In-Situ Infield Magnufluxing
Magnafluxing, developed originally by Carl E. Betz, is a method of testing ferrous metals for surface and subsurface flaws. The component being tested must be made of a ferromagnetic material such as iron, nickel or cobalt, or some of their alloys. This test is often used on industrial tools, and engine parts during maintenance inspections. It can also be used to diagnose failure, as in crash investigations.
It works by applying a magnetic field to the component under test using a permanent magnet. This will cause a high concentration of magnetic flux at surface cracks, which can be made visible by dusting iron powder or a similar magnetic material over the component.
Parts can be tested using one of two methods. The wet method consist of bathing the parts in a solution containing iron oxide particles. The wetted part is then placed in a magnetic field and inspected using a black light (ultraviolet light). The iron oxide particles are attracted to surface discontinuities or cracks, where the magnetic field is discontinuous. The particles flux around the imperfections and the patterns are visible under the black light.
The dry method is based on the same principle. Parts are dusted with iron oxide particles and charged using a yoke. The particles are attracted to the discontinuities and are visible by black light.
Magnaflux is also a trade name, used for an operating division of Illinois Tool Works that specializes in non-destructive testing equipment and chemicals.
In theory, magnetic particle inspection (MPI) is a relatively simple concept. It can be considered as a combination of two nondestructive testing methods: magnetic flux leakage testing and visual testing. Consider the case of a bar magnet. It has a magnetic field in and around the magnet. Any place that a magnetic line of force exits or enters the magnet is called a pole. A pole where a magnetic line of force exits the magnet is called a north pole and a pole where a line of force enters the magnet is called a south pole.
When a bar magnet is broken in the center of its length, two complete bar 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 will form at each edge of the crack. The magnetic field exits the north pole and reenters at the south pole. The magnetic field spreads out when it encounters the small air gap created by the crack because the air cannot support as much magnetic field per unit volume as the magnet can. When the field spreads out, it appears to leak out of the material and, thus is called a flux leakage field.
If iron particles are sprinkled on a cracked magnet, the particles will be attracted to and cluster not only at the poles at the ends of the magnet, but also at the poles at the edges of the crack. This cluster of particles is much easier to see than the actual crack and this is the basis for magnetic particle inspection.
The first step in a magnetic particle inspection is to magnetize the component that is to be inspected. If any defects on or near the surface are present, the defects will create a leakage field. After the component has been magnetized, iron particles, either in a dry or wet suspended form, are applied to the surface of the magnetized part. The particles will be attracted and cluster at the flux leakage fields, thus forming a visible indication that the inspector can detect
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