54 Where might metal foams excel

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/p which characterizes the bending-stiffness of lightweight panels (E is Young's modulus and p 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 oly/2/p which characterizes the bending-strength of lightweight panels (oy 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 apleD which measures the energy absorbed in crushing the material up to its 'densification' strain eD (apl is the plateau stress). Metal foams absorb as much energy as tubes, and do so from any direction (Chapter 11).

4. The index qE1/3/p which measures the ability of a panel to damp flexural vibrations (q 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 CppX 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 X is the thermal conductivity). In both cases low values of the index are sought; foams offer these.

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