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Uma  formulação  alternativa do método dos elementos de contorno aplicada à análise da propagação de fissuras em materiais quase frágeis  ; An alternative formulation of the boundary element method applied to crack propagation analysis in quasi-brittle materials

Oliveira, Hugo Luiz
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 25/03/2013 PT
Relevância na Pesquisa
46.29%
Este trabalho trata da análise da propagação de fissuras, independente do tempo, em domínios bidimensionais utilizando uma formulação alternativa do método dos elementos de contorno (MEC). O MEC vem sendo utilizado com sucesso na análise de diversos problemas de engenharia. Considerando problemas de mecânica da fratura, o MEC é especialmente eficiente devido à redução da dimensionalidade de sua malha, o que permite a simulação do crescimento das fissuras sem as dificuldades do processo de remalhamento. Nesta pesquisa, desenvolvem-se formulações não lineares do MEC para a análise da propagação de fissuras em materiais quase frágeis. Nesses materiais, a zona de processo à frente da ponta da fissura introduz efeitos fisicamente não lineares no comportamento estrutural. Assim, para a simulação da presença da zona de processo, modelos não lineares são necessários. Classicamente a formulação dual do MEC é utilizada para modelar propagação de fissuras na quais equações singulares e hipersingulares são escritas para elementos definidos ao longo das faces das fissuras. O presente trabalho propõe uma segunda formulação utilizando um campo de tensões iniciais para a representação da zona coesiva. Nesta formulação...

SIMULATION of NON-LINEAR BEHAVIOR of QUASI-BRITTLE MATERIALS USING FINITE ELEMENTS WITH EMBEDDED CRACKS

Manzoli, Osvaldo L.; Rodrigues, Eduardo A.; Gamino, Andre L.
Fonte: Univ Politecnica Catalunya Publicador: Univ Politecnica Catalunya
Tipo: Artigo de Revista Científica Formato: 117-128
SPA
Relevância na Pesquisa
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A combined constitutive model based on the Continuum Damage Mechanics is presented to represent the nonlinear behavior of quasi-brittle materials, which present different response when subjected to tension or compression. 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 of cracks. To model the compressive behavior, a degradation criterion based on the second invariant of the deviatoric part of the effective stress tensor (Von Mises 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. The model is able to represent 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...

Um modelo constitutivo de dano composto para simular o comportamento de materiais quase-frágeis

Rodrigues, Eduardo Alexandre
Fonte: Universidade Estadual Paulista (UNESP) Publicador: Universidade Estadual Paulista (UNESP)
Tipo: Dissertação de Mestrado Formato: 98 f.
POR
Relevância na Pesquisa
46.52%
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES); Pós-graduação em Engenharia Mecânica - FEB; 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...

On a continuum theory of brittle materials with microstructure

Duda,Fernando Pereira; Souza,Angela Cristina Cardoso de
Fonte: Sociedade Brasileira de Matemática Aplicada e Computacional Publicador: Sociedade Brasileira de Matemática Aplicada e Computacional
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/12/2004 EN
Relevância na Pesquisa
56.35%
This paper deals with a finite strain continuum theory of elastic-brittle solids with microstructure. A single scalar microstructural field is introduced, meant to represent - even if in a summary way - the concentration of microdefects within the material. A system of microforces, dual to the microstructural field, is axiomatically introduced. The corresponding balance, augmented with suitable constitutive information, yields, inter alia, a kinetic equation for the microstructural field, criteria for damage nucleation, growth and healing as well as a failure criterion based on attainment of a critical value of the microstructural field. The theory is applied for the description of the Mullins effect.

1.978 From Nano to Macro, January (IAP) 2006; From Nano to Macro

Buehler, Markus J.
Fonte: MIT - Massachusetts Institute of Technology Publicador: MIT - Massachusetts Institute of Technology
EN-US
Relevância na Pesquisa
46.29%
The objective is to introduce large-scale atomistic modeling techniques and motivate its importance for solving problems in modern engineering sciences. We demonstrate how atomistic modeling can be successfully applied to understand how materials fail under extreme loading, emphasizing on the competition between ductile and brittle materials failure. We will demonstrate the techniques in describing failure of a copper nano-crystal.

Cracking in brittle materials during low-load indentation and its relation to fracture toughness

Harding, David Scott
Fonte: Universidade Rice Publicador: Universidade Rice
ENG
Relevância na Pesquisa
56.49%
Nanoindentation is a widely recognized method for characterizing the mechanical properties of thin films and small volumes. This thesis reports the results of an investigation aimed at developing a technique by which the fracture toughness of a thin film or small volume can be determined from low-load indentation experiments. The method relies on the fact that most brittle materials form radial cracks when indented with a sharp indenter such as a Vickers indenter. The lengths of the radial cracks produced during microindentation experiments have been shown to correlate reasonably well with fracture toughness. As a result, simple semi-empirical relations have been developed to calculate fracture toughness based on the measurement of indentation crack length. The one problem encountered in applying the indentation cracking method for measuring fracture toughness to low loads is that there are threshold loads below which most materials do not form radial cracks. For Vickers and Berkovich indenters, the cracking threshold is 25 grams ($sim$250 mN) or more for most ceramic and glass materials. It is shown that the problems imposed by the cracking threshold can largely be overcome by using an indenter with the geometry of a cube corner. With a cube corner indenter...

Model methodology for studying crushing of unbound granular materials during compaction; Proceedings of the TC105 ISSMGE International Symposium on Geomechanics from micro to macro, Cambridge, UK, 1-3 September 2014

Ocampo Terreros, Manuel Santiago; Caicedo Hormaza, Bernardo; vallejo Rosero, Luis Eduardo
Fonte: CRC Press Publicador: CRC Press
Formato: 297 - 302
Relevância na Pesquisa
46.39%
A fracture and abrasion model is used in order to understand and visualize the evolution of crushing resulting from compression and shear in unbound granular materials. Modelling was done using numerical simulation with the discrete element method in two dimensions in which each particle in the granular material is represented by a disc. The particle fragmentation criterion is defined on the bases of grain fracturing under diametric compression, and it is estimated as a function of material properties using an adaptation ofWeibull’s statistical theory for fracturing of brittle materials. Stresses in discs are measured using numerical modelling in order to evaluate particle crushing in soils subjected to loads, defining the failure criterion on an individual particle subjected to an arbitrary set of contact forces. This procedure approximates the stress levels inside the grain through a simple analysis which take into account the magnitude, location, and number of contact forces on the particle. This model was validated with gyratory compaction tests in the laboratory with granular materials extracted from three locations near Bogotá, Colombia. Good agreement between the results of the model and laboratory tests was achieved.; A fracture and abrasion model is used in order to understand and visualize the evolution of crushing resulting from compression and shear in unbound granular materials. Modelling was done using numerical simulation with the discrete element method in two dimensions in which each particle in the granular material is represented by a disc. The particle fragmentation criterion is defined on the bases of grain fracturing under diametric compression...

Theory of dynamic crack branching in brittle materials

Adda-Bedia, M.; Arias, Rodrigo; Katzav, E.
Fonte: Universidade do Chile Publicador: Universidade do Chile
Tipo: Artículo de revista
EN
Relevância na Pesquisa
46.35%
Publicación ISI; The problem of dynamic symmetric branching of a tensile crack propagating in a brittle material is studied within Linear Elastic Fracture Mechanics theory. The Griffith energy criterion and the principle of local symmetry provide necessary conditions for the onset of dynamic branching instability and for the subsequent paths of the branches. The theory predicts a critical velocity for branching and a well defined shape described by a branching angle and a curvature of the side branches. The model rests on a scenario of crack branching based on reasonable assumptions and on exact dynamic results for the anti-plane branching problem. Our results reproduce within a simplified 2D continuum mechanics approach the main experimental features of the branching instability of fast cracks in brittle materials.

Brittle materials in nano-abrasive fabrication of optical mirror-surfaces

Yin, Ling; Huang, Han
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
66.44%
The objective of this investigation was to evaluate the response of a wide range of brittle materials (polycrystalline silicon carbide, single-crystal 6H-SiC (0 0 0 1) substrate, tungsten carbide composite, and amorphous glass-inlaid stabilized tetragonal

Strong, Tough and Stiff Bioinspired Ceramics from Brittle Constituents

Bouville, Florian; Maire, Eric; Meille, Sylvain; Van de Moortèle, Bertrand; Stevenson, Adam J.; Deville, Sylvain
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 30/06/2015
Relevância na Pesquisa
46.33%
High strength and high toughness are usually mutually exclusive in engineering materials. Improving the toughness of strong but brittle materials like ceramics thus relies on the introduction of a metallic or polymeric ductile phase to dissipate energy, which conversely decreases the strength, stiffness, and the ability to operate at high temperature. In many natural materials, toughness is achieved through a combination of multiple mechanisms operating at different length scales but such structures have been extremely difficult to replicate. Building upon such biological structures, we demonstrate a simple approach that yields bulk ceramics characterized by a unique combination of high strength (470 MPa), high toughness (22 MPa.m1/2), and high stiffness (290 GPa) without the assistance of a ductile phase. Because only mineral constituents were used, this material retains its mechanical properties at high temperature (600{\deg}C). The bioinspired, material-independent design presented here is a specific but relevant example of a strong, tough, and stiff material, in great need for structural, transportations, and energy-related applications.; Comment: 27 pages, 4 figures, supplementary data, 47 references

Crack Path Prediction in Anisotropic Brittle Materials

Hakim, Vincent; Karma, Alain
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 31/12/2004
Relevância na Pesquisa
56.29%
A force balance condition to predict quasistatic crack paths in anisotropic brittle materials is derived from an analysis of diffuse interface continuum models that describe both short-scale failure inside a microscopic process zone and macroscopic linear elasticity. The derivation exploits the gradient dynamics and translation symmetry properties of this class of models to define a generalized energy-momentum tensor whose integral around an arbitrary closed path enclosing the crack tip yields all forces acting on this tip, including Eshelby's configurational forces, cohesive forces, and dissipative forces. This condition is validated quantitatively by numerical simulations.

Dynamic buckling and fragmentation in brittle rods

Gladden, J. R.; Handzy, N. Z.; Belmonte, A.; Villermaux, E.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 25/10/2004
Relevância na Pesquisa
46.29%
We present experiments on the dynamic buckling and fragmentation of slender rods axially impacted by a projectile. By combining the results of Saint-Venant and elastic beam theory, we derive a preferred wavelength lambda for the buckling instability, and experimentally verify the resulting scaling law for a range of materials including teflon, dry pasta, glass, and steel. For brittle materials, buckling leads to the fragmentation of the rod. Measured fragment length distributions show two clear peaks near lambda/2 and lambda/4. The non-monotonic nature of the distributions reflect the influence of the deterministic buckling process on the more random fragmentation processes.; Comment: 4 pages, 5 figures, submitted to Physical Review Letters

Forced tearing of ductile and brittle thin sheets

Tallinen, T.; Mahadevan, L.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.42%
Tearing a thin sheet by forcing a rigid object through it leads to complex crack morphologies; a single oscillatory crack arises when a tool is driven laterally through a brittle sheet, while two diverging cracks and a series of concertina-like folds forms when a tool is forced laterally through a ductile sheet. On the other hand, forcing an object perpendicularly through the sheet leads to radial petal-like tears in both ductile and brittle materials. To understand these different regimes we use a combination of experiments, simulations and simple theories. In particular, we describe the transition from brittle oscillatory tearing via a single crack to ductile concertina tearing with two tears by deriving laws that describe the crack paths and wavelength of the concertina folds and provide a simple phase diagram for the morphologies in terms of the material properties of the sheet and the relative size of the tool.; Comment: 5 pages, 4 figures

From microstructural features to effective toughness in disordered brittle solids

Démery, Vincent; Ponson, Laurent; Rosso, Alberto
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.35%
The relevant parameters at the microstructure scale that govern the macroscopic toughness of disordered brittle materials are investigated theoretically. We focus on planar crack propagation and describe the front evolution as the propagation of a long-range elastic line within a plane with random distribution of toughness. Our study reveals two regimes: in the collective pinning regime, the macroscopic toughness can be expressed as a function of a few parameters only, namely the average and the standard deviation of the local toughness distribution and the correlation lengths of the heterogeneous toughness field; in the individual pinning regime, the passage from micro to macroscale is more subtle and the full distribution of local toughness is required to be predictive. Beyond the failure of brittle solids, our findings illustrate the complex filtering process of microscale quantities towards the larger scales into play in a broad range of systems governed by the propagation of an elastic interface in a disordered medium.; Comment: 7 pages, 4 figures

Asymptotic Expansion Homogenization of Discrete Fine-Scale Models with Rotational Degrees of Freedom for the Simulation of Quasi-Brittle Materials

Rezakhani, Roozbeh; Cusatis, Gianluca
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 16/09/2015
Relevância na Pesquisa
56.47%
Discrete fine-scale models, in the form of either particle or lattice models, have been formulated successfully to simulate the behavior of quasi-brittle materials whose mechanical behavior is inherently connected to fracture processes occurring in the internal heterogeneous structure. These models tend to be intensive from the computational point of view as they adopt an a priori discretization anchored to the major material heterogeneities (e.g. grains in particulate materials and aggregate pieces in cementitious composites) and this hampers their use in the numerical simulations of large systems. In this work, this problem is addressed by formulating a general multiple scale computational framework based on classical asymptotic analysis and that (1) is applicable to any discrete model with rotational degrees of freedom; and (2) gives rise to an equivalent Cosserat continuum. The developed theory is applied to the upscaling of the Lattice Discrete Particle Model (LDPM), a recently formulated discrete model for concrete and other quasi-brittle materials, and the properties of the homogenized model are analyzed thoroughly in both the elastic and inelastic regime. The analysis shows that the homogenized micropolar elastic properties are size-dependent...

Quasi-Static Brittle Fracture in Inhomogeneous Media and Iterated Conformal Maps: Modes I, II and III

Barra, Felipe; Levermann, Anders; Procaccia, Itamar
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 07/05/2002
Relevância na Pesquisa
46.35%
The method of iterated conformal maps is developed for quasi-static fracture of brittle materials, for all modes of fracture. Previous theory, that was relevant for mode III only, is extended here to mode I and II. The latter require solution of the bi-Laplace rather than the Laplace equation. For all cases we can consider quenched randomness in the brittle material itself, as well as randomness in the succession of fracture events. While mode III calls for the advance (in time) of one analytic function, mode I and II call for the advance of two analytic functions. This fundamental difference creates different stress distribution around the cracks. As a result the geometric characteristics of the cracks differ, putting mode III in a different class compared to modes I and II.; Comment: submitted to PRE For a version with qualitatively better figures see: http://www.weizmann.ac.il/chemphys/anders

Failing softly: A fracture theory of highly-deformable materials

Boué, Tamar Goldman; Harpaz, Roi; Fineberg, Jay; Bouchbinder, Eran
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.35%
Highly-deformable materials, from synthetic hydrogels to biological tissues, are becoming increasingly important from both fundamental and practical perspectives. Their mechanical behaviors, in particular the dynamics of crack propagation during failure, are not yet fully understood. Here we propose a theoretical framework for the dynamic fracture of highly-deformable materials, in which the effects of a dynamic crack are treated with respect to the nonlinearly deformed (pre-stressed/strained), non-cracked, state of the material. Within this framework, we derive analytic and semi-analytic solutions for the near-tip deformation fields and energy release rates of dynamic cracks propagating in incompressible neo-Hookean solids under biaxial and uniaxial loading. We show that moderately large pre-stressing has a marked effect on the stress fields surrounding a crack's tip. We verify these predictions by performing extensive experiments on the fracture of soft brittle elastomers over a range of loading levels and propagation velocities, showing that the newly developed framework offers significantly better approximations to the measurements than standard approaches at moderately large levels of external loadings and high propagation velocities. This framework should be relevant to the failure analysis of soft and tough...

Theory of dynamic crack branching in brittle materials

Katzav, E.; Adda-Bedia, M.; Arias, R.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
56.41%
The problem of dynamic symmetric branching of an initial single brittle crack propagating at a given speed under plane loading conditions is studied within a continuum mechanics approach. Griffith's energy criterion and the principle of local symmetry are used to determine the cracks paths. The bifurcation is predicted at a given critical speed and at a specific branching angle: both correlated very well with experiments. The curvature of the subsequent branches is also studied: the sign of $T$, with $T$ being the non singular stress at the initial crack tip, separates branches paths that diverge from or converge to the initial path, a feature that may be tested in future experiments. The model rests on a scenario of crack branching with some reasonable assumptions based on general considerations and in exact dynamic results for anti-plane branching. It is argued that it is possible to use a static analysis of the crack bifurcation for plane loading as a good approximation to the dynamical case. The results are interesting since they explain within a continuum mechanics approach the main features of the branching instabilities of fast cracks in brittle materials, i.e. critical speeds, branching angle and the geometry of subsequent branches paths.; Comment: 41 pages...

A recursive-faulting model of distributed damage in confined brittle materials

Pandolfi, A.; Conti, S.; Ortiz, M.
Fonte: Elsevier Publicador: Elsevier
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em /09/2006
Relevância na Pesquisa
46.35%
We develop a model of distributed damage in brittle materials deforming in triaxial compression based on the explicit construction of special microstructures obtained by recursive faulting. The model aims to predict the effective or macroscopic behavior of the material from its elastic and fracture properties; and to predict the microstructures underlying the microscopic behavior. The model accounts for the elasticity of the matrix, fault nucleation and the cohesive and frictional behavior of the faults. We analyze the resulting quasistatic boundary value problem and determine the relaxation of the potential energy, which describes the macroscopic material behavior averaged over all possible fine-scale structures. Finally, we present numerical calculations of the dynamic multi-axial compression experiments on sintered aluminum nitride of Chen and Ravichandran [1994. Dynamic compressive behavior of ceramics under lateral confinement. J. Phys. IV 4, 177–182; 1996a. Static and dynamic compressive behavior of aluminum nitride under moderate confinement. J. Am. Soc. Ceramics 79(3), 579–584; 1996b. An experimental technique for imposing dynamic multiaxial compression with mechanical confinement. Exp. Mech. 36(2), 155–158; 2000. Failure mode transition in ceramics under dynamic multiaxial compression. Int. J. Fracture 101...

Ductile mode turning of brittle materials and its practical aspects

Pramanik, Alokesh; Basak, Animesh Kumar
Fonte: Curran Associates, Inc. Publicador: Curran Associates, Inc.
Tipo: Conference paper
Relevância na Pesquisa
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This paper aims to investigate the mechanism of ductile machining of brittle material based on information available in the literature. It also explores the challenges associated with the ductile machining of brittle materials which stop the technology from being applied in practical fields. In addition, few factors that assist to improve productivity of ductile machining of brittle material have been discussed. It is found the higher tool wear is the main obstacle of this technology. The application ofmicro-laser heating,ultrasonic vibration and coolants improve the machining time and tool life significantly.