## Psy5 r is

Waffle panel

Waffle panel

ds dt ds dt

Sandwich panel c

Sandwich panel

Figure 10.15 A waffle-stiffened panel and a sandwich panel loaded in bending

Figure 10.15 A waffle-stiffened panel and a sandwich panel loaded in bending where the web depth, ds, is defined in Figure 10.15, with E0 as Young's modulus for the material comprising the panel. At equal weights of the sandwich and waffle panels, the web depth of the waffle is

Comparison with the result for c/£ in equation (10.31) for the globally optimized sandwich panel at equivalent weights gives:

ds V6

This result is stiffness independent because the sandwich and waffle panels have the identical functional dependence. Accordingly, a waffle panel made from the same material as a sandwich panel (p/ = E/ = E0) has a slightly smaller overall thickness at the same weight and stiffness. The choice between sandwich and waffle panels, therefore, depends primarily on manufacturing cost and durability.

10.8 Strength-limited designs

### Cylindrical shells

Strength-limited sandwich structures can be weight competitive with stiffener-reinforced designs (the lowest weight designs in current usage). Shells are a more likely candidate for sandwich construction than axially compressed panels or columns because both hoop and axial stresses are involved, enabling the isotropy of sandwich panels to be exploited. There are two basic requirements for sandwich shells: (1) sufficient core shear stiffness for adequate buckling strength, (2) sufficiently large yield strength of the metal foam to maintain the buckling resistance of the shell, particularly in the presence of imperfections. Numerical methods are needed to determine minimum weights of both sandwich and stringer-reinforced configurations. Some prototypical results for a cylindrical shell under axial compression illustrate configurations in which sandwich construction is preferred.

General considerations

The perfect cylindrical shell buckles axisymmetrically at a load per circumferential length, N, given by (Tennyson and Chan, 1990)

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