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Chapter 17: D. Testing Methods 185
Magnetic field lines
Magnetic particles
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N
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Crack
Figure 17.6 Magnetic particle inspection.
• Skill and training is more extensive than with some other methods.
• It normally requires a coupling medium to promote the transfer of
sound energy into the test specimen.
• Materials that are rough, irregular in shape, very small, exceptionally
thin, or not homogeneous are difficult to inspect.
• Cast iron and other coarse-grained materials are difficult to inspect
due to low sound transmission and high signal noise.
• Linear defects oriented parallel to the sound beam may go undetected.
• Reference standards are required for both equipment calibration and
the characterization of flaws.
Magnetic. Magnetic particle inspection is a nondestructive testing method that
is conducted on ferro-magnetic material* such as iron, nickel, cobalt, or some of
their alloys. The test induces a magnetic field into the ferrous object, the surface
of which is then coated with iron particles either dry or suspended in a liquid.
Surface imperfections will distort the magnetic field, causing an accumulation of
the iron particles near imperfections, thus indicating their presence in the material
(Aircraft NDT 2008). Many different industries use magnetic particle inspection
for determining a component’s fitness for use. Some examples of industries
that use magnetic particle inspection are the structural steel, automotive, petrochemical,
power generation, and aerospace industries. Underwater inspection is
another area where magnetic particle inspection may be used to test items such
as offshore structures and underwater pipelines (NDT-ED 2008). Figure 17.6 is an
example of testing using magnetic particle inspection.
Part III.D.1
* Ferromagnetic materials are materials that can be magnetized.