Magnetic Particle Testing (MT) (also known as magnetic particle inspection – MPI) is a non destructive test (NDT) method, used to detect surface and subsurface (near to surface) discontinuities. This NDT method can be used on metals which can be easily magnetized (ferromagnetic). Metals can be classified as ferromagnetic, paramagnetic, or diamagnetic. Only ferromagnetic metals can be effectively inspected by Magnetic Particle Testing.

Principle of MT

This method uses the phenomenon of a magnet by virtue of which path of magnetic lines of force gets disrupted when a surface breaking is occurred in the path. Any magnet has two poles i.e. north pole at one end and south pole at the other. Magnetic lines of force (also known as magnetic flux) travel from north pole to the south pole, as shown in the below figure (Fig. 1). If there is any distinct change in the continuity (such as surface discontinuity) of magnet, then the magnetic lines of force will get distorted, this phenomenon is known as flux leakage. During flux leakage, additional North and South poles will be created near discontinuity (Fig. 2). When fine Iron particle (ferromagnetic particle) is spread over a magnet, it gets accumulated at the poles. But in case of any discontinuity, flux leakage would occur and the Iron powder will accumulate at the discontinuity, as shown in Fig. 3. The accumulation of iron particle at the discontinuity would produce a visible indication of that discontinuity.

Particles used for Magnetic particle testing is similar to ferromagnetic particle and is called as detection media. These particles may be applied in dry form or may be mixed with liquid and spread over the area where Magnetic particle test has to be performed. Liquid like kerosene or a similar petroleum distillate may be used. Water can also be used by using suitable additives such as wetting agents and antifoam liquids. To provide better contrast with the test objects and enhanced sensitivity, these particles are coated, there are two types of coating;

1. Color contrast coating.Color contrast coating: Color contrast coatings are available in several colors such as red, black etc. Color of particle are selected so as to provide a good contrast with test object.
2. Fluorescent coating.Fluorescent coating: Fluorescent particles can be seen under a ‘Black light illumination’. These particles emit light when seen by a black light in a dark background. These particles provide excellent contrast in dark background.

To carry out Magnetic Particle Testing we need to temporarily magnetize the test object. Magnetization should be temporary in nature. To magnetize the test piece, common instruments which are used are;
1. Electromagnetic yoke
2. Permanent Magnet
3. Prod
4. Coil
Electromagnetic Yokes (Figure – 4)are also called as AC yokes, it’s very portable and commonly used in Industries. Yokes are connected with AC power source (Battery pack version is also available). Many yokes come with adjusting legs, to facilitate wide range of area profiles. These yokes produce longitudinal magnetization. Hence, for complete inspection, re-positioning of yokes in at least two 900 opposing direction is required.
Prod and coil method is also used for temporarily magnetizing the test object. Prods require good contact with test object otherwise it can damage the test object, due to electrical arcing. Coil method used to longitudinal product.

Before inspection, the yoke shall be properly checked by the flux direction indicators. The most common flux direction indicator used in the industries is ‘A Pie Gauge’. Other flux direction indicators are A Burmah Castrol strips and Quantitative quality indicator (QQI).

To carry out Magnetic Particle Testing, we need to follow the following activities：

1. Temporarily magnetize the test object
2. Suspend (spread) the magnetic particles (either dry or wet) on the test object
3. Inspection of test object either by black light (in case of wet fluorescent particle) in a dark area or inspection in sufficient light (in case of color particle).

Various techniques are available to carry out the first two steps (mentioned above), these are:

Wet Continuous: Particles are mixed with liquid and suspended on the test area, magnetization of test area and suspension of wet particles shall be done simultaneously. Liquids such as kerosene or petroleum distillates can be used for this purpose. Kerosene and petroleum distillates are costly, highly inflammable and possesses safety related issues. However, water is inexpensive, available in abundance and possesses no safety related issues, but it can initiate corrosion in the test object.