Ignacio Iturrioz

Crack propagation in elastic solids using the truss-like discrete element method

The crack propagation simulation is still an open problem in the mechanical simulation field. In the present work this problem is analyzed using a version of truss-like Discrete Element Method, that here we called DEM. This method has been used with success in several applications in solid mechanical problems where the simulation of fracture and fragmentation is relevant. The formulation of DEM explaining the way the process of rupture could be simulated in consistent form is showed. Also are described details about how the dynamical fracto-mechanical stress intensity factors are computed. The main aim of this paper is to show the ability of this method in simulating fracture and crack propagation in solids, for this, three examples with different levels of complexity are analyzed. The obtained results are presented in terms of the variation of dynamic stress intensity factor in the fracture process, the stress map and geometric configuration on different steps in the simulation of the fracture process, the crack speed and the energetic balance during all the process. These results are compared with experimental and numerical results obtained by other researchers and published in recognized scientific papers. Final commentaries about the performance of the version of lattice model considered are carried out.

Acoustic emission detection in concrete specimens: Experimental analysis and lattice model simulations

In civil engineering, a quantitative evaluation of damage in materials subjected to stress or strain states is of great importance due to the critical character of these phenomena, which may suddenly give rise to catastrophic failure. From an experimental point of view, an effective damage assessment criterion is provided by the statistical analysis of the amplitude distribution of the acoustic emission signals generated by growing microcracks. A classical way to work out the amplitude of acoustic emission signals distribution is the Gutenberg-Richter law, characterized by the b-value parameter, which systematically decreases with damage growth. The damage process is also characterized by a progressive localization that can be modeled through the fractal dimension 2b = D of the damaged domain. In the framework of continuum damage mechanics, the progressive deterioration of the material that causes formation of macro-cracks is described by means of phenomenological damage variables usually introduced in classical constitutive relationships. Nevertheless, taking into account discrete damage mechanics, lattice models are particularly suitable to reproduce the generation of acoustic emission events, arising from the materials, during the different stages of damage growth. These models are also fundamental for the application of advanced statistical methods and non-standard mathematical methods, e.g. fractal theory. Starting from these considerations, in this work a b-value analysis was conducted in laboratory on two concrete specimens loaded up to failure. One was a prismatic specimen subjected to uniaxial compressive loading and the other was a pre-cracked beam subjected to a three-point bending test. The truss-like discrete element method was used to perform numerical simulations of the testing processes. The test results and the results of the numerical analyses, in terms of load vs. time diagram and acoustic emission data, as determined through b-value and signal frequency variations, are compared and are seen to be in good agreement.

The truss‐like discrete element method in fracture and damage mechanics

Purpose – The purpose of this paper is to further develop the truss‐like discrete element method (DEM) in order to make it suitable to deal with damage and fracture problems.Design/methodology/approach – Finite and boundary elements are the best developed methods in the field of numerical fracture and damage mechanics. However, these methods are based on a continuum approach, and thus, the modelling of crack nucleation and propagation could be sometimes a cumbersome task. Besides, discrete methods possess the natural ability to introduce discontinuities in a very direct and intuitive way by simply breaking the link between their discrete components. Within this context, the present work extends the capabilities of a truss‐like DEM via the introduction of three novel features: a tri‐linear elasto‐plastic constitutive law; a methodology for crack discretization and the computation of stress intensity factors; and a methodology for the computation of the stress field components from the unixial discrete‐elem...

Study of imperfections in the cubic mesh of the truss-like discrete element method

In the truss-like discrete element method (DEM), masses are lumped at nodal points and interconnected by means of one-dimensional elements with arbitrary constitutive relations. In previous studies of nonhomogeneous concrete cubic samples subjected to nominally uniaxial tension, it was verified that numerical predictions of fracture using discrete element method models are feasible and yield results that present good correlation with the experimental evidence so far available, including the prediction of size and strain rate effects. In the discrete element method approach, material failure under compression is assumed to occur by indirect tension. In previous simulations of samples subjected to uniaxial compression, it was verified that the response is satisfactorily modeled up to the peak load, when a sudden collapse usually occurs, characteristic of fragile behavior. On the other hand, experimental stress versus displacement curves observed in small specimens subjected to compression typically present a softening branch, in part due to sliding with friction of the fractured parts of the specimens. A second deficiency of discrete element method models with a perfectly cubic mesh is that the best correlations with experimental results are obtained with material parameters that differ in tension and compression. This paper examines another cause of the fragile behavior of discrete element method predictions of the response of concrete elements subjected to nominally uniaxial compression, namely the regularity of the perfect cubic mesh, which is unable to capture nonlinear stability effects in the material at a microscale. It is shown herein that the introduction of small perturbations of the discrete element method regular mesh significantly improves the predicting capability of the model and in addition allows adopting a unique set of material properties, which are independent of the applied loading.