Utskrift från Malmö högskola - mah.se
Browsing Doctoral Theses /TS by Title
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Browsing Doctoral Theses /TS by Title
Now showing items 1-8 of 8
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Algorithms for Aggregate Information Extraction from Sequences Bengtsson, Fredrik : Luleå University of Technology (2007) DOCTORAL
THESIS
English abstract: In this thesis, we propose efficient algorithms for aggregate information extraction from sequences and multidimensional arrays. The algorithms proposed are applicable in several important areas, including large databases and DNA sequence segmentation. We first study the problem of efficiently computing, for a given range, the range-sum in a multidimensional array as well as computing the k maximum values, called the top-k values. We design two efficient data structures for these problems. For the range-sum problem, our structure supports fast update while preserving low complexity of range-sum query. The proposed top-k structure provides fast query computation in linear time proportional to the sum of the sizes of a two-dimensional query region. We also study the k maximum sum subsequences problem and develop several efficient algorithms. In this problem, the k subsegments of consecutive elements with largest sum are to be found. The segments can potentially overlap, which allows for a large number of possible candidate segments. Moreover, we design an optimal algorithm for ranking the k maximum sum subsequences. Our solution does not require the value of k to be known a priori. Furthermore, an optimal linear-time algorithm is developed for the maximum cover problem of finding k subsequences of consecutive elements of maximum total element sum. -
Approximation Algorithms for Geometric Networks Andersson, Mattias : Lund University (2007) DOCTORAL
THESIS
English abstract: The main contribution of this thesis is approximation algorithms for several computational geometry problems. The underlying structure for most of the problems studied is a geometric network. A geometric network is, in its abstract form, a set of vertices, pairwise connected with an edge, such that the weight of this connecting edge is the Euclidean distance between the pair of points connected. Such a network may be used to represent a multitude of real-life structures, such as, for example, a set of cities connected with roads. Considering the case that a specific network is given, we study three separate problems. In the first problem we consider the case of interconnected `islands' of well-connected networks, in which shortest paths are computed. In the second problem the input network is a triangulation. We efficiently simplify this triangulation using edge contractions. Finally, we consider individual movement trajectories representing, for example, wild animals where we compute leadership individuals. Next, we consider the case that only a set of vertices is given, and the aim is to actually construct a network. We consider two such problems. In the first one we compute a partition of the vertices into several subsets where, considering the minimum spanning tree (MST) for each subset, we aim to minimize the largest MST. The other problem is to construct a $t$-spanner of low weight fast and simple. We do this by first extending the so-called gap theorem. In addition to the above geometric network problems we also study a problem where we aim to place a set of different sized rectangles, such that the area of their corresponding bounding box is minimized, and such that a grid may be placed over the rectangles. The grid should not intersect any rectangle, and each cell of the grid should contain at most one rectangle. All studied problems are such that they do not easily allow computation of optimal solutions in a feasible time. Instead we consider approximation algorithms, where near-optimal solutions are produced in polynomial time. In addition to the above geometric network problems we also study a problem where we aim to place a set of different sized rectangles, such that the area of their corresponding bounding box is minimized, and such that a grid may be placed over the rectangles. The grid should not intersect any rectangle, and each cell of the grid should contain at most one rectangle. All studied problems are such that they do not easily allow computation of optimal solutions in a feasible time. Instead we consider approximation algorithms, where near-optimal solutions are produced in polynomial time. -
Multiphase oxide ceramics in the alumina-yttria system Alkebro, Jesper : Luleå tekniska universitet
Doktorsavhandling;43 (2002)DOCTORAL
THESIS
English abstract: As a means of creating dispersed multiphase oxide structures, high-energy milling has been used for pre-treating alumina-yttria powder mixtures before pressing and sintering. Processing was performed in a planetary ball mill with steel or alumina milling tools and measured effects of the treatment have served as data for a modeling study. Subsequent phase development and sintering during heat treatment of the milled powders have been examined. Two compositions likely to result in a minority phase dispersed in another phase in equilibrium were selected and subjected to the milling treatment. The two constituent powders were homogenously mixed and defects were injected into the crystal structures which were gradually destroyed. Subsequently, depending on milling parameters, there was either amorphization of the sample or formation of yttrium aluminum perovskite, an intermediate phase of the alumina-yttria system. Alumina milling tools exhibited a higher milling efficiency, but they were prone to chipping which lead to massive contamination. Steel milling tools were worn in a more controlled manner and the total amount of contamination was much lower. In heat treatment milled powders easily attained phase equilibrium and there was no metastable behavior noted. Transformation temperatures fell as a function of milling time but for longer milling times the effects of prolonged processing decreased. Sintering properties were also improved resulting in higher final density and lower sintering temperatures. Iron contamination from steel milling tools was suspected to be detrimental for the final solidification and to cause large porosity, but when oxidized the effect is inversed leading to very good densification in argon atmosphere. Relative densities as high as 96% were measured after sintering 1 h in 1500°C, but the process was sensitive to the environment resulting in poor sintering for oxidizing (air) or reducing (argon in graphite furnace) atmospheres. A dispersion of a second phase in the dominant matrix phase was observed but further improvement of the process should be needed to make it finer. The grain size could be estimated to be around 5 µm from fracture surface images. -
On crack growth under compressive stresses Isaksson, Per : Luleå University of Technology
Doctoral thesis / Luleå University of Technology;06 (2001)DOCTORAL
THESIS
English abstract: This thesis concerns fractures subjected to compressive stresses. In the four papers appended fracture behavior in brittle as well as ductile materials is studied. In the first paper, an expression for the mode II stress intensity factor at a straight extended kink has been calculated under the condition that crack opening is suppressed during crack growth. The expression has been found as a function of the mode II stress intensity factor K2 at the parent crack, the direction and length of the kink, and the difference between the remote compressive normal stresses perpendicular to, and parallel with, the plane of the parent crack. Crack growth directions have been suggested based on the result. At a sufficiently high non-isotropic compressive normal stress, so that the crack remains closed, the crack will propagate along a curved path maximizing the mode II stress intensity factor. Only at an isotropic compressive normal stress will the crack continue straight ahead in its original plane without directional change. By analyzing experimental crack growth patterns in paper two, the conclusion is that crack paths experimentally observed indicate that mode II crack growth under compression in some brittle materials follow a propagation path described by a function y=gx^b. In fact, the agreement between the experiments and the propagation path prescribed by the model, in which b=3/2, is astonishingly good since b was found in the interval [1.43-1.58] in all the experiments studied. Further, the investigation of the curvature parameter g has revealed that g also agree with the simplified model, even though not as good as the exponent b. However, the experimentally observed g differs in general less than 15% from the theoretical value predicted by the analytical model discussed in paper I. In paper three, a directional crack growth criterion in a compressed elastic perfectly-plastic material is considered. A slip-line solution is derived for evaluation of the stresses at the crack tip, which considers hydrostatic pressure and friction between the crack surfaces. Based upon the slip-line solution a projection stress based model is discussed for prediction of the direction of initiated crack growth. The opening displacement of an extended kink has been examined in paper four, using a finite element procedure. The conclusion is that an over-critical pressure in the plastic zone surrounding the crack tip suppresses crack opening regardless the direction of crack growth. The only possibility seems to be shear mode crack growth, which occur straight ahead in the crack plane if the crack is assumed to follow the plane of maximum shear stress. At a sub-critical hydrostatic pressure, or lower friction between the crack surfaces, the crack can extend via a kink subjected to local opening mode. An expression for the critical value determining fracture mode has been found as a function of hydrostatic pressure and friction between the crack surfaces assuming the fracture process to be predominantly controlled by local tensile stresses at the crack tip. The crack growth directions predicted by the projection stress based criterion in paper three are comparable with the directions maximizing the opening displacement of an extended kink computed in paper four. -
On the Fracture Behavior of Softwood Jernkvist, L. O. : Luleå University of Technology (2000) DOCTORAL
THESIS -
On the fracture of thin laminates Kao-Walter, Sharon : Blekinge Institute of Technology
Blekinge Institute of Technology Dissertation Series;7 (2004)DOCTORAL
THESIS
English abstract: This thesis concerns mechanical and fracture properties of a thin aluminium foil and polymer laminate that is widely used as packaging material. The possibility of controlling the path of the growing crack propagation by adjustment of the adhesion level and the property of the polymer layer is investigated. First, the fracture process of the aluminium foil is investigated experimentally. It is found that fracture occurs at a much lower load than what is suggested by standard handbook fracture toughness. Observations in a scanning electron microscope with a tensile stage show that small-scale stable crack growth occurs before the stress intensity factor reaches its maximum. An examination using an optical profilometric method shows almost no plastic deformation except for in a small necking region at the crack tip. However, accurate predictions of the maximum load are obtained using a strip yield model with a geometric correction. Secondly, the mechanical and fracture properties of the laminate are studied. A theory for the mechanics of the composite material is used to evaluate a series of experiments. Each of the layers forming the laminate is first tested separately. The results are analysed and compared with the test results of the entire laminate with varied adhesion. The results show that tensile strength and strain at peak stress of the laminate, with or without a crack, increase when the adhesion of the adhesive increases. It is also found that a much larger amount of energy is consumed in the laminated material at tension compare with the single layers. Possible explanations for the much higher toughness of the laminate are discussed. Finally, the behaviour of a crack in one of the layers, perpendicular to the bimaterial interface in a finite solid, is studied by formulating a dislocation superposition method. The stress field is investigated in detail and a so-called T stress effect is considered. Furthermore, the crack tip driving forces are computed numerically. The results show that the analytical methods for an asymptotically small crack extension can also be applied for a fairly large amount of crack growth. By comparing the crack tip driving force of the crack deflected into the interface with that of the crack penetrating into the polymer layer, it is shown how the path of the crack can be controlled by selecting a proper adhesion level of the interface for different material combinations of the laminate. -
Theoretical Aspects on Performance Bounds and Fault Tolerance in Para... Klonowska, Kamilla : Blekinge Institute of Technology (2007) DOCTORAL
THESIS
English abstract: This thesis consists of two parts: performance bounds for scheduling algorithms for parallel programs in multiprocessor systems, and recovery schemes for fault tolerant distributed systems when one or more computers go down. In the first part we deliver tight bounds on the ratio for the minimal completion time of a parallel program executed in a parallel system in two scenarios. Scenario one, the ratio for minimal completion time when processes can be reallocated compared to when they cannot be reallocated to other processors during their execution time. Scenario two, when a schedule is preemptive, the ratio for the minimal completion time when we use two different numbers of preemptions. The second part discusses the problem of redistribution of the load among running computers in a parallel system. The goal is to find a redistribution scheme that maintains high performance even when one or more computers go down. Here we deliver four different redistribution algorithms. In both parts we use theoretical techniques that lead to explicit worst-case programs and scenarios. The correctness is based on mathematical proofs. -
Time-resolved X-ray diffraction studies of phonons and phase transiti... 
Synnergren, Ola : Department of Physics, Lund University
Lund Reports on Atomic Physics;350 (2005)DOCTORAL
THESIS
English abstract: This thesis summarizes work in which time-resolved X-ray diffraction has been used to probe crystalline materials, thereby revealing the dynamics of phonons and phase transitions. X-ray diffraction is the standard tool in investigations of structure on the atomic scale. It has been used for a long time, and has successfully helped scientists to find the structure of a wide range of materials. The use of ultrafast time-resolved X-ray diffraction is a strongly emerging field which is still under development. Impulsive strain pulses, or coherent acoustic phonons, have been probed using optical techniques for at least two decades. Yet, optical pulses can only probe the surface of a semiconductor. X-rays penetrate deeper and can follow the phonons as they propagate into the sample. Real time studies of phase transitions have also been conducted using optical methods. These measurements are indirect in the sense that they probe the susceptibility change of the sample rather than the positions of the atoms. Again, time-resolved X-ray diffraction can give new insights into the field by probing the structural changes directly. This thesis focuses mainly on experimental work in which time-resolved X-ray diffraction has been used to probe phonons or samples undergoing phase transitions. A brief theoretical background will also be given, as well as a description of beamline D611 at MAX-lab, a synchrotron beamline for time-resolved X-ray diffraction measurements which has been developed during the work for this thesis. Swedish abstract: Denna avhandling sammanfattar arbete där tidsupplöst röntgendiffraktion har använts för att undersöka kristallina material och studera dynamiken hos fononer och fasövergångar. Röntgendiffraktion är standardverktyget för stukturbestämning på en atomär skala. Det har använts länge och har framgångsrikt hjälp vetenskapsmän att bestämma strukturen hos en stor mängd material. Användningen av ultrasnabb tidsupplöst röntgendiffration är ett starkt växande område som fortfarande utvecklas. Akustiska impulser, eller koherenta akustiska fononer, har studerats med hjälp av optiska tekniker i åtminstonde två årtionden. Optiska pulser kan dock endast underöka en halvledares yta. Röntgen penetrerar djupare in i proven och kan följa fononer då de färdas in i proven. Realtidsstudier av fasövergångar har också utförts med hjälp av optiska metoder. Dessa mätningar är indirekta då de mäter ändringar i susceptibiliteten hos provet istället för atomernas positioner. Återigen kan tidsupplöst röntgendiffraktion ge en ny insyn i området genom att den gör en direkt mätning av de strukturella ändringarna. Den här avhandlingen fokuserar huvudsaklingen på experimentellt arbete där tidsupplöst röntgendiffraktion använts för att studera fononer eller prover som genomgår en fasövergång. En kort teoretisk bakgrund finns med, liksom en beskrivning av D611, ett synkrotronljusstrålrör för tidsupplöst röntgendiffraktion som utvecklats under arbetet med den här avhandlingen. 
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