Dissertations / Theses on the topic 'Shear localizations'

In turbulent flows at low Reynolds numbers localized structures are observed which can grow or spontaneously decay. Viewed as a dynamical system, a turbulent evolution forms a path through a phase space built upon exact invariant solutions of the Navier-Stokes equations. The leading stable and unstable manifolds of these solutions organise the local dynamics. In small periodic domains many of these solutions are known. However, to understand the full spatial-temporal nature of turbulence requires localized solutions which are unstable and live in a very high dimensional system. In the first half of this thesis we consider two problems in small, periodic domains where turbulence is global. We consider the geometry of the edge of chaos, a manifold which divides phase space and how such a manifold can be understood in the context of turbulent decay. We demonstrate that the edge is not separate from the turbulent dynamics but is wrapped up into these dynamics. Next we consider how the dynamics on the edge in short pipes are affected by Reynolds number and find new high Reynolds number solutions. In this second half we attack the problem of finding and understanding the origins of localized solutions. These solutions hold the key to expanding the theory towards physically realisable systems. Building upon the short pipe research we find the origin of the first localized pipe flow solution in a bifurcation from a downstream-periodic solution. Moving to a model for plane Couette flow, we attempt to find evidence of homoclinic-snaking as a route to spanwise localization. Instead we find a different route which matches recent work in duct flow. Finally, motivated by questions of how localized structures interact, we introduce a new flow, "localized Couette flow", and investigate the stability of this flow.

A condition for the onset of shear localization for the two-dimensional plane strain case is presented. Based on the condition, shear localization in three different problems are studied. By using the NIKE2D finite element package, a punching process for thin sheet metal with frictional surfaces is analyzed and the relationship between the frictions and shear localization is discussed.

The initiation of subduction requires the formation of lithospheric plates which mostly deform at their edges. Shear heating is a possible candidate for producing such localized deformation. In this thesis we employ a 2D model of the mantle with a visco-elasto-plastic rheology and enabled shear heating. We are able to create a shear heating instability both in a constant strain rate and a constant stress boundary condition setup. For the rst case, localized deformation in our specic setup is found for strain rates of 10-15 1/s and mantle temperatures of 1300°C. For constant stress boundaries, the conditions for a setup to localize are more restrictive. Mantle motion is induced by large cold and hot temperature perturbations. Lithospheric stresses scale with the size of these perturbations; maximum stresses are on the order of the yield stress (1 GPa). Adding topography or large inhomogeneities does not result in lithospheric-scale fracture in our model. However, localized deformation does occur for a restricted parameter choice presented in this thesis. The perturbation size has little effect on the occurrence of localization, but large perturbations shorten its onset time.

Comprendre les mécanismes de rupture des verres est d'une importance cruciale en raison des vastes applications industrielles de ces matériaux, où la maîtrise de leurs propriétés mécaniques détermine leur performance et leur durabilité. L'examen approfondi des mécanismes de plasticité sous indentation dans les verres aluminoborosilicatés a mis en évidence le rôle déterminant de la composition chimique sur le comportement mécanique et à la rupture. Il est apparu que la présence et la concentration relative de modificateurs de réseau tels que les oxydes alcalino-terreux, ou bien une plus grande concentration de bore en tant que formateur de réseau, influencent significativement l'écoulement plastique localisé sous forme de bandes de cisaillement ainsi que la résistance à l'initiation et à la propagation des fissures, suggérant que des ajustements précis de la composition peuvent améliorer la résistance de ces matériaux face aux sollicitations mécaniques. D'autre part, un ensemble de caractérisations mécaniques et thermiques de ces verres a permis d'établir des premières corrélations entre leur structure et leur comportement mécanique sous indentation. Par ailleurs, l'étude de l'effet de l'irradiation électronique sur la plasticité des verres silicatés a révélé que l'exposition aux électrons peut accroître la sensibilité de ces matériaux à la déformation plastique, modifiant leur structure microscopique et leurs propriétés mécaniques. Il s'est avéré que l'irradiation électronique catalyse les réarrangements structuraux, sous contrainte, liés à la plasticité, entraînant une nette diminution de la limite d'élasticité du verre silicaté. Ces modifications ont été analysées à travers des modèles avancés de relaxation et de déformation, permettant de quantifier et de prévoir l'impact de l'irradiation sur le comportement des verres. Ces travaux contribuent à la compréhension des processus de plasticité dans les verres et ouvrent la voie à des stratégies d'optimisation de leurs propriétés mécaniques, notamment en concevant des compositions spécifiques pour renforcer leur résistance dans des environnements industriels exigeants ou soumis à des conditions sévères
Understanding the mechanisms of glass fracture is crucial due to the extensive industrial applications of these materials, where the control of their mechanical properties is key to ensuring performance and durability. In-depth examination of plasticity mechanisms under indentation in aluminoborosilicate glasses has highlighted the critical role of chemical composition in mechanical behavior and crack resistance. It has been observed that the presence and relative concentration of network modifiers, such as alkaline earth oxides, or a higher concentration of boron as a network former, significantly influence localized plastic flow in the form of shear bands, as well as the resistance to crack initiation and propagation. This suggests that precise adjustments in composition can enhance the material's resilience under mechanical stress. Additionally, a series of mechanical and thermal characterizations of these glasses have established correlations between their structure and mechanical behavior under indentation. Furthermore, the study of the effects of electron irradiation on the plasticity of silicate glasses revealed that exposure to electrons can increase these materials' susceptibility to plastic deformation, altering their microscopic structure and mechanical properties. It was found that electron irradiation catalyzes structural rearrangements under stress, leading to a marked decrease in the yield stress of silicate glasses. These changes were analyzed through advanced relaxation and deformation models, allowing for the quantification and prediction of irradiation's impact on glass behavior. This work advances the understanding of plasticity processes in glasses and paves the way for strategies to optimize their mechanical properties, particularly by designing specific compositions to enhance their resistance in demanding industrial environments or under severe conditions

We study axisymmetric deformations of depleted uranium (DU) and tungsten heavy alloy (WHA) rods impacting at normal incidence both a rigid, planar target and a thick, deformable steel target. Each deformable material is modeled as elastic thermoviscoplastic; the flow stress increases with an increase in the effective plastic strain and effective plastic strain-rate but decreases with a rise in the temperature. An objective of this work is to ascertain when and where a shear band, defined as a narrow region of rapid, intense plastic deformation, forms in each material subject to impact loading. The Taylor impact simulations show that shear bands form earlier in WHA than in DU for the material parameters used. In the penetration simulations, shear bands form continuously in the ejecta of the DU penetrator while only one shear band occurs in the WHA ejecta followed by more uniform deformations. Note: In order to view the computer animations referenced in this thesis, one must have a QuickTime movie player and download the files named Ujce.mov Uz2e.mov Uz3e.mov Wjce.mov Wz2e.mov and Wz3e.mov from the same directory the "pdf" file resides in.
Master of Science

During high strain rate deformations of metals and metallic alloys, narrow regions of intense plastic deformations have been observed experimentally. The phenomenon is termed strain localization and is usually a precursor to catastrophic failure of a structure. Similar phenomenon has been observed in glassy polymers deformed both at slow and high strain rates. Whereas strain localization is attributed to material softening due to thermal heating in metallic alloys, it is believed to be due to the reorganization of the molecular structure in polymers. Here we numerically study the strain localization in Tungsten Heavy Alloys (WHAs), and glassy polymers. WHAs are heterogeneous materials and thus inhomogeneities in deformations occur simultaneously at several places. Thus strains may localize into narrow bands at one or more places depending upon the microstructure of the alloy. We analyze the strain localization phenomenon during explosion and implosion of WHA hollow cylinders. We have developed a procedure to generate three-dimensional microstructures from planar images so that the two have the same 2-point correlation function. The WHA considered here is comprised of W particulates in a Nickel-Iron (NiFe) matrix, and each constituent is modeled as a heat conducting, strain hardening, strain-rate hardening and thermally softening elastic-plastic material. Furthermore, the porosity is taken to evolve in each constituent and the degradation of material properties due to porosity is incorporated into the problem formulation. It is found that the strain localization initiation in WHA hollow cylinders does not significantly depend on microstructural details during either explosive or implosive loading. However, the number of disconnected regions of localized deformations is influenced by the microstructure. We have generalized constitutive equations for high strain rate deformations of two glassy polymers, namely, Polycarbonate (PC) and poly (methyl methacrylate) (PMMA). These have been validated by comparing computed results with test findings in uniaxial compression at different axial strain rates, and subsequently used to study strain localization in a plate with a through-the-thickness elliptic hole at the centroid and pulled axially at a nominal strain rate of 5,000 /s. For the problems studied, the intensely deformed narrow regions have very high shear strains in WHAs, but large axial strains in glassy polymers.
Ph. D.

Thesis advisor: Seth C. Kruckenberg
In the earth’s lithosphere there exists both homogeneous and heterogeneous deformation on a variety of scales. The lower crust specifically plays a critical role in lithospheric deformation; however, the lower crust does not deform homogenously but rather heterogeneously in space and time. One of the best avenues for addressing heterogeneous lower crustal deformation is through an integrated study of shear zones. While many studies have identified factors such as strain rate and temperature as key actors in lower crustal strain localization, more studies are needed to characterize the dominant grain-scale mechanisms that accommodate the development of lower crustal shear zones. The primary aim of this research is to investigate the dominant mechanisms that lead to strain localization in the lower crust. The Capricorn Ridge Shear Zone (CRSZ), Central Australia, is an ideal location for study because it is a lower crustal shear zone that contains discrete zones of strain localization, primarily adjacent to major lithological boundaries. Previous studies conclude that competency contrast caused strain to localize at the lithologic boundaries of the CRSZ, a hypothesis that is tested in this study. Using microstructural, textural, and rheologic analysis, as well as field-based mapping and grain size piezometry, this study finds that differential stresses in Capricorn Ridge range from 17-27 MPa for quartz, 31-42 MPa for plagioclase, and 2.8-7.6 MPa for enstatite. Monophase aggregate strain rates range from 1.6 x 10-15 to 1.7 x 10-14 s-1 for quartz, 4.5 x 10-15 to 3.3 x 10-14 s-1 for plagioclase, and 6.0 x 10-20 to 1.2 x 10-18 s-1 for enstatite; corresponding effective viscosities 0.3-1.7 x 1021 Pa.s, 0.3-1.5 x 1021 Pa.s, and 0.2-1.8 x 1025 Pa.s for quartz, plagioclase, and enstatite, respectively. Data across the CRSZ show that while strain rate (viscosity) in monophase aggregates of quartz and plagioclase are generally similar across the shear zone, they do decrease at lithologic boundaries. In contrast to a previous study’s finding that competency contrast caused strain to localize at these boundaries, both quartz and plagioclase appear to record strain accumulation through grain size reduction. However, the observations made in previous studies are not negated by this study, as it is possible that grain size reduction in the mylonite zones near the boundaries caused strain to accumulate over time and therefore produce the observed pattern of increasing fabric intensity with proximity to the lithologic boundaries
Thesis (MS) — Boston College, 2021
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Earth and Environmental Sciences

In this thesis, I present field, microstructural, and Electron Backscatter Diffraction (EBSD) analyses of rock fabrics from high strain zones in exposures of lower crustal Cretaceous plutons at Breaksea Entrance, Fiordland, New Zealand. The interplay between deformation and metamorphism occurs across multiple scales at the root of a continental arc. I show a series of steps in which retrogressive metamorphism is linked to the accommodation of deformation. I define three main phases of deformation and metamorphism at Breaksea Entrance. The first phase (D1) involved emplacement of dioritic to gabbroic plutons at depths up to 60 km. The second phase (D2) is characterized by deformation and metamorphism at the granulite and eclogite facies that produced high strain zones with linear fabrics, isoclinal folding of igneous layering, and asymmetric pressure shadows around mafic aggregates. New structural analyses from Hāwea Island in Breaksea Entrance reveal the development of doubly plunging folds that define subdomes within larger, kilometer-scale gneiss domes. The development and intensification of S2 foliations within the domes was facilitated by the recrystallization of plagioclase and clinopyroxene at the micro-scale (subgrain rotation and grain boundary migration recrystallization), consistent with metamorphism at the granulite and eclogite facies and climb-accommodated dislocation creep. EBSD data show a strong crystallographic preferred orientation in plagioclase during D2 deformation. The third phase (D3) is characterized by deformation and metamorphism at the upper amphibolite facies that produced sets of discrete, narrow shear zones that wrap and encase lozenges of older fabrics. Structural analyses reveal a truncation and/or transposition relationship between the older S2 and the younger S3 foliations developed during D3. Progressive localization of deformation during cooling, hydration, and retrogression, resulted in the breakdown of garnet and pyroxene to form hornblende, biotite, fine plagioclase and quartz. EBSD data show a strong crystallographic preferred orientation in hornblende. During D3, hornblende and biotite accommodated most of the strain through fluid-assisted diffusion creep. The last two events (D2 and D3) reflect a transition in deformation and metamorphism during exhumation, as well as a focusing of strain and evolving strain localization mechanisms at the root of a continental arc. An examination of structures at multiple scales of observation reveals that fabrics seen in the field are a composite of multiple generations of deformation and metamorphism.

High entropy alloys (HEAs) are a relatively new class of solid solution alloys that contain multiple principal elements to take advantage of their high configurational entropy, sluggish diffusion, lattice distortion, and the cocktail effect. In recent development, work hardening mechanisms known as twinning induced plasticity (TWIP) and transformation induced plasticity (TRIP) have been found active in Al0.3CoCrFeNi (molar fraction) and Fe50Mn30Co10Cr10 (at %) HEA compositions. Friction-stir processing was done to increase the mechanical properties and improve the microstructure of the alloys for the purpose of high strain rate performance. Quasi-static tensile tests as well as top-hat geometry Split-Hopkinson pressure bar tests were conducted to view the mechanical properties as well as view the microstructural evolution at dynamic strain rates. Overall, the Al0.3CoCrFeNi condition after friction-stir processing and heat treatment has proved to have the best mechanical properties, and selecting from the conditions in this study, Al0.3CoCrFeNi has better shear localization resistance.

Thesis (Ph. D.)--Civil and Environmental Engineering, Georgia Institute of Technology, 2006.
Mayne, Paul, Committee Member ; Frost, David, Committee Member ; Santamarina, Carlos, Committee Chair ; Rix, Glenn, Committee Member ; Ruppel, Carolyn, Committee Member.

The rheology and strength of the lower crust play a key role in lithosphere dynamics, influencing the orogenic cycle and how plate tectonics work. Despite their geological importance, the processes that cause weakening of the lower crust and strain localization are still poorly understood. Through microstructural analysis of naturally deformed samples, this PhD aims to investigate how weakening and strain localization occurs in the mafic continental lower crust. Mafic granulites are analysed from two unrelated continental lower crustal shear zones which share comparable mineralogical assemblages and high-grade deformation conditions (T > 700 °C and P > 6 Kbar): the Seiland Igneous Province in northern Norway (case-study 1) and the Finero mafic complex in the Italian Southern Alps (case-study 2). Case-study 1 investigates a metagabbroic dyke embedded in a lower crustal metasedimentary shear zone undergoing partial melting. Shearing of the dyke was accompanied by infiltration of felsic melt from the adjacent partially molten metapelites. Findings of case-study 1 show that weakening of dry and strong mafic rocks can result from melt infiltration from nearby partially molten metasediments. The infiltrated melt triggers melt-rock reactions and nucleation of a fine-grained (< 10 µm average grain size) polyphase matrix. This fine-grained mixture deforms by diffusion creep, causing significant rheological weakening. Case-study 2 investigates a lower crustal shear zone in a compositionally-layered mafic complex made of amphibole-rich and amphibole-poor metagabbros. Findings of case-study 2 show that during prograde metamorphism (T > 800 °C), the presence of amphibole undergoing dehydration melting reactions is key to weakening and strain localization. Dehydration of amphibole generates fine-grained symplectic intergrowths of pyroxene + plagioclase. These reaction products form an interconnected network of fine-grained (< 20 µm average grain size) polyphase material that deforms by diffusion creep, causing strain partitioning and localization in amphibole-rich layers. Those layers without amphibole fail to produce an interconnected network of fine grained material. In this layers, plagioclase deforms by dislocation creep, and pyroxene by microfracturing and neocrystallization. Overall, this PhD research highlights that weakening and strain localization in the mafic lower crust is governed by high-T mineral and chemical reactions that drastically reduce grain size and trigger diffusion creep.

Employing the Finite Element Method (FEM), we numerically study three problems involving free and forced vibrations of linearly and nonlinearly elastic plates with a third order shear and normal deformable theory (TSNDT) and the three dimensional (3D) elasticity theory. We used the commercial software ABAQUS for analyzing 3D deformations, and an in-house developed and verified software for solving the plate theory equations. In the first problem, we consider trapezoidal load-time pulses with linearly increasing and affinely decreasing loads of total durations equal to integer multiples of the time period of the first bending mode of vibration of a plate. For arbitrary spatial distributions of loads applied to monolithic and laminated orthotropic plates, we show that plates' vibrations become miniscule after the load is removed. We call this phenomenon as vibration attenuation. It is independent of the dwell time during which the load is a constant. We hypothesize that plates exhibit this phenomenon because nearly all of plate's strain energy is due to deformations corresponding to the fundamental bending mode of vibration. Thus taking the 1st bending mode shape of the plate vibration as the basis function, we reduce the problem to that of solving a single second-order ordinary differential equation. We show that this reduced-order model gives excellent results for monolithic and composite plates subjected to different loads. Rectangular plates studied in the 2nd problem have points on either one or two normals to their midsurface constrained from translating in all three directions. We find that deformations corresponding to several modes of vibration are annulled in a region of the plate divided by a plane through the constraining points; this phenomenon is termed mode localization. New results include: (i) the localization of both in-plane and out-of-plane modes of vibration, (ii) increase in the mode localization intensity with an increase in the length/width ratio of a rectangular plate, (iii) change in the mode localization characteristics with the fiber orientation angle in unidirectional fiber- reinforced laminae, (iv) mode localization due to points on two normals constrained, and (iv) the exchange of energy during forced harmonic vibrations between two regions separated by the line of nearly stationary points that results in a beating-like phenomenon in a sub-region of the plate. This technique can help design a structure with vibrations limited to its small sub-region, and harvesting energy of vibrations of the sub-region. In the third problem, we study finite transient deformations of rectangular plates using the TSNDT. The mathematical model includes all geometric and material nonlinearities. We compare the results of linear and nonlinear TSNDT FEM with the corresponding 3D FEM results from ABAQUS and note that the TSNDT is capable of predicting reasonably accurate results of displacements and in-plane stresses. However, the errors in computing transverse stresses are larger and the use of a two point stress recovery scheme improves their accuracy. We delineate the effects of nonlinearities by comparing results from the linear and the nonlinear theories. We observe that the linear theory over-predicts the deformations of a plate as compared to those obtained with the inclusion of geometric and material nonlinearities. We hypothesize that this is an effect of stiffening of the material due to the nonlinearity, analogous to the strain hardening phenomenon in plasticity. Based on this observation, we propose that the consideration of nonlinearities is essential in modeling plates undergoing large deformations as linear model over-predicts the deformation resulting in conservative design criteria. We also notice that unlike linear elastic plate bending, the neutral surface of a nonlinearly elastic bending plate, defined as the plane unstretched after the deformation, does not coincide with the mid-surface of the plate. Due to this effect, use of nonlinear models may be of useful in design of sandwich structures where a soft core near the mid-surface will be subjected to large in-plane stresses.
Doctor of Philosophy

The static and dynamic response of lattice materials is investigated to disclose and control the connection between microstructure and effective behavior. The analytical methods developed in the thesis aim at providing a new understanding of material instabilities and strain localizations as well as effective tools for controlling wave propagation in lattice structures. The time-harmonic dynamics of arbitrary beam lattices, deforming flexurally and axially in a plane, is formulated analytically to analyze the influence of the mechanical parameters on the dispersion properties of the spectrum of Floquet-Bloch waves. Several forms of dynamic localizations are shown to occur for in-plane wave propagation of grid-like elastic lattices. It is demonstrated that lattices of rods, despite being `simple' structures, can exhibit a completely different channeled response depending on the characteristics of the forcing source (i.e. frequency and direction) as well as on the slenderness of the elastic links. It is also shown how the lattice parameters can be tuned to attain specific dispersion properties, such as flat bands and sharp Dirac cones. In the research field of material instabilities, a key result proposed in this thesis is the development of both static and dynamic homogenization methods capable of accounting for second-order effects in the macroscopic response of prestressed lattices. These methods, the former based on an incremental strain-energy equivalence and the latter based on the asymptotic analysis of lattice waves, allow the identification of the incremental constitutive operator capturing the macroscopic incremental response of arbitrary lattice configurations. The homogenization framework has allowed the systematic analysis of prestress-induced phenomena on the incremental response of both the lattice structure and its `effective' elastic solid, which in turn has enabled the identification of the complex interplay between microstructure, prestress, loss of ellipticity (shear band formation) and short-wavelength bifurcations. Potential new applications for the control of wave propagation are also shown to be possible by leveraging the inclusion of second-order terms in the incremental dynamics. In particular, the tunability of the prestress state in a square lattice structure has been exploited to obtain dynamic interfaces with designable transmission properties. The interface can be introduced in a material domain by selectively prestressing the desired set of ligaments and the prestress level can be tuned to achieve total reflection, negative refraction, and wave channeling. The obtained results open new possibilities for the realization of engineered materials endowed with a desired constitutive response, as well as to enable the identification of novel dynamic material instabilities.

The static and dynamic response of lattice materials is investigated to disclose and control the connection between microstructure and effective behavior. The analytical methods developed in the thesis aim at providing a new understanding of material instabilities and strain localizations as well as effective tools for controlling wave propagation in lattice structures. The time-harmonic dynamics of arbitrary beam lattices, deforming flexurally and axially in a plane, is formulated analytically to analyze the influence of the mechanical parameters on the dispersion properties of the spectrum of Floquet-Bloch waves. Several forms of dynamic localizations are shown to occur for in-plane wave propagation of grid-like elastic lattices. It is demonstrated that lattices of rods, despite being `simple' structures, can exhibit a completely different channeled response depending on the characteristics of the forcing source (i.e. frequency and direction) as well as on the slenderness of the elastic links. It is also shown how the lattice parameters can be tuned to attain specific dispersion properties, such as flat bands and sharp Dirac cones. In the research field of material instabilities, a key result proposed in this thesis is the development of both static and dynamic homogenization methods capable of accounting for second-order effects in the macroscopic response of prestressed lattices. These methods, the former based on an incremental strain-energy equivalence and the latter based on the asymptotic analysis of lattice waves, allow the identification of the incremental constitutive operator capturing the macroscopic incremental response of arbitrary lattice configurations. The homogenization framework has allowed the systematic analysis of prestress-induced phenomena on the incremental response of both the lattice structure and its `effective' elastic solid, which in turn has enabled the identification of the complex interplay between microstructure, prestress, loss of ellipticity (shear band formation) and short-wavelength bifurcations. Potential new applications for the control of wave propagation are also shown to be possible by leveraging the inclusion of second-order terms in the incremental dynamics. In particular, the tunability of the prestress state in a square lattice structure has been exploited to obtain dynamic interfaces with designable transmission properties. The interface can be introduced in a material domain by selectively prestressing the desired set of ligaments and the prestress level can be tuned to achieve total reflection, negative refraction, and wave channeling. The obtained results open new possibilities for the realization of engineered materials endowed with a desired constitutive response, as well as to enable the identification of novel dynamic material instabilities.

Les zones de cisaillement constituent des objets structuraux communs de la lithosphère. À grande échelle, elles sont le siège principal des déplacements entre plaques tectoniques, accommodant de grandes quantités de déformation. La compréhension de leur comportement mécanique dans le temps et l'espace est donc essentielle pour la connaissance générale de la dynamique de la lithosphère. La température joue un rôle majeur sur la loi de comportement rhéologique qui caractérise le domaine ductile (en profondeur), réduisant alors efficacement la résistance mécanique. Chaque roche possède en outre des propriétés mécaniques intrinsèques qui varient en fonction de sa composition minéralogique, de sa texture et de sa structure interne. Or, en l'absence de grandeurs directement mesurables en profondeur, la rhéologie de la lithosphère demeure sujette à diverses interprétations. Le comportement mécanique des zones de cisaillement est d'autant plus méconnu qu'elles sont le siège d'intenses changements de la nature des roches et de perturbations thermiques majeures. En particulier, l'énergie mécanique qui y est convertie en chaleur (shear heating) peut engendrer une étroite interrelation entre thermique et mécanique. Ce travail de thèse vise à contribuer à la connaissance générale de la rhéologie des zones de cisaillement lithosphérique. Une approche originale a été mise en place, se basant sur l'évolution thermique aux abords et au sein des zones de cisaillement. Sur la base de modèles numériques thermo-cinématiques 2-D et de développements analytiques, la variabilité de premier ordre de l'évolution et de la perturbation thermique est analysée et quantifiée au regard de l'influence des trois processus thermiques majeurs que sont la diffusion, l'advection et le shear heating. Les résultats sont confrontés aux signatures thermiques métamorphiques associées aux chevauchements intra-continentaux pour lesquels les influences des processus d'accrétion et d'érosion sont également examinées. Le cas du Main Central Thrust (Himalaya), associé à une inversion thermique métamorphique bien développée, est pris comme exemple de référence. Nos résultats quantitatifs mettent en avant le rôle crucial du shear heating, notamment de la variabilité de la résistance mécanique des zones de cisaillement. L'accent est mis sur l'importance des paramètres de fluage des roches. L'étude de zones de cisaillement centimétriques développées au sein de la granodiorite du Zillertal (fenêtre des Tauern, Alpes) à la faveur de faibles variations de la composition minéralogique révèle l'extrême sensibilité de la rhéologie des roches ignées représentatives de la croûte continentale. Les conséquences de cette variabilité intense à petite échelle sont finalement discutées au regard des rhéologies classiquement considérées dans les modèles qui s'intéressent aux processus qui régissent la dynamique de la lithosphère
Shear zones are common structural features in the lithosphere and occur at various scales (from microscopic to lithospheric). At the lithospheric scale, they concentrate most of the relative movements between tectonic plates, and therefore, accommodate a high amount of strain. Consequently, the understanding of both their spatial and temporal mechanical behaviour is crucial for the general knowledge of the lithosphe dynamics. Rheology of rocks, which define their mechanical behaviour, is controlled by physical laws that predict how they deform under some stresses. Temperature plays a major role in the creep-dislocation behaviour, which characterizes the ductile domain (in depth), decreasing efficiently the rock strength. Furthermore, each rock has intrinsic mechanical properties, which depend on its mineralogical composition, texture and internal structures. However, due to the lack of data directly measurable deeper than a few kilometres, the lithosphere rheology, and in particular the continental lithosphere remains subject to drastically different interpretations. The mechanical behaviour of major shear zones is not fully understood, as they are the location of intense changes of both the rock internal nature and major thermal perturbations. Especially, the mechanical energy, converted into heat (shear heating) causes a close interaction between thermal ad mechanical evolutions. This thesis aims to better understand the rheological state of lithospheric scale shear zones. For this purpose, we used an original approach, based on the temperature field evolution around and within such shear zones. From 2D numerical thermo-kinematic models and analytical developments, the first order variability of thermal evolution and perturbation is anal- ysed and quantified with respect to the impact of three major thermal processes, defined as diffusion, advection and shear heating. Results are compared to metamorphic thermal signatures associated to intra-continental thrust zones for which the influence of both accretion and erosion was also investigated. The case of the Main Central Thrust (MCT) in the Himalayas, whose the inverse metamorphic thermal zonation has been extensively studied, was chosen as the main natural analogue. Our quantitative results highlight the crucial role of shear heating, and more particularly of mechanical strength variability within shear zones. We thus emphasise on the importance of rock creep parameters. The study of centimetre-scale shear zones, which developed within the granodiorite of the Zillertal nappe (Tauern window, Tyrol, Alps) thanks to little local variations of the mineralogical composition, reveals the extreme sensitivity of igneous rocks rheology, representative of the continental crust. The consequences of such an intense variability, revealed at small scale are finally discussed with regard to rheologies usually considered in models that focus on processes controlling lithosphere dynamics

Most of the conventional elastic plastic models of soils are based on continuum mechanics, however, for stiff, hard soils and soft rocks discontinuities develop under load, and since the models assume continuity, they would cease to apply. These discontinuities had not been accounted for in the continuum-based elastic plastic models. On the other hand, fracture mechanical theory may be used to advantage to replicate their behaviour. The behaviour of soil commonly is interpreted from conventional triaxial apparatus, whereas, testing of soil using the plane strain device would be more useful information, as more geotechnical field problems are basically occur in these situations.The present study has dealt with the investigation on the behaviour of saturated over consolidated clay as well as partially saturated clay, which represent the stiff and hard brittle clay by the use of a new biaxial device modified from conventional triaxial apparatus. In general, the apparatus was able to produce data which are in a good accordance with known soil behaviour of stiff clay. Shear band localization occurred in all test specimens of over consolidated clay. Specimen initiated to be discontinuous upon reaching the peak stresses. It is evident that specimen of partially saturated containing fissures had weaker shear strength as well as compressive strength.From point of view of the discontinuities that take places in the stiff clay, a model based on the unified model (Lo et al, 1996) and the elasto-plastic shear fracture model (Lo, et al, 2005) was used in this study. The problem may be dealth with one of brittle fracture of a three-phase specimen, where the matric suction is disrupted by tensile or shear loading. As a result the fracture toughness of the specimen would vary according to matric suction changes. A problem of plane strain compression testing was carried out to implement the model. The crack propagation simulation was resulted the same pattern with the experimental results on partially saturated kaolin clay.

Exhumed granitoid plutons are an ideal natural research target for studying the processes of nucleation and evolution of ductile and brittle deformation structures. Granitoid plutons, unaffected by later tectono-metamorphic cycles, preserve pristine deformation structures developed during cooling from magmatic to host rock ambient temperatures, that can be assumed as representatives of structures that at different structural levels of the continental crust. The main focus of this Ph.D. project is the analysis of deformation structures of the Rieserferner pluton – one of the major Periadriatic intrusions. The aim of the study is two-fold: (i) reconstruct the tectonic framework during the different stages of the pluton structural evolution, and (ii) determine the processes controlling localized ductile strain at different scales and the environmental conditions at which they occurred. The structural evolution during pluton cooling consists of 5 main deformation stages, that have been bracketed in time and thermal conditions according to microstructural and textural analysis, literature and field data: (i) steeply dipping joints, leucocratic dykes and quartz-feldspar veins and associated ductile shear zones; (ii) shallowly dipping joints with associated epidote and quartz veins and ductile shear zones; (iii) steeply dipping mafic dykes and calcite-white mica-bearing brittle-ductile faults; (iv) steeply dipping pseudotachylyte-bearing cataclastic faults; and (v) zeolite-bearing faults. Integrating new field, microstructural and geothermo-chronological data with published data we have related the deformation sequence to the Tertiary tectonics of the Eastern Alps. (i) three main ductile deformation stages developed during Oligocene, followed by two brittle deformation stages during Miocene; (ii) paleostress inversion from kinematic analyses suggest a complex stress field variation during the structural evolution mainly due to switch in relative magnitudes of principal stress components; (iii) the described paleostress evolution reflect the sequence of tectonic processes occurred during Oligocene and Miocene at the scale of the Eastern Alps, from slab break-off to indentation and lateral escape tectonics. Microstructural investigations were mainly focused on the analysis of ductile shear zones exploiting epidote- and quartz-rich veins. Softening and localization in quartz veins was mainly controlled by grain size reduction by recrystallization. EBSD mapping and image analyses have shown that different crystallographic orientations of quartz vein crystals controlled the evolution of microstructures and crystallographic preferred orientations (CPO) during vein-parallel simple shear up to high shear strains ( ≈ 10). Recrystallization by Subgrain Rotation (SGR) lead to the development of fine-grained ultramylonitic quartz veins, in which, the observed CPO banding have been inherited from the original crystallographic orientation of the vein crystals. Localization of ductile strain within heterogeneous shear zones exploiting epidote veins was mainly obtained through myrmekite development and following shearing. EBSD investigations suggest that myrmekite induced a switch in the dominant deformation mechanism, from dynamic recrystallization by SGR to diffusion-assisted grain boundary sliding (GBS) during shearing of plagioclase + quartz aggregates. Thermodynamic modelling was aimed to define the temperature-pressure-fluid conditions under which deformation these processes occurred. Pseudosections computed for the chemical systems NaCaKFMASHO and MnNaCaKFMASHO suggest that: (i) the epidote-veining event in the RFP likely occurred at temperatures between 520°C and 490°C at water-saturated conditions; (ii) the main deformation phase likely occurred at 460±40°C and 0.35 ± 0.05 GPa, lasting probably during pluton cooling down to 350°C at slightly under-saturated water-conditions.
I plutoni granitoidi esumati sono un target di ricerca ideale per la caratterizzazione dei processi di nucleazione e sviluppo delle strutture deformative sia duttili che fragili. I plutoni granitoidi sono corpi magmatici che per definizione, sono privi della moltitudine di strutture pervasive derivanti dagli intensi processi tettono-metamorfici che caratterizzano le rocce metamorfiche in generale. Per cui, le strutture deformative sviluppate durante il raffreddamento dei plutoni da condizioni magmatiche alle temperature della roccia incassante, sono preservate nei loro stadi incipienti. Tali strutture possono essere prese come esempio per lo sviluppo di strutture deformative a differenti livelli strutturali della crosta continentale. Il principale soggetto di ricerca trattato in questa tesi di dottorato è l'analisi delle strutture deformative del plutone di Vedrette di Ries – Rieserferner – una delle più importanti intrusioni Periadriatiche. Gli obbiettivi dell'analisi sono molteplici: (i) ricostruzione del contesto tettonico durante lo sviluppo dei diversi stage dell'evoluzione strutturale del plutone, e (ii) definizione dei processi alla base della localizzazione della deformazione duttile a varie scale e definizione delle condizioni alle quali questi processi avvengono. L'evoluzione strutturale durante il raffreddamento e successiva esumazione del plutone delle Vedrette di Ries comprende 5 fasi principali di deformazione: (i) joint, filoni leucocratici e vene a quarzo-feldspato ad alto angolo e zone di taglio associate; (ii) joint a basso angolo e associate vene a quarzo e vene a epidoto e associate zone di taglio duttili; (iii)faglie duttili-fragili ad alto angolo, associate a mineralizzazione a calcite e mica bianca e all'intrusione di filoni mafici; (iv) faglie cataclastiche e pseudotachylyti ad alto angolo; (v) faglie cataclastiche a zeoliti. Tali fasi sono state vincolate in termini di temperature e cronologia assoluta in questo lavoro tramite la comparazione di analisi microstrutturali, dati di letteratura e dati di rilevamento geologico. Tale vincolo ci ha permesso di collegare l’evoluzione descritta con la tettonica del Terziario delle Alpi Orientali. I principali risultati del nostro lavoro possono essere così riassunti: (i) Tre fasi principali di deformazione duttile sono avvenute durante il raffreddamento del plutone nell’Oligocene; in seguito, due fasi fragili si sono sviluppate durante l’esumazione regionale nel Miocene; (ii) l'analisi della cinematica delle strutture e l'inversione del paleostress suggeriscono una variazione complessa del campo di sforzi, principalmente legato alla variazione delle intensità relative delle componenti principali di sforzo; (iii) l'evoluzione del paleostress riflette la sequenza di processi tettonici avvenuti durante l'Oligocene ed il Miocene alla scala delle Alpi Orientali, dai processi legati allo slab break-off, alla tettonica di indentazione e di estrusione laterale. Le indagini microstrutturali sono state principalmente indirizzate all'analisi delle zone di taglio derivanti da vene a quarzo ed epidoto. I processi di softening e localizzazione nelle vene a quarzo sono principalmente controllati da processi di grain-size reduction per ricristallizzazione dinamica. Le analisi dei campioni raccolti tramite electron-backscattered diffraction (EBSD) ed analisi di immagine hanno mostrato che l'orientazione cristallografica dei cristalli di quarzo della vena hanno controllato l'evoluzione microstrutturale e dell'orientazione cristallografica preferenziale (CPO) durante la deformazione di taglio semplice parallela alla vena fino ad elevate deformazione (>10). La ricristallizzazione tramite subgrain rotation (SGR) ha portato allo sviluppo di vene di quarzo ultramilonitiche a grana fine, nelle quali, la struttura a bande della CPO è stata ereditata della orientazione cristallografica originale dei cristalli di vena. La localizzazione della deformazione duttile su zone di taglio eterogenee nucleate sulle vene a epidoto, invece, è stata principalmente ottenuta tramite lo sviluppo di myrmekiti e successiva deformazione. Analisi EBSD suggeriscono che lo sviluppo di myrmekiti ha indotto uno scambio nei processi di deformazione dominanti, dalla ricristallizzazione dinamica per mezzo di dislocation creep, a processi di diffusion-assisted grain boundary sliding durante la deformazione degli aggregati di plagioclasio + quarzo derivanti dalle myrmekiti. La modellizzazione termodinamica ha permesso di definire le condizioni di pressione-temperatura-fluidi alle quali questi processi furono attivi. Le risultanti pseudosezioni calcolate per i sistemi chimici NaCaKFMASHO e MnNaCaKFMASHO suggeriscono che: la formazione delle vene ad epidoto avviene a temperature comprese tra 520°C e 490°C in condizioni di saturazione della fase fluida; (ii) la fase di deformazione principale probabilmente avviene a 460 ± 40 °C e 0.35 ± 0.05 GPa, perdurando durante il raffreddamento del plutone probabilmente fino a 350°C in condizioni di quasi-saturazione della fase fluida.

For more than five decades, hydraulic fracturing has been widely used to enhance oil and gas production. Hydraulic fracturing in solid materials (e.g., rock) has been studied extensively. The main goal of this thesis is a comprehensive study of the physical mechanisms of hydraulic fracturing in cohesionless sediments. For this purpose, experimental techniques are developed to quantify the initiation and propagation of hydraulic fractures in dry particulate materials. We have conducted a comprehensive experimental series by varying such controlling parameters as the properties of particulate materials and fracturing fluids, boundary conditions, initial stress states, and injection volumes and rates. In this work, we suggest principle fundamental mechanisms of hydraulic fracturing in particulate materials and determine relevant scaling relationships (e.g., the interplay between elastic and plastic processes). The main conclusion of this work is that hydraulic fracturing in particulate materials is not only possible, but even probable if the fluid leak-off is minimized (e.g., high flow rate, high viscosity, low permeability). Another important conclusion of this work is that all parts of the particulate material are likely to be in compression. Also, the scale effect (within the range of the laboratory scales) appears to be relatively insignificant, that is, the observed features of fractures of different sizes are similar. Based on the observed fracture geometries, and injection pressures we suggested three models of hydraulic fracturing in particulate materials. In the cavity expansion or ??e driving model, the fracturing fluid is viewed as a sheet pile (blade) that disjoints the host material, and the cavity expansion occurs at the fracture (blade) front. The shear banding model is also consistent with a compressive stress state everywhere in the particulate material and explains the commonly observed beveled fracture front. The model of induced cohesion is based on the fluid leak-off ahead of the fracture front. The induced cohesion may be caused by the tensile strain near the fracture tip (where the stress state is also compressive), which, in turn, induces the cavitation of the leaked-off fluid and hence capillary forces.

Les milieux granulaires sont très étudiés depuis des décennies mais la description de l'ensemble des comportements observés de ces matériaux reste une grande question ouverte. Lorsqu'ils sont soumis à une contrainte suffisamment importante, une caractéristique est de présenter de la localisation de la déformation. L'objectif du travail présenté dans ce mémoire est d'étudier expérimentalement et numériquement la déformation d'un milieu granulaire et de caractériser des comportements observés lors d'un text biaxial. La première partie est consacrée à la réalisation des tests biaxiaux en déformation plane. Pour pouvoir visualiser de très petites déformations, nous utilisons une méthode interférométrique basée sur la diffusion multiple de la lumière. La deuxième partie est dédiée à la modélisation numérique d'un test biaxial en 2D dans des conditions similaires à celles de l'expérience par la méthode des éléments discrets. Enfin, dans la dernière partie, des outils développés pour l'analyse d'images utilisés pour étudier aussi bien les expériences que les simulations numériques sont abordés. L'étude du champ plastique moyen dans les expériences montre que la localisation de la déformation est un processus progressif initié par une bifurcation qui correspond à l'apparition d'une direction bien définie. Cette direction est en accord avec l'angle de Mohr-Coulomb et son apparition a lieu avant la rupture du matériau. L'étude des fluctuations de la plasticité dans les expériences et les simulations numériques semble mettre en évidence une croissance d'une longueur caractéristique
Granular materials have been studied for decades, but the description of the behaviors observed of these materials is still an open question. They display localization of deformation when submitted to a large enough stress. The objective of this work is to study experimentally and numerically the deformation of a granular material and to characterize observed behaviors in a biaxial text. The first part is devoted to the realization of plane strain biaxial tests. In order to visualize very small deformations, we use an interferometric method based on the multiple light scattering. The second part is devoted to the numerical modeling of a 2D biaxial test under conditions similar to those of the experiment by the discrete element method. Finally, in the last part, tools developed for the analysis of images used to study as well the experiences as the numerical simulations are approached. The study of the average plastic field in the experiments shows that the localization of the deformation is a progressive process initiated by a bifurcation which corresponds to the appearance of a well defined direction. This direction is in agreement with the angle of Mohr-Coulomb and its appearance takes place before the failure of the material. The study of the fluctuations of the plasticity in the experiments and the numerical simulations seems to show an increase of a characteristic length

Ce travail présente une caractérisation expérimentale du comportement mécanique et de la rupture par localisation de la déformation dans un grès des Vosges. L'évolution temporelle de la localisation a été caractérisée par des mesures de champs. Une nouvelle cellule triaxiale vraie a été développée au Laboratoire 3SR (Grenoble), qui permet une visualisation des échantillons sous chargement pour réaliser de la corrélation d'image numérique (CIN). Les essais ont été réalisés par compression en déformation plane (confinement de 20 à 50 MPa). La transition d'une déformation diffuse à localisée a été finement étudiée. Une analyse comparative a été ensuite effectuée entre les mesures de champs et la microstructure à l'échelle des grains observée par microscope (MEB). Enfin, une étude théorique basée sur une analyse en bifurcation a été menée pour comparer observations des bandes de cisaillement et prédiction sur la localisation de la déformation
This work aims an experimental characterization of the mechanical behaviour and failure by strain localization on a Vosges sandstone. The time evolution of strain localization has been characterized by full-field measurements. A new true-triaxial apparatus has been developed at Laboratoire 3SR (Grenoble), which enables the observation of the specimens during mechanical loading for application of digital image correlation (DIC). Tests have been performed in plane strain compression (confining pressure from 20 to 50 MPa). The transition from diffuse to localised deformation regimes has been extensively studied. Then, a comparative analysis has been done between the strain fields (DIC) and microscope (SEM) observations to determine how closely the DIC fields are related to deformation mechanisms detected at the grain scale. Finally, a theoretical bifurcation analysis is presented to compare the experimental observations of shear bands with strain localization prediction

The frictional behavior of a fault and the physical and mechanical properties of the fault rocks have important implications for earthquake nucleation, propagation and arrest. To further understand the mechanical behavior of carbonate-bearing faults, low- to high-velocity experiments on calcite gouge have been conducted using three rotary-shear apparatus: ROSA, located at the Department of Geosciences of the University of Padua, Italy; SHIVA, at the Istituto Nazionale die Geofisica e Vulcanologia (INGV) in Rome, Italy, and the Phv-apparatus of the Physical Property Research Group of the Kochi Institute for Core Sample Research, Kochi, Japan. Three main topics were addressed with the experimental approach: 1) The formation of clast-cortex aggregates in natural and experimental calcite-bearing fault zones (Chapter I); 2) Localization of strain in gouge layers (Chapter II); and 3) The effect of fluids on the frictional behavior of calcite gouge (Chapter III). Clast-cortex aggregates (CCAs) are composite grains found in the slipping zones of faults hosted in calcite- and clay-rich rocks that were previously suggested to be textural evidence of seismic slip on a fault. Experimental investigation of the dependence of CCA formation in calcite gouge layers on the applied slip rate, normal stress, total displacement and ambient humidity showed that CCAs formed at all investigated slip rates (100 µm/s to 1 m/s) but only at relatively low normal stresses (≤5 MPa). The aggregates were better developed with increasing displacement (up to 5 m) and did not form in experiments with water-dampened gouges. In the experiments, aggregates formed in low-strain regions within the gouge layers, adjacent to the highest-strain slip zones. We propose that CCAs in calcite-bearing slip zones form in the shallow portions of faults during shearing in relatively dry conditions, but our experiments suggest that they cannot be used as indicators of seismic slip. Formation involves clast rotation due to granular flow accompanied by accretion of fine matrix material possibly facilitated by electrostatic forces. To further understand strain localization during seismic slip, which is a fundamental mechanical process that has implications for frictional heating and the earthquake energy budget, we performed intermediate- to high-velocity rotary-shear experiments on calcite gouge containing strain markers made of dolomite gouge. Sheared markers should provide microstructural information of the strain distribution in the gouge layer and its dependence on the applied total strain, normal stress, slip rate, and ambient conditions. Microstructural analysis revealed that in both dry and water-dampened gouges strain localization at 1 m/s occurs progressively and rapidly. The strain accommodated in the bulk of the gouge layer does not change significantly with increasing total displacement, suggesting that, once formed, the high-strain slipping zone and principal slip surface accommodate most of the ongoing displacement. This is supported by the presence of sintered and recrystallized grains and zones of calcite decarbonation adjacent to the principal slip surface, indicating localized frictional heating. Faster localization in water-dampened conditions, although suggested by the faster dynamic weakening, was not reflected in the investigated microstructures. Instead, weakening in water-dampened gouges may be enhanced by faster subcritical crack growth in the presence of fluids. When extrapolated to natural conditions, our results suggest that calcite-bearing gouge slip zones are more prone to slip in the presence of water than in relatively dry conditions. The effect of fluids on the frictional behavior of calcite gouge was further investigated by conducting intermediate- to high-velocity experiments with controlled fluid pressure. Consistent with our results from experiments with strain-markers, slip appears to be localized on one or more slip surfaces adjacent to which zones of recrystallization are found. Grey or black material covering the slip surface of several samples was identified as disordered carbon by Raman spectroscopy indicating decarbonation of the calcite. In low-velocity (1 mm/s) experiments, a lower shear stress of water-saturated gouges as compared to room-dry conditions is attributed to intergranular lubrication and to the lower calcite fracture surface energy accelerating subcritical crack growth, consistent with a high degree of compaction. At the initiation of sliding at high velocity, weakening in saturated gouges occurs abruptly, while the room-dry gouges show a pronounced strengthening phase before the onset of weakening. For a given effective normal stress, the peak stress is lower, and the strengthening phase is longer, for higher pore-fluid factors. The weakening in room-dry and water-saturated gouges sheared at high velocity likely occurs by flash heating, which is accelerated in the presence of fluids by subcritical crack growth. Consistent with flash heating, the presence of carbon on the slipping surface of our calcite samples indicated that decarbonation has occurred even though the bulk temperature of the gouge layer was lower than the decarbonation temperature. At high velocity, intense frictional heating leads to thermal pressurization and subsequent decrease of the shear stress in the experiments performed in undrained conditions. Zones of recrystallized grains adjacent to the principal slip surfaces are possibly featuring microstructures characteristic for grain boundary sliding aided by diffusion creep which suggests that strain was not only accommodated by frictional processes, but possibly by superplasticity. The experimental results suggest that the presence of water in carbonate-bearing faults facilitates earthquake nucleation and even more so if the fluids present are pressurized. This might explain the long-lasting earthquake sequences e.g. of Umbria-Marche and L’Aquila hosted on carbonate-bearing faults. Additionally, some of the slip distribution complexity during earthquakes occurring in carbonate sequences might be due to a difference in the degree of fluid saturation in different fault patches.
Le proprietà frizionali delle faglie e le proprietà fisiche e meccaniche delle rocce di faglia influenzano in modo importante la nucleazione, la propagazione e l’arresto dei terremoti. Per capire più approfonditamente il comportamento meccanico delle faglie in rocce carbonatiche, sono stati fatti esperimenti a diverse velocità usando gouge (rocce granulari) di calcite, con tre diverse macchine rotary shear: ROSA, installata presso il Dipartimento di Geoscienze dell’Università di Padova, Italia; SHIVA, presso l’Istituto Nazionale di Geofisica e Vulcanologia (INGV), Roma, Italia, e il Phv-apparatus del Physical Property Research Group del Kochi Institute of Core Sample Research, Kochi, Giappone. Tre sono gli obiettivi principali indagati con il metodo sperimentale: 1) La formazione di clast-cortex aggregates (aggregati aventi al nucleo un clasto e una corteccia composta da detrito granulare ultrafine) nelle zone di faglia ricche in calcite sia naturali che sperimentali (Capitolo I); 2) Localizzazione della deformazione nei livelli di gouge (Capitolo II); e 3) L’effetto dei fluidi (acqua) nel comportamento frizionale del gouge di calcite (Capitolo III). I Clast-cortex aggregates (CCAs) sono clasti compositi che si trovano nelle zone di slip delle faglie ricche in calcite e minerali argillosi, precedentemente candidati sulla base di evidenze tessiturali ad essere indicatori di scivolamento cosismico. Esperimenti mirati sono stati fatti per trovare la correlazione tra la formazione di CCA in gouge di calcite e velocità, sforzo normale, rigetto totale e condizioni ambientali (umidità atmosferica e saturazione in acqua). I risultati sperimentali mostrano che i CCA si formano a tutte le velocità di scivolamento (da 100 µm/s a 1 m/s) ma solo a sforzi normali relativamente bassi (<5 MPa). Gli aggregati sono più abbondanti e meglio sviluppati per grandi rigetti (massimo rigetto imposti pari a 5 m) e non si formano negli esperimenti con gouge saturo d’acqua. Negli esperimenti, gli aggregati si sono formati in regioni poco deformate del livello di gouge, ma adiacenti alle zone con elevata localizzazione della deformazione. Da queste osservazioni sperimentali concludiamo che i CCA si formino nelle parti più superficiali delle faglie durante lo la deformazione per taglio in condizioni relativamente asciutte, ma non necessariamente durante lo scivolamento cosismico. Di conseguenza i CCA non possono essere usati come indicatori di slip cosismico. Il meccanismo di formazione dei CCA è per rotazione dei clasti dovuta al flusso granulare accompagnato ad accrescimento per cattura di particelle più piccole della matrice, probabilmente a causa di forze di natura elettrostatica. Per meglio comprendere i meccanismi di localizzazione della deformazione durante lo scivolamento cosismico, che controlla, p.e., lo sviluppo di calore per attrito su faglia e il bilancio energetico di un terremoto, abbiamo condotto esperimenti imponendo velocità di scivolamento da intermedie ad elevate con macchine tipo rotary shear su gouge di calcite. All'interno dello spessore del gouge abbiamo posizionato dei marker (indicatori) di deformazione per taglio composti da gouge di dolomite. I marker, deformandosi unitamente alla matrice di calcite, consentono di misurare la distribuzione della deformazione per taglio nel livello di gouge negli esperimenti. Le analisi microstrutturali hanno dimostrato che sia in condizioni asciutte che in presenza d’acqua la deformazione a velocità di scivolamento di 1 m/s, è molto rapida e si localizza in una zona principale di scivolamento (ZPS) dallo spessore di poche decine di micrometri e sulla adiacente superficie principale di scivolamento (SPS). La deformazione per taglio accomodata nella parte rimanente del livello di gouge non cambia significativamente all’aumentare del rigetto, suggerendo che, una volta localizzata, la ZPS e la SPS accomodano la maggior parte del rigetto. Questa conclusione è supportata dalla presenza di granuli sinterizzati e ricristallizzati e zone di decarbonatazione della calcite adiacenti alla SPS, che indicano lo sviluppo, estremamente localizzato, di calore per attrito. I dati meccanici indicano che i gouge saturi in acqua si indeboliscono (l'attrito diminuisce più rapidamente con il rigetto) di quelli asciutti, ma le microstrutture sono sostanzialmente simili per quanto riguarda la velocità di localizzazione della deformazione. L'indebolimento frizionale nei gouge saturi d'acqua può essere innescato dal meccanismo di crescita sub-critica delle microfratture, più efficiente in presenza d'acqua. L'estrapolazione di questi risultati alle condizioni naturali, suggerisce che i gouge ricchi in calcite sono più favorevoli allo scivolamento se saturi in acqua, piuttosto che in condizioni relativamente più asciutte. L’effetto dei fluidi sul comportamento frizionale di gouge di calcite è stato ulteriormente studiato attraverso esperimenti in controllo di pressione di fluidi a velocità da intermedie ad elevate. Coerentemente con i nostri esperimenti con gli indicatori di deformazione, il rigetto appare localizzato su una o più superfici di scivolamento che sono spesso contornate da zone di ricristallizzazione. La microspettroscopia Raman ha evidenziato la presenza di carbonio amorfo sulla superficie di scivolamento, indicatore di processi di decarbonatazione nella calcite. In esperimenti condotti a basse velocità di scivolamento (1 mm/s), la minore resistenza al taglio dei gouge saturi d’acqua rispetto ai gouge deformati in presenza di sola umidità atmosferica, è attribuita a lubrificazione intergranulare operata dalla acqua e alla bassa energia di superficie della calcite. Quest'ultima consente l'accelerazione dei processi di crescita sub-critica delle microfratture cui corrisponde un alto grado di compattazione. Nelle prime fasi di scivolamento ad alte velocità, l’indebolimento nei gouge saturi avviene improvvisamente, mentre i gouge in presenza di umidità atmosferica mostrano una fase di aumento di resistenza al taglio prima della fase di indebolimento. Per un dato sforzo normale efficace, per rapporti più elevati di pressione di poro su sforzo normale, lo sforzo di taglio di picco è minore e la fase di aumento di resistenza che precede l'indebolimento più lunga. La riduzione della resistenza per attrito ad alte velocità di scivolamento (cosismiche, ca. 1 m/s), sia in condizioni di umidità atmosferica che sature d’acqua, occorre verosimilmente per meccanismo di "riscaldamento istantaneo" (flash heating) alla scala delle asperità (decine di micrometri). L'indebolimento per "flash heating" è accelerato in presenza di fluidi per il meccanismo di crescita subcritica delle microfratture. Coerentemente con il verificarsi di flash heating, la presenza di carbonio sulla superficie di scivolamento dei nostri campioni di calcite indica che la decarbonatazione è avvenuta nonostante le temperature medie nell'intera zona di scivolamento, misurate con termocoppia, fossero più basse di quella di decarbonatazione. Ad alte velocità di scivolamento, in esperimenti in condizioni sature non drenate, la presenza di un intenso riscaldamento frizionale comporta la pressurizzazione termica del livello di gouge, con conseguente diminuzione dello sforzo di taglio. Nella zona di scivolamento, la formazione di nanoparticelle, grani ricristallizati di calcite e microcavità adiacenti alla superficie di scivolamento principale può essere associata a processi di grain boundary sliding sostenuti da processi diffusivi dipendenti dalla granulometria. Di conseguenza, la deformazione cosismica non è accomodata da soli processi prettamente frizionali, ma anche di tipo superplastico. I risultati degli esperimenti indicano che la presenza d’acqua in faglie all’interno di litologie carbonatiche facilita l’enucleazione di terremoti, ancor più se i fluidi presenti sono in pressione. Questa potrebbe essere una possibile spiegazione delle lunghe sequenze sismiche all’interno di successioni carbonatiche, ad esempio Umbria-Marche e L’Aquila. In aggiunta, la complessa distribuzione dei rigetti durante un singolo terremoto potrebbe essere causata da differenze nel grado di saturazione in fluidi in diverse zone della faglia.

De nombreuses études ont montré l’impact de la déformation sur le système K-Ar, et donc les âges ⁴⁰Ar/³⁹ Ar. Ces études se limitent souvent à une comparaison des âges obtenus dans des roches déformées et un protolithe indemne de déformation. La première partie de cette étude a inversement consisté à étudier la distribution de la déformation à différentes échelles et à décrire finement les gradients d’intensité de la déformation. L’étude a porté sur deux protolithes de nature granitique, associés à une différence d’âge entre leur formation et les évènements tectonométamorphiques faible (< 1 Ma ; massif d’Ikaria) ou élevée (>240 Ma ; massif du Tende). Pour le premier cas, la déformation entraine une perte de 40Ar dans les clastes des phases potassiques, interprétée comme résultant de la réduction des tailles des domaines de diffusion qui n’est pas accentuée par une intensité de déformation croissante. Pour le second cas, l’héritageen 40Ar du protolithe se traduit par la circulation de fluides et de 40Ar externe au système via les structures se déformant activement, produisant parfois un vieillissement des âges grandissant dans la phengite des structures les plus localisantes, alors que sur d’autres coupes il est observé un rajeunissement plus logique. Pour les deux cas,l’interprétation des âges obtenus dans les phases néoformées pendant la déformation est ambiguë entre refroidissement,cristallisation et mélange, et nécessite un examen détaillé des données confrontées aux températures de fermeture possibles. Les interprétations indiquent pour le cas d’Ikaria une localisation de la déformation ductile en moins de 1-3Ma le long d’un gradient de second ordre d’une dizaine de mètre d’épaisseur. La localisation de la déformation à l’échelle d’une zone de cisaillement se réalise plus rapidement dans le cas de l’exhumation post-orogénique d’un MCC(~7 Ma) que dans le cas de l’exhumation de matériel continental impliqué dans un prisme de subduction (~14-10 Ma)
Numerous studies have shown the impact of deformation on the K-Ar system, and therefore ⁴⁰Ar/³⁹ Ar ages. These studies often do not provide data characterizing deformation and are limited to a comparison of the ages obtained indeformed rocks and an undeformed protolith. The first part of this study thus consisted in studying the strain distribution at different scales and finely describing strain intensity gradients. The study focused on two granitic protoliths, associated respectively with a difference in age between the formation of the protolith and the age of the tectonometamorphic events that is low (<1 Ma ; Ikaria Island) or inversely high (> 240 Ma ; Tenda massif). In the firstcase study, deformation results in a 40Ar loss in K-bearing phases, interpreted as resulting from the reduction of diffusion domains sizes which is not accentuated by an increasing strain intensity. In the second case study, the 40Arinheritance of the protolith results in fluids and extraneous 40Ar circulation through the actively deforming structures,ages in phengite being increasingly older approaching the most localizing structures in some sections, while others behave in an opposite way, more in line with the progressive strain localization in time. For both cases, interpretation of ages obtained in the newly formed phases during deformation is ambiguous between cooling, crystallization and mixing, and requires a detailed examination of the data confronted with the possible closing temperatures.Interpretations indicate for the Ikaria case study a strain localization in less than 1-3 Ma along a second order gradient of about ten meters in thickness. Strain localization at the scale of a shear zone occurs more rapidly in the case of a post-orogenic exhumation of a MCC (~ 7 Ma) than in the case of the exhumation of continental material involved in a subduction prism (~ 14-10 Ma)

Pour juger de la pertinence du concept de bande de cisaillement applique a la rupture des roches aussi bien dans le domaine fragile que ductile, deux methodes de suivi de la deformation lors d'essais homogenes ont ete mises en uvre, a savoir, la tomodensitometrie et la stereophotogrammetrie de faux relief. Appliquees a des essais triaxiaux et biaxiaux realises avec ou sans confinement, ces deux methodes permettent, chacune a leur echelle, de cerner la transition d'une deformation diffuse a un mode localise avec apparition de fissures. Avec la stereocomparaison, l'ouverture de fissures, leurs interactions et leur contribution a la deformation globale peuvent entre autres etre quantifiees. Le geomateriau cohesif qui a ete l'objet d'une telle etude est la marne de beaucaire, une roche tendre saturee finement grenue dont la pression de consolidation est de 5 mpa. La rupture de type fragile avec propagation brutale en mode de cisaillement d'une macro-fissure que l'on observe en compression simple et sous tres faible confinement (0. 05 ou 0. 1 mpa) a pu etre analysee de meme que la localisation en bandes de cisaillement contractantes obtenue sous 0. 25 mpa ou la densification du materiau observee sous plus fort confinement (1 mpa). Dans la mesure ou la notion de bande de cisaillement a ete, a partir de nos resultats experimentaux, jugee adequate pour decrire le comportement a la rupture de la marne de beaucaire dans les domaines de contraintes consideres, une analyse de bifurcation en mode localise visant a predire le seuil de localisation et l'orientation de la bande de cisaillement, a ete conduite avec une loi incrementale non lineaire particulierement souple, la loi cloe. Pour mener a bien une telle analyse sur un cas concret: le trou de forage, les parametres de cloe pour la marne de beaucaire ont d'abord ete identifies sur des chemins triaxiaux et biaxiaux elementaires

Les mécanismes d’exhumation des roches de haute-pression basse-température (HP-BT) au sein des zones de subduction sont nombreux, complexes et toujours activement débattus. L’étude des zones de subduction fossiles permet alors de mieux comprendre ces mécanismes, montrant notamment que l’exhumation des roches métamorphiques HP-BT est en grande partie accommodée le long de zones de cisaillement ductiles d’échelle crustale. Ce travail de thèse vise à contraindre la géométrie, la cinématique et la durée de l’histoire tectono-métamorphique des roches de HP-BT du complexe de subduction des Schistes Bleus Cycladiques située en Grèce. Un des objectifs est de contraindre, à différentes échelles, le calendrier de la localisation de la déformation au cours de l’exhumation dans le but de mieux comprendre le comportement mécanique des zones de subduction. Trois méthodes principales d’investigation ont été utilisées : i) une étude de terrain permettant de caractériser la géométrie, la cinématique et la distribution de la déformation, montrant notamment une localisation progressive de la déformation vers la base de l’unité des Schistes Bleus Cycladiques au cours de l’exhumation et le long de zones de cisaillement, ii) des outils de pétrologie métamorphique permettant de contraindre l’évolution P-T des roches métamorphiques au sein de la zone de subduction, et iii) des datations ⁴⁰AR/³⁹AR afin de déterminer le calendrier de l’histoire tectono-métamorphique des Schistes Bleus Cycladiques et de la localisation de la déformation au sein de zones de cisaillement d’échelle kilométrique à millimétrique associées à des degrés de rétromorphose variés. Une corrélation nette est clairement observée entre l’intensité de la déformation, le degré de rétromorphose et les âges les plus jeunes. Un des résultats de ce travail est que la préservation à l’affleurement d’éclogites et schistes bleus n’implique pas forcément une exhumation rapide. En effet, nos résultats suggèrent que l’unité des Schistes Bleus Cycladiques a enregistré une histoire d’exhumation longue d’environ 30 Ma. Le taux d’exhumation n’est donc pas le seul paramètre contrôlant le degré de rétromorphose des unités HP-BT, la remontée le long d’un gradient métamorphique froid dans le canal de subduction et la localisation progressive de la déformation ductile au cours de l’exhumation étant également des facteurs majeurs
Exhumation mechanisms of high-pressure low-temperature (HP-LT) metamorphic rocks in subduction zones are complex and actively discussed. The study of fossilized subduction zones allows a better comprehension of these mechanisms, showing that exhumation of HP-LT rocks is mainly accommodated along crustal-scale ductile shear zones. This study aims at constraining the geometry, the kinematic and the timing of the tectonometamorphic history of the HP-LT Cycladic Blueschist Unit (CBU) cropping out in Greece. A main objective is to constrain the timing of strain localization at different scales during exhumation to better understand the mechanical behaviour of subduction zones. Three principal methods of investigation have been used, including i) a structural fieldwork that allows to characterize the geometry, the kinematic and the distribution of deformations, highlighting progressive strain localization during exhumation toward the base of the CBU and along shear zones, ii) a metamorphic petrology study aiming at determining the P-T evolution of the CBU, and iii) ⁴⁰AR/³⁹AR dating used to constrain the timing of the tectonometamorphic evolution of the CBU and the timing of strain localization within kilometre- to millimetre-scale shear zones showing different degrees of retrogression. We observe an obvious correlation between the intensity of finite deformation, the degree of retrogression and youngest mica ages. A major result of this thesis work is that the preservation of eclogite and blueschist-facies rocks does not necessarily imply fast exhumation rates. Our results instead suggest that the exhumation history of the CBU is relatively long, spanning over ca. 30 Ma. Consequently, it appears that the exhumation rate is not the main parameter controlling the degree of retrogression of HP-LT metamorphic rocks in the CBU compared to progressive strain localization during exhumation along a cold retrograde P-T evolution within the subduction channel

On presente une etude experimentale de la localisation de la deformation en bandes de cisaillement dans les materiaux granulaires et une etude theorique de la sensibilite du modele cloe a certains parametres. L'etude experimentale est realisee en deux volets: 1) le premier volet de l'etude concerne le phenomene de la localisation de la deformation au triaxial axisymetrique, avec visualisation quantitative de l'apparition et du developpement de la localisation au cours de l'essai par la methode tomodensitometrique scanner a rayons x. De nouveaux elements ont ete apportes concernant l'influence des conditions aux limites et l'existence d'une densite limite dans les bandes de cisaillement en grande deformation. 2) le deuxieme volet de l'etude concerne la localisation a l'appareil biaxial en deformation plane. On analyse l'influence de quelques facteurs importants (geometrie, granulometrie, compacite) sur l'apparition et le developpement de la localisation en conditions drainees. En revanche, des elements complementaires interessants sont presentes concernant la localisation au biaxial en regime non draine. La partie theorique concerne la prediction de la localisation par la loi de comportement cloe. Une etude de la sensibilite du modele cloe a certaines constantes internes est presentee. Une modelisation des essais de deformation plane non drainee est realisee. Les resultats experimentaux presentes dans le cadre de ce travail constituent une base de donnees d'identification de la loi cloe et en particulier des modules de cisaillement

Une etude numerique et theorique de la localisation de la deformation en bandes de cisaillement dans les geomateriaux est presentee. Cette etude comprend deux parties distinctes : ' la premiere partie de la detection du moment d'apparition de la localisation et plus generalement des phenomenes de bifurcation. L'etude est realisee dans le cadre des grandes transformations en utilisant le modele cloe. Dans ce but, un modele cloe von mises et un algorithme de detection de perte d'unicite globale, permettant la resolution d'un probleme aux limites formule en vitesses, ont ete developpes. L'influence de l'initialisation de l'algorithme est plus particulierement etudiee. L'etude numerique d'un essai biaxial, a permis de mettre en evidence plusieurs modes de bifurcation (modes de flambage, bandes de cisaillement). Pour les modes localises, une comparaison entre les resultats numeriques et les predictions theoriques fournies par le critere de localisation de cloe ont permis d'illustrer la fiabilite de ce dernier. ' la seconde partie concerne le suivi de la localisation, de son declenchement, jusqu'a la ruine complete de l'echantillon. Dans ce but, un modele d'interface a ete developpe. Il presente la particularite d'assurer une transition continue entre le regime de pre et de post localisation (concept de consistance) d'une part et de decrire l'evolution specifique de la densite dans la zone localisee d'autre part. Le concept d'indice des vides critique est introduit explicitement dans ce modele. Les resultats obtenus sur un essai biaxial refletent assez fidelement les resultats experimentaux. Ce modele a ensuite ete integre dans un code de calcul par elements finis. Un element d'interface, fonctionnant en grandes transformations a ete developpe et valide. Enfin, une pre-etude du probleme de propagation des bandes de cisaillement au sein d'une structure a ete effectuee.

I present physical shear-box experiments investigating the relationship between geometrical properties of shear zones and mechanical properties of deformed rocks. Experimental methodology is also examined critically and new materials for analogue modelling of shear localization are presented. First, I tested experimentally whether meaningful rheological information can be deduced from finite geometrical shear zone data. The results predict characteristic geometrical responses for certain end-member materials. However, it will be difficult to constrain such responses in the field. In the second part physical controls on deformation in the shear box are analysed for Newtonian and power-law fluids and an elastoviscoplastic strain-softening material. Since models always represent simplifications of the natural problem, it is essential to understand fully the physics of a given simulation. I show that displacement boundary conditions, model geometry, and rheology control shear zone geometry. Practical applications of the shear box for modelling natural shear localization and limitations of isothermal physical models with displacement boundary conditions in general are discussed. In the third part, new data on the rheology of highly-filled silicone polymers are introduced. Since dynamic similarity must be satisfied in analogue models to permit scaled, quantitative simulations of deformation processes, the choice of suitable rock analogues is critical for physical experiments. In particular, we address the problem of designing power-law fluids to model rocks deforming by dislocation creep. We found that highly-filled polymers have complex rheologies. Hence, such materials must be used with care in analogue modelling and only for certain experimental stress-strain rate conditions. Finally, I investigated whether fault network geometry and topography of brittle strike-slip faults are influenced by the degree of compaction of the host rock. Analogue shear experiments with loose and dense sand imply that the degree of sediment compaction may be a governing factor in the evolution of fault network structure and topography along strike-slip faults in sedimentary basins. Therefore, models of strike-slip faults should consider potential volume changes of deformed rocks.

博士
逢甲大學
土木水利工程與建設規劃博士學位學程
108
Nowadays, the development of Internet has enabled people worldwide to access soil liquefaction images in earthquakes. These images clearly show that soil liquefaction only occurs locally in tectonic earthquakes. The actual soil liquefaction phenomenon can be examined to investigate whether the traditional soil liquefaction prevention methods adopted by scholars globally for a long time matches the actual needs in practice. To achieve the research purpose, this dissertation compares the definition, mechanism, factors, test methods, evaluation methods, disaster prevention methods, and design specifications of traditional soil liquefaction and localization of soil liquefaction. Then, the following observations are inferred: (1) according to the traditional definition of soil liquefaction, soil liquefaction occurs when the safety factor of soil liquefaction resistance is smaller than 1.0; (2) the traditional soil liquefaction mechanisms include flow liquefaction and cyclic mobility; (3) traditional soil liquefaction completely ignores the three essential constituent elements of localization of soil liquefaction in practice; (4) the traditional soil liquefaction potential distribution maps issued by the government show that all areas except the mountainous areas are potential areas for soil liquefaction, which accounts only for less than 1% of the total areas and therefore seriously deviates from the fact that soil liquefaction occurs only locally in the shear banding zone with brittle fracture. Based on the result-oriented feedback and improvement, it is found that the factors, test methods, evaluation methods, disaster prevention methods, and design specifications of traditional soil liquefaction only considers the secondary factors (ground vibration) of tectonic earthquakes, but excludes the main factors of tectonic earthquakes (shear banding). Therefore, all the research results are inconsistent with the actual needs of the localization of soil liquefaction. Therefore, by considering the actual needs of localization of soil liquefaction, the author proposes the factors, test methods, evaluation methods, and disaster prevention methods for localization of soil liquefaction, with a purpose of correcting the research direction to the localization of soil liquefaction and making all the results of future researches comply with the actual needs of the localization of soil liquefaction. Based on the conclusion of this dissertation, the author suggests that the government should pay attention to the fact that the traditional definition for soil liquefaction deviates from the localization of soil liquefaction, and they should redraw the distribution maps of potential areas of localization of soil liquefaction as well as revise the disaster prevention methods and design specifications of localization of soil liquefaction. Only in this way can the potential distribution maps and the soil liquefaction prevention methods meet the needs in practice.

碩士
國立中正大學
機械工程研究所
84
The shear localization phenomena during chip formation in orthogonal metal cutting process have been studied by using the explicit finite element analysis. A three dimensional computational model has been developed for analyzing dynamic thermomechanical deformations of a thermally softening viscoplastic workpiece material subjected to various tool cutting speeds and tool rake angles. The shear band characteristics such as temperature contour, effective plastic strain, effective plastic strain rate, propagating speed and orientation are investigated for each cases. Cutting forces and surface roughness can be estimated by this 3D model. The predictions of the finite element analysis are shown than the secondary shear of the chip on rake surface appear to be a negligible effect which indicated the chip segments can be separate completely due to extensive shear in the primary shear zone; this phenomena agreed well with the experimental observations for literature.

Our study was designed to test the hypothesis that flowfield properties such as WSS are closely related to cardiovascular disease. The spatial distribution patterns of several hemodynamic indices (gradient of WSS) were examined and compared with the (known) locations of plaque formation in human aorta. The part of the aorta on which we focused is ascending, aortic arch and descending aorta. Blood flow is influenced by vessel wall motion. Fluid Structure Interaction (FSI) is also investigated and discussed during the description of hemodynamic environment that leads to plaque formation in human aorta. Our Data were DICOM files from Computed Tomography (CT) scans. Using Vascular Modeling Toolkit (VMTK) and these scans as the input, we choose level set segmentation method to extract the geometry of the vessel needed for the simulation. ANSYS CFX Solver was used for the simulation of blood flow. The present numerical study revealed a direct correlation between low WSS values and atherosclerotic plaque localization. The results indicate also that Oscillating Shear Index (OSI) shows clearly points where the possibility of atherogenesis is high enough to be ignored. FSI provides unimportant details when we focused on plaque formation.
Η παρούσα εργασία μελετά την υπόθεση που συνδέει τις ιδιότητες του πεδίου ροής, όπως οι διατμητικές τάσεις (Wall Shear Stresses), με καρδιαγγειακές παθήσεις. Η χωρική κατανομή διάφορων δεικτών αιμοδυναμικής φύσεως (όπως η βάθμωση των διατμητικών τάσεων) μελετήθηκε και τα σημεία που εντοπίστηκαν ως ύποπτα για την ανάπτυξη αθηρωματικών πλακών συγκρίθηκαν με γνωστές από τη βιβλιογραφία περιοχές σχηματισμού τέτοιων φλεγμονών στην ανθρώπινη αορτή. Το τμήμα της αορτής στο οποίο εστιάσαμε είναι η ανιούσα, το αορτικό τόξο και η κατιούσα αορτή. Εξετάστηκε απίσης το ενδεχόμενο να επηρεάζεται η ροή του αίματος από την κίνηση του αρτηριακού τοιχώματος.

碩士
國立中正大學
機械工程學系
85
The shear localization phenomena during serrated chip formation in high speed metal cutting process have been studied by using the explicit finite element analysis. A three dimensional computational model has been developed for analyzing dynamic ther-mo-mechanical deformations of a thermally softening viscoplastic workpiece material subjected to various tool cutting speeds、tool rake angles and workpieces material hardness. The shear band characteristics such as effective plastic stress, effective plastic strain and shear banding angle are investigated for each cases. Cutting forces also can be estimated by this 3D model for the reason to reduce cost and time in experiment. The predictions of the finite element analysis are shown that above a critical high cutting speeds the Catastropic shear chip will be form due to shear localization in the primary shear zone and different kinds of cutting condition. The secondary shear zone of the chip on rake surface appear not to be a negligible effect which indicated the tool life during metal cutting process. Chip segments can be separated completely due to extensive shear in the primary shear zone; These phenomena during our metal cutting experiments in lathes agreed well with the experimental observations from literature so as to verify the accuracy of our numerical analysis. The numerical model presented here also applied to study the oblique cutting process.

Dynamic fracture of metals is a fascinating multiphysics-multiscale problem that often results in brittle and/or ductile fracture of structural components. Additionally, under high strain rates such as impact or blast loads, a failure phenomena known as shear banding may also occur, which is a common precursor to fracture. Both fracture and shear banding are instability processes leading to strong discontinuities and strain localization, respectively. Namely, shear bands are zones of highly localized plastic deformation, while brittle/ductile cracks are material discontinuities due to cleavage and/or void coalescence. Furthermore, while fracture events are mostly driven by triaxial tensile loading, shear bands are driven by shear heating caused by inelastic deformations and high temperature rise. In this work, fracture is modeled through a phase field formulation coupled to a set of equations that describe shear bands. While fracture is governed by a strong length scale that propagates at a fast time scale, shear bands are dominated by a weak length scale and propagate slower. These are two different failure modes with distinct spatial and temporal scales. This thesis is aimed at the development of analytical and numerical methods to determine the onset of both shear band localization and fracture. The main contribution of this thesis is the formulation of analytical criteria, based on the linear perturbation method, for the onset of fracture and shear band instabilities. We first propose a stability framework for shear bands that account for a non-constant Taylor Quinney coefficient. In addition, we apply the linear perturbation method to the phase field formulation of fracture to study the onset of unstable crack growth. The derivations lead to an analytical, energy based criterion for the phase field method in linear elastic and visco-plastic materials. The stability criterion not only recovers the critical stress value reported in the literature for simple elastic cases but also provides a criterion for visco-plastic materials with a general degradation function and fracture induced by cold-work. Finally, we analyze the physical stability of both failure modes and their interaction. The analysis provides insight into the dominant failure mode and can be used as a criterion for mesh refinement. Several numerical results with different geometries and a range of strain rate loadings demonstrate that the stability criterion predicts well the onset of failure instability in dynamic fracture applications. For the example problems considered, if a fracture instability precedes shear banding, a brittle-like failure mode is observed, while if a shear band instability is initiated significantly before fracture, a ductile-like failure mode is expected. In any case, fracture instability is stronger than a shear band instability and if initiated will dominate the response. Another contribution of this thesis is the development of numerical type stability methods based on the discretized model which can be employed within any finite element method. In this approach, a novel methodology to determine the onset of shear band localization is proposed, by casting the instability analysis as a generalized eigenvalue problem with a particular decomposition of the element Jacobian matrix. We show that this approach is attractive, as it is applicable to general rate dependent multidimensional cases and no special simplifying assumptions ought to be made. Furthermore, this technique is also applied to the fully coupled dynamic fracture problem and is shown to agree well with the analytical criteria. Finally, we propose an alternative for identifying the instability point following a generalized stability analysis concept. In this framework, a stability measure is obtained by computing the instantaneous growth rate of the vector tangent to the solution. Such an approach is more appropriate for non-orthogonal problems and is easier to generalize to difficult dynamic fracture problems.

Viscous and viscoelastic deformation strongly affects the mechanical behavior of the Earth. This style of deformation has consequences for a wide range of geodynamic processes from large scale processes like the formation and maintenance of plate boundaries, to smaller scale processes like postseismic deformation on and near faults. One of the key features of viscous and viscoelastic deformation in the Earth is that it is observed to be self localizing under some circumstances. This is in spite of the tendency for viscous deformation to be pervasive in a deforming system. Many processes are thought to contribute to strain localization in the Earth: (1) viscous dissipation or shear heating (e.g., Braeck and Podladchikov, 2007; Braeck et al., 2009; Kameyama et al., 1999; Kelemen and Hirth, 2007; Ogawa, 1987), (2) grain size reduction (e.g., Braun et al., 1999; Montési and Hirth, 2003; Précigout and Gueydan, 2009), (3) lattice preferred orientation development (LPO) (e.g., Poirier, 1980; Tomassi et al., 2009), mixing of phases (e.g., Skemer et al., 2010a; Toy et al., 2010; Warren and Hirth, 2006) and geometrical interconnection of weak phases and materials (e.g., Handy, 1994). Utilizing both natural samples from Oman (Chapter 2) and theoretical work based on numerical modeling (Chapters 3 and 4) each chapter of this thesis evaluates the effect of a different one of these processes on strain localization, and in the case of Chapter 4 evaluates the additional feedback between two of these processes. In Chapter 2 we examine strain localization in a natural system in which two very rheologically different materials, gabbronorite (predominantly plagioclase) and harzburgite (predominantly olivine), were juxtaposed due to volcanic intrusion and subsequently deformed. We utilized field relationships, pyroxene and amphibole/plagioclase thermometry, metamorphic phase equilibrium, grain size piezometry and electron backscatter diffraction (EBSD) in order to constrain the deformation conditions for the field area. The viscosity of gabbronorite was found to be: (1) consistent with the predicted viscosities based on the extrapolation of experimental flow laws and (2) at least two orders of magnitude lower than the harzburgite while deformation was occurring. This suggests both that a significant viscosity contrast exists at the crust-mantle boundary where the crustal lithology is dominated by plagioclase and the mantle by olivine, and wherever deformation is geometrically allowed to localize within plagioclase rich layers. In Chapter 3 we examine the theoretical effect of shear heating as well as the feedback between viscous dissipation and temperature dependant viscosity on strain localization in a one-dimensional model of a viscoelastic shear zone. This model builds on the work of Kelemen and Hirth (2007) by utilizing a complex dry olivine viscoelastic rheology that includes dislocation creep, diffusion creep, dislocation accommodated grain boundary sliding (disGBS) and low temperature plasticity (LTP). We have found that increasing either the applied strain rate or the grain size system behavior is modified in three significant ways: (1) it causes the maximum stress the system can archive to increase, (2) it results in more unstable system behavior and (3) it causes the system to accommodate more deformation in the background. One consequence of enhanced background deformation is that system exhibits distinct periods of accelerated stress relaxation accompanied by increased strain rates, that do not necessarily go unstable. Consequently, we have shown that shear heating may play an important roll both in viscous deformation in the Earth and potentially in the occurrence of intermediate depth earthquakes and slow slip events. In Chapter 4, we extend Chapter 3 and examine the feedbacks between grain size evolution, viscous dissipation and a complex temperature and grain size dependant viscosity in a one-dimensional model of a viscoelastic shear zone. We evaluated both the grain size evolution models of Austin and Evans (2007) and a modified version of Hall and Parmentier (2003). We find that Austin and Evans predicts unrealistically fine background grain sizes while the predictions based on Hall and Parmentier (2003) are more reasonable. We also find that, based on this model and the experimental work of Mei et al. (2010), LTP may not contribute to grain size reduction in viscously deforming materials. Based on this model grain size evolution does not appear to strongly affect the peak stress or stability of a system for fine initial grain sizes as grain size reduction does not significantly alter the initial viscosity structure. However, in systems with coarser initial grain sizes, grain size evolution does appear to contribute to system instability. Additionally, for both initially coarse and fine systems, grains size evolution results in the emergence of stress evolutions displaying two distinct episodes of stress reduction. Much like Chapter 3, our observations in Chapter 4 suggest that grain size evolution may play an important role in viscous deformation in the Earth and may potentially be a mechanism for some intermediate depth earthquakes and slow slip events. Taken together the chapters in this thesis explore several of the potentially important processes that affect strain localization in the Earth. Thus providing significant insight into this important phenomenon.