Dipartimento di Ingegneria Civile - Tesi di Dottorato

Permanent URI for this collectionhttp://localhost:4000/handle/10955/99

Questa collezione raccoglie le Tesi di Dottorato Dipartimento di Ingegneria Civile dell'Università della Calabria.

Browse

Author: search.filters.author.Greco, Fabrizio

Search Results

Now showing 1 - 5 of 5
  • No Thumbnail Available
    Item
    Analysis of fracture phenomena in concrete structures by means of cohesive modeling techniques
    (Università della Calabria, 2021-06-30) De Maio, Umberto; Critelli, Salvatore; Greco, Fabrizio; Nevone Blasi, Paolo
    Still today, the fracture phenomenon in cementitious materi-als is a research topic widely investigated by numerous research-ers in materials and structural engineering, since it involves many theoretical and practical aspects concerning both strength and durability properties of common concrete structures. In-deed, cracking is one of the main causes of the severe deteriora-tion of concrete structures, usually leading to an unacceptable re-duction of their serviceability time. The fracture processes, in-cluding onset, propagation, and coalescence of multiple cracks, arise in the structural members because of the low tensile strength of concrete, which is ultimately related to the existence of voids or undetected defects in the material microstructure.Such cracking processes significantly affect the global mechani-cal behavior of the concrete structures and may facilitate the in-gress of corrosive media; therefore, in the scientific community there is a strong interest in reducing cracks width to a minimum or in preventing cracking altogether. In the technical literature, several simplified numerical models, based on either linear-elas-tic or elastic-plastic fracture mechanics, are proposed to predict the fracture mechanisms during any stage of the lifetime of con-crete structures. However, the application of these models is somehow limited, due to their incapacity to capture the complex inelastic mechanical behavior of reinforced concrete members, involving multiple concrete cracking and steel yielding and their mutual interaction under the combined action of axial and bend-ing loadings. This thesis aims to develop a sophisticated numerical frac-ture model to predict the cracking processes in quasi-brittle ma-terials like concrete, and the main failure mechanisms of the re-inforced concrete structures in a comprehensive manner. The proposed methodology relies on a diffuse interface model (DIM), based on an inter-element cohesive fracture approach, where co-hesive elements are inserted along all the internal mesh bounda-ries to simulate multiple cracks initiation, propagation and coa-lescence in concrete. Such a model, is used in combination with an embedded truss model (ETM) for steel reinforcing bars in the failure analysis of reinforced concrete structures. In particular, truss elements equipped with an elastoplastic constitutive be-havior are suitably connected to the concrete mesh via a bond-slip interface, in order to capture the interaction with the sur-rounding concrete layers as well as with the neighboring propa-gating cracks. The proposed fracture model takes advantage of a novel mi-cromechanics-based calibration technique, developed and pro-posed in this thesis, to control and/or reduce the well-known mesh dependency issues of the diffuse cohesive approach, re-lated to the artificial compliance in the elastic regime. In this way, the initial stiffness parameters of the cohesive element employed in the diffuse interface model are suitably calibrated by means of a rigorous micromechanical approach, based on the concept of representative volume element. In particular, by performing sev-eral micromechanical analyses two charts have been constructed which provide the dimensionless normal and tangential stiffness parameters as functions of both the Poisson’s ratio of the bulk and the admitted reduction in the overall Young’s modulus after the insertion of the cohesive interfaces. The proposed fracture model has been firstly validated by performing numerical analysis in plain concrete elements, and secondly, employed to analyze the failure mechanisms in exter-nally strengthened reinforced concrete beams. In particular, several numerical simulations, involving pre-notched concrete beams subjected to mode-I loading conditions, have been performed to investigate the capability of the diffuse interface model to predict self-similar crack propagation and to assess the mesh-induced artificial toughening effects, also intro-ducing two new fracture models for comparison purpose. More-over, sensitivity analyses with respect to the mesh size and the mesh orientation have been performed to investigate the mesh dependency properties of the proposed fracture model. Further validation of the proposed diffuse interface model has been pro-vided for plain concrete structures subjected to general mixed-mode loading conditions. The role of the mode-II inelastic parameters (i.e. critical tangential stress and mode-II fracture en-ergy) on the nonlinear behavior of the embedded cohesive inter-faces is investigated in a deeper manner. In particular, two sen-sitivity analyses have been performed by independently varying the mode-II inelastic parameters required by the traction-separa-tion law adopted in the proposed concrete fracture model, in or-der to quantify the above-mentioned artificial toughening effects associated with mode-II crack propagation. Moreover, compari-sons with numerical and experimental results, with reference to mode-I and mixed-mode fracture tests, have been reported, highlighting the effectiveness of the adopted diffuse interface model (DIM) in predicting the failure response in a reliable man-ner. Subsequently, the integrated fracture approach is success-fully employed to predict the nonlinear response of (eventually strengthened) reinforced concrete beams subjected to general loading conditions. Firstly, the failure analysis of reinforced con-crete (RC) beams has been performed to assess the capability of the integrated fracture model to capture multiple crack initiation and propagation. Detailed stress analysis of the tensile reinforce-ment bars has been also reported to verify the capability of the embedded truss model (ETM) of capturing the tension stiffening effect. Secondly, the well-known concrete cover separation phe-nomenon has been predicted by performing complete failure simulations of FRP-strengthened RC elements. To this end, a sin-gle interface model (SIM) has been incorporated in the proposed fracture model to capture the mechanical interaction between the concrete element and the externally bonded reinforced system and to predict eventually debonding phenomena in con-crete/FRP plate interface. Suitable comparisons with available experimental results have clearly shown the reliability and the effectiveness (in terms of numerical accuracy) of the adopted fracture approach, especially in the crack pattern prediction. Fi-nally, the proposed integrated numerical model is used to pre-dict the structural response of ultra high-performance fiber-rein-forced concrete (UHPFRC) structures enhanced with embedded nanomaterials. In this case, the cohesive elements are equipped with a mixed-mode traction-separation law suitably calibrated to account for the toughening effect of the nano-reinforcement. The main numerical outcomes, presented in terms of both global structural response and final crack pattern, show the ability of the proposed approach to predict the load-carrying capacity of such structures, as well as to highlight the role of the embedded nano-reinforcement in the crack width control.
  • No Thumbnail Available
    Item
    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.
  • No Thumbnail Available
    Item
    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
  • No Thumbnail Available
    Item
    Multiscale approaches for failure analyses of composite materials
    (2013-11-25) Leonetti, Lorenzo; Olivito, Renato S.; Greco, Fabrizio
    Fiber-reinforced composite materials are being increasingly adopted in place of metallic elements in many structural applications of civil, automotive and aeronautical engineering, owing to their high stiffness-to-weight and strengthto- weight ratios, resistance to environmental deterioration and ability to form complex shapes. However, in many practical situations, composite materials experience different kinds of failure during their manufacturing processes and/or in-services, especially for laminate configurations, where damage phenomena are rather complex, involving both intralaminar mechanisms (e.g. matrix cracking, fiber splitting and interface debonding between fiber and matrix) and interlaminar mechanisms (e.g. delamination between plies). These damage mechanisms, which take place at the microscopic scale in conjunction with eventual contact interaction between crack faces, strongly influence the macroscopic structural behavior of composites, leading to a highly nonlinear post-peak response associated with a gradual loss of stiffness prior to failure. As a consequence, a proper failure analysis of a composite material subjected to such microstructural evolution should require a numerical model able to completely describe all its microscopic details; however fully microscopic models are not pursued in practice due to their large computational cost. To overcome this problem, homogenization techniques have increasingly gained in importance, based on either classical micromechanical or periodic homogenization approaches; if combined each other, these models are able to deal with both periodic and nonperiodic (e.g. random) composite microstructures. According to these approaches, also referred to as sequential multiscale methods, a “one-way” bottom-up coupling is established between the microscopic and macroscopic problems. As a consequence, such methods are efficient in determining the macroscopic behavior of composites in terms of stiffness and strength, but have a limited predictive capability for problems involving damage phenomena. To overcome these limitations, two classes of multiscale methods have been proposed in the literature: computational homogenization schemes and concurrent multilevel approaches. Computational homogenization approaches, also referred to as semiconcurrent approaches, are very efficient in many practical cases, also for only locally periodic composites, especially when implemented in a finite element setting, as in the FE2 method. The key idea of such approaches is to associate a microscopic boundary value problem with each integration point of the macroscopic boundary value problem, after discretizing the underlying microstructure. The macrostrain provides the boundary data for each microscopic problem (macro-to-micro transition or localization step). The set of all microscale problems is then solved and the results are passed back to the macroscopic problem in terms of overall stress field and tangent operator (micro-tomacro transition or homogenization step). Localization and homogenization steps are carried out within an incremental-iterative nested solution scheme, thus the two-scale coupling remains of a weak type. On the other hand, concurrent multiscale methods abandon the concept of scale transition in favor of the concept of scale embedding, according to which models at different scales coexist in adjacent regions of the domain. Such methods can be regarded as falling within the class of domain decomposition methods, since the numerical model describing the composite structure is decomposed into fine- and coarse-scale submodels, which are simultaneously solved, thus establishing a strong “two-way” coupling between different resolutions. The present thesis aims to develop a novel multiscale computational strategy for performing complete failure analyses of composite materials starting from crack initiation events, which usually occur at the microscopic level, up to the formation of macroscopic cracks, subjected to propagation and coalescence phenomena. To this end, two alternative models have been proposed, belonging to the classes of semiconcurrent and concurrent multiscale models, respectively. Firstly, a novel computational homogenization scheme is described, able to perform macroscopic failure analyses of fiber-reinforced composites incorporating the microstructural evolution effects due to crack initiation and subsequent crack propagation process. A two-scale approach is used, in which coupling between the two scales is obtained by using a unit cell model with evolving microstructure due to mixed-mode crack initiation and propagation at fiber/matrix interface. The method allows local failure quantities (fiber/matrix interfacial stresses, energy release and mode mixity for an interface crack) to be accurately obtained in an arbitrary cell from the results of the macroscale analysis, and, consequently, crack initiation and propagation at fiber/matrix interface to be predicted. Crack initiation at fiber/matrix interface is simulated by using a coupled stress and energy failure criterion, whereas crack propagation is analyzed by means of a mode mixity dependent fracture criterion taking advantage of a generalization of the J-integral technique in conjunction with a component separation method for computing energy release rate and mode mixity. The evolving homogenized constitutive response of the composite solid is determined in the context of deformation-driven microstructures, based on the crack length control scheme able to deal with unstable branches of the equilibrium path, such as snap-back and snap-through events; moreover, the micro-to-macro transition is performed by adopting periodic boundary conditions, based on the assumed local periodicity of the composite. The second approach proposed in this thesis consists in a novel concurrent multiscale model able to perform complete failure analyses of fiber-reinforced composite materials, by using a non-overlapping domain decomposition method in a finite element tearing and interconnecting (FETI) framework in combination with an adaptive strategy able to continuously update the finescale subdomain around a propagating macroscopic crack. The continuity at the micro-macro interface, characterized by nonmatching meshes, is enforced by means of Lagrange multipliers. When modeling fracture phenomena in composites, the competition between fiber/matrix interface debonding and kinking phenomena from and towards the matrix is accounted for, whereas continuous matrix cracking is described by using a shape optimization strategy, based on a novel moving mesh approach. A key point of the proposed approach is adaptivity, introduced into the numerical model by a heuristic zoom-in criterion, allowing to push the micro-macro interface far enough to avoid the strong influence of spurious effects due to interface nonmatching meshes on the structural response. It is worth noting that this heuristic zoomin criterion is uniquely based on geometric considerations. Numerical calculations have been performed by using both the proposed approaches, with reference to complete failure analyses of fiber-reinforced composite structures subjected to different global boundary conditions, involving both uniform and non-uniform macroscopic gradients. The validity of the proposed multiscale models has been assessed by comparing such numerical results with those obtained by means of a direct numerical simulation, considered as a reference solution. Numerical results have shown a good accuracy, especially for the proposed concurrent multiscale approach; moreover, this model has been proved to be more suitable for handling problems involving damage percolation in large composite structures and, at the same time, managing boundary layer effects
  • No Thumbnail Available
    Item
    Risposta macroscopica di materiali compositi in presenza di fenomeni di microfrattura e contatto
    (2007) Nevone Blasi, Paolo; Bruno, Domenico; Greco, Fabrizio
    Università della Calabria