Sandwich structures

Sandwich panels offer high stiffness at low weight. Their cores, commonly, are made of balsa-wood, foamed polymers, glue-bonded aluminum or Nomex (paper) honeycombs. These have the drawbacks that they cannot be used much above room temperature, and that their properties are moisture-dependent. The deficiencies can be overcome by using metal foams as cores. This chapter elaborates the potential of metal-foam-cored sandwich structures.

Competition exists. The conventional way of stiffening a panel is with stringers: attached strips with a profile like a Z, a ? or a top hat. They are generally stiffer for the same weight than sandwich structures, at least for bending about one axis, but they are anisotropic (not equally stiff about all axes of bending), and they are expensive. Metfoam-cored sandwiches are isotropic, can be shaped to doubly curved surfaces, and can be less expensive than attachment-stiffened structures.

Syntactic foams - foams with an integrally shaped skin - offer additional advantages, allowing cheap, light structures to be molded in a single operation. Syntactic polymer foams command a large market. Technologies are emerging for creating syntactic metfoam structures. It is perhaps here that current metal-foam technology holds the greatest promise (see Chapter 16).

10.1 The stiffness of sandwich beams

The first four sections of this chapter focus on the stiffness and strength of sandwich beams and plates (Figure 10.1). In them we cite or develop simple structural formulae, and compare them with experimental results and with the predictions of more refined finite-element calculations. Later sections deal with optimization and compare metfoam-cored sandwiches with rib-stiffened panels.

Consider a sandwich beam of uniform width b, with two identical face-sheets of thickness t perfectly bonded to a metallic foam core of thickness c. The beam is loaded in either in a four-point bend, as sketched in Figure 10.2(a), or a three-point bend as shown in Figure 10.2(b). For both loading cases, the span between the outer supports is I, and the overhang distance beyond the outer supports is H. We envisage that the beams are

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