Unconventional gas has become an important resource to help meet our future energy demands. Although plentiful, it is difficult to produce this resource, when locked in a massive sedimentary formation. Among all unconventional gas resources, tight gas sands represent a big fraction and are often characterized by very low porosity and permeability associated with their producing formations, resulting in extremely low production rate. The low flow properties and the recovery factors of these sands make necessary continuous efforts to reduce costs and improve efficiency in all aspects of drilling, completion and production techniques. Many of the recent improvements have been in well completions and hydraulic fracturing. Thus, the main goal of a hydraulic fracture is to create a long, highly conductive fracture to facilitate the gas flow from the reservoir to the wellbore to obtain commercial production rates. Fracture conductivity depends on several factors, such as like the damage created by the gel during the treatment and the gel clean-up after the treatment. This research is focused on predicting more accurately the fracture conductivity, the gel damage created in fractures, and the fracture cleanup after a hydraulic fracture treatment under certain pressure and temperature conditions. Parameters that alter fracture conductivity, such as polymer concentration, breaker concentration and gas flow rate, are also examined in this study. A series of experiments, using a procedure of “dynamical fracture conductivity test”, were carried out. This procedure simulates the proppant/frac fluid slurries flow into the fractures in a low-permeability rock, as it occurs in the field, using different combinations of polymer and breaker concentrations under reservoirs conditions. The result of this study provides the basis to optimize the fracturing fluids and the polymer loading at different reservoir conditions, which may result in a clean and conductive fracture. Success in improving this process will help to decrease capital expenditures and increase the production in unconventional tight gas reservoirs.

Polymer composites are engineered materials widely being used and yet not completely understood for their dynamic response. It is important to fully characterize material properties before using them for applications in critical industries, like that of defense or transport. In this project, the focus is on determining dynamic fracture toughness property of fiber reinforced polymer composites by using a combined numerical- experimental methodology. Impact tests are conducted on Split-Hopkinson pressure bar with required instrumentation to obtain load-history and initiation of crack propagation parameters followed by finite element analysis to determine desired dynamic properties. Single edge notch bend(SENB) type geometry is used for Mode-I fracture testing and similarly end-notched flexure (ENF) type of geometry is proposed to test the samples for Mode-II type of fracture. Two different linear elastic fracture mechanics approaches are used- crack opening displacement and strain energy release rates. Dynamic fracture toughness values of around 50 MPa[square root of m] and 100 MPa[square root of m] in Mode-I, whereas, around 40 MPa[square root of m] and 6 MPa[square root of m] in Mode-II are observed for carbon-epoxy and fiberglass-epoxy composites respectively. To provide a better estimate of material response, Hashin damage model is employed which takes into account non-linear behavior of composites. As observed in previous studies, values estimated using a non-linear response of composite laminates are nearly three times as high, therefore, using a linear elastic material model could underestimate a material's capacity to sustain dynamic loads without failure. It is concluded that fracture initiation toughness property is rate dependent and is higher when subjected to dynamic loads. Microscopic examination of damaged samples and a higher value of dynamic fracture toughness for fiberglass-epoxy laminates as compared to carbon-epoxy laminates suggest that dynamic fracture toughness is also a function of many other variables like mode of fracture, dominant damage criteria, manufacturing process, constituent materials and their ratios.

In the present work, the non-linear vibrations and the corresponding dynamic fracture mechanics of cables of cable-stayed bridges are studied. The cables are among the most critical components in cable-stayed bridges and there are different damage sources such as corrosion, vibration, fatigue and fretting fatigue that can significantly affect them, thereby reducing the cable’s service life and even producing their failure. Cable-Parametric Resonance is the specific non-linear vibration studied in this research. This type of vibration occurs due to displacements presented at the cable supports. These displacements are induced by the wind and traffic loads acting on the pylon and deck of the bridge. Under certain conditions, unstable cable-vibration of significant amplitude can be registered. Therefore, numerical and experimental analyses are carried out in order to describe this phenomenon and to determine the corresponding instability conditions. Two non-linear models of cable-parametric resonance are studied to predict the cable response. In the simulation method, the non-linear components are treated as external forces acting on the linear systems, which are represented by Single Degree of Freedom systems and described by digital filters. A clear non-linear relationship between the excitation and the cable response is observed in the simulations and the experiments. The corresponding experimental analysis is based on a scaled model (1:200) of the Öresund bridge and a good agreement between the numerical and experimental results is found. After obtaining the relationship between the cable response and the excitation, the cable instability conditions are determined. This is done by finding the minimum displacement required at the cable supports in order to induce nonlinear cable vibration of considerable amplitude. The instability conditions are determined within a wide range of excitation frequencies and conveniently expressed in a simplified and practical way by a curve. The determination process is rather fast and offers the possibility to evaluate all bridge cable stays in a rather short time. Finally, the dynamic fracture mechanics of the cable is considered by studying the fracture toughness characteristics of the material under dynamic conditions. Finite Element simulations on a pre-cracked three-point bending specimen under impact loading are performed. The observed cable instability is equivalently considered as the associated response to impact load conditions, and a crack as a defect on the wires of a cable stay. The simulations are based on an experimental work by using the Split Hopkinson pressure bar (Jiang et al). The dynamic stress intensity factor KI(t) up to crack initiation is then obtained by different methods. The numerical estimations based on the specimen’s crack tip opening displacement (CTOD) and mid-span displacement were closest to the experimental results. It is observed that a better estimation of the dynamic stress intensity factor relies on a proper formulation of the specimen’s stiffness. / Lic March 2011

<p>The focus of this work is to develop and employ numerical methods that provide characterization of granular microstructures, dynamic fragmentation of brittle materials, and dynamic fracture of three-dimensional bodies.</p><p>We first propose the fabric tensor formalism to describe the structure and evolution of lithium-ion electrode microstructure during the calendaring process. Fabric tensors are directional measures of particulate assemblies based on inter-particle connectivity, relating to the structural and transport properties of the electrode. Applying this technique to X-ray computed tomography of cathode microstructure, we show that fabric tensors capture the evolution of the inter-particle contact distribution and are therefore good measures for the internal state of and electronic transport within the electrode. </p><p>We then shift focus to the development and analysis of fracture models within finite element simulations. A difficult problem to characterize in the realm of fracture modeling is that of fragmentation, wherein brittle materials subjected to a uniform tensile loading break apart into a large number of smaller pieces. We explore the effect of numerical precision in the results of dynamic fragmentation simulations using the cohesive element approach on a one-dimensional domain. By introducing random and non-random field variations, we discern that round-off error plays a significant role in establishing a mesh-convergent solution for uniform fragmentation problems. Further, by using differing magnitudes of randomized material properties and mesh discretizations, we find that employing randomness can improve convergence behavior and provide a computational savings.</p><p>The Thick Level-Set model is implemented to describe brittle media undergoing dynamic fragmentation as an alternative to the cohesive element approach. This non-local damage model features a level-set function that defines the extent and severity of degradation and uses a length scale to limit the damage gradient. In terms of energy dissipated by fracture and mean fragment size, we find that the proposed model reproduces the rate-dependent observations of analytical approaches, cohesive element simulations, and experimental studies.</p><p>Lastly, the Thick Level-Set model is implemented in three dimensions to describe the dynamic failure of brittle media, such as the active material particles in the battery cathode during manufacturing. The proposed model matches expected behavior from physical experiments, analytical approaches, and numerical models, and mesh convergence is established. We find that the use of an asymmetrical damage model to represent tensile damage is important to producing the expected results for brittle fracture problems.</p><p>The impact of this work is that designers of lithium-ion battery components can employ the numerical methods presented herein to analyze the evolving electrode microstructure during manufacturing, operational, and extraordinary loadings. This allows for enhanced designs and manufacturing methods that advance the state of battery technology. Further, these numerical tools have applicability in a broad range of fields, from geotechnical analysis to ice-sheet modeling to armor design to hydraulic fracturing.</p> / Dissertation

Les structures de sphères creuses appartiennent à la famille des matériaux cellulaires qui ont récemment été étudiés pour leurs multiples propriétés. Dans le cas de cette thèse, le but des sphères creuses est de dissiper l’énergie d’impact d’un oiseau sur un cockpit d’avion. Elles sont développées dans le cadre du projet SAMBA (Shock Absorber Material for Birdshield Application) afin d’optimiser leur énergie spécifique absorbée (J/kg).Dans un premier temps, des essais quasi-statiques (v = 5 mm/min) et dynamiques (v = 2 m/s) de compression uni-axiale sont conduits à température ambiante sur une seule sphère creuse de diamètre 30 mm. Une propagation rapide de fissures macroscopiques est observée. Le formalisme de la Mécanique Élastique Linéaire de la Rupture (MELR) est utilisé pour estimer le taux de restitution d’énergie critique dynamique GIdc du matériau constitutif. La position du sommet de fissure est mesurée pendant la propagation de fissure à l’aide d’une caméra rapide. La Méthode des Éléments Discrets (DEM) permet de simuler la rupture dynamique en implémentant une technique de relâchement des nœuds. Le taux de restitution d’énergie GIdc peut être estimé à partir de l’histoire (position et temps) du sommet de fissure. Le modèle numérique montre que les structures sphériques dissipent une proportion importante de l’énergie par des effets dynamiques. A une même vitesse de propagation, plus l’épaisseur de coque est fine, plus les effets inertiels générés par la rupture sont importants et ce pour une même vitesse de propagation.Le modèle numérique DEM est ensuite employé pour reproduire la rupture dynamique sur une sphère creuse à l’aide d’un critère en contrainte seule ou un critère mixte en contrainte – énergie. Les bons résultats obtenus démontrent la capacité de la DEM à représenter la propagation de fissures en régime dynamique.Finalement, des essais numériques et expérimentaux multi-sphères sont réalisés afin évaluer le comportement des sphères creuses au sein d’un assemblage. / Hollow sphere structure (HSS) belongs to cellular solids that have been studied recently for its multiples properties. In our case, HSS aims to absorb soft impacts energy on an airliner cockpit. HSS is investigated through the SAMBA (Shock Absorber Material for Bird-shield Application) project because of its promises in term of specific energy dissipated (J/kg) during impact.First of all, quasi-static and dynamic (v = 5 mm/min to v = 2 m/s) uniaxial compression tests are conducted at room temperature on a single sphere (D = 30 mm). Rapid crack propagation (RCP) is observed to be predominant at macroscopic scale. The formalism of Linear Elastic Fracture Mechanics (L.E.F.M.) is therefore used to estimate the dynamic energy release rate GIdc . The crack tip location is measured during the crack propagation using a high speed camera. The Discrete Element Method (DEM) is used to simulate the dynamic fracture by implementing the node release technique. The dynamic energy release rate can be determined using an experimentally measured crack history. In spherical structures the numerical results reveal a high proportion of energy dissipated through inertial effects as well as a dependence of the thickness of the hollow sphere over the range of 0.04 mm to 1.2 mm.The DEM model Is then employed to reproduce the RCP according to two failure criterions: a stress criterion and a coupled stress-energy criterion. It reveals to be an interesting way to model the mechanical behavior of brittle materials.Eventually, experimental and numerical multi-spheres tests are performed to evaluate the behavior of brittle hollow spheres within an assembly.

Advances in dynamic fracture modelling have become more frequent due to increases in computer speed, meaning that its application to industrial problems has become viable. From this, the author has reviewed current literature in terms of graphite material properties, structural dynamics, fracture mechanics and modelling methodologies to be able to address operational issues related to the ageing of Advanced Gas-cooled Reactor (AGR) cores. In particular, the experimentally observed Prompt Secondary Cracking (PSC) of graphite moderator bricks which has yet to be observed within operational reactors, with the objective of supporting their plant life extension. A method known as eXtended Finite Element Method with Cohesive Zones (XCZM) was developed within Code_Aster open-source FEM software. This enabled the incorporation of velocity toughening, irradiation-induced material degradation effects and multiple 3D dynamic crack initiations, propagations and arrests into a single model, which covers the major known attributes of the PSC mechanism. Whilst developing XCZM, several publications were produced. This started with first demonstrating XCZM's ability to model the PSC mechanism in 2D and consequently that methane holes have a noticeable effect on crack propagation speeds. Following on from this, XCZM was benchmarked in 2D against literature experiments and available model data which consequently highlighted that velocity toughening was an integral feature in producing energetically correct fracture speeds. Leading on from this, XCZM was taken into 3D and demonstrated that it produced experimentally observed bifurcation angle from a literature example. This meant that when a 3D graphite brick was modelled that the crack profile was equivalent to an accepted quasi-static profile. As a consequence of this validation, the XCZM approach was able to model PSC and give insight into features that could not be investigated previously including: finer-scale heterogeneous effects on a dynamic crack profile, comparison between Primary and Secondary crack profiles and also, 3D crack interaction with a methane hole, including insight into possible crack arrest. XCZM was shown to improve upon previous 2D models of experiments that showed the plausibility of PSC; this was achieved by eliminating the need for user intervention and also incorporation of irradiation damage effects through User-defined Material properties (UMAT). Finally, while applying XCZM to a full-scale 3D graphite brick including reactor effects, it was shown that PSC is likely to occur under LEFM assumptions and that the Secondary crack initiates before the Primary crack arrests axially meaning that modal analysis would not be able to fully model PSC.

Cette thèse s’intéresse à la propagation dynamique de fissure dans les membranes élastomères du point de vue expérimental. Elle a pour but d’identifier les paramètres qui gouvernent la cinématique de ces fissures se propageant à grande vitesse, afin d’en prédire la trajectoire. Fondé sur l’utilisation conjointe d’une caméra à haute résolution et d’une caméra rapide, le dispositif expérimental permet de mesurer des champs à partir de la corrélation d’images au cours de la propagation de la fissure. Mis en pratique sur un polyuréthane, ce dispositif permet de retrouver la configuration de référence de l’éprouvette pendant la propagation de fissure, préalable indispensable à l’étude mécanique du problème. En plus des champs cinématiques, la densité d’énergie élastique et les contraintes sont évaluées grâce à une loi de comportement hypérélastique Les résultats de ces essais constituent une large base de données sur la rupture dynamique de membranes élastomères. La méthode permet de réaliser une analyse cinématique et énergétique de la propagation stationnaire et instationnaire, toujours dans la configuration de référence. La propagation supersonique est observée pour les hauts niveaux de déformation de la membrane. Enfin,ces observations permettent une discussion sur l’utilisation de l’approche énergétique de la rupture dynamique et de la pertinence des mesures de champs actuelles pour caractériser ce type de propagation de fissure. / This PhD thesis tackles the issue of dynamic fracture of elastomer membranes from an experimental point of view. It aims at providing some insight to predict the trajectories of high speed cracks under large strain. An experimental procedure involving high resolution and high speed cameras is developed in order to perform full-field measurements based on digital image correlation during crack propagation. Tested with a highly stretchable elastomer (polyurethane), this set-up permits to retrieve the material configurations of the sample all along crack growth, which is a crucial step toward a complete mechanical analysis of the problem. In addition to the kinematic fields,both strain energy density and stress fields are estimated thanks to a hyperelastic model, which is issued from mechanical characterisation of the material. Results of these experiments provide a comprehensive database on dynamic fracture of membranes. The method is designed to perform kinematic and energetic analyses of both steady and unsteady crack propagation in the reference configuration. Supersonic crack growth is observed for large prescribed deformation of the membranes. Finally, these observations lead to a discussion on the energetic approach in dynamic crack growth and the current applicability of full-field measurements to characterise dynamic crack growth in elastomers.

Blasting is a complex phenomenon and many parameters affect the outcome of a blast. The process of rock fragmentation by blasting is not well understood yet. Therefore, as a first step, blast-induced dynamic fractures must be studied under highly controlled conditions. The whole cycle of conducting a series of laboratory-scale blast, analyzing the results, and using them to test the validity of an advanced numerical code is reported in this thesis. Initially, the respective contributions by both shock energy and gas energy fractions in an explosive in the blasting process are explained. Then, microstructural, physical and mechanical properties of Laurentian and Barre granites as the selected rock types are investigated. Explosively driven fractures in a blast are controlled by rock and explosive properties, coupling media and coupling ratio. Sample geometries, types of explosives and coupling media used in the experiments are explored in the next step. In order to isolate the effect iii of shock energy from the gas energy in explosively driven fractures, copper liners were installed in the blast holes to prevent gas penetration into the shock induced cracks. The aim of the experiments was to study exclusively the nature of shock-driven fractures, and to contain the dynamic fractures within the samples and avoid sample fragmentation. At the same time and in order to investigate the stress field as a function of distance from the borehole, pressure gauges were installed in the samples. The measured pressures were used in a numerical-experimental procedure to estimate the attenuation properties of the rocks. Blasted samples were cut and impregnated with a mix of epoxy and fluorescent dye. Next, dynamic fracture patterns were highlighted using a strong ultraviolet source. After taking photographs, fracture patterns were manually mapped and crack densities were calculated at different depths and distances from the boreholes. The parameters that affect the development of dynamic fracture patterns are also discussed and relation between crack densities and pressures applied by explosives are investigated. Finally, the dynamic fracture patterns and measured pressures will be used for calibrating the selected equation of state, strength and failure models implemented in AUTODYN. Governing equations, the procedure for obtaining the model constants, applicability of the selected model for predicting the blast experiments and its limitations are discussed in detail.

Blasting is a complex phenomenon and many parameters affect the outcome of a blast. The process of rock fragmentation by blasting is not well understood yet. Therefore, as a first step, blast-induced dynamic fractures must be studied under highly controlled conditions. The whole cycle of conducting a series of laboratory-scale blast, analyzing the results, and using them to test the validity of an advanced numerical code is reported in this thesis. Initially, the respective contributions by both shock energy and gas energy fractions in an explosive in the blasting process are explained. Then, microstructural, physical and mechanical properties of Laurentian and Barre granites as the selected rock types are investigated. Explosively driven fractures in a blast are controlled by rock and explosive properties, coupling media and coupling ratio. Sample geometries, types of explosives and coupling media used in the experiments are explored in the next step. In order to isolate the effect iii of shock energy from the gas energy in explosively driven fractures, copper liners were installed in the blast holes to prevent gas penetration into the shock induced cracks. The aim of the experiments was to study exclusively the nature of shock-driven fractures, and to contain the dynamic fractures within the samples and avoid sample fragmentation. At the same time and in order to investigate the stress field as a function of distance from the borehole, pressure gauges were installed in the samples. The measured pressures were used in a numerical-experimental procedure to estimate the attenuation properties of the rocks. Blasted samples were cut and impregnated with a mix of epoxy and fluorescent dye. Next, dynamic fracture patterns were highlighted using a strong ultraviolet source. After taking photographs, fracture patterns were manually mapped and crack densities were calculated at different depths and distances from the boreholes. The parameters that affect the development of dynamic fracture patterns are also discussed and relation between crack densities and pressures applied by explosives are investigated. Finally, the dynamic fracture patterns and measured pressures will be used for calibrating the selected equation of state, strength and failure models implemented in AUTODYN. Governing equations, the procedure for obtaining the model constants, applicability of the selected model for predicting the blast experiments and its limitations are discussed in detail.

This work is about fracture conductivity; how to measure and model it based on experimental data. It is also about how to determine the relative importance of the factors that affect its magnitude and how to predict its magnitude based on these factors. We dynamically placed the slurry hereby simulating the slurry placement procedure in a field-scale fracture. We also used factorial and fractional factorial designs as the basis of our experimental investigation. The analysis and interpretation of experimental results take into account the stochastic nature of the process. We found that the relative importance of the investigated factors is dependent on the presence of outliers and how they are handled. Based on our investigation we concluded that the investigated factors arranged in order of decreasing impact on conductivity are: closure stress, polymer loading, flow back rate, presence of breaker, temperature and proppant concentration. In particular, we find that at high temperatures, fracture conductivity was severely reduced due to the formation of a dense proppant-polymer cake. Also, dehydration of the residual gel in the fracture at high flow back rates appears to cause severe damage to conductivity at higher temperatures. This represents a new way of thinking about the fracture cleanup process; not only as a displacement process, but also as a displacement and evaporative process. In engineering practice, this implies that aggressive flow back schemes are not necessarily beneficial for conductivity development. Also, we find that at low proppant concentrations, there is the increased likelihood of the formation of channels and high porosity fractures resulting in high fracture conductivities. The uniqueness of this work is a focus on the development of a conductivity model using regression analysis and also the illustration of a procedure that can be used to develop a conductivity model using dimensional analysis. We reviewed both methodologies and applied them to the challenge of modeling fracture conductivity from experimental studies.