Technical Diagnostics and NDT

There are several steps in a magnetic particle inspection. Clean the surface of the ferromagnetic material Magnetize the material Spray or pour the magnetic particles   Visual inspection   Demagnetization   Cleaning Demagnetization is done to disorganize the micro atomic arrangement which produce a magnetic field. After demagnetization the material will act as a normal metal. The magnetized components can cause problems such as,

One of the most important things an inspector should do while MPI is to check for defects at least twice in separate 90 degree angles. Let’s understand this by using a small example.  I have a piece of steel, where there suppose to be a defect. Let’s assume that there is a surface crack, which is quite straight and continues parallel to one of the edges. Then we apply a magnetic field as in the picture below. We clearly see that, no change happens to the magnetic flux.   Accordingly there is no change happens to the magnetic particles we are adding. Therefore the crack will be invisible via magnetic particles method.

First, lets check out how this magnetic particles inspection carries out :). It's always easy to understand something practical by watching a youtube video. Theory can be found in the last article . Another video with a more advanced MPI This one is in Spanish, though the MPI shown very well, and the crack is well visible. MPI usually find cracks which are perpendicular to the direction of the flux, so It's necessary to search for defects twice. I'll explain more about that in a future article, but first it's important to see and understand the basic chemistry of this method.

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.