magnetic flux above the surface of a part due to a crack near
Magnetizing Equipment for
Magnetic Particle Inspection
To properly inspect a part for cracks or other defects, it is
important to become familiar with the different types of magnetic
fields and the equipment used to generate them. As discussed previously,
one of the primary requirements for detecting a defect in a
ferromagnetic material is that the magnetic field induced in the
part must intercept the defect at a 45 to 90 degree angle. Flaws
that are normal (90 degrees) to the magnetic field will produce
the strongest indications because they disrupt more of the magnet
field of force that exists around a magnetic body or a current
Therefore, for proper inspection of a component,
it is important to be able to establish a magnetic field in at
least two directions. A variety of equipment exists to establish
the magnetic field for MPI. One way to classify equipment is based
on its portability. Some equipment is designed to be portable
so that inspections can be made in the field and some is designed
to be stationary for ease of inspection in the laboratory or manufacturing
facility. Portable equipment will be discussed first.
Permanent magnets Permanent
magnets are sometimes used for magnetic particle inspection as
the source of magnetism. The two primary types of permanent magnets
are bar magnets and horseshoe (yoke) magnets. These industrial
magnets are usually very strong and may require significant strength
to remove them from a piece of metal. Some permanent magnets require
over 50 pounds of force to remove them from the surface. Because
it is difficult to remove the magnets from the component being
inspected, and sometimes difficult and dangerous to place the
magnets, their use is not particularly popular. However, permanent
magnets are sometimes used by divers for inspection in underwater
environments or other areas, such as explosive environments,
where electromagnets cannot be used. Permanent magnets can also
be made small enough to fit into tight areas where electromagnets
might not fit.
most of the equipment used to create the magnetic field used in
MPI is based on electromagnetism. That is, using an electrical
current to produce the magnetic field. An electromagnetic yoke
is a very common piece of equipment that is used to establish
a magnetic field. It is basically made by wrapping an electrical
coil around a piece of soft ferromagneticsteel. A switch
is included in the electrical circuit so that the current and,
therefore, the magnetic field can be turned on and off. They
can be powered with alternating current from a wall socket or
by direct current from a battery pack. This type of magnet generates
a very strong magnetic field in a local area where the poles of
the magnet touch the part being inspected. Some yokes can lift weights
in excess of 40 pounds.
yoke with battery pack
magnetic particle kit
View a short movie on portable magnetic yokes.
are handheld electrodes that are pressed against the surface of
the component being inspected to make contact for passing electrical
current through the metal. The current passing between the prods
creates a circular magnetic field around the prods that can
be used in magnetic particle inspection. Prods are typically made
from copper and have an insulated handle to help protect the operator.
One of the prods has a trigger switch so that the current can
be quickly and easily turned on and off. Sometimes the two
prods are connected by any insulator (as shown in the image) to
facilitate one hand operation. This is referred to as a dual prod
and is commonly used for weld inspections.
If proper contact is not maintained between the
prods and the component surface, electrical arcing can occur and
cause damage to the component. For this reason, the use of prods
are not allowed when inspecting aerospace and other critical components.
To help prevent arcing, the prod tips should be inspected frequently
to ensure that they are not oxidized, covered with scale or other
contaminant, or damaged.
The following applet shows two prods used to create a current
through a conducting part. The resultant magnetic field roughly
depicts the patterns expected from an magnetic
particle inspection of an unflawed surface. The user is encouraged to manipulate
the prods to orient the magnetic field to "cut across"
Portable Coils and Conductive
Coils and conductive cables are used to establish a longitudinal
magnetic field within a component. When a preformed coil is used,
the component is placed against the inside surface on the coil.
Coils typically have three or five turns of a copper cable within
the molded frame. A foot switch is often used to energize the
coil. Conductive cables are wrapped around the component. The
cable used is typically 00 extra flexible or 0000 extra flexible.
The number of wraps is determined by the magnetizing force needed
and of course, the length of the cable. Normally, the wraps are
kept as close together as possible. When using a coil or cable
wrapped into a coil, amperage is usually expressed in ampere-turns.
Ampere-turns is the amperage shown on the amp meter times the
number of turns in the coil.
Watch these short movies showing a cable being used to establish magnetic fields in parts. Cable wrapped around part (690 KB mov). Cable wrapped through the part (425 KB mov).
Portable Power Supplies Portable power supplies are used to
provide the necessary electricity to the prods, coils or cables.
Power supplies are commercially available in a variety
of sizes. Small power supplies generally provide up to 1,500A
of half-wave direct current or alternating current when used with
a 4.5 meter 0000 cable. They are small and light enough to be
carried and operate on either 120V or 240V electrical service.
When more power is necessary, mobile power supplies can be used.
These units come with wheels so that they can be rolled where needed.
These units also operate on 120V or 240V electrical service
and can provide up to 6,000A of AC or half-wave DC when 9 meters
or less of 0000 cable is used.
The features of a portable power supply are explained in this short movie (800KB mov).