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Permeability
As previously mentioned, permeability is a material
property that describes the ease with which a magnetic flux is
established in a component. It is the ratio of the flux density
to the magnetizing force and is represented by the following
equation:
 It
is clear that this equation describes the slope of the curve at
any point on the hysteresis loop. The permeability value given
in papers and reference materials is usually the maximum permeability
or the maximum relative permeability. The maximum permeability
is the point where the slope of the B/H curve for the unmagnetized
material is the greatest. This point is often taken as the point
where a straight line from the origin is tangent to the B/H curve.
The relative permeability is arrived at by taking the ratio of
the material's permeability to the permeability in free space
(air).
m(relative)
= m(material)
/ m(air)
where: m(air)
= 1.256 x 10-6 H/m
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The shape of the hysteresis loop tells
a great deal about the material being magnetized. The hysteresis
curves of two different materials are shown in the graph.
 Relative
to other materials, a material with a wider hysteresis loop has:
- Lower Permeability
- Higher Retentivity
- Higher Coercivity
- Higher Reluctance
- Higher Residual Magnetism
Relative to other materials, a material with the narrower hysteresis loop has:
- Higher Permeability
- Lower Retentivity
- Lower Coercivity
- Lower Reluctance
- Lower Residual Magnetism.
In magnetic particle testing, the level
of residual magnetism is important. Residual magnetic fields are
affected by the permeability, which can be related to the carbon
content and alloying of the material. A component with high carbon
content will have low permeability and will retain more magnetic
flux than a material with low carbon content.
In the two B-H loops above, which one would indicative of a low
carbon steel? Answer
- The material would exhibit high
permeability because of its low carbon content.
- It would exhibit low reluctance
or resistance to the magnetic field
- It would have low retentivity.
- The weak residual magnetism would
require a low coercive force to remove it.
The B-H curve for a low carbon steel
would, therefore, resemble the slender curve above.
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