Surface Roughness

We get a wide range of questions, from the sublime to the ridiculous. But we always understand that the question, no matter how “far out” it might seem, is an important one for the person who has asked it, and we treat it that way. Our days are livened by some of the things that puzzle our readers, and we recently had a question that we had never received before. Here it is, verbatim:

What is the correlation of the sensitivity level of penetrant and surface roughness? ie: what sensitivity level would fit with what RMS range?

Did you ever think about this? Probably not, but someone did, and they asked the question. The answer, of course, is that there is no correlation between sensitivity and surface roughness. Sensitivity is measured on candidate penetrants at the Air Force Materials Lab at Wright-Patterson Air Force Base, and the sensitivity level is assigned and used in the Qualified Products List. The test method uses low cycle fatigue cracked bars, with smooth surfaces. This does not imply that surface roughness is not important, but it does mean that surface roughness has no correlation to the penetrant sensitivity. When the sensitivity levels is being determined, separate tests are made to determine the background fluorescence left by the penetrant system. The object is to have a minimum of background, so that the signal to noise ratio is high. When the criteria are met and a penetrant is listed on the QPL, a user is assured that the material will provide the assigned sensitivity level with a background that was found acceptable on a standardized sandblasted panel. In actual use, surface roughness can affect how easily surface indications can be seen and interpreted because of background fluorescence that is more prominent with rougher surfaces. In these cases, the sensitivity has not changed, but the ability to clearly see indications can be affected by the increased background.