Z-pinning is a through-thickness reinforcement technology for polymer composite materials that has been developed and commercialised over the past fifteen years. The through-thickness reinforcement of composites with thin metallic or fibrous pins aids in suppressing delamination, improving impact damage tolerance and increasing joint strength. Z-pins are applied to the composite part during its manufacture. Pins are embedded within sheets of foam and placed over the unconsolidated part. Subsequently, the foam is compacted and the pins transferred into the part, which is usually an uncured prepreg. In this manner, large numbers of pins can be inserted quickly and easily. The pinned composite is then cured using conventional processes. The use of z-pins is currently limited to several high performance composite structures, most notably Formula One racing cars and F/A-18 E/F (Superhornet) fighter aircraft, although the technology has potential applications in a d iverse variety of aerospace and non-aerospace composite structures. A limited understanding of the mechanical performance of z-pinned parts under high load and fatigue loading conditions currently hinders the application of z-pinned composites. The aim of this PhD project is to investigate the mechanical properties, strengthening mechanics and failure mechanisms of z-pinned carbon/epoxy laminates and joints. The effect of z-pin reinforcement on the tensile and flexural properties of laminates under monotonic and fatigue loading is studied. The sensitivity of these properties to the volume content and diameter of the z-pins is systematically studied by experimentation and analytical modelling. This PhD also evaluates the efficacy of z-pins in improving the load-bearing properties of carbon/epoxy lap joints. Improvements to the room temperature and elevated temperature properties of z-pinned lap joints under monotonic and fatigue tensile loading were determined. The effect of strain rate on the load-bearing properties of z-pinned lap joints was also evaluated. A further aim of the PhD project was to assess the z-pin manufacturing process and the microstructural damage caused by that process. The outcome of this study augments the analysis of the me chanical properties of z-pinned laminates and joints.

<p>Engine development is now driven by cost, performance, governmental regulations and customer demands. Several of the requirements have tribological associations. Tribological improvements which consist in lowering friction and improving wear resistance in engines, will play a major role to increase reliability and life cycle.</p><p>The components studied here are parts of the valvetrain mechanism of heavy-duty Diesel engines. The injection cam is one of the most problematic parts of the camshaft, as it is subjected to high pressures from the fuel injector. Lubrication is of significant importance in the prevention of cam failure caused by wear. However, the satisfactory lubrication of the cam and roller contact has proved to be one of the most difficult tribological design challenges to take up.</p><p>For a lubricated contact, the degree of separation between surfaces has a very strong influence on the type and amount of wear. This degree of separation is termed as specific film thickness ; its value provides a measure of the severity of asperities interaction in the lubricated contact. In this report, attention is drawn on the evaluation of oil film thickness in the cam-roller contact, in order to predict regimes of lubrication and thus to identify the probable wear zones of the injection cam. Then, confrontation with experimental results is performed(observation of worn cam surfaces). Future work to achieve is to discover the influence of the different parameters on oil film thickness, by performing a multivariate analysis. The next step will focus on modelling the wear of injection cams, and finally establishing quantified correlations between wear and specific film thickness.</p>

<p>Two new methods for calculating the double differential scattering cross-section (DDSCS) in electron energy loss spectroscopy (EELS) have been developed, allowing for simulations of sample geometries which have been unavailable to earlier methods of calculation. The new methods concerns the calculations of the <em>thickness function</em> of the DDSCS. Earlier programs have used an analytic approximation of a sum over the lattice vectors of the sample that is valid for samples with parallel entrance and exit surfaces.The first of the new methods carries out the sum explicitly, first identifying the unit cells illuminated by the electron beam, which are the ones needed to be summed over. The second uses an approach with Fourier transforms, yielding a final expression containing the <em>shape amplitude</em>, the Fourier transform of the <em>shape function</em> defining the shape of the electron beam inside the sample. Approximating the shape with a polyhedron, one can quickly calculate the shape amplitude as sums over it’s faces and edges. The first method gives fast calculations for small samples or beams, when the number of illuminated unit cells is small. The second is more efficient in the case of large beams or samples, as the number of faces and edges of the polyhedron used in the calculation of the shape amplitude does not need to be increased much for large beams. A simulation of the DDSCS for magnetite has been performed, yielding diffraction patterns for the L₃ edge of the three Fe atoms in its basis.</p>

Plastic relaxation was observed in InᵡGa₁₋ᵡN/GaN epilayers grown on c-plane sapphire substrates. The relaxation obeys the universal hyperbolic relation between the strain and the reciprocal of the layer thickness. Plastic relaxation in this material system reveals that there is no discontinuous relaxation at critical thickness and once a layer starts to relieve, it follows the same strain-thickness dependence, unconstrained by the original misfit until the material system work hardens. From x-ray diffraction calibration, the in-plane and normal relaxation constants KP0 and KN0 for the InᵡGa₁₋ᵡN/GaN grown on sapphire were found to be −0.98 ± 0.03 and +0.51 ± 0.03 nm, respectively. / Singapore-MIT Alliance (SMA)

A near infrared waveguide photodetector in Si-based ternary Si₁−x−yGexCy alloy was demonstrated for 0.85~1.06 µm wavelength fiber-optic interconnection system applications. Two sets of detectors with active absorption layer compositions of Si₀.₇₉Ge₀.₂C₀.₀₁ and Si₀.₇₀Ge₀.₂₈C₀.₀₂ were designed. The active absorption layer has a thickness of 120~450 nm. The external quantum efficiency can reach ~3% with a cut-off wavelength of around 1.2 µm. / Singapore-MIT Alliance (SMA)

Despite advances, stroke remains the largest cause of disability and fourth leading cause of death in the United States. The relationship between changes in human vasculature (atherosclerosis and arteriosclerosis) prior to clinical incident, and other risk factors for stroke remains unclear. This dissertation represents work towards the identification of imaging biomarkers for vascular change, focusing on ultrasound to characterize persons at risk, including differences among race/ethnic groups. This research contained three distinct projects. The first goal was to determine if changes within ultrasonographic measures of carotid vasculature could be found across race/ethnic groups after adjustment for risk factors. The second was to determine if those same measures were related to changes in cerebral white matter known to be associated with ongoing cerebrovascular disease; we compared ultrasound to an MRI marker of subclinical vascular disease, white matter hyperintensity volume (WMHV). Finally, we sought to investigate a known and well-studied ultrasound marker for atherosclerosis, carotid intima-medial thickness, with those same MRI markers of subclinical vascular disease (WMHV). All studies were conducted within an on-going multiethnic cohort that has been followed since 1990, The Northern Manhattan Study. The population is comprised of persons who self-identify as Hispanic (52%), Black (24%), or White (21%), with less than 3% identified as “race/other.” We found race/ethnic differences in carotid arterial stiffness and diameter; carotid diameter increases with age among Hispanics, but not among blacks or whites. A significant correlation was also found between diastolic diameter and subclinical vascular disease, and this relationship was also increased among Hispanics; neither black race nor white race was associated with corresponding increases in both MRI white matter hyperintensity and diastolic diameter. Finally, using a surrogate marker for atherosclerosis, carotid intima-medial thickness (cIMT), we document for the first time, positive associations between cIMT and WMHV. There are important developments still to be made in the field of vascular risk. Use of inexpensive and non-invasive ultrasound technology to approximate ongoing cerebrovascular disease could lead to better understanding of the effect of known risk factors, and could help stroke risk assessment and treatment modification.

Wire mesh vibration dampers have been the subject of some very encouraging experiments at the Texas A&M Turbomachinery laboratories for the past several years and have emerged as an excellent replacement for squeeze film dampers. Their capability to provide damping for a wide range of temperatures (even cryogenic), fluid free operation and ability to perform even when soaked with lubricants makes them a suitable option as a bearing damper. Experiments were conducted to investigate the effect of design parameters like axial thickness and axial compression that influence the characteristics of wire mesh as a bearing damper. Two groups of wire mesh were tested to show that the stiffness and damping are directly proportional to the axial thickness, if all the other parameters are kept constant. Tests on four wire mesh donuts of different radial thickness showed that stiffness and damping vary inversely with radial thickness. Rigorous tests were also conducted to quantify the effects of axial compression, radial interference and displacement amplitude on stiffness and damping of the wire mesh. Another novel kind of mesh damper tested was comprised of two small segments instead of a whole donut. The results showed that wire mesh exhibited good damping characteristics even when used in small segments. Empirical expressions were developed using MathCADTM worksheets, and an existing ExcelTM design worksheet was modified to include these factors. The effect of frequency variation was also included to give a comprehensive design tool for wire mesh. A new design worksheet was developed that can predict rotordynamic coefficients for a wire mesh bearing damper having a different size as well as different installation and operational conditions.

Study of liquid film dynamics is of significant importance to the understanding and control of various industrial processes that involve spray cooling (condensation), heating (boiling), coating, cleaning and lubrication. For instance, the critical heat flux (CHF) of boiling heat transfer is one of the key parameters ensuring the efficiency and safety of nuclear power plants under both operational and accident conditions, which occurs as the liquid layers (microlayer and macrolayer) near the heater wall lose their integrity. However, an experimental quantification of thin liquid film dynamics is not straightforward, since the measurement at micro-scale is a challenge, and further complicated by the chaotic nature of boiling process. The object of present study is to develop experimental methods for the diagnosis of liquid film dynamics, and to obtain data for the film instability under various conditions. A dedicated test facility was designed and constructed where micro conductive probes and confocal optical sensors were used to measure the thickness and dynamic characteristics of a thin liquid film on various heater surfaces, while a high speed camera was used to get visual observation. Extensive tests were performed to calibrate and verify the two thickness measuring systems. The micro conductive measuring system was proven to have a high reliability and repeatability with maximum system error less than 5µm, while the optical measuring system is capable of recording the film dynamics with spatial resolution of less than 1 mm. The simultaneous measurement on the same liquid film shows that the two techniques are in a good agreement with respect to accuracy, but the optical sensors have a much higher acquisition rate up to 30 kHz, which are more suitable for rapid process. The confocal optical sensors were therefore employed to measure the dynamic thickness of liquid films (ethanol, hexane and water) evaporating on various horizontal heater surfaces (aluminum, copper, silicon, stainless steel and titanium) to investigate the influences of heat flux, the surface and liquid properties on the film instability and the critical thickness. The critical thickness of water film evaporating on various surfaces was measured in the range of 60-150 mm, increasing with the increased contact angle or increased heat flux (evaporating rate) and also with the decreased thermal conductivity of the heater material. The data suggest the conjugate heat transfer nature of the evaporating liquid film dynamics at higher heat fluxes of interest to boiling and burnout. In the case of hexane on the aged titanium surface with contact angle of ~3o, the liquid film is found resilient to rupture, with film oscillations at relatively large amplitude ensuing as the averaged film thickness decreases below 15 µm. To interpret our experimental findings on liquid film evolution and its critical thickness at rupture, a theoretical analysis is also performed to analyze the dynamics of liquid films evaporating on heater surfaces. While the influences of liquid properties, heat flux, and thermal conductivity of heater surface are captured by the simulation of the lubrication theory, influence of the wettability is considered via a minimum free energy criterion. The thinning processes of the liquid films are generally captured by the simulation of the lubrication theory. For the case with ideally uniform heat flux over the heater surface, the instability of the liquid film occurs at the thickness level of tens micro meters, while for the case of non-uniform heating, the critical thicknesses for the film rupture are closer to  the experimental data but still underestimated by the lubrication theory simulation. By introducing the minimum free energy criterion to considering the influence of surface wettability, the obtained critical thicknesses have a good agreement with the experimental ones for both titanium and copper surfaces, with a maximum deviation less than ±10%. The simulations also explain why the critical thickness on a copper surface is thinner than that on a titanium surface. It is because the good thermal conductivity of copper surface leads to uniform temperature distribution on the heat surface, which is responsible for the resilience of the liquid film to rupture. A silicon wafer with an artificial cavity fabricated by Micro Electronic Mechanical System (MEMS) technology was used as a heater to investigate the dynamics of a single bubble in both a thick and thin liquid layer under low heat flux (&lt;60 kW/m2). The maximum departure diameter of an isolated bubble in a thick liquid film was measured to be 3.2 mm which is well predicted by the Fritz equation. However, in a thin liquid layer with its thickness less than the bubble departure diameter, the bubble was stuck on the heater surface with a dry spot beneath. A threshold thickness of the liquid film which enables the dry spot rewettable was obtained, and its value linearly increases with increasing heat flux. In addition, another test section was designed to achieve a constant liquid film flow on a titanium nano-heater surface which helps to successfully carry boiling in the liquid film from low heat flux until CHF. Again, the confocal optical sensor was employed to measure the dynamics of the liquid film on the heater surface under varied heat flux conditions.  A statistical analysis of the measured thickness signals that emerge in a certain period indicates three distinct liquid film thickness ranges: 0~50 µm as microlayer, 50~500 µm as macrolayer, 500~2500 µm as bulk layer. With increasing heat flux, the bulk layer disappears, and then the macrolayer gradually decreases to ~105 µm, beyond which instability of the liquid film may lose its integrity and CHF occurs. In addition, the high-speed camera was applied to directly visualize and record the bubbles dynamics and liquid film evolution. Dry spots were observed under some bubbles occasionally from 313 kW/m2 until CHF with the maximum occupation fraction within 5%.  A dry spot was rewetted either by liquid receding after the rupture of a bubble or by the liquid spreading from bubbles’ growth in the vicinity. This implies that the bubbles’ behavior (growth and rupture) and their interactions in particular are of paramount importance to the integrity of liquid film under nucleate boiling regime. / QC 20111205 / VR-2005-5729, MSWI

<p>The focus of this study is to synthesize a novel hinged polymer actuator. The linking molecule (hinge) is phenazine with interconnected dipyrrole units.</p>

Two new methods for calculating the double differential scattering cross-section (DDSCS) in electron energy loss spectroscopy (EELS) have been developed, allowing for simulations of sample geometries which have been unavailable to earlier methods of calculation. The new methods concerns the calculations of the thickness function of the DDSCS. Earlier programs have used an analytic approximation of a sum over the lattice vectors of the sample that is valid for samples with parallel entrance and exit surfaces.The first of the new methods carries out the sum explicitly, first identifying the unit cells illuminated by the electron beam, which are the ones needed to be summed over. The second uses an approach with Fourier transforms, yielding a final expression containing the shape amplitude, the Fourier transform of the shape function defining the shape of the electron beam inside the sample. Approximating the shape with a polyhedron, one can quickly calculate the shape amplitude as sums over it’s faces and edges. The first method gives fast calculations for small samples or beams, when the number of illuminated unit cells is small. The second is more efficient in the case of large beams or samples, as the number of faces and edges of the polyhedron used in the calculation of the shape amplitude does not need to be increased much for large beams. A simulation of the DDSCS for magnetite has been performed, yielding diffraction patterns for the L3 edge of the three Fe atoms in its basis.