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- Biblioteca Digitais de Teses e Dissertações da USP
- Univ Politecnica Catalunya
- Universidade Estadual Paulista (UNESP)
- Sociedade Brasileira de Matemática Aplicada e Computacional
- MIT - Massachusetts Institute of Technology
- Universidade Rice
- CRC Press
- Universidade do Chile
- Elsevier
- Universidade Cornell
- Curran Associates, Inc.
- Mais Publicadores...

## 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

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

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46.29%

#Boundary element method#Cohesive fracture model#Dipole based formulation#Formulação baseada em dipolos#Mecânica da fratura não linear#Método dos elementos de contorno#Modelo de fratura coesiva#Non-linear fracture mechanics#Operador tangente#Tangent operator

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...

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

Fonte: Univ Politecnica Catalunya
Publicador: Univ Politecnica Catalunya

Tipo: Artigo de Revista Científica
Formato: 117-128

SPA

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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...

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

Fonte: Universidade Estadual Paulista (UNESP)
Publicador: Universidade Estadual Paulista (UNESP)

Tipo: Dissertação de Mestrado
Formato: 98 f.

POR

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#Mecânica do dano contínuo#Fraturas#Continuum damage mechanics#Strong discontinuities#Finite elements#Quasi-brittle materials

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...

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## On a continuum theory of brittle materials with microstructure

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

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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.

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

Fonte: MIT - Massachusetts Institute of Technology
Publicador: MIT - Massachusetts Institute of Technology

EN-US

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#large-scale atomistic modeling techniques#modern engineering sciences#atomistic modeling#extreme loading#ductile and brittle materials failure#copper nano-crystal#141301#Engineering Science

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.

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

Fonte: Universidade Rice
Publicador: Universidade Rice

ENG

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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...

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## 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

Fonte: CRC Press
Publicador: CRC Press

Formato: 297 - 302

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#fracturamiento#materiales granulares#compactación#Weibull#PFC#compacidad#crushing#unbound granular materials#compaction#Particle Flow Code#Miner's rule

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...

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## Theory of dynamic crack branching in brittle materials

Fonte: Universidade do Chile
Publicador: Universidade do Chile

Tipo: Artículo de revista

EN

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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.

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

Fonte: Elsevier
Publicador: Elsevier

Tipo: Artigo de Revista Científica

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#Keywords: Abrasives#Amorphous silicon#Brittleness#Cladding (coating)#Fracture mechanics#Grinding (comminution)#Grinding (machining)#Grinding mills#Mirrors#Nonmetals#Optical design

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

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## Strong, Tough and Stiff Bioinspired Ceramics from Brittle Constituents

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 30/06/2015

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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

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## Crack Path Prediction in Anisotropic Brittle Materials

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 31/12/2004

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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.

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## Dynamic buckling and fragmentation in brittle rods

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 25/10/2004

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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

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## Forced tearing of ductile and brittle thin sheets

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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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

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## From microstructural features to effective toughness in disordered brittle solids

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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#Condensed Matter - Disordered Systems and Neural Networks#Condensed Matter - Materials Science#Condensed Matter - Statistical Mechanics

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

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 16/09/2015

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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...

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

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 07/05/2002

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#Condensed Matter - Statistical Mechanics#Condensed Matter - Materials Science#Nonlinear Sciences - Pattern Formation and Solitons

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

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## Failing softly: A fracture theory of highly-deformable materials

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...

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## Theory of dynamic crack branching in brittle materials

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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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...

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

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...

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## Ductile mode turning of brittle materials and its practical aspects

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.

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