The structural sandwich is generic name for a group of materials in which we build the composite using a thick, low density core material, faced on both surfaces with a thin, high-strength skin, or facing. The sandwich has to have a very strong and rigid bond which holds the facings in good contact with the core. The skins must also be as flat as possible.
The strength and rigidity of this attachment is so important that a structural sandwich made with rubber cement or other soft adhesives does not qualify as a sandwich, with predictable properties.
The reason that the sandwich form of composites has been so widely used lies in the fact that the thin, strong facings can do a great job when they are both separated and supported by the core. As an illustration, consider a laminate, and consider its stiffness, strength and weight as having a relative value of one. The sketch below shows how these values increase when a core material is used to make a sandwich out of the same total amount of skin material (laminate is split in half and core is inserted).
The values in the example have been calculated for a highly efficient honeycomb core material and adhesive system, but the principle is true for most forms of core. A sandwich is simply the ultimate that can be obtained in efficiency for structural members such as plates, beams and large surfaces that have to carry a bending or column load. The sketch points out the importance of making the core as thick as practical, and most sandwiches do have a core that is very much thicker than the facings. Because this method of design uses a large amount of material for core, it also follows that a very low density core will save much more weight in a structure when compared to a high density core.
It is normal practice to compare core materials to each other by quoting the strength at a given density. The strength most often quoted is the compressive strength, as it is the easiest to visualize and to test. It is common to also look at the shear strength and the shear modulus. (The term “modulus” is a measure of how much a material will deflect under a given load. A low modulus means that deflections, or bending, will be high, and a high modulus gives smaller deflections.)
In the sandwich structures field, the happy situation is that a great many materials make very good sandwich cores.