Magnetic Particle Testing

Magnetic Particle Testing

Magnetic particle inspection (MPI) is a non-destructive testing (NDT) process for detecting surface and subsurface discontinuities in ferrous materials. The process puts a magnetic field into the part that can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electrical current is passed through the test object and a magnetic field is formed in the material. Indirect magnetization occurs when no electrical current is passed through the test object but rather when a magnetic field is applied from an outside source. The magnetic lines of force are perpendicular to the direction of the electrical current, which may be either alternating current (AC) or some form of direct current (DC, or rectified AC).

The presence of a surface or subsurface discontinuity in the material allows the magnetic flux to leak. Ferrous iron particles are applied to the part being tested. The particles may be dry or in wet suspension. If an area of flux leakage is detected, the particles are attracted to this area.

The method is applicable to all metals which can be strongly magnetised – ferritic steels and irons, but not generally austenitic steels.

The method of magnetisation must produce a magnetic field with lines of force at a large angle to the expected direction of the cracks to be detected, so that it is usual to apply the magnetisation more than once in different directions, for example in two directions mutually at right-angles, but methods of swinging the field direction during magnetisation are available.

The magnetisation may be produced by any of the following methods:

a) applying a permanent or electro-magnet to the surface (magnetic flow);
b) passing a large current through the specimen, or locally by means of current prods (current flow);
c) putting the specimen inside a current-carrying coil, or forming a coil around the specimen;
d) making the specimen the secondary loop of a transformer – (induced current) – suitable for     ring-shaped specimens;
e) placing a current-carrying coil or loop close to the specimen surface;
f) threading a current-carrying bar through a hollow specimen.

The electric current used may be DC or AC of any waveform, but the current required to produce adequate magnetisation depends on the waveform of the supply, the magnetisation method used, and the material of the specimen. It is most important to ensure that the current used is correct for the specimen size and shape and also that the direction of the magnetic flux produced is suitable for the cracks expected.

By using a combination of two magnetic fields, a swinging or rotating magnetic flux can be produced, which will detect a crack in any orientation.

Usually the iron particles – dry powder or suspended in a liquid (magnetic ink) – are applied while the magnetising current is still flowing, but residual magnetisation is sometimes used, when the particles are applied after magnetisation. Some steels retain sufficient magnetisation for this method to be satisfactory, and in this case smaller, more portable, magnetising equipment can be used. Magnetic inks (particles suspended in a liquid) are used more widely than dry powders. They are applied by low-pressure spray, dipping or brushing; it is important to use plenty of ink and to allow time for the particles to flow over the surface and migrate to any cracks. On dark surfaces a very thin layer of white paint can be applied to give a higher contrast indication.

Fluorescent particles, which require UV-A illumination, are widely used and coloured particles are also available. The indications of cracks can be preserved by photography or by the use of peel-off transparent adhesive film. MPI methods can be applied to relatively rough and dirty surfaces, but the flaw sensitivity may be impaired. Magnetic methods for underwater applications have been developed. Only under very special conditions can sub-surface flaws be detected by MPI.