Metal Foams: A Design Guide

Sandwich structures 145
plastic yielding, which reduces the local stiffness of the shell and, in
turn, hastens collapse. Since they always exist, practical designs take this
imperfection sensitivity into account. Generally, experimental results establish
a knockdown factor on the theoretical loads that may be used as the design
limit with relative impunity. Guidelines for such an experimental protocol are
provided by two conclusions from a study of the interaction between plasticity
and imperfections in optimally designed axially compressed sandwich shells
(Hutchinson and He, 1999). When the perfect cylindrical shell is designed
such that buckling and face sheet yielding coincide, buckling in the imperfect
shell nearly always occurs prior to plastic yielding. Thus, knockdown factors
obtained from standard elastic buckling tests are still applicable to the
optimally designed shells. A similar statement pertains when core yielding
in the perfect shell is coincident with buckling.
Weight index
ψ = W/ρs 2b(103 )
12
10
8
6
4
2
0
0 2 4
Π = P/s f y b(×103 )
Load index
Buckling mode
ρ c /ρ s = 0.1
ε f y = 0.007
6
Thin sandwich
structure
Configuration
Figure 10.18 Minimum weight axially compressed panels
b
P