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

摘要

This Ph.D thesis treats the propagation of ultrasonic waves in heterogeneous materials.udIt is widely known how waves propagate in homogeneous materials like metallic ones.udBut very few aspects are known about the propagation in the heterogeneous materialsudbecause of the complex behavior regarding these materials. The heterogeneity can beudrelated with the intrinsic properties, elastic and chemical, of each phase of the material,udor with the periodic or random arrangement of inclusions of constant or differentudshapes. Concrete is a common case.udIn this work the first chapter gives a description of the main characteristics ofudheterogeneous materials.. In the second one, the methods of inspection onudheterogeneous materials are presented, and in particular a view on NDT methods isuddiscussed. The Ultrasonic Wave method is one of the more common NDT methods usedudto investigate the inner nature of materials. This method is well established in theudmedical field, but more studies are still needed for materials’ science. With the aim to investigate on heterogeneous materials, concrete is considered. In chapter three theudmain 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 byudpresenting some aspects of micro-structured materials. In chapter four the signaludprocessing is also presented, and an algorithm to process signals on heterogeneousudmaterials is discussed. A prototype machine of ultrasonic test developed during theuddoctoral studies is presented, and in the last part of the fourth chapter FEM analysisudand 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 andudrelated with elastic modules. On hardened concrete, a database of 70 specimen isudcreated. By using a cluster algorithm the Ultrasonic Test is able to predict theudresistance of concrete with an acceptable level of confidence. A theoretical model isudproposed and validated to explain the value of ultrasonic wave velocity propagating inudheterogeneous media, and a model on resonant frequency is discussed.udIn chapter six a multi-scale finite element approach is developed to simulate theudpropagation 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 betweenudnumerical simulations conducted by applying heterogeneous models and the multi-scaleudmesh-less/FEM model shows a good agreement. Conclusions and future developmentsudare given in chapter seven.udThe author is grateful to the University of Palermo and prof. G. Giambanco for theudgreat opportunity to develop in this work.