ULTIMATE TENSILE STRENGTH
The ultimate or final stress sustained by a specimen in a tension test; the stress at moment of rupture.

UNBOND
Area of a bonded surface in which bonding of adherends has failed to occur, or where two prepreg layers of a composite fail to adhere to each other; also denotes areas where bonding is deliberately prevented so as to simulate a defective bond.

UNDERCUT
Having a protuberance or indentation that impedes the withdrawal from a two-piece, rigid mold; any such protuberance or indentation, depending on the design of the mold (that is, tilting a model in design of a mold for that model may eliminate an apparent "undercut").

UNIDIRECTIONAL
Refers to fibers that are oriented in the same direction, such as unidirectional fabric, tape, or laminate, often called UD.

UV STABILIZER
A chemical compound which improves resistance to degradation from ultraviolet radiation.