Composite structures are usually designed to withstand forces in the x and y directions, with the fibre reinforcements being arranged accordingly. However, when a force is applied in the z direction, i.e. perpendicular to the surface, the fibres have little ability to resist it.
To address this problem, a sandwich structure is commonly used, which consists of two face sheets of traditional composite laminates, a core, and adhesive materials to hold the structure together. The primary function of the core is to increase stiffness and flexural strength and reduce warping of the laminate by effectively ‘thickening’ it. This can provide a dramatic increase in stiffness for very little additional weight. Thermal conductivity, sound insulation, and fire resistance can also be improved by use of the proper core material.
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, automobiles, wind turbine blades 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.
There are a number of types of core materials available, with a wide range of properties and costs: