When testing ferrous materials, what is the term for the small detectable portion of the magnetic flux that passes outside the metal when a surface discontinuity is encountered?

The term for the small detectable portion of the magnetic flux that passes outside the metal when a surface discontinuity is encountered is referred to as leakage flux. In Eddy Current Testing, when a magnetic field encounters a discontinuity, such as a crack or other surface defect, part of the magnetic flux will "leak" out of the material rather than remain contained within it. This leakage provides critical information about the presence and severity of the discontinuity, as it creates a secondary magnetic field that can be measured.

In contrast, air flux and induced flux are not the terms typically used in this context. Air flux does not specifically pertain to the interaction between magnetic fields and surface defects, while induced flux describes the magnetic flux that is generated as a result of induced currents within the material itself. Hence, the most accurate terminology to describe the phenomenon observed when evaluating ferrous materials for discontinuities is leakage flux.