Time Flying and Some Perspective on Qualification Documents
It is an interesting question – Is penetrant inspection a new technology or an old one? For newcomers, it probably is old. But some old timers can still remember the early days when it was an interesting and promising new inspection method. Thinking about this made us decide to present a bit of history about penetrants, and particularly about the specifications that were written to qualify them, and the methods that were investigated to determine the various sensitivity levels. This issue will concentrate on the specifications. One of the major problems in the early 1950s that confronted the new penetrant industry was how to classify penetrants according to certain standards. The use of penetrants in the aerospace industry was relatively new in the early to mid 1950’s, and the Air Force had entered the picture in order to establish standards for these new products. MIL-I-25135 was written to address these concerns, and the first version was issued on August 6, 1956. Under the title of “Classification”, penetrants were classified into “Type I – Inspection materials to be used with fluorescent inspection methods”, and “Type II – Inspection materials to be used with nonfluorescent (visible) inspection methods”. There was no further breakdown of the materials, either by the method to be used or the sensitivity level. Some of the qualification tests seem quaint by today’s standards. As an example, one requirement of the qualification process concerned the brightness and contrast of fluorescence for Type I penetrants. The requirement was that “…the fluorescence shall be sufficient for inspection purposes”. The penetrant had to have a fluorescent brightness and contrast that was “equal or superior to that of the standard sample”. This test was made by placing drops of the candidate penetrant and the standard on filter paper and then examining them under black light. Sensitivity was measured using the thermally cracked aluminum blocks that most of us are familiar with. Tests were made on three blocks, which were then photographed so that the indications could be compared on the photos. Although there was no separate identification of the penetrants into methods, separate water washability tests were listed for “Penetrants containing emulsifier” and “Penetrants containing no emulsifier”. There were other requirements for corrosion resistance and other characteristics that we will not describe at this time.
This first MILSPEC was superseded by the B revision On July 2, 1958, but regrettably, we have no copy of this. On October 21, 1959, the C revision was issued. It identified penetrants by Group. Groups 1, 2, and 3 identified visible penetrants, Group 4 was a fluorescent water washable penetrant, and Groups 5 and 6 were fluorescent post emulsifiable penetrants that were used with lipophilic emulsifiers. Group VI was described as “high sensitivity”. All groups were to be used as a family, with all materials from the same manufacturer. Fluorescent brightness testing now used the Coleman photofluorometer, and a test was added for the temperature stability of the penetrants.
Amendment 1 was issued on May 27, 1960 sharpening the washability requirements so that background fluorescence was reduced.
Amendment 2 arrived on July 25,1961 and it further sharpened the washability requirements.
Amendment 3 was issued on June 1, 1964. It added Group VII, which was a fluorescent penetrant inspection kit consisting of aerosol cans of materials.
Amendment 4 appeared on March 25, 1969. It added valve leakage and other requirements for aerosol cans, and expanded the toxicity requirements.
On September 12, 1979, this amended specification was changed to require the sensitivity to be tested using low cycle fatigue cracked specimens. No amendment designation was used for this change. Group VI was changed to Group VIA and Group VIB.
MIL-I-25135D was issued on June 29, 1984 and was very different from the various C revisions, in that the years of experience and the development of new products were a substantial input into the document. First, the “Group” classification for penetrant systems was replaced by “Type, Method, Sensitivity, form, and Class”. This is the present system, with the Type describing the penetrant, the Method describing the method of removal, the Sensitivity describing the sensitivity level, the form describing the type of developer, and the class describing the chemistry of the remover. The reference materials used in the qualification tests were reassigned so that they represented products from various manufacturers, and a test for ultraviolet stability was added. But more important, the method of testing sensitivity was revised to use low cycle fatigue cracked specimens that were subjected to the penetrants and then the indication brightness was compared with the standard. In addition, hydrophilic emulsifiers were added and the family concept was abandoned.
The E revision was the final revision to the MILSPEC. It was issued on June 26, 1989. The changes included the addition of sensitivity level ½ and tests for the bioresistance of soluble and suspendible developers. Beginning in the early 1980s, the government was moving to get out of the specification writing business, and SAE AMS Committee K had been formed to write and issue a new specification that would replace MIL-I-25135 and its amendments. This discussion will continue in following issues.