Magnetic Particle Testing

Effects of Discontinuities on Materials

SURFACE DISCONTINUITIES

The magnetic field surrounding subsurface discontinuities drops off very rapidly with distance below the surface. Magnetic particle movement is critical to the collection of contrastingly colored or fluorescent particles at a discontinuity site. Thus, it is desirable to have fluctuating magnetic fields on the surface of the test object. 
Certain metallurgical discontinuities produce much higher collections of magnetic particles than other kinds of discontinuities of the same size.
The important condition of surface cracks includes metal grainto-grain tearing induced by surface tensile forces that produce the highest amount of magnetic field disruption for a given size of discontinuity. A leakage field at an air gap in a longitudinally magnetized test object is shown in Figure. This disruption is
produced by the alignment of magnetic domains in the stretched metal crack site. For this reason, cracks in the formation state are highly detectable and normally produce sharp, well defined indications. Open cracks that have been subject to large thermal, chemical or mechanical forces may have had their magnetic field
disruption characteristics greatly or entirely reduced and may not be detectable using magnetic particle testing. 
For high detectability of forming surface cracks, magnetic particle testing uses induced alternating field magnetization techniques as the primary method to test ferromagnetic objects inservice. Using wet alternating current fluorescent techniques will generally produce the highest sensitivity obtainable with this
technology.

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LAW OF MAGNETISM

When an external magnetic field is applied to ferromagnetic materials, the magnetic domains align parallel to the applied fields. These domains are very small, but larger than the atoms of the material. It is the interchange action between atoms and the alignment of the magnetic domains that are the cause of increased flux density. As the magnetizing force increases, the aligned
domains increase in volumetric steps. This incremental increase is detectable, and is often referred to as the barkhausen effect. When all the domains in a material are aligned, the material is said to be magnetically saturated.

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MAGNETS AND MAGNETIC FIELDS

The theory of magnetic fields indicates that an object is magnetized when part or all of its atomic elements have their north and south poles aligned. Earth’s magnetic field is produced by the rotation and movement of the planet’s molten iron core. This produces a magnetic field that is relatively stable in direction.

Because of this stable magnetic field, early explorers could reliably use compasses built with lodestone, charged iron needles and other naturally magnetic materials. The Earth itself can be considered a bar magnet because of its two poles. Quite frequently, the magnetic field surrounding the Earth strongly magnetizes large ferromagnetic objects that lie aligned with the Earth’s poles for periods of time.
The directions of the Earth’s magnetic field may be changed by major geologic events over many thousands of years. There are many local magnetic anomalies having higher levels of magnetic attraction near the Earth’s surface. It is thought that these may be caused by iron or nickel deposits.

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CAPABILITIES OF MAGNETIC PARTICLE TESTING

Magnetic particle testing can reveal surface discontinuities, including those too small or too tight to be seen with the unaided eye. Magnetic particle indications form on an object’s surface in the area of a discontinuity and show the location and approximate size of the discontinuity. Magnetic particle tests can also reveal discontinuities that are slightly below the surface.
There are limits to this ability to locate subsurface discontinuities. These are determined by the discontinuity’s depth, size, type and shape; the strength of the applied field; and the type of current used.

In some cases, special techniques or equipment can improve the test’s ability to detect subsurface discontinuities. Magnetic particle testing cannot be used on nonmagnetic
materials, including glass, ceramics, plastics or such common metals as aluminum, magnesium, copper and austenitic stainless steel alloys. In addition, there are certain positional limitations: a magnetic field is directional, and for best results must be oriented perpendicular to the discontinuity. This generally requires two complete magnetizing operations to detect discontinuities parallel and perpendicular to the test object’s axis. Objects with large cross
sections require a very high current to generate a magnetic field adequate for magnetic particle tests. A final limitation is that a demagnetization procedure is usually required following the
magnetic particle process.

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Magnetic particle testing can detect both production discontinuities, such as seams, laps, grinding cracks and quenching cracks, and inservice damage, such as fatigue and overload cracks.
Next to visual testing, it is one of the most cost effective methods of nondestructive testing. Its major difficulty is that the test object must be magnetizable. Magnetic particle technology depends on the property of magnetism occurring in certain elements referred to as ferromagnetic materials.Magnetic particle testing is a relatively simple test method that can be applied to finished articles, billets, hot rolled bars, castings and forgings. It can also be used to confirm that the processing operations, such as heat treatment, machining and grinding, did not cause discontinuities. There are many differing modes of electrical current that are used in magnetic particle testing.

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