Magnaflux
Encyclopedia
Magnetic Particle Inspection , developed originally by Alfred Victor (A.V.) de Forest and Foster Baird (Bic) Doane, and developed further with the assistance of Carl E. Betz, is a method of testing ferrous
metal
s 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 alloy
s. 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 a company owned by Illinois Tool Works. Their web site is http://www.magnaflux.com. Magnaflux specializes in non-destructive testing equipment, accessories and chemicals for Magnetic Particle Inspection, along with other product lines such as Sonotech Couplants, Zyglo and Spotcheck Liquid Penetrants, Daraclean Aqueous Cleaners and Quasar PCRT.
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.
Ferrous
Ferrous , in chemistry, indicates a divalent iron compound , as opposed to ferric, which indicates a trivalent iron compound ....
metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
s for surface and subsurface flaws. The component being tested must be made of a ferromagnetic
Ferromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
material such as iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
, or cobalt
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
, or some of their alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
s. 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
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
to the component under test using a permanent magnet. This will cause a high concentration of magnetic flux
Magnetic flux
Magnetic flux , is a measure of the amount of magnetic B field passing through a given surface . The SI unit of magnetic flux is the weber...
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
Iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...
particles. The wetted part is then placed in a magnetic field and inspected using a black light
Black light
A black light, also referred to as a UV light, ultraviolet light, or Wood's lamp, is a lamp that emits ultraviolet radiation in the long-wave range, and little visible light...
(ultraviolet light
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
). 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 a company owned by Illinois Tool Works. Their web site is http://www.magnaflux.com. Magnaflux specializes in non-destructive testing equipment, accessories and chemicals for Magnetic Particle Inspection, along with other product lines such as Sonotech Couplants, Zyglo and Spotcheck Liquid Penetrants, Daraclean Aqueous Cleaners and Quasar PCRT.
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.