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Liquid Penetrant Panel Calibration

by Peter Stephens*

 

Aerospace specifications require that the "starburst" indications produced on test panels throughout the NDT process be compared with those produced at initial calibration. This is essential for checking the efficacy (or system performance) of the penetrant process. The reference standard can be created through a number of different means, each with its own advantages and disadvantages. The following article explores these options and is intended to open an exchange of views; it does not necessarily try to arrive at a final conclusion.

APPROACHES

There are four broad approaches to the generation of a calibration (known discontinuity) standard:

• photography
• adhesive tape
• strippable lacquer
• direct measurement.

Within these categories there are subdivisions:

• photography: digital or film
• adhesive tape: opaque or transparent
• strippable lacquers: with or without developer powder
• direct measurement: overall area or length of longest indication within a starburst.
  

There is no single approach for assessment and replication of
known discontinuity standards.

ADVANTAGES AND DISADVANTAGES

Digital Photography

There are a number of variables that can affect the utility of using digital photography to capture images of penetrant indications (Figure 1). To digitally photograph fluorescing crack indications, consideration must be given to the resolution of the camera. For instance, is a 1.3 megapixel resolution sufficient or is 5 megapixels the minimum? Another issue that arises when using digital images is that of image processing. Image processing software can be used to manipulate the image before printing. Hue, contrast and saturation can all be adjusted, but what effect does this have on the integrity of the image? Remember that the goal is to create as realistic an image as possible, not to create an idealized version of the indications.

Other factors also have to be considered. The type of paper (matte or glossy) used to print photographs may affect their utility. The resolution of the printer is also important. Images taken at high resolution are going to be of limited value if printed on a low-resolution printer. There are additional factors (discussed below) that may affect how one views the resulting images.

Figure 1 - Digital photo (6 Mb) of calibration panel for comparison purposes.

Film Photography

To capture images of fluorescing crack indications on film, consideration must be given to the resolution and quality of the camera lenses, as well as paper type and quality. Since the required image must include both the fluorescent emissions from the cracks as well as the surrounding background, some visible light must be present to illuminate the chrome surface. At the same time, reduced light levels must be maintained to achieve maximum contrast of the fluorescent indications.

With both digital and film photography, the final image size needs to be carefully controlled for the inspector to be able to usefully compare the photo with the actual cracked panel.

Finally, while ultraviolet-A light is required to view the calibration panel, white light is required to properly view a photograph. Inspectors also require some time for their eyes to adjust to the new lighting. Although it is possible to view photographs under ultraviolet-A lighting, the indications on the photograph will not fluoresce, so the comparison is not "like with like."

Opaque Adhesive Tape

Opaque tapes have been used for years as a means of lifting developer, and the indication contained with it, from the surface of the test object. This can then be stored as is or pressed onto clear acetate film to prevent movement of the indications. It must be kept in mind that this requires the use of more developer than might ordinarily be applied. It also requires careful consideration of storage and viewing options.

Images stored in this way are viewed under ultraviolet-A illumination either directly on the adhesive surface containing the developer/indications or through the acetate sheet. If the image has been pressed onto acetate film, there is a potential loss of fluorescence due to the absorption of ultraviolet radiation by the acetate.

Transparent Adhesive Tape

The use of clear adhesive tape is similar to that of opaque tape in that more developer is required. Clear tape has the advantage, however, that it can be fixed to an opaque base (such as paper or a card) and the indication viewed through the tape. Again, the absorption of the ultraviolet illumination by the tape must be taken into account when selecting a suitable medium.

Finally, since these adhesive tape approaches take the actual fluorescent penetrant from the surface, they are subject to the same problems encountered with the dyes in the in-use penetrant, in that they can fade when repeatedly exposed to ultraviolet-A illumination (and, of course, sunlight) and therefore may require regular replacement. Storing in dark conditions and viewing for the minimum time aids in the protection of the images.

Strippable Lacquer with Developer

Strippable coatings, by their very name, imply that removal from the test surface is easily achieved. This is not always the case, and a measure of patient practice is required if all the coating is to be removed from rough cast or shot-blasted surfaces. It is here that an application of developer can be of assistance. The action is similar to the use of "parting powder" in sand casting, where the application is used to assist in the separation of the cope and drag and the removal of the pattern.

However, the prime function of the developer is to create negative (or reverse) capillarity and cause the penetrant to creep out of the discontinuity and spread over the surface. This results in the enlargement of the indications, with the potential to create a replica whose indications appear larger than those on the discontinuity standard itself. The use of developer should therefore be limited to exactly the same procedure as for the daily performance test - usually an application of dry powder developer in a dust storm cabinet.

Finally, the replica is an actual sample of the penetrant contained in the indication, and therefore is subject to the same fading problems as with adhesive tape.

Strippable Lacquer without Developer

Applying the coating directly on the surface reduces the spread of the indications but does not completely eliminate it. It should also be borne in mind that the image produced by lacquer replica is a mirror image. As an aid to stripping the replica, a somewhat thicker coating may be required, but like the developer-assisted replica, it still suffers from the problems of fade and embrittlement with age. Plastic encapsulation can be used, but the type of plastic needs to be carefully chosen, as some types absorb ultraviolet-A.

Direct Measurement (Overall Area)

The actual measurement of the starburst requires ultraviolet-A illumination, a suitable, calibrated loupe (or measuring graticule) and a clear understanding of what is to be measured (Figure 2). This is not the sort of operation that would be undertaken as part of the regular performance test, but is a very useful guide for determining changes that could make the panel unsuitable for use.

Measuring the overall area of the individual starburst requires a graticule on which is etched a series of circles. These are available and are used primarily for sizing gas holes, gas pores and porosity in castings. The circle that is able to contain the whole of the starburst is taken to represent the size of that starburst. These results are then recorded and reassessed on a regular basis, the frequency of which will depend upon the process specification requirements.

Figure 2 - Individual starbursts (6 Mb digital image) for size change evaluation.

Direct Measurement (Maximum Dimension)

Once again, ultraviolet-A illumination, a suitable, calibrated loupe (or measuring graticule) and a clear understanding of what is to be measured are required. Measuring the maximum dimension of the individual starburst requires a graticule on which is etched a linear scale. These are available and are used primarily for sizing cracks, laps and linear indications. The maximum dimension of the starburst is taken to be the longest leg of that starburst. These results are then recorded and reassessed on a regular basis, the frequency of which will depend upon the process specification requirements.

The advantage of both these approaches is that the individual crack is being measured and no actual replication is required. This eliminates any enlargement problems as well as the fading and embrittlement issues.

Results

There is no single approach for assessment and replication of known discontinuity standards. Currently, all of those described above are specified in one form or other by a variety of contractors, leaving some choice to the supplier, but since each approach has its own advantages and disadvantages, a clear choice for a single approved approach is not obvious.

Conclusion

Conclusions on this topic are not easy to draw. While there are advantages and disadvantages to each of the various approaches I've discussed above, none of the possible solutions to the problem at hand stand out as definitively superior to the rest.

This is where I hope that the reader will come in. Any comments would be gratefully received, not just from the prime contractors who create the requirements but also from the inspectors who must apply these approaches and the Level IIIs who must interpret the requirements for their own company procedures. I can be reached at the e-mail address given in the contact information on the first page of this article, and I look forward to hearing from you.

 

* 54 Jordan Rd., Sutton Coldfield, West Midlands B75 5AB, England; 44 121 308 7466; fax 44 121 241 9172 e-mail p.stephens@ukonline.co.uk.

 

Copyright © 2007 by the American Society for Nondestructive Testing, Inc. All rights reserved.

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