Εντοπισμός και επιδιόρθωση βλάβης σε δομικά στοιχεία με τη χρήση ευφυών υλικών

Abstract

The research presented in this thesis develops a new methodology for the structural health monitoring of structures. The methodology is based on the use and the application of smart materials and specifically the piezoelectric materials (lead zirconate titanate PZT). These materials are installed and applied in the concrete members of a structure. Therefore, the specific characteristics of the PZTs in combination with their interaction with the structure are used in order to develop the new health monitoring technique. Physical changes in the structure cause changes in the structural mechanical impedance. Due to electromechanical coupling between the piezoelectric material and the structure, the change in structural mechanical impedance induces a change in the electrical impedance of the piezoelectric material. The proposed methodology is based on the interaction of PZT with the structure according to the Electro-Mechanical Impedance EMI or its inverse Electro-Mechanical Admittance EMA. The methodology uses the electromechanical data in frequency response with statistical techniques to damage detection. This includes two stages, in first stage the detection of damage, and if provided that this exists, in second stage the location and level of the damage. Since the widespreadest building material in the existing structures is concrete, the examined numerical application of the proposed methodology is a concrete structural element. The well-known strengthening technique of deficient concrete members using epoxy bonded Fibre Reinforced Polymer (FRP) materials is used herein in order to examine the application of smart materials and the proposed methodology in a FRP concrete component. These composite materials have experienced a continuous increase of use in structural strengthening and repair applications around the world in the last decade. High stiffness-to-weight and strength-to-weight ratios of these materials combined with their superior environmental durability have made them a competing alternative to the conventional strengthening and repair materials. From a structural mechanics point of view, an important concern regarding the effectiveness and safety of this method is the potential of brittle and premature failures due to the debonding of the FRP materials. Such failures, unless adequately considered in the design process, may significantly decrease the effectiveness of the strengthening. In this thesis, a new method that prevents FRP debonding using smart piezoelectric materials is investigated. The technique consists of the installation of system of piezoelectric (sensor - actuator) at the edge of the epoxy bonded FRP to the concrete member. The application of this system in combination with a specific optimisation method that is based on the EMA response of PZT, can produce controlled local forces and moments with opposite effect to that of a harmonic load at the middle of the beam. These forces are capable to reverse and to compensate the imminent debonding of the edge of the FRP which are based on a new repair criterion. Using the proposed method, the control of the FRP debonding in a concrete member under dynamic loading is achieved. Further, a special criterion for the “active” repair of the continuous contact between FRP and concrete is proposed. Thus, the application method of a “smart and readjusted FRP” for the repair of FRP concrete components under dynamic load is developed. Achieve The effectiveness of the proposed method is analytically investigated using the finite element program COMSOL (former FEMLAB) for the simulation of the smart structural system. The statistical process of the EMA response data and the development of the proposed optimisation methods are achieved using MATLAB environment developed for the purposes of this study.