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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
171

SIMAID : a rapid development methodology for the design of acyclic, bufferless, multi-process and mixed model agile production facilities for spaceframe vehicles

Tebaldi, Enrico January 2001 (has links)
The facility layout problem (FL) is a non-linear, NP-complete problem whose complexity is derived from the vast solution space generated by multiple variables and interdependent factors. For reconfigurable, agile facilities the problem is compounded by parallelism (simultaneity of operations) and scheduling issues. Previous work has either concentrated on conventional (linear or branched) facility layout design, or has not considered the issues of agile, reconfigurable facilities and scheduling. This work is the first comprehensive methodology incorporating the design and scheduling of parallel cellular facilities for the purpose of easy and rapid reconfiguration in the increasingly demanding world of agile manufacturing. A novel three-stage algorithm is described for the design of acyclic (asynchronous), bufferless, parallel, multi-process and mixed-model production facilities for spaceframe-based vehicles. Data input begins with vehicle part processing and volume requirements from multiple models and includes time, budget and space constraints. The algorithm consists of a powerful combination of a guided cell formation stage, iterative solution improvement searches and design stage scheduling. The improvement iterations utilise a modified (rules-based) Tabu search applied to a constant-flow group technology, while the design stage scheduling is done by the use of genetic algorithms. The objective-based solution optimisation direction is not random but guided, based on measurement criteria from simulation. The end product is the selection and graphic presentation of the best solution out of a database of feasible ones. The case is presented in the form of an executable program and three real world industrial examples are included. The results provide evidence that good solutions can be found to this new type and size of heavily constrained problem within a reasonable amount of time.
172

Determining the effect of strain rate on the fracture of sheet steel

Beaumont, Richard Adrian January 2012 (has links)
A key challenge for the automotive industry is to reduce vehicle mass without compromising on crash safety. To achieve this, it is necessary to model local failure in a material rather than design to the overly conservative criteria of total elongation to failure. The current understanding of local fracture is limited to quasi-static loading or strain rates an order of magnitude too high for automotive crash applications. This thesis studies the local fracture properties of DP800 sheet steel at the macroscopic scale from strain rates of to for the first time. Geometries for three stress states, namely plane-strain, shear and uniaxial tension, were developed to determine a fracture locus for DP800 steel using optical strain measurement. These geometries were developed using Finite Element Analysis and validated experimentally for strain rate and stress state. Thermal imaging was used to determine the effect of strain rate on temperature rise and its associated effect on fracture. Fractography was used to examine the specimens’ failure modes at different strain rates. The geometries were applied to the advanced high strength steel grade DP800. Despite prior evidence from simple tensile test data, DP800 showed no significant variation in fracture strain with strain rate in all three stress states. Non-contact thermal measurements showed that the high strain rate tests ( ) were non-isothermal with temperature rises of up to being observed. As a result of this it is difficult to decouple the effect of strain rate from the effect of temperature and requires further investigation. The test geometries were also applied to the deep draw steel DX54 and the aluminium alloy AA5754 where a strain rate effect was observed. Both materials are significantly more ductile than DP800 whish exposed a limitation in the test procedures. At high fracture strains the stress state deviates from its intended value and can invalidate the test. Therefore, a method was developed for determining the validity of a test for each geometry and material from experimental data. The preliminary data from DX54 indicates significantly greater strain rate sensitivity across one order of magnitude than was observed in five orders of magnitude in DP800.
173

Decision support for operational ERP systems implementation in small and medium enterprises

Mahmood, Ali January 2013 (has links)
Today organisations, due to increased competition, globalisation and cost saving, are seeking ways to improve their operational effectiveness and sustain their competitive advantage through effective deployment of available resources and strategically implementing business processes. It is observed that incorporating new developments in information technology with core business processes results in enhanced functioning and improved services to customers. To benefit from the available IT support, organisations are adopting application software, such as ERP systems, to improve operation efficiency and productivity. ERP system is primarily implemented to integrate business processes and enhance productivity. However, ERP system comes with a high price tag, implementation complexities, and prerequisite changes in how organisation and its staff functions. Implementing ERP is a challenging task for SMEs since it consumes a major portion of limited resources and carries a high risk of causing adverse consequences. To overcome the implementation challenges and assist SMEs in ERP implementation, an integrated decision support system for ERP implementation (DSS_ERP) is developed in this research. This decision support system consists of analytical regression models, a simulation model and nonlinear programming models, and it enables SMEs to identify the resources requirements for achieving the predetermined goals prior to ERP implementation. The key contribution from this research are: i) the DSS_ERP offers an analytical models to monitor the implementation progress and cost consumed by each critical success factor (CSF) during the implementation against time; ii) it assists in determining the priorities of CSFs, based on which it facilitates decision makings on resource allocations to achieve the predetermined target; iii) and it can be applied to evaluate the impacts of changes to the resources allocations.
174

Understanding the effects of high-pressure, high-temperature processing on the key quality parameters of green beans (Phaseolus vulgaris) with a view to assessing the potential quality benefits of the approach relative to conventional thermal processing

Leadley, Craig Edward January 2012 (has links)
Studies were conducted to explore whether high pressure (up to 700 MPa) could be used in combination with elevated temperatures (up to 90°C initial temperature) to produce ambient stable green beans (Phaseolus vulgaris) with improved quality compared with conventionally heat processed samples. Colour changes, texture change and chlorophyll retention were explored at a range of pressures, temperatures and times using a surface response methodology. Texture changes were essentially related to temperature effects; higher temperatures resulted in a greater loss in texture. Significant improvements in texture retention were possible using High Pressure Sterilisation (HPS) but sample colour was negatively affected. Colour parameters were predicted primarily by time and pressure so deterioration in green vegetable quality for a commercially sterile products appears inevitable when using HPS. The use of ohmic heating as a pre-heating method greatly reduced cook values (T\(_{ref}\) = 100°C, z = 39C°) for colour degradation (down to 0.24, 0.12, 0.35 from 3.02, 2.50, 3.70 minutes for ohmically heating and water bath heated samples respectively) which yielded significant benefits in terms of colour retention of raw materials at the start of the HPS cycle; values of a* and b* for ohmically pre-heated samples were close to that of blanched beans.
175

Design and fabrication of microstructured and switchable biological surfaces

Yeung, Chun L. January 2011 (has links)
The research presented in this thesis explores the design and fabrication of microstructured and switchable biological surfaces, which may have potential applications of nanobiotechnology. The thesis focuses on the fabrication of biological surfaces which can be controlled via external stimuli. Chapter 1 - Introduction to Nanobiotechnology - presents an introduction to the background of this research including the role of self-assembled monolayers (SAMs) in nanobiotechnology, microstructure fabrication techniques, stimuli responsive surfaces and cell migration. Chapter 2 - Surface characterization techniques - presents surface characterization techniques employed throughout this research. Chapter 3 - Study of Arp2/3 complex activity in filopodia of spreading cells using patterned biological surfaces - presents the fabrication and characterization of patterned biological (fibronectin) surfaces using patterning technology (microcontact printing) and several surface analytical techniques. This study explores the role of filopodia in the spreading of Mouse Embryonic fibroblast (MEF) cells and the function of Arp2/3 complex in this process. The results demonstrated that filopodia, produced by MEF cells interacted with the patterned fibronectin surface and guided lamellipodia protrusion. Arp2/3 complex, which is absent on the filopodia adhesion site, does not facilitate in the adhesion of filopodia on the fibronectin surface. Chapter 4 - Tuning specific biomolecular interactions using electro-switchable oligopeptide surfaces - presents the fabrication of responsive surfaces that rely on electro-switchable peptides to control biomolecular interactions on gold surfaces. This system is based upon the conformational switching of positively charged oligolysine peptides that are tethered to a gold surface. The bioactive molecular moieties (biotin) terminates on the oligolysines can be reversibly exposed (bio-active state) or concealed (bio-inactive state) on demand, as a function of surface potential. Chapter 5 - Experimental procedures, protocols and synthesis - describes the experimental techniques used during the investigations performed throughout the work described in this thesis. Experimental protocols and data analysis by various equipment are described.
176

An investigation of using micromachined silicon diaphragms in high temperature pressure sensors

Ren, Juan January 2012 (has links)
Micromachined silicon diaphragms have been widely used as sensing elements in standard pressure sensors. However, at elevated temperature, the pressurized silicon diaphragms may suffer from the plastic deformation or creep. Therefore, this project is to investigate the possibility of using silicon diaphragms in high temperature pressure sensors. A series of experiments were performed to investigate the effect of size, temperature and ion implantation on the mechanical behaviour of the micromachined silicon diaphragms. The test structures were annealed under the atmospheric pressure at a temperature of 600°C, 800°C and 900°C, respectively. The surface profiles of the diaphragms were measured with respect to the anneal time. Based on the experimental results, the suggestions of the design of the silicon diaphragms for the high temperature pressure sensors were given. Moreover, the onset of the plastic deformation was predicted by the theory of the critical resolved shear stress. The predicted behaviour was in good agreement with the experimental observation for the heavily-boron-doped silicon diaphragms. Finally, the constitutive equations were implemented in Comsol Multiphysics to simulate the evolution of the diaphragm deformation in the initial anneal stage. The model successfully predicted the deflections for the diaphragms with a radius from 0.5mm to 2.0mm at 900°C.
177

Effects of light absorber on micro stereolithography parts

Zabti, Mohamed Mohamed January 2012 (has links)
This thesis reports the results of an investigation of the effects of adding Tinuvin\(\char{msam10}{0x72}\)327 to PIC-100 acrylate resin on such important parameters as cure depth, critical energy, part density, dimension accuracy and surface quality. Initially, an experimental investigation was carried out to characterise the cure depth of resin after the addition of Tinuvin\(\char{msam10}{0x72}\)327 in five different concentrations using a white light microscope. The investigation has shown that increasing Tinuvin\(\char{msam10}{0x72}\)327 concentration reduces cure depth thickness and increases critical energy; it also found that increasing Tinuvin\(\char{msam10}{0x72}\)327 concentration increased the density of the fabricated parts but significantly reduced the tensile strength. The influence of different concentrations of Tinuvin\(\char{msam10}{0x72}\)327 in PIC-100 acrylate resin on the accuracy of the fabricated parts was investigated, and it was shown that increasing the proportion of Tinuvin\(\char{msam10}{0x72}\)327 from 0.1% to 1% (w/w) caused a significant increase in dimensional . The accuracy of Jacobs’ cure depth model in predicting cure depth after the addition of Tinuvin\(\char{msam10}{0x72}\)327 was investigated by comparing measured cure depth and predicted results using Jacobs’ model. The results show that Jacobs’ model does not keep pace with the changes that occur in the PIC-100 photo polymerisation process with changes in material characteristics due to the addition of Tinuvin\(\char{msam10}{0x72}\)327. Jacobs’ cure model was adapted using two empirically derived constants to accurately predict the cure depth. Finally, a parametric optimisation process was performed using the PARETO multi objective optimisation function of Matlab 2010. For 1% Tinuvin\(\char{msam10}{0x72}\)327 concentration and an irradiation level of 750mW/dm\( 2\), the optimum exposure time was found to be 10 seconds.
178

Equal channel angular pressing of elemental & alloyed P/M aluminium systems

Harrison, Nicholas January 2014 (has links)
Aluminium powder metallurgy provides lightweight automobile components for reduced energy consumption. Conventional press-and-sinter methods can leave porosity in final parts, which reduces strength of the alloy. Equal Channel Angular Pressing (ECAP) can be used to increase density of powder compacts after sintering via severe plastic deformation. Solid state sintering of pure aluminium and liquid phase sintering of a commercial Al-Sn bearing alloy were analysed with industrial collaborative support. The use of room temperature ECAP to these sintered materials was investigated. Cold compaction produced a porous region on the outside of all samples due to increased friction at the die wall. The application of ECAP to pure aluminium and bearing alloys caused elongation in the longitudinal section and a relatively equiaxed microstructure in the transverse section. Sintering at 500°C for 1 and lOhours and 550°C for lh was ineffective as liquid tin did not wet the aluminium effectively. After ECAP, density and hardness increased with refined grain sizes and formed two distinct regions; a denser, deformed core and a non-deformed skin, which resulted from non-uniform densification after cold compaction.
179

Design and evaluation of an encapsulated artificial disc

Alnaimat, Feras Adnan Mohammad January 2018 (has links)
Artificial discs have been developed to replace and restore motion to degenerated intervertebral discs. The most common configuration for these devices include ball and socket articulation surfaces that can induce high frictional torques and wear rates. When these particles interact with the surrounding tissues they can induce inflammations leading to osteolysis, subsidence of the implant and then revision surgery. A new device has been developed to reduce friction and eliminate wear migration that incorporates an elastomer sheath to encapsulate the disc, retaining debris and an optimised bio-lubricant. The artificial disc has been assessed with an experimental programme that compared the resistive torques of the artificial discs both with and without encapsulation, for a range of motions. Durability tests were also conducted to 2M cycles and gravimetric wear rate was measured in accordance with BS 18192-1: 2011. Encapsulating the articulating surfaces reduced resistive torques and completely eliminated debris migration. Wear rates within the sheath ranged from 10.1 to 11.3 mg/million cycles, well within acceptable levels for this type of device. The encapsulated discs successfully contained all wear debris and displayed durability in excess of 2M accelerated life cycles. The concept of an encapsulated artificial disc has been shown to be feasible and could replace current technologies.
180

Electro-mechanical behaviour of indium tin oxide coated polymer substrates for flexible electronics

Potoczny, Grzegorz A. January 2012 (has links)
Highly conductive (3.0 - 5.0 x 10 \(^{-4}\) \( \Omega\) cm) and transparent (80 – 85% ) ITO films were successfully fabricated on glass and polymer substrates (PET, PEN and PC) by pulsed laser deposition at low temperatures (24 – 150 °C). The influence of deposition conditions on the structural and physical properties of ITO-coated glass substrates was studied. The samples were investigated using Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), X-Ray Diffraction (XRD), the fourpoint probe and a spectrophotometer. Sol-gel derived ITO films dip-coated on glass substrates were also studied. The optimum film obtained at a firing temperature of 600 °C had a resistivity of 1.8 x 10 \(^{-2}\) \( \Omega\) cm, and optical transmittance of 80%. The electro-mechanical behaviour of ITO/polymer systems was investigated under uniaxial tension and controlled buckling in tension and compression. The resistance changes were monitored in situ. Cracking and buckling delamination failure modes were observed for all samples investigated at critical strains raging from 2.8 to 3.4%, and from 7.0 to 8.0%, respectively. The results showed that the dominant critical failure mode depends on the applied stress conditions. The ITO/PEN samples showed high flexibility; the samples were buckled in tension down to a 2.6 mm radius of curvature before cracks start to occur.

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