122 research outputs found
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Grain refinement and nucleation processes in Aluminium alloys through liquid shearing
This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The industrial practice of grain refinement of aluminium alloys involves the addition of inoculant particles to initiate alpha-aluminium grains at small undercoolings. This results in a uniformly fine, equiaxed as-cast microstructure and is commonly achieved using Al-Ti-B additions. The phase responsible for initiation of grains in aluminium melts inoculated with Al-Ti-B was determined during the 1990s; since that time, scientific understanding of grain refinement has advanced rapidly. However, one of the main problems of addition inoculants is impurities which is added to the melt and may affect the desired characteristics of the product. With regards to this problem other methods of refinement and the mechanisms of refining have not been fully understood and prediction of as-cast Microstructures in aluminium alloys has much scope for improvement. In this thesis:
1-Factors in establishing equiaxed microstructure were analysed and the origin of equiaxed grains were explored. Then the nucleation process and the involved mechanisms were investigated in depth and control of nucleation process to achieve a fine and uniform structure was set as target.
2-Refinement of microstructure with introduction of shearing was evaluated and the process of refinement in the mushy zone (semisolid state) as a base line was established. Then introduction of shearing above liquidus as a development was analysed and outstanding refinement was seen with shearing above liquidus which have not been investigated properly elsewhere.
3- The mechanisms of refinement by introducing shearing were investigated and the refining mechanisms below and specifically above liquidus were investigated systematically. As results an appropriate understanding about the mechanisms of nucleation and refinement above liquidus was established.
4- Finally, with simulation the most dominant factor in approaching fine grain size by applying shear was identified and the results of experimental examination was verified by simulation.UK Department of Trade and Industry (DTI
Fault-tolerant software: dependability/performance trade-offs, concurrency and system support
PhD ThesisAs the use of computer systems becomes more and more widespread in applications
that demand high levels of dependability, these applications themselves are growing in
complexity in a rapid rate, especially in the areas that require concurrent and distributed
computing. Such complex systems are very prone to faults and errors. No matter how
rigorously fault avoidance and fault removal techniques are applied, software design
faults often remain in systems when they are delivered to the customers. In fact,
residual software faults are becoming the significant underlying cause of system
failures and the lack of dependability. There is tremendous need for systematic
techniques for building dependable software, including the fault tolerance techniques
that ensure software-based systems to operate dependably even when potential faults
are present. However, although there has been a large amount of research in the area of
fault-tolerant software, existing techniques are not yet sufficiently mature as a practical
engineering discipline for realistic applications. In particular, they are often inadequate
when applied to highly concurrent and distributed software.
This thesis develops new techniques for building fault-tolerant software, addresses the
problem of achieving high levels of dependability in concurrent and distributed object
systems, and studies system-level support for implementing dependable software. Two
schemes are developed - the t/(n-l)-VP approach is aimed at increasing software
reliability and controlling additional complexity, while the SCOP approach presents an
adaptive way of dynamically adjusting software reliability and efficiency aspects. As a
more general framework for constructing dependable concurrent and distributed
software, the Coordinated Atomic (CA) Action scheme is examined thoroughly. Key
properties of CA actions are formalized, conceptual model and mechanisms for
handling application level exceptions are devised, and object-based diversity
techniques are introduced to cope with potential software faults. These three schemes
are evaluated analytically and validated by controlled experiments. System-level
support is also addressed with a multi-level system architecture. An architectural
pattern for implementing fault-tolerant objects is documented in detail to capture
existing solutions and our previous experience. An industrial safety-critical application,
the Fault-Tolerant Production Cell, is used as a case study to examine most of the
concepts and techniques developed in this research.ESPRIT
Grain refinement and nucleation processes in aluminium alloys through liquid shearing
The industrial practice of grain refinement of aluminium alloys involves the addition of inoculant particles to initiate alpha-aluminium grains at small undercoolings. This results in a uniformly fine, equiaxed as-cast microstructure and is commonly achieved using Al-Ti-B additions. The phase responsible for initiation of grains in aluminium melts inoculated with Al-Ti-B was determined during the 1990s; since that time, scientific understanding of grain refinement has advanced rapidly. However, one of the main problems of addition inoculants is impurities which is added to the melt and may affect the desired characteristics of the product. With regards to this problem other methods of refinement and the mechanisms of refining have not been fully understood and prediction of as-cast Microstructures in aluminium alloys has much scope for improvement. In this thesis: 1-Factors in establishing equiaxed microstructure were analysed and the origin of equiaxed grains were explored. Then the nucleation process and the involved mechanisms were investigated in depth and control of nucleation process to achieve a fine and uniform structure was set as target. 2-Refinement of microstructure with introduction of shearing was evaluated and the process of refinement in the mushy zone (semisolid state) as a base line was established. Then introduction of shearing above liquidus as a development was analysed and outstanding refinement was seen with shearing above liquidus which have not been investigated properly elsewhere. 3- The mechanisms of refinement by introducing shearing were investigated and the refining mechanisms below and specifically above liquidus were investigated systematically. As results an appropriate understanding about the mechanisms of nucleation and refinement above liquidus was established. 4- Finally, with simulation the most dominant factor in approaching fine grain size by applying shear was identified and the results of experimental examination was verified by simulation.EThOS - Electronic Theses Online ServiceUK Department of Trade and Industry (DTI)GBUnited Kingdo
Software Engineering and Petri Nets
This booklet contains the proceedings of the Workshop on Software Engineering and Petri Nets (SEPN), held on June 26, 2000. The workshop was held in conjunction with the 21st International Conference on Application and Theory of Petri Nets (ICATPN-2000), organised by the CPN group of the Department of Computer Science, University of Aarhus, Denmark. The SEPN workshop papers are available in electronic form via the web page:http://www.daimi.au.dk/pn2000/proceeding
A proposal for the use of blockchain in the portuguese voting system
Dissertation presented as the partial requirement for obtaining a Master's degree in Information Management, specialization in Knowledge Management and Business IntelligenceThe key objective of this proposal is to present one of the problems that the Portuguese economy, as
well as other European countries, have been facing in regard to the civil society intervention in the
democracy: the decrease of turnover rates in the voting system. The main objective is to propose the
use of Blockchain technology in the Portuguese Voting System, as a mechanism to counter this trend.
In order to understand how the possible application of a remote e-voting system succeeds, Estonia
was selected as the case of study. Its architecture, as well as the legal, social and technological issues
and challenges associated are investigated in the light of the information collected in the literature
review.
Considering the case analysis and discussion, a set of recommendations that purpose the use of a
remote electronic voting system in the Portuguese electoral system are presented and a critical
analysis about the introduction of a Blockchain algorithm is made. This dissertation concludes about
the advantages and disadvantages from the use of this decentralized system when compared with a
system involving a third-party as the one used in the Estonian I-Voting.
The validation is based on interviews and discussions with professors in the area of information
systems and law, and also with a contribution of a Digital adviser of the Estonian e-Governance
model
Replicated Computations in a Distributed Switching Environment
Replication of computations in a distributed switching environment is studied. The first topics discussed are the requirements and the other design goals that have to be met by replicated computations in a distributed switching system. The requirements on the grade of service and availability performance objectives are largely set out in the international standards. A structured probability oriented software approach to building a kernel supporting replicated computations is suggested and the functional as well as the probability properties of the replication scheme are investigated. To aid the definition and investigation of the functional properties of the replication scheme a model of computation based on the actor model of Hewitt and Agha is defined and used. The overall replication scheme consists of a loose basic scheme, the real-time computation migration tools, here designated as warm-up algorithms, and the corrective replication tools augmenting the basic scheme. Language methods which enhance the transparency of the replication scheme are also discussed. The work has been done in connection with a redesign project of a distributed digital switching system and the results have largely been implemented in that environment
Resilient Computing Curriculum Draft -- ReSIST NoE Deliverable D16
This Deliverable presents the first version of ReSIST's Curriculum in Resilient Computing, limited to the description of the syllabi for the first year (Semesters 1 and 2) and indicates the line and title for the curriculum in the second year (semesters 3 and 4) and propose it to the general discussion for improvements. The curriculum will be updated and completed in successive versions that will take advantage of a large open discussion inside and outside ReSIS
Information communication technologies and distance education in Sri Lanka: a case study of two universities
Sri Lanka's participation rates in higher education are low and have risen only slightly in the
last few decades; the number of places for higher education in the state university system
only caters for around 3% of the university entrant age cohort. The literature reveals that the
highly competitive global knowledge economy increasingly favours workers with high levels
of education who are also lifelong learners. This lack of access to higher education for a
sizable proportion of the labour force is identified as a severe impediment to Sri Lankaâs
competitiveness in the global knowledge economy.
The literature also suggests that Information and Communication Technologies are
increasingly relied upon in many contexts in order to deliver flexible learning, to cater
especially for the needs of lifelong learners in todayâs higher educational landscape. The
government of Sri Lanka invested heavily in ICTs for distance education during the period
2003-2009 in a bid to increase access to higher education; but there has been little research
into the impact of this.
To address this lack, this study investigated the impact of ICTs on distance education in Sri
Lanka with respect to increasing access to higher education. In order to achieve this aim, the
research focused on Sri Lankaâs effort from three perspectives: policy perspective,
implementation perspective and user perspective. A multiple case study research using an
ethnographic approach was conducted to observe Orange Valley Universityâs and Yellow
Fields Universityâs (pseudonymous) implementation of distance education programmes
using questionnaires, qualitative interviewing and document analysis. In total, data for the
analysis was collected from 129 questionnaires, 33 individual interviews and 2 group
interviews.
The research revealed that ICTs have indeed increased opportunities for higher education;
but mainly for people of affluent families from the Western Province. Issues identified were
categorized under the themes: quality assurance, location, language, digital literacies and
access to resources. Recommendations were offered to tackle the identified issues in
accordance with the study findings. The study also revealed the strong presence of a
multifaceted digital divide in the country. In conclusion, this research has shown that
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although ICT-enabled distance education has the potential to increase access to higher
education the present implementation of the system in Sri Lanka has been less than
successful
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