Metal Foams: A Design Guide

104 Metal Foams: A Design Guide
Strain, ε
Primary
Elastic
Time, t (s)
t R (Rupture)
•
εss Secondary
Figure 9.1 Schematic of creep strain response of a metal under constant
load
dominant; the secondary creep rate, Pε, then depends on the stress, ,raisedto
a power n>1:
� �n Pε DPε0
⊲9.1⊳
where
0
Pε0 D A exp
� �
Q
RT
Here n, 0 and Q are properties of the material ( 0 is a reference stress and Q
is the activation energy for the kinetic process controlling the rate of creep), A
has the dimensions 1/second and R is the ideal gas constant (8.314 J/mole K).
Typical values for n, 0 and Q for power-law creep of several solid metals
are listed in Table 9.1.
The time to rupture for a solid metal can be found by assuming that failure
is associated with a constant critical strain in the material so that the product
Table 9.1 Power law creep parameters for solid metals
Material n 0 (MPa) Q (kJ/mole)
Aluminum 4.4 0.12 142
Nickel 4.6 0.5 284
316 Stainless steel 7.9 33.5 270