T. N. Bittencourt

Quasi-automatic simulation of crack propagation for 2D LEFM problems

A strategy for quasi-automatic simulation of propagation of arbitrary cracks in two-dimensional, linear elastic finite element models is presented. This strategy has been implemented in FRANC2D (FRacture ANalysis Code 2D). An underlying winged-edge data structure enables automatic local modifications of the mesh along the propagation path without loss of any unaffected structural information. The finite element mesh is locally regenerated after each step of propagation by means of a robust remeshing algorithm. The propagation process is driven by linear elastic fracture mechanics concepts which are used to calculate mixed-mode stress intensity factors, predict incremental changes in trajectory, and assess local crack stability. Crack trajectories, obtained for different techniques of stress intensity factor calculation, and for different mixed-mode interaction theories, are presented and favorably compared to experimentally obtained paths.

Fatigue life and crack path predictions in generic 2D structural components

This paper proposes a reliable and cost-effective two-phase methodology to predict crack propagation life in generic two-dimensional (2D) structural components. First, the usually curved fatigue crack path and its stress-intensity factors are calculated at small crack increments in a specialized finite-element software, using automatic remeshing algorithms, special crack tip elements and appropriate crack increment criteria. Then, the computed stress-intensity factors are transferred to a powerful general-purpose fatigue-design program, which has been designed to predict both initiation and propagation fatigue lives by means of classical design methods. Particularly, its crack propagation module accepts any KI expression and any crack growth rate model, considering sequence effects such as overload-induced crack retardation to deal with 1D and 2D crack propagation under variable amplitude loading. Non-trivial application examples compare the numerical simulation results with those measured in physical experiments. � 2002 Elsevier Science Ltd. All rights reserved.

Experimental Analysis of Fracture Processes in Concrete

This paper draws on the basic problems related to the determination of parameters to characterize the structural behavior of concretes using Fracture Mechanics concepts. Experimental procedures and results are discussed.

R-curve behavior in notched beam tests of rocks

Abstract The R -curve for sandstone is obtained from the load-crack mouth opening response of notched specimens subjected to three-point-bending. This approach is used to analyze the fracture behavior under monotonic and cyclic loading. The asymptotic limit of the R -curve compares well with the fracture toughness determined through an effective crack model. The analysis of the relaxation observed before the unloading–reloading cycles in the cyclic tests leads to the conclusion that the fracture toughness remains practically constant while the crack propagates slightly during the load drop.

Adaptive simulation of fracture processes based on spatial enumeration techniques

Abstract An interactive graphics computational system with self-adaptive, integrated, two-dimensional finite element analysis capabilities is described in this work. This system is able to handle both standard structural and fracture mechanics problems. The self-adaptive strategy is based on recursive spatial enumeration techniques: a binary tree partition for the boundary and the crack-line curves definition, and a quadtree partition for domain mesh generation. The ‘ a priori ’ refinement of the curves has the advantage of generating good transition meshes at the boundary regions. The system integrates different tools: a geometric modeler to create the model geometry, a pre-processor for mesh generation and attribute assignment, a numerical analysis module to evaluate the finite element response, and a post-processor for result visualization. The system is capable of deciding where to refine an initial mesh, of redoing the analysis, and of repeating this procedure until a pre-defined convergence criterion is achieved. Cracks can be introduced arbitrarily by the user at any position in the model. The system regenerates the meshes automatically taking into account the new created crack surfaces. For linear elastic analysis, quarter-point elements are inserted around the crack tips. The self-adaptive procedure is also considered in the crack propagation process. This procedure takes into account the arbitrarily generated crack geometry and the finite element error estimation analysis.

Adaptable Strut-and-Tie Model for Design and Verification of Four-Pile Caps

Pile caps transfer the load from columns to a group of piles. Many pile caps support only one column, and the pile caps in turn are supported by only a few piles. Codes of practice do not provide uniform treatment for the design of these types of pile caps, which are typically short and deep with overall span-depth ratios of less than 1.5. These members have traditionally been designed as beams spanning between piles with the depth selected to avoid shear failures and the amount of longitudinal reinforcement selected to provide sufficient flexural capacity as calculated by the engineering beam theory. The strut-and-tie method also has been used for the design of pile caps in which the load path is envisaged to be a three-dimensional truss, with compressive forces being supported by concrete compressive struts between the column and piles and tensile forces being carried by reinforcing steel located between piles. However, neither of these models have provided uniform factors of safety against failure or been able to predict whether failure will occur by flexure or shear. This paper presents an analytical model based on the strut-and-tie approach. The proposed model has been calibrated using an extensive experimental database of pile caps subjected to compression and evaluated analytically for more complex loading conditions. Findings show that this model is applicable across a broad range of test data and can predict the failures modes, cracking, yielding, and failure loads of four-pile caps with reasonable accuracy. The proposed model is also demonstrated to be more safe and rational than the application of a sectional design method for the design of pile caps.

Failure behavior modeling of slender reinforced concrete columns subjected to eccentric load

This work presents a numerical model to simulate the failure behavior of slender reinforced concrete columns subjected to eccentric compression loads. Due to the significant influence of the lateral displacements on the loading state provided by an eccentric load, geometric nonlinearity is considered. The responses of the concrete in tension and compression are described by two scalar damage variables that reduce, respectively, the positive and negative effective stress tensors, which lead to two different damage surfaces that control the dimension of the elastic domain. To describe the behavior of the reinforcements, truss finite elements with elastoplastic material model are employed. Interaction between the steel bars and concrete is modeled through the use of interface finite elements with high aspect ratio and a damage model designed to describe the bond-slip behavior. The results showed that the numerical model is able to represent the nonlinear behavior of slender concrete columns with good accuracy, taking into account: formation of cracks steel yielding crushing of the concrete in the compressive region and interaction between rebars and concrete.

Modelos de Fissuração Distribuída em Vigas de Concreto Armado pelo Método dos Elementos Finitos

The main goal of the present work is to present a comparison between two different strategies for the computational simulation of reinforced concrete structures, both using smeared crack models to represent the behavior of the materials. As a first approach, a multidirectional smeared crack model available in DIANA has been adopted along with different softening rules for the cracked materials (brittle, linear, nonlinear of Moelands-Reinhardt and Hordijk). Additionally, the Disturbed Stress Field Model - DSFM has also been adopted to model cracked concrete as an orthotropic material with smeared rotating cracks. The finite element codes DIANA and VecTor2 have been used to evaluate the performance of the different smeared crack models to predict the behavior of reinforced concrete beams subjected primarily to flexure.

EXPERIMENTAL ANALYSIS OF INTERFACE BETWEEN CFRP AND CONCRETE USING CYLINDRICAL SPECIMENS

In this work an experimental methodology to determine the bond capacity of the interface between CFRP sheets and concrete was investigated. The test setup has been developed to reproduce in a more realistic way the pure mode I1 fracture behavior of the interface under shear and to evaluate the maximum load capacity of this interface. Cylindrical concrete specimens, also used in tests for compressive strength, have been adapted in this study. Two concrete cylinders are placed end-to-end, bonded with FRP and pulled apart. The tensile load, which separates the cylinders, subjects the FRPconcrete interface to shear. The results obtained for static, cyclic and long term loadings indicate that the method is promising and of practical interest. In this paper the results of the static loading tests are discussed.

Modelagem por elementos finitos do comportamento do concreto nas primeiras idades

Resumo In this work a computational model, based on the finite element method was implemented, to simulate the early age concrete behavior, having as a special feature, the evaluation of the cracking risk. The finite element analysis encloses the computational modeling of the following phenomena: chemical, thermal, diffusion and mechanical which occur at the first days after the concrete cast. The developed software results were compared with experimental values found in the literature, demonstrating an excellent agreement for all the implemented analysis.