Metal Foams: A Design Guide

Sandwich structures 139
Note that, as the stiffness index increases, the face sheet thicknesses needed
to achieve minimum weights increase substantially, relative to core thickness
and density. Consequently at lower stiffnesses yielding is more likely to intervene
because of the thinner face sheets and lower core densities at the global
weight minimum (equation (10.31)). For yielding to be avoided, the loads on
the structure must by limited by equations (10.37) and (10.39).
Stiffened panels
The principal competitors for sandwich systems subject to biaxial
bending are waffle-stiffened panels (Figure 10.15). For comparison, it is
convenient to re-express the result for the globally optimized sandwich beam
(equation (10.32)) in the form:
W
bℓ 2 D � � �
3/5
P/υ
15
f
bEf B 1/5
1 ⊲18˛2B2⊳ 2/5
�
⊲10.41⊳
For a waffle panel subject to bending about one of the stiffener directions, the
weight and stiffness are related by:
� �� �2 W 72 P/υ ℓ
D ⊲10.42⊳
2
bℓ 5 E0B1b
d s
Waffle panel
d s
Sandwich panel
c
ds
d t
d t
Figure 10.15 A waffle-stiffened panel and a sandwich panel loaded in
bending
M
M