The casualty rate of road vehicle accidents in South Africa is one of the highest in the world. This trend has persisted with little variation over the years, despite the efforts of local road safety organisations and research institutions to decrease them. Some of these road vehicle accidents are due to a mechanical failure of the vehicle. The main goal of this study is to establish how high the incidence of mechanical failure is in these accidents. And further to assess if these percentages of mechanical failures do coincide with trends already indicated nationally as well as internationally. Detailed information on the condition of vehicles was collected in and around the Pretoria area. Surveys were conducted to obtain local road and traffic information about vehicle conditions. The one survey was defined as Potential Mechanical Defect Tests (PMDT) where vehicles were stopped and given a brief mechanical inspection, and the other was a Minibus Survey where information was obtained about the age of the vehicle and the overall condition and pressure of tyres. Additional information was obtained from the Accident Response Unit (ARU) and the Forensic Sciences Laboratories, both of the South African Police Services in Pretoria. The findings of the study are that according to the data collected by the ARU over a period of 2,5 years, on average 3,3% of the accidents reported per year in the region were caused by mechanical failures. These identified percentages correspond with values obtained for international countries. The PMDT data indicate that 40% of the vehicles surveyed in suburban areas and 29% of the vehicles surveyed on the highway had mechanical defects that contravened current road and traffic regulations. The difference between the percentages indicates that the condition of vehicles inspected in the suburban area differs significantly from the condition of those using the highway. In the Minibus Survey, large irregularities in tyre pressure were identified as cause for concern. In general, all of the above findings indicated that maintenance on older vehicles seemed to receive less attention. It is proposed that annual vehicle inspections should be introduced, especially for vehicles carrying fare-paying passengers, to improve road safety standards on South African roads. / Dissertation (M Eng (Mechanical Engineering))--University of Pretoria, 2006. / Mechanical and Aeronautical Engineering / unrestricted

La diminution des dimensions critiques dans l’industrie du semi-conducteur requiert l’utilisation de nouveaux matériaux fragiles qui dégradent la résistance mécanique des puces. On s’intéresse plus particulièrement aux étapes précédant la mise en boîtier, à savoir les tests paramétriques qui permettent de vérifier la fonctionnalité électrique de la puce, et les assemblages tels la connexion filaire qui ont pour but d’établir les connexions avec le boîtier. Durant ces opérations, des défaillances mécaniques sont observées dans les interconnexions situées sous le pad. Des techniques expérimentales (par ex : FIB/MEB) sont mises en œuvre une fois les tests ou les assemblages avec des fils d’or et de cuivre réalisés afin de mieux comprendre les raisons d’apparition de ces défaillances ainsi que leur localisation. Des plans d’expériences sont mis en place pour évaluer l’influence des divers paramètres de tests et d’assemblage et également celle des architectures de pad. En parallèle, une nouvelle méthode d’analyse basée sur la nanoindentation est utilisée pour comparer la robustesse mécanique de divers plots de connexion. D’autre part, plusieurs modèles éléments finis complexes, prenant en compte la gestion du contact entre la pointe de test et le pad ainsi que les effets inertiels associés, sont développés dans le but de reproduire les conditions de chargement sur les pads. Finalement, un ensemble d’outils adaptés à l’étude et l’optimisation des architectures de pad, dans une optique industrielle, est présenté de même que des règles de dessin permettant d’accompagner le développement technologique. / The diminution of the critical dimensions in the semiconductor industry and the introduction of new brittle dielectric materials raise questions on the mechanical resistance of the die and the pad architectures. Nowadays, pad structures are prone to crack. More precisely, the electrical wafer sort (EWS), which allows checking the electrical functionality of the die, and the assemblies such as the wire bonding to achieve the electrical connections with the packaging, are performed at the wafer level and introduce high levels of local mechanical stresses. Indeed, during these operations, failures in the oxide layers of the interconnections are observed. Experimental techniques (e.g. profilometry, FIB/SEM) are developed after EWS and bonding with gold and copper wires to gain insight on the root causes and localization of the failures. Some designs of experiments are set up to evaluate the influence of the test and process parameters and also of the various pad designs on the mechanical robustness of the structures. In addition, a novel analysis procedure, based on nanoindentation technique, is employed to compare various pads, which are complex multilayer systems. Moreover, several finite element models, using both explicit and implicit schemes are developed to mimic the EWS test. Indeed, these models have shown their ability to reproduce the loading conditions, the contact between the testing needle tip and the pad, and some inertial effect during the test. Finally, a comprehensive set of tools to evaluate and optimize the pad architectures is presented. Guidelines for pad layouts are also given, providing integration insights in the frame of the technology development.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Power transformers are essential and a large part of the asset cost structure of the electrical power system. When they present any defect or failure, it will be required high costs to repair or replace them, due their commercial costs and by the capability loss to transmit power during a period of time. Generally, transformer failure can occur as a result of the weakness insulation caused by the electrochemical processes involving the cooling liquid (oil), by vibrations due the electromagnetic forces in normal operation, i.e., steady state, and also by winding deformation as the result of short-circuits. It is expected that a transformer will experience and survive a number of short circuits during its service life. But one such event will cause some slight winding movement, which can gradually increasing and the ability of the transformer to withstand further electromechanical forces will be then reduced. It is therefore very important to check the mechanical condition of transformers periodically, particularly for older units, to provide an early warning of an impending catastrophic failure. Thus a specialist technique is required for the monitoring and assessment of mechanical condition of winding transformer. Despite designers and manufacturers, as well as the operation and maintenance technicians of power supply companies worry about this subject, new methodologies and tools to assess the strength effects as the result of short circuits in transformers are still necessaries. So, this work proposes to know the strength characteristics caused by the radial and axial forces in winding transformers. In this context, it will be presented different strengths that can occur in winding transformer, identifying the principal failures as the result of these stresses. Besides, it will be developed an analytical method to evaluate the forces and stresses in winding three phase transformer under a three phase short circuit. Finally, the analytical method results will be compared with a computer simulation based in time domain using the software FEMM, which applies the finite element method. / Transformadores de potência são equipamentos essenciais aos sistemas elétricos e também um de seus componentes de maior preço. Quando estes equipamentos se danificam ou apresentam algum tipo de falha, o seu reparo ou até mesmo sua substituição demanda altíssimos custos financeiros, tanto pelo alto valor comercial destes equipamentos como pela perda temporária da capacidade de transmissão de energia elétrica. Muitas falhas destes equipamentos devem-se à redução da suportabilidade da isolação dos seus condutores/enrolamentos causado pelos fenômenos eletroquímicos do líquido de resfriamento (óleo), pelas vibrações produzidas pelas forças eletromecânicas durante a sua operação normal, ou seja, em regime permanente, e também pelas deformações dos enrolamentos causadas pelas altas correntes de curto-circuito. Espera-se, no entanto, que um transformador suporte um determinado número de curtos-circuitos durante seu tempo de operação. No entanto, verifica-se que cada evento poderá provocar pequenos deslocamentos relativos nos enrolamentos, os quais podem aumentar cumulativamente, reduzindo desta forma a capacidade do transformador de suportar novos esforços eletromecânicos. Neste contexto, torna-se importante a verificação periódica das suas condições mecânicas, principalmente nas unidades com muito tempo de operação, de forma a se obter subsídios para impedir falhas catastróficas. O emprego de técnicas especiais é requerido para o monitoramento e avaliação das condições mecânicas do enrolamento de um transformador. Apesar de este assunto ser uma preocupação constante dos projetistas e fabricantes de transformadores, bem como dos profissionais de manutenção e operação das empresas de energia elétrica, há certa carência de metodologias e ferramentas consistentes para se avaliar os efeitos dos esforços mecânicos decorrentes das elevadas correntes transitórias sobre estes equipamentos. Nessa perspectiva, este trabalho tem por meta o entendimento das características dos esforços mecânicos causados pelas forças radiais e axiais nos enrolamentos dos transformadores. Para tanto, esta dissertação mostrará os diferentes tipos de esforços eletromecânicos passíveis de ocorrer nos enrolamentos de um transformador, identificando os principais tipos de falhas provocadas por essas solicitações. Em seguida, será desenvolvida uma metodologia analítica para estimar as forças e estresses nos enrolamentos de um transformador trifásico submetidos a um curto-circuito trifásico. Assim, os valores obtidos pela metodologia analítica serão confrontados com resultados de uma simulação computacional no domínio do tempo utilizando-se o programa FEMM baseado na técnica de elementos finitos. / Mestre em Ciências