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Complete Guide to Composites, Part 5

Core materials

courtesy of SP Composites

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At a glance...

  • Foams
  • Honeycombs
  • Wood
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The core material used – whether that’s a foam, honeycomb or wood – has a dramatic affect on the strength of a composite. In this article we look at the advantages and disadvantages of the different types of cores.

Core Materials

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Engineering theory shows that the flexural stiffness of any panel is proportional to the cube of its thickness. The purpose of a core in a composite laminate is therefore 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. This diagram shows a cored laminate under a bending load. Here, the sandwich laminate can be likened to an I-beam, in which the laminate skins act as the I-beam flange, and the core materials act as the beam’s shear web. In this mode of loading it can be seen that the upper skin is put into compression, the lower skin into tension and the core into shear. It therefore follows that one of the most important properties of a core is its shear strength and stiffness.

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.

Foam Cores

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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.

  • PVC Foam

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 also have a large operating temperature range of typically -240 degrees C to +80 degrees C, and are resistant to many chemicals. Although PVC foams are generally flammable, there are fire-retardant grades that can be used in many fire-critical applications, such as train components.

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 and it is therefore 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, with the uncrosslinked foams sometimes being referred to as ‘linear’. The uncrosslinked foams (such as Airex R63.80) 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, less susceptible to softening or creeping in hot climates. Typical cross-linked PVC products include the Herex C-series of foams, Divinycell H and HT grades and Polimex Klegecell and Termanto products.

A new generation of toughened PVC foams is now also becoming available which trade some of the basic mechanical properties of the cross-linked PVC foams for some of the improved toughness of the linear foams. Typical products include Divincell HD grade.

  • Polystyrene Foams

Although 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, 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

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 degrees C) and good acoustic absorption. The foam can readily be cut and machined to required shapes or profiles.

  • Polymethyl methacrylamide Foams

For a given density, polymethyl methacrylamide (acrylic) foams such as Rohacell offer some of the highest overall strengths and stiffnesses of foam cores. Their high dimensional stability also makes them unique in that 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.

  • Styrene acrylonitrile (SAN) co-polymer Foams

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 PVC’s, which use plasticizers to toughen the polymer, the toughness properties of SAN are inherent in the polymer itself, and so do not change appreciably with age.

SAN foams are replacing linear PVC foams in many applications since they have much of the linear PVC’s 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 PVC’s. Typical SAN products include ATC Core-Cell’s A-series foams.

Honeycombs

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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 (such as 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 degrees 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 (eg 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

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.

  • Nomex honeycomb

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Nomex honeycomb is made from Nomex paper - a form of paper based on Kevlar, rather than cellulose fibres. The initial paper honeycomb is usually dipped in a phenolic resin to produce a honeycomb core with high strength and very good fire resistance. It is widely used for lightweight interior panels for aircraft in conjunction with phenolic resins in the skins. Special grades for use in fire retardant applications (eg public transport interiors) can also be made which have the honeycomb cells filled with phenolic foam for added bond area and insulation. Nomex honeycomb is becoming increasingly used in high-performance non-aerospace components due to its high mechanical properties, low density and good longterm stability. However, as can be seen from this diagram, it is considerably more expensive than other core materials.

  • Thermoplastic honeycomb

Core materials made of other thermoplastics are light in weight, offering some useful properties and possibly also making for easier recycling. Their main disadvantage is the difficulty of achieving a good interfacial bond between the honeycomb and the skin material, and their relatively low stiffness. Although they are rarely used in highly loaded structures, they can be useful in simple interior panels. The most common polymers used are:

ABS - for rigidity, impact strength, toughness, surface hardness and dimensional stability

Polycarbonate - for UV-stability, excellent light transmission, good heat resistance & self-extinguishing properties

Polypropylene - for good chemical resistance

Polyethylene - a general-purpose low-cost core material

Wood

Wood can be described as ‘nature’s honeycomb’, as it has a structure that, on a microscopic scale, is similar to the cellular hexagonal structure of synthetic honeycomb. When used in a sandwich structure with the grain running perpendicular to the plane of the skins, the resulting component shows properties similar to those made with man-made honeycombs. However, despite various chemical treatments being available, all wood cores are susceptible to moisture attack and will rot if not well surrounded by laminate or resin.

  • Balsa

The most commonly used wood core is end-grain balsa. Balsa wood cores first appeared in the 1940’s in flying boat hulls, which were aluminium skinned and balsa-cored to withstand the repeated impact of landing on water. This performance led the marine industry to begin using end-grain balsa as a core material in FRP construction. Apart from its high compressive properties, its advantages include being a good thermal insulator offering good acoustic absorption. The material will not deform when heated and acts as an insulating and ablative layer in a fire, with the core charring slowly, allowing the non-exposed skin to remain structurally sound. It also offers positive flotation and is easily worked with simple tools and equipment.

Balsa core is available as contoured end-grain sheets 3 to 50mm thick on a backing fabric, and rigid end-grain sheets up to 100mm thick. These sheets can be provided ready resin-coated for vacuum-bagging, prepreg or pressure-based manufacturing processes such as RTM. One of the disadvantages of balsa is its high minimum density, with 100kg/m3 being a typical minimum. This problem is exacerbated by the fact that balsa can absorb large quantities of resin during lamination, although pre-sealing the foam can reduce this. Its use is therefore normally restricted to projects where optimum weight saving is not required or in locally highly stressed areas.

  • Cedar

Another wood that is used sometimes as a core material is cedar. In marine construction it is often the material used as the ‘core’ in strip-plank construction, with a composite skin on each side and the grain of the cedar running parallel to the laminate faces. The cedar fibres run along the length of the boat giving fore and aft stiffness while the fibres in the FRP skins are laid at 45 degrees giving torsional rigidity, and protecting the wood.

Other Core Materials

Although not usually regarded as true sandwich cores, there are a number of thin, low-density ‘fabric-like’ materials which can be used to slightly lower the density of a single-skin laminate. Materials such as Coremat and Spheretex consist of a non-woven ‘felt-like’ fabric full of density-reducing hollow spheres. They are usually only 1- 3mm in thickness and are used like another layer of reinforcement in the middle of a laminate, being designed to ‘wet out’ with the laminating resin during construction. However, the hollow spheres displace resin and so the resultant middle layer, although much heavier than a foam or honeycomb core, is lower in density than the equivalent thickness of glass fibre laminate. Being so thin they can also conform easily to 2-D curvature, and so are quick and easy to use.

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Comparison of Core Mechanical Properties

These diagrams give the shear strength and compressive strength of some of the core materials described, plotted against their densities. All the figures have been obtained from manufacturers’ data sheets.

Compressive Strength v Core Density Shear Strength v Core Density

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As might be expected, all the cores show an increase in properties with increasing density. However, other factors, besides density, also come into play when looking at the weight of a core in a sandwich structure. For example, low density foam materials, while contributing very little to the weight of a sandwich laminate, often have a very open surface cell structure which can mean that a large mass of resin is absorbed in their bondlines. The lower the density of the foam, the larger are the cells and the worse is the problem. Honeycombs, on the other hand, can be very good in this respect since a well formulated adhesive will form a small bonding fillet only around the cell walls (as shown in this diagram).

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. Scrim-backed 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.

In weight-critical components the use of foam cores which are thermoformable should be considered. These include the linear PVC’s and the SAN foams which can all be heated to above their softening points and pre-curved to fit a mould shape. For honeycombs, over-expanded forms are the most widely used when fitting the core to a compound curve, since with different expansion patterns a wide range of conformability can be achieved.

Next week: composite manufacturing processes

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