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NDT - Non Destructive
Non Destructive Testing (NDT) is a noninvasive method and the testing is
carried out without impairing further usefulness of the material,
component and structure. The component can be put into use after the
Testing. The signals that are employed do not alter the properties
NDT applied in all fields such as Petro-chemicals, Refineries, Nuclear
power plants, Aero planes, Rockets, Rails, Ships etc.
The number of NDT methods that can be used to inspect components and
make measurements is large and continues to grow. Researchers continue
to find new ways of applying physics and other scientific disciplines to
develop better NDT methods. However, there are six NDT methods that are
used most often. These methods are visual inspection, penetrant testing,
magnetic particle testing, electromagnetic or eddy current testing,
radiography, and ultrasonic testing. These methods and a few others are
briefly described below.
» Visual and Optical Testing (VT)
» Penetrant Testing (PT)
» Magnetic Particle Testing (MT)
» Radiography (RT)
» Ultrasonic Testing (UT)
» Radiographic Film Interpretation (RTFI)
and Optical Testing (VT)
Visual inspection involves using an inspector's eyes to look for
defects. The inspector may also use special tools such as magnifying
glasses, mirrors, or borescopes to gain access and more closely inspect
the subject area. Visual examiners follow procedures that range from
simple to very complex.
Penetrant Testing (PT)
Test objects are coated with visible or fluorescent dye solution. Excess
dye is then removed from the surface, and a developer is applied. The
developer acts as blotter, drawing trapped penetrant out of
imperfections open to the surface. With visible dyes, vivid color
contrasts between the penetrant and developer make "bleedout" easy to
see. With fluorescent dyes, ultraviolet light is used to make the
bleedout fluoresce brightly, thus allowing imperfections to be readily
Particle Testing (MT)
This NDE method is accomplished by inducing a magnetic field in a
ferromagnetic material and then dusting the surface with iron particles
(either dry or suspended in liquid). Surface and near-surface
imperfections distort the magnetic field and concentrate iron particles
near imperfections, previewing a visual indication of the flaw.
Radiography involves the use of penetrating gamma or X-radiation to
examine parts and products for imperfections. An X-ray generator or
radioactive isotope is used as a source of radiation. Radiation is
directed through a part and onto film or other imaging media. The
resulting shadowgraph shows the dimensional features of the part.
Possible imperfections are indicated as density changes on the film in
the same manner as a medical X-ray shows broken bones.
Ultrasonic Testing (UT)
Ultrasonics use transmission of high-frequency sound waves into a
material to detect imperfections or to locate changes in material
properties. The most commonly used ultrasonic testing technique is pulse
echo, wherein sound is introduced into a test object and reflections
(echoes) are returned to a receiver from internal imperfections or from
the part's geometrical surfaces.
Radiographic Film Interpretation(RTFI)
In addition to producing high quality radiographs, the radiographer must also be skilled in radiographic interpretation. Interpretation of radiographs takes place in three basic steps : (1) detection, (2)interpretation, and (3)evaluation. All of these stepsmake use of the radiographer's visual acuity. Visual acuity is the ability to resolve a spatial pattern in an image. The ability of an individual to detect discontinuities in radiography is also affected by the lighting condition in the place of viewing, and the experience level for recognizing various features in the image. The following material was developed to help students develop an understanding of the types of defects found in weldments and how they appear in a radiograph.