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Nondestructive Testing (NDT) is a form of engineering evaluation
that detects and interprets flaws in materials, while causing no harm to
the materials or structure under examination. It is an exacting science, with
its own highly developed testing techniques. NPI is a dedicated, full-service
NDT company. Listed below are some of the commonly used NDT methods:
 | Magnetic Particle — Magnetic Particle testing consists of inducing a
magnetic field into a ferromagnetic material (such as rolled, cast, or forged
steel). The process entails the application of magnetic particles and the
interpretation of patterns formed by the particles as they are attracted by
magnetic leakage fields (magnetic poles) created at breaks in the material
continuity, such as surface cracks. To ensure visibility, the magnetic
particles are coated either with a colored dye easily seen in white light, or a
fluorescent dye visible under black ultraviolet light.
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| Liquid Penetrant — Liquid Penetrant Testing is a nondestructive means of
locating surface discontinuities based on capillarity or capillary action. A
penetrating liquid is applied to the surface of a test specimen, and is allowed
to seep into any surface discontinuities. If a discontinuity is small or
narrow, such as a crack or pinhole, capillarity assists the penetration. After
sufficient time has passed for the penetrant to enter the discontinuity, the
penetrant on the surface is cleaned off of the test material. Capillary action
is again employed to act as a blotter to draw penetrant from the discontinuity.
To ensure visibility, the liquid penetrant contains either a colored dye easily
seen in white light, or a fluorescent dye visible under black ultraviolet
light.
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| Ultrasonic — Ultrasonic is a versatile inspection technique, used to test
a variety of metallic and nonmetallic products, including welds, forgings,
plate, tubing, plastics, and ceramics. Ultrasonic tests use high frequency
sound waves introduced into the test specimen. Any returning echoes or loss of
sound energy is evaluated to determine material integrity. Ultrasonic tests are
capable of economically revealing subsurface discontinuities in a variety of
materials.
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| Radiography — Because of the penetration and absorption capabilities of X
and gamma radiation, radiography is used to test for subsurface discontinuities
in a variety of products, including welds and castings. Radiographic testing
usually requires the exposure of X-ray film to X or gamma rays that have
penetrated a specimen, processing the exposed film, and interpreting the
resultant radiograph. A radiograph is essentially a two-dimensional shadow
image of a three-dimensional test specimen.
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| Visual — Visual examination is generally used to determine the
surface discontinuities, the adequate surface condition of parts, the correct
alignment of mating surfaces, part shape correctness, and evidence of leaks.
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| Eddy Current — This method uses circulating electrical currents induced
in an electrically conductive test object by an alternating magnetic field. The
current flow in the test specimen creates its own magnetic field. Interruptions
or changes in current flow result in changes in this magnetic field, which are
detected, displayed, and interpreted. |
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