What is the term for the portion of magnetic flux that passes outside of a ferrous material when a surface discontinuity is encountered?

The term for the portion of magnetic flux that passes outside of a ferrous material when a surface discontinuity is encountered is referred to as "leakage flux." This phenomenon occurs because the presence of a discontinuity, such as a crack or surface defect, disrupts the continuous flow of magnetic lines of force. Instead of all the magnetic flux being contained within the ferrous material, some of it will "leak" out into the surrounding air or other non-ferrous materials in the vicinity.

In materials testing, particularly in magnetic particle testing, understanding leakage flux is critical as it indicates the presence and severity of flaws. When a magnetic field is applied, areas of leakage flux can indicate discontinuities, which are crucial for assessing the integrity of the material being tested.

The other terms provided do not accurately describe this concept. "Air flux" does not refer to the magnetic phenomena associated with discontinuities, and "induced flux" relates to the flux caused by external magnetic fields rather than the flux that leaks due to a flaw. Therefore, the correct terminology in this context is leakage flux, as it specifically describes the behavior of magnetic fields in the presence of surface discontinuities in ferrous materials.