 18. D !stribution Statement 
Creep Unclassified-Unlimited 
High temperature 
STAR category 39 
r----19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price . 
Unclassified Unclassified 8 A02 
'----. 
*For sale by the National Technical Information Service. Springfield, Virginia 22161. 
End of Document 


Effect of creep in titanium alloy Ti-6Al-4V at elevated temperature on aircraft design and flight test

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