Creep
Creep is slow plastic extension (strain) at elevated temperatures (T ~> 0.3 melting T) and stress. Typical strain
vs time at elevated temperature and stress is:
rupture
X
stage I: primary creep: rapid initial
deformation
stage II: secondary creep: a.k.a.
steady state creep: constant strain
rate designated "minimum strain rate"
stage III: tertiary creep: reduction in
cross section eventually leading to
fracture
T2
III
strain ε
II
minimum creep rate
dε/dt
I
T1<<T2
time
Typical creep data is usually shown as stress vs time to rupture, 0.5% strain, and 0.2% strain for a given
temperature. A set of curves derived manually from NACA TN 4112 Generalized Master Curves for Creep and
Rupture, Heimerl and McEvily Oct 1957 (Aero Library) for Aluminum 2024-T3 alloy plate is shown:
NACA TN 4112 data
creep data shown as stress (ksi) vs time (hours) parameterized by temperature - includes rupture, 0.5% strain and
0.2% strain
Creep for Aluminum 2024-T3 alloy sheet
100
stress (ksi)
T = 500 F rupture
T = 500 F 0.5 % strain
T = 500 F 0.2 % strain
T = 350 F rupture
T = 350 F 0.5 % strain
T = 350 F 0.2 % strain
10
0.1
1
10
100
3
1 .10
time (hours)
11/28/2005
1
4
1 .10
5
1 .10
6
1 .10
creep data shown as stress (ksi) vs temperature parameterized by time (hours) - rupture only shown - gets too busy
Creep for Aluminum 2024-T3 alloy sheet
100
stress (ksi) (log)
time to rupture = 1000 hours
time to rupture = 5000 hours
time to rupture = 10000 hours
10
200
250
300
350
400
450
500
temperature deg F
These curves are difficult to use so researchers proposed a parameter that collapses data empirically. E.g.these
data can be represented can be represented on a Master Curve for Creep using the Larson-Miller (or another
similar parameter). The plots lack smoothness due to manual extraction of data.
"Master Creep Curve" Al 2024-T3
50
rupture
0.5% strain
0.2% strain
stress (ksi)
40
30
20
10
13
14
15
16
17
Larson-Miller TR(17+log(time) (*10^-3)
11/28/2005
2
18
19
The Larson Miller parameter combines temperature and times (rupture, strain) as follows: (
( ))
LM = TR⋅ C1 + log tr
C1 = constant_based_on_material
C1_al_2024 := 17
C1_carbon_molybdenum_steel := 20
TR = temperature_deg_R = deg_F + 460
tr = time_to_rupture_hours
above curves
similar parameter for t ε_0.5
tε_0.5 = time_to_point_5%_strain_hours
tε_0.2 = time_to_point_2%_strain_hours
Master Creep Curves - rupture
stress (ksi)
100
10
Carbon Molybdenum steel
Aluminum 2024-T3 alloy plate
1
10
15
20
25
30
35
Larson Miller TR(C1+log(tr)) (*10^-3)
11/28/2005
3
40
45