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Inspection Trends - January 2011 - Winter

Fig. 5 — Detection of the 6.9-mm-long notch using the flat and curved probes (single skip) on 70-mm pipe. A — Flat probe. The measured notch length is 9.6 mm. B — curved probe. The measured notch length is 7.1 mm. Fig. 6 — Detection of the OD end of the 1-mm through hole with the flat and curved probes (double skip) on 38-mm pipe. A — Flat probe. The measured size is 10 mm. B — focused probe. The measured size is 3.6 mm. particularly important for smallerdiameter pipes, as beam spread was more significant. Initially, an R&D project was defined to evaluate the possible solutions to beam spread with phased array units: matrix (2-D) arrays, mechanically curved arrays, or any combinations. Both matrix and curved arrays offer practical solutions for focusing in both axial and circumferential directions. Modeling was performed using PASS to determine the relative merits (Ref. 5). Modeling showed that only two curvatures were required to cover essentially all small pipe diameters, independent of wall thickness. The larger radius curved array was manufactured and tested on known reflectors, and compared with a standard flat (unfocused) array on pipes of 70 and 38 mm diameter. The summary results follow, and are compared with flat arrays. Figure 4 shows the modeled results, with beam profiles displayed in both beam axis and cross section. Comparing the images in the third (last) row with either the first or second row, it was immediately clear that the flat, unfocused beam had significantly worse beam spread than either the matrix array or the curved array. Wall thickness had been demonstrated as relatively unimportant since multiple skips were required for thinner walls, so beam paths tended to be quite constant. (Note that PASS cannot simulate the field with a beam skip, so the OD field is calculated by ignoring the pipe ID reflection and by just considering total metal path.) There was no obvious advantage in using the matrix probe over a linear array probe with an optimized radius of curvature. Matrix arrays would be more expensive and complex to implement. Curved arrays can be implemented with no extra hardware or software, unlike matrix arrays. Subsequent modeling with a 10-MHz linear array confirmed these results for 25-, 30-, 50-, and 75-mm diameters. In summary, the modeling showed that one probe with 40-mm radius of curvature is suitable for pipe with an OD greater than 25 mm, and one probe with 30-mm radius of curvature for pipe OD less than 25 mm. Two curved arrays effectively covered all pipe diameters. Experimental Confirmation of Modeling Results Selected for testing were two pipes in sizes 2.75 in. (70 mm) and 1.5 in. (38 mm). Two wedges were contoured to match the pipe diameters, as per standard practice. Appropriate notches and holes were used as reflectors. The Inspection Trends / Winter 2011 21 A A B B


Inspection Trends - January 2011 - Winter
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