Magnetic Particle Inspection
Magnetic Particle Inspection is a method of locating surface and subsurface discontinuities is ferromagnetic materials. It depends on the fact that when the material or part under test is magnetized, magnetic discontinuities that lie in a direction generally transverse to the direction of the magnetic field will cause a leakage field, and therefore the presence of the discontinuity, is detected by the user of finely divided ferromagnetic particles applied over the surface, with some of the particles being gathered and held by the leakage field. This magnetically held collection of particles forms an outline of the discontinuity and generally indicates its location, size, shape, and extent. Magnetic particles are applied over a surface as dry particles, or as wet particles in a liquid carrier such as water or oil.
Nonferromagnetic materials cannot be inspected by magnetic particle inspection. Such materials include aluminum alloys, magnesium alloys, copper and copper alloys, lead, titanium and titanium alloys, and austenitic stainless steels.
Limitations: There are certain limitations to magnetic particle inspection the operator must be aware of; for example, thin coatings of paint and other nonmagnetic coverings, such as plating, adversely affect the sensitivity of magnetic particle inspection.
Liquid Penetrant Inspection
Liquid Penetrant Inspection is a nondestructive method if revealing discontinuities that are open to the surfaces of solid and essentially nonporous materials. Indications of a wide spectrum of flaw sizes can be found regardless of the configuration of the work piece and regardless of flaw orientations. Liquid penetrants seep into various types of minute surface openings by capillary action. Because of this, the process is well suited to the detection of all shrinkage areas, laminations, and similar discontinuities. It is extensively used for the inspection of wrought and cast products of both ferrous and nonferrous metals, powder metallurgy parts, ceramics, plastics, and glass objects.
The Liquid Penetrant method does not depend on ferromagnetism (as does, for example, magnetic particle inspection), and the arrangement of the discontinuities is not a factor. The Penetrant method is effective not only for detecting surface flaws is nonmagnetic metals but also for revealing surface flaws in a variety of other nonmagnetic materials. Liquid Penetrant Inspection is also used to inspect items made from ferromagnetic steels; generally, its sensitivity is greater than that of magnetic particle inspection.
Portable Equipment: For occasional inspections, especially in the field, where equipment portability is necessary, minimal kits for either visible or fluorescent penetrant inspection are commercially available. (Generally, portable penetrant applications are limited to localized areas or spot inspections rather than entire part surfaces.)
Such a kit for visible penetrant inspection work includes a precleaner, a penetrant, and a penetrant remover and developer, all in pressurized spray cans. Penetrant removal requires wiping with lint-free cloths or paper towels.
A similar kit is available for fluorescent work; a precleaner, a penetrant, penetrant remover and developer are likewise supplied in pressurized cans. Cleaning is accomplished by wiping with lint-free cloths or paper towels. This kit includes a small portable black light for conducting the inspection.
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