Magnetic Particle Testing

Magnetic Particle Testing is an inspection method with which defects on the surface or close to the surface (to a depth of approx. 2 mm) in a ferro-magnetic material can be traced.

It uses the principle that a discontinuity on the surface causes a magnetic leakage when apply a magnetic field to the object. A crack will be made visible by an interruption of this field.

The indications that are found are judged by their nature, size and location with the aid of an inspection standard. The shape or dimensions of the object do not generally limit it.

Here’s a detailed look at two common types of Magnetic Particle Testing techniques:

a) Electromagnetic Yoke Type

The electromagnetic yoke method uses a portable yoke to create a magnetic field in the test material.

  • Operation:
    • The yoke consists of an electromagnetic coil wrapped around a U-shaped core.
    • When energized, it creates a strong magnetic field between the poles of the yoke.
    • The yoke is placed on the material’s surface, and the magnetic field flows between the poles, magnetizing the area.
    • Dry or wet ferromagnetic particles are then applied to the surface.
    • If there is a flaw, the magnetic particles will accumulate at the site of the disruption, indicating its presence.
  • Applications:
    • Ideal for detecting surface and slightly subsurface flaws.
    • Commonly used in weld inspections, structural steel testing, and pipeline examinations.
    • Portable and easy to use in various field conditions.

b) Prod Type

The prod method involves using two electrically energized prods to induce a magnetic field in the test material.

  • Operation:
    • Two prods are placed in contact with the surface of the material.
    • An electric current is passed through the prods, creating a localized magnetic field around the area.
    • Ferromagnetic particles are then applied to the surface.
    • Flaws disrupt the magnetic field, causing the particles to gather and form visible indications.
  • Applications:
    • Suitable for larger, irregularly shaped, or localized areas where other methods may not be practical.
    • Often used in applications where the surface is not uniform or where precise, localized testing is required.
    • Common in inspections of large castings, forgings, and weldments.