This strategy is utilized for the identification of surface and close surface defects in ferromagnetic materials and is essentially utilized for crack detection. The specimen is magnetized either locally or by and large, and if the material is sound the attractive transition is transcendently inside the material. Assuming, be that as it may, there is a surface-breaking flaw, the attractive field is distorted, causing local magnetic flux leakage around the imperfection. This leakage flux is shown by covering the surface with extremely fine iron particles connected either dry or suspended in a fluid. The particles gather at the regions of flux leakage, producing a build-up which can be seen visually even when the crack opening is very narrow. Consequently, a split is indicated as a line of iron powder particles on the surface.
The technique can be to all metals which can be strongly magnetized – ferritic steels and irons, yet not for the most part austenitic steels.
The strategy for magnetization must create an attractive field with lines of force at a large angle to the expected direction of the cracks to be identified, so it is regular to apply the magnetization
more than once in various ways, for instance in two headings commonly at right-points, however techniques for swinging the field bearing during polarization are accessible.
The magnetization may be produced by any of the following methods
- Applying an electromagnet to the surface (magnetic flow)
- Passing a large current through specimen
- By means of current prods (current flow)
- introducing the specimen inside a current-carrying coil
- Forming a coil inside a specimen
- Tie a thread along a current-carrying bar through a hollow specimen
The electric flow utilized might be DC or AC of any waveform, however, the flow required to deliver satisfactory charge relies upon the waveform of the supply, the magnetization strategy utilized, and the material of the specimen. It is most imperative to guarantee that the current utilized is right for the specimen size and shape and furthermore that the course of the attractive transition delivered is appropriate for the cracks expected.
By using a combination of two magnetic fields, a rotating magnetic flux can be produced, which will detect a crack in any orientation.
Normally the iron particles – dry powder or suspended in a fluid are connected while the magnetizing current is as yet streaming, yet residual magnetization is some of the time utilized when the particles are applied after magnetizing. A few steels hold adequate magnetization for this strategy to be acceptable, and for this situation littler, progressively versatile, magnetizing equipment can be utilized. Magnetic fluids (particles suspended in a fluid) are utilized more generally than dry powders. They are connected by low-pressure spray, dipping or brushing; it is imperative to utilize a lot of magnetic fluid and to enable time for the particles to stream over the surface and move to any breaks. On dark surfaces, a dainty layer of white paint can be connected to give a higher contrast indication.
Fluorescent particles, which require UV light, are broadly utilized and coloured particles are likewise accessible. The indications of cracks can be safeguarded by photography or by the utilization of peel-off transparent film. MPI techniques can be applied to generally rough and dirty surfaces, yet the defect affectability might be weakened. Magnetic methods for submerged applications have been created. Just under extremely exceptional conditions can sub-surface defects be recognized by MPI.