In October 2015 Bill Mooz, President of Met-L-Chek, gave a talk at the Fall ASNT Conference in Salt Lake City, Utah. Bill has been closely involved with the development of the specifications govern- ing penetrant inspection materials, and used his decades of experience to present a paper on the historical development of the specifications for the qualification of penetrants.
We offer a summery of Bill’s paper, especially for the younger penetrant practitioners so they have some understanding of what has transpired in specification development.
If there was one thing that could be said about the change from the original MIL-I-25135 to the present AMS-2644, it is that “We have come a long way, baby!” In 1956, when the first version of MIL- I-25135 was published, it is not a very far stretch of imagination to say that perhaps colored water could have qualified as a penetrant and be listed on the QPL. In fact, Loy Sockman, the founder of Met- L-Chek bought a gallon of an ap- proved penetrant, cut it in half with kerosene, submitted it for qualification and it was approved!
The writers of the specification simply did not have the knowledge or the data to design a better document. As an example, the specification required that “ The penetrant shall contain dyes that fluoresce…”, and “The fluorescent brightness and contrast shall be equal or superior to that of the standard sample.” No tests were specified to evaluate these requirements.
This paper examined some of the devices and methods that were involved in developing SAE-AMS 2644, the qualifying specification
that is in use today. These early failings were recognized, and there was a conscious effort to develop more explicit requirements
across the board. Some of these made sense, and some did not. There was the thought that a standard for fluorescent penetrant
brightness had to be established, and so a device (Fig. 1) was constructed to measure and test this.
As a standard, it used a piece of fluorescent glass (Figs. 2 and 3). The candidate penetrants were diluted, and then were soaked into filter paper that was dried and then tested in the device.
This was never written into the specification and was later forgotten. A special box was designed that fluidized a bed of dry powder developer for testing. It was interesting, but not necessary and it also found its way into the museum of discarded tests.
A glass sample that had been sandblasted with various grits (Fig. 4) was suggested for testing background fluorescence, but also died.
But the key parameter evaded analysts. This was a method of measuring the sensitivity of the penetrants. The initial method was to use thermally cracked aluminum blocks that were divided into two parts. (Fig. 5) The standard penetrant was applied to one part and the candidate penetrant was applied to the other part. The block was then processed and the results compared by eye. (Fig. 6)
This was an enormously unsatisfactory method, and it resulted in great difficulties for those who purchased penetrants, because they had no way to be sure that penetrant A was as sensitive as penetrant B. The Air Force Materials Laboratory recognized this and they actively searched for a better method. When the government decided to get out of the specification writing business and turn it over to civilian organizations, AMS Committee K was formed and the search for a more scientific method of measuring sensitivity was launched. This resulted in a number of potential methods.
The Air Force had already contracted with Ohio State University to address this question, and they had developed cracked chrome panels, and were able to produce these showing coarse, medium, or fine cracks. Japanese companies were able to refine these to the place where they could make the panels with cracks of various sizes, such as 50 or 30 microns. These panels were further developed so that the cracks on a panel ranged from coarse to fine. (Figs. 7 & 8).
The Air Force then awarded a contract to Paul Packman, of the University of Tennessee to develop a test piece, and the result was the very large bar that has been nicknamed the “Hernia Bar”. (Fig. 9)
Turco, then a penetrant manufacturer, developed aluminum panels that had small circular impressions of various sizes on them that were designed to test how small a flaw could be detected. These panels were made in several variations and could be used to test a single penetrant (Figs. 10 & 11) or to make comparisons between two penetrants. (Figs. 12 & 13)
James Alburger, of Uresco, patented the meniscus method and sold the kits for making the measurements. This was an optical test that purportedly measured the dye content of a penetrant as a dark spot in the center of a glass lens. In theory the dye content was a measurement of the penetrant sensitivity. (Figs. 14, 15, & 16)
There were two problems with each of these methods. The first was that the flaws or other attributes tested did not replicate the flaws that were to be detected on real parts. The second was that the tests had no way of quantifying the results in numeric terms. The Air Force Materials Laboratory tested each of these methods and was the arbiter of the results. At one point, they prepared seven samples, and a round robin test of these by the penetrant manufacturers was made in which Grover Hardy asked the results to be arranged in order of the penetrant sensitivities. Each sample was coded by a letter, the letters being A, E, G, M, O, T, and Y. When arranged in the order that had been determined by the Air Force, the results read “YA GOT EM”.
The Air Force had examined low cycle fatigue cracked bars that had been in use at GE and determined that these satisfied the need for cracks typical of what penetrants were supposed to locate. (Fig. 17) At about the same time, a spot meter (Fig. 18) was developed that could be focused on a crack indication and measure its brightness in numerical terms. These two developments satisfied what was sought and were chosen by Committee K, with the approval of the Air Force, for use in AMS-2644.
There was one last thing that had to be dealt with. Repeated tests with this system on a single penetrant resulted
in a range of results forming a normal distribution curve. It was necessary to use this statistical information in
deciding whether a candidate penetrant was equal or superior to the standard penetrant. Without going into detail
about how this was done, suffice it to say that the question was resolved and the present specification reflects
what must be done.
This paper described some of the various methods that were suggested and tested to qualify inspection penetrants beginning with the first such specification published in 1956. A major problem was to identify those physical characteristics that were important to include in the specification, to assign values to these, and to develop methods of testing candidate products for conformance to the levels required. A number of such characteristics were initially thought to be important but were subsequently discarded, and finally one key factor was determined to be the sensitivity level, and methods of quantifying this were developed and perfected. We have come a long way, with the present availability of five sensitivity levels of fluorescent penetrants, four methods of penetrant removal, and four different forms of developer. It took 12 years to produce the first version of AMS-2644, but the vexing question of measuring sensitivity was finally laid to rest, at least for the moment. SAE AMS Committee K continues to work with the specification so as to insure that purchasers of penetrants can rely upon the QPL qualification as a measure of quality.
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