Magnetic particle test.

MAGNETIC PARTICLE TEST


AIM OF THE EXPERIMENT:-To Perform Magnetic Particle Test.

APPARATUS REQUIRED:-Specimen, Dry magnetic powder, Yokes, Magnetic Particle Testing Machine.

PRINCIPLE:- This method involves the magnetisation of an area to be examined and the application of ferromagnetic particles to the surface.  The particles gather at areas of magnetic flux leakage and form indications characteristic of the type of discontinuity detected.

In theory, magnetic particle inspection (MPI) is a relatively simple concept. It can be considered as a combination of two nondestructive testing methods: magnetic flux leakage testing and visual testing. Consider the case of a bar magnet. It has a magnetic field in and around the magnet. Any place that a magnetic line of force exits or enters the magnet is called a pole. A pole where a magnetic line of force exits the magnet is called a north pole and a pole where a line of force enters the magnet is called a south pole.

How Magnetic Particle Examination Works

When ferromagnetic material  is defect-free, it will transfer lines of magnetic flux through the material without any interruption.

But when a crack or other discontinuity is present, the magnetic flux leaks out of the material. As it leaks, magnetic flux  will collect ferromagnetic particles

However, the magnetic flux will only leak out of the material if the discontinuity is generally perpendicular to its flow. If the discontinuity, such as a crack, is parallel to the lines of magnetic flux, there will be no leakage and therefore no indication observed.

PROCEDURE:-
1.Clean the surface to be examine.
Introduce magnetic field into parts.
2.Apply ferromagnetic medium while magnetizing.
3.Remove excess ferromagnetic material.
4.Repeat the same process to get accurate value.

OBSERVATION:-


APPLICATION:-
1.To detect different type of surface diffect.
Boiler examination.
2.To determine fatigue type of discontinuities in welded sample.
3.To determine subsurface defects.

ADVANTAGES:-
1.Can detect both surface and near-surface indications.
2.Surface preparation is not as critical compared to other NDE methods.

3.Most surface contaminants will not hinder detection of a discontinuity.
4.A relatively fast method of examination.
Indications are visible directly on the surface.
5.Low-cost compared to many other NDE methods.

DISADVANTAGES:-
1.Non-ferrous materials, such as aluminum, magnesium, or most stainless steels, cannot be inspected.
2.Examination of large parts may require use of equipment with special power requirements.
3.May require removal of coating or plating to achieve desired sensitivity.
4.Limited subsurface discontinuity detection capabilities.
5.Post-demagnetization is often necessary.

PRECAUTION:-
1.Dry magnetic particle should distributed all over the surface.
2.Specimen should be clean before experiment.

REFRENCES:-
1-http://www.nationalboard.org/index.aspx?pageID=164&ID=377
2-http://www.inspection-for-industry.com/magnetic-particle-inspection.html
3.http://mech.vub.ac.be/teaching/info/Damage_testing_prevention_and_detection_in_aeronautics/PDF/magnetic.pdf