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Copyright © 2018 by The American Society
for Nondestructive Testing, Inc.
LESSON 4
Magnetic Flux Leakage Theory
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INTRODUCTION
1. Magnetic flux leakage testing (MFL) involves
magnetization of the test object by a permanent
magnet or by passing an excitation current directly
through an electromagnet.
2. The presence of a discontinuity or thickness
change on or near the surface of the material
disturbs the magnetic flux lines and results in a
local leakage field around the discontinuity.
3. Magnetic flux leakage can be detected using
noncontact sensors, such as a hall effect probe or
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INTRODUCTION
Typical flux leakage field signal for a
rectangular notch in a test object.
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INTRODUCTION
4. A hall effect probe uses an element oriented
parallel to the sample surface that is sensitive to
the normal component of the magnetic flux leakage
field so as to generate a signal.
5. Cracks in the formation state are highly detectable
and normally produce sharp, well-defined
indications.
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INTRODUCTION
6. Open cracks that have been subject to large
thermal, chemical, or mechanical forces may have
had their magnetic field disruption characteristics
greatly or entirely reduced and may not be
detectable using MFL.
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INTRODUCTION
Leakage field at an air gap in a
longitudinally magnetized test object.
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LINES OF FORCE
1. If a bar magnet is covered with a sheet of paper
and iron filings are scattered over the paper, the
filings align themselves along definite lines that
pass from the poles of the magnet.
2. The alignment of the iron particles indicates that
these lines form a field around the magnet, and
any magnetizable material that enters this field is
attracted to the magnet.
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LINES OF FORCE
Magnetic field surrounding a bar magnet.
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LINES OF FORCE
3. These lines are called lines of force. Since the
lines of force around a magnet seem to flow from
the north to the south pole, they are often called
flux lines.
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MAGNETIC POLES
1. Magnetic poles: the regions close to the ends of
bar or other magnets.
2. Lines of force are postulated to begin and end at
magnetic poles.
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LAW OF MAGNETISM
1. When like poles of magnets are brought together,
the lines of force appear to repel each other.
2. When unlike poles are brought together, the
magnets attract each other.
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MAGNETIC FLUX
Magnetic flux is the total amount of magnetism in a
specific area.
Measured in webers (Wb).
1 Wb is equal to 1 volt-second (V·s).
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FLUX DENSITY
1. The flux lines that surround a magnet are close
together near a bar magnet.
Distribution of flux around a magnet.
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FLUX DENSITY
2. The density of the flux in a particular area
determines the strength of the field in that area.
3. Flux density can be determined by measuring the
strength of the field. Flux density is defined as:
the number of lines of force of magnetism
perpendicular to a specific area.
one line of force at right angles to an area of
1 square centimeter (cm2).
4. The SI unit for magnetic flux density is the tesla (T).
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FLUX DENSITY
or
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RIGHT-HAND RULE
1. When an electrical current flows through a wire, a
magnetic field exists around the wire.
2. The direction of the magnetic field around the wire
depends on the direction of current flow through
the wire.
3. This relationship may be determined by the
right-hand rule.
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RIGHT-HAND RULE
Magnetic field surrounding a bar magnet.
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MAGNETIC FIELD INDUCED BY COIL
1. If straight wire is wound into a coil, the lines of
force encircling the wire form the magnetic field
inside and outside of the coil.
2. The strength of the magnetic field is dependent
upon two factors:
the number of turns in the coil,
the magnitude of the current.
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MAGNETIC FIELD INDUCED BY COIL
Magnetic field of a coil.
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MAGNETIC FIELD INDUCED BY COIL
3. Electric current can be used to create a magnetic
field in materials.
4. If a coil is wrapped around a bar of iron and direct
current passed through the coil, a magnetic field is
established in and around the bar due to the
magnetic field caused by the current flowing
through the coil.
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MAGNETIC PROPERTIES OF
MATERIALS
1. Ferromagnetic materials: materials, like iron, that
are magnetizable.
2. Ferromagnetic materials are capable of retaining
some part of the magnetic field induced in them.
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MAGNETIC PROPERTIES OF
MATERIALS
3. Within the ferromagnetic group of materials, some
are more easily magnetized than others.
Those that are easily magnetized retain relatively
little of the magnetic field after the current is shut off.
Those that are difficult to magnetize retain more of
the magnetic field after the current is shut off.
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MAGNETIC DOMAINS
1. Some materials that can be magnetized possess
microscopic regions called domains that are fully
magnetized (to magnetic saturation) in specific
directions that are dominated by their crystal axis
orientations and boundaries.
2. These domains have a positive and negative
polarity at opposite ends because of internal
magnetic alignment.
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MAGNETIC DOMAINS
3. If the material is not magnetized, the domains are
randomly aligned, so the net magnetization
appears to be zero from outside the material.
4. Domain walls are defined by the edges of the
microscopic crystallites or submicroscopic
anomalies within the material.
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MAGNETIC DOMAINS
Alignment of magnetic domains in:
(top) unmagnetized material;
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(bottom) magnetized material.
MAGNETIC DOMAINS
5. When the material is subjected to an external
magnetic field, some of the domains grow or shrink
depending upon whether their axes are parallel or
nonparallel to the external field strength.
6. At higher external fields, the remaining domains
begin to rotate into the field direction, and many
domains disappear as a result.
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MAGNETIC HYSTERESIS
1. All ferromagnetic materials have certain magnetic
properties that are specific to that material. Most of
these properties are described by a magnetic
hysteresis loop.
2. Data for the hysteresis loop are collected by
placing a bar of ferromagnetic material in a coil and
applying an alternating current or a direct current.
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MAGNETIC HYSTERESIS
By increasing the magnetizing field strength (H) in
small increments, and measuring the flux density
(B) at each increment, the relationship between
magnetic field strength and flux density can be
plotted.
A hysteresis loop is also referred to as a B and H
(B, H) curve.
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MAGNETIC HYSTERESIS
3. The relationship between magnetic field strength
and flux density is not linear for ferromagnetic
materials.
4. Beyond the saturation point, increases in magnetic
field strength do not increase the flux density in the
material.
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MAGNETIC HYSTERESIS
5. In diagrams of full hysteresis loops, the curve 0-A
is often drawn as a dashed line since it occurs
only during the initial magnetization of an
unmagnetized material. It is referred to as the
virgin curve of the material.
6. When the magnetic field strength is reduced to
zero the flux density slowly decreases.
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MAGNETIC HYSTERESIS
Hysteresis data for unmagnetized steel: (left) virgin
curve of a hysteresis loop and (right) hysteresis
loop showing residual magnetism.
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MAGNETIC HYSTERESIS
Hysteresis data for unmagnetized steel:
(left) hysteresis loop showing coercive force and
(right) hysteresis loop showing reverse magnetism.
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MAGNETIC HYSTERESIS
Hysteresis data for unmagnetized steel: (left)
hysteresis loop showing showing reverse residual
magnetism and (right) complete hysteresis loop.
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MAGNETIC HYSTERESIS
7. Residual magnetism or remanence: the amount
of flux density remaining in the material (line 0-B).
8. Retentivity: the ability of ferromagnetic materials
to retain a certain amount of magnetism.
9. Removal of residual magnetism requires the
application of a magnetic field strength in the
opposite or negative direction.
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MAGNETIC HYSTERESIS
10. Coercive force: the reverse field strength
required to reduce B to zero.
11. If the amount of magnetic field strength is
increased beyond point C, the magnetic flux
changes its polarity and initially increases quite
rapidly. It then gradually slows until point D (the
reverse polarity saturation point) is reached.
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MAGNETIC HYSTERESIS
12. When the reversed magnetic field strength is
reduced to zero, the flux densities of the residual
magnetism from straight and reversed polarities
are equal (that is, line 0-B is equal to line 0-E).
13. Removal of the reversed polarity residual
magnetism requires application of magnetic field
strength in the original direction.
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MAGNETIC HYSTERESIS
14. In terms of domains:
What occurs during 0-A has been previously
discussed.
From A to B, some domains rotate back out of the
direction of H.
Further disordering occurs between B and C until,
at C, the disorder is complete and the material is
apparently demagnetized.
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MAGNETIC HYSTERESIS
The area between C and D reorients in the applied
field direction.
D-E-F-A is the reverse of what occurred along
A-B-C-D.
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MAGNETIC HYSTERESIS
Positive field strength hysteresis loops: (left) hardened
steel hysteresis loop; and (right) annealed low carbon
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steel hysteresis loop.
INNER HYSTERESIS LOOPS
1. If H is not strong enough to saturate the material
with magnetic flux, an inner loop is obtained, which
exhibits smaller values of residual magnetism at B
and E, and lower values of the coercive force at C
and F.
2. Remanance: maximum values of the residual
magnetism.
3. Coercivity: the maximum values of the coercive
force.
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MAGNETIC PERMEABILITY
1. One of the most important properties of magnetic
materials is permeability.
2. Permeability can be thought of as the ease with
which materials can be magnetized. Air is assigned
a permeability of one.
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MAGNETIC PERMEABILITY
3. Permeability (µ):
the ratio between the flux density and the
magnetic field strength (B/H).
the rate of change of flux density (B) with respect
to the magnetizing force (H).
varies with position around the B and H curve.
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MAGNETIC PERMEABILITY
or
where
µ = permeability,
B = magnetic flux density, in gauss (tesla), and
H = magnetic field intensity (A/m).
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MAGNETIC PERMEABILITY
4. Magnetic properties and hysteresis loops vary
widely between materials and material conditions.
They are affected by:
chemical compositions,
microstructure, and
grain size.
