These are a type of laminar composite composed of a relatively thick, low-density core between faces of comparatively higher density. Structural sandwiches can be compared to I beams. The facings correspond to the flanges; the objective is to place a high-density, high-strength material as far from the neutral axis as possible, thus increasing the section modulus. The bulk of a sandwich is the core. Therefore, it is usually lightweight for high strength-to-weight and stiffness-to-weight ratios. However, it must also be strong enough to withstand normal shear and compressive loadings, and it must be rigid enough to resist bending or flexure.
Core Materials
Core materials can be divided into three broad groups: cellular, solid, and foam. Paper, reinforced plastics, impregnated cotton fabrics, and metals are used in cellular form. Balsa wood, plywood, fiberboard, gypsum, cement-asbestos board, and calcium silicate are used as solid cores. Plastic foam cores — especially polystyrene, urethane, cellulose acetate, phenolic, epoxy, and silicone — are used for thermal-insulating and architectural applications. Foamed inorganics such as glass, ceramics, and concrete also find some use. Foam cores are particularly useful where the special properties of foams are desired, such as insulation. And the ability to foam in place is an added advantage in some applications, particularly in areas that are difficult to reach.
Cellular Cores
Of all the core types, however, the best for structural applications are the rigid cellular cores. The primary advantages of the cellular core are that (1) it provides the highest possible strength-to-weight ratio, and (2) nearly any material can be used, thereby satisfying virtually any service condition.
There are, essentially, three types of cellular cores: honeycomb, corrugated, and waffle. Other variations include small tubes or cones and mushroom shapes. All these configurations have certain advantages and limitations. Honeycomb sandwich materials, for example, can be isotropic, and they have a high strength-to-weight ratio, good thermal and acoustical properties, and excellent fatigue resistance. Corrugated-core sandwich is anisotropic and does not have as wide a range of application as honeycomb, but it is often more practical than honeycomb for high production and fabrication into panels.
Construction
Theoretically, any metal that can be made into a foil and then welded, brazed, or adhesive-bonded can be made into a cellular core. A number of materials are used, including aluminum, glass-reinforced plastics, and paper. In addition, stainless steel, titanium, ceramic, and some superalloy cores have been developed for special environments.
One of the advantages of sandwich construction is the wide choice of facings, as well as the opportunity to use thin sheet materials. The facings carry the major applied loads and therefore determine the stiffness, stability, and, to a large extent, the strength of the sandwich. Theoretically, any thin, bondable material with a high tensile- or compressive-strength-to-weight ratio is a potential facing material. The materials most commonly used are aluminum, stainless steel, glass-reinforced plastics, wood, paper, and vinyl and acrylic plastics, although magnesium, titanium, beryllium, molybdenum, and ceramics have also been used.
Theory
The theory of sandwich materials and functions of the individual components may best be described by making an analogy to an I-beam. The high-density facings of a sandwich correspond to the flanges of the I-beam; the objective is to place a high-density, high-strength material as far from the neutral axis as possible to increase the section modulus without adding much weight. Honeycomb in a sandwich is comparable to the I-beam web that supports the flanges and allows them to act as a unit. The web of the I-beam and honeycomb of the sandwich carry the beam shear stress. Honeycomb in a sandwich differs from the web of an I-beam in that it maintains a continuous area support for the facings, allowing them to carry stresses up to or above the yield strength without crippling or buckling. The adhesive that bonds honeycomb to its facings must be capable of transmitting shear loads between these two components, thus making the entire structure an integral unit.
When a sandwich panel is loaded as a beam, the honeycomb and the bond resist the shear loads while the facings resist the moments due to bending forces, and hence carry the beam bending as tensile and compres-sive load. When loaded as a column, the facings alone resist the column forces while the core stabilizes the thin facings to prevent buckling, wrinkling, or crippling.
Advantages
The largest single reason for the use of sandwich construction and its rapid growth to one of the standard structural approaches during the past 40 years is its high strength-to-weight or stiffness-to-weight ratio. As an example consider a 0.6-m-span beam with a width of 0.3 m and supporting a load of 1634.4 kg at the midspan. This beam, if constructed of solid steel, would have a deflection of 14.2 mm and weigh 31.15 kg. A honeycomb-sandwich beam using aluminum skins and aluminum cores and carrying the same total load at the same total deflection would weigh less than 3.6 kg. As an interesting further comparison, a magnesium plate to the same specifications would weigh 11.9 kg and an aluminum plate 15.7 kg. Although such clear and simple cases of comparative strength, weight, and stiffness are not normally found in actual designs, it has generally been found that equivalent structures of sandwich construction will weigh from 5% to 80% less than other minimum weight structures and frequently possess other significant advantages.
Other advantages of sandwich construction include extremely high resistance to vibration and sonic fatigue, relatively low noise transmission, either high or low heat transmission depending on the selection of core materials, electrical transparency (varying from almost completely transparent in the case of radom structures to completely opaque in the case of metal sandwich structures), relatively low-cost tooling when producing complex aircraft parts, ability to mass-produce complicated shapes, ability to absorb damage and absorb energy while retaining significant structural strength, and flexibility of design available.
