MPI_ Magnetic Particle Inspection

1. MET312 NON
MET312 NON-
-DESTRUCTIVE
DESTRUCTIVE
TESTING
TESTING
M
MODULE
ODULE-
-3
3 –
– M
MAGNETIC
AGNETIC P
PARTICLE
ARTICLE I
INSPECTION
NSPECTION
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Sukesh O P
Assitant Professor
Department of Mechanical Engineering,
Jyothi Engineering College, Cheruthuruthy
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2. MET312 NON-DESTRUCTIVE TESTING
Introduction to NDT
Introduction to NDT-
- Visual
Visual
Inspection
Inspection-
- Liquid Penetrant
Liquid Penetrant
Inspection
Inspection-
- Magnetic
Magnetic
Particle
Particle
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Particle
Particle
Inspection
Inspection-
- Ultrasonic
Ultrasonic
Testing
Testing-
- Radiography Testing
Radiography Testing-
- Eddy
Eddy
Current Testing.
Current Testing. 2
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3. MODULE
MODULE-
-3
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4. INTRODUCTION

 MPI
MPI is governed by the laws of magnetism and is
therefore restricted to the inspection of materials that
can support magnetic flux lines, metals can be classified
as ferromagnetic, paramagnetic, or diamagnetic.
 Ferromagnetic metals are those that are strongly
attracted to a magnet and can become easily
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MPI- it is not
applicable to
non-magnetic
materials
attracted to a magnet and can become easily
magnetized. Ex: iron, nickel, cobalt.
 Paramagnetic metals such as austenitic stainless steel
are very weakly attracted by magnetic forces of
attraction and cannot be magnetized. Ex: Aluminum
 Diamagnetic metals which 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. Ex: copper, silver, gold
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5. INTRODUCTION
 The only requirement For the inspection through this
technique is that the components being inspected must
be made of a ferromagnetic material (a materials that
can be magnetized) such as iron, nickel, cobalt, or some
of their alloys.
 The method is used to inspect a variety of product forms
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 The method is used to inspect a variety of product forms
including castings, forgings, and weldments.
 Many different industries use MPI such as structural
steel, automotive, petrochemical, power generation, and
aerospace industries.
 Underwater inspection is another area where magnetic
particle inspection may be used to test items such as
offshore
offshore structures
structures and
and underwater
underwater pipelines
pipelines.
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6. ME312
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7. INTRODUCTION TO MAGNETISM
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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 the a magnetic line
of force exits or enters a material.
Magnetic field lines:
•Form complete loops.
•Do not cross.
•Follow the path of least
resistance.
•All have the same strength.
•Have a direction such that they
cause poles to attract or repel.
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Magnetic lines of
force around a bar
magnet
Opposite poles
attracting Similar poles repelling

8. FERROMAGNETIC MATERIALS
 A material is considered ferromagnetic if it can
be magnetized. Materials with a significant
Iron, nickel or cobalt content are generally
ferromagnetic.
 Ferromagnetic materials are made up of many
regions in which the magnetic fields of atoms
regions in which the magnetic fields of atoms
are aligned. These regions are call magnetic
domains.
 Magnetic domains point randomly in
demagnetized material, but can be aligned
using electrical current or an external magnetic
field to magnetize the material.
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Demagnetized Magnetized

9. HOW DOES MAGNETIC PARTICLE
INSPECTION WORK?
A ferromagnetic test specimen is magnetized with a
strong magnetic field created by a magnet or special
equipment. If the specimen has a discontinuity, the
discontinuity will interrupt the magnetic field
flowing through the specimen and a leakage field
will occur.

10. HOW DOES MAGNETIC PARTICLE
INSPECTION WORK? (CONT.)
Finely milled iron particles coated with a dye pigment
are applied to the test specimen. These particles are
attracted to leakage fields and will cluster to form an
indication directly over the discontinuity. This
indication can be visually detected under proper
lighting conditions.
lighting conditions.

11. BASIC PROCEDURE- MPI
Basic steps involved:
1.Component pre-cleaning
2.Introduction of magnetic field
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2.Introduction of magnetic field
3.Application of magnetic media
4.Interpretation of magnetic particle
indications
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12. 1. 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.
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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.

13. 2. INTRODUCTION OF THE MAGNETIC
FIELD
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
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3. Flowing an electrical current through a coil of wire around
the part or through a central conductor running near the
part.
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14. 3. Application of Magnetic Media (Wet
Versus Dry)
MPI can be performed using either
dry particles, or particles
suspended in a liquid. With
With the
the
dry
dry method,
method, the
the particles
particles are
are
lightly
lightly dusted
dusted on
on to
to the
the surface
surface.
.
With the wet method, the part is
flooded with a solution carrying the
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With the wet method, the part is
flooded with a solution carrying the
particles.
The dry method is more portable.
The wet method is generally more
sensitive since the liquid carrier
gives the magnetic particles
additional mobility.
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15. 4. INTERPRETATION OF
INDICATIONS
After applying the magnetic field, indications that
form must interpreted. This process requires that
the inspector distinguish between relevant and
non-relevant indications.
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16. CRANE HOOK WITH SERVICE INDUCED
CRACK
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Fluorescent, Wet Particle Method
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17. GEAR WITH SERVICE INDUCED CRACK
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Fluorescent, Wet Particle Method
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18. DRIVE SHAFT WITH HEAT TREATMENT
INDUCED CRACKS
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Fluorescent, Wet Particle Method
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19. SPLINED SHAFT WITH SERVICE
INDUCED CRACKS
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Fluorescent, Wet Particle Method
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20. THREADED SHAFT WITH SERVICE
INDUCED CRACK
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Fluorescent, Wet Particle Method
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21. LARGE BOLT WITH SERVICE INDUCED
CRACK
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Fluorescent, Wet Particle Method
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22. CRANK SHAFT WITH SERVICE INDUCED CRACK
NEAR LUBE HOLE
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Fluorescent, Wet Particle Method
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23. LACK OF FUSION IN SMAW WELD
Indication
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Visible, Dry Powder Method
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24. TOE CRACK IN SMAW WELD
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Visible, Dry Powder Method
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25. THROAT AND TOE CRACKS IN
PARTIALLY GROUND WELD
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Visible, Dry Powder Method
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26. BASIC PHYSICS OF
BASIC PHYSICS OF
MAGNETISM
MAGNETISM
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1. POLARITY
 In magnetism, polarity refers to the
orientation of north & south poles in
space.
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space.
 E.g., If a magnetized rod is suspended
at its centre, it will eventually align
itself with the earth’s magnetic field so
that one side points to geographic north
& other end to the south.
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2. MAGNETIC FORCE
 It’s a force of attraction or repulsion that one
body has upon another.
 E.g., When the N pole of a magnetized rod is
BASIC PHYSICS OF
BASIC PHYSICS OF
MAGNETISM
MAGNETISM
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 E.g., When the N pole of a magnetized rod is
placed closely to the S of another, they attract
each other.
3. MAGNETIC FIELD
 It’s the area around a magnet where the
magnetic forces are observed.
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
BASIC PHYSICS OF
BASIC PHYSICS OF
MAGNETISM
MAGNETISM
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5. FLUX DENSITY
 It’s defined as the no. of lines of force per unit area, measured in
‘Gauss’ & denoted by ‘B’.
BASIC PHYSICS OF
BASIC PHYSICS OF
MAGNETISM
MAGNETISM
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6. MAGNETIZING FORCE
 The total no. of lines of force making up a magnetic field determines
the strength of the force of attraction or repulsion exerted by the
magnet. This is known as magnetizing force.
 It’s the force which sets up magnetic flux in a material.
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6. COERCIVE FORCE
 It’s a measure of the ability of a ferromagnetic material to
withstand an external magnetic field without becoming
demagnetized.
BASIC PHYSICS OF
BASIC PHYSICS OF
MAGNETISM
MAGNETISM
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demagnetized.
7. RETENTIVITY
 The ability of a coil to retain some of its magnetism within the
core after the magnetization process has stopped.
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8. HYSTERESIS LOOP
 By exposing an un magnetized piece of material to
magnetizing current, we can plot the density B of the field
induced by applied magnetizing force H & the resultant curve
BASIC PHYSICS OF
BASIC PHYSICS OF
MAGNETISM
MAGNETISM
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induced by applied magnetizing force H & the resultant curve
is called Hysteresis loop.
9. RESIDUAL MAGNETISM
 It’s defined as the amount of magnetism left behind after
removing the external magnetic field from the circuit.
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32. HYSTERESIS LOOP
HYSTERESIS LOOP
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33. MAGNETIC PARTICLES/ MEDIA
 The particles used in magnetic particle testing are
made of ferromagnetic
ferromagnetic materials,
materials,
usually combinations of iron
iron and
and iron
iron oxides,
oxides,
having
having a
a high
high permeability
permeability and
and low
low retentivity
retentivity.
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 Particles having high permeability are
easily attracted to and magnetized by the low-
level leakage fields at discontinuities.
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34.  Low retentivity is required to prevent the
particles from being permanently magnetized.
 Strongly retentive particles will cling
together and to any magnetic surface,
resulting in reduced particle mobility and inc
reased background accumulation.
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MAGNETIC PARTICLES/ MEDIA
resulting in reduced particle mobility and inc
reased background accumulation.
 A common particle used to detect cracks is iron
oxide, for both dry and wet system
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35. 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.
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36. 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.
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are viewed under black light.
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37. Dry
● Common, relatively
cheap
● Generally applied to
rougher surfaces
● Particle types
Wet
● More expensive,
accurate
● Painted or sprayed
onto surfaces
● Wet dye
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● Particle types
○ Elongated - align
well with
magnetic fields
○ Rounded - move
freely across a
surface
● Wet dye
classifications
○ Light (UV or
fluorescent)
○ Removal type
(water, solvent, or
emulsifier)
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38. TYPES OF MAGNETIZATION
Two general types
types of
of magnetic
magnetic fields
fields (longitudinal
and circular) may be established within the specimen.
The type of magnetic field established is determined
by the method used to magnetize the specimen.
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•A longitudinal magnetic field
longitudinal magnetic field has magnetic
lines of force that run parallel to the long
axis of the part.
•A circular magnetic field
circular magnetic field has magnetic lines
of force that run circumferentially around
the perimeter of a part.
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39. IMPORTANCE OF MAGNETIC FIELD
DIRECTION
Being able to magnetize the part in two
directions is important because the best
detection of defects occurs when the lines of
magnetic force are established at right
angles to the longest dimension of the
defect.
Flux Leakage
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defect.
This orientation creates the largest
disruption of the magnetic field within the
part and the greatest flux leakage at the
surface of the part. An orientation of 45 to
90 degrees between the magnetic field and
the defect is necessary to form an indication.
No Flux Leakage
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40. ME312
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Since defects may occur in various and unknown directions,
each part is normally magnetized in two directions at right
angles to each other.
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41. METHODS OF MAGNETIZATION
METHODS OF MAGNETIZATION
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 The basic principle of magnetization is to produce magnetic
lines of force across the expected direction of cracks.
 If the likely crack direction is unknown, then the test must be
performed in 2 directions at right angles.
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performed in 2 directions at right angles.
1. Magnetic flow
2. Current flow
3. Induced current flow
4. Electromagnetic Induction
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1. Magnetic flow: To make the component of a magnetic circuit by
effectively using it as the bridge of a permanent or electromagnet.
2. Current flow: To pass an electric current through the specimen,
along the direction & through the region where cracks are expected.
METHODS OF MAGNETISATION
METHODS OF MAGNETISATION
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along the direction & through the region where cracks are expected.
3. Induced Current flow: Used for ring specimens by effectively
making them the secondary of a main transformer.
4. Electromagnetic induction: Pass an electric current through a
conductor which is threaded through a hollow specimen or placed
adjacent or wrapped around it.
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44. METHODS OF MAGNETIZATION
 There are a variety of methods that can be used
to establish a magnetic field in a component for
evaluation using magnetic particle inspection. It
is common to classify the magnetizing methods
as either direct or indirect.
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as either direct or indirect.
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45. M
MAGNETIZATION
AGNETIZATION U
USING
SING D
DIRECT
IRECT
I
INDUCTION
NDUCTION (D
(DIRECT
IRECT M
MAGNETIZATION
AGNETIZATION)
)
 With direct magnetization, current is passed
directly through the component. The flow of
current causes a circular magnetic field to form
in and around the conductor.
 When using the direct magnetization method,
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 When using the direct magnetization method,
care must be taken to ensure that good electrical
contact is established and maintained between
the test equipment and the test component to
avoid damage of the component (due to arcing or
overheating at high resistance points).
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46. 
 Magnetization
Magnetization Using
Using Direct
Direct Induction
Induction
(Direct
(Direct Magnetization)
Magnetization)
 Clamping the component between two
electrical contacts (HEAD
(HEAD SHOT
SHOT Method)
Method)
 Using clamps or prods
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 Using clamps or prods
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47. CLAMPING THE COMPONENT BETWEEN TWO
ELECTRICAL CONTACTS(HEAD SHOT
(HEAD SHOT
M
METHOD
ETHOD)
)
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48.  Current is passed through the component and a
circular magnetic field is established in and
around the component. When the magnetizing
current is stopped, a residual magnetic field will
remain within the component.
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remain within the component.
 The strength of the induced magnetic field is
proportional to the amount of current passed
through the component.
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49. HEAD SHOT TECHNIQUE
HEAD SHOT TECHNIQUE
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 This technique produces circular magnetization by passing
electric current through the part itself.
 A round bar is held between the heads in a wet horizontal unit.
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 As the current flows from one end to the another end of the
bar, the magnetic lines of force gets circulated around the bar.
 The circulation of magnetic flux around the bar is known as
circular magnetism & the technique is known as heat shot
technique.
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(Wet horizontal unit is a stationary unit in which longitudinal &
circular fields are produced. The unit has a fixed headstock &
sliding tailstock. The part can be placed b/w these by adjusting
the tailstock.)
HEAD SHOT TECHNIQUE
HEAD SHOT TECHNIQUE
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the tailstock.)
 One disadvantage is that, it produces heating & arcing at the
juncture due to the passage of high current through a small
contact area.
 To avoid over heating & arcing, the contact faces should be
made flexible.
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51. USING CLAMPS OR PRODS
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 which are attached or placed in contact with the
component. Electrical current flows through the
component from contact to contact.
 The current sets up a circular magnetic field around
the path of the current.
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52. M
MAGNETIZATION
AGNETIZATION U
USING
SING I
INDIRECT
NDIRECT
I
INDUCTION
NDUCTION (I
(INDIRECT
NDIRECT M
MAGNETIZATION
AGNETIZATION)
)
 Indirect magnetization is accomplished by using
a strong external magnetic field to establish a
magnetic field within the component. As with
direct magnetization, there are several ways that
indirect magnetization can be accomplished.
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indirect magnetization can be accomplished.
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53. 2
2.
. Magnetization
Magnetization Using
Using Indirect
Indirect Induction
Induction
(Indirect
(Indirect Magnetization)
Magnetization)
 Use of permanent magnets
 Electromagnets(using yokes)
 Central conductor magnetization
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 Central conductor magnetization
 Use of coils and solenoids
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54. U
USE
SE OF
OF PERMANENT
PERMANENT MAGNETS
MAGNETS
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 The use of permanent magnets is a low cost method
of establishing a magnetic field. However, their use
is limited due to lack of control of the field strength
and the difficulty of placing and removing strong
permanent magnets from the component.
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55. ELECTROMAGNETS
(USING YOKES)
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 Electromagnets in the form of an adjustable horseshoe
magnet (called a yoke) eliminate the problems associated
with permanent magnets and are used extensively in
industry.
 Electromagnets only exhibit a magnetic flux when electric
current is flowing around the soft iron core. When the
magnet is placed on the component, a magnetic field is
established between the north and south poles of the
magnet.
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56. CENTRAL CONDUCTOR
MAGNETIZATION
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 Another way of indirectly inducting a magnetic field
in a material is by using the magnetic field of a
current carrying conductor. A circular magnetic field
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current carrying conductor. A circular magnetic field
can be established in cylindrical components by using
a central conductor.
 Typically, one or more cylindrical components are
hung from a solid copper bar running through the
inside diameter. Current is passed through the copper
bar and the resulting circular magnetic field
establishes a magnetic field within the test
components.
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57. USE OF COILS AND SOLENOIDS
57
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 When the length of a component is several times larger
than its diameter, a longitudinal magnetic field can be
established in the component.
 The component is placed longitudinally in the
concentrated magnetic field that fills the center of a coil or
solenoid. This magnetization technique is often referred to
as a "coil shot".
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58. LONGITUDINAL &CIRCULAR
MAGNETIC FIELDS
Longitudinal
1. Permanent magnets
2. Electromagnetic yokes
3. Using a Coil(COIL SHOT)
Circular
1. Clamping the component
between two
electrical contacts(HEAD
SHOT)
3. Using a Coil(COIL SHOT) SHOT)
2. Using clamps or prods
3. Central conductor
magnetization

59. TYPES OF MAGNETIZING CURRENT
59
 As mentioned previously, electric current is often
used to establish the magnetic field in
components during magnetic particle inspection.
Alternating current (AC) and direct current (DC)
are the two basic types of current commonly
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are the two basic types of current commonly
used.
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60. 1. DIRECT CURRENT
60
 Direct current (DC) flows
flows continuously
continuously in
in one
one direction
direction
at
at a
a constant
constant voltage
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.
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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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61. 2. ALTERNATING CURRENT
61
 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
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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 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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62. 3. RECTIFIED ALTERNATING CURRENT
62
 Clearly, the skin effect limits the use of AC since
many inspection applications call for the
detection of subsurface defects.
 Luckily, AC can be converted to current that is
very much like DC through the process of
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very much like DC through the process of
rectification. With the use of rectifiers, the
reversing AC can be converted to a one
directional current.
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63. 63
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64. HALF WAVE RECTIFIED ALTERNATING
CURRENT (HWAC)
64
 When single phase alternating current is passed through a
rectifier, current is allowed to flow in only one direction.
 The reverse half of each cycle is blocked out so that a one
directional, pulsating current is produced.
 The current rises from zero to a maximum and then
returns to zero. No current flows during the time when the
reverse cycle is blocked out.
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returns to zero. No current flows during the time when the
reverse cycle is blocked out.
 The HWAC repeats at same rate as the unrectified current
(50 or 60 Hz). Since half of the current is blocked out, the
amperage is half of the unaltered AC. This type of current
is often referred to as half wave DC or pulsating DC. The
pulsation of the HWAC helps magnetic particle indications
form by vibrating the particles and giving them added
mobility where that is especially important when using dry
particles. HWAC is most often used to power
electromagnetic yokes.
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65. FULL WAVE RECTIFIED ALTERNATING
CURRENT (FWAC) (SINGLE PHASE)
65
 Full wave rectification inverts the negative
current to positive current rather than blocking it
out. This produces a pulsating DC with no
interval between the pulses. Filtering is usually
performed to soften the sharp polarity switching
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performed to soften the sharp polarity switching
in the rectified current.
 While particle mobility is not as good as half-
wave AC due to the reduction in pulsation, the
depth of the subsurface magnetic field is
improved.
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66. THREE PHASE FULL WAVE RECTIFIED
ALTERNATING CURRENT
66
 Three phase current is often used to power
industrial equipment because it has more
favorable power transmission and line loading
characteristics.
 This type of electrical current is also highly
desirable for magnetic particle testing because
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desirable for magnetic particle testing because
when it is rectified and filtered, the resulting
current very closely resembles direct current.
 Stationary magnetic particle equipment wired
with three phase AC will usually have the ability
to magnetize with AC or DC (three phase full
wave rectified), providing the inspector with the
advantages of each current form.
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67. EQUIPMENT'S USED FOR MPI
67
 MPI equipment serves the following purposes, it
provides
 Sufficient power of right type
 Suitable contact and coils
 Convenient means for accomplishing proper
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 Convenient means for accomplishing proper
magnetization with respect to field strength and
direction.
 Means of applying the magnetic particles
 Well lighted space for careful examination of the part
of indication.
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68. 68
1. Simple equipment:
1. Simple equipment: for occasional
testing of small casting or machine parts for
detection of surface cracks, small and easily
portable equipment is most convenient.
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69. PERMANENT MAGNETS
69
 The use of industrial magnets is not popular
because they are very strong (they require
significant strength to remove them from the
surface, about 250 N for some magnets) and thus
they are difficult and sometimes dangerous to
handle.
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handle.
 However, permanent magnets are sometimes
used by divers for inspection
inspection in
in underwater
underwater
environments
environments or
or other
other areas,
areas, such
such as
as explosive
explosive
environments,
environments, where
where electromagnets
electromagnets cannot
cannot be
be
used
used.
.
 Permanent magnets can also be made small
enough to fit into tight areas where
electromagnets might not fit.
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70. ELECTROMAGNETIC YOKES
70
 An electromagnetic yoke is a very common piece
of equipment that is used to establish a magnetic
field. A switch is included in the electrical circuit
so that the current and, therefore, the magnetic
field can be turned on and off.
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field can be turned on and off.
 They can be powered with AC from a wall socket
or by DC from a battery pack. This type of
magnet generates a very strong magnetic field in
a local area where the poles of the magnet touch
the part being inspected.
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71. 71
2. Large portable equipment: it is used where
higher power is required or heavier duty cycles
make the small kits inadequate. One of the
smallest of this series operates at 120V AC and
delivers up to 700amperes, either AC or half wave
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delivers up to 700amperes, either AC or half wave
DC
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72. PRODS
72
 Prods are handheld electrodes that are pressed
against the surface of the component being inspected
to make contact for passing electrical current (AC or
DC) through the metal.
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 Prods are typically made
made from
from copper
copper and
and have
have an
an
insulated
insulated handle
handle to
to help
help protect
protect the
the operator
operator.
. One of
the prods has a trigger switch so that the current can
be quickly and easily turned on and off. Sometimes
the two prods are connected by any insulator, to
facilitate one hand operation. This is referred to as a
dual prod and is commonly used for weld inspections.
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73. PORTABLE COILS AND CONDUCTIVE
CABLES
73
 Coils and conductive cables are used to
establish a longitudinal magnetic field within
a component. When a preformed coil is
used, the component is placed against the
inside surface on the coil. Coils typically have
three or five turns of a copper cable within the
molded frame. A foot switch is often used to
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molded frame. A foot switch is often used to
energize the coil.
 Also, flexible conductive cables can be
wrapped around a component to form a coil.
The number of wraps is determined by the
magnetizing force needed and of course, the
length of the cable. Normally, the wraps are
kept as close together as possible. When using
a coil or cable wrapped into a coil, amperage is
usually expressed in ampere-turns. Ampere-
turns is the amperage shown on the amp
meter times the number of turns in the coil.
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74. PORTABLE POWER SUPPLIES
74
 Portable power supplies are used to
provide the necessary electricity to the
prods, coils or cables. Power supplies
are commercially available in a
variety of sizes. Small power supplies
generally provide up to 1,500A of half-
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generally provide up to 1,500A of half-
wave DC or AC. They are small and
light enough to be carried and operate
on either 120V or 240V electrical
service.
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When more power is necessary, mobile power
supplies can be used. These units come with wheels
so that they can be rolled where needed. These units
also operate on 120V or 240V electrical service and
can provide up to 6,000A of AC or half-wave DC.

75. 75
3. Stationary magnetizing equipment: A large
variety of stationary , bench-type units is available,
with various characteristics to fit different testing
requirements.
The smaller size equipment is used for small parts
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The smaller size equipment is used for small parts
that can be easily transported and handled by
hand. The larger ones are used for heavy parts
such as long diesel engine crankshafts, where
handling must be crane.
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76. STATIONARY EQUIPMENT
76
 Stationary magnetic particle inspection equipment is designed for
use in laboratory or production environment. The most common
stationary system is the wet horizontal (bench) unit. Wet horizontal
units are designed to allow for batch inspections of a variety of
components. The units have head and tail stocks (similar to a lathe)
with electrical contact that the part can be clamped between. A
circular magnetic field is produced with direct magnetization.
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circular magnetic field is produced with direct magnetization.
 Most units also have a movable coil that can be moved into place so
the indirect magnetization can be used to produce a longitudinal
magnetic field. Most coils have five turns and can be obtained in a
variety of sizes. The wet magnetic particle solution is collected and
held in a tank. A pump and hose system is used to apply the particle
solution to the components being inspected. Some of the systems
offer a variety of options in electrical current used for magnetizing
the component (AC, half wave DC, or full wave DC). In some units, a
demagnetization feature is built in, which uses the coil and decaying
AC.
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77. 77
4. Large heavy duty DC equipment: very
versatile types of heavy duty stationary equipment
are those DC units designed for application of the
“overall” method of magnetizing , for the inspection
of very large and complicated castings. Rectified
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of very large and complicated castings. Rectified
three-phase AC is delivered with current values
running as high as 20,000 amperes.
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78. SENSITIVITY
78
 MPI Methods are sensitive means of locating
small and shallow surface cracks in
ferromagnetic components. Many incipient
fatigue cracks and fine grinding cracks having
size less than 0.02mm deep and surface openings
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size less than 0.02mm deep and surface openings
of one tenth of that or less can be located using
MPI.
 Detectability generally involves a relation b/w
surface opening and depth.
 If defects sought are usual cracks, comparatively
low level of magnetic force will give sufficient
build-up.
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79. 79
 However, to find minute cracks or subsurface
flaws, the flux level should be high. Theoretically,
a level just below saturation would give the most
sensitive results. But this is impractical owing to
the non-regular shapes of the components
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the non-regular shapes of the components
encountered.
 Sensitivity also depends on the type of current
used.
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80. 80
 Various tests have conclusively proved the
following information:
 AC magnetization is most effective for surface
defects.
 AC magnetization is not effective for subsurface
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 AC magnetization is not effective for subsurface
defects.
 DC( straight or half wave) must be used for
subsurface defects.
 Half wave DC gives superior penetration as
compared to straight DC
 Half wave DC dry method gives the greatest
penetration.
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81.  fast, simple and inexpensive
 direct, visible indication on surface
 unaffected by possible deposits, e.g. oil, grease or other
metals chips, in the cracks
 can be used on painted objects
ADVANTAGES OF MPI
81
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 results readily documented with photo or tape impression
 Post-cleaning generally not necessary.
 A relatively safe technique; materials generally not
combustible or hazardous.
 Indications can show relative size and shape of the
discontinuity.
 Easy to use and requires minimal amount of training.
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82. LIMITATIONS OF MPI
 Non-ferrous materials, such as aluminum, magnesium, or
most stainless steels, cannot be inspected.
 Examination of large parts may require use of equipment
with special power requirements.
 May require removal of coating or plating to achieve
desired sensitivity.
 Limited subsurface discontinuity detection capabilities.
82
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 Limited subsurface discontinuity detection capabilities.
 Post-demagnetization is often necessary.
 Alignment between magnetic flux and indications is
important.
 Each part needs to be examined in two different
directions.
 Only small sections or small parts can be examined at one
time.
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83. TESTING TECHNIQUES
83
 In MPI, various methodologies for inspections are
used some are:
 Dry particle inspection
 Wet suspension inspection
 Magnetic rubber inspection
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 Magnetic rubber inspection
 Residual and continuous magnetization technique.
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84. STEPS FOR PERFORMING DRY
PARTICLES INSPECTION:
84
 Surface preparation
 Applying the magnetizing force
 Applying dry magnetic particles
 Blowing off excess powder
Terminating the magnetizing force
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 Terminating the magnetizing force
 Inspection for indications
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85. 85
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86. STEPS FOR PERFORMING WET PARTICLE
INSPECTION:
86
 Surface preparation
 Applying suspended magnetic particles
 Applying the magnetizing force
 Inspection for indications
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87. 87
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88. RESIDUAL MAGNETIZATION TECHNIQUE
88
 In this technique , component to be inspected is
magnetized first, then before application of
magnetic particle magnetizing force is stopped.
Thus indication is produced by magnetic particle
attracted due to residual field of magnetized
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attracted due to residual field of magnetized
component. This technique is insensitive to
components or material with high permeability
and for low permeable material special care
taken to ensure the sufficient strength of residual
field so that indications may be produced.
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89. CONTINUOUS MAGNETIZATION TECHNIQUE
89
 This technique is used when great level of
sensitivity is required. In this technique,
component to be inspected is magnetized and
magnetization is continued while the magnetic
particles are applied on the surface of component.
The technique provides an advantage of strong
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 The technique provides an advantage of strong
leakage field(strong magnetic field). Thus a
strong magnetic flux is produced, which forces
more particles to vibrate and slide down towards
the crack surface or leakage surface.
 Only limitation with this method is that, it
produces heat which may sometime damages the
test component when direct magnetization is used.
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90. ME312 NDT
TASK
TASK -
-3 (MODULE
3 (MODULE-
-3) A
3) ASSIGNMENT
SSIGNMENT 1.3
1.3
90
 Important terminologies related to
magnetic properties of material.
 Write short notes on Magnetic field
indicators / Equipments
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Submit on or before : 29-06-2022(in class note)
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91. ME312 NDT
91
END OF MODULE -3
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END OF MODULE -3
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92. 92
Quality & Process Control
Magnetic rubber inspection
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