Magnetic Particle Testing (MPT), also referred to as Magnetic Particle Inspection, is a nondestructive examination (NDE) technique used to detect surface and slightly subsurface flaws in most ferromagnetic materials such as iron, nickel, and cobalt, and some of their alloys. Because it does not necessitate the degree of surface preparation required by other nondestructive test methods, conducting MPT is relatively fast and easy. This has made it one of the more commonly utilized NDE techniques.
MPT is a fairly simple process with two variations: Wet Magnetic Particle Testing (WMPT) and Dry Magnetic Particle Testing (DMPT). In either one, the process begins by running a magnetic current through the component. Any cracks or defects in the material will interrupt the flow of current and will cause magnetism to spread out from them. This will create a “flux leakage field” at the site of the damage.
The second step involves spreading metal particles over the component. If there are any flaws on or near the surface, the flux leakage field will draw the particles to the damage site. This provides a visible indication of the approximate size and shape of the flaw.
There are several benefits of MPT compared to other NDE methods. It is highly portable, generally inexpensive, and does not need a stringent pre-cleaning operation. MPT is also one of the best options for detecting fine, shallow surface cracks. It is fast, easy, and will work through thin coatings. Finally, there are few limitations regarding the size/shape of test specimens.
Despite its strengths, the method is not without its limits. The material must be ferromagnetic. Likewise, the orientation and strength of the magnetic field is critical. The method only detects surface and near-to-surface defects. Those further down require alternative methods. Large currents are sometimes required to perform this method, thus “burning” of test parts is sometimes possible. In addition, once MPT has been completed, the component must be demagnetized, which can sometimes be difficult.
An MPI inspection comprises several steps:
- Pre-cleaning
- Demagnetisation, if necessary
- Application of a background contrast paint if necessary
- Magnetisation
- Application of magnetic particle powders or inks
- Inspection of surfaces for indications of flaws
- Demagnetisation, and re-magnetisation by another method if necessary
- Recording flaws, if any
- Demagnetisation, if necessary
- Cleaning and protecting
There are several methods of magnetic particle inspection (MPI). Site methods, utilising portable equipment, include:
- Permanent magnet
- Electromagnetic Yoke
- Current flow probes
- Flexible coil
- Adjacent cable
Bench, or fixed, installations in a factory include:
- Current flow
- Threading bar
- Magnetic flow
- Fixed coil
- Induced current (as a specialised option)
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