Tuesday, April 6, 2010

Magnetic Particle Inspection

Magnetic particle test is one of the common NDT methods. As you can see, this has to do something with magnets. For a starter, it is better to remember little bit about magnets before understanding this method.
Magnet is a material produces a magnetic field. They have two poles which are known as North, and South.  Opposite poles (N+S) attracts but same poles such as N+N or S+S repels.
Figure: A magnetic field, created by iron dust
Magnetic field is invisible. Any ferromagnetic material or another magnet responds to a magnetic field. Ferromagnetic materials are materials which magnetized either by another magnet or an electric field. When magnetized, the microscopic structure of the ferromagnetic material becomes organized, and well directed to one side. Otherwise the internal structure (of each and every single microscopic element) is a mess. Such a material does not act as a magnet because in whole, the sum of all individual vectors becomes zero.  

Figure: The path of the magnetic flux
We did study those things for physics in school. But we didn’t concentrate much about the magnetic field, or flux lines behave inside of a material. We did observe that the magnetic flux travels from north to south in general. But inside of the material, the same magnetic flux travels from south to north.
If the magnet is created by using an electric electromotive force, it’s called as an electro magnet. Magnetic particles inspection is usually done by using electricity. Both AC and DC currents are used depending on the situation.
This is the basic principle of the magnetic particles test. Internal fluxes which travelers inside of a ferromagnetic material does not go through a defect, but goes around that as in the figure below.
Figures show what happens to the flux when there is a crack
If someone add magnetic particles (form of a dry or wet powder), they will try to gather in the place of that flux leakage. Often, those particles are fluorescent which allows the engineer to inspect with a black light, inside of a dark room.
There are several ways to create fluxes inside of a material. They all have connections with the Right hand grip rule, where it says that, if the direction of the current shows from the direction of the right thumb, then the direction of the magnetic flux is equivalent to the direction of the other fingers.
Figure: The right hand grip rule
Obviously, the first way of creating a flux is, to conduct electricity through the material. Magnetic flux will appear clockwise to the direction of the electric current.
Figure: Using a Central Conductor to Circularly Magnetize a Cylinder
Second one is, to conduct an electric current around the material, by using a separate conductor. Magnetic flux will go through the material, from one side to the other

Figure: Longitudinal Field Produced by the Coil Generates an Indication of Crack in Part