OPERATING STRESS: The stress to which a structural unit is subjected in service.
OPTICAL DENSITY (RT): See DENSITY.
OPTICAL PYROMETER: A temperature measuring optical device used to compare the incandescence of a heated
object with that of an electrically heated filament whose brightness can be regulated.
OPTIMUM FREQUENCY (ET, UT): That frequency, which provides the highest signal-to-noise ratio obtainable for
the detection of an individual property such as conductivity, crack, or inclusion of the test specimen. Each type of
defect in a given material may have its own optimum frequency.
ORANGE-PEEL EFFECT: A surface roughening in the form of a grain pattern where a metal of unusually coarse
grain is stressed beyond its elastic limit. Also called pebbles and alligator skin.
ORBITAL ELECTRON (SHELL ELECTRON) (RT): An electron in the extra-nuclear structure of an atom.
ORIENTATION: Position of a discontinuity or part or surface in relation to the test surface of the article or ultrasonic
ORIENTATION (CRYSTAL): Arrangement in space of the axes of a crystal with respect to a chosen reference or
OSCILLATOR (ET): A component of an electrical circuit that provides a source of current that varies in magnitude
and direction with time. In eddy current testing, the oscillator provides a source of alternating current to establish a
varying magnetic field.
OSCILLOGRAM (UT): Common term for photograph of data displayed on CRT.
OVER-DEVELOPMENT (RT): Development that is greater than that required to produce the optimum results in a
particular radiograph. It may arise from development for too long a time, or at too high a temperature, and may give
rise to excessive graininess and lack of contrast.
OVERHEATED: Steel subjected to such high temperatures that coarse grains are produced without destroying the stock
as in burning. This may be corrected by suitable heat treat.
OVERLAP: Protrusion of weld metal beyond the bond at the toe of the weld.
OVERLOAD INTERLOCK, X-RAY UNIT (RT): An X-ray machine in which the presetting of voltage, current, and
time are interlinked in such a way that if their product (i.e., the energy to be applied) exceeds the permissible loading of
the X-ray tube, the latter cannot be energized.
OVERSTRESSING: In fatigue testing, cycling at a stress level higher than that used at the end of the test.
OXIDATION: The reaction of an element to oxygen or an oxygen containing compound.
OXIDATION FOG (RT): Fog caused by exposure of a film to air during development.
PAIR PRODUCTION (RT): The conversion of very high-energy photons, when absorbed in matter, by a process
wherein the photon is converted in the electrical field of a nucleus into an electron (negative charge) and a positron
(equal but opposite positive charge).