Gabriel de Oliveira Ribeiro

Study of size effect using digital image correlation

Resumo Size effect is an important issue in concrete structures bearing in mind that it can influence many aspects of analysis such as strength, brittleness and structural ductility, fracture toughness and fracture energy, among others. Further this, ever more new methods are being developed to evaluate displacement fields in structures. In this paper an experimental evaluation of the size effect is performed applying Digital Image Correlation (DIC) technique to measure displacements on the surface of beams. Three point bending tests were performed on three different size concrete beams with a notch at the midspan. The results allow a better understanding of the size effect and demonstrate the efficiency of Digital Image Correlation to obtain measures of displacements.

Non-local constitutive modelling by the boundary element method

Rodrigo G. Peixoto Samuel S. Penna Gabriel O. Ribeiro Roque L. S. Pitangueira rodrigo.peixoto@dees.ufmg.br spenna@dees.ufmg.br gabriel@dees.ufmg.br roque@dees.ufmg.br Dep. de Eng. de Estruturas, Escola de Engenharia, Universidade Federal de Minas Gerais Avenida Antônio Carlos, 6627, CEP: 31270-901, Belo Horizonte, Minas Gerais, Brazil Abstract. The conventional (local) constitutive modelling of materials exhibiting strain softening behaviour is suceptive to a spurious mesh dependence caused by numerically induced strain localization. Also, for very refined meshes, numerical instabilities may be verified, mainly if the simulations are performed by the boundary element method. An alternative to overcome such difficulties is the adoption of the so called non-local constitutive models. In this approach, some internal variables of the constitutive model in a single point are averaged considering its values of the neighbouring points. In this paper, the implicit formulation of the boundary element method for physically non linear problems is used with non-local isotropic damage models. It is shown that the analysis become more stable in comparison to the case of a local application of the same models and that the results recover the desired objectiveness to mesh refinement.

The dependence of step size in the continuum strong discontinuity approach using the boundary element method

Abstract. The failure analysis in structures is extremely important since it allows to evaluate the safety degree and the integrity of structural systems. However, despite the increasing use of numerical methods in the study of material failures in the last years, the boundary element method is still little used when compared to domain methods. Thus, in this work it is analyzed the failure mechanics in some classical problems through the continuum strong discontinuity approach. In this way, it is used the implicit formulation of the boundary element method for nonlinear problems to shown the dependence of the step size when the cells with embedded discontinuity are generated only after step convergence or during any iteration, respectively, in the incremental-iterative process. For this purpose, the same problem was analyzed considering these two cases and also different numbers of steps. It was verified that the results are coincident for all numbers of steps considered in the simulations when cells are generated during any iteration, showing step size independence in this case, while the same is not true for the case of cells generated only after convergence, in which a large numbers of steps is required for well accuracy.

Concrete modulus of elasticity assessment using digital image correlation

This paper presents the use of the technique of digital image correlation for obtaining the elasticity modulus of concrete. The proposed system uses a USB microscope that captures images at a rate of five frames per second. The stored data are correlated with the applied loads, and a stress-strain curve is generated to determine the concrete compressive modulus of elasticity. Two different concretes were produced and tested using the proposed system. The results were compared with the results obtained using a traditional strain gauge. It was observed a difference in the range of 4% between the two methods, wherein this difference depends on some parameters in the case of the DIC results, as focal length and a video capture resolution, indicating that DIC technique can be used to obtain mechanical properties of concrete.