Aluminum honeycomb structureHoneycomb structure in nature
Honeycomb structures are natural or man-madestructures 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 1950s. 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.
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
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 3000 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 1638 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 1665 that the natural cellular structure of cork is similar to the hexagonal honeybee comb.[5] and Charles Darwin states in 1859 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 2000 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 1901[7] for decorative applications. First honeycomb structures from corrugated metal sheets had been proposed for bee keeping in 1890.[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 1878.[9] The three basic techniques for honeycomb production that are still used today—expansion, corrugation and moulding—were already developed by 1901 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 1915.[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 1914.[11] In 1934 Edward G. Budd patented a welded steel honeycomb sandwich panel from corrugated metal sheets and Claude Dornier aimed 1937 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 1938.[13] The North American XB-70 Valkyrie made extensive use of stainless steel honeycomb panels using a brazing process they developed.
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.
1638 Galileo Galilei discusses hollow solids and their increase of resistance without adding weight.
1665 Robert Hooke discovers that the natural cellular structure of cork is similar to the hexagonal honeybee comb.
1859 Charles Darwin states that the comb of the hive-bee is absolutely perfect in economizing labour and wax.
1877 F. H. Küstermann invents a honeycomb moulding process using a paper paste glue mixture.
1890 Julius Steigel invents the honeycomb production process from corrugated metal sheets.
1901 Hans Heilbrun invents the hexagonal paper honeycombs and the expansion production process.
1914 R. Höfler and S. Renyi patent the first use of honeycomb structures for structural applications.
1915 Hugo Junkers patents the first honeycomb cores for aircraft application.
1931 George Thomson proposes to use decorative expended paper honeycombs for lightweight plasterboard panels.
1934 Edward G. Budd patents welded steel honeycomb sandwich panel from corrugated metal sheets.
1937 Claude Dornier patents a honeycomb sandwich panel with skins pressed in a plastic state into the core cell walls.
1938 Norman de Bruyne patents the structural adhesive bonding of honeycomb sandwich structures.
1941 John D. Lincoln proposes the use of expanded paper honeycombs for aircraft radomes
1948 Roger Steele applies the expansion production process using fiber reinforced composite sheets.
1969 Boeing 747 incorporates extensive fire-resistant honeycombs from Hexcel Composites using DuPont's Nomex aramid fiber paper.
1980s Thermoplastic honeycombs produced by extrusion processes are introduced.
Manufacture
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 1901 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.
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 1920.[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
Composite honeycomb structures have been used in numerous engineering and scientific applications.
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
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., and , 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
Honeycomb grille used on a computer fan to cover fan blades.
Plywood is a composite material manufactured from thin layers, or "plies", of wood veneer that have been stacked and glued together. It is an engineered wood from the family of manufactured boards, which include plywood, medium-density fibreboard (MDF), oriented strand board (OSB), and particle board.
In materials science, a metal matrix composite (MMC) is a composite material with fibers or particles dispersed in a metallic matrix, such as copper, aluminum, or steel. The secondary phase is typically a ceramic or another metal. They are typically classified according to the type of reinforcement: short discontinuous fibers (whiskers), continuous fibers, or particulates. There is some overlap between MMCs and cermets, with the latter typically consisting of less than 20% metal by volume. When at least three materials are present, it is called a hybrid composite. MMCs can have much higher strength-to-weight ratios, stiffness, and ductility than traditional materials, so they are often used in demanding applications. MMCs typically have lower thermal and electrical conductivity and poor resistance to radiation, limiting their use in the very harshest environments.
Lamination is the technique/process of manufacturing a material in multiple layers, so that the composite material achieves improved strength, stability, sound insulation, appearance, or other properties from the use of the differing materials, such as plastic. A laminate is a layered object or material assembled using heat, pressure, welding, or adhesives. Various coating machines, machine presses and calendering equipment are used.
A structural insulated panel, or structural insulating panel, (SIP), is a form of sandwich panel used as a building material in the construction industry.
Ultrasonic welding is an industrial process whereby high-frequency ultrasonic acoustic vibrations are locally applied to work pieces being held together under pressure to create a solid-state weld. It is commonly used for plastics and metals, and especially for joining dissimilar materials. In ultrasonic welding, there are no connective bolts, nails, soldering materials, or adhesives necessary to bind the materials together. When used to join metals, the temperature stays well below the melting point of the involved materials, preventing any unwanted properties which may arise from high temperature exposure of the metal.
Wood–plastic composites (WPCs) are composite materials made of wood fiber/wood flour and thermoplastic(s) such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), or polylactic acid (PLA).
Syntactic foams are composite materials synthesized by filling a metal, polymer, cementitious or ceramic matrix with hollow spheres called microballoons or cenospheres or non-hollow spheres as aggregates. In this context, "syntactic" means "put together." The presence of hollow particles results in lower density, higher specific strength, lower coefficient of thermal expansion, and, in some cases, radar or sonar transparency.
High-density polyethylene (HDPE) or polyethylene high-density (PEHD) is a thermoplastic polymer produced from the monomer ethylene. It is sometimes called "alkathene" or "polythene" when used for HDPE pipes. With a high strength-to-density ratio, HDPE is used in the production of plastic bottles, corrosion-resistant piping, geomembranes and plastic lumber. HDPE is commonly recycled, and has the number "2" as its resin identification code.
In materials science, a metal foam is a material or structure consisting of a solid metal with gas-filled pores comprising a large portion of the volume. The pores can be sealed or interconnected. The defining characteristic of metal foams is a high porosity: typically only 5–25% of the volume is the base metal. The strength of the material is due to the square–cube law.
In materials science, a sandwich-structured composite is a special class of composite materials that is fabricated by attaching two thin-but-stiff skins to a lightweight-but-thick core. The core material is normally of low strength, but its greater thickness provides the sandwich composite with high bending stiffness with overall low density.
Isogrid is a type of partially hollowed-out structure formed usually from a single metal plate with integral triangular stiffening stringers. It was patented by McDonnell Douglas. Isogrids are extremely light and stiff. Compared to other materials, it is expensive to manufacture, and so it is restricted to spaceflight applications and some aerospace use.
Glass fiber reinforced concrete (GFRC) is a type of fiber-reinforced concrete. The product is also known as glassfibre reinforced concrete or GRC in British English. Glass fiber concretes are mainly used in exterior building façade panels and as architectural precast concrete. Somewhat similar materials are fiber cement siding and cement boards.
A threaded insert, also known as a threaded bushing, is a fastener element that is inserted into an object to add a threaded hole. They may be used to repair a stripped threaded hole, provide a durable threaded hole in a soft material, place a thread on a material too thin to accept it, mold or cast threads into a work piece thereby eliminating a machining operation, or simplify changeover from unified to metric threads or vice versa.
A sandwich panel is any structure made of three layers: a low-density core, and a thin skin-layer bonded to each side. Sandwich panels are used in applications where a combination of high structural rigidity and low weight is required.
Solid is one of the four fundamental states of matter along with liquid, gas, and plasma. The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structural rigidity and resistance to a force applied to the surface. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire available volume like a gas. The atoms in a solid are bound to each other, either in a regular geometric lattice, or irregularly. Solids cannot be compressed with little pressure whereas gases can be compressed with little pressure because the molecules in a gas are loosely packed.
Twin-wall plastic, specifically twin-wall polycarbonate, is an extruded multi-wall polymer product created for applications where its strength, thermally insulative properties, and moderate cost are ideal. Polycarbonate, which is most commonly formed through the reaction of Bisphenol A and Carbonyl Chloride, is an extremely versatile material. It is significantly lighter than glass, while managing to be stronger, more flexible, and more impact resistant. Twin-wall polycarbonate is used most commonly for green houses, where it can support itself in a structurally sound configuration, limit the amount of UV light due to its nominal translucence, and can withstand the rigors of daily abuse in an outdoor environment. The stagnant air in the cellular space between sheets provides insulation, and additional cell layers can be extruded to enhance insulative properties at the cost of light transmission.
Aluminium foam sandwich (AFS) is a sandwich panel product which is made of two metallic dense face sheets and a metal foam core made of an aluminium alloy. AFS is an engineering structural material owing to its stiffness-to-mass ratio and energy absorption capacity ideal for application such as the shell of a high-speed train.
Cymat Technologies is an innovative materials technology company based out of Mississauga, Ontario, Canada, and one of the world leaders in the production of stabilized aluminum foam.
Titanium foams exhibit high specific strength, high energy absorption, excellent corrosion resistance and biocompatibility. These materials are ideally suited for applications within the aerospace industry. An inherent resistance to corrosion allows the foam to be a desirable candidate for various filtering applications. Further, titanium's physiological inertness makes its porous form a promising candidate for biomedical implantation devices. The largest advantage in fabricating titanium foams is that the mechanical and functional properties can be adjusted through manufacturing manipulations that vary porosity and cell morphology. The high appeal of titanium foams is directly correlated to a multi-industry demand for advancement in this technology.
↑ Diodorus Siculus, Library of History, 1st century BC
↑ MacDonald, William L. (1976). The Pantheon: Design, Meaning, and Progeny. Cambridge, MA: Harvard University Press. p.33. ISBN0674010191. There are openings in it [the rotunda] here and there, at various levels, that give on to some of the many different chambers that honeycomb the rotunda structure, a honeycombing that is an integral part of a sophisticated engineering solution...
↑ Galilei, G., Discorsi e dimostrazioni matematiche, intorno á due nuoue scienze, Leiden, Elzeviers, 1638
↑ Hooke, R., Micrographia, London, J. Martyn and J. Allestry, 1665
↑ Darwin, C., On the Origin of Species by Means of Natural Selection, London, John Murray, 1859
↑ Julius Steigel, Verfahren zur Herstellung von Kunstwaben, DE57655, 1890
↑ Küstermann, F. H., Künstliche Bienenwaben nebst den Instrumenten zur Herstellung derselben, DE7031, 1879
↑ Hugo Junkers, Abdeckung für Flugzeugtragflächen und dergleichen, DE310040, 1915
↑ Höfler, R. and S. Renyi, Plattenförmiger Baukörper, DE355036, 1914
↑ Dornier, C. Improvements in or relating to a method for the fabrication of lightweight structural members more particularly for aircraft construction, GB515267, Dornier Metallbauten GmbH, 1937
↑ Masuda, H. and Fukuda, K., (1995), Ordered metal nanohole arrays made by a two-step replication of honeycomb structures of anodic alumina, Science, 268(5216), p. 1466.
↑ Yabu, H. and Takebayashi, M. and Tanaka, M. and Shimomura, M., 2005, Superhydrophobic and lipophobic properties of self-organized honeycomb and pincushion structures, Langmuir, 21(8), pp. 3235–3237.
↑ Gadkaree, KP, (1998), Carbon honeycomb structures for adsorption applications, Carbon, 36(7–8), pp. 981–989.
↑ Broeng, J. and Barkou, S.E. and Bjarklev, A. and Knight, J.C. and Birks, T.A. and Russell, P.S.J., (1998), Highly increased photonic band gaps in silica/air structures, Optics Communications, 156(4–6), pp. 240–244.
↑ Bradshaw, Peter; Mehta, Rabi. "Honeycomb". Wind Tunnel Design. Archived from the original on 2011-09-02.
↑ Torquato, S.; Gibiansky, L.V.; Silva, M.J.; Gibson, L.J. (January 1998). "Effective mechanical and transport properties of cellular solids". International Journal of Mechanical Sciences. 40 (1): 71–82. doi:10.1016/s0020-7403(97)00031-3. ISSN0020-7403.
↑ Zhang, Qiancheng; Yang, Xiaohu; Li, Peng; Huang, Guoyou; Feng, Shangsheng; Shen, Cheng; Han, Bin; Zhang, Xiaohui; Jin, Feng (October 2015). "Bioinspired engineering of honeycomb structure – Using nature to inspire human innovation". Progress in Materials Science. 74: 332–400. doi:10.1016/j.pmatsci.2015.05.001. ISSN0079-6425.
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