165 research outputs found
Experimental and finite element study of the hydroforming bi-layered tubular components
No full textThe application of finite element method (FEM) in the area of metal forming and material processing has been increasing rapidly during the recent years. The present study has been carried out on one of the unconventional metal forming processes called hydroforming of a multi-layered tube. The study involved both experimental and simulation work using FEA. Multi-layered tubes have extensive advantages in both domestic and industrial uses. The specimen tube consists of two different layers of materials. The outer tube material is brass and the inner tube material is copper. This project is mainly dedicated to the modelling, simulation and advanced study of one of the unconventional metal forming processes called hydroforming in which extremely high fluid pressure is used to deform the metal into desired shape. Various types of complex industrial products can be made by hydroforming. This process is suitable to produce seamless, lightweight, near net shaped industrial components. There are some complex products, which are easier to produce by hydroforming than by conventional technique.
In this research work the main forming load is hydrostatic pressure applied to the internal surface of the tube, together with an in-plane compressive load applied simultaneously. The blank is placed in a pre-shaped die block and due to the action of simultaneous internal pressure and axial load; it is formed into a complex desired shape. If the internal pressure is too high during the process without sufficient axial load it may cause the tube to burst, on the other hand too large axial load without applying sufficient internal pressure may cause wrinkling of the tube. For these reasons, a number of simulations of the hydroforming process have been carried out for different axial load and internal pressure combinations and optimum conditions have been established for the particular process. This simulated hydroforming of composite material tube and the formed product has been analysed on the basis of forming conditions and the simulated forming conditions have been verified by experiment. The simulations of hydroforming process for X or T branch have been carried by using the commercial finite element package ANSYS
An investigation into the influence of stent strut thickness on in-stent restenosis using the finite element method
No full textIn-stent restenosis represents the major limitation for stenting procedures. In-stent restenosis is the renarrowing o f the artery lumen within a stent predominantly due to excessive growth of neointimal hyperplasia. Clinical studies have found that stent design is a key determinant in the propensity o f stents to cause restenosis, indicating a vital link between the biomechanics o f stents and the development of the disease. The ISAR-STEREO Trial specifically assessed the effect o f strut thickness on restenosis outcome and found that for the same stent design, a thinner strut stent was associated with a significant reduction o f angiographic and clinical restenosis compared to the same stent with a thicker strut. The main objective o f this study is to use the finite element method to simulate these stenting procedures, and to examine the stresses induced within the stented arterial vessel walls by the stents, thus enabling the mechanical stimuli for in-stent restenosis to be identified.
Finite element models o f thin and thick strut stents were developed and the stents were deployed in various stenosed vessel geometries such that the stresses induced within the stented vessels by the two stents could be compared. The stresses were examined at the end o f stent deployment, to determine the mechanical stimuli for acute damage, and again at stent unloading, to determine the long term stimuli for in-stent restenosis. The stress analyses were used to determine the level of vascular injury caused to the artery by different strut thickness stents.
The finite element studies successfully identified differences between the mechanical loading of the arterial tissue in the vessels stented with the two different stents. The higher restenosis rate of the thicker strut stent, reported in the ISAR-STEREO clinical study, was found to be the result of the higher luminal gain achieved by the thicker strut stent, due to the lower recoil of the stent structure when both stents were expanded to the same initial lumen diameter. Further stenting analyses, however, found that the thicker strut stent resulted in a lower percentage of volume stressed at high levels compared with the thinner strut stent when it was expanded to the same final lumen diameter. This suggests that a thicker strut stent may in fact have the potential to be expanded to an optimal diameter whereby the in-stent restenosis is minimised. Therefore, it is proposed that the use of preclinical testing tools, such as finite element modelling, could be used to predetermine the deployment protocol and optimum luminal gain of a particular stent design in order to minimise the mechanical stimuli for in-stent restenosis
Bis[5-(4-methoxybenzyl)furan-3-yl]methanone
No full textThe title compound, C25H22O5, was obtained by a dehydrogenative carbonylation reaction. It crystallizes with one half-molecule in the asymmetric unit. The molecules have crystallographic C2 symmetry and the two atoms of the carbonyl group are located on the rotation axis. The methoxy groups are coplanar with the benzene ring to which they are attached [C-C-O-C = 1.0 (6)°]. The two furan rings are inclined at 17.3 (3)° with respect to each other and the dihedral angle between the furan ring and the benzene ring is 75.83 (12)°. The crystal structure is stabilized by C-H...O hydrogen bonds. Key indicators: single-crystal X-ray study; T = 183 K; mean ( σ(C–C) = 0.006 Å; R factor = 0.081; wR factor = 0.195; data-to-parameter ratio = 13.4
Synthetische und mechanistische Perspektiven homogener Gold-Katalysen
No full texta) The first chapter of this thesis manifests the exploration of homogeneous gold-catalyzed conversions of furan containing aryl-ynamides and ynol ethers. Enynes boast to be the most explored substrate structures in the realm of homogeneous gold catalysis, whereas the reactivity of ene-ynamides and ene-ynol ethers are much less explored till date.34, 45 Hashmi and co-workers recently reported the homogeneous gold-catalyzed synthesis of phenols from furan containing ynamides/ynol ethers.45e These substrates showed excellent reactivity and selectivity apparently owing to the heteroatom directly attached to the alkyne unit. Impressed with the reactivity and high selectivity of these systems, we decided to explore the catalytic activity of the aryl substituted variants of these compounds.
Gold catalysis of aryl-ynamides
The aryl-ynamide substrates turned out to be interesting candidates for gold catalysis. The mode of reactivity depended on the tether length. The substrates with two carbon tether underwent a Friedel-Crafts type reaction upon subjected to gold catalysts and furnished benzannulated arenes in moderate to excellent yields (Scheme C). A combination of Ph3PAuCl /AgBF4 (5 mol%, 1:1) in CH2Cl2 or CHCl3 was found to be the best choice of catalyst. The reaction conditions were mild and a broad spectrum of polyarenes and polyheteroarenes were synthesized. The substrates carrying an unactivated arene failed to react even under higher temperature and prolonged reaction time.
The gold catalysis of substrates 58r-58t with a longer tether took a completely different pathway and gave rise to cyclopentadiene structures (Scheme D). Similar to shorter tethered substrates, electron withdrawing groups on the arene were not tolerated.
b) The second chapter of this thesis deals with the development of gold-catalyzed transformations of furan containing alkynes that prove the potential intermediacy of a cationic intermediate in these types of reactions. The actual electronic state of the intermediate in gold-catalyzed enyne cycloisomerizations is a hot spot in recent scientific reports.20a-c A cyclopropyl "carbene" form of the intermediate has been used by most of the authors. But it is important to note that such a species is yet to be isolated or characterized spectroscopically. Recently Fürstner et al., proposed remarkable experimental evidence for the involvement of the cationic form of this intermediate in gold-catalyzed cyclizations.20a,20c The scenario demands the consideration of the actual electronic state of the intermediate as a mesomeric/tautomeric package of different canonical forms (Scheme H).
In the context of this thesis it was found that furyl-alkynes carrying electron donating substituents in the α-position of the tether led to the formation of unexpected five-membered structures (instead of the expected phenols) when subjected to gold catalysts (Scheme I).
The observed reactivity pattern for the substrates 116/118 was highly informative and served as an experimental evidence for the significant contribution of the cationic form of the intermediate AO in the mesomeric/tautomeric equilibria shown in the scheme H. A mechanistic proposal based on this intermediate was suggested for this hitherto unknown transformation in the gold catalysis of furyl-alkynes (Scheme J).
c) The third chapter of this thesis elaborates the unexpected formation of N,O-acetals from oxanorbornadienes-the potential precursors for phenols- upon exposure to gold and other Lewis acids (Scheme K). AuCl was found to be the best Lewis acid catalyst under the given conditions, and Yb(CF3SO3)3 also showed remarkable reactivity. The generality and the absence of any other products/side products confirmed that the reactions followed general acid catalysis. The observation that there is a net reversal in the connectivity of the tether in the product hinted at a complex mechanism operating in the process.
d) The fourth chapter of this thesis describes the investigations carried out on the aerobic oxidation reactions catalyzed by gold. A homogeneous catalytic system comprising of gold(I) chloride, n-butyl lithium and sodium carbonate was developed and was found to oxidize primary aromatic alcohols to aldehydes. The system was poor in activity towards aliphatic and secondary alcohols (Scheme L).a) Das erste Kapitel dieser Doktorarbeit befasst sich mit der homogenen goldkatalysierten Umsetzung von furylsubstituierten Arylinamiden und Arylinolethern. Enine gehören wohl zu den am besten erforschten Substratstrukturen auf dem Gebiet der homogenen Goldkatalyse, wohingegen die Reaktivität von Eninamiden oder Eninolethern bisher kaum untersucht wurde.34,35 Erst kürzlich berichteten Hashmi et al. von der homogenen goldkatalysierten Synthese von Phenolen aus furansubstituierten Inamiden/Inolethern.45e Diese Substrate zeigten eine exzellente Reaktivität und Selektivität, die offensichtlich auf die Heteroatome zurückzuführen sind, welche direkt an die Alkineinheit angeknüpft sind. Beeindruckt von der Reaktivität und der hohen Selektivität dieser Systeme, entschieden wir uns, die Eigenschaften der arylsubstituierten Derivate in der katalysierten Umsetzung dieser Stoffe zu untersuchen.
Goldkatalysierte Umsetzung von Arylinamiden
Die Arylinamid-Substrate stellten sich als interessante Kandidaten für die Goldkatalyse heraus, da die Art der Reaktivität von der Kettenlänge abhing. Die Substrate mit zwei Kohlenstoffeinheiten in der Kette vollzogen eine Friedel-Crafts-artigen Reaktion und lieferten benzannelierte Arene in hervorragenden bis moderaten Ausbeuten (Schema C). Eine Kombination aus Ph3PAuCl/AgBF4 (5 mol%, 1:1) in CH2Cl2 oder CHCl3 stellte sich als die geeignetste Katalysatorwahl heraus. Daduch konnte ein breites Spektrum an Polyarenen und Polyheteroarenen synthetisiert werden. Substrate, die ein nicht-aktiviertes Aren besitzen, reagierten selbst bei höheren Temperaturen und verlängerten Reaktionszeiten nicht.
Die Goldkatalyse der Substrate 58r-58t mit einer C3- Kette zwischen Furan und Inamideinheit ergab eine komplett andere Reaktivität und führte zu Cyclopentadien-Strukturen (Schema D). Wie bei den kürzerkettigen Substraten wurden elektronenziehende Gruppen am Aren nicht toleriert.
Goldkatalysierte Umsetzung von Arylinolethern
Die goldkatalysierte Umsetzung der Inoletherverbindungen 59a führte zum cyclisch konjugierten Keton 67 (Schema G). Bei Substraten mit verlängerter Alkinyl-Kette erfolgte jedoch eine Addition von Wasser an das Alkin.
b) Das zweite Kapitel dieser Arbeit behandelt die goldkatalysierte Umsetzung von Alkinylfuranen, die ein mögliches kationischen Intermediats im Reaktionverlauf nachweisen sollen. Hintergrund sind die neusten wissenschaftlichen Diskussionen um die elektronische Natur des Zwischenprodukts in der goldkatalysierten Enin-Cycloisomerisierung.20 Von der Mehrzahl der Autoren wird für den Reaktionsmechanismus hierzu ein Cyclopropyl-Carben–Intermediat postuliert. Bisher konnte jedoch eine solche Spezies nicht isoliert oder spektroskopisch analysiert werden. Kürzlich schlug Fürstner et al. einen experimentellen Beweis für die Beteiligung einer kationischen Form dieses Zwischenprodukts in goldkatalysierten Cyclisierungen vor.20 Dieses Szenario verlangt die Betrachtung des Zwischenprodukts als eine mesomere und tautomere Kombination von verschiedenen kanonischen Formen (Schema H).
Die Untersuchungen ergaben, dass Alkinylfurane mit Donorsubstituenten R in -Position der Alkinylkette in der Goldkatalyse unerwartete Fünfringstrukturen, statt der erwarteten Phenole bildeten (Schema I).
Aus dem Reaktionsmuster der Substrate 116/118 erschloss sich, dass eine kationische Form des Intermediats BB im mesomeren/tautomeren Gleichgewicht auftreten muss, wie in Schema H gezeigt wird. Der mechanistische Vorschlag hierzu basiert auf dem Zwischenprodukt für diese bislang unbekannte Umformung in der Goldkatalyse von Furylalkinen (Schema J).
c) Das dritte Kapitel der Arbeit befasst sich mit der unerwartete Formierung von N,O-Acetalen aus Oxanorbornadienen, einem Diels-Alder-Produkt aus Alkinylfuranen ,die sich unter der Einwirkung von Gold und anderen Lewissäuren bildeten (Schema L). AuCl hat sich unter den gegebenen Bedingungen als die beste Lewissäure erwiesen, wobei Yb(CF3SO3)3 ebenfalls eine bemerkenswerte Reaktivität aufzeigte. Die Untersuchungen zeigten, dass die Reaktion einem einfachen säurekatalysierten Mechanismus unterliegt. Da aber im Produkt eine Umlagerung des annelierten Heterocyclus stattfindet, deutet dies auf einen komplexen Reaktionsmechanismus hin.
d) Das vierte Kapitel dieser Doktorarbeit untersucht goldkatalysierte aerobe Oxidationsreaktionen. Das hierzu entwickelte homogene Katalysatorsystem, bestehend aus Gold(I)-chlorid, n-Butyllithium und Natriumcarbonat war in der Lage primäre aromatische Alkohole zu Aldehyden mit Luftsauerstoff in guten bis sehr guten Ausbeuten zu oxidierten (Schema M). Dieses System zeigte jedoch kaum Aktivität bezüglich aliphatischen und sekundären Alkoholen
Solidification behaviour and mechanical properties of cast Mg-alloys and Al-based particulate metal matrix composites under intensive shearing
No full textThis thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Magnesium alloys, as the lightest of all structural metallic materials, and aluminium-based particulate metal matrix composites (PMMCs), offering unified combination of metallic and ceramic properties, have attracted increased interest from the automotive, aerospace, electronic and recreation industries. Current processing technologies for PMMCs do not achieve a uniform distribution of fine-sized reinforcements and produce agglomerated particles in the ductile matrix, which are detrimental to the ductility. At the same time, molten magnesium alloys contain impurities and oxides and when cast conventionally, the final components usually exhibit a coarse and non-uniform microstructure with various casting defects. The key idea in this thesis has been to adopt a novel intensive melt conditioning process, allowing the application of sufficient shear stress that would disperse solid particles present in the melt and offer unique solidification behaviour, improved fluidity and die-filling during casting. The Melt Conditioned High Pressure Die Casting (MC-HPDC) process, where intensive shearing is directly imposed on the alloy melt, which is then cast by the conventional HPDC process, has been used to produce PMMC and magnesium alloy castings. The MC-HPDC process for PMMCs leads to a uniform dispersion of the reinforcement in the matrix, confirmed by quantitative statistical analysis, and increased mechanical performance as indicated by an increase in the hardness and the tensile properties of the composites. We describe a solidification path for aluminium containing magnesium alloys, where intensive shearing prior to casting leads to effective dispersion of solid oxide particles, which then effectively act as nucleation sites for magnesium grains, resulting in significant grain refinement. The MC-HPDC processed magnesium castings have a significantly refined microstructure, with reduced porosity levels and casting defects. Evaluation of the mechanical properties of the castings reveals the beneficial effect of intensive shearing. After careful optimization, the MC-HPDC process shows promising potential for the direct recycling of high purity magnesium die casting scrap, producing casting with mechanical properties comparable to those of primary magnesium alloys
Finite element modelling of the expanson of small tubular parts by internal pressure
No full textA stent is a device that is used to support arterial walls to alleviate the blockage of arteries by plaque. The slotted tube stent is one of the most common stent designs being widely used in many patients today. The amount of stent implantations is growing each year and for this it is becoming increasingly important in interventional cardiology. One of the reasons is that the use of stents has proved more effective in the therapy of coronary stenosis. However, it is evidenced that the success of the stent implantation is limited by re-stenosis, which is discussed almost entirely in terms of medical and biological reasons.
This work is concerned with investigating the deformation mechanism with variable conditions during slotted tube expansion processes using numerical simulation. The simulations were run and analysed using commercial finite element software. A nonlinear explicit solution method was used in each case. The processes chosen for simulation were: slotted tube expansion without balloon catheter, slotted tube expansion with balloon catheter, scaling the stent size, stent production defect and slotted tube expansion with the presence of artery and plaque. A number of process parameters were varied for balloon catheter (i.e. balloon length, thickness and friction factor) and the subsequent effects on the process were identified. Some trial experimental tests were carried out to verify the finite element results from the computer simulations.
From the results obtained it was concluded that higher rate of pressure increase results in higher stresses in stent for similar levels of expansion. Deployment of higher magnitude pressure might increase the expansion in radial direction but it does not necessarily increase the uniformity of the slotted tube. Appropriate balloon length has to be chosen to obtain a uniform expansion. Friction could reduce the foreshortening of the stent. When scaling down the stent, displacement deceases proportionally for similar deployment pressure and stent geometry. It was found from the simulations that the tubular stent is not very sensitive to imperfections. Altering sent strut configuration and number has practical effects on the clinical use of vascular stents, as the magnitude of expansion will change. The deformation characteristic of the slotted tube during deployment is affected by the presence of artery and plaque
The design, manufacture and analysis of a new implant for fracture fixation in human and veterinary orthopaedic surgery: the bone fastenerod
No full textFracture fixation in humans and animals has troubled surgeons and scientists since first it was attempted right up to the present day. At every milestone of achievement in the understanding and practice of fracture repair there has remained a significant problem left unresolved. Of paramount concern is the preservation of blood vessels and soft tissues, avoidance of stress shielding and concentration, promotion. of bone healing and a rapid return to function.
However, matching these principles with the variables of degree and site of fracture/injury, age, size and status of patient, environmental and surgical factors is complex and difficult. To be able to attempt to allow a surgeon to make decisions about every case, knowing that the implant choice does not constrain him but offers flexibility to aim for the ideal fixation for each case, the system must be modular. The objectives were to produce an implant system that would satisfy the most up to date principles of fracture repair through design optimization, mechanical evaluation and testing for specific fracture types. The design was called the bone fastenerod following the optimization and analysis procedures to indicate the origins of its basic formation.
To begin with, the design of the fastenerod had to be optimized and this was achieved using bench testing, initially of selected designs followed by finite element analysis, which allowed a greater number of designs to be processed. Once the optimum design had been found the process of manufacture had to be selected and various possible methods of manufacture were examined until the one most suitable was determined. To analyze the fastenerod, the current industry standard implants that are used in the same clinical type settings were chosen for comparative testing. Testing was performed using static and cyclic loading to failure with wood samples in four point, tensile, side, axial and torsional loading Specific fractures in dog, cat and horse bones were created and repaired using the fastenerod versus the best method currently available and tested in three point, tensile, axial, static loading to failure. Also, specific fractures were created in human mechanical bones and tested using axial, cyclic and four point bending again, comparing the fastenerod w ith the best technology available. The analysis revealed that in static loading the fastenerod was comparable to the industry standards for small implants but not comparable with the large human implants in the specific cases selected However, in the case of the cyclic loading to Mure , the fastenerod performed better than the plate system of similar size, with the ultimate load to failure being higher and no stress concentration leading to implant fracture or failure. Thus, the modular system of the bone fastenerod could now claim to provide fixation that could be flexible, less invasive and destructive to tissues, capable o f greater choice o f screw placement and stiff to level of choice whilst avoiding stress. concentration and shielding On the basis o f this analysis, the fastenerod system can proceed to fu ll c lin ica l trial
Vitamin D Status and its Association with Morbidity Including Wasting and Opportunistic Illnesses in HIV-Infected Women in Tanzania
No full textVitamin D has a potential role in preventing HIV-related complications, based on its extensive involvement in immune and metabolic function, including preventing osteoporosis and premature cardiovascular disease. However, this association has not been examined in large studies or in resource-limited settings. Vitamin D levels were assessed in 884 HIV-infected pregnant women at enrollment in a trial of multivitamin supplementation (excluding vitamin D) in Tanzania. Information on HIV related complications was recorded during follow-up (median, 70 months). Proportional hazards models and generalized estimating equations were used to assess the relationship of vitamin D status with these outcomes. Women with low vitamin D status (serum 25-hydroxyvitamin D<32-ng/mL) had 43% higher risk of reaching a body mass index (BMI) less than 18 kg/m2 during the first 2 years of follow-up, compared to women with adequate vitamin D levels (hazard ratio [HR]: 1.43; 95% confidence intervals: [1.03-1.99]). The relationship between continuous vitamin D levels and risk of BMI less than 18 kg/m2 during follow-up was inverse and linear (p=0.03). Women with low vitamin D levels had significantly higher incidence of acute upper respiratory infections (HR: 1.27 [1.04-1.54]) and thrush (HR: 2.74 [1.29-5.83]) diagnosed during the first 2 years of follow-up. Low vitamin D status was a significant risk factor for wasting and HIV-related complications such as thrush during follow-up in this prospective cohort in Tanzania. If these protective associations are confirmed in randomized trials, vitamin D supplementation could represent a simple and inexpensive method to improve health and quality of life of HIV-infected patients, particularly in resource-limited settings.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90503/1/apc-2E2011-2E0182.pd
A novel laser-doppler flowmetry assisted murine model of acute hindlimb ischemia-reperfusion for free flap research
No full textSuitable and reproducible experimental models of translational research in reconstructive surgery that allow in-vivo investigation of diverse molecular and cellular mechanisms are still limited. To this end we created a novel murine model of acute hindlimb ischemia-reperfusion to mimic a microsurgical free flap procedure. Thirty-six C57BL6 mice (n = 6/group) were assigned to one control and five experimental groups (subject to 6, 12, 96, 120 hours and 14 days of reperfusion, respectively) following 4 hours of complete hindlimb ischemia. Ischemia and reperfusion were monitored using Laser-Doppler Flowmetry. Hindlimb tissue components (skin and muscle) were investigated using histopathology, quantitative immunohistochemistry and immunofluorescence. Despite massive initial tissue damage induced by ischemia-reperfusion injury, the structure of the skin component was restored after 96 hours. During the same time, muscle cells were replaced by young myotubes. In addition, initial neuromuscular dysfunction, edema and swelling resolved by day 4. After two weeks, no functional or neuromuscular deficits were detectable. Furthermore, upregulation of VEGF and tissue infiltration with CD34-positive stem cells led to new capillary formation, which peaked with significantly higher values after two weeks. These data indicate that our model is suitable to investigate cellular and molecular tissue alterations from ischemia-reperfusion such as occur during free flap procedures
CIM implementation in developed and developing countries
No full textImplementing integrated advanced technologies is an effective approach towards solving most problems in today’s competitive global market Computer Integrated Manufacturing (CIM) technologies are presented as a solution to manufacturing organisations which need to perform well.
This research describes the major issues and problems facing developing countnes that wish to implement CIM. In this investigation, Libya is taken as an example of a developing country, while Ireland is used as a model of a more developed country, which has already implemented components of the CIM technology in some of its industrial companies. This research demonstrates the effect of CIM technology on Irish industry by investigating the cost effectiveness of implementing CIM, reduction of the production time, reduction of the product cost and management efficiency.
To carry out this study, 267 questionnaires were distributed to manufacturing companies in Ireland in order to help establish the status of CIM within manufacturing companies, to study the effect of CIM on Irish industries. To study CIM in Libyan industries, four Libyan companies were investigated to assess the CIM level in their companies and the possibility of implementing CIM technology in Libyan companies as a developing country. The mam findings of this research are that successful implementation of CIM depends on sufficient initial and continued investment from within the country and outside. Furthermore, in order to implement CIM successfully it is vital that there is sufficient expertise and trained workers
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