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.

Demagnetization

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.

Aircraft Visual Inspection

Figure: Boeing 777 aircraft Commercial aircrafts are frequently inspected visually. There is a primary inspection called "walk around check" both in the line and the base, transit checks, daily check, weekly check, A check, B check etc… Line is the more about the quick checks while aircraft is in the park or in between flights. Base is mostly the hanger, where hard checks, repairs take place. A qualified engineer walk around the aircraft and inspect structural items visually. Engine is also checked during the walk around checks. There engineer watches for oil leaks, structural damages in the engine, inlet of the engine for composite parts fixtures to the structure, fan/first stages of the compressor, last stages of the turbine for deterioration or damages , or any unusual changes of the engine. In this example shows how, engineer/Pilot carry on a walk around inspection of a Boeing 777 aircraft. Figure: Walk around inspection of a Boeing 777 1. Left Mid Fuselage...

Eddy current Theory

Eddy current theory is based on electromagnetic induction. That means, a variable magnetic field can induce an electric current on a conductor which is separate to the source of the magnetic field, and also vice versa, which is an electric current generates a magnetic field. Similar theory is used for the magnetic particles inspection as well. Electromagnetic induction is used in many applications such as, Transformers, Electric motors, Generators, etc... In the field of Non destructive testing (NDT), we manipulate with this theory which I mentioned above, and try to find surface defects or/and shallow internal defects of ferromagnetic materials.

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.

Technical Diagnostics and NDT

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