Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-09 GTBL042-Callister-v3 October 4, 2007 11:53

2nd Revised Pages

328 • Chapter 9 / Failure

Time

Creep strain

T 3 or  3

T 3 > T 2 > T 1

×

×

×

 3 >  2 >  1

T 2 or  2

T 1 or  1

T < 0.4Tm

Figure 9.36 Influence of stress

σand temperatureTon creep

behavior.

9.16 STRESS AND TEMPERATURE EFFECTS

Both temperature and the level of the applied stress influence the creep character-

istics (Figure 9.36). At a temperature substantially below 0.4Tm, and after the initial

deformation, the strain is virtually independent of time. With either increasing stress

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 9.37 is one such plot for a

nickel alloy in which a linear relationship 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  ̇s=K 1 σn (9.20)

strain rate on stress

102 103 104 105

60

40

400

300

200

100

80

60

40

30

20

30

20

10

8

6

4

3

2

427 °C (800°F)

538 °C (1000°F)

649 °C (1200°F)

Rupture lifetime (h)

Stress (MPa) Stress (10

3 psi)

Figure 9.37 Stress (logarithmic scale) versus rupture lifetime (logarithmic scale) for a low

carbon–nickel alloy at three temperatures. [FromMetals Handbook: Properties and

Selection: Stainless Steels, Tool Materials and Special-Purpose Metals,Vol. 3, 9th edition,

D. Benjamin (Senior Editor), American Society for Metals, 1980, p. 130.]