What are structural composites like laminates and sandwich panels, and why are they so stiff yet light?

In one line

Structural composites are made of layers or of face sheets and a core bonded together, and their properties depend on the geometry of the structure as well as on the materials used.

Explained simply

Plywood is many thin wood sheets glued together with grains crossing each other, so it does not split easily in any direction. A sandwich panel is like a slice of bread–cheese–bread where the bread slices are strong thin skins and the cheese is a very light filling that keeps them apart. Moving the strong skins apart makes the panel much stiffer in bending, while the light core adds very little weight. An I-beam uses the same trick.

How it works

1. Laminar composites

1. Thin layers (laminae or plies), each often a fibre-reinforced sheet with strong direction, are stacked and bonded.
2. Orientations are changed layer to layer (0°, 90°, ±45°) so the laminate has strength in several directions.
3. Examples: plywood, carbon-fibre laminates in aircraft skins, glass–epoxy PCB boards, laminated safety glass (glass + polymer interlayer).

2. Sandwich panels

1. Face sheets: thin, strong and stiff (aluminium alloy, CFRP, GFRP, steel). They carry most of the bending (tension and compression) loads.
2. Core: thick, light material (honeycomb of aluminium or aramid paper, polymer foams, balsa wood). It keeps the faces apart, resists shear and supports faces against buckling.
3. An adhesive bonds faces and core.
4. Bending stiffness increases roughly with the square of the distance between the faces, so a thicker light core greatly increases stiffness with little added weight.

Key formula

For a sandwich beam with thin faces, the bending stiffness per unit width is approximately

\( D \approx \dfrac{E_f\, t\, h^2}{2} \)

• \( E_f \) = modulus of the face sheet material (GPa)
• \( t \) = thickness of each face sheet
• \( h \) = distance between the centres of the two faces

Shown for understanding: doubling h gives about four times the stiffness.

Quick example

Aircraft cabin floors, helicopter rotor blades, satellite panels, high-speed train interiors and refrigerator walls use honeycomb or foam sandwich panels for very high stiffness-to-weight ratio, plus thermal and sound insulation from the core.

Common mistakes

• Saying the core carries the main bending load; the faces do, the core carries shear.
• Calling plywood a fibre composite only; it is classically given as a laminar structural composite.
• Forgetting the adhesive bond; core–face debonding is a major failure mode.

How MU asks it

As part of "Classify composites" (5 marks) or "Write a note on structural composites" (3 marks). For full marks: definition, laminar and sandwich types with a labelled sketch (face sheet, core, adhesive), function of each part, and two applications each.