MPT Magnetic Particle testing
This method of inspection is used
on magnetic ferrous castings for detectin g invisible surface or slightly
subsurface defects. Deeper subsurface defects are not satisfactorily detected
be cause the influence of the distorted lines of magnetic flux (o wing to a
Discontinuity) on the magnetic particles spread over the casting.
The defects commonly reveale d by
magnetic particle inspection are quenching cracks, overlaps, thermal cracks,
seams , laps, grinding cracks, fatigue cracks, hot tears Etc,
Working Principle.
When a piece of metal is place in
a magnetic field and the lines of magnetic flux get intersected by a
discontinuity such as a crack or slag inclusion in a casting, magnetic poles
are induced on either side of the discontinuity. The discontinuity causes an
abrupt change in the path of magnetic flux flowing through the casting normal
to the discontinuity, resulting in a local flux leakage field and interference
with the magnetic lines of force. This local flux disturbance can be detected
by its effect upon magnetic particles which are attracted to the region of
discontinuity and pile up and bridge over the discontinuity.
A surface crack is indicated
(under favorable conditions) by a line of fine particles following the crack
outline and a subsurface defect by a fuzzy collection of the magnetic particles
on the surface near the discontinuity. Maximum sensitivity of indication is
obtained when the discontinuity lies in a direction normal to the applied
magnetic field and when the strength of magnetic field is just enough to saturat e the section being inspect
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