In recent years, the interest in the design, construction and utilization of rubber tracks for agriculture and earth moving machinery has increased considerably. The development of such types of tracks was initiated by the efforts to invent a more environmentally friendly vehicle-terrain system. These tracks are also the result of the continuous effort to develop more cost-effective traction systems. A rubber-surfaced and friction-based prototype track was developed and mounted on the patented modification of a new Allis Chalmers four wheel drive tractor. The track is propelled by smooth pneumatic tyres by means of rubber-rubber friction and the tractive effort of the track is mainly generated by soil-rubber friction between the rubber surface of the track elements and terrain. The experimental track layer tractor, based on an Allis Chalmers 8070 tractor (141 kW) was tested on concrete and on cultivated sandy loam soil at 7.8%; 13% and 21% soil water content. The contact pressure and the tangential force on an instrumented track element, as well as the total torque input to one track, was simultaneously recorded during the drawbar pull-slip tests. Soil characteristics for pressure-sinkage and friction-displacement were obtained from the field tests by using an instrumented linear shear and soil sinkage device. By applying the approach based on the classical bevameter technique, analytical methods were implemented for modelling the traction performance of the prototype track system. Different possible pressure distribution profiles under the tracks were considered and compared to the recorded data. Two possible traction models were proposed, one constant pressure model, for minimal inward track deflection and the other a flexible track model with inward deflection and a higher contact pressure at both the front free-wheeling and rear driving tyres. For both models, the traction force was mainly generated by rubber-soil friction and adhesion with limited influence by soil shear. For individual track elements, close agreement between the measured and predicted contact pressure and traction force was observed based on the flexible track model. The recorded and calculated values of the coefficient of traction based on the summation of the traction force for the series of track elements were comparable to the values predicted from modelling. However, the measured values of drawbar pull coefficient were considerably lower than the predicted values, largely caused by internal track friction in addition to energy dissipated by soil compaction. The tractive efficiency for soft surface was also unacceptably low, probably due to the high internal track friction and the low travel speeds applied for the tests. The research undertaken identified and confirmed a model to be used to predict contact pressure and tangential stresses for a single track element. It was capable of predicting the tractive performance for different possible contact pressure values. / Thesis (PhD (Argricultural Engineering))--University of Pretoria, 2007. / Civil Engineering / Unrestricted

published_or_final_version / Dentistry / Master / Master of Dental Surgery

Les renforts fibreux secs utilisés dans les composites structuraux sont composés d'un arrangement de fils eux-même constitués de milliers de fibres. Etant données la complexe architecture multi-échelle et la faible cohésion entre leurs constituants, ils montrent des propriétés en traction de plusieurs ordres de grandeurs supérieurs à ceux en flexion. Dans ce contexte, ce travail porte sur l'étude de la traction et de la flexion de matériaux fibreux en tenant compte de leur réalité géométrique à l'échelle mésoscopique, à la fois expérimentalement et analytiquement. Les matériaux tissés, ayant une ondulation périodique, sont considérés. Une nouvelle méthodologie a été développée pour la modélisation du comportement de structures ondulées (avec une variable rapide) en re-projetant des forces, moments et déplacements sur l'axe neutre de la structure. Les solutions exactes des problèmes sur la traction et flexion d'une poutre sont obtenues. En parallèle, les tissus et fils extraits ont été testés en flexion et traction. Les rigidités des fils ont été extraites en couplant l'analyse d'image et une méthode inverse. En général les résultats expérimentaux ont démontré un lien entre l'ondulation de la structure et les propriétés résultantes. De plus, ils ont montré l'applicabilité de la méthode d'homogénéisation réalisée. Enfin, l'approche permet de poser et résoudre le problème d'optimisation de l'architecture fibreuse au travers de la géométrie des fils constitutifs. / Dry fiber reinforcements used in structural composites consist of arrangements of yarns themselves consisting of alignments of thousands of fibers. Due to the complex multi-scale architecture and low cohesion between their constituents, they exhibit tensile properties several orders of magnitude higher than the bending ones. This work aims at studying the tensile and bending behaviors of fibrous materials, taking into account their meso-scale architecture, both experimentally and analytically. Woven fabrics, owing a periodic and corrugated geometry of their inter-weaved yarns, are under consideration. A new methodology has been developed for the modeling of the stress-strain state of corrugated structures (with fastly variable parameters) re-projecting applied forces, moments, and displacements on the neutral axis of the structure. The exact solutions for the problems about tension and bending of a beam have been obtained. In parallel, fabrics and constituting yarns have been tested on a bending set-up based on the cantilever principle. Tensile tests have also been performed on yarns extracted from the considered fabrics. Tensile and bending properties of yarns have been identified from tests coupling image analysis and inverse method. In general, the experimental results have demonstrated the link between the structure with corrugated constituents and its mechanical response to loadings. Also, they have justified the applicability of the multi-scale homogenization method for the prediction of effective properties. Finally, the approach permits to set and to efficiently solve the problem of the optimal design of fibrous materials from the crimped constituents geometry.

Dans un assemblage précontraint, des efforts sont transmis entre les pièces par mobilisation du frottement suite à l'application d'une contrainte de compression normale. L'objectif de ce travail était de transposer ce principe de transmission des efforts au matériau bois. A partir de résultats existants, une étude expérimentale préliminaire a été menée sur des chaumes de bambou. L'intérêt de cette « graminée » par rapport au bois est sa résistance importante en compression transversale. Pour appliquer ce principe à des avivés de bois, il a été nécessaire de les densifier par compression localisée à froid au-delà de leur limite élastique. Une caractérisation de Picea abies en compression transversale au-delà de sa limite élastique a été réalisée. Cette analyse a abouti à une loi de comportement fonction de la densité de l'éprouvette. De plus, l'influence de la densification du bois sur sa résistance en traction longitudinale a été mesurée. Il s'avère que cette résistance est constante puis chute à partir d'un taux de densification critique fonction de la masse volumique du bois. La phase de mise en place de la précontrainte dans les assemblages bois et bambou testés a été modélisée par éléments finis. L'orthotropie des matériaux a été pris en compte par la superposition d'un modèle mousse (comportement non-linéaire transversal) et d'un modèle poutre (direction longitudinale) créant une structure dont la taille des éléments ne descend pas en dessous de celle des cernes. Ce modèle structural mésoscopique a permis de simuler les phases de densification, charge, décharge de l'assemblage et d'atteindre la limite à rupture, permettant d'aller jusqu'à une démarche d'optimisation. / In a pre-stressed fastener, forces are transmitted between two elements by friction which is made possible by the application of transversal compression forces. The aim of this work was to apply this principle to timber fasteners. Thanks to former results, a preliminary experimental study was done on stubbles of bamboo. This “grass” has a higher transversal compression resistance than wood. In order to transpose the principle of pre-stress from bamboo to timber, it was necessary to increase the compression resistance of timber. The idea proposed here is the densification of wood by localised cold compression beyond its elastic limit. Picea abies was characterized in transversal compression beyond its elastic limit. This analysis gave stress-strain curves which depend on the density of the wood. The influence of the densification on the resistance in longitudinal tension was also measured. This resistance is constant and then it decreases rapidly from a critical densification rate function of the density of the wood. The pre-stress of the tested timber and bamboo fasteners was modelled by a finite elements model. The anisotropy of the materials was taken into account by the superposition of a crushable foam model (non linear behaviour in transversal direction) and beams (longitudinal direction). This structural mesoscopic model allowed simulating the densification, the loading and unloading of the fasteners until their failure which lead to an optimization of the connectors.

O presente trabalho propôs avaliar cefalometricamente as alterações dentárias, esqueléticas e tegumentares em jovens com má oclusão de Classe II, 1ª divisão tratados com o aparelho extrabucal conjugado (splint maxilar) e com o aparelho extrabucal de tração occipital (IHG). Os grupos experimentais foram comparados à um grupo controle, que apresentavam a mesma má oclusão, pareados pelo gênero, idade e tempo de observação. Um total de 150 telerradiografias em norma lateral de 75 jovens fizeram parte da amostra, divididos em três grupos de 25. Os jovens do grupo 1 foram tratados com o aparelho extrabucal conjugado, apresentaram uma idade média inicial de 9,71 anos e foram observados pelo período médio de 1,41 anos. O grupo 2, foi submetido ao tratamento utilizando o aparelho extrabucal de tração occipital (IHG), com idade média inicial de 10,51 anos e acompanhados por um período de 1,32 anos. Os jovens do grupo 3 foram mantidos como controle, apresentaram uma idade média inicial de 10,06 anos e foram observados pelo período médio de 1,35 anos. Todas as mensurações foram submetidas à análise estatística. A análise dos resultados mostrou que o deslocamento anterior da maxila foi restringido significantemente nos grupos experimentais. A mandíbula apresentou um aumento de seu crescimento efetivo (Co-Gn) estatisticamente significante no grupo tratado com o aparelho AEB conjugado. A relação maxilomandibular melhorou significantemente nos grupos tratados. Em relação às alterações dentoalveolares superiores, o grupo tratado com o aparelho AEB conjugado apresentou uma inclinação para lingual dos incisivos superiores enquanto os outros 2 grupos apresentaram uma discreta inclinação para vestibular destes dentes. A movimentação de distalização dos molares superiores foi significante para os grupos experimentais. A movimentação de extrusão dos molares inferiores mostrou também um resultado não significante entre os grupos. Com relação ao perfil tegumentar, constatou-se que o ângulo nasolabial não apresentou modificação pelo tratamento, assim como a posição espacial do lábio inferior. Já o lábio superior demonstrou uma maior retrusão nos grupos experimentais com diferença estatística significante. Desta maneira, verificou-se que ambos protocolos de tratamento propiciaram alterações esqueléticas, dentárias e tegumentares clinicamente relevantes para a correção da má oclusão de Classe II, 1ª divisão. / This study conducted a cephalometric evaluation of the dental, skeletal and soft tissue alterations in young individuals with Class II division 1 malocclusion treated with the maxillary splint and high-pull headgear (HPH). The study groups were compared to a control group presenting the same malocclusion matched for gender, age and period of observation. A total of 150 lateral cephalograms of 75 young individuals were analyzed, which were divided into three groups with 25 individuals each. The individuals in group 1 were treated with the maxillary splint, presented an initial mean age of 10 years and were observed for 1 year and 4 months. Group 2 was treated with the highpull headgear (HPH), had an initial mean age of 10 years and was followed for 1 year and 4 months. The subjects in group 3 were kept as controls, presented an initial mean age of 10 years and were observed for a mean period of 1 year and 4 months. All measurements were submitted to statistical analysis. The results demonstrated that interception of the Class II division 1 malocclusion in the study groups was significant, presenting specific effects inherent to each appliance investigated. The anterior maxillary displacement was significantly restricted in the study groups compared to the control group, especially for the group treated with HPH. The mandible presented a statistically significant increase in effective growth (Co_Gn) in the group treated with the maxillary splint, yet its anterior displacement (SNB) did not present significant alterations. The maxillomandibular relationship was significantly improved in the study groups. The vertical components did not present statistically significant differences between groups. With regard to the dentoalveolar alterations in the maxillary arch, the group treated with the maxillary splint presented a significant restriction in buccal inclination of the maxillary incisors. Distal movement of the maxillary molars was significant for both study groups, being larger for the group treated with the HPH, especially as to the crown movement (FHP_DISTAL6). Extrusion of the mandibular incisors was more restricted (1_GoMe) in the group treated with the maxillary splint, followed by the group treated with the HPH, yet with non-statistically significant outcomes. Extrusion of the mandibular molars was also non-significant between groups. With regard to the soft tissue profile, the nasolabial angle was not changed by treatment, as well as the position of the lower lip. On the other hand, the upper lip presented more retrusion in the group treated with the maxillary splint, followed by the group treated with the HPH, with statistically significant difference between the study groups and the control group. Therefore, it was concluded that both treatment protocols provided clinically relevant skeletal, dental and soft tissue alterations for correction of the Class II division 1 malocclusion.

O presente trabalho propôs avaliar cefalometricamente as alterações dentárias, esqueléticas e tegumentares em jovens com má oclusão de Classe II, 1ª divisão tratados com o aparelho extrabucal conjugado (splint maxilar) e com o aparelho extrabucal de tração occipital (IHG). Os grupos experimentais foram comparados à um grupo controle, que apresentavam a mesma má oclusão, pareados pelo gênero, idade e tempo de observação. Um total de 150 telerradiografias em norma lateral de 75 jovens fizeram parte da amostra, divididos em três grupos de 25. Os jovens do grupo 1 foram tratados com o aparelho extrabucal conjugado, apresentaram uma idade média inicial de 9,71 anos e foram observados pelo período médio de 1,41 anos. O grupo 2, foi submetido ao tratamento utilizando o aparelho extrabucal de tração occipital (IHG), com idade média inicial de 10,51 anos e acompanhados por um período de 1,32 anos. Os jovens do grupo 3 foram mantidos como controle, apresentaram uma idade média inicial de 10,06 anos e foram observados pelo período médio de 1,35 anos. Todas as mensurações foram submetidas à análise estatística. A análise dos resultados mostrou que o deslocamento anterior da maxila foi restringido significantemente nos grupos experimentais. A mandíbula apresentou um aumento de seu crescimento efetivo (Co-Gn) estatisticamente significante no grupo tratado com o aparelho AEB conjugado. A relação maxilomandibular melhorou significantemente nos grupos tratados. Em relação às alterações dentoalveolares superiores, o grupo tratado com o aparelho AEB conjugado apresentou uma inclinação para lingual dos incisivos superiores enquanto os outros 2 grupos apresentaram uma discreta inclinação para vestibular destes dentes. A movimentação de distalização dos molares superiores foi significante para os grupos experimentais. A movimentação de extrusão dos molares inferiores mostrou também um resultado não significante entre os grupos. Com relação ao perfil tegumentar, constatou-se que o ângulo nasolabial não apresentou modificação pelo tratamento, assim como a posição espacial do lábio inferior. Já o lábio superior demonstrou uma maior retrusão nos grupos experimentais com diferença estatística significante. Desta maneira, verificou-se que ambos protocolos de tratamento propiciaram alterações esqueléticas, dentárias e tegumentares clinicamente relevantes para a correção da má oclusão de Classe II, 1ª divisão. / This study conducted a cephalometric evaluation of the dental, skeletal and soft tissue alterations in young individuals with Class II division 1 malocclusion treated with the maxillary splint and high-pull headgear (HPH). The study groups were compared to a control group presenting the same malocclusion matched for gender, age and period of observation. A total of 150 lateral cephalograms of 75 young individuals were analyzed, which were divided into three groups with 25 individuals each. The individuals in group 1 were treated with the maxillary splint, presented an initial mean age of 10 years and were observed for 1 year and 4 months. Group 2 was treated with the highpull headgear (HPH), had an initial mean age of 10 years and was followed for 1 year and 4 months. The subjects in group 3 were kept as controls, presented an initial mean age of 10 years and were observed for a mean period of 1 year and 4 months. All measurements were submitted to statistical analysis. The results demonstrated that interception of the Class II division 1 malocclusion in the study groups was significant, presenting specific effects inherent to each appliance investigated. The anterior maxillary displacement was significantly restricted in the study groups compared to the control group, especially for the group treated with HPH. The mandible presented a statistically significant increase in effective growth (Co_Gn) in the group treated with the maxillary splint, yet its anterior displacement (SNB) did not present significant alterations. The maxillomandibular relationship was significantly improved in the study groups. The vertical components did not present statistically significant differences between groups. With regard to the dentoalveolar alterations in the maxillary arch, the group treated with the maxillary splint presented a significant restriction in buccal inclination of the maxillary incisors. Distal movement of the maxillary molars was significant for both study groups, being larger for the group treated with the HPH, especially as to the crown movement (FHP_DISTAL6). Extrusion of the mandibular incisors was more restricted (1_GoMe) in the group treated with the maxillary splint, followed by the group treated with the HPH, yet with non-statistically significant outcomes. Extrusion of the mandibular molars was also non-significant between groups. With regard to the soft tissue profile, the nasolabial angle was not changed by treatment, as well as the position of the lower lip. On the other hand, the upper lip presented more retrusion in the group treated with the maxillary splint, followed by the group treated with the HPH, with statistically significant difference between the study groups and the control group. Therefore, it was concluded that both treatment protocols provided clinically relevant skeletal, dental and soft tissue alterations for correction of the Class II division 1 malocclusion.

PURPOSE: To study the long-term anatomic and visual outcomes after pars plana vitrectomy (PPV) with and without internal limiting membrane (ILM) peeling in patients with symptomatic vitreomacular traction (VMT). This study assesses the frequency of complications, changes in visual acuity, and changes in anatomical central macular thickness after macular surgery. METHODS: This retrospective, single-site, single-surgeon study reviewed 40 medical records (45 eyes) of patients at the Beth Israel Deaconess Medical Center requiring PPV with ILM peeling (n=27) or without ILM peeling (n=18) for VMT between the years of 2003 and 2016. Successful surgery was defined as the relief of anatomical traction, and the absence of a second surgery, or any post-operative complications (n=42). Visual acuity was documented for each eye prior to surgery and post surgery. RESULTS: All 27 (100%) eyes that had ILM peeling had successfully resolved macular traction following a single surgery, and 15 of the 18 (83.3%) eyes without ILM peel were successful. None of 27 (0%) eyes that had ILM peeling required a second surgery, nor did they have complications. 3 of the 18 (16.7%) eyes without ILM peeling required a second surgery. Best corrected visual acuity (BCVA, logMAR) improved significantly in both groups: BCVA improved from 0.59 ± 0.29 preoperatively to 0.37 ± 0.25 postoperatively in eyes receiving ILM peeling and from 0.77 ± 0.37 to 0.53 ± 0.37 in eyes with PPV only. Mean change in CMT pre-operatively to post-operatively was found to be greater in eyes with PPV alone, but this difference was not statistically significant. CONCLUSIONS: Our case series shows that PPV with ILM peeling for VMT relieved macular traction better than PPV alone, although there was no significant difference in visual acuity outcomes or central macular thickness between the two groups. Further research is required to validate these findings. / 2019-07-11T00:00:00Z

La conception et la réalisation des tests de qualification des engins propulsés nécessitent une connaissance du comportement mécanique des matériaux utilisés pour leur fabrication. Dans ce cadre, une large gamme de vitesses de sollicitation est à considérer, intégrant de nombreuses situations de service des propergols solides. La connaissance de leur comportement doit permettre de prédire les risques associés à une sollicitation imprévue (conditions de transport difficiles, chute). La méthodologie usuelle consiste en la caractérisation des matériaux par un ensemble d’essais de traction uniaxiale, d’essais de DMA et d’essais de traction sous pression pour identifier les paramètres du modèle numérique (HRVM). Cependant, il a été indiqué que la méthodologie usuelle ne permettait pas la réalisation de simulations numériques satisfaisantes lorsque la vitesse de déformation est supérieure à 50 s−1. Ce travail de thèse propose des nouvelles techniques de caractérisation expérimentale permettant de compléter les résultats existants pour une gamme de vitesse de l’ordre de 100 s−1. Des essais originaux assurant des modes de sollicitations dynamiques et homogènes ont été mis au point et validés. Par exemple, un montage expérimental de traction uni-axiale rapide a été conçu pour la caractérisation d’un élastomère composite à matrice p − BHT hautement chargé de particules rigides (une distribution de CaCO3 et une autre d’aluminium). Parmi les autres essais développés, un essai de cisaillement hétérogène d’impact sur tranche a été adapté, cet essai permet d’apprécier l’état d’endommagement dans le matériau pendant et suite à l’impact. Les paramètres de comportement quantifiés dans la gamme de vitesse de déformation prospectée ont été intégrés à la bibliothèque des paramètres existante. Les premières simulations numériques intégrant ces nouveaux paramètres ont permis d’étendre l’utilisation du modèle de comportement utilisé par Ariane Group. / The design and qualification tests of rocket motors require knowledge of the mechanical behavior of the materialsused to manufacture them. In this aim, a large range of strain rates have to be considered, this range integratemany solid propellant service situations. Knowledge of their behaviour should make it possible to predict the risksassociated with unexpected stress on propelled vehicles (difficult transport conditions, falls). The usual methodologyuses both numerical and experimental approaches. This methology is based on several uniaxial tesile tests, DMA andtensile tests under pressure to identify the parameters of the constitutive law (HRVM). However, it was indicatedthat the usual method did not allow satisfactory numerical predictions to be performed when the strain rate isgreater than 50 s−1. This PhD work proposes new experimental characterization techniques to complement existingresults for a range of strain rates of about 100 s−1. Original tests ensuring dynamic and homogeneous loadingshave been developed. A dynamic tensile test has been designed to characterize composites elastomers highly filled(CaCO3 and aluminum). A heterogeneous dynamic shear test of edge-on impact has been also adapted, this testmakes it possible to assess the state of damage in the material during and after the impact. The material parametersquantified with new mechanical tests have been integrated into the existing parameter library. The firsts numericalsimulations using these new parameters have made it possible to extend the practice of numerical model used byAriane Group.

Les objectifs de la thèse sont d'étudier expérimentalement et d'effectuer des simulations numériques du comportement différé du béton. La principale application concerne la prédiction du comportement des enceintes de confinement de centrales nucléaires (REP) dans le cas des épreuves décennales et accidentelles. Afin de répondre à ces besoins industriels, il est nécessaire de caractériser le comportement du béton en traction uniaxiale. Ainsi, une importante étude expérimentale du fluage en traction sur des bétons âgés de 90 jours, chargés pendant trois jours (durée des épreuves décennales) a été menée pour différents niveaux de contrainte (50 à 90 % de la résistance). Plusieurs essais ont été réalisés : mesure des propriétés élastiques, des résistances en compression et en traction directe, suivi du séchage, du retrait, du fluage propre et du fluage de dessiccation en traction directe. Des essais en compression ont permis de mettre en évidence une différence importante entre le fluage en compression et en traction. De plus, une diminution de la résistance en traction après les essais en dessiccation a été observée, ainsi qu'une rupture prématurée pour des niveaux de chargement importants lors du fluage en traction. Un modèle numérique a été proposé et développé dans le code de calcul aux éléments finis Cast3m.

Mass Rapid Transit(MRT) plays a very important role for the city development,the investment cost is very expensive. It is necessary to derive the MRT system planning by considering the service reliability and performance index according to the forecast of annual ridership. With the less ridership as compared to Taipei MRT network, Kaohsiung MRT has to be developed to achieve the most cost effective investment of power supply and rolling stock planning. This thesis is to investigate the proper expansion planning of traction substations (TSS) for an electrified mass rapid transit system. The motion equation of train sets is used to solve the mechanical power consumption at each time snapshot according to the operation timetable, the passenger ridership and various types of operation resistance. The mathematical models of power converters in traction substations for different operation modes have been derived. With all train sets operated along the main line, the AC/DC load flow analysis is performed to find power demand of all traction substations for annual system peak operation over the study period. The objective function is formulated by considering both the voltage drop of train sets and investment cost of traction substations as the equivalent cost of all feasible states of each year. By performing the dynamic programming (DP) and immune algorithm (IA), the expansion planning of traction substations to achieve the minimum overall cost has been solved by identifying the optimal capacity and locations of new traction substations to be committed at each year.