Lpi I 1k38c

Fluorescent Penetrant Inspection System Performance Test Using Known Defect Standards

Most specifications state a requirement for conducting this check, but do not provide a procedure. A chrome plated strip approximately 44.5 mm wide x 0.076 mm thick (1.75 x 0.003 in.) covers the length of the , and the remaining area is a grit blasted surface of medium roughness used for determining penetrant material's washability. The chrome plated strip contains five cracked centers evenly spaced at 25 mm (1 in.) apart (Figure 1). These cracked centers are in circular patterns, and vary in size from about 3.8 to 6.3 mm (0.015-0.25 in.) in diameter, and are arranged in order of magnitude. The cracks may disseminate from the center in a star burst or a linear pattern. manufacturers also provide Tam s for which the chrome strip has been lightly grit blasted. These types of Tam s are recommended when parts or components with rough surfaces are to be liquid penetrant inspected.

Figure 1 - Tam showing fuorescent indication sizes. Although neither MIL-STD-6866 nor ASTM E 1417 specify the type of known defect standards to be used for the system performance check, several aerospace specifications require the use of Tam s for this function. These specifications require that penetrant inspection materials exhibit a specified number of indications based on their respective sensitivity level (Table 1). Table 1 Sensitivity level and indications required for materials Sensitivity level

Minimum No. of Indications

Low

Two largest

Normal

Three largest

High

Four largest

Ultra high

All five

Discussion Some of the most common oversights sustained by penetrant inspection facilities using Tam s are:  

not qualifying (calibrating) the s prior to use, and misunderstanding their applicability.

Qualification Upon receipt, if the s have not been serialized by the manufacturer, the inspection (purchaser) facility should serialize each and designate two s (working and master) for each type and sensitivity level of penetrant in use. The certificate accompanying the Tam s from the manufacturer does not state the actual sizes of the indications. This certificate will attest only that the meets the requirements of Tam Drawing 146040. The exception is the Five Star NDT certification which does state the actual sizes. Nonetheless, the inspection facility should qualify (calibrate) the Tam by processing it with unused penetrant materials of the type and sensitivity for the 's intended use, measuring the longest line of each fluorescent indication under the appropriate ultraviolet illumination, and recording the measurement. The measurements may be direct, using an optical comparator, caliper with the tips coated with a fluorescent paint, stereoscopic microscope with a reticule and fluorescent lighting background, or any other reliable method, and shall be recorded on a suitable form for subsequent use. The same line should be measured to ensure repeatability, especially during the periodic degradation check (Table 2). The measurements will confirm that the 's indications meet the minimum sizes as required by the drawing, and should be used to establish a baseline for the periodic degradation check. This degradation check is required by several aerospace primes to determine whether the 's ability to exhibit the indications has deteriorated.

Table 2

Photographs (with a scale) of the indications have been recommended by some, but it is a less reliable method for one-to-one comparison purposes, or for degradation checks. Degradation Check ASTM E 1417 states that the "maintenance procedures shall assure 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." This can be accomplished by periodically (at least monthly) reprocessing the s with unused penetrant materials and comparing the sizes to the initial sizes taken when the was first received. A practical tolerance should be allowed for gain or loss of indication sizes. Several facilities are using a tolerance of approximately 30 percent from the initial measurements. These measurements are also to be used during the system performance tests in order to evaluate: 



a drastic change of the penetrant inspection system's Performance. This change may be due to the penetrant or the emulsifier becoming contaminated, improper use of developers, improper dwell times, improper emulsifier concentrations, improper washing parameters, or excessive temperatures. the Tam s indications (crack centers) have degraded. This degradation may be evidenced by a decrease or increase in indication size. A decrease may be due to contamination with residue penetrant materials, excessive handling (skin oils), and improper washing, after each use. An increase in crack sizes may be due to thermal shock (excessive drying, overheating) or by simply dropping the on a hard surface.

Misunderstanding The system performance is the requirement for the penetrant inspection system (materials, washing, lighting equipment, operator's techniques, etc.) to be able to demonstrate its capability to exhibit the indications (quantity, sizes, and brilliance) of the known defect standard (Tam ) on a continuing (reliable) basis. Several inspection

facilities have misinterpreted this requirement. They are under the assumption that a level 2 sensitivity penetrant is required only to exhibit the three largest indications during the system performance test; whereas during the initial qualification (calibration) of the Tam , and occasionally during daily system checks, up to five indications may have been exhibited. If the system cannot demonstrate repeatability, regardless of the number of indications required by the specifications, then the penetrant inspection system should be considered suspect. The number of indications required by the specifications are the minimums for that sensitivity level of penetrant, not the minimum for the penetrant inspection system. If the system is not, therefore, able to exhibit the same number of indications found during the initial qualification, then the should be thoroughly cleaned and reprocessed. If, after reprocessing, the required indications are still not evident, then the master for that level of penetrant should be processed and its current indications compared with its respective initial indications. If the master fails to exhibit the required indications, then an investigation and corrective action is warranted. The investigation may include an assessment of impact on inspections, processes, and parts or components. Penetrant Sensitivity Tests MIL-STD-6866 and ASTM E 1417 require inspection facilities to compare unused with in-use penetrant materials on a monthly basis by processing both materials in order to demonstrate that there is no significant difference between them. The acceptance criteria in both standards states "sensitivity of the used penetrant noticeably less than the reference is unsatisfactory." "Noticeable difference" should mean not only the number of indications seen, but also the size and brilliance of the indications. Although both standards state that the sensitivity test shall be conducted using the same procedures used for performing the system performance test, they do not state what type of known defect standards are to be used. Several NDT facilities, including certain penetrant testing laboratories, are using Tam s for conducting the sensitivity test. Concern Tam manufacturers state on the brochures accompanying the s that:  

"No two Tam s are identical. Crack patterns vary from to , as chrome plate properties and thickness cannot be precisely controlled," and "... nor is the a sensitivity comparison tool: it does not replace the NiCr s."

It is not, therefore, practical to use Tam s for this test unless the criterion adopted is simply the number of indications found with used materials compared with the number found with unused materials. If the facility conducting this test is attempting to validate the number, sizes, and brightness of the indications by using Tam s, then

it stands to reason that by comparing against the will always detect a noticeable difference and therefore is required to reject the used materials every time the test is conducted. Resolution The known defect standards which would give a more convincing evaluation (side by side comparison) of the monthly sensitivity tests are the nickel chromium (NiCr) s; (See Figure 2). These s are made from a 2 mm thick x 100 mm long ´ 71 mm wide (0.079 x 3.94 x 2.8 in.) brass plate. A nickel plating is applied to the brass plate, which is then flashed or plated with a thin layer of chromium. The 's plating is then cracked by applying tension parallel to the long dimension. The cracks do not extend into the brass base. The depth of the cracks is equal to the thickness of the plating, allowing for an accurate control of crack size. After the is cracked, it is cut lengthwise, thus allowing for two s with identical cracks running across its width.

Figure 2 - A nickle-chromium (NiCr) showing its construction. These s may be purchased with crack depth in several sizes (measured in micrometers[µm]) depending on the manufacturer and level of penetrant sensitivity to be tested. The sizes and respective sensitivity levels are given in Table 3. Table 3 Size and sensitivity levels for NiCr s Sensitivity level 1

2

3

4

40 µm

20 or 30 µm

10 µm

NiCr sizes 50 µm

One of the s is to be processed with in-use penetrant material and the accompanying is processed with the unused (reference) penetrant material and observed side by side under the proper ultraviolet illumination for overall brightness, color, and presence or absence of the indications. Any noticeable difference between the materials shall be cause for investigation. Conclusion Penetrant inspection facilities should be informed of and be required to:     

qualify (calibrate) all Tam s immediately after purchasing to confirm the number and actual sizes of the star burst/linear indications, and not use the drawing sizes for the periodic degradation check. compare the Tam s indications during the System Performance Test against those obtained during the initial calibration. not compare Tam against (working vs. master) Tam since no two s are alike; rather to compare the Tam against its respective initial calibration sizes. demonstrate repeatability of the same number of indications found during the initial Tam calibration regardless of the penetrant sensitivity level during the periodic System Performance Test. not use Tam s for the monthly sensitivity test for comparing used with unused penetrant materials.