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NDT - Non Destructive Testing
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 permanently.
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)
»
Electromagnetic
Testing (ET) or Eddy Current Testing
»
Radiography (RT)
»
Ultrasonic Testing
(UT)
»
Acoustic Emission
Testing (AE)
»
Leak Testing (LT)
Visual 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 seen.
Magnetic 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.
Electromagnetic Testing (ET) or Eddy Current Testing
Electrical currents
are generated in a conductive material by an induced alternating
magnetic field. The electrical currents are called eddy currents
because they flow in circles at and just below the surface of
the material. Interruptions in the flow of eddy currents, caused
by imperfections, dimensional changes, or changes in the
material's conductive and permeability properties, can be
detected with the proper equipment.
Radiography (RT)
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.
Acoustic Emission Testing (AE)
When a solid material is stressed, imperfections within the
material emit short bursts of acoustic energy called
"emissions." As in ultrasonic testing, acoustic emissions can be
detected by special receivers. Emission sources can be evaluated
through the study of their intensity, rate, and location.
Leak Testing (LT)
Several techniques are used to detect and locate leaks in
pressure containment parts, pressure vessels, and structures.
Leaks can be detected by using electronic listening devices,
pressure gauge measurements, liquid and gas penetrant
techniques, and/or a simple soap-bubble test. |
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