COMPOSITE MATERIALS

Composite Materials



Composite Materials

Structural sandwich is a layered composite formed by bonding two thin facings to a thick core. It is a type of stressed-skin construction in which the facings resist nearly all of the applied edgewise (in-plane) loads and flat wise bending moments. The thin spaced facings provide nearly all of the bending rigidity to the construction. The core spaces the facings and transmits shear between them so that they are effective about a common neutral axis. The core also provides most of the shear rigidity of the sandwich construction. By proper choice of materials for facings and core, constructions with high ratios of stiffness to weight can be achieved (Halpin, 1984).

A basic design concept is to space strong, thin facings far enough apart to achieve a high ratio of stiffness to weight; the lightweight core that does this also provides the required resistance to shear and is strong enough to stabilize the facings to their desired configuration through a bonding medium such as an adhesive layer, braze, or weld. The sandwich is analogous to an I-beam in which the flanges carry direct compression and tension loads, as do the sandwich facings, and the web carries shear loads, as does the sandwich core (Kujala, 2000).

In order that sandwich cores be lightweight, they are usually made of low-density material, some type of cellular construction (honeycomb-like core formed of thin sheet material), or of corrugated sheet material. As a consequence of employing a lightweight core, design methods account for core shear deformation because of the low effective shear modulus of the core. The main difference in design procedures for sandwich structural elements as compared to design procedures for homogeneous material is the inclusion of the effects of core shear properties on deflection, buckling, and stress for the sandwich.

Because thin facings can be used to carry loads in a sandwich, prevention of local failure under edgewise direct or flat wise bending loads is necessary just as prevention of local crippling of stringers is necessary in the design of sheet-stringer construction. Modes of failure that may occur in sandwich under edge load are shown in figure 1-1.

Figure 1-1

Shear crimping failure (fig. 1-1B) appears to be a local mode of failure, but is actually a form of general overall buckling (fig. 1-1A) in which the wavelength of the buckles is very small because of low core shear modulus. The crimping of the sandwich occurs suddenly and usually causes the core to fail in shear at the crimp; it may also cause shear failure in the bond between the facing and core.

Crimping may also occur in cases where the overall buckle begins to appear and then the crimp occurs suddenly because of severe local shear stresses at the ends of the overall buckle. As soon as the crimp appears, the overall buckle may disappear. Therefore, although examination of the failed sandwich indicates crimping or shear instability, failure may have begun by overall buckling that finally caused ...