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Acoustic Emission
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When a solid material is stressed, imperfections within the material
emit short bursts of energy called "emissions". In much the same
manner as ultrasonic testing, these acoustic emissions can be detected
by special receivers. The source of the "emissions" can be evaluated
through study of their strength, rate, and location.
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Eddy Current
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In eddy current testing, one of several electromagnetic testing
methods, electrical currents are generated in a conductive material by
an induced magnetic field. Interruptions in the flow of the electric
currents (eddy currents), which are caused by imperfections or changes
in a material's conductive properties, will cause changes in the
induced magnetic field. These changes when detected, indicate the
presence of a change in the test object.
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Leak Testing
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A variety of techniques are used to detect and locate leaks in
pressure containment parts and structures. Leaks can be detected by
using electronic listening devices, pressure gage measurements, liquid
and gas penetrant techniques, and/or a simple soap bubble test.
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Liquid Penetrant
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Liquid penetrant testing is probably the most widely used NDT method.
The test object or material is coated with a visible or fluorescent
dye solution. The excess dye is removed from the surface, and then a
developer is applied. the developer acts like a blotter and draws
penetrant out of imperfections which open to the surface. With
visible dyes, the vivid color contrast between the penetrant and the
developer makes the "bleedout' easy to see. With fluorescent dyes,
and ultraviolet lamp is used to make the "bleedout" fluoresce
brightly, thus allowing the imperfection to be seen readily.
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Magnetic Particle
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Magnetic particle testing is done by inducing a magnetic field in a
ferro-magnetic material and dusting the surface with iron particles
(either dry or suspended in a liquid). Surface imperfections will
distort the magnetic field and concentrate the iron particles near
imperfections, thus indicating their presence.
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Radiography
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Radiography involves the use of penetrating X or gamma radiation to
examine parts and products for imperfections. An X-ray machine or
radioactive isotope is used as a source of radiation. Radiation is
directed through a part and onto film. When the film is developed, a
shadowgraph is obtained that shows the internal soundness of a part.
Possible imperfections show up as density changes in the film, in much
the same manner as an x-ray can show broken bones.
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Ultrasonics
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Ultrasonic testing uses the transmission of high frequency sound
waves, into a material to detect imperfections within the material or
changes in material properties. The most commonly used ultrasonic
testing technique is pulse echo wherein sound is introduced into the
test object and reflections (echoes) are returned to a receiver from
interal imperfections or from geometrical surfaces of the part.
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Visual Examination
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Probably the oldest and most common method of NDT is visual
examination, which has numerous industrial and commercial
applications. Examiners follow procedures ranging from simple to very
complex, some of which involve comparison of workmanship samples with
production parts. Visual techniques are used wih all other NDT
methods.
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