As cerâmicas a base de carbeto de silício, SiC, tem muitas aplicações na engenharia devido às suas excelentes propriedade mecânicas, térmicas e químicas. Para a produção de cerâmicas de SiC com propriedades específicas, seu processamento deve ser escolhido de maneira a se produzirem microestruturas adequadas para cada aplicação. Para isso, na maioria das vezes, essas cerâmicas são sinterizadas via fase líquida, usando como aditivos formadores dessa fase misturas de óxidos de alumínio e algumas terras raras. Esta dissertação tem dois objetivos principais: implantar o método SEVNB no Departamento de Engenharia de Materiais (DEMAR), na Escola de Engenharia de Lorena (EEL), para medida de KIC, propriedades mecânicas que, muitas vezes, limita aplicações das cerâmicas, e introduzir mais dois óxidos de terras raras para formação da fase líquida na sinterização do SiC, os óxidos de disprósio e itérbio. Após a mistura e secagem das matérias primas, os pós foram prensados com dupla ação de pistões e isostaticamente, e em seguida foram sinterizados em atmosfera de argônio utilizando seis ciclos diferentes para cada sistema de aditivos. Após sinterização as amostras foram caracterizadas e entalhadas com lâmina de barbear utilizando pastas diamantadas de 6 e 0,25 ?m e flexionadas em 4 pontos. Os resultados mostraram uma maior densificação para o sistema de aditivos Al2O3/ Yb2O3, de aproximadamente 97% da densidade teórica, prensados a 400 MPa e sinterizados na temperatura de 1950ºC por 1 hora, com formação de ?-SiC, DyAG e YbAG em todas as temperaturas estudadas. A máquina fabricada para entalhamento das amostras se mostrou eficiente, produzindo raios de entalhes atendendo a norma ISSO 23146:2008. Quanto as propriedades mecânicas, o sistema com itérbia possui os maiores valores de módulo de elasticidade e dureza, porém menor tenacidade à fratura em relação a cada ciclo de sinterização, resultado que sugere a influência do raio catiônico das terras raras formadoras da fase secundária nas cerâmicas a base de SiC. / The silicon carbide, SiC, has many applications in engineering due to its excellent mechanical, thermal and chemical properties. For the production of SiC ceramics with specific properties, the processing should be chosen to produce microstructures suitable for each application. For this reason, in most cases, these ceramics are liquid phase sintered using additives such as formers of this phase mixed oxides of aluminum and certain rare-earths. This dissertation has two main objectives: to implement the method SEVNB at Department of Materials Engineering (DEMAR), Engineering School of Lorena (EEL), to measure KIC, mechanical propertie, which often limits the ceramic\'s applications, and introduce two more rare-earth oxides for the formation of liquid phase sintering SiC, dysprosium and ytterbium oxide . After mixing and drying, the powders were pressed with a double acting piston and isostatically, and then sintered in an argon atmosphere using six different cycles for each additive system. After sintered, samples were characterized and V-notched using a razor blade and diamond pastes of 6 and 0.25 ?m and four-point flexured. The results showed a higher densification for the system Al2O3 / Yb2O3, approximately 97% , pressed at 400 MPa and sintered at a temperature of 1950 ° C for 1 hour, with formation of ?-SiC, DyAG and YbAG at all temperatures studied. The machine used to notched the samples proved to be effective, producing notch root according to the standard ISO 23146:2008. As for the mechanical properties, the system with ytterbium oxide has higher values of elastic modulus and hardness, but lower fracture toughness for each sintering cycle studied, result that suggests the influence of the rare-earth\'s cationic radius that forming the secondary phase in the SiC.

This thesis reports a study of the effects on processing and properties of incorporating nano-scale reinforcements (multiwall carbon nanotubes, MWCNTs) in the matrix of epoxy- carbon fibre (CF) laminate composites to produce multi-scale composites (M-SC). The main aim of this research was to study the effects of MWCNTs on matrix toughening and the through-thickness properties of M-SCs based on a commonly used aerospace grade epoxy resin — triglycidyl-p-aminophenol (TGPAP) cured with diaminodiphenyl sulphone (DDS). In order to improve resin processing, diglycidyl ether of bisphenol F (DGEBF) was added into the TGPAP/DDS system as a reactive diluent. Factorial experimental design (FED) was used to optimize the composition of this tri-component system to obtain high Tg and low resin viscosity, which gave a TGPAP/DGEBF/DDS system with 30.56 wt.% of DGEBF and a chemical stoichiometry of 0.5. Three types of MWCNTs were used; as-received (AR-), base-washed (BW-) and amine functionalized (NH2-). These were shear-mixed with both the bi- and tri-component systems using a 3-roll mill to produce nanocomposite matrices (NCM). The curing behaviour, dispersion state of MWCNTs in the resin and processability of NCMs were studied to decide upon the preparation method for the final M-SC. The fracture toughness (KIC) and the flexural properties of NCM were affected by both MWCNTs and the matrix type; thus KIC increased by up to 8 % in TGPAP/DDS NCM but decreased by 23% in TGPAP/DGEBF/DDS NCM with 0.5 wt.% AR-CNTs. The addition of both non-functionalized and functionalized MWCNTs increased the flexural modulus. The failure mechanism of NCMs was found to be dominated by the size and distribution of CNT aggregates and the behaviour of MWCNTs, both those dispersed in the matrix and in aggregates. The addition of functionalized MWCNTs increased the interfacial bonding between MWCNT and epoxy resin and thus improved the mechanical properties. All the NCM systems were taken forward to manufacture M-SC using a hybrid resin film infusion (RFI)/hot press process. The fibre volume fraction and the void content could be controlled at 43 ± 5 % for M-SC with TGPAP/DDS NCM and 60 ± 6 % for M-SC with TGPAP/DGEBF/DDS NCM. M-SCs were characterised using a range of tests, including flexural, interlaminar shear strength (ILSS), mode-II interlaminar fracture toughness (GIIC), low velocity impact and compression after impact (CAI). The most obvious improvement occurred for the M-SC with tri-component system with 0.5 wt.% CNTs, whereILSS increased by 16 % upon adding NH2-CNTs and GIIC increased significantly on addition of 0.5 wt.% AR-CNTs and NH2-CNTs, by 85% and 184% respectively. However the effect of MWCNTs on other properties was at best marginal. For example, for the M-SC with TGPAP/DDS, the flexural modulus and ILSS only increased by 4.1 % and 2.3 % with 0.5 wt.% AR-CNT.

Progress in the fields of wearable computing, soft robotics and bio hybrid engineering depend on new classes of soft multifunctional materials that match the mechanical properties of soft biological tissue and possess high toughness, while having metal-like electrical and thermal properties.

Research in local fracture processes and micro-machining of polymers and polymer-based composites has attracted increasing attention, in development of composite materials and miniaturisation of polymer components. In this thesis, sectioning (machining) of a glassy polymer and a carbon nanotube based composite at the nanoscale was performed by an instrumented ultramicrotome. The yield stresses and fracture toughness of these materials were determined by analysing the sectioning forces. Fractographic analysis by atomic force microscopy was conducted to characterise the topographies and elastic properties of the sectioned surfaces to explore the deformation and fracture behaviour of the polymer during nanosectioning. The study reveals that a transition from homogenous to shear localised deformation occurred as the uncut chip thickness (depth of cut) or sectioning speed increased to a critical value. Analytical and finite element methods were used to model the nanosectioning process. The shear localised deformation was caused by thermal softening due to plastic dissipation. Although not considering sectioning, the tensile properties of a polymer nanocomposite were additionally investigated, where the degree of nanofibrillation and polyethylene glycol (PEG) content had significant effects.

The aim of the work presented was to investigate the effects of upper bainite on impact toughness in quench and tempered low alloy plate steels. The experimental research included construction of CCT diagrams by dilatometry, verification of phases by optical microscopy (OM), Vickers hardness, scanning electron microscopy (SEM), transmission electron microscopy (TEM) on precipitates extracted by carbon replica and by electrolytic means and finally impact testing of Charpy specimens with mixed bainite:martensite microstructures. Bainite was formed in High Chromium Low Molybdenum (HCrLMo) and in High Molybdenum Low Chromium (HMoLCr) steel samples by isothermal annealing within the bainite C-curve of the respective CCT diagrams. The isothermal kinetics of the upper bainite transformation was modelled with the Johnson Mehl Avrami Kolmogorov (JMAK) model. Avrami exponents of 1.4 and 1.3 were obtained for the HCrLMo and HMoLCr steels respectively which indicated linear growth with a considerable lengthening rate of laths and negligible thickening. The measurably slower growth kinetics in the HMoLCr steel as observed in the JMAK model and the higher hardenability with reference to its CCT diagram, suggested a strong Mo alloying element effect. The stronger effect of Mo compared to Cr was attributed to a solute drag like effect. The effect of upper bainite in a tempered martensitic matrix was investigated for the following amounts of bainite; 0%, 10%, 25%, 60%, 75%, 90% and 100%. The impact toughness of the mixed bainite:martensite samples was evaluated against the toughness of 100% bainite and 100% martensite. It was demonstrated that upper bainite reduces the total absorbed impact energy by an adverse effect on crack nucleation energy and crack propagation energy. / Dissertation (MSc)--University of Pretoria, 2013. / gm2014 / Materials Science and Metallurgical Engineering / Unrestricted

[pt] Neste trabalho apresenta-se uma análise dos principais procedimentos desenvolvidos para calcular a tenacidade à fratura sob condições elastoplásticas. São avaliadas as principais medidas de tenacidade, bem como as diferenças, características, fontes de conservadorismo e as principais referências bibliográficas que relatam diferenças nas metodologias de cálculo para prever as cargas de falha em estruturas com defeito. Também são analisados os principais procedimentos de avaliação da Integridade Estrutural de estruturas com defeito (Fitness For Service, FFS). Testes de tenacidade são feitos variando as condições de restrição em torno da trinca (geometria do CP, tamanho inicial de trinca, entalhe lateral). Foram levantadas as curvas JR para todos os CPs testados, e previsões de carga de fratura são feitas segundo os principais procedimentos FFS, e testes são realizados em placas trincadas centralmente com o objetivo de registrar as cargas de falha do defeito, para compará-las com as previsões dos procedimentos FFS e avaliar o conservadorismo de cada um deles. Como outra contribuição ao trabalho, simulações numéricas dos ensaios de tenacidade são desenvolvidas com o objetivos de reproduzir os testes feitos e calibrar os parâmetros de simulação de rasgamento dúctil em estruturas com defeito. / [en] This work presents an analysis of the main procedures developed to calculate fracture toughness under elastoplastic conditions. The main measures of toughness, differences, characteristics, and sources of conservatism are evaluated. Also, the main bibliographies reporting differences in calculation methods to predict failure loads in flawed structures are examined. Lastly, the main assessment procedures of Structural Integrity of structures with flaws (fitness for service, FFS) are also analyzed.Toughness tests are performed varying the constraint conditions near the crack tip (geometry, initial crack size, side grooves). JR curves and fracture load predictions were made according to the principal FFS procedures. Also, experimental tests are performed on centrally cracked plates in order to record the failure loads of the structure, to compare them with the predictions of FFS procedures and to evaluate their conservatism. Other contributions of this work are numerical simulations of toughness tests, developed in order to reproduce the tests and calibrate the simulation parameters of ductile tearing in structures with flaws.

Since resin composites were first presented to dentistry more than half a century ago, the composition of resin composites has developed significantly. One major change was that the reinforcing filler particles were reduced in size to generate materials of a given filler content that display better physical and mechanical properties. Resin composites may absorb water and chemicals from the surrounding environment but at the same time, composites may release constituents to their surroundings. The physical/mechanical properties of a restorative material provide an indication of how the material will function under stress in the oral environment. The aims of this research were to examine the effects of water at 37°C on the physical and mechanical properties, and the effect of food-simulating solvents of a variety of experimental and contemporary resin composites, on the surface properties. Eight representative resin composites were selected (Exp. VT, BL, NCB, TEC, GSO, XB, VDF and CXD). Due to the recent development of bulk fill materials on the market during the course of this research, the post-cure depth of cure of new bulk fill materials was also investigated. Five representative resin composites were selected: TBF, XB, FBF, VBF and SF. Water sorption and solubility were investigated at 37°C for 150 days. Sorption and solubility are affected by the degree of hydrophilicity of the resin matrix. The bulk fill materials examined showed the lowest water sorption and solubility. Laser scan micrometer (LSM) was used to investigate hygroscopic expansion. The extent of the hygroscopic expansion positively correlated with the amount of water sorption. The effect of water on fracture toughness was also examined. A self-adhesive hydrophilic resin matrix decreased in fracture toughness after 7 days of storage at 37°C. By contrast, the least water absorbed bulk fill material increased in fracture toughness over time. The effect of food-simulating solvents (distilled water, 75% ethanol/water and MEK) on surface micro-hardness, colour stability and gloss retention were investigated. The MEK solvent resulted in the lowest micro-hardness and the greatest colour change (ΔE) for most of the examined composites, while the 75% ethanol/water solution caused the greatest loss in gloss for most of the examined composites. A highly filled nano-composite showed the best result over time, regardless of the condition of storage. Surface micro-hardness profiles were used as an indirect method to assess the depth of cure of bulk fill resin composites. The examined bulk fill resin composites can be cured to an acceptable depth (4 mm).

What do we do when we realise that others with whom we interact already know something about relevant qualities of ours? Do we relying on the information that we naturally emit to get our message across, or do we take actions to try to change the situation in our favour? And, does the information that we emit, generated intentionally or unintentionally, allow us to cooperate with others and conflict to be resolved peacefully? Drawing on signalling and 'countersignalling' theories, and a recent behavioural theory, I explore these questions using two experiments in which the relevant qualities are trustworthiness and toughness, and observational data from the 1997 Survey of Inmates in State and Federal Correctional Facilities concerning violence among prisoners. In the experiments, subjects, who do not know what will follow, initially create a natural indicator of their qualities, and subsequently, have the opportunity to send another indicator, this time fully informed. In one experiment, consistent with countersignalling theory, subjects whose generosity is clear deign to send further information, while those whose generosity can be called under question choose to update their initial action the most. In the other, the toughest subjects put the most effort into getting their message across. Consistent among both, senders' actions correspond to receivers' evaluations. Finally, I find that pertinent information can allow conflicts to be resolved successfully; it is linked with less violence in prison and in a laboratory-based contest.

The purpose of the work presented in this thesis was to explore and understand the mechanisms governing toughness, ductility and ballistic performance in a class of nanostructured carbide-free bainite-austenite steels, sometimes known as ‘superbainite’. The mechanical properties of these alloys have been extensively reported, but their interpretation is not clear. The thesis begins with an introduction to both the relevant nanostructures and some of the difficulties involved in explaining observed properties, alongside a summary of the role of mixed- microstructures in alloy development. An overview of the debate regarding the mechanism of bainite formation is pre- sented in Chapter 2, in the form of a literature survey encompassing the period of explicit recognition of the bainite microstructure. Of note is the role played by the displacive theory of formation in the development of the alloy structures investigated in this thesis. A characterisation of a commonly available bainitic alloy forms the basis for Chapter 4. Observations confirm the nanoscale nature of the structure, although additional phases are found to be present, namely: cementite and martensite. This is explained as resulting from relatively low alloying additions and chem- ical segregation effects, which are modelled using thermodynamic and kinetic approaches. Chapters 5 and 6 contain a comprehensive study of the response of this alloy to the stress concentration present at the notch root of a Charpy impact sample. The work provides evidence of notch root embrittlement due to stress-induced martensite transformation. Results from synchrotron and laboratory X-ray experiments in particular reveal that machining, as well as applied stress, can initiate the austenite-martensite transformation, and methods to mitigate this effect are suggested. An innovative approach is harnessed in Chapter 7, in order to identify exper- imentally the volume fraction at which three-dimensional connectivity (‘percolation’) of austenite is lost in a superbainitic steel. Hydrogen thermal desorption techniques are applied to this problem, inspired by the tendency of such alloys to undergo tensile failure with limited or zero necking. The striking result sheds light on the importance of austenite morphology in restricting the diffusion of hydrogen into a mixed structure. The final set of experimental work is directed towards understanding the damage mechanisms that occur during projectile penetration of a coarser bainitic armour- plate alloy. The formation of adiabatic shear bands is found to be a dominant factor governing the ballistic failure of the plate. The sheared material undergoes severe high-temperature deformation, but does not change phase upon cooling, leading to the proposal of certain methods that could be implemented to improve ballistic resistance of the steel. The totality of the research presented herein is summarised in Chapter 9, which draws attention to new areas of interest that have arisen from the current work, proposing several future directions of investigation. The broader issue of understanding, common to all studies performed thus far, is that of the causes, effects, and extent, of stress-induced transformation to martensite experienced by the retained austenite that is a key feature of superbainite and similar steels.

Highly crosslinked epoxy resin systems are essential in aerospace applications due to the high operating temperatures. Although highly crosslinked epoxy resins have the required glass transition temperature (Tg) for the application, they are inherently brittle and matrix toughness is improved by incorporation of a second phase. Previous studies have focused mostly on toughening of lightly crosslinked epoxy systems, whereas this study investigates toughening of a highly crosslinked epoxy resin system using thermoplastic toughners poly(ether sulfone) (PES) and a poly(methyl methacrylate)-b- poly(butyl acrylate)-b-poly(methyl methacrylate) (MAM) block copolymer (BCP).