With the penetration procedure cracks, lacks of fusion, overlaps and pores, which are open to the surface, are indicated. It is used for example for steel, aluminum, copper, fired ceramics, forgings, castings, welded joints and other components with a relatively smooth surface.

Depending on the requirements and environmental conditions different penetration procedures are available.

A low viscosity penetrant is absorbed by capillary forces in cracks. With a developer the defects are then made visible.