BMFB 4283
NDT & FAILURE ANALYSIS
Lectures for Week 3
Prof. Qumrul Ahsan, PhD
Department of Engineering Materials
Faculty of Manufacturing Engineering
Issues to address
3.0 Magnetic Particle Testing
3.1 Introduction
3.2 Theory
3.3 Techniques and Equipment
3.4 Inspection and Application
MAGNETIC PARTICLE TESTING - Outline
• Magnetism and Ferromagnetic
Materials
• Introduction of Magnetic Particle
Inspection
• Basic Procedure and Important
Considerations
• Magnetizing methods and apparatus
• The detecting medium
• Examples of MPI Indications
• Reporting Indications
Introduction
• This module is intended to present information
on the widely used method of magnetic particle
inspection.
• Magnetic particle inspection can detect both
production discontinuities (seams, laps, grinding
cracks and quenching cracks) and in-service
damage (fatigue and overload cracks).
Magnetic Particle Testing
• Test method for the detection of surface and
slightly sub-surface indications in ferromagnetic
materials
MT
MT
Subsurface
Surface Defect
Internal
Ferromagnetic Material
CANNOT BE
DETECTED BY
Magnetic Particle
Testing
Introduction to Magnetism
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 lines of force
around a bar magnet
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.
Opposite poles attracting Similar poles repelling
Domain Theory
UNMAGNETISED STATE
DOMAINS RANDOMLY
ORIENTATED
MAGNETIZED STATE.
DOMAINS ORIENTATED IN EXTERNAL
MAGNETIC FIELD
FIELD
SATURATED STATE
DOMAINS ORIENTATED IN STRONG
EXTERNAL FIELD
FIELD
Domain Theory
RESIDUAL STATE. DOMAIN REMAINING
ORIENTATED
DEMAGNETISED STATE.
DOMAINS RANDOMLY ORIENTATED IN
OPPOSING CURVE
FIELD
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.
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.
Principle of MT : Flux Leakage
Ferromagnetic
Particles
N
S
Ring Magnet
Magnetic field is Fully
contained: No Poles
Ring Magnet
Flux Leakage occurs: Poles
created
Flux
Leakage
Attracted at
poles
Principle of MPI : Flux Leakage
No Defect
Defect
Flux Leakage
N
S
N
S
The change in permeability causes flux leakage
Principle of MPI : Flux Leakage
N
STEEL µ= 1000
No Flux Leakage because No change in
permeability
S
Principle of MPI : Flux Leakage
N
Flux Leakage
AIR µ= 1
S
STEEL µ= 1000
The change in permeability causes flux leakage
Factors Affecting Flux Leakage
•
•
•
•
•
Depth of defect
Orientation of defect shape of defect
Size of defect
Permeability of material
Amount of flux available
Factors Affecting Flux Leakage
N
S
N
S
Depth below surface
•
•
•
•
•
Depth of defect
Orientation of defect shape of defect
Size of defect
Permeability of material
Amount of flux available
Basic Procedure
Basic steps involved:
1. Component pre-cleaning
2. Introduction of magnetic field
3. Application of magnetic media
4. Interpretation of magnetic particle indications
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.
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
3. Flowing an electrical current through a coil of wire around the
part or through a central conductor running near the part.
Direction of the Magnetic Field
Two general types of magnetic 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.
•
A longitudinal magnetic field has
magnetic lines of force that run parallel
to the long axis of the part.
• External solenoidal coil
• With yoke
•
A circular magnetic field has magnetic
lines of force that run circumferentially
around the perimeter of a part.
• Head Shot
• Central conductor
• Prod
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.
 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.
Since
defects may
occur in various and
unknown directions,
each part is normally
magnetized in two
directions at right
angles to each other.
Flux Leakage
No Flux Leakage
Defect Orientation
Defect at 90 degrees to flux : maximum
indication
Defect Orientation
>60 Degrees to Flux:
Acceptable
indication
Defect Orientation
<60 Degrees to Flux
:
Weak
indication
Question
?
From the previous slide regarding the optimum test sensitivity,
which kinds of defect are easily found in the images below?
Longitudinal (along the axis)
Transverse (perpendicular the axis)
Induction methods
– Threaded bar
• Hollow object must have access both ends
• Conductor carrying current is threaded through
bore passed current through it.
• Produced circular field
– Flexible cables
• Used for a variety component shape
• Place flexible cable on or around specimen
• Current passed through coil induce magnetic field
Producing a Longitudinal Magnetic
Field Using a Coil
A longitudinal magnetic field is
usually established by placing
the part near the inside or a
coil’s annulus. This produces
magnetic lines of force that are
parallel to the long axis of the
test part.
Coil on Wet Horizontal Inspection
Unit
Portable Coil
Magnetizing methods – Induction methods
• Methods:
– Encircling coils
• Placing specimen inside coils
• Low voltage, high amperage current is
passed
• Creates longitudinal magnetic field
• Current values:
–NI = K/[L/D]
–
–
–
–
–
N = number of turns in the coils
I = current in amperes
L = the component length
D = the component diameter
L/D = ratio of geometrical
information of component
– K = source constant ( K= 32 for DC,
22 for AC, 11 for mean value)
Flexible cable technique
• Advantages:
–
–
–
–
–
AC or DC field
Large areas can be inspected
No poles to attract magnetic particles
Filed strength can be altered
Predictable field strength
• Disadvantages:
– Cumbersome long heavy cable required
– Longer setting time
– Heavy transformer required for large amperage
Producing a Longitudinal Field Using
Permanent or Electromagnetic Magnets
Permanent magnets and electromagnetic yokes are also often used to
produce a longitudinal magnetic
field. The magnetic lines of force run
from one pole to the other, and the
poles are positioned such that any
flaws present run normal to these
lines of force.
Yokes:
• Highly permeable, low
retentive steel
• Laminated to reduce
induction and prevent the
yoke from permanently
magnetized
Electromagnetic yokes
• Magnetism:
– Encircling yoke with coil through which
current is passed
– Strength of field is varied by:
• Adjusting the current (amperage) flowing
through the yoke
• Varying the distance between the pole pieces
– The field produced is longitudinal
• Depth of field depends on type of current
• Require source of electrical energy
(AC or DC)
• Surface discontinuities using AC
• Sub-surface defect using DC
Electromagnetic yokes
• Advantages:
–
–
–
–
–
AC or rectified DC operation
Controllable field strength
Can be switched on/off as required
No damage done to test piece
Relatively lightweight
• Disadvantages:
– Requires power supply
– Only small area can be examined
– Leaves only one hand free
Magnetizing methods - Permanent magnet
• Permanent magnet:
– able to maintain a magnetic field in surrounding space
– Field strength can vary considerably, depends on flux
density in magnet and shape
• Magnetic bar:
– A piece of ferromagnetic material with a magnetic pole at
each end
– Placed into a closed loop:
• create magnetic field within closed circuit and no
external field would exist
• If defect present in the loop, flux leakage occur
• Provide magnetic flow in the specimen and produce
longitudinal magnetic field between poles
Permanent magnets
• Advantages:
–
–
–
–
No power supply required
Inexpensive
No damage to the test piece from arcing
Relatively lightweight (portable)
• Disadvantages:
–
–
–
–
Deterioration with wear
Have to be pulled from the test surface
Magnetic particles attracted to poles
Limited application on awkward shapes
Circular Magnetic Fields
Magnetic Field
Electric
Current
Circular magnetic fields are produced by
passing current through the part or by
placing the part in a strong circular magnet
field.
A headshot on a wet horizontal test unit
and the use of prods are several common
methods of injecting current in a part to
produce a circular magnetic field. Placing
parts on a central conductors carrying high
current is another way to produce the field.
Magnetizing methods – Current flow
• Produce circular magnetic field by
passing current through test piece
– Prod technique
• Current is introduced using electrical
contacts (prod)
• Prod induce circular magnetic field
within specimen using current values
• Correct positioning is essential to ensure
all possible defects are detected
• Ideally the prods should be in line
parallel to or on the same axis as the
defect
Current flow - Prods
• Advantages:
– AC or DC fields
– Low voltage output
– No poles to attract magnetic particles
– Variable field strength
– Can be used in confined spaces
• Disadvantages:
– Risk of creating arc strikes
– Heavy transformer required
– Contacts and small test items can be overheated
– Careful positioning and spacing of prods are
required
Longitudinal vs. Circular Magnetic Field
Longitudinal Magnetic Field
Circular Magnetic Field
Advantages
Easy to generate
Higher field strength
No electrical contacts
Wholly contained within the part
Rapid processing of small and long
part
Penetrating power is more
Easy to demagnetize part
Disadvantages
Lower field strength than circular
Difficult to remove circular DC fields
Generates nonrelevant poles at the
part ends
Electric contact and associated with
arcs
Very little subsurface sensitivity
Types of Magnetization Current
• DC, AC, HWDC, 1FWDC, 3FWDC
• Type of current used depends primarily on the depth of the
defect from the surface, not the crack size
• AC provides a highly concentrating field at the parts
surface ( detecting surface and very near surface
defects)
– Alternating magnetic field increases particle mobility and particles
attract to leakage field(dry powder with HWDC)
– A time varying magnetic field induces eddy current within the
material. Eddy currents cause the magnetic field to decay
exponentially .
– The depth  is the skin depth, where the magnetic field is 37% of its
maximum value. = 1/πµf
• DC penetrates deeper, provides both moderate surfaces and
subsurface sensitivity.
Magnetic Particles
• Fine magnetic particles that create an indication at
a leakage field caused by a flaw in the magnetized
sample
• Depending on its characteristics and application
– Type of particle (dry or wet)
– Viewing method (color contrast or fluorescent)
– Method of application (continuous or residual)
Requirement of magnetic particles
• Fine grains to reduce the gravitational effect. The
maximum size (BS 4069): 200m for powder and 100
m for inks
• Particles are chemically treated iron oxide particles,
small in size and varying in shape (spherical to irregular
 sensitive to flaw)
– Elongated shape for easier polarization. Spherical particles
are also needed to ensure dispersal over the surface
• High magnetic permeability and low retentivity (avoid
to become permanent magnet)
– High permeability for magnetization in weak flux leakage
fields
– Low retentivity if particles are to be removed after the test
• High contrast against the background of the test
surface
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.
• May be black, grey, red, orange, fluorescent
• Usually applied to a surface by means of
puffer device:
They should be floated, not blasted on to the
area under test
the particles are lightly dusted on to the
surface
• Should be ideally be used with ac or dc
current
Because of extra mobility of the current
impart onto powder
• Must be used when MPI is being carried out on
hot and rough surfaces
Inks are not suitable
The dry method
is more portable
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.
Wet Magnetic Particles
• According to BS 4069, the composition of
inks shall be:
– Ferromagnetic particles in non-fluorescent
inks should not be less than 1.25% and not
more than 3.5% by volume
– Ferromagnetic particles in fluorescent inks
should not be less than 0.1% and not more
than 0.3% by volume
• Carrier fluid may be oil based: paraffin or
water to make up volume by 100%
– If water is used, additives shall be added to
prevent corrosion on the surface or the
particles and improve the wetting action
With the wet method, the part is flooded with a solution carrying the
particles.
The wet method is generally more sensitive since the liquid carrier
gives the magnetic particles additional mobility.
Viewing conditions
• Non-fluorescent inks and powders:
– Area under inspection must be evenly illuminated
– Min illumination level of 500 lux (daylight/artificial light)
• Fluorescent inks and powders:
– Min UV-A irradiance level: 800 W/cm2, min background
intensity 10 lux
– Degrade with exposure to ordinary light over a period of
time and high temperature
– UV intensity increases, the amount of fluorescent
increases too
UV-A light
• Generated by mercury vapor lamps
• Mercury is vaporized inside a quartz capsule by small
low current arc from auxiliary electrode
• After 5 minutes, there is sufficient mercury vapor in
the capsule to initiate between main electrode
• The lamp should be used after 15 minutes to allow
sufficient time to attain full working intensity
Safety
• UV light operate with wavelengths between 320 – 400 nm
• Shorter wavelength cause injury to the eyes
• Filter must be used which cut wavelength under 320 nm to
prevent injury
• Looking into UV-A cause temporary clouding vision
–  fluid in the eyeball fluorescing
–  will normalize with no permanent effects after few seconds
– Prolonged exposure may cause cataract
Health
• Flammability:
– Read container labels for flash points. Materials cannot be
used under flash points
• Asthmatic:
– Do not use in confined space without masks or adequate
ventilation
• Skin hazard:
– Use protective clothing
Methods of Application
• Continuous method : apply particles to all surfaces of
the part while the part is being magnetized
– Rather easy for dry powder
– For wet powder parts are magnetised several time
with short duration (0.5 sec)
• Residual Method: apply the particle after
magnetizing has been removed
– Residual force must be large
– Suitable for wet part and defects under plating or
coating
– less sensitive
Relative Penetration Sensitivity
• Current Type vs. Wet or Dry Method
AC
Penetration
Wet
Method
HW
Poor
Surface and near
surface to 0.25 mm
Dry Method Surface and near
surface to 0.25mm
Excellent
DC (1FWDC &
3FDC)
Good
Surface and
Surface and
near surface to subsurface to
0.65 mm
1.3 mm
Surface and
subsurface to
6.35 mm
Surface and
subsurface to
1.3 mm
Inspection Aids
• Controlling particle suspension
– Changes in particle concentration
 The acceptable operating range for
fluorescent particles is 0.15-0.25 ml in 100
ml of liquid
 Below 0.1 ml – too low to detect small
defects
 Above 0.4 ml –background fluorescrnce
may mask small defects
– Loss of fluorescence
– Contaminations
 Water in oil bath or vice versa
 Excessive wetting agent
 Agglomeration reduces mobility
Centrifuge tube
Inspection Aids
• Controlling magnetization and
system performance
– Hall Effect Meter: measures the
amplitude of either an AC or a
DC magnetic field
– Pie Gauge : made by brazing
together six pie-shaped pieces of
high permeability, low
retentivity material
– Ketos Test Ring : A centered hole
circular disk with several small
drilled holes at varying depth to
check leakage flux of circular
field.
• Avoiding non relevant indication
Indications
Relevant Indications - Indications due to
discontinuities or flaws
Non-Relevant Indications - Indications due
to flux leakage from design features
Spurious Indications - Indications due
incorrect inspection procedures
All surface defects form indications
But not all indications are caused by defects
Non-relevant indications
Due to flux leakage but arising from
design features or geometry
Splines
Furring



Rough
Surface
Rivet
Changes in section
Changes in permeabilityKeyway
Toe of welds
Furring
Furring
Caused by:
Sharp change of contour
Furring
Furring
Furring
Caused by:
Excessive flux on the surface or ends of component
Furring
Magnetic Writing
Caused by:
Localised polarization when magnetised object induced the
magnetic field into another object
Spurious / False Indications
Indications caused by operator errors
Not due to flux leakage



Lint
Dirt
Hairs
Relevant Indications
Indications caused by defects
Magnetic Particle Testing
Cracks indications by Fluorescent Ink
Crane Hook with
Service Induced Crack
Fluorescent, Wet Particle Method
Gear with
Service Induced Crack
Fluorescent, Wet Particle Method
Drive Shaft with
Heat Treatment Induced Cracks
Fluorescent, Wet Particle Method
Splined Shaft with
Service Induced Cracks
Fluorescent, Wet Particle Method
Threaded Shaft with
Service Induced Crack
Fluorescent, Wet Particle Method
Large Bolt with
Service Induced Crack
Fluorescent, Wet Particle Method
Crank Shaft with
Service Induced Crack Near Lube
Hole
Fluorescent, Wet Particle Method
Lack of Fusion in SMAW Weld
Indication
Visible, Dry Powder Method
Toe Crack in SMAW Weld
Visible, Dry Powder Method
Throat and Toe Cracks in
Partially Ground Weld
Visible, Dry Powder Method
Reporting
• Adequate reporting is essential for transmission of relevant
and correct information after the test
• Minimum requirement:
–
–
–
–
–
Work location
Description and identity of the component tested
Date of test
Stage of test
Reference to the written test procedure and the technique sheets
used
– Name of the company
– Name and signature of the person performing the test
– The test results
Reporting
• If written procedure is absent: these information must be
supplied:
–
–
–
–
–
–
–
–
Description of equipment used
Technique of flux generation
Indicated current values and waveform for each technique used
Distance between contact areas of dimensional details of the coils
Detection medium used and background
Surface preparation
Viewing condition
Method of recoding or marking indications
Method of recording indications
• Common method:
– Reproduce indications on a scaled diagram.
– Indications drawn with references to a datum on
test piece
– The diagrams should not be overloaded with too
much information
– Separate diagram show the magnetization
techniques
Method of recording indications
• Other methods:
– Photographs
– Clear sticky tape to peel the dried magnetic particles
indication from the test piece
– Propriety lacquers sprayed on wet; when dry the resultant
film is then peeled away with the indication
– Magneto-graph
– Magnetic sachets with light sensitive paper backings
Preservation of indications
• To maintain permanent record, one of these
methods can be used:
• Cover the indications with a transparent adhesive film. Carefully
peel off the film and the adhering indications and reapply to
either paper or card of contrasting color.
• Degrease the test surface, cover with a white matt adhesive film
and retest. After drying, if necessary, cover the indications with a
clear film as in method describe above.
• Spray the tested area with a quick-drying, strippable coating.
Strip off this coating and view the face previously in contact with
the workplace to which the indications will be transferred.
Preservation of indications
• Heat the work place to an approved temperature and, without delay,
slowly immerse in a powdered plastic material and slowly withdraw.
Allow it to drain and cure it in accordance with the manufacturers
instructions. Strip off the coating complete with the indications from
the work piece and view the face previously in contact with it
• Degrease the test surface and coat with a proprietary, strippable,
magnetic-oxide paint. Magnetize the part to saturation and peel off
the coating. If it is dipped in agitated magnetic ink, it will reveal the
flaw indications on the oxide film.
• Degrease the test area and coat the test area with a proprietary, self
curing magnetic silicone-rubber compound. Magnetize to saturation
and allow the compound to cure. The oxide in the compound will
migrate to the position of nay flaw and when removed from the work
piece, the rubber previously in contact with the surface will show the
flaw
Photographic records
• When photographic record is made, the
resulting photograph of tested surface should
be, if possible, the actual size
• If the surface is highly polished, care should be
taken to avoid highlights
• The use of matt contrast medium applied
prior to testing may be desirable
Surface preparation
• Dry powder:
– No preparation is necessary
• Wet powder:
– Drained surface or adequately dried
– Retest work piece with magnetic ink made with
volatile carrier fluid
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.
• 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.
Magnetized
Demagnetized
Demagnetization (Cont.)
• Using a solenoid is the simplest way to demagnetize a
part
90% of all parts magnetized in MPI are demagnetised using a
simple AC coil
• 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.
Advantages of
Magnetic Particle Inspection
• Can detect both surface and near sub-surface defects.
• Can inspect parts with irregular shapes easily.
• Precleaning of components is not as critical as it is for
some other inspection methods. Most contaminants
within a flaw will not hinder flaw detectability.
• Fast method of inspection and indications are visible
directly on the specimen surface.
• Considered low cost compared to many other NDT
methods.
• Is a very portable inspection method especially when
used with battery powered equipment.
Limitations of
Magnetic Particle Inspection
•Cannot inspect non-ferrous materials such as aluminum,
magnesium or most stainless steels.
•Inspection of large parts may require use of equipment
with special power requirements.
•Some parts may require removal of coating or plating to
achieve desired inspection sensitivity.
•Limited subsurface discontinuity detection capabilities.
Maximum depth sensitivity is approximately 0.6” (under ideal
conditions).
•Post cleaning, and post demagnetization is often
necessary.
•Alignment between magnetic flux and defect is
important
Glossary of Terms
• Black Light:
•
•
ultraviolet light which is filtered to produce a
wavelength of approximately 365 nanometers. Black light
will cause certain materials to fluoresce.
Central conductor: an electrically conductive bar usually
made of copper used to introduce a circular magnetic field
in to a test specimen.
Coil: an electrical conductor such a copper wire or cable
that is wrapped in several or many loops that are brought
close to one another to form a strong longitudinal
magnetic field.
Glossary of Terms
• Discontinuity:
•
•
•
an interruption in the structure of the
material such as a crack.
Ferromagnetic: a material such as iron, nickel and cobalt
or one of it’s alloys that is strongly attracted to a magnetic
field.
Heads: electrical contact pads on a wet horizontal
magnetic particle inspection machine. The part to be
inspected is clamped and held in place between the heads
and shot of current is sent through the part from the
heads to create a circular magnetic field in the part.
Leakage field: a disruption in the magnetic field. This
disruption must extend to the surface of the part for
particles to be attracted.
Glossary of Terms
• Non-relevant indications: indications produced due to
•
•
some intended design feature of a specimen such a
keyways, splines or press fits.
Prods: two electrodes usually made of copper or
aluminum that are used to introduce current in to a test
part. This current in turn creates a circular magnetic field
where each prod touches the part. (Similar in principal to
a welding electrode and ground clamp).
Relevant indications: indications produced from
something other than a design feature of a test specimen.
Cracks, stringers, or laps are examples of relevant
indications.
Glossary of Terms
• Suspension: a bath created by mixing particles with either
•
oil or water.
Yoke: a horseshoe magnet used to create a longitudinal
magnetic field. Yokes may be made from permanent
magnets or electromagnets.