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Magnetic Particle Testing (MT)
Used for finding surface/near surface defects in ferromagnetic material, Magnetic Particle testing is a versatile inspection method used for field and shop applications. Magnetic particle testing works by magnetizing a ferromagnetic specimen using a magnet or special magnetizing equipment. If the specimen has discontinuity, the magnetic field flowing through the specimen is interrupted and leakage field occurs. Finely milled iron particles coated with a dye pigment are applied to the specimen. These are attracted to leakage fields and cluster to form an indication directly over the discontinuity. The indication is visually detected under proper lighting conditions.
The basic procedure that is followed to perform magnetic particle testing consist of the following:
1. Pre-cleaning of component
2. Introduction of Magnetic field
3. Application of magnetic media
4. Interpretation of magnetic particle indications
It is essential for the particles to have an unimpeded path for migration to both strong and weak leakage fields. Therefore, the component in question should be clean and dry before beginning the inspection process. The presence of oil, grease or scale may compromise the inspection. The introduction of the magnetic field can be introduced a number of ways including use of a permanent magnet, flowing of electrical current through the specimen or flowing an electrical current through a coil of wire around the part or through a central conductor running near the part. Two types of magnetic fields can be established within the specimen. These are a longitudinal magnetic field that runs parallel to the long axis of the part or a circular magnetic field that runs circumferentially around the perimeter. Longitudinal magnetic fields are produced using a magnetic coil or a permanent magnet called a magnetic particle yoke. Circular magnetic fields are produced by passing current through the part or by placing the part in a strong circular magnetic field.
Magnetic particle inspection can use either wet or dry magnetic media. The dry method is more
portable, while the wet method is generally more sensitive since the liquid carrier gives the magnetic particles additional mobility.
Indications that are formed after applying the magnetic field must be interpreted by a skilled inspector. This requires the individual to distinguish between relevant and irrelevant indications.
The following are the advantages of magnetic particle inspection:
Can detect both surface and near subsurface indications
Can inspect parts with irregular shapes easily
Pre-cleaning is not as critical as for some other inspection methods
Fast method of inspection and indications are visible directly on the specimen surface
Considered low cost compared to many other NDT techniques
Very portable inspection especially when used with battery powered equipment