<p align='justify'>This thesis is devoted to the micromechanical study of the size-dependent strengthening in Transformation Induced Plasticity (TRIP) steels. Such grades of advanced high-strength steels are compelling for the automotive industry, due to their improved mechanical properties. Among others, they combine a good strength versus ductility balance. In this context, many research works have been carried out to study these grades of steels. In particular, from a numerical point of view, earlier studies within the framework of classical plasticity do not properly reproduce the strengthening levels characterizing TRIP steels and obtained experimentally.</p> <p align='justify'>In this study, the strain gradient plasticity theory presented by Fleck and Hutchinson (2001) is chosen to account for the strengthening effect resulting from the phase transformation. A two-dimensional embedded cell model of a simplified microstructure composed of small cylindrical metastable austenitic inclusions, partially undergoing the phase transformation, within a ferritic matrix is used.</p> <p align='justify'>First, the single-parameter version of the strain gradient plasticity theory under small strain assumption is used for the simulations. The impact of the higher order boundary conditions is assessed. It is shown that, when the plastic flow is unconstrained at the elasto-plastic boundaries, the transformation strain has no significant impact on the overall strengthening. The strengthening is essentially coming from the composite effect with a marked inclusion size effect resulting from the appearance during deformation of new boundaries (at the interface between parent and product phases) constraining the plastic flow.</p> <p align='justify'>Second, the multi-parameter version of the strain gradient plasticity theory, incorporating separately the rotational and extensional gradients in the formulation, is employed under small strain assumption. The effect of the plastic strain gradients resulting from the transformation strain is better captured. In particular, the results show a significant influence of the shear component of the transformation strain. An implicit confinement effect is revealed at the elasto-plastic boundaries which is partly responsible for the transformation strain effect. Size effects on the overall strengthening are also revealed, due to a combined size dependent effect of the transformation strain and of the evolving composite structure.</p><p align='justify'>Third, the extension of the strain gradient plasticity theory to a finite strain description is applied. A significant effect of the transformation strain is obtained with the multi-parameter version of the theory as well as an optimal austenite grain size improving the damage resistance of the martensite, in agreement with the typical grain size of the current TRIP-assisted steels (Jacques et al., 2007).</p>

Silicon Germanium (Si1-xGex) is an alloy semiconductor that has caught considerable attention of the semiconductor industry in the past decade. Effects of strain in thin films are the reason for this. Strain leads to considerable deformation of bands providing enhanced mobility for both electrons and holes. Another important aspect of SiGe is the reduction of band gap. This makes band gap engineering feasible in all silicon technology. Yet another attractive point is the adaptability and compatibility of SiGe to silicon process technology.In CMOS circuits the p-channel MOSFET needs more than double the area of the n-channel MOSFET due to the lower mobility of holes in silicon. Hence a p-channel hetero MOSFET (HMOSFET) is chosen as the object of this dissertation.A simple general device structure that can provide considerable enhancement in performance, compared to a conventional MOSFET, is selected. A one dimensional Poisson equation is solved for this hetero junction device. Using these results an Excel spreadsheet is used as a tool to design a complete analytical program that can provide internal as well as terminal parameters of this device. The analytical program is tested by comparing the results with ISE-TCAD numerical device simulator results. The results were found to match very well. This analytical program yields results in a fraction of the time compared to numerical programs. For the device of choice variable parameters are identified. It is found that these parameters are interconnected in many ways and trade offs between them need to be applied.From the front end of the spreadsheet input parameters can be varied and parameters like potentials, hole density and terminal characteristics can be plotted very easily while simultaneously computing other parameters like threshold voltage and saturation current.The main contribution of this dissertation research is(1) Development of a very efficient and accurate analytical program to interactively design and optimize a p-channel HMOSFET(2) A detailed understanding and explanation of various design parameters, their implications, interdependency and trade offs(3) Study and explanation of certain special characteristics ofp-HMOSFET like dual threshold voltage, low off-currents, structural limitations etc.

Les caoutchoucs naturels chargés sont largement utilisés pour leurs excellentes propriétés mécaniques comme matériaux structurels dans les pneumatiques. Leurs excellentes performances sont souvent associées à leur capacité à cristalliser sous tension. Dans le cas de la fatigue cyclique, la dissipation induite par la création et fusion de cristallites est souvent donnée comme responsable de la bonne résistance des caoutchoucs naturel à la propagation de fissures. Ce travail de doctorat s'est focalisé sur les divers mécanismes dissipatifs qui sont activé par les grandes déformations rencontrées près d'une pointe de fissure. La diffusion des rayons X a été utilisée pour caractériser les changements de structure à des échelles sub-microniques, tels que la cristallisation sous tension, la création de nano-cavités, ou la réorganisation de la charge. Un premier volet de l'étude s'est focalisé sur des tests uniaxiaux, afin d'étudier l'effet d'un changement de formulation du matériau (taux de charge, densité de réticulation), d'un vieillissement thermique, ou encore d'une élévation de la température sur ces modification de structure. Dans une seconde partie, une caractérisation complète de fissures de fatigue a été effectuée. La corrélation d'image numérique a été utilisée pour mesurer les déformations locales au voisinage d'une pointe de fissure, et les rayons X ont permis d'y associer une mesure locale de la structure du matériau. Les propriétés de fatigue de ces matériaux ont été mit en regard des structures observées afin de discuter leur relation. Il a été mis en évidence que le rôle de la cristallisation dépassait largement sa contribution à la dissipation locale. / Filled natural rubbers are widely used in structural parts such as tires for their outstanding mechanical properties. Their exceptional behavior is often associated to the ability of natural rubber chains to form a crystalline structure under tension. In the case of cyclic fatigue cracking, the dissipation added through crystallization and melting at the crack tip is often seen as the main reinforcing mechanism that reduces fatigue crack growth. This PhD work focused on all the dissipative mechanisms activated by the strain amplification near a crack tip. An extensive use of X-ray scattering was made to investigate sub-micronic changes in structure (strain-induced crystallization, cavitation, filler network). A study was made in uniaxial tension to understand the effects of material composition and test environment on these structure changes. The effect of filler volume fraction, crosslink density, thermal ageing and test temperature were considered. This study was followed by a complete description of several fatigue crack-tips. Digital image correlation was used to map the strain fields at the vicinity of the cracks, while X-ray mapping of the process zone gave information on the local changes in structure. The cyclic fatigue properties of the materials were then discussed through the knowledge acquired both in uniaxial tension and near crack-tips. It showed that the effect of strain induced-crystallization far outweigh the dissipation added.

An infrared radiometry setup has been developed based on a commercially available FTIR spectrometer for measuring mid-infrared thermal radiation. The setup was calibrated with a lab-built blackbody source. The setup was tested with a grating structure with 4-micron periodicity. Periodic microstructures using nickel and gold are fabricated on elastomeric substrates by use of strain-induced buckling of the nickel layer. The intrinsically low emissivity of gold in the mid-infrared regime is selectively enhanced by the surface plasmonic resonance at three different mid-infrared wavelengths, 4.5 µm, 6.3 µm, and 9.4 µm. As the thermal emission enhancement effect exists only for the polarization perpendicular to the orientation of the microstructures, substantially polarized thermal emission with an extinction ratio of close to 3 is demonstrated. Moreover, the elastically deformed plasmonic thermal emitters demonstrate strain-dependent emission peaks, which can be applied for future mechano-thermal sensing and dynamic thermal signature modulation.

No description available.

A thesis submitted to the School of Graduate Studies in partial fulfilment of the requirements of the Master of Applied Science degree / This thesis investigated structure-function relationships of materials designed to mimic the plant cell wall by comparing their mechanical properties measured using strain-induced buckling methods. The plant cell wall mimics are submicrometer-thick films composed of cellulose nanocrystals (CNCs) and various types of xyloglucan (XG), a common plant hemicellulose. Our goal was to establish links between film composition/architecture and elastic modulus, to better understand the interactions between plant cell wall components and their influence on mechanical properties. Three buckling methods for measuring mechanical properties of supported films were compared. All methods involved compressing a thin film deposited onto a shape memory polymer or an elastomeric substrate, through thermal shrinking or mechanical compression, respectively. Two thermal shrinking methods (constrained in one axis or unconstrained) and one compression method (using a mechanical strain stage) were used. Based on the mismatch of mechanical properties between the film and the substrate, the rigid thin film “buckles” upon compression to dissipate strain. The resulting surface wrinkle sizes are characteristic of the mechanical properties of the thin film. A Fourier analysis algorithm with Gaussian curve fitting was optimized to extract wrinkle sizes accurately and reproducibly from microscopy images to reliably quantify the elastic moduli of thin films. To select the most precise strain-induced buckling method, model layer-by-layer (LbL) thin films composed of CNCs and polyethylene imine were tested. All three buckling methods precisely quantified the elastic moduli of the films and helped us build connections between the mechanical properties and the film composition. Elastic moduli determined were 15-44 GPa (depending on composition) and films up to 350 nm-thick were tested. Based on sensitivity analyses, however, unconstrained thermal shrinking proved to be the most robust method for calculating the elastic modulus. We believe these buckling methods may find widespread use in the characterization and surface structuring of thin films for applications in biosensors, flexible electronics, point-of-care diagnostics, and for studying plant cell wall mimics. Using the unconstrained thermal shrinking method, plant cell wall mimics were constructed using LbL thin film assembly with various concentrations of CNCs and XG. Three types of XG were compared: (1) unmodified XG, (2) XG with a fraction of the galactosyl residues removed (degalactosylated), and (3) a fragmented lower molecular weight XG. It was inferred that molecular weight impacts the stiffness of XG-CNC based on adsorption conformation of XG onto CNCs, where lower molecular weight XG results in a higher modulus film (27 ± 1 vs. 19 ± 2 GPa). As well, saccharide residues of XG, specifically galactosyl, impact XG’s capacity for self-association and interaction with CNCs, because saccharide residues hinder association through their glucan backbone. This is evidenced by the higher elastic moduli calculated for degalactosylated XG-CNC films (75 ± 6, GPa), compared to native XG-CNC films (19 ± 2 GPa). This work highlights the importance of material structure as it relates to overall performance and therefore function in natural systems, such as the plant cell wall. These studies contribute to a greater understanding of the mechanical properties of the plant cell wall and serve as a basis to extend bio-based and biomimetic materials to applications such as drug delivery, packaging, and coatings. / Thesis / Master of Applied Science (MASc) / The plant cell wall boasts impressive mechanical properties, balancing seemingly opposing properties of structural strength with flexibility. These natural materials have been a source of inspiration for new material design, but the phenomena that govern interactions between components and how their structures translate into function, have yet to be fully understood. In this work, we have constructed thin multilayered films to mimic the plant cell wall, composed of cellulose nanocrystals (rod-shaped nanoparticles derived from plant cellulose) and xyloglucan (a common hemicellulose “glue”). When the films on flexible supports are compressed, they buckle into wrinkled surface patterns that can be used to calculate their mechanical properties. This investigation compares three buckling methods and supports the notion that the mechanical performance of the plant cell wall is strongly dependent on the structure of the different components and the way they interact.

Ce travail de thèse porte sur l'étude du comportement rhéologique de deux mélanges élastomères chargés en noir de carbone dans une gamme de températures rencontrée en extrusion (90°C à 40°C) et sur la modélisation du procédé d'extrusion. La caractérisation rhéologique des deux matériaux de l'étude a mis en évidence leur comportement rhéologique différent. En effet, le SBR et ses mélanges chargés avec différentes teneurs en noir de carbone montrent une courbe d'écoulement en deux branches stables (branche I et branche II) séparées par un plateau. La présence d'un glissement, faible en branche I et important en branche II, a été mis en évidence. Le second matériau, le NR (pur et chargé), montre, quant à lui, un comportement en deux parties distinctes : une partie sans rhéodurcissement et une autre avec rhéodurcissement dû à la mise en place d'une cristallisation sous contrainte de cisaillement. L'existence d'un temps caractéristique de cristallisation, diminué par la vitesse de cisaillement, a pu être mise en évidence. Les lois de comportement pour chacun des matériaux ont été déterminées. La réalisation d'une superposition temps/taux de charge a permis de définir une loi de comportement générale ; dans la gamme de température de 40°C à 90°C pour les mélanges SBR/noir de carbone et dans la gamme de température de 50°C à 90°C pour le NR. La loi rhéologique déterminée pour le mélange SBR chargé à 33% en masse en noir de carbone a été implémentée dans un modèle 1D et 3D. Les résultats numériques ont alors été comparés aux résultats expérimentaux, obtenus suite à des essais sur une extrudeuse instrumentée. Le modèle 1D s'est avéré insuffisant pour rendre compte des phénomènes se déroulant lors du procédé d'extrusion. L'implémentation d'un modèle 3D a été effectuée afin de prendre en compte des phénomènes tels que les effets de bords et les gradients de températures. L'importance de prendre en compte le phénomène de glissement dans le modèle 3D a été démontrée. / The present work deals with the characterization of the rheological behaviour of two rubbers filled with carbon black in a range of temperature encountered in extrusion (90°C to 40°C) and the modelling of the single screw extrusion process. The rheological characterization of the two rubbers shows behaviour totally different. The synthetic SBR rubber and SBR compounds filled with carbon black (with different amounts, from 16 to 33 wt%) showed a flow curve that can be divided in two stable parts (branch I and branch II) separated by a plateau. The occurrence of wall slip, low in branch I and important in branch II has been highlighted. The second rubber of the study, natural rubber, showed a different behaviour with a strain hardening due to the occurrence of a strain induced crystallization. The flow curve can be thus divided in two different parts: the first part without strain hardening and the second part with this phenomenon. The existence of a characteristic crystallization time, reduced by the shear rate, has been evidenced. By analysing data respectively on branch I for SBR and without strain hardening for NR, viscosity curves for all tested materials have been defined. General viscosity law have been proposed: for the SBR compound filled with carbon black valid in the range of temperature from 40°C to 90°C and for NR in the range of temperature from 50°C to 90°C. The rheological law defined for the SBR compound filled 33%wt with carbon black has been implemented in two different models: 1 D and 3D. Numerical results have been compared to experimental results obtained after experimentations on instrumented single screw extruder. The 1D approach was not sufficient to take into account all phenomena occurring during extrusion process. Then a 3D approach has been developed in order to take into account some effects as side effects and temperature gradients. The presence of the slip phenomenon in the modeling has been proved.

Le comportement en fatigue du polychloroprène, un élastomère synthétique, est fonction de : • l’endommagement lié à l’amorçage et la propagation de fissures. • le renforcement induit par la cristallisation sous tension. • l’auto-échauffement de l’éprouvette. L’équilibre entre ces trois phénomènes a été investigué en suivant l’évolution de l’aire de l’hystérésis et de la raideur au cours d’essais de fatigue à amplitude constante. Des essais après un précyclage ont permis de modifier cet équilibre en travaillant sur une éprouvette ayant un historique de chargement. La présence initiale de fissures dans une éprouvette dont l’autoéchauffement est très faible induit un effet de la cristallisation sous tension plus marqué. Une nette amélioration des caractéristiques en fatigue est ainsi observée. Des essais à amplitude variable ont également permis d’observer l’influence de la modification de l’équilibre entre ces trois phénomènes. Ces observations ont mené à la construction du diagramme raideur/hystérésis permettant l’unification des courbes à une sollicitation donnée, indépendamment de l’historique de chargement. Une approche par amorçage de fissure a permis de mettre au point une méthode issue de l’analyse de l’évolution de l’aire de l’hystérésis pour déterminer très rapidement la limite à 106 cycles en fatigue du CR. / The fatigue behavior of the polychloroprene, a synthetic rubber, relies on : Damage caused by the initiation and the propagation of the cracks The reinforcement caused by the strain-induced crystallization The self-heating of the sample The equilibrium between those three phenomena was investigated by following the evolution of the hysteresis area and the stiffness during constant amplitude fatigue tests Some pre cycling tests allowed the modification of this equilibrium by working on a sample with a loading history. The presence at the beginning of the fatigue test of cracks in the sample, which undergoes at that time of the test a very low self-heating, is subjected to a more important effect of the strain induced crystallization. The fatigue characteristic of the CR is well enhanced. Some variable amplitude tests made possible the observation of the consequences of the modification of this equilibrium. It leads to the drawing of a stiffness/hysteresis diagram allowing the unification of the curves at a given solicitation, independently of the loading history. A fast method derived from the study of the evolution of the hysteresis area to find the fatigue limit at 106 cycles using the crack initiation approach has been presented.

No description available.

L’objectif de cette thèse est de contribuer à la compréhension et à la modélisation de la cristallisation induite. Le phénomène est caractérisé pour deux matériaux : le PET et le caoutchouc naturel réticulé. Les conditions favorables au phénomène, de type caoutchoutique sont déterminées par analyses calorimétriques et spectroscopiques. La microstructure qui se développe au cours de la déformation est observée par diffraction des rayons X.Le PET est déformé en traction uni- et biaxiale. Une partie des étirages est suivie d’une relaxation des contraintes, une autre est suivie d’une trempe rapide. Il ressort de l’étude que l’étirage du PET dans ces conditions n’aboutit pas à l’obtention d’un cristal PET avec toutes les périodicités qui lui sont propres.Le caoutchouc naturel est déformé en traction uniaxiale et en cisaillement précédé d’un étirage uniaxial. En cisaillement, la phase cristalline obtenue au cours du pré-étirage ou du cisaillement tourne et tend à s’orienter comme les directions des déformations principales mais avec un retard angulaire. L’extension principale est utilisée pour étudier la phase cristalline obtenue pour les différents modes de sollicitation.Un modèle de comportement visco-hyperélastique, décrit dans le cadre de la thermodynamique des processus irréversibles, est étendu afin de reproduire le durcissement mécanique lié au développement d’une phase organisée/cristalline. Le modèle permet de reproduire les différents comportements mécaniques observés expérimentalement. / The present PhD thesis aims at a better understanding and modeling of strain-induced-crystallization. The phenomenon is characterized for two polymers: PET and crosslinked natural rubber. Strain conditions leading to strain-induced-crystallization are determined by thermal and dynamic mechanical analysis. The developing microstructure is observed by X-ray scattering.The PET is stretched in uni- and biaxial tension. A part of samples is rapidly quenched after stretching and another is submitted to a stress relaxation after stretching. The studies demonstrate that the stretching of PET does not enable the formation of a complete PET crystal with all its own families of planes.The crosslinked natural rubber is stretched in uniaxial tension and in shear preceded by uniaxial stretching. In shear, the crystalline phase, appeared during the pre-stretching or during the shear rotates and has a tendency to orient as the directions of the principal strains. The principal elongation is used to compare the crystallization under the different stresses.A constitutive modeling for visco-hyperelastic behaviors, in a complete thermodynamics framework of irreversible processes, is extended in order to reproduce le mechanical hardening related to the development of an organized/crystalline phase. The modeling successes in reproducing the experimental behaviors in uploading/unloading for various strain conditions.