THORIUM: A heavy malleable, radioactive metal used in the manufacture of thoriated tungsten target material in the
X-ray tube head.
THREE-PHASE ALTERNATING CURRENT: Commercial, electricity is commonly transmitted as three single phase
currents, that is, three separate currents following separate sine curves, each at 60 cycles (or other frequency) per
second, but with the peaks of their individual curves one-third of a cycle apart. At least three (sometimes four)
conductors are required for three-phase alternating current.
THRESHOLD: In reference to currents or magnetic fields, the minimum strength necessary to create a looked-for effect
is called the threshold value. For example, the minimum current necessary to produce a readable indication at a given
defect, is the threshold value of current for that purpose.
THRESHOLD DOSE (RT): The minimum absorbed dose or dose equivalent that will produce a specified effect.
THROUGH TRANSMISSION METHOD (UT): An inspection method in which ultrasonic energy is generated by one
search unit and received by another at the opposite surface of the test part.
3-2T RADIOGRAPHY: Quality level of radiography in which the finished radiograph displays a discernible image or a
penetrameter hole that has a diameter equal to twice the penetrameter thickness. The penetrameter thickness equals 3
percent of the material thickness.
THYLIUM-170: A radioisotope of the element thulium.
TIME DELAY (UT): See SWEEP DELAY.
TLD (RT): See THERMOLUMINESENCE DOSIMETER.
TOE CRACK: A base-metal crack at the Toe of Weld.
TOE OF WELD: The junction between the face of a weld and the base metal.
TOLERANCE: The specified permissible deviation from a specified nominal dimension, or the permissible variation in
size of a part.
TOMOGRAPH (RT): A radiograph of a specified plane of a deep structure.
TOMOGRAPHY (RT): The radiography of a predetermined interior plane of a thick material. In one method the X-
ray tube and the film are moved simultaneously in opposite directions about a pivotal point in the plane of the layer.
TORSION: A twisting action resulting in shear stresses and strains.
TOUGHNESS: Ability of a metal to absorb energy and deform plastically before fracturing. It is usually measured by
the energy absorbed in a notch impact test, but the area under the stress-strain curve in tensile testing is also a measure
TOXIC: The quality of certain materials being proportionally poisonous, as indicated by jeopardy to life, health or
TRACER: An element or compound that has been made radioactive so that it can be easily followed (traced) in
biological and industrial processes. Radiation emitted by the radioisotope pinpoints its location.
TRANSDUCER: Any device that is capable of converting energy from one form to another.