The document discusses magnetic particle inspection, which uses magnetic fields to detect surface or near-surface defects in ferromagnetic materials. It explains that direct current is preferred for deeper penetration and defect detection, while alternating current is limited to surface inspection by the skin effect. It also defines different types of materials' magnetic properties, including diamagnetic, paramagnetic, and ferromagnetic, and notes the advantages and limitations of magnetic particle inspection.

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*
*MAGNETIC PARTICLE
INSPECTION
*By:
Zoha Nasir
Assistant professor
JIT, Barabanki

2. *
Alternating current.
Direct current.
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Direct current (DC) flows continuously in one direction at a
constant voltage.
A battery is the most common source of direct current. The
current is said to flow from the positive to the negative terminal,
though electrons flow in the opposite direction.
DC is very desirable when inspecting for subsurface defects
because DC generates a magnetic field that penetrates deeper
into the material.
In ferromagnetic materials, the magnetic field produced by DC
generally penetrates the entire cross-section of the component.
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Alternating current (AC) reverses in direction at a rate of 50 or 60 cycles
per second.
Since AC is readily available in most facilities, it is convenient to make use
of it for magnetic particle inspection. However, when AC is used to induce a
magnetic field in ferromagnetic materials, the magnetic field will be limited
to a thin layer at the surface of the component.
This phenomenon is known as the "skin effect" and it occurs because the
changing magnetic field generates eddy currents in the test object.
The eddy currents produce a magnetic field that opposes the primary field,
thus reducing the net magnetic flux below the surface.
Therefore, it is recommended that AC be used only when the inspection is
limited to surface defects.
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Diamagnetic materials:
They have a weak, negative susceptibility to magnetic fields.
Diamagnetic materials are slightly repelled by a magnetic field and the
material does not retain the magnetic properties when the external field
is removed.
In diamagnetic materials all the electrons are paired so there is no
permanent net magnetic moment per atom.
 Most elements in the periodic table, including copper, silver, and
gold, are diamagnetic.
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6. Paramagnetic materials:
 They have a small, positive susceptibility to magnetic fields.
These materials are slightly attracted by a magnetic field and the
material does not retain the magnetic properties when the external
field is removed.
Paramagnetic materials have some unpaired electrons.
Examples of paramagnetic materials include magnesium,
molybdenum, lithium.
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7. Ferromagnetic Materials:
They have a large, positive susceptibility to an external magnetic
field.
They exhibit a strong attraction to magnetic fields and are able to
retain their magnetic properties after the external field has been
removed.
Ferromagnetic materials have some unpaired electrons so their
atoms have a net magnetic moment.
They get their strong magnetic properties due to the presence of
magnetic domains. In these domains, large numbers of atom's
moments are aligned parallel so that the magnetic force within the
domain is strong.
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Can detect both surface and near sub-surface defects.
Can inspect parts with irregular shapes easily.
Pre-cleaning of components is not as critical as it is for some
other inspection methods. Most contaminants within a flaw will
not hinder flaw detectability.
Fast method of inspection and indications are visible directly
on the specimen surface.
Considered low cost compared to many other NDT methods.
Is a very portable inspection method especially when used with
battery powered equipment.
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Cannot inspect non-ferrous materials such as aluminum,
magnesium or most stainless steels.
Inspection of large parts may require use of equipment with special
power requirements.
Some parts may require removal of coating or plating.
Achieve desired inspection sensitivity.
Limited subsurface discontinuity detection capabilities. Maximum
depth sensitivity is approximately 0.6” (under ideal conditions).
Post cleaning, and post demagnetization is often necessary.
Alignment (angle) between magnetic flux and defect is important.
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