Metal Foams: A Design Guide

98 Metal Foams: A Design Guide
ductile manner when the net section stress equals the uniaxial strength? Or
does it fail in a notch-sensitive, brittle manner, when the local stress at the
edge of the hole equals the uniaxial strength? We can answer this question
immediately for the case of static compression of a foam containing a circular
hole: the large compressive ductility in a uniaxial compression test makes
the foam notch insensitive. Experimental results confirm this assessment: a
panel of width W, containing a hole of diameter D fails when the net section
stress ns equals the uniaxial compressive strength pl of the foam. On noting
that ns is related to the applied stress 1 by ns D ⊲1 ⊲D/W⊳⊳ 1 the net
section failure criterion can be rewritten as
1 D pl⊲1 ⊲D/W⊳⊳ ⊲8.2⊳
Figure 8.7 confirms that this relation is obeyed for Alporas, Alulight and Alcan
aluminum foams, for panels cut in a variety of orientations with respect to the
direction of foaming.
σ ∞
σ pl
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Notch sensitive,
σ
0 0.2 0.4 0.6
D/W
∞
σ
=
pl i
kt Alporas, 11%, L, Compression W = 70mm
Alporas, 11%, T, Compression W = 70mm
Alporas, 5.7%, L, Tension W = 70mm
Alporas, 5.7%, T, Tension W = 70mm
Alcan, 5.7%, L, Compression W = 115mm
Alcan, 5.7%, L, Compression W = 115mm
Alcan, 5.7%, L, Tension W = 115mm
Alcan, 5.7%, L, Tension W = 115mm
Alulight, 25−35%, L, Compression W = 20mm
Alulight, 25−35%, L, Tension W = 20mm
Notch lnsensitive,
σ∞ σ
= 1 −
pl D
W
0.8 1 1.2
Figure 8.7 Notch strength of foams, showing behavior to be notch
insensitive in both monotonic tension and compression
For the case of compression–compression fatigue an analogous notched
strength criterion applies as follows. Define the endurance limit for a unnotched
panel or a notched panel as the gross-section value of max in a fatigue
test for which progressive shortening does not occur (NI > 10 7 cycles). Then
W
D
σ ∞