High energy particle (neutron, proton and He ions) irradiation to materials typically leads to deteriorating properties, including void swelling, blistering, embrittlement, fracture and exfoliation of surfaces. This dissertation examines size dependent radiation damage in nanostructured metallic multilayers synthesized by the magnetron sputtering technique at room temperature. It reveals the roles of interface in achieving enhanced radiation tolerance in metallic materials. The microstructure and mechanical properties of as-deposited Cu/V multilayer films are systemically investigated, providing the basis for studying radiation damage mechanisms. Sputter-deposited Cu/V multilayers are subjected to helium (He) ion irradiation at room temperature with a peak dose of 6 displacements per atom (dpa). The average helium bubble density and lattice expansion induced by radiation decrease significantly with decreasing h, where h is individual layer thickness. The magnitude of radiation hardening decreases with decreasing h, and becomes negligible when h is 2.5 nm or less. The interactions between interfaces and radiation induced point defects and the evolution of microstructurs and mechanical behavior are discussed. This study indicates that nearly immiscible Cu/V interfaces spaced a few nm apart can effectively reduce the concentration of radiation induced point defects. Dose dependent radiation damage at room temperature in these Cu/V multilayers is systematically investigated with a peak dose in the range of 1-12 dpa. Peak bubble density increases with increasing dose, but it is much lower in Cu/V 2.5 nm multilayers than that in Cu/V 50 nm specimens. A similar radiation hardening trend is observed in multilayers irradiated at different fluences. Radiation hardening increases with dose and seems to reach saturation at a peak dose of 6 dpa. Negligible hardening for fine ( h less than/equal to 2.5 nm) multilayers is observed at all dose levels. Thermal stability of Cu/V multilayers is revealed by in situ annealing inside a transmission electron microscope. During isothermal annealing at 600 degrees C grain boundary grooving occurs across layer interfaces in Cu/V 50 nm specimens, whereas Cu/V 5 nm multilayers appear rather stable. Annealing of Cu/V multilayers at 400 degrees C leads to hardening of multilayers, whereas softening occurs in Cu/V multilayers annealed at 600 degrees C. The evolution of mechanical properties during annealing is correlated to the degradation of the layer interface and the consequent reduction of interface resistance to the transmission of single dislocation.

The fatigue response of mechanical properties and life due to constant stress amplitude tension-tension(T-T)cyclic loading at elevated temperature in nanocomposite APC-2 laminates was investigated. From the basic testing the total amount of 1% by weight of SiO2 spreaded in the interfaces was proved optimally. Related experiments on unidirectional nanocomposite APC-2 laminates were completed, including static tension tests in [0]16¡B[30]16¡B[45]16¡B[60]16 and [90]16 and T-T cyclic tests in [0]16¡B[45]16 and [90]16 specimens at room temperature. After obtaining experimental data, such as ultimate strength and elastic modulus, which were found improved significantly, and then comparing with the basic theory of mechanics of composites, rule of mixtures was adopted to estimate the properties of cross-ply and quasi-isotropic nanocomposite APC-2 laminates and found the largest errors were within 25%. In the consideration of heterogeneous and anisotropic properties of the matrix and the reinforced fibers in nature, the results are reasonably acceptable. On the other hand, the S-N curves according to the experimental data of the fatigue tests were plotted. The vertical axis shows the nondimensional stress level, i.e., the applied maximum stress normalized by ultimate strength at room temperature, and the horizontal axis represents the logarithm of applied cycles. The S-N curves at room and elevated temperatures were also expressed by curve fitting from top to bottom as temperature increasing from RT to 150¢J for both cross-ply and quasi-isotropic nanocomposite laminates. However, when applied maximum stress was normalized by the corresponding ultimate strength, the positions of S-N curves were reverse, i.e., the curves were shown from bottom to top as temperature increasing from RT to 150¢J. That strongly hints us the resistance to fatigue at elevated temperature in both lay-ups of nanocomposite laminates is indeed significantly improved.

Abstract Cr-containing diamond-like carbon coatings (Cr-DLC) with gradient interlayers were studied to elucidate the effects of Cr content and substrate bias on the mechanical properties and microstructure of the deposited coatings. The coatings were deposited with a closed field unbalanced magnetron sputtering (CFUBMS) system. The Cr content and substrate bias were varied from 5 at.% to 30 at.% and -22 V to -60 V, respectively. Mechanical properties of the coatings were evaluated with nano-indenter, scratch tester, ball-on-disk tribo-tester and ball crater. Microstructures of the films were characterized by SEM, TEM, and Raman spectroscopy. Experimental results show that an increases in Cr content from 5 at.% to 30 at.% for the Cr-DLC coatings deposited at substrate bias of ¡V40V results in the increase of the hardness, Young¡¦s modulus, adhesion and friction coefficient, and the decrease of the deposition rate. A minimum abrasive wear rate was found at about 10 ~ 15 at.% Cr content. An increase in substrate bias from -22 V to -60 V for the Cr-DLC a of 10 at% Cr content results in a maximum hardness, Young¡¦s modulus and adhesion, and a minimum friction coefficient and abrasive wear rate at a substrate bias of -50 V, the although the deposition rate is decreased. TEM analysis revealed layered structure of about 35 nm period and fine CrC crystallite nanometer in size on the top layer of the Cr-DLC coatings.

Rats that undergo hindlimb unloading (HU) as a simulation for space flight experience bone changes similar to astronauts in microgravity. The purpose of this research was to assess whether an exercise countermeasure would be effective in preventing or mitigating bone degradation during HU. Controlled electrical muscle stimulation was applied to the lower left hindlimb to simulate resistive exercise. Adult 6-mo. old male rats were assigned to 3 groups of 12 each: hindlimb unloaded (HU), aging cage control (CC), and baseline (BL). The CC group was pair-fed to match the nutritional intake of HU animals during the 28 days of the study. The left leg was exercised 3 days a week for the duration of the study, with the unexercised right leg serving as a contra-lateral control. Mechanical tests were conducted to assess the strength of cancellous bone in the proximal tibia metaphysis. Although isolated specimens of cancellous bone are not feasible, reduced platen compression (RPC) was employed to directly load only the cancellous core region of each specimen. There was no significant difference in ultimate stress or elastic modulus between BL, CC, and HU-Ex (exercised). However, HU-Ex results were dramatically and significantly higher than HU-No Ex (contra-lateral unexercised control) for both ultimate stress (68%) and elastic modulus (81%). It is also notable that ultimate stress was 32% higher (but not statistically significant) for HU-Ex compared to CC. The total bone mineral density in the tibial metaphysis was significantly larger, 11%, in the HUEx compared to the HU-No Ex group's values. The results clearly demonstrate the efficacy of the exercise protocol in preventing the substantial mechanical deterioration induced by HU.

Fuel cells are the direct energy conversion devices which convert the chemical energy of a fuel to electrical energy with much greater efficiency than conventional devices. Solid Oxide Fuel Cell (SOFC) is one of the various types of available fuel cells; wherein the major components are made of inherently brittle ceramics. Planar SOFC have the advantages of high power density and design flexibility over its counterpart tubular configuration. However, structural integrity, mechanical reliability, and durability are of great concern for commercial applications of these cells. The stress distribution in a cell is a function of geometry of fuel cell, temperature distribution, external mechanical loading and a mismatch of thermo-mechanical properties of the materials in contact. The mismatch of coefficient of thermal expansion and elastic moduli of the materials in direct contact results in the evolution of thermal stresses in the positive electrode/electrolyte/negative electrode (PEN) assembly during manufacturing and operating conditions (repeated start up and shut down steps) as well. It has long been realized and demonstrated that the durability and reliability of SOFCs is not only determined by the degradation in electrochemical performance but also by the ability of its component materials to withstand the thermal stresses. In the present work, an attempt has been made to evaluate the thermal stresses as a function of thermal and mechanical properties of the component materials assuming contribution from other factors such as thermal gradient, mechanical loading and in-service loading conditions is insignificant. Materials used in the present study include the state of art anode (Ni-YSZ), electrolyte(YSZ) and cathode materials(LM and LSM) of high temperature SOFC and also the ones being suggested for intermediate temperature SOFC Ni-SCZ as an anode, GDC and SCZ as electrolyte and LSCF as the cathode. Variation of thermo-mechanical properties namely coefficient of thermal expansion, and elastic and shear moduli were studied using thermo-mechanical analyzer and resonant ultrasound spectroscope respectively in 25-900°C temperature range. A non-linear variation in elastic and shear moduli- indicative of the structural changes in the studied temperature range was observed for most of the above mentioned materials. Coefficient of thermal expansion (CTE) was also found to increase non-linearly with temperature and sensitive to the phase transformations occurring in the materials. Above a certain temperature (high temperature region- above 600°C), a significant contribution from chemical expansion of the materials was also observed. In order to determine thermal stress distribution in the positive electrode, electrolyte, negative electrode (PEN) assembly, CTE and elastic and shear moduli of the component materials were incorporated in finite element analysis at temperature of concern. For the finite element analysis, anode supported configuration of PEN assembly (of 100mm x 100mm) was considered with 1mm thick anode, 10μm electrolyte and 30μm cathode. The results have indicated that cathode and anode layer adjacent to cathode/electrolyte and electrolyte/anode interface respectively are subjected to tensile stresses at the operating temperature of HT-SOFC (900°C) and IT-SOFC (600°C). However, the magnitude of stresses is much higher in the former case (500MPa tensile stress in cathode layer) when compared with the stress level in IT-SOFC (178MPa tensile stress in cathode layer). These high stresses might have been resulted from the higher CTE of cathode when compared with the adjacent electrolyte. However, it is worth mentioning here that in the present work, we have not considered any contribution from the residual stresses arising from fabrication and the stress relaxation from softening of the glass sealant.

The mechanical properties and micro-forming of the Au-based bulk metallic glasses are reported in this thesis. The original ingots were prepared by arc melting and induction melting. The Au49Ag5.5Pd2.3Cu26.9Si16.3 bulk metallic glasses with different diameters 2 and 3 mm were successfully fabricated by conventional copper mold casting in an inert atmosphere. By the observation of transmission electron microscopy diffraction pattern, there are crystalline phases among the amorphous matrix phase. The Au49Ag5.5Pd2.3Cu26.9Si16.3 bulk metallic glass shows the high glass forming ability and good thermal stability. By the Differential scanning calorimetry (DSC) results, the values of£G£Vx and £G£Vm are 50 and 21 K. And Trg, £^, and £^m values for the Au49Ag5.5Pd2.3Cu26.9Si16.3 bulk metallic glass (BMG) at the heating rate of 0.67 K/s are 0.619, 0.430 and 0.774, respectively. The mechanical properties of Au49Ag5.5Pd2.3Cu26.9Si16.3 in terms of compression testing are examined using an Instron 5582 universal testing machine. Room temperature compression tests are conducted on specimens with various strain rates. To know the size effect, the micro-pillars were made by using a focus ion beam (FIB) technique. The micro-pillars were under the tests of compression at different strain rates, compared with macro-scale 2 mm rod specimens. In contrast to the brittle fracture in a bulk sample, these micro-pillar specimens show significant plasticity. The morphology of compressed pillar samples indicates that the number of shear bands increased with the sample size and strain rates.

Molecular dynamics theory and second reactive empirical bond order (REBO) potential are employed to determine the mechanical and dynamic properties of (10,10) and (17,0) single-wall carbon nanotubes (SWNT). According to the different simulated environment, the article can be divided into two parts and discussed. The mechanical properties of SWNT in vacuum environment are investigated by tensile process. The physical parameters can be obtained during the tensile process, for example, the yield stress and the Young¡¦s modulus. In addition, the slip vector can be used to investigate the dynamic behaviors of carbon nanotubes during the tensile process and the variation of microstructure after carbon nanotubes broken. Moreover, the mechanical properties of SWNT in the bulk water are also taken into account. In this section, we mainly investigate the effect of the structure of water molecules in the SWNT with different diameters of SWNT. Finally, the mechanical properties of SWNT influenced by water molecules inside the carbon nanotubes are investigated, and compare the results with those in vacuum environment.

<p>The drying sequence in the manufacturing process for paperboard involves evaporation of water, primarily from within the fibres. The vapour is then transported out of the web by pressure or concentration gradients. As the moisture transport from the paper web to the ambient is quicker than the moisture transport within the fibre network to the surfaces of the web, moisture gradients develop through the thickness of the web. This work concerns effects on the mechanics of paper drying from the variation in moisture through the relatively thin structures of paper and paperboard.</p><p>Distributions of inplane residual stresses through paper materials in the unloaded state after drying are believed to be caused by the varying moisture through the thickness during drying. The distributions in general exhibit compressive stress near the board surfaces and tensile stress in the interior of the board. This may be modified after drying and is also affected by structural variation in the material between different plies of multi-ply paperboards.</p><p>The stress development during drying is important because it influences the resulting material properties of the paper and because it can lead to curl, which is a quality problem. The residual stresses themselves are an error source in simulation or evaluation of the mechanical behaviour of paper.</p><p>In this work, residual stress distributions in paperboard were determined experimentally, to clarify the mechanisms of residual stress build-up. An experimental method for such tests was also developed. Based on the experimental findings, the mechanics of paper drying was modelled and the stress build-up simulated. Simulation offers a way of studying how the properties of paper develop during drying of wet paper webs.</p>

Identification of favorable production zones in hydrocarbon-bearing shale often requires the quantification of in-situ mechanical properties. These properties are also necessary for the optimal design of hydro-fracturing operations. Rock elastic properties are affected by volumetric concentrations of mineral constituents, porosity, fluid saturations, and total organic carbon (TOC). Rapid depth variations of rock properties often encountered in shale gas formations make conventional petrophysical interpretation methods inadequate to estimate volumetric concentration of mineral constituents. We introduce a new method to assess elastic properties of organic shale based on the combined quantitative interpretation of sonic, nuclear, and resistivity logs. In-situ elastic properties of organic shale are estimated by (a) improving the assessment of volumetric concentrations of mineral constituents, (b) implementing reliable rock physics models and mixing laws for organic shale, and (c) numerically reproducing wideband frequency dispersions of Stoneley and flexural waves. An example of the application of the method is described in the Haynesville shale gas formation. Estimates of mineral concentrations, porosity, and fluid saturations are in agreement with available laboratory core measurements and X-Ray Diffraction (XRD) data. Calculated layer-by-layer P- and S-wave velocities differ by less than 15% from measured velocities thus confirming the reliability of the method. Finally, based on the new interpretation method developed in this thesis, correlations are found between mineral concentrations, TOC, porosity, and rock elastic properties, which can be used in the selection of optimal production zones. / text

published_or_final_version / Mechanical Engineering / Master / Master of Philosophy