Core Materials for Sandwich Structures

Bonded sandwich structures have been a basic component of the composites industry for over 45 years. The concept of using relatively thin, strong face sheets bonded to thicker, lightweight core materials has allowed the industry to build strong, stiff, light and highly durable structures that otherwise would not be practical. This technology has been demonstrated in boats, trucks, and building panels. A 3% weight increase can increase the flexural strength and stiffness by a magnitude of 3.5 times and 7 times respectively if cores and skins are properly chosen. The structure then acts more or less monolithically.

The most common comparison made is that of a composite sandwich to an I-beam. The panel skins, like the flanges of the I-beam, carry the stresses imposed by use. The stresses are transferred between the top and bottom skins through shear stresses that run through the core or web of the I-beam. The purpose of an I-beam is to lessen the weight required to support a given load in bending. Since the highest stresses are carried at the extremities, both the top and bottom of the I-beam, the center section can be much narrower in width in relation to the flanges. In a sandwich structure, the core will generally have the same width and length dimensions as the skins, but can be much weaker than the skins since it primarily experiences shear stresses. Care must be taken in design to ensure that the shear carrying capability of the expected loads does not exceed both the core and the adhesive.

Face sheets can be of almost any material. In the composites industry, the most common face sheets are glass and carbon. The common core materials are foam, syntactic foam, honeycomb, and balsa wood.  Some core materials can be shaped, such as a waffle pattern or corrugation to achieve the desired mechanical properties.