Metal Foams: A Design Guide

196 Metal Foams: A Design Guide
allow simple, effective attachment. Typical of their use is the attachment of
face-sheets to metal foam cores in sandwich-panel construction.
Fasteners
Embedded fasteners (Figure 15.2), when strongly bonded by threads or adhesives
to the foam itself, pull out at an axial load
Ff D 2 R⊲ℓ x⊳ s ⊲15.1⊳
x
2R
F s
Figure 15.2 Embedded fasteners: wood screws and inserts
F p
where 2R is the diameter, ℓ is the embedded length of the fastener, x ³ R is a
small end correction allowing for the tapered tip of the fastener if there is one,
and s is the shear-yield strength of the foam. This last can be estimated as
� �3/2 s D 0.2
y,s
⊲15.2⊳
giving
�
Ff ³ 0.4 R⊲ℓ x⊳
s
s
� 3/2
y,s
t
F s
F p
⊲15.3⊳
where y,s is the yield strength of the solid of which the foam is made.
Figure 15.3 shows the dependence of pull-out load on foam density, on logarithmic
scales, for a range of fasteners. All scale with density in the way
described by Equation (15.3). Experiments confirm the dependence on length
and diameter.
Bolted fasteners (Figure 15.4) fail when the head of the fastener pulls
through the foam. The pull-through load is
Fp D ⊲R 2 w R 2 ⊳ c C 2 Rw ⊲15.4⊳
The first term is simply the crushing strength, c, of the foam times the contact
area of the washer, of radius Rw. The second accounts for the tearing of the