Dissertations / Theses on the topic 'Phase field modeling of brittle fracture'

Fracture is the total or partial separation of an initially intact body through the propagation of one or several cracks. Computational methods for fracture mechanics are becoming increasingly important in dealing with the nucleation and propagation of these cracks. One method is the phase field approach, which approximates sharp crack discontinuities with a continuous scalar field, the so-called phase field. The latter represents the smooth transition between the intact and broken material phases. The evolution of the phase field due to external loads describes the fracture process. An original length scale is used to govern the diffusive approximation of sharp cracks. This method further employs a degradation function to account for the loss of the material stiffness during fracture by linking the phase field to the body’s bulk energy. To prevent the development of unrealistic crack patterns and interpenetration of crack faces under compression, this study uses the anisotropic split of the bulk energy, as proposed by Amor et al. [5], to model the different fracture behavior in tension, shear and compression. This research is part of a larger project aimed at the modeling of Antarctic sea ice dynamics. One aspect of this project is the modeling of the gradual break-up of the consolidated ice during spring. As a first step, this study reviews a phase field model used for dynamic brittle fracture at finite strains. Subsequently, this model is implemented into the in-house finite element software SESKA to solve the benchmark tension and shear tests on a single-edge notched block. The implementation adopts the so-called monolithic scheme, which computes the displacement and phase field solutions simultaneously, with a Newmark time integration scheme. The results of the solved problems demonstrate the capabilities of the implemented dynamic phase field model to capture the nucleation and propagation of cracks. They further confirm that the choice of length-scale and mesh size influences the solutions. In this regard, a small value of the length-scale converges to the sharp crack topology and yields a larger stress value. On the other hand, a large length-scale parameter combined with a too coarse mesh size can yield unrealistic results.

Une bonne tenue mécanique des structures du génie civil en béton armé sous chargements dynamiques sévères est primordiale pour la sécurité et nécessite une évaluation précise de leur comportement en présence de propagation dynamique de fissures. Dans ce travail, on se focalise sur la modélisation constitutive du béton assimilé à un matériau élastique-fragile endommageable. La localisation des déformations sera régie par un modèle d'endommagement à gradient où un champ scalaire réalise une description régularisée des phénomènes de rupture dynamique. La contribution de cette étude est à la fois théorique et numérique. On propose une formulation variationnelle des modèles d'endommagement à gradient en dynamique. Une définition rigoureuse de plusieurs taux de restitution d'énergie dans le modèle d'endommagement est donnée et on démontre que la propagation dynamique de fissures est régie par un critère de Griffith généralisé. On décrit ensuite une implémentation numérique efficace basée sur une discrétisation par éléments finis standards en espace et la méthode de Newmark en temps dans un cadre de calcul parallèle. Les résultats de simulation de plusieurs problèmes modèles sont discutés d'un point de vue numérique et physique. Les lois constitutives d'endommagement et les formulations d'asymétrie en traction et compression sont comparées par rapport à leur aptitude à modéliser la rupture fragile. Les propriétés spécifiques du modèle d'endommagement à gradient en dynamique sont analysées pour différentes phases de l'évolution de fissures : nucléation, initiation, propagation, arrêt, branchement et bifurcation. Des comparaisons avec les résultats expérimentaux sont aussi réalisées afin de valider le modèle et proposer des axes d'amélioration
In civil engineering, mechanical integrity of the reinforced concrete structures under severe transient dynamic loading conditions is of paramount importance for safety and calls for an accurate assessment of structural behaviors in presence of dynamic crack propagation. In this work, we focus on the constitutive modeling of concrete regarded as an elastic-damage brittle material. The strain localization evolution is governed by a gradient-damage approach where a scalar field achieves a smeared description of dynamic fracture phenomena. The contribution of the present work is both theoretical and numerical. We propose a variationally consistent formulation of dynamic gradient damage models. A formal definition of several energy release rate concepts in the gradient damage model is given and we show that the dynamic crack tip equation of motion is governed by a generalized Griffith criterion. We then give an efficient numerical implementation of the model based on a standard finite-element spatial discretization and the Newmark time-stepping methods in a parallel computing framework. Simulation results of several problems are discussed both from a computational and physical point of view. Different damage constitutive laws and tension-compression asymmetry formulations are compared with respect to their aptitude to approximate brittle fracture. Specific properties of the dynamic gradient damage model are investigated for different phases of the crack evolution: nucleation, initiation, propagation, arrest, kinking and branching. Comparisons with experimental results are also performed in order to validate the model and indicate its further improvement

La fabrication additive attire une attention croissante en raison de ses avantages en termes de flexibilité de modélisation et de facilité de conception de microstructures complexes. Nous avons constaté qu'en manipulant la stratégie d'impression, les échantillons imprimés par dépôt de fusion de polycarbonate peuvent présenter un comportement fortement anisotrope en termes de résistance à la rupture, tout en conservant des propriétés isotropes en termes d'élasticité.Le focus de cette thèse est d'explorer le comportement en matière de rupture dans des milieux élastiques isotropes présentant une ténacité de rupture anisotrope, en utilisant une combinaison d'investigations expérimentales et de simulations numériques. Dans la partie expérimentale, nous examinons la propagation des fissures dans diverses conditions de chargement en utilisant des géométries d'échantillons variées, englobant à la fois le Mode I et le Mode I+II. Dans la partie numérique, nous adoptons la modélisation de la fissuration fragile par champ de phase basée sur l'approche variationnelle, en utilisant des données expérimentales pour l'étalonnage et l'identification des paramètres numériques. À travers ces méthodologies complètes, notre objectif est de favoriser une compréhension plus profonde de l'interaction entre les motifs d'impression et la sélection des trajectoires de fissures. Cette compréhension a des implications significatives pour guider et gérer la propagation des fissures dans les composants fabriqués par fabrication additive. De plus, nous adoptons les critères classiques basés sur le taux de restitution d'énergie maximale généralisé pour améliorer notre compréhension de la sélection des trajectoires de fissures et de la force critique correspondante.Dans la dernière partie de cette thèse, nous présentons quelques investigations préliminaires concernant l'éventuelle émergence d'un motif de fissure en Zig-Zag dans des spécimens imprimés en 3D. De plus, nous plongeons en profondeur dans le comportement de rupture des spécimens imprimés sous chargement cyclique, offrant une comparaison exhaustive entre les observations expérimentales et les prévisions numériques
Additive manufacturing is receiving increasing attention due to its advantages in terms of modelling flexibility and allowing to easily design complex micro-structures. Through the manipulation of the printing strategy, we observed that fused deposition of polycarbonate can result in printed samples showcasing a distinct anisotropic behavior in fracture toughness, all the while retaining isotropic properties in elasticity.This thesis is dedicated to investigating fracture behavior within isotropic elastic media with anisotropic fracture toughness. The approach involves a combination of fracture experiments and numerical simulations. In the experimental part, we examine crack propagation under various loading conditions using diverse sample geometries, encompassing both Mode I and Mode I+II loading condition. In the numerical part, we adopt the phase-field modeling of brittle fracture based on a variational approach, using experimental data for calibrating and identification of the numerical parameters. Through these comprehensive methodologies, our objective is to foster a deeper comprehension of the interplay between printing patterns and the selection of crack paths. This understanding holds significant implications for guiding and controlling crack propagation in additive manufacturing-produced components. Besides, we adopted the classical based criteria Generalized Maximum Energy Release Rate to enhance our understanding of crack path selection and the relevant critical force.In the last part of this thesis, we presents some preliminary investigations regarding the potential emergence of Zig-Zag crack patterns in 3D printed specimens. Additionally, we delve extensively into the fracture behavior of printed specimens under cyclic loading, offering a comprehensive comparison between experimental observations and numerical forecasts

The objective of this paper is to propose a 2-D time-independent phase-field model with validating its performance as well as applying it for simulating existing representative experiments. Firstly, the section of the literature review provides an overview of quasi-brittle material and brittle fracture behaviours, as well as the existing FE models from both discontinuous and continuous approaches for simulating fracture behaviours. Next, the governing equations of the proposed phase-field model are determined, which are based on traditional Griffith’s theory as well as a specific variational method evolved from that. The proposed model is implemented in Abaqus. In particular, the implementation is achieved by using the User Subroutine in order to take the phase-field into account. The proposed model is validated by simulating a pure-tension and a pure shear test. In this part, not only the effect of discretisation but also the effects of length parameter and energy release rate has been discussed, of which the latter effect is exclusive in phase-field method. Finally, the validated model is used for simulating two sets of existing experiments, including a mixed-mode test and a series of Brazilian disks test. The results in both validation and simulation part indicate that the proposed model can successfully simulate both crack initiation and propagation in these cases, and good qualitative agreement with theoretical or experimental results can be observed.

Les simulations numériques des fissures fragiles par les modèles d’endommagement à gradient deviennent main- tenant très répandues. Les résultats théoriques et numériques montrent que dans le cadre de l’existence d’une pre-fissure la propagation suit le critère de Griffith. Alors que pour le problème à une dimension la nucléation de la fissure se fait à la contrainte critique, cette dernière propriété dimensionne le paramètre de longueur interne.Dans ce travail, on s’attarde sur le phénomène de nucléation de fissures pour les géométries communément rencontrées et qui ne présentent pas de solutions analytiques. On montre que pour une entaille en U- et V- l’initiation de la fissure varie continument entre la solution prédite par la contrainte critique et celle par la ténacité du matériau. Une série de vérifications et de validations sur diffèrent matériaux est réalisée pour les deux géométries considérées. On s’intéresse ensuite à un défaut elliptique dans un domaine infini ou très élancé pour illustrer la capacité du modèle à prendre en compte les effets d’échelles des matériaux et des structures.Dans un deuxième temps, ce modèle est étendu à la fracturation hydraulique. Une première phase de vérification du modèle est effectuée en stimulant une pré-fissure seule par l’injection d’une quantité donnée de fluide. Ensuite on étudie la simulation d’un réseau parallèle de fissures. Les résultats obtenus montrent qu’il a qu’une seule fissure qui se propage et que ce type de configuration minimise mieux l’énergie la propagation d’un réseau de fractures. Le dernier exemple se concentre sur la stabilité des fissures dans le cadre d’une expérience d’éclatement à pression imposée pour l’industrie pétrolière. Cette expérience d’éclatement de la roche est réalisée en laboratoire afin de simuler les conditions de confinement retrouvées lors des forages.La dernière partie de ce travail se concentre sur la rupture ductile en couplant le modèle à champ de phase avec les modèles de plasticité parfaite. Grâce à l’approche variationnelle du problème on décrit l’implantation numérique retenue pour le calcul parallèle. Les simulations réalisées montrent que pour une géométrie légèrement entaillée la phénoménologie des fissures ductiles comme par exemple la nucléation et la propagation sont en concordances avec ceux reportées dans la littérature
Phase-field models, sometimes referred to as gradient damage, are widely used methods for the numerical simulation of crack propagation in brittle materials. Theoretical results and numerical evidences show that they can predict the propagation of a pre-existing crack according to Griffith’s criterion. For a one- dimensional problem, it has been shown that they can predict nucleation upon a critical stress, provided that the regularization parameter is identified with the material’s internal characteristic length.In this work, we draw on numerical simulations to study crack nucleation in commonly encountered geometries for which closed-form solutions are not available. We use U- and V-notches to show that the nucleation load varies smoothly from the one predicted by a strength criterion to the one of a toughness criterion when the strength of the stress concentration or singularity varies. We present validation and verification of numerical simulations for both types of geometries. We consider the problem of an elliptic cavity in an infinite or elongated domain to show that variational phase field models properly account for structural and material size effects.In a second movement, this model is extended to hydraulic fracturing. We present a validation of the model by simulating a single fracture in a large domain subject to a control amount of fluid. Then we study an infinite network of pressurized parallel cracks. Results show that the stimulation of a single fracture is the best energy minimizer compared to multi-fracking case. The last example focuses on fracturing stability regimes using linear elastic fracture mechanics for pressure driven fractures in an experimental geometry used in petroleum industry which replicates a situation encountered downhole with a borehole called burst experiment.The last part of this work focuses on ductile fracture by coupling phase-field models with perfect plasticity. Based on the variational structure of the problem we give a numerical implementation of the coupled model for parallel computing. Simulation results of a mild notch specimens are in agreement with the phenomenology of ductile fracture such that nucleation and propagation commonly reported in the literature

The unique electro-mechanical coupling properties of ferroelectrics make them ideal materials for use in micro-devices as sensors, actuators and transducers. Nevertheless, because of the intrinsic brittleness of ferroelectrics, the optimal design of the electro-mechanical devices is strongly dependent on the understanding of the fracture behavior in these materials. Fracture processes in ferroelectrics are notoriously complex, mostly due to the interactions between the crack tip stress and electric fields and the localized switching phenomena in this zone (formation and evolution of domains of different crystallographic variants). Phase-field models are particularly interesting for such a complex problem, since a single partial differential equation governing the phase-field accomplishes at once (1) the tracking of the interfaces in a smeared way (cracks, domain walls) and (2) the modeling of the interfacial phenomena such as domain-wall energies or crack face boundary conditions. Such a model has no difficulty for instance in describing the nucleation of domains and cracks or the branching and merging of cracks. Furthermore, the variational nature of these models makes the coupling of multiple physics (electrical and mechanical fields in this case) very natural. The main contribution of this thesis is to propose a phase-field model for the coupled simulation of the microstructure formation and evolution, and the nucleation and propagation of cracks in single crystal ferroelectric materials. The model naturally couples two existing energetic phase-field approaches for brittle fracture and ferroelectric domain formation and evolution. The finite element implementation of the theory is described. Simulations show the interactions between the microstructure and the crack under mechanical and electro-mechanical loadings. Another objective of this thesis is to encode different crack face boundary conditions into the phase-field framework since these conditions strongly affect the fracture behavior of ferroelectrics. The smeared imposition of these conditions are discussed and the results are compared with that of sharp crack models to validate the proposed approaches. Simulations show the effects of different conditions, electro-mechanical loadings and media filling the crack gap on the crack propagation and the microstructure of the material. In a third step, the coupled model is modified by introducing a crack non-interpenetration condition in the variational approach to fracture accounting for the asymmetric behavior in tension and compression. The modified model makes it possible to explain anisotropic crack growth in ferroelectrics under Vickers indentation loading. This model is also employed for the fracture analysis of multilayer ferroelectric actuators, which shows the potential of the model for future application. The coupled phase-field model is also extended to polycrystals by introducing realistic polycrystalline microstructures in the model. Inter- and trans-granular crack propagation modes are observed in the simulations. Finally and for completeness, the phase-field theory is extended for the simulation of conducting cracks and some preliminary simulations are also performed in three dimensions. Salient features of the crack propagation phenomenon predicted by the simulations of this thesis are directly compared with experimental observations.
Los materiales ferroeléctricos poseen únicas propiedades electro-mecánicas y por eso se utilizan para los micro-dispositivos como sensores, actuadores y transductores. No obstante, debido a la fragilidad intrínseca de los ferroeléctricos, el diseño óptimo de los dispositivos electro-mecánicos es altamente dependiente de la comprensión del comportamiento de fractura en estos materiales. Los procesos de fractura en ferroeléctricos son notoriamente complejos, sobre todo debido a las interacciones entre campos de tensión y eléctricos y los fenómenos localizados en zona de fractura (formación y evolución de los dominios de las diferentes variantes cristalográficas). Los modelos de campo de fase son particularmente útiles para un problema tan complejo, ya que una sola ecuación diferencial parcial que gobierna el campo de fase lleva a cabo a la vez (1) el seguimiento de las interfaces de una manera suave (grietas, paredes de dominio) y (2) la modelización de los fenómenos interfaciales como las energías de la pared de dominio o las condiciones de las caras de grieta. Tal modelo no tiene ninguna dificultad, por ejemplo en la descripción de la nucleación de los dominios y las grietas o la ramificación y la fusión de las grietas. Además, la naturaleza variacional de estos modelos facilita el acoplamiento de múltiples físicas (campos eléctricos y mecánicos en este caso). La principal aportación de esta tesis es la propuesta de un modelo campo de fase para la simulación de la formación y evolución de la microestructura y la nucleación y propagación de grietas en materiales ferroeléctricos. El modelo aúna dos modelos de campo de fase para la fractura frágil y para la formación de dominios ferroeléctricos. La aplicación de elementos finitos a la teoría es descrita. Las simulaciones muestran las interacciones entre la microestructura y la fractura del bajo cargas mecánicas y electro-mecánicas. Otro de los objetivos de esta tesis es la codificación de diferentes condiciones de contorno de grieta porque estas condiciones afectan en gran medida el comportamiento de la fractura de ferroeléctricos. La imposición de estas condiciones se discuten y se comparan con los resultados de modelos clasicos para validar los modelos propuestos. Las simulaciones muestran los efectos de diferentes condiciones, cargas electro-mecánicas y medios que llena el hueco de la grieta en la propagación de las fisuras y la microestructura del material. En un tercer paso, el modelo se modifica mediante la introducción de una condición que representa el comportamiento asimétrico en tensión y compresión. El modelo modificado hace posible explicar el crecimiento de la grieta anisotrópica en ferroeléctricos. Este modelo también se utiliza para el análisis de la fractura de los actuadores ferroeléctricos, lo que demuestra el potencial del modelo para su futura aplicación. El modelo se extiende también a policristales mediante la introducción de microestructuras policristalinas realistas en el modelo. Modos de fractura inter y trans-granulares de propagación se observan en las simulaciones. Por último y para completar, la teoría del campo de fase se extiende para la simulación de las grietas conductivas y algunas simulaciones preliminares también se realizan en tres dimensiones. Principales características del fenómeno de la propagación de la grieta predicho por las simulaciones de esta tesis se comparan directamente con las observaciones experimentales.

The investigation of failure in brittle materials, subjected to dynamic transient loading conditions, represents one of the ongoing challenges in the mechanics community. Progresses on this front are required to support the design of engineering components which are employed in applications involving extreme operational regimes. To this purpose, this thesis is devoted to the development of a framework which provides the capabilities to model how crack patterns form and evolve in brittle materials and how they affect the quantitative description of failure. The proposed model is developed within the context of diffusive interfaces which are at the basis of a new class of theories named phase field models. In this work, a set of additional features is proposed to expand their domain of applicability to the modelling of (i) rate and (ii) pressure dependent effects. The path towards the achievement of the first goal has been traced on the desire to account for micro-inertia effects associated with high rates of loading. Pressure dependency has been addressed by postulating a mode-of-failure transition law whose scaling depends upon the local material triaxiality. The governing equations have been derived within a thermodynamically-consistent framework supplemented by the employment of a micro-forces balance approach. The numerical implementation has been carried out within an updated lagrangian finite element scheme with explicit time integration. A series of benchmarks will be provided to appraise the model capabilities in predicting rate-pressure-dependent crack initiation and propagation. Results will be compared against experimental evidences which closely resemble the boundary value problems examined in this work. Concurrently, the design and optimization of a complimentary, improved, experimental characterization platform, based on the split Hopkinson pressure bar, will be presented as a mean for further validation and calibration.

The primary focus of this research is to evaluate the time-dependent crack growth in rocks using lab tests and numerical modeling and its application to geologic hazard problems. This research utilized Coconino sandstone and Columbia granite as the study materials and produced the subcritical crack growth parameters in both mode I and II loadings using the rock materials. The mode I loading test employs three different types of fracture mechanics tests: the Double Torsion (DT), the Wedge Splitting (WS), and the Double Cantilever Beam (DCB) test. Each test measured the mode I crack velocity. The DT test indirectly measured the crack velocity using the load relaxation method. The WS and DCB tests directly measured the crack velocity by monitoring using a video recording. The different mode I subcritical crack growth parameters obtained from the three tests are discussed. For the mode II loading test, this study developed a new shear fracture toughness test called the modified Punch-Through Shear (MPTS). The MPTS test conducted at different loading rates produced the mode II subcritical crack growth parameters. These fracture mechanics tests were calibrated and simulated using the distinct element method (DEM) and the finite element method (FEM). DEM analysis employed the particle flow code (PFC) to simulate the mixed mode crack growth and to match with the failure strength envelop of the triaxial compressive tests. FEM analysis employed the Phase2 program to analyze the crack tip stress distribution and the FRANC2D program to calculate the modes I and II stress intensity factors. The fracture mechanics tests and numerical modeling showed well the dependency of the mode II subcritical crack growth parameters according to confining pressure, loading rate, and the mode II fracture toughness. Finally, the UDEC modeling based on DEM is utilized in this study to forecast the long-term stability of the Coconino rock slope, as one of geologic hazards. The fracture mechanics approach is implemented in the program using the modes I and II subcritical crack growth parameters obtained from the lab tests and numerical modeling. Considering the progressive failure of rock bridges due to subcritical crack growth, the UDEC results predicted the stable condition of the Coconino rock cliff over 10,000 years. This result was validated by comparing it with the previous planar failure case.

Hydrogen embrittlement can manifest itself as hydride formation in structures when in contact with hydrogen-rich environments, e.g. in space and nuclear power applications. To supplant experimentation, modeling of such phenomena is beneficial to make life prediction reduce cost and increase the understanding. In the present work, two different approaches based on phase field theory are employed to study the precipitation kinetics of a second phase in a metal, with a special focus on the application of hydride formation in hexagonal close-packed metals. For both presented models, a single component of the non-conserved order parameter is utilized to represent the microstructural evolution. Throughout the modelling the total free energy of the system is minimized through the time-dependent Ginzburg-Landau equation, which includes a sixth order Landau potential in the first model, whereas one of fourth order is used for the second model. The first model implicitly incorporates the stress field emanating from a sharp crack through the usage of linear elastic fracture mechanics and the governing equation is solved numerically for both isotropic and anisotropic bodies by usage of the finite volume method. The second model is applied to plate and notched cantilever geometries, and it includes an anisotropic expansion of the hydrides that is caused by the hydride precipitation. For this approach, the mechanical and phase transformation aspects are coupled and solved simultaneously for an isotropic material using the finite element method. Depending on the Landau potential coefficients and the crack-induced hydrostatic stress, for the first model the second-phase is found to form in a confined region around the crack tip or in the whole material depending on the material properties. From the pilot results obtained with the second model, it is shown that the applied stress and considered anisotropic swelling induces hydride formation in preferential directions and it is localized in high stress concentration areas. The results successfully demonstrate the ability of both approaches to model second-phase formation kinetics that is triggered by flaw-induced stresses and their capability to reproduce experimentally observed hydride characteristics such as precipitation location, shape and direction.

The development of mathematical and numerical models for the study of the problem of fracture in porous media is motivated by several real-world applications. In particular, the phase-field approach to fracture, based on the regularization of the variational formulation of the Griffith's theory, seems to be one of the most promising, due to its ability to model complicated fracture processes, such as nucleation and branching, and preserve the continuity of the displacement field. The majority of the phase-field models for fracture in porous media present in the literature are mainly oriented to the study the problem of fracture in saturated porous media. Anyway, certain phenomena, such as the cracking of clayey soils during a desiccation process, suggest the importance of the extension of these models to a partially saturated framework, in which also the flow of the gaseous phase can influence the mechanical behavior of the porous medium, and thus the process of formation and evolution of fractures. Abstract The aim of this work is to develop a finite element model for the phase-field analysis of fracture in three-phase porous media, in which both the flux of the water and the flux of the dry air are taken into account. In the first part of the thesis particular attention is payed to the study of some numerical difficulties that such modeling implies, such as the errors in the evaluation of the mass conservation of the water and the occurrence of numerical locking when a volumetric-deviatoric energy split for the phase-field model is used. An original mass conservative formulation, which takes into account the deformability of the solid skeleton, and a new stabilized mixed finite element formulation for the phase-field model of fracture in saturated porous media have been proposed, and tested with different numerical applications. In the last part of the thesis the finite element discretization of the proposed three-phase model is derived and applied to the numerical simulation of two different desiccation problems, in order to to study the influence of the balance equation of the air in the development of fractures in the porous medium.
Lo sviluppo di modelli matematici e numerici per lo studio della frattura nei mezzi porosi è motivato da numerose applicazioni nel mondo reale. In particolare, lo studio della frattura con la tecnica del phase-filed, basata sulla regolarizzazione della formulazione variazionale della teoria di Griffith, sembra essere una delle più promettenti, grazie alla sua abilità di modellare fenomeni complessi, come la formazione e la ramificazione di fratture, a preservare la continuità del campo di spostamenti. La maggior parte dei modelli phase-field presenti in letteratura sono principalmente orientati allo studio della frattura in mezzi porosi saturi. D'altro canto, alcuni fenomeni, come la formazione di fratture in argille durante un processo di essicazione, indicano l'importanza di estendere questi modelli in condizione di parziale saturazione, tenendo in considerazione la possibile influenza del flusso della fase gassosa sul comportamento meccanico dello scheletro solido e, di conseguenza, sul processo di formazione e evoluzione della frattura. Lo scopo di questa tesi è la formulazione di un modello numerico agli elementi finiti per lo studio, con la tecnica del phase-field, della frattura in mezzi porosi trifase, in cui si considerino sia il flusso d'acqua che il flusso dell'aria all'interno del mezzo. Particolare attenzione è rivolta ad un approfondimento di alcune problematiche numeriche che tale modellazione comporta, come gli errori nella conservazione della massa della fase liquida e il locking numerico dovuto ad un eccesso di rigidezza volumetrica, quando lo split volumetrico-deviatorico dell'energia viene utilizzato nel modello phase-field. In particolare, vengono proposte e testate attraverso varie applicationi numeriche una nuova formulazione conservativa che tenga conto della deformabilità dello scheletro solido, e una nuova stabilizzazione per la formulazione mista del modello phase-field per la frattura in mezzi porosi saturi. Nell'ultima parte la discretizzazione agli elementi finiti del modello trifase proposto viene derivata, e applicata alla simulazione numerica di due problemi di essicazione, con l'obiettivo di studiare l'influenza dell'equazione di bilancio dell'aria sullo sviluppo di fratture nel mezzo poroso.

Le superalliage base nickel 718 est réputé pour présenter une excellente tenue à la corrosion, une très forte résistancemécanique et une bonne tenue sous irradiation. De ce fait, il s’agit d’un matériau de choix au sein d’un réacteur électronucléaire pour les pièces soumises à des sollicitations extrêmes (ressorts, systèmes de maintien. . . ).Pourtant des ruptures en service ont été observées de ce matériau sous le phénomène de corrosion sous contraintes assistée par l’irradiation. La présente thèse vise à apporter de nouveaux éléments de compréhension de ce phénomène complexe sous l’angle de la modélisation numérique. Le processus de fissuration par corrosion sous contrainte est modélisé par la méthode des champs de phase. Une implémentation unifiée, apte à traiter lesfissurations intra et intergranulaires, est proposée et permet de coupler efficacement différentes échelles de travail (du milieu continu au polycristal) et différents physiques (mécanique des milieux continus et généralisés et oxydation interne). Cette modélisation permet de proposer des simulations des étapes complexes de la corrosion sous contrainte, à savoirl’amorçage, la coalescence et la propagation
Inconel 718 alloy is renowned for having excellent corrosion resistance, very high mechanical strength and good resistance to irradiation. Thus, it is a material of choice within a nuclear power reactor for parts subjected to extreme stresses (springs, retaining systems,...). However, failures in service have been observed in this material under irradiationassisted stress corrosion cracking phenomenon. This thesis aims to bring new elements of understanding of this complex phenomenon from the point of view of numerical modeling. The stress corrosion cracking process is modeled by the phase field fracture method. A unified implementation, able to deal with inter and intergranular fracture, is proposedand allows to couple efficiently different scales of work (from continuous medium to polycrystal) and different physics (mechanics of continuous and generalized media and internal oxidation). This modeling allows to propose simulations of the complex stages of stress corrosion cracking, namely initiation, coalescence and propagation

To date, efforts to model fracture and crack propagation have focused on two broad approaches: discrete and continuum damage descriptions. The discrete approach incorporates a discontinuity into the displacement field that must be tracked and updated. Examples of this approach include XFEM, element deletion, and cohesive zone models. The continuum damage, or smeared crack, approach incorporates a damage parameter into the model that controls the strength of the material. An advantage of this approach is that it does not require interface tracking since the damage parameter varies continuously over the domain. An alternative approach is to use a phase-field to describe crack propagation. In the phase-field approach to modeling fracture the problem is reformulated in terms of a coupled system of partial differential equations. A continuous scalar-valued phase-field is introduced into the model to indicate whether the material is in the unfractured or fractured ''phase''. The evolution of the phase-field is governed by a partial differential equation that includes a driving force that is a function of the strain energy of the body in question. This leads to a coupling between the momentum equation and the phase-field equation. The phase-field model also includes a length scale parameter that controls the width of the smooth approximation to the discrete crack. This allows discrete cracks to be modeled down to any desired length scale. Thus, this approach incorporates the strengths of both the discrete and continuum damage models, i.e., accurate modeling of individual cracks with no interface tracking. The research presented in this dissertation focuses on developing phase-field models for dynamic fracture. A general formulation in terms of the usual balance laws supplemented by a microforce balance law governing the evolution of the phase-field is derived. From this formulation, small-strain brittle and large-deformation ductile models are then derived. Additionally, a fourth-order theory for the phase-field approximation of the crack path is postulated. Convergence and approximation results are obtained for the proposed theories. In this work, isogeometric analysis, and particularly T-splines, plays an important role by providing a smooth basis that allows local refinement. Several numerical simulations have been performed to evaluate the proposed theories. These results show that phase-field models are a powerful tool for predicting fracture.
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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.

In this thesis, a non-local continuum approach coupled with the phase-field theory and Jones-Wilkins-Lee (JWL) equation of state (EOS) is proposed to model and simulate blast-induced fracture in brittle materials. Also, we study the evolution of field variables in the underground explosion of steel fiber reinforced concrete (SFRC). For numerical implementation, we have used a non-ordinary state-based peridynamic theory coupled with a diffusive phase-field damage approach. Moreover, JWL EOS from the family of isentropes has been formed, and finally, the constitutive relations have been arrived with the help of the JWL EOS. As the conventional equation of motion cannot capture the discontinuities in the damaged portion of the material, derivative-free integro-differential equations of motion have been used in line with the peridynamics approach to overcome the limitation of the conventional equation of motion. The motivation for using non-ordinary state-based (NOSB) peridynamics (PD) over the bond-stretch-based (BSB) or bond-energy-based (BEB) models comes from the fact that the applicability of BSB and BEB models are only limited to materials with Poisson's ratio 1/4. We have introduced a dynamic failure mechanism due to blast-induced stress wave propagation in rock media, generated due to the pressure of a high-velocity gaseous detonator. Furthermore, an expansion of stress wave in the SFRC mass due to the underground explosion of highly pressurized detonators (Iregel 1175U) has been studied in this thesis. A program-burn algorithm inside the JWL EOS has been implemented to model rock fragmentation. A predictor-corrector explicit time integration scheme has been used to update the field variables at every time step. The phase-field damage can capture well the explosive-induced fracture in rock media and damage in the SFRC medium for the underground explosion. The current numerical approach suggests a versatile physics-oriented future model for this class of problems. The formulation proposed in the thesis has been validated against two numerical benchmark tests and has shown good predictive quality.