CORE MATERIALS

1
Lecture 4

• CORE MATERIALS

• 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)

2
Lecture 4

• CORE MATERIALS

• Core should have low density in order to add as
  little as possible to the total weight of the
  sandwich
• Youngs 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
  Youngs modulus and the shear modulus of the core

3
Lecture 4

• 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

4
Lecture 4

• HONEYCOMB CORES

• 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

5
Lecture 4

• HONEYCOMB CORES

• 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

6
Lecture 4

• HONEYCOMB CORES

• 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

7
Lecture 4

• HONEYCOMB CORES

• Various honeycomb cores may be found such as
• Aluminium alloy honeycomb
• Kraft paper honeycombs
• Non-metallic honeycombs

8
Lecture 4

• HONEYCOMB CORES

• 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 180C and the 2024 up to
  210C.

9
Lecture 4

• HONEYCOMB CORES

• 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

10
Lecture 4

• HONEYCOMB CORES

• 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 180C, polyimide 250C,
  polyester 80C

11
Lecture 4

• HONEYCOMB CORES

• Non-metallic honeycomb contd
• 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 180C at which
  its strength still approximately 75 of its room
  temperature value

12
Lecture 4

• BALSA WOOD CORE

• First material used as cores in load carrying
  sandwich structures
• Balsa is a wood but under the microscope it can
  be seen as a high-aspect-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 125.
• 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.

13
Lecture 4

• BALSA WOOD CORE contd

• 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

14
Lecture 4

• CELLULAR FOAMS

• Cellular foams do not offer the same high
  stiffness and strength-to-weight 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

15
Lecture 4

• CELLULAR FOAMS contd

• 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)

16
Lecture 4

• CELLULAR FOAMS

• Polyurethane foam (PUR)
• The urethane polymer is formed through the
  reaction between iso-cyanate 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

17
Lecture 4

• CELLULAR FOAMS

• 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

18
Lecture 4

• CELLULAR FOAMS

• 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

19
Lecture 4

• CELLULAR FOAMS

• Polystyrene foam contd
• 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

20
Lecture 4

• CELLULAR FOAMS

• 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

21
Lecture 4

• CELLULAR FOAMS

• Polyvinyl chloride foam (PVC) contd
• 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

22
Lecture 4

• CELLULAR FOAMS

• Poly-methacryl-imide (PMI)
• Acryl-imide cellular plastics are made from
  expanded imide-modified 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

23
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.