Liquid Penetrant Examination

Flaw detection process utilizing dye penetrants are extensively used in Aircraft industry.This is a logical development of the oil and chalk method popular in the early days. Briefly the method consists of immersing the part to be tested in a liquid penetrant wiping the excess panetrant and then applying a developer to bring back the dye which seeped into the flaw open to surface. The basic principle of penetrant inspection is that when a penetrant is applied over a clean surface to be inspected by the combined action of surface tension and capillary action penetrant seeps into defect which when developed, by the blotting action of the developer powder clearly shows up on a white back ground and can be conveniently inspected.

The following are the basic stages on dye-penetrant method:
Surface preparation: The surface on the part to be tested is thoroughly cleaned to remove dirt.

Application of penetrant: The penetrant is a liquid of a very low viscosity, carrying a dye - generally red or a fluorescent penetrant which easily inters into flaws because of low surface tension and capillary action.

Liquid penetrant may be applied on the parts by the following methods:

a. Dipping or immersing,

b. Spraying,

c. Brushing,

d. Pouring.

Excess penetrant removal: After the parts stand in the penetrant for desired time it is washed in clean running water & dried with air.

Developer application: The developer (dry or wet) is applied by evenly spreading or dusting over the part. The developer may be chalk powder of very fine grade or wet developer which is a fine chalk powder suspended in alcohol or spirit

Inspection: As the alcohol or spirit in the developer evaporates, the chalk powder draws back penetrant to the surface because of blotting properties of the chalk. This shows up as a red line on a white background. In case of fluorescent penetrant inspection, the part is inspected in a dark enclosure under ultra violet or black light. The blotted out fluorescent particles will give a visible glow under ultra violet light.
Fluorescent penetrant inspection is very effective for machined machined and finished parts