http://www.domson.ca/Technical_Articles/FAQ_MPI.htm

What is Magnetic Particle Inspection?
Magnetic Particle Inspection or MPI is a nondestructive testing (NDT) method that is used to detect surface or near to surface discontinuities (such as cracks) in iron or steel. This NDT method is also often referred to as "Magnafluxing."

How does it work?
As its name implies, Magnetic Particle Inspection works using magnetism. Thus it will only work on a ferromagnetic material such as iron or steel.
The part or surface to be tested is placed in a magnetic field. The field is made up of flux lines that by nature move from the north pole of a magnet to the south pole. For this to happen the flux lines can either move through the air, or they can move through the metal. It is "easier" for the flux lines to move through the metal. Thus, most of them do just that; they travel in a path that is parallel to the surface being tested.
If, there were a crack in the test surface, the path of the flux lines through the metal would be interrupted. The flux lines, in order to cross this "ditch" would be forced to "jump over" it by temporarily leaving the metal, and travel through the air, before returning to the metal. This causes what is called a flux leakage at the crack. The flux leakage causes the crack to become a mini magnet. At this point, if one were to sprinkle some iron filings over the area with the crack, the filings would immediately be attracted to the crack. They would form up along it in such a way that the filings would form the complete outline of the crack. This would be easily visible to the naked eye, especially if the iron filings were themselves red in color, and the test surface were painted white.

What are some of the advantages and disadvantages of MPI?
MPI has the following advantages:
1) It's considered to be the best method for the detection of fine, shallow surface cracks in iron and steel. Thus, it's ideally suited to look for fatigue cracks, heat cracks, and other discontinuities, in weldments, on shafts, etc.
2) It will work through thin coatings of paint. This is not true for some of the other NDT methods. Thus new paint jobs on equipment need not be marred for this method to work.
3) It is highly portable. This method can be used anywhere, such as out in the field on construction sites, and even under water.
4) It's not limited by the size and shape of the specimen being tested. Small tanks to large cranes can be inspected using this method.
5) Inexpensive. The powders and liquids used for this test method are readily available.

MPI has the following disadvantages:
1) It will only work on ferromagnetic materials such as iron or steel. For other materials such as aluminum or plastic, alternative methods such as liquid penetrant would have to be used.
2) Although it can detect problems that are close to the surface of the material, ones that are very deep in the material will not be found when using this method.

How does Domson make use of MPI?
Of all the various NDT methods, Magnetic particle inspection is the method that Domson employs the most. It is always used extensively throughout our equipment certification programmes.
MPI is used on all the critical welds on lift equipment, and other structural members. It is also used on the main bodies/sections of lifting equipment and structural steel. Examples of this include the saddle areas of lifting hooks, and the high stress corner areas of structural steels.

MPI is best suited for finding surface cracks. Why would you use it on structural steel; wouldn't you miss the internal cracks?
Lift equipment and structural steels can be subjected to continuous and/or dynamic loads (i.e. shock loads). These can cause fatigue cracks to form in the cross sections of the steel. However, whether the steel is subjected to bending or shear, or shear due to angular twisting, or any combinations of these, it is the surface of the part that is typically subjected to the highest stresses. This means that if a fatigue crack were to form on the cross section of a part, it will always form on the outer surface of the part, rather than starting somewhere in the middle or interior of the cross section. Therefore, MPI is ideally suited for finding fatigue cracks.
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Dave Weber
doublegunshop.com
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