What the HONEYCOMB is about

INTRODUCTION
A sandwich panel (or a sandwhich structure) is a composite building material consisting, in its basic feature, of two strong sheets, called skins or faces, held apart by a connecting structure named core. The core is usually a material, whose main property is to be light. Its main function is to held apart the skins. The main properties of the skins are to be thin and shear resistant. A sandwich panel is very strong in resisting to bending, depending from the strength of its skins (ideally acting as a beam of double T shape), and from their maintaining, also under stress, their relative position. The distance between the skins is the main factor in giving to a sandwich panel its resistance to bending, which tends to be nil when said distance is zero. Due to this property, of light weight and resistance to bending, sandwich panels are widely used in aerospace. Examples of their widespread use are in cardboard and door hollow panels.

SKINS
The skins in sandwich panels are usually of materials with high mechanical strength, such as a composite cloth of fiberglass, carbon or kevlar, but also thin aluminum or steel.

CORE
The core is usually of the form of bee cells, therefore named honeycomb, (or alternatively foams).
Honeycomb cells are industrially produced in many different ways:
  • Honeycomb aluminum cells are produced by gluing in alternately positioned thin stripes, sheets of aluminum which are subsequently stretched apart, perpendicularly to their plane; or by mechanically funneling sheets of aluminum and gluing said funneled sheets on top of the funnels themselves.
  • Honeycomb cells from aramidic fibers molded in a matrix of thermohardening resin.

Honeycomb cored sandwiches show a few typical problems, well known in the field, and therefore interesting to be pointed out:
crimpling, wrinkling and dimpling are surface defects of sandwich slabs, which are due to operating stresses of the material and which can be avoided, respectively, by choosing the right thickness of the cells, by increasing the resistance to compression of the sandwich, by choosing the appropriate area of the cells.

Foams are cellular materials resulting from introduction of gas bubbles inside solid plastic material during manufacturing. Foams can be:
  • with open cells, when the gas bubbles are interconnected;
  • with closed cells, when the gas bubbles are not interconnected;
  • flexible, partially stiff, hard;
  • made of materials having the property of softening or, on the contrary, hardening when heated.

Foams can be available in a liquid form, to be injected in a mould together with gas, or as solid blocks, to be cut and otherwise operated upon. Foams are easy to work and cheap, therefore their use is widespread in sandwich panels; have very good properties as thermal and acoustic insulating material, are very good in reducing and absorbing vibrations and blows; their mechanical properties are however quite lower in comparison to honeycomb.

STRUCTURAL ADHESIVE
An adhesive, or glue, is required to firmly attach the skins to the core materials. Film adhesives are used to this end, similar to the resin employed in composite materials, where polymerization must be achieved under pressure in oven. Structural adhesives are available with specific chemical formulations tailored specifically for the materials to be glued (metal and metal, composite and metal, composite and composite).

TO BE REMARKED
In the new Airbus A380, the internal panels and the wing flaps have an internal honeycomb structure in Kevlar. An equal weight of kevlar has five times the strength of steel; its use is therefore very effective in reducing structural weight.

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