Transcript Title

Core materials
for sandwich panels
John Summerscales
Outline of lecture
• Polymer foam cores
• Syntactic cores
• Wood cores
• Honeycomb and corrugated cores
• Alternative technologies
Polymer foam cores
• Wide variety of polymers:
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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
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StyreneAcryloNitrile (SAN) e.g. Corecell
• Densities from 30 kg/m3 to 400 kg/m3
PMI foam
• high overall strength and stiffness
for foam of a given density
• high dimensional stability
• high fatigue life
• can be used at elevated temperatures
• high cost limits use to aerospace components:
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helicopter rotor blades, ailerons,
stringer profiles in pressure bulkheads
PS foam
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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
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hence, normally used with epoxy resins.
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.
PVC foams I
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usually a hybrid of PVC and polyurethane
balance of static and dynamic properties
good resistance to water absorption
wide operating temperature range
typically -240°C to +80°C
• resistant to styrene which
permits their use with polyester resins.
PVC foams II
• Uncrosslinked foams
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flexible, tough
easily thermoformed
lower mechanical properties
lower resistance to elevated temperatures
lower resistance to styrene
• Cross-linked PVC
harder, relatively brittle
o 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
o typical diameter is 10 to 200 microns
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• Core:
densities ranging from 300 kg/m3 to 550 kg/m3
o 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
o closed cell structure imparts high compressive
(and hence high resistance to crushing) and
shear stiffness and strength to a sandwich panel
o in end grain configuration, balsa has the ability
to handle dynamic loads with high resistance to
fatigue.
o good thermal and acoustic insulator.
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• denser woods included to anchor attachments
Honeycomb/corrugated cores
• honeycomb cores
aluminium, paper or polymers
o made by selectively bonding layers of scored
material and then expanding the stack to
produce a regular cellular structure.
o 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.
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do remember that galvanic corrosion may occur
if carbon fibres are in contact with aluminium
in 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
o be aware: aramid is hydrophilic
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Honeycomb/corrugated cores
• Auxetic (negative Poisson’s ratio) structures
• hexagonal materials > anticlastic curvature
• auxetic structures > synclastic curvature
re-entrant cells
chiral structure
Right hand image from Rod Lakes (University of Wisconsin)
webpage http://silver.neep.wisc.edu/~lakes/chiralHny.gif
Alternative technology: Parabeam
• Parabeam® 3D glass fabric
woven E-glass yarn
o two deck-layers bonded by vertical piles
o when impregnated with a thermoset resin,
fabric absorbs resin and the capillary forces
cause fabric to rise to a preset height
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images from http://www.parabeam.nl/
Alternative technology: Lantor
• "bulker mats" lofty layer of synthetic fibre
with embedded microspheres.
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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 both
resin transfer medium and a core material.
Alternative technology: Lantor
Soric ^
< Coremat
Summary of lecture
• Polymer foam cores
• Syntactic cores
• Wood cores
• Honeycomb and corrugated cores
• Alternative technologies