Metal Foams: A Design Guide

Stress amplitude, ∆σ
+
Stress, σ
−
0
∆σe
Tension-Tension
1
∆σ
∆σ
Compression-Compression
Endurance
limit
σ max
σ min
σ min
σ max
10 2 10 4 10 6 10 8
Cycles to failure, N f
a
Time (t)
b
Design for fatigue with metal foams 89
Figure 8.1 (a) Definition of fatigue loading terms; (b) Typical S–N curve
for aluminum alloys, in the form of stress range 1 versus number of cycles
to failure, Nf. The endurance limit, 1 e, is defined, by convention, for a
fatigue life of 10 7 cycles
min D 1MPa and max D 10 MPa. The load ratio R is defined by
R
min
max
σ
σ
⊲8.1⊳
It is well known that the fatigue life of structural metals such as steels
and aluminum alloys is insensitive to the loading frequency, under ambient
conditions. This simplification does not hold in the presence of a corrosive
medium, such as a hot alkaline solution, or salt water for aluminum alloys.
These broad conclusions are expected to hold also for metallic foams.
In low-cycle fatigue testing, the usual strategy is to measure the number
of cycles to failure, Nf, for a given constant stress range 1 D max min,
and then to plot the resulting pairs of values ⊲Nf,1 ⊳ on log-linear axes. The
resulting S–N curve is used in design for finite life (Figure 8.1(b)). Many