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
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
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 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.
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 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.
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.
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
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
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 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.
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.
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.
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.
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
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.
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