Dissertations / Theses on the topic 'Plastic slip'

To better understand consequences of classical assumptions regarding deformation mechanisms at the mesoscale, experimental observations of mesoscale deformation are presented. In light of actual micrographics of deformed polycrystals, the Von Mises criterion which states that 5 independent plastic deformation sources are needed at each material point to satisfy compatibility is studied, and the consequences of violating this assumption are presented through comprehensive parametric studies. From these studies, it can be concluded that not only are 5 independent plastic deformation sources not needed or observed at each point, but if less than 5 sources are allowed to be active a new physical understanding of a mechanism for kinematic hardening emerges. Furthermore, for enhanced subgrain rotation and evolution the Von Mises criterion must be violated. The second focus of this work is looking at studies, experiments, and models of mesoscale deformation in order to better understand controlling deformation length scales, so that they can be fed into a combined top-down, bottom-up, non-uniform crystal plasticity model that captures the variability provided by the mesoscale during deformation. This can in turn be used to more accurately model the heterogeneity provided by the response of each grain. The length scale intuited from insight into mesoscale deformation mechanisms through observation of experiments and analytical models is the free slip line length of each slip system, which informs non-uniform material parameters in a crystal plasticity model that control the yielding, hardening, and subsequent softening of each individual slip system. The usefulness of this non-uniform multiscale crystal plasticity model is then explored with respect to its ability to reproduce experimentally measured responses at different strain levels for different size grains. Furthermore, a "Mantle-Core" type model which combines both the non-uniform material parameter model and the limited slip model is created, in which the majority of plastic deformation is accommodated near the grain boundary under multi-slip, and uniform plastic deformation occurs in the bulk dominated by double or triple slip. These models are compared for similar levels of hardening, and the pole figures that result from their deformation are compared to experimental pole figures. While there are other models that can capture the heterogeneity introduced by mesoscale deformation at the grain scale, this combined top-down, bottom-up multiscale crystal plasticity model is by far one of the most computationally efficient as the heterogeneity of the mesoscale is does not emerge by introducing higher order terms, but rather by incorporating the heterogeneity into a simple crystal plasticity formulation. Therefore, as computational power increases, this approach will be among the first that will be able to perform accurate polycrystal level modeling while retaining the heterogeneity introduced by non-local mesoscale deformation mechanisms at the sub-grain scale.

Many metals used in modern engineering exhibit anisotropy. A common assumption when modelling anisotropic metals is that the level of anisotropy is fixed throughout the calculation. As it is well understood that processes such as cold rolling, forging or shock loading change the level of anisotropy, it is clear that this assumption is not accurate when dealing with large deformations. The aim of this project was to develop a tool capable to predict large deformations of a single crystal or crystalline aggregate of a metal of interest and able to trace an evolution of anisotropy within the material. The outcome of this project is a verified computational tool capable of predicting large deformations in metals. This computational tool is built on the Crystal Plasticity Finite Element Method (CPFEM). The CPFEM in this project is an implementation of an existing constitutive model, based on the crystal plasticity theory (the single crystal strength model), into the framework of the FEA software DYNA3D® . Accuracy of the new tool was validated for a large deformation of a single crystal of an annealed OFHC copper at room temperature. The implementation was also tested for a large deformation of a polycrystalline aggregate comprised of 512 crystals of an annealed anisotropic OFHC copper in a uniaxial compression and tension test. Here sufficient agreement with the experimental data was not achieved and further investigation was proposed in order to find out the cause of the discrepancy. Moreover, the behaviour of anisotropic metals during a large deformation was modelled and it was demonstrated that this tool is able to trace the evolution of anisotropy. The main benefit of having this computational tool lies in virtual material testing. This testing has the advantage over experiments in time and cost expenses. This tool and its future improvements, which were proposed, will allow studying evolution of anisotropy in FCC and BCC materials during dynamic finite deformations, which can lead to current material models improvement.

A model for stress and strain accumulation in strike slip earthquake faults is presented in which a finite width cuboidal fault region is embedded between two cuboidal tectonic plates. Elasto-plastic continuum constitutive equations model the gouge in the fault and the tectonic plates are linear elastic solids obeying the generalised Hooke's law. The model predicts a velocity field which is comparable to surface deformations. The plastic behaviour of the fault material allows the velocities in the tectonic plate to increase to values which are independent of the distance from the fault. Both of the non-trivial stress and strain components accumulate most significantly in the vicinity of the fault. The release of these strains during a dynamic earthquake event would produce the most severe deformations at the fault which is consistent with observations and the notion of an epicenter. The accumulations in the model, however, are at depths larger than would be expected. Plastic strains build up most significantly at the base of the fault which is in yield for the longest length of time but additionally is subject to larger temperatures which makes the material more ductile. The speed of propagation of the elasto-plastic boundary is calculated and its acceleration towards the surface of the fault may be indicative of a dynamic earthquake type event.

Au sein de la lithosphère, la transition entre déformations fragile et plastique des roches s’effectue dans le régime semicassant. Comprendre le comportement des failles naturelles dans le régime semi-cassant est fondamental puisque d’importants séismes nucléent à la base de la zone sismogénique, à des conditions de pression et température proches de celles de la transition fragile-plastique. Pendant un séisme, l’énergie élastique accumulée lors de la période intersismique est dissipée au sein de l’interface de glissement par des processus frictionnels et de fracture, le reste étant relâché sous forme d’ondes sismiques. Ce budget énergétique est influencé par la déformation des surfaces de failles pendant des glissements lents à rapides, et plus particulièrement par des processus de chauffage, invisibles aux yeux de la sismologie. Afin d’étudier la déformation semi-cassante des roches et le budget énergétique des séismes, nous avons effectué des expériences de reproduction de séismes au laboratoire, en conditions triaxiales, à l’aide de failles expérimentales de différentes lithologies. Nous avons étudié l’influence de la pression, de la vitesse de déformation, de la température et de la rugosité sur la stabilité des failles le long de la transition fragile-plastique et exploré la dynamique des séismes au laboratoire en mesurant la quantité de chaleur produite sur une faille durant un cycle sismique. Deux conclusions principales émanent de ces travaux. D’abord, les séismes au laboratoire peuvent se déclencher au sein de roches déformées plastiquement dans le régime semi-cassant. Les glissements observés sont majoritairement contrôlés par la rugosité de la faille. Pour finir, lors d’un cycle sismique, les failles opèrent une transition depuis un stade avec de multiples aspérités radiant peu d’énergie, à un stade où elles évoluent comme une aspérité unique, radiant un maximum d’énergie
In the lithosphere, the transition from brittle to plastic rock deformation corresponds to the semi-brittle regime. Understand how natural faults behave in the semi-brittle regime is fundamental to explain why large earthquakes generally nucleate at the base of the seismogenic zone, found at pressure and temperature conditions close to the predicted brittle-plastic transition. During an earthquake, part of the released elastic strain energy stored during the interseismic period is dissipated within a fault slip zone by frictional and fracturing processes, the rest being radiated away via elastic waves. This energy balance is influenced by the deformation of fault surfaces during slow or fast sliding, especially by frictional heating processes which could not be resolved by seismology. To investigate semi-brittle deformation and the energy balance of natural earthquakes, we performed laboratory earthquakes in triaxial conditions on experimental faults of various lithologies. We studied the influence of the confining pressure, axial loading rates, temperature and fault roughness on fault stability across the brittle-plastic transition and investigate the dynamics of laboratory earthquakes by measuring frictional heat dissipated during the propagation of shear instabilities. The main conclusions are twofold. First, laboratory earthquakes may nucleate on inherited fault interfaces at brittle-plastic transition conditions and fault slip behavior is mainly influenced by roughness. Second, we conclude that during sliding, faults exhibit a transition from a weak stage with multiple strong asperities and little overall radiation, to a highly radiative stage during which the fault behaves as a single strong asperity

Plasticity in crystalline solids is controlled by the microscopic line defects known as “dislocations”. Decisive role of dislocations in crystal plasticity in addition to fundamentals of plastic deformation are presented in the current thesis work. Moreover, major features of numerical modeling method “Discrete Dislocation Dynamics (DDD)” technique are described to elucidate a powerful computational method used in simulation of crystal plasticity. First part of the work is focused on the investigation of strain rate effect on the dynamic deformation of crystalline solids. Single crystal copper is chosen as a model crystal and discrete dislocation dynamics method is used to perform numerical uniaxial tensile test on the single crystal at various high strain rates. Twenty four straight dislocations of mixed character are randomly distributed inside a model crystal with an edge length of 1 µm subjected to periodic boundary conditions. Loading of the model crystal with the considered initial dislocation microstructure at constant strain rates ranging from 103 to 105s1 leads to a significant strain rate sensitivity of the plastic flow. In addition to the flow stress, microstructure evolution of the sample crystal demonstrates a considerable strain rate dependency. Furthermore, strain rate affects the strain induce microstructure heterogeneity such that more heterogeneous microstructure emerges as strain rate increases. Anisotropic characteristic of plasticity in single crystals is investigated in the second part of the study. Copper single crystal is selected to perform numerical tensile tests on the model crystal along two different loading directions of [001] and [111] at two high strain rates. Effect of loading orientation on the macroscopic behavior along with microstructure evolution of the model crystal is examined using DDD method. Investigation of dynamic response of single crystal to the mechanical loading demonstrates a substantial effect of loading orientation on the flow stress. Furthermore, plastic anisotropy is observed in dislocation density evolution such that more dislocations are generated as straining direction of single crystal is changed from [001] to [111] axis. Likewise, strain induced microstructure heterogeneity displays the effect of loading direction such that more heterogeneous microstructure evolve as single crystal is loaded along [111] direction. Formation of slip bands and consequently localization of plastic deformation are detected as model crystal is loaded along both directions.

QC 20151015

It is becoming generally accepted that wall slip, the loss of adhesion of a polymer to a solid substrate, occurs when the shear stress exceeds a critical value. Wall slip probably plays a much more important role in plastics processing than has been previously thought. For example, the extrudate distortion called sharkskin is thought to involve slip. Recent experiments have shown that this phenomenon is highly material dependant. The surface chemistry of both the molten polymer and the bounding surface interface play important roles in both the occurrence and magnitude of wall slip. The initial objectives of the research were to study slip in further detail, concentrating on the effects of the molecular weight and molecular weight distribution of the polymer. The effects of surface properties were also to be investigated. The research was performed using a sliding plate rheometer. Erratic stress signals were observed when the shear stress exceeded a critical level, and there was strong evidence that these were related to a highly complex type of slip. Thus, a steady slip velocity, which could be simply related to the shear stress, could not be determined. It was concluded that wall slip is, in general, a chaotic process with a strong dependence on initial conditions, sample history, and boundary conditions. Further research is suggested to elucidate this phenomenon.

Experiments were performed in a sliding plate rheometer and in capillaries and slits with several polymer melts, particularly polyethylenes, to determine the conditions for the onset of slip and melt fracture. In the sliding plate rheometer various shear tests were used to determine the relationship between the slip velocity and wall shear stress, and based on the experimental observations both steady state and dynamic slip models are proposed that are consistent with experimental observations. In the capillary flow studies the dependence of the slip velocity on wall shear stress, pressure, temperature and molecular parameters of molten polyethylenes was studied and the results were used to formulate a general slip velocity model. Because the slip velocity depends on pressure and thus varies with position in capillary flow, the Mooney procedure in determining the slip velocity is not appropriate. A modified Mooney technique is proposed to analyse the capillary data in cases where the slip velocity is a function of both wall shear stress and pressure. Using the slip velocity model, the steady state and unsteady state equation of motion was solved in capillary flow, and the calculated results were found to agree with the experimental results to a satisfactory degree. The oscillating flow regime was studied in detail, and the slip flow model was found to predict several features of this flow regime very well. Finally, the effect of interface conditions (surface coatings and metal of construction of slits) on both wall slip and melt fracture was studied.

Experiments were performed on a slit die rheometer to determine the effect of pressure on the wall slip of linear polyethylene. A novel shear stress transducer was developed to measure local wall shear stress at several locations along the die. It was found that above the critical shear stress for the onset of slip, the shear stress distribution along the die was nonuniform. Furthermore, it was discovered that the local wall shear stress near the exit of the die was greater than the nominal, or length-averaged, shear stress within the die as calculated from the pressure drop. Teflon$ sp circler$ wall coatings were used to promote slip by lowering the critical shear stress for the onset of slip. Relative local slip velocities were calculated and found to be a function of pressure and wall shear stress.
A new semi-empirical model for the pressure dependence of slip was developed based on the effect of pressure on the work of adhesion and the work needed for flow. The new model indicates that pressure can both suppress and promote slip depending on the level of stress involved. At low pressures, and for a given shear stress, slip is markedly suppressed due to the increase in the work of adhesion. As pressure increases, however, the work needed for flow overcomes the work of adhesion, and slip dramatically increases. However, at higher pressure, the effect of pressure on slip becomes weaker. Numerical simulation results with the new model predict the existence of a local maximum in the shear stress distribution along the die for flow with slip.

Etude du comportement en fatigue plastique de l'acier inoxydable austenitique 316l fatigue sous vide a 20, 300 et 600c dans une gamme de deformations plastiques imposees. Analyse par microscopie electronique en transmission des sous-structures a20 et 600c, les microstructures sont constituees de bandes de glissement persistantes, de murs-canaux des labyrinthes et des structures en echelle evoluant vers la structure cellulaire. A300c, une structure de contraste en cotes de velours est observee. Cette structure est fonction de la deformation plastique et provoque un durcissement cyclique secondaire. Presentation de modeles de formation et d'evolution de microstructures de fatigue

L'apparition récente de nouveaux matériaux dans les structures de chaussée associée à une diminution de l'épaisseur des couches de surface et une augmentation du chargement des poids lourds et de leur fréquence de passage a entrainé de nouvelles pathologies de dégradation. Outre les problèmes d'orniérage bien connus, apparaissent désormais des fissures descendantes (top down cracking) ainsi que des problèmes de décohésion aux interfaces. Ces nouvelles pathologies entrainent des dépenses considérables sur l'ensemble du réseau (environ 15 milliards d'euros par an), particulièrement en zones urbanisées plus sujettes aux dégradations de surface et ne permettent pas d'estimer convenablement les durées de vie de la chaussée, le plus souvent surestimée dans les méthodes de dimensionnement actuelles. Ce travail de doctorat propose une nouvelle approche du contact pneu-chaussée permettant de mieux appréhender les contraintes principales et résiduelles dans une structure de chaussée bitumineuse. A l'aide d'un outil numérique rapide de calcul basé sur une approche semi-analytique (« Semi-Analytical Methods » (SAM)), la géométrie précise du pneumatique est intégrée afin d'obtenir une répartition de pression de contact ainsi qu'un cisaillement surfacique réelle sur la chaussée. Dans un premier temps, un modèle de contact roulant tractif élastique est implémenté pour des cas théoriques simples et validé par des résultats analytiques et numériques de la littérature. Ensuite, ce modèle est étendu pour prendre en compte le comportement élasto-plastique des corps en contact. Ce dernier est comparé à un résultat numérique basé sur la méthode des éléments finis issu de la littérature. Les résultats, pour une application contact pneu-chaussée, montrent une répartition non homogène des contraintes dans la structure et principalement dans les premiers centimètres sous la surface avec des niveaux beaucoup plus importants que peuvent le prédire les modèles actuels qui utilisent une charge uniformément répartie. La pression de contact est comparée aux mesures effectuées par un système nommé TekScan et les champs mécaniques en sous couches sont comparés à ceux d'Alizé-LCPC dans le cas d'une structure simple. Les cisaillements surfaciques sont déterminés dans le cas du roulement tractif. Une application est effectuée sur la modélisation des dégradations des chaussées en surface. Dans un premier temps, des analyses sur le comportement de la chaussée en surface sont effectuées pour une couche de béton bitumineux semi grenu (BBSG) semi-infinie supposée élastique, homogène sous conditions d'accélération, de freinage et de virage. Pour des études sur le top down cracking, des déformations et directions principales sont déterminées et analysées. Ensuite, le modèle de contact élasto-plastique est appliqué sur une couche semi-infinie de grave bitume GB3. Des déformations et contraintes résiduelles générées dans la structure sont déterminées en vue d'une analyse sur les ornières d'instabilité. Une fois validés, ces résultats permettront d'estimer plus fidèlement la durée de vie résiduelle des chaussées mais également de comprendre et d'éviter les mécanismes de dégradation en surface ou proche de la surface
The recent appearance of new materials in road structures associated with surface layers thickness decreasing and the increasing of trucks loading and their passage frequency has led to new pathologies of degradation. In addition to the well-known rutting problems, top down cracking is now appearing as well as problems of decohesion at the interfaces. These new pathologies led to considerable expenditure on the entire network (around 15 billion euros per year), particularly in urbanized areas that are more prone to surface damage and do not make it possible to adequately estimate the lifetimes of the roadway, most often overestimated in current design methods. This doctoral work proposes a new approach of the tire-road contact allowing for better apprehend of the main and residual stresses in a bituminous pavement structure. Using a fast numerical tool based on a semi-analytical approach ("Semi-Analytical Methods" (SAM)), the precise geometry of the tire is integrated in order to obtain a real contact pressure distribution as well as surface shear on the pavement surface. Initially, an elastic tractive rolling contact model is implemented for simple theoretical cases and validated by analytical and numerical results from the literature.Then, this model is extended to take into account the elastoplastic behavior of the bodies in contact. This is compared to a numerical result based on the nite element method from the literature. The application for tire-pavement contact results, show a non-uniform distribution of stresses in the structure and mainly in the rst centimeters below the surface with much higher levels than can be predicted by current models that use a uniformly distributed load. The contact pressure is compared to the measurements made by a system called TekScan and the mechanical elds in sublayers are compared to those of Alizé-LCPC in the case of a simple structure. The surface shears are determined in the case of tractive rolling. An application is carried out on the modeling of surface pavement damage. Firstly, analyzes of the behavior of the surface pavement are carried out for a semi-innite semi-grit asphalt concrete layer supposed to be elastic, homogeneous under conditions of acceleration, braking and turning. For studies on top down cracking, principals deformations and directions are determined and analyzed. Then, the elastoplastic contact model is applied on a semi-innite asphalt agragate layer. Deformations and residuals stresses generated in the structure are determined for an analysis on the instability ruts. Once validated, these results will make it possible to more accurately estimate the residual life of pavements but also to understand and avoid surface or near surfacedegradation mechanisms

Etude experimentale sur des monocristaux en superalliage à base de nickel soumis à des essais à chaud d'écrouissage cyclique à basse fréquence. Observation d'un phénomene d'instabilité plastique accompagné de la formation de lignes de glissement cristallographiques sur le fut de l'éprouvette. Modélisation du comportement élasto-plastique par une généralisation de la loi de schmid. L'extension de la loi de normalite permet de déterminer la direction de l'écoulement plastique et les systèmes actifs du glissement octaédrique. C'est la bifurcation locale de cette loi de comportement qui rend compte de l'instabilite observée

Analyse des microstructures de deformation dans l'alliage de structure l1::(2) en fonction de la temperature afin de comprendre les causes du comportement plastique anormal. A basse temperature, les superdislocations dominent. Analyse de leur structure fine et de leur role de verrou en glissement octaedrique. Etude de l'evolution des structures des dislocations lors de l'augmentation de la temperature d'essai

La modélisation de l'évolution des textures de déformation des métaux hexagonaux, tels que le titane et le magnésium, et de l'anisotropie plastique des tôles manufacturées nécessitent la connaissance de leur comportement plastique lors des opérations classiques de mise en forme. Pour ce faire, nous caractérisons expérimentalement les mécanismes de déformation mis en jeu au cours de l'essai de compression plane sur plusieurs orientations de monocristaux de titane déformés à l'ambiante et de magnésium déformés à l'ambiante et à 400°C, en conditions de glissement et de maclage multiple, que nous comparons aux simulations numériques de type Taylor-Bishop et Hill généralisées. Ainsi, ce travail se distingue par la réalisation de monocristaux de titane de grandes dimensions, obtenus par cyclage en température autour du transus α↔β. Il a également donné lieu à la mise au point d'une nouvelle technique de détermination précise et univoque des systèmes de maclage par microdiffraction EBSD et analyse des traces associées. Les résultats de nos expériences nous donnent les valeurs suivantes des rapports de cissions critiques résolues sur les différents systèmes: Pour le magnésium, en prenant la cission critique résolue pour le glissement basal égale à 1 : 20 pour les glissements prismatique et pyramidal, 3 pour le maclage {1 0 -1 2} et 10 pour les maclages {1 0 -1 1} et {1 0 -1 3}. Pour le titane, la valeur de référence étant la cission critique résolue pour le glissement prismatique: 5 pour le glissement basal, 4 pour le glissement pyramidal →a, pour le glissement pyramidal →c+→a 2 en traction et 3 en compression, 1,5 pour les maclages {1 0 -1 2} et {1 1 -2 1} et 5 pour le maclage {1 1 -2 2}. Nous avons ainsi pu corréler l'évolution des textures de déformation obtenues au comportement individuel des grains de l'agrégat polycristallin.

Etude experimentale sur le superalliage soumis a une charge cyclique uniaxiale suivant la direction de croissance 001 et les orientations 111, 011 et 123 a 650 c et 900c. Analyse des courbes d'ecrouissage cyclique. Caracterisation des systemes de glisement associes aux grandes deformations plastiques. Description par un modele de l'influence quantitative de l'orientation sur les courbes, d'ecrouissage cyclique. Mise en evidence du role preponderant de la periode de propagation des fissures sur l'endommagement. Prevision theorique de la duree de vie en fatigue en utilisant des donnees de fissuration etablies pour une orientation donnee

La these se compose de deux parties : etude du glissement des dislocations dans insb et etude de leur montee sous irradiation aux electrons. Le glissement est etudie par microscopie electronique. Mesure de la vitesse des dislocations et de la contrainte locale et comparaison avec les resultats obtenus par d'autres methodes. D'autre part, observation de l'apparition et de la croissance de defauts sous irradiation des electrons de 1 mev, dans un microscope a haute tension

Etude detaillee de l'evolution de la structure fine des dislocations dans une large gamme de temperature autour du pic de limite elastique (600-700c). Analyse cristallographique de la structure ordonnee l1::(2) et des defauts plans de cette structure. Presentation, a partir de cette analyse, des resultats des simulations atomiques de paroi d'antiphase. Calcul de forme et d'energie de dislocations en elasticite anisotrope. Analyse du mecanisme de formation des defauts d'empilement. Etude, en fonction de la temperature d'essai, de la structure fine des dislocations. Influence d'une variation de composition sur la morphologie des dislocations

L'alliage de magnésium AZ31 présente une très faible densité. Cette caractéristique en fait un matériau apprécié pour la conception de structures légères. La limitation principale de son utilisation industrielle est sa mauvaise formabilité et ce en raison de la texture cristallographique des tôles qui s'avère être peu adaptée aux procédés de mise en forme tel que l'emboutissage. Cette texture résultant du laminage initial, l'ambition de ce travail est de la modifier en utilisant la technique de laminage asymétrique et de mesurer l'impact de cette voie sur la formabilité de l'alliage. Il a été montré que l'asymétrie, produite par un différentiel de vitesses de rotation des cylindres du laminoir, induit systématiquement de fortes instabilités plastiques sous forme de bandes de cisaillement. Des techniques de cartographie sur microscope électronique en transmission (ACOM) et à balayage (EBSD) ainsi que des analyses de texture par DRX ont été utilisées pour analyser les mécanismes physiques concourant à l'émergence de cette instabilité. Il résulte de cette analyse que l'asymétrie du laminage provoque une forte activité du système de glissement basal que ne compense ni les autres systèmes ni le maclage. Ceci conduit à une localisation marquée de la déformation plastique et à la ruine du matériau.

Inspired by key experimental and analytical results regarding Shape Memory Alloys (SMAs), we propose a modelling framework to explore the interplay between martensitic phase transformations and plastic slip in polycrystalline materials, with an eye towards computational efficiency. The resulting framework uses a convexified potential for the internal energy density to capture the stored energy associated with transformation at the meso-scale, and introduces kinetic potentials to govern the evolution of transformation and plastic slip. The framework is novel in the way it treats plasticity on par with transformation.

We implement the framework in the setting of anti-plane shear, using a staggered implicit/explict update: we first use a Fast-Fourier Transform (FFT) solver based on an Augmented Lagrangian formulation to implicitly solve for the full-field displacements of a simulated polycrystal, then explicitly update the volume fraction of martensite and plastic slip using their respective stick-slip type kinetic laws. We observe that, even in this simple setting with an idealized material comprising four martensitic variants and four slip systems, the model recovers a rich variety of SMA type behaviors. We use this model to gain insight into the isothermal behavior of stress-stabilized martensite, looking at the effects of the relative plastic yield strength, the memory of deformation history under non-proportional loading, and several others.

We extend the framework to the generalized 3-D setting, for which the convexified potential is a lower bound on the actual internal energy, and show that the fully implicit discrete time formulation of the framework is governed by a variational principle for mechanical equilibrium. We further propose an extension of the method to finite deformations via an exponential mapping. We implement the generalized framework using an existing Optimal Transport Mesh-free (OTM) solver. We then model the $\alpha$--$\gamma$ and $\alpha$--$\varepsilon$ transformations in pure iron, with an initial attempt in the latter to account for twinning in the parent phase. We demonstrate the scalability of the framework to large scale computing by simulating Taylor impact experiments, observing nearly linear (ideal) speed-up through 256 MPI tasks. Finally, we present preliminary results of a simulated Split-Hopkinson Pressure Bar (SHPB) experiment using the $\alpha$--$\varepsilon$ model.

Alloys of titanium are highly preferred materials for their excellent strength to weight ratio but the tribological issues while using them has been posing challenging issues for the tribological analyst, which are still areas of active research. Ti-6Al-4V (Ti64) is the most popular alloy of titanium and our understanding of the fundamental mechanisms of wear and friction of this alloy is still not complete. Previous investigations related to the tribology of these alloys have suggested a synergistic effect of plastic deformation and tribo-oxidation. The present investigation described in this thesis explores the existence of one more mode, namely the formation of a Mechanically Mixed Layer (MML). The thesis examines the effect of these modes one by one and analyses the synergistic effect of these mechanisms, and also the effect of heat generation during sliding. The tribological condition existing have been varied by doing wear experiments using Ti64 pins sliding against alumina and SS316L (controls MML), diameter of pin (expected to control debris entrapment and thus MML formation), tribo-system (horizontal disc Vs vertical disc, which is also expected to control debris entrapment and thus MML formation), environment (ambient and vacuum, expected to control tribo-oxidation) and sliding speed (expected to control interface temperature and thus plastic deformation mechanism and tribo-oxidations). The division of the main chapters has been so made to present the findings spread over Chapters 5-8, with each chapter dealing with specific tribological test conditions. In each chapter, results from the tribological experimentations in the form of wear and friction are presented, together with the characterization methods which throw light into the tribological mechanisms. These characterization methods include Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDAX), X-Ray Diffraction (XRD) and Electron probe micro-analyzer (EPMA). Wherever possible, the debris collected from the experiments have been subjected to morphological and detailed chemical analysis, and a feature which has not been explored much in detail by tribological investigators, but having a promising potential. Experimental results from tribological testing when Ti64 pins slides against two different materials (Alumina and SS316L) in pin-on-disc tribometers under two different environmental conditions (ambient /vacuum) are analyzed. Each set of experiments looks at two different effects - (1) the effect of sliding speed on the tribological behavior while using a pin of a fixed diameter (all other parameters remaining the same) and (2) the effect of using pins of different diameters for a given set of parameters. Three different pin-sizes were employed (2.1 mm. 4.6 mm and 6.6 mm), the normal loads on these pins were changed according to the pin-size used so that all experiments were done at the same contact pressure (2.8 MPa). By performing the experiments against the ceramic disc (alumina) under vacuum conditions, the effect of this plastic deformation is studied in isolation because the possibility of the Tribo Chemical Reaction (TCR) due to oxidation is inhibited and no MML was found to be formed due to poor compatibility of mixing between the metallic pin and the ceramic disc. For the low speeds/strain rates experiments, the effect of plastic deformation as influenced by the adiabatic shear banding is seen to influence wear which progressively changes to temperature induced plastic deformation and wear. The situation is found to be different when we change the environmental conditions from vacuum to ambient for the same tribo-combination. The tests shows a reduction in wear rate with speed, and this is due to the oxide formations due to TCR as confirmed from the SEM/EDAX characterization. In contrast to previous experiments under vacuum, these permit the effect of TCR also to influence the tribological behavior. The scenario changes when the alumina disc is replaced by a metallic one (SS316L) and tests carried out in vacuum, as the MML was found to be formed with this tribo-pair. Because of the mutual affinity of the materials in the tribo-pair, the wear damage is severe in this case and the flash temperatures crossing the phase transition temperature (~880oC) for Ti64 at high speeds. The growth of the β phase with increase in the sliding (temperature) conditions is captured from the XRD spectra of the wear debris. Synergistic effect of all these mechanisms (plastic deformation, MML, and TCR) is permitted by conducting experiments with Ti64 pin against stainless steel and in ambient conditions. A comparison of the tribological response by presenting results when experiments are run over a range of speeds while using different sized pins under ambient conditions (and compared with similar results in vacuum) while using SS316L disc serve to demarcate the differences in the wear modes which are active/inactive depending on the tribological conditions. In addition a study incorporating the effect of frictional heating and its influence on the tribological phenomena is analyzed. Main conclusions from the thesis are: The wear resistance of Ti64 alloy when sliding against SS316L is found to be influenced by Strain Rate Response (SRR), Tribo Oxidation (TO), Mechanically Mixed Layer (MML) and the prevailing heat flux conditions at the contact. The wear rates were found to decrease marginally with sliding speeds (strain rates) up to a certain speed, which is ascribed to reduction in adiabatic shear band intensity with increase in strain rate. Adiabatic Shear Band (ASB), which allows easy crack propagation, intensity reduces as temperature of deformation of Ti64 is increased. From the results it can be confirmed that the propensity for formation of MML depends on compatibility of the disc and the pin material. The contribution due to of entrapment and retention of debris in the contact zone also would influence formation of the MML. The effect of frictional heating plays an influential role as it can affect the factors (TO, ASB, MML) governing the tribological response. The sensitivity to temperature, which is a marked feature of this alloy in undergoing softening, as confirmed by previous researchers, is reflected in the experimental results. Since the main factor that triggers the micro-structural instability is the energy dissipation that accompanies deformation more fundamental research which can improve the thermal transport properties of this alloy, would be the future scope of work of this thesis. Also, the unique composition of the MML which offers high wear resistance under specific operating conditions opens up the possibility of new such alloy formulations, production routes and techniques which should improve the tribological response of this alloy.

Alloys of titanium are highly preferred materials for their excellent strength to weight ratio but the tribological issues while using them has been posing challenging issues for the tribological analyst, which are still areas of active research. Ti-6Al-4V (Ti64) is the most popular alloy of titanium and our understanding of the fundamental mechanisms of wear and friction of this alloy is still not complete. Previous investigations related to the tribology of these alloys have suggested a synergistic effect of plastic deformation and tribo-oxidation. The present investigation described in this thesis explores the existence of one more mode, namely the formation of a Mechanically Mixed Layer (MML). The thesis examines the effect of these modes one by one and analyses the synergistic effect of these mechanisms, and also the effect of heat generation during sliding. The tribological condition existing have been varied by doing wear experiments using Ti64 pins sliding against alumina and SS316L (controls MML), diameter of pin (expected to control debris entrapment and thus MML formation), tribo-system (horizontal disc Vs vertical disc, which is also expected to control debris entrapment and thus MML formation), environment (ambient and vacuum, expected to control tribo-oxidation) and sliding speed (expected to control interface temperature and thus plastic deformation mechanism and tribo-oxidations). The division of the main chapters has been so made to present the findings spread over Chapters 5-8, with each chapter dealing with specific tribological test conditions. In each chapter, results from the tribological experimentations in the form of wear and friction are presented, together with the characterization methods which throw light into the tribological mechanisms. These characterization methods include Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDAX), X-Ray Diffraction (XRD) and Electron probe micro-analyzer (EPMA). Wherever possible, the debris collected from the experiments have been subjected to morphological and detailed chemical analysis, and a feature which has not been explored much in detail by tribological investigators, but having a promising potential. Experimental results from tribological testing when Ti64 pins slides against two different materials (Alumina and SS316L) in pin-on-disc tribometers under two different environmental conditions (ambient /vacuum) are analyzed. Each set of experiments looks at two different effects - (1) the effect of sliding speed on the tribological behavior while using a pin of a fixed diameter (all other parameters remaining the same) and (2) the effect of using pins of different diameters for a given set of parameters. Three different pin-sizes were employed (2.1 mm. 4.6 mm and 6.6 mm), the normal loads on these pins were changed according to the pin-size used so that all experiments were done at the same contact pressure (2.8 MPa). By performing the experiments against the ceramic disc (alumina) under vacuum conditions, the effect of this plastic deformation is studied in isolation because the possibility of the Tribo Chemical Reaction (TCR) due to oxidation is inhibited and no MML was found to be formed due to poor compatibility of mixing between the metallic pin and the ceramic disc. For the low speeds/strain rates experiments, the effect of plastic deformation as influenced by the adiabatic shear banding is seen to influence wear which progressively changes to temperature induced plastic deformation and wear. The situation is found to be different when we change the environmental conditions from vacuum to ambient for the same tribo-combination. The tests shows a reduction in wear rate with speed, and this is due to the oxide formations due to TCR as confirmed from the SEM/EDAX characterization. In contrast to previous experiments under vacuum, these permit the effect of TCR also to influence the tribological behavior. The scenario changes when the alumina disc is replaced by a metallic one (SS316L) and tests carried out in vacuum, as the MML was found to be formed with this tribo-pair. Because of the mutual affinity of the materials in the tribo-pair, the wear damage is severe in this case and the flash temperatures crossing the phase transition temperature (~880oC) for Ti64 at high speeds. The growth of the β phase with increase in the sliding (temperature) conditions is captured from the XRD spectra of the wear debris. Synergistic effect of all these mechanisms (plastic deformation, MML, and TCR) is permitted by conducting experiments with Ti64 pin against stainless steel and in ambient conditions. A comparison of the tribological response by presenting results when experiments are run over a range of speeds while using different sized pins under ambient conditions (and compared with similar results in vacuum) while using SS316L disc serve to demarcate the differences in the wear modes which are active/inactive depending on the tribological conditions. In addition a study incorporating the effect of frictional heating and its influence on the tribological phenomena is analyzed. Main conclusions from the thesis are: The wear resistance of Ti64 alloy when sliding against SS316L is found to be influenced by Strain Rate Response (SRR), Tribo Oxidation (TO), Mechanically Mixed Layer (MML) and the prevailing heat flux conditions at the contact. The wear rates were found to decrease marginally with sliding speeds (strain rates) up to a certain speed, which is ascribed to reduction in adiabatic shear band intensity with increase in strain rate. Adiabatic Shear Band (ASB), which allows easy crack propagation, intensity reduces as temperature of deformation of Ti64 is increased. From the results it can be confirmed that the propensity for formation of MML depends on compatibility of the disc and the pin material. The contribution due to of entrapment and retention of debris in the contact zone also would influence formation of the MML. The effect of frictional heating plays an influential role as it can affect the factors (TO, ASB, MML) governing the tribological response. The sensitivity to temperature, which is a marked feature of this alloy in undergoing softening, as confirmed by previous researchers, is reflected in the experimental results. Since the main factor that triggers the micro-structural instability is the energy dissipation that accompanies deformation more fundamental research which can improve the thermal transport properties of this alloy, would be the future scope of work of this thesis. Also, the unique composition of the MML which offers high wear resistance under specific operating conditions opens up the possibility of new such alloy formulations, production routes and techniques which should improve the tribological response of this alloy.

碩士
國立中興大學
生命科學系
92
Plasmid pML (2158 bp) from the moderately halophilic methanogenic archaeon- Methanohalophilus mahii SLP was sequenced. The deduced amino acid sequence of the largest ORF (328 aa) shows 34 % similarity to the putative replication protein (Rep) of pGRB1 from Halobacterium sp. and contains three sequence motifs that conserved in the Rep proteins of rolling circle replication (RCR) mechanism. Based on the numbers of tyrosine residues in motif 3, pML belongs to the superfamily I of RCR. The single-stranded intermediate form of pML was detected by southern blotting, which confirms that pML replicate through RCR mechanism. RT-PCR with total M. mahii RNA and primers specific for orf1 amplified an RNA species of 984 bp. DNA encoding ORF1 was cloned into Escherichia coli and a polypeptide with expected molecular mass of 38 kDa was expressed. Both the N-terminal amino acid sequence and western blotting with anti-His (C-term)-HRP antibody confirm that this overexpressed protein is the translational product of orf1. This protein binds to pMM3 which contains a putative double-stranded replication region and made this fragment sensitive to S1 nuclease indicating the site-specific nuclease activity of this Rep protein. Phylogenetic analysis based on the amino acid sequences of all known archaeal Rep indicated that Rep of pML is clustered to the plasmids of the aerobic extremely halophilic Euryarchaeota, which they inhabited together at hypersaline environment.

This doctoral dissertation takes the reader through a journey where applied shear rheology and flow-velocimetry are used to understand the mesoscopic factors that control the flow behavior of three microstructured fluids. Three individual protocols that measure relative physical and mechanical properties of the flow are developed. Each protocol aims to advance the particular transformation of novel soft materials into a commercial product converging in the demonstration of the real the chemical, physical and thermodynamical factors that could potentially drive their successful transformation.

First, this dissertation introduces the use of rotational and oscillatory shear rheometry to quantify the solvent evaporation effect on the flow behavior of polymer solutions used to fabricate isoporous asymmetric membranes. Three different A-B-C triblock copolymer were evaluated: polyisoprene-b-polystyrene-b-poly(4-vinylpyridine) (ISV); polyisoprene-b-polystyrene-b-poly(N,N-dimethylacrylamide) (ISD); and polyisoprene-b-polystyrene-b-poly(tert-butyl methacrylate) (ISB). The resulting evaporation-induced microstructure showed a solution viscosity and film viscoelasticity strongly dependent on the chemical structure of the triblock copolymer molecules.

Furthermore, basic shear rheometry, flow birefringence, and advanced flow-velocimetry are used to deconvolute the flow-microstructure relationships of concentrated surfactant solutions. Sodium laureth sulfate in water (SLE₁S) was used to replicate spherical, worm-like, and hexagonally packed micelles and lamellar structures. Interesting findings demonstrated that regular features of flow curves, such as power-law shear thinning behavior, resulted from a wide variety of experimental artifacts that appeared when measuring microstructured fluids with shear rheometry.

Finally, the successful integration of shear rheometry to calculate essential parameters to be used in a cost-effective visualization technique (still in development) used to calculate the dissolution time of polymers is addressed. The use of oscillatory rheometry successfully quantify the viscoelastic response of polyvinyl alcohol (PVA) solutions and identify formulations changes such as additive addition. The flow behavior of PVA solutions was correlated to dissolution behavior proving that the developed protocol has a high potential as a first screening tool.