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If the key requirements for an application are high stiffness and low-weight, core materials are probably the best and most appropriate choice. Core materials began to be used extensively in sandwich composites for their unique properties, and are mainly used for applications in aerospace, marine, wind energy, transportation and industry.
Sandwich constructions were one of the first forms of composite structures that gained great acceptance and usage. The World War II Mosquito aircraft is said to be the first major application of sandwich panels, although there were some less spectacular uses of the sandwich principles earlier.
A typical sandwich structured composite consists of two skins which are thin and strong, and a lightweight, softer core. Each skin maybe an isotropic material or a fibre-reinforced composite laminate. The primary function of the skins is to provide the required bending and in-plane shear-stiffness and to carry the axial, bending, and in-plane shear loading. The core has to be strong enough to keep the skins in their relative positions while loading and stiff in shear, so that the faces do not slide over each other. In order to force the facings and the core to cooperate and act as an integral structural part, an adhesive is used inbetween. Most of adhesives used are epoxies, modified epoxies or epoxy polyamides. The toughness of the adhesive refers to the capability to resist loads which act to separate the facings from the core under either static or dynamic conditions. Each component is relatively weak and flexible by itself, but when combined in a sandwich panel, the result is a really strong structure.
Sandwich composites are becoming very popular for their ability to increase stiffness, decrease weight and improve mechanical performance. Weight reduction results in a number of benefits, including lower fuel consumption, increased speed, load capacity and reduced carbon footprint for the environment. According to engineering theory, the stiffness of a panel is proportional to the cube of its thickness. So, adding a low density core material in order to thicken the sandwich panel, can significantly increase the composites stiffness with a very small weight penalty.

Effective flexular stiffness: (EI)eff = Ep ep (ep + ec)2 / 2
   ep = thickness of skin
   ec = thickness of core
   Ep = the modulus of elasticity of the material skin

In order to understand better core materials some basic terms about their typical properties are described below.

Stress describes the forces acting within a material due to an applied load, or in other words, stress is a measurement of the forces that are acting within a material at a certain load.
Stress σ = P/A
where P is the force applied to area A.

Strain describes the trend of deformation (it is a material’s change in geometry due to an applied load,) and is measured as deformation per unit length.
Strain ε = δI/I
where δI is the length of deformation to the original length I.

The relationship between deformation and stress determines a material’s elastic modulus.
Elastic modulus is the measure of stiffness of the material. The higher the modulus the stiffer the material is.
Elastic modulus E = σ/ε
where σ is stress to strain ε.

Different core materials provide a diversity of densities, sizes, weights, life spans, physical attributes and costs, in order to select the most suitable material for any particular job. The most commonly used high quality core materials are Nomex honeycomb and PVC foam.

Nomex Honeycomb
‘Nomex’ honeycomb (Nomex is Du Pont’s brand name for its aramid paper) is manufactured with aramid fiber paper. Nomex paper is shaped into honeycomb, a hexagonal cell structure in sheets, (like bee’s honeycomb,) that is dipped in a heat resistant phenolic resin, to improve mechanical properties and gain some stability. This structure provides an excellent combination of strength and efficiency while at the same time reduces the weight of the component. For this reason honeycomb materials are widely used where their high strength to weight ratio is valuable, in industries like aerospace, marine, military, construction, sports and automotive.
Honeycomb is also manufactured with other cheaper materials, such as other aramid paper, plain paper, metal (aluminum), and plastic (polypropylene.) On the contrary, it is manufactured with more expensive materials like fiberglass and carbon fiber reinforced plastics.
More information about Nomex honeycomb.

PVC Foam
PVC foam is made from a polymer, polyvinyl chloride (or PVC), that is filled with air bubbles. The mix of polymer and air bubbles forms a consistent material, with increased volume many times over, while the weight remains low. It consists of 50% to 95% air resulting to a density of 55-200 kg/sqm, and thereby can contribute to save valuable weight and raw material. It may be used with most types of resin like epoxy, vinylester and others, and all types of fiber reinforcement like carbon, aramid, Kevlar, fiberglass, etc. There are two types of PVC foam: cross-linked or rigid, and linear or ductile. It is used mainly in marine construction due to the low water absorbance, but the fields of applications extend to transportation, aerospace, military, wind and industrial markets due to its excellent physical properties.
More information about PVC foam.

Other, low cost core materials, maybe of lower quality, are as follows:

Wood - Plywood
Wood is a natural core, and has a similar to honeycomb structure. It is easy to find, less expensive than other core materials, and simple to work with. Plywood is a widely used wood product. It is used instead of wood because of its high strength, resistance to cracking, shrinkage and twisting. Marine plywood is much more resistant to moisture. However, all wood cores are sensitive to moisture, and will eventually rot if not surrounded by epoxy resin.

Balsa wood
Balsa is another commonly used core wood. It was first used in 1940s in flying boat hulls. It exhibits very low density (for a wood) and some water resistance. It also provides good thermal insulation and good acoustic absorption. It is easily formed with simple equipment. However, balsa wood has very high minimum density, 100 or 200 kg/m3 is a typical quality, so it is often used where weight saving is not highly required.

PET Foam
PET (Polyethylene-Terephthalate) is a lightweight, recyclable thermoplastic. PET foam offers great strength and is temperature resistant, but low in impact behavior.

Polyurethane foam
Polyurethane foams exhibit good mechanical properties but tend to deteriorate with age, leading to skin delamination. This foam also has reasonable raised temperature properties, good acoustic absorption, and can be machined to required shapes.

Pseudo cores
These cores are usually not regarded as «true» cores, they are thin, low density fabric-like materials used to lower the density of a single skin laminate. Coremat and Spheretex for example, are polyester nonwovens that contain microspheres and volumised glass. Pseudo cores are used as a thin core in fibre reinforced laminates, to increase their stiffness with only some weight added.

Polystyrene, or mostly known as Styrofoam, is also used as a low cost core material. Two types of polystyrene exist, namely expanded (or EPS) and extruded (or XPS.) It exhibits very low weight and relatively low water absorption, when compared to some other foams, like polyurethane. It is very extensively used in wind surf and surfboard industries (almost all surfboards and wind surf boards have Styrofoam inside.) It can be CNC machined and is a very handy material suitable for shaping and prototyping.

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