ADVANCED MODELING APPROACHES FOR THE FAILURE ANALYSIS OF HETEROGENEOUS MATERIALS AND STRUCTURES

Abstract

The development of new engineered materials and the introduction of innovative design techniques have led researchers in the field of structural engineering to focus their attention on the mechanical behavior of these materials to develop optimal and innovative design procedures to improve the structural performance of new and existing buildings, especially concerning their seismic behavior. The most investigated aspects are, on the one hand, the mechanical characterization of complex microstructures, such as those of composite reinforced with fibers and/or particles, to be carried out taking into account the influence of the micro-constituents on the global properties, and, on the other hand, the study of the interactions between these materials and the structural elements to which these composites are applied for mechanical reinforcement, considering the damage and fracture phenomena that can potentially occur. From a mechanical point of view, both innovative materials (e.g. composite laminates) and conventional materials (e.g. concrete, reinforced concrete, and masonry) can be considered heterogeneous materials, as they are composed of a more or less complex microstructure, made up of different constituents, usually distinguishable at very small scale compared to the dimensions of the whole structure, known as microscopic scale [1]. Being formed by the combination of distinct phases, such materials often have different (even better) mechanical properties than those of the individual constituents, but at the same time are subject to failure phenomena including: • fiber/matrix debonding, for composites, or FRP/substrate debonding for FRP-strengthened structures; • delamination between the different constituents for layered composites; • matrix cracking; • damage and plasticity phenomena; • growth of voids in the matrix phase; •As a characteristic feature of materials with heterogeneous microstructure, the different failure mechanisms may interact with each other, especially if coupled with additional effects related to unilateral contact (with or without friction) between the surfaces of the cracks or due to the presence of imperfect interfaces between the different phases [2]. As a consequence, the analysis of these non-linear phenomena and the associated structural response, results in the solution of highly non-linear problems, which make the study of the behavior of heterogeneous materials extremely challenging, requiring highly specialized theoretical and numerical knowledge as well as accurate and computationally efficient tools. In recent decades, different theoretical and numerical models have been developed to study the collapse mechanisms in heterogeneous materials and their influence on the overall properties in terms of strength and stiffness. Among these, for example, multiscale approaches that make it possible to analyze the response by considering the interaction that occurs between the various phenomena at different involved scales, or methods that use damage and fracture mechanics to describe the behavior of heterogeneous solids subject to damage phenomena. Besides the study of these issues, research interest in recent years has been focusing on structural health monitoring and damage identification within existing structures; the aim is to reduce the risk of collapse mechanisms within the materials so that the structural integrity is no longer compromised and premature and catastrophic collapses of the structures are avoided. This thesis aims to develop a series of advanced numerical methods for the failure analysis of heterogeneous materials and structures, both at the meso- and micro-scale. All the developed models use a cohesive/volumetric finite element method, based on an inter-element fracture approach [3]. In particular, two models have been developed: • A first model combining the cohesive fracture approach with a hierarchical multiscale model used to study the collapse phenomena of materials at a microscopic scale; • A second model, based exclusively on the inter-element cohesive approach, to analyze the structural behavior of FRP-strengthened reinforced concrete elements subjected to cyclic loading conditions for structural health monitoring, as well as to investigate the failure mechanisms in masonry elements. The key aspect of this work is to illustrate the models developed, to show the different strategies and procedures required to adapt them to different scales, and to the different materials and structures in the engineering fields. Chapter 1 contains the introduction and a review of the technical literature, as well as the aims and objectives of the work. Chapter 2 presents the theoretical formulation of the proposed models, while Chapters 3 and 4 review the obtained numerical results. Finally, Chapter 5 outlines the conclusions and the future perspective of the present work. microscopic and macroscopic instabilities due to finite deformations.