A Local Load Sharing (LLS) model recently developed by Curtin and co-workers for the numerical simulation of tensile stress-strain behavior in fiber-reinforced composites is used to predict the tensile strength of metal matrix composites consisting of a Titanium matrix and unidirectionally aligned SiC fibers. This model is extended to include the effects of free boundary conditions and non-constant load gradients and then used to predict the strength of a Ti-6Al-4V matrix reinforced with Sigma SiC fibers under 4-point flexure testing. The predicted tensile and flexure strengths agree very well with the values measured by Gundel and Wawner and Ramamurty et al. The composite strength of disordered spatial fiber distributions is investigated and is shown to have a distribution similar to the corresponding ordered composite, but with a mean strength that decreases (as compared to the ordered composite) with increasing Weibull modulus. A modified Batdorf-type analytic model is developed and similarly extended to the case of non-uniform loading to predict the strength of composites under tension and flexure. The flexure model is found to be inappropriate for application to the experimental materials, but the tensile model yields predictions similar to the Local Load Sharing models for the experimental materials. The ideas and predictions of the Batdorf-type model, which is essentially an approximation to the simulation model, are then compared in more detail to a simulation-based model developed by Ibnabdeljalil and Curtin to more generally assess the accuracy of the Batdorf model in predicting tensile strength and notch strength versus composite size and fiber Weibull modulus. The study shows the Batdorf model to be accurate for tensile strength at high Weibull moduli and to capture general trends well, but it is not quantitatively accurate over the full range of material parameters encountered in various fiber composite systems. / Master of Science

Heart valve disease is currently one of the leading forms of heart disease. Current literature has shown that endothelin I, angiotension II and 5-HT are vasoactive agents which cause concentration dependent contractions in aortic valve leaflet tissue. This study tested the mechanical properties of leaflets cut in the radial direction after they were exposed to the agents at varied concentrations or for 0.5, 6 or 24 hours. The elastic modulus, ultimate tensile strength and the yield strength were calculated. In the time trials the elastic modulus and the ultimate tensile strength both showed a significant increase at 24-hours. However, there were no significant differences found between the concentrations. Indicating the amount of vasoactive agent is not as significant as the length of exposure.

The effect of the chemical composition, austenitisation temperature and tempering temperature and time on the mechanical properties and on the ballistic performance of martensitic steel armour plates was studied. It was established in this study that the mechanical properties and the ballistic performance of martensitic steels can be optimised by controlling the chemical composition and the heat treatment parameters. However, it was observed that for a given chemical composition of the steel the heat treatment parameters to be applied to advanced ballistic performance armour plates were different from those required for higher mechanical properties. Such a contradiction rendered the relationship between mechanical properties and ballistic performance questionable. Systematic analysis of the microstructure and the fracture mechanism of some martensitic armour plate steels was carried out to explain the improved ballistic performance of steels whose mechanical properties were below that specificied for military and security applications. It was inferred from phase analysis and its quantification by X-ray diffraction, characterisation of the martensite using scanning electron microscopy, transmission electron microscopy and atomic force microscopy that the retained austenite located in the plate interfaces and on grain boundaries of the martensite was the main constituent resisting localised yielding during ballistic impact on thin steel plates. A part of the kinetic energy is transformed into adiabatic heat where a reaustenitisation of the plate martensite and the formation of new lath martensite was observed. Another part is used to elastically and plastically deform the ballistic impact affected region around the incidence point. Dislocation pile-ups at twinned plate interfaces suggest that the twin interfaces act as barriers to dislocation movement upon high velocity impact loading. The diameter of the affected regions, that determines the volume of the material deforming plastically upon impact, was found to vary as a function of the volume fraction of retained austenite in the martensitic steel. Upon impact, retained austenite transforms to martensite by Transformation Induced Plasticity, the “ TRIP ” effect. High volume fractions of retained austenite in the martensitic steel were found to yield low values of the ratio yield strength to ultimate tensile strength (YS/UTS) and a high resistance against localised yielding and, therefore, against ballistic perforation. A Ballistic Parameter was proposed for the prediction of ballistic performance using the volume fraction of retained austenite and the thickness of the armour plate as variables. Based on the martensite structure and the results of the ballistic testing of 13 armour plate steels a design methodology comprising new specifications was proposed for the manufacture of armour plates whose thicknesses may be thinner than 6mm. / Dissertation (MSc (Metallurgical Engineering))--University of Pretoria, 2007. / Materials Science and Metallurgical Engineering / unrestricted

Cardiovascular diseases (CVDs) are a leading cause of death worldwide, emphasizing the need for advanced and effective intervention and treatment measures. Hypertension, a significant risk factor for CVDs, is characterized by reduced vascular compliance in arterial vessels. There is a significant rise in interest in exploring the viscoelastic properties of arteries in the last few years, for the treatment of these diseases. This study aims to develop an experimental setup using 3D Printing Technology to test viscoelastic arterial models for the validation of a diagnostic device for cardiovascular diseases. The research investigates the selection of polymer-based materials that closely mimic the viscoelastic properties of arterial vessels. An experimental setup is designed and fabricated to perform mechanical tests on 3D-printed specimens. The study utilizes a mathematical model to describe the viscoelastic behavior of the materials. The model's predictions are validated using experimental data obtained from the mechanical tests. This study demonstrates the potential of 3D printing technology in fabricating specimens using elastic and flexible resin materials. These specimens closely replicate the mechanical properties of native arteries, offering a tangible platform for controlled mechanical testing. Stress relaxation tests on the3D printed specimens highlight the viscoelastic properties of fabricated materials, shedding light on their behavior under strain. The study goes further to model the mechanics of these materials, utilizing the Fractional Voigt model to capture the intricate balance between elastic and resistive behaviors under varying deformation levels. The results highlight the successful fitting of the Fractional Voigt model to the experimental data, confirming the viscoelastic behavior of the specimens. The obtained values of α and RMSE indicate a good representation of arterial mechanical properties within the viscoelastic arterial model, under different loading conditions. This research contributes to improving cardiovascular device validation and offers a practical and reliable alternative to invasive experiments. Future works include exploring different materials and conditions for arterial modeling and enhancing the precision and scope of the viscoelastic model. Overall, this study advances the understanding of cardiovascular biomechanics, contributing to the development of more effective diagnostic devices for cardiovascular diseases.

Digital image correlation techniques (DICT), a non-contact deformation measuring technique based on gray value digital images, have become increasingly used over the last years. By using the DIC technique during a tensile test, the deformation behavior of different engineering material under an applied load can be determined and analyzed. Digital images, acquired from a tensile test, can be correlated by using DICT software and from that the local or global mechanical properties can be calculated. The local or global mechanical properties determination of a flat test specimens are based on the displacements or changes in a previous stochastic sprayed or natural pattern. The used material for this purpose is cast silicon (Si) based aluminium (Al) component, designated as AlSi7Mg0.3 (Anticorodal-78 dv). The hypoeutectic Al- Si alloy is widely applicable for engine constructions, vehicle and aerospace constructions, shipbuilding, electrical engineering and constructions for food industry. There are many microstructural parameters in a binary system Al- Si alloys, which the mechanical properties can be depended on, for instance phase distribution, Secondary Dendrite Arm Spacing (SDAS), morphology of Si particles (Roundness) and microscopic defects or pores. All these parameters can contribute to enhance the proper mechanical performance (e.g. Strength and ductility) in the Al-Si cast components.

Made available in DSpace on 2017-07-24T19:22:05Z (GMT). No. of bitstreams: 1 FELIPE GUTIERREZ REYES.pdf: 2973646 bytes, checksum: 0e8c702449c97c861e9f0314e3b69e2f (MD5) Previous issue date: 2016-02-23 / Objectives: This study evaluated the effect of addition of copper nanoparticles at different concentrations into a simplified etch-and-rinse (ER) adhesive system (Ambar [AM]) on antimicrobial activity (AMA), degree of conversion of adhesives discs (DC-d) and dentin-resin interface (DC-i), the ultimate tensile strength (UTS), 28-day cumulative water sorption (WS), solubility (So) and copper release (CR) as well as immediate (IM) and 1-year (1Y) resin–dentine bond strength (μTBS) and nanoleakage (NL). Methods: Seven experimental adhesive systems were formulated according to the addition of copper nanoparticles (0 [control], 0.0075, 0.015, 0.06, 0.1, 0.5 and 1%) in AM adhesive system. We tested the antimicrobial activity of synthesized adhesives against Streptococcus mutans using agar diffusion assays. For DC-d, specimens were constructed and tested after 24 h for FTIR and micro-Raman spectroscopy. For UTS, specimens were tested after 24 h and 28 days. For WS and So, after specimens build-up, they were stored in water and the properties measured for 28 days. For CR, specimens were stored in 2% nitric acid solution and the properties were measured for 28 days. The occlusal enamel of thirty-five molars was removed and adhesives were applied to dentine surface after 37% phosphoric acid etching. After composite resin build-ups, specimens were longitudinally sectioned to obtain resin–dentine bonded sticks (0.8 mm2). Specimens were tested in tension at 0.5 mm/min in IM or 1Y (μTBS). For NL, 2 bonded sticks from each tooth were prepared and analyzed under SEM in IM or 1Y. For DC-i, 2 bonded sticks were prepared and analyzed under micro-Raman spectroscopy. The μTBS and NL data of each adhesive were subjected to two-way repeated measures ANOVA. For UTS, WS, So, copper release, DC-d and DC-i data of each adhesive were subjected to a one-way ANOVA. Tukey’s post hoc test was used for pair-wise comparisons (α = 0.05). Results: The addition of copper nanoparticles provided antimicrobial properties to the adhesives at all concentrations (p < 0,05), and did not influence UTS, WS and SO (p > 0.05). Higher CR was observed in adhesives with higher concentration of copper nanoparticles (p < 0.05). The addition of 1% of copper nanoparticles decreased the DC-d and DC-i significantly. After 1Y, significant reductions of μTBS and increases of NL were observed in the control group (p < 0.05). Reductions of μTBS and increase of NL over time were not observed for copper-containing adhesives.Conclusions: The addition of copper nanoparticles in concentrations up to 0.5% in the simplified ER Ambar adhesive system may be an alternative to provide antimicrobial properties, increase the long-term stability of resin–dentine interfaces, 12 without reducing adhesives’ mechanical properties evaluated. / Objetivos: Este estudo avaliou o efeito da adição de nanopartículas de cobre, em diferentes concentrações, em um sistema adesivo convencional (CON) simplificado Ambar [FGM] sobre a atividade antimicrobiana (AAM), grau de conversão de corpos de prova de adesivos (GC-c) e na interface de união a dentina (GC-i), resistência máxima à tração (RT), sorção cumulativa de água (SO), solubilidade (SB) e liberação de cobre (LC) durante 28 dias, bem como a resistência de união (RU) e nanoinfiltração (NI) na interface de união à dentina nos tempos imediatos (IM) e após 1 ano (1A). Material e métodos: Sete sistemas adesivos experimentais foram formulados de acordo com a adição de nanopartículas de cobre (0 [controle], 0,0075, 0,015, 0,06, 0,1, 0,5 e 1%) no sistema adesivo Ambar. Foi testada a atividade antimicrobiana dos adesivos formulados contra Streptococcus mutans por meio de ensaios de difusão em ágar. Para GC-c, as amostras foram construídas e testadas após 24 h por meio de espectrofotometria no infravermelho com Transformação de Fourier (FTIR) e espectroscopia de micro-Raman. Para RT, as amostras foram testadas depois de 24 horas e 28 dias. Para SO e SB, depois de confeccionados os espécimes, eles foram armazenados em água e as propriedades medidas por 28 dias. Para LC, as amostras foram armazenadas em solução de ácido nítrico a 2% e as propriedades medidas durante 28 dias. O esmalte oclusal de trinta e cinco molares foi removido e os adesivos foram aplicados na superfície da dentina depois de condicionamento com ácido fosfórico a 37%. Depois de restaurações de resina composta, os espécimes foram seccionados longitudinalmente para se obter espécimes (palitos) de resina-dentina (0,8 mm2). Os espécimes foram testados em tensão a 0,5 mm/min no IM ou 1A (RU). Para NI, 2 espécimes de cada dente foram preparadas e analisadas em MEV e testados no IM ou 1A. Para GC-i, 2 espécimes foram preparados e analisados sob espectroscopia de micro-Raman. Os dados do RT foram analisados por uma análise de variância de 2 fatores. Para AAM, SO, SB, e liberação de cobre, os dados foram analisados por análise de variância de fator único, e para RU e NI os dados foram submetidos a análise de variância de 2 fatores para medidas repetidas. Também foi feito o teste post hoc de Tukey para múltiplas comparações. A significância estatística foi predefinida em α = 0,05.Resultados: A adição de nanopartículas de cobre adicionou propriedades antimicrobianas aos adesivos em todas as concentrações (p < 0,05), e não influenciou a RT, SO e SB (p > 0,05). LC elevadas foram observadas em adesivos com maior concentração de nanopartículas de cobre (p < 0,05). A adição de 1% de 10 nanopartículas de cobre diminuiu significativamente o GC-c e GC-i. Após 1 ano, decréscimos significativos de RU e aumentos de NI foram observados no grupo controle (p < 0,05). Decréscimos significativos de RU e aumentos de NI não foram observados nos adesivos contendo cobre (p > 0,05). Conclusões: A adição de nanopartículas de cobre em concentrações de até 0,5% no sistema adesivo convencional simplificado Ambar pode ser uma alternativa para adicionar atividade antimicrobiana e aumentar a estabilidade a longo prazo das interfaces resina-dentina, sem comprometer as propriedades mecânicas dos adesivos.