Metal Foams: A Design Guide

5.4 Where might metal foams excel?
Design analysis for material selection 61
Material indices help identify applications in which a material might excel.
Material-selection charts like those shown in Chapters 4, 11 and 12 allow the
values of indices for metal foams to be established and compared with those
of other engineering materials. The comparison reveals that metal foams have
interesting values of the following indices:
1. The index E1/3 / which characterizes the bending-stiffness of lightweight
panels (E is Young’s modulus and the density). A foam panel is lighter,
for the same stiffness, than one of the same material which is solid. By
using the foam as the core of a sandwich structure (Chapter 10) even greater
weight saving is possible. Metal foam sandwiches are lighter than plywood
panels of the same stiffness, and can tolerate higher temperatures. Their
weight is comparable with that of waffle-stiffened aluminum panels but
they have lower manufacturing cost.
2. The index 1/2
y / which characterizes the bending-strength of lightweight
panels ( y is the elastic limit). A foam panel is stronger, for a given weight,
than one of the same material which is solid. Strength limited foam-core
sandwich panels and shells can offer weight savings over conventional
stringer-stiffened structures (Chapters 7 and 10).
3. The exceptional energy-absorbing ability of metal foams is characterized
by the index plεD which measures the energy absorbed in crushing the
material up to its ‘densification’ strain εD ( pl is the plateau stress). Metal
foams absorb as much energy as tubes, and do so from any direction
(Chapter 11).
4. The index E1/3 / which measures the ability of a panel to damp flexural
vibrations ( is the mechanical loss coefficient). High values of this index
capture both high natural flexural vibration frequencies of metal foams
(suppressing resonance in the acoustic range) and the ability of the material
to dissipate energy internally.
5. The index Cp which characterizes the time-scale for penetration of a
thermal front through an insulating layer of given thickness; it also characterizes
the total thermal energy lost in the insulation of an oven or furnace
in a thermal cycle (Cp is the specific heat and is the thermal conductivity).
In both cases low values of the index are sought; foams offer these.
References
Ashby, M.F. (1999) Materials Selection in Mechanical Design, Butterworth-Heinemann, Oxford.
Ashby, M.F. and Cebon, D. (1997) Case Studies in Materials Selection, Granta Design Ltd,
Trumpington Mews, 40B High Street, Trumpington, Cambridge CB2 2LS, UK; tel: C44 1223
518895; fax: C44 1223 506432; web site: http//www.granta.co.uk
CES (1999) Granta Design Ltd, Trumpington Mews, 40B High Street, Trumpington, Cambridge
CB2 2LS, UK; tel: C44 1223 518895; fax: C44 1223 506432; web site: http//www.granta.co.uk