2. MAGNETIC PARTICLE TESTING
This method is suitable for the detection of surface and near
surface discontinuities in magnetic material , mainly
ferromagnetic materials.
In theory, magnetic particle inspection (MPI) is a relatively
simple concept. It can be considered as a combination of
two nondestructive testing methods: magnetic flux
leakage testing and visual testing. Consider the case of a
bar magnet. It has a magnetic field in and around the
magnet.
Any place that a magnetic line of force exits or enters the
magnet is called a pole.
A pole where a magnetic line of force exits the magnet is
called a north pole and
a pole where a line of force enters the magnet is called a
south pole.
3. CONTD. PRINCIPLE
When a bar magnet is broken in the center of its length, two
complete bar magnets with magnetic poles on each end of each
piece will result.
If the magnet is just cracked but not broken completely in two, a
north and south pole will form at each edge of the crack.
The magnetic field exits the north pole and re-enters at the south
pole.
When the magnetic field encounters a break in the magnetic field
due to crack or inclusion, magnetic field disturbances are produced.
This vaiation in magnetic field can be identified by sprinkling
magnetic particles on the surface of the part to be inspected.
The particles will be attracted at the edges of crack.
Critical investigation will reveal the pattern of defect in the surface or
subsurface of the part.
4. PROCEDURE OF MAGNETIC PARTICLE TEST:
Cleaning
Demagnetization
Magnetizing
Addition of magnetic particles
Illumination
Interpretation
Documentation and reporting
5. PRE-CLEANING
When inspecting a test part with the magnetic particle
method it is essential for the particles to have an
unimpeded path for migration to both strong and
weak leakage fields alike. The part’s surface should
be clean and dry before inspection.
Contaminants such as oil,
grease, or scale may not
only prevent particles from
being attracted to leakage
fields, they may also
interfere with interpretation
of indications.
6. DEMAGNETIZATION
• Parts inspected by the magnetic particle method
may sometimes have an objectionable residual
magnetic field that may interfere with subsequent
manufacturing operations or service of the
component.
• Possible reasons for demagnetization include:
– May interfere with welding and/or machining
operations
– Can effect gauges that are sensitive to magnetic
fields if placed in close proximity.
– Abrasive particles may adhere to components
surface and cause and increase in wear to
engines components, gears, bearings etc.
7. DEMAGNETIZATION (CONT.)
• Demagnetization requires that the residual
magnetic field is reversed and reduced by the
inspector.
• This process will scramble the magnetic domains
and reduce the strength of the residual field to an
acceptable level.
DemagnetizedMagnetized
8. MAGNETIZATION
To identify defects, the part to be examined must be magnetized
Magnetization methods
1.Longitudinal Magnetization
2.Circumferential Magnetization
The required magnetic field can be introduced into a component in a
number of different ways.
1. Using a permanent magnet or an electromagnet that contacts the
test piece
2. Flowing an electrical current through the specimen
3. Flowing an electrical current through a coil of wire around the part
or through a central conductor running near the part.
9. ADDITION OF MAGNETIC PARTICLES
After magnetizing the part, magnetic particles are applied
on the surface of inspection
The magnetic particles to be used must be available in
powder form, hence called as magnetic powder
Types of magnetic particles
Dry Magnetic Particles
Magnetic particles come in a variety of colors. A color that
produces a high level of contrast against the background
should be used.
Wet Magnetic Particles
Wet particles are typically supplied as visible or fluorescent.
Visible particles are viewed under normal white light and
fluorescent particles are viewed under black light.
Iron oxide Particles:
Pure iron particle
Fluorescent magnetic particles
10. ILLUMINATION
To identify magnetic particles attracted near the
defects, illumination is necessary
Ordinary day light is sufficient, but it is possible to
manage with artificial light like strong lamp which
can be moved around to illuminate the surface of
the object
When coloured or fluorescent powder is used, a
sufficiently strong ultraviolet light is used to achieve
maximum sensitivity during inspection.
11. INTERPRETATION
After magnetization and spreading of powder,
indications are appeared on the surface due to
piling up of magnetic powders at the defective area.
It is the task of the examiner to evaluate the cause
of the indications.
A crack which is open to surface is easy to detect
but in questionable situations, light polishing and
magnifying glass can be helpful to determine the
existence and type of defects.
12. DOCUMENTATION AND REPORTING
Photographs are useful as a permanent record of
the appearance of the defect
Another way of permanent record is taking tape
impressions
When making tape impressions the defects are
transferred to a report sheet by means of tape
which is pressed against the top of each defective
indication.
13. EQUIPMENTS USED IN MAGNETIC
PARTICLE TESTING
1.Magnetization Equipment
2.Portable power supply
3. Lighting equipment
1.Magnetization Equipment
Portable Magnetization Equipment
Stationary Magnetization Equipment
Portable Magnetization Equipment
Permanent magnet
Electromagnetic yokes
Prods
Portable coils
14. APPLICATIONS
This method is suitable for the detection of surface and near
surface discontinuities in magnetic material , mainly
ferromagnetic materials
Magnetic particle testing or inspection (MT or MPI testing) is
used for quality control and materials testing in all major
industries. This includes Magnetic particle testing of castings,
forgings, plates, extruded components, weld joints, electrical
and electronic component manufacturing, production of steel,
pressure vessels, ships, bridges, motor vehicles, machinery
and jet engines.
In service MPI testing for preventive maintenance is used for
detecting impending failure of rail road rolling stock axles,
press columns, earth-moving equipment, mill rolls, mining
equipment and other machines and components.
The flaws to be detected include cracks, inclusions, pipe,
laminations, bursts and flakes. They may be inherent in the
raw materials, may result from fabrication and heat treatment,
or may occur in service from fatigue, corrosion or other
causes. MPI testing effective in detecting fatigue cracks during
in-service maintenance inspection of power plants, cement
plants, sugar plants, petroleum refinery machinery components
and structures
15. ADVANTAGES OF MPT
•Principally and relatively simple method
•Economical and easy to perform
•Portable for field testing
•Fast for production testing
•Reveal or disclose small surface flaws or cracks
which may be tight
• High sensitive and complex parts can be
inspected rapidly
• Elaborate surface preparation is not required
16. LIMITATIONS OF MPT
•Material must be ferromagnetic
•Orientation and strength of magnetic field is critical
•Detects surface and near-to-surface discontinuities
only
•Large currents are required
•This method cannot be used if a thick paint coating
is present
