The aim of this thesis is to establish the extent to which materials can be deformed under shear-dominant loadings. Custom Al-6061-T6 tubular specimens are loaded under radial and corner paths of tension and shear to failure. During the experiments, the deformation is monitored in a test section designed to have nearly uniform stress and deformation at large strains while providing minimum constraint to the development of localization that precedes failure. The recorded shear stress-rotation and axial stress-displacement responses exhibit maxima beyond which deformation localizes in a narrow band that is of the order of the 1 mm wall thickness of the test section. For the mainly shear dominated stress paths followed, deformation remained nearly planar allowing for the establishment of both the true stresses and the local deformation strictly from measurements. Results from thirteen radial path experiments as well as from four corner path experiments show the strain at failure to monotonically increase as the mean stress decreases, a result that is in contrast with previously reported results for Al alloys. Also, the measured failure strains are significantly larger than previously reported values. Analysis of corner stress paths investigates the path dependence of localization and failure. Results show little path dependence on the failure strains, but some path dependence on stress maxima and failure stresses. Furthermore, statistical grain-level strain estimates from five of the stress paths revealed a significant variation in strain across the macroscopically observed localization zone. In the neighborhood of the crack tip strains with 25-100% higher levels than the macroscopic values were recorded. This indicates that localization also occurs at a smaller scale than hitherto understood. The difference between the macro strain at failure and the average grain level values increased as the axial/shear stress ratio increased. / text

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Thesis (M.S.)--Mississippi State University. Department of Mechanical Engineering. / Title from title screen. Includes bibliographical references.

The interfacial micromechanics of both high modulus and low modulus carbon fibres have been investigated using Raman spectroscopy. The innovative step was to make low modulus carbon fibres more Raman active by coating them with SWNTs to act as as a strain sensor. Two types of SWNTs have been employed; namely HiPCO SWNTs and COOH SWNTs. Single fibre deformation tests were carried out and the Raman band shift rates with respect to fibre strain have been determined. Meanwhile, different SWNTs coating methods have been investigated. The method of adding COOH SWNTs into the silane layer and within a hot cured epoxy layer was found to generate the highest band shift rates. Furthermore, an investigation of the effect of SWNTs on the strength of the interface was also carried out. A coating of COOH SWNTs was found to significantly improve the interfacial shear strength. Micromechanical tests have been carried out and the stress transfer between the carbon fibres and an epoxy resin was monitored using three different model composite geometries; namely microdroplet-fibre, a film-fibre and a standard fragmentation approach. The result of interfacial shear stress determined from microdroplet-fibre method varied and was found to be highly dependent on the droplet size and shape; this gave the lowest values of interfacial shear stress (ISS). The method of film-fibre obtained an intermediate ISS value which is between that from the microdroplet model test and the fragmentation test. The standard fragmentation test using Raman technique gives the highest ISS and HiPCO SWNTs were found to be a better strain sensor without affecting the original interfacial properties.

For the first time, new metallophthalocyanine (MPc) complexes: (i) nanostructured MPc (nanoMPc, where M = iron or cobalt); (ii) octabutylsulphonylphthalocyanine (MOBSPc, where M = iron or cobalt); and (iii) iron (II) tetrakis(diaquaplatinum)octacarboxyphthalocyanine (PtFeOCPc) were synthesized and characterized using advanced microscopic and spectroscopic techniques such as MS, AFM, HRTEM, FESEM, and EDX. Electrochemical techniques such as cyclic voltammetry, square wave voltammetry, chronoamperometry, rotating disk electrode, and electrochemical impedance spectroscopy, were used to explore the redox chemistry, heterogeneous electron transfer kinetics (HET), and electrocatalytic properties of these MPc complexes towards oxygen reduction reaction (ORR), oxidation of formic acid, thiocyanate and nitrite on a edge plane pyrolytic graphite electrode (EPPGE) platform pre-modified with or without acid functionalized multi-walled carbon nanotubes (MWCNTs). The MWCNT-MPc platforms exhibit enhanced electrochemical response in terms of (i) HET towards an outer-sphere redox probe ([Fe(CN)6]3-/[Fe(CN) 6]4-), and (ii) catalytic activities towards the investigated analytes. The MWCNTnanoMPc electrode exhibits faster HET constant (kapp ≈ 30 – 56 x 10-2 cms-1 compared to their bulk MPc counterparts (≈ 4 – 25 x 10-2 cms-1). The MWCNT-nanoMPc exhibited enhanced electrocatalytic properties (in terms of sensitivity and limit of detection, LoD) towards the detection of thiocyanate and nitrite in aqueous solutions. ORR was a 4- electron process with very low onset potential (-5 mV vs. Ag|AgCl saturated KCl). HET and ORR at MOBSPc complexes supported on MWCNTs showed that the MWCNT–MOBSPc exhibited larger Faradaic current responses than the electrodes without MWCNTs. The rate constant at the MWCNT-MOBSPc electrodes (kapp ≈ (22 – 37) x 10-2 cms-1) is about a magnitude higher than the electrodes without MWCNT (kapp ≈ (0.2 – 93) x 10-3 cms-1). The MWCNT–FeOBSPc electrode gave the best ORR activity involving a direct 4-electron mechanism with low onset potential (0.0 mV vs. Ag|AgCl saturated KCl). The onset potential is comparable and even much lower than recent reports. The HET and electrocatalytic properties of PtFeOCPc supported on a MWCNT platform (MWCNT-PtFeOCPc) gave enhanced electrochemical response in terms of (i) HET (kapp ≈ 78 x 10-2 cms-1), (ii) catalytic rate constant (kcat ≈ 41 cm3mol-1s-1) and (iii) tolerance towards CO poisoning during formic acid oxidation. The ORR activity is a direct 4-electron transfer process at a rate constant of 2.78 x 10-2 cms-1; with a very low onset potential approximately 0.0 mV vs. Ag|AgCl saturated KCl. The electrooxidation of formic acid at MWCNT-PtFeOCPc follows the preferred ‘direct pathway’. This work clearly proves that the MWCNT-MPcs hybrid exhibit enhanced electrochemical and electrocatalytic activities towards the selected analytes compared to the MPcs alone. Considering the ease of fabrication of these electrodes (drop-dry method), these nanocomposite materials are promising platform for potential application in sensing and cataly. / Thesis (PhD)--University of Pretoria, 2011. / Chemistry / unrestricted

Stroke is a medical condition in which poor blood flow to the brain results in cell death. The current treatment options are limited and only very few patients can benefit from these treatments. Stroke causes brain swelling and often a decompressive craniectomy is performed for some of the patients to release intracranial pressure to prevent further damage. As a result, a duraplasty is implanted to replace the surgically-damaged dura mater to protect the brain. In view of that, the purpose of this project was to develop double-walled microspheres (DWMS) which can be used as a drug delivery system when incorporated into duraplasty to promote endogenous stem cell therapy to treat stroke. The DWMS were composed of poly (l-lactic acid) (PLLA) and poly (lactic-co-glycolic acid) (PLGA) using a solvent evaporation method. Bovine serum albumin (BSA), as a model protein, was entrapped within these DWMS with different core-shell thicknesses and compositions to investigate the distribution of protein, encapsulation efficiency, and in vitro release. The fabrication process parameters of DWMS were also optimized to attain higher yields, and the phase separation and surface morphology were examined by differential scanning calorimetry and scanning electron microscopy.

No description available.

It is reasonable to assume that an aircraft might experience some in-flight discrete source damage caused by various incidents. It is, thus, necessary to evaluate the impact of such damage on the performance of the aircraft. This study is focused on evaluating the effect of a simple discrete damage in an aircraft wing on its static and dynamic response. The damaged wing is modeled by a thin-walled beam with a longitudinal crack the response of which can be obtained analytically. As uncertainties are present in the location and size of the crack as well as in the applied loads, their effects are incorporated into the framework consisting of structural response, crack propagation and aeroelasticity. The first objective of this study is to examine the effect of damage represented by a crack on the wing flexibility that influences its deformation and aero-elastic divergence characteristics. To study this, the thin-walled beam is modeled by Benscoter thin-walled beam theory combined with Gunnlaugsson and Pedersen compatibility conditions to accurately account for the discontinuity at the interface of the cracked and uncracked beam segments. Instead of conducting a detailed finite element analysis, the solution is obtained in an exact sense for general distributed loads representing the wind pressure effects. This analytical approach is shown to provide very accurate values for the global beam response compared with the detailed finite element shell analysis. This analytical solution is, then, used to study the beam response probabilistically. The crack location and size are assumed to be uncertain and are, thus, characterized by random variable. For a specified limit state, the probability of failure can be conveniently calculated by the first order second moment analysis using the safety index approach. The same analytical solution is also used to study the aero-elastic divergence characteristics of a wing, the inner structure of which is represented by a thin-walled beam with a crack of uncertain size and position along the beam. The second objective of this study is to examine the time growth of a crack under dynamic gust type of loading to which a wing is likely to be exposed during flight. Damage propagating during operation further deteriorates the safety of the aircraft and it is necessary to study its time growth so that its impact on the performance can be evaluated before it reaches its unstable state. The proposed framework for the crack growth analysis is based on classical fracture mechanics where the remaining flight time is obtained by Monte Carlo simulation in which various uncertainties are taken into account. To obtain equivalent cyclic loading required for crack growth analysis, random vibration analysis of the thin-walled beam is conducted for stochastic wind load defined by a gust load spectral density function. The probability of failure represented by the crack size approaching the critical crack size within the flight duration or the remaining flight time before a crack reaches its limiting value are obtained. This study with a simple representation of a wing and damage is anticipated to provide initial guidance for future studies to examine the impact of discrete source damage on the in-flight performance of the aircrafts, with the ultimate goal of minimizing the adverse effect and enhancing the safety of aircrafts experiencing damage. / Ph. D.

<p> A general, consistent, nonlinear theory for open thin-walled elastic beams is presented. The theory takes into account geometric nonlinearities caused by large rotation of the cross section of the beam. The nonlinear differential equations of deformation and response are derived by means of application of Hamilton's principle. It is found that the set of equations reduces to the results obtained by Cullimore and Gregory in the special cases of large uniform torsion of thin-walled members. A solution of a thin-walled beam, subjected to large non-uniform torsional deformation due to application of torques at the ends, is obtained. Comparison is made on the torque - rotation characteristics of a thin-walled beam subjected to large uniform torsion and large non-uniform torsion to show the effect of end constraint from warping.</p> <p> A set of nonlinear equations to study the stability of a thin-walled beam of open cross section, under axial loading (spatial stability) and lateral loading (lateral stability), is presented. Using the derived equations, the dynamic stability of thin-walled beams of symmetrical and monosymmetrical cross sections subjected to axial loads, is investigated. The regions of parametric instability, the steady state amplitudes of oscillations, once parametric instability takes place, and the non-steady state solutions, to show the growth of the parametric oscillations, are carried out.</p> <p> The effect of viscous damping on the steady state amplitude and the growth behaviour of the parametrically excited oscillations is shown. The dynamic stability of a thin-walled beam of symmetrical I section and a monosymmetrical split ring section are worked out in detail as examples.</p> / Thesis / Doctor of Philosophy (PhD)