Magnetic particles test
Magnetic particles test is applicable to materials which can be easily magnetized. This method can identify the imperfections near and beneath the surface. First the sample is magnetized by using electromagnet or by transmitting electric current through or around it. The magnetic field which enters the sample, consists of magnetic force lines. When there is a defect which interrupts the magnetic force lines, some of the lines should exit from the sample and enter it again. These second exit and entrance points, form opposite magnetic poles. When magnetic particles are distributed on the surface of such a sample, they are attracted by the magnetic poles to form a clear mark. This mark is approximately the shape and size of the defect. Depending on the application, there are different ways of magnetizing used in magnetic particles test. This method can be classified into two groups:
- Direct electric current techniques: the electric current is transmitted through the sample and the magnetic field generated, is used to identify the imperfections.
- Magnetic flux techniques: magnetic flux is induced through the sample by using permanent magnet or transmission of electric current through a coil or conductor
The benefits are:
- Pre-cleaning operation is not needed
- The best method to identify tiny and shallow surface cracks in ferromagnetic materials
- Fast and relatively simple
- Completely cheap
- Operable through thin coatings
- Low limits for size and shape of the sample
- Excellent portability
- Fast and without time loss
Some of the limits are:
- Importance of the direction and intensity of the magnetic field
- The sample must be ferromagnetic
- Only suitable for discontinuities on the surface and close to the surface
- Sometimes needs high electrical currents
The workpieces should be usually demagnetized which may cause problems
