The basic principle upon which Penetrant Testing is based is that of capillary action. This is a surface tension phenomenon that permits liquids to be drawn into tight openings as a result of the energies that are present at the surface of the openings. PT method is used to detect surface discontinuities only.

The major advantage is that this method can be used on both ferrous and nonferrous materials. The application include the detection of surface discontinuities such as cracks, seams, laps, cold shuts, laminations and porosity. The indication can be viewed with visible daylight or using fluorescent method.