Development and testing of a Fourier trasform based experimental system for Dynamic Mechanical Thermal Analysis tests

Altough nowadays DMTA is one of the most used techniques to characterize polymers thermo-mechanical behaviour, it is only effective for small amplitude oscillatory tests and limited to a single frequency analysis (linear regime). In this thesis work a Fourier transform based experimental system has proven to give hint on structural and chemical changes in specimens during large amplitude oscillatory tests exploiting multi frequency spectral analysis turning out in a more sensitive tool than classical linear approach. The test campaign has been focused on three test typologies: Strain sweep tests, Damage investigation and temperature sweep tests.

Dynamic mechanical-thermal, microstructural and mechanical analysis of ultra-light polymer-metal composites: influence of forming

A really particular and innovative metal-polymer sandwich material is Hybrix. Hybrix is a product developed and manufactured by Lamera AB, Gothenburg, Sweden. This innovative hybrid material is composed by two relatively thin metal layers if compared to the core thickness. The most used metals are aluminum and stainless steel and are separated by a core of nylon fibres oriented perpendicularly to the metal plates. The core is then completed by adhesive layers applied at the PA66-metal interface that once cured maintain the nylon fibres in position. This special material is very light and formable. Moreover Hybrix, depending on the specific metal which is used, can achieve a good corrosion resistance and it can be cut and punched easily. Hybrix architecture itself provides extremely good bending stiffness, damping properties, insulation capability, etc., which again, of course, change in magnitude depending in the metal alloy which is used, its thickness and core thickness. For these reasons nowadays it shows potential for all the applications which have the above mentioned characteristic as a requirement. Finally Hybrix can be processed with tools used in regular metal sheet industry and can be handled as solid metal sheets. In this master thesis project, pre-formed parts of Hybrix were studied and characterized. Previous work on Hybrix was focused on analyze its market potential and different adhesive to be used in the core. All the tests were carried out on flat unformed specimens. However, in order to have a complete description of this material also the effect of the forming process must be taken into account. Thus the main activities of the present master thesis are the following: Dynamic Mechanical-Thermal Analysis (DMTA) on unformed Hybrix samples of different thickness and on pre-strained Hybrix samples, pure epoxy adhesive samples analysis and finally moisture effects evaluation on Hybrix composite structure.

Mechanical analysis and modeling of structural archetypes

This is a research B for the University of Bologna. The course is the civil engineering LAUREA MAGISTRALE at UNIBO. The main purpose of this research is to promote another way of explaining, analyzing and presenting some civil engineering aspects to the students worldwide by theory, modeling and photos. The basic idea is divided into three steps. The first one is to present and analyze the theoretical parts. So a detailed analysis of the theory combined with theorems, explanations, examples and exercises will cover this step. At the second, a model will make clear all these parts that were discussed in the theory by showing how the structures work or fail. The modeling is able to present the behavior of many elements, in scale which we use in the real structures. After these two steps an interesting exhibition of photos from the real world with comments will give the chance to the engineers to observe all these theoretical and modeling-laboratory staff in many different cases. For example many civil engineers in the world may know about the air pressure on the structures but many of them have never seen the extraordinary behavior of the bridge of Tacoma ‘dancing with the air’. At this point I would like to say that what I have done is not a book, but a research of how this ‘3 step’ presentation or explanation of some mechanical characteristics could be helpful. I know that my research is something different and new and in my opinion is very important because it helps students to go deeper in the science and also gives new ideas and inspirations. This way of teaching can be used at all lessons especially at the technical. Hope that one day all the books will adopt this kind of presentation.

Analisi sperimentale degli effetti di temperature elevate sul comportamento strutturale dei rinforzi in FRP

In questa tesi si è voluta porre l’attenzione sulla suscettibilità alle alte temperature delle resine che li compongono. Lo studio del comportamento alle alte temperature delle resine utilizzate per l’applicazione dei materiali compositi è risultato un campo di studio ancora non completamente sviluppato, nel quale c’è ancora necessità di ricerche per meglio chiarire alcuni aspetti del comportamento. L’analisi di questi materiali si sviluppa partendo dal contesto storico, e procedendo successivamente ad una accurata classificazione delle varie tipologie di materiali compositi soffermandosi sull’ utilizzo nel campo civile degli FRP (Fiber Reinforced Polymer) e mettendone in risalto le proprietà meccaniche. Considerata l’influenza che il comportamento delle resine riveste nel comportamento alle alte temperature dei materiali compositi si è, per questi elementi, eseguita una classificazione in base alle loro proprietà fisico-chimiche e ne sono state esaminate le principali proprietà meccaniche e termiche quali il modulo elastico, la tensione di rottura, la temperatura di transizione vetrosa e il fenomeno del creep. Sono state successivamente eseguite delle prove sperimentali, effettuate presso il Laboratorio Resistenza Materiali e presso il Laboratorio del Dipartimento di Chimica Applicata e Scienza dei Materiali, su dei provini confezionati con otto differenti resine epossidiche. Per valutarne il comportamento alle alte temperature, le indagini sperimentali hanno valutato dapprima le temperature di transizione vetrosa delle resine in questione e, in seguito, le loro caratteristiche meccaniche. Dalla correlazione dei dati rilevati si sono cercati possibili legami tra le caratteristiche meccaniche e le proprietà termiche delle resine. Si sono infine valutati gli aspetti dell’applicazione degli FRP che possano influire sul comportamento del materiale composito soggetto alle alte temperature valutando delle possibili precauzioni che possano essere considerate in fase progettuale.

Studio dell’adenina come indurente per resine epossidiche e produzione di materiali compositi da fonti rinnovabili

A causa di una maggiore preoccupazione ambientale, la ricerca si sta muovendo verso lo sviluppo di nuovi prodotti bio-based. Il progetto è stato incentrato sulla possibilità di produrre materiali compositi utilizzando un sistema resina-indurente interamente ottenuto da fonti rinnovabili con fibre naturali totalmente green e sintetiche. In particolare, è stata studiata la possibilità di utilizzare l’adenina, una molecola atossica e derivante da fonte rinnovabile, per la produzione di compositi con fibre di lino, juta, carbonio vergine e riciclato con resina epossidica da fonte bio. In particolare è stato ottimizzato il ciclo di cura e sono state caratterizzate le proprietà termiche e meccaniche dei materiali prodotti mediante analisi DSC (Differential Scanning Calorimetry), DMA (Dynamic Mechanical Analysis) e TGA (Termo-Gravimetric-Analysis). Per comprendere meglio la reticolazione delle resine epossidiche, si è studiato il meccanismo di reazione tra l’adenina e un precursore epossidico attraverso spettroscopia 1H-NMR (Nuclear Magnetic Resonance).

Damping and Thermomechanical behaviour of CFRP laminates modified with rubbery nanofibers

Nanofibrous membranes are a promising material for tailoring the properties of laminated CFRP composites by embedding them into the structure. This project aimed to understand the effect of number, position and thickness of nanofibrous modifications specifically on the damping behaviour of the resulting nano-modified CFRP composite with an epoxy matrix. An improvement of damping capacity is expected to improve a composites lifetime and fatigue resistance by prohibiting the formation of microcracks and consequently hindering delamination, it also promises a rise in comfort for a range of final products by intermission of vibration propagation and therefore diminution of noise. Electrospinning was the technique employed to produce nanofibrous membranes from a blend of polymeric solutions. SEM, WAXS and DSC were utilised to evaluate the quality of the obtained membranes before they were introduced, following a specific stacking sequence, in the production process of the laminate. A suitable curing cycle in an autoclave was applied to mend the modifications together with the matrix material, ensuring full crosslinking of the matrix and therefore finalising the production process. DMA was exercised in order to gain an understanding about the effects of the different modifications on the properties of the composite. During this investigation it became apparent that a high number of modifications of laminate CFRP composites, with an epoxy matrix, with thick rubbery nanofibrous membranes has a positive effect on the damping capacity and the temperature range the effect applies in. A suggestion for subsequent studies as well as a recommendation for the production of nano-modified CFRP structures is included at the end of this document.

Synthesis of Na-PSS geopolymers and analysis of the H2O/Na2O molar ratio influence over their mechanical properties

Geopolymers are solid aluminosilicate material made by mixing an activating solution and a solid precursor. This work studied the mechanisms of synthesis of metakaolin-based geopolymers and the influence of water content, described by the molar ratio H2O/Na2O, on the final product. The samples were tested using a Uniaxial Compressive Test (UCT) to define their compressive resistance. Two geopolymers series were synthetized and let them rest for 7- days and 28-days, each of them composed by six different sets. 7-day rest series showed that water addition had no relevant effect over its resistance while the 28-day rest series almost doubled the compressive resistance, although those with the highest H2O/Na2O molar ratio showed instead a drastic reduction. Two other series were synthesized by adding silt aggregate, a waste material obtained in the production of aggregate for concrete, corresponding to 10wt% and 20wt%of the metakaolin used. After 28 days of aging, these samples were tested via UCT to measure the variation of the compressive resistance after the silt addition. The aggregate has disruptive effects over the compressive resistance, but the 20wt% samples achieved a higher compressive resistance. Samples with highest and lowest compressive resistance have been chosen to carry out an XRD analysis. In all the samples it has been recognized the presence of Anatase (TiO2), a titanium oxide found in the metakaolin and Thermonatrite, a hydrated sodium carbonate [Na2CO3 • (H2O)]. Scanning Electron Microscopy was carried out on the samples with the highest compressive resistance and showed that the samples with lower water content developed a homogeneous geopolymeric texture, while those with higher water content showed instead a spongy-like texture and a higher air or pore solution bubbles presence. Silt/geopolymer composites showed a fracture system developing across the interstitial transition zone between the geopolymer matrix and the aggregate particle.

Synthesis, characterization and conformational studies of arylbenzylmaleimides

From the discoveries of Pasteur, stereochemistry has played an increasingly important role in the chemical sciences. In particular conformational study of molecules with axial chirality is object of intense research. Through Dynamic-NMR analysis and simulation of the spectra, the energy rotational barriers value of conformers are obtained. When this barrier is high sufficiently, atropisomeric stable compounds can be reached. They can be separated and used in stereo-synthesis and in packing processes. 3,4-bis-aryl maleimides, in which the aromatic groups are sufficiently bulky, generate atropisomeric stable configurations, that can be isolated at room temperature. The assignment of absolute configurations is performed through ECD analysis and comparison with computational calculations. The biological activities of maleimide derivatives widen the field of atropisomers application also in biological systems.

Misura di adesione di film polimerici su tappi di sughero

L’industria enologica oggigiorno propone numerosi modi per sigillare le bottiglie in alternativa ai tappi di sughero: tappi sintetici, tappi a vite, tappi a corona. Tuttavia si conosce la preferenza del consumatore medio per il sughero. I motivi sono molteplici: il sughero restituisce un'immagine di qualità del vino, esprime la tradizione ed è un materiale vivo e naturale, così come il vino. La naturalità del sughero però comporta dei rischi. Ad esempio si stima in 10 miliardi di dollari a livello mondiale la perdita economica dovuta al "sentore di tappo" o "cork taint". Recenti ricerche attribuiscono la responsabilità di tale difetto a composti chimici che migrano dal sughero al vino. Innovazione e tradizione si sono incontrate nell'ideazione della "tecnologia a membrana" o "membrane technology". Essa consiste nell'isolare le teste dei tappi con delle pellicole multistrato per non far diffondere nel vino alcun tipo di agente contaminante. In questo studio si espone in dettaglio un metodo per misurare l'adesione di film multistrato polimerici – strato esterno in polietilene – su tappi di sughero monopezzo. I tappi già rivestiti sono stati forniti da un'azienda del settore senza alcuna specifica di fabbricazione. In particolare la fase di trazione è stata ideata in analogia alla prova meccanica nota come pull-off test, usata nell'ambito dell'ingegneria edile. La bontà dell'adesione è indagata dal punto di vista della fisica dei materiali. Per le misurazioni si è utilizzato il Dynamic Mechanical Analyser (DMA), modello Q800, che si trova presso il laboratorio CIRI-MAM ospitato dal Dipartimento di Fisica e Astronomia di Bologna.

The influence of infill walls on seismic behavior of a historical building in Shanghai

Seismic assessment and seismic strengthening are the key issues need to be figured out during the process of protection and reusing of historical buildings. In this thesis the seismic behaviors of the hinged steel structure, a typical structure of historical buildings, i.e. hinged steel frames in Shanghai, China, were studied based on experimental investigations and theoretic analysis. How the non-structural members worked with the steel frames was analyzed thoroughly. Firstly, two 1/4 scale hinged steel frames were constructed based on the structural system of Bund 18, a historical building in Shanghai: M1 model without infill walls, M2 model with infill walls, and tested under the horizontal cyclic loads to investigate their seismic behavior. The Shaking Table Test and its results indicated that the seismic behavior of the hinged steel frames could be improved significantly with the help of non-structural members, i.e., surrounding elements outside the hinged steel frames and infilled walls. To specify, the columns are covered with bricks, they consist of I shape formed steel sections and steel plates, which are clenched together. The steel beams are connected to the steel column by steel angle, thus the structure should be considered as a hinged frame. And the infilled wall acted as a compression diagonal strut to withstand the horizontal load, therefore, the seismic capacity and stiffness of the hinged steel frames with infilled walls could be estimated by using the equivalent compression diagonal strut model. A SAP model has been constructed with the objective to perform a dynamic nonlinear analysis. The obtained results were compared with the results obtained from Shaking Table Test. The Test Results have validated that the influence of infill walls on seismic behavior can be estimated by using the equivalent diagonal strut model.

Substructuring approache in state space models for dynamic system parameters identification

In the recent decade, the request for structural health monitoring expertise increased exponentially in the United States. The aging issues that most of the transportation structures are experiencing can put in serious jeopardy the economic system of a region as well as of a country. At the same time, the monitoring of structures is a central topic of discussion in Europe, where the preservation of historical buildings has been addressed over the last four centuries. More recently, various concerns arose about security performance of civil structures after tragic events such the 9/11 or the 2011 Japan earthquake: engineers looks for a design able to resist exceptional loadings due to earthquakes, hurricanes and terrorist attacks. After events of such a kind, the assessment of the remaining life of the structure is at least as important as the initial performance design. Consequently, it appears very clear that the introduction of reliable and accessible damage assessment techniques is crucial for the localization of issues and for a correct and immediate rehabilitation. The System Identification is a branch of the more general Control Theory. In Civil Engineering, this field addresses the techniques needed to find mechanical characteristics as the stiffness or the mass starting from the signals captured by sensors. The objective of the Dynamic Structural Identification (DSI) is to define, starting from experimental measurements, the modal fundamental parameters of a generic structure in order to characterize, via a mathematical model, the dynamic behavior. The knowledge of these parameters is helpful in the Model Updating procedure, that permits to define corrected theoretical models through experimental validation. The main aim of this technique is to minimize the differences between the theoretical model results and in situ measurements of dynamic data. Therefore, the new model becomes a very effective control practice when it comes to rehabilitation of structures or damage assessment. The instrumentation of a whole structure is an unfeasible procedure sometimes because of the high cost involved or, sometimes, because it’s not possible to physically reach each point of the structure. Therefore, numerous scholars have been trying to address this problem. In general two are the main involved methods. Since the limited number of sensors, in a first case, it’s possible to gather time histories only for some locations, then to move the instruments to another location and replay the procedure. Otherwise, if the number of sensors is enough and the structure does not present a complicate geometry, it’s usually sufficient to detect only the principal first modes. This two problems are well presented in the works of Balsamo [1] for the application to a simple system and Jun [2] for the analysis of system with a limited number of sensors. Once the system identification has been carried, it is possible to access the actual system characteristics. A frequent practice is to create an updated FEM model and assess whether the structure fulfills or not the requested functions. Once again the objective of this work is to present a general methodology to analyze big structure using a limited number of instrumentation and at the same time, obtaining the most information about an identified structure without recalling methodologies of difficult interpretation. A general framework of the state space identification procedure via OKID/ERA algorithm is developed and implemented in Matlab. Then, some simple examples are proposed to highlight the principal characteristics and advantage of this methodology. A new algebraic manipulation for a prolific use of substructuring results is developed and implemented.