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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 construction
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.
slide2

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:

slide3

Bending stiffness is increased by making beams or panel thicker - with sandwich construction this can be achieved with very little increase in weight:

slide4

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

foam core comparison
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

foam core comparison6
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

foam core comparison7
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?

slide9

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).

slide10

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:

slide11

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:

slide13

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

sandwich construction14
Sandwich Construction
  • Many different possible failure modes exist, each of which has an approximate design formula.