Assessing the effect of penetration length variations on dimensional measurements with X-ray computed tomography

Abstract

From discussions in various standardization committees, testing material thickness influences on dimensional X-ray computed tomography (CT)—through variations of X-ray penetration-path lengths—has become a matter of great interest. Although this topic does not seem to be fully covered by the VDI/VDE 2630–1.3 guideline, other emerging documentary standards (ASME B89.4.23 and ISO/DIS 10360–11) are trying to address it in their guidelines. This paper evaluates variations in CT dimensional data when, to provide different penetration-path lengths to X-rays, enclosures of varying aluminum thickness are added (‘plus-shell’ condition) to surround three test workpieces during measurement. The test workpieces are two hole plates of square-shaped geometry with 28 drilled holes, one made of aluminum (48 mm × 48 mm × 8 mm) and the other made of steel (6 mm × 6 mm × 1 mm), and a multisphere artifact containing twelve ruby spheres of 4.0 mm nominal diameter. Geometrical features in the 0.5–66 mm range are studied. The main results show that while the deviations between CT and reference/calibrated data for uni-directional (center-to-center) lengths can be confined to a range of ±5 μm, with no significant differences in measurement results between no-shell and plus-shell conditions, bi-directional (edge-to-edge) lengths and form measurements have substantial differences—with deviations of up to 16 μm for form measurements in the aluminum hole plate in the no-shell condition and 42 μm when the same hole plate is housed within a highly irregular (gear-like) aluminum enclosure.