José Luiz Antunes de Oliveira e Sousa

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.

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.

Experimental investigation on the influence of temperature on the mechanical properties of reservoir rocks

Abstract This paper reports an investigation on mechanical properties of reservoir rocks obtained in laboratory at room temperature, and their pertinence to characterize the actual rock behavior under normal downhole or steam injection conditions. The investigation was based on triaxial tests performed by a servo-controlled system on samples of friable sandstones cored from a reservoir in the Potiguar Basin, Northeastern Brazil. The samples were tested at 24°C, 80°C and 150 °C, with confining pressure varying from 2.5 MPa to 20MPa. A finite element elastic-plastic analysis of wellbore stability was performed using mechanical properties obtained at these temperatures, showing quite different yielding zones. Although the results obtained so far are not sufficient to characterize completely these effects, they indicate that a more appropriate range of temperature must be used in laboratory to better represent real downhole and steam injection conditions.

A quasi-analytical model for three-dimensional analysis of hydraulic fracture propagation in reservoir rocks

Abstract The development of a quasi-analytical pseudo-three-dimensional model (Q3DFRAC) is presented. The fracture is assumed planar and elliptical. The structural response of the rock (strains, stresses, stress intensity factors, etc.), are computed assuming that the net pressure in the fracture is described by a third degree polynomial in two variables, according to analytical expressions developed by Shah and Kobayashi. These assumptions were previously used by Palmer, Luiskutty, Bouteca and Rueda, varying the number of polynomial terms. The model proposed here is based on Ruedas work, using all the terms respecting symmetry about the wellbore axis. An alternative approach is presented to model leakoff and the average apparent viscosity for non-Newtonian fluids. Simulations with two different fracture propagation criteria, based on stress intensity factors, are presented and compared.

Determination of parameters of a viscous-cohesive fracture model by inverse analysis

The quasi-brittle, loading rate dependent behaviour of the concrete, characterized by a fracture process zone (FPZ) ahead of the crack front, can be described through a viscous-cohesive model. In this paper, a viscous cohesive model proposed in a former paper is evaluated for a group of high strength concrete beams loaded at rates from 10-5 mm/s to 10+1 mm/s. A software has been developed to enable the automatic determination of the viscous-cohesive model parameters through inverse analysis on load-versus loading-point displacement (P-d) from threepoint bend tests on notched prismatic specimens. The strategy allowed the sensitivity analysis of the parameters related to viscous behaviour. The analysis of results shows that the formerly proposed model can be improved for a better simulation of the loading rate dependence on the cohesive fracture process.

Um algoritmo de Levenberg-Marquardt para ajuste de curvas σ-w a partir de ensaios de flexão em três pontos para concretos simples e reforçados com fibras

Esta nota tecnica descreve a implementacao de um algoritmo de Levenberg-Marquardt para a analise inversa de dados de ensaios de flexao em tres pontos em especimes de concreto simples ou reforcado com fibras para a determinacao de parâmetros de amolecimento (curvas σ-w). Testes indicaram que o algoritmo e suficientemente efi ciente e robusto.