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  • non destructive services
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non destructive services

magnetic particle inspection

Magnetic Particle Inspection (MPI or MT) processes are non-destructive testing methods for the detection of surface and sub-surface defects in ferrous materials. These use an externally applied magnetic field or electric current into the material, and the principle is, that the magnetic flux will break or leave the part at the area of the flaw.

The presence of a surface or near surface flaw (defect) in the material causes distortion in the magnetic flux , which in turn causes leakage of the magnetic fields at the flaw. This deformation of the magnetic field is not limited to the immediate locality of the defect but extends for a considerable distance; even through the surface and into the air if the magnetism is intense enough. The size of the distortion is much larger than that of the defect and is made visible at the surface of the part by means of the tiny particles that are attracted to the leakage fields.

The most common method of magnetic particle inspection uses tiny iron filings or magnetic iron oxide particles, held in suspension in a suitable carrier liquid and background paint (white). This applied prior to the application of the iron filings and during magnetism. For fluorescent Magnetic particle inspection, particles are colored and usually coated with fluorescent dyes that are made visible with a hand-held ultraviolet (UV) light. The suspension is sprayed or painted over the magnetized specimen during magnetization with a direct current or with an electromagnet, to localize areas where the magnetic field has protruded from the surface. The magnetic particles are attracted by the surface field in the area of the defect and hold on to the edges of the defect to reveal it as a buildup of particles.

There are two different ways of magnetizing a part, Longitudinal and Circular magnetization. Longitudinal Magnetization passes current through a coil and the magnetic flux lines go through the part. Circular magnetization passes current through the part and establishes a magnetic field around the part. The two different methods are used because cracks can only be seen 45 to 90 degrees to the magnetic flux lines.

Magnetic particle inspection cannot be used for non-ferrous materials and non-magnetic ferrous materials such as austenitic stainless steels. In such cases, other methods such as dye penetrant inspection are used.


news at wis

WIS have implemented and are now accredited to the following: EN ISO 9001:2015, EN ISO 14001:2015 and EN ISO 45001:2018.

WIS have attended courses and examinations at QSA-Global in Houston, Texas and now have technicians trained and qualified in Servicing & Maintenance of the QSA-Global equipment. WIS also have technicians trained and qualified in Source Retrieval, this entailed the actual retrieval of a live source, which other companies providing this service do not carry out. WIS intend to roll out these services to NDT Contractors nationwide.