A sandwich panel (or sandwich structure) is an element consisting of two resistant layers, called skins or faces, separated by a connective material called core.
The core is typically lightweight and resistant material, whose only function is to distance the skins, in noble material and with reduced thickness.
The axial stiffness of the core is absolutely negligible compared to that of the skins, which are responsible for the loads on the surface.
The presence of the core is instead useful to increase the value of the flexural stiffness of the panel, which depends on the distance of the sheets from the middle surface.
The use of this structure is therefore comparable to the concept of the "double T beam", where the core serves to increase the flexural stiffness in the direction of same.
By distancing the skins, you get a remarkable increase in stiffness compared to a panel consisting only of a material thickness equal to that of the two sides with a greatly reduced weight increase.
For these reasons, sandwich panels have become increasingly popular in the aerospace industry over the past forty years.
A more common example of a sandwich panel is one made of cardboard in which the flat outer layers are separated by a layer of corrugated cardboard.

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The skins are usually made of material with high mechanical strength, which can be a composite material made of fibreglass, carbon or Kevlar, or even thin aluminium or steel.

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The core involves use of structures with honeycomb cells, simply called honeycomb, (or alternative foams).
Honeycomb cells are obtained in various ways:

From thin aluminium sheets that are processed by expansion or deformation. In the first case the sheets overlap, with structural adhesive between them pre-positioned on strips at equally distant intervals, offsetting the lower layer with respect to the upper one. The structural adhesive is cured in order to obtain anchorage on these strips and the block is deformed with appropriate traction in the direction perpendicular to the sheets to plastically deform it.
In the second case, the slabs are first worked by deforming them plastically through the passage between toothed rollers with appropriate geometries and the finished product is obtained by bonding.
|*Honeycomb structures consisting of aramid fibre cells in a thermosetting resin matrix.
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You are reminded of some local or global instability issues that have honeycomb core sandwiches panels:

Crimpling is a misalignment phenomenon of the skin surfaces following shear stress that causes excessive local cell stress; to overcome this problem, it is necessary to size the thickness of the cell to withstand the expected loads.
Wrinkling is a kind of localised buckling phenomenon of the walls of a set of cells that looks like a dip in one of the skins; in this case the problem is related to the compression module of the faces and core.
Dimpling is presented as excessive bending of the skin in the empty space between the walls of the cells creating a wavy surface; sizing the area of the cells, you can monitor this form of global instability of the skin.
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Foams are defined as those cellular materials obtained by dispersing a gas into a solid plastic material.
The foam can be:

open cell, if the gas phase is continuous;
closed cell, if the gas phase is not interconnected;
flexible, semi-rigid or rigid;
in thermoplastic or thermosetting material.

The foam can be in liquid state, if it is pressed into the mould together with the gas, or in semi-finished solid blocks that are appropriately cut and processed.
Foam is easily processed and is low cost, and for this reason it is usually used to build sandwich panels, despite its mechanical characteristics being inferior to those of honeycomb.
It offers excellent thermal and acoustic insulation, excellent vibration damping and shock resistance.

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Lighweight marble - Producied by FFPANELS®

The adhesive is necessary to join the skins to the core. Film adhesives are used with characteristics very similar to those of the resins that make up the matrixes in composite materials, therefore curing is necessary in autoclave. There are different chemical formulations depending on the nature of the materials to be joined (metal-metal, metal-composite, composite-composite).

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The new Airbus A380 uses a new type of honeycomb made of Kevlar, which is used for applications such as the interior panels and wing flaps.
Of equal weight, Kevlar is five times stronger than steel and its use allows substantial weight savings.