Tesi di Dottorato
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Item Analisi del comportamento non-lineare dei materiali compositi con microstruttura periodica(2009) Sgambittera, Girolamo; Olivito, Renato Sante; Bruno, Domenico; Greco, FabrizioIn the present thesis the macroscopic non-linear behavior of composite materials with a periodic and heterogeneous microstructure is studied. There are many different kinds of phenomena that produce non-linear effects in composite materials, for example intralaminar damage, delamination and microbucking in fiber reinforced composite or micro-cracking in cellular materials. In this work attention is devoted to the mechanical modeling of nonlinear phenomena associated to the presence of micro-cracks in the context of linear elasticity and of microscopic instabilities in the framework of the finite strain theory. Applications have been developed with reference to microstructures of cellular type and with embedded inclusions. The thesis is structured according to the following chapters: -In the first chapter the fundamental concepts of the finite strains theory are recalled. The constitutive relations associated to a class of conjugate stress-strain pairs are introduced. The basic expressions of the incremental constitutive laws are shown with special reference to incrementally linear constitutive laws. Finally the stability and the uniqueness of the equilibrium solution are analyzed. -In the second chapter, after an introduction about the homogenization techniques, the micro and macro stability phenomena occurring in composite materials with a periodic microstructure are studied from a theoretical point of view in the context of the finite strains theory. The formulation starts from a variational formulation of the problem. Novel macroscopic measures of micro-structural stability are introduced corresponding to the positive definiteness of the homogenized moduli tensors relative to a class of conjugate stress-strain pairs and their effectiveness to obtain a conservative prediction of the microscopic primary instability load is pointed out. Analysis of these stability phenomena plays a fundamental role because often the collapse of composite materials with periodic microstructure is related to microstructural instabilities. In addition the microscopic stability analysis establishes the region of validity of the standard homogenization procedure based on the unit cell procedure. -In the third chapter, in the context of the small strains theory, non-linear phenomena are presented with reference to composite materials with a porous microstructure containing micro-cracks spreading from the voids. The fundamental techniques of homogenization are applied in conjunction with fracture mechanics theory and interface models. The energy release rate is evaluated through the J-integral technique. -In the fourth chapter some numerical applications carried out by means of a one-way coupled finite element code, are proposed. In the first section the numerical results will be introduced with reference to the theoretical aspects developed in the second chapter. Numerical analyses are addressed to composite materials with a periodic microstructure, namely a porous microstructure and a particle-reinforced microstructure. The adopted constitutive law is hyperelastic. Periodic boundary conditions will be used for the microstructure, and uniaxial and equibiaxial loading conditions are considered. Numerical analyses are able to show the exact region of microscopic stability, obtained by taking into account all the microstructural details, and the region of macroscopic stability, determinate by studying homogenized material properties. To elaborate macroscopic criteria able to give a conservative prediction of the microstructural stability, different measures of macroscopic instability are introduced with reference to work conjugate strain-stress measures. In the second section of this chapter a numerical analyses with reference to the micromechanical model proposed in the third chapter is developed. In this case the microstructure adopted for the composite materials is a cellular microstructure in which there is the presence of two micro-cracks advancing symmetrically from the void. The microstructure is subjected to three different boundary conditions namely respectively: linear displacements, periodic fluctuations and antiperiodic tractions and uniform tractions. The objective of this section is to verify the validity of the homogenization technique in the prediction of micro-crack evolution phenomena, for composites with locally periodic microstructure. The energy release rate obtained through the micromechanical model will be compared with a 2D composite structure composed by a regular arrangement of 5x5 unit cells. The composite structure is subjected to two different boundary conditions: the former is associated with the absence of contact between the surfaces of the micro-cracks, on the contrary in the latter case there is the presence of the contact. This type of comparison allows to investigate the accuracy of the proposed procedure in presence of macroscopic tension and strain gradients.Item Membrane crystallization for recovery of valuable compounds from waste streams(2016-02-26) Quist-Jensen, Cejna Anna; Drioli, Enrico; Macedonio, Francesca; Molinari, RaffaeleSustainable development and Process intensification strategy are guidelines for industrial processes in perspective. It is becoming more and more common that industry wants to fully exploit their resources due to environmental regulations, economic gain, sustainable standpoint, etc. In this perspective, waste streams have to be turned into resources in the most environmental friendly, economic and sustainable way. Membrane Engineering is already a key-figure to realize this objective. Novel membrane technologies such as membrane distillation (MD), membrane crystallization (MCr), pressure retarded osmosis (PRO), reverse electrodialysis (RED) and forward osmosis (FO), are evolving and are being suggested for a better exploitation of waste streams. This Ph.D. study focusses, particular, on Membrane crystallization (MCr), which is a novel technology for simultaneously production of water and minerals. It has several advantages with respect to conventional crystallizers in terms of purity, controlled kinetics and crystal morphology. Moreover, MCr is able to treat high concentration solutions, which are challenging for other traditional membrane operations. The current Ph.D. work emphasizes on various aspects of membrane crystallization for approaching zero-liquid discharge in industrial processes. Improved membranes, specifically developed for MCr applications, have to be manufactured. In this study, preliminary suggestions on membrane features are given for the requirements in MCr. Lab-made PVDF membranes with different characteristics have been tested and evaluated for their performance in MCr. This study, suggests that membranes with symmetric sponge layer structure and low thickness are favorable. Membrane of asymmetric structure with many macrovoids seems more pronounced to suffer from wetting. Moreover, it has been shown that, membrane crystallization is able to treat several kinds of feed solutions including RO brine, produced water and wastewater containing high amounts of sodium sulfate. The recovered crystals exhibit high purity, good size distribution and controlled growth. Na2SO4 can be recovered as different polymorphs and in this study it has been crystallized in the anhydrous form (Thenardite). Moreover, the process has shown excellent stability in terms of transmembrane flux and maintenance of hydrophobicity of the membrane. In some cases the treatment has been continued for more than 90 hours by only slight cleaning with distillate water. Membrane crystallization, in the direct-contact membrane distillation configuration, can normally treat solutions with very high concentrations. However, its limitations in the recovery of lithium from single salt solutions have been highlighted in this study. Vapor pressure, due to increase in concentration, is reduced significant, that it is not possible to reach LiCl saturation by this configuration. Likewise, combined direct-contact and osmotic distillation configuration have not been able to increase the driving force enough in order to exceed saturation. Instead vacuum membrane distillation has been introduced to eliminate the osmotic phenomena. This configuration has been able to recover LiCl in two different polymorph structures depending on the utilized operative conditions. Furthermore, integrated membrane system, including membrane crystallization, has shown excellent capability to treat orange juice. The quality of the juice has been maintained through ultrafiltration, membrane distillation and membrane crystallization treatment. In this study, the MD/MCr feed temperature is kept below 30 °C causing a relatively low flux. However, it has still been possible to reach from a concentration of 9 °brix to 65 °brix using MD/MCr. The advantages of MD/MCr with respect to isothermal osmotic membrane distillation configuration, is the elimination of the reconcentration stages of the draw solution. All the carried out case studies show that MD/MCr is able to reduce the volume of the waste stream significantly. The obtained results might be used as guidelines for practical application. Moreover, the low temperatures and atmospheric pressures utilized, makes it possible in real industrial processes to use waste or low-grade heat. Unlike other processes, MCr is able to produce two high quality products (i.e. water and salts) and will therefore not produce any additional waste. Hereby, the extended treatment by means of MCr will only positively influence the overall “sustainability” of the entire industrial process.Item Chemical looping desulphurization: model and applications to power systems(2016-02-26) Settino, Jessica; Molinari, Raffaele; Amelio, MarioI processi di assorbimento, sia sici che chimici a base di ammine, sono attualmente utilizzati per rimuovere e cacemente i composti dello zolfo. Nonostante l'eccellente desolforazione, questa strategia e termicamente ine ciente, in quanto richiede gas a bassa temperatura. Scopo di questo lavoro e quello di analizzare soluzioni alternative che operino a temperature pi u elevate. A tale scopo, e stato analizzato il processo del chemical looping. Si tratta di una nuova tecnologia, in cui un materiale sorbente, in contatto con il gas combustibile grezzo, viene convertito nel suo solfuro e poi rigenerato cos da ricominciare il ciclo. Il sistema e costituito da due reattori: uno per la rigenerazione e l'altro per la desolforazione. Un modello matematico di tale sistema e stato sviluppato con il software Athena Visual Studio ed i suoi risultati confrontati con quelli ottenuti dal modello proposto dal National Energy Technology Laboratory, validati sulla base di dati sperimentali. Nella fase successiva, il sistema modellato e stato applicato a tre casi studio di interesse industriale: per la produzione di energia elettrica negli impianti a ciclo combinato con gassi cazione integrata, nei processi di metanazione, nei processi per la sintesi del metanolo. Mediante simulazioni, condotte con i software commerciali Thermo ex e UniSim Design, sono stati studiati gli e etti della desolforazione a caldo sulle prestazioni dei diversi sistemi.Item Polymeric materials for biomedical applications: synthesis and characterisation(2010-12-02) Parisi, Ortensia Ilaria; Puoci, Francesco; De Cindio, BrunoItem Simplified Methods for Dynamic Analysis of Structures under Blast Loading(2007) Campidelli, Manuel; Viola, Erasmo; Bruno, DomenicoThe increasing threat of extremely severe loading conditions caused by a number of explosive sources made engineers and scientists developing, during the last half century, several methods of analysis and design of blast–resistant structures. Simple, intermediate, and advanced computational approaches have been adopted, requiring increasing computational resources. These efforts led to the publication of several manuals and guidelines for the analysis and design of blast–resistant reinforced concrete and steel structures, mostly based on simple considerations derived from Single Degree of Freedom (SDOF) models. Although the development of future guidelines based on advanced numerical techniques is desirable, typical design activities cannot be effectively carried out by applying complex methods, because of their large demand of resources. Therefore the necessity to develop simplified, low time consuming, methods of analysis, capable of supporting a daily design activity and, at the same time, takeing into account issues usually neglected, such as a strong non linear material behavior and the influence of the strain rate caused by a blast load on the structural response. The development of such design tools is the object of this study. The first part of this thesis deals with the influence of the blast load shape on the dynamic response of an undamped linear elastic oscillator. Response spectrum and pressure–impulse diagrams are shown for several shape parameters, and a sensitivity analysis of the results with respect to the computational parameters is also presented. A method validation is carried out via genetic algorithms, through a careful calibration of all the genetic parameters, such as crossover fraction and number of elite elements. Non linear material modeling and strain rate dependent constitutive laws are objects of the second part of this dissertation. A non linear oscillator made of displacement, velocity, and acceleration dependent springs and dampers, under an arbitrary dynamic load, is proposed. Spring and damper constitutive laws have no restrictions as well as the load–time function, and the dynamic analysis is accomplished by a piecewise linear approximation of any input function. Numerical problems are dealt with by applying the Newton–Raphson method, in such a way that enables the error range to be estabiv lished “a priori”. Any possible drawback of this method is carefully avoided, and a quadratic speed of convergence is always ensured. Since the model provides velocity dependent springs, strain rate effects of blast loads on the structural response are taken into account by including strain rate dependent constitutive laws within the problem definition.Item Gallium coordination compounds as multifunctional materials. Synthesis, characterization and properties(2006-11-30) De Franco, Irene; Ghedini, Mauro; Longeri, MarcelloItem 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, DomenicoRecent 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.Item Study of innovative electrochromic films(2009) Saladino, Rosa; Chidichimo, G.; Versace, C.Item Impiego di scarti industriali in materiali di natura ceramica destinati all'edilizia urbana(2008-10-28) Sansosti, Rosa; Nastro, Alfonso; D'elia, SergioItem Preparation and characterization of mesostructured functional materials with different morphologies(2008-11-17) Aiello, Daniela; Aiello, Rosario; Testa, Flaviano; Molinari, RaffaeleMesoporous materials with their good surface and structural properties and versatility can be synthesized in different morphologies (thin films, fibers, membranes, etc.) and represent excellent host matrices, highly functional and with great potentials for advanced applications. In this research work, mesoporous powders and thin films have been successfully achieved and then functionalized with special guest molecules such as organic molecules, organometallic complexes, fluorescent dyes, all having specific and interesting properties. Mesoporous matrices have been prepared by sol-gel chemistry with different mesostructures and high order degree. Chemical modifications approaches (post-synthesis grafting and one-pot synthesis) applied to porous supports highly organized have allowed to product a new class of functional materials, particularly interesting for various applications (opto-electronic, photovoltaic materials, etc.). Surface and structural characterization techniques (FTIR, UV-Vis, fluorescence, ellipsometry spectroscopies, electron transmission microscopy, XRD diffraction and porosimetry analysis) have allowed to investigate the effects of the introduction of guest species inside mesoporous matrices and to identify noteworthy changes about organization and mesostructures. Results show that mesoporous materials, both as powders and thin films, do not suffer significant reductions of surface (surface area, pore volume and diameter) and structural properties (order degree, mesostructure organization, stability) after functionalization process, representing confined environment well adapted to various guest species, with the advantage to increase their activity and physic-chemical properties. In particular, guest species are well trapped into rigid porous matrices with an increase of their functional properties and inorganic network stability. All results have demonstrated that the high structural homogeneity, the control over surface and morphological properties and also the possibility to host different molecules permit to project and engineer high potential technological materials for applications in optic and electro-optic fields.