Dye Penetrant testing is for locating and identifying surface defects. It is also used for the detection of cracking in welded joints or of fatigue cracking in stressed materials.
The surface of the material being tested is coated with a penetrant which contains a dye. The penetrant will seep into any open areas on the material. After the excess penetrant is cleaned from the surface of the sample, a developer is applied. The developer lifts the penetrant out of the defect, and the dye stains the developer, creating an outline of the
defect or crack.

Hydro-Testing
Hydro-Testing is a method of testing a vessel for leaks by filling with water and applying pressure.