What are the benefits of honeycomb design?

Natural or man-made structures that have the geometry of a honeycomb

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Aluminum honeycomb structure Honeycomb structure in nature

Honeycomb structures are natural or man-made structures that have the geometry of a honeycomb to allow the minimization of the amount of used material to reach minimal weight and minimal material cost. The geometry of honeycomb structures can vary widely but the common feature of all such structures is an array of hollow cells formed between thin vertical walls. The cells are often columnar and hexagonal in shape. A honeycomb-shaped structure provides a material with minimal density and relative high out-of-plane compression properties and out-of-plane shear properties.[1]

Man-made honeycomb structural materials are commonly made by layering a honeycomb material between two thin layers that provide strength in tension. This forms a plate-like assembly. Honeycomb materials are widely used where flat or slightly curved surfaces are needed and their high specific strength is valuable. They are widely used in the aerospace industry for this reason, and honeycomb materials in aluminum, fibreglass and advanced composite materials have been featured in aircraft and rockets since the s. They can also be found in many other fields, from packaging materials in the form of paper-based honeycomb cardboard, to sporting goods like skis and snowboards.

Introduction

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Natural honeycomb structures include beehives, honeycomb weathering in rocks, tripe, and bone.

Man-made honeycomb structures include sandwich-structured composites with honeycomb cores.[citation needed] Man-made honeycomb structures are manufactured by using a variety of different materials, depending on the intended application and required characteristics, from paper or thermoplastics, used for low strength and stiffness for low load applications, to high strength and stiffness for high performance applications, from aluminum or fiber reinforced plastics. The strength of laminated or sandwich panels depends on the size of the panel, facing material used and the number or density of the honeycomb cells within it. Honeycomb composites are used widely in many industries, from aerospace industries, automotive and furniture to packaging and logistics. The material takes its name from its visual resemblance to a bee's honeycomb &#; a hexagonal sheet structure.

History

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The hexagonal comb of the honey bee has been admired and wondered about from ancient times. The first man-made honeycomb, according to Greek mythology, is said to have been manufactured by Daedalus from gold by lost wax casting more than years ago.[2] Marcus Varro reports that the Greek geometers Euclid and Zenodorus found that the hexagon shape makes most efficient use of space and building materials. The interior ribbing and hidden chambers in the dome of the Pantheon in Rome is an early example of a honeycomb structure.[3]

Galileo Galilei discusses in the resistance of hollow solids: "Art, and nature even more, makes use of these in thousands of operations in which robustness is increased without adding weight, as is seen in the bones of birds and in many stalks that are light and very resistant to bending and breaking&#;.[4] Robert Hooke discovers in that the natural cellular structure of cork is similar to the hexagonal honeybee comb.[5] and Charles Darwin states in that "the comb of the hive-bee, as far as we can see, is absolutely perfect in economizing labour and wax&#;.[6]

The first paper honeycomb structures might have been made by the Chinese years ago for ornaments, but no reference for this has been found. Paper honeycombs and the expansion production process has been invented in Halle/Saale in Germany by Hans Heilbrun in [7] for decorative applications. First honeycomb structures from corrugated metal sheets had been proposed for bee keeping in .[8] For the same purpose, as foundation sheets to harvest more honey, a honeycomb moulding process using a paper paste glue mixture had been patented in .[9] The three basic techniques for honeycomb production that are still used today&#;expansion, corrugation and moulding&#;were already developed by for non-sandwich applications.

Hugo Junkers first explored the idea of a honeycomb core within a laminate structure. He proposed and patented the first honeycomb cores for aircraft application in .[10] He described in detail his concept to replace the fabric covered aircraft structures by metal sheets and reasoned that a metal sheet can also be loaded in compression if it is supported at very small intervals by arranging side by side a series of square or rectangular cells or triangular or hexagonal hollow bodies. The problem of bonding a continuous skin to cellular cores led Junkers later to the open corrugated structure, which could be riveted or welded together.

The first use of honeycomb structures for structural applications had been independently proposed for building application and published already in .[11] In Edward G. Budd patented a welded steel honeycomb sandwich panel from corrugated metal sheets and Claude Dornier aimed to solve the core-skin bonding problem by rolling or pressing a skin which is in a plastic state into the core cell walls.[12] The first successful structural adhesive bonding of honeycomb sandwich structures was achieved by Norman de Bruyne of Aero Research Limited, who patented an adhesive with the right viscosity to form resin fillets on the honeycomb core in .[13] The North American XB-70 Valkyrie made extensive use of stainless steel honeycomb panels using a brazing process they developed.

A summary of the important developments in the history of honeycomb technology is given below:[14]

• 60 BC Diodorus Siculus reports a golden honeycomb manufactured by Daedalus via lost wax casting.
• 36 BC Marcus Varro reports most efficient use of space and building materials by hexagonal shape.
• 126 The Pantheon was rebuilt in Rome using a coffer structure, sunken panel in the shape of a square structure, to support its dome.
• Galileo Galilei discusses hollow solids and their increase of resistance without adding weight.
• Robert Hooke discovers that the natural cellular structure of cork is similar to the hexagonal honeybee comb.
• Charles Darwin states that the comb of the hive-bee is absolutely perfect in economizing labour and wax.
• F. H. Küstermann invents a honeycomb moulding process using a paper paste glue mixture.
• Julius Steigel invents the honeycomb production process from corrugated metal sheets.
• Hans Heilbrun invents the hexagonal paper honeycombs and the expansion production process.
• R. Höfler and S. Renyi patent the first use of honeycomb structures for structural applications.
• Hugo Junkers patents the first honeycomb cores for aircraft application.
• George Thomson proposes to use decorative expended paper honeycombs for lightweight plasterboard panels.
• Edward G. Budd patents welded steel honeycomb sandwich panel from corrugated metal sheets.
• Claude Dornier patents a honeycomb sandwich panel with skins pressed in a plastic state into the core cell walls.
• Norman de Bruyne patents the structural adhesive bonding of honeycomb sandwich structures.
• John D. Lincoln proposes the use of expanded paper honeycombs for aircraft radomes
• Roger Steele applies the expansion production process using fiber reinforced composite sheets.
• Boeing 747 incorporates extensive fire-resistant honeycombs from Hexcel Composites using DuPont's Nomex aramid fiber paper.
• s Thermoplastic honeycombs produced by extrusion processes are introduced.

Manufacture

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Honeycomb crash absorption structure made of injection moulded thermoplastic polymer on a BMW i3

The three traditional honeycomb production techniques, expansion, corrugation, and moulding, were all developed by for non-sandwich applications. For decorative applications the expanded honeycomb production reached a remarkable degree of automation in the first decade of the 20th century.

Today honeycomb cores are manufactured via the expansion process and the corrugation process from composite materials such as glass-reinforced plastic (also known as fiberglass), carbon fiber reinforced plastic, Nomex aramide paper reinforced plastic, or from a metal (usually aluminum).[15]

Honeycombs from metals (like aluminum) are today produced by the expansion process. Continuous processes of folding honeycombs from a single aluminum sheet after cutting slits had been developed already around .[16] Continuous in-line production of metal honeycomb can be done from metal rolls by cutting and bending.[17]

Thermoplastic honeycomb cores (usually from polypropylene) are usually made by extrusion processed via a block of extruded profiles[18] or extruded tubes[19][20] from which the honeycomb sheets are sliced.

Recently a new, unique process to produce thermoplastic honeycombs has been implemented, allowing a continuous production[21] of a honeycomb core as well as in-line production of honeycombs with direct lamination of skins into cost efficient sandwich panel.[22]

Applications

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Composite honeycomb structures have been used in numerous engineering and scientific applications.

More recent developments show that honeycomb structures are also advantageous in applications involving nanohole arrays in anodized alumina,[23] microporous arrays in polymer thin films,[24] activated carbon honeycombs,[25] and photonic band gap honeycomb structures.[26]

Aerodynamics

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Honeycombed, screened center for Langley's first wind tunnel

A honeycomb mesh is often used in aerodynamics to reduce or to create wind turbulence. It is also used to obtain a standard profile in a wind tunnel (temperature, flow speed). A major factor in choosing the right mesh is the length ratio (length vs honeycomb cell diameter) L/d.

Length ratio < 1: Honeycomb meshes of low length ratio can be used on vehicles front grille. Beside the aesthetic reasons, these meshes are used as screens to get a uniform profile and to reduce the intensity of turbulence.[27]

Length ratio >> 1: Honeycomb meshes of large length ratio reduce lateral turbulence and eddies of the flow. Early wind tunnels used them with no screens; unfortunately, this method introduced high turbulence intensity in the test section. Most modern tunnels use both honeycomb and screens.

While aluminium honeycombs are common use in the industry, other materials are offered for specific applications. People using metal structures should take care of removing burrs as they can introduce additional turbulences. Polycarbonate structures are a low-cost alternative.

The honeycombed, screened center of this open-circuit air intake for Langley's first wind tunnel ensured a steady, non-turbulent flow of air. Two mechanics pose near the entrance end of the actual tunnel, where air was pulled into the test section through a honeycomb arrangement to smooth the flow.

Honeycomb is not the only cross-section available in order to reduce eddies in an airflow. Square, rectangular, circular and hexagonal cross-sections are other choices available, although honeycomb is generally the preferred choice.[28]

Properties

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A composite sandwich panel (A) with honeycomb core (C) and face sheets (B)

In combination with two skins applied on the honeycomb, the structure offers a sandwich panel with excellent rigidity at minimal weight. The behavior of the honeycomb structures is orthotropic, meaning the panels react differently depending on the orientation of the structure. It is therefore necessary to distinguish between the directions of symmetry, the so-called L and W-direction. The L-direction is the strongest and the stiffest direction. The weakest direction is at 60° from the L-direction (in the case of a regular hexagon) and the most compliant direction is the W-direction.[1] Another important property of honeycomb sandwich core is its compression strength. Due to the efficient hexagonal configuration, where walls support each other, compression strength of honeycomb cores is typically higher (at same weight) compared to other sandwich core structures such as, for instance, foam cores or corrugated cores.

The mechanical properties of honeycombs depend on its cell geometry, the properties of the material from which the honeycomb is constructed (often referred to as the solid), which include the Young's modulus, yield stress, and fracture stress of the material, and the relative density of the honeycomb (the density of the honeycomb normalized by that of the solid, ρ*/ρs).[29][30] The ratio of the effective elastic moduli and the solid's Young's moduli, e.g., κ &#; / E s {\displaystyle \kappa ^{*}/E_{\text{s}}} and E &#; / E s {\displaystyle E^{*}/E_{\text{s}}} , of low-density honeycombs are independent of the solid.[31] The mechanical properties of honeycombs will also vary based on the direction in which the load is applied.

In-plane loading: Under in-plane loading, it is often assumed that the wall thickness of the honeycomb is small compared to the length of the wall. For a regular honeycomb, the relative density is proportional to the wall thickness to wall length ratio (t/L) and the Young&#;s modulus is proportional to (t/L)3.[29][30] Under high enough compressive load, the honeycomb reaches a critical stress and fails due to one of the following mechanisms &#; elastic buckling, plastic yielding, or brittle crushing.[29] The mode of failure is dependent on the material of the solid which the honeycomb is made of. Elastic buckling of the cell walls is the mode of failure for elastomeric materials,[30] ductile materials fail due to plastic yielding, and brittle crushing is the mode of failure when the solid is brittle.[29][30] The elastic buckling stress is proportional to the relative density cubed, plastic collapse stress is proportional to relative density squared, and brittle crushing stress is proportional to relative density squared.[29][30] Following the critical stress and failure of the material, a plateau stress is observed in the material, in which increases in strain are observed while the stress of the honeycomb remains roughly constant.[30] Once a certain strain is reached, the material will begin to undergo densification as further compression pushes the cell walls together.[30]

Out of-plane loading: Under out-of-plane loading, the out-of-plane Young&#;s modulus of a regular hexagonal honeycombs is proportional to the relative density of the honeycomb.[29] The elastic buckling stress is proportional to (t/L)3 while the plastic buckling stress is proportional to (t/L)5/3.[29]

The shape of the honeycomb cell is often varied to meet different engineering applications. Shapes that are commonly used besides the regular hexagonal cell include triangular cells, square cells, and circular-cored hexagonal cells, and circular-cored square cells.[32] The relative densities of these cells will depend on their new geometry.

See also

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Honeycomb grille used on a computer fan to cover fan blades.

References

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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.

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 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.

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