A virtual testing strategy to determine effective yield criteria for porous pressure sensitive solids

The aim of this work is to determine an effective yield criteria for porous pressure sensitive solids by employing a virtual testing strategy. The focus is on the pressure sensitivity typically displayed by geomaterials, such as sandstone. Virtual testing strategy is based on computational homogenisation approach following a unified variational formulation, which provides bounds on the effective material properties for a given choice of the Representative Volume Element (RVE). In order to estimate the effective properties of porous solid, the constitutive behaviour of continuum matrix is assumed to follow the standard Drucker–Prager elasto-plastic model. The computationally generated effective yield criteria for porous solids are obtained for various RVE choices and compared against the recently proposed analytical estimates for Drucker–Prager type solids and the SR4 constitutive model for soft rocks. The developed virtual testing strategy is applied to estimate the effective properties of a realistic rock sample, thus illustrating a wide range of potential applications

Pore-space controlled hardening model in plasticity of porous materials: application to the analysis of indentation experiments

Based on a multi-scale approach comprising a multi-scale material model and a respective finite-element (FE) analysis tool, the indentation response of porous materials is examined in this paper. The considered material is assumed to consist of a homogeneous Drucker-Prager-type matrix-phase and spherical pores. Non-linear homogenization is employed to derive both a strength criterion and a hardening rule at the macroscopic scale without the need of any additional non-physical material parameters. Hereby, the underlying macroscopic hardening is exclusively controlled by the evolution of the porespace during loading. The material model is implemented in a FE program within the framework of elastoplasticity. The so-obtained analysis tool is applied to the analysis of indentation experiments commonly used for characterization and performance-based optimization of materials

A Virtual Testing Strategy to Determine Macroscopic Properties of Elasto-Plastic Heterogeneous Composite Materials

The objective of this work is to determine an e˙ective yield criteria for porous pressure sensitive solids and investigate the anisotropic yield behavior by employing a virtual testing strategy. The work is concerned with the pressure sensitivity typically displayed by geometarials, such as sandstone and composite materials consisting of a series of parallel layers, such as sedimentary rock and underground salt.Virtual testing strategy is based on computational homogenization approach for the definition of the elasto-plastic transition. Representative volume elements (RVEs) containing single-centered and distributed ellipsoidal voids are analyzed using three-dimensional finite element models under both small and finite strains. Yield curves are obtained following a unified variational formulation, which provides bounds on the e˙ective material properties for a given choice of the Representative Volume Element (RVE).In order to estimate the e˙ective properties of porous solid, the constitutive behavior of the continuum matrix is assumed to follow the standard Drucker-Prager elasto-plastic model. The computationally generated e˙ective yield criteria are compared against the recently proposed analytical estimates for Drucker-Prager type solids and the SR4 constitutive model for soft rocks. The developed computational approach is applied to estimate the e˙ective properties of a realistic rock sample. To illustrate a wide range of potential engineering applications, the computationally e˙ective yield surface are also obtained under the explicit finite element method.Finally, based on the simulated yield stress point of composite materials, the pa-rameters for proposed analytical models are acquired with ellipse fit by Taubin’s method

Strength properties of nanoporous materials: Theoretical analyses and Molecular Dynamics computations

Since the recent arising of advanced nano-technologies, as well as of innovative engineering design approaches, nanoporous materials have been extensively studied in the last two decades, leading to a considerable worldwide research interest in both industrial and academic domains. Generally characterised by high specific surface area, uniform pore size and rich surface chemistry, nanoporous materials have allowed for the development of challenging ultra-high performance devices with tailorable properties, finding widespread application in several technical fields, including civil and environmental engineering, petroleum and chemical industries, aeronautics and biomechanics. In order to fulfil to these promising applications, one of the most fundamental research aspect consists in characterising and predicting the strength properties of these materials, as dependent on the size of voids. Since the current lack of an exhaustive benchmarking evidence, as well as of a comprehensive theoretical modelling, the central purpose of the present thesis consisted in: Investigating strength properties of in-silico nanoporous samples via Molecular Dynamics computations. In detail, a parametric analysis with respect to the void radius and for different porosity levels has been carried out, by considering different loading paths with a wide range of triaxiality scenarios. As a result, the influence of void-size effects on the computed strength properties has been clearly quantified, also highlighting the dependence of the predicted material strength domain on the three stress invariants; Establishing engineering-oriented theoretical models able to predict macroscopic strength properties of nanoporous materials, by properly accounting for void-size effects. To this end, theoretical approaches based on both non-linear homogenization techniques and kinematic limit-analysis strategies have been proposed. As a result, closed-form macroscopic strength criteria have been derived, allowing for a consistent description of void-size effects and taking into account different local plastic behaviours

Strength properties of nanoporous materials: Theoretical analyses and Molecular Dynamics computations

Since the recent arising of advanced nano-technologies, as well as of innovative engineering design approaches, nanoporous materials have been extensively studied in the last two decades, leading to a considerable worldwide research interest in both industrial and academic domains. Generally characterised by high specific surface area, uniform pore size and rich surface chemistry, nanoporous materials have allowed for the development of challenging ultra-high performance devices with tailorable properties, finding widespread application in several technical fields, including civil and environmental engineering, petroleum and chemical industries, aeronautics and biomechanics. In order to fulfil to these promising applications, one of the most fundamental research aspect consists in characterising and predicting the strength properties of these materials, as dependent on the size of voids. Since the current lack of an exhaustive benchmarking evidence, as well as of a comprehensive theoretical modelling, the central purpose of the present thesis consisted in: Investigating strength properties of in-silico nanoporous samples via Molecular Dynamics computations. In detail, a parametric analysis with respect to the void radius and for different porosity levels has been carried out, by considering different loading paths with a wide range of triaxiality scenarios. As a result, the influence of void-size effects on the computed strength properties has been clearly quantified, also highlighting the dependence of the predicted material strength domain on the three stress invariants; Establishing engineering-oriented theoretical models able to predict macroscopic strength properties of nanoporous materials, by properly accounting for void-size effects. To this end, theoretical approaches based on both non-linear homogenization techniques and kinematic limit-analysis strategies have been proposed. As a result, closed-form macroscopic strength criteria have been derived, allowing for a consistent description of void-size effects and taking into account different local plastic behaviours

Influence des effets de forme et de taille des cavités, et de l'anisotropie plastique sur la rupture ductile

Ductile fracture of metallic alloys occurs by the nucleation, growth and coalescence of microvoids. In a first step, we study the influence of void shape effects and plastic anisotropy on the growth phase. we implement numerically in a finite element code the void growth model of madou and leblond for ellipsoidal voids embedded in an isotropic material, in order to apply the model to ductile fracture problems involving important void shape effects. We show that the consideration of void shape effects is necessary in order to reproduce shear-dominant ductile fracture. This model is then extended to plastic anisotropy, in the spirit of the models of monchiet and benzerga. In particular, we derive a macroscopic criterion for anisotropic materials containing general ellipsoidal voids, which is assessed by finite element limite analyses. In a second step, we study the effects of void size on the ductile fracture of nanoporous materials contenant spherical or spheroidal voids. The last part of the thesis is dedicated to the study of void shape effects and plastic anisotropy on the coalescence phase. We derive two new criteria of coalescence by internal necking, which are assessed numerically. Then, we derive a new criterion that permits to unify the growth and coalescence phases. Finally we study the influence of plasticy anisotropy on coalescence by internal necking.La rupture ductile des alliages métalliques survient suite à la nucléation, la croissance et la coalescence de microcavités. La première partie de cette thèse est consacrée à l'étude des effets de forme et d'anisotropie plastique sur la phase de croissance des cavités. Dans un premier temps, nous implémentons numériquement le modèle de croissance de Madou et Leblond pour des cavités ellipsoïdales générales plongées dans un matériau isotrope dans un code de calcul par éléments finis, afin d'appliquer le modèle à des cas de rupture où les effets de forme sont importants. On montre que la prise en compte des effets de forme des cavités est nécessaire afin de reproduire la rupture ductile en cisaillement. Ce modèle est ensuite étendu au cas de l'anisotropie plastique, en s'inspirant des travaux de Monchiet et Benzerga. On dérive notamment un critère de plasticité macroscopique pour les matériaux anisotropes contenant des cavités ellipsoïdales générales, que nous validons par analyse limite numérique. La seconde partie de la thèse est dédiée à l'étude des effets de taille sur la rupture ductile des matériaux nanoporeux contenant des cavités sphériques ou sphéroïdales. Enfin, la troisième partie de la thèse est consacrée à l'étude des effets de forme et d'anisotropie plastique sur la phase de coalescence des cavités. Nous dérivons deux nouveaux critères de coalescence en couche que nous validons par analyse limite numérique. Cette étude nous permet de développer un nouveau critère permettant d'unifier les phases de croissance et coalescence. Enfin nous dérivons un critère de coalescence pour les matériaux anisotropes

DEVELOPMENT, PARAMETERIZATION AND VALIDATION OF DYNAMIC MATERIAL MODELS FOR SOIL AND TRANSPARENT ARMOR GLASS

Despite the signing of several mine ban treaties in the 1990\u27s, it is widely recognized that there is a landmine crisis. The following are some of the main aspects of this crisis: (a) Millions of unexploded landmines remain deployed all over the world; (b) Thousands of civilians are killed or maimed every year by unintended detonations of the mines; (c) The cost of medical treatment of landmine injuries runs into the millions; (d) the ability of the international community to provide the humanitarian relief in terms of medical services, safe drinking water and food, etc., is greatly hampered by landmine contamination of the infrastructure in mine affected countries; and so on. To address the aforementioned landmine crisis, the research community around the world has taken upon itself the challenge of helping better understand the key phenomena associated with landmine detonation and interaction between detonation products, mine fragments and soil ejecta with the targets (people, structures and vehicles). Such improved understanding will help automotive manufacturers to design and fabricate personnel carriers with higher landmine-detonation survivability characteristics and a larger level of protection for the onboard personnel. In addition, the manufacturer of demining equipment and personnel protection gear used in landmine clearing are expected to benefit from a better understanding of the landmine detonation-related phenomena. The landmine detonation-related research activity can be broadly divided into three main categories: (a) shock and blast wave mechanics and dynamics including landmine detonation phenomena and large-deformation/high-deformation rate constitutive models for the attendant materials (high explosive, air, soil, etc.); (b) the kinematic and structural response of the target to blast loading including the role of target design and use of blast attenuation materials; and (c) vulnerability of human beings to post-detonation phenomena such as high blast pressures, spall fragments and large vertical and lateral accelerations. The present work falls primarily into the category (a) of the research listed above since it emphasizes the development of a large-deformation/high-deformation rate material model for soil. It is generally recognized that the properties of soil, into which a landmine is buried, play an important role in the overall effectiveness/lethality of the landmine regardless of the nature of its deployment (fully-buried, flush-buried or ground-laid). Therefore, in the present work, a series of continuum-level material models for soil of different types has been derived (using available public-domain data and various basic engineering concepts/principles), parameterized and validated against experimental results obtained from standard mine-blast testing techniques. Special attention is paid to improving the understanding of the effects of moisture, clay and gravel content on the different aspects of soil material behavior under blast loading conditions. Specifically, the effect of these soil constituents/conditions on the equation of state, strength and failure modes of the material response is investigated. The results obtained clearly revealed that: (a) the moisture clay and gravel contents of soil can substantially affect the response of soil under blast loading conditions as well as the extent of detonation-induced impulse transferred to the target structure/personnel; (b) over all, the models developed in the present work, when used in transient non-linear dynamics analysis of landmine detonation and detonation product/mine-fragment/ soil-ejecta interaction with the target structures/personnel, yielded results which are in reasonably good agreement with their experimental counterparts

Mechanisms of Crater Formation While Drilling a Well.

Current well control practice for land or bottom-supported marine rigs usually calls for shutting-in the well when a kick is detected if sufficient casing has been set to keep any flow underground. In addition, the working pressure of the casing and surface equipment is maintained high enough to insure that formation fracture occurs before a failure of these items. Even if high shut-in pressures are seen, an underground blowout is preferred over a surface blowout. On the other hand, an operator will put the well on a diverter if he believes that the casing is not set deep enough to keep the underground flow outside the casing from breaking through the sediments to the surface. Once the flow reaches the surface, craters are sometimes formed which can lead to loss of the rig and associated structures. Cratering also increases the difficulty and time required to kill the blowout. The principal objective of this dissertation is to examine cratering mechanisms with the purpose of better understanding the processes involved. This work reviews various failure mechanisms that can lead to cratering and the shallow sediment conditions which are favorable for each mechanism. The cratering mechanisms discussed include borehole erosion, formation liquefaction, piping, and formation caving. Several mechanisms for upward fluid migration are also discussed. Improved methods to estimate overburden pressure and fracture pressure gradient are also proposed. Several case histories are presented to illustrate some of the more common failure mechanisms and situations that should be avoided through improved well planning. Finally, suggestions and conclusions are presented

Planetary-Scale Impacts and their Geophysical Consequences

Planetary-scale impacts are thought to have been common during the final stages of planet formation. Such events may be responsible for many of the most distinguishing features of the Solar System’s celestial inhabitants, such as the stark contrast between the two hemisphere’s of Mars, known as the Martian Dichotomy, or the relatively small iron core and high angular momentum of the Moon. With such long-term consequences, the study of planetary-scale impacts requires the careful consideration of both the highly energetic, shock-inducing conditions directly after the impact and the geophysical implications that follow. In this thesis, such an approach is adopted throughout, as smoothed-particle hydrodynamics (SPH) simulations are coupled with geophysical models to study both the immediate and long-term effects of planetaryscale impacts. In Chapter 1, a general introduction to the topic is given, placing planetary-scale impacts in the broader context of star and planet formation. In Chapter 2, the SPH code used to simulate planetary-scale impacts, SPHLATCH, is described in detail, including derivations of the background continuum mechanics theory along with descriptions of any practical developments applied to the code. In Chapter 3, the application of these SPH impact simulations to geophysical investigations is described. A particular focus of this chapter is the crust distribution inferred by an impact simulation; a novel scheme that estimates postimpact crust across a body directly from SPH simulations is described, as well as a more sophisticated approach involving a long-term mantle convection code. In the remaining chapters, these methods are applied in two scientific studies: Chapter 4 investigates the feasibility of an impactinduced Martian Dichotomy through a large suite of SPH simulations coupled with the crust-production scheme of Chapter 3, and finally, Chapter 5 presents a previously undiscovered impact regime that may explain the Sputnik Planitia region of Pluto