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Title: Improving X-ray CT dimensional metrology with simple holeplates
Author: Corcoran, Hannah Clare
ISNI:       0000 0004 7659 9320
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2018
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X-ray computed tomography (XCT) is increasingly being used for the internal and external dimensional measurement of objects in many sectors. However, multiple factors affect the accuracy and precision of these dimensional measurements. Initial work found errors associated with the intensity and spectrum of the X-ray flux, both spatially and temporally over the imaging period. These variables were measured using a single ionisation chamber, an ionisation chamber array and a spectrometer, and their effects on dimensional metrology are discussed. The main focus of this work is on the apparent surface deformation of the reconstructed holes within a holeplate. Measurements of a complex holeplate with multiple holes highlighted systematic deformations in the reconstructed geometry (± 60 µm) for a 2 mm radius hole. Observed systematic deformation patterns were correlated with holeplate orientation. The simplification needed to understand the problem was achieved through practical testing and numerical simulation of simple holeplates, each containing a single hole. Similar trends in the systematic deformation patterns between complex and simple holeplates were found with deformations in the geometry of ±30 µm for a 2 mm radius hole. Algorithms using a ray tracing method were used to simulate X-ray images which were subsequently reconstructed. Results had the same trends in deformation as the experimental data but were of a smaller magnitude, probably due to scatter not being modelled in the simulation. Systematic deformation in the reconstructed geometry of the holes is caused by multiple factors: the orientation of the holeplate during imaging; the largest errors (< 60 µm) were found when the holeplate was orientated at 45° to horizontal. The path length of the X-rays through the material and errors of < 40 µm highlighted a correlation between the hole and the centre of rotation. Understanding these factors is essential to traceability in dimensional measurements using XCT.
Supervisor: Robson, S. ; Speller, R. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available