Callister - An introduction - 8th edition

8.13 Stress and Temperature Effects • 267
Creep strain
T 3 > T 2 > T 1
× 3 > 2 > 1
×
T 3 or 3
T 2 or 2
T 1 or 1
×
Figure 8.29 Influence of stress
and temperature T on creep
behavior.
T < 0.4T m
Time
or temperature, the following will be noted: (1) the instantaneous strain at the time
of stress application increases, (2) the steady-state creep rate is increased, and
(3) the rupture lifetime is diminished.
The results of creep rupture tests are most commonly presented as the logarithm
of stress versus the logarithm of rupture lifetime. Figure 8.30 is one such plot
for an S-590 alloy in which a set of linear relationships can be seen to exist at each
temperature. For some alloys and over relatively large stress ranges, nonlinearity in
these curves is observed.
Empirical relationships have been developed in which the steady-state creep
rate as a function of stress and temperature is expressed. Its dependence on stress
can be written
Dependence of
creep strain rate on
stress
# s K 1 s n
(8.19)
where K 1 and n are material constants. A plot of the logarithm of # s versus the logarithm
of yields a straight line with slope of n; this is shown in Figure 8.31 for an
S-590 alloy at four temperatures. Clearly, one or two straight line segments are
drawn at each temperature.
Figure 8.30 Stress (logarithmic scale)
versus rupture lifetime (logarithmic scale)
for an S-590 alloy at four temperatures.
[The composition (in wt%) of S-590 is as
follows: 20.0 Cr, 19.4 Ni, 19.3 Co, 4.0 W, 4.0
Nb, 3.8 Mo, 1.35 Mn, 0.43 C, and the
balance Fe.] (Reprinted with permission of
ASM International. ® All rights reserved.
www.asminternational.org)
Stress (MPa)
1000
800
600
400
200
100
80
925°C
730°C
650°C
815°C
60
40
20
10 –4 10 –3 10 –2 10 –1 1 10 10 2 10 3 10 4 10 5
Rupture lifetime (h)