Magnetic Particle Inspection

MAGNETIC MATERIALS

Classified as;  Ferromagnetic  Paramagnetic  Diamagnetic -depending on their behavior in magnetic field.

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Ferromagnetic materials

Easily magnetized and show high value of magnetic susceptibility

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Paramagnetic materials

Have magnetic permeability greater than one and small positive value of magnetic susceptibility

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Diamagnetic materials

Have magnetic permeability less than one and constant magnetic susceptibility

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Materials are magnetized by a permanent magnet or by the magnetic field produced by an electric current. Earth’s magnetic field also magnetizes materials.

Magnetization of an iron bar by a permanent magnet bar

Production of a radial field by permanent magnet bar

Production of a magnetic field by permanent magnet yoke

The choice of current depends on the; strength, direction and distribution of the desired magnetic field.  Magnetic field produced by;

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 Direct current (DC)- penetrates the cross-section

of a component  Alternating current (AC)- is largely confined to the surface of the component due to the skin effect.

In Magnetic Particle Inspection, a magnet or electrical current is used to establish a magnetic field in the object. Iron filings are then dusted on to the surface of the object. The filings should align along the magnetic lines of force. If a crack or other defect is present, the magnetic fines of force will be disrupted and the magnetic particles will cluster along the edges of the flaw.

Basic Principles

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. 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...