NDT Magnetic Particle Explained in English

Most procedures require coverage of the MT inspection in two directions and this is usually performed through the use of longitudinal or circular magnetic fields. Magnetization of parts using traditional methods occurs in one direction at a time. Cracks or voids that you are seeking can only be found perpendicular to the magnetic field because the crack has to disrupt the magnetism in order to form a leakage field. If your magnetic field is in the same direction as the flaw you will probably not see the crack.  When speaking of electrically induced magnetization it is said that cracks are found in the direction of the electrical field. This is easily explained by stating that magnetization occurs at 90 degrees to the electrical field and cracks occur at 90 degrees to the magnetic field.

MT Equipment is available in portable and stationary configuration. Portable instrument is usually much weaker than the stationary counterpart, but they serve an important purpose in magnetic particle of making the process portable. Most portable equipment is used for weld inspections.

One of the main concerns of the process is ensuring that after the inspection the technician demagnetizes the part. Magnetism on a rotating or moving part like an engine could be detrimental by attracting metal particles and causing wear or catastrophic failures.

Note: MT Magnetic particle is relatively simple to use but can be very easily done inappropriately making the inspection useless or causing harm to the inspector if performed by an untrained person.  Inspections should only be performed by qualified and certified personnel.

A critical mistake performed with magnetic particle is not ensuring that the part is appropriately magnetized with a gauss meter, also often called a field indicator. A field indicator is similar to a compass - it has a needle to show the presence of a positive or negative magnetic field. If the inspector does not verify the magnetization, the inspection could be nonexistent since magnetism is not visible to the human eye.

Another tool utilized to determine field direction is a pie gage or shim. These are good devices for quick verification of field direction, but are subject to external fields and may be misleading at times. ASTM E1444 covers these devices and how to use them.  For other procedures relevant to NDT and Magnetic particle testing you may Click Here.