1. The principles of Magnectic particle inspection:

It depends on the fact that when the material or part under test is magnetized, magnetic discontinuities that lie in a direction generally transverse to the direction of the magnetic field will cause a leakage field to be formed at and above the surface of the part.

Picture 1: A leakage field on surface of the discontinuity.

The presence of this leakage field, and therefore the presence of the discontinuity, is detected by the use of finely divided ferromagnetic particles applied over the surface, with some of the particles being gathered and held by the leakage field. 
This magnetically held collection of particles forms an outline of the discontinuity and generally indicates its location, size, shape, and extent.

2. The process of Magnectic Particle tesing:

- Accomplished by inducing a magnetic field in a ferromagnetic material. Surface flaws disrupt the magnetic field flow, forcing a part of the field to leak out at the surface.

Picture 1: Magnectize the ferromagnetic materials .

- Iron particles (either dry or suspended in a liquid; either visible or fluorescent) are applied to the surface.

Picture 2: Apply the iron particles on surface.

- Iron particles are attracted and concentrated at the magnetic flux leakage location producing a visible indication of flaws on the surface with white light (daylight) or black light (UV light).

Picture 3: Discontinuities with Iron particles fluorescent dyes by UV light.

3. Advantages and Disadvantages of MT::

- Advantages:

•  It is quick and relatively uncomplicated.
• It gives immediate indications of defects.
• It shows surface and near surface defects, and these are the most serious ones as they concentrate stresses.
• The method can be adapted for site or workshop use.
• Large or small objects can be examined.
• Elaborate pre-cleaning is not necessary.

- Disadvantages:

• It is restricted to ferromagnetic materials - usually iron and steel and cannot be used on austenitic stainless steel.
• Most methods need a supply of electricity.
• The method cannot be used if a thick paint coating is present.
• Spurious, or non-relevant indications, are probable, and thus interpretation is a skilled task.
• The object being tested should be demagnetized after finished the inspection.