As the flexural stiffness of any panel is proportional to the cube of its thickness, the purpose of a core in a composite laminate is to increase the laminate·s stiffness by effectively ·thickening· it with a low-density core material. This can provide a dramatic increase in stiffness for very little additional weight. In addition, particularly when using lightweight thin laminate skins, the core must be capable of taking a compressive loading without premature failure. This helps to prevent the thin skins from wrinkling and failing in a buckling mode.

Foams are one of the most common forms of core material. They can be manufactured from a variety of synthetic polymers including polyvinyl chloride (PVC), polystyrene (PS), polyurethane (PU), polymethyl methacrylamide (acrylic), polyetherimide (PEI) and styreneacrylonitrile (SAN). They can be supplied in densities ranging from less than 30kg/m3 to more than 300kg/m3 , although the most used densities for composite structures range from 40 to 200 kg/m3. They are also available in a variety of thicknesses, typically from 5mm to 50mm.

Closed-cell polyvinyl chloride (PVC) foams are one of the most commonly used core materials for the construction of high performance sandwich structures. Although strictly they are a chemical hybrid of PVC and polyurethane, they tend to be referred to simply as ·PVC foams·. PVC foams offer a balanced combination of static and dynamic properties and good resistance to water absorption. They have a large operating temperature range of -240C to +80C (-400F to +180F) and are resistant to many chemicals. Although PVC foams are generally flammable, fire-retardant grades that can be used in many fire-critical applications such as train components are also available. When used as a core for sandwich construction with FRP skins, its reasonable resistance to styrene means that it can be used safely with polyester resins, making it popular in many industries. It is normally supplied in sheet form, either plain, or grid-scored to allow easy forming to shape.
There are two main types of PVC foam: crosslinked and uncrosslinked. The uncrosslinked foams are referred to as ·linear·, are tougher and more flexible and are easier to heat-form around curves. However, they have some lower mechanical properties than an equivalent density of cross-linked PVC, and a lower resistance to elevated temperatures and styrene. Their cross-linked counterparts are harder but more brittle and will produce a stiffer panel that is less susceptible to softening or creeping in hot climates. Recently available new generation of toughened PVC foams trade some of the basic mechanical properties of the cross-linked PVC foams for some of the improved toughness of the linear foams.

Owing to the nature of the PVC/polyurethane chemistry in cross-linked PVC foams, these materials need to be thoroughly sealed with a resin coating before they can be safely used with low-temperature curing prepregs. Although special heat stabilization treatments are available for these foams, these treatments are primarily designed to improve the dimensional stability of the foam and reduce the amount of gassing that is given off during elevated temperature processing.

Polystyrene foams are used extensively in sail and surf board manufacture, where their light weight (40kg/m3), low cost and easy to sand characteristics are of prime importance. However, they are rarely employed in high performance component construction because of their low mechanical properties. They cannot be used in conjunction with polyester resin systems because they will be dissolved by the styrene present in the resin.

Polyurethane foams exhibit only moderate mechanical properties and have a tendency for the foam surface at the resin/core interface to deteriorate with age, leading to skin delamination. Their structural applications are therefore normally limited to the production of formers to create frames or stringers for stiffening components. However, polyurethane foams can be used in lightly loaded sandwich panels, with these panels being widely used for thermal insulation. The foam also has reasonable elevated service temperature properties (150°C/300°F) and good acoustic absorption. The foam can readily be cut and machined to required shapes or profiles.

For a given density, polymethyl methacrylamide (acrylic) foams offer some of the highest overall strengths and stiffnesses of foam cores. Their high dimensional stability also makes them unique as they can readily be used with conventional elevated temperature curing prepregs. However, they are expensive, which means that their use tends to be limited to aerospace composite parts such as helicopter rotor blades and aircraft flaps.

SAN foams behave in a similar way to toughened cross-linked PVC foams. They have most of the static properties of cross-linked PVC cores, yet have much higher elongations and toughness. They are therefore able to absorb impact levels that would fracture both conventional and even the toughened PVC foams. However, unlike the toughened PVCs, which use plasticisers to toughen the polymer, the toughness properties of SAN are inherent in the polymer itself, and hence do not change appreciably with age.
SAN foams are replacing linear PVC foams in many applications since they have much of the linear PVCs toughness and elongation, yet have a higher temperature performance and better static properties. However, they are still thermoformable, which helps in the manufacture of curved parts. Heat-stabilised grades of SAN foams can also be more simply used with low-temperature curing prepregs, since they do not have the interfering chemistry inherent in the PVCs
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As new techniques develop for the blowing of foams from thermoplastics, the range of expanded materials of this type continues to increase. Typical is PEI foam, an expanded polyetherimide/polyether sulphone, which combines outstanding fire performance with high service temperature. Although it is expensive, this foam can be used in structural, thermal and fire protection applications in the service temperature range ·194°C to +180°C (-320°F to +355°F). It is highly suitable for aircraft and train interiors, as it can meet some of the most stringent fire resistant specifications.

Honeycomb cores are available in a variety of materials for sandwich structures. These range from paper and card for low strength and stiffness, low load applications (such as domestic internal doors) to high strength and stiffness, extremely lightweight components for aircraft structures. Honeycombs can be processed into both flat and curved composite structures and can be made to conform to compound curves without excessive mechanical force or heating.
Thermoplastic honeycombs are usually produced by extrusion, followed by slicing to thickness. Other honeycombs (those made of paper and aluminium) are made by a multi-stage process. In these cases, large thin sheets of the material (usually 1.2x2.4m) are printed with alternating, parallel, thin stripes of adhesive and the sheets are then stacked in a heated press while the adhesive cures. In the case of aluminium honeycomb the stack of sheets is then sliced through its thickness. The slices (known as ·block form·) are later gently stretched and expanded to form the sheet of continuous hexagonal cell shapes.
In the case of paper honeycombs, the stack of bonded paper sheets is gently expanded to form a large block of honeycomb, several feet thick. Held in its expanded form, this fragile paper honeycomb block is then dipped in a tank of resin, drained and cured in an oven. Once this dipping resin has cured, the block has sufficient strength to be sliced into the final thicknesses required.
In both cases, by varying the degree of pull in the expansion process, regular hexagon shaped cells or over-expanded (elongated) cells can be produced, each with different mechanical and handling/drape properties. Due to this bonded method of construction, a honeycomb will have different mechanical properties in the 0 and 90 degree directions of the sheet.
While skins are usually of FRP, they may be almost any sheet material with the appropriate properties, including wood, thermoplastics (e.g. melamine) and sheet metals, such as aluminium or steel. The cells of the honeycomb structure can also be filled with a rigid foam. This provides a greater bond area for the skins, increases the mechanical properties of the core by stabilising the cell walls and increases thermal and acoustic insulation properties.
Properties of honeycomb materials depend on the size (and therefore frequency) of the cells and the thickness and strength of the web material. Sheets can range from typically 3-50 mm in thickness and panel dimensions are typically 1200 x 2400mm, although it is possible to produce sheets up to 3m x 3m.
Honeycomb cores can give stiff and very light laminates but due to their very small bonding area they are almost exclusively used with high-performance resin systems such as epoxies so that the necessary adhesion to the laminate skins can be achieved.
Aluminium honeycomb produces one of the highest strength/weight ratios of any structural material. There are various configurations of the adhesive-bonding of the aluminium foil which can lead to a variety of geometric cell shapes (usually hexagonal). Properties can also be controlled by varying the foil thickness and cell size. The honeycomb is usually supplied in the unexpanded block form and is stretched out into a sheet on-site.
Despite its good mechanical properties and relatively low price, aluminium honeycomb has to be used with caution in some applications, such as large marine structures, because of the potential corrosion problems in a salt-water environment. In this situation care also has to be exercised to ensure that the honeycomb does not come into direct contact with carbon skins since the conductivity can aggravate galvanic corrosion. Aluminium honeycomb also has the problem that it has no ·mechanical memory·. On impact of a cored laminate, the honeycomb will deform irreversibly whereas the FRP skins, being resilient, will move back to their original position. This can result in an area with an unbonded skin with much reduced mechanical properties.
Finally, consideration needs to be given to the form a core is used in to ensure that it fits the component well. The weight savings that cores can offer can quickly be used up if cores fit badly, leaving large gaps that require filling with adhesive. Scrimbacked foam or balsa, where little squares of the core are supported on a lightweight scrim cloth, can be used to help cores conform better to a curved surface. Contour-cut foam, where slots are cut part way through the core from opposite sides achieves a similar effect. However, both these cores still tend to use quite large amounts of adhesive since the slots between each foam square need filling with resin to produce a good structure.