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Introduction to Magnetic Particle Inspection

Basic Principles
History of MPI

Magnetic Domains
Magnetic Fields
Electromag. Fields
Field From a Coil
Mag Properties
Hysteresis Loop
Field Orientation
Magnetization of Mat'ls
Magnetizing Current
Longitudinal Mag Fields
Circular Mag Fields
Measuring Mag Fields

Equipment & Materials
Portable Equipment
Stationary Equipment
Multidirectional Equipment
Field Strength Indicators
Magnetic Particles
Suspension Liquids

Testing Practices
Dry Particles
Wet Suspension
Magnetic Rubber
Continuous & Residual Mag
Field Direction & Intensity
L/D Ratio

Process Control
Particle Concentration
Suspension Contamination
Electrical System
Eye Considerations

Example Indications
Visible Dry Powder
Fluorescent Wet


Dry Particle Inspection

In this magnetic particle testing technique, dry particles are dusted onto the surface of the test object as the item is magnetized. Dry particle inspection is well suited for the inspections conducted on rough surfaces. When an electromagnetic yoke is used, the AC or half wave DC current creates a pulsating magnetic field that provides mobility to the powder. The primary applications for dry powders are unground welds and rough as-cast surfaces.

Dry particle inspection is also used to detect shallow subsurface cracks. Dry particles with half wave DC is the best approach when inspecting for lack of root penetration in welds of thin materials. Half wave DC with prods and dry particles is commonly used when inspecting large castings for hot tears and cracks.

Steps in performing an inspection using dry particles

Prepare the part surface - the surface should be relatively clean but this is not as critical as it is with liquid penetrant inspection. The surface must be free of grease, oil or other moisture that could keep particles from moving freely. A thin layer of paint, rust or scale will reduce test sensitivity but can sometimes be left in place with adequate results. Specifications often allow up to 0.003 inch (0.076 mm) of a nonconductive coating (such as paint) and 0.001 inch max (0.025 mm) of a ferromagnetic coating (such as nickel) to be left on the surface. Any loose dirt, paint, rust or scale must be removed.
Apply the magnetizing force - Use permanent magnets, an electromagnetic yoke, prods, a coil or other means to establish the necessary magnetic flux.
Dust on the dry magnetic particles - Dust on a light layer of magnetic particles.
Gently blow off the excess powder -
With the magnetizing force still applied, remove the excess powder from the surface with a few gentle puffs of dry air. The force of the air needs to be strong enough to remove the excess particles but not strong enough to dislodge particles held by a magnetic flux leakage field.
Terminate the magnetizing force -
If the magnetic flux is being generated with an electromagnet or an electromagnetic field, the magnetizing force should be terminated. If permanent magnets are being used, they can be left in place.
Inspect for indications -
Look for areas where the magnetic particles are clustered.