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Title: Applied photogrammetry in orthopaedic implant manufacture
Author: Evans, S. E. K.
ISNI:       0000 0004 7428 6283
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2018
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This thesis describes work performed to develop a rapid, low complexity metrology system for the purpose of analysing geometries of open cell porous coatings produce using Additive Manufacturing (AM). AM is applied to the production of porous material, particularly for orthopaedic implants, due to an ability to produce complex geometries in a repeatable manner from digital designs, using fewer steps than existing production methods. Variations in the AM process cause each component to be manufactured differently from one another requiring inspection post-build. Existing techniques for quantifying porous materials currently do not meet the requirements needed to practically inspect large numbers of manufactured components non-destructively. To overcome these limitations a metrology system based around image analysis and an off-the-shelf digital single-lens reflex (DSLR) camera was developed. This system is capable of capturing images of the top surface of planar porous material in a consistent and repeatable manner. A methodology of automatically processing and measuring the geometries of the solid material and pores comprising the porous structure with low deviation was developed. The system was validated using simple geometries, demonstrating accurate and consistent values, with a resolution and minimum resolvable feature size of 8 µm, below the required precision. The system was also compared directly to established techniques with the image based system demonstrating a lower deviation of measurements, covering a larger area of the porous structure, at a fraction of the time of the established methods. Additionally all measurements were performed non-destructively, with minimum user input, making the technique suitable for inspecting large volumes of AM porous components. Using this system, variation in the geometry of manufactured porous material was statistically quantified with respect to part location (intra-build) and from one build to another (inter-build). Part location on the build plate was shown not to significantly affect the variation in manufacturing however, significant differences were observed between builds, from one plate to another. Additionally the system was able to measure changes in porous material when deliberately altering the laser power, with the minimum variation in power causing significant change in geometry and mechanical properties of the porous material. The relation between mechanical properties and geometry was then determined. Measuring porous specimens manufactured at different laser powers to quantify their porosity and compressive yield strength, allowed the correlation between these properties and the geometry produced. Using this data, non-destructive estimation of the mechanical properties can be performed during image based inspection of the porous components. The final application of the image analysis system investigated measuring the geometry of non-planar porous components, with photographs captured using a 5-axis Coordinate Measuring Machine (CMM) and mounted camera probe. The developed image processing methodology was adjusted to compensate for irregularities in lighting and focus on the curved surface not apparent in planar specimens, through application of histogram equalisation and stitching. This allowed measurements to be taken of the entire surface of porous coated cylinders of different diameters. Results showed values close to those seen in planar porous material were produced with reduction in error compared to simply capturing and analysing individual images.
Supervisor: Sutcliffe, Chris ; Fox, Peter Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral