| 21. |
- Euler, Marianna
(författare)
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Group theoretical methods for solving multidimensional nonlinear partial differential equations
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- New methods for constructing both exact and approximate solutions of multidimensional nonlinear partial differential equations are developed. The basis of the methods is taken from the classical Lie transformation group theory. New symmetry concepts are investigated for the construction of solutions of important nonlinear partial differential equations. Those concepts are: conditional symmetries, hidden symmetries, and approximate symmetries. Moreover, we propose a method by which a multidimensional wave equation can be related and classified with respect to a compatible system of multidimensional differential equations. The equations under investigation are: nonlinear Schrödinger equations, nonlinear heat equations, a generalized van der Pol equation, and nonlinear d'Alembert equations. The new solutions presented in this thesis could be of fundamental importance in the applications of the physical processes modelled by the above mentioned differential equations.
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| 22. |
- Fischer, Britta
(författare)
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System identification in alternative shift operators with applications and some other topics
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis mainly concentrates on two issues of system identification, namely identification for controller design and identification with orthonormal basis functions. In identification for controller design one seeks to obtain sufficient knowledge about the plant to design a high-performance controller. The frequency domain method proposed in this thesis is applicable to plants that are typical to process industry. While designing a high-performance controller it is desirable to keep the amount of identification experiments small. This has been achieved by choosing the next experimental conditions in an adaptive manner, i.e. the current experimental conditions depend on past data. The remaining parts of the thesis are concerned with identification by means of orthonormal basis functions, especially the Laguerre and Kautz functions. These particular functions have been chosen due to their simple mathematical structure and due to the fact that their dynamics are similar to the nature of a wide range of industrial processes. Linear, time-invariant, multi-input multi-output systems are considered. In system identification one usually uses the entire measurement data in order to find an appropriate plant model. In this thesis, the data is projected onto the Laguerre and Kautz functions, respectively, and only the projection coefficients, also called expansion coefficients or Laguerre/Kautz coefficients, are used for identification. As the considered orthonormal basis functions provide the user with some additional degrees of freedom in the time constants of the functions, noise and unmodelled dynamics can be attenuated through a judicious choice of these time constants. Furthermore, the projection leads to a considerable data reduction. After the projection of the measurement data, classical identification methods as subspace identification or the least squares method can be used to identify a model which, after a certain mapping, can for example be used for controller design. The suggested method has been derived for the identification of continuous and discrete models and has been successfully employed for the estimation of a coal injection process, a vibration process, subprocesses of a sugar mill, a paper machine headbox and time delay estimation. For the case of time delay estimation, error bounds on the estimation error have been derived. Results from applications show that the presented method in many cases outperforms classical linear methods and provides good linear approximations of non-linear processes. Furthermore, the excitation characteristics and numerical properties of the expansion coefficients have been studied and compared to the properties of the measurement data sets.
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| 23. |
- Forsberg, Johan
(författare)
-
Mobile robot navigation using non-contact sensors
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis concerns localization and control of mobile robots using range measurements from optronic sensors, in particular scanning time-of-flight lasers. The thesis covers the localization algorithms, on-line sensor calibration and tele-operation for semi-autonomous control of mobile robots. A real-world application of an autonomous mobile robot equipped with an arm for surface finishing operations is also developed. The Range Weighted Hough/Radon transform is introduced for robust detection of walls and extraction of geometric parameters. Thus a robot can automatically create a map of its environment and update it using an extended Kalman filter. The required association of observations with the map features uses a Bayesian classifier. New walls are added to the map as they are observed and classified as new. Navigation is performed with typical fluctuations of 1 cm and 1 degree (standard deviation) at speeds of 0.5 m/s. The navigation is robust even in cluttered environments and with several persons moving around. Sheet-of-light range cameras have also been tested for mobile robot navigation. A calibration algorithm for simultaneous calibration and localization integrates the calibration into the localization system. The only absolute reference needed by the calibration algorithm is the relative motion of the robot. The resulting system performs calibration during normal operation without lowering the localization performance. The algorithms above were applied to two applications: - A mobile robot capable of autonomously spray-plastering the walls and ceiling of a room. The prototype was successfully tested at construction sites. - A semi-autonomous robot tele-operated using tele-commands. The tele- commands include navigation along corridors, through open doors, following walking persons and automatic map generation of the remote site.
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| 24. |
- Forssén, Annika, et al.
(författare)
-
Mellan ansvar och makt : en diskussion om arbete, hälsa och ohälsa utifrån tjugo kvinnors livsberättelser
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Syftet med vår gemensamma medicinska avhandling är att skapa ny kunskap om kvinnors arbete, hälsa och ohälsa. Forskningen är kvalitativ och bygger på tjugo kvinnors livsberättelser. Feministisk teoribildning är grundläggande. Kvinnornas arbete (betalt och obetalt) och deras upplevelser av hälsa och ohälsa beskrivs och karakteriseras. Även graviditeter, födande och amning räknas som arbete. Begrepp som fått betydelse i förhållande till kvinnornas arbete/hälsa/ohälsa är; skapande, ansvar, lyhördhet, självbestämmande, "klara av", tyngd (i arbete), tid och planering av tid, ekonomi, (sam)levnadsform, maktrelationer mellan könen, könsarbetsdelning (i hem och förvärvsliv), föreställningar om kvinnlighet och manlighet, mötet med sjukvården, förändring. Vetenskapsteoretiska frågor av vikt för medicinsk (kvinno)forskning diskuteras.
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| 25. |
- Fredriksson, Christian
(författare)
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Exploratory experimental and theoretical studies of cyclone gasification of wood powder
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis describes an exploratory experimental and theoretical study of gasification of wood powder in a cyclone gasifier. The generated gas could be used to operate a gas turbine in a combined cyclone power plant. The objective has been to develop the understanding of cyclone gasification by experimental studies of the performance of a cyclone designed in principle as a separation cyclone and by comparisons between the experimental results and theoretical predictions. The experiments were carried out with commercial Swedish wood powder fuels, injected with air or steam/air mixture through two diametrically opposite tangential inlets and gasified at atmospheric pressure in cyclones of two different configurations with a volume of about 0.034 m3. The studies show that stable gasification of this fuel can be obtained for a specific fuel feeding rate of about 5 MW/m3 cyclone volume for equivalence ratios above 0.15 and that the equivalence ratio had to be kept below about 0.4 in order to avoid material temperatures above 950oC. A cyclone with a short outlet pipe, designed as a conventional separation cyclone was found to give lower char conversion than a modified cyclone with a long outlet pipe. The heating value of the gas was found to be approximately 4.5 MJ/kg. The dust load in the product gas was measured to between 1000 and 2500 mg/Nm3. It was possible to separate at least 40-60% of the potassium and 60-90% of the sodium supplied with the wood. The alkali that left the cyclone with the product gas appear to be in solid or melted phase in the unseparated char particles and consequently not vaporised during gasification. As the K and Na were assumed to remain within the particles during gasification, it was concluded that to reduce the amount of alkali metals in the product gas it would be necessary to improve the particle separation efficiency. The results of the theoretical modelling, using the existing models in the commercial software CFX 4.2, show with 3-dimensional modelling of the cyclone flow that RSM turbulence models give better agreement between predicted and observed velocity distributions than k-e models. Nevertheless, the latter give acceptable predictions as regards particle separation. Comparisons between predicted and experimental performance of cyclone gasifiers show that the heating value is over-predicted and the carbon conversion and alkali carry over are under-predicted. The results show that further development of the theoretical models is necessary.
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| 26. |
- Friborg, Johan
(författare)
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Experimental and theoretical investigations into the streaming potential phenomenon with special reference to applications in glaciated terrain
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The occurrence of an electrical potential difference between the ends of a capillary tube when a fluid flows through is known as the streaming potential phenomenon. It was reported by Quincke in 1859 and was studied by Helmholtz, among others, in the nineteenth century. Its geophysical manifestation is the development of electrical potential differences in the ground when groundwater flows through porous rocks or soils. The phenomenon has been comparatively little studied in a geophysical context. The present thesis is the outcome of the author's experimental and theoretical research in the phenomenon. Natural streaming potentials, along with other electrical potentials in the ground that are present even in the absence of an artificially injected current, are also known as self-potentials (SP) or self-potential anomalies. Small-scale field measurements in the present work have demonstrated that SP observations within areas of size 0.5 m by 0.5 m appear to be approximately normally distributed. Hence a mean of such observations can be accepted as a representative value of the potential of the "point". The experimental work in the thesis was undertaken to simulate the natural phenomenon in the laboratory. An equipment to measure the streaming potentials developed across soil samples as a function of the applied pressure was designed. The total applied pressure could be varied between approximately 15 and 400 kPa. Pressure differences and electrical potential differences could be measured with an accuracy of about 0.1 kPa and 1 mV, respectively. The streaming potential developed across a sample is generally observed to be proportional to the pressure difference and the constant of proportionality is called the streaming potential coefficient C. This was determined for a number of sand and moraine samples. Two methods to estimate C from field observations of SP have been developed. The first can be regarded as a correction to observations of potentials due to water flow in slopes ("topographic SP") and the second is an active method where pumping from a well is used as a controlled source of streaming potentials. A comparison of values of C obtained from laboratory measurements and from estimates based on field observations showed that laboratory data can give reasonable estimates of the in situ value of C. Field observations of SP at several different sites have been used to illustrate the stability of the potentials over extended periods of time as long as conditions in the ground stay the same. When the appearance of an SP-anomaly changes this generally reflects significant changes in the conditions in the ground. It appears that anomalies with an amplitude exceeding about 10 mV, and probably even smaller ones, are significant. A case history illustrates the occurrence of streaming potentials in a practical field situation. It is shown that the removal of a topographic trend enhances the appearance of any local anomaly patterns present in the data. In the case under consideration these patterns reflect both variations in the electrical resistivity and presence of self-potentials not of a streaming origin. The apparent streaming potential coefficient can be obtained from a plot of SP versus elevation but it was found to vary with time due to variation in the near-surface resistivity. The streaming potential phenomenon can be described by means of the theory of coupled flows which expresses the flow (of, e.g., charge, matter or heat) as a linear combination of driving forces (gradients of , e.g., electric potential, pressure or temperature). The formulation is well suited to numerical modelling, and a detailed examination of the generation of sources of conduction current in the streaming potential problem has been made. A numerical study illustrates the calculation of conduction current source terms in a practical example. A qualitative discussion of the generation of sources of conduction current, by flow of ground water, for some simple geological models has been made to further illustrate the physical mechanisms behind the streaming potential phenomenon. Although not strictly a modelling tool, a method to estimate the limiting depth to a streaming potential source region has also been devised using the formal analogy between streaming potentials and magnetostatics and following Smith's analysis for the determination of the maximum depth to the top of a magnetised body.
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| 27. |
- Fältholm, Ylva
(författare)
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Work, cooperation and professionalization : a multiple case study
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This dissertation, originally aimed at investigating cooperation in heterogeneous as well as homogeneous groups, is based on a multiple case study presented in six separate, but partly related papers. These papers, preceded by Part 1 with an introduction and Part II, dealing with the method used, are divided into three different parts. Part III a case study of the introduction of self-regulating work groups at a paper mill and in an office, both homogeneous groups. For various reasons, the project objectives regarding change in division of labor were not fully implemented. I argue that the reason can be seen in terms of the workers´ collective and infer that "new" ways of opposing change might be observable in the future. Additionally, by analyzing my own recollections of blue-collar work, I conclude that the worker´s collective in the future, as a result of globalization, labor-saving technology and new production concepts might take a differentiated form. This means also that the "collective identity" of the worker or the relations of workers to work and to life outside work will, instead of being uniform, be differentiated. Other changes might be understood in terms of a weakened or even obliterated workers´collective. In part IV, based on empirical studies of how midwives cooperate with other professions within heterogeneous groups, I conclude that the problems regarding different hierarchical positions of group members can be described in terms of professionalization. Additionally, by carrying out a discourse analysis of the discourse of midwifery, I investigate what role it plays in the professonalization of midwives and show that it draws upon the concept of the female midwife and the male obstetrician and that the discursive professonalization strategies used both toward the medical profession and toward groups subordinate to the midwives - which are based on the discourse - are generated. These two groups of papers focus on and investigate different types of defensive strategies used to cope with the subordinate or intermediate position of respective group. They also explore the different ways (by the worker´s collective and the discourse of midwifery) whereby a collective identity is formed and transformed. The last part of the thesis, including one paper, focuses problems related to cooperation within inter-professional teams.
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| 28. |
- Gabrielsson, Henrik
(författare)
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Ductility in high performance concrete structures : an experimental investigation and a theoretical study of prestressed hollow core slabs and prestressed cylindrical poles
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The thesis presents results from a project dealing with ductility in high performance concrete structures. The main objectives were to investigate the material and structural ductility/brittleness of prestressed structural elements of High Performance Concrete (HPC). The aim was to get a better understanding of the fracture process and to study sudden and brittle failures formed by shear stresses. The project was split into three parts: (I) Torsion of cylindrical pole elements, (II) Shear, torsion and bending of hollow core slabs, and (III) Shear, torsion and bending of cylindrical pole elements. Laboratory investigations were performed and displacements were carefully studied. The test results were compared with different code models and especially with the Modified Compression Field Theory(MCFT) proposed by Collins & Mitchell (1991). A computer program based on MCFT was written to calculate the interaction between shear, torsion and bending. One conclusion is that structures made of HPC can be as ductile as structures made of normal strength concrete. Another conclusion is that the strain-based modified compression field theory can represent failure processes in a correct way with good results.
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| 29. |
- Gamstedt, Kristofer
(författare)
-
Fatigue damage mechanisms in polymer matrix composites
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Polymer matrix composites are finding increased use in structural applications, in particular for aerospace and automotive purposes. Mechanical fatigue is the most common type of failure of structures in service. The relative importance of fatigue has yet to be reflected in design where static conditions still prevail. The fatigue behavior of composite materials is conventionally characterized by a Wöhler or S-N curve. For every new material with a new lay-up, altered constituents or different processing procedure, a whole new set of fatigue life tests has to be repeated for such a characterization. If the active fatigue damage micromechanisms and the influence of the constituent properties and interface were known, it would be possible, at least qualitatively, to predict the macroscopic fatigue behavior. A study of the fatigue damage mechanisms would also give indications of the weakest microstructural element, which is useful information in materials selection for improvement in service properties. In tensile fatigue of a multidirectional laminate, the critical elements are the longitudinal plies which are the last to fail. Although failure of neighboring off-axis plies as well as delamination will influence the fatigue process, an understanding of the behavior of the longitudinal plies forms an important foundation. Effects of plies of other directions may then be interpreted based on this foundation. Fatigue of longitudinal plies is therefore focused on in the present study. The underlying fatigue damage mechanisms were investigated for unidirectional O' carbon fiber reinforced plastics (CFRP) and glass fiber reinforced polypropylene (GF/PP) in tension-tension fatigue. By use of a surface replication technique the evolution of fatigue damage could intermittently be monitored during the course of fatigue testing. In the CFRPS, the matrix was an epoxy resin or polyetheretherketone (PEEK). In the GF/PP system, the matrix was modified with maleic anhydride (MA) to achieve a stronger fibermatrix interface. The macroscopic fatigue behavior was characterized by fatigue life diagrams. A statistical method has been devised to systematically characterize fatigue life data in terms of fatigue life diagrams. On the microscopic level, the CF/epoxy and GF/MA-PP composites have relatively strong interfaces and showed localized and scarce fiber breaks from which matrix cracks propagated perpendicular to the fiber direction. In CF/epoxy, fiber bridged cracks with squeezed fiber tips appeared. Conversely, CF/PEEK and GF/PP have weaker interfaces, and the principal mechanisms were extensive and distributed debonding or longitudinal matrix cracking followed by further fiber breakage. Macroscopically, the weak interface composites showed shorter fatigue lives and more rapid fatigue degradation. This suggests that higher interfacial strengths lead to improved fatigue performance. Modeling studies were undertaken for the two observed mechanisms; debonding from a fiber break, and fiber bridged cracking. The stochastic breakage of fibers next to a growing debond was parametrically investigated with a shear lag model. The stress profile in the surviving fibers becomes attenuated and more distributed as the debonds grow. This results in longer axial distances between fiber breaks, and hence a more jagged and uneven crack propagation. A larger variability in strength along the fibers has basically the same break distributing effect. With a more homogeneous stress distribution caused by long debonds, the variability in fiber stress at failure of the intact fibers decreases. This can explain the experimentally observed lower scatter in fatigue life of composites exhibiting a more homogeneous distribution of damage caused by debonding. Furthermore, the experimental results of fiber bridged cracking was modeled with a fracture mechanics approach. The crack growth curve can be plotted in terms of the effective stress intensity factor where the contribution of the cohesive crack surface forces from the bridging fibers are taken into account. This curve falls somewhat closer to that of the neat matrix material compared to the unbridged crack, but the difference is still considerable. Besides the fiber bridging, there should therefore be other active toughening mechanisms that slows the crack propagation down to account for the fatigue resistant behavior of the tested material. In fatigue of multidirectional laminates, tension-compression loading has shown to be more detrimental than tension-tension loading. The reason for this behavior has not been entirely clarified. The adverse effect of the compressive load excursions is partly caused by the formation of transverse cracks. This was verified by counting transverse cracks in cross-ply laminates. Since debonding is the subcritical mechanism which leads to transverse cracking and eventually influences ultimate failure, the debonding was studied in low cycle fatigue of a single transverse fiber. In tension, contact zones developed at the crack tips for sufficiently large debonds. Due to the inherent geometry and the mismatch in elastic properties of the constituents, an opening zone appeared at the crack tips of the debond in compression. This was also verified by finite element analysis. Since debond propagation is more suseptible to mode I loading, the sensitivity to tension-compression loading is explained by the effective opening zone in compression.
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| 30. |
- Granlund, Jan
(författare)
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Structural steel plasticity : experimental study and theoretical modelling
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Föreliggande avhandling behandlar konstitutiv modellering av konstruktionsstål i elastoplastiskt område. Ett koncept för tvåaxlig provning i huvudspänningsplanet har utvecklats vilket sedan har använts för att generera försöksdata för att formulera en konstitutiv modell för sambandet mellan spänningar och plastiska töjningar. Utgångspunkten har varit att söka en modell som på ett bättre sätt beskriver stålets beteende än de modeller man normalt använder i finita elementmetoder idag. De modeller som normalt används idag klarar egentligen inte av att beskriva materialets beteende vid annat än monoton pålastning. Vid påföljande avlastning och förnyad pålastning i annan riktning, såsom exempelvis kallformning, har en betydande anisotropi introducerats vilket gör att man sannolikt erhåller ett dåligt resultat. Skälen till att man i många sammanhang använder dessa modeller, t.ex. isotropt eller kinematiskt hårdnande, är bl.a. att försöksdata som underlag saknas för mer avancerade modeller. Det koncept för tvåaxlig provning av plan plåt i leveranstillstånd med korsformade provkroppar som har utvecklats, kan med anbringande av stödplåtar för att förhindra buckling ut ur planet genomföra provning i hela huvudspänningsplanet. Denna utrustning har använts för att utföra omfattande provning av två olika stålsorter, en höghållfast och en av normal hållfasthet. Konceptet klarar i det aktuella utförandet töjningar upp till drygt 3%. Initiellt flytvillkor samt påföljande flytvillkor för två olika förbelastningsnivåer samt förändringen av den plastiska modulen har studerats. Vidare har även kvalitativa studier av den plastiska flytlagen gjorts. Resultaten visar att det initiella flytvillkoret faller mellan det som förutspås av von Mises och Trescas villkor. Vad påföljande flytvillkor beträffar visar resultaten att man har en påtaglig Bauschingereffekt, dvs. en reduktion av flytgränsen i riktning rakt motsatt pålastningen. Vinkelrätt pålastningsriktningen hårdnar materialen i nästan samma omfattning som i pålastnings~ riktningen. Man tydligt se att vid återpålastning så fås en gradvis övergång från elastiskt till plastiskt tillstånd även om materialet initiellt har en skarp övergång däremellan. Ett förslag till en konstitutiv modell presenteras. Modellen använder en elastisk gränsyta samt en minnesyta. Den gradvisa övergången mellan elastiskt och plastiskt tillstånd vid återpålastning beskrivs med en teori grundad på sk "fuzzy sets". Den elastiska gränsytan beskrivs med teori för distortionshårdnande vilket ger goda möjligheter att beskriva Bauschingereffekten vid generella spänningstillstånd. Modellen kräver bara två ytterligare parameter jämfört med en enkelytemodell med blandat hårdnande varför den är enkel att använda. Jämförelser mellan experimentella data och simuleringar med den föreslagna modellen visar på god överensstämmelse generellt. Jämförelser med existerande modeller visar också att betydande förbättringar av beskrivningen erhålls. Speciellt tydligt är detta för det höghållfasta stålet där det dominerande bidraget till hårdnandet kommer från Bauschingereffekten.
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