A Brief Introduction to Magnetic Particle Testing
Magnetic particle testing (MT) is one of many non-destructive examination (NDE) techniques. MT is often used to discover surface discontinuities (like cracks and weld porosity) in ferromagnetic materials, such as steel. MT is often the method of choice for detecting surface discontinuities due to rapid testing speed, portability, and low cost.
How does it work, and how do you do it?
Here’s the general principle. Magnetic lines of force, called magnetic flux lines, travel from one end of a magnet to the opposite end. These ends are designated as the north and south poles. The magnetic flux lines form continuous loops that travel from one pole to the other in one direction only, and the lines never cross – they remain virtually parallel to one another.
Two types of field can be generated in a part: longitudinal and circular, referring to the direction of the magnetic field that is generated within the part. Since the magnetic particle testing here at VGO is done using a longitudinal field from an AC electromagnetic yoke, I will focus on that method.
In a part in contact with the magnet, surface discontinuities interrupt these magnetic flux lines. The yoke is placed onto the surface of the part and small iron particles are sprinkled onto the surface – when there is a discontinuity, it interrupts the magnetic flux lines and the particles will be held in place rather than accumulating at the poles of the magnet. The accumulation of particles in the discontinuity provides a visual indication, as shown below:
Often times, a thin coat of white powder suspended in solvent is sprayed on the surface of the part first, to provide contrast with the dyed iron particles. The particles are available in a range of different colors.
I mentioned that discontinuities interrupt the magnetic flux lines – this is true…for flaws perpendicular to the lines. Flaws at angles between 45° and 90° to the lines of flux will be revealed using MT, so in practice, we orient the yoke in one direction and then repeat the test with the magnet oriented 90° from the first chosen orientation.
MT is a great technique for discovering the location and extent of cracking in a part, revealing discontinuities not visible to the naked eye. The results of MT are easily recorded photographically, and very little cleaning must be done afterwards – especially if you didn’t spray the part with white powder first!
By P.Sumanth Naik (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
By ATG, s.r.o. (ATG, s.r.o.) [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons