2 a) possible additional sources of error (creep test)
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2 a) possible additional sources of error (creep test)
• temperature not constant over time• light (optical strain gages!) may be not constant• humidity (sample affected!) not constant• load or displacement may not be const.• machine/instrument creeping• creeping of strain gage• etc.
(day / night!)
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2 b) Analyzing a creep experiment
0 1 2 3 4 5 6 7 80
5
10
15
20
Dis
plac
emen
t [%
]
Time [min]
secondary
tertiary
primary
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0 1 2 3 4 5 6 7 80,000
0,001
0,002
0,003
0,004
0,005
Com
plia
nce
D(t)
[1/M
Pa]
Time [min]
0 1 2 3 4 5 6 7 8
1E-3
0,002
0,003
0,004
0,005
Com
plia
nce
D(t)
[1/M
Pa]
Time [min]
𝐷(𝑡)=𝜀(𝑡)𝜎 0
creep compliance
note that the values of e are given in %, so you have to divide them by 100
to evaluate creep compliance is only useful in the secondary region (marked by )D is in the range of 1/MPa
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130°C 125°C 122°C 120°C 115°C 110°C 100°C 90°C
10-3 10-1 101 103 10510-6
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Time (min)
Com
plia
nce
D(t)
(1/p
si)
2 c) i. time-temperature-shift for master curve at 120°C
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Com
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D(t)
(1/p
si)
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130°C 125°C 122°C 120°C 115°C 110°C 100°C 90°C
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Com
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D(t)
(1/p
si)
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D(t)
(1/p
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Com
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D(t)
(1/p
si)
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Time (min)
Com
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D(t)
(1/p
si)
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Time (min)
Com
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D(t)
(1/p
si)
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D(t)
(1/p
si)
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10-3 10-1 101 103 10510-6
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Time (min)
Com
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D(t)
(1/p
si)
2 c) i. result: master curve at 120°C
master curve at 120°C
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Time (min)
Com
plia
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D(t)
(1/p
si)
master curve at 120°C
2 c) ii. predict compliance after 5 days
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Time (min)
Com
plia
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D(t)
(1/p
si)
master curve at 120°C
5 d = 5*24*60 min = 7200 min
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Time (min)
Com
plia
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D(t)
(1/p
si)
master curve at 120°C
5 d = 5*24*60 min = 7200 min
D(5 d) ≈ 1/psi
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2 c) iii. possible prediction range at 120°C
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master curve at 120°C
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master curve at 120°C
40000 min == 0,9 months == 0,1 years
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Time (min)
Com
plia
nce
D(t)
(1/p
si)
master curve at 90°C
3e+9 min == 70000 months == 5700 years
for comparison: possible prediction range at 90°C
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3. Fatigue testing
a) three examples of applications cyclic stress loading
1. wings of airplanes2. turbine blades3. rotor blades
(nicht sehr schwer, wenn zwei davon schon in b) genannt sind)
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b) Wöhler curve and estimation of life time
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20
40
60
80
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120 Stress
Stre
ss [M
Pa]
Cycles N
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b) Wöhler curve and estimation of life time
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20
40
60
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100
120 Stress
Stre
ss [M
Pa]
Cycles N
i. N(40 MPa) ≈ 9000 ii. N(27 MPa) ≈ 300000
i. N(40 MPa) ≈ 9000 tlife = N/f = 9000 * 1 / 0,5 s = 18000 s = = 5 hours
ii. N(27 MPa) ≈ 300000 tlife = N/f = 300000 * 1 / 1000 s = 300 s = = 5 minutes
(aim: to show that cyclic loaded components must have huge safetyfactors, if fatigue testing is not performed)