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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Analysis of nonlinear phenomena in heterogeneous materials by means of homogenization and multiscale techniques
(Università della Calabria, 2020-06-07) Pranno, Andrea; Critelli, Salvatore; Bruno, Domenico; Greco, Fabrizio
Over the past decade, scientific and industrial communities have shared their expertise to improve mechanical and structural design favoring the exploration and development of new technologies, materials and ad-vanced modeling methods with the aim to design structures with the highest structural performances. The most promising materials used in many advanced engineering applications are fiber- or particle-rein-forced composite materials. Specifically, materials with periodically or randomly distributed inclusions embedded in a soft matrix offer excel-lent mechanical properties with respect to traditional materials (for in-stance, the capability to undergo large deformations). Recent applica-tions of these innovative materials are advanced reinforced materials in the tire industry, nanostructured materials, high-performance structural components, advanced additive manufactured materials in the form of bio-inspired, functional or metamaterials, artificial muscles, tunable vi-bration dampers, magnetic actuators, energy-harvesting devices when these materials exhibit magneto- or electro-mechanical properties. To-day the scientific community recognizes that, to develop new advanced materials capable of satisfying increasingly restrictive criteria, it is vital fully understanding the relationship between the macroscopic behavior of a material, and its microstructure. Composite materials are charac-terized by complex microstructures and they are commonly subjected also to complex loadings, therefore their macroscopic response can be evaluated by adopting advanced strategies of micro-macro bridging, such as numerical homogenization and multiscale techniques. The aim of this thesis is to provide theoretical and numerical methods able to model the mechanical response of heterogeneous materials (fiber- or particle-reinforced composite materials) in a large deformation context predicting the failure in terms of loss of stability considering also the interaction between microfractures and contact. In the past literature, several theories have been proposed on this topic, but they are preva-lently limited to the analysis of microscopic and macroscopic instabili-ties for not damaged microstructures, whereas the problem of interac-tion between different microscopic failure modes in composite materi-als subjected to large deformations in a multiscale context still has not been investigated in-depth and it represents the main aspect of novelty of the present thesis. The thesis starts with a literature review on the previously announced topic. Then, the basic hypotheses of the numerical homogenization strategy are given together with a review of the most recurring mul-tiscale strategies in the modeling of the behavior of advanced composite materials following a classification based on the type of coupling be-tween the microscopic and the macroscopic levels. In addition, a theo-retical non-linear analysis of the homogenized response of periodic composite solids subjected to macroscopically uniform strains is given by including the effects of instabilities occurring at microscopic levels and the interaction between microfractures and buckling instabilities. Subsequently, the numerical results obtained were reported and dis-cussed. Firstly, the interaction between microfractures and buckling instabili-ties in unidirectional fiber-reinforced composite materials was investi-gated by means of the nonlinear homogenization theory. In such mate-rials, the investigated interaction may lead to a strong decrease in the compressive strength of the composite material because buckling causes a large increase in energy release rate at the tips of preexisting cracks favoring crack propagation or interface debonding. Thus, mi-crocracked composite materials characterized by hyperelastic constitu-ents and subjected to macrostrain-driven loading paths were firstly in-vestigated giving a theoretical formulation of instability and bifurcation phenomena. A quasi-static finite-strain continuum rate approach in a variational setting has been developed including contact and frictionless sliding effects. It worth noting that, the above developments show that non-standard self-contact terms must be included in the analysis for an accurate prediction of microscopic failure; these terms are usually ne-glected when contact is modelled in the framework of cohesive inter-face constitutive laws. The influence of the above-mentioned non-standard contributions on the instability and bifurcation critical loads in defected fiber-reinforced composites can be estimated in light of the results which will be presented in this thesis. Thus, the role of non-standard crack self-contact rate contributions to the stability and non-bifurcation conditions was pointed out by means of comparisons with simplified formulations and it was clearly shown that these contribu-tions have a notable role in an accurate prediction of the real failure behavior of the composite solid. Secondly, two multiscale modeling strategies have been adopted to an-alyze the microstructural instability in locally periodic fiber-reinforced composite materials subjected to general loading conditions in a large deformation context. The first strategy is a semiconcurrent multiscale method consisting in the derivation of the macroscopic constitutive re-sponse of the composite structure together with a microscopic stability analysis through a two-way computational homogenization scheme. The second approach is a novel hybrid hierarchical/concurrent mul-tiscale approach able to combine the advantages inherent in the use of hierarchical and concurrent approaches and based on a two-level do-main decomposition; such a method allows to replace the computation-ally onerous procedure of extracting the homogenized constitutive law for each time step through solving a BVP in each Gauss point by means of a macro-stress/macro-strain database obtained in a pre-processed step. The viability and accuracy of the proposed multiscale approaches in the context of the microscopic stability analysis in defected compo-site materials have been appropriately evaluated through comparisons with reference direct numerical simulations, by which the ability of the second approach in capturing the exact critical load factor and the boundary layer effects has been highlighted. Finally, the novel hybrid multiscale strategy has been implemented also to predict the mechanical behavior of nacre-like composite material in a large deformation context with the purpose to design a human body protective bio-inspired material. Therefore, varying the main micro-structural geometrical parameters (platelets aspect ratio and stiff-phase volume fraction), a comprehensive parametric analysis was performed analyzing the penetration resistance and flexibility by means of an in-dentation test and a three-point bending test, respectively. A material performance metric, incorporating the performance requirements of penetration resistance and flexibility in one parameter and called pro-tecto-flexibility, was defined to investigate the role of microstructural parameters in an integrated measure. The results have been revealed that advantageous microstructured configurations can be used for the design and further optimization of the nacre-like composite material.
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
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
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.
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.
Deformabilità dei ponti di grande luce soggetti all'azione di carichi viaggianti
(2014-05-26) Cavallaro, Daniele V.; Bruno, Domenico

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