Developed during the 1930s in the United States of America, the magnetic particle testing (MPT) method of non-destructive tests was established to inspect steel components on the production lines. The main principle of MPT is magnetisation of the specimen for producing flux or magnetic lines in it. In case the flux lines encounter a discontinuity (like a crack), they create secondary magnetic poles at its faces. The places where the secondary fields appear on the metal surface can be visualised by spreading magnetic particles or liquid suspension on the surface. Once applied, the particles or suspension get attracted towards the flux leakage, clumping around it to make the flaw visible to human eye.
- In a typical MPT, the particles used are generally black or have a coating of fluorescent dye on them for ensuring maximum visibility. The lines of the magnetic flux are normally at right angles in order to give a good indication of the flaw by establishing maximum flux leakage. This factor dictates the selection of an appropriate magnetising technique, which is used in different forms to get an absolute inspection.According to the welding experts providing non destructive testing in Melbourne, an MPT encompasses the following vital steps:
- Pre-cleaning of the material
- Demagnetisation, in case it has magnetic properties
- Background contrasting paint application, if required
- Magnetisation of the component
- Application of magnetic inks or particle powders
- Investigation of the surfaces for visible discontinuities
- Demagnetisation, as well as re-magnetisation in case it’s necessary
- Recording the defects
- Cleaning and safeguarding
A defect or discontinuity on a steel component draws in more particles if it crosses a greater number of magnetic lines. As such, the ability to reveal a flaw always depends on its depth, its angle to lines of force and the induced magnetic field strength. Also, the magnetic particle testing method can be used only for ferromagnetic materials like nickel, cobalt and iron. Diamagnetic and paramagnetic materials are incapable of holding an adequate flux for attracting particles and therefore, produce no results with MPT.
The different kinds of electrical currents used for conducting MPT include alternating current (AC), full wave direct current (FWDC) and half wave direct current (HWDC). Out of them, HWDC is probably the best due to its high magnetic penetration ability. Detection of subsurface discontinuities using AC is limited because of the skin effect – tendency of the current to get distributed inside a conductor.