Sandwich-structured composites are easier to manufacturer and can provide both a higher strength against bending and a lower density when used instead of traditional materials and are commonly used in high-demand applications.
Sandwich structured composites are created by layering a thick, lightweight core between two thin, stiff sheets. The core, which is generally either open- or closed-cell foam, allows the overall composite to achieve both high stiffness and low density without the use of more expensive materials. Thermoplastics, thermosetting polymers, or sheet metal are widely used to form the outer sheets, or skin, which is then bonded together with an adhesive layer.
Sandwich structures can be categorized according to both the way the core supports the skin as well as the type of core. The core can support the skin in five different ways: homogeneously, locally, regionally, unidirectionally, and bidirectionally. Because the bidirectionally supported structure gives the best performance to weight ratio, it is used for the most mechanically demanding applications, such as applications within the aerospace industry.
Additionally there are three simple core types with 2D isotropic patterns: triangular, hexagonal, and starcell. While all three of these core types exhibit the same stiffness normal to their faces, triangular cores have a lower strength than the other two types. Starcell cores have a higher flexibility than the other two, meaning that this type of core would be ideal for applications that require a curved sandwich structure. There are many more types of cores that can be used, and the cell patterns can become quite complex for specialized applications.
Ain't That Tough Enough?
The composite’s strength depends on both the strength of the skin as well as the interface between the skin and the core. The stresses are felt in very different ways in the core material and the skin, causing the adhesive between the two layers to feel a significant shear force. Therefore, if the bond is weak, the composite can very easily delaminate and fall apart. Similarly, if the sandwiched composite is subjected to a bending force, one skin will experience compression and the other skin will experience tension. If the skins are unable to withstand these stresses, the skin can crack and will ultimately fail, leaving the soft foam core to bear the entirety of the stress. In that situation, the core will likely fail very quickly after the failure of the skin due to its low tensile strength. To combat this, the core should be as thick as is reasonable to increase the strength of the overall composite against bending.
Sandwich structured composites can provide both a higher strength against bending and a lower density when used instead of traditional materials and are commonly used in high-demand applications. Additionally, they are simpler to manufacture than conventional composites as the two phases are kept separate and do not have to be thoroughly mixed as with other composite types, such as nanocomposites.
To learn more about the three different categories of composites, check out "The Inside Scoop on Composites."