Magnetic Particle Inspection (MPI)

Magnetic Particle Inspection or MPI is a process for detecting surface and subsurface anomalies in ferromagnetic materials such as iron, nickel, and cobalt. The process involves putting a magnetic field into the test object. Flaws will be detected when the magnetic field flowing through the test object is interrupted and there is flux leakage. Ferrous iron particles are then applied to the object in a dry or wet suspension and will be attracted to the flux leakage. This step-by-step process includes Pre-cleaning of test object (to ensure it is free of rust, dust, oil, and grease), introduction of magnetic field, application of ferrous iron particles, and

interpretation of indications. MPI testing methods offered by Steel City NDT LLC include:

  • Dry Magnetic Particle – Iron Oxide
  • Wet Fluorescent Magnetic Particle (oil or water based) – Iron Oxide
  • Wet Fluorescent Magnetic Particle Inspection and Test (Crack)
  • Wet Fluorescent Magnetic Particle Test (Crack)

Magnetic Particle Inspection (MPI) also utilizes either a Direct (DC) or Alternating (AC) current. 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 (MPI) can also be performed by utilizing several different pieces of equipment. Equipment includes:

  • Wet Horizontal MPI Machine – 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.
  • Mobile Power Packs – Custom-built magnetizing power supplies which are used in wire wrapping applications such as coils.
  • Magnetic Yoke – Handheld device that induces a magnetic field between the two prods on the yoke. This is the most commonly used device for on-site Magnetic Particle Inspection (MPI). While performing the inspection, the yoke is rotated 90 degrees for each inspection area to detect both horizontal and vertical discontinuities.
  • Wet Fluorescent Magnetic Particle Inspection and Test (Crack)
  • Wet Fluorescent Magnetic Particle Test (Crack)
  • The information on this page is not all inclusive of the method but is rather a high-level overview of our processes.

What is the Basic Principle of Magnetic Particle Testing?

To properly understand what Magnetic Particle Testing/Inspection (MPT/I) 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 MPT, focusing on the example of a magnet.

For example, using a generic bar magnet implies that there is a magnetic field surrounding it. From this, we can define a pole as a place where the magnetic energy making up the field enters or exits the magnet, giving the two options of a north pole or a south pole.

No matter how many times you break up this magnet in half, there will be an infinite number of north and south poles that can be created due to each separate object retaining the characteristics that make it a magnet. However, if you were to crack a magnet without fully breaking it, this would spread out the magnetic field due to what is called a flux leakage field, a place where the air leaking through the magnetic field cannot support the weight of the field.

A way to address flux leakage fields is to sprinkle iron particles on top of a magnet, as the particles will gravitate towards the crack in addition to the North and South Poles. This is where MPT is introduced, as it is a type of testing that involves doing just that to examine if there are any defects on a magnet’s surface.

What is the Magnetic Particle Inspection Procedure?

MPI is a way to test the quality of the surface in ferromagnetic objects made up of cobalt, nickel, iron, and similar materials. It is known for being a very intuitive process, not requiring much preparation beforehand and only having two variants: Wet Magnetic Particle Testing (WMPT) and Dry Magnetic Particle Testing (DMPT). Though they’re different processes, they begin with the same premise: a magnetic current is run through the component with the goal of showing any defects in the material.

A defect can be noticed because when the current runs through it, instead of just passing it on, it will deflect it outward. After this is done, metal particles can be spread over it with the goal of them gravitating naturally towards the place where the gap is in the material.
If done properly, you’ll be able to find all of the gaps within the material pretty easily. This is why MPI is used so often, as it is the quickest way to detect flaws within any given materials.

Using MPI isn’t always possible, though, as a material will not work with the procedure if it is not ferromagnetic. In addition to this, the magnetic field being tested must also be strong on its own, or else the aforementioned particles won’t distribute themselves properly. Once you do MPI on a specific material, it must be demagnetized, making it an unappealing process for certain situations.