This method is used to reveal surface discontinuities by bleedout of a colored or fluorescent dye from the flaw.
The technique is based on the ability of a liquid to be drawn into a "clean" surface discontinuity by capillary action. After a period of time called the "dwell time", excess surface penetrant is removed and a developer applied. This acts as a blotter that draws the penetrant from the discontinuity to reveal its presence.
Materials that are commonly inspected using this method include; metals, glass, many ceramic materials, rubber and plastics. However, liquid penetrant testing can only be used to inspect for flaws that break the surface of the sample (such as surface cracks, porosity, laps, seams, lack of fusion, etc.).
Pin Holes in Weld
Lack of Fusion along the edge of bond line
Flaws that are undetectable by the unaided eye
It produces a flaw indication that is much larger and easier for the eye to detect than the flaw itself.
Better Flaw Detectability
It improves the detectability of a flaw due to the high level of contrast between the indication and the background which helps to make the indication more easily seen (such as a red indication on a white background for visible penetrant or a penetrant that glows under ultraviolet light for fluorescent penetrant).