Flux Leakage

When a magnetic field is created within a ferromagnetic specimen, lines of magnetic flux are developed and flow through and around the material, completing a circuit. Magnetic particle inspection relies on the phenomenon of flux leakage occurring within this circuit. This may be caused by a break or discontinuity in the material.

Cracks may have very irregular contours, therefore, they may sometimes be detected using MPI when the lines of force are parallel, or close to parallel, to the crack’s major axis. However, they are best detected if at 90°. It would be highly unlikely to be able to detect lack of fusion if in the same direction as the magnetic field.

Because it is a change in magnetic permeability that causes a leakage field, flux leakages may also be caused by changes in metallurgy. Alternatively, flux leakages may be caused by changes in metallurgy and dissimilar materials.

Opposite poles attract, therefore any break or discontinuity causing a flux leakage will, because of the magnetic poles, attract the ferromagnetic material in the detection media being used, such as dry powder or inks. This would accumulate at the area of the leakage field and give an indication of the discontinuity’s existence.

• Where the flux leaves the circuit, a north pole is created.
• Where the flux re-enters the circuit, a south pole is created.

For a discontinuity to be detected by MT it must interrupt the lines of force. Any linear discontinuities running parallel with the flux or small non-linear discontinuities, i.e. equiaxed discontinuities, such as porosity, do not break the lines of force, they bend around these discontinuities taking the path of least resistance; these discontinuities may remain undetected.

MPI is most effective in detecting discontinuities with their major axis at 90° to the lines of force but will usually remain effective down to about 45° of this axis (BS EN ISO 17638 and BS EN ISO 9934 quotes ± 60°). Below that it is unlikely that the discontinuity will be found, therefore in order to examine a specimen completely, the lines of force must be applied in a minimum of two directions, perpendicular to each other.

A sharp change in contour can also create a flux leakage creating an MPI indication known as ‘furring’. Furring can also occur on flat surfaces when there is an excessive amount of magnetic flux. (ASNT textbooks refer to this as ‘BEARDING’.)