Sandwich Construction

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Sandwich Construction

• Thin composite skins bonded to thicker,
  lightweight core.
• Large increase in second moment of area without
  weight penalty.
• Core needs good shear stiffness and strength.
• Skins carry tension and compression loads.

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Sandwich panels are a very efficient way of
providing high bending stiffness at low weight.
The stiff, strong facing skins carry the bending
loads, while the core resists shear loads. The
principle is the same as a traditional I beam
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Bending stiffness is increased by making beams or
panel thicker - with sandwich construction this
can be achieved with very little increase in
weight
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The stiff, strong facing skins carry the bending
loads, while the core resists shear loads.
Total deflection bending shear Bending
depends on the skin properties shear depends on
the core
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Foam core comparison

• PVC (closed cell)- linear high ductility,
  low properties- cross-linked high strength
  and stiffness, but brittle- 50 reduction of
  properties at 40-60oC- chemical breakdown (HCl
  vapour) at 200oC

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Foam core comparison

• PU- inferior to PVC at ambient temperatures-
  better property retention (max. 100oC)
• Phenolic- poor mechanical properties- good
  fire resistance- strength retention to 150oC

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Foam core comparison

• Syntactic foam- glass or polymer microspheres-
  used as sandwich core or buoyant filler- high
  compressive strength
• Balsa- efficient and low cost- absorbs water
  (swelling and rot)- not advisable for primary
  hull and deck structures OK for internal
  bulkheads, etc?

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Why honeycomb? List compiled by company
(Hexcel) which sells honeycomb!
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Sandwich constructions made with other core
materials (balsa, foam, etc) have a large surface
are available for bonding the skins. In honeycomb
core, we rely on a small fillet of adhesive at
the edge of the cell walls
The fillet is crucial to the performance of the
sandwich, yet it is very dependent on
manufacturing factors (resin viscosity,
temperature, vacuum, etc).
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Honeycomb is available in polymer, carbon, aramid
and GRP. The two commonest types in aerospace
applications are based on aluminium and Nomex
(aramid fibre-paper impregnated with phenolic
resin). Cells are usually hexagonal but
overexpanded core is also used to give extra
formability
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Core properties depend on density and cell size.
They also depend on direction - the core is much
stronger and stiffer in the ribbon or L
direction
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Aluminium generally has superior properties to
Nomex honeycomb, e.g
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Aluminum Honeycomb relatively low cost best
for energy absorption greatest
strength/weight thinnest cell walls smooth
cell walls conductive heat transfer
electrical shielding machinability
Aramid Fiber (Nomex) Honeycomb
flammability/fire retardance large selection of
cell sizes, densities, and strengths
formability and parts-making experience
insulative low dielectric properties
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Sandwich Construction

• Many different possible failure modes exist, each
  of which has an approximate design formula.

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Design Formulae for Sandwich Construction
t
c
h
core tensile modulus Ec shear
modulus Gc skin tensile modulus Es d c t
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Sandwich Construction - flexural rigidity

• Neglecting the core stiffness
• Including the core
• If core stiffness is low

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Sandwich Construction - flexural rigidity

• Shear stiffness is likely to be
  significantwhere shear stiffness Q b c Gc
• If D/L2Q lt 0.01, shear effects are small.
• If D/L2Q gt 0.1, shear effects are dominant.

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Sandwich Construction - flexural rigidity

• Plate stiffnesses can be calculated by CLA, but
  shear effects must be considered.
• Formula for plate deflection is of the
  formwhere the transverse shear stiffness is
  now Q c Gc. a is the longest side of a
  rectangular panel.

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Bending stresses in sandwich beams

• It is often assumed that the core carries no
  bending stress, but are under a constant shear
  stress. For an applied bending moment M
• Skin stress
• Core shear stresswhere S is the shear forcey
  is distance from neutral axis
• If core stiffness can be neglected

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