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:
- Clean the part
- Apply the Penetrant and allow dwelling
- Remove excess Penetrant.
- Apply a developer and allow developing
- 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)?