Permeability is a complex function of the applied magnetic field. For the material shown below, the permeability curve peaks at about 12 Oersted and a further increase in magnetization produces a decrease in permeability. A reduction in wall thickness coupled with a reduction of permeability causes the flux to flow in alternate paths. One such path is out of the material, hence the term flux leakage.

In flux leakage testing, the saturation is often used to imply that flux leakage is occurring. However, saturation has many definitions. The nondestructive testing volume of the American Society for Testing and Materials 1992 annual book of standards ^{[ ASNT ]} gives the following definition:

Magnetic Saturation - That degree of magnetization where a further increase in magnetization force produces no significant increase in the magnetic flux density (permeability) in a specimen ^{[ see also Dobmann80]}

For the material shown above, applied field levels above 12 Oersted produce a decrease in permeability. Using the above definition, saturation could be take as 12 Oersted. The magnetization curve for the same material is shown below. At applied field levels above about 40 Oersted, an increase in magnetization force produces a small increase in flux density. So, the definition given by the American Society for Testing and Materials gives two possible values for saturation.

A third, classical definition of saturation is that total magnetic saturation occurs when all of the magnetic domains are aligned and the permeability relative to that of air becomes one. For pipeline steels, this occurs at very high field levels (above 1000 Oersted) and is impractical for flux leakage in-line inspection applications.

Saturation is defined in this report as the degree of magnetization where an increase produces no significant change in flux density. This is the definition most commonly used by inspection vendors, and it refers to magnetization levels higher than the medium levels shown above.