Mumbai University > Mechanical Engineering > Sem 3 > Production Process 1

Marks: 5M

Year: May 2016

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.

When a bar magnet is broken in the center of its length, two complete bar magnets with magnetic poles on each end of each piece will result. If the magnet is just cracked but not broken completely in two, a north and south pole will form at each edge of the crack. The magnetic field exits the north pole and reenters at the south pole. The magnetic field spreads out when it encounters the small air gap created by the crack because the air cannot support as much magnetic field per unit volume 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 iron particles are sprinkled on a cracked magnet, the particles will be attracted to and cluster not only at the poles at the ends of the magnet, but also at the poles at the edges of the crack. This cluster of particles is much easier to see than the actual crack and this is the basis for magnetic particle inspection.

The first step in a 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:

• Can detect both surface and near-surface indications.
• Surface preparation is not as critical compared to other NDE methods. Most surface contaminants will not hinder detection of a discontinuity
• A relatively fast method of examination.
• Indications are visible directly on the surface.
• Low-cost compared to many other NDE methods.
• A portable NDE method, especially when used with battery-powered yoke equipment.
• Post-cleaning generally not necessary.
• A relatively safe technique; materials generally not combustible or hazardous.
• Indications can show relative size and shape of the discontinuity.
• Easy to use and requires minimal amount of training.

Disadvantages:

• Non-ferrous materials, such as aluminum, magnesium, or most stainless steels, cannot be inspected
• Examination of large parts may require use of equipment with special power requirements.
• May require removal of coating or plating to achieve desired sensitivity.
• Limited subsurface discontinuity detection capabilities.
• Post-demagnetization is often necessary.
• Alignment between magnetic flux and indications is important.
• Each part needs to be examined in two different directions.
• Only small sections or small parts can be examined at one time.