Dipartimento di Ingegneria Civile - Tesi di Dottorato
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Questa collezione raccoglie le Tesi di Dottorato Dipartimento di Ingegneria Civile dell'Università della Calabria.
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Item Analisi del comportamento non-lineare dei materiali compositi con microstruttura periodica(2009) Sgambittera, Girolamo; Olivito, Renato Sante; Bruno, Domenico; Greco, FabrizioIn the present thesis the macroscopic non-linear behavior of composite materials with a periodic and heterogeneous microstructure is studied. There are many different kinds of phenomena that produce non-linear effects in composite materials, for example intralaminar damage, delamination and microbucking in fiber reinforced composite or micro-cracking in cellular materials. In this work attention is devoted to the mechanical modeling of nonlinear phenomena associated to the presence of micro-cracks in the context of linear elasticity and of microscopic instabilities in the framework of the finite strain theory. Applications have been developed with reference to microstructures of cellular type and with embedded inclusions. The thesis is structured according to the following chapters: -In the first chapter the fundamental concepts of the finite strains theory are recalled. The constitutive relations associated to a class of conjugate stress-strain pairs are introduced. The basic expressions of the incremental constitutive laws are shown with special reference to incrementally linear constitutive laws. Finally the stability and the uniqueness of the equilibrium solution are analyzed. -In the second chapter, after an introduction about the homogenization techniques, the micro and macro stability phenomena occurring in composite materials with a periodic microstructure are studied from a theoretical point of view in the context of the finite strains theory. The formulation starts from a variational formulation of the problem. Novel macroscopic measures of micro-structural stability are introduced corresponding to the positive definiteness of the homogenized moduli tensors relative to a class of conjugate stress-strain pairs and their effectiveness to obtain a conservative prediction of the microscopic primary instability load is pointed out. Analysis of these stability phenomena plays a fundamental role because often the collapse of composite materials with periodic microstructure is related to microstructural instabilities. In addition the microscopic stability analysis establishes the region of validity of the standard homogenization procedure based on the unit cell procedure. -In the third chapter, in the context of the small strains theory, non-linear phenomena are presented with reference to composite materials with a porous microstructure containing micro-cracks spreading from the voids. The fundamental techniques of homogenization are applied in conjunction with fracture mechanics theory and interface models. The energy release rate is evaluated through the J-integral technique. -In the fourth chapter some numerical applications carried out by means of a one-way coupled finite element code, are proposed. In the first section the numerical results will be introduced with reference to the theoretical aspects developed in the second chapter. Numerical analyses are addressed to composite materials with a periodic microstructure, namely a porous microstructure and a particle-reinforced microstructure. The adopted constitutive law is hyperelastic. Periodic boundary conditions will be used for the microstructure, and uniaxial and equibiaxial loading conditions are considered. Numerical analyses are able to show the exact region of microscopic stability, obtained by taking into account all the microstructural details, and the region of macroscopic stability, determinate by studying homogenized material properties. To elaborate macroscopic criteria able to give a conservative prediction of the microstructural stability, different measures of macroscopic instability are introduced with reference to work conjugate strain-stress measures. In the second section of this chapter a numerical analyses with reference to the micromechanical model proposed in the third chapter is developed. In this case the microstructure adopted for the composite materials is a cellular microstructure in which there is the presence of two micro-cracks advancing symmetrically from the void. The microstructure is subjected to three different boundary conditions namely respectively: linear displacements, periodic fluctuations and antiperiodic tractions and uniform tractions. The objective of this section is to verify the validity of the homogenization technique in the prediction of micro-crack evolution phenomena, for composites with locally periodic microstructure. The energy release rate obtained through the micromechanical model will be compared with a 2D composite structure composed by a regular arrangement of 5x5 unit cells. The composite structure is subjected to two different boundary conditions: the former is associated with the absence of contact between the surfaces of the micro-cracks, on the contrary in the latter case there is the presence of the contact. This type of comparison allows to investigate the accuracy of the proposed procedure in presence of macroscopic tension and strain gradients.Item Simplified Methods for Dynamic Analysis of Structures under Blast Loading(2007) Campidelli, Manuel; Viola, Erasmo; Bruno, DomenicoThe increasing threat of extremely severe loading conditions caused by a number of explosive sources made engineers and scientists developing, during the last half century, several methods of analysis and design of blast–resistant structures. Simple, intermediate, and advanced computational approaches have been adopted, requiring increasing computational resources. These efforts led to the publication of several manuals and guidelines for the analysis and design of blast–resistant reinforced concrete and steel structures, mostly based on simple considerations derived from Single Degree of Freedom (SDOF) models. Although the development of future guidelines based on advanced numerical techniques is desirable, typical design activities cannot be effectively carried out by applying complex methods, because of their large demand of resources. Therefore the necessity to develop simplified, low time consuming, methods of analysis, capable of supporting a daily design activity and, at the same time, takeing into account issues usually neglected, such as a strong non linear material behavior and the influence of the strain rate caused by a blast load on the structural response. The development of such design tools is the object of this study. The first part of this thesis deals with the influence of the blast load shape on the dynamic response of an undamped linear elastic oscillator. Response spectrum and pressure–impulse diagrams are shown for several shape parameters, and a sensitivity analysis of the results with respect to the computational parameters is also presented. A method validation is carried out via genetic algorithms, through a careful calibration of all the genetic parameters, such as crossover fraction and number of elite elements. Non linear material modeling and strain rate dependent constitutive laws are objects of the second part of this dissertation. A non linear oscillator made of displacement, velocity, and acceleration dependent springs and dampers, under an arbitrary dynamic load, is proposed. Spring and damper constitutive laws have no restrictions as well as the load–time function, and the dynamic analysis is accomplished by a piecewise linear approximation of any input function. Numerical problems are dealt with by applying the Newton–Raphson method, in such a way that enables the error range to be estabiv lished “a priori”. Any possible drawback of this method is carefully avoided, and a quadratic speed of convergence is always ensured. Since the model provides velocity dependent springs, strain rate effects of blast loads on the structural response are taken into account by including strain rate dependent constitutive laws within the problem definition.Item Modellazione e analisi non lineare di travi composte acciaio-calcestruzzo(2007-11-17) Mercurio, Francesco Antonio; Aristodemo, Maurizio; Vulcano, AlfonsoItem Analisi teorico-sperimentale di pareti murarie caricate fuori dal piano(2006-11-20) Zuccarello, Francesca Anna; Bruno, Domenico; Olivito, Renato S.Item Problemi inversi nella meccanica del danneggiamento(2007-11-11) Donato, Giuseppe; Aristodemo, Maurizio; Zinno, RaffaeleItem Modelling the lateral pedestrian force on rigid and moving floors by a self-sustained oscillator(2009-11-23) Trovato, Andrea; Olivito, Renato S.; Bruno, Domenico; Argoul, PierreFor the serviceability analysis of civil engineering structures under human induced vibrations, a correct modelling of the pedestrian-structure interaction is needed. The proposed approach consists in thinking the human body as a Single Degree of Freedom oscillator: the force transmitted to the floor is the restoring force of this oscillator [1, 2]. In rigid floor conditions, such an oscillator must be able to reproduce two experimentally observed phenomena: (i) the time-history of lateral force can be approximated by a periodic signal with a “natural” frequency related with the single pedestrian characteristics; (ii) the motion of a pedestrian is self-sustained, in the sense that the pedestrian produces by itself the energy needed to walk. Accounting for these aspects, a modified Van der Pol (MVdP) oscillator is proposed here to represent the lateral pedestrian force. The suitable form of its nonlinear restoring force is inferred from experimental data concerning a sample of twelve pedestrians. The experimental and model lateral forces show an excellent agreement. For a laterally moving floor, the MVdP oscillator representing a pedestrian becomes non-autonomous. It is well-known that self-sustained oscillators in the non-autonomous regime are characterized by the so-called entrainment phenomenon. It means that under certain conditions, the vibration frequency switches from the ”natural” value to that of the external force: the response frequency is entrained by the excitation frequency. According to the physical interpretation considered here, the entrainment corresponds to the situation where the pedestrian changes its natural walking frequency and synchronizes with the floor oscillation frequency. The steady response of the MVdP oscillator subjected to a harmonic excitation is discussed in terms of non-dimensional amplitude response curves, obtained using the harmonic balance method truncated at the first harmonic. The model predictions are compared with some experimental results concerning pedestrians available in the literature and a good agreement is obtained. These topics are detailed in this thesis and also in the companion papers [3, 4] and in the report [5].Item Modellazione e analisi non lineare di pareti strutturali in calcestruzzo armato(2014-06-03) Iaccino, Rosamaria; Aristodemo, Maurizio; Vulcano, AlfonsoItem Risposta macroscopica di materiali compositi in presenza di fenomeni di microfrattura e contatto(2007) Nevone Blasi, Paolo; Bruno, Domenico; Greco, FabrizioUniversità della Calabria