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1. A suitable technique of magnetization is chosen.
2. The part is magnetized in the desired direction.
3. While the magnetizing field is present, iron powders
are applied dry or wet.
4. Iron powders bridge the leakage fields, if present, to produce
indications.
5. Powder patterns produced by flaws are identified and
accept reject decision is made.
we have an electromagnetic coil with a high field strength due to the current flowing through it, and that the ferromagnetic core material has reached its saturation point, maximum flux density. If we now open a switch and remove the magnetising current flowing through the coil we would expect the magnetic field around the coil to disappear as the magnetic flux reduced to zero.
However, the magnetic flux does not completely disappear as the electromagnetic core material still retains some of its magnetism even when the current has stopped flowing in the coil. This ability for a coil to retain some of its magnetism within the core after the magnetisation process has stopped is called Retentivity
Surface discontinuities in a magnetized ring produce leakage
fields :
A crack at the surface of a magnetized ring produces leakage field which
attracts and accumulate ferromagnetic particles.
Advantages Of MPT:
1.Detects flaw below ¼” below surface.
Limitation of MPT :
1.Applicable to only ferromagnetic material only.
2.Limited depth of detection.
3.Depth & size is not indicated.
4.Sensitivity varies with surface roughness & position.
5.Sensitivity rapidly diminishes with depth.
Detection of leakage field :
Finely divided iron particles are used for detection of leakage field. Leakage field at the surface collects and hold the iron particles together to form visible indication patterns.
Discontinuities at or near the surface produce magnetic leakage field on the surface :
Magnetic flux in a magnetized part pass through the body of the part. Discontinuities at or near the surface of the part disturb the normal flow of magnetic flux and leakage flux fields are produced on the surface above the discontinuities. These leakage fields attract iron particles and are detected by applying iron powders.
What is magnetic particle testing?
This method is used for the detection of surface and near-surface flaws in ferromagnetic materials and is primarily used for crack detection. The specimen is magnetised either locally or overall, and if the material is sound the magnetic flux is predominantly inside the material. If, however, there is a surface-breaking flaw, the magnetic field is distorted, causing local magnetic flux leakage around the flaw. This leakage flux is displayed by covering the surface with very fine iron particles applied either dry or suspended in a liquid. The particles accumulate at the regions of flux leakage, producing a build-up which can be seen visually even when the crack opening is very narrow. Thus, a crack is indicated as a line of iron powder particles on the surface.
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