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The Use of Known Discontinuity Standards
by Israel Vasquez*
| How do you know when your fluorescent penetrant testing materials are no longer performing as they should? You test them, of course! This month's author provides some insight into how this testing of the materials should be done. It is basic to nondestructive testing that we test our testing materials so that we can do the job that we think we are doing.
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The intent of this paper is to provide some insight for understanding the requirements for conducting the system performance test of a fluorescent penetrant testing system. This author has encountered a great deal of confusion throughout the aerospace nondestructive testing (NDT) industry as to what is the proper method for conducting this test. The uncertainties this paper will address are those concerning the system performance test and the proper acceptance criteria. In addition, it is intended to clarify the confusion between the system performance test and the penetrant sensitivity test.
The system performance test required by specifications such as MIL-STD-6866 (1996), ASTM E 1417 (1999) and SAE AMS 2647 (1995) require that a known discontinuity standard be processed through the penetrant testing system using standard parameters and that the indications detected be compared to those obtained with unused penetrant materials (Vasquez, 1997). This comparison is to be made with photographs (not recommended by this author), other such records of previously obtained indications or with a similar known discontinuity standard processed with the unused materials. Known discontinuity standards generally used by the aerospace industry for conducting this test are the penetrant system monitor (PSM-5) panel, the TAM test panel and recently the five star penetrant test panel. All three are recognized throughout the industry as the test panel. For all intents and purposes, the term "test panel" shall be used throughout this paper (see Figure 1).
An acceptable test would convey confidence that the system is in control to the user of the penetrant testing system
The failure mode was disbonding of the cardboard from the steel strip. The transmission lost torque. The problem was posed to my NDT group as a requirement to develop a test for adhesion of the cardboard to the steel band.
For several years, several aerospace specifications specified imprecise acceptance criteria when conducting the system performance test using the test panels. These specifications required that, on a daily basis, the test panel exhibit a specified number of indications based on the penetrant's sensitivity (see Table 1).
In addition, several aerospace specifications acknowledged this test as a penetrant sensitivity test. This only added to the quandary that we are experiencing in the industry today. The acceptance criteria prescribed by these specifications do not take into account that the methods for inducing the cracks of these panels were not collaborated with empirical data (for example, comparing different penetrant sensitivities and manufacturers against a particular number of known crack sizes). In addition, there are several manufacturing inconsistencies, such as plating thickness and varying forces used by the hardness indenter, that contribute to the test panel being an imprecise and inappropriate tool for determining a penetrant's sensitivity. The fundamental objective of the system performance test is for the penetrant testing system to be able to demonstrate its capability to reproduce the same number of indications of the known discontinuity standard on a continual basis regardless of the penetrant's sensitivity level. An acceptable test would convey confidence that the system is in control to the user of the penetrant testing system. Note that the penetrant testing system encompasses as a minimum the penetrant materials, washing equipment, lighting equipment, temperatures, pressures and the operator's techniques (an essential element). Therefore, when using the test panel, the system performance test is essentially a test by attributes (go/no go), not a test by variables.
In essence, the criteria for the penetrant testing system in reality should be stated as follows: the number of indications found during the system performance test shall be equal to those found during the initial qualification of the penetrant testing system with all parameters, equipment and techniques being equal.
If the system is not able to exhibit the same number of indications found during the initial qualification, then the panel should be thoroughly cleaned and reprocessed. If after reprocessing, the required number of indications is still not evident, then a referee (master) panel for the same level of penetrant sensitivity and method of removal (for example, water washable or postemulsifiable) should be processed and its indications compared to its respective initial indications. Note that panels should not be compared against each other, since no two panels are alike. If the master panel fails to display the required number indications, then an investigation and subsequent corrective action is warranted for the system.
Compliance
Even though most of the aerospace industry requires the use of a known discontinuity standard for the daily system performance test, a number of them, remarkably, do not require a penetrant system/test panel qualification (an initial calibration). Consequently, when conducting an audit of a laboratory's fluorescent penetrant testing system, NDT auditors need to be aware of this disparity and should therefore conduct a through review of the pertinent specification(s) prior to the audit.Although most aerospace specifications, including MIL-STD-6866, ASTM E 1417 and SAE AMS 2647, require that a comparison of the indications obtained on a daily basis to those originally obtained with unused samples of the same materials be employed as the acceptance criteria, the industry's approach has been to compare the number of indications to the penetrant's sensitivity levels (see Table 1). It is this author's assertion that the criterion has always been properly stated in MIL-STD-6866 and ASTM E 1417, but, ever since the initiation of the test panel, it has been inappropriately mandated by the aerospace industry, misunderstood by NDT laboratories/facilities and mistakenly accepted by NDT auditors.
Penetrant Sensitivity Test
Certain aerospace primary specifications, MIL-STD-6866 and ASTM E 1417 require NDT facilities to compare unused versus in use penetrant materials on either a monthly or weekly basis by processing both materials in order to demonstrate that there is no significant difference between them. The acceptance criteria referenced by these specifications is "sensitivity of the in-use penetrant noticeably less than the reference (un-used) is unsatisfactory." This author's opinion is that noticeable difference should mean not only the number of indications seen, but also the size and brilliance of the indications.Although the specifications require the sensitivity test to be conducted using the same procedures used for conducting the system performance test, they do not reference the type of known discontinuity standard to be utilized. Many NDT facilities, including certain penetrant testing laboratories, are using test panels for conducting the sensitivity test.
It is the opinion of this author that it is not reasonable to use test panels for the penetrant sensitivity test unless the criterion specified is to simply count the number of indications found and compare this number against those found with unused materials or (recommended) each indication is measured and compared to original measurements (within an appreciable tolerance). Surprisingly enough, test panels may be used for conducting the penetrant sensitivity test, if they are properly validated.
Rationale
During the initial qualification (upon receipt) of the test panel used for the system performance test, the laboratory should process it with unused samples of the same materials to be used with the penetrant testing system. This author recommends that the test panels be processed a minimum of three times for continuity during this initial qualification. Each indication should then be measured and the average size recorded. It stands to reason that if a laboratory periodically reprocesses the panel with in use penetrant (unused emulsifier, when applicable) and unused developer, then measures the indications and compares this data against the original sizes (within an appreciable tolerance), the lab is meeting the intent of the specifications for the penetrant sensitivity test. Of course, this is only possible with a properly maintained known discontinuity standard.In addition to the sensitivity test, these measurements may also be used for conducting the periodic maintenance (degradation) checks of the test panels required by certain aerospace corporations' specifications and industry specifications. ASTM E 1417-99, paragraph 7.8.3.1 states "The maintenance procedures shall ensure that cleaning of the standards between usages is adequate and that physical changes in the standard that make it unsuitable for use can be detected." By periodically reprocessing and remeasuring the indications, one is ensuring that physical changes in the standard that make it unsuitable for use is being detected.
Ideally, the preferred method for conducting the penetrant sensitivity test would be to use a known discontinuity standard which provides a more accurate test (side by side comparison under the black light) such as the NiCr twin panels (see Figure 2) or the twin known discontinuity standard panels (see Figure 3).
One of the panels is to be processed with in use penetrant material (unused emulsifier, when applicable) and unused developer. The accompanying panel is processed with unused penetrant material, (unused emulsifier, when applicable) and unused developer. Both panels are then observed side by side under the proper ultraviolet light for overall brightness, color, presence and/or absence of the indications. Any noticeable difference between the materials shall be cause for rejection of the in use penetrant material. Note that photographs of the known discontinuity standard's indications are not reliable, primarily due to the fact that the photo must be viewed under a white light while the known discontinuity standards are viewed under an ultraviolet light and, therefore, it is not an actual side by side comparison.
Conclusion
Aerospace companies, laboratories and auditors should be advised to:
verify that the users qualify (calibrate) all test panels with their respective penetrant testing system immediately after purchasing to confirm the quantity and actual sizes of the starburst/linear indications 
demonstrate repeatability on a daily basis of the same number of indications found during the initial penetrant testing system/test panel qualification (calibration) regardless of the penetrant's sensitivity level for the periodic sensitivity test, use test panels only when each indication size is known and this data used for comparison against in use materials (original measurements) or as an alternate, require the use of NiCr twin panels or the twin known discontinuity standard panels for the sensitivity test. Acknowledgments
Pertinent information used herein was provided by Amos Sherwin and Sam J. Robinson of Sherwin, Inc.
References
Army Materials Directorate, MIL-STD-6866, Military Standard Inspection Liquid Penetrant, Aeronautical Systems Center, 1996.ASTM International, ASTM E 1417, Standard Practice for Liquid Penetrant Examination, Annual Book of ASTM Standards, Metals Test Method and Analytical Procedures, Vol. 03.03, 2000, pp. 726-735.
Society of Automotive Engineers, SAE AMS 2647A, Fluorescent Penetrant Inspection Aircraft and Engine Component Maintenance, 1995.
Vasquez, Israel, "Fluorescent Penetrant Inspection System Performance Test Using Known Defect Standards," Materials Evaluation, Vol. 56, 1997, pp. 1319-1322.
* Vastek Consulting, 7290 Jasmine Dr., Hanover Park, IL 60133; (630) 213-3432; fax (630) 213-3495; e-mail <vastek@aol.com>.
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