Characteristics of the double-torsion test to determine the R-curve of ceramic materials

Double-torsion tests were carried out on a commercial ceramic floor tile to verify whether this test is suitable for determining the R-curve of ceramics. The instantaneous crack length was obtained by means of compliance calibration, and it was found that the experimental compliance underestimates the real crack length. The load vs. displacement curves were also found to drop after maximum loading, causing the stress intensity factor to decline. The R-curves were calculated by two methods: linear elastic fracture mechanics and the energetic method. It was obtained that the average values of crack resistance, R, and the double of the work of fracture, 2 · γwof, did not depend on notch length, a0, which is a highly relevant finding, indicating that these parameters were less dependent on the test specimen's geometry. The proposal was to use small notches, which produce long stable crack propagation paths that in turn are particularly important in the case of coarse microstructures.

Fadiga em compósitos 5HS carbono/epóxi processados via RTM: relação entre cargas axiais no plano e modos de delaminação

Pós-graduação em Engenharia Mecânica - FEG

Efficiency comparison of hyperbranched polymers as toughening agents for a one-part epoxy resin

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

An anisotropic linear elastic boundary element formulation for masonry wall analysis

This work presents an application of a Boundary Element Method (BEM) formulation for anisotropic body analysis using isotropic fundamental solution. The anisotropy is considered by expressing a residual elastic tensor as the difference of the anisotropic and isotropic elastic tensors. Internal variables and cell discretization of the domain are considered. Masonry is a composite material consisting of bricks (masonry units), mortar and the bond between them and it is necessary to take account of anisotropy in this type of structure. The paper presents the formulation, the elastic tensor of the anisotropic medium properties and the algebraic procedure. Two examples are shown to validate the formulation and good agreement was obtained when comparing analytical and numerical results. Two further examples in which masonry walls were simulated, are used to demonstrate that the presented formulation shows close agreement between BE numerical results and different Finite Element (FE) models. © 2012 Elsevier Ltd.

Fractal characterization of brittle fracture in ceramics under mode I stress loading

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

A finite element approach for predicting the ultimate rotation capacity of RC beams

This paper presents a numerical approach to model the complex failure mechanisms that define the ultimate rotational capacity of reinforced concrete beams. The behavior in tension and compression is described by a constitutive damage model derived from a combination of two specific damage models [1]. The nonlinear behavior of the compressed region is treated by the compressive damage model based on the Drucker-Prager criterion written in terms of the effective stresses. The tensile damage model employs a failure criterion based on the strain energy associated with the positive part the effective stress tensor. This model is used to describe the behavior of very thin bands of strain localization, which are embedded in finite elements to represent multiple cracks that occur in the tensioned region [2]. The softening law establishes dissipation energy compatible with the fracture energy of the concrete. The reinforcing steel bars are modeled by truss elements with elastic-perfect plastic behavior. It is shown that the resulting approach is able to predict the different stages of the collapse mechanism of beams with distinct sizes and reinforcement ratios. The tensile damage model and the finite element embedded crack approach are able to describe the stiffness reduction due to concrete cracking in the tensile zone. The truss elements are able to reproduce the effects of steel yielding and, finally, the compressive damage model is able to describe the non-linear behavior of the compressive zone until the complete collapse of the beam due to crushing of concrete. The proposed approach is able to predict well the plastic rotation capacity of tested beams [3], including size-scale effects.

Análise fractal de fraturas em regime linear elástico: modo I de carregamento

For the development of this graduate work of fractal fracture behavior, it is necessary to establish references for fractal analysis on fracture surfaces, evaluating, from tests of fracture tenacity on modes I, II and combined I / II, the behavior of fractures in fragile materials, on linear elastic regime. Fractures in the linear elastic regime are described by your fractal behavior by several researchers, especially Mecholsky JJ. The motivation of that present proposal stems from work done by the group and accepted for publication in the journal Materials Science and Engineering A (Horovistiz et al, 2010), where the model of Mecholsky could not be proven for fractures into grooved specimens for tests of diametric compression of titania on mode I. The general objective of this proposal is to quantify the distinguish surface regions formed by different mechanisms of fracture propagation in linear elastic regime in polymeric specimens (phenolic resin), relating tenacity, thickness of the specimens and fractal dimension. The analyzed fractures were obtained from SCB tests in mode I loading, and the acquisition of images taken using an optical reflection microscope and the surface topographies obtained by the extension focus method of reconstruction, calculating the values of fractal dimension with the use of maps of elevations. The fractal dimension was classified as monofractal dimension (Df), when the fracture is described by a single value, or texture size (Dt), which is a macroscopic analysis of the fracture, combined with the structural dimension (Ds), which is a microscopic analysis. The results showed that there is no clear relationship between tenacity, thickness and fractal values for the material investigated. On the other hand it is clear that the fractal values change with the evolution of cracks during the fracture process ... (Complete abstract click electronic access below)

A mixture theory based method for three-dimensional modeling of reinforce concrete members with embedded crack finite elements

The paper presents a methodology to model three-dimensional reinforced concrete members by means of embedded discontinuity elements based on the Continuum Strong Discontinuous Approach (CSDA). Mixture theory concepts are used to model reinforced concrete as a 31) composite material constituted of concrete with long fibers (rebars) bundles oriented in different directions embedded in it. The effects of the rebars are modeled by phenomenological constitutive models devised to reproduce the axial non-linear behavior, as well as the bond-slip and dowel action. The paper presents the constitutive models assumed for the components and the compatibility conditions chosen to constitute the composite. Numerical analyses of existing experimental reinforced concrete members are presented, illustrating the applicability of the proposed methodology.

An implicit BEM formulation to model strong discontinuities in solids

A boundary element method (BEM) formulation to predict the behavior of solids exhibiting displacement (strong) discontinuity is presented. In this formulation, the effects of the displacement jump of a discontinuity interface embedded in an internal cell are reproduced by an equivalent strain field over the cell. To compute the stresses, this equivalent strain field is assumed as the inelastic part of the total strain. As a consequence, the non-linear BEM integral equations that result from the proposed approach are similar to those of the implicit BEM based on initial strains. Since discontinuity interfaces can be introduced inside the cell independently on the cell boundaries, the proposed BEM formulation, combined with a tracking scheme to trace the discontinuity path during the analysis, allows for arbitrary discontinuity propagation using a fixed mesh. A simple technique to track the crack path is outlined. This technique is based on the construction of a polygonal line formed by segments inside the cells, in which the assumed failure criterion is reached. Two experimental concrete fracture tests were analyzed to assess the performance of the proposed formulation.

Dependência estrutural da capacidade de rotação plástic em vigas de concreto armado

Summary In this work the structural dependence of plastic rotation capacity in RC beams is evaluated using the Finite Element Method. The objective is to achieve a better understanding of the non-linear behavior of reinforced concrete members and perform extensive parameter studies, using a rational model developed by Bigaj [1] to analyze the phenomenon of plastic rotation capacity in reinforced concrete members. It is assumed that only bending failure is relevant due to sufficient member resistance against shear and torsion. The paper begins with the physical and theoretical background of the phenomenon of plastic hinge development in RC structures. Special emphasis is laid on the issue of structural dependence of deformation capacity of plastic hinges in RC members. Member size dependence and influence of properties of construction materials were emphasized as well. The essential components of the Bigajs model for calculating the plastic rotation capacity are discussed. The behaviour of the plastic hinge is analysed taking into account the strain localisation in the damage zones of the hinge region. The Fictitious Crack Model (FCM) and the Compressive Damage Zone Model (CDZ) are adopted in a Fracture Mechanics approach to model the behaviour of concrete in tension and compression, respectively. The approach is implemented in FEMOOP, a FEM in-house solver under development, and applied to evaluate ductility in 2D beams. The models were generated with GiD, a pre-processor and post-processor developed by CIMNE, and analyzed with the capabilities implemented in FEMOOP. © Universitat Politècnica de Catalunya, Barcelona, España 2010.

Fractografia quantitativa: relações entre tenacidade e o comportamento fractal em ligas metálicas

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Efeito do preenchimento e do material restaurador na resistência à fratura e distribuição de tensões de pré-molares superiores fragilizados

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Numerical solution of the FENE-CR model in complex flows

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Three-dimensional analysis of reinforced concrete members via embedded discontinuity finite elements

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

2D evaluation of crack openings using smeared and embedded crack models

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)