Liquid Penetrant Testing (PT)

Liquid penetrate inspection (LPI), also referred to as Dye penetrant inspection (DP), is both a common and low-cost NDT inspection method that can be used to identify surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). Penetrant may be applied to all non-ferrous materials, but for inspection of ferrous components magnetic particle examination is preferred for its subsurface detection capability. LPI is used to detect casting and forging defects, cracks, and leaks in new products, and fatigue cracks on in-service components.

Dye penetrant inspection involves capillarity or capillary attraction where a liquid has the ability to flow into tight or narrow spaces without assistance - and even in opposition to - natural forces such as gravity. The materials used in the dye penetrant inspection make it possible for the results of the capillary attraction to be visible, collected as data and interpreted. Ultimately, liquid penetrant testing is a very effective method of identifying the extent of surface irregularities in many materials, including those that are not immediately visible.

Advantages of (LPI):

  • Effective on most materials including metallic, non-metallic, conductive, non-conductive, magnetic and non-magnetic,

  • Compatible with complex geometric forms

  • Materials used are highly portable and affordable

  • Visual, readily apparent results

  • Can detect in-service discontinuities including fatigue cracks, HIC, SCC and SOHIC

Liquid penetrant inspection process:


The surface of the testing material is properly cleaned of any contaminate that could interfere with the procedure or result in false or irrelevant data.

Application of the penetrant to a part in a ventilated test area

A liquid penetrant is applied to the surface of the test item. Penetrant material is typically a brightly coloured fluid that possess a very low surface tension and capillary action. The penetrant is then allowed to ‘soak’ into any flaws.

Excess Penetrant Removal

Excess penetrant is removed from the surface.

Application of White Developer

The developer will draw the penetrant directly out of any defects. It will form on the surface to form a visible indication, commonly referred to as ‘bleed-out’.


For visible dye penetrant, inspectors will use light of appropriate intensity. Ultraviolet (UV-A) radiation along with low ambient light levels is used for fluorescent penetrant examinations.


After inspection and data collection, the area is cleaned.