MAGNETIC PARTICLE TESTING - MPI or MT

Name: Steel City NDT
Address: 322 Mall Blvd. Suite 270, Monroeville, PA, 15146
Telephone: 724-212-7322

Magnetic Particle Testing is a process for detecting surface and shallow subsurface anomalies in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys. MT can be considered a combination of two testing methods; Visual Testing and Magnetic Flux Leakage.

The testing process involves introducing a magnetic field into the test object. Ferrous iron particles (i.e., iron filings) are then applied to the test object in dry form or a wet suspension and will gather at any point where there is flux leakage.

Please follow this link to learn more about Dry Visible Magnetic Particle Testing

Please follow this link to learn more about Fluorescent Magnetic Particle Testing

METHODS OF MAGNETIC PARTICLE TESTING

MPI utilizes either a Direct Current (DC) or Alternating Current (AC). Alternating current is most commonly used to detect surface discontinuities. Direct current is used to detect both surface and subsurface discontinuities. Direct current, however, is limited on very large cross-sectional parts in terms of how well the part is able to be magnetized. Magnetic Particle Inspection can also be performed by utilizing several different types of equipment which includes:

Magnetic Yoke – Handheld device that induces a magnetic field between the two arms on the yoke. This is the most commonly used device for on-site MPI. While performing the inspection, the yoke is rotated 90 degrees for each inspection area to detect both transverse and longitudinal flaws.

Magnetic Benches – Mass production inspection machine that contains a head and tail stock, which has an induction coil centered between them. These are typically customized to a specific application.

Magnetic Prods - Prods are handheld electrodes that are pressed against the surface of the part being inspected to make contact for passing electrical current through the metal. The current passing between the prods creates a circular magnetic field around the prods.

Mobile Power Packs – Custom-built magnetizing power supplies which are used in wire wrapping applications such as coils.

Magnetic Particle Inspection Blog

What is the Basic Principle of Magnetic Particle Testing?

To properly understand what Magnetic Particle Testing/Inspection is, it helps to be familiar with two processes: magnetic flux leakage testing and visual testing. Both of these types of testing play a role into MPI, focusing on the example of a magnet.

Any instance where a magnetic line of force enters and exits a magnet is called a pole. When a magnet is broken in the center, two magnets with magnetic poles on each end of each piece will result. If the magnet is just cracked but not broken completely in two, a north and south pole forms at each edge of the crack.

When the ferrous iron particles are applied to a cracked magnetized test piece, for example, the particles will attract to the poles at the ends of the crack, and also at the poles at the edges of the crack. This will cause a grouping of the iron particles making the defect highly visible to the inspector.