| 1. |
- Isaksson, Per
(författare)
-
On crack growth under compressive stresses
- 2001
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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.
|
|
| 2. |
- Tharani, Soraya
(författare)
-
Immigration, security and the public debate on US language policy : A critical discourse analysis of language attitudes in the United States of America
- 2011
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The narrative of the United States is of a "nation of immigrants" in which the language shift patterns of earlier ethnolinguistic groups have tended towards linguistic assimilation through English. In recent years, however, changes in the demographic landscape and language maintenance by non-English speaking immigrants, particularly Hispanics, have been perceived as threats and have led to calls for an official English language policy. This thesis aims to contribute to the study of language policy making from a societal security perspective as expressed in attitudes regarding language and identity originating in the daily interaction between language groups. The focus is on the role of language and American identity in relation to immigration. The study takes an interdisciplinary approach combining language policy studies, security theory, and critical discourse analysis. The material consists of articles collected from four newspapers, namely USA Today, The New York Times, Los Angeles Times, and San Francisco Chronicle between April 2006 and December 2007. Two discourse types are evident from the analysis namely Loyalty and Efficiency. The former is mainly marked by concerns of national identity and contains speech acts of security related to language shift, choice and English for unity. Immigrants are represented as dehumanised, and harmful. Immigration is given as sovereignty-related, racial, and as war. The discourse type of Efficiency is mainly instrumental and contains speech acts of security related to cost, provision of services, health and safety, and social mobility. Immigrants are further represented as a labour resource. These discourse types reflect how the construction of the linguistic 'we' is expected to be maintained. Loyalty is triggered by arguments that the collective identity is threatened and is itself used in reproducing the collective 'we' through hegemonic expressions of monolingualism in the public space and semi-public space. The denigration of immigrants is used as a tool for enhancing societal security through solidarity and as a possible justification for the denial of minority rights. Also, although language acquisition patterns still follow the historical trend of language shift, factors indicating cultural separateness such as the appearance of speech communities or the use of minority languages in the public space and semi-public space have led to manifestations of intolerance. Examples of discrimination and prejudice towards minority groups indicate that the perception of worth of a shared language differs from the actual worth of dominant language acquisition for integration purposes. The study further indicates that the efficient working of the free market by using minority languages to sell services or buy labour is perceived as conflicting with nation-building notions since it may create separately functioning sub-communities with a new cultural capital recognised as legitimate competence. The discourse types mainly represent securitising moves constructing existential threats. The perception of threat and ideas of national belonging are primarily based on a zero-sum notion favouring monolingualism. Further, the identity of the immigrant individual is seen as dynamic and adaptable to assimilationist measures whereas the identity of the state and its members are perceived as static. Also, the study shows that debates concerning language status are linked to extra-linguistic matters. To conclude, policy makers in the US need to consider the relationship between four factors, namely societal security based on collective identity, individual/human security, human rights, and a changing linguistic demography, for proposed language intervention measures to be successful.
|
|
| 3. |
- Azadian, Saied
(författare)
-
Aspects of precipitation in alloy Inconel 718
- 2004
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A study was made of the microstructure of the Ni-base alloy Inconel 718 with emphasis on the precipitation and stability of intermetallic phases as affected by heat treatments. In addition the effect of the precipitation on selected mechanical properties namely hardness, creep notch sensitivity and hot ductlity were investigated. The materials studied were a spray-formed version and three wrought versions of the alloy. The spray-formed version of the alloy was of interest since it exhibited a superior compositional homogeneity to more conventional forms of the alloy. To characterise the precipitation kinetics and to determine the associated hardening, a series of isothermal ageing treatments were carried out on both spray-formed and ring rolled material at selected temperatures between 600°C and 1025°C for up to 100h. The results were presented in the form of TTH diagrams for the two alloys. The heat treated specimens were then studied using optical microscopy, scanning electron microscopy and transmission electron microscopy. Where possible these were supplemented with quantitative metallographic measurements of volume fraction and precipitates size.The three main intermetallic precipitation phases in IN 718 are (i) gamma’ having a composition Ni3(Al,Ti), a cubic crystal structure and cubic or spherical particle shape (ii) gamma” having a composition Ni3Nb, a bct crystal structure and a lens-like disc shape (iii)delta having composition Ni3Nb, an orthorhombic crystal structure and forming as grain boundary particles and films as well as thin plates extending long distances into the grains. In thermomechanically processed material the delta-phase can also occur as shorter platelets and particles resulting from fragmentation of the original plates. In this work the rates of precipitation and the temperature ranges of their existence were determined qualitatively. Moreover the coarsening of the ganmma" precipitates was determined quantitatively. The kinetics of precipitation and dimensional coarsening of the delta-phase were also established quantitatively. Similarly, the dissolution kinetics of the delta-phase were measured in the spray-formed and three wrought versions of the alloy leading to a determination of the effect of Nb-content on the delta solvus temperature. After a standard heat treatment, the spray-formed IN 718 was found to be creep notch sensitive according to an SAE-AMS standard rupture test. It was found that notch ductility could be restored to the alloy by inserting a delta-phase precipitation treatment at 875°C-900°C prior to the standard heat treatment. This could be attributed to the formation of a more favourable delta-phase morphology than that formed during the solution treatment at 954°C in the standard heat treatment cycle. A study was made of high temperature tensile properties of a wrought version of IN 718 in particular in relation to the problem of hot cracking in repair welding. Measurements were made of the strength loss temperature on heating and the strength and ductility return temperatures on cooling. These properties provide an indication of the sensitivity of the alloy to hot cracking due to liquation. The tests were made on the alloy after various heat treatments designed to produce microstructures with two different grain sizes with and without delta-phase precipitation. Hot tensile tests were also performed on the in-situ solidified alloy in order to describe the weld metal ductility and properties. The grain size and the presence of large fractions of delta-phase precipitated at 900°C had only small effects on the strength loss temperature and brittle-ductile transition of the alloy. On the other hand, a small fraction of delta-phase precipitated in the grain boundaries at 960°C reduced the brittle-ductile transition temperature significantly. It is proposed that the reduction was associated with interaction between the grain boundary delta-phase and grain boundary boron segregation that was observed in all the heat treated materials.
|
|
| 4. |
- Bengtsson, Fredrik
(författare)
-
Algorithms for aggregate information extraction from sequences
- 2007
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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.
|
|
| 5. |
- Pederson, Robert
(författare)
-
The microstructures of Ti-6Al-4V and Ti-6Al-2Sn-4Zr-6Mo and their relationship to processing and properties
- 2004
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The research described in the thesis concerns phase transformations and mechanical properties of the titanium alloys Ti-6Al-4V and Ti-6Al-2Sn-4Zr-6Mo. The need to weld the alloys for certain engine components can expose the alloy locally to non-optimum thermal cycles and it is therefore of importance to gain an understanding of the kinetics involved in the phase transformations during heating and cooling. Moreover, for the purpose of modelling and computer simulations of heat treatments and welding processes, quantitative descriptions of the transformation are necessary. One focus in this work has been to examine the feasibility of using high temperature X-ray diffraction (HT-XRD) to study the phase transformation kinetics. Another aim in this work was to develop a weld simulation model leading to a prediction of the joint structure and microstructure of an electron beam welded component of the alloy Ti-6Al-4V. Electron beam welding (EBW) and friction welding (FRW) of Ti-6Al-4V and Ti-6Al-2Sn-4Zr- 6Mo have also been investigated, including microstructural weld characterization and the effect of post weld heat treatments on selected mechanical properties. In addition, two components of Ti-6Al-4V produced by ring-rolling and closed die forging, respectively, showed unexpected differences in low cycle fatigue (LCF) under certain loading conditions. Quantitative metallographic studies and texture examinations were conducted in order to find possibly reasons for the observed difference in fatigue properties. The HT-XRD technique was successfully used to monitor the alpha-to-beta transition during heating and the beta-to-alpha transition during cooling and including the transformation kinetics during isothermal hold. From the recorded spectra the thermal expansion properties for the alpha and beta phases could be extracted up to a temperature of 1050C, enabling the determination of the overall thermal expansion for the alloy by using rule of mixture (ROM) calculations. The weld simulation model here presented proved to predict the joint structure and microstructure of an electron beam welded Ti64 component with reasonable good accuracy, both in terms of thermal history during welding and also in terms of martensitic fraction formed during cooling and the distribution of martensite through the cross section of the FZ and HAZ. All mechanically tested Ti64/Ti6246 EBWs and FRWs, independent of PWHT temperature, fractured in the Ti64 base material and not in the weld region, thus, both types of welding processes seem to be promising candidate methods for successful joining of the Ti64 alloy to the Ti6246 alloy. Quantitative metallographic studies revealed that the closed die forged material exhibited a finer primary alpha grain size and a finer Widmanstätten platelet colony size which would be expected to provide a superior resistance to fatigue crack initiation. Observed differences in the texture of the two materials as determined by electron back-scattered diffraction might also have contributed to the difference in fatigue properties.
|
|
| 6. |
- Persson, Per-Eskil
(författare)
-
Räkna med bokstäver! En longitudinell studie av vägar till en förbättrad algebraundervisning på gymnasienivå
- 2010
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Huvudsyftet med studien är att skapa insikter om algebrakunskaper och villkor för algebralärande hos gymnasieelever. Ett andra syfte är att med utgångspunkt i resultaten föreslå vägar till en förbättrad algebraundervisning inom det svenska skolsystemet. Studien bygger på ett längre forsknings- och utvecklingsprojekt, som kan indelas i fyra faser. I den empiriska grundstudien följdes två årskullar elever under sin studietid på en gymnasieskola i Sydsverige, och några huvudfaktorer för framgång i algebrastudierna identifierades. Denna delstudie har närmare presenterats i min licentiatuppsats.De tre följande faserna är beskrivna i de tre artiklar, som denna avhandling vilar på. I den andra delstudien reflekterar jag över vad min utveckling från att bara vara lärare till att även bli forskare betytt för min förståelse för vad som händer i klassrummet och på vilka sätt detta förändrat mitt sätt att undervisa. I den tredje fördjupas analysen av elevernas svar på frågan om hur ett centralt funktionssamband kan förklaras. Den fjärde delstudien är en litteraturöversikt över nyare forskning kring räknares och andra teknologiska verktygs inverkan på algebrakunskaper och –lärande.Samtliga delstudier sätts in i ett övergripande sammanhang som utgår från den didaktiska triangeln med huvuddelarna: den lärande/eleven, läraren/undervisningen och det lärda/resultatet av undervisningen. Som bakgrund för studien används ett teoretiskt ramverk bestående av fem huvudkomponenter: matematik och matematikfilosofi, teorier om kunskap och lärande, symbolernas betydelse, representationsformer och verktyg för lärande. En utförlig översikt över tidigare forskning inom området algebraundervisning ges, med ett multidimensionellt perspektiv och med speciellt fokus på delområden av betydelse för studien.En rad resultat från de olika delstudierna presenteras och sammanställs, utgående från såväl det teoretiska ramverket som den didaktiska triangeln. Dessa resultat bildar sedan utgångspunkt för viktiga implikationer för undervisningspraktiken i matematik inom områdena: kunskap och kunskapsutveckling, symboler och representationsformer, algebra som en röd tråd i matematikundervisningen, teknologi i matematikundervisningen och de affektiva faktorernas betydelse. Dessutom ges förslag till lärare om olika metoder och tillvägagångssätt för lokala utvecklingsarbeten på skolorna, samt förslag till vidare forskning.
|
|
