Dye penetrant testing (DP) also known at Penetrant Testing (PT) or Liquid penetrant Inspection (LPI) is a fairly low cost non-destructive testing method used to check surface defects in all non-porous materials.  This makes this method different to Magnetic Particle inspection as it is also able to test non-magnetic materials (which MPI cannot).  Dye Penetrant testing is used to detect surface defects such as cracks and porosity in castings, forgings, piping and welded components as well as fatigue cracks on in-service components. Dye Penetrant testing works via a capillary action whereby low surface tension fluid penetrates into clean and dry surface discontinuities (aka defects).  the penetrant may be applied via dipping, spraying or brushing a test sample after it has been prepared or cleaned.  After sufficient time has passed, the excess penetrant is removed and the developer is applied.  The developer acts to draw the remaining penetrant (residing in the flaw) to the surface thereby identifying its location on the samples surface