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This research focused on a comprehensive microstructural and mechanical property characterisation study of the Ni-Fe-Cr alloys 718 and 945X. The aim of the project was to better understand the relationship between performance and microstructure of existing (Alloy 718) and newly developed (Alloy 945X) high strength nickel alloys focusing on downhole applications. The main difference between the two alloys is that alloy 945X has lower Nb content than alloy 718, which may minimise the tendency to form delta when combined with correct processing. Previous studies have related the hydrogen embrittlement in alloy 718 with the collection of hydrogen by delta phase. Microstructural characterisation of the new alloy 945X after long term isothermal exposure up to 120 hours in the temperature range 650◦C to 900◦C was conducted with scanning electron microscopy (SEM), to generate a time-temperature-transformation (TTT) diagram. The TTT diagram was used as a road map for designing two isothermal heat treatments of alloy 945X on tensile specimens. Then, the effect of hydrogen charging on the tensile properties and microstructure of the 'as-received' and these two variant heat treatments was investigated. Fractographic analysis showed that, in the presence of hydrogen, intergranular fracture occurred for all the heat treatments, regardless the presence of delta phase at grain boundaries. There was no simple correlation between the volume fraction of delta-phase and susceptibility to hydrogen assisted embrittlement. Rather, it was demonstrated that the morphology and distribution of delta-phase along grain boundaries plays a key role and the other precipitate phases also have an influence through their effect on the ease of strain localisation. This study also examined the hydrogen embrittlement sensitivity of nickel alloy 718 given four different heat treatments to obtain various microstructural states. Each heat treatment leads to differences in the precipitate morphologies of γ', γ'' and delta phases. Material characterisation and fractography of the examined heat treatments were performed using a high resolution FEG-SEM. Three specimens of each condition were pre- charged with hydrogen and tensile properties were compared with those of non-charged specimens. It was observed that hydrogen embrittlement was associated with intergranular and transgranular microcrack formation, leading to an intergranular brittle fracture. delta phase may assist the intergranular crack propagation, and this was shown to be particularly true when this phase is coarse enough to produce crack initia- tion, but this is not the only factor determining embrittlement. Other microstructural features play a role, as does the strength of the material. Finally, the evolution of delta-(Ni3Nb) phase in alloy 718 from the early stages of precipitation, with a particular focus on identifying the grain boundary characteristics that favour precipitation of grain boundary delta phase was investigated. Results showed that delta phase was firstly formed on Σ3 boundaries after 5 hours at the examined temperature (800◦C). Increasing ageing time at 800◦C was observed to lead to an increase in size and precipitation of phases γ'-γ''-delta, an increase in fraction of the special CSL boundaries and an evolution in the morphology of twins and the growth of grains.

Les CMC oxyde/oxyde sont de bons candidats pour des applications thermostructurales. Le comportement mécanique et les mécanismes d’endommagement de deux composites alumine/alumine à renforts tissés bi- et tridimensionnels ont été étudiés et comparés. La microstructure de ces CMC à matrice faible a été caractérisée à partir de porosimétrie et de CND, tel que thermographie IR, scan ultrasonore et tomographie X, ce qui a permis de mettre en évidence la présence de défauts initiaux. Le comportement mécanique en traction, ainsi qu’en compression dansle cas du CMC à renfort bidimensionnel, dans la direction des fibres ainsi que dans la direction ±45°, aété étudié à température ambiante. Afin d’exploiter pleinement ces essais, nous avons eu recours à plusieurs méthodes d’extensométrie et de suivi d’endommagement, telles que la thermographie IR et l’émission acoustique. Les propriétés mécaniques à rupture ainsi que le module de Young du CMC à renfort bidimensionnel développé à l’Onera se sont avérées supérieures à celles disponibles dans la littérature. Les mécanismes d’endommagement des matériaux ont été déterminés à partir d’observations post mortem au MEB et d’essais in situ dans un MEB, ce qui a permis d’évaluer la nocivité des défauts initiaux. Enfin, l’étude du comportement mécanique de ces composites a permisde proposer un modèle d’endommagement tridimensionnel qui permettra de poursuivre le développement de ces matériaux grâce à du calcul de structure. A l’issue de cette thèse, des pistes d’amélioration des procédés d’élaboration et de choix d’instrumentation à utiliser pour les futures études, notamment en ce qui concerne le suivi d’endommagement, ont également été proposées. / Oxide/oxide CMCs are good candidates for thermostructural applications. Themechanical behaviour and damage mechanisms of two alumina/alumina composites with two andthree dimensional woven reinforcements were studied and compared. The microstructure of theseweak matrix CMCs was characterized by porosimetry and NDT methods, such as IR thermography,ultrasound scanning and X-ray tomography, which highlighted initial defects. The mechanicalbehaviour was studied through tensile tests, as well as compression tests in the case of the twodimensionalreinforced CMC. These tests were conducted at room temperature, in the fibres directionsand in the ±45° direction. In order to fully exploit these tests, several extensometry and damagemonitoring methods, such as IR thermography and acoustic emission, were used. Young’s moduli andmaximum stresses and strains of the two-dimensional reinforced CMC developed at Onera appearedto be higher than those available in the literature. The damage mechanisms of the materials weredetermined by post mortem SEM observations and in situ testing in a SEM, which made it possible toassess the nocivity of initial defects. Studying the mechanical behaviour of these composites finallyenabled the development of a three-dimensional damage model that will facilitate the furtherdevelopment of such materials, through finite element analysis. Finally, some improvements regardingthe manufacturing processes and the instrumentation for damage monitoring were suggested forfuture studies.

Thermo-stabilized lignin fibers from pH-fractionated softwood kraft lignin were carbonized to various temperatures during thermomechanical analysis (TMA) under static and increasing load and different rates of heating. The aim was to optimize the carbonization process to obtain suitable carbon fiber material with good mechanical strength potential (high tensile strength and high E-modulus). The carbon fibers were therefore mainly evaluated of mechanical strength in Dia-Stron uniaxial tensile testing. In addition, chemical composition, in terms of functional groups, and elemental (atomic) composition was studied in Fourier transform infrared spectroscopy (FTIR) and in energy-dispersive X-ray spectroscopy (EDS), respectively. The structure of carbon fibers was imaged in scanning electron microscope (SEM) and light microscopy. Thermogravimetrical analysis was performed on thermo-stabilized lignin fibers to evaluate the loss of mass and to calculate the stress-changes and diameter-changes that occur during carbonization. The TMA-analysis of the deformation showed, for thermo-stabilized lignin fibers, a characteristic behavior of contraction during carbonization. Carbonization temperatures above 1000°C seemed most efficient in terms of E-modulus and tensile strength whereas rate of heating did not matter considerably. The E-modulus for the fibers was improved significantly by slowly increasing the load during the carbonization. The tensile strength remained however unchanged. The FTIR-analysis indicated that many functional groups, mainly oxygen containing, dissociate from the lignin polymers during carbonization. The EDS supported this by showing that the oxygen content decreased. Accordingly, the relative carbon content increased passively to around 90% at 1000°C. Aromatic structures in the carbon fibers are thought to contribute to the mechanical strength and are likely formed during the carbonization. However, the FTIR result showed no evident signs that aromatic structures had been formed, possible due to some difficulties with the KBr-method. In the SEM and light microscopy imaging one could observe that porous formations on the surface of the fibers increased as the temperature increased in the carbonization. These formations may have affected the mechanical strength of the carbon fibers, mainly tensile strength. The carbonization process was optimized in the sense that any heating rate can be used. No restriction in production speed exists. The carbonization should be run to at least 1000°C to achieve maximum mechanical strength, both in E-modulus and tensile strength. To improve the E-modulus further, a slowly increasing load can be applied to the lignin fibers during carbonization. The earlier the force is applied, to counteract the lignin fiber contraction that occurs (namely around 300°C), the better. However, in terms of mechanical performance, the lignin carbon fibers are still far from practical use in the industry.

abstract: The future grid will face challenges to meet an increased power demand by the consumers. Various solutions were studied to address this issue. One alternative to realize increased power flow in the grid is to use High Temperature Low Sag (HTLS) since it fulfills essential criteria of less sag and good material performance with temperature. HTLS conductors like Aluminum Conductor Composite Reinforced (ACCR) and Aluminum Conductor Carbon Composite (ACCC) are expected to face high operating temperatures of 150-200 degree Celsius in order to achieve the desired increased power flow. Therefore, it is imperative to characterize the material performance of these conductors with temperature. The work presented in this thesis addresses the characterization of carbon composite core based and metal matrix core based HTLS conductors. The thesis focuses on the study of variation of tensile strength of the carbon composite core with temperature and the level of temperature rise of the HTLS conductors due to fault currents cleared by backup protection. In this thesis, Dynamic Mechanical Analysis (DMA) was used to quantify the loss in storage modulus of carbon composite cores with temperature. It has been previously shown in literature that storage modulus is correlated to the tensile strength of the composite. Current temperature relationships of HTLS conductors were determined using the IEEE 738-2006 standard. Temperature rise of these conductors due to fault currents were also simulated. All simulations were performed using Microsoft Visual C++ suite. Tensile testing of metal matrix core was also performed. Results of DMA on carbon composite cores show that the storage modulus, hence tensile strength, decreases rapidly in the temperature range of intended use. DMA on composite cores subjected to heat treatment were conducted to investigate any changes in the variation of storage modulus curves. The experiments also indicates that carbon composites cores subjected to temperatures at or above 250 degree Celsius can cause permanent loss of mechanical properties including tensile strength. The fault current temperature analysis of carbon composite based conductors reveal that fault currents eventually cleared by backup protection in the event of primary protection failure can cause damage to fiber matrix interface. / Dissertation/Thesis / Fault current temperature relationship program in C / Current temperature relationship program in C / M.S. Electrical Engineering 2014

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Electroconductive cellulose-based fibres were fabricated by depositing multi-walled carbon nanotubes (MWNTs) on the surface using a simple and scalable dip coating. The morphology, mechanical properties and conductive properties of the resultant MWNT–cellulose fibres were investigated by scanning electron microscopy, tensile testing and electrical resistance measurement, respectively. The resistance (RL) of the single MWNT–cellulose fibre can be controlled in a wide range of 50–200 000 kΩ cmˉ¹ by varying the conditions of dip coating. The sensing behaviour of these fibres to liquid water was investigated in detail. The results showed that they exhibit rapid response, high sensitivity and good reproducibility to water, with a relative electrical resistance change of about 100–8000% depending on the initial resistance. It was proposed that the disconnection of MWNT networks caused by swelling effects of the cellulose fibres is the dominant mechanism. Moreover, the sensitivity of the MWNT–cellulose fibres to an electrolyte solution was also investigated.

Indiana University-Purdue University Indianapolis (IUPUI) / Understanding the mechanical properties of peripheral nerves is essential for chronically implanted device design. The work in this thesis aimed to understand the relationship between local deformation responses to global strain changes in peripheral nerves. A custom-built mechanical testing rig and sample holder enabled an improved cyclical uniaxial tensile testing environment on rabbit sciatic nerves (N=5). A speckle was placed on the surface of the nerve and recorded with a microscope camera to track local deformations. The development of a semi-automated digital image processing algorithm systematically measured local speckle dimension and nerve diameter changes. Combined with the measured force response, local and global strain values constructed a stress-strain relationship and corresponding elastic modulus. Preliminary exploration of models such as Fung and 2-Term Mooney-Rivlin confirmed the hyperelastic nature of the nerve. The results of strain analysis show that, on average, local strain levels were approximately five times smaller than globally measured strains; however, the relationship was dependent on global strain magnitude. Elastic modulus values corresponding to ~9% global strains were 2.070 ± 1.020 MPa globally and 10.15 ± 4 MPa locally. Elastic modulus values corresponding to ~6% global strains were 0.173 ± 0.091 MPa globally and 1.030 ± 0.532 MPa locally.

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