Please provide information about Dye Penetrant Inspection (DPI) – Liquid Penetrant Inspection (LPI) – Liquid Penetrant Testing (PT)

Dye Penetrant Inspection (DPI), also called Liquid Penetrant Inspection (LPI) or Liquid Penetrant Testing (PT), is a widely applied and low-cost inspection method used to locate surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). For applications where a greater sensitivity to smaller defects is required, the fluorescent penetrant method is preferred.

The Penetrant may be applied to all non-ferrous materials and ferrous materials, but for inspection of ferrous components magnetic-particle inspection may be preferred for its subsurface detection capability.

Commonly, DPI is used to detect cracks, surface porosity, lack of penetration in welds and defects resulting from in-service conditions (e.g. fatigue cracks of components or welds) in castings, forgings, and welding surface defects.

The effectiveness of this method relies on many factors including the training/skill of the technician, how clean the part is, and the procedure that is being used to perform the test. DPI is a relatively cost-effective method, considering the amount of training required, and the cost of materials used. DPI can be used in both manufacturing and in-service inspections. As with other inspection methods, DPI requires that a known defect standard has been defined using standard parameters. Thus indications can be compared to defined allowable limits.

The basic steps are:

  1. Clean the part 
  2. Apply the Penetrant and allow dwelling
  3. Remove excess Penetrant.
  4. Apply a developer and allow developing
  5. Read the part for indications. 6) Clean the part.

This may sound easy, BUT, here comes the tricky part. There are several variations to every step – i.e. what type of Penetrant? What method (of excess Penetrant removal) of Penetrant? What sensitivity of Penetrant? What form of a developer? What class of solvent (to remove excess Penetrant)?