The first step when conducting an MPI NDT test is where an NDT technician will magnetise the material that they need to inspect. This can be done either by direct or indirect magnetisation. Direct magnetisation involves an electromagnetic current being passed directly through the test specimen, forming a magnetic field within the material. The lines of magnetic force are at a direct 90 degree right angle to the direction of the electric current, of which both AC (alternating current) or DC (direct current) can be used. Indirect magnetisation is where no electromagnetic current is passed directly through the test object, but a magnetic field is applied from an outside source.

If the magnetised test object is sound, without any defects or surface discontinuities, the magnetic field applied to the object will transfer throughout the material without interruption. However, any cracks or defects will cause the magnetic field to be interrupted, creating what is known as a flux leakage field around the location of the defective area. After the material has been thoroughly magnetised and these leakage fields have been created, an NDT technician will spread magnetic particles, usually made from iron, over the test piece. These particles will then gravitate to the areas where the leakage field is present, thus alerting the NDT technician to the crack or defect. This can be done in two ways, sometimes a dry powder made up of these particles can be used, which is known as Dry Magnetic Particle Testing (DMPT), and sometimes a water based solution or a fluorescent dye is used to complain the magnetic particles, known as Wet Magnetic Particle Testing (WMPT). These are often used interchangeably, depending on which method will help the cracks and defects the most visible in the environment in which they are being tested. These are most often applied during the magnetisation process while the test subject is still magnetised, but some materials such as forms of steel are able to hold magnetisation for longer, allowing for residual magnetic NDT to be used, which can offer smaller and more portable solutions for magnetic non destructive testing. 

Sometimes, if a crack runs parallel to the magnetic field, there is little disturbance to the magnetic field, meaning that the crack is less likely to be detected during an MPI NDT test. Using a combination of two magnetic fields can produce a magnetic flux that swings and rotates, which can help detect a crack in any orientation of the material, which can be very helpful when carrying out an MPI NDT test.

To learn more about about why Magnetic Particle Inspection is so important, along with other NDT techniques, read our guide on “Why Is Non Destructive Testing Important?”.