Core materials for sandwich panels

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Core materials for sandwich panels

• John Summerscales

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Outline of lecture

• Polymer foam cores
• Syntactic cores
• Wood cores
• Honeycomb and corrugated cores
• Alternative technologies

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Polymer foam cores

• Wide variety of polymers
• Polyethylene terephthalate (PET) , e.g. Airex
• Polymethacrylimide (PMI) e.g. Rohacell
• Polystyrene (PS)
• Polyurethane (PU) e.g. Kapex
• Polyvinylchloride (PVC)
• linear/uncrosslinked e.g. Airex
• rigid/crosslinked e.g. Divinycell or Herex
• StyreneAcryloNitrile (SAN) e.g. Corecell
• Densities from 30 kg/m3 to 400 kg/m3

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PMI foam

• high overall strength and stiffnessfor foam of a
  given density
• high dimensional stability
• high fatigue life
• can be used at elevated temperatures
• high cost limits use to aerospace components
• helicopter rotor blades, ailerons,stringer
  profiles in pressure bulkheads

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PS foam

• light weight, from 40 kg/m3
• low cost
• easy to shape
• extensive use in surf board manufacture
• mechanical properties are relatively poor
• styrene in polyester resin acts as a solvent
• hence, normally used with epoxy resins.

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PU foam

• available as thermoplastic or thermoset
• isocyanate and polyol mixture free foams to
  fill hollow mouldings
• generally inexpensive
• poor mechanical properties
• health and safety issues with isocyanate. 

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PVC foams I

• usually a hybrid of PVC and polyurethane
• balance of static and dynamic properties
• good resistance to water absorption
• wide operating temperature range typically
  -240C to 80C
• resistant to styrene which permits their use
  with polyester resins.

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PVC foams II

• Uncrosslinked foams
• flexible, tough
• easily thermoformed
• lower mechanical properties
• lower resistance to elevated temperatures
• lower resistance to styrene
• Cross-linked PVC
• harder, relatively brittle
• less susceptible to softening when hot. 

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Syntactic cores

• Syntactic foam lightweight composite of hollow
  spheres in a resin matrix.
• spheres are normally polymeric or glass
• typical diameter is 10 to 200 microns
• Core
• densities ranging from 300 kg/m3 to 550 kg/m3
• high compressive strength-to-weight ratio

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Wood cores

• End Grain Balsa (EGB) ultra-light wood
• high strength to weight ratio for a core
• closed cell structure imparts high compressive
  (and hence high resistance to crushing) and shear
  stiffness and strength to a sandwich panel
• in end grain configuration, balsa has the ability
  to handle dynamic loads with high resistance to
  fatigue.
• good thermal and acoustic insulator.
• denser woods included to anchor attachments

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Honeycomb/corrugated cores

• honeycomb cores
• aluminium, paper or polymers
• made by selectively bonding layers of scored
  material and then expanding the stack to produce
  a regular cellular structure.
• alternative manufacture includes corrugation
  followed by bonding
• extrusion of thermoplastics. 

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Honeycomb/corrugated cores

• Aluminium yields on expansion to give permanent
  deformation when expanded.
• do remember that galvanic corrosion may occur if
  carbon fibres are in contact with aluminiumin a
  moist environment.
• Short fibre paper cores are expanded and
  impregnated with resin to hold shape
• Nomex (a DuPont aramid fibre)
• used for aerospace quality honeycombs
• be aware aramid is hydrophilic

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Honeycomb/corrugated cores

• Auxetic (negative Poissons ratio) structures
• hexagonal materials gt anticlastic curvature
• auxetic structures gt synclastic curvature
• re-entrant cells chiral structure

Right hand image from Rod Lakes (University of
Wisconsin) webpage http//silver.neep.wisc.edu/la
kes/chiralHny.gif
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Alternative technology Parabeam

• Parabeam 3D glass fabric
• woven E-glass yarn
• two deck-layers bonded by vertical piles
• when impregnated with a thermoset resin,fabric
  absorbs resin and the capillary forcescause
  fabric to rise to a preset height
• images from http//www.parabeam.nl/

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Alternative technology Lantor

• "bulker mats" lofty layer of synthetic fibre with
  embedded microspheres.
• can build laminate thickness quickly
• reduced resin and glass usage
• hence, save both weight and labour.
• Lantor Coremat random polyester fabric
• Lantor SORIC for use in closed molding(resin
  infusion) processes, acting as bothresin
  transfer medium and a core material.

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Alternative technology Lantor

• Soric
• lt Coremat

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Summary of lecture

• Polymer foam cores
• Syntactic cores
• Wood cores
• Honeycomb and corrugated cores
• Alternative technologies