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A UK Perspective on Known Discontinuity Standards
by Peter Stephens*
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As the old saying goes, "another country has been heard from." That would seem to be the introduction for this month's article. I did not open Pandora's box with the publication of an earlier article on penetrant standards, but it certainly has gotten notice and comments. Good!
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Introduction
In his paper "The Use of Known Discontinuity Standards," Israel Vasquez rightly points out that the phrase "noticeably less than the reference (unused)" is open to a great deal of subjective interpretation (2002). This author supports his assertion that not only is the number of indications (starbursts) important but so are the dimensions of each one.This piece offers a view from a UK perspective and from the author's experience in meeting the need for control of the penetrant process.
Codes and Standards
Vasquez's paper refers back to US specifications such as MIL-STD-6866 and ASTM E 1417, both of which are occasionally called upon in the UK. However, for the general engineering industries, a more common specification would be PREN 571-1-91 (European Committee for Standardization, 1991). In the aerospace industry, it is the primary manufacturers' requirements which take precedence. Here, such standards as Rolls-Royce RPS 702 (1999) and Lucas Aerospace 981-060-021 (1993) are some of the controlling documents. It is the approach of these two companies which will be reviewed here, since this author believes they address some of the points raised by Vasquez.
The initial calibration should be by replica rather than by photography.
Calibration and Wear of Test Panels
Appendix B of the Lucas Aerospace Standard 981-060-021 (1993) describes the mandated system for the annual calibration of the TAM 146040 panel. This involves processing the panel and a physical measurement of the indications that are produced. Appendix M then contains a specimen process log pro forma for the recording of the results obtained.It is the experience of the author (having complied with the above standard for some years) that the cracks do grow and that the TAM panels require replacement in surprisingly short time periods. This seems to relate to the frequency of processing and handling. The frequency of processing will be mandated by the relevant primary manufacturer and therefore cannot be limited, but the handling is under the control of the relevant laboratory or testing facility. It is the author's experience that it is handling which has the most significance in terms of life and durability of the panels. In many instances, fresh cracks emanating from the edges of the plates have been noted, indicating impact damage during processing. The cumulative effect of these changes makes the panel unsuitable for use.
Recording Initial Calibrations
Assume that during initial calibration four starbursts are found. This becomes the base standard for the specific process and its panel. Should the daily check reveal five starbursts, then this will require immediate review to determine what has changed (for example, the operator, the technique, the system or the TAM panel).If, however, during initial calibration all five starbursts are recorded, crack growth will not be immediately highlighted until a comparison with the results of this initial calibration is made. Even then (as Vasquez points out) we may only be looking to see if it is noticeably different.
This leads to a need for precise records of initial calibration. This author again supports Vasquez in his view that photographs are unreliable in that they need to be viewed in white light, may not even be the same size in reproduction and are not present in the same media or form. A better approach is the transfer lacquer replica which can be viewed simultaneously alongside the panel, in the same booth under the same lighting conditions.
Some years ago, Rolls-Royce designed a test piece which was supplied in a lined case which included the replica for direct comparison with the test piece (Figure 1). Since the test panel was made from a steel plate and the edges were chamfered, the risk of handling damage was greatly reduced and the potential for crack growth was minimized. This system ideally met the needs of a reliable reference standard. On the negative side, there were only three starbursts, which resulted in limited sensitivity testing.
Comparator Blocks
A further device worth consideration is the ASME cracked aluminum comparator block (1998). Here a cracked aluminum plate (Figure 2) is marked across the center to assign two similar halves. One half is then covered in unused penetrant and the other half is covered in used penetrant. The whole plate is then processed to completion in the relevant way. The indications from each half can be tested under identical illumination on the same alloy with the same surface condition, resulting in a true comparison of relative sensitivity. The slight potential for error with this device arises with the requirement that the discontinuities in both halves be as identical as possible. An advantage of the method is that no replica is required, since, in a similar approach of the matched pairs of Ni/Cr panels, comparison is made on the test plate itself.
Conclusion
This author supports the view expressed in Vasquez's article that users should carry out an initial calibration (qualification) of all test panels prior to first use. The initial calibration should be by replica rather than by photography. Dimensions of all starbursts should be confirmed at least annually and growth in excess of 25% is cause for rereplication if the growth is uniform and replacement if the growth is nonuniform or the plate is damaged.Consideration is given to the reintroduction of a thicker based test panel, similar to the one described, but with more (perhaps five) starbursts.
References
ASME, ASME Boiler & Pressure Vessel Code, T-653.2, Liquid Penetrant Examination, Fairfield, New Jersey, ASME, 1998.European Committee for Standardization, PREN 571-1-91, Nondestructive Testing - Penetrant Testing - Part 1: General Principles for the Examination, Brussels, Belgium, 1991.
Lucas Aerospace, Lucas Aerospace Standard 981-060-021, Penetrant Inspection, Hemel Hempstead, Great Britain, 1993.
Rolls-Royce, RPS 702, Non-destructive Testing, Fluorescent Penetrant Inspection, Derby, United Kingdom, 1999.
Vasquez, Israel, "The Use of Known Discontinuity Standards," Materials Evaluation, Vol. 60, 2002, pp. 141-145.
* NDTplus, 54 Jordan Road, Sutton Coldfield, West Midlands B75 5AB, England; 44 121 308 7466; fax 44 121 241 9172; e-mail <peter@ndtplus.com>.
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