The Evolution of Penetrant Specifications

A Few More Thoughts On The Evolution of Penetrant Specifications

The past two issues of the PENETRANT PROFESSOR have concentrated on the historical path of the relevant qualifying documents and the journey to develop a method of measuring absolute sensitivity. While these have been extremely important in the creation of a uniform system of qualification, progress was also being made in other areas besides sensitivity measurement.

We previously pointed out that an important contribution to the measurement of sensitivity was the development of the spotmeter. While this found its place measuring sensitivity, it also became important in the measurement of background fluorescence. The first edition of the MILSPEC merely stated that after water washing, the penetrants “shall leave no residual fluorescence”. The
test relied upon sandblasted panels and eyeball examination. Now, of course, the test is still carried out on sandblasted panels, but the residual fluorescence is measured with the spotmeter.

  • Originally, the flash point of products was merely required to be tested and reported. Since then, OSHA and other regulations have limited the flash point of materials to be used in open tanks to 200°F.
  • Corrosion tests were initially only performed on magnesium and aluminum at relatively low temperature. Now steel, titanium, and nickel have been added to the list, and the temperature range has been extended, reflecting the demands on the materials used in modern jet engines.
  • Toxicity has always been an important subject, and the initial testing was not insignificant. This morphed into a system whereby manufacturers were required to submit their formulations to the Air Force for toxicological approval. Later this requirement was changed to require the submission of an MSDS for each product.
  • Water tolerance of water washable penetrants was added to the list of qualification requirements.
  • The ability of emulsifiers to continue to perform after they had been contaminated with penetrant was added.
  • Water suspendible and water soluble developers appeared upon the scene, and bioresistance criteria were added to the qualification specification. Hydrophilic emulsifiers were developed and added to the specification.
  • Black light intensity criteria were added, and penetrants were required to have stable indications under conditions of temperature and also exposure to UV-A.
  • Chlorinated solvent removers came and then went, as they were found to be ozone depleting substances.

So, to sort of put it in context, initially one could qualify a penetrant if a cracked aluminum block showed indications under black light illumination with unmeasured intensity, and the indications were judged to be adequate in intensity and contrast. Now, the test is performed on low cycle fatigue cracked specimens, the indications are measured with a spotmeter under controlled black light intensity, and the background is also measured and held to a high standard. One is also assured that the indications will not fade in the drying oven or under prolonged exposure to the black light, that the materials are non corrosive, not ozone depleting, and have reasonable bioresistance. And it has only taken a bit more than 50 years!