INTRODUCTION

Magnetic particle testing (MPT) method is used for the detection of surface and near-surface flaws in
ferromagnetic materials and is primarily used for crack detection.
Magnetic particle inspection can detect both production discontinuities (seams, laps, grinding cracks and
quenching cracks) and in-service damage (fatigue and overload cracks).
Magnetism & Inspection
Magnetism is the ability of matter to attract other matter to itself. Objects that possess the property of
magnetism are said to be magnetic or magnetized and magnetic lines of force can be found in and around the
objects. A magnetic pole is a point where a magnetic line of force exits or enters a material.
Because magnetization of certain metals is possible, it is possible to reveal discontinuities by using a medium
(iron powder) having magnetic attraction.
The medium is applied to the surface of the test object after or during induction of a magnetic field.
The sketch below shows a build-up of the magnetic particle medium over the discontinuity in the magnetized
part.

Magnetic particle testing is a relatively easy and simple test method that can be applied at various stages of
manufacture and processing operations.
The objective of magnetic particle testing is to ensure product reliability by providing a means of:
A. Obtaining a visual image of an indication on the surface of a material.
B. Disclosing the nature of discontinuities without impairing the material.
C. Separating acceptable and unacceptable material in accordance with predetermined standards.

THEORY OF MAGNETIZATION

An object is magnetized when part or all of its magnetic domains have their north and south poles oriented as
in the sketch below.

The ability of a magnet to attract or repel is concentrated at the local areas called poles. The north and south
poles exhibit attraction and repulsion as shown in the sketch below.
With all of the magnetic domains lined up, the magnetic bar develops a total force equal to the sum of all of the
magnetic domains.
These are the magnetic lines of force which form a closed loop or circuit. Do not cross. Follow the path of least
resistance. All have the same strength.
All of the lines of force make up the magnetic field.

Magnetic Field

The force that attracts other magnetizable materials to the magnetic poles is known as magnetic flux.
Magnetic flux is made up of all of the lines of force.
The horseshoe magnet will attract other magnetizable material only where the lines of force leave or enter the
magnet.

If a magnet is bent into a complete loop as shown below, the magnetic field is entirely within, thus no external
force.
However, a crack in the circular magnet will disrupt the flow of lines of force and create a flux leakage.

Leakage fields (flux leakage) are actually magnetic lines of force that leave the part and pass through air from
one pole to the other of opposite polarity.
Whenever the leakage field is forced out of the part, iron particles would be attracted showing an indication of
a discontinuity. Even some subsurface discontinuities may be detected if the leakage field is strong enough as
shown below.

VECTOR FIELD
When two magnetizing forces are imposed simultaneously in the same part, the object is not magnetized in two
directions at the same time. A vector field is formed which is the resultant direction and strength of the two
imposed fields.
This is illustrated below, where Fa is the first magnetizing force, Fb is the second force, and Fa+b equals the
resultant magnetizing force.

MAGNETIC MATERIALS
If an object is placed in a magnetic field a force is exerted on it and it is said to become magnetized.
The intensity of magnetization depends upon the susceptibility of the material to become magnetized.
Diamagnetic metals- have a small and negative susceptibility to magnetization (slightly repelled).
Copper, silver, and gold are examples of diamagnetic materials.
Paramagnetic metals- have a small and positive susceptibility to magnetization (slightly attracted).
Magnesium, molybdenum, lithium, and tantalum are examples.
Ferromagnetic metals- have a large and positive susceptibility to magnetization. They have a strong attraction
and are able to retain their magnetization after the magnetizing field has been removed.
Iron, cobalt, and nickel are examples of ferromagnetic metals.
Ferromagnetic materials are the only metals commonly inspected with the magnetic particle testing method.
Magnetic flux- the total number of magnetic lines of force existing in a magnetic circuit is called magnetic flux.
The lines of force in a magnetic circuit are always closed loops, therefore, a magnetic circuit is always closed as
shown below.
Flux density or induction is usually designated in "gauss" units and refers to the flux-per-unit area at right
angles to the direction of the flux.
Magnetic Field
RIGHT-HAND RULE VS LEFT-HAND RULE
To find the direction of an electrically induced magnetic field, place your thumb on the conductor in the
direction of "current flow" and your fingers will then point in the direction of the lines of force.
A circular magnetic field is produced in the sketch below.

The general dynamics handbook uses the "current flow" theory which utilizes the right-hand rule. In this older
convention electricity is considered to flow from + to -.
If the more commonly accepted "electron flow" theory were used, it would be necessary to use the left-hand
rule. The "electron flow" theory considers electricity to flow from -to +.

IMPORTANT
The two methods of determining the flow of electricity should not become confusing. They both result in a
magnetic field flowing in the same direction.

The sketch below shows how a magnetic field is produced utilizing a coil. The field is circular around the cable
but produces a longitudinal field in the specimen.
Using figure "A" below, try the right-hand rule (current flow + to -) to demonstrate the direction of the magnetic
field with the "current flow" theory.

Figure A

Also, try the left-hand rule on figure "B" below to demonstrate that the "electron flow" theory (-to+) will
produce a magnetic field in the same direction.

Figure B

The following properties of a metal can determine how effective the magnetic particle method will be in
evaluating a part.
These properties will be discussed in greater detail in the next lesson.
1. Permeability- this refers to the ease with which a magnetic flux is established in the article being inspected.
2. Reluctance- this is the opposition of a magnetic material to the establishment of a magnetic flux. A material
with high permeability will have a low reluctance.
3. residual magnetism- this refers to the amount of magnetism retained after the magnetizing force is removed.
4. Retentivity- refers to the ability of the material to retain a certain amount of residual magnetism.
5. Coercive force- refers to the reverse magnetizing force necessary to remove the residual magnetism from the
part.
For example: if a piece of high carbon steel were placed in a magnetizing field, it would exhibit the following:
A. It would have low permeability because it would be hard to magnetize.
B. It would be highly reluctant to accept a magnetic flux because of the high carbon content.
C. It would have a high residual magnetic field. The high carbon steel is reluctant to accept a magnetic flux but
is also reluctant to give it up once it has been accepted.
D. It would be highly retentive of the magnetic field that it has accepted.
E. It would take a high coercive force to remove the residual magnetism from the high carbon steel part.

Advantages of the Magnetic Particle method

1. It is quick and relatively uncomplicated.

2. Testing is possible up 300°C using dry powder.
3. It gives immediate indications of defects.
4. Surface preparation is less critical than it is in penetrant inspection.
5. It shows surface and near surface defects, and these are the most serious ones as they concentrate stresses,
Detects flaw below 1/4" below surface.
6. The method can be adapted for site or workshop use.
7. Detects flaw filled with foreign material.
8. It is inexpensive compared to radiography.
9. Large or small objects can be examined.
10. Magnetic particle indications are produced directly on the surface of the part where the flaw is located.

Disadvantages of the Magnetic Particle method

1. It is restricted to ferromagnetic materials - usually iron and steel, and cannot be used on austenitic stainless
steel.
2. Large currents may be needed for very large parts.
3. It is sometimes unclear whether the magnetic field is sufficiently strong to give good indications.
4. Depth & size is not indicated.
5. Sensitivity varies with surface roughness & position; Sensitivity rapidly diminishes with depth.
6. The method cannot be used if a thick paint coating is present.
7. Spurious, or non-relevant indications, are probable, and thus interpretation is a skilled task.
8. Some of the paints and particle suspension fluids can give a fume or fire problem, particularly in a confined
space.
Steps for Magnetic Particle Inspection

1. Clean Surface (Preparation):

All surfaces and adjacent areas (within 1-1.5 inch) that will be examined must be free from scale, sand, rust,
grease, slag, paint, oily or other interfering conditions. Rough surfaces may interfere with magnetic particle
powder and making interpretations difficult.
2. Inducing a Magnetic Field:

This is the very important step in the magnetic particle inspection procedure. In this step, place the equipment
on the area to be tested and induce a magnetic field. Various types of magnetic particle inspection equipment
are available. Generally used industrial equipment are Yoke, Permanent magnets, Central conductors, Head
shot, Prods, Adjacent cable, coils etc. Magnetization technique can be Longitudinal, Circular, or Multidirectional
Magnetization. Equipment spacing in the inspection area is normally kept in between 3 inches to 6 inches. An
ASME Pie Gauge can be used to verify direction of magnetism in the part.

3. Applying Magnetic Particles on the Test Surface:

Both dry and wet magnetic particles can be used, it’s also available in either fluorescent or non-fluorescent
(visible, color contrast) and are available in a variety of colors to contrast with the tested material.

4. Examine the component surface for defects:

Remove the excess particles using light airflow (Bubbler) and inspect the Specimen for defects as per
acceptance criteria.

5. Repeat the test by changing the magnetic field:

Remember that indications that are perpendicular to Magnetic flux lines can be easily found out So two
separate examinations are carried out on each area to be tested. The second examination is performed with the
lines of flux perpendicular to those used for the first examination in that area.
6. Demagnetization and Cleaning:

The presence of Residual magnetism in the component may interfere with the subsequent usage. Hence, the
demagnetization shall always be performed on the parts once the magnetic particle inspection is over. The
presence of residual magnetism can be verified using a calibrated Gaussmeter, Magnetic Field Meter, or a hall
Probe Gauss meter. Residual magnetism must not exceed (+/-) 3 gausses.

After that, the parts shall be cleaned to remove all residual magnetic particle materials. If wet fluorescent MPI
was performed, the part shall be scanned with the backlight to assure that the cleaning is adequate.