Research Outputs (School of Statistics and Actuarial Science)

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    Maintenance policies based on time-dependent repair cost limits
    (Academy of Science of South Africa (ASSAf), 2006-11) Beichelt, F.; Nkadimeng, R.M.; Yadavalli, S.S.
    This paper considers two replacement policies for systems which, during their useful life, are subject to deterioration. Strategy 1: after a failure, the repair cost is estimated. If the repair cost exceeds a given limit, the system is not repaired, but replaced with a new one. So far, this policy has been analysed only for constant repair cost limits. This paper investigates the effect of applying time-dependent repair cost limits on the long-run maintenance cost rate. Examples show that, compared to the application of constant repair cost limits, a reduction of the maintenance cost rate between 5% and 10% can be expected. Strategy 2: the system is replaced as soon as the total repair costs arising during its running time exceed a given limit. Compared to the economic lifetime method, which is based on the average repair cost development and that requires the same data input, maintenance cost savings up to 20% could be achieved.
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    Cortical Structure of Hallucal Metatarsals and Locomotor Adaptations in Hominoids
    (Public Library of Science, 2015-01-30) Jashashvili, T.; Dowdeswell, M.R.; Lebrun, R.; Carlson, K.J.
    Diaphyseal morphology of long bones, in part, reflects in vivo loads experienced during the lifetime of an individual. The first metatarsal, as a cornerstone structure of the foot, presumably expresses diaphyseal morphology that reflects loading history of the foot during stance phase of gait. Human feet differ substantially from those of other apes in terms of loading histories when comparing the path of the center of pressure during stance phase, which reflects different weight transfer mechanisms. Here we use a novel approach for quantifying continuous thickness and cross-sectional geometric properties of long bones in order to test explicit hypotheses about loading histories and diaphyseal structure of adult chimpanzee, gorilla, and human first metatarsals. For each hallucal metatarsal, 17 cross sections were extracted at regularly-spaced intervals (2.5% length) between 25% and 65% length. Cortical thickness in cross sections was measured in one degree radially-arranged increments, while second moments of area were measured about neutral axes also in one degree radially-arranged increments. Standardized thicknesses and second moments of area were visualized using false color maps, while penalized discriminant analyses were used to evaluate quantitative species differences. Humans systematically exhibit the thinnest diaphyseal cortices, yet the greatest diaphyseal rigidities, particularly in dorsoplantar regions. Shifts in orientation of maximum second moments of area along the diaphysis also distinguish human hallucal metatarsals from those of chimpanzees and gorillas. Diaphyseal structure reflects different loading regimes, often in predictable ways, with human versus non-human differences probably resulting both from the use of arboreal substrates by non-human apes and by differing spatial relationships between hallux position and orientation of the substrate reaction resultant during stance. The novel morphological approach employed in this study offers the potential for transformative insights into form-function relationships in additional long bones, including those of extinct organisms (e.g., fossils).
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    Thermal excitation of gadolinium-based contrast agents using spin resonance.
    (Public Library of Science, 2016-06) Dinger, S.C.; Fridjhon, P.; Rubin, D.M.
    Theoretical and experimental investigations into the thermal excitation of liquid paramagnetic contrast agents using the spin resonance relaxation mechanism are presented. The electronic spin-lattice relaxation time ole of gadolinium-based contrast agents, which is estimated at 0.1 ns, is ten orders of magnitude faster than the relaxation time of protons in water. The shorter relaxation time is found to significantly increase the rate of thermal energy deposition. To the authors knowledge this is the first study of gadolinium based contrast agents in a liquid state used as thermal agents. Analysis shows that when ô1e and other experimental parameters are optimally selected, a maximum theoretical heating rate of 29.4 °C.s.1 could be achieved which would suffice for clinical thermal ablation of neoplasms. The experimental results show a statistically significant thermal response for two out of the four contrast agents tested. The results are compared to the simulated estimates via analysis of a detailed model of the system. While these experimentally determined temperature rises are small and thus of no clinical utility, their presence supports the theoretical analysis and strongly suggests that the chemical structure of the selected compounds plays an important role in this mechanism of heat deposition. There exists an opportunity for the development of alternative gadolinium-based compounds with an order of magnitude longer τ1e in a diluted form to be used as an efficient hyperthermia agent for clinical use.