Magnetism
Materials will strongly attract pieces of iron to
themselves
Phenomenon discovered in the ancient Greek city
of Magnesia
Magnets utilised in navigation
Oersted discovered the link between electricity and
magnetism
Faraday revealed that electrical and magnetic
energy could be interchanged
Magnetism
Magnetic lines of force
around a magnet Opposite force attract
Magnetic Particle Inspection
magnetic flux leakage
Test method for the detection of surface and sub-
surface indications in ferromagnetic materials
Only suitable for ferrous material (iron content)
Not suitable for non ferrous material (non iron )
Used AC & DC current
AC-surface identify , DC- sub surface identify
Defects – crack & porosity
Depth – 3 to 6mm
Basic principle
Magnetic flux leakage :
When the magnetic field is created with in ferro
magnetic specimen, lines of flux are developed and
flow through around the material and completing
circuit.
If there is any abrupt change in contour magnetic
flux is disrupted and poles are produced at he point
of disruption is known as flux leakage.
Basic Principle
Advantages
Mpi is sensitive means of locating small and shallow
surface crack in ferromagnetic material.
If the discontinuity is fine, sharp and close to the
surface, a clear indication can be produced.
If the indication is deeper, the indication is less
distinct.
Limitation
This method can be used only on ferromagnetic
material.
Non ferrous metal cannot be inspected by MPI such as
Aluminium, copper and titanium.
Demagnetization is always necessary.
Care is necessary to avoid local heating of finished
part.
Magnets
N S
Lines of force / Lines of flux
Lines of force or Magnetic flux lines
Magnetic lines existing in a magnetic circuit called
Magnetic flux lines
By convention they flow from North to South outside
and South to North inside
Form closed loops
Never cross
Field is strongest where most numerous
Follow path of least resistance
Definitions
Magnetic field Region in which
magnetic forces exist
• Flux Total number of lines
existing in a magnetic
circuit
• Flux Density Magnetic flux per unit
area (measured in
Teslas)
Definitions
Magnetic Poles:
Poles are the point near each end of the a magnet
where magnetism appears to be concentrated. The
ability of magnet to attract or repel iron is not
uniform over the surface but concentrated at a
localized area is known as poles.
Magnetic field is more dense at the poles.
Like poles repel unlike poles attract.
Other Forms of Magnet
Horseshoe Circular Magnet
Magnet
N S
Electromagnetism
A current flows through a conductor and
sets up a magnetic field around it
Field is at 90o to the direction of the
electrical current
Direction
of current
flow
Direction of magnetic field
Coil Magnetisation
• Changes circular filed into longitudinal
• Increases the strength of the field
Coil Magnetisation
• Changes circular filed into longitudinal
• Increases the strength of the field
Principle of MPI : Flux Leakage
No Defect Defect
N S N S
Lines of flux follow the path of least resistance
Visibility of Flux Leakage
Depth of defect
Orientation of defect shape of defect
Size of defect
Permeability of material
Depth below surface
N S N S
Defect Orientation
Defect at 90 degrees to flux : maximum
indication
Defect Orientation
>45 Degrees to Flux: Acceptable
indication
Defect Orientation
<45 Degrees to Flux : Unacceptable
indication
Defect Orientation
Test 1 Test 2
Defects
FIELD Defects Defects
Defects
MPI requires 2 tests at 90o to one another
NB All surface defects form indications
But not all indications are caused by defects
Non-relevant indications Spurious indications
Due to flux leakage but Not due to flux leakage
arising from design Lint
features Scale
• Changes in section Dirt
• Changes in Hairs
permeability Magnetic writing
• Grain boundaries
• Forging flow lines
Permeability of Material
Paramagnetic: Weakly attracted by magnets
Examples Aluminium, Tungsten
• Diamagnetic: Slightly repelled by magnets
• Examples Gold, Copper, Water
• Ferromagnetic: Strongly attracted
• Examples Iron, Cobalt, Nickel
Permeability
The ease with which a material can be
magnetised
Opposite of reluctance (difficulty with
which a material can be magnetised)
Represented by the symbol
=B/H
Measured in Henries / metre
Permeability
Paramagnetic Slightly > 1
Diamagnetic Slightly < 1
Ferromagnetic 240 +
Susceptibility
Magnetic susceptibility is defined as the degree of
magnetization of a material in response to an applied
magnetic field
If magnetic susceptibility is positive then the material
can be paramagnetic, ferromagnetic.
Definition
Magnetization :
It is a vector field that express the density of induced
magnetic dipole moment in a magnetic material.
Reluctance:
It is a measure of resistance offered by the material
to the applied magnetic field, measured in inverse
henries.
Definition
Residual magnetism:
It is the amount of magnetization left behind in a
ferromagnetic material after an magnetic field is
removed.
Retentivity:
Ability of the material to retain certain amount of
residual magnetism.
High permeability = Low retentivity
Low permeability = High retentivity
Definition
Coercive Force:
It is related to reverse magnetizing force which is
necessary to remove residual magnetism for
demagnetization of a part.
Coercivity is also known as coercive force.
