Creep and Elevated Report - First Draft
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1
Creep and Elevated Temperature Properties of Aluminum
1.0 Experimental Procedure
1.1 Material
Two types of aluminum alloy were supplied by Virginia Tech for the purposes of determining
high temperature static and creep properties.
One type of aluminum alloy was formed via extrusion, and is referred herein as Type A. This
material was supplied in the form of extruded strips of dimensions 6.60 mm, 50 mm (width)
and 600 mm (length).
The other type of aluminum alloy had the same composite as Type A, but was formed by
strain hardening (i.e. hot rolled) and in this report is refereed to as Type B. This material was
supplied as rectangular plates of dimensions 600 mm × 300 mm and two thicknesses. One
plate had a thickness of 7.60 mm (referred to herein as B-Thin) and the other with a thickness
of 9.90 mm (referred to herein as B-Thick).
1.2 Methodology
1.2.1 Specimen Dimensions
All specimens for high temperature tests were machined into dog-bone shaped coupons.
Extruded samples (Type A) had a necked gauge section of 10 mm wide, 6 mm thick and 300
mm long. The strain-hardened aluminum specimen; B-Thin had a necked gauge section of 10
mm wide, 7.60 mm thick and 300 mm long. Specimen from sample B-Thick had a necked
gauge section of 10.0 mm wide, 9.90 mm thick and 300 mm long. Figure 1 shows the dog-
bone shaped test specimens used in this study. The extrusion direction (for Type A) and
rolling direction (for Type B) was aligned in the length-wise direction of the specimens.
Figure 1: Dog-bone specimen used for static tests.
2
1.2.2 High Temperature Static Tests
Elevated temperature static tests were performed in tension loading for temperatures ranging
between 20 and 460°C. The specimens were centrally heated over a 100 mm long section of
the gauge region using a temperature-controllable heating device. Prior to loading in tension
at a loading rate of 0.5 mm/min, the specimens were allowed to equilibrate at the set test
temperature for 10 min. Figure 2 shows the experimental set-up of the high temperature tests.
The ends of the specimen show dimples made by the grips of the testing machine. From the
calculated stress-strain relationships, the Young’s modulus and 0.2% proof strength of the
aluminum alloys were determined.
Figure 2: Experimental set-up for tensile and creep testing.
1.2.3 Creep Tests
Using the same test set-up as the elevated temperature static test shown in Figure 2, high
temperature creep tests were performed at constant temperature and constant stress on the
dog-bone shaped specimens. The specimens were held at the creep test temperature until they
had reached thermal equilibrium, after which they were pre-loaded at a rate of 3 kN/min to the
pre-determined applied creep stress. Creep tests were performed for constant stress levels
ranging between 15 and 200 MPa at constant temperatures ranging from 20 to 460°C for both
types of aluminum alloy. The creep behavior (i.e. strain evolution as a function of time) was
measured for the entire test until the specimen failed via stress rupture. For Type A
specimens, the creep behaviour was measured at temperature ranges and stress levels that
allowed the specimens to fail within a short period of time realistic of the duration of a fire on
naval vessel (i.e. less than several hours). However, creep tests for sample B-Thick were
performed in two sets: (1) constant temperature with varying applied stress and (2) constant
applied stress and varying temperature. This procedure was adopted since it allows the
3
extraction of creep parameters for use in thermo-mechanical (analytical and numerical)
models.
2.0 Results
2.1 Elevated Temperature Static Properties
2.1.1 Sample A: Extruded Aluminum Alloy
Figure 3 shows the tensile stress-strain curves for selected elevated temperature tests
performed for sample A; i.e. the extruded aluminum alloy. For this sample, the test
temperatures were varied from 25 to 430°C.
Figure 3: Stress-strain curves for sample A.
The elastic moduli and 0.2% proof strength of specimens from sample A calculated for
different test temperatures are shown in Figures 4 and 5. The mechanical properties decrease
with increasing test temperatures. A fitting tanh function was used to mathematically fit the
elastic moduli and proof strength to test temperatures using equations (1) and (2),
respectively.
( ) ( )( )%50tanh22
TTkEE
TE RTRT −−= (1)
( ) ( ) ( ) ( )( )%50
%2.0%2.0
%2.0 tanh22
TTkTRTRT
−−=σσ
σ (2)
where ( )TE and ( )RT%2.0σ are the Young’s modulus and the 0.2% proof strength at room
temperature, respectively; k is a material fitting constant describing the breath of the property-
0.000 0.002 0.004 0.006 0.008 0.0100
50
100
150
200
250
300
430 oC
363 oC
326 oC
290 oC
202 oC
110 oC
25 oC
Str
ess (
MP
a)
Strain (mm/mm)
4
temperature dependency and %50T is the temperature at which the room temperature measured
property value has dropped by 50%. These values are given for respective property-
temperature dependencies in Figures 4 and 5.
Figure 4: The effect of temperature on the Young’s modulus of the extruded sample (A).
Figure 5: The effect of temperature on the 0.2% proof strength of the extruded sample (A).
The experimental data obtained from elevated static tests for sample A (elastic moduli and
0.2% proof strength) were compared to a tanh fitting functioning describing the Eurocode 9
data [1,2] and Langhelle and co-workers derived data for aluminum alloy 6082-T651 which is
contained in an article published by Fogle et al. [3]. These results are shown in Figures 6 and
7. The degradation in elastic moduli as a function of temperature for the extruded sample is
similar to that observed for 6082-T651. However, these data are significantly different from
0 100 200 300 400 500 6000
10
20
30
40
50
60
70
80
ERT
=78.0 GPa
k = 0.007207
T50%
= 373.61 oC
Ela
stic
Modulu
s (
GP
a)
Temperature (oC)
0 100 200 300 400 500 6000
50
100
150
200
250
300
σ0.2%,RT
=285.1 MPa
k = 0.008444
T50%
= 283.79 oC
0.2
% O
ffset Y
ield
Str
ess (
MP
a)
Temparature (oC)
5
0 100 200 300 400 500 6000.0
0.2
0.4
0.6
0.8
1.0
Eurocode 9 data
Aluminum Type A data
No
rma
lise
d E
lastic M
od
ulu
s
Temperature (oC)
0 100 200 300 400 500 6000.0
0.2
0.4
0.6
0.8
1.0
Aluminum Type A data
Eurocode 9 data
No
rma
lise
d 0
.2%
Off
se
t Y
ield
Str
ess
Temperature (oC)
observations made for 5083-H116 alloy specimens previously tested in our research facilities.
The proof strength comparison (see Figure 7) of the extruded sample and 6082-T651,
however, reveals significant differences in some of their mechanical properties as function of
temperature.
Figure 6: Comparison of experimental Type A Young’s modulus data from this study to
Eurocode 9-derived data for the 6082-T651 aluminum alloy [1,2].
Figure 7: Comparison of experimental proof strength data from this study to Eurocode 9-
derived data for the 6082-T651 aluminum alloy [1,2].
2.1.2 Sample B: Strain-Hardened (Hot-Rolled) Aluminum Alloy
Tensile stress-strain curves for sample B (both thick and thin) are shown in Figure 8. The
elevated temperature static property data for sample B was collected from tests performed at
temperatures in the range of 20 to 460°C. The elastic moduli and 0.2% proof strength of
6
0.000 0.002 0.004 0.006 0.008 0.0100
50
100
150
200
250
460 oC
340 oC
260 oC
220 oC
150 oC
20 oC
Str
ess (
MP
a)
Strain (mm/mm)
sample B-Thin and B-Thick tested are shown in Figures 9 and 10, respectively. Similar to
observations made for the extruded sample, the mechanical properties (elastic modulus and
0.2% proof strength) decrease with increase in temperature. The same tanh functions used to
mathematically describe the relationship between the elastic moduli or the proof strength with
temperature (equations 1 & 2) were also used for this data. The rate of mechanical property
(elastic modulus and proof strength) degradation with temperature for both B-Thin and B-
Thick specimens is the same, Figure 9.
Figure 8: Stress-strain curves for sample B-Thin.
Figure 9: Effect of temperature on Young’s modulus of samples B-Thick and B-Thin.
0 100 200 300 400 500 6000
10
20
30
40
50
60
70
ERT
=70.41 GPa
k=0.005350
T50%
=285.41 oC
B Thin
B Thick
Ela
stic
Modulu
s (
GP
a)
Temperature (oC)
7
0 100 200 300 400 500 6000
50
100
150
200
250
σ0.2%,RT
=234 MPa
k=0.009133
T50%
=271.97 oC
B-Thin
B-Thick
0.2
% O
ffse
t Y
ield
Str
ess (
MP
a)
Temperature (oC)
0 100 200 300 400 500 6000
10
20
30
40
50
60
70
80
Sample B (Thin)
Sample A (Extruded)
Sample B (Thick)Ela
stic
Modulu
s (
GP
a)
Temperature (oC)
Figure 10: Effect of temperature on 0.2% proof strength of sample-Thick and B-Thin.
The experimental data for the elastic modulus and 0.2% proof strength for the strain-hardened
sample (B) are compared to that of the extruded sample (A) in Figures 11 and 12,
respectively. In both cases, the mechanical properties of the extruded sample (A) are superior
to those of the strain-hardened sample (B) over the entire temperature range over which
elevated temperature static tests were conducted.
Figure 11: Comparison of effect of temperature on Young’s modulus of aluminum alloys A, B-
Thin and B-Thick.
8
0 100 200 300 400 500 6000
50
100
150
200
250
300
Sample B (Thick)
Sample A (Extruded)
Sample B (Thin)0.2
% O
ffset Y
ield
Str
ess (
MP
a)
Temperature (oC)
1 10 100 1000 10000
0.00
0.15
0.30
0.45
0.60
50 M
Pa-3
63 C
15 M
Pa-4
32 C25 M
Pa-4
00 C
50
MP
a-3
41
C
17
.5 M
Pa-4
37 C
100
MP
a-2
90
C
40
MP
a-3
63
C
30 M
Pa-3
90 C
20
MP
a-4
30
C75
MP
a-3
01 C
20
0 M
Pa
-200
C
Str
ain
(m
m/m
m)
Log (Time)
Figure 12: Comparison of effect of temperature on 0.2% proof strength of aluminum alloys A,
B-Thin and B-Thick.
2.2 High Temperature Creep Properties
2.2.1 Sample A: Extruded Aluminum Alloy
Selected creep strain curves for the extruded sample (A) at different temperature and stress
conditions are shown in Figure 13. The creep strain is plotted against the logarithm of time.
Figure 13: Creep curves for the extruded sample (Type A).
The rupture times were converted into Larson-Miller parameters by using the relationship:
T(log(t) + C) = LMP (3)
where T is the absolute isothermal exposure temperature, t the exposure time in hours, and C a
material constant; set at a value of 18 in this report. Figure 14 shows the variation of the
9
0 5000 10000 15000 20000 250000.0
0.1
0.2
0.3
0.4
125 M
Pa
95 M
Pa
70 M
Pa
105 M
Pa
115 M
Pa
Cre
ep
str
ain
Time (s)
8000 9000 10000 11000 12000 13000 140001.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
Log(σ) = 0.32 + 9.90exp(-LMP/5488.76)
Lo
g (
σ)
Larson-Miller Parameter
natural logarithm of applied stress as a function of the Larson-Miller Parameter (LMP). This
relationship has been successfully demonstrated to offer a reliable means of constructing
creep curves performed within and outside the test matrix; i.e. interpolation and extrapolation
[4].
Figure 14: Relationship between natural logarithm of applied stress and the Larson-Miller
parameter.
2.2.2 Sample B: Strain-Hardened (Hot-Rolled) Aluminum Alloy
Creep curves collected for sample B-Thick at a constant temperature of 350°C under varying
stress levels in the range 70 to 125 MPa are shown in Figure 15.
Figure 15: Creep curves for the Type B-Thick alloy at a constant temperature of 350°C under
different stress level loadings.
10
4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9-4.2
-4.1
-4.0
-3.9
-3.8
-3.7
-3.6
-3.5
-3.4
ln (
str
ain
at ze
ro)
ln (σ)
From the curves shown in Figure 15, the steady creep strain rate ( )IIεɺ was determined. The
natural logarithm of the ( )IIεɺ was then plotted against the logarithm of stress at 350°C, Figure
16. There is a linear dependency of ln ( )IIεɺ on the natural logarithm of the applied stress, ln(σ)
from which a value of n = –3.96 as contained in ( ) )/exp( RTQA n
II −= σεɺ is obtained from the
slope.
Figure 16: Plot of ln ( )IIεɺ versus ln(σ ) at 350°C for the Type B -Thick alloy.
In addition, the natural logarithm of the strain at time t = 0, ( )0ε , was also plotted against the
natural logarithm of the applied stress, Figure 17. The data is scattered and no mathematical
expression could be used to describe this data set. The same observation was made in our
previous studies wherein a different alloy 5083-H116 was investigated [5].
Figure 17: Plot of ln ( )0ε versus ln(σ) for the extruded Type B-Thick alloy.
4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9-14
-13
-12
-11
-10
-9
-8
ln (strain rate) = 3.96*ln (σ) - 28.73
ln (
str
ain
rate
)
ln (σ)
11
0 5000 10000 15000 20000 250000.0
0.1
0.2
0.3
0.4
0.5
40
0oC
36
5oC
41
0oC
40
5oC
35
0oC
Cre
ep
str
ain
Time (s)
From another set of experiments, creep curves obtained at a constant stress of 70 MPa at
different temperatures in the range of 350 to 420°C are shown in Figure 18. Due to the limited
sample size and the variability in the behaviour of the test specimens, only a limited number
of creep test data sets were usable.
Figure 18: Creep curves for the Type B-Thick alloy at a constant stress of 70 MPa at different
temperatures.
From the curves shown in Figure 18, the steady creep strain rate ( )IIεɺ was determined. The
natural logarithm of the ( )IIεɺ was then plotted against the inverse of the absolute test
temperatures, (1/T), Figure 19. There is a linear dependency of ln ( )IIεɺ on the inverse of
absolute test temperature (1/T). The activation energy, Q, calculated from the slope of the
curve is 163 kJ/mol. By taking the natural logarithm of ( ) )/exp( RTQA n
II −= σεɺ , the intercept of
the plot of ln(strain rate) versus ln(σ) can be used to calculated the frequency factor, A. A
value of 16.76 s-1
was determined for the parameter A.
12
0.00148 0.00152 0.00156 0.00160-12.5
-12.0
-11.5
-11.0
-10.5
-10.0
-9.5
-9.0
-8.5
ln(strain rate) = 19.5377 - 19655.3347*(1/T)
ln(s
tra
in r
ate
)
1/T(K-1)
0.00148 0.00152 0.00156 0.00160
-6.5
-6.0
-5.5
-5.0
-4.5
-4.0
-3.5
-3.0
ln(s
tra
in a
t zero
)
1/T (K-1)
Figure 19: Plot of ln ( )IIεɺ versus 1/T for the Type B-Thick alloy.
The natural logarithm of the strain at time t = 0, ( )0ε , was also plotted against 1/T, Figure 20.
Figure 20: Plot of ln ( )0ε versus1/T for the Type B-Thick alloy.
The data is scattered and no mathematical expression could be used to describe this data set.
The same observation was made in our previous studied wherein a different ally 5083-H116
was investigated [5].
13
References
1. EN 1999:1-1, 2009, ‘Eurocode 9 – Design of Aluminum Structures – Part 1-1: General
Rules,’ British Standard.
2. EN 1999-1-2:2007, 2009, ‘Eurocode 9 – Design of Aluminum Structures – Part 1-2:
Structural Fire Design,’ British Standard.
3. Fogle, E.J., Lattimer, B.Y., Feih, S., Kandare, E., Mouritz, A.P., Case, S.W. ‘Compression
load failure of aluminum plates due to fire’, Engineering Structures Journal, Submitted
2011.
4. Feih, S., Kandare, E., Lattimer, B.Y., Mouritz, A.P. ‘Structural analysis of compression
deformation and failure of aluminium in fire’, Journal of Structural Engineering.
http://dx.doi.org/10.1061/(ASCE)ST.1943-541X.0000313.
5. Feih, S., Kandare, E., Mouritz, A.P. ‘Aluminium creep data and modelling approaches’,
CRC-ACS TM 10025, June 2010.