Ply

A composite element built up from multiple bonded layers (plies) of reinforcement and matrix, consolidated and cured so that they act as a single structural material. Its stiffness and strength depend on the fibre orientations and stacking sequence of the plies.

The specified order and orientation of the plies through the thickness of a laminate, usually written as a shorthand code of ply angles. It governs the laminate's stiffness, strength and coupling behaviour, and is fixed on the drawing as part of the laminate schedule.

Reinforcement in which virtually all fibres run in a single direction, supplied as dry tape, lightly stitched or bound fabric, or prepreg. Unidirectional plies give the highest stiffness and strength along the fibre direction but need supporting plies in other orientations.

Reinforcement fibre pre-impregnated with a controlled quantity of resin, usually a partially cured (B-staged) thermoset, supplied in rolls and stored chilled or frozen until layup. Prepregs give precise fibre and resin content and are the standard material form for high-performance aerospace and motorsport laminates.