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.Critelli, SalvatoreStart date: 2020

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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.
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    La valutazione della vulnerabilità sismica degli edifici storici in muratura mediante diversi approcci
    (Università della Calabria, 2020-04-16) Porzio, Saverio; Critelli, Salvatore; Oliverio, Renato Sante
    Le costruzioni in muratura rappresentano gran parte del tessuto costruito e la loro salvaguardia riveste un ruolo sociale e culturale primario. Basti pensare che molti di questi edifici – quali chiese, palazzi, castelli, torri – si pongono come simboli delle città in cui riconoscersi e riconoscere le città stesse. L’interesse di studiosi e ricercatori è, dunque, rivolto alla definizione di strumenti utili alla valutazione della vulnerabilità sismica delle costruzioni storiche in muratura. Vari metodi sono attualmente in uso per la valutazione sismica dei manufatti murari, così come diversificate sono le strategie per simulare il comportamento meccanico dei materiali costituenti. Ai consolidati metodi grafici per la valutazione della sicurezza statica degli archi, volte e cupole, si sono aggiunti nuovi modelli di analisi favoriti dall’introduzione del calcolo numerico. Questo lavoro di tesi mira a valutare il comportamento delle costruzioni storiche in muratura attraverso alcuni dei diversi approcci attualmente impiegati e convalidati dalla comunità scientifica. Gli studi eseguiti partono dall’analisi di alcuni degli elementi costitutivi maggiormente rappresentativi in un edificio, quali volte e pareti, per proseguire con analisi globali attuate con differenti strategie di modellazione. Relativamente alle analisi locali, le indagini sulle volte composte – vale a dire quelle originate dall’intersezione di due volte a botte – sono state svolte in termini statici applicando le teorie dell’analisi membranale, mentre per le pareti murarie si è valutata la loro risposta nei confronti delle azioni fuori dal piano al fine di evidenziarne il contributo nella risposta sismica d’insieme del fabbricato. Riguardo alle analisi globali, uno dei principali strumenti per la valutazione della risposta sismica è rappresentato dall’analisi statica non lineare, chiamata anche analisi pushover, la quale abbina accuratezza dei risultati ad un non eccessivo tempo di calcolo. Tuttavia, nelle strutture più irregolari, l’utilizzo degli approcci canonici – che richiedono la lettura degli spostamenti solo di alcune parti del fabbricato – può portare a risultati completamente inesatti, sia a causa dell’insorgenza dei meccanismi locali di collasso che alla differente risposta della costruzione in relazione alle sue capacità duttili a livello locale. Quest’ultimo aspetto compete al tracciamento della curva di capacità della struttura che avviene, generalmente, considerando un unico punto di controllo: se questo si sposta poco, relativamente breve sarà il ramo della curva elasto-plastico dell’oscillatore equivalente; e viceversa. È per tale ragione che si è sviluppata una metodologia consistente nel considerare diversi punti di controllo, non scelti a priori, ma suggeriti dallo stato di danneggiamento individuato dalle simulazioni numeriche. All’interno della metodologia proposta, è stata definito un nuovo strumento grafico di rappresentazione degli spostamenti dei punti di controllo: l’evoluzione del danno è mostrata utilizzando delle sfere, i cui raggi sono proporzionali agli spostamenti rilevati ed il cui baricentro ha le stesse coordinate del punto di controllo che rappresenta. Le dimensioni delle sfere possono fornire informazioni sul danno occorso e sulla posizione dei punti deboli della struttura investigata, diventando così uno strumento utile per orientare le decisioni sulla tecnica di rinforzo strutturale più adeguata. La validazione della metodologia proposta è avvenuta confrontando – per un caso studio reale consistente in una costruzione di forma triangolare realizzata esclusivamente in muratura – i valori di accelerazione spettrale ottenuti mediante tutte le tipologie di approcci impiegati: dall’individuazione del moltiplicatore dei carichi mediante il teorema cinematico dell’analisi limite, applicato sul meccanismo di collasso fuori piano ritenuto più significativo, all’analisi dinamica non lineare eseguita prendendo in considerazione un accelerogramma artificiale spettro-compatibile, passando per la già citata analisi statica non lineare. I risultati mostrano una comparabilità di valori per gli approcci numerici evidenziando, invece, una discrepanza con quelli analitici a causa di diversi fattori, fra cui la non-raffinatezza dei metodi semplificati. Tuttavia, si sono dedotte informazioni dettagliate sul comportamento strutturale generale dell’edificio, nonché sulla sua sicurezza sismica. Il sommario della tesi comprende quanto segue: Capitolo 1 – Introduzione (argomenti trattati dalla tesi, revisione della letteratura, obiettivi e campo di applicazione); Capitolo 2 – illustra alcune applicazioni effettuate mediante le trattazioni analitiche discusse nello studio dello stato dell’arte; Capitolo 3 – riporta le investigazioni sismiche di alcuni casi studio basate sulla modellazione a telaio equivalente, con un’ultima parte dedicata all’utilizzo di tale strategia di modellazione per le analisi di vulnerabilità su scala territoriale attraverso l’utilizzo delle schede CARTIS-ReLUIS; Capitolo 4 – riporta le analisi numeriche basate sull’approccio FEM e la metodologia pushover a punti di controllo multipli messa a punto per l’analisi delle costruzioni con geometria irregolare in pianta; Note conclusive – presenta le conclusioni più importanti a cui si è giunti attraverso questa tesi, tra cui alcune tabelle utili ad orientare il professionista verso la scelta della strategia di valutazione più indicata per il particolare caso studio da analizzare. Masonry buildings are the main part of the building heritage and their preservation has a primarily social and cultural role. Many of these buildings – such as churches, palaces, castles, and towers – are recognizable and representative symbols of their cities. Therefore, practitioners and researchers are interested in defining useful tools for the evaluation of the seismic vulnerability of historic masonry buildings. Various methods are currently being used for the seismic assessment of masonry artifacts, as well as several strategies for simulating the mechanical behavior of materials being available. The introduction of numerical calculation has led to new analysis models, which support the graphical methods used for evaluating the static safety of arches, vaults, and domes. This thesis aims to evaluate the behavior of historic masonry structures by using some of the different approaches currently used and validated by the scientific community. The studies start from the analysis of some typical elements of a building, such as vaults and walls. Afterwards, global analyses are implemented with different modeling strategies. Regarding the local analyses: the investigations on compound vaults – namely those originating from the intersection at right angles of two barrel vaults – are carried out in a static framework by applying the membrane theory; while the out-of-plane response of masonry walls is evaluated in order to highlight their contribution in the overall seismic response of the building. Among the global analyses, the non-linear static analysis – also called pushover analysis – is one of the main tools for the evaluation of the seismic response of a building because it combines results accuracy with a reduced computational burden. However, the use of canonical approaches - which require the reading of the displacements of only some building points - can lead to inaccurate results in the most irregular structures. This is due both to the onset of local collapse mechanisms and to the different building response concerning its local ductile capabilities. These aspects are related to the capacity curve of the structure, which plots the displacements of a single control point: a short elastoplastic branch of the bilinear curve in the case of small displacements; and vice-versa. For this reason, a coupled numerical-geometrical methodology – to represent the results arising from pushover analysis – is developed by considering an appropriate number of control points, not set a priori but suggested by the state of damage detected through numerical simulations. A new graphic tool is defined to represent the displacements of the control points, and the damage evolution is shown by using spheres in which their radiuses are proportional to displacements detected, whereas each centroid has the same coordinates as the control point which it represents. The spheres’ dimensions can provide information about the damage occurred and the position of weak points of the investigated structure, so becoming a useful tool to orientate decisions about structural strengthening technique. In order to validate the proposed methodology, a comparison between the spectral acceleration values obtained through all approaches used is carried out, taking into account a real case study consisting of a triangular construction entirely made in masonry. These accelerations are based on:  the load multiplier obtained from the most significant out-of-plane collapse mechanism is defined by means of the kinematic theorem of the limit analysis;  the nonlinear dynamic analysis performed by considering an artificial spectrum-compatible accelerogram;  the above nonlinear static analysis. The results showed comparable values for numerical approaches, highlighting a discrepancy instead with the analytical ones due to various factors, including the non-refinement of simplified methods. However, detailed information on the structural behavior of the building, as well as its seismic safety, are drawn clearly. The (summary) thesis comprises the following: Chapter 1 - Introduction (thesis topics, literature review, aims and scope); Chapter 2 - illustrates some analytical applications on compound vaults and out-of-plane mechanisms of masonry façades; Chapter 3 - reports the seismic investigations of some case studies based on equivalent frame modeling, with the last part dedicated to the use of this modeling strategy in the seismic vulnerability assessment at the territorial scale by using of CARTIS-ReLUIS forms; Chapter 4 - reports the numerical analyses based on the FEM approach and the multi-control point pushover methodology developed to assess irregular buildings; Concluding remarks - presents the most important conclusions reached through this thesis, including some useful tables to guide the practitioner towards the choice of the most suitable evaluation strategy for a particular case study.
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    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.