Unidirectional Fabric
A unidirectional (UD) ply is one in which all of the fibres run in one direction only. A small amount of fibre or other material may run in other directions with the main intention being tohold the primary fibres in position. Uni directionals usually have their fibres in the 0° direction along the roll on which they are supplied (a warp UD) but can also have them at 90° to the roll length (a weft UD). Frequently, these materials are supplied in prepreg (resin-impregnated)unidirectional tape, where only the resin system holds the fibres in place.