Materials for engineering, 3rd Edition - (Malestrom)

Determination of mechanical properties 53
C
A
Strain
B
Time (log scale)
2.12 Showing various strain–time curves for creep.
˙ ε = ˙ ε0 ( σ/ σo) n exp – ( QRT / )
[2.19]
Considerable time and thus expense is involved in determining creep data
in this form, and design data are often given as a series of curves relating
stress and time at a given test temperature to produce a given constant creep
strain (1%, 2%, etc.) or time to rupture (or ‘creep life’), both are shown in
the example in Fig. 2.13 for a nickel-based alloy in single crystal form (due
to W. Schneider, J. Hammer and H. Mughrabi, in Superalloys 1992, edited
by S.D. Antolovich et al., TMS, Warrendale, PA, 1992, 589).
Acquisition of creep life data requires no strain measurement to be carried
out, but it clearly can given no indication of the time spent in the various
stages of the creep curve. Engineering components may thus be designed on
1000
800°C
Stress (MPa)
500
300
ε pl (%)
1
2
3
4
Rupture
950°C
0.1 1 10 100 1000 5000
Time (h)
2.13 Creep–rupture diagrams for a single crystal Ni-based superalloy
(CMSX-4) for 800 and 950 °C.