ULTRASONIC WAVES IN HETEROGENEOUS MATERIALS - MECHANICAL CHARACTERIZATION OF HETEROGENEOUS MATERIALS BY MEANS OF ULTRASONIC TESTS AND NUMERICAL MODELS: THE CONCRETE THE CONCRETE

This Ph.D thesis treats the propagation of ultrasonic waves in heterogeneous materials. It is widely known how waves propagate in homogeneous materials like metallic ones. But very few aspects are known about the propagation in the heterogeneous materials because of the complex behavior regarding these materials. The heterogeneity can be related with the intrinsic properties, elastic and chemical, of each phase of the material, or with the periodic or random arrangement of inclusions of constant or different shapes. Concrete is a common case. In this work the first chapter gives a description of the main characteristics of heterogeneous materials.. In the second one, the methods of inspection on heterogeneous materials are presented, and in particular a view on NDT methods is discussed. The Ultrasonic Wave method is one of the more common NDT methods used to investigate the inner nature of materials. This method is well established in the medical field, but more studies are still needed for materials’ science. With the aim to investigate on heterogeneous materials, concrete is considered. In chapter three the main characteristics of concrete are presented: the chapter discusses about the main constituents, namely aggregates, cements, water and additives. Fresh and hardened phases of concrete are also described. Chapter four presents the theoretical aspect related with the propagation of wave in homogeneous and heterogeneous continuum by presenting some aspects of micro-structured materials. In chapter four the signal processing is also presented, and an algorithm to process signals on heterogeneous materials is discussed. A prototype machine of ultrasonic test developed during the doctoral studies is presented, and in the last part of the fourth chapter FEM analysis and laboratory tests are discussed to validate the right working of the instrument Chapter five presents the results of laboratory tests conducted at DICAM department fresh and hardened concrete, and the results of the investigation by mean of ultrasonic tests. Curve of velocity of UW in concrete during the curing time are determined and related with elastic modules. On hardened concrete, a database of 70 specimen is created. By using a cluster algorithm the Ultrasonic Test is able to predict the resistance of concrete with an acceptable level of confidence. A theoretical model is proposed and validated to explain the value of ultrasonic wave velocity propagating in heterogeneous media, and a model on resonant frequency is discussed. In chapter six a multi-scale finite element approach is developed to simulate the propagation of waves in heterogeneous materials. The model is based on the definition of an elementary cell(representative volume element or RVE) whose equilibrium equation are written based on a mesh-less approach. Follows a procedure that transforms the heterogeneous material in an anisotropic one. The comparison between numerical simulations conducted by applying heterogeneous models and the multi-scale mesh-less/FEM model shows a good agreement. Conclusions and future developments are given in chapter seven. The author is grateful to the University of Palermo and prof. G. Giambanco for the great opportunity to develop in this work.