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Magnetic particle inspection - Coggle Diagram
Magnetic particle inspection
Principles
uses magnetic particles to detect surface discontinuities in a magnetised test piece.
some circumstances sub-surface
Only materials - strongly ferromagnetic.
contrast between magnetic particles caught in the magnetic flux leakage around discontinuities and the background colour
flux leakage and the contrast depends on
A large difference in magnetic permeability between discontinuity and parent material.
Magnetic permeability
ratio of flux density (B) to applied magnetic field (H)
(air) gap which breaks the magnetic circuit
permeability of air is unity
permeability of steel 800
leakage from the surface - creating poles attract the magnetic particles.
Applications
Weld testing - in particular of fillet welds in T joints.
Underwater weld testing - of offshore structures and jetties.
Last castings - in the foundry using flexible cables and prods.
Small forged components.
Advantages/ disadvantages
Advantages
The method is simple to operate.
It is very sensitive to surface cracks.
Will detect some sub-surface defects.
Pre-cleaning not as critical as with PT.
It can be used on components with awkward shapes.
A rapid rate of inspection is attainable.
A permanent record can be made using lacquers instead of inks.
The method is suitable for difficult on site applications, for example underwater.
Disadvantages
Test materials must be ferromagnetic.
High magnetising currents may cause arcing or burning.
Magnetisation must be in at least two directions 90º to each other.
Spurious indications can make interpretation difficult.
Demagnetisation may be required.
Equipment
Electro magnets
power source required
Bench units,
longitudinal fields
circular fields
up to 10,000A.
circular fields
Prods
arcing or burning at the prod contacts
no not use - aircraft structures
flexible cables
looped to produce longitudinal fields.
permanent magnets
no power source required
standards restrict use
agreements between contracting parties
Directing medium
suspensions in
paraffin
water
dry powders
colours
Black
Red
fluorescent
Accessories
Flux indicators are used to indicate the strength and direction of the applied magnetic field. They do not indicate the flux density inside the test piece and therefore the strength of any flux leakage. Since they ignore the nature of the test material they give misleading indications on non-magnetic materials.
Field indicators will detect any residual magnetic poles that are a consequence of longitudinal magnetisation. But they do not detect residual circular fields.
Flux meters give an accurate measure of the applied magnetic field.
Demagnetisers are often necessary to remove residual magnetic fields during a magnetising sequence or before most MPI processes.
Standard test blocks - are used to assess the longitudinal and circular fields generated by a magnetising bench unit.
Sediment flasks are necessary for measuring ink concentrations.
Black lights to be used with fluorescent inks should be checked regularly with a black light monitor in accordance with BS EN ISO 3059.