Dye Penetrant Test (DPT) is also known as Liquid Penetrant Inspection (LPI) or Penetrant Test (PT). It is one of the most widely used Non Destructive Test (NDT) methods. It can be used to inspect almost all non-porous materials such as metals, plastics, ceramics etc. We can detect any opened surface discontinuity (or flaw) such as surface cracks, porosity, pinholes etc. by this test method.
Principle: DPT is based upon a natural phenomenon called as CAPILLARY ACTION. By the virtue of capillary action, a liquid tends to flow or seep in narrow spaces (or openings) even against the external forces like gravity. This phenomenon occurs due to the molecular attraction.

STEPS TO BE FOLLOWED FOR DYE PENETRANT TEST:
Procedure for Dye Penetrant Test can vary depending upon the factors such as the penetrant system being used and the condition and environment under which the inspection is performed. However, the general steps which are to be followed are as below:
1. Surface Preparation: The surface must be thoroughly cleaned, to make it free from dirt, oil, paint, grease, water, or other contaminants. For cleaning one can use dry cloth, solvents, cleaner, rust remover etc. depending on the condition of the surface to be inspected.
2. Penetrant Application: After thorough cleaning, Penetrant is applied. Penetrant is a red colored Liquid. It can be applied on the surface by spraying (most common) or by brushing or by immersing the entire surface in a penetrant bath. Temperature of the surface shall be between 5C to 52C.
3. Dwell period: Leave the penetrant, as it is, on the surface for a minimum period of time (Known as Dwell Time or Dwell Period). During dwell period, penetrant seeps into the flaws (if present on the surface being inspected), due to Capillary action. Dwell period varies from 5 minutes to 60 minutes or even more than that depending upon the material and its service condition. Penetrant manufacturer also mentions the dwell period on the product itself. However, there is no harm in going beyond the minimum dwell period, provided that the temperature shall be ambient, because at elevated temperature chances of penetrant getting evaporated will be always there. Hence at elevated temperature, one should strictly restrict to the recommended dwell time.
4. Removal of excess penetrant: After leaving the surface for recommended dwell period, the penetrant shall be cleaned, for cleaning the excess penetrant cloth and penetrant removal shall be used.
5. Application of Developer: After thorough cleaning, a thin layer of developer is applied. Two things shall be taken care of while spraying the developer.
6. Inspection: After application of developer wait for 10 to 60 minutes and then perform the inspection. Appropriate lighting is necessary to detect the indications from any flaws which may be present. Minimum light required is 1000 Lux or 100 foot candles.