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Handbook for Damage Tolerant Design

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Section Magnetic Particle Inspection

Magnetic particle inspection is effective in the detection of surface and near-surface cracks in ferromagnetic parts.  The inspection is accomplished by inducing a magnetic field in the part and applying either a dry magnetic powder or a liquid suspension of iron particles to the surface being inspected.  Defects in the part cause local bipolar perturbations in the magnetic field which attract the magnetic particles, producing visible indications by color contrast or by fluorescence under “black light”.  The magnetically-held particles form the outline of the discontinuity and generally indicate its location, size, shape, and extent to an experienced inspector.

The magnetic particle method is a relatively fast and inexpensive method for locating small and shallow surface cracks in ferromagnetic materials.  Discontinuities that do not break the surface are detectable, but deeper cracks must be larger to be found.  Elaborate pre-cleaning is not necessary, but thin coatings of paint or other non-magnetic coverings, such as plating, adversely affect the sensitivity of this inspection technique.  Following the inspection, the material must often be de-magnetized, and post-cleaning to remove the clinging magnetic particles is usually necessary.  This NDI method can be used only on ferromagnetic materials, which include most of the iron, nickel and cobalt alloys.  Many of the precipitation-hardening steels, such as 17-4PH, 17-7PH, and 15-4PH stainless steels, are magnetic after aging.  Non-ferromagnetic materials that cannot be inspected by this method include aluminum, magnesium, copper, and titanium alloys and austenitic stainless steels.