Transcript EBB 324 /3 - USM :: Universiti Sains Malaysia

CORE MATERIALS
Lecture 4
The cores used in load carrying sandwich constructions can be
divided into four main groups:
•Corrugated
•Honeycomb (Various shapes and materials)
•Balsa wood
•Cellular foams (Polymeric, metallic and Ceramic)
CORE MATERIALS
Lecture 4
•Core should have low density in order to add as little as
possible to the total weight of the sandwich
•Young’s modulus perpendicular to the faces should be fairly
high to prevent a decrease in the core thickness and therefore a
rapid decrease in the flexural rigidity
•The core is mainly subjected to shear so that the core shear
strains produce global deformations and core shear stresses
•Thus, a core must be chosen that would not fail under the
applied transverse load and with a shear modulus high enough to
give the required shear stiffness
The critical wrinkling load depends on both Young’s modulus and
the shear modulus of the core
CORE MATERIALS
•The properties of primary interest for the core may be
summarised as:
•Low density
•Shear modulus
•Shear strength
•Stiffness perpendicular to the faces
•Thermal insulation
Lecture 4
HONEYCOMB CORES
Lecture 4
•Core materials of honeycomb type have been developed and used
mainly in aerospace applications
•However, cheap honeycomb materials made from impregnated
paper are also used in building applications
•Honeycomb cores can be manufactured in a variety of cell shapes
but the most commonly used shape is the hexagonal
•Others are square, over-expanded hexagonal, flex-core.
•Over-expanded and flex-core are mainly used when the core
needs to be curved in the manufacturing of the sandwich element
HONEYCOMB CORES
Lecture 4
•Over-expanded hexagonal and flex-core shapes reduce the
anticlastic bending and cell wall buckling when curved
•There are other cell shapes used such as rectangular, and
reinforced hexagonal.
•The manufacturing of metal honeycombs is performed in two
different ways: Corrugating and expansion processes
•Corrugating implies that pre-corrugated metal sheets are bonded
together and stacked into blocks
•When the adhesive has cured, blocks with the required thickness
can be cut from the stack
•The process is commonly used in manufacture of high-density metal
honeycombs
HONEYCOMB CORES
Lecture 4
•The expansion process begins with the stacking of thin plane sheets
of web material on which adhesive nodes have been printed
•By stacking many thin layers in this way a block is made
•Each block may then be cut into desired thickness (T-direction).
•When the adhesive has cured it may be expanded by pulling in the
W-direction until a desired cell shape has been achieved
HONEYCOMB CORES
Various honeycomb cores may be found such as:
•Aluminium alloy honeycomb
•Kraft paper honeycombs
•Non-metallic honeycombs
Lecture 4
HONEYCOMB CORES
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Aluminium alloy honeycomb
•Extensivly used in aerospace applications during the past
decades
•They are commonly made of the aluminium alloys 5052, 5056,
and 2024
•5052 is a general purpose alloy, 5056 a high strength version of
5052 and 2024 a heat treated aluminium alloy with good
properties even at elevated temperature
•The 5052 and 5056 alloy honeycombs can be used in
environments up to 180°C and the 2024 up to 210°C.
HONEYCOMB CORES
Lecture 4
Kraft paper honeycombs
•Manufactured by impregnating paper with resin to make it
water resistant
•This provides cheap, but still mechanically very good sandwich
core
•Some manufacturers can even fill the cells of Kraft paper
honeycomb with a light weight foam (usually PUR or phenolic)
for improved thermal insulation
HONEYCOMB CORES
Lecture 4
Non-metallic honeycomb
•Similar to fibre-reinforced plastics but with honeycomb shape
•Produced by impregnating a pre-fabricated cell-shaped fabric in a
bath of resin
•Different honeycombs are available with glass, aramid or even
carbon fibre fabric reinforcement
•The matrix which the fabric is impregnated with usually phenolic,
heat resistant phenolic, polyimide or polyester
•Phenolic impregnated have maximum working temperature up to
180°C, polyimide 250°C, polyester 80°C
HONEYCOMB CORES
Lecture 4
Non-metallic honeycomb cont’d
•A well-known type of fibre-impregnated honeycomb is made of
NOMEX paper, which is an aramid fibre based fabric expanded in
much the same way as aluminium alloy honeycomb before being
coated with resin
•It is widely used because of its high toughness and damage
resistance and since it has almost as high mechanical properties as
aluminium alloy honeycomb.
•Nomex honeycomb can be used up to 180°C at which its strength
still approximately 75% of its room temperature value
BALSA WOOD CORE
Lecture 4
•First material used as cores in load carrying sandwich structures
•Balsa is a wood but under the microscope it can be seen as a highaspect-ratio closed-cell structure
•The fibres or grains are oriented in the direction of growth
producing cells with a typical length of 0.5-1.0 mm and with a
diameter of about 0.05 mm, thus giving the cell ratio of
approximately 1:25.
•The properties of balsa are therefore high in direction of growth
but much lower in the others
•Balsa exists in different qualities with densities in the regime 100
to 300 kgm-3.
BALSA WOOD CORE cont’d
Lecture 4
•Balsa is also very sensitive to humidity with the properties rapidly
declining with the water content
•To overcome the above problem balsa is most commonly utilised in
its “end-grain” shape.
•This means that the balsa wood is cut up in cubic pieces and
bonded together edge wise so that a block is produced where the
fibre direction is located perpendicular to the plane of the block.
•In this way, principal direction of stiffness is perpendicular to the
faces, and humidity is spread along the fibres and hence damage
would only cause localised humidity damage
•The drawback is that all the small balsa blocks have different
densities and the design limit must be taken from the piece of having
the lowest properties
CELLULAR FOAMS
Lecture 4
•Cellular foams do not offer the same high stiffness and strength-toweight ratios as honeycombs but have other very important
advantages
•Firstly, cellular foams are in general less expensive than
honeycombs but more importantly, a foam is a solid on a
macroscopic level making the manufacturing of sandwich element
easier; the foam surface is easy to bond to, surface preparation and
shaping is simple and connections of block are easily performed by
adhesive bonding
•In addition, cellular foams offer high thermal insulation, acoustal
damping, and the closed cell structure of most foams ensure that the
structure will become bouyant and resistant to water penetration
CELLULAR FOAMS cont’d
There exist a variety of foams, with different advantages and
disadvantages. Some of these are (polymer-based):
•Polyurethane foam (PUR)
•Polystyrene foam (PS)
•Polyvinyl chloride foam (PVC)
•Poly-methacryl-imide foam (PMI)
Lecture 4
CELLULAR FOAMS
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Polyurethane foam (PUR)
•The urethane polymer is formed through the reaction between isocyanate and polyol, and tri-chloro-fluoro-methane or carbon dioxide
used as blowing agent
•Produced in many variations from soft with more or less open cells
to rigid types with predominantly closed cells and in a wide range of
density
•They can be made fire resistant by using additive containing
phosphorous
•Due to high molecular weight, PUR foams have low thermal
conductivity and diffusion coefficients giving them very good
insulation properties
CELLULAR FOAMS
Lecture 4
Polyurethane foam (PUR)
•Rigid PUR foams generally have quite brittle cell walls and hence
the PUR core has low toughness and low ultimate elongation
•The mechanical properties are lower than most other cellular
plastic core but PUR foams are probably the cheapest of all available
core materials
•The primary use of PUR is for insulation purposes or in less critical
load bearing elements
•An advantage is that PUR foam can be produced in finite size blocks
as well as being formed in-situ thus giving an integrated
manufacturing process in conjunction with the manufacturing of
sandwich elements
CELLULAR FOAMS
Lecture 4
Polystyrene foam
•Produced either by extrusion or by expansion in closed moulds
•In both cases the plastic is mixed with the blowing agent which
then expands at elevated temperature
•A major obstacle was that CFC was used as blowing agent, but
recently PS foams have been expanded without the use of
environmentally dangerous CFC-gases
•PS has closed cells and is available in densities ranging from 15 to
300 kgm-3.
•Ps foam has quite good mechanical and thermal insulation
properties, and its cheap
CELLULAR FOAMS
Lecture 4
Polystyrene foam cont’d
•A drawback is its sensitivity to solvents, particularly styrene, and
hence ester-based matrices can not be used as adhesives
•PS is primarily used as thermal insulation material but lately it has
also been used in load carrying structures such as refrigerated tanks
and containers
CELLULAR FOAMS
Lecture 4
Polyvinyl chloride foam (PVC)
•Exists in two different forms; one purely thermoplastic also called
linear PVC foam, and one cross-linked iso-cyanide modified type
•The linear PVC has great ductility, quite good mechanical properties
but softens at elevated temperatures
•The cross-linked PVC is more rigid, has higher mechanical
properties, is less heat sensitive, but more brittle.
•Still, even cross-linked PVC has an ultimate elongation of about 10%
in tension which is much higher than PUR foam
CELLULAR FOAMS
Lecture 4
Polyvinyl chloride foam (PVC) cont’d
•PVC foam is available in finite size blocks with densities from 30 to
400 kgm-3
•The mechanical properties of PVC are higher than those of both
PUR and PS, but is also expensive than those
•It is non-flammable foam but when burnt a hydrochloric acid gas is
released
•PVC foam are used in almost every type of application varying from
pure insulation applications to aerospace structures and hence the
almost widely used of all foams and perhaps of all core materials
•PVC has about 95% closed cells for the lower densities and almost
entirely closed cell for higher, which is much appreciated in
applications where water absorption is a problem
CELLULAR FOAMS
Lecture 4
Poly-methacryl-imide (PMI)
•Acryl-imide cellular plastics are made from expanded imidemodified polyacrylates
•The mechanical properties are good, perhaps the best of all
commercially available cellular foams, but the price is also the
highest
•PMI is fairly brittle with an ultimate elongation in tension of
approximately 3% in tension.
•The main advantage is the temperature resistance making it
possible to use PMI foam in conjunction with epoxy prepregs in
autoclave manufacturing in up to 180C environments
•The cell structure is very fine with closed cells and the densities
available are from 30 to 300 kgm-3
Lecture 4
In most cases, an efficient sandwich panel is obtained when the weight
of the core is almost equivalent to the combined weight of the faceplates
[2]. By separating the faceplates using a low density core, the moment
of inertia of the panel is increased and hence resulted in improved
bending stiffness. Therefore, the bending stiffness of a sandwich
structure greatly exceeds that of a solid structure having the same total
weight and made of the same material as the facings. Furthermore, due
to the porous nature of the core material, sandwich structure has
inherent exceptional thermal insulation and acoustic damping properties.