Rozprawy doktorskie: „Brittle damage”

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Narayan, Sooraj. "A gradient-damage theory for quasi brittle fracture". Thesis, Massachusetts Institute of Technology, 2019. https://hdl.handle.net/1721.1/122236.

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Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2019
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 73-77).
Phase-field modeling of brittle fracture of linear elastic solids has been the subject of several studies in the past 25 years. An attractive feature of this approach to model fracture is its seamless ability to simulate the complicated fracture processes of nucleation, propagation, branching and merging of cracks in arbitrary geometries. While most existing models have focussed on fracture of "ideal brittle" materials, we consider fracture of "quasi-brittle" materials. The material is considered to be quasi-brittle in the sense that it does not lose its entire load-carrying capacity at the onset of damage. Instead there is a gradual degradation of the strength of the material, which is the result of microscale decohesion/damage micromechanisms. In this thesis we discuss the formulation of our gradient-damage theory for quasi-brittle fracture using the virtual-power method. The macro- and microforce balances, obtained from the virtual power approach, together with a standard free-energy imbalance law under isothermal conditions, when supplemented with a set of thermodynamically-consistent constitutive equations will provide the governing equations for our theory. We have specialized our general theory to formulate a simple continuum model for fracture of concrete - a quasi-brittle material of vast importance. We have numerically implemented our theory in a finite element program, and simulated numerical examples which show the ability of the simulation capability to reproduce the macroscopic characteristics of the failure of concrete in several technically relevant geometries reported in the literature..
by Sooraj Narayan.
S.M.
S.M. Massachusetts Institute of Technology, Department of Mechanical Engineering

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Eberhardt, Erik Brian. "Brittle rock fracture and progressive damage in uniaxial compression". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape17/PQDD_0008/NQ27403.pdf.

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DOMINGUES, STELLA MARIS PIRES. "ANALYSIS OF BRITTLE ELASTIC MATERIALS THROUGH A CONTINUOUS DAMAGE MODEL". PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 1996. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=33202@1.

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COORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIOR
CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO
Este trabalho lida com uma teoria de dano contínuo, desenvolvida em um contexto termodinâmico, capaz de realizar uma descrição macroscópica da degradação de um material induzida pela deformação em estruturas elásticas frágeis (isto é, falha ocorre sem deformações permanentes). Na modelagem, supõe-se que a energia livre de Helmholtz não depende apenas da deformação e da temperatura absoluta, mas também da variável dano e de seu gradiente. Além disso, para levar em conta os efeitos microscópicos, a potência dos esforços internos não depende apenas da velocidade e de seu gradiente, mas também da taxa de evolução do dano e de seu gradiente. Apesar da sofisticação mecânica da teoria, uma técnica numérica simples, baseada no método dos elementos finitos, é proposta para aproximar a solução dos problemas matemáticos não lineares resultantes. Nestes problemas o acoplamento entre as variáveis dano e deformação é contornado por meio da técnica de partição dos operadores. Para validar o modelo e investigar as características principais do método numérico, diversos exemplos são apresentados para mostrar que os algoritmos utilizados não são sensíveis à malha (mesh dependent).
The present work deals with a continuum damage theory, developed within a thennodynamical framework, able to perform a macroscopic description of material degradation induced by deformation in brittle elastic structures (i.e. failure occurs without permanent deformations). In the modeling, the Helmholtz free energy is supposed to depend not only on the strain and on the absolute temperature but on a damage variable and its gradient as well. Besides, to account for microscopic effects, the power of internal forces depends not only on the velocity and its gradient, but also on the damage velocity and its gradient. Despite the mechanical sophistication of the theory, a simple numerical technique, based on the únite element method, is proposed to approximate the solution of the resulting non linear mathematical problems. The coupling between damage and strain variables in these problems is circumvented by means of a splitting technique. In order to analyse the physical coherence of the model and to access the main features of the numerical method, a number of examples is presented showing that the numerical computations are not mesh dependent.

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Berthier, Estelle. "Quasi-brittle failure of heterogeneous materials : damage statistics and localization". Thesis, Paris 6, 2015. http://www.theses.fr/2015PA066588/document.

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Nous proposons une nouvelle approche inspirée des modèles d'endommagement non-locaux pour décrire la ruine des matériaux quasi-fragiles désordonnés. Les hétérogénéités matériaux sont introduites à une échelle continue mésoscopique via des variations spatiales de la résistance à l'endommagement alors que le mécanisme de redistribution des contraintes est décrit à travers une fonction d'interaction que l'on peut faire varier. L'évolution spatio-temporelle de l'endommagement est déterminée à partir du principe de conservation d'énergie et caractérisée via une étude statistique des précurseurs à la rupture. Cette approche nous permet de prédire la valeur des seuils de localisation et de rupture en fonction de la nature des redistributions. A l'approche de la rupture, nous mettons également en évidence une augmentation en loi de puissance du taux d'énergie dissipée ainsi qu'une longueur de corrélation, supportant l'interprétation de la rupture quasi-fragile comme un phénomène critique. En effet, nous démontrons que notre model d'endommagement s'apparente à la loi d'évolution d'une interface élastique évoluant dans un milieu désordonné. Cette analogie nous permet d'identifier les paramètres d'ordre et de contrôle de cette transition critique et d'expliquer les invariances d'échelle des fluctuations dans la limite champ moyen. Enfin, nous appliquons ces concepts théoriques à travers l'étude expérimentale de la compression d'un empilement bidimensionnel de cylindres élastiques. Notre approche permet de décrire de façon quantitative la réponse mécanique non-linéaire du matériau, et en particulier la statistique des précurseurs ainsi que la localisation des déformations
We propose a novel approach inspired from non-local damage continuum mechanics to describe damage evolution and quasi-brittle failure of disordered solids. Heterogeneities are introduced at a mesoscopic continuous scale through spatial variations of the material resistance to damage. The central role played by the load redistribution during damage growth is analyzed by varying the interaction function used in the non-local model formulation. The spatio-temporal evolution of the damage field is obtained from energy conservation arguments, so that the formulation is thermodynamically consistent. We analytically determine the onsets of localization and failure that appear controlled by the redistribution function. Damage spreading is characterized through a complete statistical analysis of the spatio-temporal organization of the precursors to failure. The power law increase of the rate of energy dissipated by damage and an extracted correlation length close to failure supports the interpretation of quasi-brittle failure as a critical phenomena. Indeed, we establish a connection between our damage model and the evolution law of an elastic interface driven in a disordered medium. It allows to identify the order and control parameters of the critical transition, and capture the scale-free statistical properties of the precursors within the mean field limit. Finally, we experimentally investigate the coaction of localized dissipative events and elastic redistributions in disordered media via compression experiments of two-dimensional arrays of hollow soft cylinders. Our experimental observations show a quantitative agreement with the predictions derived following our approach

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Wang, Xiaofeng. "Computational technology for damage and failure analysis of quasi-brittle materials". Thesis, University of Manchester, 2015. https://www.research.manchester.ac.uk/portal/en/theses/computational-technology-for-damage-and-failure-analysis-of-quasibrittle-materials(a7c91eb6-5058-4e73-95de-b2f3efd645d2).html.

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The thesis presents the development and validation of novel computational technology for modelling and analysis of damage and failure in quasi-brittle materials. The technology is demonstrated mostly on concrete, which is the most widely used quasi-brittle material exhibiting non-linear behaviour. Original algorithms and procedures for generating two-dimensional (2D) and three-dimensional (3D) heterogeneous material samples are developed, in which the mesoscale features of concrete, such as shape, size, volume fraction and spatial distribution of inclusions and pores/voids are randomised. Firstly, zero-thickness cohesive interface elements with softening traction-separation relations are pre-inserted within solid element meshes to simulate complex crack initiation and propagation. Monte Carlo simulations (MCS) of 2D and 3D uniaxial tension tests are carried out to investigate the effects of key mesoscale features on the fracture patterns and load-carrying capacities. Size effect in 2D concrete is then investigated by finite element analyses of meso-structural models of specimens with increasing sizes. Secondly, a 3D meso-structural damage-plasticity model for damage and failure analysis of concrete is developed and applied in tension and compression. A new scheme for identifying interfacial transition zones (ITZs) in concrete is presented, whereby ITZs are modelled by very thin layers of solid finite elements with damage-plasticity constitutive relations. Finally, a new coupled method named non-matching scaled boundary finite element-finite element coupled method is proposed to simulate crack propagation problems based on the linear elastic fracture mechanics. It combines the advantage of the scaled boundary finite element method in modelling crack propagation and also preserves the flexibility of the finite element method in re-meshing. The efficiency and effectiveness of the developed computational technology is demonstrated by simulations of crack initiation and propagation problems.

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Gbetchi, Kokouvi. "Multi-scale modeling of thermo-mechanical dynamic damage in quasi-brittle materials". Electronic Thesis or Diss., Université de Lorraine, 2020. http://www.theses.fr/2020LORR0049.

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Sous l’effet des impacts mécaniques, les structures constituées de matériaux fragiles peuvent être exposés à la rupture dynamique. La modélisation appropriée des mécanismes de rupture à différentes échelles d’observation et la prédiction de l’endommagement thermomécanique dans ces matériaux sont essentielles pour la conception de structures fiables. Des observations expérimentales sur la rupture dynamique des matériaux fragiles montrent des effets de refroidissement et d’échauffement importants à proximité d’une pointe de fissure. La modélisation du couplage thermomécanique lors de la rupture fragile a été entreprise, en général, sans tenir compte des aspects microstructuraux. L’objectif de cette thèse est de développer une procédure pour obtenir des lois d’endommagement thermomécaniques dans lesquelles l’évolution de l’endommagement est déduite à partir de la propagation des microfissures et des effets thermiques associés à l’échelle petite du matériau. Nous utilisons la méthode d’homogénéisation asymptotique pour obtenir la réponse macroscopique thermomécanique et d’endommagement du solide. Pour la propagation des microfissures, en mode I ou II, un critère de type Griffith est adopté. Des sources de chaleur sont considérés aux pointes des microfissures en mouvement, en lien avec l’énergie dissipée pendant la propagation. Nous considérons aussi des sources de chaleur représentant la dissipation par frottement sur les lèvres des microfissures qui se propagent en mode de cisaillement. Grâce à une analyse énergétique combinée avec la méthode d’homogénéisation nous obtenons des lois d’endommagement macroscopiques. Dans le système thermoélastique et d’endommagement ainsi obtenu, on identifie de forts couplages entre les champs mécaniques et thermiques. Le calcul des coefficients effectifs nous a permis d’étudier la réponse locale prédite par les nouveaux modèles. Cette réponse montre des effets de vitesse de déformation, de taille de la microstructure, de dégradation des propriétés thermoélastiques et des évolutions thermiques spécifiques engendrées par la microfissuration et le frottement à l’échelle petite du matériau. Dans l’équation macroscopique de la température, on retrouve des termes sources de chaleur distribuées en lien avec les dissipations d’endommagement et de frottement. L’implémentation de modèles d’endommagement dans un logiciel d’éléments finis nous a permis d’effectuer des simulations numériques à l’échelle des structures. Nous avons reproduit numériquement certains tests expérimentaux publiés dans la littérature concernant la rupture rapide d’échantillons de PMMA sous sollicitation d’impact. Les résultats des simulations obtenus sont en bon accord avec les observations expérimentales
Under impact mechanical loadings, structural components made of brittle materials may be exposed to dynamic failure. The appropriate modeling of the failure mechanisms at different scales of observation and the prediction of the corresponding thermomechanical damage evolution in such materials is essential for structural reliability predictions. Experimental observations on dynamic failure in brittle materials report important cooling and heating effects in the vicinity of the crack tip. Theoretical modeling of the thermo-mechanical coupling during fracture have been generally undertaken without accounting for microstructural aspects. The objective of the present thesis is to develop a procedure to obtain macroscopic thermo-mechanical damage laws in which the damage evolution is deduced from the propagation of microcracks and the associated small-scale thermal effects in the material. We use the asymptotic homogenization method to obtain the macroscopic thermo-mechanical and damage response of the solid. A Griffith type criterion is assumed for microcracks propagating in modes I or II. Heat sources at the tips of microcracks are considered as a consequence of the energy dissipated during propagation. Frictional heating effects are also considered on the lips of microcracks evolving in the shear mode. An energy approach is developed in combination with the homogenization procedure to obtain macroscopic damage laws. The resulting thermoelastic and damage system involves strong couplings between mechanical and thermal fields. Computation of the effective coefficients allowed us to study the local response predicted by the new models. The macroscopic response exhibits strain-rate sensitivity, microstructural size effects, degradation of thermoelastic properties and specific thermal evolutions due to microcracking and frictional effects at the small scale. Distributed heat sources are present in the macroscopic temperature equation linked to damage and frictional dissipations. The implementation of the proposed damage models in a FEM software allowed us to perform numerical simulations at the structural level. We reproduced numerically experimental tests reported in the literature concerning the rapid failure of PMMA samples impact. The results obtained in the simulations are in good agreement with the experimental observations

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Li, Shanhu. "Extended voronoi cell finite element model for damage in brittle matrix composites". The Ohio State University, 2006. http://rave.ohiolink.edu/etdc/view?acc_num=osu1135317411.

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Li, Tianyi. "Gradient-damage modeling of dynamic brittle fracture : variational principles and numerical simulations". Thesis, Université Paris-Saclay (ComUE), 2016. http://www.theses.fr/2016SACLX042/document.

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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

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Xiao, Jing. "Damage and fracture of brittle viscoelastic solids with application to ice load models". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape11/PQDD_0019/NQ47510.pdf.

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Das, Sagar. "A strain-rate dependent tensile damage model for brittle materials under impact loading". Thesis, The University of Sydney, 2016. http://hdl.handle.net/2123/15612.

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Brittle materials are often subjected to high strain rate impact load, which could be imposed due to intentional demolition purposes or during ballistic impact on protective structures. Fragments of different sizes are generally observed by such impact, which are directly related to the strain rate experienced by the material at different locations. This thesis presents a rate-dependent constitutive model to predict such dynamic behaviour of brittle solid under tensile loading. A three-parameter rate dependent tensile damage model, under continuum mechanics framework, is developed for simulating the fragmentation of brittle materials subjected to high strain-rate loading. The damage model is formulated under the assumption that the isotropic and homogeneous material contains initial microcracks and the microcrack induced damage increases when a critical volumetric strain is exceeded. Considering the microcrack induced damage and energy into account, a quantitative and direct method is developed to determine the fragment size under a constant strain rate loading. In this model, instead of assuming spherical fragment, more realistic prolate spheroid fragment is assumed, which eventually determines the more accurate surface energy from a fragment. In addition, complete strain energy (until the fracture of the material) is considered which improves the global energy balance in predicting the size of a fragment. The parameters of this model can be conveniently calibrated by experimental data on fracture strength and strain rate. The proposed rate-dependent model is validated by a spall experiment of concrete and with a dynamic Brazilian disc experiment of sandstone. Both of these experiments are numerically simulated with the proposed model and the experimental observations are compared with the simulation. The predicted strain rate, fracture strength, fracture location and fragment size are in very good agreement with those obtained in the experiments.

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Trivellato, Edoardo. "Softening damage contributions to the failure zone around deep tunnels in quasi-brittle rocks". Thesis, Paris Est, 2018. http://www.theses.fr/2018PESC1170.

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L'Agence nationale pour la gestion des déchets radioactifs (Andra) gère un laboratoire souterrain de recherche au Centre de la Meuse/Haute-Marne (CMHM), pour étudier la faisabilité d’un stockage géologique profonde dans l’argilite du Callovo-Oxfordien. Les galeries suivent les deux contraintes horizontales principales majeure et mineure. Tandis que certaines galeries possèdent un état de contraintes presque isotrope dans leur section, les autres montrent une anisotropie plus importante.Ces travaux étudient les phénomènes de rupture et fracturation à court terme, induits par l’excavation autour des ouvrages. L’endommagement fragile est considéré comme mécanisme fondamental de rupture. En effet, on retrouve dans la littérature scientifique une estimation de cette zone à partir d'un post-traitement du champ des contraintes en élasticité ou basée sur un calcul élasto-plastique. Si la première méthode ne considère pas la redistribution des contraintes due aux phénomènes dissipatifs, les approches élasto-plastiques semblent parfois insuffisantes pour expliquer la géométrie de cette zone dans certains cas des excavations en roches quasi-fragiles (Pouya et al 2016). Deux phénomènes sont étudiés, avec des simulations numériques aux Eléments Finis 2d : le développement d’une rupture diffusé, autour de la section des galeries, et l’apparition des fractures le long de l’excavation. Alors qu’une modélisation en déformations planes simule le premier cas, le deuxième est étudié en axisymétrie.Pour la première approche, deux décharges isotropes, en élasto-plasticité ou élasto-endommagement radoucissant, démontrent une instabilité globale liée au premier modèle, alors que des ruptures localisées se produisent avec le deuxième. Ceci, enrichi pour considérer les anisotropies d’élasticité, de résistance et d’endommagement du matériau, permet une bonne estimation de la zone de rupture à court terme, avec des valeurs des convergences conformes aux données, pour une galerie testée comme cas d’étude. Ensuite, la transition de la rupture du matériau de fragile à ductile, avec le confinement, est aussi prise en compte, avec les différentes conditions aux limites d’une deuxième galerie. Les zones endommagées à court terme estiment correctement la forme et l'extension de la rupture et les valeurs des convergences instantanées sont comparables aux mesures (Trivellato et al 2018).Le deuxième axe de recherche concerne la prédiction de la longueur des fractures, simplifiées comme des discontinuités finies et parallèles, ainsi que leur espacement. Numériquement, ces sont assimilées aux éléments joints (Goodman 1976). Le modèle de la fracture cohésive (Pouya et Bemani 2015) a été choisi pour ces éléments. Avec une seule discontinuité, on observe une initiation instable de la fracture, possiblement suivi par une évolution stable, d’une longueur réduite. Plusieurs séquences des joints ont été employés pour calculer la périodicité des fractures. Ces travaux ont permis aussi l’étude de différentes méthodes numériques qui simulent l’avancement d’un front d’excavation. Selon un choix précis des paramètres, on calcule des longueurs de fracturation comparables aux mesures, ainsi que la possibilité d’obtenir l’activation d’une seule fracture parmi plusieurs (Trivellato et al 2018).Ces travaux constituent une nouvelle approche de prédiction des effets du creusement sur l’argilite du Callovo-Oxfordien. Ils étudient une rupture à court terme due au comportement fragile sous faible confinement. Le mécanisme de dissipation en endommagement est intégré par les effets à la fois de l’anisotropie intrinsèque du matériau et de sa transition fragile-ductile. Les résultats obtenus par les deux axes de recherche favorisent l’utilisation de ce modèle comme complément aux études des excavations. En perspective, on peut envisager d’intégrer ces modèles avec les effets de la plasticité/fluage du matériau ainsi qu’avec la poro-élastique en comptant les effets hydrauliques
The French National Radioactive Waste Management Agency (Andra) manages an Underground Research Laboratory (URL) at the Meuse / Haute-Marne Center to study the feasibility of a deep geological repository in the Callovo-Oxfordian claystone (COx). The galleries follow the major and minor principal stress directions. Some galleries show a quasi-isotropic stress state in their cross-section, while others show a greater anisotropy.These works study the short-term failure and fracturing phenomena induced by the underground structures’ excavation. Brittle softening damage is considered as a fundamental failure mechanism. The major part of scientific literature reports an estimation of this area from an elastic post-treatment or based on elastic-plastic analyses. If the first method does not consider the stresses’ redistribution due to dissipative phenomena, elastic-plastic approaches sometimes seem insufficient to explain the geometry of these zones in some cases of deep excavations in quasi-brittle rocks (Pouya et al 2016). Two phenomena are studied, through numerical simulations by 2d Finite Elements: the development of a diffused failures, around the galleries’ cross-section, and the fractures occurrence at their perimeter, along the excavation. While a plane strain analysis is suitable for the first problem, the second one is studied in axial symmetry.For the first part of the research, two isotropic unloading processes, in softening elastic-plasticity or elastic-damage, reproduce a global instability related to the first modelling, whereas localized failure occurs with the second one. The latter model, upgraded to consider intrinsic anisotropies in terms of elasticity, resistance and damage dissipative law, allows a consistent estimation of the short-term failure zone, with values of convergences in accordance to the data, for a gallery considered as a case study. Then, the transition of material’s failure from a brittle to ductile behaviour, with the confinement, is also simulated, according to different boundary conditions of a second gallery. The short-term damaged zones well reproduce the shape and extension of failure systems and the values of the instantaneous convergences are comparable to in-situ measurements (Trivellato et al 2018).The second research axis concerns the elongation and mutual spacing of fractures, simplified as a system of finite and parallel discontinuities. Numerically, they are assimilated to joint elements (Goodman 1976). The cohesive fracture model, based on damage, (Pouya and Bemani 2015) was chosen for these elements. In presence of a single potential fracture, an unstable initiation is observed, possibly followed by a stable evolution, of reduced length. Then, models with a sequences of several joints were used to analyse the fracture periodicity. This work also allowed the study of different numerical techniques simulating the advancement of an excavation front. According to a precise choice of parameters, fractures’ lengths are comparable to the geological and geophysical surveys. Similarly, the activation of one potential fracture among different discontinuities was calculated, showing a periodic occurrence (Trivellato et al 2018).This dissertation constitutes a new approach to reproduce the immediate effects of deep excavations in the Callovo-Oxfordian claystone. They study a short-term failure due to the material’s brittleness, under low confinement. Damage is adopted as the only dissipation mechanism and is integrated by the effects of material’s intrinsic anisotropy as well as its brittle-ductile post-peak transition. Results obtained by every research axis appear favourable to employ these models as complements to excavation studies. In perspective, the integration of plasticity / creep effects, as well as a poro-elastic framework accounting for hydraulic effects, may be considered

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Mühlich, Uwe. "Generalised continuum approach for modelling quasi-brittle failure". Doctoral thesis, Technische Universitaet Bergakademie Freiberg Universitaetsbibliothek "Georgius Agricola", 2014. http://nbn-resolving.de/urn:nbn:de:bsz:105-qucosa-137217.

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A proper description of quasi-brittle failure within the frame of continuum Mechanics can only be achieved by models based on so-called generalised continua. This thesis focuses on a strain gradient generalised continuum and provides a specific methodology to derive corresponding models which account for the essential features of quasi-brittle failure. This methodology is discussed by means of four peer-reviewed journal articles. Furthermore, an extensive overview of the state of the art in the field of generalised continua is given at the beginning of the thesis. This overview discusses phenomenological extensions of standard Continuum Mechanics towards generalised continua together with corresponding homogenisation strategies for materials with periodic or random microstructure
Eine geeignete, kontinuumsmechanische Beschreibung quasi-spröden Versagens ist nur unter Verwendung verallgemeinerter Kontinuumstheorien möglich. In dieser Habilitationsschrift stehen sogenannte Gradientenkontinua im Vordergrund. Für diese wird eine Methodik vorgeschlagen, welche die Herleitung von Modellen erlaubt, die in der Lage sind, quasi-sprödes Versagen adäquat abzubilden. Diese Methodik wird anhand von vier Publikationen dargestellt und diskutiert. Ein umfangreicher Überblick über den Stand der Forschung auf dem Gebiet der veralgemeinerten Kontinuumstheorien wird am Anfang der Habilitationschrift gegeben. Dabei werden neben phänomenologischen Ansätzen zur Ableitung verallgemeinerter Kontinuumstheorien auch die entsprechenden Homogenisierungskonzepte dargestellt. Letztere werden für Materialien mit periodischer Mikrostruktur und für Materialien mit zufälliger Mikrostruktur diskutiert

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Chia, Julian Yan Hon. "A micromechanics-based continuum damage mechanics approach to the mechanical behaviour of brittle matrix composites". Thesis, University of Glasgow, 2002. http://theses.gla.ac.uk/2856/.

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The thesis describes the development of a new continuum damage mechanics (hereafter, CDM) model for the deformation and failure of brittle matrix composites reinforced with continuous fibres. The CDM model is valid over sizes scales large compared to the spacing of the fibres and the dimensions of the damage. The composite is allowed to sustain damage in the form of matrix micro-cracking, shear delamination, tensile delamination and fibre failure. The constitutive equations are developed by decomposing the composite compliance into terms attributable to the fibre and matrix, and modelling the competing failure modes by intersecting failure surfaces based on maximum stress theory. The fibres are treated as being weakly bonded to the matrix so that the fibres only transmit axial loads, and fail in tension. The matrix is modelled as isotropic linear elastic and is treated as transversely-isotropic after damage has initiated. The effect of multiple matrix cracking on the stiffness was determined from experimental data, while failure was modelled by a rapid decay in the load bearing capacity. Although the model is motivated largely to proportional loading, matrix unloading and damage closure has been modelled by damage elasticity. During compression, the matrix stiffness is identical to the undamaged state with the exception that the fibres are assumed not to transmit compressive loads. The model was implemented computationally through a FORTRAN subroutine interfaced with the ABAQUS/Standard finite element solver. The CDM model was validated by comparing experimental and computational results of test specimens with unidirectional and balanced 0°-90° woven fibres of a brittle matrix composite, fabricated from polyester fibres in a polyester matrix. This composite system exhibits low elastic mismatch between fibres and matrix, and has similar non-dimensionalised stress-strain response to a SiC/SiC composite proposed for the exhaust diffuser unit of the Rolls-Royce EJ200 aero-engine.

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Kim, Jinkoo. "Micromechanical model for damage and failure of brittle materials : application to polycrystalline ice and concrete". Thesis, Massachusetts Institute of Technology, 1995. http://hdl.handle.net/1721.1/11389.

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Chen, Wei. "Damage characteristics of brittle rocks inside the pre-failure range: numerical simulation and lab testing". Doctoral thesis, Technische Universitaet Bergakademie Freiberg Universitaetsbibliothek "Georgius Agricola", 2016. http://nbn-resolving.de/urn:nbn:de:bsz:105-qucosa-193998.

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The time-independent and -dependent damage characteristics of brittle rocks inside the pre-failure range have been investigated using numerical simulations and lab testing. Grain-based discrete element models have been developed to simulate both, time-independent and -dependent damage evolution leading to ultimate failure of sandstone and granite, respectively. The models take into account elastic grain and elasto-plastic contact deformation, inter- and intra-granular fracturing and lifetime prediction on the basis of subcritical crack growth. The time-independent mechanical behavior of Coconino sandstone and Lac du Bonnet granite during uniaxial compression tests, Brazilian splitting tests and fracture toughness tests was simulated. Triaxial compression tests and fracture toughness tests for Kirchberg II granite and fracture patterns tests for Eibenstock II granite were carried out in laboratory to perform time-independent damage and failure criterion analysis. The corresponding simulations showed reasonable damage phenomena compared with experimental results. Damage indices were deduced and were applied for different time-independent simulations. Based on calibrations of the time-independent damage simulations of selected brittle rocks, Charles equation and Hillig-Charles equation, which are generally used to describe subcritical crack growth, were implemented into the numerical code to simulate time-dependent damage. One-edged crack growth in Coconino sandstone specimen due to stress corrosion has been analyzed theoretically and numerically. Uniaxial compressive creep tests for Lac du Bonnet granite were simulated and time-dependent behavior in terms of the damage process during primary, secondary and tertiary creep until final failure characterized by macroscopic fracturing was discussed in detail. Subsequent to this, the time-dependent Mode-I crack growth tests and uniaxial compressive creep tests for Kirchberg II granite were carried out and the corresponding simulations were performed. Simulation results are in good agreement with experimental observations. In addition, damage indices and time-dependent fracture development were monitored and illustrated. The developed approach was applied to two potential practical applications: the damage analysis of a sandstone landscape arch and a tunnel. Finally, the results are summarized and recommendations for future work are proposed.

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Vongbandit, Pratip. "Morphology of surface damage resulting from static and dynamic contacts". Thesis, Brunel University, 2008. http://bura.brunel.ac.uk/handle/2438/3215.

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Contact fatigue damages resulting either from static or dynamic contact are of interest for understanding the failure modes and mechanisms leading to improvement of the components’ performances in tribological applications. The objective of this research was to ascertain how and to what extent the counterface materials, loading conditions, contact configuration, lubrication, and the environment affect the failure behaviours of material under static and dynamic contact fatigue loading. An experimental ball-on-flat test configuration was employed for both static and dynamic contact fatigue testing. In house designed test rig was used to study static cyclic loading contact fatigue behaviours of brittle polymethylmethacrylate (PMMA) in contact with balls made of different materials, i.e. Si3N4, steel, aluminium, bronze and PMMA in dry and oil-lubricated conditions. A modified four ball test machine was used to study dynamic rolling contact fatigue behaviours of thermally sprayed molybdenum and titanium coatings in contact with steel balls in dry and seawater conditions. The static contact fatigue and the dynamic contact fatigue test results revealed that counterface material, loading magnitude, lubricant and the environment play a vital role in controlling failure modes and the extent of damage. In static contact fatigue, adhesive strength of the interface was the key factor controlling damage of the PMMA plate in both dry and oil-lubricated conditions. In dry conditions, three failure modes, i.e. adhesive wear, ring cracks, and radial cracks controlled the damage of PMMA to a different degree for each combination of materials. Whereas, the damage of each combination in oil-lubricated conditions was affected by the extent of three failure modes, i.e. adhesive wear, radial cracks and abrasive wear. In dynamic contact fatigue tests, adhesive wear and inter-lamellar cracking were the major failure modes controlling damage of molybdenum coating and titanium coating in dry contact conditions while abrasive wear, corrosion and lubrication controlled damage processes in seawater conditions.

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Iurlano, Flaviana. "An Approximation Result for Generalised Functions of Bounded Deformation and Applications to Damage Problems". Doctoral thesis, SISSA, 2013. http://hdl.handle.net/20.500.11767/4831.

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Luther, Torsten Verfasser], i Carsten [Akademischer Betreuer] [Könke. "Adaptation of atomistic and continuum methods for multiscale simulation of quasi-brittle intergranular damage / Torsten Luther ; Betreuer: Carsten Könke". Weimar : Institut für Strukturmechanik, 2010. http://d-nb.info/1115806416/34.

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Sicsic, Paul. "Modeling and simulation of the nucleation and propagation of damage in quasi-brittle materials: Contribution of the variational approach". Palaiseau, Ecole polytechnique, 2013. http://pastel.archives-ouvertes.fr/docs/00/90/33/60/PDF/13_these_sicsic.pdf.

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Cette thèse explore l'utilisation de modèles d'endommagement pour prédire la nucléation et la propagation de la rupture de manière cohérente. Les résultats sont basés sur la donnée d'une énergie, qui définit le matériau, et d'une loi d'évolution construite sur un principe de stabilité, de conservation d'énergie et d'irreversibilité. Dans un premier temps, on étudie l'initiation de fissures dans une structure contenant un coin. Le chargement limite se réduit à trois composantes : un facteur d'échelle, une composante géométrique fonction de l'angle, et une composante propre au modèle. Ces modèles donnent naissance à des fissures dont le profil est caractéristique et dont la largeur est de l'ordre de la longeur interne du modèle. Cette dernière étant petite devant les dimensions de la structure, dans le cadre d'une séparation d'échelles et en utilisant un principe de minimum local, on montre que le modèle d'endommagement considéré converge vers la loi de propagation de Griffith. Ce résultat fondamental étend ceux existants, basés sur la minimisation globale, mais avec une base physique plus forte. Une étude approfondie donne une meilleure compréhension de la phase d'initiation dans le cas d'un choc thermique et on établit la propriété globale qu'est l'éemergence d'une solution periodique. Ces résultats s'appuient sur le cadre variationnel, les propriétées seraient probablement perdues pour un modèle d'endommagement développé dans un autre cadre. Dans un dernier temps, les résultats numériques basés sur un algorithme de minimisation alternée capturent une initiation contrôlée par la contrainte critique, ainsi que la propagation des fissures controlée par la densité d'énergie de fissuration. Des effets d'échelle en deux et trois dimensions sont mis en évidence. Les simulations sont alors confrontées à des résultats expérimentaux
This thesis explores the use of damage models to predict the onset and propagation of cracks in a coherent manner. The results are based on the definition of a bulk energy density and a stability principle. Firstly, we study the nucleation of cracks in a notched domain. The limit loading can be decomposed as the product of three stress intensity factors: a scale effect, a geometry induced factor, function of the angle of the notch, and one due to the damage model. The cracks that appear have a characteristic profile whose width is of the order of the internal length. When the latter is small in front of the dimensions of the structure, by separating scales, and using a local minimum principle, we prove that the length of these damage bands follow Griffith's law. This fundamental results extends those based on global minimization but with a sounder physical base. A thorough investigation of the thermal shock problem leads to a better understanding of the nucleation of cracks. Especially the global property of crack periodicity is exhibited. These results are based on the variational approach and the properties would probably be lost for models developed in an other framework. Finally, numerical results based on an alternate minimization algorithm are established. The nucleation phase is controlled by the critical stress whereas the propagation is governed by the toughness. Size effects in two and three dimensions are captured. These numerical simulations are then confronted to experimental results

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20

Yu, Jianguo. "A contact interaction framework for numerical simulation of multi-body problems and aspects of damage and fracture for brittle materials". Thesis, Swansea University, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.580168.

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Zohora, Fatematuz. "Evaluation of material crack using acoustic emission technique". Thesis, Queensland University of Technology, 2016. https://eprints.qut.edu.au/94663/1/Fatematuz_Zohora_Thesis.pdf.

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Cracks in civil structures can result in premature failure due to material degradation and can result in both financial loss and environmental consequences. This thesis reports an effective technique using Acoustic Emission (AE) technique to assess the severity of the crack propagation in steel structures. The outcome of this work confirms that combination of AE parametric analysis and signal processing techniques can be used to evaluate crack propagation under different loading configurations. The technique has potential application to assess and monitor the condition of civil structures.

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22

Hütter, Geralf. "Multi-scale simulation of crack propagation in the ductile-brittle transition region". Doctoral thesis, Technische Universitaet Bergakademie Freiberg Universitaetsbibliothek "Georgius Agricola", 2013. http://nbn-resolving.de/urn:nbn:de:bsz:105-qucosa-121281.

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In the present thesis the crack propagation in the ductile-brittle transition region is studied on two scales with deterministic models. In the macroscopic model the ductile failure is described by a non-local Gurson-model whereas the discrete void microstructure is resolved around the crack tip in the microscopic model. The failure by cleavage is not evaluated by means of a post-processing criterion but is modeled equivalently using a cohesive zone model on both scales. Thus, cleavage is not a priori identified with unstable crack propagation but the transition between stable and unstable mode of propagation is a result of the simulation. The problem of handling completely failed material within the framework of non-local damage models is pointed out. A method to overcome this problem is proposed and successfully applied. The case of contained plastic yielding at the crack tip is addressed with a modified-boundary layer model. The macroscopic simulations reproduce many features which are known from experiments like the formation of stretch zones, cleavage after initial ductile tearing, pop-ins with crack arrest, among others. The microscopic simulations substantiate the understanding of the macroscopically observed behavior. Systematic parameter studies are performed. Starting with considerations on the limit cases like pure ductile failure or the lower-ductile brittle transition region allows to separate the effects of the different constitutive parameters. Based on these results, a methodology is proposed to extract the macroscopic material parameters from experiments. This scheme is successfully applied to experimental data from literature. The results show that the behavior of a low-constraint specimen can be reliably predicted with the parameters extracted from a high-constraint specimen
In der vorliegenden Arbeit wird die Rissausbreitung im spröd-duktilen Übergangsbereich auf zwei Skalen mittels deterministischer Modelle untersucht. Das duktile Versagen wird im makroskopischen Modell durch ein nichtlokales Gurson-Modell beschrieben, während im mikroskopischen Modell die Porenmikrostruktur im Bereich um die Rissspitze diskret aufgelöst wird. Das mögliche Versagen durch Spaltbruch wird nicht, wie üblich, nachträglich durch ein spannungsbasiertes Kriterium bewertet. Stattdessen wird der Spaltbruch auf beiden Skalen durch ein Kohäsivzonenmodell abgebildet. Somit wird die Spaltbruchinitiierung nicht a priori mit instabiler Rissausbreitung gleichgesetzt. Vielmehr ist die Stabilität der Rissausbreitung ein Ergebnis der Simulationen. Außerdem wird das Problem der der Handhabung vollständig ausgefallenen Materials im Rahmen nichtlokaler Schädigungsmodelle herausgestellt. Es wird eine Methode vorgestellt, dieses Problem zu behandeln und erfolgreich angewendet. In den Simulationen wird der Fall vollständig eingebetteten, plastischen Fließens untersucht. Die Simulationen mit dem makroskopischen Modell geben viele Effekte wieder, die aus Experimenten bekannt sind. Dazu zählen die Ausbildung von Stretchzonen, die Spaltbruchinitiierung nach anfänglichem, duktilem Reißen oder lokale Instabilitäten mit Rissarrest. Die mikroskopischen Simulationen tragen zum Verständnis des makroskopisch beobachteten Verhaltens bei. In der vorliegenden Arbeit werden systematische Parameterstudien durchgeführt. Zunächst werden Grenzfälle wie das rein duktile Versagens oder der Spaltbruch in Abwesenheit der Mikroporen untersucht, um die Einflüsse der einzelnen Materialparameter abzugrenzen. Ausgehend von diesen Ergebnissen wird eine Prozedur vorgeschlagen, die Materialparameter des makroskopischen Modells Schritt für Schritt aus Experimenten zu bestimmen. Diese Prozedur wird erfolgreich auf experimentelle Daten aus der Literatur angewendet. Die Ergebnisse zeigen, dass es das entwickelte Modell erlaubt, das Verhalten einer Bruchmechanikprobe mit geringer Dehnungsbehinderung an der Rissspitze mit denjenigen Materialparametern vorherzusagen, die an Proben mit einer hohen Dehnungsbehinderung ermittelt wurden

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Liu, Dongxin. "Numerical Investigation on Dynamic Responses of Granular Materials under Impact Loading Using the Material Point Method". Thesis, The University of Sydney, 2015. http://hdl.handle.net/2123/14398.

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This study illustrates the dynamic responses of brittle granular materials under impact with the MPM and the comparison with experiments, helping us understand how a brittle particle chain fails under the loading of high rate stress wave. High strain rate experiments of chains of glass beads under impact demonstrate that first failure develops in either the first or second bead, depending on the impact velocity and associated stress propagation. In the elastic model simulation, the results show that depending on the ratio of impact-induced maximum tensile stress to the fracture strength of a material, the failure might first form in either the first or the second brittle bead, which can be verified from the experiments. Further dimensionless analysis in the material strength and impact velocity reveals that there exists a transition failure stress line, above which the second brittle bead will fail first and otherwise damage will first occur in the first bead. In the elasto-damage model simulation, similar results show that the second brittle bead will become the most severely damaged one, despite of the initial damage in the first bead due to high impact velocity. Usually, most failure points will appear in the contact area between the spheres and extend to interior conically. It is found that if the damage threshold decreases, the surface of the bead tend to fail simultaneously because of the tension stresses generated by reflected waves from the ball surface. In addition, with a lower damage threshold or fracture energy, less damage is developed in some beads after a period of time. This is because more damage at the beginning dissipates excessive stress wave energy to the extent where the reflected wave will not be able to cause more damage in the local system. Future work will pay attention to the friction function if the particles are not aligned. In addition, a more realistic situation where multiple grains are randomly arranged will be investigated.

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Parrinello, Antonino. "A rate-pressure-dependent thermodynamically-consistent phase field model for the description of failure patterns in dynamic brittle fracture". Thesis, University of Oxford, 2017. https://ora.ox.ac.uk/objects/uuid:c6590f4f-f4e2-40e3-ada1-49ba35c2a594.

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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.

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Amorim, David Leonardo Nascimento de Figueiredo. "On the lumped damage mechanics for nonlinear structural analyses: new developments and applications". Universidade de São Paulo, 2016. http://www.teses.usp.br/teses/disponiveis/18/18134/tde-06042016-112414/.

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The accurate description of the nonlinear structural behaviour is an important issue in engineering science. Usually, classic nonlinear theories, such as fracture and damage mechanics, applied to finite element programmes are used to fulfil that purpose. Classic fracture mechanics describes the structural deterioration process by a few discrete cracks. This theory presents good precision for structures with simple geometries, few cracks and homogeneous materials. Classic damage mechanics measures the deterioration process by an internal variable called damage. This theory has been successful in the description of several deterioration mechanisms in continuum media. Despite their accuracy, classic fracture and damage mechanics present some drawbacks. Firstly, regarding civil engineering problems, both theories are not suitable for some practical applications. Secondly, fracture mechanics demands the consideration of initial cracks to begin the analysis. Lastly, classic damage models may present an issue known as localisation, what essentially leads to ill-posed problems and mesh-dependent numerical algorithms. Alternatively, a recent theory, called lumped damage mechanics, was proposed in order to achieve good accuracy in actual engineering problems. Such theory applies key concepts from fracture and damage mechanics in plastic hinges. In the light of the foregoing, the main goal of this thesis is the extension of the lumped damage mechanics framework to analyse different engineering problems. So far, lumped damage mechanics was characterised as a simplified methodology to analyse reinforced concrete frames under seismic and monotonic loadings; even with a few contributions on the analysis of local buckling in metallic structures. Therefore, this work extends the lumped damage mechanics framework to analyse reinforced concrete arches, unreinforced concrete structures, high cycle fatigue and continuum problems. The application examples show the accuracy of the proposed methodologies.
A descrição acurada do comportamento não linear de estruturas é um problema importante na engenharia. Usualmente, teorias não lineares clássicas, tais como as mecânicas da fratura e do dano, aplicadas a programas de elementos finitos são utilizadas a fim de cumprir aquele propósito. A mecânica da fratura clássica descreve o processo de deterioração estrutural por meio de um pequeno número de fissuras discretas. Esta teoria apresenta boa precisão para estruturas com geometrias simples, poucas fissuras e materiais homogêneos. A mecânica do dano clássica tem sido exitosa na descrição de diversos mecanismos de deterioração em meios contínuos. Apesar de precisas, as abordagens clássicas em fratura e dano apresentam alguns entraves. Primeiramente, tratando-se de problemas da engenharia civil, ambas teorias não são adequadas para aplicações práticas. Em segundo lugar, os modelos clássicos de fratura demandam a consideração de fissuras iniciais para iniciar a análise. Por fim, os modelos clássicos de dano podem apresentar um problema conhecido como localização, o que essencialmente implica em problemas mal colocados e algoritmos com dependência de malha. Alternativamente, uma teoria recente, chamada teoria do dano concentrado, foi proposta a fim de obter boa precisão em problemas reais de engenharia. Tal teoria aplica conceitos-chave das mecânicas da fratura e do dano em rótulas plásticas. À luz do exposto, o principal objetivo desta tese é a extensão da teoria do dano concentrado para analisar diferente problemas da engenharia. Até então, a teoria do dano concentrado era caracterizada como uma metodologia simplificada para analisar pórticos de concreto armado sob solicitações monotônicas ou sísmicas; mesmo com algumas poucas contribuições na análise de instabilidade local em estruturas metálicas. Desta forma, este trabalho estende a teoria do dano concentrado a fim de analisar arcos de concreto armado, estruturas de concreto simples, fadiga de alto ciclo e problemas contínuos. Os exemplos de aplicação mostram a acurácia das metodologias propostas.

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Rodrigues, Eduardo Alexandre [UNESP]. "Um modelo constitutivo de dano composto para simular o comportamento de materiais quase-frágeis". Universidade Estadual Paulista (UNESP), 2011. http://hdl.handle.net/11449/97142.

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Made available in DSpace on 2014-06-11T19:28:35Z (GMT). No. of bitstreams: 0 Previous issue date: 2011-03-21Bitstream added on 2014-06-13T20:37:41Z : No. of bitstreams: 1 rodrigues_ea_me_bauru.pdf: 1602991 bytes, checksum: 7f755b87b5be84900b2d054f02413197 (MD5)
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No presente trabalho desenvolve-se um modelo constitutivo baseado na mecânica do dano contínuo para representar o comportamento de materiais que apresentam diferentes respostas quando solicitados à tração ou à compreensão. obtem-se uma representação constitutiva através da composição de modelos simples e específicos para tratar cada tipo de solicitação. Este modelo combinado é capaz inclusive de lidar com carregamentos alternados (tração e compreensão), envolvendo fechamento e reabertura de fissuras existentes. Para modelar o comportamento em compreensão emprega-se o modelo constitutivo que tem como critério de degradação o segundo invariante do tensor de tensão desviador (critério de Von Mises ou J2). Para simular o aparecimento de fissuras de tração, usa-se o modelo de dano com critério de degradação baseado na energia de deformação da parte positiva do tensor efetivas. A integração dos modelos é feita com base em tensões efetivas associadas a duas escalas distintas (escala grosseira e refinada). O modelo é apto para representar a formação de descontinuidades no campo de deslocamento (descontinuidades fortes) em materiais quase-frágeis. Nesse caso, a região de localização de deformação (zona de processo da fatura) pode ser descrita pelo modelo de dano combinado, com lei de abrandamento de tensões (softening) exponencial, que estabelece dissipação compatível com a energia de fratura. A região contínua pode ser descrita pelo modelo de dano J2, com parâmetros ajustados com base no comportamento não linear à compreensão. Valida-se o modelo proposto mediante testes básicos, focando a capacidade do modelo em representar os principais aspectos do comportamento de materiais quase-frágeis. A aplicabilidade do modelo é demonstrada através do estudo da capacidade de rotação plástica de vigas de concreto armado, confrontando-se os resultados numéricos com os experimentais
A combined constitutive model based on the Continuum Damage Mechanics (CDM) is presented to represent the nonlinear behavior of quasi-brittle materials, which present different response when subjected to tension or compreession. The constitutive model is a composition of two simple and specific models designed to treat each type of behavior. The combined model is able to deal with alternating load (tension-compression), involving formation, closure and reopening cracks. To model the compressive behavior, a degradation criterion based on the second invariant of the deviatoric part of the effective stress tensor (Von Miser or J2 criterion) is used. To simulate cracking, a damage model with degradation criterion based on the strain energy associated to the positive part the effective stress tensor is adopted. The combination of the models is made on the basis of the effective stresses associated to two distinct scales (coarse and fine scales) The model is able to represented the formation of discontinuities in the displacement field (strong discontinuities) for quasi-brittle materials. The region of strain localization (fracture process zone) is described by a softening law which establishes dissipation energy compatible with the fracture energy. The continuous region is described by the J2 damage model, with parameters ajusted to describle the compressive nonlinear behavior in compression. Some basic tests are performed to asses the ability of the model to represent the main aspects of the behavior of quasi-brittle materials. The applicability of the model is demonstrated by the study of the plastic rotation capacity of reinforced concrete beams, comparing the numerical responses with the experimental ones

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Tanne, Erwan. "Variational phase-field models from brittle to ductile fracture : nucleation and propagation". Thesis, Université Paris-Saclay (ComUE), 2017. http://www.theses.fr/2017SACLX088/document.

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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

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Shaikh, Adnan Muzaffar. "Study of the fracture behavior of concrete by the development and application of the nonlinear finite element code incorporating the composite damage mechanics theory for brittle materials". Diss., Connect to online resource, 2006. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:1435225.

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Chen, Wei [Verfasser], Heinz [Akademischer Betreuer] Konietzky, Heinz [Gutachter] Konietzky, Thomas [Gutachter] Kohl i Bo [Gutachter] Liu. "Damage characteristics of brittle rocks inside the pre-failure range: numerical simulation and lab testing / Wei Chen ; Gutachter: Heinz Konietzky, Thomas Kohl, Bo Liu ; Betreuer: Heinz Konietzky". Freiberg : Technische Universitaet Bergakademie Freiberg Universitaetsbibliothek "Georgius Agricola", 2016. http://d-nb.info/1220912360/34.

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Jebahi, Mohamed. "Discrete-continuum coupling method for simulation of laser-inducced damage in silica glass". Thesis, Bordeaux 1, 2013. http://www.theses.fr/2013BOR14911/document.

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Une méthode de couplage continu-discret a été développée pour simuler les mécanismes complexes d'endommagement de la silice soumise à un choc laser de haute puissance. Dans un premier temps, une classification des méthodes numériques existantes a été faite pour choisir celles les mieux adaptées à la simulation du comportement sous choc de la silice. Comme résultat de cette classification, deux méthodes ont été retenues: la méthode des éléments discrets (DEM) et la méthode des éléments naturels contraints (CNEM). Ces méthodes sont alors couplées en se basant sur la technique dite "Arlequin". Puis, un modèle numérique permettant de tenir compte des différents phénomènes qui caractérise le comportement de la silice sous haute pression a été développé. Pour bien caractériser les mécanismes de fissuration de la silice à l’échelle microscopique, un nouveau modèle de rupture a été développé dans ce travail. Finalement, ces deux modèles, modèle de comportement et modèle de rupture, ont été intégrés dans la méthode du couplage pour simuler d'un point de vue mécanique le choc laser sur un échantillon en silice
A discrete-continuum coupling approach has been developed to simulate the laser-induced damage in silica glass. First, a classification of the different numerical methods has been performed to select the ones that best meet the objectives of this work. Acting upon this classification, the Discrete Element Method (DEM) and the Constrained Natural Element Method (CNEM) have been retained. Subsequently, a coupling approach between these methods has been proposed. This approach is based on the Arlequin technique. In the second part, a numerical model of the silica glass mechanical behavior has been developed to better characterize the silica glass response under highly dynamic loadings and particularly loading generated by a laser beam. To correctly characterize the silica glass cracking mechanisms, a new fracture model has been proposed in this work. Finally, all these developments have been used to simulate the laser-induced damage in silica glass

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Rodrigues, Eduardo Alexandre. "Um modelo constitutivo de dano composto para simular o comportamento de materiais quase-frágeis /". Bauru : [s.n.], 2011. http://hdl.handle.net/11449/97142.

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Resumo: No presente trabalho desenvolve-se um modelo constitutivo baseado na mecânica do dano contínuo para representar o comportamento de materiais que apresentam diferentes respostas quando solicitados à tração ou à compreensão. obtem-se uma representação constitutiva através da composição de modelos simples e específicos para tratar cada tipo de solicitação. Este modelo combinado é capaz inclusive de lidar com carregamentos alternados (tração e compreensão), envolvendo fechamento e reabertura de fissuras existentes. Para modelar o comportamento em compreensão emprega-se o modelo constitutivo que tem como critério de degradação o segundo invariante do tensor de tensão desviador (critério de Von Mises ou J2). Para simular o aparecimento de fissuras de tração, usa-se o modelo de dano com critério de degradação baseado na energia de deformação da parte positiva do tensor efetivas. A integração dos modelos é feita com base em tensões efetivas associadas a duas escalas distintas (escala grosseira e refinada). O modelo é apto para representar a formação de descontinuidades no campo de deslocamento (descontinuidades fortes) em materiais quase-frágeis. Nesse caso, a região de localização de deformação (zona de processo da fatura) pode ser descrita pelo modelo de dano combinado, com lei de abrandamento de tensões (softening) exponencial, que estabelece dissipação compatível com a energia de fratura. A região contínua pode ser descrita pelo modelo de dano J2, com parâmetros ajustados com base no comportamento não linear à compreensão. Valida-se o modelo proposto mediante testes básicos, focando a capacidade do modelo em representar os principais aspectos do comportamento de materiais quase-frágeis. A aplicabilidade do modelo é demonstrada através do estudo da capacidade de rotação plástica de vigas de concreto armado, confrontando-se os resultados numéricos com os experimentais
Abstract: A combined constitutive model based on the Continuum Damage Mechanics (CDM) is presented to represent the nonlinear behavior of quasi-brittle materials, which present different response when subjected to tension or compreession. The constitutive model is a composition of two simple and specific models designed to treat each type of behavior. The combined model is able to deal with alternating load (tension-compression), involving formation, closure and reopening cracks. To model the compressive behavior, a degradation criterion based on the second invariant of the deviatoric part of the effective stress tensor (Von Miser or J2 criterion) is used. To simulate cracking, a damage model with degradation criterion based on the strain energy associated to the positive part the effective stress tensor is adopted. The combination of the models is made on the basis of the effective stresses associated to two distinct scales (coarse and fine scales) The model is able to represented the formation of discontinuities in the displacement field (strong discontinuities) for quasi-brittle materials. The region of strain localization (fracture process zone) is described by a softening law which establishes dissipation energy compatible with the fracture energy. The continuous region is described by the J2 damage model, with parameters ajusted to describle the compressive nonlinear behavior in compression. Some basic tests are performed to asses the ability of the model to represent the main aspects of the behavior of quasi-brittle materials. The applicability of the model is demonstrated by the study of the plastic rotation capacity of reinforced concrete beams, comparing the numerical responses with the experimental ones
Orientador: Osvaldo Luís Manzoli
Coorientador: André Luís Gamino
Banca: Leonardo José do Nascimento Guimarães
Banca: Edson Antonio Capello Sousa
Mestre

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Pröchtel, Patrick. "Anisotrope Schädigungsmodellierung von Beton mit Adaptiver Bruchenergetischer Regularisierung". Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2008. http://nbn-resolving.de/urn:nbn:de:bsz:14-ds-1224751435667-29771.

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Der Gegenstand der vorliegenden Arbeit ist die Simulation von Betonstrukturen beliebiger Geometrie unter überwiegender Zugbelastung. Die Modellierung erfolgt auf Makroebene als Kontinuum und zur Lösung des mechanischen Feldproblems wird die Finite-Elemente-Methode verwendet. Ein neues Materialmodell für Beton und eine Erweiterung der Bruchenergetischen Regularisierung werden vorgestellt. Die Arbeit ist in zwei Teile gegliedert. Im ersten Teil wird ein lokales, anisotropes Schädigungsmodell abgeleitet, wobei als Schädigungsvariable ein symmetrischer Tensor zweiter Stufe gewählt wird. Die Verwendung einer Normalenregel im Raum der dissipativen Kräfte zur Bestimmung der Schädigungsevolution und die Definition der Schädigungsgrenzflächen im Raum der dissipativen Kräfte gewährleisten die Gültigkeit der Hauptsätze der Thermodynamik und des Prinzips der maximalen Dissipationsrate. Vorteilhaft ist die Symmetrie der Materialtangente, die sich aus diesem Vorgehen ergibt. Eine Formulierung mit drei entkoppelten Schädigungsgrenzflächen wird vorgeschlagen. Eine wichtige Forderung bei der Ableitung des Materialmodells war die Verwendung einer möglichst geringen Anzahl von Materialparametern, welche darüber hinaus aus wenigen Standardversuchen bestimmbar sein sollten. Das Schädigungsmodell enthält als Materialparameter den Elastizitätsmodul, die Querdehnzahl, die Zugfestigkeit und die auf eine Einheitsfläche bezogene Bruchenergie. Im zweiten Teil der Arbeit stehen Lokalisierung und Regularisierung im Fokus der Betrachtungen. Aufgrund der lokalen Formulierung des Materialmodells tritt bei Finite-Elemente Simulationen eine Netzabhängigkeit der Simulationsergebnisse auf. Um dieser Problematik zu begegnen und netzunabhängige Simulationen zu erreichen, werden Regularisierungstechniken angewendet. In dieser Arbeit wird die Bruchenergetische Regularisierung eingesetzt, die durch die Einführung einer äquivalenten Breite in ein lokal formuliertes Stoffgesetz gekennzeichnet ist. Die spezielle Wahl eines Wertes für die äquivalente Breite beruht auf der Forderung, dass in der Simulation die korrekte Bruchenergie je Einheitsfläche für den Bruchprozess verbraucht wird, d.h. die Energiedissipation der Realität entspricht. In vorliegender Arbeit wird die neue These aufgestellt, dass die Energiedissipation nur für den Fall korrekt abgebildet wird, wenn die im Stoffgesetz enthaltene äquivalente Breite in jedem Belastungsinkrement der Breite des Bereiches entspricht, in dem in der Simulation Energie dissipiert wird. In einer Simulation wird in den Bereichen Energie dissipiert, in denen die Schädigung im aktuellen Belastungsinkrement zunimmt. In vorliegender Arbeit werden die energiedissipierenden Bereiche daher als Pfad der Schädigungsrate bezeichnet. Um Erkenntnisse über die Entwicklung des Pfades der Schädigungsrate über den Belastungsverlauf zu erhalten, wurden umfangreiche Untersuchungen anhand von Simulationen eines beidseitig gekerbten Betonprobekörpers unter kombinierter Zug-Schubbeanspruchung durchgeführt, wobei die gewählten Werte für die äquivalente Breite variiert wurden. Es wurde stets eine Diskretisierung mit linearen Verschiebungselementen verwendet, wobei die Bereiche mit zu erwartender Schädigung feiner und regelmäßig mit Elementen quadratischer Geometrie diskretisiert wurden. Die Ergebnisse der Untersuchungen zeigen, dass die Breite des Pfades der Schädigungsrate abhängig ist von der Schädigung am betrachteten Materialpunkt, dem von Schädigungsrichtung und Elementkante eingeschlossenen Winkel, der Elementgröße und den Materialparametern. Um die geforderte Übereinstimmung von äquivalenter Breite und der Breite des Pfades der Schädigungsrate zu erreichen, werden neue Ansätze für die äquivalente Breite vorgeschlagen, die die erwähnten Einflüsse berücksichtigen. Simulationen unter Verwendung der neuen Ansätze für die äquivalente Breite führen zu einer guten Übereinstimmung von äquivalenter Breite und der Breite des Pfades der Schädigungsrate in der Simulation. Die Ergebnisse der Simulationen, wie z.B. Last-Verformungsbeziehung und Rissverläufe, sind netzunabhängig und stimmen gut mit den experimentellen Beobachtungen überein. Basierend auf den gewonnenen Erkenntnissen wird eine Erweiterung der Bruchenergetischen Regularisierung vorgeschlagen: die Adaptive Bruchenergetische Regularisierung. Im abschließenden Kapitel der Arbeit werden mit der vorgeschlagenen Theorie, dem neuen Schädigungsmodell und der Adaptiven Bruchenergetischen Regularisierung, noch zwei in der Literatur gut dokumentierte Versuche simuliert. Die Simulationsergebnisse entsprechen den experimentellen Beobachtungen
This doctoral thesis deals with the simulation of predominantly tensile loaded plain concrete structures. Concrete is modeled on the macro level and the Finite Element Method is applied to solve the resulting mechanical field problem. A new material model for concrete based on continuum damage mechanics and an extended regularization technique based on the fracture energy approach are presented. The thesis is subdivided into two parts. In the first part, a local, anisotropic damage model for concrete is derived. This model uses a symmetric second-order tensor as the damage variable, which enables the simulation of orthotropic degradation. The validity of the first and the second law of thermodynamics as well as the validity of the principle of maximum dissipation rate are required. Using a normal rule in the space of the dissipative forces, which are the thermodynamically conjugated variables to the damage variables, and the definition of the loading functions in the space of the dissipative forces guarantee their validity. The suggested formulation contains three decoupled loading functions. A further requirement in the derivation of the model was the minimization of the number of material parameters, which should be determined by a small number of standard experiments. The material parameters of the new damage model are the Young’s modulus, the Poisson’s ratio, the tensile strength and the fracture energy per unit area. The second part of the work focuses on localization and regularization. If a Finite Element simulation is performed using a local material model for concrete, the results of the Finite Element simulation are mesh-dependent. To attain mesh-independent simulations, a regularization technique must be applied. The fracture energy approach, which is characterized by introducing a characteristic length in a locally formulated material model, is used as regularization technique in this work. The choice of a value for the characteristic length is founded by the requirement, that the fracture energy per unit area, which is consumed for the fracture process in the simulation, must be the same as in experiment, i.e. the energy dissipation must be correct. In this dissertation, the new idea is suggested that the correct energy dissipation can be only attained if the characteristic length in the material model coincides in every loading increment with the width of the energy-dissipating zone in the simulation. The energy-dissipating zone in a simulation is formed by the integration points with increasing damage and obtains the name: damage rate path. Detailed investigations based on simulations of a double-edge notched specimen under mixed-mode loading are performed with varying characteristic lengths in order to obtain information concerning the evolution of the damage rate path during a simulation. All simulations were performed using displacement-based elements with four nodes. The range with expected damage was always finer and regularly discretized. The results of the simulations show that the width of the damage rate path depends on the damage at the specific material point, on the angle between damage direction and element edges, on the element size and on the material parameters. Based on these observations, new approaches for the characteristic length are suggested in order to attain the coincidence of the characteristic length with the width of the damage rate path. Simulations by using the new approaches yield a sufficient coincidence of the characteristic length with the width of the damage rate path. The simulations are mesh-independent and the results of the simulation, like load-displacement curves or crack paths, correspond to the experimental results. Based on all new information concerning the regularization technique, an extension of the fracture energy approach is suggested: the adaptive fracture energy approach. The validity and applicability of the suggested theory, the new anisotropic damage model and the adaptive fracture energy approach, are verified in the final chapter of the work with simulations of two additional experiments, which are well documented in the literature. The results of the simulations correspond to the observations in the experiments

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Rojas, Solano Laura Beatriz. "Endommagement non-local, interactions et effets d’échelle". Thesis, Pau, 2012. http://www.theses.fr/2012PAUU3032/document.

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Cette thèse porte sur la description du processus de fissuration du béton soumis à des sollicitations mécaniques. L'objectif principal est d'améliorer la description macroscopique à l'aide d'un modèle continu. Un modèle décrivant de façon cohérente le comportement à la rupture du béton devrait au moins représenter : (i) la transition continu/discret et l'effet d'écran induit par une macrofissure, (ii) la discontinuité du déplacement, (iii) l'interaction entre le processus de fissuration et un bord libre (iv) il doit aussi être capable de reproduire la réponse mécanique obtenue expérimentalement. Dans un premier temps, nous avons fait une analyse comparative entre le modèle d'endommagement non-local classique et différents modèles continus améliorés proposés dans la littérature. Des outils de comparaison ont été proposés pour cette analyse : (i) du point de vue numérique, deux exemples considérant la rupture dynamique d'une barre (barre en traction et test d'écaillage) et (ii) du point de vue expérimental, une base de données issue d'une série d'essais sur des poutres homothétiques entaillées et non-entaillées en flexion trois points. Nous avons conclu que seule une combinaison entre différentes formulations peut rendre compte de tous les mécanismes mis en jeu lors du processus de fissuration. Elle inclue à la fois la façon dont l'information non-locale est transmise, la croissance de défauts et la description des effets de bord. Nous avons mis en évidence que son implémentation 2D ou 3D reste complexe et donc la comparaison avec des données expérimentales s'avère impossible. Dans un deuxième temps, nous avons choisi de changer l'échelle d'analyse pour connaitre en détail les mécanismes ayant lieu au sein de la mésostructure du béton (pâte, granulat, interface) à l'aide d'un modèle mésoscopique basé sur des éléments lattice. Cette analyse a permis de conclure que la prise en compte des interactions entre les composants de la mésostructure du béton fournit des résultats numériques plus proches de la réalité que ceux obtenus avec le modèle non-local macroscopique classique. Le mésomodèle est capable de représenter aussi bien la charge maximale (effet d'échelle) que la phase adoucissante pour toutes les tailles de poutre et pour toutes les géométries d'entaille. Nous avons transposé la prise en compte des interactions de l'échelle mésoscopique à l'échelle macroscopique au travers de la fonction poids d'un nouveau modèle non-local. Elle est estimée en décrivant le matériau comme étant un ensemble d'inclusions qui interagissent entre elles lors du chargement. Ces inclusions sont dilatées élastiquement et successivement afin de caractériser le transfert d'information au sein du matériau et de reconstruire la fonction poids du modèle proposé. Ce nouveau modèle est capable de décrire la transition continu/discret et l'effet d'écran, la discontinuité du déplacement et de retrouver un effet de bord cohérent avec les résultats de la micromécanique. Son implémentation en 2D est présentée et les premiers résultats de calculs illustrent la démarche. Finalement, nous revenons sur la modélisation mésoscopique du comportement du béton. Sa richesse en information peut conduire à une compréhension plus fine du processus de fissuration et de la création puis l'évolution de la zone d'élaboration
This work focuses on the description of the process of cracking of concrete subjected to mechanical stresses. The main objective is to improve the understanding of the mechanisms involved using a continuous macroscopic model. A model describing consistently the fracture behavior of concrete should at least represent: (i) the continuous / discrete transition and the shielding effect induced by a macrocrack, (ii) the discontinuity of displacement, (iii) the interaction between the cracking process and a free boundary, (iv) it must also be able to reproduce the mechanical response obtained experimentally. At first, we made a comparative analysis of the classical non-local damage model and others improved continuous models proposed in the literature. Comparison tools have been proposed for this analysis: (i) from a numerical point of view, two examples considering the dynamic rupture of a bar (tensile test and spalling test) and (ii) from an experimental point of view, a database obtained from three-point bending test on notched and unnotched geometrically similar beams made from the same concrete formulation. We found that only a combination of this formulations may account for the different mechanisms involved in the process of cracking. It includes the transmission of the non-local information, the growing of voids and the description of boundary effects. We shown that its implementation in 2D or 3D remains complex and thus comparison with experimental results are impossible. In a second step, we decided to change the scale of analysis to precise the mechanisms which are taking place within the mesostructure of concrete using a mesomodel based on lattice elements. This analysis shown that since the mesomodel intrinsically took into account the interactions evolution within the structure, it is able to provide relevant results when classical macroscopic non-local models failed. It is able to represent both the maximum load (size effect) and the softening regime whatever the beam size or the pre-notch geometry. In addition, we proposed a new non-local framework where the interactions were upscale from the mesoscale to the macroscale through a new weight function. This function is estimated by describing the material as a set of inclusions that interact upon loading. These inclusions are successively elastically dilated to characterize the transfer of information within the material and rebuild the non-local weight function. This new model is able to describe the continuous / discrete transition, the shielding effect and the discontinuity of displacement. The model has been implemented in 2D in a finite element code and first results shown its capabilities to reproduce experimental results in term of maximum loads. In a third step, the richness of the mesoscopic approach has been used to describe precisely the local process of failure in term of fracture process zone evolution

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Dib, Dayana. "Analyse théorique et numérique de l'endommagement par micro-fissuration descomposites à matrice quasi-fragile". Thesis, Paris Est, 2015. http://www.theses.fr/2015PEST1099.

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Le problème initial traité dans cette thèse relève du cadre général de la modélisation des tunnels profonds. Pour cela, on a adopté l'approche basée sur la mécanique linéaire de la rupture. L'étude s'est appuyée sur le critère mixte de Leguillon. Suite à cette étude, on a pu tirer que ce n'est pas le critère mixte qui est insuffisant mais plutôt la façon d'aborder le problème. D'où le passage à la prise en compte de l'hétérogénéité du matériau constitutif et la possibilité d'amorçage d'une fissure sous une contrainte de compression. Une première approche a été entreprise par l'étude d'une bicouche périodique sous contrainte de compression verticale. La couche de grande raideur s'est apparue le siège d'une traction transversale. Effectivement la possibilité d'amorçage d'une fissure est tout à fait probable grâce toujours à la vérification des critères d'énergie et de contrainte. Une deuxième approche consistait à observer au plus près la microstructure du matériau ; on a considéré le problème d'une inclusion elliptique dans une matrice infinie. Par la méthode des variables complexes et la technique de la transformation conforme, on a analysé le champ de contrainte autour de l'inclusion et on a mis en évidence la présence d'une traction qui dépend fortement des paramètres choisis. Par la méthode des éléments finis étendus, on a calculé la variation de l'énergie potentielle mise en jeu par la création d'une fissure. Par une démarche semblable à l'approche précédente, à savoir la vérification des critères d'énergie et de contrainte, on a conclu à la possibilité d'amorçage d'une fissure. Mots clefs : mécanique linéaire de la rupture, critère mixte de Leguillon, énergie potentielle, taux de restitution d'énergie, méthode des éléments finis étendus, bicouche périodique, méthode des variables complexes
The initial problem treated in this thesis falls within the general framework of modeling deep tunnels. For this reason, the approach based on linear fracture mechanics was adopted. The study was based on the mixed criterion of Leguillon. Following This study, the mixed criterion was not insufficient but the way to approach the problem was. Where the transition to the consideration of the heterogeneity of the material component and the possibility of initing a crack under a compressive stress. A first approach was undertaken the study of periodic bilayer under the stress of vertical compression. The layer of the highest stiffness has appeared the seat of a transverse traction. Indeed the possibility to initiate a crack is quite likely always through the verification of the energy and the stress criteria. A second approach was to observe more closer the microstructure of the material; we have considered the problem of elliptic inclusion in an infinite matrix. By the method of complex variables and the technique of conformal mapping, we analyzed the stress field around the inclusion and were revealed the presence of a traction which depends strongly of the selected parameters. By the extended finite element method, we calculated the variation of the potential energy involved by creating a fracture. In a similar approach to the previous one, namely verification of the energy and the stress criteria, we concluded the possibility of initiating a crack. Keywords: linear fracture mechanics, mixed criterion of Leguillon, potential energy, energy release rate, extended finite element method, periodic bilayer, method of complex variables

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Saloustros, Savvas. "Tracking localized cracks in the computational analysis of masonry structures". Doctoral thesis, Universitat Politècnica de Catalunya, 2017. http://hdl.handle.net/10803/461714.

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Numerical methods aid significantly the engineering efforts towards the conservation of existing masonry structures and the design of new ones. Among them, macro-mechanical finite element methods based on the smeared crack approach are commonly preferred as an affordable choice for the analysis of large masonry structures. Nevertheless, they usu-ally result in a non-realistic representation of damage as smeared over large areas of the structure, which hampers the correct interpretation of the damage pattern. Additionally, a more critical pathology of this approach is the mesh-dependency, which influences nota-bly the safety and stability predictions. To overcome these limitations, this thesis proposes a novel computational tool based on the {enrichment} of the classical smeared crack approach with a local tracking algorithm. The objective of this localized damage model is the realistic and efficient non-linear anal-ysis of masonry structures with an enhanced representation of cracking. The non-linear behaviour of masonry is simulated through the adoption of a continuum damage mechanics model with two damage indices, allowing the differentiation between the tensile and compressive mechanical responses of masonry. In this context, a novel explicit formulation for the evolution of irreversible strains is proposed and implemented. Two new expressions are derived for the regularization of the tensile and compressive softening responses according to the crack-band approach, ensuring the mesh-size objec-tivity of the damage model. The simulation of the structural behaviour of masonry structures under versatile loading and boundary conditions necessitates some developments in the context of local tracking algorithms. To this end, this thesis presents the enhancement of local tracking algorithms with novel procedures that make possible the simulation of multiple, arbitrary and inter-secting cracking under monotonic and cyclic loading. Additionally, the effect of different crack propagation criteria is investigated and the selection among more than one potential failure planes is tackled. The proposed localized damage model is validated through the simulation of a series of structural examples. These vary from small-scale tests on concrete specimens with few dominant cracks, to medium and large-scale masonry structures with multiple tensile, shear and flexural cracking. The analyses are compared with analytical, experimental and numerical results obtained with alternative methods available in the literature. Overall, the localized damage model developed in this thesis largely improves the mesh-independency of the classical smeared crack approach and reproduces crack patterns and collapse mech-anisms in an efficient and realistic way.
Los métodos numéricos son decisivos en la ingeniería para la conservación de estructuras de mampostería existentes y el diseño de estructuras nuevas. Entre ellos, los métodos macro-mecánicos de elementos finitos, basados en el concepto de fisuras distribuidas, son habitualmente los preferidos como opción asequible para el análisis de grandes estructuras de mampostería. Sin embargo, suelen resultar en a una representación poco realista del daño, distribuido en grandes áreas de la estructura, lo que impide la correcta interpretación del patrón de daño. Además, esta metodología presenta una patología más crítica, la dependencia de la malla, que influye notablemente en las predicciones de seguridad y estabilidad. Para superar estas limitaciones, esta tesis propone una nueva herramienta numérica basada en el enriquecimiento del clásico enfoque de fisuras distribuidas con un algoritmo de trazado local. El objetivo de este modelo de daño localizado es el análisis no-lineal de las estructuras de mampostería de manera realista y eficiente con una representación mejora-da de fisuras. El comportamiento no lineal de la mampostería se simula a través de la adopción de un modelo de mecánica de daño continuo con dos índices de daño, permitiendo la diferenciación entre las respuestas mecánicas de tensión y compresión de la mampostería. En este contexto, se propone e implementa una nueva formulación explícita para la evolución de deformaciones irreversibles. Se derivan dos nuevas expresiones para la regularización del ablandamiento de tracción y compresión según el ancho de banda de la fisura, garantizan-do la objetividad del modelo de daño al respecto del tamaño de la malla. La simulación del comportamiento estructural de las estructuras de mampostería en condiciones de carga y contorno generales precisa de algunos desarrollos en el contexto de los algoritmos locales de trazado. Con este objetivo, se presenta la mejora de los algoritmos locales de trazado con nuevos procedimientos que posibilitan la simulación de fisuración múltiple, arbitraria e secante bajo cargas monótonas y cíclicas. Además, se investiga el efecto de diferentes criterios de propagación de fisuras y se aborda la selección entre más de un plano de falla posible. El modelo de daño localizado propuesto se valida mediante la simulación de una serie de ejemplos estructurales. Éstos van desde pruebas a pequeña escala en probetas de hormigón, con pocas fisuras dominantes, hasta estructuras de mampostería de mediana y gran escala con fisuración múltiple de tracción, de cortante y de flexión. Los análisis se comparan con los resultados analíticos, experimentales y numéricos obtenidos con métodos alternativos disponibles en la literatura. El modelo de daño localizado mejora en gran medida la independencia de la malla del clásico método de fisuras distribuidas y reproduce patrones de daño y mecanismos de colapso de una manera eficiente y realista

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Gómez, Martínez Fernando. "FAST simplified vulnerability approach for seismic assessment of infilled RC MRF buildings and its application to the 2011 Lorca (Spain) earthquake". Doctoral thesis, Universitat Politècnica de València, 2015. http://hdl.handle.net/10251/54780.

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[EN] A simplified analytical method ("FAST") for the estimation of large-scale vulnerability of Reinforced Concrete (RC) Moment Resisting Frames with masonry infills is proposed and subsequently tested by using real damage scenario caused by the 2011 Lorca earthquake as a benchmark. FAST is a spectral-based approach that allows predicting the average non-structural Damage State expected for each class of building (defined by number of storeys, age of construction, infills ratio in plan and location) for a given demand level. It accounts for non-uniformity of infills in elevation, i.e. a reduction of infills ratio of the ground floor. FAST is based on: (i) the definition of approximated capacity curves of the infilled building, assuming that the RC frame is designed according to the corresponding seismic code; and on (ii) the assumption of "a priori" deformed shapes in accordance with the attainment of each non-structural damage state at 1st storey, estimated through experimental and numerical correlations. Two versions of FAST are proposed: a "simplified" approach aimed at the evaluation of uniformly infilled frames; and a "generalised" version which can account for any intermediate situation between uniformly infilled frames and pilotis frames (i.e. without infills at 1st storey). Also, some extensions of the method are highlighted. Aimed at testing FAST, the real damage scenario after the earthquake of Lorca (2011) is used as a benchmark, despite its impulsivity and directivity. In order to define the specific input parameters for the case study, information regarding ground motion, post-earthquake damage scenario and also building design practice must be collected. Hence, a detailed review of historical Spanish seismic codes and a critical analysis of current Spanish seismic code NCSE-02 in comparison with current reference performance-based codes such as Eurocode 8 are provided. Special emphasis is placed on provisions which can prevent a proper capacity design and that, in turn, can cause brittle failures or favour the interaction with infills. Also, the prescription of lower behaviour factor for wide-beam frames with respect to deep-beam frames -which is not present in most codes¿ is discussed; outcomes of several case studies suggest that such prescription is obsolete. Finally, FAST is applied to Lorca earthquake and predicted damage scenarios are obtained, considering different assumptions for input values. Results show proper agreement between predicted and real damages. Structural collapses were rarely observed, even though the PGA was three times higher than the typical acceleration of design, so FAST proves that masonry infills provided additional strength to RC frames.
[ES] Se propone un método analítico simplificado ("FAST") para la estimación de la vulnerabilidad a gran escala de edificios porticados de hormigón armado con tabiquería de fábrica, posteriormente testeado mediante la adopción del escenario de daño real correspondiente al terremoto de Lorca de 2011 como patrón de comparación. FAST es un procedimiento espectral que permite predecir el nivel de daño no estructural medio esperado para cada clase de edificio (definido por su número de plantas, año de construcción, densidad de tabiquería en planta y localización geográfica), considerando un nivel de demanda dado. El método tiene en cuenta la irregularidad de la tabiquería en alzado, es decir, la posible reducción relativa de tabiquería en planta baja. FAST se basa en: (i) la definición de curvas de capacidad aproximadas para los edificios tabicados, asumiendo que la estructura de HA se ha proyectado según la norma sísmica correspondiente en cada caso; y en (ii) la asunción de deformadas "apriorísticas" coherentes con cada grado de daño (suponiendo que éste se alcanza siempre en planta baja), estimadas a través de correlaciones experimentales y numéricas. Se proponen dos versiones de FAST: una "simplificada" para la evaluación de edificios uniformemente tabicados en altura, y otra "generalizada", que es capaz de tener en cuenta cualquier situación intermedia entre el prototipo uniformemente tabicado y el de planta baja diáfana. Además, se proponen ciertas extensiones al método. A fin de validar FAST, se elige el escenario de daño real correspondiente al terremoto de Lorca (2011) como patrón de comparación, a pesar de su impulsividad y directividad. Para definir los parámetros de input correspondientes al caso de estudio, es necesario recopilar previamente la información concerniente a la señal sísmica, el escenario de daño y las características del parque construido. Por tanto, se lleva a cabo una revisión exhaustiva de las normas sísmicas históricas en España y un análisis crítico de la norma sísmica española actual NCSE-02 en comparación con otras normas actuales de referencia basadas en el desempeño, como el Eurocódigo 8, haciendo énfasis en las provisiones que no garantizan el diseño por capacidad y que por tanto pueden provocar mecanismos frágiles o favorecer la excesiva influencia de la tabiquería. Además, se discute sobre la restricción del coeficiente de ductilidad en estructuras de vigas planas, cuestión que no se refleja en otras normas. Los resultados obtenidos mediante análisis de casos de estudio muestran que dicha prescripción resulta obsoleta para normas actuales. Finalmente, FAST se aplica al caso del terremoto de Lorca, obteniéndose predicciones de daño medio para diferentes asunciones. Los resultados muestran una coincidencia aceptable entre la predicción y los daños reales. FAST confirma que la causa principal de la práctica ausencia de colapsos (ante un terremoto con PGA triple que la típica de proyecto) hay que buscarla en la contribución estructural de la tabiquería de fábrica.
[CAT] Es proposa un mètode analític simplificat ("FAST") per a l'estimació de la vulnerabilitat a gran escala d'edificis porticats de formigó armat amb envans de fàbrica. Posteriorment, el mètode ha estat testejat mitjançant l'adopció de l'escenari de dany real corresponent al terratrèmol de Lorca de 2011 com a patró de comparació. FAST és un procediment espectral que permet predir el nivell de dany no estructural mitjà esperat per a cada classe d'edifici (definit pel seu nombre de plantes, any de construcció, densitat d'envans en planta i localització geogràfica), considerant un determinat nivell de demanda. El mètode té en compte la irregularitat de la distribució de envans al llarg de les diferents plantes del edifici. Es a dir, es pot tenir en compte que, freqüentment, hi ha una menor quantitat de d'envans a la planta baixa. FAST es fonamenta en: (i) la definició de corbes de capacitat aproximades que tenen en compte no sols la estructura del edifici sinó també els envans i assumint que l'estructura de HA s'ha projectat segons la norma sísmica corresponent en cada cas; (ii) l'assumpció de deformades "apriorístiques" coherents amb cada grau de dany (suposant que aquest es dona sempre a la planta baixa) que han estat estimades a través de correlacions experimentals i numèriques. Es proposen dues versions de FAST: una "simplificada" per a l'avaluació d'edificis amb envans uniformement repartits per totes les plantes, i una altra "generalitzada", que és capaç de tenir en compte qualsevol situació intermèdia entre el prototip uniformement paredat i el de planta baixa diàfana. A més, es proposen certes extensions al mètode. Per tal de validar FAST, es tria l'escenari de dany real corresponent al terratrèmol de Lorca (2011) com a patró de comparació, malgrat la seva impulsivitat i directivitat. Per definir els paràmetres de entrada corresponents al cas d'estudi, cal recopilar prèviament la informació concernent al senyal sísmica, l'escenari de dany i les característiques del parc construït. Per tant, es porta a terme una revisió exhaustiva de les normes sísmiques històriques a Espanya i una anàlisi crítica de la norma sísmica espanyola actual (NCSE-02) comparant-la amb altres normes actuals de referència, com l'Eurocodi 8, fonamentat en el concepte d'acompliment. També es fa èmfasi a les provisions que no garanteixen el disseny per capacitat i que, per tant, poden provocar mecanismes de col·lapse fràgils o afavorir la interacció de la estructura amb els envans. A més, es discuteix sobre la restricció del coeficient de ductilitat de les estructures de bigues planes ja que es una qüestió que no aborden la majoria de les normes. Els resultats obtinguts mitjançant l'anàlisi de casos d'estudi mostren que aquesta restricció resulta obsoleta a les normes actuals. Finalment, FAST s'aplica al cas del terratrèmol de Lorca, obtenint prediccions de dany mitjà per a diferents combinacions del paràmetres de entrada. Els resultats mostren una coincidència acceptable entre la predicció i els danys reals. FAST confirma que la causa principal de la pràctica absència de col·lapses (davant un terratrèmol amb PGA triple que la típica de projecte) cal buscar-la en la contribució estructural dels envans.
Gómez Martínez, F. (2015). FAST simplified vulnerability approach for seismic assessment of infilled RC MRF buildings and its application to the 2011 Lorca (Spain) earthquake [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/54780
TESIS
Premiado

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Wrzesniak, Aleksandra. "Modélisation double-échelle de la rupture des roches : influence du frottement sur les micro-fissures". Thesis, Grenoble, 2012. http://www.theses.fr/2012GRENI104/document.

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Propagation des fissures microscopiques, est représentée par des variables d’endommagement. L’évolution de la variable d’endommagement est généralement formulée sur la base d’observations expérimentales. De nombreux modèles phénoménologiques d’endommagement ont été proposés dans la littérature. L’objet de cette thèse est de développer une nouvelle procédure pour obtenir des lois d’évolution macroscopique d’endommagement,dans lesquelles l’évolution de l’endommagement est entièrement déduite de l’analyse de la microstructure. Nous utilisons une homogénéisation basée sur des développements asymptotiques pour décrire le comportement global à partir de la description explicite d’un volume élémentaire microfissuré.Nous considérons d’une part un critère quasi-fragile (indépendant du temps) puis un critère sous-critique(dépendant du temps) pour décrire la propagation des microfissures. De plus, le frottement entre les lèvres des microfissures est pris en compte. Une analyse énergétique est proposée, conduisant à une loi d’évolution d’endommagement qui intègre une dégradation de la rigidité, un adoucissement du comportement du matériau, des effets de taille et d’unilatéralité, mettant en avant un comportement différent à la rupture en contact avec et sans frottement. L’information sur les micro-fissures est contenue dans les coefficients homogénéisés et dans la loi d’évolution de l’endommagement. Les coefficients homogénéisés décrivent la réponse globale en présence de micro-fissures (éventuellement statiques), tels qu’ils sont calculées avec la(quasi-) solution microscopique statique. La loi d’endommagement contient l’information sur l’évolution des micro-fissures, résultant de l’équilibre énergétique dans le temps pendant la propagation microscopique.La loi homogénéisée est formulée en incrément de contrainte. Les coefficients homogénéisés sont calculées numériquement pour des longueurs de fissures et des orientations différentes. Cela permet la construction complète des lois macroscopiques. Une première analyse concerne le comportement local macroscopique, pour des trajets de chargement complexes, afin de comprendre le comportement prédit par le modèle à deux échelles et l’influence des paramètres micro structuraux, comme par exemple le coefficient de frottement. Ensuite, la mise en œuvre en éléments finis des équations macroscopiques est effectuée et des simulations pour différents essais de compression sont réalisées. Les résultats des simulations numériques sont comparés avec les résultats expérimentaux obtenus en utilisant un nouvel appareil triaxial récemment mis au point au Laboratoire 3SR à Grenoble (France)
In continuum damage models, the degradation of the elastic moduli, as the results of microscopic crackgrowth, is represented through damage variables. The evolution of damage variable is generally postulatedbased on the results of the experimental observations. Many such phenomenological damage modelshave been proposed in the literature. The purpose of this contribution is to develop a new procedurein order to obtain macroscopic damage evolution laws, in which the damage evolution is completelydeduced from micro-structural analysis. We use homogenization based on two-scale asymptotic developmentsto describe the overall behaviour starting from explicit description of elementary volumes withmicro-cracks. We consider quasi-brittle (time independent) and sub-critical (time dependent) criteria formicro-cracks propagation. Additionally, frictional contact is assumed on the crack faces. An appropriatemicro-mechanical energy analysis is proposed, leading to a damage evolution law that incorporates stiffnessdegradation, material softening, size effect, and unilaterality, different fracture behaviour in contactwithout and with friction. The information about micro-cracks is contained in the homogenized coefficientsand in the damage evolution law. The homogenized coefficients describe the overall response inthe presence of (possibly static) micro-cracks, as they are computed with the (quasi-) static microscopicsolution. The damage law contains the information about the evolution of micro-cracks, as a result ofthe energy balance in time during the microscopic propagation. The homogenized law is obtained in therate form. Effective coefficients are numerically computed for different crack lengths and orientations.This allows for the complete construction of the macroscopic laws. A first analysis concerns the localmacroscopic behaviour, for complex loading paths, in order to understand the behaviour predicted bythe two-scale model and the influence of micro structural parameters, like for example friction coefficient.Next, the FEM implementation of the macroscopic equations is performed and simulations for variouscompression tests are conducted. The results of the numerical simulations are compared with the experimentalresults obtained using a new true-triaxial apparatus recently developed at the Laboratory 3SRin Grenoble (France)

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Atiezo, Megbeme Komla. "Modélisation multi-échelle de l'endommagement dynamique des matériaux fragiles sous chargements complexes". Electronic Thesis or Diss., Université de Lorraine, 2019. http://www.theses.fr/2019LORR0212.

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Dans cette thèse, la modélisation de l’endommagement et de la rupture dynamique des matériaux quasi-fragiles est proposée en utilisant une approche double-échelle basée sur la méthode d’homogénéisation asymptotique. Des lois d’endommagement dynamique sont établies et des simulations numériques du comportement associé sont effectuées dans les cas de sollicitations correspondants aux trois modes classiques de la Mécanique de la Rupture. Le premier modèle d’endommagement dynamique est proposé pour le chargement en mode de cisaillement antiplan (mode III). La loi d’endommagement est déduite à partir d’un critère énergétique de type Griffith décrivant la propagation des microfissures, en utilisant la méthode de changement d’échelle basée sur des développements asymptotiques. Une étude locale de la réponse macroscopique prédite par ce nouveau modèle est faite pour mettre en évidence l’influence des paramètres, comme la taille de la microstructure et la vitesse de chargement, sur l’évolution de l’endommagement. Les résultats des simulations macroscopiques de rupture dynamique et les instabilités de branchement associées sont présentés et comparés à ceux des observations expérimentales. Le modèle est implémenté dans un code Éléments finis/Différences finies en utilisant le logiciel de calcul Matlab. Des simulations numériques de rupture rapide en mode d’ouverture (mode I) en utilisant une loi d’endommagement dynamique sont ensuite présentées. Le modèle utilisé pour ces simulations, est déduit à partir d’un critère microscopique de type Griffith en mode I en utilisant la méthode d’homogénéisation asymptotique. La loi d’endommagement obtenue est sensible à la vitesse de chargement qui influence le mode de rupture macroscopique. Des simulations numériques sont effectuées afin d’identifier les prédictions du modèle et les résultats obtenus sont comparés aux résultats expérimentaux. Différents tests, comme ceux de traction directe et de l’échantillon en forme de L pour les bétons, les essais d’impact sur des échantillons CCS en PMMA et le test d’impact de Kalthoff pour des roches calcaires sont reproduits numériquement. Ces simulations montrent que la vitesse de chargement détermine essentiellement la trajectoire de rupture macroscopique et la formation des branches associées, en accord avec les résultats expérimentaux. La loi a été implémentée dans le code d’éléments finis Abaqus/Explicit via une sub-routine VUMAT. Un troisième modèle d’endommagement est obtenu pour le mode de cisaillement plan (Mode II) par une démarche de modélisation double-échelle similaire à celle utilisée dans les deux premiers modèles, en tenant compte du contact unilatéral avec frottement sur les lèvres des microfissures. Une étude locale concernant l’effet du chargement de compression et du coefficient du frottement sur les fissures est faite. L’influence des paramètres comme la taille de la microstructure et la vitesse de déformation sur l’évolution de l’endommagement sont étudiés. Ces études sont complétées par des simulations des essais de rupture/frottement sur des échantillons en PMMA en utilisant le logiciel Abaqus/Explicit
In this thesis, the modeling of dynamic damage and failure of quasi-materials is addressed using a two-scale approach based on the asymptotic homogenization method. Dynamic damage laws are obtained and numerical simulations of the associated behavior are performed for loadings corresponding to the classical three modes of Fracture Mechanics. The first dynamic model of damage is proposed for the anti-plane shear loading case (Mode III). The damage evolution law is deduced from the Griffith’s energy criterion governing the dynamic propagation of microcracks, by using the homogenization method based on asymptotic expansions. A study of the local macroscopic response predicted by the new model is conducted to highlight the influence of parameters, like the size of the microstructure and the loading rate, on the evolution of damage. Results of macroscopic simulations of dynamic failure and the associated branching instabilities are presented and compared with those reported by experimental observations. The model is implemented in a Finite-Elements/Finite-Differences code using the Matlab software environment. Numerical simulations of rapid failure in opening mode (Mode I) are using a dynamic damage law are presented subsequently. The model is deduced from a microscopic Griffith type criterion describing the dynamic mode I propagation of microcracks, using the asymptotic homogenization approach. The resulting damage law is sensitive to the rate of loading that determines the macroscopic failure mode. Numerical simulations are performed in order to identify the model predictions and the obtained numerical results are compared with the experimental ones. Different tests, like the compact tension and L-shape specimen tests for concrete, the compact compression test for the PMMA brittle polymer and the Kalthoff impact test for limestone rocks, are considered in the numerical simulations. These simulations show that the loading rate essentially determines the macroscopic crack trajectory and the associated branching patterns, in agreement with the experimental results. The law has been implemented in a finite element code Abaqus/Explicit via a VUMAT subroutine. A third model of damage is obtained for the in-plane shear mode (Mode II) through a similar double-scale approach by considering unilateral contact with friction conditions on the microcracks lips. A local study concerning the effects of normal compression and of the friction coefficient is carried out. The influence of the size of the microstructure and the rate of loading on damage evolution is analyzed at the local level. These studies are completed by structural failure simulations of PMMA specimens using the Abaqus/Explicit finite element software

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Campagnolo, Alberto. "Local approaches applied to fracture and fatigue problems". Doctoral thesis, Università degli studi di Padova, 2016. http://hdl.handle.net/11577/3424240.

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Intentionally designed or accidentally caused, notches, cracks or defects are inevitably present in engineering components and can induce high stress gradients when a far field loading is applied. Then, structural strength assessments are often based on the local stress and strain state in the close neighbourhood of the stress raisers. The present PhD thesis is divided into six Chapters corresponding to different research topics, all related to new applications of important and widely employed local approaches to notched or cracked structural components. In the first Chapter, the adopted local approaches, namely the Notch Stress Intensity Factor-based approach (NSIF), the averaged Strain Energy Density (SED) criterion and the Peak Stress Method (PSM), are briefly introduced and described along with their theoretical frameworks. The second Chapter deals with brittle fracture under mixed mode static loading. A wide experimental campaign has been carried out on PMMA as well as on graphite cracked and notched specimens subjected to mixed mode I+II and I+III loading. Then, all experimental results have been reanalysed by means of the SED approach. The third Chapter deals with multiaxial fatigue loadings. First, the fatigue strength of severely notched titanium grade 5 alloy, Ti-6Al-4V, has been investigated. Then, the SED criterion has been applied for the first time to an industrial case study, that is the multiaxial fatigue strength assessment of steel welded rollers produced by Rulmeca S.p.a. Finally, some remarks about the phase angle effect on sharp V-notched components under multiaxial fatigue have been drawn on the basis of a proposed analytical frame. The fourth Chapter addresses the numerical study of 3D effects in notched and cracked components. Initially, the attention has been focused on coupled modes and on the effect of different boundary conditions in 3D cracked discs and plates subjected to nominal mode III or mode II loading. Then, the presence of three-dimensional effects has been investigated both theoretically and numerically in blunt notched components under cyclic plasticity conditions. The fifth Chapter, instead, is related to the comparison between different fracture criteria. The SED approach and that based on the Finite Fracture Mechanics (FFM) have been compared considering sharp V-notches under pure mode I or mode II loading. Finally the sixth Chapter address the link between the SED approach and the Peak Stress Method (PSM). Cracks under in-plane mixed mode I+II and out-of-plane mixed mode I+III loading have been investigated. A method to rapidly evaluate the averaged SED based on the peak stresses at the crack tip has been proposed. On the basis of the derived link, some practical applications related to the fatigue strength assessment of aluminium and steel butt welded joints and of tube-to-flange steel welded joints have been carried out.
Variazioni geometriche, come intagli, cricche o difetti in generale, sono comunemente presenti nella maggior parte dei componenti meccanici e possono indurre elevati gradienti di tensione per effetto dei carichi esterni. La valutazione della resistenza strutturale dei componenti meccanici è perciò generalmente basata sullo stato di tensione e deformazione locale, nelle adiacenze di tali variazioni geometriche. La presente tesi di dottorato è divisa in sei Capitoli corrispondenti a diversi argomenti di ricerca, tutti relativi a nuove applicazioni di importanti e ampiamente diffusi approcci locali a componenti strutturali intagliati o criccati. Nel primo Capitolo, sono brevemente introdotti e descritti gli approcci locali adottati, cioè l’approccio basato sul Notch Stress Intensity Factor (NSIF), il criterio basato sulla densità di energia di deformazione mediata (SED) e il Peak Stress Method (PSM), assieme alle loro basi teorico-analitiche. Il secondo Capitolo si occupa della frattura fragile sotto carichi statici di modo misto. Una campagna sperimentale estesa è stata eseguita su provini intagliati e criccati in PMMA ed in grafite soggetti a carichi di modo misto I+II e I+III. In seguito, tutti i dati sperimentali sono stati rianalizzati per mezzo dell’approccio SED. Il terzo Capitolo tratta il tema della fatica multiassiale. Inizialmente, è stata investigata la resistenza a fatica di una lega di titanio grado 5, Ti-6Al-4V, severamente intagliata. In seguito, il criterio SED è stato applicato per la prima volta ad un caso studio di interesse industriale: la valutazione della resistenza a fatica multiassiale di rulli saldati in acciaio, prodotti da Rulmeca S.p.a. e caratterizzati da cedimenti alla radice del cordone di saldatura. Infine, prendendo in esame componenti indeboliti da intagli a V acuti soggetti a carichi di fatica multiassiale, sono state tratte alcune osservazioni sull’effetto dell’angolo di fase sulla base di un nuovo approccio analitico. Il quarto Capitolo tratta lo studio numerico e teorico degli effetti 3D in componenti intagliati e criccati. Inizialmente, l’attenzione è stata focalizzata sui modi accoppiati e sull’effetto di diverse condizioni al contorno in dischi e piastre criccate, tridimensionali e soggette ad un carico nominale di modo III o modo II. Infine la presenza di effetti 3D è stata investigata sia dal punto di vista teorico che numerico in componenti indeboliti da intagli blandi e in condizioni di plasticità ciclica. Il quinto Capitolo, invece, è relativo al confronto tra diversi criteri di cedimento. Sono stati confrontati l’approccio SED e quello basato sulla teoria della Finite Fracture Mechanics (FFM), considerando intagli a V acuti soggetti a puro modo I o puro modo II. Infine, nel sesto Capitolo è stato investigato un legame tra il criterio SED ed il Peak Stress Method (PSM). Sono stati presi in esame componenti strutturali criccati soggetti a carichi di modo misto nel piano I+II e fuori piano I+III. È stato proposto un metodo per calcolare rapidamente il SED a partire dalle tensioni di picco valutate all’apice di cricca. Il legame ottenuto tra PSM e SED è stato poi impiegato nella stima della resistenza a fatica di giunti saldati testa a testa in acciaio ed alluminio e di giunti saldati tubo-su-flangia in acciaio sottoposti a carichi torsionali.

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40

Goda, Ibrahim. "Micromechanical models of network materials presenting internal length scales : applications to trabecular bone under stable and evolutive conditions". Thesis, Université de Lorraine, 2015. http://www.theses.fr/2015LORR0055/document.

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Des méthodes micromécaniques spécifiques ont été développées pour la détermination du comportement effectif de matériaux cellulaires dotés d’une architecture discrète à l’échelle microscopique. La méthode d’homogénéisation discrète a été appliquée à des structures tissées monocouches ainsi qu’à l’os trabéculaire. La topologie discrète initiale de ces milieux est remplacée à l’échelle mésoscopique par un milieu effectif anisotrope micropolaire, qui rend compte des effets d’échelles observés. Ces méthodes d’homogénéisation permettent d’accéder à des propriétés classiques et non classiques dont la mesure expérimentale est souvent difficile. Des modèles 3D ont été développé afin de décrire la rupture fragile et ductile de l’os trabéculaire, incorporant des effets de taille des surfaces d’écoulement plastique. Nous avons construit par des analyses éléments finis de la microstructure de l’os trabéculaire un milieu de substitution 3D homogène, orthotrope de type couple de contraintes, sur la base d’une équivalence en énergie. Les tissus osseux ont la capacité d’adapter leur densité locale et leur taille et forme aux stimuli mécaniques. Nous avons développé des modèles de remodelage interne et externe dans le cadre de la thermodynamique des processus irréversibles, aux échelles cellulaire et macroscopique. Finalement, le remodelage interne anisotrope a été couplé à l’endommagement de fatigue, dans le cadre de la théorie continue de l’endommagement
A methodology based on micromechanics has been developed to determine the effective behavior of network materials endowed with a discrete architecture at the microscopic level. It relies on the discrete homogenization method, which has been applied to textile monolayers and trabecular bones. The initially discrete topology of the considered network materials results after homogenization at the mesoscopic level in anisotropic micropolar effective continuum, which proves able to capture the observed internal scale effects. Such micromechanical methods are useful to remedy the difficulty to measure the effective mechanical properties at the intermediate mesoscopic level scale. The bending and torsion responses of vertebral trabecular bone beam specimens are formulated in both static and dynamic situations, based on the Cosserat theory. 3D models have been developed for describing the multiaxial yield and brittle fracture behavior of trabecular bone, including the analysis of size-dependent non-classical plastic yield. We have constructed by FE analyses a homogeneous, orthotropic couple-stress continuum model as a substitute of the 3D periodic heterogeneous cellular solid model of vertebral trabecular bone, based on the equivalent strain energy approach. Bone tissues are able to adapt their local density and load bearing capacities as well as their size and shape to mechanical stimuli. We have developed models for combined internal and external bone remodeling in the framework of the thermodynamics of irreversible processes, at both the cellular and macroscopic levels. We lastly combined anisotropic internal remodeling with fatigue continuum damage

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41

Caisso, Camille. "Caractérisation et modélisation de la rupture dans le domaine de la transition ductile-fragile de matériaux tubulaires utilisés dans la fabrication de générateurs de gaz pour airbags". Thesis, Brest, École nationale supérieure de techniques avancées Bretagne, 2021. http://www.theses.fr/2021ENTA0014.

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Ce travail porte sur la caractérisation des mécanismes de rupture, ductile et fragile, de matériaux tubulaires utilisés pour la fabrication de générateurs de gaz pour airbag. Lors d’un accident, le coussin d’un airbag est gonflé en quelques millisecondes par un générateur de gaz. En cas de défaillance de ce dernier, un mode de rupture ductile doit être assuré jusqu’à des températures de -60°C. Un essai Charpy sur éprouvette anneau a été proposé pour quantifier le risque de rupture fragile des matériaux des générateurs de gaz. Cependant, cette modification de l’essai Charpy soulève deux problématiques : dans quelle mesure cet essai permet de caractériser la transition ductile-fragile et comment déduire le risque de rupture fragile d’un générateur de gaz en fonctionnement à partir des résultats des essais Charpy ? Pour répondre à ces questions, une approche combinant essais et simulations numériques a été mise en place. Différents essais ont été développés afin de caractériser le comportement et l’endommagement ductile du matériau de l’étude. Une approche locale de la rupture ductile a été adoptée pour la modélisation. Ensuite, une campagne d’essais Charpy sur anneau effectuée pour des températures comprises entre -160°C et 23°C a permis de caractériser la transition ductile-fragile. Une étude numérique de cet essai a été menée puis couplées aux essais Charpy, cela a permis de mettre en place une modélisation de la rupture fragile. Le risque de rupture fragile des éléments structuraux de générateurs de gaz est enfin évalué. L’influence du procédé de fabrication sur le risque de rupture fragile a aussi été étudiée
This work deals with the characterization of the ductile and brittle failure mechanisms of tubular materials used for the manufacture of airbag gas generators. During a crash, airbag cushion is inflated in a few milliseconds by a gas generator. In case of gas generator failure, a ductile failure mode must be ensured up to temperatures of -60°C. A Charpy ring test has been proposed to quantify the risk of brittle failure of gas generator materials. However, this modification of the Charpy test raises two issues: to what extent does this test allow to characterize the ductile-fragile transition and how can the risk of brittle failure of an operating gas generator be deduced from the results of the Charpy test? To answer these questions, an approach combining experiments and numerical simulations was implemented. Various experimental configurations were developed in order to characterize material behavior and ductile damage. The ductile failure is modeled with a local approach. Then, ductile to brittle transition was characterized by a Charpy ring test campaign performed for temperatures between -160°C and 23°C. A numerical study of this test was carried out. Coupled with the Charpy tests, it allowed to set up a model of the brittle failure. The risk of gas generators brittle failure is finally evaluated. The influence of the manufacturing process on the risk of brittle failure was also studied

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42

Hu, Song-Ru, i 胡淞儒. "A study on the deformation and the damage of grinding brittle material". Thesis, 2010. http://ndltd.ncl.edu.tw/handle/14238408222345052302.

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43

Chen, Wei-Chih, i 陳韋志. "Application of Coupled Nondestructive Technique on Fluid-driven Damage of Quasi-brittle Rock". Thesis, 2014. http://ndltd.ncl.edu.tw/handle/77388966366810472543.

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博士
國立臺灣科技大學
營建工程系
102
Concerning the crucial geo-mechanics projects such as pressure tunnel, grouting injection for jointed rock, geological formation of dike, exploitation of natural resources, and underground nuclear waste repository etc., different types of fluid-driven fracture are of the world-wide interests. This study reviewed the theoretical fluid-driven fracture model under the microscopic view. By establishing novel setup consisted of fluid-driven fracturing apparatus with coupling nondestructive techniques of acoustic emission (AE) and speckle-shearing interferometry (SSI), experimental examination will be conducted to identify this conceptual model. In this research, a new loading instrument was designed to supply the fluid-driven motion, which can adjust the injection rate automatically according to the feedback from the extensometer. The stable crack propagation within material therefore was controlled and complete loading history can also be obtained. The specimens were made of cement mortar sand as a quasi-brittle rock. Some effects of factors such as grain size, permeability in solids; and viscosity in fluids were also investigated in this study. Test results in macroscopic view show that the stiffness, peak strength, and the post-peak behavior can be investigate from complete loading curve and the peak strength will increase with decreasing grain size and permeability and increasing viscosity. In microscopic view, the localization of AE events and first fringe occurrence was found ranging between 99% to 95% at post peak stage and 99% to 100% at pre peak stage, respectively. This phenomenon shows that the fracture development of fluid-driven in brittle material will occur rapidly when load pressure reached the peak value. Coupled analysis of the AE event distribution and measured out-of-plane displacement can be used to trace the position of crack tip and estimate the crack opening width. The obtained crack opening width from experiment was used to compare with the one from theoretical solution.

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44

Liu, Yi-Hung, i 劉益宏. "Effects of Viscosity on the MechanicalBehavior of Fluid-driven Damage for Quasi-brittle Solids". Thesis, 2015. http://ndltd.ncl.edu.tw/handle/88838808629938380551.

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碩士
國立臺灣科技大學
營建工程系
103
In the past, such as the tunnels, reservoirs; exploitation of natural resources in underground drilling and carbon sequestration,and underground nuclear waste repository etc. shall be concidering fluid-driven fracture causing of the derivative of the crack to be understood. This study tested the impermeably quasi-brittle rock, in order to expand the amount of test substance for feedback control signal of fluid-driven fracture, apparatus with coupling e non-destructive testing.acoustic emission (Acoustic Emission) and speckle shearing interferometry (SSI)

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45

Liou, Jyh-Chau, i 劉智超. "A Study on the Damage and Acoustic Emission Characteristics of Brittle Rocks under Loading". Thesis, 1994. http://ndltd.ncl.edu.tw/handle/49215312652539335085.

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碩士
國立交通大學
土木工程研究所
82
The microfractures in a brittle rock may expand when the material is subjected to loading. Microfracturing process in a material results in energy dissippation and acoustic emission. The gradual loss of integrity due to microfracturing is also the cause of material damage due to loading. This thesis investigate the damaging process and the acoustic emissionof brittle rocks subjected to various loading conditions. Uniaxial tests with acoustic emission measurement on brittle rocks quartzite and marble) were performed. Conclusions were drawn from as follows: (1) For the samples used in the study, the Kaiser effect is valid for stress level lower than 60% of the peak strength. (2) Acoustic emission tends to recover if the time interval between unloading and reloading is long and the loading high enough. (3) Duing stress-controlled uniaxial compression, axial strain rate gradually reduced to a constant. The lateral rate remains constant in the early stage of loading, but increases significantly when the hit rate of acoustic rises. (4) Back calculation of the stress-strain curves of unixail compression tests based on the Walsh''s damaging model fromacoustic emission measurement data was attempted. The and the measured results agree satisfactorily only in the pre-prak range of stress-strain curve.

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46

Akdag, Selahattin. "Experimental investigation of damage evolution during strain burst in brittle rocks for deep mines". Thesis, 2019. http://hdl.handle.net/2440/120726.

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The increasing demand for resources and depletion of near ground mineral resources caused deeper mining operations under high-stress and high-temperature rock mass conditions. As a results of this, strain burst, which is the sudden and violent release of stored strain energy during dynamic brittle failure of rocks, has become more prevalent and created considerable safety risks damaging underground infrastructures. This research focuses on the development of experimental methodologies to better understand the fundamental knowledge concerning the failure mechanism of strain burst and the influence of thermal damage, high confining pressure and various loading rate on the overall mechanical behaviour of highly brittle granitic rocks leading to strain burst. Strain burst is related to the elastic stored strain energy and how this stored energy is released during the unstable spontaneous failure. Therefore, it is significant to investigate the energy state during strain burst from the viewpoint of energy theory. In this sense, circumferential strain controlled quasi-static tests on Class II rocks over a wide range of confining pressures at different heat-treatment temperatures were conducted to capture the snap-back behaviour and calculate excess strain energy that is responsible for the spontaneous instability. A new energy calculation method associated with acoustic emission (AE) was developed to express the propensity of strain burst and investigate the post-peak energy distribution characteristics for brittle rocks under the coupling influence of confinement and temperature. In order to quantify the micro-crack density and reveal the micro-fracture characteristics of the brittle rocks exposed to various temperatures, scanning electron microscopy (SEM) analysis was also conducted. This is highly relevant to link the excess strain energy and the main failure mechanism triggering strain burst under high-temperature condition. The failure process of strain burst is the outcome of the unstable growth and coalescence of secondary micro-cracks. If the dissipative energy to grow pre-existing cracks and the secondary cracks is smaller than the elastic stored strain energy in rock masses, the residual strain energy will be released suddenly in the form of kinetic energy, resulting in ejecting high-velocity rock fragments. Therefore, understanding the crack initiation and propagation in rocks is of great concern for engineering stability and security. As an intrinsic property of rocks to resist crack initiation and propagation, the rock fracture toughness is the most significant material property in fracture mechanics. In this respect, the three-point bending method was applied using cracked chevron notched semi-circular bend (CCNSCB) granite specimens subjected to different temperatures under a wide range of loading rates in pure mode I. A suitable relation for the dimensionless stress intensity factor (𝑌∗) of SCB with chevron notch samples were presented based on the normalised crack length (𝛼) and half-distance between support rollers (𝑆/2). The minimum dimensionless stress intensity factor (𝑌𝑚𝑖𝑛∗) of CCNSCB specimens were determined using an analytical method, i.e., Bluhm’s slice synthesis method. In this study, the influence of thermal damage and loading rate on the quasi-static mode I fracture toughness and the energy-release rate using CCNSCB method was investigated. In the deep mining process, the rock mass is subjected to a dynamic disturbance caused by blasting, and mechanical drilling resulting in dynamic fractures in the forms of strain burst, slabbing, and spalling. The dynamic rock fracture parameters, including dynamic initiation fracture toughness and fracture energy which are an important manifestation of dynamic rock failure (strain burst) in deep underground engineering and they are of great practical significance to assess the dynamic fracture behaviour of deep rock masses. Since deep rock engineering operations in high temperature and high pressure environment is prone to strain burst, the influence of thermally induced damage on the dynamic failure parameters of granite specimens was investigated. The damage evolution of granitic rocks were studied over a wide range of loading rates to reveal the rate dependency of strain burst. Dynamic fracture toughness tests were carried out on granite under different temperatures and impact loadings using a Split Hopkinson Pressure Bar (SHPB) apparatus at Monash University. With dynamic force balance achieved in the dynamic tests, the stable-unstable transition of the crack propagation crack was observed and the dynamic initiation fracture toughness was calculated from the dynamic peak load. The thermal damage influence on strain burst characteristics of brittle rocks under true-triaxial loading-unloading conditions was investigated using the AE and kinetic energy analyses. A unique strain burst testing system enabling to simulate the creation of excavation at the State Key Laboratory for Geomechanics and Deep Underground Engineering in Beijing (China) was used to replicate strain burst condition. Time-domain and frequency-domain responses AE waves related to strain burst were studied, and the damage evolution was quantified by b-values, cumulative AE energy and events rates that can be used as warning signals to rock failure. The ejection velocities of the rock fragments from the free face of the granite specimens were used to calculate kinetic energies which can be used as an indicator for quantitatively evaluating the intensity of strain burst. Based on the energy evolution characteristics of brittle rocks under uniaxial and triaxial compression, true-triaxial loading-unloading and three-point bending, new strain burst proneness indexes and strain burst criterion were proposed. The effects of temperature, confinement and loading rate on strain burst proneness were discussed. This study aims to advance the understanding on underlying processes that govern the macro-behaviour of brittle rocks during strain burst and make use of this insight to further advance our current predictive capabilities of strain burst with references to large-scale underground mining. Using the developed experimental methodologies in this study, fractures around an excavation to reduce the amount of excess strain energy leading to strain burst can be determined and ultimately incipient strain burst in deep mines can be predicted avoiding potential hazards. Using the methodology for forecasting of strain burst in this research can be used for enhanced understanding of the design of rock support in strain burst-prone areas in deep mining activities. The findings of this study will facilitate achieving a better and comprehensive understanding of the damage process during strain burst in deep mines. This study underpins the development of better and more efficient prediction methods for strain burst which will lead to better planning guidelines and ultimately safer deep underground working conditions.
Thesis (Ph.D.) -- University of Adelaide, School of Civil, Environmental and Mining Engineering, 2019

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47

Wanne, Toivo. "Bonded-particle Modeling of Thermally Induced Damage in Rock". Thesis, 2009. http://hdl.handle.net/1807/17840.

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The objective of the research presented in this thesis is to validate the parallel-bonded modeling method in the context of coupled thermo-mechanical simulations. The simulation results were compared with analytical and experimental data, in the attempt to assess the usability of this particular modeling method. Previous studies of numerical approaches that related to the thermal fracturing of hard rock had used continuum-based models with constitutive relations. The simulations in the thesis were conducted using Particle Flow Code (PFC) which was chosen for the research because of its several benefits. The code has unique features such as spontaneous damage development without imposed conditions, and emergent properties such as material heterogeneity, and dynamic behavior giving possibility to monitor synthetic seismic events. The basic code has been available since 1995 and research using the code has produced hundreds of publications. The thermal option for the code is a recent addition and lacked verification, validation and applications. The thesis is the answer for that. In the course of the research work new particle clustering and grouping routines were developed and tested. Three modeling studies were conducted varying from laboratory to field scales. The 2D modeling study of the heated cylinder experiment yielded similar results both in fracture-behavioral and acoustic emission (AE) magnitude ranges when compared with the laboratory data. The 3D cubic numerical specimens, created with breakable particle clusters, were heated, and the induced damage was observed by P wave velocity measurements. The results showed trends comparable to the laboratory data: P wave velocity decreases with rising temperatures of up to 250°C and cluster-boundary cracking occurs, comparable to grain-boundary cracking in the heated rock samples. The large 2D tunnel models captured the phenomena observed in-situ displaying the difference in the damage to the roof and floor regions, respectively. This damage was due to the filling material confinement of about 100 kPa on the tunnel floor. In general, the results of the thermo-mechanical simulations were in accordance with the experimental data. The modeled temperature evolutions during the heating and cooling periods were also in accordance with the experimental and analytical data.

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48

Chen, Wei. "Damage characteristics of brittle rocks inside the pre-failure range: numerical simulation and lab testing". Doctoral thesis, 2015. https://tubaf.qucosa.de/id/qucosa%3A23015.

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The time-independent and -dependent damage characteristics of brittle rocks inside the pre-failure range have been investigated using numerical simulations and lab testing. Grain-based discrete element models have been developed to simulate both, time-independent and -dependent damage evolution leading to ultimate failure of sandstone and granite, respectively. The models take into account elastic grain and elasto-plastic contact deformation, inter- and intra-granular fracturing and lifetime prediction on the basis of subcritical crack growth. The time-independent mechanical behavior of Coconino sandstone and Lac du Bonnet granite during uniaxial compression tests, Brazilian splitting tests and fracture toughness tests was simulated. Triaxial compression tests and fracture toughness tests for Kirchberg II granite and fracture patterns tests for Eibenstock II granite were carried out in laboratory to perform time-independent damage and failure criterion analysis. The corresponding simulations showed reasonable damage phenomena compared with experimental results. Damage indices were deduced and were applied for different time-independent simulations. Based on calibrations of the time-independent damage simulations of selected brittle rocks, Charles equation and Hillig-Charles equation, which are generally used to describe subcritical crack growth, were implemented into the numerical code to simulate time-dependent damage. One-edged crack growth in Coconino sandstone specimen due to stress corrosion has been analyzed theoretically and numerically. Uniaxial compressive creep tests for Lac du Bonnet granite were simulated and time-dependent behavior in terms of the damage process during primary, secondary and tertiary creep until final failure characterized by macroscopic fracturing was discussed in detail. Subsequent to this, the time-dependent Mode-I crack growth tests and uniaxial compressive creep tests for Kirchberg II granite were carried out and the corresponding simulations were performed. Simulation results are in good agreement with experimental observations. In addition, damage indices and time-dependent fracture development were monitored and illustrated. The developed approach was applied to two potential practical applications: the damage analysis of a sandstone landscape arch and a tunnel. Finally, the results are summarized and recommendations for future work are proposed.:1 Introduction 2 State of the art 3 Time-independent damage analysis 4 Time-dependent damage analysis 5 Applications of numerical models . 6 Conclusions and outlook References

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49

Ghazvinian, Ehsan. "Modelling and Testing Strategies for Brittle Fracture Simulation in Crystalline Rock Samples". Thesis, 2010. http://hdl.handle.net/1974/6083.

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The failure of brittle rocks around deep underground excavations due to the high induced stress is controlled by the crack accumulation in the rock. The study shows that the damage initiation strength, CI, corresponds to the long-term strength, and the short-term strength of the brittle rocks in-situ is the crack interaction strength, CD. Therefore the damage thresholds that are being used for the calibration and validation of numerical models are important parameters in the design of underground structures. The accurate detection of the damage thresholds is important as they define the in-situ behaviour of the brittle rocks. The two most common methods of detecting damage thresholds are the Acoustic Emission method and the strain measurement method. Apparent discrepancy that exists between the accuracy of these methods was the author’s motivation for comparing these two methods on Stanstead and Smaland granites. The author introduced two new parameters based on the measured strains for improving the strain measurement method. Based on the comparisons, the author is of the opinion that the Acoustic Emission method is a more accurate method of detecting damage thresholds. Numerical models are an important tool in the design of underground structures. The numerical methods that are able to simulate fractures explicitly have the ability to predict the brittle failure, the density and the extension of the microcracks around the opening. Itasca’s Particle Flow Code (PFC) was used in this study due to its potential to simulate fractures explicitly. Calibration of PFC models to Unconfined Compressive Strength properties of the rock does not mean that the model will behave correctly under other confining stresses or in tension. The author has tried to solve this problem by different methods and developing new procedures. Improvements in the model behaviour have been achieved but more work is required. The definition, and detection and calibrated simulation of rock damage thresholds for calibration of numerical models is helpful for a successful design of underground excavations and long term, lower bound strength, a critical design parameter for deep geological repositories for the storage of nuclear wastes, for example.
Thesis (Master, Geological Sciences & Geological Engineering) -- Queen's University, 2010-09-23 13:59:28.795

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50

Mühlich, Uwe. "Generalised continuum approach for modelling quasi-brittle failure". Doctoral thesis, 2013. https://tubaf.qucosa.de/id/qucosa%3A22912.

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A proper description of quasi-brittle failure within the frame of continuum Mechanics can only be achieved by models based on so-called generalised continua. This thesis focuses on a strain gradient generalised continuum and provides a specific methodology to derive corresponding models which account for the essential features of quasi-brittle failure. This methodology is discussed by means of four peer-reviewed journal articles. Furthermore, an extensive overview of the state of the art in the field of generalised continua is given at the beginning of the thesis. This overview discusses phenomenological extensions of standard Continuum Mechanics towards generalised continua together with corresponding homogenisation strategies for materials with periodic or random microstructure.:1 Introduction 7 2 Generalised Continua - a journey 9 2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.2 Principal classes of generalised continua . . . . . . . . . . . . . . . . . 10 2.2.1 Polar field theories and their relatives . . . . . . . . . . . . . . 10 2.2.2 Non-local continua . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3 Generalised continua by explicit homogenisation . . . . . . . . . . . . 15 2.3.1 Random micro-structures . . . . . . . . . . . . . . . . . . . . . 15 2.3.2 Periodic micro-structures . . . . . . . . . . . . . . . . . . . . . 18 2.3.3 Generalised homogenisation based on polynomials . . . . . . 20 3 Modelling of quasi-brittle failure 25 3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 3.2 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 3.3 Discussion of main results . . . . . . . . . . . . . . . . . . . . . . . . . 26 3.4 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Bibliography 29 4 Collection of articles reflecting the author’s contribution 35
Eine geeignete, kontinuumsmechanische Beschreibung quasi-spröden Versagens ist nur unter Verwendung verallgemeinerter Kontinuumstheorien möglich. In dieser Habilitationsschrift stehen sogenannte Gradientenkontinua im Vordergrund. Für diese wird eine Methodik vorgeschlagen, welche die Herleitung von Modellen erlaubt, die in der Lage sind, quasi-sprödes Versagen adäquat abzubilden. Diese Methodik wird anhand von vier Publikationen dargestellt und diskutiert. Ein umfangreicher Überblick über den Stand der Forschung auf dem Gebiet der veralgemeinerten Kontinuumstheorien wird am Anfang der Habilitationschrift gegeben. Dabei werden neben phänomenologischen Ansätzen zur Ableitung verallgemeinerter Kontinuumstheorien auch die entsprechenden Homogenisierungskonzepte dargestellt. Letztere werden für Materialien mit periodischer Mikrostruktur und für Materialien mit zufälliger Mikrostruktur diskutiert.:1 Introduction 7 2 Generalised Continua - a journey 9 2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.2 Principal classes of generalised continua . . . . . . . . . . . . . . . . . 10 2.2.1 Polar field theories and their relatives . . . . . . . . . . . . . . 10 2.2.2 Non-local continua . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3 Generalised continua by explicit homogenisation . . . . . . . . . . . . 15 2.3.1 Random micro-structures . . . . . . . . . . . . . . . . . . . . . 15 2.3.2 Periodic micro-structures . . . . . . . . . . . . . . . . . . . . . 18 2.3.3 Generalised homogenisation based on polynomials . . . . . . 20 3 Modelling of quasi-brittle failure 25 3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 3.2 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 3.3 Discussion of main results . . . . . . . . . . . . . . . . . . . . . . . . . 26 3.4 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Bibliography 29 4 Collection of articles reflecting the author’s contribution 35

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