SANDWICH CONSTRUCTION:

BASIC DESIGN PRINCIPLES:

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 flatwise 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.

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.

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 flatwise 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.

igure 1-1. Possible modes of failure of sandwich composite under edgewise loads: General buckling, shear crimping, dimpling of facings, and wrinkling of facings either away from or into the core.

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 crimping.

If the core is of cellular (honeycomb) or corrugated material, it is possible for the facings to buckle or dimple into the spaces between core walls or corrugations as shown in figure 1-1C. Dimpling may be severe enough so that permanent dimples remain after removal of load and the amplitude of the dimples may be large enough to cause the dimples to grow across the core cell walls and result in a wrinkling of the facings.

Wrinkling, as shown in figure 1-1D, may occur if a sandwich facing subjected to edgewise compression buckles as a plate on an elastic foundation. The facing may buckle inward or outward, depending on the flatwise compressive strength of the core relative to the flatwise tensile strength of the bond between the facing and core. If the bond between facing and core is strong, facings can wrinkle and cause tension failure in the core. Thus, the wrinkling load depends upon the elasticity and strength of the foundation system; namely, the core and the bond between facing and core. Since the facing is never perfectly flat, the wrinkling load will also depend upon the initial eccentricity of the facing or original waviness.

The local modes of failure may occur in sandwich panels under edgewise loads or normal loads. In addition to overall buckling and local modes of failure, sandwich is designed so that facings do not fail in tension, compression, shear, or combined stresses due to edgewise loads or normal loads, and cores and bonds do not fail in shear, flatwise tension, or flatwise compression due to normal loads.

The basic design principles can be summarized into four conditions as follows:

1. Sandwich facings shall be at least thick enough to withstand chosen design stresses under design1 loads.

2. The core shall be thick enough and have sufficient shear rigidity and strength so that overall sandwich buckling, excessive deflection, and shear failure will not occur under design1loads.

3. The core shall have high enough moduli of elasticity, and the sandwich shall have great enough flatwise tensile and compressive strength so that wrinkling of either facing will not occur under design— loads.

4. If the core is cellular (honeycomb) or of corrugated material and dimpling of the facings is not permissible, the cell size or corrugation spacing shall be small enough so that dimpling of either facing into the core spaces will not occur under design1 loads.

The choice of materials, methods of sandwich assembly, and material properties used for design shall be compatible with the expected environment in which the sandwich is to be utilized. For example, facing to core bonding shall have sufficient flatwise tensile and shear strength to develop the required sandwich panel strength in the expected environment. Included as environment are effects of temperature, water or moisture, corrosive atmosphere and fluids, fatigue, creep, and any condition that may affect material properties.

Certain additional characteristics, such as thermal conductivity, resistance to surface abrasion, dimensional stability, permeability, and electrical properties of the sandwich materials should be considered in arriving at a thoroughly efficient design for the intended purpose.

Detailed procedures giving formulas and graphs for use in structural design are given in subsequent sections of this Handbook. The formulas and graphs can be used to determine dimensions of facings and core as well as necessary core properties for sandwich components under various types of loads. Graphs and formulas are presented in terms of general parameters, and are not for specific materials. Design procedures involving buckling are based on theoretical buckling coefficients. These coefficients are in fair agreement with average test results, but allowance can be made in the final design to account for the scatter characteristic of buckling test results, perhaps by choosing a slightly thicker core, so that buckling of the sandwich component does not occur at design load.

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