Metal Foams: A Design Guide

Axial strain (%)
5
4
3
2
1
0
0
σmax = 1
σpl 0.92
At σ max
0.83
At σ min
0.2 0.4 0.6 0.8 1.0
Cycles (×10 6 )
Design for fatigue with metal foams 91
0.67
Figure 8.2 Progressive lengthening in tension–tension fatigue of Alporas,
at various fixed levels of stress cycle (R D 0.1; relative density 0.11; gauge
length 100 mm)
manner (see Figure 8.3). The underlying mechanism is thought to be a combination
of distributed cracking of cell walls and edges, and cyclic ratcheting
under non-zero mean stress. Both mechanisms lead to the progressive crushing
of cells. Three types of deformation pattern develop:
1. Type I behavior. Uniform strain accumulates throughout the foam, with no
evidence of crush band development. This fatigue response is the analogue
of uniform compressive straining in monotonic loading. Typical plots of
the observed accumulation of compressive strain with cycles, at constant
stress range 1 , are shown in Figure 8.4(a) for the Duocel foam Al-6101-
T6. Data are displayed for various values of maximum stress of the fatigue
cycle max normalized by the plateau value of the yield strength, pl.
2. Type II behavior. Crush bands form at random non-adjacent sites, causing
strain to accumulate as sketched in Figure 8.3(b). A crush band first forms
at site (1), the weakest section of the foam. The average normal strain in
the band increases to a saturated value of about 30% nominal strain, and
then a new crush band forms elsewhere (sites (2) and (3)), as is sometimes
observed in monotonic tests. Type II behavior has been observed for
Alporas of relative density 0.08 and for Alcan Al–SiC foams. A density
gradient in the loading direction leads to the result that the number of crush
bands formed in a test depends upon stress level: high-density regions of the
material survive without crushing. Consequently the number of crush bands
and the final strain developed in the material increases with increasing stress
(Figure 8.4(b)) for an Alcan foam of relative density 0.057.