Permeability (µ)

For magnetic particle inspection, the only materials of interest are those which are ferromagnetic.
Within this group, some materials are more easily magnetised than others, that is to say, more permeable.

To permeate means to spread through. In this context it refers to the ease by which the magnetic lines of force are spread through the material.

Soft iron / low carbon steels have a higher permeability (low retentivity) i.e. they are easier to magnetise than hard iron / high carbon steel that have a lower permeability (high retentivity) i.e. they are more difficult to magnetise. Whereas pure iron (ferrite), which has almost no carbon content, would have an even higher permeability.

An alternative description favoured in the USA would be, the ability to concentrate magnetic fields and it is shown on the ‘slope’ of the B/H curve which varies continuously.
Magnetic force (H) may also be termed ‘magnetic field strength’ or ‘tangential field strength’ and is a measure of the electrical current applied to a material.

Flux density (B) may be termed ‘magnetic induction’. Magnetic flux per unit area of a section normal to the direction of flow. Measured in Tesla/Gauss, it is a way of defining the induction field as the force line number per unit area.

Permeability (µ) may be calculated by dividing the flux density (B) achieved by the magnetising force applied (H) =

Relative Permeability = $\mu = \frac{B}{H}$

Effective Permeability = $\mu\text{eff} = 6^{L/D} - 5$

Note: Effective permeability relates to the shape of the object.

The permeability of a material may be given a value based on a ratio when compared with free space. These values vary depending on alloy composition, heat treatment and any working applied. The reciprocal of permeability is reluctivity or reluctance, this being the amount of opposition to the alignment of magnetic domains.