Penetrant System Performance Verification Requires Matched Known Discontinuities Standards
ASTM E-1417 recently superseded MIL-STD-6866 as the specification governing the use of liquid inspection penetrants. With this change there are new demands on penetrant users. One is a more exacting procedure for verifying penetrant system performance.

It has been an established practice since the 1970s to check the performance of production penetrant systems at scheduled intervals, such as at the beginning of each shift, by processing a test piece having known discontinuities. Substandard results observed on the known discontinuity test piece would be cause for shutting down the system until the reason for the substandard results was found and remedied.

Like its predecessor, ASTM E-1417 includes the scheduled processing of a test piece, referred to as a known discontinuity standard (KDS). However, it also requires the results of the processing to be compared to the results of processing with unused penetrant material. The comparison can be made to a photograph, a record of previous processing with unused material, or to a second KDS processed with unused penetrant materials in the same time frame.

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"ASTM E-1417 includes the scheduled processing of a test piece, referred to as a known discontinuity standard (KDS)."

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 The rationale behind the KDS processing requirement is not only to prevent running a suspect penetrant system but also to establish a time period of possible nonperformance. Should the KDS, after processing, indicate that the system is not functioning properly, a period of possible malfunction is outlined as being between the last successful KDS test and the failed test. It is important to establish this time slot so that parts processed during that period can be reinspected, thus eliminating any doubts as to their integrity.

The authors have encountered situations where failing to process a known discontinuity standard has placed an organization in a quandary as to when the penetrant system began to malfunction, how many parts went through the system while its performance was inadequate, and the whereabouts of such parts. For example, during weekend building repair activity, an overhead water pipe ruptured into a tank of water washable (method A) fluorescent penetrant contaminating the penetrant solution to the point of inversion. In this mode, the solution lacked brilliance, had minimum capillary action, and had no staying power during the wash cycle. This malfunction was not recognized until days later when it was noticed that penetrant system rejects were inordinately low. If a KDS had been processed with the first shift on the Monday morning following the pipe rupture, no parts would have been processed until the penetrant had been replaced. The problem would have remained a controllable one.

 

New Requirements Under ASTM E-1417
As mentioned, the NDT industry, because of incidents such as above, has recognized for some time the value of processing a known discontinuity standard. But ASTM E-1417 has expanded this requirement to read:

7.8.3 System Performance - The penetrant system shall be checked daily for performance. The check shall be made with known discontinuity standards. The check shall be performed by processing the known discontinuity standard through the system using appropriate processing parameters and comparing the indications thus obtained to those obtained with unused samples of the same materials. This comparison may be made with photographs or other such records of previously obtained indications or with a similar known discontinuity standard processed with the unused materials ... (ASTM-1417 Paragraph 7.8.3.1)

ASTM E-1417 differs from its predecessor, MIL-STD-6866, by calling for a com-parison of the KDS reading to that of a photograph, to an equivalent KDS, or to records of processing with new, unused penetrant material.

Until this new comparison requirement appeared, detecting a nonperforming or underperforming system depended solely on the eye and memory of the conscientious technician. This technician was sufficiently familiar with the KDS being used to notice the system's sudden inability to reveal known discontinuities with the usual intensity and form. Such a change would signal the sudden deterioration or misapplication of the penetrant materials - penetrant, emulsifier or developer - to an out of control oven or wash cycle, or some other malfunction. With reliance on the eye and memory of the technician, the KDS delineated the sudden change. Calling attention to the unforeseen was the primary duty of the KDS prior to ASTM E-1417.

Under MIL-STD-6866, detecting gradual shifts in performance, such as a gradually diminishing fluorescent brightness, was not expected by observation of the KDS after daily processing. Such detection is now deemed important and, for this reason, the ASTM paragraph cited above requires a comparison of the results of the daily check to a photograph, to records, or to a similar test piece.

Furthermore, although there are some allowances for avoiding these tests, ASTM E-1417 calls for a weekly test of the penetrant's sensitivity as well as a monthly test to monitor deterioration of the surface penetrant removal function of the penetrant system. This is true whether the penetrant is water washable (method A) or postemulsifiable (methods B and D). These tests rely on comparison of known discontinuity standard results, one processed with in use penetrant or emulsifier, and one with new, unused penetrant or emulsifier, as the case may be. Again, comparison can be made to a photograph, to records, or to an equivalent KDS. If the performance of the in use material is noticeably less than the new or reference material, the in-use material is classified as unsatisfactory.

 

Photographs and Records Not Meaningful
The authors argue that comparing a processed KDS to a photograph or to records is not truly meaningful. Comparison to a photograph is not a realistic choice. Black light photography requires time exposure whose length affects the size and definition of the indication image. Moreover, the color in the photograph will not match the color of the indications on the KDS; the color will depend upon the filter system. Comparison would be under black light, and photos are meant for viewing under white light. But more obvious, the discontinuity marks in a photograph do not fluoresce. How can fluorescent brightness or degradation be judged and compared in a photograph that does not respond to black light?

Using records for comparison also has limitations. How can the most critical factor in determining the integrity of a fluorescent penetrant system, indication brightness, be recorded by any other than subjective observation?

The third choice, comparison to a similar test piece, is realistic only if the two test pieces are virtual twins. As KDS test pieces traditionally have been produced separately, it is not easy to find two with induced cracks sufficiently identical to fulfill the comparison function.

The known discontinuity standard of choice has been the PSM-5, penetrant system monitor panel, also referred to as the TAM panel. It meets the requirements of ASTM E-1417. The PSM-5 panel was introduced in 1975 and conforms to P&W drawing TAM 146040.

PSM-5 panels have cracks, varying in magnitude from 0.4 to 6.4 mm (0.015 to 0.25 in.), induced in a strip of hard chrome plating about 0.08 to 0.1 mm (3 x 10^-3 to 4 x 10^-3 in.) in thickness. Adjacent to and paralleling the chrome plated strip is an area of the 321 stainless steel substrate which has been lightly roughened by grit blasting. The cracks are induced with pressure exerted from a Brinnell hardness tester. Unlike past panels, such as twin NiCr panels, these panels are sufficiently rugged to take the abuse of processing. The panels are produced individually; no two panels are identical.

Although the PSM-5 panel has served the penetrant industry for over 20 years, it does have limitations. The depth of the plating, 0.08 mm (3 x 10^-3 in.) or more, and the inherent splintered form with which hard chrome cracks makes cleaning between tests very difficult. And ASTM E-1417 mandates that "the maintenance procedures shall ensure that cleaning of the standards between uses is adequate." (ASTM-1417 Paragraph 7.8.3.1) Penetrant retained in the induced discontinuity from previous tests lowers the confidence that PSM-5 readings accurately portray the penetrant system's condition.

Another weakness of the PSM-5 panel is its insufficient sensitivity to malfunctions. The deep plating makes the induced crack indications less susceptible to heat fade from an overheated drying oven, or to wash out from an overactive removal system, such as a too concentrated hydrophilic emulsifier bath. The depth of plating and the other described factors limit the ability of the test piece to discriminate between a marginal system and one that performs satisfactorily.

 

New Specification, New Requirement, New Panel
Complying with the spirit of ASTM E-1417 calls for a new test piece, an improved known discontinuity standard. The improved KDS must be supplied with a matched twin for comparing in-use materials to new, unused materials. It must meet the ASTM E-1417 stipulation that cleaning - such as clearing the cracks of penetrant from previous testing - is assured. Further, it should be more sensitive to deviations from normal parameters than past test pieces. Finally, it must be rugged; able withstand the punishment of daily processing down a penetrant system line. Born of necessity, a new test panel was recently introduced to the NDT industry, known as the Twin KDS Panel. The new KDS panels differ from PSM-5 panels in these ways:

• Two panels are produced simultaneously so they are precisely matched as to plating, cracks, and surface roughness. They are virtual twins.

• The plating is not hard chrome with its highly reflective, glaring, and distracting surface.

• The plating thickness is 0.1 mm (1 x 10^-3 in.), not 0.08 to 0.1 mm (3 x 10^-3 to 4 x 10^-3 in.) or more, making it more sensitive to process anomalies.

• Cracks are not induced by pressure from a hardness tester, but by a patent pending, more precise method.

The following figures describe the making of the new KDS panels. Figure 1 illustrates the appearance of the panels as they are produced from a single sheet of stainless steel. The center section is grit blasted to specification roughness. The outer edges have a 25 mm (1 in.) strip of brittle plating with induced cracks of varying dimension, each strip is virtually identical to the other. Figure 2 illustrates the sheet after it is sheared into two equal sections called virtual twins.

Figure 3 illustrates how the two sections can be placed side by side so each strip receives an equivalent amount of black light. The juxtaposition of the new KDS panels gives a complete comparison picture with assurance of equivalent light exposure at one viewing.

Figure 1 - New KDS panel.

 

Figure 2 - New KDS panel sheared in two equal sections - virtual twins.

 

New KDS Panels Permit Reliable Penetrant Comparisons
The new KDS panels are ideal for highlighting differences in penetrant performance, both of penetrants and of penetrant systems. The following figures illustrate this point.

Figure 4 illustrates how the twins can be placed adjacent to one another to compare penetrant performance; in this example, developer versus no developer.

Figures 5 and 6 show how the new KDS panels can be used to compare penetrants. Figure 5 shows how the new KDS panels may be used to verify that a method D, level 3, fluorescent penetrant is more sensitive than its level 2 counterpart. Similarly, Figure 6 shows the comparison between a new water based fluorescent penetrant and a conventional, oil-based penetrant having the same sensitivity.

Figures 7 demonstrates how the new KDS panels can be used to highlight differences in penetrant system performance. It shows what happens when hydrophilic emulsion time is overextended.

Figure 3 - Virtual twins side by side for equal blacklight exposure.

 

Figure 4 - Level 2, method A: no developer versus developer.

 

Figure 5 - Level 2, method D fluorescent penetrant versus level 3, method D penetrant.

 

Figure 6 - Level 2 water based penetrant versus level 2 oil based penetrant.

 

Figure 7 - Level 3, method D emulsification time - 60 s versus 240 s.

 

Conclusions
The new ASTM E-1417 places an additional requirement on penetrant users to compare system performance against unused materials using a known discontinuity standard, or KDS. Prior to this new requirement, penetrant users routinely validated system performance using such test panels as the PSM-5 panel. However, the PSM-5 panel and the other means of validating performance - photos and subjective estimates- are not well suited to satisfying the requirements of ASTM E-1417.

To satisfy ASTM E-1417 requirements and to permit reliable penetrant performance comparisons, new KDS panels were developed. The panels come in pairs having known discontinuities in known sizes at known locations. The panels permit the monitoring of in use penetrants as well as side by side comparisons. They are rugged and withstand daily system processing.

*   Sherwin Incorporated, 5530 Borwick Ave., South Gate, CA 90280; (562) 861-6324; fax (562) 923-8370; e-mail sherwininc@aol.com.

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