Honeycomb structures are a lattice of hollow, thin-walled cells with relatively high compression and shear properties out-of-plane while boasting a low density.
Although the shape of honeycomb structures can vary widely, the common feature in all honeycomb structures is a lattice of hollow, thin-walled cells which are often hexagonal and columnar. Honeycomb structures allow for the minimization of materials to save on both weight and cost during the design process. Additionally, honeycomb structures have relatively high compression and shear properties out-of-plane while boasting a low density, meaning that they have very high specific strengths. For that reason, honeycomb structure materials are widely used in the aerospace industry.
Honeycomb Mesh Materials
A common use of honeycomb materials in the aerospace industry is to employ a honeycomb mesh to either reduce or create wind turbulence as the situation demands. An important factor when designing the mesh is the length ratio (length vs. diameter of the honeycomb cell). For length ratios less than one, turbulence intensity is decreased, making these materials ideal for use on the front grill of a vehicle. On the other hands, length ratios much larger than one tend to reduce the lateral turbulence and eddies of the wind flow. However, when used without screens, they increase the turbulence intensity and for that reason both honeycombs and screens are generally used in modern wind tunnels.
Natural Examples of Honeycomb Structures
Natural examples of honeycomb structures include beehives and honeycomb weathering in rocks, tripe, and bone. Generally, man-made honeycomb structures are sandwich-structure composites with thin plates surrounding honeycomb cores. Depending on the situation, many different materials can be sued to construct the core including paper and thermoplastics for low loads or aluminum or fiber reinforced plastics for high loads.
Mechanical Properties of Honeycomb Structures
The mechanical properties of honeycomb structures are orthotropic, meaning that their values change when the orientation of the stress with respect to the material changes. Therefore, the two planes of symmetry must be identified and distinguished. The L-direction is the strongest direction, and the W-direction (located 60º from the L-direction if the honeycomb is a regular hexagon) is the most compliant direction.
There are three traditional techniques for producing honeycomb materials: expansion, corrugation, and molding. Today, composite honeycomb materials are produced using expansion and corrugation. Metal (usually aluminum) honeycomb materials are produced solely through the expansion process. In contrast, thermoplastic honeycomb materials are generally produced through extrusion processes which are sliced to form honeycomb sheets.
Although honeycomb structures are mainly known for their use in the aerospace industry, they can be found in a variety of other applications such as packing materials, racing boat shells, and even construction. In the future, we may even see the use of honeycomb materials expand further into industries they are already used in, like the sporting goods industry or into industries that have yet to adopt them into use.