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In this work, the evaluation of brake disc fixation bolt integrity, used at high speed train wheels, is presented. Inspection procedure is detailed, and it is shown how progressive focusing technique (ProFoc®) ensures the best lateral resolution at any inspection depth. The goal of the inspection is to detect three artificial notches that emulate a broken thread at different depths. Figure 2 shows resultant images with three configurations: (a) fixed focus at 20mm, (b) fixed focus at 40 mm and (c) progressive focusing (ProFoc®). A quite short deep of field (DOF) is obtained when a single focus is applied, which prevents to evaluate the whole component with a single image. Progressing focusing technique overcomes this problem, because it maintains a constant angular resolution at any inspection depth.
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