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.

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:

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

Total deflection = bending + shear Bending depends on the skin properties; shear depends on the core The stiff, strong facing skins carry the bending loads, while the core resists shear loads.

Foam core comparison

PVC (closed cell) -

‘linear’ – high ductility, low properties ‘cross-linked’ – high strength and stiffness, but brittle - ~ 50% reduction of properties at 40-60 o C - chemical breakdown (HCl vapour) at 200 o C

Foam core comparison

PU -

inferior to PVC at ambient temperatures - better property retention (max. 100 o C)

Phenolic

- poor mechanical properties - good fire resistance - strength retention to 150 o C

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?

Why honeycomb?

List compiled by company (Hexcel) which sells honeycomb!

Material Property

Foam includes – polyvinyl chloride (PVC) – polymethacrylimide – polyurethane – polystyrene – phenolic – polyethersulfone (PES) Relatively low crush strength and stiffness Increasing stress with increasing strain Friable Limited strength Fatigue Cannot be formed around curvatures

Honeycomb Advantages

Excellent crush strength and stiffness Constant crush strength Structural integrity Exceptionally high strengths available High fatigue resistance OX-Core and Flex-Core cell configurations for curvatures Wood-based includes – plywood – balsa – particleboard Very heavy density Subject to moisture degradation Flammable Excellent strength-to-weight ratio Excellent moisture resistance Self-extinguishing, low smoke versions available

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).

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:

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:

5056 aluminium honeycomb

600 500 400 300 200 100 0 30 40 50 60

density (kg/m 3 )

70 80 L' direction 'W' direction

Aluminium generally has superior properties to Nomex honeycomb, e.g:

'L' direction plate shear modulus

600 500 400 300 200 100 0 20 40 60

density (kg/m 3 )

80 Nomex Al

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 Construction

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