Explain Laminar and sandwich panel of structural composite.

Structural composites are made of homogeneous and composite layers. Their properties depend on the constituent materials and on the geometric design of the parts.

1. Laminar composites are built from two-dimensional sheets or panels (laminae, plies) stacked and bonded together. In each ply the high-strength direction is different, e.g. 0°/90°/±45° orientations of fibres.

   ----------- ply 1 (fibres 0°)
   ||||||||||| ply 2 (fibres 90°)
   ----------- ply 3 (0°)
   ||||||||||| ply 4 (90°)   -> bonded laminate

• The layered build gives fairly high, nearly uniform strength in all directions within the plane.
• Examples: plywood (wood veneers with grain directions alternated), laminated glass-fibre or carbon-fibre sheets, laminated safety glass, Formica.

Sandwich panel (a structural composite)

A sandwich panel has two thin, strong and stiff outer face sheets (skins) bonded to a thick, lightweight core.

   ===========================  <- face sheet (Al alloy / FRP / steel / plywood)
   |/\/\/\/\/\/\/\/\/\/\/\/\/|  <- lightweight core (honeycomb / foam / balsa)
   ===========================  <- face sheet
        (adhesive bonds core to faces)

• Faces: aluminium alloys, fibre-reinforced plastics, titanium, steel or plywood. They carry the in-plane loads and the bending stresses (tension on one face, compression on the other).
• Core: low-density material such as honeycomb (aluminium or aramid paper), rigid polymer foams (PU, PVC) or balsa wood. It keeps the faces apart, resists shear and stops the faces buckling. It also gives thermal and sound insulation.
• The result is a panel with high bending stiffness and strength for very low weight.

Applications: aircraft floors, wings, rotor blades and fuselage panels; walls and roofs of buildings; ship hulls and decks; truck and railway-coach bodies; refrigerated containers; skis and surfboards; satellite panels.