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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Author: search.filters.author.Bruno, DomenicoStart date: 2006

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    Analisi del comportamento non-lineare dei materiali compositi con microstruttura periodica
    (2009) Sgambittera, Girolamo; Olivito, Renato Sante; Bruno, Domenico; Greco, Fabrizio
    In 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.
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    Modelling of edge debonding in beams strengthened with composite meterials
    (2016-01-28) Lo Feudo, Stefania; Bruno, Domenico; Olivito, Renato Sante; Greco, Fabrizio; Blasi, Paolo Nevone
    L'oggetto principale della presente tesi di dottorato, è costituito dallo studio dei fenomeni di scollamento d'interfaccia in sistemi di rinforzo composti da elementi strutturali rinforzati da piastre in materiale composito brorinforzato (FRP). L'argomento è inizialmente introdotto in termini generali attraverso un'attenta ricerca bibliogra ca, concentrata sulla de nizione delle principali proprietà dei materiali compositi e sulla loro modellazione. Un'innovativa formulazione multistrato è poi presentata e adattata al caso oggetto di studio, e un criterio di frattura accoppiato è esteso al caso di delaminazione in presenza di condizioni di carico di modo misto. Il sistema strutturale considerato è quindi costituito da tre componenti sici, ossia la trave, lo strato di adesivo e la piastra incollata esternamente, ciascuno dei quali è modellato attraverso uno o più strati deformabili a taglio. Il problema è considerato in primo luogo da un punto di vista analitico, attraverso la formulazione delle equazioni governanti il problema nel caso in cui ad ogni componente sico corrisponde un solo strato matematico. La formulazione multistrato è poi implementata numericamente, utilizzando degli elementi niti (FE) multivariabili monodimensionali. In particolare, per modellare le interfacce tra gli strati sici e matematici sono considerate sia delle equazioni costitutive di interfaccia forte che debole. Le tensioni interfacciali e le energie di frattura sono quindi calcolate, ottenendo un'accettabile corrispondenza con i risultati di un modello continuo FE e riducendo di molto gli oneri computazionali. L'innesco dello scollamento è poi valutato grazie all'innovativo criterio di frattura di modo misto, il quale permette di prendere in considerazione sia le tensioni interfacciali che l'energia di frattura, consentendo allo stesso tempo di studiare di erenti posizioni dello scollamento lungo lo spessore dell'adesivo. La propagazione del danno è quindi studiata utilizzando un criterio classico di frattura in modo misto. Uno studio parametrico, condotto al variare dei parametri critici dell'interfaccia quali la tenacità e la resistenza, ha in ne permesso di valutare l'in uenza di tali proprietà sul fenomeno dello scollamento. Gli studi condotti hanno evidenziato che la tecnica di modellazione proposta permette sia di modellare tali sistemi di rinforzo, sia di predire lo scollamento d'estremità. Inoltre, nonostante emerga che l'accuratezza della soluzione può essere migliorata aumentando il numero di strati matematici e adottando delle interfacce miste forti/deboli, è possibile concludere che l'utilizzo di pochi strati nella modellazione di ogni componente sico permette di predire lo scollamento con ragionevole precisione
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    Uno studio sul comportamento statico non-lineare dei ponti di grande luce
    (2012-11-29) Bianchi, Elisabetta; Olivito, Renato S.; Bruno, Domenico; Blasi, Paolo Nevone
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    Simplified Methods for Dynamic Analysis of Structures under Blast Loading
    (2007) Campidelli, Manuel; Viola, Erasmo; Bruno, Domenico
    The 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.
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    Analisi teorico-sperimentale di pareti murarie caricate fuori dal piano
    (2006-11-20) Zuccarello, Francesca Anna; Bruno, Domenico; Olivito, Renato S.
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    Ultrasonic Guided Waves for Structural Health Monitoring and Application to Rail Inspection Prototype for the Federal Railroad Administration
    (2007-11) Coccia, Stefano; Viola, Erasmo; Lanza di Scalea, Francesco; Bruno, Domenico
    Recent train accidents, associated direct and indirect costs, as well as safety concerns, have reaffirmed the need for developing rail defect detection systems more effective than those used today. Current methods for detecting internal flaws in rails rely primarily on ultrasonic pulse-echo technology operated in a water-filled wheel or sled. Presence and loss of echoes along each tested direction are analyzed in parallel to map surface and internal cracks in the rail. While this technology has served the industry well, several inherent weaknesses exist. Contact heads and cross-sectional inspection limit the speed of the measurement; liquid couplant is required to maintain efficient wave/echo transmission through the contact patch. Despite the acoustic couplant, significant transmission loss results from the pulse/echo passing through the contact patch twice. More importantly, ultrasonic beams launched vertically from the top of the rail head can miss internal defects located under horizontal shelling; this was the case, in the disastrous train derailment at Superior, WI in 1991. As a proposal to address these issues, the use of ultrasonic guide waves appears promising. One objective of this work is extending the fundamental knowledge of the guided v wave propagation in rails by predicting modal and forced solutions in the high frequency (<500 kHz) range. The selection of guided wave features sensitive to the presence of the different type of defects is essential for a successful defect detection performance. Another accomplishment of this work is the development of a rail defect detection prototype based on a laser/air-coupled ultrasonic technique. The prototype has been successfully tested twice in the field for the Federal Railroad Administration in the United States of America and it shows promise for implementation in rail inspection cars.
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    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, Pierre
    For 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].
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    La meccanica della frattura nelle applicazioni ai calcestruzzi fibrorinforzati
    (2014-06-13) Colosimo, Emanuela; Ombres, Luciano; Bruno, Domenico
    The advent of modern computing technology over the last 20 years has permitted to the researchers and engineers to solve very difficult structural problems using sophisticated techniques. The use of finite element method (FEM) in the analysis and design of reinforced concrete structures is one of these various successful techniques. As a matter of fact, the FEM is a powerful computational tool which can be used to simulate the response of structures, structural components and materials when submitted to a specified load. In particular its ability to capture the nonlinear behaviour of RC has made the finite element method a very powerful tool in understanding the behaviour of structures and to quantify its load carrying capacity, stress distribution and cracking path. The fiber reinforced concrete (FRC) have been introduced recently in buildings materials to improve the durability of conventional RC structures. Actually, it is well known that the concrete is a relatively brittle material, that could not support tensile strength. As a consequence, concrete member reinforced with continuous reinforcing bars were used to withstand tensile stresses but they don’t provide a concrete with homogeneous tensile properties. The additional of steel reinforcement significantly increases the strength of concrete but without producing an homogeneous material due to the developments of microcracks and voids in the body. The introduction of fibres randomly distributed throughout the concrete can overcome cracks more effectively. The formation of cracks is undoubtedly one of the most important non linear phenomena which govern the behaviour of concrete structures. Ever since the finite element method has been applied to concrete, the formation of cracks has received much attention. The nonlinear fracture mechanics theory has been used to simulate the quasi-brittle fracture of concrete. The discrete and the smeared crack models are the most used in the literature to model the concrete fracture. The first is especially suitable to simulate the failure in concrete structures where fracture is governed by the occurrence of a small number of cracks with a path that can be predicted. The second crack approach is more appropriate than the first to simulate fracture in concrete structures with a reinforcement ratio that ensures crack stabilization. The main purpose of this research is to analyze short fibres in structural concrete; in particular, according to a nonlinear fracture mechanics approach based on smeared crack models, comparisons in terms of fiber amount between concrete containing no fibres and concrete with fibres are proposed. Starting from an investigation in the fracture mechanics frameworks (LEFM and NLFM), and on fiber reinforced concrete concepts, the work presents the results based on finite methodology, performed consistently with the distributed-crack concept and implemented Diana FE code. In particular the results of simulation of four point bending tests on polymeric and steel fiber reinforced concrete beams were presented in this work of thesis. A 2D plane stress model for analysing the development of cracks is employed; this approach, named rotating crack model, within the smeared crack concept, is based on the total strain crack model. The total strain crack model describes the tensile and compressive behaviour of concrete beams with one stress-strain relationship. With the use of Diana FE program, a standard nonlinear, incremental, iterative approach is performed. The employed theory of the smeared crack concept describes the deterioration of simple concrete and of fiber reinforced concrete characterized by the tensile strength , energy release rate , crack band width , and the shape of the stress-strain curve of the material in the crack band during softening. The smeared crack models are modelled based on concepts using linear and exponential tension-softening constitutive law. tffGh The cracks are defined in the integration points of the elements, i.e., discrete points to compute the elemental mechanical behaviour, are numerically simulated by an adjustment of the compliance matrix at the integration point level and then are modelled according to the rotating model in which the direction of the crack may change during the loading process. The loading process is considered as a sequence of quasi-static loading steps (increments). The theory of linearity is assumed until the maximum local principal stress reached the strength limit of the material, whereafter initiation of mode I cracking in the plane normal to the maximum principal stress will occur. The model is able to simulate multiple crack propagation predicted cracking processes as well as distributed crack pattern, in agreement with experimental observations. Moreover, load-deflection curves are accurately predicted and as well as these corresponding to a linear tension softening assumption of the model. The results show how the proposed approach predicts accurately the maximum loads for the two different class of beam employed as well as it is able to make reasonably good predictions of load and displacement throughout the bending tests. The advantages provided by the short fibers to the properties of concrete improves mainly its post-cracking behaviour (ductility,cracking control and performance under dynamic loading) and can also alter tensile strength. These advantages vary according to the type and volume of fiber added to the matrix. The characteristics of the concrete are strictly dependent from the amount of fiber volume fraction. Fibres have the ability to prevent crack formation and to increase the ultimate load capacity of the concrete structures. KEYWORDS : fracture mechanics, smeared crack models, tension-softening; fiber reinforced concrete, steel fibres, polymeric fibres; four point bending test.
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    Impiego strutturale dei calcestruzzi fibro rinforzati modellazioni teoriche e verifiche sperimentali
    (2014-06-04) Rizzuti, Lidia; Spadea, Giuseppe; Bencardino, Francesco; Bruno, Domenico
    Recently researches, in the field of civil engineering, have been addressed to study high performance concrete materials. In particular Fiber Reinforced Concrete (FRC) materials are very promising. With the addition of fibers into the concrete matrix, a brittle material like concrete can be modified towards a composite material with some ductility properties, able to absorb notable impact energy due to the dynamic or cyclic actions, like the seismic action, in the “during and post” cracking stage. Although, several studies were developed further research is needed to investigate some topics. The present work deals with the study of FRC materials used for structural applications with reference to the material and structural behaviour. Initially, the compressive and the tensile behaviour of the FRC material were analyzed by experimental, theoretical and numerical approaches. Specifically, an experimental investigation and a numerical analysis were carried out with the aim to identify the parameters like fiber types and contents, that are able to improve the performance of FRC material with reference to the structural behaviour. The implications on the use of fibers added into the concrete matrix on the workability at the fresh state and on the toughness at the hardened state were also considered. The aim of the experimental investigation was to analyze and compare the mechanical and the fracture properties of FRC materials by varying some main parameters like matrix compressive strength, fiber types (steel/polypropylene), fiber volume content (Vf) and the length of steel fibers (Lf). Through the numerical analysis it was possible to investigate the influence of high steel fiber content on the tensile post-peak behaviour. The numerical results obtained were compared with the experimental ones and the reliability of the numerical procedure was checked. A comparative study was carried out between experimental and theoretical stress-strain relationships available in literature, with particular reference to compressive behaviour of FRC. The aim was to evaluate the reliability of the proposed models and their range of validity. Several experimental data available in literature were analyzed and collected in a database. The above database was integrated with further experimental results obtained in the Laboratory of the Department of Structural Engineering at the University of Calabria. At the same time, the analytical uniaxial stress-strain relationship available in literature were analyzed and collected. Each theoretical model was critically analyzed and the reliability was checked through a comparison with the experimental curves of the same author and other available in literature. Subsequently, the structural behaviour was studied. With reference to the behaviour of eccentrically loaded FRC columns interaction diagrams axial load (N) – bending moment (M) of the cross section were computed to highlight the role of some main parameters on the strength. The theoretical models proposed in the Italian guideline CNR-DT 204 (2006) and other models available in literature were used in the analysis. The reliability of the theoretical models to describe the real behaviour of fiber reinforced concrete elements was checked through a comparison with some experimental data on steel fiber reinforced concrete columns subjected to eccentric loads available in literature. With reference to a rectangular FRC cross section, symmetrically reinforced, interactions diagrams, using the relationships proposed in the CNR-DT 204 (2006), were computed by varying some parameters like fiber geometrical properties and contents. The aim was to provide interaction diagrams which can be useful to design/check the strength of the concrete members reinforced with fibers and traditional steel reinforcement. Keywords: Experimental investigation; Fiber reinforced concrete; Post-peak behaviour; Eccentric loads; Stress-strain relationships.
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    Deformabilità dei ponti di grande luce soggetti all'azione di carichi viaggianti
    (2014-05-26) Cavallaro, Daniele V.; Bruno, Domenico