Metal Foams: A Design Guide

94 Metal Foams: A Design Guide
Cumulative strain
Axial stiffness, E/E o
0
−0.1
−0.2
−0.3
−0.4
−0.6
R = 0.1
0 102104 106 −0.5
R = 0.5
σmax σ
= 0.94
pl
1
0.9
0.8
0.7
0.6
0.5
0.4
1.2
Cycles, N
Cyclic
0.3 0 0.1 0.2 0.3
Figure 8.4 (continued)
(c)
1.0
0.89
Monotonic
Nominal compressive strain, ε
(d)
0.74
0.67
In designing with metal foams, different fatigue failure criteria are
appropriate for tension–tension loading and compression–compression
loading. Material separation is an appropriate failure criterion for
tension–tension loading, while the initiation period for progressive shortening
is appropriate for compression–compression loading. Often, a distinct knee
on the curve of strain versus cycles exists at a compressive strain of 1–2%,
and the associated number of cycles, NI, is taken as the fatigue life of the
material.
8.3 S–N data for metal foams
Test results in the form of S–N curves are shown in Figure 8.5 for a number of
aluminum alloy foams. Tests have been performed at constant stress range, and