|NDT.net - February 2003, Vol. 8 No.2|
Apparently that the type testing procedure should be conducted under the conditions ensuring reproducibility of its outcomes. We investigated various factors effecting upon the reproducibility. To obtain quantitative characteristics of indication’s visibility the image-processing hard-and software were used. It was established that there are two main groups of these factors. The first depends on the differences between various test panels, which are used for type testing. The second one concerns the reproducibility of some stages of penetrant testing procedure (excess penetrant removing and developer application). The recommendations to increase the reproducibility of type testing are developed and illustrated experimentally.
The determination of penetrant system sensitivity as a part of type testing should be conducted ensuring reproducibility of the result. We investigated some factors effecting the reproducibility. To obtain quantitative characteristics of indication’s visibility, an image processing system was used . We used 3 different sets of reference blocks (rb1, rb2, rb3) corresponding to type 1 of EN ISO 3452-3 . Two fluorescent penetrants with different sensitivities and the same wet solvent developer were used.
It was established that the reproducibility of type testing is influenced by mainly two principal factors. First there are the differences between reference blocks. Second one concerns the reproducibility of penetrant testing procedure, especially excess penetrant removal and developer application.
According to EN ISO 3452-3 a reference block for determination of sensitivity level of fluorescent penetrants consists of a set of three nickel-chrome plated panels with 10, 20 and 30 µm plating thickness. Transverse cracks are made in each panel by stretching the panels in the longitudinal direction and width to depth ratio of the cracks should be approximately 1/20. Each panel presents a certain sensitivity level, which is characterised by the level of visibility of the indication. The highest level 3 corresponds to the panel with 10 µm thickness plating. Each panel is cut longitudinally (perpendicular to the crack) so that 2 sheets (panels A and B) with nearly the same cracks will be produced. These panels A and B of a reference block are used for relative evaluation, for example to compare sensitivities of two penetrants. It is considered that the application of the same penetrant to both panels with subsequent adequate development results in the same visibility of the indications on these panels.
Quantitative correlations between the abilities of the reference blocks used for determining of the sensitivity were obtained in our investigations. We used the relations between the measured values of light-fluxes from the indications of similar comparing areas on two panels. These indications correspond to certain cracks’ parts of the same total length on both comparing areas. The relations of the steps between the panel are responsible for the definition of the sensitivity groups. From our investigations it follows that the steps of the different sets are different. Besides it was established that the indication‘s visibility for the same penetrant applied on different reference blocks might have a considerable difference depending on the penetrant sensitivity: the higher penetrant’ sensitivity, the smaller is such a difference between the steps 30/20 and 20/10. For example, for rb3 the relations 30/20 and 20/10 for lower sensitive penetrant (I1) reached factor 3 comparing with factor 2 for higher sensitive penetrant (I2).
Some results of the comparison between the 3 sets (rb1, rb2, rb3) investigations are summarised in Table 1 with rb1 as a reference.
In spite of the fact that the panel pairs A and B are produced similarly, differences between the panel pairs A and B sometimes occur. Our investigations have shown that for some reference blocks the date spread for indication’s visibility between A and B reached factor 3, which has to be taken into account. Averaged values of the results of 3-5 experiments conducted for the same panels were compared (Table 2).
|Table 1: Relations between different panel sets.|
|Table 2: Relations between the panel pairs A and B (A/B) for penetrant I2.|
Certain conclusions can follow from these results. Firstly, perhaps, some test panels (like the rb1 in our experiments) might be hardly used for absolute evaluation of penetrant sensitivity because their usage may result in obtaining the result, which corresponds to the highest level 3 even for the penetrants of level 2. Secondly, it seems reasonable to establish quantitative correlation between the abilities of various test panels used for determining of product family sensitivity by the independent laboratories, which are engaged in type testing. This may be introduced by a cooperation of the institutions interested in type testing, for example, by round robin action.
One more conclusion concerns the necessity of taken into account the difference between A and B panel, which requires appropriate equipment even for relative evaluation. For example, image-processing hard-and software seems being the optimal one.
Another factors, which considerably effect on the type testing reproducibility are the conditions (method, characteristics of a procedure) of the excess penetrant removal and developer application. For example in some our experiments even practically hardly noticeable difference in carrying out of excess penetrant removal resulted in the considerable (up to 50%) change of indications’ visibility (Fig. 1). Moreover, when we change the characteristics of excess penetrant removal procedure, then a sharp difference in penetrant testing outcomes takes place. For example, in our experiments the accurate rinse of test panels in water during 4 sec resulted in full absence of a hindering background (Fig. 2, a). However the penetrant was partially washed out from the cracks, which resulted in the dimming of the indications, obtained at a development, and also decreasing their area. Decreasing the rinsing and wiping time by the factor 2 has resulted in considerable increase of the light-flux from indications on the evaluated area. As a result, in spite of the presence of the background, the indications are revealed much better in this case (Fig. 2, b). Such a difference in the visibility of indications between Fig. 2, a and Fig. 2, b demonstrates the crucial importance of the maintenance of the conditions of realization of excess penetrant removal procedure as reproducible as possible to provide the reliable outcomes of the sensitivity estimation.
Fig 1: Indications’ images corresponding to two penetrant testing procedures
carried out using the panel of reference block rb1 (maximum crack’s opening 0,5 µm)
and penetrant I2 at the same technological parameters.
Fig 2: Indications resulting on the panel of reference block rb3 (maximum crack’s
opening 1 µm) after 4 s (a) and 2 s (b) excess penetrant removal at other parameters
The developer application has two main characteristics ensuring its reproducibility. These are the homogeneity and the thickness of developer layer. Fig. 3 illustrates a dependence of the indication‘s visibility on the developer layer thickness. During developer application two zones of different thickness were formed on the panel A of rb1.
|Fig 3: Indications‘ visibility at different developer layer thickness.|
Developer thickness of upper zone on Fig. 3 is about 3 times smaller than for the layer of lower zone. As one can see, the indications’ visibility is very different on these two areas.
Furthermore our experiments show that using spraying gun we get notably better outcomes comparing with the application of a spray from a can. Averaging the results of 3 times repeated and carefully conducted procedures we obtain acceptable results for the evaluation of the sensitivity. Besides both ultra-violet and visible light levels on test panel area should be always kept constant during all procedures.
Federal Ministry of Economics of Germany has supported the work.
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