5195 - INFLUENCE OF THE MATERIAL MESO-STRUCTURE ON 2D AND 3D LATTICE RESPONSE

Heterogeneous lattice models are receiving increasing recognition as one of the most suitable models for studying the mechanisms of fracture in disordered materials such as concrete. In concrete failure is induced by formation of micro-cracks, which propagate and coalesce to form macro-cracks. Micro and macro cracking correspond in the load-displacement diagram to the preand post-peak regime, respectively. In lattice models concrete is schematized as a network of beams (or springs) that have a linear elastic brittle behavior until failure, and removing at each loading step the element that violates the adopted fracture criterion simulates cracking. Despite the local brittleness, the overall response of the lattice presents softening. Thus, lattice models are useful not only for investigating the factors that influence the strength, but also for understanding the mechanism of softening in the structure. Though, these models are still controversial. The reason is that 2D lattice analyses, although able to reliably predict crack patterns for a wide range of laboratory experiments, give always a too brittle response in terms of load-displacement diagrams in comparison to the experiments. In this paper it is shown that the brittle lattice response is consequence of the rough schematization of the material, which is usually adopted, and the neglect of 3D effect. 3D Lattice analyses performed with varying particle density show that by increasing the particle density an increasing ductility of the lattice response can be obtained. The same type of analysis conducted with a 2D lattice shows a less remarkable influence of the particle density on the overall lattice response. The reason for this may be that percolation of the interface occurs in 2D lattices already for relatively sparse particle distributions. In fact, the same type of response can be obtained with a sparse particle distribution by increasing the thickness of the interface.