Nature and the bees beat us to it with real honeycomb and as usual, Mother Nature came up with a great design concept (also found in stomach lining and animal bone). Nature uses a honeycomb structure because it is a highly efficient design. Us humans copied her and honeycomb is the word we now use to describe a form of material structure often used in composite sandwich panels. The term ‘periodic cellular material’ is also used for honeycomb, though this can also include 3D cellular designs, whereas honeycomb is only 2D.
Aerospace, yacht and motor racing designers use the phrase ‘Build lightness in to your design’. It is a powerful mantra and honeycomb sandwich enables a designer to do just that. The honeycomb form provides stiffness, strength and light weight, whilst still offering a degree of flexibility. The technical terms for this are ‘high out-of-plane shear strength’ and ‘high out-of-plane compression properties’ in relation to other design options.
Real bee honeycomb has a hexagonal cell shape, but man-made structural honeycomb has a wide variety of cell shape options which can be tailored to specific applications. These cell shapes range from square and triangular to complex geometric designs.
The earliest recorded man-made use of the design was in the late 19th century for ornamental paper and then a metallic form for bee keeping.
The first recorded use for industrial honeycomb in a sandwich structure was in 1915 by Hugo Junkers who founded the German airplane company of that name. In 1938 an adhesive with the right viscosity to form resin fillets on the honeycomb core was patented. This led the way to modern sandwich panel designs.
How Honeycomb is Made
When you look at honeycomb core, it looks as if it might be difficult to fabricate, but there are three main methods in use today: expansion, corrugation and molding. The three materials most commonly used are thermoplastics, paper/aramid and a variety of metals including aluminium, stainless steel and even titanium.
Molding is probably the simplest to envision (and make at home), best suited to thermoplastics. Thermoplastics can also be extruded as pipes and sliced, and the latest technique offers continuous extrusion and bonding to a sheet – co-extrusion.
Expansion is simple too. Sheets of the material are laid down with lines of adhesive between them. When the adhesive has cured, the block (known as HOBE – honeycomb before extraction) is sliced and the sheets are pulled apart, resulting in the honeycomb. Think of an expanding paper chain – that’s the basic principle. Instead of lines of adhesive, welds can be used for metals sheets.
Corrugation lends itself to materials such as metals which can be rolled over a corrugated roller and stacked. Adhesives or resistance welding is used to attach each layer to the next. Finally, the block is sliced into sheets ready to be bonded into the surface layer sheets.
A fourth method is also in use. This utilizes slotted strips of the material being set up in a grid assembly.
Besides aerospace, yacht, ship and motor sport, the outstanding performance that this structural design offers is used wherever there lightweight but strong bulkheads, doors and partitions are needed. Low cost, low weight interior doors are made from a simple wooden frame, cardboard honeycomb and wood veneers.
In the trucking industry, using honeycomb material in a 53’ trailer can reduced its weight by almost 1.5 tons, saving over 1% in fuel costs.
Metal honeycomb offers safety features too. Explosion-proof fuel tanks have used metal honeycomb.
The capability of this versatile structure is being exploited with new uses and materials being developed regularly. Sheets of honeycomb can be profiled before covering. This tailors its limited flexibility properties and with modern laser cutting technology this technique is in the early days of development.
General Dynamics has announced an external airplane fuel tank design which uses a honeycomb core made of urethane foam-filled aramid and an all composite shell. This all-composite construction delivers the structural stiffness needed for aircraft carrier survivability requirements and will replace the metal honeycomb tanks which came into use following a disastrous fire on the USS Forrestal.