Ffyc 4t of

for both three- and four-point bending. The error incurred by taking Ffy instead of Ffy is small for typical sandwich panels with a metallic foam core. For example, upon taking the representative values t/c = 0.1 and oy/of = 0.02, we obtain an error of 15% according to equation (10.11). It is safer to design on Ffy than on FJ} because a low-ductility core will fracture almost as soon as it yields, causing catastrophic failure.

Indentation

The indentation mode of collapse involves the formation of four plastic hinges within the top face sheet adjacent to each indenter, with compressive yield of the underlying core, as sketched in Figure 10.3. Early studies on the indentation of polymer foams (for example, Wilsea et al., 1975) and more recently on metal foams (Andrews et al., 1999) reveal that the indentation pressure is only slightly greater than the uniaxial compressive strength. The underlying cause of this behavior is the feature that foams compress with little transverse expansion (see Chapter 7 and Gibson and Ashby, 1997).

Plastic hinge

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