Friday, April 16, 2010

Important facts about the MPI

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.

Because of this reason, it is required to inspect this place once again, in a separate angle as in the picture below.

This time magnetic flux goes around the defect, therefore magnetic particles will gather on the top of the crack. 
After finishing the inspection, it is utmost important to demagnetize the machine part. Otherwise, it can act as a magnet while the machine is on work and will result inefficiency or it can attract unwanted ferromagnetic particles or objects and cause destruction.
You will find more details about demagnetization in a future article.