Magnetic Particle Inspection
This method of NDT may detect surface, and in certain cases, slight sub- surface discontinuities up to 2-3 mm below the surface. MPI can be used on ferromagnetic materials only.
A magnetic field is introduced into a specimen to be tested, fine particles of ferromagnetic powder, or ferromagnetic particles in a liquid suspension, are then applied to the test area. Any discontinuity which interrupts the magnetic lines of force will create a leakage field, which has a north and south pole on either side of it. This attracts the ferromagnetic particles in great numbers. The discontinuity may show as a black indication against the contrasting background – usually white contrast paint – or as a fluorescent indication which is usually green/yellow against a dark violet background.
- Sensitivity is effected by two factors, contrast and definition, if either/or both are increased then the overall sensitivity is increased, since there is a higher contrast with fluorescent detecting mediums they are considered more sensitive.
When MPI is carried out using fluorescent inks, the use of an ultraviolet (UV-A) light is necessary to cause fluorescence of the particles, although there is no need to apply a contrast paint.
Fluorescent ink methods are more sensitive* than black ink methods. There are many ways to apply a magnetic field, e.g. a permanent magnet, coils, prods, cables and threading bar.
Listed below is a sequence of operations to inspect a weld using a permanent magnet with black ink:
- Clean area using wire brush and a cloth plus solvent if necessary.
- Apply a thin layer of white contrast paint.
- When the paint is dry, straddle the magnet over the weld.
- Apply ink (1.25 to 3.5% ferrous oxide particles to a paraffin base).
- Interpret area.
- To look for transverse defects, turn magnet approximately 90° and re-apply the ink.
- Interpret area.