The test is very effective in detecting porosity, cracks, fractures, laps, seams and other flaws that are open to the surface of the test piece and may be caused by fatigue, impact, quenching, machining, grinding, forging, bursts, shrinkage or overload. The very first step is a thorough surface cleaning to be sure the test piece is free of oil, grease, water, heat-treat scale, paint, plating and other contaminants that may prevent liquid penetrant from entering flaws. Liquid penetrant testing depends mainly on a liquid’s effectively wetting the surface of a solid work piece of specimen, flowing over the migrating into cavities that are open to the surface. Penetrants can be washable with water, removable with a solvent or require treatment with an emulsifier that is lipophilic (oil-based) or hydrophilic (water-based). A national aerospace standard specifies the requirements for the certification and qualification of lpi personnel for aerospace applications. Lpi offers flexibility in performing inspections because it can be applied in a large variety of applications ranging from automotive spark plugs to critical aircraft components.
This test is named for the liquid, called penetrant, that is applied to the sample during testing in order to make any surface flaws more visible. Parts of almost any shape and size can be inspected, although the capacity of equipment and the facility used for testing can create limitations. Liquid penetrant examination is one of the most popular nondestructive examination (nde) methods in the industry.