Metal Foams: A Design Guide

92 Metal Foams: A Design Guide
Cumulative
strain,
ε
Cumulative
strain,
ε
0
0
ε max
ε max
ε min
ε min
Log cycles, N
Log cycles, N
(a)
(b)
1
3
2
β
∆σ,(∆ε)
Band spreading
∆σ,(∆ε)
Sequential
discrete
bands
Figure 8.3 Typical behaviors in compression–compression fatigue of
metallic foams at a fixed stress range. (a) Progressive shortening by
broadening of a single crush band with increasing cycles; (b) sequential
formation of crush bands
3. Type III behavior. A single crush band forms and broadens with increasing
fatigue cycles, as sketched in Figure 8.3(a). This band broadening event
is reminiscent of steady-state drawing by neck propagation in a polymer.
Eventually, the crush band consumes the specimen and some additional
shortening occurs in a spatially uniform manner. Type III behavior has
been observed for Alulight of composition Al–1Mg–0.6Si (wt%) and of
relative density 0.35, and for Alporas of relative density 0.11. Data for the
Alporas are presented in Figure 8.4(c). In both materials, the normal to the
crush bands is inclined at an angle of about 20° to the axial direction, as
sketched in Figure 8.3(a). The strain state in the band consists of a normal
strain of about 30% and a shear strain of about 30%.
A significant drop in the elastic modulus can occur in fatigue, as shown in
Figure 8.4(d) for Alporas. This drop in modulus is similar to that observed in
static loading, and is a result of geometric changes in the cell geometry with
strain, and cracking of cell walls. The precise details remain to be quantified.
A comparison of Figures 8.4(a)–(c) shows that all three types of shortening
behavior give a rather similar evolution of compressive strain with the number
of load cycles. Large compressive strains are achieved in a progressive manner.
We anticipate that this high ductility endows the foams with notch insensitivity
in compression–compression fatigue (see Section 8.4 below).