Dissertations / Theses on the topic 'Multiscale deformations'

Ni-base superalloys are widely used in hot sections of gas turbine engines due to the high resistance to fatigue and creep at elevated temperatures. Due to the demands for improved performance and efficiency in applications of the superalloys, new and improved higher temperature alloy systems are being developed. Constitutive relations for these materials need to be formulated accordingly to predict behavior of cracks at notches in components under cyclic loading with peak dwell periods representative of gas turbine engine disk materials. Since properties are affected by microstructure at various length scales ranging from 10 nm tertiary γ' precipitates to 5-30 μm grains, hierarchical multiscale modeling is essential to address behavior at the component level. The goal of this work is to develop a framework for hierarchical multiscale modeling network that features linkage of several fine scale models to incorporate relevant microstructure attributes into the framework to improve the predictability of the constitutive model. This hierarchy of models is being developed in a collaborative research program with the Ohio State University. The fine scale models include the phase field model which addresses dislocation dissociation in the γ matrix and γ' precipitate phases, and the critical stresses from the model are used as inputs to a grain scale crystal plasticity model in a bottom-up fashion. The crystal plasticity model incorporates microstructure attributes by homogenization. A major task of the present work is to link the crystal plasticity model, informed by the phase field model, to the macroscale model and calibrate models in a top-down fashion to experimental data for a range of microstructures of the improved alloy system by implementing a hierarchical optimization scheme with a parameter clustering strategy. Another key part of the strategy to be developed in this thesis is the incorporation of polycrystal plasticity simulations to model a large range of virtual microstructures that have not been experimentally realized (processed), which append the experimentally available microstructures. Simulations of cyclic responses with dwell periods for this range of virtual (and limited experimental) polycrystalline microstructures will be used to (i) provide additional data to optimize parameter fitting for a microstructure-insensitive macroscopic internal state variable (ISV) model with thermal recovery and rate dependence relevant to the temperatures of interest, and (ii) provide input to train an artificial neural network that will associate the macroscopic ISV model parameters with microstructure attributes for this material. Such microstructure sensitive macroscopic models can then be employed in component level finite element studies to model cyclic behavior with dwell times at smooth and cracked notched specimens.

The mechanical and physical properties of polymers are determined primarily by the underlying nano-scale structures and characteristics such as entanglements, crystallites, and molecular orientation. These structures evolve in complex manners during the processing of polymers into useful articles. Limitations of available and foreseeable computational capabilities prevent the direct determination of macroscopic properties directly from atomistic computations. As a result, computational tools and methods to bridge the length and time scale gaps between atomistic and continuum models are required. In this research, an internal state variable continuum model has been developed whose internal state variables (ISVs) and evolution equations are related to the nano-scale structures. Specifically, the ISVs represent entanglement number density, crystal number density, percent crystallinity, and crystalline and amorphous orientation distributions. Atomistic models and methods have been developed to investigate these structures, particularly the evolution of entanglements during thermo-mechanical deformations. A new method has been created to generate atomistic initial conformations of the polymer systems to be studied. The use of the hyperdynamics method to accelerate molecular dynamics simulations was found to not be able to investigate processes orders of magnitude slower that are typically measurable with traditional molecular dynamics simulations of polymer systems. Molecular dynamics simulations were performed on these polymer systems to determine the evolution of entanglements during uniaxial deformation at various strain rates, temperatures, and molecular weights. Two methods were evaluated. In the first method, the forces between bonded atoms along the backbone are used to qualitatively determine entanglement density. The second method utilizes rubber elasticity theory to quantitatively determine entanglement evolution. The results of the second method are used to gain a clearer understanding of the mechanisms involved to enhance the physical basis of the evolution equations in the continuum model and to derive the models material parameters. The end result is a continuum model that incorporates the atomistic structure and behavior of the polymer and accurately represents experimental evidence of mechanical behavior and the evolution of crystallinity and orientation.

Resin infusion-based processes are good candidates for manufacturing thin composite materials parts such as those used in aeronautics for instance. These processes consist in infusing a liquid resin into a stacking of fibrous preforms under the action of a mechanical pressure field applied onto this stacking where a stiff- distribution medium is also placed to create a resin feeding. Both physical and mechanical properties of the final pieces are rather difficult to predict and control. Numerical simulation are perfectly suited to master these processes. In the present work a numerical finite element modeling framework is proposed to simulate infusion processes. The flow of the assumed Newtonian resin is described in the distribution medium, a highly porous medium, through Stokes' equations and through Darcy's equations in the fibrous preforms, very low permeability media. This coupled Stokes-Darcy flow is modeled in a monolithic approach which consists in using a single mesh for both media. The mixed velocity- pressure formulation is then discretized by linear-linear finite elements, stabilized by a so-called ASGS multi-scale approach. Both Stokes-Darcy interface and fluid front are represented individually thanks to "Level-Set" functions, and some specific coupling conditions are prescribed on the interface separating both fluid and porous media. During the process, orthotropic preforms undergo finite strains, either during the compaction stage when resin is not yet present, or during resin infusion. Resin pressure then tends to make the preforms swell. Preforms deformations are represented through an updated Lagrangian formulation for finite deformations. Dry preforms possess a non-linear elastic behaviour in their transverse direction - across their thickness- given by existing experimental measurements. The effect of the presence of resin in the wet preforms is accounted for using a Terzaghi's equivalent model. Also, when preforms deform their porosity will change, and so will their permeability, modifying the resin flow. A Carman-Kozeny model is then used to relate porosity and permeability. After the Stokes-Darcy coupling is validated both on numerous tests cases and using the method of manufactured solutions, various 2D and 3D simulations of injection and infusion-based processes are analyzed.The latter includ- ing preform deformations along with resin flow. Comparisons with existing experimental measurements permit to validate the approach on a simple geometry. Last, some ex- tensions to more complex 3D cases are proposed as outlooks, including curvatures and thickness variations.

Le domaine intracontinental est éloigné des limites des plaques tectoniques sur lesquelles sont localisées l’essentiel des déformations. La déformation intraplaque est une conséquence de la transmission de la contrainte depuis les limites de plaques vers le domaine intraplaque. Elle peut s’exprimer par un flambage lithosphérique, par des déformations régionales et des réseaux de fractures à micro- et méso-échelle. Ces déformations reflètent la nature des régimes de contraintes aux limites des plaques continentales. Elles ont une distribution hétérogène et à l'échelle régionale, révèlent souvent les directions d'un cadre structural préexistant. Le rôle de cet héritage structural à l'échelle micro et méso reste cependant mal compris. Ce travail de thèse se concentre sur les déformations cassantes à micro- (millimétrique à centimétrique) et méso-échelle (centimétrique à métrique) du domaine intra-plaque.Dans les zones urbaines avec des infrastructures souterraines importantes, il est essentiel de mieux comprendre la géométrie et la distribution des réseaux de fractures. Ces réseaux contrôlent, dans une certaine mesure, la dynamique de la circulation des fluides souterrains. Dans le cadre de cette thèse, la distribution, les géométries, les cinématiques et les chronologies relatives et absolues de ces déformations intraplaques ont été étudiées dans le Bassin de Paris. Pour ce faire, nous avons utilisé une approche multi-technique combinant travail de terrain et de laboratoire, afin d'aborder à deux échelles différentes : la micro-échelle et la méso-échelle. Les différentes méthodes utilisées sont l’inversion des données microtectoniques pour remonter à des paleo-contraintes, la datation absolue U-Pb des calcites de veines et de faille, et l'inversion des données de susceptibilité magnétique et de vitesse des ondes P afin de caractériser les microstructures internes des échantillons de roches.Les résultats nous permettent de proposer un modèle de déformations intraplaques dans le Bassin de Paris et de discuter du lien avec les événements géodynamiques de l'Europe occidentale tels que les orogénèses pyrénéenne et alpine et le système de rifting cénozoïque européen (ECRIS).Les données montrent que les déformations intraplaques dans le Bassin de Paris sont plus qu'une simple réactivation des discontinuités du socle. Ces déformations s’expriment via un vaste réseau de joints cassants, et sous forme de réseaux de failles, bien que plus rares. Enfin, grâce à la datation U-Pb de la calcite, nous montrons que la chronologie du réseau de failles spatialement hétérogène est surtout contemporaine des phases paroxysmales de déformation de l'orogenèse pyrénéenne. Ainsi, les champs de paléocontraintes déduits de l’inversion des plans de failles datés indiquent davantage l'orientation de la compression pyrénéenne que celle de la compression alpine ou de l'extension de l'ECRIS. Enfin, l'anisotropie de la vitesse des ondes P suggère que le réseau interne de fractures microstructurales des échantillons de craie reflète la nature géométrique des joints mesurés sur le terrain
The intracontinental domain is located far from tectonic plate margins, where significant stress can accumulate. Deformation of the intraplate domain is a consequence of stress transmission from plate boundaries. It results in lithospheric buckling, regional deformations, and meso to microscale fracture networks. These intraplate deformations reflect the nature of stress regimes at continental plate boundaries. They often have a heterogeneous distribution, and on a regional scale, are frequently impacted by the directions of pre-existing faults or weaknesses. The role of a structural inheritance remains to be shown on a micro and mesoscale.Understanding intraplate deformation is crucial for assessing geological hazards and fluid circulation in the context of subsurface solicitation. This is especially true in densely populated areas with substantial underground infrastructure. In this PhD, we investigate the geometries, distribution, kinematics, and timing of these intraplate deformations within the intracontinental Paris Sedimentary Basin, with the city of Paris at its center.To achieve this and to attempt to link the different scales of structures, we use multiple techniques, combining fieldwork and laboratory work, in order to approach the problem from two different scales: the mesoscale (metric to centimetric) and microscale (centimetric to millimetric). The structural analyses include calculating paleostress fields from microtectonic data collected in the field and in-situ U-Pb absolute dating of synkinematic calcites and calcitic veins. The microscale is investigated through the inversion of magnetic susceptibility and P-wave velocity data to characterize the internal microstructures of rock samples.The data show that an extensive network of multi-directional brittle joints exists and is expressed at different scales. In chalk samples for example, the measured anisotropy of P-wave velocity reflects the directions of mesoscale joints measured in the field. When the applied shear stress exceeds the shear strength of the joint, failure occurs. This can manifest as sliding or fracturing along the joint plane. Thus, some of the joints are reactivated later. Evidences of faulting, while less common than joints, also exist in the Paris Basin. Calculated paleostress tensors indicate mostly strike-slip faulting regimes with maximal principal stress axes (sigma1) roughly N-S. This direction is concordant with the N-S orientation of Pyrenean compression more so than Alpine compression or Tertiary extension at the origin of the European rifted continental basins. Furthermore, through in-situ U-Pb dating of calcite mineralized along fault planes or within veins, we show that the Late Cretaceous to Eocene timing of the fault network is more aligned with the Pyrenean Orogeny than the Alpine Orogeny or the European Cenozoic Rift System (ECRIS)

Les études récentes de la déformation plastique à l'aide de techniques expérimentales à haute résolution témoignent que les processus de déformation sont souvent caractérisés par des effets collectifs qui émergent à une échelle mésoscopique, intermédiaire entre celle de défauts cristallins et celle d'une éprouvette macroscopique. Notamment, la méthode de l'émission acoustique (EA) révèle, dans divers conditions expérimentales, l'intermittence de la déformation plastique, qui se manifeste par une propriété de l'invariance d'échelle, caractéristique de phénomènes d'auto-organisation. L'objectif de la thèse a été d'étudier la structure inhérente de l'EA pour différents mécanismes de déformation plastique, d'examiner sa dépendance à la vitesse de déformation et à l'écrouissage du matériau, et d'appréhender les liens entre les petites échelles de temps, liées à l'organisation des défauts, et celles qui relèvent de l'approche continue de la plasticité. L'étude a été réalisée sur des alliages AlMg et des alliages base Mg, dont la déformation plastique est accompagnée d'une forte activité acoustique et contrôlée par différents mécanismes physiques : l'effet Portevin-Le Chatelier (PLC) dans les premiers et une combinaison du maclage et du glissement des dislocations dans les deuxièmes. L'utilisation de la technique d'enregistrement continue de l'EA ("data streaming") a permis de montrer que le comportement apparent - discrète ou continue - de l'EA accompagnant l'effet PLC dépend de l'échelle de temps d'observation et du paramètre physique étudié. Cependant, contrairement à une vision traditionnelle, il se trouve que l'EA a un caractère intermittent pendant l'écoulement macroscopiquement lisse tant que pendant l'instabilité macroscopique de la déformation plastique. Grace aux méthodes d'analyse issues de la théorie des systèmes dynamiques non linéaires, telles que l'analyse multifractale, une tendance à la transition entre la dynamique invariante d'échelle et les comportements caractérisés par des échelles intrinsèques a été trouvée lors de l'écrouissage des matériaux. Enfin, nous avons prouvé que les distributions statistiques en loi puissance persistent dans des larges intervalles de variation des paramètres, conventionnellement utilisés pour individualiser les événements acoustiques. Ce résultat est d'une importance générale car il s'applique à tous les processus avalancheux émergeant dans différents systèmes dynamiques
Recent studies of plastic deformation using high-resolution experimental techniques testify that deformation processes are often characterized by collective effects that emerge on a mesoscopic scale, intermediate between the scale of individual crystal defects and that of the macroscopic sample. In particular, the acoustic emission (AE) method reveals intermittency of plastic deformation in various experimental conditions, which is manifested by the property of scale invariance, a characteristic feature of self-organized phenomena. The objective of the dissertation was to study the inherent structure of AE for different mechanisms of plastic deformation, to examine its dependence on the strain rate and strain hardening of the material, and to understand the relationships between short time scales related to organization of defects and those relevant to the continuous approach of plasticity. The study was performed on AlMg and Mg-based alloys, the plastic deformation of which is accompanied by a strong acoustic activity and controlled by different physical mechanisms: the Portevin-Le Chatelier (PLC) effect in the first case and a combination of twinning and dislocation glide in the second case. Application of a technique of continuous AE recording ("data streaming") allowed proving that the apparent behavior, discrete or continuous, of AE accompanying the PLC effect depends on the time scale of observation and the physical parameters surveyed. However, unlike the traditional view, it appears that AE has an intermittent character during both stress serrations and macroscopically smooth flow. Using methods of the theory of nonlinear dynamical systems, such as the multifractal analysis, a tendency to a transition between the scale-invariant dynamics and the behaviors characterized by intrinsic scales was detected during work hardening. Finally, we proved that the power-law statistical distributions persist in wide ranges of variation of parameters conventionally used to individualize acoustic events. This result is of general importance because it applies to all avalanche-like processes emerging in dynamical systems

The mechanical behavior of the arterial wall is determined by the composition and structure of its internal constituents as well as the applied traction-forces, such as pressure and axial stretch. The purpose of this work is to develop new theoretical frameworks and experimental methodologies to further the understanding of arterial mechanics and role of the various intrinsic and extrinsic mechanically motivating factors. Specifically, residual deformation, matrix organization, and perivascular support are investigated in the context of their effects on the overall and local mechanical behavior of the artery. We propose new kinematic frameworks to determine the displacement field due to residual deformations previously unknown, which include longitudinal and shearing residual deformations. This allows for improved predictions of the local, intramural stresses of the artery. We found distinct microstructural differences between the femoral and carotid arteries from non-human primates. These arteries are functionally and mechanically different, but are geometrically and compositionally similar, thereby suggesting differences in their microstructural alignments, particularly of their collagen fibers. Finally, we quantified the mechanical constraint of perivascular support on the coronary artery by mechanically testing the artery in-situ before and after surgical exposure.

Superplastic forming (SPF) is a valuable near net shape fabrication method, used to produce very complex, contoured and monolithic structures that are often lighter, stronger and safer than the assemblies they replace. However, the widespread industrial use of Superplastic (SP) alloys is hindered by a number of issues including low production rate and limited predictive capabilities of stability during deformation and failure. Failure during SPD may result from geometrical macroscopic instabilities and/or microstructural aspects. However, the available failure criteria are either based on geometrical instabilities or microstructural features and do not account for both failure modes. The present study presents a generalized multi-scale stability criterion for SP materials, accounting for both aspects of failure under various loading conditions. A combined model accounting for cavity nucleation and plasticity controlled cavity growth along with a grain growth model and a modified microstructure based constitutive equation for SP materials is incorporated into Harts stability analysis to develop the proposed stability criterion for different loading conditions. Effects of initial grain size, initial levels of cavitation, nucleation strain, strain-rate sensitivity, and grain-growth exponent on the optimum forming curves of different SP alloys are investigated, for different loading conditions.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2009.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 116-121).
Alpha-helical (AH) protein structures are critical building blocks of life, representing the key constituents of biological materials such as cells, hair, hoof and wool, where they assemble to form hierarchical structures. AHs play an important mechanical role in biological processes such as mechanotransduction, cell mechanics, tissue mechanics and remodeling. Whereas the mechanics of engineered materials has been widely investigated, the deformation and failure mechanisms of biological protein materials remain largely unknown, partly due to a lack of understanding of how individual protein building blocks respond to mechanical load and how the hierarchical features participate in the function of the overall biological system. In this Thesis, we develop, calibrate, validate and apply two computational models to predict the elasticity, deformation, strength and failure mechanisms of AH protein arrangements and eukaryotic cells over multiple orders of magnitude in time- and lengthscales. Our AH protein model is based on the formulation of tensile double-well mesoscale potentials and intermolecular adhesion Lennard-Jones potentials derived directly from results of full atomistic simulations. We report a systematic analysis of the influence of key parameters on the strength properties and deformation mechanisms, including structural and chemical parameters, and compare it with theoretical strength models. We find a weakening effect as the length of AH proteins increases, followed by an asymptotic regime in which the strength remains constant. We also show that interprotein sliding is a dominating mechanism that persists for a variety of geometries and realistic biologically occurring amino acid sequences. The model reported here is generally applicable to other protein filaments that feature a serial array of domains that unfold under applied strain. Although simple, our coarse-grained cell model agrees well with experiments and illustrates how the multiscale approach developed here can be used to describe more complex biological structures. We further show that cytoskeletal intermediate filaments contribute to cell stiffness and deformation and thus play a significant role to maintain cell structural integrity in response to stress. These studies lay the foundation to improve our understanding of pathological pathways linked to AH proteins such as muscular dystrophies.
by Jeremie Bertaud.
S.M.

Thesis (m.s.)--University of Kentucky, 2004.
Title from document title page (viewed Jan. 5, 2005). Document formatted into pages; contains x, 112p. : ill. Includes abstract and vita. Includes bibliographical references.

Parmi les nouveaux matériaux émergents à base de carbone, le graphène est rapidement devenu un candidat idéal pour plusieurs applications en nanoélectronique. Dans ce contexte, différentes méthodes ont été proposées pour transformer ses propriétés électriques, et notamment supprimer sont point de dégénérescence de Dirac. L’ouverture d’un gap d’énergie peut ainsi conduire à l’usage du graphène dans des nano-transistors. Dans cette thèse, nous appliquons un modèle compact semi-analytique pour étudier deux types de nanotransistors à base de graphène: les transistors à nanorouban et les transistors à nanomesh. Un modèle de type thight-binding est utilisé pour déterminer les expressions analytiques des bandes d'énergie d'un nanorouban de graphène. Des comparaisons sont montrées avec des approches ab initio, et avec des mesures effectuées sur des transistors du même type mais à plus grande échelle. Dans le contexte de l'électronique pour applications souples, les contraintes mécaniques sur les circuits et les déformations géométriques des composants à base de graphène peuvent constituer un problème important. Nous étudions ces effets sur les propriétés de conduction des transistors à nanorubans (dans les régimes balistique et partiellement balistique). En supposant la présence de petites déformations, une mise à l'échelle spectrale et un décalage spectral dû à la présence d'une déformation peuvent être pris en compte de manière analytique. Ce modèle conduit à définir sous forme analytique les quantités effectives (masses, densités d’états) utilisées pour calculer numériquement les potentiels et les courants dans le nano-dispositif. Les résultats numériques sont présentés à la fois sous un régime balistique et partiellement balistique, avec ou sans contacts de Schottky. Les résultats proposés dans le Chapitre 2 illustrent de manière très simple comment la déformation du nanoribbon de graphène influence les caractéristiques I-V du transistor. Une autre solution pour réaliser un nanotransistor de graphène est la gravure de nano-trous dans une feuille de graphène (réalisant ainsi un nanomesh). Si le graphène nanomesh est correctement formé, le rapport de courant On / Off du transistor devrait être amélioré. Dans le Chapitre 3, la méthode semi-analytique est utilisée pour évaluer les performances d'un nanomesh à transistors à nanorubans. Les résultats sont à nouveau comparés à une méthode ab-initio. Les caractéristiques I-V du graphène nanomesh transistor sont présentées et comparées aux résultats expérimentaux. Les résultats proposés montrent comment la taille des nanomesh de graphène influence les caractéristiques I-V du transistor. Compte tenu de la simplicité et du temps de calcul réduit de l'approche proposé, ces résultats peuvent permettre des analyses paramétriques, des optimisations et des caractérisations de nano-transistor à graphène dans des circuits à plus grande échelle
Among emerging carbon materials, graphene has rapidly become an ideal candidate for nano-electronics. In this context, different methods have been proposed to transform its electric properties and remove the Dirac degeneracy point, leading to application to nano-transistors. In this thesis we apply a semi-analytical compact model to study two kinds of graphene-based nanotransistors: nanoribbon graphene transistor and nanomesh transistor. A tight-binding model is used to determine analytical expressions for the energy bands of a graphene nanoribbon. Comparisons are shown with ab-initio approaches and with measurements done on larger-scale transistors of the same kind. In the context of flexible electronics, mechanical stresses on circuits and subsequent geometric deformations of graphene-based components is an important issue. We investigate these effects on the conduction properties of nanoribbon transistors (both in ballistic and partially ballistic regimes). By assuming the presence of small deformations, a spectral scaling and a spectral shift due to the presence of a deformation can be taken into account analytically. This model leads to define in closed form effective quantities (masses, densities of states) used to numerically calculate potentials and currents in the nano-device. Numerical results are shown both in a ballistic and partially-ballistic regime, with and without the presence of Schottky contacts. The proposed results in Chapter 2 illustrate in a very simple way how the deformation of graphene nanoribbon influences the I-V characteristics of transistor. Another solution to realize graphene nanotransistor is the etching of nanoholes in a graphene sheet (thus realizing a nanomesh). If graphene nanomesh is properly shaped, the On/Off current ratio of transistor is expected to be enhanced. In Chapter 3, the semi-analytic method is used to evaluate the performance of nanomesh transistor with nanoribbon ones. The results are again compared with an ab-initio method. I-V characteristics of graphene nanomesh transistor are presented and compared with experimental results. The proposed results show how graphene nanomesh size influences the I-V characteristics of transistor. Given the simplicity and the reduced computation time of the approach, these results can lead to perform parametric analyses, optimizations and characterization of graphene nano-transistor when applied in larger-scale circuits

Le deformazioni lente di versante in roccia (DGPV e grandi frane) sono fenomeni diffusi che interessano interi versanti e mobilizzano volumi di roccia anche di miliardi di metri cubi. La loro evoluzione è legata a processi di rottura progressiva sotto forzanti esterne e di accoppiamento idromeccanico, rispecchiate da un complesso processo di creep. Sebbene caratterizzate da bassi tassi di spostamento (fino a pochi cm / anno), queste instabilità di versante danneggiano infrastrutture e ospitano settori potenzialmente soggetti a differenziazione e collasso catastrofico. È quindi necessaria una robusta caratterizzazione del loro stile di attività per determinare il potenziale impatto sugli elementi a rischio e anticipare un eventuale collasso. Tuttavia una metodologia di analisi finalizzata a questo scopo è ancora mancante. In questa prospettiva, abbiamo sviluppato un approccio multiscala che integra dati morfostrutturali, di terreno e tecniche DInSAR, applicandoli allo studio di un inventario di 208 deformazioni lente di versanti mappate in Lombardia. Su questo dataset abbiamo eseguito una mappatura geomorfologica e morfostrutturale di semi dettaglio tramite immagini aeree e DEM. Abbiamo quindi sviluppato un pacchetto di procedure oggettive per lo screening su scala di inventario delle deformazioni lente di versante integrando dati di velocità di spostamento, cinematica e di danneggiamento dell’ammasso roccioso per ogni frana. Utilizzando dataset PS-InSAR e SqueeSAR, abbiamo sviluppato una procedura mirata a identificare in maniera semiautomatica la velocità InSAR rappresentativa, il grado di segmentazione e l'eterogeneità interna di ogni frana mappata identificando la presenza di possibili fenomeni secondari. Utilizzando la tecnica 2DInSAR e tecniche di machine learning, abbiamo inoltre sviluppato un approccio automatico caratterizzare la cinematica di ciascuna frana. I dati così ottenuti sono stati integrati tramite analisi di PCA e K-medoid per identificare gruppi di frane caratterizzati da stili di attività simili. Partendo dai risultati della classificazione su scala regionale, ci siamo poi concentrati su 3 casi di studio emblematici, le DGPV di Corna Rossa, Mt. Mater e Saline, rappresentativi di problematiche tipiche delle grandi frane (segmentazione spaziale, attività eterogenea, sensibilità alle forzanti idrologiche). Applicando un approccio DInSAR mirato abbiamo indagato la risposta del versante a diverse baseline temporali per evidenziare le eterogeneità spaziali e, tramite un nuovo approccio di stacking su basline temporali lunghe abbiamo estrattoi segnali di spostamento permanenti ed evidenziato i settori e le strutture con evoluzione differenziale. Lo stesso approccio DInSAR è stato utilizzato per studiare la sensibilità delle deformazioni lente di versante alle forzanti idrologiche. Il confronto tra i tassi di spostamento stagionale e le serie temporali di precipitazioni e scioglimento neve per il monte. Mater e Saline hanno delineato complessi trend di spostamento stagionale. Queste tendenze, più evidenti per i settori più superficiali, evidenziano una risposta maggiore a periodi prolungati di precipitazione modulati dagli effetti dello scioglimento della neve. Ciò suggerisce che le DGPV, spesso considerate non influenzate dalla forzante climatica a breve termine (pluriennale), sono sensibili a input idrologici, con implicazioni chiave nell'interpretazione del loro fallimento progressivo. I nostri risultati hanno dimostrato l'efficacia della metodologia multi-scala proposta, che sfrutta i prodotti DInSAR e l'analisi mirata per identificare, classificare e caratterizzare l'attività delle deformazioni lente di versante includendo dati geologici in tutte le fasi dell'analisi. Il nostro approccio, è applicabile a diversi contesti e dataset e fornisce gli strumenti per indagare processi chiave in uno studio finalizzato alla definizione del rischio connesso alle deformazioni lente di versante.
Slow rock slope deformations (DSGSDs and large landslides) are widespread, affect entire hillslopes and displace volumes up to billions of cubic meters. They evolve over long time by progressive failure processes, under variable climatic and hydro-mechanical coupling conditions mirrored by a complex creep behaviour. Although characterized by low displacement rates (up to few cm/yr), these slope instabilities damage sensitive structures and host nested sectors potentially undergoing rockslide differentiation and collapse. A robust characterization of the style of activity of slow rock slope deformations is required to predict their interaction with elements at risk and anticipate possible failure, yet a comprehensive methodology to this aim is still lacking. In this perspective, we developed a multi-scale methodology integrating geomorphological mapping, field data and different DInSAR techniques, using an inventory of 208 slow rock slope deformations in Lombardia (Italian Central Alps), for which we performed a geomorphological and morpho-structural mapping on aerial images and DEMs. On the regional scale, we developed an objective workflow for the inventory-scale screening of slow-moving landslides. The approach is based on a refined definition of activity that integrates the displacement rate, kinematics and degree of internal damage for each landslide. Using PS-InSAR and SqueeSAR datasets, we developed an original peak analysis of InSAR displacement rates to characterize the degree of segmentation and heterogeneity of mapped phenomena, highlight the occurrence of sectors with differential activity and derive their characteristic displacement rates. Using 2DInSAR velocity decomposition and machine learning classification, we set up an original automatic approach to characterize the kinematics of each landslides. Then, we sequentially combine PCA and K-medoid cluster analysis to identify groups of landslides characterized by consistent styles of activity, accounting for all the relevant aspects including velocity, kinematics, segmentation, and internal damage. Starting from the results of regional-scale classification, we focused on the Corna Rossa, Mt. Mater and Saline DSGSDs, that are emblematic case studies on which apply DInSAR analysis to investigate typical issues in large landslide studies (spatial segmentation, heterogenous activity, sensitivity to hydrological triggers). We applied a targeted DInSAR technique on multiple temporal baselines to unravel the spatial heterogeneities of complex DSGSDs and through a novel stacking approach on raw long temporal baseline interferograms, we outlined the permanent displacement signals and sectors with differential evolution as well as individual active structures. We then used DInSAR to investigate the possible sensitivity of slow rock slope deformations to hydrological triggers. Comparison between seasonal displacement rates, derived by interferograms with targeted temporal baselines, and time series of precipitation and snowmelt at the Mt. Mater and Saline ridge outlined complex temporally shifted seasonal displacement trends. These trends, more evident for shallower nested sectors, outline dominant controls by prolonged precipitation periods modulated by the effects of snowmelt. This suggests that DSGSDs, often considered insensitive to short-term (pluri-annual) climatic forcing, may respond to hydrological triggering, with key implication in the interpretation of their progressive failure. Our results demonstrated the effectiveness of the proposed multi-scale methodology that exploits DInSAR products and targeted processing to identify, classify and characterize the activity of slow rock slope deformation at different levels of details by including geological data in all the analysis stages. Our approach, readily applicable to different settings and datasets, provides the tools to solve key scientific issues in a geohazard-oriented study of slow rock slope deformations.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008.
Includes bibliographical references.
The development of nanoscale polymeric materials for mechanical applications necessitates advances in small-volume experimental techniques and analyses that reflect the viscoelastoplastic behavior of such materials. In this thesis, the time-dependence and response of homogeneous engineering polymers under confined contact loading are characterized as a function of polymer physical and structural properties. The validity of the time-independent metric indentation hardness Hi is evaluated through the combination of nanoindentation and atomic force microscopy imaging. In addition, the classic, time-dependent metric creep compliance J(t) is used to establish the experimental conditions necessary for linear elastic behavior for a set of thermoplastic and thermoset materials. For large indentations (hmax > 1 um), properties are tacitly assumed to reflect the properties of bulk polymer; however, this assumption does not hold within 100 nm of a free surface or interface of amorphous polymers such as polystyrene and polycarbonate. The contact deformation mechanism near an amorphous polymer surface is found to scale with the surface area of contact, suggesting the dynamic formation of a structural interphase region. Chemical probe functionalization experiments are developed to explore the effects of probe surface charge on the probe-polymer interface and contribute to the understanding of the interphase that dominates nanocomposite material response. A technique to rapidly screen mechanical response of combinatorial polymer libraries is presented, to establish structure-property-processing relationships of such chemomechanically defined interfaces before nanoscale deformation mechanisms in confined polymers are fully understood.
(cont.) Finally, material design for elastic, viscoelastic, and viscoelastoplastic mechanical properties is discussed in terms of polymer physical length and time scales.
by Catherine Anne Tweedie.
Ph.D.

Les méthodes conçues pour être incorporées dans des polycristaux de modélisation multi-échelles sont présentées dans ce travail en deux tâches. Ce travail contient des méthodes à moyenne échelle pour capturer les effets des interactions de dislocations de glissement rencontrant des joints de grains maclage et la croissance simultanée de plusieurs fractions de volume de grains maclage sur le durcissement mécanique et l’évolution de la texture. Celles-ci sont mises en œuvre dans un cadre de plasticité cristalline utilisant le code visco-plastic-self consistent de Los Alamos, VPSC-7. Présentés ici, les effets de la croissance simultanée de multiples variantes maclage sur l’évolution de la texture sont suivis à l’aide d’un schéma de transfert de volume double de type Kalidindi. Dans la tâche 1, la mise en œuvre de ce schéma afin de simuler la texture des aciers à plasticité induite par maclage (TWIP) soumis au pressage angulaire à canal égal (ECAP) est résumée. Dans la tâche 2, les effets de durcissement de deux types d’interaction entre les dislocations de glissement et les joints de grain maclage rencontrés, à savoir la transmutation et la dissociation de dislocation, sont capturés au moyen de la modification du modèle de durcissement basé sur la densité de dislocation de [11]. Les interactions du premier type sont présentées dans une relation constitutive calculant la quantité de densité de dislocations attribuée à un système de glissement donné contenu dans la fraction de volume maclage rencontrée à partir de chaque système de glissement en interaction dans la fraction de volume mère. La quantité transmutée à partir de chaque système de glissement en interaction décrit à l’aide de la méthode de correspondance, sur la cartographie des systèmes de glisse- ment d’un grain parent à des systèmes de glissement dans des grains maclage considérés. Des interactions du second type sont ensuite introduites dans cette relation constitutive en tant que paramètre de dissociation, dont la valeur est établie par les observations tirées des résultats des simulations de dynamique moléculaire de [8] et [53]. Ces méthodes sont implantées pour simuler le comportement de durcissement anisotrope du magnésium HCP sous plusieurs chemins de charge
Methods designed for incorporation into multiscale modeling polycrystals are presented in this work in two tasks. This work contains mesoscale methods for capturing the effects of both the interactions of slip dislocations encountering twin grain boundaries and the simultaneous growth of multiple twin grain volume fractions on mechanical hardening and texture evolution. These are implemented in a crystal plasticity framework using the Los Alamos viscoplastic self-consistent code, VPSC-7. Presented here, the effects of simultaneous growth in multiple twin variants on textural evolution is tracked using a Kalidindi-type twin volume transfer scheme. In Task 1, the implementation of this scheme in order to simulate the texture of Twinning Induced Plasticity steels (TWIP) subjected to Equal Channel Angular Pressing (ECAP) are summarized. In Task 2, the hardening effects of two types of interactions between slip dislocations and encountered twin grain boundaries, namely dislocation transmutation and dissociation, are captured by way of modifying the dislocation density based hardening model of [11]. Interactions of the first type are presented in a constitutive relation calculating the amount of dislocation density apportioned to a given slip system contained within the encountered twin volume fraction from each interacting slip system in the parent volume fraction. The amount transmuted from each interacting slip system described using the Correspondence Method, an on to mapping of slip systems in a parent grain to slip systems in considered twin grains. Interactions of the second type are then introduced into this constitutive relation as a disassociation parameter, the value of which is established by observations gleaned from the results of the molecular dynamics simulations of [8] and [53]. These methods are implanted to simulate the anisotropic hardening behavior of HCP magnesium under multiple load paths

This research project is a step forward in predicting the bulk-scale deformations in food-plant materials during drying in a computationally efficient and accurate manner. In the proposed numerical modelling method, a novel force interaction field was introduced to represent the mechanics of a whole plant cell during drying through its centroid. By utilising this novel modelling approach, the distinct deformation profiles of several food-plant materials were revealed along with the impact of drying situations and features in the microstructure. The developed modelling framework has potential applications in the field of food engineering in controlling quality attributes of dried food-plant products.

Articular cartilage is a complex structure with an architecture in which fluid-swollen proteoglycans constrained within a 3D network of collagen fibrils. Because of the complexity of the cartilage structure, the relationship between its mechanical behaviours at the macroscale level and its components at the micro-scale level are not completely understood. The research objective in this thesis is to create a new model of articular cartilage that can be used to simulate and obtain insight into the micro-macro-interaction and mechanisms underlying its mechanical responses during physiological function. The new model of articular cartilage has two characteristics, namely: i) not use fibre-reinforced composite material idealization ii) Provide a framework for that it does probing the micro mechanism of the fluid-solid interaction underlying the deformation of articular cartilage using simple rules of repartition instead of constitutive / physical laws and intuitive curve-fitting. Even though there are various microstructural and mechanical behaviours that can be studied, the scope of this thesis is limited to osmotic pressure formation and distribution and their influence on cartilage fluid diffusion and percolation, which in turn governs the deformation of the compression-loaded tissue. The study can be divided into two stages. In the first stage, the distributions and concentrations of proteoglycans, collagen and water were investigated using histological protocols. Based on this, the structure of cartilage was conceptualised as microscopic osmotic units that consist of these constituents that were distributed according to histological results. These units were repeated three-dimensionally to form the structural model of articular cartilage. In the second stage, cellular automata were incorporated into the resulting matrix (lattice) to simulate the osmotic pressure of the fluid and the movement of water within and out of the matrix; following the osmotic pressure gradient in accordance with the chosen rule of repartition of the pressure. The outcome of this study is the new model of articular cartilage that can be used to simulate and study the micromechanical behaviours of cartilage under different conditions of health and loading. These behaviours are illuminated at the microscale level using the socalled neighbourhood rules developed in the thesis in accordance with the typical requirements of cellular automata modelling. Using these rules and relevant Boundary Conditions to simulate pressure distribution and related fluid motion produced significant results that provided the following insight into the relationships between osmotic pressure gradient and associated fluid micromovement, and the deformation of the matrix. For example, it could be concluded that: 1. It is possible to model articular cartilage with the agent-based model of cellular automata and the Margolus neighbourhood rule. 2. The concept of 3D inter connected osmotic units is a viable structural model for the extracellular matrix of articular cartilage. 3. Different rules of osmotic pressure advection lead to different patterns of deformation in the cartilage matrix, enabling an insight into how this micromechanism influences macromechanical deformation. 4. When features such as transition coefficient were changed, permeability (representing change) is altered due to the change in concentrations of collagen, proteoglycans (i.e. degenerative conditions), the deformation process is impacted. 5. The boundary conditions also influence the relationship between osmotic pressure gradient and fluid movement at the micro-scale level. The outcomes are important to cartilage research since we can use these to study the microscale damage in the cartilage matrix. From this, we are able to monitor related diseases and their progression leading to potential insight into drug-cartilage interaction for treatment. This innovative model is an incremental progress on attempts at creating further computational modelling approaches to cartilage research and other fluid-saturated tissues and material systems.

L’objectif de cette thèse est d’établir le scénario multi-échelles de déformation du PEEK lorsqu’il est sollicité en traction uniaxiale. Préalablement à la mise en oeuvre d’échantillons de deux grades commerciaux de PEEK, les propriétés thermiques et mécaniques de ces matériaux ont été caractérisées. La température d’oubli thermodynamique ainsi que la sensibilité aux vitesses de refroidissement ont été établies. Des éprouvettes de traction ont été obtenues à partir de plaques thermocompressées, procédé choisi pour obtenir des morphologies les plus isotropes possibles. Les propriétés mécaniques en traction ont ensuite été caractérisées au-dessus et au-dessous de la transition vitreuse de la phase amorphe (Tg). Grâce à un dispositif expérimental fabriqué sur mesure, des essais de traction à deux températures distinctes au-dessous et au-dessus de Tg ont été suivis par diffusion des rayons X aux petits (SAXS) et grands angles (WAXS) pour caractériser les déformations à l’échelle des empilements lamellaires et à l’échelle de la maille cristalline. Simultanément, le champ de déformation a été mesurée par corrélation d’images (DIC) afin de comparer la déformation macroscopique et microscopique. Pour les deux températures, les lamelles tendent à s’orienter perpendiculairement à la direction de traction (TD). Ce mécanisme d’orientation local (que nous appelons « modèle de réseau de chaînes ») est induit par la transmission des contraintes par les chaînes amorphes reliant les lamelles cristallines adjacentes. Au-dessus de Tg, l’allongement local est plus faible que l’allongement macroscopique dans les lamelles perpendiculaire à TD, ce qui implique que les lamelles inclinées doivent être cisaillées. L’évolution de la distribution d’orientation des lamelles appuie ce résultat. Une morphologie fortement orientée est finalement obtenue quelle que soit la température. Cependant, le profil d’endommagement est différent. En-dessous de Tg, le profil de diffusion centrale indique l’existence de petites entités (lamelles ou crystallites) orientées aléatoirement. A hautes température, le matériau est fibrillaire et présente des cavités
The aim of this PhD work is accessing the microscopic deformation mechanisms of bulk poly-ether-ether-ketone (PEEK) under tensile stretching. Beforehand, the thermal and mechanical properties of two commercial grades of PEEK were characterized. Tensile specimens were then compression-molded to obtain morphologies as isotropic as possible and characterized below and above the glass transition temperature. Deformations at the scales of lamellar stacks and of the crystalline unit cell have been characterized by small and wide-angle X-ray scattering (SAXS and WAXS) performed in-situ during tensile tests. Simultaneously, the strain field within the samples was followed by digital image correlation (DIC) in order to compare microscopic and macroscopic strains. At both temperatures, lamellae tend to orient perpendicular to the tensile direction (TD). This orientation mechanism (which we denote as ‘Chain Network model’) is driven by the amorphous chains which transmit the stress between adjacent lamellae. The tensile strain in lamellar stacks perpendicular to TD is lower than the macroscopic tensile strain, which must be compensated by increased shear in inclined stacks. Some differences of behavior have been observed depending on the test temperature, especially at high deformation. A highly oriented morphology is ultimately obtained in all cases. However, the central scattering profiles changes with testing temperatures. Below Tg, the presence of small entities randomly oriented is indicated. Above Tg, the material is fibrillar and contains cavities

Cette thèse est dédiée à la caractérisation de la déformation plastique sévère et des évolutions de la microstructure engendrées dans les rails en service, conduisant à leur fissuration par Fatigue de Contact de Roulement. L’amorçage de ces fissures en surface et leur propagation en profondeur mettent en jeu des phénomènes à l’échelle de la microstructure qui peuvent entrainer à l’échelle macroscopique des écaillages de surface ou même conduire à des ruptures brutales de rails en cours de fonctionnement. Pour améliorer la compréhension de ces différents phénomènes en sous surface, une méthodologie expérimentale multi-échelles et multi-techniques a été conduite sur des rails prélevés en cours de service. Dans un premier temps, la présence d’un gradient tridimensionnel de microstructure, de cristallographie et de propriétés mécaniques engendré par les contacts répétés avec les roues a été mise en évidence dans la tête du rail au cours de son fonctionnement. Par le biais d’une campagne de prélèvement de rails en circulation à différents chargements, les stades de mise en place de ces gradients et la déformation plastique accumulée dans la tête de rail ont ensuite pu être évalués, de même que leurs évolutions par rapport aux passages des roues sur le rail et au développement des fissures. Cette étude contribue ainsi à une meilleure appréhension des mécanismes d’endommagement en fatigue de contact de roulement des rails en fonctionnement et pourra fournir une base de données solide pour les travaux à venir dans le domaine
This work is dedicated to the characterization of severe plastic deformation and microstructure evolution induced in rails in service, leading to cracks initiation by Rolling Contact Fatigue. Initiation of these surface cracks and in-depth propagation involve several phenomena at the microstructure scale which can lead to surface spalling at the macroscopic scale or even to brutal failure of the rail during its service. To improve understanding of these various phenomena beneath the rail surface, an experimental, multi-scales and multi-techniques methodology has been followed on rails removed from service. In the first part of results, the presence of a three-dimensional gradient of microstructure, of crystallography and of mechanical properties induced by the repeated contacts with wheels has been highlighted in a rail head during its service. Then, by means of a field analysis campaign of rails removed from service at several accumulated loads, the different stages of in-depth gradients development and plastic deformation accumulated in the rail head have been estimated in relation with total accumulated tonnage and cracks initiation. This study contributes to improve the understanding of the damage mechanisms in rolling contact fatigue of rails in service and the modeling of rail plasticity and crack propagation by including anisotropy of the running band and effect of in-depth microstructure evolution

Les nanomatériaux sont actuellement largement utilisés dans le domaine bio-médical et jouent un rôle crucial dans les stratégies modernes pour remédier aux dysfonctionnements des tissus souples naturels tels que les tendons, les ligaments et les disques intervertébraux. Par ailleurs, les progrès de la biomécanique sont étroitement liés à l'élaboration de nouveaux biomatériaux tout en répondant à certains besoins spécifiques. Au cours des dernières décennies, une attention toute particulière a été portée à la combinaison de la nanotechnologie à d'autres domaines scientifiques dans le but d'obtenir de nouveaux biomatériaux avancés. Les matériaux souples renforcés par des nanoparticules inorganiques sont un exemple d'une telle combinaison entre la nanotechnologie et la science des biomatériaux. Ces biomatériaux peuvent imiter les propriétés chimiques, mécaniques, électriques et biologiques des tissus naturels. La présente thèse aborde le problème de la représentation constitutive multi-échelle du comportement inélastique multiaxial des matériaux souples renforcés par des nanoparticules inorganiques. La principale réalisation de cette thèse concerne le développement d'un modèle entièrement tridimensionnel, dans le cadre d'un traitement micromécanique, pour analyser la rupture, la capacité d'auto-guérison et les mécanismes de renforcement des nanoparticules tout en tenant compte des effets environnementaux. La représentation constitutive du système matériau est traitée à l'aide d'une cellule unitaire cubique contenant neuf nanoparticules ; une nanoparticule centrale relie huit autres placées aux sommets du cube via un certain nombre de chaînes polymères afin de tenir compte du rôle effectif des nanoparticules sur le comportement macroscopique non linéaire en grandes transformations. Les interactions directes en champ proche entre les nanoparticules et le réseau de chaînes sont physiquement décrites à l'aide d'une transition d'échelle micro-macro dans le cadre de la théorie de l'inclusion d'Eshelby. Le modèle considère explicitement le réseau de chaînes ainsi que les mécanismes réversibles de détachement / ré-attachement des liaisons dynamiques pour décrire de manière cohérente l'extensibilité extrême dépendante de la vitesse de sollicitation et certaines caractéristiques inélastiques, notamment la forte hystérésis lors des phases d'étirement-rétraction et la relaxation continue. Une évaluation quantitative de notre modèle est présentée à l'aide de comparaisons à des données expérimentales disponibles pour une variété de systèmes matériaux nanocomposites contenant une large gamme de concentrations de nanoparticules et pour différents modes de déformation lors de séquences de chargement monotones et cycliques. Le modèle s'avère capable de reproduire avec succès les différentes caractéristiques de la réponse multiaxiale macroscopique. Il est enfin utilisé pour mettre en évidence certaines informations clés sur les mécanismes de renforcement des nanoparticules et leur rôle sur la dissipation multiaxiale, la rupture multiaxiale et la capacité d'auto-guérison à température ambiante tout en tenant compte des effets de gonflement
Nanomaterials are currently widely used in bio-applications and play a crucial role in modern strategies to remedy malfunctions of natural soft tissues such as tendons, ligaments and intervertebral discs. Besides, progress in biomechanics is closely related to the elaboration of new biomaterials tailored to suit certain specifications. The combination of nanotechnology with other fields of science has attracted increasing attention during the past decades to get improved biomaterials. Soft materials reinforced by inorganic nanoparticles are an example of such a combination between nanotechnology and biomaterial science. These biomaterials can mimic the chemical, mechanical, electrical, and biological properties of native tissues. The present PhD dissertation addresses the problem of the multiscale constitutive representation of the multiaxial inelastic behavior of soft materials reinforced by inorganic nanoparticles. The main achievement of this PhD concerns the development of a fully three-dimensional model within a micromechanical treatment to analyze the failure, the self-healing facility and the nanofiller reinforcement mechanisms considering the environmental effects. The material system is representatively regarded as a cubic unit cell containing nine nanoparticles; a central nanoparticle connects eight nanoparticles placed at the cube vertices via a number of polymer chains to account for the effective role of nanoparticles on the nonlinear and finite-strain macro-behavior. The near-field direct interactions between the nanoparticles and the chains network are physically described using a micro-macro scale transition within the Eshelby inclusion theory. The model explicitly considers the chains network with dynamic reversible detachable/re-attachable mechanisms of bonds to coherently capture the rate-dependent extreme stretchability and some inelastic features including strong hysteresis upon stretching-retraction and continuous relaxation. A quantitative evaluation of our model is presented by comparisons to available experimental data of a variety of nanocomposite material systems over a wide range of nanoparticle concentrations for different modes of deformation upon monotonic and cyclic loading sequences. The model is found being able to successfully reproduce the significant features of the multiaxial macro-response. It is finally used to highlight some important insights on the nanoparticle reinforcement mechanisms and their role on the multiaxial dissipation, multiaxial failure and room temperature self-healing facility considering the swelling effects

This study introduces a multiscale model for analyzing nonlinear thermo-viscoelastic responses of particulate composites. A simplified micromechanical model consisting of four sub-cells, i.e., one particle and three matrix sub-cells is formulated to obtain the effective thermal and mechanical properties and time-dependent response of the composites. The particle and matrix constituents are made of isotropic homogeneous viscoelastic bodies undergoing small deformation gradients. Perfect bonds are assumed along the sub-cell⁰́₉s interfaces. The coupling between the thermal and mechanical response is attributed to the dissipation of energy due to the viscoelastic deformation and temperature dependent material parameters in the viscoelastic constitutive model. The micromechanical relations are formulated in terms of incremental average field quantities, i.e., stress, strain, heat flux and temperature gradient, in the sub-cells. The effective mechanical properties and coefficient of thermal expansion are derived by satisfying displacement- and traction continuities at the interfaces during the thermo-viscoelastic deformations. The effective thermal conductivity is formulated by imposing heat flux- and temperature continuities at the subcells⁰́₉ interfaces. The expression of the effective specific heat at a constant stress is also established. A time integration algorithm for simultaneously solving the equations that govern heat conduction and thermoviscoelastic deformations of isotropic materials is developed. The algorithm is then incorporated within each sub-cell of the micromechanical model together with the macroscopic energy equation to determine the effective coupled thermoviscoelastic response of the particulate composite. The numerical formulation is implemented within the ABAQUS, general purpose displacement based FE software, allowing for analyzing coupled heat conduction and deformations of composite structures. Experimental data on the effective thermal properties and time dependent responses of particulate composites available in the literature are used to verify the micromechanical model formulation. The multiscale model capability is also examined by comparing the field variables, i.e., temperature, displacement, stresses and strains, obtained from heterogeneous and homogeneous composite structures, during the transient heat conduction and deformations. Examples of coupled thermoviscoelastic analyses of particulate composites and functionally graded structures are also presented. The present micromechanical modeling approach is found to be computationally efficient and shows good agreement with experiments in predicting the effective thermo-mechanical response of particulate composites and functionally graded materials. Our analyses forecast a better design for creep resistant and less dissipative structures using particulate composites and functionally graded materials.

碩士
國立臺灣大學
土木工程學研究所
96
Microcantilever-based biosensors are rapidly becoming an enabling sensing technology for a variety of label-free biological applications due to their wide applicability, versatility and low cost. It is thus imperative for us to reveal the physical origin of adsorption-induced deformation, and to further analyze its implication of microscopic mechanisms on macroscopic deformation. The objective of this work is to develop a multi-scale theory that can analyze deformation of micro-cantilever beam subjected to bio-adsorption mechanisms calculated by ab- initio simulation and classical molecular dynamics. The multi-scale theory developed herein has successfully correlated atomistic information (the mechanism of bio-adsorption) and continuum description (bending behavior of a cantilever beam). We have studied the adsorption mechanisms of bio-molecules for SAM (self-assembly monolayer, alkanethiolic molecular for n=1~14) adsorbed on gold through ab-initio and molecular dynamics simulation. The ab-initio simulation results are in a good agreement with the literature, and the error of calculated absorption energy is less than 13％. We then extend to longer SAM simulation by molecular dynamics and the calculated absorption energy is less than 7％ when comparing with the ab-initio results. Adsorption-induced stresses for different SAMs (for n=4, 6, 8, 12 and 14) are calculated by the multi-scale method. Calculated deflection based on the adsorption-induced stress agrees well with experimental measurements. Physical origin of adsorption induced deformation is revealed through the change of atomic positions and forces.

O presente trabalho pretende investigar os mecanismos de desenvolvimento estrutural do PET e nanocompósitos de PET, durante a aplicação de deformações uniaxiais. São objectivos de trabalho estudar: • a influência estatística das variáveis de estiramento no desenvolvimento estrutural do PET em deformações efectuados a temperaturas acima de Tg (patamar de borracha), • o efeito da morfologia inicial do PET no seu desenvolvimento após deformação a frio, • o efeito da utilização de nanocargas (montemorilonite, MMT, dióxido de titânio, TiO2, e dióxido de sílica, SiO2) na morfologia e propriedades do PET reforçado (propriedades térmicas e mecânicas), • a influência de diferentes tipos de nanocargas (MMT, TiO2, e SiO2) no comportamento à deformação do PET e na sua evolução estrutural durante deformações a frio. Esta investigação procura propor modelos estruturais multi-escala adequados à compreensão da evolução estrutural do PET e dos seus nanocompósitos durante a aplicação de esforços uniaxiais. A influência estatística das variáveis de estiramento: (temperatura, Tst, taxa de deformação, st ε& , e razão de estiramento, λst) no desenvolvimento estrutural do PET em deformações efectuadas no patamar de borracha do material (acima de Tg) foi investigada através das técnicas de WAXD, birrefringência e DSC. Concluiu-se que: i) a transformação da fase amorfa em mesofase é controlada maioritariamente por λst e a interacção entre λst e st ε& ; ii) o fenómeno de cristalização induzida por deformação é governado pela Tst, λst e a interacção entre ambos, o mesmo acontecendo para os nível de orientação molecular; iii) a temperatura de transição vítrea é governada pela λst e a sua interacção com Tst, no entanto, a contribuição individual de Tst é estatisticamente baixa; iv) a influência sobre a temperatura de cristalização a frio, Tcc, está associada a Tst seguida de λst e a interacção entre ambas. O estudo sobre a influência do estado morfológico inicial na subsequente evolução estrutural do PET durante o estiramento uniaxial, a frio, foi efectuado usando dois tipos de amostras: (i) quasi-amorfa, QA, e (ii) semicristalina, SC, neste caso, com ordem cristalina 2D e 3D, respectivamente. A evolução estrutural foi acedida através da deformação uniaxial das amostras em simultâneo com a sua caracterização in situ por WAXS. As amostras QA e SC com ordem 2D evoluem seguindo 3 etapas: i) Etapa I, a um nível de orientação quase constante, uma pequena quantidade da fase amorfa desenvolve-se para mesofase; ii) Etapa II, há um rápido aumento da orientação do polímero, acompanhado pelo aumento significativo da mesofase. Na amostra QA há também a formação de uma mesofase periódica. Finalmente, iii) na Etapa III, a orientação média do polímero atinge um patamar máximo, verificando-se a existência de relaxação parcial da mesofase periódica e da mesofase, no caso da amostra QA, enquanto que a amostra semicristalina evolui para uma estrutura 3D. A evolução da amostra semicristalina de ordem 3D consiste na contínua, mas lenta, orientação da fase cristalina e no aumento da fracção mássica de mesofase. Os nanocomposites de PET com MMT, TiO2, e SiO2 foram preparados através de diferentes processos de mistura por fusão, respectivamente: i) moldação por injecção, IM, ii) moldação por extrusão seguida de injecção, MEI e iii) utilizado um mini-misturador assimétrico de polímeros, MAP. Pela análise de SAXS, os nanocompósitos de PET com 3wt% de MMT, apresentam a existência de uma morfologia intercalada do MMT quando as técnicas de IM e MEI são utilizadas e uma melhor dispersão/desaglomeração do TiO2 e do SiO2 para MEI, comparativamente com MI. Os resultados de TEM sobre PET com 0.3wt% de nanopartículas, processados por MAP, mostram que a estrutura dos MMTs no compósito é dependente do tamanho dos aglomerados em pó, isto é, para aglomerados de pequena dimensão é obtida uma estrutura tactoide enquanto que para aglomerados de maiores dimensões é obtida uma estrutura intercalada. No caso de compósitos com TiO2 ou SiO2 é obtida uma boa dispersão. Comparativamente ao PET virgem, a incorporação de nanopartículas teve como efeito: i) o aumento da degradação do PET; ii) a redução de Tg do polímero; iii) actuam como agente nucleante, independentemente do seu tipo; iv) reduzem a deformabilidade do PET para nanocompósitos com 3wt% e aumentam para os de 0.3wt%. Neste último caso, o aumento é mais significativo quanto menor for a dimensão das nanocargas incorporadas e também se estas forem esféricas. O estudo da evolução estrutural multi-escala de nanocompósitos de PET foi efectuado a partir de experiências in situ de WAXS/SAXS. Concluiu-se que, independentemente do tipo de nanopartículas usadas, existem três etapas de desenvolvimento: Etapa I, antes da formação de pescoço, em que uma pequena percentagem de fase amorfa é transformada em mesofase, a um nível quase constante de orientação; Etapa II, coincidente com a propagação do pescoço, onde existe um rápido aumento da orientação molecular e da mesofase e o aparecimento de mesofase periódica. Simultaneamente é observado o aparecimento de fissuras na matriz e vazios entre os aglomerados de partículas. Na Etapa III, pescoço, corresponde ao aumento de fissuras e vazios e à estabilização da orientação molecular num patamar máximo. Há um ligeiro aumento da mesofase e é atingido o máximo de mesofase periódica. Em comparação com o PET, todos os tipos de nanocompositos mostraram: o aumento da quantidade de mesofase e o máximo da mesofase periódica formada a mais baixa deformação; o atraso no crescimento de fissuras no polímero; igual orientação máxima no final da deformação. Baseado nestes resultados foram sugeridos modelos estruturais multi-escala.
The present work deals with the investigation of the structural evolution mechanisms of PET and its nanocomposites under uniaxial deformation. The study is focused on: • the statistical influence of the stretching variables on the structure development of PET upon deformation in the rubbery state, • the effect of initial morphological state on the structural evolution under deformation in the solid state, • the effect of nanofillers (montmorillonite, MMT, titanium dioxide, TiO2, and silica dioxide, SiO2) on the morphology and final properties of reinforced PET (thermal and mechanical properties), and • the influence of different types nanofillers on the deformation behaviour of PET and its structure evolution during solid state stretching. This investigation aims at proposing multiscale structural models adequate to understand the morphological evolution of PET and its nanocomposites under uniaxial stretching. The statistical influence of the stretching variables: (temperature, Tst, rate, st ε& , and ratio, λst) on the structural development of PET during the rubbery state uniaxial stretching is investigated by means of WAXS, optical birefringence, BIR, and DSC. It is concluded that: i) the transformation of amorphous phase into mesophase is mainly controlled by λst and its interaction with st ε& ; ii) the strain-induced crystallization is governed by Tst, λst and the interaction among them. The same is happening with the level of molecular orientation; iii) the glass transition temperature is governed by the λst and its interaction with Tst, however the individual contribution of Tst is statistically low; iv) the influence over the cold crystallization temperature, Tcc, is associated to Tst, followed by λst and the interaction among them. The study of the influence of the initial morphological state on further structure evolution under solid state step uniaxial stretching was carried out using two types of samples: (i) a quasiamorphous, QA, and (ii) a semi-crystalline with 2D and 3D crystalline order, SC, respectively. The structural evolution was assessed by in situ WAXS performed simultaneously to the uniaxial stretching of the samples. The initial QA and 2D crystalline order SC samples evolve following three stages, described as: Stage I, before neck, at almost constant orientation level, the amorphous phase evolves into mesophase; Stage II, neck formation, there is a fast increase of polymer molecular orientation accompanied by large formation of mesophase. In the case of QA sample it is also observed the formation of periodical mesophase. Finally, Stage III, necking propagation, is characterized by the leveling off of the average polymer molecular orientation. In the case of the QA sample, a partial relaxation of periodical mesophase and mesophase is observed during this stage, while the SC sample evolves into a 3D crystalline order. The evolution of SC sample with 3D crystalline order mainly feature the continuous, but slow, increase of crystalline phase orientation and mesophase mass fraction. PET nanocomposites with MMT, TiO2, and SiO2 were prepared via different melt blending techniques, namely: i) direct injection moulding, DIM, ii) extrusion blending and injection moulding, EIM, and iii) asymmetric batch minimixer, ABM. SAXS analyses of PET 3 wt% nanocomposites revealed an intercalated MMT morphology for DIM and EIM, and also a better dispersion/deagglomeration of TiO2 and SiO2 for EIM than for DIM. The TEM results of PET 0.3 wt% nanocomposites processed via ABM shows that the structure of MMTs on the composite are dependent on the size of the powder agglomerate size, i.e., for agglomerates of small dimension, a tactoid morphologies is obtained while for larger dimensions of agglomerates an intercalated structure is observed. A good dispersion is obtained in the case of TiO2 and SiO2. The comparison of the nanocomposites with neat PET, revealed that the incorporation of the nanofillers: i) increases the polymer matrix degradation, ii) reduces the glass transition temperature of the polymer; iii) act as nucleating agents, regardless of their type; iv) reduces the deformability of the nanocomposite when 3wt% of filler is used but enhances it with 0.3wt% of filler. In this last case a greater improvement on deformation is observed when nanofillers of smaller size and spherical shape are incorporated in the PET matrix. Multiscale structure evolution of PET and its 0.3wt% nanocomposites during solid state uniaxial stretching were studied via in situ WAXS and SAXS. Despite the type of nanoreinforcements, three common stages were indentified: Stage I, before necking, is characterized by a small amount of amorphous phase evolving into mesophase at almost constant molecular orientation level; Stage II, at neck propagation, where a rapid increase of polymer molecular orientation is accompanied by a sharp increase of the mesophase and by the formation of a periodical mesophase; it is also observed the appearance of crazes in the polymer matrix, and voids within the nanoparticles agglomerates. Stage III, during necking, corresponds to the transformation of crazes and voids into micro-voids, at a plateau of average molecular orientation. The highest periodical mesophase content is achieved together with a slight increment of mesophase. In comparison to the neat PET structure evolution, all kind of nanocomposites showed: i) improved amount of mesophase and maximum periodical mesophase formed at earlier deformations; ii) retarded crazes widening/growing within the polymer bulk, and iii) similar maximum orientation level are achieved. Multiscale structures modelling are suggested based on the results obtained.

abstract: Commercially pure (CP) and extra low interstitial (ELI) grade Ti-alloys present excellent corrosion resistance, lightweight, and formability making them attractive materials for expanded use in transportation and medical applications. However, the strength and toughness of CP titanium are affected by relatively small variations in their impurity/solute content (IC), e.g., O, Al, and V. This increase in strength is due to the fact that the solute either increases the critical stress required for the prismatic slip systems ({10-10}<1-210>) or activates another slip system ((0001)<11-20>, {10-11}<11-20>). In particular, solute additions such as O can effectively strengthen the alloy but with an attendant loss in ductility by changing the behavior from wavy (cross slip) to planar nature. In order to understand the underlying behavior of strengthening by solutes, it is important to understand the atomic scale mechanism. This dissertation aims to address this knowledge gap through a synergistic combination of density functional theory (DFT) and molecular dynamics. Further, due to the long-range strain fields of the dislocations and the periodicity of the DFT simulation cells, it is difficult to apply ab initio simulations to study the dislocation core structure. To alleviate this issue we developed a multiscale quantum mechanics/molecular mechanics approach (QM/MM) to study the dislocation core. We use the developed QM/MM method to study the pipe diffusion along a prismatic edge dislocation core. Complementary to the atomistic simulations, the Semi-discrete Variational Peierls-Nabarro model (SVPN) was also used to analyze the dislocation core structure and mobility. The chemical interaction between the solute/impurity and the dislocation core is captured by the so-called generalized stacking fault energy (GSFE) surface which was determined from DFT-VASP calculations. By taking the chemical interaction into consideration the SVPN model can predict the dislocation core structure and mobility in the presence and absence of the solute/impurity and thus reveal the effect of impurity/solute on the softening/hardening behavior in alpha-Ti. Finally, to study the interaction of the dislocation core with other planar defects such as grain boundaries (GB), we develop an automated method to theoretically generate GBs in HCP type materials.
Dissertation/Thesis
Doctoral Dissertation Mechanical Engineering 2014

博士
國立臺灣大學
應用力學研究所
101
Nanomechanical sensors, which are usually cantilever-shaped, have attracted increasing interests in the last decade as a promising tool for real-time and label-free detection of chemical gas and biomolecules. These adsorbates introduce surface stress and additional mass upon the detective layer of the sensors and sequentially transduce to a static displacement or a resonant frequency shift of the nanomechanical system. The induced surface stress is the key element to design the performance of the microcantilever sensors. The surface stress can be compressive or tensile, which will result in opposite deflection at the free end of the microcantilever beam. Understanding the physical phenomena of stress change and knowing how much of the changes are needed to design the nanomechanical sensors. In this study, first-principles calculations were employed to investigate the adsorption-induced surface stress of self-assembled alkanethiolate monolayers on a sqrt(3)*sqrt(3)R30 Au(111) surface. A recently developed fully nonlocal van der Waals density functional was used to accurately account for the chain-chain interactions. Our results show that surface charge redistribution produces compressive surface stress, while chain-chain interactions produce tensile surface stress. The stress induced by surface charge redistribution is about one order of magnitude greater than that of chain-chain interactions. We observed that the chain-chain interactions play an important role in determining the molecular configuration during adsorptions, and also contribute significantly to the induced anisotropic tensile (positive) surface stress. As the chain length increases the tensile stress increases at a rate of ~0.32 (~0.18) N/m for the direction perpendicular (parallel) to the chain tilt direction. We also propose a multiscale modeling framework based on density functional theory calculation and finite element method analysis. The framework has been verified with the Stoney formula. The macroscopic surface stress has also derived from local anisotropic surface stresses. The deflection of microcantilever sensors subjected to randomly distributed SAM domains has shown to be similar to that under the macroscopic isotropic surface stress, endorsed this proposed framework. For coverage effect, the average cantilever deflection has a proportional relationship with the coverage of the surface stress. For chain length effect, the deflections due to the adsorption of fully covered alkanethiolate on Au(111) decrease as the chain length increase. This framework can be used not only for alkanethiolates SAMs on Au(111) in microcantilever, but also for other molecular adsorptions on general substrates in nanomechanical sensors.

abstract: Interstitial impurity atoms can significantly alter the chemical and physical properties of the host material. Oxygen impurity in HCP titanium is known to have a considerable strengthening effect mainly through interactions with dislocations. To better understand such an effect, first the role of oxygen on various slip planes in titanium is examined using generalized stacking fault energies (GSFE) computed by the first principles calculations. It is shown that oxygen can significantly increase the energy barrier to dislocation motion on most of the studied slip planes. Then the Peierls-Nabbaro model is utilized in conjunction with the GSFE to estimate the Peierls stress ratios for different slip systems. Using such information along with a set of tension and compression experiments, the parameters of a continuum scale crystal plasticity model, namely CRSS values, are calibrated. Effect of oxygen content on the macroscopic stress-strain response is further investigated through experiments on oxygen-boosted samples at room temperature. It is demonstrated that the crystal plasticity model can very well capture the effect of oxygen content on the global response of the samples. It is also revealed that oxygen promotes the slip activity on the pyramidal planes. The effect of oxygen impurity on titanium is further investigated under high cycle fatigue loading. For that purpose, a two-step hierarchical crystal plasticity for fatigue predictions is presented. Fatigue indicator parameter is used as the main driving force in an energy-based crack nucleation model. To calculate the FIPs, high-resolution full-field crystal plasticity simulations are carried out using a spectral solver. A nucleation model is proposed and calibrated by the fatigue experimental data for notched titanium samples with different oxygen contents and under two load ratios. Overall, it is shown that the presented approach is capable of predicting the high cycle fatigue nucleation time. Moreover, qualitative predictions of microstructurally small crack growth rates are provided. The multi-scale methodology presented here can be extended to other material systems to facilitate a better understanding of the fundamental deformation mechanisms, and to effectively implement such knowledge in mesoscale-macroscale investigations.
Dissertation/Thesis
Doctoral Dissertation Mechanical Engineering 2018

This thesis presents a multiscale structural analysis of upper crustal deformation at a passive continental margin, using the Jurassic - Quaternary Otway Basin along Australia’s southern margin as a case study. Techniques of structural analyses across the micro (calcite twin, magnetic and porefabric analyses), meso (wellbore and outcrop natural fracture analysis) and macroscales (three-dimensional seismic interpretation) providing an effective means of characterising stress and strain across space and time. The integration of these investigative methods at a passive continental margin for the first time, has assisted in reducing structural uncertainty for basin evolution models, delivering original insights into the evolution of stress within these tectonic environments. The results of this study show magnitudes of maximum differential stress as high as 69MPa during extension and continental breakup, in contrast to magnitudes as low as 13MPa during basin inversion. The influence of high extensional stresses during continental break up, resulting in layer parallel stretching (LPSt), a microstructural strain which may develop in layered rock, characterised by an azimuth of stretching or thinning, orthogonal to the orientation of regional extensional faults. LPSt occurs in the early stages of extension, prior to the development of calcite twins, natural fractures, and faults which occur progressively as the intensity and duration of extension increases. This is evidenced in the Otway Basin, where Late Cretaceous aged NE-SW and N-S oriented LPSt is co-axial with extensional azimuths during that time, derived from the stress inversion of seismic scale faults, calcite twins and natural fractures from the outcrop and wellbore. The neotectonic preservation of LPSt in the Otway Ranges, an uplifted section of Early Cretaceous sediments in the Otway Basin, suggests that early grain-scale extensional strain can be preserved during ensuing phases of inversion at continental margins. As during the process of inversion, stress is primarily released through the reactivation of previously formed extensional fault and detachment systems. A process of deformation that results in low levels of coupling between the basement and cover, an observation that is supported by the low magnitudes of compressional stress (13MPa) calculated during the same period. Additionally, the results of this study have improved our understanding of sub-surface fluid flow in the Otway Basin. Geomechanical modelling demonstrating that low contemporary magnitudes of effective normal stress, acting on NW-SE oriented faults, striking parallel to the orientation of maximum horizontal stress, results in a high risk of fault dilation. This suggests that future efforts of exploration for conventional oil and gas systems within the Otway Basin, are best focused where E-W, N-S and NE-SW striking faults interact with the major NW-SE fabric, or where the influence of basin inversion is most pronounced. A major outcome of this study is a new structural framework for the Otway Basin, one that is defined by a consistent pattern of NW-SE striking faults across much of the basin, in contrast to the previous structural model of opposing fault trends in the west and east. The new framework characterises a structural trend that is consistent with faulting patterns in sedimentary provinces to the west and east along Australia’s southern margin.
Thesis (Ph.D.) -- University of Adelaide, Australian School of Petroleum, 2019