Titânio e suas ligas são materiais estruturais atrativos para várias aplicações em engenharia devido a alta resistência específica, boa resistência a corrosão e alto ponto de fusão. Em particular, o titânio grau 2 exerce um importante papel nas industrias automotivas e aeroespaciais. Entretanto, a afinidade com o oxigênio e a baixa resistência ao desgaste são fatores que limitam a aplicação destas ligas como materiais estruturais em altas temperaturas. Os tratamentos termoquímicos tem sido uma eficiente alternativa para melhorar as propriedades tribológicas e o comportamento mecânico desta classe de ligas. Neste contexto, o principal objetivo deste trabalho foi avaliar o efeito da nitretação a gás na resistência a fluência do titânio grau 2. A nitretação foi realizada a 850ºC por um período de 9 h. As amostras foram caracterizadas por medidas de dureza e rugosidade, análises metalográficas e difração de raios X. Testes de curta duração foram realizados sob carga constante a 500ºC. A nitretação gerou uma camada com Ti2N and TiN, espessura de 4,5 ?m com dureza de 1125± 82,26 HV. A nitretação aumentou em 60% a rugosidade superficial. As propriedades em fluência foram significativamente melhoradas pela nitretação. Os testes mostraram que a nitretação reduziu a taxa de fluência estacionária e aumentou o tempo de vida. Com base nos valores do expoente de tensão, o mecanismo predominante foi controlado por escalagem de discordâncias. Após a nitretação do titânio, observou-se um amento do parâmetro de tolerância ao dano. As análises fractográficas revelaram que os mecanismos de fratura foram associados à nucleação e coalescimento de microcavidades. / Titanium and its alloys are attractive structural materials for various engineering applications due to its high specific resistance, good corrosion resistance and high melting point. In particular, titanium grade 2 plays an important role in the automotive and aerospace industries. However, the affinity of titanium for oxygen and low wear resistance are factors that limit the application of titanium alloys as structural materials at high temperatures. The thermochemical treatments has been an efficient alternative for improving the tribological conditions and the mechanical behavior of these alloys. In this context, the main objective of this study was to evaluate the effect of gas nitriding on the creep resistance of titanium grade 2. The gas nitriding was performed at 850ºC during 9h. The samples were characterized using hardness and roughness measurements, metallographic analysis and Xray diffraction. Short-term creep tests were performed under constant tensile load in air at 500ºC.The nitriding treatment produced a layer compound layer of Ti2N and TiN, thickness of 4,5 µm and hardness of 1125± 82,26 HV. There was a 60% increase in the average roughness after nitriding. The creep properties of nitrided specimens were significantly improved in comparison with those of unnitrided Ti.The tests showed that the nitriding reduced the steady-state creep and increased the creep life. Based on the values of stress exponent, it was estimated that the dominant creep mechanism was controlled by dislocation climb. There was an increase in the damage tolerance after nitriding of titanium. The failure analysis revealed that the fracture mechanisms were associated with the nucleation and coalescence of microvoids.

The aims of the thesis were to study how washboarding (the undulations present on the surface of corrugated cardboard used to manufacture boxes) relates to the mechanical properties of paper and the board manufacturing conditions and to examine the impact of washboarding upon the structural integrity and printability of corrugated cardboard packaging. A digital image profilometry technique was developed to measure the washboarding profiles of corrugated board. This technique was used to measure the washboarding depth and profiles for a range of corrugated boards, some constructed manually and some machine manufactured. This enabled a study into how a change in the mechanical properties of paper and glue affect washboarding depth. The effect that the speed of machine manufacturing had upon the degree of washboarding was also determined. A study of how environmental conditions affect washboarding geometry was undertaken. The effects of the extent of washboarding upon a range of board performance measures were tested empirically and modelled using Finite Element Analysis. These were edgewise compression testing (ECT), three-point bend, and MD-Shear (an Amcor Ltd. proprietary test). A method was developed to measure full-tone print coverage of corrugated board and was used to study how washboarding affects the printing quality of corrugated board.

Surface characteristics such as topography and critical dimensions serve as important indicators of product quality and manufacturing process performance especially at the micrometer and the nanometer scales. This paper first reviews different technologies used for obtaining high precision 3-D images of surfaces, along with some selected applications. Atomic force microscopy (AFM) is one of such methods. These images are commonly distorted by convolution effects, which become more prominent when the sample surface contains high aspect ratio features. In addition, data artifacts can result from poor dynamic response of the instrument used. In order to achieve reliable data at high throughput, dynamic interactions between the instrument's components need to be well understood and controlled, and novel image deconvolution schemes need to be developed. Our work aims at mitigating these distortions and achieving reliable data to recover metrology soundness. A summary of our findings will be presented. / Singapore-MIT Alliance (SMA)

A new dynamic model is presented for piezoelectric tube actuators commonly used in high-precision instruments. The model captures coupling between motions in all three axes such as bending motion due to a supposedly pure extension of the actuator. Both hysteresis and creep phenomena are included in the overall actuator model permitting modeling nonlinear sensitivity in the voltage to displacement response. Experimental data on hysteresis and creep are presented to support the modeling. Experiments and model predictions show that due to coupling a voltage Vz corresponding to vertical displacement will produce lateral displacement that acts as a disturbance to the main lateral response. The resonance frequency for the lateral dynamics is inherently lower than that of the longitudinal dynamics. Therefore, Vz is expected to contain frequencies that may excite the lateral resonance. Accordingly, this out of bandwidth disturbance will not be well compensated for either in open or closed loop control of the actuator. In order to preserve performance in open loop actuator control and stability and performance in closed loop control, a large reduction in the bandwidth of vertical motion would be required to avoid exciting the first bending mode. / Singapore-MIT Alliance (SMA)

The mechanical property of polymeric materials changes over time, especially when they are subjected to long-term loading scenarios. To predict the time-dependent viscoelastic behaviors of epoxy-based adhesive materials, it is imperative that reliable accelerated tests be developed to determine their long-term performances under different exposed environments. A neat epoxy resin system and a commercial structural adhesive system for bonding aluminum substrates are investigated. A series of moisture diffusion tests have been performed for more than three months in order to understand the influence of the absorbed moisture on creep behavior. The material properties, such as elastic modulus and glass transition temperature, are also studied under different environmental conditions. The time-temperature superposition method produces a master curve allowing the long-term creep compliance to be estimated. The physics-based Coupling model is found to fit well the long-term creep master curve. The equivalence of the temperature and moisture effect on the creep compliance of the epoxy adhesives is also addressed. Finally, a methodology for predicting the long-term creep behavior of epoxy adhesives is proposed.

Corrosion of steel reinforcement in concrete structures results in significant repair and rehabilitation costs. In the past several years, new fiber reinforced polymer (FRP) reinforcing bars have been introduced as an alternative to steel reinforcing bars. Several national and international organizations have recently developed standards based on preliminary test results. However, limited validation testing has been performed on the recommendations of these standards. High variability of the tensile properties, degradation of tensile strength, direct shear capacity, predicted deflections due to creep, cracking behavior of FRP-reinforced concrete flexural members, bond behavior and development length, and effects of thermal expansion on cracking of FRP reinforced concrete have all been reported, but are areas that need further investigation and validation. The objective of this study is to evaluate the characteristics of glass FRP reinforcing bars and provide recommendations on the design and construction of concrete structures containing these bar types with regard to the areas described. The recently developed ACI 440 design guidelines were analyzed and modifications proposed.

The current research is part of a wider experimental program on creep modeling of glass mat reinforced polypropylene composites which are increasingly being used in molding automotive parts. This specific study is focused on the dimensional and thermal stability of chopped fibre mat and long fibre mat composites. The objective of the study is two-fold. First, to characterize in-situ the micro-failure mechanisms associated with damage accumulation during creep at room temperature and at service temperature (80°C) for stresses up to 67% of the ultimate tensile strength. Second, to characterize the effects of prolonged exposure at elevated temperature on the crystallinity and chemical degradation of the polypropylene matrix. In the first part of the investigation, micro-failure mechanisms including fibre-matrix interface, matrix yielding and cracking during the creep process have been captured in-situ using reflection microscopy. Specimens with 12 mm gauge length were mounted onto a Minimat tensile tester. The applied stress levels of interest were 33% and 67% of the ultimate tensile strength (UTS) at room temperature (RT) and high temperature (HT), respectively. It was found that the deformation mechanisms do not change with temperature but creep in the chopped fibre material is substantially higher than that in the long-fibre. Creep deformation is typically associated with multiple transverse crack initiation at the fibre-matrix interface, crack crazing and rapid coalescence of the small cracks leading to abrupt fracture. Debonding of the fibres is usually detected at the loading stage of the test but fibre breakage is minimal even at high temperature. The change in creep strain at room temperature is similar for both composites but creep strains are highly sensitive to the fibre-mat type at higher temperature. Long-fibre mat structures offer greater creep resistance. Micro-indentations on the matrix-rich regions showed elongation along the loading direction but shear yielding (distortion of indentations) was not noticeable. Using scanning electron microscopy (SEM), the fibre pullout was observed to be pronounced thus suggesting poor adhesion at the fibre-matrix interface. In the second part of this study, the effects of elevated temperature aging on the microstructural changes of isotactic polypropylene matrix in a composite have been studied using wide-angle X-ray scattering (WAXS) and Fourier-transform infrared spectroscopy (FTIR). The objective was to quantify small and slow changes in crystallinity due to thermal aging. To minimize sample variability, polypropylene resin was extracted from the molded composite plaque. Changes in crystallinity level and crystalline form were detected using WAXS after prolonged aging at 90 and 140 °C. FTIR was utilized to monitor in-situ crystallinity changes and to detect oxidation products due to thermal decomposition. The level of crystallinity was monitored by changes in the absorbance ratio of A997/A973 and A841/A973; the former ratio was found to be more sensitive for detecting crystallinity changes. Aging at 140°C resulted in oxidation. The kinetics of secondary crystallization for the aging conditions studied was characterized using Avrami plots.

The current research is part of a wider experimental program on creep modeling of glass mat reinforced polypropylene composites which are increasingly being used in molding automotive parts. This specific study is focused on the dimensional and thermal stability of chopped fibre mat and long fibre mat composites. The objective of the study is two-fold. First, to characterize in-situ the micro-failure mechanisms associated with damage accumulation during creep at room temperature and at service temperature (80°C) for stresses up to 67% of the ultimate tensile strength. Second, to characterize the effects of prolonged exposure at elevated temperature on the crystallinity and chemical degradation of the polypropylene matrix. In the first part of the investigation, micro-failure mechanisms including fibre-matrix interface, matrix yielding and cracking during the creep process have been captured in-situ using reflection microscopy. Specimens with 12 mm gauge length were mounted onto a Minimat tensile tester. The applied stress levels of interest were 33% and 67% of the ultimate tensile strength (UTS) at room temperature (RT) and high temperature (HT), respectively. It was found that the deformation mechanisms do not change with temperature but creep in the chopped fibre material is substantially higher than that in the long-fibre. Creep deformation is typically associated with multiple transverse crack initiation at the fibre-matrix interface, crack crazing and rapid coalescence of the small cracks leading to abrupt fracture. Debonding of the fibres is usually detected at the loading stage of the test but fibre breakage is minimal even at high temperature. The change in creep strain at room temperature is similar for both composites but creep strains are highly sensitive to the fibre-mat type at higher temperature. Long-fibre mat structures offer greater creep resistance. Micro-indentations on the matrix-rich regions showed elongation along the loading direction but shear yielding (distortion of indentations) was not noticeable. Using scanning electron microscopy (SEM), the fibre pullout was observed to be pronounced thus suggesting poor adhesion at the fibre-matrix interface. In the second part of this study, the effects of elevated temperature aging on the microstructural changes of isotactic polypropylene matrix in a composite have been studied using wide-angle X-ray scattering (WAXS) and Fourier-transform infrared spectroscopy (FTIR). The objective was to quantify small and slow changes in crystallinity due to thermal aging. To minimize sample variability, polypropylene resin was extracted from the molded composite plaque. Changes in crystallinity level and crystalline form were detected using WAXS after prolonged aging at 90 and 140 °C. FTIR was utilized to monitor in-situ crystallinity changes and to detect oxidation products due to thermal decomposition. The level of crystallinity was monitored by changes in the absorbance ratio of A997/A973 and A841/A973; the former ratio was found to be more sensitive for detecting crystallinity changes. Aging at 140°C resulted in oxidation. The kinetics of secondary crystallization for the aging conditions studied was characterized using Avrami plots.

This thesis describes an experimental study on the long-term creep behaviour of adhesive anchors under sustained tensile loads in combination with different environmental exposures. A comprehensive background and literature review is presented, focusing on various bond stress models for adhesive anchors, factors affecting their bond behavior, and an overview of available testing standards and evaluation criteria. The experimental program comprises of 82 test specimens. The specimens consist of a cylindrical shaped concrete block of 300 mm (12 inch) in diameter and 200mm (8 inch) in depth, with 15M (No. 5) deformed steel bar post-installed to an embedment depth of six times the bar diameter or 125mm (5 inch). Three types of adhesives were used for anchor installation: Type-A a fast setting two component methyl methacrylate adhesive, Type-B a fast setting two part epoxy adhesive, and Type-C a standard set two part epoxy adhesive. The study is divided into four phases. Phase I consists of 27 static pullout tests to determine the yield strength (fy) and the maximum tensile capacity of each anchor system under three exposure conditions. Phase II and Phase III consist of 36 specimens tested under sustained load levels of 40%fy (32kN) and 60%fy (48kN)under normal laboratory conditions (room temperature) and moisture exposure, respectively. Phase IV consists of 9 specimens tested under sustained load with a load level of 40%fy (32kN) with exposure to freeze/thaw cycling. All sustained load tests lasted for a period of at least 90 days. The results of the static pullout testing showed that specimens with epoxy based adhesive exhibited stronger bond strength, forcing the anchor to fail by rupture prior to bond failure. Under sustained load testing, specimens with standard set epoxy based adhesive showed insignificant creep displacement under room conditions, however, when exposed to moisture noticeable creep displacements were recorded. Specimens with both fast setting epoxy and methyl methacrylate based adhesives showed higher creep displacements under environmental exposure (moisture, freeze/thaw) versus those kept at room temperature. Displacement data from creep testing were analysed and projected over a service life span of 50 years for room temperature exposure, and for 10 years for moisture and freeze/thaw exposures. Based on the analysis results, the service life of different anchor systems was estimated. An integrated qualification and testing protocol is proposed for the creep behavior of adhesive anchors under various environmental exposures.

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