Metal Foams: A Design Guide

Design for fatigue with metal foams 97
compression-compression results for R D 0.5 are in good agreement with
the corresponding results for R D 0.1, when max is used as the loading
parameter.
3. There is a drop in fatigue strength for tension–tension loading compared
with compression–compression fatigue. The fatigue strength is summarized
in Figure 8.6 for the various aluminum foams by plotting the value
of max at a fatigue life of 10 7 cycles versus relative density, over a
wide range of mean stresses. The values of max have been normalized by
the plateau value of the yield strength, pl, in uniaxial compression. The
fatigue strength of fully dense aluminum alloys has also been added: for
tension–tension loading, with R D 0.1, the value of max at the endurance
limit is about 0.6 times the yield strength.
endurance
limit
σ max/σ p
1
0.8
0.6
ERG
0.4
Fully dense
Aluminum alloys Alcan
x
0.2
Compression (R = 0.1, 0.5)
x Tension (R = 0.1)
0
0.01 0.1 1
x
Alporas
Relative density, ρ/ρ s
Alulight
Figure 8.6 Ratio of max at the endurance limit to the monotonic yield
strength pl for foams, compared with that for tension–tension fatigue of fully
dense aluminum alloys at R D 0.1
We conclude from Figure 8.6 that the fatigue strength of aluminum foams
is similar to that of fully dense aluminum alloys, when the fatigue strength has
been normalized by the uniaxial compressive strength. There is no consistent
trend in fatigue strength with relative density of the foam.
8.4 Notch sensitivity in static and fatigue loading
A practical concern in designing with metallic foams is the issue of damage
tolerance: in the presence of a notch or hole, does it fail in a notch-insensitive,
x