Browsing by Author "Cloete,Trevor"

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    An experimental investigation of interspecies variation in mechanical properties of cortical bone
    (2018) Welgemoed, Lee-Anne; Cloete,Trevor
    Cortical bone has been found to display properties that vary greatly, with most previous work focused on human or bovine bone. The main aim of this research was to investigate mechanical properties of cortical bone from various species of animal (ostrich, baboon, crocodile and sheep) to allow for comparison of mechanical properties across species, as well as investigate the relationship of material properties to strain rate within each species. This information is valuable for use in modelling. Testing was performed quasistatically and at the high end of the intermediate strain rate regime. These higher rate tests were performed on a modified Split Hopkinson bar setup. A Cone-in-Tube striker was used to provide a near constant strain rate during testing. The use of a momentum trapping system was investigated, but was not practically useful for the scale of specimens and strain rates investigated. It was found that properties of bone from all species displayed a clear dependence on strain rate. The relationship between the properties and strain rate were very similar across all species investigated, although the ultimate values differed. The apparent modulus showed a distinct increase when testing at a higher strain rate than when testing quasistatically. The compressive strength (both yield and ultimate) displayed an increase over the quasistatic range tested and then levelled out at the higher rate. There was greater difference seen in the values of apparent modulus than in the compressive strengths for all species tested. Baboon bone consistently displayed the greatest properties for both apparent modulus and compressive strength, and crocodile the weakest. It was also found that, despite whole bones and specimens being frozen at -32◦C, with the specimens frozen in normal saline, a change in the material properties of the bone occurred over time. Apparent moduli decreased, and compressive strengths stayed constant or increased slightly. Less change was seen in the strength than in the apparent modulus.
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    The effect of boundary conditions and architecture on the response of cancellous bone
    (2018) Hilton, Kelsey; Cloete,Trevor
    The investigation presented herein was performed to determine the effect of boundary conditions and architectural parameters on the mechanical properties of bovine cancellous bone. 124 cylindrical cancellous samples (7.5 mm height) were harvested from a total of 9 bovine humeri. Mechanical properties of these samples were determined using compression tests performed at three, quasi-static strain rates, namely: 10−3 , 10−2 and 10−1 s −1 .The specimen conditions investigated were standard specimens (with marrow, 10 mm diameter), defatted specimens (without marrow, 10 mm diameter), structurally confined specimens (with marrow and a confining collar, 10 mm diameter) and bone confined specimens (with marrow, 20 mm and 28 mm diameters). Each specimen was scanned using a µCT scanner (Phoenix, voxel size 80 µm, 70 kV, 350 µA, acquisition time of 500 ms per image). The images were used to determine the architectural parameters for each specimen, which were calculated using algorithms developed during the study. These algorithms were validated against existing software (BoneJ) which is available to calculate cancellous bone architectural parameters. The results of the compression testing showed little dependence of mechanical properties on strain rate. The results of the defatted and structurally confined specimens showed a decrease in scatter with the elimination and reduction, respectively, of flow of the marrow within the trabecular network. This suggests that although marrow does not strengthen bone at quasi-static strain rates, the flow of marrow disrupts the trabecular network. The bone confinement results showed significantly increased mechanical strength of the inner 10 mm core compared to the whole sample. Apparent modulus was found to be 58% and 60% higher in the central core of the sample for 20 mm and 28 mm samples respectively. This suggests that doubling the diameter effectively removes the edge effects, with any additional diameter increase having no effect. Inner core yield strength was 58% higher in the 20 mm samples, and roughly 96% higher in the 28 mm samples compared to full specimen yield strength. This suggested that post-yield behaviour requires a further increase in overall diameter to sufficiently remove the edge effects due to the boundary condition. University of Cape Town Department of Mechanical Engineering The results of the architectural parameters suggested a linear correlation between the mechanical properties and parameters bone volume versus total volume and trabecular number. An exponential relationship was found to exist between the mechanical properties and mean trabecular separation. No correlation was found between mechanical properties and mean trabecular thickness. It was also concluded that specimen condition affects the relationship between mechanical properties and architectural parameters. Therefore, to effectively predict the response of cancellous bone, specimen condition should be used in conjunction with at least two architectural parameters, preferably bone volume versus total volume and mean trabecular separation.