Use of a Developer
Brittain reported that the thick film output from a fluorescent penetrant could be multiplied by up to seven times when a suitable powder developer was used. This study concluded that the developer should be transparent to visible and UV light, have a low refractory index, and have a particle size above one micron (0.0004 inch). The author noted that the extent of spread of a penetrant material through a powder developer likely depends on both the penetrant and the developer used. To obtain optimum sensitivity, developers may need to be selected to suit particular penetrants.
- Brittain, P. I., The Amplifying Action of Developer Powders, QUALTEST 3 Conference, Cincinnati OH, Oct 1984.
Rummel shows that the use of developer can have a dramatic effect on the POD of an inspection. When a Haynes Alloy 188, flat panel specimen with a low-cycle fatigue crack was inspected without a developer, a 90 percent POD was never reached with crack lengths as long as 19 mm (0.75 inch). The operator detected only 86 of 284 cracks and had 70 false calls. When a developer was used, a 90-percent POD was reached at 2 mm (0.077 inch), with the inspector identifying 277 of 311 cracks with no false calls.
- Rummel, W. D., Probability of Detection as a Quantitative Measure of Nondestructive Testing End-To-End Process Capabilities, Materials Evaluation, January 1998, pp. 35.
It is pointed out in a paper titled, ?Evaluation of High-Sensitivity Water-Washable Fluorescent Penetrants,? that in special situations the use of a developer may actually reduce sensitivity. The specific example used in this report was the inspection of jet engine turbine blades for leading edge cracks. The blades had a crazed coating and some surface porosity that entrapped the penetrant resulting in numerous nonrelevant indications.
- Fricker, R. T., Evaluation of High-Sensitivity, Water-Washable Fluorescent Penetrants, Materials Evaluation, Vol. 30, No. 9, September 1972, pp. 200-203.
In a round-robin experiment organized by McDonnell Douglas, it was concluded that a developer might not always be necessary. Ten aluminum specimens were used in the study. Two of the specimens were said to be production parts with manufacturing defects. The other eight specimens were produced by placing electrical discharge machined notches in a machined surface groove; compressing the grooves closed with a load frame; and then machining the surface smooth. The ten specimens contained 22 defects total. The size of the defects was not reported. The specimens were processed by inspectors at ten companies using a fluorescent penetrant with a sensitivity level of 2 or 3. It is reported that 216 indications were recorded for the 220 find opportunities. It was also noted that in all cases, flaws detected with the application of a dry developer were also detected without a developer. It is not clear whether the author meant that the technique with no developer used was equivalent to the technique employing the dry developer. The author did caution that this study was aimed at certain manufacturing defects and the results should not be extended to parts with possible fatigue cracks.
- Smith, D. S., Penetrant Performance With and Without Developer, Paper Summaries of the ASNT 1996 Spring Conference/ 5th Annual Research Symposium, American Society for Nondestructive Testing, Columbus, OH, March 1996, pp. 139-141.
Gram reported that indications could be produced in the absence of a developer if the defect is large such that the penetrant seepage from it forms a fairly thick layer. He reports that cracks 0.025 mm (0.001 inch) in width were detectable without a developer but only a few microns wide were not detectable because the indication thickness was below the dimensional threshold of fluorescence. Therefore, developers must be used for highly sensitive inspection involving fine cracks.
- Gram, B., Mechanisms Contributing to Fluorescence and Visibility of Penetrants, Proceedings of the Fifth International Conference on Nondestructive Testing, May 1967, pp 225-233.
Type of Developer Used
In a POD study reported on Martian Marietta Aerospace and NASA, researchers produced a set of Inconel 718 specimens with a total of 281 fatigue cracks ranging in length from 0.254 to 6.350 mm (0.010 to 0.250 inch). They found that flaw detectability for a sensitivity level II inspection improved from 69 percent detection to 75.1 percent when dry powder developer was replaced with a nonaqueous wet developer. In addition, the influence of developers on sensitivity was studied. It was shown that the type of developer used has a considerable influence on the sensitivity attained. It was reported that wet suspension and dry powder developer produced a nearly equal improvement in sensitivity over no developer, and that solvent suspension produced significantly better results than the other developers.
- Christner, B. K.; Long, D. L., and Rummel, W. D., NDE Detectability of Fatigue-Type Cracks in High-Strength Alloys: NDI Reliability Assessments, Final Report, NAS126183539, MCR881044, NASACR183539, September 1988, pp. 3-1 to 5-17.
As part of a study to develop an improved FPI process for detecting small flaws in aircraft turbine blades, Pratt & Whitney reported that a water-soluble developer (ZP-13A) provided higher sensitivity and less background fluorescence when compared to a dry developer (ZP-4B). The water-soluble developer was reported to produce brighter indications, which is not a critical factor in detectability for relatively large cracks. However, for small cracks, a slightly brighter indication may mean that an otherwise missed indication will be seen.
- Malpani, J.K. and Cargill, J.S., Methods Improvement of the Fluorescent Penetrant Inspection (FPI) Process, US Air Force Technical Report #AFWAL-TR-80-4161, October 1980.
- Cargill, J.S. and Smith, K.D., Improved Penetrant Process Evaluation Criteria, US Air Force Report #AFWAL-TR-81-4124, October 1981.
In a paper presented by Mooz at the 12th World Conference for Non-Destructive Testing, the selection of the proper developer is reviewed. The paper primarily discusses how different developer types function and when they should be used or not used. The paper ranks the relative sensitivities of the various types as follows from low to high: dry developer, soluble developer, suspendible developer, and nonaqueous developer. Dry developers are least sensitive because they depend on a certain amount of penetrant to be present at the flaw opening for the developer to stick and they provide no contrasting background. Soluble developers form a very thin continuous coating over the part to absorb the penetrant but are opaque and provide little in the way of a contrasting background. Suspendible developers produce a relatively thick coating that provides good blotting action and a good contrasting background. Nonaqueous developers wet the surface with a solvent that carries the powder particles. The solvent enters the flaw and dilutes the penetrant so it is less viscous and can quickly be blotted up by the developer particles. Once the solvent evaporates, a full strength penetrant indication is left on the surface. The advantages and disadvantages of each developer type are summarized in Table 5.
- Mooz, W., Technical Differences Among Inspection Penetrant Developers, Proceedings of the 12th World Conference for Nondestructive Testing, 1989, pp. 417- 420.
MacCracken points out several other potential problems involving the use of wet soluble developers, which could affect sensitivity. These problems could also occur when a wet suspendible developer is used. When components with complex shapes are being inspected, it is sometimes difficult to get an even coating when a wet soluble or suspendible developer is used. Pooling can take place if excess solution is not removed properly. The parts must also be handled very carefully before drying as fingerprints, water drops, or other marks can hide defects. Finally, lint and other particles can be trapped in the coating and can result as a false call.
- MacCracken, B., Wet Soluble Developers: The Negative View, Materials Evaluation, Vol. 51, No. 8, August 1993, pp. 932-934.
Lord and Holloway rated the relative effectiveness of various developers at producing indications for several common defects in titanium. Both a level 2 post-emulsifiable fluorescent penetrant (Magnaflux ZL-2A penetrant and ZE-3 emulsifier) and a level 2 water washable penetrant (Tracer-Tech P-133A penetrant) were included in the study. Nonaqueous wet, aqueous wet and dry developers were studied. Specimens were also processed using no developer. The specimen defects included stress corrosion cracks, fatigue cracks and porosity. The study was qualitative and conclusions were arrived at by making side-by side caparisons of the indications. The following table summarizes their findings.
|Post-emulsifiable Penetrant||Water Washable Penetrant|
|Large Fatigue Cracks||1) All Developers Equally Effective
2) No Developer
|1) All Developers Equally Effective
2) No Developer
2) Nonaqueous Wet
3) No Developer
|1) All Equally Effective|
|Small Stress Corrosion Cracks||1) Nonaqueous Wet and Dry - Equally Effective
2) No Developer
|1) Nonaqueous Wet and Dry - Equally Effective
2) No Developer
- Lord, R. J. and Holloway, J. A., Choice of Penetrant Parameters for Inspecting Titanium, Materials Evaluation, October 1975, pp. 249-256.