MPI:
Magnetic Particle Inspection (MPI) also known as Magnetic Test (MT) is a Non-destructive test method to detect surface and sub-surface discontinuities in Ferromagnetic Materials.
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A Ferromagnetic Material is the materials that can be magnetized or strongly attracted by a magnetic field.
How Magnet Particle Examination Works?
Magnetic Particle Inspection is a relatively simple concept. 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 exists or enters the magnet is called pole. A pole where a magnetic line of force exits the magnet is called North pole and a pole where a line of force Enters the magnet is called South pole.
When a Magnet bar is broken into two pieces, each piece will become a Magnet bar with its own north and south pole. If the magnet is just cracked but not broken, a north and south pole will form at each edge of the crack. as the magnetic field exists the north pole and renters at the south pole. The magnetic field spreads out when it encounters the small air gap created by the crack because the air can't support as much magnetic field as the magnet can. When the field spreads out, it Appears to leak out of the Material and, thus is called a flux leakage field.
If the magnet is cracked, and iron particles are sprinkled on a cracked magnet, the magnetic flux will leak out of the magnet. as it leaks, the magnetic flux will collect ferromagnetic particles (i.e. Iron powder), making the size and shape of the discontinuity easily visible.
If the Magnet is defect-free, it will transfer lines of magnetic flux through the material without any interruption.
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 line of magnetic flux, there will be no leakage and therefore no Indication observed. to resolve this issue, each area needs to be examined twice. The second examination needs to be perpendicular to the first so discontinuities in any direction are detected.
Magnetize the Component:
The first step in Magnetic Particle Inspection is to Magnetize the component that is to be inspected. If any defects on or near the surface are present, the defects will create a leakage field. After the component has been magnetized, iron particles (either in a dry or wet suspended form) are applied to the surface of the magnetized part. The Particles will be attracted and cluster at the flux leakage fields, thus forming a visible indication that the inspector can detect.
Advantages of MPI:
Magnetic Particle testing is very popular, low-cost method to perform Non-destructive testing (NDT) of ferromagnetic material. It is one of the most widely utilized NDT methods since it is fast and relatively easy to use. Some advantages of using Magnetic particle examination are-
Disadvantages of MPI:
» We can't Inspect Non-ferrous Materials, such as Aluminium, magnesium, stainless steels etc.
» Only small sections or small parts can be examined at one time.
» It may require removal of coating or painting to make inspection.
» Each part needs to be examined in two different directions.
» Alignment between magnetic flux & indications is important.
» Post-demagnetization is often necessary.
In this tutorial, we discuss how the MPI technique work exactly. in next tutorials, we will look at some useful MPI techniques. so stay connected, and enjoy your learning.