Magnetic Particle Inspection (MPI) - The Basics
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Magnetic particle inspection (MPI) is a non-destructive testing (NDT) method that is used primarily for detecting surface or near-surface defects in ferromagnetic materials.

The technique emerged first emerged in the 1920s and followed the recognition of X-ray technology as a viable testing method for inanimate materials. Radiography had been found to be extremely useful for detecting defects that were below the surface or deeper inside the component, but industry needed a reliable and cost-effective solution for locating surface flaws in large quantities of machine and structural parts - and hence MPI was developed.

MPI is now one of the most widely used NDT methods and is used to inspect a variety of products across many different industries including mining, structural steel, automotive, marine, aerospace, underwater inspection, power generation and oil & gas.

The basic test method of MPI involves the application of magnetic field either externally or by applying electric current through the material under test, and then applying very fine magnetic particles (eg iron oxide particles between 0.125 and 60 microns) to the test surface either in a dry powder form (non fluorescent) or in a liquid containing a magnetic powder suspension. The liquid methods can either be fluorescent or non-fluorescent.

In the dry method, particles that have been chemically dyed to provide a clear contrast on the test surface are applied with a squeeze bulb or spray gun. The dry method gives improved sensitivity for detecting sub-surface flaws.

In the wet method, the particles are suspended in an oil or water-based liquid. When the wet method is used, operators need to add conditioners as well as a wetting agent and corrosion-inhibitors. The wet method gives improved sensitivity for detecting very fine surface flaws.

Flux lines of the magnetic particles will distort at the site of a surface or near sub-surface discontinuity, and this distortion in the induced magnetic field causes leakage of the magnetic fields. The magnetic particles are attracted by the surface field and adhere to the edges of the defect, giving a visible indication of its shape and size.

Here are some of the basics about MPI testing.

It is only effective on ferromagnetic materials which include most of the iron, nickel and cobalt alloys. It cannot be used on non ferrous parts such as austenitic stainless steel, brass, copper, aluminium, titanium or any of the precious metals.
It is used to locate fine, shallow surface discontinuities which are invisible to the naked eye, without causing any harm to the component itself.
It is quick, relatively uncomplicated and highly sensitive.
Surface preparation is not as critical as for some other NDT methods.
It can be done either in a workshop or on site on items that are in service.
Both large and small objects can be examined using MPI.
It is inexpensive compared to other more sophisticated quality assurance methods such as radiography.
It is used to test for linear indications (ie those which are at least three times as long as they are wide L=3 x >W).
In some instances, MPI can also work through layers of contaminants and/or coatings.
It is highly effective for testing high volumes of products at speed.

MPI does have limitations however. As mentioned earlier, it can only be used on ferrous materials and to be fully effective, the direction of the magnetic field has to intercept the major dimension of the discontinuity. It is not always effective on parts that have complex surface geometry.
Care must be also taken to avoid burning or arcing of the surface of the test part at the points of electrical contact and it is often necessary to demagnetise the test part once inspection is complete.

It is crucial that the inspectors know the advantages and disadvantages of each method as well as the different types of magnetising currents that can be used, and it is advantageous to source technical equipment from a supplier who understands the industry so that the right choices are made from the outset.

Nexxis is a preferred supplier of NDT and remote visual inspection solutions to all major industries in Australia, with a reputation for delivering cost-effective and tailored solutions. The Nexxis team works closely with clients to find the most relevant and practical testing solutions which add value to their bottom lines, and if you are interested in benefiting from their genuine commitment to customer service, give them a call on 08 9418 4952 or visit nexxis.com.au.