The penetrant method of NDE was developed from the 19th century 'oil and chalk' inspection technique. Cracks in railway engines and rolling stock were found by immersing (or painting) components in thin oil for several hours, cleaning the surface, then covering the component with chalk dust. Oil, absorbed into cracks, was drawn out by the chalk and showed as a brown smear.
Penetrant inspection was introduced in the USA during World War 2. Very thin hydrocarbons with fluorescent dyes were used to find flaws during mass production of aero engines. Colour contrast systems were developed in the UK in the immediate post war years.
Penetrant inspection involves a number of steps:
- Pre-cleaning the component, so that no grease, paint or dirt clogs up surface breaking defects
- Application of the penetrant
- Removal of surface penetrant, taking care not to wash the penetrant from flaws. Solvents or water are used
- Development with an absorbent coating
- Inspection, after a development time of at least ten minutes, using natural light or ultra violet radiation
- Recording flaws
- Post cleaning and protection
Books on the subject are:
- R Hamshaw, Introduction to the Non-Destructive Testing of Welded Joints, 2nd edition, Abington Publishing, Cambridge, UK, 1996 (ISBN 1 85573 314 5)
- David Lovejoy, Penetrant Examination. A practical guide, published by Chapman and Hall, 1991
- C E Betz, Principles of Penetrants, published by the Magnaflux Corporation, 1963
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