Liquid penetrant inspection is a non-destructive method for finding discontinuities that are open to the surface of parts fabricated from essentially nonporous materials.  After cleaning the surface, the penetrant is applied and will seep or be drawn into various types of minute surface openings.  The excess penetrant is removed and a developer is applied which highlights the cracks under ultraviolet light.  The process is well-suited for the detection of all types of surface cracks, laps, porosity, shrinkage area, laminations, and similar discontinuities.

Indications of cracks can be found regardless of size, configuration, internal structure, or chemical composition of the workpiece being inspected and regardless of the orientation of the crack to the workpiece.

Liquid penetrant inspections are relatively simple and inexpensive (as compared to the other NDI methods) and can be applied to a broad range of materials.  Very small cracks can be found.  However, they can only detect surface cracks and their effectiveness can be adversely influenced by surface coatings, surface roughness, and porosity.  Extreme care is required in pre- and post-inspection cleaning and, in some cases, etching may be required prior to inspection.