Metal Foams: A Design Guide

Sandwich structures 147
within the stringers, as well as the panels, of a stringer-stiffened configuration.
This construction has lowest weight at small levels of load index.
Further minimization with core density leads to more pronounced weight
savings (Figure 10.19). In this case, even flat sandwich panels can have lower
weight than stringer-stiffened panels, especially at lower levels of load index.
The failure modes governing the weight change as the load index changes and
the minimum weights coincide with simultaneous occurrences of either two
or three modes, as in the case of the optimally designed cylindrical shells.
The challenge in taking advantage of the potential weight savings arises in
manufacturing and relates to the low relative densities required to realize these
performance levels (Figure 10.19) and the need for acceptable morphological
quality.
Results for columns (Figure 10.20) indicate that thin-walled sandwich tubes
are lighter than foam-filled and conventional tubes, but the beneficial load
ranges are small.
Weight Index
ψ = W /ρ 3 4
s (×10 )
8
6
4
2
0
0
Elastic
ρ c /ρ s = 0.1
ε f y = 0.007
1 2 3
Empty tube
Π = P/s f y 2 (×10 6 )
Load index
Porous annulus
Buckling mode
4 5
Figure 10.20 Axially compressed columns
Configuration
P
P