Utskrift från Malmö högskola - mah.se
Doctoral Theses /TS
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Doctoral Theses /TS
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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. -
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. -
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. -
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. -
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.
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