The fundamental issue of determining acoustic properties of porous media from their local geometry is examined in this PhD dissertation thesis, thanks to a sample of open-cell aluminum foam analyzed by axial computed microtomography. Various geometric properties are measured to characterize the experimental sample at the cell size level. This is done in order to reconstruct a porous medium by means of idealized three- and two- dimensional unit-cells.The frequency dependant thermal and velocity fields governing the propagation and dissipation of acoustic waves through rigid porous media are computed by Brownian motion simulation and the finite element method, respectively. Macroscopic behavior is derived by spatial averaging of the local fields. Our results are compared to experimental data obtained from impedance tube measurements. Firstly, this approach leads to the identification of the macroscopic parameters involved in Pride and Lafarge semiphenomenological models. Secondly, it yields a direct access to thermal and viscous dynamic permeabilities. However, the bi-dimensional model underestimates the static viscous permeability as well as the viscous characteristic length; what thus require a three-dimensional implementation.

Cette thèse décrit l’incorporation de fer dans l’hétérostructure InGaAsP/InP par implantation ionique à haute énergie (MeV) suivi d’un recuit thermique rapide. L’alliage quaternaire InGaAsP est tout indiqué pour fabriquer des couches photoconductrices qui peuvent absorber dans le proche-infrarouge, à 1.3 µm ou 1.55 µm. Ce procédé vise à développer de nouveaux matériaux de forte résistivité pour l’holographie photoréfractive et la spectroscopie térahertz pulsée. À notre connaissance, cette investigation représente les premiers essais détaillés de l’implantation de fer dans le matériau InGaAsP/InP. Les principaux paramètres de fabrication, tels la fluence d’ions de fer, la température d’implantation et la température de recuit ont été explorés. Les propriétés physiques des matériaux produits ont été étudiées avec des mesures électriques (résistivité et effet Hall avec l’analyse de Van der Pauw), optiques (photoluminescence, absorption et réflectivité différentielle résolue en temps) et structurales (diffraction de rayons X, canalisation de la rétrodiffusion Rutherford et microscopie électronique en transmission). Pour fabriquer des couches à forte résistivité pour des applications holographiques à 1.3 µm, nos résultats ont montré qu’il est préférable d’éviter l’amorce de l’amorphisation lors de l’implantation du quaternaire pour maintenir une bonne qualité cristalline après recuit. Ceci favoriserait une compensation par l’activation du fer comme impureté profonde. Une résistivité de l’ordre de 10[indice supérieur 4] Ωcm est mesurée après recuit. Pour fabriquer des couches à forte résistivité pour des applications de spectroscopie térahertz pulsée à 1.55 µm, nous privilégions l’amorphisation par implantation froide et la recristallisation, ce qui réduit le temps de recombinaison des photoporteurs sous la picoseconde. L’émission d’ondes térahertz par ce matériau est démontrée sur une largeur de bande de 2 THz. L’évidence expérimentale montre la formation d’une microstructure polycrystalline dans la couche d’InGaAsP, ayant une forte densité de fautes planaires et une taille de grains nanométrique qui varient avec la température de recuit, ce qui suggère une connexion avec les propriétés optoélectroniques du matériau.

It is perhaps paradoxical that many material properties arise from the absence of material rather than the presence of it. For example, the strength, stiffness, and toughness of a concrete are related to its pore structure. Furthermore, the volume, size distribution, and interconnectivity of porosity is important for understanding permeability, diffusivity, and capillary action occurring in concrete, which are necessary for predicting service lives in aggressive environments. This research advances the state-of-the-art of multiscale characterization of cement-based materials, and uses this characterization information to model the material behavior under competing durability concerns. In the first part of this research, a novel method is proposed to characterize the entrained air void system. In the second and third parts of this research, microstructural characterization is used in tandem with mechanical models to investigate the behavior of cementitious materials when exposed to rapid rates of loading and to cyclic freezing and thawing. First, a novel analytical technique is presented which reconstructs the 3D entrained air void distribution in hardened concrete using 2D image analysis. This method proposes a new spacing factor, which is believed to be more sensitive to microstructural changes than the current spacing factor commonly utilized in practiced, and specified in ASTM C457, as a measure of concrete's ability to resist to damage under cyclic freeze/thaw loading. This has the potential to improve economy by improving the quality of petrographic assessment and reducing the need for more expensive and time-consuming freeze/thaw tests, while also promoting the durability of concrete. Second, quantitative measurements of the sizes, shapes, and spatial arrangements of flaws which are through to drive failure at strain rates above 100/s were obtained in order to model mortar subjected to high strain-rate loading (i.e., extremes in load rate). A micromechanics model was used to study the ways in which flaw geometry and flaw interaction govern damage. A key finding suggests that dynamic strength may be multimodal, with larger flaws shifting the dynamic strength upwards into the highest strength failure mode. Third, a robust theoretical approach, based upon poroelasticity, is presented to further validate the utility of the novel spacing factor proposed this research. The model is truly multiscale, using in its formulation pore size data ranging from the nanoscale to the micro-scale, entrained air data from the micro-scale to the millimeter scale, and infers a representative volume element on the centimeter scale. The results provide an underlying physical basis for the performance of the novel spacing factor. Furthermore, the framework could be used as a forensic tool, or as a tool to optimize the entrained air void system against freeze/thaw damage.

Hot section components of land-based gas turbines are subject to extremely harsh, high temperature environments and require the use of advanced materials. Directionally solidified Ni-base superalloys are often chosen as materials for these hot section components due to their excellent creep resistance and fatigue properties at high temperatures. These blades undergo complex thermomechanical loading conditions throughout their service life, and the influences of blade geometry and variable operation can make life prediction difficult. Accurate predictions of material response under thermomechanical loading conditions is essential for life prediction of these components. Complex crystal viscoplasticity models are often used to capture the behavior of Ni-base superalloys. While accurate, these models are computationally expensive and are not suitable for all phases of design. This work involves the calibration of a previously developed reduced-order, macroscale transversely isotropic viscoplasticity model to a directionally solidified Ni-base superalloy. The unified model is capable of capturing isothermal and thermomechanical responses in addition to secondary creep behavior. An extreme reduced order microstructure-sensitive constitutive model is also developed using an artificial neural network to provide a rapid first-order approximation of material response under various temperatures, rates of loading, and material orientation from the axis of solidification.

A novel approach to microfabrication based on stereolithography was presented. This fabrication process is referred to as, ‘Exposure Controlled Projection Lithography’ (ECPL). In the ECPL process, incident radiation, patterned by a dynamic mask, passes through a transparent substrate to cure photopolymer resin. By controlling the amount of exposure, the height field of the cured film can be controlled. An ECPL system was designed and assembled. Factors affecting the accuracy of the ECPL process in fabricating micron shaped features were identified and studied. A real-time in-situ photopolymerization monitoring system was designed and assembled within the ECPL system to identify the sources of variations present in the system. Parts are fabricated from the ECPL process because of polymerization (or cross-linking) of monomer resin using light energy. Photopolymerization is a complex process involving coupling between several phenomena. This process was modeled by utilizing an understanding of the known polymerization reaction kinetics with incorporating the effects of oxygen inhibition and diffusion. A material response model and a simulation tool to estimate the shape of a cured part resulting from photopolymerization was created. This model was used to formulate a process-planning method to estimate the manufacturing process inputs required to cure a part of desired shape and dimensions. The process planning method was validated through simulations and experiments.

Al 7050-T7451 alloy with good combinations of strength, stress corrosion cracking resistance and toughness, is used broadly in the aerospace/aviation industry for fatigue-critical airframe structural components. However, it is also considered as a highly anisotropic alloy as the crack growth behavior along the short transverse direction is very different from the one in the long transverse direction, due to the inhomogeneous microstructure with the elongated grains distributed in the work material used in the sheet/plate applications. Further processes on these materials are needed to improve its mechanical and material properties and broaden its applications. The material with ultra-fine or nano grains exhibits improved wear and corrosion resistance, higher hardness and better fatigue life, compared to the one with coarse grains. In recent times, the development of novel processing technologies has gained great attention in the research community to enhance the properties of the materials employed in the aerospace, biomedical, precision instrument, automotive, nuclear/power industries. These novel processing technologies modify the microstructure of this alloy and improve the properties. The aim of this dissertation is to investigate the effects of cryogenic processes, including friction stir processing (FSP), machining and burnishing, on Al 7050-T7451 alloy to solve the inhomogeneity issue and improve its surface integrity. FSP is applied to modify the microstructure of Al 7050-T7451 alloy for achieving more homogeneous structure with near ultra-fine grains (UFG) which were less than 2 µm, particularly in cryogenic FSP with liquid nitrogen as the coolant. Approximately 10% increase could be observed from the hardness measurement from the samples processed by cryogenic FSP, in contrast to dry FSP. Also, the texture change from Al (200) to Al (111) could be achieved in all the samples processed by dry and cryogenic FSP. Cryogenic machining and burnishing processes were also applied to enhance the surface integrity of the manufactured components with near-UFG structure. The highest cutting temperature was reduced by up to 44.7% due to the rapid cooling effect of liquid nitrogen in cryogenic machining, compared with dry machining. Nano grains were produced in the refined layers induced by cryogenic burnishing. And, up to 35.4% hardness increase was obtained within the layer depth of 200 µm in the cryogenically-burnished surface. A numerical finite element method (FEM) model was developed for predicting the process performance in burnishing. Less than 10% difference between the experimental and predicted burnishing forces was achieved in the simulation of cryogenic burnishing, and reasonable predictions were also achieved for temperatures, severe plastic deformation (SPD) layers.

In this paper, processing effects of electroplating and post- plating annealing on via extrusion are investigated. The study is based on two TSV structures with identical geometry but different processing conditions. Via extrusion, stress and material behaviors of the TSV structures were first compared. Electron backscatter diffraction (EBSD) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) were used to characterize the microstructure of TSVs and the additives incorporated during electroplating. Based on the results, processing effects on via extrusion and its mechanism are discussed, including grain growth, local plasticity, and diffusional creep.

Thesis (DLitt (Afrikaans and Dutch))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: This dissertation is an attempt to design a theoretical model for an encyclopaedic dictionary for Gabonese languages. It makes specific reference to Yilumbu, a developing language of the Bantu phylum (B44, according to Guthrie’s classification) spoken in Gabon and in Congo. All existing metalexicographic studies on Gabonese languages (including Yilumbu) pursue the compilation of comprehensive language dictionaries. Most of them are bilingual languages. On the other hand, a number of encyclopaedic dictionaries exist for Gabonese languages. Most of them were compiled by Catholic and Protestant missionaries and colonial administrators. However, none of these reference works was compiled on the basis of a prior metalexicographic orientation. Ultimately, the present study is the first of its kind to propose an encyclopaedic dictionary for Gabonese languages on the basis of a comprehensive metalexicographic analysis. This dissertation also shows that the planned encyclopaedic dictionary reflects the language planning needs of Gabon. Developing and documenting the Gabonese languages on all levels of communication in order to address all the roles and user characteristics are a great challenge. As far as encyclopaedic dictionary compilation is concerned, acquaintance with state-of-the-art developments in the theory and practice of lexicography is necessary. This dissertation comprises nine chapters. Chapter 1 mainly focuses on the specific aims, the purpose and the motivation of the study. The research problem is also demarcated in this chapter. Furthermore, the chapter presents the theoretical model that provides the framework for this research as well as the methodological approach adopted. The potential impact of the study, the reasons for and the position of the source and target languages as well as the sociolinguistic situation of Yilumbu are also introduced in the first chapter. Chapter 2 is a review of the dictionary typology with reference to the proposed model. It aims to present a typological framework for the planned encyclopaedic dictionary. It also introduces various dictionary typologies and contrasts encyclopaedic lexicography and general lexicography. The chapter concludes with a typological profile of the planned encyclopaedic dictionary for Yilumbu before outlining specific principles of the proposed model for an encyclopaedic dictionary. Chapter 3 is a review of encyclopaedic lexicography in Gabon. It introduces the concept of encyclopaedic lexicography. Then it provides a literature review of lexicographic surveys in the Gabonese languages by presenting existing encyclopaedic works in Gabon. The chapter also outlines recent trends in Gabonese lexicography in order to explain the situation of encyclopaedic lexicography in Gabonese lexicographic studies. Finally, the need for research in encyclopaedic lexicography in the Gabonese languages is underlined and specific advantages of encyclopaedic research for Yilumbu lexicography are introduced. Chapter 4 focuses on the target users of the planned encyclopaedic dictionary and its lexicographic functions. The chapter starts with a broad outline of target users and the activity of dictionary compilation. It also reviews what is known as target users and outlines user characteristics. The lexicographic functions of the planned encyclopaedic dictionary are also outlined in this chapter. Chapter 5 gives a comprehensive presentation of the frame structure of the planned encyclopaedic dictionary of Yilumbu. More specifically, the chapter provides a definition of the concept of frame structure and schematises the frame structure, highlighting its components. It outlines the accessory texts and the outer texts of the planned dictionary. This chapter also contains an outline of the front matter and the back matter strategies used for the Yilumbu encyclopaedic dictionary project. The central word list is also dealt with in this chapter. Finally, the chapter reports on the proposed frame structure functions. Chapter 6 focuses on the macrostructure of the planned dictionary. In this chapter, a general overview of the concept of macrostructure is first given. Then the chapter presents the dictionary basis and the selection of the lemma candidate list. Further macrostructural issues are also described. In Chapter 7, different aspects of the microstructure are discussed, focusing on a number of contributions. The chapter presents the concept of microstructure according to Hausmann and Wiegand (1989) and enumerates the different types of article to be included in the microstructure. Data to be included in the microstructure are also presented. Chapter 8 discusses the notion of access structure and its related concepts, namely outer access structure, inner access structure and search zones. The effectiveness and success of any dictionary is measured, among other things, by the accessibility and retrievability of the desired information. The chapter contains an overview of the concept of access structure and also deals with the outer access structure. Moreover, the chapter presents the inner access structure and the rapid access structure. The mediostructure is finally dealt with as a way of gaining access to data. Chapter 9 provides a comprehensive conclusion of the dissertation. This chapter also outlines the contribution of the research findings to lexicographic research, to the emerging Gabonese lexicography and to the fuller promotion of the Yilumbu language. It provides conclusions and formulates suggestions for further research. / AFRIKAANSE OPSOMMING: Hierdie proefskrif is 'n poging om 'n teoretiese model te ontwerp vir 'n ensiklopediese woordeboek vir Gaboenese tale. Spesifieke verwysing word gemaak na Yilumbu, 'n ontwikkelende taal van die Bantoefilum (B44 volgens Guthrie se klassifikasie) wat in Gaboen en die Kongo gepraat word.Alle bestaande metaleksikografiese studie oor die Gaboenese tale (insluitende Yilumbu) is gerig op die samestelling van omvattende taalwoordeboeke; meestal tweetalige woordeboeke. Daar bestaan wel 'n aantal ensiklopediese woordeboeke vir Gaboenese tale. Die meeste daarvan is deur Katolieke en Protestantse sendelinge en koloniale administrateurs saamgestel. Geeneen van hierdie woordeboeke is op 'n voorafbeplande metaleksikografiese basis saamgestel nie. Die huidige studie is die eerste van sy soort om 'n ensiklopediese woordeboek vir die Gaboenese tale voor te stel op die basis van 'n omvattende metaleksikografiese analise. Hierdie proefskrif wil aantoon dat die voorgestelde ensiklopediese woordeboek die taalbeplanningsbehoeftes van Gaboen reflekteer. Ontwikkeling en dokumentering van die Gaboenese tale op alle vlakke van kommunikasie, ten einde aan al die rolle en gebruikerseienskappe aandag te gee, is ʼn groot uitdaging. So ver dit die samestelling van ’n ensiklopediese woordeboek aangaan, is bekendheid met die allerjongste ontwikkelings in die teorie en praktyk van leksikografie nodig. Die proefskrif bevat nege hoofstukke. Hoofstuk 1 bied die besondere oogmerke, die doel en die motivering van die studie. Die navorsingsprobleem word in hierdie hoofstuk afgebaken. Die hoofstuk bevat ook ʼn uiteensetting van die teoretiese model wat die raamwerk vir hierdie navorsing asook die metodologiese benadering wat ingeneem is, verskaf. Daarbenewens word die potensiële trefkrag van die studie, die redes vir en die posisie van die bron- en doeltale asook die sosiolinguistiese situasie van Yilumbu aangebied. Hoofstuk 2 bied 'n oorsig van woordeboektipologie met verwysing na die voorgestelde model. Dit beoog om ʼn tipologiese raamwerk vir die beplande ensiklopediese woordeboek te bied. Dit stel ook verskeie woordeboektipologieë bekend en kontrasteer ensiklopediese leksikografie en algemene leksikografie. Die hoofstuk skets die tipologiese profiel van die beplande ensiklopediese woordeboek vir Yilumbu voordat dit spesifieke beginsels van die voorgestelde model vir ʼn ensiklopediese woordeboek skets. Hoofstuk 3 bied 'n oorsig oor die ensiklopediese leksikografie in Gaboen. Dit stel die begrip ensiklopediese leksikografie aan die orde. Daarna verskaf dit ʼn literatuuroorsig van leksikografiese opnames in die Gaboenese tale deur die aanbieding van bestaande ensiklopediese werke in Gaboen. Die hoofstuk beskryf ook onlangse tendense in Gaboenese leksikografie ten einde die situasie van ensiklopediese leksikografie in Gaboenese leksikografiese studies te verduidelik. Laastens word die behoefte aan navorsing in ensiklopediese leksikografie in die Gaboenese tale onderstreep en spesifieke voordele van ensiklopediese navorsing vir Yilumbu leksikografie word bekend gestel. Hoofstuk 4 fokus op die teikengebruikers van die beplande ensiklopediese woordeboek en die leksikografiese funksies daarvan. Die hoofstuk begin met ʼn algemene oorsig van teikengebruikers en die aktiwiteit van woordeboeksamestelling. Dit kyk ook na wat bekend is as teikengebruikers en skets eienskappe van gebruikers. Die leksikografiese funksies van die beplande ensiklopediese woordeboek word ook in hierdie hoofstuk beskryf. Hoofstuk 5 verskaf ʼn omvattende aanbieding van die raamstruktuur van die beplande ensiklopediese woordeboek van Yilumbu. Meer spesifiek, die hoofstuk verskaf ʼn definisie van die begrip raamstruktuur en bied die raamstruktuur skematies aan, en belig die komponente daarvan. Dit skets die hulptekste en die buitetekste van die beplande woordeboek. Hierdie hoofstuk bevat ook ʼn skets van die voor- en die agtertekstestrategieë wat vir die Yilumbu- ensiklopediese woordeboekprojek gebruik word. Die sentrale woordelys kom ook in hierdie hoofstuk aan bod. Laastens doen die hoofstuk verslag oor die voorgestelde raamstruktuurfunksies. Hoofstuk 6 fokus op die makrostruktuur van die beplande woordeboek. In hierdie hoofstuk word eerstens ʼn algemene oorsig van die begrip makrostruktuur gegee. Daarna bied die hoofstuk die woordeboekbasis en die keuse van die lemmakandidaatlys aan. Verdere makrostrukturele kwessies word ook beskryf. In Hoofstuk 7 word verskillende aspekte van die mikrostruktuur bespreek, met die fokus op ʼn aantal bydraes. Die hoofstuk bied die begrip mikrostruktuur aan soos in Hausmann en Wiegand (1989), en noem die verskillende soorte artikels wat in die mikrostruktuur ingesluit moet word. Data wat by die mikrostruktuur ingesluit moet word, word ook aangebied. Hoofstuk 8 bespreek die gedagte van toegangstruktuur en begrippe wat daarmee verband hou, naamlik eksterne toegangstruktuur, interne toegangstruktuur en soeksones. Die doeltreffendheid en sukses van enige woordeboek word onder andere gemeet deur die toeganklikheid en herwinbaarheid van die gewenste inligting. Die hoofstuk bevat ʼn oorsig van die begrip toegangstruktuur en betrek ook die eksterne toegangstruktuur. Verder bied die hoofstuk die interne toegangstruktuur en die kitstoegangstruktuur. Die mediostruktuur kom uiteindelik aan bod as ʼn wyse om toegang tot data te verkry. Hoofstuk 9 verskaf ʼn omvattende samevatting van die proefskrif. Hierdie hoofstuk skets ook die bydrae van die navorsingsbevindings tot leksikografiese navorsing, tot die opkomende Gaboenese leksikografie en tot die groter bevordering van die Yilumbu-taal. Dit verskaf gevolgtrekkings en formuleer voorstelle vir verdere navorsing.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

Within industrial product development processes there is an increasing demand towards reliable predictions of the material behavior, which aims to promote a property driven development that can reduce the lead times. The implementation of simulation based product development with integrated casting simulation may enable the design engineers to gain an early understanding of the products with relation to castability, and orient the subsequent design refinement so as to achieve the desired mechanical properties. This work investigates the suitability of three commercial casting simulation softwares –MAGMA 5.2, NovaFlow & Solid 4.7.5 (NFS) and Click2Cast 3.0 (C2C)–, with respect to the needs of design engineers, such as prediction of shrinkage porosity and mechanical properties with relation to the design. Simplified solidification simulations suitable for this stage were thus performed for three high pressure die cast components with different geometrical constraints. The comparability between the solidification and cooling behaviour predicted by the three softwares was studied, and showed that a reasonably good agreement between predicted solidification times by MAGMA and NFS could be obtained, albeit not between predictions by MAGMA and C2C. Predictions by the three softwares of the hot spot/porosity areas showed to have a good agreement. The calculation times by each software were compared, and MAGMA was seen to have the best performance, yielding significantly shorter times than NFS and C2C. The results obtained were also compared to experimental investigations of porosity, microstructural coarseness, and mechanical properties. There was a good agreement between the predicted hot spot areas –i.e. areas in the geometry that solidify last– and the findings of porosities in the actual castings, meaning that solidification simulations might be able to provide important information for the prediction of most of shrinkage related porosity locations that are related to the casting geometry. However, the lack of a detailed knowledge at the design stage of the casting process limits the possibilities to predict all porosities. The predicted microstructure and mechanical properties by MAGMA non-ferrous were seen to have a good agreement in trend with the experimental data, albeit the predicted values showed large differences in magnitude with the experimental data. Although, the MAGMA non-ferrous module was not developed for HPDC components, it was interesting to study if it could be applied in this context. However, the models seem to need adoption to the HPDC process and alloys. In conclusion, with a limited knowledge of the manufacturing parameters, simplified solidification simulations may still be able to provide reasonably reliable and useful information during early development stages in order to optimise the design of castings.