Dye Penetrant Techniques for Flaw Detection: A Discussion of their Nature and Limitations

FLAW detection processes utilizing dye penetrants are now extensively used throughout the aircraft industry. They are a logical development of the lard oil and chalk method popular in the early days, a system which is still occasionally used for economic reasons and where only the more obvious flaws are of significance. Briefly, this method consists of immersing the parts to be tested in hot lard oil, wiping off the excess, and then applying a film of chalk; oil which has seeped into surface flaws eventually exudes out and is absorbed into the chalk, revealing itself as typical oily stains. It was reasoned that if lard oil were substituted by a penetrating medium which was highly coloured, the exudations would then become much more obvious and from then on intensive research programmes were started which have culminated in the very efficient processes now available.