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Weight reduction in automobiles is amongst the most economical ways of reducing greenhouse gas emissions and increasing fuel efficiency. The recently invented ablation casting process is a novel technique of producing high performance light weight parts which meet this objective. In this technique a water jet demolds the water soluble sand mold and subsequently impinges upon the solidifying metal, thereby producing high cooling rates in the casting which in turn leads to microstructural refinement and higher mechanical properties. The objective of this study was to develop a comprehensive understanding of the effect of various parameters involved in the casting of a thin walled part using the HiPerMag casting process for the wrought aluminum alloy AA 7050. The study is divided into three major parts that deal with the composition of the sand binder system, optimization of the sand mold thickness, various aspects of the water jet parameters and the desired microstructural parameters which will result in a defect free part. In first phase of the study, sand mold properties such as the green and dry strengths of the water soluble sand binder system used in the study were tested to ensure that they meet the molding requirements. An average green strength of 160 kPa and an average dry strength of 3825 kPa were found for the water soluble sand binder system. These values were similar to those reported in the literature for clay bonded sands and were sufficient to make molds for casting in the current study. Secondly, a heat transfer model was developed to find a minimum mold thickness required to design a mold for the HiPerMag casting process such that the liquid metal remained sufficiently insulated before being quenched. Based on the model, for a mold with a cope thickness of 12.9 cm, the heat flux losses to the surroundings were reduced by up to 90 % versus a case where a thinner mold was used. In addition, an analytical solution was derived for the mold thickness problem from which it was found that at a distance of 10 cm from the mold cavity there was a negligible increase in temperature of the sand at that location at large times. Further, the minimum mold thickness was determined based on the temperature profile in the sand mold during the HiPerMag casting process. This study showed that a thin mold of about 2 cm thickness was sufficient to provide insulation to the hot metal during the HiPerMag casting process. Thirdly, it was found that, based on cooling curve data and microstructural analysis, that a jet spacing of 15.3 cm and a time delay of 7.4 s between successive jet activations starting from the farthest jet (located near the edge of the casting), was necessary to obtain a single solidification front throughout the casting. This also ensured that grain size variation in the casting was less than 10 μm for having uniform mechanical properties. Also, it was found for a thin walled casting, the amount of solid present in the solidifying casting at the time of water jet impingement had a negligible effect on the movement of the solidification interface. Finally, the effect of jet momentum on surface defects was examined. It was determined that the maximum jet momentum resulting in no surface defects at temperatures close to the liquidus for Al AA 6061 alloy was approximately 2 kg.m/s. / Thesis / Master of Applied Science (MASc)

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Magister Scientiae Dentium - MSc(Dent) / Restorative dentistry aims to replace lost or damaged tooth structure with durable and life-like alternatives. To accommodate the inherent limitations and weakness of the restorative materials, preparation techniques often require the sacrifice of healthy tooth structure to create enough restorative space. This can lead to weakening of the remaining tooth structure, with subsequent damage or catastrophic failure. When using indirect restoratives, the development of adhesive luting agents (adhesive cements) and stronger allporcelain restorations (lithium disilicate) has contributed to the development of “minimally invasive” preparation techniques and concepts such as cavity design optimization (CDO) and bio-substitution. With these techniques, resin materials are combined with ceramic restoratives in an attempt to not only produce strong restorations, but also increase the longevity of the remaining tooth. The clinician needs to therefore find the ideal preparation design that combine such materials to produces a clinically performing restoration while increasing the strength and longevity of the underlying tooth.

The aim of this thesis is to propose a more general understanding of the influence of the thickness of the plate and of the microstructural and mechanical properties of the material on the resistance to ductile tearing in thin metallic plates. The objective is to attempt unifying different observations made in the literature together with the results of a new extensive experimental campaign. The final goal is to develop predictive simulation tools with a micromechanics-based foundation. In order to reach this objective, a detailed experimental campaign has been performed concerning the fracture behavior of the aluminium alloy 6082, complemented by experiments on a stainless steel A316L and on a set of 14 other materials. In a first modelling effort, we propose very simple closed-form models in order to separate the different contributions to the total work of fracture in thin plates: the work of necking and the work of damage and material separation. The respective contributions are compared and an unique explanation of the different behaviors observed experimentally is proposed. In a second modelling step, we develop a full 3D numerical tool based on cohesive elements for simulating crack propagation in thin ductile plates. Three different methods are proposed to calibrate the parameters of the model in order to reproduce the experimental data and to extrapolate the results to other material properties or geometric conditions. Finally, the parameters of the cohesive zone model are justified using micromechanics-based arguments. / Le but de cette thèse est de proposer un modèle général à base micromécanique permettant de comprendre l’influence de l’épaisseur de la tôle ainsi que de la microstructure et des propriétés mécaniques du matériau sur la résistance à la rupture ductile de plaques minces métalliques. L’objectif est d’essayer d’unifier les différentes observations de la littérature ainsi que les résultats d’une nouvelle campagne expérimentale afin d’aboutir au développement d’outils numériques prédictifs. Pour atteindre cet objectif, nous avons réalisé une campagne d’essais concernant le comportement à la rupture de différents matériaux. Cette campagne a été menée en profondeur sur l’alliage d’aluminium 6082 et de manière moins approfondie sur un acier inoxydable A316L ainsi que sur 14 autres matériaux. Dans un premier temps, nous présentons une série de modèles semi-analytiques simples dont le but est de séparer les différentes contributions au travail de rupture total dans les tôles minces : le travail de striction et le travail d’endommagement du matériau. Ces deux contributions sont ensuite comparées et nous proposons une explication qui reprend les différents comportements observés expérimentalement. Dans un deuxième temps, nous développons un outil numérique 3D complet destiné à simuler la propagation de fissures dans les tôles minces ductiles et qui utilise des éléments cohésifs. Trois méthodes différentes sont proposées pour calibrer les paramètres du modèle de manière à reproduire les données expérimentales et à permettre l’extrapolation des résultats à d’autres matériaux ou d’autres épaisseurs de tôles. Finalement, les paramètres du modèle de zone cohésive sont justifiés grâce à des arguments à fondement micromécanique.

Ultrasound elevation beamwidth is the out of plane thickness causing image artefacts normally appearing around anechoic areas in the medium. These artefacts could also cause uncertainties in localizing objects (such as a surgical needle) in the ultrasound image slices. This thesis studies the clinical significance of elevation beamwidth artefacts in needle insertion procedures. A new measurement device was constructed to measure the transrectal ultrasound elevation beamwidth. The beam profiles of various lateral and axial distances to the transducer were generated. It is shown that the ultrasound elevation beamwidth converges to a point within its focal zone close to the transducer. This means that the transrectal ultrasound images have the best resolution within the focal zone of the ultrasound close to the transducer. It is also shown that the ultrasound device settings have a considerable impact on the amount of beamwidth artefacts. Needle tip localization error was examined for a curvilinear transrectal ultrasound transducer. Beveled prostate brachytherapy needles were inserted through all holes of a grid template orthogonal to the axial beam axis. The effects of device imaging parameters were also investigated on the amount of localization error. Based on the developed results, it was found that the imaging parameters of an ultrasound device have direct impact on the amount of object localization error from 0.5 mm to 4 mm. The smallest localization error occurs laterally close to the center of the grid template, and axially within the beam’s focal zone. Similarly, the largest localization error occurs laterally around both sides of the grid template, and axially within the beam’s far field. Using the ultrasound device with appropriate imaging settings could minimize the effects of these artefacts. We suggest to reduce the gain setting of the ultrasound device. This will reduce the energies assigned to the off-axis beams and as a result, the elevation beamwidth artefacts are minimized. / Thesis (Master, Computing) -- Queen's University, 2011-09-01 15:27:43.098

This study is concerned with flow and stability of thin liquid films flowing over the surface of a disc rotating about a vertical axis. The work consists of a theoretical and experimental investigation into the flow of the steady-state film, and the waves which occur on it. The theoretical model is investigated in its steady-state form (no waves present) using both asymptotic and numerical techniques. The unsteady problem is also examined, using asymptotic methods, for the inception and propagation of waves of small amplitude with respect to the mean film thickness. The experimental investigation employs a light absorption technique to obtain accurate film thickness measurements across individual three dimensional wave profiles. The results are used to test the validity of the small amplitude assumption of the theoretical model, which is found to be restrictive, and to give both qualitative and quantitative data about the large amplitude waves that are often observed.

Thesis (Ph. D.)--University of Washington, 2007. / Vita. Includes bibliographical references (leaves 113-116).