Magnetic particle testing (MT).
This testing method consists of establishing a magnetic field in the part to be tested and applying magnetic probes to its surface, which will attract any particles to the magnet ends or poles. Accumulations of these particles indicate discontinuities.
It’s important to note that magnets will attract materials only where the lines of force enter and leave the magnet at the poles. If a magnet is bent and the two poles are joined so as to form a closed loop, no external poles will exist, and consequently, it will have no attraction to magnetic material.
This is the basic principle of magnetic particle testing . If the part has no cracks or other discontinuities, magnetic particles will not be attracted, and you’ll know your weld is without surface cracks.
Only ferromagnetic materials can be tested by this method. Ferromagnetic parts that have been magnetized during testing may retain a certain amount of residual magnetism. Certain parts may require demagnetization if they are to function properly in service.
Magnetic particle testing is used to detect cracks, porosity, seams, inclusions, lack of fusion, and other discontinuities in ferromagnetic materials. Surface discontinuities and shallow subsurface discontinuities are also detected using magnetic particle testing .