Honeycomb - Copied from Nature

Achieving maximum benefit with the lowest-possible mass - a technological challenge which is becoming increasingly important with the discussion about resource conservation.

© Wolfram Schroll

Honeycomb structures in daily life

Do you remember your last move? When the heavy load of books found its place on the shelves of your new home, how little the transport carton weighed - and yet, despite its load, it was not torn during transport, even though it was only made of cardboard! A glance to the edge of the cardboard lid explains the favourable ratio of weight to stability - the core of the cardboard, which is just a few millimetres thick and mostly hollow. To be more precise, it consists of a simple layer of corrugated cardboard glued between the thin layers of cardboard on the carton surface. Although the material is rather unstable, how much stability should be achieved by replacing the simple corrugations with a hexagonal structure? Natural sciences are teaching the enormous stability of this structure - it is not for nothing that it is found in the structure of numerous molecules.

But there it is not visible to the naked eye, as it is elsewhere in nature: in honeycombs. The name "Honeycomb" has therefore established itself for the technical application of this structure in the production of components. Thanks to their hexagonal structure, honeycombs have an incredible resilience, even though they are only made of the extremely soft material wax. A beekeeper, for example, can easily extract a whole kilo of honey from a honeycomb area of just 0.03 square metres under the high mechanical stress of centrifuging without the honeycombs breaking! 

Not only the packaging industry has learned from this long ago. The cardboard material, which is also not particularly robust but, like the beeswax, completely biodegradable, is also produced in honeycomb structures. From filling material for shock-sensitive packaged goods to edge protectors and even as intermediate plates for the stacking of heavy loads on transport pallets. The lesson of learning from nature's proven functional principles for technical applications by humans is called bionics.


Bionics: Key discipline of technological progress

The honeycomb and also honeycomb materials are by far not the only examples that explain bionics. The oldest example of this principle is probably Leonardo da Vinci's idea of transferring the flight of birds to flying machines. Even though the multitalented 15th-century man was never privileged to fly himself up into the air, he nevertheless created decisive foundations. A look at his wing drawings reveals striking similarities to the designs with which the aviation pioneer Otto Lilienthal succeeded for the first time at the end of the 19th century.

Our daily lives nowadays are soaked with the fruits of bionics. A seemsingly banal example are the velcro fasteners which are so common in our shoes and clothing today. Other examples are modern building glazings whose surface structure allows rain to roll off as if they were lotus leaves. At this point, at the latest, it becomes apparent that the achievements of bionics with which we are confronted on a daily basis usually remain invisible to us. And Honeycomb is an example par excellence of this "hidden achievement". The Dresden-based Elbe Flugzeug Werke (EFW), for example, advertises the fact that every flight today is on Saxon ground. The interior of commercial aircraft today is mainly made of honeycomb composite material - right up to the sandwich panels of the floor, which has to absorb considerable compressive loads in a fully loaded Airbus and at the same time be ultra-light and highly rigid. EFW not only supplies these floor panels for commercial aircraft, but also converts them into cargo aircraft using honeycomb elements and has even supplied components for the super cargo aircraft Beluga to the southern French Airbus assembly site in Toulouse.


 
 

Composite sructures and interiors for aircrafts ©EFW

The compressive strength of the panels made of modern fibre composites is provided by the honeycomb core. The bending stiffness is given by the bond with the very thin, flexurally but not break-resistant bonded cover panels. Combined with each other, this results in panels which, like the technical boards mentioned above, are very thin, light and yet highly stable.


Wide range of materials and production options

When honeycomb sandwich components are used in modern material mixes of glass and carbon fibre even in the exterior of aircraft, the light metal aluminium nevertheless remains a proven material in the aerospace industry. And even from this material with its favourable ratio of mass to stability, an astonishing weight saving is achieved by honeycomb construction. Hexagonal honeycomb structures made of aluminium with density weights between 160 kilograms and less than 30 kilograms per cubic meter are commonplace in industrial mass production today. For comparison: one cubic metre of the solid light metal weighs in at 2.7 tonnes! Advanced manufacturing techniques make it possible to produce thin honeycomb "foils" even in the micrometer range. If a cubic meter of a 5 millimeter "thick" foil with a weight of only 30 kilograms had to be used to achieve the greatest possible surface benefit, almost 250 square meters could be covered!

Due to the good deformability of the material, aluminium honeycomb cores can be brought into almost any geometric shape. This characteristic also enables a high absorption capacity of mechanical energy. Even if the foils in the micron range allow compressive loads of almost 1.5 kg/mm2 at higher densities of 160 kg/m3, stronger aluminium honeycomb cores are often used for crash elements. The fact that the material is corrosion- and temperature-resistant and thermally conductive makes Honeycomb even interesting for a wide range of technical applications from this classic material. If other technical properties are desired, the honeycomb structure is also made of plastics such as polypropylene.


Multiple Areas of Application

The variety of different materials, some of them combinable with each other, leads to almost unlimited application possibilities. The hexagonal honeycomb structures are no longer the domain of aerospace, road and rail construction or ships. In defence technology, mechanical engineering and even in architecture, parts made from a wide variety of materials using this design principle have also established themselves. They are supplied both as open structures and as cores in sandwich components, the shape requirements of the customer can be met by various hot and cold forming processes and even chemical high-tech fibers such as Kevlar® or Nomex® from Dupont make it possible to manufacture highly stable but ultra-lightweight claddings even for fire protection. Some manufacturers combine aluminium honeycombs with stainless steel panels or supply aluminium sandwich panels with a plastic core, so that honeycomb sandwiches can even be found in acoustic construction and interior design. There are hardly any limits to the variety of combinations, shapes and applications of honeycombs.

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